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.