Reaction Mechanism of 1,4-Dimethoxy Benzene as an Overcharge Protection Additive

The reaction mechanism of 1,4-dimethoxybenzene (p-DMOB) as an overcharge protection additive for lithium ion batteries was determined by theoretical calculation of density functional theory (DFT) at the level of B3LYP/6-311+G(d,p) and MP2/6-311+G(d,p). It was found that p-DMOB is oxidized prior to the solvents, ethyl methyl carbonate, dimethyl carbonate, and ethylene carbonate, when the lithium ion battery is overcharged. The calculated oxidative potentials of p-DMOB by B3LYP and MP2 methods are well in agreement at 4.12 and 4.05 V (vs Li/Li^+·), respectively. The initial oxidation of p-DMOB involves a one-electron transfer resulting in a radical cation p-DMOB^+·. The corresponding energy variations were 701.24 and 728.27 kJ·mol^-1 from B3LYP and MP2 calculations, respectively. The p-DMOB^+· species then loses one proton forming a radical p-DMOB·through the breaking of a C―H bond on the benzene ring, with the corresponding energy variations of 1349.78 and 1810.99 kJ·mol^-1 for B3LYP and MP2, respectively. The p-DMOB·species is unstable and copolymerizes forming an insulated polymer with the corresponding energy variations of -553.37 and -1331.20 kJ·mol^-1 for B3LYP and MP2, respectively.