Positive electrode and lithium-ion secondary battery

Abstract

To provide a positive electrode with reduced resistance.SOLUTION: A positive electrode for a lithium-ion secondary battery includes a positive electrode active material layer. The positive electrode active material layer includes a positive electrode active material and carbon nanotubes. The carbon nanotubes have an aspect ratio equal to or more than 400. The carbon nanotubes cover at least a part of the surface of the positive electrode active material. The percentage content of the carbon nanotubes in the positive electrode active material layer is 0.3 mass% or more and 1.5 mass% or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Patent Document 1 (JP 2020-184490 A) discloses a positive electrode for a lithium ion secondary battery that contains at least a positive electrode active material and a conductive material, and that the conductive material contains carbon nanotubes with a diameter of 12 nm or less and an aspect ratio of 250 or less, and that the carbon nanotubes cover at least a portion of the surface of the positive electrode active material. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2020-184490 Summary of the Invention [Problem to be solved by the invention]

[0004] The positive electrode of a battery contains a positive electrode active material. The positive electrode active material tends to have poor electronic conductivity. To compensate for the electronic conductivity of the positive electrode active material, a conductive material is used. Generally, the conductive material contains a conductive carbon material such as carbon nanotubes.

[0005] Lithium-ion secondary batteries are being developed with increasing capacity. As capacity increases, the amount of conductive material contained in the positive electrode decreases. A decrease in the amount of conductive material contained in the positive electrode can increase the resistance of the positive electrode. Patent Document 1 discloses a technique for suppressing an increase in the resistance of the positive electrode even when the amount of conductive material contained in the positive electrode decreases. However, the technique disclosed in Patent Document 1 leaves room for improvement.

[0006] It is therefore an object of the present disclosure to provide a positive electrode with reduced resistance. [Means for solving the problem]

[0007] The technical configuration and effects of the present disclosure will be described below. However, the mechanism of action in this specification includes speculation. The mechanism of action does not limit the technical scope of the present disclosure.

[0008] [1] A positive electrode for a lithium ion secondary battery, a positive electrode active material layer, the positive electrode active material layer contains a positive electrode active material and carbon nanotubes, The carbon nanotubes have an aspect ratio of 400 or more, the carbon nanotubes cover at least a portion of the surface of the positive electrode active material, A positive electrode, wherein the content of the carbon nanotubes in the positive electrode active material layer is 0.3% by mass or more and 1.5% by mass or less.

[0009] The larger the aspect ratio of carbon nanotubes, the more easily they can creep between the cathode active material and contact it. Therefore, the area of ​​contact between the carbon nanotubes and the cathode active material is larger than when the same amount of carbon nanotubes with a smaller aspect ratio is contained, and this is expected to reduce resistance. Furthermore, as the carbon nanotube content decreases, the energy density of lithium-ion secondary batteries is expected to increase.

[0010] On the other hand, if the carbon nanotube content is reduced too much, it becomes difficult to form conductive paths, which may increase resistance. Therefore, it is considered necessary to keep the carbon nanotube content within a certain range.

[0011] [2] The diameter of the carbon nanotube is 10 nm or less, The positive electrode according to [1], wherein the carbon nanotubes have a length of 4 μm or more.

[0012] [3] The positive electrode according to [1] or [2], wherein the coverage of the surface of the positive electrode active material with the carbon nanotubes is 40% or more.

[0013] [4] A lithium ion secondary battery comprising the positive electrode according to any one of [1] to [3]. [5] A positive electrode for a lithium ion secondary battery, a positive electrode active material layer, the positive electrode active material layer contains a positive electrode active material and carbon nanotubes, The carbon nanotubes have an aspect ratio of 400 or more, the carbon nanotubes cover at least a portion of the surface of the positive electrode active material, the content of the carbon nanotubes in the positive electrode is 0.5% by mass or more and 1.3% by mass or less, The diameter of the carbon nanotubes is 10 nm or less, The length of the carbon nanotubes is 4 μm or more, A positive electrode, wherein the coverage of the surface of the positive electrode active material with the carbon nanotubes is 50% or more. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic diagram showing an example of the lithium ion secondary battery of this embodiment. [Figure 2] FIG. 2 is a schematic diagram showing an example of the electrode assembly of this embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, an embodiment of the present disclosure (hereinafter may be abbreviated as "the present embodiment") and an example of the present disclosure (hereinafter may be abbreviated as "the present example") will be described. However, the present embodiment and the example do not limit the technical scope of the present disclosure. In this disclosure, a lithium ion secondary battery will be simply referred to as a "battery."

