Positive electrode active material and secondary battery

Abstract

The present invention provides a secondary battery positive electrode capable of reducing direct current resistance of a secondary battery. This positive electrode active material is characterized by including: a mixture including a first lithium transition metal composite oxide having a number average particle diameter (D50) in a range of 10-30 μm and a second lithium transition metal composite oxide having a number average particle diameter (D50) in a range of 0.1-8 μm; and a carbonaceous film formed on at least one of the surface of the first lithium transition metal composite oxide and the surface of the second lithium transition metal composite oxide. The positive electrode active material is characterized in that the carbonaceous film contains at least one of an alkali metal excluding Li and an alkaline earth metal.

Description

Positive electrode active material and secondary battery

[0001] The present disclosure relates to a positive electrode active material and a secondary battery.

[0002] BACKGROUND ART In recent years, secondary batteries that include a positive electrode, a negative electrode, and an electrolyte and are charged and discharged by transferring Li ions or the like between the positive electrode and the negative electrode have been widely used as high-power, high-capacity secondary batteries.

[0003] For example, Patent Documents 1 and 2 disclose the use of a composite in which a carbonaceous coating is formed on the surface of an active material body as an active material for a secondary battery.

[0004] JP 2009-140876 A JP 2001-015111 A

[0005] Meanwhile, with regard to secondary batteries, there is a demand for a reduction in DC resistance due to the increasing demand for rapid charging.

[0006] Therefore, an object of the present disclosure is to provide a positive electrode active material that can reduce the DC resistance of a secondary battery.

[0007] A positive electrode active material according to one aspect of the present disclosure includes a mixture containing a first lithium transition metal composite oxide having a number average particle size (D50) in the range of 10 μm or more and 30 μm or less and a second lithium transition metal composite oxide having a number average particle size (D50) in the range of 0.1 μm or more and 8 μm or less, and a carbonaceous coating formed on at least one of a surface of the first lithium transition metal composite oxide and a surface of the second lithium transition metal composite oxide, wherein the carbonaceous coating contains at least one of an alkali metal and an alkaline earth metal excluding Li.

[0008] A secondary battery according to one aspect of the present disclosure is characterized by including a positive electrode containing the above-described positive electrode active material, a negative electrode, and an electrolyte.

[0009] According to the positive electrode active material of one embodiment of the present disclosure, it is possible to reduce the direct current resistance of a secondary battery.

[0010] 1 is a cross-sectional view of a secondary battery according to an embodiment;

[0011] An example of an embodiment of a secondary battery according to the present disclosure will be described below.

[0012] FIG. 1 is a cross-sectional view of a secondary battery according to an embodiment. The secondary battery 10 shown in FIG. 1 includes a wound electrode assembly 14 formed by winding a positive electrode 11 and a negative electrode 12 with a separator 13 interposed therebetween, an electrolyte, insulating plates 18 and 19 disposed above and below the electrode assembly 14, respectively, and a battery case 15 for accommodating the above components. The battery case 15 is composed of a cylindrical case body 16 with a bottom and a sealing body 17 that closes the opening of the case body 16. Note that, instead of the wound electrode assembly 14, other electrode bodies may be used, such as a laminated electrode body formed by alternately stacking positive and negative electrodes with separators interposed therebetween. Examples of the battery case 15 include a cylindrical, rectangular, coin-shaped, or button-shaped metal case, and a resin case formed by laminating a resin sheet (a so-called laminate type).

[0013] The electrolyte has, for example, ion conductivity (for example, lithium ion conductivity). The electrolyte may be a liquid electrolyte (electrolytic solution) or a solid electrolyte.

[0014] The liquid electrolyte (electrolytic solution) contains, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixed solvents of these. The non-aqueous solvent may contain a halogen-substituted compound (e.g., fluoroethylene carbonate) in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine. Examples of the electrolyte salt include LiPF ６ Lithium salts such as

[0015] Furthermore, examples of the solid electrolyte that can be used include solid or gel-like polymer electrolytes, inorganic solid electrolytes, and the like. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt, and a matrix polymer. For example, a polymer material that absorbs a non-aqueous solvent and gels is used as the matrix polymer. Examples of the polymer material include fluororesin, acrylic resin, and polyether resin. For example, the inorganic solid electrolyte can be a material known in all-solid-state lithium ion secondary batteries (e.g., oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, and the like). While the above-exemplified electrolytes are non-aqueous electrolytes, the electrolyte is not limited to non-aqueous electrolytes and may be an aqueous electrolyte.

