Fluorine-doping-induced phosphorus vacancy engineering in NiCoP@NC for enhanced sodium storage performance

Abstract

Transition metal phosphides (TMPs), particularly NiCoP composites, have emerged as promising anode candidates for sodium ion batteries (SIBs). However, their poor intrinsic conductivity and significant volume changes during sodiation/desodiation hinder reaction kinetics and limit capacity utilization. Vacancy defect engineering can become an effective strategy to address these challenges. Herein, a F-NiCoP@NC composite was successfully synthesized via a hydrothermal method, followed by in situ phosphidation and fluorine doping to induce significant phosphorus vacancies. The obtained F-NiCoP@NC electrode demonstrated a higher reversible discharge specific capacity (430.9 mA h g⁻¹ after 200 cycles at 0.1 A g⁻¹) and excellent long-term cycling stability (267.2 mA h g⁻¹ after 1000 cycles at 1.0 A g⁻¹). The ex situ XRD results combined with the ex situ EIS results reveal the conversion reaction mechanism, while Density Functional Theory (DFT) calculations confirmed that phosphorus vacancies induced by fluorine doping enhance the intrinsic electrical conductivity and the storage of Na⁺. Furthermore, a full cell based on the F-NiCoP@NC anode and Na₃V₂(PO₄)₃ cathode was assembled to demonstrate promising potential for practical applications.