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.