JP7765221B2 - Positive electrode for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery - Google Patents

Description

The present invention relates to a positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery.

In non-aqueous electrolyte secondary batteries, ions in the electrolyte are responsible for electrical conduction. Lithium secondary batteries, one type of non-aqueous electrolyte secondary battery, are used as power sources in compact electronic devices such as digital cameras and laptop computers, as well as in vehicles. Lithium secondary batteries are composed of a positive electrode, a negative electrode, and a separator. The positive electrode comprises a positive electrode current collector and a positive electrode mixture layer coated on the surface of the positive electrode current collector and containing a positive electrode active material, a conductive material, and a binder.

Positive electrode active materials for nonaqueous electrolyte secondary batteries include _{LiFePO4 or an olivine compound in which a portion of the LiFePO4} is substituted with Mn; _LiCoO2 , LiNiO2, _{LiMnO2 , LiCoNiO2} , LiCoMO2, and _{LiNiMO2 ,} in which some elements of these positive electrode active materials are substituted with different elements; and ternary layered compounds containing Co, Ni, or Mn. Layered compounds, in particular, are suitable for applications where energy density is important because of their high capacity and high voltage. However, layered compounds have poor thermal stability in the charged state, making it difficult to suppress heat generation during short-circuiting when used in nonaqueous electrolyte secondary batteries. Therefore, to ensure the safety of nonaqueous electrolyte secondary batteries, a technique has been developed in which layered compounds are mixed with _LiFePO4 or an olivine compound in which a portion of the LiFePO4 is substituted with Mn. Among these, it is known that an olivine compound in which part of _LiFePO4 is substituted with Mn has an operating potential that is not significantly different from that of a layered compound, and therefore when mixed with other compounds, it is easier to ensure safety without significantly reducing the energy density.

For example, in Patent Document 1, to achieve thermal stability, a layered compound such as lithium metal oxide is mixed with an olivine compound that accounts for 5 to 100% by volume of the positive electrode active material and has particle diameters of approximately 0.1 to 3 μm. Also, in Patent Document 2, a layered compound is mixed with an olivine compound that has an average particle diameter smaller than that of the layered compound in a positive electrode active material.

Patent No. 6813487 Patent No. 5574239

However, after further investigations, the inventors of the present application found that these prior art technologies may not be sufficient in terms of the safety against sudden short circuits that is required of recent non-aqueous electrolyte secondary batteries. In particular, when multiple non-aqueous electrolyte secondary batteries are connected in parallel to form a battery pack, the resistance of the entire battery pack is low, and therefore, even if the same short circuit occurs, short-circuit current tends to flow more suddenly than when a single non-aqueous electrolyte secondary battery is used. Therefore, countermeasures have been sought. Specifically, if the temperature of a non-aqueous electrolyte secondary battery becomes so high that separator meltdown is certain to occur, further short circuits will occur within the non-aqueous electrolyte secondary battery, leading to a chain reaction of heat generation. Therefore, there is a need for the development of a non-aqueous electrolyte secondary battery that can suppress heat generation even when a sudden short circuit occurs.

The present invention was made in light of the above, and aims to provide a positive electrode for a nonaqueous electrolyte secondary battery and a nonaqueous electrolyte secondary battery that suppresses heat generation even when a sudden short circuit occurs.

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 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 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 material, wherein a ratio of the specific surface area of the first positive electrode active material to the specific surface area of the second positive electrode active material is 0.005 or more and 0.025 or less.
Li _a Ni _x Co _y M1 _1-x-y O ₂ ...(1)
LiMn _z M2 _b Fe _1-z-b PO ₄ ...(2)
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, and satisfies 0<a≦1.2, 0<x≦0.9, 0<y≦0.5, and 0<x+y<1;
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, and satisfies 0<z≦0.9, 0≦b≦0.1, and 0<z+b<1.

Furthermore, in the positive electrode for a nonaqueous electrolyte secondary battery according to the present invention, in the above invention, the second positive electrode active material may have a tap density of 0.7 g/cc or more and 1.00 g/cc or less.

Furthermore, in the positive electrode for a nonaqueous electrolyte secondary battery according to the present invention, in the above invention, the median diameter of the second positive electrode active material may be 1/100 or more and 1/5 or less of the median diameter of the first positive electrode active material.

Furthermore, in the positive electrode for a nonaqueous electrolyte secondary battery according to the present invention, in the above invention, the median diameter of the second positive electrode active material may be 0.1 μm or more and 1.0 μm or less.

Furthermore, in the positive electrode for a nonaqueous electrolyte secondary battery according to the present invention, in the above invention, the weight ratio 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 may be 10% or more and 30% or less.

The nonaqueous electrolyte secondary battery according to the present invention also includes the positive electrode for a nonaqueous electrolyte secondary battery according to the above invention, a negative electrode, a separator, and a nonaqueous electrolyte solution containing a lithium salt and a nonaqueous solvent.

The present invention makes it possible to obtain a positive electrode for a nonaqueous electrolyte secondary battery and a nonaqueous electrolyte secondary battery that suppresses heat generation even when a sudden short circuit occurs.

