Single-crystal sodium nickel phosphate nanoparticles as ultra-high capacitance and rate-performance cathode of supercapacitor

Abstract

Sodium nickel phosphate (NaNiPO4, NNP) material is an attractive cathode for high performance supercapacitors due to its the abundant active sites of oxidation/reduction, highly stable framework structure etc. However, the disadvantage of low electric conductivity, disturbance of impure crystalline phase, numerous pores/gaps produced by agglomerated polycrystalline morphologies in this cathode often limited its electrochemical performances. Herein, the single-crystalline NNP rod-like nanoparticles with high purity-phase have been prepared by spontaneous combustion combined with subsequent solid-phase calcination. In order to enhance the conductivity of phosphate material, the surface of NNP naonoparticles was coated with the highly graphitized carbon, while the internal NNP nanoparticles still maintained the single-crystal morphology. It is noteworthy that the obtained NNP@C composite cathode displayed a recorded-up 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 single-crystalline morphology and graphitized carbon overlayer enhanced the interface electronic conductivity and interface/bulk ion diffusion velocity of NNP@C cathode. In addition, the assembled AC//NNP@C asymmetric capacitor has an ultrahigh energy density of 40.5 Wh kg-1 at a power density of 800 W kg-1 under the voltage window of 0∼1.6 V. This design could provide a new insight into designing the highly stabilized and high-conductivity polyanionic cathodes for supercapacitor.