Electrode material manufacturing method, electrode material and secondary battery

Abstract

To provide a manufacturing method for an electrode material, an electrode material, and a secondary battery, capable of achieving excellent charge and discharge characteristics even with a small amount of conductivity aid for an electrode active material.SOLUTION: The manufacturing method for an electrode material includes mixing in water a first mixture including active material particles and a first material having an ionic group and a second mixture including carbon particles and a second material having an ionic group opposite to the ionic group of the first material.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a method for producing an electrode material, an electrode material, and a secondary battery. [Background technology]

[0002] Secondary batteries such as lithium-ion batteries are used as power sources in a variety of fields, including mobile devices and electric vehicles. In recent years, research into secondary batteries that use sulfur as the positive electrode active material (lithium-sulfur batteries) has been actively conducted. Sulfur has a theoretical capacity (1672 mAhg) that is more than five times that of rare metal oxides, which are commonly used as the positive electrode active material in lithium-ion batteries. -1 However, when sulfur is used as an active material, lithium polysulfide, a reaction intermediate, is produced during the charge and discharge process of secondary batteries, and this leaches into the electrolyte, causing a decrease in cycle performance.

[0003] As a measure to improve the cycle characteristics of secondary batteries that use sulfur as an active material, Patent Document 1 proposes using sulfur chemically bonded to carbon as an active material to suppress the elution of sulfur components into the electrolyte. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6441462 Summary of the Invention [Problem to be solved by the invention]

[0005] While the secondary battery using the active material described in Patent Document 1 has improved cycle characteristics, it is necessary to use a conductive aid such as carbon powder in combination to compensate for the conductivity of the active material. In order to increase the energy density of a secondary battery, it is desirable to reduce the amount of conductive additive and increase the proportion of active material in the electrode. Therefore, an object of the present disclosure is to provide a manufacturing method for an electrode material, an electrode material, and a secondary battery that can achieve excellent charge-discharge characteristics even when the amount of conductive additive relative to the active material is small. [Means for solving the problem]

[0006] Specific means for achieving the above object are as follows. <1> A method for producing an electrode material, comprising mixing, in water, a first mixture containing active material particles and a first substance having an ionic group, and a second mixture containing carbon particles and a second substance having an ionic group opposite to the ionic group of the first substance. <2> At least one of the active material particles and the carbon particles is treated with a surfactant. <1> A method for producing the electrode material described in <3> the active material particles contain sulfur; <1> or <2> A method for producing the electrode material described in <4> the active material particles contain sulfur, carbon, and oxygen; <1> ~ <3> 10. A method for producing the electrode material according to claim 1 . <5> The mass ratio of the active material particles to the carbon particles contained in the electrode material is 50:50 to 95:5. <1> ~ <4> 10. A method for producing the electrode material according to claim 1 . <6> An electrode material comprising: active material particles; carbon particles; a first substance having ionic groups present on the surfaces of the active material particles; and a second substance having ionic groups opposite to the ionic groups of the first substance present on the surfaces of the carbon particles. <7> A secondary battery comprising the electrode material according to claim 6. [Effects of the Invention]

[0007] According to the present disclosure, there are provided a method for producing an electrode material, an electrode material, and a secondary battery that can achieve excellent charge-discharge characteristics even when the amount of conductive additive relative to the active material is small. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a graph showing zeta potential measurement data of electrode materials. [Figure 2] 1 is an SEM image of an electrode obtained in an example. [Figure 3] 1 is an SEM image of an electrode obtained in a comparative example. [Figure 4] 1 is a graph showing charge / discharge curves of a test battery. [Figure 5] 1 is a graph showing charge / discharge curves of a test battery. [Figure 6] 1 is a graph showing the cycle stability of the test battery. [Figure 7] 1 is a graph showing the cycle stability of the test battery. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments for carrying out the present invention will be described in detail. However, the present invention is not limited to the following embodiments. In the following embodiments, components (including element steps, etc.) are not essential unless otherwise specified. The same applies to numerical values ​​and their ranges, and do not limit the present invention. In the present disclosure, numerical ranges indicated using "to" include the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In the numerical ranges described in stages in this disclosure, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. Furthermore, in the numerical ranges described in this disclosure, the upper or lower limit value of that numerical range may be replaced with a value shown in the examples. In the present disclosure, when a composition contains multiple substances corresponding to each component, the content of each component in the composition means the total content of the multiple substances present in the composition, unless otherwise specified.

