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

Abstract

Provided is a positive electrode for a lithium-ion secondary battery that uses an olivine-type compound and a layered rock salt-type oxide as positive electrode active materials and has excellent cycle characteristics. 【Solution means】The positive electrode for a lithium-ion secondary battery includes a positive electrode current collector layer and a positive electrode composite material layer containing an olivine-type compound and a layered rock salt-type oxide as positive electrode active materials, and the layered rock salt-type oxide contains a single crystal oxide represented by the following formula (1). LiNi 1 Co b Mn c M 1 w O2 ··· Formula (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, and w satisfy 0.25 ≤ a < 0.80, 0 < b ≤ 0.74, 0 < c ≤ 0.74, 0 ≤ w ≤ 0.34, and 3a + 3b + 3c + (valence of M 1 ) × w = 3. )

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, layered rock salt oxides such as nickel-cobalt-manganese composite oxide (NCM) have the advantage of high energy density, enabling the construction of high-power and high-capacity secondary batteries. Olivine-type compounds such as manganese iron lithium phosphate (LMFP) also possess low cost and high energy density. The combination of these positive electrode active materials according to specific purposes is described (Patent Document 1). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2022-138053 [Overview of the project] [Problems that the invention aims to solve]

[0005] Olivine-type compounds and layered rock salt-type oxides each possess excellent properties as cathode active materials. However, olivine-type compounds containing manganese, such as LMFPs, may be prone to manganese leaching during high-temperature charge-discharge cycles. Therefore, increasing the content of this type of olivine-type compound can significantly degrade cycle performance. There is a need to maintain cycle performance even during high-temperature cycle operation while using olivine-type compounds.

[0006] This specification provides a positive electrode and a lithium-ion secondary battery with excellent cycle characteristics, using an olivine-type compound and a layered rock salt-type oxide as positive electrode active materials. [Means for solving the problem]

[0007] This specification provides a positive electrode for lithium-ion secondary batteries. The positive electrode comprises a positive electrode current collector layer and a positive electrode composite layer containing an olivine-type compound and a layered rock salt-type oxide as the positive electrode active material, wherein the layered rock salt-type oxide contains a single crystal oxide represented by the following formula (1). 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.

[0008] According to this positive electrode, a single-crystal oxide represented by equation (1) with a Ni molar ratio of less than 80 mol% exhibits excellent electronic conductivity and durability. Therefore, this single-crystal oxide reacts preferentially, reducing the load on the olivine-type compound. Furthermore, even if the load on the single-crystal oxide increases due to degradation of the olivine-type oxide caused by high-temperature charge-discharge cycles, the excellent durability of the single-crystal oxide suppresses the breakdown of the particle structure, such as particle cracking of the single-crystal oxide. Therefore, a rapid deterioration of cycle characteristics is suppressed.

[0009] The present specification also provides a lithium secondary battery including this positive electrode, a negative electrode, and a separator. According to this lithium ion secondary battery, since it includes a positive electrode that contributes to excellent cycle characteristics, a lithium ion secondary battery with excellent cycle characteristics is provided.

Brief Description of the Drawings

[0010] [Figure 1] It is a diagram showing an example of a cell of a lithium ion secondary battery. [Figure 2] It is a diagram showing an example of a positive electrode composite layer of a lithium ion secondary battery. [Figure 3] It is a diagram showing another example of a positive electrode composite layer of a lithium ion secondary battery. [Figure 4] It is a diagram showing another example of a positive electrode composite layer of a lithium ion secondary battery.

Modes for Carrying Out the Invention

[0011] The present specification relates to a positive electrode for a lithium ion secondary battery and a secondary battery. One aspect of the positive electrode disclosed in the present specification includes a positive electrode current collector layer and a positive electrode composite layer containing an olivine-type compound and a layered rock salt-type oxide as positive electrode active materials, and the layered rock salt-type oxide may contain a single crystal oxide represented by the following formula (1). LiNi a Co b Mn c M 1 w O2 ··· Formula (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 numbers satisfying 0.25 ≦ a < 0.80, 0 < b ≦ 0.74, 0 < c ≦ 0.74, 0 ≦ w ≦ 0.34, and 3a + 3b + 3c + (valence of M 1 ) × w = 3.)

