Ultrathin TiO₂-B nanowires as an anode material for Mg-ion batteries based on a surface Mg storage mechanism

Ultrathin TiO₂-B nanowires with a naked (-110) surface were prepared by a hydrothermal process and used as the anode material for Mg-ion batteries. The material delivered a reversible Mg²⁺ ion capacity of 110 mA h g^-1 at the 0.1C rate. Excellent cycling stability was achieved with a small capacity-fading rate of 0.08% per cycle. In addition, a discharge capacity of 34 mA h g^-1 was obtained at the 50C rate, demonstrating the material's excellent high rate capability. First-principles calculations showed that Mg²⁺ ions hardly penetrated into the TiO₂-B lattice because of a very large Mg²⁺ ion diffusion barrier of 0.63 eV. Instead, the Mg²⁺ ions were stored at the 4-coordinated vacancies of TiO₂-B nanowire (-110) surfaces. The adsorbed Mg²⁺ ions were bonded with unpaired surface oxygen atoms. Meanwhile, a small amount of electrons were transferred from the O-2p state to the Ti-3d state.