secondary batteries

Abstract

The secondary battery comprises: a positive electrode having a positive electrode active material layer, the positive electrode active material layer comprising a positive electrode active material, a positive electrode binder, and a positive electrode conductive agent; a negative electrode comprising a negative electrode active material; and an electrolyte. The positive electrode active material comprises a lithium cobalt composite oxide, the positive electrode binder comprises a vinylidene fluoride polymer having a melting point above 160°C and below 170°C, the positive electrode conductive agent comprises carbon black having a hollow structure, and the negative electrode active material comprises a carbon material. The weight ratio of the positive electrode active material to the sum of the weights of the positive electrode active material, the positive electrode binder, and the positive electrode conductive agent is 97.9% by weight or more and 98.5% by weight or less; the weight ratio of the positive electrode binder to the sum of the weights of the positive electrode active material, the positive electrode binder, and the positive electrode conductive agent is 0.8% by weight or more and 1.4% by weight or less; the weight ratio of the positive electrode conductive agent to the sum of the weights of the positive electrode active material, the positive electrode binder, and the positive electrode conductive agent is 0.5% by weight or more and 1.1% by weight or less; and the bulk density of the positive electrode active material layer is 4.15 g / cm³. 3 The concentration of fluorine atoms, as measured by surface analysis of the positive electrode active material layer using X-ray photoelectron spectroscopy, is 1.9% or more and 3.0% or less.

Description

Technical Field

[0001] This invention relates to a secondary battery. Background Technology

[0002] With the widespread use of mobile phones and other electronic devices, secondary batteries are being developed as small, lightweight power sources capable of achieving high energy density. These secondary batteries possess a positive electrode, a negative electrode, and an electrolyte, and their structure has been the subject of various studies.

[0003] Specifically, to obtain excellent storage properties, a LiCoO2-based compound is used as the positive electrode active material, and fluorine atoms are detected in XPS analysis of the positive electrode surface (see, for example, Patent Document 1). To improve cycle performance, a portion (first region) of the positive electrode active material contains lithium cobalt oxide, and a range for the fluorine concentration measured by X-ray photoelectron spectroscopy is specified (see, for example, Patent Document 2). To improve the adhesion between the electrode and the separator, the oxygen atom ratio measured using XPS on the surface of the vinylidene fluoride copolymer particles is specified (see, for example, Patent Document 3). The vinylidene fluoride copolymer particles contain vinylidene fluoride and a compound having functional groups containing oxygen atoms.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Publication No. 2002-093405

[0007] Patent Document 2: Japanese Patent Application Publication No. 2018-206747

[0008] Patent document 3: Japanese Patent Application Publication No. 2018-172596. Summary of the Invention

[0009] Various studies have been conducted to improve the performance of secondary batteries, but there is still room for improvement in balancing energy density and resistance reduction.

[0010] This technology was proposed in view of the above-mentioned problems, and its purpose is to provide a secondary battery that can achieve both increased energy density and reduced resistance.

[0011] One embodiment of the secondary battery of this technology comprises: a positive electrode having a positive electrode active material layer, the positive electrode active material layer comprising a positive electrode active material, a positive electrode binder, and a positive electrode conductive agent; a negative electrode comprising a negative electrode active material; and an electrolyte. The positive electrode active material comprises a lithium cobalt composite oxide, the positive electrode binder comprises a vinylidene fluoride polymer having a melting point of 160°C or higher and 170°C or lower, the positive electrode conductive agent comprises carbon black having a hollow structure, and the negative electrode active material comprises a carbon material. The weight ratio of the positive electrode active material to the sum of the weights of the positive electrode active material, the positive electrode binder, and the positive electrode conductive agent is 97.9% by weight or more and 98.5% by weight or less; the weight ratio of the positive electrode binder to the sum of the weights of the positive electrode active material, the positive electrode binder, and the positive electrode conductive agent is 0.8% by weight or more and 1.4% by weight or less; the weight ratio of the positive electrode conductive agent to the sum of the weights of the positive electrode active material, the positive electrode binder, and the positive electrode conductive agent is 0.5% by weight or more and 1.1% by weight or less; and the bulk density of the positive electrode active material layer is 4.15 g / cm³. 3 The concentration of fluorine atoms, as measured by surface analysis of the positive electrode active material layer using X-ray photoelectron spectroscopy, is 1.9% or more and 3.0% or less.

[0012] According to one embodiment of the present technology, the secondary battery comprises a positive electrode active material containing a lithium cobalt composite oxide, a positive electrode binder containing a vinylidene fluoride polymer having the aforementioned melting point, a positive electrode conductive agent containing carbon black having a hollow structure, and a negative electrode active material containing a carbon material. Furthermore, the weight ratios of the positive electrode active material, the positive electrode binder, and the positive electrode conductive agent, the bulk density of the positive electrode active material layer, and the elemental concentration of fluorine atoms measured by surface analysis of the positive electrode active material layer using X-ray photoelectron spectroscopy are within the aforementioned ranges. Therefore, it is possible to achieve both increased energy density and reduced resistance.

[0013] Here, "lithium-cobalt composite oxide" is a general term for oxides containing lithium and cobalt as constituent elements, and "vinylidene fluoride polymer" is a general term for polymers containing vinylidene fluoride as a polymerization unit. It should be noted that details regarding lithium-cobalt composite oxide and vinylidene fluoride polymer will be described later.

[0014] It should be noted that the effects of this technology are not necessarily limited to those described herein, but can be any of the series of effects related to this technology described later. Attached Figure Description

[0015] Figure 1 This is a perspective view showing the structure of a secondary battery in one embodiment of the present technology.

[0016] Figure 2 It is shown Figure 1 The diagram shows a cross-sectional view of the structure of the battery element.

[0017] Figure 3 This is a block diagram illustrating the structure of a suitable example of a secondary battery. Detailed Implementation

[0018] Hereinafter, an embodiment of the present technology will be described in detail with reference to the accompanying drawings. It should be noted that the description is presented in the following order.

[0019] 1. Secondary battery

[0020] 1-1. Structure

[0021] 1-2. Actions

[0022] 1-3. Manufacturing Method

[0023] 1-4. Functions and Effects

[0024] 2. Variations

[0025] 3. Uses of secondary batteries

[0026] <1. Secondary Battery>

[0027] First, a secondary battery according to one embodiment of this technology will be described.

[0028] The secondary battery described herein is a secondary battery in which battery capacity is obtained by the intercalation and deintercalation of electrode reactants, and it includes a positive electrode, a negative electrode, and an electrolyte in liquid form. In this secondary battery, in order to prevent the electrode reactants from depositing on the surface of the negative electrode during charging, the charging capacity of the negative electrode is greater than the discharging capacity of the positive electrode. That is, the electrochemical capacity per unit area of ​​the negative electrode is set to be greater than the electrochemical capacity per unit area of ​​the positive electrode.

[0029] There are no particular restrictions on the types of substances used in the electrode reactions; specifically, they are light metals such as alkali metals and alkaline earth metals. Alkali metals include lithium, sodium, and potassium, while alkaline earth metals include beryllium, magnesium, and calcium.

[0030] The following example uses lithium as the electrode reactant. A secondary battery that utilizes the insertion and extraction of lithium to obtain battery capacity is called a lithium-ion secondary battery. In this lithium-ion secondary battery, lithium is inserted and extracted in an ionic state.

[0031] <1-1. Structure>

[0032] Figure 1 The three-dimensional structure of a secondary battery is shown. Figure 2 It shows Figure 1 The cross-sectional structure of the battery element 10 is shown. Additionally, Figure 1The battery element 10 and the outer membrane 20 are shown in a state where they are separated from each other. Figure 2 Only a portion of battery element 10 is shown.

[0033] like Figure 1 As shown, the secondary battery includes a battery element 10, an outer film 20, a positive electrode lead 31, and a negative electrode lead 32. The secondary battery described here is a laminated film type secondary battery that uses a flexible (or soft) outer packaging component (outer film 20) to house the battery element 10.

[0034] [Exterior film]

[0035] like Figure 1 As shown, the outer casing 20 is a film-shaped component that can be folded in the direction of arrow R (dotted line). As described above, since the outer casing 20 houses the battery element 10, it also houses the electrolyte along with the positive electrode 11 and negative electrode 12, which will be described later. A recessed portion 20U (so-called deep-drawn portion) for accommodating the battery element 10 is provided on the outer casing 20.

[0036] Specifically, the outer film 20 is a laminated film consisting of three layers stacked sequentially from the inside: a welding layer, a metal layer, and a surface protective layer. When the outer film 20 is folded, the outer peripheries of the opposing welding layers are welded together. Thus, the outer film 20 has a pouch-like structure capable of sealing the battery element 10 inside. The welding layer contains a polymer compound such as polypropylene. The metal layer contains a metallic material such as aluminum. The surface protective layer contains a polymer compound such as nylon.

[0037] Furthermore, there are no particular restrictions on the structure (number of layers) of the outer membrane 20; it can be one layer, two layers, or four or more layers. That is, the outer membrane 20 is not limited to a laminated membrane; it can also be a single-layer membrane.

