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

Abstract

The present invention provides a means capable of improving cycle characteristics in a lithium secondary battery including a positive electrode active material layer that contains a solid electrolyte. In the positive electrode active material layer that contains a solid electrolyte, a combination of a lithium transition metal composite oxide having a large particle diameter and a lithium transition metal composite oxide having a small particle diameter is used as a positive electrode active material, and the composite oxides are coated with a lithium ion conductive compound. When doing so, the compound covering the latter composite oxide is more electrochemically stable than the compound covering the former composite oxide.

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 cathode 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 cathode 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 cathode active material having such a configuration to the cathode 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 cycle characteristics may not be obtained.

[0005] Therefore, the present invention aims to provide a means for improving the cycle characteristics of a lithium secondary battery equipped with a positive electrode active material layer containing a solid electrolyte.

[0006] The inventors of this invention conducted intensive studies in view of the above-mentioned problems. In the process, they discovered that the above-mentioned problems could be solved by using a combination of a large-particle lithium transition metal composite oxide and a small-particle lithium transition metal composite oxide as the positive electrode active material, and coating each of these composite oxides with a lithium-ion conductive compound. In this process, they found that the compound coating the latter composite oxide was more electrochemically stable than the compound coating the former composite oxide, and thus completed 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 and containing a first lithium ion conductive compound; 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 containing a second lithium ion conductive compound; and a solid electrolyte. In this positive electrode for a lithium secondary battery, 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 is located on the outermost surface of the first particles, and the second coating layer is located on the outermost surface of the second particles. Furthermore, the oxidation potential of the second lithium ion conductive compound is higher than the oxidation potential of the first lithium ion conductive compound.

[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 and containing a first lithium ion conductive compound; 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 containing a second lithium ion conductive compound; and a solid electrolyte, wherein 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 is located on the outermost surface of the first particles, the second coating layer is located on the outermost surface of the second particles, and the oxidation potential of the second lithium ion conductive compound is higher than the oxidation potential of the first lithium ion conductive compound. According to the positive electrode for a lithium secondary battery of this embodiment (hereinafter also simply referred to as "the positive electrode of this embodiment"), the cycle characteristics can be improved in a lithium secondary battery equipped with a positive electrode active material layer containing a solid electrolyte.

[0010] Conventionally, in positive electrode active material layers containing a solid electrolyte, when a lithium transition metal composite oxide is used as the positive electrode active material, it is known that as charging and discharging progresses, a high-resistance layer is formed at the contact interface between the solid electrolyte and the lithium transition metal composite oxide, increasing interfacial resistance and gradually decreasing battery capacity. To address this problem, a technique has been proposed to suppress the formation of a high-resistance layer by coating the surface of the lithium transition metal composite oxide with a coating layer to prevent contact between the solid electrolyte and the lithium transition metal composite oxide. Separately, a technique has been proposed to improve the energy density of lithium secondary batteries by using both large-particle (hereinafter also referred to as "large particles") and small-particle (hereinafter also referred to as "small particles") as positive electrode active materials in the positive electrode active material layer, filling the voids between the large particles with small particles. Therefore, the present inventors prepared large and small particles of lithium transition metal composite oxide and fabricated a positive electrode active material layer using these particles coated with a coating layer. However, it was found that in lithium secondary batteries to which this positive electrode active material layer was applied, sufficient cycle characteristics could not be obtained in some cases. Therefore, further investigations were conducted by changing the materials of the coating layer in various ways. In the process, it was discovered that by using a lithium-ion conductive compound as the material for the coating layer, and by forming the coating layer of small particles using a lithium-ion conductive compound with higher electrochemical stability than the lithium-ion conductive compound used for the large-particle coating layer, the cycle characteristics were improved, leading to the completion of the present invention. The inventors speculate that the mechanism by which these effects are achieved is as follows: The diffusion resistance of lithium ions in lithium transition metal composite oxides is greater the larger the particle size of the lithium transition metal composite oxide. Therefore, the charge-discharge reaction proceeds more easily and degrades faster in small particles with lower diffusion resistance than in large particles with higher diffusion resistance. By employing a lithium-ion conductive compound with high electrochemical stability in the coating layer of small particles, the degradation of small particles is suppressed, and the difference between the degradation state of small particles and the degradation state of large particles becomes smaller. As a result, the charge-discharge reaction proceeds more uniformly in the positive electrode active material layer, and it is speculated that this improves the cycle characteristics.It should be noted that the mechanism described above is based solely on speculation, and its accuracy does not affect the technical scope of the present invention.

