Dual Lithium Insertion and Conversion Mechanisms in a Titanium-Based Mixed-Anion Nanocomposite

The electrochemical reaction of lithium with a vacancy-containing titanium hydroxyfluoride was studied. On the basis of pair distribution function analysis, NMR, and X-ray photoelectron spectroscopy, we propose that the material undergoes partitioning upon initial discharge to form a nanostructured composite containing crystalline LixTiO2, surrounded by a Ti-0 and LiF layer. The Ti-0 is reoxidized upon reversible charging to an amorphous TiF3 phase via a conversion reaction. The crystalline LixTiO2 is involved in an insertion reaction. The resulting composite electrode, Ti-0-LiF/LixTiO2 double left right arrow TiF3/ LiyTiO2, allows reaction of more than one Li per Ti, providing a route to higher capacities while improving the energy efficiency compared to pure conversion chemistries.