Positive electrode for lithium secondary battery, and lithium secondary battery using same

Abstract

An objective of the present invention is to provide a means capable of achieving both rapid charging characteristics and cycle characteristics in a lithium secondary battery including a positive electrode active material layer that contains a solid electrolyte. Particles provided with a coat layer containing a specific lithium ion conductive compound on the surface of a polycrystal lithium transition metal composite oxide having a large particle diameter, and particles provided with a coat layer containing a specific lithium ion conductive compound on the surface of a single crystal lithium transition metal composite oxide having a small particle diameter are contained in a positive electrode active material layer together with a solid electrolyte.

Description

Positive electrode for lithium secondary batteries and lithium secondary batteries using the same

[0001] This invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery using the same.

[0002] In recent years, research and development on all-solid-state lithium secondary batteries using oxide-based or sulfide-based solid electrolytes has been actively pursued. Solid electrolytes are materials mainly composed of ion conductors capable of ion conduction in a solid state. Therefore, in all-solid-state lithium secondary batteries, various problems caused by flammable organic electrolytes, as seen in conventional liquid-based lithium secondary batteries, do not occur in principle. In addition, generally, using high-potential, high-capacity positive electrode materials and high-capacity negative electrode materials can significantly improve the power density and energy density of the battery.

[0003] Here, Japanese Patent Publication No. 2023-079219 (corresponding to U.S. Patent Application Publication No. 2023 / 0170479) discloses a composite positive electrode active material comprising a first core containing a first lithium transition metal oxide, a second core containing a second lithium transition metal oxide, and a shell disposed along one or more surfaces of the first and second cores, comprising a specific first metal oxide and a carbon-based material. The composite positive electrode active material is also characterized in that the first lithium transition metal oxide and the second lithium transition metal oxide have different particle sizes, and the second lithium transition metal oxide contains primary particles with a particle size of 1 μm or larger. According to the above document, applying a composite positive electrode active material having such a configuration to the positive electrode of a lithium battery improves the high-temperature cycle characteristics of the lithium battery, suppresses the increase in internal resistance, and improves high-efficiency characteristics.

[0004] However, our own investigations have revealed that when the technology described in the above-mentioned patent document is applied to a lithium secondary battery equipped with a positive electrode active material layer containing a solid electrolyte, sufficient performance may not be obtained in either the rapid charging characteristics (QC characteristics) or the cycle characteristics.

[0005] Therefore, the present invention aims to provide a means for achieving both rapid charging characteristics and cycle characteristics in a lithium secondary battery equipped with a positive electrode active material layer containing a solid electrolyte.

[0006] In view of the above problems, the inventors conducted thorough research. In the process, they discovered that the above problems could be solved by incorporating particles having a coating layer containing a specific lithium ion conductive compound on the surface of a polycrystalline, large-particle-grain lithium transition metal composite oxide, and particles having a coating layer containing a specific lithium ion conductive compound on the surface of a single-crystal, small-particle-grain lithium transition metal composite oxide, together with a solid electrolyte in the positive electrode active material layer, thereby completing the present invention.

[0007] In other words, one embodiment of the present invention relates to a positive electrode for a lithium secondary battery having a positive electrode active material layer comprising: first particles comprising a first lithium transition metal composite oxide and a first coating layer disposed on the surface of the first lithium transition metal composite oxide; second particles comprising a second lithium transition metal composite oxide and a second coating layer disposed on the surface of the second lithium transition metal composite oxide; and a solid electrolyte. In this positive electrode for a lithium secondary battery, the first lithium transition metal composite oxide is polycrystalline, and the second lithium transition metal composite oxide is single-crystal. The average particle diameter X of the first lithium transition metal composite oxide is greater than the average particle diameter Y of the second lithium transition metal composite oxide. The first coating layer and the second coating layer are each independently of the general formula (2): Li ｘ AO ｙ The compound includes a lithium-ion conductive compound represented as follows: (A is Al, Zr, Nb, Ti, Ta, La, P, B, C, Si, S, Mo, or W, and x and y each represent independent positive integers).

[0008] Figure 1 is a schematic cross-sectional view showing the overall structure of a stacked (internal parallel connection type) all-solid-state lithium secondary battery (stacked secondary battery) according to one embodiment of the present invention. Figure 2 is an enlarged cross-sectional view of the positive electrode active material layer constituting the stacked secondary battery according to one embodiment of the present invention.

[0009] One embodiment of the present invention is a positive electrode for a lithium secondary battery having a positive electrode active material layer comprising: first particles comprising a first lithium transition metal composite oxide and a first coating layer disposed on the surface of the first lithium transition metal composite oxide; second particles comprising a second lithium transition metal composite oxide and a second coating layer disposed on the surface of the second lithium transition metal composite oxide; and a solid electrolyte, wherein the first lithium transition metal composite oxide is polycrystalline, the second lithium transition metal composite oxide is single-crystal, the average particle diameter X of the first lithium transition metal composite oxide is greater than the average particle diameter Y of the second lithium transition metal composite oxide, and the first coating layer and the second coating layer each independently contain a material of the general formula (2): Li ｘ AO ｙ This is a positive electrode for a lithium secondary battery containing a lithium-ion conductive compound represented by (A being Al, Zr, Nb, Ti, Ta, La, P, B, C, Si, S, Mo, or W, and x and y each independently representing positive integers). The positive electrode for a lithium secondary battery according to this embodiment (hereinafter also simply referred to as "the positive electrode according to this embodiment") can provide a lithium secondary battery with excellent rapid charging characteristics and cycle characteristics.

[0010] The positive electrode for a lithium secondary battery and a lithium secondary battery to which this positive electrode is applied will be described below with reference to the drawings. However, the technical scope of the present invention should be determined based on the claims and is not limited to the following embodiments. Note that the dimensional ratios in the drawings are exaggerated for illustrative purposes and may differ from the actual ratios.

[0011] Figure 1 is a schematic cross-sectional view showing the overall structure of a stacked (internal parallel connection type) all-solid-state lithium secondary battery (hereinafter also simply referred to as "stacked secondary battery"), which is one embodiment of the present invention. Figure 1 shows a cross-section of the stacked secondary battery during charging. The stacked secondary battery 10a shown in Figure 1 has a structure in which a substantially rectangular power generation element 21, where the charge and discharge reaction actually proceeds, is sealed inside a laminate film 29, which is the battery casing. Here, the power generation element 21 has a configuration in which a negative electrode, a solid electrolyte layer 17 containing a solid electrolyte, and a positive electrode are stacked.

[0012] The negative electrode has a structure in which a negative electrode current collector 11' and a negative electrode active material layer 13 made of lithium metal deposited on the surface of the negative electrode current collector 11' are laminated. Further, a negative electrode intermediate layer 14 is disposed so as to be in contact with the negative electrode active material layer 13 and the solid electrolyte layer 17, respectively.

