Positive electrode for secondary batteries, and secondary battery

Abstract

A positive electrode mixture layer of this positive electrode for secondary batteries contains a positive electrode active material, a binder, a conductive material, and an ion-binding compound. The positive electrode active material contains lithium-containing transition metal oxide particles and a boron compound that is present on the surfaces of the lithium-containing transition metal oxide particles. The binder contains a polyvinylidene fluoride. The difference in electronegativity between the cation of the ion-binding compound and oxygen is 1.7 or more. The content ratio of the polyvinylidene fluoride is 1 mass% or less. When the positive electrode mixture layer is subjected to elemental analysis by EPMA, the ratio of the number of aggregates of a specific element indicating the ion-binding compound detected overlapping elemental F detected in a region from a current collector to 50% of the total thickness of the positive electrode mixture layer to the total number of aggregates of the specific element indicating the ion-binding compound detected in the region is 70% or more.

Description

Positive electrode for secondary batteries and secondary batteries

[0001] This disclosure relates to the technology of a positive electrode for a secondary battery and a secondary battery.

[0002] In recent years, secondary batteries that provide high power output and high energy density, comprising a positive electrode, a negative electrode, and an electrolyte, and that charge and discharge by moving lithium ions between the positive and negative electrodes, have become widely used.

[0003] For example, Patent Document 1 discloses a positive electrode for a non-aqueous electrolyte secondary battery, comprising a first particle and a second particle as the positive electrode, wherein the first particle comprises an electrochemically active positive electrode material, the positive electrode active material comprises a lithium-containing transition metal oxide, the second particle comprises an electrochemically inert metal oxide, and an electrochemically inert phosphate is attached to the surface of the second particle.

[0004] Furthermore, for example, Patent Document 2 discloses a positive electrode for a non-aqueous electrolyte secondary battery, comprising a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector, wherein the positive electrode mixture layer contains a positive electrode active material comprising a lithium-containing transition metal composite oxide having a layered rock salt structure and a phosphate, the lithium-containing transition metal composite oxide contains at least Ni, Al, and at least one of Ca or Sr, the Ni content in the lithium-containing transition metal composite oxide is 75 mol% or more with respect to the total amount of metal elements excluding Li, and the phosphate content in the positive electrode mixture layer is 0.1 to 5 parts by mass when the content of the positive electrode active material is 100 parts by mass.

[0005] International Publication No. 2018 / 123603, International Publication No. 2022 / 138031, Japanese Patent Publication No. 2021-72193

[0006] Incidentally, the positive electrode mixture layer that constitutes the positive electrode is manufactured by applying a positive electrode mixture slurry containing, for example, a positive electrode active material, polyvinylidene fluoride (PVDF) as a binder, and a conductive material onto the positive electrode current collector. However, when lithium-containing transition metal oxide particles with a boron compound attached to the particle surface are used as the positive electrode active material, PVDF becomes difficult to adhere to the particle surface. Also, in the positive electrode mixture slurry, PVDF is trapped inside aggregates of the conductive material. As a result of these factors, the adhesion area between the positive electrode active material and PVDF decreases, and the peel strength of the positive electrode mixture layer formed on the positive electrode current collector decreases. To simply improve the peel strength, it is conceivable to increase the PVDF content, but doing so may increase the initial viscosity of the positive electrode mixture slurry, leading to coating defects such as uneven coating. Therefore, increasing the PVDF content is not desirable.

[0007] Therefore, the present disclosure aims to provide a positive electrode for secondary batteries in which the peel strength of the positive electrode mixture layer is improved even with a low PVDF content.

[0008] A positive electrode for a secondary battery according to one aspect of the present disclosure comprises a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector, wherein the positive electrode mixture layer comprises a positive electrode active material, a binder, a conductive material, and an electrochemically inert ionic compound, the positive electrode active material comprises lithium-containing transition metal oxide particles and a boron compound present on the surface of the lithium-containing transition metal oxide particles, the binder comprises polyvinylidene fluoride (PVDF), and the ionic compound comprises a cation and oxygen, wherein the difference in electronegativity between the cation and oxygen is 1.7 or greater. The average particle size of the ionic compound is 3 μm or less, the content of polyvinylidene fluoride is 1% by mass or less relative to the total mass of the positive electrode active material, and when the positive electrode mixture layer is elementally analyzed by an electron beam microanalyzer (EPMA), the ratio of the number of aggregates of the specific element indicating the ionic compound detected in overlap with the F element detected in the region up to 50% of the total thickness of the positive electrode mixture layer relative to the current collector is 70% or more.

[0009] Furthermore, a secondary battery according to one aspect of this disclosure is characterized by comprising the positive electrode for the secondary battery.

[0010] According to one aspect of this disclosure, it is possible to provide a positive electrode for secondary batteries in which the peel strength of the positive electrode mixture layer is improved even with a low PVDF content.

[0011] This is a cross-sectional view of a secondary battery, which is an example of an embodiment.

