CN100544074C - 表现出改善强度和稳定性的多层隔板 - Google Patents

表现出改善强度和稳定性的多层隔板 Download PDF

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CN100544074C
CN100544074C CNB2007100903592A CN200710090359A CN100544074C CN 100544074 C CN100544074 C CN 100544074C CN B2007100903592 A CNB2007100903592 A CN B2007100903592A CN 200710090359 A CN200710090359 A CN 200710090359A CN 100544074 C CN100544074 C CN 100544074C
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罗纳德·W·考尔
石烈
张正铭
小仓静雄
魏祥云
普瑞曼纳恩德·罗摩达斯
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Abstract

一种表现出改善强度和稳定性的多层隔板,即多层微孔电池隔板,其包括:具有对层测量时≤1.2熔体流动指数的高分子量聚丙烯层;聚乙烯层;具有对层测量时≤1.2熔体流动指数的高分子量聚丙烯层。所得由干法拉伸工艺形成的多孔电池隔板产生在保持13-25秒Gurley数和≤25微米厚度的同时具有≤37%孔隙率的微孔电池隔板。

Description

表现出改善强度和稳定性的多层隔板
发明领域
本发明为一种电池隔板和制造这种隔板的方法。当与通过干法拉伸工艺或者溶剂抽提工艺制造的其它隔板相比时,本发明的隔板在混合穿透测试和收缩减少方面表现出提高。出乎意料地,本发明的隔板即使在小于等于37%的孔隙率下也具有13至25秒的Gurley数。
发明背景
使用微孔多层膜作为电池隔板是已知的。见例如美国专利US5,480,745、US5,691,047、US5,667,911、US5,691,077和US5,952,120。
美国专利US5,480,745公开了通过共挤压多层前体或者通过在152℃下加热-熔接预成形前体层而形成多层薄膜。然后,通过退火和拉伸,将由两种方法中的任一种所形成的多层前体制成微孔性的。这种由干法拉伸工艺制成的膜具有100%到300%的优选净拉伸量。
美国专利US5,691,047公开了通过共挤压多层前体或者通过在加热(120-140℃)和加压(1-3kg/cm2)下使三层或更多前体层组合而形成多层薄膜。在加热和加压下形成的前体,在0.5至8m/min(1.6-26.2ft/min)的速度下,具有3至60g/15mm(0.2-4g/mm)范围内的剥离强度。在实施例中,一种34μm的隔板具有1g/mm的剥离强度,而另一个约为0.5g/mm。然后,通过退火和拉伸,将由两种方法中的任一种所形成的多层前体制成微孔性的。在显示出较高Gurley数的同时,这些隔板的孔隙率大于本发明。
美国专利US5,667,911公开了通过将交叉叠层的微孔薄膜组合成(通过加热和加压或通过粘合)多层微孔薄膜而形成多层薄膜。利用加热(110℃-140℃)和加压(300-450psi)并以15-50ft/min(4.6-15.2m/min)的线速度将微孔薄膜层叠在一起。此参考文献教导了较低的Gurley值,这是这些薄膜孔隙率高的良好指示。
美国专利US5,691,077公开了通过组合、通过加热和加压(压延)、或者通过粘合、或者通过图案熔接使微孔薄膜形成多层微孔薄膜而形成多层薄膜。压延在125℃至130℃下以2至10分钟的停留时间进行。四层(4)层叠的多层微孔前体在单压料辊间压延。在显示出较高Gurley数的同时,这些隔板的孔隙率大于本发明。
