JP4832430B2 - Lithium ion secondary battery separator and lithium ion secondary battery - Google Patents

Lithium ion secondary battery separator and lithium ion secondary battery Download PDF

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JP4832430B2
JP4832430B2 JP2007516360A JP2007516360A JP4832430B2 JP 4832430 B2 JP4832430 B2 JP 4832430B2 JP 2007516360 A JP2007516360 A JP 2007516360A JP 2007516360 A JP2007516360 A JP 2007516360A JP 4832430 B2 JP4832430 B2 JP 4832430B2
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separator
lithium ion
ion secondary
secondary battery
nonwoven fabric
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聡 西川
博行 本元
高弘 大道
弘樹 佐野
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Teijin Ltd
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Description

本発明はリチウムイオン二次電池に用いるセパレータに関するものである。特に、リチウムイオン二次電池の安全性向上を目的としたセパレータ技術に関する。   The present invention relates to a separator used for a lithium ion secondary battery. In particular, the present invention relates to separator technology for the purpose of improving the safety of lithium ion secondary batteries.

リチウムイオンのドープ・脱ドープにより起電力を得るリチウムイオン二次電池は高いエネルギー密度を有するという特徴があり、携帯電話、ノートパソコン等の携帯用電子機器の電源として広く普及している。また、高出力化がなされ電動工具等のパワー用途への適用も始まっている。
近年、地球環境問題からハイブリット電気自動車（ＨＥＶ）の関心が高まっており、現状は電池としてニッケル水素電池が一般に適用されているが、リチウムイオン二次電池はニッケル水素電池に比べメモリー効果がないといった使い易さがあり、また高エネルギー、高出力密度なので小型化が可能といった利点があるためＨＥＶ用電源として検討されている。ＨＥＶに用いる電池を考えた場合、当然携帯用電子機器とはその要求が異なる。例えば、ＨＥＶにおいては電池が高温に曝される可能性が高く高温環境下における安全性確保も非常に重要な特性の１つである。このように携帯用電子機器とＨＥＶで電池に求められる特性が異なるわけであるから、電池の構成部材に求められる特性も当然異なってくる。
現状のリチウムイオン二次電池にはポリエチレン製微多孔膜がセパレータとして用いられている。このセパレータはシャットダウン機能を有しており、電池の安全性確保に寄与している。このシャットダウン機能は熱で微多孔膜が溶融し孔が閉塞することを利用したものであり、熱ヒューズ温度とショート温度によって特徴づけられる。熱ヒューズ温度は孔閉塞によってセパレータ抵抗が上がり始める温度で、ショート温度はセパレータが破膜し、セパレータ抵抗が急激に降下する温度である。この熱ヒューズ温度とショート温度の間ではセパレータ抵抗は非常に高く、電流をシャットアウトすることができる。この機能は外部短絡等の安全性確保に有効と言われている。
ＨＥＶの場合、外部から積極的に加熱されるケースが想定される。このような用途においてシャットダウン機能を考えた場合、熱ヒューズは高温に曝されたとき性能低下につながる可能性があり、破膜は正極と負極の内部短絡による発火につながる可能性がある。そのため、ＨＥＶにおいてはシャットダウン機能を有するセパレータが必ずしも有効とは言えないのが現状であり、十分な耐熱性を有するセパレータの方が好適であるとも考えられる。
耐熱性の高いセパレータの構成は既にいくつかの提案がなされている。例えば、芳香族ポリアミド繊維のような耐熱性の高い繊維からなる不織布シートが日本国特許第３１４２６９３号等で提案されている。ただし、不織布のような形態は目開きが大きく正負極の短絡を防止し電解液を保持するといったセパレータの本質に関る特性が不十分であり、目開きを小さくするためには細い繊維を用いればよいが、現状の技術レベルにおいてはリチウムイオン二次電池用セパレータとして好適なものを得ることは非常に困難であり実用化されていない。
また、芳香族ポリアミドのような耐熱性の高いポリマーからなる多孔膜もＷＯ０１／０１９９０６号明細書等で提案されている。この系は破膜によるショートという観点での耐熱性は十分である。ただし、過充電のような内部から積極的に発熱が起こるケースにおいて電池は暴走反応で発火する可能性があるが、このようなケースにおける安全性確保が不十分であり、別の安全対策が煩雑になるという課題を有している。また芳香族ポリアミドからなる多孔膜は強度の観点から生産性が低いという課題も有している。
上記のような耐熱性の高いポリマーからなる多孔膜にシャットダウン機能を付与するという技術も提案されている。例えば特開２００１−２３６０２号公報にシャットダウン機能を有する多孔質フィルムに高耐熱性樹脂からなる多孔質層をコーティングする技術が開示されている。しかし、熱ヒューズ前の抵抗に対し熱ヒューズ後の抵抗が１０倍程度までしか上昇しておらず、電池の安全性を確保する上で十分なシャットダウン機能とは言い難い。また、特開平１０−６４５３号公報では耐熱性樹脂からなる多孔膜中にポリエチレンからなる微粒子を混合させた構成が提案されている。この系も同様に十分なシャットダウン機能は得られていない。
ＷＯ０１／０６７５３６号明細書では耐熱性の高い繊維からなる不織布にポリフッ化ビニリデン共重合体からなる多孔質層をコーティングした構成が提案されている。この明細書にはセパレータのモロホロジーを適切に制御することで過充電防止機能防止を可能とする技術要素が記載されている。また、このモロホロジー制御に関して適切な製造方法が特開２００３−１７１４９５号公報に開示されている。ただし、電解液に膨潤したポリフッ化ビニリデン共重合体の耐熱性は決して高いものではなく、高温では溶融してしまうので、この系の耐熱性は不織布によって確保されていると考えられるが、日本国特許第３１４２６９３号の議論と同様に不織布での正負極の短絡防止は確実性に欠けるという課題がある。
特開平１０−３２４７５８号公報では、繊維またはパルプからなる基材の表面及び空隙が多孔質のパラアラミドポリマーで覆われたセパレータが開示されている。日本国特許第３１７５７３０号は特開平１０−３２４７５８号公報の系に加え多孔質層にセラミックを分散させた系である。これら特許文献で記載されているセパレータの製法は不織布をキャリアーシート上に置いてパラアラミドポリマードープを上から塗り、適切な湿度、温度環境下でパラアラミドポリマーを析出させるという方法で多孔膜を得ている。この方法では実質的に不織布の両面に多孔質のパラアラミドポリマーをコーティングすることは出来ず、明らかに片面コーティングである。また、析出速度も表裏で異なるため表裏で多孔質層のモロホロジーも大きく非対称となっている。このような顕著に表裏差があるセパレータは適切な電極／セパレータ界面の形成が困難で電池性能上課題があり実用的ではない。また、カールしハンドリング性が悪いという課題もある。さらに、パラアラミドポリマーはドープ調整や成形が難しいという課題もある。具体的には、孔が連続的に成形され難く、また非常に小さい孔しか成形されないため、十分なイオン透過性が得られない。加えてこの技術は製造法方法が煩雑であるという課題も有している。A lithium ion secondary battery that obtains an electromotive force by doping and dedoping of lithium ions has a high energy density, and is widely used as a power source for portable electronic devices such as mobile phones and laptop computers. In addition, the output has been increased and application to power applications such as electric tools has begun.
In recent years, interest in hybrid electric vehicles (HEV) has increased due to global environmental problems. Currently, nickel-metal hydride batteries are generally used as batteries, but lithium-ion secondary batteries have less memory effect than nickel-metal hydride batteries. It is considered as a power source for HEV because it is easy to use and has the advantage of being able to be miniaturized because of its high energy and high output density. When considering a battery used for HEV, the requirement is naturally different from that of a portable electronic device. For example, in HEV, there is a high possibility that a battery is exposed to a high temperature, and ensuring safety in a high temperature environment is one of the very important characteristics. As described above, since the characteristics required for the battery are different between the portable electronic device and the HEV, the characteristics required for the constituent members of the battery are naturally different.
In the current lithium ion secondary battery, a polyethylene microporous membrane is used as a separator. This separator has a shutdown function and contributes to ensuring the safety of the battery. This shutdown function utilizes the fact that the microporous film melts and closes the pores due to heat, and is characterized by the thermal fuse temperature and the short circuit temperature. The thermal fuse temperature is a temperature at which the separator resistance starts to increase due to hole closure, and the short-circuit temperature is a temperature at which the separator resistance is abruptly lowered because the separator is broken. Between this thermal fuse temperature and the short circuit temperature, the separator resistance is very high and the current can be shut out. This function is said to be effective for ensuring safety such as external short circuit.
In the case of HEV, a case where it is actively heated from the outside is assumed. When considering a shutdown function in such applications, thermal fuses can lead to performance degradation when exposed to high temperatures, and film breakage can lead to ignition due to an internal short circuit between the positive and negative electrodes. Therefore, in HEV, a separator having a shutdown function is not always effective, and it is considered that a separator having sufficient heat resistance is more suitable.
Several proposals have already been made for the construction of a separator having high heat resistance. For example, a nonwoven fabric sheet made of highly heat-resistant fibers such as aromatic polyamide fibers has been proposed in Japanese Patent No. 3142893. However, the shape of the nonwoven fabric has a large opening and prevents the short circuit between the positive and negative electrodes and retains the electrolyte, and the characteristics related to the essence of the separator are insufficient. To reduce the opening, thin fibers are used. However, at the current technical level, it is very difficult to obtain a suitable separator for a lithium ion secondary battery, and it has not been put into practical use.
A porous film made of a polymer having high heat resistance such as aromatic polyamide has also been proposed in WO01 / 019906. This system has sufficient heat resistance in terms of short-circuiting due to membrane breakage. However, in cases where heat is actively generated from the inside, such as overcharging, the battery may ignite due to a runaway reaction. However, in such cases, safety is not sufficiently secured, and other safety measures are complicated. Has the problem of becoming. In addition, a porous membrane made of an aromatic polyamide has a problem that productivity is low from the viewpoint of strength.
A technique for providing a shutdown function to a porous film made of a polymer having high heat resistance as described above has also been proposed. For example, Japanese Patent Application Laid-Open No. 2001-23602 discloses a technique for coating a porous film having a shutdown function with a porous layer made of a high heat resistant resin. However, the resistance after the thermal fuse has increased only to about 10 times the resistance before the thermal fuse, and it is difficult to say that the shutdown function is sufficient for ensuring the safety of the battery. Japanese Patent Laid-Open No. 10-6453 proposes a structure in which fine particles made of polyethylene are mixed in a porous film made of a heat resistant resin. Similarly, this system does not have a sufficient shutdown function.
WO 01/067536 proposes a configuration in which a non-woven fabric made of highly heat-resistant fibers is coated with a porous layer made of a polyvinylidene fluoride copolymer. This specification describes a technical element that enables prevention of an overcharge prevention function by appropriately controlling the morphology of the separator. Moreover, a manufacturing method suitable for this morphology control is disclosed in Japanese Patent Application Laid-Open No. 2003-171495. However, the heat resistance of the polyvinylidene fluoride copolymer swollen in the electrolyte is not high at all, and it melts at high temperatures, so it is considered that the heat resistance of this system is ensured by the nonwoven fabric. Similar to the discussion of Japanese Patent No. 3142893, there is a problem that the prevention of short circuit between the positive and negative electrodes in the non-woven fabric lacks certainty.
Japanese Patent Application Laid-Open No. 10-324758 discloses a separator in which the surface and voids of a substrate made of fibers or pulp are covered with a porous para-aramid polymer. Japanese Patent No. 3175730 is a system in which ceramic is dispersed in a porous layer in addition to the system disclosed in JP-A-10-324758. The manufacturing method of the separator described in these patent documents is obtained by placing a non-woven fabric on a carrier sheet, applying para-aramid polymer dope from above, and depositing para-aramid polymer in an appropriate humidity and temperature environment to obtain a porous film. ing. In this method, the porous para-aramid polymer cannot be substantially coated on both sides of the nonwoven fabric, and is obviously a single-sided coating. In addition, since the deposition rate differs between the front and back surfaces, the morphology of the porous layer on the front and back surfaces is greatly asymmetric. Such a separator having a significant difference between the front and the back is not practical because it is difficult to form an appropriate electrode / separator interface and there are problems in battery performance. Another problem is curling and poor handling. Furthermore, the para-aramid polymer has a problem that it is difficult to adjust the dope and to be molded. Specifically, since it is difficult to form holes continuously and only very small holes are formed, sufficient ion permeability cannot be obtained. In addition, this technique has a problem that the manufacturing method is complicated.

