Ion Transport in Separator Membranes of Lithium Secondary Batteries

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Ion Transport in Separator Membranes of Lithium Secondary Batteries Yuria Saito, Wataru Morimura, Rika Kuratani, and Satoshi Nishikawa J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b00085 • Publication Date (Web): 12 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

Ion Transport in Separator Membranes of Lithium Secondary Batteries Yuria Saito,a* Wataru Morimura a Rika Kuratani,b and Satoshi Nishikawab

a

National Institute of Advanced Industrial Science and Technology

1-8-31, Midorigaoka, Ikeda, Osaka 563-8577 Japan

b

Teijin Limited, 2-1, Hinode-cho, Iwakuni, Yamaguchi 740-8511 Japan

Corresponding Author Yuria Saito* 1-8-31, Midorigaoka, Ikeda, Osaka 563-8577 Japan Tel. +81-72-751-4527 e-mail: [email protected]

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ACS Paragon Plus Environment


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ABSTRACT The migration properties of cations and anions in lithium electrolyte solutions through separator membranes were evaluated on the basis of diffusion coefficients. The observed diffusion coefficients for the cationic (DLi), anionic (DF), and solvent species (DH) of the electrolyte solution in the membrane are lower than those of the free electrolyte solution. Two main effects are responsible for the reduction in D. One is the physical barrier effect of the membrane substrate. The magnitude of this effect depends on the size, configuration, and the total volume of the pore spaces which hold the solution. The other is a chemical interaction effect, which is associated with polar groups or sites in the membrane substrate. For example, DF values are anomalously lower than DLi values for solutions in polyvinylidene difluoride (PVDF)-based membranes, leading to an increase in the apparent cation transport number. This would indicate that the anions selectively interact with the membrane substrate through a Coulombic effect contributed by the PVDF chemical structure. These results suggest that separator membranes could be intentionally designed to control the structural stability and mobility of ionic species in the electrolyte, which underpin the output performance of battery systems.

KEYWORDS:

Lithium electrolyte solution, Separator, Diffusion coefficient, Microviscosity,

Coulombic and van der Waals interactions 2


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INTRODUCTION

In a lithium secondary battery system, the separator membrane electrically insulates the anode and cathode, while also containing the electrolyte which is responsible for ion transport between the electrodes.1,2 Both the quantity and ionic mobility of the electrolyte held within the separator membrane dominate the output performance of a battery equipped with the membrane. A separator membrane typically has a porous morphology to efficiently retain the electrolyte solution. Ion migration properties in the membrane are associated with the porosity, size, and configuration of its pores, as well as its structural changes upon swelling (if any). The ionic conductivity and mobility of the electrolyte in the membrane increase with increasing porosity and pore size; with higher porosity, the membrane can hold more carrier ions, and larger pore sizes expand the pathways for ion transport and decrease resistance. When the membrane is swollen with electrolyte, a fine network of solution held around the fibers or polymer chains comprising the membrane is the dominant ion transport pathway; this is different from the coarse network of linked sub-micron- to micron-sized pores in porous membranes. Furthermore, the chemical structure of the membrane also affects the ion migration properties. Polar sites on the membrane substrate can interact via Coulombic interactions with ionic species in the electrolyte, possibly enabling a selective change in ionic mobility. This is analogous to the case of polymer gel 3



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electrolytes in which specific polar groups on the polymer chain can control the mobility of a desired ion.3,4 We conjectured that it would be significant to investigate the mechanism of ion migration for an electrolyte in a separator membrane in terms of its chemical and morphological features in order to design and optimize electrolyte systems (electrolyte + separator) for lithium secondary batteries. We studied the diffusion coefficients of mobile species as an index for the systematic evaluation of ion transport properties in the membranes. We previously confirmed that the diffusion behavior of an ionic species in a membrane reflects its porous morphology.5 The diffusion coefficient of the electrolyte solution in a membrane showed a distribution characterized by an average diffusion value D0 with a standard deviation σ. This was attributed to the distributions of the uneven pore size and the tortuosity of the migration pathway that is determined by the way of pore connection in the membrane; a species’ diffusion rate depends on the size and configuration of the pores it encounters. We propose herein a new approach for the evaluation of ion transport properties in separator membranes based on diffusion coefficient ratios. We reveal that the behavior of the ions and their mobility in the membrane depend not only on the micron-scale morphology but also the nano-scale chemical structure of the membrane associated with site-specific interactions with the 4


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mobile ions. It is expected that the systematic control of the physical and chemical structure of the membrane would facilitate increased degrees of salt dissociation, ionic mobility, and chemical stability of the electrolyte in the membrane, which would directly lead to enhanced battery performance.

