Secondary battery positive electrode, production method therefor, and secondary battery

Abstract

A secondary battery positive electrode comprising a positive electrode collector and a positive electrode active material layer supported on the positive electrode collector, wherein the positive electrode active material layer contains active material particles, a binder, and a heat-decomposable additive, and the heat-decomposable additive contains an acetamidobenzoic acid.

Description

Positive electrode for secondary battery, method for producing the same, and secondary battery

[0001] The present disclosure relates primarily to a positive electrode for a secondary battery.

[0002] The higher the energy density of a secondary battery, the more safety is required. In particular, reducing heat generation due to internal short circuits is important. When a battery has an internal short circuit, even if the short circuit is small, the separator melts due to Joule heat generated by the short circuit current, forming a larger short circuit. As the short circuit expands and the short circuit current increases, the battery temperature rises at an accelerated rate. The higher the energy density of a secondary battery, the greater the Joule heat generated by the short circuit current.

[0003] Patent Document 1 discloses a composite oxide having a layered crystal structure and containing lithium and nickel as main components, which is represented by the general formula: Li a Ni 1-b-c M1 b M2 c O2, 0.95≦a≦1.05, 0.01≦b≦0.10, 0.10≦c≦0.20 (wherein M1 is one or more elements selected from Al, B, Y, Ce, Ti, Sn, V, Ta, Nb, W, and Mo, and M2 is one or more elements selected from Co, Mn, and Fe), and the powder has a compressed density of 4.0 g / cm when compacted under pressure. 3 Electrical conductivity of the powder compact at 25 ° C.: σ is 5 × 10 -2 ≧σ≧5×10 -4 The present inventors have proposed a positive electrode active material characterized by having a capacitance within the range of [S / cm].

[0004] Japanese Patent Application Laid-Open No. 2000-315502

[0005] Patent Document 1 states that the above-mentioned positive electrode active material improves thermal stability in a charged state, suppresses Joule heat generation due to short-circuit current even in a situation where an internal short circuit occurs in the battery, and makes it easier to ensure safety.

[0006] However, when the conductivity of the powder compact is controlled as in Patent Document 1, the battery as a whole has a high resistance, which leads to a decrease in battery performance. Furthermore, Patent Document 1 is a measure that is specific to a composite oxide mainly composed of lithium and nickel, and is therefore not very versatile.

[0007] One aspect of the present disclosure relates to a positive electrode for a secondary battery, including a positive electrode current collector and a positive electrode active material layer supported on the positive electrode current collector, the positive electrode active material layer including active material particles, a binder, and a thermally decomposable additive, and the thermally decomposable additive including acetamidobenzoic acid.

[0008] Another aspect of the present disclosure relates to a secondary battery including the above-described positive electrode for secondary batteries, a negative electrode, a non-aqueous electrolyte, and a separator interposed between the positive electrode and the negative electrode.

[0009] Yet another aspect of the present disclosure relates to a method for manufacturing a positive electrode for a secondary battery, the method including: preparing a positive electrode slurry containing active material particles, a binder, a heat-decomposable additive, and a dispersion medium; preparing a positive electrode current collector; applying the positive electrode slurry to a surface of the positive electrode current collector to form a coating film; drying the coating film to form an unrolled layer; and rolling the unrolled layer to form a positive electrode active material layer, wherein the heat-decomposable additive contains acetamidobenzoic acid and the dispersion medium contains an organic solvent.

[0010] According to the present disclosure, it is possible to suppress an increase in resistance at room temperature while increasing resistance at high temperatures when an internal short circuit occurs in the battery, thereby achieving both high safety and good battery performance.

[0011] The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0012] 1 is a longitudinal cross-sectional view schematically illustrating an internal structure of a secondary battery according to an embodiment of the present disclosure.

[0013] The following describes examples of embodiments of a positive electrode for a secondary battery according to the present disclosure and a secondary battery using the same, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the term "numerical value A to numerical value B" includes numerical value A and numerical value B and can be interpreted as "numerical value A or greater and numerical value B or less." In the following description, when lower and upper limits of numerical values ​​related to specific physical properties or conditions are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit. Furthermore, the present disclosure encompasses combinations of the features set forth in two or more claims arbitrarily selected from the multiple claims set forth in the appended claims. In other words, as long as no technical contradiction arises, any of the features set forth in two or more claims arbitrarily selected from the multiple claims set forth in the appended claims can be combined.

[0014] A positive electrode for a secondary battery according to an embodiment of the present disclosure includes a positive electrode current collector and a positive electrode active material layer supported on the positive electrode current collector. The positive electrode active material layer is formed on the surface of the positive electrode current collector.

[0015] [Positive electrode current collector] The positive electrode current collector is made of a sheet-like conductive material. As the positive electrode current collector, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet) is used.

[0016] [Positive Electrode Active Material Layer] The positive electrode active material layer is supported on one or both surfaces of the positive electrode current collector. The positive electrode active material layer is typically a positive electrode mixture layer composed of a positive electrode mixture, and is in the form of a membrane or film. The positive electrode mixture contains active material particles (particles of positive electrode active material), a binder, and a thermally decomposable additive as essential components. The active material particles may contain a lithium-containing transition metal oxide. The binder contains at least one selected from the group consisting of fluororesin and hydrogenated nitrile butadiene rubber. The thermally decomposable additive contains acetamidobenzoic acid.

[0017] Even if acetamidobenzoic acid is present in the positive electrode active material layer, it has almost no effect on battery performance. Acetamidobenzoic acid may be in an acid form in which its COOH group is bound to a proton, in a zwitterionic form, or in a salt form in which the proton of the COOH group is replaced with a cation. The cation may be an ammonium cation, a metal cation (such as Na, K, or Li), or the like. The acetamidobenzoic acid may be 3-acetamidobenzoic acid or 4-acetamidobenzoic acid, with 3-acetamidobenzoic acid being more preferable. The content of 3-acetamidobenzoic acid in the total amount of acetamidobenzoic acid is preferably 98% by mass or more, and may be 100%.

