Positive electrode layer

Abstract

To provide a positive electrode capable of suppressing the initial resistance of a battery.SOLUTION: A positive electrode layer contains a positive electrode active material 10 and carbon nanotubes 11. The positive electrode active material contains a lithium-nickel-based complex oxide and tungsten that presents at least in the interior or on a surface of the lithium-nickel-based complex oxide, and the positive electrode active material is secondary particles 2 including a plurality of primary particles 1 and voids formed between the plurality of primary particles. The positive electrode layer contains, as the carbon nanotubes, first carbon nanotubes of which at least some are included in the voids of the secondary particles, and spectrums at rising positions (10,195 eV-10,206 eV) of peaks of L absorption edges of tungsten measured by X-ray absorption fine structure analysis (XAFS) satisfy formula (1): (a-b) / (c-b)≤0.79.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a positive electrode layer used in a lithium ion secondary battery. [Background technology]

[0002] Lithium-ion secondary batteries are increasingly being put to practical use not only as small power sources for mobile phones and laptops, but also as medium- and large-sized power sources for automobiles and power storage.

[0003] Research has been conducted focusing on positive electrode active materials to improve the performance of lithium ion secondary batteries. For example, Patent Document 1 discloses a nonaqueous electrolyte secondary battery with a positive electrode containing, as a positive electrode active material, porous particles of a lithium composite oxide having a specific porosity and containing two or more specific voids, the porous particles having a coating on the surface of the porous particles containing tungsten oxide (WO3, hexavalent tungsten) and lithium tungstate, with the aim of providing a nonaqueous electrolyte secondary battery with low initial resistance and suppressed increase in resistance during repeated charge and discharge.

[0004] Furthermore, Patent Document 2 discloses a positive electrode for a lithium ion secondary battery that uses a positive electrode active material containing tungsten, which has a valence of 4. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2021-018895 [Patent Document 2] Japanese Patent Publication No. 2023-082743 Summary of the Invention [Problem to be solved by the invention]

[0006] Although a battery using a positive electrode active material containing tungsten is expected to have the effect of suppressing the initial resistance of the battery, there is still room for further improvement in suppressing the initial resistance.

[0007] The present disclosure has been made in view of the above circumstances, and has as its main object to provide a positive electrode that can suppress the initial resistance of a battery. [Means for solving the problem]

[0008] [1] A positive electrode layer used in a lithium ion secondary battery, the positive electrode layer contains a positive electrode active material and carbon nanotubes, The positive electrode active material contains a lithium nickel-based composite oxide containing at least Li, Ni, O, and an X element (excluding Li and Ni) having a valence of 1 or 2, and tungsten present in at least one of the interior and the surface of the lithium nickel-based composite oxide, and the positive electrode active material is a secondary particle having a plurality of primary particles and voids formed between the plurality of primary particles, The positive electrode layer contains, as the carbon nanotubes, first carbon nanotubes at least partially enclosed in the voids of the secondary particles, and The spectrum of the tungsten L absorption edge peak rise position (10195 eV to 10206 eV) measured by X-ray absorption fine structure analysis (XAFS) is Formula (1): (ab) / (cb)≦0.79 [In formula (1), a represents the energy (eV) at which the spectral slope is greatest in the range of 10195 eV to 10206 eV, When the spectral intensity at a (eV) is A, b represents the energy (eV) at which the spectral intensity of WO2 (tungsten (IV) oxide) becomes A in the range of 10195 eV to 10206 eV, and c represents the energy (eV) at which the spectral intensity of WO3 (tungsten (VI) oxide) becomes A in the range of 10195 eV to 10206 eV.].

[0009] [2] The positive electrode layer according to [1], wherein the lithium nickel composite oxide contains, as the X element, at least one of Na, K, Rb, Cs, Be, Mg, Ca, Sr, and Ba.

[0010] [3] The positive electrode layer according to [1] or [2], wherein in the lithium nickel-based composite oxide, the molar ratio of the X element to the Ni element is 0.05 or more and 0.13 or less.