[0013] The positive electrode has a structure in which a positive electrode active material layer 15 is disposed on the surface of a positive electrode current collector 11". Thus, the negative electrode current collector 11', the negative electrode active material layer 13, the negative electrode intermediate layer 14, the solid electrolyte layer 17, the positive electrode active material layer 15, and the positive electrode current collector 11" constitute a single battery layer 19. Therefore, it can be said that the laminated secondary battery 10a shown in FIG. 1 has a configuration in which a plurality of single battery layers 19 are laminated and electrically connected in parallel.

[0014] A negative electrode current collector plate 25 and a positive electrode current collector plate 27 that are electrically connected to the respective electrodes (negative electrode and positive electrode) are attached to the negative electrode current collector 11' and the positive electrode current collector 11", respectively, and have a structure in which they are led out to the outside of the laminate film 29 so as to be sandwiched between the ends of the laminate film 29. A restraining pressure is applied to the laminated secondary battery 10a in the stacking direction of the power generation element 21 by a pressing member. Therefore, the volume of the power generation element 21 is kept constant.

[0015] FIG. 2 is an enlarged cross-sectional view of the positive electrode active material layer 15 that constitutes the laminated secondary battery 10a according to an embodiment of the present invention. As shown in FIG. 2, the positive electrode active material layer 15 of the laminated secondary battery 10a according to the present embodiment includes first particles 30, second particles 40, and a solid electrolyte (for example, Li ６ PS ５ Cl) 50. The first particles 30 are composed of a first lithium transition metal composite oxide (for example, LiNi ０．８ Mn ０．１ Co ０．１ O ２ ) 31 that is polycrystalline, and a first coat layer 32 disposed on the surface of the first lithium transition metal composite oxide 31. The second particles 40 are composed of a second lithium transition metal composite oxide (for example, LiNi ０．８ Mn ０．１ Co ０．１ O ２) 41 and a second coating layer 42 disposed on the surface of the second lithium transition metal composite oxide 41 are composed of a first coating layer 32 and a second coating layer 42. In the positive electrode active material layer 15 shown in Figure 2, both the first coating layer 32 and the second coating layer 42 are composed of a lithium ion conductive compound Li ２ ZrO ３ The cathode active material layer 15 shown in Figure 2 is composed of the first lithium transition metal composite oxide 31, the entire surface of which is covered by the first coating layer 32, and the entire surface of the second lithium transition metal composite oxide 41, the entire surface of which is covered by the second coating layer 42. The average particle diameter of the first lithium transition metal composite oxide 31 is larger than the average particle diameter of the second lithium transition metal composite oxide, and in the cathode active material layer 15 shown in Figure 2, the average particle diameter of the first particles 30 is larger than the average particle diameter of the second particles 40. The average particle diameter of the solid electrolyte 50 is smaller than the average particle diameter of the second lithium transition metal composite oxide 41. As a result, the average particle diameter of the solid electrolyte 50 is smaller than the average particle diameter of the second particles 40. Because the average particle diameters of the first particles, second particles, and solid electrolyte are in the relationship described above, in the cathode active material layer 15, the second particles 40, which have a small particle size, fill the gaps between the first particles 30, which have a large particle size. Furthermore, the voids formed by the first particles 30 and the second particles 40 are filled with a solid electrolyte 50. In the positive electrode active material layer 15 shown in Figure 2, each particle of the first particles 30, the second particles 40, and the solid electrolyte 50 are bound to a binder (not shown). In addition, conductivity is ensured between the first particles 30 and the second particles 40 by a conductive additive (not shown).

[0016] The main components of the lithium secondary battery according to this embodiment will be described below.

[0017] [Current Collector] The current collector (negative electrode current collector, positive electrode current collector) has the function of mediating the movement of electrons from the electrode active material layer (negative electrode active material layer, positive electrode active material layer). There are no particular restrictions on the materials that make up the current collector. Examples of materials that can be used to make up the current collector include metals such as aluminum, nickel, iron, stainless steel, titanium, and copper, as well as conductive resins. There are no particular restrictions on the thickness of the current collector, but one example is 10 to 100 μm.

[0018] Furthermore, if the electrode active material layers (negative electrode active material layer, positive electrode active material layer) themselves possess conductivity and can perform a current-collecting function, it is not necessary to use a current collector as a separate component from these active material layers. In such a configuration, the negative electrode active material layer directly constitutes the negative electrode, and the positive electrode active material layer directly constitutes the positive electrode.

[0019] [Positive Electrode Active Material Layer] In the positive electrode for a lithium secondary battery according to this embodiment, the positive electrode active material layer contains a lithium transition metal composite oxide as the positive electrode active material. The lithium transition metal composite oxide is a compound belonging to space group R3m, and preferably has a layered structure (layered rock salt type structure) in which lithium atomic layers and transition metal atomic layers are stacked alternately. By using such a positive electrode active material, the battery capacity of the lithium secondary battery can be improved.

[0020] Examples of lithium transition metal composite oxides include LiCoO ２ LiNiO ２ LiMnO ２ , Li(Ni-Mn-Co)O ２ , Li(Ni-Co-Al)O ２ Examples include those in which some of these transition metals are substituted with other elements. In some cases, two or more lithium transition metal composite oxides may be used in combination. More preferably, a composite oxide containing lithium and nickel is used, and even more preferably Li(Ni-Mn-Co)O ２ and those in which some of these transition metals are substituted with other elements (hereinafter also simply referred to as "NMC composite oxides") or Li(Ni-Co-Al)O ２ These transition metals, or those in which some of these transition metals are substituted with other elements (hereinafter also simply referred to as "NCA composite oxides"), are used, and NMC composite oxides are particularly preferred.

[0021] As described above, NMC composite oxides and NCA composite oxides also include composite oxides in which some of the transition metal elements are substituted with other metal elements. Examples of other elements in this case include Al, Sn, Nb, Ti, Zr, Mg, W, P, V, Ca, Sr, Cr, Fe, B, Ga, In, Si, Mo, Y, V, Cu, Ag, Zn, and the like, preferably Al, Sn, Nb, Ti, Zr, Mg, W, P, V, Ca, Sr, Cr, more preferably Al, Sn, Nb, Ti, Zr, Mg, P, Cr, and even more preferably Al, Sn, Nb, Ti, Zr, Mg, P, Cr from the viewpoint of improving cycle characteristics. However, other metal elements that can substitute for the transition metal elements in NCA composite oxides are those other than Al.

[0022] Lithium transition metal composite oxides are preferred as positive electrode active materials because they have a high theoretical discharge capacity, and the general formula (1): Li ｘ Ni ａ M ｂ N ｃ O ２ The material has a composition represented by (wherein x, a, b, and c satisfy 0.8 ≤ x ≤ 1.1, a + b + c = 1, 0.33 ≤ a ≤ 0.95, 0.05 ≤ b ≤ 0.67, and 0 ≤ c ≤ 0.10. M is one or more elements selected from the group consisting of Mn and Co, and N is one or more elements selected from the group consisting of Al, Sn, Nb, Ti, Zr, and Mg). Here, x represents the atomic ratio of Li, a represents the atomic ratio of Ni, b represents the atomic ratio of M, and c represents the atomic ratio of N. The composition of each element can be measured, for example, by plasma (ICP) emission spectrometry. From the viewpoint of achieving a higher discharge capacity, it is preferable that in general formula (1), 0.80 ≤ a ≤ 0.95 (i.e., a high nickel composite oxide).