前述のように、芳香族ポリアミド等の耐熱性樹脂を活用した耐熱性の高いセパレータが提案はされているが、この耐熱性を特徴にして有意にリチウムイオン二次電池の安全性を向上させ生産性良好なセパレータ構成は見出されておらず実用化に至っていないのが現状である。十分に耐熱性の高いセパレータにおいて、リチウ厶イオン二次電池の安全性確保の観点から不足しているのは、内部からの自己発熱で暴走反応に至る過充電対策であると本発明者らは考えた。そこで、耐熱性が十分に高く、過充電対策にも有効でハンドリング性の良好なセパレータを提供することを本発明の目的とする。
本発明者らが上記に課題に対し鋭意検討した結果、不織布の表裏両面に主として芳香族ポリアミドからなる多孔質層がコーティングされたセパレータがリチウムイオン二次電池の安全性確保に有効であることを見出し本発明に至った。すなわち本発明は、不織布の表裏両面に主としてメタ芳香族ポリアミドからなる多孔質層が形成されていることを特徴とするリチウムイオン二次電池用セパレータを提供する。さらに本発明は以下の発明も提供する。
１．該セパレータの膜厚が１５〜４０μｍ、ガーレ値（ＪＩＳ Ｐ８１１７）が１０〜５０秒／１００ｃｃであることを特徴とする上記発明記載のリチウムイオン二次電池用セパレータ。
２．該多孔質層に陽イオン性界面活性剤、陰イオン性界面活性剤、両性界面活性剤、非イオン性界面活性剤からなる群から選ばれる少なくとも１種を含む界面活性剤が付着されていることを特徴とする上記発明または１のいずれかに記載のリチウムイオン二次電池用セパレータ。
３．該界面活性剤の付着量が０．００５〜０．７５０ｇ／ｍ２であることを特徴とする２記載のリチウムイオン二次電池用セパレータ。
４．該不織布が主としてポリエチレンテレフタレートからなる不織布であることを特徴とする上記発明または１から３いずれかに記載のリチウムイオン二次電池用セパレータ。
５．該主としてメタ芳香族ポリアミドからなる多孔質層の重量が４〜１０ｇ／ｍ２であることを特徴とする４記載のリチウムイオン二次電池用セパレータ。
６．該不織布が主としてメタ芳香族ポリアミドからなる不織布であることを特徴とする上記発明または１から３いずれかに記載のリチウムイオン二次電池用セパレータ。
７．該不織布がメタ芳香族ポリアミド短繊維とパラ芳香族ポリアミドパルプからなることを特徴とする上記発明または１から３いずれかに記載のリチウムイオン二次電池用セパレータ。
８．該メタ芳香族ポリアミドがポリメタフェニレンイソフタルアミドであることを特徴とする上記発明または１から５いずれかに記載のリチウムイオン二次電池用セパレータ。
９．該多孔質層に平均粒子径０．０５〜２μｍのセラミック微粒子が含まれており、多孔資質層の重量に対してセラミック微粒子が３０〜８０重量％となっていることを特徴とする上記発明または１から８いずれかに記載のリチウムイオン二次電池用セパレータ。
１０．メタ芳香族ポリアミドと該メタ芳香族ポリアミドに対し良溶媒である溶媒を主成分とする高分子溶液を不織布の表裏両面に塗工し、次いで塗工された不織布を該メタ芳香族ポリアミドに対し貧溶媒である溶媒と良溶媒である溶媒から主としてなる混合液中で凝固させ、次いで水洗、乾燥することを特徴とするリチウムイオン二次電池用セパレータの製造方法。
１１．正極、負極、非水系電解液、セパレータを具備し、リチウムイオンのドープ・脱ドープにより起電力を得るリチウムイオン二次電池において、セパレータが不織布の表裏両面に主としてメタ芳香族ポリアミドからなる多孔質層が形成されていることを特徴とするリチウムイオン二次電池。
１２．該セパレータの膜厚が１５〜４０μｍ、ガーレ値（ＪＩＳ Ｐ８１１７）が１０〜５０秒／１００ｃｃであることを特徴とする１１記載のリチウムイオン二次電池。
１３．該多孔質層に陽イオン性界面活性剤、陰イオン性界面活性剤、両性界面活性剤、非イオン性界面活性剤からなる群から選ばれる少なくとも１種を含む界面活性剤が付着されていることを特徴とする１１または１２いずれかに記載のリチウムイオン二次電池。
１４．該界面活性剤の付着量が０．００５〜０．７５０ｇ／ｍ２であることを特徴とする１３記載のリチウムイオン二次電池用セパレータ。
１５．該不織布が主としてポリエチレンテレフタレートからなる不織布であることを特徴とする１１から１４いずれかに記載のリチウムイオン二次電池。
１６．該主としてメタ芳香族ポリアミドからなる多孔質層の重量が４〜１０ｇ／ｍ２であることを特徴とする１５記載のリチウムイオン二次電池。
１７．電解液の電解質がＬｉＰＦ６を主体とし、溶媒全重量に対しビニレンカーボネートまたはビニルアセテートを０．５〜５重量％含むことを特徴とする１５記載のリチウムイオン二次電池。
１８．該不織布が主としてメタ芳香族ポリアミドからなる不織布であることを特徴とする１１から１４いずれかに記載のリチウムイオン二次電池。
１９．該メタ芳香族ポリアミドがポリメタフェニレンイソフタルアミドであることを特徴とする１１から１８いずれかに記載のリチウムイオン二次電池。As mentioned above, separators with high heat resistance using heat-resistant resins such as aromatic polyamides have been proposed, but this heat resistance is a feature that significantly improves the safety of lithium ion secondary batteries and produces them. At present, a separator structure with good properties has not been found and has not been put into practical use. In the sufficiently high heat-resistant separator, the present inventors lacked from the viewpoint of ensuring the safety of the lithium ion secondary battery is an overcharge countermeasure that leads to a runaway reaction due to self-heating from the inside. Thought. Accordingly, it is an object of the present invention to provide a separator having sufficiently high heat resistance, effective for overcharge countermeasures, and good handling properties.
As a result of intensive studies on the above problems by the present inventors, it is confirmed that a separator in which a porous layer mainly made of an aromatic polyamide is coated on both front and back surfaces of a nonwoven fabric is effective for ensuring the safety of a lithium ion secondary battery. The headline has led to the present invention. That is, the present invention provides a separator for a lithium ion secondary battery, in which a porous layer mainly composed of a metaaromatic polyamide is formed on both front and back surfaces of a nonwoven fabric. The present invention also provides the following inventions.
1. The separator for a lithium ion secondary battery according to the invention, wherein the separator has a thickness of 15 to 40 μm and a Gurley value (JIS P8117) of 10 to 50 seconds / 100 cc.
2. A surfactant containing at least one selected from the group consisting of a cationic surfactant, an anionic surfactant, an amphoteric surfactant and a nonionic surfactant is attached to the porous layer. A separator for a lithium ion secondary battery according to any one of the above inventions or 1.
3. 3. The separator for a lithium ion secondary battery according to 2, wherein the surfactant is attached in an amount of 0.005 to 0.750 g / m 2 .
4). The separator for a lithium ion secondary battery according to the above invention or any one of 1 to 3, wherein the nonwoven fabric is a nonwoven fabric mainly composed of polyethylene terephthalate.
5). 5. The separator for a lithium ion secondary battery according to 4, wherein the weight of the porous layer mainly composed of metaaromatic polyamide is 4 to 10 g / m 2 .
6). The separator for a lithium ion secondary battery according to the above invention or any one of 1 to 3, wherein the nonwoven fabric is a nonwoven fabric mainly composed of metaaromatic polyamide.
7). The separator for a lithium ion secondary battery according to the above invention or any one of claims 1 to 3, wherein the nonwoven fabric is composed of a meta-aromatic polyamide short fiber and a para-aromatic polyamide pulp.
8). The separator for a lithium ion secondary battery according to the above invention or any one of 1 to 5, wherein the metaaromatic polyamide is polymetaphenylene isophthalamide.
9. The above invention, wherein the porous layer contains ceramic fine particles having an average particle size of 0.05 to 2 μm, and the ceramic fine particles are 30 to 80% by weight based on the weight of the porous material layer or The separator for lithium ion secondary batteries in any one of 1-8.
10. A polymer solution mainly composed of a metaaromatic polyamide and a solvent that is a good solvent for the metaaromatic polyamide is applied to both the front and back surfaces of the nonwoven fabric, and then the coated nonwoven fabric is poor against the metaaromatic polyamide. A method for producing a separator for a lithium ion secondary battery, comprising coagulating in a mixed liquid mainly composed of a solvent as a solvent and a solvent as a good solvent, followed by washing with water and drying.
11. In a lithium ion secondary battery comprising a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator, and obtaining an electromotive force by doping and dedoping lithium ions, the separator is a porous layer mainly composed of metaaromatic polyamide on both front and back surfaces of the nonwoven fabric. Is formed, a lithium ion secondary battery.
12 11. The lithium ion secondary battery according to 11, wherein the separator has a thickness of 15 to 40 μm and a Gurley value (JIS P8117) of 10 to 50 seconds / 100 cc.
13. A surfactant containing at least one selected from the group consisting of a cationic surfactant, an anionic surfactant, an amphoteric surfactant and a nonionic surfactant is attached to the porous layer. The lithium ion secondary battery according to any one of 11 and 12, wherein:
14 14. The separator for a lithium ion secondary battery according to 13, wherein the surfactant is attached in an amount of 0.005 to 0.750 g / m 2 .
15. The lithium ion secondary battery according to any one of 11 to 14, wherein the nonwoven fabric is a nonwoven fabric mainly composed of polyethylene terephthalate.
16. 16. The lithium ion secondary battery according to 15, wherein the weight of the porous layer mainly composed of metaaromatic polyamide is 4 to 10 g / m 2 .
17. 16. The lithium ion secondary battery according to 15, wherein the electrolyte of the electrolytic solution is mainly composed of LiPF 6 and contains 0.5 to 5% by weight of vinylene carbonate or vinyl acetate with respect to the total weight of the solvent.
18. 15. The lithium ion secondary battery according to any one of 11 to 14, wherein the non-woven fabric is a non-woven fabric mainly composed of a metaaromatic polyamide.
19. The lithium ion secondary battery according to any one of 11 to 18, wherein the metaaromatic polyamide is polymetaphenylene isophthalamide.

図１は過充電試験の結果である。   FIG. 1 shows the result of the overcharge test.

以下、本発明の形態について説明する。
［セパレータ］
本発明リチウムイオン二次電池用セパレータは、不織布の表裏両面に主として、メタ芳香族ポリアミドからなる多孔質層が形成されていることを特徴とする。不織布を用いることで芳香族ポリアミドからなる多孔膜の強度及び寸法安定性を改善し、ハンドリング性及び生産性を向上させることができる。