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 a first embodiment of the present invention. 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 a second embodiment of the present invention.

The following describes an embodiment of the present invention, but the present invention is not limited to the following description. Furthermore, various modifications and improvements can be made to this embodiment, and such modifications and improvements may also be included in the present invention.

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 also not particularly limited and known materials such as laminate film, aluminum, aluminum alloy, and stainless steel can be used.

(Embodiment 1)
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, a negative electrode, and a separator.

The non-aqueous electrolyte secondary battery 1 includes a bag-shaped exterior body 2 made of laminate film. The exterior body 2 houses a laminated electrode group 3. 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 consists of two laminate films stacked together with the heat-sealable resin films facing each other. The electrode group 3 and non-aqueous electrolyte are housed between these laminate films, and 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.

The electrode group 3 includes 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 in the outermost layer and the separator 6 is located between the negative electrode 5 and the inner surface of the exterior body 2.

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 sides of the positive electrode current collector 41.

The positive electrode current collector 41 is made of aluminum, nickel, stainless steel, titanium, or other alloys. Among these, aluminum is preferred from the standpoint of electronic conductivity and battery operating potential.

The positive electrode mixture layer 42 includes a first positive electrode active material, a second positive electrode active material, a conductive material, and a binder.
The first and second positive electrode active materials are each capable of absorbing and desorbing lithium.

The first positive electrode active material is a layered compound represented by the following general formula (1): The layered compound is a lithium (Li), nickel (Ni), and cobalt (Co)-containing composite metal oxide composed of layers of sheet-like particles.
Li _a Ni _x Co _y M1 _1-x-y O ₂ ...(1)
In the general 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), yttrium (Y), or 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), and the following conditions are satisfied: 0<a≦1.2, 0<x≦0.9, 0<y≦0.5, 0<x+y<1.

The second positive electrode active material is represented by the general formula shown in the following formula (2), and is a compound in which a coating film made of a carbon material is formed on the surface of a phosphate compound (olivine-based compound) having an olivine structure.
LiMn _z M2 _b Fe _1-z-b PO ₄ ...(2)
In the general formula (2), M2 is at least one selected from nickel (Ni), cobalt (Co), titanium (Ti), copper (Cu), zinc (Zn), magnesium (Mg), zirconium (Zr), calcium (Ca), yttrium (Y), molybdenum (Mo), barium (Ba), lead (Pb), bismuth (Bi), lanthanum (La), cerium (Ce), neodymium (Nd), gadolinium (Gd), aluminum (Al), gallium (Ga), and strontium (Sr), and satisfies 0<z≦0.9, 0≦b≦0.1, and 0<z+b<1.

The carbon material is an electrically conductive carbon material. The second positive electrode active material has at least some of the surfaces of the primary particles coated with a coating of the carbon material present on the surface of the second positive electrode active material. The purity of the carbon and the thickness of the carbon coating can be selected as desired, but it is preferable to control the weight ratio of the carbon material to the total weight of the second positive electrode active material so that it is 0.1% or more and 5% or less.

In prior art such as those described in Patent Documents 1 and 2, attention has often been focused on the particle size of the first and second positive electrode active materials to suppress heat generation during a short circuit. However, through extensive research, the inventors of the present application have discovered that the specific surface area, rather than the particle size, of each positive electrode active material is important. Generally, even positive electrode active materials with roughly equivalent particle sizes can have significantly different specific surface areas depending on their shape. Therefore, simply controlling particle size alone does not produce the desired results. Here, assuming that the specific surface area of the first positive electrode active material is S1 and the specific surface area of the second positive electrode active material is S2, the ratio of the specific surface area S1 to the specific surface area S2 (S1/S2) is 0.005 or greater and 0.025 or less. More preferably, it is 0.010 or greater and 0.020 or less. When the ratio S1/S2 is 0.025 or less, i.e., when the specific surface area of the second positive electrode active material is significantly larger than that of the first positive electrode active material, the second positive electrode active material tends to surround the first positive electrode active material in the positive electrode mixture layer 42. Therefore, even during a sudden short circuit, the exothermic reaction between the first positive electrode active material and the nonaqueous electrolyte can be minimized, thereby suppressing heat generation in the nonaqueous electrolyte secondary battery. Note that the specific surface area is the surface area per unit area. When the ratio S1/S2 is less than 0.005, i.e., when the specific surface area of the second positive electrode active material is significantly larger than that of the first positive electrode active material, the dispersibility of the second positive electrode active material decreases and the second positive electrode active material is more likely to aggregate during the preparation of the slurry for the positive electrode mixture layer 42. Because aggregation deteriorates the mixed state of the first and second positive electrode active materials, it is believed that the second positive electrode active material is unable to surround the first positive electrode active material as tightly as conventional materials, and heat generation during a short circuit cannot be sufficiently suppressed. Furthermore, when the ratio S1/S2 is greater than 0.025, it is believed that the second positive electrode active material is unable to surround the first positive electrode active material as tightly as conventional materials, and heat generation during a short circuit cannot be sufficiently suppressed.

