Positive electrode plate and nonaqueous electrolyte secondary battery including the same

Abstract

To provide a positive electrode plate capable of providing excellent rapid charging capabilities to a nonaqueous electrolyte secondary battery even if a porosity of an active material layer is reduced.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 first layer and the second layer each have a porosity of 22% or less. The first layer has a thickness smaller than that of the second 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 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] To improve the capacity of a secondary battery, it is desirable to form the active material layer of the positive electrode plate at a high density. However, if the porosity of the active material layer is reduced by forming the active material layer at a high density, the rapid charging performance of the secondary battery may be reduced.

[0005] The present disclosure aims to provide a positive electrode plate that can impart excellent rapid charging performance to a nonaqueous electrolyte secondary battery even when the porosity of the active material layer is reduced, 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 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, The porosity of each of the first layer and the second layer is 22% or less; The thickness of the first layer is smaller than the thickness of the second 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 3.5:6.5. [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 porosity of each of the first layer and the second layer is 10 to 22%. [6] The positive electrode plate according to any one of [1] to [5], wherein the first layer forms a surface of the active material layer on the side opposite to the current collector side. [7] The positive electrode plate according to any one of [1] to [6], wherein the second layer is in contact with the current collector. [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 provide a nonaqueous electrolyte secondary battery with excellent rapid charging performance. [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 mainly composed of a first active material represented by the following formula (I), and a second layer 12 mainly composed of a second active material represented by the following formula (II). The second layer 12 is disposed closer to the current collector 15 than the first layer 11. The porosity of both the first layer 11 and the second layer 12 is 22% or less. The thickness of the first layer 11 is smaller than the thickness of the second layer 12. 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. Compared to the second active material, the first active material is more likely to exhibit high capacity but has poorer Li diffusibility within the solid state. The second active material has a lower Li content than the first active material and therefore has a lower capacity but excellent Li diffusibility within the solid state. The positive electrode plate 1 has a first layer 11 mainly composed of the first active material and a second layer 12 mainly composed of the second active material, making it easy to provide a secondary battery with high capacity and good Li diffusibility within the solid state.

[0013] In the positive electrode plate 1, the porosity of both the first layer 11 and the second layer 12 is 22% or less, which allows for a secondary battery with a higher capacity. However, because the Li diffusibility in the solid state of the first active material is low, reducing the porosity of the first layer 11 and the second layer 12 makes it difficult for a reaction to occur throughout the first layer 11 during charging of the secondary battery, which tends to reduce rapid charging performance. In the positive electrode plate 1, the first layer 11 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 "opposite side of the active material layer 10"). This facilitates contact between the first active material, which has poor Li diffusibility in the solid state, and the electrolyte when the positive electrode plate 1 is used in a secondary battery, which tends to improve rapid charging performance. Furthermore, because the thickness of the first layer 11 is smaller than the thickness of the second layer 12, the entire first layer 11 is likely to come into contact with the electrolyte, and the second layer 12, which is disposed on the current collector 15 side of the active material layer 10, is also likely to come into contact with the electrolyte. Therefore, during charging of the secondary battery, the reaction is prevented from concentrating on the surface of the first layer 11 on the side opposite the current collector of the active material layer 10, and the reaction is likely to proceed throughout the first layer 11 and in the second layer 12 on the current collector 15 side. In this way, the positive electrode plate 1 has a structure that makes it easy for the reaction to proceed uniformly throughout the active material layer 10, and therefore a secondary battery using the positive electrode plate 1 has excellent rapid charging performance.

[0014] The current collector 15 is, for example, a metal foil made of an aluminum material such as aluminum or an aluminum alloy.

[0015] 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. In addition to the positive electrode active material, the active material layer 10 contains at least one of a conductive additive and a binder, and preferably contains both a conductive additive and a binder.

