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

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 FSO 2 NSO 2 –2 with the addition of 2e – from the Li-metal decomposes into SO 2 –2 and NFSO 2 –2 . SO 2 –2 deposits on the Li surface and decomposes into Li 2 O and Li 2 S. The remaining NFSO 2 –2 defluorinates, losing F – ion to the lithium surface, resulting in LiF and the remaining NSO 2 –1 deposits on the lithium surface and decomposes in the following picoseconds, forming several binary compounds such as Li 3 N, Li 2 S, and Li 2 O. 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 FSO 2 NSO 2 –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.