Patent Application
20160365556
The present application relates to an electrospun nonwoven nanofiber separator and a method of improving physical stability of a battery separator with improved performance.
Flexible thin film battery market is an attractive and fast growing market characterized by very high production volumes. To sustain its growth, manufacturers have tried to make them extremely reliable and low in cost Thin-film flexible lithium-ion batteries (LIB) are ideally suited for a variety of applications where small-power sources are needed. They can be manufactured in a variety of shapes and sizes as required by customers. By using the available space within a device, the battery can provide the required power while occupying otherwise wasted space and adding negligible mass. While flexible batteries are frequently bent and stretched, battery separators need to have superior mechanical strength, in addition to chemical stability, to withstand the stress.
Polymeric nonwovens have been used as separators in flexible lithium-ion batteries. They are fibrous mats made by bonding numerous fibers directionally or randomly together through chemical, physical or mechanical methods. Both natural and synthetic materials have been used to manufacture the fibers for separators in lithium-ion batteries. They are generally uniform in weight, easily controlled in thickness, and have high porosity and resistance to degradation by electrolyte.
Materials such as polyethylene (PE), polypropylene (PP), polyamide (PA), poly(tetrafluoroethylene) (PTFE), poly(vinylidene fluoride) (PVDF) and poly(vinyl chloride) (PVC) are commonly used in fabrication of nanofiber separators. They can be single formulated polymers or complexes of polymers formulated as composites. However, it is difficult to make a thin nonwoven fabric mat with acceptable physical properties, mechanical stabilities and comparable battery performances as compared with microporous polymer films separator.
Therefore, there is a need to provide a polymeric nonwoven nanofabric separator for lithium-ion batteries showing high porosity, good wettability, high puncture strength and comparable dimension stability with microporous thin film separators.
In one aspect, the present application provides a nonwoven nanofiber separator, including a composite of a first polymer material and a second polymer material, wherein:
In another embodiment, in conjunction with any of the above and below embodiments, the first polymer material may include poly(vinylidene fluoride) and the second polymer material may include poly(vinylidene fluoride-hexafluoropropylene).
In another embodiment, in conjunction with any of the above and below embodiments, the second polymer material may have a melting temperature lower than a melting temperature of the first polymer material.
In another embodiment, in conjunction with any of the above and below embodiments, the first and second polymer materials may be in a weight ratio ranging from about 3:1 to about 1:1, preferably about 3:1 to about 2:1, more preferably about 3:1 to about 3:2, and most preferably about 3:1.
In another embodiment, in conjunction with any of the above and below embodiments, the nonwoven nanofiber separator may further include at least one additive selected from the group consisting of tetramethyl orthosilicate and tetraethyl orthosilicate.
In another embodiment, in conjunction with any of the above and below embodiments, the nonwoven nanofiber separator may be prepared by electrospinning a polymer formulation on aluminum foil to give a freestanding separator, wherein the polymer formulation may include the first polymer material, the second polymer material, at least one solvent, optionally the at least one additive and optionally lithium chloride.
In another embodiment, in conjunction with any of the above and below embodiments, the polymer formulation may include the first and second polymer materials in a total amount of about 15-25 wt. % of the formulation, preferably about 15-20 wt. %, and more preferably about 16.5 wt. %.
In another embodiment, in conjunction with any of the above and below embodiments, the polymer formulation may further include about 1-5 wt. % of the at least one additive, preferably about 2-5 wt. %, more preferably about 3-5 wt. %, and most preferably about 5 wt. %.
In another embodiment, in conjunction with any of the above and below embodiments, the polymer formulation may include about 0.1-0.6 μg of lithium chloride solution, preferably about 0.1-0.5 μg, more preferably about 0.2-0.4 μg, and most preferably about 0.3-0.4 μg.
In another embodiment, in conjunction with any of the above and below embodiments, the at least one solvent may be selected from the group consisting of N-methyl-2-pyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide, acetone and tetrahydrofuran.
In another embodiment, in conjunction with any of the above and below embodiments, the at least one solvent may include N,N-dimethylacetamide and acetone.
In another embodiment, in conjunction with any of the above and below embodiments, the at least one solvent may include N,N-dimethylacetamide and acetone in a weight ratio ranging from about 3:1 to about 1:1, preferably about 2:1 to about 1:1, more preferably about 1.5:1 to about 1:1, and most preferably about 1.25:1.