[0011] [4] The positive electrode layer according to any one of [1] to [3], wherein in the lithium nickel-based composite oxide, the average valence of the Ni element is 2.90 or more and 3.50 or less.

[0012] [5] The positive electrode layer according to any one of [1] to [4], wherein the lithium nickel composite oxide contains at least Be as the X element. [Effects of the Invention]

[0013] The present disclosure has an effect of providing a positive electrode that can suppress an increase in battery resistance. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a schematic cross-sectional view illustrating a positive electrode active material according to the present disclosure. [Figure 2] FIG. 2 is a diagram for explaining formula (1) in the present disclosure. [Figure 3] FIG. 2 is a diagram for explaining formula (1) in the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0015] The positive electrode layer in the present disclosure will be described in detail below. In this specification, the symbol "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower limit and upper limit.

[0016] 1. Positive electrode layer The positive electrode layer in the present disclosure is a positive electrode layer for use in a lithium-ion secondary battery, and contains a positive electrode active material and carbon nanotubes. The positive electrode active material contains a lithium-nickel composite oxide containing at least Li, Ni, O, and an X element (excluding Li and Ni) with a valence of 1 or 2, and tungsten present at least in the interior and on the surface of the lithium-nickel composite oxide. The positive electrode active material is a secondary particle having a plurality of primary particles and voids formed between the plurality of primary particles. The positive electrode layer contains, as the carbon nanotubes, first carbon nanotubes at least partially enclosed in the voids of the secondary particles. The positive electrode layer satisfies the formula (1): (ab) / (cb)≦0.79 when measured by X-ray absorption fine structure (XAFS). In this specification, the "voids formed between primary particles" may also be referred to as "voids of secondary particles." Furthermore, the "voids of secondary particles" usually refer to the internal spaces of secondary particles surrounded by a plurality of primary particles.

[0017] According to the present disclosure, the positive electrode active material contains a lithium nickel-based composite oxide containing a predetermined X element, and tungsten, and therefore the positive electrode layer containing the positive electrode active material can suppress the initial resistance of the battery.

[0018] It is known that lithium-nickel composite oxides such as NCM and NCA are used as positive electrode active materials. 2+ ) is a lithium ion (Li +), there is a risk of cation mixing occurring between lithium ions and nickel ions. If cation mixing occurs, the movement of lithium ions during battery charging and discharging may be hindered, and battery resistance may increase.

[0019] Also, a positive electrode active material containing tungsten is known. As will be described in detail later, when the above formula (1) is satisfied, the tungsten contained in the positive electrode active material has a valence of tetravalent or an average valence between tetravalent and hexavalent, which can suppress battery resistance such as initial resistance.

[0020] In this regard, the present inventors have found that there is a risk that cation mixing may be promoted in a positive electrode active material containing tungsten and a lithium nickel-based composite oxide, and that there is room for further improvement in suppressing battery resistance. This is because tungsten may cause insufficient oxidation of Ni. In contrast, in the positive electrode active material of the present disclosure, the lithium nickel-based composite oxide contains an X element with a valence of 1 or 2, so the valence of Ni can be increased (Ni can be sufficiently oxidized). As a result, cation mixing can be suppressed, and an increase in battery resistance can be suppressed. Note that LiNi 0.9 X 0.1 In the lithium nickel-based composite oxide represented by O2, when the valence of X is tetravalent, the valence (average valence) of Ni is 2.89, but when the valence of X is trivalent, the valence of Ni is 3.00.

[0021] (1) Positive electrode active material The positive electrode active material in the present disclosure contains a lithium nickel-based composite oxide containing at least Li, Ni, O, and an X element (excluding Li and Ni) whose valence is 1 or 2. The X element is a so-called typical element that has no valence other than 1 or 2.

[0022] The lithium nickel-based composite oxide may contain, as the X element, only an element with a valence of 1, or only an element with a valence of 2, or may contain both an element with a valence of 1 and an element with a valence of 2. The lithium nickel-based composite oxide preferably contains, as the X element, at least an element with a valence of 2.

