The electrolyte in square lithium - ion batteries serves as the medium for lithium - ion transport between the anode and the cathode. It plays a vital role in determining the battery's performance, safety, and lifespan.

Liquid electrolytes are the most commonly used type in commercial square lithium - ion batteries. They typically consist of a lithium - salt dissolved in an organic solvent. Lithium hexafluorophosphate (LiPF₆) is the most widely used lithium - salt. It dissociates in the organic solvent to release lithium ions, which can then move through the electrolyte to facilitate the charge - discharge process. The organic solvents used are usually a mixture of carbonate - based solvents, such as ethylene carbonate (EC), propylene carbonate (PC), and dimethyl carbonate (DMC). EC has a high dielectric constant, which helps in the dissociation of LiPF₆, while DMC has a low viscosity, allowing for better lithium - ion mobility. The combination of these solvents in the right ratio can optimize the electrolyte's properties, such as its conductivity, stability, and compatibility with the electrode materials. However, liquid electrolytes also have some drawbacks. They are flammable, which poses a safety risk, especially in high - temperature or over - charging situations.

To address the safety issues associated with liquid electrolytes, solid - state electrolytes are being developed as an alternative. Solid - state electrolytes can be inorganic or organic. Inorganic solid - state electrolytes, such as lithium - garnet - type (LLZO) and lithium - phosphorous - oxynitride (LIPON), have high ionic conductivity and good thermal stability. They are non - flammable, which significantly improves the safety of lithium - ion batteries. Organic solid - state electrolytes, on the other hand, are often based on polymers, such as polyethylene oxide (PEO). PEO - based solid - state electrolytes can be easily processed and have good flexibility. However, their ionic conductivity at room temperature is generally lower compared to liquid electrolytes. Researchers are working on improving the ionic conductivity of solid - state electrolytes through various methods, such as doping with other elements or creating composite materials.

The interface between the electrolyte and the electrode materials is also an important consideration. A stable and low - resistance interface is crucial for efficient lithium - ion transfer. During the charge - discharge process, a solid - electrolyte - interphase (SEI) layer forms on the surface of the anode. The SEI layer is a thin, protective film that allows lithium ions to pass through while blocking the solvent molecules. The formation and properties of the SEI layer can be influenced by the electrolyte composition. For example, additives can be added to the electrolyte to promote the formation of a more stable and conductive SEI layer. Some additives can also improve the compatibility of the electrolyte with the cathode material, enhancing the overall performance of the battery. In summary, the development of advanced electrolytes is an ongoing area of research in the field of square lithium - ion batteries, aiming to improve battery performance, safety, and lifespan.