[0012] An example of an embodiment will be described in detail below. The drawings referenced in the description of the embodiment are schematic representations, and the dimensional ratios of the components depicted in the drawings may differ from those of the actual objects.

[0013] Figure 1 is a cross-sectional view of a secondary battery, which is an example of an embodiment. The secondary battery 10 shown in Figure 1 comprises a wound electrode body 14 in which a positive electrode 11 and a negative electrode 12 are wound around a separator 13, an electrolyte, insulating plates 18 and 19 arranged above and below the electrode body 14, respectively, a battery case 15, a positive electrode lead 20, and a negative electrode lead 21. The battery case 15 is composed of a case body 16 having an opening and housing the electrode body 14, etc., and a sealing body 17 that closes the opening of the case body 16. In addition, other forms of electrode bodies may be used instead of the wound electrode body 14, such as a laminated electrode body in which the positive electrode and negative electrode are alternately stacked with a separator. Furthermore, the battery case 15 can be exemplified by metal cases such as cylindrical, rectangular, coin-shaped, or button-shaped cases, or resin cases (so-called pouch type) formed by laminating resin sheets.

[0014] The electrolyte, for example, has lithium ion conductivity. The electrolyte may be a liquid electrolyte (electrolyte solution) or a solid electrolyte.

[0015] A liquid electrolyte (electrolyte solution) comprises a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and mixtures of two or more of these. Examples of non-aqueous solvents include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixtures thereof. The non-aqueous solvent may also contain halogen-substituted solvents (e.g., fluoroethylene carbonate) in which at least some of the hydrogen atoms in the solvent are replaced with halogen atoms such as fluorine. Examples of electrolyte salts include LiPF4. ６ Lithium salts such as these are used.

[0016] As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc., can be used. As the inorganic solid electrolyte, materials known for all-solid-state lithium-ion secondary batteries, etc. (for example, oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, etc.) can be used. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. Examples of polymer materials include fluororesins, acrylic resins, polyether resins, etc. Although the electrolytes exemplified above are non-aqueous electrolytes, the electrolyte is not limited to non-aqueous electrolytes and may also be an aqueous electrolyte.

[0017] The case body 16 is, for example, a metal container in the shape of a bottomed cylinder. A gasket 28 is provided between the case body 16 and the sealing body 17 to ensure airtightness inside the battery. The case body 16 has, for example, a protruding portion 22 that supports the sealing body 17, which is a part of the side surface that protrudes inward. The protruding portion 22 is preferably formed in an annular shape along the circumferential direction of the case body 16, and its upper surface supports the sealing body 17.

[0018] The sealing body 17 has a structure in which a filter 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each component except the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective centers, with the insulating member 25 interposed between their respective peripheral edges. When the internal pressure of the secondary battery 10 rises due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 towards the cap 27, thus interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure rises further, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0019] In the secondary battery 10 shown in Figure 1, the positive electrode lead 20 attached to the positive electrode 11 extends through a through-hole in the insulating plate 18 towards the sealing body 17, and the negative electrode lead 21 attached to the negative electrode 12 extends outside the insulating plate 19 towards the bottom of the case body 16. The positive electrode lead 20 is connected by welding or the like to the lower surface of the filter 23, which is the bottom plate of the sealing body 17, and the cap 27, which is the top plate of the sealing body 17 and is electrically connected to the filter 23, becomes the positive electrode terminal. The negative electrode lead 21 is connected by welding or the like to the inner surface of the bottom of the case body 16, and the case body 16 becomes the negative electrode terminal.

[0020] The positive electrode 11, negative electrode 12, and separator 13 are described in detail below.

[0021] [Positive Electrode] The positive electrode 11 comprises a positive electrode current collector and a positive electrode mixture layer provided on the positive electrode current collector. The positive electrode mixture layer contains a positive electrode active material, a binder, a conductive material, and an electrochemically inert ionic compound. The positive electrode mixture layer may be provided on one side of the positive electrode current collector or on both sides of the positive electrode current collector.

[0022] The positive electrode current collector can be made of a metal foil that is stable within the positive electrode potential range, such as aluminum or an aluminum alloy, or a film with the metal arranged on its surface. The positive electrode current collector has a thickness of, for example, about 1 μm to 100 μm.

[0023] The positive electrode mixture layer is formed by, for example, applying and drying a positive electrode mixture slurry containing a positive electrode active material, binder, conductive material, and ionic bonding compound onto a positive electrode current collector, and then rolling it under a predetermined pressure. The method for preparing the positive electrode mixture slurry will be described later.

[0024] The positive electrode active material includes lithium-containing transition metal oxide particles and boron compounds present on the surface of the lithium-containing transition metal oxide particles.

