Negative electrode mixture layer and liquid battery

Abstract

To provide a negative electrode mixture layer that can make the ion conductivity of a liquid battery with a negative electrode mixture layer high.SOLUTION: The negative electrode mixture layer used in a liquid battery having an electrolyte contains a negative electrode active material and polyvinylidene fluoride having a degree of crystallization of 43% or more and 93% or less. The average thickness of the negative electrode mixture layer is 150 μm or more.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a negative electrode composite layer and a liquid-based battery.

Background Art

[0002] Conventionally, as a negative electrode composite layer used in a battery, a negative electrode composite layer containing various binders has been studied.

[0003] For example, Patent Document 1 discloses an electrode containing at least an active material and a binder, the binder containing polyvinylidene fluoride, and in the FT-IR spectrum of the binder, the intensity ratio (A / B) of peak A near 1210 cm -1 and peak B near 1275 cm -1 is 3.60 or more and 5.92 or less, and the thickness of the electrode is 205 μm or more and 994 μm or less.

[0004] Also, Patent Document 2 discloses a method for manufacturing a positive electrode, which includes a step of mixing phosphorous acid (H3PO3) with a positive electrode active material to prepare a positive electrode mixture slurry, and a step of applying the positive electrode mixture slurry to a positive electrode current collector to form a positive electrode active material layer.

[0005] Also, Patent Document 3 discloses a lithium-ion battery having a positive electrode capable of occluding and releasing lithium ions, a negative electrode capable of occluding and releasing lithium ions, a separator disposed between the positive electrode and the negative electrode, and an electrolytic solution. The binder of the negative electrode composite layer contains polyvinylidene fluoride, and the crystallinity of polyvinylidene fluoride in the negative electrode composite layer is higher on the current collector side than on the separator side. The value of R1 calculated from the absorbance (A736) at 736 cm -1 and the absorbance (A840) at 840 cm -1 by formula 1: R1 = A763 / A840 is greater than 1.5 on the current collector side and 1.5 or less on the separator side.

Prior Art Documents

Patent Documents

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2020-115441 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-087885 [Patent Document 3] Japanese Patent Application Laid-Open No. 2012-150972 [Summary of the Invention] [Problems to be Solved by the Invention]

[0007] In a conventional liquid-based battery, for example, a negative electrode composite material layer using styrene-butadiene rubber (SBR) as a binder has been used. However, when SBR is used as the binder, aggregation occurs, making it difficult for ion diffusion to occur, and the ion conductivity after repeated charge and discharge may be poor.

[0008] The present disclosure has been made in view of the above circumstances, and an object thereof is to provide a negative electrode composite material layer capable of obtaining high ion conductivity in a liquid-based battery provided with the negative electrode composite material layer, and a liquid-based battery provided with the negative electrode composite material layer. [Means for Solving the Problems]

[0009] Means for solving the above problems include the following aspects. <1> A negative electrode composite material layer used in a liquid-based battery having an electrolytic solution, comprising a negative electrode active material, polyvinylidene fluoride having a crystallinity of 43% or more and 93% or less, and having an average thickness of the negative electrode composite material layer of 150 μm or more. <2> The negative electrode composite material layer according to <1>, wherein the content of the polyvinylidene fluoride with respect to the negative electrode active material is 0.11% by mass or less. <3> The negative electrode composite material layer according to <1> or <2>, wherein the average thickness of the negative electrode composite material layer is 150 μm or more and 355 μm or less. <4> A current collector foil, The positive electrode composite layer disposed on the current collector foil and the negative electrode composite layer according to any one of <1> to <3>, an electrolytic solution, and a liquid-based battery having the same.

Advantages of the Invention

[0010] According to the present disclosure, there are provided a negative electrode composite layer capable of obtaining high ionic conductivity in a liquid-based battery including the negative electrode composite layer, and a liquid-based battery including the negative electrode composite layer.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0012] Hereinafter, embodiments which are an example of the present disclosure will be described. These descriptions and examples are illustrative of the embodiments and do not limit the scope of the invention. In the numerical ranges described step by step in this specification, the upper limit value or the lower limit value described in one numerical range may be replaced with the upper limit value or the lower limit value of another numerically described range. Further, in the numerical ranges described in this specification, the upper limit value or the lower limit value of the numerical range may be replaced with the value shown in the examples.

