Positive electrode layer

Abstract

To provide a positive electrode layer that can improve cycle characteristics of lithium ion secondary batteries.SOLUTION: A positive electrode layer contains a positive electrode active material and carbon nanotubes. The positive electrode active material contains tungsten and is secondary particles including a plurality of primary particles and voids formed between the plurality of primary particles. The positive electrode layer contains first carbon nanotubes of which at least some are included in the voids of the secondary particles, and spectrums at rising positions of peaks of L absorption edges of tungsten measured by X-ray absorption fine structure analysis satisfy formula (1): (a-b) / (c-b)≤0.79 [In formula (1), a represents an energy (eV) when the slope of the spectrum is the largest, and when a spectral intensity at a (eV) is set as A, b represents an energy (eV) at which the spectral intensity of tungsten (IV) oxide becomes the A, and c represents an energy (eV) at which the spectral intensity of tungsten (VI) oxide becomes the A.].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 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 resistance increase during repeated charge and discharge. Note that Patent Document 1 does not specify the valence of tungsten in the lithium tungstate.

[0004] Furthermore, Patent Document 2 discloses that a part of the surface of a positive electrode active material is covered with carbon nanotubes, which are a conductive material. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2021-018895 [Patent Document 2] Japanese Patent Application Publication No. 2020-184490 Summary of the Invention [Problem to be solved by the invention]

[0006] From the viewpoint of improving the performance of a lithium ion secondary battery, it is preferable that the cycle characteristics are good. The present disclosure has been made in view of the above-mentioned circumstances, and a main object of the present disclosure is to provide a positive electrode layer that can improve the cycle characteristics of a lithium ion secondary battery. [Means for solving the problem]

[0007] [1] A positive electrode layer for use in a lithium ion secondary battery, the positive electrode layer containing a positive electrode active material and carbon nanotubes, the positive electrode active material containing tungsten and secondary particles having a plurality of primary particles and voids formed between the plurality of primary particles, the positive electrode layer containing first carbon nanotubes as the carbon nanotubes, at least a portion of which is enclosed in the voids of the secondary particles, and a rise position (10195 eV to 10206 eV) of a peak at the L absorption edge of tungsten measured by X-ray absorption fine structure analysis (XAFS) a positive electrode layer in which the spectrum of WO2 (tungsten (IV) oxide) satisfies 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, and 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).

[0008] [2] The positive electrode layer according to [1], wherein the carbon nanotubes contained in the positive electrode layer are second carbon nanotubes that are not contained in the voids of the secondary particles.

[0009] [3] The positive electrode layer according to [1] or [2], wherein the formula (1) satisfies 0.22≦(ab) / (cb)≦0.79.

[0010] [4] The positive electrode layer according to any one of [1] to [3], wherein the positive electrode active material contains a lithium metal composite oxide. [Effects of the Invention]

[0011] The present disclosure provides an effect of improving the cycle characteristics of a lithium ion secondary battery. [Brief explanation of the drawings]

[0012] [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

[0013] 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.

[0014] 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 tungsten and 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. Furthermore, the positive electrode layer in the present disclosure satisfies 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 be referred to as "voids of secondary particles." The "voids of secondary particles" generally refers to the internal spaces of secondary particles surrounded by a plurality of primary particles.

[0015] According to the present disclosure, the positive electrode layer contains first carbon nanotubes at least some of which are enclosed in the voids of the secondary particles, and therefore when used in a lithium ion secondary battery, the cycle characteristics can be improved.

[0016] When a battery is repeatedly charged and discharged, the positive electrode active material may expand and contract, resulting in cracks. There is a particular concern that cracks in the positive electrode active material may be more likely to occur when the battery is charged and discharged under high load conditions. If cracks occur, the contact between the positive electrode active material and a conductive material such as carbon nanotubes may be severed, potentially disrupting the maintenance of a good electron conduction path. This may result in an increase in battery resistance. In contrast, the positive electrode layer in the present disclosure contains first carbon nanotubes at least partially enclosed in the voids of the secondary particles of the positive electrode active material. Therefore, even if cracks occur in the positive electrode active material, fragments of the broken positive electrode active material may be able to contact the first carbon nanotubes, thereby maintaining the electron conduction path. As a result, it is believed that a decrease in the cycle characteristics of the lithium-ion secondary battery can be suppressed.

