Post-annealing tailored 3D cross-linked TiNb2O7 nanorod electrode: towards superior lithium storage for flexible lithium-ion capacitors

TiNb2O7 anode materials (TNO) have unique potential for applications in Li-ion capacitors (LICs) due to their high specific capacity of ca. 280 mA h g−1 over a wide anodic Li-insertion potential window. However, their high-rate capability is limited by their poor electronic and ionic conductivity. In particular, studies on TNO for LICs are lacking and that for flexible LICs have not yet been reported. Herein, a unique TNO porous electrode with cross-linked nanorods tailored by post-annealing and its application in flexible LICs are reported. This binder-free TNO anode exhibits superior rate performance (~66.3% capacity retention as the rate increases from 1 to 40 C), which is ascribed to the greatly shortened ion-diffusion length in TNO nanorods, facile electrolyte penetration and fast electron transport along the continuous single-crystalline nanorod network. Furthermore, the TNO anode shows an excellent cycling stability up to 2000 cycles and good flexibility (no capacity loss after continuous bending for 500 times). Model flexible LIC assembled with the TNO anode and activated carbon cathode exhibits increased gravimetric and volumetric energy/power densities (~100.6 W h kg−1/4108.8 W kg−1; 10.7 mW h cm−3/ 419.3 mW cm−3), more superior to previously reported hybrid supercapacitors. The device also efficiently powers an LED light upon 180° bending.