Positive electrode for secondary batteries, and secondary battery

Abstract

The present invention provides a positive electrode for secondary batteries capable of reducing a DC resistance of a secondary battery. The positive electrode for the secondary batteries is characterized by including a positive electrode current collector and a positive electrode mixture layer that is disposed on the positive electrode current collector, wherein the positive electrode mixture layer contains a positive electrode active material and a conductive material; the positive electrode active material contains a lithium transition metal composite oxide and a carbonaceous film that is formed on the surface of the lithium transition metal composite oxide; the carbonaceous film contains at least one of an alkali metal excluding Li and an alkaline earth metal; and the conductive material contains a fibrous conductive material.

Description

Positive electrode for secondary battery, and secondary battery

[0001] The present disclosure relates to a positive electrode for a secondary battery 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 for a secondary battery that can reduce the DC resistance of the secondary battery.

[0007] A positive electrode for a secondary battery according to one aspect of the present disclosure includes a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector, the positive electrode mixture layer including a positive electrode active material and a conductive material, the positive electrode active material including a lithium transition metal composite oxide and a carbonaceous coating formed on a surface of the lithium transition metal composite oxide, the carbonaceous coating including at least one of an alkali metal and an alkaline earth metal excluding Li, and the conductive material including a fibrous conductive material.

[0008] A secondary battery according to one aspect of the present disclosure includes the above-described positive electrode for secondary batteries, a negative electrode, and an electrolyte.

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

[0010] 1 is a cross-sectional view of a secondary battery according to an embodiment of the present invention; 2 is a diagram showing the results of TEM-EDX analysis of the positive electrode active materials of Examples; 3 is a diagram showing the results of XPS analysis of the positive electrode active materials of Examples and Comparative Example 1;

[0011] Hereinafter, an example of an embodiment of a nonaqueous electrolyte secondary battery according to the present disclosure will be described.

[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 and a conductive material. The positive electrode mixture layer may also contain a binder. 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] The conductive material contained in the positive electrode mixture layer contains a fibrous conductive material. Examples of the fibrous conductive material include carbon nanotubes (CNTs), carbon nanofibers (CNFs), vapor-grown carbon fibers, electrospun carbon fibers, polyacrylonitrile (PAN)-based carbon fibers, pitch-based carbon fibers, and other fibrous carbons, fibrous metals obtained by fiberizing metals such as stainless steel, conductive fibers in which the surface of organic fibers is coated with a conductive material such as a metal or carbon material, or conductive fibers in which the surface of organic fibers is coated with a resin containing a conductive material. By including a fibrous conductive material in the positive electrode mixture layer, for example, the particles of the positive electrode active material are bridged by the fibrous conductive material, thereby improving the electronic conductivity of the positive electrode mixture layer and thereby reducing the DC resistance of the secondary battery.

[0022] The fibrous conductive material preferably contains fibrous carbon, and more preferably contains carbon nanotubes, in order to reduce the DC resistance of the secondary battery. The length (fiber length) of the carbon nanotubes is preferably 0.1 μm or more, more preferably 0.1 μm or more and 20 μm or less, more preferably 1 μm or more and 10 μm or less, and more preferably 1 μm or more and 5 μm or less, in order to reduce the DC resistance of the secondary battery. The outermost diameter of the carbon nanotubes is preferably 1 nm or more and 20 nm or less, and more preferably 1.5 nm or more and 10 nm or less, in order to improve the conductivity of the carbon nanotubes and ensure a conductive path in the positive electrode mixture layer by adding a small amount of carbon fiber due to the improved conductivity. The aspect ratio of the carbon nanotubes (length of the carbon nanotube / outermost diameter) is preferably 5 or more, and more preferably 100 or more. The length and outermost diameter of a carbon nanotube refer to the length and outermost diameter of the fiber alone, and can be determined by measuring the lengths and outer diameters (outermost diameters) of 50 random carbon nanotubes using a field emission scanning electron microscope (FE-SEM) or a transmission electron microscope (TEM) and taking the arithmetic average.

[0023] Examples of carbon nanotubes include single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes. Single-walled carbon nanotubes (SWCNTs) are carbon nanostructures in which one layer of graphene sheets forms a cylindrical shape. Double-walled carbon nanotubes are carbon nanostructures in which two layers of graphene sheets are concentrically stacked to form a cylindrical shape. Multi-walled carbon nanotubes are carbon nanostructures in which three or more layers of graphene sheets are concentrically stacked to form a cylindrical shape. Note that a graphene sheet refers to a layer in which carbon atoms in sp2 hybrid orbitals that form a graphite crystal are located at the vertices of a regular hexagon.