[0023] Examples of the X element include alkali metal elements and alkaline earth metal elements. Examples of the X element include Na, K, Rb, Cs, Be, Mg, Ca, Sr, and Ba. The lithium nickel-based composite oxide may contain one or more of the above elements as the X element.

[0024] In the lithium nickel-based composite oxide, the molar ratio of the X element to the Ni element is, for example, 0.05 or more, or may be 0.06 or more, or may be 0.07 or more, while the molar ratio of the X element to the Ni element is, for example, 0.13 or less, or may be 0.12 or less, or may be 0.10 or less.

[0025] In the lithium nickel-based composite oxide, the molar ratio of X element to Li element is, for example, 0.01 or more, and may be 0.03 or more, while the molar ratio of X element to Li element is, for example, 0.1 or less, and may be 0.07 or less, or may be 0.05 or less.

[0026] The lithium nickel-based composite oxide may further contain transition metal elements such as Mn, Co, and Al. In this case, the molar ratio of Ni to all transition metal elements in the lithium nickel-based composite oxide is preferably 0.5 or more. The molar ratio of Ni to all transition metal elements may be 0.6 or more, or may be 0.7 or more. On the other hand, the molar ratio of Ni to all transition metal elements is, for example, 0.9 or less.

[0027] In the lithium nickel-based composite oxide, the average valence of the Ni element is preferably high. The average valence of the Ni element is, for example, 2.90 or more, or may be 2.95 or more, or may be 3.00 or more. On the other hand, the average valence of the Ni element is, for example, 3.50 or less, or may be 3.30 or less, or may be 3.10 or less.

[0028] The composition of the lithium nickel composite oxide is, for example, Li[Ni a Me b X (1-a-b) ]O2. In the above formula, a satisfies 0.5≦a<1.0, and b satisfies 0≦b<0.5. Me is at least one element selected from Mn, Co, and Al, and X is the X element described above.

[0029] The lithium nickel composite oxide constitutes the primary particles of the positive electrode active material, which will be described later.

[0030] The positive electrode active material in the present disclosure contains tungsten.

[0031] The tungsten contained in the positive electrode active material may be tetravalent tungsten, or may be a mixture of tetravalent tungsten and hexavalent tungsten in which the spectrum including the rising position (10195 eV to 10206 eV) of the peak at the L absorption edge of tungsten satisfies the formula (1) described below.

[0032] Examples of tetravalent tungsten compounds include WO2, lithium tungstate containing tetravalent tungsten, and lithium nickel cobalt manganese composite oxides containing tetravalent tungsten.

[0033] Examples of hexavalent tungsten compounds include lithium tungstates containing hexavalent tungsten, such as WO3 and Li2WO4, and lithium nickel cobalt manganese composite oxides containing hexavalent tungsten.

[0034] The tungsten content (the tungsten content in the entire positive electrode active material) is not particularly limited, but may be, for example, 0.1% by mass or more, 0.3% by mass or more, or 0.5% by mass or more. Meanwhile, the tungsten content may be, for example, 1.0% by mass or less, 0.8% by mass or less, or 0.6% by mass or less. If the tungsten content is within the above range, the battery resistance of the lithium-ion secondary battery can be further reduced. The tungsten content can be determined by elemental analysis using ICP (inductively coupled plasma) atomic emission spectroscopy.

[0035] Furthermore, the tungsten is present at least in the interior and on the surface of the lithium-nickel-based composite oxide. In the positive electrode active material, tungsten may be present in the interior of the lithium-nickel-based composite oxide. For example, tungsten may be present as a component constituting the lithium-nickel-based composite oxide, or may be present dispersedly in the interior of the lithium-nickel-based composite oxide. In this way, tungsten may constitute the primary particles of the positive electrode active material together with the lithium-nickel-based composite oxide. In addition, tungsten may be present on the surface of the lithium-nickel-based composite oxide in the positive electrode active material. That is, tungsten may be present so as to coat the surfaces of the primary particles (primary particles of the lithium-nickel-based composite oxide) or the secondary particles (secondary particles of the lithium-nickel-based composite oxide) in the positive electrode active material. In addition, tungsten may be present both in the interior and on the surface of the lithium-nickel-based composite oxide.

