A Strongly Binding Lithium Salt with Stepwise Interphase Formation Mechanism Enables Stable High-Voltage Lithium Metal Batteries

Stabilizing high-voltage cathodes in lithium metal batteries (LMBs) remains a key challenge due to severe interfacial degradation. Although anion-derived, inorganic-rich cathode-electrolyte interphases (CEIs) offer a promising solution, most conventional anions are chemically inert and lack the multifunctionality required to undergo both chemical and electrochemical decomposition across a wide potential window. Existing strategies to enhance anion reactivity often involve trade-offs in salt concentration, anodic stability, or environmental concern, highlighting the need for novel anion design with intrinsic and synergistic interfacial activity. In this study, we designed a multifunctional anion, 1,1,1-trifluoro-2,5,8-trioxa-1-borate (FTOB), by integrating a chelating polyethylene glycol backbone with a terminal -BF₃ group as a CEI precursor. The reactive B-O bond facilitates a stepwise interphase formation mechanism: chemical decomposition of FTOB and PF₆^- at lower potentials (<4.5 V vs Li⁺/Li) via their mutual interactions, followed by direct electrochemical oxidation of FTOB at higher potentials. These dual pathways enable the construction of LiF- and borate-rich CEIs, supporting stable cycling of LMBs with both 4.3-V high-nickel layered cathode and 5-V cobalt-free spinel cathode. This work highlights the potential of rational anion design to integrate multiple interfacial formation mechanisms, advancing interphase engineering for high-voltage LMBs.