Under Graduate Dissertation Work

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DEVELOPMENT OF NON-AQUEOUS

ASYMMETRIC HYBRID SUPERCAPACITORS

BASED ON Li-ION INTERCALATED

COMPOUNDS

A PROJECT REPORT

Submitted by

A. NAKKIRAN

in partial fulfillment for the award of the degree

of

BACHELOR OF TECHNOLOGY

in

CHEMICAL AND ELECTROCHEMICAL ENGINEERING

CENTRAL ELECTROCHEMICAL RESEARCH INSTITUTE

KARAIKUDI – 630 006

ANNA UNIVERSITY :: CHENNAI 600 025

MAY 2007

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ANNA UNIVERSITY :: CHENNAI 600 025

BONAFIDE CERTIFICATE

Certified that this project report titled “DEVELOPMENT OF NON-

AQUEOUS ASYMMETRIC HYBRID SUPERCAPACITORS

BASED ON Li-ION INTERCALATED COMPOUNDS”, is the

bonafide work of “A. NAKKIRAN”, who carried out the project

work under my supervision.

V.Nandakumar Dr.D.Kalpana PROJECT CO-ORDINATOR PROJECT SUPERVISOR Dean (Academics) Scientist Centre For Education EEC Division CECRI CECRI Karaikudi Karaikudi

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ABSTRACT

Modern power sources concentrate on electric vehicle’s

applications. Supercapacitors are also finding wider applications in

that field too. In our present study, four hybrid supercapacitor cells

were fabricated using LiCo1-xAlxO2 (x = 0, 0.2, 0.4 and 0.6) as

cathodes respectively and carbon nano foam as anode. The electrolyte

here used was 1M LiClO4 in 50-50 volume mixture of Ethylene

carbonate and Propylene carbonate. Polypropylene is used as

separator. The cathode materials were synthesized using soft

combustion method from their respective nitrate precursors. Glycine

was used as fuel for the combustion reaction. The synthesized cathode

materials were characterized physically by Thermal analysis, XRD

and FTIR. The fabricated supercapacitors were characterized

electrochemically using cyclic voltammetry, impedance spectroscopy

and galvanostatic charge-discharge. The performance of the

supercapacitors was evaluated using the above characterizations. The

specific parameters of the supercapacitors were also calculated.

Among the four supercapacitors the one, which is having the

LiCo0.6Al0.4O2 composition as cathode, is having good capacitance

behaviour.

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TABLE OF CONTENTS

CHAPTER TITLE

PAGE NO

NO.

ABSTRACT 3

LIST OF TABLES v

LIST OF FIGURES vi

LIST OF ABBREVIATIONS ix

LIST OF SYMBOLS x

1. INTRODUCTION 15

1.1 Different types of energy storage devices 16

1.1.1 Conventional Capacitors 16

1.1.2 Batteries 18

1.1.2.1 Primary Batteries 19

1.1.2.2 Secondary Batteries 19

1.1.2.3 Emergence of Lithium Batteries 19

1.1.2.4 Cathode Materials for Li Ion Batteries 22

1.1.3 Supercapacitors 23

1.1.3.1 Advantages of Supercapacitors 29

1.1.3.2 Limitations of Supercapacitors 30

1.1.3.3 Electrodes for Supercapacitors 30

1.1.3.4 Classification of Supercapacitor 32

REFERENCES 37

2. LITERATURE REVIEW 41

2.1 Review on Carbon Electrodes 41

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2.2 Review on Electrolytes 44

2.3 Review on Lithium Compound Electrodes 47

2.4 Review on Hybrid Supercapacitors 49

REFERENCES 50

3. SCOPE AND OBJECTIVE 55

3.1 Advantages of Hybrid Supercapacitors 56

3.2 Advantages of LiCoO2 57

3.3 Advantages of CNF 58

3.4 Advantages of Non-Aqueous Electrolyte 59

REFERENCES 59

4. EXPERIMENTAL 61

4.1 Synthesis of Cathode Material 61

4.1.1 Existing Methodologies for the Synthesis of

Cathode materials 61

4.1.2 Need to Identify Suitable Synthesis Methods 44

4.1.3 Soft Combustion Method 62

4.1.4 Synthesis Procedure 65

4.1.5 Flow chart for synthesis 66

4.1.6 Annealing 67

4.2 Physical Characterization Techniques 67

4.2.1 Thermal Analysis 67

4.2.1.1 Thermo-Gravimetric Analysis 67

4.2.1.2 Differential Thermal Analysis 68

4.2.2 Fourier Transform Infra Red Spectroscopy 70

4.2.3 X-Ray Diffraction 72

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4.3 Cell Fabrication 74

4.3.1 Composition for Positive Electrode 74

4.3.2 Composition for Negative Electrode 75

4.3.3 Steps Involved in Cell Fabrication 75

4.4 Electrochemical Characterization Techniques 77

4.4.1 Cyclic Voltammetry 77

4.4.2 Electrochemical Impedance Spectroscopy 79

4.4.3 Galvanostatic Charge-Discharge Cycle Tests84

REFERENCES 84

5. RESULTS AND DISCUSSION 87

5.1 Thermal Analysis 87

5.1.1 Thermo-Gravimetric Analysis 87

5.1.2 Differential Thermal Analysis 88

5.2 FTIR Spectroscopy 88

5.3 X-Ray Diffraction 91

5.4 Cyclic Voltammetry 93

5.5 Electrochemical Impedance Spectroscopy 100

5.6 Galvanostatic Charge-Discharge Cycle Tests 102

6. CONCLUSIONS 110

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LIST OF TABLES

TABLE TITLE

PAGE NO.

NO.

1.1 Parametric comparison among capacitor, supercapacitor and batteries

4.1 Typical materials prepared by combustion method 64

4.2 Composition for synthesis of LiCo(1-x)AlxO2

(x = 0, 0.2, 0.4, 0.6) 66

5.1 Specific capacitance (F/g) values of LiCo(1-x)AlxO2

(x = 0, 0.2,0.4, 0.6) as calculated from cyclic

voltammograms 99

5.2 Solution resistance (Rs) and double layer capacitance

(Cdl) values of LiCo(1-x)AlxO2 (x = 0, 0.2, 0.4, 0.6)

as calculated from electrochemical

impedance spectrographs 102

5.3 Various parameters of supercapacitors fabricated using

the synthesized cathode materials as calculated

from galvanostatic charge-discharge tests 108

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LIST OF FIGURES

FIGURE TITLE

PAGE NO.

NO.

1.1 Construction of a conventional capacitor 17

1.2 Schematic representation of conventional capacitor 17

1.3 The formation of double layer during the charging 23

1.4 Ragone’s plot for different energy storage systems 24

1.5 Charge/Discharge profile comparison 27

1.6 Classification of supercapacitors 33

1.7 Charging at electrode surface 34

1.8 Ragone’s plot 36

3.1 Layered structure of LiCoO2 58

4.1 Diagram of Thermo Gravimetric Analyzer 68

4.2 Construction of Differential Thermal Analyzer 69

4.3 Layout of FTIR spectrometer 71

4.4 Binder, Spatula, Mortar and Pestle 75

4.5 A dried single electrode 76

4.6 Electrodes tied using nylon thread 77

4.7 A fully fabricated supercapacitor cell 77

4.8 Schematic explanation of a cyclic voltammetry

experiment in the absence of a redox couple 79

4.9 Magnitude of the resultant sine wave as a function of

frequency for a representative standard test cell with one

layer of Celgard 2500 80

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4.10 Phase difference between the incident and resultant wave

forms 80

4.11 The real portion of the resultant wave form as a function of

frequency 81

4.12 The imaginary portion of the resultant wave form as a

function of frequency 81

4.13 The resistance of the resultant waveform as a function 83

4.14 The capacitance of the resultant waveform as a function of

frequency 83

5.1 TGA curves of base LiCoO2 and aluminium doped LiCoO2 87

5.2 DTA curves of base LiCoO2 and aluminium doped LiCoO2 88

5.3 FTIR spectrum of LiCoO2 89

5.4 FTIR spectrum of LiCo0.8Al0.2O2 90



5.7 XRD spectra of cathode materials 92

5.8 XRD spectra of cathode materials (2θ range from 35 to 41) 92

5.9 CV of LiCoO2/CNF supercapacitor as fabricated 95

5.10 CV of LiCo0.8Al0.2O2/CNF supercapacitor as fabricated 96



5.13 CV of LiCoO2/CNF supercapacitor after 500

Charge/discharge cycles 97

5.14 CV of LiCo0.8Al0.2O2/CNF supercapacitor after 500

Charge/discharge cycles 98


charge/discharge cycles 98

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charge/discharge cycles 99

5.17 Electrochemical impedance spectroscopes of supercapacitors

before cycles 101

5.18 Electrochemical impedance spectroscopes of supercapacitors after 500 cycles

5.19 2nd charge/discharge cycle of LiCoO2/CNF supercapacitor 104

5.20 500th charge/discharge cycle of LiCoO2/CNF supercapacitor 104

5.21 2nd charge/discharge cycle of LiCo0.8Al0.2O2/CNF

supercapacitor 105

5.22 500th charge/discharge cycle of LiCo0.8Al0.2O2/CNF

supercapacitor 105


supercapacitor 106


supercapacitor 106


supercapacitor 107


supercapacitor 107

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LIST OF ABBREVIATIONS

EDLC - Electrochemical Double Layer Capacitor

DLC - Double Layer Capacitor

DC - Direct Current

RF - Radio Frequency

ESR - Equivalent Series Resistance

EC - Electrolytic Capacitor

CNF - Carbon Nano Foam

EV - Electric Vehicles

IR - Internal Resistance

TGA - Thermo-Gravimetric Analysis

DTA - Differential Thermal Analysis

FTIR - Fourier Transform Infra-Red

XRD - X-Ray Diffraction

SEM - Scanning Electron Microscope

IR - Infra-Red

NMP - n-Methyl Pyrrolidine

EC - Ethylene Carbonate

PC - Propylene Carbonate

EIS - Electrochemical Impedance Spectroscopy

CV - Cyclic Voltammetry/Voltammogram

AC - Activated Carbon

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LIST OF SYMBOLS

C - Capacitance

Q - Charge

V - Voltage or Volt

ε - Dielectric constant

εo - Permittivity of vacuum

A - Area of electrode or Absorbance or Ampere

D - Distance between electrodes 0C - Degree Celsius

µ - micro (10^-6)

F - Farad

W - Watt

h - Hour

k - Kilo (10^3)

g - Gram

tc - Charging time

td - Discharge time

m - Milli (10^-3) or Meter

Tm - Melting point

Tb - Boiling point

Tf - freezing point

G - Gibbs energy

T - Transmittance

I - Intensity of radiation or Magnitude of current

λ - Wavelength

d - Inter-atomic spacing

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θ - Angle

E - Voltage drop / magnitude of voltage

v - Scan rate

ω - Frequency

t - Time

Z - Impedance

ZI - Real part of impedance

ZII - Imaginary part of impedance

R - Resistance

j - Imaginary Coefficient

c - Centi (10^-2)

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Chapter-1

INTRODUCTION

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1. Introduction

Energy and its storage devices are one amongst the principal concerns

of present day research. It is so important because, research in energy storage

devices indirectly solves a lot of problems by providing an effective

alternative for non-renewable resources like petroleum, diesel, etc. and also

contributes towards pollution control by developing battery powered vehicles

which are ecofriendly. Modern power electronics began with the advent of the

first power semiconductor devices in the 1950’s. Since then many inventions

of devices, converter circuits, controls, and applications have enriched the

field. Since the early 1980’s power electronics has emerged as an important

discipline in engineering due to the introduction of new devices and

increasing popularity [1]. Power electronics systems or power processors

almost in every topology, with the exception of very special cases such as

matrix converters, have the need of energy storage devices to provide energy

backup and decoupling between different power conversion stages.

Traditionally due to their low cost the most commonly used storage devices

are capacitors and batteries, but they have some drawbacks such as size,

performance and life. There are several new energy storage technologies

available today that appear to be very promising options to the traditional

energy storage devices [2]. Among these new energy storage devices

supercapacitors appear to be a good option for applications that require high

power densities and fast transient response, and all this in a reduced volume.