[0018] The thermally decomposable additive thermally decomposes and interrupts the conductive path when an internal short circuit occurs in the battery, increasing the short-circuit current and raising the battery temperature. That is, the thermally decomposable additive has the effect of barely increasing resistance at room temperature but selectively increasing the internal resistance of the battery at high temperatures. Because acetamidobenzoic acid thermally decomposes at temperatures of approximately 250°C or higher, the internal resistance of the battery increases significantly before the battery temperature rises excessively. Although the mechanism by which the conductive path is interrupted is unclear, it is speculated that the thermal decomposition of the thermally decomposable additive generates gas within the positive electrode active material layer, causing local structural changes in the positive electrode active material layer, resulting in an increase in resistance.

[0019] Acetamidobenzoic acid is believed to be compatible with binders containing at least one selected from the group consisting of fluororesin and hydrogenated nitrile butadiene rubber (H-NBR). Fluororesin and H-NBR are soluble in organic solvents (e.g., N-methyl-2-pyrrolidone) used as a dispersion medium when forming the positive electrode active material layer. Acetamidobenzoic acid also dissolves moderately in organic solvents used as dispersion mediums. Therefore, acetamidobenzoic acid, which dissolves moderately in organic solvents and is dispersed in the organic solvent as fine particles or aggregates, is believed to be able to be uniformly dispersed in the positive electrode active material layer together with the binder. Furthermore, acetamidobenzoic acid is believed to be stable and unlikely to undergo side reactions in the presence of organic solvents. Incidentally, when water is used as a dispersion medium for the positive electrode slurry, the water may degrade the positive electrode active material, resulting in a decrease in capacity.

[0020] At least a portion of the thermally decomposable additive is preferably present in the positive electrode active material layer as particles having a particle diameter of 10 μm or more. Here, the term "particle" encompasses primary particles, secondary particles, and aggregates formed by aggregation of these particles. It also encompasses concepts such as aggregation and agglomeration in addition to general particles. The particle diameter (or aggregate diameter) can be considered as the diameter of a circle having the same area as the area enclosed by the outline of the particle (or aggregate) when observing a cross-sectional sample, as described below. It is presumed that when the thermally decomposable additive is contained in the positive electrode active material layer at such a size, local structural changes in the positive electrode active material layer due to thermal decomposition of the thermally decomposable additive are significant, resulting in a more significant increase in resistance. The particle diameter of the thermally decomposable additive may be 20 μm or more, or may be 30 μm or more. The maximum particle diameter of the thermally decomposable additive is preferably 100 μm or less, and may be 60 μm or less, or may be 50 μm or less. If the maximum value is within the above range, the effect on the battery performance can be sufficiently reduced.

[0021] The particle size can be measured as follows. (1) Preparation of a Cross-Sectional Sample of a Positive Electrode First, a positive electrode to be measured is prepared. Next, the positive electrode active material layer and the positive electrode current collector are simultaneously cut along the thickness direction of the positive electrode to form a cross section. At this time, a thermosetting resin may be filled into the positive electrode active material layer and cured. For example, a cross-sectional sample of the positive electrode is obtained by a CP (cross-section polisher) method, an FIB (focused ion beam) method, or the like.

[0022] The positive electrode to be measured is taken from a secondary battery with a depth of discharge (DOD) of 90% or more. Depth of discharge (DOD) is the ratio of the amount of discharged electricity to the amount of electricity in a fully charged battery. Note that the amount of electricity charged (i.e., the fully charged amount) when a battery in a fully discharged state (DOD = 100%) is charged to a fully charged state (SOC = 100%, DOD = 0%) corresponds to the rated capacity. The voltage of a battery in a fully charged state corresponds to the end-of-charge voltage. The voltage of a battery in a fully discharged state corresponds to the end-of-charge voltage.

[0023] (2) Imaging of the Cross Section of the Positive Electrode with a Scanning Electron Microscope (SEM) Next, the cross-sectional sample is observed with an SEM. The SEM observation is performed at a low magnification (e.g., 200 to 1000 times). The SEM image is taken so that a region of 300 μm or more (preferably 400 μm or more) in length in the plane direction of the positive electrode active material layer is observed.

[0024] (3) Elemental analysis by electron probe microanalyzer (EPMA) Elemental analysis by EPMA is performed using the SEM image of the cross-sectional sample. Components derived from the thermally decomposable additive (e.g., nitrogen element) are extracted from the EPMA analysis data of the cross-sectional sample to obtain a map of the thermally decomposable additive. The measurement conditions for nitrogen mapping are an acceleration voltage of 8.0 kV and a probe current of 1.0 × 10 -7 A is defined as the number of detected particles or aggregates of the thermally decomposable additive. A threshold value is set to 1 / 3 of the maximum count number in the positive electrode active material layer, and pixels with a detection level equal to or greater than the threshold value are considered to be locations where the thermally decomposable additive is present. Particles or aggregates of the thermally decomposable additive are identified from the resulting map. The diameter of an equivalent circle having the same area as the area of ​​the identified particle or aggregate is defined as the particle size of the thermally decomposable additive.

[0025] It is sufficient that at least one particle (or aggregate) of the thermally decomposable additive having a particle diameter of 10 μm or more is observed in a rectangular observation field defined by the length in the plane direction of the positive electrode active material layer (300 μm) × the thickness T of the positive electrode active material layer (if a positive electrode active material layer is provided on both sides of the positive electrode current collector, the thickness T of the positive electrode active material layer on one side (hereinafter the same)). The greater the number of particles of the thermally decomposable additive having a particle diameter of 10 μm or more observed in the observation field, the greater the effect of selectively increasing the internal resistance of the battery at high temperatures. The upper limit of the number of particles of the thermally decomposable additive having a particle diameter of 10 μm or more observed in the observation field is, for example, 40 or less. Each of the above numbers may be the average value of the number of particles of the thermally decomposable additive having a particle diameter of 10 μm or more counted in multiple (e.g., three or more) observation fields (300 μm × T).

