A Selective Interaction Between Cation and Separator Membrane in

Article Cite This: J. Phys. Chem. C 2017, 121, 23926-23930

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A Selective Interaction between Cation and Separator Membrane in Lithium Secondary Batteries Yuria Saito,*,† Sahori Takeda,† Wataru Morimura,‡ Rika Kuratani,‡ and Satoshi Nishikawa‡ †

National Institute of Advanced Industrial Science and Technology, 1-8-31, Midorigaoka, Ikeda, Osaka 563-8577 Japan Teijin Limited, 2-1, Hinode-cho, Iwakuni, Yamaguchi 740-8511 Japan

‡

ABSTRACT: The presence of a specific Coulombic interaction between the lithium cation and the separator membrane in lithium secondary batteries was proven in this study, through evaluating the mobilities and microviscosities of the mobile ions in electrolyte solutions outside and within the separator membrane. The magnitude of the interaction depends on the solvation structure of the lithium cation, whose net charge is affected by the type and number of solvating species due to their shielding effect. Lithium cations with larger or more strongly coordinated solvating species are less attracted to the membrane; therefore, they display higher mobility compared to weakly solvated cations that are more strongly attracted to the membrane. We confirmed that the mobility of lithium cations in the separator membrane is controlled by their solvated structure in the electrolyte, as well as by the surface charge of the separator membrane. This knowledge could lead to the systematic design of battery performance appropriate for the battery loading system.

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INTRODUCTION Porous separator membranes are used in lithium secondary battery devices, in order to separate the cathode from the anode to prevent short circuits and to keep the electrolyte inside the battery to act as a medium for lithium cation transport. To improve the battery power, the mobility of lithium cations in both the electrolyte and the separator membrane must be increased, because the ionic mobility governs the high-rate charge−discharge performance of the battery system.1,2 To increase the cation mobility, the influence of the membrane on ion migration within its porous spaces must be systematically investigated. In a previous research, we hypothesized that the ionic mobility is dominated by interactions between the ion and the separator membrane, which in turn are associated with the chemical and morphological features of the membrane.3 Specifically, the cation mobility is affected by the surface charge within the pores in the membrane,4 and the anion mobility is influenced by the tortuosity of the ion transport pathways composed of linked pores. A Coulombic interaction between the surface charge of the pore walls and the cationic species was observed in membranes with higher porosities, contrary to the effect of the pathway tortuosity observed for anionic species in membranes with lower porosities. As a result, the difference between the two ion mobilities (Dcation and Danion) was increased in the more porous regions of the membrane, leading to a reduced cation transport number. Therefore, it is expected that the charged layer on the walls of the membrane pores would be negative, due to its selective attraction of cations. It is accepted that, in the equilibrium state of lithium salt dissociation, lithium cations are solvated rather than isolated. © 2017 American Chemical Society

The solvation structure of lithium depends on the salt concentration, the type of solvent, and the solute composition in the cases of binary or ternary solvents.5−7 The strength of the Coulombic interaction between the cation and the membrane would depend on the solvation structure of the cationic species as characterized by the solvated cation size and the solvation number.8,9 This is because the solvation provides different levels of shielding to the cationic charge, depending on the number and size of the solvating species as well as the strength of their coordination. In this research, we evaluated the ionic mobilities and microviscosities to reveal the interactions of ions in the electrolyte solutions and polyethylene (PE) separator membranes by changing the solvation structure of lithium. The results proved the presence of a Coulombic interaction between the cations and the PE membrane, which affected the cation mobility in the membrane. This is part of a series of studies of the Coulombic interactions in separator membranes, an issue that has not been examined before. The results will enable the control of ionic mobilities in the separator, with logical design of the electrolyte solution and membrane structures.