EXPERIMENTAL SECTION

We used four types of polymer membranes: polypropylene (PP) purchased from Cellugard Co., and polyethylene (PE), polyvinylidene difluoride-coated polyethylene (PVDF-PE), and polyvinylidene difluoride (PVDF) single membranes, which were produced by Teijin Co. The fundamental properties of the membranes are listed in Table 1. For diffusion coefficient measurements, samples were prepared as follows. Around 10 to 20 membrane sheets were stacked and punched into disks, 4 mm in diameter and 5 mm in height, and then dried in air atmosphere at 60 °C for 24 h. A dried sample was placed into a 5 mm NMR tube such that the plane of the film was perpendicular to the longitudinal direction of the tube, in order to measure the diffusion coefficient in the direction perpendicular to the plane of the membrane. The electrolyte solution (1 M LiPF6 in 1:1 (v/v) ethylene carbonate (EC):diethyl carbonate (DEC)) was then introduced into the membrane in the NMR tube in two ways. The first involved dropping a fixed quantity of the 5



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electrolyte solution (volumes of around 100 or 200% versus the total pore space of the membranes in the NMR tube) on the membrane stack to allow the solution to absorb naturally into the membrane. We refer to these samples as the 100% and 200% solution samples hereinafter. In the preparation process, the membrane stack was separated into three to four sub-stacks, and almost the same amount of the solution was dropped on each sub-stack to avoid the solution localization in the membranes. The solution volume in the membrane stack in a NMR tube was estimated from the weight change between the dry and solution-penetrated membrane stack and the solution density. In the second, the membrane stack with a large excess of electrolyte solution was compressed under vacuum and restored to atmospheric pressure several times in the sample tube to accelerate solution penetration and complete the filling of the pore spaces in the membranes. We refer to this sample as the excess solution sample hereinafter. These treatments were performed in a dry room with a −60 °C dew point to restrict the admixture of water, which is a principal factor in electrolyte degradation.

6


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Table 1. Physical Properties of the Investigated Membranes polypropylene (PP)

polyethylene

PVDF-coated

(PE)

PE (PVDF-PE)

Thickness / µm

25

8.7

10.9

10.5

Basis Weight / gm-2

14.1

5.40

7.42

18.4

34 0.12

37 0.10

property

Porosity / % Average pore size

41 0.043

PVDF

7 -

/ µm

Diffusion coefficients DLi, DF, and DH were measured at 25 °C for the probed nuclear species 7Li (116.8 MHz),

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F (282.7 MHz), and 1H (300.5 MHz), respectively, using the pulsed gradient

spin-echo (PGSE) NMR technique with a JNM-ECP300W wide-bore spectrometer (JEOL Co. Ltd.).6 DH was calculated from the 1H peak of the DEC component of the binary solvent. A Hahn-echo pulse sequence was used for the measurements. A half sine-shaped gradient pulse was applied twice in the sequence after the 90° and 180° pulses to detect attenuation of the echo intensity, according to the diffusion of the probed species.7,8 The diffusion coefficients were measured in the direction perpendicular to the plane of the membranes, which corresponds to the dominant direction of ion migration in a battery system. The typical values of the parameters for the pulse sequence were g = 2–4 T/m for the strength of the gradient pulse, δ = 0–7 ms for the pulse

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width, and ∆ = 50–100 ms for the diffusion time corresponding to the interval between the two gradient pulses. THEORETICAL DERIVATION—Rationale for the Diffusion Coefficient Changes by Solution Insertion into the Separator Membrane In order to logically illustrate the differences in the diffusion coefficients of the species between the inside and outside of the separator membrane, we examined the microviscosities of the mobile species in the electrolyte solution responsible for ionic mobility.4,9 According to the Stokes-Einstein relationship, the inherent diffusion coefficients, Dsolv, Dca, and Dan, of an electrolyte solution without any restriction can be represented as =