[0016] The case body 16 is, for example, a cylindrical metal container with a bottom. A gasket 28 is provided between the case body 16 and the sealing body 17 to ensure airtightness inside the battery. The case body 16 has, for example, a protruding portion 22, which is a portion of the side surface that protrudes inward and supports the sealing body 17. The protruding portion 22 is preferably formed in an annular shape along the circumferential direction of the case body 16, and supports the sealing body 17 on its upper surface.

[0017] The sealing body 17 has a structure in which a filter 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in this order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disk or ring shape, and all components except for the insulating member 25 are electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. If the internal pressure of the secondary battery 10 increases due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 24 may deform and rupture, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure further increases, the upper valve body 26 may rupture, and gas may be discharged from the opening of the cap 27.

[0018] In the secondary battery 10 shown in Fig. 1, a positive electrode lead 20 attached to the positive electrode 11 passes through a through-hole in the insulating plate 18 and extends toward the sealing body 17, and a negative electrode lead 21 attached to the negative electrode 12 passes outside the insulating plate 19 and extends toward the bottom side of the case body 16. The positive electrode lead 20 is connected by welding or the like to the underside of a filter 23, which is the bottom plate of the sealing body 17, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the filter 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected by welding or the like to the inner bottom surface of the case body 16, and the case body 16 serves as the negative electrode terminal.

[0019] The positive electrode 11, the negative electrode 12, and the separator 13 that constitute the secondary battery 10 will be described in detail below.

[0020] [Positive Electrode] The positive electrode 11 has a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector. The positive electrode mixture layer may be disposed on one side or both sides of the positive electrode current collector. The positive electrode current collector may be a foil of a metal, such as aluminum or an aluminum alloy, that is stable within the potential range of the positive electrode 11, or a film having such a metal disposed on its surface. The positive electrode mixture layer contains a positive electrode active material. The positive electrode mixture layer may also contain a binder and a conductive material. The positive electrode 11 can be produced, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive material, a binder, etc. to the surface of the positive electrode current collector, drying the coating, and then rolling the coating using a roller or the like.

[0021] Examples of the conductive material contained in the positive electrode mixture layer include carbon materials such as carbon black, acetylene black, ketjen black, graphite, carbon nanotubes, carbon nanofibers, graphene, etc. The content of the conductive material is, for example, 0.01% by mass to 10% by mass, and preferably 0.1% by mass to 5% by mass, relative to the total mass of the positive electrode mixture layer.

[0022] Examples of the binder contained in the positive electrode mixture layer include fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefin. Carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), and the like may also be used. The content of the binder is, for example, 0.1% by mass to 10% by mass, and preferably 0.5% by mass to 5% by mass, relative to the total mass of the positive electrode mixture layer.

[0023] The positive electrode active material contained in the positive electrode mixture layer includes a mixture containing a first lithium transition metal composite oxide having a number-average particle size (D50) in the range of 10 μm to 30 μm and a second lithium transition metal composite oxide having a number-average particle size (D50) in the range of 0.1 μm to 8 μm, and a carbonaceous coating formed on at least one of the surfaces of the first lithium transition metal composite oxide and the second lithium transition metal composite oxide. The number-based particle size distribution of the mixture preferably has peaks of 2% or more in relative particle amount in the particle size range of 10 μm to 30 μm and in the particle size range of 0.1 μm to 8 μm, respectively. The carbonaceous coating also contains at least one of an alkali metal and an alkaline earth metal excluding Li. The use of such a positive electrode active material improves the electronic conductivity of the positive electrode mixture layer, thereby reducing the DC resistance of the secondary battery.

