Positive electrode for secondary batteries, and secondary battery

Abstract

Provided is a positive electrode for secondary batteries, which comprises a positive electrode collector and a positive electrode mixture layer that is provided on the surface of the positive electrode collector, wherein: the positive electrode mixture layer contains a positive electrode active material and a binder; the positive electrode active material contains a first metal composite oxide that has a first particle size distribution and a second metal composite oxide that has a second particle size distribution; the volume-based median diameters D1 and D2 of the first metal composite oxide and the second metal composite oxide satisfy D1>D2; the average primary particle size d1 of the first metal composite oxide and the average primary particle size d2 of the second metal composite oxide satisfy d1<d2; the ratio of the mass of the first metal composite oxide W1 to the total mass of the mass W1 and the mass of the second metal composite oxide W2 contained in the positive electrode mixture layer is 50% or more; and the absolute value |R1-R2| of the difference (R1-R2) between the powder resistances R1 and R2 (Ω∙cm) of the first metal composite oxide and the second metal composite oxide is 15 or less.

Description

Positive electrode for secondary battery and secondary battery Cross-reference to related applications

[0001] This disclosure claims the benefit of priority of Japanese Patent Application No. 2025-008593, filed on January 21, 2025, with the Japan Patent Office, and the entire contents of the said patent application are incorporated herein by reference.

[0002] This disclosure relates to a positive electrode for a secondary battery and a secondary battery.

[0003] Patent Document 1 discloses a positive electrode material containing a first positive electrode active material represented by the following Chemical Formula 1 and a second positive electrode active material represented by the following Chemical Formula 2. The positive electrode material has a bimodal particle size distribution including large particle size particles and small particle size particles, and the difference in average particle size (D ｂ , ｅ , ２ , ｄ , ｚ , ｃ , ｘ , ａ , １ , ｂ , ２ , ２ , ａ , ｕ , １ , ｖ , ｂ , ｗ , ５０ , ｙ , ２ , ａ ) is 3 μm or more, [Chemical Formula 1] Li ａ [Ni ｂ Co ｃ M １ ｄ M ａ ｅ O ２ In the Chemical Formula 1, M １ is Mn, Al, Zr or Mg, and M ａ is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, In, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, 0.9 ≦ a ≦ 1.2, 0.8 ≦ b < 1, 0.01 ≦ c < 0.2, 0.01 ≦ d < 0.2, 0 ≦ e ≦ 0.02, [Chemical Formula 2] Li<00000\alpha>[Ni ｙ Co ｚ Mn ｗ M ２ <00000\alpha>M ｂ ｕ O ２ In the Chemical Formula 2, M ２ is Al, Mg, Zr or Ti, and M ｂThe proposed "cathode material" is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, In, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and satisfies the following conditions: 0.9 ≤ x ≤ 1.2, 0.8 ≤ y < 1, 0.01 ≤ z < 0.2, 0.01 ≤ w < 0.2, 0.01 ≤ v < 0.2, and 0 ≤ u ≤ 0.02.

[0004] Special table 2021-501982 publication

[0005] By using a positive electrode active material having a bimodal particle size distribution containing both large and small particles, it is possible to increase the capacity of the positive electrode. On the other hand, if such a secondary battery is stored in a high-temperature environment while charged, its capacity may deteriorate rapidly.

[0006] One aspect of this disclosure comprises a positive electrode current collector and a positive electrode mixture layer provided on the surface of the positive electrode current collector, wherein the positive electrode mixture layer comprises a positive electrode active material and a binder, wherein the positive electrode active material comprises a first metal composite oxide having a first particle size distribution and a second metal composite oxide having a second particle size distribution, wherein the volume-based median diameter D1 of the first metal composite oxide and the volume-based median diameter D2 of the second metal composite oxide satisfy D1 > D2, and the average primary particle size d of the first metal composite oxide The present invention relates to a positive electrode for a secondary battery, wherein the average primary particle size d2 of the second metal composite oxide satisfies d1 < d2, the ratio of the mass W1 of the first metal composite oxide to the total mass of the first metal composite oxide and the second metal composite oxide W2 contained in the positive electrode mixture layer is 50% or more, and the absolute value of the difference (R1 - R2) between the powder resistance R1 (Ω・cm) of the first metal composite oxide and the powder resistance R2 (Ω・cm) of the second metal composite oxide is 15 or less.

[0007] Another aspect of this disclosure relates to a secondary battery comprising the above-mentioned positive electrode for a secondary battery, a separator, a negative electrode facing the positive electrode via the separator, and a non-aqueous electrolyte.

[0008] The positive electrode for secondary batteries and secondary batteries comprising the same according to this disclosure exhibit excellent storage characteristics at high temperatures. Novel features of the present invention are described in the appended claims, but the present invention, both in terms of its structure and content, and in conjunction with other objects and features of the present invention, will be better understood by the following detailed description in conjunction with the drawings.

[0009] This is a schematic longitudinal section of a secondary battery according to one embodiment of the present disclosure.

[0010] The embodiments relating to this disclosure will be described below with examples, but this disclosure is not limited to the examples described below. In the following descriptions, specific numerical values ​​and materials may be given as examples, but other numerical values ​​and materials may be applied as long as the effects of this disclosure are obtained. In this specification, the description "numerical value A to numerical value B" includes numerical value A and numerical value B, and can be read as "greater than or equal to numerical value A and less than or equal to numerical value B". In the following descriptions, when lower and upper limits of numerical values ​​relating to specific physical properties or conditions are given as examples, either the given lower limit and either the given upper limit may be arbitrarily combined, as long as the lower limit does not exceed the upper limit. In the following descriptions, when examples of components or methods are listed, unless otherwise specified, only one of the listed examples may be used, or multiple of the listed examples may be used in combination.

[0011] Furthermore, this disclosure encompasses any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims. In other words, any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims, is possible, provided that no technical inconsistency arises.

[0012] The secondary battery according to this disclosure is typically a non-aqueous electrolyte secondary battery comprising a wound electrode body. The wound electrode body is constructed by winding a positive electrode and a negative electrode with a separator in between. The electrode body, together with the non-aqueous electrolyte, is housed in, for example, a bottomed cylindrical case. The opening of the bottomed cylindrical case is sealed with a sealing body. The bottomed cylindrical case and the sealing body constitute the outer casing.

[0013] Non-aqueous electrolyte secondary batteries include lithium-ion secondary batteries, lithium metal secondary batteries, and solid-state batteries containing gel electrolytes or solid electrolytes. In other words, non-aqueous electrolyte secondary batteries may be liquid-type secondary batteries containing an electrolyte, or all-solid-state secondary batteries containing a solid electrolyte.

[0014] (Positive electrode for secondary battery) The positive electrode for a secondary battery according to this embodiment (hereinafter also referred to as "positive electrode (P)") comprises a positive electrode current collector and a positive electrode mixture layer. The positive electrode mixture layer is provided on the surface of the positive electrode current collector. The positive electrode mixture layer is composed of a positive electrode mixture, which includes a positive electrode active material and a binder, and may also include a conductive additive.

[0015] The positive electrode mixture layer is formed by applying a positive electrode slurry, in which the positive electrode mixture is dispersed in a liquid component (dispersion medium), to the surface of the positive electrode current collector, drying the coating of the positive electrode slurry, and then rolling the coating.

