Positive electrode for lithium-ion secondary battery, lithium-ion secondary battery, and method for manufacturing a positive electrode for a lithium-ion secondary battery

Abstract

To provide a positive electrode for lithium-ion secondary batteries, a lithium-ion secondary battery, and a method for manufacturing a positive electrode for lithium-ion secondary batteries that suppresses minute short circuits caused by the inclusion of metallic foreign matter. [Solution] The positive electrode plate 10 comprises a positive electrode current collector 11 and a positive electrode composite material layer 20 formed on the positive electrode current collector 11. The positive electrode composite material layer 20 comprises a first layer 21 located on the positive electrode current collector 11 side and a second layer 22 located on the side furthest from the positive electrode current collector 11. Both the first layer 21 and the second layer 22 contain positive electrode active material particles 30 with a hollow structure having a shell portion 31, a hollow portion 32 formed inside the shell portion 31, and through holes 33 that connect the outside of the shell portion and the hollow portion 32. The second layer 22 contains a first conductive material 41 having an average diameter larger than the average diameter of the through holes 33 and arranged to block the through holes 33.

Description

[Technical Field]

[0001] This disclosure relates to a positive electrode for lithium-ion secondary batteries, a lithium-ion secondary battery, and a method for manufacturing a positive electrode for lithium-ion secondary batteries. [Background technology]

[0002] Rechargeable batteries are widely used as so-called portable power sources for personal computers and mobile devices, as well as power sources for vehicle propulsion. In particular, lithium-ion rechargeable batteries, which are lightweight and offer high energy density, are suitably used as high-output power sources for vehicles such as electric vehicles, hybrid vehicles, and plug-in hybrid vehicles. A lithium-ion rechargeable battery is a rechargeable battery that can be charged and discharged by the movement of lithium ions in an electrolyte between positive and negative electrodes containing an active material that absorbs and releases lithium ions.

[0003] For example, Patent Document 1 discloses an anode for a lithium-ion secondary battery comprising a current collector and an active material-containing layer formed on the current collector. The active material-containing layer of this anode is composed of a top surface layer located on the side furthest from the current collector and a lower layer consisting of one or more layers located between the top surface layer and the current collector, wherein the curvature of the top surface layer is greater than that of the lower layer. The curvature here is a value obtained by L' / L, where L is the vertical straight line A in the cross-section of each layer, and L' is the sum of the maximum diameters in the lateral direction (perpendicular to straight line A) of the active material particles that overlap with straight line A. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2008-251250 [Overview of the project] [Problems that the invention aims to solve]

[0005] When metallic foreign matter is introduced into the positive electrode of a lithium-ion secondary battery, metal ions originating from the foreign matter may migrate to the negative electrode, form dendrites, and cause a small chemical short circuit. Therefore, to suppress small short circuits caused by the incorporation of metallic foreign matter, it is desirable to suppress the migration of metal ions to the negative electrode. To suppress the migration of metal ions to the negative electrode, it is conceivable to promote the diffusion of metal ions in the in-plane direction of the separator within the separator by increasing the curvature of the separator interposed between the positive and negative electrodes or by increasing the thickness of the separator.

[0006] However, increasing the curvature of the separator or increasing the thickness of the separator presents a trade-off: the film resistance of the separator increases. Therefore, to suppress the movement of metal ions toward the negative electrode, it is considered effective to promote the diffusion of metal ions in the in-plane direction of the positive electrode.

[0007] Incidentally, the positive electrode of lithium-ion secondary batteries is expected to improve lithium ion diffusion and thereby increase power output by using hollow-structured positive electrode active material particles. Furthermore, in order to further increase the power output of the positive electrode, hollow-structured positive electrode active material particles may be provided with through-holes that connect the inside and outside of the particle. When hollow-structured positive electrode active material particles with through-holes are used, the diffusion of lithium ions into the interior of the positive electrode active material particle is promoted, and the solid-liquid interface inside the positive electrode active material particle can be made into a reaction surface, thereby improving the power output of the positive electrode.

[0008] On the other hand, when using positive electrode active material particles with a hollow structure and through-holes, it is thought that metal ions not only move outside the positive electrode active material particles but also pass through the interior of the positive electrode active material particles via the through-holes. If metal ions can pass through the interior of the positive electrode active material particles via these through-holes, then relying solely on the curvature of the active material-containing layer (curvature ratio of the positive electrode composite layer) as an indicator may result in insufficient diffusion of metal ions in the in-plane direction of the positive electrode. Therefore, there was a problem in that minute short circuits due to the inclusion of metal foreign matter were likely to occur.

[0009] This disclosure is made to solve such problems and aims to provide a positive electrode for lithium-ion secondary batteries, a lithium-ion secondary battery, and a method for manufacturing a positive electrode for lithium-ion secondary batteries that suppresses minute short circuits caused by the inclusion of metallic foreign matter. [Means for solving the problem]

[0010] A positive electrode for a lithium-ion secondary battery according to one embodiment comprises a positive electrode current collector and a positive electrode composite layer formed on the positive electrode current collector, wherein the positive electrode composite layer comprises a first layer located on the positive electrode current collector side and a second layer located on the side furthest from the positive electrode current collector, and both the first and second layers contain positive electrode active material particles having a shell portion, a hollow portion formed inside the shell portion, and through holes connecting the outside of the shell portion and the hollow portion, and the second layer contains a first conductive material having an average diameter larger than the average diameter of the through holes and arranged to block the through holes.

[0011] A lithium-ion secondary battery according to one embodiment has the above-described positive electrode for lithium-ion secondary batteries.

