What are the classifications of lithium-ion battery electrolytes?
The electrolyte in a lithium-ion battery is as vital to the human body as blood is to the human body. It is the medium through which lithium ions move back and forth between the lithium cathode and the cathode. Without it, there would be no electron flow, and the battery would not exist. Therefore, its importance is self-evident. (Explanation of the original text regarding electrolyte properties.)
The electrolyte serves to transfer charge between the positive and negative electrodes, and should be able to conduct ions while insulating electrons. It has a significant impact on the battery's cycle performance, operating temperature range, and battery durability. For lithium-ion batteries, the electrolyte composition includes at least two components: a solvent and a lithium salt.
A: Liquid electrolyte
The selection of a solvent is primarily based on three performance requirements: dielectric constant, viscosity, and electron-donating ability. Generally, a high dielectric constant facilitates the dissociation of lithium salts, while strong electron-donating ability promotes the solubility of electrolyte salts. The electron-donating property of a solvent refers to the inherent electron-losing ability of solvent molecules, which determines the solvation ability of electrolyte cations. Low viscosity increases ion mobility and helps improve conductivity.
Currently, binary and multi-component mixed solvents are commonly used, consisting of two or more solvents. Common organic solvents include ethers, alkyl carbonates, lactones, and ketones.
Lithium is primarily used to supply an efficient carrier. The selection of lithium salts generally follows these principles:
It exhibits good stability (compatibility) with both positive and negative electrode materials, meaning that the electrochemical reaction rate between the electrolyte and the active material is very low during storage, minimizing the self-discharge capacity loss of the battery; the solution has high specific conductivity and low ohmic voltage drop; it is highly safe, non-toxic, and pollution-free.
Several commonly used lithium salts include lithium hexafluoroarsenide (LiPF6), which releases toxic arsenic compounds during charging and discharging and is relatively expensive. LiPF6 is widely used in commercial batteries due to its good conductivity and compatibility with carbon materials. Its disadvantages include high price, poor solid-state stability, and sensitivity to water. Lithium trifluoromethanesulfonate has good stability, but its conductivity is only half that of LiPF6-based liquid electrolytes. Lithium tetrafluoroborate (LiBF4) and lithium perchlorate (LiClO4) are widely used inorganic salts. However, lithium salts containing lithium perchlorate, especially lithium difluorosulfonyl phthalate (LiN(CF3SO2)2), have higher stability than FLiCF3SO2 and their conductivity is comparable to that of LiPF6 electrolytes.
b. Solid electrolyte
Solid electrolytes, also known as superionic conductors or fast ion conductors, refer to a class of solid ionic conductive materials whose ionic conductivity is close to (or in some cases exceeds) that of melting and electrolyte solutions. They are a peculiar solid substance existing between a solid and a liquid. This is an unusual state of matter where some atoms (ions) exhibit near-liquid mobility, while others maintain their spatial structure (arrangement). This liquid-solid two-phase property and its broad application prospects in energy (including production, storage, and energy conservation), metallurgy, environmental protection, electrochemistry, and other fields have attracted widespread attention from physicists, chemists, and materials scientists.
Polymer solid electrolytes are solid electrolyte materials composed of polymers and salt-containing soluble polar groups. Besides exhibiting the performance characteristics of common conductivity systems such as semiconductors and ionic solutions, they also possess plasticity not found in inorganic solid electrolytes, giving polymer solid electrolytes three major advantages in applications:
Thin films of any shape or thickness are possible. Therefore, although the room-temperature conductivity of polymer electrolytes is not high—it is 2-3 orders of magnitude lower than that of inorganic electrolytes—the internal resistance of the battery is greatly reduced due to the thin-film processing, which compensates for the low conductivity by increasing the area/thickness ratio. Excellent contact with the electrodes increases the charge/discharge current; responsiveness allows it to withstand pressure changes and adapt to changes in electrode volume during charge and discharge. Polymer solid electrolytes and their electrolyte solutions are lightweight, pressure-resistant, shock-resistant, fatigue-resistant, non-toxic, and non-corrosive.
