Multi-functional zwitterionic compounds as additives for lithium battery electrolytes

A series of multi-functional zwitterionic compounds having both ester and sulfonate groups were synthesized and their electrochemical properties were investigated. The effect of added zwitterionic compounds on the cycling performance of the cell containing 1 M LiPF6 in EC, DMC, and EMC (1/1/1 by vol.) was also examined. The cell capacity was not varied much at 1/5 C up to 50 cycles with the addition of either 2.25 wt% N-methylpyrrolidinium-N-(propyl sulfonate) (MePyS) or N-methylpiperidinium-N-(propyl sulfonate) (MePipS) as an additive, but dropped significantly at higher C rate of 1 C. Such a sharp decrease of the performance at higher C rate was not observed when MePyS or MePipS was replaced by N-(2-acetoxyethyl) pyrrolidinium-N-(propyl sulfonate) (EsPyS) or N-(2-acetoxyethyl) piperidinium-N-(propyl sulfonate) (EsPipS), implying the positive role of the ester functional group. FT-IR study clearly demonstrates that ester-containing zwitterionic compounds are able to interact with Li+ ions through both sulfonate and ester functional groups.