Novel Ni–Ge–P anodes for lithium-ion batteries with enhanced reversibility and reduced redox potential

Abstract

Nickel phosphides as anode materials for lithium-ion batteries (LIBs) have attracted much attention due to their large theoretical capacities and great phase diversity. However, the obstacles of large volume expansions, sluggish Li diffusion kinetics, and poor reversibility hinder their practical applications. Herein, we introduce Ge into the Ni–P system using a mechanical alloying method to synthesize Ni–Ge–P composites as novel anodes for LIBs. The results show that the Ge-substituted Ni₂Ge_0.5P_0.5, NiGe_0.5P_0.5, and NiGeP₂ compounds of the Ni–Ge–P composites possess higher crystal symmetry and larger cell volumes than the structurally similar Ni₂P, NiP, and NiP₃, respectively. When used as anodes, the Ni–Ge–P anodes exhibit enhanced electrochemical reversibility and reduced redox potential compared to the Ni–P anodes. Furthermore, the Li-storage mechanism of a selected NiGe_0.5P_0.5/Ni₂Ge_0.5P_0.5 composite was investigated, and the result shows that it undergoes a conversion-type reaction, along with the generation of the Ni, Li₂Ni₁₂P₇, LiP, Li₃P, and Li_xGe phases. In addition, a topological conversion reaction mechanism between the Ni₂P-type structure and Li₂Ni₁₂P₇ was proposed. Moreover, a Ni–Ge–P₂/C composite was synthesized to improve the cycling performance of Ni–Ge–P₂. This method of regulating the electrochemical properties of materials by Ge substitution for P offers new insight into designing high-performance anodes.