In another embodiment, in conjunction with any of the above and below embodiments, the nonwoven nanofiber separator may be prepared by the following steps:
In another embodiment, in conjunction with any of the above and below embodiments, the nonwoven nanofiber separator may be thermal annealed after the electrospinning.
In another embodiment, in conjunction with any of the above and below embodiments, the annealing temperature may be lower than the melting points of the first and second polymer materials.
In another embodiment, in conjunction with any of the above and below embodiments, the nonwoven nanofiber may be thermal annealed at around 100-165° C. for about 1-10 hour.
In another aspect, the present application provides a method of improving physical stability of a battery separator, including:
In another embodiment, in conjunction with any of the above and below embodiments, the at least one polymer may be poly(vinylidene fluoride-hexafluoropropylene).
In another embodiment, in conjunction with any of the above and below embodiments, the polymer solution may include the at least one polymer in an amount of about 10-40 wt. %, preferably about 10-30 wt. %, more preferably about 10-20 wt. %, and most preferably about 10-15 wt. %.
In another embodiment, in conjunction with any of the above and below embodiments, the at least one solvent may be selected from the group consisting of N-methyl-2-pyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide, acetone and tetrahydrofuran.
The embodiments of the present application are described with reference to the attached figures, wherein:
Reference will now be made in detail to preferred embodiments of nonwoven nanofiber separators and methods of improving physical stability of a battery separator, examples of which are also provided in the following description. Exemplary embodiments of the methods disclosed in the present application are described in detail, although it will be apparent to those skilled in the relevant art that some features that are not particularly important to an understanding of the present application may not be shown for the sake of clarity.
Before the present application is described in further detail, it is to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present application will be limited only by the appended claims.
Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range is encompassed within the application. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges is also encompassed within the application, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the application.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present application, a limited number of the exemplary methods and materials are described herein.
It must be noted that as used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise.
In the first aspect, the present application provides an electrospun nonwoven nanofiber separator including a composite of a first polymer material and a second polymer material. The first polymer material may include poly(vinylidene fluoride) (PVDF), polyimide (PI), polyamide (PA) or polyacrylonitrile (PAN). The second polymer material may include polyethylene glycol (PEG), polyacrylonitrile (PAN), poly(ethylene terephthalate) (PET), poly(vinylidene fluoride) (PVDF), poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) or poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-co-CTFE), wherein the second polymer material is different from the first polymer material.
PVDF has been widely used for making ultrafiltration and microfiltration membranes owing to its excellent chemical resistance and good thermal stability. Pure PVDF polymer has high melting point and crystallinity, and good mechanical properties. However, it is only soluble in a limited number of solvents, such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAC), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and shows low swelling ability when soaked in common electrolytes.
Copolymer PVDF-HFP has been studied for the application in rechargeable lithium ion batteries. It can improve the solubility, making the polymer soluble in common organic solvents, such as acetone and tetrahydrofuran (THF), which can further improve the process capability. In addition, it has very high swelling ability which can enhance the electrolyte uptake for the separator layer.
The inventors of the present application found that the composite of PVDF and PVDF-HFP offers many advantages over other polymers in the fabrication of separators, such as:
Accordingly, in a preferred embodiment, the electrospun nonwoven nanofiber separator of the present application includes a composite of PVDF and PVDF-HFP.
In another embodiment, in conjunction with any of the above and below embodiments, the first polymer material may have a melting temperature greater than about 170° C. In another embodiment, in conjunction with any of the above and below embodiments, the second polymer material has a melting temperature lower than that of the first polymer material such that the second polymer material is melted and fused together during thermal runaway which will reduce the porosity in the layer and slow down the Li-ion transport or prevent the electrochemical reaction.
In another embodiment, in conjunction with any of the above and below embodiments, the first and second polymer materials may be in the weight ratio ranging from about 3:1 to about 1:1, preferably about 3:1 to about 2:1, more preferably about 3:1 to about 3:2, and most preferably about 3:1.
In another embodiment, in conjunction with any of the above and below embodiments, the electrospun nonwoven nanofiber separator of the present application may further include at least one additive selected from the group consisting of tetramethyl orthosilicate (TMOS) and tetraethyl orthosilicate (TEOS). These additives can be hydrolyzed into ceramic components, thereby improving the surface texture and enhancing dimensional stability of the separator.
