Li-Metal Anode in Dilute Electrolyte LiFSI/TMP: Electrochemical Stability Using Ab Initio Molecular Dynamics

Abstract

Ab initio molecular dynamics simulations were performed for Li+-conducting electrolytes based on trimethyl phosphates (TMP) and lithium bis­(fluorosulfonyl)­imide (Li+FSI–) salt in contact with a Li-metal electrode. We focused on the transient-state behavior at the electrolyte, interfacial electrolyte–Li-metal electrode, and lithium reference electrode–electrolyte–Li-metal electrode to study dynamics and activation energy barriers of the Li+ ion, electrochemical and thermal stability of the interface electrode–electrolyte, and potential behavior of the Li-metal electrode, respectively. Our results show that in the most stable state, Li+ ions are tetrahedrally coordinated to three TMP and one FSI–. The inner solvation shell of a Li ion is composed of three TMP and one FSI– in one contact ion pair and four TMP in a solvent-separated ion pair. On the other hand, Li ions transporting through electrolyte cages take place when they are coordinated with three or less molecules that could be a combination of TMP and FSI–. The decomposition pathway of the LiFSI salt when in direct contact with the Li-metal anode starts with defluorination of FSI–, rapidly losing F– to the lithium surface, forming LiF species. The remaining FSO2NSO2 –2 with the addition of 2e– from the Li-metal decomposes into SO2 –2 and NFSO2 –2. SO2 –2 deposits on the Li surface and decomposes into Li2O and Li2S. The remaining NFSO2 –2 defluorinates, losing F– ion to the lithium surface, resulting in LiF and the remaining NSO2 –1 deposits on the lithium surface and decomposes in the following picoseconds, forming several binary compounds such as Li3N, Li2S, and Li2O. In contrast, when the salt is solvated by the TMP molecules, avoiding a direct contact with the Li-metal electrode, only one defluorination occurs, decomposing the FSI– into FSO2NSO2 –2 and F–. The two anions remain stable as they are solvated by the TMP molecules. We also analyzed the open-circuit potential energy (OCPE) of the Li-metal electrode during the solid-electrolyte interphase (SEI) formation. OCPE is calculated from the average local potential profile difference within the Li-metal electrode and a pristine Li-crystal reference electrode (LRE). When no SEI is formed, the Li-metal electrode has an average OCPE of +0.36 eV vs LRE. Due to the formation of an SEI, the Li-metal electrode has an average OCPE between −0.07 and −0.21 eV vs LRE. The OCPE of the Li-metal electrode decreases by ∼0.42 eV when an SEI is formed.

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