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 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 in the next-generation high energy density and sustainable batteries, due to the high theoretical energy density of ∼650 Wh kg-1, elimination of toxic Co element and intrinsic safety brought by LNMO spinel structure compared with the unstable layered oxides LiNixCoyMnzO2. However, their development is critically limited by the incompatibility between state-of-the-art carbonate electrolytes and the two aggressive electrodes. Herein, we synthesize a new electrolyte additive, i.e. 2,2-difluoroethyl methyl sulfone (FS), which could enable the ultrahigh-voltage lithium metal batteries to stably cycle in conventional carbonate electrolytes. On the cathode side, different from conventional electrolyte additives, FS could be selectively adsorbed on the LNMO surface and form a special assembled FS “buffer” layer, which could efficiently exclude the free carbonate molecules away from the cathode surface. Therefore, upon charging, the -CF2H group of FS is preferably anodically decomposed to form inorganics-rich CEI, which efficiently suppresses micro-fracture and transition metal dissolution of LNMO. On the anode side, the FS could also be preferably cathodically decomposed, resulting in an inorganics-rich SEI for the stable cycling of Li metal anode. Thus, carbonate electrolyte with FS additive enables the unprecedented high performance for cobalt-free 5V-class lithium metal batteries, i.e. 40 μm-Li/LNMO (loading=7 mg cm−2) full cell with high capacity retention of 84% even over 600 cycles at 1 C using commercial 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 capacity retention of 99% after 100 cycles at 0.25 C. Moreover, the Li/LNMO pouch cells, which has not been reported before to the best of our knowledge, is assembled and can operate stably for over 150 cycles.