[0016] The liquid component (dispersion medium) is not particularly limited and may include water, organic solvents, or mixtures thereof. Examples of organic solvents include protic solvents such as alcohols (e.g., ethanol), aprotic solvents such as ethers (e.g., tetrahydrofuran), amides (e.g., dimethylformamide), and N-methyl-2-pyrrolidone (NMP).

[0017] The positive electrode current collector is, for example, a long sheet, and a non-porous conductive substrate (such as metal foil) or a porous conductive substrate (such as mesh, net, or punched sheet) can be used. The thickness of the positive electrode current collector may be, for example, 3 μm to 20 μm. Examples of materials for the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium. The positive electrode mixture layer may be formed on only one side of the positive electrode current collector, but it is preferable from the viewpoint of increasing capacity to form it on both sides of the positive electrode current collector.

[0018] (Positive electrode active material) The positive electrode active material includes a first metal composite oxide having a first particle size distribution (hereinafter also referred to as the "first particle group") and a second metal composite oxide having a second particle size distribution (hereinafter also referred to as the "second particle group"), wherein the volume-based median diameter D1 of the first metal composite oxide and the volume-based median diameter D2 of the second metal composite oxide satisfy D1 > D2.

[0019] The first metal composite oxide constitutes large active material particles, and the second metal composite oxide constitutes small active material particles. When large and small active material particles are mixed, the packing of the positive electrode active material in the positive electrode mixture layer is significantly improved. This is because, during rolling, the small active material particles fill the gaps between the large active material particles. As a result, the gaps in the positive electrode mixture layer are reduced, the content of the positive electrode active material in the positive electrode mixture layer is increased, and a high-capacity positive electrode can be obtained.

[0020] On the other hand, the average primary particle size d1 of the first metal composite oxide and the average primary particle size d2 of the second metal composite oxide satisfy d1 < d2. In other words, the relationship between the average primary particle sizes (d) is the inverse of the relationship between the median diameters (D). Thus, by making the average primary particle size d2 of the smaller active material particles larger than the average primary particle size d1 of the larger active material particles, the charge-discharge cycle characteristics under high-temperature conditions are improved. This is because increasing the primary particle size of the smaller active material particles reduces the interface between primary particles, suppressing degradation from the interface between primary particles during charge-discharge and also suppressing particle cracking between primary particles.

[0021] On the other hand, when using a positive electrode active material that satisfies D1 > D2 and d1 < d2, if the secondary battery is stored in a high-temperature environment while charged, its capacity may rapidly deteriorate after storage. The main cause of this capacity deterioration is thought to be the deterioration of the first metal composite oxide.

[0022] The reaction resistance of the first metal composite oxide, which has a relatively smaller average primary particle size, tends to be lower than that of the second metal composite oxide. Furthermore, the specific surface area of ​​the first metal composite oxide tends to be larger than that of the second metal composite oxide. Therefore, the charging depths of the first and second metal composite oxides in the charging reaction are not the same, resulting in a difference. In other words, the charging reaction proceeds more rapidly in the first metal composite oxide, which has lower reaction resistance, and more Li ions are preferentially extracted from it.

[0023] In batteries stored in a high-temperature environment while in a charged state, side reactions between the positive electrode active material and the non-aqueous electrolyte are likely to proceed. These side reactions tend to occur more readily on the surface of active material particles with a deeper charge depth. When side reactions are accelerated, the amount of gas generated increases, leading to gas entrapment between the primary particles of the positive electrode active material and subsequent particle splitting (hereinafter also simply referred to as "particle splitting"). Such phenomena preferentially occur in primary metal composite oxides, which have low reaction resistance, a large specific surface area, and are therefore more susceptible to side reactions.

[0024] As described above, the first metal composite oxide is subjected to a greater load than the second metal composite oxide, making it more susceptible to particle cracking. As particle cracking progresses, the specific surface area of ​​the first metal composite oxide increases further, accelerating its degradation and causing a rapid decrease in its capacity.

[0025] The smaller the proportion of the first metal composite oxide in the positive electrode active material and the larger the proportion of the second metal composite oxide, the greater the load on the first metal composite oxide, and the more its degradation is accelerated. Furthermore, the degree of capacity degradation increases with greater negative electrode expansion during charging and higher internal pressure of the electrode body during charging. This is thought to be because the increased pressure applied to the positive electrode composite layer promotes particle cracking of the degraded first metal composite oxide. Moreover, the degree of capacity degradation increases with the thickness of the positive electrode composite layer. This is because the thicker the positive electrode composite layer, the more likely the potential distribution and reactivity within the positive electrode composite layer are to become non-uniform, increasing the probability of locally increased load on the first metal composite oxide. Additionally, because the charging reaction proceeds faster on the surface side of the positive electrode composite layer compared to the positive electrode current collection side, the difference in charging depth between the first and second metal composite oxides becomes particularly pronounced on the surface side.

[0026] In contrast, the positive electrode (P) has the following characteristics: (A) The ratio of the mass W1 of the first metal composite oxide to the total mass of the second metal composite oxide W2 contained in the positive electrode composite layer (hereinafter also referred to as "ratio (Rw1)") is 50% or more.

[0027] (B) The absolute value of the difference (R1-R2) between the powder resistance R1 (Ω·cm) of the first metal composite oxide and the powder resistance R2 (Ω·cm) of the second metal composite oxide is 15 (Ω·cm) or less.

[0028] By setting the ratio (Rw1) to 50% or more, the load on the first metal composite oxide per unit mass is reduced. Even if the potential distribution and reactivity within the positive electrode mixture layer become non-uniform, it is less likely that the charging depth will become excessively deep locally, and particle cracking is suppressed.

[0029] The ratio (Rw1) should be 50% or more, preferably 60% or more, and may also be 70% or more. However, from the viewpoint of increasing the packing rate of the positive electrode active material in the positive electrode mixture layer and sufficiently improving the charge-discharge cycle characteristics, the ratio (Rw1) should preferably be 90% or less, may also be 85% or less, and may also be 80% or less. That is, the ratio of the mass of the first metal composite oxide (W1) contained in the positive electrode mixture layer to the mass of the second metal composite oxide (W2) contained in the positive electrode mixture layer may be, for example, W1:W2 = 9:1 to 5:5, 8:2 to 6:4, or 8:2 to 7:3.

[0030] Next, if the absolute value of the difference in powder resistance between the first metal composite oxide and the second metal composite oxide |R1-R2| is 15 or less, then in the charging reaction, a difference in charging depth between the first metal composite oxide and the second metal composite oxide becomes less likely. Therefore, excessive load is less likely to be placed on the first metal composite oxide, and particle cracking of the first metal composite oxide is suppressed. In other words, in a battery in a charged state stored under high temperature conditions, side reactions on the surface of the first metal composite oxide are suppressed, gas entrapment between primary particles of the active material becomes less likely, and particle cracking is less likely to progress.

[0031] The effect of suppressing capacity degradation due to particle cracking becomes more pronounced when the negative electrode expands significantly during charging and the internal pressure of the electrode body is high during charging. In other words, if the negative electrode does not expand significantly during charging, a rapid capacity degradation does not necessarily occur. However, even if the negative electrode expands only slightly during charging, the load should not be concentrated on the first metal composite oxide, as prolonged conditions can lead to significant capacity degradation in the long term.

[0032] Furthermore, the effect of suppressing capacity degradation due to particle cracking becomes more pronounced as the positive electrode mixture layer thickens. As the positive electrode mixture layer thickens, the load on the first metal composite oxide tends to increase locally, so there is a strong need to homogenize the reaction by setting the absolute value of |R1-R2| to 15 or less. Also, as the positive electrode mixture layer thickens, the degradation on the surface side of the positive electrode mixture layer tends to become more pronounced, and rapid capacity degradation is likely to occur, so there is a strong need to avoid the concentration of load on the first metal composite oxide in the surface layer by setting the absolute value of |R1-R2| to 15 or less.

