Secondary battery

Abstract

A secondary battery includes: a positive electrode containing a lithium-nickel composite oxide; a negative electrode containing a lithium-titanium composite oxide; and an electrolyte containing a carboxylic acid ester containing at least one of ethyl acetate, propyl acetate, ethyl propionate, and propyl propionate. In surface analysis of the positive electrode using X-ray photoelectron spectroscopy, a first oxygen spectrum having a peak in a range of a binding energy of 528 eV or more and 531 eV or less, and a second oxygen spectrum having a peak in a range of a binding energy of more than 531 eV and 535 eV or less are detected, and a ratio of an intensity of the first oxygen spectrum to an intensity of the second oxygen spectrum is 0.30 or more and 0.80 or less. In surface analysis of the negative electrode using X-ray photoelectron spectroscopy, a third oxygen spectrum having a peak in a range of a binding energy of 528 eV or more and 531 eV or less, and a fourth oxygen spectrum having a peak in a range of a binding energy of more than 531 eV and 535 eV or less are detected, and a ratio of an intensity of the third oxygen spectrum to an intensity of the fourth oxygen spectrum is 0.82 or more and 1.35 or less.

Description

Technical Field

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

[0002] With the widespread adoption of mobile phones and other electronic devices, the development of secondary batteries—small, lightweight power sources capable of delivering high energy density—is underway. These secondary batteries possess a positive electrode, a negative electrode, and an electrolyte, and various studies have been conducted regarding their structure.

[0003] Specifically, to obtain excellent cycle characteristics, when using Ni-based compounds as the positive electrode active material, the surface analysis results (intensity ratio of the 1s spectrum of oxygen) of the Ni-based compounds obtained by X-ray photoelectron spectroscopy are optimized (for example, see Patent Document 1). To prevent internal short circuits, when using a composite oxide containing lithium and transition metal M as the positive electrode active material and forming a thin film on the surface of the positive electrode, the surface analysis results (intensity ratio of the 2p spectrum of transition metal M) of the positive electrode obtained by X-ray photoelectron spectroscopy are optimized (for example, see Patent Document 2). To suppress expansion, when using titanium oxide as the negative electrode active material and forming a coating on the surface of the negative electrode, the analysis results (ratio of oxygen atoms) of the coating obtained by X-ray photoelectron spectroscopy are optimized (for example, see Patent Document 3).

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2005-251700

[0007] Patent Document 2: Japanese Patent Application Publication No. 2003-338277

[0008] Patent Document 3: Japanese Patent Application Publication No. 2017-168265 Summary of the Invention

[0009] Although various studies have been conducted on the performance of secondary batteries, the expansion characteristics, cycle characteristics, and load characteristics are still insufficient, and therefore there is room for improvement.

[0010] Therefore, there is a need for secondary batteries that can achieve excellent expansion characteristics, excellent cycle characteristics, and excellent load characteristics.

[0011] One embodiment of the secondary battery of this technology comprises: a positive electrode containing a lithium-nickel composite oxide; a negative electrode containing a lithium-titanium composite oxide; and an electrolyte containing a carboxylic acid ester, wherein the carboxylic acid ester contains at least one selected from ethyl acetate, propyl acetate, ethyl propionate, and propyl propionate. In surface analysis of the positive electrode using X-ray photoelectron spectroscopy, a first oxygen spectrum with peaks in the range of binding energies of 528 eV and 531 eV and a second oxygen spectrum with peaks in the range of binding energies greater than 531 eV and 535 eV and below are detected, wherein the intensity ratio of the first oxygen spectrum to the intensity of the second oxygen spectrum is 0.30 and 0.80 or less. In the surface analysis of the negative electrode using X-ray photoelectron spectroscopy, a third oxygen spectrum with a peak in the range of binding energy above 528 eV and below 531 eV, and a fourth oxygen spectrum with a peak in the range of binding energy above 531 eV and below 535 eV were detected. The ratio of the intensity of the third oxygen spectrum to the intensity of the fourth oxygen spectrum was above 0.82 and below 1.35.

[0012] Here, "lithium-nickel composite oxides" refers to the general term for oxides containing both lithium and nickel as constituent elements, while "lithium-titanium composite oxides" refers to the general term for oxides containing both lithium and titanium as constituent elements. It should be noted that detailed information about lithium-nickel composite oxides and lithium-titanium composite oxides will be described later.

[0013] According to one embodiment of the present technology, the secondary battery contains a lithium-nickel composite oxide as the positive electrode, a lithium-titanium composite oxide as the negative electrode, and a carboxylic acid ester in the electrolyte, wherein the carboxylic acid ester contains ethyl acetate, etc. Furthermore, the surface analysis results of the positive electrode using X-ray photoelectron spectroscopy (the ratio of the intensity of the first oxygen spectrum to the intensity of the second oxygen spectrum) satisfy the above-mentioned conditions, and the surface analysis results of the negative electrode using X-ray photoelectron spectroscopy (the ratio of the intensity of the third oxygen spectrum to the intensity of the fourth oxygen spectrum) also satisfy the above-mentioned conditions. Therefore, excellent expansion characteristics, excellent cycle characteristics, and excellent load characteristics can be obtained.

[0014] It should be noted that the effect of this technology is not limited to the effect described herein, but can be any of the series of effects associated with this technology described later. Attached Figure Description

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

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

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

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

[0019] 1. Secondary battery

[0020] 1-1. Structure

[0021] 1-2.Physical properties

[0022] 1-3. Actions

[0023] 1-4. Manufacturing Method

[0024] 1-5. Functions and Effects

[0025] 2. Variations

[0026] 3. Uses of secondary batteries

[0027] <1. Secondary Battery>

[0028] First, a secondary battery according to one embodiment of this technology will be described.

[0029] The secondary battery described herein is a secondary battery in which battery capacity is obtained by the intercalation and deintercalation of electrode reactants, and it includes a positive electrode, a negative electrode, and an electrolyte in liquid form. In this secondary battery, in order to suppress the deposition of electrode reactants on the surface of the negative electrode during charging, the charging capacity of the negative electrode is greater than the discharging capacity of the positive electrode. That is, the electrochemical capacity per unit area of ​​the negative electrode is set to be greater than the electrochemical capacity per unit area of ​​the positive electrode.

[0030] There are no particular restrictions on the types of substances used in the electrode reactions. Specifically, they are light metals such as alkali metals and alkaline earth metals. Alkali metals include lithium, sodium, and potassium, while alkaline earth metals include beryllium, magnesium, and calcium.

[0031] The following example uses lithium as the electrode reactant. A secondary battery that utilizes the insertion and extraction of lithium to obtain battery capacity is called a lithium-ion secondary battery. In this lithium-ion secondary battery, lithium is inserted and extracted in an ionic state.

[0032] <1-1. Structure>

[0033] Figure 1 The three-dimensional structure of the secondary battery is shown, and at the same time Figure 2 It shows Figure 1 The cross-sectional structure of the battery element 20 is shown. Additionally, Figure 1 The outer packaging film 10 and the battery element 200 are shown separated from each other. Figure 2The image shows only a portion of the battery element 20, while a cross-section of the battery element 20 along the XZ plane is shown in dashed lines.

[0034] like Figure 1 as well as Figure 2 As shown, the secondary battery includes an outer packaging film 10, a battery element 20, a positive electrode lead 31 and a negative electrode lead 32, and sealing films 41 and 42. The secondary battery described here is a laminated film type secondary battery that uses a flexible (or supple) outer packaging film 10 as an outer packaging component for housing the battery element 20.

[0035] [Outer packaging film and sealing film]

[0036] like Figure 1 As shown, the outer packaging film 10 is a flexible outer packaging component that houses the battery element 20, and has a bag-like structure that is sealed when the battery element 20 is housed inside. Therefore, the outer packaging film 10 houses the positive electrode 21 and the negative electrode 22, as well as the electrolyte, which will be described later.

[0037] Here, the outer packaging film 10 is a film-shaped component that can be folded in the direction R. A recess 10U (so-called deep stretch portion) for accommodating the battery element 20 is provided on the outer packaging film 10.

[0038] Specifically, the outer packaging film 10 is a laminated film consisting of three layers stacked sequentially from the inside: a welding layer, a metal layer, and a surface protective layer. When the outer packaging film 10 is folded, the outer peripheries of the opposing welding layers are welded together. The welding layer contains a polymer compound such as polypropylene. The metal layer contains a metal material such as aluminum. The surface protective layer contains a polymer compound such as nylon.

[0039] In addition, there is no particular limitation on the structure (number of layers) of the outer packaging film 10; it can be 1 layer, 2 layers, or more than 4 layers.

[0040] Sealing film 41 is inserted between outer packaging film 10 and positive lead 31, and sealing film 42 is inserted between outer packaging film 10 and negative lead 32. Alternatively, one or both of sealing films 41 and 42 may be omitted.

[0041] The sealing film 41 is a sealing component that prevents external gases from entering the interior of the outer packaging film 10. Furthermore, the sealing film 41 contains a polymer compound such as a polyolefin, which has a sealing effect on the positive electrode lead 31. This polyolefin is polypropylene, etc.

[0042] Aside from being a sealing component that provides a tight seal for the negative electrode lead 32, the structure of the sealing membrane 42 is the same as that of the sealing membrane 41. That is, the sealing membrane 42 contains a polymer compound such as polyolefin that provides a tight seal for the negative electrode lead 32.

