TW201916443A - Positive electrode active material coating material, positive electrode, and secondary battery - Google Patents

Abstract

The purpose of the present invention is to provide a positive electrode active material coating material that can provide a secondary battery having excellent battery characteristics, as well as a positive electrode and a secondary battery. A positive electrode active material coating material according to the present invention comprises a positive electrode active material, a conductive auxiliary agent, a dispersion medium, and a dispersing agent, and is characterized in that the dispersion medium is N-methylpyrrolidone, the dispersing agent contains a polyvinyl alcohol-polyvinylpyrrolidone graft copolymer, and the content of the polyvinyl alcohol is 10-90 mass% with respect to 100 mass% for the polyvinyl alcohol-polyvinylpyrrolidone graft copolymer.

Description

Positive active material coating, positive electrode, and secondary battery

The present invention relates to a positive electrode active material coating material, a positive electrode, and a secondary battery.

In recent years, as a secondary battery, a lithium ion secondary battery capable of achieving high energy density and high output density has been attracting attention. In a solvent formulation used for the production of an electrode for a secondary battery, N-methylpyrrolidone is used as a solvent, and polyvinylidene fluoride (hereinafter sometimes referred to as PVDF) functions as a current collector and an active material. The function of the binder. However, the adhesion of PVDF to the current collector is weak, and it must be used in a large amount relative to the active material in comparison with the amount of the binder of the water-based positive electrode or the negative electrode. Therefore, in order to promote the high capacitance of the battery, PVDF must be reduced. The amount added. Further, in the positive electrode, in order to secure the conductive path in accordance with the volume change of the active material, a conductive auxiliary agent having the same amount as the active material in volume is required, and therefore it is necessary to sufficiently disperse it. Patent Document 1 proposes a binder which uses a polyacrylonitrile-based resin as a binder and further imparts thixotropy which is difficult to achieve in the system. Patent Document 2 proposes to use polyvinylpyrrolidone (hereinafter also referred to as PVP) for dispersion of a conductive auxiliary agent. [Prior Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open Publication No. 2013-122913 (Patent Document 2) Japanese Patent Laid-Open Publication No. 2004-281096

[The problem that the invention wants to solve]

However, when a polyacrylonitrile-based resin is used as a binder, sufficient dispersion performance cannot be exhibited for dispersion of a highly hydrophobic conductive auxiliary agent (Patent Document 1). Further, since PVP is dissolved in the electrolytic solution and the internal resistance is increased, it is not suitable (Patent Document 2). Therefore, an object of the present invention is to provide a positive electrode active material coating material, a positive electrode, and a nonaqueous secondary battery which can provide a secondary battery excellent in battery characteristics. [Technical means to solve the problem]

According to a first embodiment of the present invention, a positive electrode active material coating material comprising a positive electrode active material, a conductive auxiliary agent, a dispersion medium, and a dispersing agent, wherein the dispersion medium is N-methylpyrrolidone, is dispersed. The agent contains a polyvinyl alcohol-polyvinylpyrrolidone graft copolymer, and the content of the polyvinyl alcohol is 10% by mass or more and 90% by mass based on 100% by mass of the polyvinyl alcohol-polyvinylpyrrolidone graft copolymer. the following.

In a preferred embodiment, the content of the dispersant is 0.1% by mass or more and 20% by mass or less based on 100% by mass of the total amount of the positive electrode active material, the conductive auxiliary agent, and the dispersing agent.

According to another embodiment of the present invention, a positive electrode comprising a current collector and a positive electrode active material mixture layer formed on the current collector, wherein the positive electrode active material mixture layer is a positive electrode active material The paint is dried.

According to another embodiment of the present invention, there is provided a secondary battery comprising a negative electrode and a positive electrode, a separator disposed between the negative electrode and the positive electrode, and an electrolyte, wherein the positive electrode is the positive electrode. [Effects of the Invention]

The present invention can provide a positive electrode active material coating material, a positive electrode, and a nonaqueous secondary battery which can provide a secondary battery excellent in battery characteristics.