美国专利US5,952,120公开了通过挤压无孔前体、将无孔前体粘合在一起、使粘合的无孔前体退火和将粘合的无孔前体拉伸成多层微孔薄膜而形成多层薄膜。至少四个(4)三层前体同时通过粘合、退火和拉伸步骤。在128℃(在125℃-135℃范围)下,在压料辊轧辊间以30ft/min(9.1m/min)的线速度进行粘合,产生5.7g/in(0.2g/mm)的剥离强度,而在128℃-130℃下,在压料辊轧辊间以40ft/min(12.2m/min)的线速度产生30g/in(1.2g/mm)的剥离强度。这些隔板的净拉伸均趋于至少100%或更高,同时Gurley值在高的一边。
尽管上述工艺已经生产出在商业上可行的适合用作电池隔板的多层微孔薄膜,但就隔板制造商和电池制造商双方而言,仍然都需要生产具有更大加工性能的隔板。为改善加工性能,需要隔板在制造过程中能更抗破损。困扰电池制造的两大问题是隔板的渗漏和收缩。当隔板处于电池在使用中将经受的热环境时,会发生收缩。过去,用来测试隔板渗漏的一个办法是通过击穿强度测试。但是,已经得知,一种称为混合穿透的新测试是一种比击穿强度测试好得多的表明隔板在制造过程中如何表现的指标。当测试收缩时,需要将隔板在高热下暴露一段时间。电池制造仍将要求隔板具有所需范围的Gurley数。
因此,需要提供一种改进的多层微孔薄膜来用作隔板,该隔板在仍保持低收缩值并且仍表现出所需范围的Gurley数的同时,在混合穿透强度方面显示出增强。
发明概述
本发明为一多层微孔电池隔板,其具有一通过对层测量时熔体流动指数≤1.2所指征的高分子量聚丙烯层、一聚乙烯层、和一具有对层测量时≤1.2的熔体流动指数的高分子量聚丙烯层。所得微孔电池隔板是通过干法拉伸工艺形成的。该微孔电池隔板对≤25微米厚的隔板在保持13至25秒Gurley数的同时,具有≤37%的孔隙率。
附图简述
通过参考下面的详细说明和附图,以上内容将变得更明显,在附图中:
图1是在混合穿透测试中的多层隔板的侧视图。
图2是显示施压后的电极和隔板的侧视图。
图3是显示隔板离子电阻斜率的图。
图4是测量隔板膜离子电阻的四探针AC阻抗法的示意图。
图5是由当前多层干法拉伸膜工艺制成的相同厚度的多层膜在混合穿透强度方面的百分增量图。
发明详述
电池隔板是指用在电化学电池或电容器中的微孔薄膜或膜。电化学电池包括初级(不可再充电的)和二次(可再充电的)电池,如基于锂化学的电池。这些薄膜通常由聚烯烃制成,例如聚乙烯、聚丙烯、聚丁烯、聚甲基戊烯、其混合物及其共聚物。聚丙烯(包括全同立构和无规立构的)和聚乙烯(包括LDPE、LLDPE、HDPE、和UHMWPE)及其混合体和它们的共聚物是用于制造可商购的用于这些用途的薄膜的优选聚烯烃。可通过工艺(也称干法工艺,即挤压-退火-拉伸)或通过溶剂抽提工艺(也称湿法工艺或相转化工艺或TIPS—热诱导相分离-工艺)或通过颗粒拉伸工艺来制造这些薄膜。这些薄膜中的一些,由干法工艺所制造的那些,通常是多层薄膜。由于其具有断路能力(即,在发生短路时可以终止离子流),多层薄膜是优选的。常见的多层薄膜是三层薄膜。通行的三层薄膜具有聚丙烯(PP)/聚乙烯(PE)/聚丙烯(PP)结构,另一种结构是PE/PP/PE。
本发明针对具有三层的多层微孔电池隔板。第一层为具有对该层测量时小于等于(≤)1.2熔体流动指数的高分子量聚丙烯层,第二聚乙烯层,和具有对该层测量时≤1.2的熔体流动指数的第三高分子量聚丙烯层。此微孔电池隔板通过干法拉伸工艺形成。本发明的方法生产出在保持13至25秒Gurley数和小于等于≤25微米厚度的同时具有小于等于≤37%孔隙率的微孔电池隔板。
与同样厚度的三层干法拉伸的微孔电池隔板相比,此多层微孔电池隔板在混合穿透强度方面表现出5%或更多的增强。在105℃下暴露6小时后,此微孔电池隔板的净收缩小于5%。此微孔电池隔板的离子电阻小于2.5Ω-cm2。此隔板的聚乙烯层为高密度聚乙烯。
