Reaction mechanism of methyl trifluoroacetate (CH3TFA) with lithium polysulfides (Li2S6) in gas and solvent phase

In this DFT study, we have evaluated the reaction mechanism of lithium polysulfide (Li2S6) with the electrolyte additive methyl trifluoroacetate (CH3TFA) in the gas and solvent (dimethoxyethane (DME)) phase at room temperature (298 K) by locating transition states (TS) for the methyl group transfer from CH3TFA to Li2S6, which is reported to produces organosulfur ((CH3)2S6). All the reported methyl transfer reactions that lead to the formation of organosulfur are having high barrier energy. The barrier energy difference between gas and solvent phase is maximum of 7 kcal/mol, and both the reactions are in extremely slow regime, therefore, the methyl transfer reaction for the formation of organosulfur implausible at room temperature.


Introduction
The focus on lithium-sulfur battery has increased due to its high theoretical capacity (1675 mAh/g) and specific energy (2600 Wh/kg), which is higher than all other lithium batteries [1][2][3][4][5].In addition, the lithium-sulfur battery produces higher voltage with low-cost and environmentally friendly materials.However, the shuttling of lithium polysulfides between the electrodes in the electrolyte has curbed the development of the Li-S (lithium-sulfur) battery [6].In addition, many technological challenges, including poor cyclability, low discharging efficiency, and high selfdischarge rate, are reported due to the formation of these lithium polysulfides [7][8][9].Alternatively, the use of additives in the electrolyte and implementation of cathode scaffolds to an extent can overcome the problems related to the shuttle phenomenon [10][11][12][13].
Recently, Manthiram et al.Have proposed CH 3 TFA (methyl trifluoroacetate) as an electrolyte in Li-S battery, leading to the formation of organosulfur compounds during the interaction with lithium polysulfides (Li 2 S 6 ) [32].CH 3 TFA react with lithium polysulfides to form lithium trifluoroacetate (LiTFA) and dimethyl polysulfides (organosulfur compound).From the above, it is imperative that the reaction mechanism of CH 3 TFA with lithium polysulfides is critical for the organosulfur formation and lithium-sulfur batteries.Therefore, understanding the reaction mechanism of the CH 3 TFA with lithium polysulfide (Li 2 S 6 ) molecules is essential.These results will help improve organic solvents like CH 3 TFA as the electrolyte in the Li-S battery.In the present work, study on reaction mechanics of the gas and solvent (dimethoxyethane (DME) between the electrolyte 97 Page 2 of 8 additive, CH 3 TFA and the lithium polysulfide, Li 2 S 6 to form LiTFA and organosulfur ((CH 3 ) 2 S 6 ) [33,34] is performed in room temperature (298 K).

Computational details
Molecular geometries of reactants, transition states and products are optimized using 6-311 + G(d,p) basis set and the Minnesota hybrid functional (M06) [35,36].M06 functional has been found to perform well in a variety of tests against accurate and experimental data, which is considered as the best functional for the calculations of thermochemical kinetics.Vibrational frequency analysis was executed to ascertain the stability of structures.(All the structures are local minima, since PES was not performed.)Here, the transition state (NIMF = 1) possesses one imaginary frequency [37,38].Intrinsic reaction coordinate (IRC) calculations were performed to identify the transition state (TS) that connects the selected reactant and product [39,40].The solvent parameters for the solvation model based on density (SMD) calculation for dimethoxyethane (DME) solvent is obtained from the study of Zhang et al., and the dielectric constant is set to be 7.22.The macroscopic surface tension at a liquid-air interface as 33.18 cal.mol −1 Å −2 , Abraham's hydrogen bond basicity as 0.41, and hydrogen bond aromaticity, acidity and electronegative halogenicity are all set to zero [41].The computations were performed using Gaussian 09 software.Topological analysis (electron density  and Laplacian of electron density) is performed using the single-point calculation of the optimized geometry for the interactions using atom in a molecule analysis (AIM) with Multiwfn software [42,43].

Results and discussion
In the present work, we study the gas-and solvent-based reaction mechanism between the electrolyte additive CH 3 TFA and the lithium polysulfide Li 2 S 6 , which leads to the formation of LiTFA and organosulfur ((CH 3 ) 2 S 6 ).