Next, embodiments of the present invention will be described in detail.

The positive electrode active material coating material (hereinafter sometimes referred to simply as a coating material) of the present embodiment contains a positive electrode active material, a conductive auxiliary agent, a dispersion medium, and a dispersing agent, and the dispersion medium is N-methylpyrrolidone, and the dispersing agent contains poly a vinyl alcohol-polyvinylpyrrolidone graft copolymer (hereinafter sometimes referred to simply as a graft copolymer) having a polyvinyl alcohol content of 100% by mass based on the above polyvinyl alcohol-polyvinylpyrrolidone graft copolymer It is 10% by mass or more and 90% by mass or less.

The positive electrode active material is not particularly limited as long as it can insert and detach lithium ions. For example, examples of the positive electrode active material include metal oxides such as CuO, Cu ₂ O, MnO ₂ , MoO ₃ , V ₂ O ₅ , CrO ₃ , MoO ₃ , Fe ₂ O ₃ , Ni ₂ O ₃ , and CoO ₃ , and Li. _a composite oxide of lithium and a transition metal such as _x CoO ₂ , Li _x NiO ₂ , Li _x Mn ₂ O ₄ , or a metal chalcogenide such as TiS ₂ , MoS ₂ or NbSe ₃ , polyacene, polyparaphenylene, poly A conductive polymer compound such as pyrrole or polyaniline.

Among these, it is generally preferable that the composite oxide of lithium or a transition metal selected from a high voltage system selected from the group consisting of a transition metal of cobalt, nickel, and manganese and lithium is preferable in terms of lithium ion release property and high voltage. Specific examples of the composite oxide of cobalt, nickel, manganese and lithium include LiCoO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiNi _x Co _{(1 -x)} O ₂ , and LiMn _a Ni _b Co _{c .} (a+b+c=1) and so on.

The conductive auxiliary agent is not particularly limited as long as it is an electron conductive material that does not adversely affect the battery performance. Usually, carbon black such as acetylene black or ketjen black is used, but it can also be natural graphite (scaly graphite, flaky graphite, earthy graphite, etc.), artificial graphite, carbon whisker, carbon fiber or metal (copper, nickel, aluminum). , silver, gold, etc.) conductive materials such as powder, metal fiber, and conductive ceramic materials. Among these conductive agents, graphite, acetylene black, carbon black, ketjen black, a carbon nanotube or a derivative thereof, and carbon fiber may be mentioned. These may also be used as a mixture of 2 or more types. The amount of addition is preferably 0.1% by mass or more and 30% by mass or less, and particularly preferably 0.2% by mass or more and 20% by mass or less based on 100% by mass of the mass of the positive electrode active material.

The above dispersion medium is N-methylpyrrolidone, but other dispersion medium may be used in combination as long as the effects of the present invention are not impaired. The other dispersion medium is not particularly limited, and specific examples thereof include N-ethylpyrrolidone, N,N-dimethylformamide, tetrahydrofuran, dimethylacetamide, and dimethylammonium. Hexamethylsulfonamide, tetramethylurea, acetone, methyl ethyl ketone, and the like.

The dispersant contains a polyvinyl alcohol-polyvinylpyrrolidone graft copolymer, and the content of the polyvinyl alcohol is 10% by mass or more and 90% by mass based on 100% by mass of the polyvinyl alcohol-polyvinylpyrrolidone graft copolymer. Below mass%.

The lower limit of the degree of polymerization of the polyvinyl alcohol is preferably 200 or more, more preferably 500 or more, still more preferably 1,000 or more. The upper limit is preferably 4,000 or less, more preferably 3,000 or less, and still more preferably 2,000 or less. When the degree of polymerization of the polyvinyl alcohol is within the above range, the positive electrode active material coating material having excellent battery characteristics can be obtained by dispersing the positive electrode active material and the dispersing aid with a small amount of a dispersing agent.

