Enhanced Wettability and Thermal Stability of a Novel Polyethylene

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Enhanced Wettability and Thermal Stability of a Novel Polyethylene Terephthalate-Based Poly(Vinylidene Fluoride) Nanofiber Hybrid Membrane for the Separator of Lithium-Ion Batteries Chunhong Zhu,† Tomoki Nagaishi,‡ Jian Shi,§ Hoik Lee,‡ Pok Yin Wong,∥ Jianhua Sui,⊥ Kenji Hyodo,‡ and Ick Soo Kim*,‡ †

Faculty of Textile Science and Technology, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan Nano Fusion Technology Research Group, Division of Frontier Fibers, Institute for Fiber Engineering (IFES), Interdisciplinary Cluster for Cutting Edge Research (ICCER), Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan § Faculty of Systems Science and Technology, Akita Prefectural University, 84-4 Aza Ebinokuchi Tsuchiya, Yurihonjo, Akita 015-0055, Japan ∥ Guangdong Jundong Technology Co., Ltd., Sino-German Cooperative Innovation Base, Sino-German Metal Eco City, Jiedong District, Jieyang, Guangdong Province 515557, China ⊥ College of Textile and Clothing Engineering, Soochow University, No. 178 Ganjiang East Road, Suzhou, Jiangsu Province 215021, China ‡

ABSTRACT: In this study, a novel membrane for the separator in a lithium-ion (Li-ion) battery was proposed via a mechanically pressed process with a poly(vinylidene fluoride) (PVDF) nanofiber subject and polyethylene terephthalate (PET) microfiber support. Important physical properties, such as surface morphology, wettability, and heat stability were considered for the PET-reinforced PVDF nanofiber (PRPN) hybrid separator. Images of scanning electron microscopy (SEM) showed that the PRPN hybrid separator had a homogeneous pore size and high porosity. It can wet out in battery electrolytes completely and quickly, satisfying wettability requirements. Moreover, the electrolyte uptake was higher than that of dry-laid and wet-laid nonwovens. For heat stability, no shrink occurred even when the heating temperature reached 135 °C, demonstrating thermal and dimensional stability. Moreover, differential scanning calorimetry (DSC) showed that the PRPN hybrid separator possessed a shutdown temperature of 131 °C, which is the same as conventional separators. Also, the meltdown temperature reached 252 °C, which is higher than the shutdown temperature, and thus can protect against internal cell shorts. The proposed PRPN hybrid separator is a strong candidate material for utilization in Li-ion batteries. KEYWORDS: hybrid separator, PVDF nanofiber membrane, wettability, thermal stability, Li-ion battery electrolyte.3 A separator is an indispensable component and acts as an electrically insulating unit between the anode and the cathode. This allows for the effective transport of ionicconductor carriers between both electrodes.3−6 A safety mechanism for the separator requires the following characteristics: (1) insulation, (2) high affinity with the organic electrolytic solution, (3) continuous pores, (4) mechanical strength for manufacturing and using, (5) Li-ion permeation resistance that is as low as possible, and (6) a function in which, under abnormal conditions, the micropores of the separator become clogged to block Li-ion permeation and to shut down

1. INTRODUCTION As fossil energy sources are limited and their utilization inevitably causes environmental issues, there is increasing interest in developing clean, alternative, and sustainable energy, such as solar energy and tidal energy. However, these energies have some limitations because of their uncontrollability. This has led to the rapid development of new technologies, especially lithium-ion (Li-ion) batteries that possess high energy and power densities, and a long cycle life.1,2 Li-ion batteries have received much attention as a major energy source for portable electronic devices. A Li-ion battery system can be mainly divided into the anode, cathode, electrolyte, and separator. The separator is a film or sheet with a porous structure interposed between both electrodes, and the pores in the separator are filled by an ionically conductive liquid © XXXX American Chemical Society