[0025] Lithium-containing transition metal oxide particles are, for example, composite oxides containing lithium (Li) and transition metal elements such as cobalt (Co), manganese (Mn), and nickel (Ni). Lithium-containing transition metal oxide particles may also contain other additive elements besides Co, Mn, and Ni, such as aluminum (Al), zirconium (Zr), magnesium (Mg), scandium (Sc), yttrium (Y), titanium (Ti), iron (Fe), copper (Cu), zinc (Zn), chromium (Cr), lead (Pb), tin (Sn), sodium (Na), potassium (K), barium (Ba), strontium (Sr), calcium (Ca), tungsten (W), molybdenum (Mo), niobium (Nb), antimony (Sb), and silicon (Si).

[0026] Lithium-containing transition metal oxide particles preferably contain Ni, for example, in terms of increasing battery capacity and suppressing the deterioration of charge-discharge cycle characteristics. The Ni content in lithium-containing transition metal oxide particles is preferably in the range of 50 mol% to 97 mol%, preferably 65 mol% to 97 mol%, and more preferably 65 mol% to 95 mol%, relative to the total number of moles of metal elements excluding Li. The lithium-containing transition metal oxide particles are, for example, those with the general formula Li ａ Ni ｂ Co ｃ Mn ｄ Al ｅ M ｆ O ｇ(In the formula, M is at least one element selected from B, Zr, Mg, Sc, Y, Ti, S, Fe, Cu, Pb, Sn, Na, K, Ba, Sr, Ca, W, Mo, Nb, Sb, Si, and P; 0.8 ≦ a ≦ 1.2, 0.5 ≦ b ≦ 0.97, 0 ≦ c, 0 ≦ d, 0 ≦ e, 0 ≦ f, 1.8 ≦ g ≦ 2.2).

[0027] The content of the transition metal element and additive element contained in the lithium-containing transition metal oxide particles can be measured, for example, by an inductively coupled plasma atomic emission spectrometer (ICP-AES).

[0028] The lithium-containing transition metal oxide particles are, for example, secondary particles in which primary particles are aggregated. The average particle size of the lithium-containing transition metal oxide particles is preferably in the range of 3 μm or more and 25 μm or less, and more preferably in the range of 7 μm or more and 20 μm or less. The average particle size of the lithium-containing transition metal oxide particles is the volume average particle size measured by the laser diffraction method and is the median diameter at which the volume integration value is 50% in the particle size distribution. The average particle size of the lithium-containing transition metal oxide particles can be measured by the laser diffraction method using, for example, Microtrac Bell Corporation MT3000II.

[0029] The boron compound may be any compound containing B. Examples of the boron compound include boric acid (H ３ BO ３ ), boron oxide (B ２ O ３ ), lithium borate (LiBO ２ , Li ３ BO ３ , Li ２ B ４ O ７ ), etc. The content rate of the boron compound is preferably in the range of 0.1 mol% or more and​​When lithium-containing transition metal oxide particles are secondary particles, the boron compound is present on the surface of the secondary particles. Alternatively, the boron compound may be present on the surface of primary particles or at grain boundaries within the secondary particles of the lithium-containing transition metal oxide particles. For example, the boron compound can be attached to the surface of the lithium-containing transition metal oxide particles by mechanically mixing them. Furthermore, heat treatment may be performed after the boron compound has been attached. The heat treatment temperature is, for example, 200°C or higher and 400°C or lower.

[0031] Lithium-containing transition metal oxide particles with boron compounds attached to their surface are preferably present in an amount of 80% by mass or more, and more preferably 90% by mass or more, relative to the total mass of the positive electrode active material, from the viewpoint of improving battery characteristics. The positive electrode active material may also contain lithium-containing transition metal oxide particles without boron compounds attached. The presence of boron compounds and the measurement of their content can be performed using energy-dispersive X-ray spectroscopy (EDX) and ICP-AES.

[0032] The binder contains polyvinylidene fluoride (PVDF). The polyvinylidene fluoride content is 1% by mass or less relative to the total mass of the positive electrode active material. The low concentration of polyvinylidene fluoride in the positive electrode mixture layer (1% by mass or less) is to suppress the increase in viscosity of the positive electrode mixture slurry and prevent coating defects such as uneven coating. The polyvinylidene fluoride content in the positive electrode mixture slurry is 1% by mass or less relative to the total mass of the positive electrode active material. The polyvinylidene fluoride content may be 0.1% by mass or more, or 0.5% by mass or more, relative to the total mass of the positive electrode active material, for example, from the standpoint of peel strength of the positive electrode mixture layer.

[0033] The binder may contain binders other than polyvinylidene fluoride. Examples include polytetrafluoroethylene (PTFE), polyacrylonitrile (PAN), polyimide resins, acrylic resins, polyolefin resins, styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC) or its salts, polyacrylic acid (PAA) or its salts, and polyvinyl alcohol (PVA). These may be used individually or in combination of two or more. The content of binders other than polyvinylidene fluoride may be, for example, 0.5% by mass or less, 0.1% by mass or less, or 0% by mass, relative to the total mass of the positive electrode active material.

[0034] Examples of conductive materials include carbon black such as acetylene black and Ketjenblack, carbon nanotubes (CNTs), graphene, and carbon-based particles such as graphite. These may be used individually or in combination of two or more types.

