Role of Solvent Size in Ordered Ionic Structure Formation in

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Role of Solvent Size in Ordered Ionic Structure Formation in Concentrated Electrolytes for Lithium-Ion Batteries Michiru Sogawa, Saki Sawayama, Jihae Han, CoCo Satou, Koji Ohara, Masaru Matsugami, Hideyuki Mimura, Masayuki Morita, and Kenta Fujii J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01038 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 17, 2019

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The Journal of Physical Chemistry

Role of Solvent Size in Ordered Ionic Structure Formation in Concentrated Electrolytes for LithiumIon Batteries Michiru Sogawa,

a

Saki Sawayama,

a

Jihae Han,a CoCo Satou,a Koji Ohara,

b

Masaru

Matsugami,c Hideyuki Mimura,d Masayuki Morita, a and Kenta Fujii a* a

Graduate School of Sciences and Technology for Innovation, Yamaguchi University, 2-16-1

Tokiwadai, Ube, Yamaguchi 755-8611. b Japan Synchrotron Radiation Institute (JASRI), Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan. c Faculty of Liberal Studies, National Institute of Technology, Kumamoto College, 2659-2 Suya, Koshi, Kumamoto 861-1102, Japan. FINECHEM Corporation, 4988 Kaisei-cho, Shunan, Yamaguchi 746-0006, Japan.

ACS Paragon Plus Environment

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d

TOSOH

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ABSTRACT

The structural and electrochemical properties of lithium (Li) ion complexes in concentrated electrolytes based on acetonitrile (AN) and tris(2,2,2-trifluoroethyl) phosphate (TFEP) as solvents and LiTFSA [TFSA: bis(trifluoromethanesulfonyl)amide] as a Li salt were investigated by employing electrochemical measurements, vibrational spectroscopy, and high-energy X-ray total scattering (HEXTS) with all-atom molecular dynamics (MD) simulations. Via electrochemical measurements, reversible Li-ion insertion/deinsertion into/from the graphite electrode was observed in concentrated LiTFSA/AN solutions but not in concentrated LiTFSA/TFEP solutions. The experimental radial distribution functions [Gexp(r)] derived from HEXTS were successfully represented by the corresponding MD-derived values [GMD(r)] for both AN- and TFEP-based electrolyte systems. We found that: (1) in the dilute system, Li ions were solvated with only solvent molecules in AN-based solutions to form a completely dissociated [Li(AN)4]+ complex, while contact ion pairs exhibiting Li+TFSA− interactions were formed in the TFEP-based solutions. (2) In the concentrated system, a specific Li+Li+ correlation was observed for shorter r values (~ 3 Å) in the AN-based solutions, suggesting ordered ionic structure formation based on multinuclear Li-ion complexes. However, no ordered ionic structure formation was found in the TFEP-based solutions. We discussed the relation between the ordered ionic structure and graphite electrode reaction at the molecular level, particularly focusing on the solvent size; i.e., the smaller AN more easily forms a compact solution structure (ordered structure) in the concentrated solutions, while bulky TFEP causes steric repulsion among the coordinated species (TFEP and TFSA) in the Li-ion complexes, preventing such an ordered formation.


