negative electrode

Abstract

To provide a negative electrode that is less prone to peeling or cracking. [Solution] A negative electrode 20 for a secondary battery having a negative electrode current collector 22 and a negative electrode composite layer 21 laminated on the negative electrode current collector 22 and containing an active material, comprising a first layer 21a and a second layer 21c laminated so as to cover the layer surface and side surface of the first layer 21a, wherein in the first layer 21a the longitudinal direction of the active material contained in the first layer is oriented toward the thickness direction of the negative electrode current collector 22, and the second layer 21c has a higher peel strength than the first layer 21a.

Description

[Technical Field]

[0001] This disclosure relates to the negative electrode of a secondary battery. [Background technology]

[0002] Patent Document 1 discloses that in the negative electrode active material layer, the orientation degree of the surface layer is 0.01% or more and 0.4% or less, and the orientation degree of the layer on the negative electrode current collector foil is 0.5% or less. Patent Document 2 discloses a layer in which the binder concentration on the current collector foil side is higher than the binder concentration on the opposite side of the current collector foil. Patent Document 3 discloses that the first positive electrode active material has a layer surrounding it, and the basis weight of the second positive electrode active material is larger. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2015-138644 [Patent Document 2] Japanese Patent Publication No. 2014-107182 [Patent Document 3] Japanese Patent Publication No. 2015-138730 [Overview of the project] [Problems that the invention aims to solve]

[0004] Conventional negative electrode active material layers suffered from problems such as peeling due to stress caused by loads during the manufacturing process, and cracking of the electrode surface due to expansion and contraction during durable charge and discharge.

[0005] Therefore, the purpose of this disclosure is to provide a negative electrode that is less prone to peeling or cracking. [Means for solving the problem]

[0006] This application discloses a negative electrode for a secondary battery, comprising a negative electrode current collector and a negative electrode composite layer laminated to the negative electrode current collector and containing an active material, wherein the negative electrode includes a first layer and a second layer laminated to cover the layer surface and side surface of the first layer, the first layer being oriented such that the longitudinal direction of the active material contained in the first layer is oriented in the thickness direction of the negative electrode current collector, and the second layer having a higher peel strength than the first layer.

[0007] The second layer may be oriented such that the longitudinal direction of the active material contained in the second layer intersects with the thickness direction of the negative electrode current collector.

[0008] The second layer may contain 25% or more less binder than the first layer. [Effects of the Invention]

[0009] According to this disclosure, it is expected that the diffusibility of lithium ions will be maintained, and peeling due to stress such as pressing and handling during the process, as well as cracking of the electrode surface due to expansion and contraction during durable charge and discharge, will be suppressed. [Brief explanation of the drawing]

[0010] [Figure 1] Figure 1 illustrates the layer structure of the secondary battery 10. [Figure 2] Figure 2 illustrates the configuration of the negative electrode 20 in Example 1. [Figure 3] Figure 3 illustrates the configuration of the negative electrode 30 in Example 2. [Modes for carrying out the invention]

[0011] [Nonaqueous electrolyte secondary battery] Figure 1 is a schematic diagram showing an example of the configuration of a non-aqueous electrolyte secondary battery 10 (sometimes referred to as "secondary battery 10") according to one example of its form. The secondary battery 10 shown in Figure 1 is a lithium-ion secondary battery, in which a negative electrode 20 and a positive electrode 40 are stacked via a separator 11, and together with a non-aqueous electrolyte, are sealed in an outer casing.

[0012] <Negative electrode (Morphological Example 1)> Fig. 2 shows a schematic cross-sectional view of an example of the configuration of the negative electrode 20 according to Morphological Example 1. As can be seen from Fig. 2, the negative electrode 20 includes a negative electrode composite material layer 21 and a negative electrode current collector 22. A separator 11 is laminated on one surface side of the negative electrode composite material layer 21, and a negative electrode current collector 22 is laminated on the other surface side.

[0013] The negative electrode current collector 22 is laminated on the negative electrode composite material layer 21 to collect current from the negative electrode composite material layer 21. In this embodiment, the negative electrode current collector 22 is in the form of a foil and can be made of, for example, stainless steel, copper, nickel, carbon, aluminum, and alloys thereof. Alternatively, it may be those plated or vapor-deposited with nickel, chromium, or carbon.

