Positive electrode for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery

Abstract

To provide a positive electrode for a nonaqueous electrolyte secondary battery capable of achieving both high volume energy density and excellent charge rate characteristics, and to provide a nonaqueous electrolyte secondary battery.SOLUTION: The positive electrode mixture layer contains a first positive electrode active material represented by the following formula (1), a second positive electrode active material represented by the following formula (2), and a conductive assistant, the conductive assistant contains a carbon-based material having a specific surface area of 100m2 / g or more in an amount of 0. 4wt% or more and 3. 0wt% or less based on the total weight of the positive electrode mixture layer, and a value represented by the following formula (3) is 0.2 or more and 100 or less. LiaNixCoyM11-x - yO2 (where 0 <a ≤ 1.2, 0 <x ≤ 0.9, 0 <y ≤ 0.5, 0 <x + y <1) (1) LiMnzM2bFe1 - z - bPO4 (where 0 <z ≤ 0.9, 0 ≤ b ≤ 0.1, 0 <z + b <1) (2) (wL / (tL * DL503)) / (wN / (tN * DN503)) (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] In non-aqueous electrolyte secondary batteries, ions in the electrolyte are responsible for electrical conduction. Lithium secondary batteries, a type of non-aqueous electrolyte secondary battery, are used as power sources for compact electronic devices such as digital cameras and laptop computers, as well as vehicles. Lithium secondary batteries consist of a positive electrode and a negative electrode. The positive electrode comprises a positive electrode current collector and a positive electrode composite layer coated on the surface of the positive electrode current collector, which contains a positive electrode active material, a conductive additive, and a binder. The positive electrode active material is the heart of energy storage, and various research and development efforts are underway. In particular, research is being conducted on positive electrodes that combine two different active materials to exploit the characteristics of each active material, thereby achieving superior performance compared to when each active material is used alone.

[0003] For example, Patent Document 1 describes the blending of a layered lithium (Li), nickel (Ni), cobalt (Co), and manganese (Mn) composite oxide (NCM) with an olivine-type lithium manganese iron phosphate (LMFP). NCM is known as a cathode active material with high output and capacity, while LMFP is known as a cathode active material with high safety characteristics similar to lithium iron phosphate (LFP) but with a higher operating potential than LFP. Blending these two active materials allows the production of a cathode active material for lithium-ion secondary batteries that can sufficiently enhance both the energy density per unit volume and the discharge rate characteristics. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2022-94473 Summary of the Invention [Problem to be solved by the invention]

[0005] However, Patent Document 1 does not describe charge rate characteristics, and it is unclear whether the rate characteristics have truly been improved by mixing. Furthermore, Patent Document 1 still has room for improvement in the energy density per unit volume. The simplest method for increasing the energy density per unit volume is to increase the electrode density after electrode pressing. However, when amorphous carbon materials such as acetylene black are used as conductive additives, the electrode density after pressing cannot be increased. Furthermore, if an attempt is made to forcibly increase the electrode density by, for example, significantly increasing the pressing pressure, the electrolyte does not sufficiently penetrate into the voids between the composite layers. As a result, the diffusion of lithium ions in the electrolyte becomes the rate-limiting step in the charge / discharge reaction, leading to a deterioration in charge rate characteristics. Another method for increasing the energy density per unit volume is to increase the proportion of positive electrode active material in the positive electrode composite layer. However, this reduces the proportion of conductive additive, making it impossible to establish a conductive path within the positive electrode composite layer.

