Cathode for non-aqueous electrolyte secondary batteries and non-aqueous electrolyte secondary battery

Abstract

To provide a cathode for non-aqueous electrolyte secondary batteries capable of suppressing a rise of internal resistance even in a cathode using a high Ni containing NCM-based cathode active material, and a non-aqueous electrolyte secondary battery.SOLUTION: A cathode for non-aqueous electrolyte secondary batteries includes a cathode collector and a cathode mixture layer. In the cathode for non-aqueous electrolyte secondary batteries, the cathode mixture layer contains a first cathode active material expressed by the following general formula (1): LiaNixCoyM11-x-yO2 (which leads 0<a≤1.2, 0.6≤x≤0.8, 0.1≤y≤0.2 and 0.7≤x+y≤0.9), a second cathode active material expressed by the following general formula (2): LiMnzM2bFe1-z-bPO4 (which leads 0<z≤0.9, 0≤b≤0.1 and 0<z+b<1) and in which a coating consisting of a carbon material is formed on a surface of a phosphate compound having an olivine structure, and a conductive assistant, and satisfies the following formula (3): secondary particle diameter of first cathode active material / secondary particle diameter of second cathode active material≥(2.5p)1 / 3.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries have become widely used due to their high energy density and other reasons, and are installed as power sources in small portable devices such as mobile phones, digital cameras, laptop computers, etc. Furthermore, in view of issues such as energy resource depletion and global warming, non-aqueous electrolyte secondary batteries are being developed for large-scale industrial applications such as hybrid vehicles, electric vehicles, and power storage using natural energy sources such as solar and wind power.

[0003] In recent years, non-aqueous electrolyte secondary batteries have been attracting attention as power sources for drones, robots, electric vehicles, hybrid electric vehicles, etc., and their applications are expected to continue to expand. Such power sources are required to have even higher densities, and measures have been taken to address this, such as using lithium nickel oxide (NCA), which has a high unit capacity, as the positive electrode active material, or a high-Ni-content NCM system, which is composed mainly of nickel, cobalt, and manganese (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2019-50105 Summary of the Invention [Problem to be solved by the invention]

[0005] However, when using a high-Ni NCM-based cathode active material, there are problems specific to other NCM-based cathode active materials, such as high reactivity with the electrolyte due to low chemical stability, which can lead to the formation of a highly insulating substance on the surface of the cathode active material. As a result, when using a high-Ni NCM-based cathode active material, the internal resistance increases during or after cell fabrication, which is a major barrier to practical application.

[0006] The present invention has been made in view of the above, and an object of the present invention is to provide a positive electrode for a nonaqueous electrolyte secondary battery and a nonaqueous electrolyte secondary battery that can suppress an increase in internal resistance even in a positive electrode that uses an NCM-based positive electrode active material with a high Ni content. [Means for solving the problem]

[0007] In order to solve the above-mentioned problems and achieve the object, a positive electrode for a non-aqueous electrolyte secondary battery according to the present invention is, in a first aspect, a positive electrode for a non-aqueous electrolyte secondary battery including a positive electrode current collector and a positive electrode composite layer formed on a surface of the positive electrode current collector, wherein the positive electrode composite layer includes a first positive electrode active material that is a layered compound represented by the general formula shown in the following formula (1), a second positive electrode active material that is represented by the general formula shown in the following formula (2) and in which a coating made of a carbon material is formed on a surface of a phosphate compound having an olivine structure, and a conductive additive, and satisfies the following formula (3): Li a Ni x Co y M1 1-x-y O2 (However, 0 <a≦1.2、0.6≦x≦0.8、0.1≦y≦0.2、0.7≦x+y≦0.9)···(1) LiMn z M2 b Fe 1-z-b PO4 (However, 0 <z≦0.9、0≦b≦0.1、0<z+b<1)···(2) 0.8≦(n×p) 2 / 3 ≦13 (3) Here, n is the theoretical minimum particle number of the second positive electrode active material, and p is the ratio of the weight of the first positive electrode active material to the weight of the second positive electrode active material. In the above formula (1), M1 is at least one selected from Ti, Zr, Nb, W, P, Al, Mg, V, Mn, Ca, Sr, Cr, Fe, B, Ga, In, Si, Mo, Y, Sn, Cu, Ag, Ce, Pr, Ge, Bi, Ba, Er, La, Sm, Yb, Sb, Bi, S, and Zn; In the above formula (2), M2 is at least one selected from Ni, Co, Ti, Cu, Zn, Mg, Zr, Ca, Y, Mo, Ba, Pb, Bi, La, Ce, Nd, Gd, Al, Ga, and Sr.

[0008] In addition to the first aspect, the positive electrode for a non-aqueous electrolyte secondary battery according to the present invention is characterized in that, as a second aspect, n is calculated by the following formula (4): n=4×(r N / r L ) 2 ···(4) where r N is the median diameter of the secondary particles of the first positive electrode active material, and r L is the median diameter of the particles of the second positive electrode active material.

[0009] In addition to the first or second aspect, the positive electrode for a non-aqueous electrolyte secondary battery according to the present invention is characterized in that p is 4 or more and 9 or less.

[0010] Furthermore, in addition to the first to third aspects, the positive electrode for a non-aqueous electrolyte secondary battery according to the present invention is characterized in that the first positive electrode active material has a secondary particle diameter D50 of 3 μm or more and 30 μm or less, and the second positive electrode active material has a secondary particle diameter D50 of 1 μm or more and 10 μm or less.

[0011] In addition, as a fifth aspect, a nonaqueous electrolyte secondary battery according to the present invention is characterized by comprising: the positive electrode for a nonaqueous electrolyte secondary battery according to any one of the first to fourth aspects; a negative electrode; a separator; and a nonaqueous electrolyte solution containing a lithium salt and a nonaqueous solvent.