η

=

=

(ηα)

(1)

(ηα)

where C = kT/6π (k, Boltzmann constant; T, absolute temperature); rsolv, ran, and rca are the radii of the solvent, anion, and cation species, respectively; and h = ran/rca.4 η and α are the microviscosities contributed from the van der Waals interactions with the surrounding species and the Coulombic interactions between the cation and anion species, respectively.4,9 When the

8


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solution stays in the pore spaces and/or penetrates the membrane during swelling, the ions can migrate through solution pathways in the linked pore network and/or the swollen regions of the membrane substrate. The ionic mobility in the membrane is generally lower than that in the free solution due, in general, to the physical barrier effect and chemical interactive effect of the insulating membrane substrate present in the migration field. From a microscopic point of view, it is reasonable to suppose that the membrane effects are reflected in the microviscosities acting on the migrating species. As a result, the diffusion values of the species would be affected as ′ =

η

′ =

′ =

(ηα)

(2)

(ηα)

η′ and α′ are the microviscosities of the species in the membrane, and differ from η and α in the free solution. The diffusion value ratios, D′solv/Dsolv, D′an/Dan, or D′ca/Dca, allow convenient evaluation of the microviscosity changes upon solution insertion in the membrane. From eqs. 1 and 2,

= η η

η α

= ηα

(3)

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ηα

= η α

We can find from these relationships that the diffusion value ratios are functions of the microviscosities of the species. The differences in the ratios can be represented as −

= η − η α = η − α × ηα

−

= η − α × η α (4)

−

η α

η

η

η

α

α

α

α

= (" − 1)ηα η − α η

α

where h = ran/rca and is almost equal to 0.5 for LiPF6 (based on the van der Waals radii of Li(EC)4+ (4.95 Å) and PF6− (2.56 Å)).10,11 These results show that order among the ratios D′solv/Dsolv, D′an/Dan, and D′ca/Dca depends on the order between η/η′ and α/α′. We then consider the correlation between η/η′ and α/α′. The causes of the microviscosity changes of the species upon solution insertion in the membrane (η → η′, α → α′) can be categorized into two elements. One is the generation of new interactive forces between the ions and the membrane substrate, and the other is the change in the original interactive forces between the species due to their location change, from the free solution to the restricted space of the membrane. For the former, the physical barrier of the pore walls acts as a new component of the van der Waals effect on the species, adding to the 10


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microviscosity of the species in the membrane. If polar sites are present on the membrane surface, electrostatic interactions can act on specific ions, adding to the microviscosity through the Coulombic effect. On the other hand, the spatial constraints for the species in the membrane affect the existing (original) microviscosities, η and α in the free solution. This is because the average distance between the species decreases in the membrane due to the restriction of their movement in the pore spaces. This situation increases the original microviscosities derived from van der Waals and Coulombic interaction components because the interaction forces vary with the distance between the species. Here, we assume that the membrane substrate does not have specific polar sites (i.e., it is chemically inert) and only supplies a physical barrier to the species in solution in the membrane. Such would be the case for the polyolefin and PVDF-related membranes used in this research, in which the new microviscosity component, derives only from the van der Waals interaction responsible for the physical barrier, leads to η/η′ < α/α′. On the other hand, the microviscosity change due to the reduced distance between the species in the membrane has contributions from both changes in the van der Waals force (Fv) and Coulombic force (FC). According to Stokes’ law, η and α are in proportion to Fv (= 6πrην,) and FC (= 6πrαν) where ν is the drift velocity, which are proportional to the inverse seventh power 11



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and the inverse square power of the distance between the species, respectively. Therefore, the changes in microviscosity with distance, η(r)/η(r − ∆r) and α(r)/α(r − ∆r), can be represented as,

η($)

η($ − ∆$)

=

& /($ − ∆$) ($ − ∆$)( = &′ /$ $( (5)

)($) & /($ − ∆$) ($ − ∆$)* = = &′ /$ $* )($ − ∆$)

As (r − ∆r)/r < 1, we can accept the relationship for the change in the original microviscosities as

η(r)/η(r − ∆r) < α(r)/α(r − ∆r)

(6)

That is, the existing microviscosity change from the van de Waal interaction is larger than that from the Coulombic interaction. As a result, considering the contributions from the generation of the new component and the change in the original component, the rate of η′ increase is larger than that of α′, and η/η′ is smaller than α/α′. This leads to the correlation between the diffusion value ratios of eq. 4 as

D′H/DH < D′ca/Dca < D′an/Dan

(7)

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Conversely, if polar sites attractive to the ionic species are present on the membrane substrate, eq. 7 would not hold.

