Quantifying the reaction mechanisms of a high-capacity CuP2/C composite anode for potassium ion batteries

Introducing metals into phosphorus to form metal phosphide materials as anodes for potassium ion batteries (PIBs) is an effective strategy to improve the electronic conductivity and alleviate the volume change during cycling, although with a compromise of capacity. Herein, we explore a CuP2/C composite as a novel anode for PIBs, which delivers a high reversible capacity of >450 mA h g(-1). Unexpectedly, our results reveal that the POx components existing in the prepared composite are reversible, through a quantitative analysis via high-resolution solid-state P-31 NMR and synchrotron X-ray diffraction tests. Their potassiation products K3PO4 and K4P2O7 can react with K-P alloys and turn back to POx during depotassiation, which probably accounts for the high capacity of the prepared material. The results also illustrate a crystallization-amorphization evolution process during cycling involving nanocrystalline alpha-K4P6, K4P3 and KP, and amorphous K4P6, KP and K3P phases, among which, the amorphous phases are identified for the first time.