ここで、不織布の表裏表面に主としてメタ芳香族ポリアミドからなる多孔質層が形成されているとは、セパレータの表裏を観察したとき表裏全面が芳香族ポリアミドからなる多孔質層に覆われていることであり、不織布を構成する繊維が見えないことである。これは走査型電子顕微鏡（ＳＥＭ）で容易に観察できる。表及び裏どちらか一方もしくは両方において不織布を構成する繊維が露出しているようなセパレータでは、電極とセパレータとの接合界面が不均一になり電池性能に不具合が生じる。これは電極／セパレータ界面における電解液保持性が不十分であることが要因であり、サイクル経過に伴い電極／セパレータ界面に存在する電解液が枯渇し、サイクル特性やサイクル経過の放電特性が不良になる。
本発明セパレータでは不織布表面に形成する層はメタ芳香族ポリアミドからなる多孔質層である。多孔質層においては空孔が連続的に形成されている必要があり、この多孔質層の構造はガーレ値（ＪＩＳ Ｐ８１１７）を指標とすることができる。本発明セパレータのガーレ値は１０〜５０秒／１００ｃｃが好適である。ガーレ値が１０秒／１００ｃｃより低いと不織布形成繊維が露出していたりピンホールがあったりする欠陥部分が存在する確率が高く好ましくない。また、５０秒／１００ｃｃより大きいとイオン透過性が十分でなくレート特性が低下するので好ましくない。さらに、ＷＯ０１／０６７５３６号明細書記載の過充電防止機能を良好に得ることが困難となり、過充電時の安全性確保の観点から好ましくない。
本発明セパレータにおいて膜厚は１５〜４０μｍが好適である。膜厚が１５μｍより薄いとセパレータ本来の短絡防止という機能が不十分となる。また、４０μｍより厚いとイオン伝導に伴う抵抗が高くなりレート特性が十分でなくなったり、電池のエネルギー密度が高くならなかったりという問題が生じ好ましくない。
上記のようなセパレータ厚みを実現するためには不織布厚みとしては１０〜３９μｍであり、メタ芳香族ポリアミドからなる多孔質層の厚みは表裏合計で１〜１０μｍ程度が好ましい。本発明セパレータの実質的な強度は不織布によって決定されるが、不織布厚みが１０μｍより薄いとリチウムイオン電池用セパレータとして十分な強度を確保することが困難となる。また３９μｍより厚いとセパレータ厚みを４０μｍ以下とすることが実質的に困難となる。また、該多孔質層の厚みが表裏合計で１μｍより薄いと実質的に不織布表裏全面を覆うことが困難となり好ましくない。該多孔質層はセパレータのイオン伝導抵抗を概ね律速し１０μｍより厚くなると十分な放電性確保という観点から好ましくない。
本発明セパレータに用いる不織布は目開きが出来る限り細かい方が好ましく、このような不織布を得るためには繊維径は細かい方が好ましい。このような観点から、不織布を構成する繊維の繊維径は１０μｍ以下が好適であり、さらに５μｍ以下が好ましい。
不織布を成形する場合、主繊維と主繊維を結着するためのバインダーが必要である。この不織布を成形するためのバインダーは繊維またはパルプが好ましい。
該不織布を製造する方法は公知の方法を適用することが可能である。具体的には乾式法、ウォーターニードル法、湿式抄造法、スパンボンド法、メルトブロー法、エレクトロスピニング法等が挙げられる。薄葉化と目開きの均一性を考えると湿式抄造法が特に好適である。
該不織布を構成する材質は十分な耐熱性と電解液に対する耐性があれば特に限定はされず、具体的にはポリエチレンテレフタレート（ＰＥＴ）に代表されるポリエステル、芳香族ポリアミド、ポリスルホン、ポリエーテルスルホン、ポリフェニレンサルファイド、ポリイミド等が挙げられる。特に、より細い繊維を成形し易く耐熱性が高いという観点からＰＥＴが好ましい。また、高強度・高耐熱性という観点では芳香族ポリアミド、特にメタ芳香族ポリアミドが好ましく、特に成形性の観点からポリメタフェニレンイソフタルアミドが好適である。該不織布において不織布の耐熱性を損ねない範疇でポリエチレン、ポリプロピレン等のポリオレフィン系繊維を混合させても問題なく、これら材料を少量混入させると安全性が向上する場合もある。この場合のポリオレフィン繊維の添加量は該不織布の重量に対し、３０重量％以下が好ましい。
特に、薄膜・高強度という観点ではメタ芳香族ポリアミドからなる短繊維とパラ芳香族ポリアミドパルプを用いた不織布が好適である。該不織布は多孔質層を形成するメタ芳香族ポリアミドと親和性が高く、複合化することで高い強度が得られる。また、上記の構成の不織布は薄膜化が容易である。
本発明セパレータにおいて、多孔質層を形成する材質は主にメタ芳香族ポリアミドが好適である。芳香族ポリアミドはポリパラフェニレンテレフタルアミドに代表されるパラ芳香族ポリアミドとポリメタフェニレンイソフタルアミドに代表されるメタ芳香族ポリアミドがあるが、本発明セパレータにおいてはメタ芳香族ポリアミド、特にポリメタフェニレンイソフタルアミドが好ましい。パラ芳香族ポリアミドは溶媒への溶解性が低く、低濃度においても高粘度となるため該多孔質層を強度、イオン透過性が十分なものに形成することが非常に難しい。具体的には小さい孔しか形成せず、これらが非連続となるので、イオン透過性が不十分となる。それに対し、メタ芳香族ポリアミドは溶媒に十分溶解し、濃度・粘度の観点で適切な高分子溶液を調整することが容易である。また、孔径制御も容易で十分なイオン透過性を確保できる。特に、ＷＯ０１／０６７５３６号明細書の過充電防止機能を発現させるためには該多孔層の孔径を適切に制御する必要があり、このような機能を付加するという観点からメタ芳香族ポリアミドはパラ芳香族ポリアミドに比べ好適である。
該多孔質層を形成する材質は主にメタ芳香族ポリアミドが好ましく、メタ芳香族ポリアミドの特徴である耐熱性と多孔質層の構造制御を損ねない範疇で他の材質が混合されていても構わない。具体的には、パラ芳香族ポリアミド、ポリスルホン、ポリエーテルスルホン、ポリフッ化ビニリデン、ポリフッ化ビニリデン共重合体、ポリアクリロニトリル、ポリメチルメタクリレート、ポリエチレンオキサイド、ポリプロピレンオキサイド、ポリビニルピロリドン等が挙げられ、これらの成分は多孔質層を形成するメタ芳香族ポリアミドに対して、３０重量％以下が好適である。
該不織布にＰＥＴを用いる場合、該多孔質層を形成するメタ芳香族ポリアミドの重量は４〜１０ｇ／ｍ２が好適である。ＰＥＴをリチウムイオン二次電池用セパレータの材質に用いる場合、特殊な環境における耐久性に課題があるが、メタ芳香族ポリアミドと複合化させることでこの耐久性を顕著に改善することができる。メタ芳香族ポリアミドの重量が４ｇ／ｍ２未満であると耐久性改善が十分でなく好ましくない。重量が１０ｇ／ｍ２より多いとイオン透過性を悪化させるといった問題が生じる。
本発明におけるメタ芳香族ポリアミドは、Ｎ−メチル−２−ピロリドンに溶解した場合に、下式（１）の対数粘度で表して、０．８〜２．５ｄｌ／ｇ、好ましくは１．０〜２．２ｄｌ／ｇの範囲のポリマーであることが好ましい。対数粘度が０．８ｄｌ／ｇより低いと十分な機械強度が得られず、対数粘度が２．５ｄｌ／ｇを超えると安定なポリマー溶液を得ることが困難となり、均一な多孔質層を形成するためには好ましくない。
対数粘度（単位：ｄｌ／ｇ）＝ｌｎ（Ｔ／ＴＯ）／Ｃ （１）
Ｔ：メタ芳香族ポリアミド０．５ｇをＮ−メチル−２−ピロリドン１００ｍｌに溶解した溶液の３０℃における毛細管粘度計の流動時間
ＴＯ：Ｎ−メチル−２−ピロリドンの３０℃にける毛細管粘度計の流動時間
Ｃ：ポリマー溶液中のポリマー濃度（ｇ／ｄｌ）
本発明のセパレータにおいて、帯電防止の観点から該多孔質層に界面活性剤を付着させたものも好適である。該界面活性剤は特に限定されないが、例えば陽イオン系、陰イオン系、両性イオン系、非イオン系の界面活性剤を使用することができる。陽イオン系界面活性剤としては、高級アミンハロゲン酸塩、ハロゲン化アルキルピリジニウム、第四級アンモニウム塩等が挙げられる。陰イオン系界面活性剤としては、高級脂肪酸アルカリ塩、ポリオキシエチレンアルキルエーテルスルホン酸エステル塩、ポリオキシエチレンアルキルエーテルホスホン酸塩、アルキル硫酸塩、アルキルスルホン酸塩、アルキルアリールスルホン酸塩、スルホコハク酸エステル塩等が挙げられる。両性イオン系界面活性剤としては、アルキルベタイン系化合物、イミダゾリン系化合物、アルキルアミンオキサイド、ビスオキシボレート系化合物等が挙げられる。非イオン系界面活性剤としては、ポリオキシエチレンアルキルエーテル類、ポリオキシエチレンアルキルフェニルエーテル類、ポリオキシエチレンアルキルアリルエーテル類、グリセリン脂肪酸エステル、ポリオキシエチレンソルビタン脂肪酸エステル、ソルビタン脂肪酸エステル等が挙げられる。
特に、陽イオン系界面活性剤、陰イオン系界面活性剤、両性イオン系界面活性剤は帯電防止効果が強いため、使用量を低く抑えることができ、使用が望ましい。また、これらを混合することによって、芳香族アミドと界面活性剤の親和性を高め、帯電防止効果を向上させることもできる。
界面活性剤の付量は０．００５〜０．７５０／ｍ２であることが望ましい。０．００５ｇ／ｍ２より少ないと充分な帯電防止効果が得られず、０．７５０ｇ／ｍ２より多いと電池の性能に悪影響が出ることがある。界面活性剤の付量は、界面活性剤塗工後９０℃で１０時間真空乾燥した電池用セパレータと、界面活性剤可溶溶媒に浸漬させた後９０℃で１０時間真空乾燥した電池用セパレータの質量差や、界面活性剤可溶溶媒で抽出した溶液を乾固させた抽出成分をＨ−ＮＭＲやガスクロマトグラフィーなどにより分析し質量を測定することなどから求めることができる。
静電気を評価する手法としては、ＪＩＳ Ｌ １０８４の摩擦帯電圧測定法を用いる。摩擦帯電圧測定法によって静電気の半減期が３０秒以下であることが好適である。半減期が３０秒以上であると、静電気の減衰が遅いため、帯電防止効果が充分とは言えない。
［セパレータの製造法］
本発明のリチウムイオン二次電池用セパレータはさまざまな方法で製造でき、製造法により限定されるものではない。例えば、不織布の両面にメタ芳香族ポリアミド多孔膜をプレス加工で圧着させる方法、不織布の両表面にメタ芳香族ポリアミドからなる高分子溶液を塗工し、該塗工層を乾式法、乾湿式法、湿式法いずれかの方法でミクロ相分離させる方法等が挙げられる。
特に、メタ芳香族ポリアミドとこれに対し良溶媒である溶媒を主成分とする高分子溶液を不織布の表裏両面に塗工し、次いで塗工された不織布をメタ芳香族ポリアミドに対し貧溶媒である溶媒と良溶媒である溶媒から主としてなる混合液（凝固液）中で凝固させ、次いで水洗、乾燥する製造法（湿式ミクロ相分離法）が好適である。該製造法を採用することによってメタ芳香族ポリアミド多孔質層の多孔質層の構造制御が容易となる。また、表裏両面に塗工することによって高分子溶液の含浸不良による不織布構成繊維の露出を無くすことで品位のよいセパレータを製造することが可能となる。
上記の製造法について具体的には特許文献６記載の製造装置や概念が適用できる。具体的には、不織布の表裏両面に高分子溶液を塗工する塗工方式として不織布の両面から過剰な高分子溶液を供給し、対峙した１対のマイヤーバーやダイの間に不織布を通過させることで計量する方式が挙げられ、さらにこの高分子溶液が塗工された不織布を表裏両面が凝固液と接するように凝固液に浸漬することで凝固された多孔質層が不織布の表裏両面に成形される。
該製造法において、該良溶媒はアミド系溶媒が適切であり、例えばジメチルアセトアミド、Ｎ−メチル−２−ピロリドン等が好ましい。また、該貧溶媒は具体的にアルコール類、水等が挙げられ、特に水が好適である。
該高分子溶液の高分子濃度としては、使用するメタ芳香族ポリアミドの種類や重合度に大きく依存するため一概には決められないが、例えばポリメタフェニレンイソフタルアミドを使用する場合は５〜２０重量％の範囲が好適である。
高分子濃度が低い場合はイオン透過性に特に優れたセパレータが得られるが、高分子溶液の粘度が低いため不織布に塗工するときピンホール等の観点から生産性は十分なものでない。高分子溶液の高分子濃度を下げイオン透過性に特に優れたものを得ようとする場合は該高分子溶液にセラミック微粒子を添加する手法が好適である。セラミック微粒子の添加によって該高分子溶液の粘度が増加し低高分子濃度であっても不織布へ容易に塗工できるようになる。
該セラミック微粒子として具体的に、シリカ、アルミナ、ジルコニア、マグネシア、チタニア、チタン酸バリウム、窒化アルミニウム、酸化カルシウム、炭酸カルシウム、フッ化リチウム、酸化リチウム等が挙げられる。特に、アルミナ、ジルコニア、マグネシアが好適である。
該セラミック微粒子の平均粒子径は０．０５〜２μｍが好適であり、特に０．１〜１μｍの範囲が好ましい。セラミック微粒子が０．０５μｍ以下となると凝集等の観点からハンドリング性が難しくなる。また、２μｍ以上となると塗工の際にダイ筋を発生しやすくなり好ましくない。ここで平均粒子径はレーザー回折測定法にて測定可能であり、体積粒度分布における中心粒径（Ｄ５０）を意味する。また、本発明での平均粒子径とは１次粒子の平均粒子径のことである。
該セラミック微粒子の添加量は高分子溶液の高分子濃度との兼ね合いで好適に決定されるものであるが、概ね該多孔質を形成するメタ芳香族ポリアミド（他の有機高分子を含む場合はこれも含む）とセラミック微粒子の重量に対し３０〜８０重量％が好ましい。３０重量％よりセラミック微粒子の量が少ないと十分な増粘効果が得られず効果が不十分である。また、８０重量％よりセラミック微粒子の量が多いとセパレータを成形した際に粉落ち等の問題が生じ好ましくない。
この製造法上のセラミック微粒子添加の効果は該高分子溶液中の高分子濃度が１０重量％以下のときに特に好適である。
また、セラミック微粒子添加の効果は上記の製造法における効果だけではなく、以下に述べるように構成上の特徴も有する。セラミック微粒子は一般にメタ芳香族ポリアミドに比べ耐熱性が高く、メタ芳香族ポリアミドが熱分解する４００℃付近まで電池温度が上昇したときにもセラミック微粒子を添加した構成ではセパレータとして機能する。さらに、セラミック微粒子は滑剤としても機能し帯電防止効果または静電気が発生した場合のハンドリング性向上にも寄与する。
該高分子溶液に多孔構造を制御する目的で、相分離剤を添加しても良い。相分離剤はメタ芳香族ポリアミドに対して貧溶媒であり、凝固液に相溶化するものであれば用いることが可能である。具体的には、水やアルコール類が好適であり、特に重合体を含むプロピレングリコール、エチレングリコール、ジエチレングリコール、トリプロピレングリコール、１，３−ブタンジオール、１，４−ブタンジオール、ポリエチレングリコールモノエチルエーテル、メタノール、エタノール、グリセリン等多価アルコール等が好適に選ばれる。該高分子溶液中の相分離剤の濃度は、該良溶剤と相分離剤の混合液中に対して０〜４０重量％の範囲で好適に選ばれる。
該凝固液は前述の良溶媒と貧溶媒の混合液が好ましい。ここで高分子溶液に相分離剤を適用した場合においては適切な比率で凝固液中へ該相分離剤を混合した方がプロセス管理上好ましい。具体的には、該高分子溶液における該良溶媒と相分離剤の比率に凝固液での比率も一致させるのが好適である。
該凝固浴における該貧溶媒の比率は、該貧溶媒に水を適用した場合、１０〜８０重量％の範囲から好適に選ばれる。
凝固させた多孔質層が形成された不織布は次に水洗工程に移され、次いで乾燥工程において水を乾燥することで本発明セパレータを得ることができる。ここで、乾燥工程は加熱ロールに接触させて乾燥させる方式が好適に選ばれる。
本発明セパレータの製造法において界面活性剤を付着させる場合は、その方法は特に限定されないが、界面活性剤を溶媒に溶かし、多孔膜にスプレーして乾燥させる方法や、多孔膜を浸漬させ乾燥させる方法などが挙げられる。
［リチウムイオン二次電池］
一般にリチウムイオン二次電池はセパレータを介して正極と負極とが対向し接合された電池エレメントに電解液が含浸され外装に封入された構成となっている。本発明のリチウムイオン二次電池は前述した本発明セパレータを用いることが特徴であり、他の構成要素においては公知技術を適用することができ、他の構成要素において本質的に限定されるものではない。
負極は、負極活物質、バインダー、導電助剤から成形された層が集電体上に塗工されたものが一般的に用いられる。これは負極活物質、バインダー、導電助剤に溶剤を加え混練してスラリーを作製し、これを集電体上へ塗工し、乾燥・プレスを行うことで作製する。負極活物質、バインダー、導電助剤の合計重量を１００％としたとき、負極活物質の重量は８０〜９８重量％、バインダーは２〜２０重量％、導電助剤は０〜１０重量％の範囲が好適である。負極活物質としては炭素材料、シリコン、スズ等が挙げられる。炭素材料としてはメソカーボンマイクロビーズやマイクロカーボンファイバーのような黒鉛化し易いピッチ等を前駆体として得たもの、フェノール樹脂のような黒鉛化し難いものを前駆体としたものが挙げられる。バインダーとしてはポリフッ化ビニリデンやカルボキシメチルセルロース等が挙げられる。導電助剤は黒鉛粉末、アセチレンブラック、ケッチェンブラック、気相成長カーボンファイバー等が好適に用いられる。集電体には銅箔、ステンレススチール等が好適である。
正極も負極同様に、正極活物質、バインダー、導電助剤から形成された層が集電体上に塗工されたものが一般的に用いられる。これは正極活物質、バインダー、導電助剤に溶剤を加え混練してスラリーを作製し、これを集電体上へ塗工し、乾燥・プレスを行うことで作製する。正極活物質、バインダー、導電助剤の合計重量を１００％としたとき、正極活物質の重量は８０〜９８重量％、バインダーは２〜２０重量％、導電助剤は０〜１０重量％の範囲が好適である。正極活物質としてはＬｉＣｏＯ２、ＬｉＮｉＯ２、スピネルタイプのＬｉＭｎ２Ｏ４、オリビンタイプのＬｉＦｅＰＯ４等及びこれらに異種元素を固溶化したものが挙げられ、これらは混合して用いてもよい。バインダーとしてはポリフッ化ビニリデンが好適に用いられる。導電助剤は黒鉛粉末、アセチレンブラック、ケッチェンブラック、気相成長カーボンファイバー等が好適に用いられる。集電体にはアルミ箔、ステンレススチール等が好適である。
電解液はリチウム塩を非水系溶媒に溶解させた非水系電解液が用いられる。リチウム塩としては、ＬｉＰＦ６、ＬｉＢＦ４、ＬｉＣｌＯ４等が好適に用いられる。非水溶媒はプロピレンカーボネート（ＰＣ）、エチレンカーボネート（ＥＣ）、ジメチルカーボネート（ＤＭＣ）、ジエチルカーボネート（ＤＥＣ）、エチルメチルカーボネート（ＥＭＣ）等が挙げられる。これらリチウム塩及び非水溶媒は単独で用いても２種類以上混合して用いても構わない。リチウム塩の濃度は０．５〜２．０Ｍの範囲が好適である。また、電解液にビニレンカーボネート（ＶＣ）、ビニルアセテート（ＶＡ）を添加した方が耐久性の観点から好適である。
本発明のリチウムイオン二次電池において、上記正極、負極、セパレータからなる電池エレメントは捲回して円筒状または扁平状にしたり、積層構造としたりして外装中に封入される。過充電防止機能を良好に発現させるという観点では、折り曲げ構造を有していた方がよく、特に捲回し扁平状の構成とすることが好ましい。
外装は金属ケース、アルミラミネートフィルムケース等の如何なる形態において実施可能である。Hereinafter, embodiments of the present invention will be described.
