Positive electrode for lithium-ion battery and lithium-ion battery

Abstract

Provided is a positive electrode for a lithium ion battery that can achieve a high energy density and improve durability. The positive electrode (21) for a lithium ion battery includes a positive electrode current collector (21A) and a positive electrode composite material layer (21B) formed on the positive electrode current collector (21A). The positive electrode composite material layer (21B) contains a lithium nickel composite oxide and a metal phosphate covering the lithium nickel composite oxide. The lithium nickel composite oxide is represented by the chemical formula LiNi x Co y M z O2 (where M is one or more elements selected from the group consisting of Mn, Al, Mg, and W, x + y + z = 1, and 0.6 ≤ x < 1.0). The metal phosphate is one or more materials selected from the group consisting of VPO4, VP2O7, and VPO4F. The mass ratio of the metal phosphate to the lithium nickel composite oxide is 0.01% by mass or more and 20% by mass or less.

Description

Technical Field

[0001] The present invention relates to a positive electrode for a lithium ion battery and a lithium ion battery. Background Art

[0002] In recent years, with the high performance and multi-functionality of electric and electronic devices, there have been demands for high energy density, high capacity, high output, and improved durability of lithium ion batteries that supply power to various devices. As one of the methods for achieving this, research is being conducted on positive electrode active materials as components of the positive electrode.

[0003] As a conventional positive electrode active material, for example, a composite positive electrode active material containing a metal phosphate and a lithium composite oxide has been proposed (Patent Document 1).

[0004] The above metal phosphate is represented by the chemical formula (a): M x P y O z (wherein M is one or more elements selected from vanadium (V), niobium (Nb), and tantalum (Ta), and has a range of 1 ≤ y / x ≤ 1.33 and 4 ≤ z / y ≤ 5).

[0005] In addition, the above lithium composite oxide is a compound represented by any one of the following chemical formulas (b) to (e).

[0006] Chemical formula (b): LiM2O4 (wherein M is one or more elements selected from the group consisting of nickel (Ni), manganese (Mn), and cobalt (C o )

[0007] Chemical formula (c): Li 1+x M 1-x O2 (wherein M is one or more elements selected from the group consisting of nickel (Ni), cobalt (Co), manganese (Mn), titanium (Ti), vanadium (V), iron (Fe), niobium (Nb), and molybdenum (Mo), and 0 < x ≤ 0.3)

[0008] Chemical formula (d): Li a Ni b Co c Mn d M e O2 (wherein M is one or more elements selected from the group consisting of titanium (Ti), vanadium (V), iron (Fe), niobium (Nb), and molybdenum (M o ) and 1.1 ≤ a < 1.5, 0 < b < 1, 0 ≤ c < 1, 0 < d < 1, 0 ≤ e < 1, and 0 < b + c + d + e < 1)

[0009] Chemical formula (e): Li 1+x1 M 1-x1O₂ (where M is one or more elements selected from the group consisting of nickel (Ni), cobalt (Co), manganese (Mn), titanium (Ti), vanadium (V), iron (Fe), niobium (Nb), and molybdenum (Mo), and 0.1 ≤ x1 ≤ 0.3)

[0010] In addition, as another conventional positive electrode active material, a positive electrode active material for a non-aqueous electrolyte secondary battery including particles of a lithium nickel composite oxide represented by the following chemical formula (f) has been proposed (Patent Document 2). In this positive electrode active material, a coating film with a thickness of 1 to 200 nm containing tungsten (W) and lithium (Li) is present on the surface of the particles of the lithium nickel composite oxide. In addition, the length of the c-axis in the crystal of the lithium nickel composite oxide obtained from the Rietveld analysis of X-ray diffraction is 14.183 Å or more and 14.205 Å or less.

[0011] Chemical formula (f): Li b Ni 1-x-y Co x M y O₂

[0012] (where M is at least one element selected from magnesium (Mg), aluminum (Al), calcium (Ca), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), niobium (Nb), zirconium (Zr), and molybdenum (Mo). b satisfies 0.95 ≤ b ≤ 1.03, x satisfies 0 < x ≤ 0.15, y satisfies 0 < y ≤ 0.07, and x + y satisfies x + y ≤ 0.16.)