[0033] The absolute value |R1 - R2| should be 15 or less, but it is preferable that the absolute value |R1 - R2| be small, more preferably 10 or less, and even more preferably 5 or less. It is difficult to make the absolute value |R1 - R2| 0, but it is possible to satisfy, for example, the absolute value |R1 - R2| ≤ 4.

[0034] The powder resistance R1 (Ω·cm) of the first metal composite oxide and the powder resistance R2 (Ω·cm) of the second metal composite oxide are measured by separating the positive electrode active material from the positive electrode mixture layer. Specifically, the positive electrode active material may be separated by peeling the positive electrode mixture layer from the positive electrode (P), immersing it in a suitable solvent to dissolve or swell components other than the active material particles, such as conductive additives and binders, and then separating them by centrifugation one or more times. From the obtained positive electrode active material, samples of the first metal composite oxide and the second metal composite oxide are separated by 0.5 g or more each, and their powder resistances are determined. The powder resistance is measured by the four-probe method. The powder resistance may also be measured using, for example, a powder resistivity measuring device (e.g., Loresta SP from Nitto Seikou Analytech Co., Ltd.). With this device, the first metal composite oxide or the second metal composite oxide can be measured at 204 kgf / cm². ２ The resistance is determined under pressure. The difference between the powder resistance R1 of the first metal composite oxide and the powder resistance R2 of the first metal composite oxide (R1-R2) is determined, and its absolute value |R1-R2| is measured. The absolute value |R1-R2| obtained in this way must be 15 (Ω・cm) or less.

[0035] With the above configuration, it is possible to improve both the charge-discharge cycle characteristics and storage characteristics under high-temperature conditions of the secondary battery.

[0036] D1 is preferably 6.5 μm or larger, for example, 7 μm or larger, 9 μm or larger, 11 μm or larger, or 13 μm or larger. D1 is preferably 20 μm or smaller, for example, 15 μm or smaller, or 14 μm or smaller. The first particle size distribution preferably satisfies 6.5 μm ≤ D1 ≤ 20 μm, for example, 7 μm ≤ D1 ≤ 20 μm, for example, 7 μm ≤ D1 ≤ 16 μm, or 13 μm ≤ D1 ≤ 15 μm.

[0037] D2 is preferably 6 μm or less, may be 5.5 μm or less, or 5 μm or less. D2 is preferably 1 μm or more, may be 1.5 μm or more, may be 2 μm or more, may be 2.5 μm or more, or may be 3 μm or more. The second particle size distribution is preferably 1 μm ≤ D2 ≤ 6 μm, may be 1.5 μm ≤ D2 ≤ 5.5 μm, or may be 2 μm ≤ D2 ≤ 5 μm.

[0038] The ratio of the median diameter D1 to the median diameter D2 (D1 / D2) is preferably, for example, 2.0 ≤ D1 / D2 ≤ 4.5, but may also be 2.5 ≤ D1 / D2 ≤ 4, 3.0 ≤ D1 / D2 ≤ 4.5, or 3.0 ≤ D1 / D2 ≤ 3.5.

[0039] The average primary particle size d1 is, for example, 0.8 μm or less, or 0.5 μm or less, and may be 0.3 μm or less. The average particle size d1 may be, for example, 0.1 μm or more. In other words, the first metal composite oxide is a particle in an aggregated state. A particle in an aggregated state refers to a particle formed by, for example, 20 or more primary particles coming together. The aggregated particle may consist of 10,000 to 5,000,000 primary particles. The first metal composite oxide in an aggregated state has a small average primary particle diameter d1 and tends to be easily crushed, but it has a large reaction area with the non-aqueous electrolyte, excellent output characteristics, and is easy to obtain high volume.

[0040] The average primary particle size d2 is, for example, 1 μm or more, and may be 1.5 μm or more. The average particle size d2 may be, for example, 3 μm or less. Note that d2 may be approximately the same as D2. In other words, the second metal composite oxide may be particles in a non-aggregated state. Non-aggregated particles include particles that exist as a single primary particle, as well as particles formed by the aggregation of several to more than ten primary particles (for example, 2 to 19 particles). Non-aggregated particles are difficult to crush and have a suppressed increase in surface area, thus exhibiting excellent durability.

[0041] The volume-based median diameter D1 of the first metal composite oxide and the volume-based median diameter D2 of the second metal composite oxide may be measured by separating the positive electrode active material from the positive electrode mixture layer, or by image analysis of a cross-sectional SEM image of the positive electrode mixture layer. Either method yields approximately the same (without significant difference) median diameter (D).

[0042] When separating the positive electrode active material from the positive electrode mixture layer, the positive electrode mixture layer is peeled off from the positive electrode (P), the positive electrode active material is separated using the method described above, and the separated sample of positive electrode active material is analyzed with a laser diffraction scattering particle size distribution analyzer to obtain a volume-based particle size distribution. The volume-based particle size distribution can be measured by the laser diffraction scattering method. For example, the "LA-750" manufactured by HORIBA, Ltd. can be used as the measuring device.

[0043] The particle diameter of the peak with the largest area in the obtained particle size distribution is the median diameter of either the first or second particle group, and the particle diameter of the peak with the second largest area is the median diameter of the other. The larger median diameter is D1, and the smaller one is D2. If the peaks overlap, peak separation can be performed by image analysis. Furthermore, the volume V1 of the first particle group and the volume V2 of the second particle group can be calculated from the area of ​​each peak.

[0044] When performing image analysis of a cross-sectional SEM image of a positive electrode mixture layer, first, the positive electrode mixture layer and the positive electrode current collector are simultaneously cut along the width direction of the positive electrode to obtain a cross-sectional sample of the positive electrode in the thickness direction. At this time, the cross section may be processed with a cross-section polisher (CP) to obtain the cross-sectional sample. Next, the cross section of the positive electrode mixture layer in the cross-sectional sample is observed using a scanning electron microscope (SEM).

[0045] From the contour images of the active material particles in the SEM image, the area enclosed by the contour is determined. The diameter of the equivalent circle having the same area as the area enclosed by the contour of the active material particle is determined and is taken as the particle size of each particle i. Then, the volume of a sphere having the same diameter as the equivalent circle is considered as the volume Vi of each particle i. By determining the diameter and volume of the equivalent circle for any 100 or more (preferably 1000 or more) particles, a volume-based particle size distribution can be obtained. From the obtained particle size distribution, the median diameters D1 and D2, and the volumes V1 of the first particle group and V2 of the second particle group can be calculated, similar to the case in which the positive electrode active material is separated from the positive electrode mixture layer.

[0046] The average primary particle diameters d1 and d2 are determined by observing the grain boundaries of the primary particles based on SEM images of the particles of each metal composite oxide. The SEM images used to determine the average primary particle diameters d1 and d2 may be the same SEM images used to determine the median D1 and D2. The contours of the grain boundaries in the cross-sections of multiple (e.g., 20 or more) arbitrarily selected primary particles are identified, and the diameter of the equivalent circle with the same area as each primary particle is determined from these contours, and the average value of these is taken as the average primary particle diameter.

