Correlation between Solvation Structure and Ion-Conductive Behavior

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Correlation Between Solvation Structure and Ion-Conductive Behavior of Concentrated Poly(ethylene carbonate)-Based Electrolytes Kento Kimura, Joh Motomatsu, and Yoichi Tominaga J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03277 • Publication Date (Web): 27 May 2016 Downloaded from http://pubs.acs.org on June 3, 2016

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The Journal of Physical Chemistry

A paper contributed to The Journal of Physical Chemistry C as an article (revised)

Correlation Between Solvation Structure and Ion-Conductive Behavior of Concentrated Poly(ethylene carbonate)-Based Electrolytes

Kento Kimura, Joh Motomatsu, Yoichi Tominaga*

Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan Tel: +81-42-388-7058

*Corresponding author: [email protected]

ACS Paragon Plus Environment


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ABSTRACT:

Solid polymer electrolytes (SPEs) are important materials in realizing safe

and flexible energy storage devices. solvation

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structure

and

the

The present study looks at correlation between

ion-conductive

behavior of poly(ethylene carbonate)

(PEC)/lithium bis(fluorosulfonyl)imide (LiFSI) electrolytes which have high Li transference number (t+) and show unusual salt-concentration dependence of conductivity.

From FT-IR

and Raman spectroscopy, we determined that Li ions interact with carbonyl (C=O) groups and also with FSI ions, which can be referred to as contact ion pair or aggregate.

7

Li

magic-angle-spinning (MAS) NMR spectroscopy and density functional theory (DFT) calculations for model species suggest that a loose coordination structure, in which Li ions interact with C=O groups and FSI ions with appropriate strength, allows the electrolytes to have both reasonable conductivity and high t+ with a flexible and transparent character.

A

high salt dissociation rate is generally considered essential in SPEs, but the presence of aggregated ions having the loose coordination structure gives rise to favorable performance in highly-concentrated PEC-based solid polymer electrolytes.


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INTRODUCTION Ion-conductive solid polymer electrolytes (SPEs) are mixtures of polar polymers and metal salts.1,2

They are noteworthy soft ionics materials because of their light weight,

flexibility, and non-volatility.

SPEs are promising new materials, particularly as alternatives

to flammable liquid-based counterparts of Li rechargeable batteries,3,4 including Li-O2 (air) and Li-S systems.5

Therefore, SPE is rated as a key technology in the furthering of electric

vehicles (EVs) and the utilization of renewable energies by facilitating energy storage.6 Since the first report of a poly(ethylene oxide) (PEO)-based electrolyte,7 most attention has been directed to polyethers.1,2,5

The polar ether oxygen atoms induce dissociation of salt;

then Li ions are transported by the segmental motion of polymer chains, mainly in an amorphous region.

Practical application of SPEs has so far been hindered by their limited

Li-ion conductivity, i.e., the product of the overall conductivity and Li transference number (t+).

It is difficult to improve the Li-ion conductivity because there is a very tight

coordination structure between the Li ions and the ether oxygen atoms.

This structure

induces a strong coupling between the ionic conduction and slow segmental motion, and gives rise to an extremely low value of t+ (generally 0.1-0.2).

Addition of Li salts slows the

segmental motion, and in turn causes an increase in glass transition temperature (Tg).

For

this reason, most research has focused on SPEs with low salt concentration (i.e., [Li]/[monomer unit] ratio is 1/20 to 1/10), in which the conductivity is high.



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In the present study, we focus on poly(ethylene carbonate) (PEC) as a polymer host for SPEs.

Various poly(alkylene carbonate)s, including PEC, are generated from CO2/epoxide

alternating copolymerization.

This copolymerization was first discovered by Inoue and

co-workers in 1968,8 and has become popular in polymer chemistry and catalytic chemistry9-12 because large portion of polymer chain is derived from CO2.

Despite

interesting features, such as biodegradability,13 however, no particular application has yet been established.

