Introduction

A lithium-ion battery (sometimes Li-ion battery or LIB) is a member of a family of rechargeable

battery types in which lithium ions move from the negative electrode to the positive electrode

during discharge and back when charging. Li-ion batteries use

anintercalated lithium compound as one electrode material, compared to the metallic lithium used

in a non-rechargeable lithium battery. The electrolyte which allows for ionic movement, and the

two electrodes are the consistent components of a lithium-ion cell.

Lithium-ion batteries are common in consumer electronics. They are one of the most popular

types of rechargeable batteries forportable electronics, with a high energy density, no memory

effect, and only a slow loss of charge when not in use. Beyond consumer electronics, LIBs are

also growing in popularity for military, electric vehicle and aerospace applications.[6] For example,

lithium-ion batteries are becoming a common replacement for the lead acid batteries that have

been used historically for golf carts and utility vehicles. Instead of heavy lead plates and

acid electrolyte, the trend is to use lightweight lithium-ion battery packs that can provide the

same voltage as lead-acid batteries, so no modification to the vehicle's drive system is required.

Chemistry, performance, cost and safety characteristics vary across LIB types. Handheld

electronics mostly use LIBs based on lithium cobalt oxide (LiCoO

2), which offers high energy density, but presents safety risks, especially when damaged. Lithium

iron phosphate(LFP), lithium manganese oxide (LMO) and lithium nickel manganese cobalt oxide

(NMC) offer lower energy density, but longer lives and inherent safety. Such batteries are widely

used for electric tools, medical equipment and other roles. NMC in particular is a leading

contender for automotive applications. Lithium nickel cobalt aluminum oxide (NCA) and lithium

titanate (LTO) are specialty designs aimed at particular niche roles.

Lithium-ion batteries can be dangerous under some conditions and can pose a safety hazard

since they contain, unlike other rechargeable batteries, a flammable electrolyte and are also kept

pressurised. Because of this the testing standards for these batteries are more stringent than

those for acid-electrolyte batteries, requiring both a broader range of test conditions and

additional battery-specific tests.[7][8] This is in response to reported accidents and failures, and

there have been battery-related recalls by some companies.

ConstructionThe three primary functional components of a lithium-ion battery are the positive and negative

electrodes and electrolyte. Generally, the negative electrode of a conventional lithium-ion cell is

made from carbon. The positive electrode is a metal oxide, and the electrolyte is alithium salt in

an organic solvent.[36] The electrochemical roles of the electrodes reverse between anode and

cathode, depending on the direction of current flow through the cell.

The most commercially popular negative electrode is graphite. The positive electrode is generally

one of three materials: a layered oxide(such as lithium cobalt oxide), a polyanion (such as lithium

iron phosphate) or a spinel (such as lithium manganese oxide).[37]

The electrolyte is typically a mixture of organic carbonates such as ethylene carbonate or diethyl

carbonate containing complexes of lithium ions.[38] These non-aqueous electrolytes generally use

non-coordinating anion salts such as lithium hexafluorophosphate (LiPF

6), lithium hexafluoroarsenate monohydrate (LiAsF

6), lithium perchlorate (LiClO

4), lithium tetrafluoroborate (LiBF

4) and lithium triflate (LiCF

3SO

3).

Depending on materials choices, the voltage, energy density, life and safety of a lithium-ion

battery can change dramatically. Recently, novel architectures using nanotechnology have been

employed to improve performance.

Pure lithium is highly reactive. It reacts vigorously with water to form lithium

hydroxide and hydrogen gas. Thus, a non-aqueous electrolyte is typically used, and a sealed

container rigidly excludes moisture from the battery pack.

Lithium ion batteries are more expensive than NiCd batteries but operate over a wider

temperature range with higher energy densities. They require a protective circuit to limit peak

voltage.

For notebooks or laptops, lithium-ion cells are supplied as part of a battery pack with temperature

sensors, voltage converter/regulator circuit, voltage tap, battery charge state monitor and the

main connector. These components monitor the state of charge and current in and out of each

cell, capacities of each individual cell (drastic change can lead to reverse polarities which is

dangerous),[39] temperature of each cell and minimize the risk of short circuits.[40]

ElectrochemistryThe participants in the electrochemical reactions in a lithium-ion battery are the negative and

positive electrodes with the electrolyte providing a conductive medium for Lithium-ions to move

between the electrodes.

Both electrodes allow lithium ions to move in and out of their interiors.

During insertion (or intercalation) ions move into the electrode. During the reverse

process, extraction (ordeintercalation), ions move back out. When a lithium-ion based cell

is discharging, the positive Lithium ion moves from the negative electrode (usually graphite) and

enters the positive electrode (lithium containing compound). When the cell is charging, the

reverse occurs.

Useful work is performed when electrons flow through a closed external circuit. The following

equations show one example of the chemistry, in units of moles, making it possible to use

coefficient  .

The positive electrode half-reaction is:[44]

The negative electrode half reaction is:

The overall reaction has its limits. Overdischarge supersaturates lithium cobalt oxide, leading to

the production of lithium oxide,[45] possibly by the following irreversible reaction:

Overcharge up to 5.2 volts leads to the synthesis of cobalt(IV) oxide, as evidenced by x-ray

diffraction:[46]

In a lithium-ion battery the lithium ions are transported to and from the positive or negative

electrodes by oxidizing the transition metal, cobalt (Co).