Furthermore, the tap density of the second positive electrode active material is preferably 0.7 g/cc or more and 1.00 g/cc or less, and more preferably 0.8 g/cc or more. When the tap density is 1.00 g/cc or less, the primary particles of the second positive electrode active material do not form high-density granules or secondary particles but have a bulky shape. This is thought to enable them to surround the first positive electrode active material more tightly than conventional particles, thereby suppressing heat generation due to thermal runaway during short circuit. As a result, not only is the temperature of the non-aqueous electrolyte secondary battery less likely to rise during short circuit, but swelling, cracking, and rupture due to electrolyte decomposition on the surface of the first positive electrode active material caused by temperature rise are also less likely to occur. On the other hand, when the tap density is greater than 1.00 g/cc, the second positive electrode active material is in a high-density granular state, making it difficult for it to surround the first positive electrode active material. This makes it difficult to prevent temperature rise in the non-aqueous electrolyte secondary battery during short circuit, and makes swelling, cracking, and rupture more likely to occur. Furthermore, if the tap density is less than 0.70 g/cc, the dispersibility of the second positive electrode active material decreases when the slurry for the positive electrode mixture layer 42 is prepared, making it more likely to aggregate, which can lead to streaks during positive electrode preparation, making it difficult to produce positive electrodes of uniform quality. Furthermore, if the tap density is 0.80 g/cc or higher, the dispersibility of the second positive electrode active material during slurry preparation is improved.

The median diameter of the second positive electrode active material is preferably 1/100 to 1/5 of the median diameter of the first positive electrode active material. Furthermore, the median diameter of the second positive electrode active material is preferably 0.1 μm to 1.0 μm. When these requirements are met, it is believed that the second positive electrode active material can surround the first positive electrode active material more tightly than conventional materials, further suppressing heat generation due to thermal runaway during a short circuit. Therefore, not only is the temperature of the nonaqueous electrolyte secondary battery less likely to rise during a short circuit, but swelling, cracking, or rupture due to electrolyte decomposition on the surface of the first positive electrode active material due to temperature rise is also less likely to occur. If the median diameter of the second positive electrode active material is significantly smaller than that of the first positive electrode active material, the second positive electrode active material will be less dispersible during the preparation of the slurry for the positive electrode mixture layer 42, and will likely aggregate, leading to streaking during the preparation of the positive electrode, making it difficult to produce a positive electrode of uniform quality. Furthermore, if the median diameter of the second positive electrode active material is less than 0.1 μm, the crystallinity of the second positive electrode active material itself will likely be extremely low, making material synthesis itself difficult. If the median diameter of the second positive electrode active material is 1/20 or more (1/5 or less) of the median diameter of the first positive electrode active material, or if the median diameter of the second positive electrode active material is 0.5 μm or more, the dispersibility of the second positive electrode active material during the preparation of the slurry will be better. Note that the median diameter is sometimes referred to as D50 throughout this specification.

Furthermore, the weight ratio 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 is preferably 10% or more and 30% or less. If the weight ratio of the second positive electrode active material exceeds 30%, the charge/discharge curve of the non-aqueous electrolyte secondary battery 1 will tend to have more steps (the number of inflection points will increase), making it difficult to estimate the depth of charge and state of degradation from the voltage and current values during charge/discharge, reducing practicality. Furthermore, if the weight ratio of the second positive electrode active material is less than 10%, it will be difficult for the second positive electrode active material to sufficiently surround the first positive electrode active material.

The above-mentioned median diameter, tap density, and specific surface area refer to the values within the positive electrode mixture layer 42 after the nonaqueous electrolyte secondary battery has been fabricated. However, these values are roughly equivalent to the median diameter, tap density, and specific surface area of the first and second positive electrode active materials before the positive electrode is fabricated. Therefore, if the median diameter, tap density, and specific surface area of the first and second positive electrode active materials before fabrication satisfy the requirements of the present invention, the fabricated positive electrode can be considered to also satisfy the requirements of the present invention.

Whether the median diameter and tap density of the first and second positive electrode active materials in the fabricated positive electrode mixture layer satisfy the requirements of the present invention can be confirmed, for example, by the following method. The nonaqueous electrolyte secondary battery is disassembled in an argon-filled glove box to remove the positive electrode 4. The positive electrode 4 is washed with an appropriate solvent (e.g., dimethyl carbonate) and then vacuum-dried to remove the solvent. The positive electrode is then immersed in a solvent such as N-methyl-2-pyrrolidone and subjected to ultrasonic waves to separate 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 is centrifuged to separate the individual substances within the positive electrode mixture layer 42. The median diameter of the separated first and second positive electrode active materials can be measured, for example, by laser diffraction/scattering. The tap density can be measured, for example, by placing the materials in a container, tapping the container, and measuring the weight divided by the volume of the particles. The nonaqueous electrolyte secondary battery used above may have undergone initial activation or charge/discharge cycling in any step, and it is preferable to set the charge rate of the nonaqueous electrolyte secondary battery to 0% before removing the positive electrode 4.