For other polycarbonate species, some groups have investigated their

suitability as Li battery electrolytes.14-18

Motivated by these facts, we proposed to evaluate

various polycarbonates derived from CO2 as novel SPE polymer hosts.19,20

During studies of

poly(alkylene carbonate)-based electrolytes, we noticed the following properties: i) decrease in Tg and increase in ionic conductivity with increasing salt content, and ii) a rather high t+ of the PEC-based electrolytes.21,22 We have already confirmed the operation of prototype Li batteries combined with an unusually-concentrated PEC-based electrolyte23 and a composite containing plasticizing and reinforcing additives.24 To further improve the electrochemical performance of poly(alkylene carbonate)-based electrolytes, more study is necessary to understand their unusual behavior.

In the present

study we performed FT-IR, Raman, and solid-state MAS NMR spectroscopy in combination with a computational study, along with a more detailed investigation of the salt-concentration dependence

of

the

thermal

and

electrochemical

properties


of

PEC/lithium

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bis(fluorosulfonyl)imide (LiFSI) electrolyte, so as to reveal correlations between solvation structure and the unusual ion-conductive behavior.

The present study is intended to provide

new insight into developing SPEs having both high conductivity and high t+.

EXPERIMENTAL SECTION Materials and electrolyte preparation Poly(ethylene carbonate) (PEC, QPAC®25, Empower Materials, Mn=1.0×105, Mw/Mn=2.7) was precipitated from acetonitrile into methanol for rinsing, and dried under vacuum at 60 oC for 24 h before use.

Lithium bis(fluorosulfonyl)imide (LiFSI, battery grade,

Kishida Chemical) was used as received. follows.

Electrolytes were prepared by a casting method, as

PEC and LiFSI were mixed with acetonitrile and stirred overnight.

The resulting

mixtures were kept in a circulation chamber filled with dry Ar or N2 at 60 oC for several hours to remove excess solvent, and finally dried under vacuum at 60 oC for more than 24 h in order to eliminate all volatile residues.

The concentration of LiFSI was set in the range from 10 to

90 mol% (x mol%: x = ([LiFSI]/[EC unit])×100).

A poly[(ethylene oxide)-ran-(propylene

oxide)] (P(EO/PO), [EO]:[PO]=89:11, Mn=2.8×104, Mw/Mn=2.0, donated from ZEON and used without further purifications) electrolyte with 10 mol% LiFSI was also prepared for comparison. Thermal characterization



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Thermogravimetric analysis (TGA) was carried out using a Discovery (TA Instruments) under dry He gas flow at a scan rate of 10 oC min-1.

The measurement was

performed in a dry room (Battery Research Platform of the National Institute for Materials Science (NIMS), Japan), where the dew point is controlled to be below -35 oC.

Differential

scanning calorimetry (DSC) was performed using a DSC7020 (Hitachi High-Tech).

Small

samples were sealed in aluminum pans and were heated from 30 oC to 70 oC and then cooled to -110 oC, followed by second heating scans to 110 oC under dry N2 gas flow at a scan rate of 10 oC min-1. Electrochemical characterization The ionic conductivity of the electrolytes was measured by electrochemical impedance spectroscopy (EIS) for symmetric cells with two stainless steel blocking electrodes, using a potentiostat/galvanostat SP-150 (Bio-Logic).

These measurements were performed in a

frequency range from 300 kHz to 10 Hz with a voltage amplitude of 30 mV.

The Li

transference number (t+) was estimated by combining EIS and chronoamperometry for symmetric cells with two Li metal non-blocking electrodes using the SP-150, following a previous report.25

t+ =

The value of t+ can be calculated as

I SS (∆ V − R0 I 0 ) I 0 (∆ V − RSS I SS )

(1)

where I0 is the initial current, Iss is the steady state current, ∆V is the applied voltage (10 mV


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in this study), and R0 and Rss are the electrolyte/electrode interfacial resistances before and after the polarization.

EIS was performed within a frequency range 300 kHz to 0.1 Hz with

a voltage amplitude of 30 mV.

In these electrochemical measurements, exposure of the

samples to the air was prevented by using a dry Ar-filled glovebox. FT-IR and Raman spectroscopy Fourier transfer-infrared (FT-IR) spectra were acquired at room temperature using a Nicolet iS50 spectrometer (Thermo Scientific) equipped with an attenuated total reflection (ATR) unit (Ge lens) at a resolution of 8 cm−1.

Laser Raman spectroscopy was performed at

room temperature using a RamanTouch-VIS-NIR system (Nanophoton) with a laser wavelength of 532 nm.

