Positive electrode plate and non-aqueous electrolyte secondary battery

Abstract

To provide a positive electrode plate and a battery having good cycle characteristics and output resistance.SOLUTION: A positive electrode plate 10 includes an active material layer 12 and a core body 11, the active material layer includes active material particles, the active material particles include large particles 23 and small particles 24, the large particles 23 are more abundant near the surface of the active material layer 12 on the core body side than near the surface of the active material layer 12 opposite the core body 11.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a positive electrode plate and a non-aqueous electrolyte secondary battery (hereinafter also referred to as a battery) including the same. [Background technology]

[0002] International Publication No. 2019 / 069459 (Patent Document 1) discloses an electrode plate in which a mixture layer and a coating layer each containing particles with different particle diameters are stacked. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2019 / 069459 Summary of the Invention [Problem to be solved by the invention]

[0004] The electrode plate described in Patent Document 1 did not have sufficient cycle characteristics and output resistance. An object of the present invention is to provide a positive electrode plate and a battery having good cycle characteristics and output resistance. [Means for solving the problem]

[0005] The present invention provides the following positive electrode plate and battery. [1] A positive electrode plate including an active material layer and a core body, wherein the active material layer contains active material particles, the active material particles include large particles and small particles, and the large particles are present in greater amounts near the surface of the active material layer on the core body side than near the surface of the active material layer opposite the core body. [2] The positive electrode plate according to [1], wherein in a cross section of the active material layer, the ratio of the total area of ​​the large particles present near the surface on the side opposite to the substrate side to the total area of ​​the large particles present near the surface on the substrate side is 0.2 or more and less than 1.0. [3] The positive electrode plate according to [1] or [2], wherein the small particles include single-crystal particles or polycrystalline particles. [4] The positive electrode plate according to any one of [1] to [3], wherein the large particles have grain boundaries. [5] The positive electrode plate according to any one of [1] to [4], wherein the large particles include secondary particles formed by aggregation of primary particles. [6] The particle packing density of the active material layer is 3.0 to 3.8 g / cm 3 The positive electrode plate according to any one of [1] to [5], wherein [7] The positive electrode plate according to any one of [1] to [6], wherein the active material layer has a deep layer disposed on the substrate side and a surface layer disposed on the opposite side of the deep layer from the substrate side. [8] The positive electrode plate according to any one of [1] to [7], wherein the mass ratio of the large particles in the surface layer and the deep layer (surface layer / deep layer) is greater than 1 / 9 and less than 1 / 1. [9] A non-aqueous electrolyte secondary battery comprising the positive electrode plate according to any one of [1] to [8]. [Effects of the Invention]

[0006] According to the present invention, a positive electrode plate and a battery having good cycle characteristics and output resistance are provided. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic diagram showing an example of the configuration of the positive electrode in this embodiment. [Figure 2] FIG. 2 is a schematic diagram showing another example of the configuration of the positive electrode in this embodiment. [Figure 3] FIG. 3 is a schematic flow chart showing a method for producing a positive electrode. [Figure 4] FIG. 4 is a schematic diagram showing an example of the configuration of a battery according to this embodiment. [Figure 5] FIG. 5 is a schematic diagram showing an example of the configuration of the electrode body in this embodiment. [Figure 6] FIG. 6 shows a cross-sectional image of the active material layer of Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to the following embodiments. In all of the drawings, the scales of the components are appropriately adjusted to make them easier to understand, and the scales of the components shown in the drawings do not necessarily match the scales of the actual components.

[0009] FIG. 1 is a schematic diagram showing an example of the configuration of a positive electrode plate in this embodiment. The positive electrode plate 10 includes a core 11 and an active material layer 12. The core 11 is a conductive sheet. The core 11 may be, for example, an aluminum alloy foil. The core 11 may have a thickness of, for example, 10 μm to 30 μm. The thickness of the core 11 may be measured using a constant pressure thickness gauge. The active material layer 12 may be disposed on the surface of the core 11. The active material layer 12 may be disposed on, for example, one side or both sides of the core 11. The active material layer 12 may have a thickness of, for example, 10 μm to 200 μm. The thickness of the active material layer 12 is measured using a cross-sectional SEM image of the active material layer 12. The observation surface may be parallel to the thickness direction of the active material layer 12. The thickness of each active material layer 12 is measured at five or more locations. The thickness of the active material layer 12 is defined as the arithmetic average of the thicknesses measured at the five or more locations. The particle packing density of the active material layer 12 is, for example, 3.0 to 3.8 g / cm 3 It can be more than that.

