Types of lithium-ion battery electrolytes
1. Liquid electrolyte
The choice of electrolyte has a significant impact on the performance of lithium-ion batteries. It must possess good chemical stability, especially at high potentials and temperatures, and be resistant to decomposition. It must also have high ionic conductivity (10⁻³ S/cm) and be inert to the anode and cathode materials, preventing corrosion. Because lithium-ion batteries have high charge/discharge potentials and the anode material contains chemically active lithium ions, the electrolyte must be an organic compound and cannot contain water. However, organic compounds generally have poor ionic conductivity, so soluble conductive salts are added to the organic solvent to improve it. Currently, liquid electrolytes are commonly used in lithium-ion batteries, with anhydrous organic solvents such as EC (ethylcarbonate), PC (propylene carbonate), DMC (dimethylcarbonate), and DEC (diethylcarbonate). Mixed solvents are also commonly used, such as EC₂DMC and PC₂DMC. Conductive salts include LiClO₄, LiPF₆, LibF₆, LiAsF₆, and LiOSO₂CF₃, with their conductivity in the following order: LiAsF₆, LiPF₆, LiClO₄, LibF₆, LiOSO₂CF₃. LiClO4, due to its high oxidizing power, is prone to explosions and other safety issues, and is generally limited to experimental research. LiAsF6 has high ionic conductivity, is easy to purify, and has good stability, but it contains toxic As, which restricts its use. LibF6 has poor chemical and thermal stability and low conductivity, while LiiOSO2CF3 has poor conductivity and corrosive effects on electrodes, so it is rarely used. Although LiPF6 undergoes decomposition, it has high ionic conductivity, so it is currently the primary electrolyte used in lithium-ion batteries. Most commercially available lithium-ion batteries currently use LiPF6 EC2DMC as the electrolyte, which has high ionic conductivity and good electrochemical stability.
2. Solid electrolyte
Using metallic lithium ions directly as the anode material offers a high reversible capacity, with a theoretical capacity as high as 3862 mAh g⁻¹, more than ten times that of graphite materials. Its lower price makes it a highly attractive anode material for next-generation lithium-ion batteries, but it also leads to the formation of lithium dendrites. Using a solid electrolyte as the ion conductor suppresses the growth of lithium dendrites, making the use of metallic lithium ions as the anode material possible. Furthermore, using a solid electrolyte prevents the leakage problems of liquid electrolytes and allows for thinner (only 0.1 mm thick), higher energy density, and smaller size high-energy batteries. Destructive testing shows that solid-state lithium-ion batteries have high safety performance. After destructive tests such as nail penetration, heating (200℃), short circuit, and overcharging (600%), liquid electrolyte lithium-ion batteries exhibit leakage and explosion safety issues, while solid-state batteries show only a slight increase in internal temperature (<20℃) without any other safety problems. Solid polymer electrolytes possess good flexibility, film-forming properties, stability, and low cost, and can be used as both a separator between positive and negative electrodes and an electrolyte for ion transport.
Solid polymer electrolytes (SPEs) can generally be classified into dry solid polymer electrolytes (SPEs) and gel polymer electrolytes (GPEs). SPEs are primarily based on polyethylene oxide (PEO), but their disadvantage is low ionic conductivity, reaching only 10⁻⁴⁰ cm⁻¹ at 100°C. In SPEs, ionic conduction mainly occurs in the amorphous regions, relying on the movement of polymer chains for transport. PEO readily crystallizes due to the high regularity of its molecular chains, and crystallization reduces ionic conductivity. Therefore, to improve ionic conductivity, one approach is to reduce the crystallinity of the polymer, increasing chain mobility, and another is to increase the solubility of the conductive salt in the polymer. Using grafting, block polymerization, crosslinking, and copolymerization to disrupt the crystallinity of polymers can significantly improve ionic conductivity. Furthermore, adding inorganic complex salts can also enhance ionic conductivity. Adding a high-dielectric-constant, low-molecular-weight liquid organic solvent, such as PC, to a solid polymer electrolyte can greatly increase the solubility of the conductive salt. The resulting electrolyte is the GPE gel polymer electrolyte, which exhibits high ionic conductivity at room temperature, but may experience liquid separation and failure during use. Gel polymer lithium-ion batteries have been commercialized.
