Positive electrode for lithium-ion secondary batteries and lithium-ion secondary batteries

Abstract

To provide a positive electrode for lithium-ion secondary batteries and a lithium-ion secondary battery that have excellent electronic conductivity and reduce the internal resistance of the battery. [Solution] A cathode composite layer containing a layered rock salt type oxide and an olivine type compound as the cathode active material, The positive electrode composite layer comprises a first region that is proximal to a first surface facing the current collector in the thickness direction of the positive electrode composite layer, a second region that is proximal to a second surface facing the separator in the thickness direction, and a third region that is between the first region and the second region in the thickness direction. The positive electrode comprises the first and second regions containing the layered rock salt type oxide at a higher concentration than the third region.

Description

[Technical Field]

[0001] The technologies disclosed herein relate to cathodes for lithium-ion secondary batteries and lithium-ion secondary batteries. [Background technology]

[0002] The positive electrode of lithium-ion secondary batteries often uses a positive electrode active material that is a metal oxide containing lithium ions. For example, layered rock salt type oxides and olivine type compounds are used.

[0003] For example, Patent Document 1 describes that a positive electrode with high energy density and excellent safety can be provided by combining an olivine-type compound, lithium iron manganese phosphate (LMFP), and a layered rock salt-type compound, nickel cobalt manganese complex acid compound (NCM), in a predetermined mass ratio as the positive electrode active material. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] International Publication No. 20 / 261879 [Overview of the project] [Problems that the invention aims to solve]

[0005] However, it was found that the positive electrode described in Patent Document 1 has a problem in that it has low electronic conductivity.

[0006] This specification provides a positive electrode for a lithium-ion secondary battery and a lithium-ion secondary battery that have excellent electronic conductivity and reduce the internal resistance of the battery. [Means for solving the problem]

[0007] According to this specification, the following means are provided:

[0008] [1] A cathode composite layer containing a layered rock salt type oxide and an olivine type compound as the cathode active material, Equipped with, The positive electrode composite layer comprises a first region that is proximal in the thickness direction of the positive electrode composite layer to a first surface facing the current collector, a second region that is proximal in the thickness direction to a second surface facing the separator, and a third region that is between the first region and the second region in the thickness direction. The positive electrode comprises the first and second regions containing the layered rock salt type oxide at a higher concentration than the third region. [2] The olivine-type compound D in the first region 50 D of the layered rock salt type oxide relative to (volume basis) 50 The first particle size ratio, which is the ratio (by volume), is the ratio of the olivine-type compound D in the second region. 50 D of the layered rock salt type oxide relative to (volume basis) 50 The positive electrode described in [1] is greater than the second particle size ratio, which is the ratio (by volume). [3] The positive electrode according to [1] or [2], wherein the first region is a region from the first surface that is 10% or less of the thickness of the positive electrode composite layer, and the second region is a region from the second surface that is 10% or less of the thickness of the positive electrode composite layer. [4] The positive electrode according to any one of [1] to [3], wherein the third region comprises a fourth region containing only the olivine-type compound as the positive electrode active material. A positive electrode for a lithium-ion secondary battery as described in any of [5][1]~[4], The negative electrode and, Separator and, A lithium-ion secondary battery equipped with these features.

[0009] In the positive electrode described above, the positive electrode composite layer contains the layered rock salt type oxide, which has excellent electronic conductivity, at a higher concentration in the first and second regions than in the third region. Therefore, the positive electrode composite layer can exhibit excellent DC internal resistance (DCIR). Furthermore, a lithium-ion secondary battery equipped with this positive electrode exhibits excellent discharge characteristics (e.g., 3C discharge rate). In addition, by including the layered rock salt type oxide at a high concentration in the first and second regions, it may be possible to improve the peel strength between the current collector and separator and the positive electrode. Therefore, a secondary battery with excellent positive electrode integrity is provided.

[0010] Furthermore, this specification provides a lithium secondary battery comprising a positive electrode, a negative electrode, and a separator. This lithium-ion secondary battery provides a lithium-ion secondary battery with excellent cycle characteristics because it includes a positive electrode that contributes to excellent cycle characteristics. [Brief explanation of the drawing]

[0011] [Figure 1] This diagram schematically shows an example of a lithium-ion secondary battery cell. [Figure 2] This diagram schematically shows an example of a positive electrode composite layer in a lithium-ion secondary battery. [Figure 3] This diagram schematically illustrates an example of the manufacturing process for a positive electrode. [Modes for carrying out the invention]

[0012] This specification relates to a positive electrode for lithium-ion secondary batteries (hereinafter also simply referred to as secondary batteries) and secondary batteries.