Gas phase
The reaction process is illustrated in four steps in Figs. 1, 2, 3 and 4 for gas phase.
Step 1 Formation of reaction complex (RC) in gas phase.The gas phase reaction in Fig. 1 shows the formation of reactant complex (RC) by the interaction of CH 3 TFA with Li 2 S 6 (Step 1).The reactant Li 2 S 6 and CH 3 TFA transforms to the reactant complex RC without any barrier (barrierless reaction).Due to the higher affinity between lithium and oxygen atoms, strong Li8-O13 bond are formed between Li 2 S 6 and CH 3 TFA.A strong Li-O bond induces elongation of other bonds (Li-S) in the vicinity.The electron density (ρ) value for the newly formed Li8-O13 bond is 0.0095 a.u.(Table S1) which is greater than electron density values for the Li-S bond in the RC.
The reactions are exothermic and spontaneous from the enthalpy and Gibbs free energy values (Given in Fig. 1).
Step 2 Formation of intermediate complex (IC1) in the gas phase.
In Step2, the transformation from RC to intermediate complex (IC1) occurs with transferring of the methyl group from the CH 3 TFA (RC) to Li 2 S 6 to form IC1 through transition state TS1 (Barrier energy, 42.85 kcal/mol).In Fig. 2, the LiTFA molecule binds with the CH 3 LiS 6 molecule to form in IC1 through a strong Li7-O13 bond.Notably, from Table S1, the electron density for the Li7-S4 bond in RC (0.0022 a.u.) is reduced in IC1 (0.0016 a.u) due to the strong Li-O bond formed between LiTFA and CH 3 LiS 6 .
Step 3 Formation of intermediate complex (IC2) in gas phase.
Further in Step 3, elimination of the LiTFA molecule occurs through a barrierless reaction between the intermediate complex IC1 with CH 3 TFA resulting in another intermediate complex IC2.During this reaction, the bond length of Li7-S4 (2.306 Å) increases to 2.483 Å in IC2, shown in Fig. 3. Step 4 Formation of product molecules (P1) in gas phase.In Step 4 (Fig. 4), the internal methyl group transfer occurs in IC2 and forms a product P1.The transition from IC2 to P1 occurs via transition state TS2 (Barrier energy 39.49 kcal/mol).The results from AIM analysis (Table S2) shows the existence of bond critical points (BCP) between the LiTFA and the (CH 3 ) 2 S 6 for Li-S bonds.However, these bonds (Li-S) evaluated using NBO (natural bonding orbital) show no charge transfer between them.Hence, after Step 4, the LiTFA molecules detach from the organosulfur molecule.
The IRC (intrinsic reaction coordinates) diagrams are plotted in Figure S1 to validate the transition state TS1 and TS2 in the gas phase.
To understand the thermodynamics of the reaction in Step 2 (Fig. 2) and Step 4 (Fig. 4), the activation energy, reaction energy, enthalpy, Gibbs free energy ΔG for the reaction and enthalpy (ΔH) were calculated and are given in Table 1.The enthalpy value of 42.83 also indicates extremely slow reaction mechanics (ΔH = 35-50 kcal/mol indicates extremely slow mechanism) [44].On the contrary, the enthalpy value is reduced by 3 kcal/mol for the second methyl transfer, which indicates a comparatively faster mechanism.From the theoretical report for the interaction between DME with the reactive intermediates of lithium sulfur battery proposed by Assary et.al, it is clear that potentiality of the reaction in extremely slow regime are on the order of months to years [44].Hence, the presence of CH 3 TFA do not pose any threat to the stability of the Li 2 S 6 at room temperature.The formation of organosulfur by the interaction between Li 2 S 6 with CH 3 TFA is improbable in gas phase.