[0010] The active material layer 12 contains active material particles. The active material particles include a first particle group and a second particle group. The first particle group is composed of large particles 23. The second particle group can be composed of small particles 24. The large particles 23 are present in greater numbers near the surface of the active material layer 12 on the substrate 11 side than near the surface on the opposite side from the substrate 11. The inventors have found that the deterioration in cycle characteristics is due to cracks occurring in the active material particles due to the insertion and extraction of lithium (Li) during cycle testing, and that small particles tend to be relatively resistant to cracking, while large particles tend to be relatively prone to cracking. Furthermore, they have found that output characteristics tend to improve as the packing density of the active material particles increases, and that small particles tend to be relatively difficult to improve packing density, while large particles tend to be relatively easy to improve packing density. Therefore, by arranging a large number of small particles, which are less likely to crack, on the side opposite to the core 11 (surface side) where Li insertion and de-insertion frequently occurs, and by arranging a large number of large particles on the core 11 side (deep side), we succeeded in achieving both good cycle characteristics and output resistance.

[0011] In the cross section of the active material layer 12, the ratio of the total area of ​​the large particles 23 present near the surface on the side opposite to the substrate 11 side to the total area of ​​the large particles 23 present near the surface on the substrate 11 side may be, for example, 0.2 or more and less than 1.0, preferably 0.25 or more and 0.9 or less, and more preferably 0.3 or more and 0.8 or less. The total area of ​​the large particles 23 in the cross section of the active material layer 12 is measured according to the method described in the Examples section below.

[0012] The vicinity of the surface of active material layer 12 opposite to core 11 is a region including the surface of active material layer 12 opposite to core 11, and may be a region that exists in the thickness direction of active material layer 12 from the surface of active material layer 12 opposite to core 11 to 1 / 5, 2 / 5, 1 / 2, 3 / 5, or 4 / 5 of the thickness of active material layer 12. The vicinity of the surface on the core 11 side of active material layer 12 is a region including the surface of active material layer 12 on the core 11 side, and may be a region other than the vicinity of the surface of active material layer 12 opposite to core 11.

[0013] The large particles may be secondary particles (agglomerated particles) formed by aggregation of primary particles. The large particles may have grain boundaries. The small particles may be single particles. The small particles preferably include single crystal particles or polycrystalline particles. Single crystal particles or polycrystalline particles have few grain boundaries (or no grain boundaries), so they tend to be less likely to crack due to Li insertion and deintercalation. Single crystal particles or polycrystalline particles tend to have high resistance, so they tend to be difficult to use as large particles. By arranging a large number of small particles that are single crystal particles or polycrystalline particles with few grain boundaries, which are less likely to crack, on the side opposite to the core side (surface side) where Li insertion and deintercalation frequently occurs, and arranging a large number of large particles with grain boundaries on the deeper side, it is possible to suppress particle cracking due to Li insertion and deintercalation, improve cycle characteristics, and suppress an increase in plate resistance.

[0014] The average particle diameter (D50) of the first particle group (large particles) may be, for example, 10 μm or more and 20 μm or less, preferably 14 μm or more and 18 μm or less. The average primary particle diameter may be, for example, 0.1 μm or more and 3 μm or less, preferably 0.5 μm or more and 2.5 μm or less. The average particle diameter (D50) of the second particle group (small particles) may be, for example, 2 μm or more and 6 μm or less, preferably 3 μm or more and 6 μm or less. In this specification, the average particle diameter (D50) refers to the particle diameter (hereinafter also referred to as D50) at which the cumulative particle volume from the small particle side in the volume-based particle size distribution is 50% of the total particle volume. The average particle diameter (D50) can be measured by a laser diffraction / scattering method. The average primary particle diameter is the average of the distances between the two most distant points on the outlines of 10 primary particles randomly sampled on an SEM image. The BET specific surface area of ​​the active material particles is, for example, 0.5 m 2 / g to 1.5m 2 / g.

