Effects of Nonsolvent Molecular Structure and Its Content on the

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Effects of Nonsolvent Molecular Structure and Its Content on the Formation of the Macroporous Polyarylate Layer Coated onto the Polyethylene Separator Sang Chul Roh, Kwon Won Song, and C. K. Kim* School of Chemical Engineering and Materials Science, Chung-Ang University, 221, Heukseok-dong, Dongjak-gu, Seoul 156-756, Korea ABSTRACT: A macroporous polyarylate (PAR) layer was formed on a polyethylene (PE) separator from coating solutions containing PAR, tetrahydrofuran (THF) as the solvent, and various alcohols (or diols) as nonsolvents via a nonsolvent-induced phase separation (NIPS) process. The effects of the nonsolvent molecular structure and its content on the morphology of the coating layer were explored. Droplets grew by a coarsening process after the onset of phase separation by solvent evaporation, and then leveled off at a fixed droplet size. The final pore size increased with the molecular weight of the alcohol (or diol) when the composition of the coating solution was fixed. It increased as the nonsolvent content in the coating solution increased, while it decreased as the PAR content in the coating solution increased. The meltdown temperature of the separator increased with the macroporous PAR coating, while air permeability of the separator was reduced with the coating. Changes in the meltdown temperature and air permeability depended on the morphology of the coating layer.

’ INTRODUCTION Lithium ion batteries are widely used as power sources in portable electronic devices such as mobile phones and laptop computers due to their high energy density and power, excellent cyclic properties, and long life cycle.17 Their primary application has gradually moved to the powering of large scale devices such as hybrid electric vehicles and energy reservoirs. Lithium ion batteries consist of an anode, a cathode, a porous film separator interposed between the anode and the cathode, and a liquid electrolyte. The separator offers protection against internal short circuits and safe deactivation of the cell under overcharge conditions. Microporous membranes fabricated from high density polyethylene (PE) via a thermally induced phase separation (TIPS) process are used as separators for commercially available lithium-ion batteries.813 Battery failure mainly stems from internal short circuits, which occur because of battery overcharge and electrode contact caused by defects formed on the electrode surface.26 When a lithiumion battery is overcharged, it starts to self-heat because of exothermic reactions occurring within the components of the cell. Separator shutdown, which takes place when the porous polyolefin separator turns into a nonporous insulating film around the melting temperature of PE crystallines, is a useful safety feature for preventing thermal runaway reactions.11,1417 When the mechanical properties of the PE separator deteriorate greatly after shutdown, a safety hazard stemming from a separator meltdown can occur. Since the shutdown temperature of a PE separator is close to its meltdown temperature, separators offering mechanical and thermal stability above the shutdown temperature of PE are required for battery safety. To enhance the meltdown temperature of separators, a porous PE layer sandwiched between two porous polypropylene (PP) layers has been developed.11 A PE separator coated with crosslinked polymers synthesized from various ethylene diol dimethacrylate monomers and a bilayered separator coated onto the PE r 2011 American Chemical Society

separator with macroporous polyarylate (PAR) have been previously examined in our laboratory.14,18,19 Bilayered separators prepared from a coating solution containing PAR, tetrahydrofuran (THF) as a solvent, and ethylene glycol butylether (EGBE) as a nonsolvent via a nonsolvent-induced phase separation (NIPS) process exhibited increased meltdown temperatures, while the air permeability (or electrolyte permeability) was reduced. Since the meltdown temperature and air permeability of the coated separator depend on the morphology of the macroporous PAR layer, its morphology should be optimized to maximize the meltdown temperature and to minimize the reduction in air permeability. For this purpose, the morphologies of the macroporous PAR coated onto the PE separator were controlled by varying the nonsolvent and the composition of the coating solutions. Changes in the meltdown temperature and air permeability were also explored.