[0024] The content of the fibrous conductive material is, for example, preferably 0.01% by mass to 10% by mass, and more preferably 0.1% by mass to 0.5% by mass, relative to the total mass of the positive electrode mixture layer, from the viewpoint of reducing the DC resistance of the secondary battery.

[0025] The conductive material may contain, in addition to the fibrous conductive material, a conductive material other than the fibrous conductive material. Examples of conductive materials other than the fibrous carbon include particulate conductive materials such as carbon black, acetylene black, ketjen black, and graphite.

[0026] 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.

[0027] The positive electrode active material contained in the positive electrode mixture layer includes a lithium transition metal composite oxide and a carbonaceous coating formed on the surface of the lithium transition metal composite oxide, and the carbonaceous coating contains at least one of an alkali metal and an alkaline earth metal excluding Li. This improves the electronic conductivity of the positive electrode active material. In addition, combined with the effect of adding the fibrous conductive material described above, the electronic conductivity of the positive electrode mixture layer is improved, thereby reducing the DC resistance of the secondary battery.

[0028] The lithium transition metal composite oxide includes, 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 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 lithium transition metal composite oxide, or may be present so as to cover the entire surface of the secondary particles. The carbonaceous coating may also be present at the interface where the primary particles of the lithium transition metal composite oxide come into contact with each other.

[0029] The particle size of the primary particles constituting the secondary particles of the lithium transition metal composite oxide is, for example, 0.02 μm to 2 μm. The particle size of the primary particles is measured as the diameter of the circumscribed circle in a particle image observed with a scanning electron microscope (SEM). The average particle size of the secondary particles of the lithium transition metal composite oxide is, for example, 2 μm to 30 μm. Here, the average particle size refers to the volume-based median diameter (D50). D50 refers to the particle size at which the cumulative frequency in the volume-based particle size distribution is 50% from the smallest particle size, and is also called the median diameter. The particle size distribution of the secondary particles of the lithium transition metal composite oxide can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II, manufactured by Microtrac-Bell Corporation) using water as a dispersion medium.

[0030] The lithium transition metal composite oxide has a layered rock salt structure belonging to, for example, space group R-3m, space group C2 / m, etc. From the viewpoints of high capacity and stability of the crystal structure, it is preferable that the 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 reversible movement of Li ions present in the Li layer causes the charge / discharge reaction of the battery to proceed.

[0031] The content of Co in the 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 %, based on 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] The lithium transition metal composite oxide has the general formula Li ａ Ni １－ｘ－ｙ Mn ｘ M ｙ O ２＋ｂHowever, 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 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).

[0035] 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).

[0036] The positive electrode mixture layer may contain other positive electrode active materials in addition to the lithium transition metal composite oxide having a carbonaceous coating formed on its surface. Examples of other positive electrode active materials include lithium transition metal composite oxides that do not have a carbonaceous coating on their surfaces. The content of the lithium transition metal composite oxide having a carbonaceous coating formed on its surface is preferably 90% by mass or more, and more preferably 99% by mass or more, based on the total mass of the positive electrode active material.

[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] [Preparation of Positive Electrode Active Material] <Example> Sodium lauryl sulfate as a surfactant was dissolved in water to prepare a 10 mass % aqueous solution. ０.８ Mn ０．２ O ２ A lithium transition metal composite oxide containing 1000 g of 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 resulted in a lithium transition metal composite oxide having a carbonaceous coating formed on its surface. This was used as the positive electrode active material of the example. Observation using a transmission electron microscope (TEM) revealed that the carbonaceous coating had a thickness of 10 nm. Furthermore, as shown in FIG. 2, 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, as shown in FIG. 3, XPS analysis of the positive electrode active material of the example confirmed the presence of Na in the carbonaceous coating.

[0045] [Preparation of Positive Electrode] Positive electrode active material, acetylene black, carbon nanotubes (length: 2.5 μm, aspect ratio: 300), and polyvinylidene fluoride were mixed in a solids mass ratio of 100:1:0.5:1, 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 aluminum foil with a thickness of 15 μm, 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 area on the periphery of the positive electrode lead was covered with an insulating film.

[0046] [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.

[0047] [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).

[0048] [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. ３ After 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.

[0049] [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.

