JP2007273355A - Negative electrode for lithium secondary battery, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a negative electrode for a lithium secondary battery that is effective for exerting more superior cycle characteristic while a high discharge capacity is maintained, and to provide its manufacturing method. <P>SOLUTION: This is the negative electrode for the lithium secondary battery which has a foil state current collector containing at least one selected from a group consisting of Fe, Ni, and Cu, and an active material layer formed in a part or whole of one face or both faces of the collector, and in which the active material layer has a following composition. (1) A metal component containing at least one kind from Si and Sn:86 to 97.9 wt.%, (2) an aqueous thickener:0.1 to 1 wt.%, (3) at least one kind of resin component selected from a group consisting of tetrafluoro ethylene-hexafluoro propylene copolymer, polystyrene-maleic acid copolymer, and polytetrafluoro ethylene:1 to 10 wt.%, and (4) carbon component:1 to 3 wt.%. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a negative electrode for a lithium secondary battery and a method for producing the same.

  A lithium secondary battery such as a lithium ion battery using a metal lithium negative electrode has characteristics that it exhibits a high voltage, a high energy density, and a low self-discharge. Utilizing such advantages, lithium secondary batteries are increasingly used as main power sources for mobile communication devices, portable electronic devices, and the like.

  In recent years, further increase in energy density and output of lithium secondary batteries has been desired, and for this purpose, for example, improvement of electrodes has been proposed. Currently, a carbon material is generally used as a negative electrode material for a lithium secondary battery. The negative electrode can be obtained, for example, by applying and drying a paste containing a carbon material and a binder on a conductive core (current collector) such as a metal foil.

  By using such a carbon material, the life and safety of the lithium secondary battery are improved. On the other hand, compared with the case of using a metal lithium negative electrode, the capacity per weight / volume is less than a fraction, and the negative electrode has room for further improvement in capacity.

  From the above viewpoint, research and development have been carried out using a metal (particularly an alloy) as a negative electrode. Examples of the metal include at least one of Si and Sn, and alloys of these with other metals. Patent Document 1 describes Si, Ge, Mg, Sn, Pb, Ag, Al, Zn, Cd, Sb, Bi, In, Ca, Fe, Ni, Co, and Mn as metal powders. And as a manufacturing method of a metal negative electrode, the method of manufacturing a thin film by sputtering, for example, the method of apply | coating and drying to a collector using a metal powder as a paste is known.

  When the electrode is formed by applying and drying the paste, a binder is used as necessary. Examining the necessity of a binder for each type of secondary battery is as follows. For example, in a lead storage battery, no binder is required. In a nickel / cadmium secondary battery or a nickel / hydrogen secondary battery, no binder is required when the nickel positive electrode is formed by a sintering method or a foaming method. On the other hand, when the nickel positive electrode is formed from a paste, a binder is necessary, and the type of the binder is selected from the viewpoint of alkali resistance, oxidation resistance, binding property, coatability, etc., and polyvinyl alcohol is used. The

  As a binder in forming a negative electrode using a carbon material in a lithium secondary battery, for example, there is polyvinylidene fluoride as shown in Examples of Patent Document 2, and N-methylpyrrolidone (NMP) is used. Dissolved in a paste. And it is supposed that the outstanding cycling characteristics will be acquired by using this binder.

  However, using such an organic substance as a solvent increases the cost and requires consideration for the environment. Therefore, recently, as a binder, styrene butadiene rubber (SBR) having excellent binding properties is being used in an emulsion state (so-called aqueous state).

  In the case of using a metal material other than the carbon material as the negative electrode of the lithium secondary battery, for example, in JECS, 151 (6) A867 (2004), polyvinylidene fluoride (PVdF) is used as a binder in an organic solution state. It is said that excellent discharge characteristics and cycle life can be obtained. However, when the binder is changed to the SBR emulsion, the discharge capacity and the cycle life are lowered. The reason for this is that when a metal material is used, lithium ions are not intercalated like graphite (carbon material), but the metal material occludes lithium (alloyed with lithium) during charging. However, it is considered that the SBR emulsion cannot follow the volume change of the alloy and cannot sufficiently bind with the surroundings of the alloy and the alloy falls off.

  Patent Document 3 describes that carboxymethyl cellulose (CMC) and polyvinyl alcohol (PVA) are used as a binder in the form of an aqueous solution to form a paste, which is applied to a copper foil. This disclosure also shows that the binder has shifted from organic to aqueous. However, CMC is generally poor in chemical resistance and easily oxidatively decomposes during discharge. Therefore, when a large amount of CMC is used, the charge and discharge characteristics may be adversely affected by the oxidation product. In addition, after oxidative decomposition, the ability to suppress swelling of the negative electrode is reduced.