[0036] The positive electrode active material in the present disclosure is a secondary particle having a plurality of primary particles and voids formed between the plurality of primary particles.

[0037] The primary particles may have voids. In other words, the primary particles may be porous particles. The voids are the same as those in the secondary particles described below.

[0038] Average particle size of primary particles (D50 ) is, for example, 0.01 μm or more and 100 μm or less. 50 ) refers to the cumulative 50% particle size in the volume-based particle size distribution measured by a laser diffraction particle size analyzer.

[0039] The secondary particles have voids. The proportion of voids (porosity) is not particularly limited, but is, for example, 20% or more and 50% or less. The void ratio can be determined, for example, by observing the cross section of the positive electrode active material using a scanning electron microscope (SEM). The void ratio can also be determined by measuring the pore distribution using a mercury porosimeter.

[0040] The voids may have a predetermined average pore size (radius), for example, 1 nm or more and 500 nm or less. The average pore size can be determined, for example, by mercury porosimeter measurement.

[0041] Average particle diameter of secondary particles (D 50 ) is, for example, 0.1 μm or more and 1000 μm or less. 50 ) is the same as above.

[0042] The proportion of the positive electrode active material in the positive electrode layer is not particularly limited, but is, for example, 50 mass % or more and 90 mass % or less.

[0043] The method for producing the positive electrode active material in the present disclosure is not particularly limited as long as it can produce the above-described positive electrode active material, and examples thereof include the methods described in the examples below.

[0044] (2) Carbon nanotubes (CNTs) The positive electrode layer in the present disclosure contains carbon nanotubes, which function as a conductive material.

[0045] The positive electrode layer contains, as carbon nanotubes, first carbon nanotubes at least a portion of which is enclosed in the voids of secondary particles. The location of the carbon nanotubes will now be described with reference to FIG. 1. FIG. 1 is a schematic cross-sectional view illustrating an example of a positive electrode active material according to the present disclosure. As shown in FIG. 1(a), in a positive electrode active material 10, the first carbon nanotubes 11 may all be enclosed in the voids α (inside) of secondary particles 2 formed by aggregation of primary particles 1. On the other hand, as shown in FIG. 1(b), the first carbon nanotubes 11 may be partially present in the voids α (inside) of secondary particles 2, and partially present outside (on the outer surface) of the secondary particles.

[0046] As a method for incorporating the first carbon nanotubes into the voids of the secondary particles, for example, the methods described in the examples below can be mentioned.

[0047] The positive electrode layer may also contain, as the carbon nanotubes, second carbon nanotubes that are not enclosed in the voids of the secondary particles.

[0048] The second carbon nanotubes can be contained in the positive electrode layer by adding the carbon nanotubes when preparing the positive electrode mixture in the examples described below.

[0049] The ratio of the first carbon nanotubes to the second carbon nanotubes in the positive electrode layer is not particularly limited. When the mass of the first carbon nanotubes is X1 and the mass of the second carbon nanotubes is X2, X1 / X2 is, for example, 0.1 or more and 3 or less.

[0050] The proportion of carbon nanotubes in the positive electrode layer is not particularly limited, but is, for example, 0.5 mass % or more and 20 mass % or less.

[0051] (3) Positive electrode layer The positive electrode layer in the present disclosure has a spectrum measured by X-ray absorption fine structure analysis (XAFS) at the rising position (10195 eV to 10206 eV) of the peak at the L absorption edge of tungsten, Formula (1): (ab) / (cb)≦0.79 [In formula (1), a represents the energy (eV) at which the spectral slope is greatest in the range of 10195 eV to 10206 eV, When the spectral intensity at a (eV) is A, b represents the energy (eV) at which the spectral intensity of WO2 (tungsten (IV) oxide) becomes A in the range of 10195 eV to 10206 eV, and c represents the energy (eV) at which the spectral intensity becomes A in the range of 10195 eV to 10206 eV of WO3 (tungsten (VI) oxide).