The conductive material assists in the conduction of electrons in the positive electrode. There are no particular limitations on the conductive material, and any known material can be used. Examples of conductive materials include conductive carbon powders such as acetylene black and carbon black (e.g., ketjen black), carbon nanotubes, carbon nanofibers, graphene, activated carbon, and graphite. The conductive material may consist of a single type of material, or multiple types of materials (e.g., a first conductive material and a second conductive material).

The binder binds the positive electrode current collector, positive electrode active material, and conductive material together. 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 materials include polyvinyl alcohol (PVA), carboxymethylcellulose (CMC), butyl rubber, poly(meth)acrylate (PMMA), polyethylene oxide (PEO), polypropylene oxide (PO), polyepichlorohydrin, polyphosphazene, polyacrylonitrile, hexafluoropropylene (HFP), and copolymers thereof, or a mixture of two or more thereof.

The 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 or metallic lithium (not shown) formed on one or both sides of the negative electrode current collector 51.

The negative electrode current collector 51 is not particularly limited, but it is preferable to use a metal. Suitable metals include aluminum foil and copper, and depending on the application, porous aluminum current collectors may also be used. Among these, copper is preferred from the standpoint of electronic conductivity and battery operating potential.

The negative electrode mixture layer 52 contains, as an active material, at least one selected from the group consisting of _lithium , a lithium alloy _, a titanium- _niobium alloy, graphite, amorphous carbon, a transition metal composite oxide (e.g., _Li4Ti5O12 or _TiNb2O7 ), an alloy 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. The negative electrode mixture layer 52 also contains a conductive material and a binder. The conductive material and binder may be the same materials as those used in the positive electrode 4.

Separator 6 is disposed between the positive and negative electrodes and is porous enough to allow the components of the non-aqueous electrolyte to pass through. Separator 6 is constructed using, for example, a porous sheet separator made of polymer or fiber, or a nonwoven fabric separator. Separator 6 may be made of materials such as polyethylene, polypropylene, aramid, or polyimide, or may have multiple layers of different materials including these. However, it is preferable for separator 6 to have a layer containing polyethylene in order to provide a shutdown function in the event of heat generation. Separator 6 preferably has a pore size of 0.01 to 10 μm and a thickness of 5 to 30 μm. Separator 6 may also be constructed by laminating a ceramic layer as a heat-resistant insulating layer on a porous substrate. If a solid electrolyte is used for the non-aqueous electrolyte, separator 6 may not be present.

The positive electrode leads 7 each extend from the positive electrode mixture layer 42, for example, downward as shown in Figure 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 mixture layer 42. The positive electrode leads 7 are bundled and joined together at the end opposite the positive electrode mixture layer 42 side within the exterior body 2.

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

The negative electrode leads 9 each extend upward from the negative electrode mixture layer 52, for example, as shown in Figure 1. They may extend in the same direction as the positive electrode lead 7, provided 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 mixture layer 52. The negative electrode leads 9 are bundled and joined together at the end opposite the negative electrode mixture layer 52 side within the exterior body 2.

One end of the negative electrode tab 10 is joined to the negative electrode lead 9, and the other end extends to the outside through the sealing portion of the outer casing 2.

A nonaqueous electrolyte solution or a solid electrolyte can be used as the nonaqueous electrolyte. This section will focus on nonaqueous electrolyte solutions. The nonaqueous electrolyte solution is sealed within the exterior case 2. The nonaqueous electrolyte solution injection portion of the exterior case 2 is sealed after the nonaqueous electrolyte solution is injected. The nonaqueous electrolyte solution contains an electrolyte and a nonaqueous solvent.

The electrolyte is not particularly limited, and lithium salts commonly used in nonaqueous electrolyte secondary batteries can be used. For example, _LiPF6 , _LiAsF6 , LiBF4, _LiCF3SO3 , _LiN ( _{CmF2m + 1SO2} )( _{CnF2n + 1SO2} ) (m and n are integers of 1 or more), LiC( _{CpF2p + 1SO2} )(CqF2q _{+ 1SO2} )( _{CrF2r + 1SO2} ) (p, _q , r are integers of 1 or more), lithium difluoro(oxalato)borate, lithium bisoxalatoborate, etc. can be used. These electrolytes can be used alone or in combination of two or _more . In addition, from the viewpoint of lithium ion conductivity, viscosity of the electrolyte, 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.

The non-aqueous solvent contains a cyclic carbonate and/or a chain carbonate as its 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.

Furthermore, additives other than the lithium salt may be contained 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 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, Li ₂ PO ₂ F ₂ , and the like. 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.

In the present embodiment 1, the positive electrode 4 of the nonaqueous electrolyte secondary battery 1 includes a positive electrode mixture layer 42 having a first positive electrode active material represented by general formula (1) and a second positive electrode active material containing a compound represented by general formula (2), with a specific surface area S1 and a ratio S1/S2 of the specific surface area S1 of the first positive electrode active material to the specific surface area S2 of the second positive electrode active material, both of which are 0.005 or more and 0.025 or less. By satisfying the above conditions in the positive electrode mixture layer 42, the second positive electrode active material coats the first positive electrode active material, suppressing electrolyte decomposition on the surface of the first positive electrode active material, thereby reducing the likelihood of thermal runaway during a short circuit. According to the present embodiment 1, a positive electrode 4 and a nonaqueous electrolyte secondary battery 1 can be obtained that suppress heat generation even when a sudden short circuit occurs.