’ MATERIALS AND PROCEDURE Polyethylene lithium battery separators made of high density polyethylene (grade: 320s) were provided by SK Energy Co. (Seoul, Korea). According to the supplier, the separators had a porosity of 46%, pore sizes in the range 0.070.12 μm, and thicknesses of 20 μm. Polyarylate (PAR, Tg = 200 °C) purchased from Unitika (grade: U-100, Osaka, Japan) was used as the polymer to coat the PE separator surface. According to the supplier, this PAR is based on bisphenol A with an equimolar mixture of isophthalic and terephthalic acids. Tetrahydrofuran (THF) was used as a solvent. Various normal alcohols (propanol, pentanol, octanol, and decanol) and diols (ethanediol, 1,3-propanediol, 1,4-butanediol, and 1,5-petanediol) purchased from Aldrich Chemical Co (Milwaukee, WI) were used as nonsolvents. Received: August 3, 2011 Accepted: October 12, 2011 Revised: October 6, 2011 Published: October 12, 2011 12596

dx.doi.org/10.1021/ie201716m | Ind. Eng. Chem. Res. 2011, 50, 12596–12605

Industrial & Engineering Chemistry Research

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Figure 1. Phase diagrams of ternary mixtures containing PAR, THF, and various alcohols.

Separator coating with porous PAR was performed by a wet phase inversion process, i.e., a nonsolvent induced phase separation (NIPS) process. The PAR was completely dissolved in THF by stirring for 10 h at 30 °C, and then the proper amounts of nonsolvent were mixed in to prepare the casting solutions. Note that ternary mixtures, which are formed from single-phase mixtures, were used as casting solutions. The homogeneous polymer mixture was left without stirring until all bubbles were completely removed. Casting with the resulting polymer solution was performed in a clean room at 30 °C. The polymer solution was cast at a thickness of 100 μm onto a separator using a steel doctor blade. The casted separator was kept in a clean room at 30 °C for a specific time to induce phase separation by evaporating the volatile solvent (THF). The resulting separator was immersed in a water bath for 24 h until the remaining nonsolvent and solvent were completely removed. The separator was then dried in a vacuum oven at 60 °C for one day. The phase boundary of PAR/THF/nonsolvent ternary mixtures was determined by observing solution turbidity with the naked eye. A series of THF solutions containing different amounts of PAR (PAR/THF = 3/97, 5/95, 8/92, and 10/90 by weight) were prepared in vials with a Teflon-lined cap. Different amounts of nonsolvent were added to these solutions and mixed in an ultrasonic bath at 30 °C for 1 h, and the resulting solutions were kept at 30 °C for 24 h. Thereafter, if the solution was clear, it was judged to be thermodynamically stable and to have formed a homogeneous mixture. If the solution was visually turbid, it was phase separated into two phases. The morphologies of the separators were observed with a field emission scanning electron microscope (FE-SEM, model: JSM6700F, JEOL, Japan). The average pore size of the PAR coating layer was calculated from the SEM photos with an image analyzer (Bummi Co., Model: I-Top, Korea). Five SEM photos of each coated separator were analyzed to determine the average pore size. The porosity of the coated separator, which is defined as the volume of the pores divided by the total volume of the porous membrane, was obtained as described previously.20 The dry separator was dipped in ethanol for 24 h. Then, the ethanol on the surface of the resulting separator was wiped up with filter

paper, and the separator was weighted to determine the porosity of the coated separator. Changes in the air permeabilities of the coated separators were measured using the Gurley method (Technical Association of the Pulp and Paper Industry (TAPPI) T-460 method). A circular separator with an effective area of 6.45 cm2 was placed in the testing equipment (model: G-B3C, Toyoseiki Co., Japan). The time required for 100 cm3 of air to pass through the separator under a pressure differential of 1.22 kPa was measured. The shutdown temperature at which the pores on the PE separator completely disappeared from heating was measured by an annealing technique.18 Separators mounted onto a cover glass were annealed on a hot plate (Linkam THMS 600) equipped with a temperature controller (Limkam, TMS 92) at a specific temperature around the melting temperature of PE (120140 °C) for 5 min. The resulting separators were quenched to room temperature, and then changes in the morphology were observed by FE-SEM. The temperature at which meltdown started to occur was measured with a thermomechanical analyzer (TMA, model: Q400 TA Instruments, New Castle, DE). The displacement of the specimen (width length = 6 mm 15 mm) was monitored as a function of temperature at a scanning rate of 5 °C/min when a constant force of 0.015 N was applied. The temperature at which the specimen started to elongate was defined as the meltdown temperature. Five specimens for each separator were tested, and their results were averaged. To confirm the meltdown behavior of the separator, changes in the specimen shape as a function of temperature were observed using a digital camera as the specimen was heated on a hot stage at a scanning rate of 5 °C/min.