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INTRODUCTION

The salt concentration is the simplest parameter for controlling the physicochemical properties (density, viscosity, and ion transportability) of electrolyte solutions. In electrolytes for lithium (Li) ion batteries (LIBs), the solubility of Li salts depends on both the solvent and counter-anion (X−); therefore, Li-ion solvation, which is directly related to both the ionic conductivity and electrochemical reactions in electrolytes, can be controlled by varying the Li salt concentration (cLi) in a solvent/LiX combination. Amide anions, such as bis(trifluoromethanesulfonyl)amide (TFSA)1 and bis(fluorosulfonyl)amide (FSA),2 are often used as counter-ions for Li salts because they resist coordination with Li ions in electrolyte solutions and thus avoid ion-pair formation, resulting in much higher salt solubilities. Using such a non-coordinating anion, a novel concept of practical LIB electrolytes with high-voltage operation, i.e., super-concentrated electrolytes, was recently proposed.3-7 Suo et al. reported that considerably increasing the LiTFSA salt concentration, up to 21 mol kg−1, in aqueous solution widened the electrochemical stability window from 1.23 V (dilute region) to ~3 V. This contributed to the development of safer (nonflammable) higher voltage aqueous Li-ion batteries,8 which can be applied to organic electrolyte systems using various organic solvents, such as carbonate solvents [e.g., ethylene carbonate (EC), dimethyl carbonate (DMC), and vinylene carbonate (VC)], oligoethers (glymes), acetonitrile (AN), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), and N,N-dimethylformamide (DMF).5-6 In commercialized LIB systems, the electrolyte requirements are: (1) low cLi around 0.5–1.0 mol dm−3 to maintain high ionic conductivity, and (2) limited solvent species, i.e., carbonate solvents, to form a solid electrolyte interphase (SEI) to protect the graphite electrode during


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multiple charge-discharge cycling.9-11 It was recently established that concentrated organic electrolytes with cLi above ca. 3 mol dm−3 allow a reversible graphite electrode reaction (Li-ion insertion/deinsertion), resulting in excellent LIB performances with high-voltage operation even in carbonate-free systems.3,

12-16

Sodeyama et al. reported a DFT-MD simulation study [DFT:

density functional theory, MD: molecular dynamics] on a concentrated electrolyte system based on AN (solvent) and LiTFSA (salt).17 They presented a unique solution structure based on multiple Li+ TFSA− networks, which was key to the enhanced electrochemical stability of the concentrated electrolyte solutions.3, 12, 17-18 We reported a structural-electrochemical study on Liion complexes formed in a highly concentrated LiTFSA electrolyte using DMF as a model solvent.19-20 Based on a combined experimental and theoretical investigation, we revealed that the Li ions were coordinated by both DMF and TFSA− in a highly concentrated region to form long-range ordered ionic structures based on extended multiple Li-ion complexes, which was consistent with the above-mentioned DFT-MD results. In such an ordered ionic structure, the coordination of solvent molecules and TFSA anions with Li ions may be an important factor in controlling the ordered structure and its relation with the electrochemical properties;21 however, knowledge of this relation at a molecular level remains limited. The ordered ionic structure formation in the highly concentrated electrolytes may depend on several factors, e.g., (1) the electron-pair donating ability of the solvent (i.e., the Gutmann donor number, DN), (2) the salt dissociativity depending on the X− species, and (3) the solvent bulkiness (i.e., “steric repulsion” in the solvated complexes22-24) as well as the salt concentration cLi. Among these, in this work, we focused on the “solvent-size effect” on the ordered ionic structure formation in a concentrated electrolyte system. We therefore chose two solvents: AN and tris(2,2,2-trifluoroethyl) phosphate (TFEP) as model solvents; i.e., the molecular size of AN


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is much smaller than that of bulky TFEP (molecular volumes: AN = 60.5 Å3, TFEP = 288.6 Å3; see Figure 1), while DN is similar for both solvents (AN: 14.1,25 TFEP: 12.922). The Li salt examined herein was LiTFSA for both AN- and TFEP-based electrolyte systems. By comparing the LiTFSA/AN and LiTFSA/TFEP systems, we could accurately determine the solvent-size effect on the ordered ionic structure formation in the concentrated electrolyte system due to the negligible effects of DN and the X-coordination power. To address this, the structures of ordered Li-ion complexes were investigated in both the concentrated electrolyte systems via vibrational spectroscopy and high-energy X-ray total scattering (HEXTS) using computational techniques (all-atom MD simulations and DFT calculations).