[0013] The positive electrode and secondary battery disclosed herein will be described below with reference to the drawings as appropriate. In this specification, "secondary battery" refers to a battery that can be repeatedly charged and discharged by the movement of a charge carrier between a positive electrode and a negative electrode. In this specification, "lithium-ion secondary battery" refers to a secondary battery that uses lithium ions as a charge carrier and achieves charging and discharging by the movement of charge associated with lithium ions between the positive and negative electrodes. A secondary battery is equipped with a positive electrode terminal and a negative electrode terminal on an electrode body and is usually housed in a battery case. A secondary battery may be equipped with a liquid or gel-like electrolyte, or it may be an all-solid-state secondary battery equipped with a solid electrolyte. The shape of the secondary battery is also not particularly limited.

[0014] (Positive electrode for secondary batteries) Figure 1 schematically shows an example of a cell 2 of a secondary battery 100. As shown in Figure 1, the cell 2 as a unit structure of the secondary battery 100 comprises a positive electrode 4, a separator 6, and a negative electrode 8. The cell 2 further comprises a positive electrode current collector 10 and a negative electrode current collector 12. The secondary battery 100 usually has a structure in which many cells 2 are stacked. The separator 6 may hold, for example, a liquid or gel-like electrolyte.

[0015] (Positive electrode composite layer) The positive electrode 4 comprises a positive electrode composite layer 20. In cell 2, one side of the positive electrode composite layer 20 is equipped with a positive electrode current collector 10, and the other side is equipped with a separator 6. The so-called positive electrode 4 may consist of the positive electrode composite layer 20 alone, or it may include the positive electrode composite layer 20 and the positive electrode current collector 10.

[0016] The positive electrode composite layer 20 consists of a positive electrode composite. The positive electrode composite contains a positive electrode active material. As the positive electrode active material, olivine-type compounds and layered rock salt-type oxides can be used.

[0017] Next, the distribution of layered rock salt oxides and olivine-type compounds in the positive electrode composite layer 20 will be described. The positive electrode composite layer 20 has a specific distribution of layered rock salt oxides and olivine-type compounds in the thickness direction of the positive electrode composite layer 20.

[0018] Figure 2 schematically shows an example of the positive electrode composite layer 20, and Figure 3 schematically shows an example of the manufacturing process of the positive electrode 4. In Figures 2 and 3, only layered rock salt type oxides are shown as active materials in order to emphasize the distribution of layered rock salt type oxides in the first region 22, the second region 24, and the third region 26.

[0019] As shown in Figure 2, the positive electrode composite layer 20 comprises a first region 22, a second region 24, and a third region 26. The first region 22 is the region in the thickness direction of the positive electrode composite layer 20 that is proximal to the first surface 10a facing the current collector 10. That is, it is the region in the positive electrode composite layer 20 that is closest to the current collector 10. The second region 24 is the region in the thickness direction that is proximal to the second surface 10b facing the separator 6, and is the region that is closest to the separator 6. The third region 26 is the region between the first region 22 and the second region 24.

[0020] The positive electrode composite layer 20 contains layered rock salt oxides at a higher concentration in the first region 22 and the second region 24 than in the third region 26. The high concentration of layered rock salt oxides in the first region 22 and the second region 24 contributes to the excellent DC internal resistance (DCIR) of the secondary battery 100 due to the high electronic conductivity based on the layered rock salt oxides. In particular, the high concentration of layered rock salt oxides in the second region 24 can contribute to a reduction in DCIR.

[0021] Furthermore, by including a high concentration of layered rock salt oxide in the first region 22, it may be possible to improve the peel strength between the positive electrode composite layer 20 and the positive electrode current collector 10. For example, because layered rock salt oxide tends to have a small surface area due to its shape, it tends to be easier to ensure peel strength between it and the positive electrode current collector 10 even with a small amount of binder.

[0022] Region 1 22 and Region 24 should contain a higher concentration of layered rock salt type oxides than Region 3 26. While not particularly limited, for example, Region 1 22 and Region 24 may each contain layered rock salt type oxides in a ratio of 5% to 40% by mass relative to the total mass of layered rock salt type oxides and olivine type compounds. The ratio of olivine type compounds is the remainder, for example, 60% to 95% by mass. The ratio of layered rock salt type oxides may also be, for example, 8% or more by mass, 10% or more by mass, 12% or more by mass, 15% or more by mass, 20% or more by mass, 35% or less by mass, 30% or less by mass, and 25% or less by mass. The range of the ratio of layered rock salt type oxides can be set as appropriate, for example, 8% to 35% by mass and 10% to 30% by mass. In the first region 22 and the second region 24, the concentrations of layered rock salt-type oxides may be the same or different.

