High-Energy-Density Sodium-Ion Hybrid Capacitors Enabled by Interface-Engineered Hierarchical TiO₂ Nanosheet Anodes

Sodium-ion hybrid capacitors are known for their high power densities and superior cycle life compared to Na-ion batteries. However, low energy densities (<100 Wh kg^-1) due to the lack of high-capacity (>150 mAh g^-1) anodes capable of fast charging are delaying their practical implementation. Herein, we report a high-performance Na-ion hybrid capacitor based on an interface-engineered hierarchical TiO₂ nanosheet anode consisting of bronze (∼15%) and anatase (∼85%) crystallites (∼10 nm). This pseudocapacitive dual-phase anode demonstrated exceptional specific capacity of 289 mAh g^-1 at 0.025 A g^-1 and excellent rate capability (110 mAh g^-1 at 1.0 A g^-1). The Na-ion hybrid capacitor integrating a dual-phase hierarchical TiO₂ nanosheet anode and an activated carbon cathode exhibited a high energy density of 200 Wh kg^-1 (based on the total mass of active materials in both electrodes) and power density of 6191 W kg^-1. These values are in the energy and power density range of Li-ion batteries (100-300 Wh kg^-1) and supercapacitors (5000-15 000 W kg^-1), respectively. Furthermore, exceptional capacity retention of 80% is observed after 5000 charge-discharge cycles. Outstanding electrochemical performance of the demonstrated Na-ion hybrid capacitor is credited to the enhanced pseudocapacitive Na-ion intercalation of the two-dimensional TiO₂ anode resulting from nanointerfaces between bronze and anatase crystallites. Mechanistic investigations evidenced Na-ion storage through intercalation pseudocapacitance with minimal structural changes. This approach of nanointerface-induced pseudocapacitance presents great opportunities toward developing advanced electrode materials for next-generation Na-ion hybrid capacitors.