[0012] <N Another aspect of the positive electrode is that the positive electrode composite layer may include a first region containing the single crystal oxide at a high concentration on a side closer to a first surface which is a surface opposite to the surface facing the positive electrode current collector layer in the positive electrode composite layer. By doing so, since the single crystal oxide reacts preferentially in the first region closer to the separator, the load on the olivine-type compound is effectively reduced.

[0013] Another aspect of the positive electrode is that in the formula (1), the single crystal oxide may have 0.60 ≦ a ≦ 0.70. By using a single crystal oxide having a molar ratio of Ni within this range, the decrease in cycle characteristics at high temperatures can be effectively suppressed.

[0014] Another aspect of the positive electrode is that the positive electrode composite layer may include a second region containing the single crystal oxide at a low concentration or not containing it on a side closer to a second surface which is the surface facing the positive electrode current collector layer in the positive electrode composite layer. By doing so, it is possible to suppress the decrease in cycle characteristics while reducing the amount of the single crystal oxide used in the entire positive electrode composite layer.

[0015] Another aspect of the positive electrode is that the olivine-type compound may include a compound represented by the following formula (2). In the formula (2), 0.20 ≦ h ≦ 1.2 may be satisfied. By doing so, a positive electrode excellent in the cost, energy density, and safety of the positive electrode is provided. Li g Mn h Fe i M 2 y PO4···(2) (In the 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) × g + (valence of Fe) × h + (valence of M 2 represents a number that satisfies × x = 3.)

[0016] Another embodiment of the positive electrode is that in formula (2), 0.20 ≤ h ≤ 1.2. When the molar ratio of Mn is 0.20 or greater, using the single-crystal oxide can suppress the dissolution of Mn and thus suppress the deterioration of cycle characteristics.

[0017] In another embodiment of the positive electrode, the positive electrode composite layer may contain 20% to 40% by mass of the single crystal oxide relative to the total mass of the single crystal oxide and the olivine-type compound. This reduces the cost of the positive electrode.

[0018] One embodiment of a lithium-ion secondary battery disclosed herein may comprise a positive electrode, a negative electrode, and a separator as described above.

[0019] The positive electrode and the secondary battery of the lithium-ion secondary battery (hereinafter simply referred to as "secondary battery") disclosed herein will be described below with reference to the drawings as appropriate. Figure 1 shows an example of a cell of secondary battery 2.

[0020] 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. Furthermore, 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 through the movement of charge associated with lithium ions between the positive and negative electrodes.

[0021] Furthermore, the secondary battery is equipped with a positive electrode terminal and a negative electrode terminal on the electrode body and is housed in a battery case. The secondary battery is a lithium-ion secondary battery equipped with a non-aqueous electrolyte. The shape of the secondary battery is not particularly limited and may be cylindrical, laminated, or the like.

[0022] (Positive electrode for secondary batteries) As shown in Figure 1, the cell, as a unit structure of the secondary battery 2, comprises a positive electrode 4, a separator 6, and a negative electrode 8. The secondary battery 2 typically has a structure in which many cells are stacked. The separator 6 is impregnated with an electrolyte. The positive electrode 4 comprises a sheet-like positive electrode current collector 10 and a positive electrode composite layer 12. The positive electrode 4 has a configuration corresponding to the form of the secondary battery.

[0023] (Positive electrode current collector) The positive electrode current collector 10 is made of a conductive metal such as aluminum, aluminum alloy, nickel, or stainless steel, although this is not particularly limited. The positive electrode current collector 10 is generally in the form of a sheet with a thickness of about 100 μm or less. The positive electrode current collector 10 is typically a metal foil, more specifically an aluminum foil. The positive electrode current collector 10 is an example of a positive electrode current collector layer disclosed herein.

[0024] (Positive electrode composite layer) The positive electrode composite layer 12 is provided by being fixed to at least one surface of the positive electrode current collector 10. The positive electrode composite layer 12 may be provided on both sides of the positive electrode current collector 10. The positive electrode composite layer 12 has a layered form along the surface of the positive electrode current collector 10.

[0025] The positive electrode composite layer 12 consists of a positive electrode composite material. The positive electrode composite material contains a positive electrode active material. As the positive electrode active material, olivine-type compounds and layered rock salt-type oxides can be used. Below, we will first describe the layered rock salt-type oxides and olivine-type compounds which are positive electrode active materials, and then describe the distribution morphology of single-crystal oxides in the positive electrode composite layer 12.

