2,5-Difluoro-1,4-dimethoxybenzene for overcharge protection of secondary lithium batteries

Based on the voltammetric behaviour of a series of halogen-substituted dimethoxybenzene in 1 M LiPF6/EC:DEC electrolyte, 2,5-difluoro-1,4-dimethoxybenzene (F2DMB) was selected and tested as an electrolyte additive for overcharge protection of Li/Li4Ti5O12 cell. From the galvanostatic study of the cells at different overcharge current (C/20, C/50, C/100 and C/200) in the presence of F2DMB, it was found that the shuttle additive can be adsorbed at the cathode surface and form a dense layer which prevents the intercalation of Li+ ion in the positive electrode. At low overcharge current (C/200 rate) the voltage of the cell levelled off at the oxidation potential of the shuttle molecule for more than 50 cycles, but at higher charge rates (C/50 and C/100), the voltage of the cell was levelled off for only 16 cycles. The reason is that the F2DMB molecules remaining in solution after the formation of the layer at the cathode cannot carry the current even at charge rates as low as C/100.