EXPERIMENTAL SECTION

Materials. AN solvent (Wako Chemicals, super-dehydrated) and TFEP (TOSOH FINECHEM, battery grade) were used without further purification. LiTFSA salt (Kanto Chemical, battery grade) was vacuum-dried at 373 K for 100 h before use. The sample electrolyte solutions were prepared by weighing the solvent and Li salt to the required molar ratio (Li salt:solvent) in an Ar-filled glovebox; the corresponding molarities (Li salt concentration, cLi) were then calculated using the solution density (g cm−3). The densities were measured using an SVMTM 3000 Stabinger viscometer (Anton Paar) at 298 K and are listed in Tables S1 and S2. The water content in the sample solution was determined by Karl Fischer titration as below 50 ppm. The chemical structures of AN and TFEP are shown in Figure 1.


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Figure 1. The chemical structures of (a) acetonitrile (AN) and (b) tris(2,2,2-trifluoroethyl) phosphate (TFEP). The molecular volumes of AN and TFEP are 60.5 Å3 and 288.6 Å3, respectively, according to DFT calculations based on their optimized geometries.

Measurements. Cyclic voltammetry (CV, HZ-5000; Hokuto Denko) was performed using a conventional three-electrode cell with graphite (1.6 mAh cm−2, Piotrek) as the working electrode (0.78 cm2) and Li foil as both the counter (3.75 cm2) and reference (0.30 cm2) electrodes. The scan rate was 0.2 mV s−1. Raman spectra of the LiTFSA/AN solutions were measured using dispersion Raman spectroscopy (Jasco, NRS-3100) with a 532.2 nm laser at 298 K. Attenuated total reflection (ATR)-infrared (ATR-IR) spectra of the LiTFSA/TFEP solutions were obtained with a FT-IR spectrometer (JASCO, FT/IR-6100) fitted with a KBr beam splitter and single-reflection ATR cell (298 K). The details are similar to those described in our previous Raman and IR studies.2223, 26-27

In ATR-IR spectroscopy, the penetration depth dp of the evanescent wave per reflection

by a diamond prism was estimated by the following equation: dp = /[2n1(sin2 − n22/n12)1/2], where  represents the wavelength,  is the incident angle, n1 and n2 are the refractive indexes of the diamond prism (2.42) and sample solutions, respectively. The reflective indexes of the samples at 298 K were determined using an Abbe refractometer (NAR-1T Solid, Atago), which are listed in the Supporting Information (Table S1). The observed absorbance A was thus corrected using the estimated dp value, i.e., A/dp.22, 28 Peak deconvolution of the observed Raman and IR spectra was conducted using a least-squares curve-fitting analysis based on a pseudo-


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Voigt function, as in our previous work.19, 29 For the Raman spectra, the integrated intensity of the single band for free and bound solvent species was represented by If = Jfcf and Ib = Jbcb, respectively, where Jf and cf are the Raman scattering coefficient and the concentration for free species and Jb and cb are those for bound species. The mass balance equation was: cf = cT − cb = cT − ncLi, where cT and cLi are the concentrations of total solvent and Li ions, respectively, and n is the solvation number for solvent molecules in the first Li-ion solvation sphere. Therefore, we obtained the following equation: If/cT = −nJf(cLi/cT) + Jf. Plots of If/cT vs. cLi/cT are straight lines with slope  (= −nJf) and intercept  (= Jf). The value of n was obtained from n = −/. The IR peaks were analyzed using the equation: If/cT = −nf(cLi/cT) + f, where f is the molar absorption coefficient for a single band. HEXTS measurements were conducted at room temperature with a high-energy X-ray diffraction apparatus (BL04B2 beamline at SPring-8, JASRI, Japan).30 Monochromatized X-ray radiation of 61.4 keV was obtained with a Si(220) monochromator. The observed X-ray scattering intensities were corrected for absorption, polarization, and incoherent scattering to determine coherent scattering intensities, Icoh(q).31-33 The experimental X-ray structure factor, Sexp(q), per stoichiometric volume and radial distribution function, Gexp(r),34 were calculated using Eqs (1) and (2): 𝑆

