Fluorinated Ether Co-Solvents for Lithium-Metal Batteries

Lithium (Li)-metal batteries (LMBs) have been developed for next-generation energy storages with higher energy density than the current lithium-ion batteries (LIBs). It is attributed to the metallic Li having around a ten-fold higher specific capacity (3,860 mAh g^-1) than the graphite. However, interfacial reactions at Li surface are complicated and significantly depend on electrolyte solutions. Carbonate-based electrolyte solutions, which were generally used in the LIBs, were incompatible with the Li and remained poor solid electrolyte interphase (SEI).^[1]. Alternatively, Ether-based electrolyte solutions, for example, dimethoxyethane (DME), showed better performance owing to their high reductive potential (&lt; 0.5 V vs. Li/Li⁺) and chemical stability against the metallic Li.^[2],[3] Nonetheless, the low oxidative potential (&lt; 4 V vs. Li/Li⁺) of ether-based electrolyte solution was not applicable for the NCM cathode requiring chemical and electrochemical stability over 4.5 V vs. Li/Li⁺. These limitations have restricted the development of LMBs toward high energy density.<br/>We investigated the improved oxidative stability of electrolyte solutions using fluorinated ethers in the LMBs. Recently, fluorinated 1,4-dimethoxylbutane (FDMB) was designed and tested for the LMBs and anode-free Li batteries.^[4] The inductive effect of fluorine in FDMB significantly raised the oxidative potential up to 6.14 V vs. Li/Li⁺ at the Al electrode, which was correlated with the low HOMO energy level (-7.75 eV).^[4] However, the ionic conductivity of FDMB was lower (5.0 cp at 25 ^oC) than DME with 1 M LiFSI due to the high intrinsic viscosity and the weak Li⁺ solvation affinity of fluorines. We added 1,2-diethoxyethane (DEE) solvents to FDMB with different ratios and investigated their ionic conductivities, oxidative potentials, and chemical stability with Li. The ionic conductivities and voltage polarizations of symmetric Li cells were improved with increasing DEE concentrations. However, the oxidative potentials shift more positively at a higher FDMB amounts. In this presentation, I will show physicochemical properties of FDMB with different DEE ratios and their feasibility for symmetric Li cells, Li|LFP, and Li|NCM full cells.<br/><br/><b>References</b><br/>[1] Xu, K. Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries. Chem. Rev. <b>2004</b>, 104, 4303−4418.<br/>[2] Chibueze, V. A.; Zhiao, Y.; Xian K.; Jian, Q.; Yi, C.; Zhenan, B. A New Class of Ionically Conducting Fluorinated Ether Electrolytes with High Electrochemical Stability. <i>J. Am. Chem. Soc.</i> <b>2020</b>, <i>142</i>, 7393-7403.<br/>[3] Jiao, S.; Ren, X.; Cao, R.; Engelhard, M. H.; Liu, Y.; Hu, D.; Mei, D.; Zheng, J.; Zhao, W.; Li, Q.; et al. Stable Cycling of High-Voltage Lithium Metal Batteries in Ether Electrolytes. <i>Nat. Energy.</i> <b>2018</b>, <i>3</i>, 739−746.<br/>[4] Yu, Z.; Wang, H.; Kong, X., et al. Molecular design for electrolyte solvents enabling energy-dense and long-cycling lithium metal batteries. <i>Nat. Energy</i> <b>2020</b>, <i>5</i>, 526–533.