[0023] The positive electrode for lithium secondary batteries according to this embodiment is characterized in that the positive electrode active material layer includes a polycrystalline lithium transition metal composite oxide and a single-crystal (single-particle) lithium transition metal composite oxide (hereinafter, this characteristic will also be referred to as "Feature 1"). In this specification, the polycrystalline lithium transition metal composite oxide is referred to as the "first lithium transition metal composite oxide," and the single-crystal lithium transition metal composite oxide is referred to as the "second lithium transition metal composite oxide," but the ordinal numbers themselves do not have any particular meaning (the same applies to the "first coat layer," "second coat layer," "first particle," and "second particle" described later). Whether the lithium transition metal composite oxide is "polycrystalline" or "single-crystal" can be determined by crystal analysis using SEM-EBSD. More specifically, crystal orientation maps were obtained for lithium transition metal composite oxides, and for each particle, it was determined whether or not grain boundaries were observed. Those in which grain boundaries were observed (crystals with different crystal orientations were observed inside the contour of the particle) were classified as "polycrystalline," and those in which no grain boundaries were observed (the particle consisted only of crystals with a single crystal orientation) were classified as "single crystals."

[0024] The first lithium transition metal composite oxide and the second lithium transition metal composite oxide may be the same compound or different compounds. Furthermore, two or more compounds may be used in combination for each of the first lithium transition metal composite oxide and the second lithium transition metal composite oxide. From the viewpoint of achieving a higher discharge capacity, it is preferable that the first lithium transition metal composite oxide and the second lithium transition metal composite oxide are compounds having a composition represented by the above general formula (1), more preferably that in general formula (1), 0.80 ≤ a ≤ 0.95 (i.e., a high nickel composite oxide), and even more preferably that the first lithium transition metal composite oxide and the second lithium transition metal composite oxide are high nickel composite oxides having the same composition.

[0025] Furthermore, in addition to Feature 1 described above, the positive electrode for lithium secondary batteries according to this embodiment is also characterized in that the average particle diameter X of the first lithium transition metal composite oxide is larger than the average particle diameter Y of the second lithium transition metal composite oxide (i.e., the ratio of the average particle diameter X of the first lithium transition metal composite oxide to the average particle diameter Y of the second lithium transition metal composite oxide (X / Y) is greater than 1) (hereinafter, this feature will also be referred to as "Feature 2"). In this specification, "average particle diameter" refers to the value calculated as the 50% cumulative diameter (D50) based on volume, measured by a particle size distribution analyzer using the laser diffraction / scattering method. Because the average particle diameter X of the polycrystalline first lithium transition metal composite oxide is larger than the average particle diameter Y of the single-crystal second lithium transition metal composite oxide (i.e., by using a combination of a polycrystalline, large-particle lithium transition metal composite oxide and a single-crystal, small-particle lithium transition metal composite oxide as the positive electrode active material), a lithium secondary battery with excellent rapid charging characteristics and cycle characteristics can be provided. According to the inventors' studies, when a single-crystal, large-particle lithium transition metal composite oxide is used in combination with a single-crystal, small-particle lithium transition metal composite oxide, the rapid charging characteristics may deteriorate (see Comparative Example 1 described later). This is thought to be because the diffusion resistance of lithium ions is high in the single-crystal, large-particle lithium transition metal composite oxide. Furthermore, when a polycrystalline, large-particle lithium transition metal composite oxide is used in combination with a polycrystalline, small-particle lithium transition metal composite oxide, the cycle characteristics may deteriorate (see Comparative Example 2 described later). This is thought to be because cracks occur in the polycrystalline, small-particle lithium transition metal composite oxide when pressure is applied. The ratio (X / Y) is preferably 1.5 or more, and more preferably 2 or more. When the ratio (X / Y) is within this range, the second particles containing the second lithium transition metal composite oxide can more easily fill the gaps between the first particles containing the first lithium transition metal composite oxide, and the energy density can be further improved. The ratio (X / Y) is preferably less than 3, and more preferably 2.5 or less.If the ratio (X / Y) is within this range, the diffusion resistance of lithium ions in the first lithium transition metal composite oxide does not become too large, thus suppressing an increase in internal resistance and unevenness in the battery reaction. As a result, the rapid charging characteristics and cycle characteristics of the lithium secondary battery can be further improved. The ratio (X / Y) is preferably 1.5 or more and less than 3, and more preferably 2 or more and 2.5 or less.

[0026] The average particle size X of the first lithium transition metal composite oxide is not particularly limited, but is preferably 1 μm or more and 12 μm or less. If the average particle size X is within the above range, the diffusion resistance of lithium ions is further reduced, and the increase in internal resistance and the occurrence of unevenness in the battery reaction can be further suppressed. As a result, the rapid charging characteristics and cycle characteristics of lithium secondary batteries can be further improved. The average particle size X may be 1 μm or more and 8 μm or less, 1 μm or more and 7 μm or less, 1 μm or more and 6 μm or less, 5 μm or more and less than 7 μm, or 7 μm or more and 12 μm or less.

[0027] The average particle diameter Y of the second lithium transition metal composite oxide is not particularly limited as long as it is smaller than the average particle diameter X, but is preferably 1 μm or more and 4 μm or less, more preferably 1 μm or more and 3.5 μm or less, and even more preferably 1 μm or more and 3 μm or less. If the average particle diameter Y is within the above range, further improvement in energy density can be achieved.

[0028] The mixing ratio of the first lithium transition metal composite oxide and the second lithium transition metal composite oxide in the positive electrode active material layer is not particularly limited, but it is preferable that the proportion of the first lithium transition metal composite oxide is large. That is, in one preferred embodiment, the ratio (x / y) of the mass x of the first lithium transition metal composite oxide to the mass y of the second lithium transition metal composite oxide in the positive electrode active material layer is greater than 1. By setting the ratio (x / y) within the above range, the rapid charging characteristics and cycle characteristics of the lithium secondary battery can be further improved. From the viewpoint of further improving the rapid charging characteristics and cycle characteristics of the lithium secondary battery, when the average particle diameter X of the first lithium transition metal composite oxide is 5 μm or more and less than 7 μm, the above ratio (x / y) is preferably 4 or more, more preferably 6 or more, and particularly preferably 6 or more and 20 or less. From a similar viewpoint, when the average particle size X of the first lithium transition metal composite oxide is 7 μm or more and 12 μm or less, the above ratio (x / y) is preferably 3 or less, more preferably greater than 1 and 3 or less, and even more preferably 2 or more and 3 or less.

