Characteristics of Lithium Ions and Superoxide Anions in EMI-TFSI

Article pubs.acs.org/JPCA

Characteristics of Lithium Ions and Superoxide Anions in EMI-TFSI and Dimethyl Sulfoxide Sun-ho Jung,*,† Filippo Federici Canova,*,‡ and Kazuto Akagi† †

Advanced Institute for Materials Research, Tohoku University, Katahira, Sendai, Japan Aalto Science Institute, Aalto University, PO Box 15500, FI-00076 Aalto, Finland

‡

S Supporting Information *

ABSTRACT: To clarify the microscopic effects of solvents on the formation of the Li+-O−2 process of a Li−O2 battery, we studied the kinetics and thermodynamics of these ions in dimethyl sulfoxide (DMSO) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI-TFSI) using classical molecular dynamics simulation. The force field for ions− solvents interactions was parametrized by force matching first-principles calculations. Despite the solvation energies of the ions are similar in both solvents, their mobility is much higher in DMSO. The free-energy profiles also confirm that the formation and decomposition rates of Li+-O−2 pairs are greater in DMSO than in EMI-TFSI. Our atomistic simulations point out that the strong structuring of EMI-TFSI around the ions is responsible for these differences, and it explains why the LiO2 clusters formed in DMSO during the battery discharge are larger than those in EMI-TFSI. Understanding the origin of such properties is crucial to aid the optimization of electrolytes for Li−O2 batteries.

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INTRODUCTION Li−O2 batteries are attracting attention worldwide because their theoretical specific energy is ∼10 times greater than that of conventional Li-ion batteries.1,2 Schematically, a simple Li− O2 battery comprises a pure lithium metal anode, a porous cathode, and a nonaqueous electrolyte.2 During the discharge process, oxygen is reduced at the cathode producing superoxide (O−2 ), and Li+ is ionized at the anode. Ultimately, the two ions recombine in the electrolyte, forming lithium peroxide (Li2O2).1 Typically, in nonaqueous systems, Li2O2 forms in the cathode’s pores, impeding the discharge process and reducing the lifetime of the battery.3 To recharge the battery, it is necessary to split Li2O2 back into Li+ and O2, and the efficiency of this process depends on the nanoscale growth details of Li2O2. Thus, the optimal electrolyte would be chemically stable in the presence of highly reactive O−2 and allow fast diffusion of Li+ and O2, giving high specific capacity, energy, and power density.4 Low volatility is also important for safety. Additionally, the electrolyte should facilitate the decomposition of Li2O2 in the charging process. The identification of such a material is currently an open problem. Alkyl carbonates are commonly used as electrolytes for Li-ion batteries;5 hence, they have also been tested for Li−O2 batteries. Because of its low vapor pressure and adequate Li+ mobility compared to other organic solvents, propylene carbonate (PC) was another early candidate for Li−O2 batteries.6 However, by using Fourier-transform infrared spectroscopy (FTIR) and mass spectroscopy, it was found that side products such as Li2CO3, CH3CO2Li, HCO2Li, and C3H6(OCO2Li)2 formed after only six charge−discharge cycles, indicating a reaction between PC and O−2 .7 Other families of organic solvents, such as tetraethylene glycol dimethyl ether8,9 and dimethyl sulfoxide10−15 (DMSO; Figure S1a), have been © 2015 American Chemical Society

thoroughly investigated because their specific capacity is greater than that of other compounds, and well resistant to O2− attack.4,12 DMSO-based electrolytic systems were proven to remain intact even after 100 charge−discharge cycles.13 Ionic-liquid (IL) electrolytes are also widely investigated because of their negligible vapor pressure, low flammability, and wide electrochemical window.16−19 Despite their oxygen solubility and Li+ conductivity being typically lower than organic solvents such as PC and DMSO, recent transmission electron microscopy imaging showed how Li2O2 particles that form in an IL are an order of magnitude smaller than those that form in organic solvents.16 This way, small Li2O2 particles form better contact with the cathode surface and can be decomposed much easier during the battery recharge process. The origin of this characteristic of Li2O2 formation and decomposition kinetics in ILs is still not understood. To date, various ILs have been tested for use in Li−O2 batteries, of which 1-ethyl-3-methylimidazolium (EMI+; Figure S1b) and bis(trifluoromethylsulfonyl)imide (TFSI−; Figure S1c) are examples of cation and anion. This IL has particularly low viscosity compared with other ILs, due to the delocalized charge on the anion, which makes the molecular structure less stiff than BF−4 or PF−6 anions.20 The absence of long alkyl chains on the cation also prevents them from aggregating into domains, slowing the diffusion.21,22 Previous studies show that batteries using the EMI-TFSI electrolyte with gold18 or singlewalled carbon-nanotube17 cathodes have reasonable cycle lifetimes. In the present study, we use theoretical methods to investigate the discharge process of Li−O2 batteries featuring Received: October 4, 2015 Revised: December 21, 2015 Published: December 21, 2015 364

