Secondary batteries

Abstract

Provided is a secondary battery with which it is possible to obtain excellent battery characteristics. The secondary battery comprises a positive electrode including a positive electrode active material layer, a negative electrode, and an electrolytic solution. The positive electrode active material layer includes a plurality of positive electrode active material particles, and the positive electrode active material particles include a central portion including a lithium complex oxide and a coating portion provided on the surface of the central portion. The lithium complex oxide has a layered rock salt type crystal structure and contains lithium, nickel and other elements as constituent elements, and if the sum of the nickel content in the lithium complex oxide and the content of the other elements in the lithium complex oxide is defined as 100 mol parts, the nickel content is 80 mol parts or more and 100 mol parts or less. By performing a depth-direction analysis of the positive electrode active material layer using time-of-flight secondary ion mass spectrometry, first negative secondary ions derived from NiO2 - and second negative secondary ions derived from LiBO2F- are detected, and a first depth profile representing a change in the ion intensity of the first negative secondary ions in the depth direction, and a second depth profile representing a change in the ion intensity of the second negative secondary ions in the depth direction are acquired. In the first depth profile, the ion intensity of the first negative secondary ions increases in the depth direction, and in the second depth profile, the ion intensity of the second negative secondary ions decreases in the depth direction, and the second depth profile has a step region in which the ion intensity of the second negative secondary ions temporarily stops decreasing in the depth direction while the ion intensity of the first negative secondary ions increases in the depth direction.

Description

[Technical Field]

[0001] This technology relates to secondary batteries. [Background technology]

[0002] With the widespread use of various electronic devices such as mobile phones, development of rechargeable batteries is progressing as a power source that is small, lightweight, and provides high energy density. These rechargeable batteries contain an electrolyte along with a positive electrode and a negative electrode, and various studies are being conducted on the configuration of these batteries.

[0003] Specifically, by charging and discharging the assembled battery under heating conditions, a coating component derived from the electrolyte (LiPF6 and LiBF4) is made present on the surface of the positive electrode, and this coating component is analyzed using time-of-flight secondary ion mass spectrometry (TOF-SIMS) (see, for example, Patent Document 1). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2013-062026 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] Although various studies have been conducted on the configuration of secondary batteries, their battery characteristics are still not satisfactory, and there is room for improvement.

[0006] There is a need for a secondary battery that can achieve excellent battery characteristics. [Means for solving the problem]

[0007] A secondary battery according to one embodiment of this technology comprises a positive electrode including a positive electrode active material layer, a negative electrode, and an electrolyte. The positive electrode active material layer contains a plurality of positive electrode active material particles, each of which includes a central part containing a lithium composite oxide and a coating portion provided on the surface of the central part. The lithium composite oxide has a layered rock salt type crystalline structure and contains lithium, nickel, and other elements as constituent elements. If the sum of the nickel content and the content of other elements in the lithium composite oxide is 100 moles, the nickel content is between 80 moles and 100 moles. Analysis of the depth direction of the positive electrode active material layer using time-of-flight secondary ion mass spectrometry revealed that NiO2 - The first negative secondary ion derived from and LiBO2F - A second negative secondary ion originating from the first negative secondary ion is detected, and a first depth profile representing the change in ionic intensity of the first negative secondary ion in the depth direction and a second depth profile representing the change in ionic intensity of the second negative secondary ion in the depth direction are obtained. In the first depth profile, the ionic intensity of the first negative secondary ion increases in the depth direction, and in the second depth profile, the ionic intensity of the second negative secondary ion decreases in the depth direction. The second depth profile has a step region where, while the ionic intensity of the first negative secondary ion is increasing in the depth direction, the ionic intensity of the second negative secondary ion temporarily stops decreasing in the depth direction. [Effects of the Invention]

[0008] According to one embodiment of this technology, the positive electrode active material layer contains a plurality of positive electrode active material particles, the positive electrode active material particles include a core (lithium composite oxide) and a coating, the lithium composite oxide has a layered rock salt type crystalline structure and contains lithium, nickel and other elements as constituent elements, the nickel content in the lithium composite oxide is 80 moles to 100 moles, and analysis of the depth direction of the positive electrode active material layer using time-of-flight secondary ion mass spectrometry reveals that the first negative secondary ion (NiO2) - The first depth profile and the second negative secondary ion (LiBO2F) represent the change in ionic strength. -A second depth profile representing the change in ionic strength of ) is obtained. In the first depth profile, the ionic strength of the first negative secondary ions increases in the depth direction. In the second depth profile, the ionic strength of the second negative secondary ions decreases in the depth direction. The second depth profile has a step region where the ionic strength of the second negative secondary ions no longer temporarily decreases in the depth direction while the ionic strength of the first negative secondary ions increases in the depth direction. Therefore, excellent battery characteristics can be obtained.

[0009] Note that the effects of this technology are not necessarily limited to the effects described here, and may be any of the series of effects related to this technology described later.

Brief Description of Drawings

[0010] [Figure 1] FIG. 1 is a perspective view showing the configuration of a secondary battery in one embodiment of this technology. [Figure 2] FIG. 2 is a cross-sectional view showing the configuration of the battery element shown in FIG. 1. [Figure 3] FIG. 3 is a cross-sectional view showing the configuration of the positive electrode active material particles. [Figure 4] FIG. 4 is a diagram showing the analysis results in the depth direction of the positive electrode active material layer using time-of-flight secondary ion mass spectrometry. [Figure 5] FIG. 5 is a block diagram showing the configuration of an application example of a secondary battery. [Figure 6] FIG. 6 is a cross-sectional view showing the configuration of a test secondary battery.

Modes for Carrying Out the Invention

[0011] Hereinafter, one embodiment of this technology will be described in detail with reference to the drawings. The order of description is as follows. 1. Secondary battery 1-1. Configuration 1-2. Physical properties 1-3. Operation 1-4. Manufacturing method 1-5. Action and effect 2. Variations 3. Applications of rechargeable batteries

[0012] <1. Secondary battery> First, we will describe a secondary battery according to one embodiment of this technology.

[0013] The secondary battery described here is a secondary battery that obtains its capacity by utilizing the intercalation and deintercalation of electrode reactants, and is equipped with an electrolyte along with a positive electrode and a negative electrode.

[0014] The charging capacity of the negative electrode is preferably greater than the discharge capacity of the positive electrode. That is, the electrochemical capacity per unit area of ​​the negative electrode is preferably greater than the electrochemical capacity per unit area of ​​the positive electrode. This is to suppress the deposition of electrode reactants on the surface of the negative electrode during charging.

[0015] The types of electrode reactants are not particularly limited, but specifically, they are light metals such as alkali metals and alkaline earth metals. Specific examples of alkali metals include lithium, sodium, and potassium, while specific examples of alkaline earth metals include beryllium, magnesium, and calcium.

[0016] In the following example, we will consider the case where lithium is the electrode reactant. A secondary battery that obtains battery capacity by utilizing the intercalation and deintercalation of lithium is a so-called lithium-ion secondary battery. In this lithium-ion secondary battery, lithium is intercalated and deintercalated in an ionic state.

[0017] <1-1. Structure> Figure 1 shows a perspective view of the secondary battery. Figure 2 shows a cross-sectional view of the battery element 20 shown in Figure 1. Figure 3 shows a cross-sectional view of the positive electrode active material particles 210.

[0018] However, Figure 1 shows the outer film 10 and the battery element 20 separated from each other, and the cross-section of the battery element 20 along the XZ plane is shown with a dashed line. Figure 2 shows only a part of the battery element 20.

[0019] As shown in Figures 1 and 2, this secondary battery comprises an outer film 10, a battery element 20, a positive electrode lead 31, a negative electrode lead 32, and sealing films 41 and 42.

[0020] As described above, the secondary battery described here uses a flexible or pliable outer film 10 as an outer material for housing the battery element 20 inside. Therefore, the secondary battery shown in Figure 1 is a so-called laminate film type secondary battery.

[0021] [Exterior film] As shown in Figure 1, the outer film 10 has a bag-like structure that is sealed with the battery element 20 housed inside. Thus, the outer film 10 houses the positive electrode 21, negative electrode 22, and separator 23, which will be described later.

[0022] Here, the outer film 10 is a single film-like component that is folded in the folding direction F. The outer film 10 is provided with a recessed portion 10U (a so-called deep-drawn portion) for housing the battery element 20.

[0023] Specifically, the outer film 10 is a three-layer laminate film in which a fusion layer, a metal layer, and a surface protection layer are laminated in this order from the inside out. When the outer film 10 is folded, the outer edges of the opposing fusion layers are fused together. The fusion layer contains a polymer compound such as polypropylene. The metal layer contains a metallic material such as aluminum. The surface protection layer contains a polymer compound such as nylon.

[0024] However, the composition (number of layers) of the outer film 10 is not particularly limited; it may consist of one or two layers, or four or more layers.

[0025] [Battery element] The battery element 20 is housed inside the outer film 10. This battery element 20 is a so-called power generation element and includes a positive electrode 21, a negative electrode 22, and a separator 23, as shown in Figures 1 and 2.

[0026] Here, the battery element 20 is a so-called wound electrode body, and the positive electrode 21 and negative electrode 22 are wound around a winding axis P, facing each other via a separator 23. This winding axis P is a virtual axis extending in the Y-axis direction.

[0027] The three-dimensional shape of the battery element 20 is not particularly limited. Here, since the battery element 20 has a flattened three-dimensional shape, the shape of the cross-section of the battery element 20 intersecting the winding axis P (cross-section along the XZ plane) is a flattened shape defined by the major axis J1 and the minor axis J2.

[0028] The major axis J1 is a virtual axis extending in the X-axis direction and has a length greater than the length of the minor axis J2. The minor axis J2 is a virtual axis extending in the Z-axis direction intersecting the X-axis direction and has a length less than the length of the major axis J1. Here, since the three-dimensional shape of the battery element 20 is a flattened cylinder, the cross-sectional shape of the battery element 20 is a flattened, approximately elliptical shape.

[0029] (positive electrode) As shown in Figure 2, the positive electrode 21 includes a positive electrode current collector 21A and a positive electrode active material layer 21B.