  On the other hand, since PVA has high film-forming properties and binding properties, it tends to hinder lithium insertion at the beginning of the charge / discharge cycle. However, repeated charge / discharge tends to cause oxidative decomposition during discharge. Therefore, when PVA is used in a large amount, the charge / discharge characteristics may be adversely affected like CMC, and the ability to suppress swelling of the negative electrode after oxidative decomposition is reduced.

  Conventionally, fluorine resins other than PVdF have been widely disclosed as binders. Patent Document 4, Patent Document 5, and the like describe fluororesin powders such as polytetrafluoroethylene (PTFE) and polyethylene tetrafluoroethylene (PETFE). Patent Document 6 describes a tetrafluoroethylene-hexafluoropropylene copolymer as a binder together with many other fluororesin copolymers.

  In addition, ethylene-vinyl acetate copolymer, ethylene-vinyl chloride copolymer, urethane resin, etc., which have excellent binding properties and film-forming ability, are all inferior in charge / discharge characteristics compared to PVdF binder. ing. That is, it has been found that the discharge capacity is extremely likely to decrease with a small number of charge / discharge cycles. This is because when a large amount of a binder having excellent binding properties is added, lithium insertion is inhibited (charging is inhibited) and the discharge capacity is reduced, and when a small amount of binder is added, lithium ion insertion / release is caused. This is considered to be caused by insufficient suppression of expansion / contraction of the negative electrode.

From the above, for the negative electrode for lithium secondary batteries, it is necessary to select a binder etc. from a different point of view as compared to conventional general paste type electrodes, and simply select from the viewpoint of electrolyte resistance, binding property, etc. However, it cannot be said that it exhibits good characteristics.
JP 2004-220911 A JP-A-10-223208 JP 2000-311681 A JP 2001-176548 A JP 2002-289162 A

  An object of this invention is to provide the negative electrode for lithium secondary batteries, and its manufacturing method effective in order to exhibit the outstanding cycling characteristics, maintaining a high discharge capacity.

  As a result of intensive studies to achieve the above object, the present inventors have found that the above object can be achieved when the active material layer is formed by drying a paste film having a specific composition, and the present invention has been completed. It came to do.

  That is, this invention relates to the following negative electrode for lithium secondary batteries, and its manufacturing method.

1. A negative electrode for a lithium secondary battery having a foil-like current collector containing at least one selected from the group consisting of Fe, Ni and Cu and an active material layer formed on a part or all of one side or both sides thereof A negative electrode for a lithium secondary battery, wherein the active material layer has the following composition:
(1) Metal component containing at least one of Si and Sn: 86-97.9 wt%,
(2) Water-soluble thickener: 0.1 to 1% by weight,
(3) At least one resin component selected from the group consisting of tetrafluoroethylene-hexafluoropropylene copolymer, polystyrene-maleic acid copolymer and polytetrafluoroethylene: 1 to 10% by weight,
(4) Carbon component: 1 to 3% by weight.

  2. Item 2. The negative electrode for a lithium secondary battery according to Item 1, wherein the metal component further contains at least one selected from the group consisting of Al, Cu, Ag, Sb, and Bi (metal component A).

  3. The metal component further contains at least one selected from the group consisting of B, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Y, Zr, Nb, Mo, and W (metal component B). Item 3. The negative electrode for a lithium secondary battery according to Item 1 or 2.

  4). Item 4. The negative electrode for a lithium secondary battery according to any one of Items 1 to 3, wherein a part or all of the surface of the foil-shaped current collector is subjected to an antioxidant treatment.

  5. Item 5. The negative electrode for a lithium secondary battery according to any one of Items 1 to 4, wherein the water-soluble thickener is at least one of carboxymethylcellulose and xanthan gum.

  6). Item 6. The negative electrode for a lithium secondary battery according to any one of Items 1 to 5, wherein the carbon component is carbon black.

  7). Item 7. The lithium secondary battery according to any one of Items 1 to 6, wherein the active material layer is a dry film formed from an aqueous paste containing a metal component, a water-soluble thickener, a resin component, and a carbon component. Negative electrode.