DOI: 10.1021/acs.jpca.5b09692 J. Phys. Chem. A 2016, 120, 364−371

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The Journal of Physical Chemistry A

solvent fixing the position of isolated ions, and (3) restrained relaxation of the full system. In step (3), large repulsive interaction is applied between Li+ and O−2 to avoid their clustering before the production run. Optimization of the Force Field. We employ the force fields from refs 36 and 37 to describe EMI-TFSI and those from ref 38 to describe DMSO, respectively. These force fields are based on CHARMM39 and reproduce the equilibrium properties of the solvents quite well. Since no appropriate force field between the solute (Li+, O−2 , O2) and solvent are found in literature, we newly build a set of parameters to provide reasonable forces among them. Most pairwise ions−solvents interactions are described by Lennard-Jones potentials; however, we use the Born−Mayer− Huggins (BMH) potential40 to model the interaction between O−2 and positively charged species in EMI-TFSI, as well as Li+O−2 :

EMI-TFSI and DMSO, to characterize how the atomic-scale properties of the electrolytes affect the performance of the device. Earlier theoretical studies focused mainly on the mobility of Li+ and its solvated structure in ILs23−28 and DMSO.29−31 However, to our knowledge, the kinetics and thermodynamics of O−2 in ILs or DMSO have not yet been studied; they will be discussed in this paper. A clear difference in Li+-O−2 aggregation dynamics in the two electrolytes is observed in our simulations. This is explained by the dynamics of DMSO and EMI-TFSI molecules in proximity of Li+ and O−2 ions. The kinetics of O2, which play an important role in the battery discharge process, is also subject of our investigation.

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METHODS Classical Molecular Dynamics Calculation. Classical MD simulations are performed with the LAMMPS code.32 The cutoff distance for short-range and Coulomb interactions is 1 nm; tail corrections are applied to the 1/r6 term in the potentials, while long-range electrostatics are calculated with the particle−particle particle-mesh method33 (PPPM) with root-mean-square accuracy of 0.0001. The time step and temperature are 1 fs/step and 400 K, respectively. This temperature is slightly higher than the operational range of common batteries (300−350 K);34 however, the available force fields are more accurate at 400 K than at 300 K. Modeling of the System. Model systems are based on either EMI-TFSI or DMSO liquid in cubic cells, eventually including the ions, as indicated in Table 1. Systems IL0 and D0