[0030] The positive electrode current collector 21A has a pair of surfaces on which the positive electrode active material layer 21B is provided. This positive electrode current collector 21A contains a conductive material such as a metal material, a specific example of which is aluminum.

[0031] The positive electrode active material layer 21B contains one or more types of positive electrode active materials that intercalate and deintercalate lithium. However, the positive electrode active material layer 21B may further contain one or more types of other materials such as positive electrode binders and positive electrode conductive agents. The method for forming the positive electrode active material layer 21B is not particularly limited, but specifically includes methods such as coating.

[0032] Here, the positive electrode active material layer 21B is provided on both sides of the positive electrode current collector 21A. However, the positive electrode active material layer 21B may be provided on only one side of the positive electrode current collector 21A on the side where the positive electrode 21 faces the negative electrode 22.

[0033] Specifically, the positive electrode active material layer 21B contains multiple particulate positive electrode active material particles (hereinafter referred to as "multiple positive electrode active material particles 210"), as shown in Figure 3. These positive electrode active material particles 210 include a central part 210X and a coating part 210Y.

[0034] The central 210X contains one or more lithium composite oxides that intercalate and deintercalate lithium. These lithium composite oxides have a layered rock salt-type crystalline structure and contain lithium, nickel, and other elements as constituent elements. These other elements are one or more of the elements other than lithium and nickel.

[0035] However, in lithium composite oxides, the nickel content is set to be sufficiently large. Specifically, if the sum of the nickel content and the content of other elements in the lithium composite oxide is 100 moles, then the nickel content is between 80 and 100 moles. As is clear from the upper limit of 100 moles for the nickel content, lithium composite oxides may or may not contain other elements as constituent elements.

[0036] Furthermore, if a lithium composite oxide contains two or more other elements as constituent elements, the content of those other elements in the lithium composite oxide is the sum of the individual content amounts of the two or more other elements contained in that lithium composite oxide.

[0037] In other words, if the nickel content in the lithium composite oxide is C1 (mol) and the content of other elements in the lithium composite oxide is C2 (mol), then the nickel content C, calculated using the formula C = [C1 / (C1+C2)] × 100, is between 80 mol% and 100 mol%.

[0038] The reason why the content ratio C is 80 mol% to 100 mol% is that, compared to when the content ratio C is less than 80 mol%, the potential for intercalation and deintercalation of lithium decreases, resulting in a higher battery capacity.

[0039] Other elements are not particularly limited, but specifically include cobalt, aluminum, manganese, zirconium, titanium, molybdenum, tantalum, chromium, niobium, iron, copper, zinc, vanadium, magnesium, tungsten, sulfur, strontium, boron, sodium, and fluorine. This is because sufficient battery capacity can be obtained.

[0040] More specifically, lithium composite oxides contain one or more of the compounds represented by formula (1). The compounds shown in formula (1) contain the element E as a constituent element.

[0041] Li a Ni b E 1-b O2···(1) (E is at least one of the following: Co, Al, Mn, Zr, Ti, Mo, Ta, Cr, Nb, Fe, Cu, Zn, V, Mg, W, S, Sr, B, Na, and F. a and b satisfy 0.8 ≤ a ≤ 1.05 and 0.8 ≤ b ≤ 1.0.)

[0042] Incidentally, the positive electrode active material layer 21B may further contain any one or two or more of other positive electrode active materials that occlude and release lithium. The other positive electrode active materials contain a lithium-containing compound, and the above-described lithium composite oxide is excluded from the lithium-containing compounds described herein.

[0043] The lithium-containing compound is a compound containing lithium and one or two or more transition metal elements as constituent elements, and may further contain one or two or more additional elements as constituent elements. The types of additional elements (excluding lithium and transition metal elements) are not particularly limited, but specifically, they are elements belonging to Groups 2 to 15 in the long-period type periodic table. The types of lithium-containing compounds are not particularly limited, but specifically, they are oxides, phosphate compounds, silicate compounds, borate compounds, and the like.

[0044] Specific examples of the oxide are LiNiO2, LiCoO2, LiCo 0.98 Al 0.01 Mg 0.01 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, and LiMn2O4, etc. Specific examples of the phosphate compound are LiFePO4, LiMnPO4, and LiFe 0.5 Mn 0.5 PO4, etc.

[0045] Since the coating portion 210Y is provided on the surface of the central portion 210X, it covers the surface of the central portion 210X.

[0046] Here, the coating portion 210Y covers the entire surface of the central portion 210X. However, the coating portion 210Y may cover only a part of the surface of the central portion 210X. In this case, a plurality of coating portions 210Y separated from each other may cover the surface of the central portion 210X.

[0047] This coating portion 210Y is LiBO2F described later -It contains a material formed using second negative secondary ions derived from [the specified source]. The composition of the material forming the coating portion 210Y is not particularly limited as long as the coating portion 210Y is formed using second negative secondary ions.

[0048] A coating portion 210Y is provided on the surface of the central part 210X, and this coating portion 210Y contains a material formed using second negative secondary ions because even if lithium composite oxide (content ratio C ≥ 80 mol%) is used as the forming material for the central part 210X, the increase in the electrical resistance of the positive electrode 21 is suppressed.

[0049] In detail, as described above, a high battery capacity can be obtained by using lithium composite oxide (content ratio C ≥ 80 mol%) as the forming material for the central 210X.

[0050] However, when lithium composite oxide (containing C ≥ 80 mol%) is used, the surface activity of the positive electrode 21 increases during charging, making the electrolyte more susceptible to oxidative decomposition on its surface. When the electrolyte is oxidatively decomposed, decomposition products that act as resistance components are more likely to form, and these decomposition products tend to accumulate on the surface of the central part 210X. As a result, charge transfer reactions are more easily inhibited in the central part 210X, and the electrical resistance of the positive electrode 21 tends to increase. Consequently, the battery capacity tends to decrease with repeated charging and discharging.

[0051] However, if a coating portion 210Y is provided on the surface of the central portion 210X, and the three physical property conditions described later are met regarding the physical properties of the positive electrode active material layer 21B, the surface of the central portion 210X is electrochemically protected by the coating portion 210Y. This coating portion 210Y has high oxidation resistance and is ionic conductive but not electronically conductive. As a result, the coating portion 210Y suppresses the formation of decomposition products and prevents these decomposition products from accumulating on the surface of the central portion 210X. Therefore, the inhibition of charge transfer reactions is suppressed, and the oxidative decomposition of the electrolyte on the surface of the central portion 210X is also suppressed. Consequently, the electrical resistance of the positive electrode 21 is less likely to increase, and the battery capacity is less likely to decrease even after repeated charging and discharging.

[0052] As described above, the coating portion 210Y contains a material formed using second-order negative ions, and therefore it is preferable that it contains lithium, boron, and fluorine as constituent elements. This is because the surface of the central part 210X is more easily protected electrochemically by the coating portion 210Y.

[0053] In particular, it is more preferable that the coating portion 210Y contains lithium borate fluoride (LiBOF2). This is because it facilitates the formation of the coating portion 210Y, and the surface of the central portion 210X is sufficiently electrochemically protected by the coating portion 210Y.

[0054] Here, the coating portion 210Y is formed on the surface of the central part 210X using a coating source material during the manufacturing process of the secondary battery, more specifically during the stabilization treatment of the assembled secondary battery (initial charge / discharge treatment), as will be described later. In this case, since the coating source material decomposes and reacts during the stabilization treatment, the coating portion 210Y is formed to cover the surface of the central part 210X.

[0055] This coating source material is a material containing the constituent elements of the coating portion 210Y, and more specifically, it is one or more materials that serve as a source of boron and fluorine, which are included as constituent elements in the coating portion 210Y. The location where the coating source material is present is not particularly limited, as long as it is inside the secondary battery before the stabilization treatment.

[0056] The coating source material may be a material containing boron as a constituent element (boron-containing material) and a material containing fluorine as a constituent element (fluorine-containing material), or a material containing both boron and fluorine as constituent elements (boron-fluorine-containing material).

[0057] Details regarding the location and type of the coating source material will be described later.

[0058] The positive electrode binder contains one or more materials, such as synthetic rubber and polymer compounds. Specific examples of synthetic rubber include styrene-butadiene rubber, fluorine-based rubber, and ethylene propylene diene. Specific examples of polymer compounds include polyvinylidene fluoride, polyimide, and carboxymethylcellulose.

[0059] The positive electrode conductive agent contains one or more conductive materials, such as carbon materials, metallic materials, and conductive polymer compounds. Specific examples of carbon materials include graphite, carbon black, acetylene black, and Ketjen black.

[0060] Furthermore, the other materials included in the positive electrode active material layer 21B may also include one or more of the materials other than the positive electrode binder and positive electrode conductive agent described above.

[0061] Specifically, the other materials are dispersants used in the secondary battery manufacturing process (the preparation process for the positive electrode mixture slurry), which will be described later. These dispersants are materials that improve the dispersibility of multiple positive electrode active material particles 210 and other elements in the positive electrode mixture slurry, and specific examples of such dispersants include polyvinylpyrrolidone.

[0062] Furthermore, the other materials are one or more of the coating source materials. A specific example of a coating source material used in the positive electrode 21 is lithium metaborate (LiBO2), which is a boron-containing material. As described above, this coating source material is used to form the coating portion 210Y in the stabilization process of the secondary battery after assembly.

[0063] In this secondary battery, certain conditions are met regarding the physical properties of the positive electrode active material layer 21B in order to improve the battery characteristics. Details of the physical properties of the positive electrode active material layer 21B will be described later.

[0064] (Negative electrode) As shown in Figure 2, the negative electrode 22 includes a negative electrode current collector 22A and a negative electrode active material layer 22B.

[0065] The negative electrode current collector 22A has a pair of surfaces on which the negative electrode active material layer 22B is provided. This negative electrode current collector 22A contains a conductive material such as a metallic material, a specific example of which is copper.