[0023] The first region 22 and the second region 24 contain layered rock salt type oxides within this concentration range, and the third region 26 contains layered rock salt type oxides at a lower concentration, thereby efficiently contributing to a reduction in DCIR and an improvement in peel strength between the separator 6 and the positive electrode current collector 10.

[0024] The third region 26 only needs to contain layered rock salt type oxides at a lower concentration than the first region 22 and the second region 24, and the concentration of layered rock salt type oxides is not particularly limited. For example, the concentration of layered rock salt type oxides is 20% by mass or less, 15% by mass or less, 10% by mass or less, 5% by mass or less, 3% by mass or less, 1% by mass or less, 0.5% by mass or less, 0.1% by mass or less, and 0.01% by mass or less, relative to the total mass of layered rock salt type oxides and olivine type compounds. The olivine type compounds make up the remainder. Even if the concentration of layered rock salt type oxides in the third region 26 is 0.01% by mass or less or not present, the DCIR can be reduced by the presence of layered rock salt type oxides in the first region 22 and the second region 24, thus reducing the amount of layered rock salt type oxides used.

[0025] The third region 26 may include a fourth region 28 containing only olivine-type compounds without containing layered rock salt-type oxides, or the entire third region 26 may be such a fourth region 28. This reduces the amount of layered rock salt-type oxides used.

[0026] Furthermore, the concentration of layered rock salt-type oxides in the positive electrode composite layer 20 may change stepwise in the thickness direction throughout these regions 22, 24, 26, and 28, or it may change gradually in a gradient manner.

[0027] The positive electrode composite layer 20 may contain 5% to 20% by mass of layered rock salt oxide and 80% to 95% by mass of olivine-type compound, based on the total mass of the layered rock salt oxide and olivine-type compound. By having the above-described distribution of layered rock salt oxide, the positive electrode composite layer 20 can achieve excellent DCIR in the secondary battery 100 and excellent integration with the positive electrode current collector 10, while maintaining the above-described content of layered rock salt oxide throughout the layered rock salt oxide layer 20. The ratio of layered rock salt oxide may be 7% to 20% by mass.

[0028] The content of layered rock salt type oxides and olivine type compounds in the positive electrode mixture layer 20 is, for example, 85% to 98% by mass, 90% to 97% by mass, and 90% to 95% by mass of the total amount of positive electrode mixture used in the positive electrode mixture layer 20.

[0029] The distribution of layered rock salt-type oxides and / or olivine-type compounds in the first region 22, the second region 24, and the third region 26 can be obtained, for example, by an energy-dispersive X-ray fluorescence spectrometer. Alternatively, it can be obtained by sampling samples from each region 22, 24, and 26 and measuring the concentration of layered rock salt-type oxides and / or olivine-type compounds.

[0030] The thickness of the first region 22 and the second region 24 depends on the total thickness of the positive electrode composite layer 20, but may be, for example, 1% or more and 40% or less of the total thickness. For example, 2% or more, 3% or more, 4% or more, 5% or more, 6% or more, 7% or more, 8% or more, 9% or more, 10% or more, and 15% or more. Also, the thickness may be 35% or less, 30% or less, 25% or less, 20% or less, and 15% or less. The first region 22 and the second region 24 can be set as appropriate, but may also be, for example, 3% or more and 35% or less, and 5% or more and 30% or less. The thicknesses of the first region 22 and the second region 24 may be the same or different.

[0031] For example, the thicknesses of the first region 22, the second region 24, and the third region 26 in the positive electrode composite layer 20 are not particularly limited. For the first region 22, a thickness of several μm to several tens of μm from the first surface 10a may be sufficient. Similarly, for the second region 24, a thickness of several μm to several tens of μm from the second surface 10b may be sufficient.

[0032] The thickness of the third region 26 is not particularly limited. The thickness of the third region 26 can be the remainder of the thickness of the first region 22 and the second region 24, which are effective in ensuring the contribution of these regions 22 and 24 to DCIR.

[0033] The thicknesses of the first region 22, the second region 24, and the third region 26 are obtained based on the distribution of layered rock salt-type oxides in an energy-dispersive X-ray fluorescence analyzer, and these regions 22, etc. can also be obtained during the manufacturing of cell 2 based on the interface during lamination.