[0012] In addition, according to a sixth aspect of the present invention, in addition to the fifth aspect, the nonaqueous electrolyte secondary battery is characterized in that the nonaqueous solvent contains dimethyl carbonate. [Effects of the Invention]

[0013] According to the present invention, it is possible to obtain a positive electrode for a nonaqueous electrolyte secondary battery that can suppress an increase in internal resistance even when the positive electrode uses an NCM-based positive electrode active material with a high Ni content, and a nonaqueous electrolyte secondary battery using the positive electrode. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a cross-sectional view illustrating the configuration of a nonaqueous electrolyte secondary battery according to Embodiment 1 of the present invention. [Figure 2] FIG. 2 is an exploded perspective view illustrating the configuration of a nonaqueous electrolyte secondary battery according to Embodiment 2 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following description. Furthermore, various modifications and improvements can be made to the present embodiments, and such modifications and improvements can also be included in the present invention.

[0016] Although details will be described later, FIG. 1 is a cross-sectional view of a laminate-type nonaqueous electrolyte secondary battery, and FIG. 2 is an exploded perspective view for explaining the configuration of a coin-type nonaqueous electrolyte secondary battery. 1 and 2 show configuration examples of a laminate-type nonaqueous electrolyte secondary battery and a coin-type nonaqueous electrolyte secondary battery as examples of embodiments, but the shape of the nonaqueous electrolyte secondary battery in the present invention is not particularly limited and may be flat, cylindrical, prismatic, coin-type, etc. Furthermore, the exterior body of the nonaqueous electrolyte secondary battery is not particularly limited either and known materials such as laminate film, aluminum, aluminum alloy, and stainless steel can be used.

[0017] (Embodiment 1) Fig. 1 is a cross-sectional view illustrating the configuration of a nonaqueous electrolyte secondary battery according to embodiment 1 of the present invention. The nonaqueous electrolyte secondary battery 1 shown in Fig. 1 is a stacked type nonaqueous electrolyte secondary battery formed by stacking a plurality of pairs of a positive electrode 4, a negative electrode 5, and a separator 6.

[0018] The non-aqueous electrolyte secondary battery 1 includes a bag-shaped exterior body 2 made of laminate film. An electrode group 3 having a laminated structure is housed within the exterior body 2. The laminate film has a structure in which, for example, multiple (e.g., two) plastic films are stacked together with a metal foil such as aluminum foil sandwiched between adjacent plastic films. One of the two plastic films is a heat-sealable resin film. The exterior body 2 is made by stacking two laminate films with the heat-sealable resin films facing each other, and housing the electrode group 3 and non-aqueous electrolyte between these laminate films. The two laminate film portions around the electrode group 3 are heat-sealed to seal them together, thereby hermetically housing the electrode group 3 and non-aqueous electrolyte.

[0019] The electrode group 3 has a positive electrode 4, a negative electrode 5, a separator 6, a positive electrode lead 7, a positive electrode tab 8, a negative electrode lead 9, and a negative electrode tab 10. The separator 6 is interposed between the positive electrode 4 and the negative electrode 5. The electrode group 3 has a multi-layer structure in which the negative electrode 5 is located as the outermost layer and the separator 6 is located between the negative electrode 5 and the inner surface of the exterior body 2. It is also desirable that the separator 6 be located between the negative electrode 5 and the inner surface of the exterior body 2.

[0020] (positive electrode) The positive electrode 4 is composed of a positive electrode current collector 41 and a positive electrode mixture layer 42 formed on one or both surfaces of the positive electrode current collector 41 .

[0021] The positive electrode current collector 41 is made of copper, aluminum, nickel, stainless steel, titanium, other alloys, etc. Among them, it is preferable to use aluminum from the viewpoint of electronic conductivity and battery operating potential.

[0022] Positive electrode mixture layer 42 contains a positive electrode active material, a conductive additive, and a binder. The positive electrode active material includes a first positive electrode active material and a second positive electrode active material, and is formed into a film by, for example, applying it to positive electrode current collector 41 and drying it. The density of positive electrode mixture layer 42 is adjusted by, for example, pressing. The first and second positive electrode active materials are each capable of absorbing and desorbing lithium.

[0023] The first positive electrode active material is preferably secondary particles of a layered compound represented by the general formula (1) below, with a secondary particle diameter (D50) of 3 μm to 30 μm. The layered compound is a lithium (Li), cobalt (Co), and nickel (Ni)-containing composite metal oxide (hereinafter sometimes referred to as NCM) composed of layers of sheet-like particles. The secondary particles are particles formed by the aggregation of multiple single particles (primary particles). Li a Ni x Co y M1 1-x-y O2 (However, 0 <a≦1.2、0.6≦x≦0.8、0.1≦y≦0.2、0.7≦x+y≦0.9) ···(1) In the above formula (1), M1 is titanium (Ti), zirconium (Zr), niobium (Nb), tungsten (W), phosphorus (P), aluminum (Al), magnesium (Mg), vanadium (V), manganese (Mn), calcium (Ca), strontium (Sr), chromium (Cr), iron (Fe), boron (B), gallium (Ga), indium (In), silicon (Si), molybdenum (Mo), or the like. The element is at least one selected from the group consisting of silicon (Mo), yttrium (Y), tin (Sn), copper (Cu), silver (Ag), cerium (Ce), praseodymium (Pr), germanium (Ge), bismuth (Bi), barium (Ba), erbium (Er), lanthanum (La), samanium (Sm), ytterbium (Yb), antimony (Sb), bismuth (Bi), sulfur (S), and zinc (Zn). D50 refers to the median diameter, which is the diameter value corresponding to the median value of the particle size distribution. Moreover, an NCM containing 60% or more of Ni, such as that used in the present invention, is called a high-Ni-content NCM-based positive electrode active material.