[0024] The first lithium transition metal composite oxide and the second lithium transition metal composite oxide include, for example, secondary particles formed by aggregation of primary particles. Here, the surface of the lithium transition metal composite oxide refers to the surface of the secondary particles of the lithium transition metal composite oxide. That is, the carbonaceous coating is present at least on the surface of the secondary particles of the first lithium transition metal composite oxide and the surface of the secondary particles of the second lithium transition metal composite oxide. The carbonaceous coating may be present in a dotted form so as to cover at least a portion of the surface of the secondary particles of the first lithium transition metal composite oxide and the surface of the second lithium transition metal composite oxide, or may be present so as to cover the entire surface of the secondary particles. Note that the carbonaceous coating may be present at the interface where primary particles of the first lithium transition metal composite oxide contact each other or the interface where primary particles of the second lithium transition metal composite oxide contact each other.

[0025] The primary particles constituting the secondary particles of the first lithium transition metal composite oxide and the second lithium transition metal composite oxide each have a particle size of, for example, 0.02 μm to 2 μm, and are measured as the diameter of a circumscribed circle in a particle image observed with a scanning electron microscope (SEM).

[0026] The number average particle size (D50) of the first lithium transition metal composite oxide may be in the range of 10 μm or more and 30 μm or less, and preferably in the range of 10 μm or more and 25 μm or less. The number average particle size (D50) of the second lithium transition metal composite oxide may be in the range of 0.1 μm or more and 8 μm or less, and preferably in the range of 0.5 μm or more and 5 μm or less. When the D50 of the first lithium transition metal composite oxide and the second lithium transition metal composite oxide satisfy the above range, for example, in the positive electrode mixture layer, gaps between large particles of the first lithium transition metal composite oxide are filled with small particles of the second lithium transition metal composite oxide, ensuring many contact points between the particles, which leads to improved electronic conductivity of the positive electrode mixture layer and ultimately reduced DC resistance of the secondary battery. Although the particle size distribution of the mixture of the first lithium transition metal composite oxide and the second lithium transition metal composite oxide is not particularly limited, using a mixture having peaks of 2% or more in relative particle amount in the particle size range of 10 μm or more to 30 μm or less and in the particle size range of 0.1 μm or more to 8 μm or less reduces the variation in particle size within each particle size range, thereby significantly improving the packing density of the positive electrode active material. This ensures that the first lithium transition metal composite oxide and the second lithium transition metal composite oxide are present in the mixture in amounts greater than or equal to a certain level, ensuring many contact points between the particles.

[0027] In this disclosure, the number-average particle size (D50) refers to the particle size at which the cumulative frequency in a number-based particle size distribution is 50% from the smallest particle size, and is also referred to as the median particle size in the number-based particle size distribution. The number-based particle size distribution can be measured using a laser diffraction particle size analyzer (e.g., MT3000II manufactured by Microtrac-Bell Corporation) with water as the dispersion medium. The number-based particle size distribution of a mixture is a particle size distribution in which the horizontal axis represents particle size and the vertical axis represents the number-based relative particle amount (%), where the total number of particles is 100%. In other words, the peak in the number-based particle size distribution of a mixture is the apex of a mountain observed in a particle size distribution in which the horizontal axis represents particle size and the vertical axis represents the number-based relative particle amount (%), where the total number of particles is 100%.

[0028] The content of the second lithium transition metal composite oxide in the mixture is preferably 10% by mass or more and 50% by mass or less, and more preferably 20% by mass or more and 40% by mass or less, relative to the total mass of the mixture, from the viewpoint of, for example, reducing the DC resistance of the secondary battery. Furthermore, the ratio of the first lithium transition metal composite oxide to the second lithium transition metal composite oxide in the mixture is preferably, for example, in the range of 50:50 to 90:10, and more preferably 60:40 to 80:20, in terms of, for example, reducing the DC resistance of the secondary battery. The mixture may also contain a lithium transition metal composite oxide having a D50 different from that of the first lithium transition metal composite oxide or the second lithium transition metal composite oxide. For example, the mixture may contain a lithium transition metal composite oxide having a number average particle size (D50) of more than 30 μm or a lithium transition metal composite oxide having a number average particle size (D50) of less than 0.1 μm.

