Positive electrode plate and lithium ion secondary battery

Abstract

To provide a positive electrode plate which improves chemical fine short resistance, and a lithium ion secondary battery including the positive electrode plate.SOLUTION: A positive electrode plate includes a positive electrode current collector, and a positive electrode active substance layer which is stacked on the positive electrode current collector and has a multilayer structure, wherein the layer positioned on the most distant side from the positive electrode current collector in the positive electrode active substance layer has hollow positive electrode active substance secondary particles in which particles of the positive electrode active substance are coagulated in a hollow shape, and a coating layer which contains a first binding resin and coats the surface of the hollow positive electrode active substance secondary particles at a coating ratio of 60% or more.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present disclosure relates to a positive electrode plate and a lithium ion secondary battery. [Background technology]

[0002] Lithium-ion secondary batteries are widely used in electric vehicles, small electronic devices (smartphones, personal computers), energy facilities, and the like.

[0003] Many studies have been made to improve the performance of such lithium ion secondary batteries. For example, a lithium ion secondary battery anode comprising a current collector and an active material-containing layer formed on the current collector as an electrode for the lithium ion secondary battery, the active material-containing layer being composed of an outermost layer disposed on the side farthest from the current collector and a lower layer consisting of one or more layers disposed between the outermost layer and the current collector, the outermost layer having a smaller degree of bending than the lower layer, has been disclosed (Patent Document 1).

[0004] The electrode includes an active material-containing layer composed of a top surface layer and a lower layer with different degrees of curvature, so that the lower layer with a larger degree of curvature contributes to improving the electric capacity, while the top surface layer with a smaller degree of curvature ensures a flow path for the electrolyte, preventing the retention of lithium ions on the electrode surface and suppressing the formation of dendrites (branched crystals). Therefore, when such an anode is used in a lithium ion secondary battery, it is possible to sufficiently ensure safety (suppression of dendrites) while increasing the capacity (increasing the density of the electrode), and it is possible to obtain excellent high-rate discharge characteristics by ensuring the flow path for the electrolyte in the top surface layer. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2008-251249 A Summary of the Invention

Problems to be Solved by the Invention

[0006] For the positive electrode plate of a lithium-ion secondary battery, by using hollow positive electrode active material secondary particles, an improvement in the diffusibility of lithium ions and thereby an increase in output are expected. However, the positive electrode active material layer of a lithium-ion secondary battery contains lithium ions and may also contain other metal foreign substances mixed therein derived from the raw materials of the active material layer. Then, the metal ions eluted from this metal foreign substance move within the positive electrode active material layer and accumulate near the separator, which may cause chemical short-circuiting. At that time, if the positive electrode active material is hollow positive electrode active material secondary particles, since metal ions can easily pass through the inside of the hollow structure, the moving distance of the metal ions in the direction of the separator can be short, and the metal ions are likely to accumulate near the separator. As a result, there is a concern that chemical short-circuiting is likely to occur.

[0007] The problem to be solved by one embodiment of the present disclosure is to provide a positive electrode plate with improved chemical short-circuit resistance and a lithium-ion secondary battery including the positive electrode plate.

Means for Solving the Problems

[0008] The means for solving the above problems include the following aspects. <1> A positive electrode current collector and a positive electrode active material layer having a multilayer structure laminated on the positive electrode current collector, wherein the layer located on the side farthest from the positive electrode current collector in the positive electrode active material layer includes hollow positive electrode active material secondary particles in which particles of the positive electrode active material are aggregated in a hollow shape, and a coating layer including a first binder resin and covering the surface of the hollow positive electrode active material secondary particles with a coverage rate of 60% or more. The positive electrode plate includes coated hollow positive electrode active material secondary particles. <2> The layers other than the layer located on the side farthest from the positive electrode current collector in the positive electrode active material layer include hollow positive electrode active material secondary particles in which particles of the positive electrode active material are aggregated in a hollow shape and a second binder resin, and the coverage rate of the hollow positive electrode active material secondary particles by the second binder resin is 0% to 40%. The positive electrode plate according to <1>. <3> The average thickness of the coating layer of the coated hollow cathode active material secondary particles contained in the layer located on the side farthest from the cathode current collector in the cathode active material layer is 10 μm to 30 μm, and the cathode plate according to <1> or <2>. <4> The first binder resin contains polyvinylidene fluoride (PVDF), and the cathode plate according to any one of <1> to <3>. <5> The average thickness of the layer located on the side farthest from the cathode current collector in the cathode active material layer is 60 μm to 80 μm, and the cathode plate according to any one of <1> to <4>. <6> A lithium-ion secondary battery including the cathode plate according to any one of <1> to <5>.

Advantages of the Invention

[0009] According to an embodiment of the present disclosure, a cathode plate with improved chemical micro short resistance and a lithium-ion secondary battery including the cathode plate are provided.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

Figure 4

Modes for Carrying Out the Invention

[0011] Hereinafter, embodiments of the present disclosure will be described. The description is illustrative of the embodiments and does not limit the scope of the present disclosure.