[0026] The amount of the thermally decomposable additive contained in the positive electrode active material layer may be small. The content of acetamidobenzoic acid contained in the positive electrode active material layer may be, for example, 0.1% by mass to 5% by mass, 0.3% by mass to 3% by mass, or 0.5% by mass to 2% by mass. Furthermore, as described above, when a cross section of the positive electrode active material layer is analyzed using a scanning electron microscope and an electron beam microanalyzer, the area of ​​the location where acetamidobenzoic acid is present may be, for example, 0.5% by area to 27% by area, 1% by area to 15% by area, or 3% by area to 10% by area in a rectangular observation field defined by a length of 300 μm in the surface direction of the positive electrode active material layer and a thickness T of the positive electrode active material layer. The acetamidobenzoic acid contained in the above content does not significantly affect the battery capacity. Furthermore, acetamidobenzoic acid contained in the above content can be uniformly dispersed in the positive electrode active material layer as particles or aggregates having the above-mentioned appropriate particle size.

[0027] The thermally decomposable additive may contain an acetic acid component. The acetic acid component has the effect of suppressing precipitation of the positive electrode mixture in the positive electrode slurry prepared when forming the positive electrode active material layer. The acetic acid component may be acetic acid (acid type) having an OH group, or may be a salt type in which the proton of the OH group is replaced with a cation. The cation may be an ammonium cation, a metal cation (Na, K, Li, etc.), or the like. The acetic acid component is preferably contained in a proportion of 500 μg to 5,000 μg per gram of the thermally decomposable additive or acetamidobenzoic acid. The amount of the acetic acid component contained in the thermally decomposable additive or acetamidobenzoic acid can be measured, for example, by peeling the positive electrode active material layer from the positive electrode, extracting the acetamidobenzoic acid from the positive electrode active material layer with a solvent, and analyzing the solvent containing the acetamidobenzoic acid by ion chromatography. N-methyl-2-pyrrolidone (NMP) may be used as the solvent.

[0028] The thermally decomposable additive preferably covers a portion of the surface of the active material particles. It is believed that the thermally decomposable additive present on the surface of the active material particles efficiently blocks the conductive paths between the active material particles, between the active material particles and the conductive material, or between the active material particles and the positive electrode current collector at high temperatures. For example, the thermally decomposable additive present on the surface of the active material particles and interposed between the active material particles reduces contact between the active material particles due to a state change such as gasification. On the other hand, if the thermally decomposable additive covers only a portion of the surface of the active material particles, but not the entire surface, there is almost no increase in resistance to the battery reaction.

[0029] The fact that the thermally decomposable additive covers only a portion of the surface of the active material particles can be confirmed by observing elemental mapping by EPMA of an SEM image of a cross-sectional sample of the positive electrode obtained by the above-mentioned method.

[0030] It is desirable that the thermally decomposable additive (especially a particulate thermally decomposable additive (especially particles with a particle diameter of 10 μm or more)) be unevenly distributed on the outermost surface side (i.e., the separator side) of the positive electrode active material layer, rather than on the positive electrode current collector side. This is because internal short circuits in batteries often begin to expand on the separator side of the positive electrode active material layer. Internal short circuits occur, for example, when conductive foreign matter penetrates the separator. By unevenly distributing the thermally decomposable additive on the separator side of the positive electrode active material layer, a high-resistance region can be formed on the separator side of the positive electrode active material layer in the early stages after a short circuit occurs. This makes it possible to suppress the expansion of the short-circuited portion.

[0031] More specifically, when the thickness of the positive electrode active material layer is T, the presence probability Pb ​​of the thermally decomposable additive present in the region of the positive electrode active material layer from the surface of the positive electrode current collector to 0.5 T and the presence probability Pt of the thermally decomposable additive present in the region of the positive electrode active material layer from a position 0.5 T from the surface of the positive electrode current collector to the outermost surface (i.e., the surface of the positive electrode active material layer on the separator side) may, for example, satisfy 1.35≦Pt / Pb, and may also satisfy 1.5≦Pt / Pb≦40. Note that when Pt / Pb<1.35, it cannot be said that the thermally decomposable additive is substantially unevenly distributed on the outermost surface side of the positive electrode active material layer relative to the positive electrode current collector side.

[0032] Hereinafter, the region of the positive electrode active material layer from the surface of the positive electrode current collector to 0.5 T will be referred to as the "lower layer region," and the region of the positive electrode active material layer from the position 0.5 T from the surface of the positive electrode current collector to the outermost surface will be referred to as the "upper layer region."

[0033] Pb and Pt can be measured by analyzing a cross-sectional sample of the positive electrode obtained by the above-mentioned method using SEM and EPMA. Specifically, in the elemental map of the thermally decomposable additive obtained from the EPMA analysis data of the cross-sectional sample, the area occupied by the thermally decomposable additive in the lower layer region and the upper layer region is measured. At this time, all thermally decomposable additives are counted regardless of the particle diameter of the thermally decomposable additive. The proportion of the area occupied by the thermally decomposable additive in the area of ​​the lower layer region and the upper layer region is regarded as Pb and Pt, respectively. Pb and Pt are measured in a rectangular observation field defined by a length of 300 μm in the surface direction of the positive electrode active material layer × a thickness T of the positive electrode active material layer. Pb and Pt may each be an average value of Pb and Pt determined in multiple (e.g., three or more) observation fields.

[0034] Preferably, when the thickness of the positive electrode active material layer is T, the existence probability Pb(q) of the thermally decomposable additive present in the region of the positive electrode active material layer from the surface of the positive electrode current collector to 0.25 T and the existence probability Pt(q) of the thermally decomposable additive present in the region of the positive electrode active material layer from a position 0.75 T from the surface of the positive electrode current collector to the outermost surface (i.e., the surface of the positive electrode active material layer on the separator side) may satisfy 1.5≦Pt(q) / Pb(q)≦50.