[0029] The mixture containing the first lithium transition metal composite oxide and the second lithium transition metal composite oxide preferably accounts for 90 mass% or more, and more preferably 99 mass% or more, of the total mass of the lithium transition metal composite oxides contained in the positive electrode mixture layer.

[0030] The first lithium transition metal composite oxide and the second lithium transition metal composite oxide have a layered rock salt structure belonging to, for example, space group R-3m, space group C2 / m, etc. From the viewpoints of high capacity, stability of the crystal structure, etc., it is preferable that at least one of the first lithium transition metal composite oxide and the second lithium transition metal composite oxide has a layered rock salt structure belonging to space group R-3m. The layered rock salt structure of the lithium transition metal composite oxide may include a transition metal layer, a Li layer, and an oxygen layer. The charge / discharge reaction of the battery progresses as Li ions present in the Li layer reversibly enter and exit.

[0031] The Co content in at least one of the first lithium transition metal composite oxide and the second lithium transition metal composite oxide is preferably 5 mol % or less, more preferably 2 mol % or less, even more preferably 1 mol % or less, and particularly preferably 0.1 mol %, relative to the total number of moles of metal elements excluding Li. Since Co is a rare metal and expensive, it is preferable to use a small amount.

[0032] At least one of the first lithium transition metal composite oxide and the second lithium transition metal composite oxide is represented by the general formula Li ａ Ni １－ｘ－ｙ Mn ｘ M ｙ O ２＋ｂ However, this general formula satisfies 0.9≦a≦1.1, 0≦x≦0.5, 0≦y≦0.1, 0.5≦1-x-y≦0.95, and -0.05≦b≦0.05, and the element M is an element other than Li, Ni, Mn, and oxygen. The value of a, which indicates the molar ratio of lithium, increases or decreases with charge and discharge. From the viewpoint of stabilizing the crystal structure of the lithium transition metal composite oxide, the element M is preferably at least one element selected from the group consisting of Co, Nb, Al, Zr, B, Fe, Cu, Zn, Sn, W, Mo, Si, Ti, Fe, and Cr.

[0033] The content of elements constituting the lithium transition metal composite oxide can be measured by an inductively coupled plasma atomic emission spectrometer (ICP-AES), an electron probe microanalyzer (EPMA), or the like.

[0034] The carbonaceous coating may be formed on the surface of the first lithium transition metal composite oxide, may be formed on the surface of the second lithium transition metal composite oxide, or may be formed on the surface of the first lithium transition metal oxide and the surface of the second lithium transition metal oxide. The carbonaceous coating improves the electronic conductivity of the positive electrode active material, which leads to improved electronic conductivity of the positive electrode mixture layer and ultimately to reduced DC resistance of the battery.

[0035] The carbonaceous coating may contain at least one of an alkali metal and an alkaline earth metal excluding Li, but preferably contains at least one of Na and K in order to enhance the electronic conductivity of the positive electrode active material. Furthermore, the carbonaceous coating preferably further contains S in order to enhance the electronic conductivity of the positive electrode active material. The carbonaceous coating may also contain Li. The presence of S in the carbonaceous coating can be confirmed by energy dispersive X-ray spectroscopy (TEM-EDX). The presence of alkali metals and alkaline earth metals such as Na and K in the carbonaceous coating can be confirmed by X-ray photoelectron spectroscopy (XPS).

[0036] The thickness of the carbonaceous coating is, for example, 30 nm or less. The lower limit of the thickness of the carbonaceous coating is, for example, 1 nm. The thickness of the carbonaceous coating is measured from an image of a cross section of the positive electrode active material observed with a transmission electron microscope (TEM).

[0037] An example of a method for forming a carbonaceous coating on the surface of a lithium transition metal composite oxide is described below. (1) An anionic surfactant is dissolved in water to prepare an aqueous solution. (2) A powder of a lithium transition metal composite oxide is added to the aqueous solution and stirred to prepare a suspension in which the lithium transition metal composite oxide is dispersed in the aqueous solution. (3) The suspension is filtered, and the resulting residue is calcined, thereby forming a carbonaceous coating on the surface of the lithium transition metal composite oxide.