[0066] The negative electrode active material layer 22B contains a negative electrode active material that intercepts and deintercepts lithium. However, the negative electrode active material layer 22B may further contain one or more other materials such as a negative electrode binder and a negative electrode conductive agent. The method for forming the negative electrode active material layer 22B is not particularly limited, but specifically, it is one or more of the following: coating, gas phase, liquid phase, thermal spraying, and firing (sintering).

[0067] Here, the negative electrode active material layer 22B is provided on both sides of the negative electrode current collector 22A. However, the negative electrode active material layer 22B may be provided on only one side of the negative electrode current collector 22A on the side where the negative electrode 22 faces the positive electrode 21.

[0068] The type of negative electrode active material is not particularly limited, but specifically, it includes carbon materials and metallic materials, because they allow for high energy density to be obtained.

[0069] Specific examples of carbon materials include readily graphitizable carbon, non-graphitizable carbon, and graphite (natural graphite and artificial graphite).

[0070] Metallic materials are materials that contain one or more metallic elements and metalloid elements capable of forming alloys with lithium as constituent elements. Specific examples of these metallic and metalloid elements include silicon and tin. These metallic materials may be elements, alloys, compounds, mixtures of two or more of these, or materials containing two or more of these phases. However, the "elements" described here may contain arbitrary amounts of impurities, so the purity of the "element" is not necessarily limited to 100%. Specific examples of metallic materials are TiSi2 and SiO2. x (0 <x≦2、または0.2<x<1.4)などである。

[0071] Details regarding the negative electrode binder are the same as those regarding the positive electrode binder, and details regarding the negative electrode conductive agent are the same as those regarding the positive electrode conductive agent.

[0072] (Separator) As shown in Figure 2, the separator 23 is an insulating porous membrane interposed between the positive electrode 21 and the negative electrode 22, allowing lithium to pass through in an ionic state while preventing short circuits caused by contact between the positive electrode 21 and the negative electrode 22. This separator 23 contains a polymer compound such as polyethylene.

[0073] (electrolyte) The electrolyte is a liquid electrolyte. This electrolyte is impregnated into the positive electrode 21, the negative electrode 22, and the separator 23, and contains a solvent and an electrolyte salt.

[0074] Here, the solvent contains one or more non-aqueous solvents (organic solvents), and the electrolyte containing these non-aqueous solvents is a so-called non-aqueous electrolyte. These non-aqueous solvents include esters and ethers, and more specifically, carbonate ester compounds, carboxylic acid ester compounds, and lactone compounds. This is because it improves the dissociation of the electrolyte salt and also improves the mobility of ions.

[0075] Carbonate ester compounds include cyclic carbonate esters and linear carbonate esters. Specific examples of cyclic carbonate esters include ethylene carbonate and propylene carbonate, while specific examples of linear carbonate esters include dimethyl carbonate, diethyl carbonate, and ethylmethyl carbonate.

[0076] Carboxylate ester compounds include chain-like carboxylic acid esters, with specific examples being ethyl acetate, ethyl propionate, propyl propionate, and trimethylethyl acetate. Lactone compounds include lactones, with specific examples being γ-butyrolactone and γ-valerolactone. Ethers may also include 1,2-dimethoxyethane, tetrahydrofuran, 1,3-dioxolane, and 1,4-dioxane.

[0077] The electrolyte salt contains one or more types of light metal salts, such as lithium salts. Specific examples of lithium salts include lithium hexafluoride phosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(fluorosulfonyl)imide (LiN(FSO2)2), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF3SO2)2), lithium tris(trifluoromethanesulfonyl)methide (LiC(CF3SO2)3), lithium bis(oxalato)borate (LiB(C2O4)2), lithium monofluorophosphate (Li2PFO3), and lithium difluorophosphate (LiPF2O2). This is because it allows for high battery capacity.

[0078] The electrolyte salt content is not particularly limited, but specifically, it is between 0.3 mol / kg and 3.0 mol / kg relative to the solvent. This is because it allows for high ionic conductivity.

[0079] This electrolyte may further contain one or more of the additives, as this improves the electrochemical stability of the electrolyte. The types of additives are not particularly limited, but specifically include unsaturated cyclic carbonates, fluorinated cyclic carbonates, sulfonic acid esters, phosphate esters, acid anhydrides, nitrile compounds, and isocyanate compounds.

[0080] Specific examples of unsaturated cyclic carbonate esters include vinylene carbonate, vinylethylene carbonate, and methyleneethylene carbonate. Specific examples of fluorinated cyclic carbonate esters include monofluoroethylene carbonate and difluoroethylene carbonate. Specific examples of sulfonic acid esters include propanesultone and propensultone. Specific examples of phosphate esters include trimethyl phosphate and triethyl phosphate. Specific examples of acid anhydrides include succinic anhydride, 1,2-ethanedisulfonic anhydride, and 2-sulfobenzoic anhydride. Specific examples of nitrile compounds include succinonitrile. Specific examples of isocyanate compounds include hexamethylene diisocyanate.

[0081] The electrolyte may contain one or more of the coating source materials. Specific examples of coating source materials used in the electrolyte include lithium metaborate (LiBO2), a boron-containing material; lithium hexafluoride phosphate (LiPF6), a fluorine-containing material; and lithium tetrafluoroborate (LiBF4), a boron-fluorine-containing material. As described above, these coating source materials are used to form the coating portion 210Y in the stabilization process of the assembled secondary battery.

[0082] [Positive lead] The positive electrode lead 31, as shown in Figures 1 and 2, is a positive electrode wire connected to the positive electrode current collector 21A and is led out to the outside of the outer film 10. This positive electrode lead 31 contains a conductive material such as a metal material, a specific example of which is aluminum. The shape of the positive electrode lead 31 is either a thin plate shape or a mesh shape.

[0083] [Negative lead] The negative electrode lead 32, as shown in Figures 1 and 2, is a negative electrode wire connected to the negative electrode current collector 22A and is led out to the outside of the outer film 10. Here, the direction of lead generation of the negative electrode lead 32 is the same as the direction of lead generation of the positive electrode lead 31. This negative electrode lead 32 contains a conductive material such as a metal material, a specific example of which is copper. Details regarding the shape of the negative electrode lead 32 are the same as details regarding the shape of the positive electrode lead 31.

[0084] [Sealing film] The sealing film 41 is inserted between the outer film 10 and the positive lead 31, and the sealing film 42 is inserted between the outer film 10 and the negative lead 32. However, one or both of the sealing films 41 and 42 may be omitted.

[0085] The sealing film 41 is a sealing member that prevents outside air and other elements from entering the interior of the outer film 10. This sealing film 41 contains a polymer compound such as polyolefin that has good adhesion to the positive electrode lead 31, and a specific example of such a polymer compound is polypropylene.

[0086] The structure of the sealing film 42 is the same as that of the sealing film 41, except that it is a sealing member that adheres to the negative electrode lead 32. That is, the sealing film 42 contains a polymer compound such as a polyolefin that adheres to the negative electrode lead 32.

[0087] <1-2. Physical Properties> In this secondary battery, as described above, certain conditions are met regarding the physical properties of the positive electrode active material layer 21B in order to improve the battery characteristics. Below, we will explain the case in which the coating portion 210Y contains lithium boroate fluoride (LiBOF2).

[0088] Figure 4 shows the analysis results for the depth direction D of the positive electrode active material layer 21B using time-of-flight secondary ion mass spectrometry (TOF-SIMS). In Figure 4, the horizontal axis represents the sputtering time (seconds), and the vertical axis represents the ion intensity (counts).

[0089] [Physical properties when positive electrode active material particles include a coating] The positive electrode active material layer 21B is analyzed in the depth direction D using TOF-SIMS. The TOF-SIMS analysis described here is a so-called depth analysis. The depth direction D corresponds to the thickness direction of the positive electrode active material layer 21B, as shown in Figure 2, and more specifically, it is the direction from the surface of the positive electrode active material layer 21B toward the interior of the positive electrode active material layer 21B.

[0090] In this depth analysis, conventional ion analysis using primary ions and sputter etching, which uses sputter ions to penetrate the positive electrode active material layer 21B, are alternately repeated in the depth direction D. As a result, the amount of various ions detected from the positive electrode active material layer 21B in the depth direction D is measured, and the analysis results for the positive electrode active material layer 21B in the depth direction D are obtained, as shown in Figure 4.

[0091] As described above, the horizontal axis represents the sputtering time during sputter etching, and therefore corresponds to the internal position (so-called depth) of the positive electrode active material layer 21B in the depth direction D. Furthermore, as described above, the horizontal axis represents the ionic intensity, and therefore corresponds to the detection amount of various ions in the depth direction D.

[0092] Here, the positive electrode active material particles 210, as shown in Figure 3, contain a central part 210X (a lithium composite oxide with a content of C of 80 mol% or more) and a coating part 210Y (lithium borate fluoride). In this case, two types of negative secondary ions are detected by analysis of the depth direction D of the positive electrode active material layer 21B using TOF-SIMS. These two types of negative secondary ions are NiO2 - The first negative secondary ion derived from and LiBO2F - It is a second negative secondary ion derived from [the source].

[0093] As a result, as shown in Figure 4, a first depth profile 4A representing the change in ionic intensity of the first negative secondary ion in the depth direction D, and a second depth profile 4B representing the change in ionic intensity of the second negative secondary ion in the same depth direction D are obtained.

[0094] When the coating portion 210Y covers the surface of the central portion 210X, and the coating portion 210Y contains lithium borate fluoride, the following three physical property conditions are met.

[0095] (First physical property condition) In the first depth profile 4A, the amount of detected first negative secondary ions increases as the sputtering time increases, and therefore the ionic intensity of these first negative secondary ions increases in the depth direction D. In this case, the ionic intensity of the first negative secondary ions may increase sharply midway through the depth direction D, i.e., while the ionic intensity of the first negative secondary ions is increasing.

[0096] The reason why the ionic strength of the first negative secondary ion increases in the depth direction D is as follows.

