The Evolution of Electrochemical and Safety Properties of Nonflammable Triethyl Phosphate Based Electrolyte Under Salt Concentration Control

ABSTRACT Phosphates exhibit excellent flame retardancy. Phosphate‐based electrolytes with high‐salt‐concentration possess unique solvation structure and superior cycling stability, making them a potential alternative to conventional carbonate‐based electrolytes. However, using high‐concentration lithium salts increases the cost and viscosity of the electrolyte. Furthermore, research on the safety properties of cells based on phosphate‐based electrolytes lags behind the extensive studies of their electrochemical performance. This study systematically investigates the evolution of the electrochemical and safety performance of triethyl phosphate (TEP) electrolytes containing different concentrations of lithium bis (fluorosulfonyl) imide (LiFSI) (1, 2, and 3 M). The results indicate that an increased concentration of LiFSI promotes the formation of a stable solid electrolyte interphase (SEI) film enriched with lithium fluoride (LiF), thereby enhancing cycling stability. However, excessively high salt concentration leads to increased battery polarization and reduced reversible capacity. The LiFePO 4 (LFP)||Li cells based on the 2 M LiFSI/TEP with 2% fluoroethylene carbonate (FEC) electrolyte are found to exhibit optimal electrochemical performance, achieving a capacity retention rate of 99.42% after 50 cycles. In terms of safety performance, the thermal stability of the electrolyte, the electrolyte interphase, and the electrolyte/electrode composite system under the regulation of salt concentration are systematically revealed. High lithium salt concentration lowers the initial heat generation temperature of the electrolyte, enhancing the thermal stability of the electrode interphase film and effectively preventing the coupled reactions between the electrolyte and the lithium metal anode. This work establishes a clear reference for controlling the salt concentration of phosphate‐based nonflammable electrolytes.