[0013] Moreover, as the constitution of another conventional positive electrode active material, a positive electrode active material including particles of a lithium nickel composite oxide and particles of lithium vanadium phosphate covering the surface of the particles of the lithium nickel composite oxide has been proposed (Patent Document 3). In this positive electrode active material, the mass ratio of the lithium vanadium phosphate particles to the lithium nickel composite oxide particles is in the range of 5:85 to 60:30.

[0014] Prior Art Documents

[0015] Patent Documents

[0016] Patent Document 1: Japanese Patent Application Laid-Open No. 2016-127024

[0017] Patent Document 2: International Publication No. 2017-073246

[0018] Patent Document 3: Japanese Patent Application Laid-Open No. 2013 / 77420 Summary of the Invention

[0019] Problems to be Solved by the Invention

[0020] However, in Patent Document 1, on the premise that the lithium composite oxide has an overlithiated layered structure (OLO: overlithiated layered oxide), a structure in which a prescribed metal phosphate is coated on the OLO is adopted, and an improvement in durability can be predicted to some extent. However, there are problems such as a decrease in the open circuit voltage (OCV: Open Circuit Voltage) due to a change in the crystal structure of the positive electrode active material and a decrease in the durability of the lithium ion battery.

[0021] Patent Document 2 has a structure in which the crystal structure in a lithium nickel composite oxide with an increased nickel ratio is controlled and a coating film with a prescribed thickness containing tungsten (W) is formed on the particles of the lithium nickel composite oxide. The coating film containing tungsten (W) contributes to the high capacity, high output, and low resistance of the lithium ion battery, but it is difficult to contribute to an improvement in durability.

[0022] In addition, Patent Document 3 has a structure in which the lithium vanadium phosphate particles are 30% by mass or more and 85% by mass or less with respect to the lithium nickel composite oxide particles, so that the surface of the lithium nickel composite oxide particles is hardly exposed and the oxidative decomposition of the lithium nickel composite oxide particles can be suppressed. However, since lithium vanadium phosphate also functions as a positive electrode active material, the discharge capacity of lithium vanadium phosphate greatly affects the overall discharge capacity of the positive electrode active material, and the overall discharge capacity of the positive electrode active material decreases, resulting in a decrease in energy density.

[0023] An object of the present invention is to provide a positive electrode for a lithium ion battery and a lithium ion battery that can achieve a high energy density and improve durability.

[0024] Means for Solving the Problem

[0025] As a result of repeated intensive studies by the inventors, it was found that by covering a lithium nickel composite oxide with an increased nickel ratio to a specific range with a metal phosphate and setting the mass ratio of the metal phosphate to the lithium nickel composite oxide to a specific range, a high energy density of the lithium ion battery can be achieved by a high nickel ratio, and oxygen detachment occurring from the surface of the lithium nickel composite oxide can be sufficiently suppressed by a metal phosphate with a specific mass ratio lower than before. Non-active nickel oxide is not easily generated on the surface of the lithium nickel composite oxide, and thus the durability of the lithium ion battery can be improved.

[0026] That is, the main structure of the present invention is as follows.

[0027] [1] A positive electrode for a lithium ion battery, wherein

[0028] the positive electrode for the lithium ion battery has a positive electrode current collector and a positive electrode composite material layer formed on the positive electrode current collector,

[0029] The positive electrode composite material layer contains a lithium nickel composite oxide and a metal phosphate covering the lithium nickel composite oxide.

[0030] The lithium nickel composite oxide is represented by the chemical formula LiNi x Co y M z O2 (where M is one or more elements selected from the group consisting of Mn, Al, Mg, and W, and x + y + z = 1, 0.6 ≤ x < 1.0).

[0031] The metal phosphate is one or more materials selected from the group consisting of VPO4, VP2O7, and VPO4F.

[0032] The mass ratio of the metal phosphate to the lithium nickel composite oxide is 0.01% by mass or more and 20% by mass or less.

[0033] [2] The positive electrode for a lithium ion battery according to [1] above, wherein, in the chemical formula, 0 < y ≤ 0.2.