[0012] A method for manufacturing a positive electrode for a lithium-ion secondary battery according to one embodiment includes a mixing step of obtaining a mixture by mixing positive electrode active material particles having a hollow structure with a shell portion, a hollow portion formed inside the shell portion, and through holes connecting the outside of the shell portion and the hollow portion, and a first conductive material having an average diameter larger than the average diameter of the through holes, and a coating step of forming a second layer on the first layer by coating a second layer forming composition containing the mixture onto a first layer containing positive electrode active material particles formed on a positive electrode current collector. [Effects of the Invention]

[0013] This disclosure provides a positive electrode for lithium-ion secondary batteries, a lithium-ion secondary battery, and a method for manufacturing a positive electrode for lithium-ion secondary batteries that suppresses minute short circuits caused by the inclusion of metallic foreign matter. [Brief explanation of the drawing]

[0014] [Figure 1] It is a diagram schematically showing a cross section of a positive electrode plate according to Embodiment 1. [Figure 2] It is a diagram schematically showing the appearance of positive electrode active material particles included in the positive electrode plate shown in FIG. 1. [Figure 3] It is a flowchart showing a method for manufacturing a positive electrode plate according to Embodiment 1. [Figure 4] It is a diagram schematically showing a cross section of a lithium ion secondary battery according to Embodiment 1. [Figure 5] It is a diagram for explaining the configuration of the first layer in the example. [Figure 6] It is a diagram for explaining the configuration of the second layer in the example. [Figure 7] It is a diagram schematically showing the periphery of positive electrode active material particles included in the second layer of the positive electrode plate. [Figure 8] It is a diagram showing the measurement results of the electronic resistance of the positive electrode plate, the measurement results of the battery resistance, and the calculation results of the surface diffusion distance of metal ions. [Figure 9] It is a cross-sectional view of a positive electrode plate for explaining the surface diffusion distance of metal ions.

Mode for Carrying Out the Invention

[0015] Embodiment 1 Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. However, the present disclosure is not limited to the following embodiments. What is shown in the figures is a part of the whole, and many other configurations not shown are actually included. Also, for clarity of explanation, the following description and drawings are appropriately simplified. In the following description, the same or equivalent elements are denoted by the same reference numerals, and overlapping descriptions are omitted.

[0016] (Positive electrode plate) Referring to FIG. 1, an example of a positive electrode plate 10, which is a positive electrode for a lithium ion secondary battery according to Embodiment 1, will be described. FIG. 1 is a diagram schematically showing a cross section of a positive electrode plate according to Embodiment 1.

[0017] As shown in Figure 1, the positive electrode plate 10 includes a positive electrode current collector 11 and a positive electrode composite layer 20 formed on the positive electrode current collector 11. The positive electrode current collector 11 is made of a conductive material such as a metal with good conductivity. The material of the positive electrode current collector 11 is not particularly limited, but examples include aluminum, aluminum alloy, copper, copper alloy, nickel, titanium, stainless steel, etc. Among these, aluminum is preferably used. The shape of the positive electrode current collector 11 may be, for example, foil-like or mesh-like. The thickness of the positive electrode current collector 11 may be 10 μm or more and 15 μm or less.

[0018] The positive electrode composite layer 20 has a first layer 21 located on the positive electrode current collector 11 side and a second layer 22 located on the side furthest from the positive electrode current collector 11. The first layer 21 may be a single layer or a multi-layer structure of two or more layers. The thickness of the positive electrode composite layer 20 may be 10 μm or more and 50 μm or less.

[0019] The first layer 21 and the second layer 22 both contain hollow positive electrode active material particles 30. The positive electrode active material particles 30 contain a lithium transition metal oxide having a layered crystalline structure. The lithium transition metal oxide contains one or more predetermined transition metal elements in addition to Li (lithium). Preferably, the transition metal elements contained in the lithium transition metal oxide are at least one of Ni, Co, and Mn. A preferred example of the lithium transition metal oxide is a lithium transition metal oxide containing all of Ni, Co, and Mn.

[0020] The positive electrode active material particles 30 may contain, in addition to transition metal elements (i.e., at least one of Ni, Co, and Mn), one or more additional elements. The additional elements may include any of the elements belonging to Group 1 (alkali metals such as sodium), Group 2 (alkaline earth metals such as magnesium and calcium), Group 4 (transition metals such as titanium and zirconium), Group 6 (transition metals such as chromium and tungsten), Group 8 (transition metals such as iron), Group 13 (metalloid elements such as boron or metals such as aluminum), and Group 17 (halogens such as fluorine) of the periodic table.

[0021] In a preferred embodiment, the positive electrode active material particles 30 may have a composition (average composition) represented by the following general formula (1). Li1+ x Ni y Co z Mn (1-y-z) MA α MB β O2…Formula (1)

[0022] In the above formula (1), x can be a real number satisfying 0 ≦ x ≦ 0.2. y can be a real number satisfying 0.1 < y < 0.6. z can be a real number satisfying 0.1 < z < 0.6. MA is at least one metal element selected from W, Cr, and Mo, and α is a real number satisfying 0 < α ≦ 0.01 (typically 0.0005 ≦ α ≦ 0.01, for example 0.001 ≦ α ≦ 0.01). MB is one or more elements selected from the group consisting of Zr, Mg, Ca, Na, Fe, Zn, Si, Sn, Al, B, and F, and β can be a real number satisfying 0 ≦ β ≦ 0.01. β may be substantially 0 (that is, an oxide substantially free of MB). In the chemical formula showing the layered lithium transition metal oxide, for the sake of convenience, the composition ratio of O (oxygen) is shown as 2, but this value should not be strictly interpreted and some compositional variations (typically included in the range of 1.95 or more and 2.05 or less) can be tolerated.

[0023] The positive electrode active material particles 30 have a particle form of a hollow structure. The positive electrode active material particles 30 can typically be substantially spherical, slightly distorted spherical, etc. The positive electrode active material particles 30 have a shell portion 31, a hollow portion 32 formed inside the shell portion 31, and through holes 33 that communicate the outside of the shell portion 31 and the hollow portion 32.