[0010] <Electrode material manufacturing method> The method for producing an electrode material of the present disclosure includes mixing, in water, a first mixture containing active material particles and a first substance having an ionic group, and a second mixture containing carbon particles and a second substance having an ionic group opposite to the ionic group of the first substance.

[0011] The electrode material produced by the above method exhibits excellent charge-discharge characteristics even when the amount of carbon particles as a conductive additive relative to the active material particles is small. The reason for this is presumed to be as follows. A first substance having ionic groups is attached to the surfaces of active material particles contained in the first mixture, and a second substance having ionic groups opposite to those of the first substance is attached to the surfaces of carbon particles contained in the second mixture. Therefore, when the first and second mixtures are mixed in water, electrostatic interaction occurs between the ions of the first substance attached to the surfaces of the active material particles and the ions of the second substance attached to the surfaces of the carbon particles, causing the carbon particles to adhere to the surfaces of the active material particles by electrostatic adsorption.

[0012] The above method allows carbon particles to be attached to active material particles more efficiently than methods in which carbon particles are attached to active material particles using a binder, etc. As a result, the conductivity of the active material particles can be improved with a smaller amount of carbon particles (i.e., the proportion of active material particles in the electrode can be increased), and excellent battery performance can be achieved.

[0013] As a result of investigations by the present inventors, it has been found that the application of the electrode material produced by the above method to a secondary battery can effectively increase the energy density of the secondary battery. Furthermore, it has been found that the above method enables high-speed charging and discharging even for active material particles that have low conductivity and cannot be charged and discharged at high speeds.

[0014] The type of active material particles used in the method of the present disclosure is not particularly limited. Specific examples of active material particles include lithium transition metal compounds such as lithium transition metal oxides and lithium transition metal phosphates, sulfur, graphite, carbon materials such as hard carbon and soft carbon, metallic silicon, silicon oxides, and polymer active materials. The active material particles used in the method of the present disclosure may be of one type or two or more types. According to the method of the present disclosure, carbon particles can be efficiently attached to active material particles that have no conductivity or only low conductivity, thereby improving charge-discharge characteristics.

[0015] The active material particles may contain sulfur, or may contain sulfur, carbon, and oxygen. When the active material particles contain sulfur, it is possible to obtain a secondary battery with a high capacity that cannot be achieved with conventional lithium ion secondary batteries.

[0016] The active material particles containing sulfur, carbon, and oxygen can be obtained, for example, by heat treating a mixture containing sulfur and an organic compound under conditions that carbonize the organic compound. The active material particles obtained by the above method are in a state in which the carbon and sulfur of the carbide formed by heat-treating the organic compound are chemically bonded, which effectively suppresses the elution of intermediate products generated during charge and discharge, thereby achieving excellent cycle characteristics.

[0017] The type of carbon particles used in the method of the present disclosure is not particularly limited as long as they function as a conductive aid for the active material particles. Specific examples of carbon particles include graphite, carbon black such as acetylene black, carbon nanotubes, and carbon nanofibers. The carbon particles used in the method of the present disclosure may be of one type or two or more types.

[0018] The first substance having an ionic group and the second substance having an ionic group opposite to the ionic group of the first substance used in the method of the present disclosure are not particularly limited as long as they are substances that become ionic in water. In this disclosure, "ionic group" refers to an anionic group or a cationic group. "A second substance having an ionic group opposite to the ionic group of a first substance" refers to a substance having an anionic group when the first substance has a cationic group, and refers to a substance having a cationic group when the first substance has an anionic group.

[0019] The type of ionic group contained in the substance having an ionic group is not particularly limited. Specific examples of the ionic group include anionic groups such as a sulfo group, a sulfonimide group, a sulfate group, a phosphonic acid group, a phosphate group, and a carboxy group, cationic groups such as an amino group and an ammonium group, and groups that are salts of these groups.

[0020] Specific examples of substances having cationic groups include poly(diallylmethylammonium chloride), polyethyleneimine, polyvinylamine, and poly(vinylpyrrolidone·N,N-dimethylaminoethyl acrylic acid) copolymer. Specific examples of substances having an anionic group include polystyrene sulfonic acid, polyvinyl sulfate, polyacrylic acid, polymethacrylic acid, and salts thereof.