’ RESULTS AND DISCUSSION Phase Behavior of PAR/THF/Nonsolvent Mixtures. The miscibility of ternary mixtures composed of PAR, THF, and various nonsolvents such as alcohols and diols were examined. Figure 1 shows the phase diagrams for PAR/THF/alcohol ternary mixtures. Only PAR/THF solutions containing less than or equal to 10 wt % of PAR were examined because PAR did not completely dissolve in THF when the solutions contained more 12597

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than 12 wt % PAR. Phase diagrams for PAR/THF/alcohol ternary mixtures were similar regardless of kinds of alcohols used as nonsolvents. The miscible region is very narrow, and nonsolvent tolerance gradually decreased as PAR content in the ternary mixture increased. When the mixing ratio of the PAR/ THF solutions in the ternary mixtures was fixed, the miscible region of the ternary mixtures gradually increased as the molecular weight of the alcohol increased, as shown in Figure 1. The observed phase behavior of the ternary blends can be interpreted with FloryHuggins theory.21,22 The free energy of mixing per unit volume, g, is given by Δgm ¼ Δgnc þ Δgc

ð1Þ

where Δgc is the combinatorial entropy ϕ ln ϕ3 ϕ ln ϕ1 ϕ ln ϕ2 þ 2 þ 3 Δgc ¼ RT 1 V~ 1 V~ 2 V~ 3

ð2Þ

and Δgnc is the noncombinatorial free energy represented by the Van-Laar form Δgnc ¼ ϕ1 ϕ2 B12 þ ϕ1 ϕ3 B13 þ ϕ2 ϕ3 B23

ð3Þ

ϕi and V~ i are the volume fraction and molar volume of component i, respectively (1, PAR; 2, THF; 3, alcohol), and Bij is the interaction energy between components i and j. For a ternary mixture, the stability condition is given by2325 g11 g 0

ð4Þ

g g ðg12 Þ2 11 12 det g0 ¼ g11 g21 g22 g22

ð5Þ

where the subscripts 1 and 2 indicate partial derivatives with respect to ϕ1 or ϕ2. Each second partial derivative is given by ! 1 1 þ g11 ¼ 2B13 þ RT ð6Þ ϕ1 V~ 1 ϕ3 V~ 3 g12 ¼ g21 ¼ ðB12 B13 B23 Þ þ

RT ϕ3 V~ 3

ð7Þ

! g22 ¼ 2B23

1 1 þ RT þ ~ ϕ2 V 2 ϕ3 V~ 3

ð8Þ

The first condition of ternary stability (eq 4 or 6) is composed of two terms: the interaction energy between PAR and alcohol (B13) (a positive interaction energy harms miscibility) and the combinatorial entropy, which always aids miscibility. This means that one phase mixture is formed when the combinatorial entropy is greater than the interaction energy. Figure 2 shows the solubility parameters of PAR, THF, and various nonsolvents calculated by the Van Krevelen method.26 The solubility parameters of PAR (δ1 = 10.4 J1/2/cm3/2) and THF (δ2 = 18.0 J1/2/cm3/2) are smaller than those of various alcohols, which are decreasing in molecular weight. This indicates that the unfavorable interaction energy between PAR (or THF) and the alcohol gradually decreases as the molecular weight of the alcohol is increased. Note that interaction energy (B12) is nearly the same with [(δ1 δ2)2]. However, reducing the molar volume (molecular weight) of the alcohol increases the favorable combinatorial entropy. This means that the miscibility of the ternary mixtures is determined