[0006] The present invention has been made in view of the above, and has an object to provide a positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery that can achieve both high volumetric energy density and good charge rate characteristics. [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 mixture layer formed on a surface of the positive electrode current collector, wherein the positive electrode mixture layer includes a first positive electrode active material which is a layered compound represented by the general formula shown in the following formula (1), a second positive electrode active material which is represented by the general formula shown in the following formula (2) and which is a phosphate compound having an olivine structure and on the surface of which a coating film made of a carbon material is formed, and a conductive additive, wherein the conductive additive has a specific surface area of ​​100 m 2 / g or more of a carbon-based material is contained in an amount of 0.4 wt% or more and 3.0 wt% or less of the total weight of the positive electrode mixture layer, and the weight ratio of the first positive electrode active material in the positive electrode mixture layer is w N , the median diameter is D N50 , tap density is t N , the weight ratio of the second positive electrode active material in the positive electrode mixture layer is w L , the median diameter is D L50 , tap density is t L When the value expressed by the following formula (3) is 0.2 or more and 100 or less, Li a Ni x Co y M1 1-x-y O2 (However, 0 <a≦1.2、0≦x≦0.9、0<y≦1、0<x+y≦1)···(1) LiMn z M2 b Fe 1-z-b PO4 (However, 0 <z≦0.9、0≦b≦0.1、0<z+b<1)···(2) (w L / (t L ×D L50 3 )) / (w N / (t N ×D N50 3 ))···(3) 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, 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, in a second aspect, the positive electrode for a non-aqueous electrolyte secondary battery according to the present invention is characterized in that, in addition to the first aspect, the conductive additive contains carbon nanotubes.

[0009] Furthermore, as a third aspect, 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 the positive electrode mixture layer is coated on the positive electrode current collector, and the electrode density is 2.5 g / cc or more and 3.2 g / cc 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 provides, as a fourth aspect, the positive electrode mixture layer has a basis weight on the positive electrode current collector of 5 mg / cm. 2 More than 30mg / cm 2 The present invention is characterized in that:

[0011] Furthermore, as a fifth aspect, in addition to the second aspect, the positive electrode for a non-aqueous electrolyte secondary battery according to the present invention is characterized in that the carbon nanotubes have an average fiber outer diameter of 5 nm or more and 11 nm or less, an average length of 5 μm or more and 30 μm or less, and a specific surface area of ​​250 m 2 / g or more 350m 2 / g or less.

[0012] In addition, as a sixth 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 fifth aspects; a negative electrode; a separator; and a nonaqueous electrolyte solution containing a lithium salt and a nonaqueous solvent. [Effects of the Invention]

[0013] According to the present invention, a non-aqueous electrolyte secondary battery can achieve both a high volumetric energy density and good charge rate characteristics. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a cross-sectional view illustrating the configuration of a nonaqueous electrolyte secondary battery including a positive electrode for 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 including a positive electrode for 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 including a positive electrode for 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 each other to seal, 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.

[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 aluminum, nickel, stainless steel, titanium, other alloys, etc. Among these, 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 secondary particles of a layered compound represented by the general formula (1) below. The layered compound is a lithium (Li), cobalt (Co), and nickel (Ni)-containing composite metal oxide composed of layers of sheet-like particles. 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≦x≦0.9、0<y≦1、0<x+y≦1) ···(1) In the above formula (1), M1 is at least one selected from 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), 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), sulfur (S), and zinc (Zn). In the following description, the positive electrode active material represented by the above formula (1) is referred to as NCM.

[0024] The second positive electrode active material is a phosphate compound (olivine-based compound) having an olivine structure, represented by the following general formula (2): This phosphate compound is a compound composed of manganese, iron (Fe), and lithium. 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. In the following description, the positive electrode active material represented by the above formula (2) is referred to as LMFP.

[0025] Compounds with an olivine structure that can absorb and desorb lithium are generally inexpensive and have excellent thermal stability, so their use as positive electrode active materials or addition to the positive electrode mixture layer can improve the safety of batteries.

[0026] A coating made of a carbon material is formed on the second positive electrode active material. The carbon material is a conductive carbon material. The coating made of the carbon material is present on at least a portion of the surface of the primary particles of the second positive electrode active material. The purity of the carbon and the thickness of the carbon coating can be selected as desired.

[0027] The conductive additive has a specific surface area of ​​100m 2 It is desirable to use a carbonaceous material with a specific surface area of ​​100 m / g or more. Generally, among the materials used in the positive electrode composite layer, the conductive additive has the lowest density. 2 By using a carbon-based material with a specific surface area of ​​1 / g or more, the amount of conductive additive required per active material particle can be reduced. This reduction in the amount of conductive additive can improve the compression limit density. However, at such a compression limit density, the electrolyte cannot sufficiently enter the gaps between the composite layers. As a result, the diffusion of lithium ions in the electrolyte becomes the rate-limiting step in the charging reaction, resulting in a decrease in charge rate characteristics. In other words, when comparing a conductive additive with a large specific surface area and a conductive additive with a small specific surface area under the same electrode density of the positive electrode composite layer, the former, which leaves more gaps between the composite layers, exhibits better charge rate characteristics.

