Space-confined engineering boosted high-performance of ultrafine nickel selenide nanocomposites for sodium-ion capacitors

Sodium-ion hybrid capacitors (SIHCs) are desirable energy storage devices due to the combined advantages of their battery- and capacitive-type electrodes. However, their further development is mainly handicapped by the battery-type electrodes, which suffer from poor cycling stabilities and rate capacities caused by dramatic volume changes and Na⁺ ion's intrinsically sluggish diffusion, respectively. Hence, we synthesized a Ni-based framework encapsulated with graphene oxide (Ni-MOF@GO) composites containing hexa(4-carboxyl-phenoxy)-cyclotriphosphazene (HCTP-COOH) ligands. The reported one-step selenization of Ni-MOF@GO composites realizes the space-confined growth of ultrafine nickel selenide wrapped in a multi-heteroatom-doped carbon matrix (NiSe@C/rGO). The synergic combination of downsizing, carbon encapsulation, and multi-heteroatom doping strategy on the specific NiSe@C/rGO composites improves cycling stability and superior rate performance for sodium-ion storage. The NiSe@C/rGO electrode displays a high specific capacity of 448.9 mAh g⁻¹ at 0.1 A g⁻¹, excellent rate capability with 272.3 mAh g⁻¹ at 10 A g⁻¹, and a remaining capacity of 263.9 mAh g⁻¹ with 1000 testing cycles at 1.0 A g⁻¹. In addition, the battery-type electrode NiSe@C/rGO with improved rate capacity and cycling stability enables an optimal SIHC delivering a maximum energy density of 60.5 Wh kg⁻¹, a power density of 13,340 W kg⁻¹ at 37 Wh kg⁻¹, and capacity retention of 75% after 5000 cycles at 2.0 A g⁻¹.