[0038] An adhesive film 21 is inserted between the outer membrane 20 and the positive electrode lead 31, and an adhesive film 22 is inserted between the outer membrane 20 and the negative electrode lead 32. Adhesive films 21 and 22 are components used to prevent external air from entering the interior of the outer membrane 20, and contain one or more polymeric compounds such as polyolefins, which are adhesive to each of the positive and negative electrode leads 31 and 32. The polyolefin is polyethylene, polypropylene, modified polyethylene, or modified polypropylene, etc. Alternatively, one or both of the adhesive films 21 and 22 may be omitted.

[0039] [Battery Components]

[0040] like Figure 1 as well as Figure 2As shown, the battery element 10 is housed inside the outer membrane 20 and includes a positive electrode 11, a negative electrode 12, a separator 13, and an electrolyte (not shown). The electrolyte is immersed in the positive electrode 11, the negative electrode 12, and the separator 13, respectively.

[0041] Here, the battery element 10 is a structure (wound electrode body) in which the positive electrode 11 and the negative electrode 12 are stacked on top of each other with a separator 13 in between, and the positive electrode 11, the negative electrode 12 and the separator 13 are wound around a winding axis (an imaginary axis extending along the Y-axis direction) as the center. Therefore, the positive electrode 11 and the negative electrode 12 are opposite to each other with a separator 13 in between.

[0042] The three-dimensional shape of the battery element 10 is flat. That is, the shape of the cross-section (along the XZ plane) of the battery element 10 intersecting the winding axis is a flat shape defined by the major axis and the minor axis, more specifically, a flat, approximately elliptical shape. The major axis is an imaginary axis extending along the X-axis direction and having a relatively large length, and the minor axis is an imaginary axis extending along the Z-axis direction intersecting the X-axis direction and having a relatively small length.

[0043] (positive electrode)

[0044] like Figure 2 As shown, the positive electrode 11 has a positive electrode active material layer 11B. Here, the positive electrode 11 has the aforementioned positive electrode active material layer 11B and a positive electrode current collector 11A supporting the positive electrode active material layer 11B.

[0045] Specifically, the positive electrode 11 includes a positive current collector 11A with one side facing each other and positive active material layers 11B disposed on both sides of the positive current collector 11A. Therefore, the positive electrode 11 has two positive active material layers 11B. In addition, since the positive active material layer 11B is disposed on only one side of the positive current collector 11A, the positive electrode 11 may have only one positive active material layer 11B.

[0046] The positive current collector 11A comprises one or more conductive materials, such as metals, aluminum, nickel, and stainless steel. The positive active material layer 11B comprises a positive active material, a positive binder, and a positive conductive agent. The method for forming the positive active material layer 11B is not particularly limited; specifically, it can be one or more methods, such as coating.

[0047] (Positive electrode active material)

[0048] The positive electrode active material contains lithium-containing compounds capable of lithium insertion / extraction, and more specifically, contains any one or more of lithium-cobalt composite oxides. As mentioned above, "lithium-cobalt composite oxides" is a general term for oxides containing lithium and cobalt as constituent elements, and has a layered rock-salt type crystal structure. This is because high energy density can be obtained.

[0049] The type (composition) of lithium-cobalt composite oxides is any oxide containing lithium and cobalt as constituent elements, without any particular restrictions. Specifically, lithium-cobalt composite oxides contain lithium, cobalt, and other elements as constituent elements. The other elements are any one or more elements belonging to groups 1 to 17 of the long-period periodic table (excluding lithium, cobalt, and oxygen).

[0050] More specifically, the lithium cobalt composite oxide comprises one or more compounds represented by formula (1). This is because a high energy density can be stably obtained.

[0051] Li x Co 1-y M y O 2-z X z …(1)

[0052] (M is at least one of Ti, V, Cr, Mn, Fe, Ni, Cu, Na, Mg, Al, Si, Sn, K, Ca, Zn, Ga, Sr, Y, Zr, Nb, Mo, Ba, La, W, and B. X is at least one of F, Cl, Br, I, and S. x, y, and z satisfy 0.8 < x < 1.2, 0 ≤ y < 0.15, and 0 ≤ z < 0.05. Furthermore, the composition of Li varies depending on the charge / discharge state, and the value of x is for the fully discharged state.)

[0053] As shown in equation (1), the lithium-cobalt composite oxide is an oxide containing lithium, cobalt, a first other element (M), and a second other element (X) as constituent elements. Furthermore, from the range of possible values ​​for y (y≥0), it can be seen that the lithium-cobalt composite oxide may or may not contain the first other element (M) as a constituent element. Additionally, from the range of possible values ​​for z (z≥0), it can be seen that the lithium-cobalt composite oxide may or may not contain the second other element (X) as a constituent element.

[0054] Specific examples of lithium-cobalt composite oxides are LiCoO2 and LiCo. 0.90 Al 0.10 O2, LiCo 0.98 Al 0.02 O2, LiCo 0.98 Al 0.01 Mg 0.01 O2, LiCo 0.98 Al 0.01 Mg 0.01 O 1.98 F 0.02 LiCo 0.98 Mn0.02 O2, LiCo 0.98 Zr 0.02 O2 and LiCo 0.98 Ti 0.02 O2.

[0055] It should be noted that the positive electrode active material only needs to contain the above-mentioned lithium cobalt composite oxide, and may also contain any one or more other lithium-containing compounds.

[0056] There are no particular restrictions on the types of other lithium-containing compounds; specifically, they include lithium transition metal compounds. The term "lithium transition metal compounds" is a general term for compounds containing lithium and one or more transition metal elements as constituent elements, and may also contain other elements. Details regarding other elements are as described above. Furthermore, the lithium transition metal compounds described herein do not include the aforementioned lithium-cobalt composite oxides.

[0057] There are no particular restrictions on the types of lithium transition metal compounds; specifically, they include oxides, phosphoric acid compounds, silicate compounds, and borate compounds. Specific examples of oxides are LiNiO2 and LiNi. 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.8 Co 0.15 Al 0.05 O2, LiNi 0.33 Co 0.33 Mn 0.33 O2, Li 1.2 Mn 0.52 Co 0.175 Ni 0.1 O2, Li 1.15 (Mn 0.65 Ni 0.22 Co 0.13 O2 and LiMn2O4, etc. Specific examples of phosphoric acid compounds are LiFePO4, LiMnPO4, and LiFe 0.5 Mn 0.5 PO4 and LiFe 0.3 Mn 0.7 PO4, etc.

[0058] (Positive electrode binder)

[0059] The positive electrode binder includes an adhesive material, and more specifically, includes any one or more of vinylidene fluoride polymers with low melting points. As mentioned above, "vinylidene fluoride polymer" is a general term for polymers containing vinylidene fluoride as a polymerization unit, and the melting point of the polymer is 160°C to 170°C.

[0060] This is because, as described later, by compressing the positive electrode 11 during the manufacturing process of the secondary battery (the manufacturing process of the positive electrode 11), a mixed film of positive electrode binder and positive electrode conductive agent covers the surface of the positive electrode active material. As a result, the friction between the positive electrode active materials (interparticle friction) is reduced, thus the positive electrode active material is less likely to break during the compression molding of the positive electrode active material layer 11B. The term "breakage of the positive electrode active material" includes not only rupture of the positive electrode active material but also the formation of cracks on the positive electrode active material.

[0061] To distinguish between the aforementioned vinylidene fluoride polymers with low melting points (160°C to 170°C) and those with high melting points (above 170°C, more specifically, above 170°C but below 175°C), the former will be referred to as "low-melting-point vinylidene fluoride polymers," and the latter as "high-melting-point vinylidene fluoride polymers." As mentioned above, the melting point of a high-melting-point vinylidene fluoride polymer is a temperature above 170°C, and more specifically, a temperature within the range above 170°C but below 175°C.

[0062] The structure of low-melting-point vinylidene fluoride polymers is not particularly limited as long as they are polymers with low melting points and contain vinylidene fluoride as the polymerization unit. Therefore, low-melting-point vinylidene fluoride polymers can be homopolymers, copolymers, or both.

[0063] Low-melting-point vinylidene fluoride polymers, which are homopolymers, are called polyvinylidene fluoride (PVDF). PVDF, as a low-melting-point polymer, is a polymer obtained by introducing one or more functional groups into ordinary PVDF, which is a high-melting-point polymer. In other words, PVDF, as a low-melting-point polymer, is a polymer obtained by modifying ordinary PVDF with one or more functional groups, and therefore has a low melting point.

[0064] The low-melting-point vinylidene fluoride polymer as a copolymer contains vinylidene fluoride and one or more monomers (excluding vinylidene fluoride) as polymerization units, and is therefore a polymer copolymerized from vinylidene fluoride and one or more monomers. That is, the low-melting-point vinylidene fluoride polymer as a copolymer not only contains vinylidene fluoride, but also contains one or more monomers as polymerization units, and therefore has a low melting point.

[0065] The type of monomer is not particularly limited as long as it can achieve the melting point (160℃~170℃) of the low-melting-point vinylidene fluoride polymer. Specifically, hexafluoropropylene, etc. It should be noted that there is no particular limitation on the copolymerization amount of monomers in the copolymer (low-melting-point vinylidene fluoride polymer) and it can be set arbitrarily.

[0066] It should be noted that the positive electrode binder only needs to contain the aforementioned low-melting-point vinylidene fluoride polymer, and may also contain any one or more other binder materials. Furthermore, the other binder materials described herein do not include low-melting-point vinylidene fluoride polymers.