[0012] In this specification, a numerical range indicated using "~" indicates a range including the numerical values described before and after "~" as the minimum value and the maximum value, respectively. In a numerical range described stepwise in this specification, the upper limit value or the lower limit value described in one numerical range may be replaced with the upper limit value or the lower limit value of another numerically described range. Also, in the numerical ranges described in this specification, the upper limit value or the lower limit value of the numerical range may be replaced with the value shown in the examples.

[0013] In this specification, when an embodiment is described with reference to the drawings, the configuration of the embodiment is not limited to the configuration shown in the drawings. Also, the sizes of the members in each drawing are conceptual, and the relative relationships of the sizes between the members are not limited thereto.

[0014] In this specification, each component may contain a plurality of corresponding substances. When referring to the amount of each component in a composition, when there are a plurality of substances corresponding to each component in the composition, unless otherwise specified, it means the total amount of the plurality of substances present in the composition.

[0015] In this specification, the layer located on the side farthest from the positive electrode current collector in the positive electrode active material layer of the positive electrode plate may be simply referred to as the "outermost layer".

[0016] <Positive electrode plate> The positive electrode plate of the present disclosure includes a positive electrode current collector and a positive electrode active material layer having a multilayer structure laminated on the positive electrode current collector. The layer located on the side farthest from the positive electrode current collector in the positive electrode active material layer includes coated hollow positive electrode active material secondary particles having hollow positive electrode active material secondary particles in which particles of the positive electrode active material are aggregated in a hollow shape and a coating layer containing a first binder resin and covering the surface of the hollow positive electrode active material secondary particles with a coverage rate of 60% or more. The positive electrode plate of the present disclosure has an outermost layer containing coated hollow positive electrode active material secondary particles in which hollow positive electrode active material secondary particles are coated with a coating layer containing a first binder resin. Therefore, in this outermost layer, metal ions cannot easily pass through the inside of the coated hollow positive electrode active material secondary particles. As a result, when metal ions move in the vicinity of the hollow positive electrode active material secondary particles in the outermost layer, they move around the periphery of these secondary particles, and the moving distance of the metal ions in the direction of the separator becomes longer. As a result, it becomes difficult for metal ions to accumulate near the separator, and the chemical short circuit resistance is improved.

[0017] Figure 1 is a conceptual diagram for explaining the movement of metal ions in the outermost layer of the positive electrode active material layer in the positive electrode plate of the present disclosure. As shown in Figure 1, in the outermost layer of the positive electrode active material layer, it is difficult for metal ions 30 to pass through the inside of the hollow positive electrode active material secondary particles 10 (coated hollow positive electrode active material secondary particles) having a coating layer 20. Therefore, since the metal ions 30 move around the periphery of the coated hollow positive electrode active material secondary particles, the moving distance of the metal ions 30 in the direction of the separator becomes longer. As a result, the metal ions 30 are less likely to stay on the downstream side (that is, accumulate near the separator), and dendrites are less likely to be formed. Therefore, chemical short circuit is less likely to occur. On the other hand, Figure 2 is a conceptual diagram for explaining the movement of metal ions in the positive electrode active material layer in a positive electrode plate other than the positive electrode plate of the present disclosure. As shown in Figure 2, the metal ions 30 easily pass through the inside of the hollow positive electrode active material secondary particles 10 that do not have a coating layer 20. Therefore, the moving distance of the metal ions 30 in the direction of the separator becomes shorter, the metal ions 30 are likely to stay on the downstream side, and dendrites are likely to be formed. Therefore, chemical short circuit is likely to occur.

[0018] -Positive electrode current collector- Examples of the positive electrode current collector include those made of Cu, Al, Fe, Co, Ni, Cr, Ni-plated steel, stainless steel, etc. Among them, those made of Al are preferred.

[0019] The thickness of the current collector may be 5 μm to 30 μm.

[0020] - Positive electrode active material layer - In the positive electrode plate of the present disclosure, the positive electrode active material layer has a multilayer structure laminated on the positive electrode current collector. Note that the multilayer structure is a structure having two or more layers. The layer located on the farthest side from the positive electrode current collector in the positive electrode active material layer includes hollow positive electrode active material secondary particles in which particles of the positive electrode active material are aggregated in a hollow shape, and a first binder resin, and the surface of the hollow positive electrode active material secondary particles is covered with a coverage rate of 60% or more. It contains coated hollow positive electrode active material secondary particles having a coating layer.

[0021] (Hollow positive electrode active material secondary particles) In the hollow positive electrode active material secondary particles, particles (primary particles) of the positive electrode active material are aggregated in a hollow shape. Examples of the positive electrode active material include particles made of a lithium composite oxide. Examples of the lithium composite oxide include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganate (LiMn2O4), lithium nickel cobalt aluminum oxide (NCA), lithium nickel cobalt manganate (NCM), and lithium iron phosphate (LiFePO4).

