Positive electrode plate and non-aqueous electrolyte secondary battery having the same

Abstract

To provide a positive electrode plate that has excellent fusing properties and can impart excellent cycle characteristics to a non-aqueous electrolyte secondary battery.SOLUTION: A positive electrode plate has an active material layer on a current collector. The active material layer has a first layer and a second layer in this order from the current collector side. The first layer is mainly composed of a first active material represented by Formula (I) and contains carbon nanotubes. The second layer is mainly composed of a second active material represented by Formula (II). Formulas (I) and (II) are as defined in the claims. The content of carbon nanotubes in the first layer is 0.15 to 0.65 mass% with respect to the total amount of the first layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a positive electrode plate and a non-aqueous electrolyte secondary battery having the same. [Background technology]

[0002] It is known that secondary batteries use a positive electrode plate containing a lithium transition metal composite oxide as a positive electrode active material on a current collector. Patent Document 1 discloses a positive electrode for secondary batteries that includes two layers of positive electrode active material particles with different aggregation forms. Patent Document 2 discloses a positive electrode for secondary batteries that includes two layers of lithium transition metal composite oxide with different molar ratios of lithium to transition metal. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2022-112207 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-115244 Summary of the Invention [Problem to be solved by the invention]

[0004] To increase the capacity of the positive electrode plate, it is considered to use a positive electrode active material with a high lithium to transition metal molar ratio in combination with a positive electrode active material with a high Ni content. However, when such a positive electrode active material is used, the fusing property of the positive electrode plate is reduced, and the cycle characteristics of the secondary battery are sometimes deteriorated.

[0005] An object of the present disclosure is to provide a positive electrode plate that has excellent fusing properties and can impart excellent cycle characteristics to a nonaqueous electrolyte secondary battery, and a nonaqueous electrolyte secondary battery having the same. [Means for solving the problem]

[0006] [1] A positive electrode plate having an active material layer on a current collector, the active material layer has a first layer and a second layer in this order from the current collector side, The first layer contains a first active material represented by the following formula (I) as a main component and contains carbon nanotubes: The second layer contains a second active material represented by the following formula (II) as a main component, The positive electrode plate, wherein the content of the carbon nanotubes in the first layer is 0.15 to 0.65 mass % with respect to the total amount of the first layer. Li(Li a1 Ni x1 Mn y1 M1 z1 )O2(I) Li(Li a2 Ni x2 Mn y2 M2 z2 )O2(II) [In formula (I) and formula (II), M1 and M2 are each independently one or more metal elements selected from the group consisting of Co, Al, Mg, Ca, Ti, V, Cr, Fe, Cu, Zn, Zr, Nb, Mo, Ta, and W; 0.1≦a1≦0.33, 0≦x1≦0.5, 0.5≦y1≦0.7, 0≦z1≦0.2, and a1+x1+y1+z1=1 is satisfied; -0.1≦a2≦0.1, 0.6≦x2≦1, 0≦y2≦0.3, 0≦z2≦0.3, and a2+x2+y2+z2=1. [2] The positive electrode plate according to [1], wherein the thickness ratio of the first layer to the second layer is first layer:second layer=0.8:9.2 to 4.5:5.5. [3] The positive electrode plate according to [1] or [2], wherein the thickness ratio of the first layer to the second layer is first layer:second layer=1:9 to 3:7. [4] The positive electrode plate according to any one of [1] to [3], wherein the active material layer has a thickness of 10 to 200 μm. [5] The positive electrode plate according to any one of [1] to [4], wherein the carbon nanotubes are multi-walled carbon nanotubes. [6] The positive electrode plate according to any one of [1] to [5], wherein the first layer is in contact with the current collector. [7] The positive electrode plate according to any one of [1] to [6], wherein the second layer forms a surface of the active material layer on the side opposite to the current collector side. [8] A non-aqueous electrolyte secondary battery having the positive electrode plate according to any one of [1] to [7]. [Effects of the Invention]

[0007] The positive electrode plate of the present disclosure can have excellent fusing properties, and can provide a nonaqueous electrolyte secondary battery with excellent cycle characteristics. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 2 is a cross-sectional view schematically illustrating an example of a positive electrode plate according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] In this specification, unless otherwise specified, a numerical range such as "x to y" includes both the upper and lower limits. That is, "x to y" represents a numerical range of "greater than or equal to x and less than or equal to y." A numerical value arbitrarily selected from within the numerical range may be set as a new upper or lower limit. For example, a new numerical range may be set by arbitrarily combining a numerical value within the numerical range with a numerical value described elsewhere in this specification, in a table, or in a figure.

