High-Capacitance γ-rGO/MXene Cathode and Rapid Na+-Transfer Dynamics Sodium Titanate Anode for a Quasi-Solid-State Sodium-Ion Capacitor

Sodium-ion capacitors (SICs) bear the advantages of secondary batteries and supercapacitors and are regarded as promising energy-storage devices. However, the matching of the respective Na+-transfer kinetics of the anode and the cathode and the low energy density are still a challenge. Herein, to achieve a high-capacity cathode, 2D V2C MXene nanosheets were introduced into γ-ray-reduced graphene oxide (γ-rGO), and the γ-rGO/V2C MXene foam (GMF) was fabricated. The GMF provides a high specific capacitance of 391.4 mF cm–2 (130.5 F g–1) at a high current density of 60 mA cm–2 (20 A g–1). Meanwhile, the structure-directing strategy combining multiple nanocarbon composites accelerates the Na+-transfer kinetics of sodium titanate (NTO). The as-fabricated CNT film-supported sodium titanate nanowires encapsulated by the graphene (CNTF@NTO-G) anode deliver a specific capacity of 109 mA h g–1 at a high current density of 10 A g–1. As a result, the assembled quasi-solid-state SIC displays an energy density of 5.61 mW h cm–3 (56.1 W h kg–1) at a power density of 1 W cm–3 (10 kW kg–1) with excellent cycle stability (87.3% capacitance retention after 10,000 cycles).