[0052] Here, the above formula (1) will be explained using FIGS. 2 and 3. FIG. 2 shows XAFS measurement spectra of the positive electrode layer (positive electrode sample), WO2, and WO3 in the present disclosure. Specifically, FIG. 2 shows spectra including the rising position of the peak at the L absorption edge of tungsten (10195 eV to 10206 eV) obtained by XAFS measurement of the positive electrode sample, WO2 (tungsten(IV) oxide), and WO3 (tungsten(VI) oxide). FIG. 3 shows an enlarged view of a portion of the rising position of the peak at the L absorption edge of tungsten in the XAFS measurement spectra of the positive electrode sample, WO2, and WO3.

[0053] The rising position of the peak at the L absorption edge of tungsten in the XAFS measurement spectrum is said to represent the valence of the element tungsten. In the present disclosure, the energy (eV) at which the slope of the peak at the L absorption edge of tungsten in the XAFS measurement spectrum of the positive electrode sample is greatest (10195 eV to 10206 eV) is defined as a, and this is used as an index of the rising position of the peak. a can be determined by differentiating the spectrum in the range of 10195 eV to 10206 eV and determining the energy at which the differential peak top occurs. When the spectral intensity at a (eV) is A, the energy (eV) at which the spectral intensity of WO2 (tungsten (IV) oxide) in the range of 10195 eV to 10206 eV becomes A is defined as b, and the energy (eV) at which the spectral intensity of WO3 (tungsten (VI) oxide) in the range of 10195 eV to 10206 eV becomes A is defined as c. Here, WO2 is used as a standard sample of tetravalent tungsten, and WO3 is used as a standard sample of hexavalent tungsten.

[0054] In formula (1), if (ab) / (cb)=1, the tungsten in the positive electrode layer (positive electrode active material) is interpreted as hexavalent. Furthermore, if (ab) / (cb)=0 in formula (1), the tungsten in the positive electrode layer is interpreted as tetravalent. Furthermore, if formula (1) satisfies (ab) / (cb)≦0.79, the tungsten in the positive electrode layer is interpreted as tetravalent or having an average valence between tetravalent and hexavalent.

[0055] Tungsten with an average valence between tetravalent and hexavalent is considered to be a mixture of tetravalent and hexavalent tungsten. Compared to hexavalent tungsten, its lower average valence provides high conductivity and reduces activation energy. Therefore, when tungsten in the positive electrode layer is tetravalent or has an average valence between tetravalent and hexavalent, tungsten exhibits high conductivity and reduces activation energy, which is thought to reduce cell resistance. Furthermore, low-valence tungsten is easily dissolved in Ni, Co, Mn, and other elements used in positive electrode active materials. When dissolved, it is thought that the diffusion resistance of lithium ions can be reduced by widening the crystal axes of the crystal structure of the positive electrode active material.

[0056] The ratio (ab) / (cb) in formula (1) may be 0.70 or less, 0.60 or less, or 0.50 or less. On the other hand, the ratio (ab) / (cb) in formula (1) may be, for example, 0.20 or more, 0.30 or more, or 0.40 or more. The XAFS measurement for determining the ratio (ab) / (cb) in formula (1) will be described in the Examples below.

[0057] The positive electrode layer in the present disclosure may contain at least one of an electrolyte and a binder in addition to the above-described positive electrode active material and carbon nanotubes. The positive electrode layer may also contain a conductive material other than the above-described carbon nanotubes.

[0058] The electrolyte may be a liquid electrolyte or a solid electrolyte. Examples of liquid electrolytes include electrolytes containing an organic solvent such as ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC), and a supporting salt such as LiPF6. Examples of solid electrolytes include inorganic solid electrolytes such as oxide solid electrolytes and sulfide solid electrolytes. Examples of binders include rubber-based binders and fluoride-based binders. Examples of conductive materials other than carbon nanotubes include particulate carbon materials such as acetylene black (AB) and ketjen black (KB), and fibrous carbon materials such as carbon nanofibers (CNF).

[0059] The thickness of the positive electrode layer is not particularly limited, but is, for example, 0.1 μm or more and 1000 μm or less.