[0050] Comparative Example 1: A battery for evaluation was fabricated in the same manner as in Example 1, except that sodium lauryl sulfate was not added in the preparation of the positive electrode active material, carbon nanotubes were not added in the preparation of the positive electrode, and the amount of acetylene black was changed from 1 part by mass to 1.5 parts by mass. Note that, as shown in Figure 3, XPS analysis of the positive electrode active material of Comparative Example 1 did not confirm the presence of Na.

[0051] Comparative Example 2 A battery for evaluation was produced in the same manner as in Example, except that in the production of the positive electrode, no carbon nanotubes were added and the amount of acetylene black was changed from 1 part by mass to 1.5 parts by mass.

[0052] Comparative Example 3 A battery for evaluation was produced in the same manner as in Example, except that sodium lauryl sulfate was not added in the production of the positive electrode active material.

[0053] The evaluation results of DCIR for the batteries of the Examples and Comparative Examples are shown in Table 1. In Table 1, the DCIR value for Comparative Example 1 is set to 100, and the DCIR values ​​for the Examples and other Comparative Examples are shown as relative values.

[0054]

[0055] As shown in Table 1, the batteries of the examples had lower DCIR than the batteries of Comparative Examples 1 to 3. These results show that by using, as the positive electrode active material, a lithium transition metal composite oxide having a carbonaceous coating containing at least one of an alkali metal and an alkaline earth metal excluding Li formed on its surface, and by using a fibrous conductive material as the conductive material contained in the positive electrode mixture layer, it is possible to reduce the DC resistance of the battery.

[0056] The present disclosure is further described by the following embodiments. Configuration 1: A positive electrode for a secondary battery, comprising: a positive electrode current collector; and a positive electrode mixture layer disposed on the positive electrode current collector, wherein the positive electrode mixture layer comprises a positive electrode active material and a conductive material, wherein the positive electrode active material comprises a lithium transition metal composite oxide and a carbonaceous coating formed on the surface of the lithium transition metal composite oxide, the carbonaceous coating containing at least one of an alkali metal and an alkaline earth metal excluding Li, and the conductive material containing a fibrous conductive material. Configuration 2: The positive electrode for a secondary battery according to Configuration 1, wherein the fibrous conductive material comprises carbon nanotubes having a length of 0.1 μm or more and an aspect ratio of 5 or more. Configuration 3: The positive electrode for a secondary battery according to Configuration 1 or 2, wherein the carbonaceous coating further contains S. Configuration 4: The positive electrode for a secondary battery according to any one of Configurations 1 to 3, wherein the carbonaceous coating contains at least one of Na and K. Configuration 5: A positive electrode for a secondary battery according to any one of configurations 1 to 4, wherein the content of Co in the lithium transition metal composite oxide is 5 mol % or less relative to the total number of moles of metal elements excluding Li. Configuration 6: The lithium transition metal composite oxide is represented by the general formula Li ａ Ni １－ｘ－ｙ Mnｘ 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 7: The positive electrode for a secondary battery according to any one of configurations 1 to 6, wherein the carbonaceous coating has a thickness of 30 nm or less. Configuration 8: A secondary battery comprising the positive electrode for a secondary battery according to any one of configurations 1 to 7, a negative electrode, and an electrolyte.

[0057] 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 for a secondary battery comprising: a positive electrode current collector; and a positive electrode mixture layer disposed on the positive electrode current collector; the positive electrode mixture layer comprises a positive electrode active material and a conductive material; the positive electrode active material comprises a lithium transition metal composite oxide and a carbonaceous coating formed on a surface of the lithium transition metal composite oxide, the carbonaceous coating containing at least one of an alkali metal and an alkaline earth metal excluding Li; and the conductive material contains a fibrous conductive material.

2. The positive electrode for a secondary battery according to claim 1, wherein the fibrous conductive material comprises carbon nanotubes having a length of 0.1 μm or more and an aspect ratio of 5 or more.

3. The positive electrode for a secondary battery according to claim 1 or 2, wherein the carbonaceous coating further contains S.

4. The positive electrode for a secondary battery according to claim 1 or 2, wherein the carbonaceous coating contains at least one of Na and K.

5. The positive electrode for a secondary battery according to claim 1 or 2, wherein the content of Co in the lithium transition metal composite oxide is 5 mol % or less based on the total number of moles of metal elements excluding Li.

6. The lithium transition metal composite oxide has 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). The positive electrode for secondary batteries according to claim 1 or 2.

7. The positive electrode for a secondary battery according to claim 1 or 2, wherein the carbonaceous coating has a thickness of 30 nm or less.

8. A secondary battery comprising the positive electrode for secondary batteries according to claim 1 or 2, a negative electrode, and an electrolyte.

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