The lower limit of the degree of saponification of the polyvinyl alcohol is preferably 70 mol% or more, more preferably 80% or more, and on the other hand, the upper limit is preferably 100 mol% or less, more preferably 90 mol% or less. When the degree of saponification is in the above range, the dispersibility of the conductive auxiliary agent becomes excellent.

The lower limit of the content of the polyvinyl alcohol in 100% by mass of the polyvinyl alcohol-polyvinylpyrrolidone graft copolymer is 10% by mass or more, preferably 20% by mass or more, and more preferably 40% by mass or more. On the other hand, the upper limit is 90% by mass or less, preferably 80% by mass or less, and more preferably 60% by mass or less. When the content is in the above range, the positive electrode active material and the dispersing aid can be dispersed by using a small amount of a dispersing agent, and a positive electrode active material coating material having excellent peeling strength and positive electrode characteristics can be obtained.

As an embodiment of the above graft copolymer, it can be produced by the method described in the following examples.

The above dispersing agent may contain other dispersing agents within a range not inhibiting the effects of the present invention. The other dispersing agent is not particularly limited, and specific examples thereof include styrene-butadiene rubber, polyvinylpyrrolidone, poly(meth)acrylonitrile, ethylene-vinyl alcohol copolymer, and polyvinylpyrrolidone. - a polymer such as a polyacrylonitrile graft copolymer or a vinyl acetate polymer; a fluorine-based polymer such as polyvinylidene fluoride, tetrafluoroethylene or pentafluoropropylene. These may be used alone or in combination of two or more.

In the positive electrode active material paint, the content of the dispersant is 0.1% by mass or more and 20% by mass or less based on 100% by mass of the total amount of the positive electrode active material, the conductive auxiliary agent, and the dispersing agent. The lower limit of the content is preferably 1% by mass or more. Further, the lower limit of the content is preferably 10% by mass or less. When the content is in the above range, the positive electrode active material coating material can be obtained by dispersing the positive electrode active material and the dispersing aid with a small amount of a dispersing agent.

The positive electrode active material coating material can be prepared by adding a positive electrode active material, a conductive auxiliary agent, a dispersing agent, and other additives, mixing them, and further diluting them with a dispersion medium. The above mixing and dispersion are not particularly limited, and can be carried out by a usual production method using a planetary mixer or a dispersing machine. The viscosity of the positive electrode active material coating material can be appropriately set. Specifically, the lower limit is 1,000 mPa·s (25 ° C) or more, and preferably 2,000 mPa·s (25 ° C) or more. On the other hand, the upper limit is preferably 10,000 mPa·s (25 ° C) or less, more preferably 7,000 mPa·s (25 ° C) or less. When the viscosity is within the above range, it is preferably a coating material having excellent coatability.

The positive electrode is formed by applying the positive electrode active material coating material to a current collector and volatilizing the dispersion medium to form a positive electrode active material layer.

Any of the above-described current collectors may be any electronic conductor that does not adversely affect the battery to be formed. For example, in addition to copper, stainless steel, nickel, aluminum, titanium, calcined carbon, conductive polymer, conductive glass, Al-Cd alloy, etc., copper, etc. can be used for the purpose of improving adhesion, conductivity, and oxidation resistance. The surface is treated with carbon, nickel, titanium or silver. The collector materials can also be oxidized to the surface. Further, the shape thereof may be a molded body such as a film, a sheet, a mesh, a punched or expanded, a lath body, a porous body, or a foam, in addition to a foil shape. The thickness is not particularly limited, and usually 1 to 100 μm is used.

The basis weight per unit area of the positive electrode active material layer is appropriately set, and is usually 3 to 20 mg/cm ² or more.

The secondary battery of the present invention comprises the above positive electrode as a positive electrode. The structure of the secondary battery of the embodiment is not particularly limited, and examples thereof include a positive electrode, a negative electrode, a separator, and an electrolyte, and the negative electrode is the electrode of the above-described embodiment. In one embodiment, the battery may be provided with a laminate obtained by alternately stacking a positive electrode and a negative electrode via a separator, a container for accommodating the laminate, and an electrolyte such as an electrolyte injected into the container. [Examples]

Next, the examples and the comparative examples will be described together. However, the invention is not limited to the embodiments. Further, in the example, "%" means a quality standard unless otherwise specified.