[0017] Furthermore, the positive electrode layer of the present disclosure satisfies the predetermined formula (1) when measured by X-ray absorption fine structure analysis (XAFS), and therefore battery resistance can also be reduced. As will be described in detail later, when tungsten in the positive electrode layer is tetravalent or has an average valence between tetravalent and hexavalent, it is believed that tungsten exhibits high conductivity and the effect of reducing activation energy. As a result, when the positive electrode layer of the present disclosure is used in a lithium-ion battery, it is believed that battery resistance can be reduced.

[0018] (1) Positive electrode active material The positive electrode active material in the present disclosure contains tungsten.

[0019] Tungsten may be present inside the primary particles of the positive electrode active material. That is, tungsten may be present as one of the components constituting the composition of the primary particles. Tungsten may also be present on the surface of the primary particles. That is, tungsten (tungsten compound) may be present so as to coat the surfaces of the primary particles (e.g., primary particles of a lithium metal composite oxide described later). In this case, tungsten may be present between the primary particles. Tungsten may also be present on the surface of the secondary particles of the positive electrode active material. That is, tungsten (tungsten compound) may be present so as to coat the surfaces of the secondary particles (e.g., secondary particles of a lithium metal composite oxide described later). For example, after the secondary particles are produced, the surfaces of the secondary particles may be coated with tungsten (tungsten compound). In this case, tungsten is usually not present at the contact interface between adjacent primary particles constituting the secondary particles.

[0020] 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.

[0021] Examples of tetravalent tungsten compounds include WO2, lithium tungstate containing tetravalent tungsten, and lithium nickel cobalt manganese composite oxides containing tetravalent tungsten.

[0022] 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.

[0023] 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.

[0024] The positive electrode active material may further contain a lithium metal composite oxide as an essential component, or may contain a lithium metal composite oxide with a layered structure.

[0025] Examples of lithium metal composite oxides include lithium nickel composite oxides, lithium manganese composite oxides, lithium cobalt composite oxides, lithium nickel cobalt aluminum composite oxides, lithium iron nickel manganese composite oxides, lithium nickel cobalt manganese composite oxides, etc. Among these, lithium nickel cobalt manganese composite oxides are preferred because they have more excellent resistance characteristics.

[0026] In this specification, the term "lithium nickel cobalt manganese composite oxide" encompasses oxides containing Li, Ni, Co, Mn, and O as constituent elements, as well as oxides containing one or more additional elements. Examples of such additional elements include transition metal elements and typical metal elements such as Mg, Ca, Al, Ti, V, Cr, Si, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. The additional element may also be a metalloid element such as B, C, Si, or P, or a nonmetal element such as S, F, Cl, Br, or I. The content of these additional elements is preferably 0.1 moles or less relative to lithium. This also applies to the lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide described above.

[0027] 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.

[0028] The primary particles preferably contain the above-mentioned tungsten, and more preferably contain the above-mentioned tungsten and lithium metal composite oxide.

[0029] 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.

[0030] Average particle size of primary particles (D 50 ) 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.

[0031] The secondary particles may be particles consisting only of the above-mentioned tungsten-containing primary particles, or may be particles containing the above-mentioned tungsten-containing primary particles and other primary particles (for example, particles containing a lithium metal composite oxide without containing tungsten).

[0032] 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.

[0033] 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.

[0034] Average particle diameter of secondary particles (D 50 ) is, for example, 0.1 μm or more and 1000 μm or less. The average particle size is the same as above.

[0035] 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.

[0036] 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.

[0037] (2) Carbon nanotubes (CNTs) The positive electrode layer in the present disclosure contains carbon nanotubes, which function as a conductive material.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] The second carbon nanotubes can be contained in the positive electrode layer by adding carbon nanotubes when preparing the positive electrode mixture, which will be described later.

[0042] 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.

[0043] 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.

[0044] (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).

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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 lowers 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 lowers activation energy, which is believed 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 believed that the crystal axes of the crystalline structure of the positive electrode active material are broadened, thereby reducing the diffusion resistance of lithium ions. For the reasons described above, the use of the positive electrode layer of the present disclosure in a lithium-ion battery is believed to reduce battery resistance.

[0049] 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.

[0050] 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.