Thus the energy storage devices may be divided into three types such as,

1) Conventional capacitors

2) Batteries and

3) Supercapacitors

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1.1. Different types of energy storage devices

1.1.1 Conventional capacitors

Conventional capacitors consist of two electrodes separated by a thin

layer of dielectric material. Due to electronic and ionic polarisation of the

dielectric medium upon application of an electric field, electrical charge can

be stored at the electrodes [3]. The capacitance (C) can be calculated with

C = ∆Q/∆V……………………….………1.1

Where Q is the electrical charge stored at the electrodes and V the

applied potential difference between the electrodes. In an ideal capacitor the

capacitance does not change upon variation in the potential difference. The

capacitance can be increased by making use of a dielectric medium with a large

dielectric constant, increasing the electrode surface area or decreasing the

distance between the electrodes, since

C = (εεo A)/D…………………………….1.2

where ξr (also referred to as k) is the relative dielectric constant of the di-

electric medium, ξ the permittivity of vacuum, A the geometric

electrode surface area and D the distance between the electrodes. By

decreasing the distance between the electrodes, the electric field strength is

increased. If the electric field strength exceeds a certain threshold value called

the dielectric strength, the material will lose its insulating property.

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Fig 1.1: Construction of a conventional capacitor

The above theory can be simply represented by the following schematic

diagram.

Fig 1.2: Schematic representation of conventional capacitor

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1.1.2. Batteries

A battery is a device that converts chemical energy into electrical

energy by means of an electrochemical reduction-oxidation (redox) reaction.

With the ever-increasing market for portable electronic devices, e.g. cell-

phones, laptop computers, there is always an increased need for improved

energy sources. Most of the electronic products today use “state-of-the-art”

batteries, and yet the performance of these leaves much to be desired. Another

aspect is the environmental threat posed by the heavy metals used in many of

today’s battery concepts. As society is becoming more aware of these

problems, the desire for environmentally friendly battery components is

growing. Batteries may be classified into many types based on the cathode,

anode and the electrolyte used. The historical battery development has gone

from Copper/Zinc cell to Li-ion cell [4-6].

1. Copper/zinc (Volta, 1800)

2. Lead-acid (Planté, 1859)

3. Zinc/manganese-oxide (Leclanché, 1866) over zinc-air

(1878),

4. Ni-Cd (Jungner, 1899)

5. Sodium-sulphur (Yao & Kummer, 1966)

6. Li primary cells (1960’s) to the present technology of Ni-MH

(early 1980’s)

7. Li- ion secondary cells (early 1990’s).

Batteries may also be classified into two types based on their

electrode reactions. They are:

1.1.2.1. Primary batteries

1.1.2.2. Secondary batteries

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1.1.2.1. Primary batteries

In primary batteries, the electrode reactions are not reversible and the

cells are therefore not rechargeable, i.e. after one discharge, they are

discarded.

For example: - Dry cell (Zn-MnO2), Ni-MH.

1.1.2.2. Secondary batteries

In secondary batteries, the electrode reactions are reversible and the

cells are rechargeable. For example:-Lead acid battery, Li-ion battery.

A battery comprises three main components: cathode, electrolyte and

anode. The cathode is characterized as the electrode where a reduction-

reaction occurs (i.e. electrons are accepted from an outer circuit), while an

oxidation-reaction occurs at the anode (i.e. electrons are donated to an outer

circuit) [7]. The electrolyte is an electronic insulator, but a good ionic

conductor; its main function is to provide a transport-medium for ions to

travel from one electrode to the other. It must also prevent short-circuiting by

acting as a physical barrier between the electrodes, either alone in the case of

a polymer or in the matrix it impregnates (the separator) if a liquid. The

voltage and capacity of a cell are functions of the electrode materials used.

1.1.2.3. Emergence of lithium batteries

One of the last century’s most revolutionary battery-related discoveries

was the interaction between alkali-metal-salts and polar polymers. These

compounds exhibit significant ionic conductivity and prompted Armand et al

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[8] to make a more detailed electrical characterization, which led to the idea

of the all-solid-state lithium polymer battery concept. Originally, Li-metal

was used as anode in the secondary Li batteries, and an inorganic intercalation

or insertion compound as cathode. The first secondary Li/insertion-compound

system was the Li/TiS2 system, announced to be commercialized by Exxon in

the mid 1970’s. There are advantages with using metallic Li as anode in the

battery: the redox potential becomes very low and the equivalent weight is

also low. There are problems, however, associated with the use of metallic Li:

e.g. corrosion of Li by reaction with electrolyte,

Safety aspects where dendrite formation can cause short-circuiting of

the cell with a following thermal runaway and possible explosion. The

concept of Li ion transfer cells was proposed to solve these problems.

Nowadays, these are usually referred to as Li-ion cells. In these cells, an

insertion electrode, usually carbon-based, replaces the Li metal anode. In this

way, the active lithium is always present as an ion rather than as a metal.

Metallic lithium is highly reactive with oxygen, nitrogen and moisture,

making it a difficult material to handle; glove-boxes or dry-rooms becomes a

necessity in production.

Battery production can be simplified by using insertion electrodes as

both anode and cathode, since the battery components can be produced in

ambient atmosphere and then swelled with electrolyte in a controlled

atmosphere. Electrode materials other than insertion compounds have also

showed promising properties, e.g. polymeric cathodes and nano-particle

anodes [9-10]. To make a complete Li-ion cell, one of the electrodes will have

to contain Li-ions, which are then shuttled reversibly between the electrodes

during charge/discharge. A variety of electrodes have been tested over the

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years but, for commercial purposes, the LiCoO2 electrode or variations

thereof have been the most exploited [11-13].

A big problem with the Li-ion concept is that any Li consumed by

secondary reactions cannot be retrieved, and the specific capacity is totally

dependent on the amount of Li available for the reversible redox process in

the cell. Some of the processes known to lead to capacity loss in Li-ion cells

are: Li deposition (in cell over-charge), electrolyte decomposition, active

material dissolution, phase changes in the insertion electrode materials, and

passive film formation on the electrode and current collector surfaces.

The distinguishing features of today’s commercial Li-ion batteries are

as follows.

High operating voltage: a single cell has an average operating potential

of approx. 3.6 V, three times the operating voltage of both Ni-Cd and

Ni-MH batteries and about twice that of sealed Pb-acid batteries.

Compact, lightweight, and high energy density: the energy density is

about 1.5 times and specific energy is about twice that of high-capacity

Ni-Cd batteries.

Fast charging potential; batteries can be charged to about 80-90% of

full capacity in one hour.

High discharge rate: up to 3C are attainable.

Wide range of operating temperature: from –20 to +600C.

Superior cycle life: service life of a battery exceeds 500 cycles.

Excellent safety: United States Department of Transportation,

Dangerous Materials Division has declared Li-ion batteries exempt

from dangerous materials regulations.

Low self-discharge: only 8-12% per month.

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Long shelf-life: no reconditioning required up to approximately 5 years

(Ni-Cd: 3 months; Ni-MH: 1 month).

No memory-effect: can be recharged at any time.

Non-polluting: does not use toxic heavy

1.1.2.4. Cathode materials for Li-ion batteries

LiCoO2 is the most commonly used cathode material in commercial Li-

ion batteries today by virtue of its high working voltage, structural stability

and long cycle life. However, Co is an expensive metal and much effort has

been made in recent years to find a cheaper alternative.

LiNiO2 (isostructural with LiCoO2) and spinel type LiMn2O4 are

promising materials in this respect, where LiNiO2 is the more attractive

alternative because of its high specific capacity and better elevated-

temperature performance.

However, LiNiO2 has not been commercialized successfully for several

reasons: i) difficult synthesis conditions, ii) poor structural stability on cycling

and iii) poor thermal stability in the delithiated state as a result of the unstable

Ni4+ ion. One way to circumvent these problems is to partially substitute

nickel by other cations. Many recent studies focus on substituted compounds

of this general type [14-19].

Co substituted LiNiO2, LiNi1-xCoxO2 (V), has the advantage of

combining the favorable properties of LiNiO2 and LiCoO2. LiNi1-xCoxO2 is

known to have much higher structural stability than pure nickel oxide and

combined with its potentially lower cost than LiCoO2, is highly promising

material for practical application.

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1.1.3. Supercapacitors

Supercapacitors are an advanced version of capacitors with unique

ability to combine energy storage capabilities of batteries and power storage

behavior of capacitor. Hence fill the gap between batteries and conventional

capacitors such as the electrolyte capacitors in terms of specific energy as

well as specific power.

Supercapacitor or Electrochemical Double Layer Capacitor (EDLC) is

complementary to secondary batteries for applications in hybrid power

systems like electric vehicles and memory backup systems etc [20]. The

supercapacitor resembles a regular capacitor with exception that it offers very

high capacitance in a small package.

Energy storage is by means of static charge rather than of an electro-

chemical process that is inherent to the battery. Applying a voltage

differential on the positive and negative plates charges the supercapacitor.

This concept is similar to an electrical charge that builds up when walking on

a carpet. The supercapacitor concept has been around for a number of years.

Newer designs allow higher capacities in a smaller size.Fig 1.4. Shows the

formation of double layer during charging.

Fig 1.3: Formation of double layer during the charging.

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Fig 1.4: Ragone’s plot for different energy storage systems

Supercapacitor devices consist of two electrodes to allow a potential to

be applied across the cell, and there are therefore two double-layers present,

one at each electrode/electrolyte interface. An ion-permeable separator is

placed between the electrodes in order to prevent electrical contact, but still

allows ions from the electrolyte to pass through. The electrodes are made of

high effective surface-area materials such as porous carbon or carbon aerogels

in order to maximize the surface-area of the double-layer. High energy

densities are therefore achievable in EDLCs due to their high specific

capacitance, attained because of a high electrode/electrolyte interface surface-

area and a small charge layer separation of atomic dimensions. In addition to

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the capacitance that arises from the separation of charge in the double-layer, a

contribution to capacitance can be made from reactions that can occur on the

surface of the electrode. The charge required to facilitate these reactions is

dependent on the potential, resulting in a Faradaic ‘pseudo capacitance’. Due

to the fundamental differences between double-layer capacitance and pseudo

capacitance, the two topics shall be discussed separately later.

The understanding of the electrical processes that occur at the boundary

between a solid conductor and an electrolyte was developed gradually.

Various models have been developed over the years to explain the phenomena

observed by chemical scientists. The widely accepted explanation that was

proposed by Bockris, Devanathan and Muller in 1963 [21]. They suggested

that a layer of water was present within the inner Helmholtz plane at the

surface of the electrode. The dipoles of these molecules would have a fixed

alignment because of the charge in the electrode. Some of the water

molecules would be displaced by specifically adsorbed ions. Other layers of

water would follow the first, but the dipoles in these layers would not be as

fixed as those in the first layer.

Pseudocapacitance arises from reversible faradaic reactions occurring

at the electrode, and is denoted as ‘pseudo’-capacitance in order to

differentiate it from electrostatic capacitance. The charge-transfer that takes

places in these reactions is voltage dependent, so a capacitive phenomenon

occurs. There are two types of reactions that can involve a charge transfer that

is voltage dependent. They are redox reactions and ion sorption reactions. In

summary, the high values of specific capacitance attainable through EDLC

technology are a result of double-layer capacitance, and often

pseudocapacitance. Double-layer capacitance offers good charge storage

capabilities thanks to possessing high surface-area materials as electrodes,

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and the fact that charge separation occurs at atomic dimensions.

Pseudocapacitance that arises from redox or ion sorption reactions further

improves the achievable capacitance.

A regular capacitor consists of conductive foils and a dry separator; the

supercapacitor crosses into battery technology by using special electrodes and

some electrolyte. There are three types of electrode materials suitable for the

supercapacitor. They are: high surface area activated carbons, metal oxides

and conducting polymers. The high surface electrode material, also called

Double Layer Capacitor (DLC), is least costly to manufacture and is the most

common. It stores the energy in the double layer formed near the carbon

electrode surface. The electrolyte may be aqueous or organic. The aqueous

variety offers low internal resistance but limits the voltage to one volt. In

contrast, the organic electrolyte allows 2.5 volts of charge, but the internal

resistance is higher.

To operate at higher voltages, supercapacitors are connected in series.

On a string of more than three capacitors, voltage balancing is required to

prevent any cell from reaching over-voltage. The amount of energy a

capacitor can hold is measured in microfarads or µF. (1µF = 0.000,001 farad).

While small capacitors are rated in nano-farads (1000 times smaller than 1µF)

and pico-farads (1 million times smaller than 1µF), supercapacitors come in

farads.

The gravimetric energy density of the supercapacitor is 1 to 10Wh/kg.

This energy density is high in comparison to a regular capacitor but reflects

only one-tenth that of the nickel-metal-hydride battery. Whereas the electro-

chemical battery delivers a fairly steady voltage in the usable energy

spectrum, the voltage of the supercapacitor is linear and drops evenly from

full voltage to zero volts. Because of this, the supercapacitor is unable to

27

deliver the full charge. If, for example, a 6V battery is allowed to discharge to

4.5V before the equipment cuts off, the supercapacitor reaches the threshold

within the first quarter of the discharge cycle. The remaining energy slips into

an unusable voltage range. A DC-to-DC converter could correct this problem

but such a regulator would add costs and introduce a 10 to 15 percent

efficiency loss.