[0035] <Method for Manufacturing Positive Electrode> Next, an example of a method for manufacturing a positive electrode for a secondary battery according to an embodiment of the present disclosure will be described. The method includes the steps of: (I) preparing a positive electrode slurry containing active material particles, a binder, a thermally decomposable additive, and a dispersion medium; (II) preparing a positive electrode current collector; (III) applying the positive electrode slurry to the surface of the positive electrode current collector to form a coating film; (IV) drying the coating film to form an unrolled layer; and (V) rolling the unrolled layer to form a positive electrode active material layer.

[0036] (I) Slurry Preparation Step The positive electrode slurry is prepared by mixing a positive electrode mixture containing active material particles, a binder, and a thermally decomposable additive with a liquid dispersion medium to disperse the positive electrode mixture in the dispersion medium. The positive electrode mixture may further contain other components (e.g., a conductive material). As the liquid dispersion medium, an organic solvent having excellent affinity with both the binder and the thermally decomposable additive is used.

[0037] As the organic solvent, it is preferable to use N-methyl-2-pyrrolidone (NMP), but alcohols such as ethanol, ethers such as tetrahydrofuran, amides such as dimethylformamide, ketones such as cyclohexanone, etc. may also be used.

[0038] At least acetamidobenzoic acid is used as the thermally decomposable additive. The acetamidobenzoic acid may be mixed with a dispersion medium in advance to dissolve a portion of the acetamidobenzoic acid. For example, a dispersion of particulate acetamidobenzoic acid having an average particle diameter d1 (d1<D1) may be prepared by mixing particulate acetamidobenzoic acid having an average particle diameter D1 with a dispersion medium. Here, the average particle diameter refers to the median diameter (D) at which the cumulative volume is 50% in a volume-based particle size distribution obtained using a laser diffraction particle size distribution analyzer. 50 The average particle diameter D1 is preferably 100 μm or less, and it is more preferable to use acetamidobenzoic acid that passes through a 100 μm mesh. The average particle diameter d1 is preferably 50 μm or less.

[0039] The binder is at least one selected from the group consisting of fluororesins and hydrogenated nitrile butadiene rubber. Examples of fluororesins include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), perfluoroalkoxyalkane (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), ethylene-tetrafluoroethylene copolymer (ETFE), and vinylidene fluoride-hexafluoropropylene copolymer. These are easily soluble in organic solvents and are suitable for use in combination with sulfamic acid. Of these, PVDF is preferred.

[0040] (II) Step of Preparing a Positive Electrode Current Collector As the positive electrode current collector, a sheet-like conductive material (metal foil, mesh, net, punched sheet, etc.) is used. Among these, metal foil is preferred. Examples of the material for the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium. The thickness of the positive electrode current collector is not particularly limited, but may be, for example, 1 to 50 μm, or 5 to 30 μm.

[0041] (III) Step of Forming a Coating Film of Positive Electrode Slurry The coating film is formed by applying the positive electrode slurry to the surface of the positive electrode current collector. Examples of the coating device for the positive electrode slurry include a bar coater, gravure coater, blade coater, roll coater, comma coater, die coater, and lip coater.

[0042] In the positive electrode slurry applied to the surface of the positive electrode current collector, it is desirable that at least a portion of the thermally decomposable additive be present in the form of particles having a particle diameter of 10 μm or more. This makes it easy to obtain a positive electrode active material layer containing the thermally decomposable additive having a particle diameter of 10 μm or more. In addition, the thermally decomposable additive can easily coat only a portion of the surface of the active material particles.

[0043] (IV) Coating Drying Process Next, the coating is dried to volatilize the dispersion medium, forming an unrolled coating. By appropriately controlling the drying conditions, the thermally decomposable additive can be migrated from the lower layer region to the upper layer region (i.e., from the positive electrode current collector side to the separator side) together with the dispersion medium. As a result, the thermally decomposable additive is unevenly distributed on the separator side of the positive electrode active material layer relative to the positive electrode current collector side. For example, when the coating is dried at a temperature of 150°C or higher, or even 200°C or higher, the thermally decomposable additive is likely to be unevenly distributed on the separator side of the positive electrode active material layer relative to the positive electrode current collector side.

[0044] For example, when the thickness of the unrolled layer is T2, the probability P2b of the presence of the thermally decomposable additive present in the region from the surface of the positive electrode current collector of the unrolled layer to 0.5T2 and the probability P2t of the presence of the thermally decomposable additive present in the region from the position 0.5T2 from the surface of the positive electrode current collector of the unrolled layer to the outermost surface may satisfy 1.35≦P2t / P2b.

[0045] (V) Rolling Step of Unrolled Layer Next, the unrolled layer is rolled to form a positive electrode active material layer. The conditions for rolling are not particularly limited, but the density of the active material particles (positive electrode active material) in the positive electrode active material layer is, for example, 3.3 g / cm ３ 4.0g / cm or more ３ or less, and 3.5 g / cm ３ 4.0g / cm or more ３ The following is also acceptable.

[0046] Although the method for controlling P2t / P2b by controlling the drying conditions of the coating film has been described here, the method for controlling P2t / P2b is not limited to this. For example, a positive electrode active material layer composed of two or more layers with different contents of the thermally decomposable additive may be formed by applying two or more types of positive electrode slurries in a layered manner.

[0047] B. Secondary Battery A secondary battery according to an embodiment of the present disclosure includes the above-described secondary battery positive electrode, negative electrode, a lithium ion conductive electrolyte, and a separator interposed between the positive electrode and the negative electrode. The secondary battery may be a liquid secondary battery containing an electrolytic solution as the lithium ion conductive electrolyte, and the electrolytic solution may be a nonaqueous electrolytic solution or an aqueous electrolytic solution. A gel electrolyte or a solid electrolyte in which the electrolytic solution is held in a matrix material may be used. An all-solid-state secondary battery containing a solid electrolyte as the lithium ion conductive electrolyte may also be used. Secondary batteries include lithium ion secondary batteries that use a material that reversibly absorbs and releases lithium ions as the negative electrode active material, lithium secondary batteries in which lithium metal precipitates at the negative electrode during charging and dissolves during discharge, and solid-state batteries containing a gel electrolyte or a solid electrolyte.