[0060] The method for producing the positive electrode layer is not particularly limited, but may include, for example, a method in which a positive electrode mixture containing at least the above-described positive electrode active material (positive electrode active material in which first carbon nanotubes are enclosed in the voids of secondary particles) is applied to a substrate such as a positive electrode current collector and then dried. At least one of carbon nanotubes (second carbon tubes), a binder, a conductive material, and an electrolyte may be added to the positive electrode mixture.

[0061] 2. Lithium-ion secondary battery The positive electrode layer of the present disclosure is used in a lithium-ion secondary battery. Therefore, the present disclosure can also provide a lithium-ion secondary battery having a positive electrode layer, a negative electrode layer, and an electrolyte layer disposed between the positive electrode layer and the negative electrode layer, where the positive electrode layer is the above-described positive electrode layer. Furthermore, a lithium-ion secondary battery typically includes a positive electrode current collector that collects current from the positive electrode layer and a negative electrode current collector that collects current from the negative electrode layer. The lithium-ion secondary battery may be a liquid-based battery containing a liquid electrolyte as the electrolyte, or a solid-state battery containing a solid electrolyte as the electrolyte.

[0062] The positive electrode layer is as described in "1. Positive electrode layer." The negative electrode layer, electrolyte layer, positive electrode current collector, and negative electrode current collector may be conventionally known materials used in lithium ion secondary batteries.

[0063] Lithium ion secondary batteries can be used as power sources for vehicles such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), electric vehicles (BEVs), gasoline-powered vehicles, and diesel-powered vehicles. They are particularly preferably used as driving power sources for hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), or electric vehicles (BEVs). The batteries may also be used as power sources for mobile objects other than vehicles (for example, trains, ships, and aircraft), and may also be used as power sources for electrical appliances such as information processing devices.

[0064] The present disclosure is not limited to the above-described embodiments. The above-described embodiments are merely examples, and any configuration that is substantially identical to the technical idea described in the claims of the present disclosure and that provides similar effects is included within the technical scope of the present disclosure. [Example]

[0065] [Example 1] (Preparation of Positive Electrode Active Material) A positive electrode active material containing a lithium nickel composite oxide and tungsten was prepared as follows.

[0066] First, a raw material aqueous solution (concentration 30 wt%) was prepared by dissolving a Ni element source (nickel sulfate), a Co element source (cobalt sulfate), a Mn element source (manganese sulfate), and an X element source (beryllium sulfate) in ion-exchanged water. A reaction solution with adjusted pH using sulfuric acid and aqueous ammonia was prepared in a reaction vessel. A sodium hydroxide aqueous solution was also prepared as a pH adjuster. Next, the raw material aqueous solution was added to the reaction solution at a predetermined rate while stirring, and neutralized with the pH adjuster. The crystallized product was washed with water, filtered, and dried to obtain composite hydroxide particles (precursor particles). Drying was performed at 120°C for 16 hours.

[0067] The obtained precursor particles were mixed with lithium carbonate. The molar ratio of lithium to the total of nickel, cobalt, manganese, and beryllium was 1.1. This mixture was fired in an electric furnace at 870°C for 15 hours. After cooling to room temperature in the furnace, it was subjected to a crushing treatment to obtain a lithium-nickel composite oxide (Li(Ni 0.8 Co 0.1 Mn 0.05 Be 0.05 )O2) was obtained. When the obtained lithium nickel composite oxide was observed under a microscope, it was confirmed that there were sufficient gaps between the primary particles (voids in the secondary particles).

[0068] The resulting lithium-nickel composite oxide was mixed with tungsten (IV) oxide (WO2) and tungsten (VI) oxide (WO3) at a tungsten ratio of 0.5 mass% (W / (lithium-nickel composite oxide + WO2 + WO3)). The mixture was treated in a mechanochemical device at 3000 rpm for 30 minutes and then heat-treated at 150°C for 1 hour to obtain a positive electrode active material in the form of a lithium-nickel composite oxide coated with tungsten oxide (WO2, WO3).

