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 Ti 4+ and Ti 3+ . 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 Na 3 V 2 (PO 4 ) 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.