In another embodiment, in conjunction with any of the above and below embodiments, the electrospun nonwoven nanofiber separator of the present application may be prepared by electrospinning a polymer formulation on aluminum foil to give a freestanding separator. The polymer formulation may include the first and second polymer materials in a total amount of about 15-25 wt. % of the formulation, preferably about 15-20 wt. %, and more preferably about 16.5 wt. %.
In another embodiment, in conjunction with any of the above and below embodiments, the polymer formulation for electrospinning the nonwoven nanofiber separator of the present application may further include about 1-5 wt. % of the at least one additive, preferably about 2-5 wt. %, more preferably about 3-5 wt. %, and most preferably about 5 wt. %.
In another embodiment, in conjunction with any of the above and below embodiments, the polymer formulation for electrospinning the nonwoven nanofiber separator of the present application may further include lithium chloride (LiCl) to facilitate ion transfer across the separator and facilitate the electrospinning process by increasing the conductivity of the polymer solution. In a preferred embodiment, the polymer formulation may include about 0.1-0.6 μg of LiCl, preferably about 0.1-0.5 μg, more preferably about 0.2-0.4 μg, and most preferably about 0.3-0.4 μg.
In another embodiment, in conjunction with any of the above and below embodiments, the polymer formulation for electrospinning the nonwoven nanofiber separator of the present application may include at least one solvent selected from the group consisting of N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAC), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetone and tetrahydrofuran (THF). In a preferred embodiment, the polymer formulation may include DMF and acetone. In another preferred embodiment, the polymer formulation may include DMF and acetone in a weight ratio ranging from about 3:1 to about 1:1, preferably about 2:1 to about 1:1, more preferably about 1.5:1 to 1:1, and most preferably about 1.25:1.
In another embodiment, in conjunction with any of the above and below embodiments, the nonwoven nanofiber separator of the present application may be electrospun by the following steps:
In another embodiment, in conjunction with any of the above and below embodiments, electrospinning of the nonwoven nanofiber separator of the present application may be performed under the following parameters:
In another embodiment, in conjunction with any of the above and below embodiments, the nanofiber has a diameter between about 100 nm to about 300 nm. The nonwoven nanofiber separator of the present application may have a porosity of about 60% to about 90% as compared to solid thin film prepared by polyethylene (PE)/polypropylene (PP) materials. The average pore size may be less than 1μm. The nonwoven nanofiber separator may have mix puncture strength of >500 N.
Many reports have indicated that electrospun PVDF fiber mats have poorer physical properties than other polymer films, which is mainly attributed to the nonwoven nature and low interconnection between each fiber. While not wishing to be bound by theory, it is believed that thermal annealing of electrospun nanofiber separator can enhance the interconnection between fibers, thereby improve the physical stability, tensile strength, elongation at break as well as tensile modulus, thus the stability of the electrospun nanofiber mats. Therefore, in another embodiment, in conjunction with any of the above and below embodiments, electrospinning of the nonwoven nanofiber separator of the present application may further include thermal annealing the electrospun nonwoven nanofiber mat. In a preferred embodiment, the annealing temperature may be lower than the melting points of the first and second polymer materials. In another preferred embodiment, the nonwoven nanofiber separator of the present application may be thermal annealed at around 100-165° C. for about 1-10 hours. In still another preferred embodiment, the electrospun nonwoven nanofiber mat may be thermal annealed at around 160° C. for about 5 hours.
In another aspect, the present application provides a method of improving physical stability of a battery separator, including:
In another embodiment, in conjunction with any of the above and below embodiments, the polymer solution for electrospinning as nanofiber coating may include the at least one polymer in an amount of about 10-40 wt. %, preferably about 10-30 wt. %, more preferably about 10-20 wt. %, and most preferably about 10-15 wt. %.
In another embodiment, in conjunction with any of the above and below embodiments, the polymer solution for electrospinning as nanofiber coating may include at least one solvent selected from the group consisting of N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAC), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetone and tetrahydrofuran (THF). In a preferred embodiment, the polymer formulation may include DMF.
