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 TiNb<inf>2</inf>O<inf>7</inf> nanoparticles through a facile solvothermal method and annealing treatment. Then a homogeneous three-dimensional (3D) self-supported reduced graphene oxide (rGO)-coated TiNb<inf>2</inf>O<inf>7</inf> (TiNb<inf>2</inf>O<inf>7</inf>/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 TiNb<inf>2</inf>O<inf>7</inf>/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 TiNb<inf>2</inf>O<inf>7</inf>/rGO is mainly dominated by capacitive behavior, making it perfectly match with the capacitor-type activated carbon (AC) cathode. By using pre-lithiated TiNb<inf>2</inf>O<inf>7</inf>/rGO as anode material and AC as cathode material, a high-rate TiNb<inf>2</inf>O<inf>7</inf>/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.