JP2010232077A - Electrode for nonaqueous secondary battery and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode for a nonaqueous secondary battery, capable of suppressing deterioration of battery performance accompanied with pulverization of active material caused by repetition of charge and discharge, and to provide a method for manufacturing the same. <P>SOLUTION: The electrode is obtained by a preparing process of an electrode mixture for obtaining an electrode mixture by mixing an active material containing Si contained powder, composed of silicon and/or silicon compound, an electric conduction assistant composed of carbon powder containing aggregated particles with a knitting ball shape formed, by entangling carbon fibers which have an average fiber diameter of 5 nm or more to 100 nm or less and an average fiber length of 1 μm or more to 6 μm or less, and a binder, and by a coating process for coating a collector with the electrode mixture. In the electrode, there exist the active material, the electrically conducting assistants, and the binder binding the active material and the electrically conducting assistants, and the carbon fibers partially exist in an aggregated state. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a non-aqueous secondary battery, and particularly to an electrode used for a non-aqueous secondary battery.

  Secondary batteries such as lithium ion secondary batteries are small and have a large capacity, and are therefore used in a wide range of fields such as mobile phones and notebook computers. A lithium ion secondary battery has an active material capable of inserting and removing lithium (Li) in each of a positive electrode and a negative electrode. And it operate | moves because Li ion moves in the electrolyte solution provided between both electrodes.

The performance of the secondary battery depends on the materials of the positive electrode, the negative electrode, and the electrolyte constituting the secondary battery. In particular, research and development of electrode materials constituting electrodes are actively conducted. For example, lithium metal or a lithium alloy is often used as an active material for an electrode material of a lithium secondary battery because a battery having a high energy density is obtained. Recently, an active material made of silicon (Si), which is an element capable of forming an alloy with lithium, has attracted attention. For example, Patent Document 1 describes a non-aqueous electrolyte secondary battery using Li _x Si (0 ≦ x ≦ 5) as a negative electrode active material.

However, an active material containing silicon such as Li _x Si (silicon-based active material) expands and contracts due to a charge / discharge cycle. If the expansion / contraction is large, the electrode expands greatly in the thickness direction, thereby causing problems such as a decrease in current collecting performance of the current collector, bending of the electrode, and further expansion of the battery itself. In addition, due to expansion / contraction, there is a problem that the conductive path in the electrode is broken and the capacity is remarkably lowered or the internal resistance is increased.

  Therefore, in Patent Document 2, a current collector made of a metal foam or a fibrous metal sintered body is used so that the expansion / contraction of the electrode does not increase even when charging and discharging are repeated. Further, Patent Document 3 includes an electrode active material, carbon fiber as a conductive additive, and a binder in order to obtain a good conductive path, and a mixture of lithium two-piece formed so as not to aggregate the carbon fiber. A secondary battery electrode is disclosed. That is, in the electrode described in Patent Document 3, since the electrode active material and the carbon fiber are uniformly dispersed in the electrode, many electrode active materials form a conductive path through one carbon fiber. . However, no specific example using a silicon-based active material as an electrode active material is described.

Japanese Patent Laid-Open No. 7-29602 JP 2003-308831 A JP 2009-16265 A

  By the way, it is known that the volume of silicon becomes about four times at maximum by forming a compound with lithium. That is, the volume difference between charging and discharging is extremely large. Therefore, when a silicon-based active material containing silicon is used, large distortion occurs in the active material due to repeated charging and discharging, and the active material is pulverized. This can be seen by observing the surface of the Si coating before and after the charge / discharge test. For example, with respect to a crystalline Si thin film (film thickness: 5 μm) formed on the surface of a copper foil by a general chemical vapor deposition method, a condition of a charge end voltage of 2 V and a discharge end voltage of 0 V under a constant current of 0.2 mA FIG. 10 shows the result of the charge / discharge test conducted and the surface of the Si thin film before and after the test observed with a scanning electron microscope (SEM). (A) is the surface of the Si thin film before charging and discharging, and (b) is the surface of the Si thin film after charging and discharging a plurality of times. From FIG. 10B, it can be seen that the surface of the Si thin film was split into the same size of about 5 to 10 μm by multiple times of charge and discharge. If you look at each of the splits further, it is splitting smaller. By repeating charging and discharging in this way, the silicon-based active material particles are also finely divided and pulverized. Since the silicon-based active material has low conductivity, when the active material is pulverized, the conductive path is broken, and the portion that cannot participate in the electrochemical reaction may increase, and the charge / discharge capacity may decrease.