[Separator]
The separator for a lithium ion secondary battery of the present invention is characterized in that a porous layer mainly composed of a metaaromatic polyamide is formed on both front and back surfaces of a nonwoven fabric. By using a non-woven fabric, the strength and dimensional stability of the porous membrane made of aromatic polyamide can be improved, and handling properties and productivity can be improved.
Here, the porous layer mainly made of meta-aromatic polyamide is formed on the front and back surfaces of the nonwoven fabric, when the front and back surfaces of the separator are observed, the entire front and back surfaces are covered with the porous layer made of aromatic polyamide. The fibers constituting the nonwoven fabric are not visible. This can be easily observed with a scanning electron microscope (SEM). In the separator in which the fibers constituting the nonwoven fabric are exposed on one or both of the front and back surfaces, the bonding interface between the electrode and the separator becomes non-uniform, resulting in a problem in battery performance. This is due to insufficient electrolyte retention at the electrode / separator interface, and as the cycle progresses, the electrolyte present at the electrode / separator interface is depleted, resulting in poor cycle characteristics and cycle discharge characteristics. Become.
In the separator of the present invention, the layer formed on the nonwoven fabric surface is a porous layer made of metaaromatic polyamide. In the porous layer, it is necessary that pores are continuously formed, and the structure of the porous layer can use the Gurley value (JIS P8117) as an index. The Gurley value of the separator of the present invention is preferably 10 to 50 seconds / 100 cc. If the Gurley value is lower than 10 seconds / 100 cc, the probability that a non-woven fabric-forming fiber is exposed or has a pinhole is high, which is not preferable. On the other hand, if it is greater than 50 seconds / 100 cc, the ion permeability is not sufficient and the rate characteristics are deteriorated. Furthermore, it becomes difficult to obtain the overcharge prevention function described in WO01 / 067536 well, which is not preferable from the viewpoint of ensuring safety during overcharge.
In the separator of the present invention, the film thickness is preferably 15 to 40 μm. If the film thickness is less than 15 μm, the separator's original function of preventing short circuit becomes insufficient. On the other hand, if it is thicker than 40 μm, the resistance associated with ion conduction is increased, resulting in problems that the rate characteristics are not sufficient and the energy density of the battery is not increased.
In order to realize the separator thickness as described above, the nonwoven fabric thickness is 10 to 39 μm, and the total thickness of the porous layer made of the metaaromatic polyamide is preferably about 1 to 10 μm. The substantial strength of the separator of the present invention is determined by the nonwoven fabric, but if the thickness of the nonwoven fabric is less than 10 μm, it is difficult to ensure sufficient strength as a separator for a lithium ion battery. On the other hand, when the thickness is larger than 39 μm, it becomes substantially difficult to make the separator thickness 40 μm or less. In addition, if the thickness of the porous layer is thinner than 1 μm in total, it is difficult to substantially cover the entire surface of the nonwoven fabric. If the porous layer has a rate limiting ion conduction resistance of the separator and is thicker than 10 μm, it is not preferable from the viewpoint of ensuring sufficient discharge performance.
The non-woven fabric used in the separator of the present invention is preferably as fine as possible, and in order to obtain such a non-woven fabric, the fiber diameter is preferably fine. From such a viewpoint, the fiber diameter of the fibers constituting the nonwoven fabric is preferably 10 μm or less, and more preferably 5 μm or less.
When forming a nonwoven fabric, a binder for binding the main fiber to the main fiber is required. The binder for forming the nonwoven fabric is preferably fiber or pulp.
A known method can be applied to the method for producing the nonwoven fabric. Specific examples include a dry method, a water needle method, a wet papermaking method, a spunbond method, a melt blow method, and an electrospinning method. The wet papermaking method is particularly suitable in view of the thinning and uniformity of the openings.
The material constituting the nonwoven fabric is not particularly limited as long as it has sufficient heat resistance and resistance to an electrolytic solution. Specifically, polyesters represented by polyethylene terephthalate (PET), aromatic polyamide, polysulfone, polyethersulfone, Examples include polyphenylene sulfide and polyimide. In particular, PET is preferable from the viewpoint that it is easy to form thinner fibers and has high heat resistance. Further, aromatic polyamides, particularly metaaromatic polyamides are preferable from the viewpoint of high strength and high heat resistance, and polymetaphenylene isophthalamide is particularly preferable from the viewpoint of moldability. In the non-woven fabric, there is no problem even if polyolefin fibers such as polyethylene and polypropylene are mixed within a range that does not impair the heat resistance of the non-woven fabric, and if these materials are mixed in a small amount, the safety may be improved. In this case, the amount of the polyolefin fiber added is preferably 30% by weight or less based on the weight of the nonwoven fabric.
In particular, from the viewpoint of thin film and high strength, short fibers made of meta-aromatic polyamide and non-woven fabric using para-aromatic polyamide pulp are suitable. The nonwoven fabric has a high affinity with the metaaromatic polyamide that forms the porous layer, and a high strength can be obtained by combining the nonwoven fabric. In addition, the nonwoven fabric having the above structure can be easily thinned.
In the separator of the present invention, the material that forms the porous layer is preferably metaaromatic polyamide. Aromatic polyamides include para-aromatic polyamides typified by polyparaphenylene terephthalamide and meta-aromatic polyamides typified by polymetaphenylene isophthalamide. In the separator of the present invention, meta-aromatic polyamides, particularly polymetaphenylene isophthalate are used. Amides are preferred. Para-aromatic polyamide has low solubility in a solvent and high viscosity even at a low concentration, so that it is very difficult to form the porous layer with sufficient strength and ion permeability. Specifically, only small pores are formed and these are discontinuous, resulting in insufficient ion permeability. On the other hand, the metaaromatic polyamide is sufficiently dissolved in a solvent, and it is easy to prepare an appropriate polymer solution in terms of concentration and viscosity. Further, the pore diameter can be easily controlled and sufficient ion permeability can be ensured. In particular, in order to develop the overcharge prevention function of WO01 / 067536, it is necessary to appropriately control the pore diameter of the porous layer. From the viewpoint of adding such a function, metaaromatic polyamides are para-aromatic. It is preferable compared to the group polyamide.
The material for forming the porous layer is mainly preferably a metaaromatic polyamide, and other materials may be mixed in a category that does not impair the heat resistance, which is a characteristic of the metaaromatic polyamide, and the structure control of the porous layer. Absent. Specific examples include para-aromatic polyamide, polysulfone, polyethersulfone, polyvinylidene fluoride, polyvinylidene fluoride copolymer, polyacrylonitrile, polymethyl methacrylate, polyethylene oxide, polypropylene oxide, polyvinylpyrrolidone, and the like. Is preferably 30% by weight or less based on the metaaromatic polyamide forming the porous layer.
When PET is used for the nonwoven fabric, the weight of the metaaromatic polyamide forming the porous layer is preferably 4 to 10 g / m 2 . When PET is used as a material for a lithium ion secondary battery separator, there is a problem in durability in a special environment, but this durability can be remarkably improved by combining with a metaaromatic polyamide. If the weight of the metaaromatic polyamide is less than 4 g / m 2 , the durability is not sufficiently improved, which is not preferable. When the weight is more than 10 g / m 2 , there arises a problem that the ion permeability is deteriorated.
When the metaaromatic polyamide in the present invention is dissolved in N-methyl-2-pyrrolidone, it is represented by the logarithmic viscosity of the following formula (1), and is 0.8 to 2.5 dl / g, preferably 1.0 to A polymer in the range of 2.2 dl / g is preferred. If the logarithmic viscosity is lower than 0.8 dl / g, sufficient mechanical strength cannot be obtained. If the logarithmic viscosity exceeds 2.5 dl / g, it becomes difficult to obtain a stable polymer solution, and a uniform porous layer is formed. Therefore, it is not preferable.
Logarithmic viscosity (unit: dl / g) = ln (T / TO) / C (1)
T: Flow time of capillary viscometer at 30 ° C. in solution of 0.5 g of metaaromatic polyamide in 100 ml of N-methyl-2-pyrrolidone TO: Capillary viscometer of N-methyl-2-pyrrolidone at 30 ° C. Flow time C: Polymer concentration (g / dl) in the polymer solution
In the separator of the present invention, the one obtained by attaching a surfactant to the porous layer is also preferable from the viewpoint of antistatic. The surfactant is not particularly limited, and for example, a cationic, anionic, zwitterionic or nonionic surfactant can be used. Examples of the cationic surfactant include higher amine halogenates, alkylpyridinium halides, quaternary ammonium salts and the like. Anionic surfactants include higher fatty acid alkali salts, polyoxyethylene alkyl ether sulfonate esters, polyoxyethylene alkyl ether phosphonates, alkyl sulfates, alkyl sulfonates, alkyl aryl sulfonates, sulfosuccinic acid. Examples include ester salts. Examples of zwitterionic surfactants include alkylbetaine compounds, imidazoline compounds, alkylamine oxides, and bisoxyborate compounds. Nonionic surfactants include polyoxyethylene alkyl ethers, polyoxyethylene alkyl phenyl ethers, polyoxyethylene alkyl allyl ethers, glycerin fatty acid esters, polyoxyethylene sorbitan fatty acid esters, sorbitan fatty acid esters, and the like. .
In particular, cationic surfactants, anionic surfactants, and zwitterionic surfactants have a strong antistatic effect, so that the amount used can be kept low, and it is desirable to use them. Also, by mixing these, the affinity between the aromatic amide and the surfactant can be increased, and the antistatic effect can be improved.
The amount of the surfactant is preferably 0.005 to 0.750 / m 2 . 0.005 g / m less than 2 and a sufficient antistatic effect can not be obtained, may be adversely affected the performance of the battery is more than 0.750 g / m 2. The amount of the surfactant applied is that of the battery separator vacuum-dried at 90 ° C. for 10 hours after the surfactant coating, and the battery separator vacuum-dried at 90 ° C. for 10 hours after being immersed in the surfactant-soluble solvent. It can be determined by analyzing the mass difference or the extracted components obtained by drying the solution extracted with the surfactant-soluble solvent by H-NMR or gas chromatography and measuring the mass.
As a method for evaluating static electricity, the friction band voltage measurement method of JIS L 1084 is used. It is preferable that the half-life of static electricity is 30 seconds or less by the frictional voltage measurement method. If the half-life is 30 seconds or longer, the static electricity decay is slow, so that the antistatic effect is not sufficient.
[Separator manufacturing method]
The separator for a lithium ion secondary battery of the present invention can be produced by various methods, and is not limited by the production method. For example, a method of pressing a meta-aromatic polyamide porous film on both surfaces of a nonwoven fabric by press working, a polymer solution composed of a meta-aromatic polyamide is coated on both surfaces of a nonwoven fabric, and the coating layer is dry-processed or dry-wet-processed And a method of microphase separation by any of the wet methods.
In particular, a polymer solution mainly composed of a metaaromatic polyamide and a solvent that is a good solvent for the metaaromatic polyamide is applied to both front and back surfaces of the nonwoven fabric, and then the coated nonwoven fabric is a poor solvent for the metaaromatic polyamide. A production method (wet microphase separation method) in which coagulation is carried out in a mixed liquid (coagulation liquid) mainly comprising a solvent and a good solvent, followed by washing with water and drying is preferred. By adopting this production method, the structure control of the porous layer of the metaaromatic polyamide porous layer becomes easy. Moreover, it becomes possible to manufacture a high-quality separator by eliminating the exposure of the non-woven fabric constituting fiber due to poor impregnation of the polymer solution by coating on both front and back surfaces.
Specifically, the manufacturing apparatus and concept described in Patent Document 6 can be applied to the above manufacturing method. Specifically, as a coating method in which a polymer solution is applied to both the front and back surfaces of a nonwoven fabric, an excess polymer solution is supplied from both sides of the nonwoven fabric, and the nonwoven fabric is passed between a pair of opposed Meyer bars and dies. In addition, a non-woven fabric coated with this polymer solution is immersed in a coagulating liquid so that both the front and back surfaces are in contact with the coagulating liquid. Is done.
In the production method, the good solvent is suitably an amide solvent, and for example, dimethylacetamide, N-methyl-2-pyrrolidone and the like are preferable. Specific examples of the poor solvent include alcohols and water, and water is particularly preferable.
The polymer concentration of the polymer solution cannot be determined unconditionally because it largely depends on the type and degree of polymerization of the metaaromatic polyamide to be used. For example, when polymetaphenylene isophthalamide is used, it is 5 to 20% by weight. % Range is preferred.