[0043] [Battery Components]

[0044] like Figure 1 as well as Figure 2 As shown, the battery element 20 is a power generation element containing a positive electrode 21, a negative electrode 22, a separator 23 and an electrolyte (not shown), and is housed inside the outer packaging film 10.

[0045] The battery element 20 is a so-called wound electrode body. That is, in the battery element 20, the positive electrode 21 and the negative electrode 22 are stacked on top of each other with a separator 23 in between, and the positive electrode 21, the negative electrode 22 and the separator 23 are wound around a winding axis P. The winding axis P is an imaginary axis extending in the Y-axis direction. Thus, the positive electrode 21 and the negative electrode 22 are wound opposite each other with the separator 23 in between.

[0046] The three-dimensional shape of the battery element 20 is not particularly limited. Here, since the battery element 20 is flat, the shape of the cross-section (along the XZ plane) of the battery element 20 intersecting the winding axis P is a flat shape defined by the major axis J1 and the minor axis J2. The major axis J1 is an imaginary axis extending in the X-axis direction and having a length greater than the minor axis J2, while the minor axis J2 is an imaginary axis extending in the Z-axis direction intersecting the X-axis direction and having a length less than the major axis J1. Here, the three-dimensional shape of the battery element 20 is a flattened cylindrical shape, therefore the cross-sectional shape of the battery element 20 is a flattened, approximately elliptical shape.

[0047] (positive electrode)

[0048] The positive electrode 21 contains a lithium-nickel composite oxide. More specifically, such as... Figure 2 As shown, the positive electrode 21 includes a positive current collector 21A, a positive active material layer 21B, and a coating 21C. The positive active material layer 21B contains lithium nickel composite oxide.

[0049] The positive current collector 21A has one side with a positive active material layer 21B disposed thereon. The positive current collector 21A contains a conductive material such as a metal, which is aluminum, etc.

[0050] Since the positive electrode active material layer 21B is disposed on both sides of the positive electrode current collector 21A, the positive electrode 21 includes two positive electrode active material layers 21B. Each positive electrode active material layer 21B contains any one or more positive electrode active materials capable of lithium insertion / extraction. Alternatively, the positive electrode active material layer 21B may be disposed only on one side of the positive electrode current collector 21A opposite the negative electrode 22. Furthermore, the positive electrode active material layer 21B may also contain a positive electrode binder and a positive electrode conductive agent, etc.

[0051] The positive electrode active material contains lithium-containing compounds, more specifically, it contains any one or more of the aforementioned lithium-nickel composite oxides. "Lithium-nickel composite oxides" refers to a general term for oxides containing both lithium and nickel as constituent elements, exhibiting a layered rock salt-type crystal structure. The reason for containing lithium-nickel composite oxides in the positive electrode active material is to achieve high energy density, thereby increasing battery capacity.

[0052] The type (structure) of lithium-nickel composite oxides is not particularly limited as long as it contains lithium and nickel as constituent elements. Specifically, lithium-nickel composite oxides contain lithium, nickel, and other elements as constituent elements. The other elements are any one or more elements belonging to Groups 2 to 15 of the long-period periodic table (excluding nickel). Alternatively, lithium-nickel composite oxides may not contain any other elements as constituent elements.

[0053] More specifically, the lithium-nickel composite oxide contains one or more of the compounds represented by formula (1) below. Furthermore, the composition of lithium varies depending on the charge / discharge state, and the x value in formula (1) is the value for the fully discharged state.

[0054] Li x Ni (1-y) M1 y O2…(1)

[0055] (M1 is at least one element belonging to groups 2 to 15 of the long-period periodic table (excluding Ni). x and y satisfy 0.8 ≤ x ≤ 1.2 and 0 ≤ y < 1.0.)

[0056] As can be seen from equation (1), the Ni content in the lithium-nickel composite oxide is determined by the content of other elements (M1). Furthermore, from the range of possible values ​​for y, it can be seen that the lithium-nickel composite oxide may or may not contain M1 as a constituent element. If the lithium-nickel composite oxide contains Ni as a constituent element, then the Ni content in the lithium-nickel composite oxide is not particularly limited and can therefore be arbitrarily set.

[0057] In this preferred embodiment, the Ni content in the lithium-nickel composite oxide is sufficiently high. More specifically, the ratio of the number of moles of Ni to the sum of the number of moles of Ni and the number of moles of M1 (Ni ratio) is preferably 80% or higher. This is because a high battery capacity (capacity per positive electrode active material) can be obtained due to the increased capacity density. This Ni ratio is calculated using the formula: Ni ratio (%) = [number of moles of Ni / (number of moles of Ni + number of moles of M1)] × 100.

[0058] That is, the lithium-nickel composite oxide preferably contains any one or more of the compounds represented by the following formula (5). This is because a higher energy density can be obtained.

[0059] Li x Ni (1-y) M5 y O2…(5)

[0060] (M5 is at least one element belonging to groups 2 to 15 of the long-period periodic table (excluding Ni). x and y satisfy 0.8 ≤ x ≤ 1.2 and 0 ≤ y ≤ 0.2.)

[0061] Specific examples of lithium-nickel composite oxides are LiNiO2 and LiNi. 0.70 Co 0.30 O2, LiNi 0.80 Co 0.15 Al 0.05 O2, LiNi 0.82 Co 0.14 Al 0.04 O2, LiNi 0.50 Co 0.20 Mn 0.30 O2, LiNi 0.80 Co 0.10 Al 0.05 Mn 0.05 O2, LiNi 0.80 Co 0.20 O2, LiNi 0.82 Co 0.18 O2, LiNi 0.85 Co 0.15 O2, LiNi 0.90 Co 0.10 O2, LiNi 0.33 Co 0.33 Mn 0.33 O2, Li 1.2 Mn 0.52 Co 0.175 Ni 0.1 O2 and Li 1.15 (Mn 0.65 Ni 0.22 Co 0.13 O2, etc. Among them, LiNi with a Ni ratio of 80% or higher is preferred. 0.80 Co 0.15 Al 0.05 O2, LiNi 0.80 Co 0.10 Al 0.05 Mn 0.05 O2, LiNi 0.80 Co0.20 O2, LiNi 0.82 Co 0.18 O2, LiNi 0.85 Co 0.15 O2 and LiNi 0.90 Co 0.10 O2, etc.

[0062] It should be noted that the positive electrode active material only needs to contain the above-mentioned lithium-nickel composite oxide, and may also contain any one or more of other positive electrode active materials (other lithium-containing compounds).

[0063] Other positive electrode active materials are not specifically limited in type; specifically, they include lithium transition metal compounds. "Lithium transition metal compounds" refers to a general term for compounds containing lithium and one or more transition metal elements as constituent elements, and may also contain one or more other elements. The types of other elements are not specifically limited; they can be any elements other than transition metal elements. Specifically, they can be any one or more elements belonging to Groups 2 to 15 of the long-period periodic table. Furthermore, the aforementioned lithium-nickel composite oxides are not included in the lithium transition metal compounds described herein.

[0064] The types of lithium transition metal compounds are not particularly limited, but specifically include oxides, phosphoric acid compounds, silicate compounds, and borate compounds. Specific examples of oxides are LiCoO2 and LiMn2O4. Specific examples of phosphoric acid compounds are LiFePO4, LiMnPO4, and LiFe... 0.5 Mn 0.5 PO4 and LiFe 0.3 Mn 0.7 PO4, etc.

[0065] The positive electrode binder contains one or more of the following: synthetic rubber and polymeric compounds. Synthetic rubbers include styrene-butadiene rubber, fluorinated rubber, and ethylene propylene diene monomer (EPDM) rubber. Polymeric compounds include polyvinylidene fluoride (PVDF), polyimide, and carboxymethyl cellulose.

[0066] The positive electrode conductive agent contains one or more of the following conductive materials: carbon materials, such as graphite, carbon black, acetylene black, and Ketjen black. Alternatively, the conductive material can also be a metallic material or a polymer compound.

[0067] There is no particular limitation on the method for forming the positive electrode active material layer 21B. Specifically, it can be any one or more of the following methods: coating method, etc.

[0068] The coating 21C is a positive electrode coating disposed on the surface of the positive electrode active material layer 21B, covering the surface of the positive electrode active material layer 21B.

[0069] Here, since the coating 21C is provided in such a way that it covers the surface of each of the two positive electrode active material layers 21B, the positive electrode 21 includes two coatings 21C. Alternatively, the coating 21C may be provided in such a way that it covers only one surface of the two positive electrode active material layers 21B, so the positive electrode 21 includes one coating 21C.

[0070] The coating 21C can cover the entire surface of the positive electrode active material layer 21B, or it can cover only a portion of the surface of the positive electrode active material layer 21B. In the latter case, multiple coatings 21C can cover the surface of the positive electrode active material layer 21B at multiple mutually isolated locations. Figure 2 The diagram shows the situation where the coating 21C covers the entire surface of the positive electrode active material layer 21B.

[0071] Here, as described later, in the manufacturing process of the secondary battery, a coating 21C is formed on the surface of the positive electrode active material layer 21B by stabilizing the assembled secondary battery (initial charge-discharge treatment). This coating 21C mainly contains one or more of the reactants and decomposition products of carboxylic acid esters contained in the electrolyte, as described later, and is therefore a coating derived from these carboxylic acid esters. Thus, the coating 21C contains oxygen as a constituent element.