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EXPERIMENTAL AND THEORETICAL METHODS Experimental Methods. We prepared two lithium electrolyte solutions: L1 and L3 contained 1 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) mixed in volume ratios of EC:DEC = 1:1 and 1:6.9, respectively (Kishida Chemical Co. Ltd.). We used 11 different PE membranes with Received: July 17, 2017 Revised: September 15, 2017 Published: October 10, 2017 23926

DOI: 10.1021/acs.jpcc.7b07056 J. Phys. Chem. C 2017, 121, 23926−23930

Article

The Journal of Physical Chemistry C

experiment, which probes an averaged environment. As a result, the measured diffusion coefficients (DLi, DF, and DH; probed by the 7Li, 19F, and 1H nuclear species, respectively) can be defined with their inherent diffusion coefficients (Dcation, Danion, and Dpair) as shown in eq 1,

various porosities and pore sizes. The PE membranes were prepared based on the general wet process, using liquid paraffin to form the porous structure.10,11 The porosity of the membrane was estimated from the ratio between the bulk density and true density of PE. The pore size was determined using the specific surface area measured by the Brunauer− Emmett−Teller (BET) method, assuming that the pores were columnar in structure.12,13 The range of pore size was 0.04−8.4 μm, and the porosity was 34−85%. With increasing porosity, the pore size may have increased as shown in the previous report.3 For measuring the NMR spectra and diffusion coefficients of the species in the solution within the membrane, dried and stacked membrane sheets were placed in an NMR sample tube (ϕ = 5 mm), such that the plane of the films was perpendicular to the longitudinal direction of the sample tube. Then, an electrolyte solution was introduced into the membrane, followed by several repetitions of compression under vacuum and restoring to atmospheric pressure in order to completely fill the pore spaces with the electrolyte solution. The detailed composition of the solution used for filling the pore space is explained in the previous report.3 The diffusion coefficients DLi, DF, and DH of the probed nuclear species7Li (116.8 MHz), 19F (292.7 MHz), and 1H (300.5 MHz), respectivelywere measured at 25 °C using the pulsed gradient spin-echo (PGSE) NMR technique with a JNM-ECP300W wide-bore spectrometer (JEOL Co. Ltd.).14 A Hahn-echo pulse sequence was used for the measurements. A half sine-shaped gradient pulse was applied twice in succession after the 90° and 180° pulses, in order to determine the attenuation of the echo intensity with the diffusion of the probed species.15,16 The diffusion coefficients were measured in the direction perpendicular to the plane of the membranes, i.e., the dominant ion migration direction in a battery system. The typical pulse width (δ) and diffusion time (Δ) for the pulse sequence were 0−7 and 20 ms, respectively. This diffusion time was fairly short compared to those determined from measurements of conventional electrolyte solutions and gel electrolytes, because the relaxation time of the target species was fairly short here due to the influence of the wall composed of linked pores along the ion migration pathways. The ionic conductivity of the solution in the membrane was measured by the impedance method using a frequency analyzer (model 1250) combined with a potentiostat (model 1287, Solartron). An ac voltage of 20 mV was applied in the frequency range of 1 mHz to 65 kHz. The conductivity cell was prepared by stacking a prescribed number of sheets (2−12) containing the solution, and sandwiching them between stainless steel (SUS) electrodes with a diameter of 15 mm. The sandwich was finally laminated and sealed. The cell resistance was plotted as a function of the number of membrane sheets, and the plot was found to be linear. Then, from the slope of the straight line passing through the origin, we estimated the ionic conductivity of the solution in the membrane. Outline of Theoretical Derivation. The diffusion coefficient of each species (Dcation, Danion, and Dpair) and the microviscosities originating from the interactions of the mobile species (η, α, and β) were estimated as follows. Under lithium salt dissociation equilibrium in the lithium electrolyte solution, there are dissociated cations and anions as well as associated ion pairs. Due to their rapid interconversion, they are not detected individually on the time scale of the NMR