[0051] 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).

[0052] 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.

[0053] 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-mentioned positive electrode active material (positive electrode active material in which first carbon nanotubes are encapsulated in the voids of secondary particles) and a solvent 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.

[0054] 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 solid-state lithium-ion secondary battery having a positive electrode layer, a negative electrode layer, and a solid 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.

[0055] 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.

[0056] 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.

[0057] 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]

[0058] [Example 1] (Preparation of Positive Electrode Active Material) A raw material aqueous solution containing nickel sulfate, cobalt sulfate, and manganese sulfate in a molar ratio of 1:1:1 was prepared. Meanwhile, a reaction solution with the pH adjusted using sulfuric acid and aqueous ammonia was prepared in a reaction vessel. A sodium hydroxide aqueous solution was also prepared as a pH adjusting solution. Next, the raw material aqueous solution was added to the reaction solution at a predetermined rate while stirring, and neutralized with the pH adjusting solution. The crystallized product was washed with water, filtered, and dried to obtain composite hydroxide particles (precursor particles).

[0059] The obtained precursor particles were mixed with lithium carbonate. The molar ratio (Li / Me) of lithium (Li) to the total (Me) of nickel, cobalt, and manganese 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 crushed to obtain a lithium metal composite oxide (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2) was obtained. When the obtained lithium composite oxide was observed under a microscope, it was confirmed that there were sufficient gaps between the primary particles (voids in the secondary particles).

[0060] The resulting lithium metal composite oxide was mixed with tungsten (IV) oxide (WO2) and tungsten (VI) oxide (WO3) at a tungsten ratio of 0.5% by mass (W / (lithium metal 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 lithium metal composite oxide coated with tungsten oxide (WO2, WO3) as a positive electrode active material.

[0061] The obtained positive electrode active material was analyzed using a transmission electron microscope with energy dispersive X-ray analysis (TEM-EDX), which revealed that tungsten was incorporated into the primary particles of the lithium metal composite oxide.

[0062] (Preparation of positive electrode layer) The obtained positive electrode active material, a conductive material (carbon nanotubes), and a binder (polyvinylidene fluoride) were weighed out in a mass ratio of positive electrode active material:conductive material:binder = 88:10:2 and mixed. A dispersion medium was added to the obtained mixture and stirred to obtain a positive electrode slurry. The obtained positive electrode slurry was applied to a positive electrode current collector (Al foil) using a film applicator (with film thickness adjustment function, All Good 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.

[0063] (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.

[0064] 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.

[0065] Next, a non-aqueous electrolyte was poured into the battery case through the inlet, which was then sealed airtight. The non-aqueous electrolyte was a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 3:4:3, with LiPF6 dissolved as a supporting electrolyte at a concentration of 1.0 mol / L. In this manner, a test battery (lithium ion secondary battery) was obtained.

[0066] [Examples 2 to 4] A positive electrode active material and a battery for evaluation were prepared in the same manner as in Example 1, except that in preparing the positive electrode active material, the proportions of the various materials were adjusted so that the amount of tungsten (W / (lithium metal composite oxide + WO2 + WO3)) was the value shown in Table 1.

[0067] [Comparative Example 1] The preparation conditions for the lithium composite oxide were changed to prepare a lithium composite oxide with no or very little gaps between primary particles (voids in secondary particles). Furthermore, only tungsten (VI) oxide (WO) was used as the tungsten oxide. A positive electrode active material and a test battery were prepared in the same manner as in Example 1, except for the above.

[0068] [Comparative Examples 2 and 4] The preparation conditions for the lithium composite oxide were changed to prepare a lithium composite oxide with no or very few gaps between primary particles (voids in secondary particles). The ratio of WO2 and WO3 added was also changed, and at least one of the heat treatment temperature (100°C to 200°C) and the heat treatment time (0.5 hours to 2.0 hours) was changed so that the (ab) / (cb) ratio in the positive electrode active material was the value shown in Table 1. Other than these, the positive electrode active material and the evaluation battery were prepared in the same manner as in Example 1.

[0069] Comparative Example 3 The positive electrode active material and the evaluation battery were prepared in the same manner as in Example 1, except that the ratio of WO2 to WO3 added was changed so that (ab) / (cb) in the positive electrode active material was the value shown in Table 1, and at least one of the heat treatment temperature (100°C to 200°C) and the heat treatment time (0.5 hours to 2.0 hours) was changed.