Received: May 5, 2017 Accepted: July 19, 2017

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DOI: 10.1021/acsami.7b06303 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces the current.7,8 The main parameters influencing separator performance include porosity/pore size, permeability, absorption and retention of electrolyte, and chemical, mechanical, and thermal stability.9,10 In order to achieve the mentioned characteristics, the general requirements for separators in a Li-ion battery system are as follows:8−11 (1) Thickness: A low thickness is essential for high energy and power density. (2) Pore size: It should be smaller than the particle size of the electrode parts, and it should be less than 1 μm. (3) Porosity: It is required to keep enough electrolyte for ionic conductivity. Porosity for current separators is about 40%. (4) Air permeability: Low gurley values are needed for good electrical performance. (5) Wettability: It is essential for a separator to wet sufficiently in liquid electrolyte and retain it durably. (6) Thermal stability: Thermal shrinkage is required to be not more than 5% after 1 h at 90 °C. (7) Dimensional stability: The shape of the separator should not be changed when put into liquid electrolyte. Moreover, dimensions of the separator should keep stability over a wide temperature range. (8) Shutdown:12,13 In order to ensure cell safety, the temperature difference between shutdown and meltdown should be as large as possible. (9) Mechanical property: Separators should satisfy tensile and puncture strength requirements. (10) Chemical stability: It is required to be stable in the electrolyte for a long time. In the above requirements, pore size, porosity, wettability, thermal stability, and shutdown are important parameters for evaluating whether a material is suitable as a separator in a Liion battery system. Currently, polyolefin-based microporous membranes, mainly composed of polypropylene (PP), polyethylene (PE), or PP/ PE composite are the most widely used separators because they possess excellent chemical stability, thickness, and high mechanical strength. However, these separators are known to have low porosity (about 40%), insufficient electrolyte wettability, and poor thermal stability.14−16 In these membranes, it is difficult to ensure electrical isolation and facile ionic transport between anode and cathode.4 Therefore, it is necessary to develop novel separators with sufficient porosity, good wettability, and improved thermal stability to enable high performance Li-ion batteries with enhanced safety. Nonwoven separators are being considered to replace microporous membranes because they are low cost and are able to easily form microstructures2 (e.g., porosity of about 60− 90%17). However, the large pore size of nonwoven separators limits their application. Electrospinning is an effective technique to fabricate nanofibers using polymer solutions; therefore, it has been investigated over recent decades.18−20 Using this technique, nanofibers can be fabricated to form a nonwoven membrane on the substrate. In comparison with conventional nonwovens, electrospun fiber membranes possess the following properties, such as smaller pore size (several hundred nm), higher porosity (about 80%), and improved air permeability; therefore, they are selected as excellent separator candidates for Li-ion batteries. Electrospun poly(vinylidene fluoride) (PVDF),21−25 polyacrylonitrile (PAN),17,26,27 and polyvinyl chloride (PVC)28 nanofiber-based membranes have been

reported in a Li-ion battery system due to their large capacity and high-rate capability. Recently, polyethylene terephthalate (PET), a well-known membrane material, has attracted much attention as a separator of Li-ion batteries because of its good mechanical properties, thermal stability, and excellent electronic insulation.29 In our previous study, the tensile strength of the PET was measured as 115 and 52 MPa in machine and transverse directions, respectively. This is similar to the strength of current commercial products, satisfying the needs of tensile strength during battery assembly. The puncture strength was 350 gf/mil, which shows a higher value than required (300 gf/mil) for Liion batteries. Therefore, in this study, a novel PET-reinforced PVDF nanofiber (PRPN) hybrid separator was proposed, consisting of a PVDF nanofiber and a PET microfiber membrane. The PVDF nanofiber contributed to controlling pore size for the PRPN hybrid separator, which can improve porosity and specific surface area to improve wettability. Meanwhile, the PET microfiber acted as a mechanical support that was helpful to improving mechanical strength, heat, and dimensional stability of the separator. As the PVDF and PET membranes are low-priced, it is possible for the proposed PRPN hybrid separator to be applied widely in Li-ion battery systems. Therefore, in our study, a PRPN hybrid membrane for the separator was obtained under controlled conditions. The morphologies of both materials were observed. In addition, electrolyte wettability, electrolyte uptake, shutdown and meltdown temperature, and heat stability were also investigated as these parameters reflect the physical properties of a separator used in a Li-ion battery.