⎛σ − r ⎞ C D + 8 E BMH = A exp⎜ ⎟− ⎝ ρ ⎠ r6 r

The first term is excluded (A = 0), and we fit only the second and third terms (C and D), representing van der Waals attraction and short-range repulsion, respectively. The choice of this special functional form is purely due to the final accuracy of the fit. Using Lennard-Jones, it is not possible to independently tune the attractive and repulsive terms. This appeared to be required to obtain a good fit for O−2 -EMI-TFSI interactions. Li+ ions and each oxygen atom of O−2 carry a charge of +1.0;−0.5, respectively. O atoms in O2 are charge neutral, and its interaction with the solvents is modeled by Lennard-Jones potential. To build force optimized parameters, we prepare a (1.6 × 1.6 × 1.6) nm3 unit cell with periodic boundary conditions with one Li+or O−2 and 30 DMSO molecules or 10 pairs of EMITFSI. First, we run a classical MD simulation with nonoptimized potentials and extract a sample of 100 snapshots from the 10 ns trajectory for each system, to be used as a reference set. Atomic forces in the reference set are evaluated with first-principles calculations based on density functional theory (DFT)41 using the Vienna ab initio simulation package (VASP).42,43 Plane-wave cutoff energy (400 eV), Γ-point sampling, and the projector-augmented-wave (PAW) scheme are used. van der Waals interactions are taken into account by the revised vdW-DF2 functional.44−46 Then, force matching is performed to reproduce the DFT results with the classical model, using genetic algorithm.47 We run another classical MD simulation using these improved parameters and obtain a new reference set of configurations. These are evaluated with DFT, and classical parameters are further improved by repeating the force matching optimization. The fitted parameters are listed in Tables S1−S3, giving an average force discrepancy of 0.33;0.21;0.24;0.27 eV/Å for Li+/ DMSO, O2−/DMSO, Li+/EMI-TFSI, and O2−/EMI-TFSI, respectively. We accept the parameters based on the fact that the force discrepancy for N in pure EMI-TFSI and H in DMSO are 0.54;0.45 eV/Å respectively. This means that the systematic DFT/CHARMM mismatch is already larger in the solvents, and we should not expect the force fields for the ions to perform much better. Force acting on O2 in DMSO and EMITFSI is fairly accurate, and the discrepancy is smaller than that of Li+ or O−2 .

Table 1. Content of the Systemsa Studied with Classical MD system

Li+

O−2

O2

EMI+

TFSI−

IL0 IL1 IL2 IL3 IL4 IL5 IL6 IL7 D0 D1 D2 D3 D4 D5

0 1 0 50 0 0 50 1 0 1 0 0 50 1

0 0 1 0 50 0 50 1 0 0 1 0 50 1

0 0 0 0 0 50 0 0 0 0 0 50 0 0

500 499 500 450 500 450 450 499

500 500 499 500 450 450 450 499

(1)

DMSO

1574 1428 1428 1574 1428 1574

a

The composition of each system is designed to preserve the charge neutrality (except D1,2).

correspond to pure EMI-TFSI and DMSO, respectively. While only one Li+ or O−2 is introduced in IL/D 1−2, 50 Li+ or O−2 are present in IL3−4, giving an ionic concentration of 0.4 mol/L. This value corresponds to a saturated solution of Li+ in EMITFSI.35 Neutral O2 molecules are introduced in IL5 and D3. These systems are used for evaluation of the diffusion coefficient and analysis of the solvation structure. Prior to the production runs, all DMSO-based systems are relaxed in NPT ensemble at 1 atm for 5 ns, while EMI-TFSI-based systems required 10 ns due to slow dynamics. Systems IL6 and D4 including 50 Li+ and 50 O−2 are used for observation of the growth process of LiO2 clusters. The initial configurations are prepared as follows: (1) random generation of solvent molecules and fully isolated ions, (2) relaxation of 365


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The Journal of Physical Chemistry A Interactions between Li+ and O−2 are modeled with BMH potential and tuned based on MP2 calculation using gaussian09 code48 with 6-311++G(d,p) basis set. In our preliminary studies, the growth rate of Li+-O−2 clusters (see Figure 4b,c) strongly depends on the depth of the potential-energy well between Li+ and O−2 . DFT calculations, however, give a too shallow potential curve. Therefore, we evaluate only this interaction using MP2 method. Finally, electrostatic repulsion in Li+-Li+ and O−2 -O−2 pairs is the dominating interaction; therefore, no short-range potential is required. Evaluation of the Diffusion Coefficient. The diffusion coefficients for molecules and ions are obtained by computing the long-time limit slope of the mean-square displacement (MSD) over 100 ns MD simulations as follows:

Atomic structure renderings in Figures 2 and 4 are made with VESTA50 and VMD,51 respectively. Data analysis and plots are done with Wolfram Mathematica.52

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RESULTS AND DISCUSSION Diffusion Properties. We first estimate the self-diffusion coefficients D of pure EMI-TFSI and DMSO at 400 K (systems IL0 and D0). The results are reported in Table 2 together with

Table 2. Comparison of Diffusion Coefficient (m2/S) Obtained by NMR53,54 and by MD Simulations at 400 K and at the Dilute Limit TFSI−

