Liquid Electrolytes For Li-batteries

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Nano Energy Lab

1. Electrolytes

2. Requirement of Liquid Electrolytes

3. Components of Liquid Electrolytes

3. Characteristics of Liquid Electrolytes

4. Electrolytes Additives

5. Development of Liquid Electrolytes

Outline

Electrolytes

i. Electrolytes-acts as a medium for the movement of ions and commonly consist of solvents and salt.

ii. Electrolytes - Liquid electrolytes-organic solvents - Solid electrolytes - Inorganic compounds or polymers while polymers are prepared from the polymers and salt.

In general “electrolyte solution” is used to refer to liquid electrolyte

Role of electrolyte:- transports the lithium ions from anode to cathode during charging, and vice versa.

Requirements of Liquid Electrolytes

Liquid electrolytes = Lithium salt + organic solvent.

I. The electrolyte should have high ionic conductivity-The movement of lithium ions at the electrodes and diffusion within the electrolyte

are important when lithium secondary batteries are rapidly charged and discharged (~10-

3 S/cm).

II. The electrolyte should exhibits high chemical and electrochemical stability towards electrodes.

- should be electrochemically stable within potential range of redox reaction. - chemically stable towards various metals and polymers constituting the

cathode ,anode and battery.

III. The electrolyte should be used over a wide temperature range -20 to 600C –temeprature range. - Electrochemical stability drops , while ionic conductivity increases.

IV. The electrolyte should be highly safe - Organic solvents used in electrolytes are flammable, need to use nonflammable materials should be used. - low toxicity in case of leakage or disposal.

V. The electrolyte should be low cost - for commercialization

Components of Liquid Electrolytes

Organic Solvents – i. Since lithium secondary batteries have a high working voltage , organic solvents

are used instead of aqueous electrolytes. Examples: EC, PC,DMC,DEC etc.ii. Greatest disadvantage of organic solvents is the low dielectric constant.iii. Electrolyte should possess high ionic conductivity to dissolve the lithium salts

and exits as polar aprotic solvents to avoid chemical reaction with lithium.

iv. Solvent viscosity should be low 1cP or less.v. Solvents should exhibits a high boiling point and low vapor pressure..

High dielectric constant and low viscosity are required for electrolytes to possess the high ionic conductivity.

But higher dielectric constant leads to increased polarity and viscosity.

Lithium Salt

Table: Physicochemical properties of lithium salts

L0-limiting molar conductivity

Anions with larger radii are favored since lithium salt having localized anions tends to dissociate more readily.

Li(CF3SO2)2N > LiAsF6 > LiPF6 > LiClO4 > LiBF4

On the other hand, an increase in ionic radius leads too less movement if anions.

Size of the anions is the important factor that determines the properties of the lithium salt.

Lithium salt Tm(0C) Anion diameter(nm) L0 in PC(Scm2/mol)

LiBF4 > 300 0.229 28.9

LiClO4 236 0.237 27.4

LiPF6 194 0.254 26.3

LiAsF6 >300 0.260 26.0

LiCF3SO3 >300 0.270 2.3

Li(CF3SO2)2N 228 0.325 22.8

LiC4F9SO3 >300 0.339 21.5

Li(CF3SO2)3C 263 0.375 20.2

Fig.3.125 Space-filling model and ionic radii of lithium salts.

LiClO4 is used as a lithium salt in lithium primary batteries but not in lithium secondary batteries due to safety issue arising from the oxidation environment during charging.

LiBF4 and LiPF6 –commonly used in lithium ion batteries-solubility and chemical stable.

But LiBF4-lower conductivity than LiPF6 and LiClO4 –affects the high rate characteristics LiPF6-high ionic conductivity and low thermal stability.

Characteristics of Liquid Electrolytes

s=NAeS|Zi| Ci mi

NA -the Avogadro constantse- elementary electric chargeZi- charge no. of ionic species iC- concentrationm- mobility.

I. Ionic conductivity becomes higher with an increase in the

number of dissociated free ions and more rapid migration of such ions.(10-3 S/cm).

II. Ionic conductivity can be measured using -conductivity meter - based on the resistance of an electrode having a known cell constant - ionic conductivity= distance between electrodes/(solvent resistance x

electrode surface area).

Ionic Conductivity

• Electrochemical stability

I. Do not Participate in redox reaction.II. LSV –used to scan the potential of working electrode(Pt) with reference

electrode(Li metal). A rapid increase or decrease in current corresponds to the decomposition voltage.

Ethylene carbonate

(EC)

Propylene carbonate

(PC)

Dimethyl carbonate

(DMC)

Diethyl carbonate

(DEC)

Ethylmethyl carbonate

(EMC)

1,2-Dimethoxye

thane(DME)

Tetrahydrofuran

1,3-Dioxolane

Methyl formate

(MF)

6.2 6.6 6.7 6.7 6.7 5.1 5.2 5.2 5.4

Eox- Oxidative potential(Scan rate: 5 mV/S); reference electrode:Li

As shown in above table , alkyl carbonates or ester solvents are able to withstand oxidation at 1V higher than ether solvents.

