Positive electrode plate and nonaqueous electrolyte secondary battery including the same

Abstract

To provide a positive electrode plate capable of achieving both higher capacity and lower resistance of a nonaqueous electrolyte secondary battery.SOLUTION: A positive electrode plate includes an active material layer on a current collector. The active material layer has a first layer containing, as main components, a first active material represented by formula (I), and a second layer arranged closer to the current collector than the first layer and containing, as main components, a second active material represented by formula (II). Formula (I) and formula (II) are defined in the scope of patent claims. The content ratio of the first active material and the second active material in the active material layer is first active material:second active material=2.5:7.5-6.5:3.5 (weight ratio). The first layer has a porosity of 20-45%, and the second layer has a porosity of 18-41%.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 a secondary battery uses 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 a secondary battery that includes two layers with different molar ratios of lithium to transition metal. [Prior art documents] [Patent documents]

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

[0004] Secondary batteries are required to have high capacity and low resistance. The present disclosure aims to provide a positive electrode plate that can achieve both high capacity and low resistance in non-aqueous electrolyte secondary batteries, and a non-aqueous electrolyte secondary battery having the same. [Means for solving the problem]

[0005] [1] A positive electrode plate having an active material layer on a current collector, The active material layer is A first layer mainly composed of a first active material represented by the following formula (I): a second layer disposed closer to the current collector than the first layer and containing a second active material represented by the following formula (II) as a main component, a content ratio of the first active material to the second active material in the active material layer is first active material:second active material=2.5:7.5 to 6.5:3.5 (weight ratio); The porosity of the first layer is 20 to 45%; The positive electrode plate, wherein the second layer has a porosity of 18 to 41%. 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, 0.5≦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 content ratio of the first active material and the second active material in the active material layer is first active material:second active material=3:7 to 6:4 (weight ratio). [3] The positive electrode plate according to [1] or [2], wherein the content ratio of the first active material and the second active material in the active material layer is first active material:second active material=3:7 to 5:5 (weight ratio). [4] The positive electrode plate according to any one of [1] to [3], wherein the porosity of the first layer and the porosity of the second layer are both 20 to 40%. [5] The positive electrode plate according to any one of [1] to [4], wherein the first layer forms a surface of the active material layer on the side opposite to the current collector side. [6] The positive electrode plate according to any one of [1] to [5], wherein the second layer is in contact with the current collector. [7] The positive electrode plate according to any one of [1] to [6], wherein the second active material is a mixture of the following [p1] and the following [p2]: [p1] At least one of a single particle and a secondary particle formed by agglomeration of 2 to 10 primary particles [p2] Secondary particles formed by agglomeration of 50 or more primary particles [8] A non-aqueous electrolyte secondary battery having the positive electrode plate according to any one of [1] to [7]. [Effects of the Invention]

[0006] The positive electrode plate of the present disclosure can achieve high capacity and low resistance in a nonaqueous electrolyte secondary battery. [Brief explanation of the drawings]

[0007] [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

[0008] 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.

[0009] (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.

[0010] 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 containing a first active material represented by the following formula (I) as a main component, and a second layer 12 containing a second active material represented by the following formula (II) as a main component. The second layer 12 is disposed closer to the current collector 15 than the first layer 11. The content ratio of the first active material to the second active material in the active material layer 10 is first active material:second active material=2.5:7.5 to 6.5:3.5 (weight ratio). The porosity of the first layer is 20 to 45%, and the porosity of the second layer is 18 to 41%. 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, 0.5≦x2≦1, 0≦y2≦0.3, 0≦z2≦0.3, and a2+x2+y2+z2=1.