[0011] 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.

[0012] 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.

[0013] 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 stacked. Furthermore, a negative electrode intermediate layer 14 is arranged so as to be in contact with the negative electrode active material layer 13 and the solid electrolyte layer 17, respectively.

[0014] The positive electrode has a structure in which a positive electrode active material layer 15 is arranged on the surface of the positive electrode current collector 11''. As a result, 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 cell layer 19. Therefore, the stacked secondary battery 10a shown in Figure 1 can also be said to have a configuration in which multiple single cell layers 19 are stacked and electrically connected in parallel.

[0015] A negative current collector 11' and a positive current collector 11" are each attached with a negative current collector plate 25 and a positive current collector plate 27 that are electrically connected to the respective electrodes (negative electrode and positive electrode), and have a structure that is led out to the outside of the laminate film 29 so as to be sandwiched between the ends of the laminate film 29. In the laminated secondary battery 10a, a restraining pressure is applied in the lamination direction of the power generation element 21 by a pressing member. Therefore, the volume of the power generation element 21 is kept constant.

[0016] 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 that is single crystal, and a second coat layer 42 disposed on the surface of the second lithium transition metal composite oxide 41. The first coat layer 32 and the second coat layer 42 are present on the outermost surfaces of the first particles 30 and the second particles 40, respectively. In the positive electrode active material layer 15 shown in FIG. 2, the first coat layer 32 contains Li ２ ZrO ３ as a lithium ion conductive compound, and the second coat layer 42 contains Li ３ BO ３ as a lithium ion conductive compound. The oxidation potential of Li ３ BO ３ is Li ２ ZrO ３It is higher than the oxidation potential and more electrochemically stable. In the positive electrode active material layer 15 shown in Figure 2, the entire surface of the first lithium transition metal composite oxide 31 is covered by the first coating layer 32. Also, the entire surface of the second lithium transition metal composite oxide 41 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 positive electrode 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 positive electrode active material layer 15, the small particle size second particles 40 fill the gaps between the large particle size first particles 30. 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).

[0017] The main components of the lithium secondary battery according to this embodiment will be described below.

[0018] [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.

[0019] 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.

[0020] [Positive Electrode Active Material Layer] In the positive electrode for a lithium secondary battery according to this embodiment, the positive electrode active material layer comprises: 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 and containing a first lithium ion conductive compound; 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 containing a second lithium ion conductive compound; and a solid electrolyte. In this specification, ordinal numbers such as "first," "second," and "third" are used to describe the components, but the numbers themselves do not have any particular meaning.

[0021] The first and second particles (hereinafter, the first and second particles together will also be referred to as "composite particles") have a configuration in which a first coating layer and a second coating layer are arranged on the surface of a first lithium transition metal composite oxide and a second lithium transition metal composite oxide, respectively.

[0022] Lithium transition metal composite oxides function as positive electrode active materials. These lithium transition metal composite oxides are compounds belonging to space group R3m, and preferably have a layered structure (layered rock salt type structure) in which lithium atomic layers and transition metal atomic layers are alternately stacked. Using such a positive electrode active material can improve the battery capacity of lithium secondary batteries.

[0023] 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 ２Examples 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.

[0024] 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.

[0025] 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 ２The 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).

[0026] 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.

[0027] In the positive electrode for a lithium secondary battery according to this embodiment, it is essential 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 (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 greater than 1). 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. By making the average particle diameter X of the first lithium transition metal composite oxide larger than the average particle diameter Y of the second lithium transition metal composite oxide (i.e., by using a lithium transition metal composite oxide with a large particle size and a lithium transition metal composite oxide with a small particle size in combination as the positive electrode active material), the second particles containing the second lithium transition metal composite oxide can more easily fill the voids between the particles of the first particles containing the first lithium transition metal composite oxide, thereby improving the energy density. The ratio (X / Y) is preferably 2 or more, and more preferably 3 or more. If the ratio (X / Y) is within this range, the energy density can be further improved. Also, the battery resistance can be further reduced. The ratio (X / Y) is preferably 10 or less, and more preferably 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, so an increase in internal resistance and unevenness in the battery reaction can be suppressed. The ratio (X / Y) is preferably 2 or more and 10 or less, and more preferably 3 or more and 10 or less.