[0067] Other adhesive materials include synthetic rubbers and polymers. Specific examples of synthetic rubbers include styrene-butadiene rubber, fluorinated rubbers, and ethylene propylene diene monomer (EPDM) rubber. Specific examples of polymers include polyvinylidene fluoride (PVDF), a common polymer of high-melting-point vinylidene fluoride (melting point = above 170°C and below 175°C), polyimide, and carboxymethyl cellulose.

[0068] (Positive conductive agent)

[0069] The positive electrode conductive agent comprises conductive materials, and more specifically, any one or more types of carbon black having a hollow structure. This is because, as described above, during the manufacturing process of the secondary battery, when the positive electrode 11 is compressed and molded, a mixed film of the positive electrode binder and the positive electrode conductive agent covers the surface of the positive electrode active material, which can reduce the friction between the positive electrode active materials, thus making the positive electrode active material less prone to damage.

[0070] A specific example of carbon black with a hollow structure is Ketjen black. This is because the mixed film of the positive electrode binder and the positive electrode conductive agent can easily cover the surface of the positive electrode active material, thus reducing the friction between the positive electrode active materials.

[0071] It should be noted that the positive electrode conductive agent only needs to contain carbon black with the hollow structure described above, and may also contain any one or more other conductive materials. Furthermore, the other conductive materials described herein do not include carbon black with a hollow structure.

[0072] Other conductive materials are carbon materials, such as graphite and acetylene black. Other conductive materials can also be metals and polymer compounds.

[0073] (additive)

[0074] It should be noted that the positive electrode active material layer 11B may also contain any one or more additives. The type of additive can be arbitrarily selected according to its function, etc. Specific examples of additives are polyvinylpyrrolidone, etc. This is because the dispersibility of the positive electrode active material, etc., can be promoted in the preparation process of the positive electrode slurry described later. That is, even if there are agglomerates of the positive electrode active material, etc., these agglomerates are easily dispersed, thus improving the dispersibility of the positive electrode active material, etc. As a result, the coatability of the positive electrode slurry is improved, and the adhesion of the positive electrode active material layer 11B to the positive electrode current collector 11A is improved. There is no particular limitation on the content of polyvinylpyrrolidone in the positive electrode active material layer 11B, specifically, it is 0.01% to 0.05% by weight.

[0075] (mixing ratio)

[0076] The mixing ratio of the positive electrode active material, the positive electrode binder, and the positive electrode conductive agent is set within a predetermined range. In particular, the mixing ratio of each of the positive electrode binder and the positive electrode conductive agent is set to be sufficiently small relative to the mixing ratio of the positive electrode active material, and conversely, the mixing ratio of the positive electrode active material is set to be sufficiently large relative to the mixing ratio of each of the positive electrode binder and the positive electrode conductive agent.

[0077] Specifically, the ratio R1 of the weight M1 of the positive electrode active material to the sum of the weights M1 of the positive electrode active material, M2 of the positive electrode binder, and M3 of the positive electrode conductive agent is 97.9% to 98.5% by weight. This ratio R1 can be calculated by R1 = [M1 / (M1+M2+M3)] × 100.

[0078] The ratio R2 of the weight M2 of the positive electrode binder to the sum of the weights M1, M2, and M3 of the positive electrode active material is 0.8% to 1.4% by weight. This ratio R2 can be calculated by R2 = [M2 / (M1+M2+M3)] × 100.

[0079] The ratio R3 of the weight M3 of the positive electrode conductive agent to the sum of the weights M1, M2, and M3 of the positive electrode active material is 0.5% to 1.1% by weight. This ratio R3 can be calculated by R3 = [M3 / ×(M1+M2+M3)]×100.

[0080] The ratios R1, R2, and R3 are within the aforementioned ranges because this allows for a more balanced relationship between the ratios R1, R2, and R3, while relatively increasing the weight M1 of the positive electrode active material and decreasing the weights M2 of the positive electrode binder and M3 of the positive electrode conductive agent. Therefore, firstly, since the proportion of the positive electrode active material in the positive electrode active material layer 11B increases with increasing ratio R1, a high energy density can be obtained. Secondly, since the mixed film of the positive electrode binder and the positive electrode conductive agent easily and uniformly covers the surface of the positive electrode active material, the friction between the positive electrode active materials is stably reduced, thus ensuring the stability and resistance to breakage of the positive electrode active material layer 11B during compression molding. Thirdly, even if the ratios R2 and R3 are relatively small, the positive electrode active materials can easily bond to each other via the mixed film, and the positive electrode active materials can easily be electrically connected to each other via the mixed film.

[0081] The steps for determining the proportions R1, R2, and R3 are as follows. First, the secondary battery is disassembled to recover the positive electrode 11. Next, the positive electrode 11 (positive electrode active material layer 11B) is analyzed using thermogravimetric analysis, and the weights M1, M2, and M3 of the positive electrode active material, binder, and conductive agent are measured. Finally, based on the weights M1, M2, and M3, the proportions R1, R2, and R3 are calculated respectively.

[0082] (bulk density)

[0083] As described above, if the ratios R1, R2, and R3 are within a predetermined range, the friction between the positive electrode active materials is reduced, thus the positive electrode active materials are less likely to break during the compression molding of the positive electrode active material layer 11B. Therefore, in the manufacturing process of the positive electrode 11, the positive electrode active material layer 11B can be fully compressed and molded while suppressing the breakage of the positive electrode active material.

[0084] Specifically, if the ratios R1, R2, and R3 are within predetermined ranges, the friction between the positive electrode active materials decreases; if the ratios R1, R2, and R3 are outside the predetermined ranges, the friction between the positive electrode active materials does not decrease. In the former case, compared to the latter, the bulk density of the positive electrode active material layer 11B is sufficiently increased. Specifically, the bulk density of the positive electrode active material layer 11B is 4.15 g / cm³. 3 The preferred value is 4.15 g / cm³. 3 ~4.20g / cm 3 .

[0085] (Physical properties)

[0086] When the surface of the positive electrode active material layer 11B was analyzed (elemental analysis) using X-ray photoelectron spectroscopy (XPS), the elemental concentration of fluorine atoms measured on the surface of the positive electrode active material layer 11B became sufficiently low. Specifically, the elemental concentration of fluorine atoms measured by surface analysis of the positive electrode active material layer 11B using XPS was 1.9% to 3.0%. This is because the amount of fluorine reactants (LiF) formed in the positive electrode active material layer 11B is reduced.

[0087] In detail, as described above, the positive electrode binder comprises a low-melting-point vinylidene fluoride polymer, and the proportion R2 of this positive electrode binder is sufficiently small relative to the proportion R1 of the positive electrode active material. Therefore, when the positive electrode active material layer 11B is heated during the secondary battery manufacturing process, fluorine reactants are difficult to form due to the fluorine atoms in the positive electrode binder. Consequently, even though the positive electrode binder contains fluorine as a constituent element, fluorine reactants are difficult to form when the positive electrode active material layer 11B is heated, and thus the elemental concentration of fluorine atoms measured by surface analysis of the positive electrode active material layer 11B using XPS becomes sufficiently small.

[0088] (negative electrode)

[0089] like Figure 2 As shown, the negative electrode 12 is opposite to the positive electrode 11 across a separator 13. The negative electrode 12 includes a negative current collector 12A having one opposite side and two negative active material layers 12B disposed on both sides of the negative current collector 12A. Alternatively, the negative active material layer 12B may be disposed on only one side of the negative current collector 12A.

[0090] The negative electrode current collector 12A comprises any one or more conductive materials, such as copper, aluminum, nickel, and stainless steel. The negative electrode active material layer 12B comprises any one or more negative electrode active materials capable of lithium insertion and extraction, and may also include a negative electrode binder and a negative electrode conductive agent. The method for forming the negative electrode active material layer 12B is not particularly limited; specifically, it can be any one or more methods such as coating.

[0091] The negative electrode active material contains active material, more specifically, any one or more carbon materials. This is because a high energy density can be obtained. The carbon material is graphite, easily graphitized carbon, and difficult-to-graphitize carbon, etc., and the graphite is natural graphite and artificial graphite, etc. Preferably, the carbon material contains one or two of artificial graphite and natural graphite. This is because the charge and discharge reaction can proceed smoothly and stably in the negative electrode 12.

[0092] In addition to the carbon materials mentioned above, the negative electrode active material can also contain any one or more silicon-containing materials. This is because the energy density is further increased. The term "silicon-containing material" is a general term for materials containing silicon as a constituent element; it can be silicon monomers, alloys, or compounds, or a mixture of two or more of them, or a material containing two or more phases of them. There are no particular restrictions on the mixing ratio of carbon materials and silicon-containing materials; it can be set arbitrarily.

[0093] Specific examples of silicon-containing materials include SiB4, SiB6, Mg2Si, Ni2Si, TiSi2, MoSi2, CoSi2, NiSi2, CaSi2, CrSi2, Cu5Si, FeSi2, MnSi2, NbSi2, TaSi2, VSi2, WSi2, ZnSi2, and SiO. x (0<x≤2) and LiSiO, etc. Additionally, SiO x x can also satisfy 0.2 < x < 1.4.