  In view of the above problems, an object of the present invention is to provide an electrode for a non-aqueous secondary battery and a method for producing the same that can suppress a decrease in battery performance due to pulverization of an active material due to repeated charge and discharge. To do.

  In general, in an electrode composed of an electrode active material, a conductive additive and a binder, a state where the active material and the conductive additive are uniformly dispersed has been considered preferable from the viewpoint of conductivity. That is, even if the active material is a silicon-based active material and the conductive additive is carbon fiber, it is a conventional desirable state that both are uniformly dispersed. However, the present inventors have found that such a uniform dispersion state is not suitable for a silicon-based active material. Since the silicon-based active material has low conductivity, even if most of the active material is in contact with the carbon fiber in the initial state, the proportion of the active material that does not come into contact with the powder is increased by charging and discharging, and the conductivity is increased. This is because of a decrease. The inventor has developed this idea and completed the invention described below.

An electrode for a non-aqueous secondary battery according to the present invention includes an active material containing Si-containing powder made of silicon and / or a silicon compound, and a carbon fiber having an average fiber diameter of 5 nm to 100 nm and an average fiber length of 1 μm to 6 μm An electrode mixture preparation step of obtaining an electrode mixture by mixing a conductive additive made of carbon powder containing fluff-shaped aggregated particles intertwined with each other, and a binder,
An application step of applying the electrode mixture to a current collector;
The carbon fiber is partially agglomerated and comprises the active material, the conductive additive, the active material, and the binder that binds the conductive additive. And

  The electrode for a non-aqueous secondary battery of the present invention is produced from an electrode mixture obtained by using, as a conductive additive, carbon powder made of “flooded” aggregated particles in which carbon fibers are intertwined. In the electrode mixture preparation step, carbon fibers adhere to the Si-containing powder by mixing the carbon powder and the Si-containing powder. The flocculated aggregated particles are formed by entanglement of ultrafine carbon fibers having an average fiber diameter of 5 nm to 100 nm and relatively short carbon fibers having an average fiber length of 1 μm to 6 μm. Such ultrafine and relatively short carbon fibers are very likely to agglomerate. Therefore, even if carbon powder is mixed with other components in the electrode mixture preparation step, the carbon fibers are difficult to uniformly disperse. Therefore, the carbon fibers are partially aggregated and present in the electrode mixture and further in the active material layer of the electrode obtained after the coating step. As a result, the Si-containing powder having a relatively large particle size is entangled with the carbon fibers loosened from the aggregated particles, but the Si-containing powder having a small particle size or finely pulverized by charging / discharging is a carbon fiber. Are captured in the part where they coagulate. In particular, carbon fibers having an average fiber diameter and an average fiber length in the above-mentioned range are aggregated with a good mesh size to hold the finely divided Si-containing powder. Therefore, the Si-containing powder is retained by the carbon fiber regardless of its particle size, and forms a good conductive path. And the fall of the battery performance accompanying the pulverization of the active material by repeated charging / discharging is suppressed.