[0024] The second positive electrode active material is a phosphate compound having an olivine structure (hereinafter, sometimes referred to as LMFP) represented by the following general formula (2). LiMn z M2 b Fe 1-z-b PO4 (However, 0 <z≦0.9、0≦b≦0.1、0<z+b<1) ···(2) In the above formula (2), M2 is at least one selected from Ni, Co, Ti, Cu, Zn, Mg, Zr, Ca, Y, Mo, Ba, Pb, Bi, La, Ce, Nd, Gd, Al, Ga, and Sr.

[0025] The second positive electrode active material is, for example, formed by forming a coating made of a carbon material on the surface of this phosphate compound (primary particles). The particle diameter (D50) of the secondary particles of the second positive electrode active material is preferably 1 μm or more and 10 μm or less. Note that, like the first positive electrode active material, the second positive electrode active material may be, for example, an aggregate (secondary particle) formed by aggregation of a plurality of single particles (primary particles).

[0026] The substance ratio of the metal elements in the first positive electrode active material and the second positive electrode active material and the elements contained as M1 and M2 may be the same or different as long as they satisfy the above ranges.

[0027] The ratio of the weight of the second positive electrode active material to the total weight of the first positive electrode active material and the second positive electrode active material should preferably be 10% or more and 20% or less. Here, if the weight ratio of the second positive electrode active material to the total weight of the first and second positive electrode active materials is less than 10%, the abundance ratio of the first positive electrode active material becomes high, and it becomes difficult to suppress the increase in internal resistance by covering the first positive electrode active material particles, which have low chemical stability and high reactivity with the electrolyte, with the second positive electrode active material, which has high chemical stability and low reactivity with the electrolyte, under specific conditions, thereby reducing direct contact with the electrolyte. Furthermore, if the weight ratio of the second positive electrode active material to the total weight of the first and second positive electrode active materials exceeds 20%, the abundance ratio of the second positive electrode active material becomes high, which may result in a decrease in volumetric energy density. To suppress the increase in internal resistance, it is preferable that the weight ratio of the second positive electrode active material to the total weight of the first and second positive electrode active materials be 15% to 20%.

[0028] In the present embodiment 1, the positive electrode composite layer 42 contains a first positive electrode active material which is a layered compound, a second positive electrode active material in which a coating film made of a carbon material is formed on the surface of a phosphate compound having an olivine structure, and a conductive additive, and satisfies the following formula (3): 0.8≦(n×p) 2 / 3 ≦13 (3) Here, n is the theoretical minimum particle number based on the ratio of the secondary particle diameter of the first positive electrode active material to the secondary particle diameter of the second positive electrode active material, and p is the ratio of the weight of the first positive electrode active material to the weight of the second positive electrode active material (weight of the first positive electrode active material / weight of the second positive electrode active material). The secondary particle diameter in the above formula (3) is a value actually measured using an SEM image.

[0029] Specifically, n is calculated by the following equation (4). n=4×(r N / r L ) 2 ···(4) where r N is the secondary particle diameter of the first positive electrode active material, and r L is the secondary particle diameter of the second positive electrode active material. The secondary particle diameter in the above formula (4) is a value actually measured using an SEM image. In this case, p is preferably 4 or more and 9 or less. For example, when p=4, the weight mixing ratio of the first positive electrode active material to the second positive electrode active material is 20 wt %, and when p=9, the weight mixing ratio of the first positive electrode active material to the second positive electrode active material is 10 wt %.

[0030] For example, when p=4 (the weight ratio of LMFP is 20 wt%), r N / r L By satisfying the condition ≧2.16, particles of the second positive electrode active material can geometrically surround particles of the first positive electrode active material.

[0031] Here, the above formula (4) is derived as follows: When the second positive electrode active material surrounds the first positive electrode active material and the first positive electrode active material is completely filled with the second positive electrode active material, the relationship in the following formula (5) holds. 4πr N 2 ≦n×πr L 2 ···(5) From the above equation (5), the above equation (4) is derived.

[0032] Moreover, it is preferable that the above formula (3) satisfies the following formula (3)' in terms of suppressing an increase in internal resistance. 0.8≦(n×p) 2 / 3 <11 ···(3)´

[0033] Whether or not the above formula (3) (or (3)') is satisfied can be confirmed by an EDS image obtained by energy dispersive X-ray spectroscopy (EDS) of a cross section of positive electrode composite layer 42. This field of view is assumed for electrodes that can be seen at least twice when cross-sectional views are taken three times at random.

[0034] As a basic premise, it was well known that the composition ratio of materials was important when considering the composition of the positive electrode composite layer, but analysis had not progressed to the point of determining how important the materials were configured and arranged inside the composite layer. The present invention focuses on the configuration and arrangement. The reason why the effects of the configuration of the present invention are confirmed is not clear, but it is assumed as follows.

[0035] When active materials with different particle sizes are added in equal amounts, the larger the ratio of the secondary particle size of the first positive electrode active material to the secondary particle size of the second positive electrode active material, the greater the proportion of the surface of the first positive electrode active material that is surrounded by the second positive electrode active material. By setting the ratio of the secondary particle size of the first positive electrode active material to the secondary particle size of the second positive electrode active material at a certain value or higher, it is natural that the contact area with the carbon material (hereinafter sometimes referred to as C) and the second positive electrode active material is larger and the contact area with the electrolyte is smaller than when the ratio is less than that value. When the ratio is less than that value, the surface of the first positive electrode active material cannot be sufficiently covered with the second positive electrode active material, causing an increase in internal resistance. The present inventors have discovered that the surface of the first positive electrode active material can be sufficiently covered with the second positive electrode active material and the effect of suppressing an increase in internal resistance can be expected when the above formula (3) is satisfied.