[0014] The negative electrode composite material layer 21 includes a first layer 21a disposed on the side of the negative electrode current collector 22 and a second layer 21c disposed on the side of the separator 11. Further, as can be seen from Fig. 2, the second layer 21c is disposed so as to cover the side surface of the first layer 21a. Thereby, the peel strength of the second layer 21c can be made higher than the peel strength of the first layer 21a.

[0015] Both the first layer 21a and the second layer 21c include a negative electrode active material and a binder. In each layer, the negative electrode active material is bound by the binder.

[0016] The negative electrode active material is preferably a graphite-based negative electrode active material, and either natural graphite or artificial graphite may be used. However, from the viewpoints of theoretical capacity and raw material cost, it is preferable to use natural graphite. The particle shape is not particularly limited, but flaky graphite is preferable from the viewpoints of easy orientation and easy alignment of the expansion and contraction directions. Also Li + From the viewpoint of acceptance, the average particle diameter of the graphite-based negative electrode active material is preferably about 2 μm to 15 μm, for example, about 10 μm. In this specification, the "average particle diameter" refers to the median diameter (d50) measured by the laser diffraction / scattering method.

[0017] As the binder, for example, styrene-butadiene rubber (SBR), polytetrafluoroethylene (PTFE), polyethylene oxide (PEO), acrylic rubber (ACR), carboxymethyl cellulose (CMC), or the like can be used. Note that CMC can also function as a thickener in the negative electrode composite paste. The content ratio of the binder in the first layer 21a and the content ratio of the binder in the second layer 21c are not particularly limited, but are preferably 1% by mass to 5% by mass. Among them, in Embodiment 1, it is preferable that the content ratio of the second layer 21c is larger than that of the first layer 21a. For example, the content ratio of the second layer 21c is 25% or more larger than that of the first layer 21a. Thereby, the peel strength of the second layer 21c can be further increased compared to the peel strength of the first layer 21a.

[0018] The mass ratio (areal ratio) of the first layer 21a and the second layer 21c is not particularly limited, but is preferably the first layer: the second layer = 1:9 to <9.5:0.5>, and more preferably the first layer: the second layer = 6:4 to <9.5:0.5>.

[0019] Furthermore, in Embodiment 1, the longitudinal direction of the active material 21b of the first layer 21a is oriented so as to be in the thickness direction of the negative electrode current collector 22. Here, "being in the thickness direction of the negative electrode current collector 22" means a direction inclined at an average value of 0 degrees to 65 degrees, preferably 0 degrees to 45 degrees, with respect to the direction parallel to the thickness direction. Thereby, the strain generated at the interface between the first layer 21a and the second layer 21c due to the expansion and contraction of the graphite-based negative electrode active material can be suppressed, and thus excellent cycle characteristics can be obtained.

[0020] The orientation is as known, and can be obtained by measuring the degree of orientation. For example, it can be expressed as the percentage of the ratio I(110) / I(002) of the peak intensity I(110) derived from the (110) plane of the graphite crystal and the peak intensity I(002) derived from the (002) plane by powder X-ray diffraction (XRD) measurement. Such peak intensities can be measured using a conventionally known XRD apparatus.

[0021] As described above, the first layer 21a in which the negative electrode active material 21b has a specific orientation can be fabricated by the following method. First, a negative electrode mixture paste is prepared by a conventionally known method. The negative electrode mixture paste can be prepared, for example, by kneading a graphite-based negative electrode active material and a binder in water. The negative electrode composite paste is then coated using a conventionally known method (for example, the die-coating method). By applying a magnetic field before the negative electrode composite paste dries (solidifies), the graphite-based negative electrode active material contained in the negative electrode composite paste can be oriented. When a magnetic field is applied to the negative electrode composite paste before drying, the graphite-based negative electrode active material 21b contained in the negative electrode composite paste is oriented so that the graphite layer is parallel to the magnetic field lines. Therefore, orientation can be controlled by adjusting the magnetic field lines in the orientation direction. The strength of the magnetic field and the duration of application of the magnetic field are not particularly limited and can be set as appropriate to obtain the desired orientation. For example, the strength of the magnetic field can be set so that the magnetic flux density of the entire first layer 21a is about 500mT to 1000mT, and the duration of application of the magnetic field can be about 1 second to 20 seconds for the entire first layer 21a.

