PNNL

Abstract

Rutile TiO2 nanorods were synthesized by hydrolysis of TiCl4 followed by a hydrothermal method. Lithium insertion into the rutile nanorods was achieved by a chemical lithium intercalation process. The structural evolution of nano-structured rutile upon lithium intercalation was characterized by several experimental techniques, namely, XRD, TEM and 6Li MAS NMR. The XRD and TEM studies indicate the formation of a new lithium titanate phase (LixTiO2) during lithium intercalation. Additionally, SAED patterns show that the lithium titanate phase has cubic symmetry. Finally, ultra-high magnetic field (21.1T) 6Li MAS NMR reveals that the lithium titanate phase adopts two different structures depending on lithium content. Taken together, the three techniques consistently show that the intercalation of lithium into rutile TiO2 nanorods causes two consecutive structural phase transformations to lithium titanate phases with spinel (Fd m) and rocksalt (Fm m) structures at x=0.46 and 0.88, respectively. In addition, the broad line widths in the 6Li MAS NMR spectrum of the rocksalt phase are indicative of a disordered structure. Density functional theory calculations of the rutile, spinel and rocksalt bulk phases as a function of lithium content corroborate the observed phase transformations. These phase transitions could account for the large irreversible capacity loss of nano-structured rutile anodes observed in electrochemical cycling experiments.