High performance Na3V2(PO4)3 with nitrogen-chlorine co-doped carbon matrix in-situ synthesized in chitosan quaternary ammonium hydrogel for sodium ion batteries

Recently, Na3V2(PO4)3 (NVP) has attracted extensive attention as ideal cathode. However, the low electronic conductivity restricts its further application. Herein, a novel strategy is proposed to in-situ synthesize NVP based on chitosan quaternary ammonium salt hydrogel (CHACC) substrate by a sol–gel method. The beneficial CHACC inhibits the agglomeration of active grains, providing shortened pathway for Na+ migration. Moreover, the unique hydrogel surrounding NVP particle converts to the favorable nitrogen-chlorine co-doped carbon layers to construct an efficient conductive network to accelerate the electronic transportation. Notably, the doping of nitrogen is beneficial to improve the disorder degree of the carbon layer. Furthermore, ClO4- can reversibly insert/extract in the carbon layer during charging and discharging processes, suggesting it can act as an additional charge carrier to accelerate electronic transport and provide additional capacity, which is demonstrated by ex-situ FT-IR and XPS spectra. Distinctively, the CHACC-NVP-3 delivers superior sodium storage property. It submits a capacity of 129.3 mAh/g at 0.1 C, surpassing the theoretical value of 117.6 mAh/g. It reveals a value of 93.1 mAh/g and remains 82.3 mAh/g at 60 C after 3000 cycles, corresponding to a high retention of 88.40 %. Even at 200 C, it still possesses a high capacity of 81.8 mAh/g, remaining 60.6 mAh g−1 after 1000 cycles.