[0035] Ionic compounds are electrochemically inert materials. Here, electrochemical inertness means that they do not contribute to reversible charge-discharge reactions on the positive electrode side. Furthermore, ionic compounds are compounds that contain a cation and oxygen (O), and the difference in electronegativity between the cation and oxygen is 1.7 or greater. Examples of cations constituting ionic compounds include Li. ＋ Na ＋ _K ＋ Alkali metal ions such as Mg ２＋ Ca ２＋ Alkaline earth metal ions such as Al ３＋ , Zr ４＋ Ti ４＋ Examples include at least one metal ion selected from such as the following. The oxygen constituting the ionic compound is, for example, an oxygen ion (O). ２－ ), and oxygen contained in anions such as phosphate ions, carbonate ions, sulfate ions, and nitrate ions.

[0036] Examples of ionic compounds include lithium phosphate, sodium phosphate, potassium phosphate, magnesium phosphate, calcium phosphate, aluminum oxide, zirconium oxide, and titanium oxide. The presence of ionic compounds and their content can be confirmed and measured using energy-dispersive X-ray spectroscopy (EDX) and ICP-AES. The average particle size of ionic compounds is 3 μm or less. The average particle size of ionic compounds is the median diameter as described above and is measured by the method described above.

[0037] Although the factors contributing to the effects obtained by this embodiment of the invention have not been clearly identified, the following elements are considered to be possible contributors. Ionic compounds in which the difference in electronegativity between the cation and oxygen is 1.7 or more adhere specifically to polyvinylidene fluoride. Therefore, in the positive electrode slurry, polyvinylidene fluoride adheres closely to the above-mentioned ionic compounds, making it less likely to be trapped inside the aggregates of the conductive material. Furthermore, the average particle size of the ionic compounds being 3 μm or less further enhances adhesion to polyvinylidene fluoride, further suppressing trapping into the conductive material. In addition, polyvinylidene fluoride with which the ionic compounds adhere closely has higher adhesion to lithium-containing transition metal oxide particles with boron compounds present on the particle surface compared to polyvinylidene fluoride without which the ionic compounds adhere.

[0038] The positive electrode mixture layer of this embodiment has the following characteristics. In this embodiment, when the positive electrode mixture layer is elementally analyzed by electron beam microanalyzer (EPMA), the total number of aggregates of specific elements indicating ionic compounds detected (N ｔ The number of aggregates of a specific element that shows an ionic bond compound detected overlapping with the detected element F (N ｉ The proportion of ) (hereinafter, the proportion will be N ｉ / N ｔThe percentage of (sometimes referred to as) is 70% or more, preferably 80% or more. The specific element exhibiting ionic bonding can be any element that constitutes the ionic bonding compound, but it is desirable to select an element different from the elements that constitute other materials (e.g., positive electrode active material). Element F is an element derived from polyvinylidene fluoride. Furthermore, if at least a portion of the aggregate of the specific element exhibiting the detected ionic bonding compound overlaps with at least a portion of the region of element F, the number of aggregates (N) is determined. ｉ It counts as one of the items.

[0039] In the positive electrode mixture layer, N ｉ / N ｔ The fact that the polyvinylidene content is 70% or more indicates that the majority of the ionic compounds in the positive electrode mixture layer are in close contact with polyvinylidene fluoride. As mentioned above, polyvinylidene fluoride with closely attached ionic compounds exhibits high adhesion to lithium-containing transition metal oxide particles on which boron compounds are present on the particle surface. Therefore, because the majority of the ionic compounds are in close contact with polyvinylidene fluoride, the adhesion area between the positive electrode active material and polyvinylidene fluoride increases, thereby improving the peel strength of the positive electrode mixture layer. Furthermore, this embodiment exhibits an unprecedented and excellent effect in that it improves the peel strength of the positive electrode mixture layer even at a low polyvinylidene fluoride content of 1% by mass or less in the positive electrode mixture layer.

[0040] Elemental analysis of the positive electrode mixture layer using EPMA is performed as follows. First, the positive electrode is embedded in the resin, and the cross-section of the positive electrode is exposed by processing such as cross-section polishing (CP). Then, elemental mapping is performed on the exposed cross-section of the positive electrode using EPMA (electron beam microanalyzer). The conditions for EPMA elemental analysis are an acceleration voltage of 10 kV, irradiation current of 30 nA, beam diameter of <1 μm, step size of 0.16 μm, number of measurement points of 256 × 192, analysis area of ​​50 μm × 50 μm, and integration time of 50 milliseconds. Then, from the cross-section of the positive electrode analyzed by EPMA, 10 analysis areas (50 μm × 50 μm areas) are randomly selected from the region up to 50% of the total thickness of the positive electrode mixture layer from the current collector, and elemental mapping (mapping of element F and specific elements indicating ionic bonding compounds) is performed on each analysis area. ｉ / N ｔ The N of the positive electrode mixture layer is calculated and its average value is used. ｉ / N ｔ Let's assume that.