Fig 1.5: Charge/Discharge profile comparison

Rather than operate as a main battery, supercapacitors are more

commonly used as memory backup to bridge short power interruptions.

Another application is improving the current handling of a battery. The

supercapacitor is placed in parallel to the battery terminal and provides

current boost on high load demands. The supercapacitor will also find a ready

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market for portable fuel cells to enhance peak-load performance. Because of

its ability to rapidly charge, large supercapacitors are used for regenerative

braking on vehicles. Up to 400 supercapacitors are connected in series to

obtain the required energy storage capacity. The charge time of a

supercapacitor is about 10 seconds. The ability to absorb energy is, to a large

extent, limited by the size of the charger. The charge characteristics are

similar to those of an electrochemical battery. The initial charge is very rapid;

the topping charge takes extra time. Provision must be made to limit the

current when charging an empty supercapacitor.

In terms of charging method, the supercapacitor resembles the lead-

acid battery. Full charge occurs when a set voltage limit is reached. Unlike the

electrochemical battery, the supercapacitor does not require a full-charge

detection circuit. Supercapacitors take as much energy as needed. When full,

they stop accepting charge. There is no danger of overcharge or 'memory'.

The supercapacitor can be recharged and discharged virtually an

unlimited number of times. Unlike the electrochemical battery, there is very

little wear and tear induced by cycling and age does not affect the

supercapacitor much. In normal use, a supercapacitor deteriorates to about 80

percent after 10 years.

The self-discharge of the supercapacitor is substantially higher than that

of the electro-chemical battery. Supercapacitors with an organic electrolyte

are affected the most. In 30 to 40 days, the capacity decreases from full

charge to 50 percent. In comparison, a nickel-based battery discharges about

10 percent during that time.

The comparison between supercapacitors, conventional capacitors and

batteries can be clearly explained by the table 1.1.

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Table 1.1: Parametric comparison among capacitor, supercapacitor and

batteries.

1.1.3.1. Advantages of supercapacitors

Low impedance - enhances load handling when put paralleled

with a battery.

Rapid charging -supercapacitors charge in seconds.

Simple charge methods - no full-charge detection is

needed; no danger of overcharge.

Extends battery run time.

Enables design to meet current specifications.

Cuts pulse current noise.

Lessens RF noise by eliminating DC/DC

Allows low/high temperature operation

Minimizes space requirements

50-300 0.5-5 <0.01 Specific energy (Wh/kg)

<500 1000-3000 > 10,000 Specific power (W/kg)

200-1000 106 - 108

106 - 108 Cycle life

minutes– months m s – minute µ s – m s Discharge time

hours m s - minute µ s – m s Charging time

Battery EDLC Capacitors Device

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Reduces battery size

Enhances load balancing when used in parallel with a battery

Meets environmental standards

Virtually unlimited cycle life - can be cycled millions of time.

1.1.3.2. Limitations of supercapacitors

Linear discharge voltage prevents use of the full energy

spectrum.

Low energy density - typically holds one-fifth to one-tenth the

energy of an electrochemical battery.

Cells have low voltages - serial connections are needed to obtain

higher voltages. Voltage balancing is required if more than three

capacitors are connected in series.

High self-discharge - the rate is considerably higher than that of

an electrochemical battery.

1.1.3.3. Electrodes for supercapacitors

The following are used as electrode materials for Supercapacitor.

Carbon

Metal oxides

Conducting Polymer

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Carbon

Carbon-Carbon supercapacitors use the cheapest technology due to the

low price of activated carbon. These system functions on the basis of Gouy-

Chapmann and Stern-Geary electrochemical double layer theory [22]. On the

basis of the mechanism for double layer formation, it is generally true that the

layer surface area that an EDLC can provide for adsorption of electrolytes on

electrodes, the more energy can be stored in the EDLC. Activated carbons

with large surface area and porosity are suitable candidates as materials for

the polarizable electrodes in EDLC. There is no charge-transfer reaction

occurring during the charge-discharge process. Cyclability of these

supercapacitors is very high. Both non-aqueous and aqueous electrolyte can

be used. Organic electrolyte leads to a larger electrochemical window than the

aqueous one. It is then possible to increase the voltage up to 3V but the

Equivalent series resistance is usually high that precludes to reach easily high

powers. Supercapacitor voltage is limited to 1V the thermodynamic window

of water is 1.23V with a relatively low Equivalent Series Resistance (ESR)

with aqueous electrolyte based supercapacitors.

Metal oxides

In metal oxide, pseudocapacitance is developed due to the redox

reaction. As a result, metal oxide supercapacitors generally have a very high

capacitance. Conducting metal oxides like RuO2 or IrO2 were the favorable

electrode a material in early EC’s used for space for military applications. The

32

high capacitance in combination with low resistance resulted in very high

powers [23].

Conducting polymer

Conducting polymers are low cost materials and they have been found

as electrode materials for supercapacitor applications. Polymeric materials,

such as p and n doped poly (3-arylthiopene), p-doped poly (pyrrole), poly 93-

methylthiophene) or poly (1, 5 – daaminoan thraquinone) have been

suggested by many authors. The cyclic voltammogram of a polymer exhibits

a current peak at the respective redox potential of the polymer. These types

of electrodes are used in both aqueous and non-aqueous media and are

reported to yield high energy density and power density [24]. The drawbacks

linked with these conducting polymer, are slow electrochemical kinetics and

swelling or shrinking of electrode materials in solution during the charge-

discharge process.

1.1.3.4. Classification of supercapacitors

Based on the electrode materials listed above the supercapacitors may

be classified as:

Electrochemical double layer capacitors

Pseudocapacitors or ultra capacitors

Hybrid capacitors

33

The detailed classification is explained in Fig 1.6.

Fig 1.6: Classification of supercapacitors

Electrochemical double layer capacitors

The term electrochemical double layer capacitor is most commonly

used for carbon based double layer capacitors because of its high

capacitance value. It generally denotes the supercapacitor having non-

faradaic reactions at both electrodes

34

Error!

Fig 1.7: Charging at electrode surface

Advantages

High surface area and double layer of charge allows for much

higher densities than conventional capacitors, comparable

power densities.

No chemical or structural change during charge storage—106

cycles for supercapacitors compared to 103 for batteries

Work in extreme temperatures and are very safe

35

Nanostructured carbon are relatively cheap and have well

developed fabrication techniques

can achieve wide range of pore distribution, thus energies and

powers

Disadvantages

cannot match energy densities of mid level batteries

Pseudocapacitors or ultra capacitors

In a pseudocapacitor, there are two basic reactions, which lead to

electrochemical cell. Both occur at the interface between a conductor and an

electrolyte and both benefits form very high specific surface areas at the

electrode. The first mechanism commonly referred to as charge separation,

which is well documented as non-faradaic mechanism and is the basis for

EDLC. The second reaction commonly referred to as an oxidation –

reduction reaction due faradaic mechanism.

Advantages

High surface area and fast faradic reactions allow for higher

energy densities than EDLCs

Hydrous ruthenium oxide can achieve extraordinary

capacitances

Disadvantages

36

Generally lower energy densities than EDLCs

Cycle life can be limited by mechanical stress caused during

reduction oxidation reactions

Negatively charged conducting polymer electrodes are not very

efficient

The best metal oxide electrodes are very expensive and require

an aqueous electrolyte which means lower voltage.

Hybrid supercapacitors

Hybrid power system is a new highly reliable energy storage device. It

is a combination of EDLC and a battery. (e.g. C and Li-ion).

Fig 1.8: Ragone’s plot

37

Advantages

Most flexible performance characteristics of supercapacitor—fit

the widest range of applications.

can achieve very high energy and power densities without

sacrifices in cycling stability and affordability

Disadvantages

Relatively new and unproven technologies

More research is needed to understand the full potential of

hybrid capacitors.

References

1. N. Mohan, T.M. Underland and W.P. Robbins, Power Electronics,

Converters, Applications and Design, New York: John Wiley &

Sons, 1995.

2. M. Stanley Whittingham et al, Chemical Reviews, 104, 10, (2004)

P.4243-4244.

3. R. Kötz and M Carlen, Electrochimica Acta, 45 (2000) P.2483.

4. Handbook of Batteries, 2nd edition, D. Linden, Ed., McGraw-Hill,

Inc., New York (1995).

38

5. T. Nagaura and K. Tozawa, Progress in Batteries and Solar Cells, 9

(1990) P.209.

6. P.G. Bruce, Solid State Electrochemistry, P.G. Bruce, Ed.,

Cambridge University Press, Cambridge, 1995.

7. M.S. Whittingham, Science, 192 (1976) P.1126.

8. M.B. Armand, Materials for Advanced Batteries, D.W. Murphy, J.

Broadhead, B.C.H. Steel, Eds., Plenum, New York, 1980, P.145.

9. A.S. Gozdz, C.N. Schmutz, and J.M. Tarascon, U.S. Pat. 5,296,318

(1996).

10. N Oyama, T. Tatsuma, T. Sato, and T. Sotomura, Nature, 373 (1995)

P.598.

11. D.W. Murphy and P.A. Christian, Science, 205 (1979) P.651.

12. M. Winter, J.O. Besenhard, M.E. Spahr, and P. Novák, Advanced

Materials, 10 (1998) P.725.

13. R. Koksbang, J. Barker, H. Shi, and M.Y. Saïdi, Solid State Ionics,

84 (1996) P.1.

14. A. Manthiram and J. Kim, Chem. Mater., 10 (1998) P.2895.

39

15. D. Abraham, presentation at CMM-MRL-UIUC, 28 Feb. 2001.

16. T. Ohzuku and A. Ueda, Solid State Ionics, 69 (1994) P.201.

17. M.M. Thackeray, Prog. Solid. St. Chem., 25 (1997) P.1.

18. H. Berg, Comprehensive Summaries of Uppsala Dissertations from

the Faculty of Science and Technology, P.485, Uppsala, 1999.

19. K. Kanamura, S. Shiraishi, H.T akezawa, and Z. Takehara, Chem.

Mater., 9 (1997) P.1797.

20. B.E. Conway, Electrochemical Supercapacitors, Klewer Academic

Plenum Publishers Chapter 7, P.125 (1999)

21. A.S. Fialkov, Russian J Electrochem., 36, 4, (2000) P.345.

22. M. Ue, A.Marakami and S.Nakamura, J. Electrochem. Soc., 149,

10, (2002) P.A1385.

23. M. Ue, M. Takeda, A. Toriumi, A. Kominato, R. Higiwara and Y.

Ito, J. Electrochem. Soc., 150, 4, (2003) P.A499.

24. Y.J. Kim, Y. Matsuzawa, S. Ozaki, et al, J. Electrochem. Soc., 152,

4, (2005) P. E135.

40

Chapter-2

LITERATURE REVIEW

41

2. Literature review

2.1. Review on carbon electrodes

The concept of the double layer have been studied by chemists since

the 19th century when von Helmholtz first developed and modeled the

double layer concept in investigations on colloidal suspensions [1]. This

work was subsequently extended to the surface of metal electrodes in the

late 19th and early- 20th centuries [2–6]. In 1957, the practical use of a

double-layer capacitor, for the storage of electrical charge, was demonstrated

and patented by General Electric [7]. This early patent utilized crude porous

carbon electrodes in an aqueous electrolyte. Not until the granting of a patent

to SOHIO in 1966 [8] was it acknowledged that these devices actually store

energy in the electrical double-layer, at the interphase between electrode and

solution. The first commercial double-layer supercapacitors originated from

SOHIO [9] who went on to patent a disc-shaped device that consisted of

carbon paste electrodes, formed by soaking porous carbon in an electrolyte

separated by an ion-permeable separator [10]. SOHIO also utilized

nonaqueous electrolytes in their early devices, but, a lack of sales saw them

license their technology to NEC in 1971; who further developed and

successfully marketed double-layer supercapacitors, primarily for memory

backup applications [11]. These early devices typically had a low voltage

and a high internal resistance [12]. By the 1980s a number of companies

were producing double-layer capacitors, e.g., Matsushita (Gold capacitor),

Elna (Dynacap) and PRI (PRI ultracapacitor), although the last-mentioned

incorporated relatively expensive metal oxide electrodes, primarily targeted

for military applications. Today, a number of high-performance EDLC

42

devices, based on porous carbons, are commercially available from a range

of manufacturers and distributors around the world [13]

The attraction of carbon as a supercapacitor electrode material arises

from a unique combination of chemical and physical properties, namely:

• High conductivity,

• High surface-area range (1 to 2000m2/g),

• Good corrosion resistance,

• High temperature stability,

• Controlled pore structure,

• Processability and compatibility in composite

materials,

• Relatively low cost.

