Internal-Parallel Hybrid Hard Carbon/TiO2 Electrode with a Boosted High-Rate Capability and Volumetric Capacity for Sodium-Ion Capacitors

Low-cost hybrid sodium-ion capacitors (SICs) can deliver high energy density at high power, showing tremendous potential for application in large-scale grids. The negative electrode using hard carbon (HC) faces the limitations of poor rate capability and low volumetric capacity. Herein, we report the design of internal-parallel hybrid electrodes composed of HC microparticles (MPs) and anatase TiO2 nanoparticles (TiO2(A) NPs) for sodium-ion storage. Taking advantage of the rapid surface-redox properties of TiO2 NPs and the pseudocapacitive Na+ adsorption of HC in the slope region (1.5–0.1 V vs Na+/Na), the hybrid electrodes deliver enhanced high-rate capacities. Additionally, by utilization of the geometric features of the MPs and NPs, the compaction densities and volumetric capacities of the hybrid HC/TiO2 electrodes are much higher than those of HC electrodes. A series of fine controls demonstrates that the C60T40 electrode (HC/TiO2 = 60:40 in wt %) exhibits excellent comprehensive electrochemical performance, including high volumetric/gravimetric capacities, high-rate capability, and long-term cyclability. Even at a high mass loading of 10 mg cm–2, the HC/TiO2 thick-film electrode delivers a high volumetric capacity of 162 mAh cm–3 at 5 mA cm–2, surpassing that of the HC electrode (64 mAh cm–3). This work highlights the design of internal-parallel hybrid electrodes with a comprehensive electrochemical performance for advanced SICs.