A range of methyl 2-fluorocyanoester derivatives were synthesized from dimethyl 2-fluoromalonate ester, and their efficacy as additives in lithium-ion battery (LIB) electrolytes was determined. The role played by the 2-fluorocyanoester additives on battery performance was explored by linear sweep cyclic voltammetry, NMR, GCMS, and XPS techniques. For all fluorocyanoester additives studied, initial reduction of the carbonyl group occurs which is then followed by formation of the corresponding radical anion. Possible degradation routes arising from loss of fluoride ion, loss of methyl radicals, and cleavage of the αβ carbon-carbon bond were observed, and all affect battery performance. Electrode protection upon addition of fluorocyanoesters to the electrolyte is the main contribution to the improvement of battery stability, but improvements on the electrode protection are somewhat offset by free radical processes initiated at the anode. Longer alkyl-chain fluorocyanoesters showed the best LIB improvement with effective cathode protection.
Keywords: electrolyte additive; fluorination; fluorocyanoester; lithium-ion battery; solid electrolyte interface.