Ultralong Bistable, Electrolytic MnO 2 -Based, Electrochromic Battery Enabled by Porous, Low-Barrier, Hydroxylated TiO 2 Interface.

Electrochromic (EC) battery technology shows great potential in future "zero-energy building" by controlling outdoor solar transmission to tune heat gain as well as storing the consumed energy to reuse across other building systems. However, challenges still exist in exploring an electrochemical system to satisfy requirements on both ultra-long optical memory (also called bistability) without continuous power supply and high energy density. Herein, an EC battery is proposed to demonstrate ultra-long bistability (>760 h) based on the reversible deposition and dissolution of manganese oxide (MnO 2 ) without the addition of any mediators. A porous low-barrier hydroxylated titanium dioxide (TiO 2 ) interface is incorporated to synergistically enrich Mn 2+ -affinity active sites for deposition and effectively reduce the electron transport barrier of MnO 2 for dissolution, thereby significantly improving the reversibility, high optical modulation (60.2% at 400 nm), and energy density (352 mAh m -2 ). The modification strategy is also verified on the cathode-less button cells with a much higher average coulombic efficiency (99.9%) compared to the batteries without the porous hydroxylated TiO 2 interface (74.6%). These achievements lay a foundation for advancements in both electrochromism and Zn-Mn aqueous batteries.