Coated organic positive electrode active material particles for secondary batteries

Abstract

The present invention aims to provide coated organic cathode active material particles for secondary batteries that can stably produce secondary batteries with a low rate of increase in internal resistance and excellent cycle characteristics. [Solution] Coated organic cathode active material particles for secondary batteries, wherein at least a portion of the surface of the organic cathode active material particles is coated with a coating layer, the coating layer comprising an ion-conductive polymer composition and a conductive additive.

Description

Technical Field

[0001] The present invention relates to coated organic cathode active material particles for secondary batteries.

Background Art

[0002] In recent years, with the progress of electronics engineering, portable electronic devices such as mobile phones, notebook computers, digital cameras, etc. have rapidly spread. Along with this, the demand for secondary batteries and the like as power sources for portable electronic devices has been increasing. In addition, in the trend of forming a sustainable society, there is a major trend of replacing conventional fossil fuel-based power sources with power sources using renewable energy such as sunlight and wind power, and the demand for secondary batteries necessary for stably using the power of sunlight and wind power, which are variable power sources, as power sources is expected to increase in the future. As secondary batteries, lithium-ion batteries that can achieve high energy density and high output density have attracted attention.

[0003] In lithium-ion batteries, lithium-containing transition metal oxides are often used as the cathode active material. Also, carbon materials are used as the anode active material, and charge and discharge are performed by utilizing the insertion reaction and desorption reaction of lithium ions with respect to these electrode active materials. However, in the above-mentioned lithium-ion batteries, there is a problem that since the density of the lithium-containing transition metal oxide is relatively large, the theoretical capacity per unit mass remains at a small value. Also, since the movement speed of lithium ions in the cathode is slower than that in the electrolyte and the anode, there is also a problem that the charge and discharge speed is limited, so that high output and shortening of the charge and discharge time cannot be achieved.

[0004] In order to solve the above problems, research and development of next-generation secondary batteries using organic materials with a lower density than transition metal oxides, such as conductive polymers, organic radical compounds, and quinone compounds, as electrode active materials has been actively carried out. For example, Patent Document 1 discloses a secondary battery that uses a conductive polymer as the material for the positive or negative electrode. Prior art documents that use organic radical compounds as electrode active materials include Patent Document 2 and Non-Patent Documents 1 and 2. An example of using quinone compounds as electrode active materials is one that utilizes the redox reaction of paraquinone (Non-Patent Document 3). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] U.S. Patent Publication No. 4442187 [Patent Document 2] Japanese Patent Publication No. 2004-207249 [Non-patent literature]

[0006] [Non-Patent Document 1] Electrochimica Acta, 2004, No. 50, p.827 [Non-Patent Document 2] Journal of The Electrochemical Society, 1986, No. 133, pp. 836-841. [Non-Patent Document 3] Chemical Physics Letters, 2002, No. 359, p.351 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, it has been pointed out that when the conductive polymer described in Patent Document 1 is used as an electrode active material, the doping and dedoping reactions of electrolyte ions in the conductive polymer may cause undesirable interactions through the π-electron conjugated system of the conductive polymer, which limits the amount of electrolyte ion doping, and thus the secondary battery capacity. While it is said that using organic radical compounds described in Patent Document 2 and Non-Patent Documents 1 and 2 as electrode active materials achieves good cycle characteristics and a high average operating voltage that contributes to improved energy density, the charge-discharge reaction is limited to a one-electron reaction involving only one electron. This is because if a multi-electron reaction involving two or more electrons is induced in the organic radical compound, it leads to irreversible decomposition, and the reversibility of the charge-discharge reaction is lost. For this reason, batteries using the aforementioned organic radical compounds as electrode active materials have a capacity per unit mass that remains small compared to lithium-ion secondary batteries using transition metal oxides, making it difficult to achieve a high energy density. Furthermore, when benzoquinone or anthraquinone derivatives described in Non-Patent Document 3 are used as electrode active materials, it is known that the electrode active materials gradually undergo electrochemical changes and dissolve into the electrolyte as repeated charging and discharging occurs. As the amount of usable electrode active material decreases, there is a problem that the cycle characteristics deteriorate as a result.