In general terms, the first two of these properties are critical to the

construction of supercapacitor electrodes. As will be seen, the properties of

carbon allow both conductivity and surface area to be manipulated and

optimized. Such activities continue to be the subject of a considerable

amount of research. Prior to reviewing the results of this research, it is useful

first to consider in more detail other aspects of carbon, e.g., its structural

diversity and chemical behavior, so as to establish a better understanding of

the role of carbon materials in supercapacitors.

Carbon has four crystalline (ordered) allotropes: diamond (sp3

bonding), graphite (sp2), carbines (sp1) and fullerenes (‘distorted’ sp2). While

two carbon allotropes are naturally found on earth as minerals, namely,

43

natural graphite and diamond, the other forms of carbon are synthetic.

Carbon is considered unusual in the number of its allotropic structures and

the diversity of structural forms, as well as in its broad range of physical

properties [15, 16]. Due to the wide range of carbon materials, and to avoid

confusion, the term ‘carbon’ is typically used to describe the element rather

than its form. To describe a carbon-based material, it is best coupled with a

qualifier such as ‘carbon black’, ‘activated carbon’, ‘vitreous carbon’ and

others. A comprehensive guide to the terminology and description of carbon

solids, as used in the science and technology of carbon and graphite

materials, is available [17].

The majority of commercial carbons used today can be conveniently

described as ‘engineered carbons’. These are manufactured carbons that

have an amorphous structure with a more or less disordered microstructure

based on that of graphite [15]. Amorphous carbons can be considered as

sections of hexagonal carbon layers with very little order parallel to the

layers. The process of graphitization consists essentially of the ordering and

stacking of these layers and is generally achieved by high-temperature

treatment (>25000C). Between the extremes of amorphous carbon and

graphite, a wide variety of carbon materials can be prepared and their

properties tailored, to some extent, for specific applications.

The majority of carbon materials are derived from carbon rich organic

precursors by heat treatment in inert atmospheres (a process referred to as

carbonization). The ultimate properties of these carbons are dependent on a

number of critical factors, e.g., the carbon precursor, its dominant

aggregation state during carbonization (i.e., gas, liquid or solid), processing

conditions, and the structural and textural features of the products [18].

44

Summaries of the key processing conditions that lead to specific carbon

products and their features are outlined in Table 3.

It is usually anticipated that the capacitance of a porous carbon

(expressed in F/g) will be proportional to its available surface-area (in m2/g).

Whilst this relationship is sometimes observed [25, 21], in practice it usually

represents an oversimplification [14, 22, 23]. The major factors that

contribute to what is often a complex (non-linear) relationship are

1. Assumptions in the measurement of electrode surface-area

2. Variations in the specific capacitance of carbons with differing

morphology;

3. Variations in surface chemistry (e.g., wettability and

pseudocapacitive contributions discussed above);

4. Variations in the conditions under which carbon capacitance is

measured.

Most notably, the double-layer capacitance of the edge orientation of

graphite is reported to be an order of magnitude higher than that of the basal

layer [20, 24]. One determinant of specific double-layer capacitance could

therefore be the relative density of edge and basal plane graphitic structures

in carbon materials. Carbons with a higher percentage of edge orientations

(i.e., high Lc/La ratio) could be expected to exhibit a higher capacitance.

2.2. Review on electrolytes

Most compositions of lithium electrolytes are based on solutions of

one or more lithium salts in mixtures of two or more solvents, and single-

45

solvent formulations are very rare, if there are any. The rationale behind this

mixed solvent formulation is that the diverse and often contradicting

requirements of battery applications can hardly be met by any individual

compound, for example, high fluidity versus high dielectric constant;

therefore, solvents of very different physical and chemical natures are often

used together to perform various functions simultaneously. A mixture of

salts, on the other hand, is usually not used, because anion choice is usually

limited, and performance advantages or improvements are not readily

demonstrated.

In accordance with the basic requirements for electrolytes, an ideal

electrolyte solvent should meet the following minimal criteria

1. It should be able to dissolve salts to sufficient concentration. In

other words, it should have a high dielectric constant (ε).

2. It should be fluid (low viscosity), so that facile ion transport can

occur.

3. It should remain inert to all cell components, especially the

charged surfaces of the cathode and the anode, during cell

operation.

4. It should remain liquid in a wide temperature range. In other

words, its melting point (Tm) should be low and its boiling point

(Tb) high.

5. It should also be safe (high flash point Tf), nontoxic, and

economical.

For lithium-based batteries, the active nature of the strongly reducing

anodes (lithium metal or the highly lithiated carbon) and the strongly

46

oxidizing cathodes (transition metal based oxides) rules out the use of any

solvents that have active protons despite their excellent power in solvating

salts, because the reduction of such protons and/or the oxidation of the

corresponding anions generally occurs within 2.0- 4.0 V versus Li, while the

charged potentials of the anode and the cathode in the current rechargeable

lithium devices average 0.0-0.2 V and 3.0-4.5 V, respectively. On the other

hand, the nonaqueous compounds that qualify as electrolyte solvents must be

able to dissolve sufficient amounts of lithium salt; therefore, only those with

polar groups such as carbonyl, nitrile, sulfonyl, and ether-linkage (-O-) merit

consideration.

Since the inception of nonaqueous electrolytes, a wide spectrum of

polar solvents has been investigated, and the majority of them fall into either

one of the following families: organic esters and ethers.

An ideal electrolyte solute for ambient rechargeable lithium batteries

should meet the following minimal requirements

1. It should be able to completely dissolve and dissociate in the

nonaqueous media, and the solvated ions (especially lithium

cation) should be able to move in the media with high mobility.

2. The anion should be stable against oxidative decomposition at the

cathode.

3. The anion should be inert to electrolyte solvents.

4. Both the anion and the cation should remain inert toward the other

cell components such as separator, electrode substrate, and cell

packaging materials.

47

5. The anion should be nontoxic and remain stable against thermally

induced reactions with electrolyte solvents and other cell

components.

These salts include lithium perchlorate (LiClO4) and various lithium

borates, arsenates, phosphates, and antimonates, LiMXn (where M = B or

As, P, and Sb and n = 4 or 6, respectively).

2.3. Review on lithium compound electrodes

An insertion compound is a solid host network incorporating guest

ions. It has two specific properties which makes it different from a normal

solid structure: the guest ions are mobile between sites in the host network,

and the guest-ions can be removed from or added to the host network, thus

varying the guest-ion concentration. An insertion compound is an ionic and

electronic conductor, and the uptake or release of electrons compensates for

a change in guest-ion concentration. In Li-ion batteries, Li+ is the guest ion

and the host network comprises transition-metal-oxides (TMO’s), other

transition metal- chalcogenides (sulphides, selenides, or tellurides) or

transition-metal salts with oxoanions (e.g. phosphate, sulphate or arsenate).

For transition-metal oxides and other chalcogenides, positive guests

(like Li+) occupy sites surrounded by negative oxygen or chalcogen ions.

The guest ions strive to stay as far away as possible from the positive

transition-metal ions. The sites available to the Li-ion are determined by the

host structure. The first experimental investigation of a lithium insertion

compound was on the LixTiS2 structure [12]. It has later been established

that TMO’s are more attractive candidates for insertion electrodes [24]. This

48

is mainly due to their higher potential vs. Li/Li+ giving a high specific

energy and their excellent reversibility.

The term “intercalation” is sometimes used in the literature; it implies

a special case where “insertion” occurs into a layered host matrix, which

retains its structural integrity during the intercalation process.

The simplest and most studied insertion-compound structures contain

close-packed arrays of oxygen with the transition-metal (M) atoms

occupying half of the octahedral sites. These compounds are either layered

(LiMO2) or framework (LiM2O4) type. The ideal properties for an insertion

electrode in a Li-ion cell are [25, 26]

1. Large ∆G for the total cell reaction to provide a high cell voltage.

Limited change in ∆G over the useful range of inserted Li-ions to

ensure a stable operating voltage.

2. Minimal changes in the host network to ensure good reversibility.

3. Light host structure that is able to accommodate a significant

amount of Li to provide a high capacity.

4. Good electronic and ionic conductivity to provide high rate

capability.

5. Chemically and structurally stable over the whole voltage range

and insoluble in the electrolyte.

6. Inexpensive and non-toxic.

7. Low oxidation potential for a fully charged positive electrode (high

voltage vs. Li/Li+); high oxidation potential for a fully charged

negative electrode.

49

2.4. Review on hybrid supercapacitors

Hybrid electrochemical supercapacitors which consist of a battery-

type electrode and a capacitor-type electrode offer the advantages of both

supercapacitors (rate, cycle life) and the advanced batteries (energy density).

It shows higher energy density and the same time maintain the extended

cycle life and fast charge capability. Supercapacitors coupled with batteries

and fuel cells are considered as the most promising mid-term and long-term

solutions for low- and zero-emission transport vehicles. It has been rapidly

becoming the state-of-art of the energy storage devices. For example, in

AC/KOH/ Ni(OH)2 system [32], OH- in the electrolyte will transport to the

Ni(OH)2 cathode to form NiOOH, and K+ will drift to the carbon anode to

adsorb; in the Li4Ti5O12/LiPF6 ECDMC/ AC system [33], simultaneous with

the intercalation reaction of cation of Li-ion in the Li4Ti5O12 negative

electrode, anion of PF6- reversibly adsorbs/de-adsorbs on the surface of AC

positive electrode. Recently, we developed a new concept hybrid

electrochemical supercapacitor in which lithium intercalated compound

spinel structure LiMn2O4 was used as a positive electrode in combination

with an activated carbon negative electrode in a Li2SO4 aqueous electrolyte.

The new state-of-the-art hybrid aqueous supercapacitor technology

overcomes the drawbacks of electrolyte depletion during charge process of

conventional hybrid supercapacitors. It showed increased energy density,

maintained excellent cycling life, and also solved the major problems of

poor cycling life of lithium-ion battery electrode materials in aqueous

electrolyte [34].

50

References

1. H. von Helmholtz, Ann. Phys. (Leipzig) 89 (1853) 211.

2. G. Gouy, J. Phys. 9 (1910) 457.

3. D.L. Chapman, Phil. Mag. 25 (1913) 475.

4. O. Stern, Zeit. Elektrochem. 30 (1924) 508.

5. D.C. Grahame, Chem. Rev. 41 (1947) 441.

6. J.O. Bockris, M.A. Devanathan, K. Muller, Proc. R. Soc. A274

(1963)55.

7. H.I. Becker, Low voltage electrolytic capacitor, United States

Patent2,800,616 (1957).

8. R.A. Rightmire, Electrical energy storage apparatus, United States

Patent 3,288,641 (1966).

9. D.L. Boos, S.D. Argade, International Seminar on Double Layer

Supercapacitors and Similar Energy Storage Devices, Florida

Educational Seminars, Deerfield Beach, FL, 1991, p. 1.

10. D.L. Boos, Electrolytic capacitor having carbon paste electrodes,

United States Patent 3,536,963 (1970).

51

11. M. Endo, T. Takeda, Y.J. Kim, K. Koshiba, K. Ishii, Carbon Sci.

1(2001) 117.

12. M.F. Rose, Proceedings of the 33rd International Power Sources

Symposium, Pennington, NJ, 1988, p. 572.

13. A.M. Namisnyk, B.E. Thesis, University of Technology, Sydney,

Australia, 2003.

14. D. Qu, H. Shi, J. Power Sources 74 (1998) 99.

15. B. McEnaney, T.D. Burchell (Eds.), Carbon Materials for Advanced

Technologies, Pergamon, 1999, p. 1.

16. H.O. Pierson, Handbook of Carbon, Graphite, Diamond and

Fullerenes, Noyes Publications, NJ, USA, 1993.

17. E. Fitzer, K.-H. K¨ochling, H.P. Boehm, H. Marsh, Pure Appl.

Chem.67 (1995) 473.

18. M. Inagaki, L.R. Radovic, Carbon 40 (2002) 2263.

19. R.C. Bansal, J.B. Donnet, F. Stoeckli, Active Carbon, Marcel

Dekker, New York, 1988 (Chapter 2).

20. D. Qu, J. Power Sources 109 (2002) 403.

52

21. A. Yoshida, S. Nonaka, I. Aoki, A. Nishino, J. Power Sources

60(1996) 213.

