Amorphous and crystalline TiO₂ nanoparticle negative electrodes for sodium-ion batteries

Titanium dioxide (TiO₂) is a promising negative electrode for sodium ion batteries (SIBs). Although TiO₂ materials with amorphous (A-TiO₂) and single-phase crystalline structures (C–TiO₂) have been separately explored, the study to compare the fundamental electrochemistry of A-TiO₂ and C–TiO₂ is limited. In this work, we investigated A-TiO₂ and C–TiO₂ nanoparticles with identical chemical composition and morphology. C–TiO₂ exhibits enhanced electrochemical performance than A-TiO₂ in terms of rate capability and cycle life. Cyclic voltammetry (CV) analysis suggests reversible Na ion insertion/extraction in C–TiO₂. However, such process is irreversible in the case of A-TiO₂. The charge storage mechanisms in both samples were studied to show that diffusion-controlled intercalation process becomes significant in C–TiO₂ sample. The C–TiO₂ sample has a better Na⁺ diffusivity measured through the galvanostatic intermittent titration technique (GITT) compared to A-TiO₂, which corroborates well with the rate capability study. Furthermore, the evolution of local structure of the TiO₂ samples was analyzed by ex situ pair distribution function (PDF) to understand the variation in electrochemical properties. It reveals that the corner-shared Ti–Ti distance along Na ion diffusion pathway increases with the increase of crystallinity, leading to the expanded diffusion channels and therefore more active sites and faster diffusion.