These measurements were carried out in the dry room.

deconvolutions of the spectra were obtained via the Voigt function.

Peak

In the FT-IR and the

assignment of aggregates of FSI ions in the Raman spectroscopy, the wavenumbers and Raman shift values of the peaks were automatically adjusted for best fit. Solid-state MAS NMR spectroscopy Solid-state magic angle spinning (MAS) NMR spectra were acquired at room temperature using a JNM-ECX400 spectrometer equipped with a 4 mm MAS probe (JEOL Resonance). 1

A MAS frequency of about 10 kHz was used for all signal acquisitions.

H-13C cross-polarization (CP)/MAS spectra were acquired using a dipolar-decoupling (DD)

technique with a 1H 90o pulse of 3 µs, a contact time of 3 ms and a recycle delay of 2 s.


7

Li


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spectra were acquired on a single-pulse sequence with a 90o pulse of 3.3 µs and a recycle delay of 0.5 s.

The

13

C and 7Li chemical shifts were calibrated using adamantane (δ=29.5

ppm) and a 1 M LiCl aqueous solution (δ=0 ppm) as external references, respectively. DFT calculations Density functional theory (DFT) calculations were performed using the Gaussian09 program.26

The geometrical parameters were optimized at the B3LYP/6-31++G** level.

The 7Li NMR chemical shift was calculated based on the optimized structures at the same level to evaluate the chemical shielding tensors for the Li atom.

The chemical shift was

calibrated using [Li-(H2O)5]+ (δ=0 ppm) as a reference.

RESULTS AND DISCUSSION Thermal and ion-conductive properties We adopted LiFSI concentrations in the range 10 to 90 mol% for preparation of PEC/LiFSI electrolytes.

The resulting electrolytes prepared are flexible and transparent

membranes (Figure S1).

At about 50 mol%, the salt/polymer weight ratio exceeds unity

(Table S1), above which the electrolytes may be referred to as ‘Polymer-in-Salt’ system27 in terms of weight.

The 90 mol% electrolyte contains LiFSI of nearly double the PEC polymer

host by weight. With increasing salt concentration, the membrane gradually becomes sticky and soft.

Thermal stabilities of PEC/LiFSI electrolytes up to about 170 oC were confirmed


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by thermogravimetric analysis (TGA) (Figure S2), although the addition of LiFSI appears to expedite the thermal decomposition of PEC. We investigated this concentrated electrolyte system not only because of the salt-dissolving ability of PEC, but because of a unique dependence on the salt concentration of its thermal and ion-conductive properties, as summarized in Figures 1 and 2.

Differential

scanning calorimetry (DSC) curves (Figure S3) clearly reflect typical amorphous behavior and glass-transition events of neat PEC and the PEC/LiFSI electrolytes. Figure 1a shows the salt concentration dependence of glass transition temperature (Tg) for the electrolytes.

The

value of Tg for conventional polyether-based electrolytes is expected to rise with increasing salt concentration.

This behavior is commonly believed to be due to a quasi-cross-linking

effect, which is a result of a tight coordination structure between ether oxygen atoms and Li ions.1,2

This property makes it impossible to improve the conductivity by increasing the salt

concentration in polyether-based systems.

The slight increase in Tg of the PEC-based

electrolyte with 10 mol% LiFSI seen in Figure 1a may be attributed to the same effect, probably due to interactions between carbonyl (C=O) groups and Li ions. addition, however, unexpectedly causes a decrease in Tg.

Further salt

Lower Tg is basically favorable for

achieving high ionic conductivity, because it is correlated with fast segmental motion of polymer chains. concentration.

As shown in Figure 1b, the conductivity increases with increasing LiFSI This is reasonable in view of the decrease in Tg, although this behavior is



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rarely reported in other SPE systems including similar polycarbonate-based ones.17

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Note

that electrolytes with salt concentrations greater than 90 mol% may have better conductivities, as we have reported previously;23 they are excluded from the present study as being too sticky to act as a membrane.

The temperature dependence of the conductivity for PEC/LiFSI 10 to

90 mol% electrolytes is shown in Figure 2.

