Atomic Insights into Aluminium-Ion Insertion in Defective Anatase for Batteries

Aluminium batteries constitute a safe and sustainable high-energy-density electrochemical energy-storage solution. Viable Al-ion batteries require suitable electrode materials that can readily intercalate high-charge Al³⁺ ions. Here, we investigate the Al³⁺ intercalation chemistry of anatase TiO₂ and how chemical modifications influence the accommodation of Al³⁺ ions. We use fluoride- and hydroxide-doping to generate high concentrations of titanium vacancies. The coexistence of these hetero-anions and titanium vacancies leads to a complex insertion mechanism, attributed to three distinct types of host sites: native interstitial sites, single vacancy sites, and paired vacancy sites. We demonstrate that Al³⁺ induces a strong local distortion within the modified TiO₂ structure, which affects the insertion properties of the neighbouring host sites. Overall, specific structural features induced by the intercalation of highly polarising Al³⁺ ions should be considered when designing new electrode materials for polyvalent batteries.