Breaking Solubility Limitation via Molecule Design to Build Localized Ultrahigh‐Concentration Electrolytes for Lithium Metal Batteries

The increase in salt concentration of electrolytes has been known as a simple but effective approach to restrain solvent activity and regulate solvation structure, thus enabling the long-term interfacial stability of batteries. Nevertheless, the ion electrostatic intercalations and finite coordination states have frustrated the efforts to further elevate the salt solubility. Here, we break the upper limit of lithium-salt dissolution through a molecule engineering strategy, in which the solvent with three ether-oxygen groups provides multi-coordination sites and the intrinsically inert diluents with low steric hindrance are activated to shield electrostatic repulsion. As a result, a series of localized ultrahigh-concentration electrolytes (LUCEs) are prepared with a molar ratio of Li⁺ to the solvent as high as 1.8. The LUCEs are endowed with high Li⁺ transference number of 0.682, high Coulombic efficiency for lithium plating/stripping up to 99.97%, and high oxidation stability over 6.5 V. Meanwhile, the scarce free solvent promotes the formation of a robust inorganic-rich interphase on both the lithium anode and a high-voltage cathode, which enables the operation of Li||LiNi_0.8Co_0.1Mn_0.1O₂ batteries over 180 cycles (>80% capacity retention) under a lean lithium source (20 µm) and high-loading cathode (3.885 mAh cm^-2). Our work elucidates the underlying mechanism of salt dissolution chemistry and offers an affordable method for stabilizing energy-dense electrochemical storage devices.