[0021] The weight-average molecular weight of the substance having an ionic group used in the method of the present disclosure is not particularly limited. For example, the weight-average molecular weight measured by gel permeation chromatography may be in the range of 5,000 to 500,000, 10,000 to 300,000, or 50,000 to 250,000. From the viewpoint of efficiently adhering carbon particles to the surfaces of active material particles, the substance having an ionic group is preferably a polymer.

[0022] The active material particles and carbon particles contained in the first mixture and the second mixture may each independently be treated with a surfactant. Treating the active material particles and carbon particles with a surfactant improves the dispersibility of the active material particles or carbon particles in the first mixture or the second mixture, making it possible to suppress the occurrence of aggregation, precipitation, and the like.

[0023] Specific examples of surfactants include sodium deoxycholate, sodium cholate, sodium dodecyl sulfate, and cetyltrimethylammonium bromide.

[0024] The method for obtaining the first mixture and the second mixture is not particularly limited. For example, the active material may be obtained by dissolving a substance having an ionic group in water and mixing the resulting aqueous solution with active material particles or carbon particles. The first mixture and the second mixture may each independently be in a liquid state (including a paste state) or a solid state.

[0025] The method for mixing the first mixture and the second mixture in water is not particularly limited. From the viewpoint of efficiently adhering the carbon particles to the active material particles, the first mixture and the second mixture are preferably mixed so that the solids concentration in water is in the range of 0.1 mass % to 5 mass %, more preferably in the range of 0.2 mass % to 3 mass %, and even more preferably in the range of 0.25 mass % to 2 mass %.

[0026] The active material particles contained in the first mixture may be pre-treated with a substance having an ionic group opposite to the ionic group of the first substance. For example, if the first mixture contains a substance having a cationic group, the active material particles may be pre-treated with a substance having an anionic group. By pretreating the active material particles contained in the first mixture with a substance having an ionic group opposite to the ionic group of the first substance, it is possible to reduce unevenness in the charge density on the surface of the active material particles, and to attach the carbon particles more evenly.

[0027] The carbon particles contained in the second mixture may be pre-treated with a substance having ionic groups opposite to the ionic groups of the second substance, for example, if the second mixture contains a substance having anionic groups, the carbon particles may be pre-treated with a substance having cationic groups. By treating the carbon particles with a substance that has an ionic group opposite to the ionic group of the second substance, the unevenness in the charge density on the surface of the carbon particles can be reduced, allowing them to adhere more evenly to the active material particles.

[0028] In the method of the present disclosure, the mass ratio of the active material particles to the carbon particles contained in the electrode material is not particularly limited. For example, the mass ratio of the active material particles to the carbon particles (active material particles:carbon particles) may be 50:50 to 95:5, 60:40 to 90:10, or 70:30 to 85:15.

[0029] The mass ratio between the active material particles and the carbon particles contained in the electrode material can be adjusted by the mixing ratio between the first mixture and the second mixture, etc.

[0030] <Electrode material> The electrode material of the present disclosure includes active material particles, carbon particles, a first substance having ionic groups present on the surfaces of the active material particles, and a second substance having ionic groups opposite to the ionic groups of the first substance present on the surfaces of the carbon particles.

[0031] The electrode material of the present disclosure exhibits excellent charge-discharge characteristics even when the amount of carbon particles as a conductive additive relative to the active material particles is small. The details and preferred embodiments of the active material particles, carbon particles, and substance having an ionic group contained in the electrode material of the present disclosure are the same as the details and preferred embodiments of the active material particles, carbon particles, and substance having an ionic group used in the above-described manufacturing method. The electrode material of the present disclosure can be produced, for example, by the above-described method for producing an electrode material.

[0032] <Secondary battery> The secondary battery of the present disclosure includes the electrode material of the present disclosure described above. In the present disclosure, the term "secondary battery" refers to any battery that can be repeatedly used by charging. There are no particular limitations on the type of secondary battery, and examples include lithium ion secondary batteries, sodium ion secondary batteries, magnesium ion secondary batteries, calcium ion secondary batteries, and aluminum ion secondary batteries.

[0033] In the secondary battery of the present disclosure, the electrode material is used, for example, in a state mixed with a binder to form an electrode layer on a current collector such as a metal foil. The content of the electrode material in the electrode layer is not particularly limited. From the viewpoint of increasing the capacity of the secondary battery, the content of the active material particles in the electrode layer is preferably 60 mass % or more, more preferably 65 mass % or more, and even more preferably 70 mass % or more. In the secondary battery of the present disclosure, the electrode material may be used as a positive electrode material or a negative electrode material.