[0013] Each component may contain a plurality of corresponding substances. When referring to the amount of each component in the composition, when there are a plurality of substances corresponding to each component in the composition, unless otherwise specified, it means the total amount of the plurality of substances present in the composition.

[0014] <Negative electrode composite layer> The negative electrode composite layer according to an embodiment of the present disclosure is a negative electrode composite layer used in a liquid-based battery having an electrolytic solution. This negative electrode composite layer contains a negative electrode active material and polyvinylidene fluoride (hereinafter also simply referred to as "PVDF") having a crystallinity of 43% or more and 93% or less. And the thickness of the negative electrode composite layer is 150 μm or more.

[0015] Note that the thickness of the negative electrode composite layer described above means the single-sided thickness in the liquid-based battery, that is, the average thickness of only the negative electrode composite layer formed on one side of the current collector foil in the negative electrode composite layer.

[0016] By satisfying the above configuration, the negative electrode composite layer according to an embodiment of the present disclosure can obtain high ionic conductivity in a liquid-based battery provided with this negative electrode composite layer. The reason is speculated as follows.

[0017] In a conventional liquid-based battery, for example, a negative electrode composite layer using styrene-butadiene rubber (SBR) as a binder is used. However, when forming the negative electrode composite layer by coating and drying the negative electrode composite material in the form of a current collector foil, a migration phenomenon occurs in which the binder SBR floats and aggregates on the surface side of the coating layer. And when SBR aggregation occurs on the surface or inside of the negative electrode composite layer, it becomes difficult for ion diffusion to occur, and the ionic conductivity after repeated charge and discharge may be inferior.

[0018] In contrast, the negative electrode composite material layer according to the embodiment of the present disclosure contains polyvinylidene fluoride (PVDF) having a crystallinity of 43% or more and 93% or less as a binder. Also in the case of PVDF as a binder, migration occurs to a certain extent as in the case of SBR, that is, aggregation is considered to occur on the surface or inside of the negative electrode composite material layer. However, PVDF has the property of swelling while retaining the electrolytic solution in the liquid-based battery, and ion conductivity is exhibited by the retained electrolytic solution. Therefore, unlike the case where aggregates of SBR are present in the negative electrode composite material layer, even if there is an aggregation value of PVDF, it does not become a blocked pore, and a decrease in ion diffusivity is suppressed. In addition, since the amorphous portion of PVDF particularly has the property of retaining and swelling the electrolytic solution, the ratio of the amorphous portion of PVDF is important for the manifestation of ion conductivity. Therefore, by using PVDF having a crystallinity of 43% or more and 93% or less, that is, a large ratio of the amorphous portion, the electrolytic solution is appropriately retained by the binder. As a result, high ion conductivity can be obtained in a liquid-based battery provided with this negative electrode composite material layer.

[0019] ·Crystallinity of PVDF In the negative electrode composite material layer, PVDF contained as a binder has a crystallinity of 43% or more and 93% or less. By having a crystallinity of 93% or less, PVDF with a large ratio of the amorphous portion is obtained, and by appropriately retaining the electrolytic solution by the binder, high ion conductivity can be obtained in the liquid-based battery. Also, by having a crystallinity of 43% or more, high ion conductivity can be obtained in the liquid-based battery.

[0020] From the viewpoint of obtaining high ion conductivity in the liquid-based battery, the crystallinity of PVDF is preferably 50% or more and 85% or less, and more preferably 60% or more and 75% or less.

[0021] The crystallinity of PVDF is measured by the following method. After vacuum drying the electrode without the positive electrode composite layer at 45 °C, a part of the primer layer was scraped off with a razor, and powder NMR (nuclear magnetic resonance) was measured. The analysis method is as follows. After the NMR measurement, the areas of the crystalline and amorphous peaks at the main peak of PVDF were determined respectively, and then the crystallinity was determined by calculating the area percentage of the crystalline part from the sum of these areas.

