Low cost Na₂FeSiO₄/H–N-doped hard carbon nanosphere hybrid cathodes for high energy and power sodium-ion supercapacitors

Na₂FeSiO₄ (NFS)/H–N-doped hard carbon nanospheres (HN-HCNSs) hybrid cathodes have been synthesized by using ferrous gluconate as template and carbon source via sol-gel method for the first time. In the structure of this hybrid cathode, the ultrathin NFS nanosheets are uniformly anchored in the mesoporous network structure of HN-HCNSs coating, forming the fast conductive transport pathways for electrons and Na+-ions. The NFS/HN-HCNSs hybrid cathode shows a hybrid energy storage mechanism with high initial discharge capacity of 218.4 mAh g−1 at 0.1 C and in the voltage range of 1.2–4.6 V versus Na/Na+. It also shows excellent long-term cycling stability (the capacity retention rates of 73.8% at 1 C after the 3300 cycles and 56.8% at 5 C after the 750 cycles in the voltage range of 1.5–4.6 V). Moreover, the unique mesoporous carbon-coated structural features endow the hybrid cathode with a maximum energy density of 331.99 W h kg−1 and a maximum power density of 2431.87 W kg−1 within working voltage range of 1.5–4.6 V.