22. S. Shiraishi, H. Kurihara, A. Oya, Carbon Sci. 1 (2001) 133.

23. M. Endo, Y.J. Kim, T. Takeda, T.H.T. Maeda, K. Koshiba, H. Hara,

M.S. Dresselhaus, J. Electrochem. Soc. 148 (2001) A1135–A1140.

24. R.L. McCreery, K.K. Cline, P.T. Kissinger, W.R. Heineman

(Eds.),Laboratory Techniques in Electroanalytical Chemistry, 2nd

ed., Marcel Dekker, New York, 1995, p. 293.

25. D. Lozano-Castell´o, D. Cazorla-Amor´os, A. Linares-Solano,

S.Shiraishi, H. Kurihara, A. Oya, Carbon 41 (2003) 1765.

26. Ue, M. J. Electrochem. Soc. 1995, 142, 2577.

27. Ravdel, B.; Abraham, K. M.; Gitzendanner, R.; Marsh, C.Extended

Abstracts of the 200th Electrochemical Society Meeting,San

Francisco, CA, Sept 2-7, 2001; Abstract No. 97; Electrochemical

Society: Pennington, NJ.

28. Xu, W.; Angell, C. A. Electrochem. Solid State Lett. 2001, 4, E1.

29. Hossain, S. Handbook of Batteries, 2nd ed.; Linden, D.,

Ed.;McGraw-Hill: New York, 1995; Chapter 36.

53

30. Schmidt, M.; Heider, U.; Kuehner, A.; Oesten, R.; Jungnitz,

M.;Ignat’ev, N.; Sartori, P. J. Power Sources 2001, 97/98, 557.

31. Walker, C. W.; Cox, J. D.; Salomon, M. J. Electrochem. Soc. 1996,

143, L80.

32. S.M. Lipka, D.E. Reisner, J. Dai, R. Cepulis, in: Proceedings of

the11nternational Seminar on Double Layer Capacitors, Florida

Educational Seminars Inc., 2001.

33. G.G. Amatucci, F. Badway, A.D. Pasquier, T. Zheng, J.

Electrochem.Soc. 148 (2001) A930.

34. Y.Y. Xia, Y.G. Wang, A hybrid lithium-ion aqueous cell, Patent CN

200510025269.6.

54

Chapter-3

SCOPE &

OBJECTIVE

55

3. Scope and objective

This project aims at developing a Hybrid supercapacitor based on

LiCo1-xAlxO2 (x = 0, 0.2, 0.4 and 0.6) cathode materials synthesized by soft

combustion method from their respective nitrate precursors using Glycine as

fuel. Carbon NanoFoam has been used as anode material with Non-

aqueous electrolyte to characterize their physical and electrochemical

properties to be used for hybrid supercapacitors.

The commercially available carbon based supercapacitors and

pseudocapacitors based on metal oxides have many disadvantages say, high

voltages cannot be reached, low energy densities and poor charge retention.

All these problems have been addressed in this work.

In the case of conventional supercapacitors or pseudocapacitors the

power obtained is only due to the charge separation and no redox behavior

may be observed and as a result they may have high power densities but

their energy density value is too low. This problem has been eliminated in

this work by replacing one of electrodes with a battery material. Thus with

the use of battery material, the charge retention property and the energy

density value may be improved effectively.

Hence the main objectives of our project is to

Synthesis LiCoO2 and to dope with Aluminium

Fabricate hybrid supercapacitors

Study its physical and electrochemical characterizations

56

Although variety of battery materials are available, we have chosen

LiCoO2 because of its advantages like high energy density, ease of synthesis

etc. and aluminium to dope with the parent material. The reason for

choosing the above material for this study is due to its various advantages

which may be listed as follows.

3.1. Advantages of hybrid supercapacitors

In supercapacitor two symmetric capacitors are connected in series

and the total capacitance is halved.

1/Ctotal = 1/C + 1/C

Ctotal = C/2.

But in a hybrid supercapacitor, one of the electrodes is replaced by a

battery electrode. So we can get the total capacitance of the single capacitor

electrode with the added advantages of battery electrode.

Electrochemical capacitors are currently receiving great attention for

their use as energy-storage devices in high-power applications with their

potential applications ranging from mobile devices to electric vehicles (EV).

It has virtually unlimited cycle life, but modest energy densities. At the other

end of the spectrum, Li-ion batteries benefit from the highest energy densities,

suffer limited cycle life and lack the ability to safely accept the fast charging

without Li metal deposition.

To merge the advantages of both systems, Amatucci et al [1] invented

a new class of nonaqueous hybrid devices which utilizes Li-intercalation

57

anode and an acetonitrile-LiBF4 electrolyte, combined with an activated

carbon double-layer cathode. The combined effect of a higher output voltage

and an anode of greater specific capacity results in higher energy densities

than carbon/carbon supercapacitors, while maintaining a high power density

and robustness.

Recently, Xia et al [2] developed new concept hybrid electrochemical

supercapacitors in which a lithium intercalated compound was used as a

positive electrode in combination with an activated carbon negative

electrode in a Li2SO4 aqueous electrolyte. The new state-of-the-art hybrid

aqueous supercapacitor technology overcomes the drawbacks of electrolyte

depletion during charge process of conventional hybrid supercapacitors. It

showed increased energy density, maintained excellent cycling life, and also

solved the major problems of poor cycling life of lithium-ion battery

electrode materials in aqueous electrolyte.

In the present work, we reported the composition and the

electrochemical performance of such type hybrid supercapacitors based on

Pure and doped LiMn2O4 cathode material.

3.2. Advantages of LiCoO2

LiCoO2 has a layered structure where lithium and transition metal

cations occupy alternate layers of octahedral sites in a distorted cubic close-

packed oxygen ion lattice. The synthesis of LiCoO2 on industrial scale is

easier than LiNiO2. It also shows better reversibility, long cycle life and very

high energy density. The substance has been doped with Al to increase the

potential window.

58

The layered structure of LiCoO2 is clearly represented in

Fig 3.1.

Fig 3.1: Layered structure of LiCoO2

3.3. Advantages of CNF

Carbon Nano Foam is having the following positive properties

compared to other carbons.

High surface area (1500 m2/g)

Low electrical resistance

No participation in Faradaic reactions at the applied

voltage

High capacity (100 - 200 F/g)

Unlike Activated Carbon, CNF combine high surface

area with high bulk density to give large capacitance

values

59

3.4. Advantages of non-aqueous electrolyte

The reason for this inability to reach higher voltages arises from the

use of aqueous electrolyte because at voltages higher than 1.2V, problems of

H2 and O2 evolution occur. This problem has been solved by the replacement

of aqueous electrolyte with non-aqueous electrolyte where the problem of

hydrogen and oxygen evolution at higher voltages may easily be eliminated.

The non-aqueous electrolyte used in this present study is 1M LiClO4 in

Ethylene carbonate-Propylene Carbonate.

References

1. G.G.Amatucci, F.Badway, A.D.Pasquier, T.Zheng, J.Electrochem.

Soc. 148 (2001) P.A930.

2. Y.Y.Xia, T.Sakai, T.Fujieda, X.Q.Yang, X.Su, Z.F.Ma, J.McBreen,

M.Yoshio, J. Electrochem. Soc. 148 (2001) P.A723.

60

Chapter-4

EXPERIMENTAL

61

4. Experimental

4.1. Synthesis of cathode material

4.1.1. Existing methodologies for the synthesis of cathode materials

A number of methods are available to synthesize cathode materials for

their use in rechargeable lithium batteries and in hybrid supercapacitors

involving Li intercalated compounds as one of the electrodes. Of all, the

conventional solid - state fusion method is the one which has gained a wide

popularity after Sony’s first Li-ion cell with LiCoO2 as the active material.

But the very same method in view of homogeneity and time requirement to

end up with the product has led to the blooming of various other

technologies such as sol gel, combustion and precursor methods.

The general methods of synthesis may be grouped under the following

three categories:

Solid-state synthesis

Sol-gel method

Soft combustion process

4.1.2. Need to identify suitable synthesis methods

Literature is replete with the methods of synthesis of these materials

that employ various precursors and heating procedures with variation in

temperature, reaction time, atmosphere, etc. The standard approach towards

synthesizing the cathode samples namely, the solid solution method usually

required a long heating time and high temperature. Firing at high

62

temperature includes crystalline growth which causes adverse effects on the

electrochemistry of the materials. In other words, the formation of large

crystallites tends to lower the kinetics of diffusion. By contrast, smaller

particles result in better cyclability, charge retention, and smaller IR drop.

Therefore, particle-size has a direct influence on the electrochemical

performance of the cell. High temperature processing may lead to Li loss

and there by affects the stoichiometry and structural characteristics of the

compound.

Hence it is essential to identify an ideal methodology that can result in

the formation of stoichiometric compounds with good control over

chemistry, purity, morphology etc., of the crystals. Generally, the electrode

materials are synthesized in nano and micron levels. Nano structured

electrode materials can either improve the energy density of batteries or

enhance the rate capability of Lithium–based batteries. But the synthesis of

nano structured materials requires stringent conditions. Hence we opted for

the synthesis of novel electrode materials in micron level. We have chosen

soft combustion method and our choice may be justified as follows:

4.1.3. Soft combustion method

Combustion or self propagating high-temperature synthesis is a

versatile method used for the synthesis of a variety of solids. The method

makes use of a highly exothermic reaction between the reactants to produce

a flame due to spontaneous combustion which then yields the desired

product or its precursor in finely divided form. Borides, carbides, oxides,

chalcogenides and other metal derivatives have been prepared by this

method and the topic has been reviewed recently by Merzhanov [1]. In order

63

for the combustion to occur, one has to ensure that the initial mixture of

reactants is highly dispersed and has high chemical energy. For example,

one may add a fuel and an oxidizer when preparing oxides by the

combustion method, both these additives being removed during combustion

to yield only the product or its precursor. Thus, one can take a mixture of

nitrates (oxidizer) of the desired metals along with a fuel (e.g. hydrazine,

glycine or urea) in solution, evaporate the solution to dryness and heat the

resulting solid to around 423K to obtain spontaneous combustion, yielding

an oxidic product in fine particulate form. Even if the desired product is not

formed immediately after combustion, the fine particulate nature of the

product facilitates its formation on further heating. In order to carry out

combustion synthesis, the powdered mixture of reactants (0.1 – 100 µm

particle size) is generally placed in an appropriate gas medium which favors

an exothermic reaction on ignition. The combustion temperature is anywhere

between 1500 and 3500K, depending on the reaction. Reaction times are

very short since the desired product results soon after the combustion. The

gas medium is not always necessary. This is so in the synthesis of borides,

silicides and carbides where the elements are quite stable at high

temperatures (e.g. Ti + 2B – TiB2). Combustion in a nitrogen atmosphere

yields nitrides. Nitrides of various metals have been prepared in this manner.

Azides have been used as sources of nitrogen. The following are some

typical combustion reactions

MoO3 + 2SiO2+ 7Mg ( MoSi2+7MgO)

WO3+C+2Al ( WC+Al2O3)

TiO2+B2O3+5Mg ( TiB2+ 5MgO)

64

Recently, MoS2 and other refractories have been prepared from

halides [2].

Use of the combustion method in an atmosphere of air or oxygen to

prepare complex metal oxides seems obvious. In the last three to four years,

a variety of oxides have been prepared using nitrate mixtures with a fuel

such as glycine or urea [3]. It seems that almost any ternary or quaternary

oxide can be prepared by this method [4]. All the superconducting cuprates

have been prepared by this method, although the resulting products in fine

particulate form have to be heated to an appropriate high temperature in a

desired atmosphere to obtain the final cuprate. The typical materials

prepared by the combustion method are listed in table 4.1 [5].

Table 4.1: Typical materials prepared by combustion method.

Oxides BaTiO3, LiNbO3, PbMoO4,

Bi4Ti3O12, BaFe12O19, YBa2Cu3O7

Carbides TiC, Mo2C, NbC

Borides TiB2, CrB2, MoB2, FeB

Silicides MoSiO2, TiS2, ZrSi2

Phosphides NbP, MnP, TiP

Chalcogenides WS2, MoS2, MoSe2, TaS2, LaTaS3

Hydrides TiH3, NdH2

65

The important advantages of soft combustion method are listed

below

Single step process with good product yield.

Short processing time at moderate temperature

High chemical homogeneity

Mono-dispersed particles of submicron size

Large surface area which leads to better electrochemical

performance

4.1.4. Synthesis procedure

The LiDxM1-xO2 (x =0, 0.2, 0.4 and 0.6; M – Co; D – Al) active

materials were synthesized by adopting soft combustion method.

Stoichiometric proportions of respective high purity metal nitrates (Merck,

India) were dissolved in distilled water. The details of composition are

shown in Table 4.2 based on the following reaction.