8). A method for producing a negative electrode for a lithium secondary battery, wherein a film made of an aqueous paste containing a raw material mixture and water is formed on part or all of one or both sides of a current collector, and then the film is dried.
Manufacturing method in which the raw material mixture has the following composition:
(1) Metal component containing at least one of Si and Sn: 86-97.9 wt%,
(2) Water-soluble thickener: 0.1 to 1% by weight,
(3) At least one resin component selected from the group consisting of tetrafluoroethylene-hexafluoropropylene copolymer, polystyrene-maleic acid copolymer and polytetrafluoroethylene: 1 to 10% by weight,
(4) Carbon component: 1 to 3% by weight.

Hereinafter, the present invention will be described in detail.

1. Negative electrode for lithium secondary battery The negative electrode for a lithium secondary battery of the present invention comprises a foil-like current collector containing at least one selected from the group consisting of Fe, Ni and Cu and a part or all of one or both sides thereof. And an active material layer formed on the negative electrode for a lithium secondary battery, wherein the active material layer has the following composition.
(1) Metal component containing at least one of Si and Sn: 86-97.9 wt%,
(2) Water-soluble thickener: 0.1 to 1% by weight,
(3) At least one resin component selected from the group consisting of tetrafluoroethylene-hexafluoropropylene copolymer, polystyrene-maleic acid copolymer and polytetrafluoroethylene: 1 to 10% by weight,
(4) Carbon component: 1-3% by weight
Hereinafter, each element of the negative electrode for a lithium secondary battery of the present invention will be described.

≪Foil current collector≫
As the foil current collector used in the present invention, a foil current collector containing at least one selected from the group consisting of Fe, Ni and Cu is used. Examples of the foil-shaped current collector include those composed of single metals (for example, copper foil) of Fe, Ni, and Cu or alloys of these metals. The form of the foil current collector is not limited as long as it is a foil shape, and may be, for example, a screen shape (including a net shape) or an expanded metal shape. The contact area with the active material layer can be increased efficiently.

  The thickness of the foil current collector is not limited, but is preferably about 7 to 35 μm, more preferably about 8 to 18 μm.

  It is preferable that a part or all of the surface of the foil-like current collector is subjected to an antioxidant treatment. In the case where the anti-oxidation treatment is performed, when the active material layer is formed from an aqueous paste or the like, the current collector surface can be prevented from being oxidized, leading to an improvement in the life of the negative electrode.

  The antioxidant can use the processing agent conventionally used for antioxidant of metal foil. Examples of the antioxidant treatment agent include benzotriazole, isopropyl triisostearoyl titanate, and γ-mercaptopropyltrimethoxysilane. The antioxidant treatment can be performed by applying and adhering the antioxidant treatment agent as it is or after dilution to the surface of the current collector.

≪Active material layer≫
An active material layer is formed on part or all of one surface or both surfaces of the foil-shaped current collector.

  The thickness of the active material layer is not limited, but is preferably about 5 to 100 μm, and more preferably about 10 to 50 μm. Although details of the method for forming the active material layer will be described later, in the present invention, the active material layer is preferably a dry coating film formed from an aqueous paste. When formed from an aqueous paste, the environmental load can be reduced as compared with the case where a solvent-based paste is used, and the process of forming the active material layer can be simplified.

The active material layer has the following composition.
(1) Metal component containing at least one of Si and Sn: 86-97.9 wt%,
(2) Water-soluble thickener: 0.1 to 1% by weight,
(3) At least one resin component selected from the group consisting of tetrafluoroethylene-hexafluoropropylene copolymer, polystyrene-maleic acid copolymer and polytetrafluoroethylene: 1 to 10% by weight,
(4) Carbon component: 1-3% by weight
Hereinafter, each component will be described.

Metal component As a metal component, the metal component containing at least 1 sort (s) of Si and Sn contains 86-97.9 weight%. By making the composition and content of the metal component within the above range, the discharge capacity per volume of the lithium secondary battery can be increased, and the lithium secondary battery can be prevented from falling off and a good charge / discharge cycle life can be obtained. easy.

  The form of the metal component is not limited, but is preferably in the form of powder. The powder usually has a structure in which microcrystals having a size of about several tens of nm are gathered. The powder is composed of primary particles of metal particles or aggregates thereof, and the average particle diameter of the powder (measured value by laser diffraction method) is not limited, but is preferably about 0.1 to 50 μm, preferably 0.1 to 20 μm. The degree is more preferable.

  What is necessary is just to contain at least 1 sort (s) of Si and Sn as a metal component. Si and Sn both have the property of reacting with Li to form a compound. In the present invention, in addition to Si and Sn, it is preferable to further contain a metal component A and / or a metal component B. From the performance of improving the charge / discharge cycle, it contains at least one of Si and Sn and a metal component A. It is desirable to do.