[0022] ·Average thickness of the negative electrode composite layer The average thickness of the negative electrode composite layer is 150 μm or more. When the average thickness of the negative electrode composite layer is 150 μm or more, the influence on the ionic conductivity due to the migration of the binder becomes large. However, for the negative electrode composite layer according to the embodiment of the present disclosure, it is PVDF with a large ratio of the amorphous part, and the electrolyte is appropriately held by the binder, so that high ionic conductivity can be obtained in the liquid-based battery. In addition, the average thickness of the negative electrode composite layer is preferably further 150 μm or more and 355 μm or less. The average thickness means the arithmetic mean value of the thicknesses at arbitrarily selected 10 locations.

[0023] ·Content of PVDF The content of PVDF with respect to the negative electrode active material is preferably 0.11 mass% or less. By the content of PVDF being 0.11 mass% or less, high ionic conductivity can be obtained in the liquid-based battery. The content of PVDF with respect to the negative electrode active material is more preferably further 0.09 mass% or less.

[0024] ·Components of the negative electrode composite layer Note that the negative electrode composite layer contains a negative electrode active material, and examples include metals such as metallic lithium, metallic indium, metallic aluminum, metallic silicon, metallic tin, etc., metals that can form an alloy with metallic lithium, oxides of these metals, alloys of these metals and metallic lithium, etc. Examples of the oxide include oxide active materials such as Li4Ti5O 12 and the like.

[0025] The negative electrode composite layer contains a binder in addition to the negative electrode active material. Examples of the binder include rubbers such as styrene-butadiene copolymer (SBR) in addition to PVDF. The negative electrode active material layer may further contain other components such as a thickening agent. Examples of the thickening agent include celluloses such as carboxymethyl cellulose (CMC).

[0026] <Battery> Next, each component constituting the liquid battery according to the embodiment of the present disclosure will be described.

[0027] (Positive electrode composite layer) The positive electrode composite layer contains a positive electrode active material and may further contain, for example, a binder. Examples of the positive electrode active material include lithium nickel cobalt manganese composite oxide (hereinafter sometimes simply referred to as "LNCM"). The simplest LNCM has the following general formula: LiNi x Co y Mn z O2 (where x, y, z satisfy 0 < x < 1, 0 < y < 1, 0 < z < 1, and x + y + z = 1). In addition to Li, Ni, Co, and Mn, LNCM may contain other additive elements such as transition metal elements other than Ni, Co, and Mn, and typical metal elements other than Li. LNCM has a layered crystal structure. LNCM may exceed 50% by mass of the entire positive electrode active material and may account for, for example, 80 to 100% by mass. The positive electrode active material may be composed of only LNCM. Examples of other positive electrode active materials include lithium nickel composite oxide, lithium cobalt composite oxide, lithium nickel manganese composite oxide, and the like.

[0028] Examples of the binder contained in the positive electrode composite layer include vinyl halide resins such as polyvinylidene fluoride (PVdF). The positive electrode composite layer may further contain other components such as a conductive material. Examples of the conductive material include graphitizable carbon such as non-graphitizable carbon and carbon black, and graphite.

[0029] (Negative electrode active material layer) The negative electrode active material layer uses the aforementioned negative electrode composite material layer. Since the details have already been described, they are omitted here.

[0030] (Positive electrode current collector) The positive electrode composite material layer is formed on the positive electrode current collector. As the positive electrode current collector, a conductive member made of a metal with good conductivity (for example, aluminum) is suitable.

[0031] (Negative electrode current collector) The negative electrode composite material layer is formed on the negative electrode current collector. As the negative electrode current collector, a conductive member made of a metal with good conductivity (for example, copper) is suitable.

[0032] (Separator) The separator is a porous film with electrical insulation. The separator electrically isolates the positive electrode and the negative electrode. The separator may have a thickness of, for example, 5 to 30 μm. The separator can be composed of, for example, a porous polyethylene (PE) film, a porous polypropylene (PP) film, etc. The separator may have a multilayer structure. For example, the separator may be composed of a porous PP film, a porous PE film, and a porous PP film laminated in this order. The separator may have a heat-resistant layer on its surface. The heat-resistant layer contains a heat-resistant material. Examples of the heat-resistant material include metal oxide particles such as alumina, and high melting point resins such as polyimide.

[0033] (Electrolyte) The battery according to the embodiment of the present disclosure further has an electrolyte. As the electrolyte, either a solid electrolyte or an electrolytic solution can be adopted. Here, the electrolytic solution is taken as an example to explain the electrolyte. In particular, a non-aqueous electrolytic solution is preferred.