2. EXPERIMENTAL SECTION 2.1. Preparation of PRPN Hybrid Separator. A 10 wt % polymer solution was prepared by dissolving PVDF (Sigma-Aldrich Co. LLC.) into a DMAC/MEK solution at a concentration of 7:3 (w/ w) by homogenized mixing for 24 h. For electrospinning, the apparatus was applied as introduced in previous study.18,19,30−32 The prepared solution was loaded into a plastic syringe using a sharp conducting tip. A high voltage resulted in the molecular ionization of charge redistribution when it was applied to the solution. The needle was electrified by a high-voltage direct-current supply at 11 kV, and the distance between the capillary tip and metal collector was 12 cm. A PVDF nanofiber with a thickness of 4 μm was fabricated. The hybrid PRPN separator was mechanically pressed by the asspun PVDF nanofiber and PET microfiber nonwoven (Tentok Paper. Ltd.). PET microfiber nonwoven was fabricated using an average diameter of 2.5 μm and average fiber length of 4 mm PET fiber. It was 12 μm in thickness with an area density of 10 g/m2. The pressing process was carried out in air at a temperature of 110 °C, under 10.2 kPa load for 30 s. During the process, a kind of low-melting polymer (melting point: 110 °C) was applied for bonding between the PVDF nanofiber and PET substrate, with an area density of 0.1 g/m2. 2.2. Characterization of As-Fabricated Hybrid Separator. Surface morphology, wettability, electrolyte uptake, shutdown and meltdown temperature, and thermal shrinkage can majorly define separators. The morphologies of as-spun nanofiber and PET microfiber were observed by scanning electron microscopy (SEM, JSM-6010LA, JEOL, Japan). In a single SEM image, 100 points were randomly selected to obtain the average diameters of the PVDF nanofiber and PET microfiber. Pore size and porosity were also obtained by the SEM image analysis. The wettability of the PRPN hybrid separator was evaluated on the basis of Byreck and a drop test [JIS L 1907], using a liquid electrolyte (commercial) instead of water. The Byreck test measured the migration distance by dipping the end of sample into liquid electrolyte B

DOI: 10.1021/acsami.7b06303 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces for 1 min. The drop test measured the diffusion area of the liquid electrolyte when a drop of electrolyte was dropped on a sample. Moreover, the wettability of two commercial separators was measured and evaluated for comparison. One is dry-laid nonwoven (A), using PP as material, and the other is wet-laid nonwoven (B), using PE as material. Liquid electrolyte uptake and retention of the PRPN hybrid separator were measured by immersing the membrane in liquid electrolyte for 5 min. The liquid electrolyte-absorbed separator was weighed quickly after removing the superfluous solution using wipes. Samples used in this experiment were cut into a piece of 5 × 5 cm square. The electrolyte uptake was calculated by Eq 1:

uptake (%) =

W1 − W0 × 100% W0

thermal shrinkage (%) =

L0 − L1 × 100% L0

(2)

where L0 is the length of sample before heat treatment, and L1 is the length of sample after heat treatment. The thermal shrinkage of the mentioned commercial separators was also measured and compared. Differential scanning calorimetry (DSC) thermal analysis was applied using a PerkinElmer Pyris-1 analyzer to determine the melting point of the membrane. A standard indium reference was used to calibrate both temperature and heat flow. All thermal analyses were performed under a dry nitrogen atmosphere. Samples were first heated to 290 °C at a rate of 10 °C/min and then maintained at this temperature for 10 min to ensure complete melting.

(1)

3. RESULTS AND DISCUSSION 3.1. Morphological Analysis. The morphology of a separator has an effect on battery performance as well as the efficiency during the assembling and handling process due to mechanical and physical properties. Figure 2 shows the scheme of the PRPN hybrid separator, which is multilayer mechanically pressed by PVDF nanofiber and PET microfiber. The crosssectional image shown in Figure 2 demonstrates a two-layer structure. The top layer is the PVDF nanofiber and the bottom layer is the PET microfiber. In the top layer, finer fibers can be found than the fibers in the bottom layer. SEM images of PVDF nanofiber and PET microfiber are shown in Figure 3. Image (a) and (d) were recorded at magnification of 5000, and (b) and (c) were of 30 000 and 1000 respectively. Figure 3a,b show that the PVDF membrane consists of nanofibers oriented randomly, with an average diameter of 184 nm. A highly uniform, porous structure can be observed, ensuring a uniform current distribution throughout the separator. The pore-size distribution of the PVDF nanofiber shown in Figure 4 demonstrates a uniform pore size with average size of 0.28 μm, and 89.3% of the pore size was distributed from 0.25 to 0.30 μm, which shows a uniform poresize distribution. The pore size of this structure is so small that it can prevent shorts caused by dendrite growth or particle migration between the anode and the cathode. Moreover, uniform pore-size distribution is helpful to avoiding nonuniform current density, which may accelerate the formation of Li dendrites in the Li-ion battery system. The porosity of PVDF was about 80%, twice that of current separators (about 40%9), which is beneficial for ionic conductivity between electrodes. The nano structure is favorable for maintaining battery voltage from self-discharge and also can breakdown at high discharge rates or under vigorous conditions.6 The SEM images of the PET microfiber shown in Figure 3c,d, indicate that the PET fibers are randomly arranged,

