High-Capacitance ?-rGO/MXene Cathode and Rapid Na plus -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 gamma-ray-reduced graphene oxide (gamma- rGO), and the gamma-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).