[0038] Anionic surfactants can be adsorbed onto the surface of lithium transition metal composite oxides in water to form a substantially uniform carbonaceous coating. The surfactant is not particularly limited as long as it contains C and at least one of an alkali metal and an alkaline earth metal excluding Li. The surfactant preferably contains at least one of Na and K. The surfactant may further contain S. Examples of surfactants include sodium lauryl sulfate and potassium lauryl sulfate. The concentration of the surfactant in the aqueous solution is, for example, 0.1% by mass to 10% by mass, and preferably 1% by mass to 10% by mass.

[0039] [Negative Electrode] The negative electrode 12 has, for example, a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector. The negative electrode mixture layer may be disposed on one side of the negative electrode current collector or on both sides of the negative electrode current collector. The negative electrode current collector may be a foil of a metal stable within the potential range of the negative electrode 12, such as copper or a copper alloy, or a film with such a metal disposed on the surface layer. The negative electrode mixture layer includes, for example, a negative electrode active material and a binder. The negative electrode 12 can be produced, for example, by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, etc. to the surface of the negative electrode current collector, drying the coating, and then rolling the coating using a roller or the like.

[0040] The negative electrode active material contained in the negative electrode mixture layer is not particularly limited as long as it can reversibly absorb and release lithium ions, and generally, a carbon material such as graphite is used. The graphite may be any of natural graphite such as flake graphite, lump graphite, and amorphous graphite, or artificial graphite such as lump artificial graphite and graphitized mesophase carbon microbeads. In addition, metals that alloy with Li such as Si and Sn, metal compounds containing Si, Sn, and the like, and lithium titanium composite oxides may also be used as the negative electrode active material. For example, SiO ｘ (0.5≦x≦1.6) or Li ２ｙ SiO （２＋ｙ） A Si-containing compound in which fine particles of Si are dispersed in a lithium silicate phase represented by (0<y<2) may be used in combination with graphite.

[0041] Examples of the binder contained in the negative electrode mixture layer include styrene butadiene rubber (SBR), nitrile butadiene rubber (NBR), carboxymethyl cellulose (CMC) or its salts (CMC-Na, CMC-K, CMC-NH ４ Examples of such binders include polyacrylic acid (PAA) or its salts (PAA-Na, PAA-K, etc., which may also be partially neutralized salts), polyvinyl alcohol (PVA), etc. These may be used alone or in combination of two or more. The content of the binder is, for example, 0.1 to 10 parts by mass, preferably 0.5 to 5 parts by mass, relative to 100 parts by mass of the negative electrode active material. The negative electrode mixture layer may also contain a conductive material, similar to the positive electrode.

[0042] [Separator] The separator 13 may be, for example, a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin-based resin. Alternatively, the separator 13 may be a multilayer separator including a polyethylene layer and a polypropylene layer, and the surface of the separator 13 may be coated with a material such as an aramid-based resin or ceramic.

[0043] Hereinafter, the present disclosure will be further described with reference to examples and comparative examples, but the present disclosure is not limited to the following examples.

[0044] <First Lithium Transition Metal Composite Oxide> The first lithium transition metal composite oxide has a number average particle size (D50) of 17 μm and a composition of LiNi ０.８ Mn ０．２ O ２ A lithium transition metal composite oxide was prepared.

[0045] <First lithium transition metal composite oxide having a carbonaceous coating formed on its surface> Sodium lauryl sulfate as a surfactant was dissolved in water to prepare a 10 mass % aqueous solution. ０.８ Mn ０．２ O ２ The first lithium transition metal composite oxide was added and stirred to prepare a suspension with a concentration of 1000 g / L. The suspension was filtered, and the resulting residue was vacuum-baked. This yielded a first lithium transition metal composite oxide having a carbonaceous coating formed on its surface.

[0046] <Second lithium transition metal composite oxide> The second lithium transition metal composite oxide has a number average particle size (D50) of 4 μm and a composition of LiNi ０.８ Mn ０．２ O ２ A lithium transition metal composite oxide was prepared.