[0097] As described above, the positive electrode active material layer 21B contains not only multiple positive electrode active material particles 210, but also positive electrode binders and positive electrode conductive agents. A portion of these positive electrode active material particles 210 (the central part 210X) contains a large amount of nickel as a constituent element, while the positive electrode binder and positive electrode conductive agents contain almost no nickel as a constituent element. In this case, analysis of the positive electrode active material layer 21B in the depth direction D using TOF-SIMS reveals that first negative secondary ions are detected in regions where nickel is sufficiently present, while first negative secondary ions are not detected in regions where nickel is almost absent.

[0098] As a result, when the positive electrode active material layer 21B is analyzed in the depth direction D using TOF-SIMS, the amount of detected first negative secondary ions increases in the depth direction D, as described above, and therefore the ionic strength of these first negative secondary ions increases in the depth direction D.

[0099] In particular, when there are almost no positive electrode active material particles 210 near the outermost surface of the positive electrode active material layer 21B, the ionic strength of the first negative secondary ions increases gradually. However, since there are many positive electrode active material particles 210 inside the positive electrode active material layer 21B, the ionic strength of the first negative secondary ions increases rapidly. As a result, the ionic strength of the first negative secondary ions increases gradually in the depth direction D before increasing rapidly, and as described above, the ionic strength of the first negative secondary ions may increase rapidly in the middle of the depth direction D.

[0100] (Second physical property condition) In the second depth profile 4B, the amount of detected second negative secondary ions decreases as the sputtering time increases, and therefore the ionic intensity of these second negative secondary ions decreases in the depth direction D.

[0101] The reason why the ionic strength of the second negative secondary ion decreases in the depth direction D is as follows.

[0102] During the stabilization process of a rechargeable battery after assembly, not only does the coating source material decompose and react, but several types of compounds present inside the rechargeable battery after assembly also decompose and react. As a result, a coating containing a variety of decomposition products and reactants is formed on the surface of the positive electrode active material layer 21B. Specific examples of these several types of compounds include solvents and electrolyte salts contained in the electrolyte.

[0103] The various decomposition products and reactants contained in the coating may contain large amounts of boron and fluorine as constituent elements. In contrast, the positive electrode binder and positive electrode conductive agent contained in the positive electrode active material layer 21B do not contain as much boron and fluorine as constituent elements as the coating described above. In this case, analysis of the depth direction D of the positive electrode active material layer 21B using TOF-SIMS reveals that second negative secondary ions are detected in regions where boron and fluorine are sufficiently present, while second negative secondary ions are not detected in regions where boron and fluorine are almost absent.

[0104] As a result, when the positive electrode active material layer 21B is analyzed in the depth direction D using TOF-SIMS, the amount of detected second negative secondary ions decreases in the depth direction D, as described above, and therefore the ionic intensity of these second negative secondary ions decreases in the depth direction D.

[0105] (Third physical property condition) However, the second depth profile 4B has a step region S in which, while the ionic intensity of the first negative secondary ion is increasing in the depth direction D, the ionic intensity of the second negative secondary ion temporarily stops decreasing in the depth direction D.

[0106] In this stepped region S, the ionic strength of the second negative secondary ion may be constant in the depth direction D, or it may increase in the depth direction D, or a region in which the ionic strength of the second negative secondary ion is constant in the depth direction D and a region in which the ionic strength of the second negative secondary ion increases in the depth direction D may be mixed.

[0107] The reason why the second depth profile 4B has a step region S is explained below.

[0108] As described above, during the stabilization treatment of the secondary battery after assembly, a film is formed on the surface of the positive electrode active material layer 21B due to the decomposition and reaction of not only the coating source material but also multiple types of compounds, resulting in a decrease in the ionic strength of the second negative secondary ions in the depth direction D.

[0109] However, since a portion of the positive electrode active material particles 210 (coating portion 210Y) contains lithium boro fluoride, it contains a large amount of boron and fluorine as constituent elements. In this case, analysis of the depth direction D of the positive electrode active material layer 21B using TOF-SIMS reveals that second negative secondary ions are detected in regions where boron and fluorine are sufficiently present, while second negative secondary ions are not detected in regions where boron and fluorine are almost absent.

[0110] As a result, when the positive electrode active material layer 21B is analyzed in the depth direction D using TOF-SIMS, the ionic intensity of the second negative secondary ion decreases in the depth direction D, but when the analysis position in the depth direction D reaches the coating portion 210Y, the ionic intensity of the second negative secondary ion temporarily increases. This temporarily prevents the ionic intensity of the second negative secondary ion from decreasing. After this, when the analysis position in the depth direction D reaches the center 210X, the ionic intensity of the second negative secondary ion decreases again. Therefore, the second depth profile 4B has a stepped region S in the middle of the depth direction D where the ionic intensity of the second negative secondary ion temporarily stops decreasing.

[0111] [Physical properties when positive electrode active material particles do not contain a coating] If the positive electrode active material particles 210 do not contain the coating portion 210Y (lithium boroate fluoride) and only contain the central portion 210X (lithium composite oxide with a content of C of 80 mol% or more), then the first depth profile 4A is obtained, and instead of the second depth profile 4B, the third depth profile 4C is obtained.

[0112] In Figure 4, the first depth profile 4A and the second depth profile 4B are shown as solid lines, while the third depth profile 4C is shown as a dashed line.

[0113] In the third depth profile 4C, since there is no coating portion 210Y inside the positive electrode active material layer 21B, the ionic strength of the second negative secondary ions decreases in the depth direction D. That is, since the ionic strength of the second negative secondary ions does not temporarily increase midway in the depth direction D, it decreases continuously in the depth direction D without temporarily decreasing midway in the depth direction D. As a result, unlike the second depth profile 4B, the third depth profile 4C does not have a step region S.

[0114] Furthermore, even if the positive electrode active material particles 210 contain a coating portion 210Y, if the amount of the coating portion 210Y is insufficient, a third depth profile 4C is obtained instead of a second depth profile 4B, similar to the case where the positive electrode active material particles 210 do not contain a coating portion 210Y.

[0115] [Analysis Procedure] As described above, by analyzing the positive electrode active material layer 21B in the depth direction D using TOF-SIMS, it is possible to retrospectively identify (after the secondary battery is completed) that the positive electrode active material particles 210 contain a central part 210X (lithium composite oxide with a content C of 80 mol% or more) and a coating part 210Y (lithium borate fluoride).

[0116] In other words, by analyzing the positive electrode active material layer 21B in the depth direction D using TOF-SIMS, if the three physical property conditions described above are met, it can be demonstrated that the positive electrode active material particles 210 contain a central part 210X (lithium composite oxide with a content of C of 80 mol% or more) and a coating part 210Y (lithium borate fluoride).

[0117] For TOF-SIMS analysis, the ION-TOF TOF V analyzer can be used. The analytical conditions are: primary ion = Bi 3+ Ion gun acceleration voltage = 25 keV, analysis mode = depth profiling by High Current Bunched, irradiation ion current (measured with pulsed beam) = 0.2 pA, pulse frequency = 10 kHz, mass range = 1 amu to 800 amu, scanning range = 200 μm × 200 μm, sputtered ions = Ar + The sputter ion gun's acceleration voltage is 1kV, emission current is 200mA, and sputter area is 500μm × 500μm.

[0118] The procedure for confirming whether three types of physical properties are met by analyzing the positive electrode active material layer 21B in the depth direction D using TOF-SIMS is as follows.

[0119] First, discharge the rechargeable battery until the voltage reaches 2.0V. The discharge current is not particularly limited and can be set arbitrarily.

[0120] Next, the positive electrode 21 is recovered by disassembling the secondary battery inside the glove box, and then the positive electrode 21 is cleaned using a cleaning solvent. This removes the electrolyte impregnated in the positive electrode 21. The type of cleaning solvent is not particularly limited, but specifically, it is one or more types of organic solvents such as dimethyl carbonate.

[0121] Next, the cleaned positive electrode 21 is attached to the sample holder using adhesive tape. The type of adhesive tape is not particularly limited, but specifically, carbon tape is used. Subsequently, the positive electrode active material layer 21B is analyzed in the depth direction D using TOF-SIMS to obtain the analysis results shown in Figure 4.

[0122] Finally, based on the analysis results, we visually confirm whether the three types of physical property conditions are met.

[0123] Specifically, since the ionic strength of the first negative secondary ion increases in the depth direction D, when the first depth profile 4A is obtained, the first physical property condition is met because multiple positive electrode active material particles 210 (a central part 210X containing lithium composite oxide with a content C of 80 mol% or more) are present inside the positive electrode active material layer 21B.

[0124] Furthermore, since the ionic strength of the second negative secondary ion decreases in the depth direction D, the second physical property condition is met when the second depth profile 4B is obtained.

[0125] Furthermore, even if the ionic strength of the second negative secondary ion decreases in the depth direction D, if the second depth profile 4B has a step region S, the third physical property condition is met because multiple positive electrode active material particles 210 (coating portion 210Y containing lithium borate fluoride) are present inside the positive electrode active material layer 21B.

[0126] Furthermore, if a third depth profile 4C, which does not have a step region S, is obtained instead of a second depth profile 4B that has a step region S, then the second physical property condition is not met because the positive electrode active material particles 210 do not contain a coating portion 210Y, or even if the positive electrode active material particles 210 contain a coating portion 210Y, the amount of coating portion 210Y formed is insufficient.

[0127] [Reasons for using TOF-SIMS as the analysis method] The reason for using TOF-SIMS (depth analysis) as an analytical method to investigate the physical properties of the positive electrode active material layer 21B is as follows.

[0128] Firstly, the thickness of the coating portion 210Y is assumed to be extremely thin, at only a few nanometers. In this case, it is effective to employ TOF-SIMS (depth analysis), which has extremely high depth resolution, in order to investigate the physical properties of the positive electrode active material layer 21B in the depth direction D.

[0129] Secondly, when multiple types of compounds are present within the positive electrode active material layer 21B, it is more effective to use TOF-SIMS, which allows for the individual analysis of the composition of these multiple types of compounds, rather than using an analytical method that allows for the analysis of the average composition of the positive electrode active material layer 21B.

[0130] In detail, analytical methods such as X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (TEM-EELS) are generally used to investigate the composition of the coating.

