Quasi-equilibrium ion insertion/extraction kinetics in nanocomposite films of 1-3 nm amorphous titania centers embedded in mesoporous silica: unlocking Li plus intercalation from aqueous media

Abstract

True reversible lithiation behavior of TiO2 from aqueous electrolytes has remained obscured by the parasitic hydrogen evolution reaction (HER), even for 10 nm thin films. In this work, this is addressed using a mesoporous nano-TiO2/SiO2 composite film synthesized via a sol-gel process. The SiO2 matrix stabilizes 1–3 nm amorphous TiO2 nanoparticles, which are embedded in a percolative-like network allowing continuous electronic transport. The mesoporous nature of the nanocomposite material, with interconnected pores of 2–10 nm and 30% porosity, facilitates enhanced accessibility to reaction sites. Remarkably, in the aqueous system, the nanocomposite film displays distinguishable and reversible lithiation, well separated from HER, a distinction not previously observed except under highly basic conditions. While the nano-TiO2/SiO2 composite film exhibited similar rate performance as that of previously reported amorphous TiO2 thin films in non-aqueous media, it showed exceptional ultrafast kinetics in aqueous electrolyte, achieving 93% capacity at rates exceeding 300 C. It is shown that the ion insertion happens at quasi-equilibrium kinetics. Its operation in aqueous media is explained by the instantaneous equilibration of Li+ state-of-charge over the 1–3 nm TiO2 centers. This work establishes a framework for fabrication of mixed-oxide nanocomposites as ion-insertion electrodes for applications in aqueous and non-aqueous media.