Unlocking Fast and Stable Lithium Insertion in TiNb₂O₇ via Bi-Induced Lattice Compliance

TiNb₂O₇ (TNO) is a promising anode material for next-generation safe lithium-ion batteries owing to its high theoretical capacity and intrinsically safe operating potential. However, its practical application is limited by the difficulty in simultaneously achieving high-rate capability and structural stability, arising from the energetic cost associated with cooperative lattice distortion during lithium insertion. Herein, we demonstrate that Bi doping enables a more adaptable lattice response in TNO by modulating local metal-oxygen bonding at the electronic level. The readily deformable electron cloud of Bi³⁺ facilitates adaptive charge redistribution under electrochemical perturbation, thereby lowering the rigidity of Ti/Nb-O bonding environments while preserving structural integrity. This reduces the deformation energy cost associated with cooperative octahedral rearrangement during lithiation and enables a strain-accommodating insertion process, as supported by highly reversible lattice evolution observed by in situ X-ray diffraction and accelerated reaction kinetics. As a result, the Bi_0.02-TNO delivers a high specific capacity of 270 mAh g^-1 at 0.2C, retains 145 mAh g^-1 at an ultrahigh rate of 30C, and maintains 130 mAh g^-1 (80% retention, 0.016% per-cycle decay) after 1250 cycles at 10C. This work established electronic-structure-mediated lattice response modulation as a viable design principle for reconciling rate capability and structural stability in crystallographic shear oxide anodes.