[0072] In this secondary battery, the physical properties of the positive electrode 21, i.e., the properties of the coating 21C, meet specified conditions in order to improve the expansion characteristics, cycle characteristics, and load characteristics, respectively. Details regarding the physical properties of the positive electrode 21 (coating 21C) will be described later.

[0073] (negative electrode)

[0074] The negative electrode 22 contains lithium-titanium composite oxide. More specifically, such as... Figure 2 As shown, the negative electrode 22 includes a negative electrode current collector 22A, a negative electrode active material layer 22B, and a coating 22C. The negative electrode active material layer 22B contains lithium titanium composite oxide.

[0075] The negative current collector 22A has one side with a negative active material layer 22B disposed thereon. The negative current collector 22A contains a conductive material such as a metal, which is copper, etc.

[0076] Since the negative electrode active material layer 22B is disposed on both sides of the negative electrode current collector 22A, the negative electrode 22 includes two negative electrode active material layers 22B. Each negative electrode active material layer 22B contains any one or more negative electrode active materials capable of lithium insertion / extraction. Alternatively, the negative electrode active material layer 22B may be disposed only on one side of the negative electrode current collector 22A on the side of the negative electrode 22 opposite to the positive electrode 21. Furthermore, the negative electrode active material layer 22B may also contain a negative electrode binder and a negative electrode conductive agent. The details regarding the negative electrode binder and the negative electrode conductive agent are the same as the details regarding the positive electrode binder and the positive electrode conductive agent.

[0077] The negative electrode active material contains any one or more of the aforementioned lithium-titanium composite oxides. "Lithium-titanium composite oxide" is a general term for oxides containing lithium and titanium as constituent elements, and has a spinel-type crystal structure. The reason for containing lithium-titanium composite oxides in the negative electrode active material is that the discharge capacity is not easily reduced even after repeated charge-discharge cycles, thus improving cycle characteristics.

[0078] The type (structure) of lithium-titanium composite oxides is any oxide containing lithium and titanium as constituent elements; there are no particular restrictions. Specifically, lithium-titanium composite oxides contain lithium, titanium, and other elements as constituent elements. These other elements are any one or more elements belonging to Groups 2 to 15 of the long-period periodic table (excluding titanium). Furthermore, oxides containing nickel as a constituent element along with lithium and titanium are not lithium-nickel composite oxides, but rather lithium-titanium composite oxides.

[0079] More specifically, the lithium-titanium composite oxide contains any one or more of the compounds represented by formulas (2), (3), and (4) below. M2, as shown in formula (2), is a metallic element capable of becoming a divalent ion. M3, as shown in formula (3), is a metallic element capable of becoming a trivalent ion. M4, as shown in formula (4), is a metallic element capable of becoming a tetravalent ion. This is because, even with repeated charging and discharging, the discharge capacity is not easily reduced sufficiently, thus significantly improving cycle characteristics.

[0080] Li[Li x M2 (1-3x) / 2 Ti (3+x) / 2 ]O4…(2)

[0081] (M2 is at least one of Mg, Ca, Cu, Zn, and Sr. x satisfies 0 ≤ x ≤ 1 / 3.)

[0082] Li[Li y M3 1-3y Ti 1+2y ]O4…(3)

[0083] (M3 is at least one of Al, Sc, Cr, Mn, Fe, Ga, and Y. y satisfies 0 ≤ y ≤ 1 / 3.)

[0084] Li[Li 1 / 3 M4 z Ti (5 / 3)-z ]O4…(4)

[0085] (M4 is at least one of V, Zr, and Nb. z satisfies 0 ≤ z ≤ 2 / 3.)

[0086] From the range of possible values ​​for x in equation (2), it can be seen that the lithium-titanium composite oxide shown in equation (2) may or may not contain other elements M2 as constituent elements. From the range of possible values ​​for y in equation (3), it can be seen that the lithium-titanium composite oxide shown in equation (3) may or may not contain other elements M3 as constituent elements. From the range of possible values ​​for z in equation (4), it can be seen that the lithium-titanium composite oxide shown in equation (4) may or may not contain other elements M4 as constituent elements.

[0087] A specific example of the lithium-titanium composite oxide shown in equation (2) is Li 3.75 Ti 4.875 Mg 0.375 O 12 Etc. Specific examples of lithium-titanium composite oxides shown in equation (3) are LiCrTiO4, etc. Specific examples of lithium-titanium composite oxides shown in equation (4) are Li4Ti5O, etc. 12 and Li4Ti 4.95 Nb 0.05 O 12 wait.

[0088] It should be noted that the negative electrode active material only needs to contain the above-mentioned lithium titanium composite oxide, and may also contain any one or more of other negative electrode active materials.

[0089] The types of other negative electrode active materials are not particularly limited. Specifically, they are one or both of carbon materials and metal-based materials, etc. This is because a high energy density can be obtained. The carbon materials are easily graphitizable carbon, difficult-to-graphitize carbon, and graphite (natural graphite and artificial graphite), etc. The metal-based materials are a general term for materials containing any one or two or more of metal elements and metalloid elements that can form an alloy with lithium, and the metal elements and metalloid elements are one or both of silicon and tin, etc. In addition, the metal-based material can be a monomer, an alloy, a compound, a mixture of two or more of them, or a material containing two or more phases of them. In addition, the above-mentioned lithium titanium composite oxide is not included in the metal-based materials described herein. Specific examples of the metal-based material are TiSi2 and SiO x (0 < x ≤ 2, or 0.2 < x < 1.4), etc.

[0090] The method for forming the negative electrode active material layer 22B is not particularly limited. Specifically, it is any one or two or more of a coating method, a vapor phase method, a liquid phase method, a spraying method, and a firing method (sintering method), etc.

[0091] The coating film 22C is a negative electrode coating film provided on the surface of the negative electrode active material layer 22B, covering the surface of the negative electrode active material layer 22B.

[0092] Here, since the coating film 22C is provided so as to cover the surface of each of the two negative electrode active material layers 22B, the negative electrode 22 includes two coating films 22C. In addition, it may be that the coating film 22C is provided so as to cover only one surface of the two negative electrode active material layers 22B, and thus the negative electrode 22 includes one coating film 22C.

[0093] The coating film 22C can cover the entire surface of the negative electrode active material layer 22B, or can cover only a part of the surface of the negative electrode active material layer 22B. In addition, in the latter case, a plurality of coating films 22C can cover the surface of the positive electrode active material layer 22B at a plurality of mutually isolated portions. Figure 2 The case where the coating film 22C covers the entire surface of the negative electrode active material layer 22B is shown.

[0094] Here, the coating film 22C is formed by the same steps as those for forming the coating film 21C. That is, as described later, in the manufacturing process of the secondary battery, using the stabilization treatment (initial charge and discharge treatment) of the assembled secondary battery, the coating film 22C is formed on the surface of the negative electrode active material layer 22B. Similar to the coating film 21C, the coating film 22C mainly contains any one or two or more of the reactants and decomposition products of the carboxylic acid ester contained in the electrolyte described later, and thus is a coating film derived from the carboxylic acid ester. Therefore, the coating film 22C contains oxygen as a constituent element.

[0095] In this secondary battery, the physical properties of the negative electrode 22, i.e., the properties of the coating 22C, can meet specified conditions in order to improve the expansion characteristics, cycle characteristics, and load characteristics, respectively. Details regarding the physical properties of this negative electrode 22 (coating 22C) will be described later.

[0096] (Diaphragm)

[0097] like Figure 2 As shown, the separator 23 is an insulating porous membrane located between the positive electrode 21 and the negative electrode 22, which prevents contact (short circuit) between the positive electrode 21 and the negative electrode 22 while allowing lithium ions to pass through. The separator 23 contains polymer compounds such as polyethylene.

[0098] (electrolyte)

[0099] The electrolyte contains a carboxylic acid ester, which may contain one or more of ethyl acetate, propyl acetate, ethyl propionate, and propyl propionate. This electrolyte is impregnated in each of the positive electrode 21, the negative electrode 22, and the separator 23. The reason for containing the carboxylic acid ester in the electrolyte is that it can suppress the decomposition reaction of the electrolyte during charging and discharging, thereby suppressing the generation of gas due to the decomposition reaction of the electrolyte.

[0100] The chain carboxylic ester is preferably one or both of ethyl propionate and propyl propionate. This is because the decomposition reaction of the electrolyte can be sufficiently suppressed, thereby effectively suppressing the generation of gas due to the decomposition reaction of the electrolyte.

[0101] Here, the electrolyte contains a solvent and an electrolyte salt, the solvent of which contains a carboxylic acid ester. The solvent may also contain any one or more non-aqueous solvents (organic solvents). Furthermore, the aforementioned carboxylic acid esters are not included in the non-aqueous solvents described herein.

[0102] The content of carboxylic acid esters in the solvent is not particularly limited, but is preferably 50% to 90% by weight. This is because the decomposition reaction of the electrolyte can be sufficiently suppressed, thereby effectively suppressing the generation of gas due to the decomposition reaction of the electrolyte.