[0026] (Layered rock salt oxides) As the layered rock salt type oxide, one or more single-crystal oxides selected from lithium-containing transition metal oxides represented by the following formula (1) can be used. This type of layered rock salt type oxide exhibits excellent electronic conductivity.

[0027] LiRing a Co b Mn c M1 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.

[0028] Single-crystal oxides are a type of so-called Li-Ni-Co-Mn oxide (NCM-based oxide or NCM).

[0029] In single-crystal 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].

[0030] The molar ratio of Ni in single-crystal oxides, a, is 0.25 ≤ a < 0.80. If the molar ratio of Ni is 0.80 or higher, the degradation of the NCM-based oxide itself increases, and the improvement effect on cycle characteristics decreases. Considering cycle characteristics at high temperatures (capacity retention rate), for example, a can be 0.30 or higher, 0.40 or higher, 0.50 or higher, 0.55 or higher, 0.60 or higher, 0.70 or higher, or for example, 0.78 or lower, 0.74 or lower, 0.70 or lower, 0.60 or lower, 0.64 or lower. The range of a can be selected from these lower and upper limits, for example, 0.40 or higher and 0.74 or lower, 0.50 or higher and 0.74 or lower, 0.55 or higher and 0.74 or lower, 0.60 or higher and 0.70 or lower. Note that in this specification, the molar ratio is expressed as a percentage (mol%).

[0031] The molar ratio of Co in the single crystal oxide, b, satisfies 0 < b ≤ 0.74. For example, b 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.

[0032] The molar ratio of Mn in the single crystal oxide, c, satisfies 0 < c ≤ 0.74. For example, c 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.

[0033] The single crystal oxide is not particularly limited. From the viewpoint of cycle characteristics and the like, for example, LiNi 0.70 Co 0.10 Mn 0.20 O2, LiNi 0.60 Co 0.20 Mn 0.20 O2, LiNi 0.50 Co 0.30 Mn 0.20 O2, LiNi 0.33 Co 0.31 Mn 0.33 Mg 0.03 O2, LiNi 0.33 Co 0.31 Mn 0.33 Zn 0.03 O2 can be mentioned. Among them, for example, LiNi 0.70 Co 0.10 Mn 0.20 O2, LiNi 0.60 Co 0.20 Mn 0.20 O2, LiNi 0.50 Co 0.30 Mn 0.20 O2 may be preferred. Also, for example, LiNi 0.70 Co 0.10 Mn0.20 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).

[0034] Single-crystal oxides can contribute to the cycle characteristics of secondary batteries because they are single-crystal materials. Polycrystalline materials tend to develop cracks due to particle splitting during charging and discharging. The single-crystal nature of a single-crystal oxide can be confirmed by X-ray diffraction spectroscopy and optical microscopy.

[0035] Single-crystal oxides, depending on the manufacturing method, are generally spherical or irregularly shaped particles. The average particle size (D) of single-crystal 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 12 so that the intended properties can be obtained. For example, it may be 50 nm to 20 μm, 1 μm to 10 μm, or 1 μm to 5 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).

[0036] 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.

[0037] 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, and 3a + 3b + 3c + (valence of M) × w = 3. 3 (Here, the numbers satisfying the above conditions are shown.)

[0038] The content of the single crystal oxide in the layered rock salt type oxide is not particularly limited. From the viewpoint of cycle characteristics and the like, for example, it can be 70% by mass or more and 100% by mass or less, 80% by mass or more and 100% by mass or less, 90% by mass or more and 100% by mass or less, 95% by mass or more and 100% by mass or less, 99% by mass or more and 100% by mass, 100% by mass, etc.

[0039] (Olivine type compound) As the olivine type compound, various known olivine type compounds can be used without particular limitation. As the olivine type compound, for example, one or more selected from compounds represented by oxides represented by the following formula (2) can be used. This type of lithium manganese iron phosphate has an olivine type and excellent structural stability, so it can contribute to safety. In addition, it may contribute to an improvement in the energy density per unit area.

[0040] Li g Mn h Fe i M 2 y PO4 ··· (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) × g + (valence of Fe) × h + (valence of M 2 × x = 3. (Here, the numbers satisfying the above conditions are shown.)

[0041] Examples of the olivine type compound represented by formula (2) include LMFP and LMP. Preferably, it is LMFP. In formula (2), M 2From the viewpoint of increasing the energy density per unit volume, Mg, Al, Ti, Zn, Nb, Co, Zr, or Gd is preferable.