LiNO3 + (1-x) Co(NO3)2.6H2O + x Al(NO3)2.9H2O LiCo1-xAlxO2

The contents in the beaker were allowed to cook and a calculated

quantity of glycine was added as a fuel. The solution obtained initially was

stirred well using a magnetic stirrer and then heat treated at 1000C for 6 h.

The product obtained in this stage was ground to yield finer particles and

was subjected to TG/DTA analysis to know the stability of the compound

with increase in temperature.

66

Table 4.2: Composition for synthesis of LiCo(1-x)AlxO2 (x = 0, 0.2, 0.4, 0.6)

Requirement

Precursor

X=0 X=0.2 X=0.4 X=0.6

LiNO3 13.8g 13.8 g 13.8g 13.8

Al(NO3)2.9H2O 0 15 g 30g 45g

Co(NO3)2.6H2O 58.2g 46.56 g 34.92g 23.28g

Glycine ( C2H5NO2) 30g 30 g 30g 30g

Distilled Water 100ml 100 ml 100ml 100ml

4.1.5. Flow chart for synthesis of LiCoO2

Weighing of required chemicals

Dissolve in 100ml distilled water

Stir well at 600C

Heat the mixture at 1000C for 6 hours

Product is formed following a soft combustion

67

4.1.6. Annealing

The temperature for annealing the sample prepared can be found by

thermo gravimetric analysis. The temperature was found to be around

8500C. Therefore the samples are annealed at 8500C for duration of 4 hours.

After annealing the samples are packed separately and given for physical

characterization.

4.2. Physical characterization techniques

TG/DTA is used to find the drying temperature. The degree of cation

ordering and local cation structure are identified through FTIR studies. XRD

patterns of the synthesized materials are used to identify the formation,

purity and crystallinity of the samples individually. SEM images indicate the

surface morphology possessed by the synthesized powders.

4.2.1. Thermal analysis

4.2.1.1. Thermo-gravimetric analysis

TGA is generally used to find the optimum temperature ranges for

drying a sample to remove the moisture and impurities from it. Here the

apparatus used is having a thermo balance. In this analysis the weight of a

sample is followed over a period of time while its temperature is increased

linearly. A simple layout of TGA apparatus is shown in Fig 4.1.

68

Fig 4.1: Diagram of Thermo Gravimetric Analyzer

TGA has been conducted for all four samples in the instrument SDT

Q600 V8.2 Build 100 at a heating rate of 200C per minute in Nitrogen

atmosphere.

4.2.1.2. Differential thermal analysis

In this analysis phase transitions or chemical reactions are followed

through observation of heat absorbed or liberated. It is especially suited to

study structural changes within a solid at elevated temperatures, where few

other methods are available. The temperature difference between a sample

69

and an inert reference material is monitored while both are subjected to a

linearly increasing environmental temperature. A typical schematic

representation of DTA apparatus is shown in Fig 4.2.

Fig 4.2: Construction of Differential Thermal Analyzer

With constant heating, any transition or thermally induced reaction in

the sample will be recorded as a peak or dip in an otherwise straight line. An

endothermic process will cause the thermocouple junction in the sample to

lag behind the junction in the reference material, and hence develop a

voltage, whereas an exothermic event will produce a voltage of opposite

sign. It is customary to plot exotherms upward and endotherms downward.

The usual mode of operation is to supply heat to the samples, and therefore

endothermic events are more likely to occur than exothermic. When an

exotherm does occur, it is often caused by a secondary process. DTA has

70

been conducted for all our four samples in the instrument SDT Q600 V8.2

Build 100 at a heating rate of 200C per minute in Nitrogen atmosphere.

4.2.2. Fourier transform infra red spectroscopy

The FTIR spectroscopy deals with the study of the interaction of

matter with infrared radiation. The absorption by a sample requires that the

energy content of radiation should correspond to the energy difference

between the vibrational states; there should be strong coupling reaction

between the sample and the radiation. This coupling interaction takes place

only if there is a change in dipole moment during the absorption process.

If there is no change in dipole moment during the absorption process,

there will be no coupling interaction between the sample molecules and

radiation, and therefore no absorption is possible, even if the first condition

is satisfied.

Radiation in the near-IR region (4000-12,500 cm-1) excites harmonics

of molecular vibrations as well as low energy electronic transitions in

molecules. Generally employed source is a Quartz-enveloped tungsten-

halogen lamp, with detection by photomultiplier or photodiodes. The mid-IR

region (200-4000 cm-1) is the one where most stretching and bending

fundamental vibrations of molecules occur.

Radiation source is Nernst glowers or globars and detectors are

thermocouples or photoconductive materials like HgCdTe or InSb. The far-

IR (10-200 cm-1) is where rotational in gaseous molecules and lattice modes

are observed. A simple layout of FTIR spectrometer is shown in Fig 4.3.

71

Fig 4.3: Layout of FTIR spectrometer

Radiation source is high pressure mercury arc and doped-germanium

bolometers are useful detectors. Solids can be observed directly as thin

sections; as mulls in mineral or fluorocarbon oils; as finely ground

dispersion in discs of an IR-transparent salt such as CCl4 or CS2. The

usefulness of water is limited by its strong and broad IR-absorptions.

Infrared spectra are usually plotted as percentage transmittance (% T) or

absorbance (A) on a scale, linear in wave numbers (v). Transmittance is the

72

ratio of the intensity of radiation transmitted by the sample (I) to the incident

sample (I0), expressed as a percentage, so that T=100(I/I0) and, T and A are

related as

A=log10 (I0/I) =log10 (100/T) =2-log10T

By employing a mathematical procedure called as the “Fourier

Transform (FT)”, the desired spectrum i.e. the source intensity as a function

of wave number can be obtained.

FTIR spectra were recorded employing a Perkin-Elmer paragon-500

FTIR spectrophotometer using KBr pellets in the region of 400-3500 cm-1.

4.2.3. X-ray diffraction

When X-rays interact with a crystalline substance (phase), one gets a

diffraction pattern. In 1919 A.W.Hull gave a paper titled, “A New Method

of Chemical Analysis”. Here he pointed out that “Every crystalline

substance gives a pattern; the same substance always gives the same pattern;

and in a mixture of substances each produces its pattern independently of the

others”. The X-ray diffraction pattern of a pure substance is, therefore, like a

fingerprint of the substance. The powder diffraction method is thus ideally

suited for characterization and identification of polycrystalline phases.

Today for about 50,000 inorganic and 25,000 organic single component,

crystalline phases, diffraction patterns have been collected and stored on

magnetic or optical media as standards.

The main use of powder diffraction is to identify components in a

sample by a search/match procedure. Furthermore, the areas under the peak

are related to the amount of each phase present in the sample. In crystalline

73

substances the atoms are arranged in a regular pattern, and there is as

smallest volume element that by repetition in three dimensions describes the

crystal. For example we can describe a wall by the shape and orientation of a

single brick. This smallest volume element is called a unit cell. The

dimensions of the unit cell are described by three axes a, b, c and the angles

between them alpha, beta, and gamma.

An electron in an alternating electromagnetic field will oscillate with

the same frequency as the field. When an X-ray beam hits an atom, the

electrons around the atom start to oscillate with the same frequency as the

incoming beam. In almost all directions we will have destructive

interference, that is, the combining waves are out of phase and there is no

resultant energy leaving the solid sample. However the atoms in a crystal are

arranged in a regular pattern, and in a very few directions we will have

constructive interference. The waves will be in phase and there will be well

defined X-ray beams leaving the sample at various directions. Hence, a

diffracted beam may be described as a beam composed of a large number of

scattered rays mutually reinforcing one another. There is a definite relation

between the angle of diffraction (θ), wavelength of X-radiation (λ and the

inter-atomic spacing (d) of the crystal; popularly known as the “Bragg’s

law” which is

nλ=2d sin θ.

This model is complex to handle mathematically, and in day to day

work we talk about X-ray reflections from a series of parallel planes inside

the crystal. The orientation and interplanar spacing of these planes are

defined by the three integers h, k, l called indices. A given set of planes with

indices h, k, l cut the a-axis of the unit cell in h sections, the b axis in k

sections and the c axis in l sections. A zero indicates that the planes are

74

parallel to the corresponding axis. e.g. the (2, 2, 0) planes cut the a– and the

b– axes in half, but are parallel to the c– axis.

Phase characterization was done by powder X-Ray diffraction

technique on a Philips 1830 X-Ray diffractometer using Ni filtered Cu-Kα

radiation (λ=1.5406) in the 2Ө range of 10-120° at a scan rate of 0.1˚/s.

4.3. Cell fabrication

After the completion of physical characterizations, a cell has to be

fabricated to proceed with electro-chemical characterization. The

constituents of a cell may be listed as follows:

Current collector - Mild steel

Positive material - LiCo(1-x)AlxO2 (x = 0, 0.2, 0.4, 0.6)

Negative material - Carbon Nano Foam

Binder - N- Methyl Pyrrolidine

Separator - Polypropylene

Electrolyte - 1M LiClO4 in EC-PC

4.3.1. Composition for positive electrode

LiCo(1-x)AlxO2 (x = 0, 0.2, 0.4, 0.6)-------- 80%

CNF------------------------------------------- 15%

Binder----------------------------------------- 5%

A small amount of CNF is added to the positive electrode to increase

its conductivity.

75

4.3.2. Composition for negative electrode

CNF--------------------------------------------- 95%

Binder------------------------------------------- 5%

4.3.3. Steps involved in cell fabrication

Initial polishing and weighing of the current collectors

Two current collector samples, each of size 1cm2 are taken, polished

manually and weighed initially to fabricate a single cell.

Pasting of electrode materials

The electrode materials are weighed based on the above composition

and ground well uniformly in a mortar and pestle which is shown on Fig 4.4.

Fig 4.4: Binder, Spatula, Mortar and Pestle.

76

After grinding the electrode materials, NMP binder is added and the

powder was made into a paste. This paste was applied on the polished

surface of the current collector (MS).

Drying the pasted electrode

The electrodes were then dried in an oven at 2000C for 30 minutes to

allow the binder material to evaporate and cooled to room temperature. It is

weighed finally to calculate the weight of active material present in it. A

single electrode after taken out from the oven is shown in Fig 4.5.

Fig 4.5: A dried single electrode

Fabrication of supercapacitor cell

Both positive and negative electrodes were prepared as above and

they are tied together using a nylon thread after keeping a polypropylene

separator in between. Care should be taken to avoid any contact between the

electrode materials. The pair of electrodes tied together is shown in the fig

4.6. Then the electrodes are dipped in a beaker containing EC-PC

electrolyte. A completely fabricated supercapacitor cell is shown in Fig 4.7.

77

Fig 4.6: Electrodes tied using Fig 4.7: A fully fabricated

nylon thread supercapacitor cell

4.4. Electrochemical characterization techniques

4.4.1. Cyclic voltammetry

Cyclic Voltammetry technique is used to determine the capacitance of

an electrochemical capacitor. An electrode-elctrolyte interface in the absence

of a redox couple is not a pure parallel-plate capacitor, it behaves rather like

one and a parallel-plate capacitor model is often adequate to describe

electrochemical systems. Use of this model allows us to learn about the

behavior of electrodes in the absence of a redox couple. For a simple parallel

plate capacitor, charge on the capacitor, Q, is proportional to the voltage

drop E, across the capacitor.

Q = CE …………...……………………………4.1

78

The proportionality constant C is the capacitance of the medium. The

simplest description of electrochemical capacitance is the Helmholtz model

given by:

Where ε is the dielectric constant of the material separating the

parallel plates, εo is the permittivity of free space, l is the separation between

the plates, and A is the area of the electrode. This model does not adequately

describe all electrochemical interfaces as the capacitance can depend on both

potential and the supporting electrolyte. Still it is a helpful construct.

Capacitance is a crucial factor in electrochemical experiments because

it gives rise to current during the charging of the capacitor. Rather logically

(and without imagination), we term this as charging current. To calculate the

magnitude of this current, we differentiate equation 4.1 with respect to t

assuming that the capacitance is constant

Recognizing that dQ/dt is an expression for current and dE/dt is the

potential scan rate ν, we obtain

……………………………………….4.4

From this very simple derivation, we have an expression for the

charging current at steady state when applying a ramping voltage. Thus by

…………………...................................4.2

…………………...................................4.3

79

measuring the charging current at a given scan rate, we can determine the

capacitance of the system. If there is no possibility for electron transfer

between the solution and the electrode (we don’t add a redox couple) this is

the only current that we will observe. Let’s now consider the very simple

cyclic voltammetry (no active redox couple present) experiment shown in

Fig 4.8. We apply the potential form shown in the figure. Initially, we have a

sharp rise in current because of a sharp change in the scan rate ν. The current

then reaches steady state as constantly varying the voltage. At reversal of the

scan rate, the current changes sign, and when we stop scanning, current goes

to zero.