𝐺exp(𝑟) ― 1 =

1

exp

(𝑞) =

∫

2𝜋2𝑟0

𝑞max 0

𝐼coh(𝑞) ― ∑ 𝑛𝑖𝑓𝑖(𝑞)2 [∑ 𝑛𝑖𝑓𝑖(𝑞)]2

(1)

𝑠𝑖𝑛 (𝑞𝜋/𝑞max) 𝑞[𝑆exp(𝑞) ― 1]𝑠𝑖𝑛(𝑞𝑟) 𝑑𝑞 𝑞𝜋/𝑞max

(2)

Here, ni is the number of atoms i per stoichiometric volume. The parameters fi and 0 represent the atomic scattering factor for atom i and the number density of atoms, respectively. As shown in Eq. 2, the Lorch window function was used for inverse Fourier transformation.34


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Computational Study. An all-atom MD simulation was conducted using the GROMACS 4.5.5 program35 under the NTP ensemble condition controlled by a Nosè–Hoover thermostat36-37 and a Parrinello–Rahman barostat.38 The former was set at 298 K and the latter was at 1 atm during the simulation. The composition (i.e., number of LiTFSA ion pairs and solvent molecules) in the simulation box, the resulting box size, and density at equilibrium are listed in Table S3. The long-range interactions were determined using the smooth particle mesh Ewald (PME) method39 with a real-space cutoff distance of 12 Å. The total simulation time was set at 15.0 ns for all the systems, and the detailed procedures are given in the Supporting Information. The data collected at 0.1 ps intervals from the last 500 ps were analyzed to determine 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 (LJ) and Coulombic interactions and intramolecular interactions with (1) bond stretching, (2) angle bending, and (3) torsion of dihedral angles, were used for TFSA,40 AN,41-42 and TFEP.43-44 The LJ parameter for Li ions reported by Soetens et al. (Li salt/carbonate solvent system) was used in this work.45 The original partial charges (q+ and q−) for TFSA,40 AN,46 and TFEP are given in Figure S1; the values for TFEP were then calculated based on the current DFT calculations (ChelpG method).47 For the Coulombic term, we used modified partial charges based on the original q+ and q− to successfully represent the experimental Gexp(r) by the theoretical GMD(r); the detailed calculation is given in the Supporting Information. The SMD(q) functions were calculated using the trajectory from the MD simulations as follows: 𝑆

MD

(𝑞) =

∑𝑖∑𝑗𝑤𝑖𝑗(𝑞)

∫

[∑𝑘𝑛𝑘𝑓𝑘(𝑞)/𝑁]2

𝑤𝑖𝑗(𝑞) ≡

{

𝑟max 0

𝑠𝑖𝑛 (𝑞𝑟) 4𝜋𝑟20(𝑔𝑖𝑗MD(𝑟) ― 1) 𝑑𝑟 + 1 𝑞𝑟

𝑛𝑖(𝑛𝑗 ― 1)𝑓𝑖(𝑞)𝑓𝑗(𝑞)/𝑁(𝑁 ― 1) (𝑖 = 𝑗) 2𝑛𝑖𝑛𝑗𝑓𝑖(𝑞)𝑓𝑗(𝑞)/𝑁2

(𝑖 ≠ 𝑗)


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(4)

(3)

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Here, ni and N are the number of i atoms and the total number of atoms in the simulation box, and gijMD(r) is the atom–atom pair correlation function between atoms i and j. GMD(r) was obtained from the calculated SMD(q) by inverse Fourier transform. DFT calculations were performed using Gaussian 09 software.48 The geometries of the complexes were fully optimized at the B3LYP/6-311G** level, followed by normal frequency analysis.