[0016] <Positive electrode> The positive electrode includes a positive electrode current collector foil and a positive electrode active material layer. The positive electrode current collector foil may include, for example, aluminum (Al) foil.

[0017] The positive electrode active material layer contains a positive electrode active material. The positive electrode active material may contain, for example, a lithium composite oxide having a layered structure. Examples of the lithium composite oxide having a layered structure include lithium nickel composite oxide, lithium manganese composite oxide, lithium cobalt composite oxide, lithium nickel cobalt aluminum composite oxide, lithium iron nickel manganese composite oxide, and lithium nickel cobalt manganese composite oxide. Among these, lithium nickel cobalt manganese composite oxide is preferred because of its particularly excellent resistance characteristics.

[0018] The lithium-nickel-cobalt-manganese composite oxide may further contain additional elements such as transition metal elements and typical metal elements in addition to lithium (Li), nickel (Ni), cobalt (Co), and manganese (Mn). The additional element is at least one element selected from the group consisting of aluminum (Al), zirconium (Zr), boron (B), magnesium (Mg), iron (Fe), copper (Cu), zinc (Zn), tin (Sn), sodium (Na), potassium (K), barium (Ba), strontium (Sr), calcium (Ca), tungsten (W), molybdenum (Mo), niobium (Nb), titanium (Ti), silicon (Si), vanadium (V), chromium (Cr), and germanium (Ge). The content of the additional element is preferably 0.1 mol or less relative to Li.

[0019] The lithium nickel cobalt manganese based composite oxide preferably has a composition represented by the following formula (1).

[0020] Li 1+z Ni 1-x-y Co x Mn y M a O2(1) In formula (1), z, x, y, and a are each -0.3 <z<0.3、0<x≦0.4、0<y≦0.5、1-x> y satisfies 0≦a≦0.1, and M represents the above-mentioned additional element.

[0021] The positive electrode active material may be secondary particles (aggregates of primary particles). The secondary particles may have an average particle size (D50) of, for example, 1 to 50 μm, or may have a D50 of 5 to 20 μm. The primary particles may have a maximum Feret diameter of, for example, 0.1 to 3 μm. Here, D50 refers to the particle size at which the cumulative frequency from the smaller particle size side reaches 50% in the volume-based particle size distribution. D50 can be measured by a laser diffraction method.

[0022] The positive electrode active material may be, for example, porous particles or solid particles, and is preferably porous particles because it increases the contact area with carbon nanotubes (CNTs) described later.

[0023] The content of the positive electrode active material in the positive electrode active material layer is 95% by mass or more, optionally 96% by mass or more, or optionally 97% by mass or more. The content of the positive electrode active material in the positive electrode active material layer is 99% by mass or less, optionally 98.5% by mass or less, or optionally 98% by mass or less.

[0024] The positive electrode active material layer contains CNT. CNT is a conductive material. The CNT has an aspect ratio of 400 or more. When the CNT has an aspect ratio of 400 or more, the area of ​​contact between the CNT and the positive electrode active material increases, and a reduction in resistance is expected. The CNT may have an aspect ratio of 450 or more.

[0025] The aspect ratio is the ratio of length to diameter. In this specification, the "aspect ratio" is calculated by dividing the average length of CNTs by the average diameter of CNTs. The average length and average diameter can each be the arithmetic mean of measurements taken on 25 CNTs. The length and diameter of individual CNTs are measured in SEM (scanning electron microscope) images. The magnification of the SEM image is about 5000 times.

[0026] The CNTs have an average length of, for example, 4 μm or more. When the average length of the CNTs is 4 μm or more, the area of ​​contact with the positive electrode active material increases, and it is expected that the CNTs will be effective even at a low content. The CNTs have an average diameter of, for example, 10 nm or less.

