Cyclic carbonate for highly stable cycling of high voltage lithium metal batteries

The lithium metal battery (LMB) is one of the most promising next-generation battery systems due to its ultrahigh energy density. However, problematic dendrite formation and low Coulombic efficiency (CE) greatly limit its practical application. Carbonate electrolyte solvents are still indispensable for the operation of LMBs using a transition metal oxide cathode. We determined the impact of different cyclic carbonates, which actively participate in the formation of the solid-electrolyte interface (SEI), on the stable cycling of LMBs using a nickel-rich layered cathode LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622). The substitution of fluorine atoms in the cyclic carbonate profoundly enhances the stability of the lithium metal anode while fluoroalkyl and alkoxy substituents are detrimental. Cyclic carbonate trans-difluoroethylene carbonate (DFEC) was identified as a novel SEI enabler on the lithium metal anode, facilitating the formation of a protective SEI with relatively high lithium fluoride content. A Li/NMC622 cell utilizing DFEC electrolyte solvent as SEI enabler displayed a capacity retention larger than 82% after 400 cycles and an average CE of 99.95%. In contrast, the cycling retention after 400 cycles for a Li/NMC622 cell using monofluoroethylene carbonate was only 31% with an average CE of 99.73%. Other fluoroalkyl or alkoxy cyclic carbonates do not provide improved stabilization of the lithium metal anode over ethylene carbonate. The fundamental studies in this work provide critical insight for the further development of advanced electrolytes in LMBs.