When the polymer concentration is low, a separator having particularly excellent ion permeability can be obtained. However, since the viscosity of the polymer solution is low, productivity is not sufficient from the viewpoint of pinholes and the like when applied to a nonwoven fabric. In order to obtain a polymer solution having a particularly excellent ion permeability by lowering the polymer concentration, a method of adding ceramic fine particles to the polymer solution is suitable. The addition of the ceramic fine particles increases the viscosity of the polymer solution, so that it can be easily applied to the nonwoven fabric even at a low polymer concentration.
Specific examples of the ceramic fine particles include silica, alumina, zirconia, magnesia, titania, barium titanate, aluminum nitride, calcium oxide, calcium carbonate, lithium fluoride, lithium oxide and the like. In particular, alumina, zirconia, and magnesia are suitable.
The average particle size of the ceramic fine particles is preferably 0.05 to 2 μm, particularly preferably 0.1 to 1 μm. When the ceramic fine particles are 0.05 μm or less, handling properties are difficult from the viewpoint of aggregation and the like. On the other hand, when the thickness is 2 μm or more, die streaks are likely to occur during coating, which is not preferable. Here, the average particle diameter can be measured by a laser diffraction measurement method, and means the central particle diameter (D50) in the volume particle size distribution. Moreover, the average particle diameter in this invention is an average particle diameter of a primary particle.
The amount of the ceramic fine particles to be added is suitably determined in view of the polymer concentration of the polymer solution, but is generally a metaaromatic polyamide that forms the porous material (if other organic polymers are included, this is not the case). 30 to 80% by weight based on the weight of the ceramic fine particles. If the amount of ceramic fine particles is less than 30% by weight, a sufficient thickening effect cannot be obtained and the effect is insufficient. On the other hand, if the amount of the ceramic fine particles is larger than 80% by weight, problems such as powder falling when the separator is molded are not preferable.
The effect of adding ceramic fine particles to this production method is particularly suitable when the polymer concentration in the polymer solution is 10% by weight or less.
Further, the effect of adding ceramic fine particles is not only the effect of the above manufacturing method, but also has structural features as described below. Ceramic fine particles generally have higher heat resistance than metaaromatic polyamide, and function as a separator in the configuration in which ceramic fine particles are added even when the battery temperature rises to around 400 ° C. where the metaaromatic polyamide is thermally decomposed. Furthermore, the ceramic fine particles also function as a lubricant and contribute to an antistatic effect or improved handling properties when static electricity is generated.
A phase separation agent may be added to the polymer solution for the purpose of controlling the porous structure. The phase separation agent is a poor solvent for the metaaromatic polyamide and can be used as long as it is compatible with the coagulating liquid. Specifically, water and alcohols are preferred, and in particular, propylene glycol, ethylene glycol, diethylene glycol, tripropylene glycol, 1,3-butanediol, 1,4-butanediol, polyethylene glycol monoethyl ether containing a polymer. Polyhydric alcohols such as methanol, ethanol and glycerin are preferably selected. The concentration of the phase separation agent in the polymer solution is suitably selected in the range of 0 to 40% by weight with respect to the mixture of the good solvent and the phase separation agent.
The coagulation liquid is preferably a mixed liquid of the aforementioned good solvent and poor solvent. Here, when the phase separation agent is applied to the polymer solution, it is preferable in terms of process management to mix the phase separation agent into the coagulation liquid at an appropriate ratio. Specifically, it is preferable to match the ratio of the coagulating liquid with the ratio of the good solvent and the phase separation agent in the polymer solution.
The ratio of the poor solvent in the coagulation bath is suitably selected from the range of 10 to 80% by weight when water is applied to the poor solvent.
The non-woven fabric on which the solidified porous layer is formed is then transferred to a water washing step, and then water is dried in the drying step to obtain the separator of the present invention. Here, the drying process is preferably selected to be brought into contact with a heating roll and dried.
In the production method of the separator of the present invention, when the surfactant is attached, the method is not particularly limited, but the surfactant is dissolved in a solvent and sprayed onto the porous film to dry, or the porous film is immersed and dried. The method etc. are mentioned.
[Lithium ion secondary battery]
Generally, a lithium ion secondary battery has a configuration in which a battery element in which a positive electrode and a negative electrode are opposed to each other and bonded via a separator is impregnated with an electrolytic solution and enclosed in an exterior. The lithium ion secondary battery of the present invention is characterized by the use of the separator of the present invention described above, and a known technique can be applied to other components, which are not essentially limited to other components. Absent.
A negative electrode is generally used in which a layer formed from a negative electrode active material, a binder, and a conductive additive is coated on a current collector. This is prepared by adding a solvent to a negative electrode active material, a binder, and a conductive additive to knead to prepare a slurry, which is coated on a current collector, dried and pressed. When the total weight of the negative electrode active material, the binder, and the conductive additive is 100%, the negative electrode active material has a weight of 80 to 98% by weight, the binder is 2 to 20% by weight, and the conductive auxiliary is in the range of 0 to 10% by weight. Is preferred. Examples of the negative electrode active material include carbon materials, silicon, and tin. Examples of the carbon material include those obtained by using, as a precursor, pitches that are easily graphitized such as mesocarbon microbeads and microcarbon fibers, and those that are difficult to graphitize such as phenol resins. Examples of the binder include polyvinylidene fluoride and carboxymethyl cellulose. As the conductive additive, graphite powder, acetylene black, ketjen black, vapor grown carbon fiber, and the like are preferably used. The current collector is preferably copper foil, stainless steel, or the like.
As in the case of the negative electrode, a positive electrode in which a layer formed of a positive electrode active material, a binder, and a conductive additive is coated on a current collector is generally used. This is prepared by adding a solvent to a positive electrode active material, a binder, and a conductive additive to prepare a slurry, applying the slurry onto a current collector, drying and pressing. When the total weight of the positive electrode active material, the binder, and the conductive additive is 100%, the positive electrode active material is in the range of 80 to 98% by weight, the binder is in the range of 2 to 20% by weight, and the conductive auxiliary is in the range of 0 to 10% by weight. Is preferred. Examples of the positive electrode active material include LiCoO 2 , LiNiO 2 , spinel-type LiMn 2 O 4 , olivine-type LiFePO 4, and the like, and those obtained by solidifying different elements therein, and these may be used in combination. As the binder, polyvinylidene fluoride is preferably used. As the conductive additive, graphite powder, acetylene black, ketjen black, vapor grown carbon fiber, and the like are preferably used. For the current collector, aluminum foil, stainless steel or the like is suitable.
As the electrolytic solution, a non-aqueous electrolytic solution in which a lithium salt is dissolved in a non-aqueous solvent is used. As the lithium salt, LiPF 6 , LiBF 4 , LiClO 4 and the like are preferably used. Nonaqueous solvents include propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and the like. These lithium salts and non-aqueous solvents may be used alone or in combination of two or more. The concentration of the lithium salt is preferably in the range of 0.5 to 2.0M. In addition, it is preferable from the viewpoint of durability that vinylene carbonate (VC) or vinyl acetate (VA) is added to the electrolytic solution.