[0022] As long as the hollow positive electrode active material secondary particles have a hollow structure, their shape is not particularly limited, and they may have a shape such as a spherical shape (for example, a true spherical shape, an elliptical spherical shape, etc.).

[0023] Hereinafter, examples of the hollow structure that can be formed by the hollow positive electrode active material secondary particles will be described. The hollow positive electrode active material secondary particles can form a hollow structure in the form shown in FIGS. 4(A) to 4(D), for example. Each positive electrode active material particle 50 (primary particle) shown in FIGS. 4(A) to 4(D) has a shell portion in which the positive electrode active material particles 50 (primary particles) are gathered together, and a hollow portion formed inside the shell portion. It takes the form of hollow positive electrode active material secondary particles 40. As shown in FIGS. 4(A) to 4(D), the positive electrode active material particles 50 (primary particles) can mainly form four types of hollow positive electrode active material secondary particles 40 depending on the direction in which the positive electrode active material particles 50 (primary particles) gather together and the number of particles that gather together. The positive electrode active material particles 50 (primary particles) shown in Fig. 4(A) are oriented to form hollow positive electrode active material secondary particles 40 such that the region of contact with each other is minimized, that is, the specific surface area is increased, and the unevenness on the surface of the shell portion is suppressed to be small. The positive electrode active material particles 50 (primary particles) shown in Fig. 4(B) are oriented to form hollow positive electrode active material secondary particles 40 such that the region of contact with each other is minimized, that is, the specific surface area is increased, and the unevenness on the surface of the shell portion is increased. The positive electrode active material particles 50 (primary particles) shown in Fig. 4(C) are oriented to form hollow positive electrode active material secondary particles 40 such that they overlap and contact each other, that is, the specific surface area is small, and the unevenness on the surface of the shell portion is suppressed to be small. The positive electrode active material particles 50 (primary particles) shown in Fig. 4(D) are oriented to form hollow positive electrode active material secondary particles 40 such that they overlap and contact each other, that is, the specific surface area is small, and the unevenness on the surface of the shell portion is increased.

[0024] The morphology of the hollow positive electrode active material secondary particles preferably takes the forms shown in Fig. 4(A) and Fig. 4(B), and more preferably takes the form of Fig. 4(A).

[0025] The BET specific surface area of the hollow positive electrode active material secondary particles is preferably 1.70 m 2 / g to 2.00 m 2 / g. When the BET specific surface area of the positive electrode active material secondary particles is 1.70 m 2 / g or more, the positive electrode active material secondary particles have a high specific surface area, so the reaction area increases, the internal resistance decreases, and the battery is easily made to have a high output. On the other hand, when the BET specific surface area of the positive electrode active material secondary particles is 2.00 m 2 / g or less, it is easy to suppress the manufacturing cost of the positive electrode. The BET specific surface area of the positive electrode active material secondary particles is more preferably 1.72 m 2 / g to 1.98 m 2 / g, and even more preferably 1.74 m 2 / g to 1.96 m 2 / g.

[0026] The BET specific surface area of the hollow cathode active material secondary particles can be determined, for example, by analyzing the adsorption / desorption isotherm obtained by the adsorption / desorption measurement of nitrogen gas (temperature: -196°C) using the BET (Brenauer-Emmet-Teller) multi-point method. The measurement can be carried out using a known measuring device (for example, a high-precision automatic gas / vapor adsorption amount measuring device "BELSORP MAX" manufactured by BEL Japan, Inc.).

[0027] Also, the particle size of the hollow cathode active material secondary particles may be from 0.1 μm to 30 μm, and may further be from 10 μm to 20 μm.

[0028] The particle size of the hollow cathode active material secondary particles can be determined as follows. A cross-section cut along the thickness direction of the cathode plate is observed with a scanning electron microscope (SEM), and image analysis is performed on any 10 hollow cathode active material secondary particles. Then, the straight-line distance that becomes the longest when connecting one end to the other end in the longitudinal direction of the outer peripheral surface of the hollow cathode active material secondary particles is defined as the particle size of the hollow cathode active material secondary particles.

[0029] (Coating layer) The coating layer contains a first binder resin and coats the surface of the hollow cathode active material secondary particles with a coating rate of 60% or more. Examples of the first binder resin include polyvinylidene fluoride (PVDF), modified polyvinylidene fluoride (modified PVDF), polytetrafluoroethylene (PTFE), etc. Among them, from the viewpoints of excellent adhesiveness, chemical stability, excellent electrolyte swelling property, and suppressing the retention of metal ions on the downstream side (that is, the accumulation near the separator), it is preferable to contain polyvinylidene fluoride (PVDF). The first binder resin may contain other binder resins in addition to the above polyvinylidene fluoride (PVDF).