  Examples of the metal component A include at least one of Al, Cu, Ag, Sb, and Bi.

  Examples of the metal component B include at least one of B, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Y, Zr, Nb, Mo, and W.

The content of these metal components is not limited. For example, Si or Sn may be contained in a microcrystalline state, these may be combined (alloyed), or metal component A It may be contained in a state such as Ag ₃ Sn in a composite form with Ag. That is, as long as it contains at least one of Si and Sn, the element constituting the metal component may exist in any state such as a simple substance, a mixture, a composite (alloy or intermetallic compound). These states may be a mixture of a plurality of states.

  Particles or powders in which multiple metal components are mixed or compounded (alloyed) can be prepared, for example, by mechanical alloying (MA) treatment. In this method, it is possible to easily form fine primary particles having a particle size of about 0.1 to 50 μm. As a specific method, a raw material containing at least one of Si and Sn and a metal component A and / or a metal component B is mixed, subjected to mechanical alloying, and the primary particle size is 0.1 to 50 μm. The target composite powder can be obtained by adjusting the degree. The centrifugal acceleration (input energy) in the mechanical alloying process is preferably about 5 to 20G, and more preferably about 7 to 15G.

  For the MA processing itself, a known method may be applied as it is. For example, the target composite powder can be obtained by compounding (partially alloying) the raw material mixture while repeating mixing and adhesion by mechanical joining force. As an apparatus to be used, a mixer, a disperser, a pulverizer and the like generally used in the powder field can be used as they are. Specific examples include a reiki machine, a ball mill, a vibration mill, an agitator mill, and the like. In particular, in order to reduce the powder stacking, it is necessary to efficiently disperse the powder that has been overlapped or agglomerated during the compounding operation one by one, so use a mixer that can give shearing force. Is desirable. The operating conditions of these devices are not particularly limited.

  The fine primary particles obtained in this way are usually agglomerated to form secondary agglomerates. When an alloy is produced by the mechanical alloying method, the secondary agglomerates are examined by laser diffraction. The maximum particle size is about 1 to 105 μm. The secondary aggregate may be used after reducing the degree of aggregation as necessary.

  The particle size of the secondary agglomerates is not particularly limited, but when used as a negative electrode material, 90% or more, preferably 99.9% or more of the secondary agglomerates are in the range of about 1 to 50 μm in particle size. Preferably, it exists in the range of about 1-40 micrometers. Moreover, about the average particle diameter of a secondary aggregate, it is preferable that it is about 2-20 micrometers, and it is more preferable that it is about 2-10 micrometers. The particle size of the secondary aggregate is determined by laser diffraction method or scanning electron microscope observation.

  The metal component A and the metal component B are less reactive with Li than Si and Sn. That is, it is difficult to form a compound or the like with Li. The degree of reactivity decreases in the order of Si or Sn> metal component A> metal component B. The metal component A is referred to as “a metal having low reactivity with Li”, and the metal component B is also referred to as “a metal that does not easily react with Li”. In the present invention, by using the metal component A and / or the metal component B in addition to Si and Sn as the metal component, volume change when lithium is occluded in the metal component (during charging) can be reduced. In other words, in the case of a graphite negative electrode or the like, lithium ions enter the interlayer during charging, whereas in the case of a metal negative electrode using Si or Sn, lithium does not enter the interlayer during charging. Will increase, and in extreme cases it will increase 3-4 times. When lithium is released by discharge, it shrinks. In order to repeat such high-magnification expansion and contraction, in the case of a metal negative electrode, the active material is easily pulverized by repetition of expansion and contraction, and the discharge capacity is thereby easily decreased. In the present invention, in addition to Si and Sn, the volume change can be alleviated by using the metal component A and / or the metal component B (preferably the metal component A) in combination. This also contributes to an improvement in cycle life.

  When the metal components A and / or B are used in combination, the content of Si and Sn in all metal components is preferably 40 to 95 atomic%, and more preferably 45 to 90 atomic%. When the metal component A and the metal component B are used in combination (that is, when three components are used in combination), the metal component A and the metal component B are used in a ratio of 5 to 35 atomic%: 5 to 20 atomic%. Is preferred.

