Sodiation-driven amorphous Co-based species in slope-dominated hard carbon with ultralong cycling life for sodium-ion hybrid capacitors

Hard carbon is a promising anode for commercial sodium-ion batteries. However, it suffers from significant plateau capacity loss at high current densities, which limits its application in sodium-ion hybrid capacitors. To overcome this limitation, a high-rate T10-Co₃O₄@NC700 anode has been developed. This anode comprises tilted hard carbon (NC700) and Co₃O₄ nanoparticles, exhibiting a slope-dominated Na⁺ storage behavior with minimal plateau capacity. The formation of Co-based amorphous species during sodiation promotes the electronic conductivity and the adsorption energy of NC700 for Na⁺, thereby enhancing the pseudocapacitive Na ion storage. The NC700 shell and substrate further facilitate Na⁺ diffusion within Co₃O₄ and reduce mechanical stress during cycling. Accordingly, T10-Co₃O₄@NC700 exhibits excellent rate capability and electrochemical stability, retaining 97% of its initial capacity after 24 000 cycles even at a current density of 10 A g⁻¹. When incorporated into a sodium ion hybrid capacitor (SIC, T10-Co₃O₄@NC700//AC) with activated carbon (AC) as the cathode, the device achieves a maximum energy density of 59 W h kg⁻¹ and a maximum power density of 4.1 kW kg⁻¹, and retains 50% of its capacity after 5000 cycles. This work provides a promising strategy for designing high-rate, long-cycle hard carbon anodes for sodium ion hybrid capacitors.