Moreover, this invention can also be grasped | ascertained as a manufacturing method of the electrode for nonaqueous secondary batteries. That is, the method for producing an electrode for a non-aqueous secondary battery of the present invention includes an active material containing Si and / or a silicon-containing powder composed of a silicon compound, an average fiber diameter of 5 nm to 100 nm, and an average fiber length of 1 μm to 6 μm. An electrode mixture preparation step of obtaining an electrode mixture by mixing a conductive additive made of carbon powder containing fluffy aggregated particles formed by intertwining carbon fibers, and a binder,
An application step of applying the electrode mixture to a current collector;
A non-aqueous secondary battery comprising the active material, the conductive additive, the active material, and the binder that binds the conductive additive, wherein the carbon fibers are partially aggregated An electrode for use is obtained.

It is a drawing substitute photograph which shows the carbon powder suitable for the electrode for non-aqueous secondary batteries of this invention, Comprising: It is the low magnification microscope picture which observed the carbon powder which consists of aggregated particles. It is a drawing substitute photograph which shows the carbon powder suitable for the electrode for non-aqueous secondary batteries of this invention, Comprising: It is the microscope picture of the high magnification which observed the aggregated particle in detail. It is explanatory drawing which shows the structure of the electrode group of a laminate cell. It is a scanning electron microscope (SEM) image of the surface of the active material layer of electrode # C1 (comparative example). It is a SEM image of the surface of the active material layer of electrode # C2 (comparative example). It is a SEM image of the surface of the active material layer of electrode # C3 (comparative example). It is a SEM image of the surface of the active material layer of electrode # 01 (Example). It is the SEM image which observed the surface of the active material layer of electrode # 01 (Example) at high magnification. It is a graph which shows the cycle characteristic of the laminate cell (Example) which used the electrode # 01 as the positive electrode, and the laminate cell (Comparative example) which used the electrode # C0 as the positive electrode, Comprising: The current capacity at the time of charge with respect to the cycle number is shown. It is the SEM image which observed the surface of the Si thin film before and after a charging / discharging test, (a) is before charging / discharging, (b) shows the surface of the Si thin film after charging / discharging.

  The best mode for carrying out the electrode for a non-aqueous secondary battery and the method for producing the same according to the present invention will be described below with reference to the drawings. Unless otherwise specified, the numerical range “x to y” described in this specification includes the lower limit x and the upper limit y. And a numerical range can be comprised by combining arbitrarily the upper limit value and lower limit value as described in this specification.

  The electrode for a non-aqueous secondary battery of the present invention comprises an active material, a conductive additive and a binder that binds both, and is obtained mainly through an electrode mixture preparation step and a coating step. Below, each process is demonstrated.

<1. Electrode compound preparation process>
The electrode mixture preparation step is a step of obtaining an electrode mixture by mixing an active material, a conductive additive and a binder. Below, the active material, the conductive support material, and the binder as raw materials to be mixed in the electrode mixture preparation step will be described.

(Active material)
The active material includes Si-containing powder made of silicon and / or a silicon compound. Examples of the Si-containing powder include powders such as a simple substance of Si, an oxide containing Si, a nitride containing Si, and an alloy containing Si. If it is an alloy, it is good to contain at least 1 sort (s) selected from the group which consists of Ti, Fe, Ni, Mo, Mn, Cu, and Al as an alloy element. Si may be crystalline or amorphous. These Si-containing powders can be produced using methods known in the art.

  The average particle size of the Si-containing powder is preferably 0.01 to 10 μm, more preferably 0.01 to 5 μm. Therefore, prior to the electrode mixture preparation step, the Si-containing powder may be classified (screened) to 10 μm or less, further 5 μm or less.

  Further, the active material may contain Si-containing powder as a main component, and other known active materials may be added and used. Specifically, graphite, Sn, Al, Ag or the like. Among these, one kind or a mixture of two or more kinds can be used.

