Lithium Polymer Gel Electrolytes Designed to Control Ionic Mobility

Mar 3, 2014 - We prepared new lithium polymer gel electrolytes with Lewis acid ionic groups on the PVB polymer chain (ionic PVB gel) with the goal of ...
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Lithium Polymer Gel Electrolytes Designed to Control Ionic Mobility Yuria Saito,*,† Miki Okano,† Tetsuo Sakai,† and Taisuke Kamada‡ †

National Institute of Advanced Industrial Science and Technology, 1-8-31, Midorigaoka, Ikeda, Osaka 563-8577, Japan Kuraray Company Ltd., 2045-1 Sakazu, Kurashiki, Okayama 710-0801, Japan

‡

ABSTRACT: We prepared new lithium polymer gel electrolytes with Lewis acid ionic groups on the PVB polymer chain (ionic PVB gel) with the goal of attracting anions to restrict their mobility. The cation and anion diffusion coefficients (Dcation and Danion), dissociation degree of the salt (x), interactive forces between the ion and polar groups on the polymer (βcation and βanion), and interactive forces between the cation and anion (α) were estimated based on measurements of the diffusion coefficients (DLi, DF, and DH) and ionic conductivity (σ). The Dcation and Danion of the ionic PVB gel increased and decreased, respectively, compared to the values in the original PVB gel that did not contain an ionic group, which led to the enhanced cation transport number. The interactions between the mobile ions and polar groups depended on the solvation structure of the lithium cation, Li(EC)n+, which is associated with the polarity of the gel solvent. The solvating EC species around Li+ is the barrier for weakening Li+/OH− and Li+/TFSI− interactions and consequently activating the TFSI−/ionic group interaction.

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INTRODUCTION To replace electrolyte solutions in lithium secondary battery systems, the development of reliable solid electrolyte materials free from liquid leakage and flammability is required. Polymer gel electrolytes are promising materials, which have high conductivity and self-supporting structure.1,2 The characteristic feature of the polymer electrolyte is that the ionic mobility and ion transport number can be systematically controlled by taking advantage of the effect of the chemical interactions between the polar sites on the polymer chains on the mobile ionic species.3−5 We previously discussed that the interactions responsible for ionic mobility in lithium gel electrolytes can be categorized into several effects, and we proposed a new approach for individual evaluation of the interactive forces.5 These effects are the van der Waals interaction between the mobile ion and surrounding neutral species acting as the solvent, Coulombic interaction between the cation and anion, and Coulombic interaction between the ions and polar groups of the polymer. For example, in the case of the lithium polymer gel electrolyte composed of LiTFSI/(ethylene carbonate (EC) + dimethyl carbonate (DMC)) and polyvinyl butyral (PVB), the lithium cations were selectively attracted by the OH− groups of the PVB.5 The Coulombic force between the lithium cation and OH− group (Li+/OH−) was proportional to the OH− group concentration of the gel. Furthermore, the dissociation degree of the salt also increased with the concentration of the OH− group. Because of the investigation into the microscopic interactive situation in the PVB gel electrolytes responsible for the dynamic properties, we were confident that the ionic conductivity or mobility could be systematically controlled via modification of the polar site structure of the polymer. Lewis © 2014 American Chemical Society

acidic polar groups could selectively attract the anion to restrict anion mobility, which would result in the increase of the cation transport number. Furthermore, the dissociation degree of the salt would increase with increasing polar group content based on the OH− group effect on salt dissociation.5 On the basis of this, we prepared a new polymer with Lewis acidic ionic groups on the PVB polymer (ionic PVB polymer). The reason we selected PVB as the base polymer for modification is because PVB has some branch groups, hydroxide (OH−), butyral (BA), and acetyl (OAc) groups, which are useful for substitution by another polar group. In addition, OH− groups could be the effective cardinal points for cross-linking the polymer chains in the gelation process to form a stable gel network. In this research, we prepared gels with electrolyte solutions of different solvent polarities. This is because the polarity of the solvent affects the solvation structure of the lithium (Li(EC)n+) and, consequently, affects the interactive situation between the ions and polar groups (ions/polar groups) or cation and anion (Li+/ TFSI−).5 In order to evaluate the inherent ionic diffusion coefficients (Dcation and Danion) corresponding to the ionic mobilities and the interactive situation between the species, we measured the diffusion coefficients (DLi and DF), ionic conductivity (σ), and density (d) of the gel electrolytes. Using these values, we analytically estimated Dcation and Danion, the dissociation degree of the salt (x), the interactive forces attributed to the van der Waals interaction (γ), and the Coulombic interactions between Li+/TFSI− (α) and ion/polar group (β) in the gel electrolyte. This research is the first challenge in the evaluation of the interactive effects of two Received: December 25, 2013 Revised: February 27, 2014 Published: March 3, 2014 6064

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simultaneous but contrasting polar sites that are responsible for ionic mobility.