In the lithium ion secondary battery of the present invention, the battery element composed of the positive electrode, the negative electrode, and the separator is wound into a cylindrical or flat shape or a laminated structure and enclosed in an exterior. From the viewpoint of satisfactorily expressing the overcharge prevention function, it is better to have a folded structure, and it is particularly preferable to have a wound and flat configuration.
The exterior can be implemented in any form such as a metal case or an aluminum laminated film case.

以下、実施例により本発明を詳述する。ただし、本発明は以下の実施例に限定されるものではない。
［実施例１］
繊度０．１１ｄｔｅｘ（平均繊維径約３．２μｍ）のＰＥＴ短繊維と繊度０．３３ｄｔｅｘ（平均繊維径約５．５μｍ）のＰＥＴ短繊維、および繊度０．２２ｄｔｅｘ（平均繊維径約４．５μｍ）のバインダー用ＰＥＴ短繊維を３／２／５の重量比でブレンドし、湿式抄造法により目付１２．６ｇ／ｍ２で製膜し、１４０℃でカレンダーを施し、膜厚１８μｍのＰＥＴ不織布を得た。
ポリメタフェニレンイソフタルアミド（帝人テクノプロダクツ株式会社製；商品名「コーネックス」）をジメチルアセトアミド：トリプロピレングリコール＝８５：１５（重量比）である混合溶媒に９重量％となるように溶解し、高分子溶液を調整した。この高分子溶液を該ＰＥＴ不織布の両面に塗工し、この塗工物をジメチルアセトアミド：水＝５５：４５（重量比）の組成である３０℃の凝固浴に６０秒間浸漬し凝固膜を得た。この凝固膜を５０℃の水浴中で１０分間水洗し、次いで乾燥することで本発明のリチウムイオン二次電池用セパレータを得た。これを本発明セパレータ１とする。本発明セパレータ１は、膜厚２１μｍ、目付１４．７ｇ／ｍ２、ガーレ値（ＪＩＳ Ｐ８１１７）２３秒／１００ｃｃであった。
本サンプルを走査型電子顕微鏡（ＳＥＭ）で表裏表面を観察した結果、不織布を構成する繊維の露出は認められなかった。
［実施例２］
繊度０．９ｄｔｅｘ（平均繊維径約１０μｍ）のポリメタフェニレンイソフタルアミド短繊維を用い、乾式法により目付１５．０ｇ／ｍ２で製膜して、３２０℃にてカレンダーを施し、膜厚３０μｍの不織布を得た。
該不織布を用い、実施例１と同様に高分子溶液を用い、同様の方法で本発明のリチウムイオン二次電池用セパレータを得た。これを本発明セパレータ２とする。本発明セパレータ２は、膜厚３４μｍ、目付１８．１ｇ／ｍ２、ガーレ値（ＪＩＳ Ｐ８１１７）３５秒／１００ｃｃであった。
［比較例１］
実施例１と同様の不織布をＰＥＴフィル厶上に固定し、その上から実施例１で用いた高分子溶液を塗工した。実施例１と同様の凝固液中に浸漬し凝固膜を得た。この凝固膜を５０℃の水浴中で１０分間水洗し、次いで乾燥した。その後、ＰＥＴフィルムを剥がし、比較セパレータ１を得た。比較セパレータ１は、膜厚２０μｍ、目付け１３．９ｇ／ｍ２、ガーレ値（ＪＩＳ Ｐ８１１７）１５秒／１００ｃｃであった。
本サンプルを走査型電子顕微鏡（ＳＥＭ）で表裏表面を観察した結果、ＰＥＴフィル
ムと接していた方の面では有意に不織布を構成する繊維の露出が認められた。
［比較例２］
繊度０．３３ｄｔｅｘ（平均繊維径５．５μｍ）のＰＥＴ短繊維と繊度０．２２ｄｔｅｘ（平均繊維径４．５μｍ）のバインダー用ＰＥＴ繊維を６／４の重量比でブレンドし、湿式抄造法により目付１２．０ｇ／ｍ２で製膜して、２００℃でカレンダーを施し、膜厚１８μｍのＰＥＴ不織布を得た。
フッ化ビニリデン：ヘキサフロロプロピレン：クロロトリフロロエチレン＝９５．５：２．３：２．２（重量比）であるポリフッ化ビニリデン共重合体を、ジメチルアセトアミド：トリプロピレングリコール＝７０：３０（重量比）である混合溶媒に１２重量％となるように溶解し高分子溶液を調整した。
この高分子溶液を該ＰＥＴ不織布の両面に塗工し、この塗工物をジメチルアセトアミド：水＝５５：４５（重量比）の組成である３０℃の凝固浴に６０秒間浸漬し凝固膜を得た。この凝固膜を５０℃の水浴中で１０分間水洗し、次いで乾燥することで比較セパレータ２を得た。比較セパレータ２は、膜厚２４μｍ、目付１７．３ｇ／ｍ２、ガーレ値（ＪＩＳ Ｐ８１１７）１２秒／１００ｃｃであった。
［耐熱性評価１］
上記、本発明セパレータ１及び２、比較セパレータ１及び２、市販のリチウムイオン二次電池用セパレータであるポリプロピレン製微多孔膜（セルガード社製；商品名「セルガード＃２４００」）をそれぞれ１０ｃｍ×１０ｃｍに切り出した。電解液を７０ｃｃサンプル瓶にとった。ここで電解液は１Ｍ ＬｉＢＦ４ ＰＣ／ＥＣ（１／１重量比）を用いた。切り出したセパレータを該サンプル瓶に入れ、１５０℃にて２時間処理した。その後、サンプル瓶からセパレータを取り出しセパレータを観察した。結果を表１に示す。
［耐熱性評価２］
上記、本発明セパレータ１及び２、比較セパレータ１及び２、市販のリチウムイオン二次電池用セパレータであるポリプロピレン製微多孔膜（セルガード社製；商品名「セルガード＃２４００」）をそれぞれ１０ｃｍ×１０ｃｍに切り出した。これを４方向固定できる枠に固定し、２００℃にて３０分間熱処理した。熱処理後の形態及び寸法を測定した。結果を表１に示す。
［実施例３］
コバルト酸リチウム（ＬｉＣｏＯ２；日本化学工業社製）粉末８９．５重量部、アセチレンブラック４．５重量部、ポリフッ化ビニリデン６重量部となるようにＮ−メチル−２ピロリドン溶媒を用いてこれらを混練し、スラリーを作製した。得られたスラリーを厚さが２０μｍのアルミ箔上に塗布乾燥後プレスし、１００μｍの正極を得た。
メソフェーズカーボンマイクロビーズ（ＭＣＭＢ：大阪瓦斯化学社製）粉末８７重量部、アセチレンブラック３重量部、ポリフッ化ビニリデン１０重量部となるようにＮ−メチル−２ピロリドン溶媒を用いてこれらを混練し、スラリーを作製した。得られたスラリーを厚さが１８μｍの銅箔上に塗布乾燥後プレスし、９０μｍの負極を得た。
上記正極及び負極を実施例１で作製した本発明セパレータ１を介して対向させた。これに電解液を含浸させアルミラミネートフィルムからなる外装に封入して本発明リチウムイオン二次電池を作製した。ここで、電解液には１Ｍ ＬｉＰＦ６ ＥＣ／ＥＭＣ（３／７重量比）を用いた。この電池を本発明電池１とする。
［比較例３］
セパレータに比較セパレータ１を用い実施例１と同様の方法にてリチウムイオン二次電池を作製した。本リチウムイオン二次電池を比較電池１とする。
［比較例４］
セパレータに市販のリチウムイオン二次電池用セパレータであるポリプロピレン製微多孔膜（セルガード社製；商品名「セルガード＃２４００」）を用い、実施例１と同様の方法にてリチウムイオン二次電池を作製した。本リチウムイオンに二次電池を比較電池２とする。
［サイクル試験］
本発明電池１及び比較電池１、２について、１Ｃ、４．２Ｖ、２時間の定電流定電圧充電、１Ｃ、２．７５Ｖの定電流放電を１００サイクル行った。それぞれの電池に対し、（容量維持率）＝（１００サイクル目の放電容量）／（１サイクル目の放電容量）を求めた。これを表２に示す。不織布の露出が確認された比較セパレータ１を用いた比較電池１のみ有意にサイクル特性が悪く、両面にコーティングを施してある本発明セパレータ１を用いた本発明電池１は市販セパレータと同等の特性である。
［過充電試験］
本発明電池１及び比較電池２について１Ｃで１０時間の定電流充電（本来の充電に対し１０倍の充電を行う過充電試験）を行った。ただし、電圧が６Ｖに到達したら強制的に受電終了とした。電圧が６Ｖに到達し１０時間経つ前に充電が強制的に終了した場合を過充電特性不十分とし、そうでなかった場合を過充電特性十分とした。過充電試験の結果を図１に示すが、本発明セパレータを用いた本発明電池１では過充電特性十分であるが、市販のセパレータを用いた比較電池２では不十分である。
［実施例４］
エマルゲン１２０（花王製；非イオン系界面活性剤）をメタノールに溶解し、１重量％溶液を作製した。実施例１で作製した本発明セパレータ１を該界面活性剤メタノール溶液に浸漬し乾燥させることで界面活性剤を付着させ、本発明セパレータ３を得た。ここで本発明セパレータ３に付着した界面活性剤の量は０．１５ｇ／ｍ２であった。
［実施例５］
エレクトロストリッパーＡＣ（花王製；両イオン系界面活性剤）をメタノールに溶解し、１重量％溶液を作製した。実施例１で作製した本発明セパレータ１を該界面活性剤メタノール溶液に浸漬し乾燥させることで界面活性剤を付着させ、本発明セパレータ４を得た。ここで本発明セパレータ４に付着した界面活性剤の量は０．０２ｇ／ｍ２であった。
［実施例６］
コータミン６０Ｗ（花王製；陽イオン系界面活性剤）をメタノールに溶解し、１重量％溶液を作製した。実施例１で作製した本発明セパレータ１を該界面活性剤メタノール溶液に浸漬し乾燥させることで界面活性剤を付着させ、本発明セパレータ５を得た。ここで本発明セパレータ５に付着した界面活性剤の量は０．０４ｇ／ｍ２であった。
［実施例７］
エレクトロストリッパーＦ（花王製；陰イオン系界面活性剤）をメタノールに溶解し、１重量％溶液を作製した。実施例１で作製した本発明セパレータ１を該界面活性剤メタノール溶液に浸漬し乾燥させることで界面活性剤を付着させ、本発明セパレータ６を得た。ここで本発明セパレータ６に付着した界面活性剤の量は０．１０ｇ／ｍ２であった。
［帯電圧測定］
本発明セパレータ１、３〜６の帯電圧半減期をスタティックオネストメータＨ−０１１０（シシド静電気製）を用いて測定した。その結果を表３に示す。表３より界面活性剤を付着させることが帯電防止に有効であることが分かる。
［実施例８］
繊度０．９ｄｔｅｘ（平均繊維径約１０μｍ）のポリメタフェニレンイソフタルアミド短繊維を主繊維とし、パラアラミドからなるパルプをバインダーとし、これらを主繊維／バインダー＝１／１（重量比）で混合させ、湿式抄造法により目付２９．９ｇ／ｍ２で製膜して、カレンダーを施し、膜厚３０μｍのアラミド不織布（アラミドペーパー）を得た。
ポリメタフェニレンイソフタルアミド（帝人テクノプロダクツ株式会社製；商品名「コーネックス」）をジメチルアセトアミド：トリプロピレングリコール＝８５：１５（重量比）である混合溶媒に９重量％となるように溶解し、高分子溶液を調整した。この高分子溶液を該アラミド不織布の両面に塗工し、この塗工物をジメチルアセトアミド：水＝６０：４０（重量比）の組成である４０℃の凝固浴に６０秒間浸漬し凝固膜を得た。この凝固膜を３０℃の水浴中で１０分間水洗し、次いで乾燥することで本発明のリチウムイオン二次電池用セパレータを得た。これを本発明セパレータ７とする。本発明セパレータ７は、膜厚３９μｍ、目付３４．５ｇ／ｍ２、ガーレ値（ＪＩＳ Ｐ８１１７）４０秒／１００ｃｃであった。なお、ポリメタフェニレンイソフタルアミドからなる多孔質層の重量は４．６ｇ／ｍ２であった。
［実施例９］
繊度０．１１ｄｔｅｘ（平均繊維径約３．２μｍ）のＰＥＴ短繊維と繊度０．３３ｄｔｅｘ（平均繊維径約５．５μｍ）のＰＥＴ短繊維、および繊度０．２２ｄｔｅｘ（平均繊維径約４．５μｍ）のバインダー用ＰＥＴ短繊維を３／２／５の重量比でブレンドし、湿式抄造法により目付１３．０ｇ／ｍ２で製膜し、カレンダーを施し、膜厚１８μｍのＰＥＴ不織布を得た。
該ＰＥＴ不織布を用いて実施例８と同様の手法で本発明のリチウムイオン二次電池用セパレータを得た。これを本発明セパレータ８とする。本発明セパレータ８は、膜厚２９μｍ、目付１８．３ｇ／ｍ２、ガーレ値（ＪＩＳ Ｐ８１１７）２８秒／１００ｃｃであった。なお、ポリメタフェニレンイソフタルアミドからなる多孔質層の重量は５．３ｇ／ｍ２であった。
［実施例１０］
実施例９と同様の手法で、塗工クリアランスを変化させ、膜厚３７μｍ、目付け２２．１ｇ／ｍ２、ガーレ値（ＪＩＳ Ｐ８１１７）３２秒／１００ｃｃである本発明セパレータ９を得たなお、ポリメタフェニレンイソフタルアミドからなる多孔質層の重量は９．１ｇ／ｍ２であった。
［実施例１１］
実施例９と同様の手法で、塗工クリアランスを変化させ、膜厚２２μｍ、目付け１６３ｇ／ｍ２、ガーレ値（ＪＩＳ Ｐ８１１７）３２秒／１００ｃｃである本発明セパレータ１０を得たなお、ポリメタフェニレンイソフタルアミドからなる多孔質層の重量は３．３ｇ／ｍ２であった。
［実施例１２］
実施例９と同様の手法で、塗工クリアランスを変化させ、膜厚２４μｍ、目付け１７３ｇ／ｍ２、ガーレ値（ＪＩＳ Ｐ８１１７）３２秒／１００ｃｃである本発明セパレータ１１を得たなお、ポリメタフェニレンイソフタルアミドからなる多孔質層の重量は４．３ｇ／ｍ２であった。
［突刺強度測定］
本発明セパレータ２、７〜１１について突刺強度の測定を行った。突刺強度は１１．３ｍｍΦの固定枠にセパレータをセットし、先端部半径０．５ｍｍの針をセパレータの中央に垂直に突き立て、２ｍｍ／秒の一定速度で針を押し込むことで測定した。針が５ｍｍ移動する間でセパレータにかかっている最大荷重を突刺強度とした。その結果を表４に示す。不織布の材質としてアラミド系材料を適用した場合、塗工前後における突刺強度の増加量が大きい。アラミド系材料からなる不織布は多孔質層を形成するポリメタフェニレンイソフタルアミドと親和性が高く、高い補強効果が得られるため高強度のセパレータを得るとう観点において有効である。
［実施例１３〜１６］
実施例９〜１２で得られた本発明セパレータ８〜１１を用いて実施例３と同様に本発明電池２〜５を得た。
［実施例１７〜２０］
実施例９〜１２で得られた本発明セパレータ８〜１１を用い、電解液に１Ｍ ＬｉＰＦ６ＥＣ／ＥＭＣ／ＶＣ（２９／７０／１重量比）を用いたこと以外は実施例３と同様に本発明電池６〜９を得た。
［実施例２１〜２４］
実施例９〜１２で得られた本発明セパレータ８〜１１を用い、電解液に１Ｍ ＬｉＰＦ６ＥＣ／ＥＭＣ／ＶＡ（２９／７０／１重量比）を用いたこと以外は実施例３と同様に本発明電池１０〜１３を得た。
［比較例５］
セパレータに実施例９で作製したＰＥＴ不織布を用いた以外は実施例３と同様の方法で電池を作製した。この電池を比較電池３とする。
［電池耐久性評価］
実施例１３〜２４で作製した本発明電池２〜１３と比較電池３を４．２Ｖまで充電し、８０℃で４日間保存した。その後電池を解体し中のセパレータを取り出して観察した。その結果を表５に示す。表５より、ポリメタフェニレンイソフタルアミドの塗布量を適切なものとした場合にＰＥＴの劣化を十分に抑制する効果があることが分かる。また、このＰＥＴ劣化を防止するためには電解液中にＶＣやＶＡを添加することも有効であることが分かる。
［実施例２５］
繊度０．１１ｄｔｅｘ（平均繊維径約３．２μｍ）のＰＥＴ短繊維と繊度０．３３ｄｔｅｘ（平均繊維径約５．５μｍ）のＰＥＴ短繊維、および繊度０．２２ｄｔｅｘ（平均繊維径約４．５μｍ）のバインダー用ＰＥＴ短繊維を３／２／５の重量比でブレンドし、湿式抄造法により目付１２．６ｇ／ｍ２で製膜し、１４０℃でカレンダーを施し、膜厚１８μｍのＰＥＴ不織布を得た。
ポリメタフェニレンイソフタルアミド（帝人テクノプロダクツ株式会社製；商品名「コーネックス」）をジメチルアセトアミド：トリプロピレングリコール＝６０：３０（重量比）である混合溶媒に６重量％となるように溶解し、高分子溶液を調整した。この高分子溶液へ平均粒子径０．８μｍのα−アルミナ微粒子（岩谷化学工業社製；ＳＡ−１）をポリメタフェニレンイソフタルアミドと同重量部分散させ塗工用スラリーを作製した。この塗工用スラリーを該ＰＥＴ不織布の両面に塗工し、この塗工物をジメチルアセトアミド：水＝５０：５０（重量比）の組成である４０℃の凝固浴に６０秒間浸漬し凝固膜を得た。この凝固膜を５０℃の水浴中で１０分間水洗し、次いで乾燥することで本発明のリチウムイオン二次電池用セパレータを得た。これを本発明セパレータ１２とする。本発明セパレータ１は、膜厚２３μｍ、目付１６．６ｇ／ｍ２、ガーレ値（ＪＩＳ Ｐ８１１７）２０秒／１００ｃｃであった。なお、本発明セパレータ１２を目視で観察した結果、ピンホールは見られなかった。
［参考例１］
ポリメタフェニレンイソフタルアミド（帝人テクノプロダクツ株式会社製；商品名「コーネックス」）をジメチルアセトアミド：トリプロピレングリコール＝６０：３０（重量比）である混合溶媒に６重量％となるように溶解し、高分子溶液を調整した。
この高分子溶液を実施例２５で得たＰＥＴ不織布の両面に塗工し、この塗工物をジメチルアセトアミド：水＝５０：５０（重量比）の組成である４０℃の凝固浴に６０秒間浸漬し凝固膜を得た。この凝固膜を５０℃の水浴中で１０分間水洗し、次いで乾燥することで本発明セパレータを作製したが、ピンホールが多発し十分なものが得られなかった。
［参考例２］
α−アルミナ微粒子の量をポリメタフェニレンイソフタルアミドに対して１０重量％とした以外は実施例２５と同様に本発明セパレータを作製したが、ピンホールが多発し十分なものを得ることができなかった。
［参考例３］
α−アルミナ微粒子の量をポリメタフェニレンイソフタルアミドに対して９００重量％とした以外は実施例２５と同様に本発明セパレータを作製したが、凝固・水洗工程においてα−アルミナ微粒子が欠け落ちハンドリング性十分なセパレータを得ることができなかった。
［膜抵抗測定］
電解液をセパレータに含浸させこれをアルミ箔からなる一対の電極（サイズ：２ｃｍ×１．４ｃｍ＝２．８ｃｍ２）挟み、アルミラミネートフィルム中へ封入することで測定用セルを作製した。ここで電解液には１Ｍ ＬｉＢＦ４ ＰＣ／ＥＣ（１／１重量比）を用いた。該セルにおいてセパレータを１枚、２枚、３枚としたものをそれぞれの抵抗を交流インピーダンス法により測定し、この抵抗をセパレータの枚数に対してプロットしたときの傾きからセパレータ１枚の抵抗を求めた。なお交流インピーダンス測定は４端子法で行い、振幅１０ｍＶ、周波数１００ｋＨｚとした。また、測定温度は２０℃とした。
上記の膜抵抗測定を本発明セパレータ１及び１２について行った結果、膜抵抗はそれぞれ６．０２６ｏｈｍ・ｃｍ２、５．２１１ｏｈｍ・ｃｍ２であった。
上記膜抵抗の測定結果より高分子溶液中のポリメタフェニレンイソフタルアミド濃度を低減することが有効であることが分かる。しかし、このような高分子溶液を用いて塗工する場合、参考例１、２にあるように粘度が低すぎてピンホールが多発し十分なものを得ることが困難である。実施例２５にあるように適切にセラミック微粒子を添加することでこの課題を解決することができる。ただし、参考例３にあるようにセラミック微粒子を添加しすぎると粉落ちの問題から十分な塗膜を得ることが困難となる。

Figure 0004832430
Figure 0004832430
Figure 0004832430
Figure 0004832430
Figure 0004832430
 Hereinafter, the present invention will be described in detail by way of examples. However, the present invention is not limited to the following examples.