[0034] [3] The positive electrode for a lithium ion battery according to [1] above, wherein the metal phosphate covers the entire surface of the lithium nickel composite oxide.

[0035] [4] The positive electrode for a lithium ion battery according to [1] above, wherein the mass ratio of the metal phosphate to the lithium nickel composite oxide is 0.1% by mass or more and 10% by mass or less.

[0036] [5] The positive electrode for a lithium ion battery according to any one of [1] to [4] above, wherein the lithium nickel composite oxide is the chemical formula LiNi x Co y Mn z O2 (x + y + z = 1, 0.6 ≤ x < 1.0).

[0037] [6] The positive electrode for a lithium ion battery according to any one of [1] to [4] above, wherein the metal phosphate is VP2O7.

[0038] [7] A lithium ion battery, wherein the lithium ion battery includes the positive electrode for a lithium ion battery according to any one of [1] to [6] above.

[0039] Advantages of the Invention

[0040] According to the present invention, it is possible to achieve a high energy density of the lithium ion battery and at the same time improve the durability. Brief Description of the Drawings

[0041] Figure 1It is a perspective view showing the overall structure of a lithium-ion battery according to an embodiment of the present invention.

[0042] Figure 2 Among them, (a) is a partial cross-sectional view briefly showing Figure 1 the internal structure of the lithium-ion battery, Figure 2 and (b) among them is a partial enlarged cross-sectional view briefly showing Figure 2 the structure of the positive electrode for a lithium-ion battery in (a) among them, Figure 2 and (c) among them is a cross-sectional view showing the structure of the positive electrode active material. Detailed Embodiments

[0043] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0044] [Structure of Positive Electrode for Lithium-Ion Battery and Lithium-Ion Battery]

[0045] Figure 1 It is a perspective view showing the overall structure of a lithium-ion battery according to an embodiment of the present invention, Figure 2 and (a) among them is a partial cross-sectional view briefly showing Figure 1 the internal structure of the lithium-ion battery, Figure 2 and (b) among them is a partial enlarged cross-sectional view briefly showing Figure 2 the positive electrode structure for a lithium-ion battery in (a) among them. It should be noted that in the following description, the drawings used sometimes enlarge the parts that become features expediently for easy understanding of the features, and the shapes, dimensional ratios, etc. of each component are not limited to those shown in the drawings.

[0046] As Figure 1 shown, the lithium-ion battery 1 includes a laminate 2 containing electrodes, an outer package 4 housing the laminate, and a lid 5 sealing the outer package 4. The lithium-ion battery 1 is, for example, a square lithium-ion secondary battery. The outer package 4 is composed of, for example, a metal frame.

[0047] The laminate 2, as Figure 2 shown in (a) and Figure 2 shown in (b) among them, includes a positive electrode 21 for a lithium-ion battery (hereinafter also simply referred to as a positive electrode), a negative electrode 22 for a lithium-ion battery (hereinafter also simply referred to as a negative electrode), and a separator 23 sandwiched between the positive electrode 21 and the negative electrode 22. An electrolytic solution is impregnated in the positive electrode 21, the negative electrode 22, and the separator. The positive electrode current collector 21A is connected to a positive electrode current collecting portion (not shown), and the negative electrode current collector 22A is connected to a negative electrode current collecting portion (not shown).

[0048] The positive electrode 21 has a positive electrode current collector 21A and a positive electrode composite material layer 21B formed on the positive electrode current collector 21A and containing a positive electrode active material.

[0049] The positive electrode current collector 21A is, for example, a plate-like body or a film-like body formed of a conductive material. As the conductive material, metals such as aluminum (Al) or nickel (Ni) can be used, for example. In the case where the conductive material is aluminum (Al), an Al-Fe alloy such as JIS A8021 or pure aluminum such as JIS A1085 can be used. The thickness of the positive electrode current collector 21A is, for example, 8 μm or more and 15 μm or less.

[0050] The positive electrode composite material layer 21B contains, as shown in (c) of Figure 2 , a lithium nickel composite oxide 21a and a metal phosphate 21b covering the lithium nickel composite oxide 21a. In addition, preferably, the positive electrode composite material layer 21B contains a binder 21c and a conductive additive 21d.