(Conductive aid)
The conductive aid is made of carbon powder containing fluffy aggregated particles in which carbon fibers are intertwined. The carbon powder is mainly composed of aggregated particles, and carbon powder such as carbon black, acetylene black, ketjen black, and graphite may be added and used. Moreover, you may add powder other than carbon, such as a metal generally used with the electrode for non-aqueous secondary batteries as a conductive support material, and a conductive polymer to a conductive support material. These powders may be used alone or in combination of two or more.

  The carbon fiber preferably has an average fiber diameter of 5 to 100 nm and an average fiber length of 1 to 6 μm. Such ultra-fine and relatively short carbon fibers tend to aggregate. Therefore, even if they are mixed, the flocculated aggregated particles are not easily loosened, and the aggregated carbon fibers remain after mixing. If carbon fibers with a small fiber diameter and a short fiber length are aggregated and entangled, even if the Si-containing powder is pulverized, the fine powder is caught in a fine mesh formed by the entanglement of carbon fibers and the conductivity is maintained. In addition, the fine powder is prevented from falling off because it enters the agglomerated carbon fiber. The average fiber diameter of the carbon fibers forming such agglomeration is preferably 50 nm or less, more preferably 20 nm or less. The lower limit of the average fiber diameter is not particularly limited, but is 5 nm or more, further 10 nm or more. A particularly preferable average fiber diameter of the carbon fibers is 13 to 18 nm. The average fiber length of the carbon fibers is preferably 5.5 μm or less, more preferably 5 μm or less. Moreover, although there is no limitation in particular in the minimum of average fiber length, it is 1.5 micrometers or more, Furthermore, it is 2 micrometers or more. The average fiber length of particularly preferable carbon fibers is 3 to 5 μm. In the present specification, “average fiber diameter” and “average fiber length” of a carbon fiber are a number average fiber diameter and a number average fiber length calculated by measuring a plurality of locations in a micrograph.

  Further, the carbon fibers that can be aggregated to such an extent that the finely divided Si-containing powder can be satisfactorily retained can also be defined by the aspect ratio (average fiber length / average fiber diameter). The aspect ratio of the carbon fiber is preferably 10 or more, 100 or more, and more preferably 200 or more. The upper limit of the aspect ratio is not particularly limited, but is preferably 1200 or less, more preferably 500 or less. A particularly preferred aspect ratio is 250 to 300.

  The average particle diameter of the aggregated particles is preferably 1 μm or more and 1000 μm or less, and more preferably 300 μm or more and 800 μm or less. When the average particle diameter of the aggregated particles exceeds 1000 μm, it is not preferable because unevenness is likely to occur on the electrode surface in the coating process described later, and it is difficult to achieve a uniform coating thickness. In the present specification, the “average particle diameter of the aggregated particles” means the interval between the parallel lines when a plurality of aggregated particles are sandwiched between two parallel lines in a micrograph obtained by observing the carbon powder under a microscope. The number average value calculated by measuring the maximum value is used.

  One aggregated particle is preferably composed of 1000 or more and 2 trillion or less, more preferably 40 billion or more and 1 trillion or less carbon fibers. The number of carbon fibers constituting one aggregated particle is described in detail in the column of [Example], but can be calculated from a micrograph of the aggregated particle. When the number of carbon fibers is less than 1000, the carbon fibers of the aggregated particles are easily loosened, and the carbon fibers are less likely to aggregate in the electrode mixture and further in the electrode. Further, if the number of carbon fibers exceeds 2 trillion, it is not preferable because the particle size of the aggregated particles becomes too large.

The carbon powder composed of the agglomerated particles described above can be produced using a method known in the art. Specifically, it is obtained by granulating a vapor-grown carbon fiber produced by a vapor phase method and then compression-molding it as necessary. Commercially available carbon fiber granules can also be used. For example, carbon fiber VGCF-X (fiber diameter: 15 nm, fiber length: 4 μm, bulk density: 0.1 g / cm ³ , pill-shaped) manufactured by Showa Denko KK Is preferred.