Figure 2. Solubility parameters of PAR, THF, and various alcohols (or diols) calculated by the Van Krevelen method.

by the competition of these two terms. As a consequence, the miscible region of PAR/THF/alcohol ternary mixtures gradually increases as the molecular weight of the alcohol increases. The decrease in the nonsolvent tolerance with increasing PAR content in the ternary mixture can be explained by eq 4. An increase in the polymer content of a solution always reduces the combinatorial entropy because the molar volume of PAR (V~ i) is much higher than that of the solvent (or nonsolvent). Reduction in the combinatorial entropy of the mixture always harms miscibility.23,24 The phase behaviors of PAR/THF/diol ternary mixtures are similar to those of PAR/THF/alcohol ternary mixtures, as shown in Figure 3. The miscible region of the PAR/THF/diol mixtures gradually increased as the molecular weight of the diol increased. The solubility parameters of diols are greater than those of PAR and THF, and they gradually decrease as the molecular weight of the diol increases, as shown in Figure 2. Since differences in the solubility parameter between PAR (or THF) and the diols also increase as the molecular weight of the diol decreases, the miscible region of the ternary mixture is reduced as the molecular weight of the diol decreases. Comparing the phase diagram of PAR/THF/pentanol mixtures with that of PAR/ THF/1,5-pentanediol mixtures, the miscible region of the latter is narrower than that of the former (see Figures 1 and 3). This means that the hydroxyl groups in the alcohol and diol are energetically unfavorable for the miscibility of the ternary mixture. To summarize, miscibility of ternary mixtures containing PAR, THF, and alcohols (or diols) gradually increase as the molecular weight of the alcohol (or diol) increases because of the unfavorable interactions between the alcohol (or diol) and PAR (or THF) can be reduced by increasing the molecular weight of the alcohol (or diol). Morphology Changes in the Coating Layer. The morphology of the PAR layer coated on the separator is influenced by factors such as the composition of the casting solution, solvent evaporation time and temperature, air-flow rate, and casting thickness. To understand the effects of solution composition, type of nonsolvent, and solvent evaporation time on the morphology of the coated layer, the following parameters were fixed 12598



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Figure 3. Phase diagrams of ternary mixtures containing PAR, THF, and various diols.

unless otherwise specified: evaporation temperature (30 °C), casting thickness (100 μm), and air-flow rate (natural convection). Figure 4 shows changes in the structure of the coating layer as a function of solvent evaporation time when the solution composition was fixed at (PAR/THF = 8/92)//pentanol = 93//7. Droplet aggregates consisting of small droplets about 1 μm in diameter and large droplets about 715 μm in diameter were observed after 1 min of solvent evaporation (Figure 4a). The solution coated onto the PE separator might undergo a phase separation when the system enters a two-phase region. A gradual reduction in the number of droplets and an increase in their size are observed as evaporation time increases after the development of the domain-matrix structure (Figure 4bd). The number of small droplets around the large droplets fell with evaporation time, consistent with the so-called Oswald ripening process (see the high magnitude microphotographs in Figure 4).27 Small droplets are more apt to evaporate than large droplets because of the chemical potential difference that is proportional to the curvature of the droplet.27,28 The evaporated polymer-lean phase diffuses through the polymer matrix and is then partly condensed on the surface of the larger droplets. As shown in Figure 4, large droplets collide with each other and then coalesce into a bigger droplet. When ternary mixtures containing PAR, THF, and other kinds of alcohols (or diols) were used as coating solutions, coarsening by the Oswald ripening process in small droplets and by the coalescence mechanism in large droplets was also observed. The results indicate that droplet growth in PAR/THF/ nonsolvent mixtures occurs by both coalescence and the Oswald ripening process regardless of the type of nonsolvent used. The droplets initially grow with evaporation time and then level off at a fixed droplet size. The average droplet size, which does not change after a given evaporation time, was defined as the final droplet size. When the composition of the coating solution was fixed at (PAR/THF = 8/92)//nonsolvent = 93//7, the time required to reach the final droplet size depended on the