[0042] Also, g which is the molar ratio of Li in formula (2) is more than 0 and 1.2 or less, 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.

[0043] h which is the molar ratio of Mn in formula (2) is more than 0 and 1.2 or less. 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.

[0044] i which is the molar ratio 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.

[0045] In formula (2), M 2 The molar ratio y is 0 or more and 0.3 or less, for example, 0 or more and 0.20 or less, 0 or more and 0.15 or less, 0 or more and 0.10 or less.

[0046] The olivine-type compound is not particularly limited, for example, 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 Fe0.37 Examples include PO4, among others. 0.60 Fe 0.40 PO4 may be preferable from a cost and other perspectives.

[0047] 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 12 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 / scattering is D 50 It can be measured as follows.

[0048] The positive electrode composite layer 12 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.

[0049] The positive electrode 4 and the positive electrode composite layer 12 can be manufactured by supplying a slurry prepared by mixing a positive electrode active material, a binder, a conductive additive, and the necessary solvent to the positive electrode current collector 10 in a known manner, and then drying, pressing, etc.

[0050] Next, the distribution of single-crystal oxides and olivine-type compounds in the positive electrode composite layer 12 will be described. The distribution state of single-crystal oxides and olivine-type compounds in the positive electrode composite layer 12 is not particularly limited and can take various forms.

[0051] For example, as shown in Figure 2, the single-crystal oxide can be unevenly distributed so that it is contained at a higher concentration on the side away from the positive electrode current collector 10. In Figure 2, for illustrative purposes, the single-crystal oxide is shown as particles 22. In the positive electrode composite layer 12 shown in Figure 2, there is a region 20 containing a high concentration of single-crystal oxide particles 22 on the side closer to surface 14, which is the surface opposite to the surface 16 facing the positive electrode current collector 10. Surface 14 is an example of the first surface disclosed herein, and surface 16 is an example of the second surface disclosed herein. Region 20 is an example of the first region disclosed herein.

[0052] By unevenly distributing the single-crystal oxide on surface 14, that is, on the region 20 on the separator 6 side of the secondary battery 2, the single-crystal oxide preferentially receives and releases electrons, allowing the positive electrode composite layer 12 to move rapidly. As a result, the load on the olivine-type compound is reduced. Therefore, even in charge-discharge cycles at high temperatures, such as near or above 60°C, the leaching of Mn in the olivine-type compound and the resulting decrease in capacity and cycle characteristics are suppressed. Furthermore, because the single-crystal oxide is a single crystal, even under excessive load, particle cracking and other problems are less likely to occur, suppressing rapid degradation. Note that unevenly distributing the single-crystal oxide on the separator 6 side may be more effective in suppressing the decrease in cycle characteristics than unevenly distributing the single-crystal oxide on the side closer to the positive electrode current collector 10, as described later.

[0053] Having such a region 20 means that, in the thickness direction of the positive electrode composite layer 12, the particles 22 are unevenly distributed, with a higher concentration on the side closer to the surface 14 than on the side closer to the surface 16. Therefore, in addition to cases where single-crystal oxide is contained only on the side closer to the surface 14, there are also cases where single-crystal oxide is contained at a first concentration on the side closer to the surface 14 and at a second concentration lower than the first concentration on the side closer to the surface 16. On the other hand, region 20 may also be a region in which the concentration of single-crystal oxide increases continuously or stepwise as it approaches the surface 14 of the positive electrode composite layer 12. Such region 20 may extend over the entire thickness of the positive electrode composite layer 12.

[0054] Furthermore, from the viewpoint of electronic conductivity, region 20 is preferably located in close proximity to surface 14, and preferably is provided in the positive electrode composite layer 12 including surface 14. In terms of the distribution of particles 22 in the thickness direction of the positive electrode composite layer 12, it is preferable that surface 14 or the area including surface 14 has the highest concentration.

[0055] Region 20 is a region containing both an olivine-type compound and a single-crystal oxide. Region 20 does not necessarily have to be layered in the positive electrode composite layer 12, but it is preferable that it exists in a layered manner over almost the entire surface 14 of the positive electrode composite layer 12. This allows the effect of unevenly distributed single-crystal oxide to be obtained throughout the positive electrode 4.

