Synergistic Effects of LiTFSI and LiPF ₆ in Localized High‐Concentration Electrolytes for Nickel‐Rich Cathodes in Li‐Metal Batteries

High‐voltage operation of Ni‐rich cathodes such as LiNi _0.8 Mn _0.1 Co _0.1 O ₂ (NMC811) in lithium–metal batteries (LMBs) is hindered by interfacial instability, excessive impedance growth, and poor cycling performance. To address these challenges, we introduce a dual‐salt localized high‐concentration electrolyte (D‐LHCE) containing LiPF ₆ and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), diluted with 1,1,2,2‐tetrafluoroethyl‐2,2,3,3‐tetrafluoropropyl ether (TTE). Among the various electrolytes we examined, the D‐LHCE exhibited the most outstanding performance, delivering an initial discharge capacity of 229 mAh/g and 69.1% capacity retention (vs. 25.5% for the Standard) after 450 cycles at C/3. Systematic postmortem analyses using scanning electron microscopy (SEM), X‐ray photoelectron spectroscopy (XPS), and distribution of relaxation times (DRT) analysis revealed that D‐LHCE produces thinner, LiF‐rich electrode‐electrolyte interphase layers on both NMC811 cathode and Li‐metal anode. These interphases arise from solvation structure of D‐LHCE, promoting contact ion pairs (CIP) and ion aggregates (AGG). As a result, D‐LHCE effectively minimized cell impedance growth during cycling, prevented NMC811 particle cracking, and maintained relatively smooth Li–metal surface. Our results demonstrate a promising electrolyte design strategy for next‐generation Li–metal batteries with high energy densities and cycle life.