Demands for rechargeable batteries have been significantly increasing not only in portable electronics (e.g. cell phones and laptops), but also in large-scale energy storage systems (e.g. grids).^1-3 TiO₂ as anode materials is electrochemically active in both lithium and sodium ion batteries. It has drawn significant attentions due to its nontoxicity, stability, low production cost and high capacity. Indeed, many works have been conducted to study the impact of morphology, particle size, facet control and electronic structure manipulation on the electrochemical performance of TiO₂ materials in either lithium or sodium ion batteries.^4-9 In addition, in terms of crystalline structures, TiO₂ materials with various polymorphs, for example, anatase (tetragonal, I4₁/amd), rutile (tetragonal, P4₂/mnm), TiO₂-B (monoclinic, C2/m), brookite (orthorhombic, Pbcv) and amorphous TiO₂ have been extensively investigated.^10-14

Previously, Xiong et al. observed a phase transformation from amorphous to face-centered cubic phase when the amorphous TiO₂ nanotube electrode first discharged under 1 V vs. Li/Li⁺, significantly self-improving the long-term reversibility and specific capacity.¹⁵ Xiong et al. also investigated the electrochemical performance of amorphous TiO₂ nanotubes as anode materials in sodium cell.¹⁴Though capacity increased gradually with charge-discharge cycling, the phase transition from amorphous to cubic phase was not observed.

Herein, we demonstrate the impact of various degree of crystallinity on pure TiO₂ anode materials for both Li and Na ion batteries. XRD, Raman, selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM) were conducted to confirm the gradually crystalized TiO₂ nanoparticles. Electrochemical performance of TiO₂ nanoparticles was tested by voltage profile, rate capability, cycle life, cyclic voltammetry (CV) and galvanostatic intermittent titration technique (GITT). Structure evolution during cycling was tested by ex situRaman spectroscopy and soft XAS.

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