[0010] (positive electrode plate) 1 is a cross-sectional view schematically illustrating an example of a positive electrode plate according to an embodiment. The positive electrode plate according to this embodiment is used in a non-aqueous electrolyte secondary battery (hereinafter also referred to as a "secondary battery") such as a lithium ion battery.

[0011] The positive electrode plate 1 has an active material layer 10 on a current collector 15. The active material layer 10 has a first layer 11 and a second layer 12 in this order from the current collector 15 side. The first layer is mainly composed of a first active material represented by the following formula (I) and contains carbon nanotubes (hereinafter also referred to as "CNTs"). The second layer 12 is mainly composed of a second active material represented by the following formula (II). The content of CNTs in the first layer 11 is 0.15 to 0.65 mass % with respect to the total amount of the first layer 11. Li(Li a1 Ni x1 Mn y1 M1 z1 )O2(I) Li(Li a2 Ni x2 Mn y2 M2 z2 )O2(II) [In formula (I) and formula (II), M1 and M2 are each independently one or more metal elements selected from the group consisting of Co, Al, Mg, Ca, Ti, V, Cr, Fe, Cu, Zn, Zr, Nb, Mo, Ta, and W; 0.1≦a1≦0.33, 0≦x1≦0.5, 0.5≦y1≦0.7, 0≦z1≦0.2, and a1+x1+y1+z1=1 is satisfied; -0.1≦a2≦0.1, 0.6≦x2≦1, 0≦y2≦0.3, 0≦z2≦0.3, and a2+x2+y2+z2=1.

[0012] The first active material and the second active material are both lithium transition metal composite oxides. The first active material is a lithium-excess positive electrode active material with a high Li content. The first active material utilizes an oxygen redox reaction, which makes it easy to achieve high capacity. The first active material has high resistance and excellent thermal stability, which can improve the fusing property of the positive electrode plate, but it has poor cycle characteristics. The second active material has a lower Li content than the first active material, but a higher Ni content, which makes it easy to achieve high capacity. The second active material has a lower resistance than the first active material, which makes it easy to reduce the fusing property of the positive electrode plate 1.

[0013] In the positive electrode plate 1, a first layer 11 containing a first active material as a main component and CNTs is disposed on the current collector 15 side of the active material layer 10. A second layer 12 containing a second active material as a main component is disposed on the surface side of the active material layer 10 (hereinafter also referred to as the "anti-current collector side of the active material layer 10"), which is farther from the current collector 15 side than the first layer 11. As described above, the first layer 11 disposed on the current collector 15 side contains the first active material, which has a higher resistance and superior thermal stability than the second active material. The first layer 11 can protect the vicinity of the current collector 15, thereby suppressing the flow of a large current during a nail penetration test (described below) for evaluating fusing property. This suppresses heat generation in the positive electrode plate 1 and improves fusing property.

[0014] In the positive electrode plate 1, the second layer 12 is arranged on the anti-collector side of the active material layer 10, which is more likely to come into contact with the electrolyte, and the first layer 11 is arranged on the current collector 15 side. This prevents the reaction from concentrating on the anti-collector side of the active material layer 10, thereby improving the cycle characteristics of the secondary battery.

[0015] The first layer 11 contains CNT. CNT is a fibrous conductive additive, and therefore can form a mesh-like network of conductive paths between the first active materials. The first active material may generate gas during charging and discharging of the secondary battery, but the mesh-like network of CNT in the first layer 11 can prevent the conductive paths from being broken, thereby improving the cycle characteristics of the secondary battery. When carbon black is used as the conductive additive, it is more difficult to prevent the conductive paths from being broken during charging and discharging of the secondary battery than when CNT is used, and therefore it is more difficult to improve the cycle characteristics.

[0016] The current collector 15 is, for example, a metal foil made of an aluminum material such as aluminum or an aluminum alloy.