[0048] The configuration of a secondary battery will be specifically described below using a lithium ion secondary battery as an example.

[0049] [Positive Electrode] The positive electrode for a secondary battery having the above-described characteristics is used as the positive electrode. The positive electrode active material layer is composed of a positive electrode mixture. The positive electrode mixture contains active material particles (particles of positive electrode active material), a binder, and a thermally decomposable additive (at least acetamidobenzoic acid) as essential components, and may also contain optional components. The optional components may include a conductive material, a thickener, etc.

[0050] The thickness of the positive electrode active material layer is not particularly limited, but may be, for example, 50 μm to 150 μm, or 75 μm to 125 μm. A single positive electrode active material layer may be formed by a plurality of layers having different morphologies. For example, two or more layers containing active material particles having different average particle sizes may be stacked, or two or more layers containing positive electrode active materials of different types or compositions may be stacked.

[0051] The average particle diameter of the active material particles (particles of the positive electrode active material) is, for example, 3 μm or more and 30 μm or less, and may be 5 μm or more and 25 μm or less. Here, the average particle diameter is also the median diameter (D 50 The active material particles can be separated and collected from the positive electrode. For example, an LA-750 manufactured by Horiba Ltd. can be used as the measuring device.

[0052] The average particle diameter of the active material particles may be measured from a cross-sectional sample of the positive electrode as described above. An SEM image of the cross-section may be taken so that 10 or more active material particles are observed, and the diameters of the circles having the same area as the cross-sections of the 10 or more active material particles may be determined by image processing, and the average of these diameters may be used as the average particle diameter.

[0053] The positive electrode active material constituting the active material particles may contain a lithium-containing transition metal oxide. From the viewpoint of increasing capacity, the lithium-containing transition metal oxide preferably contains lithium nickel oxide (composite oxide N) containing lithium and Ni and having a layered rock salt crystal structure. The proportion of the composite oxide N in the positive electrode active material is, for example, 70 mass % or more, or may be 90 mass % or more, or may be 95 mass % or more. The proportion of Ni in the metal elements other than Li contained in the composite oxide N may be 50 atomic % or more.

[0054] The composite oxide N is, for example, a compound represented by the formula (1): Li α Ni ｘ１ M1 ｘ２ M2 （１－ｘ１－ｘ２） O ２＋βHere, element M1 is at least one selected from the group consisting of V, Co, and Mn. Element M2 is at least one selected from the group consisting of Mg, Al, Ca, Ti, Cu, Zn, and Nb. However, formula (1) satisfies the following conditions: 0.95≦α≦1.05, −0.05≦β≦0.05, 0.5≦x1<1, 0≦x2≦0.5, 0<1−x1−x2≦0.5. α increases or decreases with charge and discharge.

[0055] The composite oxide N contains Ni and may contain at least one element selected from the group consisting of Co, Mn, and Al as the element M1 and the element M2. Co, Mn, and Al contribute to stabilizing the crystal structure of the composite oxide N.

[0056] From the viewpoint of reducing costs and increasing capacity, the proportion of Co in the metal elements other than Li contained in the composite oxide N is preferably 0 atomic % or more and 20 atomic % or less, and more preferably 0 atomic % or more and 15 atomic % or less.

[0057] The proportion of Mn in the metal elements other than Li may be 10 atomic % or less, or 5 atomic % or less, or 1 atomic % or more, or 3 atomic % or more, or 5 atomic % or more.

[0058] The proportion of Al in the metal elements other than Li may be 10 atomic % or less, or 5 atomic % or less. The proportion of Al in the metal elements other than Li may be 1 atomic % or more, or 3 atomic % or more, or 5 atomic % or more.

[0059] The composite oxide N is, for example, a compound represented by the formula (2): Li α Ni(1-y1-y2-y3-z)Co ｙ１ Mn ｙ２ Al ｙ３ M ｚ O ２＋βThe element M is an element other than Li, Ni, Co, Mn, Al, and oxygen, and may be at least one selected from the group consisting of Ti, Zr, Nb, Mo, W, Fe, Zn, B, Si, Mg, Ca, Sr, Sc, and Y. However, formula (2) satisfies 0.95≦α≦1.05, -0.05≦β≦0.05, 0≦y1≦0.1, 0≦y2≦0.1, 0≦y3≦0.1, and 0≦z≦0.10. 1-y1-y2-y3-z (=v), which indicates the atomic ratio of Ni, is, for example, 0.8 or more, or may be 0.85 or more, 0.90 or more, or 0.95 or more. Furthermore, v, which indicates the atomic ratio of Ni, may be 0.98 or less, or may be 0.95 or less.

[0060] Examples of conductive materials that can be optionally contained in the positive electrode active material layer include carbon nanotubes (CNTs), carbon fibers other than CNTs, and conductive particles (for example, carbon black and graphite).

[0061] [Negative Electrode] The negative electrode includes at least a negative electrode current collector. The negative electrode includes, for example, a negative electrode current collector and a negative electrode active material layer. The negative electrode active material layer is supported on one or both surfaces of the negative electrode current collector.

[0062] The negative electrode active material layer may be a negative electrode mixture layer composed of a negative electrode mixture. The negative electrode mixture layer is in the form of a membrane or film. The negative electrode mixture contains particles of the negative electrode active material as an essential component, and may contain optional components such as a binder, a conductive agent, and a thickener. Alternatively, a lithium metal foil or a lithium alloy foil may be attached to the negative electrode current collector as the negative electrode active material layer.

[0063] The negative electrode mixture layer can be formed, for example, by applying a negative electrode slurry, which is a negative electrode mixture containing particles of a negative electrode active material, a binder, etc., dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying the applied film. The dried coating film may be rolled, if necessary.

