Secondary battery

Abstract

The secondary battery comprises: a positive electrode containing a positive electrode active material layer comprising a layered rock salt type lithium-nickel composite oxide represented by formula (1) below; a negative electrode containing a lithium-titanium composite oxide; and an electrolyte comprising a dinitrile compound and a carboxylic acid ester. The ratio of the capacity of the positive electrode per unit area to the capacity of the negative electrode per unit area is 100% or more and 120% or less. When analyzing the positive electrode active material layer using X-ray photoelectron spectroscopy, the ratio X of the atomic concentration of Al to the atomic concentration of Ni satisfies the condition represented by formula (2) below. When analyzing the positive electrode active material layer inside the positive electrode active material layer (depth = 100 nm) using X-ray photoelectron spectroscopy, the ratio Y of the atomic concentration of Al to the atomic concentration of Ni satisfies the condition represented by formula (3) below. The ratio Z of X to Y satisfies the condition represented by formula (4) below. Li a Ni 1‑b‑c‑d Co b Al c M d O e …(1)(M is at least one of Fe, Mn, Cu, Zn, Cr, V, Ti, Mg and Zr. a, b, c, d and e satisfy 0.8<a<1.2, 0.06≤b≤0.18, 0.015≤c≤0.05, 0≤d≤0.08, 0<e<3, 0.1≤(b+c+d)≤0.22 and 4.33≤(1-b-c-d) / b≤15.0.) 0.30≤X≤0.70…(2)0.16≤Y≤0.37…(3)1.30≤Z≤2.52…(4).

Description

Technical Field

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

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

[0003] Specifically, in order to obtain excellent thermal stability, a layer containing LiAlO2 is provided on the surface of the lithium transition metal composite oxide particles, and Al from the LiAlO2 is dissolved in the vicinity of the surface of the lithium transition metal composite oxide particles (for example, see Patent Document 1).

[0004] Furthermore, to improve low-temperature output characteristics, the operating voltage of the negative electrode is 1.2V or higher relative to the lithium potential, and the electrolyte contains carboxylic acid esters such as methyl acetate (see, for example, Patent Documents 2 and 3). To suppress secondary battery expansion, the negative electrode contains spinel-type lithium titanate, and the electrolyte contains ethyl acetate (see, for example, Patent Document 4). To improve electrochemical characteristics over a wider temperature range, the negative electrode contains lithium titanate as the negative electrode active material, and the electrolyte contains isocyanate compounds (see, for example, Patent Document 5). To reduce gas generation during high-temperature use, the negative electrode contains titanium oxide, and the electrolyte contains dinitrile compounds (see, for example, Patent Document 6).

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2010-129471

[0008] Patent Document 2: Japanese Patent Application Publication No. 2010-205563

[0009] Patent Document 3: International Publication No. 2009 / 110490

[0010] Patent Document 4: Japanese Patent Application Publication No. 2013-229341

[0011] Patent Document 5: International Publication No. 2015 / 030190

[0012] Patent Document 6: International Publication No. 2015 / 033620

[0013] Various studies have been conducted to improve the battery characteristics of secondary batteries, but since their battery characteristics are still insufficient, there is room for improvement.

[0014] This technology was developed in view of the above-mentioned problems, and its purpose is to provide a secondary battery that can achieve excellent battery characteristics. Summary of the Invention

[0015] One embodiment of the secondary battery of this technology comprises: a positive electrode containing a positive electrode active material layer comprising a layered rock salt type lithium-nickel composite oxide represented by formula (1) below; a negative electrode containing a lithium-titanium composite oxide; and an electrolyte comprising a dinitrile compound and a carboxylic acid ester. The ratio of the capacity of the positive electrode per unit area to the capacity of the negative electrode per unit area is 100% or more and 120% or less. When the positive electrode active material layer is analyzed by X-ray photoelectron spectroscopy, the ratio X of the atomic concentration of Al to the atomic concentration of Ni satisfies the condition represented by formula (2) below. When the positive electrode active material layer is analyzed by X-ray photoelectron spectroscopy, the ratio Y of the atomic concentration of Al to the atomic concentration of Ni satisfies the condition represented by formula (3) below. The ratio Z of X to Y satisfies the condition represented by formula (4) below.

[0016] Li a Ni 1-b-c-d Co b Al c M d O e …(1)

[0017] (M is at least one of Fe, Mn, Cu, Zn, Cr, V, Ti, Mg, and Zr. a, b, c, d, and e satisfy 0.8 < a < 1.2, 0.06 ≤ b ≤ 0.18, 0.015 ≤ c ≤ 0.05, 0 ≤ d ≤ 0.08, 0 < e < 3, 0.1 ≤ (b + c + d) ≤ 0.22, and 4.33 ≤ (1 - bcd) / b ≤ 15.0.)

[0018] 0.30≤X≤0.70…(2)

[0019] 0.16≤Y≤0.37…(3)

[0020] 1.30≤Z≤2.52…(4)

[0021] It should be noted that the details of the measurement steps for the ratio of the positive electrode capacity to the negative electrode capacity, as well as the analysis steps for the positive electrode active material layer using X-ray photoelectron spectroscopy (the determination steps for the ratios of X, Y, and Z), will be described later.

[0022] According to one embodiment of the present technology, the secondary battery comprises a positive electrode (positive electrode active material layer) containing a layered rock-salt type lithium-nickel composite oxide, a negative electrode containing a lithium-titanium composite oxide, and an electrolyte containing a dinitrile compound and a carboxylic acid ester. Furthermore, the ratio of the positive electrode capacity to the negative electrode capacity satisfies the aforementioned conditions, and the analysis results (ratio X, ratio Y, and ratio Z) of the positive electrode active material layer using X-ray photoelectron spectroscopy also satisfy the aforementioned conditions. Therefore, excellent battery characteristics can be obtained.

[0023] It should be noted that the effects of this technology are not limited to those described herein, but can also be any of the series of effects related to this technology described later. Attached Figure Description

[0024] Figure 1 This is a perspective view showing the structure of a secondary battery in one embodiment of the present technology.

[0025] Figure 2 It means Figure 1 The diagram shows a cross-sectional view of the structure of the battery element.

[0026] Figure 3 It is an enlarged representation Figure 2 The cross-sectional view of the positive electrode structure shown.

[0027] Figure 4 This is a block diagram illustrating the structure of a secondary battery application example. Detailed Implementation

[0028] Hereinafter, an embodiment of the present technology will be described in detail with reference to the accompanying drawings. It should be noted that the order of description is as follows.

[0029] 1. Secondary battery

[0030] 1-1. Structure

[0031] 1-2.Physical properties

[0032] 1-3. Actions

[0033] 1-4. Manufacturing Method

[0034] 1-5. Functions and Effects

[0035] 2. Variations

[0036] 3. Uses of secondary batteries

[0037] <1. Secondary Battery>

[0038] First, a secondary battery according to one embodiment of this technology will be described.

[0039] The secondary battery described herein is a secondary battery in which battery capacity is obtained by the intercalation and deintercalation of electrode reactants. It includes a positive electrode, a negative electrode, and an electrolyte in liquid form. In this secondary battery, to prevent the electrode reactants from depositing on the surface of the negative electrode during charging, the charging capacity of the negative electrode is greater than the discharging capacity of the positive electrode. That is, the electrochemical capacity per unit area of ​​the negative electrode is set to be greater than the electrochemical capacity per unit area of ​​the positive electrode.

[0040] 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.

[0041] 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.

[0042] <1-1. Structure>

[0043] Figure 1 The three-dimensional structure of a secondary battery is shown. Figure 2 It shows Figure 1 The cross-sectional structure of the battery element 20 is shown. Additionally, Figure 1 The outer membrane 10 and the battery element 20 are shown in a state where they are separated from each other. Figure 2 Only a portion of battery element 20 is shown.

[0044] like Figure 1 as well as Figure 2 As shown, the secondary battery includes an outer casing 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 soft) outer casing (outer casing 10) to house the battery element 20.

[0045] [Exterior film]

[0046] like Figure 1 As shown, the outer membrane 10 is a flexible outer component that houses the battery element 20 (i.e., the positive electrode 21, negative electrode 22, and electrolyte, etc., described later) and has a pouch-like structure.

[0047] Here, the outer film 10 is a film-shaped component that can be folded in the direction of arrow R (single-dot line). A recess 10U (so-called deep drawing portion) for accommodating the battery element 20 is provided on the outer film 10.

[0048] The structure (material and number of layers, etc.) of the outer membrane 10 is not particularly limited, so it can be a single-layer membrane or a multi-layer membrane.

[0049] Here, the outer film 10 is a laminated film consisting of three layers: a welding layer, a metal layer, and a surface protective layer, layered sequentially from the inside. When the outer film 10 is folded, the outer periphery portions of the opposing outer films 10 (welding layers) are bonded together. Thus, the outer film 10 has a pouch-like structure capable of sealing the battery element 20 inside. The welding layer contains a polymer compound such as polypropylene. The metal layer contains a metallic material such as aluminum. The surface protective layer contains a polymer compound such as nylon.

[0050] [Sealing film]

[0051] like Figure 1 As shown, sealing membranes 41 and 42 are sealing components used to prevent external air from entering the interior of the outer membrane 10. Sealing membrane 41 is inserted between the outer membrane 10 and the positive lead 31, and sealing membrane 42 is inserted between the outer membrane 10 and the negative lead 32. Alternatively, one or both of sealing membranes 41 and 42 may be omitted.

[0052] Specifically, the sealing film 41 contains a polymer compound such as a polyolefin that has a sealing effect on the positive electrode lead 31, and the polyolefin is polypropylene or the like.

[0053] The structure of the sealing membrane 42 is the same as that of the sealing membrane 41, except that it provides a tight seal for the negative electrode lead 32. That is, the sealing membrane 42 contains a polymer compound such as polyolefin that provides a tight seal for the negative electrode lead 32.

[0054] [Battery Components]

[0055] like Figure 1 as well as Figure 2 As shown, the battery element 20 is a power generation element housed inside the outer membrane 10, and includes a positive electrode 21, a negative electrode 22, a separator 23, and an electrolyte (not shown).

[0056] Here, the battery element 20 is a so-called wound electrode body. Therefore, 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 (an imaginary axis extending along the Y-axis direction). That is, the positive electrode 21 and the negative electrode 22 are wound opposite each other with a separator 23 in between.

[0057] Because the battery element 20 has a flat, three-dimensional shape, the shape of the cross-section (along the XZ plane) of the battery element 20 intersecting the aforementioned winding axis is a flat shape defined by the major axis and the minor axis. The major axis is an imaginary axis extending in the X-axis direction and having a length greater than the minor axis, and the minor axis is an imaginary axis extending in the Z-axis direction intersecting the X-axis direction and having a length less than the major axis. Here, the cross-sectional shape of the battery element 20 is a flat, approximately elliptical shape.

[0058] Here, in battery element 20, the capacity ratio of the positive electrode 21 to the negative electrode 22 is optimized. Specifically, the capacity C1 (mAh / cm²) of the positive electrode 21 per unit area is optimized. 2 The capacity C2 (mAh / cm²) relative to the negative electrode 22 per unit area 2 The capacity ratio (CR) is 100%–120%. This is because a high energy density can be obtained. The capacity ratio CR is calculated using CR(%) = (capacity C1 / capacity C2) × 100.

