In Situ Species Analysis of a Lithium-Ion Battery Electrolyte Containing LiTFSI and Propylene Carbonate

In this study, we analyzed the species in a model electrolyte consisting of a lithium salt, lithium bis(trifluoromethane sulfone)imide (LiTFSI), and a widely used neutral solvent propylene carbonate (PC) with excess infrared (IR) spectroscopy, ab initio molecular dynamics simulations (AIMD), and quantum chemical calculations. Complexing species including the charged ones [Li⁺(PC)₄, TFSI^-, TFSI^-(PC), TFSI^-(PC)₂, and Li(TFSI)₂^-] are identified in the electrolyte. Quantum chemical calculations show strong Li⁺···O(PC) interaction, which suggests that Li⁺ would transport in the mode of solvation-carriage. However, the interaction energy of each hydrogen bond in TFSI^-(PC) is very weak, suggesting that TFSI^- would transport in hopping mode. In addition, the concentration dependences of the relative population of the species were also derived, providing a scenario for the dissolving process of the salt in PC. These in-depth studies provide physical insights into the structural and interactive properties of the electrolyte of lithium-ion batteries.