[0034] · Solvent The non-aqueous electrolytic solution contains a solvent (non-aqueous solvent) and an electrolyte. Examples of the solvent (non-aqueous solvent) include N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(fluorosulfonyl)imide (DEME), 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMI), 1-ethyl-2,3-dimethylimidazolium bis(fluorosulfonyl)imide (DEMI-FSI), and the like.

[0035] ·Electrolyte Examples of the electrolyte in the electrolytic solution include Li salts. Examples of the Li salts include lithium bis(fluorosulfonyl)imide (LiFSI), LiPF6 (lithium hexafluorophosphate), lithium tetrafluoroborate (LiBF4), Li[N(CF3SO2)2], and the like. The amount of the electrolyte may be, for example, 1.0 to 2.0 mol / L, and preferably 1.0 to 1.5 mol / L.

[0036] In addition to the solvent and the electrolyte, the electrolytic solution may contain various additives, such as a thickener, a film-forming agent, a gas-generating agent, and the like. The electrolyte is typically a non-aqueous electrolytic solution that is liquid at room temperature (for example, 25 ± 10 °C). The electrolytic solution typically remains liquid under the operating conditions of the battery (for example, in a temperature environment of -20 to +60 °C).

[0037] (Applications) Examples of the applications of the battery according to the embodiments of the present disclosure include power sources for hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), battery electric vehicles (BEVs), and the like.

Examples

[0038] Hereinafter, the present disclosure will be described based on examples, but the present disclosure is not limited to these examples in any way.

[0039] [Prototype of the electrode] In the formation of the positive electrode, first, [active material] LiNi 0.8 Co 0.1 Mn 0.1O2: [Conductive agent] Single-walled carbon nanotubes (MWCNT): [Binder] PVDF = 97.8:0.7:1.4 (unit: mass%) were used to prepare a slurry. This slurry was applied onto a 30-μm Al foil (current collector) using a doctor blade, dried under the conditions of 100 °C for 30 minutes, and pressed by a roll press to a density of 3.2 g / cc to form a positive electrode composite layer. For the formation of the negative electrode, first, a slurry was prepared with [active material] artificial graphite: [binder] CMC: [binder] PVDF = 97:0.6:2.4 (unit: mass%). This slurry was applied onto a 12-μm Cu foil (current collector) using a doctor blade, dried under the conditions of 100 °C for 30 minutes, and pressed by a roll press to a density of 1.3 g / cc to form a negative electrode composite layer. The weight of the negative electrode was adjusted so that the charge capacity of the negative electrode / the charge capacity of the positive electrode = 1.1. After that, the positive electrode / separator (a three-layer structure of PP / PE / PP with an average thickness of 16 μm) / negative electrode were stacked to fabricate a laminated cell. As the separator, a separator with a three-layer structure of porous polypropylene (PP) / porous polyethylene (PE) / PP (average thickness 16 μm) was used. As the electrolyte solution, an electrolyte containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of EC:DMC:EMC = 30:40:30, and 1.1 M (mol / l) of LiPF6 as the electrolyte salt was used.

[0040] The crystallinity of PVDF was measured by the method described above.

[0041] [Activation and Characterization] For the obtained electrodes, the initial charge current was set to a constant current - constant voltage method. Constant current charging was performed at a current value of 0.1C up to 4.25V, followed by constant voltage charging until the constant voltage charging time reached 3 hours. Discharging was then performed at a current value of 0.1C up to 2.5V by the constant current method to activate the battery.

[0042] [Discharge Characteristic Evaluation] The charge and discharge of the electrode was carried out in a constant current mode, and the current rate was set to 0.1C or 1.0C. The 1C discharge rate was calculated by 0.1C-CC discharge capacity / 1.0C-CC discharge capacity × 100%.

[0043] [Method for calculating the degree of bending] A symmetric laminate cell of negative electrode / separator / negative electrode was prototyped to obtain the degree of bending. After impregnating the electrolyte for 12 hours, AC imp. measurement was carried out at 25°C. The measurement frequency was set to 10 mHz to 1,000,000 Hz. From the equivalent circuit shown in Figure 1, the ionic resistance [Ω] (resistance of W01 in the equivalent circuit) of each electrode was calculated, and the degree of bending was obtained from the following formula. Degree of bending τ=(R ion ×A×κ×ε) / 2d (In the formula, R ion : ionic resistance, A: electrode area, κ: ionic conductivity, ε porosity, d: membrane thickness.)