[0015] The active material particles may be particles containing a lithium transition metal composite oxide. The crystal structure of the lithium transition metal composite oxide is not particularly limited and may be a layered structure, a spinel structure, an olivine structure, or the like. The lithium transition metal composite oxide is preferably a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element. Specific examples include lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide. When the active material particles contain nickel, the nickel content ratio relative to the metal elements excluding lithium in the active material particles may be, for example, 60 mol % or less. The active material particles may also be surface-coated particles.

[0016] The active material particles may include, for example, a layered metal oxide having the formula: Li 1-a1 Ni x1 Me 1 1-x1 In formula (1), "a1" satisfies the relationship "-0.3≦a1≦0.3". "x1" satisfies the relationship 0.1≦x1≦0.95. "Me 1 " indicates at least one selected from the group consisting of Co, Mn, Al, Zr, Ti, V, Cr, Fe, Cu, Zn, Mo, Sn, Ge, Nb, and W.

[0017] The content of active material particles in the active material layer 12 may be, for example, 70 wt% or more, preferably 80 to 99 wt%, and more preferably 90 to 99 wt% or less, based on 100 wt% of the total solid content of the active material layer 12. The mass ratio (surface layer / deep layer) of large particles present near the surface of the active material layer 12 opposite the core 11 (or in the surface layer described below) to large particles present near the surface of the active material layer 12 facing the core 11 (or in the deep layer described below) is, for example, more than 1 / 9 and less than 1, preferably 1 / 4 to 2 / 3, and more preferably 3 / 7. The mass ratio (small particles / large particles) of small particles to large particles present near the surface of the active material layer 12 facing the core 11 (or in the deep layer described below) may be, for example, more than 1 / 9 and less than 1, preferably 1 / 4 to 2 / 3, and more preferably 3 / 7. The mass ratio of small particles to large particles (small particles / large particles) near the surface of the active material layer 12 opposite the core 11 (or in the surface layer described below) may be, for example, 1 or more and 9 or less, preferably 1.5 or more and 4 or less, and more preferably 7 / 3.

[0018] The active material layer 12 may contain a binder and a conductive additive. Examples of binders include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyimide (PI), polyamide (PA), polyamideimide (PAI), butadiene rubber (BR), styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), styrene-ethylene-butylene-styrene block copolymer (SEBS), carboxymethyl cellulose (CMC), and combinations thereof. The content of the binder in the active material layer 12 may be, for example, 0.5 to 10 wt %, and preferably 1.0 to 5 wt %, based on 100 wt % of the total solid content of the active material layer 12.

[0019] The conductive additive may include, for example, carbon black such as acetylene black (AB), other carbon materials (e.g., graphite), carbon nanotubes, etc. The content of the conductive additive in the active material layer 12 may be, for example, 0.05 to 2.0 wt %, and preferably 0.10 to 1.0 wt %, based on 100 wt % of the total solid content of the active material layer 12.

[0020] As shown in Fig. 2, the active material layer 12 may include a deep layer 31 disposed on the core 11 side and a surface layer 32 disposed on the opposite side of the deep layer from the core 11 side. The active material layer 12 may have a two-layer structure consisting of a deep layer and a surface layer. The deep layer 31 and the surface layer 32 may have the same thickness or different thicknesses. The thickness ratio of the deep layer 31 to the surface layer 32 (deep layer / surface layer) is preferably 4 / 5 to 5 / 4.