[0017] The active material layer 10 is formed on the current collector 15 and contains positive electrode active materials such as a first active material and a second active material. The active material layer 10 may be formed on only one surface of the current collector 15, or on both surfaces thereof.

[0018] The porosity of the active material layer 10 may be, for example, 10 to 45%, 15 to 40%, 18 to 30%, or 18 to 25%. The porosity of the first layer 11 and the second layer 12 can also be within the above range. The porosity of the first layer 11 and the porosity of the second layer 12 may be the same as each other or different from each other. The porosity of the first layer 11 may be greater than or smaller than the porosity of the second layer 12. By having the porosity within the above-described range, it is easy to achieve a high capacity of the secondary battery. The porosity of the active material layer 10, the first layer 11, and the second layer 12 can be determined by image analysis of a cross-sectional image of the positive electrode plate 1 obtained using a scanning electron microscope (SEM).

[0019] The first active material may be a compound represented by the above formula (I). In formula (I), M1 may contain one or more selected from the group consisting of Co, Al, Mg, Ti, Nb, and Mo, and preferably contains Co. In formula (I), a1 may be 0.11 ≦ a1 ≦ 0.3, 0.11 ≦ a1 ≦ 0.25, or 0.12 ≦ a1 ≦ 0.20. In formula (I), x1 may be 0 < x1 ≦ 0.4, 0.1 ≦ x1 ≦ 0.4, or 0.2 ≦ x1 ≦ 0.3. In formula (I), y1 may be 0.50 ≦ y1 ≦ 0.65, 0.52 ≦ y1 ≦ 0.60, or 0.55 ≦ y1 ≦ 0.60. In formula (I), z1 may be 0 < z1 ≦ 0.2, 0.05 ≦ z1 ≦ 0.18, or 0.08 ≦ z1 ≦ 0.15. The composition of the first active material can be determined, for example, by ICP (inductively coupled plasma) emission spectroscopy.

[0020] The first active material may contain two or more lithium transition metal composite oxides having different compositions from each other within the range of the composition represented by formula (I). The first active material may contain two or more lithium transition metal composite oxides having at least one of different particle shapes and particle diameters from each other.

[0021] The second active material may be a compound represented by the above formula (II). In formula (II), M2 may contain one or more selected from the group consisting of Co, Al, Mg, Ti, Nb, and Mo, and preferably contains Co. In formula (II), a2 may be -0.05 ≤ a2 ≤ 0.05, may be -0.03 ≤ a2 ≤ 0.03, or may be -0.02 ≤ a2 ≤ 0.02. In formula (II), x2 may be 0.65 ≤ x2 ≤ 1.0, may be 0.70 ≤ x2 ≤ 0.90, or may be 0.75 ≤ x2 ≤ 0.85. In formula (II), y2 may be 0 < y2 ≤ 0.3, may be 0.01 ≤ y2 ≤ 0.2, or may be 0.05 ≤ y2 ≤ 0.15. In formula (II), z2 may be 0 < z2 ≤ 0.3, may be 0.01 ≤ z2 ≤ 0.2, or may be 0.05 ≤ z2 ≤ 0.15. The composition of the second active material can be determined, for example, by ICP (Inductively Coupled Plasma) emission spectroscopy.

[0022] The second active material may contain two or more lithium transition metal composite oxides having different compositions within the range of the composition represented by formula (II). The second active material may contain two or more lithium transition metal composite oxides in which at least one of the particle shape and particle diameter is different from each other.

[0023] The CNT may be a single-walled carbon nanotube (SWCNT) or a multi-walled carbon nanotube such as a double-walled carbon nanotube (DWCNT) (hereinafter also referred to as "multi-layer CNT"). The number of layers of the multi-layer CNT is, for example, 2 to 25 layers, may be 3 to 20 layers, may be 5 to 15 layers, or may be 6 to 10 layers. The CNT contained in the first layer 11 is preferably a multi-layer CNT.

[0024] First layer 11 may contain a first active material as a main component and may further contain CNTs. "Containing the first active material as a main component" means that first layer 11 contains 50% by weight or more of the first active material relative to the total weight of first layer 11. The content of the first active material in first layer 11 may be 60% by weight or more, 70% by weight or more, 80% by weight or more, 50 to 99% by weight, 60 to 98% by weight, 70 to 95% by weight, or 80 to 92% by weight.

