Liquid Electrolyte for Lithium Batteries

1. Liquid Electrolytes For Li-batteries
SAIFUL ISLAM
ID: 156086
Nano Energy Lab

2. 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

3. 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.

4. 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

5. 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.

6. 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

7. 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.

8. Characteristics of Liquid Electrolytes
s=NAeS|Zi| Ci mi
NA -the Avogadro constants
e- elementary electric charge
Zi- charge no. of ionic species i
C- concentration
m- 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

9. • 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)
Tetrahydro
furan
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.

10. • Interfacial characteristics of electrodes/Electrolytes
I. 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.

11. 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.

12.  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.

13.  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
LiPF6 - liquid electrolytes

14. Development of Liquid Electrolytes
Organic 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.

15.  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

16.  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