DMSO

3.08 × 10−10 1.17 × 10−10

8.91 × 10−9 7.31 × 10−9

EMI+ −10

5.08 × 10 1.75 × 10−10

Dexp DMD

1 d ⟨|r(t0 + τ ) − r(t0)|2 ⟩ (2) 6 dt The models with 50 solute ions or molecules (IL3−5, D3) show good convergence to linear MSD slopes using 100 ns of MD trajectories. However, the slopes of MSD for Li+ and O−2 in the models with only one ion (IL1−2, D1−2) did not converge well even using 500 ns of MD trajectories because of insufficient sampling. Hence, the corresponding diffusion coefficients would not be so accurate as the models IL3−5 and D3. Evaluation of the Solvation Energy. To evaluate the solvation energies of Li+ and O−2 , we perform the box-standard umbrella sampling and weighted histogram analysis49 using the cell as shown in Figure 1. The solvent is confined within D = lim

τ→∞

experimental values.53,54 The ordering of diffusion coefficients from simulation, DDMSO ≫ DEMI+ > DTFSI− is consistent with that of the experimental results. While the diffusion of DMSO agrees well with experiments, EMI-TFSI in our simulations diffuses ∼3 times slower than measured. Despite the calculated D is accurate (statistical error is negligible), the discrepancy with experiments is large, and it is a known issue for classical, nonpolarizable models of ILs. Table 3 shows the diffusion coefficients of O2, Li+, O−2 , and those of solvent molecules in the corresponding system. At first, Table 3. Diffusion Coefficienta (m2/S) of Li+, O−2 , and O2 in EMI-TFSI and in DMSO Dions

system IL1 IL2 IL3 IL4 IL5 D1 D2 D3 a

Figure 1. Schematic representation of the system used for separatesolvation free energy calculation.

3.98 7.24 3.21 6.88 2.37 2.66 3.77 1.86

× × × × × × × ×

DEMI+ −11

10 10−11 10−11 10−11 10−9 10−9 10−9 10−8

1.68 1.81 1.18 1.41 1.98

× × × × ×

DTFSI− −10

10 10−10 10−10 10−10 10−10

1.11 1.09 6.63 1.06 1.31

× × × × ×

DDMSO −10

10 10−10 10−11 10−10 10−10 7.21 × 10−9 6.97 × 10−9 7.57 × 10−9

Compositions of each system are described in Table 1.

we notice that the diffusion of ions is ∼2 orders of magnitude slower in EMI-TFSI than in DMSO. Although we must pay attention to the quantitative accuracy on MSD of Li+ or O−2 of the one ion models as mentioned in the Methods section, the discrepancy of diffusion coefficients of ions between in EMITFSI and DMSO much larger than the range of statistical error. Among the species in EMI-TFSI, the ordering of diffusion coefficients is DEMI+ > DTFSI− > DLi+, and it shows the same trend observed in the NMR studies on PYR14-TFSI,55 BMPTFSI,56 and MPI-TFSI.57 Interestingly, the inclusion of 50 Li+TFSI− pairs in EMI-TFSI (system IL3) causes the slow-down of all molecular species in the system. This phenomenon is seen also in simulations with polarizable force fields,25 and our nonpolarizable model seems to overestimate it. The slow down is not observed for high concentration of O−2 -EMI+ (IL4). Neutral O2 molecules in either solvent show the fastest diffusion (systems IL5 and D3), followed by O−2 , while Li+ is the slowest species. Structural Properties. To better understand the difference in mobility, we calculated solvation energy, coordination number, and coordination lifetime (Table 4). The solvation

artificial walls, which have no interaction with the ions. The umbrella sampling is done between the vacuum region and the center of solvent region, then the solvation energy is obtained as free energy difference. Since a counterion is introduced in the solvent region to keep the charge neutral, the change in free-energy has a linear tail in the vacuum region. Therefore, we set the free energy reference with the ion at 0.4 nm away from the wall inside the vacuum. Other Details. Coordination numbers of each ion are estimated by integrating the normalized radial distribution functions (RDF; Figure 3). The lifetime of coordination between Li+/TFSI− or O−2 /EMI+ is calculated as the average of lifetimes of each coordination. The free-energy profile for Li+O−2 aggregation is also calculated with box-standard umbrella sampling and weighted histogram analysis method.49 The evaluation of solvation energy and free energy profile for Li+O−2 aggregation are performed in the NVT ensemble using the lattice parameters obtained from the NPT calculation. 366


