Electrochemical and structural investigations of different polymorphs of TiO2 in magnesium and hybrid lithium/magnesium batteries

Commercial nanocrystalline TiO₂ anatase and self-prepared TiO₂(B) bronze synthesized by hydrothermal method were used as the cathode materials for Mg-ion and hybrid Li/Mg-ion batteries. The TiO₂ anatase delivers high initial discharge/charge capacities of 225 mAh g⁻¹/204 mAhg⁻¹ in a mixture of Phenylmagnesium chloride-AlCl₃/LiCl in tetrahydrofuran (APC/LiCl) hybrid electrolyte, while the TiO₂(B) shows high initial discharge/charge capacities of 260 mAh g⁻¹/231 mAh g⁻¹ in the same hybrid electrolyte. Low discharge capacities of 67 and 57 mAh g⁻¹ and strong polarization are observed for both TiO₂ anatase and TiO₂(B) samples in pure APC electrolyte, respectively. In situ synchrotron diffraction strongly identifies that in APC/LiCl hybrid electrolyte, the TiO₂ anatase undergoes structural evolution via a solid solution and a two-phase reaction mechanism, while TiO₂(B) exhibits a solid solution reaction during the discharge-charge processes. The analysis of X-ray photoelectron spectra for the TiO₂ anatase and TiO₂(B) cathodes in the discharged/charged states reveals a change of Mg/Ti as well as Li/Ti ratios at different stages, which may indicate a contribution of both Mg and Li to the charge storage mechanism in the hybrid electrolyte.