[0131] However, in XPS, the spectra derived from boron and phosphorus overlap, making it difficult to investigate the chemical bonding state of boron, and even difficult to determine the presence or absence of boron at all.

[0132] Furthermore, in TEM-EELS (microparticle cross-sectional analysis), when line analysis of the coating in the depth direction D, the average composition including lithium, boron, oxygen, and fluorine is analyzed in a state where not only lithium boroate fluoride but also other compounds are present, making it difficult to analyze only lithium boroate fluoride.

[0133] In contrast, TOF-SIMS analyzes the average composition of lithium boroate, including lithium, boron, oxygen, and fluorine, making it possible to analyze only lithium boroate.

[0134] Thirdly, while surface analysis using TOF-SIMS can analyze a variety of ions present on the surface of a coating, it is difficult to analyze the variety of ions present inside that coating.

[0135] In contrast, depth analysis using TOF-SIMS makes it possible to analyze not only the diverse ions present on the surface of the coating, but also the diverse ions present inside the coating. This makes it possible to analyze the diverse ions related to the positive electrode active material layer 21B, and more specifically, to analyze the diverse ions present inside the coating portion 210Y provided on the surface of the central part 210X.

[0136] <1-3. Operation> This secondary battery operates as follows in the battery element 20.

[0137] During charging, lithium is released from the positive electrode 21 and absorbed into the negative electrode 22 via the electrolyte. Conversely, during discharging, lithium is released from the negative electrode 22 and absorbed into the positive electrode 21 via the electrolyte. During both discharging and charging, lithium is absorbed and released in an ionic state.

[0138] <1-4. Manufacturing method> When manufacturing a secondary battery, the positive electrode 21 and negative electrode 22 are prepared and the electrolyte is prepared according to the example procedure described below. The secondary battery is then assembled using the positive electrode 21, negative electrode 22, and electrolyte, and the assembled secondary battery is subjected to stabilization treatment.

[0139] The following describes a method for manufacturing a secondary battery when the coating portion 210Y contains lithium borate fluoride.

[0140] [Fabrication of the positive electrode] First, several core 210X containing lithium composite oxide (content C ≥ 80 mol%) are prepared. This lithium composite oxide contains residual alkali components, which include alkali compounds such as lithium hydroxide (LiOH) and lithium carbonate (Li2CO3). The residual alkali components are mainly present at grain boundaries within the lithium composite oxide.

[0141] Next, multiple cores 210X are heated in a pressurized environment. Heating conditions such as the pressure in the environment (Pa), heating temperature (°C), and heating time (hours) can be set arbitrarily. As a result, the residual alkaline components contained in the cores 210X are heated, and these residual alkaline components diffuse onto the surface of the cores 210X. Therefore, the residual alkaline components tend to be uniformly distributed on the surface of the cores 210X.

[0142] The reason for heating the central part 210X to diffuse the remaining alkaline components onto its surface is that, in the stabilization treatment of the assembled secondary battery described later, the reaction between the remaining alkaline components and the coating source material facilitates the formation of the coating portion 210Y.

[0143] The content (by weight) of residual alkali components in the central part 210X is not particularly limited, as long as the coating 210Y is formed using these residual alkali components. This residual alkali component content is calculated based on the formula: Residual alkali component content = (Weight of residual alkali components / Weight of central part 210X) × 100. In this case, the amount of residual alkali components diffused to the surface of the central part 210X can be controlled by changing the heating conditions described above.

[0144] The procedure for calculating the residual alkali content in the central 210X is as follows: First, the weight M1 of multiple central 210X samples is measured. Next, the weight M2 of the residual alkali content contained in the multiple central 210X samples is measured using the two-stage neutralization titration method (Walder method) described later. Finally, the residual alkali content is calculated based on the calculation formula described above.

[0145] The procedure for measuring the weight M2 of the remaining alkaline component using a two-stage neutralization titration method is as follows.

[0146] A residual alkaline aqueous solution is prepared by adding multiple 210X cores to pure water and then stirring the pure water. The weight M2 of the residual alkaline component is the weight of the residual alkaline component contained in the supernatant of the residual alkaline aqueous solution, and more specifically, it is the total weight of lithium hydroxide and lithium carbonate.

[0147] The supernatant of the residual alkaline aqueous solution is an aqueous solution containing residual alkaline components, and therefore, the weight M2 of these residual alkaline components can be measured based on the amount of acid required to titrate the supernatant until the neutralization point is reached. In this case, the residual alkaline aqueous solution may be diluted with pure water. The neutralization point in the neutralization titration is determined using electrotitration. In this electrotitration method, the point where the rate of change of the measured potential is greatest with respect to the amount of acid titrated is the first neutralization point (final neutralization point), and the point where the rate of change of the measured potential is second greatest with respect to the amount of acid titrated is the first neutralization point.

[0148] In neutralization titration, hydrochloric acid (concentration = 0.1 mol / L = 0.1 mol / dm³) is used as the acid to titrate the remaining alkaline aqueous solution. 3 )) may also be used. In this case, the neutralization reaction shown in reaction equations (1) to (3) proceeds, and the weight M2 of the remaining alkaline component can be calculated using calculation formula (4). Reaction equations (1) and (2) show the neutralization reaction that proceeds up to the first neutralization point, while reaction equation (3) shows the neutralization reaction that proceeds from the first neutralization point to the second neutralization point.

[0149] LiOH+HCl → LiCl+H2O ···(1) Li2CO3+HCl → LiCl+LiHCO3···(2) LiHCO3++HCl → LiCl+CO2+H2O ···(3) M2=cf(V2-V1)m1+cf(2V1-V2)m2 ···(4)

[0150] In formula (4), M2 is the weight of the remaining alkaline component, c is the concentration of hydrochloric acid used in the neutralization titration, f is the factor value of the hydrochloric acid used in the neutralization titration (a coefficient for correcting the concentration of hydrochloric acid), V1 is the volume of hydrochloric acid required to reach the first neutralization point, V2 is the volume of hydrochloric acid required to reach the second neutralization point, m1 is the molecular weight of lithium carbonate, and m2 is the molecular weight of lithium hydroxide.

[0151] Next, the heated positive electrode active material particles 210, the positive electrode binder, and the positive electrode conductive agent are mixed together to form a positive electrode mixture. Subsequently, the positive electrode mixture is added to a solvent to prepare a paste-like positive electrode mixture slurry. This solvent may be an aqueous solvent or an organic solvent.

[0152] Next, a precursor layer (not shown) is formed by applying a positive electrode mixture slurry to both sides of the positive electrode current collector 21A. This precursor layer has the same configuration as the positive electrode active material layer 21B, except that a coating portion 210Y is not provided on the surface of the central part 210X and that it is not impregnated with electrolyte. After this, the precursor layer may be compressed and molded using a roll press or the like. In this case, the precursor layer may be heated, or the compression molding of the precursor layer may be repeated multiple times.

[0153] Finally, as will be described later, after assembling the secondary battery, a stabilization treatment is performed using the assembled secondary battery. This forms a coating portion 210Y on the surface of the central part 210X, thus forming positive electrode active material particles 210. As a result, a positive electrode active material layer 21B containing multiple positive electrode active material particles 210 is formed, and the positive electrode 21 is manufactured.

[0154] Furthermore, if a coating source material is used in the manufacturing process of the positive electrode 21, the positive electrode 21 is manufactured according to the procedure described below.

[0155] First, multiple cores 210X and powdered coating source material are mixed together. This coating source material contains boron-containing material such as lithium metaborate, as described above. As a result, the powdered coating source material adheres to the surface of the cores 210X. This coating source material adhering to the surface of the cores 210X is used in a subsequent process to form the coating portion 210Y, as described above.

[0156] Next, using multiple central portions 210X with powdered coating material attached to their surfaces, a positive electrode mixture slurry is prepared according to the procedure described above, and then a precursor layer is formed using the positive electrode mixture slurry.

[0157] Finally, after assembling the secondary battery, a stabilization treatment is performed using the assembled secondary battery. This utilizes the powdery coating source material attached to the surface of the central part 210X to form a coating portion 210Y on the surface of the central part 210X, thus forming a positive electrode active material layer 21B. As a result, a positive electrode active material layer 21B containing multiple positive electrode active material particles 210 is formed, and the positive electrode 21 is manufactured.

[0158] [Fabrication of the negative electrode] First, the negative electrode active material, negative electrode binder, and negative electrode conductive agent are mixed together to form a negative electrode mixture. Next, the negative electrode mixture is added to a solvent to prepare a paste-like negative electrode mixture slurry. Details regarding the solvent are as described above. Finally, the negative electrode mixture slurry is applied to both sides of the negative electrode current collector 22A to form the negative electrode active material layer 22B. After this, the negative electrode active material layer 22B may be compression-molded using a roll press or the like. In this case, the negative electrode active material layer 22B may be heated, or the compression molding of the negative electrode active material layer 22B may be repeated multiple times. This produces the negative electrode 22.

[0159] [Preparation of electrolyte solution] The electrolyte salt is added to the solvent. This disperses or dissolves the electrolyte salt in the solvent, thus preparing the electrolyte solution.

[0160] Furthermore, when a coating source material is used in the electrolyte preparation process, an electrolyte salt that also serves as the coating source material may be used. As mentioned above, this electrolyte salt that also serves as the coating source material contains a fluorine-containing material such as lithium hexafluoride phosphate.

[0161] Furthermore, when a coating source material is used in the electrolyte preparation process, the electrolyte salt may be added to the solvent, and then the coating source material may be added to the solvent. As described above, this coating source material contains a boron-containing material such as lithium tetrafluoroborate. As a result, the coating source material is dispersed or dissolved in the solvent, and an electrolyte containing the coating source material is prepared. The coating source material contained in this electrolyte is used to form the coating portion 210Y in the stabilization treatment of the assembled secondary battery.

[0162] [Assembly of rechargeable batteries] First, the positive electrode lead 31 is connected to the positive electrode current collector 21A of the positive electrode 21 using a joining method such as welding, and the negative electrode lead 32 is connected to the negative electrode current collector 22A of the negative electrode 22 using a joining method such as welding.