[0029] Furthermore, in the positive electrode for lithium secondary batteries according to this embodiment, in addition to the above features 1 and 2, a first coating layer is disposed on the surface of the first lithium transition metal composite oxide, and a second coating layer is disposed on the surface of the second lithium transition metal composite oxide. The first coating layer and the second coating layer are each independently of the general formula (2): Li ｘ AO ｙThe present invention is also characterized by containing a lithium-ion conductive compound represented by (A is Al, Zr, Nb, Ti, Ta, La, P, B, C, Si, S, Mo, or W, and x and y each represent independently positive integers) (hereinafter, this characteristic will also be referred to as "characteristic 3"). In the positive electrode according to this embodiment, the positive electrode active material layer contains a solid electrolyte, but when the lithium transition metal composite oxide and the solid electrolyte come into contact, a reaction occurs at the contact point, a high-resistance layer is formed at the interface, the interfacial resistance increases, and the battery capacity may gradually decrease. By placing a coating layer containing a lithium-ion conductive compound on the surfaces of the first lithium transition metal composite oxide and the second lithium transition metal composite oxide, the decrease in battery capacity can be suppressed, and both rapid charging characteristics and cycle characteristics can be achieved. In this specification, "placing B on the surface of A" includes both a form in which the surface of A and B are in direct contact, and a form in which another member exists between the surface of A and B. In the positive electrode for a lithium secondary battery according to this embodiment, it is preferable that the first coating layer is arranged so as to be in direct contact with the surface of the first lithium transition metal composite oxide, and the second coating layer is arranged so as to be in direct contact with the surface of the second lithium transition metal composite oxide. Furthermore, in the positive electrode for a lithium secondary battery according to this embodiment, it is preferable that the first coating layer is located on the outermost surface of the first particle, and the second coating layer is located on the outermost surface of the second particle.

[0030] Specific examples of lithium-ion conductive compounds represented by general formula (2) include, for example, Li ２ ZrO ３ LiNbo ３ LiAlO ２ Li ２ TiO ３ Li ４ Ti ５ O １２ Li ２ Ti ２ O ５ , LiTaO ３ Li ３ PO ４ Li ３ BO ３ LiBO ２ Li ２ CO ３Li ４ SiO ４ Li ２ SiO ３ Li ２ SO ４ Li ２ MoO ４ and Li ２ WO ４ These are some examples. In particular, from the perspective of being especially excellent in improving cycle characteristics, Li ２ ZrO ３ LiNbo ３ and LiAlO ２ It is preferable that at least one of the following is used: ２ ZrO ３ Or LiNbo ３ It is more preferable that at least one of the following is used, Li ２ ZrO ３ It is particularly preferable that the following be used. The lithium-ion conductive compound contained in the first coat layer and the lithium-ion conductive compound contained in the second coat layer may be the same or different.

[0031] Preferably, the coating layers (first and second coating layers) are substantially free of conductive carbon material. By not including conductive carbon material in the coating layers, degradation of the solid electrolyte due to conductive carbon material is suppressed, and the rapid charging characteristics and cycle characteristics can be further improved. In this specification, "substantially free" means that a certain material is not intentionally added, and does not exclude cases where it is unintentionally included, for example, due to unintentional mixing or insufficient removal. In one embodiment, when a certain material is "free" or "substantially free," the content of that material may be 1000 ppm by mass or less, 100 ppm by mass or less, 10 ppm by mass or less, 1 ppm by mass or less, or 0.1 ppm by mass or less, relative to the total mass of the coating layer.

[0032] The coating layers (first coating layer and second coating layer) only need to be arranged on at least a portion of the surface of the lithium transition metal composite oxide (first lithium transition metal composite oxide and second lithium transition metal composite oxide). However, from the viewpoint of further improving the cycle characteristics, it is preferable that a wider area of ​​the surface of the lithium transition metal composite oxide is covered by the coating layer. That is, when the "coverage rate" is defined as the percentage of the surface area of ​​the lithium transition metal composite oxide that is covered by the coating layer, a higher coverage rate is preferable. For each of the first and second particles, the coverage rate is preferably 50% or more, more preferably 70% or more, even more preferably 80% or more, particularly preferably 90% or more, and most preferably 95% to 100%. A coverage rate of 100% means that the coating layer is arranged on the entire surface of the lithium transition metal composite oxide. The "coverage rate" can be calculated from the observation image obtained when the positive electrode active material layer is observed using a scanning electron microscope (SEM).

[0033] The thickness of the coating layers (first coating layer and second coating layer) is not particularly limited, but is preferably 1 to 100 nm, more preferably 2 to 50 nm, and even more preferably 3 to 20 nm. The thickness of the coating layers (first coating layer and second coating layer) can be determined by measuring the thickness at 50 or more locations on the coating layer in the observation image when the positive electrode active material layer is observed using a scanning electron microscope (SEM), and calculating the arithmetic mean.

[0034] The means for arranging the coating layers (first coating layer and second coating layer) on the surface of the lithium transition metal composite oxide (first lithium transition metal composite oxide and second lithium transition metal composite oxide) are not particularly limited. However, as shown in the examples described below, a raw material (element of A or a compound containing A) containing metallic lithium and A (Al, Zr, Nb, Ti, Ta, La, P, B, C, Si, S, Mo, or W) in the above general formula (2) is dissolved in a solvent to obtain a desired composition to prepare a raw material solution. By mixing this raw material solution with the lithium transition metal composite oxide and removing the solvent, particles (first particles and second particles) with a coating layer arranged on the surface of the lithium transition metal composite oxide can be obtained.

[0035] The content (total amount) of the first and second particles in the positive electrode active material layer is not particularly limited, but it is preferably more than 50% by mass, more preferably in the range of 50% to 95% by mass, and even more preferably in the range of 60% to 90% by mass, relative to 100% by mass of the total solid content contained in the positive electrode active material layer.

[0036] In the positive electrode for a lithium secondary battery according to this embodiment, the positive electrode active material layer includes a solid electrolyte. By including a solid electrolyte in the positive electrode active material layer, the lithium ion conductivity of the positive electrode active material layer can be improved. Examples of solid electrolytes include sulfide solid electrolytes and oxide solid electrolytes. In this specification, the term "solid electrolyte" refers to a material mainly composed of an ion conductor capable of ion conduction in a solid state, and in particular, a lithium ion conductivity of 1 × 10⁻¹⁶ at room temperature (25°C) is －５ This refers to a material with a lithium ion conductivity of S / cm or higher, and this lithium ion conductivity is preferably 1 × 10⁻⁶. －４ The value is S / cm or higher. Here, the ionic conductivity can be measured by the AC impedance method.

[0037] The solid electrolyte preferably is a sulfide solid electrolyte containing S element from the viewpoint of exhibiting excellent lithium ion conductivity and being more capable of following the volume change of the electrode active material accompanying charge and discharge. More preferably, it contains Li element, M element and S element, and the M element is a sulfide solid electrolyte containing at least one element selected from the group consisting of P, Si, Ge, Sn, Ti, Zr, Nb, Al, Sb, Br, Cl and I. Even more preferably, it is a sulfide solid electrolyte containing S element, Li element and P element.

