Phase transformation of spinel Li₄Ti₅O₁₂ to anatase TiO₂ by catalytic delithiation

The metal oxides were derived from lithium metal oxides through the phase transformation of a lithium metal oxide to metal oxide by catalytic delithiation using silesquioxane-coated lithium metal oxide as a precursor. The metal oxide was gradually formed from the surface of the lithium metal oxide contacting with silesquioxane through the phase transformation. Lithium silicate (Li₂SiO₃) was subsequently formed by the reaction between lithium and silesquioxane. Detailed characterizations were performed using spinel Li₄Ti₅O₁₂. Li₄Ti₅O₁₂/TiO₂ composites with three identifiable areas: a core (Li₄Ti₅O₁₂), shell (TiO₂), and coating layer (Li₂SiO₃) were derived by the simple thermal treatment of silesquioxane-coated lithium metal oxide in inert gas. During the thermal treatment, the silesquioxane coating layer was formed owing to the phase transformation of spinel Li₄Ti₅O₁₂ to anatase TiO₂. In addition, it sequentially reacted with lithium while forming an Li₂SiO₃ coating layer. TiO₂ hydrogenation was further induced, which had Ti³⁺ sites and oxygen vacancies. The Li₂SiO₃-coated Li₄Ti₅O₁₂/TiO₂ composite exhibited significantly improved specific capacity, rate capability, and cycling stability, in comparison to pristine Li₄Ti₅O₁₂. The improved electrochemical properties of the composite can be attributed to the following factors. (1) The phase transformation of spinel Li₄Ti₅O₁₂ to anatase TiO₂ increased the specific capacity of the composite because anatase TiO₂ has a higher theoretical capacity than spinel Li₄Ti₅O₁₂. (2) TiO₂ hydrogenation increased the electronic and ionic conductivities by introducing Ti³⁺ sites and oxygen vacancies. (3) The Li₂SiO₃ coating layer suppressed gas production by protecting the composite surface from the electrolytes.