[0028] Furthermore, the conductive additive is preferably contained in an amount of 0.4 wt% to 3.0 wt% of the total weight of the positive electrode composite layer. If the amount is less than 0.4 wt%, a conductive path cannot be established between the composite layers, resulting in a decrease in charge rate characteristics. If the amount is more than 3.0 wt%, the ratio of the positive electrode active material in the positive electrode composite layer decreases, resulting in a smaller capacity. Furthermore, the increased amount of conductive additive reduces the electrode density and the volumetric energy density. To achieve both a high volumetric energy density and good charge rate characteristics, the conductive additive is preferably contained in an amount of 0.4 wt% to 3.0 wt% of the total weight of the positive electrode composite layer.

[0029] The conductive additive preferably contains carbon nanotubes. The carbon nanotubes have a specific surface area of ​​250 m 2 / g or more 350m 2 / g or less, and is a carbon-based material with high electronic conductivity. Therefore, by adding carbon nanotubes as a conductive additive, even a small amount can provide good conductivity to the positive electrode composite layer. The average fiber outer diameter, average length, and specific surface area of ​​the carbon nanotubes used are 5 nm to 11 nm, 5 μm to 30 μm, and 250 m 2 / g or more 350m 2 / g or less. If the average fiber outer diameter is within the above range, the electronic conductivity of the carbon nanotubes is ensured. If the average length is within the above range, a conductive path can be established between the active material particles. If the specific surface area is within the above range, a stable positive electrode slurry that has good conductivity with a small amount can be produced.

[0030] Furthermore, the weight ratio of the first positive electrode active material in the positive electrode mixture layer is w N , the median diameter is D N50 , tap density is t N , the weight ratio of the second positive electrode active material in the positive electrode mixture layer is w L , the median diameter is D L50 , tap density is t L When this is the case, the value expressed by the following formula (3) satisfies the range of 0.2 to 100. (w L / (t L ×D L50 3 )) / (w N / (t N ×D N50 3 )) ···(3) Here, the numerator and denominator represent the number of particles per unit weight of the first and second positive electrode active materials in the positive electrode mixture layer. Here, the structure of the formula will be explained only for the first positive electrode active material. Weight ratio w N The reciprocal of the tap density, 1 / t N The volume [cc / g] occupied by the first positive electrode active material per unit weight is then calculated by multiplying the volume [cc / g] by the median diameter D N50 Let us assume that the sphere has a volume of 4π×(D N50 / 2) 3 Dividing by 1 / 3 (cc) gives the number of first positive electrode active material particles per unit weight (g). A similar calculation was performed for the second positive electrode active material. The ratio of the first positive electrode active material to the second positive electrode active material was calculated, and the resulting equation is the above equation (3). Therefore, this equation can be said to represent the number of second positive electrode active material particles per first positive electrode active material particle. If the value obtained from this equation is less than 0.2, the first positive electrode active material particles are arranged around the second positive electrode active material particles. In this case, the conductive path formed by the carbon coating on the second positive electrode active material particles is interrupted by the surrounding first positive electrode active material particles. As a result, a good conductive path cannot be formed, and the charge rate characteristics deteriorate. If the value obtained from this equation is 0.1 or greater, the second positive electrode active material particles are arranged around the first positive electrode active material particles.

[0031] However, if the value obtained from this equation is greater than 100, there are three main possible states: (1) The median diameter of the second positive electrode active material is clearly smaller than the median diameter of the first positive electrode active material. (2) The tap density of the second positive electrode active material is clearly smaller than the tap density of the first positive electrode active material. (3) The weight ratio of the second positive electrode active material is significantly greater than the weight ratio of the first positive electrode active material. In the case of (1), the total specific surface area of ​​the particles becomes larger, and the amount of conductive additive required increases. This reduces the availability of the active material compared to the optimal value, leading to a decrease in volumetric energy density. In addition, the obviously small particles tend to fill the gaps between the composite layers, resulting in a decrease in charge rate characteristics. In the case of (2), the use of a second positive electrode active material with a low tap density makes it impossible to increase the electrode density, and therefore the volumetric energy density decreases. In the case of (3), the proportion of the first positive electrode active material having high rate characteristics decreases, and therefore the charge rate characteristics deteriorate. In order to achieve both a high volumetric energy density and good charge rate characteristics, it is desirable that the value expressed by the above formula (3) be 0.2 or more and 100 or less.