In another embodiment, in conjunction with any of the above and below embodiments, electrospinning nanofiber coating on a separator may be performed under the following parameters:
To a 250 ml round bottom flask, 20 ml AR grade acetone, 25.2 ml DMF and 40 μl lithium chloride solution (25 mg LiCl dissolved in 250 ml DMF) were mixed well by magnetic stirrer for about 10-15 minutes. 7.83 g polymer power of PVDF (Solef® PVDF 6000 series from Solvay) and PVDF-HFP (Solef® PVDF-HFP from Solvay) in the weight ratio of 3:1 was added to a container of THINKY mixer and mixed at 100-500 rpm for about 1-2 minutes. After stirring process was completed, the PVDF/PVDF-HFP powder mixture was added to the as prepared acetone/DMF solution. The solution was kept at around 85° C. for about 5 hours and the resulting solution was allowed to cool down to room temperature before further process.
A formulation of PVDF/PVDF-HFP similar to that of Example 1 was prepared, except that after heating, 3 ml of TMOS was added into the as prepared polymer solution by syringe and heated at around 80-85° C. for another 5 hours.
To a 50 ml round bottom flask, 5.0 g polymer power of PVDF and PVDF-HFP in the weight ratio of 3:1 was dissolved in 24.2 ml DMF and was heated at 80-100° C. with stiffing at 100-500 rpm for 2 hours. Then, the as prepared polymer solution was allowed to cool down to room temperature. The polymer solution was added with 3 ml TEOS, stirred at room temperature for 20-30 minutes, and heated at 80-100° C. with stirring at 100-500 rpm for another 5 hours.
To a 50 ml round bottle flask, 5.0 g polymer powder (PVDF-HFP) was dissolved in 24.2 ml DMF. The solution was heated at 80-100° C. with stiffing at 100-500 rpm for 2 hours.
The polymer formulations from Examples 1 and 2 were separately loaded into a 30 ml syringe for electrospinning using an electrospinning machine Model No: NANON-01A from Mechanics Electronics Computer Corporation (MECC). The syringe was connected to the spinner head of the machine via a rubber pipe. A roll-to-roll collector with a 200-mm wide aluminium foil as substrate was used to collect the electrospun nanofiber. After electrospinning, the whole nonwoven nanofiber mat with aluminium substrate was treated at 160° C. for about 1-5 h in a convection oven. Table 1 summarizes the parameters of electrospinning and post-treatment.
SEM was used to characterize the as prepared nanofiber separators.
The polymer formulation from Example 3 was loaded into a carriage of an electrospinning machine Model No: NS LAB from Elmarco NanospiderTM for electrospinning. A roll-to-roll collector with a 200-mm wide aluminium foil as substrate was used to collect the electrospun nanofiber. After electrospinning, the whole nonwoven nanofiber mat with aluminium substrate was treated at 160° C. for lh in a convection oven and Separator V was obtained. Table 2 summarizes the parameters of electrospinning and post-treatment.
Similar to the previous study, SEM was used to characterize and evaluate the electrospinning result. The fibres were formed in nonwoven manner with no polymer beans found in this study with fibres diameter of 200-300 nm. While not wishing to be bound by theory, it is believed that the nanofibers were interconnected to each other after the thermal annealing process.
The polymer formulation from Example 4 was loaded into a carriage of an electrospinning machine Model No: NS LAB from Elmarco NanospiderTM for electrospinning. A roll-to-roll collector with a trilayer polypropylene-polyethylene-polypropylene (PP-PE-PP) separator, Celgard®_2325) as substrate was used to collect the electrospun nanofiber. Table 3 summarizes the parameters of electrospinning. Separator VI with only one side coated with nanofiber was obtained.
Similar to the previous study, SEM was used to characterize and evaluate the electrospinning result. It was found that the nanofibers were able to formed and developed on the commercial separators. No destructive effect was found on the commercial separator after the nanofiber coating process.
The mechanical performances of Separators I-IV have been confirmed by MTS® mechanical tester. All the test samples were prepared in the size of 305 mm (W)×1000 mm (L) with the effective testing size of each sample is 305 mm (W)×600 mm (L).
The pulling rate of each sample is 10 mm/min with all the separators were evaluated under the same testing condition. The mechanical performances are illustrated in
As showed in Table 4, it was found that Separator I showed the lowest tensile strength (56 kgfcm−2) within the study while Separator II showed a double increment in tensile strength (110 kgfcm−2) after thermal annealing for lh. Upon increase of annealing time from 1 h to 5 h, a 3-fold of increment in tensile strength (150 kgfcm−2) was observed. It was found that thermal treatment at 160° C. can improve the tensile strength of the separator in machine direction (MD) which is three times higher than that of non-annealed separator.