[0030] The coating layer covers the surface of the hollow cathode active material secondary particles with a coverage rate of 60% or more. Since the coating layer covers the surface of the hollow cathode active material secondary particles with a coverage rate of 60% or more, metal ions cannot easily pass through the inside of the hollow cathode active material secondary particles. Therefore, metal ions are less likely to stay on the downstream side (i.e., accumulate near the separator), and dendrite formation is suppressed. Accordingly, the chemical short-circuit resistance is improved. The coating layer preferably covers the surface of the hollow cathode active material secondary particles with a coverage rate of 65% or more, more preferably with a coverage rate of 70% or more, still more preferably with a coverage rate of 75% or more, and particularly preferably with a coverage rate of 80% or more.

[0031] The coverage rate is obtained by analyzing the cross-section of the positive electrode plate using FE-EPMA (field emission type electron beam microanalyzer, "JXA-8530F" manufactured by JEOL Ltd.) to obtain the concentration distribution of F element. Then, using image analysis software ("Image J"), the ratio of the hollow cathode active material secondary particle part and the ratio of the F element part are calculated respectively, and by dividing the ratio of the F element part by the ratio of the hollow cathode active material secondary particle part, the coverage rate (%) by the first binder resin can be obtained. When the first binder resin is a resin that does not contain F element, the concentration distribution can be obtained from FE-EPMA by performing a pretreatment called Os (osmium) staining method, and the coverage rate can be calculated.

[0032] As a method for forming a coating layer on the surface of the hollow cathode active material secondary particles, for example, a method of kneading the hollow cathode active material secondary particles and the first binder resin contained in the coating layer can be mentioned. In order to make the coverage rate of the hollow cathode active material secondary particles by the coating layer within the above range, when preparing the slurry for forming the outermost layer, it is preferable to knead the hollow cathode active material secondary particles and the first binder resin in advance in a solvent before adding other components (such as conductive agents, etc.) (hereinafter, this pre-kneaded kneading is referred to as "pre-kneading"). Also, the coverage rate can be controlled by adjusting the degree of kneading during the pre-kneading, for example, the kneading time and the kneading strength, etc.

[0033] The average thickness of the coating layer is preferably 10 μm to 30 μm, more preferably 11 μm to 25 μm, and even more preferably 12 μm to 20 μm. When the average thickness of the coating layer is 10 μm or more, in the layer located on the side farthest from the positive electrode current collector in the positive electrode active material layer, it is difficult for metal ions to pass through the inside of the hollow positive electrode active material secondary particles, and it is easy to suppress the retention of metal ions on the downstream side (that is, accumulation near the separator). Therefore, the chemical micro short resistance is likely to be improved. On the other hand, when the average thickness of the coating layer is 30 μm or less, the manufacturing cost of the coated hollow positive electrode active material secondary particles is likely to be reduced.

[0034] The average thickness of the coating layer can be obtained using a scale bar for an image obtained by FE-EPMA (field emission type electron beam microanalyzer, "JXA-8530F" manufactured by JEOL Ltd.). The average thickness of the coating layer can be controlled, for example, by adjusting the ratio of the addition of the first binder resin to the hollow positive electrode active material secondary particles when kneading the hollow positive electrode active material secondary particles and the first binder resin contained in the coating layer.

[0035] The content C of the hollow positive electrode active material secondary particles in the outermost layer A The content C of the first binder resin relative to R The ratio (C R / C A ×100 (mass%)) is preferably 0.5 mass% to 5.0 mass%, and more preferably 1.0 mass% to 4.0 mass%.

[0036] (Outermost layer) The outermost layer may contain other additives in addition to the coated hollow positive electrode active material secondary particles. For example, it may contain a conductive agent, and examples of the conductive agent include acetylene black, ketjen black, vapor grown carbon fiber (VGCF (registered trademark)), carbon nanotube (CNT), etc. The content rate of the conductive agent may be, for example, 0.5 mass% to 1.0 mass% with respect to the total mass of the outermost layer.

[0037] The average thickness of the outermost layer (the layer located on the side farthest from the positive electrode current collector among the positive electrode active material layers) is preferably 60 μm to 80 μm. When the average thickness of the outermost layer is 60 μm or more, in the positive electrode active material layer, the region where metal ions hardly pass through the inside of the hollow positive electrode active material secondary particles becomes larger, and the moving distance of the metal ions in the direction of the separator becomes longer. As a result, the retention of metal ions on the downstream side (that is, accumulation near the separator) is easily suppressed, and the chemical micro short circuit resistance is easily improved. Also, when the average thickness of the outermost layer is 80 μm or less, it is easy to suppress the manufacturing cost.

[0038] In the outermost layer, metal ions cannot easily pass through the inside of the coated hollow positive electrode active material secondary particles and move around the coated hollow positive electrode active material secondary particles. As a result, the moving distance of the metal ions in the direction of the separator becomes longer, and it becomes difficult for metal ions to accumulate near the separator. Here, the degree of bending (that is, the degree to which the moving distance becomes longer) when the metal ions move around the hollow positive electrode active material secondary particles in the outermost layer is cited as an index. The degree of bending in the outermost layer is preferably 2.2 to 10.0, more preferably 2.2 to 7.5, and even more preferably 2.2 to 5.5. When the degree of bending in the outermost layer is 2.2 or more, since metal ions cannot easily pass through the inside of the hollow active material secondary particles, it is difficult for metal ions to stay on the downstream side (that is, accumulate near the separator), and the chemical micro short circuit resistance is easily improved. On the other hand, when the degree of bending is 10.0 or less, it is easy to suppress the manufacturing cost.

