Deciphering the\nDecomposition Mechanisms of Ether\nand Fluorinated Ether Electrolytes on Lithium Metal Surfaces: Insights\nfrom CMD and AIMD Simulations

The performance of lithium metal batteries can be significantly\nenhanced by incorporating fluorinated ether-based electrolytes, yet\nthe solid electrolyte interphase (SEI) formation mechanism on lithium\nmetal surfaces remains elusive. This study employs classical and ab\ninitio molecular dynamics simulations to investigate the decomposition\nmechanisms of lithium bis(fluoromethanesulfonyl)imide (LiFSI) in 1,2-diethoxyethane\n(DEE) and its fluorinated analogues, F5DEE and F2DEE, when in contact\nwith lithium metal. Our findings indicate that F5DEE-based electrolytes\nfavor the formation of a FSI-rich primary solvation shell around Li<sup>+</sup>, while F2DEE-based electrolytes yield a solvent-rich environment.\nThe normalized number density at the Li/electrolyte/Li interface shows\na depletion of FSI anions in the electrochemical double layer (EDL)\nstructure near the Li anode upon charging, with the distance between\nthe first main peak of the FSI anion and Li anode following the order\nF5DEE < DEE < F2DEE. Analysis of the electronic projected density\nof states and charge transfer dynamics unveils the reductive dissociation\npathways of FSI anions and fluorinated DEE solvents on the lithium\nmetal surface, taking into account the influence of the EDL structure.\nDEE is identified as the most reduction-stable solvent, leading to\nthe selective dissociation of FSI anions and the formation of an entirely\ninorganic SEI. In contrast, F2DEE displays a pronounced reduction\ntendency, forming an organic-rich SEI due to the solvent-dominated\nlowest unoccupied molecular orbital at the interface. F5DEE, competing\nwith FSI anions for reduction, results in the formation of an inorganic-rich\nhybrid SEI with the highest LiF content. The simulation results correlate\nwell with experimental observations and underscore the pivotal role\nof various fluorinated functional groups in the formation of EDL and\nSEI near the lithium metal surface.