[0044] (Example 1) In the above-mentioned [Prototype of electrode], an electrode in which PVDF as a binder was changed to SBR (2.4 mass%) and an electrode in which the amount of PVDF was changed to 3.5 mass% were prepared. Then, the degree of bending was measured when the average thickness (negative electrode thickness) of the negative electrode composite layer was changed. The graph of the results is shown in Figure 2. The tortuosity is a kind of parameter representing ion diffusion in the electrode. The lower the tortuosity, the easier it is for ions to diffuse in the electrode. Theoretically, as long as the composition in the negative electrode composite layer remains unchanged even if the thickness is increased, the tortuosity is constant. However, as shown in Fig. 2, since the tortuosity tends to increase as the negative electrode thickness increases, it can be seen that the influence of migration is increasing. For both the electrode using SBR as the binder and the electrode using PVDF, the tortuosity tends to increase as the negative electrode thickness increases, but the sensitivity of the electrode using PVDF is lower. Even when PVDF is used as the binder, migration phenomena and aggregation occur, but it is presumed that the sensitivity is low because, unlike the case of using SBR, the binder itself has an ion conductivity function. And since the ion conductivity of this PVDF is expressed by the swelling of PVDF in the electrolyte, the crystallinity of PVDF, which has a correlation with the swelling property, is considered important.

[0045] (Example 2) Next, the difference in tortuosity due to the difference in the crystallinity of PVDF was confirmed. In the above-mentioned [Prototype of Electrode], electrodes were prepared using PVDF with different crystallinities as the binder. The negative electrode thickness was 177 μm. The results are shown in Table 1. The one with 100% crystallinity of PVDF cannot retain the electrolyte. If the PVDF binder migrates and aggregates, it is expected that ions cannot pass through in the vicinity, and it is presumed that the tortuosity deteriorates. Also, when the crystallinity is extremely low, that is, 37% or less, it is expected that the swelling degree of the binder becomes too high, resulting in less contact between the active materials and a decrease in voids, and it is presumed that the tortuosity decreases. By setting the crystallinity of PVDF as the binder within the above-mentioned range, a high discharge rate can be exhibited even in a thick-film electrode compared with the case of using a conventional SBR binder.

[0046]

Table 1

[0047] (Example 3) Next, when SBR was used as the binder and when PVDF was used, the difference in discharge rate due to the difference in negative electrode thickness was confirmed. The crystallinity of PVDF was set to 63%. If the negative electrode thickness is less than 150 μm, there is no difference in the discharge rate whether SBR or PVDF is used as the binder. On the other hand, when the negative electrode thickness is 150 μm or more, the discharge rate significantly decreases when SBR is used, while the decrease in the discharge rate is gentle when PVDF is used.

[0048] [Table 1]

[0049] (Example 4) Next, the difference in discharge rate when the negative electrode thickness was 177 μm and the content ratio of PVDF (crystallinity 63%) in the negative electrode composite layer was changed was confirmed. The results are shown in Figure 3. As the PVDF ratio (binder content rate in the composite) in the negative electrode composite layer increases, the 1C discharge rate tends to decrease. It can be seen that it is desirable that the ratio of the PVDF binder in the composite is 0.11% or less.

Claims

**Claim 1** A negative electrode composite material layer used in a liquid-based battery having an electrolyte, comprising a negative electrode active material, and polyvinylidene fluoride having a crystallinity of 43% or more and 93% or less, wherein the average thickness of the negative electrode composite material layer is 150 μm or more. The negative electrode composite material layer. **Claim 2** The negative electrode composite material layer according to claim 1, wherein the content of the polyvinylidene fluoride with respect to the negative electrode active material is 0.11% by mass or less. **Claim 3** The negative electrode composite material layer according to claim 1, wherein the average thickness of the negative electrode composite material layer is 150 μm or more and 355 μm or less. **Claim 4** A current collector foil, a positive electrode composite material layer disposed on the current collector foil, and the negative electrode composite material layer according to any one of claims 1 to 3, an electrolyte, and a liquid-based battery having the same.

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