在本发明的另一实施方案中,多层微孔电池隔板包括三层干法拉伸的微孔电池隔板,其具有一外聚烯烃层、一内聚烯烃层、和一外聚烯烃层。隔板的总厚度为≤25微米。外聚烯烃层是高分子量聚丙烯。内聚烯烃层是聚乙烯。与同样厚度的三层干法拉伸的微孔电池隔板相比,此三层干法拉伸的微孔电池隔板在混合穿透强度方面表现出5%或更多的增强。此多层微孔电池隔板在保持13至25秒Gurley数的同时具有≤37%的孔隙率。出乎意料地,在105℃下6小时测量的此微孔电池隔板的净收缩小于5%。此多层微孔电池隔板具有小于2.5Ω-cm2的离子电阻。
本发明的另一实施方案是包括三层干法拉伸的微孔电池隔板的多层微孔电池隔板。此隔板具有一外聚烯烃层、一内聚烯烃层和一外聚烯烃层。外聚烯烃层是高分子量聚丙烯。内聚烯烃层是聚乙烯。此三层干法拉伸的微孔电池隔板具有≤25微米的厚度、37%或更低的孔隙率、和13至25秒的Gurley数。与同样厚度的三层干法拉伸的微孔电池隔板相比,此隔板在混合穿透强度方面表现出5%的增强。此多层微孔电池隔板出人意料地表现出在105℃下6小时测量的小于5%的净收缩。此多层微孔电池隔板也具有小于2.5Ω-cm2的离子电阻。
本发明的隔板可通过下述多层微孔电池隔板的制备工艺来制备。提供聚丙烯和聚乙烯,该聚丙烯在对加工前的颗粒测量时的MFI≤1.0。将聚丙烯,其为高分子量聚丙烯,挤压成前体聚丙烯薄膜。然后,提供聚乙烯并将其挤压成前体聚乙烯薄膜。之后,在前体聚乙烯薄膜的每一面上层叠上前体聚丙烯薄膜,以形成无孔三层前体。然后,将此无孔三层前体退火。在退火之后,将无孔三层前体拉伸成拉伸的微孔三层薄膜。随后,使拉伸的微孔三层薄膜回缩成微孔三层薄膜。此过程中的净拉伸小于90%。净拉伸是通过给予薄膜的拉伸减去回缩量的百分比确定的。拉伸可在热或冷、或者热和冷的混合下完成。回缩也可在热或冷、或者热和冷的混合下完成。
在本发明的另一实施方案中,提供由微孔聚烯烃制成的电池隔板,其具有≤25微米的总厚度,其中在105℃下6小时测量的隔板净收缩小于5%。此电池隔板具有37%或更低的孔隙率。仍出乎意料地,此电池隔板具有13至25秒的Gurley数。惯常地,为获得在13至25秒范围内的Gurley数,隔板必须具有大于37%的孔隙率,并且在大多数情况下,孔隙率为至少40%或更多。已知对于隔板,即使孔隙率上的微小变化也倾向于对Gurley数产生大的影响。对由湿法工艺制成的隔板以及由干法工艺制成的隔板来说,具有≤25微米总厚度的由微孔聚烯烃制成的电池隔板,其中在105℃下6小时测量的隔板净收缩小于5%,其中在保持13至25秒Gurley数的同时隔板具有≤37%的孔隙率是出乎意料的。
在本发明的另一实施方案中,多层电池隔板由具有一高分子量聚丙烯层外层的微孔聚烯烃制成,在加工后对外层测量时,其具有≤1.2的熔体流动指数。由于许多会被认为是高密度的聚丙烯在加工之后在熔体流动性能方面有显著下降,因此对层的测量是重要的。过去,当使用熔体流动指数时,它总是指加工之前的熔体流动。
参考下面给出的实施例来进一步说明本发明。在下列实施例中,Gurley数由ASTM D-726(B)方法测定。本文所用的Gurley数是由Gurley透气度测定仪(例如4120型)测定的对空气流的阻力。本文列出的Gurley值表示在12.2英寸水柱压力下,使10cc空气通过一平方英寸产品所需的以秒计的时间。
沿MD和TD的拉伸强度用ASTM D-638法测定。沿抗撕裂性由ASTMD-1004测定。
电池隔板的厚度按由纸浆和纸工业技术协会主持开发的T411om-83法测定。使用有1/2英寸直径、圆形底板的精密千分尺,在7psi压力下接触样品来测量厚度。对横跨样品宽度读取的10个独立的千分尺读数取平均。
微孔薄膜的孔隙率按ASTM D-2873测定。
