Yttrium doping stabilizes the structure of Ni3(NO3)2(OH)4 cathodes for application in advanced Ni–Zn batteries

Abstract

Ni₃(NO₃)₂(OH)₄ has a high theoretical specific capacitance, low cost, and environmental friendliness, making it a promising electrode material. Specifically, Ni₃(NO₃)₂(OH)₄ electrodes have a larger layer spacing (c = 6.898 Å) than Ni(OH)₂ electrodes since NO₃⁻ has a much larger ionic radius than OH⁻. The larger layer spacing stores more electrolyte ions, significantly improving the electrochemical activity of the electrodes. Additionally, the interlayer NO₃⁻ can enhance the structural stability of Ni₃(NO₃)₂(OH)₄. However, since Ni₃(NO₃)₂(OH)₄ has a higher molar mass than Ni(OH)₂, it has a lower theoretical specific capacity. Consequently, Ni₃(NO₃)₂(OH)₄ has not been used in zinc-based alkaline batteries. Studies showed that doping could enhance the electrochemical performance of electrode materials. Therefore, this study used a simple solvothermal reaction to synthesize yttrium-doped Ni₃(NO₃)₂(OH)₄ (Y-Ni₃(NO₃)₂(OH)₄), assembling a Y-Ni₃(NO₃)₂(OH)₄//Zn battery for electrochemical testing. Y-Ni₃(NO₃)₂(OH)₄ served as the cathode in the battery. The analysis of Y-Ni₃(NO₃)₂(OH)₄ showed that yttrium (Y) doping increased the specific surface area and pore size of Ni₃(NO₃)₂(OH)₄ significantly. The increased specific surface area improved the active material utilization, and the abundant mesopores facilitated OH⁻ transport, substantially enhancing the battery's specific capacity and energy density. Ultimately, the specific discharge capacity of the advanced Y-Ni₃(NO₃)₂(OH)₄//Zn battery reached 177.97 mA h g⁻¹ at a current density of 4 A g⁻¹, nearly doubling the capacity of the earlier Ni₃(NO₃)₂(OH)₄//Zn battery (103.59 mA h g⁻¹).