[0024] The shell portion 31 is formed, for example, by a plurality of primary particles 34. More specifically, the shell portion 31 is formed in a substantially spherical shell shape by the aggregation of a plurality of primary particles 34 of lithium transition metal oxide. That is, the positive electrode active material particle 30 is a secondary particle formed by the aggregation of primary particles 34. Here, primary particles 34 refer to particles that can be considered as unit particles (ultimate particles) based on their apparent geometric form. In the positive electrode active material particle 30 disclosed herein, the primary particles 34 are typically aggregates of crystallites of lithium transition metal oxide. The shape of the positive electrode active material particle 30 can be observed using SEM (Scanning Electron Microscope) images. The hollow portion 32 is a space larger than the gaps between adjacent secondary particles.

[0025] The average particle size of the positive electrode active material particles 30 is preferably about 2 μm or more, and more preferably 3 μm or more. If the average particle size of the positive electrode active material particles 30 is too small, the volume of the hollow portion 32 will also be small, and the amount of electrolyte stored in the hollow portion 32 will also be small. Furthermore, from the viewpoint of productivity, the average particle size of the positive electrode active material particles 30 is preferably 25 μm or less, and more preferably 15 μm or less. In one preferred embodiment, the average particle size of the positive electrode active material particles 30 is 3 μm or more and 10 μm or less.

[0026] In this embodiment, "average particle size" refers to the median diameter (D) corresponding to 50% of the cumulative particle size distribution based on volume. 50 The average particle size can be determined as (50% volume average particle size). For average particle sizes of approximately 1 μm or more, it can be determined by laser diffraction / light scattering. For average particle sizes of approximately 1 μm or less, it can be determined by dynamic light scattering (DLS).

[0027] The through-hole 33 is formed to penetrate the shell portion 31. The through-hole 33 allows electrolyte to flow back and forth between the hollow portion 32 and the outside of the shell portion 31. The through-hole 33 is formed by gaps provided between a plurality of primary particles 34 that make up the shell portion 31. By providing the through-hole 33 in the shell portion 31, external electrolyte can easily flow into the hollow portion 32 through the through-hole 33, and electrolyte in the hollow portion 32 can easily flow out to the outside through the through-hole 33. As a result, the electrolyte in the hollow portion 32 is replaced appropriately.

[0028] Furthermore, since electrolyte is stored in the hollow portion 32, electrolyte depletion, which would result in a shortage of electrolyte in the positive electrode composite layer 20, is less likely to occur. Since lithium-ion secondary batteries charge and discharge by the movement of lithium ions, by facilitating the exchange of electrolyte between the hollow portion 32 and the outside of the shell portion 31, the primary particles 34 facing the hollow portion 32 can be more actively utilized for charging and discharging.

[0029] The number of through-holes 33 in the positive electrode active material particles 30 is preferably about 1 to 10 (for example, 1 to 5) on average per particle of the positive electrode active material 30. If the average number of through-holes 33 is too large, it may become difficult to maintain the hollow shape. Also, while a larger average number of through-holes allows the electrolyte to circulate more easily, a larger average number of through-holes 33 reduces the amount of positive electrode active material per unit volume, thus lowering the energy density. For this reason, it is preferable to keep the number of through-holes 33 to the minimum necessary.

[0030] The average diameter of the through-holes 33 provided in the positive electrode active material particles 30 is preferably, for example, 10 nm or more and 100 nm or less. If the average diameter of the through-holes 33 is 10 nm or more, the through-holes 33 can function more effectively as flow paths for the electrolyte. If the average diameter of the through-holes 33 is larger than 100 nm, the porosity of the shell portion 31 relative to the whole increases, which may reduce the strength of the shell portion 31. The average diameter of the through-holes 33 can be determined by taking the average of the diameters of the narrowest parts of about 10 through-holes 33 arbitrarily selected from an SEM image of the cross-section of the positive electrode active material particles 30.

[0031] Thus, the positive electrode composite layer 20 of the positive electrode plate 10 has positive electrode active material particles 30 with a hollow structure and through holes 33. Within such a positive electrode plate 10, ions in the electrolyte can pass through the hollow portion 32 via the through holes 33, which can reduce the curvature of the ion transfer path P shown by the black arrow in Figure 1. The curvature represents the degree of bending of the ion transfer path P.

[0032] On the other hand, if metallic foreign matter is mixed into the positive electrode plate 10, the smaller the curvature of the ion transfer path P, the shorter the distance that metal ions travel to the surface side of the positive electrode composite layer 20. As a result, the diffusion of metal ions in the in-plane direction of the positive electrode plate 10 may become insufficient. Consequently, there is a problem that minute chemical short circuits due to the mixing of metallic foreign matter are likely to occur.

[0033] Therefore, the second layer 22 of the positive electrode composite layer 20, which is located on the side furthest from the positive electrode current collector 11, includes a first conductive material 41 that has an average diameter (average particle size) larger than the average diameter of the through-hole 33 and is arranged to block the through-hole 33.

[0034] The first conductive material 41 blocks the through-holes 33, increasing the curvature of the ion movement path P within the second layer 22 compared to the case where the through-holes 33 are not blocked. When metal ions move along the ion movement path P, they cannot easily pass through the interior of the positive electrode active material particles 30 within the second layer 22, and instead move in a bent manner between the positive electrode active material particles 30 on the outside of the positive electrode active material particles 30. As a result, the distance over which metal ions move toward the surface side of the positive electrode composite layer 20 increases, promoting the diffusion of metal ions in the in-plane direction of the positive electrode plate 10. Consequently, the positive electrode plate 10 according to this embodiment can suppress minute short circuits caused by the inclusion of metallic foreign matter.

[0035] The first conductive material 41 is a material that forms conductive paths in the positive electrode composite layer 20. The first conductive material 41 can electrically connect the positive electrode active material particles 30 and the second conductive material 42, which will be described later, by adhering to the surface of the positive electrode active material particles 30 so as to block the through holes 33.

[0036] The average particle size of the first conductive material 41 is preferably, for example, 100 nm or more and 1 μm or less. If the average particle size of the first conductive material 41 is larger than 1 μm, the efficiency of forming conductive paths in the positive electrode composite layer 20 may decrease. On the other hand, if the average particle size of the first conductive material 41 is less than 100 nm, the first conductive material 41 may pass through the through-hole 33 and penetrate into the hollow portion 32.