where W1 refers to the weight of the electrolyte-absorbed separator, and W0 is the weight of dry separator sample. For comparison, commercial nonwovens A and B were also utilized and the electrolyte uptake was calculated. The thermal shrinkage of the PRPN hybrid separator was evaluated by measuring the dimensional changes of separator before and after heat treatment at several setting temperatures (105, 115, 125, and 135 °C) for 1 h in an oven and then cooled for 10 min. Sample sizes were 14 cm in the machine direction (MD) and 6 cm in the transverse direction (TD). Before heat treatment, two 10 and 4 cm long lines were marked in the MD and TD on the surface of samples,

Figure 1. Thermal stability experiment for a separator.

respectively, as shown in Figure 1. Thermal shrinkage can be obtained by Eq 2.

Figure 2. Scheme of the PRPN hybrid separator, showing the composition and the cross-section of the separator. C

DOI: 10.1021/acsami.7b06303 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. SEM images of (a,b) PVDF nanofiber and (c,d) PET microfiber.

Figure 4. Pore-size distribution of PVDF nanofiber.

Figure 5. Electrolyte migration distances and drop areas of three kinds of separator, with the inset showing images of the drop-area test.

exhibiting a three-dimensional porous network structure. The PET microfibers have a smooth surface and an average diameter of about 2.88 μm. Both PVDF nanofiber and PET microfiber have a smooth surface and are almost free of defects such as beads; therefore, it is of essential significance to gain good mechanical property and prevent the internal short circuits within the cells.2 3.2. Wettability, Uptake, and Retention of Liquid Electrolyte. The wettability of the separator is an important factor because a separator with high wettability can retain electrolyte effectively and facilitate smooth electrolyte diffusion into the cell assembly.8 The improvement in the electrolyte wettability is partly ascribed to the nanoporous structure.33 Electrolyte migration distances and drop areas are shown in Figure 5 in comparison to two commercial separators A and B. For the diffusion distance, the PRPN hybrid separator is 4 times longer than current commercial ones. Moreover, the Figure 5 inset shows an image of a drop-area test, with the drop area of the PRPN hybrid separator being 18 times larger compared to that of current separators A and B. Therefore, PRPN hybrid separators can completely and quickly wet out in battery electrolytes, which satisfies the wettability needed for separators. In addition to wettability, liquid electrolyte uptake is considered as an major parameter for evaluating the absorption ability of a separator. After 5 min of electrolyte absorption and removing superfluous solution, the liquid electrolyte retention area and dried area are shown in Figure 6, along with the

commercial separators A and B. The dried area shows liquid electrolyte that is unabsorbed by the separators, marked by a red circle in Figure 6c. The PRPN hybrid separator absorbed the most liquid electrolyte compared to the other two separators, and the dried area of the PRPN hybrid separator has only some small points, smaller than the commercial others. The PRPN hybrid separator demonstrates improved performance of retaining the electrolyte permanently, which increases the cycle life of battery. These results match well with that of migration distance and drop area. Moreover, the electrolyte uptakes of three different separators are shown in Figure 6c. The electrolyte uptake for PRPN hybrid separator was 270%, higher than dry-laid and wet-laid nonwovens. The high wettability and absorption ability of the PRPN hybrid separator is attributed to its excellent porosity. The pore structure of a PRPN hybrid separator has a porosity of 80%, higher than separators A and B (both are about 40%). 3.3. Thermal Stability. The heat stability is another important factor for separators as it influences battery safety. The heat shrink of the separators is shown in Figure 7 at different temperatures after 1 h heating. The PRPN hybrid separator showed heat stability at these setting temperatures, as the separator did not shrink significantly, while at 135 °C, separators A and B obviously shrank. Over a wide temperature range from 105−135 °C, the PRPN hybrid separator remained constant in length and width, showing dimensional stability. The thermal shrinkage of the three separators is shown in D

DOI: 10.1021/acsami.7b06303 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. (a) Schematic diagram, (b) real image, and (c) results of uptake and retention of liquid electrolyte, inset in (c) showing the retention of separators, with the red marks indicating some dried parts on the separators.