[0047] The compositions of the first metal composite oxide and the second metal composite oxide are independent of each other and may be the same or different. Even with the same composition, the volume-based median diameter (D) can be arbitrarily controlled by the synthesis conditions of the positive electrode active material.

[0048] The compositions of the first metal composite oxide and the second metal composite oxide are, independently, given by the general formula: Li ｙ Ni ｘ M （１－ｘ） O ２It may also be expressed as follows: However, the above general formula satisfies 0.8 ≤ x and 0 < y ≤ 1.2, and element M is at least one selected from the group consisting of Co, Mn, Al, Fe, Zr, Ti, Sr, Ca, and B.

[0049] The composite oxide having the composition represented by the above general formula has a high Ni content and is promising as a high-capacity positive electrode active material. It is more preferable that the above general formula satisfies 0.85 ≤ x.

[0050] Such composite oxides containing Ni and Li (hereinafter also referred to as "composite oxide (N)") are advantageous for increasing capacity and reducing cost. From the viewpoint of obtaining high capacity, the proportion of Ni among the metal elements other than Li in the composite oxide (N) is preferably 80 atomic percent or more, may be 85 atomic percent or more, or may be 90 atomic percent or more, as described above. The proportion of Ni among the metal elements other than Li is preferably 95 atomic percent or less, for example.

[0051] The composite oxide (N) may be a lithium transition metal composite oxide having a layered rock salt-type crystal structure. The composite oxide (N) may contain Co as element M, or at least one of Mn and Al. Co, Mn, and Al contribute to stabilizing the crystal structure of the composite oxide (N) with a high Ni content.

[0052] However, from the viewpoint of reducing manufacturing costs, element M preferably contains at least Mn, and a lower Co content is preferable; it does not need to contain Co. From the viewpoint of reducing manufacturing costs, it is desirable that the proportion of Co among metal elements other than Li in the composite oxide (N) be kept to less than 5 atomic percent.

[0053] A composite oxide (N) is, for example, one with the general formula: Li ｙ Ni1-x1-x2-x3-zCo ｘ１ Mn ｘ２ Al ｘ３ Me ｚ O ２＋βIt may also be expressed as follows: However, the general formula satisfies 0.95 ≤ y ≤ 1.05, 0 ≤ x1 ≤ 0.1, 0 ≤ x2 ≤ 0.5, 0 ≤ x3 ≤ 0.1, 0 ≤ z ≤ 0.1, 0.8 ≤ 1-x1-x2-x3-z and -0.05 ≤ β ≤ 0.05, where Me is an element other than Li, Ni, Mn, Al, Co and oxygen. x1, which represents the Co content, may satisfy 0.01 ≤ x1 ≤ 0.2 and 0.05 ≤ x1 ≤ 0.15. x2, which represents the Mn content, may satisfy 0.01 ≤ x2 ≤ 0.2 and 0.03 ≤ x2 ≤ 0.1.

[0054] As for Me, at least one selected from the group consisting of Nb, Zr, B, Mg, Fe, Cu, Zn, Sn, Na, K, Ba, Sr, Ca, W, Mo, Si, Ti, Fe, and Cr may be used, from the viewpoint of stabilizing the crystal structure of the composite oxide (N).

[0055] The elemental content of the active material particles can be measured using an inductively coupled plasma atomic emission spectrometer (ICP-AES), an electron probe microanalyzer (EPMA), or an energy dispersive X-ray spectrometer (EDX).

[0056] (Binder) The binder in the positive electrode mixture layer is not limited, but preferably contains a vinylidene fluoride polymer. The vinylidene fluoride polymer has a strong effect in binding active material particles together or between active material particles and the positive electrode current collector. The vinylidene fluoride polymer contains vinylidene fluoride units as monomer units. The vinylidene fluoride polymer may also be a copolymer of vinylidene fluoride and other monomers. The vinylidene fluoride resin polymer includes polyvinylidene fluoride (PVDF) and polymers of vinylidene fluoride units and other monomer units (tetrafluoroethylene, hexafluoropropylene, etc.). The content of vinylidene fluoride units in the polymer is, for example, 30 mol% or more.

[0057] The weight-average molecular weight of the vinylidene fluoride polymer may be 1 million or more, preferably 1.1 million or more, and may be 1.2 million or 1.3 million or more. The weight-average molecular weight of the vinylidene fluoride resin may be 2 million or less, or 1.8 million or less. By setting the weight-average molecular weight to 1 million or more (more preferably 1.3 million or more), a higher binder effect can be obtained with a smaller amount. The weight-average molecular weight can be measured using, for example, a gel permeation chromatography (GPC) device with N-methyl-2-pyrrolidone (NMP) as the solvent.

[0058] The binder content in the positive electrode mixture layer is preferably, for example, 0.1% by mass or more and 1.0% by mass or less. From the viewpoint of kneading addition and peel strength during positive electrode slurry preparation, a higher binder content in the positive electrode mixture layer is preferable, and may be 0.15% by mass or more, or 0.2% by mass or more. On the other hand, from the viewpoint of ensuring a high positive electrode capacity, a lower binder content in the positive electrode mixture layer is preferable, and may be 0.8% by mass or less, 0.6% by mass or less, or 0.5% by mass or less. The binder content in the positive electrode mixture layer may be 0.1% by mass to 0.6% by mass, or 0.2% by mass to 0.5% by mass.

[0059] (Conductive additive) The positive electrode mixture may contain a conductive additive. Examples of conductive additives include carbon nanotubes (hereinafter also referred to as "CNT") and carbon black.

[0060] Examples of carbon black include acetylene black, Ketjen black, and furnace black, and the type of carbon black is not particularly limited. Among carbon blacks, acetylene black is preferred because it can impart particularly high conductivity to the positive electrode mixture layer.

[0061] Carbon nanotubes (CNTs) are generally classified into single-walled (Single-Walled CNTs), double-walled (Double-Walled CNTs), and multi-walled (Multi-Walled CNTs). CNTs may include single-walled CNTs or multi-walled CNTs.

[0062] The content of the conductive additive in the positive electrode mixture layer is preferably, for example, 0.1% by mass or more and 1.1% by mass or less. From the viewpoint of improving the conductivity of the positive electrode, the content of the conductive additive in the positive electrode mixture layer is preferably 0.2% by mass or more, more preferably 0.3% by mass or more, and even more preferably 0.4% by mass or more. On the other hand, from the viewpoint of ensuring a high capacity of the positive electrode, the lower the content of the conductive additive in the positive electrode mixture layer, the better, and it may be 1.0% by mass or less, 0.9% by mass or less, or 0.8% by mass or less. From the viewpoint of ensuring sufficient conductivity, it is preferable to use a small amount of CNTs. The content of the conductive additive in the positive electrode mixture layer may be 0.1% by mass to 1.0% by mass, 0.3% by mass to 1.0% by mass, 0.5% by mass to 1.0% by mass, or 0.3% by mass to 0.8% by mass.

[0063] The content, composition, and type of binders and conductive additives other than the positive electrode active material contained in the positive electrode mixture layer can be determined using a sample of the positive electrode mixture. A sample of the positive electrode mixture is obtained by the following procedure: First, a discharged secondary battery is disassembled and the positive electrode is removed. Next, the positive electrode is washed with an organic solvent, then vacuum dried, and then only the positive electrode mixture is removed. The sample of the positive electrode mixture can be analyzed using TG-DTA, NMR, pyrolysis GC-MS, micro-Raman spectroscopy, etc.