[0032] Furthermore, the electrode density of the positive electrode composite layer 42 on the positive electrode current collector 41 is preferably 2.5 g / cc or more and 3.2 g / cc or less. If the electrode density is less than 2.5 g / cc, the volumetric energy density decreases, and the contact between the conductive additive and the active materials becomes insufficient, resulting in a decrease in charge rate characteristics. If the electrode density is more than 3.2 g / cc, the gap between the composite layers decreases, which becomes the diffusion rate limiting factor for lithium ions, resulting in a decrease in charge rate characteristics.

[0033] The positive electrode mixture layer 42 has a weight per unit area of ​​5 mg / cm 2 on the positive electrode current collector 41. 2 More than 30mg / cm 2 The weight per unit area is preferably 5 mg / cm or less. 2 If the weight is less than 30 mg / cm, not only will the amount of active material be insufficient, but the electrode density during coating (pressing) will not be sufficient, and the volumetric energy density will decrease. 2 If it is larger, the thickness of positive electrode mixture layer 42 becomes thick, the distance from positive electrode current collector 41 to the non-aqueous electrolyte interface becomes long, the electrode resistance increases, and the input / output characteristics deteriorate. In addition, cracks are likely to occur after positive electrode mixture layer 4 is dried after coating, and the handling of the electrode deteriorates.

[0034] Furthermore, whether the median diameter, tap density, mixing ratio, electrode density, and basis weight of the first and second positive electrode active materials in the prepared positive electrode composite layer satisfy the requirements of the present invention can be confirmed by the method described below. First, 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. Next, the positive electrode 4 is immersed in a solvent such as N-methyl-2-pyrrolidone and ultrasonically applied, thereby removing the positive electrode composite layer 4 from the positive electrode current collector 41. The removed positive electrode composite layer 42 can be confirmed by measuring the particle size distribution on a volume basis using a laser diffraction particle size distribution analyzer to measure the particle size and mixing ratio of each active material contained in the positive electrode composite layer 42. Tap density refers to the "tap bulk density" defined in JIS R 1628, "Method for Determining the Bulk Density of Fine Ceramic Powders," and is measured by a constant mass measurement method. Electrode density can be measured using a cleaned positive electrode 4. A punch capable of punching out a desired area is used to punch out a positive electrode 4, and the weight of the punched positive electrode 4 is measured using an electronic balance or the like. The thickness of the punched positive electrode 4 can be measured using a micrometer or film thickness meter. The basis weight can be determined from the weight and area of ​​the punched positive electrode 4 during electrode density measurement. The nonaqueous electrolyte secondary battery used above may have undergone initial activation or charge / discharge cycling in an optional process. It is preferable to fully discharge the positive electrode 4 to the minimum voltage expected by the manufacturer before removing it. In the following description, D50 refers to the median diameter, which is the diameter value corresponding to the center value of the particle size distribution.

[0035] 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.

[0036] (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 or metallic lithium (not shown).

[0037] 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.

[0038] The negative electrode composite layer 52 is made of, for example, metallic lithium, lithium, titanium-niobium alloy, or graphite, amorphous carbon, transition metal composite oxide (e.g., Li4Ti5O 12 The negative electrode composite layer 52 contains at least one active material selected from the group consisting of graphite, an alloy capable of absorbing and releasing lithium, and silicon (e.g., TiNb2O7), an alloy capable of absorbing and releasing 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. The negative electrode composite layer 52 also contains a binder. The negative electrode composite layer 52 may also contain a conductive additive. The binder and conductive additive may be the same materials as those used in the positive electrode 4.

[0039] (separator) The separator 6 is provided between the positive electrode and the negative electrode and has a porosity that allows 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] The nonaqueous electrolyte may be a nonaqueous electrolytic solution or a solid electrolyte. The following describes the nonaqueous electrolytic solution in particular. The nonaqueous electrolytic solution is enclosed within the exterior body 2. The nonaqueous electrolytic solution injection portion of the exterior body 2 is sealed after the nonaqueous electrolytic solution is injected. The nonaqueous electrolytic solution contains an electrolyte and a nonaqueous solvent.

