Free-Standing Electrode of Core–Shell-Structured NiO@Co₃S₄ for High-Performance Hybrid Zn–Co/Air Batteries

Hybrid Zn batteries, which couple two electrochemical reactions in one cell, attract research interests owing to the combination of high energy density and energy efficiency, and the electrode material is usually focused on metal oxides, such as nickel and cobalt oxides, which are restricted by the poor electric conductivity and limited cycling stability. In this work, a free-standing NiO@Co₃S₄ electrode with a core–shell structure is successfully fabricated and used in a hybrid Zn battery for the first time. As a result of the high electric conductivity of metal sulfides, synergistic effects between NiO and Co₃S₄, and unique core–shell structure, this electrode exhibits improved performance in both pseudocapacitance and oxygen electrocatalysis compared to the pristine Co₃O₄ electrode. A hybrid Zn–Co/air battery with this electrode delivers the discharge voltage plateaus of 1.75 and 1.28 V and charge voltage plateaus of 1.80 and 1.95 V at the current density of 1 mA cm^–2, which correspond to the Zn–Co and Zn–air reactions, respectively. Moreover, it operates for 100 cycles at 2 mA cm^–2 with stable voltages of over 200 h. This work fabricates a high-performance electrode for hybrid Zn batteries and provides a promising design strategy for electrochemical storage materials.