[0051] The lithium nickel composite oxide 21a is represented by the chemical formula LiNixCo y M z O2 (where M is one or more elements selected from the group consisting of Mn, Al, Mg, and W, and x + y + z = 1, 0.6 ≤ x < 1.0). In this way, by increasing the nickel ratio in the lithium nickel composite oxide 21a, high energy density can be achieved. In addition, preferably, the lithium nickel composite oxide 21a is represented by the chemical formula LiNi x Co y Mn z O2 (x + y + z = 1, 0.6 ≤ x < 1.0).

[0052] Here, it is well known that regardless of the Ni concentration, the crystal structure near the surface of the positive electrode active material in the contact region between the positive electrode composite material layer and the electrolyte deteriorates, hindering the insertion / extraction reaction of lithium. Therefore, by covering the surface of the positive electrode active material with a stable material, that is, a specific metal phosphate described later, direct contact between the positive electrode active material and the electrolyte can be effectively prevented. Thus, for example, even if the lithium nickel composite oxide is a lithium nickel cobalt manganese composite oxide (ternary active material), that is, any of NCM811, NCM622, and NCM523, by using the above-mentioned specific metal phosphate, direct contact between the composite oxide and the electrolyte can be prevented.

[0053] In addition, in the above chemical formula, the lithium nickel composite oxide 21a can use, for example, a compound represented by 0.6 ≤ x ≤ 0.95, 0 ≤ y 0.2, 0 ≤ z ≤ 0.4.

[0054] The lithium nickel composite oxide 21a has a particle shape, for example. The lithium nickel composite oxide 21a can be primary particles or secondary particles obtained by aggregating primary particles with each other.

[0055] The metal phosphate 21b is one or more materials selected from the group consisting of VPO4, VP2O7, and VPO4F. From the viewpoint of further improving durability, VPO4F is preferred. In addition, the metal phosphate 21b does not necessarily need to cover the entire surface of the lithium nickel composite oxide 21a. For example, when the lithium nickel composite oxide 21a has a particle shape, it is preferred that the metal phosphate 21b covers the surface of the particles of the lithium nickel composite oxide 21a. In Figure 2 In the example of (c) in

[0056] the metal phosphate 21b covers the entire surface of the lithium nickel composite oxide 21a, but it does not necessarily need to cover the entire surface of the lithium nickel composite oxide 21a. It is sufficient that the metal phosphate 21b covers at least a part of the surface of the lithium nickel composite oxide 21a.

[0057] In addition, the mass ratio of the metal phosphate 21b to the lithium nickel composite oxide 21a is 0.01 mass% or more and 20 mass% or less. Thus, the nickel ratio in the lithium nickel composite oxide 21a is high, and high energy density of the lithium ion battery is achieved by the high nickel ratio. In addition, even when the metal phosphate 21b in the range of a mass ratio lower than before is covered on the lithium nickel composite oxide 21a, oxygen desorption occurring from the surface of the lithium nickel composite oxide 21a can be sufficiently suppressed during charge-discharge cycles, and inactive nickel oxide (NiO) is not easily generated on the surface of the lithium nickel composite oxide 21a, thereby improving the durability of the lithium ion battery.

[0058] The mass ratio of the metal phosphate 21b to the lithium nickel composite oxide 21a is preferably 0.1 mass% or more and 10 mass% or less. Thereby, high energy density of the lithium ion battery can be achieved while further improving durability.

[0059] In addition, preferably, in the lithium nickel composite oxide 21a in the above chemical formula LiNi x Co y M z O2, 0 < y ≤ 0.2. Cobalt ions (Co 3+) Different from nickel ions (Ni 3+ ), it does not generate crystallization strain accompanied by the Jahn-Teller effect. Therefore, by increasing cobalt ions (Co 3+ ), the formation of inactive nickel oxide on the surface of the lithium nickel composite oxide 21a can be suppressed. Thus, by setting the ratio of cobalt (Co) in the lithium nickel composite oxide 21a within the above range, oxygen detachment can be further suppressed, and the layered structure of the positive electrode active material can be more stabilized. In the above chemical formula LiNi x Co y M z O2, when 0.2 < y, the discharge capacity of the lithium nickel composite oxide decreases.