[0027] The CNTs cover at least a portion of the surface of the positive electrode active material. The CNTs may cover the entire surface of the positive electrode active material. The coverage may be, for example, 30% or more, 40% or more, or 50% or more. A higher coverage is expected to result in a reduction in initial resistance, for example.

[0028] The coverage rate is calculated from an SEM image of an arbitrary cross section of the positive electrode active material layer. The SEM image is taken of a cross section parallel to the thickness direction of the positive electrode active material layer. The magnification of the SEM image is approximately 5000 times. The SEM image is binarized to distinguish between the positive electrode active material and the portions of the positive electrode active material that are not covered with CNTs (voids). The binarization is performed using a threshold value that can properly separate the positive electrode active material and the voids. In the SEM image, areas with dark contrast are voids. The coverage rate is calculated from the area ratio of the voids on the surface of the positive electrode active material.

[0029] The CNT content in the positive electrode active material layer is 0.3% by mass or more and 1.5% by mass or less. If the CNT content in the positive electrode active material layer is less than 0.3% by mass, it becomes difficult to form a conductive path, which may increase resistance. If the CNT content in the positive electrode active material layer exceeds 1.5% by mass, the energy density may decrease. The CNT content in the positive electrode active material layer is preferably 0.3% by mass or more and 1.4% by mass or less, and more preferably 0.5% by mass or more and 1.3% by mass or less.

[0030] The positive electrode active material layer may further contain, for example, a conductive material other than CNT, a binder, etc. The conductive material other than CNT may include, for example, acetylene black (AB), etc. The binder may include, for example, polyvinylidene fluoride (PVdF), etc. The blending amount of the conductive material other than CNT and the binder in the positive electrode active material layer may be, for example, 0.1 mass % or more and 5 mass % or less.

[0031] <Lithium-ion secondary battery> FIG. 1 is a schematic diagram showing an example of a lithium-ion secondary battery according to this embodiment. The battery 100 includes a case 90. The case 90 may have any shape. For example, the case 90 may be rectangular or cylindrical. The case 90 may be made of metal, such as a pouch made of an aluminum (Al) laminate film. A positive electrode terminal 91 and a negative electrode terminal 92 may be provided on the case 90.

[0032] The case 90 houses the electrode assembly 50 and an electrolyte. The electrolyte is impregnated into the electrode assembly 50. The electrode assembly 50 is connected to a positive electrode terminal 91 and a negative electrode terminal 92.

[0033] FIG. 2 is a schematic diagram showing an example of an electrode assembly of this embodiment. The electrode assembly 50 includes a positive electrode 20, a separator 40, and a negative electrode 30. The electrode assembly 50 has any structure. For example, the electrode assembly 50 may be a wound type. The positive electrode 20, the separator 40, and the negative electrode 120 may all be strip-shaped sheets. The electrode assembly 50 may be formed, for example, by stacking the positive electrode 20, the separator 40 (first sheet), the negative electrode 30, and the separator 40 (second sheet) in this order. After winding, the electrode assembly 50 may be formed into a flat shape.

[0034] 《Negative electrode》 The negative electrode 30 includes a negative electrode current collector foil and a negative electrode active material layer. The negative electrode current collector foil may include, for example, copper (Cu) foil.

[0035] The negative electrode active material layer contains a negative electrode active material. The negative electrode active material may contain, for example, at least one selected from the group consisting of graphite, soft carbon, and hard carbon. The content of the negative electrode active material in the negative electrode active material layer is 90% by mass or more, and may be 95% by mass or more and 98% by mass or less.

[0036] The negative electrode active material layer may further contain, for example, a thickener, a binder, etc. The thickener may include, for example, carboxymethyl cellulose (CMC), methyl cellulose (MC), etc. The binder may include, for example, styrene butadiene rubber (SBR), PVdF, etc. The content of the thickener and binder in the negative electrode active material layer may be, for example, 0.1 mass % or more and 10 mass % or less.