[0056] As shown in Figure 2, the positive electrode composite layer 12 may have a region 30 in addition to region 20, on the side closer to the surface 16, which does not contain single-crystal oxide or contains it at the second concentration. Region 30 contains an olivine-type compound and does not contain single-crystal oxide or contains it at a low concentration. Region 30 does not necessarily have to be layered in the positive electrode composite layer 12, but it is preferable that it exists in a layered manner over almost the entire surface 16. This makes it possible to obtain the effect of unevenly distributed single-crystal oxide throughout the positive electrode 4. The thickness of region 30 is not particularly limited, but generally it is the remaining thickness of region 20 in the thickness direction of the positive electrode composite layer 12. Region 30 is an example of the second region disclosed herein.

[0057] In region 20, the mass ratio of single-crystal oxide to olivine-type compound is not particularly limited, and it is sufficient that the single-crystal oxide is concentrated on the side closer to the surface 14. In region 20 where particles 22 are contained at a generally constant concentration, the content of single-crystal oxide relative to the total mass of single-crystal oxide and olivine-type compound is, for example, 20% to 70% by mass, 30% to 65% by mass, or 30% to 60% by mass, from the viewpoint of cycle characteristics, etc.

[0058] Region 20 may consist of two or more regions with different single-crystal oxide concentrations in the thickness direction of the positive electrode composite layer 12.

[0059] On the other hand, in region 30, the concentration of single-crystal oxide should be lower than in region 20. For example, the content of single-crystal oxide relative to the total mass of single-crystal oxide and olivine-type compound can be, from the viewpoint of cycle characteristics, for example, 0% to 30% by mass, 0% to 20% by mass, 0% to 10% by mass, 0% to 5% by mass, or 0% by mass.

[0060] Furthermore, as shown in Figure 3, the single-crystal oxide can be unevenly distributed so that it is contained in a higher concentration on the side closer to the positive electrode current collector 10. In the positive electrode composite layer 12 shown in Figure 3, there is a region 40 containing a high concentration of single-crystal oxide particles 22 on the side closer to the surface 16 facing the positive electrode current collector 10. By doing so, the single-crystal oxide can be reacted preferentially in the region closer to the positive electrode current collector 10, and as a result, the load on the olivine-type compound is reduced.

[0061] In the case of the distribution morphology shown in Figure 3, contrary to the distribution morphology shown in Figure 2, there may be a region 50 on the side closer to the surface 14 that does not contain single-crystal oxide or contains it at a lower concentration than region 40.

[0062] The mass ratio of single-crystal oxide to olivine-type compound in region 40 is not particularly limited, but various embodiments of the mass ratio of the above compound in region 20 can be applied. Similarly, the mass ratio in region 50 can also be various embodiments of the mass ratio of the above compound in region 30.

[0063] Furthermore, as shown in Figure 4, the single-crystal oxide may be distributed almost uniformly throughout the entire cathode composite layer 12. In the cathode composite layer 12 shown in Figure 4, this also allows the single-crystal oxide to react throughout the entire cathode composite layer 12, thereby reducing the load on the olivine-type compound and suppressing the deterioration of cycle characteristics.

[0064] In the case of the distribution morphology shown in Figure 4, the content of single-crystal oxide relative to the total mass of single-crystal oxide and olivine-type compound in the positive electrode composite layer 12 (overall) is, for example, 15% to 44% by mass, or 20% to 40% by mass, from the viewpoint of cycle characteristics, etc.

[0065] Furthermore, in the distribution (uneven distribution) morphology of single-crystal oxides shown in Figures 2 and 3, the content of single-crystal oxides relative to the total mass of single-crystal oxides and olivine-type compounds in the entire positive electrode composite layer 12 is, for example, 15% to 44% by mass and 20% to 40% by mass.

[0066] Those skilled in the art can produce a layer in which single-crystal oxide particles 22 are dispersed in a desired distribution in a positive electrode composite layer 12 by laminating a slurry containing single-crystal oxide particles 22 at a desired concentration.

[0067] In this way, by distributing single-crystal oxides in various forms in the positive electrode composite layer 12 of the secondary battery 2, the load on the olivine-type compound can be suppressed by the single-crystal oxide, which has excellent electronic conductivity and durability, thereby suppressing the deterioration of cycle characteristics at high temperatures.

[0068] (Secondary battery) As shown in Figure 2, the secondary battery 2 disclosed in the present invention comprises a positive electrode 4, a negative electrode 8, and a separator 6. Because this secondary battery 2 includes a positive electrode 4 with excellent input / output characteristics, it is possible to provide a secondary battery with superior input / output characteristics.