[0064] Negative electrode active materials include materials that electrochemically absorb and release lithium ions, lithium metal, lithium alloys, etc. Materials that electrochemically absorb and release lithium ions include carbon materials and alloy-based materials. Examples of carbon materials include graphite, easily graphitized carbon (soft carbon), and hardly graphitized carbon (hard carbon). Of these, graphite is preferred because of its excellent charge / discharge stability and low irreversible capacity. Examples of alloy-based materials include those containing at least one metal that can form an alloy with lithium, specifically silicon, tin, silicon alloys, tin alloys, and silicon compounds. Silicon oxide, tin oxide, etc., may be used, or other silicon-containing materials may be used.

[0065] The negative electrode current collector may be a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0066] As the binder, for example, styrene-butadiene rubber can be used, but there is no particular limitation.

[0067] Examples of conductive materials include carbon nanotubes (CNTs), carbon fibers other than CNTs, and conductive particles (for example, carbon black and graphite).

[0068] Examples of thickeners include carboxymethyl cellulose (CMC) and modified products thereof (including salts such as the Na salt), cellulose derivatives such as methyl cellulose (cellulose ethers, etc.); saponified polymers having vinyl acetate units such as polyvinyl alcohol; and polyethers (polyalkylene oxides such as polyethylene oxide, etc.).

[0069] [Separator] The separator is interposed between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulation. Examples of the separator include a microporous thin film, a woven fabric, and a nonwoven fabric. The separator is made of a polyolefin such as polypropylene or polyethylene. The separator may have a heat-resistant insulating layer on at least one surface layer. The heat-resistant insulating layer may contain an inorganic oxide filler as a main component (e.g., 80% by mass or more) or a heat-resistant resin as a main component (e.g., 40% by mass or more). The heat-resistant resin may be a polyamide resin such as aromatic polyamide (aramid), a polyimide resin, or a polyamideimide resin.

[0070] [Electrolyte] The electrolyte may be a liquid electrolyte (electrolytic solution), a gel electrolyte, or a solid electrolyte. The gel electrolyte contains a lithium salt and a matrix polymer, or a lithium salt, a non-aqueous solvent, and a matrix polymer. For example, a polymer material that absorbs the non-aqueous solvent and gels is used as the matrix polymer. Examples of the polymer material include fluororesin, acrylic resin, polyether resin, and polyethylene oxide.

[0071] The solid electrolyte may be an inorganic solid electrolyte. For example, a material known in all-solid-state lithium ion secondary batteries (e.g., oxide-based solid electrolyte, sulfide-based solid electrolyte, halide-based solid electrolyte, etc.) may be used as the inorganic solid electrolyte. The electrolyte may be, for example, a nonaqueous electrolyte solution containing a nonaqueous solvent and a lithium salt dissolved in the nonaqueous solvent. The concentration of the lithium salt in the nonaqueous electrolyte solution is, for example, 0.5 mol / L or more and 2 mol / L or less. The nonaqueous electrolyte solution may contain a known additive.

[0072] Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, and cyclic carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC) and ethylene carbonate (EC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0073] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 etc.), lithium salts of fluorine-containing acids (LiPF6, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2 etc.), lithium salts of fluorine-containing acid imides (LiN(SO ２ F) ２ , LiN(CFSO)2, LiN(CFSO)(CFSO), LiN(CFSO)2, etc.), lithium halides (LiCl, LiBr, LiI, etc.), etc. The lithium salt may be used alone or in combination of two or more.

[0074] An example of the structure of a secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween is housed in an exterior body together with an electrolyte solution. However, this is not limited thereto, and other forms of electrode groups may also be applied. For example, a stacked electrode group in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween may also be used. The shape of the secondary battery is also not limited, and may be, for example, a cylindrical type, a prismatic type, a coin type, a button type, a laminate type, or the like.

[0075] The structure of the secondary battery will be described below with reference to Fig. 1. Fig. 1 is a longitudinal cross-sectional view of a cylindrical nonaqueous secondary battery 10 that is an example of this embodiment. However, the present disclosure is not limited to the following configuration.

[0076] The secondary battery 10 includes an electrode group 18, an electrolyte (not shown), and a cylindrical battery can 22 with a bottom that accommodates these. A sealing body 11 is crimped to the opening of the battery can 22 via a gasket 21, thereby sealing the battery. The sealing body 11 includes a valve body 12, a metal plate 13, and an annular insulating member 14 interposed between the valve body 12 and the metal plate 13. The valve body 12 and the metal plate 13 are connected to each other at their respective centers. A positive electrode lead 15a extending from a positive electrode plate 15 is connected to the metal plate 13. Thus, the valve body 12 functions as an external terminal for the positive electrode. A negative electrode lead 16a extending from a negative electrode plate 16 is connected to the inner bottom surface of the battery can 22. An annular groove 22a is formed near the open end of the battery can 22. A first insulating plate 23 is disposed between one end face of the electrode group 18 and the annular groove portion 22a. A second insulating plate 24 is disposed between the other end face of the electrode group 18 and the bottom of the battery can 22. The electrode group 18 is formed by winding a positive electrode plate 15 and a negative electrode plate 16 with a separator 17 interposed therebetween.

[0077] Hereinafter, the present disclosure will be specifically described based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0078] Example 1 [Preparation of Positive Electrode] The positive electrode active material constituting the active material particles was a composite oxide N (LiNi) having an average particle diameter of 13 μm. 0.85 Co 0.10 Al 0.05 O2) was used. Furthermore, an aluminum foil having a thickness of 15 μm was prepared as a positive electrode current collector.

[0079] A positive electrode mixture containing active material particles, acetylene black, and polyvinylidene fluoride (PVDF) in a mass ratio of 95:2.5:2.5 and containing 1.3 mass% of 3-acetamidobenzoic acid (average particle size 100 μm) was added with NMP and stirred to prepare a positive electrode slurry. The stirring conditions were controlled so that the particle size of the 3-acetamidobenzoic acid would be in the range of 10 μm to 50 μm.