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The coordination lifetime is quite different in the two solvents, indicating that despite the ions appear better solvated and coordinated in DMSO than in EMI-TFSI, the solvation structure is stiffer in EMI-TFSI. Effectively, the energy barrier preventing ion−solvent separation (i.e., moving an ion away from solvent molecules in equilibrium) is greater for EMI-TFSI than for DMSO, as shown by the potential of mean force profiles in Figure S3. The weaker cohesive strength and lower energy barrier for solvent−solute separation leads to the greater mobility of the ions in DMSO. Compared to the ions, however, neutral O2 evolves in a shallower potential well and is held by a lower energy barrier, causing its higher mobility. The ion−solvent and ion−ion RDFs clarify the spatial structure of solvation. First, we can see both the Li+-TFSI− and Li+-DMSO RDFs give a sharp peak at ∼0.3 nm (Figure 3a,b). It means Li+ forms a strong solvation shell as discussed above. However, the first peak for O−2 -EMI+ and O−2 -DMSO is lower reflecting the larger size of O−2 and its weaker interaction with solvent. We also observe a strong Li+-Li+ peak at ∼0.5 nm at high concentration (Figure 3c). This indicates that Li+ ions share the solvation shell and form a large effective particle. It would lead to the smaller diffusion constant of Li+ in the system IL3. The weak O−2 -O−2 peak shows each O−2 can move around independently without affecting the motion of solvent molecules even at high concentration. Finally, we should pay attention to the character of nonpolarizable force fields used here. Previous MD studies with polarizable force fields and our own ab initio calculations gave a coordination number of 4 between O atoms in TFSI− and Li+, while our classical MD gives 5 coordination.23,25,58 This indicates that the fixed charge model overestimates the interactions, resulting in large discrepancy of diffusion coefficients with respect to experimental measurements. However, this does not cause the change of order relation of diffusion coefficient between in EMI-TFSI and DMSO. Li+-O−2 Clusters Formation. Pairing of Li+ and O−2 is the first process occurring during discharge, eventually leading to the formation of Li2O2.2 Using umbrella sampling and weighted histogram analysis,49 we calculated the free-energy profile for the aggregation of Li+ and O−2 into LiO2 in the solvents (systems IL7 and D5). The results in Figure 4a show that Li+O−2 aggregates become energetically favored once the ions overcome an energy barrier of 0.27 eV in EMI-TFSI, and 0.16 eV in DMSO. As discussed above, Li+ in EMI-TFSI has a stiffer and larger solvation shell compared with that in DMSO. When the two ions approach, this strong solvation structure must break, raising the energy barrier. Since the solvation structure in DMSO is more easily undone, the free-energy barrier becomes lower. The Li+-O−2 pairing dynamics is expected to proceed

Table 4. Solvation Energy, Coordination Number, and Coordination Lifetime in EMI-TFSI and DMSO solvation energy (eV) coordination number lifetime of coordination

Li+ O−2 Li+ O−2 Li+ O−2

in EMI-TFSI

in DMSO

−4.5 −2.4 3.4 (TFSI−) 4.6 (EMI+) 12.0 ns (TFSI−) 2.0 ns (EMI+)

−5.5 −2.2 4.1 (DMSO) 6.3 (DMSO) 1.0 ns 0.1 ns

energy of Li+ is about twice that of O−2 in both solvents, but the ion appears less coordinated with the solvents than O−2 . Visual inspection of the solvation structures (Figure 2) and analysis of

Figure 2. Solvated structure of (a) Li+/TFSI−, (b) O−2 /EMI+, (c) Li+/ DMSO, and (d) O−2 /DMSO.

interatomic distances (Figure S2) confirm that O in TFSI− (or DMSO) surround Li+ as much as H atoms in EMI+ (or DMSO) surround O−2 . The charges involved, however, are quite different. Li+ has charge 1 e, O (in O−2 , TFSI−, DMSO) has ca. −0.5 e, and H atoms in the solvents have less than 0.1 e. Interactions between Li+ and O in solvent will then be much stronger than interactions between O−2 and H. Magnitude relation of the solvation energies of Li+ and O−2 in EMI-TFSI and DMSO shows an agreement with that of the diffusion constant. However, there is no strong correlation between solvation energy and coordination number. This implies that intuitive understanding based on the local and time-averaged information such as coordination number could fail to give a correct picture.