[0011] 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 is more likely to exhibit high capacity but has high resistance compared to the second active material. The second active material is a positive electrode active material with a lower Li content compared to the first active material, resulting in a lower capacity but also a lower resistance. In the positive electrode plate 1, the first layer 11, which is primarily composed of the first active material, is disposed on the surface side of the active material layer 10, which is farther from the current collector 15 side than the second layer 12 (hereinafter also referred to as the "anti-current collector side of the active material layer 10"). Therefore, when the positive electrode plate 1 is applied to a secondary battery, the first active material, which has a relatively high resistance, easily comes into contact with the electrolyte. Because the porosity of the first layer 11 and the porosity of the second layer 12 are within the above-described ranges, the first active material and the second active material easily come into contact with the electrolyte. When the first active material and the second active material are in a state where they can easily come into contact with the electrolyte, Li ions are easily diffused, and charge / discharge reactions in the secondary battery are easily promoted, thereby suppressing an increase in the resistance of the secondary battery. Furthermore, since the active material layer 10 contains the high-capacity first active material and the first layer 11 contains the first active material as a main component, the capacity of the secondary battery can also be increased.

[0012] The current collector 15 is, for example, a metal foil made of an aluminum material such as aluminum and aluminum alloy.

[0013] The active material layer 10 is formed on the current collector 15 and contains a positive electrode active material such as a first active material and a second active material. The active material layer 10 may be formed only on one side of the current collector 15 or may be formed on both sides. The active material layer 10 contains at least one of a conductive assistant and a binder in addition to the positive electrode active material, and preferably contains both the conductive assistant and the binder.

[0014] 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, may be 0.12 ≦ a1 ≦ 0.25, or may be 0.15 ≦ a1 ≦ 0.2. In formula (I), x1 may be 0 < x1 ≦ 0.4, may be 0.05 ≦ x1 ≦ 0.3, or may be 0.1 ≦ x1 ≦ 0.2. In formula (I), y1 may be 0.50 ≦ y1 ≦ 0.68, may be 0.52 ≦ y1 ≦ 0.65, or may be 0.55 ≦ y1 ≦ 0.6. In formula (I), z1 may be 0 < z1 ≦ 0.2, may be 0.05 ≦ z1 ≦ 0.18, or may be 0.08 ≦ z1 ≦ 0.15. The composition of the first active material can be determined, for example, by ICP (Inductively Coupled Plasma) emission spectroscopic analysis.

[0015] The first active material may contain two or more lithium transition metal composite oxides having different compositions 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. The first active material is preferably a secondary particle in which 50 or more primary particles are aggregated. When the first active material is such a secondary particle, it is easy to improve battery performance such as the resistance of the secondary battery. The fact that the first active material has a particle shape such as a secondary particle can be determined by scanning electron microscope (SEM) observation of the first active material.

[0016] The particle diameter (D50) of the first active material is preferably 5 to 15 μm, may be 6 to 14 μm, may be 7 to 12 μm, or may be 8 to 11 μm. In this specification, the particle diameter (D50) is the particle diameter at which the cumulative frequency from the smaller particle diameter in the volume-based particle size distribution reaches 50%. The volume-based particle size distribution can be measured by a laser diffraction particle size distribution measuring device.

[0017] 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 -0.08 ≦ a2 ≦ 0.05, may be -0.07 ≦ a2 ≦ 0.03, may be -0.05 ≦ a2 ≦ 0.02, or may be -0.02 ≦ a2 ≦ 0.02. In formula (II), x2 may be 0.52 ≦ x2 ≦ 1.0, may be 0.55 ≦ x2 ≦ 0.8, may be 0.57 ≦ x2 ≦ 0.75, or may be 0.6 ≦ x2 ≦ 0.7. In formula (II), y2 may be 0 < y2 ≦ 0.3, may be 0.05 ≦ y2 ≦ 0.25, or may be 0.1 ≦ y2 ≦ 0.23. In formula (II), z2 may be 0 < z2 ≦ 0.3, may be 0.05 ≦ z2 ≦ 0.25, or may be 0.1 ≦ z2 ≦ 0.23. The composition of the second active material can be determined by, for example, ICP (inductively coupled plasma) emission spectroscopy.

[0018] 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 the particle diameter is different from each other. The second active material is preferably a mixture of [p1] and [p2] shown below, and more preferably, the second active material in the second layer is a mixture of [p1] and [p2]. [p1] At least one of single particles and secondary particles aggregated from 2 to 10 primary particles [p2] Secondary particles formed by agglomeration of 50 or more primary particles

[0019] [p1] may be a single particle, a secondary particle, or a mixture of a single particle and a secondary particle. Both [p1] and [p2] are compounds represented by formula (II), and the composition of [p1] and the composition of [p2] may be the same or different. The particle shapes of [p1] and [p2] of the second active material can be determined by observing the second active material with a scanning electron microscope (SEM).