The plots comprise convex trends, representing

typical Vogel-Tammann-Fulcher (VTF)-type temperature dependence, and suggesting a correlation between segmental motion of polymer chains and ionic migration.


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Figure 1 (a) Glass transition temperature (Tg) and (b) ionic conductivity at 70 oC of PEC/LiFSI electrolytes as a function of salt concentration

Figure 2

Temperature dependence of conductivity for PEC/LiFSI electrolytes.

The low Li transference number (t+) is a major issue for the practical use of SPEs.

A

low t+ may cause large cell polarizations and accumulations of anion-decomposed products on the electrodes during the charge/discharge process of the envisaged batteries.28

As an

example of the high t+ of the PEC/LiFSI electrolyte, we show t+ measurement for PEC/LiFSI 90 mol% in Figure S4.

The value of t+ can be estimated by a technique proposed by Evans

et al.,25 involving chronoamperometry (DC polarization) and electrochemical impedance spectroscopy (EIS) for a symmetric Li/Li cell.

The chronoamperometric trend (Figure S4a)

exhibited a relatively small drop of the current, and t+ was calculated to be as high as 0.60 ACS Paragon Plus Environment


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based on Eq. 1. FT-IR and Raman spectroscopy Deeper study of the salt solvation structure is the key to a better understanding of this extraordinary ion-conductive ability.

First, to detect the C=O…Li+ interaction, we used

FT-IR, which shows a particular stretching vibration peak for C=O at around 1740 cm-1. Upon addition of Li salt, a new peak is expected to appear at lower wavenumbers because of the C=O…Li+ interaction, as seen in a common carbonate/Li salt solution.29

As expected,

the new peak appears at around 1720 cm-1 upon addition of LiFSI to PEC, as shown in Figure 3a.

The peak corresponding to C=O interacting with Li ions gradually becomes larger with

increasing concentration, along with the remaining peak of the free C=O. We performed a peak deconvolution to quantify the proportions of the two kinds of C=O groups (Figure 3b and c).

There is an obvious unassigned peak at high wavenumbers, shown by the black

curves in Figure 3b, which is ignored in the estimation of peak fraction shown in Figure 3c. As is clear from Figure 3c, the proportion for C=O…Li+ interaction is nearly saturated above 50 mol%.

This is quite surprising, given the drastic increase in the conductivity at higher

concentrations. Raman spectroscopy provides information on the dissociation of LiFSI from the S-N-S stretching vibrational mode that appears as a band around 700-800 cm-1 (Figure 4a).

At first

the peak undergoes an upward shift with salt addition, but above 50 mol% there is almost no


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further change.

The peak can be divided into three different dissociation states: free FSI ions

at 720 cm-1, contact ion pairs at 732 cm-1 (CIPs, FSI ions interacting with a single Li ion), and aggregates at 746 cm-1 (AGGs, FSI ions interacting with two or more Li ions) (see Table S2 for details).30

As shown in the peak deconvolution results in Figures 4b and c, even in the

less-concentrated 10 mol% electrolyte the majority of FSI ions interact with one or more Li ions.

Upon increasing the LiFSI concentration, AGG gradually predominates as the species

of FSI ion.

Above 50 mol%, all the FSI ions exist as AGG.

The peak-top shifts gradually

to higher frequency above 50 mol%, probably due to a transition in the solvation structure; we assumed that the peak represents the aggregates even when the peak-top is higher than 746 cm-1.

Figure 3 (a) FT-IR spectra of neat PEC and PEC/LiFSI electrolytes. (b) Deconvolution of peaks of the C=O stretching vibrational mode. (c) Peak fraction of free C=O and C=O interacting with Li ion as a function of salt concentration.



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Figure 4 (a) Raman spectra of PEC/LiFSI electrolytes. (b) Deconvolution of peaks of the S-N-S stretching vibrational mode. (c) Peak fraction of free FSI ion, contact ion pair (CIP), and aggregate (AGG) as a function of salt concentration.

As a summary of the above spectroscopic investigations on the PEC/LiFSI electrolytes, Li ions interact constantly with both carbonyl groups (C=O) and FSI ions.

In conventional

polyether-based electrolytes, Li ions are tightly surrounded by the ether chains, which inhibits effective migration.

It is probable that PEC-based electrolytes form no tight coordination

structure between polymer chains and cations which induces an increase in Tg in contrast to polyether-based systems.