[0025] The CNT content in the first layer 11 may be 0.15 to 0.65 mass %, 0.2 to 0.6 mass %, or 0.3 to 0.55 mass % relative to the total amount of the first layer 11. By ensuring that the CNT content is within the above range, a positive electrode plate 1 with excellent fusing properties can be obtained, and a secondary battery with excellent cycle characteristics can be obtained. If the CNT content is low, the conductive path is more likely to be cut during charging and discharging of the secondary battery, and the cycle characteristics are more likely to deteriorate. If the CNT content is high, a large current flows through the first layer 11 in a nail penetration test described below, which makes the positive electrode plate 1 more likely to heat up, and the fusing properties of the positive electrode plate 1 are more likely to deteriorate.

[0026] The first layer 11 may contain a positive electrode active material other than the first active material, for example, the second active material, or a positive electrode active material other than the first active material and the second active material. The first layer 11 may contain a binder.

[0027] The second layer 12 may contain the second active material as a main component. "Containing the second active material as a main component" means that the second active material accounts for 50% by weight or more of the total weight of the second layer 12. The content of the second active material in the second layer 12 may be 60% by weight or more, 70% by weight or more, 80% by weight or more, 50 to 99% by weight, 60 to 98% by weight, 70 to 95% by weight, or 80 to 92% by weight.

[0028] The second layer 12 may contain a positive electrode active material other than the second active material, for example, the first active material, or a positive electrode active material other than the second active material and the first active material. The second layer 12 may further contain at least one of a conductive additive and a binder. The second layer 12 may contain graphite as a conductive additive.

[0029] The thickness of the first layer 11 is smaller than the thickness of the second layer 12. The thickness ratio of the first layer 11 to the second layer (first layer:second layer) is preferably 0.8:9.2 to 4.5:5.5, or may be 1:9 to 4:6, or more preferably 1:9 to 3:7, or may be 1.5:8.5 to 3.5:6.5, or 1.5:8.5 to 2.5 to 7.5, or may be 2:8 to 3:7. When the thickness ratio is within the above range, a positive electrode plate 1 having excellent fusing properties can be obtained, and a secondary battery having excellent cycle characteristics can be obtained.

[0030] The thickness of the active material layer 10 is, for example, 10 to 200 μm, preferably 30 to 200 μm, and may be 50 to 180 μm or 60 to 150 μm. The thickness of the first layer 11 is, for example, 5 to 100 μm, or may be 8 to 60 μm, 10 to 50 μm, 15 to 40 μm, or 20 to 35 μm. The thickness of the second layer 12 is, for example, 10 to 180 μm, or may be 30 to 160 μm, 40 to 140 μm, 50 to 130 μm, or 60 to 120 μm.

[0031] The porosity of the active material layer 10, the first layer 11, and the second layer 12, the thickness ratio of the first layer 11 and the second layer, and the thicknesses of the active material layer 10, the first layer 11, and the second layer 12 can all be adjusted by the amount of the first active material and the second active material applied when forming the active material layer 10, the application speed, the press pressure and number of presses when rolling the coated layer formed by applying the first active material and the second active material, the particle properties (particularly particle diameter) of the first active material and the second active material, the mixing ratio of particles with different particle diameters and / or particle shapes, or the content of the conductive additive and binder contained in the first layer 11 and the second layer 12, etc.

[0032] Of the first active materials contained in active material layer 10, the first active material contained in first layer 11 is preferably 80 to 100 wt %, more preferably 90 to 100 wt %, or may be 95 to 99 wt %. Of the second active materials contained in active material layer 10, the second active material contained in second layer 12 is preferably 80 to 100 wt %, more preferably 90 to 100 wt %, or may be 95 to 99 wt %. This allows a larger amount of the first active material to be disposed on the current collector side of active material layer 10, and a larger amount of the second active material to be disposed on the side opposite the current collector of active material layer 10. Using positive electrode plate 1 with excellent fusing properties, a secondary battery with excellent cycle characteristics can be obtained.

