Lithium-Ion Battery Electrolyte Overview

Lithium-ion battery electrolytes are the ion-conducting medium that enables lithium ions to move between the cathode and anode during charge and discharge. A conventional electrolyte is typically composed of a lithium salt, organic carbonate solvents, and small amounts of functional additives.

Common lithium salts include LiPF6, which is widely used because it offers good ionic conductivity and compatibility with commercial electrode materials. However, LiPF6 is sensitive to moisture and heat, producing acidic species such as HF that can degrade cell components. Alternative salts such as LiFSI, LiTFSI, and LiBOB are used or studied to improve thermal stability, high-voltage performance, or solid–electrolyte interphase formation.

The solvent system usually combines cyclic carbonates, such as ethylene carbonate, with linear carbonates, such as dimethyl carbonate, diethyl carbonate, or ethyl methyl carbonate. Ethylene carbonate helps form a stable solid–electrolyte interphase on graphite anodes, while linear carbonates reduce viscosity and improve low-temperature conductivity.

Electrolyte additives are essential for performance and lifetime. Vinylene carbonate and fluoroethylene carbonate are commonly used to improve anode interphase stability, especially for graphite and silicon-containing anodes. Other additives can reduce gas generation, suppress metal dissolution, improve flame resistance, or stabilize high-voltage cathodes.

Key performance requirements include high ionic conductivity, wide electrochemical stability window, low viscosity, good wetting of electrodes and separators, thermal stability, low flammability, and compatibility with both anode and cathode surfaces. The electrolyte must also support formation of stable interphases: the solid–electrolyte interphase on the anode and the cathode–electrolyte interphase on high-voltage cathodes.

Current development trends include high-concentration electrolytes, localized high-concentration electrolytes, fluorinated solvent systems, nonflammable electrolytes, gel polymer electrolytes, and solid-state electrolytes. These approaches aim to improve fast charging, safety, cycle life, low-temperature operation, and compatibility with high-energy materials such as lithium metal anodes, silicon-rich anodes, and nickel-rich cathodes.