What is the electrolyte in a lithium battery?
electrolyte
The electrolyte in a lithium-ion battery is the carrier of ions in the battery. It is generally composed of lithium salts, organic solvents, and additives, as shown in Figure 7-4. The electrolyte plays a crucial role in conducting ions between the positive and negative electrodes of a lithium-ion battery, ensuring its advantages such as high voltage and high specific energy. Electrolytes are typically prepared under specific conditions and in specific proportions from high-purity organic solvents, lithium salts, and necessary additives. While electrode materials determine the battery's energy density, the electrolyte fundamentally determines its cycle life, high and low temperature performance, and safety. The basic composition of the electrolyte remains relatively unchanged; innovation mainly lies in the development of novel lithium salts and additives, as well as a deeper understanding of the interfacial chemical processes and mechanisms involved in lithium-ion batteries.
There are many types of lithium salts, as shown in Figure 7-5, but very few are used in commercially available lithium-ion batteries. An ideal lithium salt should possess the following properties:
1) Low association degree, readily soluble in organic solvents, ensuring high ionic conductivity of the electrolyte.
2) Anions with antioxidant and reduction resistance; reduction products facilitate the formation of a stable, low-resistance SEI film.
3) Good chemical stability, without causing harmful side reactions with electrode materials, electrolytes, or separators.
4) Simple preparation process, low cost, non-toxic and pollution-free.
LiPF6 is the most widely used lithium salt. While its individual properties are not its most outstanding, it exhibits relatively optimal overall performance in carbonate mixed solvent electrolytes. LiPF6 has the following key advantages:
1) Suitable solubility and high ionic conductivity in non-aqueous solvents.
2) It can form a stable passivation film on the surface of aluminum foil current collectors.
3) It synergistically forms a stable SEI film on the graphite electrode surface with carbonate solvents.
However, LiPF6 has poor thermal stability and is prone to decomposition reactions. Byproducts can damage the SEI film on the electrode surface, dissolve the positive electrode active components, and lead to capacity decay during cycling.
LiBF is also a commonly used lithium salt additive. Compared to LiPF6, LiBF has a wider operating temperature range, better high-temperature stability, and superior low-temperature performance. LiBF possesses high conductivity, a wide electrochemical window, and good thermal stability. Its greatest advantage lies in its film-forming properties, as it can directly participate in the formation of the SEI film.
Structurally, LiDFOB is composed of half-molecules of LiBOB and LiBF, combining the advantages of good film-forming properties of LiBOB and the good low-temperature performance of LiBF4. Compared with LiBOB, LiDFOB has higher solubility in linear carbonate solvents and higher electrolyte conductivity. Its high-temperature and low-temperature performance is better than LiPF4, and it has good compatibility with the battery cathode, forming a passivation film on the aluminum foil surface and inhibiting electrolyte oxidation.
The CF₃SO₂ groups in the LiTFSI structure have a strong electron-withdrawing effect, which exacerbates the delocalization of the negative charge and reduces ion association pairing, resulting in high solubility of the salt. Furthermore, LiTFSI has high electrical conductivity, a high thermal decomposition temperature, and is not easily hydrolyzed; however, it will severely corrode aluminum current collectors at voltages above 3.7V.
The fluorine atoms in the LiFSI molecule have strong electron-withdrawing properties, which delocalize the negative charge on N, resulting in weak ion association and easy dissociation of Li+, thus leading to high conductivity.
LiPO2F2 exhibits good low-temperature performance and also improves the high-temperature performance of the electrolyte. As an additive, it can form an SEI film rich in LixPOyFz and LiF on the negative electrode surface, which helps reduce battery interfacial impedance and improve battery cycle performance. However, LiPO2F2 also suffers from low solubility.
The main component of the liquid electrolyte is the organic solvent, which dissolves lithium salts and provides a carrier for lithium ions. An ideal organic solvent for a lithium-ion battery electrolyte needs to meet the following conditions:
1) High dielectric constant and strong dissolving ability for lithium salts.
2) Low melting point and high boiling point, maintaining a liquid state over a wide temperature range.
3) Low viscosity, facilitating lithium-ion transport.
4) Good chemical stability, does not damage the positive and negative electrode structure or dissolve the positive and negative electrode materials.
5) High flash point, good safety, low cost, non-toxic and non-polluting.
