The Role of Solid Electrolyte Interphase Enabler in Li Metal Batteries
Authors: Su Chi Cheung , He Meinan , Shi Jiayan , Amine Rachid , Amine Khalil 
Published: July 2021

To facilitate the massive commercialization of electric vehicles, it is imperative to enhance the energy density of the state-of-the-art lithium battery technology. [1] Lithium metal battery (LMB) is considered as a promising energy storage system because of the exceptionally low electrochemical potential and ultrahigh specific capacity of the lithium metal anode. [2-4] Yet, the extensive application of LMBs is still obstructed by the extreme reactivity of lithium metal, which leads to severe safety problems. [5-6] To tackle this issue, the re-design of the electrolyte is viewed as the most practical approach due to its cost effectiveness. [7-8]

In this presentation, we successfully establish a solvation rule for the SEI enabler to achieve stable cycling of the lithium metal anode through a variety of electrochemical and solvation studies such as a Li/Cu cell test and IR-DOSY analysis. Fluoroethylene carbonate (FEC) was used as the exemplary solid-electrolyte interphase (SEI) enabler because it is the most common SEI enabler in the LMB electrolyte, and it is able to form a desirable SEI on the lithium metal surface. The results of all electrochemical studies point to the existence of a critical ratio between FEC and a co-solvent (e.g., DMC, EMC, and EA) for the stable cycling of LMBs. If the FEC/co-solvent ratio is lower than the critical ratio, not only will an unstable SEI be formed, but the SEI will also be gradually deteriorated by the significant decomposition of lithium complexes, with Li+ solvated solely by the co-solvent/non-SEI enabler. We discovered that the solvation number of FEC must be ≥ 1 to ensure the formation of a stable SEI, which mainly resulted from the sacrificial reduction of the SEI enabler and subsequent repair of the SEI to facilitate stable cycling of LMBs. This finding is crucial for the future development of functional electrolytes in that the ratio of FEC to co-solvent must exceed the critical volume/molar ratio for the solvation number of FEC to be ≥ 1.

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