[0036] When the D50, which indicates the secondary particle diameter of the first positive electrode active material, is 3 μm or more and 30 μm or less, and the D50, which indicates the secondary particle diameter of the second positive electrode active material, is 1 μm or more and 10 μm or less, the surface of the first positive electrode active material can be sufficiently covered with the second positive electrode active material, making it possible to suppress an increase in internal resistance. Note that, to control the D50, which indicates the secondary particle diameter, a sieve with a mesh size of about 1500, targeting particles of 5 μm to 10 μm, can be used.

[0037] To control the positive electrode composite layer as described above, first, the ratio of the particle size of the first positive electrode active material to the particle size of the second positive electrode active material must be devised, and second, the mixing method used to prepare the positive electrode composite layer slurry must be devised.

[0038] In the manufacturing process for the positive electrode composite slurry, a first positive electrode active material with a particle diameter of 10 μm, a second positive electrode active material, and a conductive additive are first mixed in a dry state. A binder and a solvent are then added to the mixture so that the solids content is in the range of 80 to 85% and mixed. At this stage, the slurry is immobile, solidified, and no lumps are visible. The mixture is then reduced in viscosity with a solvent to a level suitable for coating (solids content of 75 to 70%). Mixing is performed using a planetary centrifugal mixer (Thinky Corporation) as an example. However, the appropriate equipment may be changed, for example, to a high-speed mixer equipped with a rotating stirring blade inside a stirring vessel. Hereinafter, in this specification, a method that first incorporates mixing in a dry state is referred to as "hard kneading," and a method that first incorporates mixing in a wet state is referred to as "liquid kneading."

[0039] The density of the positive electrode mixture layer 42 is 2.3 g / cm 3 from the viewpoint of energy density. 3 It is preferable that the concentration is 2.5 g / cm or more. 3 Furthermore, in order to ensure an appropriate void inside the positive electrode mixture layer 42, maintain good electrolyte impregnation, and obtain good cycle characteristics, the density is preferably 3.5 g / cm. 3 Preferably, it is 2.9 g / cm or less. 3 It is even more preferable that:

[0040] The positive electrode mixture layer 42 is applied to one or both surfaces of the positive electrode current collector 41, and the amount of application per surface is 75 g / m 2 More than 200g / m 2 It is preferable that the coating amount of positive electrode mixture layer 42 is within the above range. By setting the coating amount of positive electrode mixture layer 42 to a value within the above range, it is possible to ensure sufficient energy density and maintain good output characteristics and cycle characteristics. Furthermore, it is preferable to leave a region on positive electrode current collector 41 where positive electrode mixture layer 42 is not coated in order to form positive electrode lead 7.

[0041] Furthermore, in this specification, the secondary particle diameter D50 represents the particle diameter of each particle determined using a laser diffraction particle size analyzer. The secondary particle diameter D50 indicates a relative particle amount of 50% measured using the laser diffraction / scattering method described in JIS standard Z8825:2013. The measured value of the secondary particle diameter represents the average particle diameter of each particle determined using a scanning electron microscope (SEM). For example, 20 particles visible in an SEM image can be randomly selected and the average long diameter can be used. Similar values ​​are used in the examples and comparative examples. In the examples, the secondary particle diameter D50 of the first positive electrode active material was nearly identical to the measured value of the secondary particle diameter of the first positive electrode active material.

[0042] Furthermore, whether the particle sizes and mixing ratios of the first and second positive electrode active materials in the prepared positive electrode mixture layer satisfy the requirements of the present invention can be confirmed by the following method. The nonaqueous electrolyte secondary battery is disassembled in a glove box filled with an inert gas such as argon, and the positive electrode 4 is removed. The positive electrode 4 is washed with an appropriate nonaqueous organic solvent (e.g., dimethyl carbonate) and then dried to remove the solvent. The positive electrode 4 is then immersed in a solvent such as N-methyl-2-pyrrolidone and ultrasonically removed, thereby removing the positive electrode mixture layer 42 from the positive electrode current collector 41. The solvent in which the separated positive electrode mixture layer 42 is dispersed can be centrifuged to separate the individual substances in the positive electrode mixture layer 42. In particular, the particle size and mixing ratio of the first positive electrode active material can be measured by measuring the particle size distribution on a volume basis using a laser diffraction particle size distribution analyzer. The laser diffraction particle size distribution analyzer may be an SALD-2300 (Shimadzu Corporation). The coating density can be measured using a cleaned positive electrode 4. The positive electrode 4 is punched using a punching tool capable of punching out a desired area, the weight of the punched positive electrode 4 is measured using an electronic balance, and the thickness of the punched positive electrode 4 is measured using a micrometer or film thickness gauge. The coating weight can be determined from the weight and area of ​​the positive electrode 4 punched out during coating density measurement. The nonaqueous electrolyte secondary battery used above may be subjected to initial activation or charge / discharge cycling in any step. It is preferable to fully discharge the positive electrode 4 to the minimum voltage assumed by the manufacturer before removing it. Furthermore, there is no significant difference in density between a positive electrode before assembly into a nonaqueous electrolyte secondary battery and a positive electrode fully discharged to the minimum voltage of a nonaqueous electrolyte secondary battery.