[0020] When the second active material in the second layer is a mixture of [p1] and [p2], the packing density of the second active material in the second layer is likely to be improved. This facilitates contact between the second active materials, which reduces the resistance of the second layer and also helps prevent cracking of the secondary particles of [p2], thereby improving the durability of the secondary battery. The mixing ratio of [p1] to [p2] (weight ratio) may be 3:7 to 7:3, 3.5:6.5 to 6.5:3.5, or 4:6 to 6:4.

[0021] The particle size (D50) of the above [p1] is preferably 1 to 10 μm, and may be 2 to 8 μm, or 3 to 6 μm. The particle size (D50) of the above [p2] is preferably 10 to 25 μm, and may be 12 to 20 μm, or 14 to 18 μm.

[0022] The particle size (D50) of the above [p1] is preferably smaller than the particle size (D50) of the above [p2]. The particle size (D50) of the second active material [p2] may be larger or smaller than the particle size (D50) of the first active material.

[0023] The weight ratio of the first active material to the second active material in the active material layer 10 may be 2.5:7.5 to 6.5:3.5 (first active material:second active material), preferably 3:7 to 6:4, more preferably 3:7 to 5:5, and may be 3.5:6.5 to 5.5:4.5 or 3.7:6.3 to 5.2:4.8. When the first active material contains two or more lithium transition metal composite oxides, the total weight of the first active material is defined as the weight of the first active material. When the second active material contains two or more lithium transition metal composite oxides, the total weight of the second active material is defined as the weight of the second active material. When the content ratio of the first active material to the second active material is within the above range, it is easy to achieve both high capacity and low resistance in the secondary battery. When the content ratio of the first active material is lower than the above range and the content ratio of the second active material is higher than the above range, it becomes difficult to achieve high capacity in the secondary battery. If the content of the first active material is greater than the above range and the content of the second active material is less than the above range, the resistance of the secondary battery tends to increase.

[0024] The first layer 11 may contain the first active material as a main component. "Containing the first active material as a main component" means that the first layer 11 contains the first active material in an amount of 50% by weight or more relative to the total weight of the first layer 11. The content of the first active material in the 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. The first layer 11 may contain a positive electrode active material other than the first active material, such as a 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 further contain at least one of a conductive additive and a binder.

[0025] The second layer 12 may contain the second active material as a primary component. "Containing the second active material as a primary component" means that the second layer 12 contains the second active material in an amount of 50% by weight or more relative to 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. The second layer 12 may contain a positive electrode active material other than the second active material, such as 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.

[0026] The porosity of the first layer 11 may be 20 to 45%, may be 20 to 42%, preferably 20 to 40%, may be more than 20% but not more than 40%, may be 21 to 45%, or may be 25 to 42%. The porosity of the second layer 12 may be 18 to 41%, preferably 20 to 40%, may be more than 20% but not more than 40%, may be 21 to 41%, or may be 25 to 40%. The porosity of the first layer 11 and the porosity of the second layer 12 may be the same or different. When the porosities of the first layer 11 and the second layer 12 are within the above-mentioned ranges, it is easy to achieve both high capacity and low resistance in the secondary battery. When the porosities of the first layer 11 and the second layer 12 are outside the above-mentioned ranges, it becomes difficult to achieve high capacity and low resistance in the secondary battery.

[0027] The porosity of the first layer 11 and the second layer 12 can be adjusted by the application amounts of the first and second active materials when forming the active material layer 10, the application speed, the press pressure and number of presses when rolling the coating layer formed by applying the first and second active materials, the particle properties (particularly particle diameter) of the first and second active materials, the mixing ratio of particles with different particle diameters and / or particle shapes, or the contents of the conductive additive and binder contained in the first layer 11 and the second layer 12. The porosity of the first and second layers can be determined by image analysis of a cross-sectional image of the positive electrode plate 1 obtained using a scanning electron microscope (SEM).