[Synthesis of Graft Copolymer] (Graft Copolymer 1) Polyvinyl alcohol was dissolved in 800 parts by mass of pure water (degree of polymerization: 1700, completely saponified, product name: PVA-117, manufactured by Kuraray Co., Ltd.) 100 In parts by mass, 100 parts by mass of N-vinylpyrrolidone was added and mixed, and deoxidation was carried out by driving nitrogen gas. Then, the temperature of the reaction system was adjusted to 70 °C. The polymerization was started by adding 0.01 parts by mass of 1% by weight of copper sulfate, 1 part by mass of 28% by weight of ammonia water, and 1.5 parts by mass of 30% by mass of hydrogen peroxide water (concentration of 450 ppm with respect to the system). During the polymerization, the temperature was maintained at 70 to 80 ° C, the pH was maintained at 5.5 to 6.5 by ammonia, and 1.5 parts by mass of 30% by weight of hydrogen peroxide water was added every 15 minutes for 10 times, whereby the polymerization rate was 90% or more. . In the meantime, the hydrogen peroxide concentration is 600 ppm or less. Then, as a residual N-vinylpyrrolidone treatment step, 4 parts by mass of 30% by mass of hydrogen peroxide water is added, and a pH of 5 or more is maintained by ammonia water to carry out a total of 210 minutes to obtain a graft copolymer. An aqueous solution of 1. The obtained aqueous solution was spray-dried to obtain a graft copolymer 1 (polyvinyl alcohol content: 50% by mass).

(Graft Copolymer 2) The polyvinyl alcohol was changed to partially saponified polyvinyl alcohol (degree of polymerization: 1700, saponification degree: 88%, product name: PVA-217, manufactured by Kuraray Co., Ltd.), 160 parts by mass, and N- The graft copolymer 2 (polyvinyl alcohol content: 80% by mass) was obtained in the same manner as in the graft copolymer 1 except that the vinylpyrrolidone was changed to 40 parts by mass.

(Graft Copolymer 3) The polyvinyl alcohol was changed to partially saponified polyvinyl alcohol (degree of polymerization: 1700, saponification degree: 88%, product name: PVA-217, manufactured by Kuraray Co., Ltd.), and The branched copolymer 1 was produced in the same manner to obtain a graft copolymer 3 (polyvinyl alcohol content of 50% by mass).

(Graft Copolymer 4) The polyvinyl alcohol was changed to partially saponified polyvinyl alcohol (degree of polymerization: 1700, saponification degree: 88%, product name: PVA-217, manufactured by Kuraray Co., Ltd.), 40 parts by mass, and N- The graft copolymer 4 (polyvinyl alcohol content: 20% by mass) was obtained in the same manner as in the graft copolymer 1 except that the vinylpyrrolidone was changed to 160 parts by mass.

[Preparation of the positive electrode active material paint] (Coating material 1) 4 parts by mass of a dispersing agent (graft copolymer 1) and 66.7 parts by mass of a dispersion medium (N-methylpyrrolidone) were added to a high-profile beaker and mixed. By stirring at 1000 rpm by a dispersing machine, 92 parts by mass of _a positive electrode active material (LiMn _a Ni _b Co _c (a+b+c=1)) previously mixed with powder-powder was gradually added and a conductive auxiliary agent (conductive carbon black) In the product name: SuperP, manufactured by Imerys GC Japan Co., Ltd., 4 parts by mass, stirred at 2000 rpm for 90 minutes, and further allowed to stand overnight to obtain the coating material 1.

(Coatings 2 to 6) Coatings 2 to 6 were obtained in the same manner as in the coating 1 except that the dispersing agent was changed as shown in the following Table 1.

