Rational molecular design of electrolyte additive endows stable cycling performance of cobalt-free 5 V-class lithium metal batteries

Ultrahigh-voltage lithium metal batteries based on a cobalt-free LiNi0.5Mn1.5O4 (LNMO) cathode (5 V-class, vs. Li+/Li) and lithium metal anode (-3.04 V vs. the standard hydrogen electrode) have attracted extensive attention in recent years as promising candidates for the next-generation high energy density and sustainable batteries owing to their high theoretical energy density of similar to 650 W h kg-1, elimination of toxic Co elements and intrinsic safety caused by the LNMO spinel structure compared with the unstable layered oxide LiNixCoyMnzO2. However, their development is critically limited by the incompatibility between state-of-the-art carbonate electrolytes and the two aggressive electrodes. Herein, we synthesized a new electrolyte additive, i.e. 2,2-difluoroethyl methyl sulfone (FS), which could enable ultrahigh-voltage lithium metal batteries to stably cycle in conventional carbonate electrolytes. On the cathode side, unlike conventional electrolyte additives, FS could be selectively adsorbed on the LNMO surface and form a special assembled FS "buffer" layer, which could efficiently exclude free carbonate molecules away from the cathode surface. Therefore, upon charging, the -CF2H group of FS is preferably anodically decomposed to form an inorganic-rich CEI, which efficiently suppresses the micro-fracture and transition metal dissolution of LNMO. On the anode side, FS could also be preferably cathodically decomposed, resulting in an inorganic-rich SEI for the stable cycling of the Li metal anode. Thus, carbonate electrolyte with the FS additive enables the unprecedented high performance of cobalt-free 5 V-class lithium metal batteries, i.e. a 40 mu m-Li/LNMO (loading = 7 mg cm-2) full cell with a high capacity retention of 84% over 600 cycles at 1C using commercial a carbonate-based low-concentration electrolyte. The full cell consisting of a high-loading cathode (20 mg cm-2) and an ultrathin Li anode (40 mu m) achieves a capacity retention of 99% after 100 cycles at 0.25C. Moreover, Li/LNMO pouch cells, which have not been reported before to the best of our knowledge, are assembled and can operate stably for over 150 cycles. We rationally designed a semi-fluorinated sulfone electrolyte additive, i.e. 2,2-difluoroethyl methyl sulfone (FS), which can meet the harsh demand for Li/LNMO 5V class lithium metal batteries.