Breathable Carbon‐Free Electrode: Black TiO 2 with Hierarchically Ordered Porous Structure for Stable Li–O 2 Battery

This paper introduces oxygen‐deficient black TiO 2 with hierarchically ordered porous structure fabricated by a simple hydrogen reduction as a carbon‐ and binder‐free cathode, demonstrating superior energy density and stability. With the high electrical conductivity derived from oxygen vacancies or Ti 3+ ions, this unique electrode features micrometer‐sized voids with mesoporous walls for the effective accommodation of Li 2 O 2 toroid and for the rapid transport of reaction molecules without the electrode being clogged. In the highly ordered architecture, toroidal Li 2 O 2 particles are guided to form with a regular size and separation, which induces the most of Li 2 O 2 external surface to be directly exposed to the electrolyte. Therefore, large Li 2 O 2 toroids (≈300 nm) grown from solution can be effectively charged by incorporating a soluble catalyst, resulting in a very small polarization (≈0.37 V). Furthermore, disordered nanoshell in black TiO 2 is suggested to protect the oxygen‐deficient crystalline core, by which oxidation of Ti 3+ is kinetically impeded during battery operation, leading to the enhanced electrode stability even in a highly oxidizing environment under high voltage (≈4 V).