(Invited) High Coulombic Efficiency Electrolyte Enables Li||Span Batteries with Superior Cycling Performance
Authors: Liu Ping , Liu Haodong , Holoubek John , Zhou Hongyao , Xing Xing 
Published: December 2020

Lithium metal is being extensively studied as an anode to replace graphite due to its high capacity and the low electrochemical potential.Current research focus is to mitigate lithium dendrite growth and to increase coulomb efficiency during electrochemical cycling by using various surface coatings and electrolyte additives.1
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2 In addition, a 3D host can be beneficial due to the reduced volume change.3 While sulfur or metal oxide intercalation cathodes can both in principle achieve full-cell energy densities of 500 Wh kg-1, sulfur-based cathodes are particularly appealing due to their low cost, abundant supply, and high storage capacity. Among these, the SPAN (sulfurized polyacrylonitrile) cathode is particularly promising due to the physical confinement of the small molecular sulfur in the conductive polymer network which provides a high specific capacity of > 550 mAh g-1 and an average discharge potential of ~ 1.8 V. We have recently focused on designing novel electrolyte solution chemistry that enables Li||SPAN batteries with superior cycling performance.
In the first example, we designed a high-concentration, ether-based electrolyte with LiTFSI and LiNO3 as the co-salts. In addition to providing excellent protection for lithium metal anodes by forming the solid electrolyte interface (SEI), the electrolyte promotes the formation of a crystalline cathode electrolyte interface (CEI) on the SPAN surface composed of LiF and LiNO2. The CEI effectively prevents the formation of soluble polysulfide species and enables stable cycling of the Li||SPAN battery. The benefit of having effective CEI and SEI layers is also demonstrated in Li||SPAN cells with limited lithium supply, which exhibit lithium cycling efficiency values consistent with or exceeding Li||Cu test results, reaching 99.5%. The development of electrolyte chemistries to enable the formation of effective CEI layers is a promising approach to long-life lithium metal batteries.
In another example, we formulated a highly concentrated salt/ether electrolyte diluted in a fluorinated ether, which realized an average lithium coulombic efficiency of 99.37% at 0.5 mA cm−2 and 1 mAh cm−2 for more than 900 cycles. This electrolyte also maintained a record coulombic efficiency of 98.7% at 10 mA cm−2, indicative of its ability to provide fast-charging with high cathode loadings. Morphological studies reveal dense, dendrite free Li depositions after prolonged cycling, while surface analyses confirmed the formation of a robust LiF-rich SEI layer on the cycled Li surface. Moreover, this ether-based electrolyte is highly compatible with the SPAN cathode, where the constructed Li||SPAN cell exhibited reversible cathode capacity of 579 mAh g-1 and no capacity decay after 1200 cycles. A cell where a high areal loading SPAN electrode (3.5 mAh cm−2) is paired with only one-fold excess Li was constructed and cycled at 1.75 mA cm−2, maintaining a coulombic efficiency of 99.30% for the lithium metal. This study provides a path to enable high energy density Li||SPAN battery with stable cycling.

Acknowledgements

This work was supported by the Office of Vehicle Technologies of the U.S. Department of Energy through the Advanced Battery Materials Research Program (Battery 500 Consortium) under Contract DE-EE0007764. Part of the work used the UCSD-MTI Battery Fabrication Facility and the UCSD-Arbin Battery Testing Facility.

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