[0059] To determine the capacity ratio CR, calculate the capacities C1 and C2 respectively by following the steps described below, and then calculate the capacity ratio CR.

[0060] First, the positive electrode 21 and the negative electrode 22 are recovered by disassembling the secondary battery.

[0061] Next, a test secondary battery (coin type) was fabricated using positive electrode 21 as the test electrode and a lithium metal plate as the counter electrode. As described later, the positive electrode 21 contains lithium nickel composite oxide as the positive electrode active material.

[0062] Next, the capacity (mAh) of the positive electrode 21 was measured by charging and discharging the secondary battery used in the experiment. During charging, a constant current of 0.1C was applied until the voltage reached 4.3V, followed by constant voltage charging at that 4.3V until the total charging time reached 15 hours. During discharging, a constant current of 0.1C was applied until the voltage reached 2.5V. 0.1C refers to the current value required to fully discharge the battery's theoretical capacity within 10 hours.

[0063] Next, based on the area (cm²) of the positive electrode 21 2 ), calculate capacity C1 (mAh / cm³) 2 The capacity C1 is calculated by C1 = capacity of positive electrode 21 / area of ​​positive electrode 21.

[0064] Next, a test secondary battery (coin type) was fabricated using negative electrode 22 as the test electrode and a lithium metal plate as the counter electrode. As described later, negative electrode 22 contains lithium titanium composite oxide as the negative electrode active material.

[0065] Next, the capacity (mAh) of the negative electrode 22 was measured by charging and discharging the secondary battery used in the experiment. During charging, a constant current of 0.1C was used to charge until the voltage reached 2.7V, and then a constant voltage of 2.7V was used to charge until the total charging time reached 15 hours. During discharging, a constant current of 0.1C was used to discharge until the battery voltage reached 1.0V.

[0066] Next, based on the area (cm²) of the negative electrode 22 2 ), calculate capacity C2 (mAh / cm³) 2 The capacity C2 is calculated by C2 = capacity of negative electrode 22 / area of ​​negative electrode 22.

[0067] Finally, based on capacities C1 and C2, the capacity ratio CR is calculated. As mentioned above, this capacity ratio CR is calculated using CR = (capacity C1 / capacity C2) × 100.

[0068] (positive electrode)

[0069] like Figure 2 As shown, the positive electrode 21 includes a positive electrode active material layer 21B. Here, the positive electrode 21 includes a positive electrode active material layer 21B and a positive electrode current collector 21A supporting the positive electrode active material layer 21B.

[0070] 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.

[0071] The positive electrode active material layer 21B contains a positive electrode active material capable of lithium insertion and extraction, and is disposed on both sides of the positive electrode current collector 21A. Alternatively, the positive electrode active material layer 21B may be disposed on only one side of the positive electrode current collector 21A on the side 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. The method for forming the positive electrode active material layer 21B is not particularly limited; specifically, it may be a coating method, etc.

[0072] Specifically, the positive electrode active material layer 21B contains one or more of the layered rock salt type lithium-nickel composite oxides represented by the following formula (1) as the positive electrode active material. This is because a high energy density can be obtained.

[0073] Li a Ni 1-b-c-d Co b Al c M d O e …(1)

[0074] (M is at least one of Fe, Mn, Cu, Zn, Cr, V, Ti, Mg, and Zr. a, b, c, d, and e satisfy 0.8 < a < 1.2, 0.06 ≤ b ≤ 0.18, 0.015 ≤ c ≤ 0.05, 0 ≤ d ≤ 0.08, 0 < e < 3, 0.1 ≤ (b + c + d) ≤ 0.22, and 4.33 ≤ (1 - bcd) / b ≤ 15.0.)

[0075] From the conditions for a to e shown in equation (1), it can be seen that the lithium-nickel composite oxide is a composite oxide containing Li, Ni, Co, and Al as constituent elements, and has a layered rock salt-type crystal structure. That is, the lithium-nickel composite oxide contains two transition metal elements (Ni and Co) as constituent elements.

[0076] Furthermore, given the range of possible values ​​for d (0 ≤ d ≤ 0.08), lithium-nickel composite oxides can also contain an additional element M as a constituent element. The additional element M can be any one or more of the aforementioned Fe, Mn, Cu, Zn, Cr, V, Ti, Mg, and Zr; there are no particular limitations.

[0077] In particular, from the range of possible values ​​for (b+c+d) (0.1 ≤ (b+c+d) ≤ 0.22), we know that the range of possible values ​​for (1-bcd) is 0.78 ≤ (1-bcd) ≤ 0.9. Therefore, the lithium-nickel composite oxide contains Ni as the main component of two transition metal elements (Ni and Co). This is because a high energy density can be obtained.

[0078] Furthermore, the range of possible values ​​for (1-bcd) / b (4.33 ≤ (1-bcd) / b ≤ 15.0) indicates that in lithium-nickel composite oxides containing two transition metal elements (Ni and Co) as constituent elements, the molar ratio of Ni (1-bcd) relative to the molar ratio of Co (b) is sufficiently large. That is, the ratio of the molar ratio of Ni to the molar ratio of Co (NC ratio = (1-bcd) / b) is sufficiently large within an appropriate range. This is because, while ensuring energy density, the discharge capacity is not easily reduced even with repeated charge-discharge cycles. It should be noted that the NC ratio value is rounded to the third decimal place.

[0079] Here, since the molar ratio (d) of the added element M satisfies d≥0, the lithium-nickel composite oxide may or may not contain the added element M as a constituent element. Since d>0, the lithium-nickel composite oxide preferably contains the added element M as a constituent element. This is because lithium ions can be easily and smoothly input and output in the positive electrode active material (lithium-nickel composite oxide) during charging and discharging.

[0080] The specific composition of the lithium-nickel composite oxide is not particularly limited as long as it meets the conditions shown in equation (1). The specific composition of the lithium-nickel composite oxide will be described in detail in the examples described later.

[0081] It should be noted that, in addition to the lithium-nickel composite oxides mentioned above, the positive electrode active material may also include any one or more other substances capable of lithium intercalation / deintercalation. The types of other substances are not particularly limited; specifically, they include lithium compounds, etc. Furthermore, the lithium compounds described here do not include the lithium-nickel composite oxides already described.

[0082] Lithium compounds are a general term for compounds containing lithium as a constituent element. More specifically, they are compounds containing lithium and one or more transition metal elements as constituent elements. The types of lithium compounds are not particularly limited, but specifically include oxides, phosphoric acid compounds, silicate compounds, and borate compounds. Specific examples of oxides are LiNiO2, LiCoO2, and LiMn2O4, and specific examples of phosphoric acid compounds are LiFePO4 and LiMnPO4.

[0083] The positive electrode binder includes any one or more of synthetic rubber and polymer compounds. The synthetic rubber is styrene-butadiene rubber, etc., and the polymer compound is polyvinylidene fluoride, etc. The positive electrode conductive agent includes any one or more of conductive materials such as carbon materials, including graphite, carbon black, acetylene black, and Ketjen black, etc. Alternatively, the conductive material can also be a metallic material or a polymer compound, etc.

[0084] Here, in order to improve the battery characteristics of the secondary battery, the physical properties of the positive electrode 21 (positive electrode active material layer 21B), which contains the positive electrode active material (lithium-nickel composite oxide), meet the specified physical property conditions. Details of these physical property conditions will be described later.

[0085] (negative electrode)

[0086] like Figure 2 As shown, the negative electrode 22 includes a negative electrode current collector 22A and a negative electrode active material layer 22B.

[0087] 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.

[0088] The negative electrode active material layer 22B comprises one or more negative electrode active materials capable of lithium insertion / extraction, and is disposed on both sides of the negative electrode current collector 22A. Alternatively, the negative electrode active material layer 22B may be disposed on only one side of the negative electrode current collector 22A on the side opposite the positive electrode 21 to the negative electrode 22. Furthermore, the negative electrode active material layer 22B may also comprise a negative electrode binder and a negative electrode conductive agent. Details regarding the negative electrode binder and negative electrode conductive agent are the same as those regarding the positive electrode binder and positive electrode conductive agent. The method for forming the negative electrode active material layer 22B is not particularly limited; specifically, it may be one or more of the following: coating method, vapor phase method, liquid phase method, spraying method, and firing method (sintering method).

[0089] Specifically, the negative electrode active material layer contains any one or more of lithium-titanium composite oxides as the negative electrode active material. As mentioned above, the "lithium-titanium composite oxide" is a general term for oxides containing lithium and titanium as constituent elements, and has a spinel-type crystal structure. This is because the decomposition reaction of the electrolyte in the negative electrode 22 can be suppressed, thereby suppressing the generation of gas due to the decomposition reaction of the electrolyte.

[0090] 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 must be one or more elements belonging to groups 2 to 15 of the long-period periodic table (excluding titanium). Furthermore, oxides containing lithium, titanium, and nickel as constituent elements are not lithium-nickel composite oxides, but rather lithium-titanium composite oxides.

[0091] More specifically, the lithium-titanium composite oxide contains any one or more of the compounds represented by formulas (5), (6), and (7) below. M1 in formula (5) is a metallic element capable of becoming a divalent ion. M2 in formula (6) is a metallic element capable of becoming a trivalent ion. M3 in formula (7) is a metallic element capable of becoming a tetravalent ion. This is because the decomposition reaction of the electrolyte in the negative electrode 22 can be sufficiently suppressed, and therefore the generation of gas due to the decomposition reaction of the electrolyte can also be sufficiently suppressed.

[0092] Li[Li x M1 (1-3x) / 2 Ti (3+x) / 2 ]O4…(5)

[0093] (M1 is at least one of Mg, Ca, Cu, Zn, and Sr. x satisfies 0 ≤ x ≤ 1 / 3.)

[0094] Li[Li y M21-3y Ti 1+2y ]O4…(6)

[0095] (M2 is at least one of Al, Sc, Cr, Mn, Fe, Ga, and Y. y satisfies 0 ≤ y ≤ 1 / 3.)

[0096] Li[Li 1 / 3 M3 z Ti (5 / 3)-z ]O4…(7)

[0097] (M3 is at least one of V, Zr, and Nb. z satisfies 0 ≤ z ≤ 2 / 3.)

[0098] From the range of possible values ​​for x in equation (5), it can be seen that the lithium-titanium composite oxide shown in equation (5) may contain other elements (M1) as constituent elements, or it may not contain other elements (M1) as constituent elements. From the range of possible values ​​for y in equation (6), it can be seen that the lithium-titanium composite oxide shown in equation (6) may contain other elements (M2) as constituent elements, or it may not contain other elements (M2) as constituent elements. From the range of possible values ​​for z in equation (7), it can be seen that the lithium-titanium composite oxide shown in equation (7) may contain other elements (M3) as constituent elements, or it may not contain other elements (M3) as constituent elements.

[0099] A specific example of the lithium-titanium composite oxide shown in equation (5) is Li 3.75 Ti 4.875 Mg 0.375 O 12 Etc. Specific examples of lithium-titanium composite oxides shown in equation (6) are LiCrTiO4, etc. Specific examples of lithium-titanium composite oxides shown in equation (7) are Li4Ti5O, etc. 12 and Li4Ti 4.95 Nb 0.05 O 12 wait.