[0045] 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.

[0046] 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.

[0047] 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 salt. The additives are not particularly limited, but examples include vinylene carbonate, 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.

[0048] In the present embodiment 1, in the positive electrode 4 of the nonaqueous electrolyte secondary battery 1, the positive electrode composite layer 42 is made of a first positive electrode active material which is a layered compound represented by general formula (1), a second positive electrode active material which is a phosphate compound represented by general formula (2) having an olivine structure and in which a coating made of a carbon material is formed on the particle surface, and a conductive additive having a specific surface area of ​​100 m 2 / g or more of a carbon-based material is contained in an amount of 0.4 wt% or more and 3.0 wt% or less of the total weight of the positive electrode mixture layer, and (w L / (t L ×D L503 )) / (w N / (t N ×D N50 3 )) (Equation (3)) is set to satisfy the range of 0.2 to 100. According to the first embodiment, it is possible to achieve both a high volumetric energy density and good charge rate characteristics.

[0049] (Embodiment 2) 2 is an exploded perspective view illustrating the configuration of a nonaqueous electrolyte secondary battery including a positive electrode for 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] The nonaqueous electrolyte may be the same as that of the first embodiment.

[0054] The negative electrode 115 has the same configuration as the negative electrode 5 .

[0055] In the present second embodiment, in the positive electrode 118 of the nonaqueous electrolyte secondary battery 1A, the positive electrode composite layer 113 is made of a first positive electrode active material which is a layered compound represented by general formula (1), a second positive electrode active material which is a phosphate compound represented by general formula (2) having an olivine structure and in which a coating made of a carbon material is formed on the particle surface, and a conductive additive having a specific surface area of ​​100 m 2 / g or more of a carbon-based material is contained in an amount of 0.4 wt% or more and 3.0 wt% or less of the total weight of the positive electrode mixture layer, and (w L / (t L ×D L50 3 )) / (w N / (t N ×D N50 3 )) (Equation (3)) is set to satisfy the range of 0.2 to 100. According to the second embodiment, it is possible to achieve both a high volumetric energy density and good charge rate characteristics. [Example]

[0056] 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.

[0057] <How to make the positive electrode> Li as the first cathode active material (NCM) 1.08 Ni 0.5 Co 0.2 Mn 0.3 O2, LiMn as the second cathode active material (LMFP) 0.45 Fe 0.55 Positive electrode active material slurry was prepared by mixing appropriate amounts of PO4, acetylene black (AB) and conductive carbon or carbon nanotubes as a conductive additive, and N-methyl-2-pyrrolidone (NMP) as a binder and viscosity adjusting solvent.

[0058] The obtained positive electrode active material slurry was applied to a positive electrode current collector and dried to form a positive electrode mixture layer, thereby producing a positive electrode.

[0059] <Method for producing the negative electrode> A 0.5 mm thick lithium metal foil was used as the negative electrode active material, and the lithium metal foil was punched using a 16 mm diameter punch to prepare a negative electrode.

[0060] <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 1.0 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.

[0061] <Fabrication of Non-Aqueous Electrolyte Secondary Battery> The fabricated positive electrode was pressed to a predetermined electrode density using a benchtop roll press. The pressed electrode was then punched into a circle and vacuum-dried together with coin cell components at 60°C for 12 hours. The materials were placed in a glove box filled with inert gas, and a coin cell consisting of the fabricated electrode, separator, and electrolyte was fabricated as a nonaqueous electrolyte secondary battery for testing.

[0062] <Particle size measurement> The particle sizes of NCM and LMFP were determined by the particle size (D50) at which the relative particle amount was 50%, as measured by the laser diffraction / scattering method described in JIS standard Z8825: 2013. Measurements were performed using a laser diffraction particle size distribution analyzer SALD-2300 (Shimadzu Corporation).

[0063] <Measurement of electrode density> The gap setting of the benchtop roll press was set to an arbitrary value, and electrodes were pressed to achieve the desired electrode density. The pressed electrodes were punched using a φ13.5 mm punch, and the thickness and weight of the punched electrodes, including the current collector, were measured to calculate the electrode density. The density of all current collectors was assumed to be constant, set at 2.7 g / cc. When removing electrodes from the cells after testing to measure electrode density, the removed electrodes were washed with an electrolyte solvent (e.g., DMC), dried, and punched using a φ13.5 mm punch. The thickness and weight of the punched electrodes were measured to calculate the density. At this time, the state of charge (SOC) of the cells was set to 0%.

