Fluorine-donating electrolytes enable highly reversible 5-V-class Li metal batteries
Significance Rechargeable lithium metal battery (RLMB) is the holy grail of high-energy-density batteries. If lithium metal anode (LMA) could be combined with 5-V LiNi 0.5 Mn 1.5 O 4 cathode, energy density could exceed 600 Wh/kg based on the cathode and anode electrode mass. Despite such promises, 5-V RLMB is still a vacant research space so far due to the unavailability of electrolytes which simultaneously satisfy a wide enough electrochemical stability window, good compatibility with LiNi 0.5 Mn 1.5 O 4 , and superior reversibility of LMA. In this work, a class of full-fluoride (FF) electrolyte is invented for 5-V RLMB which not only has good compatibility with cathode and a wide stability window but also possesses the capability to make LMA more stable and reversible. Lithium metal has gravimetric capacity ∼10× that of graphite which incentivizes rechargeable Li metal batteries (RLMB) development. A key factor that limits practical use of RLMB is morphological instability of Li metal anode upon electrodeposition, reflected by the uncontrolled area growth of solid–electrolyte interphase that traps cyclable Li, quantified by the Coulombic inefficiency (CI). Here we show that CI decreases approximately exponentially with increasing donatable fluorine concentration of the electrolyte. By using up to 7 m of Li bis(fluorosulfonyl)imide in fluoroethylene carbonate, where both the solvent and the salt donate F, we can significantly suppress anode porosity and improve the Coulombic efficiency to 99.64%. The electrolyte demonstrates excellent compatibility with 5-V LiNi 0.5 Mn 1.5 O 4 cathode and Al current collector beyond 5 V. As a result, an RLMB full cell with only 1.4× excess lithium as the anode was demonstrated to cycle above 130 times, at industrially significant loading of 1.83 mAh/cm 2 and 0.36 C. This is attributed to the formation of a protective LiF nanolayer, which has a wide bandgap, high surface energy, and small Burgers vector, making it ductile at room temperature and less likely to rupture in electrodeposition.
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