Vanadium diphosphide as a negative electrode material for sodium secondary batteries

The abundance of sodium resources has sparked interest in the development of sodium-ion batteries for large-scale energy storage systems, amplifying the need for high-performance negative electrodes. Although transition metal phosphide electrodes have shown remarkable performance and great versatility for both lithium and sodium batteries, their electrochemical mechanisms in sodium batteries, particularly vanadium phosphides, remain largely elusive. Herein, we delineate the performance of VP₂ as a negative electrode alongside ionic liquids in sodium-ion batteries. The polycrystalline VP₂ is synthesized via one-step high energy ball-milling and characterized using X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Electrochemical tests ascertained improved performance at intermediate temperatures, where the initial cycle was conducted at 100 mA g⁻¹ yielded a significantly higher discharge capacity of 243 mAh g⁻¹ at 90 °C compared to the limited capacity of 49 mAh g⁻¹ at 25 °C. Enhanced rate and cycle performance are also achieved at 90 °C. Electrochemical impedance spectroscopy and scanning electron microscopy further reveal a reduced charge transfer resistance at 90 °C and the formation of a uniform and stable solid electrolyte interface (SEI) layer after cycling. X-ray diffraction and nuclear magnetic resonance spectroscopy are used to confirm the conversion-based mechanism forming Na₃P after charging.