The molar ratio of Li, OH−, and the substituted ionic group of each gel is represented in Table 1.

EXPERIMENTAL SECTION PVB polymer was synthesized according to a procedure presented in a previous report.5,6 Electrolyte solutions of different solvent polarity (EC:DMC = 0:10, 1:9, 2:8, 3:7 (v/v %); EC, ethylene carbonate; DMC, dimethyl carbonate) were used. Ionic PVB polymer was prepared as follows. In a 500 mL four-necked separable flask with a reflux condenser, an anchor impeller, thermometer, 212 g of deionized water, 50.5 g of thiol-substituted PVA M-105 (viscosity average molecular weight, 1500; Kuraray Co., Ltd.), and 6.2 g of vinylbenzyl trimethyl ammonium chloride (AGC Seimichemical Co., Ltd.) were placed under N2. The mixture was stirred at 90 °C for 1 h, and 5.6 mL of 2,2-azo-bis[2-methyl-N-(2-hydroxyethyl)propionamide] aqueous solution (2.0%) was added for 1.5 h. The reaction mixture was stirred for 4 h and an ionic PVA solution (20 wt %) was obtained. In a 500 mL four-necked separable flask with a reflux condenser, an anchor impeller, thermometer, 200 g of ionic PVA solution (20 wt %), and 22.7 g of n-butylaldehyde were placed under N2. Then, 26.7 mL of 20 wt % HCl aq. was added for 12 min, and the mixture was stirred for 4 h at 70 °C. After the reaction, a white slurry was filtered by polyethylene terephthalate (PET) mesh filter, washed by water, and dried under vacuum, and an ionic PVB resin was obtained. The ratio of ion substituent was found to be 1.4 mol % using 1H NMR (DMSO-d6). We also prepared the PVB with 6.2 mol % ion substituent by changing the mixing content of the materials. The structure of ionic PVB is shown in Figure 1.

Table 1. Molar Ratio of Li, OH, Butyral (BA), and Acetyl (OAc) Groups and Ionic Groups in the PVB and Ionic PVB Gel Electrolytes Composed of 5 w/v% PVB or Ionic PVB and 1 M LiTFSI/(EC + DMC) Solution

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polymer type

[Li]

[OH]

[BA]

[OAc]

[ionic group]

PVB 1.4 mol % ionic PVB 6.2 mol % ionic PVB

10 10 10

2.99 2.31 1.15

4.88 5.27 5.13

0.244 0.008 0.006

0 0.106 0.391

The ion conductivity of the gel electrolyte was measured by the impedance method using an LCR meter 6440B (Wayne Kerr Electronics Ltd.) in the frequency range of 1 mHz to 1 MHz and a conductivity cell with platinum electrodes at 25 °C. The density of the gel was measured using the Archimedes’ method at 25 °C. Diffusion coefficients DLi, DF, and DH were measured at 25 °C for the probed nuclear species 7Li (116.8 MHz), 19F (282.7 MHz), and 1H (300.5 MHz), respectively, using the pulsed gradient spin−echo (PGSE) NMR technique with a JNMECP300W wide bore spectrometer.7 DH was obtained using the 1 H peak of the DMC species of the binary solvent. Hahn-echo pulse sequence was used for measurements. The half sineshaped gradient pulse was applied twice in the sequence after the 90° and 180° pulses to detect attenuation of the echo intensity according to the diffusive migration of the probed species.8,9 The typical values of the parameters for the pulse sequence are g = 2−4 T/m for the strength of the gradient pulse, δ = 0−5 ms for the pulse width, and Δ = 50 ms for the diffusion time corresponding to the interval between the two gradient pulses.

