JP4115006B2 - Lithium ion battery - Google Patents

Description

上記表１より明らかなように、実施例の電池Ａは漏液個数が０個であったのに対して、比較例の電池Ｂは３Ｃ以上の高率充電においては漏液個数が増大したことが分かる。これは、比較例の電池Ｂのように正極活物質が正極芯体に直接接触していると、３Ｃ以上の高率充電になると過電流によるジュール熱で電池温度が急激に上昇して、樹脂製のセパータの溶融点に達して、セパレータの微細孔が溶融物で塞がれてイオンの移動が阻止されるまでは、充電電流が流れ続けて、やがてはガス排出弁６７が作動して、電解液が電池外に漏れ出したためと考えられる。
【００１９】
一方、実施例の電池Ａのように正極１０が導電層１２と活物質層１３との二層構造となっていると、正極活物質が直接正極芯体１１に接触していないため、３Ｃ以上の高率充電となって、過充電により高電位となった場合には、ポリエチレンオキシド（ＰＥＯ）は、過充電状態の高電位（例えば、４．５〜５．５Ｖ）で酸化分解されて、エーテル結合部の主鎖が切断されてガスを発生する。
【００２０】
すると、発生したガスが第１層となる導電層１２を構造破壊するとともに、導電層１２と活物質層１３との界面破壊をするように作用するため、活物質が存在する第２層となる活物質層１３と正極芯体（正極集電体）１１との電気的接触が遮断されるようになる。この結果、電池の内部抵抗が上昇するため、急激な温度上昇を生じることなく、充電電流を遮断することができるようになったと考えられる。
【００２１】
上述したように、本発明においては、第１層となる導電層１２に過充電状態での高電位で分解する物質（エーテル結合部を有する結着剤）を備えているので、過充電により高電位（例えば、４．５〜５．５Ｖ）となった場合には、エーテル結合部の主鎖が切断されるポリエチレンオキシド（ＰＥＯ）あるいはその誘導体が高電位（例えば、４．５〜５．５Ｖ）により分解されてガスを発生して、電池の内部抵抗が上昇して充電電流を遮断することができるようになる。
【００２２】
なお、上述の実施形態においては、過充電状態の高電位（例えば、４．５〜５．５Ｖ）により分解されてガスを発生する高分子結着剤として、ポリエチレンオキシド（ＰＥＯ）のような主鎖にエーテル結合部を有する高分子結着剤を用いる例について説明したが、側鎖にエーテル結合部を有する高分子結着剤を用いても、その側鎖のエーテル結合部が高電位（例えば、４．５〜５．５Ｖ）により分解されてガスを発生するため、同様な効果を得ることができる。
【００２３】
なお、上述の実施形態においては、負極活物質として天然黒鉛（ｄ＝３．３６Å）を用いる例について説明したが、天然黒鉛以外に、リチウムイオンを吸蔵・脱離し得るカーボン系材料、例えば、グラファイト、カーボンブラック、コークス、ガラス状炭素、炭素繊維、またはこれらの焼成体等が好適である。
【００２４】
また、上述の実施形態においては、正極活物質としてＬｉＣｏＯ₂を用いる例について説明したが、ＬｉＣｏＯ₂以外に、リチウムイオンをゲストとして受け入れ得るリチウム含有遷移金属化合物、例えば、ＬｉＮｉＯ₂、ＬｉＣｏ_XＮｉ_(1-X)Ｏ₂、ＬｉＣｒＯ₂、ＬｉＶＯ₂、ＬｉＭｎＯ₂、αＬｉＦｅＯ₂、ＬｉＴｉＯ₂、ＬｉＳｃＯ₂、ＬｉＹＯ₂、ＬｉＭｎ₂Ｏ₄等が好ましいが、特に、ＬｉＮｉＯ₂、ＬｉＣｏ_XＮｉ_(1-X)Ｏ₂を単独で用いるかあるいはこれらの二種以上を混合して用いるのが好適である。
【００２５】
さらに、電解液としては、有機溶媒に溶質としてリチウム塩を溶解したイオン伝導体であって、イオン伝導率が高く、正・負の各電極に対して化学的、電気化学的に安定で、使用可能温度範囲が広くかつ安全性が高く、安価なものであれば使用することができる。例えば、上記した有機溶媒以外に、プロピレンカーボネート（ＰＣ）、スルフォラン（ＳＬ）、テトラハイドロフラン（ＴＨＦ）、γブチロラクトン（ＧＢＬ）等あるいはこれらの混合溶媒が好適である。また、溶質としては電子吸引性の強いリチウム塩を使用し、上記したＬｉＰＦ₆以外に、例えば、ＬｉＢＦ₄、ＬｉＣｌＯ₄、ＬｉＡｓＦ₆、ＬｉＣＦ₃ＳＯ₃、Ｌｉ（ＣＦ₃ＳＯ₂）₂Ｎ、Ｌｉ（Ｃ₂Ｆ₅ＳＯ₂）₂Ｎ、ＬｉＣ₄Ｆ₉ＳＯ₃等が好適である。
【図面の簡単な説明】
は図１の正極板を用いてセパレータを介して負極板を重ね合わせて卷回した渦巻状電極体を外装缶内に収納した状態を示すリチウムイオン電池の断面を示す図である。
【図１】 正極板を示す斜視図であり、図１（ａ）は本発明の正極板を示し、図１（ｂ）は従来例の正極板を示す。
【図２】 図１の正極板を用いてセパレータを介して負極板を重ね合わせて卷回した渦巻状電極体を外装缶内に収納した状態を示すリチウムイオン電池の断面を示す図である。
【符号の説明】
１０…正極板、１１…正極芯体、１２…導電層（第１層）、１３…活物質層（第２層）、１０ａ…正極集電タブ、２０…正極板、２１…正極芯体、２２…活物質層、２０ａ…正極集電タブ、３０…負極板、３０ａ…負極集電タブ、４０…セパレータ、５０…外装缶、５１…スペーサ、５２…絶縁ガスケット、６０…封口体、６７…ガス排出弁[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a lithium ion battery including a positive electrode, a negative electrode capable of occluding and desorbing lithium ions, and a non-aqueous electrolyte, and in particular, it does not leak even in an overcharged state and has safety. The present invention relates to an improvement in positive electrode structure that can be secured.