[0070] [evaluation] (SEM observation) The positive electrode layers obtained in each example and comparative example were observed by cross-section SEM to confirm the location of the carbon nanotubes in the positive electrode active material. The results are shown in Table 1. In Table 1, "surface only" means that the first carbon nanotubes were not observed, and only the second carbon nanotubes were observed. The reason that the first carbon nanotubes were not observed in Comparative Examples 1, 2, and 4 is presumably because the produced lithium composite oxide did not have sufficient voids for the carbon nanotubes to enter the inside of the positive electrode active material (secondary particles).

[0071] (Measurement of tungsten content in positive electrode active material) 1 g of the resulting positive electrode active material was weighed out and heated at 300°C using a heater in a mixture of 5 ml of commercially available nitric acid and 10 ml of hydrogen peroxide solution until complete dissolution was confirmed visually. The residue was filtered and the volume was adjusted to 100 ml with pure water, and the tungsten content (mass%) was measured using ICP atomic emission spectroscopy. The results are shown in Table 1. The ICP atomic emission spectroscopy used was an ICP atomic emission spectroscopy analyzer manufactured by Hitachi High-Tech Science Corporation.

[0072] (X-ray absorption fine structure (XAFS) measurement of the positive electrode layer) XAFS measurements were performed on the positive electrodes obtained in each Example and Comparative Example 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 (10195 eV to 10206 eV) of the peak at the L absorption edge of tungsten. The results of (ab) / (cb) for Examples 1 to 4 and Comparative Examples 1 to 4 are shown in Table 1. 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.

[0073] (Initial resistance evaluation) The evaluation batteries prepared in each example and comparative example were subjected to the activation treatment described below, and then the 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.

[0074] 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 value 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 ratio of the initial resistance of each Example and Comparative Example, assuming the initial resistance of Comparative Example 1 to be 1.00, was determined. The results are shown in Table 1.

[0075] (Resistance increase rate evaluation (cycle test)) Each test battery whose initial resistance had been measured was placed in a 60°C environment and subjected to 500 cycles of charge / discharge, with one cycle consisting of a constant-current charge at 10 C to 4.2 V and a constant-current discharge at 10 C to 3.3 V. The battery resistance at the 500th cycle was measured using the same method as above. The resistance increase rate was calculated as an index of resistance increase using the formula: (battery resistance at the 500th charge / discharge cycle - initial resistance) / initial resistance. The ratio of the resistance increase rate for each Example and Comparative Example, where the resistance increase rate for Comparative Example 1 was set to 1, was then calculated. The results are shown in Table 1. After the cycle test, the positive electrode layer was removed from the test battery and examined by cross-section SEM. It was confirmed that cracks had occurred in the positive electrode active material in each Example and Comparative Example.

[0076] [Table 1]

[0077] Comparative Examples 2 and 3 and each Example showed that when (ab) / (cd) was 0.79 or less and CNTs were present inside the active material (in the voids between secondary particles) (i.e., the positive electrode layer contained first carbon nanotubes), the initial resistance was significantly reduced. This is thought to be because electrons were sufficiently conducted to the inside of the active material without cracks occurring in the active material. Furthermore, Examples 1 to 4 and Comparative Example 3 showed a reduced resistance increase rate after cycle testing compared to Comparative Examples 1, 2, and 4. This is thought to be because the electron conduction path was maintained even when cracks occurred in the active material due to cycle testing under harsh conditions. This indicates that using a positive electrode layer according to the present disclosure in a lithium ion battery can reduce the resistance (initial resistance) of the lithium ion secondary battery and exhibit good cycle characteristics. [Explanation of symbols]

[0078] 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 is a secondary particle containing tungsten and 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): 0.22≦(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 represents the energy (eV) at which the spectral intensity of tungsten(VI) oxide reaches the above A in the range of 10195 eV to 10206 eV.

2. The positive electrode layer according to claim 1 , wherein the carbon nanotubes in the positive electrode layer are second carbon nanotubes that are not enclosed in the voids of the secondary particles.

3. The positive electrode layer according to claim 1 or 2, wherein the positive electrode active material contains a lithium metal composite oxide.

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