[0021] The manufacturing method of the positive electrode plate 10 includes the steps of preparing a positive electrode slurry (A1), applying it (B1), drying it (C1), and compressing it (D1), as shown in FIG. 3. In the preparation of the positive electrode slurry (A1), a positive electrode slurry containing active material particles and CNTs is prepared using any stirring device. The positive electrode slurry is prepared by dispersing the active material particles in a dispersion medium (an organic solvent such as N-methyl-2-pyrrolidone (NMP), tetrahydrofuran (THF), dimethylformamide (DMF), methyl ethyl ketone (MEK), or dimethyl sulfoxide (DMSO)). The positive electrode slurry may have a solids concentration of, for example, 40% to 80%.

[0022] In coating (B1), the positive electrode slurry can be applied to the surface of the substrate using any coating device to form a coating film. When the active material layer includes a deep layer and a surface layer, the first coating film for the deep layer and the second coating film for the surface layer can be formed sequentially or substantially simultaneously. For example, the deep layer slurry can be applied to one surface of the core to form a first coating film, and then the surface layer slurry can be applied on top of that to form a second coating film. In this specification, the term "coating film" can refer to both the first coating film and the second coating film. Drying (C1) can include heating and drying the coating film, for example, using a hot air dryer, to form a dried coating film. Compression (D1) can include compressing the dried coating film using any compression device to form the active material layer 12 and obtain the positive electrode plate 10. The positive electrode plate 10 can be cut to a predetermined planar size depending on the battery specifications. The positive electrode plate 10 can also be cut into a strip-like planar shape, for example. The positive electrode plate 10 may be cut to have, for example, a rectangular planar shape.

[0023] The battery may be a lithium-ion battery. FIG. 4 is a schematic diagram showing an example of a lithium-ion battery in this embodiment. The battery 200 shown in FIG. 4 may be, for example, a lithium-ion battery used as a main power source for an electric vehicle or a power source for power assist. The battery 200 houses an electrode assembly 50 and a non-aqueous electrolyte (not shown) in an outer casing 90. The electrode assembly 50 is connected to a positive terminal 91 by a positive current collecting member 81. The electrode assembly 50 is connected to a negative terminal 92 by a negative current collecting member 82. FIG. 5 is a schematic diagram showing an example of an electrode assembly in this embodiment. The electrode assembly 50 is a wound type. The electrode assembly 50 includes a positive electrode plate 10, a separator 70, and a negative electrode plate 60. That is, the battery 200 includes the positive electrode plate 10. The positive electrode plate 10 includes an active material layer 12 and a core 11. The negative electrode plate 60 includes a negative electrode active material layer 62 and a negative electrode substrate 61. [Example]

[0024] The present invention will be described in more detail below with reference to examples. In the examples, "%" and "parts" are by mass % and mass parts unless otherwise specified.

[0025] <Measurement of the average particle size (D50) of the first and second lithium composite oxide particles> The volumetric particle size distribution of the first and second lithium composite oxide particles was measured using a commercially available laser diffraction / scattering particle size distribution analyzer, and the particle size corresponding to a cumulative frequency of 50 volume % from the side of fine particles with smaller particle sizes was determined as the average particle size (D50) of the first and second lithium composite oxide particles.

[0026] <Measurement of the Mixing Ratio of the First and Second Lithium Composite Oxide Particles Obtained from the Cross Section of the Electrode Plate> A sample for cross-sectional observation of the positive electrode sheet was prepared by cross-section polisher processing. An SEM image of this sample was obtained using an SEM. Using the image processing software "GIMP," the first and second lithium composite oxide particles were colored separately in the SEM image to determine the area, and the area ratio of the first and second lithium composite oxide particles was measured. Figure 6 shows a cross-sectional SEM image of the active material layer of Example 1 and an image after coloring.

[0027] <Measurement of BET specific surface area of ​​first and second lithium composite oxide particles> The BET specific surface areas of the first and second lithium composite oxide particles were measured by a nitrogen adsorption method using a commercially available specific surface area measuring device (Macsorb Model-1208, manufactured by Mountech Co., Ltd.). The calculated BET value was determined by calculating the total BET specific surface area of ​​the active material present near the surface (surface layer) on the side opposite to the core side of the active material layer from the mass ratio of the first and second lithium composite oxide particles.