[Example 1]
PET short fibers having a fineness of 0.11 dtex (average fiber diameter of about 3.2 μm), PET short fibers having a fineness of 0.33 dtex (average fiber diameter of about 5.5 μm), and fineness of 0.22 dtex (average fiber diameter of about 4.5 μm) The PET short fibers for binder were blended at a weight ratio of 3/2/5, formed into a film with a basis weight of 12.6 g / m 2 by a wet papermaking method, and calendered at 140 ° C. to obtain a PET nonwoven fabric with a film thickness of 18 μm. It was.
Polymetaphenylene isophthalamide (manufactured by Teijin Techno Products Co., Ltd .; trade name “Conex”) was dissolved in a mixed solvent of dimethylacetamide: tripropylene glycol = 85: 15 (weight ratio) to be 9% by weight, A polymer solution was prepared. This polymer solution was applied to both sides of the PET nonwoven fabric, and the coated product was immersed in a coagulation bath at 30 ° C. having a composition of dimethylacetamide: water = 55: 45 (weight ratio) for 60 seconds to obtain a coagulated film. It was. This solidified film was washed with water in a 50 ° C. water bath for 10 minutes and then dried to obtain a separator for a lithium ion secondary battery of the present invention. This is the separator 1 of the present invention. The separator 1 of the present invention had a film thickness of 21 μm, a basis weight of 14.7 g / m 2 , and a Gurley value (JIS P8117) of 23 seconds / 100 cc.
As a result of observing the front and back surfaces of this sample with a scanning electron microscope (SEM), exposure of fibers constituting the nonwoven fabric was not recognized.
[Example 2]
Using a polymetaphenylene isophthalamide short fiber having a fineness of 0.9 dtex (average fiber diameter of about 10 μm), a dry method is used to form a film with a basis weight of 15.0 g / m 2 , a calender is applied at 320 ° C., and a film thickness is 30 μm. A nonwoven fabric was obtained.
Using the nonwoven fabric, a polymer solution was used in the same manner as in Example 1 to obtain a lithium ion secondary battery separator of the present invention in the same manner. This is the separator 2 of the present invention. The separator 2 of the present invention had a film thickness of 34 μm, a basis weight of 18.1 g / m 2 , and a Gurley value (JIS P8117) of 35 seconds / 100 cc.
[Comparative Example 1]
The same non-woven fabric as in Example 1 was fixed on a PET fill bag, and the polymer solution used in Example 1 was applied thereon. The film was immersed in the same coagulating liquid as in Example 1 to obtain a coagulated film. The coagulated film was washed with water in a 50 ° C. water bath for 10 minutes and then dried. Thereafter, the PET film was peeled off to obtain a comparative separator 1. The comparative separator 1 had a film thickness of 20 μm, a basis weight of 13.9 g / m 2 , and a Gurley value (JIS P8117) of 15 seconds / 100 cc.
As a result of observing the front and back surfaces of this sample with a scanning electron microscope (SEM), exposure of fibers constituting the nonwoven fabric was significantly observed on the surface in contact with the PET film.
[Comparative Example 2]
A PET short fiber having a fineness of 0.33 dtex (average fiber diameter of 5.5 μm) and a PET fiber for binder having a fineness of 0.22 dtex (average fiber diameter of 4.5 μm) are blended at a weight ratio of 6/4, and the basis weight is obtained by a wet papermaking method. A film was formed at 12.0 g / m 2 and calendered at 200 ° C. to obtain a PET nonwoven fabric having a film thickness of 18 μm.
A polyvinylidene fluoride copolymer of vinylidene fluoride: hexafluoropropylene: chlorotrifluoroethylene = 95.5: 2.3: 2.2 (weight ratio) was converted into dimethylacetamide: tripropylene glycol = 70: 30 (weight). The polymer solution was prepared by dissolving in a mixed solvent having a ratio of 12% by weight.
This polymer solution was applied to both sides of the PET nonwoven fabric, and the coated product was immersed in a coagulation bath at 30 ° C. having a composition of dimethylacetamide: water = 55: 45 (weight ratio) for 60 seconds to obtain a coagulated film. It was. This solidified film was washed with water in a 50 ° C. water bath for 10 minutes and then dried to obtain a comparative separator 2. The comparative separator 2 had a film thickness of 24 μm, a basis weight of 17.3 g / m 2 , and a Gurley value (JIS P8117) of 12 seconds / 100 cc.
[Heat resistance evaluation 1]
The above-mentioned separators 1 and 2 of the present invention, comparative separators 1 and 2, and a commercially available polypropylene microporous membrane (manufactured by Celgard; trade name “Celgard # 2400”), each 10 cm × 10 cm. Cut out. The electrolyte was taken in a 70 cc sample bottle. Here, 1 M LiBF 4 PC / EC (1/1 weight ratio) was used as the electrolytic solution. The cut separator was placed in the sample bottle and treated at 150 ° C. for 2 hours. Then, the separator was taken out from the sample bottle and the separator was observed. The results are shown in Table 1.
[Heat resistance evaluation 2]
The above-mentioned separators 1 and 2 of the present invention, comparative separators 1 and 2, and a commercially available polypropylene microporous membrane (manufactured by Celgard; trade name “Celgard # 2400”), each 10 cm × 10 cm. Cut out. This was fixed to a frame that can be fixed in four directions, and heat-treated at 200 ° C. for 30 minutes. The form and dimensions after heat treatment were measured. The results are shown in Table 1.
[Example 3]
Lithium cobaltate (LiCoO 2 ; manufactured by Nippon Chemical Industry Co., Ltd.) powder 89.5 parts by weight, acetylene black 4.5 parts by weight, polyvinylidene fluoride 6 parts by weight using N-methyl-2pyrrolidone solvent It knead | mixed and produced the slurry. The obtained slurry was applied onto an aluminum foil having a thickness of 20 μm, dried and pressed to obtain a positive electrode having a thickness of 100 μm.
These are kneaded using N-methyl-2pyrrolidone solvent so as to be 87 parts by weight of mesophase carbon microbeads (MCMB: Osaka Gas Chemical Co., Ltd.), 3 parts by weight of acetylene black and 10 parts by weight of polyvinylidene fluoride, and slurry Was made. The obtained slurry was applied onto a copper foil having a thickness of 18 μm, dried and pressed to obtain a negative electrode having a thickness of 90 μm.
The positive electrode and the negative electrode were opposed to each other through the separator 1 of the present invention produced in Example 1. This was impregnated with an electrolytic solution and sealed in an exterior made of an aluminum laminate film to produce a lithium ion secondary battery of the present invention. Here, 1M LiPF 6 EC / EMC (3/7 weight ratio) was used as the electrolytic solution. This battery is referred to as a battery 1 of the present invention.
[Comparative Example 3]
A lithium ion secondary battery was produced in the same manner as in Example 1 using the comparative separator 1 as the separator. This lithium ion secondary battery is referred to as comparative battery 1.
[Comparative Example 4]
Using a polypropylene microporous membrane (manufactured by Celgard; trade name “Celgard # 2400”), which is a commercially available separator for lithium ion secondary batteries, as the separator, a lithium ion secondary battery was produced in the same manner as in Example 1. did. The secondary battery is referred to as a comparative battery 2 for this lithium ion.
[Cycle test]
The battery 1 of the present invention and the comparative batteries 1 and 2 were subjected to 100 cycles of 1C, 4.2V, 2 hours constant current and constant voltage charge, 1C and 2.75V constant current discharge. For each battery, (capacity maintenance ratio) = (discharge capacity at the 100th cycle) / (discharge capacity at the first cycle) was determined. This is shown in Table 2. Only the comparative battery 1 using the comparative separator 1 in which the exposure of the nonwoven fabric was confirmed has significantly poor cycle characteristics, and the inventive battery 1 using the inventive separator 1 coated on both sides has the same characteristics as a commercially available separator. is there.
[Overcharge test]
The battery 1 of the present invention and the comparative battery 2 were subjected to constant current charging at 1 C for 10 hours (overcharge test in which charging was 10 times the original charging). However, when the voltage reached 6V, the power reception was forcibly terminated. The case where charging was forcibly terminated 10 hours before the voltage reached 6 V was regarded as insufficient overcharge characteristics, and the case where it was not satisfied was regarded as sufficient overcharge characteristics. The result of the overcharge test is shown in FIG. 1. The present battery 1 using the separator of the present invention has sufficient overcharge characteristics, but the comparative battery 2 using a commercially available separator is insufficient.
[Example 4]
Emulgen 120 (manufactured by Kao; nonionic surfactant) was dissolved in methanol to prepare a 1% by weight solution. The separator 1 of the present invention produced in Example 1 was immersed in the surfactant methanol solution and dried to attach the surfactant, thereby obtaining the separator 3 of the present invention. Here, the amount of the surfactant adhered to the separator 3 of the present invention was 0.15 g / m 2 .
[Example 5]
Electro stripper AC (manufactured by Kao; amphoteric surfactant) was dissolved in methanol to prepare a 1 wt% solution. The separator 1 of the present invention produced in Example 1 was immersed in the surfactant methanol solution and dried to attach the surfactant, whereby the separator 4 of the present invention was obtained. Here, the amount of the surfactant adhered to the separator 4 of the present invention was 0.02 g / m 2 .
[Example 6]
Coatamine 60W (manufactured by Kao; cationic surfactant) was dissolved in methanol to prepare a 1% by weight solution. The separator 1 of the present invention produced in Example 1 was immersed in the surfactant methanol solution and dried to attach the surfactant, thereby obtaining the separator 5 of the present invention. Here, the amount of the surfactant adhered to the separator 5 of the present invention was 0.04 g / m 2 .
[Example 7]
Electro stripper F (manufactured by Kao; anionic surfactant) was dissolved in methanol to prepare a 1 wt% solution. The separator 1 of the present invention produced in Example 1 was immersed in the surfactant methanol solution and dried to attach the surfactant, whereby the separator 6 of the present invention was obtained. Here, the amount of the surfactant adhered to the separator 6 of the present invention was 0.10 g / m 2 .
[Electrostatic voltage measurement]
The charged voltage half-lives of the separators 1 and 3 to 6 of the present invention were measured using a Static Honestometer H-0110 (manufactured by Sisid Electric). The results are shown in Table 3. From Table 3, it can be seen that the attachment of the surfactant is effective for antistatic.
[Example 8]
A polymetaphenylene isophthalamide short fiber having a fineness of 0.9 dtex (average fiber diameter of about 10 μm) is used as a main fiber, pulp made of para-aramid is used as a binder, and these are mixed at a main fiber / binder = 1/1 (weight ratio). A wet papermaking method was used to form a film with a basis weight of 29.9 g / m 2 and calendering to obtain an aramid nonwoven fabric (aramid paper) having a thickness of 30 μm.
Polymetaphenylene isophthalamide (manufactured by Teijin Techno Products Co., Ltd .; trade name “Conex”) was dissolved in a mixed solvent of dimethylacetamide: tripropylene glycol = 85: 15 (weight ratio) to be 9% by weight, A polymer solution was prepared. This polymer solution was coated on both sides of the aramid nonwoven fabric, and this coated material was immersed in a coagulation bath at 40 ° C. having a composition of dimethylacetamide: water = 60: 40 (weight ratio) for 60 seconds to obtain a coagulated film. It was. This solidified film was washed with water in a 30 ° C. water bath for 10 minutes and then dried to obtain a separator for a lithium ion secondary battery of the present invention. This is the separator 7 of the present invention. The separator 7 of the present invention had a film thickness of 39 μm, a basis weight of 34.5 g / m 2 , and a Gurley value (JIS P8117) of 40 seconds / 100 cc. The weight of the porous layer made of polymetaphenylene isophthalamide was 4.6 g / m 2 .
[Example 9]
PET short fibers having a fineness of 0.11 dtex (average fiber diameter of about 3.2 μm), PET short fibers having a fineness of 0.33 dtex (average fiber diameter of about 5.5 μm), and fineness of 0.22 dtex (average fiber diameter of about 4.5 μm) The PET short fibers for binder were blended at a weight ratio of 3/2/5, formed into a film with a basis weight of 13.0 g / m 2 by a wet papermaking method, and calendered to obtain a PET nonwoven fabric having a film thickness of 18 μm.
A separator for a lithium ion secondary battery of the present invention was obtained in the same manner as in Example 8 using the PET nonwoven fabric. This is the separator 8 of the present invention. The separator 8 of the present invention had a film thickness of 29 μm, a basis weight of 18.3 g / m 2 , and a Gurley value (JIS P8117) of 28 seconds / 100 cc. The weight of the porous layer made of polymetaphenylene isophthalamide was 5.3 g / m 2 .
[Example 10]
In the same manner as in Example 9, the coating clearance was changed to obtain the present separator 9 having a film thickness of 37 μm, a basis weight of 22.1 g / m 2 and a Gurley value (JIS P8117) of 32 seconds / 100 cc. The weight of the porous layer made of metaphenylene isophthalamide was 9.1 g / m 2 .
[Example 11]
In the same manner as in Example 9, the coating clearance was changed to obtain the separator 10 of the present invention having a film thickness of 22 μm, a basis weight of 163 g / m 2 and a Gurley value (JIS P8117) of 32 seconds / 100 cc. The weight of the porous layer made of isophthalamide was 3.3 g / m 2 .
[Example 12]
In the same manner as in Example 9, the coating clearance was changed to obtain the present separator 11 having a film thickness of 24 μm, a basis weight of 173 g / m 2 and a Gurley value (JIS P8117) of 32 seconds / 100 cc. The weight of the porous layer made of isophthalamide was 4.3 g / m 2 .
[Puncture strength measurement]
The puncture strength of the present separators 2 and 7 to 11 was measured. The puncture strength was measured by setting a separator on a fixed frame of 11.3 mmφ, pushing a needle with a tip radius of 0.5 mm perpendicularly to the center of the separator, and pushing the needle at a constant speed of 2 mm / second. The maximum load applied to the separator during the movement of the needle by 5 mm was defined as the puncture strength. The results are shown in Table 4. When an aramid material is applied as the material of the nonwoven fabric, the amount of increase in puncture strength before and after coating is large. Nonwoven fabric made of an aramid material has a high affinity with polymetaphenylene isophthalamide forming the porous layer and is effective in terms of obtaining a high-strength separator because a high reinforcing effect is obtained.