[0069] The secondary battery 2 consists of a positive electrode 4, a negative electrode 8, an electrolyte (liquid 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. The electrolyte solution is, for example, an organic solvent in which a supporting salt is dissolved. The organic solvent is not particularly limited as long as it is an organic solvent commonly used in the electrolyte of lithium-ion secondary batteries; for example, carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones, oxolane compounds, etc., can be used. The type of supporting 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 polyolefin polymers (polyethylene (PE), polypropylene (PP)), can be used. Furthermore, the solid electrolyte can be any material that electrically insulates the positive and negative electrodes and exhibits high lithium-ion conductivity; any known solid electrolyte can be used as appropriate.

[0070] The shape of the secondary battery 2 having the above configuration is not particularly limited and may be various shapes such as coin-type, cylindrical, or prismatic, or it may be an irregular shape enclosed in a laminated casing. [Examples]

[0071] The following describes examples that embody the disclosures of this specification, but the disclosures of this specification are not limited to the following examples.

[0072] (1) Preparation of the positive electrode As the first positive electrode active material, LMFP (average particle size 0.5 μm), which is an olivine-type compound, and as the second positive electrode active material, NCM (average particle size 3.5 μm), which is a single crystal layered rock salt-type oxide (single crystal oxide), were used to prepare a positive electrode composite material slurry and fabricate a positive electrode.

[0073] As LMFP, LiMn 0.60 Fe 0.20 PO4 in which the molar ratios (%) of Mn and Fe are 60 mol% and 40 mol%, respectively, was used. As NCM, three types (LiNi 0.60 Co 0.20 Mn 0.20 O2, LiNi 0.70 Co 0.10 Mn 0.20 O2, LiNi 0.80 Co 0.10 Mn 0.10 O2) with different molar ratios of Ni, Co, and Mn were used.

[0074] Using one type of the first active material (LMFP) and three types of the second active material (NCM), carbon nanotubes (CNT) as a conductive assistant and polyvinylidene fluoride (PVDF) as a binder, a plurality of types of slurry I on the current collector side and slurry II on the separator side were prepared respectively. Each slurry had the contents of CNT and PVDF fixed at 1.5% by mass and 4.0% by mass respectively, and the remainder was formulated so as to be the combination and mass ratio (%) of the first active material and the second active material shown in Table 1, and was prepared as a slurry together with a solvent.

[0075]

Table 1

[0076] Using slurry I and slurry II in the combinations shown in Table 1, positive electrodes having a positive electrode composite material layer of Examples 1 to 4 and Comparative Examples 1 to 2 were fabricated. First, a predetermined amount of slurry I was applied with a doctor blade onto an Al foil with a thickness of 30 μm to a basis weight of 12.5 mg / cm 2 and then slurry II was applied onto slurry I in the same amount as above to a basis weight of 12.5 mg / cm2 The material was applied using a doctor blade to achieve the desired consistency. Afterward, it was dried at 100°C for 10 minutes, and then pressed to create each positive electrode with a density of 2.5 g / cc.

[0077] (2) Fabrication of the negative electrode Artificial graphite (average particle size 22 μm) was used as the negative electrode active material, and styrene-butadiene rubber (SBR) and carboxymethylcellulose (CMC) were used as binders. The active material:SBR:CMC were mixed in a ratio of 96:3:1 (mass%) to prepare a negative electrode mixture slurry. This negative electrode mixture slurry was applied to a 15 μm Cu foil with a basis weight of 13 mg / cm² on one side. 2 The material was applied using a doctor blade so that the ratio of positive electrode capacity to negative electrode capacity was 1.1. It was then dried at 100°C for 10 minutes and pressed to a density of 1.25 g / cc.

[0078] (3) Manufacturing of secondary battery cells A 3-layer laminate of PP / PE / PP (16 μm) was used as the separator, and laminate cells were prepared by laminating the positive electrode, separator, and negative electrode from Examples 1-4 and Comparative Examples 1-2. 1.1 M LiPF6 (ethylene carbonate (EC):dimethyl carbonate (DMC):ethyl methyl carbonate (EMC), 30:40:30 (vol%)) was used as the electrolyte, and the cell confinement pressure was set to 500 kPa.