[0037] The first conductive material 41 is composed of a conductive carbon material. Examples of carbon materials that make up the first conductive material 41 include carbon black such as acetylene black (AB), Ketzen black, and furnace black; particulate carbon materials such as graphite; and fibrous carbon materials such as vapor-grown carbon fiber (VGCF), carbon nanotubes (CNT), and carbon nanofibers (CNF). These carbon materials may be used individually as the first conductive material 41, or two or more may be used in combination.

[0038] The carbon material constituting the first conductive material 41 may be primary particles, or aggregated particles (secondary particles) formed by the aggregation of primary particles. In a preferred embodiment, the first conductive material 41 includes aggregated particles of at least one of particulate carbon material and fibrous carbon material. This ensures that the through-holes 33 are reliably blocked by the first conductive material 41, thereby further promoting the diffusion of metal ions in the in-plane direction of the positive electrode plate 10.

[0039] In order to keep the average particle size of the first conductive material 41 within the above range, the average particle size of the primary particles of the particulate carbon material is preferably, for example, 10 nm or more and 100 nm or less. The shape of the particulate carbon material is not particularly limited as long as it is particulate, and examples include spherical, elliptical, etc. In order to exhibit high conductivity and form good conductive paths, the particulate carbon material is preferably AB, and the fibrous carbon material is preferably CNT.

[0040] Whether or not the first conductive material 41 contains aggregated particles can be determined by measuring the particle size distribution spectrum of a sample in which the first conductive material 41 is dispersed in a suitable solvent. For example, a particle size distribution spectrum containing two or more peaks indicates the presence of aggregated particles. On the other hand, a particle size distribution spectrum containing one peak indicates the absence of aggregated particles.

[0041] In one preferred embodiment, the second layer 22 includes a second conductive material 42 having an average diameter smaller than the average diameter of the through-holes 33 and positioned outside the shell portion 31.

[0042] The second conductive material 42 within the second layer 22 forms a conductive path between the positive electrode active material particles 30. The second conductive material 42 within the second layer 22 is electrically connected to the first conductive material 41, which is positioned to block the through-hole 33.

[0043] Here, the first layer 21 may contain a conductive material. The conductive material in the first layer 21 forms conductive paths between the positive electrode active material particles 30 and also forms conductive paths between the positive electrode active material particles 30 and the positive electrode current collector 11. Preferably, the conductive material included in the first layer 21 is a second conductive material 42. Since the second conductive material 42 in the first layer 21 is arranged not only outside the shell portion 31 but also in the hollow portion 32, it also forms conductive paths inside the positive electrode active material particles 30 included in the first layer 21. As a result, the electronic conductivity inside the positive electrode active material particles 30 included in the first layer 21 is improved. Consequently, the increase in the electronic resistance of the positive electrode plate 10 can be suppressed.

[0044] The second conductive material 42 is composed of a conductive carbon material. The second conductive material 42 may be one of the carbon materials exemplified in the first conductive material 41, or two or more may be mixed. It is preferable that the second conductive material 42 includes a fibrous carbon material because it can efficiently form conductive paths between the positive electrode active material particles 30 with a smaller amount of material compared to particulate carbon material. By arranging the fibrous carbon material as the second conductive material 42 between the positive electrode active material particles 30, a dense conductive network is formed between the positive electrode active material particles 30 and between the positive electrode composite layer 20 and the positive electrode current collector 11. As a result, electron transfer between the positive electrode active material particles 30 and between the positive electrode composite layer 20 and the positive electrode current collector 11 is performed efficiently, improving the electronic conductivity of the positive electrode composite layer 20. As a result, the increase in the electronic resistance of the positive electrode plate 10 can be suppressed.

[0045] Furthermore, by using a fibrous carbon material as the second conductive material 42, the proportion of the second conductive material 42 in the positive electrode composite layer 20 can be reduced. This allows for a higher proportion of the positive electrode active material in the positive electrode composite layer 20, resulting in a positive electrode plate 10 with a higher energy density. To exhibit high conductivity and form good conductive paths, the second conductive material 42 is preferably carbon nanotubes (CNTs). CNTs can be prepared by methods such as chemical vapor deposition (CVD), arc discharge, or laser evaporation. Commercially available CNTs may also be used.

[0046] The average diameter of the second conductive material 42 is preferably, for example, 5 nm or more and 30 nm or less. If the average diameter of the second conductive material 42 is greater than 30 nm, the efficiency of forming conductive paths in the positive electrode composite layer 20 may decrease. On the other hand, if the average diameter of the second conductive material 42 is less than 5 nm, the mechanical strength and dispersibility may decrease. The average diameter of the second conductive material 42 can be determined as the average value of the outer diameters of 10 arbitrarily selected second conductive material 42s obtained by SEM observation of the second conductive material 42.

[0047] The length of the fibrous carbon material is not particularly limited. The longer the fibrous carbon material, the more efficiently conductive paths can be formed in the positive electrode composite layer 20. On the other hand, the longer the fibrous carbon material, the lower its dispersibility. From these viewpoints, the average length of the second conductive material 42 is preferably, for example, 1 μm or more and 10 μm or less. The average length of the second conductive material 42 can be measured, for example, based on SEM images of the second conductive material 42. For example, the average length of the second conductive material 42 can be determined as the average value of 10 lengths of the second conductive material 42 arbitrarily selected from SEM images obtained by observing the second conductive material 42 with an SEM.

[0048] The first layer 21 and the second layer 22 may contain a binder. As the binder, a polymer that is soluble or dispersible in the solvent used can be used. Examples of binders include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid, and polyacrylate.

[0049] The first layer 21 and the second layer 22 may contain additives such as dispersants. Examples of dispersants include polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinylpyrrolidone (PVP), polyacrylate, polymethacrylate, polyoxyethylene alkyl ether, polyalkylene polyamine, and benzimidazole.