It was also found that the use of ceramic additives, such as TMOS, can also improve the tensile strength of separator from 56 kgf/cm2 (Separator I-MD) to 166 kgf/cm2 (Separator IV-MD). In addition, the thermal annealed separator showed an improved in modulus which indicated the deformation stability.
Moreover, it was found that both Separators III and IV showed similar tensile strength in both mechanical (MD) and transverse directions (TD), which indicated that these separators are strong in both directions. On the contrary, the maximum deformation of separators decreased (Separator I>Separator II>Separator III) with the increase in tensile strength (Separator III>Separator II>Separator I) which indicated that the nanofibers are bonded or interconnected with each other after thermal annealing process.
As shown in Table 5, the dimension stabilities (MD) Separators I to III increase with the annealing time. Separators I and II showed deformation (>2%) when 10 kgfcm−2 was applied while Separator III showed 1.8% deformation only. Once 20 kgfcm−2 force was applied, about 5% deformation was found for Separators I and II while Separator III only showed deformation value of 3.7%. Upon further increase of loading force to 30 kgfcm−2, serious deformation was found in Separator I (>50%) which indicated the separator deform dramatically and is not suitable to use for the battery packing with 30 kgfcm−2.
In addition, the TMOS modified Separator IV showed the highest dimension stability within the study. 1% deformation was observed when 10 kgfcm−2 was loaded. Once the loading force was increased to 30 kgfcm−2, a 5% deformation was observed which is similar to 20 kgfcm−2 being applied to Separator II.
The melting points of Separators I, III and IV were analyzed by DSC analysis. The thermal analysis was performed from 25° C. to 300° C. with a heating rate of 10° C./min and the whole experiment was protected under nitrogen.
From the DSC analysis shown in
In order to have a better understanding on thermal stability of Separators I, III and IV, TGA was used to analyze their decomposition temperature. The sample was heated from 25° C. to 800° C., with a heating rate of 10° C./min, protected under nitrogen.
From the TGA study as shown in
The affinity of the battery electrolyte to Separators I-IV and a commercial separator, Celgard®, was studied by contact angle experiment. Lithium ion battery electrolyte was loaded into a small syringe. The contact angle image was taken when a droplet of electrolyte was dropped on the separator.
As shown in
Contact angle study was also carried out on nanofiber coated Separator VI (see
Porosity of Separators I-VI was monitored. The porosity test was performed by AutoPore IV 9510. As shown in Table 6, it was found that freestanding separator showed a porosity of 81-86% which is much higher than that of the commercial separator. In addition, nanofiber coated Separator VI also showed slight improvement in porosity which is of about 7.9%. The improved in porosity indicated that the separator can absorb more electrolyte which accounted for the improvement in performance stability during charge and discharge processes.
The mix puncture strengths of Separators I, III, IV and VI have been characterized by MTS tester. Anode and cathode of 3 cm×3 cm in size were prepared while the separators were prepared with slightly larger than the electrodes. Multi-meter was used to monitor the short circuit condition.
It was found that Separators III and IV showed comparable puncture strength with Celgard® while Separator VI showed higher puncture strength than others. Among all the separators, Separator I showed the lowest mix puncture strength as it was not thermally treated, thus, the nanofibers are not interconnected. Thermal annealing can enhance the mean puncture strength from 563.4 N (Separator I) to 875.9 N (Separator III). Upon addition of additive to the polymer formulations, the mix puncture strength can further improve to 980.7 N which accounted for the best puncture result within the study.
Nanofiber coated Separator VI (1093 N) showed an improved in puncture strength as compared with Celgard® (898.4 N). This may attribute to the additional nanofiber layer which acts as a sponge layer, thus, an increased in puncture strength was found.
To study the effect on battery performance, nanofiber freestanding Separator V and Celgard® were separately packed into a 70 mAh lithium ion battery for charge-discharge performance test. The test conditions are listed in Table 8 and the battery performance is shown in
Another charge-discharge performance test was performed under the conditions provided in Table 9. As shown in
Thus, specific nanofiber separators and methods of improving physical stability of a battery separator have been disclosed. It should be apparent, however, to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive subject matter, therefore, is not to be restricted except in the spirit of the disclosure. Moreover, in interpreting the disclosure, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms “includes”, “including”, “comprises” and “comprising” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced.
This application claims the benefit of U.S. Provisional Patent Application No. 62/230,648 filed on Jun. 11, 2015, the entire content of which is hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 62230648 | Jun 2015 | US |