This loose coordination structure offers an explanation for the

unusual increase in conductivity with increasing salt concentration; the plasticizing effect of the FSI ions is predominant against the quasi-cross-linking effect caused by the C=O…Li+ interaction. value.

Furthermore, the highly-aggregated ionic structure probably causes the high t+

Similarly, Brandell et al.31 have reported that a less stable coordination structure

between poly(trimethylene carbonate) (PTMC) chains and Li ions causes a significant


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ion-pairing, which leads to high value of t+.

A similar ‘Polymer-in-Salt’ strategy was

pioneered by Angell et al.27 to achieve super-conductivity like ion-conductive glasses dominated by cationic transports.

Typically, Polymer-in-Salt electrolytes are connected with

the decoupling ion-conductive mechanism,32 in which the mobility of ions is less correlated with the segmental motion of the polymers, because salts dominate the system quantitatively. Our recent study using dielectric relaxation spectroscopy suggests a clear correlation between ion transport and segmental relaxation of PEC, however.33

Further efforts are necessary to

clarify the correlations between ionic migration and segmental dynamics.

We assume that

the aggregated ions may migrate in the electrolyte like an ionic liquid as previously suggested in a polyacrylonitrile-based Polymer-in-Salt system,34,35 with a moderate interaction with carbonyl groups, via the segmental motion of polymer chains.

Similar highly-concentrated

electrolytes have recently been studied in standard liquid electrolyte research in view of the efficient operations of many types of batteries.30,36-39

These effects may arise from

aggregated ions which induce high t+, and a different solvation sheath structure and solid electrolyte interphase (SEI) composition.

We believe that concentrated PEC/LiFSI

electrolytes have different features from conventional SPE systems with low salt concentration.

Solid-state MAS NMR spectroscopy



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To investigate the solvation structure further, we performed solid-state MAS NMR spectroscopy on the PEC-based electrolytes.

Figure 5a shows 13C CP/MAS NMR spectra of

PEC/LiFSI 10 and 50 mol% electrolytes, respectively. CH2 peak at around 67 ppm.

There is no significant change in the

In contrast, the strong peak at around 156 ppm, which is

assigned to the carbonate carbon, shifts to low magnetic field (high chemical shift value) with increasing salt concentration (at 155.5 ppm for 10 mol% and at 156.8 ppm for 50 mol%). This shift matches the tendency observed in NMR spectra of carbonate/Li salt solutions,40 and represents the electron-withdrawing effect resulting from coordination of Li ions.

This result

indicates the presence of C=O…Li+ interactions similar to those in typical carbonate-based liquid electrolytes, which are also shown in the above FT-IR spectra. Down-field shifts in NMR spectra of alkali metal nuclei in solutions are expected when the electron donor ability of the solvation shell molecules is strong.41,42

The chemical shift is

influenced by interactions between cations and anions, as well as those between cations and the solvent.

Figure 5b shows 7Li MAS NMR spectra of PEC/LiFSI 10 and 50 mol%

electrolytes, and also P(EO/PO)/LiFSI 10 mol% electrolyte, i.e., a typical polyether-based electrolyte, for comparison.

In a situation in which the donor ability of the anions alters the

solvation shell structure to facilitate interaction with cations, the chemical shift is expected to depend on salt concentration.43,44

As Figure 5b shows, the chemical shift of the spectra for

the PEC-based electrolytes is strongly dependent on the concentration, suggesting the


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formation of CIPs and AGGs as the concentration increases, as shown by Raman spectroscopy. This shift of the peak was not observed in a less-concentrated polyether-based electrolyte,45 perhaps because of tight complexations to separate cations and anions.

Consequently,

according to Figure 5, the Lewis basicity induced by the contributions of both C=O groups and FSI ions in the PEC/LiFSI 50 mol% electrolyte can be taken to be nearly identical to or slightly weaker than that of the ether oxygen atoms of P(EO/PO).



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Figure 5 (a) 13C CP/MAS NMR spectra of PEC/LiFSI 10 and 50 mol% electrolytes. (b) 7 Li MAS NMR spectra of PEC/LiFSI 10 and 50 mol%, and P(EO/PO)/LiFSI 10 mol% electrolytes.

