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Cite This: J. Phys. Chem. C 2017, 121, 22720-22726

Long-Range Ion-Ordering in Salt-Concentrated Lithium-Ion Battery Electrolytes: A Combined High-Energy X‑ray Total Scattering and Molecular Dynamics Simulation Study Kenta Fujii,*,† Masaru Matsugami,‡ Kazuhide Ueno,§ Koji Ohara,∥ Michiru Sogawa,† Takashi Utsunomiya,⊥ and Masayuki Morita† †

Graduate School of Sciences and Technology for Innovation, Yamaguchi University, 2-16-1 Tokiwadai, Ube, Yamaguchi 755-8611, Japan ‡ Faculty of Liberal Studies, National Institute of Technology, Kumamoto College, 2659-2 Suya, Koshi, Kumamoto 861-1102, Japan § Department of Chemistry and Biotechnology, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Kanagawa 240-8501, Japan ∥ Japan Synchrotron Radiation Institute (JASRI), Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan ⊥ Technical Research Center, Mazda Motor Corporation, 3-1 Shinchi, Fuchu-cho, Hiroshima 730-8670, Japan S Supporting Information *

ABSTRACT: Herein, we report on a structural study for characterizing unique solution structures in the salt-concentrated electrolytes, which are promising new lithium (Li)-ion battery electrolytes. A combination of high-energy X-ray total scattering (HEXTS) experiments with all-atom molecular dynamics (MD) simulations was performed on the salt-concentrated electrolytes that were based on Li bis(trifluoromethanesulfonyl)amide (LiTFSA) and N,N-dimethylformamide (DMF). The radial distribution functions obtained from the HEXTS and MD approaches were in good agreement in the current LiTFSA/DMF solutions. We found that in the local structure: (1) the Li-ions were coordinated with both the DMF molecules and the TFSA anions in the concentrated solutions and (2) specific Li+···Li+ correlations were present in the radial distribution function over the r range of 3 Å−15 Å. The Li+···Li+ correlations originated from the extended multiple Li-ion complexes, that is, polymerized [Li+···TFSA−···Li+]n complexes so that they were highly structurally ordered. We concluded that this type of an ion-ordered structure plays a crucial role in the electrochemical stability and the ion-conducting mechanism, resulting in a unique LIB performance employing these salt-concentrated electrolytes.

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INTRODUCTION Superconcentrated electrolyte solutions (i.e., “solvent-in-salt” electrolytes in the liquid state) have attracted considerable attention as new conceptual lithium (Li)-ion battery (LIB) electrolytes and alternatives to conventional LIB electrolytes based on carbonate-type solvents, such as ethylene carbonate (EC), and linear carbonates, such as dimethyl carbonate (DMC).1−6 In conventional electrolytes with an Li salt concentration (cLi) of approximately 1 M (herein, dilute system), carbonate solvents (particularly EC) are required for stable and reversible working LIBs with a graphite negative electrode. This is because such carbonates are reductively decomposed on graphite during the first charging process to form a stable passivation film [i.e., solid electrolyte interphase (SEI)], which plays a crucial role in allowing reversible Li-ion insertion/desertion into and out of graphite and suppressing further decomposition of the electrolytes.7,8 However, in an extremely high cLi range (>2−3 M), the concentrated electrolytes exhibit excellent LIB performance even in “carbonate solvent-free” conditions, which is comparable to © 2017 American Chemical Society

the current commercialized systems (e.g., LiPF6 in a mixture of EC and DMC).9 The noticeable characteristics of the concentrated electrolytes are as follows:5,6 (1) All solvent molecules coordinate with the Li-ions in the solution, that is, there are no free solvent molecules in the bulk phase; (2) The Li-ions can thus form complicated multiple ion pairs in the solution; (3) The anion species [e.g., bis(trifluoromethanesulfonyl)amide (TFSA)] in multiple ion-pair complexes contributes to the formation of an SEI on the graphite electrode by altering the solvent decomposition to an anion decomposition, resulting in an improvement in the reductive stability. From the structural viewpoint, these findings suggest that the specific ion-ordering structure fabricated with multiple ion-pair complexes are essential for understanding the concentrated electrolyte system and its LIB performance,9,10 which may lead to its application in practical LIBs. Several Received: August 18, 2017 Revised: September 19, 2017 Published: September 25, 2017 22720