[0041] The content of the ionic bonding compound is preferably 1% by mass or less relative to the total mass of the positive electrode active material, for example, in order to suppress the decrease in battery capacity. Furthermore, the content of the ionic bonding compound is preferably 0.1% by mass or more relative to the total mass of the positive electrode active material, for example, in order to further improve the adhesion between the PVDF and the positive electrode active material.

[0042] The ionic bonding compound preferably contains at least one of lithium phosphate, aluminum oxide, and zirconium oxide, and is particularly preferably lithium phosphate, in that it can further improve the adhesion between PVDF and the positive electrode active material.

[0043] The content of the conductive material may be 0.01% by mass or more, preferably 0.1% by mass or more, and more preferably 0.5% by mass or more, relative to the total mass of the positive electrode active material, for example, in terms of improving battery capacity. Furthermore, the content of the conductive material may be 5% by mass or less, and more preferably 2% by mass or less, in terms of suppressing the trapping of PVDF.

[0044] Below, N ｉ / N ｔ An example of a method for preparing a cathode mixture slurry for creating a cathode mixture layer with a concentration of 70% or more will be described.

[0045] One example of a method for preparing a positive electrode mixture slurry (hereinafter sometimes referred to as preparation method A) involves putting predetermined amounts of positive electrode active material, an ionic bonding compound, polyvinylidene fluoride, NMP, and a conductive material into a kneader and kneading them to obtain a positive electrode mixture slurry. Although not particularly limited, it is preferable to put the positive electrode active material, ionic bonding compound, and polyvinylidene fluoride into the kneader and knead them first, and then add the NMP and conductive material to the kneader and knead them. The positive electrode mixture slurry thus obtained is then coated onto a positive electrode current collector to obtain a positive electrode mixture layer.

[0046] Another example of a method for preparing a positive electrode mixture slurry (hereinafter sometimes referred to as preparation method B) involves pre-mixing predetermined amounts of an ionic compound, polyvinylidene fluoride, and NMP in a kneader. Next, predetermined amounts of positive electrode active material, NMP, and conductive material are added to the kneader and mixed to obtain a positive electrode mixture slurry. The positive electrode mixture slurry thus obtained is then coated onto a positive electrode current collector to obtain a positive electrode mixture layer. Note that the kneader used for the pre-mixing of the ionic compound, polyvinylidene fluoride, and NMP may be different from the kneader used to add and mix the positive electrode active material, NMP, and conductive material afterward.

[0047] Examples of mixing machines include disper mixers, planetary mixers, rotating / revolving mixers, and ball mills. Mixing and stirring conditions can be set appropriately by adjusting the rotation speed and mixing time. For example, in the case of a rotating / revolving mixer, the rotation speed is set in the range of 500 rpm to 2000 rpm, and the mixing time is set in the range of 5 minutes to 20 minutes.

[0048] By using the positive electrode mixture slurry prepared by the above preparation method A, N ｉ / N ｔ It becomes possible to produce a positive electrode mixture layer in which 70% or more is obtained. Furthermore, by using the positive electrode mixture slurry prepared by the above preparation method B, N is obtained compared to when using the positive electrode mixture slurry prepared by the above preparation method A. ｉ / N ｔThis makes it possible to produce a positive electrode mixture layer with high N. ｉ / N ｔ It becomes possible to produce a positive electrode mixture layer in which 80% or more of the material is N. ｉ / N ｔ The method for preparing a cathode mixture slurry to obtain a cathode mixture layer having 70% or more of N is not limited to the above. However, as in Comparative Example 6 described later, in a cathode mixture slurry prepared by a method in which an ionic compound is first compounded onto the particle surface of the cathode active material using a ball mill or the like, and then polyvinylidene fluoride, NMP, and conductive material are added sequentially while kneading, N ｉ / N ｔ It is difficult to produce a positive electrode mixture layer in which the content is 70% or more.

[0049] [Negative electrode] The negative electrode 12 comprises, for example, a negative electrode current collector and a negative electrode mixture layer provided on the negative electrode current collector. The negative electrode mixture layer includes, for example, a negative electrode active material, a binder, etc. The negative electrode mixture layer may be provided on one side of the negative electrode current collector or on both sides of the negative electrode current collector.

[0050] The negative electrode current collector can be made of a metal foil that is stable in the negative electrode potential range, such as copper, or a film with the metal arranged on its surface. The negative electrode current collector has a thickness of, for example, 1 μm to 100 μm.

[0051] The negative electrode mixture layer is formed, for example, by applying and drying a negative electrode mixture slurry containing a negative electrode active material, a binder, etc., onto a negative electrode current collector, and then rolling it at a predetermined pressure.

[0052] The negative electrode active material is, for example, a material capable of intercalating and releasing lithium ions. For example, the negative electrode active material can be metallic lithium, lithium-aluminum alloy, lithium-lead alloy, lithium-silicon alloy, lithium-tin alloy, and other lithium alloys, as well as carbon materials such as graphite, coke, and calcined organic materials, and SnO ２ SnO, TiO ２ Examples include metal oxides such as those listed above. These may be used individually or in combination of two or more.