[0037] <Separator> The separator 40 is porous. The separator 40 is permeable to the electrolyte. The separator 40 separates the positive electrode 20 and the negative electrode 30. The separator 40 is electrically insulating. The separator 40 may contain, for example, a polyolefin resin such as polyethylene (PE) or polypropylene (PP). The separator 40 may have, for example, a single-layer structure or a multi-layer structure. The separator 40 may, for example, be essentially composed of a PE layer, or may be formed by laminating a PP layer, a PE layer, and a PP layer in this order. A heat-resistant layer, for example, may be formed on the surface of the separator 40.

[0038] 《Electrolyte》 The electrolyte solution includes a solvent and a Li salt. The solvent is aprotic. The solvent may include any component. For example, the solvent may include at least one selected from the group consisting of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC).

[0039] The Li salt is a supporting electrolyte. The Li salt is dissolved in a solvent. The Li salt may include, for example, at least one selected from the group consisting of LiPF6 and LiBF4. The Li salt may have a molar concentration of, for example, 0.5 mol / L or more and 2.0 mol / L or less.

[0040] The electrolytic solution may further contain an optional additive. For example, the electrolytic solution may contain 0.01% by mass or more and 5% by mass or less of the additive. The additive may include, for example, at least one selected from the group consisting of vinylene carbonate (VC) and vinyl ethylene carbonate (VEC). [Example]

[0041] The present embodiment will be described below using examples, but the present embodiment is not limited to these.

[0042] Example 1 (positive electrode) The positive electrode current collector foil is Al foil (thickness: 15 μm), and the positive electrode active material is LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (porous particles), CNTs having the outer diameter, length, and aspect ratio listed in Table 1 as the conductive material, PVdF as the binder, and N-methyl-2-pyrrolidone (NMP) as the dispersion medium were prepared. The positive electrode active material, CNTs, binder, and dispersion medium were mixed using a planetary mixer to prepare a positive electrode paste. The mass ratio of the positive electrode active material to CNTs was as listed in Table 1, and the mass ratio of the binder was 1.5 mass%. The positive electrode paste was applied to both sides of a positive electrode current collector foil using a die coater, dried, and pressed to prepare a positive electrode sheet. The outer diameter and length of the CNTs listed in Table 1 were measured using the method described above. The coverage rates listed in Table 1 were also calculated using the method described above.

[0043] (Negative electrode) A Cu foil (thickness: 10 μm) was prepared as the negative electrode current collector foil, natural graphite as the negative electrode active material, CMC as the thickener, SBR as the binder, and ion-exchanged water as the dispersion medium. The negative electrode active material, thickener, binder, and dispersion medium were mixed in ion-exchanged water to prepare a negative electrode paste. The mixing ratio (mass ratio) of the negative electrode active material, thickener, and binder was 98:1:1. The negative electrode paste was applied to both sides of the negative electrode current collector foil using a die coater, dried, and then pressed to produce a negative electrode sheet.

[0044] (separator) Two 16 μm thick separator sheets (porous membranes) were prepared. These separators had a three-layer structure consisting of a PP porous layer, a PE porous layer, and a PP porous layer stacked in this order.

[0045] (electrolyte) A mixed solvent was prepared by mixing EC, DMC, and EMC. The mixing ratio (volume ratio) of EC, DMC, and EMC was 3:4:3. An electrolyte was prepared by dissolving LiPF6 in the solvent. LiPF6 was dissolved so that the concentration in the electrolyte was 1.0 mol / L.

[0046] (lithium-ion secondary battery) A positive electrode sheet, a negative electrode sheet, and two separator sheets were stacked and wound to produce a wound electrode body. Electrode terminals were attached to the positive electrode sheet and negative electrode sheet of the wound electrode body by welding, and the resultant was housed in a case with a liquid inlet. Electrolyte was poured into the case. After the electrolyte was poured, the case was sealed. Thus, the battery of Example 1 was produced.

[0047] Examples 2 and 3 A positive electrode sheet was produced in the same manner as in Example 1, except that the conductive material was changed to CNTs having the outer diameter, length, and aspect ratio shown in Table 1, and the mass ratio of the positive electrode active material to the CNTs was changed to the ratio shown in Table 1. Then, the batteries of Examples 2 and 3 were produced in the same manner as in Example 1.

