Charge-Discharge with Rocking-Chair-Type Li⁺ Migration Characteristics in a Zwitterionic Radical Copolymer Composed of TEMPO and Trifluoromethanesulfonylimide with Carbonate Electrolytes for a High-Rate Li-Ion Battery

Redox-active copolymer containing organic robust radical, 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO), and charge neutralizing anion, trifluoromethanesulfonylimide (TFSI^-) was synthesized as a cathode material of a Li-ion battery. The copolymer, poly(2,2,6,6-tetramethylpiperidin-1-oxy-4-yl methacrylate-co-styrenesulfonyl(trifluoromethanesulfonyl)imide) (P(TMA-co-TFSI)), was designed to give rise to the Li⁺ migration during its charge-discharge process, based on the self-charge compensation of TEMPO with TFSI^- bound to the polymer chain in a widely used electrolyte system for Li-ion battery, organic carbonate mixtures. Copolymerization was performed to achieve efficient self-charge compensation with uniformly distributed TFSI^- units. The P(TMA-co-TFSI) layer electrode exhibited reversible redox reaction at 0.73 V vs Ag/AgCl. Electrochemical measurements combined with quartz crystal microbalance analysis evidenced that the redox reaction involved the Li⁺ migration in binary system of ethylene carbonate and diethyl carbonate. A test cell fabricated with the P(TMA-co-TFSI) cathode exhibited high discharging voltage of 3.7 V and high-rate charge-discharge capability at 30 C (i.e., full charging in 2 min).