Unveiling Dynamic Structural Evolution Mechanism of Titanium Oxide Anode for High-Performance Sodium-Ion Capacitors

Abstract Anatase TiO2 derived rock-salt NaTiO2 has emerged as a superior anode material for efficient sodium ion storage, but the dynamic structural evolution mechanism has yet been fully revealed. Herein, based on comprehensive electrochemical analyses and in situ structural characterizations, an overall structural evolution mechanism is established. Specifically, upon repeated sodiation and desodiation, considerable sodium ions are irreversibly intercalated into the anatase TiO2, straightening initially distorted TiO6 octahedra and producing a cubic rock-salt NaTiO2. Notably, the in situ formed rock-salt NaTiO2 delivers an ultrahigh specific capacity of 256 mA h g–1 at 0.1 A g–1 and maintains a high value of 165 mA h g–1 even at 10 A g–1. When coupled with a graphene-based capacitive cathode, the as-built sodium-ion capacitor achieves a high energy density of 102 W h kg–1, a high power density of 6.8 kW kg–1, and a prolonged cycling stability of 40 000 cycles, achieving collaborative improvements in comprehensive performances.