Unraveling the Lithium/Sodium-Ion Diffusion Mechanism in Alloyed Phosphides for Lithium/Sodium Storage

Phosphorus is a promising candidate for high energy density lithium/sodium-ion batteries due to its high capacity. However, the dynamics of carrier shuttling in the alloyed phosphorus anode are quite complex due to the multiple phase transitions during the lithiation/sodiation process. Here, we identify Li(Na)P and Li(Na)3P as stable phases from both ex situ and in situ X-ray diffraction measurements of black phosphorus (BP). Our results regarding the dynamics of lithiation/sodiation indicate that the entire reaction could be limited by sluggish lithium/sodium ion migration in the stable phases, Li(Na)P and Li(Na)3P, supported by ab initio molecular dynamics (AIMD) simulations. Additionally, Li3P has higher activation energies and correspondingly lower ion conductivity than LiP, making it the key role in the whole dynamics of lithium-ion transport during the lithiation process. In comparison, the dynamics of sodiation exhibit quite active behavior due to lower energy barriers for sodium-ion transport. However, the electrochemical performance of sodium-ion batteries suffers from a large charge transfer resistance at the electrode/electrolyte interface due to the generation of corrosive Na3P. Therefore, the construction of a stable solid electrolyte interphase (SEI) is crucial to improving the electrochemical performance of phosphorus anodes.