Urchin-like Ni2P yolk-shell microspheres with N-doped carbon shells for high-rate and ultralong cycle life K-ion batteries

Abstract The Ni2P anode own high theoretical capacity but still suffer from big volume expansion thus resulting sluggish electrochemical kinetics in K-ion batteries. Herein, we design and synthesize the urchin-like Ni2P yolk-shell microspheres encapsulated into N-doped carbon shells (Ni2P@N-C) for the first time through one-step hydrothermal followed a phosphidation/carbonization process. The unique urchin-like morphology provides fast charge transfers to the active sites for improved K-ion storage at high current densities. The yolk-shell structure couples with the nitrogen-doping carbon layer to effectively relieve the volume expansions while enhancing the conductivity of the whole anode composite. As a result, Ni2P@N-C electrode delivers an ultrahigh capacity of 172.6 mAh g−1 at 2000 mA g−1 after long time of 3000 cycles and shows outstanding rate performance of 136.6 mAh g−1 at 5000 mA g−1. Lastly, ex situ X-ray diffraction analysis of K+ ion insertion/extraction into/from the Ni2P@N-C electrode shows that potassium ion storage mechanism was to be Ni2P + 3K++3e-↔K3P + 2Ni.