[0060] As the binder 21c, for example, polyvinylidene fluoride (PVDF) can be used. In addition, as the conductive additive 21d, for example, a carbon material can be used. As the carbon material, one or more selected from the group consisting of acetylene black, carbon nanotubes, graphene, and graphite particles can be used. As the carbon nanotubes, for example, VGCF synthesized by the chemical vapor deposition (CVD) method can be used.

[0061] The mixing ratio of the binder in the positive electrode composite material layer 21B can be set, for example, as (positive electrode active material) : (conductive additive) : (binder) = 90 - 95 : 3 - 5 : 2 - 5.

[0062] The negative electrode 22 has a negative electrode current collector 22A and a negative electrode composite material layer 22B formed on the negative electrode current collector 22A and containing a negative electrode active material. The negative electrode composite material layer 22B may also contain a binder, a conductive additive, a thickener, etc. not shown.

[0063] The negative electrode current collector 22A is, like the positive electrode current collector 21A, for example, a plate-like body or a film-like body formed of a conductive material. As the conductive material, for example, metals such as copper (Cu) or nickel (Ni) can be used. When the above conductive material is copper, for example, tough-pitch copper such as JIS C1100 can be used. The thickness of the negative electrode current collector 22A is, for example, 5 μm or more and 10 μm or less.

[0064] The negative electrode active material is not particularly limited, and for example, it may contain one or more selected from the group consisting of natural graphite, artificial graphite, hard carbon, activated carbon, silicon (Si), silicon oxide (SiOx), tin (Sn), and tin oxide (SnOx).

[0065] As the binder for the negative electrode composite material layer 22B, for example, one or more selected from the group consisting of polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC) can be used. Additionally, as the conductive aid for the negative electrode composite material layer 22B, for example, either or both of acetylene black and carbon nanotubes can be used. As the carbon nanotubes, for example, VGCF synthesized by the vapor phase method (CVD) can be used.

[0066] The mixing ratio of the binder in the negative electrode composite material layer 22B can be set, for example, as (negative electrode active material)∶(conductive aid)∶(binder)∶(thickener) = 96 - 98∶0 - 1∶1 - 2∶0.5 - 1.

[0067] The positive electrode current collector portion electrically connects a plurality of positive electrode current collectors 21A to the positive electrode terminal 6. The positive electrode current collector portion is made of, for example, aluminum (Al) or an aluminum alloy.

[0068] The negative electrode current collector portion electrically connects a plurality of negative electrode current collectors 22A to a negative electrode terminal (not shown). The negative electrode current collector portion is made of, for example, copper (Cu) or a copper alloy.

[0069] The lithium ion battery 1 is square-shaped, but is not limited thereto. It can also be a laminated battery type or a cylindrical type. Additionally, the exterior body 4 of the lithium ion battery 1 is, for example, a metal frame, but is not limited thereto. The exterior body can also be a laminated film.

[0070] When the exterior body of the lithium ion battery 1 is a laminated film, the laminated film can have a base material, a protective layer, and an adhesive layer.

[0071] The base material is made of, for example, aluminum (Al) or stainless steel such as SUS. The protective layer is made of, for example, one or more selected from the group consisting of polyethylene terephthalate (PET), polyethersulfone (PES), and nylon. The adhesive layer is made of, for example, a polyolefin resin. As the polyolefin resin, for example, either maleic anhydride-modified polyethylene or polypropylene (PP) can be used.

[0072] The separator 23 is an insulating thin film, for example, a porous body formed of materials such as polyethylene resin, polypropylene resin, or aromatic polyamide resin. Additionally, the separator 23 can also have a porous body and a coating formed on the surface of the porous body. As the coating, for example, ceramics composed of silicon oxide (SiOx), aluminum oxide (Al2O3), etc., or aromatic polyamide resin can be used.

[0073] The electrolytic solution can contain, for example, a solvent, a lithium salt, and an additive.

[0074] As the solvent, one or more selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and γ-butyrolactone (γBL) can be used, for example.

[0075] As the lithium salt, one or more selected from the group consisting of LiPF6, lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(oxalato)borate (LiBOB), lithium difluorophosphate (LiDFP), and lithium difluoro(oxalato)borate (LiDFOB) can be used, for example.