RESULTS AND DISCUSSION

Electrode Reaction. Figure 2a shows cyclic voltammograms (CVs) for the graphite electrode in the concentrated electrolyte using AN as a solvent with cLi = 4.0 mol dm−3, (corresponding to a Li salt:AN molar ratio of 1:2). AN-based electrolytes with lower cLi ( 0.5 mol dm−3 implied that nAN gradually decreased with cLi. We therefore concluded that the coordination structure of Li ions changed at cLi = 0.5 mol dm−3, leaving a large amount of free AN in the bulk to form certain ion pairs, such as [Lin(AN)m(TFSA)l] complexes. In our previous work, we reported that, in the LiTFSA/DMF system, where DMF has a large electron-pair donating ability (DN = 26.6), contact ion pair (Li+TFSA−) formation was observed in highly concentrated solutions with cLi > 2.0 mol dm−3 (Li salt:DMF = 1:4); conversely, Li ions exist as ion-pair-free complexes, [Li(DMF)4]+, in the cLi solutions up to 2.0 mol dm−3.19-20 It is therefore expected that contact ion pairs are easily formed in the AN electrolyte system around cLi ~0.5 mol dm−3 due to the low solvation power of AN (DN = 14.1). The TFSA-coordination characteristics in the ion-pair complexes are discussed in detail later based on the HEXTS and MD simulation data. Solvation in TFEP-Based Electrolytes: IR Spectroscopy. As with the spectroscopic investigation conducted on the LiTFSA/AN system described above, we performed vibrational spectroscopy for the LiTFSA/TFEP system to investigate the Li-ion solvation structure. Based on our previous research on LiTFSA/TFEP electrolytes, it was established that ion-pair formation, such as [Li(TFEP)2(TFSA)1], easily occurs even in very dilute systems (the whole cLi range from 0–1.0 mol dm−3 was examined).22 We suggested that such ion-pair complex formation is due to (1) the low electron-pair donating ability of TFEP (DN = 12.9) and (2) solvation steric effects due to the bulkiness of TFEP. Herein, we extended our IR study of a dilute system to a highly concentrated system (up to 2.1 mol dm−3; Li salt:TFEP = 1:1.6). Figure 4a shows IR spectra in the range 750–1000 cm−1 observed for the LiTFSA/TFEP solutions over a wide cLi range from dilute to concentrated systems. A broad peak in the range 860–930 cm−1,


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Figure 4. (a) IR spectra for LiTFSA/TFEP solutions with varying cLi, (b) a typical curve-fitting result for the cLi = 1.5 mol dm−3 solution (Li:TFEP = 1:2.5), and (c) If/cT vs. cLi/cT plots using the deconvoluted bands of the free TFEP.

which was assigned to the O–P–O asymmetric stretching mode of TFEP in the bulk (free TFEP),22 decreased in intensity as cLi increased, while the upshifted peak (903.5 cm−1) assigned to bound TFEP increased in intensity with cLi. We conducted curve-fitting analysis to deconvolute the observed IR spectra to individual bands. A typical result for the cLi = 1.5 mol dm−3 solution is shown in Figure 4b. As shown in the figure, the spectrum was deconvoluted into two free TFEP bands (887.0 and 907.2 cm−1) and one bound TFEP (903.5 cm−1), which is consistent with the result for the TFEP-based electrolyte system studied in our previous work.22 The If/cT vs. cLi/cT plots (Figure 4c), where If is the sum of the integrated intensities of the two free bands (i.e., If = I887 + I907), showed a straight line in the cLi range 0–1.8 mol dm−3 (corresponding to cLi/cT = 0–0.5 in the figure), yielding nTFEP = 2.1  0.1 according to the equation: If/cT = −nf(cLi/cT) + f. The ratio of molar absorption coefficients (f/b) was determined to be 0.7. Above 1.8 mol dm−3, the free TFEP disappeared in the bulk, and subsequently all the TFEP molecules bound to Li ions, leading to If/cT = 0 as shown in the figure. To obtain more insight into Li-ion complex coordination structures in the AN- and TFEPbased concentrated electrolytes, we performed HEXTS experiments combined with MD


