A chiral salen-Co(II) complex as soluble redox mediator for promoting the electrochemical performance of Li-O₂batteries

Low discharge capacity and poor cycle stability are the major obstacles hindering the operation of Li-O₂ batteries with high-energy-density. These obstacles are mainly caused by the cathode passivation behaviours and the accumulation of by-products. Promoting the discharge process in solution and accelerating the decomposition of discharge products and by-products are able to alleviate above problems to some extent. Herein, chiral salen-Co(II) complex, (1R,2R)-(-)-N,N-bis(3,5-di-t-butylsalicylidene)-1,2-cyclohexanediaminocobalt(II) (Co(II)) as a multi-functional redox mediator was introduced into electrolyte to induce solution phase formation of Li₂O₂ and catalyze the oxidation of Li₂O₂ and main by-products Li₂CO₃. Due to the Co(II) has the solvation effect towards Li⁺, it can drive solution phase formation of Li₂O₂, to prevent electrode from passivation and then increase the discharge capacity with a high Li₂O₂ yield of 96.09 %. Furthermore, the Co(II) possesses suitable redox couple potentials, and it does so while simultaneously boosting the oxidization of Li₂O₂ and the decomposition of Li₂CO₃, reducing charge overpotential, and promoting cycle lifespan. Thereby, a cell with Co(II) achieved a long cycling stability at low charge plateau (3.66 V) over 252 cycles with a specific capacity of 500 mAh·g_carbon⁻¹.