(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 mixture layer 113, separator 114, negative electrode 115, gasket 116, and cap 117. Positive electrode current collector 112 and positive electrode mixture layer 113 form positive electrode 118.

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

The positive electrode current collector 112 is made of the same material as the positive electrode current collector 41 .
The positive electrode mixture layer 113 has the same configuration as the positive electrode mixture layer 42 .

Separator 114 is disposed between the positive electrode and negative electrode 115 and is a porous, disc-shaped separator. Separator 114 has the same structure as separator 6.

The nonaqueous electrolyte described in embodiment 1 can be used as the nonaqueous electrolyte.

Anode 115 has the same configuration as cathode 5.

In the present second embodiment, the positive electrode 118 of the nonaqueous electrolyte secondary battery 1A includes a positive electrode mixture layer 113 having a first positive electrode active material represented by general formula (1) and a second positive electrode active material containing a compound represented by general formula (2), with a specific surface area S1 and a ratio S1/S2 of the specific surface area S1 of the first positive electrode active material to the specific surface area S2 of the second positive electrode active material of 0.005 or more and 0.025 or less. According to the second embodiment, as in the first embodiment, it is possible to obtain a positive electrode 118 and a nonaqueous electrolyte secondary battery 1A that suppress heat generation even when a sudden short circuit occurs.

The present invention will be explained in more detail below using examples, but the present invention is not limited to these examples in any way.

<Method for producing positive electrode>
A positive electrode active material slurry was prepared by _{mixing 75.2 wt % of LiNi0.5Co0.2Mn0.3O2 ( NCM} ) as the first positive _electrode active material, 18.8 wt % _{of LiMn0.7Fe0.3PO4} (LMFP) as the second positive electrode active material, 2 wt % of graphite as the _first conductive material, 3 wt % of acetylene black as the second conductive material, 1 wt % of polyvinylidene fluoride (PVDF) as the binder, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) as a viscosity adjusting solvent.

The obtained positive electrode active material slurry was applied to both sides of a 20 μm thick aluminum foil serving as a positive electrode current collector and dried to form a positive electrode mixture layer. The amount of the positive electrode mixture layer applied per side was 99 g/m ^2. The positive electrode was then pressed to set the density of the positive electrode mixture layer to 2.7 g/cm ^3. Thereafter, the positive electrode mixture layer was cut so that the uncoated portion protruded in a rectangular shape from one side of the rectangular portion coated with the positive electrode mixture layer as a positive electrode lead. The protruding portion did not have a positive electrode mixture layer formed thereon and functioned as a positive electrode lead.

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

The prepared negative electrode active material slurry was applied to both sides of a 10 μm thick copper foil serving as a negative electrode current collector and dried to form a negative electrode mixture layer, thereby producing a negative electrode. The coating amount of the negative electrode mixture layer per side at this time was 58 g/m ^2. Next, the negative electrode was subjected to press working to set the density of the negative electrode mixture layer to 1.2 g/cm ^3. Thereafter, the uncoated portion was cut so that a rectangular protrusion was formed from one side of the rectangular portion where the negative electrode mixture layer was formed. The protruding portion did not have a negative electrode mixture layer formed thereon and functioned as a negative electrode lead.

<Preparation of non-aqueous electrolyte>
A nonaqueous 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.

<Fabrication of electrode element>
Next, an electrode element was fabricated by alternately stacking a positive electrode having a positive electrode current collector lead and a negative electrode having a negative electrode current collector lead on a separator connected in a zigzag pattern. The separator used was a polyethylene substrate layer with a polypropylene surface layer on both sides (PE/PP/PE). The separator had a thickness of 20 μm. The positive and negative electrode leads were then bundled together, and a positive electrode terminal was connected to the bundled positive electrode lead by ultrasonic welding, and a negative electrode terminal was connected to the bundled negative electrode lead by ultrasonic welding. The fabricated electrode element had a thickness of 3.0 mm and a rated capacity of 4.8 Ah. The term "rated capacity" refers to the discharge capacity measured when a constant-current-constant-voltage charge (cutoff current: 0.05 C) was performed at an upper limit voltage of 4.2 V and a current value of 0.5 C, followed by a constant-current discharge at a lower limit voltage of 2.7 V and a current value of 0.2 C.

<Fabrication of Non-Aqueous Electrolyte Secondary Battery>
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 accommodated in the two accommodating recesses. The electrode groups were arranged so that the portions of the heat-sealable resin portions of each terminal passed between the edges of the two laminate films, leaving portions of each terminal exposed to the outside. In this state, the heat-sealable resin layers of the laminate films were heat-sealed along three edges, including the two edges from which the tabs of the laminate films extended. The electrolyte solution prepared above was then injected into the unheat-sealed edge of the exterior body. The remaining edge of the exterior body was then heat-sealed under reduced pressure to produce a nonaqueous electrolyte secondary battery (cell).

