Theoretical and Experimental Study of the Structure and Electrochemical Properties of V-Doped TiNb2O7 Anode for Lithium-Ion Batteries

Vanadium-doped TiNb2O7 anode material with a Wadsley–Roth crystallographic shear structure was prepared by a mechanochemically assisted solid-state synthesis. Partial substitution of Ti4+ or Nb5+ with V5+ ions was studied by using theoretical and experimental approaches. The crystal structure and morphology of the samples were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Possible interstitial sites, the energy of V5+ doping, and the electronic structure of TiNb2O7 doped with vanadium were investigated using theoretical modeling. According to electron paramagnetic resonance spectroscopy (EPR), the substitution of Ti4+ by V5+ leads to a higher content of reduced Ti and Nb atoms, compared to the case of Nb5+ substitution by V5+. Electrochemical performance was studied by galvanostatic cycling and cyclic voltammetry (CV). The lithium diffusion coefficient for Ti0.99V0.01Nb2O7 and TiNb1.99V0.01O7 determined by CV was an order of magnitude higher than those for TiNb2O7, Ti0.95V0.05Nb2O7, and TiNb1.95V0.05O7. It was shown that Ti1–xVxNb2O7 samples have superior electrochemical performance compared to TiNb2–xVxO7 and pristine samples due to the charge compensation of V5+ when substituting Ti4+. Improved electrochemical performance of Ti0.99V0.01Nb2O7 (reversible capacity of 194 mAh g–1 at 5C) is associated with improved ionic conductivity due to an increase in the unit cell volume as a result of interstitial V5+ doping.