[0064] <Measurement of basis weight> The electrodes were punched using a φ13.5 mm punch, and the electrode density was calculated by weighing the punched electrode including the current collector. The density of all current collectors was assumed to be constant, 2.7 g / cc. When removing the electrodes from the cells after testing to measure the electrode density, the removed electrodes were washed with an electrolyte solvent (e.g., DMC), dried, and punched using a φ13.5 mm punch. The punched electrodes were then weighed to calculate the density. At this time, the state of charge (SOC) of the cells was set to 0%.

[0065] <Charging rate test> As shown in Table 1, the fabricated coin cells were left in a constant temperature bath at 25°C for 24 hours, and then subjected to an initial charge / discharge at 0.1C. Subsequently, activation was performed by repeating charge / discharge at 0.2C four times. After activation, a charge rate test was initiated. In the charge rate test, the discharge rate was fixed, and charge / discharge was performed while varying the charge rate. The discharge rate was 0.5C, and the charge rates were 0.2C, 0.5C, 1.0C, 2.0C, 3.0C, 4.0C, and 5.0C. Furthermore, for each rate (1.0C to 5.0C), the capacity retention rate (%) was calculated based on the capacity (Ah) relative to the initial capacity (Ah). [Table 1]

[0066] <Volumetric energy density calculation> The discharge capacity of the produced coin cell was measured at 0.1 C. Then, the volumetric energy density was calculated using the following formula (4). Volumetric energy density of the positive electrode active material at 25°C (Wh / L) = Discharge capacity (mAh / g) at 25°C and 0.1C x average voltage (V) × electrode density of the positive electrode composite layer (g / cm 3 ) × total ratio of active material in the positive electrode mixture layer ···(4)

[0067] Table 2 shows the physical properties of the NCM and LMFP used in the examples and comparative examples. [Table 2]

[0068] Table 3 shows the physical properties of the conductive additives used in the examples and comparative examples. [Table 3]

[0069] (Examples 1 to 18, Comparative Examples 1 to 15) In Examples 1 to 18 and Comparative Examples 1 to 15, cells were fabricated using positive electrodes having the compositions shown in Tables 4 and 5 as the positive electrode composite layer. Table 4 shows the physical property values ​​and test results for Examples 1 to 18, and Table 5 shows the physical property values ​​and test results for Comparative Examples 1 to 15. [Table 4] [Table 5]

[0070] Here, in Comparative Example 1, the weight ratio of the conductive additive carbon nanotubes was set to 0.2 wt% compared to Examples 1 to 3. In Comparative Example 1, the capacity ratio at 5C was 1.7%, which was lower than in Examples 1 to 3. This was because a conductive path could not be established between the composite layers due to a lack of conductive additive.

[0071] In Comparative Example 2, the LMFP particles were changed to particles 2-2 in Example 1. The volumetric energy density of Comparative Example 2 was 1780 Wh / L, which was lower than that of Example 1. This is because the median diameter of the LMFP particles was clearly smaller than that of the NCM particles, which increased the total specific surface area of ​​the particles and reduced the usability of the active material.

[0072] In Comparative Example 3, the NCM particles in Example 1 were changed to Particles 1-2. Compared to Example 1, the volumetric energy density and charge capacity ratio at 5 C in Comparative Example 3 were lower at 1631 Wh / L and 26.8%, respectively. This is because the median diameter of the NCM particles became smaller, resulting in an arrangement in which NCM particles were present around LMFP particles, and the conductive path provided by the coating of the LMFP particles was interrupted by the surrounding NCM particles, making it impossible to form a good conductive path.