[0163] Next, a laminate (not shown) is formed by stacking a positive electrode current collector 21A, on which a precursor layer has been formed, and a negative electrode current collector 22A, on which a negative electrode active material layer 22B has been formed, via a separator 23. Subsequently, a wound body (not shown) is produced by winding the laminate, and then the wound body is shaped into a flattened shape by pressing it with a press or the like. This shaped wound body has the same configuration as the battery element 20, except that it contains a precursor layer instead of the positive electrode active material layer 21B and is not impregnated with electrolyte.

[0164] Next, after housing the wound body inside the recessed portion 10U, the outer film 10 (fusion layer / metal layer / surface protection layer) is folded so that the outer films 10 face each other. Subsequently, using an adhesive method such as heat fusion, the outer edges of two sides of the opposing fusion layers are bonded together, thereby housing the wound body inside the bag-shaped outer film 10.

[0165] Finally, after injecting the electrolyte into the bag-shaped outer film 10, the outer edges of the remaining sides of the opposing fused layers are bonded together using an adhesive method such as heat fusion. In this case, a sealing film 41 is inserted between the outer film 10 and the positive electrode lead 31, and a sealing film 42 is inserted between the outer film 10 and the negative electrode lead 32.

[0166] As a result, the winding material is impregnated with electrolyte, and the winding material is sealed inside the bag-shaped outer film 10, thus assembling the secondary battery.

[0167] [Stabilization treatment for rechargeable batteries after assembly] 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 set arbitrarily.

[0168] As a result, a coating portion 210Y is formed on the surface of the central portion 210X, thus forming positive electrode active material particles 210. Therefore, a positive electrode active material layer 21B containing multiple positive electrode active material particles 210 is formed, and the positive electrode 21 is fabricated.

[0169] Furthermore, since a coating is formed on the surface of the positive electrode 21, the positive electrode 21 is electrochemically stabilized, and since a coating is formed on the surface of the negative electrode 22, the negative electrode 22 is electrochemically stabilized.

[0170] In this case, when a boron-containing material such as lithium metaborate is used as the coating source material (boron-containing material) in the manufacturing process of the positive electrode 21, and lithium hexafluoride phosphate is used as the electrolyte salt that also serves as the coating source material (fluorine-containing material) in the electrolyte preparation process, the coating portion 210Y containing lithium borate fluoride is formed by utilizing the reaction between the coating source materials described below.

[0171] LiBO2 + LiPF6 → LiBOF2 + POF3 + LiF

[0172] Furthermore, when lithium borate tetrafluoride is used as the coating source material (boron-fluorine-containing material) in the electrolyte preparation process, the coating portion 210Y containing lithium borate fluoride is formed by utilizing the reaction between the coating source material and the remaining alkaline components, as described below.

[0173] LiBF4+Li2CO3→LiBOF2+2LiF+CO2 LiBF4 + 2LiOH → LiBOF2 + 2LiF + H2O

[0174] As a result of these steps, the battery element 20 is fabricated and then sealed inside the bag-shaped outer film 10, thus completing the secondary battery.

[0175] <1-5. Mechanism and Effects> According to this secondary battery, the positive electrode active material layer 21B contains multiple positive electrode active material particles 210, each of which includes a central part 210X and a coating part 210Y, with the central part 210X containing lithium composite oxide (content ratio C ≥ 80 mol%). Furthermore, the analysis results of the depth direction D of the positive electrode active material layer 21B using TOF-SIMS show that the three physical property conditions described above are met.

[0176] In this case, since the central 210X contains lithium composite oxide (content ratio C ≥ 80 mol%), a high battery capacity can be obtained as described above.

[0177] Furthermore, since a coating portion 210Y is provided on the surface of the central part 210X, and three types of physical property conditions are satisfied regarding the physical properties of the positive electrode active material layer 21B, as described above, even if the central part 210X contains lithium composite oxide (content ratio C ≥ 80 mol%), the surface of the central part 210X is electrochemically protected by the coating portion 210Y. Therefore, while the inhibition of the charge transfer reaction is suppressed, the oxidative decomposition of the electrolyte on the surface of the central part 210X is also suppressed, making it less likely for the electrical resistance of the positive electrode 21 to increase. As a result, the battery capacity is less likely to decrease even after repeated charging and discharging.

[0178] From these findings, even when lithium composite oxide (content C ≥ 80 mol%) is used as the forming material for the central part 210X, not only is a high battery capacity obtainable, but the electrical resistance of the positive electrode 21 is less likely to increase by utilizing the coating part 210Y, thus enabling the acquisition of excellent battery characteristics.

[0179] In particular, if the coating portion 210Y contains lithium, boron, and fluorine as constituent elements, the coating portion 210Y is more easily formed, thus achieving a higher effect. In this case, if the coating portion 210Y contains lithium boroate fluoride, the surface of the central part 210X is sufficiently electrochemically protected by the coating portion 210Y, thus achieving an even higher effect.

[0180] Furthermore, if the lithium composite oxide contains one or more of the following elements as other elements—cobalt, aluminum, manganese, zirconium, titanium, molybdenum, tantalum, chromium, niobium, iron, copper, zinc, vanadium, magnesium, tungsten, sulfur, strontium, boron, sodium, and fluorine—sufficient battery capacity can be obtained, resulting in a higher performance.

[0181] Furthermore, if the secondary battery is a lithium-ion secondary battery, a sufficient battery capacity can be stably obtained by utilizing the intercalation and deintercalation of lithium, thus achieving a higher level of efficiency.

[0182] <2. Variant> The configuration of the secondary battery can be modified as appropriate, as described below. However, any two or more of the variations described below may be combined with each other.

[0183] [Example 1] The case where the positive electrode 21 or the electrolyte contains a coating source material has been described. However, the location of the coating source material is not particularly limited, and it may also be in the negative electrode 22, or two or more of the positive electrode 21, negative electrode 22, and electrolyte.

[0184] In this case as well, since the covering portion 210Y is formed during the stabilization process of the secondary battery after assembly, the same effect can be obtained.

[0185] [Differentiation 2] The case described above involves forming the coating portion 210Y after the assembly of the secondary battery by using a stabilization treatment of the secondary battery after assembly. However, the coating portion 210Y may also be formed before the assembly of the secondary battery by forming the coating portion 210Y without using a stabilization treatment.

[0186] The procedure for manufacturing the positive electrode 21 when forming the coating portion 210Y before assembly of the secondary battery without using a stabilization treatment is as follows.

[0187] First, a treatment solution is prepared by mixing the material for forming the coating portion 210Y with a dispersion solvent. The material for forming the coating portion 210Y is a material that enables the satisfaction of three types of physical property conditions regarding the physical properties of the positive electrode active material layer 21B, and specifically, is lithium borate fluoride as described above. The type of dispersion solvent is not particularly limited, and may be an aqueous solvent or an organic solvent.

[0188] Furthermore, when preparing the processing solution, a binder may be included in the solution. Details regarding the binder are the same as those regarding the positive electrode binder.

[0189] Next, the treatment solution is applied to the surface of the multiple central parts 210X, and then the treatment solution is dried to form the coated part 210Y. In this case, instead of applying the treatment solution to the surface of the multiple central parts 210X, the multiple central parts 210X may be immersed in the treatment solution, and then the multiple central parts 210X may be removed from the treatment solution and dried.

[0190] As a result, a coating portion 210Y is formed on the surface of multiple central portions 210X, thus forming multiple positive electrode active material particles 210.

[0191] Finally, a positive electrode active material layer 21B is formed using multiple positive electrode active material particles 210 according to the procedure described above. This allows the positive electrode 21 to be manufactured without the need for stabilization treatment of the secondary battery after assembly.

[0192] In this case as well, since positive electrode active material particles 210 including the central part 210X and the coating part 210Y are obtained, the same effect can be obtained.

[0193] [Difference 3] A porous membrane separator 23 was used. However, although not specifically shown in the diagram here, a laminated separator containing a polymer compound layer may be used instead of the porous membrane separator 23.

[0194] Specifically, the laminated separator includes a porous membrane having a pair of surfaces and a polymer compound layer provided on one or both sides of the porous membrane. This improves the adhesion of the separator to the positive electrode 21 and the negative electrode 22, thereby suppressing misalignment of the battery element 20, i.e., misalignment of the windings of the positive electrode 21, the negative electrode 22, and the separator. As a result, swelling of the secondary battery is suppressed even if a decomposition reaction of the electrolyte occurs. The polymer compound layer contains a polymer compound such as polyvinylidene fluoride. Polyvinylidene fluoride has excellent physical strength and is electrochemically stable.

[0195] Furthermore, one or both of the porous membrane and the polymer compound layer may contain one or more types of insulating particles from a plurality of insulating particles. This is because the plurality of insulating particles dissipate heat when the secondary battery generates heat, thereby improving the safety (heat resistance) of the secondary battery. The plurality of insulating particles include one or more types of insulating materials such as inorganic materials and resin materials. Inorganic materials include aluminum oxide, aluminum nitride, boehmite, silicon oxide, titanium oxide, magnesium oxide, and zirconium oxide. Resin materials include acrylic resin and styrene resin.

[0196] When fabricating a laminated separator, a precursor solution containing a polymer compound and an organic solvent is prepared, and then the precursor solution is applied to one or both sides of a porous membrane. In this case, the precursor solution may contain multiple insulating particles.

[0197] Even when using this stacked separator, lithium can move in an ionic state between the positive electrode 21 and the negative electrode 22, thus achieving a similar effect. In this case, in particular, as described above, swelling of the secondary battery is further suppressed as the displacement of the battery element 20 is suppressed, thus achieving an even greater effect.

[0198] [Differentiation Example 4] A liquid electrolyte solution was used. However, although not specifically illustrated here, a gel-like electrolyte layer may also be used.

[0199] In the battery element 20 using an electrolyte layer, the positive electrode 21 and the negative electrode 22 are wound facing each other via a separator 23 and the electrolyte layer. This electrolyte layer is interposed between the positive electrode 21 and the separator 23, and also between the negative electrode 22 and the separator 23.