[0028] <Preparation of negative electrode plate> Graphite (C) as the negative electrode active material, styrene butadiene rubber (SBR) as the binder, and carboxymethyl cellulose (CMC) as the thickener were mixed in ion-exchange water in a mass ratio of C:SBR:CMC = 98:1:1 to prepare a slurry for forming a negative electrode active material layer. This slurry for forming a negative electrode active material layer was applied to copper foil and dried to form a negative electrode active material layer. The negative electrode active material layer was roll-pressed with a rolling roller to achieve a predetermined density, and then cut to a predetermined size to prepare a negative electrode sheet.

[0029] <Preparation of Lithium-ion Secondary Batteries for Evaluation> A porous polyolefin sheet was prepared as a separator. The positive electrode plate prepared in each of the examples and comparative examples and the negative electrode plate prepared above were stacked with the separator interposed between them to produce a stacked electrode assembly. An electrode terminal was attached to the stacked electrode assembly, which was then inserted into a battery case made of an aluminum laminate sheet, and a nonaqueous electrolyte was poured into it. The nonaqueous electrolyte used was a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of EC:EMC:DMC = 30:70, in which LiPF6 was dissolved at a concentration of 1 mol / L as a supporting electrolyte, and vinylene carbonate was added to a concentration of 0.3 mass%. The battery case was then sealed to obtain a lithium-ion secondary battery for evaluation.

[0030] <Output resistance (internal resistance ratio measurement) and cycle characteristics evaluation> For the initial charge, each evaluation lithium-ion secondary battery was charged at 0.2 mA / cm in a temperature environment of 25°C. 2 The lithium-ion secondary batteries were charged at a constant current of 0.2 mA / cm2 to 4.25 V, and then at a constant voltage of 4.25 V until the current density reached 0.04 mA / cm2. After a 10-minute rest, each of the lithium-ion secondary batteries was charged at a constant current of 0.2 mA / cm2. 2 Each lithium-ion secondary battery for evaluation was adjusted to SOC 50% and 80%, and the internal resistance ratio was measured. Each lithium-ion secondary battery for evaluation was placed in a temperature environment of 25°C and discharged at a constant current density of 0.2 mA / cm 2 The battery was charged at a constant current density of 0.04 mA / cm to 4.18 V, and then the current density was increased to 0.04 mA / cm. 2 The battery was then charged at a constant voltage of 0.2 mA / cm 2 The battery was discharged at a constant current density of 0.05 V to 3.48 V. The discharge capacity at this time was determined. This charge / discharge cycle was counted as one cycle, and each lithium-ion secondary battery for evaluation was subjected to 400 cycles of charge / discharge. The discharge capacity at the 400th cycle was determined. The capacity retention rate (%) was calculated as an index of cycle characteristics according to the formula: (discharge capacity at the 400th charge / discharge cycle / discharge capacity at the 1st charge / discharge cycle) x 100.

[0031] Example 1 The first lithium composite oxide particles (small particles) have an average diameter (D50) of 3.9 μm and a BET specific surface area of ​​0.60 m 2 / g of single particle LiNi 0.6 Co 0.2 Mn 0.2 The second lithium composite oxide particles (large particles) had an average primary particle diameter of 1.8 μm, an average particle diameter (D50) of 16.5 μm, and a BET specific surface area of ​​0.20 m. 2 / g secondary particle LiNi 0.55 Co 0.20 Mn 0.25 O2 was prepared. The first lithium composite oxide particles and the second lithium composite oxide particles were mixed in a mass ratio of 30:70 to prepare active material particles 1. The active material particles 1, carbon black (CNT) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were mixed in a mass ratio of active material particles:CNT:PVDF=97.5:1.5:1.0, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to the resulting mixture to prepare slurry A for forming a positive electrode active material layer. Active material particles 2 were prepared by mixing first lithium composite oxide particles and second lithium composite oxide particles at a mass ratio of 70:30. Active material particles 2, AB as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were mixed at a mass ratio of active material particles 2:AB:PVDF=97.5:1.5:1.0, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to the resulting mixture to prepare slurry B for forming a positive electrode active material layer. Slurry A for forming a positive electrode active material layer was applied to both sides of an aluminum foil core and dried to form a deep-dried coating. Slurry B for forming a positive electrode active material layer was then applied to the deep-dried coating to a weight equivalent to that of the deep-dried coating, and dried to form a surface-dried coating. The two-layer active material layer was roll-pressed using a rolling roller, and then cut to the specified dimensions to produce a positive electrode plate having an active material layer with a deep layer and a surface layer. The results are shown in Table 1.