[0069] The obtained positive electrode active material was also analyzed using a transmission electron microscope with energy dispersive X-ray analysis (TEM-EDX). The results revealed that tungsten was incorporated into the primary particles of the lithium metal composite oxide. In other words, tungsten was present both inside and on the surface of the lithium nickel composite oxide in the obtained positive electrode active material.

[0070] (Preparation of positive electrode layer) The obtained positive electrode active material, a conductive material (carbon nanotubes), and a binder (polyvinylidene fluoride) were weighed in a mass ratio of 88:10:2 (positive electrode active material:conductive material:binder) and mixed to obtain a positive electrode composite. Mixing was performed for 10 minutes by adding the positive electrode composite and 1% by mass of zirconia balls (3.0 mm diameter) to a kneading machine (Thinky Corporation). The obtained positive electrode composite was applied to a positive electrode current collector (Al foil) using a film applicator (with film thickness adjustment function, Allgood Co., Ltd.), and then dried at 80°C for 5 minutes. This resulted in a positive electrode structure having a positive electrode current collector and a positive electrode layer.

[0071] (Production of evaluation battery (lithium ion secondary battery)) A paste for forming a negative electrode active material layer was prepared by mixing natural graphite (C) as a negative electrode active material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener in a mass ratio of C:SBR:CMC = 98:1:1 in ion-exchanged water. This paste was applied to both sides of a 10 μm-thick negative electrode current collector (Cu foil), dried, and then pressed to prepare a negative electrode structure having negative electrode layers on both sides of the negative electrode current collector.

[0072] Two 24 μm-thick porous polyolefin sheets with a three-layer structure of PP / PE / PP were prepared as separator sheets. The prepared positive electrode structure, negative electrode structure, and two prepared separator sheets were stacked and wound together to prepare a wound electrode assembly. Electrode terminals were attached to the positive and negative electrodes of the wound electrode assembly by welding, and the assembly was housed in a battery case with a liquid inlet.

[0073] [Examples 2 and 3] Except for using magnesium sulfate or calcium sulfate as the X element source to prepare the positive electrode active material, a test battery was fabricated in the same manner as in Example 1. The ratios and valences of Ni element, Co element, Mn element, and X element in the lithium nickel-based composite oxide are as shown in Table 1.

[0074] [Comparative Example] Except for preparing the positive electrode active material without using the X element source, a test battery was fabricated in the same manner as in Example 1. The ratios and valences of Ni element, Co element, and Mn element in the lithium nickel-based composite oxide are as shown in Table 1.

[0075] [evaluation] (SEM observation) The positive electrode layers obtained in the examples and comparative examples were cross-sectioned using a cross polisher. The cross-sections obtained were observed using a field emission scanning electron microscope (FE-SEM) to confirm the locations of the carbon nanotubes in the positive electrode layer. The results are shown in Table 2. As shown in Table 2, in all examples and comparative examples, the carbon nanotubes were present both on the surface and inside of the active material. In other words, the positive electrode layers in the examples and comparative examples contained both the first carbon nanotubes and the second carbon nanotubes.

[0076] (X-ray absorption fine structure (XAFS) measurement of the positive electrode layer) XAFS measurements were performed on the positive electrodes obtained in the examples and comparative examples using the following equipment. XAFS measurements were also performed on standard samples of tungsten (IV) oxide (WO2) and tungsten (VI) oxide (WO3) using the following equipment. The value of (ab) / (cb) in the above formula (1) was calculated from the XAFS spectrum at the rising position of the peak at the tungsten L absorption edge (10195 eV to 10206 eV). The results are shown in Table 2. Equipment: Hard X-ray XAFS at the Aichi Synchrotron Light Center, Science and Technology Foundation Measurement range: 9897-11297 eV (peak position of the L absorption edge of tungsten) Measurement method: The positive electrode was measured by the fluorescence method, and the tungsten compound was measured by the transmission method. The transmission method for measuring tungsten compounds in standard materials detects transmitted X-rays when irradiated with incident X-rays, while the fluorescence method for measuring positive electrodes detects fluorescent X-rays emitted when irradiated with incident X-rays, so even though the measurement methods are different, they can be expressed as the same spectrum. Normalizing the XAFS measurement data using the analysis software Athena makes it possible to compare the tungsten in tungsten compounds in standard materials with the tungsten in the positive electrode. To ensure the reproducibility of equation (1), a standard sample was measured for each measurement of the positive electrode sample, and any slight deviations were corrected.