[0038] The sulfide solid electrolyte may have a Li ３ P S ４ skeleton, and may have a Li ４ P ２ S ７ skeleton, and may have a Li ４ P ２ S ６ skeleton. Examples of the sulfide solid electrolyte having a Li ３ P S ４ skeleton include, for example, LiI-Li ３ P S ４ , LiI-LiBr-Li ３ P S ４ , Li ３ P S ４ . Further, examples of the sulfide solid electrolyte having a Li ４ P ２ S ７ skeleton include, for example, a Li-P-S based solid electrolyte called LPS. Further, as the sulfide solid electrolyte, for example, Li （４－ｘ） Ge （１－ｘ） P ｘ S ４ (where x satisfies 0 < x < 1), such as LGPS, may be used. More specifically, for example, LPS (Li ２ S-P ２ S ５ ), Li ７ P ３ S １１ , Li ３．２ P ０．９６ S, Li ３．２５ Ge ０．２５ P ０．７５ S ４ , Li １０GeP ２ S １２ , or Li ６ PS ５ Examples include X (where X is Cl, Br, or I). ２ S-P ２ S ５ The description of " is Li ２ S and P ２ S ５ This refers to a sulfide solid electrolyte made using a raw material composition containing the above, and the same applies to other descriptions. In particular, the sulfide solid electrolyte is preferably LPS (Li) because it has high ionic conductivity and a low bulk modulus, and can therefore follow the volume change of the electrode active material associated with charging and discharging. ２ S-P ２ S ５ ), Li ６ PS ５ X (where X is Cl, Br, or I), Li ７ P ３ S １１ Li ３．２ P ０．９６ S and Li ３ PS ４ It is selected from the group consisting of the following.

[0039] Examples of solid electrolyte shapes include spherical, ellipsoidal, or other particulate forms, as well as thin films. When the solid electrolyte is particulate, its average particle size Z is preferably smaller than the average particle size Y of the second lithium transition metal composite oxide. This configuration increases the contact points between the first and second particles and the solid electrolyte (increases the interface area), thereby reducing reaction resistance and further improving rapid charging and cycling characteristics.

[0040] The content of the solid electrolyte in the positive electrode active material layer is preferably in the range of 1 to 60% by mass, and more preferably in the range of 7 to 50% by mass.

[0041] The positive electrode active material layer may further contain, in addition to the first and second particles and the solid electrolyte, at least one of a binder and a conductive additive. Examples of binders include polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), and carboxymethylcellulose (CMC). Examples of conductive additives include fibrous conductive additives and particulate conductive additives. Examples of fibrous conductive additives include carbon fibers such as carbon nanotubes, carbon nanohorns, carbon nanofibers, carbon nanofilaments, carbon fibrils, and vapor-grown carbon fibers. Examples of particulate conductive additives are not particularly limited and include carbon powders such as acetylene black, Ketjen black (furnace black), channel black, and thermal black.

[0042] The thickness of the positive electrode active material layer varies depending on the configuration of the lithium secondary battery, but is preferably in the range of 0.1 to 1,000 μm, and more preferably 40 to 100 μm.

[0043] [Negative Electrode Active Material Layer] In the lithium secondary battery according to the above embodiment, the negative electrode active material layer 13 contains a negative electrode active material. The type of negative electrode active material is not particularly limited, but examples include carbon materials, metal oxides, and metal active materials. In addition, a lithium-containing metal may be used as the negative electrode active material. Such a negative electrode active material is not particularly limited as long as it is a lithium-containing active material, and examples include lithium metal and lithium-containing alloys. Examples of lithium-containing alloys include alloys of Li and at least one of In, Al, Si, and Sn. In some cases, two or more negative electrode active materials may be used in combination. Of course, negative electrode active materials other than those mentioned above may also be used. The negative electrode active material preferably contains lithium metal or lithium-containing alloy, silicon-based negative electrode active material, or tin-based negative electrode active material, and it is particularly preferable that it contains lithium metal or lithium-containing alloy. Furthermore, when lithium metal or a lithium-containing alloy is used as the negative electrode active material, the lithium secondary battery as an electrical device may be a so-called lithium deposition type, in which lithium metal as the negative electrode active material is deposited on the negative electrode current collector during the charging process. Therefore, in this configuration, the thickness of the negative electrode active material layer increases as the charging process progresses, and decreases as the discharge process progresses. The negative electrode active material layer does not need to be present during complete discharge, but in some cases, a negative electrode active material layer consisting of a certain amount of lithium metal may be present during complete discharge.

[0044] The shape of the negative electrode active material can be, for example, particulate (spherical, fibrous), thin film, etc. When the negative electrode active material is in particulate form, its average particle diameter is preferably in the range of 1 nm to 100 μm, more preferably in the range of 10 nm to 50 μm, even more preferably in the range of 100 nm to 20 μm, and particularly preferably in the range of 1 to 20 μm.

[0045] The content of the negative electrode active material in the negative electrode active material layer is not particularly limited, but is preferably in the range of 40 to 99% by mass, and more preferably in the range of 50 to 90% by mass. The negative electrode active material layer may further contain a solid electrolyte, a conductive additive and / or a binder, and the specific forms and preferred forms of these can be similar to those described in the section on the positive electrode active material layer above.

[0046] [Negative Electrode Intermediate Layer] If the lithium secondary battery according to the above embodiment is a lithium deposition type in which lithium metal or lithium-containing alloy as a negative electrode active material is deposited on the negative electrode current collector 11' during the charging process, it is preferable that the power generation element includes a negative electrode intermediate layer containing a lithium-reactive material between the negative electrode active material layer and the solid electrolyte layer. Examples of lithium-reactive materials include materials that can intercept and deintercept lithium ions during charging, and metals that can alloy with lithium during charging. The presence of such a negative electrode intermediate layer suppresses the growth of dendrites from the lithium metal layer (negative electrode active material layer) when lithium metal is deposited between the negative electrode intermediate layer and the negative electrode current collector during charging, thereby preventing short circuits in the battery and the resulting decrease in capacity.

[0047] While there are no particular limitations on the material capable of intercalating and deintercalating lithium ions, carbon materials are preferred. Specific examples of carbon materials include carbon black (specifically, acetylene black, Ketjenblack®, furnace black, channel black, thermal lamp black, etc.), carbon nanotubes (CNTs), graphite, and hard carbon. Among these, carbon black is preferred, and it is more preferable that it be at least one selected from the group consisting of acetylene black, Ketjenblack®, furnace black, channel black, and thermal lamp black.

[0048] Examples of metals that can be alloyed with lithium include In, Al, Si, Sn, Mg, Au, Ag, and Zn. Among these, In, Si, Sn, and Ag are preferred, with Ag being more preferred.

[0049] The lithium-reactive material may be used alone or in combination of two or more types. As a form of using two or more types in combination, a preferred embodiment is to use a material capable of intercalating and deintercalating lithium ions in combination with a metal capable of alloying with lithium. This ensures sufficient strength and lithium-ion conductivity of the negative electrode intermediate layer. More specifically, it is preferable to use nanoparticles made of In, Si, Sn, and Ag in combination with carbon black, and more preferably to use nanoparticles made of Ag in combination with carbon black. When using a material capable of intercalating and deintercalating lithium ions in combination with a metal capable of alloying with lithium, the mixing ratio (mass ratio) of these materials is not particularly limited, but the ratio of material capable of intercalating and deintercalating lithium ions to metal capable of alloying with lithium is preferably 10:1 to 1:1, and more preferably 5:1 to 2:1.