[0034] In an embodiment, the secondary battery of the present disclosure may be a secondary battery in which the electrode material is used as a lithium ion positive electrode active material and contains sulfur (lithium-sulfur battery). [Example]

[0035] The present disclosure will be described in more detail below with reference to examples, but the present disclosure is not limited to the following examples.

[0036] <Example> (1) Preparation of active material particles According to "Example 13" of Japanese Patent No. 6441462, active material particles containing sulfur and polyethylene glycol carbide (SPEG) were prepared.

[0037] (2) Preparation of the first mixture As a pretreatment, SPEG (0.20344 g) was added to 30 mL of sodium deoxycholate solution (SDC, Fujifilm Wako Pure Chemical Industries, Ltd., 9.99 mg / mL) and stirred for 10 minutes. The mixture was then ultrasonically dispersed for 10 minutes, centrifuged, and the supernatant was removed. This washing procedure, which consisted of adding 30 mL of distilled water, stirring, dispersing, and centrifuging, was repeated twice. An aqueous solution of polydiallyldimethylammonium chloride (PDDA, Sigma Aldrich, weight average molecular weight = 100,000 to 200,000, 9.60 mg / mL) (30 mL) was added to the obtained solid, followed by stirring, dispersion, and centrifugation, and the supernatant was removed to prepare a first mixture.

[0038] (3) Preparation of the second mixture Acetylene black (AB, Denka Co., Ltd., pressed product) (0.04985 g) was pretreated with an aqueous SDC solution in the same manner as in the first mixture. Next, an aqueous solution of polydiallyldimethylammonium chloride (PDDA, Sigma Aldrich, weight average molecular weight=100,000 to 200,000, 9.60 mg / mL) (30 mL) was added to the obtained solid, followed by stirring, dispersion and centrifugation, and the supernatant was removed. Next, an aqueous solution of poly(sodium 4-styrenesulfonate) (PSS, Alfa Aesar, weight average molecular weight = 70,000, 10.07 mg / mL) (30 mL) was added to the obtained solid, followed by stirring, dispersion, and centrifugation, and the supernatant was removed to prepare a second mixture.

[0039] (4) Preparation of SPEG / AB complex The first and second mixtures were mixed so that the SPEG and AB contents in the electrode were the values ​​listed in Table 1, and the mixture was stirred for 1 hour with a stirrer. After that, the supernatant was removed by centrifugation, and the same washing procedure as in (2) was repeated twice to obtain the SPEG / AB complex.

[0040] (5) Measurement of surface charge Using a measuring device (HORIBA, SZ-100), the zeta potentials of the SPEG in the first mixture, the AB in the second mixture, and the SPEG / AB complex were measured. As shown in Figure 1, the SPEG in the first mixture was positively charged (73 mV), and the AB in the second mixture was negatively charged (-128 mV). The zeta potential of the SPEG / AB complex was -40 mV. This indicated that the SPEG and AB in the SPEG / AB complex were electrostatically adsorbed.

[0041] (6) Preparation of electrodes The SPEG / AB complex and carboxymethyl cellulose (CMC) as a binder were mixed at a mass ratio of 9:1, distilled water was added, and the container was sealed. The mixture was then stirred using a planetary mixer (ARE-310, manufactured by Thinky Corporation) to obtain an electrode slurry. The resulting electrode slurry was applied to aluminum foil (thickness: 20 μm) and dried under reduced pressure at 50°C for 12 hours to obtain an electrode. An SEM (scanning electron microscope) image of the electrode surface is shown in Figure 2. It was observed that AB particles were attached to the surface of the SPEG particles, and that the AB particles were attached so as to fill the gaps between the SPEG particles.

[0042] <Comparative Example> Untreated SPEG and AB (not subjected to the treatments (2) to (4) above) and a binder (polyimide resin) were mixed in the proportions shown in Table 1, and N-methylpyrrolidone (NMP) was added and the mixture was pulverized and mixed in a ball mill to obtain an electrode slurry. The obtained electrode slurry was applied to aluminum foil (thickness 20 μm) and dried under reduced pressure at 180°C for 12 hours to obtain electrodes of Comparative Examples 1 to 3. Figure 3 shows an SEM image of the surface of the electrode prepared in Comparative Example 1. Unlike Figure 2, no AB particles were observed adhering to the surface of the SPEG particles, and no AB particles were observed between the SPEG particles.

