Reversible magnesium and aluminium ions insertion in cation-deficient anatase TiO₂

In contrast to monovalent lithium or sodium ions, the reversible insertion of multivalent ions such as Mg²⁺ and Al³⁺ into electrode materials remains an elusive goal. Here, we demonstrate a new strategy to achieve reversible Mg²⁺ and Al³⁺ insertion in anatase TiO₂, achieved through aliovalent doping, to introduce a large number of titanium vacancies that act as intercalation sites. We present a broad range of experimental and theoretical characterizations that show a preferential insertion of multivalent ions into titanium vacancies, allowing a much greater capacity to be obtained compared to pure TiO₂. This result highlights the possibility to use the chemistry of defects to unlock the electrochemical activity of known materials, providing a new strategy for the chemical design of materials for practical multivalent batteries.