Understanding the Critical Role of Binders in Phosphorus/Carbon Anode for Sodium‐Ion Batteries through Unexpected Mechanism

Polymer binders that combine active materials with conductive agents have played a critical role in maintaining the structural integrity of phosphorus anodes with a huge volume change upon sodiation/desodiation. Herein, the role of binders on the structural/chemical stability of phosphorus/carbon anode is spectroscopically uncovered through unexpected mechanism. Surprisingly, the selection of different binders is found to determine the oxidation degree of active phosphorus in various electrodes, which correlate well with their electrochemical properties. At a high oxidation degree, the electrode applying a conventional poly(vinylidene difluoride) binder displays the worst electrochemical properties, while the electrode using a sodium alginate binder delivers the best electrochemical performance (a highly reversible capacity of 1064 mAh g(-1)with a 90.1% capacity retention at 800 mA g(-1)after 200 cycles and an outstanding rate capability of 401 mAh g(-1)at 8000 mA g(-1)) for its negligible oxidation. Additionally, the emergence/decomposition of surface intermediates, including (PO2)(3-)and (PO4)(3-)species, are observed in the discharging/charging processes via the ex situ P K-edge X-ray absorption spectroscopies. This novel discovery of the unique role of binders in phosphorus anodes, not only provides an opportunity to ameliorate their electrochemical properties, but also enables their practical applications in high-energy sodium-ion batteries.