[0053] For example, the binder may be the material exemplified in the positive electrode 11. The negative electrode mixture layer may also contain the conductive material mentioned above.

[0054] [Separator] For the separator 13, for example, a porous sheet having ion permeability and insulating properties can be used. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator include olefin resins such as polyethylene and polypropylene, and cellulose. The separator 13 may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin resin. Alternatively, it may be a multilayer separator containing a polyethylene layer and a polypropylene layer, or a separator with a material such as aramid resin or ceramic coated on its surface may be used.

[0055] The present disclosure will be further illustrated below with reference to examples, but the present disclosure is not limited to these examples.

[0056] <Example 1> [Preparation of positive electrode mixture slurry] A positive electrode mixture slurry was prepared based on the above preparation method A. Specifically, 100 parts by mass of positive electrode active material and lithium phosphate (Li) which is an ionic compound were used. ３ PO ４ A solution of NMP containing 1 part by mass of (average particle size 2.5 μm), 1 part by mass of PVDF as a binder, NMP, and 1 part by mass of acetylene black as a conductive material were put into a kneader and kneaded to obtain a positive electrode mixture slurry. The positive electrode active material was Li １．０ Ni ０．８９ Co ０．０５ Mn ０．０６ O ２ Lithium borate (Li) is present on the secondary particle surface of lithium-containing transition metal oxide particles represented by ３ BO ３ LiBO ２ Materials with ) attached were used. The lithium borate content was set to 1 mol% based on the boron element relative to the total number of moles of metal elements excluding Li in the lithium-containing transition metal oxide particles. Furthermore, a Shinki Awatori Rentaro was used as the kneading machine, and the mixture was kneaded at a rotation speed of 1000 rpm for 10 minutes. Lithium phosphate (Li ３ PO ４The difference in electronegativity between the cation of ) and oxygen is 2.4, which is greater than or equal to 1.7.

[0057] [Preparation of the positive electrode] The above positive electrode mixture slurry was applied to both sides of a positive electrode current collector made of aluminum foil, and after the coating film was dried, it was rolled using a rolling mill. In this way, a positive electrode was prepared in which positive electrode mixture layers were formed on both sides of the positive electrode current collector.

[0058] From the obtained positive electrode, a cross-section of the positive electrode mixture layer was prepared by the cross-section polishing (CP) process described above. Then, elemental analysis of the cross-section of the positive electrode mixture layer was performed using EPMA under the conditions described above, and N ｉ / N ｔ The result of this calculation was 77%.

[0059] <Example 2> The cathode was prepared in the same manner as in Example 1, except that the amount of lithium phosphate added to the cathode slurry was 0.5 parts by mass. Elemental analysis was then performed by EPMA in the same manner as in Example 1, and N ｉ / N ｔ The figure was 79%.

[0060] <Example 3> The cathode was prepared in the same manner as in Example 1, except that the amount of lithium phosphate added to the cathode slurry was 0.1 parts by mass. Elemental analysis was then performed by EPMA in the same manner as in Example 1, and N ｉ / N ｔ The figure was 78%.

[0061] <Example 4> In the preparation of the cathode mixture slurry, aluminum oxide (Al) was used as the ionic compound. ２ O ３ A positive electrode was prepared in the same manner as in Example 1, except that a particle with an average particle size of 0.1 μm was used and the amount of aluminum oxide added was 0.25 parts by mass. Elemental analysis by EPMA was then performed in the same manner as in Example 1, and N ｉ / N ｔ It was 74%. Note that aluminum oxide (Al ２ O ３ The difference in electronegativity between the cation of ) and oxygen is 1.8, which is greater than or equal to 1.7.

[0062] <Example 5> In the preparation of the cathode mixture slurry, zirconium oxide (ZrO) was used as the ionic bonding compound. ２ A cathode was prepared in the same manner as in Example 1, except that a particle with an average particle size of 2.8 μm was used and the amount of zirconium oxide added was 1 part by mass. Elemental analysis by EPMA was then performed in the same manner as in Example 1, and N ｉ / N ｔ The percentage was 78%. ２ The difference in electronegativity between the cation of ) and oxygen is 2.1, which is greater than or equal to 1.7.

[0063] <Example 6> The cathode was prepared in the same manner as in Example 1, except that the amount of lithium phosphate added to the cathode slurry was 0.05 parts by mass. Elemental analysis by EPMA was then performed in the same manner as in Example 1, and N ｉ / N ｔ The figure was 78%.