[0043] <Charge / discharge test> CR2032-type batteries were fabricated using the electrodes fabricated in the Examples and Comparative Examples and lithium metal as the positive and negative electrodes, respectively. The electrolyte was a solution of lithium trifluoromethanesulfonylamide (LiTFSA), tetraglyme (G4), and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) mixed in a molar ratio of 1:1:4. A polypropylene film was used as the separator. Charge-discharge tests were conducted using the fabricated batteries at a temperature of 30°C, a voltage range of 1 to 3 V, and rates of 0.01 C and 0.05 C (1 C = 1672 mA / g, constant current mode) (start of discharge). The results for the first, second, fifth, and tenth cycles (eighth cycle for Example 1) are shown in Table 1.

[0044] [Table 1]

[0045] As shown in Table 1, the initial discharge capacity of the batteries fabricated in the examples was 922 mAh / g at 0.01 C (Example 1) and 444 mAh / g at 0.05 C (Example 2). The battery of Comparative Example 1, which contained untreated SPEG in a proportion (75 mass%) almost the same as 72 mass% in the Example, did not function as a battery under high-rate charge / discharge conditions (0.05 C). In order to achieve an initial discharge capacity similar to that of the Examples using untreated SPEG at 0.05 C, it was necessary to reduce the proportion of SPEG to 46 mass % (Comparative Example 2). When the proportion of untreated SPEG was 80% by mass, the initial discharge capacity at 0.01 C was 533 mAh / g (Comparative Example 3). Although the proportion of SPEG was larger than in the Examples, the energy density (calculated from the product of the average voltage and the battery capacity) was smaller than in the Examples. When the energy densities calculated using the discharge capacities after 5 cycles of Example 1 and Comparative Example 2 were compared, the energy density of Example 1 was about three times that of Comparative Example 2.

[0046] <Battery performance comparison> The charge-discharge curves for the first and eighth cycles in Example 1 are shown in Figure 4. As shown in Figure 4, the average voltage (the intersection of the discharge curve and the charge curve) for the first cycle was approximately 1.9 V. In addition, the overvoltage (polarization) caused by the internal resistance of the battery was approximately 0.3 V after eight cycles. The charge-discharge curves for the first and tenth cycles in Comparative Example 2 are shown in Figure 5. As shown in Figure 5, the average voltage for the first cycle was approximately 1.6 V, and almost no plateau region was observed. The discharge capacity was approximately half that of the Example. After 10 cycles, although an increase in the average voltage was observed, the discharge capacity was 300 mAh / g or less. In addition, the overvoltage caused by the internal resistance of the battery was approximately 0.7 V.

[0047] <Comparison of cycle stability and rate characteristics> The cycle stability of Example 1 and Comparative Example 3, which were charged and discharged at 0.01 C, is shown in Figure 6. As shown in Figure 6, Example 1 had a higher energy density than Comparative Example 3, even though the blending ratio of SPEG was lower. The cycle stability of Example 2, Comparative Example 1, and Comparative Example 2, which were charged and discharged at 0.05 C, is shown in Figure 7. As shown in Figure 7, Comparative Example 1, which contained the same amount of SPEG as Example 2, did not work as a battery.

Claims

1. a first mixture containing active material particles and a first substance having an ionic group; a second mixture including carbon particles and a second material having ionic groups opposite to the ionic groups of the first material; in water.

2. The method for producing an electrode material according to claim 1 , wherein at least one of the active material particles and the carbon particles is treated with a surfactant.

3. The method for producing an electrode material according to claim 1 or 2, wherein the active material particles contain sulfur.

4. The method for producing an electrode material according to any one of claims 1 to 3, wherein the active material particles contain sulfur, carbon, and oxygen.

5. The method for producing an electrode material according to any one of claims 1 to 4, wherein a mass ratio of the active material particles to the carbon particles contained in the electrode material is 50:50 to 95:

5.

6. active material particles; Carbon particles, a first substance having an ionic group present on the surface of the active material particles; an ionic group of the first substance present on the surface of the carbon particle, and a second substance having an ionic group opposite to the ionic group of the first substance, wherein the active material particles and the carbon particles are in an electrostatically adsorbed state.

7. A secondary battery comprising the electrode material according to claim 6.

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