[0043] The conductive additive assists the conduction of electrons in the positive electrode. The conductive additive is not particularly limited, and known conductive additives can be used. Examples of the conductive additive include conductive carbon powders such as carbon blacks such as acetylene black and ketjen black, carbon nanotubes, carbon nanofibers, graphene, activated carbon, and graphite. The conductive additive may be made of a single material or multiple materials (e.g., a first conductive additive and a second conductive additive). The weight ratio of the conductive additive to the positive electrode mixture layer is preferably 0.5% to 10% because if it is less than 0.5%, it is insufficient to form a conductive path, and if it exceeds 10%, the relative amount of active material in the positive electrode mixture layer is insufficient.

[0044] The binder binds the positive electrode current collector, the positive electrode active material, and the conductive additive. There are no particular limitations on the binder, and known or commercially available binders can be used. Examples of binders include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinylpyrrolidone (PVP), polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), ethylene-propylene copolymer, and styrene-butadiene rubber (Styrene-Butadiene Examples of such rubbers include SBR, butadiene rubber, polyvinyl alcohol (PVA), carboxymethyl cellulose (CMC), butyl rubber, poly(meth)acrylate (PMMA), polyethylene oxide (PEO), polypropylene oxide (PO), polyepichlorohydrin, polyphosphazene, and polyacrylonitrile.

[0045] (Negative electrode) Negative electrode 5 is composed of a negative electrode current collector 51 and a negative electrode mixture layer 52 containing a negative electrode active material formed on one or both surfaces of negative electrode current collector 51. Negative electrode 5 may be composed of metallic lithium.

[0046] Although there is no particular limitation on the negative electrode current collector 51, it is preferable to use a metal. Suitable metals include aluminum foil and copper, and depending on the application, a porous aluminum current collector may also be used. Among these, copper is preferable from the viewpoint of electronic conductivity and battery operating potential.

[0047] The negative electrode composite layer 52 is made of, for example, a lithium alloy, a titanium-niobium alloy, or a material such as graphite, amorphous carbon, or a transition metal composite oxide (e.g., Li4Ti5O 12 The active material contains at least one selected from the group consisting of lithium ion batteries (e.g., TiNb2O7), alloys capable of absorbing and desorbing lithium, and silicon. Among these, graphite is preferred because it has an operating potential very close to that of metallic lithium, can be charged and discharged at a high operating voltage, and has excellent cycle characteristics. Graphite may also be used in combination with other negative electrode active materials.

[0048] Negative electrode mixture layer 52 may also contain a binder. The binder and conductive additive may be the same materials as those used in positive electrode 4.

[0049] The negative electrode mixture layer 52 is preferably applied to one or both surfaces of the negative electrode current collector 51. The amount of application per surface is 40 g / m 2 More than 120g / m 2 Preferably, the coating amount of negative electrode mixture layer 52 is equal to or less than 1.01. By setting the coating amount of negative electrode mixture layer 52 to a value within the above range, it is possible to ensure sufficient energy density and maintain good output characteristics and cycle characteristics. Furthermore, from the viewpoint of suppressing lithium metal dendrite precipitation during charging, the coating amount of negative electrode mixture layer 52 is preferably set so that the positive / negative electrode capacity ratio is 1.01 or more, and more preferably set so that it is 1.1 or more. Here, the positive / negative electrode capacity ratio is calculated by the following formula (6). (Positive and negative electrode capacity ratio) = (negative electrode capacity per surface area) / (single-sided cathode capacity per area) (6) However, if the value of the positive and negative electrode capacity ratio is too large, the amount of negative electrode composite layer 52 that does not contribute to the charge / discharge reaction increases, resulting in a decrease in energy density. Therefore, the value of the positive and negative electrode capacity ratio is preferably 1.3 or less, and more preferably 1.2 or less.

[0050] The density of the negative electrode mixture layer 52 is 1.0 g / cm 3 from the viewpoint of energy density. 3 It is preferable that the concentration is 1.2 g / cm or more.3 Furthermore, in order to obtain good cycle characteristics by ensuring an appropriate amount of voids inside negative electrode mixture layer 52 and maintaining good electrolyte impregnation, the density of negative electrode mixture layer 52 is preferably 1.7 g / cm or more. 3 Preferably, it is 1.5 g / cm or less. 3 It is even more preferable that:

[0051] (separator) The separator 6 is disposed between the positive electrode 4 and the negative electrode 5 and has a porosity that allows the components of the non-aqueous electrolyte to pass through. The separator 6 is formed, for example, using a porous sheet separator made of polymer or fiber, a nonwoven fabric separator, or the like. The separator 6 may be made of a material such as polyethylene, polypropylene, aramid, or polyimide, or may have multiple layers of different materials including these. However, from the viewpoint of providing a shutdown function in the event of heat generation, it is preferable that the separator 6 has a layer containing polyethylene. The pore size of the separator 6 is preferably 0.01 to 10 μm, and the thickness is preferably 5 to 30 μm. The separator 6 may also be formed by laminating a ceramic layer as a heat-resistant insulating layer on a porous substrate. When a solid electrolyte is used as the non-aqueous electrolyte, the separator 6 may not be present.

[0052] (others) The positive electrode leads 7 extend downward from the positive electrode composite layer 42, for example, as shown in FIG. 1. As an example, each positive electrode lead 7 is a portion of the positive electrode current collector 41 that is not coated with the positive electrode composite layer 42. The positive electrode leads 7 are bundled and joined to each other at the end opposite to the positive electrode composite layer 42 side within the exterior body 2.

[0053] One end of the positive electrode tab 8 is joined to the positive electrode lead 7, and the other end passes through the sealing portion of the exterior body 2 and extends to the outside.