[0033] The active material layer 10 includes at least a first layer 11 and a second layer 12. In the active material layer 10, the first layer 11 is disposed closer to the current collector than the second layer 12, and the second layer 12 is disposed closer to the opposite current collector side of the active material layer 10 than the first layer 11. The first layer 11 is preferably in contact with the current collector 15. The second layer 12 preferably forms the surface of the active material layer 10 on the surface side opposite to the current collector 15 side (i.e., the surface on the opposite current collector side of the active material layer 10). This makes it possible to obtain a positive electrode plate 1 with excellent fusing properties, and a secondary battery with excellent cycle characteristics.

[0034] The active material layer 10 may have a two-layer structure of a first layer 11 and a second layer 12, or may have a multi-layer structure of three or more layers including layers other than the first layer 11 and the second layer 12 (hereinafter also referred to as "other layers"). The arrangement positions of the other layers in the active material layer 10 are not particularly limited and may be set depending on the type and content of the positive electrode active material contained in the other layers. For example, when the other layers include a first active material and / or a second active material, the arrangement of the first layer 11, the second layer 12, and the other layers may be adjusted so that the content of the first active material increases or the content of the second active material decreases from the anti-current collector side of the active material layer 10 toward the current collector 15 side.

[0035] The first layer 11 may contain a conductive aid other than CNT, and may contain, for example, graphite in addition to CNT. The second layer 12 may contain a conductive aid. Examples of the conductive aid that the second layer 12 may contain include carbon materials such as fibrous carbon and graphite. Examples of graphite include one or more types selected from the group consisting of carbon black (acetylene black, ketjen black, etc.), coke, and activated carbon. Examples of fibrous carbon include CNT. Examples of CNT include the above-mentioned CNT. The conductive aid may include one or more types of the above-mentioned conductive aids.

[0036] Examples of binders that may be contained in the first layer 11 or the second layer 12 include known materials such as fluororesins such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene (PTFE), cellulose-based resins such as carboxymethyl cellulose (CMC), methyl cellulose (MC), and hydroxypropyl cellulose, and styrene butadiene rubber (SBR). The binder may contain one or more of the above-mentioned binders.

[0037] The positive electrode plate 1 can be manufactured, for example, by forming a first layer 11 and a second layer 12 in this order on a current collector 15. For example, a first coating layer is formed by applying a first slurry containing a first active material to the current collector 15 and drying it. Next, a second coating layer is formed by applying a second slurry containing a second active material to the first coating layer and drying it. The positive electrode plate 1 can be obtained by compressing the first coating layer and the second coating layer formed in this order on the current collector 15. The first slurry can contain, in addition to the first active material and CNTs, a binder and a solvent such as N-methyl-2-pyrrolidone (NMP). The second slurry can contain, in addition to the second active material, a conductive additive, a binder, and a solvent such as NMP.

[0038] (Nonaqueous electrolyte secondary battery) The nonaqueous electrolyte secondary battery of this embodiment (hereinafter also referred to as "the battery") has a positive electrode plate 1. As described above, the positive electrode plate 1 has an active material layer 10 including a first layer 11 and a second layer 12. The battery has excellent fusing properties of the positive electrode plate 1 and excellent cycle characteristics.

[0039] The battery may include an electrode assembly including a positive electrode plate 1 and a non-aqueous electrolyte, and may have a battery case that houses the electrode assembly and the non-aqueous electrolyte. The battery case may include an exterior body having an opening and a sealing plate that seals the opening. The exterior body and the sealing plate are preferably made of metal and can be formed using aluminum, an aluminum alloy, iron, an iron alloy, or the like. A resin sheet serving as an electrode holder may be disposed between the electrode assembly and the exterior body. The battery case may also be a laminate film. The laminate film has a layered structure in which, for example, a metal layer and a resin layer are stacked. A pouch-shaped battery case can be formed by overlapping and welding the edges of the laminate film.

[0040] The electrode body may include a positive electrode plate 1, a negative electrode plate, and a separator. In the electrode body, the active material layer 10 of the positive electrode plate 1 and the negative electrode active material layer of the negative electrode plate face each other via the separator. The electrode body may be a laminated type in which the positive electrode plate 1, the negative electrode plate, and the separator are laminated, or a wound type in which a laminate in which the positive electrode plate 1, the negative electrode plate, and the separator are laminated is wound. The wound type electrode body may have a flat shape that is pressed after the laminate is wound.