[0050] The proportion of positive electrode active material particles 30 in the entire positive electrode composite layer 20 is preferably 50% by mass or more (typically 90% by mass or more and 99% by mass or less), and preferably 95% by mass or more and 99% by mass or less. The proportion of binder in the entire positive electrode composite layer 20 is preferably, for example, 5% by mass or less, and preferably 1% by mass or less (for example, approximately 0.5% by mass or more and 1% by mass or less, for example 0.8% by mass). The combined proportion of the first conductive material 41 and the second conductive material 42 in the entire positive electrode composite layer 20 is preferably 10% by mass or less, and preferably 9% by mass or less (for example, approximately 8% by mass). If the combined proportion of the first conductive material 41 and the second conductive material 42 is too high, the mass per unit volume of positive electrode active material will decrease, and the desired energy density may not be obtained. On the other hand, if the combined ratio of the first conductive material 41 and the second conductive material 42 is too small, the electronic conductivity of the positive electrode composite layer 20 will decrease, which may increase the electronic resistance of the positive electrode plate 10.

[0051] Furthermore, the mass ratio of the first conductive material 41 to the second conductive material 42 (mass of the first conductive material 41 / mass of the second conductive material 42) is preferably, for example, 0.1 or more and 0.5 or less. If the mass ratio of the first conductive material 41 to the second conductive material 42 is 0.5 or more, the conductive paths between the positive electrode active material particles 30 formed by the second conductive material 42 may be insufficient. On the other hand, if the mass ratio of the first conductive material 41 to the second conductive material 42 is 0.1 or less, it may be difficult to seal the through-holes 33 with the first conductive material 41.

[0052] Here, Figure 2 is a schematic diagram showing the appearance of the positive electrode active material particles contained in the positive electrode plate shown in Figure 1. In one preferred embodiment, the positive electrode plate 10 satisfies the following formula (a) when the average particle size of the primary particles 34 is D1, the average diameter of the through holes 33 is D2, and the average particle size of the first conductive material 41 is D3. D3≧D1+D2...Formula (a)

[0053] Since the average particle size of the first conductive material 41 is greater than or equal to the sum of the average particle size of the primary particles 34 and the average diameter of the through-holes 33, the through-holes 33 can be reliably blocked by the first conductive material 41, thereby further promoting the diffusion of metal ions in the in-plane direction of the positive electrode plate 10.

[0054] (Manufacturing method for positive electrode plates) The method for manufacturing the positive electrode plate 10 according to Embodiment 1 will be described with reference to Figure 3. Figure 3 is a flowchart of the method for manufacturing the positive electrode plate according to Embodiment 1. As shown in Figure 3, the method for manufacturing the positive electrode plate 10 includes, for example, the following steps S1 to S2.

[0055] Step S1 is a first layer formation step in which a first layer 21 is formed on the positive electrode current collector 11. For example, the first layer formation step is a step in which a first layer 21 is formed on the positive electrode current collector 11 by coating the positive electrode current collector 11 with a first layer formation composition obtained by mixing positive electrode active material particles 30 having a hollow structure having a shell portion 31, a hollow portion 32 formed inside the shell portion 31, and a through hole 33 that connects the outside of the shell portion 31 and the hollow portion 32, a second conductive material 42, and a solvent as needed. Note that if a positive electrode current collector 11 with the first layer 21 already provided can be prepared, the first layer formation step can be omitted.

[0056] The composition for forming the first layer may contain a binder. Furthermore, the composition for forming the first layer may contain additives such as a dispersant. The first layer forming step may include a drying step for drying the first layer 21. Furthermore, the first layer forming step may include a pressing step for pressing the first layer 21.

[0057] Step S2 is a second layer formation step in which a second layer 22 is formed on the first layer 21. For example, the second layer formation step includes steps S2-1 to S2-3.

[0058] Step S2-1 is a first mixing step in which a mixture is obtained by mixing a positive electrode active material particle 30 having a hollow structure with a shell portion 31, a hollow portion 32 formed inside the shell portion 31, and a through hole 33 connecting the outside of the shell portion 31 and the hollow portion 32, with a first conductive material 41 and a solvent as needed. The mixture may contain a binder. Furthermore, the mixture may contain additives such as a dispersant.

[0059] Here, the first conductive material 41 needs to be positioned so as to block the through-holes 33. During kneading to obtain the mixture, the first conductive material 41 is fixed to the primary particles 34 arranged around the through-holes 33 by at least one of the intermolecular forces between the particles, the binding force of the first conductive material 41 itself, and the action of the dispersant. In this way, the through-holes 33 are blocked by the first conductive material 41 in the mixture.

[0060] Step S2-2 is a second mixing step in which a composition for forming the second layer is obtained by mixing the mixture with the second conductive material 42. The composition for forming the second layer may contain a dispersant for dispersing the second conductive material 42.

[0061] Here, the positive electrode active material particles 30 in the mixture have their through-holes 33 blocked by the first conductive material 41. Therefore, the second conductive material 42 in the second layer-forming composition obtained by mixing the mixture with the second conductive material 42 is positioned outside the shell portion 31.

[0062] Step S2-3 is a coating step in which a second layer 22 is formed on the first layer 21 formed on the positive electrode current collector 11 by coating the second layer forming composition containing the mixture onto the first layer 21. The second layer forming step may include a drying step for drying the second layer 22. Furthermore, the second layer forming step may include a pressing step for pressing the second layer 22. Note that if a second layer 22 that does not contain the second conductive material 42 is to be formed, the second mixing step can be omitted. In this case, the mixture can be used as the second layer forming composition.

[0063] The solvent used in the first and second layer formation steps only needs to be able to uniformly dissolve or disperse the binder, and can be appropriately selected depending on the binder used. Examples of solvents that can be used include non-aqueous solvents such as N-methyl-2-pyrrolidone (NMP), methyl ethyl ketone (MEK), dimethylformamide (DMF), and toluene, mixed solvents combining non-aqueous solvents, water, and aqueous solvents mainly composed of water.