[0100] It should be noted that the negative electrode active material only needs to include the aforementioned lithium-titanium composite oxide, and may also include any one or more other materials capable of lithium intercalation / deintercalation. The types of other materials are not particularly limited; specifically, they include carbon materials and metallic materials. Furthermore, the metallic materials described here do not include the lithium-titanium composite oxide already described.

[0101] Carbon materials include easily graphitized carbon, difficult-to-graphitize carbon, and graphite, which can be natural or artificial graphite. Metallic materials are materials containing one or more metallic and semi-metallic elements capable of forming alloys with lithium. The types of metallic and semi-metallic elements are not particularly limited; specifically, they include silicon and tin. These metallic materials can be monomers, alloys, compounds, mixtures of two or more of them, or materials containing two or more of their phases.

[0102] Specific examples of metallic materials include SiB4, SiB6, Mg2Si, Ni2Si, TiSi2, MoSi2, CoSi2, NiSi2, CaSi2, CrSi2, Cu5Si, FeSi2, MnSi2, NbSi2, TaSi2, VSi2, WSi2, ZnSi2, SiC, Si3N4, Si2N2O, and SiO. v (0<v≤2), LiSiO, SnO w (0<w≤2), SnSiO3, LiSnO, and Mg2Sn, etc. Among them, SiO v The value of v can satisfy 0.2 < v < 1.4.

[0103] It should be noted that when the positive electrode 21 and the negative electrode 22 are fabricated separately, the capacity ratio CR can be adjusted by changing the relationship between the amounts of the positive electrode active material and the negative electrode active material. More specifically, in the process of fabricating the positive electrode 21 and the negative electrode 22 separately, the capacity ratio CR can be adjusted by fixing the thickness of the positive electrode active material layer 21B and changing the thickness of the negative electrode active material layer 22B.

[0104] The "thickness of the negative electrode active material layer 22B" mentioned here refers to the total thickness of the negative electrode active material layer 22B. Therefore, since the negative electrode active material layer 22B is disposed on both sides of the negative electrode current collector 22A, when the negative electrode 22 contains two negative electrode active material layers 22B, the thickness of the negative electrode active material layer 22B is the sum of the thickness of one negative electrode active material layer 22B and the thickness of the other negative electrode active material layer 22B.

[0105] In this case, as described above, the capacity ratio CR is 100% to 120%. Therefore, even if the thickness of the negative electrode active material layer 22B is relatively thin, as will be explained later, the decomposition reaction of the electrolyte can be suppressed, thus suppressing the generation of gas due to the decomposition reaction of the electrolyte. The thickness of the negative electrode active material layer 22B is not particularly limited; specifically, it is 130 μm or less.

[0106] (Diaphragm)

[0107] like Figure 2As shown, the separator 23 is an insulating porous membrane located between the positive electrode 21 and the negative electrode 22, which allows lithium ions to pass through while preventing contact (short circuit) between the positive electrode 21 and the negative electrode 22. The separator 23 contains a polymer compound such as polyethylene.

[0108] (electrolyte)

[0109] The electrolyte permeates into each of the positive electrode 21, the negative electrode 22, and the membrane 23, and contains solvent and electrolyte salt.

[0110] The solvent includes any one or more non-aqueous solvents (organic solvents), and the electrolyte containing such non-aqueous solvents is called a non-aqueous electrolyte. Specifically, non-aqueous solvents include dinitrile compounds and carboxylic acid esters.

[0111] Dianitrile compounds are chain-like compounds with nitrile groups (-CN) at both ends, thus containing two nitrile groups. These dinitrile compounds enhance the antioxidant properties of carboxylic acid esters when used in combination with them.

[0112] There is no particular limitation on the types of dinitrile compounds. Specifically, they are compounds formed by two nitrile groups bonded together by a straight-chain alkylene group. Specific examples of dinitrile compounds include malononitrile (1 carbon atom), succinic anionitrile (2 carbon atom), glutaronitrile (3 carbon atom), adiponitrile (4 carbon atom), heptacyanide (5 carbon atom), and octanoic anionitrile (6 carbon atom). The carbon atom number in parentheses above refers to the number of carbon atoms in the alkylene group.

[0113] The alkylene group preferably has 2 to 4 carbon atoms, therefore the dinitrile compound is preferably any one or more of succinic anionibacterium, glutaronitrile, and adiponitrile. This is because the dinitrile compound improves the solubility and compatibility, and also significantly enhances the antioxidant properties of the carboxylic acid ester.

[0114] Carboxylic acid esters are straight-chain esters of saturated fatty acids. Specific examples of carboxylic acid esters include methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, and ethyl trimethylacetate.

[0115] The carboxylic acid ester is preferably one or both of ethyl propionate and propyl propionate. Because the decomposition reaction of the carboxylic acid ester can be sufficiently suppressed during charging and discharging, the generation of gas due to the decomposition reaction of the carboxylic acid ester can also be sufficiently suppressed.

[0116] Furthermore, the content of the dinitrile compound is set within a specified range relative to the content of the carboxylic ester. Specifically, the molar ratio (MR) of the dinitrile compound (R1) to the carboxylic ester (R2) is 1% to 4%. This is to optimize the content of the dinitrile compound relative to the carboxylic ester. Therefore, even when the dinitrile compound and the carboxylic ester are used together, the decomposition reaction of the carboxylic ester can be suppressed, and thus the generation of gas due to the decomposition reaction of the carboxylic ester can also be suppressed. This molar ratio MR is calculated by MR(%) = (molar R1 / molar R2) × 100.

[0117] The content of carboxylic acid ester in the solvent is not particularly limited, but is preferably 50% to 90% by weight. Because the decomposition reaction of the carboxylic acid ester can be sufficiently suppressed during charging and discharging, the generation of gas due to the decomposition reaction of the carboxylic acid ester can also be sufficiently suppressed.

[0118] It should be noted that the solvent only needs to contain the aforementioned dinitrile compounds and carboxylic acid esters, and may also contain any one or more other substances.

[0119] The types of other substances are not particularly limited, but specifically they include esters and ethers, and more specifically, carbonate compounds and lactone compounds. This is because it can improve the dissociation of electrolyte salts and result in high ion mobility.

[0120] 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.

[0121] Lactone compounds are lactones, etc. Specific examples of lactones are γ-butyrolactone and γ-pentanolactone, etc. It should be noted that ethers, in addition to the lactone compounds mentioned above, can also be 1,2-dimethoxyethane, tetrahydrofuran, 1,3-dioxolane, and 1,4-dioxane, etc.

[0122] In addition, other substances can be unsaturated cyclic carbonates, halocarbonates, sulfonates, phosphate esters, acid anhydrides, mononitrile compounds, and isocyanate compounds, etc. This is because they can improve the chemical stability of the electrolyte.

[0123] The electrolyte salt is any one or more of light metal salts such as lithium salts. The lithium salt includes 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.

[0124] There is no particular limitation on the content of the electrolyte salt, but specifically, it is 0.3 mol / kg to 3.0 mol / kg relative to the solvent. This is because high ionic conductivity can be obtained.

[0125] The steps for determining the composition of the electrolyte (including the molar ratio MR and the content of carboxylic acid esters in the solvent) are as follows.

[0126] When investigating the composition of components (solvents) in an electrolyte, one or more analytical methods, such as gas chromatography and high-speed liquid gas chromatography, are used to analyze the electrolyte. This allows determination of the types of solvents present in the electrolyte.

[0127] In investigating the content of components (solvents) in the electrolyte, firstly, the battery element 20 is recovered by disassembling the secondary battery, and then the electrolyte is recovered from the battery element 20. This electrolyte is used as a reference solution in subsequent processes. Next, the battery element 20, from which no electrolyte was recovered, is immersed in an organic solvent (dimethyl carbonate) (immersion time = 24 hours). Thus, the electrolyte immersed in the battery element 20 is extracted into the organic solvent, thereby obtaining an electrolyte extract. Finally, the electrolyte extract is analyzed using gas chromatography. In this case, the electrolyte recovered in the previous process is used as a reference solution. Furthermore, the peak areas of each component (each solvent contained in the electrolyte extract) are standardized based on the peak area of ​​propylene carbonate, thereby determining the residual amount of each component. Thus, the content of solvents contained in the electrolyte can be determined.

[0128] When investigating the content of carboxylic acid esters in the solvent, the content of the carboxylic acid ester is calculated based on the content of the solvent contained in the electrolyte. The content of the carboxylic acid ester is calculated using the formula: Carboxylic acid ester content (wt%) = (Weight of carboxylic acid ester / Weight of solvent) × 100. The "weight of solvent" is the sum of the weights of all solvents contained in the electrolyte.

[0129] In investigating the molar ratio MR, based on the content of solvents (dianitronic compounds and carboxylic esters) contained in the electrolyte, the molar number R1 of carboxylic esters and the molar number R2 of dinitrile compounds are determined, and then the molar ratio MR is calculated based on these molar numbers R1 and R2.

[0130] [Positive and negative leads]

[0131] like Figure 1 As shown, the positive lead 31 is the positive terminal connected to the battery element 20 (positive electrode 21) and is led out from the inside of the outer casing 10 to the outside. The positive lead 31 contains a conductive material such as aluminum, and the shape of the positive lead 31 is either a thin plate or a mesh.

[0132] like Figure 1 As shown, the negative electrode lead 32 is the negative terminal connected to the battery element 20 (negative electrode 22), and it extends from the inside of the outer casing 10 to the outside in the same direction as the positive electrode 21. The negative electrode lead 32 contains a conductive material such as copper, and the details of the shape of the negative electrode lead 32 are the same as the details of the shape of the positive electrode lead 31.

[0133] <1-2.Physical Properties>

[0134] In this secondary battery, as described above, in order to improve battery characteristics, the physical properties of the positive electrode 21 (positive electrode active material layer 21B) containing the positive electrode active material (lithium nickel composite oxide) meet the specified physical property conditions.

[0135] Specifically, the analytical results (physical properties) of the positive electrode active material layer 21B using X-ray photoelectron spectroscopy (XPS) simultaneously meet the three conditions described below (physical property conditions 1 to 3).

[0136] Before explaining the physical property conditions 1 to 3, the prerequisites for explaining these conditions will be explained.

[0137] Figure 3 Enlarged Figure 2 The cross-sectional structure of the positive electrode 21 is shown. Figure 3The positions P1 and P2 shown represent two analytical positions when using XPS to analyze the positive electrode active material layer 21B. When observing the positive electrode active material layer 21B from the surface in the depth direction (Z-axis direction), position P1 is the position on the surface of the positive electrode active material layer 21B. When observing the positive electrode active material layer 21B from the surface in the same direction, position P2 is the position inside the positive electrode active material layer 21B, more specifically, it is the position at a depth D of 100 nm from the surface of the positive electrode active material layer 21B (depth D = 100 nm).

[0138] [Physical Properties]

[0139] As described above, the positive electrode active material layer 21B contains a layered rock salt type lithium-nickel composite oxide as the positive electrode active material, and the lithium-nickel composite oxide contains Ni and Al as constituent elements.

