Revisited Ti₂Nb₂O₉ as an Anode Material for Advanced Li-Ion Batteries

Ti₂Nb₂O₉ with a tunnel-type structure is considered as a perspective negative electrode material for Li-ion batteries (LIBs) with theoretical capacity of 252 mAh g^-1 corresponding to one-electron reduction/oxidation of Ti and Nb, but only ≈160 mAh g^-1 has been observed practically. In this work, highly reversible capacity of 200 mAh g^-1 with the average (de)lithiation potential of 1.5 V vs Li/Li⁺ is achieved for Ti₂Nb₂O₉ with pseudo-2D layered morphology obtained via thermal decomposition of the NH₄TiNbO₅ intermediate prepared by K⁺→ H⁺→ NH₄⁺ cation exchange from KTiNbO₅. Using operando synchrotron powder X-ray diffraction (SXPD), single-phase (de)lithiation mechanism with 4.8% unit cell volume change is observed. Operando X-ray absorption near-edge structure (XANES) experiment revealed simultaneous Ti⁴⁺/Ti³⁺ and Nb⁵⁺/Nb⁴⁺ reduction/oxidation within the whole voltage range. Li⁺ migration barriers for Ti₂Nb₂O₉ along [010] direction derived from density functional theory (DFT) calculations are within the 0.15-0.4 eV range depending on the Li content that is reflected in excellent C-rate capacity retention. Ti₂Nb₂O₉ synthesized via the ion-exchange route appears as a strong contender to widely commercialized Ti-based negative electrode material Li₄Ti₅O₁₂ in the next generation of high-performance LIBs.