[0050] The content of lithium-reactive material in the negative electrode intermediate layer (referring to the total content of two or more materials when used in combination; the same applies hereinafter) is not particularly limited, but is preferably in the range of 50 to 100% by mass, more preferably in the range of 70 to 100% by mass, even more preferably in the range of 85 to 100% by mass, and particularly preferably in the range of 90 to 99% by mass.

[0051] The negative electrode intermediate layer may consist solely of lithium-reactive material if a self-supporting film can be fabricated using only lithium-reactive material, but may also contain a binder as needed. The type of binder is not particularly limited, and any known binder in the art can be used as appropriate; one example is described above.

[0052] The binder content in the negative electrode intermediate layer is not particularly limited, but is preferably in the range of 1 to 15% by mass, and more preferably in the range of 5 to 10% by mass. If the binder content is 1% by mass or more, a negative electrode intermediate layer with sufficient strength can be formed. If the binder content is 15% by mass or less, a negative electrode intermediate layer with sufficient lithium ion conductivity can be formed.

[0053] The thickness of the negative electrode intermediate layer is not particularly limited, but is preferably 1 to 50 μm, more preferably 5 to 40 μm, and even more preferably 10 to 30 μm. When the thickness of the negative electrode intermediate layer is 1 μm or more, the functions of the negative electrode intermediate layer can be fully exhibited. When the thickness of the negative electrode intermediate layer is 50 μm or less, the decrease in energy density can be suppressed.

[0054] [Positive electrode current collector plate and negative electrode current collector plate] The material constituting the current collector plates (25, 27) is not particularly limited, and known highly conductive materials conventionally used as current collector plates for secondary batteries can be used. Preferred materials for the current collector plates are metallic materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof. From the viewpoint of lightness, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The positive electrode current collector plate 27 and the negative electrode current collector plate 25 may be made of the same material, or different materials may be used.

[0055] [Positive and Negative Leads] Although not shown in the diagram, the current collectors (11', 11") and the current collector plates (25, 27) may be electrically connected via positive and negative leads. The materials used for the positive and negative leads may be the same as those used in known secondary batteries. It is preferable to cover the parts that are removed from the casing with heat-resistant insulating heat shrink tubing or the like to prevent leakage current from coming into contact with peripheral equipment or wiring and affecting the product (for example, automotive parts, especially electronic equipment).

[0056] [Battery casing material] As the battery casing material, known metal can cases can be used, or, as shown in Figure 1, a bag-shaped case made of aluminum-containing laminate film 29 that can cover the power generation elements can be used. For example, a three-layer laminate film made by laminating PP, aluminum, and nylon in that order can be used, but there are no limitations to these. Laminate film is preferable from the viewpoint of being able to increase output power and have excellent cooling performance, and can be suitably used for batteries in large equipment for EVs and HEVs. Furthermore, since the group pressure applied to the power generation elements from the outside can be easily adjusted, an aluminum-containing laminate film is more preferable for the casing.

[0057] The stacked secondary battery according to this embodiment has a configuration in which multiple single cell layers are connected in parallel, resulting in high capacity and excellent cycle durability. Therefore, the stacked secondary battery according to this embodiment is suitable for use as a power source for EVs and HEVs.

[0058] Although one embodiment of a lithium secondary battery has been described above, the present invention is not limited to the configuration described in the above-described embodiment, and can be modified as appropriate based on the description of the claims.

[0059] For example, one type of battery to which the lithium secondary battery according to the present invention is applied is a bipolar battery that includes a bipolar electrode having a positive electrode active material layer electrically coupled to one side of a current collector and a negative electrode active material layer electrically coupled to the opposite side of the current collector.

[0060] Furthermore, the lithium secondary battery according to this embodiment does not have to be all-solid type. That is, the solid electrolyte layer may further contain a conventionally known liquid electrolyte (electrolyte solution). There are no particular restrictions on the amount of liquid electrolyte (electrolyte solution) that can be contained in the solid electrolyte layer, but it is preferable that the amount is such that the shape of the solid electrolyte layer formed by the solid electrolyte is maintained and leakage of the liquid electrolyte (electrolyte solution) does not occur. As the liquid electrolyte (electrolyte solution), a solution having the form of a conventionally known lithium salt dissolved in a conventionally known organic solvent is used. The liquid electrolyte (electrolyte solution) may further contain additives other than the organic solvent and lithium salt. These additives may be used individually or in combination of two or more. Also, the amount of additives used in the electrolyte can be adjusted as appropriate.

[0061] Furthermore, the following embodiments are also included in the scope of the present invention: a positive electrode for a lithium secondary battery according to claim 1 having the features of claim 2; a positive electrode for a lithium secondary battery according to claim 1 or 2 having the features of claim 3; a positive electrode for a lithium secondary battery according to any one of claims 1 to 3 having the features of claim 4; a positive electrode for a lithium secondary battery according to any one of claims 1 to 4 having the features of claim 5; a positive electrode for a lithium secondary battery according to any one of claims 1 to 5 having the features of claim 6; a positive electrode for a lithium secondary battery according to any one of claims 1 to 6 having the features of claim 7; a positive electrode for a lithium secondary battery according to any one of claims 1 to 7 having the features of claim 8; a positive electrode for a lithium secondary battery according to any one of claims 1 to 8 having the features of claim 9; a positive electrode for a lithium secondary battery according to any one of claims 1 to 9 having the features of claim 10; and a lithium secondary battery according to claim 11 having a positive electrode for a lithium secondary battery according to claim 11.

[0062] The present invention will be described in more detail below with reference to examples. However, the technical scope of the present invention is not limited to the following examples.

[0063] <Preparation of the first and second particles> [Particle a] Solution A was prepared by dissolving 0.013 g of lithium metal in 8.8 g of ethanol. Solution B was prepared by mixing 1 g of zirconium(IV) propoxide (70%, 1-propanol solution) and 6 g of ethanol. Solution C was prepared by mixing 6.9 g of solution A and 2.4 g of solution B. Solution C and NMC composite oxide (LiNi) as a lithium transition metal composite oxide ０．８ Mn ０．１ Co ０．１ O ２ 5.0 g of single crystal (average particle size 12 μm) was placed in a flask and stirred for 1 hour. The solvent was removed by immersing the flask in 50°C water using an evaporator. The resulting solid was calcined at 300°C for 1 hour in an air atmosphere. This allowed Li to form on the entire surface of the NMC composite oxide. ２ ZrO ３ A particle a was obtained in which a coating layer (thickness 5 nm) containing the above was arranged.

[0064] [Particle b] NMC composite oxide (LiNi) as a lithium transition metal composite oxide ０．８ Mn ０．１ Co ０．１ O ２ Particle b was obtained using the same method as for [particle a] above, except that a single crystal with an average particle diameter of 4 μm was used.

[0065] [Particle c] NMC composite oxide (LiNi) as a lithium transition metal composite oxide ０．８ Mn ０．１ Co ０．１ O ２ Particle c was obtained using the same method as for [particle a] above, except that polycrystalline material with an average particle size of 8 μm was used.