[0076] As the additive, one or more selected from the group consisting of vinylene carbonate (VC), fluoroethylene carbonate (FEC), PS (propane sultone), and PRS (propylene sultone) can be used, for example.

[0077] The embodiments of the present invention have been described in detail above, but the present invention is not limited to the above embodiments, and various modifications and changes can be made within the scope of the gist of the present invention described in the technical solution.

[0078] Examples

[0079] Examples of the present invention will be described below. However, the present invention is not limited only to the following examples.

[0080] (Example 1-1)

[0081] A specified amount of vanadium tetraacetate (C8H 12 O8V) was dissolved in distilled water and stirred for 30 minutes to obtain Solution A. In addition, NH4H2PO4 in the same molar amount as vanadium tetraacetate was dissolved to obtain Solution B1. Solution B1 was dropped into Solution C obtained by dispersing nickel composite oxide (LiNi 0.8 Co 0.1 Mn 0.1 O2) in Solution A and stirred for 3 hours to obtain Solution D1. After drying Solution D1 in an oil bath at 60°C, heat treatment was performed at 300°C for 5 hours to obtain a positive electrode active material in which VPO4 was covered on the surface of the lithium nickel composite oxide. The mass ratio of the metal phosphate to the lithium nickel composite oxide in the obtained positive electrode active material is shown in Table 1.

[0082] Next, 94% by mass of the obtained positive electrode active material, 3% by mass of a carbon material as a conductive aid, and 3% by mass of a PVDF binder as a binder were mixed to adjust a positive electrode mixture slurry, which was then coated on an aluminum foil with a thickness of 15 μm. The coating amount of the positive electrode mixture slurry was 21.2 mg / cm 2After that, it is dried and rolled to obtain a positive electrode. The size of the positive electrode is 40 mm × 40 mm.

[0083] In addition, 97% by mass of natural graphite, 1% by mass of a carbon material as a conductive aid, 1% by mass of SBR as a binder, and 1% by mass of CMC as a thickener are mixed to adjust the negative electrode mixture paste, which is then coated on a rolled copper foil with a thickness of 8 μm. The coating amount of the negative electrode mixture paste is 12.5 mg / cm 2 After that, it is dried and rolled to obtain a negative electrode. The size of the negative electrode is 44 mm × 44 mm.

[0084] Next, the positive electrode and negative electrode obtained as described above, and a porous polyolefin separator are prepared, and the positive electrode, the porous separator, and the negative electrode are laminated in sequence, and they are wound to form a laminate. Next, the laminate is housed in an outer package, and the positive electrode current collector portion and the negative electrode current collector portion are connected to a positive terminal and a negative terminal, respectively. After that, an electrolyte is adjusted by mixing 1.2 M of LiPF6 in EC: 30 wt%, EMC: 40 wt%, DMC: 30 wt%, and the electrolyte is filled into the outer package, and the outer package is sealed with a lid to obtain a lithium-ion battery.

[0085] (Example 1-2)

[0086] A positive electrode and a lithium-ion battery are obtained in the same manner as in Example 1-1, except that the mass ratio of metal phosphate to lithium nickel composite oxide in the positive electrode active material is changed.

[0087] (Example 2-1)

[0088] A specified amount of vanadium tetraacetate is dissolved in distilled water and stirred for 30 minutes to obtain Solution A. In addition, NH4H2P2O7 in the same molar amount as vanadium tetraacetate is dissolved to obtain Solution B2. In Solution C obtained by dispersing nickel composite oxide (LiNi 0.8 Co 0.1 Mn 0.1 O2) in Solution A, Solution B2 is dropped and stirred for 3 hours to obtain Solution D2. After drying Solution D2 in an oil bath at 60 °C, heat treatment is performed at 300 °C for 5 hours to obtain a positive electrode active material in which VP2O7 is covered on the surface of the nickel composite oxide positive electrode. After that, a positive electrode and a lithium-ion battery are obtained in the same manner as in Example 1-1.