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THEORETICAL DERIVATION Fundamental parameters, inherent diffusion coefficient of the ionic species, dissociation degree of the salt, and the microscopic viscosity responsible for ionic mobility were estimated from the measured diffusion coefficients and ionic conductivities. The lithium salt, LiTFSI, dissolved in the gel has the following equilibrium state:

Figure 1. Chemical structure of the ionic PVB polymer.

Sodium and chlorine impurities in the polymer were measured by ICP atomic emission spectrometry and confirmed to be less than 200 ppm during the preparation process. Proton content was measured by neutralization titration using KOH. As a result, we confirmed that the molar ratios [Na]/[Li], [Cl]/ [Li], and [H]/[Li] were 1.9 × 10−3, 1.3 × 10−3, and 5.5 × 10−4, respectively, in the polymer-rich gel with 15 wt % of PVB. These results suggest that the contribution of the ionic impurities to the conductivity is negligible for the gels (with 5 w/v% PVB and ionic PVB) in this research. Lithium polymer gel electrolytes were prepared in the glovebox as follows. Two milliliters of a 1 M LiTFSI solution (EC/DMC), 100 mg of ionic PV resin, and a magnetic stirrer bar were placed in a glass vial. The mixture was stirred at 60 °C, and an ionic PVB gel of 5 w/v% was obtained. The polarity of the electrolyte solution was changed by changing the mixing ratio of EC to DMC (EC:DMC = 0:10, 1:9, 2:8, and 3:7 v/v%). This is because the polarity of the solvent affects the solvation structure of the lithium ion, Li(EC)n+ and therefore affects the interactive forces between Li+/TFSI− and the ion/ionic group.5

LiTFSI + n(EC) ⇆ Li(EC)n+ + TFSI−

(1)

In the binary solvent, EC + DMC, EC is the dominant species contributing to the lithium solvation.10 When the solvent is DMC, DMC species weakly coordinate to the lithium and support lithium salt dissociation because solvated lithium with DMC has been reported,11 and the ionic conductivity of the gel was practically observed. The dissociation degree of the salt, x, depends on the mixing ratio of EC to DMC in the binary solvent and the consequent solvation structure. Therefore, the inherent diffusion coefficients of Li(EC)n+ (Dcation), TFSI− (Danion), and isolated solvent species (DDMC) correlated with the observed diffusion values, DLi, DF, and DH, which was probed by 7Li, 19F, and 1H, respectively, as follows. DLi = xDcation + (1 − x)Dpair DF = xDanion + (1 − x)Dpair DH = DDMC 6065

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where Dpair is the inherent diffusion coefficient of the associated ion pair, LiTFSI. However, molar ionic conductivity, Λ, can be represented with Dcation, Danion, and x according to the Nernst− Einstein equation as Λ = Λcation + Λanion =

F2 x(Dcation + Danion) RT

(3)

where F is the Faraday constant, R is the gas constant, and T is the absolute temperature. The neutral species in the gel, ion pair, and DMC are isolated without any specific Coulombic interactions with other species and follow the Stokes−Einstein relationship. Therefore, the diffusion coefficients (Dpair and DDMC ) and radii (rpair and rDMC) have the following relationship: Dpair /DDMC = rDMC/rpair

DDMC =

kT 6πrDMCη

Danion =

kT , η′ = η + α + βanion 6πranionη′

Dcation =

r kT , η″ = η + anion α + βcation rcation 6πrcationη″

(5)

where η is the microviscosity from the van der Waals force and is estimated from the observed DH (= DDMC) and van der Waal’s size, rDMC = 2.67 Å. α, βcation, and βanion are the microviscosities from the Coulombic forces between the cation and anion, the cation and polar group of the ionic PVB, and the anion and polar group of the ionic PVB, respectively. The prefactor, ranion/rcation on α of Dcation, means that the viscosity is proportional to F/rion (F: Coulombic force between the cation and anion) according to the Stokes equation. From eq 5, we can derive βcation and βanion as a function of α as follows:

(4)

where DDMC is obtained by 1H (DH) probe, and rDMC and rpair are estimated as 2.67 and 4.0 Å, respectively, according to the van der Waals size of the atomic species.12,13 Here, we assumed for simplification that the ion pair is the contact ion pair. Applying the measured values, DLi, DF, and DH, to these three equations, we can numerically evaluate Dcation, Danion, Dpair, and x individually. At the equilibrium state, the resistance the ions in the gel feel during migration is the microviscosity derived from the van der Waals and Coulombic interactions with the surrounding molecules, ions, and specific sites on the polymers. We have already discussed and estimated each interaction in the PVB gel electrolyte in a previous report.5 The interactive situation of the ionic PVB gel electrolyte is illustrated in Figure 2. We have to