[Examples 13 to 16]
Inventive batteries 2 to 5 were obtained in the same manner as in Example 3, using the inventive separators 8 to 11 obtained in Examples 9 to 12.
[Examples 17 to 20]
Similar to Example 3 except that the separators 8 to 11 of the present invention obtained in Examples 9 to 12 were used and 1 M LiPF 6 EC / EMC / VC (29/70/1 weight ratio) was used as the electrolyte. Invention batteries 6 to 9 were obtained.
[Examples 21 to 24]
Similar to Example 3 except that the separators 8 to 11 of the present invention obtained in Examples 9 to 12 were used, and 1 M LiPF 6 EC / EMC / VA (29/70/1 weight ratio) was used as the electrolyte. Invention batteries 10 to 13 were obtained.
[Comparative Example 5]
A battery was produced in the same manner as in Example 3 except that the PET nonwoven fabric produced in Example 9 was used as the separator. This battery is referred to as a comparative battery 3.
[Battery durability evaluation]
Invention batteries 2 to 13 and Comparative battery 3 prepared in Examples 13 to 24 were charged to 4.2 V and stored at 80 ° C. for 4 days. Thereafter, the battery was disassembled and the separator inside was taken out and observed. The results are shown in Table 5. From Table 5, it can be seen that there is an effect of sufficiently suppressing the deterioration of PET when the amount of polymetaphenylene isophthalamide applied is appropriate. It can also be seen that it is effective to add VC or VA to the electrolytic solution in order to prevent this PET deterioration.
[Example 25]
PET short fibers having a fineness of 0.11 dtex (average fiber diameter of about 3.2 μm), PET short fibers having a fineness of 0.33 dtex (average fiber diameter of about 5.5 μm), and fineness of 0.22 dtex (average fiber diameter of about 4.5 μm) The PET short fibers for binder were blended at a weight ratio of 3/2/5, formed into a film with a basis weight of 12.6 g / m 2 by a wet papermaking method, and calendered at 140 ° C. to obtain a PET nonwoven fabric with a film thickness of 18 μm. It was.
Polymetaphenylene isophthalamide (manufactured by Teijin Techno Products Ltd .; trade name “Conex”) was dissolved in a mixed solvent of dimethylacetamide: tripropylene glycol = 60: 30 (weight ratio) so as to be 6% by weight, A polymer solution was prepared. In this polymer solution, α-alumina fine particles (manufactured by Iwatani Chemical Industry Co., Ltd .; SA-1) having an average particle diameter of 0.8 μm were dispersed in the same weight part as polymetaphenylene isophthalamide to prepare a coating slurry. This coating slurry was applied to both sides of the PET nonwoven fabric, and this coated product was immersed in a coagulation bath at 40 ° C. having a composition of dimethylacetamide: water = 50: 50 (weight ratio) for 60 seconds to form a coagulated film. Obtained. This solidified film was washed with water in a 50 ° C. water bath for 10 minutes and then dried to obtain a separator for a lithium ion secondary battery of the present invention. This is the separator 12 of the present invention. The separator 1 of the present invention had a film thickness of 23 μm, a basis weight of 16.6 g / m 2 , and a Gurley value (JIS P8117) of 20 seconds / 100 cc. In addition, as a result of visually observing the separator 12 of the present invention, no pinhole was observed.
[Reference Example 1]
Polymetaphenylene isophthalamide (manufactured by Teijin Techno Products Ltd .; trade name “Conex”) was dissolved in a mixed solvent of dimethylacetamide: tripropylene glycol = 60: 30 (weight ratio) so as to be 6% by weight, A polymer solution was prepared.
This polymer solution was applied to both sides of the PET nonwoven fabric obtained in Example 25, and this coated product was immersed in a coagulation bath at 40 ° C. having a composition of dimethylacetamide: water = 50: 50 (weight ratio) for 60 seconds. A coagulated film was obtained. This solidified film was washed with water in a 50 ° C. water bath for 10 minutes and then dried to produce the separator of the present invention. However, pinholes frequently occurred, and a sufficient product could not be obtained.
[Reference Example 2]
A separator of the present invention was produced in the same manner as in Example 25 except that the amount of α-alumina fine particles was changed to 10% by weight with respect to polymetaphenylene isophthalamide. However, pinholes occurred frequently and a sufficient product could not be obtained. It was.
[Reference Example 3]
A separator of the present invention was prepared in the same manner as in Example 25 except that the amount of α-alumina fine particles was 900% by weight with respect to polymetaphenylene isophthalamide. A sufficient separator could not be obtained.
[Membrane resistance measurement]
A measurement cell was prepared by impregnating an electrolyte with a separator, sandwiching the separator between a pair of electrodes (size: 2 cm × 1.4 cm = 2.8 cm 2 ) made of aluminum foil, and enclosing it in an aluminum laminate film. Here, 1M LiBF 4 PC / EC (1/1 weight ratio) was used as the electrolytic solution. The resistance of one, two, and three separators in the cell was measured by the AC impedance method, and the resistance of one separator was obtained from the slope when this resistance was plotted against the number of separators. It was. The AC impedance measurement was performed by the 4-terminal method, and the amplitude was 10 mV and the frequency was 100 kHz. The measurement temperature was 20 ° C.
Results performed above the membrane resistance measurement to the present invention the separator 1 and 12, respectively, membrane resistance 6.026ohm · cm 2, was 5.211ohm · cm 2.
From the measurement results of the film resistance, it can be seen that it is effective to reduce the polymetaphenylene isophthalamide concentration in the polymer solution. However, when coating is performed using such a polymer solution, as in Reference Examples 1 and 2, the viscosity is too low and pinholes occur frequently, making it difficult to obtain a sufficient product. As described in Example 25, this problem can be solved by appropriately adding ceramic fine particles. However, if the ceramic fine particles are added too much as in Reference Example 3, it is difficult to obtain a sufficient coating film due to the problem of powder falling.
Figure 0004832430
Figure 0004832430
Figure 0004832430
Figure 0004832430
Figure 0004832430

本発明リチウムイオン二次電池用セパレータは耐熱性が高く過充電対策にも有効であるためこれを用いることでリチウムイオン二次電池の安全性を向上させることができる。本発明セパレータを用いた本発明リチウムイオン二次電池は、高温での安全性や性能の確保が要求されるＨＥＶ用途に適する。   Since the separator for a lithium ion secondary battery of the present invention has high heat resistance and is effective for overcharge countermeasures, the use of this separator can improve the safety of the lithium ion secondary battery. The lithium ion secondary battery of the present invention using the separator of the present invention is suitable for HEV applications that require safety and performance at high temperatures.

Claims (17)

不織布の表裏両面に主としてメタ芳香族ポリアミドからなる多孔質層が形成されたリチウムイオン二次電池用セパレータであって、
該セパレータの膜厚が１５〜４０μｍ、ガーレ値（ＪＩＳ Ｐ８１１７）が１０〜５０秒／１００ｃｃであることを特徴とするリチウムイオン二次電池用セパレータ。A separator for a lithium ion secondary battery in which a porous layer mainly composed of a metaaromatic polyamide is formed on both front and back surfaces of a nonwoven fabric ,
A separator for a lithium ion secondary battery, wherein the separator has a thickness of 15 to 40 μm and a Gurley value (JIS P8117) of 10 to 50 seconds / 100 cc . 該多孔質層に陽イオン性界面活性剤、陰イオン性界面活性剤、両性界面活性剤、非イオン性界面活性剤からなる群から選ばれる少なくとも１種を含む界面活性剤が付着されていることを特徴とする請求項１に記載のリチウムイオン二次電池用セパレータ。A surfactant containing at least one selected from the group consisting of a cationic surfactant, an anionic surfactant, an amphoteric surfactant and a nonionic surfactant is attached to the porous layer. The separator for a lithium ion secondary battery according to claim 1. 該界面活性剤の付着量が０．００５〜０．７５０ｇ／ｍ  Adhesion amount of the surfactant is 0.005 to 0.750 g / m ２2 であることを特徴とする請求項２に記載のリチウムイオン二次電池用セパレータ。The separator for a lithium ion secondary battery according to claim 2, wherein: 該不織布が主としてポリエチレンテレフタレートからなる不織布であることを特徴とする請求項１から３のいずれか１項に記載のリチウムイオン二次電池用セパレータ。  The separator for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the nonwoven fabric is a nonwoven fabric mainly composed of polyethylene terephthalate. 該主としてメタ芳香族ポリアミドからなる多孔質層の重量が４〜１０ｇ／ｍ  The weight of the porous layer mainly composed of metaaromatic polyamide is 4 to 10 g / m. ２2 であることを特徴とする請求項４に記載のリチウムイオン二次電池用セパレータ。The lithium ion secondary battery separator according to claim 4, wherein the separator is a lithium ion secondary battery separator. 該不織布が主としてメタ芳香族ポリアミドからなる不織布であることを特徴とする請求項１から３のいずれか１項に記載のリチウムイオン二次電池用セパレータ。  The separator for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the nonwoven fabric is a nonwoven fabric mainly composed of a metaaromatic polyamide. 該不織布がメタ芳香族ポリアミド短繊維とパラ芳香族ポリアミドパルプからなることを特徴とする請求項１から３のいずれか１項に記載のリチウムイオン二次電池用セパレータ。  The separator for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the non-woven fabric is composed of a meta-aromatic polyamide short fiber and a para-aromatic polyamide pulp. 該メタ芳香族ポリアミドがポリメタフェニレンイソフタルアミドであることを特徴とする請求項１から７のいずれか１項に記載のリチウムイオン二次電池用セパレータ。  The separator for a lithium ion secondary battery according to any one of claims 1 to 7, wherein the metaaromatic polyamide is polymetaphenylene isophthalamide. 該多孔質層に平均粒子径０．０５〜２μｍのセラミック微粒子が含まれており、多孔質層の重量に対してセラミック微粒子が３０〜８０重量％となっていることを特徴とする請求項１から８のいずれか１項に記載のリチウムイオン二次電池用セパレータ。  2. The ceramic fine particles having an average particle diameter of 0.05 to 2 [mu] m are contained in the porous layer, and the ceramic fine particles are 30 to 80% by weight based on the weight of the porous layer. The separator for lithium ion secondary batteries of any one of 1-8. メタ芳香族ポリアミドと該メタ芳香族ポリアミドに対し良溶媒である溶媒を主成分とする高分子溶液を不織布の表裏両面に塗工し、次いで塗工された不織布を該メタ芳香族ポリアミドに対し貧溶媒である溶媒と良溶媒である溶媒から主としてなる混合液中で凝固させ、次いで水洗、乾燥することを特徴とする請求項１〜９のいずれか１項に記載のリチウムイオン二次電池用セパレータの製造方法。  A polymer solution mainly composed of a metaaromatic polyamide and a solvent that is a good solvent for the metaaromatic polyamide is applied to both the front and back surfaces of the nonwoven fabric, and then the coated nonwoven fabric is poor against the metaaromatic polyamide. The separator for a lithium ion secondary battery according to any one of claims 1 to 9, wherein the separator is solidified in a mixed liquid mainly composed of a solvent which is a solvent and a solvent which is a good solvent, and then washed with water and dried. Manufacturing method. 正極、負極、非水系電解液、セパレータを具備し、リチウムイオンのドープ・脱ドープにより起電力を得るリチウムイオン二次電池において、  In a lithium ion secondary battery comprising a positive electrode, a negative electrode, a non-aqueous electrolyte, a separator, and obtaining an electromotive force by doping and dedoping of lithium ions,
該セパレータが不織布の表裏両面に主としてメタ芳香族ポリアミドからなる多孔質層が形成されており、  The separator is formed with a porous layer mainly made of a metaaromatic polyamide on both front and back surfaces of the nonwoven fabric,
該セパレータの膜厚が１５〜４０μｍ、ガーレ値（ＪＩＳ Ｐ８１１７）が１０〜５０秒／１００ｃｃであることを特徴とするリチウムイオン二次電池。  A lithium ion secondary battery, wherein the separator has a thickness of 15 to 40 μm and a Gurley value (JIS P8117) of 10 to 50 seconds / 100 cc. 該多孔質層に陽イオン性界面活性剤、陰イオン性界面活性剤、両性界面活性剤、非イオン性界面活性剤からなる群から選ばれる少なくとも１種を含む界面活性剤が付着されていることを特徴とする請求項１１に記載のリチウムイオン二次電池。  A surfactant containing at least one selected from the group consisting of a cationic surfactant, an anionic surfactant, an amphoteric surfactant and a nonionic surfactant is attached to the porous layer. The lithium ion secondary battery according to claim 11. 該界面活性剤の付着量が０．００５〜０．７５０ｇ／ｍ ２ であることを特徴とする請求項１２記載のリチウムイオン二次電池。 12. The lithium ion secondary battery according to the amount deposited of the surfactant is characterized by a 0.005~0.750g / m 2. 該不織布が主としてポリエチレンテレフタレートからなる不織布であることを特徴とする請求項１１から１３のいずれか１項に記載のリチウムイオン二次電池。  The lithium ion secondary battery according to any one of claims 11 to 13, wherein the nonwoven fabric is a nonwoven fabric mainly composed of polyethylene terephthalate. 該主としてメタ芳香族ポリアミドからなる多孔質層の重量が４〜１０ｇ／ｍ  The weight of the porous layer mainly composed of metaaromatic polyamide is 4 to 10 g / m. ２2 であることを特徴とする請求項１４記載のリチウムイオン二次電池。The lithium ion secondary battery according to claim 14, wherein: 該不織布が主としてメタ芳香族ポリアミドからなる不織布であることを特徴とする請求項１１から１３のいずれか１項に記載のリチウムイオン二次電池。  The lithium ion secondary battery according to any one of claims 11 to 13, wherein the nonwoven fabric is a nonwoven fabric mainly composed of a metaaromatic polyamide. 該メタ芳香族ポリアミドがポリメタフェニレンイソフタルアミドであることを特徴とする請求項１１から１６のいずれか１項に記載のリチウムイオン二次電池。  The lithium ion secondary battery according to any one of claims 11 to 16, wherein the metaaromatic polyamide is polymetaphenylene isophthalamide.
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