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simulations to determine the specific ordered Li-ion formation that occurs in the AN system but not in the TFEP system, due to differences in the solvent size (less bulky AN molecule vs. bulky TFEP molecule). Combined HEXTS Experiments with MD Simulations. Figure 5 shows X-ray structure factors [Sexp(q) values] in the q-range < 5 Å−1 obtained from HEXTS measurements for (a) the LiTFSA/AN solutions [Li:AN = 1:35 (cLi = 0.5 mol dm−3), 1:2 (4.0 mol dm−3), and neat AN] and (b) LiTFSA/TFEP solutions [Li:TFEP = 1:4 (cLi = 1.0 mol dm−3), 1:1.6 (2.0 mol dm−3), and neat TFEP]. The corresponding Sexp(q)s over the entire q-range (~25 Å−1) are shown in Figure S3. A significant change in the Sexp(q) profiles was observed in the LiTFSA/AN solutions (Figure 5a); i.e., the peak at 0.85 Å−1 increased, while that at 1.8 Å−1 decreased in intensity as cLi increased. Generally, the peaks for q < 2 Å−1 are attributed to intermolecular interactions in solution,54-55 so the result indicated that the liquid structures, including the Li-ion solvated complexes, were significantly different in the dilute and concentrated LiTFSA/AN solutions. However, almost no

Figure 5. Sexp(q) profiles obtained from high-energy X-ray total scattering (HEXTS) measurements for dilute and concentrated (a) LiTFSA/AN solutions (Li:AN = 1:35 and 1:2) and (b) LiTFSA/TFEP solutions (Li:TFEP = 1:4, and 1:1.6), together with the corresponding neat solvents.


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change was found in the LiTFSA/TFEP solutions, irrespective of cLi (Figure 5b), although a small peak at 0.6 Å−1 appeared for the LiTFSA/TFEP system but not the neat TFEP system (i.e., without Li salt). To clarify the different intermolecular interactions between the AN and TFEP systems, especially Lisolvent and Lianion interactions, the X-ray radial distribution function [Gexp(r)], calculated by Fourier transformation of Sexp(q), was investigated by comparing it with the GMD(r) function from the MD simulations performed on the corresponding electrolyte systems. Figure 6 shows Gexp(r) and GMD(r) as an r-weighted difference form, r2[G(r) − 1], for the LiTFSA/AN and LiTFSA/TFEP systems. It was clear that the GMD(r) values successfully reproduced the Gexp(r) values from the short to long r for both AN- and TFEP-based solutions with various salt concentrations. The potential parameters used in the current MD simulations were therefore valid representations of the experimental data, which allowed us to investigate the

Figure 6. X-ray radial distribution functions in the r2[G(r) − 1] form derived from HEXTS experiments (open circles) and MD simulations (solid lines) for the LiTFSA/AN solutions (left) and LiTFSA/TFEP solutions (right).