[0064] The greater the mass of the positive electrode mixture layer supported per unit area of ​​the positive electrode current collector, the more likely the positive electrode active material is to deteriorate during high-temperature storage. In other words, the greater the mass of the positive electrode mixture layer per unit area, the more apparent the effect of the above configuration on improving the high-temperature storage characteristics of the secondary battery becomes. For example, the mass of the positive electrode mixture layer per unit area is 250 g / m². ２ The above is also acceptable, 260 g / m ２ The above is also acceptable, 280 g / m ２ Anything above that is also acceptable; for example, 250 g / m ２ 350g / m or more ２ The following range is also acceptable. Here, "mass of positive electrode mixture layer per unit area" refers to the mass of the positive electrode mixture layer on one side if the positive electrode mixture layer is formed on both sides of the positive electrode current collector.

[0065] Similarly, the greater the thickness of the positive electrode mixture layer, the more pronounced the effect of the above configuration on improving the storage characteristics of the secondary battery at high temperatures becomes. The thickness of the positive electrode mixture layer may be, for example, in the range of 50 μm to 250 μm, or it may be 100 μm or more and 200 μm or less. Here, "thickness of the positive electrode mixture layer" refers to the thickness on one side when the positive electrode mixture layer is formed on both sides of the positive electrode current collector.

[0066] Similarly, the higher the density of the positive electrode mixture layer, the more pronounced the effect of the above configuration on improving the storage characteristics of the secondary battery at high temperatures becomes. For example, the density of the positive electrode mixture layer is 3.5 g / cm³. ３ The above is preferable, with 3.6 g / cm³. ３ It may be greater than or equal to the above. The density of the positive electrode mixture layer can be determined from the thickness and mass of a positive electrode mixture layer of a predetermined size.

[0067] The longer the positive electrode current collector is in the longitudinal direction, the greater the internal pressure of the electrode body obtained by winding the positive electrode. In other words, the longer the positive electrode current collector is in the longitudinal direction, the more apparent the effect of the above configuration on improving the high-temperature storage characteristics of the secondary battery becomes. The length of the positive electrode current collector in the longitudinal direction may be, for example, 3000 mm or more.

[0068] (Secondary Battery) A secondary battery comprises a positive electrode for secondary batteries as described above, a separator, a negative electrode facing the positive electrode via the separator, and a non-aqueous electrolyte. The secondary battery may also include an outer casing. Examples of secondary batteries include lithium-ion secondary batteries and lithium metal secondary batteries. The components other than the positive electrode mixture layer are not particularly limited, and known components may be used. Examples of components of a secondary battery other than the positive electrode are described below.

[0069] (Negative electrode) The negative electrode typically includes a negative electrode mixture layer containing a negative electrode active material. The negative electrode may also include a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector. However, in the case of lithium metal secondary batteries, a negative electrode current collector capable of depositing lithium metal or lithium alloy is used for the negative electrode. The negative electrode of a lithium metal secondary battery does not necessarily have a negative electrode mixture layer.

[0070] The negative electrode mixture layer is composed of a negative electrode mixture, which contains a negative electrode active material as an essential component. The negative electrode mixture may also contain optional components such as binders, thickeners, and conductive additives. These optional components may be those exemplified as components of the positive electrode.

[0071] The negative electrode mixture layer may be formed by dispersing the components of the negative electrode mixture in a dispersion medium, applying the resulting negative electrode slurry to the surface of the negative electrode current collector, and drying it. The dried coating may be rolled if necessary. The dispersion medium may be one of the dispersion media exemplified for the positive electrode slurry. The negative electrode mixture layer may be formed on one side of the negative electrode current collector or on both sides of the negative electrode current collector.

[0072] (Negative electrode active material) The negative electrode active material is selected according to the type of secondary battery. An example of a negative electrode active material is a material capable of intercalating and releasing lithium ions. Examples of such materials include carbonaceous materials and Si-containing materials.

[0073] Examples of carbonaceous materials include graphite, easily graphitizable carbon (soft carbon), and difficult-to-graphitize carbon (hard carbon). Carbonaceous materials may be used individually or in combination of two or more. Graphite is preferred because of its excellent charge-discharge stability and low irreversible capacity. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles.

[0074] Graphite is preferred among carbonaceous materials due to its excellent charge-discharge stability and low irreversible capacity. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles. The graphite particles may partially contain amorphous carbon, easily graphitizable carbon, and difficult-to-graphitize carbon.

[0075] Graphite is a carbonaceous material in which a graphite-type crystal structure is well-developed. The average interplanar spacing d002 of the (002) planes of graphite, as measured by X-ray diffraction, may be, for example, 0.340 nm or less, or 0.3354 nm or more and 0.340 nm or less.

[0076] Examples of Si-containing materials include elemental Si, silicon alloys, silicon compounds (such as silicon oxides), and composite particles in which a silicon (Si) phase is dispersed within a lithium-ion conducting phase (matrix).

[0077] The lithium ion conducting phase (matrix) of composite particles containing the Si phase is SiO ２ It is preferable to use at least one selected from the group consisting of a phase, a silicate phase, and a carbon phase. Specific examples of composite particles include SiO ２ Phase and SiO ２ Examples include composite particles containing a Si phase dispersed within the phase (silicon oxide composite particles), composite particles containing a silicate phase and a Si phase dispersed within the silicate phase (silicate composite particles), and composite particles containing a carbon phase and a Si phase dispersed within the carbon phase (carbon composite particles).

[0078] Silicon oxide composite particles SiO ２ The phase is, for example, an amorphous phase containing 95% or more by mass of silicon dioxide. SiO ２ Composite particles in which the Si phase is dispersed within the phase are SiO ｘ It can be expressed as follows, and may satisfy, for example, 0.5 ≤ x < 2 and 0.8 ≤ x ≤ 1.6. SiO ｘ For example, silicon monoxide is heat-treated and undergoes a disproportionation reaction to produce SiO ２ It is obtained by separating the phase and the fine Si phase. Using a transmission electron microscope (TEM), SiO ｘ When observing the particle cross-section, SiO ２ The Si phase dispersed within the phase can be observed.

[0079] The silicate phase preferably contains at least one of an alkali metal element and a group 2 element of the long-period periodic table. Alkali metal elements include lithium (Li), potassium (K), sodium (Na), etc. Group 2 elements include magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), etc. The lithium silicate phase has the formula: Li ２ｙ SiO ２＋ｙThe composition may be represented by (0 < y < 2), where y may be 1 / 2 or 1. Silicate composite particles can be obtained, for example, by grinding a mixture of silicate and raw silicon while stirring in a ball mill or the like to produce fine particles, and then heat-treating the mixture in an inert atmosphere.

[0080] The Si phase content in the silicate composite particles may be 30% by mass or more and 95% by mass or less, or 35% by mass or more and 75% by mass or less.

[0081] Because the carbon phase is electrically conductive, even if cracks occur in the composite particles due to the expansion and contraction of the Si phase, they are less likely to become isolated, and the contact points between the carbon composite particles and their surroundings are easily maintained. Therefore, the cycle characteristics are less likely to deteriorate.

[0082] The carbon phase can consist of, for example, amorphous carbon (i.e., crystalline carbon) and crystalline carbon. Amorphous carbon may be hard carbon, soft carbon, or something else. Generally, amorphous carbon refers to a carbonaceous material in which the average interplanar spacing d002 of (002) planes, as measured by X-ray diffraction, exceeds 0.340 nm.