Figure 4. Changes in the structure of the coating layer as a function of solvent evaporation time when the solution composition was fixed at (PAR/THF = 8/92)//pentanol = 93//7: (a) after 1 min, (b) 2 min, (c) 4 min, (d) 5 min.

nonsolvent. The droplet size reached its final value after about 5 min with pentanol and after about 7 min with decanol. The 12599



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Figure 5. Final morphologies of the coating layers fabricated from coating solutions containing various alcohols (or diols) as nonsolvents when the composition of the coating solution was fixed at (PAR/THF = 5/95)//nonsolvent = 91//9.

droplet growth rate increased as the diffusion coefficient and interfacial tension increased.2730 Since the diffusion rate increases as the medium viscosity and the molecular weight of a diffusing component decrease, and interfacial tension increases as interaction energy increases, droplets formed from a coating solution containing pentanol might reach their final size earlier than those formed from a coating solution containing decanol. Note that the diffusion coefficient and interfacial tension of the pentanol solution are greater than those of the decanol solution. Decanol, which had the highest molecular weight among the nonsolvents used in this study, required the longest time (about 7 min) for the droplets to reach their final size. Thus, the solvent evaporation time was fixed to 7 min unless otherwise specified. The final morphology of the coating layer depended on the nonsolvent used and the composition of the coating solution. The final morphologies of the coating layers fabricated from coating solutions containing various alcohols (or diols) as nonsolvents were investigated with the composition fixed at (PAR/ THF = 5/95)//nonsolvent = 91//9. As shown in Figure 5a, a coating layer containing macropores and polymer precipitates was formed regardless of the type of alcohol, and the pore size gradually increased with increasing molecular weight of the alcohol. Figure 6 quantitatively illustrates the change of pore size and porosity as a function molecular weight of the nonsolvent. The pore size and the porosity gradually increased with increasing molecular weight of the nonsolvent; the pore size increased from about 110 to 310 μm and the porosity increased from about 52% to 58% when the nonsolvent was changed from propanol to decanol. Since the evaporation time required to reach the final droplet size increases as the molecular weight of the alcohol increases, the time period at which coarsening occurs also increases as the molecular weight of the alcohol increases. This means that a large droplet can be formed in the coating layer with a nonsolvent having a high molecular weight. When various diols were used as the nonsolvent, macropores in the coating layer were also formed and their size also increased as the molecular weight of the diol increased (see Figure 5b). When the mixing ratio of the PAR and THF was fixed (PAR/ THF = 5/95), the morphology of the coating layer changed with the nonsolvent content in the coating solution. Figure 7 shows changes in the morphology of the coating layers as a function of

Figure 6. Changes in the porosity of the separator and the pore size of the coating layer fabricated from coating solutions containing various alcohols as nonsolvents when the composition of the coating solution was fixed at (PAR/THF = 5/95)//nonsolvent = 91//9.

nonsolvent content. With (PAR/THF = 5/95)//pentanol = 96//4, pore aggregates consisting of small macropores were observed. Note that macropores were not observed when the solution contained less than or equal to 3 wt % pentanol. When the coating solution contained about 510 wt % nonsolvent, large macropores were formed from pore aggregates by a coarsening process. As the nonsolvent content continuously increased, the macropores continuously collapsed. As a result, the network structure formed by the PAR changed to discontinuous precipitates when the coating solution contained greater than or equal to 11 wt % of nonsolvent. Figure 8 shows changes in the morphologies of the coating layers formed from coating solutions containing 1,3-propanediol as nonsolvents. Macropores were not observed when the solution contained less than or equal to 3 wt % of 1,3-propanediol. When the nonsolvent content increased to 5 wt %, large macropores formed. As the nonsolvent content continuously increased, large macropores collided with each other and coalesced into larger macropores, which then continuously collapsed. Finally, the continuous network structure changed to discontinuous precipitates when the 12600



Figure 7. Changes in the morphology of the coating layers formed from (PAR/THF = 5/95)//pentanol solutions as a function of nonsolvent content: (a) (PAR/THF = 5/95)//pentanol = 96//4; (b) (PAR/THF = 5/95)//pentanol = 93//7; (c) (PAR/THF = 5/95)//pentanol = 89// 11; (d) (PAR/THF = 5/95)//pentanol = 88//12.