Solvent phase
Step 1 Formation of reactant complex (RC) in solvent phase.
The reaction mechanism is studied in solvent phase (DME) in four steps as illustrated in Figs. 5, 6, 7 and 8.
Step 1: Reactant complex RCDME is formed between Li2S6 and CH3TFA through the Li-O bond.From Table S3, the electron density for Li-O bond in DME is 0.0011 a.u.greater than in the gas phase.This indicates the strong binding between CH 3 TFA and Li 2 S 6 in the solvent DME.Similar to the gas phase, the formation of reactant complex RC DME is also barrierless.The reactions are exothermic and spontaneous from the enthalpy and Gibbs free energy values (Given in Fig. 5).
Step 2 Formation of intermediate complex (IC1) in solvent phase.
In step 2, RC DME transforms to IC1 DME through the methyl transfer from CH 3 TFA via transition state TS1 DME (Barrier energy, 44.88 kcal/mol) as shown in Fig. 6.Unlike gas phase, in intermediate complex IC1 DME between LiTFA and CH 3 LiS 6 , only weak interaction (through Li-S bond) is observed from the BCPs and low electron density values (Table S3).Therefore, in solvent phase, LiTFA can detach from the first intermediate complex IC1 DME with ease compared to the gas phase.
Step 3 Formation of intermediate complex (IC2) in the solvent phase.
In step 3, IC2 DME is formed through the reaction of the second CH 3 TFA with IC1 DME (Fig. 7).Similar to the gas phase, in IC2 DME , the CH 3 TFA shows weaker interaction with the intermediate complex IC1 DME .The reaction is barrierless.Step 4 Formation of product molecules (P1) in the solvent phase.
In step 4, internal methyl group transfer occurs within IC2 DME through transition state TS2 DME with barrier energy 46.33 kcal/mol.Subsequently, the product P1 DME (Fig. 8).From Table S4, both the LiTFA molecules in P1 DME shows weaker interaction with the organosulfur molecule.
The IRC (intrinsic reaction coordinates) diagram for interaction in DME is plotted in Figure S2 to validate the transition state TS1 DME and TS2 DME .
From Table 2, unlike gas phase, the first methyl transfer holds lower activation energy than the second methyl transfer in DME.Further the enthalpy values for Step 2 and 4 confirm a slow reaction mechanism during the first and second methyl transfer.The energy profile for the transitions in gas phase and DME are plotted in Figure S3 (Supporting information).The reaction energies of the reactions in both gas phase and DME are validated using coupled-cluster single-, double-and perturbative triple-excitations [CCSD(T)] method (Table S5 and S6, supporting information).The methyl transfer reaction between CH 3 TFA with Li 2 S 6 in both gas and solvent phase is found to be having higher barrier energy in 298 K. Hence, the presence of CH 3 TFA do not pose any threat to the stability of the Li 2 S 6 in the presence of DME.

Conclusion
The reaction mechanism of electrolyte additive CH 3 TFA with lithium polysulfide Li 2 S 6 is enumerated in the gas and solvent phase to understand organosulfur formation.All the reaction steps leading to the formation of organosulfur possess high barrier energy in gas and DME solvent.For methyl transfer reactions, the barrier energies are comparatively lower in gas than DME.Nevertheless, the barrier energy for the reactions in the gas and solvent phase is in an extremely slow regime (ΔH = 35-50 kcal/mol indicates extremely slow mechanism).The probability of happening this methyl transfer between CH3TFA and Li2S6 is very low at room temperature.

Fig. 1
Fig. 1 Formation of reactant complex RC during the interaction between Li 2 S 6 and CH 3 TFA with ΔH and ΔG values (kcal/mol) are optimized using 6-311 + g(d,p) basis set and M06 level of theory

Fig. 2 Fig. 3
Fig. 2 Schematic diagram for the transformation from RC to IC1 are optimized using 6-311 + g(d,p) basis set and M06 level of theory

Fig. 4
Fig. 4 Schematic diagram for the transformation from IC2 to P1 using 6-311 + g(d,p) basis set and M06 level of theory

Fig.Fig. 6
Fig. Formation of reactant complex RC DME during the interaction between Li 2 S 6 and CH 3 TFA optimized using 6-311 + g(d,p) basis set and M06 level of theory

Fig. 7
Fig. 7 Formation of reactant complex IC2 DME during the interaction between P1 DME and CH 3 TFA optimized using 6-311 + g(d,p) basis set and M06 level of theory

Fig. 8
Fig. 8 Schematic diagram for the transformation from IC2 DME to P1 DME is optimized using 6-311 + g(d,p) basis set and M06 level of theory

Table 1
Thermodynamical properties of Step 2 andStep 4 in the gas phase