  By setting it as said component range, when using as a lithium secondary battery negative electrode material, at the time of lithium occlusion by charge, a lithium type compound LixASi phase, a LixASn phase, etc. can be mainly mainly produced | generated. In LixASi and LixASn, these phases have no alloy phase separation in the range of x = 1.5 to 5, and can maintain an amorphous structure and occlude lithium. As a result, it is effective in suppressing volume change during lithium occlusion, and good charge / discharge cycle characteristics can be achieved while maintaining a high capacity. In the composite phase, a compound containing lithium is formed at the first charge (lithium occlusion) and returns to the original composite phase at the next discharge (lithium release), but a part remains as an irreversible compound containing lithium. As a result, insertion / extraction of lithium is performed via the lithium-containing compound, and an irreversible lithium compound formed in the first charge (first time) is present as a skeleton. By inserting and extracting lithium therein, volume change due to charge / discharge can be reduced, pulverization can be suppressed, electrode deterioration can be prevented, and cycle life can be improved. As described above, in the negative electrode for a lithium secondary battery of the present invention, when the lithium occlusion / release process is performed through the lithium compound LixASi phase, LixASn phase, etc., the volume increase is extremely small, and the swelling and fineness of the electrode This is advantageous in that the cycle life is improved by suppressing a decrease in capacity due to crystallization.

Water-soluble thickener The water-soluble thickener is contained in an amount of 0.1 to 1% by weight (preferably 0.5 to 1% by weight).

  As the water-soluble thickener, for example, any of carboxymethyl cellulose (CMC) and xanthan gum or a combination thereof can be used. By containing such a water-soluble thickener, it becomes easy to knead and disperse the metal component, resin component and carbon component in the preparation of the active material layer due to the thickening effect, the viscosity of this aqueous paste is stabilized, and the coating is performed. The film thickness can be made uniform, and after drying, the metal component and the carbon component described later can be prevented from falling off from the foil-like current collector, and a good active material layer can be formed.

Resin component As the resin component, at least one selected from the group consisting of a tetrafluoroethylene-hexafluoropropylene copolymer, a polystyrene-maleic acid copolymer and polytetrafluoroethylene is 1 to 10% by weight ( Preferably 2 to 10% by weight).

  The resin component is resistant to electrolyte and has almost no binding property. By using such a resin component, extreme expansion of the negative electrode can be suppressed during lithium occlusion (during charging), which contributes to an improvement in cycle life. In this invention, when this resin component acts as a binder, the other component is entangled and the active material layer is formed.

  The content of the resin component is as described above. If it is less than 1% by weight, it is difficult to suppress the expansion of the metal component during charging, and the cycle life may be reduced. If it exceeds 10% by weight, the connectivity between the metal components, between the carbon components, or between these and the current collector may be lowered, and the function of the negative electrode may be lowered.

Carbon component The carbon component is contained in an amount of 1 to 3% by weight.

The carbon component is not limited, but carbon black (particularly ketjen black, which is conductive carbon black) is preferred. In particular, those having a primary particle diameter of about 10 to 100 nm and a BET specific surface area of 10 to 50 m ² / g are preferable because good electrical conductivity can be obtained by mixing a small amount.

  When the content of the carbon component is less than 1% by weight, the conductivity between the metal component or these and the current collector may be insufficient, and the negative electrode function may not be sufficiently achieved. When it exceeds 3% by weight, the utilization factor does not improve, but rather the discharge capacity tends to decrease.

2. The negative electrode of lithium secondary battery The present invention may be a positive electrode, a separator, and a lithium secondary battery in combination with the electrolyte solution or the like. Such positive electrodes, separators, electrolytic solutions and the like are not limited, and the same materials as those used for known lithium secondary batteries can be used. Further, the shape of the lithium secondary battery is not limited, and a rectangular shape, a cylindrical shape, a coin shape, or the like can be appropriately selected depending on how to assemble. What is necessary is just to follow a conventional method about the assembly method of a lithium secondary battery.

  The lithium secondary battery of the present invention is excellent in cycle characteristics while maintaining a high discharge capacity because it is about 1.2 to 2.5 times when increasing the thickness of the negative electrode by repeated charge and discharge is preferable. To do.

3. Method for Producing Lithium Secondary Battery Negative Electrode The method for producing the lithium secondary battery negative electrode is not limited, but in the present invention, a method of drying a film made of an aqueous paste containing the constituent material of the active material layer is preferable. More specifically,
A method for producing a negative electrode for a lithium secondary battery, wherein a film made of an aqueous paste containing a raw material mixture and water is formed on a part or all of one side or both sides of a current collector, and then the film is dried. A production method in which the mixture has the following composition is preferred.
(1) Metal component containing at least one of Si and Sn: 86-97.9 wt%,
(2) Water-soluble thickener: 0.1 to 1% by weight,
(3) At least one resin component selected from the group consisting of tetrafluoroethylene-hexafluoropropylene copolymer, polystyrene-maleic acid copolymer and polytetrafluoroethylene: 1 to 10% by weight,
(4) Carbon component: 1 to 3% by weight.