[0034] The thickness of the positive electrode composite layer 20 is not particularly limited, but for example, it can be 50 μm or more and 200 μm or 50 μm or more and 100 μm or less.

[0035] The positive electrode composite layer 20 contains layered rock salt type oxides and olivine type compounds, but the particle size D of these compounds 50 However, when the following relationship is satisfied, it contributes to improving the discharge characteristics of the secondary battery 100, as well as the adhesion and peel strength between the positive electrode composite layer 20 and the positive electrode current collector 10.

[0036] That is, D of the olivine-type compound in the first region 22 50 (by volume, the same hereinafter)(d12) to D of the layered rock salt-type oxide 50 The first particle size ratio R1 (Dd11 / d12), which is the ratio of (d11), is D of the olivine-type compound in the second region 24 50 to D of the layered rock salt-type oxide with respect to (d22) 50 (d21) may preferably be larger than the second particle size ratio R2 (d21 / d22).

[0037] By doing so, when pressing the laminate including the current collector 10 and the separator 6 in the positive electrode composite layer 20, the first region 22 close to the current collector 10 tends to be more compressed than the second region 24. This is because the particle size ratio R1 in the first region 22 is larger than the particle size ratio R2 in the second region 24. As a result, the first region 22 is crimped to the current collector 10, and the adhesion and peel strength are improved. Also, since the second region 24 close to the separator 6 is less likely to be compressed than the first region 22, the lithium ion diffusivity is improved in the vicinity of the second surface 10b of the second region 24, contributing to excellent discharge characteristics.

[0038] [[ID=第十九]]The first particle size ratio R1 and the second particle size ratio R2 are not particularly limited. For example, R1 is 10 or more and 20 or less, 12 or more and 18 or less, and R2 is less than 10, 9 or less, 8 or less, 7 or less.

[0039] [[ID=二十二]] D 50 can be measured as the particle size corresponding to 50% by volume in the volume-based particle size distribution (cumulative distribution) based on the laser diffraction / scattering method, from the smaller side (fine particle side) of the particle size.

[0040] (layered rock salt-type oxide) The layered rock salt oxide is not particularly limited, and known layered rock salt oxides can be used. For example, the layered rock salt oxide can be one or more oxides selected from lithium-containing transition metal oxides represented by the following formula (1). This type of layered rock salt oxide can be a single-crystal oxide and / or a polycrystalline oxide. This type of layered rock salt oxide has excellent electronic conductivity.

[0041] LiRing a Co b Mn c M 1 w O2...Equation (1) (In formula (1), M 1 represents one or more elements selected from Mg, Ti, Nb, Fe, Cr, Si, Al, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi, and Ge. a, b, c, w are 0.25 ≤ a < 0.80, 0 <b≦0.74、0<c≦0.74、0≦w≦0.34、かつ3a+3b+3c+(M 1 Show the number that satisfies (valence of) × w = 3.

[0042] This oxide is a type of so-called Li-Ni-Co-Mn oxide (NCM-based oxide or NCM).

[0043] In NCM oxides, Ni, Co, and Mn are known to have excellent electronic conductivity and contribute to battery capacity and output characteristics. Furthermore, from the viewpoint of cycle characteristics, some of these transition elements are found to be other metallic elements M 1 It may be preferable that it be replaced by [another substance].

[0044] a, which is the molar ratio of Ni in the oxide, satisfies 0.25 ≦ a < 0.80. When the molar ratio of Ni is 0.80 or more, the deterioration of the NCM-based oxide itself also increases, and the effect of improving the cycle characteristics becomes small. Considering cycle characteristics (capacity retention rate), etc., for example, it is 0.30 or more, 0.40 or more, 0.50 or more, 0.55 or more, 0.60 or more, 0.70 or more, and for example, 0.78 or less, 0.74 or less, 0.70 or less, 0.60 or less, 0.64 or less. The range of a can be set by selecting these lower and upper limits. For example, it can be 0.40 or more and 0.74 or less, 0.50 or more and 0.74 or less, 0.55 or more and 0.74 or less, 0.60 or more and 0.70 or less, etc. Note that when the molar ratio in this specification is marked as a percentage, it becomes the molar ratio (mol%).

[0045] b, which is the molar ratio of Co in the NCM-based oxide, satisfies 0 < b ≦ 0.74. For example, it is 0.10 or more, 0.15 or more, 0.20 or more, and for example, 0.70 or less, 0.50 or less, 0.40 or less, 0.34 or less, 0.30 or less, 0.24 or less, 0.20 or less. The range of b can be set by selecting these lower and upper limits. For example, it can be 0.10 or more and 0.40 or less, 0.10 or more and 0.30 or less, 0.10 or more and 0.24 or less, etc.