DLi = xDcation + (1 − x)Dpair DF = xDanion + (1 − x)Dpair DH = DDEC = Dsolv

(1)

where x is the dissociation degree of the lithium salt in the solution. Dcation and Danion, which characterize the diffusion of single cationic and anionic species, are directly related to the cation and anion mobilities, respectively, according to the Einstein relation.17 On the other hand, ionic conductivity is the sum of cation and anion conductivities, which are functions of the carrier concentration (xN) and ionic diffusion coefficient, as shown in eq 2. σ=

e2 xN (Dcation + Danion) kT

(2)

where N is the salt concentration in the solution. As DH directly reflects the diffusion coefficient of the DEC species (Dsolv), the diffusion coefficient of the neutral ion pair (Dpair) is estimated using the sizes of the ion pair (rpair) and DEC (rDEC) based on the relationship of Dsolv/Dpair = rpair/rDEC, according to the van der Waals size and Stokes−Einstein equation.18,19 As a result, Dcation, Danion, and the dissociation degree of the salt (x) can be determined by solving the simultaneous eqs 1 and 2. Each of the inherent diffusion coefficients (Dcation, Danion, and Dsolv) is a function of the size and microviscosity of the corresponding species, according to the Stokes−Einstein relation as follows (eq 3). Dsolv =

kT 6πrDECη

Danion =

kT 6πranionη′

η′ = η + α

Dcation =

kT 6πrcationη″

η″ = η +

ranion α + βcation rcation

(3)

where η, α, and βcation are the respective microviscosities attributed to (1) the van der Waals interactions with the surrounding ionic and neutral species, (2) the Coulombic interaction between the cation and anion species, and (3) the selective interaction between the cation and attractive sites on the membrane, as shown in the diagram of Figure 1. When the anion interacts more strongly than the cation with the membrane, βanion has to be included in the equation for Danion, whereas βcation has to be removed from the equation for Dcation (eq 3). Therefore, by solving the simultaneous equations in eq 3 with the estimated Dcation, Danion, and Dpair, we could estimate the values of η, α, and βcation (or βanion).

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RESULTS AND DISCUSSION It is generally accepted that the most stable structure of solvated lithium requires tetrahedral coordination.5,6,20,21 In the case of typical binary solvents such as EC+DEC or propylene carbonate (PC)+dimethyl carbonate (DMC), the lithium cation is preferentially coordinated to the more polar solvent molecules (i.e., EC and PC), due to their lower coordination 23927

DOI: 10.1021/acs.jpcc.7b07056 J. Phys. Chem. C 2017, 121, 23926−23930

Article

The Journal of Physical Chemistry C

higher in L1, while, in the low-temperature region, Dcation became lower in L1 than in L3. This reversal can be attributed to the abrupt increase in the size of solvated cation (rcation) of L1 in the lower-temperature region, shown in Figure 2c, which reflects change in the solvation number of the lithium cation in L1 with temperature. In L1, it is reasonable to assume that secondary and tertiary solvation layers are formed when the temperature and mobility of the species are reduced. As the solvation size of the lithium cation increases, the Coulombic interaction between the cation and anion, characterized by α, is reduced due to the charge shielding effect of the solvating species, resulting in α(L1) < α(L3). On the other hand, the rcation value of L3 attributed to the Li(EC)(DEC)3+. The reason is that, after the solvation of the lithium cations, the free solvent in L3 is mostly the almost nonpolar DEC species, which cannot contribute to further solvation of the lithium cation. As a result, Li(EC)(DEC)3+ and Li(EC)2(DEC)2+ have weaker solvation coordination than Li(EC)4+, but interact more strongly with their surroundings. Therefore, L3 has a larger α value than L1, with the difference increasing at lower temperatures. Figure 3 represents the estimated values of Dcation, Danion, η, α, and the microviscosity attributed to the ion−membrane interaction (β) for L1 and L3 within the PE separator membranes with different porosities and pore sizes. The trends are very similar for L1 and L3. That is, Dcation reaches a maximum at around 50% porosity, while Danion shows a monotonous increase with increasing porosity. On the other hand, η and α of both solutions decrease with increasing porosity, as the van der Waals effect and cation−anion Coulombic effect on the ionic species both decrease when there is more space for ion migration and less frequent