[0079] (4) Activation and evaluation of capacity retention rate at 60°C 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 constant voltage charging was performed for 3 hours. Then, the battery was discharged to 3.0V at a current of 0.1C using the constant current method to activate the battery. After the activation treatment, the battery was charged to 4.30V at a current of 0.1C using the constant current-constant voltage method at 25°C, and discharged to 3.0V at a current of 0.1C using the constant current-constant voltage method to calculate the initial discharge capacity. For the cycle test, at a 60°C environment, the pattern of charging to 4.3V at a current of 1C using the constant current-constant voltage method and discharging to 3.0V at a current of 1C using the constant current-constant voltage method was repeated 50 times. After that, the discharge capacity after 50 cycles was checked at a 25°C environment using the same method as the initial capacity, and the capacity retention rate before and after the cycle was calculated. The results are shown in Table 1.

[0080] As shown in Table 1, Examples 1 and 2 were configured such that the current collector foil side contained only LMFP, and the separator side contained 60% by mass of NCM with molar ratios of Ni of 60 mol% and 70 mol% (the entire positive electrode composite layer contained 30% by mass of NCM, and the entire amount was contained in the separator side). The capacity retention rates of these cells at 60°C were good, at 89.0% and 87.5%, respectively.

[0081] In contrast, Comparative Example 1, which had the same configuration as Examples 1 and 2 except for using NCM with a molar ratio of 80 mol% Ni, showed a decrease in the 60°C volume retention rate to 81.4%. Furthermore, Comparative Example 2, which had the same configuration as Example 2 except for using polycrystalline NCM, showed a decrease in the same volume retention rate to 82.9%.

[0082] These results show that using single-crystal NCM with a Ni molar ratio of less than 80 mol%, particularly 70 mol% or less, can suppress the decrease in capacity retention even when Mn elution from LMFP is accelerated at high temperatures. NCM with a Ni molar ratio of 80 mol% is thought to be more susceptible to accelerated degradation of the NCM itself, resulting in a smaller effect in suppressing capacity retention. Furthermore, single-crystal NCM was found to be effective in maintaining capacity. This is thought to be because single-crystal NCM is less prone to particle cracking due to charging and discharging than polycrystalline NCM, thus reducing degradation due to particle cracking.

[0083] Example 3 has a configuration in which the current collector foil side contains 60% by mass of NCM with a molar ratio of 60 mol% Ni (the entire positive electrode composite layer contains 30% by mass of NCM, and the entire amount is contained in the current collector foil side), and the separator side consists only of LMFP. In other words, Example 3 has a configuration in which the current collector foil side and the separator side are arranged in the opposite direction compared to Example 1. The capacity retention rate of Example 3 at 60°C was a good 86.2%.

[0084] The results from Examples 1-3 showed that by incorporating NCM unevenly on either the separator side or the current collector foil side, the NCM, which has excellent electronic conductivity, reacts preferentially, thereby reducing the load on the LMFP. Furthermore, since Examples 1-2, in which the NCM was unevenly distributed on the current collector foil side, showed a higher capacity retention rate, it was found that incorporating NCM unevenly on the separator side is more effective in reducing the load on the LMFP and is effective in suppressing the decrease in capacity retention rate.

[0085] Furthermore, in Example 4, where NCM was distributed to a similar extent on both the current collector foil side and the separator side, the capacity retention rate was 84.0%. From these results, it was found that even when Ni is dispersed throughout the entire positive electrode composite layer at a molar ratio of 60-70 mol%, it is effective in suppressing the decrease in capacity retention rate.

[0086] Furthermore, these results indicate that by incorporating approximately 30% by mass of single-crystal NCM throughout the cathode composite layer, the deterioration of cycle characteristics at high temperatures can be effectively suppressed.

[0087] From the above, it was found that when using olivine-type compounds such as LMFP and layered rock salt-type oxides such as LCM as positive electrode active materials, using single-crystal NCM with a Ni molar ratio of less than 80 mol%, for example, 70 mol% or less, can suppress the amount of single-crystal NCM used while also suppressing the decrease in capacity retention even after high-temperature charge-discharge cycles. In particular, it was found that unevenly distributing the single-crystal NCM on the separator side or the current collector foil side is effective.

