Introduction to Electrolytes for Lithium Ion Batteries

 

Electrolytes play a role in ion conduction and electronic insulation between the positive and negative electrodes of a battery.

In the secondary lithium battery, the nature of electrolyte has an important impact on the cycle life, operating temperature range, charging and discharging efficiency, battery safety, power density and other performance of the battery.

 

1、 The electrolyte material for secondary lithium batteries should have the following properties:

(1) High lithium ion conductivity

 

(2) High electrochemical stability, maintaining temperature over a wide potential range.

 

(3) It has good compatibility with the electrode and can effectively form a stable SEI film on the negative electrode. On the positive electrode, it has sufficient antioxidant decomposition ability under high potential conditions.

 

(4) Good contact with the electrode, sufficient wetting of the electrode for liquid electrolytes.

 

(5) Good low-temperature performance, able to maintain high conductivity and low viscosity in a lower temperature range (-20~20 ℃), in order to maintain good electrode surface wettability during the charging and discharging process.

 

(6) Wide voltage range.

 

(7) Good thermal stability, no thermal decomposition occurs over a wide temperature range.

 

(8) Good chemical stability. During the long-term cycling and storage process of the battery, it does not undergo chemical reactions on its own, nor does it react with materials such as positive electrode, negative electrode, current collector, adhesive, conductive agent, separator, packaging material, sealing agent, etc.

 

(9) Non toxic, pollution-free, safe to use, preferably biodegradable.

 

(10) Easy preparation and low cost.

 

2、 Classification of electrolyte materials for lithium-ion batteries

1. Non aqueous organic liquid electrolytes

 

2. Polymer electrolytes (all solid and colloidal)

 

3. Inorganic solid electrolyte

 

III Characteristics of high-performance electrolyte lithium salts

1. Lithium salts have a sufficiently high solubility in organic solvents and are not associated. The solvated cations should have a high mobility

 

2. Anions shall not undergo redox Chemical decomposition during positive charging

 

3. Anions and cations should not react with electrodes, membranes, and packaging materials

 

4. Lithium salts should be non-toxic and the decomposition products should have minimal environmental pollution

 

5. High thermal stability, easy preparation, and low cost

 

4、 Electrolyte lithium salt

Lithium perchlorate: good comprehensive performance, but poor safety due to strong oxidation

 

Lithium hexafluoroarsenate: Its comprehensive performance is relatively good, but its toxicity is too high

 

Lithium tetrafluoroborate: good low-temperature performance, but expensive and low solubility

 

Lithium hexafluorophosphate: good comprehensive performance, easy absorption and hydrolysis

 

Lithium diborate ester: It has good high-temperature performance, especially in inhibiting solvent insertion damage to the negative electrode, but its solubility is too low

 

5、 Selection criteria for organic solvents

① Organic solvents should be inert to the electrodes and do not undergo electrochemical reactions with the positive and negative electrodes during the charging and discharging process of the battery, resulting in good stability;

 

② Organic solvents should have high dielectric constant and low viscosity to ensure sufficient solubility of lithium salts and high conductivity;

 

③ Low melting point, high boiling point, and low steam pressure, resulting in a wider working temperature range;

 

④ It has good compatibility with electrode materials and can exhibit excellent electrochemical performance in the electrolyte composed of it;

 

⑤ Consideration of battery cycle efficiency, cost, environmental factors, etc.

 

6、 Non aqueous organic solvents

1. Carbonates

 

Ring: EC (Ethylene carbonate), PC (propylene carbonate)

 

Chain shape: DMC (Dimethyl carbonate), DEC (Diethyl carbonate), EMC (methyl ethyl carbonate), MPC (methyl propyl carbonate)

 

2. Ethers

 

Ring: tetrahydrofuran (THF)

 

Chain: DME (1,2-di Methoxy group ethane), DMM (dipropylene glycol dimethyl ether), DMP (Dimethyl phthalate)

 

3. Carboxylates

 

Circular: γ- Butyrolactone

 

Chain: MF (methyl formate), EA (ethyl acetate), MA (maleic anhydride)

 

7、 Electrolyte additive

1. SO2/CO2/VC (ethylene carbonate)

 

Improving SEI membrane performance

 

2. Trimethyl phosphate (TMP)

 

Improving electrolyte safety

 

3. Crown ether

 

Improving the conductivity of electrolyte

 

4. Carbonates of Al2O2, MgO, lithium or calcium

 

Control the content of water and acid

 

Due to the shortcomings of liquid electrolytes such as leakage, flammability, volatility, and instability—— Hope: Solid electrolyte (with both electrolyte and diaphragm functions)

 

Advantages of polymer electrolytes: flexibility of polymer materials, good film-forming properties, viscoelasticity, stability, lightweight, low cost, mechanical properties, and electrochemical stability.