[0008] Based on the above circumstances, the present invention aims to provide coated organic cathode active material particles for secondary batteries that can stably produce secondary batteries with a low rate of increase in internal resistance and excellent cycle characteristics. [Means for solving the problem]

[0009] The inventors of this invention arrived at this present invention after diligently studying to solve these problems. In other words, the present invention relates to coated organic positive electrode active material particles for secondary batteries, wherein at least a portion of the surface of the organic positive electrode active material particles is coated with a coating layer, and the coating layer comprises an ion-conductive polymer composition and a conductive additive. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide coated organic cathode active material particles for secondary batteries that can stably produce secondary batteries with a low rate of increase in internal resistance and excellent cycle characteristics. [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 is a schematic diagram showing an example of the structure of coated organic cathode active material particles for secondary batteries. [Modes for carrying out the invention]

[0012] The present invention will be described in detail below. The coated organic positive electrode active material particles for secondary batteries of the present invention have at least a portion of their surface covered with a coating layer, the coating layer comprising an ion-conductive polymer composition and a conductive additive. By coating at least a portion of the surface of the organic cathode active material particles with a coating layer, the opportunity for the electrolyte to directly contact the organic cathode active material is reduced, thereby preventing deterioration of the battery's cycle characteristics. Furthermore, by including an ion-conductive polymer composition in the coating layer, the exchange of carrier ions with the electrolyte can be ensured, and by including a conductive additive, an electron conduction path with the current collector can be ensured.

[0013] The organic cathode active material particles are not particularly limited as long as they are organic compounds capable of multi-electron reactions, but from the viewpoint of energy density, they are preferably benzoquinone, naphthoquinone, anthraquinone, or phenazine, and more preferably benzoquinone or anthraquinone.

[0014] An ionic conductive polymer composition is a polymer composition containing ions that act as an electrolyte, and refers to a polymer composition in which carrier ions of a secondary battery, such as sodium or lithium, can move within the polymer composition according to their potential difference. As a method for confirming whether or not a sample has ion conductivity, there is a method in which a metal of carrier ions is arranged at both ends of the sample, and after forming a configuration of a symmetric cell, an electric current is applied, and it is confirmed whether or not the closed-circuit voltage continues at a constant value. When there is no ion conductivity, the carrier for flowing an electric current is immediately depleted, so the closed-circuit voltage immediately improves and it becomes impossible to flow an electric current. On the other hand, when there is ion conductivity, carrier ions move in the ion-conductive resin in response to electrons moving in the external circuit during energization, so the ion concentration between the symmetric cell electrodes becomes a steady state, and the closed-circuit voltage converges to a constant value.

[0015] The polymer composition having ion conductivity preferably contains a fluororesin, a polyester resin, a polyether resin, a vinyl resin, a urethane resin, a polyamide resin, an epoxy resin, an acrylic resin, a polyimide resin, or a mixture thereof.

[0016] As the polymer composition having ion conductivity, a polymer composition having a liquid absorption rate of 10% or more when immersed in an electrolytic solution and a tensile elongation at break of 10% or more in a saturated liquid absorption state is preferable.

[0017] The liquid absorption rate when immersed in an electrolytic solution is obtained by measuring the weights of the coating resin before and after immersion in the electrolytic solution and using the following formula. Liquid absorption rate (%) = [(weight of coating resin after immersion in electrolytic solution - weight of coating resin before immersion in electrolytic solution) / weight of coating resin before immersion in electrolytic solution] × 100 [[ID=十六]]As the electrolytic solution for obtaining the liquid absorption rate, an electrolytic solution in which LiTFSI is dissolved at a concentration of 1 mol / L in a mixed solvent obtained by mixing 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) at a volume ratio of 1:1 is used. The immersion in the electrolytic solution for obtaining the liquid absorption rate is performed at 50°C for 3 days. By performing the immersion at 50°C for 3 days, the coating resin becomes a saturated liquid absorption state. The saturated liquid absorption state means a state in which the weight of the coating resin does not increase even when further immersed in the electrolytic solution. Note that the electrolytic solution used when manufacturing a secondary battery is not limited to the above electrolytic solution, and other electrolytic solutions may be used.

[0018] If the liquid absorption rate is 10% or more, the polymer has sufficiently absorbed the electrolyte, and carrier ions can easily permeate the coating layer, so the movement of carrier ions between the active material and the electrolyte is not hindered. If the liquid absorption rate is less than 10%, the electrolyte does not easily penetrate the coating layer, resulting in low conductivity of carrier ions, and the performance of the secondary battery may not be fully realized. The liquid absorption rate is preferably 20% or more, and more preferably 30% or more. Furthermore, a preferred upper limit for the liquid absorption rate is 400%, and a more preferred upper limit is 300%.