[0140] In this case, when XPS analysis was performed on the positive electrode active material layer 21B, two XPS spectra (Ni2p3 / 2 spectrum and Al2s spectrum) were detected as results. The Ni2p3 / 2 spectrum is the XPS spectrum of Ni atoms in the lithium nickel composite oxide, and the Al2s spectrum is the XPS spectrum of Al atoms in the lithium nickel composite oxide.

[0141] Therefore, the atomic concentration (atomic %) of Ni was calculated based on the spectral intensity of the Ni2p3 / 2 spectrum, and the atomic concentration (atomic %) of Al was calculated based on the spectral intensity of the Al2s spectrum.

[0142] (Physical property condition 1)

[0143] On the surface of the positive electrode active material layer 21B (position P1), when XPS analysis is used on the positive electrode active material layer 21B, the ratio of the atomic concentration of Al to the atomic concentration of Ni, that is, the concentration ratio X (= atomic concentration of Al / atomic concentration of Ni), satisfies the condition expressed by the following equation (2).

[0144] 0.30≤X≤0.70…(2)

[0145] The concentration ratio X is a parameter representing the relationship between the amount of Ni atoms and the amount of Al atoms at position P1. As shown in equation (2), on the surface of the positive electrode active material layer 21B (position P1), the amount of Al atoms is appropriately reduced compared to the amount of Ni atoms.

[0146] (Physical property condition 2)

[0147] Inside the positive electrode active material layer 21B (position P2), when XPS analysis is used on the positive electrode active material layer 21B, the ratio of the atomic concentration of Al to the atomic concentration of Ni, that is, the concentration ratio Y (= atomic concentration of Al / atomic concentration of Ni), satisfies the condition expressed by the following equation (3).

[0148] 0.16≤Y≤0.37…(3)

[0149] The concentration ratio Y is a parameter representing the relationship between the amount of Ni atoms and the amount of Al atoms at position P2. As shown in equation (3), the amount of Al atoms in the interior (position P2) of the positive electrode active material layer 21B is appropriately reduced compared to the amount of Ni atoms. Furthermore, a comparison of property conditions 1 and 2 shows that the amount of Al atoms is appropriately increased at the surface (position P1) compared to the interior (position P2), and conversely, appropriately reduced inside (position P2) compared to the surface (position P1).

[0150] (Physical property condition 3)

[0151] Regarding the above concentration ratios X and Y, the ratio of concentration ratio X to concentration ratio Y, that is, the relative ratio Z (=concentration ratio X / concentration ratio Y), satisfies the condition expressed by the following equation (4).

[0152] 1.30≤Z≤2.52…(4)

[0153] The relative ratio Z is a parameter representing the relationship between the amount of Al atoms present at position P1 and the amount of Al atoms present at position P2. As shown in equation (4), in the positive electrode active material layer 21B, the amount of Al atoms gradually decreases from the surface (position P1) to the interior (position P2), thus creating an appropriate concentration gradient regarding the amount of Al atoms (atomic concentration).

[0154] (Reasons for satisfying physical property conditions 1-3)

[0155] The reason why physical property conditions 1 to 3 are satisfied simultaneously is that, while achieving high energy density, the reduction in discharge capacity and the generation of gas can be suppressed even with repeated charge and discharge. Furthermore, the lithium-ion input and output performance can be improved not only during the initial charge and discharge but also thereafter. It should be noted that the detailed reasons for simultaneously satisfying physical property conditions 1 to 3 will be described later.

[0156] [Analysis Steps]

[0157] The analytical procedures for the positive electrode active material layer 21B using XPS (the procedures for determining the concentration ratios X, Y, and relative ratio Z) are as follows.

[0158] First, the secondary battery is discharged, and then the positive electrode 21 (positive electrode active material layer 21B) is recovered by disassembling the secondary battery. Next, the positive electrode 21 is washed with pure water and then dried. Next, the positive electrode 21 is cut into rectangular shapes (10mm × 10mm) to obtain a sample for analysis.

[0159] Next, the sample was analyzed using an XPS analysis apparatus. In this case, the XPS analysis apparatus used was the PHI Quantera SXM scanning X-ray photoelectron spectroscopy analyzer manufactured by ULVAC-PHI Corporation. Furthermore, the analytical conditions were: light source = monochromatic Al Kα rays (1486.6 eV), vacuum level = 1 × 10⁻⁶. -9 Torr (approximately 133.3 × 10⁻⁶) - 9 Pa), analysis range (diameter) = 100 μm, analysis depth = several nm, presence or absence of neutralization gun = present.

[0160] Therefore, the Ni 2p3 / 2 spectrum and Al 2s spectrum were detected on the surface (position P1) of the positive electrode active material layer 21B, and the atomic concentrations (atomic %) of Ni and Al were calculated respectively. Based on these atomic concentrations, the concentration ratio X was calculated.

[0161] Next, repeat the concentration ratio X calculation process 20 times, and then calculate the average of the 20 concentration ratios X as the final concentration ratio X (used to determine whether the concentration ratio X meets property condition 1). Using the average value as the concentration ratio X is to improve the calculation accuracy (reproducibility) of the concentration ratio X.

[0162] Next, except for changing the analytical depth in the analytical conditions from several nm to 100 nm, and converting the accelerating voltage = 1 kV and sputtering rate = SiO2 to 6 nm to 7 nm as new analytical conditions, the same analytical steps as when calculating the concentration ratio X were performed. Therefore, within the positive electrode active material layer 21B (position P2), the atomic concentrations (atomic %) of Ni and Al were calculated, and the concentration ratio Y was calculated based on these Ni and Al atomic concentrations. In this case, by using the average value as the final concentration ratio Y, the accuracy (reproducibility) of the calculated concentration ratio Y can be improved.

[0163] Finally, the relative ratio Z is calculated based on the concentration ratios X and Y. Thus, the concentration ratios X and Y are determined, and the relative ratio Z is also determined.

[0164] <1-3. Actions>

[0165] During charging of the secondary battery, lithium is deintercalated from the positive electrode 21 in battery element 20 and intercalated into the negative electrode 22 via the electrolyte. Conversely, during discharging of the secondary battery, lithium is deintercalated from the negative electrode 22 in battery element 20 and intercalated into the positive electrode 21 via the electrolyte. During these charging and discharging processes, lithium is intercalated and deintercalated in an ionic state.

[0166] <1-4. Manufacturing Method>

[0167] After manufacturing the positive electrode active material (lithium-nickel composite oxide), the positive electrode active material is used to make a secondary battery.

[0168] [Manufacturing of positive electrode active materials]

[0169] The positive electrode active material (lithium-nickel composite oxide) is manufactured using the co-precipitation method and the firing method (one-time firing process) through the steps described below.

[0170] First, as raw materials, prepare the supply sources of Ni (nickel compounds) and Co (cobalt compounds).

[0171] Nickel compounds are any one or more compounds containing Ni as a constituent element, specifically oxides, carbonates, sulfates, and hydroxides. Details regarding cobalt compounds are the same as those for nickel compounds, except that Co is used instead of Ni as a constituent element.

[0172] Next, a mixed aqueous solution is prepared by adding a mixture of nickel and cobalt compounds to an aqueous solvent. The type of aqueous solvent is not particularly limited; specifically, it can be pure water, etc. Details regarding the type of aqueous solvent described here will be repeated later. The mixing ratio of the nickel and cobalt compounds (the molar ratio of Ni to Co) can be arbitrarily set according to the composition of the final positive electrode active material (lithium-nickel composite oxide).

[0173] Next, one or more alkali compounds are added to the mixed aqueous solution. The type of alkali compound is not particularly limited, but specifically, hydroxides, etc. As a result, multiple particulate precipitates are granulated (coprecipitation method), thus obtaining secondary particles (nickel-cobalt co-precipitated hydroxide) that are precursors for the synthesis of lithium-nickel composite oxides. In this case, as detailed in the examples described later, secondary particles designed using a Bi-model containing two types of particles (large-diameter particles and small-diameter particles) can also be used. The precursor is then washed with an aqueous solvent.

[0174] Next, as other raw materials, a supply source of Li (lithium compound) and a supply source of Al (aluminum compound) are prepared. In this case, a supply source of additional element M (additional compound) can also be prepared.

[0175] Lithium compounds are any one or more compounds containing Li as a constituent element, specifically oxides, carbonates, sulfates, and hydroxides. Details regarding aluminum compounds are the same as those regarding lithium compounds, except that Al replaces Li as a constituent element. Details regarding additional compounds are the same as those regarding lithium compounds, except that an additional element M replaces Li as a constituent element.

[0176] Next, a precursor mixture is obtained by mixing the precursor, lithium compound, and aluminum compound together. In this case, a precursor mixture containing an additional compound can also be obtained by mixing the precursor, etc. The mixing ratio of the precursor, lithium compound, and aluminum compound (molar ratio of Ni, Co, Li, and Al) can be arbitrarily set according to the composition of the final manufactured positive electrode active material (lithium-nickel composite oxide). The same applies to the mixing ratio of the additional compound (molar ratio of the additional element M).

[0177] Finally, the precursor mixture is sintered in an oxygen atmosphere (sintering method). The sintering temperature and time can be arbitrarily set. Thus, the precursor, lithium compound, and aluminum compound react with each other, thereby synthesizing a lithium-nickel composite oxide containing Li, Ni, Co, and Al as constituent elements. Therefore, a positive electrode active material (lithium-nickel composite oxide) can be obtained. Of course, if the precursor mixture contains additional compounds, a positive electrode active material (lithium-nickel composite oxide) further containing an additional element M as a constituent element can be obtained.

[0178] In this case, during the sintering process of the precursor mixture, Al atoms in the aluminum compound diffuse sufficiently into the interior of the precursor, thus creating a concentration gradient in which the amount of Al atoms present (atomic concentration) gradually decreases from the surface (position P1) to the interior (position P2).

[0179] It should be noted that when manufacturing positive electrode active material (lithium-nickel composite oxide), by changing the calcination temperature and other conditions during the calcination of the precursor mixture, the concentration ratios X and Y can be adjusted separately, and therefore the relative ratio Z can also be adjusted.

[0180] [Manufacturing of secondary batteries]

[0181] The above-described positive electrode active material (lithium-nickel composite oxide) is used to manufacture a secondary battery by following the steps described below.

[0182] (The production of the positive electrode)

[0183] A positive electrode active material (including lithium-nickel composite oxide), a positive electrode binder, and a positive electrode conductive agent are mixed to form a positive electrode mixture. This mixture is then added to an organic solvent to prepare a paste-like positive electrode mixture slurry. Subsequently, the positive electrode mixture slurry is coated onto both sides of the positive electrode current collector 21A to form a positive electrode active material layer 21B. It should be noted that the positive electrode active material layer 21B can be compressed and molded using a roller press or similar device. In this case, the positive electrode active material layer 21B can be heated, or the compression molding process can be repeated multiple times. Thus, the positive electrode active material layer 21B is formed on both sides of the positive electrode current collector 21A, forming the positive electrode 21.

[0184] (Making the negative electrode)

[0185] The negative electrode 22 is manufactured using the same steps as those described for the positive electrode 21. Specifically, a negative electrode active material (containing lithium-titanium composite oxide), a negative electrode binder, and a negative electrode conductive agent are mixed to form a negative electrode mixture. This mixture is then added to an organic solvent to prepare a paste-like negative electrode mixture slurry. This slurry is then coated onto both sides of the negative electrode current collector 22A to form a negative electrode active material layer 22B. Alternatively, the negative electrode active material layer 22B can be compressed and molded. Thus, the negative electrode active material layer 22B is formed on both sides of the negative electrode current collector 22A, thus producing the negative electrode 22.