[0016] The first active material may be a compound represented by the above formula (I). In the 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 the formula (I), a1 may be 0.11 ≦ a1 ≦ 0.3, may be 0.11 ≦ a1 ≦ 0.25, or may be 0.12 ≦ a1 ≦ 0.20. In the formula (I), x1 may be 0 < x1 ≦ 0.4, may be 0.1 ≦ x1 ≦ 0.4, or may be 0.2 ≦ x1 ≦ 0.3. In the formula (I), y1 may be 0.50 ≦ y1 ≦ 0.65, may be 0.52 ≦ y1 ≦ 0.60, or may be 0.55 ≦ y1 ≦ 0.60. In the 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 spectroscopy.

[0017] 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 the formula (I). The first active material may contain two or more lithium transition metal composite oxides having at least one of the particle shape and the particle diameter different from each other.

[0018] 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 spectroscopic analysis.

[0019] 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 having at least one of different particle shapes and particle diameters from each other.

[0020] The first layer 11 only needs to contain the first active material as a main component. Containing the first active material as a main component means that the first active material is contained at 50% by weight or more based on 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, may be 70% by weight or more, may be 80% by weight or more, may be 50 to 99% by weight, may be 60 to 98% by weight, may be 7 to 95% by weight, or may be 80 to 92% by weight. The first layer 11 may contain a cathode active material other than the first active material, for example, may contain the second active material, or may contain a cathode active material other than the first active material and the second active material. The first layer 11 can further contain at least one of a conductive assistant and a binder.

[0021] 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.

[0022] The porosity of the first layer 11 and the second layer 12 is independently 22% or less, preferably 20% or less, and may be less than 20%, 10 to 22%, 12 to 21%, 15 to 20%, or 15% or more but less than 20%. The porosity of the first layer 11 and the porosity of the second layer 12 may be the same or different. The porosity of the first layer 11 may be greater or smaller than the porosity of the second layer 12. When the porosities of the first layer 11 and the second layer 12 are within the above-described ranges, a high capacity secondary battery can be easily achieved. The porosities of the first layer and the second layer can be determined by image analysis of a cross-sectional image of the positive electrode plate 1 obtained using a scanning electron microscope (SEM).

[0023] The thickness of the first layer 11 is smaller than the thickness of the second layer 12. In a secondary battery, when the thickness of the first layer 11 is large, the first layer 11 on the side opposite the current collector of the active material layer 10 is more likely to come into contact with the electrolyte, but the first layer 11 on the side opposite the current collector is less likely to come into contact with the electrolyte. On the other hand, when the thickness of the first layer 11 is small, the entire first layer 11 is more likely to come into contact with the electrolyte, and the electrolyte is more likely to reach the second layer 12. This makes it easier for a reaction to occur throughout the entire first layer 11 when the secondary battery is charged, and makes it easier for the reaction to proceed throughout the entire active material layer 10, thereby improving the rapid charging performance of the secondary battery.

[0024] The thickness ratio of the first layer 11 to the second layer is preferably first layer:second layer = 0.8:9.2 to 4.5:5.5, more preferably 1:9 to 4:6, or may be 1:9 to 3.5:6.5, or even more preferably 1:9 to 3:7, or may be 1.5:8.5 to 2.5 to 7.5. When the thickness ratio is within the above range, the rapid charging performance of the secondary battery can be further improved.

[0025] 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, preferably 15 to 100 μm, and may be 5 to 80 μm, 7 to 60 μm, or 10 to 59 μm. The thickness of the second layer 12 is, for example, 7 to 180 μm, preferably 20 to 180 μm, and may be 20 to 150 μm, 30 to 120 μm, or 50 to 100 μm.

[0026] The porosity of 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 applied 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.

[0027] 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 side opposite the current collector 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, which facilitates improving the rapid charging performance of the secondary battery.

[0028] 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, which tends to improve the rapid charging performance of the secondary battery.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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).

[0033] (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. Because the battery has the positive electrode plate 1, it has excellent rapid charging performance.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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]

[0042] The present disclosure will be described in more detail below with reference to Examples, Comparative Examples, and Reference Examples.