[0039] The degree of bending in the outermost layer is obtained as follows. FIG. 3 is an explanatory diagram for explaining the degree of bending in the outermost layer of the positive electrode plate of the present disclosure. As shown in FIG. 3, the outermost layer includes hollow positive electrode active material secondary particles 10 (coated hollow positive electrode active material secondary particles) coated with a coating layer 20. The degree of bending L is obtained from L = A × (σ / h), where A is the average thickness of the outermost layer, h is the average thickness per particle of the hollow positive electrode active material secondary particles 10 (coated hollow positive electrode active material secondary particles) coated with the coating layer 20 (the average thickness in the direction parallel to the thickness direction of the outermost layer), and σ is the average value of the conduction paths obtained by following the outer periphery of the coating layer of the hollow positive electrode active material secondary particles 10 (coated hollow positive electrode active material secondary particles) coated with the coating layer 20.

[0040] When calculating the degree of bending in the outermost layer, a scanning electron microscope (SEM) photograph of the cross-section of the outermost layer of the positive electrode plate is taken, and any sample of the coated hollow positive electrode active material secondary particles 10 is used as the measurement target. The SEM photograph is taken so that the entire thickness direction of the outermost layer is included. The average thickness A of the outermost layer is calculated by measuring the thickness of the outermost layer at any five locations in the taken SEM photograph and obtaining the average value. The average thickness h per particle of the hollow positive electrode active material secondary particles 10 (coated hollow positive electrode active material secondary particles) coated with the coating layer 20 is calculated by measuring the length in the direction parallel to the thickness direction of the outermost layer for any sample of the coated hollow positive electrode active material secondary particles 10 that is the measurement target in the SEM photograph, that is, the distance from one end to the other end of the coating layer 20 in the thickness direction of the outermost layer, and obtaining the average value. The average value σ of the conduction paths obtained by following the outer periphery of the coating layer of the hollow positive electrode active material secondary particles 10 (coated hollow positive electrode active material secondary particles) coated with the coating layer 20 is calculated by calculating half of the outer edge length (that is, 1 / 2 of the outer periphery length of the secondary particles 10 in the SEM photograph) for any sample of the coated hollow positive electrode active material secondary particles 10 that is the measurement target in the SEM photograph and obtaining the average value.

[0041] (Layers other than the outermost layer) In the positive electrode active material layer, a layer other than the outermost layer (the layer located on the positive electrode current collector and farthest from the positive electrode current collector among the positive electrode active material layers) is applied with a layer containing a positive electrode active material. Examples of the positive electrode active material contained in the layer other than the outermost layer include particles composed of a lithium composite oxide. Examples of the lithium composite oxide include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganate (LiMn2O4), lithium nickel cobalt aluminum oxide (NCA), lithium nickel cobalt manganate (NCM), lithium iron phosphate (LiFePO4), and the like. The positive electrode active material contained in the layer other than the outermost layer is preferably secondary particles in which particles (primary particles) of the positive electrode active material are aggregated, and more preferably hollow positive electrode active material secondary particles in which particles (primary particles) of the positive electrode active material are aggregated in a hollow shape.

[0042] The layer other than the outermost layer may contain a second binder resin in addition to the positive electrode active material. The layer other than the outermost layer preferably contains hollow positive electrode active material secondary particles and a second binder resin, and more preferably, the coverage rate of the hollow positive electrode active material secondary particles by the second binder resin is 0% to 40%.

[0043] Examples of the second binder resin include the same ones as those exemplified for the first binder resin. From the viewpoint of not hindering the diffusibility of lithium ions, the coverage rate of the hollow positive electrode active material secondary particles by the second binder resin in the layer other than the outermost layer is preferably 0% to 40%, more preferably 0% to 25%, and still more preferably 0% to 15%. The coverage rate of the hollow positive electrode active material secondary particles by the second binder resin can be determined by the same method as the method described above.

[0044] In the layer other than the outermost layer, as a method of setting the coating rate of the hollow cathode active material secondary particles with the second binder resin within the above range, for example, a method of adjusting the degree of kneading of the hollow cathode active material secondary particles and the second binder resin can be mentioned. Specifically, when preparing a slurry for forming a layer other than the outermost layer, instead of performing pre-kneading in which only the hollow cathode active material secondary particles and the second binder resin are pre-kneaded in a solvent, kneading is performed in a state where other components (for example, a conductive agent, etc.) are also added, whereby the coating rate can be set within the above range. Further, the coating rate can also be controlled by adjusting the degree of kneading during the kneading, for example, the kneading time and the kneading strength, etc.