[0083] Carbon composite particles can be obtained, for example, by grinding a mixture of a carbon source and raw silicon while stirring in a ball mill or the like to produce fine particles, and then heat-treating the mixture in an inert atmosphere. Examples of carbon sources include petroleum resins such as coal pitch, petroleum pitch, and tar, as well as carboxymethylcellulose (CMC), polyvinylpyrrolidone, cellulose, sugars such as sucrose, and water-soluble resins. When mixing the carbon source and raw silicon, for example, the carbon source and raw silicon may be dispersed in a dispersion medium such as alcohol. After drying the milled mixture, the carbon phase is formed by heating it in an inert gas atmosphere, for example, at a temperature of 600°C or higher and 1000°C or lower, to carbonize the carbon source.

[0084] The Si phase content in the carbon composite particles is, for example, 30% by mass or more and 80% by mass or less, and may be 40% by mass or more and 70% by mass or less.

[0085] Since composite particles containing the Si phase expand and contract in volume during charging and discharging, a higher content of Si in the negative electrode mixture layer leads to greater expansion of the negative electrode during charging, resulting in a significantly higher internal pressure in the electrode body. It is preferable to use a combination of a carbonaceous material such as graphite and composite particles containing the Si phase as the negative electrode active material.

[0086] In contrast, when the electrode body is composed of a positive electrode (P) having the aforementioned characteristics (A) and (B), the negative electrode expands significantly during charging, and even if the internal pressure of the electrode body increases during charging, capacity degradation due to particle cracking of the positive electrode active material is less likely to occur. Therefore, the negative electrode mixture layer can contain a high percentage of Si-containing material.

[0087] The content of Si-containing material (e.g., composite particles containing Si phase) in the negative electrode mixture layer may be, for example, 5% to 15% by mass, 5% to 10% by mass, 6% to 10% by mass, or 7% to 10% by mass. In this case, the content of silicon phase in the negative electrode mixture layer may be, for example, 1% to 15% by mass, 1% to 10% by mass, 1.5% to 10% by mass, or 2% to 10% by mass.

[0088] The crystallite size of the Si phase dispersed within the matrix phase is, for example, 500 nm or less, and may also be 30 nm or less. The lower limit of the crystallite size of the Si phase is not particularly limited, but is, for example, 5 nm or more. The crystallite size is calculated from the full width at half maximum of the diffraction peaks attributed to the Si(111) plane in the X-ray diffraction (XRD) pattern of the Si phase using Scherrer's formula.

[0089] The Si phase content in composite particles containing the Si phase can be measured, for example, by Si-NMR. The following describes the desirable measurement conditions for Si-NMR.

[0090] (Binder) Examples of binders for the negative electrode mixture layer include fluororesins (e.g., polytetrafluoroethylene, polyvinylidene fluoride), polyolefin resins, polyamide resins, polyimide resins, acrylic resins, and rubber-like materials (e.g., styrene-butadiene copolymer rubber (SBR)).

[0091] (Thickener) Examples of thickeners include cellulose derivatives. CMC and its modified forms are preferably used as cellulose derivatives. Modified forms of CMC include CMC salts. The salt may be an alkali metal salt (e.g., a sodium salt) or an ammonium salt.

[0092] The negative electrode current collector is, for example, a long sheet, and a non-porous conductive substrate (such as metal foil) or a porous conductive substrate (such as mesh, net, or punched sheet) can be used. The thickness of the negative electrode current collector may be, for example, 3 μm to 20 μm. Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, and copper alloy. The negative electrode composite layer may be formed on only one side of the negative electrode current collector, but it is preferable from the viewpoint of increasing capacity to form it on both sides of the negative electrode current collector.

[0093] (Non-aqueous electrolytes) Non-aqueous electrolytes may be liquid electrolytes (electrolyte solutions), gel electrolytes, or solid electrolytes.

[0094] The gel-like electrolyte comprises a lithium salt and a matrix polymer, or a lithium salt, a non-aqueous solvent, and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. Examples of polymer materials include fluororesins, acrylic resins, polyether resins, and polyethylene oxide.

[0095] The solid electrolyte may be an inorganic solid electrolyte. As an inorganic solid electrolyte, for example, materials known for use in all-solid-state lithium-ion secondary batteries, etc. (e.g., oxide-based solid electrolytes, sulfide-based solid electrolytes, halide-based solid electrolytes, etc.) can be used.

[0096] The liquid electrolyte (electrolyte solution) contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. The electrolyte salt contains at least a lithium salt. The concentration of the lithium salt in the non-aqueous electrolyte is, for example, 0.5 mol / L or more and 2 mol / L or less.

[0097] Any known material can be used as the non-aqueous solvent. Examples of non-aqueous solvents include cyclic carbonate esters, linear carbonate esters, cyclic carboxylic acid esters, and linear carboxylic acid esters. Examples of cyclic carbonate esters include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC). Examples of linear carbonate esters include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of linear carboxylic acid esters include non-aqueous solvents such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). The non-aqueous solvent may be used alone or in combination of two or more types.

[0098] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO2). ４ LiAlCl ４ LiB １０ Cl １０ (e.g.), lithium salts of fluorine-containing acids (LiPF) ６ LiPF ２ O ２ LiBF ４ LiSbF ６ LiAsF ６ LiCF ３ SO ３ LiCF ３ CO ２ (etc.), lithium salts of fluorine-containing acidimides (LiN(FSO) ２ ) ２ ,LiN(CF ３ SO ２ ) ２ ,LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ ), LiN(C ２ F ５ SO ２ ) ２These include lithium halides (LiCl, LiBr, LiI, etc.), etc. Lithium salts may be used individually or in combination of two or more types.

[0099] The concentration of lithium salt in the electrolyte may be 1 mol / L or more and 2 mol / L or less, or 1 mol / L or more and 1.5 mol / L or less. By setting the lithium salt concentration within the above range, an electrolyte with excellent ionic conductivity and appropriate viscosity can be obtained.

[0100] (Separator) The separator is placed between the positive electrode and the negative electrode. The separator preferably has high ion permeability and appropriate mechanical strength and insulating properties. As the separator, a microporous thin film, woven fabric, nonwoven fabric, etc., can be used. Polyolefin is preferably used as an example of the material of the separator.

[0101] (Outer casing) The outer casing (battery case) houses the electrode body and the non-aqueous electrolyte. The outer casing is not particularly limited, and known outer casings may be used. The configuration of the electrode body is not particularly limited, but this disclosure is suitable for wound-type electrode bodies. A wound-type electrode body is formed by winding a positive electrode and a negative electrode with a separator in between. The form of the secondary battery may be, for example, cylindrical, prismatic, laminated, etc.

[0102] Figure 1 is a longitudinal cross-sectional view of a cylindrical secondary battery 10 (hereinafter also simply referred to as "battery 10") according to the present disclosure. However, the present disclosure is not limited to the following configuration.