[0046] c, which is the molar ratio of Mn in the NCM-based oxide, satisfies 0 < c ≦ 0.74. For example, it is 0.10 or more, 0.15 or more, and for example, 0.70 or less, 0.50 or less, 0.34 or less, 0.30 or less, 0.24 or less, 0.20 or less, 0.15 or less. The range of c can be set by selecting these lower and upper limits. For example, it can be 0.10 or more and 0.34 or less, 0.15 or more and 0.34 or less, etc.

[0047] The NCM-based oxide is not particularly limited. From the perspective of cycle characteristics, etc., for example, LiNi 0.70 Co 0.10 Mn 0.20 O2, LiNi 0.60 Co 0.20 Mn 0.20 [[ID=2I4]]O2, LiNi 0.50 Co0.30 Mn 0.20 O2, LiLiLi 0.33 Co 0.31 Mn 0.33 Mg 0.03 O2, LiLiLi 0.33 Co 0.31 Mn 0.33 Zn 0.03 O2 is one example. In particular, LiNi 0.70 Co 0.10 Mn 0.20 O2, LiLiLi 0.60 Co 0.20 Mn 0.20 O2, LiLiLi 0.50 Co 0.30 Mn 0.20 O2 may be preferable in some cases. Also, for example, LiNi 0.70 Co 0.10 Mn 0.20 O2, LiLiLi 0.60 Co 0.20 Mn 0.20 NCM-based oxides with an O2 composition may be preferable from the viewpoint of high-temperature cycling characteristics (capacity retention rate).

[0048] NCM-based oxides, being single crystals, can contribute to the cycle characteristics of secondary batteries. Polycrystalline oxides tend to be prone to cracking due to particle splitting during charging and discharging. The crystallinity of single crystals can be confirmed by X-ray diffraction spectroscopy and optical microscopy.

[0049] NCM oxides, depending on the manufacturing method, are generally spherical or irregularly shaped particles. The average particle size (D) of NCM oxides is... 50 The primary particle size (D) is not particularly limited. It is appropriately set within a range that allows for dispersion in the positive electrode composite layer 20 so that the intended properties can be obtained. For example, it may be 50 nm to 20 μm, 1 μm to 20 μm, 3 μm to 20 μm, or 3 μm to 10 μm. 50 In a volume-based particle size distribution (cumulative distribution) based on laser diffraction and scattering methods, the particle size can be measured as the particle size corresponding to a cumulative 50% of the total volume, starting from the smallest particle size (fine particle side).

[0050] As layered rock salt type oxides, in addition to NCM-type oxides, one or more types from the so-called NCA-type oxides represented by the following formula (3), and other known lithium-containing transition metal oxides can be appropriately selected or combined.

[0051] LiRing d Co e Al f M 3 x O2...Equation (3) (In formula (1), M 3 represents one or more elements selected from Mg, Ti, Nb, Fe, Cr, Si, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi, and Ge. d, e, f, x are 0.4 ≤ d < 1, 0 <e≦0.5、0<f≦0.3、0≦x≦0.3、かつ3d+3e+3f+(M 3 Show the number that satisfies (valence of x) × x = 3.

[0052] The content of NCM-based oxides in layered rock salt-type oxides is not particularly limited, but from the viewpoint of cycle characteristics, for example, it can be 70% to 100% by mass, 80% to 100% by mass, 90% to 100% by mass, 95% to 100% by mass, 99% to 100% by mass, 100% by mass, etc.

[0053] (Olivine-type compounds) Various known olivine-type compounds can be used as the olivine-type compound without particular limitations. For example, one or more compounds selected from those represented by the following formula (2) can be used as the olivine-type compound. This type of lithium manganese iron phosphate contributes to safety because its olivine form provides excellent structural stability. It may also contribute to improving the energy density per unit area.

[0054] Li g Mn h Fe i M 2 yPO4···(2) (In formula (2), M 2 represents one or more elements selected from Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, and Gd. g, h, i, and y satisfy 0 < g ≤ 1.2, 0 < h ≤ 1.2, 0 ≤ i ≤ 1.2, provided that h + i is not 0, 0 ≤ y ≤ 0.30, and g + (valence of Mn) × h + (valence of Fe) × i + (valence of M 2 × y = 3.)