[0064] For the kneading performed in the first and second layer formation steps, kneading equipment such as a planetary mixer, disper, ball mill, extrusion kneader, or mixer can be used. For the coating performed in the first and second layer formation steps, coating equipment such as a die coater, comma coater, knife coater, or gravure coater can be used. The drying performed in the first and second layer formation steps may be natural drying or heated drying. In the case of heated drying, appropriate drying equipment such as a hot air dryer or infrared heating device can be used. For the pressing performed in the first and second layer formation steps, pressing equipment such as a roll press can be used.

[0065] By following the above steps, the positive electrode plate 10 shown in Figure 1 can be manufactured.

[0066] (Lithium-ion rechargeable battery) Figure 4 is a schematic diagram showing a cross-section of a lithium-ion secondary battery according to Embodiment 1. The lithium-ion secondary battery 100 shown in Figure 4 has the positive electrode plate 10 described above. As shown in Figure 4, the lithium-ion secondary battery 100 has an electrode body 50 and an electrolyte. The lithium-ion secondary battery 100 may also have an outer casing such as a battery case that houses the electrode body 50 and the electrolyte.

[0067] The electrode body 50 includes a positive electrode plate 10, a negative electrode plate 60, and a separator 70 interposed between the positive electrode plate 10 and the negative electrode plate 60. The electrode body 50 has a structure in which the positive electrode plate 10 and the negative electrode plate 60 are laminated with the separator 70 in between. The electrode body 50 may be a wound electrode body in which a strip-shaped positive electrode plate 10 and a strip-shaped negative electrode plate 60 are overlapped with two strip-shaped separators 70 and wound in the longitudinal direction, or it may be a laminated electrode body in which a rectangular positive electrode plate 10 and a rectangular negative electrode plate 60 are laminated with a rectangular separator 70 in between.

[0068] The negative electrode plate 60 has a negative electrode current collector 61 and a negative electrode composite material layer 62 formed on the negative electrode current collector 61. The negative electrode current collector 61 is made of a conductive material such as a metal with good conductivity. The material of the negative electrode current collector 61 is not particularly limited, but examples include those similar to those exemplified in the positive electrode plate 10. Examples of the shape of the negative electrode current collector 61 include foil shape and mesh shape.

[0069] The negative electrode composite layer 62 contains a negative electrode active material. Examples of negative electrode active materials include carbon-based active materials such as graphite, and lithium titanate (e.g., Li4Ti5O4). 12 Examples include oxide-based active materials such as ) and Si-based active materials such as elemental Si. The particle size of the negative electrode active material may be 0.1 μm or more and 100 μm or less.

[0070] The separator 70 is placed between the positive electrode composite layer 20 and the negative electrode composite layer 62 so as to insulate the positive electrode plate 10 and the negative electrode plate 60. As the separator 70, a porous resin sheet with insulating properties such as polyethylene (PE) or polypropylene (PP) is used. Such a porous resin sheet may have a single-layer structure or a laminated structure of two or more layers. Furthermore, a porous heat-resistant layer may be provided on a part of the surface of the resin sheet.

[0071] The electrolyte is a composition in which a supporting salt is contained in a non-aqueous solvent. As the non-aqueous solvent, one or more solvents selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), etc. can be used. Further, as the supporting salt, one or more lithium compounds (lithium salts) selected from the group consisting of LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, LiC4F9SO3, LiN(CF3SO2)2, LiC(CF3SO2)3, LiI, etc. can be used.

Example

[0072] Hereinafter, embodiments of the present disclosure will be described in more detail with reference to examples. Note that the examples do not limit the present disclosure. FIG. 5 is a diagram for explaining the configuration of the first layer in the example. FIG. 6 is a diagram for explaining the configuration of the second layer in the example. Examples 1 to 2 and Comparative Examples 1 to 3 will be described with reference to FIGS. 5 and 6.

[0073] (Example 1) According to the flow shown in FIG. 3, the positive electrode plate 10 was manufactured. First, as shown in FIG. 5, in the first layer forming step, hollow structure positive electrode active material particles 30 which are secondary particles in which primary particles 34 having a composition represented by LiNi 0.33 Co 0.33 Mn 0.33 O2 are aggregated, CNT as a conductive material, PVDF as a binder, and NMP as a solvent were prepared. The average particle diameter of the primary particles 34 of the positive electrode active material particles 30 was 0.3 μm, the average diameter of the through holes 33 was 0.1 μm, and the average particle diameter of the secondary particles was 5 μm. The average diameter of the CNT was 15 nm, and the fiber length was 500 nm to 2000 nm.

[0074] Next, a paste-like composition for forming the first layer was obtained by adding positive electrode active material particles 30 and CNTs to a binder solution in which PVDF was dissolved in NMP, and then kneading the mixture using a kneading apparatus. In this case, the mass ratio of positive electrode active material particles 30, CNTs, and PVDF was set to 98% by mass of positive electrode active material particles 30, 1% by mass of CNTs, and 1% by mass of PVDF.

[0075] Next, the obtained first layer forming composition is applied to one side of an Al foil (thickness 10 μm) serving as the positive electrode current collector 11, with a basis weight of 6 mg / cm². 2 After coating using a coating apparatus, drying was performed using a drying apparatus and pressing was performed using a pressing apparatus. This formed the first layer 21 on the Al foil. The first layer 21 had a thickness of 15 μm and a curvature ratio of 3.0.