[0080] The positive electrode slurry was applied to the surface of an aluminum foil serving as a positive electrode current collector, and the coating was dried to form unrolled layers on both sides of the aluminum foil. At this time, the drying temperature was controlled to 200°C so that the distribution of 3-acetamidobenzoic acid in the positive electrode active material layer (described later) was Pt / Pb ≈ 1.5.

[0081] Next, the unrolled layer was rolled to obtain a positive electrode active material having a density of 3.6 g / cm ３ The total thickness of the positive electrode after rolling was 160 μm.

[0082] [Fabrication of Negative Electrode] A negative electrode mixture containing graphite, sodium carboxymethyl cellulose (CMC-Na), and styrene-butadiene rubber (SBR) as negative electrode active materials in a mass ratio of 96:2:2 was added with water and stirred to prepare a negative electrode slurry. Next, the negative electrode slurry was applied to the surface of copper foil as a negative electrode current collector, and the coating was dried and then rolled to form a negative electrode active material layer on both sides of the copper foil. The density of the negative electrode active material in the negative electrode active material layer was 1.6 g / cm ３ The total thickness of the negative electrode was 170 μm.

[0083] [Preparation of Nonaqueous Electrolyte] A nonaqueous electrolyte was prepared by dissolving LiPF6 at a concentration of 1.0 mol / L in a mixed solvent containing ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7.

[0084] [Preparation of Secondary Battery] A tab was attached to each electrode, and the positive and negative electrodes were spirally wound with a separator interposed therebetween so that the tabs were positioned at the outermost periphery to prepare an electrode assembly. The electrode assembly was inserted into an exterior case made of aluminum laminate film, and vacuum dried at 105°C for 2 hours. After that, an electrolyte solution was poured into the exterior case, and the opening of the exterior case was sealed to obtain a secondary battery A1.

[0085] Example 2 A secondary battery A2 was fabricated in the same manner as in Example 1, except that the drying temperature was controlled to room temperature so that the distribution of 3-acetamidobenzoic acid in the positive electrode active material layer would satisfy Pt / Pb≈1.

[0086] Example 3 A secondary battery A3 was fabricated in the same manner as in Example 1, except that 3-acetamidobenzoic acid was completely dissolved in NMP beforehand when preparing the positive electrode slurry. Specifically, 3-acetamidobenzoic acid in an amount equivalent to 1.3 mass% of the positive electrode mixture was added to NMP at 50°C and dissolved over 3 hours. Thereafter, active material particles, acetylene black, and polyvinylidene fluoride (PVDF) were added to the NMP solution of 3-acetamidobenzoic acid in a mass ratio of 95:2.5:2.5 and stirred to prepare a positive electrode slurry. The drying temperature was controlled to 200°C so that the distribution of 3-acetamidobenzoic acid in the positive electrode active material layer was Pt / Pb ≈ 1.5.

[0087] Example 4 A secondary battery A4 was fabricated in the same manner as in Example 3, except that the drying temperature was controlled to room temperature so that the distribution of 3-acetamidobenzoic acid in the positive electrode active material layer would satisfy Pt / Pb≈1.

[0088] Comparative Example 1 A secondary battery B1 was fabricated in the same manner as in Example 1, except that the positive electrode mixture did not contain 3-acetamidobenzoic acid (the content of 3-acetamidobenzoic acid was 0%).

[0089] [Evaluation 1] The secondary batteries obtained in Examples 1 to 4 and Comparative Example 1 were evaluated as follows. (a) In an environment of 25°C, the batteries were charged at a constant current of 0.3 It until the voltage reached 4.2 V, and then at a constant voltage of 4.2 V, they were charged at a constant voltage of 0.05 It until the current reached 0.05 It. (b) In an environment of 25°C, the tip of a round nail (diameter 2.7 mm) was contacted with the center of the battery charged in (a) and pierced at a rate of 1 mm / sec. Immediately after detecting a battery voltage drop (Δ50 mV) due to an internal short circuit, the nail's piercing motion was stopped. The surface temperature of the battery was measured 1 minute after the battery shorted. The surface temperatures of Batteries A1 to A4 and Battery B1 are shown in Table 1.

[0090]

[0091] Table 1 shows that batteries A1 to A4 had improved safety in the event of an internal short circuit compared to battery B1. Furthermore, when comparing examples with the same Pt / Pb ratio, the improvement in safety was significant when particles or aggregates of 3-acetamidobenzoic acid of 10 μm or larger were contained in the positive electrode active material layer.

[0092] Example 5 A secondary battery A4 was produced in the same manner as in Example 1, except that the thermally decomposable additive was changed from 3-acetamidobenzoic acid to 4-acetamidobenzoic acid (average particle size: 100 μm) and the drying temperature was controlled so that the distribution of 4-acetamidobenzoic acid in the positive electrode active material layer was Pt / Pb≈1.35.

[0093] Example 6 A secondary battery A6 was fabricated in the same manner as in Example 2, except that the thermally decomposable additive was changed from 3-acetamidobenzoic acid to 4-acetamidobenzoic acid (average particle size: 100 μm).

[0094] Example 7 A secondary battery A7 was produced in the same manner as in Example 3, except that the thermally decomposable additive was changed from 3-acetamidobenzoic acid to 4-acetamidobenzoic acid, and the drying temperature was controlled so that the distribution of 4-acetamidobenzoic acid in the positive electrode active material layer was Pt / Pb ≈ 1.35.

[0095] Example 8 A secondary battery A8 was produced in the same manner as in Example 4, except that the thermally decomposable additive was changed from 3-acetamidobenzoic acid to 4-acetamidobenzoic acid. [Evaluation 2] The secondary batteries obtained in Examples 5 to 8 were evaluated in the same manner as in Evaluation 1. The surface temperatures of batteries A5 to A8 are shown in Table 2.

