Fe3+-substitutional Doping of Nanostructured Single-crystal TiNb2O7 for Long-stable Cycling of Ultra-fast Charging Anodes

Abstract

Titanium niobate (TiNb₂O₇, TNO) has emerged as a promising lithium-ion battery (LIB) anode option for fast charging applications. However, the cycling durability of TNO under extremely fast charging is still limited, and the corresponding structural alteration mechanism remains unclear. This research reports an ultra-fast charging anode with long-term cycling stability enabled by Fe substitution in single-crystal TNO nanostructures. The underlying mechanism via which Fe substitution affects TNO’s electronic properties, ionic diffusion kinetics, and structural stability is revealed through combined theoretical modeling and experimental characterization. The optimal Fe³⁺-doped TNO monocrystalline material (Fe_0.05Ti_0.95Nb₂O_6.975) (Fe5-TNO) provides a remarkable charge capacity of 238 mAh/g under a 10 C (6 min charging time only) extreme fast-charging protocol (coupled with 1 C discharge), and a high capacity of 200 mAh/g at 5 C with high cycling retention of 85% after 1000 cycles. Our calculations suggest that Fe³⁺ substitutional doping leads to a lowering of the band gap coupled with a reduction in the Li⁺ diffusion energy barrier. Overall, these factors contribute to reduced capacity decay and extreme fast charging, together promoting durable cycling performance suitable for LIB usage. Reflection electron energy loss spectroscopy (REELS) reveals that Fe³⁺ doping narrows the band gap from 3.75 eV of TNO to approximately 3.40 eV for Fe5-TNO; after initial lithiation, both TNO and Fe³⁺-doped TNO are transformed into a higher-conductivity phase, in agreement with density functional theory (DFT) predictions. Meanwhile Fe³⁺ doping is shown exhibited to decrease the Li⁺ diffusion energy barrier, boosting the Li⁺ diffusion coefficient by one order of magnitude, from 10^-13 to 10^-12 cm²/s. This research provides new insights into the design of next-generation fast-charging LIB anodes via DFT-guided substitutional doping.