Conversion of hydroxide into carbon-coated phosphide using plasma for sodium ion batteries

Transition metal phosphides (TMPs) are promising candidates for sodium ion battery anode materials because of their high theoretical capacity and earth abundance. Similar to many other P-based conversion type electrodes, TMPs suffer from large volumetric expansion upon cycling and thus quick performance fading. Moreover, TMPs are easily oxidized in air, resulting in a surface phosphate layer that not only decreases the electric conductivity but also hinders the Na ion transport. In this work, we present a general electrode design that overcomes these two major challenges facing TMPs. Using metal hydroxide and glucose as precursors, we show that the metal hydroxide can be converted into phosphide whereas the glucose simultaneously decomposes and forms carbon shell on the phosphide particles under a plasma ambient. Ni₂P@C core shell structures as a proof-of-concept are designed and synthesized. The in situ formed carbon shell protects the Ni₂P from oxidation. Moreover, the high-energy plasma introduces porosity and vacancies to the Ni₂P and more importantly produces phosphorus-rich nickel phosphides (NiP_x). As a result, the Ni₂P@C electrodes achieve high sodium capacity (693 mAh·g⁻¹ after 50 cycles at 100 mA·g⁻¹) and excellent cyclability (steady capacity maintained for at least 1, 500 cycles). Our work provides a general strategy for enhancing the sodium storage performance of TMPs, and in general many other conversion type electrode materials that are unstable in air and suffer from large volumetric changes upon cycling.