击穿强度按下述测定:在跨越拉伸制品的宽度内作十次测量并取平均。使用Mitech Stevens LFRA组织分析仪。针为1.65毫米直径,具有0.5毫米半径。下降速率为2mm/sec,偏转量为6mm。薄膜被紧固在具有11.3mm中心孔的夹紧装置中。记录相对于由所测试薄膜产生的阻力(以克力计)被针刺的薄膜的位移(以mm计)。最大阻力为击穿强度。
混合穿透是由于混合穿透而通过隔板产生短路所需要的力。在此测试中,从图1中金属板10的底部开始,在该板的上面放置阴极材料片15,在阴极的上面放置多层隔板20,并在多层隔板20的上面放置阳极材料片25。然后,提供一与测力计35相连的3mm的球头30。球头30通过电阻计40与金属板10相连。如图2所示,向球头30施加压力45,该压力被记录在图1中的测力计35上。一旦施加力,在隔板20的两边就形成图2中的阳极混合50和阴极混合55。当电阻急剧下降时,表明由于混合穿透而通过隔板的短路。
混合穿透测试隔板的强度和对混合穿透的电阻。已经发现,这更精确地模拟了实际电池的行为。它是比击穿强度更好的表明隔板在电池中如何表现的指标。此测试用来表明隔板在电池组装期间容许短路的趋势。
熔体指数根据ASTM DS 1238测定;PE:190℃/2.16Kg;PP:230℃/2.16Kg。其以g/10分钟测量。
收缩在105℃下测量6小时。在所述热处理前后测量隔板膜的宽度和长度。通过下列公式计算净收缩:
净收缩%=100*((L0-L1)/L0+(W0-W1)/W0)
其中,L0是热处理前的长度,L1是热处理后的长度,W0是热处理前的宽度,而W1是热处理后的宽度。
由于隔板对电性能的影响,对现有电池制造来说,测量用特定电解液浸泡的隔板的离子电阻是非常重要的。离子电阻是比Gurley数更全面的穿透性度量,因为该测量是在用于实际电池应用中的实际的电解质溶液中进行的。多孔膜的离子电阻实质上是内含在隔板孔中的电解液的离子电阻。典型地,浸在电解液中的微孔隔板具有它所置换的相应体积电解液电阻约6-7倍的电阻。它是膜的孔隙率、弯曲度、电解液的电阻、膜厚以及电解液对膜孔润湿程度的函数。
通过从成品切下小片隔板,然后,将其置于两个块电极之间来测定隔板电阻。用3:7体积比EC/EMC溶剂中1.0M LiPF6盐的电池电解液使隔板完全饱和。通过4探针AC阻抗法测量隔板的电阻R(Ω)。为降低在电极/隔板界面上的测量误差,需要通过增加更多的隔板层进行多次测量。
根据多层测量,然后通过公式计算用电解液饱和的隔板的离子电阻Rs(Ω):
R s = ρ s l A - - - ( 1 )
其中,ps是以Ω-cm计的隔板的离子电阻率,A是以cm2计的电极面积,l是以cm计的隔板膜的厚度。比值ps/A是计算出的隔板电阻随多重隔板层变化的斜率,由公式给出:
Figure C200710090359D00101
其中,ΔR和Δδ在图3中定义。图3中斜率的计算用来评价使用多层测量法的隔板膜的离子电阻。
隔板膜的离子电阻通过使用四探针AC阻抗法测量。图4显示了用于测量电阻的电池的示意图60。从电池顶部65和底部70探针引出的每根导线具有双线75、80,一根用来感测电流,另一根感测电压。所有电阻测量所用的电解液为3:7体积比EC:EMC溶剂中1.0M的LiPF6盐。将隔板样品置于底电极85上。隔板应当完全覆盖底电极并且隔板应被电解液完全润湿。然后,将第二电极90滑到底电极85上并测量阻抗值。用来自Potentiostat的阻抗计95测量阻抗。为降低测量误差,开始加上更多隔板层并测量总电阻。通过加上一可放置在底部探针70上的有孔中心105的特氟隆隔膜100,能够只测定电解液的电阻。随后加入电解液至装满有孔中心105,然后,将顶部探针65置于隔膜100上。
实施例