Common organic solvents used in lithium-ion battery electrolytes are mainly divided into carbonate solvents and organic ether solvents, as shown in Figure 7-6. To obtain a high-performance lithium-ion battery electrolyte, a mixed solvent containing two or more organic solvents is usually used, allowing them to complement each other and achieve better overall performance. The physical properties of common carbonate solvents are shown in Table 7-1.
Table 7-1 Physical Properties of Common Carbonate Solvents
| Organic Solvent | Relative Dielectric Constant | Melting Point/°C | Boiling Point/°C | Viscosity Coefficient |
|---|---|---|---|---|
| Ethylene Carbonate (EC) | 89.6 | 37 | 243 | 1.86 |
| Propylene Carbonate (PC) | 64.4 | -55 | 240 | 2.53 |
| Dimethyl Carbonate (DMC) | 0.59 | 2 | 91 | 0.59 |
| Diethyl Carbonate (DEC) | 2.8 | -43 | 126 | 0.75 |
| Ethyl Methyl Carbonate (EMC) | 3.0 | -53 | 108 | 0.65 |
Organic ether solvents mainly include chain ethers such as 1,2-dimethoxypropane (DMP), dimethoxymethane (DMM), and ethylene glycol dimethyl ether (DME), and cyclic ethers such as tetrahydrofuran (THF) and 2-methyltetrahydrofuran (2-Me-THF). For chain ether solvents, the longer the carbon chain, the better the chemical stability, but the higher the viscosity and the lower the lithium-ion migration rate. Ethylene glycol dimethyl ether can form a relatively stable chelate (LiPF6·DME) with lithium hexafluorophosphate, exhibiting strong dissolving power for lithium salts and resulting in high electrolyte conductivity. However, DME has poor chemical stability and cannot form a stable passivation film on the surface of the negative electrode material.
Carbonate solvents include cyclic carbonates such as propylene carbonate (PC) and ethylene carbonate (EC), and chain carbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC). Cyclic carbonates have a high dielectric constant, making lithium salts more soluble, but they also have high viscosity, resulting in a lower lithium-ion migration rate. Chain carbonates have a low dielectric constant and weak lithium salt solubility, but low viscosity and good flowability, facilitating lithium-ion migration.
The types of flame-retardant additives for lithium-ion electrolytes are shown in Figure 7-7. Additives, used in small amounts, have significant effects and are an economical and practical method for improving the performance of lithium-ion batteries. By adding a small dose of additives to the electrolyte of lithium-ion batteries, certain battery performance characteristics can be specifically improved, such as reversible capacity, electrode/electrolyte compatibility, cycle performance, rate performance, and safety performance, playing a crucial role in lithium-ion batteries. An ideal lithium-ion battery electrolyte additive should possess the following four characteristics:
1) High solubility in organic solvents.
2) A small amount can significantly improve one or more performance characteristics.
3) No harmful side reactions with other battery components that affect battery performance.
4) Low cost, non-toxic or low toxicity.
Based on their function, additives can be categorized into conductive additives, overcharge protection additives, flame retardant additives, SEI film-forming additives, cathode material protectants, LiPF6 stabilizers, and other functional additives.
Conductive additives improve the rate performance of lithium-ion batteries by coordinating with electrolyte ions, promoting lithium salt dissolution, and increasing electrolyte conductivity. Because conductive additives work through coordination reactions, they are also called ligand additives, and are classified into anionic ligands, cationic ligands, and neutral ligands based on the interacting ion.
Overcharge protection additives provide overcharge protection or enhance overcharge tolerance. They are functionally classified into redox additives and monomer additives. Currently, redox additives are mainly anisole series, which have high redox potentials and good solubility. Monomer additives undergo polymerization reactions under high voltage, releasing gases, and the polymer coats the cathode material surface, interrupting charging. Monomer additives mainly include aromatic compounds such as xylene and phenylcyclohexane.
Flame retardant additives function by raising the ignition point of the electrolyte or terminating the free radical chain reaction that inhibits combustion. Their types are shown in Figure 7-8. Adding flame retardants is one of the important ways to reduce the flammability of the electrolyte, broaden the operating temperature range of lithium-ion batteries, and improve their performance. The mechanisms of action of flame retardant additives are mainly twofold:
1) By creating an insulating layer between the gas phase and the condensed phase, they prevent combustion in both the condensed and gas phases.
2) They capture free radicals during the combustion reaction process, terminating the free radical chain reaction that inhibits combustion reactions between the gas phases.