RESULTS AND DISCUSSION Behavior of the Solution and Ion Migration in the Membrane. Figure 1 is the 7Li spectra of PP membranes exposed to the electrolyte solution at 100 and 200% volumes against the total pore space, as well as a large excess volume following the deaeration process described in the experimental section. These spectra were typical of a non-swollen porous membrane. The 7Li spectrum of the 100% solution sample showed a broad peak. As the amount of solution increased to 200%, the shape of the spectrum and the peaks were sharpened, and appeared similar to those of the excess solution sample. The apparent spectral differences between the 100% and greater than 100% volumes may be attributed to differences in the absolute quantity and the manner in which the solution occupies the pore membranes. In other words, in the 100% solution sample, the solution is distributed but localized in the larger pore spaces of the membrane, leading to peak broadening. In contrast, most of the pore spaces in the 200% and excess solution samples are fully occupied by solution, forming a stabilized, filled membrane. It was expected that the repeated evacuation process for the excess solution sample would accelerate occupation of the 13



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pores and achieve an equilibrium, solution-filled membrane structure. We assigned the highest peaks for the 200% and excess solution samples to the solution in the pore spaces, and other sub-peaks to external solution, such as in the membrane interlayer spaces, because the diffusion values estimated from the two peaks were apparently different, reflecting a difference in the environment around the species. It is expected that the connectivity through the pores filled with solution is poor in the 100% solution sample, compared with those in the larger solution samples in which most pores are linked to form a solution network appropriate for long range and fast ion transport.

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100

(a) 100 % 50 0

Intensity/a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200

(b) 200 %

100 0 400

(c)deaerated

300 200 100 0 2

1

0

-1

-2

-3

-4

chemical shift/ppm

Figure 1.

7

Li NMR spectra of the lithium electrolyte in the PP membrane for different volumes

of introduced solution: (a) 100%, (b) 200%, and (c) a large excess of solution following several decompression/restoration cycles to achieve complete pore filling. The solution volumes are based on the pore space volume of the membrane.

Figure 2 shows the aging profiles in the 7Li spectra of the PP and PVDF membranes in the presence of excess solution after the deaeration process. For the PP membrane, the spectral shape 15



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did not change even after 8 d, in contrast to the apparent change in the spectral shape of the PVDF membrane sample over the same period. This can be attributed to the difference in the absorbency properties between the two membranes. The PP membrane exhibits no swelling, and the solution is held in the pore spaces without any significant change in pore size or morphology. For the less porous PVDF membrane, on the other hand, the solution infiltrates into the nanoscale spaces between the polymer chains and swells the membrane substrate. It is reasonable to assume that a specific amount of time would be necessary to reach the equilibrium state of the swollen structure because the process is accompanied by morphological changes in the cross-linked polymer chain networks. These differences would account for the variations observed with aging between the PP and PVDF membranes. Based on these results, we measured the diffusion values for all the samples ~8 d after preparation in order to ensure sufficient equilibration.

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70

600

1day

60 50

500

40

400

30

300

20

200

Intensity/a.u.

Intensity/a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10 0 50

8day

40 30

1day

100 0 2500

8 days

2000 1500

20

1000

10

500

0 2

1

0

-1

-2

-3

-4

0 2

chemical shift/ppm

1

0

-1

-2

-3

-4

chemical shift/ppm 2a

2b

Figure 2. Changes in the 7Li spectra of the solution upon aging in the (a) PP and (b) PVDF membranes.