[0103] In Figure 1, the battery 10 comprises an electrode body 18, a non-aqueous electrolyte (not shown), and a bottomed cylindrical battery case (metal can) 22 that houses these components. A sealing body 11 is crimped and fixed to the opening of the battery case 22 via a gasket 21. This seals the inside of the battery 10. The sealing body 11 is an internal pressure-operated safety valve that cuts off the current when the internal pressure of the battery rises excessively and ruptures if necessary. Specifically, the sealing body comprises a valve body 12 having a thin-walled portion, a metal plate 13, and an annular insulating member 14 interposed between the valve body 12 and the metal plate 13. The valve body 12 and the metal plate 13 are electrically connected to each other at their respective centers. The positive electrode lead 15L, which is led out from the positive electrode 15, is connected to the metal plate 13. Therefore, the valve body 12 functions as both an external terminal for the positive electrode 15 and a safety valve. When the internal pressure of the battery rises, the connection between the valve body 12 and the metal plate 13 is broken, interrupting the current. Furthermore, when the thin-walled portion ruptures, gas is released to the outside, ensuring safety. The negative electrode lead 16L, which is led out from the negative electrode 16, is connected to the inner surface of the bottom of the battery case 22. An annular groove 22a is formed near the open end of the battery case 22. A first insulating plate 23 is placed between one end face of the electrode body 18 and the annular groove 22a. A second insulating plate 24 is placed between the other end face of the electrode body 18 and the bottom of the battery case 22. The electrode body 18 is formed by winding the positive electrode 15 and the negative electrode 16 in a cylindrical shape via a separator 17. The outermost circumference of the electrode body 18 is composed of the exposed portion 16D of the negative electrode current collector on the winding end side of the negative electrode 16.

[0104] (Note) The following technologies are disclosed in accordance with the above description. (Technology 1) A positive electrode current collector and a positive electrode mixture layer provided on the surface of the positive electrode current collector, wherein the positive electrode mixture layer comprises a positive electrode active material and a binder, wherein the positive electrode active material comprises a first metal composite oxide having a first particle size distribution and a second composite metal oxide having a second particle size distribution, wherein the volume-based median diameter D1 of the first metal composite oxide and the volume-based median diameter D2 of the second metal composite oxide satisfy D1 > D2, the average primary particle size d1 of the first metal composite oxide and the average primary particle size d2 of the second metal composite oxide satisfy d1 < d2, and the ratio of the mass W1 of the first metal composite oxide to the total mass of the first metal composite oxide and the second metal composite oxide W2 contained in the positive electrode mixture layer is 50% or more. A positive electrode for a secondary battery, wherein the absolute value of the difference (R1-R2) between the powder resistance R1 (Ω·cm) of the first metal composite oxide and the powder resistance R2 (Ω·cm) of the second metal composite oxide is 15 or less. (Technology 2) The positive electrode for a secondary battery according to Technology 1, wherein the first particle size distribution satisfies 6.5 μm ≤ D1. (Technology 3) The positive electrode for a secondary battery according to Technology 1 or 2, wherein the second particle size distribution satisfies D2 ≤ 6 μm. (Technology 4) The first metal composite oxide and the second metal composite oxide are independent of each other, and the general formula is Li ｙ Ni ｘ M （１－ｘ） O ２ (Technology 5) A positive electrode for a secondary battery according to any one of Technology 1 to 3, wherein the general formula satisfies 0.8 ≤ x and 0 < y ≤ 1.2, and M is at least one selected from the group consisting of Co, Mn, Al, Fe, Zr, Ti, Sr, Ca, and B. (Technology 5) A positive electrode for a secondary battery according to any one of Technology 1 to 4, wherein the binder contains a vinylidene fluoride polymer, and the weight-average molecular weight of the vinylidene fluoride polymer is 1.1 million or more. (Technology 6) The mass per unit area of ​​the positive electrode mixture layer is 250 g / m². ２The positive electrode for a secondary battery according to any one of Technologies 1 to 5 described above. (Technology 7) A secondary battery comprising the positive electrode for a secondary battery according to any one of Technologies 1 to 6, a separator, a negative electrode facing the positive electrode through the separator, and a non-aqueous electrolyte. (Technology 8) The negative electrode includes a negative electrode current collector and a negative electrode mixture layer provided on the surface of the negative electrode current collector, the negative electrode mixture layer contains a Si-containing material, and the content of the Si-containing material in the negative electrode mixture layer is 5% by mass to 15% by mass. The secondary battery according to Technology 7.

[0105] Hereinafter, the present disclosure will be specifically described based on examples, but the present disclosure is not limited by the following examples. In this example, a plurality of non-aqueous electrolyte secondary batteries with different positive electrodes were manufactured and evaluated.

[0106] 《Battery A1》 Battery A1 was manufactured by the following method. (1) Manufacture of the negative electrode Composite particles containing a Si phase and graphite were mixed at a mass ratio of composite particles containing a Si phase: graphite = 7:93 and used as a negative electrode active material. The negative electrode active material, sodium carboxymethyl cellulose (CMC-Na) as a thickener, styrene-butadiene rubber (SBR) as a binder, and water were mixed at a predetermined mass ratio to prepare a negative electrode slurry. Next, the negative electrode slurry was applied to the surface of a copper foil (negative electrode current collector) to form a laminate including the copper foil and the coating film formed on the copper foil. Next, after drying the coating film, the laminate was rolled. In this way, a negative electrode including the copper foil and the negative electrode mixture layer formed on both surfaces of the copper foil was formed.

[0107] (2) Manufacture of the positive electrode A positive electrode mixture containing a positive electrode active material, a conductive assistant (acetylene black (AB)), and a binder (polyvinylidene fluoride (PVDF)) at a predetermined mass ratio was dispersed in N-methyl-2-pyrrolidone (NMP (dispersion medium)) to prepare a positive electrode slurry.

[0108] The positive electrode active material has a compositional formula LiNi ０．９０ Co ０．０５ Mn ０．０５ O ２ The first metal composite oxide (NCM1) represented by and the compositional formula LiNi ０．９０ Co ０．０５ Mn ０．０５ O２ Particles of a second metal composite oxide (NCM2) represented by [formula] were used. Specifically, the molar ratio of Ni:Co:Mn in NCM1 is 90:5:5 (NCM ratio = 90 / 5 / 5), and the molar ratio of Ni:Co:Mn in NCM2 is also 90:5:5 (NCM ratio = 90 / 5 / 5).

[0109] The first metal composite oxide consists of a first particle group with a median diameter D1 of 10 μm and an average primary particle diameter d1 of 0.5 μm, and the second metal composite oxide consists of a second particle group with a median diameter D2 of 5 μm and an average primary particle diameter d2 of 2 μm (D1 / D2 = 2). The first and second particle groups were mixed in a mass ratio of 8:2 (W1 / W2 = 8 / 2). The powder resistance R1 of the first metal composite oxide was 41 Ω·cm, the powder resistance R2 of the second metal composite oxide was 50 Ω·cm, and the absolute value |R1 - R2| was 9 Ω·cm.

[0110] The content of acetylene black (AB) in the positive electrode mixture layer was set to 0.75% by mass.

[0111] The content of polyvinylidene fluoride (PVDF) in the positive electrode mixture layer was set to 0.6% by mass. The weight-average molecular weight of polyvinylidene fluoride was set to 1.2 million.

[0112] A coating film was formed on both sides of a 900 mm long sheet of aluminum foil (positive electrode current collector) by applying a positive electrode slurry. After drying the coating film, the laminate of the aluminum foil and the dried coating film was rolled with a rolling roller at a predetermined linear pressure. In this way, a positive electrode was manufactured, containing the aluminum foil and the positive electrode mixture layers formed on both sides thereof. The mass per unit area of ​​the positive electrode mixture layer was 280 g / m². ２ That's what I decided.

[0113] (3) Preparation of electrolyte: LiPF in a non-aqueous solvent ６ The electrolyte was prepared by adding (lithium salt). LiPF in the electrolyte ６ The concentration was set to 1.0 mol / L. As the non-aqueous solvent, a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of EC:EMC = 3:7 was used.

