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 Al ions. Here, we investigate the Al intercalation chemistry of anatase TiO and how chemical modifications influence the accommodation of Al 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 Al induces a strong local distortion within the modified TiO structure, which affects the insertion properties of the neighbouring host sites. Overall, specific structural features induced by the intercalation of highly polarising Al ions should be considered when designing new electrode materials for polyvalent batteries.