[0028] The crystal morphology of the first lithium transition metal composite oxide and the second lithium transition metal composite oxide is not particularly limited and may be either "polycrystalline" or "single crystal," but it is preferable that the former be polycrystalline and the latter be single crystal. This configuration can further improve cycle characteristics and reduce resistance. The mechanism by which these effects are obtained is thought to be that by making the first lithium transition metal composite oxide, which has a large grain size, polycrystalline, the increase in the diffusion resistance of lithium ions is suppressed, and by making the second lithium transition metal composite oxide, which has a small grain size, single crystal, cracking when pressure is applied is suppressed. 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."

[0029] The average particle size X of the first lithium transition metal composite oxide is not particularly limited, but is preferably 1 μm to 12 μm, more preferably 1 μm to 10 μm, and even more preferably 1 μm to 8 μm. 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 cycle characteristics of the lithium secondary battery can be further improved.

[0030] 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 to 6 μm, more preferably 1 μm to 5 μm, even more preferably 1 μm to 4 μm, and particularly preferably 1 μm to 3 μm. If the average particle diameter Y is within the above range, further improvement in energy density can be achieved. In addition, the resistance of the battery can be further reduced.

[0031] 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, it is more preferable that the ratio (x / y) is 1.5 or more, even more preferable that it is 2 or more, and particularly preferable that it is between 2 and 20.

[0032] Next, the first and second coating layers will be described. The first coating layer is disposed on the surface of the first lithium transition metal composite oxide and is located on the outermost surface of the first particle. Similarly, the second coating layer is disposed on the surface of the second lithium transition metal composite oxide and is located on the outermost surface of the second particle. By being located on the outermost surfaces of the first and second particles, the first and second coating layers prevent contact between the lithium transition metal composite oxide and the solid electrolyte, and have the function of suppressing the formation of a high-resistance layer due to the reaction of the solid electrolyte. In this specification, "displaying 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 component exists between the surface of A and B (for example, a form in which a third coating layer exists between the surface of the second lithium transition metal composite oxide and the second coating layer).

[0033] The positive electrode for lithium secondary batteries according to this embodiment is characterized in that the oxidation potential of the second lithium-ion conductive compound contained in the second coat layer is higher than the oxidation potential of the first lithium-ion conductive compound contained in the first coat layer. In this specification, the oxidation potential of the lithium-ion conductive compound is measured as follows. First, a test cell for measuring the oxidation potential was prepared in a glove box with an argon atmosphere and a dew point of -68°C or lower. A stainless steel cylindrical convex punch (10 mm diameter, working electrode) was inserted into one side of a Macol cylindrical tube jig (10 mm inner diameter, 23 mm outer diameter, 20 mm height), and 80 mg of the sample to be measured (lithium-ion conductive compound) was placed in from the top of the cylindrical tube jig. Then, another stainless steel cylindrical convex punch was inserted to sandwich the sample to be measured, and a pellet consisting of the sample to be measured with a diameter of 10 mm and a thickness of approximately 0.6 mm was formed in the cylindrical tube jig by pressing with a hydraulic press at a pressure of 300 MPa for 3 minutes. The lower stainless steel cylindrical convex punch was removed, indium foil (9 mm diameter) and lithium foil (8 mm diameter, counter electrode / reference electrode) were inserted from the bottom of the cylindrical tube jig, the stainless steel cylindrical convex punch was reinserted, and the cell was pressed at a pressure of 75 MPa for 3 minutes to create a test cell in which the working electrode (stainless steel), the object to be measured, and the counter electrode / reference electrode (metallic lithium) were stacked in this order. Next, the oxidation potential was measured using cyclic voltammetry. The test cell was placed in a constant temperature bath set to 25°C, and after the cell temperature stabilized, the cycle of oxidation from the open-circuit voltage (OCV) to 5 V relative to lithium, and then reduction to 1 V relative to lithium was repeated 5 times with a sweep rate of 1 mV / s. The oxidation potential was defined as the potential that shows a current value of 5% of the maximum absolute value of the current value in the 5th oxidation cycle.

