Phase evolution governed by phosphorization thermodynamics enabling an integrated NiS/NiP2 electrode with enhanced pseudocapacitive performance

Transition metal sulfides (TMSs) and phosphides (TMPs) composites are considered promising electrode materials for pseudocapacitive energy storage due to their high energy storage capacity and rapid charge/discharge kinetics. However, the precise and efficient synthesis of well-integrated TMSs/TMPs heterostructures remains a significant challenge, primarily due to the competitive coordination behavior between sulfur and phosphorus atoms during their reaction with transition metals. In this study, Ni₉S₈ was employed as a precursor, and through precisely controlling the phosphorization temperature (490–570 °C), phosphorus source (NaH₂PO₂), and reaction time (1 h), a one-step synthesis of an integrated NiS/NiP₂ electrode was successfully achieved. Comprehensive experimental investigations reveal that the selection of an optimal temperature range is critical that temperatures below 490 °C impede phosphorus diffusion, while temperatures above 570 °C promote the undesired formation of NiO due to reactions with oxygen-containing intermediates generated during the decomposition of the phosphorus source. Electrochemical analyses demonstrate that the resulting NiS/NiP₂ composite exhibits a specific capacitance of 1721.8 F g⁻¹ at 1 A g⁻¹, outperforming most previously reported TMSs and TMPs-based electrodes. Furthermore, the fabricated NiS/NiP₂//NC asymmetric supercapacitor delivers an energy density of up to 89.41 Wh kg⁻¹ at a power density of 749.60 W kg⁻¹ and retains 91.32 % of its initial capacitance after 10,000 cycles at 1 A g⁻¹, indicating excellent cycling stability. This work provides valuable experimental insights into phosphorization conditions for the development of integrated sulfide-phosphide electrode materials with enhanced electrochemical performance.