[0002]
[Prior art]
2. Description of the Related Art In recent years, electronic devices have been remarkably reduced in size and weight, and accordingly, there is a great demand for reduction in size and weight of a battery serving as a power source. In the field of primary batteries, small and lightweight batteries such as lithium batteries have already been put into practical use. However, since these are primary batteries, they cannot be used repeatedly, and their applications are limited. On the other hand, in the field of secondary batteries, lead storage batteries, nickel-cadmium storage batteries, nickel-hydrogen storage batteries, and the like have been conventionally used, but these have great problems in terms of reduction in size and weight.
[0003]
Therefore, non-aqueous electrolyte secondary batteries such as lithium ion batteries have come into practical use as small, lightweight, high-capacity chargeable / dischargeable batteries, and portable electronic devices such as small video cameras, mobile phones, and notebook computers.・ It has been used for communication equipment.
[0004]
A lithium ion battery uses a carbon-based material capable of inserting and extracting lithium ions as a negative electrode active material, and a lithium-containing transition metal oxide such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiFeO ₂ as a positive electrode active material. A battery that uses a non-aqueous electrolyte solution in which a lithium salt is dissolved as a solute in an organic solvent and is assembled as a battery, and then lithium ions emitted from the positive electrode active material by the first charge enter the carbon particles and can be charged and discharged. It is.
[0005]
In such a lithium ion battery, when overcharging is performed, excessive lithium is extracted from the positive electrode and excessive insertion of lithium occurs in the negative electrode as the overcharged state occurs. Both poles become thermally unstable. When both the positive and negative electrodes become thermally unstable, it eventually acts to decompose the organic solvent in the electrolyte, causing a sudden exothermic reaction and causing the battery to generate abnormally heat. The problem was that safety was compromised. Such a situation becomes more important as the energy density of the lithium ion battery increases.