[0055] Examples of the olivine-type compound represented by formula (2) include LMFP, LMP, and LFP. Preferably, it is LMFP. In formula (2), M 2 is preferably Mg, Al, Ti, Zn, Nb, Co, Zr, or Gd from the viewpoint of increasing the energy density per unit volume.)

[0056] Also, the molar ratio g of Li in formula (2) is greater than 0 and less than or equal to 1.2, for example, 0.60 or more and 1.20 or less, 0.65 or more and 1.15 or less, 0.70 or more and 1.10 or less.)

[0057] The molar ratio h of Mn in formula (2) is greater than 0 and less than or equal to 1.2. In the case of LMFP, from the viewpoint of reducing the elution of Mn, for example, it is 0.15 or more, 0.20 or more, 0.25 or more, 0.30 or more, 0.35 or more, 0.40 or more, and also for example, it is 0.84 or less, 0.80 or less, 0.74 or less, 0.70 or less. The range of h is, for example, 0.20 or more and 0.80 or less, 0.30 or more and 0.80 or less, 0.40 or more and 0.80 or less, 0.35 or more and 0.74 or less, 0.40 or more and 0.70 or less, 0.50 or more and 0.70 or less.)

[0058] The molar ratio i of Fe in formula (2) is 0 or more and 1.2 or less. In the case of LMFP, for example, it is 0.30 or more and 0.60 or less, and also for example, it is 0.30 or more and 0.50 or less.)

[0059] In formula (2), M 2The molar ratio y is between 0 and 0.3, but for example, it can be between 0 and 0.20, between 0 and 0.15, or between 0 and 0.10.

[0060] Olivine-type compounds are not particularly limited, but examples include LiMnPO4, LiFePO4, LiMn 0.20 Fe 0.80 PO4, LiMn 0.30 Fe 0.70 PO4, LiMn 0.40 Fe 0.60 PO4, LiMn 0.50 Fe 0.50 PO4, LiMn 0.60 Fe 0.40 PO4, Li 1.2 Mn 0.53 Fe 0.37 Examples include PO4, among others. 0.60 Fe 0.40 PO4 may be preferable from a cost and other perspectives.

[0061] Olivine-type compounds such as LMFPs generally consist of spherical or irregularly shaped particles, depending on the manufacturing method. The average particle diameter (primary particle diameter) and particle diameter distribution range of olivine-type compounds such as LMFPs are not particularly limited. They are appropriately set within a range that allows for dispersion in the positive electrode composite layer 20 to obtain the desired properties. For example, the average particle diameter of olivine-type compounds such as LMFPs is 1 nm to 10 μm, 1 nm to 2 μm, 1 nm to 1 μm, or 1 nm to 0.5 μm. A smaller primary particle diameter improves lithium ion diffusion within the particles. Furthermore, olivine-type compounds such as LMFPs may be granulated bodies formed from granulated primary particles. In this case, it is preferable that the granulated body is formed from primary particles with a particle diameter of 100 nm or less. Regarding the average particle diameter of LMFPs, similar to the single-crystal oxides described above, the volume-based particle size distribution (integrated distribution) based on laser diffraction and scattering is D 50 It can be measured as follows.

[0062] The positive electrode composite layer 20 may appropriately contain a binder, a conductive additive, and other additives in addition to the positive electrode active material. Examples of binders include one or more types of fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefin. Examples of conductive additives include one or more types of carbon materials such as carbon black, acetylene black, Ketjenblack, graphite, and carbon nanotubes.

[0063] (Method for manufacturing the positive electrode composite layer and the positive electrode) The positive electrode composite layer 20 is prepared by mixing a positive electrode active material, a binder, a conductive additive, and a necessary solvent. For example, as shown in Figure 3, a composite slurry with a composition corresponding to the first region 22 is supplied to the current collector 10 in a known manner to form the first region precursor layer 22'. Next, a composite slurry with a composition corresponding to the third region 26 is supplied to the first region precursor layer 22' to laminate the third region precursor layer 26'. Furthermore, a composite slurry with a composition corresponding to the second region 24 is supplied to the third region precursor layer 26' to form the second region precursor layer 24'. The laminate, which has the first region precursor layer 22', the third region precursor layer 26', and the second region precursor layer 24' in this order on the current collector 10, is dried and pressed or otherwise subjected to a process to manufacture the positive electrode composite layer 20 and the positive electrode 4, which has the current collector 10 on the positive electrode composite layer 20, simultaneously.

[0064] Furthermore, by integrating a negative electrode 8, consisting of a separator 6, a negative electrode composite layer 30, and a negative electrode current collector 12, with the positive electrode 4, a cell 2 can be obtained.