Figure 3. RDF between the ions and (a) EMI-TFSI, (b) DMSO molecules, and (c) between each other when higher concentration is included. The ion−solvent distances are calculated with respect to the center of mass of the molecules. The radial resolution is 0.01 nm. 367


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As shown in Figure 5, single ions remain abundant even at 200 ns in EMI-TFSI, and most of clusters consist of {m,n} ≤ {3,3}. The majority of clusters are charged, and even a (Li+)5(O2−)10 cluster was obtained. However, aggregation of Li+ and O−2 proceeds faster in DMSO. All the single ions have been consumed during the first 50 ns, and combination of clusters becomes dominant process to form larger clusters after that. Most clusters are neutral (m = n) or occasionally have a small charge (m = n ± 1). There are only 10 clusters larger than (Li+)2(O2−)2 at 200 ns. From the analysis of single ions in the solvents, we can infer that the solvation structure surrounding the clusters in DMSO should be weaker than that in EMI-TFSI. For this reason, clusters in DMSO can easily combine not only with isolated Li+/O−2 ions but also other clusters. Since the solvation of Li+ is stronger than that of O−2 in EMI-TFSI, O−2 ions are easily captured, and negatively charged clusters are grown. Therefore, growth by combination of clusters would be prevented in EMITFSI. Difference in solvent leads to change in growth mode of (Li+)m(O2−)n clusters. This result is in agreement with previous experimental studies showing that lithium peroxide clusters grow much larger in organic solvent than in ionic liquid.16 Finally, conversion from LiO2 to Li2O2 (self-redox reaction) is not taken into account in the current classical MD simulation, but such a conversion was partially observed even for (Li+)6(O2−)6 in our preliminary DFT MD calculations in DMSO. It implies that competition between growth of (Li+)m(O2−)n and conversion into Li2O2 becomes nonnegligible with increase in the cluster size. It is worth pointing out that the system size and time scale in this study was designed for discussion on the initial stage of discharging process in a Li−O2battery.

Figure 4. (a) Free energy as a function of Li+-O−2 aggregation and separation in EMI-TFSI and in DMSO under dilute condition (IL7, D5). Snapshots of Li+-O−2 clusters formed in (b) EMI-TFSI (IL6) and (c) DMSO (D4) after 200 ns.

faster in DMSO than in EMI-TFSI. Unpairing can also occur, but in both solvents the barrier is much higher: 0.76 eV for EMI-TFSI and 0.60 eV for DMSO. If we consider that the solubility of lithium salt in DMSO is experimentally determined to be over twice as large as in EMI-TFSI,13,35 aggregation of Li+-O−2 in DMSO should dominate over separation. The snapshots after the 200 ns time evolution of either solvent system with 0.4 mol/L of Li+ and O−2 ions (systems IL6 and D4) indicate that the aggregation proceeds beyond the single Li+-O−2 pairs, and larger clusters are formed (Figure 4b,c). The distribution of (Li+)m(O2−)n clusters in EMI-TFSI and DMSO at t = 50, 100, 150, and 200 ns are shown in Figures 5 and 6, respectively. The two horizontal axes in each plot represent the number of Li+ and O−2 {m,n} forming a cluster. The numerical label on each column shows the height, that is, the number of clusters with the corresponding composition.