[0047] <Second lithium transition metal composite oxide having a carbonaceous coating formed on its surface> Sodium lauryl sulfate as a surfactant was dissolved in water to prepare a 1 mass % aqueous solution. ０.８ Mn ０．２ O ２ The second lithium transition metal composite oxide was added and stirred to prepare a suspension with a concentration of 1000 g / L. The suspension was filtered, and the resulting residue was vacuum-baked. This yielded a second lithium transition metal composite oxide having a carbonaceous coating formed on its surface.

[0048] The first lithium transition metal composite oxide having a carbonaceous coating formed on its surface and the second lithium transition metal composite oxide having a carbonaceous coating formed on its surface were observed using a transmission electron microscope (TEM), and the thickness of the carbonaceous coating was found to be 10 nm. Furthermore, observation using energy dispersive X-ray spectroscopy (TEM-EDX) revealed a peak attributable to S near 2.2 keV, confirming the presence of S in the carbonaceous coating. Furthermore, XPS analysis confirmed the presence of Na in the carbonaceous coating.

[0049] Example 1 [Fabrication of Positive Electrode] A mixture was prepared by mixing a first lithium transition metal composite oxide having a carbonaceous coating formed on its surface with a second lithium transition metal composite oxide in a mass ratio of 70:30. This mixture was used as a positive electrode active material. The number-based particle size distribution of the mixture was measured, and peaks of 2% or more in relative particle amount were confirmed in the particle size range of 10 μm or more and 30 μm or less, and in the particle size range of 0.1 μm or more and 8 μm or less. Similar peaks were also confirmed in the mixtures used in Examples 2 and 3 and the Comparative Example below.

[0050] 100 parts by mass of this positive electrode active material, 1.1 parts by mass of acetylene black, and 0.9 parts by mass of polyvinylidene fluoride were mixed, and this was mixed with N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to one side of a positive electrode current collector made of 15 μm thick aluminum foil, and after drying the coating, the coating was rolled with a rolling roller and cut to a predetermined electrode size to prepare a positive electrode for evaluation. The positive electrode was provided with a 20 mm × 20 mm region that functions as a positive electrode and a 5 mm × 5 mm region for connection with the lead. Thereafter, the positive electrode mixture layer formed on the connection region was scraped off to expose the positive electrode current collector. Thereafter, the exposed portion of the positive electrode current collector was connected to a positive electrode lead, and a predetermined region around the periphery of the positive electrode lead was covered with an insulating film.

[0051] [Preparation of Negative Electrode] A lithium metal foil (thickness 300 μm) was cut into a 25 mm × 25 mm piece to obtain a negative electrode for evaluation. The lithium metal foil was connected to a negative electrode lead via a 5 mm × 5 mm Ni mesh, and a predetermined area around the periphery of the negative electrode lead was covered with an insulating film.

[0052] [Preparation of non-aqueous electrolyte] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 20:5:75. Lithium hexafluorophosphate (LiPF ６ ) was dissolved in the solution to a concentration of 1.3 mol / L to prepare a non-aqueous electrolyte (non-aqueous electrolytic solution).

[0053] [Preparation of Evaluation Battery] A battery for evaluation was prepared using a positive electrode and a negative electrode for evaluation. First, the positive electrode and the negative electrode were opposed to each other with a separator interposed between them so that the positive electrode mixture layer and the negative electrode lithium metal layer overlapped to obtain an electrode plate assembly. Next, an Al laminate film (thickness 100 μm) cut into a rectangle of 60 × 90 mm was folded in half, and the end of the 60 mm long side was heat-sealed at 230 ° C. to form a 60 × 45 mm cylindrical shape. Then, the prepared electrode plate assembly was placed in the cylinder, and the end face of the Al laminate film was aligned with the insulating film of each lead and heat-sealed at 230 ° C. Next, nonaqueous electrolyte was poured 0.3 cm from the short side of the Al laminate film that was not heat-sealed. ３After the injection, the mixture was left standing for 3 minutes under a reduced pressure of 0.06 MPa to impregnate each mixture layer with the electrolyte. Finally, the end face of the Al laminate film on the injected side was heat-sealed at 230 ° C. to prepare a battery for evaluation. The battery for evaluation was prepared in a dry environment with a dew point of −50 ° C. or less.

