Modest modulation on the electronic structure of Co9S8 by vanadium doping for high-performance rechargeable Zn–air batteries

Rechargeable Zn-air batteries (ZABs) have been attracting research interests worldwide owing to their high energy density, as well as the good safety and low cost. However, the lack of highly active bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at air cathode seriously hinders its developments. Herein, a vanadium doped Co9S8 (V-Co9S8) nanoparticles encapsulated in the nitrogen-doped porous carbon nanoflowers (NFs) has been prepared by an innovative double-solvent approach (DSA) that can precisely control the level of vanadium doping. The magnetic measurements and the theoretical calculation consistently demonstrate that the spin state of Co can be optimized by tuning the vanadium doping level. The spin-state modulation raises the d-band center of Co and enables the enhanced Co-S covalency in V-Co9S8, which balances the adsorption/desorption of oxygen intermediates and finally accelerates the oxygen redox kinetic. As a result, V-Co9S8 serving as a cost-effectively bifunctional catalyst for the cathode of a rechargeable ZAB, supplies a superb power density of 345 mW cm−2, an energy density of 814 mAh gZn−1 and a super durability with a stable operation of 840 cycles, which outperforms the noble metal catalyst (Pt/C and RuO2) and the most Co based catalysts reported recently.