[0096]

[0097] Table 2 shows that batteries A5 to A8 were able to improve safety in the event of an internal short circuit compared to battery B1 in Table 1. Furthermore, when comparing examples with the same Pt / Pb ratio, the improvement in safety was significant when particles or aggregates of 4-acetamidobenzoic acid of 10 μm or larger were contained in the positive electrode active material layer. Tables 1 and 2 also show that the effect of improving safety was greater with 3-acetamidobenzoic acid than with 4-acetamidobenzoic acid.

[0098] The positive electrode for a secondary battery according to the present disclosure and a secondary battery including the same are useful as main power sources for mobile communication devices, portable electronic devices, electric vehicles, and the like.

[0099] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0100] 10: Secondary battery, 11: Sealing body, 12: Valve body, 13: Metal plate, 14: Insulating member, 15: Positive electrode plate, 15a: Positive electrode lead, 16: Negative electrode plate, 16a: Negative electrode lead, 17: Separator, 18: Electrode group, 21: Gasket, 22: Battery can, 22a: Groove portion, 23: First insulating plate, 24: Second insulating plate

Claims

1. a positive electrode current collector; and a positive electrode active material layer supported on the positive electrode current collector, the positive electrode active material layer includes active material particles, a binder, and a thermally decomposable additive; The thermally decomposable additive comprises acetamidobenzoic acid.

2. 2. The positive electrode for a secondary battery according to claim 1, wherein at least a portion of the thermally decomposable additive is present in the positive electrode active material layer as particles having a particle diameter of 10 μm or more.

3. 3. The positive electrode for a secondary battery according to claim 2, wherein, in a cross-sectional observation of the positive electrode active material layer, five or more particles of the thermally decomposable additive having a particle diameter of 10 μm or more are observed in a rectangular observation field defined by a length of 300 μm in a plane direction of the positive electrode active material layer and a thickness of the positive electrode active material layer.

4. 4. The positive electrode for a secondary battery according to claim 1, wherein the content of the acetamidobenzoic acid contained in the positive electrode active material layer is 0.1% by mass or more and 5% by mass or less.

5. 4. The positive electrode for a secondary battery according to claim 1, wherein, when a cross section of the positive electrode active material layer is analyzed with a scanning electron microscope and an electron beam microanalyzer, an area of ​​a portion where the acetamidobenzoic acid is present is 0.5 area % to 27 area % in a rectangular observation field defined by a length of 300 μm in a plane direction of the positive electrode active material layer and a thickness T of the positive electrode active material layer.

6. 4. The positive electrode for a secondary battery according to claim 1, wherein the acetic acid component is contained in an amount of 500 μg or more and 5000 μg or less per gram of the thermally decomposable additive.

7. 4. The positive electrode for a secondary battery according to claim 1, wherein the thermally decomposable additive coats a part of the surface of the active material particles.

8. 4. The positive electrode for a secondary battery according to claim 1, wherein the thermally decomposable additive is unevenly distributed on the outermost surface side of the positive electrode active material layer relative to the positive electrode current collector side.

9. When the thickness of the positive electrode active material layer is T, an existence probability Pb ​​of the thermally decomposable additive present in a region of the positive electrode active material layer from the surface of the positive electrode current collector to 0.5 T; 9. The positive electrode for a secondary battery according to claim 8, wherein an existence probability Pt of the thermally decomposable additive present in a region of the positive electrode active material layer from a position 0.5 T from the surface of the positive electrode current collector to the outermost surface satisfies 1.35≦Pt / Pb.

10. the active material particles contain a lithium-containing transition metal oxide, The lithium-containing transition metal oxide includes a lithium nickel oxide that contains lithium and Ni and has a layered rock salt crystal structure, 4. The positive electrode for a secondary battery according to claim 1, wherein the proportion of Ni in the metal elements other than Li contained in the lithium nickel oxide is 50 atomic % or more.

11. The lithium nickel oxide is Formula: Li α Ni ｘ１ M1 ｘ２ M2 （１－ｘ１－ｘ２） O ２＋β is expressed as The element M1 is at least one selected from the group consisting of V, Co, and Mn, The element M2 is at least one selected from the group consisting of Mg, Al, Ca, Ti, Cu, Zn, and Nb, ０．９５≦α≦１．０５、 －０．０５≦β≦０．０５、 0.5≦x1<1, 0≦x2≦0.5, 0<1-x1-x2≦0.5 The positive electrode for a secondary battery according to claim 10 , which satisfies the above.

12. 4. The positive electrode for a secondary battery according to claim 1, wherein the binder is at least one selected from the group consisting of fluororesin and hydrogenated nitrile butadiene rubber.

13. A secondary battery comprising the positive electrode for secondary batteries according to any one of claims 1 to 3, a negative electrode, a lithium ion conductive electrolyte, and a separator interposed between the positive electrode and the negative electrode.

14. preparing a positive electrode slurry containing active material particles, a binder, a thermally decomposable additive, and a dispersion medium; preparing a positive electrode current collector; applying the positive electrode slurry to a surface of the positive electrode current collector to form a coating film; a step of drying the coating film to form an unrolled layer; rolling the unrolled phase to form a positive electrode active material layer; Equipped with the thermally decomposable additive comprises acetamidobenzoic acid; The method for producing a positive electrode for a secondary battery, wherein the dispersion medium contains an organic solvent.

15. 15. The method for producing a positive electrode for a secondary battery according to claim 14, wherein at least a portion of the thermally decomposable additive is present in the positive electrode slurry as particles having a particle diameter of 10 μm or more.

16. When the thickness of the positive electrode active material layer is T, an existence probability Pb ​​of the thermally decomposable additive present in a region of the positive electrode active material layer from the surface of the positive electrode current collector to 0.5 T; 16. The method for producing a positive electrode for a secondary battery according to claim 14 or 15, wherein an existence probability Pt of the thermally decomposable additive present in a region of the positive electrode active material layer from a position 0.5 T from the surface of the positive electrode current collector to the outermost surface satisfies 1.35≦Pt / Pb.