[Evaluation of Coatings] The coatings 1 to 6 were evaluated according to the following evaluation criteria. The results are shown in Table 1 below.

(Viscosity) The viscosity of the positive electrode active material coating at 25 ° C was measured in accordance with JIS Z8803 using a BM type viscometer (single cylinder type rotational viscometer). In this case, (a) when the number of revolutions of the rotor is 60 rpm, and (b) when the measured value in the above (a) is 8000 mPa·s or more, the number of revolutions of the rotor is changed to 30 rpm, and the measurement is performed. (c) When the measured value in the above (b) is 16,000 mPa·s or more, the number of revolutions of the rotor is changed to 12 rpm.

(Dispersion state) The dispersion state of the positive electrode active material and the conductive auxiliary agent of the positive electrode active material coating material was evaluated according to the following evaluation criteria. ◎: A cured product of a positive electrode active material or a conductive auxiliary agent which was small due to poor dispersion was not observed by visual observation. ○: A small amount of a positive electrode active material or a cured product of a conductive auxiliary agent due to poor dispersion was observed by visual observation. △: A cured product of a large positive electrode active material or a conductive auxiliary agent which is clearly known to be poorly dispersed was observed by visual observation.

[Table 1]

According to Table 1, coatings having a good dispersion state were obtained using the coatings 1 to 4 of the graft copolymers 1 to 4. On the other hand, the dispersion of the coating material 5 using polyvinylidene fluoride in place of the graft copolymers 1 to 4 was poor, and the coating material 6 in which polyvinyl alcohol and polyvinylpyrrolidone were used together was gelated.

[Preparation of positive electrode] (Positive electrode 1) The coating material 1 was adjusted so as to have a thickness of 100 μm, and an applicator was applied to an aluminum foil (thickness: 10 μm), pre-dried at 100 ° C, and vacuum-dried at 130 ° C for 8 hours. The electrode obtained by drying was subjected to press molding by a roll press, whereby an electrode sheet having a positive electrode mixture layer having a single-sided electrode density of 2.1 g/cm ³ of a copper foil was obtained. Thereafter, the above electrode sheet is utilized A 12 mm punching machine was punched into a circular shape to obtain a positive electrode 1 for evaluation.

(Positive Electrodes 2 to 5) The positive electrodes 2 to 5 were obtained in the same manner as in the case of the positive electrode 1 except that the coating material was changed as shown in Table 2. Further, since the coating material 6 was gelled after standing overnight, the positive electrode was not produced.

[Evaluation of Positive Electrode] The positive electrodes 1 to 5 were evaluated according to the following evaluation criteria. The evaluation results are shown in Table 2.

(Measurement of peel strength) The electrode sheet was cut into a short strip of 18 cm × 2 cm, and the coated surface was attached to the double-sided tape by using a double-sided tape and a steel plate having a thickness of 1 mm on the collector side. The celluloid adhesive tape was attached to the current collector, and the stress at the time of peeling to the 180° direction at a speed of 50 mm/min was measured by a tensile tester (Shimadzu Corporation, Autostereograph AGS-X).

(Electrode State) The electrode states of the positive electrode active material and the conductive material of the positive electrode active material coating material were evaluated in accordance with the following evaluation criteria. ◎: In the electrode slice of 5 cm × 5 cm, it was visually observed that there was no cured product of the positive electrode active material or the conductive auxiliary agent which was judged to be poorly dispersed and which was convex upward on the surface of the electrode. In the electrode slice of 5 cm × 5 cm, it was visually observed that there was one or less cured product of the positive electrode active material or the conductive auxiliary agent which was judged to be poorly dispersed on the surface of the electrode and which became convex upward: In the electrode slice of 5 cm × 5 cm, it was visually observed that there were two or more cured products of the positive electrode active material or the conductive auxiliary agent which were judged to be poorly dispersed and which became convex upward on the surface of the electrode ×: In the electrode slice of 5 cm × 5 cm, it was visually observed that there were more than 10 positive electrode active materials or cured products of the conductive auxiliary agent which were judged to be poorly dispersed and which became convex upward on the surface of the electrode.