[0103] An electrolyte containing one or more of the following non-aqueous solvents: carbonate compounds and lactone compounds, is called a non-aqueous electrolyte. This is because it can improve the dissociation of electrolyte salts and also improve ion mobility.

[0104] Specifically, carbonate compounds include cyclic carbonates and chain carbonates. Specific examples of cyclic carbonates are ethylene carbonate and propylene carbonate, while specific examples of chain carbonates are dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. Specific examples of lactone compounds are γ-butyrolactone and γ-valerolactone.

[0105] The non-aqueous solvent preferably contains one or more of cyclic carbonates. That is, the solvent preferably contains carboxylic acid esters and cyclic carbonates. This is because, while ensuring both the dissociation of the electrolyte salt and the ion mobility, the decomposition reaction of the electrolyte can be sufficiently suppressed.

[0106] Electrolyte salts include any one or more of light metal salts such as lithium salts. Specific examples of lithium salts are lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(fluorosulfonyl)imide (LiN(FSO2)2), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF3SO2)2), lithium tri(trifluoromethanesulfonyl)methylide (LiC(CF3SO2)3), lithium difluorooxalateborate (LiBF2(C2O4)), and lithium bis(oxalate)borate (LiB(C2O4)2), etc.

[0107] It should be noted that the electrolyte may also contain one or more additives. There are no particular limitations on the types of additives, but specifically, they may include unsaturated cyclic carbonates, halocarbonates, sulfonates, phosphate esters, acid anhydrides, nitrile compounds, and isocyanate compounds, etc. This is because they can improve the chemical stability of the electrolyte.

[0108] [Positive and negative leads]

[0109] like Figure 1 As shown, the positive lead 31 is the positive terminal connected to the battery element 20 (positive electrode 21), and more specifically, to the positive current collector 21A. This positive lead 31 extends from the inside of the outer packaging film 10 to the outside and contains a conductive material such as aluminum. The shape of the positive lead 31 is not particularly limited; specifically, it can be any of the following: a thin plate shape or a mesh shape.

[0110] like Figure 1 As shown, the negative electrode lead 32 is connected to the negative terminal of the battery element 20 (negative electrode 22), and more specifically, to the negative current collector 22A. This negative electrode lead 32 extends from the inside of the outer packaging film 10 to the outside and contains a conductive material such as copper. Here, the lead-out direction of the negative electrode lead 32 is the same as that of the positive electrode lead 31. It should be noted that the details regarding the shape of the negative electrode lead 32 are the same as the details regarding the shape of the positive electrode lead 31.

[0111] <1-2.Physical properties>

[0112] In this secondary battery, as described above, in order to improve each of the expansion characteristics, cycle characteristics, and load characteristics, the following conditions must be met regarding the physical properties of the positive electrode 21 and the physical properties of the negative electrode 22.

[0113] [Conditions regarding the physical properties of the positive electrode]

[0114] Regarding the physical properties of the cathode 21, the specified conditions are met. That is, as explained below, the physical properties of the cathode 21, as specified by the surface analysis results using X-ray photoelectron spectroscopy (XPS), are appropriated.

[0115] Specifically, in the surface analysis of the positive electrode 21 using XPS, two XPS energy dispersive spectroscopy methods were detected. Here, since the positive electrode 21 includes a positive electrode active material layer 21B and a coating 21C, the coating 21C was analyzed in the surface analysis of the positive electrode 21 using XPS.

[0116] The first type of XPS spectrum originates from the XPS spectrum of oxygen atoms contained as constituent elements in the coating 21C. More specifically, it is the first oxygen (O1s) spectrum with a peak in the binding energy range of 528 eV to 531 eV. It can be considered that this first oxygen spectrum is mainly detected from the composition of the positive electrode active material layer 21B (the lithium-nickel composite oxide that serves as the positive electrode active material), the bonding state of oxygen atoms in the crystal structure of the positive electrode active material, and the composition of the coating 21C.

[0117] The second type of XPS spectrum also originates from other XPS spectra of oxygen atoms in the coating 21C. More specifically, it is a second oxygen spectrum with peaks in the range of binding energies greater than 531 eV and less than 535 eV. Similar to the first oxygen spectrum described above, this second oxygen spectrum is primarily detected based on the composition of the positive electrode active material layer 21B (positive electrode active material), the bonding state of oxygen atoms in the crystal structure of the positive electrode active material, and the composition of the coating 21C.

[0118] In this case, the ratio specified based on the intensity of each of the two XPS spectra (first oxygen spectrum and second oxygen spectrum), namely the ratio of the intensity of the first oxygen spectrum to the intensity of the second oxygen spectrum (cathode intensity ratio), is 0.30 to 0.80. This cathode intensity ratio is calculated as cathode intensity ratio = intensity of the first oxygen spectrum / intensity of the second oxygen spectrum. It should be noted that the value of the cathode intensity ratio is rounded to the third decimal place.

[0119] The reason why the physical properties (positive electrode strength ratio) of the positive electrode 21 meet the above conditions is that when the positive electrode active material layer 21B (positive electrode active material) contains lithium nickel composite oxide and a coating 21C derived from the carboxylic acid ester in the electrolyte is formed on the surface of the positive electrode active material layer 21B, the physical properties of the positive electrode 21 (coating 21C) are appropriate, thereby suppressing the oxidative decomposition reaction of the electrolyte in the positive electrode 21.

[0120] Therefore, firstly, gas is less likely to be generated due to the oxidative decomposition reaction of the electrolyte, thus the secondary battery is less likely to expand. In particular, even if highly reducing byproducts are generated during the oxidative decomposition reaction of the electrolyte, the generation of gas due to the reduction and decomposition of these byproducts in the negative electrode 22 can be suppressed, thus the secondary battery is significantly less likely to expand. Secondly, since the oxidative decomposition reaction of the electrolyte is suppressed during charging and discharging, the amount of electrolyte is less likely to decrease excessively, so the discharge capacity is less likely to decrease even with repeated charging and discharging. Thirdly, since lithium ions can easily enter and exit the positive electrode 21 during charging and discharging, lithium ions can easily be inserted and extracted in the positive electrode 21.

[0121] By changing the composition of the positive electrode active material (lithium-nickel composite oxide) (the types and amounts of constituent elements other than lithium and oxygen), the intensity of the first oxygen spectrum and the intensity of the second oxygen spectrum can be adjusted separately. Specific examples of constituent elements other than lithium and oxygen are nickel, cobalt, aluminum, and manganese. Furthermore, when covering the surface of the positive electrode active material with a coating containing any element, changing the amount of coating coverage also allows for adjusting the intensity of the first and second oxygen spectra. Additionally, changing the composition of the electrolyte (the type of solvent and the type of electrolyte salt, etc.) also allows for adjusting the intensity of the first and second oxygen spectra. Therefore, the positive electrode intensity ratio can be controlled to the desired value.

[0122] It should be noted that the steps for determining the positive electrode intensity ratio are as follows. In this case, the XPS analysis device used is a scanning X-ray photoelectron spectroscopy device such as the PHI Quantera manufactured by ULVAC-PHI Corporation.

[0123] First, the positive electrode 21 is recovered by disassembling the completed secondary battery. Next, the positive electrode 21 is cleaned with an organic solvent and then dried. The type of organic solvent is not particularly limited, but dimethyl carbonate, etc., are examples. Next, the surface of the positive electrode 21 is analyzed using an XPS analyzer, and the intensities of the first and second oxygen spectra are measured. Finally, the positive electrode intensity ratio is calculated based on the intensities of the first and second oxygen spectra, respectively.

[0124] [Conditions regarding the physical properties of the negative electrode]

[0125] In addition, the physical properties of the negative electrode 22 meet the specified conditions.

[0126] Specifically, in the surface analysis of the negative electrode 22 using XPS, two XPS energy dispersive spectroscopy methods were detected. Here, since the negative electrode 22 includes a negative electrode active material layer 22B and a coating 22C, the coating 22C was analyzed in the surface analysis of the negative electrode 22 using XPS.

[0127] The first type of XPS spectrum originates from the XPS spectrum of oxygen atoms contained as constituent elements in the coating 22C. More specifically, it is the third oxygen spectrum, which has peaks in the binding energy range of 528 eV to 531 eV. The main reason for the detection of this third oxygen spectrum is the same as that for the first oxygen spectrum, except that it is detected from oxygen atoms in the coating 22C rather than the coating 21C.

[0128] The second type of XPS spectrum also originates from other XPS spectra of oxygen atoms in the coated 22C. More specifically, it is the fourth oxygen spectrum, which has peaks in the range of binding energies greater than 531 eV and less than 535 eV. The main reason for the detection of this fourth oxygen spectrum is the same as that for the second oxygen spectrum, except that it originates from oxygen atoms in the coated 22C rather than the coated 21C.

[0129] In this case, the ratio specified based on the intensity of each of the two XPS spectra (third oxygen spectrum and fourth oxygen spectrum), i.e., the ratio of the intensity of the third oxygen spectrum to the intensity of the fourth oxygen spectrum (negative electrode intensity ratio), is not particularly limited, but is preferably 0.82 to 1.35. This negative electrode intensity ratio is calculated as: negative electrode intensity ratio = intensity of the third oxygen spectrum / intensity of the fourth oxygen spectrum. It should be noted that, like the positive electrode intensity ratio, the value of the negative electrode intensity ratio is rounded to the third decimal place.