[0045] In the layer other than the outermost layer, the content C of the hollow cathode active material secondary particles AA with respect to the content C of the second binder resin RR of the ratio (C RR / C AA × 100 (mass %)) is preferably 0.2 mass % to 3.0 mass %, and more preferably 0.5 mass % to 2.0 mass %.

[0046] Further, the layer other than the outermost layer may contain other additives. For example, it may contain a conductive agent, and examples of the conductive agent include the same ones as the conductive agents listed as additives that may be contained in the outermost layer. The content rate of the conductive agent in the layer other than the outermost layer may also be in the same range as the content rate in the outermost layer.

[0047] The layer other than the outermost layer may be a single layer or a layer composed of a plurality of layers.

[0048] <Method for manufacturing a positive electrode plate> A method for manufacturing the positive electrode plate of the present disclosure will be described. For example, when manufacturing a positive electrode plate having a two-layer positive electrode active material layer with an outermost layer and a layer other than the outermost layer as the positive electrode plate of the present disclosure, an example will be described. First, a layer other than the outermost layer is formed on the positive electrode current collector. Specifically, a first slurry containing a positive electrode active material (preferably secondary particles in which primary particles of the positive electrode active material are aggregated, more preferably hollow positive electrode active material secondary particles in which the particles of the positive electrode active material are aggregated in a hollow shape) and a solvent is prepared. The first slurry may contain a second binder resin, and may further contain other additives (for example, a conductive agent). By coating, drying, and pressing the first slurry onto the positive electrode current collector, a layer other than the outermost layer is formed. Next, a second slurry containing hollow positive electrode active material secondary particles in which the particles of the positive electrode active material are aggregated in a hollow shape, a first binder resin, and a solvent is prepared. The second slurry may contain other additives (for example, a conductive agent). By further coating, drying, and pressing the second slurry onto the layer other than the outermost layer, the outermost layer is formed. Thereby, the positive electrode plate of the present disclosure is obtained.

[0049] The first slurry can be prepared by kneading a positive electrode active material (preferably secondary particles of the positive electrode active material, more preferably hollow positive electrode active material secondary particles), a solvent, optionally the second binder resin, and optionally other additives. The second slurry can be prepared by kneading the hollow positive electrode active material secondary particles, the first binder resin, a solvent, and optionally other additives.

[0050] In addition, when controlling the coating rate of the hollow positive electrode active material secondary particles by the second binder resin in the layer other than the outermost layer within the range of 0% to 40%, for example, when preparing the first slurry, it is preferable to knead without performing pre-kneading in which only the hollow positive electrode active material secondary particles and the second binder resin are pre-kneaded in a solvent, and to knead in a state where other components (for example, a conductive agent, etc.) are also added. In addition, in the outermost layer, in order to form a coating layer on the surface of the hollow cathode active material secondary particles, that is, to control the coating rate of the hollow cathode active material secondary particles with the first binder resin within a range of 60% or more, for example, when preparing the second slurry, it is preferable to perform preliminary kneading in a solvent by kneading only the hollow cathode active material secondary particles and the first binder resin in advance before adding other components (such as conductive agents, etc.). Also, the coating rate can be controlled by adjusting the degree of kneading during the preliminary kneading, such as the kneading time and the kneading strength, etc.

[0051] Examples of the solvent include N-methylpyrrolidone (NMP), dimethylformamide (DMF), dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), etc. Among them, N-methylpyrrolidone (NMP) is preferable as the solvent. In addition, the first slurry and the second slurry may contain conductive agents such as acetylene black, ketjen black, vapor-grown carbon fiber (VGCF (registered trademark)), carbon nanotube (CNT), etc. as other additives.

[0052] Kneading can be carried out using a planetary mixer, sand mill, ball mill, planetary mill, roll mill, extruder, disperser, etc. The kneading time can be appropriately selected, but when the kneading time is lengthened, various coating rates can be improved.

[0053] Coating can be carried out by methods such as knife coating, gravure coating, etc. The coating weight per unit area (g / cm 2 ) is not particularly limited as long as the effects of the present disclosure are not hindered. For example, it may be 1 g / cm 2 ~10 g / cm 2 . Also, the coating may be carried out so that the electrode density of the positive electrode plate is 1.00 g / cm 3 ~3.00 g / cm 3 .

[0054] Drying can be appropriately selected from natural drying, vacuum drying, and heat drying. For example, when the solvent contained in the first slurry or the second slurry is NMP, it may be heat drying at 80°C to 135°C.

[0055] Drying may be performed by roll pressing, cold isostatic pressing (CIP), etc.

[0056] As described above, the positive electrode plate of the present disclosure can be manufactured.

[0057] <Lithium-ion secondary battery> The lithium-ion secondary battery of the present disclosure includes the positive electrode plate.

[0058] The lithium-ion secondary battery of the present disclosure preferably has a structure in which the positive electrode plate and the negative electrode plate are laminated via a separator.

[0059] The negative electrode plate may include a negative electrode current collector and a negative electrode active material layer laminated on the negative electrode current collector. Examples of the negative electrode current collector include the same ones as those exemplified for the positive electrode plate. Among them, those made of Cu are preferable. The negative electrode active material layer may include a negative electrode active material, a conductive agent, and a binder resin.