[0094] It should be noted that the negative electrode active material contains the aforementioned carbon material. If required, it may also contain carbon and silicon-containing materials, and possibly any one or more other active materials. Furthermore, the other active materials described herein do not include either carbon or silicon-containing materials.

[0095] Other active materials are any one or more of the following metallic materials. These metallic materials contain any one or more of a metallic element and a half-metallic element capable of forming an alloy with lithium, such as tin. Furthermore, metallic materials can be monomers, alloys, or compounds, or mixtures of two or more of them, or materials containing two or more of their phases.

[0096] A specific example of a metallic material is SnO. w (0<w≤2), SnSiO3, LiSnO, and Mg2Sn, etc. Additionally, materials containing tin and silicon as constituent elements are not classified as silicon-containing materials, but rather as metallic materials.

[0097] The negative electrode binder includes any one or more of synthetic rubbers and polymeric compounds. Synthetic rubbers include styrene-butadiene rubber, fluorinated rubbers, and ethylene propylene diene monomer (EPDM) rubber. Polymeric compounds include polyvinylidene fluoride (PVDF) as a low-melting-point PVDF polymer, PVDF as a high-melting-point PVDF polymer, polyimide, and carboxymethyl cellulose.

[0098] The negative electrode conductive agent includes any one or more conductive materials such as carbon materials, including graphite, carbon black, acetylene black, and Ketjen black. Alternatively, the conductive material can also be a metallic material or a polymer compound.

[0099] (Septum)

[0100] like Figure 2 As shown, the separator 13 is an insulating porous membrane located between the positive electrode 11 and the negative electrode 12, preventing contact between the positive electrode 11 and the negative electrode 12 while allowing lithium ions to pass through. The separator 13 contains any one or more of the following polymer compounds: polytetrafluoroethylene, polypropylene, and polyethylene.

[0101] In the positive electrode 11 and the separator 13, since the positive electrode active material layer 11B is located between the positive electrode current collector 11A and the separator 13, the positive electrode active material layer 11B is in close contact with the positive electrode current collector 11A and the separator 13 respectively.

[0102] Here, as described later, in the manufacturing process of the positive electrode 11, a positive electrode active material layer 11B is formed by coating a positive electrode slurry onto the surface of the positive electrode current collector 11A. Therefore, in the completed secondary battery, the adhesion strength S1 of the positive electrode active material layer 11B relative to the positive electrode current collector 11A is greater than the adhesion strength S2 of the positive electrode active material layer 11B relative to the separator 13. This is because the positive electrode active material layer 11B is sufficiently adhered to the positive electrode current collector 11A, thus improving the current collecting capacity of the positive electrode 11.

[0103] When checking the relationship between the adhesion strengths S1 and S2, the secondary battery is disassembled to recover the positive electrode 11 and the separator 13 that are in close contact with each other. Then, the separator 13 is peeled off from the positive electrode 11. Thus, when the positive electrode active material layer 11B remains on the positive electrode current collector 11A and is not peeled off together with the separator 13, the adhesion strength S1 is greater than the adhesion strength S2. On the other hand, when the positive electrode active material layer 11B is peeled off from the positive electrode current collector 11A together with the separator 13, the adhesion strength S1 is less than the adhesion strength S2.

[0104] Of course, the adhesion strengths S1 and S2 can also be measured separately using a peel tester (180° peel method) to investigate the relationship between the magnitudes of the adhesion strengths S1 and S2.

[0105] (electrolyte)

[0106] The electrolyte contains a solvent and an electrolyte salt.

[0107] The solvent includes any one or more non-aqueous solvents (organic solvents), and the electrolyte containing such non-aqueous solvents is called a non-aqueous electrolyte. The non-aqueous solvent is an ester or ether, and more specifically, a carbonate compound, a carboxylic acid ester compound, or a lactone compound. This is because the electrolyte salt exhibits increased dissociation and can achieve high ion mobility.

[0108] Specifically, carbonate compounds include cyclic carbonates and chain carbonates. Specific examples of cyclic carbonates are ethylene carbonate and propylene carbonate, while specific examples of chain carbonates are dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate.

[0109] Carboxylic acid esters are compounds that contain carboxylic acid esters. Specific examples of carboxylic acid esters include ethyl acetate, ethyl propionate, propyl propionate, and ethyl trimethylacetate.

[0110] Lactone compounds include lactones, etc. Specific examples of lactones are γ-butyrolactone and γ-pentanolactone, etc. It should be noted that ethers, in addition to the lactone compounds mentioned above, can also be 1,2-dimethoxyethane, tetrahydrofuran, 1,3-dioxolane, and 1,4-dioxane, etc.

[0111] In addition, non-aqueous solvents can also be unsaturated cyclic carbonates, halocarbonates, sulfonates, phosphates, acid anhydrides, nitrile compounds, and isocyanate compounds, etc. This is because the chemical stability of the electrolyte is improved.

[0112] Specific examples of unsaturated cyclic carbonates include vinylene carbonate (1,3-dioxacyclopenten-2-one), vinylene carbonate (4-vinyl-1,3-dioxacyclopenten-2-one), and methyleneene carbonate (4-methylene-1,3-dioxacyclopenten-2-one). Specific examples of halocarbonates include fluoroethylene carbonate (4-fluoro-1,3-dioxacyclopenten-2-one) and difluoroethylene carbonate (4,5-difluoro-1,3-dioxacyclopenten-2-one). Specific examples of sulfonates include 1,3-propanesulfonolactone and 1,3-propenesulfonolactone. Phosphate esters include trimethyl phosphate and triethyl phosphate.

[0113] Acid anhydrides include cyclic dicarboxylic acid anhydrides, cyclic disulfonic acid anhydrides, and cyclic carboxylic acid sulfonic anhydrides. Specific examples of cyclic dicarboxylic acid anhydrides include succinic anhydride, glutaric anhydride, and maleic anhydride. Specific examples of cyclic disulfonic acid anhydrides include 1,2-ethanedisulfonic anhydride and 1,3-propanedisulfonic anhydride. Specific examples of cyclic carboxylic acid sulfonic anhydrides include sulfobenzoic anhydride, sulfopropionic anhydride, and sulfobutyric anhydride.

[0114] Nitrile compounds include mononitrile compounds and dinitrile compounds. Specific examples of mononitrile compounds are acetonitrile, etc. Specific examples of dinitrile compounds are succinate, glutaronitrile, and adiponitrile, etc. Specific examples of isocyanate compounds are hexamethylene diisocyanate, etc.

[0115] Electrolyte salts include any one or more of light metal salts such as lithium salts. Specific examples of lithium salts are lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(fluorosulfonyl)imide (LiN(FSO2)2), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF3SO2)2), lithium tri(trifluoromethanesulfonyl)methyl (LiC(CF3SO2)3), lithium difluorooxalate borate (LiBF2(C2O4)), and lithium bis(oxalate)borate (LiB(C2O4)2), etc.

[0116] There are no particular restrictions on the content of the electrolyte salt, specifically, it should be 0.3 mol / kg to 3.0 mol / kg relative to the solvent. This is because high ionic conductivity can be obtained.

[0117] [Positive lead and negative lead]

[0118] The positive lead 31 is the positive terminal connected to the positive electrode 11 (positive current collector 11A), and includes any one or more conductive materials such as aluminum. The shape of the positive lead 31 is not particularly limited, but specifically, it is any one or more of the following: a thin plate shape and a mesh shape.

[0119] The negative lead 32 is the negative terminal connected to the negative electrode 12 (negative current collector 12A), and contains one or more conductive materials such as copper, nickel, and stainless steel. The details of the shape of the negative lead 32 are the same as the details of the shape of the positive lead 31 described above.

[0120] Here, as Figure 1 As shown, the positive electrode lead 31 and the negative electrode lead 32 are led out from the inside of the outer membrane 20 in the same direction. Alternatively, the positive electrode lead 31 and the negative electrode lead 32 can be led out in different directions.

[0121] Furthermore, the number of positive electrode leads 31 is one. However, there is no particular limitation on the number of positive electrode leads 31, so it can be two or more. In particular, when the number of positive electrode leads 31 is two or more, the resistance of the secondary battery decreases. Here, the explanation regarding the number of positive electrode leads 31 also applies to the number of negative electrode leads 32; therefore, the number of negative electrode leads 32 is not limited to one, and can also be two or more.

[0122] <1-2 Actions>

[0123] During the charging of the secondary battery, lithium is deintercalated from the positive electrode 11 and intercalated into the negative electrode 12 via the electrolyte. Conversely, during the discharging of the secondary battery, lithium is deintercalated from the negative electrode 12 and intercalated into the positive electrode 11 via the electrolyte. During these charging and discharging processes, lithium is intercalated and deintercalated in an ionic state.

[0124] <1-3. Manufacturing Method>

[0125] In the case of manufacturing a secondary battery, a positive electrode 11 and a negative electrode 12 are fabricated according to the steps described below, and an electrolyte is prepared. The secondary battery is then manufactured using the positive electrode 11, negative electrode 12, and electrolyte. The following instructions will be consulted at any time. Figure 1 as well as Figure 2 .

