Electrochemical Analysis of the Hard Carbon/Electrolyte Interface in Lithium-Ion Batteries at Low Temperature: Fluoroethylene Carbonate as an Electrolyte Additive

The propylene carbonate (PC)-based electrolyte system is characterized by a wide liquid range and favorable Li+ migration kinetics, which endows it with excellent low-temperature applicability when paired with hard carbon (HC) anodes in lithium-ion batteries (LIBs). However, the PC-induced SEI structure is loose, which reduces the cycle stability and reversibility of LIBs. Here, the film-forming additive fluoroethylene carbonate (FEC) is introduced into the electrolyte to optimize the SEI. The results show that after adding 2% FEC, FHC-2 exhibits higher specific capacity (170.22 mAh/g at −20 °C, 82.24 mAh/g at −40 °C) and lower interface impedance at low temperature. FEC has a lower LUMO energy level, which can preferentially undergo reductive decomposition at the anode interface, playing a role in SEI construction. LiF forms a new grain boundary in the SEI, which provides a new migration path for the migration of Li+ at low temperature, promoting rapid Li+ migration. Furthermore, the fluorine atom substitution of FEC weakens the electron cloud density of the carbonyl group, which makes the binding strength of FEC and Li lower than that of the main solvent, and Li+ is easier to desolventize. This work offers guidance on future commercial HC anodes for LIBs, as well as on selection and design of low-temperature electrolytes.