[0073] In Comparative Examples 4 and 5, the conductive additive was changed to acetylene black and conductive carbon compared to Examples 1 to 3. The volumetric energy densities of Comparative Examples 4 and 5 were low, at 1770 and 1626 Wh / L, respectively. This is because the use of a material with a small specific surface area as the conductive additive increased the amount of conductive additive required to obtain charge rate characteristics equivalent to those of Examples 1 to 3, resulting in a decrease in the weight ratio of the active material in the positive electrode composite layer. Furthermore, Comparative Example 4 had an electrode density of 3.1 g / cc, consistent with Example 1. However, using a material with a small specific surface area prevented sufficient electrolyte from penetrating the voids between the composite layers, resulting in a decrease in charge rate characteristics. In Comparative Example 5, where the electrode density was set to 2.90 g / cc, the increased voids improved the charge rate characteristics, but the volumetric energy density was significantly reduced, making both improvements impossible.

[0074] In Comparative Example 6, the NCM particles were changed to particles 1-2 and the LMFP particles were changed to particles 2-2 in Example 1. In Comparative Example 6, the volumetric energy density and the charge capacity ratio at 5 C were lower, at 1594 Wh / L and 13.3%, respectively, than in Example 1. This is because the median diameter of the LMFP particles was clearly smaller than the median diameter of the NCM particles, which increased the total specific surface area of ​​the particles and reduced the usability of the active material.

[0075] In Comparative Example 7, the LMFP particles were changed to particles 2-2 and the weight ratio of the positive electrode active material was changed compared to Example 4. In Comparative Example 7, the volumetric energy density and charge capacity ratio at 5 C were lower, at 1680 Wh / L and 48.0%, respectively, than in Example 4. This is because the median diameter of the LMFP particles was clearly smaller than the median diameter of the NCM particles, which increased the total specific surface area of ​​the particles and reduced the usability of the active material.

[0076] In Comparative Example 8, the NCM particles were changed to Particles 1-2 and the weight ratio of the positive electrode active material was changed compared to Example 4. In Comparative Example 8, the volumetric energy density and charge capacity ratio at 5 C were lower, at 1554 Wh / L and 37.1%, respectively, compared to Example 4. This is because the median diameter of the NCM particles became smaller, resulting in an arrangement in which NCM particles were present around LMFP particles, and the conductive path provided by the coating of the LMFP particles was interrupted by the surrounding NCM particles, making it impossible to form a good conductive path.

[0077] In Comparative Example 9, the NCM particles were changed to Particles 1-2, the LMFP particles were changed to Particles 2-2, and the weight ratio of the positive electrode active material was changed compared to Example 4. In Comparative Example 9, the volumetric energy density and charge capacity ratio at 5 C were lower at 1515 Wh / L and 12.4%, respectively, compared to Example 4. This is because the median diameter of the LMFP particles was clearly smaller than the median diameter of the NCM particles, which increased the total specific surface area of ​​the particles and reduced the usability of the active material.

[0078] In Comparative Example 10, the LMFP particles were changed to particles 2-2 and the weight ratio of the positive electrode active material was changed compared to Example 5. In Comparative Example 10, the capacity ratio at 5C was 42.7%, which was lower than in Example 5. This is because the median diameter of the LMFP particles was clearly smaller than the median diameter of the NCM particles, which increased the total specific surface area of ​​the particles and reduced the usability of the active material.

[0079] In Comparative Example 11, the NCM particles were changed to Particles 1-2 and the weight ratio of the positive electrode active material was changed compared to Example 5. In Comparative Example 11, the volumetric energy density and charge capacity ratio at 5C were lower, at 1554 Wh / L and 37.1%, respectively, compared to Example 5. This is because the median diameter of the NCM particles became smaller, resulting in an arrangement in which NCM particles were present around LMFP particles, and the conductive path provided by the coating of the LMFP particles was interrupted by the surrounding NCM particles, making it impossible to form a good conductive path.

[0080] In Comparative Example 12, the NCM particles were changed to Particles 1-2, the LMFP particles were changed to Particles 2-2, and the weight ratio of the positive electrode active material was changed compared to Example 5. In Comparative Example 12, the volumetric energy density and the charge capacity ratio at 5C were lower at 1515 Wh / L and 12.4%, respectively, compared to Example 5. This is because the median diameter of the LMFP particles was clearly smaller than the median diameter of the NCM particles, which increased the total specific surface area of ​​the particles and reduced the usability of the active material.