[0130] The reason why the physical properties (negative electrode strength ratio) of the negative electrode 22 satisfy the above conditions is that, when the negative electrode active material layer 22B (negative electrode active material) contains lithium titanium composite oxide and a coating 22C derived from carboxylic acid esters in the electrolyte is formed on the surface of the negative electrode active material layer 22B, the physical properties of the negative electrode 22 (coating 22C) are appropriate, thereby suppressing the reduction and decomposition reaction of the electrolyte in the negative electrode 22. Therefore, while suppressing the expansion of the secondary battery, the reduction in charge and discharge capacity is suppressed even with repeated charge and discharge, and the insertion and extraction properties of lithium ions during charge and discharge are improved.

[0131] The intensities of both the third and fourth oxygen spectra can be adjusted by changing the type of the negative electrode active material (lithium-titanium composite oxide) and the composition of the negative electrode active material layer 22B (such as the type of additives). Furthermore, changing the composition of the electrolyte (such as the type of solvent and the type of electrolyte salt) can also adjust the intensities of both the third and fourth oxygen spectra. Therefore, the negative electrode intensity ratio can be controlled to the desired value.

[0132] It should be noted that the steps for determining the negative electrode strength ratio are the same as those for determining the positive electrode strength ratio, except that the negative electrode 22 is used instead of the positive electrode 21 for surface analysis.

[0133] <1-3 Actions>

[0134] During charging, lithium is deintercalated from the positive electrode 21 in the battery element 20, and simultaneously intercalated into the negative electrode 22 via the electrolyte. Conversely, during discharging, lithium is deintercalated from the negative electrode 22 in the battery element 20, and simultaneously intercalated into the positive electrode 21 via the electrolyte. During these charging and discharging processes, lithium is intercalated and deintercalated in an ionic state.

[0135] <1-4. Manufacturing Method>

[0136] In the case of manufacturing a secondary battery, a positive electrode 21 and a negative electrode 22 are made by following the steps described below, and an electrolyte is prepared. Then, the positive electrode 21, the negative electrode 22, and the electrolyte are used to manufacture a secondary battery.

[0137] [The production of the positive electrode]

[0138] First, a mixture of positive electrode active material containing lithium-nickel composite oxide, positive electrode binder, and positive electrode conductive agent (positive electrode slurry) is added to a solvent to prepare a paste-like positive electrode slurry. Next, the positive electrode slurry is coated onto both sides of the positive electrode current collector 21A to form a positive electrode active material layer 21B. Thereafter, the positive electrode active material layer 21B can be compressed and molded using a roller press or the like. In this case, the positive electrode active material layer 21B can be heated, or the compression molding process can be repeated multiple times.

[0139] Thus, a positive electrode active material layer 21B is formed on both sides of the positive electrode current collector 21A, thereby fabricating a positive electrode precursor (not shown). This positive electrode precursor has the same structure as the positive electrode 21, except that a coating 21C is not formed on the surface of the positive electrode active material layer 21B. Finally, using the stabilization treatment (initial charge-discharge treatment) of the assembled secondary battery described later, a coating 21C is formed on the surface of the positive electrode active material layer 21B. Thus, the positive electrode 21 is fabricated.

[0140] [Making the negative electrode]

[0141] The negative electrode 22 is formed using the same steps as those described for the positive electrode 21. Specifically, first, a mixture (negative electrode slurry) containing a negative electrode active material with lithium titanium composite oxide, a negative electrode binder, and a negative electrode conductive agent is added to a solvent to prepare a paste-like negative electrode slurry. Next, the negative electrode slurry is coated onto both sides of the negative electrode current collector 22A to form a negative electrode active material layer 22B. Afterward, the negative electrode active material layer 22B can be compressed and molded.

[0142] Thus, a negative electrode active material layer 22B is formed on both sides of the negative electrode current collector 22A, thereby producing a negative electrode precursor (not shown). This negative electrode precursor has the same structure as the negative electrode 22, except that a coating 22C is not formed on the surface of the negative electrode active material layer 22B. Finally, using the stabilization treatment (initial charge-discharge treatment) of the assembled secondary battery described later, a coating 22C is formed on the surface of the negative electrode active material layer 22B. Thus, the negative electrode 22 is produced.

[0143] (Preparation of electrolyte)

[0144] An electrolyte salt is added to a solvent containing a carboxylic acid ester. The electrolyte salt is then dispersed or dissolved in the solvent, thus preparing an electrolyte solution.

[0145] (Assembly of a secondary battery)

[0146] First, the positive lead 31 is connected to the positive electrode precursor (positive current collector 21A) using a soldering method or the like, and the negative lead 32 is connected to the negative electrode precursor (negative current collector 22A).

[0147] Next, the positive electrode precursor and the negative electrode precursor are stacked on top of each other with the diaphragm 23 in between, and then the positive electrode precursor, the negative electrode precursor, and the diaphragm 23 are wound around the winding shaft P to create a wound body. Next, the wound body is pressed using a press or the like to shape it into a flat shape.

[0148] Next, after the wound body is housed inside the recess 10U, the outer packaging film 10 is folded in the direction R, thereby placing the outer packaging films 10 opposite each other. Next, using a heat-sealing method or the like, the outer peripheral portions of two sides of the opposite outer packaging films 10 (welded layers) are joined together, thereby housing the wound body inside the bag-shaped outer packaging film 10.

[0149] Finally, after injecting the electrolyte into the inside of the pouch-shaped outer packaging film 10, the outer periphery of the remaining edge of the outer packaging film 10 (welded layer) is joined together using a heat fusion method or the like. In this case, a sealing film 41 is inserted between the outer packaging film 10 and the positive electrode lead 31, and a sealing film 42 is inserted between the outer packaging film 10 and the negative electrode lead 32. Thus, the electrolyte is impregnated into the wound body, thereby creating a battery element 20 as a wound electrode body, and the battery element 20 is sealed inside the pouch-shaped outer packaging film 10, thereby assembling a secondary battery.

[0150] (Stabilization of secondary batteries)

[0151] This allows the assembled secondary battery to be charged and discharged. Various conditions, such as ambient temperature, number of charge / discharge cycles, and charging / discharging conditions, can be set arbitrarily.

[0152] Thus, as described above, a coating 21C is formed on the surface of the positive electrode active material layer 21B to form the positive electrode 21, and a coating 22C is formed on the surface of the negative electrode active material layer 22B to form the negative electrode 22. In this case, since the surface of the positive electrode active material layer 21B is protected by the coating 21C, and the surface of the negative electrode active material layer 22B is also protected by the coating 22C, the state of the secondary battery is electrochemically stabilized.

[0153] Thus, a secondary battery using outer packaging film 10, namely a laminated film type secondary battery, is completed.

[0154] <1-5. Functions and Effects>

[0155] According to this secondary battery, the positive electrode 21 contains lithium-nickel composite oxide, the negative electrode 22 contains lithium-titanium composite oxide, and the electrolyte contains carboxylic acid esters such as ethyl acetate. Furthermore, the surface analysis results (positive electrode strength ratio) of the positive electrode 21 using XPS meet the above conditions (positive electrode strength ratio = 0.30–0.80), and the surface analysis results (negative electrode strength ratio) of the negative electrode 22 using XPS meet the above conditions (negative electrode strength ratio = 0.82–1.35).

[0156] In this case, as described above, since the physical properties (positive electrode intensity ratio) of the positive electrode 21 defined by the two XPS spectra (first oxygen spectrum and second oxygen spectrum) are appropriate, the oxidative decomposition reaction of the electrolyte is suppressed in the positive electrode 21. Therefore, since the generation of gas due to the oxidative decomposition reaction of the electrolyte is suppressed in the positive electrode 21, and the generation of gas due to the reductive decomposition reaction of highly reducing byproducts is suppressed in the negative electrode 22, the secondary battery is significantly less prone to expansion. Furthermore, since the amount of electrolyte is less likely to decrease excessively during charging and discharging, the discharge capacity is less likely to decrease even with repeated charging and discharging. Simultaneously, since lithium ions can easily enter and exit the positive electrode 21 during charging and discharging, lithium ions are easily inserted and extracted in the positive electrode 21.

[0157] Furthermore, as described above, the physical properties (anode intensity ratio) of the negative electrode 22, as defined by the two XPS spectra (third and fourth oxygen spectra), are optimized, thus suppressing the reduction and decomposition reaction of the electrolyte in this negative electrode 22. Consequently, while further suppressing the expansion of the secondary battery, the reduction in discharge capacity is further suppressed even with repeated charge-discharge cycles, and the lithium-ion insertion / extraction properties during charge-discharge are further improved.

[0158] Therefore, while significantly suppressing the expansion of the secondary battery, the reduction in discharge capacity is also suppressed even with repeated charging and discharging. At the same time, the insertion and extraction properties of lithium ions during charging and discharging are improved, thus achieving excellent expansion characteristics, excellent cycle characteristics, and excellent load characteristics.

[0159] In particular, if the lithium-nickel composite oxide contains the compound shown in formula (1), a sufficiently high energy density can be obtained, thus achieving a higher performance. In this case, if the Ni ratio is 80% or higher, an even higher energy density can be obtained, thus further achieving a high performance.