<Calculation of specific surface area>
The specific surface area of each positive electrode active material was calculated by applying the BET equation to the values measured by the nitrogen adsorption method. The specific surface area of NCM was designated as S1, and the specific surface area of LMFP was designated as S2.

<Tap density>
As described in JIS standard Z2504:2020, powdered LMFP was placed in a container. The container was then tapped 100 times to measure the weight divided by the volume of the particles to fill the gaps between them, and this value was used as the tap density. A shaking specific gravity measuring instrument was used for the measurement.

<Measurement of particle size>
The median diameters 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. A laser diffraction particle size distribution analyzer SALD-2300 (manufactured by Shimadzu Corporation) was used for the measurement.

<Nail penetration test>
The fabricated nonaqueous electrolyte secondary battery (cell) was previously subjected to constant-current-constant-voltage charging (cutoff current: 0.05 C) at a maximum voltage of 4.2 V and a current value of 0.5 C. A nail (stainless steel, 3 mm diameter) was inserted into the center of the cell at a nail penetration rate of 0.1 mm/s and a nail penetration depth of 3.0 mm, just before penetration. The maximum surface temperature of the cell after nail penetration (hereinafter referred to as "surface temperature") was measured. The cell's appearance was also checked one hour after nail penetration to confirm the presence or absence of cracking other than the nail penetration area. This test was based on the international standard IEC TR 62660-4, but the nail penetration depth was increased to facilitate heat generation compared to the international standard IEC TR 62660-4, resulting in a simultaneous short circuit of more positive and negative electrodes. This test simulated a very severe short circuit condition.

Example 1
A nonaqueous electrolyte secondary battery was fabricated using a positive electrode having the physical properties shown in Table 1 as the positive electrode mixture layer. The ratio of the specific surface areas of NCM and LMFP (hereinafter simply referred to as the "ratio") (S1/S2) was 0.018. After a nail penetration test, the cell did not split, and the surface temperature was 78°C.

Example 2
A nonaqueous electrolyte secondary battery was fabricated using a positive electrode having the physical properties shown in Table 1 as the positive electrode mixture layer. The battery was the same as in Example 1 except that the specific surface area S1 was 0.23 m ² /g and the D50 of the NCM was 14.0 μm. The ratio (S1/S2) was 0.012. After the nail penetration test, the cell did not split and the surface temperature was 69°C.

Example 3
A nonaqueous electrolyte secondary battery was fabricated using a positive electrode having the physical properties shown in Table 1 as the positive electrode mixture layer. The battery was the same as in Example 1 except that the specific surface area S2 was 23.80 m ² /g, the tap density of the LMFP was 0.78 g/cc, and the D50 of the LMFP was 0.80 μm. The ratio (S1/S2) was 0.015. After the nail penetration test, the cell did not cleave, and the surface temperature was 81°C.

Example 4
A nonaqueous electrolyte secondary battery was fabricated using a positive electrode having the physical properties shown in Table 1 as the positive electrode mixture layer. The battery was the same as in Example 1 except that the specific surface area S2 was 18.5 m ² /g, the tap density of the LMFP was 1.04 g/cc, and the D50 of the LMFP was 1.00 μm. The ratio (S1/S2) was 0.019. After a nail penetration test, the cell split open, and the surface temperature was 124°C.

Example 5
A nonaqueous electrolyte secondary battery was fabricated using a positive electrode having the physical properties shown in Table 1 as the positive electrode mixture layer. The battery was the same as in Example 1 except that the specific surface area S1 was 0.39 m ² /g, the tap density of the LMFP was 0.90 g/cc, and the D50 of the NCM was 4.9 μm. The ratio (S1/S2) was 0.020. After the nail penetration test, the cell did not cleave, and the surface temperature was 86°C.

Example 6
A nonaqueous electrolyte secondary battery was fabricated using a positive electrode having the physical properties shown in Table 1 as the positive electrode mixture layer. The battery was the same as in Example 1 except that the specific surface area S1 was 0.42 m ² /g, the specific surface area S2 was 20.6 m ² /g, and the D50 of the NCM was 4.3 μm. The ratio (S1/S2) was 0.020. After a nail penetration test, the cell split open, and the surface temperature was 119°C.

Example 7
A nonaqueous electrolyte secondary battery was fabricated using a positive electrode having the physical properties shown in Table 1 as the positive electrode mixture layer. The battery was the same as in Example 1, except that the specific surface area S2 was 22.3 m ² /g, the tap density of the LMFP was 0.91 g/cc, the D50 of the NCM was 21.3 μm, and the D50 of the LMFP was 1.50 μm. The ratio (S1/S2) was 0.016. After a nail penetration test, the cell split open, and the surface temperature was 135°C.

(Example 8)
A nonaqueous electrolyte secondary battery was fabricated using a positive electrode having the physical properties shown in Table 1 as the positive electrode mixture layer. The battery was the same as in Example 1 except that the specific surface area S1 was 0.50 m ² /g and the D50 of the NCM was 8.1 μm. The ratio of the specific surface areas of the NCM and the LMFP (S1/S2) was 0.025. After the nail penetration test, the cell did not split and the surface temperature was 106°C.