[0043] Example 1 (Preparation of positive electrode plate) Li as the first active material 1.14 Ni 0.29 Mn 0.57O2, graphite as a conductive additive, and polyvinylidene fluoride (PVdF) powder as a binder were prepared in a weight ratio of first active material: graphite: PVdF = 100:1:1, and these were mixed with an appropriate amount of N-methyl-2-pyrrolidone (NMP) to obtain a first slurry.

[0044] 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.

[0045] The second slurry was applied to both sides 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 coating thicknesses of the second slurry and the first slurry were set to a ratio of 7:3. The second coating layer and the first coating layer 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 active material layers on both sides of the current collector, and each active material layer had a two-layer structure consisting of layer (2) and layer (1) from the current collector side.

[0046] (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.

[0047] [Comparative Examples 1, 5, and 6, Reference Examples 1 and 3] The second slurry prepared in the procedure described in Example 1 was applied to an aluminum foil current collector, dried, and rolled to obtain a positive electrode plate. In each comparative example and reference example, the porosity, which will be described later, was adjusted by adjusting the thickness of the second slurry applied. The positive electrode plate had a single active material layer on the current collector, and this active material layer contained a second active material. Test batteries were fabricated using the obtained positive electrode plates.

[0048] Comparative Example 2 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 coating thicknesses of the first and second slurries were set to a ratio of 3:7 (first slurry:second slurry). 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 an active material layer on the current collector, and the active material layer had a two-layer structure consisting of layers (1) and (2) from the current collector side.

[0049] [Comparative Example 4, Examples 2 to 6, Reference Examples 2 and 4] A positive electrode plate and a test battery were produced in the same manner as in Example 1, except that the coating thicknesses of the first slurry and the second slurry were adjusted to achieve the porosity and the thickness ratio of the layer (1) and the layer (2) described below.

[0050] [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.

[0051] The obtained SEM images were analyzed, and the porosity of Layer (1) and Layer (2) was calculated according to the following formula. 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. The results are shown in Table 1.

[0052] [Evaluation of fast charging performance] The test battery's state of charge (SOC) was set to 0% (3.0V). It was charged at a constant current (CC) rate of 2C until the SOC reached 100% (4.2V). After reaching the cutoff voltage and charging was complete, a 30-minute rest period was allowed, and the test battery's voltage was measured. The SOC value corresponding to this voltage was calculated, and this value was used as the SOC when charging at a 2C rate. The results are shown in Table 1.

[0053] [Table 1]

[0054] A comparison between Example 1 and Comparative Examples 1 to 3 reveals that a secondary battery with excellent rapid charging performance can be obtained by using a two-layer active material layer, with layer (1) mainly composed of a first active material on the opposite side to the current collector and layer (2) mainly composed of a second active material on the opposite side to the current collector. A comparison between Examples 1 to 4 and Comparative Example 4 reveals that a secondary battery with excellent rapid charging performance can be obtained by making the thickness of layer (1) on the opposite side to the current collector thinner than the thickness of layer (2) on the current collector. The results of Examples 1 to 6 and Comparative Examples 1, 3, 5, and 6 reveal that when the porosity of the active material layer is 22% or less, a two-layer active material layer can achieve better rapid charging performance than a single-layer active material layer. The results of Reference Examples 1 to 4 reveal that when the porosity of the active material layer exceeds 22%, good rapid charging performance can be achieved regardless of whether the active material layer has a single-layer or two-layer structure. This shows that when the porosity of the active material layer is 22% or less, rapid charging performance can be improved by using a two-layer structure having a first layer (layer (1)) and a first layer (layer (1)) from the current collector side. [Explanation of symbols]

[0055] 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, The porosity of each of the first layer and the second layer is 22% or less; The thickness of the first layer is smaller than the thickness of the second 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 3.5:6.

5.

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 porosity of each of the first layer and the second layer is 10 to 22%.

6. 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.

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

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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