[0054] The negative electrode leads 9 each extend upward from the negative electrode composite material layer 52, for example, as shown in FIG. 1. They may extend in the same direction as the positive electrode lead 7, provided that they do not come into contact with the positive electrode lead 7. As an example, each negative electrode lead 9 is a portion of the negative electrode current collector 51 that is not coated with the negative electrode composite material layer 52. The negative electrode leads 9 are bundled and joined to each other at the end opposite to the negative electrode composite material layer 52 side within the exterior body 2.

[0055] One end of the negative electrode tab 10 is joined to the negative electrode lead 9, and the other end passes through the sealing portion of the exterior body 2 and extends to the outside.

[0056] (non-aqueous electrolyte) The nonaqueous electrolyte may be a nonaqueous electrolytic solution or a solid electrolyte. Here, the nonaqueous electrolytic solution, which is a liquid, will be described in particular. The nonaqueous electrolytic solution is sealed in the exterior body 2. The portion of the exterior body 2 into which the nonaqueous electrolytic solution is to be injected is sealed after the nonaqueous electrolytic solution is injected. The nonaqueous electrolytic solution contains an electrolyte and a nonaqueous solvent.

[0057] The electrolyte is not particularly limited, and any lithium salt commonly used in non-aqueous electrolyte secondary batteries can be used. For example, LiPF, LiAsF, LiBF, LiCF, SO, LiN(C m F 2m+1 SO2)(C n F 2n+1 SO2) (m and n are integers of 1 or more), LiC(C s F 2s+1 SO2)(C t F 2t+1 SO2)(C u F 2u+1 Examples of electrolytes that can be used include lithium difluoro(oxalato)borate (SO2) (s, t, and u are integers of 1 or greater). These electrolytes may be used alone or in combination of two or more. From the viewpoints of lithium ion conductivity, viscosity of the electrolyte solution, temperature characteristics of conductivity, etc., it is desirable that the electrolyte has a concentration of 0.1 to 3 mol / L, preferably 0.5 to 1.5 mol / L.

[0058] The non-aqueous solvent contains a cyclic carbonate and / or a chain carbonate as a main component. The cyclic carbonate is preferably at least one selected from ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC). The chain carbonate is preferably at least one selected from dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). The cyclic carbonate is related to the degree of dissociation of the electrolyte components, and the chain carbonate is related to the viscosity of the electrolyte solution.

[0059] In the present invention, DMC is most suitable. It is generally known that the addition of DMC reduces the viscosity of the electrolyte. It is also known that a decrease in viscosity improves the electrolyte's permeability into the electrode, thereby generally improving battery performance. However, the present inventors have found that this tendency is weakened with high-Ni-content NCM-based positive electrode active materials. The high-Ni-content NCM-based positive electrode active material used in the present invention has low chemical stability, resulting in the formation of a substance that is highly reactive with the electrolyte and highly insulating. The present inventors have discovered that direct contact between a high-Ni-content NCM-based positive electrode active material and DMC further deteriorates performance. Therefore, by adjusting the composition of the LMFP as in the present invention, the performance of the DMC and high-Ni-content NCM-based positive electrode active material can be fully exploited.

[0060] Furthermore, for the purpose of forming a high-quality coating on the surface of the negative electrode active material by reductive decomposition during charge and discharge, the nonaqueous electrolyte may contain additives other than the lithium salts. The additives are not particularly limited, but examples include vinylene carbonate (VC), fluoroethylene carbonate, 1,3,2-dioxathiolane 2,2-dioxide (MMDS), 1,5,2,4-dioxadithiane 2,2,4,4-tetraoxide, tris(trimethylsilyl) phosphite, 1-propene 1,3-sultone, and Li2PO2F2. These additives may be used alone or in combination. They may also be used in combination with other additives. Furthermore, other additives may also be used alone. However, if these additives are added in excess, gas is likely to be generated during high-temperature cycling, causing a decrease in capacity, so for example, the ratio of each additive to the total weight of the nonaqueous electrolyte is preferably 3% or less, and more preferably 1% or less. Furthermore, when using these additives, they must be used in combination with VC to suppress gas generation.

[0061] In the present embodiment 1, in the positive electrode 4 of the nonaqueous electrolyte secondary battery 1, the positive electrode mixture layer 42 includes a first positive electrode active material that is a layered compound, a second positive electrode active material in which a coating film made of a carbon material is formed on the surface of a phosphate compound having an olivine structure, and a conductive additive, and 0.8≦(n×p) 2 / 3 By satisfying the formula (3) expressed as ≦13, it is possible to suppress an increase in internal resistance in the nonaqueous electrolyte secondary battery 1. According to the first embodiment, by satisfying the condition of the formula (3), it is possible to obtain a nonaqueous electrolyte secondary battery capable of suppressing an increase in internal resistance even in a positive electrode using an NCM-based positive electrode active material with a high Ni content.

[0062] (Embodiment 2) 2 is an exploded perspective view illustrating the configuration of a nonaqueous electrolyte secondary battery according to Embodiment 2 of the present invention. Nonaqueous electrolyte secondary battery 1A includes case 110, leaf spring 111, positive electrode current collector 112, positive electrode composite layer 113, separator 114, negative electrode 115, gasket 116, and cap 117. Positive electrode current collector 112 and positive electrode composite layer 113 form positive electrode 118.

[0063] In nonaqueous electrolyte secondary battery 1A, case 110 and cap 117 are fixed together by crimping or the like, and the battery is filled with a nonaqueous electrolyte. Nonaqueous electrolyte secondary battery 1A is liquid-tightly sealed by case 110, gasket 116, and cap 117. Furthermore, positive electrode current collector 112, positive electrode composite layer 113, separator 114, and negative electrode 115 are biased toward cap 117 by leaf spring 111. This keeps the components in close contact with each other.

