Crystallographic engineering to reduce diffusion barrier for enhanced intercalation pseudocapacitance of TiNb2O7 in fast-charging batteries

Improving the ion/electron transport of TiNb2O7 composite is of great significance for achieving fast-charging lithium-ion batteries. Herein, we firstly report a rare-earth element engineering to tailor the bandgap and crystallographic structure for the dual function of electronic and ionic conductivity. Tb-doped TiNb2O7 (denoted as Tbx-TNO, x=0, 0.005, 0.010, 0.015) are successfully fabricated through a one-step solid-state reaction strategy. The Rietveld refinement technology of X-ray diffraction (XRD) demonstrates an effective substitution of Tb in the central sites of Nb-O octahedral. Significantly, in-situ XRD and ex-situ TEM techniques reveal the positive influence of Tb doping on the underlying ion transport behavior, which is verified by the increased unit cell volume, decreased mechanic effects, and promoted Li+-diffusion kinetics. DFT calculations demonstrate a narrow down of bandgap (from insulators to semiconductors) and reduction of ion-diffusion barrier in various migration modes, which lead to substantially enhanced ion/electron conductivities of Tb-TiNb2O7 microrod. Benefiting from the structural merits, the Tb0.01-TiNb2O7 presented outstanding rate capability (a reversible capacity of 192.8 mA h g−1 at 50 C) and an impressive cycle lifespan, corresponding to an ultra-high retention ratio of 98.9% (203.2 mA h g−1) over 1000 cycles. A pouch cell based on LiNi0.8Co0.1Mn0.1O2 cathodes and the Tb0.01-TiNb2O7 anodes exhibits potential application with superior rate capability and cycling stability. This work provides a simple strategy to enhance the rate-performance of TiNb2O7, making a guideline to construct fast-charging batteries with long cycling life.