Unveiling the Interfacial Instability of the Phosphorus/Carbon Anode for Sodium-Ion Batteries

As a competitive anode material for sodium-ion batteries (SIBs), a commercially available red phosphorus, featured with a high theoretical capacity (2596 mA h g^-1) and a suitable operating voltage plateau (0.1-0.6 V), has been confronted with a severe structural instability and a rapid capacity degradation upon large volumetric change. In particular, the fundamental determining factors for phosphorus anode materials are yet poorly understood, and their interfacial stability against ambient air has not been explored and clarified. Herein, a high-performance phosphorus/carbon anode material has been fabricated simply through ball-milling the carbon black and red phosphorus, delivering a high reversible capacity of 1070 mA h g^-1 at 400 mA g^-1 after 200 cycles and a superior rate capability of 479 mA h g^-1 at 3200 mA g^-1. More importantly, we first reveal the significance of inhibiting the exposure of phosphorus/carbon electrode materials to air, even for a short period, for achieving a good electrochemical performance, which would sharply decrease the reversible capacities. With the assistance of synchrotron-based X-ray techniques, the formation and accumulation of insulating phosphate compounds can be spectroscopically identified, leading to the decay of electrochemical performance. At the same time, these passivation layers on the surface of electrode were found to occur via a self-oxidation process in ambient air. To maintain the electrochemical advantages of phosphorus anodes, it is necessary to inhibit their contact with air through a rational coating or an optimal storage condition. Additionally, the employment of a fluoroethylene carbonate (FEC) additive facilitates the decomposition of the electrolyte and favors the formation of a robust solid electrolyte interphase layer, which may suppress the side reactions between the active Na-P compounds and the electrolyte. These findings could help improve the surface protection and interfacial stability of phosphorus anodes for high-performance SIBs.