[0054] [Evaluation of Direct Current Resistance (DCIR)] In a temperature environment of 25°C, the battery was charged at a constant current of 0.2 C to 4.5 V, and then charged at a constant voltage of 4.5 V to 0.05 C. Thereafter, a constant current discharge was performed at a constant current of 0.2 C until the cell voltage reached 2.5 V. Next, a constant current charge was performed at 0.2 C until the state of charge (SOC) reached 50%. After a 2-hour break, the battery was discharged at a current of 0.3 C for 30 seconds, and the resistance value was calculated from the voltage drop before and after the discharge, and this value was taken as the DCIR.

[0055] Example 2 A battery for evaluation was fabricated in the same manner as in Example 1, except that a mixture obtained by mixing a first lithium transition metal composite oxide and a second lithium transition metal composite oxide having a carbonaceous coating formed on the surface thereof in a mass ratio of 70:30 was used as the positive electrode active material.

[0056] Example 3 A battery for evaluation was fabricated in the same manner as in Example 1, except that a mixture obtained by mixing a first lithium transition metal composite oxide having a carbonaceous coating formed on its surface and a second lithium transition metal composite oxide having a carbonaceous coating formed on its surface in a mass ratio of 70:30 was used as the positive electrode active material.

[0057] Comparative Example A battery for evaluation was fabricated in the same manner as in Example 1, except that a mixture of the first lithium transition metal composite oxide and the second lithium transition metal composite oxide in a mass ratio of 70:30 was used as the positive electrode active material.

[0058] The evaluation results of DCIR for the batteries of Examples 1 to 3 and the Comparative Example are shown in Table 1. In Table 1, the DCIR value of the Comparative Example is set to 100, and the DCIR values ​​of Examples 1 to 3 are shown as relative values.

[0059]

[0060] As shown in Table 1, the batteries of Examples 1 to 3 had lower DCIR than the battery of the Comparative Example. These results demonstrate that the DC resistance of a battery can be reduced by using a positive electrode active material that includes a mixture containing a first lithium transition metal composite oxide having a number average particle size (D50) in the range of 10 μm to 30 μm and a second lithium transition metal composite oxide having a number average particle size (D50) in the range of 0.1 μm to 8 μm, and a carbonaceous coating formed on at least one of the surfaces of the first lithium transition metal composite oxide and the second lithium transition metal composite oxide, wherein the carbonaceous coating contains at least one of an alkali metal and an alkaline earth metal excluding Li.

[0061] Among Examples 1 to 3, the DC resistance of the battery decreased in the following order: Example 1, in which a carbonaceous coating was formed only on the surface of the first lithium transition metal composite oxide; Example 2, in which a carbonaceous coating was formed only on the surface of the second lithium transition metal composite oxide; and Example 3, in which a carbonaceous coating was formed on the surfaces of both the first and second lithium transition metal composite oxides.