(Binder)
The binder is not particularly limited, and for example, a known material such as a fluorine-containing resin such as polytetrafluoroethylene or polyvinylidene fluoride, or a thermoplastic resin such as polypropylene or polyethylene may be used. Also suitable are silica hybrid resins such as polyimide-silica, polyamide-imide-silica, epoxy-silica, acrylic-silica, phenol-silica, polyurethane-silica. These binders can be used as they are in a liquid state or mixed with an active material and a conductive aid in a state dissolved or dispersed in water or an organic solvent such as N-methyl-2-pyrrolidone. Good.

(Other)
As described above, the active material, the conductive additive, and the binder have been described. However, there are no particular limitations on the mixing ratio of each, and it is determined according to the type of active material and the type and amount of additive powder added to the conductive additive. It is good. For example, when the total electrode mixture is 100% by mass, it contains 66 to 98% by mass, further 85 to 89% by mass of active material, and 0.1 to 20% by mass and further 1 to 5% by mass of conductive additive. The remainder is preferably mainly composed of a binder. If necessary, an appropriate amount of solvent may be added to the electrode mixture to adjust the viscosity. The amount of the solvent added is a viscosity suitable for the electrode mixture obtained to be applied to the current collector, specifically, a value by a rotary (B-type) viscometer at room temperature (25 ° C.), 3000 to It is desirable to select so as to be 5000 mPa · s. Specific examples of the solvent include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, methylformamide, dimethylamine, acetone, and cyclohexanone, and may be appropriately selected depending on the type of the binder. .

  Moreover, what is necessary is just to use general mixing apparatuses, such as a planetary mixer, a defoaming kneader, a ball mill, a paint shaker, a vibration mill, a reiki machine, an agitator mill, for mixing of electrode mixture. There are no particular limitations on the mixing conditions, but carbon fibers are appropriately aggregated and present once mixing is performed at 1000 to 20000 rpm, further 2000 to 10000 rpm for 0.1 to 20 minutes, and further 0.5 to 9 minutes. The electrode mixture to be obtained is easily obtained.

<2. Application process>
In the electrode for a non-aqueous secondary battery of the present invention, an active material layer obtained by binding the above active material and conductive additive with a binder is generally attached to a current collector. Therefore, in the application step, the electrode mixture is applied to the current collector. A metal mesh or metal foil can be used for the current collector. If the present invention is used for the positive electrode, a current collector such as aluminum, nickel, and stainless steel; if it is the negative electrode, porous or non-porous made of a metal material such as stainless steel, titanium, nickel, aluminum, or copper, or a conductive resin These conductive substrates can be mentioned. Examples of the porous conductive substrate include a mesh body, a net body, a punching sheet, a lath body, a porous body, a foamed body, a fiber group molded body such as a nonwoven fabric, and the like. Examples of the non-porous conductive substrate include a foil, a sheet, and a film. As a coating method, a conventionally known method such as a doctor blade or a bar coater may be used.

<3. Additional steps>
Depending on the type of binder used, the current collector coated with the electrode mixture is dried by a known method and then heated as necessary to cure the binder. That is, you may provide the heating process of heating the electrode compound material after an application | coating process, and hardening a binder. Further, after drying or heating, it may be further molded by a known method such as a roll press or a pressure press so as to have a desired thickness and density. The electrode after the coating step is a sheet-like electrode, and is cut into dimensions according to the specifications of the non-aqueous secondary battery to be produced and used as a battery electrode.

<Configuration of electrode for non-aqueous secondary battery>
The electrode for a non-aqueous secondary battery of the present invention obtained through the above steps comprises an active material, a conductive additive, a binder for binding the active material and the conductive additive, and a carbon fiber. Are partially agglomerated. By using carbon powder consisting of fluffy aggregated particles in which carbon fibers having a fiber diameter and fiber length in the above range are intertwined as a conductive additive and mixing with other components, the carbon fibers are naturally aggregated in the electrode mixture. To do. Even in an electrode produced using such an electrode mixture, aggregated carbon fibers exist.