[0019] The content of the polymer composition in the coated organic positive electrode active material particles for secondary batteries of the present invention is preferably 1 to 50% by weight, more preferably 1.5 to 25% by weight, and particularly preferably 2 to 10% by weight, based on the weight of the coated organic positive electrode active material particles, from the viewpoint of ensuring ion paths and balancing energy density.

[0020] There are no particular restrictions on the conductive additive, as long as it is a conductive material. Examples of conductive additives include metals [aluminum, stainless steel (SUS), silver, gold, copper, and titanium, etc.], carbon [graphite (flaky graphite (UP)), carbon black (acetylene black, Ketjen black, furnace black, channel black, and thermal lamp black, etc.), carbon nanofibers (CNF), carbon nanotubes (CNT), etc.], and mixtures thereof. These conductive additives may be used individually or in combination of two or more. Alloys or metal oxides of these additives may also be used. From the viewpoint of electrical stability, carbon-based conductive additives are preferred, with Ketjenblack and acetylene black being more preferred.

[0021] The content of the conductive additive in the coated organic positive electrode active material particles for secondary batteries of the present invention is preferably 1 to 10% by weight, and more preferably 1 to 5% by weight, based on the weight of the coated organic positive electrode active material particles, from the viewpoint of balancing energy density and ensuring sufficient electron conduction paths.

[0022] The mixing ratio of the polymer composition to the conductive additive is not particularly limited, but it is preferable that the weight ratio of polymer (resin solids weight):conductive additive = 1:0.2 to 3.0. [Examples]

[0023] The present invention will be further described below with reference to examples and comparative examples, but the present invention is not limited to these. In the examples, "parts" refers to parts by mass unless otherwise specified. "M" indicates molar concentration (mol / l). Furthermore, unless otherwise specified, the reaction temperature refers to the internal temperature within the reaction system.

[0024] The structural formulas of the various compounds obtained in the following synthesis examples were confirmed by analyzing spectra such as ES-MS (electrospray mass spectrometry) and proton nuclear magnetic resonance spectroscopy, as needed. The measuring instruments used for these spectra are as follows. ES-MS spectrum: Gas chromatograph mass spectrometer (Shimadzu Corporation, model "GCMS-QP2010SE") Proton nuclear magnetic resonance spectroscopy (hereinafter, as appropriate, 1 (H-NMR spectrum): Nuclear Magnetic Resonance Spectrometer (manufactured by JEOL Ltd., model name "JNM-Lambda 400")

[0025] <Example 1> Preparation of coated organic cathode active material (A1) (Step 1) Preparation of coating resin solution (a) Fifty parts of trimethylolpropane triglycidyl ether, ten parts of butyl glycidyl ether, and 30 parts of an electrolyte solution prepared by dissolving LiTFSI at a ratio of 1 mol / l in a mixed solvent of 1,2-dimethoxyethane (DME) and 1,3-dioxolane (volume ratio 1:1) were mixed. Furthermore, ten parts of San-Aid SI-60 (manufactured by Sanshin Chemical Co., Ltd.), a thermoacid generator, were added, and the mixture was stirred while cooling to 0°C to obtain coating resin solution (a).

[0026] (Step 2) Preparation of coated organic cathode active material (A1) 90 parts of anthraquinone (AQ) as the organic cathode active material were placed in a universal mixer high-speed mixer FS25 [manufactured by Earth Technica Co., Ltd.], and while stirring at room temperature and 720 rpm, 10 parts of coating resin solution (a) were added dropwise over 2 minutes, and the mixture was stirred for a further 5 minutes. Next, while stirring, 3.14 parts of acetylene black [Denka Black®, manufactured by Denka Co., Ltd.], a conductive additive, were added in portions over 2 minutes, and stirring was continued for 30 minutes. Subsequently, the temperature was raised to 100°C while maintaining stirring, and the temperature was maintained for 10 minutes while stirring to remove volatile components. The obtained powder was classified using a sieve with a mesh size of 200 μm to obtain coated organic cathode active material (A1).

[0027] <Examples 2-4 and Comparative Examples 1,2> Except for changing the type of organic cathode active material and the amount of coating resin solution (a) as shown in Table 1, coated organic cathode active materials (A2) to (A4), (A'1) and (A'2) for Examples 2 to 4 and Comparative Examples 1 and 2 were obtained in the same manner as in Example 1. Note that "DBQ" in Table 1 represents dibenzoquinone.