[0160] Furthermore, if the solvent of the electrolyte contains carboxylic acid esters, and the content of carboxylic acid esters in the solvent is 50% to 90% by weight, the decomposition reaction of the electrolyte can be sufficiently suppressed, thus achieving even higher performance. In this case, if the solvent also contains cyclic carbonates, the decomposition reaction of the electrolyte can be sufficiently suppressed while ensuring both the dissociation of the electrolyte salt and the ion mobility, thus further achieving high performance.

[0161] In addition, the positive electrode 21 includes a positive electrode active material layer 21B (containing lithium-nickel composite oxide) and a coating 21C (containing oxygen as a constituent element). If the coating 21C is analyzed in the surface analysis of the positive electrode 21 using XPS, a higher effect can be obtained because the decomposition reaction of the electrolyte can be sufficiently suppressed in the positive electrode 21.

[0162] In addition, if the lithium titanium composite oxide contains any one or more of the compounds shown in formula (2), formula (3) and formula (4), the discharge capacity will not be easily reduced even after repeated charging and discharging, thus achieving a higher effect.

[0163] In addition, the negative electrode 22 includes a negative electrode active material layer 22B (containing lithium titanium composite oxide) and a coating 22C (containing oxygen as a constituent element). If the coating 22C is analyzed in the surface analysis of the negative electrode 22 using XPS, a higher efficiency can be obtained because the decomposition reaction of the electrolyte can be sufficiently suppressed in the negative electrode 22.

[0164] Furthermore, if the secondary battery has a flexible outer packaging film 10, even if the outer packaging film 10 is easily deformed due to the increase in internal pressure, the secondary battery can be effectively prevented from expanding, thus achieving a higher effect.

[0165] In addition, if the secondary battery is a lithium-ion secondary battery, sufficient battery capacity can be stably obtained by utilizing the insertion and extraction of lithium, thus achieving higher performance.

[0166] <2. Variations>

[0167] As explained below, the structure of the secondary battery can be appropriately modified. Furthermore, any two or more of the variations described below can be combined with each other.

[0168] [Variation Example 1]

[0169] A membrane 23 as a porous membrane was used. However, although not specifically illustrated here, a laminated membrane 23 comprising layers of polymer compounds may be used instead of a porous membrane 23.

[0170] Specifically, the laminated separator 23 includes a porous membrane with one and two faces and a polymer compound layer disposed on one or both faces of the porous membrane. This is because, due to the improved adhesion of the separator 23 to each of the positive electrode 21 and the negative electrode 22, the positional shift (winding shift) of the battery element 20 is less likely to occur. Therefore, even if electrolyte decomposition reactions occur, the secondary battery is less likely to expand. The polymer compound layer contains polymers such as polyvinylidene fluoride (PVDF). This is because PVDF and similar compounds have excellent physical strength and electrochemical stability.

[0171] It should be noted that one or both of the porous membrane and the polymer compound layer may contain any one or more types of insulating particles. This is because multiple insulating particles dissipate heat when the secondary battery heats up, thus improving the safety (heat resistance) of the secondary battery. Insulating particles include inorganic particles and resin particles. Specific examples of inorganic particles include alumina, aluminum nitride, boehmite, silicon dioxide, titanium dioxide, magnesium oxide, and zirconium oxide. Specific examples of resin particles include acrylic resin and styrene resin.

[0172] In the case of fabricating a laminated membrane 23, a precursor solution containing a polymer compound and an organic solvent is prepared, and then the precursor solution is coated onto one or both sides of the porous membrane. Alternatively, the porous membrane can be immersed in the precursor solution. In this case, multiple insulating particles can also be added to the precursor solution as needed.

[0173] When using this layered separator 23, lithium ions can also move between the positive electrode 21 and the negative electrode 22, thus achieving the same effect.

[0174] [Variation Example 2]

[0175] An electrolyte solution, which is a liquid electrolyte, was used. However, although not specifically illustrated here, an electrolyte layer, which is a gel electrolyte, can also be used instead of an electrolyte solution.

[0176] In the battery element 20 using an electrolyte layer, the positive electrode 21 and the negative electrode 22 are stacked on top of each other with the separator 23 and the electrolyte layer in between, and the positive electrode 21, the negative electrode 22, the separator 23 and the electrolyte layer are wound together. The electrolyte layer is located between the positive electrode 21 and the separator 23, and between the negative electrode 22 and the separator 23.

[0177] Specifically, the electrolyte layer contains an electrolyte and a polymer compound, in which the electrolyte is held in place by the polymer compound. This is to prevent electrolyte leakage. The structure of the electrolyte is as described above. The polymer compound includes polyvinylidene fluoride, etc. In the case of forming the electrolyte layer, after preparing a precursor solution containing the electrolyte, polymer compound, and organic solvent, the precursor solution is coated on one or both sides of each of the positive electrode 21 and the negative electrode 22.

[0178] When this electrolyte layer is used, lithium ions can also move between the positive electrode 21 and the negative electrode 22 via the electrolyte layer, thus achieving the same effect.

[0179] <3. Uses of Secondary Batteries>

[0180] There are no particular limitations on the uses (application examples) of secondary batteries. Secondary batteries used as a power source can be the main power source for electronic devices and electric vehicles, or they can be an auxiliary power source. The main power source is the power source used preferentially, regardless of the availability of other power sources. An auxiliary power source is a power source used in place of the main power source, or a power source switched from the main power source.

[0181] Specific examples of applications for rechargeable batteries are as follows: Electronic devices such as camcorders, digital still cameras, mobile phones, laptops, stereo headphones, portable radios, and portable information terminals. Backup power supplies and storage devices such as memory cards. Power tools such as electric drills and chainsaws. Battery packs integrated into electronic devices. Medical electronic devices such as pacemakers and hearing aids. Electric vehicles such as electric cars (including hybrid vehicles). Power storage systems such as household or industrial battery systems that pre-store power in preparation for emergencies. In these applications, one or multiple rechargeable batteries can be used.

[0182] Battery packs can use single cells or battery arrays. Electric vehicles are vehicles that operate (drive) using secondary batteries as a power source, and can also be hybrid vehicles that have a power source other than the secondary battery. In home power storage systems, electricity stored in secondary batteries, which serve as power storage sources, can be used to operate household electrical products, etc.

[0183] Here, we will specifically illustrate one application example of a secondary battery. The structure of the application example described below is only one example and can therefore be modified as appropriate.

[0184] Figure 3 The frame structure of the battery pack is shown. The battery pack described here is a battery pack (so-called a pouch) that uses a secondary battery and is installed in electronic devices such as smartphones.

[0185] like Figure 3 As shown, the battery pack includes 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.

[0186] The power supply 51 includes a secondary battery. In this secondary 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 can be connected to an external source via the positive terminal 53 and the negative terminal 54, it can be charged and discharged. The circuit board 52 includes a control unit 56, a switch 57, a PTC element 58, and a temperature detection unit 59. Alternatively, the PTC element 58 may be omitted.

[0187] The control unit 56 includes a central processing unit (CPU) and memory, etc., and controls the operation of the battery pack as a whole. The control unit 56 detects and controls the usage status of the power supply 51 as needed.

[0188] It should be noted that when the voltage of the power supply 51 (secondary battery) reaches the overcharge detection voltage or the over-discharge detection voltage, the control unit 56 cuts off the switch 57, thereby preventing the charging current from flowing through the current path of the power supply 51. The overcharge detection voltage and the over-discharge detection voltage are not particularly limited. For example, the overcharge detection voltage is 4.2V ± 0.05V, and the over-discharge detection voltage is 2.4V ± 0.1V.

[0189] Switch 57 includes a charging control switch, a discharging control switch, a charging diode, and a discharging diode, etc., and switches the connection between power supply 51 and external devices according to the instructions of control unit 56. Switch 57 includes a field-effect transistor (MOSFET) using metal-oxide-semiconductor, and the charging / discharging current is detected based on the on-resistance of switch 57.

[0190] The temperature detection unit 59 includes a temperature detection element such as a thermistor, 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 for charging and discharging control by the control unit 56 when abnormal heating occurs, and for correction processing by the control unit 56 when calculating the remaining capacity.

[0191] Example

[0192] The embodiments of this technology are described below.

[0193] <Examples 1-12 and Comparative Examples 1-11>

[0194] As explained below, it was made Figure 1 as well as Figure 2 The laminated film type secondary battery (lithium-ion secondary battery) is shown, and its performance is then evaluated.

[0195] [Making a Secondary Battery]

[0196] The secondary battery was made using the following steps.

[0197] (The production of the positive electrode)

[0198] First, 98 parts by mass of LiNi, used as the positive electrode active material (lithium-nickel composite oxide (LiNi composite oxide)), are... 0.86 Co 0.10 Al 0.03 Mn 0.01A positive electrode binder (polyvinylidene fluoride) and a positive electrode conductive agent (carbon black) are mixed to prepare a positive electrode additive. Next, the positive electrode additive is added to a solvent (N-methyl-2-pyrrolidone as an organic solvent), and the solvent is stirred to prepare a paste-like positive electrode additive slurry. Next, the positive electrode additive slurry is coated onto both sides of a positive electrode current collector 21A (a strip of aluminum foil with a thickness of 12 μm) using a coating apparatus, and then dried to form a positive electrode active material layer 21B. Next, the positive electrode active material layer 21B is compressed and molded using a roller press to obtain a positive electrode precursor. Finally, in the secondary battery stabilization treatment described later, a coating 21C is formed on the surface of the positive electrode active material layer 21B to produce the positive electrode 21.