[0006]
In order to solve such a problem, when the battery temperature rises, the separator melts and the fine pores of the separator are closed with the melt, thereby eliminating the overcurrent state. It was proposed in the publication No. 23594. In the one proposed in Japanese Patent Application Laid-Open No. 60-23594, when a synthetic resin film having fine holes is used as a separator member and the melting point of the film material is reached by Joule heat due to overcurrent, the synthetic resin film The micropores of the film are closed with a melt to prevent the movement of ions, and the synthetic resin film functions as an insulator so as to block the current. This prevents the battery temperature from rising further, prevents abnormal battery heat generation, and improves battery safety.
[0007]
[Problems to be solved by the invention]
However, in the one proposed in Japanese Patent Application Laid-Open No. 60-23954, the positive electrode core (positive electrode current collector) is directly filled with the positive electrode active material, so that the overcharged state is caused by the overcurrent. The battery temperature rises due to Joule heat, and the charging current continues to flow until the melting point of the film material is reached and the pores of the synthetic resin film are blocked by the melt and the movement of ions is prevented. . For this reason, when the charging current is too large, the battery temperature rapidly increases, the organic solvent of the electrolytic solution is decomposed, and the battery internal pressure increases. As a result, there was a problem that the gas discharge valve was activated and the electrolyte leaked out of the battery.
Therefore, the present invention has been made in view of the above problems, and even when the battery temperature is suddenly increased due to an overcharged state, the charging current can be reliably cut off to ensure the safety of the battery. It was made for the purpose of doing.
[0008]
[Means for solving the problems and their functions and effects]
For this reason, in the lithium ion battery of the present invention, as the positive electrode ,
A first layer formed as a conductive layer comprising at least a conductive filler, polyethylene oxide (PEO) decomposed at a potential of 4.5 to 5.5 V and a derivative thereof on a positive electrode current collector , and a positive electrode on the first layer A two-layered positive electrode having a second layer formed as an active material layer composed of an active material, a conductive agent, and a binder was employed.
[0010]
Thus, when polyethylene oxide (PEO) and its derivatives that decompose at a voltage of 4.5 to 5.5 V are used in the first layer , polyethylene oxide (PEO) and its derivatives are generated by overcharge. Since it is a substance that is easily decomposed at a high potential of 5.5 V , it is easily decomposed to generate gas when it is overcharged. Then, the generated gas structurally destroys the first layer and acts to break the interface between the first layer and the second layer, so that the electric current between the second layer in which the active material exists and the positive electrode current collector is present. Contact is blocked. For this reason, when polyethylene oxide (PEO) and its derivatives are used, the internal resistance of the battery is reliably increased, and the charging current can be cut off without causing a rapid temperature increase.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a preferred embodiment of the lithium ion battery of the present invention will be described with reference to FIGS. 1 is a perspective view showing a positive electrode plate, FIG. 1 (a) shows a positive electrode plate of the present invention, and FIG. 1 (b) shows a conventional positive electrode plate. FIG. 2 is a cross-sectional view of a lithium ion battery showing a state in which a spiral electrode body obtained by stacking and winding a negative electrode plate through a separator using the positive electrode plate of FIG. 1 is housed in an outer can.
1. Preparation of positive electrode plate (1) Examples Conductive fillers such as graphite and aluminum (for example, 60% by weight), polyethylene oxide (PEO) (for example, 40% by weight), and the like in an organic solvent made of N-methylpyrrolidone, etc. The dissolved material is mixed to obtain a slurry or paste. The slurry or paste is applied to both surfaces of the positive electrode core (for example, an aluminum foil or an aluminum mesh having a thickness of 20 μm) 11 by using a die coater, a doctor blade or the like in the case of a slurry, or by a roller coating method in the case of a paste The conductive layer 12 which becomes the first layer of the positive electrode is formed by uniformly coating. Here, polyethylene oxide (PEO) is a substance that generates gas by being oxidatively decomposed at a high potential in an overcharged state (for example, 4.5 to 5.5 V), and the main chain of the ether bond is cleaved. .
[0012]
On the other hand, a positive electrode active material (for example, 90% by weight) made of LiCoO ₂ , a carbon-based conductive agent (for example, 5% by weight) such as acetylene black or graphite, and a binder (for example, 5%) made of polyvinylidene fluoride (PVDF). (% By weight) and the like dissolved in an organic solvent made of N-methylpyrrolidone or the like is mixed to obtain an active material slurry or an active material paste. These active material slurries or active material pastes are applied uniformly on both surfaces of the conductive layer 12 as the first layer by using a die coater, a doctor blade or the like in the case of a slurry, or by a roller coating method in the case of a paste. Then, the positive electrode plate coated with the active material layer 13 to be the second layer is formed.