[0062] The present disclosure is further described by the following embodiments. Aspect 1: A cathode active material comprising: a mixture including a first lithium transition metal composite oxide having a number average particle size (D50) in the range of 10 μm or more and 30 μm or less, and a second lithium transition metal composite oxide having a number average particle size (D50) in the range of 0.1 μm or more and 8 μm or less; and a carbonaceous coating formed on at least one of the surfaces of the first lithium transition metal composite oxide and the second lithium transition metal composite oxide, wherein the carbonaceous coating contains at least one of an alkali metal and an alkaline earth metal excluding Li. Aspect 2: The cathode active material according to Aspect 1, wherein the carbonaceous coating is formed on the surface of the first lithium transition metal composite oxide. Aspect 3: The cathode active material according to Aspect 1, wherein the carbonaceous coating is formed on the surface of the second lithium transition metal composite oxide. Aspect 4: The cathode active material according to Aspect 1, wherein the carbonaceous coating is formed on the surface of the first lithium transition metal composite oxide and the surface of the second lithium transition metal composite oxide. Configuration 5: The cathode active material according to any one of Configurations 1 to 4, wherein the content of the second lithium transition metal composite oxide is 10 mass % or more and 50 mass % or less, relative to the total mass of the mixture. Configuration 6: The cathode active material according to any one of Configurations 1 to 5, wherein the carbonaceous coating further contains S. Configuration 7: The cathode active material according to any one of Configurations 1 to 6, wherein the carbonaceous coating contains at least one of Na and K. Configuration 8: The cathode active material according to any one of Configurations 1 to 7, wherein the content of Co in at least one of the first lithium transition metal composite oxide and the second lithium transition metal composite oxide is 5 mol % or less, relative to the total number of moles of metal elements excluding Li. Configuration 9: At least one of the first lithium transition metal composite oxide and the second lithium transition metal composite oxide is a compound represented by the general formula Li ａ Ni １－ｘ－ｙ Mn ｘ M ｙ O ２＋ｂ(wherein 0.9≦a≦1.1, 0≦x≦0.5, 0≦y≦0.1, 0.5≦1−x−y≦0.95, and −0.05≦b≦0.05 are satisfied, and element M is an element other than Li, Ni, Mn, and oxygen). Configuration 10: The cathode active material of any one of configurations 1 to 9, wherein the carbonaceous coating has a thickness of 30 nm or less. Configuration 11: A secondary battery comprising: a cathode containing the cathode active material of any one of configurations 1 to 10, a negative electrode, and an electrolyte.

[0063] 10 Secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 15 Battery case, 16 Case body, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Protruding portion, 23 Filter, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket.

Claims

1. A positive electrode active material comprising a mixture including a first lithium transition metal composite oxide having a number average particle size (D50) in the range of 10 μm or more and 30 μm or less, and a second lithium transition metal composite oxide having a number average particle size (D50) in the range of 0.1 μm or more and 8 μm or less, and a carbonaceous coating formed on at least one of a surface of the first lithium transition metal composite oxide and a surface of the second lithium transition metal composite oxide, wherein the carbonaceous coating contains at least one of an alkali metal and an alkaline earth metal excluding Li.

2. The positive electrode active material according to claim 1, wherein the carbonaceous coating is formed on a surface of the first lithium transition metal composite oxide.

3. The positive electrode active material according to claim 1, wherein the carbonaceous coating is formed on a surface of the second lithium transition metal composite oxide.

4. The positive electrode active material according to claim 1, wherein the carbonaceous coating is formed on a surface of the first lithium transition metal composite oxide and a surface of the second lithium transition metal composite oxide.

5. A positive electrode active material according to any one of claims 1 to 4, wherein the content of the second lithium transition metal composite oxide is 10 mass % or more and 50 mass % or less with respect to the total mass of the mixture.

6. The positive electrode active material according to any one of claims 1 to 4, wherein the carbonaceous coating further contains S.

7. The positive electrode active material according to any one of claims 1 to 4, wherein the carbonaceous coating contains at least one of Na and K.

8. The positive electrode active material according to any one of claims 1 to 4, wherein the content of Co in at least one of the first lithium transition metal composite oxide and the second lithium transition metal composite oxide is 5 mol % or less with respect to the total number of moles of metal elements excluding Li.

9. At least one of the first lithium transition metal composite oxide and the second lithium transition metal composite oxide is represented by the general formula Li ａ Ni １－ｘ－ｙ Mn ｘ M ｙ O ２＋ｂ (wherein 0.9≦a≦1.1, 0≦x≦0.5, 0≦y≦0.1, 0.5≦1-x-y≦0.95, and −0.05≦b≦0.05 are satisfied, and element M is an element other than Li, Ni, Mn, and oxygen).

10. The positive electrode active material according to any one of claims 1 to 4, wherein the carbonaceous coating has a thickness of 30 nm or less.

11. A secondary battery comprising a positive electrode containing the positive electrode active material according to any one of claims 1 to 4, a negative electrode, and an electrolyte.

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