[0199] (Making the negative electrode)

[0200] First, 98 parts by mass of Li4Ti5O4, used as the negative electrode active material (lithium-titanium composite oxide (LiTi composite oxide)), were... 12 A negative electrode binder (polyvinylidene fluoride) and a negative electrode conductive agent (carbon black) are mixed to prepare a negative electrode mixture. Next, the negative electrode mixture is added to a solvent (N-methyl-2-pyrrolidone as an organic solvent), and the solvent is stirred to prepare a paste-like negative electrode mixture slurry. Next, the negative electrode mixture slurry is coated onto both sides of a negative electrode current collector 22A (a strip of copper foil with a thickness of 15 μm) using a coating apparatus, and then dried to form a negative electrode active material layer 22B. Next, the negative electrode active material layer 22B is compressed and molded using a roller press to obtain a negative electrode precursor. Finally, in the secondary battery stabilization treatment described later, a coating 22C is formed on the surface of the negative electrode active material layer 22B to form the negative electrode 22.

[0201] (Preparation of electrolyte)

[0202] An electrolyte salt (lithium hexafluorophosphate (LiPF6)) was added to a solvent, and the solvent was stirred. Then, an additive (ethylene carbonate as an unsaturated cyclic carbonate) was added to the solvent, and the solvent was stirred again. A mixture of propylene carbonate (PC) as a cyclic carbonate and propyl propionate (PrPr), ethyl propionate (EtPr), propyl acetate (PrAc), or ethyl acetate (EtAc) as a carboxylic acid ester was used as the solvent. The mixing ratio (by weight) of the solvent was cyclic carbonate:carboxylic acid ester = 30:70 (the content of carboxylic acid ester in the solvent = 70% by weight). The content of the electrolyte salt relative to the solvent was 1 mol / kg. The content of the unsaturated cyclic carbonate in the electrolyte was 1% by weight. Thus, the electrolyte salt was dispersed or dissolved in the solvent, thereby preparing the electrolyte.

[0203] It should be noted that, for the purpose of comparison, the electrolyte was prepared using the same steps, except that a chain carbonate was used instead of a carboxylic acid ester. Dimethyl carbonate (DMC) or diethyl carbonate (DEC) was used as the chain carbonate.

[0204] In addition, for comparison, an electrolyte was prepared using the same steps except that another carboxylic acid ester (methyl propionate (MtPr)) was used instead of the carboxylic acid ester.

[0205] (Assembly of a secondary battery)

[0206] First, the aluminum positive lead 31 is soldered to the positive electrode 21 (positive current collector 21A), and the copper negative lead 32 is soldered to the negative electrode 22 (negative current collector 22A).

[0207] Next, the positive electrode precursor and the negative electrode precursor are stacked together with a separator 23 (a microporous polyethylene membrane with a thickness of 15 μm) in between. Then, the positive electrode precursor, the negative electrode precursor, and the separator 23 are wound around the winding shaft P to create a wound body. Next, the wound body is stamped using a press to form a flat wound body.

[0208] Next, the wound body is housed inside the recess 10U provided in the outer packaging film 10. The outer packaging film 10 is an aluminum laminate film in which a welding layer (a 30 μm thick polypropylene film), a metal layer (a 40 μm thick aluminum foil), and a surface protective layer (a 25 μm thick nylon film) are sequentially stacked. Next, the outer packaging film 10 is folded with the welding layer disposed on the inside and the wound body held in place. Then, the outer periphery portions of two sides of the outer packaging film 10 (welding layer) are thermally welded together, thereby housing the wound body inside the bag-shaped outer packaging film 10.

[0209] Finally, after injecting the electrolyte into the pouch-shaped outer packaging film 10, the outer periphery of the remaining edge of the outer packaging film 10 (welded layer) is thermally fused together under reduced pressure. In this case, a sealing film 41 (a 5 μm thick polypropylene film) is inserted between the outer packaging film 10 and the positive electrode lead 31, and a sealing film 42 (a 5 μm thick polypropylene film) is inserted between the outer packaging film 10 and the negative electrode lead 32. Thus, the electrolyte is impregnated into the wound body, thereby fabricating the battery element 20. The battery element 20 is then sealed inside the outer packaging film 10, thereby assembling a secondary battery.

[0210] (Stabilization of secondary batteries)

[0211] First, the secondary battery is charged at room temperature (23°C). Under this condition, it is charged at a constant current of 0.5C until the voltage reaches 2.7V, i.e., the State of Charge (SOC) reaches 100%. Next, the charged secondary battery is stored in a high-temperature environment (40°C) for 7 days. Finally, the secondary battery is discharged at room temperature (23°C). Under this condition, it is discharged at a constant current of 0.2C until the voltage reaches 1.0V. It should be noted that 0.5C refers to the current value that completely discharges the battery capacity (theoretical capacity) in 2 hours, while 0.2C refers to the current value that completely discharges the battery capacity in 5 hours.

[0212] Thus, a coating 21C is formed on the surface of the positive electrode active material layer 21B in the positive electrode precursor, thereby fabricating the positive electrode 21, and a coating 22C is formed on the surface of the negative electrode active material layer 22B in the negative electrode precursor, thereby fabricating the negative electrode 22. This electrochemically stabilizes the state of the secondary battery, thus completing the laminated film type secondary battery.

[0213] In the case of manufacturing this secondary battery, the positive electrode strength ratio is changed by changing the ambient temperature during charging during the stabilization treatment of the secondary battery, and the negative electrode strength ratio is changed by changing the charging current during the stabilization treatment of the secondary battery.

[0214] After the secondary battery was completed, the positive electrode 21 was recovered by disassembling the secondary battery, and then surface analysis of the positive electrode 21 (coated 21C) was performed using XPS. Based on the surface analysis results of the positive electrode 21, the intensities of two XPS spectra (first oxygen spectrum and second oxygen spectrum) were measured, and then the positive electrode intensity ratio was calculated based on these measurement results. The calculated positive electrode intensity ratio is shown in Table 1.

[0215] In addition to performing surface analysis on the negative electrode 22 instead of the positive electrode 21, the intensities of two XPS spectra (third oxygen spectrum and fourth oxygen spectrum) were measured using the same procedure, and the negative electrode intensity ratio was calculated based on the measurement results. The calculated results of the negative electrode intensity ratio are shown in Table 1.

[0216] [Performance Evaluation]

[0217] The performance of the secondary battery (expansion characteristics, cycle characteristics, and load characteristics) was evaluated, and the results are shown in Tables 1 and 2.

[0218] (Expansion characteristics)

[0219] First, the secondary battery was charged at room temperature (temperature = 23°C), and its thickness (thickness before storage) was measured. Under these conditions, it was charged at a constant current of 0.5C until the voltage reached 2.7V, and then charged at a constant voltage of 2.7V until the current reached 0.01C. It should be noted that 0.01C refers to the current value required to fully discharge the battery capacity within 100 hours. Next, the charged secondary battery was stored in a high-temperature environment (temperature = 60°C) for one month, and its thickness (thickness after storage) was measured again. Finally, the expansion rate (%) was calculated as: [(thickness after storage - thickness before storage) / thickness before storage] × 100.

[0220] (Cyclic Characteristics)

[0221] First, the discharge capacity (discharge capacity of the first cycle) was measured by charging and discharging the secondary battery once at room temperature (temperature = 23°C). Under these conditions, constant current charging was performed at 0.5C until the voltage reached 2.7V, followed by constant voltage charging at that 2.7V until the current reached 0.01C. Then, constant current discharging was performed at 0.5C until the voltage reached 1.0V.

[0222] Next, the secondary battery was repeatedly charged and discharged in a high-temperature environment (temperature = 50°C) until a total of 100 cycles were reached, and the discharge capacity (discharge capacity of the 100th cycle) was measured. The charge and discharge conditions were the same as those of the first cycle, except that the charging current was changed to 6.0C and the discharging current was changed to 2.0C. It should be noted that 6.0C refers to the current value at which the battery capacity is fully discharged in 1 / 6 hour, and 2.0C refers to the current value at which the battery capacity is fully discharged in 0.5 hours.

[0223] Finally, the cycle maintenance rate (%) was calculated as (discharge capacity of the 100th cycle / discharge capacity of the 1st cycle) × 100.

[0224] (Load characteristics)

[0225] First, the discharge capacity (discharge capacity of the first cycle) was measured by performing one charge-discharge cycle on the secondary battery at room temperature (temperature = 23°C). Under these conditions, constant current charging was performed at 0.5C until the voltage reached 2.7V, followed by constant voltage charging at that 2.7V until the current reached 0.01C. Then, constant current discharging was performed at 1.0C until the voltage reached 1.0V. It should be noted that 1.0C refers to the current value required to fully discharge the battery within one hour.

[0226] Next, the discharge capacity (discharge capacity of the second cycle) was determined by subjecting the secondary battery to another charge-discharge cycle in the same environment. The charge-discharge conditions were the same as those of the first cycle, except that the discharge current was changed to 6.0C.

[0227] Finally, the load maintenance rate (%) was calculated as follows: (Discharge capacity of the second cycle (discharge current = 6.0C) / Discharge capacity of the first cycle (discharge current = 1.0C)) × 100.

