Reaction mechanisms of aromatic compounds as an overcharge protection agent for 4 V class lithium-ion cells

Aromatic compounds such as biphenyl (BP), cyclohexylbenzene (CHB), and partially hydrogenated m-terphenyl (H-mTP) are used in commercial lithium-ion cells as a non-redox shuttle type overcharge protection agent, where they are electrochemically polymerized to form passivative films on the positive electrode under overcharge conditions. The reaction mechanisms of these aromatic compounds were investigated. The oxidation products of these aromatic compounds on the positive electrode were identified by electrochemical and surface analysis techniques including SEM, TPD–MS and MALDI-TOF-MS. They were the oligomers having 6–12 benzene rings, where the bond formation occurs mainly at ortho-positions of phenyl group. Their formation was not dependent on the monomer structure (BP, CHB, or H-mTP) and temperature (25 or 60 °C). It was considered that the cyclohexane structure in CHB or H-mTP was converted to the benzene structure by dehydrogenation after the polymerization.