[0126] [The production of the positive electrode]

[0127] First, a positive electrode mixture is prepared by mixing a positive electrode active material containing lithium cobalt composite oxide, a positive electrode binder containing a low-melting-point vinylidene fluoride polymer, and a positive electrode conductive agent containing carbon black with a hollow structure. In this case, the mixing ratio of the positive electrode active material, the positive electrode binder, and the positive electrode conductive agent is adjusted so that the proportion of the positive electrode active material (R1) is 97.9% to 98.5% by weight, the proportion of the positive electrode binder (R2) is 0.8% to 1.4% by weight, and the proportion of the positive electrode conductive agent (R3) is 0.5% to 1.1% by weight. It should be noted that, if necessary, additives such as polyvinylpyrrolidone can also be added to the positive electrode mixture.

[0128] Next, a paste-like positive electrode slurry is prepared by adding a positive electrode agent to an organic solvent or the like. Then, the positive electrode slurry is coated onto both sides of the positive electrode current collector 11A to form a positive electrode active material layer 11B.

[0129] Next, the positive electrode active material layer 11B is compressed and molded using a roller press or similar device. In this process, the positive electrode active material layer 11B is compressed and molded to a bulk density of 4.15 g / cm³. 3 That's all. Additionally, compression molding can be performed simultaneously with heating the positive electrode active material layer 11B, or this compression molding process can be repeated multiple times.

[0130] Finally, the positive electrode active material layer 11B is heated in a vacuum environment. In this case, the heating temperature is set such that the elemental concentration of fluorine atoms, as measured by surface analysis of the positive electrode active material layer 11B using XPS, is 1.9% to 3.0%. The heating temperature can be arbitrarily set, specifically above 100°C.

[0131] Therefore, the positive electrode active material layer 11B is disposed on both sides of the positive electrode current collector 11A to form the positive electrode 11. In this case, since the positive electrode active material layer 11B is in close contact with the positive electrode current collector 11A, in the completed secondary battery, the adhesion strength S1 of the positive electrode active material layer 11B relative to the positive electrode current collector 11A is greater than the adhesion strength S2 of the positive electrode active material layer 11B relative to the separator 13.

[0132] [Making the negative electrode]

[0133] The negative electrode 12 is made using roughly the same steps as those used to make the positive electrode 11 described above.

[0134] Specifically, a negative electrode active material containing carbon, a negative electrode binder, and a negative electrode conductive agent are mixed to form a negative electrode mixture. This mixture is then added to an organic solvent to prepare a paste-like negative electrode slurry. It should be noted that, if needed, a silicon-containing material can also be added to the negative electrode mixture as the negative electrode active material. Next, the negative electrode slurry is coated onto both sides of the negative electrode current collector 12A to form a negative electrode active material layer 12B. Then, the negative electrode active material layer 12B can be compressed and molded.

[0135] Thus, the negative electrode active material layer 12B is disposed on both sides of the negative electrode current collector 12A to form the negative electrode 12.

[0136] [Preparation of Electrolyte]

[0137] An electrolyte salt is added to a solvent. The electrolyte salt is then dispersed or dissolved in the solvent, thus preparing an electrolyte solution.

[0138] [Assembly of a secondary battery]

[0139] First, the positive lead 31 is connected to the positive electrode 11 (positive current collector 11A) using a soldering method or the like, and the negative lead 32 is connected to the negative electrode 12 (negative current collector 12A) using a soldering method or the like.

[0140] Next, the positive electrode 11 and the negative electrode 12 are stacked on top of each other with the separator 13 in between, and then the positive electrode 11, the negative electrode 12, and the separator 13 are wound together to form a wound body. This wound body has the same structure as the battery element 10, except that the positive electrode 11, the negative electrode 12, and the separator 13 are not impregnated with electrolyte. Next, the wound body is pressed into a flat shape using a press or the like.

[0141] Next, after housing the wound body inside the recess 20U, the outer film 20 (welding layer / metal layer / surface protective layer) is folded so that the outer films 20 are facing each other. Next, the outer periphery portions of two sides of the facing outer films 20 (welding layers) are bonded together using a heat-sealing method or the like, thereby housing the wound body inside the bag-shaped outer film 20.

[0142] Finally, after injecting the electrolyte into the pouch-shaped outer film 20, the outer periphery of the remaining side of the outer film 20 (welded layer) is bonded together using a heat-sealing method or the like. In this case, the adhesive film 21 is inserted between the outer film 20 and the positive electrode lead 31, and the adhesive film 22 is inserted between the outer film 20 and the negative electrode lead 32. Thus, the electrolyte is impregnated into the wound body, thereby forming a battery element 10 as a wound electrode body. Therefore, the battery element 10 is sealed inside the pouch-shaped outer film 20 and assembled into a secondary battery.

[0143] Stabilization of secondary batteries

[0144] The assembled secondary battery is charged and discharged. Various conditions, such as ambient temperature, number of charge / discharge cycles, and charge / discharge conditions, can be arbitrarily set. This forms a coating on the surface of the negative electrode 12, thereby stabilizing the electrochemical state of the secondary battery. Therefore, a secondary battery using the outer membrane 20, i.e., a laminated membrane type secondary battery, is completed.

[0145] <1-4. Functions and Effects>

[0146] According to this secondary battery, the positive electrode active material comprises lithium cobalt composite oxide, the positive electrode binder comprises low-melting-point vinylidene fluoride polymer, the positive electrode conductive agent comprises carbon black with a hollow structure, and the negative electrode active material comprises carbon material. Furthermore, the proportions of the positive electrode active material R1 are 97.9% to 98.5% by weight, the proportions of the positive electrode binder R2 are 0.8% to 1.4% by weight, the proportions of the positive electrode conductive agent R3 are 0.5% to 1.1% by weight, and the bulk density of the positive electrode active material layer 11B is 4.15 g / cm³. 3 The concentration of fluorine atoms, as measured by surface analysis of the positive electrode active material layer 11B of XPS, is 1.9% to 3.0%.

[0147] In this case, the positive electrode 11 (positive electrode active material) contains lithium cobalt composite oxide and the negative electrode 12 (negative electrode active material) contains carbon material. Therefore, in the positive electrode 11 and the negative electrode 12, lithium is smoothly and stably inserted and extracted during charging and discharging.

[0148] Furthermore, since the ratio R1 is sufficiently large relative to each of the ratios R2 and R3, the content of the positive electrode active material in the positive electrode active material layer 11B is sufficiently increased. As a result, the energy density per unit volume increases compared to the case where the ratio R1 is not sufficiently large relative to each of the ratios R2 and R3, i.e., the ratio R1 is less than 97.9% by weight.

[0149] Here, if the ratio R2 is small enough relative to the ratio R1, the content of the positive electrode binder in the positive electrode active material layer 11B will be excessively reduced, resulting in insufficient positive electrode binder. Therefore, the positive electrode active materials will be difficult to bond with each other through the positive electrode binder.

[0150] In addition, if the ratio R3 is small enough relative to the ratio R1, the content of the positive electrode conductive agent in the positive electrode active material layer 11B will be reduced excessively, resulting in insufficient positive electrode conductive agent. Therefore, the internal resistance of the positive electrode 11 (the resistance of the positive electrode active material layer 11B) is likely to increase.

[0151] Furthermore, when the ratio R1 is sufficiently large relative to each of the ratios R2 and R3, the excessive increase in the content of the positive active material in the positive active material layer 11B leads to an increase in the friction (interparticle friction) between these positive active materials. Consequently, during the compression molding of the positive active material layer 11B, the positive active material is prone to breakage due to collisions between the positive active materials, thus making it easier for the internal resistance of the positive electrode 11 to increase.

[0152] However, when the positive electrode binder contains a low-melting-point vinylidene fluoride polymer and the positive electrode conductive agent contains carbon black with a hollow structure, if the proportions R1, R2, and R3 are within the above ranges, then as described above, the mixed film of the positive electrode binder and the positive electrode conductive agent covers the surface of the positive electrode active material.

[0153] In this situation, because the positive electrode active materials are bonded to each other through the mixing film, even if the ratio R2 is sufficiently small relative to the ratio R1, the positive electrode active materials easily bond to each other through the mixing film. Therefore, when the positive electrode active material layer 11B is heated, it is difficult for fluorine reactants (LiF) to form through fluorine atoms in the low-melting-point vinylidene fluoride polymer. Specifically, as described above, the elemental concentration of fluorine atoms, measured by surface analysis of the positive electrode active material layer 11B using XPS, is reduced to 1.9% to 3.0%.

[0154] Furthermore, due to the presence of the mixed film, the friction between the positive electrode active materials is reduced, so even if the positive electrode active materials collide with each other during the compression molding of the positive electrode active material layer 11B, the positive electrode active materials are not easily damaged. Therefore, even if the ratio R3 is small enough relative to the ratio R1, the internal resistance of the positive electrode 11 is not easily increased.

[0155] Furthermore, corresponding to the fact that the positive electrode active material is not easily damaged, the positive electrode active material layer 11B can be easily compressed and molded, thus significantly increasing its bulk density while suppressing damage to the positive electrode active material. Specifically, as described above, the bulk density of the positive electrode active material layer 11B increases to 4.15 g / cm³. 3 Therefore, the energy density per unit volume increases further.

[0156] Based on the above, when the positive electrode active material contains lithium-cobalt composite oxide and the negative electrode active material contains carbon material, it is possible to increase the energy density per unit volume while suppressing the increase in the internal resistance of the positive electrode 11. Therefore, it is possible to achieve both increased energy density and reduced resistance.

