Microstructure controlled synthesis of Ni, N-codoped CoP/carbon fiber hybrids with improving reaction kinetics for superior sodium storage

Transition-metal phosphides (TMPs)-based hybrid structure have received considerable attention for efficient sodium storage owing to their high capacity and decent reversibility. However, the volume expansion & the poor electronic conductivity of TMPs, the poor-rate capability, and fast capacity decay greatly hinder its practical application. To address these issues, a low-cost and facile strategy for the synthesis of Ni, N-codoped graphitized carbon (C) and cobalt phosphide (CoP) embedded in carbon fiber (Ni-CoP@C-N⊂CF) as self-supporting anode material is demonstrated for the first time. The graphitized carbon and carbon fiber improve the electrical conductivity and inhibit the volume expansion issues. In addition to that, the microporous structure, and ultrasmall sized Ni-CoP offer a high surface area for electrolyte wettability, short Na-ion diffusion path and fast charge transport kinetics. As a result, outstanding electrochemical performance with an average capacity decay of 0.04% cycle⁻¹ at 2000 mA g⁻¹, an excellent rate capability of 270 mAh g⁻¹@2000 mA g⁻¹ and a high energy density of ~231.1 Wh kg⁻¹ is achieved with binder-free self-supporting anode material. This work shows a potential for designing binder-free and high energy density sodium-ion batteries.