[0013]
Thereafter, the positive electrode plate coated with the conductive layer 12 as the first layer and the active material layer 13 as the second layer is passed through the dryer to remove the organic solvent necessary for slurry or paste preparation. And dry. After drying, this dry positive electrode plate is rolled by a roll press to obtain a positive electrode plate 10 having a thickness of 0.17 mm and having a two-layer structure.
(2) Comparative Example A positive electrode active material (for example, 90% by weight) made of LiCoO ₂ , a carbon-based conductive agent (for example, 5% by weight) such as acetylene black and graphite, and a binder made of polyvinylidene fluoride (PVDF). (For example, 5% by weight) or the like dissolved in an organic solvent made of N-methylpyrrolidone is mixed to obtain an active material slurry or an active material paste.
[0014]
These active material slurries or active material pastes, in the case of slurry using a die coater, a doctor blade, etc., in the case of paste, a positive electrode core (for example, an aluminum foil or aluminum mesh having a thickness of 20 μm) by a roller coating method or the like The positive electrode plate with the active material layer 22 applied thereon is formed by uniformly applying to both surfaces of 21. Thereafter, the positive electrode plate coated with the active material layer 22 is passed through a dryer to remove the organic solvent necessary for slurry or paste preparation and dry. After drying, this dry positive electrode plate is rolled by a roll press to obtain a positive electrode plate 20 having a thickness of 0.17 mm and having a single layer structure.
2. Production of negative electrode plate On the other hand, a negative electrode active material made of natural graphite (d = 3.36 mm) and a binder made of polyvinylidene fluoride (PVDF) dissolved in an organic solvent made of N-methylpyrrolidone, etc. To make a slurry or paste. These slurries or pastes are uniformly applied over the entire surface of the negative electrode core (for example, a copper foil having a thickness of 20 μm) by using a die coater, a doctor blade or the like in the case of slurry, or by a roller coating method in the case of paste. The negative electrode plate which apply | coated and applied the active material layer is formed.
[0015]
Thereafter, the negative electrode plate coated with the active material layer is passed through a dryer to remove the organic solvent necessary for slurry or paste preparation and dry. Then, this dry negative electrode plate is rolled by a roll press to obtain a negative electrode plate 30 having a thickness of 0.14 mm.
3. Production of Electrode Body (1) Example A microporous membrane made of a polyolefin-based resin having a low reactivity with an organic solvent and an inexpensive positive electrode plate 10 and negative electrode plate 30 of an example produced as described above, Preferably, the microporous membranes made of polyethylene (for example, the thickness is 0.025 mm) 40 are sandwiched therebetween, and the electrode plates 10 and 30 are overlapped with the center lines in the width direction being matched. Then, it winds with the winder which is not illustrated. Thereafter, the outermost periphery is taped to obtain the spiral electrode body a of the example. In the case of a prismatic battery, the prismatic battery is molded into a shape that can be inserted into a prismatic outer can with a press machine.
(2) Comparative Example The positive electrode plate 20 and the negative electrode plate 30 of the comparative example prepared as described above are microporous films made of a polyolefin-based resin that is low in reactivity with an organic solvent and inexpensive, preferably polyethylene. A microporous membrane (for example, thickness of 0.025 mm) 40 is interposed therebetween, and the electrode plates 20 and 30 are overlapped with the center line in the width direction being coincident. Then, it winds with the winder which is not illustrated. Thereafter, the outermost periphery is taped to obtain a spiral electrode body b of a comparative example. In the case of a prismatic battery, the prismatic battery is molded into a shape that can be inserted into a prismatic outer can with a press machine.
4). Next, as shown in FIG. 2, the insulating plates 51 are respectively disposed above and below the electrode bodies a and b produced as described above, and then formed into a cylindrical shape by pressing from a single plate. These electrode bodies a and b are inserted from the opening of the steel outer can 50 that also serves as the negative electrode terminal. Next, the negative electrode current collecting tab 30a extending from the negative electrode plate 30 of the electrode bodies a and b is welded to the inner bottom portion of the outer can 50, and the positive electrode current collecting tab 10a extending from the positive electrode plate 10 (20) of the electrode body. (20a) is welded to the bottom of the bottom plate 64 of the sealing body 60.