[0157] In particular, if the lithium cobalt composite oxide contains a compound represented by formula (1), a high energy density can be stably obtained, thus enabling higher performance.

[0158] In addition, if the carbon black with a hollow structure contains Ketjen black, the mixed film of the positive electrode binder and the positive electrode conductive agent can easily cover the surface of the positive electrode active material, thereby reducing the friction between the positive electrode active materials and thus achieving a higher performance.

[0159] In addition, if the carbon material contains artificial graphite, the charge and discharge reaction in the negative electrode 12 can proceed smoothly and stably, thus achieving higher performance.

[0160] In addition, if the negative electrode active material also contains silicon, the energy density per unit volume increases further, thus achieving even higher performance.

[0161] Furthermore, if the positive electrode active material layer 11B also contains polyvinylpyrrolidone as an additive, it can promote the dispersion of the positive electrode active material and the like in the positive electrode slurry. Therefore, the coatability of the positive electrode slurry is improved, and the adhesion of the positive electrode active material layer 11B to the positive electrode current collector 11A is improved, thus achieving a higher performance.

[0162] Furthermore, if the separator 13 is located between the positive electrode 11 (positive current collector 11A and positive active material layer 11B) and the negative electrode 12, and the adhesion strength S1 of the positive active material layer 11B relative to the positive current collector 11A is greater than the adhesion strength S2 of the positive active material layer 11B relative to the separator 13, then compared with the case where the adhesion strength S1 is less than the adhesion strength S2, the current collection performance of the positive electrode 11 is improved, and thus a higher effect can be obtained.

[0163] In addition, if the secondary battery is a lithium-ion secondary battery, sufficient battery capacity can be stably obtained by utilizing the insertion and extraction of lithium, thus achieving higher performance.

[0164] <2. Variations>

[0165] Next, variations of the aforementioned secondary battery will be described. As explained below, the structure of the secondary battery can be appropriately modified. Furthermore, any two or more of the variations described below can be combined with each other.

[0166] [Variation 1]

[0167] The aforementioned secondary battery uses a membrane 13 as a porous membrane. However, although not specifically illustrated here, a laminated membrane comprising layers of polymer compounds can be used instead of the porous membrane 13.

[0168] Specifically, the laminated separator comprises a porous membrane with one and two faces and a polymer compound layer disposed on one or both faces of the porous membrane. This is because, due to the close adhesion of the separator to each of the positive electrode 11 and the negative electrode 12, the positional displacement of the battery element 10 is less likely to occur. Therefore, even if electrolyte decomposition reactions occur, the secondary battery is less likely to expand. The polymer compound layer comprises a polymer compound such as polyvinylidene fluoride, which has excellent physical strength and is electrochemically stable.

[0169] It should be noted that one or both of the porous membrane and the polymer compound layer may contain any one or more types of insulating particles. This is because multiple insulating particles dissipate heat when the secondary battery heats up, thus improving the safety (heat resistance) of the secondary battery. Insulating particles include inorganic particles and resin particles. Specific examples of inorganic particles include particles of alumina, aluminum nitride, boehmite, silicon dioxide, titanium dioxide, magnesium oxide, and zirconium oxide. Specific examples of resin particles include particles of acrylic resin and styrene resin.

[0170] In the case of fabricating a layered membrane, a precursor solution containing a polymeric compound and an organic solvent is prepared, and then the precursor solution is coated onto one or both sides of a porous membrane. Alternatively, the porous membrane can be immersed in the precursor solution. In this case, multiple insulating particles can also be added to the precursor solution as needed.

[0171] With the use of this layered separator, lithium ions can also move between the positive electrode 11 and the negative electrode 12, thus achieving the same effect.

[0172] [Variation 2]

[0173] The aforementioned secondary battery uses an electrolyte as a liquid electrolyte. However, although not specifically illustrated here, an electrolyte layer as a gel electrolyte can also be used instead of an electrolyte.

[0174] In the battery element 10 using an electrolyte layer, the positive electrode 11 and the negative electrode 12 are wound together with the electrolyte layer and the separator 13 in between. The electrolyte layer is located between the positive electrode 11 and the separator 13, and between the negative electrode 12 and the separator 13.

[0175] Specifically, the electrolyte layer comprises an electrolyte and a polymer compound, and the electrolyte is held in place by the polymer compound within the electrolyte layer. This is to prevent electrolyte leakage. The composition of the electrolyte is as described above. The polymer compound includes polyvinylidene fluoride, etc. In the case of forming the electrolyte layer, after preparing a precursor solution comprising the electrolyte, the polymer compound, and an organic solvent, the precursor solution is coated on one or both sides of each of the positive electrode 11 and the negative electrode 12.

[0176] Even when this electrolyte layer is used, lithium ions can move between the positive electrode 11 and the negative electrode 12 via the electrolyte layer, thus achieving the same effect.

[0177] <3. Uses of Secondary Batteries>

[0178] Next, the uses (applicable examples) of the above-mentioned secondary batteries will be explained.

[0179] The application of a secondary battery is limited to any machinery, equipment, appliance, device, or system (a collection of multiple devices) that can primarily use it as a power source for driving or as a power storage source for energy accumulation; there are no particular restrictions. A secondary battery used as a power source can be either a main power source or an auxiliary power source. The main power source is the preferred power source, regardless of the availability of other power sources. An auxiliary power source can either replace the main power source or be switched from the main power source as needed. When using a secondary battery as an auxiliary power source, the type of main power source is not limited to secondary batteries.

[0180] Specific examples of applications for rechargeable batteries include: electronic devices such as camcorders, digital still cameras, mobile phones, laptops, cordless phones, stereo headphones, portable radios, portable televisions, and portable information terminals (including portable electronic devices); portable household appliances such as electric shavers; backup power supplies and storage devices such as memory cards; power tools such as electric drills and chainsaws; battery packs used as detachable power sources in laptops, etc.; medical electronic devices such as pacemakers and hearing aids; electric vehicles such as electric cars (including hybrid vehicles); and power storage systems such as home battery systems that pre-store power for emergencies. In these applications, one or multiple rechargeable batteries can be used.

[0181] Battery packs are effective for use in electric vehicles, energy storage systems, and larger equipment such as power tools. Battery packs can use single cells or multiple battery banks. Electric vehicles are vehicles that operate (drive) using a secondary battery as a power source; as mentioned above, they can also be automobiles (hybrid vehicles, etc.) that have a power source other than a secondary battery. Energy storage systems are systems that use secondary batteries as a source of stored electricity. In household energy storage systems, since electricity is stored in the secondary battery that serves as the energy storage source, this electricity can be used to operate household electrical products, etc.

[0182] Here, we will specifically illustrate one example of the applicability of a secondary battery. The structure of the applicability example described below is only one example and can therefore be modified appropriately.

[0183] Figure 3 This describes the frame structure of the battery pack. The battery pack described here is a simplified type (so-called pouch) that uses a single rechargeable battery and is used in electronic devices such as smartphones.

[0184] like Figure 3 As shown, the battery pack includes a power supply 41 and a circuit board 42. The circuit board 42 is connected to the power supply 41 and includes a positive terminal 43, a negative terminal 44, and a temperature sensing terminal 45 (so-called T terminal).

[0185] The power supply 41 includes a secondary battery. In this secondary battery, the positive lead is connected to the positive terminal 43, and the negative lead is connected to the negative terminal 44. Since the power supply 41 can be connected to an external source through the positive terminal 43 and the negative terminal 44, it can be charged and discharged through these terminals. The circuit board 42 includes a control unit 46, a switch 47, a PTC (Positive Temperature Coefficient) element 48, and a temperature detection unit 49. Alternatively, the PTC element 48 may be omitted.

[0186] The control unit 46 includes a central processing unit (CPU) and memory, and controls the overall operation of the battery pack. The control unit 46 detects and controls the operating status of the power supply 41 as needed.

[0187] It should be noted that when the voltage of the power supply 41 (secondary battery) reaches the overcharge detection voltage or the over-discharge detection voltage, the control unit 46 cuts off the switch 47, thereby preventing the charging current from flowing through the current path of the power supply 41. Additionally, when a large current flows during charging or discharging, the control unit 46 cuts off the charging current by cutting off the switch 47. There are no particular limitations on the overcharge detection voltage and the over-discharge detection voltage. For example, the overcharge detection voltage is 4.2V ± 0.05V, and the over-discharge detection voltage is 2.4V ± 0.1V.

[0188] Switch 47 includes a charging control switch, a discharging control switch, a charging diode, and a discharging diode, etc., and switches the connection between power supply 41 and external devices according to the instructions of control unit 46. Switch 47 includes a metal-oxide-semiconductor field-effect transistor (MOSFET), etc., and detects charging and discharging current based on the on-resistance of switch 47.

[0189] The temperature detection unit 49 includes a temperature detection element such as a thermistor, measures the temperature of the power supply 41 using the temperature detection terminal 45, and outputs the temperature measurement result to the control unit 46. The temperature measurement result measured by the temperature detection unit 49 is used for charging and discharging control by the control unit 46 when abnormal heating occurs, and for correction processing by the control unit 46 when calculating the remaining capacity.