[0041] The negative electrode plate typically includes a negative electrode current collector and a negative electrode active material layer, and the negative electrode current collector is, for example, a metal foil made of a copper material such as copper or a copper alloy. The negative electrode active material layer contains a negative electrode active material and may further contain a conductive additive, a binder, and the like.

[0042] Examples of negative electrode active materials include carbon-based active material particles and metal-based active material particles. Examples of carbon-based active material particles include one or more particles selected from the group consisting of graphite (e.g., natural graphite and artificial graphite), hard carbon, soft carbon, and carbon (C) such as amorphous coated graphite. Examples of metal-based active material particles include particles of metal elements such as simple metals or metal oxides containing elements selected from the group consisting of silicon (Si), tin (Sn), antimony (Sb), bismuth (Bi), titanium (Ti), and germanium (Ge). Examples of metal-based active material particles include one or more particles selected from the group consisting of Si, SiOx (x=0.5 to 1.5), a composite of Si and C, and Sn.

[0043] Examples of the conductive additive include the conductive additives described above as the conductive additives that may be contained in the active material layer 10. The conductive additive may include one or more of the conductive additives described above. Examples of the binder include cellulose-based resins, polyacrylic acid, styrene butadiene rubber (SBR), and the like, which are described above as the binders that may be contained in the active material layer 10. The binder may include one or more of the binders described above.

[0044] The negative electrode plate can be obtained, for example, by forming a negative electrode active material layer on a negative electrode current collector. For example, a negative electrode mixture slurry containing the negative electrode active material is applied to the negative electrode current collector, dried, and compressed to obtain the negative electrode plate. The negative electrode mixture slurry may contain, in addition to the above-mentioned negative electrode active material, a conductive additive, a binder, and a solvent such as water.

[0045] The separator may have a substrate and a functional layer on at least one side of the substrate. The substrate may be a film or a porous sheet such as a nonwoven fabric made of a resin such as a polyolefin (e.g., polyethylene or polypropylene), polyester, cellulose, or polyamide. The substrate may have a single-layer structure or a multi-layer structure. Examples of the functional layer include an adhesive layer and a heat-resistant layer, and the separator may have one or both of these. The adhesive layer may be formed, for example, with an adhesive. The heat-resistant layer may contain, for example, a filler and a binder.

[0046] The non-aqueous electrolyte preferably contains an electrolyte in a non-aqueous solvent such as an organic solvent. Examples of the electrolyte include LiPF6, LiBF4, LiClO4, LiFSO3, and LiBOB (lithium bis(oxalato)borate). The non-aqueous electrolyte may contain one or more of these electrolytes. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), propylene carbonate (PC), butylene carbonate (BC), and diethyl carbonate (DEC). The non-aqueous electrolyte may contain one or more of these non-aqueous solvents. The non-aqueous electrolyte may further contain an additive such as vinylene carbonate (VC), vinyl ethylene carbonate (VEC), or fluoroethylene carbonate. [Example]

[0047] The present disclosure will be described in more detail below with reference to Examples, Comparative Examples, and Reference Examples.

[0048] Example 1 (Preparation of positive electrode plate) Li as the first active material 1.14 Ni 0.29 Mn 0.57O2, multi-walled carbon nanotubes as CNTs, and polyvinylidene fluoride (PVdF) powder as a binder were prepared in a weight ratio of first active material:CNT:PVdF = 100:0.5:1, and these were mixed with an appropriate amount of N-methyl-2-pyrrolidone (NMP) to obtain a first slurry.

[0049] LiNi as the second active material 0.8 Co 0.1 Mn 0.1 O2, graphite as a conductive additive, and polyvinylidene fluoride (PVdF) powder as a binder were prepared in a weight ratio of second active material: graphite: PVdF = 100:1:1, and these were mixed with an appropriate amount of N-methyl-2-pyrrolidone (NMP) to obtain a second slurry.

