Rational Lithium Salt Molecule Tuning for Fast Charging/Discharging Lithium Metal Battery

Abstract

AbstractThe electrolytes for lithium metal batteries (LMBs) are plagued by a low Li+ transference number (T+) of conventional lithium salts and inability to form a stable solid electrolyte interphase (SEI). Here, we synthesized a self‐folded lithium salt, lithium 2‐[2‐(2‐methoxy ethoxy)ethoxy]ethanesulfonyl(trifluoromethanesulfonyl) imide (LiETFSI), and comparatively studied with its structure analogue, lithium 1,1,1‐trifluoro‐N‐[2‐[2‐(2‐methoxyethoxy)ethoxy)]ethyl]methanesulfonamide (LiFEA). The special anion chemistry imparts the following new characteristics: i) In both LiFEA and LiETFSI, the ethylene oxide moiety efficiently captures Li+, resulting in a self‐folded structure and high T+ around 0.8. ii) For LiFEA, a Li−N bond (2.069 Å) is revealed by single crystal X‐ray diffraction, indicating that the FEA anion possesses a high donor number (DN) and thus an intensive interphase “self‐cleaning” function for an ultra‐thin and compact SEI. iii) Starting from LiFEA, an electron‐withdrawing sulfone group is introduced near the N atom. The distance of Li−N is tuned from 2.069 Å in LiFEA to 4.367 Å in LiETFSI. This alteration enhances ionic separation, achieves a more balanced DN, and tunes the self‐cleaning intensity for a reinforced SEI. Consequently, the fast charging/discharging capability of LMBs is progressively improved. This rationally tuned anion chemistry reshapes the interactions among Li+, anions, and solvents, presenting new prospects for advanced LMBs.