[0064] The positive electrode current collector 112 is made of the same material as the positive electrode current collector 41 . Positive electrode mixture layer 113 has a configuration similar to that of positive electrode mixture layer 42. Positive electrode current collector 112 and positive electrode mixture layer 113 form positive electrode 118 .

[0065] Separator 114 is a porous disk-shaped material and is provided between positive electrode 115 and negative electrode 115. Separator 114 has the same structure as separator 6.

[0066] The nonaqueous electrolyte may be the same as that of the first embodiment.

[0067] The negative electrode 115 has the same configuration as the negative electrode 5 .

[0068] In the same manner as in the first embodiment, in the positive electrode 118 of the nonaqueous electrolyte secondary battery 1 in the second embodiment, the positive electrode composite layer 113 includes a first positive electrode active material that is a layered compound, a second positive electrode active material in which a coating made of a carbon material is formed on the surface of a phosphate compound having an olivine structure, and a conductive additive, and by satisfying the above formula (3), it is possible to suppress an increase in internal resistance in the nonaqueous electrolyte secondary battery 1. According to the second embodiment, by satisfying the condition of the above formula (3), it is possible to obtain a nonaqueous electrolyte secondary battery that can suppress an increase in internal resistance even in a positive electrode that uses an NCM-based positive electrode active material with a high Ni content. [Example]

[0069] The present invention will be described in more detail below by way of examples, but the present invention is not limited to the following examples in any way.

[0070] Example 1 <How to make the positive electrode> The first positive electrode active material is NCM Li with a secondary particle diameter D50 of 3.0 μm. 1.1 Co 0.1 Ni 0.8 Mn 0.1 O2, as the second positive electrode active material, LiMn, a LMFP with a secondary particle diameter D50 of 6.7 μm 0.6 Fe 0.4 PO4 was used. The first and second positive electrode active materials were mixed with appropriate amounts of carbon nanotubes (CNT) as a conductive additive, N-methyl-2-pyrrolidone (NMP) as a binder and viscosity adjusting solvent, and the mixture was kneaded to prepare a positive electrode active material slurry. The weight ratio of the positive electrode active material slurry was (NCM+LMFP):CNT:binder = 97.3:0.7:2. Li 1.1 Co 0.1 Ni 0.8 Mn 0.1 O2 is commonly referred to as NCM811.

[0071] The obtained positive electrode active material slurry was applied to both sides of a positive electrode current collector and dried to form a positive electrode mixture layer, thereby producing a positive electrode.

[0072] <Method for producing the negative electrode> A negative electrode slurry was prepared by mixing 96.7 wt% graphite as the negative electrode active material, 0.3 wt% acetylene black as a conductive additive, 1.5 wt% styrene-butadiene rubber as a binder, 1.5 wt% carboxymethyl cellulose as a thickener, and an appropriate amount of ion-exchanged water as a viscosity adjusting solvent.

[0073] The prepared negative electrode active material slurry was applied to both sides of a negative electrode current collector and dried to form a negative electrode mixture layer, thereby producing a negative electrode.

[0074] <Preparation of non-aqueous electrolyte> A non-aqueous electrolyte solution was prepared by dissolving 1.3 mol / L of LiPF6 as a lithium salt and 3 wt% of vinylene carbonate as an additive in a mixed solvent of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 2:5:3.

[0075] <Fabrication of Non-Aqueous Electrolyte Secondary Battery> The prepared electrode was pressed to a predetermined coating density using a bench-top roll press with the gap set to an arbitrary value. Two laminate films were prepared as exterior bodies, each having a structure in which a heat-sealable resin layer made of polyolefin, a metal layer made of aluminum foil, and a protective layer made of nylon resin and polyester resin were laminated in this order. The heat-sealable resin layers of the two laminate films were arranged facing each other, and the laminate films were overlapped with their adhesive surfaces facing each other so that electrode groups could be housed in the two housing recesses. The electrode groups were arranged so that the portions where the heat-sealable resin parts of each terminal would be formed passed between the edges of the two laminate films, leaving a portion of each terminal exposed to the outside. In this state, the heat-sealable resin layers of the laminate films were heat-sealed along the three edges, including the two edges from which the tabs of the laminate films extended. Next, the electrolyte solution prepared above was injected into the one edge of the exterior body that was not heat-sealed. Next, the remaining edge of the exterior body was heat-sealed under reduced pressure to produce a nonaqueous electrolyte secondary battery (cell).

[0076] <Secondary particle size measurement> The D50 of the secondary particles of NCM and LMFP was confirmed using cross-sectional SEM images of the positive electrode composite layer. The method for capturing images of the expected field of view is as follows: The electron microscope used was a scanning electron microscope (SEM), and the electron microscope image acquisition conditions were secondary electron images, with an acceleration voltage of 15 kV, a magnification of 2500x relative to the particles, and a probe current of PC30 to PC70. The electrode surface after coating and pressing was observed. Elemental analysis was also performed using an energy dispersive X-ray analyzer (EDS). Elemental analysis makes it possible to distinguish between NCM and LMFP.

[0077] A cell was fabricated using a positive electrode having NCM and LMFP in the mass ratios shown in Table 2 as the positive electrode composite layer. After the injection and the CS process, generally called the chemical conversion process, the AC resistance (ACR) was measured using a resistance meter manufactured by HIOKI Corporation after leaving the battery to stand for 48 hours. In this specification, a custom-made product was used, but RM3544 is also expected to be suitable in general. Table 1 shows the charge / discharge process during battery fabrication. In the CS process, as shown in Table 1, charge / discharge was performed for 2 to 5 cycles, with both the charge rate and discharge rate at 0.2C. After disassembling the battery, the positive electrode was washed with DMC, and the secondary particle size was confirmed using a cross-sectional SEM image of the positive electrode composite layer. The composition and physical properties of Example 1 are shown in Table 2. In Table 2, (r N / r L ) indicates the ratio of the measured secondary particle diameter. The particle diameters in the table are in μm units, and the percentages are weight percentages (%).