[0050] The first slurry was applied to both sides of an aluminum foil serving as a current collector and dried to form a first coating layer. Subsequently, the second slurry was applied to the first coating layer and dried to form a second coating layer. The coating thicknesses of the first and second slurries were set to a ratio of 3:7. The first and second coating layers formed on the current collector were rolled using a rolling roller to form layer (1) from the first coating layer and layer (2) from the second coating layer, thereby obtaining a positive electrode plate. The positive electrode plate had active material layers on both sides of the current collector, and each active material layer had a two-layer structure consisting of layer (1) and layer (2) from the current collector side.

[0051] (Preparation of test battery) A test battery was fabricated using the positive electrode plate obtained above, a negative electrode plate serving as a counter electrode to the positive electrode plate, a separator, and the like.

[0052] Comparative Example 1 The second slurry prepared in the manner described in Example 1 was applied to an aluminum foil current collector, dried, and rolled to obtain a positive electrode plate. The positive electrode plate had a single active material layer on the current collector, and this active material layer contained a second active material. A test battery was fabricated using the obtained positive electrode plate.

[0053] Comparative Example 2 The first and second slurries were prepared according to the procedure described in Example 1. The second slurry was applied to an aluminum foil current collector and dried to form a second coating layer. Subsequently, the first slurry was applied to the second coating layer and dried to form a first coating layer. The coating thicknesses of the second and first slurries were set to a ratio of 7:3 (coating thickness of second slurry:coating thickness of first slurry). The second and first coating layers formed on the current collector were rolled using a rolling roller to form layer (2) from the second coating layer and layer (1) from the first coating layer, thereby obtaining a positive electrode plate. The positive electrode plate had an active material layer on the current collector, and the active material layer had a two-layer structure consisting of layer (2) and layer (1) from the current collector side. A test battery was fabricated using the obtained positive electrode plate.

[0054] Comparative Example 3 The first slurry prepared in the manner described in Example 1 was applied to an aluminum foil current collector, dried, and rolled to obtain a positive electrode plate. The positive electrode plate had a single active material layer on the current collector, and this active material layer contained the first active material. A test battery was fabricated using the obtained positive electrode plate.

[0055] Examples 2 and 3 A positive electrode plate and a test battery were produced in the same manner as described in Example 1, except that the coating thicknesses of the first slurry and the second slurry were adjusted so that the thickness ratios of the layer (1) and the layer (2) were as shown in Table 1.

[0056] Comparative Example 4 A first slurry was prepared in the same manner as in Example 1, except that graphite (carbon black) was used instead of CNT. A positive electrode plate and a test battery were prepared in the same manner as in Example 1, except that this first slurry was used.

[0057] [Example 4, Comparative Examples 5 and 6] A first slurry was prepared in the same manner as in Example 1, except that the CNT content was changed to the content shown in Table 1. A positive electrode plate and a test battery were produced in the same manner as in Example 1, except that this first slurry was used.

[0058] [Measurement of thickness ratio and void ratio] A scanning electron microscope (SEM) image was taken of the active material layer of a cross section obtained by cutting the positive electrode plate in the thickness direction. From the SEM image, the thicknesses of Layer (1) and Layer (2) were determined, and the thickness ratio was calculated. The results are shown in Table 1.

[0059] The obtained SEM images were analyzed, and the porosity of the active material layer was calculated according to the following formula. The porosity of each of the examples and comparative examples was in the range of 19 to 20%. Porosity [%] = [1 - {(content ratio of active material / true density of active material) + (content ratio of conductive additive / true density of conductive additive) + (content ratio of binder / true density of binder)} x weight of active material layer per unit area}] / volume of active material layer per unit area In the above formula, the content ratios of the active material, conductive material, and binder are the contents [mass %] of the active material, conductive material, and binder relative to the total amount of the active material layer, respectively.

[0060] [Nail penetration test] The test battery was charged and connected to a data logger. The data logger has voltage and current measurement functions. A nail (Dydo Hunt, round nail, body diameter: 3 mm) was driven into the test battery, and the driving of the nail was stopped when a voltage drop was confirmed. After the nail was stopped, observation was continued until a voltage increase was detected. The presence or absence of gas generation after the voltage increase was confirmed and evaluated according to the following criteria. A voltage drop is considered to indicate the occurrence of a short circuit. A voltage increase after the voltage drop is considered to indicate that the current collector (aluminum foil) around the nail melted and spread due to Joule heat generated by the short circuit, thereby interrupting the current. A positive electrode plate with excellent fusing properties is less likely to generate gas because the reaction stops when the current is interrupted, while a positive electrode plate with poor fusing properties is more likely to generate gas. The results are shown in Table 1. (Evaluation criteria) A: No gas generation was confirmed. B: Gas generation was confirmed.