  Here, the description of the metal component, the water-soluble thickener, the resin component, and the carbon component contained in the raw material mixture is the same as described above. In particular, from the viewpoint of inclusion in the raw material mixture, the metal component and the carbon component are preferably in a powder form. The description of the current collector is also the same as described above.

  As for the water-soluble thickener, if the content is less than 0.1% by weight, the paste viscosity becomes low, and it becomes difficult to form a uniform film on the current collector. On the other hand, when the content exceeds 1% by weight, the paste viscosity increases and the composition of the active material layer tends to be non-uniform. Therefore, in the present invention, a content of 0.1 to 1% by weight is adopted. By setting it within this range, the paste viscosity is optimized and a uniform film containing a metal component and a carbon component can be easily formed.

  When using a tetrafluoroethylene-hexafluoropropylene copolymer or polytetrafluoroethylene as the resin component, it is preferable to include it in the paste in the form of an emulsion (dispersion). Moreover, when using a polystyrene-maleic acid copolymer, it is preferable to include in a paste in the state of aqueous solution. It should be noted that polytetrafluoroethylene may be fiberized during paste preparation. The resin component can form a uniform film by setting the content (solid content) within the range of 1 to 10% by weight. In addition, it is easy to prevent the resulting dried film from falling off the current collector.

  In the production method of the present invention, an aqueous paste containing a raw material mixture having the above composition and water (or an aqueous solvent or dispersion medium) is prepared. By using an aqueous paste, there is no environmental load due to volatilization of organic solvents. In addition, the case where a small amount of organic solvent is contained and used as an aqueous solvent or a dispersion medium is also included in this as needed.

  The water content in the aqueous paste is not limited, and can be appropriately set according to the film forming performance of the aqueous paste. Usually, it can select widely within the range of 10-50 weight part with respect to 100 weight part of raw material mixtures.

  When preparing an aqueous paste, it can prepare by mixing the said raw material mixture and water with well-known kneading machines, such as a mixer.

  In order to form a film made of an aqueous paste on a part or all of one side or both sides of the current collector, for example, an aqueous paste is applied or sprayed. The thickness of the coating (wet) is adjusted so that the thickness of the dry coating is about 5 to 100 μm. In addition, after application | coating, film thickness can be equalize | homogenized with a doctor blade etc., or it can pressurize with a roller press etc.

  The drying conditions of the coating (wet) are not limited. The drying temperature is preferably room temperature (20 ° C.) to about 130 ° C. The drying time can be appropriately set according to the drying temperature, but is usually about 0.15 to 0.3 hours. The dry atmosphere may be not only an inert gas atmosphere but also normal air.

  A lithium secondary battery incorporating the negative electrode for a lithium secondary battery of the present invention has a large initial charge / discharge capacity and good cycle characteristics. In particular, since a resin component having almost no binding property is used as the resin component, it is considered that the factor is that extreme expansion of the negative electrode can be suppressed during lithium insertion / extraction during charge / discharge. Examples of the lithium secondary battery include a lithium ion battery and a lithium polymer battery.

  The negative electrode can also form an active material layer from an aqueous paste, and in that case, there is no environmental load due to volatilization of an organic solvent or the like.

It is sectional drawing of a coin type model battery (cell for evaluation).

  Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples. However, the present invention is not limited to the examples.

Examples 1-5
(Preparation of aqueous paste)
Each powder of Fe, Ag, and Sn was sized to an average particle size of 20 μm or less. After mixing each powder so that it might become the ratio shown below, the composite powder (alloy powder) was obtained by performing a mechanical alloying (MA) process in a ball mill.

The composite powder had an average primary particle size of 8.3 μm, and the ratio of each element was Fe / Ag / Sn = 26/22/52 (atomic%). More specifically, the composite powder was composed of Ag ₃ Sn, Ag, FeSn, Sn, and Fe microcrystals, and the size of each microcrystal was 10 to 30 nm.

  An aqueous paste was prepared by adding ketjen black (KB), a carboxymethyl cellulose (CMC) aqueous solution, and a tetrafluoroethylene-hexafluoropropylene copolymer dispersion to the composite powder. The composite powder is a metal component, KB is a carbon component, CMC is a water-soluble thickener, and the copolymer is a resin component (binder).

  The ratio of each component of the aqueous paste (excluding solvent and dispersion medium) is shown in Table 1 below.

