Thermodynamically spontaneously intercalated crystal H3O+ enables long-lasting LiMn2O4 cathodes with enhanced proton tolerance in aqueous battery chemistry

Abstract LiMn2O4 (LMO) is an attractive and inexpensive green cathode 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 the inability to improve its structural stability and achieve a breakthrough in cycle life. 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. To reveal the interaction mechanism and construct a conceptual framework for extensive studies, we demonstrate for the first time 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 in LMO, 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 propose to 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. This study will facilitate the development of advanced ALIBs with extended lifespan and enhanced energy density.