[0186] (Electrolyte preparation)

[0187] An electrolyte salt is added to a solvent (including carboxylic acid esters), and then another solvent (dianitronidium compound) is added to that solvent. The electrolyte salt is thus dispersed or dissolved in the solvent, thereby preparing an electrolyte solution.

[0188] It should be noted that, in the preparation of the electrolyte, the amount of dinitrile compound and carboxylic acid ester added is adjusted so that the molar ratio MR is 1% to 4%.

[0189] (Assembly of a secondary battery)

[0190] First, the positive lead 31 is connected to the positive electrode 21 (positive current collector 21A) using a soldering method or the like, and the negative lead 32 is connected to the negative electrode 22 (negative current collector 22A) using a soldering method or the like.

[0191] Next, the positive electrode 21 and the negative electrode 22 are stacked on top of each other with the separator 23 in between, and then the positive electrode 21, the negative electrode 22, and the separator 23 are wound together to form a wound body. This wound body has the same structure as the battery element 20, except that no electrolyte permeates into each of the positive electrode 21, the negative electrode 22, and the separator 23. Next, the wound body is pressed into a flat shape using a press or the like.

[0192] Next, the wound body is housed inside the recess 10U, and then the outer film 10 is folded so that the outer films 10 are facing each other. Next, using a heat-sealing method or the like, the outer peripheral portions of two sides of the facing outer films 10 (welded layers) are fused together, thereby housing the wound body inside the bag-shaped outer film 10.

[0193] Finally, electrolyte is injected into the pouch-shaped outer membrane 10, and then the outer periphery of the remaining side of the outer membrane 10 (welded layer) is fused together using a heat fusion method or the like. In this case, sealing film 41 is inserted between the outer membrane 10 and the positive electrode lead 31, and sealing film 42 is inserted between the outer membrane 10 and the negative electrode lead 32. Thus, by impregnating the winding body with electrolyte, a battery element 20 as a winding electrode body is produced, and the battery element 20 is sealed inside the pouch-shaped outer membrane 10 to assemble a secondary battery.

[0194] (Stabilization of secondary batteries)

[0195] The assembled secondary battery is charged and discharged. Various conditions, such as ambient temperature, number of charge / discharge cycles, and charge / discharge conditions, can be arbitrarily set. As a result, a coating is formed on the surface of the negative electrode 22, thereby stabilizing the state and electrochemical properties of the secondary battery.

[0196] Thus, a secondary battery using an outer membrane 10, namely a laminated membrane type secondary battery, was completed.

[0197] <1-5. Functions and Effects>

[0198] According to this secondary battery, the positive electrode 21 (positive electrode active material layer 21B) contains layered rock salt type lithium-nickel composite oxide, the negative electrode 22 contains lithium-titanium composite oxide, and the electrolyte contains dinitrile compounds and carboxylic acid esters. Furthermore, the capacity ratio of the positive electrode 21 to the negative electrode 22 (capacity ratio CR) satisfies the above conditions, and the XPS analysis results of the positive electrode active material layer 21B (concentration ratios X, Y, and relative ratio Z) also satisfy the above conditions. Specifically, the capacity ratio CR is 100%–120%, the concentration ratio X satisfies 0.30 ≤ X ≤ 0.70 (property condition 1), the concentration ratio Y satisfies 0.16 ≤ Y ≤ 0.37 (property condition 2), and the relative ratio Z satisfies 1.30 ≤ Z ≤ 2.52 (property condition 3).

[0199] In this case, with the positive electrode 21 containing lithium nickel composite oxide, the negative electrode 22 containing lithium titanium composite oxide, and the electrolyte containing dinitrile compound and carboxylic acid ester, the capacity ratio CR, concentration ratio X, Y, and relative ratio Z are optimized, thus obtaining a series of advantages as described below.

[0200] First, the positive electrode active material (lithium-nickel composite oxide) contains the transition metal element Ni as the main component, thus achieving high energy density.

[0201] Second, the Al contained in lithium-nickel composite oxides, as a constituent element, exists in the form of pillars within the layered rock-salt type crystal structure (transition metal layer), and does not contribute to redox reactions. Therefore, Al possesses the property of suppressing crystal structure changes but not participating in charge-discharge reactions.

[0202] Here, since the analysis results (concentration ratio X) of the positive electrode active material layer 21B using XPS meet property condition 1, there is an appropriate and sufficient amount of Al atoms on the surface (position P1) of the positive electrode active material layer 21B. Under these circumstances, during charging and discharging (when lithium ions are inserted and extracted), the crystal structure of the lithium-nickel composite oxide remains almost unchanged near the surface of the positive electrode active material layer 21B, thus the positive electrode active material layer 21B is not prone to expansion or contraction. It should be noted that changes in the crystal structure of the lithium-nickel composite oxide also include unexpected Li pull-out phenomena. Therefore, the positive electrode active material is not prone to breakage during charging and discharging, and thus highly reactive new surfaces are not easily generated in this positive electrode active material. Therefore, the electrolyte is not prone to decomposition on the new surfaces of the positive electrode active material, so even with repeated charging and discharging, the discharge capacity is not easily reduced, and gas is not easily generated due to electrolyte decomposition reactions during charging and discharging.

[0203] In this situation, especially when using (charging, discharging, or storing) a secondary battery in a high-temperature environment, the discharge capacity is not easily reduced sufficiently, and gas is not easily generated sufficiently. Furthermore, in the positive electrode active material, it is difficult to form a resistive coating because new surfaces are not easily formed, and it is also difficult to undergo changes in crystal structure (such as a change from hexagonal to cubic crystals). This change in crystal structure is the main reason for the increase in resistance.

[0204] Third, the analysis results (concentration ratio Y) of the positive electrode active material layer 21B using XPS meet property condition 2. Therefore, the amount of Al atoms present in the interior (position P2) of the positive electrode active material layer 21B is appropriately and sufficiently reduced compared to the surface (position P1). In this case, not only during the initial charge and discharge but also after charge and discharge, lithium ions can be easily inputted and output in the inner portion near the surface of the positive electrode active material layer 21B without being excessively affected by Al atoms. As a result, the charge and discharge reaction can proceed smoothly and fully, thereby ensuring energy density, and lithium ions can be easily and stably inserted and extracted during charge and discharge.

[0205] Fourth, the analysis results (relative to Z) of the positive electrode active material layer 21B using XPS satisfy property condition 3. Therefore, in the positive electrode active material layer 21B, the amount of Al atoms present in the interior (position P2) is appropriately reduced compared to the surface (position P1). More specifically, the amount of Al present gradually decreases from the surface (position P1) to the interior (position P2) without a sharp decrease. In this case, the advantages related to the first effect based on property condition 1 and the advantages related to the second effect based on property condition 2 can be obtained in a good balance in the positive electrode active material layer 21B. Thus, compared with the case where property condition 3 is not satisfied, there is no compromise where obtaining one advantage of the two would mean losing the other, and therefore the advantages of both can be effectively obtained.

[0206] Fifth, the electrolyte contains both a dinitrile compound and a carboxylic acid ester, thus the dinitrile compound improves the redox resistance of the carboxylic acid ester. Consequently, compared to electrolytes containing only carboxylic acid esters and no dinitrile compound, the potential window on the oxidation side is significantly expanded. Therefore, even when using a lithium-nickel composite oxide with high oxidizing power as the positive electrode active material, the decomposition reaction of the electrolyte (especially the carboxylic acid ester) during charging and discharging can be suppressed, thereby preventing the generation of gas in the positive electrode 21 due to electrolyte decomposition.

[0207] Sixth, the decomposition reaction of the electrolyte in the positive electrode 21 is suppressed. Therefore, even if lithium titanium composite oxide is used as the negative electrode active material, the formation of highly reducing byproducts caused by the decomposition reaction of the electrolyte in the positive electrode 21 can be suppressed. As a result, the reduction reaction of the byproducts in the negative electrode 22 can be suppressed, and thus the generation of gas due to the reduction reaction of these byproducts can be suppressed.

[0208] Seventh, since the dinitrile compound acts as a protective film, the thickness of the negative electrode 22 can be relatively thin. As a result, even when charging with a large current, the concentration distribution of the electrolyte is uniform inside the negative electrode 22, making it easier for lithium ions to intercalate and deintercalate in the negative electrode 22.

[0209] Based on the above, even when the positive electrode 21 contains a lithium-nickel composite oxide and the negative electrode 22 contains a lithium-titanium composite oxide, high energy density can be obtained. Furthermore, the reduction in discharge capacity and gas generation can be suppressed even with repeated charge-discharge cycles. Moreover, lithium-ion input and output performance can be improved not only during the initial charge-discharge cycle but also subsequently. Therefore, excellent battery characteristics can be achieved.

[0210] In this case, in particular, the manufacturing method of the positive electrode active material, by using co-precipitation and firing (one firing process), is different from the case of using co-precipitation and firing (two firing processes), and substantially satisfies physical property conditions 1 to 3 at the same time, thus improving battery characteristics.

[0211] In detail, as explained in the embodiments described later, when using the co-precipitation method and the firing method (two firing processes), similar to the case of using the co-precipitation method and the firing method (one firing process), the amount of Al atoms present in the positive electrode active material layer 21B is reduced in the interior (position P2) than in the surface (position P1). However, the amount of Al atoms present in the surface (position P1) is excessively increased, and the amount of Al atoms present in the interior (position P2) is excessively reduced, thus failing to satisfy both physical property conditions 1 and 2. Alternatively, the amount of Al atoms present in the interior (position P2) is drastically reduced compared to the surface (position P1), thus failing to satisfy physical property condition 3. Therefore, due to the inability to simultaneously satisfy physical property conditions 1 to 3, a compromise occurs, making it difficult to improve battery characteristics.

[0212] In contrast, when using co-precipitation and firing (one firing step), unlike when using co-precipitation and firing (two firing steps), the amount of Al atoms present on the surface (position P1) and the amount of Al atoms present in the interior (position P2) of the positive electrode active material layer 21B is appropriately increased, thus satisfying both physical property conditions 1 and 2. Furthermore, the amount of Al atoms gradually decreases from the surface (position P1) to the interior (position P2), thus satisfying physical property condition 3. Therefore, by simultaneously satisfying physical property conditions 1 to 3, the compromise relationship can be broken, thereby improving battery characteristics.

[0213] Furthermore, since d in equation (1) satisfies d > 0, if the lithium nickel composite oxide contains an additional element M as a constituent element, lithium ions can be easily and smoothly input and output in the positive electrode active material (lithium nickel composite oxide) during charging and discharging, thus achieving higher performance.

[0214] In addition, if the lithium titanium composite oxide contains any one or more of the compounds shown in formulas (5) to (7), the expansion of the secondary battery can be sufficiently suppressed, thus achieving a higher effect.