[0076] Next, in the second layer formation process, as shown in Figure 6, LiNi 0.33 Co 0.33 Mn 0.33 A hollow positive electrode active material particle 30, which is a secondary particle formed by the aggregation of primary particles 34 having a composition represented by O2, was prepared. AB was used as the first conductive material 41, CNTs as the second conductive material 42, PVDF as the binder, and NMP as the solvent. The positive electrode active material particle 30 was the same as that used in the first layer formation process, with an average particle size of 0.3 μm for the primary particles 34, an average diameter of 0.1 μm for the through-holes 33, and an average particle size of 5 μm for the secondary particles. AB had an average particle size of 2 μm to 4 μm. The CNTs were the same as those used in the first layer formation process, with an average diameter of 15 nm and a fiber length of 500 nm to 2000 nm.

[0077] Next, the positive electrode active material particles 30 and AB were added to a binder solution in which PVDF was dissolved in NMP, and the mixture was kneaded using a kneading apparatus to obtain a paste-like mixture. Here, the mass ratio of the positive electrode active material particles 30, AB, and PVDF was set to 88% by mass for the positive electrode active material particles 30, 10% by mass for AB, and 2% by mass for PVDF.

[0078] Next, a paste-like conductive material composition in which CNTs were dispersed in NMP was added to the mixture and kneaded using a kneading apparatus to obtain a composition for forming the second layer. In this composition, the positive electrode active material particles 30 accounted for 90% by mass, AB for 8% by mass, CNTs for 1% by mass, and PVDF for 1% by mass.

[0079] Next, the obtained second layer forming composition is applied to the surface of the first layer 21 formed on the Al foil at a basis weight of 6 mg / cm². 2 After coating using a coating apparatus, drying was performed using a drying apparatus and pressing was performed using a pressing apparatus. This formed a second layer 22 on the first layer 21. The second layer 22 had a thickness of 25 μm and a curvature ratio of 5.9. The positive electrode plate 10 was manufactured in this manner.

[0080] (Example 2) As shown in Figures 5 and 6, a positive electrode plate 10 was manufactured in the same manner as in Example 1, except that aggregated CNT particles were used as the first conductive material 41. The aggregated CNT particles were aggregated CNTs with an average diameter of 15 nm and a fiber length of 500 nm to 2000 nm, and had an average particle size of 3 μm to 10 μm.

[0081] Here, Figure 7 is a schematic diagram showing the area around the positive electrode active material particles contained in the second layer of the positive electrode plate. Observation of SEM images of each positive electrode plate 10 of Examples 1 and 2 revealed that, as shown in Figure 7, the second layer 22 of each positive electrode plate 10 contained aggregated particles of AB or CNT as a first conductive material 41 arranged to block the through-holes 33 of the positive electrode active material particles 30, and CNTs as a second conductive material 42 dispersed outside the shell portion 31.

[0082] (Comparative Example 1) A positive electrode plate was manufactured in the same manner as in Example 1, except that the thickness of the first layer 21 was changed to 40 μm, and the second layer 22 was not provided due to the omission of the second layer formation process.

[0083] (Comparative Example 2) A positive electrode plate was manufactured in the same manner as in Example 1, except that the conductive material was changed to AB, the thickness of the first layer 21 was changed to 40 μm and the curve ratio to 5.9, and the second layer 22 was not provided due to the omission of the second layer formation process.

[0084] (Comparative Example 3) A positive electrode plate was manufactured in the same manner as in Example 1, except that the conductive material was changed to aggregated CNT particles, the thickness of the first layer 21 was changed to 40 μm and the curvature ratio to 5.2, and the second layer 22 was not provided due to the omission of the second layer formation process.

[0085] (result) Figure 8 shows the measured electronic resistance of the positive electrode plate, the measured battery resistance, and the calculated surface diffusion distance of metal ions. The electronic resistance of the positive electrode plate shown in Figure 8 is (Ω·cm 2 The measurement results for the electronic resistance and battery resistance (Ω) are shown for each positive electrode plate of Examples 1-2 and Comparative Examples 1-3, respectively. The measured values ​​for the electronic resistance and battery resistance of the positive electrode plate of Comparative Example 1 are shown as indexed values ​​with the reference value (1.0). In addition, the calculation results for the surface diffusion distance (μm) of metal ions shown in Figure 8 are shown for each positive electrode plate of Examples 1-2 and Comparative Examples 1-3, respectively. The calculated value for the surface diffusion distance of metal ions of the positive electrode plate of Comparative Example 1 is shown as an indexed value with the reference value (1.0).

[0086] Here, Figure 9 is a cross-sectional view of the positive electrode plate to illustrate the surface diffusion distance of metal ions. Figure 9 shows how metal ions diffuse from metal foreign matter M mixed into the positive electrode composite layer 20 of the positive electrode plate 10. As shown in Figure 9, the surface diffusion distance r of metal ions originating from metal foreign matter M was calculated under the condition that metal foreign matter M is located at a distance D of 10 μm from the interface with the positive electrode current collector 11 in the positive electrode composite layer 20.

[0087] Specifically, the curvature ratio was first calculated using the following formula (b). τ={Rion(κεS) / L}...Equation (b)

[0088] Here, τ is the curvature and Rion is the ionic resistance ( / Ω·cm). 2 ), L is the thickness of the positive electrode composite layer (μm), κ is the conductivity of the electrolyte (S / m), ε is the porosity of the positive electrode composite layer (%), and S is the area of ​​the reaction interface (μm). 2 ) The curvature τ is such that a smaller value indicates a more linear ion transport path P and lower resistance, while a larger value indicates a more bent ion transport path P and higher resistance.

[0089] Then, the surface diffusion distance r was calculated from the calculated curvature ratio using the following formula (c). r = √{(τL / 2)} 2 -(L / 2) 2 )...Equation (c)

[0090] Here, r is the surface diffusion distance (μm), τ is the curvature ratio, and L is the thickness of the positive electrode composite layer (μm). The surface diffusion distance r is the diffusion distance of ions in the in-plane direction of the positive electrode plate 10 on the surface of the positive electrode composite layer 20 adjacent to the separator 70. A larger surface diffusion distance r indicates that metal ions diffuse more easily in the in-plane direction of the positive electrode plate 10, making it less likely for minute short circuits to occur. In other words, a larger surface diffusion distance r increases the resistance to chemical minute short circuits (hereinafter referred to as chemical short circuit resistance). On the other hand, a smaller surface diffusion distance r indicates that metal ions diffuse less easily in the in-plane direction of the positive electrode plate 10, making it more likely for minute short circuits to occur. In other words, a smaller surface diffusion distance r results in lower chemical short circuit resistance.