[0060] Examples of the negative electrode active material include carbon-based active materials such as graphite, oxide-based active materials such as lithium titanate (e.g., Li4Ti5O 12 ), and Si-based active materials such as elemental Si. The particle size of the negative electrode active material may be 0.1 μm to 100 μm. The BET specific surface area of the negative electrode active material may be 0.1 m 2 / g to 1,500 m 2 / g.

[0061] Examples of the conductive agent include the same ones as those described above for the positive electrode plate. The content of the conductive agent may be 3% by mass to 5% by mass based on the negative electrode active material.

[0062] Examples of the binder resin are the same as those described above for the positive electrode plate. The content of the binder resin may be 0.1% to 5% by mass based on the negative electrode active material.

[0063] The separator may be a resin sheet such as polyethylene (PE) or polypropylene (PP).

[0064] The lithium-ion secondary battery of the present disclosure may contain an electrolytic solution, and the electrolytic solution may contain a solid electrolyte and a solvent. Examples of the solid electrolyte include LiPF6, LiBF4, LiAsF6, Li(CF3SO2)2N, Li(C2F5SO2)2N, LiTaF6, LiClO4, LiCF3SO3, etc. Examples of the solvent include cyclic carbonate solvents such as ethylene carbonate (EC) and propylene carbonate (PC); chain carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). The concentration of the electrolytic solution may be 0.1 to 1 mol / L.

Example

[0065] Hereinafter, the embodiments of the present disclosure will be described in more detail by way of examples. However, the embodiments of the present disclosure are not limited to the following examples.

[0066] <Example 1> First, a mixture of lithium nickel cobalt manganese oxide (⇒ positive electrode active material) (manufactured by Sumitomo Metal Mining Co., Ltd.), which is a positive electrode active material having hollow-structured secondary particles, carbon nanotubes (CNT) (multi-walled carbon nanotubes manufactured by Sigma Aldrich), and PVDF (Solef 5130 manufactured by Solvay) (positive electrode active material: CNT: PVDF = 48.5: 0.4: 0.5 (mass ratio)) was added with NMP (manufactured by Mitsubishi Chemical Corporation), and adjusted so that the solid content (the amount of components other than NMP) was 62% by mass, and kneaded with a disperser to prepare a first slurry.

[0067] Next, to a mixture of lithium nickel cobalt manganese oxide (⇒ cathode active material) (manufactured by Sumitomo Metal Mining Co., Ltd.) having a hollow structure secondary particle and PVDF (Solef 5130 manufactured by Solvay) (cathode active material:PVDF = 48.5:1.0 (mass ratio)), NMP (manufactured by Mitsubishi Chemical Corporation) was added and adjusted so that the solid content (the amount of components other than NMP) became 70% by mass, and kneaded (pre-kneaded) with a ball mill (Magnetic Ball Mill φ105×140 mm manufactured by AS ONE Corporation) to prepare a second slurry (intermediate). The kneading conditions were: ball material: ceramic, ball particle size: φ15 mm, ball amount: 30% by mass based on the mass of the cathode active material and PVDF, mill rotation speed: 300 r / min, and the kneading time was 8 minutes. Next, to this second slurry (intermediate), carbon nanotubes (CNT) (multi-walled carbon nanotubes manufactured by Sigma Aldrich) were added so that the ratio of cathode active material:CNT:PVDF = 48.5:0.4:1.0 (mass ratio), and kneaded with a disper to prepare a second slurry (final).

[0068] Next, the first slurry was coated (coating weight: 2.25 g / cm 2 ) on an Al foil (thickness 12 μm), dried, and pressed with a press machine to form layers other than the outermost layer. Then, on top of that, the second slurry (final) was coated (coating weight: 2.31 g / cm 2 ), dried, and pressed with a press machine to form the outermost layer, obtaining a cathode plate.

[0069] <Example 2> A cathode plate was obtained in the same manner as in Example 1, except that the kneading time in the pre-kneading with a ball mill when preparing the second slurry (intermediate) was 10 minutes.

[0070] <Comparative Example 1> A cathode plate was obtained in the same manner as in Example 1, except that the kneading time in the pre-kneading with a ball mill when preparing the second slurry (intermediate) was 4 minutes.

[0071] <Comparative Example 2> Lithium nickel cobalt manganese oxide (⇒ positive electrode active material) (manufactured by Sumitomo Metal Mining Co., Ltd.) as a positive electrode active material having hollow-structured secondary particles, carbon nanotubes (CNT) (manufactured by Sigma Aldrich, "multi-walled carbon nanotubes"), and a mixture of PVDF (manufactured by Solvay, "Solef 5130") (positive electrode active material: CNT: PVDF = 97.5: 0.8: 1.5 (mass ratio)), NMP (manufactured by Mitsubishi Chemical Corporation) was added and adjusted so that the solid content (the amount of components other than NMP) was 62% by mass, and kneaded with a disper to prepare a first slurry. A positive electrode plate was obtained in the same manner as in Example 1 except that the first slurry was changed to the above and the outermost layer (the second slurry (final)) was not provided.

