context: In addition, the long-term cycling test at 5C was performed to investigate the cycling performance of all the NVPF-based materials shown in Fig. 3d. The NVPF@5% rGO gave the highest initial discharge capacity of 83.2 mA h g−1, compared with the NVPF (74.6 mA h g−1), NVPF@2.5% rGO (76.4 mA h g−1), and NVPF@7.5% rGO (79.9 mA h g−1). It also exhibited an excellent capacity retention of 94.06% after 1000 cycles, achieving a high average coulombic efficiency (CE) of 99.6% throughout. Therefore, apart from its outstanding rate capability, the NVPF@5% rGO also exhibited an extremely durable cycling performance. Furthermore, due to the high conductivity and uniformity of the rGO carbon network, the ΔV values of NVPF@5% rGO were smaller than that of others in each cycle (inset Fig. 3d). The NVPF@7.5% rGO had a slight increase in polarization compared to the NVPF@5% rGO at 1000 cycles, which could be caused by the uneven carbon network. But the polarization of NVPF increased almost linearly during cycling, which leads to a low CE and poor cycling performance. Moreover, although NVPF@2.5% rGO with an incomplete carbon network can inhibit the increase of polarization before 400 cycles, it still exhibits high polarization after more cycles. The schematic diagram of the Na+ and e− transport pathways in NVPF@5% rGO is shown in Fig. 3e. As shown, because of the robust 3D continuous conductive network structure, the NVPF@5% rGO could not only achieve a highly efficient, ultrafast and continuous transmission of Na+ and e−, but could also build an effective buffer to accommodate volume changes during charge (desodiation)/discharge (sodiation) processes. In addition, the large specific surface area of the rGO carbon network in NVPF@5% rGO could serve as electrolyte storage micro-pools and transport channels, which could provide sufficient and timely electrolyte replenishment for ultrafast charging/discharging.
answer: NVPF@5% rGO