[0064] <Example 7> A cathode mixture slurry was prepared based on preparation method B. Specifically, lithium phosphate (Li) is an ionic compound. ３ PO ４ A solution of NMP containing 0.5 parts by mass of (average particle size 2.5 μm) and 1 part by mass of PVDF as a binder was dissolved in it. A predetermined amount of NMP was then placed in a planetary ball mill manufactured by Fritsch. Next, 1 mm diameter balls were added in 30% of the pot volume of the ball mill, and the mixture was dispersed at a rotation speed of 500 rpm for 10 minutes to obtain a dispersion paste of the ionic compound and PVDF. Next, the dispersion paste, 100 parts by mass of positive electrode active material, NMP, and 1 part by mass of acetylene black were placed in a kneader and kneaded to obtain a positive electrode mixture slurry. In addition, a mixer manufactured by Shinkie was used, and the mixture was kneaded in the same manner as in Example 1.

[0065] The positive electrode was prepared in the same manner as in Example 1, except that the positive electrode mixture slurry obtained above was used. Elemental analysis by EPMA was then performed in the same manner as in Example 1, and N ｉ / N ｔ The figure was 89%.

[0066] <Comparative Example 1> A positive electrode was produced in the same manner as in Example 1, except that lithium phosphate was not used in the preparation of the positive electrode active material slurry.

[0067] <Comparative Example 2> A positive electrode was produced in the same manner as in Example 1, except that silicon oxide (SiO ２ , average particle size 1.5 μm) was used and the amount of silicon oxide charged was 1 part by mass. When elemental analysis by EPMA was performed in the same manner as in Example 1, N ｉ / N ｔ was 48%. The difference in electronegativity between the cation of silicon oxide (SiO ２ ) and oxygen is 1.5, which is less than 1.7.

[0068] <Comparative Example 3> A positive electrode was produced in the same manner as in Example 1, except that lithium phosphate with an average particle size of 4 μm was used and the amount of lithium phosphate charged was 0.5 part by mass. When elemental analysis by EPMA was performed in the same manner as in Example 1, N ｉ / N ｔ was 40%.

[0069] <Comparative Example 4> A positive electrode was produced in the same manner as in Comparative Example 1, except that the amount of PVDF charged was 1.4 parts by mass.

[0070] <Comparative Example 5> A positive electrode was produced in the same manner as in Example 1, except that the amount of PVDF charged was 1.4 parts by mass. When elemental analysis by EPMA was performed in the same manner as in Example 1, N ｉ / N ｔ in the positive electrode active material layer was 76%.

[0071] <Comparative Example 6> 100 parts by mass of the same positive electrode active material as in Example 1, lithium phosphate (Li ３ PO ４, 1 part by mass of the average particle diameter D50: 2.5 μm was put into a planetary ball mill manufactured by Fritsch. Next, balls with a diameter of 5 mm were put into 30% by volume of the pot volume of the ball mill, and mixed at a rotational speed of 600 rpm for 10 minutes to complex an ionically bonded compound on the surface of the positive electrode active material. Next, an NMP solution in which 1 part by mass of PVDF, which is the above complex material and a binder, was dissolved, an appropriate amount of N-methyl-2-pyrrolidone (NMP), and 1 part by mass of acetylene black were put into a kneader and kneaded to obtain a positive electrode mixture slurry. Further, as the kneader, Awatori Renk太郎 manufactured by Shinki Co., Ltd. was used, and kneading was performed in the same manner as in Example 1.

[0072] A positive electrode was produced in the same manner as in Example 1 except that the above-obtained positive electrode mixture slurry was used. Then, when elemental analysis by EPMA was performed in the same manner as in Example 1, N ｉ / N ｔ was 41%.

[0073] [Measurement of initial viscosity of positive electrode mixture slurry] The initial viscosity of the positive electrode mixture slurry prepared in each example and each comparative example was measured. Specifically, using a rheometer "MCR302" (manufactured by Anton Paar), the viscosity of the positive electrode mixture slurry was measured under the conditions of a shear rate of 10 s －１ and 25°C. This was used as the initial viscosity, and the results are summarized in Table 1. However, in Table 1, the initial viscosity of the positive electrode mixture slurry of Comparative Example 1 was used as a reference (100), and the initial viscosities of the positive electrode mixture slurries of other examples and comparative examples were shown as relative values.

[0074] [Measurement of peel strength of positive electrode mixture layer] The peel strength of the positive electrode mixture layer in the positive electrodes produced in each example and each comparative example was measured as follows. First, on a

[0075]

[0076] Comparative Examples 4 and 5 both had a PVDF content exceeding 1% by mass relative to the total mass of the positive electrode active material. In positive electrodes with such high PVDF content, the peel strength of the positive electrode mixture layer is high. However, on the other hand, the initial viscosity of the positive electrode mixture slurry was very high, resulting in uneven coating of the positive electrode mixture slurry.