[0065] (Secondary battery) The secondary battery 100 disclosed in the present invention is composed of multiple cells 2 stacked together. The secondary battery 100 consists of a positive electrode 4, a negative electrode 8, an electrolyte (liquid, gel, or solid), and a separator 6. The negative electrode 8, electrolyte, and separator 6 are not particularly limited, and known materials and configurations can be appropriately applied. For example, the negative electrode 8 can be made of lithium metal, graphite, silicon-based materials (Si, SiOx), lithium titanate, or amorphous carbon or other carbon materials. The electrolyte solution is, for example, an organic solvent in which a support salt is dissolved. The organic solvent is not particularly limited as long as it is an organic solvent that is normally used in the electrolyte of lithium-ion secondary batteries, and for example, carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones, oxolane compounds, etc. can be used. The type of support salt is not particularly limited, but known organic salts such as LiPF6, LiBF4, or their derivatives can be used. As the separator 6, for example, a porous synthetic resin membrane, particularly a porous membrane of a polyolefin polymer (polyethylene (PE), polypropylene (PP)), can be used. Furthermore, the solid electrolyte can be any known solid electrolyte that electrically insulates the positive and negative electrodes and exhibits high lithium-ion conductivity.

[0066] The shape of the secondary battery 100 having the above configuration is not particularly limited, and it may be various shapes such as coin-type, cylindrical, or prismatic, or it may be an irregular shape enclosed in a laminated casing. [Examples]

[0067] The following describes examples to illustrate the disclosures of this specification. Therefore, the disclosures of this specification are not limited to the following examples. [Examples]

[0068] In this example, evaluation cells were fabricated, and the 3C discharge capacity retention rate and peeling strength were evaluated. The configuration of the fabricated positive electrode is shown in Table 1.

[0069] (1) Preparation of the positive electrode The positive electrode of Example 1 was prepared as follows: The positive electrode active material was an olivine-type compound LMFP (Mn 60 mol%, Fe 40 mol%) (D 50 NCM (Ni 60 mol%, Co 20 mol%, Mn 20 mol%) (D), a polycrystalline layered rock salt type oxide (0.5 μm), 50 Using 3.5 μm foil, a slurry for the first region (current collector side) was prepared, consisting of LMFP:NCM:carbon nanotubes (CNT):polyvinylidene fluoride (PVDF) = 66.15:28.35:1.5:4.0 (mass%). This slurry contained 30 mass% NCM as the active material, relative to the total mass of NCM and LMFP. This slurry was then applied to a 30 μm aluminum foil at a basis weight of 8.7 mg / cm³. 2 It was applied using a doctor blade to achieve the desired result.

[0070] Next, a slurry for the third region (central part) was prepared, consisting of LMFP:CNT:PVDF = 94.5:1.5:4.0 (mass%). This slurry contained only LMFP as the active material, and no NCM. This slurry was layered on top of the coating layer of the slurry for the first region to achieve a basis weight of 8.7 mg / cm². 2 I applied it with a doctor blade to achieve this result.

[0071] Furthermore, a slurry for the second region (separator side) was prepared, consisting of LMFP:NCM:carbon nanotubes (CNT):polyvinylidene fluoride (PVDF) = 66.15:28.35:1.5:4.0 (mass%). This slurry contained 30% by mass of NCM as the active material, relative to the total mass of NCM and LMFP. This slurry was layered on top of the slurry coating for the third region to achieve a basis weight of 8.7 mg / cm². 2 It was applied using a doctor blade to achieve the desired result.

[0072] The resulting laminate was dried at 100°C for 10 minutes and pressed to a density of 2.6 g / cc to produce the positive electrode of Example 1.

[0073] The positive electrode of Example 2 was fabricated in the same manner as in Example 1, except that the compositions of LMFP and LCM in the first to third regions were as shown in Table 1. Also, the positive electrode of Example 3 was fabricated in the same manner as in Example 1, except that the NCM used in the slurry of the first region (current collector side) was replaced with NCM with D 50 being 8 μm, and the compositions of LMFP and LCM in the first and third regions were as shown in Table 1.

[0074] The positive electrodes of Comparative Example 1 and Comparative Example 2 were fabricated in the same manner as in Example 1, except that the compositions of LMFP and LCM in the first to third regions were as shown in Table 1.

