Pseudocapacitive TiNb₂O₇/reduced graphene oxide nanocomposite for high-rate lithium ion hybrid capacitors

Lithium ion hybrid capacitors (LIHCs) have a capacitor-type cathode and a battery-type anode and are a prospective energy storage device that delivers high energy/power density. However, the kinetic imbalance between the cathode and the anode is a key obstacle to their further development and application. Herein, we prepared TiNb₂O₇ nanoparticles through a facile solvothermal method and annealing treatment. Then a homogeneous three-dimensional (3D) self-supported reduced graphene oxide (rGO)-coated TiNb₂O₇ (TiNb₂O₇/rGO) nanocomposite was constructed by freeze-drying, followed by a high-temperature reduction, which demonstrates an enhanced pseudocapacitive lithium ions storage performance. Benefiting from the improved electrical conductivity, ultrashort ions diffusion paths, and 3D architecture, the TiNb₂O₇/rGO nanocomposite exhibits a high specific capacity of 285.0 mA h g^-1, excellent rate capability (73.6% capacity retention at 8 A g^-1), and superior cycling stability. More importantly, quantitative kinetics analysis reflects that the capacity of TiNb₂O₇/rGO is mainly dominated by capacitive behavior, making it perfectly match with the capacitor-type activated carbon (AC) cathode. By using pre-lithiated TiNb₂O₇/rGO as anode material and AC as cathode material, a high-rate TiNb₂O₇/rGO//AC LIHC device can be fabricated, which delivers an ultrahigh energy density of 127 Wh kg^-1 at the power density of 200 W kg^-1, a maximum power density of 10 kW kg^-1 at the energy density of 56.4 Wh kg^-1, and durable service life.