So, low viscosity ether solvents are widely used in 3V lithium primary batteries, while carbonate solvents are common in 4V lithium secondary batteries.

• Interfacial characteristics of electrodes/ElectrolytesI. SEI layer greatly affects the charge/discharge cycle characteristics.II. As the lithium diffuses with cycle progression, SEI layer has a direct

influence over additional reaction between the electrolyte and electrodes.

For example:-For a graphite cathode, EC is favored over PC, which destroy graphene layer and hinders SEI formation.

• Operating Temperature(-20 to 600C)i. melting point and boiling point of solvents must be carefully

considered.ii. solvents with low melting point(DEC,DME and PC) are mixed with

EC,DMC that exist as a solid at 00C.iii. when lithium salt is precipitated due to weak dissociation by the

solvent ,this temp. becomes lower limit for electrolyte.

• Cation Transport Number

t+ =s+/s++s- = m+/m+ + m-Measuring methods-alternating current impedance , Tubandt’s method,

Hittorf’s method The Cation number influence by Temperature, salt concentration in electrolyte, ionic radius and electric charge.

Electrolytes Additives

Main function of additive to enhance the ionic conductivity, battery life and safety.

The additives engage in different reactions with electrodes and the electrolyte.

Characteristics of Different Additives SEI formation additives Overcharge inhibitors Ionic conductivity enhancers Flame retardants

SEI formation additives• Vinylene carbonate(VC) is common additives used to form and maintain

an SEI layer at the surface of a carbon anode.• VC produces a stable SEI layer during the initial charging process and

enhances battery life by preventing carbon exfoliation and direct reaction with the electrolyte.

• VC does not affect the cathode and maintains SEI stability at high temperatures.

Overcharge Inhibitors The battery assembled with Positive temperature coefficient(PTC),

protection circuit module and safety vents.

• Redox Shuttle Type The redox shuttle type restrict high voltage by allowing the excess

charge to dissipate within the cell. The addition of n-butylferrocene to 2V Li/TiS2 battery. When cell voltage exceeds to cut off voltage- n-butylferrocene at the

cathode surface is oxidized.

Ionic Conductivity Enhancers

-to improve ionic conductivity ,the dissociation of lithium salt must be promoted and the dissociated ions should remains in the ionic state and ions must have high mobility.

-Crown ether additives allows greater dissociation of lithium salt by isolating Li+ based on the ion dipole interaction and separating from anion.

- When boron based additive combined with ploy(ethylene glycol)(PEG) ,ionic conductivity increases with greater dissociation of lithium salt within the

electrolyte.

Flame Retardants - organic solvents should have high boiling point and form a protective layer

during thermal decomposition to block out oxides and combustible gases LiPF 6 - liquid electrolytes

Development of Liquid ElectrolytesOrganic Solvents PC and EC are being proposed to allow more reversible chemical

reactions in the graphite anode.

The alkyl carbonate were chosen due to their acceptable anodic stability for the 4 V cathodes used in Li-ion battery as well as lithiated graphite.

Due to its high dielectric constant it has the ability to dissolve a wide variety of lithium salts.

PC solvent does not show good reversibility toward the graphite anode, treating the solvent with fluorine slows down decomposition reactions and facilitates lithium intercalation/deintercalation.

There are a number of properties in choosing the appropriate lithium salt for use in lithium batteries:1. The ability to completely dissolve and dissociate in a

nonaqueous medium and the solvated ions (especially the lithium cation) should be able to move in the medium with high mobility.

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

3. The anion should be inert to electrolyte solvents.

Ex: LiPF6, LiAsF6, LiClO4, LiBF4

LiPF6 was used by Sony in 1990 in the first generation lithium ion cell.

Other commercially available Li salts have too many disadvantages: LiAsF6 is poisonous, LiClO4 is explosive, and LiBF4 is problematic on the negative side (reactions of the BF4 - anion on the anode’s surface interfere badly with passivation).

Lithium Salts

Development of Liquid Electrolytes

The most used salt in the commercial liquid electrolytes is indeed LiPF6 due to several reasons:1. When dissolved in alkyl carbonates, LiPF6 shows high

conductivity, which may exceed 10 mS cm-1 at room temperature.

2. Shows good solubility properties in various solvents.3. Its oxidation stability reaches ~ 5V.

At elevated temperature, due to the poor thermal stability, LiPF6 exists in various forms, while absorbing the trace amount of H2O (in ppm level) that exists in the electrolyte solution.

LiPF6

Latter, P-F bonds are highly susceptible to hydrolysis even at room-temperature to produce HF. At temperatures PF5 tends to undergo hydrolysis, resulted the following dangerous by products

Development of Liquid Electrolytes

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