[0034] The lithium-ion conductive compound is not particularly limited as long as it is a compound having lithium-ion conductivity, but in one preferred embodiment, the first lithium-ion conductive compound contains the element oxygen (O), and the second lithium-ion conductive compound contains the element oxygen (O). Such lithium-ion conductive compounds are not particularly limited, but it is preferable that they include compounds represented by the following general formula (2). That is, in one more preferred embodiment, the first lithium-ion conductive compound and the second lithium-ion conductive compound are each independently represented by the general formula (2): Li ｘ AO ｙ The compounds include those represented by (A being Al, Zr, Nb, Ti, Ta, La, P, B, C, Si, S, Mo, or W, and x and y each representing an independent positive integer).

[0035] 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 ３ Li 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 ３and 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 this be used.

[0036] The first lithium-ion conductive compound and the second lithium-ion conductive compound can be appropriately selected such that the former has a relatively low oxidation potential and the latter has a relatively high oxidation potential. Specific combinations include the following: Combination 1: The second lithium-ion conductive compound contains boron (B) or niobium (Nb), and the first lithium-ion conductive compound contains zirconium (Zr) (more preferably, the second lithium-ion conductive compound contains Li ３ BO ３ Or LiNbo ３ It contains, and the first lithium ion conductive compound is Li ２ ZrO ３ (This includes...) Combination 2: The second lithium-ion conductive compound contains boron (B), and the first lithium-ion conductive compound contains niobium (Nb) or zirconium (Zr) (more preferably, the second lithium-ion conductive compound contains Li ３ BO ３ It contains, and the first lithium ion conductive compound is LiNbO ３ or Li ２ ZrO ３ (This includes...) Preferred combinations include the following: The second lithium-ion conductive compound contains boron (B) element, and the first lithium-ion conductive compound contains zirconium (Zr) element (more preferably, the second lithium-ion conductive compound contains Li... ３ BO ３ It contains, and the first lithium ion conductive compound is Li ２ ZrO ３(including such). According to such a combination, the cycle characteristics can be further improved.

[0037] As described above, as long as the first coating layer exists on the outermost surface, the first particle may have another member (for example, another coating layer) between the first lithium transition metal composite oxide and the first coating layer. Similarly, as long as the second coating layer exists on the outermost surface, the second particle may have another member (for example, another coating layer) between the second lithium transition metal composite oxide and the second coating layer. In particular, in the second particle, by providing another coating layer between the second lithium transition metal composite oxide and the second coating layer, it is possible to improve battery performance such as further improvement of cycle characteristics and further reduction of resistance. According to a preferred embodiment, the second lithium ion conductive compound contains the boron (B) element, and the first lithium ion conductive compound contains the zirconium (Zr) element (more preferably, the second lithium ion conductive compound is Li ３ BO ３ and the first lithium ion conductive compound is Li ２ ZrO ３ In the case of including), the second particle further has a third coating layer containing a third lithium ion conductive compound between the second lithium transition metal composite oxide and the second coating layer, and the third lithium ion conductive compound contains the zirconium (Zr) element (more preferably, the third lithium ion conductive compound is Li ２ ZrO ３ and is included).

[0038] Preferably, the coating layers (first coating layer, second coating layer, and third coating layer) 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 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.

[0039] The coating layers (first coating layer, second coating layer, and third 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).

[0040] The thickness of the coating layers (first coating layer, second coating layer, and third 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, second coating layer, and third 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.

[0041] The means of providing a coating layer (first coating layer, second coating layer, and third coating layer) on the surface of a lithium transition metal composite oxide (first lithium transition metal composite oxide and second lithium transition metal composite oxide) are not particularly limited. For example, as a lithium ion conductive compound, the above general formula (2): Li ｘ AO ｙ When using an oxide represented by the formula, as shown in the examples 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 placed on the surface of the lithium transition metal composite oxide can be obtained.