[0028] <Fabrication of positive electrodes for lithium-ion batteries> A cathode slurry was prepared by mixing 98.50 parts of the fabricated coated organic cathode active material with 2.06 parts of carbon fiber [Donacarbo Milled S-243, manufactured by Osaka Gas Chemical Co., Ltd.: average fiber length 500 μm, average fiber diameter 13 μm: electrical conductivity 200 mS / cm] and 1.03 parts of Ketjenblack [EC300J, manufactured by Lion Specialty Chemicals Co., Ltd.]. The prepared positive electrode slurry is placed on a Φ15 mold, with a basis weight of 10 mg / cm³ of positive electrode active material particles. 2 Fill to this size and press with a press machine (HANDTAB-100T15, manufactured by Ichihashi Seiki Co., Ltd.) at a rate of 1 ton / cm². 2 A positive electrode active material layer (50 μm thick) was formed by tablet molding under pressure, and this layer was laminated onto one side of a current collector to produce a positive electrode for a lithium-ion battery (circular, 15 mm in diameter).

[0029] <Manufacturing of lithium-ion batteries> A lithium-ion battery was fabricated by combining the positive electrode for the lithium-ion battery and lithium metal as the negative electrode via a separator (Cellguard #3501), and injecting an electrolyte (1 mol / L LiTFSI in DME / DOL = 1 / 1 (by vol.)).

[0030] <Measurement of the rate of increase in internal resistance> Lithium-ion batteries were evaluated at 25°C using the charge / discharge measurement device "HJ-SD8" [manufactured by Hokuto Denko Co., Ltd.] by the following method. The battery was charged to 3.5V using a constant current / constant voltage method (0.1C), then left to rest for 60 minutes, and finally discharged to 1.5V using a constant current method (0.1C). The voltage and current at 0 seconds and 10 seconds after discharge were measured using the constant current / constant voltage method (also called CCCV mode) at 0.1C, and the internal resistance was calculated using the following formula. A smaller internal resistance indicates better battery characteristics. Note that the voltage at 0 seconds after discharge refers to the voltage measured simultaneously with the discharge (also called the discharge voltage). [Internal resistance (Ω cm 2)] = [(Voltage after 0 seconds of discharge at 0.1C) - (Voltage after 10 seconds of discharge at 0.1C)] ÷ [(Current after 0 seconds of discharge at 0.1C) - (Current after 10 seconds of discharge at 0.1C)] × [Opposite area of ​​electrodes (cm²)] 2 )] To measure the internal resistance, a 20-cycle repeated test was performed. The internal resistance at the 2nd cycle (initial internal resistance) was compared with the internal resistance at the 20th cycle (internal resistance at the 20th cycle / internal resistance at the 2nd cycle) to determine the "rate of increase in internal resistance (%)".

[0031] <Measurement of volume retention rate> At 25°C, charge-discharge tests were performed on lithium-ion batteries using the charge-discharge measurement device "HJ-SD8" [manufactured by Hokuto Denko Co., Ltd.] according to the following method. The battery was charged to 3.5V using a constant current / constant voltage method (0.1C), then discharged to 1.5V using a constant current method (0.1C) after a 60-minute rest period. The discharged capacity was defined as [Discharge Capacity (mAh)]. A 20-cycle repeated test was performed, and the 20-cycle capacity retention rate (%) was determined.

[0032] [Table 1] [Industrial applicability]

[0033] The coated organic cathode active material particles of the present invention are particularly useful as cathode active material particles for secondary batteries used in stationary power supplies, mobile phones, personal computers, hybrid vehicles, and electric vehicles, etc. [Explanation of Symbols]

[0034] 1 Organic cathode active material 2. Polymers having ionic conductivity 3 Conductive additives 4 Electrolyte

Claims

1. Coated organic positive electrode active material particles for secondary batteries, wherein at least a portion of the surface of the organic positive electrode active material particles is coated with a coating layer, The aforementioned coating layer comprises an ionic conductive polymer composition and a conductive additive, comprising coated organic cathode active material particles for secondary batteries.

2. The coated organic positive electrode active material particles for a secondary battery according to claim 1, wherein the organic positive electrode active material particles are any of benzoquinone, naphthoquinone, anthraquinone, or phenazine.

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