Engineering interfacial chemistry and solvation structure via a novel ionic additive for ⁓5 V-cFlass LiNi_0.5Mn_1.5O₄ batteries

Despite the high energy density and voltage plateau offered by ∼5 V-class lithium-metal batteries using spinel LiNi_0.5Mn_1.5O₄ (LNMO) cathodes and lithium metal anodes, their practical application is hindered by rapid capacity degradation due to severe electrolyte decomposition, transition-metal dissolution, and lithium dendrite growth. To tackle these challenges, we introduce a novel ionic electrolyte additive, 1-chloromethyl-4-fluorodiazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (Selectfluor). The incorporation of only 1 % addition into a conventional-concentration electrolyte, Selectfluor significantly enhances the cycling stability of both the Li||LNMO full cells and Li||Li symmetric cells. Multimodal characterization and theoretical simulations reveal that Selectfluor (i) favorably modifies the Li⁺ solvation structure by displacing solvent molecules and promoting anion participation, (ii) facilitates the formation of the hybrid interphases rich in LiF and incorporating LiBO₂ on both electrode surfaces, and (iii) yields thin yet robust electrode/electrolyte interphases with high Li⁺ ionic conductivity, which collectively suppresses side reactions and ensures uniform lithium deposition. This work presents a cost-effective and scalable electrolyte design strategy to advance the industrialization of cobalt-free LNMO-based batteries.