DOI: 10.1021/acs.jpcc.7b08243 J. Phys. Chem. C 2017, 121, 22720−22726

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room temperature. Monochromatized X-ray radiation of 61.4 keV was obtained using a Si(220) monochromator. The observed X-ray scattering intensities were corrected for absorption, polarization, and incoherent scattering to obtain coherent scattering intensities [Icoh(q)].25−27 The experimental X-ray structure factor [Sexp(q)] per stoichiometric volume was obtained by taking into account the atomic scattering factors as follows:

combinations of Li salts and solvents have been examined in terms of LIB electrochemistry/technology to optimize their compositions;2,11−16 however, currently, the fundamental knowledge at a molecular or an atomistic level is still limited. To address this issue at the molecular level, theoretical investigations can serve as powerful tools. In particular, the use of molecular dynamics (MD) simulations and quantum chemical calculations, such as density functional theory (DFT), can aid in obtaining the static solution structure and the electronic state related to the reduction/oxidation behavior of the electrolyte solutions.17−20 Sodeyama et al. reported a DFT-based MD simulation (DFT-MD) study of a concentrated electrolyte system using acetonitrile (AN) as a solvent and LiTFSA as a salt.10 They reported that the enhanced reductive stability of the salt-concentrated electrolytes was attributed to the formation of a specific network structure involving multiple Li+ and TFSA− ions. However, their DFT-MD study was performed on limited size and time scales in a 4.2-M LiTFSA/ AN solution system, and the numbers of LiTFSA and AN in the simulation box were only 10 and 20, respectively. The total simulation time was 10 ps (shorter than the solvent-exchange rate of Li-ions in a dilute solution). To elucidate an extended network structure that is formed in the salt-concentrated electrolytes, larger-scale and longer-time simulations are needed, which should allow for a comparison of the simulated results with the corresponding experimental results. Thus, herein, we report on a structural study that combines highenergy X-ray total scattering (HEXTS) experiments with allatom MD simulations to reveal the short- and long-range solution structures of the salt-concentrated electrolytes. Such a study is useful for evaluating the solvation structures of solutes (from metal ions to polymers21,22) in solutions using aqueous and nonaqueous solvents and ionic liquids. In this research, we initially applied this technique to the salt-concentrated LIB electrolyte system and discussed the structural characteristics of this system, which were mainly based on the radial distribution functions G(r)s that were derived from both experimental and theoretical data. In this study, N,N-dimethylformamide (DMF) was used as a model solvent, which has an larger electron-pair-donating ability (Gutmann’s donor number (DN) = 26.6).23 The DN value is considerably larger than the values of the commonly used organic solvents in LIBs, such as EC (16.4), PC (15.1), and AN (14.1).23 In contrast, TFSA− is well accepted as a “noncoordinating anion”. This large contrast in coordination power between DMF and TFSA− allowed us to simplify the cLi dependence of the Li-ion complex formation. In other words, according to our previous research,20 the Li-ions are solvated with DMF molecules and exist as [Li(DMF)4]+ complexes in the dilute region, where cLi < 2.0 M, whereas complicated Li-ion complexes, including both DMF and TFSA− [Lil(DMF)m(TFSA)n] are formed in the concentrated region.

S exp(q) =

Icoh(q) − ∑ nifi (q)2 {∑ nifi (q)}2

+1 (1)

where ni and f i(q) correspond to the number and atomic scattering factor of atom i, respectively. The radial distribution function (Gexp(r)) was obtained by the inverse Fourier transform of Sexp(q) as follows: 1 2π 2rρ0

Gexp(r ) − 1 =

∫0

qmax

q{S exp(q) − 1}sin(qr )

