Unraveling the reaction mechanism of high reversible capacity CuP2/C anode with native oxidation PO component for sodium-ion batteries

Phosphorus-based anode is a promising anode for sodium-ion batteries (SIBs) due to its high specific capacity, however, suffers from poor electronic conductivity and unfavorable electrochemical reversibility. Incorporating metals such as copper (Cu) into phosphorus has been demonstrated to not only improve the electronic conductivity but also accommodate the volume change during cycling, yet the underline sodiation mechanism is not clear. Herein, take a copper phosphide and reduced graphene oxide (CuP2/C) composite as an example, which delivers a high reversible capacity of >900 mAh/g. Interestingly, it is revealed that the native oxidation POx components of the CuP2/C composite show higher electrochemical reversibility than the bulk CuP2, based on a quantitative analysis of high-resolution solid-state 31P NMR, ex-situ XPS and synchrotron X-ray diffraction characterization techniques. The sodiation products Na3PO4 and Na4P2O7 derived from POx could react with Na-P alloys and regenerate to POx during charge process, which probably accounts for the high reversible capacity of the CuP2/C anode. The findings also illustrate that the phosphorus transforms into nanocrystalline Na3P and NaxP alloys, which laterally shows crystallization-amorphization evolution process during cycling.