Fig 4.8: Schematic explanation of a cyclic voltammetry experiment in the

absence of a redox couple.

4.4.2. Electrochemical impedance spectroscopy

Electrochemical Impedance Spectroscopy (EIS) is a characterization

technique where by a sinusoidal voltage wave of known amplitude and

frequency is passed through the test subject (usually a standard test cell).A

80

sine wave is utilized because it has a characteristic magnitude (E) and

frequency (ω)

e = E sin ωt …………………………….....…4.5

Where t is the indication of time. This voltage induces a flow of

current when it is applied to an electrochemical system. This current (I) can

be described as

i = I sin (ωt + θ) ……………………………....4.6

The sinusoidal wave is phase shifted (θ) depending on the nature of

the intrafacial region next to the test electrode. Conversion of the measured

current into a voltage allows for the comparison between the excitation and

resultant waveforms. The magnitude and phase differences can be measured

and compared resulting in two pieces of information.

Fig 4.9: Magnitude of the resultant

sine wave as a function of frequency

for a representative standard test cell

with one layer of Celgard 2500.

Fig 4.10: Phase difference

between the incident and resultant

wave forms

81

The magnitude and phase shift can be alternately designated as the

real and imaginary impedances, ZI and ZII, respectively. The relationship

between the two sets is given by

|Z| = |E|/|I| = √ (ZI2 + ZII

2) ……………….………..….4.7

and

tan θ = ZII/ZI …………………………………..4.8

The real impedance is measured in phase, and the imaginary

impedance is measured 90° out of phase with the excitation waveform

Fig 4.11: The real portion of the Fig 4.12: The imaginary portion

resultant wave form as a function of the resultant wave form as a

of frequency. function of frequency.

82

A convenient and largely accurate, model for a supercapacitor is a

simple Resistor-Capacitance series circuit. The resistance (R) portion of the

circuit affects only the magnitude of the resultant signal. No phase shift is

observed. Hence

ZI = R ……….……………………...….4.9

In contrast the capacitance (C) induces a phase shift of -90°. The

effect is frequency dependant and does not contribute to the real impedance.

Hence

ZII = 1/ (ωC) …………………………….…...4.10

The magnitude can be calculated by substituting equations 4.9 and 4.10

into 4.7, thus

|Z| = √ (R2 + 1/ (ω

2C2)) …………………….....4.11

Similarly the phase angle can be calculated by substitution of

equations 4.9 and 4.10 into 4.8.

θ = arc tan (1/ωRC) …………………….…….4.12

The phase angle varies from 0° to -90° depending on the frequency.

This phase angle variation is indicative of the dominant circuit element. At

low frequencies the phase angle is -90° and is dominated by the

capacitance.

High frequencies are dominated by the resistance and have a phase

angle of 0°. The change between the phase angles are an analogue shift. At

83

a phase angle of 45° the resistance and capacitance are equally dominant.

The frequency dependant impedance can be written as

Z (ω) = R + (1/jωC) ……………………….….4.13

where the symbol j indicates √(-1)

The first phase of the characterization involved performing EIS as a

function of frequency. This gives data such as the Fig 4.13 and Fig4.14

shown below

Fig 4.14: Capacitance

the resultant waveform as a

. function of frequency

. The most convenient form of these is the resistance

and capacitance data shown as an example in figures 4.13 and 4.14 above.

A frequency sweep is one method of utilizing the EIS. A second method

involves an EIS test at a fixed frequency. Frequency of 1 kHz was chosen,

Fig 4.13: The resistance of

the resultant waveform as a

function of frequency.

84

because it is an industry standard for measuring equivalent series resistance

(ESR). It is generally considered that the phase angle is zero, or very closes

to zero, at this point. The voltage is changed in 0.25V increments, from 0V

bias to 2.5 volts. The experiment is continued as the voltage is ramped

back down to 0V bias and then back up to 2.5V. A voltage hold phase

preconditions the cell to the voltage that is being tested in the next step.

This phase extends for one minute and then the test runs for two minutes,

collecting 120 evenly spaced data points during this time. This small time

fame is the rationalization for the fact that the cell sees slightly higher then

the nominally accepted limit for a device of this configuration.

4.4.3. Galvanostatic charge-discharge cycle tests

This is used to evaluate performance of the supercapacitors. The

instrument Won-A-Tech WBCS 3000 was used for automatic

charge/discharge of supercapacitors. Here a charge/discharge current density

of 500µA/cm2 was used.

References

1. A.G. Merrzhanov, C.N.R. Rao (ed.) (1992) ‘chemistry of advanced

materials’, IUPAC 21st century monograph series, Blackwell,

oxford, p.19

2. M. Mohanrao, M. Jeyalakshmi, O.Schoff, U. Guth, Wulff, F. Scholz

(1999) Journel of Solid State Electrochemistry, P.17

85

3. P.R. Bonneau, R.F. Jarvis, Jr and R.B. Kaner (1991) Nature, P.510.

4. R. Mahesh, VA. Pavate, Om prakash and C.N.R. Rao (1992)

Superconductors Science and Technology, P.174.

5. W. LI, J.N. Reimers, J.R. Dahn (1993) Solid State Ionics, p.123.

86

Chapter-5

RESULTS &

DISCUSSIONS

87

5. Results and discussions

5.1. Thermal analysis

5.1.1. Thermo-gravimetric analysis

0 200 400 600 800 1000 1200 1400

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Weig

ht fraction

Temperature ( 0C)

LiCoO2

LiCo0.8

Al0.2

O2

LiCo0.6

Al0.4

O2

LiCo0.4

Al0.6

O2

Fig 5.1: TGA curves of base LiCoO2 and aluminium doped

Lithium Cobaltate.

In Fig 5.1, the initial weight drop from 300C-1500C is due to moisture

removal from the sample. And the subsequent weight loss from 1500C to

3000C corresponds to the elimination of organic compounds from the

samples. The weight drop observed in the temperature range of 3000C-5000C

is as a result of the reaction of unreacted precursors to give the final product.

The stabilization temperature for these samples mostly lay after

8000C. Hence the samples are heated at 8000C for 4 hours. It is also known

88

from the figure that when the aluminum substitution exceeds the fraction of

cobalt the decomposition is very low.

5.1.2. Differential thermal analysis

Fig 5.2 shows that all base and doped substances are having the

similar behavior in DTA performance, that involves the exothermic

reactions corresponding to the elimination of compounds listed above.

0 200 400 600 800 1000 1200

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Temperature difference(0

C)

Temperature(0C)

LiCoO2

LiCo0.8

Al0.2

O2

LiCo0.6

Al0.4

O2

LiCo0.4

Al0.6

O2

Fig 5.2: DTA curves of base LiCoO2 and aluminium doped

Lithium Cobaltate.

5.2. FTIR spectroscopy

Figs 5.3–5.6 show the FTIR spectrum of LiCoO2, LiCo0.8Al0.2O2,

LiCo0.6Al0.4O2, and LiCo0.4Al0.6O2 respectively. These data indicate the

89

formation of compressed CoO6 and LiO6 elongated octahedra. Upon sp

element substitution, the vibrational mode of LiO6 units observed in the far-

infrared region appears quite stable.

The systematic change in frequency of the high-wave number IR

bands as a function of Al doping is not surprising, while a blue shift is

expected with the increase of Al content. The intensity of the stretching

mode (Eu symmetry) located at 558 cm−1 increases significantly for y (Al) =

0.4, which compares with the shape of the IR feature at 510–570 cm−1 for

LiAlO2. These results show that, by doping, the local environment of lithium

ions surrounded by oxygen anions is not strongly affected and the CoO2

layer covalency increases slightly. For high level of Al substitution, the

broadening of the infrared peaks can be interpreted as an increase in CoO6

distortion due to the incorporation of Al3+ in the CoO2 layers. However,

because the stability of the LiO6 stretch, we can conclude that Al3+ cations

were not incorporated in the Li sites of the interlayer space.

500 1000 1500 2000 2500 3000 3500

0

20

40

60

80

100

% Transmittance

Wave number (cm-1)

Fig 5.3: FTIR spectrum of LiCoO2

90

500 1000 1500 2000 2500 3000 3500

0

20

40

60

80

100

% T

ransmittance

Wave number(cm-1)

Fig 5.4: FTIR spectrum of LiCo0.8Al0.2O2

500 1000 1500 2000 2500 3000 3500

0

20

40

60

80

100

% Transmittance

Wave number(cm-1)


91

500 1000 1500 2000 2500 3000 3500

0

20

40

60

80

100

% Transmittance

Wave number(cm-1)


5.3. X-ray diffraction

Fig 5.7 displays the XRD patterns of LiAlxCo1−xO2 (0.0 ≤ x ≤ 0.6)

oxides. These data show that most of the compounds are single phase when

obtained through careful selection of precursors. As the substitution of Al3+

for Co3+ ions occurs in the LiCoO2 rhombohedral structure, a solid solution

of lithium cobaltate and lithium aluminate is obtained for the LiAlxCo1−xO2

materials up to x = 0.6. The XRD patterns of the samples synthesized for

this study were indexed in the rhombohedral R3m space group. Miller

indices (h k l) are indexed in the hexagonal setting. No impurity phase was

detected in the XRD patterns.

92

10 20 30 40 50 60 70 80 90 100

% inte

nsity

LiCoO2

LiCo0.8

Al0.2

O2

LiCo0.6

Al0.4

O2

LiCo0.4

Al0.6

O2

(201)

(113)

(110)

(108)

(107)

(105)

(104)

(012)(0

06)

(101)

(003)

2 theta

Fig 5.7: XRD spectra of cathode materials

36 37 38 39 40

% Intensity

LiCoO2

LiCo0.8

Al0.2

O2

LiCo0.6

Al0.4

O2

LiCo0.4

Al0.6

O2

2 theta

Fig 5.8: XRD spectra of cathode materials

93

Fig5.8 shows the portions of XRD patterns (in the 2θ range from 35 to

410) from LiAlxCo1−xO2 for 0.0 ≤ x ≤ 0.6 illustrating the lattice parameter

shifts in the solid solution. As the aluminium content increases a broadening

of the diffraction peaks are observed. The (0 0 6) and (1 0 8) peaks shift

toward lower 2θ angles, resulting in a wider split of doublets corresponding

to the (0 0 6) (1 0 2) and (1 0 8) (1 1 0) Bragg lines compared with the native

oxide LiCoO2.

5.4. Cyclic voltammetry

The cyclic voltammetry experiments were carried out at various scan

rates in BAS 100A and BAS 100B instruments. In hybrid electrochemical

supercapacitors or double-layer capacitors, the cations and anions seperate

during the charge process. Charges stored at both electrodes should be

balanced and equal the charge of ions consumed from the electrolyte. The

electrolyte depletion problems limit greatly their energy density. In this

work, (LiCoO2/CNF) Li ions adsorb on the surface of the carbon nanofoam

electrode. The reverse process occurs during discharge. The Li ions transfer

from one electrode to another during the charge and discharge cycles. The

electrolyte mainly functions as the ionic conductor and is not consumed.

This new hybrid technology overcomes the drawbacks of electrolyte

depletion during the charge process of conventional hybrid supercapacitors

and it shows increased energy density and power density.

The CVs of as prepared LiCoO2 and CNF in electrolyte of 1M LiClO4

in EC:PC (50:50) at various scan rates are given in Fig.5.9. In order to

improve the electronic conductivity 15% of CNF was added to LiCoO2. The

94

Oxygen evolution potential was also investigated and occurred at 1.8V

which is higher than 1.3V as the pure LiMn2O4 electrode reported else where

[1].

The curves also reveal that the CNF electrode in the non-aqueous

solution exhibits rectangular shape behaviour within the potential range of

1.5V to -1.5V, which is the characteristic of double layer capacitance,

corresponding to the non faradaic reversible reaction of Li ion

adsorption/desorption on the surface of the carbon nano foam.

In the non aqueous solution, it is assumed that oxygen evolution

occurs on the positive electrode when the Li ion is extracted from LiCoO2

(charging process) and hydrogen evolution occurs at the CNF negative

electrode when a Li ion is adsorbed (charge process). The lithium ion can be

completely extracted from layered frame work in the non aqueous solution,

but with electronic conductivity of carbon, the oxygen evolution potential

shifts to a high value of 1.5V. Therefore safe charge potential of layered is

below 1.5V. It is interesting that the lithium ion can reversibly adsorb-desorb

on the CNF in the non aqueous solution containing the lithium ion and

exhibits a good capacitance profile before hydrogen evolution as shown in

the Fig 5.9.