[0016]
In addition, the sealing body 60 is comprised from the stainless steel positive electrode cap 61 formed in the reverse dish shape (cap shape), and the stainless steel baseplate 64 formed in the dish shape. The positive electrode cap 61 includes a convex portion 62 that bulges toward the outside of the battery and a flat flange portion 63 that forms the bottom side of the convex portion 62, and a gas vent (not shown) is provided at a corner of the convex portion 62. A hole is provided. On the other hand, the bottom plate 64 includes a concave portion 65 that bulges toward the inside of the battery, and a flat-plate-like flange portion 66 that constitutes the bottom side portion of the concave portion 65. A gas vent hole (not shown) is provided at the center of the recess 65. A gas discharge valve 67 is housed between the convex portion 62 of the positive electrode cap 61 and the concave portion 65 of the bottom plate 64. The gas discharge valve 67 is deformed when the gas pressure inside the battery rises and exceeds a predetermined pressure.
[0017]
Next, an electrolyte solution in which 1 M LiPF ₆ was added and mixed as an electrolyte salt into a mixed solvent composed of 40 parts by weight of ethylene carbonate (EC) and 60 parts by weight of diethyl carbonate (DEC) was poured into the opening of the outer can 50, The sealing body 60 is placed in the opening portion 50 through the insulating gasket 52 for exterior can made of polypropylene (PP), and the upper end portion of the opening portion of the exterior can 50 is caulked to the sealing body 60 side to seal it in a liquid-tight manner. 100 cylindrical lithium ion batteries A of the example having a nominal capacity of 1350 mAh and 100 cylindrical lithium ion batteries B of the comparative example are produced.
5. Test a. Overcharge test Each of the 100 lithium ion batteries A and B produced as described above was charged with a 1350 mA (1 C) charging current until the battery voltage reached 4.1 V, and then a constant voltage of 4.1 V. Charge for 3 hours until fully charged. Overcharging is performed by flowing a charging current of 1350 mA (1 C), 2700 mA (2 C), 4050 mA (3 C), and 5400 mA (4 C) between the positive and negative terminals of the lithium ion batteries A and B fully charged in this way. When the number of leaks after a predetermined time (for example, 3 hours) from the start of overcharge was measured, the results shown in Table 1 below were obtained.
[0018]
[Table 1]

As is clear from Table 1 above, the number of leaks was 0 in the battery A of the example, whereas the number of leaks increased in the battery B of the comparative example at a high rate charge of 3C or higher. I understand. This is because, when the positive electrode active material is in direct contact with the positive electrode core body as in the battery B of the comparative example, the battery temperature rapidly rises due to Joule heat due to overcurrent when charging at a high rate of 3C or more, and the resin The charging current continues to flow until the melting point of the manufactured separator is reached and the fine pores of the separator are blocked by the melt and the movement of ions is stopped. This is probably because the electrolyte leaked out of the battery.
[0019]
On the other hand, when the positive electrode 10 has a two-layer structure of the conductive layer 12 and the active material layer 13 as in the battery A of the example, the positive electrode active material is not in direct contact with the positive electrode core body 11, so In the case of a high rate of charging, and when it becomes a high potential due to overcharging, polyethylene oxide (PEO) is oxidatively decomposed at a high potential in an overcharged state (eg, 4.5 to 5.5 V) The main chain of the ether bond is broken to generate gas.
[0020]
Then, the generated gas structurally destroys the conductive layer 12 that becomes the first layer and acts to cause interface destruction between the conductive layer 12 and the active material layer 13, so that it becomes the second layer in which the active material exists. Electrical contact between the active material layer 13 and the positive electrode core (positive electrode current collector) 11 is interrupted. As a result, since the internal resistance of the battery is increased, it is considered that the charging current can be interrupted without causing a rapid temperature increase.
[0021]
As described above, in the present invention, the conductive layer 12 serving as the first layer is provided with a substance that decomposes at a high potential in an overcharged state (a binder having an ether bond). When the potential (for example, 4.5 to 5.5 V) is reached, polyethylene oxide (PEO) or a derivative thereof whose main chain at the ether bond is cleaved has a high potential (for example, 4.5 to 5.5 V). ) To generate gas, and the internal resistance of the battery increases to cut off the charging current.