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solution structure at the molecular level based on the simulation results. To extract the intermolecular interactions between Li ions and other components, we separated the total GMD(r) into GMDintra(r) and GMDinter(r) as shown in Figure S4. For GMDintra(r) (Figure S4a and b), we could confirm the extent of intramolecular bulkiness in the total GMD(r); i.e., GMDintra(r) of bulky TFEP was distributed widely in the r range 0−9.6 Å, while the less bulky TFSA anion and small AN were in the ranges 0−7.9 Å and 0−4.2 Å, respectively. The extracted GMDinter(r) [i.e., = GMDtotal(r) − GMDintra(r)] comprised all the atom–atom pair correlation functions, gMDX-Y(r) values, as six intermolecular contributions, including Li+–solvent, Li+–TFSA−, Li+–Li+, solvent–TFSA− TFSA−–TFSA−, and solvent–solvent correlations. Focusing on the Li-ion solvated complexes, we collected the gMDX-Y(r) functions corresponding to the Li+–O, Li+–N, and Li+–Li+ correlations from the total GMDinter(r). Figure 7a and b show the Li+–N and Li+–O pair correlation functions [gMDLi-Nan(r) and gMDLi-Otfsa(r)] for the Li+–AN and Li+–TFSA− interactions, respectively, in the dilute and concentrated LiTFSA/AN solutions, where Nan and Otfsa represent the coordinating N atom in AN and O atom in TFSA, respectively. In gMDLi-Nan(r), a peak appeared at 1.98 Å for both dilute (Li:AN = 1:35) and concentrated (Li:AN = 1:2) systems, which was attributed to the nearest neighbor Li+–AN interaction in the first solvation shell. However, in gMDLi-Otfsa(r), there was a slight peak at r < 5 Å in dilute solution, while an intense peak was observed at 2.03 Å for the concentrated solution. This suggested that Li ions are solvated with only AN in the dilute solution, followed by ion-pair complex formation with both AN and TFSA coordination in the concentrated solution. The average coordination number N(r), calculated by integrating the corresponding gMDX-Y(r) up to a given r, for Li+–Nan correlations was ~4 (plateau in gMDLi-Nan(r) at r = 2.5 ~ 3.5 Å) in dilute AN solution with Li:AN = 1:35. N(r) for the Li+–Nan correlations decreased to ~2.4 in concentrated solution; however, that for the Li+–Otfsa correlations was ~1.7.


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Figure 7. Atom-atom pair correlation functions [gMDLi-X(r): left axis, solid lines] for (a) the N atoms of AN and (b) the O atoms of TFSA around the Li ions and their integrated profiles [coordination number N(r): right axis, dashed lines] for LiTFSA/AN solutions with Li:AN = 1:35 (black) and 1:2 (red). The gMDLi-X(r) values of the O atoms of (c) TFEP and (d) TFSA around the Li ions and their N(r) values for LiTFSA/TFEP solutions with Li:TFEP = 1:4 (black), and 1:1.6 (blue).

These results were in agreement with that from the current Raman study described above: i.e., [Li(AN)4]+ in diluted solution and [Lin(AN)m(TFSA)l] in concentrated solution. For the LiTFSA/TFEP system (Figure 7c and d), a sharp peak appeared at 1.93 Å in gMDLi-Otfep(r) corresponding to the nearest neighbor Li+–TFEP interactions with both dilute (Li:TFEP = 1:4) and concentrated (Li:TFEP = 1:1.6) solutions. With respect to the Li+–TFSA− interactions, there was a peak at similar r (1.98 Å) in gMDLi-Otfsa(r) for both dilute and concentrated systems. N(r) for the nearest neighbor peak (Li+–O correlation) was estimated as ~2.4 for the Li+–TFEP interactions and ~2.2 for the Li+–TFSA− interactions in dilute TFEP solution with Li:TFEP = 1:4.


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The resulting N(r) values are consistent with the coordination numbers reported in our previous study employing spectroscopic experiments and DFT calculations on the dilute LiTFSA/TFEP system (cLi < 1.0 mol dm−3). As the salt concentration increased to Li:TFEP = 1:1.6, N(r) for Li+–Otfep decreased from ~2.4 to ~1.4 and then that of Li+–Otfsa increased from ~2.2 to ~3.0. The N(r) variation was also consistent with the above-mentioned current IR results. Figure 8a shows the Li+–Li+ correlations based on gMDLi-Li(r) functions determined for dilute and concentrated LiTFSA/AN solutions. For the dilute solution (Li:AN = 1:35), no peak (or a very small peak) was found for r < 5.0 Å, followed by broad and unclear peaks at around 8 and 12 Å. This was ascribed to the mononuclear [Li(AN)4]+ complex formed in the dilute solutions, resulting in no correlation in the short-r range and much reduced correlation among the mononuclear complexes in the long-r range. Here, we note that for the concentrated AN solution (Li:AN = 1:2), the closest Li+–Li+ correlation was observed at 3.2 Å. Additionally, sharper subsequent peaks were found at 6.2, 8.2, and 11.3 Å. Based on our previous work (concentrated

Figure 8. Atom-atom pair correlation functions between Li ions [gMDLi-Li(r)] for (a) LiTFSA/AN solutions [Li:AN = 1:35 (black), and 1:2 (red)] and (b) LiTFSA/TFEP solutions [Li:TFEP = 1:4 (black), and 1:1.6 (blue)].