[0200] Specifically, the electrolyte layer contains a polymer compound along with the electrolyte, and the electrolyte is held in place by the polymer compound. This prevents leakage of the electrolyte. The composition of the electrolyte is as described above. The polymer compound includes polyvinylidene fluoride, etc. When forming the electrolyte layer, a precursor solution containing the electrolyte, polymer compound, and solvent is prepared, and then the precursor solution is applied to one or both sides of the positive electrode 21 and to one or both sides of the negative electrode 22.

[0201] Even when this electrolyte layer is used, lithium can move in an ionic state between the positive electrode 21 and the negative electrode 22 via the electrolyte layer, thus achieving a similar effect. In this case, in particular, as described above, leakage of the electrolyte is prevented, thus achieving an even greater effect.

[0202] <3. Applications of rechargeable batteries> Finally, I will explain the applications (examples of use) of secondary batteries.

[0203] The uses of secondary batteries are not particularly limited. Secondary batteries used as power sources may be the primary power source or the auxiliary power source in electronic devices and electric vehicles, etc. A primary power source is a power source that is used preferentially regardless of the presence or absence of other power sources. An auxiliary power source may be a power source used in place of the primary power source, or a power source that can be switched from the primary power source.

[0204] Specific examples of secondary battery applications are described below: Electronic devices such as video cameras, digital still cameras, mobile phones, notebook computers, headphone stereos, portable radios, and portable information terminals; backup power supplies and storage devices such as memory cards; power tools such as electric drills and electric saws; battery packs installed in electronic devices; medical electronic devices such as pacemakers and hearing aids; electric vehicles (including hybrid vehicles); and power storage systems such as household or industrial battery systems that store power in preparation for emergencies. In these applications, one secondary battery may be used, or multiple secondary batteries may be used.

[0205] A battery pack may consist of individual cells or a battery pack. An electric vehicle is a vehicle that runs using a secondary battery as a power source, and may also be a hybrid vehicle that has a power source other than the secondary battery. In a household power storage system, household electrical appliances can be used by utilizing the electricity stored in the secondary battery, which is the power storage source.

[0206] Here, we will specifically explain one example of a secondary battery application. The configuration described below is merely an example and can be modified as needed.

[0207] Figure 5 shows the block configuration of a battery pack, which is an example of a secondary battery application. The battery pack described here is a single secondary battery pack (a so-called soft pack) and is installed in electronic devices such as smartphones.

[0208] As shown in Figure 5, this battery pack comprises a power supply 51 and a circuit board 52. The circuit board 52 is connected to the power supply 51 and includes a positive terminal 53, a negative terminal 54, and a temperature detection terminal 55.

[0209] The power supply 51 includes one rechargeable battery. In this rechargeable battery, the positive lead is connected to the positive terminal 53, and the negative lead is connected to the negative terminal 54. Since the power supply 51 is connected to the outside via the positive terminal 53 and the negative terminal 54, it is rechargeable and dischargeable. The circuit board 52 includes a control unit 56, a switch 57, a PTC element 58 which is a thermal resistance element, and a temperature detection unit 59. However, the PTC element 58 may be omitted.

[0210] The control unit 56 includes a central processing unit (CPU) and memory, and controls the operation of the entire battery pack. This control unit 56 also detects and controls the usage status of the power supply 51.

[0211] Furthermore, when the voltage of the power supply 51 (secondary battery) reaches the overcharge detection voltage or over-discharge detection voltage, the control unit 56 disconnects the switch 57 to prevent charging current from flowing through the current path of the power supply 51. The overcharge detection voltage is not particularly limited, but specifically it is 4.20V ± 0.05V, and the over-discharge detection voltage is not particularly limited, but specifically it is 2.40V ± 0.10V.

[0212] Switch 57 includes a charge control switch, a discharge control switch, a charging diode, and a discharge diode, and switches the connection between the power supply 51 and external equipment according to the instructions of the control unit 56. This switch 57 includes a field-effect transistor (MOSFET) using a metal oxide semiconductor, and the charging current and discharge current are detected based on the ON resistance of the switch 57.

[0213] The temperature detection unit 59 includes a temperature detection element such as a thermistor. This temperature detection unit 59 measures the temperature of the power supply 51 using the temperature detection terminal 55 and outputs the temperature measurement result to the control unit 56. The temperature measurement result measured by the temperature detection unit 59 is used when the control unit 56 performs charge / discharge control in the event of abnormal heat generation and when the control unit 56 performs correction processing when calculating the remaining capacity. [Examples]

[0214] An example of this technology will be described below.

[0215] <Examples 1-4 and Comparative Examples 1-3> As described below, after manufacturing the secondary batteries, their battery characteristics were evaluated. Here, as will be explained later, test secondary batteries were used to evaluate the battery characteristics.

[0216] [Manufacturing of secondary batteries] Figure 6 shows the cross-sectional configuration of a test secondary battery, which is a so-called coin-type secondary battery (lithium-ion secondary battery).

[0217] This secondary battery comprises a test electrode 61, a counter electrode 62, a separator 63, an outer cup 64, an outer can 65, a gasket 66, and an electrolyte (not shown).

[0218] The test electrode 61 is housed in an outer cup 64, and the counter electrode 62 is housed in an outer can 65. The test electrode 61 and the counter electrode 62 are stacked on top of each other via a separator 63, and the electrolyte is impregnated into the test electrode 61, the counter electrode 62, and the separator 63. The outer cup 64 and the outer can 65 are crimped together via a gasket 66, so the test electrode 61, the counter electrode 62, and the separator 63 are sealed by the outer cup 64 and the outer can 65.

[0219] The secondary battery shown in Figure 6 was fabricated using the procedure described below.

[0220] (Preparation of test electrode) First, several cores containing lithium composite oxide (content C ≥ 80 mol%) were prepared. The composition of the lithium composite oxide, its content C (mol%), and the content of residual alkali components (weight%) in the lithium composite oxide are shown in Table 1.

[0221] Next, multiple cores were heated in a pressurized environment (pressure = 1 MPa) (heating temperature = 650°C and heating time = 10 hours).

[0222] Next, 95.5 parts by mass of heated positive electrode active material particles, 2.5 parts by mass of positive electrode binder (polyvinylidene fluoride), 1.9 parts by mass of positive electrode conductive agent (carbon black), and 0.1 parts by mass of dispersant (polyvinylpyrrolidone) were mixed together to prepare a positive electrode mixture. Subsequently, the positive electrode mixture was added to a solvent (the organic solvent N-methyl-2-pyrrolidone), and the solvent was stirred to prepare a paste-like positive electrode mixture slurry.

[0223] Next, a positive electrode mixture slurry was applied to one side of a positive electrode current collector (aluminum foil with a thickness of 15 μm), and then the slurry was heated and dried (heating temperature = 100°C and heating time = 15 minutes) to form a precursor layer (thickness = 40 μm). Subsequently, the positive electrode current collector with the precursor layer formed on it was punched out into a disc shape (diameter = 16.5 mm).

[0224] Finally, as will be described later, after assembling the secondary battery, a stabilization treatment was performed using the assembled secondary battery. As a result, a coating containing lithium borate fluoride (LiBOF2) was formed on the central surface, thus forming positive electrode active material particles.

[0225] Therefore, a positive electrode active material layer containing multiple positive electrode active material particles was formed, and the test electrode 61 was fabricated.

[0226] In the process of preparing test electrode 61, when using a coating source material, after heating multiple centers, powdered boron-containing material (lithium metaborate) was added to the centers before obtaining the positive electrode mixture using these centers. After obtaining the mixture, it was stirred. As a result, the powdered boron-containing material adhered to the surface of the centers. The content of the coating source material (boron-containing material) in the mixture is shown in Table 1. Subsequently, a positive electrode mixture slurry was prepared using the multiple centers to which the boron-containing material had adhered, and then a precursor layer was formed using this positive electrode mixture slurry.

[0227] (Preparation of the opposing pole) A lithium metal plate was punched out into a disc shape (diameter = 17 mm). This yielded the counter electrode 62.

[0228] (Preparation of electrolyte solution) An electrolyte salt (lithium hexafluoride phosphate (LiPF6)), which also serves as a coating source material (fluorine-containing material), was added to a solvent (ethylene carbonate, a cyclic carbonate ester, and dimethyl carbonate, a chain carbonate ester), and the solvent was then stirred. The solvent mixing ratio (by weight) was ethylene carbonate:dimethyl carbonate = 30:70. The electrolyte salt content was 1 mol / l (= 1 mol / dm³) relative to the solvent. 3 This was done. The electrolyte solution was then prepared.

[0229] When a coating source material was used in the electrolyte preparation process, a boron-containing material (lithium tetrafluoroborate (LiBF4)) was added to the electrolyte, and then the electrolyte was stirred. The content (by weight %) of the coating source material (boron-containing material) in the electrolyte is shown in Table 1.

[0230] (Assembly of secondary batteries) First, the test electrode 61, including the precursor layer, was placed in the outer cup 64, and the counter electrode 62 was placed in the outer can 65. Next, the test electrode 61 in the outer cup 64 and the counter electrode 62 in the outer can 65 were stacked on top of each other via a separator 63 (a disc-shaped polyethylene film with a diameter of 17.5 μm) impregnated with electrolyte. In this case, the test electrode 61 was positioned so that the precursor layer and the counter electrode 62 faced each other via the separator 63. Then, with the test electrode 61 and the counter electrode 62 stacked on top of each other via the separator 63, the outer cup 64 and the outer can 65 were crimped together via a gasket 66. As a result, the test electrode 61 and the counter electrode 62 were sealed by the outer cup 64 and the outer can 65, and the secondary battery was assembled.

[0231] (Stabilization treatment of secondary batteries after assembly) The assembled rechargeable battery was subjected to one charge-discharge cycle.

[0232] During charging, first, constant current charging was performed at a current of 0.1C in a room temperature environment (temperature = 25°C) until the voltage reached 4.0V. Next, constant current charging was performed at a current of 0.1C in a high temperature environment (temperature = 70°C) until the voltage reached 4.3V. Finally, constant voltage charging was performed at a voltage of 4.3V with a current of 0.01C in a high temperature environment.