[0072] <Measurement and Evaluation> For each positive electrode plate obtained in the examples and comparative examples, the first binder resin coating rate (%) in the outermost layer, the second binder resin coating rate (%) in the layers other than the outermost layer, the average thickness (μm) of the coating layer in the outermost layer, the degree of bending in the outermost layer, and the chemical short-circuit resistance were measured and evaluated as follows.

[0073] - First binder resin coating rate (%) and second binder resin coating rate (%) - Using FE-EPMA (field emission type electron beam microanalyzer, "JXA-8530F" manufactured by JEOL Ltd.), the cross-section of the positive electrode plate was analyzed to obtain the concentration distribution of F element. Then, using image analysis software ("Image J"), the ratio of the hollow positive electrode active material secondary particle portion and the ratio of the F element portion were calculated respectively. The value obtained by dividing the ratio of the F element portion by the ratio of the hollow positive electrode active material secondary particle portion was calculated as the first binder resin coating rate (%) and the second binder resin coating rate (%) respectively. The results are shown in Table 1.

[0074] - Average thickness (μm) of the coating layer - The average thickness (μm) of the coating layer by the first binder resin was obtained using a scale bar for the image obtained by FE-EPMA and used as the value. The results are shown in Table 1.

[0075] - Degree of bending - A scanning electron microscope (SEM) photograph of the cross-section of the outermost layer of the positive electrode plate was taken using a scanning electron microscope (SEM) (manufactured by JEOL Ltd., model "JSM-7401F"). For each of 10 samples of arbitrarily selected coated hollow positive electrode active material secondary particles, the average thickness of the outermost layer was designated as A, the average thickness per particle (average thickness in the direction parallel to the thickness direction of the outermost layer) of the coated hollow positive electrode active material secondary particles 10 (coated hollow positive electrode active material secondary particles) coated with the coating layer 20 was designated as h, and the average value of the conduction paths obtained by following the outer periphery of the coating layer of the coated hollow positive electrode active material secondary particles 10 (coated hollow positive electrode active material secondary particles) was designated as σ. The degree of bend L = A × (σ / h) was determined. The results are shown in Table 1.

[0076] -Chemical micro short circuit resistance- For each positive electrode plate obtained in the examples and comparative examples, a fine stainless steel foil was embedded as a foreign object, CCCV (constant current constant voltage) charging was performed at 4.0 V, and aging was performed at 60°C for 20 hours. The chemical micro short circuit resistance was evaluated based on the following criteria for the voltage drop during aging. The results are shown in Table 1. ○... Less than 0.01 V △... 0.01 V or more and 0.1 V or less ×... More than 0.1 V

[0077]

Table 1

[0078] As shown in Table 1, the positive electrode plate of the present disclosure has a large degree of bend in the outermost layer and is excellent in chemical micro short circuit resistance. In the positive electrode plate in which the first binder resin coating rate (%) of the hollow positive electrode active material secondary particles contained in the outermost layer is low or the coating layer by the first binder resin is not present, the degree of bend is low and the chemical micro short circuit resistance is also poor.

Description of reference numerals

[0079] 10, 40 Hollow positive electrode active material secondary particles 20 Coating layer 30 Metal ions 50 Positive electrode active material particles (primary particles) A Average thickness (outermost layer) h Average thickness (coated hollow cathode active material secondary particles) σ Conductive path

Claims

1. A positive electrode current collector, and a positive electrode active material layer having a multilayer structure laminated on the positive electrode current collector, wherein the layer located on the side farthest from the positive electrode current collector in the positive electrode active material layer includes hollow positive electrode active material secondary particles in which particles of the positive electrode active material are aggregated in a hollow shape, and a coating layer containing a first binder resin and coating the surface of the hollow positive electrode active material secondary particles with a coverage rate of 60% or more. A positive electrode plate.

2. Layers other than the layer located on the side farthest from the positive electrode current collector in the positive electrode active material layer include hollow positive electrode active material secondary particles in which particles of the positive electrode active material are aggregated in a hollow shape, and a second binder resin, and the coverage rate of the hollow positive electrode active material secondary particles by the second binder resin is 0% to 40%. The positive electrode plate according to Claim 1.

3. The average thickness of the coating layer of the coated hollow positive electrode active material secondary particles contained in the layer located on the side farthest from the positive electrode current collector in the positive electrode active material layer is 10 μm to 30 μm. The positive electrode plate according to Claim 1.

4. The first binder resin includes polyvinylidene fluoride (PVDF). The positive electrode plate according to Claim 1.

5. The average thickness of the layer located on the side farthest from the positive electrode current collector in the positive electrode active material layer is 60 μm to 80 μm. The positive electrode plate according to Claim 1.

6. A lithium ion secondary battery including the positive electrode plate according to any one of Claims 1 to 5.

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