[0061] [Evaluation of cycle characteristics] The test battery was subjected to 300 charge / discharge cycles at 0.5 C under a temperature condition of 25°C, with the voltage range set to 4.2 to 3.0 V. The battery capacity measured after the first charge / discharge cycle was defined as the initial battery capacity, and the battery capacity measured after the 300th charge / discharge cycle was defined as the battery capacity after the cycle test. The capacity retention rate [%] in the cycle test was calculated according to the following formula. The results are shown in Table 1. Capacity retention rate [%] = (battery capacity after cycle test / initial battery capacity) x 100

[0062] [Table 1]

[0063] A comparison between Example 1 and Comparative Examples 1 to 3 reveals that a secondary battery with excellent positive electrode plate fusing properties and excellent cycle characteristics can be obtained by forming the active material layer into a two-layer structure, with a layer (1) containing a first active material as a main component and CNTs arranged on the current collector side and a layer (2) containing a second active material as a main component arranged on the opposite current collector side. A comparison between Examples 1 and 2 and Example 3 reveals that the thickness ratio affects the cycle characteristics, with Examples 1 and 2 exhibiting better cycle characteristics than Example 3. A comparison between Examples 1 and 4 and Comparative Example 4 reveals that the inclusion of CNTs in the layer (1) on the current collector side leads to good cycle characteristics. A comparison between Examples 1 and 4 and Comparative Examples 5 and 6 reveals that an increase in the CNT content in the layer (1) on the current collector side leads to a decrease in fusing properties, and a decrease in the CNT content leads to a decrease in cycle characteristics. [Explanation of symbols]

[0064] 1 positive electrode plate, 10 active material layer, 11 first layer, 12 second layer, 15 current collector.

Claims

1. A positive electrode plate having an active material layer on a current collector, the active material layer has a first layer and a second layer in this order from the current collector side, The first layer contains a first active material represented by the following formula (I) as a main component and contains carbon nanotubes: The second layer contains a second active material represented by the following formula (II) as a main component: The content of the carbon nanotubes in the first layer is 0.15 to 0.65 mass % with respect to the total amount of the first layer. Li(Li ａ１ Ni ｘ１ Mn ｙ１ M1 ｚ１ )O ２ (I) Li(ii ａ２ ii ｘ２ 7N ｙ２ 72 ｚ２ )9 ２ (99) [In formula (I) and formula (II), M1 and M2 each independently represent one or more metal elements selected from the group consisting of Co, Al, Mg, Ca, Ti, V, Cr, Fe, Cu, Zn, Zr, Nb, Mo, Ta, and W; 0.1≦a1≦0.33, 0≦x1≦0.5, 0.5≦y1≦0.7, 0≦z1≦0.2, and a1+x1+y1+z1=1 is satisfied; -0.1≦a2≦0.1, 0.6≦x2≦1, 0≦y2≦0.3, 0≦z2≦0.3, and a2+x2+y2+z2=1 is satisfied.

2. The positive electrode plate according to claim 1, wherein a thickness ratio of the first layer to the second layer is first layer:second layer=0.8:9.2 to 4.5:5.

5.

3. The positive electrode plate according to claim 1, wherein a thickness ratio of the first layer to the second layer is first layer:second layer=1:9 to 3:

7.

4. 2. The positive electrode plate according to claim 1, wherein the active material layer has a thickness of 10 to 200 μm.

5. The positive electrode plate according to claim 1 , wherein the carbon nanotubes are multi-walled carbon nanotubes.

6. The positive electrode plate according to claim 1 , wherein the first layer is in contact with the current collector.

7. The positive electrode plate according to claim 1 , wherein the second layer forms a surface of the active material layer opposite to the current collector side.

8. A non-aqueous electrolyte secondary battery comprising the positive electrode plate according to any one of claims 1 to 7.

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