[0028] The thickness of the first layer 11 is preferably smaller than the thickness of the second layer 12. In a secondary battery, if the thickness of the first layer 11 is large, the first active material is less likely to come into contact with the electrolyte, but if the thickness of the first layer 11 is small, the first active material is more likely to come into contact with the electrolyte, making it easier to exchange lithium ions, and therefore making it easier to reduce the resistance of the secondary battery.

[0029] 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 more of the first active material to be disposed on the anti-current collector side of active material layer 10, and more of the second active material to be disposed on the current collector side of active material layer 10, making it easier to reduce the resistance of the secondary battery.

[0030] 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 on the anti-current collector side of the active material layer 10 relative to the second layer 12, and the second layer 12 is disposed on the current collector side of the active material layer 10 relative to the first layer 11. The first layer 11 preferably forms the surface of the active material layer 10, on the side opposite to the current collector 15 side (i.e., the anti-current collector side of the active material layer 10). The second layer 12 is preferably in contact with the current collector 15. This facilitates contact of the first active material with the electrolyte and the second active material with the current collector 15 in the secondary battery, making it easier to reduce the resistance of the secondary battery.

[0031] 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, the second layer, and the other layers may be adjusted so that the content of the first active material decreases or the content of the second active material increases from the surface side of the active material layer 10 opposite the current collector 15 side toward the current collector 15 side.

[0032] Examples of the conductive additive that may be contained in the active material layer 10, the first layer 11, or the second layer 12 include carbon materials. Examples of the carbon material include 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 the fibrous carbon include carbon nanotubes (CNTs). The CNTs may be single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes such as double-walled carbon tubes (DWCNTs). The conductive additive may include one or more types of the conductive additives described above.

[0033] Examples of binders that may be contained in the active material layer 10, 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.

[0034] The positive electrode plate 1 can be manufactured, for example, by forming the second layer 12 and the first layer 11 in this order on the current collector 15. For example, a second coating layer is formed by applying a second slurry containing a second active material to the current collector 15 and drying it. Next, a first coating layer is formed by applying a first slurry containing a first active material to the second coating layer and drying it. The positive electrode plate 1 can be obtained by compressing the second coating layer and the first coating layer formed in this order on the current collector 15. The first and second slurries may contain, in addition to the above-mentioned positive electrode active material, a conductive additive, a binder, and a solvent such as N-methyl-2-pyrrolidone (NMP).

[0035] (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 can achieve both high capacity and low resistance.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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 (hereinafter also referred to as "SiC composite"), and Sn.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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]

[0044] The present disclosure will be described in more detail below with reference to examples and comparative examples.

[0045] Example 1 (Preparation of positive electrode plate) Li as the first active material 1.17 Ni 0.13 Co 0.13 Mn 0.57O2, graphite as a conductive additive, and polyvinylidene fluoride (PVdF) as a binder were prepared in a weight ratio of 100:1.5:1 (first active material: graphite: PVdF), and these were mixed with an appropriate amount of N-methyl-2-pyrrolidone (NMP) to obtain a first slurry. The first active material was secondary particles formed by agglomeration of more than 50 primary particles.

[0046] LiNi as the second active material 0.6 Co 0.2 Mn 0.2 O2, graphite as a conductive additive, and polyvinylidene fluoride (PVdF) as a binder were prepared in a weight ratio of 100:1.5:1 (second active material:graphite:PVdF), which was then mixed with an appropriate amount of N-methyl-2-pyrrolidone (NMP) to obtain a second slurry. The second active material was a mixture of the above-mentioned [p1] and [p2] in a ratio of [p1]:[p2]=1:1, where both [p1] and [p2] had the above-mentioned compositions.

[0047] The second slurry was applied to one side of an aluminum foil serving as a 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 second coating layer and the first coating layer formed on the current collector were rolled to a predetermined thickness using a rolling mill, forming 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. The amounts of the second slurry and the first slurry applied were adjusted so that the content ratio of the first active material to the second active material in the active material layer was first active material:second active material = 3:7.