[0077] Next, we compare each example and Comparative Examples 1-3 and 6 in which the PVDF content is 1% by mass or less relative to the total mass of the positive electrode active material. In all of Examples 1-7, the peel strength of the positive electrode mixture layer was improved compared to Comparative Example 1. On the other hand, in Comparative Examples 1-3 and 6, the peel strength of the positive electrode mixture layer was equivalent to or lower than that of Comparative Example 1. Considering these results, in a positive electrode using an active material in which a boron compound exists on the surface of lithium-containing transition metal oxide particles as the positive electrode active material, a compound is used as the ionic bonding compound in which the difference in electronegativity between the cation and oxygen is 1.7 or more, and the average particle size is 3 μm or less, and the N of the positive electrode mixture layer is improved. ｉ / N ｔ By setting the PVDF content to 70% or more, it is possible to improve the peel strength of the positive electrode mixture layer even with a low PVDF content of 1% by mass or less. Furthermore, the initial viscosity of the positive electrode mixture slurries in Examples 1-6 and Comparative Examples 1-4 and 7 was low, and no uneven coating occurred in the positive electrode mixture slurry.

[0078] [Note] Configuration 1: The device comprises a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector, the positive electrode mixture layer comprising a positive electrode active material, a binder, a conductive material, and an electrochemically inert ionic compound, the positive electrode active material comprising lithium-containing transition metal oxide particles and a boron compound present on the surface of the lithium-containing transition metal oxide particles, the binder comprising polyvinylidene fluoride (PVDF), the ionic compound comprising a cation and oxygen, wherein the difference in electronegativity between the cation and oxygen is 1.7 or more, the average particle size of the ionic compound is 3 μm or less, and the content of polyvinylidene fluoride is 1% by mass or less relative to the total mass of the positive electrode active material. A positive electrode for a secondary battery, wherein when the positive electrode mixture layer is elementally analyzed by an electron beam microanalyzer (EPMA), the ratio of the number of aggregates of the specific element indicating the ion-bonding compound detected in overlap with element F detected in the region up to 50% of the total thickness of the positive electrode mixture layer from the current collector is 70% or more. Configuration 2: The positive electrode for a secondary battery according to Configuration 1, wherein the content of the ion-bonding compound is 1% by mass or less with respect to the total mass of the positive electrode active material. Configuration 3: The positive electrode for a secondary battery according to Configuration 1 or 2, wherein the content of the ion-bonding compound is 0.1% by mass or more with respect to the total mass of the positive electrode active material. Configuration 4: A positive electrode for a secondary battery according to any one of Configurations 1 to 3, wherein, when the positive electrode mixture layer is elementally analyzed by an electron beam microanalyzer (EPMA), the ratio of aggregates of the specific element indicating the ionic bonding compound detected at the same location as the detected element F is 80% or more of the total number of aggregates of the specific element indicating the ionic bonding compound detected. Configuration 5: A positive electrode for a secondary battery according to any one of Configurations 1 to 3, wherein the ionic bonding compound contains lithium phosphate. Configuration 6: A secondary battery comprising the positive electrode for a secondary battery according to any one of Configurations 1 to 5.

[0079] 10 Secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 15 Battery case, 16 Case body, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Protruding part, 23 Filter, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket.

Claims

1. The electrode comprises a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector, wherein the positive electrode mixture layer comprises a positive electrode active material, a binder, a conductive material, and an electrochemically inert ionic compound, the positive electrode active material comprises lithium-containing transition metal oxide particles and a boron compound present on the surface of the lithium-containing transition metal oxide particles, the binder comprises polyvinylidene fluoride (PVDF), the ionic compound comprises a cation and oxygen, the difference in electronegativity between the cation and oxygen is 1.7 or more, the average particle size of the ionic compound is 3 μm or less, and the content of polyvinylidene fluoride is 1% by mass or less relative to the total mass of the positive electrode active material. A positive electrode for a secondary battery, wherein when the positive electrode mixture layer is elementally analyzed by an electron beam microanalyzer (EPMA), the ratio of the number of aggregates of the specific element indicating the ionic compound detected in the region up to 50% of the total thickness of the positive electrode mixture layer from the current collector to the region up to 50% of the total thickness of the positive electrode mixture layer, to the number of aggregates of the specific element indicating the ionic compound detected in overlapping with element F detected in the region is 70% or more.

2. The positive electrode for a secondary battery according to claim 1, wherein the content of the ionic compound is 1% by mass or less with respect to the total mass of the positive electrode active material.

3. The positive electrode for a secondary battery according to claim 1 or 2, wherein the content of the ionic compound is 0.1% by mass or more relative to the total mass of the positive electrode active material.

4. The positive electrode for a secondary battery according to claim 1 or 2, wherein when the positive electrode mixture layer is elementally analyzed by an electron beam microanalyzer (EPMA), the ratio of the number of aggregates of the specific element indicating the ionic compound detected in the same location as the F element detected in the region up to 50% of the total thickness of the positive electrode mixture layer from the current collector is 80% or more of the total number of aggregates of the specific element indicating the ionic compound detected in the region up to 50% of the total thickness of the positive electrode mixture layer from the current collector is 80% or more.

5. The positive electrode for a secondary battery according to claim 1 or 2, wherein the ionic bonding compound comprises lithium phosphate.

6. A secondary battery comprising the positive electrode for a secondary battery as described in claim 1 or 2.

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