⎛D ⎞ r βcation = ⎜ DMC DMC − 1⎟η − cα ⎝ Dcationrcation ⎠

(6a)

⎛D ⎞ r βanion = ⎜ DMC DMC − 1⎟η − α ⎝ Danionranion ⎠

(6b)

where c = ranion/rcation. We need to estimate at least one parameter from another relationship to individually estimate α, βcation, and βanion. We assume that βcation is proportional to the concentration ratio of [Li]/[OH−] and is independent of the presence and concentration of the substituent ionic groups. In order to confirm this assumption, we plotted the OH concentration dependence of η, α, and βcation of the PVB gels (5 and 10 wt % PVB gels for three types of PVB with different OH concentrations) in Figure 3 using the results from ref 5.

Figure 3. Microviscosities, η, α, and βcation as a function of [OH]/[Li] in the PVB gels (red marks, 5 wt % PVB gels; green marks, 10 wt % PVB gels).

Figure 2. Interactive situation in the ionic PVB gel electrolyte. η is the microviscosity from the van der Waals interaction between a species with the surrounding neutral species, and α, βcation, and βanion are from the Coulombic interactions between the cation and anion, cation and OH− group, and anion and ionic group, respectively.

We can see for each gel that η and α are independent of [OH]/ [Li]. However, βcation is proportional to [OH]/[Li]. The difference in η or α between the 5 and 10 wt % gels does not reflect the OH concentration but instead reflects the polymer fraction closely associated with the microviscosity from the van der Waals interaction. On the basis of this result, we first estimated βcation of the ionic PVB gels based on the OH concentration relative to that of the PVB gel and then estimated α and βanion according to eqs 6a and 6b for the ionic PVB gel electrolytes.

consider a new Coulombic interaction between the ionic group/TFSI− as responsible for βanion in addition to the interactions between Li+/TFSI− and OH−/Li+ for α and βcation, respectively. That is, the inherent diffusion coefficients of the solvent and ionic species can be represented as follows: 6066

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RESULTS AND DISCUSSION Figure 4 represents the ionic conductivity of the PVB and ionic PVB gel electrolytes. There is no apparent difference in

This is later discussed quantitatively based on the estimation of the individual interactive force on the ionic species. The second feature is that the Dcation and Danion of the PVB and ionic PVB gels were almost the same at EC = 0 and Danion > Dcation with the EC fraction. As a result, tLi decreased with EC, as shown in Figure 6. This change in the relative value of Dcation

Figure 4. Ionic conductivity of the PVB gel (○) and ionic PVB gels (▲, 1.4 mol %; ■, 6.2 mol %).

conductivity between the gels with and without ionic groups. This suggests the need for a more detailed examination of the changing manner of each ion as the conductivity has contributions from both the cation and anion species. We also found that the conductivity of both gels increased with an increase in the EC fraction of the solvent in the gels. The solvent polarity affects the dissociation degree of the salt, x, which determines the ion concentration, and the structure of the solvated lithium ion, Li(EC)n+, as the solvation number, n, changes with the EC fraction in the range used for this research. The solvated lithium structure would influence the interactive situations between Li+/TFSI− and Li+/OH− and, consequently, the ionic mobility.5 To evaluate this, we estimated Dcation, Danion, and x. Figure 5 represents the diffusion coefficients of the cation (Dcation) and anion (Danion), which are related to the ionic

Figure 6. Dissociation degree of the salt (x; red marks) and the lithium transport number (tLi; blue marks) of the PVB gel (○) and ionic PVB gels (▲, 1.4 mol %; ■, 6.2 mol %).