[0190] Example

[0191] An embodiment of this technology will be described.

[0192] (Experimental Examples 1-25)

[0193] After the secondary battery was manufactured, its performance was evaluated.

[0194] [Making a Second-hand Battery]

[0195] The following steps were followed to create the product. Figure 1 as well as Figure 2 The image shows a laminated film type secondary battery (lithium-ion secondary battery).

[0196] (The production of the positive electrode)

[0197] First, the positive electrode active material (lithium cobalt composite oxide), the positive electrode binder (low melting point vinylidene fluoride polymer), and the positive electrode conductive agent (carbon black with a hollow structure) are mixed to form a positive electrode mixture.

[0198] As a lithium-cobalt composite oxide, LiCoO2(LOC) and LiCo were used. 0.98 Al0.02 O2 (LCOA). As a low-melting-point vinylidene fluoride polymer, low-melting-point polyvinylidene fluoride (LMPVDF, electrode binder Kynar HSV1800 manufactured by Arkema Corporation (registered trademark), melting point = 160℃~170℃) was used. Ketjen black (KB) was used as a carbon black with a hollow structure. With the positive electrode mixture obtained, the mixing ratios of the positive electrode active material, positive electrode binder, and positive electrode conductive agent were adjusted so that the ratios R1, R2, and R3 were the values ​​shown in Tables 1 and 2, respectively.

[0199] Next, the positive electrode agent was added to an organic solvent (N-methyl-2-pyrrolidone), and the organic solvent was stirred to prepare a paste-like positive electrode agent slurry. Then, the positive electrode agent slurry was coated onto both sides of the positive electrode current collector 11A (aluminum foil with a thickness of 12 μm) using a coating device, and the positive electrode agent slurry was dried to form the positive electrode active material layer 11B.

[0200] Next, the positive electrode active material layer 11B is compressed and molded using a roller press. The bulk density (g / cm³) of the compressed positive electrode active material layer 11B is shown below. 3 As shown in Tables 1 and 2, this volume density is the maximum value of the volume density of the positive electrode active material layer 11B after compression molding.

[0201] Finally, the positive electrode active material layer 11B is heated in a vacuum environment (heating temperature = 100°C). Thus, the positive electrode active material layer 11B is disposed on both sides of the positive electrode current collector 11A to form the positive electrode 11.

[0202] After completing the positive electrode 11, the surface of the positive electrode active material layer 11B was analyzed by XPS to measure the elemental concentration (%) of fluorine atoms, and the results are shown in Table 1 and Table 2.

[0203] It should be noted that, for comparison, the positive electrode 11 was manufactured using the same steps, except that a high-melting-point vinylidene fluoride polymer was used instead of a low-melting-point vinylidene fluoride polymer as the negative electrode binder. As the high-melting-point vinylidene fluoride, polyvinylidene fluoride (HMPVDF, Kureha KF polymer #7300 (registered trademark), a high-performance electrode binder manufactured by Kureha Corporation, with a melting point exceeding 170°C and below 175°C) was used.

[0204] In addition, for comparison, positive electrode 11 was prepared by the same steps as described above, except that carbon black without a hollow structure (acetylene black (AB)) was used instead of carbon black with a hollow structure as the negative electrode conductive agent.

[0205] (Making the negative electrode)

[0206] First, a negative electrode mixture is prepared by mixing 98 parts by weight of the negative electrode active material and 2 parts by weight of the negative electrode binder. As the negative electrode active material, artificial graphite and natural graphite are used as carbon materials, and silicon dioxide (SiO₂) is used as a silicon-containing material. x In the case of using both artificial graphite and silicon-containing materials, the mixing ratio (by weight) was set to artificial graphite: silicon-containing material = 80:20. Polyvinylidene fluoride (PVDF), a high-melting-point PVDF polymer described above, was used as the negative electrode binder.

[0207] Next, the negative electrode agent was added to an organic solvent (N-methyl-2-pyrrolidone), and the organic solvent was stirred to prepare a paste-like negative electrode agent slurry. Then, the negative electrode agent slurry was coated onto both sides of the negative electrode current collector 12A (a copper foil with a thickness of 15 μm) using a coating apparatus, and then the negative electrode agent slurry was dried to form the negative electrode active material layer 12B.

[0208] Finally, the negative electrode active material layer 12B is compressed and molded using a roller press. Thus, the negative electrode active material layer 12B is disposed on both sides of the negative electrode current collector 12A, forming the negative electrode 12.

[0209] (Preparation of electrolyte)

[0210] An electrolyte salt (LiPF6 as a lithium salt) is added to a solvent (ethylene carbonate as a cyclic carbonate and diethyl carbonate as a chain carbonate), and the solvent is then stirred. In this case, the mixing ratio (by weight) of the solvent is ethylene carbonate:diethyl carbonate = 30:70, and the content of the electrolyte salt relative to the solvent is 1 mol / kg. Thus, the electrolyte salt dissolves or disperses in the solvent, thereby preparing an electrolyte solution.

[0211] (Assembly of a secondary battery)

[0212] First, the positive lead 31, made of aluminum, is soldered to the positive electrode 11 (positive current collector 11A), and the negative lead 32, made of copper, is soldered to the negative electrode 12 (negative current collector 12A).

[0213] Next, the positive electrode 11 and the negative electrode 12 are stacked on top of each other with a separator 13 (a microporous polyethylene membrane with a thickness of 15 μm) in between, and then the positive electrode 11, the negative electrode 12 and the separator 13 are wound together to form a wound body. Next, the wound body is pressed using a press to form a flat wound body.

[0214] Next, the wound body is housed inside the recess 20U provided in the outer film 20. The outer film 20 uses an aluminum laminate film, which consists of a weld layer (a 30 μm thick polypropylene film), a metal layer (a 40 μm thick aluminum foil), and a surface protective layer (a 25 μm thick nylon film) stacked sequentially. Next, the outer film 20 is folded so that the wound body is sandwiched between the outer film 20 and the weld layer is located on the inner side of the outer film 20. Then, the outer periphery portions of two sides of the outer film 20 (weld layer) are heat-fused together, thereby housing the wound body inside the bag-shaped outer film 20.

[0215] Finally, after injecting the electrolyte into the pouch-shaped outer membrane 20, the outer periphery of the remaining edge of the outer membrane 20 (welded layer) is thermally fused together under reduced pressure. In this case, a bonding film 21 (a 5 μm thick polypropylene film) is inserted between the outer membrane 20 and the positive electrode lead 31, and a bonding film 22 (a 5 μm thick polypropylene film) is inserted between the outer membrane 20 and the negative electrode lead 32. Thus, the electrolyte is impregnated into the winding, thereby forming the battery element 10. Therefore, the battery element 10 is sealed inside the outer membrane 20 and assembled into a secondary battery.

[0216] (Stabilization of secondary batteries)

[0217] The secondary battery was subjected to one charge-discharge cycle at room temperature (temperature = 23℃). During charging, a constant current of 0.1C was used until the voltage reached 4.2V, followed by constant voltage charging at that 4.2V until the current reached 0.05C. During discharging, a constant current of 0.1C was used until the voltage reached 2.5V. 0.1C refers to the current value required to fully discharge the battery (theoretical capacity) in 10 hours, and 0.05C refers to the current value required to fully discharge the battery (theoretical capacity) in 20 hours.

[0218] Thus, a coating is formed on the surface of the negative electrode 12, thereby stabilizing the state of the secondary battery. Therefore, a laminated film secondary battery is completed.

[0219] It should be noted that after the secondary battery is completed, the positive electrode 11 and the separator 13 are recovered, and the separator 13 is peeled off from the positive electrode 11. As a result, the positive electrode active material layer 11B remains on the positive electrode current collector 11A and is not peeled off with the separator 13. This confirms that the adhesion strength S1 of the positive electrode active material layer 11B to the positive electrode current collector 11A is greater than the adhesion strength S2 of the positive electrode active material layer 11B to the separator 13.

[0220] [Performance Evaluation]

[0221] The performance (energy characteristics and resistance characteristics) of the secondary battery was evaluated, and the results are shown in Tables 1 and 2. The evaluation steps for each characteristic are described below.

[0222] (Energy properties)

[0223] The discharge capacity (battery capacity (mAh)) of the secondary battery was measured by charging and discharging it at room temperature. The charging and discharging conditions were the same as those for stabilizing the secondary battery described above.

[0224] (Resistance characteristics)

[0225] First, the secondary battery was charged at room temperature. The charging conditions were the same as those used for stabilizing the secondary battery. Next, the secondary battery was subjected to a constant current discharge at 0.1C for 5 hours, thereby adjusting the depth of charge to 50%. Immediately after adjusting the depth of charge to 50%, the secondary battery was subjected to a constant current discharge at 1.0C for 1 second, and the voltage change ΔV before and after this constant current discharge was measured. 1.0C is the current value required to fully discharge the battery capacity in 1 hour. Finally, the DC resistance of the secondary battery was measured based on the formula: DC resistance (mΩ) = voltage change ΔV / current value (1.0C).