[0032] <Comparative Examples 1 to 3> Positive electrode plates of Comparative Examples 1 to 3 were produced in the same manner as in Example 1, except that the mass ratios of the first lithium composite oxide particles (small particles) to the second lithium composite oxide particles (large particles) in active material particles 1 used in positive electrode active material layer-forming slurry A were 70:30, 50:50, and 10:90, and the mass ratios of the first lithium composite oxide particles (small particles) to the second lithium composite oxide particles (large particles) in active material particles 2 used in positive electrode active material layer-forming slurry B were 30:70, 50:50, and 90:10. The results are shown in Table 1.

[0033] [Table 1]

[0034] As shown in Table 1, improved cycle characteristics were obtained in Example 1. This is presumably because the more single particles (first lithium composite oxide particles) there were near the surface of the active material layer opposite the core side, the less cracking occurred over cycles. Furthermore, in Example 1, the output resistance tended to improve. This is presumably because the BET specific surface area of ​​the active material present near the surface of the active material layer opposite the core side increased. Although improved cycle characteristics were not obtained in Comparative Example 3, this is presumably because the BET specific surface area of ​​the active material present near the surface of the active material layer opposite the core side increased, resulting in a lack of AB due to expansion and contraction during charge and discharge. [Explanation of symbols]

[0035] 10 positive electrode plate, 11 core body, 12 active material layer, 23 large particles, 24 small particles, 50 electrode body, 60 negative electrode plate, 61 negative electrode substrate, 62 negative electrode active material layer, 70 separator, 81 positive electrode current collecting member, 82 negative electrode current collecting member, 90 exterior body, 91 positive electrode terminal, 92 negative electrode terminal, 200 battery.

Claims

1. A positive electrode plate including an active material layer and a core body, the active material layer contains active material particles, the active material particles include large particles and small particles, the large particles are present in greater amounts near the surface on the substrate side of the active material layer than near the surface on the opposite side to the substrate, the vicinity of the surface on the substrate side is a region of the active material layer including the surface on the substrate side, and is a region of the active material layer other than the vicinity of the surface on the opposite side to the substrate, the active material layer has a deep layer disposed on the core body side and a surface layer disposed on the opposite side of the deep layer from the core body side, a ratio of the mass of the large particles in the surface layer to the mass of the large particles in the deep layer (surface layer / deep layer) is 3 / 7 or more and less than 1; the small particles include single-crystal particles or polycrystalline particles; the large grains have grain boundaries; the active material particles contain a layered metal oxide, The layered metal oxide is Represented by the formula (1): Li 1-a1 Ni x1 Me 1 1-x1 O 2 , In formula (1), "a1" satisfies the relationship "-0.3≦a1≦0.3", The "x1" satisfies the relationship 0.1≦x1≦0.6, The "Me 1 " is at least one selected from the group consisting of Co, Mn, Al, Zr, Ti, V, Cr, Fe, Cu, Zn, Mo, Sn, Ge, Nb, and W.

2. 2. The positive electrode plate according to claim 1, wherein in a cross section of the active material layer, a ratio of a total area of ​​the large particles present near the surface opposite to the substrate side to a total area of ​​the large particles present near the surface on the substrate side is 0.2 or more and less than 1.

0.

3. The positive electrode plate according to claim 1 , wherein the large particles include secondary particles formed by aggregation of primary particles.

4. The particle packing density of the active material layer is 3.0 to 3.8 g / cm ３ The positive electrode plate according to claim 1 ,

5. A non-aqueous electrolyte secondary battery comprising the positive electrode plate according to claim 1.

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