[0089] (Example 2-2)

[0090] A positive electrode and a lithium-ion battery are obtained in the same manner as in Example 2-1, except that the mass ratio of metal phosphate to lithium nickel composite oxide in the positive electrode active material is changed.

[0091] (Example 3-1)

[0092] A solution A was obtained by dissolving a specified amount of vanadium tetraacetate in distilled water and stirring for 30 minutes. In addition, NH4H2PO4 with the same molar number as that of vanadium tetraacetate was dissolved, and PTFE was further dispersed to obtain a solution B3. Solution B3 was dropped into a solution C obtained by dispersing a nickel composite oxide (LiNi 0.8 Co 0.1 Mn 0.1 O2) in solution A, and the mixture was stirred for 3 hours to obtain a solution D3. After drying solution D3 in an oil bath at 60 °C, a heat treatment at 450 °C for 5 hours was carried out to obtain a positive electrode active material with VPO4F covering the surface of the nickel composite oxide. Thereafter, a positive electrode and a lithium-ion battery were obtained in the same manner as in Example 1-1.

[0093] (Example 3-2)

[0094] A positive electrode and a lithium-ion battery were obtained in the same manner as in Example 3-1, except that the mass ratio of metal phosphate to lithium nickel composite oxide in the positive electrode active material was changed.

[0095] (Comparative Example 1)

[0096] A positive electrode and a lithium-ion battery were obtained in the same manner as in Example 1-1, except that the surface of the lithium nickel composite oxide was not covered with metal phosphate.

[0097] Next, the obtained lithium-ion battery was measured and evaluated by the following method.

[0098] [Initial resistance]

[0099] Regarding the initial resistance, it was adjusted to SOC 50% at an ambient temperature of 25 °C, and a 10-second 3C discharge was carried out. The measured values of the voltage and current at this time were measured, and the resistance value was calculated according to the following formula (1) to obtain the initial resistance.

[0100] Initial resistance (R) = (OCV - voltage at the 10th second) / discharge current ··· (1)

[0101] [Capacity retention rate]

[0102] It was set at an ambient temperature of 45 °C, a charging condition of 0.6C, a termination voltage of 4.2V, a discharging condition of 1.2C, and a termination voltage of 2.7V, and this cycle was carried out 600 times. The capacity retention rate (%) was calculated according to the following formula (2).

[0103] Capacity retention rate (%) = (discharge capacity of the 600th cycle / discharge capacity of the 1st cycle) × 100 ··· (2)

[0104] [Resistance increase rate]

[0105] Regarding the resistance increase rate (%), the initial resistance is set as R0, and the resistance after the time (t) elapsed for 600 cycles is set as R t , and the resistance increase rate (%) is calculated according to the following formula (3).

[0106] Resistance increase rate (%) = R t / R0 × 100 ··· (3)

[0107] The measurement results are shown in Table 1.

[0108] [Table 1]

[0109]

[0110] From the results in Table 1, it can be seen that in Example 1-1, when the lithium nickel composite oxide is LiNi 0.8 Co 0.1 Mn 0.1 O2 (x = 0.8) and the mass ratio of VPO4 to the lithium nickel composite oxide is 0.5% by mass, the initial resistance becomes 1.05 Ω, and high energy density can be achieved through a high nickel ratio. In addition, it can be seen that the capacity retention rate is higher than that of Comparative Example 1, and the resistance increase rate is lower than that of Comparative Example 1, improving durability.

[0111] It can be seen that in Example 1-2, the mass ratio of VPO4 to the lithium nickel composite oxide is 5.0% by mass. The initial resistance is larger than that of Example 1-1, but the capacity retention rate is higher than that of Example 1-1, and the resistance increase rate is lower than that of Example 1-1, further improving durability.

[0112] It can be seen that in Example 2-1, when the lithium nickel composite oxide is LiNi 0.8 Co 0.1 Mn 0.1 O2 (x = 0.8) and the mass ratio of VP2O7 to the lithium nickel composite oxide is 0.5% by mass, the initial resistance becomes 1.06 Ω, and high energy density can be achieved through a high nickel ratio. In addition, it can be seen that the capacity retention rate is higher than that of Comparative Example 1, and the resistance increase rate is lower than that of Comparative Example 1, improving durability.