[0226]

[0227]

[0228] [Inspection]

[0229] As shown in Tables 1 and 2, in a secondary battery in which the positive electrode 11 (positive electrode active material) contains lithium cobalt composite oxide and the negative electrode 12 (negative electrode active material) contains carbon material, the energy characteristics and resistance characteristics vary according to the composition of the positive electrode active material layer 11B (the type of positive electrode binder, the type of positive electrode conductive agent and the ratio R1, R2, R3).

[0230] Specifically, when the positive electrode binder contains high-melting-point vinylidene fluoride polymer (HMPVDF) (Experimental Examples 20-22) and the positive electrode conductive agent contains carbon black (AB) without a hollow structure (Experimental Examples 23-25), good results are not obtained in both battery capacity and DC resistance, regardless of the ratios R1, R2, and R3. That is, sufficient battery capacity cannot be obtained in almost all cases, and DC resistance increases.

[0231] In contrast, when the positive electrode binder contains a low-melting-point vinylidene fluoride polymer (LMPVDF) and the positive electrode conductive agent contains carbon black (KB) with a hollow structure (Experimental Examples 1-19), good results were obtained for both battery capacity and DC resistance depending on the ratios R1, R2, and R3.

[0232] That is, when the three conditions of proportional R1 being 97.9% to 98.5% by weight, proportional R2 being 0.8% to 1.4% by weight, and proportional R3 being 0.5% to 1.1% by weight are met simultaneously (Experimental Examples 2 to 7, 10 to 15, 17 to 19), the battery capacity is significantly increased and the DC resistance is significantly reduced, unlike the cases where these three conditions are not met simultaneously (Experimental Examples 1, 8, 9, 16).

[0233] In particular, when all three conditions are met simultaneously, a series of tendencies are observed, as described below.

[0234] First, if the ratios R1, R2, and R3 are within the aforementioned ranges, the bulk density of the positive electrode active material layer 11B increases to 4.15 g / cm³. 3 Furthermore, the elemental concentration of fluorine atoms is reduced to 1.9%–3.0%.

[0235] Second, when the type of lithium cobalt composite oxide was changed (Experimental Example 17) and the type of carbon material was changed (Experimental Example 18), the battery capacity was significantly increased and the DC resistance was significantly reduced.

[0236] Third, when the negative electrode active material contains both carbon and silicon materials (Experimental Example 19), the battery capacity is increased compared to when the negative electrode active material contains only carbon materials (Experimental Example 4).

[0237] (Experimental Example 26)

[0238] As shown in Table 3, secondary batteries were fabricated using the same procedures, except that an additive (polyvinylpyrrolidone (PVP)) was added to the positive electrode mixture, and the performance of these secondary batteries was evaluated. In this case, the amount of additive added relative to the positive electrode mixture was 0.03% by weight.

[0239]

[0240] As shown in Table 3, when the positive electrode active material layer 11B contains additives (PVP) (Experimental Example 26), the battery capacity is increased and the DC resistance is reduced compared with the case where the positive electrode active material layer 11B does not contain additives (Experimental Example 4).

[0241] (Experimental Examples 27 and 28)

[0242] As shown in Table 4, except for changing the heating temperature (°C) of the positive electrode active material layer 11B in the positive electrode fabrication process (after compression molding), a secondary battery was fabricated using the same steps, and the performance of the secondary battery was evaluated.

[0243] Table 4

[0244] R1 = 98.3% by weight, R2 = 1.0% by weight, R3 = 0.7% by weight

[0245]

[0246] As shown in Table 4, the concentration of fluorine atoms varies with the heating temperature. In this case, when the heating temperature is below 150°C, the concentration of fluorine atoms decreases to below 3.0%, thus significantly increasing the battery capacity and significantly reducing the DC resistance.

[0247] [Summarize]

[0248] As shown in Tables 1-4, if the positive electrode active material contains lithium cobalt composite oxide, the positive electrode binder contains low-melting-point vinylidene fluoride polymer, the positive electrode conductive agent contains carbon black with a hollow structure, and the negative electrode active material contains carbon material, and the proportions of the positive electrode active material (R1) are 97.9%–98.5% by weight, the proportions of the positive electrode binder (R2) are 0.8%–1.4% by weight, and the proportions of the positive electrode conductive agent (R3) are 0.5%–1.1% by weight, and the elemental concentration of fluorine atoms measured by surface analysis of the positive electrode active material layer 11B using XPS is 1.9%–3.0%, then the bulk density of the positive electrode active material layer 11B increases to 4.15 g / cm³. 3 In addition to the above, the battery capacity is significantly increased, and the DC resistance is significantly reduced. Therefore, in rechargeable batteries, it is possible to achieve both increased energy density and reduced resistance.

[0249] The present invention has been described above by listing one implementation method and embodiment, but the structure of the present invention is not limited to the structure described in one implementation method and embodiment, and various modifications are possible.

[0250] Although the case of a laminated film type battery structure for secondary batteries has been described, there are no particular limitations on this battery structure. Specifically, the battery structure can be cylindrical, square, coin-shaped, or button-shaped, etc.

[0251] Furthermore, although the case of a wound battery element structure has been described, there are no particular limitations on the structure of the battery element. Specifically, the element structure can be a stacked type with electrodes (positive and negative electrodes) stacked on top of each other, or a zigzag type with electrodes (positive and negative electrodes) folded into a Z-shape, etc.

[0252] Furthermore, while the case where lithium is used as the electrode reactant has been described, there are no particular limitations on the electrode reactant. Specifically, as mentioned above, the electrode reactant can be other alkali metals such as sodium and potassium, or alkaline earth metals such as beryllium, magnesium, and calcium. Additionally, the electrode reactant can also be other light metals such as aluminum.

[0253] The effects described in this specification are merely illustrative, and therefore the effects of this technology are not limited to those described in this specification. Thus, this technology can also achieve other effects.

Claims

1. A secondary battery, comprising: A positive electrode has a positive electrode active material layer, wherein the positive electrode active material layer comprises a positive electrode active material, a positive electrode binder, and a positive electrode conductive agent; The negative electrode contains the negative electrode active material; and Electrolyte The positive electrode active material comprises lithium cobalt composite oxide. The positive electrode binder comprises a vinylidene fluoride polymer having a melting point above 160°C and below 170°C. The positive electrode conductive agent comprises carbon black with a hollow structure. The negative electrode active material contains carbon materials. The weight of the positive electrode active material relative to the sum of the weights of the positive electrode active material, the positive electrode binder, and the positive electrode conductive agent is 97.9% by weight or more and 98.5% by weight or less. The weight of the positive electrode binder relative to the sum of the weight of the positive electrode active material, the weight of the positive electrode binder, and the weight of the positive electrode conductive agent is 0.8% by weight or more and 1.4% by weight or less. The weight of the positive electrode conductive agent relative to the sum of the weight of the positive electrode active material, the weight of the positive electrode binder, and the weight of the positive electrode conductive agent is 0.5% by weight or more and 1.1% by weight or less. The bulk density of the positive electrode active material layer is 4.15 g / cm³. 3 above, The fluorine concentration, measured by surface analysis of the positive electrode active material layer using X-ray photoelectron spectroscopy, is 1.9% or more and 3.0% or less. The manufacturing process of the positive electrode includes the steps of preparing a positive electrode slurry, forming a positive electrode active material layer by coating the positive electrode slurry on both sides of the positive electrode current collector, compressing the positive electrode active material layer, and heating the positive electrode active material layer in a vacuum environment, wherein the heating temperature in the vacuum environment is above 100°C and below 150°C.

2. The secondary battery according to claim 1, wherein, The weight of the positive electrode conductive agent relative to the sum of the weight of the positive electrode active material, the weight of the positive electrode binder, and the weight of the positive electrode conductive agent is more than 0.5% by weight and less than 0.8% by weight.

3. The secondary battery according to claim 1 or 2, wherein, The lithium-cobalt composite oxide comprises a compound represented by the following formula (1): Li x Co 1-y M y O 2-z X z …(1) M is at least one of Ti, V, Cr, Mn, Fe, Ni, Cu, Na, Mg, Al, Si, Sn, K, Ca, Zn, Ga, Sr, Y, Zr, Nb, Mo, Ba, La, W, and B; X is at least one of F, Cl, Br, I, and S; x, y, and z satisfy 0.8 < x < 1.2, 0 ≤ y < 0.15, and 0 ≤ z < 0.05; and the composition of Li varies depending on the charge / discharge state, with the value of x being the value of the fully discharged state.

4. The secondary battery according to claim 1 or 2, wherein, The hollow carbon black comprises Ketjen black.

5. The secondary battery according to claim 1 or 2, wherein, The carbon material includes at least one of artificial graphite and natural graphite.

6. The secondary battery according to claim 1 or 2, wherein, The negative electrode active material also includes silicon-containing materials.

7. The secondary battery according to claim 1 or 2, wherein, The positive electrode active material layer also contains polyvinylpyrrolidone.

8. The secondary battery according to claim 1 or 2, wherein, It also has a separator between the positive electrode and the negative electrode. The positive electrode also includes a positive electrode current collector that supports the positive electrode active material layer. The positive electrode active material layer is in close contact with the positive electrode current collector and the separator, respectively. The adhesion strength of the positive electrode active material layer to the positive electrode current collector is greater than the adhesion strength of the positive electrode active material layer to the separator.

9. The secondary battery according to claim 1 or 2, wherein, The secondary battery is a lithium-ion secondary battery.

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