Silicon substituted Na3V2(PO4)3/C nanocomposites enwrapped on conducting graphene for high-rate and long-lifespan sodium ion batteries

Abstract Na3V2(PO4)3 (NVP) has been intensively pursed to be a promising cathode material due to its high potential platform and large capacity. Nevertheless, the poor intrinsic conductivity seriously restricts its further applications. Herein, we firstly propose a facile strategy to improve the conductivity and electrochemical performance by introducing beneficial silicon substitution combined with the assistant of conducting reduced graphene oxide (rGO). Due to the large ionic radius of Si4+, the substitution of Si can expand the channels for Na+ migration, facilitating the ionic conductivity effectively. Meanwhile, Si doping is in favor of forming a porous structure, improving the interconnection between active materials and electrolyte. Besides, the introduced rGO can suppress the agglomeration of active particles, constructing a carbon network and enhancing the electronic conductivity significantly. Compared with the undoped NVP/C sample, Na3.1V2(PO4)2.9(SiO4)0.1/C@rGO composite displays a superior electrochemical performance: it delivers an initial discharge capacity of 113.6 mA h g−1 (close to the theoretical capacity of 117 mA h g−1). A high reversible capacity of 101.6 mA h g−1 is obtained and it retains 79.0% after 1000 cycles at 1 C. As for the higher current density of 6 C, it retains 76.7% of the initial capacity of 95.7 mA h g−1 after 2000 cycles. In addition, the DNa+ of Na3.1V2(PO4)2.9(SiO4)0.1/C@rGO is much higher than that of NVP/C, confirmed by CV and EIS measurements. The kinetic characteristics for Na3.1V2(PO4)2.9(SiO4)0.1/C@rGO are deeply explored by GITT. The minimum value of DNa+ appears at around 3.4 V, resulting from the intense interactions between the Na+ and host framework during the phase transition reaction. Briefly, Si doping combined with conducting rGO is an effective way to develop high performance of NVP cathode material for sodium ion batteries.