Size Particle Effects on Lithium Insertion into Sn-doped TiO₂ Anatase

Lithium insertion into Sn-doped TiO2 anatase was studied in order to clarify the mechanism responsible for the first plateau observed at 1.75 V vs Li. One of the other aims of this study was to get deeper insight into the process responsible for the appearance of a first domain observed at a small amount of lithium (before the plateau) and a third domain between 1.7 and 1.2 V (after the plateau). Pure and Sn-substituted anatase were synthesized by several synthetic methods with different precursors and solvents. These materials are expected to have different electrochemical properties because crystallite size modifies the Li-insertion process. Their electrochemical behavior is discussed in order to establish a relationship between the materials properties and the electrochemical performances. Three processes have been identified from both X-ray diffraction and 119Sn Mössbauer spectroscopy: (i) topotactic insertion into the Li-poor compound LixTiO2, with the x value depending on the particle size; (ii) a two-phase system mechanism leading to the phase transition LixTiO2 (Anatase, I41/amd) → LiyTiO2 (orthorhombic distortion, Imma); and (iii) another topotactic insertion into the Li-rich compound LiyTiO2. Crystallite size governs the topotactic mechanism but does not improve the overall electrochemical capacity of the material.