Figure 1. Diagram of interactions of the ionic species in the pore space of the separator membrane. η is the microviscosity from the van der Waals interaction, α is from the Coulombic interaction between the charged ions, and βca is from the cation−membrane interaction which may be due to the charged layer on the wall of the pores.

energy.20,21 Considering the different molar factions of the solvent species against Li (Li:EC:DEC = 1:7.2:4.1 for L1 and 1:1.8:6.9 for L3), Li(EC)4+ would be the dominant solvated cation in L1, while Li(EC)(DEC)3+ and Li(EC)2(DEC)2+ would be preferred in L3. In L1, the solvating EC displays stronger coordination than that of DEC in L3, due to the higher polarity of the former. Because of the abundant EC in L1, the solvating EC species could rapidly exchange with free EC species at the equilibrium state of lithium salt dissociation. On the other hand, the Li-EC coordination in L3 would be maintained for a longer duration, because there is a shortage of free EC species for exchange. This difference in EC coordination condition affects the solvation size and consequently the mobility of the lithium cation. Figure 2 represents the estimated values of Dcation, Danion, and the microviscosities attributed to the van der Waals and cation−anion Coulombic interactions (η and α, respectively) in free L1 and L3 solutions. In the high-temperature region, both ionic mobilities were

Figure 2. Temperature dependences of (a) Dcation and Danion, (b) η and α, and (c) rcation and x in L1 and L3 solutions without the separator membrane. 23928

DOI: 10.1021/acs.jpcc.7b07056 J. Phys. Chem. C 2017, 121, 23926−23930

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The Journal of Physical Chemistry C

Figure 3. (a) Dcation and Danion, (b) η and α, (c) βcation and βanion of L1 and L3, and (d) βcation of L3 in PE separator membranes as functions of porosity (a−c) and pore size (d). A porosity of 100% means the free electrolyte solution without membrane.

collisions among the mobile species. The higher value of α for L3 than for L1 is due to the smaller size of the solvated lithium cations in L3, similar to the observations in the free electrolyte solutions. The anomalous decrease in Dcation after the maximum value reflects the specific interaction between the membrane and cationic species, which leads to the microviscosity of the cation (βca). It is also notable that Dcation is lower in L3 than in L1 across the entire porosity range. This difference is likely associated with the different changes in the β values with increasing membrane porosity. In our previous research, we found that the specific interaction between the anion and the membrane in L1 (characterized by βan) could be attributed to the high tortuosity of the pathways that are composed of linked pores, while βca was related to the interaction between the cations and the charged layer covering the inner walls of these pathways.3 In this research, we found a clear difference between the β values of L1 and L3 in PE membranes, as shown in Figure 3c. In L1, βan was dominant in the less porous membranes, but as the porosity increased, βca became dominant. On the other hand, in L3, only βca was detected, while βan was not observed across the whole porosity range. This would mean that the Coulombic cation−membrane interaction in L3 surpassed the anion− membrane interaction, even in the low-porosity PE membranes. This reveals that the weakly solvated lithium cations in L3 interact more strongly with the pore walls of the PE separator membranes. It is reasonable to think that the magnitude of βca will depend on the number of ions colliding with a unit area of the charged pore wall, if the charge density on the pore wall is unaffected by the pore size and membrane porosity. According to the

assumption for the estimation of pore size by the BET measurement, we suppose the pore has a columnar structure, with 2r and d being the averaged diameter and length, respectively. The area of the pore wall would be 2πrd. The number of pores in a unit volume of membrane is p/(πr2d), with p being the porosity of the membrane (%). Then, the total area of pore walls in a membrane of fixed size should be p 2

πr d

× 2πrd =

2p r

(4)