[0042] 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.

[0043] In the positive electrode for a lithium secondary battery according to this embodiment, the positive electrode active material layer contains 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 the solid electrolyte include sulfide solid electrolytes and oxide solid electrolytes. In this specification, the solid electrolyte refers to a material mainly composed of an ion conductor capable of ionic conduction in a solid, and in particular, the lithium ion conductivity at room temperature (25 °C) is 1 × 10 －５ S / cm or more, and this lithium ion conductivity is preferably 1 × 10 －４ S / cm or more. Here, the value of the ion conductivity can be measured by the alternating current impedance method.

[0044] From the viewpoint that the solid electrolyte exhibits excellent lithium ion conductivity and can better follow the volume change of the electrode active material accompanying charge and discharge, it is preferably a sulfide solid electrolyte containing the S element, more preferably a sulfide solid electrolyte containing the Li element, the M element, and the S element, and the M element is at least one element selected from the group consisting of P, Si, Ge, Sn, Ti, Zr, Nb, Al, Sb, Br, Cl, and I, and even more preferably a sulfide solid electrolyte containing the S element, the Li element, and the P element.

[0045] The sulfide solid electrolyte may have a Li ３ PS 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 PS ４ skeleton include, for example, LiI - Li ３ PS ４ , LiI - LiBr - Li ３ PS ４ , Li ３ PS ４ . Also, Li ４ P ２ S ７Examples of sulfide solid electrolytes having a framework include Li-P-S system solid electrolytes called LPS. Also, as sulfide solid electrolytes, for example, Li （４－ｘ） Ge （１－ｘ） P ｘ S ４ You may also use LGPS, etc., which is represented as (where x satisfies 0 < x < 1). 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 １０ GeP 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.

[0046] 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 lithium-2 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 cycle characteristics.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] [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.

[0051] 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.

[0052] 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.

[0053] [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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] [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.

[0062] [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).

[0063] [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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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 2 having the features of claim 3; a positive electrode for a lithium secondary battery according to claim 2 or 3 having the features of claim 4; a positive electrode for a lithium secondary battery according to claim 2 or 3 having the features of claim 5; a positive electrode for a lithium secondary battery according to claim 2 or 3 having the features of claim 6; a positive electrode for a lithium secondary battery according to any one of claims 2 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.

[0069] 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.

[0070] <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 polycrystalline material (average particle size 8 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.

[0071] [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 polycrystalline material with an average particle size of 4 μm was used.

[0072] [Particle c] Solution A was prepared by dissolving 0.012 g of lithium metal in 8.0 g of ethanol. Solution B was prepared by mixing 0.1 g of boric acid and 16 g of ethanol. Solution C was prepared by mixing 10.0 g of Solution A and 7.1 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 polycrystalline material (average particle size 8 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. ３ BO ３A particle c was obtained in which a coating layer (thickness 5 nm) containing the above was arranged.

[0073] [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 c] above, except that polycrystalline material with an average particle size of 4 μm was used.

[0074] [Particle e] Solution A was prepared by dissolving 0.012 g of lithium metal in 8.0 g of ethanol. Solution B was prepared by mixing 0.1 g of boric acid and 16 g of ethanol. Solution C was prepared by mixing 10.0 g of Solution A and 7.1 g of Solution B. Solution C and 5.0 g of the above particle b were 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 obtained solid was calcined at 300°C for 1 hour in an air atmosphere. As a result, Li was deposited on the entire surface of the NMC composite oxide. ２ ZrO ３ A coating layer (5 nm thick) containing Li ３ BO ３ A particle e was obtained in which a coating layer (thickness 5 nm) containing the above was stacked in this order.

[0075] [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 c] above, except that a single crystal with an average particle diameter of 4 μm was used.

[0076] [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 c] above, except that polycrystalline material with an average particle size of 2.5 μm was used.

[0077] <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 ５ Cl (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).

[0078] (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 ５A 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.

[0079] (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.

[0080] (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.

[0081] [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 b as the second particle were weighed and mixed in a mass ratio of 70:30.

[0082] [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 a as the first particle and particle d as the second particle were weighed and mixed in a mass ratio of 70:30.

