A chiral salen-Co(II) complex as soluble redox mediator for promoting the electrochemical performance of Li-O2 batteries

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·gcarbon−1.