[0228] [Table 1]

[0229]

[0230] [Table 2]

[0231]

[0232] [Inspection]

[0233] As shown in Tables 1 and 2, when the positive electrode 21 contains lithium nickel composite oxide, the negative electrode 22 contains lithium titanium composite oxide, and the electrolyte contains carboxylic acid ester (Examples 1-5, 9-12 and Comparative Examples 1-8), the expansion rate, cycle retention rate and load retention rate vary according to the positive electrode strength ratio and the negative electrode strength ratio, respectively.

[0234] Specifically, when the appropriate condition of a positive electrode strength ratio of 0.30 to 0.80 is not met (Comparative Examples 1 to 5), a compromise relationship arises where any one of the expansion rate, cycle maintenance rate, and load maintenance rate improves while the others deteriorate. Therefore, the expansion rate, cycle maintenance rate, and load maintenance rate are not all improved.

[0235] Furthermore, even when the appropriate condition of a negative electrode strength ratio of 0.82 to 1.35 is not met (Comparative Examples 6 to 8), a compromise still occurs, and therefore the expansion rate, cycle maintenance rate, and load maintenance rate are not all improved.

[0236] In contrast, when the positive electrode strength ratio meets the above-mentioned appropriate conditions and the negative electrode strength ratio meets the above-mentioned appropriate conditions (Examples 1-5, 9-12), the above-mentioned compromise relationship is broken, and thus the expansion rate, cycle maintenance rate and load maintenance rate are all improved.

[0237] In particular, when the positive electrode strength ratio and negative electrode strength ratio meet appropriate conditions (Examples 6-8), the same results can be obtained even if the type of carboxylic acid ester is changed.

[0238] It should be noted that when the electrolyte contains chain carbonates instead of carboxylic acid esters (Comparative Examples 9 and 10), due to the aforementioned trade-offs, the expansion rate, cycle maintenance rate, and load maintenance rate are not all improved.

[0239] In addition, when the electrolyte contained other carboxylic acid esters (Comparative Example 11), the same tendency was observed as when the electrolyte contained chain carbonates (Comparative Examples 9 and 10), so the expansion rate, cycle maintenance rate and load maintenance rate were not all improved.

[0240] <Examples 13-18>

[0241] As shown in Table 3, secondary batteries were fabricated using the same procedures, except for a change in the Ni ratio, and their performance (expansion characteristics, cycle characteristics, and load characteristics) was evaluated. Here, the battery capacity (Ah) was further measured as a measure of the secondary battery's performance.

[0242] [Table 3]

[0243]

[0244] As shown in Table 3, when the Ni ratio is 80% or higher (Examples 4, 15-18), compared with the case where the Ni ratio is less than 80% (Examples 13, 14), the battery capacity increases while achieving a low expansion rate, high cycle retention rate, and high load retention rate.

[0245] <Examples 19-22>

[0246] As shown in Table 4, except for changing the content of carboxylic acid esters in the solvent, a secondary battery was fabricated using the same steps, and the performance (expansion characteristics, cycle characteristics, and load characteristics) of the secondary battery was evaluated.

[0247] [Table 4]

[0248]

[0249] As shown in Table 4, when the content of carboxylic acid ester is 50% by weight or more (Examples 4, 20-22), compared with the case where the content of carboxylic acid ester is less than 50% by weight (Example 19), the expansion rate is suppressed within the allowable range, while the cycle retention rate and load retention rate are further increased. In this case, in particular, when the content of carboxylic acid ester is 80% by weight or less (Examples 4, 20, 21), the expansion rate is significantly suppressed.

[0250] [Summarize]

[0251] As shown in Tables 1-4, the positive electrode 21 contains lithium-nickel composite oxide, the negative electrode 22 contains lithium-titanium composite oxide, and the electrolyte contains carboxylic acid esters such as ethyl acetate. Furthermore, the surface analysis results (positive electrode strength ratio) of the XPS-treated positive electrode 21 meet appropriate conditions (positive electrode strength ratio = 0.30–0.80). When the surface analysis results (negative electrode strength ratio) of the XPS-treated negative electrode 22 meet appropriate conditions (negative electrode strength ratio = 0.82–1.35), the expansion rate decreases, while the cycle retention rate and load retention rate increase. Therefore, the expansion rate, cycle retention rate, and load retention rate are all improved, resulting in excellent expansion characteristics, excellent cycle characteristics, and excellent load characteristics in the secondary battery.

[0252] The above description of the present invention, while illustrating one implementation method and example, does not limit the structure of the present invention to the structure described in one implementation method and example, and therefore allows for various modifications.

[0253] Specifically, the case of a laminated film type battery structure for secondary batteries is described, but this battery structure is not particularly limited, so it can be other battery structures such as cylindrical, square, coin-shaped, and button-shaped.

[0254] Furthermore, although the case of a wound battery element structure has been described, the structure of the battery element is not particularly limited, and other element structures such as a stacked type and a repeatedly folded type can also be used. In the stacked type, the electrodes (positive and negative electrodes) are stacked, and in the repeatedly folded type, the electrodes (positive and negative electrodes) are folded into a Z-shape.

[0255] Furthermore, while the use of lithium as the electrode reactant has been described, it is not particularly limited. Specifically, as mentioned above, the electrode reactant can be other alkali metals such as sodium and potassium, or alkaline earth metals such as beryllium, magnesium, and calcium. Additionally, other light metals such as aluminum can also be used as the electrode reactant.

[0256] The effects described in this specification are merely illustrative, and therefore the effects of this technology are not limited to those described herein. Thus, other effects can also be obtained with this technology.

Claims

1. A secondary battery, comprising: The positive electrode contains lithium-nickel composite oxide; The negative electrode contains lithium-titanium composite oxide; and The electrolyte contains carboxylic acid esters. The carboxylic acid ester contains at least one of ethyl acetate, propyl acetate, ethyl propionate, and propyl propionate. In the surface analysis of the cathode using X-ray photoelectron spectroscopy, a first oxygen spectrum with peaks in the binding energy range of 528 eV to 531 eV and a second oxygen spectrum with peaks in the binding energy range of 531 eV to 535 eV were detected. The ratio of the intensity of the first oxygen spectrum to the intensity of the second oxygen spectrum is greater than 0.30 and less than 0.

80. In the surface analysis of the negative electrode using the X-ray photoelectron spectroscopy method, a third oxygen spectrum with peaks in the binding energy range of 528 eV and 531 eV, and a fourth oxygen spectrum with peaks in the binding energy range of 531 eV and 535 eV were detected. The ratio of the intensity of the third oxygen spectrum to the intensity of the fourth oxygen spectrum is greater than 0.82 and less than 1.

35. The electrolyte contains a solvent and an electrolyte salt. The solvent contains the carboxylic acid ester. The content of the carboxylic acid ester in the solvent is more than 50% by weight and less than 90% by weight.

2. The secondary battery according to claim 1, wherein, The lithium-nickel composite oxide contains a compound represented by the following formula (1), Yes x You (1-y) M1 y O2…(1), M1 is at least one of the elements belonging to Groups 2 to 15 of the long-period periodic table, excluding Ni, and x and y satisfy 0.8≤x≤1.2 and 0≤y<1.

0.

3. The secondary battery according to claim 2, wherein, The ratio of the number of moles of Ni to the sum of the number of moles of Ni and the number of moles of M1 is more than 80%.

4. The secondary battery according to claim 1, wherein, The solvent also contains cyclic carbonates.

5. The secondary battery according to any one of claims 1 to 4, wherein, The positive electrode includes: The positive electrode active material layer contains the lithium-nickel composite oxide; and The positive electrode coating, disposed on the surface of the positive electrode active material layer, contains oxygen as a constituent element. The cathode coating was analyzed in the surface analysis of the cathode using the X-ray photoelectron spectroscopy method.

6. The secondary battery according to any one of claims 1 to 4, wherein, The lithium-titanium composite oxide contains at least one of the compounds represented by formulas (2), (3), and (4) respectively. Li[Li x M2 (1-3x) / 2 Ti (3+x) / 2 ]O4…(2), M2 is at least one of Mg, Ca, Cu, Zn, and Sr, and x satisfies 0 ≤ x ≤ 1 / 3. It[It y M3 1-3y Small 1+2y ]O4…(3), M3 is at least one of Al, Sc, Cr, Mn, Fe, Ga, and Y, where y satisfies 0 ≤ y ≤ 1 / 3. Li[Li 1 / 3 M4 z You (5 / 3)-z ]O4…(4), M4 is at least one of V, Zr, and Nb, and z satisfies 0 ≤ z ≤ 2 / 3.

7. The secondary battery according to any one of claims 1 to 4, wherein, The negative electrode includes: The negative electrode active material layer contains the lithium-titanium composite oxide; and The negative electrode coating, disposed on the surface of the negative electrode active material layer, contains oxygen as a constituent element. The negative electrode coating was analyzed in the surface analysis of the negative electrode using the X-ray photoelectron spectroscopy method.

8. The secondary battery according to any one of claims 1 to 4, wherein, The secondary battery also has a flexible outer packaging component for housing the positive electrode, the negative electrode, and the electrolyte.

9. The secondary battery according to any one of claims 1 to 4, wherein, The secondary battery is a lithium-ion secondary battery.

Images