An Mn²⁺ Cross-Linked Gel Electrolyte Enables Reversible Quasi-Solid-State Manganese Metal Batteries

Aqueous manganese metal batteries (AMMBs) have emerged as promising alternatives for stationary energy storage applications owing to their higher energy density and higher cost efficiency compared to Zn metal batteries. However, the higher reactivity of Mn metal results in severe parasitic reactions, hampering the development of AMMBs. Here, we design an ionic cross-linking gel electrolyte (SA@Mn) via the cross-linking reaction between sodium alginate (SA) and manganese cations (Mn²⁺). The hydrophilic polymer chains reduce the free water content, inhibiting water-related parasitic reactions. Moreover, the unique ionic transport channels facilitate orderly Mn²⁺ migration to suppress dendrite growth. With optimized concentration, the 3M SA@Mn displays a high ionic conductivity (172.5 mS cm^-1) and transference number (0.89). Therefore, the Mn||Mn symmetric cell achieves a high plating/stripping reversibility over 450 h, and the Mn||AgVO full cell could operate over 400 cycles. More importantly, a quasi-solid-state Mn metal pouch cell marks progress toward secure AMMBs for future smart-grid energy storage.