当考察表A中的实施例时,以上说明将是清楚的。样品A和B代表通过干法拉伸工艺制造的有竞争力的三层隔板。样品A为20微米隔板,样品B代表25微米隔板。实施例C300和C500代表通过本发明工艺制备的本发明隔板。C300为20微米隔板,C500代表25微米隔板。在表中:IR代表离子电阻,P.S.为击穿强度,MP是混合穿透,TD为相对于加工方向横向。
表A
 
       种类                        A             C300          B             C500
厚度,微米                         20            21            25            24.5
Gurley数                           15            19            21            18
IR,Ω-cm<sup>2</sup>                          1.7           2.1           2.0           2.3
P.S.,克                           337           367           412           424
自2300的MP%偏差                   -10           -2            0             5
TD拉伸强度kgf/cm<sup>2</sup>                  165           180           168           174
孔隙率%                           43            35            42            37
净收缩@,105℃下,6hr%            6.4           3.0           6.4           2.7
在混合穿透测试中,本发明的材料与通过
Figure C200710090359D00111
工艺制造的三层隔板作对比,这种隔板没有使用高分子量聚丙烯并且也不是依照本发明的方法制造的。在图5中,可以看出在混合穿透强度方面的改进。还见表A,其中与标准25微米三层隔板相比,标准20微米隔板在混合穿透强度方面显示出10%的降低。通过本发明工艺制造的20微米本发明的隔板在混合穿透强度方面显示仅有2%的降低。标准25微米隔板在混合穿透强度方面没有显示出变化,其中通过本发明工艺制造的25微米隔板在混合穿透强度方面显示出5%的提高。

Claims (2)

1、一种多层微孔电池隔板,其包括:
由具有<1.0g/10分钟的熔体流动指数(MFI)的高分子量聚丙烯制成的外层;
聚乙烯内层;
由具有<1.0g/10分钟的熔体流动指数的高分子量聚丙烯制成的外层;
微孔电池隔板通过干法拉伸工艺形成,其中,所述微孔电池隔板在保持13至25秒Gurley数和≤25微米厚度的同时,具有≤37%的孔隙率;所述微孔电池隔板与相同厚度的三层干法拉伸的微孔电池隔板相比,在混合穿透强度方面表现出5%或更高的增强,并且不采用所述高分子量聚丙烯;所述微孔电池隔板在105℃下6小时测量的净收缩小于5%;所述微孔电池隔板的离子电阻小于2.5Ω-cm2
2、如权利要求1所述的多层微孔电池隔板,其中,聚乙烯层是高密度聚乙烯。
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JP5065737B2 (ja) 2012-11-07
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US20070238017A1 (en) 2007-10-11
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