[0048] Comparative Examples 1 to 10 Positive electrode sheets were produced in the same manner as in Example 1, except that the conductive material was changed to CNTs having the outer diameter, length, and aspect ratio shown in Table 1, and the mass ratio of the positive electrode active material to the CNTs was changed to the ratio shown in Table 1. Then, batteries of Comparative Examples 1 to 10 were produced in the same manner as in Example 1.

[0049] <Evaluation> (initial resistance) Each battery prepared as described above was placed in an environment at 25°C. Activation (initial charging) was performed using a constant current-constant voltage method, with each 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 battery was then discharged at a constant current of 1 / 3C down to 3.0V. Note that "C" is the unit of current rate. "1C" indicates the current rate at which the state of charge (SOC) reaches 0-100% after one hour of charging.

[0050] Each activated battery was adjusted to an open circuit voltage of 3.70 V. It was then placed in a temperature environment of -10°C. It was discharged at a current value of 10 C for 10 seconds, and the voltage drop ΔV was determined. Next, the voltage drop ΔV was divided by the discharge current value (10 C) to calculate the battery resistance, which was taken as the initial resistance. For Examples 1 and 3 and Comparative Examples 1 to 10, the ratio of the initial resistance of the other Examples and Comparative Examples was determined, assuming that the initial resistance of Example 2 was 1. The results are shown in Table 1.

[0051] (Resistance of the positive electrode active material layer) The positive electrode pastes prepared in Examples 1 to 3 and Comparative Examples 1 to 10 were applied to one side of a positive electrode current collector foil using a die coater, dried, and then pressed to prepare positive electrode sheets. The resistance of the positive electrode active material layer of each positive electrode sheet was measured using an electrode resistance measurement system "RM2610" manufactured by Hioki E.E. Corporation. For Examples 1 and 3 and Comparative Examples 1 to 10, the resistance ratio of the positive electrode active material layer of Example 2 was determined as 1, and the results are shown in Table 1. The resistance of the positive electrode active material layer represents the dispersion state of each material in the positive electrode active material; if the materials are uniformly dispersed without aggregation, the resistance will be low.

[0052] [Table 1]

[0053] <Result> In Examples 1 to 3, the initial resistance and the resistance of the positive electrode active material layer were low.

[0054] On the other hand, in Comparative Examples 1 to 10, at least one of the initial resistance and the resistance of the positive electrode active material layer was high.

[0055] The present embodiment and examples are illustrative in all respects. The present embodiment and examples are not limiting. The technical scope of the present disclosure encompasses all modifications within the meaning and scope equivalent to the claims. For example, it is also intended from the beginning that any configuration may be extracted from the present embodiment and examples and that they may be combined in any desired manner. [Explanation of symbols]

[0056] 20 positive electrode, 30 negative electrode, 40 separator, 50 electrode body, 90 case, 91 positive electrode terminal, 92 negative electrode terminal, 100 lithium ion secondary battery.

Claims

1. A positive electrode for a lithium ion secondary battery, a positive electrode active material layer, the positive electrode active material layer contains a positive electrode active material and carbon nanotubes, The carbon nanotubes have an aspect ratio of 400 or more, the carbon nanotubes cover at least a portion of the surface of the positive electrode active material, the content of the carbon nanotubes in the positive electrode active material layer is 0.9% by mass or more and 1.5% by mass or less, The diameter of the carbon nanotubes is 10 nm or less, The length of the carbon nanotubes is 4 μm or more and 4.6 μm or less, A positive electrode, wherein the coverage of the surface of the positive electrode active material with the carbon nanotubes is 40% or more.

2. 2. The positive electrode according to claim 1, wherein a coverage of the surface of the positive electrode active material with the carbon nanotubes is 40% or more and 54.1% or less.

3. The positive electrode of claim 1 , wherein the positive electrode active material is porous particles.

4. A lithium ion secondary battery comprising the positive electrode according to any one of claims 1 to 3.

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