Figure 8. Changes in the morphology of the coating layers formed from (PAR/THF = 5/95)//1,3-propanediol solutions as a function of nonsolvent content: (a) (PAR/THF = 5/95)//1,3-propanediol =95//5, (b) (PAR/ THF = 5/95)//1,3-propanediol = 93//7, (c) (PAR/THF = 5/95)//1,3propanediol = 90//10, (d) (PAR/THF = 5/95)//1,3-propanediol = 89//11.

coating solution contained more than 10 wt % of 1,3-propanediol (Figure 8d). The morphology of the coating layer also changed with polymer content in the coating solution when the nonsolvent content was fixed. Figure 9 shows the morphology changes in the

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Figure 9. Changes in the morphology of the coating layer as a function of PAR content of the casting solution when the pentanol content was fixed at 7 wt %: (a) (PAR/THF = 3/97)//pentanol = 93//7, (b) (PAR/ THF = 5/95)//pentanol = 93//7, (c) (PAR/THF = 8/92)//pentanol = 93//7, (d) (PAR/THF = 10/90)//pentanol = 93//7.

coating layer as a function of polymer content in the coating solution. Granule-type as well as branch-type polymer precipitates attached to the separator surface were only observed when the (PAR/THF = 3/97)//pentanol = 93//7 coating solution was used, while closed macropores dispersed in the polymer matrix were observed when the PAR content was fixed to either 5 or 8 wt %. However, a dense coating layer without macropores formed when the PAR content was fixed to 10 wt %. Morphology changes of the (PAR/THF)/diol coating as a function of PAR content were similar to those of the (PAR/THF)/alcohol coating. The results obtained here indicate that the morphology of the coating layer can be controlled by changes in the composition of the casting solution and the nonsolvent used. Formation of a PAR coating layer containing closed macropores on the PE separator might be desirable for a lithium ion battery separator having a high thermal resistance. Coating layers with desirable morphologies were formed when the coating solutions contained about 58 wt % of PAR and 510 wt % of nonsolvent. The size of the macropores increased as PAR content decreased and as the molecular weight of the nonsolvent and its content in the coating solution increased. Separators coated with macroporous PAR were used for further experiments measuring the thermal resistance and air permeability, unless otherwise specified. Characteristics of Coated Separators. Separator shutdown usually takes place when the porous PE separator turns into a nonporous insulating film around the melting temperature of the PE crystallines. Figure 10 shows the surface morphologies of an uncoated separator and a separator coated with macroporous PAR after annealing at 130, 135, and 140 °C for 10 min. Both separators exhibited similar shutdown behavior regardless of the macroporous PAR coating. The separators maintained their pores at 130 °C, while the pores were nearly closed when the separators were annealed at 135 °C. Finally, the porous polymer 12601



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Figure 11. TMA thermograms of (a) an uncoated PE separator, (b) a PE separator coated with a (PAR/THF = 3/97)//1,5-pentanediol = 93//7 coating solution, (c) a PE separator coated with a (PAR/THF = 5/95)//1,5-pentanediol = 93//7 coating solution, and (d) a PE separator coated with a (PAR/THF = 8/92)//1,5-pentanediol = 93// 7 casting solution.