Diffusion Distribution In the random walk model, a diffusing species, measured by the NMR spin-echo method, follows the relationship, + = +, -./0−1 2 3 2 42 (4∆ − 3)5 62 7

(7)

where M is the measured NMR echo intensity, M0 is the initial echo intensity, γ is the gyromagnetic ratio, δ is the gradient pulse width, g is the strength of the gradient pulse, and ∆ is the diffusion time, which are the parameters of the pulse sequence applied for diffusion 17



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measurement.14 As the variable parameters δ and ∆ increase, M is attenuated. According to eq. 7, a plot of log M vs. δ2(4∆ −δ) should be linear, with the slope corresponding to the diffusion coefficient, D. However, most plots for the solution in the membrane show curves that deviate from the expected linear behavior. A curve in the plot of echo attenuation reflects the distribution of the diffusion values according to the pore size distribution of the membrane.5 By adding the distribution function to the fundamental echo attenuation of the normal diffusion of eq. 7, we can represent the migration of the species in the membrane as follows. + = +, 8 9()-./0−1 2 42 3 2 (4∆ − 3)5 62 7:

(8)

( − , )2 1 9() = ; -./ >− ? 25= 2 2= 2 where f(D) is the Gaussian distribution function, D0 is the average diffusion coefficient corresponding to the maximum point of the diffusion distribution function, and σ is the standard deviation indicating the width of the distribution function. We can then apply eq. 8 to evaluate the NMR echo attenuations of all species in the separator membranes.

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f(D) / 1E10

3

100 % 200 % deaerated

2

1

0 -1

0

1

2

D / 1E-10 m2s-1

3a

100 % 200 % deaerated

3

f(D) / 1E10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

1

0 -1

0

1

2

3

D / 1E-10 m2s-1

3b Figure 3. DLi distribution functions of the solutions in a) PP and b) PE membranes with 100% and 200% solution volumes, as well as the deaerated sample with excess solution.

Figure 3 shows the diffusion distribution functions of the lithium species for varying solution volumes against the pore space of the membranes in the sample tubes. For the PP and PE

19



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membranes, D0 and σ increase concomitantly with the solution volume. The diffusive behavior of the species in the membrane represented by D0 and σ reflects, as also evident from the spectral shape, the solution occupation behavior. In the sample with 100% solution insertion, the larger pores are preferentially and partially occupied by the solution, forming a diffusion domain for mobile species. In this case, D0 would be lower because the solution filling the larger pores is isolated with poor connectivity, which is disadvantageous for the long-range migration and high diffusivity of the mobile species. The diffusion distribution σ would be narrower because the diffusion domain is limited to the specific larger-sized pores containing the solution. As additional solution is introduced into the membrane, most pores will become fully occupied regardless of size, forming an expanded network of linked, solution-filled pores. Then, D0 will increase as the expanded network of long-range pathways promotes the fast transport of carrier species. At the same time, σ will grow wider, reflecting the differently sized pores filled with the solution and the different degrees of tortuosity in the migration pathways. Factors Determining the Diffusion Coefficient of the Solution in the Membranes. To clarify the rationale behind the diffusion coefficient change upon solution insertion into the membrane, we compared the diffusion coefficient ratios, D′Li/DLi, D′F/DF, and D′H/DH, based on the theoretical considerations in the previous section. We applied the observed diffusion values, 20


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DLi and DF, in place of the inherent diffusion values, Dca and Dan, for the evaluation of the diffusion ratios. In practice, it would be best to use Dca and Dan to explicitly evaluate the membrane effects on ionic mobility, because DLi and DF have contributions from the associated ion pair represented by DLi = xDca + (1 − x)Dpair, where x is the dissociation degree of the salt.3 However, we know empirically that Dca and Dan are correlated with DLi and DF, respectively. Furthermore, the discussion based on the diffusion coefficient ratios is significant for the samples for which it is impossible to directly estimate Dca and Dan, which lead to the microviscosities η and α.4,9 Table 2 shows the average diffusion values (DLi0, DF0, and DH0) for each sample, obtained by fitting the NMR echo attenuation data to eq. 8. For the PVDF-PE and PVDF single membranes, we obtained two DF0 values as a result of the split peaks of 19F, as shown in Figure 4. The DF0 from the additional side peak was apparently lower than that from the original main peak. The new side peak may be attributed to the anion species affected by the PVDF substrate.

