Impact of nanosizing on lithiated rutile TiO2

The structural behavior of both micro- and nanosized Ti02 rutile upon Li-ion insertion was determined from neutron diffraction measurements. In the solid solution regime in rnicrosized rutile, which extends up to x approximate to 0.07 Li/Ti, the Li ions mainly reside at the tetrahedral position at low temperatures and at the octahedral position at higher temperatures. A rationale of this effect was found in the influence of lattice dynamics, illustrated by molecular dynamics simulations of the Li-ion diffusion in the rutile structure. Maximum (chemically) lithiated microrutile (similar to 0.43 Li/Ti at room temperature) reveals the formation of a layered morphology, having a monoclinic symmetry (P2/m, space group 10) similar to the previously suggested layered hexagonal structure. In addition, abundant lattice strains were induced. Consistent with recent electrochemical studies, the nanosized rutile (needle-shaped similar to 11 nm x I I nm x 43 nm) material was able to host a significantly higher amount of lithium, leading to a maximum composition of similar to 0.85 Li/Ti. In the nanomaterial, the solid solution domain extended up to x = 0.15 Li/Ti (i.e., further than in the micromaterial). This provides more evidence that reducing the crystallite size reduces the miscibility gap of two-phase reactions upon lithiation, especially when lattice mismatches lead to strains between the phases, such as recently was demonstrated for anatase TiO2 (Wagemaker et a]. J. Am. Chem. Soc. 2007, 129, 4323). Further lithiation of the nanomaterial up to Li/Ti = 0.53 resulted in a phase transition toward an intermediate phase, very similar to the original rutile phase but slightly deformed, reducing the symmetry to the monoclinic P-2/m space group. Upon further Li-ion insertion, up to similar to 0.85 Li/Ti, this phase transformed toward another structure, again indexed by the monoclinic P2/m space group but now similar to the layered hexagonal R (3) over barm space group previously suggested to form in rutile TiO2 upon lithiation. The presented structural results provide a consistent picture of Li-ion insertion in micro- and nanosized rutile TiO2, relating structure, crystallite size, and electrochemical performance.