× exp(−Bq2)dq

(2)

where ρ0 is the number density and B is the damping factor (0.008 Å2 in this work). The qmax was set to 23 Å. Molecular Dynamics (MD) Simulations. The MD simulations were performed using the GROMACS 4.5.5 program.28 The system in a cubic cell under NTP ensemble condition controlled by a Nosé−Hoover thermostat29,30 and a Parrinello−Rahman barostat.31 The former was a target to be 298 K and the latter to be 1 atm during the simulation. The composition (i.e., the number of LiTFSA ion pairs and DMF molecules) in the simulation box was as follows: LiTFSA/DMF = 0/1000 (cLi = 0 M, neat DMF), 80/920 (cLi = 1.0 M), 230/ 770 (cLi = 2.5 M), and 320/680 (cLi = 3.2 M), and the resulting box size and density at the equilibrium state are listed in Table S1 (Supporting Information). The long-range interactions were evaluated using the smooth particle mesh Ewald (PME) method32 with real-space cutoff distance of 12 Å. The total simulation time was set as 15 ns for all the systems. The system was equilibrated for the first 10 ns with an interval of 0.5 fs, and the data collected at 5 ps intervals during the last 5 ns were analyzed to obtain the X-ray weighted structure factors and radial distribution functions [SMD(q) and GMD(r), respectively]. CLaP and OPLS-AA force fields, including intermolecular Lennard−Jones and Coulombic interactions and intramolecular interactions with (1) bond stretching, (2) angle bending, and (3) torsion of dihedral angles, were used for LiTFSA and DMF, respectively.33−35 The SMD(q) functions were calculated by using the trajectory obtained from the simulations as follows: ⎧ ∑ ∑ {ni(nj − 1)f (q)f (q)/N (N − 1)} i j ⎪ i j ⎪ {∑k (nk fk (q)/N )}2 ⎪ r sin qr ⎪ × 4πr 2ρ0 (gij MD(r ) − 1) dr + 1 (i = j) 0 qr ⎪ MD S (q) = ⎨ ⎪ ∑i ∑j {2ninjfi (q)f j (q)/N2} r ⎪ 4πr 2ρ0 (gij MD(r ) − 1) 0 ⎪ {∑k (nk fk (q)/N )}2 ⎪ sin qr dr + 1 (i ≠ j) ⎪ × qr ⎩

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∫

EXPERIMENTAL SECTION Materials. Solvent DMF (Wako Chemicals, Battery grade) was used without further purification. The water content was checked by Karl Fischer titration to be less than 50 ppm. LiTFSA salt (Kanto Chemical, Battery grade) was vacuumdried at 100 °C for 100 h. High-Energy X-ray Total Scattering (HEXTS) Experiments. The HEXTS measurements for LiTFSA/DMF solutions were performed using a high-energy X-ray diffraction apparatus24 (BL04B2 beamline at SPring-8, JASRI, Japan) at

∫

(3)

where N is the total number of atoms in the simulation box given by N = Σknk and gijMD(r) is the atom−atom pair correlation function between atoms i and j. The GMD(r) functions were obtained from the corresponding SMD(q)s using 22721

DOI: 10.1021/acs.jpcc.7b08243 J. Phys. Chem. C 2017, 121, 22720−22726

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The Journal of Physical Chemistry C a procedure similar to the experimental Gexp(r) described above.36

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RESULTS AND DISCUSSION Walden Plots: Ionic Conductivity and Viscosity. Figure 1 shows the cLi dependence of the Walden plots for the

Figure 2. (a) Sexp(q)s and (b) Gexp(r)s obtained by high-energy X-ray total scattering (HEXTS) measurements for LiTFSA/DMF solutions (cLi = 1.0, 2.5, and 3.2 M), together with neat DMF (dashed line).