Although it is difficult to define the state in which the carbon fibers are aggregated by numerical values, the density at which the carbon fibers are aggregated is preferably 100 to 1500 / μm ^3, more preferably 500 to 1000 / μm ³ . The density can be calculated by measuring a plurality of locations in the micrograph, and the measurement method will be described in the [Example] column.

  Immediately after the production of the electrode, the carbon fiber has an aggregated portion and a partially frayed portion. The Si-containing powders are intertwined with the frayed carbon fibers and in contact with each other. When the Si-containing powder is pulverized by charging / discharging, the pulverized particles are trapped in the fine mesh of aggregated carbon fibers or enter the interior of the aggregated carbon fibers. Therefore, the pulverized particles are prevented from falling off, and form a conductive path by contacting the carbon fiber at the aggregation site.

<Battery provided with electrode for non-aqueous secondary battery>
In the non-aqueous secondary battery including the electrode for the non-aqueous secondary battery of the present invention, in addition to the positive electrode and the negative electrode, a separator sandwiched between the positive electrode and the negative electrode, a non-aqueous electrolyte, as in a general secondary battery Is provided. The separator separates the positive electrode and the negative electrode and holds the electrolytic solution, and a thin microporous film such as polyethylene or polypropylene can be used. The non-aqueous electrolyte is obtained by dissolving an alkali metal salt as an electrolyte in an organic solvent. Examples of the organic solvent include aprotic organic solvents such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. Or a mixture of two or more thereof can be used. As the electrolyte to be dissolved, an alkali metal salt soluble in an organic solvent such as LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiI, LiClO ₄ , NaPF ₆ , NaBF ₄ , NaAsF _6, or the like can be used.

  In addition, the electrode for non-aqueous secondary batteries of this invention can be used as a positive electrode or a negative electrode. When used as a negative electrode, for example, an electrode using a lithium-containing transition metal compound or the like as an active material may be used as a positive electrode to constitute a non-aqueous secondary battery. On the other hand, when used as a positive electrode, for example, an electrode using an active material such as an alkali metal such as Li or Na, an alloy of an alkali metal, or the like may be used as a negative electrode to constitute a non-aqueous secondary battery.

  The shape of the non-aqueous secondary battery is not particularly limited, and various shapes such as a cylindrical shape, a stacked shape, and a coin shape can be employed. Regardless of the shape, a separator is sandwiched between the positive electrode and the negative electrode to form an electrode body, and the space between the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal is used for current collection. After connecting using a lead or the like, the electrode body is sealed in a battery case together with a non-aqueous electrolyte to form a battery.

  As mentioned above, although embodiment of the electrode for non-aqueous secondary batteries of this invention and its manufacturing method was described, this invention is not limited to the said embodiment. Without departing from the scope of the present invention, the present invention can be implemented in various forms with modifications and improvements that can be made by those skilled in the art.

  Hereinafter, the present invention will be described in detail by way of examples of the nonaqueous secondary battery electrode and the method for producing the same of the present invention.

<Preparation of electrode for lithium ion secondary battery>
As an active material, a commercially available Si powder (made by Fukuda Metals, particle size of 10 μm or less) having a purity of 99.9% or more was prepared. As conductive aids, five types of carbon powder, that is, ketjen black (KB, average particle size: 30 to 50 nm) and four types of carbon fibers (Showa Denko Co., Ltd. carbon fibers VGCF, VGCF-H, VGCF-S) And VGCF-X: both pills were prepared. Moreover, a polyimide-silica hybrid resin binder was prepared as a binder for binding the active material and the conductive additive.

