Defect structures, interfacial polarization mechanisms, and energy storage performance in pure, doped, and co-doped polycrystalline TiO2 ceramics for advanced dielectric applications

Titanium dioxide (TiO 2 ) is a simple and abundant oxide, yet it has recently attracted strong interest as a high dielectric material when its defect structure is carefully engineered by aliovalent doping and donor–acceptor co-doping. Many co-doped rutile TiO 2 ceramics exhibit very large dielectric permittivity together with relatively low dielectric loss, good thermal stability, and enhanced energy storage capability, making them promising for advanced capacitor and dielectric energy storage applications. These unusual properties are generally not intrinsic to the TiO 2 lattice but arise from defect-controlled interfacial polarization mechanisms, such as internal and surface barrier layer capacitance, electron-pinned defect dipoles, and space-charge accumulation at grain boundaries. At the same time, synthesis route, sintering conditions, grain-boundary characteristics, grain size, and dopant chemistry play decisive roles in controlling dielectric response and energy storage efficiency. In this review, a comparative and critical analysis of pure, doped, and co-doped TiO 2 ceramics is presented with particular emphasis on the relationship between defect chemistry, microstructural evolution, dielectric behavior, and energy storage performance. The review summarizes recent progress on doped and co-doped TiO 2 dielectrics, focusing on microstructure–property relationships, defect engineering strategies, grain-boundary effects, and interfacial polarization models responsible for giant dielectric behavior. We also discuss soft-chemistry synthesis approaches, processing strategies, and co-doping techniques aimed at optimizing dielectric permittivity and recoverable energy density while suppressing excessive dielectric loss and electronic conduction. Furthermore, special attention is given to the influence of oxygen vacancies, Ti 3 ⁺ defect centers, and electrically heterogeneous microstructures on dielectric relaxation, charge storage behavior, and thermal stability. Finally, current challenges and future directions toward stable, low loss, thermally robust, and high-energy density TiO 2 -based dielectric ceramics are critically discussed.