[0075] <​​​​​​​​​​​​​​​​​The initial charging was performed using a constant current-constant voltage method. Constant current charging was performed at a current of 0.1C up to 4.30V, and then constant voltage charging was performed for 3 hours. Finally, the battery was discharged to 3.0V at a current of 0.1C using the constant current method to activate it. After the activation process, the battery was charged to 4.30V at a current of 0.1C using the constant current-constant voltage method in a 25°C environment, and then discharged to 3.0V at a current of 0.1C using the constant current method to calculate the initial discharge capacity.

[0079] The discharge rate test was conducted at 25°C. The battery was charged to 4.30V using a constant current-constant voltage method with a current of 0.1C, and then discharged to 3.0V using a constant current method with a current of 3C. The retention rate of the discharge capacity at 3C relative to the discharge capacity at 0.1C was calculated. The results are shown in Table 1.

[0080] (6) Peel strength A peel test was conducted under a 90°C environment, and the peel strength was measured. The results are shown in Table 1.

[0081] [Table 1]

[0082] As shown in Table 1, the DC internal resistance of Example 1 was lower than that of Comparative Example 1 (uniform dispersion), indicating that the uneven distribution of NCM contributed to the DC internal resistance. Furthermore, while the 3C discharge rate of Examples 1-3 all exceeded 70%, that of Comparative Examples 1-2 was in the 60% range. Examples 1-3 showed superior peel strength and 3C discharge rate compared to Comparative Example 1, which was simply a mixture of LMFP and LCM laminated. From this, it was found that uneven distribution of NCM at high concentrations on the current collector side and the separator side contributes to peel strength and 3C discharge performance. Comparing Examples 1-3 with Comparative Example 2, which did not include NCM on the current collector side, it was found that uneven distribution of NCM at high concentrations on the current collector side significantly contributes to peel strength and 3C discharge performance.

[0083] Also, from Examples 1 to 3, it was found that on both the current collector side and the separator side, the peel strength and the 3C discharge performance were reliably improved when the concentration of NCM was in the range of 10% to 30% by mass. Further, from the comparison between Examples 1 to 2 and Example 3, the ratio of D 50 of NCM on the current collector side to D 50 of LMFPP (8 / 0.5 = 16), which is larger than that on the separator side (3.5 / 0.5 = 7), was found to contribute to the improvement of the peel strength. This is considered to be because the larger D 50 of NCM on the current collector side to D 50 of LMFP makes the region more easily compressed during pressing, thus improving the peel strength. On the other hand, the smaller D 50 of NCM on the separator side to D 50 of LMFP makes it difficult to be compressed even during pressing, and is considered to contribute to the improvement of the 3C discharge rate by improving the lithium ion diffusivity.

[0084] As described above, specific examples of the technology disclosed in this specification have been described in detail, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above. The technical elements described in this specification or the drawings exhibit technical utility alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. The technology illustrated in this specification or the drawings can achieve a plurality of purposes simultaneously, and has technical utility by achieving one of those purposes itself.

Explanation of Reference Numerals

[0085] 100 Lithium ion secondary battery, 2 Cell, 4 Positive electrode, 6 Separator, 8 Negative electrode, 10 Positive electrode current collector, 12 Positive electrode composite layer, 10a First surface, 10b Second surface, 20 Positive electrode composite layer, 22 First region, 24 Second region, 26 Third region

Claims

1. A cathode composite layer containing layered rock salt-type oxide and olivine-type compound as the cathode active material, Equipped with, The positive electrode composite layer comprises a first region that is proximal in the thickness direction of the positive electrode composite layer to a first surface facing the current collector, a second region that is proximal in the thickness direction to a second surface facing the separator, and a third region that is between the first region and the second region in the thickness direction. The first and second regions contain the layered rock salt type oxide at a higher concentration than the third region, and this is the positive electrode.

2. The olivine-type compound D in the first region ５０ D of the layered rock salt type oxide relative to (volume basis) ５０ The first particle size ratio, which is the ratio (by volume), is the D of the olivine-type compound in the second region. ５０ D of the layered rock salt type oxide relative to (volume basis) ５０ The positive electrode according to claim 1, wherein the ratio is greater than the second particle size ratio, which is a ratio (based on volume).

3. The positive electrode according to claim 1, wherein the first region is a region extending from the first surface to a thickness of 10% or less of the positive electrode composite layer, and the second region is a region extending from the second surface to a thickness of 10% or less of the positive electrode composite layer.

4. The positive electrode according to claim 1, wherein the third region includes a fourth region containing only the olivine-type compound as the positive electrode active material.

5. A positive electrode for a lithium-ion secondary battery according to any one of claims 1 to 4, The negative electrode and, Separator and, A lithium-ion secondary battery equipped with these features.

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