Resolving the phase instability of a fluorinated ether, carbonate-based electrolyte for the safe operation of an anode-free lithium metal battery

Abstract

The anode-free full cell architecture (Cu||NMC111) is an essential milestone for boosting the energy density of lithium metal batteries (LMBs). The LiPF6 dissolved in fluorinated carbonate (fluoroethylene carbonate (FEC)) and partially fluorinated ether (1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE)) solvents has been reported to be a fluorinated electrolyte in the development of LMBs because of their wide electrochemical windows, nonflammable characteristics, and stable LiF-rich solid electrolyte interface. However, its phase instability and high viscosity limit its practical application. In this work, an advanced ethyl methyl carbonate (EMC)-based fluorinated electrolyte (1 M LiPF6 in FEC/ TTE/EMC (3:5:2 by vol.) is developed, which is free of phase instability and has higher ionic conductivity, oxidative stability, and nonflammability. It has a higher oxidation potential of >5.3 V and better rate capabilities than the EMC-free electrolyte for lithium metal batteries (Li||NCM111). Meanwhile, the ternary electrolyte also enhances the cycling performance of anode-free lithium metal batteries (AFLMBs) within the potential range of 2.5-4.5 V at room temperature. The Cu||NMC111 cell with 1 M LiPF6 in FEC/TTE/EMC (3:5:2 by vol.) electrolyte delivers superior capacity retention of 40% and average Coulombic efficiency (av CE) of 98.30% for 80 cycles with a cutoff voltage of 4.5 V at the charge and discharge current densities of 0.2 and 0.5 mA/cm2, respectively. Hence, we develop a robust nonflammable electrolyte free of phase instability having wider oxidative stability, high rate capability, and good cyclic performance using an anode-free full cell configuration (Cu||NMC111).