As one of the four main materials in lithium-ion batteries, electrolytes primarily serve as carriers for ion migration, ensuring the transport of ions between the positive and negative electrodes.
Electrolyte has a certain impact on battery performance indicators such as safety, cycle life, charge/discharge rate, high and low temperature performance, and energy density.
Electrolytes are generally composed of high-purity organic solvents, lithium salts, and additives in a certain proportion. By mass, solvents account for 80% to 90%, lithium salts for 10% to 15%, and additives for about 5%; by cost, lithium salts account for about 40% to 50%, solvents for about 30%, and additives for about 10% to 30%.
Compared to the other three materials, lithium batteries have the most complex requirements for electrolytes, which must possess multiple properties:
Good ionic conductivity and low ion migration resistance are required;
It exhibits high chemical stability and will not undergo harmful side reactions with electrode materials, electrolytes, or diaphragms.
It has a low melting point and a high boiling point, and remains liquid over a wide temperature range;
It has good safety, simple preparation process, low cost, and is non-toxic and pollution-free.
Currently, lithium hexafluorophosphate (LiPF6) is the mainstream lithium salt solute due to its good performance and low cost. It exhibits good solubility and high conductivity in various non-aqueous solvents, is chemically stable, has good safety, and causes minimal environmental pollution. However, its drawbacks are also significant: lithium hexafluorophosphate is sensitive to moisture, has poor thermal stability, and may begin to decompose at temperatures as low as 60°C, leading to rapid performance degradation in batteries. Its cycling performance at low temperatures is also relatively poor, and its operating temperature range is narrow.
In addition, lithium hexafluorophosphate has very high requirements for purity and stability, and the production process involves harsh conditions such as low temperature, strong corrosion, and water and dust-free conditions, making production quite difficult.
Among the new generation of lithium salts, lithium bisfluorosulfonylimide (LiFSI) is considered a promising replacement for lithium hexafluorophosphate. Compared to traditional lithium salts, LiFSI has higher thermal stability and advantages in conductivity, cycle life, and low-temperature performance.
However, due to limitations in production processes and capacity, LiFSI is too expensive, far exceeding the cost of lithium hexafluorophosphate. To control costs, LiFSI is still primarily used as an electrolyte additive in practical commercial applications, rather than as a lithium salt solute.