[0022] <Negative electrode (Example of form 2)> Figure 3 shows a schematic cross-sectional view of an example of the configuration of the negative electrode 30 according to Embodiment Example 2. As can be seen from Figure 3, the negative electrode 30 comprises a negative electrode composite layer 31 and a negative electrode current collector 22. A separator 11 is laminated on one side of the negative electrode composite layer 31, and the negative electrode current collector 22 is laminated on the other side. The negative electrode current collector 22 can be considered in the same way as in Embodiment Example 1.

[0023] The negative electrode composite layer 31 comprises a first layer 21a positioned on the negative electrode current collector 22 side and a second layer 31a positioned on the separator 11 side. Furthermore, the second layer 31a is positioned to cover the sides of the first layer 21a. This makes it possible to increase the peel strength of the second layer 31a above that of the first layer 21a.

[0024] Both the first layer 21a and the second layer 31a include a negative electrode active material and a binder. The negative electrode active material is bound by the binder. The materials of the negative electrode active material and the binder can be considered in the same way as in the first exemplary form. However, in the second exemplary form, unlike the first exemplary form, the content ratio of the binder in the second layer 31a may be smaller than that in the first layer 21a. For example, the content ratio of the second layer 31a is 25% or more smaller than that of the first layer 21a. Thereby, the permeability of the electrolytic solution is improved and the reaction resistance of the battery cell can be reduced.

[0025] In this exemplary form, the orientation of the active material 21b in the first layer 21a can be considered in the same way as in the first exemplary form described above.

[0026] In the second exemplary form, the longitudinal direction of the active material 31b in the second layer 31a is oriented in a direction intersecting with the thickness direction of the negative electrode current collector 22. Here, the "direction intersecting with the thickness direction of the negative electrode current collector 22" is a direction inclined at an average value of 35 degrees to 90 degrees, preferably 45 degrees to 90 degrees with respect to the direction parallel to the thickness direction. Thereby, the effect of suppressing peeling, cracking due to expansion and contraction can be further enhanced. The method of orientation and the like can be considered in the same way as in the first exemplary form.

[0027] <Positive Electrode> The positive electrode 40 is formed by fixing a positive electrode composite layer 41 including a positive electrode active material, a conductive auxiliary material, and a binder on one surface of a positive electrode current collector 42. As the positive electrode active material, for example, LiCoO2, LiNiO2, LiNi a Co b O2 (a + b = 1, 0 < a < 1, 0 < b < 1), LiMnO2, LiMn2O4, LiNi a Co b Mn c O2 (a + b + c = 1, 0 < a < 1, 0 < b < 1, 0 < c < 1), LiFePO4, etc. can be used. As the conductive auxiliary material, for example, acetylene black (AB) etc. can be used, and as the binder, for example, polyvinylidene fluoride (PVdF) etc. can be used. The positive electrode current collector 42 is in the form of a foil, and the foil material can be, for example, stainless steel, nickel, chromium, gold, platinum, aluminum, iron, titanium, and zinc. These metal foils may also be plated or vapor-deposited with nickel, chromium, carbon, etc.

[0028] <Separator> Separator 11 is Li + The purpose is to allow light to pass through while preventing electrical contact between the positive electrode 40 and the negative electrode 20 (or negative electrode 30). As the separator 11, a microporous membrane made of a polyolefin material is preferred from the viewpoint of mechanical strength and chemical stability. Here, as the polyolefin material, for example, polyethylene (PE), polypropylene (PP), etc. can be used, and these can also be used in combination.

[0029] <Non-aqueous electrolytes> For the non-aqueous electrolyte, a lithium salt dissolved in an aprotic solvent can be used. Examples of aprotic solvents include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), γ-butyrolactone (γBL), and vinylene carbonate (VC), as well as linear carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC). The solvent composition is, for example, EC:EMC:DEC = 3:5:2 (volume ratio). Examples of lithium salts include LiPF6, LiBF4, LiClO4, LiAsF6, Li(CF3SO2)2N, and Li(CF3SO3). The salt concentration is, for example, about 0.5 mol / L to 2.0 mol / L. The non-aqueous electrolyte may be in gel or solid form. [Examples]

[0030] The present invention will be described in more detail below using examples, but this disclosure is not limited to these examples.