be observed that S(q) strongly depends on cLi, that is, the peak at 0.8 Å−1 (prepeak) increased, whereas that at 1.4 Å−1 decreased in intensity with increasing cLi. Furthermore, the broad peak (shoulder) at 2.0 Å−1 in neat DMF decreased in intensity with cLi and it was no longer present in the concentrated solutions with cLi = 2.5 and 3.2 M. Therefore, it can be concluded that the S(q) profile for the 1.0 M LiTFSA/ DMF solution largely changed with addition of the Li salt up to 2.5 M; however, almost no change was observed with further increases in cLi from 2.5 to 3.2 M. These results imply that the liquid structures, including the Li-ion solvation complexes were significantly different in the diluted and concentrated solution systems. Recently, Yamaguchi et al. reported that the prepeak was observed at 0.8 Å−1 in highly concentrated LiClO4 (3 mol kg−1) in propylene carbonate solutions and pointed out that the prepeak was ascribed to the chain-like structures of ions according to neutron diffraction with isotopic substitution method and MD simulations.43 In the current LiTFSA/DMF system, the similar prepeak (at 0.8 Å−1) was also found in the concentrated region, which was related to a long-range ionordering structure as discussed later in detail. Figure 2b shows the X-ray radial distribution functions G(r)s in the r-range < 5 Å, which represent the Fourier transformations of the corresponding S(q)s. It is plausible that the major peaks at 1.3−1.4 Å are completely assigned to intramolecular correlations, such as covalent bonds (C−C, C−N, C−O, and C−S), within DMF and TFSA. According to our previous HEXTS study on solution systems,36,42 the intramolecular and intermolecular correlations were considerably overlapped in the r-range of 2−6 Å. Thus, we examined the separation of the total G(r)s into two intramolecular and intermolecular components using all-atom MD simulations, which are comprehensively discussed later in this study. Furthermore, the use of MD simulations allows us to discuss interactions between Li and the other atoms in detail; that is, it is difficult to evaluate the Li−X interactions using HEXTS experiments only owing to a small atomic scattering factor of Li. Consequently, we found that based on the MD results, the intermolecular G(r) value in the 2 Å−5 Å range was the key for characterizing the current concentrated electrolyte system, which included the Li+···O (DMF or TFSA) interactions in the first Li-ion solvation sphere (∼2 Å) and nearest neighbor Li+···Li+ correlations (∼3 Å). The latter correlations in such a short r-range have never been observed in conventional electrolyte solutions at a cLi value of ∼1.0 M. MD Simulations. Figure 3 shows the X-ray radial distribution functions in the differential form r2[GMD(r) − 1]

Figure 1. cLi dependence of Walden plots for lithium bis(trifluoromethanesulfonyl)amide (LiTFSA)/N,N-dimethylformamide (DMF) solutions at 298 K.

LiTFSA/DMF electrolyte solutions, which were obtained from both the molar conductivity (Λ) and viscosity (η) values20 at temperature T = 298 K. It has been established that the vertical deviation of the experimental plots from the ideal line is regarded as one of good indicators for predicting the saltdissociativity in the solution,37 that is, the distance is large in the vertical direction when ions are associated with form ion pairs, whereas the reverse situation indicates that the completely dissociating salts are the solvated ions in the solution. It is evident from Figure 1 that the Walden plots are close to the ideal line for all cLi and that there is no change in the vertical distance between the plots and the ideal line, even with increasing cLi. In the LiTFSA/DMF system, we already reported that the Li-ions are solvated with DMF molecules to form the [Li(DMF)4]+ complex in solutions with a salt concentration less than 2.0 M (cLi = 2.0 M corresponds to LiTFSA/DMF ≈ 1:4 by mol.), whereas the TFSA anions begin to coordinate with the Li-ions to become complex contact ion pairs [Lil(DMF)m(TFSA)n] in solutions with salt concentrations above 2.0 M.20 The higher cLi solutions employing DMF might deviate from the Walden plots in the vertical direction; however, this is not the case in our study. The similar behavior was also seen in some salt-concentrated electrolyte systems;38−41 that is, the maximum of the ionicity is found in the highly concentrated region. Consequently, it was anticipated that the ion-conductive mechanism in the concentrated solution system was considerably different from that in the dilute system, which might originate from specific liquid structures in the highly concentrated solutions. To elucidate this at the atomistic level, we performed an HEXTS analysis with the aid of all-atom MD simulations. HEXTS Experiments. Figure 2a shows the X-ray structure factors S(q)s in the q-range < 5 Å−1 that were obtained from the HEXTS measurements of the LiTFSA/DMF solutions with cLi = 0 (neat DMF), 1.0, 2.5, and 3.2 M. The solutions with cLi of 1.0, 2.5, and 3.2 M corresponded to the Li salt mole fractions of x LiTFSA = 0.08, 0.23, and 0.32, respectively. The corresponding S(q)s over the entire q-range (∼23 Å−1) are also shown in Figure S1. Focusing on q < 2 Å−1, which is often attributed to intermolecular interactions in solutions,36,42 it can 22722