[0091] As can be seen from the results shown in Figure 8, when comparing Example 1, Example 2, and Comparative Example 1, which have similar electronic and battery resistances, Example 1 and Example 2 had a larger surface diffusion distance than Comparative Example 1. This is thought to be because AB blocks the through-holes 33 of the positive electrode active material particles 30 contained in the second layer 22, allowing for the formation of a positive electrode composite layer 20 with a greater curvature than when the through-holes 33 are not blocked, thereby promoting the diffusion of metal ions in the in-plane direction of the positive electrode plate 10. Thus, it was confirmed that each positive electrode plate 10 obtained in Example 1 and Example 2 has high chemical short circuit resistance.

[0092] Furthermore, comparing Example 1 and Comparative Example 2, which have similar surface diffusion distances, Example 1 exhibited lower electronic resistance and battery resistance than Comparative Example 2. This is thought to be because, in Example 1, AB blocked the through-holes 33 of the positive electrode active material particles 30 contained in the second layer 22, and the CNTs positioned outside the shell portion 31 formed suitable conductive paths between the positive electrode active material particles 30 contained in the positive electrode composite layer 20. This promoted the diffusion of metal ions in the in-plane direction of the positive electrode plate 10 while ensuring conductivity between the positive electrode active material particles 30. Thus, it was confirmed that the positive electrode plate 10 obtained in Example 1 possessed high chemical short circuit resistance and high electronic conductivity.

[0093] Furthermore, comparing Example 2 and Comparative Example 3, which have similar surface diffusion distances, Example 2 exhibited lower electronic resistance and battery resistance than Comparative Example 3. This is thought to be because, in Example 2, the aggregated CNT particles blocked the through-holes 33 of the positive electrode active material particles 30 contained in the second layer 22, and the CNTs positioned outside the shell portion 31 formed suitable conductive paths between the positive electrode active material particles 30 contained in the positive electrode composite layer 20. This promoted the diffusion of metal ions in the in-plane direction of the positive electrode plate 10 while ensuring conductivity between the positive electrode active material particles 30. Thus, it was confirmed that the positive electrode plate 10 obtained in Example 2 possessed high chemical short circuit resistance and high electronic conductivity.

[0094] Based on the above, this disclosure provides a positive electrode for lithium-ion secondary batteries, a lithium-ion secondary battery, and a method for manufacturing a positive electrode for lithium-ion secondary batteries that suppresses minute short circuits caused by the inclusion of metallic foreign matter.

[0095] This disclosure is not limited to the embodiments described above, and may be modified as appropriate without departing from its intent. [Explanation of Symbols]

[0096] 10 Positive electrode plate 11 Positive electrode current collector 20 Positive electrode composite layer 21 1st layer 22 2nd layer 30 Positive electrode active material particles 31 Shell portion 32 Hollow portion 33 Through hole 34 Primary particle 41 First conductive material 42 Second conductive material 50 Electrode body 60 Negative electrode plate 61 Negative electrode current collector 62 Negative electrode composite material layer 70 Separators 100 Lithium-ion rechargeable batteries D: Distance M: Metal foreign matter P: Ion transport path r: Surface diffusion distance

Claims

1. Positive electrode current collector and The positive electrode current collector has a positive electrode composite layer formed on it, The aforementioned positive electrode composite layer is The first layer located on the positive electrode current collector side, It has a second layer located on the side furthest from the positive electrode current collector, The first and second layers are both, The positive electrode active material particle comprises a shell portion, a hollow portion formed inside the shell portion, and a hollow structure having a through hole connecting the outside of the shell portion and the hollow portion. The aforementioned second layer is, A positive electrode for a lithium-ion secondary battery comprising a first conductive material having an average diameter larger than the average diameter of the through-holes and arranged to block the through-holes.

2. The aforementioned shell portion is Formed by multiple primary particles, A positive electrode for a lithium-ion secondary battery according to claim 1, which satisfies the following formula (a) when the average particle size of the primary particles is D1, the average diameter of the through-holes is D2, and the average particle size of the first conductive material is D3. D3≧D1+D2...Formula (a)

3. The positive electrode for a lithium-ion secondary battery according to claim 1, wherein the first conductive material comprises aggregated particles of at least one of particulate carbon material and fibrous carbon material.

4. The positive electrode for a lithium-ion secondary battery according to claim 3, wherein the particulate carbon material is acetylene black.

5. The positive electrode for a lithium-ion secondary battery according to claim 3, wherein the fibrous carbon material is a carbon nanotube.

6. The aforementioned second layer is, The positive electrode for a lithium-ion secondary battery according to claim 1, comprising a second conductive material having an average diameter smaller than the average diameter of the through-holes and disposed outside the shell portion.

7. The positive electrode for a lithium-ion secondary battery according to claim 6, wherein the second conductive material comprises a fibrous carbon material.

8. The positive electrode for a lithium-ion secondary battery according to claim 7, wherein the fibrous carbon material is a carbon nanotube.

9. A lithium-ion secondary battery having a positive electrode for a lithium-ion secondary battery according to any one of claims 1 to 8.

10. A mixing step to obtain a mixture by mixing positive electrode active material particles having a hollow structure with a shell portion, a hollow portion formed inside the shell portion, and through holes connecting the outside of the shell portion and the hollow portion, and a first conductive material having an average diameter larger than the average diameter of the through holes, A coating step of forming a second layer on a first layer by coating a second layer-forming composition containing the mixture onto a first layer containing positive electrode active material particles formed on a positive electrode current collector, A method for manufacturing a positive electrode for a lithium-ion secondary battery having the properties of a positive electrode.

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