[0215] Furthermore, if the molar ratio MR is 1%–4%, the dinitrile compound at the interface between the negative electrode 22 (lithium-titanium composite oxide) and the electrolyte will not hinder the movement of lithium ions (Li / Li + The extent of the charge-movement reaction is selectively coordinated with respect to titanium in the lithium-titanium composite oxide. Consequently, the dinitrile compound acts as a protective film suppressing the reduction reaction of the electrolyte at potentials below 1.5V relative to the lithium potential. Therefore, even with a capacity ratio (CR) of 100% or higher, gas generation due to the reduction reaction of the electrolyte can be sufficiently suppressed. Thus, the expansion of the secondary battery can be effectively suppressed, resulting in superior performance.

[0216] Furthermore, if the dinitrile compound contains succinate or the like, and the carboxylic acid ester contains ethyl propionate or the like, the expansion of the secondary battery can be sufficiently suppressed, thus achieving even better performance. In this case, especially when using ethyl propionate, which has a higher ionic conductivity than propyl propionate but is more prone to gas generation due to decomposition reactions, gas generation can be suppressed by succinate or the like, thus achieving both improved lithium-ion input performance and suppression of secondary battery expansion.

[0217] Furthermore, if the electrolyte solvent contains carboxylic acid esters at a concentration of 50% to 90% by weight, the decomposition reaction of the carboxylic acid esters can be sufficiently suppressed during charging and discharging, thus effectively suppressing the generation of gas due to this decomposition reaction. In other words, even with a large amount of carboxylic acid ester (concentration in the solvent = 50% to 90% by weight), the generation of gas due to the decomposition reaction of the carboxylic acid esters can be suppressed by the dinitrile compound, thus preventing the secondary battery from swelling. Therefore, the swelling of the secondary battery can be effectively suppressed, resulting in higher performance.

[0218] Furthermore, if the secondary battery has a positive electrode 21, a negative electrode 22, and a flexible outer membrane 10 for containing electrolyte, the expansion of the secondary battery can be effectively suppressed even when a flexible outer membrane 10 that is prone to deformation (expansion) is used, thus achieving a higher performance.

[0219] 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.

[0220] <2. Variations>

[0221] Next, a modified example of the above-mentioned secondary battery will be described.

[0222] As explained below, the structure of the secondary battery described above can be modified appropriately. Furthermore, any two or more of the variations described below can be combined with each other.

[0223] [Variation Example 1]

[0224] The aforementioned secondary battery uses a membrane 23 as a porous membrane. However, although not specifically illustrated here, a laminated membrane comprising layers of polymer compounds can be used instead of the porous membrane 23.

[0225] Specifically, the laminated separator comprises a porous membrane with one and two faces and a polymer compound layer disposed on one or both faces of the porous membrane. This is because, due to the improved adhesion of the separator to each of the positive electrode 21 and the negative electrode 22, positional shift of the battery element 20 (winding shift of each of the positive electrode 21, negative electrode 22, and separator) is less likely to occur. Therefore, even if electrolyte decomposition reactions occur, the secondary battery is less prone to expansion. The polymer compound layer comprises a polymer compound such as polyvinylidene fluoride (PVDF). This is because PVDF and similar compounds possess excellent physical strength and electrochemical stability.

[0226] It should be noted that one or both of the porous membrane and the polymer compound layer may contain any one or more of a variety 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, etc. Specific examples of inorganic particles are particles of alumina, aluminum nitride, boehmite, silicon dioxide, titanium dioxide, magnesium oxide, and zirconium oxide. Specific examples of resin particles are particles of acrylic resins and styrene resins, etc.

[0227] In the case of fabricating a layered membrane, 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 a porous membrane. In this case, multiple insulating particles can also be added to the precursor solution as needed.

[0228] With the use of this layered separator, lithium ions can also move between the positive electrode 21 and the negative electrode 22, thus achieving the same effect.

[0229] [Variation Example 2]

[0230] The aforementioned secondary battery uses an electrolyte as a liquid electrolyte. However, although not specifically illustrated here, an electrolyte layer as a gel electrolyte can also be used instead of the liquid electrolyte.

[0231] 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 then the positive electrode 21, the negative electrode 22, the separator 23 and the electrolyte layer are wound around each other. The electrolyte layer is located between the positive electrode 21 and the separator 23, and between the negative electrode 22 and the separator 23.

[0232] Specifically, the electrolyte layer comprises an electrolyte and a polymer compound, and the electrolyte is held in place by the polymer compound within the electrolyte layer. This is to prevent leakage. The composition of the electrolyte is as described above. The polymer compound includes polyvinylidene fluoride, etc. In the case of forming the electrolyte layer, after preparing a precursor solution comprising an electrolyte, a polymer compound, and an organic solvent, the precursor solution is coated on one or both sides of each of the positive electrode 21 and the negative electrode 22.

[0233] Even when this electrolyte layer is used, lithium ions can move between the positive electrode 21 and the negative electrode 22 via the electrolyte layer, thus achieving the same effect.

[0234] <3. Uses of Secondary Batteries>

[0235] Next, the uses (application examples) of the above-mentioned secondary batteries will be explained.

[0236] There are no particular limitations on the uses of secondary batteries. As a power source, a secondary battery can be the main power source for electronic devices and electric vehicles, or it can be an auxiliary power source. The main power source is the preferred power source, regardless of the availability of other power sources. An auxiliary power source is used to replace the main power source, or it is switched from the main power source.

[0237] Specific examples of applications for rechargeable batteries are as follows: Electronic devices (including portable 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.

[0238] Battery packs can use single cells or battery banks. Electric vehicles are vehicles that operate (drive) using secondary batteries as a power source, as mentioned above; they can also be hybrid vehicles that have a power source other than secondary batteries. In home electricity storage systems, electricity stored in secondary batteries, which serve as electricity storage sources, can be used to operate household electrical products, etc.

[0239] Here, a specific example of the application of secondary batteries is explained. The structure of the application example described below is only one example and can therefore be modified appropriately.

[0240] Figure 4 The frame structure of the battery pack is shown. The battery pack described here is a battery pack (so-called pouch) that uses a secondary battery and is installed in electronic devices such as smartphones.

[0241] like Figure 4 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.

[0242] 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 through 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 thermistor (PTC) element 58, and a temperature detection unit 59. Alternatively, the PTC element 58 may be omitted.

[0243] The control unit 56 includes a central processing unit (CPU) and memory, and controls the overall operation of the battery pack. The control unit 56 detects and controls the operating status of the power supply 51 as needed.

[0244] It should be noted that when the battery voltage of the power supply 51 (secondary battery) reaches the overcharge detection voltage or 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.

[0245] There are no particular limitations on the overcharge detection voltage and the over-discharge detection voltage. For example, the overcharge detection voltage is 4.2V ± 0.05V, and the over-discharge detection voltage is 2.4V ± 0.1V.

[0246] 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 and discharging current is detected based on the on-resistance of switch 57.

[0247] 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.

[0248] Example

[0249] An embodiment of this technology will be described.

[0250] <Examples 1-8 and Comparative Examples 1-7>

[0251] As described below, a positive electrode active material was manufactured, and a secondary battery was manufactured using the positive electrode active material. The battery characteristics of the secondary battery were then evaluated.

[0252] [Preparation of the positive electrode active material in Examples 1-8 and Comparative Examples 1-6]

[0253] The positive electrode active material (lithium-nickel composite oxide) was manufactured using the co-precipitation method and the firing method (one-time firing process) as described below.

[0254] First, powdered nickel compound (nickel sulfate (NiSO4)) and powdered cobalt compound (cobalt sulfate (CoSO4)) were prepared as raw materials. Next, the nickel and cobalt compounds were mixed to obtain a mixture. In this case, the mixing ratio of the nickel and cobalt compounds was adjusted so that the molar ratio of Ni to Co was 85.4:14.6. Furthermore, the mixing ratio of the nickel and cobalt compounds was changed by varying the molar ratio of Co according to the molar ratio of Ni.

[0255] Next, the mixture is added to an aqueous solvent (pure water), and then the aqueous solvent is stirred to obtain a mixed aqueous solution.

[0256] Next, while stirring the mixed aqueous solution, an alkaline compound (sodium hydroxide (NaOH) and ammonium hydroxide (NH4OH)) was added to the mixed aqueous solution (coprecipitation method). As a result, multiple particulate precipitates were granulated in the mixed aqueous solution, thus obtaining secondary particles of the precursor (nickel-cobalt composite coprecipitated hydroxide). The composition of this precursor is shown in Table 1. In this case, in order to finally obtain secondary particles of the positive electrode active material with two different average particle sizes (median particle size D50 (μm)) (including a Bi-model design of large and small particle sizes), two types of secondary particles with different average particle sizes were granulated by controlling their average particle size.

[0257] Next, as other raw materials, powdered lithium compound (lithium hydroxide monohydrate (LiOH·H2O)) and powdered aluminum compound (aluminum hydroxide (Al(OH)3)) were prepared.

[0258] Next, a precursor mixture is obtained by mixing the precursor, aluminum compound, and lithium compound together. In this case, the mixing ratio of the precursor to the aluminum compound is adjusted so that the mixing ratio (molar ratio) of Ni, Co, and Al is 82.0:14.0:4.0, and the amount of aluminum compound added relative to the precursor (wt%) is 1.12 wt%. Furthermore, the mixing ratio of the precursor and the aluminum compound to the lithium compound is adjusted so that the mixing ratio (molar ratio) of Ni, Co, and Al to Li is 103:100. It should be noted that by changing the mixing ratio (molar ratio) of Ni and Co according to the mixing ratio (molar ratio) of Al, the mixing ratio of the precursor to the aluminum compound is changed. Furthermore, by changing the mixing ratio (molar ratio) of Ni, Co, and Al according to the mixing ratio (molar ratio) of Li, the mixing ratio of the precursor and the aluminum compound to the lithium compound is changed.

[0259] Table 1 shows the "Addition Period" column, indicating the periods during which aluminum compounds were added in the manufacturing process of the positive electrode active material. "After co-precipitation" refers to the addition of aluminum compounds to the precursor after obtaining it using the co-precipitation method and before the calcination process described later. In Table 1, for simplicity, aluminum compounds are referred to as "Al compounds".

[0260] Finally, the precursor mixture was calcined in an oxygen atmosphere. The calcination temperatures (°C) are shown in Table 1. Thus, the powdered layered rock salt type lithium-nickel composite oxide shown in formula (1) was synthesized.

[0261] Table 1 shows the number of firing steps performed during the manufacturing process of the positive electrode active material in the column "Number of Firings". Here, since the firing step is performed after the precursor is formed using the co-precipitation method, the number of firings is one.

[0262] Thus, the positive electrode active material (lithium-nickel composite oxide) is obtained. The composition and NC ratio of this lithium-nickel composite oxide are shown in Table 2. In Table 2, for the sake of simplicity, the lithium-nickel composite oxide is referred to as "LiNi composite oxide".

[0263] It should be noted that, in the case of manufacturing the positive electrode active material, powdered manganese compound (manganese sulfate (MnSO4)) was further prepared as other raw materials, and then the manganese compound was further mixed in the precursor to obtain a precursor mixture. In addition, lithium nickel composite oxide containing manganese as an additional element M as a constituent element was synthesized by the same steps.

[0264] Table 2 shows the presence or absence of additional element M in the "Additional Element M" column, and in cases where the lithium nickel composite oxide contains additional element M as a constituent element, the type of additional element M is shown.