[Table 2]

According to Table 2, the positive electrodes 1 to 4 which were produced by using the coating materials 1 to 4 of the graft copolymers 1 to 4 obtained a good positive electrode which did not cause dry cracking. On the other hand, the positive electrode 5 prepared by using polyvinylidene fluoride instead of the coating material 5 of the graft copolymers 1 to 4 is a result that the peel strength is remarkably low.

[Production of Lithium Ion Secondary Battery] (Example 1) The positive electrode 1 obtained as described above, a separator (Celgard 2325 manufactured by THANK-METAL Co., Ltd.), and lithium metal as a working electrode ( 15 mm) is sequentially placed in a specific position in the TJ-AC coin battery manufactured by Tomcell Japan. Further, a mixed solution of ethylene carbonate and ethylene carbonate containing 1 mol/L of LiPFp ₆ was added to an electrolyte solution in which ethylene carbonate was added to prepare a secondary battery.

(Examples 2 to 4) A secondary battery was obtained in the same manner as in Example 1 except that the positive electrode 1 was changed to the positive electrode 2 to 4, respectively. Further, since the coating material 5 was not ideal in the dispersion state, and the mixture layer was broken and powdered when the electrode was punched, the secondary battery was not produced.

[Evaluation of Battery Characteristics] The secondary batteries obtained in Examples 1 to 4 were evaluated by the following evaluation methods and evaluation criteria. The results are shown in Table 3.

(First charge and discharge characteristics) Under a temperature of 20 ° C, the current value of 0.1 C obtained based on the effective capacitance of the above positive electrode was charged under constant current and constant voltage until the voltage value became 0.01 V, and the current value was lowered to The charging is stopped at the time point of 0.05 C. Then, discharge was performed under the condition of a current value of 0.1 C until the voltage of the metal Li became 1.0 V, and the initial discharge capacitance was measured.

(Discharge rate characteristic) Under a temperature of 20 ° C, the current value of 0.2 C obtained based on the second discharge capacitance is charged under a constant current constant voltage condition until the voltage value becomes 0.01 V, and the current value is lowered to The charging is stopped at the time point of 0.05 C. Then, discharge was performed under the condition of a current value of 1 C until the voltage of the metal Li became 1.0 V, and the discharge capacitance of 1 C was measured. Further, charging was performed in the same manner, and discharge was performed under the condition of a current value of 3 C, and a discharge capacitance of 3 C was measured. The discharge capacitance of 3 C when the discharge capacitance of 1 C is taken as 100% is set to the 3 C capacitor retention ratio.

[table 3] According to Table 3, it was confirmed that the positive electrode obtained in Examples 1 to 4 was operated as a secondary battery. [Industrial availability]

The positive electrode active material paint of the present invention, and a positive electrode and a secondary battery using the same can be widely used in portable machines and the like.

Claims (4)

A positive electrode active material coating material comprising a positive electrode active material, a conductive auxiliary agent, a dispersion medium, and a dispersing agent, wherein the dispersion medium is N-methylpyrrolidone, and the dispersing agent is polyvinyl alcohol-polyvinylpyrrole The ketone ketone graft copolymer has a content of polyvinyl alcohol of 10% by mass or more and 90% by mass or less. The positive electrode active material coating material of claim 1, wherein the content of the dispersant is 0.1% by mass or more and 20% by mass or less based on 100% by mass of the total of the positive electrode active material, the conductive auxiliary agent, and the dispersing agent. A positive electrode comprising a current collector and a positive electrode active material mixture layer formed on the current collector, wherein the positive electrode active material mixture layer causes the positive electrode active material coating material according to claim 1 or 2 to be dried. Adult. A secondary battery comprising a negative electrode and a positive electrode, a separator disposed between the negative electrode and the positive electrode, and an electrolyte, wherein the positive electrode is the positive electrode of claim 3.