[0113] It can be seen that in Example 2-2, the mass ratio of VP2O7 to the lithium nickel composite oxide is 5.0% by mass. The initial resistance is larger than that of Example 2-1, but the capacity retention rate is higher than that of Example 2-1, and the resistance increase rate is lower than that of Example 2-1, further improving durability.

[0114] It can be seen that in Example 3-1, when the lithium nickel composite oxide is LiNi 0.8 Co 0.1 Mn 0.1 O2 (x = 0.8) and the mass ratio of VPO4F to the lithium nickel composite oxide is 0.5% by mass, the initial resistance is 1.04 Ω, and high energy density can be achieved through a high nickel ratio. In addition, it can be seen that the capacity retention rate is higher than that of Comparative Example 1, and the resistance increase rate is lower than that of Comparative Example 1, improving the durability.

[0115] It can be seen that in Example 3-2, the mass ratio of VPO4F to the lithium nickel composite oxide is 5.0% by mass. The initial resistance and the resistance increase rate are respectively larger than those in Example 3-1, but the capacity retention rate is higher than that in Example 1-1, improving the durability.

[0116] On the other hand, in Comparative Example 1, the lithium nickel composite oxide is LiNi 0.8 Co 0.1 Mn 0.1 O2 (x = 0.8), but the lithium nickel composite oxide is not covered with metal phosphate, resulting in a low capacity retention rate, a high resistance increase rate, and poor durability.

[0117] Industrial Applicability

[0118] The positive electrode for a lithium ion battery of the present invention can be applied to lithium ion batteries such as primary batteries and secondary batteries. In addition, the lithium ion battery of the present invention can be applied to electric vehicles (EVs) such as two-wheelers and four-wheelers, and is particularly suitable for electric motor vehicles and hybrid vehicles.

[0119] Explanation of Reference Numerals:

[0120] 1 Lithium ion battery

[0121] 2 Laminate

[0122] 4 Outer package

[0123] 5 Cover

[0124] 6 Positive electrode terminal

[0125] 21 Positive electrode

[0126] 21A Positive electrode current collector

[0127] 21B Positive electrode composite material layer

[0128] 21a Lithium nickel composite oxide

[0129] 21b Metal phosphate

[0130] 21c Binder

[0131] 21d Conductive additive

[0132] 22 Negative electrode

[0133] 22A Negative electrode current collector

[0134] 22B Negative electrode composite material layer

[0135] 23 Separator.

Claims

1. A positive electrode for a lithium-ion battery, wherein, the positive electrode for the lithium-ion battery has a positive electrode current collector and a positive electrode composite material layer formed on the positive electrode current collector, the positive electrode composite material layer contains a lithium nickel composite oxide and a metal phosphate covering the lithium nickel composite oxide, The lithium nickel composite oxide is represented by the chemical formula LiNi x Co y M z O2, where M in the formula is one or more elements selected from the group consisting of Mn, Mg, and W, and x + y + z = 1, 0.6 ≤ x < 1.0, 0.1 ≤ y ≤ 0.2, the metal phosphate is one or more materials selected from the group consisting of VP2O7 and VPO4F, the mass ratio of the metal phosphate to the lithium nickel composite oxide is 0.01% by mass or more and 20% by mass or less.

2. The positive electrode for a lithium-ion battery according to claim 1, wherein, the metal phosphate covers the entire surface of the lithium nickel composite oxide.

3. The positive electrode for a lithium-ion battery according to claim 1, wherein, the mass ratio of the metal phosphate to the lithium nickel composite oxide is 0.1% by mass or more and 10% by mass or less.

4. The positive electrode for a lithium-ion battery according to any one of claims 1 to 3, wherein, The lithium nickel composite oxide has the chemical formula LiNi x Co y Mn z O2, where x + y + z = 1, 0.6 ≤ x < 1.0, and 0.1 ≤ y ≤ 0.

2.

5. The positive electrode for a lithium-ion battery according to any one of claims 1 to 3, wherein, the metal phosphate is VP2O7.

6. A lithium-ion battery, wherein, the lithium-ion battery includes the positive electrode for a lithium-ion battery according to any one of claims 1 to 5.

Images