than separators A and B. As separators A and B are fabricated by low melting-temperature polymers, shrink occurred easily when heat treatment reached near to their melting temperature. Similar to other batteries, a Li-ion battery generates heat when overcharged.34 In Li-ion batteries, the “shutdown” feature of separator is useful for preventing thermal runaway reactions, restricting temperature, and preventing ventilation in shortcircuited cells.13,35 Usually shutdown occurs near to the melting temperature, and at shutdown temperature, a significant increase in cell impedance occurs and electrical current passage through the cell is restrained. This can prevent a further electrochemical reaction and, thus, shut the cell down. The PE separator has a shutdown temperature at around 130 °C and PP melts at about 170 °C3. However, even after shutdown, the cell temperature continues to increase before actually cooling down. Therefore, the meltdown temperature of separators are expected to be above the shutdown temperature, with the difference being as large as possible to protect from internal shorts. In order to ensure cell safety, it is necessary to make the difference between the shutdown and meltdown temperature as large as possible. Thus, the superior thermal stability of the PRPN hybrid separator is important. For the PRPN hybrid separator, the difference between the shutdown and meltdown temperature was 121 °C, ensuring that the Li-ion battery is safe for use.

Figure 7. Thermal shrinkage of three separators (A: Dry-laid, B: Wetlaid, and C: PRPN separator) after 1 h heating from 105 to 135 °C.

4. CONCLUSION In this study, a PRPN hybrid separator, composed of PET microfiber-reinforced PVDF nanofiber, was proposed and successfully fabricated. Surface morphology, liquid electrolyte wettability and uptake, thermal shrinkage and shutdown, and meltdown temperature were discussed to evaluate the characteristics of the new separator. The PRPN hybrid separator had a homogeneous pore size (average 0.28 μm) and high porosity of about 80%, demonstrating higher ionic conductivities than those of current separators. The proposed separator was demonstrated as achieving wet out in battery electrolytes completely and quickly, which satisfies wettability requirements. Moreover, the electrolyte uptake was evaluated as up to 270%, which is higher than that of dry-laid and wet-laid nonwovens. Both wettability and electrolyte uptake showed that the proposed PRPN hybrid separator had a high absorption ability for a liquid electrolyte solution, which

Figure 8 at four different temperature conditions for 1 h at the MD and TD, respectively. At the MD, in the four setting temperatures, both dry-laid and wet-laid nonwovens shrank, while the PRPN hybrid separator showed no change in length. Moreover, at the TD direction, the wet-laid nonwoven begun to shrink when the heat temperature reached 115 °C, while the PRPN hybrid separator maintained stability after 1 h of heating from 105 to 135 °C, and heat shrink did not occur at all setting temperatures. The phenomenon can be explained by the high melting point of PET as shown in Figure 9, which shows the DSC curve of the PRPN hybrid separator. It is clearly noted that a PVDF nanofiber has a endothermic peak at 131 °C, and the PET microfiber gives a endothermic peak at 252 °C. That is to say, the shutdown temperature for the PRPN hybrid separator is around 130 °C, which is the same as the PE separator. However, the meltdown temperature is about 250 °C, higher E

DOI: 10.1021/acsami.7b06303 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 8. Thermal shrinkage of separators at different temperatures for (a) MD and (b) TD .

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Figure 9. DSC traces of separator under N2 atmosphere.

satisfies the requirement for Li-ion battery. For heat stability, there was no shrink even when the heating temperature reached 135 °C, demonstrating high thermal stability. Moreover, the DSC showed that the PRPN hybrid separator possessed a shutdown temperature of 131 °C, which is the same as conventional separators. Also, the meltdown temperature was 252 °C, which is higher than the shutdown temperature. This can prevent the cell temperature from increasing before actually starting to cool down after cell shutdown. This is important to protect against internal cell shorts. In summary, the PRPN hybrid separator has high porosity, wettability and electrolyte uptake, outstanding heat stability, high meltdown temperature, and mechanical strength. Thus, it is possible to use the proposed PRPN hybrid separator as a candidate material for safe and high-performance Li-ion batteries.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*I.S.K.: Tel: (+81)-268-21-5439; E-mail: [email protected] ORCID

Chunhong Zhu: 0000-0001-9251-7899 Funding

The authors received no financial support for the research, authorship, and/or publication of this article. Notes

The authors declare no competing financial interest. F

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