In case of aluminium doped lithium cobaltate, the supercapacitors

with aluminium fraction x=0.2 and 0.6 possess meager specific capacitance

values such as 16 F/g and 8 F/g respectively. But doping aluminium at a

fraction of x=0.4 shows very good results with maximum specific

capacitance value of 27 F/g.

The CV curves after 500 cycles for all the doped and bare LiCoO2

show good reversibility. Base LiCoO2 offers some resistance to degradation

on cycling whereas the cathode materials doped at x=0.2 and 0.6 fail

95

miserably on cycling. But aluminium doping at x=0.4 shows very good

specific capacitance value even after 500 cycles. Thus CV studies prove

LiCo0.6Al0.4O2 as the best candidate for supercapacitor applications. Prof.

Ceder has already proved this for lithium batteries and the same has been

effectively reproduced in this supercapacitor studies.

The specific capacitance of the hybrid supercapacitor cell

(LiCoO2/CNF) from CV is calculated by using the following equation 5.1.

C = i /(v x m) (F/g)……………………………...5.1

Where i = current value calculated from the CV graph(A)

v = scan rate (mV/s)

m = mass of the cathode material (g)

The specific capacitance values calculated from CV for the base and

Al doped materials are tabulated in Table 5.1.

2000 1000 0 -1000 -2000

-0.0008

-0.0006

-0.0004

-0.0002

0.0000

0.0002

0.0004

Current(A)

Voltage(mV)

1mV/s

2mV/s

5mV/s

Fig 5.9: CV of LiCoO2/CNF supercapacitor as fabricated

96

1500 1000 500 0 -500 -1000 -1500

-0.0004

-0.0002

0.0000

0.0002

Current(A)

Voltage(mV)

1mV/s

2mV/s

5mV/s

Fig 5.10: CV of LiCo0.8Al0.2O2/CNF supercapacitor before

cycling

1500 1000 500 0 -500 -1000 -1500

-0.0006

-0.0004

-0.0002

0.0000

0.0002

0.0004

Current(A)

Voltage(mV)

1mV/s

2mV/s

5mV/s

Fig 5.11: CV of LiCo0.6Al0.4O2/CNF supercapacitor as

fabricated

97

1500 1000 500 0 -500 -1000 -1500

-0.0002

-0.0001

0.0000

0.0001

0.0002

Current(A)

Voltage(mV)

1mV/s

2mV/s

5mV/s

Fig 5.12: CV of LiCo0.4Al0.6O2/CNF supercapacitor as

fabricated

1500 1000 500 0 -500 -1000 -1500

-0.00015

-0.00010

-0.00005

0.00000

0.00005

0.00010

0.00015

Current(A)

Voltage(mV)

1mV/s

2mV/s

5mV/s

Fig 5.13: CV of LiCoO2/CNF supercapacitor after 500

Charge/discharge cycles.

98

2000 1000 0 -1000 -2000

-0.0006

-0.0004

-0.0002

0.0000

0.0002

0.0004

0.0006

Current(A)

Voltage(mV)

1mV/s

2mV/s

5mV/s

Fig 5.14: CV of LiCo0.8Al0.2O2/CNF supercapacitor after 500

Charge/discharge cycles.

1500 1000 500 0 -500 -1000 -1500

-0.0004

-0.0002

0.0000

0.0002

0.0004

Current(A)

Voltage(mV)

1mV/s

2mV/s

5mV/s

Fig 5.15: CV of LiCo0.6Al0.4O2/CNF supercapacitor after

500 charge/discharge cycles.

99

1500 1000 500 0 -500 -1000 -1500

-0.00010

-0.00005

0.00000

0.00005

0.00010

Current(A)

Voltage(mV)

1mV/s

2mV/s

Fig 5.16: CV of LiCo0.6Al0.4O2/CNF supercapacitor after

500 charge/discharge cycles.

Table 5.1: Specific capacitance (F/g) values of LiCo(1-x)AlxO2 (x = 0, 0.2,

0.4, 0.6) as calculated from cyclic voltammograms

Specific Capacitance (F/g) Composition

5mV/s 2mV/s 1mV/s

0 15.93 18.75 20.09 0.2 11.6 15.25 16.3 0.4 21.74 26.93 27.61

Before cycles

0.6 6.1 7.63 8.3

0 4.113 5.29 11.95 0.2 8.274 10.33 12.93 0.4 16.225 19.74 21.51

After cycles

0.6 - 5.1 6.4

100

5.5. Electrochemical impedance spectroscopy

The capacitance of pure LiCoO2 / CNF and doped LiCo1-xAlxO2 (x =

0.2, 0.4 and 0.6)/CNF supercapacitor cells are mainly due to lithium ion

insertion/extraction into/out of the electrode material. The electrochemical

impedance measurement was carried out in the frequency range of 100 Hz to

10 kHz. The spectra in the Nyquist plots (ZI Vs ZII) for supercapacitor cells

having the pure and doped material as cathodes are shown in Fig 5.17 and

Fig 5.18. The plot displays the semicircle and Warburg impedance at low

and high frequencies. The semicircle observed in the high frequency range is

attributed to the resistance offered to Li migration through the bulk of the

electrode (Rs) with the associated capacitance (Cdl). The straight line in the

impedance spectra at low frequency is the impedance related with the

diffusion of the lithium ion (Zw). In the case of aluminium doped LiCoO2

material the straight line is shifted towards horizontal axis except when the

aluminium composition is 0.4.

This suggests that the capacitance of the LiCo0.6Al0.4O2/CNF cell

mainly resulted from the pseudocapacitance caused by the lithium ion

insertion/extraction into/out of the electrode. Table 5.2 shows the impedance

parameter obtained for the supercapacitor cells having the pure and doped

material as cathodes.

The solution resistance of Rs = 4.5Ω was obtained for x = 0.4 and it is

almost constant after 500 cycles. The shifting of the straight line in Nyquist

plot is also minimum for x = 0.4 even after 500 cycles. These results show

that Al substitution is good for electrochemical capacitance when x = 0.4.

101

0 20 40 60 80 100

0

-20

-40

-60

-80

ZIm

ZRe

LiCoO2

LiCo0.8

Al0.2

O2

LiCo0.6

Al0.4

O2

LiCo0.4

Al0.6

O2

Fig 5.17: Electrochemical impedance spectroscopes of

supercapacitors before cycles.

0 50 100 150 200 250

0

-50

-100

-150

-200

-250

ZIm

ZRe

LiCoO2

LiCo0.8

Al0.2

O2

LiCo0.6

Al0.4

O2

LiCo0.4

Al0.6

O2

Fig 5.18: Electrochemical impedance spectroscopes of

supercapacitors after 500 cycles.

102

Table 5.2: Solution resistance(Rs) and double layer capacitance(Cdl) values

of LiCo(1-x)AlxO2 (x = 0, 0.2, 0.4, 0.6) as calculated from

Electrochemical impedance spectrographs.

Properties

Composition

Rs (Ohm)

Cdl (mF)

0 3.747 0.6194

0.2 2.392 0.5518 0.4 4.551 0.5491

Before cycles

0.6 5.649 0.6328

0 4.721 0.6567

0.2 6.253 0.5778 0.4 4.782 0.621

After cycles

0.6 6.211 0.711

5.6. Galvanostatic charge-discharge cycle tests

The charge-discharge behavior of the hybrid supercapacitor cell was

examined by chrono-amperometry from 0V to 2.0V at 500µA/cm2 current

density. The E-t response behaved in a mirror fashion during the charge

discharge process which means that a reversible reaction occurs among the

electrode material. During the charging and discharging step curves of

charge/discharge obviously display a sloping variation of the time

dependence of the potential that indicates typical pseudocapacitance

behavior in agreement with the analysis of CV measurement. This behavior

resulted from the electrochemical adsorption/ desorption or redox reaction at

an interface between electrode and electrolyte.

103

The capacitance of the electrode can be calculated from charge-

discharge curves according to the following equation 5.2.

C (F) = Q/∆V = (i x t)/ ∆V…………..…………5.2

The specific capacitance and specific energy of the electrode can be

calculated from the equations 5.3 and 5.4 respectively.

C (F/g) = (i x t)/(∆V x m)………………………5.3

E (kWh/kg) = (i x t x ∆V)/m………...…………5.4

Where i = current of charge/discharge (A)

t = time of discharge (s)

∆V= operating potential (V)

m = amount of active material in one electrode (g)

Table 5.3 shows the specific capacitance, specific energy and specific

power of the bare and doped LiCoO2 at 500 µA Cm2 current density. As can

be seen through the bare LiCoO2 has high specific capacitance it degrades

after some cycles. The maximum specific power of 333 kW/kg was obtained

for x = 0.4 and it is also having the highest specific energy of 12 kWh/kg.

This result indicates that substituted materials especially

LiCo0.6Al0.4O2 for electrochemical capacitor possess excellent cycle life

which probably ascribed to stable Li ion insertion and exertion behavior.

104

350 400 450 500 550 600

0.0

0.4

0.8

1.2

1.6

2.0

Voltage(V

)

Time(s)

Fig 5.19: 2nd charge/discharge cycle of LiCoO2/CNF

supercapacitor

5600 5610 5620 5630 5640 5650

0.0

0.7

1.4

2.1

Voltage(V

)

Time(s)

Fig 5.20: 500th charge/discharge cycle of LiCoO2/CNF

supercapacitor

105

26 28 30 32 34 36 38 40 42

0.0

0.8

1.6

Voltage(V

)

Time(s)

Fig 5.21: 2nd charge/discharge cycle of LiCo0.8Al0.2O2/CNF

supercapacitor

1337 1338 1339 1340 1341 1342 1343 1344

0.0

0.4

0.8

1.2

1.6

2.0

Voltage(V

)

Time(s)

Fig 5.22: 500th charge/discharge cycle of LiCo0.8Al0.2O2/CNF

supercapacitor

106

600 650 700 750 800 850 900

0.0

0.4

0.8

1.2

1.6

2.0

Voltage(V

)

Time(s)


supercapacitor

9120 9140 9160 9180 9200

0.0

0.4

0.8

1.2

1.6

2.0

Voltage(v)

Time(s)


supercapacitor

107

105 110 115 120 125 130 135 140 145

0.0

0.4

0.8

1.2

1.6

2.0

Voltage(V

)

Time(s)


supercapacitor

11732 11736 11740 11744 11748 11752

0.0

0.4

0.8

1.2

1.6

2.0

Voltage(V

)

Time(s)


supercapacitor

108

Table 5.3: Various parameters of supercapacitors fabricated using the

synthesized cathode materials as calculated from Galvanostatic

charge-discharge tests

Properties

Composition of Aluminium Specific

capacitance

(F/g)

Specific

Power

(kW/kg)

Specific

Energy

(kWh/kg)

0 11.17 312.5 12.41

0.2 0.415 303.03 0.44

0.4 11.41 333.3 12.68 Before cycles

0.6 1.53 322.58 1.075

0 1.8 312.5 2.01

0.2 0.303 303.03 0.336

0.4 3.83 333.33 4.25 After cycles

0.6 0.88 322.58 0.986

References

1. Yong Gang Wang, J. Electrochemical society 153, 2 (2006) P. A450-

A454.

109

Chapter-6

CONCLUSIONS

110

6. Conclusions

The LiCo1-xAlxO2 (x=0, 0.2, 0.4 and 0.6) cathode materials were

synthesized using soft combustion method from their respective nitrate

precursors using Glycine as fuel. The synthesized cathode materials were

characterized physically by Thermal analysis, XRD and FTIR. From this

physical characterizations we can conclude that the soft combustion method

can be adopted to synthesize LiCo1-xAlxO2 (x=0, 0.2, 0.4 and 0.6) materials.

Hybrid supercapacitors were fabricated using LiCo1-xAlxO2 (x=0, 0.2, 0.4

and 0.6) respectively as cathodes and CNF as anodes. The fabricated

supercapacitors were characterized electrochemically by using cyclic

voltammetry, electrochemical impedance spectroscopy and galvanostatic

charge-discharge cycle tests. Among the four supercapacitor cells the one

which is having the LiCo0.6Al0.4O2 as cathode is having good capacitance

behaviour and retains the first place among the four, letting the

supercapacitor having LiCoO2 as cathode next to it. The other two

aluminium doped compounds didn’t show even a moderate behaviour. From

this we can arrive a conclusion that doping LiCoO2 with aluminium sounds

good for supercapacitor application only if the aluminium fraction is 0.4. We

can also conclude that LiCoO2 itself is a good cathode material for hybrid

supercapacitor since it gives a specific capacitance of 11F/g.

Under Graduate Dissertation Work

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