Modest modulation on the spin states of Co3O4 nanoframe through vanadium doping for solid-state rechargeable Zn-air/iodide hybrid batteries

The rational construction of bifunctional air cathode electrocatalysts with high intrinsic reversibility and durability is critical for practical applications of rechargeable solid Zn-air batteries (ZABs). Unlike traditional nanomorphology construction methods, this work focus on the enhancement of intrinsic catalytic properties by heteroatom vanadium doping strategy to modulate the spin state of Co³⁺ ions active sites at the octahedral sites of spinel Co₃O₄. The magnetic measurements and the theoretical calculation consistently reveal that V atom incorporation induce the spin state transition of Co³⁺ from low spin state (LS, $t_{2g}^6{e}_g^0$) to intermediate spin state (IS, $t_{2g}^5{e}_g^1$), and reduces the energy gap between the Co 3d and O 2p band centers, which balance the adsorption/desorption of oxygen intermediates and further enhance the oxygen reaction kinetics. Consequently, the optimized V-Co₃O₄ demonstrated a small potential gap value of 0.74 V between the ORR half-wave potential and the OER potential at 10 mA m⁻². More importantly, the introduction of KI in solid-state electrolyte provides a more thermodynamically favorable pathway. Thereby, the assembled ZAB displays excellent long-term durability (180 h), offering a promising power source for the next-generation electronics. This work advances a feasible design concept to tune catalytic activity for ZABs by optimizing the spin states and developing novel solid-state electrolytes.