Interface Structure in Li-Metal/[Pyr₁₄][TFSI]-Ionic Liquid System from ab Initio Molecular Dynamics Simulations

Abstract

Ionic liquids (ILs) are promising materials for application in a new generation of Li-batteries. They can be used as electrolyte, interlayer, or incorporated into other materials. ILs have ability to form a stable Solid Electrochemical Interface (SEI) which plays an important role, preventing Li-based electrode from oxidation and electrolyte from extensive decomposition. Experimentally, it is hardly possible to elicit fine details of the SEI structure. To remedy this situation, we have performed a comprehensive computational study (DFT-MD) to determine the composition and structure of the SEI compact layer formed between Li anode and [Pyr14][TFSI] IL. We found that the [TFSI] anions quickly reacted with Li and decomposed, unlike the [Pyr14] cations which remained stable. The obtained SEI compact layer structure is non-homogeneous and consists of the atomized S, N, O, F, and C anions oxidized by Li atoms.

The (In‐)Stability of the Ionic Liquids [(TMEDA)BH2][TFSI] and −[FSI] on the Li(001) Surface

“…The decomposition pathway of the [FSI] − anion reported in this manuscript is similar to those identified for [NNBH 2 ][FSI] [19] and [Pyr 13 ][FSI] [15b] . Similarly, the breakdown of the [TFSI] − anion proceeds in a similar manner to [NNBH 2 ][TFSI] [19] and [Pyr 14 ][TFSI] [15a,c] on the Li(001) surface at 298 K [15,19] . Additionally, the binding energy of the [(TMEDA)BH 2 ][TFSI], [NNBH 2 ][TFSI] [19] and [Pyr 14 ][TFSI] [15c] cations to the Li(001) surface are similar in magnitude, hence, the presence of the IL cation in the ion‐pair does not seem to modify or influence the decomposition pathway of the [TFSI] − anion at the Li(001) surface.…”

“…The reaction pathway seen for this system is in good agreement with what has previously been found for [NNBH 2 ][TFSI] [19] and [Pyr 14 ][TFSI] [15c] on Li(001), where breaking of the S−C bond is the initial reaction step. Alternatively, breaking of the S−N bond followed by S−C bond has previously been shown to occur during AIMD simulations of [Pyr 14 ][TFSI]/Li [15a,c] . Yet, the pathway is absent for the [(TMEDA)BH 2 ][TFSI]/Li(001) system in this work.…”

“…This particular product has the potential to enable polymerized species to form which have been reported to imbue desirable elastomeric properties to the SEI layer [18] . Computational modelling of the [Pyr 13 ][FSI] [15b] and [Pyr 14 ][TFSI] [15a,c] ILs on the Li(001) surface further supported the domination of chemical species attributed to the anion forming the inner inorganic component of the SEI layer. Additionally, these studies predicted the pyrrolidinium cations to remain intact during the early stages of SEI layer formation [15]…”

Design of a Multifunctional Interlayer for NASCION‐Based Solid‐State Li Metal Batteries

“…The F 1s spectra (Figure S2, Supporting Information) from the Li surface before argon ion sputtering indicates that the main breakdown product in electrolytes with FSI − is LiF (≈685.0 eV) while in TFSI − based electrolytes more C–F species (≈689.0 eV) are found compared to LiF, which is consistent with previous publications. [ 33–35 ] Depth profiling reveals that the decomposition products are present throughout the SEI layer formed with IL electrolytes, as shown in Figure 1d,e and Figure S2 (Supporting Information). In particular, the SEI layer on the Li metal anode cycled with 0.1LiFSI/0.9BMIM‐FSI has a higher concentration of LiF compared to the layers formed with the other electrolytes.…”

Fluorinated Boron-Based Anions for Higher Voltage Li Metal Battery Electrolytes

“…The repeated cycling of the Li anode can facilitate the propagation of Li dendrites and a continuous reaction between Li metal and the electrolyte, resulting in the loss of active material and a shortened cycle life [ 4 , 5 , 6 , 7 , 8 ]. One strategy that can prevent dendrite growth and prolong the cycle life of LMBs is the formation of a stable (in situ) solid electrolyte interphase (SEI) layer, which consists of a thin film of reaction products immediately formed after contact between the electrolyte and the Li anode [ 2 , 8 , 9 , 10 ]. Ideally, the SEI layer is compact and remains intact during cycling to prevent the loss of active materials and to minimise Li dendrite growth [ 4 , 11 ].…”