Delocalized Redox Framework of Indanthrone Enables Low‐Strain and Durable Mn ²⁺ /H ⁺ Storage in Aqueous Batteries

Aqueous manganese batteries (AMBs) are promising alternatives to lithium‐ion systems due to their safety and cost‐effectiveness. However, organic cathodes often face challenges regarding material dissolution and slow ion kinetics. In this work, we introduce indanthrone (IDT) as a strategic organic cathode for high‐performance Mn 2+ /H + batteries. IDT features a massive eight‐ring π‐conjugated framework that enables extensive electronic delocalization. Unlike systems with excessively concentrated active groups, the IDT framework provides a more dispersed distribution of redox‐active sites. This arch promotes smooth ion transport through enhanced charge delocalization across the extended aromatic system. The IDT electrode delivers a high specific capa of 194.8 mAh/g at 0.1 A/g, while the Mn‐IDT full‐cell exhibits 85.5% capacity retention after 3000 cycles at 0.3 A/g. Spectroscopic and crystallographic analyses confirm a low‐strain pathway, where b ‐axis expansion (2.09%) is effectively compensated by minor contractions in the a and c ‐axes. This unique anisotropic response leads to minimal overall volume fluctuation, ensuring structural integrity during prolonged operation. This work provides a molecular‐level design principle for constructing low‐strain organic cathodes for high‐voltage aqueous multivalent batteries.