[0265] [Preparation of the positive electrode active material in Comparative Example 7]

[0266] For comparison, the positive electrode active material (lithium-nickel composite oxide) was manufactured by using the co-precipitation method and the firing method (two firing processes) instead of the co-precipitation method and the firing method (one firing process) through the steps described below as the manufacturing method.

[0267] In this case, firstly, through the steps described above, a precursor (secondary particles of nickel-cobalt composite coprecipitated hydroxide) is obtained using a coprecipitation method. Next, a mixture of the precursor and a powdered lithium compound (lithium hydroxide monohydrate) is obtained, and this mixture is then calcined (first calcination step). The mixing ratio (molar ratio) of the precursor to the lithium compound is as described above, and the calcination temperature (°C) in the first calcination step is shown in Table 1. Thus, a powdered composite oxide is obtained as the calcined product.

[0268] Next, a mixture of the composite oxide and powdered aluminum compound (aluminum hydroxide) was obtained, and then the mixture was calcined in an oxygen atmosphere (second calcination step). In this case, the amount of aluminum compound added relative to the composite oxide was 0.41% by weight. The calcination temperature (°C) in the second calcination step is shown in Table 1. Thus, a powdered layered rock salt type lithium-nickel composite oxide (lithium nickel cobalt oxide with Al coating on the surface) was synthesized, and thus a positive electrode active material was obtained. The composition and NC of this lithium-nickel composite oxide are shown in Table 2.

[0269] Here, since the aluminum compound is added after the first firing process and before the second firing process, as shown in the "Addition Period" column of Table 1, the aluminum compound is added after the first firing. Furthermore, since the manufacturing method of the positive electrode active material involves two firing processes, as shown in the "Number of Firings" column of Table 1, the number of firings is two.

[0270]

[0271]

[0272] [Manufacturing of the secondary batteries in Examples 1-8 and Comparative Examples 1-7]

[0273] The following steps were followed to manufacture the product. Figures 1-3 The image shows a laminated film type secondary battery (lithium-ion secondary battery).

[0274] (The production of the positive electrode)

[0275] First, 95.5 parts by mass of positive electrode active material (lithium-nickel composite oxide), 1.9 parts by mass of positive electrode binder (polyvinylidene fluoride), 2.5 parts by mass of positive electrode conductive agent (carbon black), and 0.1 parts by mass of dispersant (polyvinylpyrrolidone) are mixed together to prepare a positive electrode mixture. Next, the positive electrode mixture is added to an organic solvent (N-methyl-2-pyrrolidone) and stirred to prepare a paste-like positive electrode mixture slurry. Next, the positive electrode mixture slurry is coated onto both sides of the positive electrode current collector 21A (a strip of aluminum foil with a thickness of 15 μm) using a coating device, and then the positive electrode mixture slurry is dried to form a positive electrode active material layer 21B. Finally, the positive electrode active material layer 21B is compressed and molded using a roller press. Thus, the positive electrode 21 is produced.

[0276] The results of XPS analysis of the physical properties (concentration ratios X, Y, and relative ratio Z) of the positive electrode 21 (positive electrode active material layer 21B) are shown in Table 2. It should be noted that the analytical procedures for the positive electrode active material layer 21B using XPS are as described above.

[0277] (Making the negative electrode)

[0278] First, 90 parts by mass of the negative electrode active material (Li4Ti5O as a lithium-titanium composite oxide) was added. 12 The negative electrode binder (polyvinylidene fluoride) and 10 parts by mass of the negative electrode binder are mixed together to prepare a negative electrode mixture. Next, the negative electrode mixture is added to an organic solvent (N-methyl-2-pyrrolidone) and stirred to prepare a paste-like negative electrode mixture slurry. Next, the negative electrode mixture slurry is coated onto both sides of the 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 the negative electrode active material layer 22B. Finally, the negative electrode active material layer 22B is compressed and molded using a roller press. Thus, the negative electrode 22 is produced. In Table 2, for simplicity, the lithium-titanium composite oxide is described as "LiTi composite oxide".

[0279] In particular, when fabricating the negative electrode 22, as shown in Table 2, the thickness (μm) of the negative electrode active material layer 22B is adjusted according to the coating amount of the negative electrode slurry to achieve a capacity ratio CR of 110%.

[0280] (Preparation of electrolyte)

[0281] First, a solvent was prepared. As this solvent, a mixture of propylene carbonate (a cyclic carbonate) and propyl propionate (PrPr) (a carboxylic acid ester) was used. In this case, the content of the carboxylic acid ester in the solvent was 75% by weight.

[0282] Next, an electrolyte salt (LiPF6 as a lithium salt) is added to the solvent, and the solvent is then stirred. In this case, the electrolyte salt concentration is 1 mol / kg relative to the solvent.

[0283] Finally, the dinitrile compound (succinate (SN)) was added to a solvent containing the electrolyte salt, and the solvent was then stirred. In this case, the molar ratio MR was adjusted to 1% by adjusting the amount of dinitrile compound added.

[0284] Thus, the electrolyte salt and dinitrile compound are dissolved or dispersed in the solvent, thereby preparing the electrolyte.

[0285] (Assembly of a secondary battery)

[0286] First, the positive lead 31 (a strip of aluminum foil) is soldered to the positive electrode 21 (positive current collector 21A), and the negative lead 32 (a strip of copper foil) is soldered to the negative electrode 22 (negative current collector 22A).

[0287] Next, the positive electrode 21 and the negative electrode 22 are stacked on top of each other with a separator 23 (a microporous polyethylene film with a thickness of 25 μm) in between, and then the positive electrode 21, the negative electrode 22 and the separator 23 are wound together to form a wound body. Next, the wound body is pressed using a press to form a flat wound body.

[0288] Next, the outer film 10 is folded while the wound body housed in the recess 10U is sandwiched between the two outer periphery portions of the outer film 10 (welding layer), and then the outer periphery portions of the two sides of the outer film 10 (welding layer) are heat-fused together, thereby housing the wound body inside the bag-shaped outer film 10. As the outer film 10, an aluminum laminate film is used, which consists of a welding layer (a polypropylene film with a thickness of 30 μm), a metal layer (an aluminum foil with a thickness of 40 μm), and a surface protective layer (a nylon film with a thickness of 25 μm) stacked sequentially from the inside.

[0289] Finally, after injecting the electrolyte into the pouch-shaped outer membrane 10, the outer periphery of the remaining edge of the outer membrane 10 (welded layer) is thermally fused together under reduced pressure. In this case, a sealing film 41 (a polypropylene film with a thickness of 5 μm) is inserted between the outer membrane 10 and the positive electrode lead 31, and a sealing film 42 (a polypropylene film with a thickness of 5 μm) is inserted between the outer membrane 10 and the negative electrode lead 32. Thus, the electrolyte is impregnated into the winding body, thereby forming a battery element 20 as a wound electrode body, and the battery element 20 is sealed inside the pouch-shaped outer membrane 10, assembling a secondary battery.

[0290] (Stabilization of secondary batteries)

[0291] The secondary battery was subjected to one charge-discharge cycle at room temperature (temperature = 25℃). During charging, a constant current of 0.1C was used until the voltage reached 4.2V, followed by constant voltage charging at that 4.2V until the current reached 0.005C. During discharging, a constant current of 0.1C was used until the voltage reached 2.5V. 0.1C refers to the current value required to fully discharge the battery (theoretical capacity) in 10 hours, and 0.005C refers to the current value required to fully discharge the battery (theoretical capacity) in 200 hours.

[0292] Thus, a coating is formed on the surface of the negative electrode 22, thereby stabilizing the state of the secondary battery. This completes the laminated film type secondary battery.

[0293] [Evaluation of Battery Characteristics]

[0294] The battery characteristics (initial capacity characteristics, cycle characteristics, load characteristics, and expansion characteristics) of the secondary battery were evaluated, and the results are shown in Table 2.

[0295] (Initial capacity characteristics)

[0296] The discharge capacity (initial capacity) was measured by subjecting the secondary battery to one charge-discharge cycle at room temperature. The charge-discharge conditions were the same as those used for stabilizing the secondary battery as described above. It should be noted that the initial capacity values ​​shown in Table 2 are normalized to 100 based on the initial capacity value in Example 1.

[0297] (Cyclic Characteristics)

[0298] First, the discharge capacity (discharge capacity of the first cycle) was measured by charging and discharging the secondary battery in a high-temperature environment (temperature = 60°C). Next, the secondary battery was repeatedly charged and discharged in the same environment 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 used for stabilizing the secondary battery. Finally, the cycle retention rate (%) was calculated as (discharge capacity of the 100th cycle / discharge capacity of the first cycle) × 100.

[0299] (Load characteristics)

[0300] Initially, the discharge capacity (discharge capacity of the first cycle) was measured by charging and discharging the secondary battery at room temperature. The charging and discharging conditions were the same as those used for stabilizing the secondary battery, except that the charging current and discharging current were changed from 0.1C to 0.2C. Next, the discharge capacity (discharge capacity of the second cycle) was measured by charging and discharging the secondary battery again in the same environment. The charging and discharging conditions were the same as those used for stabilizing the secondary battery, except that the discharging current was changed from 0.1C to 10C. 0.2C refers to the current value at which the battery capacity is fully discharged within 5 hours, and 10C refers to the current value at which the battery capacity is fully discharged within 0.1 hours. Finally, the load maintenance rate (%) was calculated as follows: (Discharge capacity of the second cycle (discharge current = 10C) / Discharge capacity of the first cycle (discharge current = 0.2C)) × 100.

[0301] (Expansion characteristics)

[0302] First, the secondary battery was charged at room temperature, and then its volume (volume before storage) was measured using the Archimedes method. The charging conditions were the same as those used for stabilizing the secondary battery described above. Next, the secondary battery was stored in a high-temperature environment (storage period = 1 week), and then its volume (volume after storage) was measured again using the Archimedes method. Finally, the expansion rate (%) was calculated as (volume after storage / volume before storage) × 100. It should be noted that the expansion rate values ​​shown in Table 2 are normalized to 100 using the expansion rate values ​​from Example 1.

[0303] [Inspection]

[0304] As shown in Table 2, the battery characteristics of the secondary battery vary based on the analysis results (concentration ratios X, Y, and relative ratio Z) of the positive electrode active material layer 21B using XPS.

[0305] Specifically, when physical property conditions 1 to 3 regarding concentration ratios X, Y, and relative ratios Z are not simultaneously satisfied (Comparative Examples 1 to 7), a compromise occurs: when any one of the initial capacity, cycle maintenance rate, load maintenance rate, and expansion rate is increased, the other conditions deteriorate. Therefore, it is not possible to increase the initial capacity, cycle maintenance rate, load maintenance rate, and expansion rate separately.

[0306] In particular, when the positive electrode active material (lithium-nickel composite oxide) was manufactured using the co-precipitation method and the firing method (one-time firing process) (Comparative Example 7), the aforementioned trade-off relationship was significantly generated due to the excessive increase in relative Z.

[0307] In contrast, when the physical property conditions 1 to 3 regarding the concentration ratio X, Y and the relative ratio Z are satisfied simultaneously (Examples 1 to 8), the above-mentioned compromise relationship is broken, and thus the initial capacity, cycle maintenance rate, load maintenance rate and expansion rate can be improved respectively.