21



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Table 2. Observed Diffusion Coefficients, Dli, DF, And DH, Associated with the Cation, Anion, and Solvent Species, Respectively, of the Free Lithium Electrolyte Solution and the Solution in Each Membrane The numbers in parentheses represent the ratio of the diffusion coefficient in the membrane to that in the free solution.

σF

0.33

-

4.5E-11

0.35

34

2.45E-10 (0.81)

6.5E-11

0.33

31

2.06E-10 (0.70)

3.1E-11

0.37

37

Electrolyte solution

1.43E-10

PP

8.56E-11

4.2E-11

1.57E-10

4.7E-11

2.39E-10

PE

7.53E-11 (0.54)

5.0E-11

1.51E-10 (0.65)

6.9E-11

PVDF coated PE

1.00E-10 (0.69)

3.8E-11

1.66E-10 (0.58) 1.30E-10 (0.46)

1.5E-11

1.42E-10 (0.61) 1.25E-10 (0.54)

5.1E-11

1.20E-10 (0.86)

DF

porosity

DLi

PVDF

σLi

DLi/(DLi+DF)

Sample

2.85E-10

5.1E-11

DH

σH

2.93E-10

8.1E-11

4.6E-11

0.43 2.55E-10 (0.85)

22


6.1E-11

0.46 0.49

7

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Intensity / a.u.

Page 23 of 31

a

electrolyte solution

b

PP

c PE

derived from PVDF

PVDF-PE

d derived from PVDF

PVDF

e

-74

-76

-78

-80

-82

chemical shift/ppm

Figure 4.

19

F spectra of the (a) free electrolyte solution, and electrolyte-infiltrated (b) PP, (c) PE,

(d) PVDF-PE, and (e) PVDF single membranes.

The apparent lithium transport numbers, tLiapp (= DLi0/(DLi0 + DF0)), associated with the PVDF substrates were larger compared with those of the free solution, PP, and PE-based membranes. It is

23



reasonable to assume that there is some specific interaction between the PVDF and ionic species in PVDF-related membranes, as reported for PVDF-based polymer gel electrolytes.15 To investigate this phenomenon in more detail, it would be useful to compare the diffusion coefficient ratios of the cation, anion, and solvent species, as shown in Figure 5. In the case of PP and PE membranes without PVDF (Figure 5(a)), D′H/DH is apparently larger than D′Li/DLi and D′F/DF. In accordance with the predicted theoretical derivation, this result suggests a specific increase in the Coulombic interaction between the cation and anion species in the membrane. The increased Coulombic interaction seems to indicate a decrease in the dissociation degree of the lithium salt dissolved in the solution in the membrane.

0.85

PE PP

0.80 0.75

D' / D

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.70 0.65 0.60 0.55

DF/DF0

F

DLi/DLi0

Li

A

DH/DH0

H

5a

24


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0.90 0.85

PVDF-PE PVDF

0.80

D' / D

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.75 0.70 0.65 0.60 0.55 0.50

DF/DF0

F

DLi/DLi0

DH/DH0

Li A

H

5b

Figure 5. Diffusion coefficient ratios of the cation (Li), anion (F), and solvent (H) species of the solution in the membrane to that in the free solution, D′/D, for a) PE and PP membranes and b) PVDF-PE and PVDF membranes.

In the cases of diffusivity in the PVDF-PE and PVDF single membranes, represented in Figure 5(b), D′F/DF is anomalously lower than D′Li/DLi and D′H/DH, and deviates from that expected from eq.7. This behavior is also different from that observed with the polyolefin membrane. The lower values of D′F/DF compared to D′Li/DLi cannot be explained by changes in the Coulombic interaction between the cation and anion. It is reasonable, then, to consider that the anions 25