[0022]
In the above-described embodiment, a main binder such as polyethylene oxide (PEO) is used as a polymer binder that is decomposed by a high potential in an overcharged state (for example, 4.5 to 5.5 V) to generate gas. Although an example using a polymer binder having an ether bond portion in the chain has been described, even if a polymer binder having an ether bond portion in the side chain is used, the ether bond portion of the side chain has a high potential (for example, , 4.5 to 5.5 V) to generate gas, and the same effect can be obtained.
[0023]
In the above-described embodiment, an example in which natural graphite (d = 3.36 mm) is used as the negative electrode active material has been described. However, in addition to natural graphite, a carbon-based material capable of inserting and extracting lithium ions, for example, graphite Carbon black, coke, glassy carbon, carbon fiber, or a fired body thereof is preferable.
[0024]
In the above-described embodiment, an example in which LiCoO ₂ is used as the positive electrode active material has been described. However, in addition to LiCoO ₂ , a lithium-containing transition metal compound that can accept lithium ions as a guest, for example, LiNiO ₂ , LiCo _X Ni _{( 1-X)} O ₂ , LiCrO ₂ , LiVO ₂ , LiMnO ₂ , αLiFeO ₂ , LiTiO ₂ , LiScO ₂ , LiYO ₂ , LiMn ₂ O _4, etc. are preferred, and in particular, LiNiO ₂ , LiCo _X Ni _(1-X) It is preferable to use O ₂ alone or a mixture of two or more of these.
[0025]
Furthermore, the electrolyte is an ionic conductor in which a lithium salt is dissolved as a solute in an organic solvent, has high ionic conductivity, is chemically and electrochemically stable for both positive and negative electrodes, and is used. If the possible temperature range is wide, the safety is high, and the cost is low, it can be used. For example, in addition to the organic solvent described above, propylene carbonate (PC), sulfolane (SL), tetrahydrofuran (THF), γ-butyrolactone (GBL), or a mixed solvent thereof is suitable. Further, a lithium salt having a strong electron-withdrawing property is used as the solute. In addition to LiPF ₆ described above, for example, LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, Li (C ₂ F ₅ SO ₂ ) ₂ N, LiC ₄ F ₉ SO _{3 and the} like are suitable.
[Brief description of the drawings]
FIG. 2 is a view showing a cross section of a lithium ion battery showing a state in which a spiral electrode body obtained by stacking and winding a negative electrode plate through a separator using the positive electrode plate of FIG. 1 is housed in an outer can.
FIG. 1 is a perspective view showing a positive electrode plate, FIG. 1 (a) shows a positive electrode plate of the present invention, and FIG. 1 (b) shows a positive electrode plate of a conventional example.
2 is a cross-sectional view of a lithium ion battery showing a state in which a spiral electrode body obtained by stacking and winding a negative electrode plate through a separator using the positive electrode plate of FIG. 1 is housed in an outer can.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 ... Positive electrode plate, 11 ... Positive electrode core, 12 ... Conductive layer (1st layer), 13 ... Active material layer (2nd layer), 10a ... Positive electrode current collection tab, 20 ... Positive electrode plate, 21 ... Positive electrode core, 22 ... Active material layer, 20a ... Positive electrode current collecting tab, 30 ... Negative electrode plate, 30a ... Negative electrode current collecting tab, 40 ... Separator, 50 ... Exterior can, 51 ... Spacer, 52 ... Insulating gasket, 60 ... Sealing body, 67 ... Gas discharge valve

Claims (1)

A lithium ion battery comprising a positive electrode, a negative electrode capable of inserting and extracting lithium ions, and a non-aqueous electrolyte,
As the positive electrode , a first layer formed on a positive electrode current collector as a conductive layer composed of at least a conductive filler, polyethylene oxide (PEO) that decomposes at a potential of 4.5 to 5.5 V, and a derivative thereof; 2. A lithium ion battery comprising a two-layered positive electrode having a second layer formed as an active material layer comprising a positive electrode active material, a conductive agent and a binder on the layer .

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