High-energy-density Li metal batteries (LMBs) with Nickel (Ni)-rich cathode and Li-metal anode have attracted extensive attention in recent years. However, commercial carbonate electrolytes bring severe challenges including poor cycling stability, severe Li dendrite growth and cathode cracks, and narrow operating temperature window, especially hardly work at below -40 °C. In this work, a 2.4 m lithium difluoro(oxalato)borate (LiDFOB) in ethyl acetate (EA) solvent with 20 wt% fluorocarbonate (FEC) (named 2.4m-DEF) is designed to solve Li+ transport dynamic at low temperature and improve interfacial stability between electrolyte with Li anode or Ni-rich cathode. Beneficial lower freezing point, lower viscosity, and higher dielectric constant of EA solvent, the electrolyte exhibits excellent Li+ transport dynamic. Relying on the unique Li+ solvation structure, more DFOB- anions and FEC solvents are decomposed to establish a stable solid electrolyte interface at electrolyte/electrode. Therefore, LiNi0.9 Co0.05 Mn0.05 O2 (NCM90)/Li LMB with 2.4m-DEF enables excellent rate capability (184 mA h g-1 at 30 C) and stable cycling performance with ≈93.7% of capacity retention after 200 cycles at 20 C and room temperature. Moreover, the NCM90/Li LMB with 2.4m-DEF exhibits surprising ultra-low-temperature performance, showing 173 mA h g-1 at -40 °C and 152 mA h g-1 at -60 °C, respectively.
Keywords: Li metal batteries; ethyl acetate; high rate; nickel-rich cathodes; wide temperatures.
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