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selectively interact with the membrane substrate, such that the anion migration is restricted in the membrane. This is supported by our previous result that the cation transport numbers of the lithium polymer gel electrolytes using the PVDF with lower porosity showed tLi > 0.5 which is larger than tLi of the lithium electrolyte solution15. Although the effect of the PVDF substrate on salt dissociation is not clear, the comparable values of D′H/DH and D′Li/DLi, in contrast to D′F/DF, show that the cation/anion interaction is not strong enough to prevent the salt dissociation as observed in the polyolefin membranes. From these results, we can assume that the ionic mobility of the species in the membrane is influenced by several factors. Restricted pore space in the membrane would reduce the distance for the random walk migration of species, leading to reduced mobility, which in turn, lowers the extent of salt dissociation. Polar membrane sites may interact with a specific ion, selectively reducing the ionic mobility in the membrane. The evaluation and comparison of the diffusion coefficient ratios, D′/D, for the polyolefin and PVDF membranes demonstrated characteristic interactions that support this idea. In conclusion, the electrolyte for a battery system should be evaluated in the presence of the separator membrane. This is because the ion transport properties of the electrolyte are influenced by the morphological and chemical features of the membrane substrate. This significantly 26


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suggests that the combination of a membrane with high porosity and an electrolyte with high conductivity does not always afford the best performance for lithium secondary batteries.

27



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REFERENCES

1. Arora, P.; Zhang, Z. Battery Separators. Chem. Rev. 2004, 104, 4419–4462.

2. Zhang, S.-S. A Review on the Separators of Liquid Electrolyte Li-Ion Batteries. J. Power Sources 2007, 164, 351–364.

3. Saito, Y.; Kataoka, H.; Murata, S.; Uetani, Y.; Kii, K. Designing of a Urea-Containing Polymer Gel Electrolyte Based on the Concept of Activation of the Interaction between the Carrier Ion and Polymer. J. Phys. Chem. B 2003, 107, 8805–8811.

4. Saito, Y.; Okano, M.; Kubota, K.; Sakai, T.; Fujioka, J.; Kawakami, T. Evaluation of Interactive Effects on the Ionic Conduction Properties of Polymer Gel Electrolytes. J. Phys. Chem. B 2012, 116, 10089–10097.

5. Saito, Y.; Hirai, K.; Emori, H.; Murata, S.; Kii, K. Carrier Diffusivity in Porous Membranes. J. Phys. Chem. B 2004, 108, 1137–1142.

6. Saito, Y.; Kataoka, H.; Capiglia, C.; Yamamoto, H. Ionic Conduction Properties of PVDF-HFP Type Gel Polymer Electrolytes with Lithium Imide Salts. J. Phys. Chem. B 2000, 104, 2389–2192.

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7. Tanner, J. E. Use of the Stimulated Echo in NMR Diffusion Studies. J. Chem. Phys. 1970, 52, 2523–2526.

8. Price, W. S.; Kuchel, P. K. Effect of Nonrectangular Field Gradient Pulses in the Stejskal and Tanner (Diffusion) Pulse Sequence. J. Magn. Reson. 1991, 94, 133–139.

9. Saito, Y.; Okano, M.; Sakai, T.; Kamada, T. Lithium Polymer Gel Electrolytes Designed to Control Ionic Mobility. J. Phys. Chem. C 2014, 118, 6064–6048.

10. Bondi, A. van der Waals Volumes and Radii. J. Phys. Chem. 1964, 68, 441–451.

11. Ue, M.; Murakami, A.; Nakamura, S. A Convenient Method to Estimate Ion Size for Electrolyte Material Design. J. Electrochem. Soc. 2002, 149, A1385–A1388.

12. Atkins, P. W. In Physical Chemistry; Oxford University Press: Oxford, UK, 2000, p. 661.

13. Bocris, J. O.; Reddy, A. K. N. In Modern Electrochemistry; Bockris, J. O., Ed.; Plenum Press: New York, NY, USA, 1998; p. 453.

14. Price, W. S.; Hayamizu, K.; Ide, H.; Arata, Y. Strategies for Diagnosing and Alleviating Artifactual Attenuation Associated with Large Gradient Pulses in PGSE NMR Diffusion Measurements. J. Magn. Reson. 1999, 139, 205–212. 29



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15. Saito, Y.; Kataoka, H.; Quartarone, E.; Mustarelli, P. Carrier Migration Mechanism of Physically Cross-Linked Polymer Gel Electrolytes Based on PVDF Membranes. J. Phys. Chem. B 2002, 106, 7200–7204.

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TABLE OF CONTENTS IMAGE

10

Diffusivity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Li

5

Li 0

anode

cathode electrolyte in separator

PP

PE PVDF-PE

31


Ion Transport in Separator Membranes of Lithium Secondary Batteries

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