The Role of Tunnel-Structured Hollandite Material Family for Advancing Energy Storage Applications

Rising demand for clean energy is driving the need for advanced materials for sustainable energy storage. Among the rising candidates storing energy via their open framework, the hollandite material family, represented by Mn-/Ti-/Fe-based metal dioxides, is particularly promising due to their unique sub-nanoscale tunnel structure that enables reversible ion diffusion. However, challenges like structural instability and ambiguous charge storage mechanisms largely reduce their commercial potential. Thus, this article timely reviews recent advancements of hollandite materials in terms of their tunnel structural features, energy storage mechanisms, properties, and performances. The focus on their crystalline framework, where ion diffusion/storage takes place, and emphasize on the structural stability and the resulted performance in both organic and aqueous electrolyte systems; particularly, this review covers their intercalation/de-intercalation mechanisms when working as electrodes in lithium-ion and sodium-ion batteries, as well as their charge storage behaviors in supercapacitors and zinc batteries. In addition, strategies like ion doping, composite fabrication, and nanostructure optimization are also included in terms of their effectiveness in enhancing electrochemical performance. This review is concluded by discussing the role andthe expectation of tunnel-structured hollandite materials in future battery technologies, emphasizing the urgent need for framework optimization toward more efficient and stable energy storage servicing.