Fluoroethylene Carbonate As an Promising Additive for γ-Butyrolactone based Electrolytes

Compared to the actual state of the art electrolyte (SOTA) for lithium ion batteries (LIBs), a novel electrolyte with better safety and lower costs and at the same time equal electrochemical performance was developed and investigated in this work. This electrolyte consists of a mixture of γ-Butyrolactone (GBL) as a solvent with fluoroethylene carbonate (FEC) as solid electrolyte interphase (SEI) additive and lithium tetrafluro borate (LiBF₄) as electrolyte salt. Raman spectroscopy measurements indicated that the salt is completely dissociated by the solvent GBL which is the reason for the enhanced ionic conductivity compared to the SOTA electrolyte. The oxidation stability of this mixture is sufficient for the use in actual LIBs. Cyclic voltammetry with a T44 graphite electrode revealed the positive influence of the SEI additive FEC. The addition of FEC (1 % by wt.) enabled fully reversible de-/ intercalation of lithium ions into graphite. Furthermore, 1 M LiBF₄ in GBL + FEC (1 % by wt.) improved the coulombic efficiency of T44 graphite in the first cycle of constant current cycling experiments and in addition, no capacity fading was observed after 50 cycles. On NCM electrodes a stable cycling was observed as well, obtaining similar performance compared to the SOTA electrolyte, even at an increased cut-off potential of 4.6 V vs. Li/Li⁺.