Table 1 shows the number average fiber diameter (nm), number average fiber length (μm), and bulk density (g / cm ³ ) of the carbon fibers. In addition, although the nominal value is described in Table 1, for example, the average fiber diameter is determined from the SEM image (for example, FIG. 2) obtained by observing the carbon fiber using a scanning electron microscope (SEM). It is possible to measure the fiber diameter and obtain the number average of them. The average fiber length can be obtained by measuring the fiber length by image processing for a plurality of SEM images obtained by dispersing carbon fibers in ethanol and then observing with SEM, and calculating the number average of them.

  Moreover, the result of having observed VGCF-X by SEM is shown in FIG. 1 and FIG. From FIG. 1, the average particle diameter of the aggregated particles of VGCF-X was 600 μm. In addition, the average particle diameter is determined based on the SEM image (for example, FIG. 1) obtained by observing the carbon powder. Value) was measured, and they were obtained by number averaging.

Further, from FIG. 2, the number of carbon fibers contained in one aggregated particle was obtained. In the SEM image of FIG. 2, it was assumed that carbon fibers existing between the surface of the aggregated particles and 75 nm were observed. The reason why the thickness was set to 75 nm is that the distance between adjacent carbon fibers on the outermost surface of the aggregated carbon fibers was measured to be 150 nm at the longest. The carbon fibers observed deeper than the outermost surface were assumed to exist from the outermost surface to 75 nm (that is, half the length of 150 nm). 50 carbon fibers were observed in the SEM image of FIG. 2 surrounded by □ (dotted line), assuming that each was an independent carbon fiber. Since there are 50 carbon fibers in the range of 500 nm × 500 nm × 75 nm, the density of carbon fibers per 1 μm ³ is 50 / (0.5 × 0.5 × 0.075) ≈2700 [lines / μm ³ ]. Since the average particle diameter of the aggregated particles was 600 μm, assuming that the aggregated particles are spherical, the volume is 1.1 × 10 ⁸ μm ³ . Therefore, the number of carbon fibers contained in one aggregated particle is 1.1 × 10 ⁸ × 2700 = 300 billion [pieces].

  An electrode mixture was prepared by mixing the Si powder weighed in a predetermined amount, any one of the five types of carbon powders, and the binder. These blending ratios were set to Si powder: 85% by mass, carbon powder: 5% by mass, and binder: 10% by mass. In addition, “Awatori Nertaro” ARE-250 type manufactured by Shinky Corporation or “TK Fillmix” 30-25 type manufactured by Primix Co., Ltd. was used for mixing. When “Awatori Neritaro” was used, mixing was performed several times at 2000 rpm for 8 minutes. When using “TK Fillmix”, mixing for 30 seconds at 10000 rpm was performed several times.

  Next, this electrode mixture was applied to the surface of a copper foil having a thickness of 18 μm so as to have a thickness of 80 μm, dried and pressed, and then punched into a predetermined shape. Thereafter, the binder was cured by heating at a predetermined temperature and time in a vacuum furnace to obtain five types of electrodes. The structure of the obtained electrode will be described with reference to FIG. FIG. 3 is an explanatory view showing the structure of the electrode plate group of the laminate cell, which will be described in detail later, and the electrode produced by the above procedure corresponds to the electrode 11 of FIG. The electrode 11 includes a sheet-shaped current collector foil 12 made of a copper foil, and an active material layer 13 formed on the surface of the current collector foil 12. The current collector foil 12 includes a rectangular (25 mm × 30 mm) coating portion 12a and a tab weld portion 12b extending from a corner of the coating portion 12a. An active material layer 13 is formed on one surface of the coating part 12a. As described above, the active material layer 13 is made of Si powder, carbon powder, and a binder that binds both. On the other hand, a nickel tab 14 is resistance-welded to the tab weld 12 b of the current collector foil 12. Furthermore, the resin film 15 is adhered to the tab weld portion 12b.