Example 9
A nonaqueous electrolyte secondary battery was fabricated using a positive electrode having the physical properties shown in Table 1 as the positive electrode mixture layer. The battery was the same as in Example 1, except that the specific surface area S1 was 0.13 m ² /g, the specific surface area S2 was 23.8 m ² /g, the tap density of the LMFP was 0.78 g/cc, the D50 of the NCM was 18.3 μm, and the D50 of the LMFP was 0.80 μm. The ratio of the specific surface areas of the NCM and the LMFP (S1/S2) was 0.005. After the nail penetration test, the cell did not cleave, and the surface temperature was 103°C.

Example 10
A nonaqueous electrolyte secondary battery was fabricated using a positive electrode having the physical properties shown in Table 1 as the positive electrode mixture layer. The battery was the same as in Example 1, except that the specific surface area S2 was 25.6 m ² /g, the tap density of the LMFP was 0.79 g/cc, and the D50 of the LMFP was 0.50 μm. The ratio of the specific surface areas of the NCM and the LMFP (S1/S2) was 0.014. After the nail penetration test, the cell did not cleave, and the surface temperature was 86°C.

Example 11
A nonaqueous electrolyte secondary battery was fabricated using a positive electrode having the physical properties shown in Table 1 as the positive electrode mixture layer. The battery was the same as in Example 1, except that the proportion of LMFP was 5%. The ratio of the specific surface areas of NCM and LMFP (S1/S2) was 0.018. After a nail penetration test, the cell did not split, and the surface temperature was 136°C.

Example 12
A nonaqueous electrolyte secondary battery was fabricated using a positive electrode having the physical properties shown in Table 1 as the positive electrode mixture layer. The battery was the same as in Example 1, except that the proportion of LMFP was 10%. The ratio of the specific surface areas of NCM and LMFP (S1/S2) was 0.018. After a nail penetration test, the cell did not split, and the surface temperature was 107°C.

Example 13
A nonaqueous electrolyte secondary battery was fabricated using a positive electrode having the physical properties shown in Table 1 as the positive electrode mixture layer. The battery was the same as in Example 1, except that the proportion of LMFP was 30%. The ratio of the specific surface areas of NCM and LMFP (S1/S2) was 0.018. After a nail penetration test, the cell did not split, and the surface temperature was 69°C.

Example 14
A nonaqueous electrolyte secondary battery was fabricated using a positive electrode having the physical properties shown in Table 1 as the positive electrode mixture layer. The battery was the same as in Example 1, except that the specific surface area S1 was 0.23 m ² /g, the specific surface area S2 was 24.5 m ² /g, the tap density of the LMFP was 1.02 g/cc, the D50 of the NCM was 14.0 μm, and the D50 of the LMFP was 5.20 μm. The ratio of the specific surface areas of the NCM and the LMFP (S1/S2) was 0.009. After a nail penetration test, the cell split open, and the surface temperature was 151°C.

Example 15
A nonaqueous electrolyte secondary battery was fabricated using a positive electrode having the physical properties shown in Table 1 as the positive electrode mixture layer. The battery was the same as in Example 1, except that the specific surface area S1 was 0.23 m ² /g, the specific surface area S2 was 22.7 m ² /g, the tap density of the LMFP was 1.05 g/cc, the D50 of the NCM was 14.0 μm, and the D50 of the LMFP was 4.80 μm. The ratio of the specific surface areas of the NCM and the LMFP (S1/S2) was 0.010. After a nail penetration test, the cell split open, and the surface temperature was 146°C.

Example 16
A nonaqueous electrolyte secondary battery was fabricated using a positive electrode having the physical properties shown in Table 1 as the positive electrode mixture layer. The battery was the same as in Example 1, except that the specific surface area S1 was 0.23 m ² /g, the specific surface area S2 was 16.2 m ² /g, the tap density of the LMFP was 1.06 g/cc, the D50 of the NCM was 14.0 μm, and the D50 of the LMFP was 3.70 μm. The ratio of the specific surface areas of the NCM and the LMFP (S1/S2) was 0.014. After a nail penetration test, the cell split open, and the surface temperature was 139°C.

(Example 17)
A nonaqueous electrolyte secondary battery was fabricated using a positive electrode having the physical properties shown in Table 1 as the positive electrode mixture layer. The battery was the same as in Example 1, except that the specific surface area S1 was 0.23 m ² /g, the specific surface area S2 was 12.4 m ² /g, the tap density of the LMFP was 1.09 g/cc, the D50 of the NCM was 14.0 μm, and the D50 of the LMFP was 3.20 μm. The ratio of the specific surface areas of the NCM and the LMFP (S1/S2) was 0.019. After a nail penetration test, the cell split open, and the surface temperature was 143°C.