[0308] In this case, especially when the positive electrode active material (lithium-nickel composite oxide) contains an additional element M (Mn) as a constituent element, the initial capacity increases, although the load retention rate decreases slightly, compared to the case where the lithium-nickel composite oxide does not contain an additional element M as a constituent element. Furthermore, even when using a flexible outer film 10 that is prone to deformation (expansion), the expansion rate can be sufficiently suppressed.

[0309] <Examples 9-28 and Comparative Examples 8-14>

[0310] As shown in Tables 3 and 4, except for changing the capacity ratio CR (%), secondary batteries were manufactured using the same steps, and then the battery characteristics of the secondary batteries were evaluated.

[0311] Here, in addition to changing the capacity ratio CR, the types of dinitrile compounds and carboxylic acid esters, the molar ratio MR (%), and the content of carboxylic acid esters in the solvent (weight %) were also changed as needed. When the molar ratio MR was changed, the amount of dinitrile compound added was changed, and when the content of carboxylic acid esters in the solvent was changed, the amount of carboxylic acid ester added was also changed.

[0312] As carboxylic acid esters, methyl propionate (MtPr), ethyl propionate (EtPr), methyl acetate (MtAc), and ethyl acetate (EtAc) were newly used.

[0313] As dinitrile compounds, malononitrile (MN), glutaronitrile (GN), adiponitrile (AN), heptanonitrile (PN), and octanoic acid dinitrile (SBN) were newly used.

[0314] It should be noted that, for the purpose of comparison, the electrolyte was prepared using the same steps, except that a dinitrile compound was not used.

[0315] In addition, for comparison, an electrolyte was prepared using the same steps, except that a chain carbonate was used instead of a carboxylic acid ester. Diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) were used as the chain carbonate.

[0316] In Table 4, for convenience, chain carbonates (DEC and EMC) are shown in the "Carboxylic Acid Esters" column. Additionally, to clarify that DEC and EMC are not carboxylic acid esters, an asterisk (*) is added before each DEC and EMC.

[0317] Furthermore, for comparison, negative electrode 22 was fabricated using the same steps, except that carbon material (graphite) was used instead of lithium titanium composite oxide as the negative electrode active material. The procedure for determining the capacity ratio CR when using carbon material as the negative electrode active material was the same as when using lithium titanium composite oxide as the negative electrode active material, except that the upper limit voltage during charging was changed to 0V and the lower limit voltage during discharging was changed to 1.5V while charging and discharging the test secondary battery to determine the capacity of negative electrode 22.

[0318] In Table 4, for convenience, graphite is shown in the column “Negative Electrode Active Material (LiTi Composite Oxide)”. Additionally, to clarify that graphite is not a LiTi composite oxide, an asterisk (*) is added before it.

[0319]

[0320]

[0321] As shown in Tables 3 and 4, even if the physical property conditions 1 to 3 regarding the concentration ratio X, Y and the relative ratio Z are met simultaneously, the battery characteristics of the secondary battery will further vary according to the capacity ratio CR.

[0322] Specifically, when the capacity ratio CR is less than 100% (Comparative Example 8) and when the capacity ratio CR is greater than 120% (Comparative Example 9), a trade-off occurs, so the initial capacity, cycle maintenance rate, load maintenance rate and expansion rate cannot be increased respectively.

[0323] It should be noted that since the electrolyte does not contain dinitrile compounds, a compromise still occurs when the molar ratio MR is 0% (Comparative Example 10), so the initial capacity, cycle maintenance rate, load maintenance rate and expansion rate cannot be improved respectively.

[0324] In contrast, when the capacity ratio CR is 100% to 120% (Examples 1, 9, 10), the initial capacity, cycle maintenance rate, load maintenance rate, and expansion rate can be improved respectively because the trade-off is broken.

[0325] In particular, when the physical property conditions 1 to 3 related to the concentration ratios X and Y and the relative ratio Z are satisfied simultaneously, and the capacity ratio CR is 100% to 120%, the following tendencies can be obtained.

[0326] First, when the molar ratio MR is 1% to 4% (Examples 1, 11 to 13), the load maintenance rate increases compared to when the molar ratio MR is greater than 4% (Examples 14 and 15).

[0327] Second, when the content of carboxylic ester in the solvent is 50% to 90% by weight (Examples 1, 17, 18), compared with the case where the content is less than 50% by weight (Example 16) and the case where the content is greater than 90% by weight (Example 19), the cycle maintenance rate and load maintenance rate are increased respectively.

[0328] Third, even when the type of dinitrile compound is changed (Examples 20-24), the trade-off is broken, thus enabling improvements in initial capacity, cycle retention, load retention, and expansion rate, respectively. In particular, when succinic anionyl, glutaronitrile, and adiponitrile are used as dinitrile compounds (Examples 1, 21, 22), the cycle retention rate increases and the expansion rate decreases.

[0329] Fourth, even when the type of carboxylic acid ester is changed (Examples 25-28), the trade-off is broken, thus enabling improvements in initial capacity, cycle retention rate, load retention rate, and expansion rate, respectively. In particular, when ethyl propionate and propyl propionate are used as carboxylic acid esters (Examples 1, 26), the cycle retention rate increases and the expansion rate decreases.

[0330] It should be noted that when carbon materials are used as the negative electrode active material (Comparative Examples 13 and 14), physical property conditions 1 to 3 regarding the concentration ratios X and Y and the relative ratio Z are satisfied simultaneously. Even if the appropriate conditions regarding the capacity ratio CR are satisfied, a compromise occurs, so the initial capacity, cycle maintenance rate, load maintenance rate, and expansion rate cannot be improved separately.

[0331] In contrast, when lithium titanium composite oxide is used as the negative electrode active material (Example 1, etc.), when the physical property conditions 1 to 3 related to the concentration ratios X, Y and the relative ratio Z are satisfied at the same time, and the appropriate conditions related to the capacity ratio CR are satisfied, as described above, the trade-off relationship is broken, and thus the initial capacity, cycle maintenance rate, load maintenance rate and expansion rate can be improved respectively.

[0332] [Summarize]

[0333] As shown in Tables 2-4, when the positive electrode 21 (positive electrode active material layer 21B) contains layered rock salt-type lithium-nickel composite oxide, the negative electrode 22 contains lithium-titanium composite oxide, the electrolyte contains dinitrile compounds and carboxylic acid esters, and the ratio of the capacity of the positive electrode 21 to the capacity of the negative electrode 22 (capacity ratio CR) and the analysis results of the positive electrode active material layer 21B using XPS (concentration ratios X, Y, and relative ratios Z) respectively satisfy the above-mentioned conditions, the initial capacity, cycle retention, load retention, and expansion rate can be improved respectively. Therefore, excellent battery characteristics (initial capacity characteristics, cycle characteristics, load characteristics, and expansion characteristics) can be obtained in secondary batteries.

[0334] The above description, while illustrating the present technology with an example of one implementation method and embodiment, does not limit the structure of the present technology to the structure described in one implementation method and embodiment, and various modifications are possible.

[0335] Specifically, while the case of a laminated film type battery structure for secondary batteries has been described, the battery structure of secondary batteries is not particularly limited, and can therefore be cylindrical, square, coin-shaped, button-shaped, etc.

[0336] Furthermore, the case where the battery element structure is a wound type has been explained. However, the battery element structure is not particularly limited, so a stacked type with electrodes (positive and negative electrodes) stacked on top of each other, or a ninety-nine-fold type where the electrodes (positive and negative electrodes) are folded into a Z-shape, etc., can be used.

[0337] Furthermore, while the case where lithium is used as the electrode reactant has been described, this electrode reactant 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, the electrode reactant can also be other light metals such as aluminum.

[0338] It should be noted that the above-mentioned positive electrode is not limited to secondary batteries, so it can also be used in other electrochemical devices such as capacitors.

[0339] The effects described in this specification are merely illustrative, and therefore the effects of this technology are not limited to those described in this specification. Thus, this technology can also achieve other effects.

Claims

1. A secondary battery, comprising: A positive electrode, comprising a positive electrode active material layer, wherein the positive electrode active material layer comprises a layered rock salt type lithium-nickel composite oxide represented by the following formula (1); The negative electrode comprises lithium-titanium composite oxide; and The electrolyte contains dinitrile compounds and carboxylic acid esters. The ratio of the capacity of the positive electrode per unit area to the capacity of the negative electrode per unit area is more than 100% and less than 120%. When analyzing the surface of the positive electrode active material layer using X-ray photoelectron spectroscopy, the ratio X of the atomic concentration of Al to the atomic concentration of Ni satisfies the condition expressed by the following equation (2). Inside the positive electrode active material layer, i.e., at a depth of 100 nm, when the positive electrode active material layer is analyzed using the X-ray photoelectron spectroscopy method, the ratio Y of the atomic concentration of Al to the atomic concentration of Ni satisfies the condition expressed by the following equation (3). The ratio Z of X to Y satisfies the condition expressed by the following equation (4). Li a Ni 1-b-c-d Co b Al c M d O e (1) M is at least one of Fe, Mn, Cu, Zn, Cr, V, Ti, Mg, and Zr; a, b, c, d, and e satisfy 0.8 < a < 1.2, 0.06 ≤ b ≤ 0.18, 0.015 ≤ c ≤ 0.05, 0 ≤ d ≤ 0.08, 0 < e < 3, 0.1 ≤ (b + c + d) ≤ 0.22, and 4.33 ≤ (1 - bcd) / b ≤ 15.

0. 0.30≤X≤0.70 (2) 0.16≤Y≤0.37 (3) 1.30≤Z≤2.52 （4） 。 2. The secondary battery according to claim 1, In the formula (1), the d satisfies d > 0.

3. The secondary battery according to claim 1, The lithium titanium composite oxide contains at least one of compounds represented by the following formula (5), formula (6), and formula (7), respectively, Li[Li x M1 （1-3x） / 2 Ti （3＋x） / 2 ]O4 (5) M1 is at least one of Mg, Ca, Cu, Zn, and Sr; x satisfies 0 ≤ x ≤ 1 / 3, Li[Li y M2 1-3y Ti 1＋2y ]O4 (6) M2 is at least one of Al, Sc, Cr, Mn, Fe, Ga, and Y; y satisfies 0 ≤ y ≤ 1 / 3, Li[Li 1 / 3 M3 z Ti （5 / 3）-z ]O4 (7) M3 is at least one of V, Zr, and Nb; z satisfies 0 ≤ z ≤ 2 / 3.

4. The secondary battery according to any one of claims 1 to 3, The ratio of the number of moles of the dicarbonitrile compound to the number of moles of the carboxylate is 1% or more and 4% or less.

5. The secondary battery according to any one of claims 1 to 3, The dicarbonitrile compound contains at least one of succinonitrile, glutaronitrile, and adiponitrile, The carboxylate contains at least one of ethyl propionate and propyl propionate.

6. The secondary battery according to any one of claims 1 to 3, The electrolytic solution contains a solvent, The solvent contains the carboxylate, The content of the carboxylate in the solvent is 50% by weight or more and 90% by weight or less.

7. The secondary battery according to any one of claims 1 to 3, The secondary battery further has a flexible exterior member that accommodates the positive electrode, the negative electrode, and the electrolytic solution.

8. The secondary battery according to any one of claims 1 to 3, The secondary battery is a lithium ion secondary battery.

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