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

Abstract

Ultrahigh-voltage lithium metal batteries based on a cobalt-free LiNi_0.5Mn_1.5O₄ (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 ∼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 LiNi_xCo_yMn_zO₂. 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 μm-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 μm) 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.