[0081] In Comparative Example 13, the LMFP particles were changed to particles 2-2 and the weight ratio of the positive electrode active material was changed compared to Example 6. In Comparative Example 13, the capacity ratio at 5C was 40.0%, which was lower than in Example 6. This is because the median diameter of the LMFP particles was clearly smaller than the median diameter of the NCM particles, which increased the total specific surface area of ​​the particles and reduced the usability of the active material.

[0082] In Comparative Example 14, the NCM particles were changed to particles 1-2 and the weight ratio of the positive electrode active material was changed compared to Example 6. Comparative Example 14 had a lower volumetric energy density of 1284 Wh / L compared to Example 6. This is because the median diameter of the NCM particles was reduced, resulting in an arrangement in which NCM particles were present around the LMFP particles, and the conductive path provided by the coating of the LMFP particles was interrupted by the surrounding NCM particles, making it impossible to form a good conductive path.

[0083] In Comparative Example 15, the NCM particles were changed to Particles 1-2, the LMFP particles were changed to Particles 2-2, and the weight ratio of the positive electrode active material was changed compared to Example 6. In Comparative Example 15, the volumetric energy density and charge capacity ratio at 5 C were lower at 1275 Wh / L and 11.8%, respectively, compared to Example 6. This is because the median diameter of the LMFP particles was clearly smaller than the median diameter of the NCM particles, which increased the total specific surface area of ​​the particles and reduced the usability of the active material.

[0084] Furthermore, Example 9 has a higher electrode density than Examples 1, 7, and 8. Compared to Example 6, Example 9 has a higher volumetric energy density, but the charge capacity ratio at 5C is slightly lower at 45.5%.

[0085] Examples 10 and 18 have lower electrode densities than Examples 1, 7, and 8. Examples 10 and 18 have higher volumetric energy densities than Example 6, but the charge capacity ratios at 5 C are slightly lower at 48.8% and 46.0%, respectively.

[0086] Furthermore, Examples 11 and 12 have a lower basis weight than Example 1. Compared to Example 1, Examples 11 and 12 have the same volumetric energy density and a higher charge capacity ratio at 5C.

[0087] Furthermore, Examples 15 and 16 have a higher basis weight than Examples 1, 13, and 14. Although Examples 15 and 16 have the same volumetric energy density as Examples 1, 13, and 14, they have a lower charge capacity ratio at 5C.

[0088] Examples 1 to 18 have relatively high volumetric energy density and high charge capacity retention at 5 C compared to Comparative Examples 1 to 15. That is, it can be said that Examples 1 to 18 achieve both high volumetric energy density and good charge rate characteristics.

[0089] The present invention provides a positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery that can achieve both high volumetric energy density and good charge rate characteristics, and therefore shows industrial applicability in the manufacture and sale of non-aqueous electrolyte secondary batteries. [Explanation of symbols]

[0090] 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≦x≦0.9, 0<y≦1, 0<x+y≦1) ... (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, The conductive additive has a specific surface area of ​​100 m ２ / g or more of a carbon-based material in an amount of 0.4 wt % or more and 3.0 wt % or less relative to the total weight of the positive electrode mixture layer, The weight ratio of the first positive electrode active material in the positive electrode mixture layer is w Ｎ , the median diameter is D Ｎ５０ , tap density is t Ｎ , the weight ratio of the second positive electrode active material in the positive electrode mixture layer is w Ｌ , the median diameter is D Ｌ５０ , tap density is t Ｌ When the value represented by the following formula (3) is 0.2 or more and 100 or less, (w Ｌ / (t Ｌ ×D Ｌ５０ ３ )) / (w Ｎ / (t Ｎ ×D Ｎ５０ ３ ))・・・(3) A positive electrode for a non-aqueous electrolyte secondary battery. 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, 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 conductive additive includes carbon nanotubes.

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

3. The positive electrode mixture layer is The coating is formed on the positive electrode current collector, The electrode density is 2.5 g / cc or more and 3.2 g / cc or less.

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

4. The positive electrode mixture layer has a weight per unit area of ​​5 mg / cm ２ 30mg / cm or more ２ Below is the 2. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1.

5. The carbon nanotubes have an average fiber outer diameter of 5 nm to 11 nm, an average length of 5 μm to 30 μm, and a specific surface area of ​​250 m ２ / g or more 350m ２ / g or less, 3. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 2.

6. The positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 5, 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:

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