[0077] (Initial resistance evaluation) The evaluation batteries prepared in the examples and comparative examples were subjected to the activation treatment described below, and then their initial resistance was measured. The evaluation batteries were placed in a 25°C environment. Activation (initial charging) was performed using a constant current-constant voltage method, with each evaluation battery being charged at a constant current of 1 / 3C up to 4.2V, followed by constant voltage charging until the current reached 1 / 50C, and then fully charged. Each evaluation battery was then discharged at a constant current of 1 / 3C down to 3.0V. In this manner, each evaluation battery was activated.

[0078] Each activated test battery was adjusted to an open circuit voltage of 3.70 V. It was then placed in a temperature environment of -28°C. It was discharged at a current of 20 C for 8 seconds, and the voltage drop ΔV was determined. Next, the voltage drop ΔV was divided by the discharge current value (20 C) to calculate the battery resistance, which was taken as the initial resistance. The initial resistance of the comparative example was set to 100%, and the initial resistance of each example was evaluated relative to it. The results are shown in Table 2.

[0079] (Evaluation of Ni mixing ratio) The positive electrode layers prepared in the examples and comparative examples were evaluated for metal content by ICP-MS and crystalline structure by XRD. Using the results, Rietveld analysis was performed to calculate the Ni mixing ratio. The results are shown in Table 2.

[0080] [Table 1]

[0081] [Table 2]

[0082] As shown in Table 2, the Ni mixing ratio was smaller in all Examples than in the Comparative Examples, and the initial resistance was suppressed. This indicates that the positive electrode layer of the present disclosure can suppress the resistance of the battery. Furthermore, as shown in Table 2, in the Examples and Comparative Examples, (ab) / (cd) was 0.79 or less, indicating that the tungsten in the positive electrode layer was tetravalent or had an average valence between tetravalent and hexavalent. [Explanation of symbols]

[0083] 1…Primary particle 2...Secondary particles 10...Cathode active material 11...First carbon nanotube

Claims

1. A positive electrode layer used in a lithium ion secondary battery, the positive electrode layer contains a positive electrode active material and carbon nanotubes, the positive electrode active material contains a lithium-nickel-based composite oxide containing at least a Li element, a Ni element, an O element, and an X element (excluding Li element and Ni element) having a valence of 1 or 2, and tungsten present in at least one of the interior and the surface of the lithium-nickel-based composite oxide; The lithium nickel-based composite oxide contains at least Be as the X element, and the positive electrode active material is a secondary particle having a plurality of primary particles and voids formed between the plurality of primary particles, the positive electrode layer contains, as the carbon nanotubes, first carbon nanotubes at least partially enclosed in the voids of the secondary particles, and The spectrum of the rising position (10195 eV to 10206 eV) of the peak at the L absorption edge of tungsten measured by X-ray absorption fine structure analysis (XAFS) is Formula (1): (a-b) / (c-b)≦0.79 [In formula (1), a represents the energy (eV) at which the spectral slope is greatest in the range of 10195 eV to 10206 eV, When the spectral intensity at a (eV) is A, b is WO ２ represents the energy (eV) at which the spectral intensity of tungsten (IV) oxide reaches the above A in the range of 10195 eV to 10206 eV, c is WO ３ (tungsten(VI) oxide) represents the energy (eV) at which the spectral intensity becomes A in the range of 10195 eV to 10206 eV.

2. 2. The positive electrode layer according to claim 1, wherein in the lithium nickel-based composite oxide, a molar ratio of the X element to the Ni element is 0.05 or more and 0.13 or less.

3. 2. The positive electrode layer according to claim 1, wherein the average valence of the Ni element in the lithium nickel-based composite oxide is 2.90 or more and 3.50 or less.

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