[0233] During discharge, a constant current discharge was performed at a current of 0.1C in a room temperature environment until the voltage reached 2.5V.

[0234] Note that 0.1C is the current value that completely discharges the battery's capacity (theoretical capacity) in 10 hours, while 0.01C is the current value that completely discharges the battery's capacity in 100 hours.

[0235] As a result, coatings were formed on the surfaces of multiple central regions, thus creating multiple positive electrode active material particles. Therefore, a positive electrode active material layer containing multiple positive electrode active material particles was formed, and the test electrode 61 was fabricated, thus completing the secondary battery.

[0236] For comparison, a secondary battery was fabricated using the same procedure, except that a coating source material was not used (excluding the electrolyte salt which also serves as the coating source material). In this case, the coating portion 210Y was not formed.

[0237] Here, after the completion of the secondary battery, the physical properties (whether or not the three types of physical property conditions were met) were investigated based on the analysis results of depth analysis of test electrode 61 using TOF-SIMS, and the results are shown in Table 1. Table 1 shows the types of coating source materials other than the electrolyte salt which also serves as the coating source material, the location in which the coating source material is contained, and the content (weight %) of the coating source material.

[0238] [Evaluation of battery characteristics] The electrical resistance characteristics of the battery were evaluated using the procedure described below, and the results shown in Table 1 were obtained.

[0239] To evaluate the electrical resistance characteristics, the secondary battery was first charged at room temperature (temperature = 25°C) with a current of 0.1C until the voltage reached 4.25V. Next, an EIS (Electrochemical Impedance Spectroscopy) measurement was performed on the charged secondary battery using an impedance analyzer (Solartron Analytical AC Impedance analyzer 1255WB) to obtain a Nyquist plot. In this case, the applied voltage was 10mV and the frequency range was 100kHz to 0.1Hz. Finally, based on the Nyquist plot (low-frequency arc), the charge transfer resistance of the test electrode 61 (charge transfer resistance before cycling (Ω)) was measured.

[0240] Next, in the same environment, the secondary battery was repeatedly charged and discharged until the total number of cycles reached 100. During charging, constant current charging was performed at a current of 1C until the voltage reached 4.25V, and then constant voltage charging was performed at that voltage of 4.25V until the current reached 0.01C. During discharging, constant current discharge was performed at a current of 5C until the voltage reached 2.5V. 1C is the current value that completely discharges the battery capacity in 1 hour, and 5C is the current value that completely discharges the battery capacity in 0.2 hours.

[0241] Next, the secondary battery was charged in the same environment with a current of 0.1C until the voltage reached 4.25V. Then, the charge transfer resistance of the test electrode 61 (charge transfer resistance after cycle (Ω)) was calculated using the procedure described above.

[0242] Finally, the increase rate, an index for evaluating electrical resistance characteristics, was calculated based on the formula: Increase Rate = [(Charge Transfer Resistance after Cycle - Charge Transfer Resistance before Cycle) / Charge Transfer Resistance before Cycle] × 100. This increase rate is a characteristic value that represents how many times the charge transfer resistance after the cycle has increased compared to the charge transfer resistance before the cycle.

[0243] [Table 1]

[0244] [Consideration] As shown in Table 1, when lithium composite oxide (content C ≥ 80 mol%) was used as the core forming material, the rate of increase varied greatly depending on the physical properties of the positive electrode active material layer.

[0245] Specifically, even when a coating was formed using the coating source material, if the amount of coating formed was insufficient, the first and second physical property conditions were met, but the third physical property condition was not met (Comparative Examples 1 and 2), the rate of increase increased significantly.

[0246] Furthermore, in the case where the coating was not formed due to the absence of a coating source material, the first and second physical property conditions were met, but the third physical property condition was not met (Comparative Example 3), the rate of increase also increased significantly.

[0247] In contrast, when a coating was formed using the coating source material, and the amount of the coating was sufficient, the first and second physical property conditions were met, as well as the third physical property condition (Examples 1-4), the rate of increase decreased significantly.

[0248] In this case, the following trends were observed in particular. Firstly, the rate of increase decreased sufficiently even when the content ratio C was changed. Secondly, the rate of increase decreased sufficiently even when the composition of the lithium composite oxide (types of other elements) was changed. Thirdly, the rate of increase decreased significantly when the coating contained lithium borate fluoride.

[0249] [summary] As shown in Table 1, the increase rate decreased when the positive electrode active material layer contained multiple positive electrode active material particles, these particles included a central part and a coating part, the central part contained lithium composite oxide (content ratio C ≥ 80 mol%), and three types of physical property conditions were met in the analysis results of the depth direction D of the positive electrode active material layer 21B using TOF-SIMS. Therefore, the electrical resistance characteristics were improved, resulting in excellent battery characteristics in the secondary battery.

[0250] Although the present technology has been described above with reference to one embodiment and one example, the configuration of the present technology is not limited to the configuration described in the one embodiment and one example, and can be modified in various ways.

[0251] Specifically, the explanation focused on cases where the battery structure of the rechargeable battery is laminate film type and coin type. However, the battery structure of the rechargeable battery is not particularly limited, and cylindrical, rectangular, and button-type batteries are also acceptable.

[0252] Furthermore, the case where the element structure of the battery element is of the wound type has been described. However, the element structure of the battery element is not particularly limited, and may also be of the stacked type or the zigzag type. In the stacked type, the positive electrode and negative electrode are stacked alternately with a separator in between, while in the zigzag type, the positive electrode and negative electrode are folded in a zigzag pattern with a separator in between, facing each other.

[0253] Furthermore, while the case where the electrode reactant is lithium has been described, the electrode reactant is not particularly limited. Specifically, as mentioned above, the electrode reactant may be other alkali metals such as sodium and potassium, or alkaline earth metals such as beryllium, magnesium, and calcium. In addition, the electrode reactant may be other light metals such as aluminum.

[0254] The effects described herein are illustrative only, and therefore the effects of this technology are not limited to those described herein. Accordingly, other effects may be obtained with respect to this technology.

[0255] Furthermore, this technology can also be configured as follows: <1> A positive electrode containing a positive electrode active material layer, The negative electrode and, Electrolyte and Equipped with, The positive electrode active material layer includes a plurality of positive electrode active material particles, The positive electrode active material particles are, A core containing lithium composite oxide, The covering portion provided on the surface of the central part Includes, The aforementioned lithium composite oxide has a layered rock salt-type crystalline structure and contains lithium, nickel, and other elements as constituent elements. If the sum of the nickel content and the other element content in the lithium composite oxide is 100 mole parts, then the nickel content is 80 mole parts or more and 100 mole parts or less. By analyzing the depth direction of the positive electrode active material layer using time-of-flight secondary ion mass spectrometry, NiO2 - the first negative secondary ions derived from LiBO2F - the second negative secondary ions derived from are detected, and a first depth profile representing the change in the ion intensity of the first negative secondary ions in the depth direction, a second depth profile representing the change in the ion intensity of the second negative secondary ions in the depth direction are obtained, in the first depth profile, the ion intensity of the first negative secondary ions increases in the depth direction, in the second depth profile, the ion intensity of the second negative secondary ions decreases in the depth direction, the second depth profile has a step region where the ion intensity of the second negative secondary ions does not temporarily decrease in the depth direction while the ion intensity of the first negative secondary ions increases in the depth direction, secondary battery. <2> The coating portion contains lithium, boron, and fluorine as constituent elements, The secondary battery according to <1>. <3> The coating portion contains lithium borate fluoride (LiBOF2), The secondary battery according to <2>. <4> The other element contains at least one of cobalt, aluminum, manganese, zirconium, titanium, molybdenum, tantalum, chromium, niobium, iron, copper, zinc, vanadium, magnesium, tungsten, sulfur, strontium, boron, sodium, and fluorine, The secondary battery according to any one of <1> to <3>. <5> It is a lithium ion secondary battery, The secondary battery according to any one of <1> to <4>.

Explanation of Reference Numerals

[0256] 21...Positive electrode, 21B...Positive electrode active material layer, 22...Negative electrode, 210...Positive electrode active material particles, 210X...Center, 210Y...Coating

Claims

1. A positive electrode containing a positive electrode active material layer, The negative electrode and, Electrolyte and Equipped with, The positive electrode active material layer includes a plurality of positive electrode active material particles, The positive electrode active material particles are, A core containing lithium composite oxide, The covering portion provided on the surface of the central part Includes, The aforementioned lithium composite oxide has a layered rock salt-type crystalline structure and contains lithium, nickel, and other elements as constituent elements. If the sum of the nickel content and the other element content in the lithium composite oxide is 100 mole parts, then the nickel content is 80 mole parts or more and 100 mole parts or less. Analysis of the positive electrode active material layer in the depth direction using time-of-flight secondary ion mass spectrometry revealed that NiO 2 - The first negative secondary ion derived from, LiBO 2 F - The second negative secondary ion derived from and As soon as it was detected, A first depth profile representing the change in ionic strength of the first negative secondary ion in the depth direction, A second depth profile representing the change in ionic strength of the second negative secondary ion in the depth direction, It was obtained, In the first depth profile, the ionic strength of the first negative secondary ion increases in the depth direction. In the second depth profile, the ionic strength of the second negative secondary ion decreases in the depth direction. The second depth profile has a stepped region in which, while the ionic strength of the first negative secondary ion is increasing in the depth direction, the ionic strength of the second negative secondary ion temporarily stops decreasing in the depth direction. Secondary battery.

2. The coating portion contains lithium, boron, and fluorine as constituent elements. The secondary battery according to claim 1.

3. The coating portion is made of lithium borate (LiBOF 2 ) including, The secondary battery according to claim 2.

4. The aforementioned other elements include at least one of the following: cobalt, aluminum, manganese, zirconium, titanium, molybdenum, tantalum, chromium, niobium, iron, copper, zinc, vanadium, magnesium, tungsten, sulfur, strontium, boron, sodium, and fluorine. A secondary battery according to any one of claims 1 to 3.

5. Lithium-ion rechargeable batteries, A secondary battery according to any one of claims 1 to 3.

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