DFT-ReaxFF hybrid molecular dynamics investigation of the decomposition effects of localized high-concentration electrolyte in lithium metal batteries: LiFSI/DME/TFEO

Due to its low electrochemical potential and high theoretical specific energy, lithium-metal batteries (LMBs) have been considered as a promising advanced energy storage system for portable applications such as electric vehicles (EVs). However, the uncontrolled growth of lithium dendrites during cycling has remained a challenge. By utilizing an inert solvent to dilute the high concentration electrolytes, the concept of localized high-concentration electrolytes (LHCEs) has recently been demostrated as an effective solution to enable the dendrite-free cycling of LMBs. In this work, we investigated the reactions of 2 M lithium bis(fluorosulfonyl)imide (LiFSI) in a mixture of dimethoxyethane (DME)/tris(2,2,2-trifluoroethyl) orthoformate (TFEO) electrolyte at a Li metal anode. The SEI formation mechanism is investigated using a hybrid ab initio and reactive force field (HAIR) method. The 1n reactive HAIR trajectory reveals the important initial reduction reactions of LiFSI, TFEO, and DME. Particularly, both FSI anions and TFEO decompose quickly to release a considerable amount of F-, which leads to a LiF-rich SEI inorganic inner layer (IIL). Furthermore, TFEO produces a significant amount of unsaturated carbon products, such as thiophene, which can potentially increase the conductivity of SEI to increase the battery performance. Meanwhile, XPS analysis is utilized to further investigate the evolution of the atomic environment in SEI. Future designs of better electrolytes can be greatly aided by these results.

Automated summary

Due to their potential use in portable applications such as electric vehicles, lithium-metal batteries (LMBs) have attracted attention for their low electrochemical potential and high theoretical specific energy. However, the uncontrolled growth of lithium dendrites during cycling has been a challenge. Recently, the concept of using localized high-concentration electrolytes (LHCEs), achieved by diluting high concentration electrolytes with inert solvents, has shown promise in enabling dendrite-free cycling of LMBs. In this study, the reactions of lithium bis(fluorosulfonyl)imide (LiFSI) in a mixture of dimethoxyethane (DME)/tris(2,2,2-trifluoroethyl) orthoformate (TFEO) electrolyte at a lithium metal anode were investigated. The formation mechanism of the solid electrolyte interface (SEI) was studied using a hybrid ab initio and reactive force field (HAIR) method. The results revealed important initial reduction reactions of LiFSI, TFEO, and DME, leading to the formation of a LiF-rich SEI inorganic inner layer (IIL) and unsaturated carbon products from TFEO. These findings can provide valuable insights for the future design of improved electrolytes.