Figure 10. Surface morphologies of the uncoated separator and separator coated with macroporous PAR after annealing at 130,135, and 140 °C for 10 min: (a) PE separator, (b) separator coated with a (PAR/ THF = 5/95)//pentanol = 93//7 solution.

film turned into a nonporous insulating film when the separators were annealed at 140 °C. This result indicates that the shutdown temperature of the separator, which was found to be around 135 °C, did not change with the macroporous PAR coating. Figure 11 shows the dimension changes of the selected separators as a function of temperature examined by TMA. Note that TMA experiments were performed on the samples in the transverse direction. As shown in Figure 11a for an uncoated separator, reduction in the sample length (shrinkage), which started at around 110 °C, continued until 138 °C (55% reduction). The sample length reached a minimum at this temperature and then began to revert to ambient. The sharp rise in sample length at around 138 °C indicates that the meltdown of the uncoated separator occurs at this temperature. When the separator was coated with macroporous PAR formed from coating solutions containing 7 wt % of nonsolvent, the meltdown temperature increased with increasing PAR content in the coating solution. When separators were coated with PAR formed from a (PAR/THF = 8/92)//1,5-pentanediol = 93//7 solution, the meltdown temperatures of the coated separators increased up to about 194 °C (Figure 11d). The temperature at which a reduction in the sample length (shrinkage) started increased as PAR content in the coating solution increased, and changes in the sample length (shrinkage) were reduced as the PAR content in

the coating solution increased. These results indicate that the thermal stability of the coated separator was gradually enhanced as the PAR content of the coating solution was increased. When the mixing ratio of PAR and THF in the coating solution was fixed to PAR/THF = 5/95, the size of the macropores in the PAR matrix increased as the nonsolvent content increased. The continuous PAR matrix then changed to discontinuous precipitates when the coating solution contained greater than or equal to 11 wt % of nonsolvent, as shown in Figures 7 and 8. Figure 12 shows changes in the meltdown temperature as a function of nonsolvent content in the coating solution when the mixing ratio of PAR and THF in the coating solution was fixed at PAR/THF = 5/95. The meltdown temperature, which was nearly constant (about 175 °C) when the PAR coating layer formed a continuous matrix, decreased to about 158 °C when the PAR coating layer formed discontinuous precipitates. The meltdown temperatures were nearly constant (about 194 °C) when the mixing ratio of PAR and THF in the coating solution was fixed at PAR/THF = 8/92. Note that the PAR coating layer formed a continuous matrix at this PAR/THF mixing ratio regardless of nonsolvent content. Changes in separator shape with temperature were observed using a digital camera to confirm the meltdown behavior of the separators. A specimen placed on a cover glass was mounted on a hot stage and then heated at a scanning rate of 5 °C/min. Figure 13 shows changes in the image as a function of temperature for the uncoated separator and for the separators coated with macroporous PAR formed from (PAR/THF = 5/95)//1,5pentanediol = 92//8 and (PAR/THF = 8/92)//1,5-pentanediol = 92//8 solutions, respectively. For the uncoated separator, a reduction in the sample size, which started at approximately 125 °C, continued as the temperature was increased until the specimen started to melt at approximately 140 °C. In contrast, the coated specimens formed from the (PAR/THF = 5/95)// pentanediol = 92//8 and (PAR/THF = 8/92)//pentanediol = 12602



Figure 12. Changes in the meltdown temperature as a function of nonsolvent content in the coating solution when the mixing ratio of PAR and THF in the coating solution was fixed at PAR/THF = 5/95. Note that differences in the melting temperatures measured with five specimens for each separator were not exceeding (2 °C.

Figure 13. Changes in the separator shape as a function of temperature: (a) an uncoated PE separator, (b) separators coated with a (PAR/THF = 5/95)//1,5-pentanediol = 92//8 solution, (c) (PAR/THF = 8/92)// 1,5-pentanediol = 92//8 solution.

92//8 solutions maintained their original shapes until approximately 140 and 150 °C, respectively, at which point they started to shrink. A reduction in the sample size of the latter was smaller than that of the former at the same heating temperature. These results indicate that the coated separator has a higher meltdown temperature than the uncoated separator, and the thermal stability of the coated separator was enhanced by increasing the PAR content of the coating solution. A decrease in the air permeability (an increase in the Gurley number) was observed when the separator was coated with porous PAR. Note that the Gurley number of the uncoated PE separator was measured to be 250 s.