[0066] [Particle d] NMC composite oxide (LiNi) as a lithium transition metal composite oxide ０．８ Mn ０．１ Co ０．１ O ２ Particle d was obtained using the same method as for [particle a] above, except that polycrystalline material with an average particle size of 4 μm was used.

[0067] [Particle e] NMC composite oxide (LiNi) as a lithium transition metal composite oxide０．８ Mn ０．１ Co ０．１ O ２ Particle e was obtained using the same method as for [particle a] above, except that polycrystalline material with an average particle size of 7 μm was used.

[0068] [Particle f] NMC composite oxide (LiNi) as a lithium transition metal composite oxide ０．８ Mn ０．１ Co ０．１ O ２ Particle f was obtained using the same method as for [particle a] above, except that a single crystal with an average particle diameter of 3 μm was used.

[0069] [Particle g] NMC composite oxide (LiNi) as a lithium transition metal composite oxide ０．８ Mn ０．１ Co ０．１ O ２ Particle g was obtained using the same method as for [particle a] above, except that polycrystalline material with an average particle size of 12 μm was used.

[0070] [Particle h] NMC composite oxide (LiNi) as a lithium transition metal composite oxide ０．８ Mn ０．１ Co ０．１ O ２ Particle h was obtained using the same method as for [particle a] above, except that polycrystalline material with an average particle size of 6 μm was used.

[0071] <Example of evaluation cell preparation> [Comparative Example 1] (Preparation of positive electrode active material layer) Particle a as the first particle and particle b as the second particle were weighed in a mass ratio of 70:30 and mixed to obtain a mixture. The mass ratio of lithium transition metal composite oxide in the first particle and lithium transition metal composite oxide in the second particle contained in the mixture was 70:30. Algyrodite-type sulfide solid electrolyte (Li ６ PS ５Cl (average particle size 0.6 μm), carbon nanofiber (CNF) (manufactured by Showa Denko K.K., VGCF®) as a conductive additive, and polytetrafluoroethylene (PTFE) as a binder were prepared. In a glove box with an argon atmosphere and a dew point of -68°C or lower, the above mixture, solid electrolyte, conductive additive, and PTFE were weighed in a mass ratio of 89:9.8:0.7:0.5 and kneaded in an agate mortar. After confirming that the binder had fibrillated, the obtained powder composition (mixture for forming the positive electrode active material layer) was supplied to the powder inlet set in a roll press machine. The powder composition was then rolled using the roll press machine (conditions are shown below) to form a sheet. The obtained sheet was folded in half, and the folded sheet was rolled again using the roll press machine (conditions are shown below) (folded rolling process). By repeating this folding and rolling process three times, a positive electrode active material layer with a thickness of 100 μm was obtained. In the folding and rolling process, the direction in which the sheet was folded in half and the direction in which the rolling process was applied to the folded sheet were randomized. (Conditions for the roll press machine) Roll size: 250 mmφ × 400 mm Roll rotation speed: 1 m / min Roll spacing (gap): 100 μm Pressure: 10 kN (linear pressure: 25 kN / m).

[0072] (Preparation of the solid electrolyte layer) In a glove box with an argon atmosphere and a dew point of -68°C or lower, an argyrodite-type sulfide solid electrolyte (Li) is prepared as the solid electrolyte. ６ PS ５ A solid electrolyte slurry was prepared by mixing 95 parts by mass of Cl with a binder solution (5 parts by mass of styrene-butadiene rubber (SBR) dissolved in mesitylene as a solvent). The obtained solid electrolyte slurry was coated onto the surface of a stainless steel foil support using an applicator and dried to obtain a solid electrolyte layer with a thickness of 40 μm.

[0073] (Preparation of the negative electrode intermediate layer) Acetylene black (AB) and silver (Ag) nanoparticles were weighed and mixed in a mass ratio of AB:Ag = 3:1. To 86 parts by mass of the resulting mixture, 14 parts by mass of polyvinylidene fluoride (PVDF) as a binder was added, and N-methylpyrrolidone was added as a solvent and mixed to prepare a negative electrode intermediate layer slurry. The negative electrode intermediate layer slurry was coated onto the surface of stainless steel foil, which was used as the negative electrode current collector, and dried to obtain a negative electrode intermediate layer with a thickness of 4 μm.

[0074] (Preparation of evaluation cell) The positive electrode active material layer prepared above was placed on top of the aluminum foil used as the positive electrode current collector. Then, the solid electrolyte layer formed on the surface of the stainless steel foil prepared above was placed on top of the positive electrode active material layer so that the exposed surface of the solid electrolyte layer faced the positive electrode active material layer, and the solid electrolyte layer was transferred onto the positive electrode active material layer by cold isostatic pressing (CIP). After peeling off the stainless steel foil adjacent to the solid electrolyte layer, the negative electrode intermediate layer formed on the surface of the stainless steel foil (negative electrode current collector) was placed on top of the transferred solid electrolyte layer so that the exposed surface of the solid electrolyte layer and the exposed surface of the negative electrode intermediate layer faced each other, and pressed by cold isostatic pressing (CIP). As a result, a laminate was obtained in which the positive electrode current collector, positive electrode active material layer, solid electrolyte layer, negative electrode intermediate layer, and negative electrode current collector were stacked in this order. Finally, an aluminum positive electrode tab and a nickel negative electrode tab were joined to the aluminum foil (positive electrode current collector) and stainless steel foil (negative electrode current collector) respectively using an ultrasonic welding machine. The resulting laminate was then placed inside an aluminum laminate film and vacuum-sealed to obtain an evaluation cell, which is a lithium deposition type all-solid-state lithium secondary battery of this comparative example.

[0075] [Comparative Example 2] An evaluation cell for this comparative example was obtained using the same method as in Comparative Example 1, except that, as the mixture used in the above (preparation of the positive electrode active material layer), particle c as the first particle and particle d as the second particle were weighed and mixed in a mass ratio of 70:30.

[0076] [Example 1] An evaluation cell for this example was obtained using the same method as in Comparative Example 1, except that, as the mixture used in the above (preparation of the positive electrode active material layer), particle c as the first particle and particle b as the second particle were weighed and mixed in a mass ratio of 70:30.

[0077] [Example 2] An evaluation cell for this example was obtained using the same method as in Comparative Example 1, except that, as the mixture used in the above (preparation of the positive electrode active material layer), particle e as the first particle and particle f as the second particle were weighed and mixed in a mass ratio of 70:30.

[0078] [Example 3] An evaluation cell for this example was obtained using the same method as in Comparative Example 1, except that, as the mixture used in the above (preparation of the positive electrode active material layer), particle g as the first particle and particle b as the second particle were weighed and mixed in a mass ratio of 70:30.

[0079] [Example 4] An evaluation cell for this example was obtained using the same method as in Comparative Example 1, except that, as the mixture used in the above (preparation of the positive electrode active material layer), particle e as the first particle and particle f as the second particle were weighed and mixed in a mass ratio of 80:20.

[0080] [Example 5] An evaluation cell for this example was obtained using the same method as in Comparative Example 1, except that, as the mixture used in the above (preparation of the positive electrode active material layer), particles h as the first particle and particles f as the second particle were weighed and mixed in a mass ratio of 70:30.