As a result, the volume of the solution (which is proportional to the number of cations in it) contacting per unit area of the pore wall is p r = 2p 2 (5) r

That is, when increasing the pore size r, the number of cations per unit area of pore wall increases, leading to a reduction of βca if the density of the interacting active sites on the pore wall is independent of r and p. In practice, however, the plot of βca vs r of Figure 3d showed a tendency of βca increasing with r, contrary to the expectation from the change of interacting active site density on the pore wall for a cation species. We could assume several reasons for the result in Figure 3d. One is an anomalous morphology in the highly porous membranes that is different from the simple porous structure of those with lower porosity. In the SEM observations, the membranes with p ≥ 80% showed a fibrous bulk structure like a nonwoven membrane. It is probable that the electrolyte solution is held in the space between the fibers, which is nanometers in size, as well as in the large pore spaces estimated 23929

DOI: 10.1021/acs.jpcc.7b07056 J. Phys. Chem. C 2017, 121, 23926−23930

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The Journal of Physical Chemistry C

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by the average pore size r. As a result, the real number of interacting active sites on the PE membrane would be larger than that simply estimated from the apparent pore size and porosity. Another possibility for the unexpected change in βca against r is the effects of porosity and pore size on the collision frequency of the cations on the wall of the membrane. From the low values of η and α of the membranes with p ≥ 80% (which are close to those in the free electrolyte solution), the ionic mobilities would be essentially higher in the larger space if the cation/membrane interaction responsible for βca does not exist. This means that, in the case of a larger pore space, the collision frequency of the cations on the pore wall could be higher, which leads to a larger βca due to the higher efficiency of the interacting active site effect per unit time.

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CONCLUSIONS The ionic mobilities and microviscosities that determine the mobilities of the lithium electrolyte solution in a PE separator membrane were compared, by using solutions with differently sized solvated lithium structures and membranes with different pore sizes and porosities. We confirmed that the lithium cation mobility in the membranes was selectively restricted, possibly due to the Coulombic effect from the surface charge on the pore walls. This effect depended on the structure of the solvated lithium cation. The types and number of the solvating species affect the strength of the Coulombic force by altering the shielding of the net charge on Li+. This result suggests a possibility of systematically controlling the cation mobility in the battery device, by designing the appropriate lithium solvation structure and surface charge on the pore walls of the separator membrane because the morphological structure and the charged condition of both the mobile species and medium providing the transport pathways for the species determine the ionic mobility the battery system.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-72-751-4527. E-mail: [email protected]. ORCID

Yuria Saito: 0000-0002-9616-6309 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Teijin Limited for financially supporting this research. REFERENCES

(1) Jow, R. T.; Ku, K.; Borodin, O.; Ue, M. Electrolytes for Lithium and Lithium-Ion Batteries; Springer: New York, 2014; p 5. (2) Braun, P. V.; Cho, J.; Pikul, H.; King, W. P.; Zhang, H. High Power Rechargeable Batteries. Curr. Opin. Solid State Mater. Sci. 2012, 16, 186−198. (3) Saito, Y.; Morimura, W.; Kuratani, R.; Nishikawa, S.; Kuse, S. Influence of the Morphological Characteristics of Separator Membranes on Ionic Mobility in Lithium Secondary Batteries. J. Phys. Chem. C 2017, 121, 2512−2520. (4) Huang, W.-Y.; Zink, J. I. Effect of Pore Wall Charge and Probe Molecule Size on Molecular Motion Inside Mesoporous Silica Nanoparticles. J. Phys. Chem. C 2016, 120, 23780−23787. (5) Borodin, O.; Smith, G. D. LiTFSI Structure and Transport in Ethylene Carbonate from Molecular Dynamics Simulations. J. Phys. Chem. B 2006, 110, 4971−4977. 23930

DOI: 10.1021/acs.jpcc.7b07056 J. Phys. Chem. C 2017, 121, 23926−23930