Boosted Surface‐Redox Pseudocapacitance in 2D Mesoporous TiN for High‐Power Sodium‐Ion Capacitors

Pseudocapacitive materials with surface‐redox reactions are capable of realizing high capacities at ultrahigh rates; however, it remains a challenge in the synthesis of active components with high surface area to boost surface‐redox sodiation but restrain side reactions. Herein, a two‐step, topochemical synthesis of 2D mesoporous TiN (2D‐meso‐TiN) with high surface area and rich mesoporosities is presented. It is demonstrated that the sodium‐ion storage mechanism of TiN anode is based on the existence of surficial titanium oxides via redox reactions between Ti4+ and Ti3+. The interconnected, highly conductive 2D‐meso‐TiN with high surface area largely increases the pseudocapacitive capacities, leading to a high capacity of 160/93 mAh g−1 at 0.1/10 A g−1, which is much higher than 2D‐TiN (120/72 mAh g−1) and commercial TiN nanoparticles (57/30 mAh g−1). The surface‐redox (de)sodiation undergoes no destruction of crystalline TiN, which enables high initial coulombic efficiency and long‐term cycles. Furthermore, a novel hybrid sodium‐ion capacitor consisting of 2D‐meso‐TiN anode and Na3V2(PO4)3 cathode is assembled without any presodiation treatments. The hybrid capacitor delivers both high energy density (94 Wh kg−1 at 64 W kg−1) and high power density (38 Wh kg−1 at 4.4 kW kg−1), as well as long cycling stability.