Single -crystal sodium nickel phosphate nanoparticles as supercapacitor cathodes with ultra-high capacitance and rate-performance

Sodium nickel phosphate (NaNiPO₄, NNP) is an attractive cathode material for high performance supercapacitors due to its abundance of active sites for oxidation/reduction, highly stable framework structure, etc. However, its disadvantages of low electric conductivity, disturbances of its impure crystalline phase, and the numerous pores/gaps produced by agglomerated polycrystalline morphologies in this cathode often limit its electrochemical performance. Herein, single-crystalline NNP rod-like nanoparticles with high phase purity have been prepared by spontaneous combustion combined with subsequent solid-phase calcination. To enhance the conductivity of the phosphate material, the surface of the NNP nanoparticles was coated with highly graphitized carbon, while the internal NNP nanoparticles still maintained a single-crystal morphology. It is noteworthy that the obtained NNP@C composite cathode displayed a record discharge specific capacitance of 1163 F g^-1, an excellent rate capacitance of 861 F g^-1 at 25 A g^-1, and outstanding cycling performance (94% capacitance retention after 5000 cycles). A series of measurements indicated that the synergistic effect of the single-crystalline morphology and graphitized carbon overlayer enhanced the interface electronic conductivity and interface/bulk ion diffusion velocity of the NNP@C cathode. In addition, an AC//NNP@C asymmetric capacitor has an ultrahigh energy density of 40.5 W h kg^-1 at a power density of 800 W kg^-1 in a voltage window of 0-1.6 V. This design could provide new insight into the design of highly stabilized and high-conductivity polyanionic cathodes for supercapacitors.