[0031] [Fabrication of the negative electrode] The characteristics of the negative electrodes fabricated in each example are shown in Table 1, and the negative electrodes of each example are basically fabricated as follows. Specifically, a slurry containing an artificial graphite-based negative electrode active material and SBR was used, and a two-layer coated electrode of DRY&WET (coating of the first layer → drying of the first layer → coating of the second layer → drying of the second layer) was fabricated using a die coater. When the second layer is applied only to the upper surface of the first layer, the slurry discharge width of the die coater is 90 mm for the first layer and 90 mm for the second layer. When the second layer is applied to the upper surface and the side surface of the first layer, the slurry discharge width of the die coater is 88 mm for the first layer and 90 mm for the second layer. (In addition to this example, it can be fabricated in the same manner with the first layer ranging from 38 mm to 148 mm and the second layer ranging from 40 mm to 150 mm.) The basis weight ratio between the first layer and the second layer was 9:1. Also, the total basis weight, which is the sum of the first layer and the second layer, was 35 mg / cm 2 was used. Note that the total basis weight can be configured in the same manner if it is 25 mg / cm 2 or more. The content ratio of the binder is shown in Table 1. Also, in Table 1, an example where the second layer is also arranged on the side surface is marked as ○, and an example where it is not arranged on the side surface is marked as ×. The orientation treatment was performed by a magnetic field as described above. In Table 1, "Yes" for the orientation treatment of the first layer is the orientation described with the active material 21b above, and "Yes" for the orientation treatment of the second layer is the orientation described with the active material 31b.

[0032] [Fabrication of the Positive Electrode] The positive electrode was evaluated using NCM623 (Ni:Co:Mn = 6:2:3) as the positive electrode active material.

[0033] [Performance Evaluation] The evaluation was carried out with the battery fabricated as described above as a laminated battery with a capacity of 6000 mAh, and "initial resistance", "resistance after durability", and "cracking after durability" were examined. The resistance was the resistance value calculated from the voltage drop when discharging for 10 seconds at 3C (C rate) with the SOC set to 50% at an environmental temperature of 25°C. The durability was performed by charging and discharging 100 cycles at 0.5C between SOC of 0% and 100% at an environmental temperature of 45°C. Note that "cracking after durability" was visually inspected and evaluated as good, acceptable, or unacceptable according to the degree. The results are shown in Table 1. In Table 1, the resistance increase rate is the value obtained by dividing the resistance after durability by the initial resistance.

[0034] [Table 1]

[0035] [result] Comparative Examples 2 and 3, which had a second layer but did not have the second layer on the side of the first layer, showed cracking after durability and a greater increase in resistance compared to Comparative Example 1, which only had the first layer. In Example 1, by providing the second layer on the side surface of the first layer, the resistance increase rate was lower compared to Comparative Examples 2 and 3, and cracking after durability was also improved. Compared to Example 1, Example 2 shows a lower resistance increase rate and improved cracking after durability by increasing the proportion of the binder in the second layer. Examples 3-5 also underwent orientation treatment of the second layer, resulting in a resistance increase rate similar to that of Example 2, and improved cracking after durability. In particular, Examples 4 and 5 allowed for a reduction in the binder content, which also reduced both the initial resistance and the resistance after durability.

[0036] Although this embodiment and examples have been described above, the embodiments and examples disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is indicated by the claims rather than the foregoing description, and all modifications within the meaning and scope equivalent to the claims are intended to be included. [Explanation of Symbols]

[0037] 10...Secondary battery, 11...Separator, 20...Negative electrode, 21...Negative electrode composite layer, 22...Negative electrode current collector, 30...Negative electrode, 31...Negative electrode composite layer, 32...Negative electrode current collector, 40...Positive electrode, 41...Positive electrode composite layer, 42...Positive electrode current collector

Claims

1. A negative electrode for a secondary battery comprising a negative electrode current collector and a negative electrode composite layer containing an active material laminated to the negative electrode current collector, The first layer, It includes a second layer laminated so as to cover the layer surface and the side surface of the first layer, In the first layer, the longitudinal direction of the active material contained in the first layer is oriented in the thickness direction of the negative electrode current collector. The second layer has higher peel strength than the first layer. Negative electrode.

2. The negative electrode according to claim 1, wherein the second layer is oriented such that the longitudinal direction of the active material contained in the second layer intersects with the thickness direction of the negative electrode current collector.

3. The negative electrode according to claim 2, wherein the second layer contains a binder amount that is 25% or less less than that of the first layer.

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