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LiTFSA/DMF solutions), the closest Li+–Li+ correlation peak ( Li-TFSA (1:1) complex > Li-TFSA (2:1) complex. This suggested that the LUMO energy levels located on the TFSA were stabilized further by forming multinuclear complexes, including the closest Li+-Li+ correlation. These lower LUMOs dominate the reduction reaction during charging. We therefore concluded that the


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Figure 10. ELUMO values for the Li-TFSA complexes calculated using DFT calculations.

unusual electronic state of the TFSA, which is trapped by a multinuclear complex structure comprising a unit such as complex 3, is the molecular origin of the stable SEI in the AN-based concentrated solutions. Indeed, Li-ion insertion/deinsertion at the graphite electrodes was clearly observed in this AN system (CV profile in Figure 2a). Conversely, this specific solution structure may not be formed in the TFEP-based concentrated solutions due to the bulkiness of TFEP; consequently, no Li-ion insertion reaction was observed in the TFEP system (Figure 2b).

CONCLUSIONS

A comprehensive experimental (Raman/IR spectroscopy and HEXTS) and computational (MD simulations and DFT calculations) study revealed the molecular-level characteristics of a multinuclear ordered Li-ion structure in a concentrated electrolyte system (LiTFSA/AN and


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LiTFSA/TFEP electrolytes examined herein) as follows: (1) in the LiTFSA/AN system, extended ionic order was formed in the concentrated solutions, which was significantly different from the mononuclear Li-ion complexes (i.e., [Li(AN)4]+ as a major species) in the dilute solutions. (2) In the concentrated solutions, a short-range Li+–Li+ correlation (r ~ 3 Å) via the O atoms of TFSA− in the gMDLi-Li(r) may be a good indicator of an ordered ionic structure. The TFSA anions trapped by the ordered Li ions exhibited LUMO energy levels, allowing reductive decomposition on the graphite electrodes during charging to form a TFSA-derived SEI. (3) In the LiTFSA/TFEP system, there was no ordered ionic structure formation in the concentrated solutions according to both experimental and theoretical investigations. Indeed, no Li-ion insertion into the graphite electrodes was observed in either dilute or concentrated solutions. Based on these results, we proposed that solvent size plays a crucial role in the ordered ionic structure formation in the concentrated systems; i.e., Li-ion coordination complexes formed only with less bulky components (in this case, AN and TFSA) adopt a compact solution structure, resulting in a close Li+–Li+ correlation at much shorter r (~ 3 Å), while this did not occur in the TFEP system due to the bulkiness of TFEP.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publication website at DOI: cLi, d, n, and the Li:TFEP molar ratios for the LiTFSA/TFEP solutions (Table S1); cLi, d, and the Li:AN molar ratios for the LiTFSA/AN solutions (Table S2); cLi, xLi, d, compositions (number of ion pairs and solvents), and box length of the systems for the MD simulations (Table S3); The original partial charges (Figure S1); CVs in LiTFSA/TFEP with cLi = 1.0 mol dm−3 (Figure S2);


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Sexp(q)s over the entire q range (Figure S3); GMDintra(r) and GMDinter(r) (Figure S4); The optimized geometry in a single TFSA anion according to DFT calculations (Figure S5); SDF in the concentrated and dilute LiTFSA/TFEP system (Figure S6).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACKNOWLEDGMENT This work was financially supported by the Grant-in-Aid for Scientific Research No. 15K17877 to K.F. from the Japan Society for the Promotion of Science (JSPS). The HEXTS experiment was performed at BL04B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Numbers: 2017B1547 and 2018B1472). REFERENCES

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Role of Solvent Size in Ordered Ionic Structure Formation in

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