[0114] (4) Fabrication of the secondary battery Leads were attached to each of the positive and negative electrodes. Next, an electrode assembly was fabricated by winding the positive electrode, negative electrode, and separator in a spiral manner such that the separator was disposed between the positive and negative electrodes. After vacuum drying the electrode assembly at 105 °C for 2 hours, it was housed in a bottomed cylindrical battery case that also served as the negative electrode terminal. An iron case was used as the battery case. After injecting an electrolyte into the battery case, the opening of the battery case was closed using a metal sealing body that also served as the positive electrode terminal. At this time, a resin gasket was interposed between the sealing body and the opening end of the battery case. The positive electrode lead was connected to the sealing body, and the negative electrode lead was connected to the inner bottom surface of the battery case. In this manner, battery A1 of Example 1 was fabricated.

[0115] [Evaluation] <Initial capacity (C ０ )> For the fabricated battery, in a temperature environment of 25 °C, constant current charging was performed at a constant current of 0.2 It until the voltage reached 4.2 V, followed by constant voltage charging until the current value reached 1 / 50 It at 4.2 V. Thereafter, the operation of constant current discharging at a constant current of 0.2 It until the voltage reached 2.5 V was repeated twice, and the initial capacity was confirmed.

[0116] <Capacity retention rate at 25 °C (Ra ２５ )> In a temperature environment of 25 °C, constant current charging was performed at a constant current of 0.2 It until the voltage reached 4.2 V, followed by constant voltage charging until the current value reached 1 / 50 It at 4.2 V. Thereafter, the operation of constant current discharging at a constant current of 0.2 It until the voltage reached 2.5 V was repeated 300 times, and the retention rate of the capacity with respect to the initial capacity was confirmed.

[0117] <Capacity retention rate at 45 °C (Ra ４５ )> Except for changing the temperature environment to 45 °C, charge and discharge were repeated 300 times in the same manner as the "capacity retention rate at 25 °C", and the retention rate of the capacity with respect to the initial capacity was confirmed.

[0118] <Storage characteristics at 55 °C (Rb 55Under a temperature environment of 25°C, constant current charging was performed with a constant current of 0.2 It until the voltage reached 4.2V. Then, constant voltage charging was performed until the current value was 1 / 50 It at 4.2V, and the charged battery was stored at 55°C for 10 months. After that, constant current discharge was performed with a constant current of 0.2 It until the voltage reached 2.5V.

[0119] Subsequently, under a temperature of 25°C, constant current charging was performed with a constant current of 0.2 It until the voltage reached 4.2V, then constant voltage charging was performed until the current value was 1 / 50 It at 4.2V, and then constant current discharge was performed with a constant current of 0.2 It until the voltage reached 2.5V, and the capacity retention rate relative to the initial capacity was confirmed.

[0120] The results are shown in Table 2. Note that the following battery evaluation results are relative values, with the result for battery A1 or A2 set to 100.

[0121] 《Batteries A2-A4, Batteries B1-8》 As shown in Table 1, the NCM ratio of the second metal composite oxide, D1, D2, D1 / D2 ratio, d1, d2, R1, R2, W1 / W2 ratio, mass per unit area of ​​the positive electrode composite layer (area density SD), and the content of composite particles containing Si phase in the negative electrode composite layer (X Ｓｉ The following changes were made: In batteries B1 and B2, only the first metal composite oxide was used, and the second metal composite oxide was not used. Except for these points, batteries A2 to A4 of the examples and batteries B1 to B8 of the comparative examples were manufactured in the same manner as battery A1 of Example 1, and the same evaluations were performed for each example. The results are shown in Table 2.

[0122]

[0123]

[0124] From Tables 1 and 2, if D1 > D2, d1 < d2, the W1 / W2 ratio is 5 / 5 or greater, and the absolute value |R1 - R2| is 15 or less, then the 55°C storage characteristics (Rb 55 This shows that a high maintenance rate can be ensured.

[0125] Furthermore, the larger the mass per unit area of ​​the positive electrode mixture layer, the better the 55°C storage characteristics (Rb 55 It is clear that it is difficult to maintain a high level of this.

[0126] Furthermore, a comparison of batteries B4 to B6 shows that the mass per unit area (area density SD) of the positive electrode mixture layer is large, while the content of composite particles containing the Si phase in the negative electrode mixture layer (X Ｓｉ It can be seen that the higher the value of |R1-R2|, the more significantly the high-temperature storage characteristics deteriorate. Furthermore, it can be understood that this problem can be resolved by setting the absolute value of |R1-R2| to 15 or less for batteries A1 to A4.

[0127] This disclosure can be applied to high-capacity, high-performance cathodes for secondary batteries. The secondary batteries according to this disclosure are useful as main power sources for mobile communication devices, portable electronic devices, electric vehicles, and the like.

[0128] Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention.

[0129] 10 Secondary battery 11 Sealing body 12 Valve body 13 Metal plate 14 Insulating material 15 Positive electrode 15L Positive electrode lead 16 Negative electrode 16L Negative electrode lead 17 Separator

Claims

1. The positive electrode comprises a positive electrode current collector and a positive electrode mixture layer provided on the surface of the positive electrode current collector, wherein the positive electrode mixture layer includes a positive electrode active material and a binder, the positive electrode active material includes a first metal composite oxide having a first particle size distribution and a second metal composite oxide having a second particle size distribution, the volume-based median diameter D1 of the first metal composite oxide and the volume-based median diameter D2 of the second metal composite oxide satisfy D1 > D2, the average primary particle size d1 of the first metal composite oxide and the average primary particle size d2 of the second metal composite oxide satisfy d1 < d2, and the ratio of the mass W1 of the first metal composite oxide to the total mass of the first metal composite oxide and the second metal composite oxide W2 contained in the positive electrode mixture layer is 50% or more. A positive electrode for a secondary battery, wherein the absolute value of the difference (R1-R2) between the powder resistance R1 (Ω·cm) of the first metal composite oxide and the powder resistance R2 (Ω·cm) of the second metal composite oxide is 15 or less.

2. The positive electrode for a secondary battery according to claim 1, wherein the first particle size distribution satisfies 6.5 μm ≤ D1.

3. The positive electrode for a secondary battery according to claim 1, wherein the second particle size distribution satisfies D2 ≤ 6 μm.

4. The first metal composite oxide and the second metal composite oxide are independent of each other and have the general formula: Li ｙ Ni ｘ M （１－ｘ） O ２ The positive electrode for a secondary battery according to claim 1, wherein the general formula satisfies 0.8 ≤ x and 0 < y ≤ 1.2, and M is at least one selected from the group consisting of Co, Mn, Al, Fe, Zr, Ti, Sr, Ca, and B.

5. The positive electrode for a secondary battery according to claim 1, wherein the binder contains a vinylidene fluoride polymer, and the weight-average molecular weight of the vinylidene fluoride polymer is 1.1 million or more.

6. The mass per unit area of ​​the positive electrode mixture layer is 250 g / m². ２ The positive electrode for a secondary battery as described in claim 1.

7. A secondary battery comprising a positive electrode for a secondary battery as described in claim 1, a separator, a negative electrode facing the positive electrode via the separator, and a non-aqueous electrolyte.

8. The secondary battery according to claim 7, wherein the negative electrode comprises a negative electrode current collector and a negative electrode mixture layer provided on the surface of the negative electrode current collector, the negative electrode mixture layer contains a Si-containing material, and the Si-containing material content in the negative electrode mixture layer is 5% by mass to 15% by mass.

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