[0088] The disclosures herein include the following embodiments: [1] A positive electrode for a lithium-ion secondary battery, The device comprises a positive electrode current collector layer and a positive electrode composite layer containing an olivine-type compound and a layered rock salt-type oxide as the positive electrode active material. The layered rock salt type oxide contains a single crystal oxide represented by the following formula (1) as the positive electrode. 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. [2] The positive electrode according to [1], wherein the positive electrode composite layer comprises a first region containing the single crystal oxide in a high concentration on the side of the positive electrode composite layer that is closer to the first surface, which is the surface opposite to the surface facing the positive electrode current collector layer. [3] The single crystal oxide is the positive electrode according to [1] or [2], where 0.60 ≦ a ≦ 0.70 in the formula (1). [4] The positive electrode mixture layer includes a second region that contains the single crystal oxide at a low concentration or does not contain it on the side closer to the second surface, which is the surface of the positive electrode mixture layer facing the positive electrode current collector layer. The positive electrode is as described in any one of [1] to [3]. [5] The olivine-type compound includes a compound represented by the following formula (2). The positive electrode is as described in any one of [1] to [4]. Li g Mn h Fe i M 2 y PO4···(2) (In the 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) × g + (valence of Fe) × h + (valence of M 2 ) × x = 3. [6] In the formula (2), 0.20 ≦ h ≦ 1.2. The positive electrode is as described in [5]. [7] The positive electrode mixture layer contains 20% by mass or more and 40% by mass or less of the single crystal oxide with respect to the total mass of the single crystal oxide and the olivine-type compound. The positive electrode is as described in any one of [1] to [6]. [8] A lithium-ion secondary battery, [1] to [7] any one of the described positive electrodes, a negative electrode, a separator, and includes a lithium-ion secondary battery.

[0089] The specific examples of the technology disclosed in this specification have been described in detail above, but these are merely illustrative and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes to the specific examples described above. The technical elements described in this specification or in the drawings exhibit technical usefulness individually 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 in the drawings can achieve multiple objectives simultaneously, and achieving even one of these objectives itself constitutes technical usefulness. [Explanation of symbols]

[0090] 2 Lithium-ion secondary battery, 4 Positive electrode, 6 Separator, 8 Negative electrode, 10 Positive electrode current collector, 12 Positive electrode composite layer, 14, 16 planes, 20, 30 regions

Claims

1. The device comprises a positive electrode current collector layer and a positive electrode composite layer containing an olivine-type compound and a layered rock salt-type oxide as the positive electrode active material. The layered rock salt type oxide contains a single crystal oxide represented by the following formula (1), and is a positive electrode for a lithium-ion secondary battery. LiNi ａ Co ｂ Mn ｃ M １ ｗ O ２ ··· Formula (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, and w are 0.25 ≤ a < 0.80, 0 < b ≤ 0.74, 0 < c ≤ 0.74, 0 ≤ w ≤ 0.34, and 3a + 3b + 3c + (M １ This shows the number that satisfies (valence of) × w = 3.

2. The positive electrode according to claim 1, wherein the positive electrode composite layer includes a first region containing the single crystal oxide in a high concentration on the side of the positive electrode composite layer that is closer to the first surface, which is the surface opposite to the surface facing the positive electrode current collector layer.

3. The positive electrode according to claim 2, wherein the single-crystal oxide is such that 0.60 ≤ a ≤ 0.70 in formula (1).

4. The positive electrode according to claim 3, wherein the positive electrode composite layer comprises a second region in the positive electrode composite layer that contains or does not contain the single crystal oxide at a low concentration on the side of the positive electrode composite layer that is closer to the second surface which is the surface facing the positive electrode current collector layer.

5. The positive electrode according to claim 4, wherein the olivine-type compound comprises a compound represented by the following formula (2). Li ｇ Mn ｈ Fe ｉ M ２ ｙ PO ４ ・・・(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 are 0 ≤ g ≤ 1.2, 0 < h ≤ 1.2, 0 ≤ i ≤ 1.2, where h + i is not 0, and 0 ≤ y ≤ 0.30, g + (valence of Mn) × g + (valence of Fe) × h + (M ２ Show the number that satisfies (valence of x) × x = 3.

6. The positive electrode according to claim 5, wherein in formula (2), 0.20 ≤ h ≤ 1.

2.

7. The positive electrode according to claim 6, wherein the positive electrode composite layer contains 20% by mass or more and 40% by mass or less of the single crystal oxide with respect to the total mass of the single crystal oxide and the olivine-type compound.

8. Lithium-ion secondary battery, A positive electrode for a lithium-ion secondary battery according to any one of claims 1 to 7, The negative electrode and, Separator and, A lithium-ion secondary battery equipped with these features.

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