High-Specific Capacity Cathodes Based on Amorphous Trimetallic NiCoMn-P Phosphide Nanowire Arrays

A nickel foam-supported trimetallic phosphide nanowire array electrode (NiCoMn-P/NF) is fabricated by a hydrothermal reaction followed by phosphidation of the NiCoMn-OH hydroxide precursor. The NiCoMn-P nanowires are composed of composite components of amorphous NiCoP and Mn x Co 1– x P with a small number of dispersed NiCoP and Mn x Co 1– x P nanocrystallines adhered to the surface. The electrode reaction mechanism is investigated based on the findings. It is found that the as-prepared phosphide electrode has a very negative open-circuit potential after immersion in alkaline electrolyte, and the electrode material is converted to metal hydroxide species via spontaneous chemical reactions during the immersion and the next electrochemical reactions upon the first charge from the open-circuit potential to a given potential. Thereafter, the converted metal hydroxide active material continues charging to 0.5 V (vs Hg/HgO) and then discharging to 0.1 V, where a reversible redox reaction takes place in the potential range of 0.1–0.5 V, and hence, it can serve as a cathode for rechargeable alkaline batteries. This electrode exhibits an ultrahigh discharge specific capacity of 822.7 mAh g –1 at 0.3 A g –1 and a retained value of 518.4 mAh g –1 even at 10 A g –1 within 0.1–0.5 V. After 800 cycles at 5 A g –1, the capacity retention is 73.9%. The high specific capacity is believed to be related to its particular trimetallic composition and amorphous nanostructure, which facilitate the two-electron transfer reaction. Moreover, the assembled Zn/6 M KOH/NiCoMn-P full cell exhibits excellent long-term cycling stability over a voltage range of 1.4–1.9 V.