[0083] [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 a as the first particle and particle e as the second particle were weighed and mixed in a mass ratio of 70:30.

[0084] [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 a as the first particle and particle f as the second particle were weighed and mixed in a mass ratio of 70:30.

[0085] [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 a as the first particle and particle g as the second particle were weighed and mixed in a mass ratio of 70:30.

[0086] <Evaluation of Discharge Capacity Retention Rate> 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 limit voltages were set to 2.5 to 4.3 V at a temperature of 25°C, and the discharge rate was 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.

[0087] <Evaluation of Resistance During Charging> Positive and negative leads were connected to the positive and negative current collectors of the evaluation cell (before initial charging), and the cell was charged to SOC = 50% while applying a constraining pressure of 3 MPa in the stacking direction of the evaluation cell using a pressurizing member. Subsequently, charging was repeated in the order of (1) to (7) below, and the relationship between current and voltage rise was plotted. The resistance during charging (Ω / cm) was determined from the slope of this plotted graph. ２ The following calculations (1) to (7) were all performed at a temperature of 25°C. The results are shown in Table 1 below. (1) 0.05C CC charging 15 sec (2) 2-hour pause (3) 0.1C CC charging 15 sec (4) 2-hour pause (5) 0.2C CC charging 15 sec (6) 2-hour pause (7) 0.5C CC charging 15 sec.

[0088]

[0089] As shown in Table 1, according to the present invention, it is possible to improve the cycle characteristics by incorporating specific first and second particles together with a solid electrolyte in the positive electrode active material layer.

[0090] Furthermore, the results shown in Table 1 also show the following: A comparison between Example 1 and Example 2 shows that by providing a specific third coat layer between the second lithium transition metal composite oxide and the second coat layer, resistance is reduced and cycle characteristics are further improved. A comparison between Example 1 and Example 3 shows that by making the first lithium transition metal composite oxide polycrystalline and the second lithium transition metal composite oxide single crystal, resistance is reduced and cycle characteristics are further improved. A comparison between Example 1 and Example 4 shows that by making the ratio (X / Y) 3 or more, resistance is reduced.

[0091] 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 and containing a first lithium ion conductive compound; 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 containing a second lithium ion conductive compound; and a solid electrolyte, wherein 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 is located on the outermost surface of the first particles; the second coating layer is located on the outermost surface of the second particles; and the oxidation potential of the second lithium ion conductive compound is higher than the oxidation potential of the first lithium ion conductive compound.

2. The positive electrode for a lithium secondary battery according to claim 1, wherein the first lithium-ion conductive compound contains the element oxygen (O), and the second lithium-ion conductive compound contains the element oxygen (O).

3. The first lithium-ion conductive compound and the second lithium-ion conductive compound are each independently of the general formula (2): Li ｘ AO ｙ A positive electrode for a lithium secondary battery according to claim 2, comprising an oxide represented by (A is Al, Zr, Nb, Ti, Ta, La, P, B, C, Si, S, Mo, or W, and x and y each independently represent positive integers).

4. The positive electrode for a lithium secondary battery according to claim 2, wherein the second lithium-ion conductive compound comprises boron (B) or niobium (Nb), and the first lithium-ion conductive compound comprises zirconium (Zr).

5. The positive electrode for a lithium secondary battery according to claim 2, wherein the second lithium-ion conductive compound contains the element boron (B), and the first lithium-ion conductive compound contains the element niobium (Nb) or the element zirconium (Zr).

6. The positive electrode for a lithium secondary battery according to claim 2, wherein the second lithium-ion conductive compound contains the element boron (B), and the first lithium-ion conductive compound contains the element zirconium (Zr).

7. The positive electrode for a lithium secondary battery according to claim 6, wherein the second particle further has a third coat layer containing a third lithium ion conductive compound between the second lithium transition metal composite oxide and the second coat layer, and the third lithium ion conductive compound contains the element zirconium (Zr).

8. The positive electrode active material layer according to claim 1, wherein the first lithium transition metal composite oxide is polycrystalline and the second lithium transition metal composite oxide is single-crystal.

9. The positive electrode active material layer 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 or more.

10. The positive electrode active material layer 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 3 or more.

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.

Images