[0048] (Preparation of battery (1) for specific capacity measurement) The positive electrode plate obtained above and a metallic lithium foil as a counter electrode for the positive electrode plate were cut to a predetermined size. A two-layer separator having a heat-resistant layer on one side of a polyethylene substrate was prepared. The positive electrode plate, the separator, and the metallic lithium foil were laminated to obtain an electrode body. The aluminum foil of the positive electrode plate was exposed in the electrode body. The positive electrode plate and the separator were laminated so that the active material layer of the positive electrode plate and the heat-resistant layer of the separator were in contact with each other. The aluminum foil of the positive electrode plate of the electrode body was welded to an aluminum plate for external current collection, and the metallic lithium foil of the electrode body was welded to a copper plate for external current collection. The electrode body was then inserted into an exterior body made of aluminum laminate film, and the film was welded to form a liquid injection port. After the nonaqueous electrolyte was injected through the liquid injection port, the liquid injection port was sealed to obtain a battery (1).

[0049] The non-aqueous electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF6) as an electrolyte at a concentration of 1.15 mol / L in a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of EC:EMC:DMC = 3:3:4.

[0050] (Preparation of battery (2) for resistance measurement) Battery (2) was obtained according to the same procedure as for producing battery (1), except that a negative electrode plate having an active material layer containing graphite was used as the counter electrode to the positive electrode plate instead of the metallic lithium foil.

[0051] Example 2 A positive electrode plate, a battery (1), and a battery (2) were produced in the same manner as in Example 1, except that the amounts of the second slurry and the first slurry applied were changed so that the content ratios of the first active material and the second active material in the active material layer were as shown in Table 1.

[0052] Comparative Example 1 The second slurry prepared by the procedure described in Example 1 was applied to an aluminum foil current collector, dried, and rolled to a predetermined thickness 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. Batteries (1) and (2) were prepared by the procedure described in Example 1 except for using this positive electrode plate.

[0053] Comparative Example 2 The first slurry prepared by the procedure described in Example 1 was applied to an aluminum foil current collector, dried, and rolled to a predetermined thickness 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 first active material. Batteries (1) and (2) were prepared by the procedure described in Example 1 except for using this positive electrode plate.

[0054] Comparative Example 3 Li as the first active material 1.17 Ni 0.13 Co 0.13 Mn 0.57 O2 and LiNi as the second active material 0.6 Co 0.2 Mn 0.2 O2, graphite as a conductive additive, and polyvinylidene fluoride (PVdF) as a binder were prepared in a weight ratio of 30:70:1.5:1 (first active material: second active material: graphite: PVdF). These were then mixed with an appropriate amount of N-methyl-2-pyrrolidone (NMP) to obtain a slurry. The first active material was a secondary particle formed by agglomeration of 50 or more primary particles. The second active material was a mixture of the aforementioned [p1] and [p2] in a ratio of [p1]:[p2]=1:1, where both [p1] and [p2] had the above-described composition of the second active material. This slurry was applied to an aluminum foil current collector, dried, and rolled to a predetermined thickness to obtain a positive electrode plate. Except for using this positive electrode plate, batteries (1) and (2) were fabricated according to the procedure described in Example 1.

[0055] Comparative Examples 4 and 5 A positive electrode plate, a battery (1), and a battery (2) were produced in the same manner as in Example 1, except that the amounts of the second slurry and the first slurry applied were changed so that the content ratios of the first active material and the second active material in the active material layer were as shown in Table 1.

[0056] Comparative Example 6 The first and second slurries were prepared according to the procedure described in Example 1. The first slurry was applied to an aluminum foil 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 first and second coating layers formed on the current collector were rolled to a predetermined thickness using a rolling mill, forming 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 an active material layer on the current collector, and the active material layer had a two-layer structure consisting of layer (1) and layer (2) from the current collector side. The amounts of the first and second slurries applied were adjusted so that the content ratio of the first active material to the second active material in the active material layer was first active material:second active material = 3:7.

[0057] Batteries (1) and (2) were fabricated in the same manner as in Example 1, except that the positive electrode plate obtained above was used.

[0058] Comparative Examples 7 and 8 A positive electrode plate, a battery (1), and a battery (2) were produced in the same manner as in Example 1, except that the conditions for rolling the second coating layer and the first coating layer using a rolling mill were changed and the porosity of each layer was changed.

