Thermodynamically spontaneously intercalated H₃O⁺ enables LiMn₂O₄ with enhanced proton tolerance in aqueous batteries

LiMn₂O₄ (LMO) is an attractive positive electrode material for aqueous lithium-ion batteries (ALIBs), but its inferior cycle performance limits the practical application. The degradation mechanism of LMO in ALIBs is still unclear, resulting in inability to predictably improve its structural stability. The electrode/electrolyte interface is believed to play an important role in electrode degradation. However, the interactions of the water-containing electrode/electrolyte interface of LMO are underexplored. In this work, we demonstrate the insertion of H₃O⁺ into LMO during cycling in aqueous electrolyte and elucidate the paradoxical effects of H₃O⁺. The crystal H₃O⁺ enhances the structural stability of LMO by forming a gradient Mn⁴⁺-rich protective shell, but an excess amount of crystal H₃O⁺ leads to poor Li⁺ conductivity, resulting in rapid capacity fading. Combining electrochemical analyses, structural characterizations, and first-principles calculations, we reveal the intercalation of H₃O⁺ into LMO and its associated mechanism on the structural evolution of LMO. Furthermore, we regulate the crystal H₃O⁺ content in LMO by modifying the hydrogen bond networks of aqueous electrolyte to restrict H₂O molecule activity. This approach utilizes an appropriate amount of crystal H₃O⁺ to enhance the structural stability of LMO while maintaining sufficient Li⁺ diffusion.