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(Figure 4a), which comprise intramolecular bonding and nonbonding correlations of DMF and TFSA, exhibited sharp peaks in the r range < 8 Å for all cLi solutions. This indicates that in the total Gexp(r)s shown in Figures 2 and 3, the observed peaks above 8 Å are completely assigned to the intermolecular interactions in the solutions. In the GMDinter(r) functions (Figure 4b), the broad peaks appeared at around 5, 10, and 15 Å and the shape and intensity were dependent on cLi. Here, we focus on the short r range ( 2.5 M. Thus, the resulting coordination number of Otfsa (=1) in the N(r) profile at cLi = 2.5 M suggests that the Li-ions exist as the [Lil(DMF)m(monoTFSA)n] complex with l/m/n = 1:3:1 as the averaged species r atio. At cLi = 3.2 M, [Lil(DMF)m(mono-TFSA)n] with l/m/n = 1:2:2 and [Lil(DMF)m(bi-TFSA)n] with l/m/n = 1:2:1 coexist with each other in the solution. Figure 6 shows the gMDLi−Li(r) functions for the LiTFSA/DMF solutions with cLi = 1.0, 2.5, and 3.2 M. In the dilute solution (cLi = 1.0 M), no peak was found in the r range < 5 Å, followed by broad and unclear Li+− Li+ correlations above 5 Å. This was because the Li-ions were solvated with DMF molecules forming only the [Li(DMF)4]+ complex in the diluted solutions with cLi < 2.0 M, resulting in no correlations in the short r range. Interestingly, at cLi = 2.5 M, the closest Li+−Li+ correlation was clearly found at r = 3.1 Å. Furthermore, the correlations appeared at 5.6, 6.8−7.6

Figure 3. X-ray radial distribution functions in the r2[G(r) − 1] form derived from HEXTS experiments (open circles) and molecular dynamics (MD) simulations (solid lines) for LiTFSA/DMF solutions (cLi = 0, 1.0, 2.5, and 3.2 M).

for the LiTFSA/DMF solutions derived from the MD simulations, together with corresponding r2[Gexp(r) − 1] obtained from the HEXTS experiments. It is clear that the GMD(r)s well reproduced the Gexp(r)s over the entire r range for all examined cLi values. As mentioned above, it was expected that the considerable overlapping of the intramolecular and intermolecular interactions was observed in the G(r) value at around r = 2−6 Å. Therefore, we separated total GMD(r) into its intramolecular and intermolecular components [i.e., GMDtotal(r) = GMDintra(r) + GMDinter(r)] to comprehensively discuss the intermolecular interactions. The extracted total GMDintra(r) and GMDinter(r) profiles for the respective cLi solutions are shown in Figure 4. The GMDintra(r) functions

Figure 4. Partial X-ray GMD(r)s for (a) intramolecular and (b) intermolecular components for LiTFSA/DMF solutions (cLi = 1.0, 2.5, and 3.2 M), together with neat DMF (dashed line). 22723

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Figure 5. Atom−atom pair correlation functions [gMDatom−atom(r)s: left axis] for the O atoms of DMF (solid black line) and TFSA− (solid red line) around the Li-ions and their integrated profiles [coordination number N(r): right axis, dashed lines] for LiTFSA/DMF solutions at (a) cLi = 1.0 M, (b) cLi = 2.5 M, and (c) cLi = 3.2 M.