The cell's energy is equal to the voltage times the charge. Each gram of lithium

represents Faraday's constant/6.941 or 13,901 coulombs. At 3 V, this gives 41.7 kJ per gram of

lithium, or 11.6 kWh per kg. This is a bit more than the heat of combustion of gasoline, but does

not consider the other materials that go into a lithium battery and that make lithium batteries

many times heavier per unit of energy.

Charge and discharge[edit]

During discharge, lithium ions Li+

 carry the current from the negative to the positive electrode, through the non-

aqueous electrolyte and separator diaphragm.[53]

During charging, an external electrical power source (the charging circuit) applies an over-

voltage (a higher voltage but of the same polarity) than that produced by the battery, forcing the

current to pass in the reverse direction. The lithium ions then migrate from the positive to the

negative electrode, where they become embedded in the porous electrode material in a process

known as intercalation.

Charging procedure

The charging procedures for single Li-ion cells, and complete Li-ion batteries, are slightly

different.

A single Li-ion cell is charged in two stages:[39]

1. Constant current  (CC)

2. Voltage source  (CV)

A Li-ion battery (a set of Li-ion cells in series) is charged in three stages:

1. Constant current

2. Balance (not required once a battery is balanced)

3. Voltage source

During the constant current phase, the charger applies a constant current to the battery at a

steadily increasing voltage, until the voltage limit per cell is reached.

During the balance phase, the charger reduces the charging current (or cycle the charging on

and off to reduce the average current) while the state of charge of individual cells is brought to

the same level by a balancing circuit, until the battery is balanced. Some fast chargers skip this

stage. Some chargers accomplish the balance by charging each cell independently.

During the constant voltage phase, the charger applies a voltage equal to the maximum cell

voltage times the number of cells in series to the battery, as the current gradually declines

towards 0, until the current is below a set threshold of about 3% of initial constant charge current.

Periodic topping charge about once per 500 hours. Top charging is recommended to be initiated

when voltage goes below 4.05 V/cell.

Failure to follow current and voltage limitations can result in an explosion.[54]

Specifications[edit]

Specific energy density: 100 to 250 W·h/kg (360 to 900 kJ/kg)[60]

Volumetric energy density: 250 to 620 W·h/L (900 to 1900 J/cm³)[2]

Specific power density: 300 to 1500 W/kg (@ 20 seconds and 285 W·h/l)[1][not in citation given]

Because lithium-ion batteries can have a variety of positive and negative electrode materials, the

energy density and voltage vary accordingly.

The open circuit voltage is higher than aqueous batteries (such as lead acid, nickel-metal

hydride and nickel-cadmium).[61] Internal resistance increases with both cycling and age.[61]

[62] Rising internal resistance causes the voltage at the terminals to drop under load, which

reduces the maximum current draw. Eventually increasing resistance means that the battery can

no longer operate for an adequate period.

Batteries with a lithium iron phosphate positive and graphite negative electrodes have a nominal

open-circuit voltage of 3.2 V and a typical charging voltage of 3.6 V. Lithium nickel manganese

cobalt (NMC) oxide positives with graphite negatives have a 3.7 V nominal voltage with a 4.2 V

maximum while charging. The charging procedure is performed at constant voltage with current-

limiting circuitry (i.e., charging with constant current until a voltage of 4.2 V is reached in the cell

and continuing with a constant voltage applied until the current drops close to zero). Typically,

the charge is terminated at 3% of the initial charge current. In the past, lithium-ion batteries could

not be fast-charged and needed at least two hours to fully charge. Current-generation cells can

be fully charged in 45 minutes or less.

Comparison of battery types

Four figures help choose which battery type is best for a given application: The energy/weight ratio, the energy/volume ratio, the power to weight ratio, and the cost in watt-hours per dollar. In comparison to the common lead-acid battery, which is probably the least desirable to run an electric vehicle, the numbers are

BatteryType

Energy/weightWatt-hours/Kg

Energy/volumewatt-hours/L

Power/weightwatt/Kg

Energy/US$watt-hr/$

Lead-acid 30-40 60-75 180 4-10

NIckel-Zinc 60-70 170 900 2-3

Lithium-Ion 160 270 1800 3-5

Lithium-Polymer 130-200 300 to 2800 3-5

Two other values should also be considered, the self-discharge rate, which causes the charge to diminish over time, and the cycle life of the batteries, the number of times the batteries can undergo a deep discharge and still accept charge. The batteries listed above perform well in those categories.

Lithium-ion battery

A Nokia Li-ion battery used on the Nokia 3310mobile phone.

Specific energy 100–265 W·h/kg[1][2]

(0.36–0.95 MJ/kg)

Energy density250–730 W·h/L[2]

(0.90–2.23 MJ/L)

Specific power ~250-~340 W/kg[1]

Charge/discharge efficiency 80–90%[3]

Energy/consumer-price 2.5 W·h/US$

Self-discharge rate 8% at 21 °C

15% at 40 °C

31% at 60 °C

(per month)[4]

Cycle durability400–1200 cycles

[5]

Nominal cell voltage NMC 3.6 / 3.7 V,LiFePO4 3.2 V

Adaptable for both the ASC and WSC regulations. Extra batteries and fixing holes can be included so thatthe solar car can be raced at the World’s major events. The nominal battery voltage for the ASC is 100.8V(28 cells) and for the WSC 122.4V (34 cells).

2.3 BATTERY CAPACITY: Motor energy needed, EMO = (P * (d)) / V where

Total power, P

d – total dist.

Topspeed, V

The car however is expected to accelerate numerous times during the journey which will deplete more energy from the battery. Thus a battery capacity of roughly 3 times higher must be selected.