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

Abstract

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 internalparallel hybrid electrodes composed of HC microparticles (MPs) and anatase TiO 2 nanoparticles (TiO 2 (A) NPs) for sodium-ion storage. Taking advantage of the rapid surface-redox properties of TiO 2 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/TiO 2 electrodes are much higher than those of HC electrodes. A series of fine controls demonstrates that the C60T40 electrode (HC/TiO 2 = 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/TiO 2 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.