which is useful in confirming the formation of the ion-ordering structures. In the highly concentrated system, a specific ion ordering based on multiple Li+···TFSA− interactions was clearly evident. In contrast, the Li-ions were surrounded only by DMF molecules in the diluted system so that there were no shortrange correlations between Li-ions. Consequently, herein, we proposed that such extended ion-ordering in the saltconcentrated solution contributes to the unusual Walden plots in the high-cLi region (Figure 1); hence, the ionconducting mechanism might be considerably different from that in the low-cLi region. In general, in dilute electrolyte solutions, the ion diffusion (dissociated Li-ions and anions) based on a Stokes−Einstein law is dominant in the ion conduction process in solution. We assumed that this mechanism cannot be applied to the highly concentrated solutions with no free solvents because all of the Li-ions bind to counter-anions to form contact ion pairs. In other words, there were no dissociated ion species in the solutions. Hence, a different type of ion-transport mechanism is required to understand the fundamental features of the salt-concentrated solution system, for example, the Li-ion transport utilizing the extended ion-ordering structure like the Grotthuss mechanism44,45 proposed as proton transportation in a polymer electrolyte fuel cell. Indeed, such ionic conduction based on the Grotthuss mechanism was recently reported according to the MD simulation study for the salt-concentrated electrolytes using organic solvents and ionic liquids.46,47

Figure 6. Atom−atom pair correlation functions between Li-ions [gMDLi−Li(r)s] for LiTFSA/DMF solutions at (a) cLi = 1.0 M, (b) cLi = 2.5 M, and (c) cLi = 3.2 M.

(shoulder), 8.9, and 12.0 Å. This strongly suggests that a multiple (or polymeric) Li-ion complex was formed in a solution with cLi = 2.5 M, which is direct evidence for the longrange ordering structure in the highly concentrated electrolyte solutions. In more concentrated solutions (cLi = 3.2 M), we found that the peak positions in gMDLi−Li(r) were similar to those at cLi = 2.5 M; however, the peaks became considerably sharper. This indicates that the structural ordering was enhanced by an increase in cLi. In the concentrated region, as discussed above, both neutral DMF molecules and negatively charged TFSA anions coordinated with the Li-ions to form a complicated [Lil(DMF)m(TFSA)n] complex. In a solution with cLi = 2.5 M, the Li+−Li+ correlations were found in gMDLi−Li(r) at five r-positions, indicating the presence of a multiple [Li5(DMF)15(TFSA)5] complex (l/m/n = 1:3:1) in the solution. It is plausible that this complex was extended at cLi = 3.2 M to become a more ordered structure because of the Li+−Li+ correlations at the seven r positions [3.1, 4.8 (shoulder), 5.7, 7.7, 9.0, 11, and 17 Å]. Figure 7 displays a typical MD snapshot obtained for solutions with cLi = 3.2 M,

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CONCLUSION We conducted the research by combining experiments and simulations to characterize the local and long-range structures in salt-concentrated electrolytes for Li-ion batteries. Using the experimental and theoretical radial distribution functions [Gexp(r) and GMD(r), respectively] covering a wide r range up to 20 Å, we demonstrated that (1) the LiTFSA/DMF electrolyte solutions are highly structured in the high-cLi region to form long-range ion-ordering structures based on multiple Li-ion complexes (including both TFSA and DMF) and that (2) the extent of structural ordering in the high-cLi solutions is enhanced by further increasing the cLi value.

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Figure 7. Snapshot of a typical ion-ordering structure found in a solution with cLi = 3.2 M, obtained from the current MD simulations. DMF molecules are not displayed in the snapshot for a clear view of the structure.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b08243. 22724

DOI: 10.1021/acs.jpcc.7b08243 J. Phys. Chem. C 2017, 121, 22720−22726

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Composition of the systems and density values (Table S1); Sexp(q) functions in the entire q-range (Figure S1; PDF).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kenta Fujii: 0000-0003-0057-1295 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Grant-in-Aids for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology (No. 15K17877 to K.F.). The HEXTS experiment was performed at BL04B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI; Proposal Numbers: 2015B1429 and 2016A1375).

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DOI: 10.1021/acs.jpcc.7b08243 J. Phys. Chem. C 2017, 121, 22720−22726