Suppressing surface passivation of bimetallic phosphide by sulfur for long-life alkaline aqueous zinc batteries

Bimetallic phosphides may potentially benefit from the synergetic effects of two metallic elements for enhanced performances. For example, a nickel cobalt phosphide (NiCoP), in which Ni promotes the electrochemical activity of the materials while Co can reduce the charge transfer resistance, may potentially achieve remarkable performance in an alkaline aqueous zinc battery (AZB). However, the main challenge of bimetallic phosphides lies in the unsatisfactory electrochemical stability caused by their easy oxidization. Here we found sulfur-doping to bimetallic nickel cobalt phosphide (S-NiCoP) can effectively get its surface passivation suppressed. As a result, the S-NiCoP electrode exhibits a remarkable area specific capacity (1.32 mAh cm⁻²) and an ultra-long lifespan (capacity retention of 160% after 8000 cycles) in an AZB. The reaction mechanism is further revealed to be a reversible redox reaction from NiCoP to NiP_xOH and CoP_1-xOH. Furthermore, a quasi-solid-state S-NiCoP//Zn battery is fabricated. It performs well under various conditions (bending, washing, cutting) and delivers high energy density (21.5 mW h cm⁻³) and power density (336.84 mW cm⁻³). This work provides a general approach to stabilize bimetallic phosphides and opens a door to utilize the synergetic effects of various bimetallic phosphides for aqueous batteries.