[Unit:% by weight]
(Preparation of negative electrode)
A copper foil having a thickness of 18 μm prepared by an electrolytic method was prepared. Each paste was applied to the entire surface of one side, and the coating film (about 4 to 5 mg / cm ² ) was almost uniformized with a doctor blade.

  The coating was then dried at 120 ° C. for 20 minutes to remove water. Finally, the coating film (active material layer) and the copper foil were brought into close contact with each other by passing the coating film through a roll press machine, and a sheet-like negative electrode with an average thickness of the active material layer of 20 μm was produced.

Comparative Examples 1-2
Instead of the tetrafluoroethylene-hexafluoropropylene copolymer dispersion, prepare an N-methylpyrrolidone (NMP) solution of polyvinylidene fluoride (PVdF). Except for the above, the paste was prepared in the same manner as in Example 1 to prepare a sheet-like negative electrode. In addition, the paste prepared in Comparative Examples 1 and 2 is a solvent-based paste.

[Unit:% by weight]
Comparative Examples 3-7
Example except that a styrene-butadiene rubber (SBR) emulsion was prepared instead of the tetrafluoroethylene-hexafluoropropylene copolymer dispersion, and the ratio of each component of the paste was as shown in Table 2 above. A paste was prepared in the same manner as in Example 1 to prepare a sheet-like negative electrode. In addition, the paste prepared in Comparative Examples 3 to 7 is an aqueous paste.

Test example 1
The sheet-like negative electrodes prepared in Examples 1 to 5 and Comparative Examples 1 to 7 were extracted with a circular punch having a diameter of 1 cm ² and dried under reduced pressure at 120 ° C. for 3 hours to obtain test electrodes (negative electrodes). In order to identify each test electrode, the electrode names are A to E and a to g, and the corresponding relationships are also shown in Tables 1 and 2.

Using a test electrode as a negative electrode, metallic lithium as a counter electrode, a 1 mol lithium PF ₆ / ethylene carbonate (EC) + diethyl carbonate (DEC) (EC: DEC = 1: 1 (volume ratio)) solution as an electrolyte, and dry A coin-type model battery (CR2032 type) shown in FIG. 1 was produced in the box. The counter metal lithium has a capacity about 20 times the calculated capacity of the test electrode (capacity that can sufficiently supply Li to the test electrode). The separator is a microporous thin film made of polyethylene, and the glass filter is a glass mat having a thickness of about 200 μm used for holding an electrolyte and preventing a short circuit.

  The negative electrode in each model battery (evaluation cell) was evaluated by the following method.

First, the model battery was discharged (Li absorbing) to reach 0V at a constant current density of 0.2 mA / cm ^2, after 10 minutes of rest, reaches 1.0V at a constant current density of 0.2 mA / cm ² Until it was charged (Li released). This was made into 1 cycle, charging / discharging was performed repeatedly and the discharge capacity was investigated.

  For each model battery, the relationship between the number of charge / discharge cycles and the discharge capacity density is shown in Table 3, and the model batteries corresponding to Comparative Examples 5 to 7 (negative electrode: e, f, g) all have 25 cycles. The results are not listed in Table 3 because the discharge is no longer possible.

  Table 3 shows the following. That is, the SBR emulsion useful as a binder for the carbon material negative electrode (c, d) is not suitable as a binder for the metal material negative electrode of the present invention and cannot exhibit excellent cycle characteristics. The electrodes (A to E) of the present invention exhibit the same or better cycle characteristics as compared with the electrodes (a, b) using an NMP solution of PVdF, which is a general-purpose binder. The electrode of the present invention can be formed from an aqueous paste, thereby reducing the environmental burden and simplifying the electrode manufacturing process as compared with the case of using a solvent-based paste.

Examples 6-9
(Preparation of aqueous paste)
Each powder of Sb, Sn, and Si was sized to an average particle size of 20 μm or less. After mixing each powder so that it might become the ratio shown below, MA treatment was performed in the ball mill, and composite powder (alloy powder) was obtained.

  The composite powder had an average primary particle size of 8.3 μm, and the ratio of each element was Sb / Sn / Si = 45/5/50 (atomic%). More specifically, the composite powder was composed of SnSb, Sn, and Si microcrystals, and the size of each microcrystal was 10 to 100 nm.

  An aqueous paste was prepared by adding KB, CMC aqueous solution and polystyrene-maleic acid copolymer aqueous solution to the composite powder. The composite powder is a metal component, KB is a carbon component, CMC is a water-soluble thickener, and the copolymer is a resin component (binder).

