Amorphous Vanadium Titanates as a Negative Electrode for Lithium-ion Batteries

Amorphous vanadium titanates (aVTOs) are examined for use as a negative electrode in lithium-ion batteries. These amorphous mixed oxides are synthesized in nanosized particles (<100 nm) and flocculated to form secondary particles. The V5+ ions in aVTO are found to occupy tetrahedral sites, whereas the Ti4+ ions show fivefold coordination. Both are uniformly dispersed at the atomic scale in the amorphous oxide matrix, which has abundant structural defects. The first reversible capacity of an aVTO electrode (295 mA h g(-1)) is larger than that observed for a physically mixed electrode (1: 2 aV(2)O(5) | aTiO(2), 245 mA h g(-1)). The discrepancy seems to be due to the unique four-coordinated V5+ ions in aVTO, which either are more electron-accepting or generate more structural defects that serve as Li+ storage sites. Coin-type Li/aVTO cells show a large irreversible capacity in the first cycle. When they are prepared under nitrogen (aVTO-N), the population of surface hydroxyl groups is greatly reduced. These groups irreversibly produce highly resistive inorganic compounds (LiOH and Li2O), leading to increased irreversible capacity and electrode resistance. As a result, the material prepared under nitrogen shows higher Coulombic efficiency and rate capability.