[0059] [Calculation of void ratio] Scanning electron microscope (SEM) images were taken of at least five locations on the active material layer of a cross section of the positive electrode plate cut in the thickness direction. Each SEM image was analyzed and binarized into void and non-void areas, and the area fraction ((Sv / St) × 100 [%]) of the total void area Sv within each layer constituting the active material layer relative to the total area St of the layer was calculated. The area fractions calculated for each SEM image were averaged to calculate the porosity [%]. The results are shown in Table 1.

[0060] [Measurement of specific capacitance] Using a charge / discharge tester, the battery (1) was charged at a current value of 0.1 C under a temperature condition of 25°C until the upper limit voltage reached 4.7 V. After a 10-minute rest, the battery (1) was discharged at a current value of 0.05 C until the lower limit voltage reached 2.5 V. The discharge capacity [mAh] at this time was divided by the weight [g] of the active material contained in the active material layer of the positive electrode plate to obtain the specific capacity [mAh / g].

[0061] [Resistance measurement] Battery (2) was charged at a current of 0.33 C at a temperature of 25°C until the state of charge (SOC) reached 7%. After a 10-minute rest, it was discharged at a current of 0.33 C for 10 seconds, and the voltage and discharged current were measured 0.1 and 10 seconds after the start of discharge. It was then charged at a current of 0.33 C for 10 seconds. The voltage and current were measured using the same procedure as above, except that the current rate was changed to 0.5 C, 1 C, and 1.5 C. The discharge resistance was calculated from the voltage and current values ​​obtained at each current rate from 0.1 to 10 seconds after the start of discharge.

[0062] The above operation was performed at a state of charge (SOC) of 50% and 90%, and the discharge resistance was calculated using the procedure described above. The discharge resistances at each state of charge (7%, 50%, and 90%) were averaged. The average discharge resistance of Comparative Example 1 was set to 1, and the average discharge resistance of each Example and Comparative Example was calculated as a ratio to the average discharge resistance of Comparative Example 1, which was used as the resistance of each Example and Comparative Example. The results are shown in Table 1.

[0063] [Confirmation of particle shape, measurement of particle diameter (D50)] A measurement sample was prepared by dispersing the first or second active material in approximately 1 g of water. The measurement sample was introduced into a laser diffraction particle size distribution analyzer (MT3000II, manufactured by Microtrac-Bell Corporation) to obtain a volumetric particle size distribution. The particle size (D50) at which the cumulative frequency of the smallest particle size in the volumetric particle size distribution reached 50% was measured. The results are shown in Table 2.

[0064] [Table 1]

[0065] [Table 2] [Explanation of symbols]

[0066] 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 is a first layer containing a first active material represented by the following formula (I) as a main component; a second layer disposed closer to the current collector than the first layer and containing a second active material represented by the following formula (II) as a main component, a content ratio of the first active material to the second active material in the active material layer is first active material:second active material=2.5:7.5 to 6.5:3.5 (weight ratio); The porosity of the first layer is 20 to 45%; A positive electrode plate, wherein the porosity of the second layer is 18 to 41%. 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, 0.5≦x2≦1, 0≦y2≦0.3, 0≦z2≦0.3, and a2+x2+y2+z2=1 is satisfied.]

2. 2. The positive electrode plate according to claim 1, wherein the content ratio of the first active material and the second active material in the active material layer is first active material: second active material = 3:7 to 6:4 (weight ratio).

3. 2. The positive electrode plate according to claim 1, wherein the content ratio of the first active material and the second active material in the active material layer is first active material: second active material = 3:7 to 5:5 (weight ratio).

4. The positive electrode plate according to claim 1, wherein the porosity of the first layer and the porosity of the second layer are both 20 to 40%.

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

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

7. The positive electrode plate according to claim 1, wherein the second active material is a mixture of the following [p1] and the following [p2]: [p1] At least one of a single particle and a secondary particle formed by agglomeration of 2 to 10 primary particles [p2] Secondary particles formed by agglomeration of 50 or more primary particles

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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