(Example 18)
A nonaqueous electrolyte secondary battery was fabricated using a positive electrode having the physical properties shown in Table 1 as the positive electrode mixture layer. The battery was the same as in Example 1, except that the specific surface area S1 was 0.23 m ² /g, the specific surface area S2 was 11.2 m ² /g, the tap density of the LMFP was 1.11 g/cc, the D50 of the NCM was 14.0 μm, and the D50 of the LMFP was 2.90 μm. The ratio of the specific surface areas of the NCM and the LMFP (S1/S2) was 0.021. After a nail penetration test, the cell split open, and the surface temperature was 152°C.

Example 19
A nonaqueous electrolyte secondary battery was fabricated using a positive electrode having the physical properties shown in Table 1 as the positive electrode mixture layer. The battery was the same as in Example 1, except that the specific surface area S2 was 25.2 m ² /g, the tap density of LMFP was 0.69 g/cc, and the D50 of LMFP was 0.80 μm. The ratio of the specific surface areas of NCM and LMFP (S1/S2) was 0.014. After a nail penetration test, the cell did not split, and the surface temperature was 87°C. However, the positive electrode had numerous streaks, making it unsuitable for continuous production and industrially undesirable.

(Example 20)
A nonaqueous electrolyte secondary battery was fabricated using a positive electrode having the physical properties shown in Table 1 as the positive electrode mixture layer. The specific surface area S1 was 0.21 m ² /g, the specific surface area S2 was 25.6 ^{m 2} /g, the tap density of LMFP was 0.79 g/cc, the D50 of NCM was 51.4 μm, and the D50 of LMFP was 0.50 μm. The ratio of the specific surface areas of NCM and LMFP (S1/S2) was 0.008. After a nail penetration test, the cell did not split, and the surface temperature was 75°C. However, the positive electrode had numerous streaks, making it unsuitable for continuous production and industrially undesirable.

(Comparative Example 1)
A nonaqueous electrolyte secondary battery was fabricated using a positive electrode having the physical properties shown in Table 1 as the positive electrode mixture layer. The battery was the same as in Example 1 except that the specific surface area S1 was 0.57 m ² /g and the D50 of the NCM was 6.2 μm. The ratio (S1/S2) was 0.029. After a nail penetration test, the cell split open and the surface temperature was 522°C.

(Comparative Example 2)
A nonaqueous electrolyte secondary battery was fabricated using a positive electrode having the physical properties shown in Table 1 as the positive electrode mixture layer. The battery was the same as in Example 1 except that the specific surface area S2 was 13.5 m ² /g, the tap density of the LMFP was 1.08 g/cc, and the D50 of the LMFP was 1.10 μm. The ratio (S1/S2) was 0.027. After a nail penetration test, the cell split open, and the surface temperature was 451°C.

(Comparative Example 3)
A nonaqueous electrolyte secondary battery was fabricated using a positive electrode having the physical properties shown in Table 1 as the positive electrode mixture layer. The battery was the same as in Example 1, except that the specific surface area S1 was 0.13 m ² /g, the specific surface area S2 was 30.1 m ² /g, the D50 of the NCM was 18.3 μm, the tap density of the LMFP was 1.04 g/cc, and the D50 of the LMFP was 2.40 μm. The ratio (S1/S2) was 0.004. After a nail penetration test, the cell split open, and the surface temperature was 498°C.

As shown in Table 1, in Examples 1 to 20, the surface temperature after nail penetration was 152°C or lower. On the other hand, in Comparative Examples 1 to 3, the cells split and the surface temperature was high, at 451°C or higher. It is believed that in Comparative Examples 1 to 3, a separator meltdown occurred, causing a short circuit inside the cell and resulting in a chain reaction of heat generation. From these results, it can be said that Examples 1 to 20 suppressed heat generation even when a sudden short circuit occurred and exhibited thermal stability. Among these, Examples 1 to 3, 5, 8 to 10, 12, and 13 did not pose any industrial problems and no cell splitting was observed, so they can be said to have particularly excellent thermal stability.

REFERENCE SIGNS LIST 1, 1A non-aqueous electrolyte secondary battery 2 exterior body 3 electrode group 4, 118 positive electrode 5, 115 negative electrode 6, 114 separator 7 positive electrode lead 8 positive electrode tab 9 negative electrode lead 10 negative electrode tab 41, 112 positive electrode current collector 42, 113 positive electrode mixture layer 51 negative electrode current collector 52 negative electrode mixture layer 110 case 111 leaf spring 116 gasket 117 cap

Claims (6)

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 (0<a≦1.2, 0<x≦0.9, 0<y≦0.5, 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 PO ₄ (0<z≦0.9, 0≦b≦0.1, 0<z+b<1)...(2)
A conductive material;
Including,
a ratio of the specific surface area of the first positive electrode active material to the specific surface area of the second positive electrode active material is 0.005 or more and 0.025 or less;
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, 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. The second positive electrode active material has a tap density of 0.7 g/cc or more and 1.00 g/cc or less.
2. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1. the median diameter of the second positive electrode active material is 1/100 or more and 1/5 or less of the median diameter of the first positive electrode active material;
3. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2. The median diameter of the second positive electrode active material is 0.1 μm or more and 1.0 μm or less.
4. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1. a 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 is 10% or more and 30% or less;
5. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1. 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: