Systematic understanding of titanium suboxides and their energy applications

Titanium suboxides, denoted as Ti_nO_m (0 < m/n < 2), represent a typical class of non-stoichiometric compounds characterized by a wide range of structures and physicochemical properties dependent on their variable titanium-to-oxygen ratios. This review systematically summarizes the crystallographic features, electronic structures, physical properties, and energy-related applications of Ti_nO_m. As oxygen atoms are progressively removed from TiO₂, the emergence of Ti-Ti metallic bonds transforms the compound from a semiconductor to a metallic conductor. The diverse configurations of Ti 3d orbital electrons impart unique physical properties to Ti_nO_m, including superconductivity, charge density waves, and ferromagnetism. This article provides an in-depth analysis of how various synthesis methods influence the crystal structures of the obtained Ti_nO_m. Due to their distinct energy band structures and physical characteristics, titanium suboxides demonstrate outstanding performance in applications such as electrochemical energy storage, thermoelectric conversion, electronic devices, catalysis, and microwave absorption. This review not only comprehensively presents the fundamental structures and properties of titanium suboxides, but also offers valuable insights to guide future research in this field.