  The ratio of each component of the aqueous paste (excluding solvent and dispersion medium) is shown in Table 4 below.

[Unit:% by weight]
(Preparation of negative electrode)
A copper foil having a thickness of 18 μm prepared by an electrolytic method was prepared. Each paste was applied to the entire surface of one side, and the coating film (about 4 to 5 mg / cm ² ) was almost uniformized with a doctor blade.

  The coating was then dried at 120 ° C. for 20 minutes to remove water. Finally, the coating film (active material layer) and the copper foil were brought into close contact with each other by passing the coating film through a roll press machine, and a sheet-like negative electrode with an average thickness of the active material layer of 20 μm was produced.

Comparative Example 8
Example 6 except that an NMP solution of PVdF was prepared instead of the polystyrene-maleic acid copolymer aqueous solution, and the ratio of each component of the paste (excluding the solvent and dispersion medium) was as shown in Table 5 below. A paste was prepared in the same manner to produce a sheet-like negative electrode. Note that the paste prepared in Comparative Example 8 is a solvent-based paste.

[Unit:% by weight]
Test example 2
The sheet-like negative electrodes prepared in Examples 6 to 9 and Comparative Example 8 were extracted with a circular punch having a diameter of 1 cm ² and dried under reduced pressure at 120 ° C. for 3 hours to obtain each test electrode (negative electrode). In order to identify each test electrode, the electrode names are F to I and h, and the corresponding relationships are shown in Tables 4 and 5.

  A model battery was prepared in the same manner as in Test Example 1 using the test electrode as the negative electrode, and the negative electrode in each model battery (evaluation cell) was evaluated in the same manner as in Test Example 1.

  Table 6 shows the relationship between the number of charge / discharge cycles and the discharge capacity density for each model battery.

Table 6 shows the following. That is, the electrodes (F to I) of the present invention exhibit the same or better cycle characteristics than the electrode (h) using the NMP solution of PVdF, which is a general-purpose binder. Since the electrode of the present invention can be formed from an aqueous paste, the environmental load can be reduced and the electrode manufacturing process can be simplified as compared with the case of using a solvent-based paste. As a further comparative example, an electrode was produced by changing the amount of addition using a binder containing ethylene-vinyl alcohol having a very good binding property, but discharge was impossible in 50 cycles.

Claims (8)

A negative electrode for a lithium secondary battery having a foil-like current collector containing at least one selected from the group consisting of Fe, Ni and Cu and an active material layer formed on a part or all of one side or both sides thereof A negative electrode for a lithium secondary battery, wherein the active material layer has the following composition:
(1) Metal component containing at least one of Si and Sn: 86-97.9 wt%,
(2) Water-soluble thickener: 0.1 to 1% by weight,
(3) At least one resin component selected from the group consisting of tetrafluoroethylene-hexafluoropropylene copolymer, polystyrene-maleic acid copolymer and polytetrafluoroethylene: 1 to 10% by weight,
(4) Carbon component: 1 to 3% by weight.   2. The negative electrode for a lithium secondary battery according to claim 1, wherein the metal component further contains at least one selected from the group consisting of Al, Cu, Ag, Sb, and Bi.   The metal component further contains at least one selected from the group consisting of B, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Y, Zr, Nb, Mo, and W. Or the negative electrode for lithium secondary batteries of 2.   The negative electrode for a lithium secondary battery according to claim 1, wherein a part or all of the surface of the foil-shaped current collector is subjected to an antioxidant treatment.   The negative electrode for a lithium secondary battery according to any one of claims 1 to 4, wherein the water-soluble thickener is at least one of carboxymethyl cellulose and xanthan gum.   The negative electrode for a lithium secondary battery according to any one of claims 1 to 5, wherein the carbon component is carbon black.   The lithium secondary battery according to claim 1, wherein the active material layer is a dry film formed from an aqueous paste containing a metal component, a water-soluble thickener, a resin component, and a carbon component. Negative electrode. A method for producing a negative electrode for a lithium secondary battery, wherein a film made of an aqueous paste containing a raw material mixture and water is formed on a part or all of one side or both sides of a current collector, and then the film is dried. Manufacturing method wherein the mixture has the following composition:
(1) Metal component containing at least one of Si and Sn: 86-97.9 wt%,
(2) Water-soluble thickener: 0.1 to 1% by weight,
(3) At least one resin component selected from the group consisting of tetrafluoroethylene-hexafluoropropylene copolymer, polystyrene-maleic acid copolymer and polytetrafluoroethylene: 1 to 10% by weight,
(4) Carbon component: 1 to 3% by weight.

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