DFT calculations To discuss the 7Li chemical shift further, we combined the DFT calculations for model low-molecular species (Table 1).

Figure S5 shows schematics of all optimized structures of


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coordinated species calculated.

We firstly focused on the calculated 7Li NMR chemical

shift values of Li-carbonate solvent coordinated species.

An increase in the number of

dimethyl carbonate (DMC) that coordinate with Li leads to a down-field shift ((a)-(c)), which demonstrates that the chemical shift is greatly affected by an interaction with C=O groups. This result for carbonate solvent-based model systems is consistent with the above notion that an NMR peak shows a down-field shift with a stronger Lewis basicity of the solvation sheath. This solvent-only coordination, however, is not the case with the PEC/LiFSI electrolytes as shown in the result of Raman spectroscopy.

If FSI ions are involved in the coordination in

the Li-DMC system, then the situation changes.

With increasing [FSI-]/[DMC] ratio, the

chemical shift shows an up-field shift, suggesting a weakened total electron donor ability of the C=O group and FSI ions ((c)-(f)).

This up-field shift is consistent with the experimental

result of 7Li MAS NMR for PEC/LiFSI electrolytes.

In conclusion, we believe that FSI ions

withdraw Li ions from strong interaction with C=O groups in the aggregated system.

This

weakened coordination of polymer chains in which Li ions constantly contact with FSI ions is probably related to the high t+ value of PEC-based electrolytes. We suppose that C=O groups play a role such that the large amount of Li and FSI ions remain amorphous and migrate in the PEC-based electrolytes.

This particular solvation structure, in which C=O

groups and ions interact maintaining an appropriate strength, may enable both high t+ and reasonable conductivity, and facilitating the formation of flexible and transparent



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ion-conductive membranes even at extraordinarily-high concentration.

Calculated 7Li chemical shift of model coordinated species.

Table 1

Coordinated species

δ (ppm)

(a) [Li-DMC]+

-1.262

(b) [Li-(DMC)2]+

1.131

(c) [Li-(DMC)3]+

1.376

(d) Li-(DMC)2-FSI

0.829

(e) [Li-(DMC)2-(FSI)2]-

0.306

(f) [Li-DMC-(FSI)2]-

0.165

CONCLUSIONS We have studied the correlation between solvation structure and dependence on salt concentration of the ion-conductive behavior of PEC/LiFSI electrolyte, making use of thermal, electrochemical, and spectroscopic studies.

Thermal and electrochemical tests indicated that

the ionic conductivity of the PEC/LiFSI electrolyte increases with increasing salt concentration, while Tg continues to decrease in the concentration range from 10 to 90 mol%. While FT-IR and

13

C CP/MAS NMR spectroscopy demonstrated the expected C=O…Li+

interaction, Raman spectroscopy indicated that most FSI ions exist as aggregates at high


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concentrations above 50 mol%.

7

Li MAS NMR spectroscopy and DFT calculations suggest

that interaction between C=O groups and Li ions in the aggregate state is weakened by contacts between Li ions and FSI ions. electrolytes to have high t+.

This coordination structure may allow the

Low salt concentration is generally chosen for typical

polyether-based SPEs because of an increase in Tg caused by addition of salt.

The present

results nevertheless indicate that an increase in salt concentration is a way to improve the Li-ion conductivity for the PEC-based electrolyte system in which the coordination of polymer chain is loosed by the contact between Li ions and anions, and the interaction of aggregated ions is relatively weak.

ASSOCIATED CONTENT Supporting Information Available: Photographic images, LiFSI/PEC weight ratio, TGA and DSC traces, chronoamperometry and EIS for t+ measurement, assignment of Raman bands, and schematics of optimized coordinated species by the DFT calculations.

This

material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGEMENTS This work was supported financially by a Grant-in-Aid for Scientific Research (B) of JSPS KAKENHI (No. 25288095), Japan.

We thank Dr. Keiichi Noguchi and Dr. Nobuyuki



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Akai (Tokyo University of Agriculture and Technology) for their technical support of the MAS NMR and DFT calculations, respectively.

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TOC graphic 50x25mm (200 x 200 DPI)


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Correlation between Solvation Structure and Ion-Conductive Behavior

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