In situ formation of circular and branched oligomers in a localized high concentration electrolyte at the lithium-metal solid electrolyte interphase: a hybrid ab initio and reactive molecular dynamics study

Abstract

Developing advanced electrolytes has been considered as a promising approach to stabilize the lithium (Li) metal anode via the formation of a stable solid electrolyte interphase (SEI) that can protect the Li anode to enable long-term cycling stability in rechargeable Li metal batteries (LMBs). Recently, the concept of localized high-concentration electrolyte (LHCE) is emerging as an efficient state-of-the-art strategy in developing advanced electrolytes for LMBs. However, the underlying reaction mechanism of SEI formation in LHCEs with the Li anode remains far from clear. In this work, a hybrid ab initio and reactive molecular dynamics (HAIR) scheme is employed to investigate the detailed reactions of a typical LHCE system that consists of lithium bis(fluorosulfonyl)-imide (LiFSI) in dimethoxyethane (DME) with 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropylether (TTE). The initial reaction involves the release of the fluorine atoms of dilute 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropylether (TTE) initiated by Li⁰ followed by C–O bond cleavages. Polymerization processes are captured in an extended 2.8 ns HAIR simulation, with both circular and branched oligomers identified as the organic part of the SEI layer; meanwhile, the LiF forms a clear inorganic SEI, as confirmed by simulated radial distribution function (RDF) and X-ray photoelectron spectroscopy (XPS). Overall, the simulated results reveal that TTE releases the F to facilitate the formation and enrichment of LiF and provide an unsaturated carbon-chain as the backbone for in situ polymerization. Experimentally, these elastic in situ oligomers with LiF offer improved SEI performance. These theoretical results depict the basic chemical mechanism between LHCE and the electrode, providing useful information for the development of sophisticated electrolyte systems.