Phase transformation of spinel Li4Ti5O12 to anatase TiO2 by catalytic delithiation

Abstract

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.