[0078] [Table 1]

[0079] [Table 2]

[0080] Examples 2 to 11 In Examples 2 to 11, the NCM and the ratio p, the theoretical minimum particle number n, (n × p) as shown in Table 2 were used.2 / 3 A positive electrode having the above structure was used. Specifically, in Examples 3 and 5, the secondary particle diameter D50 of the LMFP was 6.7 μm, and in Examples 2, 4, 6, 7, 8, and 9, the secondary particle diameter D50 of the LMFP was 9.1 μm. In Examples 10 and 11, the ratio of the amount of auxiliary agent was changed. The compositions and physical properties of Examples 2 to 11 are shown in Table 2.

[0081] (Comparative Examples 1 to 42) Comparative Examples 1 to 42 are NCM and ratio p, theoretical minimum particle number n, (n × p) as shown in Table 2. 2 / 3 A positive electrode having the following structure was used. Specifically, in Comparative Examples 1, 2, 3, 4, 15, 16, 17, 18, 29, 30, 31, and 32, the secondary particle diameter D50 of the LMFP was 1.0 μm, in Comparative Examples 5, 6, 7, 8, 19, 20, 21, 22, 33, 34, 35, 36, and 37, the secondary particle diameter D50 of the LMFP was 1.9 μm, in Comparative Examples 9, 10, 11, 23, 24, 25, 38, 39, and 40, the secondary particle diameter D50 of the LMFP was 6.7 μm, and in Comparative Examples 12, 13, 14, 26, 27, 28, and 41, the secondary particle diameter D50 of the LMFP was 9.1 μm. The compositions and physical properties of Comparative Examples 1 to 42 are shown in Tables 3 and 4, respectively. [Table 3] [Table 4]

[0082] In Examples 1 to 11, which satisfied the above formula (3), the ACR was less than 25.7 mΩ. In contrast, in Comparative Examples 1 to 42, which did not satisfy the above formula (3), the ACR was 26.6 mΩ or more. This indicates that satisfying the above formula (3) can suppress an increase in internal resistance.

[0083] The present invention provides a positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery that can suppress an increase in internal resistance even when the positive electrode uses an NCM-based positive electrode active material with a high Ni content. Therefore, the present invention contributes to the manufacture and sale of non-aqueous electrolyte secondary batteries and has industrial applicability. [Explanation of symbols]

[0084] 1, 1A non-aqueous electrolyte secondary battery 2. Exterior body 3 electrode groups 4, 118 positive electrode 5, 115 negative electrode 6, 114 Separator 7 Positive lead 8 Positive tab 9 Negative lead 10 Negative electrode tab 41, 112 Positive electrode current collector 42, 113 Positive electrode composite layer 51 Negative electrode current collector 52 Negative electrode composite layer 110 cases 111 Leaf spring 116 Gasket 117 Cap

Claims

1. A positive electrode for a non-aqueous electrolyte secondary battery, comprising: a positive electrode current collector; and a positive electrode mixture layer formed on a surface of the positive electrode current collector, The positive electrode mixture layer is a first positive electrode active material that is a layered compound represented by the following general formula (1); Li ａ Ni ｘ Co ｙ M1 １－ｘ－ｙ O ２ (where 0<a≦1.2, 0.6≦x≦0.8, 0.1≦y≦0.2, 0.7≦x+y≦0.9) ... (1) a second positive electrode active material in which a coating film made of a carbon material is formed on a surface of a phosphate compound having an olivine structure, the second positive electrode active material being represented by the following general formula (2): LiMn ｚ M2 ｂ Fe １－z－ｂ P.O. ４ (where 0<z≦0.9, 0≦b≦0.1, 0<z+b<1) ... (2) A conductive additive; Including, A positive electrode for a non-aqueous electrolyte secondary battery, characterized by satisfying the following formula (3): 0.8≦(n×p) ２／３ ≦13 ・・・(3) Here, n is the theoretical minimum particle number of the second positive electrode active material, and p is the ratio of the mass of the first positive electrode active material to the mass of the second positive electrode active material. In the above formula (1), M1 is at least one selected from Ti, Zr, Nb, W, P, Al, Mg, V, Mn, Ca, Sr, Cr, Fe, B, Ga, In, Si, Mo, Y, Sn, Cu, Ag, Ce, Pr, Ge, Bi, Ba, Er, La, Sm, Yb, Sb, Bi, S, and Zn; In the above formula (2), M2 is at least one selected from Ni, Co, Ti, Cu, Zn, Mg, Zr, Ca, Y, Mo, Ba, Pb, Bi, La, Ce, Nd, Gd, Al, Ga, and Sr.

2. The n is calculated by the following formula (4):

2. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1. n=4×(r Ｎ / r Ｌ ) ２ ・・・(4) Here, r Ｎ is the median diameter of the secondary particles of the first positive electrode active material, and r Ｌ is the median diameter of the particles of the second positive electrode active material.

3. The p is 4 or more and 9 or less.

2. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1.

4. D50, which indicates the secondary particle diameter of the first positive electrode active material, is 3 μm or more and 30 μm or less, D50, which indicates the primary particle diameter of the second positive electrode active material, is 1 μm or more and 10 μm or less.

2. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1.

5. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 ; a negative electrode; A separator; a non-aqueous electrolyte solution containing a lithium salt and a non-aqueous solvent; A non-aqueous electrolyte secondary battery comprising:

6. The non-aqueous solvent contains dimethyl carbonate.

6. The non-aqueous electrolyte secondary battery according to claim 5.

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