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CONCLUSIONS In conclusion, our simulations show a clear diffusion slow-down of Li+ in EMI-TFSI at high concentration, which is not seen in DMSO. Analysis of the atomic-scale details of the systems reveal that the strong ion−ion interactions between Li+ and

Figure 5. Distribution of (Li+)m(O2−)n clusters in EMI-TFSI at (a) 50, (b) 100, (c) 150, and (d) 200 ns. The horizontal coordinates represent the number of O−2 and Li+. Histogram bars are colored red for positively charged, while for neutral and blue for negatively charged clusters, respectively. The dashed line marks the composition for charge neutral clusters. 368


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Figure 6. Distribution of (Li+)m(O2−)n clusters in DMSO at (a) 50, (b) 100, (c) 150, and (d) 200 ns. The horizontal coordinates represent the number of O−2 and Li+. Histogram bars are colored red for positively charged, while for neutral and blue for negatively charged clusters, respectively. The dashed line marks the composition for charge neutral clusters.

TFSI− drives the formation of a stiff anion structure around Li+. Ion−dipole interactions in DMSO are weaker, allowing molecules to diffuse fast, regardless of the high concentration of ions. The dynamics of O−2 is always faster than Li+, possibly because its charge is spread over the molecule, allowing them to diffuse through multiple states of similar energy. This argument was used to explain the higher viscosity of IL with small, spherical anions.20 The diffusion of O2 is fast in both liquids, compared to Li+ and O−2 . This is an important factor for the battery since diffusing O2 away from Li2O2 after its formation from LiO2 clusters ensures its stability in solution, and O2 becomes readily available for further reduction at the cathode. The fast diffusion is caused by the lack of electrostatic interactions with the solvents and, consequently, of a solvation structure. O2 diffusion is then mostly dictated by the density and viscosity of the solvent. The solvation energy of the ions and their coordination number in the solvents are deceptive. The ions appear better solvated in DMSO, surrounded by more solvent molecules; however, the relative strength of ion−solvent interactions has a dramatic influence on the coordination lifetime. The coordination number between ions-(EMI-TFSI) is smaller than that of ions-DMSO, but it persists for an order of magnitude longer time, indicating that the ion−solvent binding energy is greater in EMI-TFSI. Therefore, the low mobility of ions in EMI-TFSI is not just due to the higher viscosity of EMI-TFSI but also to the stronger ion-EMI-TFSI Coulomb interaction. Moreover, the stronger interactions in EMI-TFSI also explain why the free energy barriers for Li+-O−2 pair formation and separation are larger in EMI-TFSI than in DMSO. These results are consistent with the size distribution of LiO2 clusters growing in a 0.4 mol/L Li+-O−2 solution. Our simulations point out that the Li+-O−2 aggregation dynamics is much faster in DMSO than in EMI-TFSI. This reaction is a required step in the formation of Li2O2 and it is thus important for it to proceed as fast as possible, ensuring larger discharge capacity, and better performance of the battery. However, these results cannot be extrapolated to the time scale of experiments; thus, they cannot, alone, explain why

Li2O2 clusters grown in EMI-TFSI are smaller. Experiments and simulations of ILs on surfaces revealed how the solid−IL interface can exhibit large dipole.59,60 It is then expected that, once Li2O2 grow larger, ILs structure on their surfaces and the interface dipole causes an additional repulsive interaction that prevents further cluster growth. These findings establish a criterion for predicting the fundamental properties of optimal solvents for the Li−O2 battery, ideally exhibiting the fast Li+-O2− recombination dynamics seen in DMSO, with the growth stopping mechanism observed in ILs. Future studies are required to elucidate the role of the cathode−electrolyte interface in the formation Li+O−2 clusters and their growth.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b09692. Illustrated structures of DMSO, EMI+, and TFSI−, radial distribution functions, force-field parameters between the ions (Li+and O−2 ) and the solvents, and potential of mean force between them. (ZIP)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (S.-H.J.) *E-mail: filippo.federici@aalto.fi. (F.C.) Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The computation was done using the SR16000 supercomputing resources from the Center for Computational Materials Science of the Institute for Materials Research, Tohoku Univ., and PRIMERGY CX400 from the Research Institute for Information Technology, Kyushu Univ. The authors also acknowledge the computational resources provided by the Aalto Science-IT project. This study was supported by the Core Research for 369


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Evolutional Science and Technology (CREST) program of the Japan Science and Technology Agency (JST), the Strategic Programs for Innovative Research (SPIRE)/the Computational Materials Science Initiative (CMSI) and the World Premier Research Center Initiative (WPI) promoted by the MEXT of Japan.

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