Bis(trimethylsilyl) 2-fluoromalonate derivatives as electrolyte additives for high voltage lithium ion batteries

Three trimethylsilyl based malonate esters, bis(trimethylsilyl) 2-methyl-2-fluoromalonate (BTMSMFM), bis(trimethylsilyl) 2-ethyl-2-fluoromalonate (BTMSEFM) and bis(trimethylsilyl) 2-propyl-2-fluoromalonate (BTMSPFM), have been used as additives in 1.0 M LiPF₆/ethylene carbonate (EC)-dimethyl carbonate (DMC)-diethyl carbonate (DEC) (1-1-1, by v) baseline electrolyte for LiNi_0.80Co_0.15Al_0.05O₂ (NCA) based high voltage lithium ion batteries. The NCA half-cells with 5 wt% BTMSMFM exhibit higher capacity retention than that in the baseline electrolyte at different upper cut off voltages, that is, 4.2, 4.3, 4.4 and 4.5 V vs. Li/Li⁺. Scanning electron microscope (SEM) show that the additive successfully prevents the formation of thick solid electrolyte interphase (SEI) films on the surface of the NCA electrodes. X-ray diffraction (XRD) further reveals that the crystal structure of NCA is also maintained in the electrolyte with 5 wt% BTMSMFM at high cut off voltages. Besides beneficial to NCA cathode, the BTMSMFM additive also ensures better cycling performance of the graphite based half-cells and NCA/graphite full-cells, and thus is a promising additive for application in rechargeable lithium ion batteries.