Li-Metal Anode in Dilute Electrolyte Lifsi/TMP: Electrochemical Stability Using Ab-Initio and Classical MD Simulations
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Ab initio and classical molecular dynamics simulations are 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 electrolyte, interfacial electrolyte Li-metal electrode, and lithium reference electrode electrolyte Li-metal electrode to study diffusion 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 TMPs and one FSI. The decomposition pathway of the LiFSI salt when in direct contact with the Li-metal anode starts with defluorination of FSI , rapidly losing Fto the lithium surface, forming LiF species. The remaining FSO2NSO22 with the addition of 2e from the Limetal decomposes into SO22 and NFSO22. SO22 deposits on the Li-surface and decomposes into Li2O and Li2S. The remaining NFSO22 defluorinates, losing Fion to the lithium surface, resulting in LiF and the remaining NSO21 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 Limetal electrode, only one defluorination occurs, decomposing the FSI into FSO2NSO22 and a F. A similar mechanism is followed when a second FSI salt is in contact with this new formed SEI. 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 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 a 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 a SEI is formed. Using the data obtained from the ab initio simulations and using machine learning algorithms such as classification and clustering a classical MD model was developed to study the SEI evolution as the charging process takes place in a battery using Li-metal as anode and the TMP/LiFSI as electrolyte.Figure 1