[0081] [Example 6] An evaluation cell for this example was obtained using the same method as in Comparative Example 1, except that the mixture used in the above (preparation of the positive electrode active material layer) was a mixture of first particle h and second particle f weighed in a mass ratio of 80:20.

[0082] [Example 7] An evaluation cell for this example was obtained using the same method as in Comparative Example 1, except that, as the mixture used in the above (preparation of the positive electrode active material layer), particles h as the first particle and particles f as the second particle were weighed and mixed in a mass ratio of 90:10.

[0083] <Evaluation of rapid charging characteristics> A constraining pressure of 3 MPa was applied to the stacking direction of the evaluation cells using a pressurizing member. Under these conditions, the upper and lower voltage limits were set to 2.5 to 4.25 V at a temperature of 60°C, and the current / cm² was set to 0.45 mA / cm². ２ and 9.0 mA / cm ２ CC charging and discharging were performed at the specified current values, and the charging capacity at each current value was measured. 0.45 mA / cm² ２ 9.0 mA / cm² relative to the charging capacity at the current value. ２ The charging capacity ratio, expressed as a percentage of the current value, was calculated, and the rapid charging characteristics were evaluated based on this value. A higher charging capacity ratio indicates superior rapid charging characteristics. The results are shown in Table 1 below.

[0084] <Evaluation of Cycle Characteristics> A constraining pressure of 3 MPa was applied to the stacking direction of the evaluation cells using a pressurizing member. Under these conditions, the upper and lower voltage limits were set to 2.5 to 4.3 V at a temperature of 25°C, and the current / cm² was set to 2.25 mA / cm². ２ The CC charge-discharge cycle was repeated 200 times at the specified current value. The ratio of the discharge capacity after 200 cycles to the initial discharge capacity (discharge capacity retention rate) was calculated as a percentage, and the cycle characteristics were evaluated from this value. A higher discharge capacity retention rate indicates superior cycle characteristics. The results are shown in Table 1 below.

[0085]

[0086] As shown in Table 1, according to the present invention, in a lithium secondary battery equipped with a positive electrode active material layer containing a solid electrolyte, it is possible to achieve both rapid charging characteristics and cycle characteristics by incorporating into the positive electrode active material layer first particles containing a polycrystalline, large-particle-sized first lithium transition metal composite oxide and a first coating layer, and second particles containing a single-crystal, small-particle-sized second lithium transition metal composite oxide and a second coating layer.

[0087] A comparison between Example 1 and Example 3 shows that by setting the ratio (X / Y) of the average particle diameter X of the first lithium transition metal composite oxide to the average particle diameter Y of the second lithium transition metal composite oxide to less than 3, the rapid charging characteristics and cycle characteristics are further improved. Examples 1 to 7 show a tendency for cycle characteristics to improve as the average particle diameter X of the first lithium transition metal composite oxide decreases.

[0088] Comparing Examples 5 to 7 (examples where the average particle size X of the first lithium transition metal composite oxide is 6 μm), it can be seen that as the ratio of the mass x of the first lithium transition metal composite oxide to the mass y of the second lithium transition metal composite oxide (x / y) increases, the rapid charging characteristics and cycle characteristics also improve further.

[0089] 10a Stacked secondary battery, 11' Negative electrode current collector, 11" Positive electrode current collector, 13 Negative electrode active material layer, 14 Negative electrode intermediate layer, 15 Positive electrode active material layer, 17 Solid electrolyte layer, 19 Single cell layer, 21 Power generation element, 25 Negative electrode current collector plate, 27 Positive electrode current collector plate, 29 Laminate film, 30 First particle, 31 First lithium transition metal composite oxide, 32 First coating layer, 40 Second particle, 41 Second lithium transition metal composite oxide, 42 Second coating layer, 50 Solid electrolyte.

Claims

1. A positive electrode for a lithium secondary battery having a positive electrode active material layer comprising: first particles comprising a first lithium transition metal composite oxide and a first coating layer disposed on the surface of the first lithium transition metal composite oxide; second particles comprising a second lithium transition metal composite oxide and a second coating layer disposed on the surface of the second lithium transition metal composite oxide; and a solid electrolyte, wherein the first lithium transition metal composite oxide is polycrystalline, the second lithium transition metal composite oxide is single-crystal, the average particle diameter X of the first lithium transition metal composite oxide is greater than the average particle diameter Y of the second lithium transition metal composite oxide, and the first coating layer and the second coating layer each independently contain a material that conforms to the general formula (2): Li ｘ AO ｙ A positive electrode for a lithium secondary battery comprising a lithium-ion conductive compound represented as follows: (A is Al, Zr, Nb, Ti, Ta, La, P, B, C, Si, S, Mo, or W, and x and y each represent independent positive integers).

2. The positive electrode for a lithium secondary battery according to claim 1, wherein the average particle size Z of the solid electrolyte is smaller than the average particle size Y of the second lithium transition metal composite oxide.

3. The positive electrode for a lithium secondary battery according to claim 1, wherein the average particle size X of the first lithium transition metal composite oxide is 1 μm or more and 12 μm or less.

4. The positive electrode for a lithium secondary battery according to claim 1, wherein the ratio (X / Y) of the average particle diameter X of the first lithium transition metal composite oxide to the average particle diameter Y of the second lithium transition metal composite oxide is less than 3.

5. The positive electrode for a lithium secondary battery according to claim 1, wherein the ratio (X / Y) of the average particle diameter X of the first lithium transition metal composite oxide to the average particle diameter Y of the second lithium transition metal composite oxide is 2.5 or less.

6. The positive electrode for a lithium secondary battery according to claim 1, wherein the ratio (x / y) of the mass x of the first lithium transition metal composite oxide to the mass y of the second lithium transition metal composite oxide in the positive electrode active material layer is greater than 1.

7. The positive electrode for a lithium secondary battery according to claim 1, wherein the average particle size X of the first lithium transition metal composite oxide is 5 μm or more and less than 7 μm, and the ratio (x / y) of the mass x of the first lithium transition metal composite oxide to the mass y of the second lithium transition metal composite oxide in the positive electrode active material layer is 4 or more.

8. The positive electrode for a lithium secondary battery according to claim 1, wherein the average particle size X of the first lithium transition metal composite oxide is 5 μm or more and less than 7 μm, and the ratio (x / y) of the mass x of the first lithium transition metal composite oxide to the mass y of the second lithium transition metal composite oxide in the positive electrode active material layer is 6 or more.

9. The positive electrode for a lithium secondary battery according to claim 1, wherein the average particle size X of the first lithium transition metal composite oxide is 7 μm or more and 12 μm or less, and the ratio (x / y) of the mass x of the first lithium transition metal composite oxide to the mass y of the second lithium transition metal composite oxide in the positive electrode active material layer is 3 or less.

10. The positive electrode for a lithium secondary battery according to claim 1, wherein the first coating layer and the second coating layer substantially do not contain conductive carbon material.

11. A lithium secondary battery comprising a power generation element having a positive electrode for a lithium secondary battery according to any one of claims 1 to 10, a negative electrode, and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte.

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