Phase Transformations and Charge Ordering during Li+ Intercalation into Hollandite-Type TiO2 Studied by Operando Synchrotron X-ray Powder Diffraction

Hollandite‐type TiO₂ with uniform submicron‐sized particles was prepared using the treatment of K_1.2Ti₈O₁₆ by KBrO₃ in an H₂SO₄ solution. The electrode material exhibited a 150 mAh g^–1 reversible capacity with an average working potential of 1.75 V vs. Li/Li⁺. Phase transformations taking place during Li⁺ (de)intercalation were studied by means of synchrotron X‐ray powder diffraction in operando regime. The lowering of symmetry from tetragonal I4/m to monoclinic I2/m at the beginning of the lithiation was observed. The monoclinic distortion was driven by the Ti³⁺/Ti⁴⁺ charge ordering and the location of the Li⁺ cations at the hollandite tunnel walls in the five‐fold oxygen coordination. The opposite variation of a and b unit cell parameters during charge‐discharge resulted in a negative overall volume change of –1 % at the lithiated state compared to the initial oxide phase. This value is several times lower than the values for other TiO₂ polymorphs such as rutile, anatase or TiO₂(B) are.