Synergistic Regulation of Electrolyte and Electrode Structures Enhance Ammonium Vanadate Mg‐storage Performance for 100 mAh‐Level Mg‐ion Pouch Cells

Abstract Aqueous Mg‐ion batteries (AMIBs) have attracted increasing interest due to their safety, fast kinetics, and sustainability. However, the development of AMIBs is hindered by the narrow electrochemical stability window (ESW) of electrolytes and electrode degradation. Here, a synergistic strategy, regulating both electrolyte and electrode structures, is proposed to enable high‐performance AMIBs. Incorporating a polyethylene glycol crowding agent into Mg(ClO 4 ) 2 /H 2 O electrolyte regulates the solvation structure, which suppresses hydrogen evolution, and expands the ESW beyond 3.1 V. Meanwhile, an ammonium vanadate (NHVO) material with enlarged interlayer spacing is synthesized, and a freestanding NHVO/graphene oxide/carbon nanotube (NHVO/GO/CNT) electrode is constructed to enhance Mg 2+ diffusion and mitigate vanadium dissolution. Under the synergistic regulation, NHVO/GO/CNT exhibits a high capacity of 284.0 mAh g −1 and the longest cycling lifespan (16 000 cycles with 95.6% capacity retention). The solvation structure of electrolytes and interfacial stabilization mechanisms are elucidated through theory calculations. The Mg‐storage mechanism of NHVO based on a single‐phase insertion/extraction reaction with NH 4 + /Mg 2+ displacement phenomenon is revealed. The constructed multilayer Mg‐ion pouch cell achieves a record‐high capacity of 103.7 mAh. The integrated solar cell‐pouch cell device demonstrates the feasibility of photo‐charging AMIBs for the first time. This work offers a viable strategy toward practical, high‐performance multivalent‐ion batteries.