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

As a competitive anode material for sodium-ion batteries, a commercially available red phosphorus, featured with a high theoretical capacity (2596 mAh g-1) and a suitable operative voltage plateau (0.1~0.6 V), has confronted with a severe structural instability and a rapid capacity degradation upon the large volumetric change. In particular, the fundamental determining factors for phosphorus anode materials are yet poorly understood, and their interfacial stability against the ambient air has not been explored and clarified. Herein, a high-performance phosphorus/carbon anode material has simply been fabricated through ball-milling the carbon black and red phosphorus, delivering a high reversible capacity of 1070 mAh g-1 at 400 mA g-1 after 200 cycles and a superior rate capability of 479 mAh g-1 at 3200 mA g-1. More importantly, we firstly reveal the significance of inhibiting phosphorus/carbon electrode materials from exposing to air, even for a short period, on 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 to air through a rational coating or an optimal storage condition. Additionally, the employment of a FEC (fluorinated ethylene carbonate) additive facilitates the decomposition of 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 electrolyte. These findings could enlighten the surface protection and interfacial stability of phosphorus anodes for high-performance sodium-ion batteries.