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

Abstract

Vanadium-doped TiNb 2 O 7 anode material with a Wadsley−Roth crystallographic shear structure was prepared by a mechanochemically assisted solid-state synthesis. Partial substitution of Ti 4+ or Nb 5+ with V 5+ 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 V 5+ doping, and the electronic structure of TiNb 2 O 7 doped with vanadium were investigated using theoretical modeling. According to electron paramagnetic resonance spectroscopy (EPR), the substitution of Ti 4+ by V 5+ leads to a higher content of reduced Ti and Nb atoms, compared to the case of Nb 5+ substitution by V 5+ . Electrochemical performance was studied by galvanostatic cycling and cyclic voltammetry (CV). The lithium diffusion coefficient for Ti 0.99 V 0.01 Nb 2 O 7 and TiNb 1.99 V 0.01 O 7 determined by CV was an order of magnitude higher than those for TiNb 2 O 7 , Ti 0.95 V 0.05 Nb 2 O 7 , and TiNb 1.95 V 0.05 O 7 . It was shown that Ti 1−x V x Nb 2 O 7 samples have superior electrochemical performance compared to TiNb 2−x V x O 7 and pristine samples due to the charge compensation of V 5+ when substituting Ti 4+ . Improved electrochemical performance of Ti 0.99 V 0.01 Nb 2 O 7 (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 V 5+ doping.