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Figure 14. Changes in the Gurley number (or air permeability) by type of nonsolvent in the coating solution when the composition of the coating solution was fixed at (PAR/THF = 8/92)//nonsolvent = 93//7.

Figure 15. Changes in the Gurley number (or air permeability) as a function of nonsolvent content in the coating solution when the PAR/ THF ratio was fixed at 5/95.

As shown in Figure 14, the air permeability increased with increasing molecular weight of the nonsolvent when the composition of the coating solution was fixed. Figure 15 shows that air permeability increased with increasing nonsolvent content of the casting solution when the polymer content was fixed. It was also observed that the air permeability decreases as PAR content in the coating solution increases for a fixed nonsolvent content. Macropore size increased with increasing molecular weight of the nonsolvent and its content in the coating solution when the PAR content was fixed, while the size of the macropores decreased with increasing PAR content in the coating solution. This result indicates that the formation of large macropores is favorable in increasing air permeability. To summarize, the meltdown temperature of the coated separator increases with increasing PAR content in the coating solution regardless of the macropore size formed in the continuous PAR coating layer. Air permeability 12603


Industrial & Engineering Chemistry Research increased with increasing macropore size in the PAR when the PAR content in the coating solution was fixed. The higher molecular weight of the nonsolvent and the more miscibility (nonsolvent tolerance) the coating solution exhibits, the larger the macropores are. Therefore, PAR-coated separators with a desirable performance balance in terms of thermal stability and air permeability can be fabricated with proper nonsolvents such as decanol and 1,5-pentanediol. Note that the meltdown temperature of the coated separator was enhanced up to 194 °C with a reduction in air permeability (about 50% air permeability of the PE separator) when the separator was coated with (PAR/THF = 8/92)//decanol = 92//8.

’ SUMMARY Polyethylene separators were coated with macroporous PAR fabricated from coating solutions containing PAR, THF as a solvent, and an alcohol (or diol) as a nonsolvent via a nonsolvent induced phase separation process. To enhance the thermal resistance of the coated separator above the shutdown temperature and to minimize the reduction in air permeability, morphologies of the coating layers were controlled by changing the nonsolvent in the coating solutions. The size of the macropores increased with decreasing PAR content and increasing nonsolvent content. When the mixing ratio of PAR and THF was fixed, the morphology of the coating layer changed with increasing nonsolvent content in the following order: pore aggregates consisting of small pores in the PAR matrix, macropores in the PAR matrix, and discontinuous polymer precipitates on the PE separator. When the composition of the coating solution was fixed, the size of the macropores formed in the PAR matrix gradually increased as the molecular weight of the alcohol (or diol), i.e., the number of methylene groups in the alcohol (or diol) molecule, increased. An increase in the meltdown temperature and a decrease in the air permeability were observed with the macroporous PAR coating on the PE separator. The meltdown temperature increased with increasing PAR content. Larger macropores in the PAR layer led to higher air permeability. As a consequence, when the separator was coated with (PAR/THF = 8/92)//decanol = 92//8, the meltdown temperature of the coated separator was enhanced up to 194 °C with an acceptable reduction in the air permeability (about 50%). ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: 822-824-3495.

’ ACKNOWLEDGMENT This work was supported by a research grant funded by LG Chemicals. ’ REFERENCES (1) Lithium Ion Battery Technology; Brodd, R. J., Friend, H. M., Nardi, J. C., Eds.; ITE-JEC Press: Brunswick, OH, 1995. (2) Prisyazhnyi, V. D.; Lisin, V. I.; Lee, E. S. In New Promising Electrochemical Systems for Rechargeable Batteries; Barukov, V., Beck, F., Eds.; Kluwer Acadrmic Publishers: Dordrecht, The Netherlands, 1996; Section 2. (3) Lithium Ion Batteries, Fundamentals and Performance; Wakihara, M., Yamamoto, O., Eds.; Wiley-VCH: New York, 1998.

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