to Danion is also associated with a change in the solvated structure of Li(EC)n+. With a change in the EC fraction from 0 to 30%, the solvation number, n, changes from 0 to ∼3.5,15 That is, the cation size increases, contrary to keeping a constant anion size with the EC fraction in the gel. The cation size increase with the EC fraction restricted the Dcation increase relative to the Danion increase because of changes in the interactive forces on the ions with an EC increase. The third and most impressive feature is that the Danion of the ionic PVB gel became smaller than that of the PVB gel (Danionic PVB < DanPVB), and the Dcation of the ionic PVB gel became larger than that of the PVB gel (Dcaionic PVB > DcaPVB) for each EC fraction. The difference between Danionic PVB and DanPVB or Dcaionic PVB and DcaPVB increased with increasing EC fraction. This was more pronounced in the 6.2 mol % ionic PVB gel. As a result, the lithium transport number, tLi, of the ionic PVB gel became larger than that of the PVB gel, shown in Figure 6. These results prove that the substituted ionic group plays the part of enhancing Dcation as well as reducing Danion. The dissociation degree of the salt, x, was also increased by the substitution of the ionic group on the PVB polymer, shown in Figure 6. We then evaluated the individual interactive force responsible for the ionic mobility of the gels, van der Waals force between the ion (cation or anion) and surrounding neutral species (η), Coulombic force between the cation and anion (α), and Coulombic forces between the cation and OH− group (βcation) and anion and substituted ionic group (βanion). Figure 7 represents the microviscosities acting on the mobile ions in the PVB and ionic PVB gels as a function of the EC fraction of the gels. In both gels with and without the ionic group, η increased and α and βcation decreased with the EC fraction. An increase in η reflects the increase in the EC concentration because the van der Waals force would be dominant for the interaction between the mobile ion (cation or anion) and polar EC species.16 η was independent of the presence of the ionic group on the PVB polymer of the gel. This is consistent with the result that η is independent of the OH− group concentration in the PVB for

Figure 5. Diffusion coefficients of the cation (red marks) and anion (blue marks) of the PVB gel (○) and ionic PVB gels (▲, 1.4 mol %; ■, 6.2 mol %).

mobility by Einstein’s relationship14 of gel electrolytes. This result shows three characteristic features. One is that Dcation and Danion increased with the EC fraction of the solvent for both types of gels. This feature reflects a decrease in the interactive force on the mobile ions that determine the ionic mobility based on changes of the solvated lithium structure, Li(EC)n+. 6067

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REFERENCES

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Figure 7. Microviscosities attributed from the van der Waals interaction (η, black marks) and Coulombic interactions between the mobile cation and anion (α, blue marks) and the ion and polar group (βcation, red marks; βanion, orange marks) in the PVB gels (○) and ionic PVB gels (▲, 1.4 mol %; ■, 6.2 mol %). The inset is an expansion of the range of 10 to 30 vol% of EC/(EC + DMC).

the PVB gel electrolytes in Figure 3. However, decreases in α and βcation with the EC content for both gels are associated with the solvation condition of the lithium in the gel. The solvating layer of EC around the Li+ ion would be a barrier, suppressing the Coulombic interactions from the TFSI− anion (responsible for α) and OH− site (responsible for βcation) on the Li+. The number of the solvating species changes from 0 to 3 or 4 with an EC fraction change from 0 to 30 vol%.5,15,17 The larger the number of the solvating EC species, the stronger the shielding effects against the Coulombic interactions of the polar groups and anions become. It should be noted that the α of the ionic PVB gels became lower than that of the PVB gels with increases in the EC fraction. This change is in response to the increase of βanion, i.e., the attraction between TFSI−/ionic group reduced α. The simultaneous attractions between Li(EC)n+/OH− and TFSI−/ionic group lead to increased independency of the cation and anion. As a result, Dcation increased because of a decrease in α, and the Danion decreased because of a new attractive effect: βanion in the ionic PVB gel compared to the Dcation and Danion of the original PVB gel. It is reasonable that βanion and α of the 6.2 mol % ionic PVB gel are respectively larger and smaller than those of the 1.4 mol % ionic PVB gels because the higher content of the ionic group could strongly attract the anion by promoting α reduction. In conclusion, we prepared a new polymer gel electrolyte, ionic PVB gel, by substituting the ionic group on the PVB. The Lewis acidic ionic group selectively attracted the anion to restrict Danion. Furthermore, Dcation of the ionic PVB gel increased compared to the PVB gel without the ionic group. This is attributed to the decrease in the cation/anion interaction owing to the simultaneous attractions between Li(EC)n+/OH− and the TFSI−/ionic group in the ionic PVB gel.

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AUTHOR INFORMATION

Corresponding Author

*(Y.S.) Tel: +81-72-751-4527. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 6068

dx.doi.org/10.1021/jp412611a | J. Phys. Chem. C 2014, 118, 6064−6068