<Evaluation of active material layer>
The conductivity (S / cm (S = Ω ⁻¹ )) was measured by the four-probe method for five types of electrodes # C0 to # C3 and # 01 produced using the different carbon powders according to the above procedure. The results are shown in Table 2. In Table 2, the conductivity of # C3 was even smaller than the conductivity of # C2 and could not be measured. Moreover, the surface of the active material layer of electrodes # C1, # C2, # C3, and # 01 using carbon fiber among the five types of electrodes was observed by SEM. The results are shown in FIGS.

  In the # C0 electrode, since particulate KB was used as a conductive additive, the Si particles of the active material formed a conductive path at one point with KB. On the other hand, in the electrodes of # C1 to # C3, as is apparent from the SEM images of FIGS. 4 to 6, no aggregation of carbon fibers is observed in the active material layer, and the particles are uniformly dispersed together with the Si particles. Many Si particles formed conductive paths through the carbon fibers. However, many Si particles that were not in contact with the carbon fiber were also observed. Therefore, the conductivity is less than 0.1 S / cm, which is not desirable as an electrode. In particular, in the # C3 electrode, since the VGCF-S fibers are thin, the carbon fibers are uniformly dispersed in the active material layer, so that the carbon fibers and the Si particles are not in good contact with each other, and a conductive path is almost formed. There wasn't.

  FIG. 7 is an SEM image of the surface of the active material layer of electrode # 01, and FIG. 8 is an SEM image observed at a higher magnification. From the SEM image of FIG. 7, in the active material layer of electrode # 01, a plurality of portions where a plurality of carbon fibers were aggregated were observed. The state in which the carbon fibers are aggregated is the same as in the case of observing the aggregated particles before mixing (FIG. 2), and a fine mesh is observed. In addition, it was observed that the carbon fibers adhered to the Si particles. This electrode # 01 exhibited a desirable conductivity (conductivity: 0.16 S / cm) as an electrode.

From the SEM image of FIG. 8 in which the portion where the carbon fibers are aggregated was further observed, the density of the aggregated carbon fibers was determined. In the SEM image of FIG. 8, it was assumed that carbon fibers existing between the surface of the aggregated particles and 0.15 μm were observed. The reason why the thickness is 0.15 μm is that the distance between adjacent carbon fibers on the outermost surface of the aggregated carbon fibers is measured to be 300 nm at the longest. It was assumed that the carbon fibers observed deeper than the outermost surface existed up to 150 nm (ie, half the length) from the outermost surface. Within the SEM image of FIG. 8 surrounded by □ (dotted line), 20 carbon fibers were observed on the assumption that each was an independent carbon fiber. Since there are 20 carbon fibers in the range of 0.5 μm × 0.5 μm × 0.15 μm, the density of carbon fibers per 1 μm ³ is 20 / (0.5 × 0.5 × 0.15) ≈ 530 [lines / μm ³ ].

<Production of lithium ion secondary battery>
A laminate cell was prepared using lithium (Li) foil as a counter electrode. The laminate cell will be described with reference to FIG. The laminate cell is composed of an electrode plate group 10 composed of the electrode 11, the counter electrode 16 and the separator 19 of any of # C0 to # C3 and # 01, and a laminate film (not shown) that encloses the electrode plate group 10 and seals it. ) And an electrolyte injected into the laminate film.

  The electrode plate group 10 is configured by laminating one electrode 11 and one counter electrode 16 and interposing a separator 19 therebetween. The configuration of the electrode 11 is as already described. The counter electrode 16 includes a 200 μm thick Li foil that is pressure-bonded to a copper foil. The counter electrode 16 includes a rectangular (26 mm × 31 mm) main body portion 16a to which a Li foil is bonded, and a tab weld portion 16b extending from a corner of the main body portion 16a, both of which are made of copper foil. Nickel tabs 17 are resistance welded to the tab welds 16b. Further, a resin film 18 is attached to the tab weld 16b. The separator 19 is a rectangular sheet (27 mm × 32 mm, thickness 27 μm) made of polypropylene resin. The electrode plate group 10 is laminated in the order of the coating portion 12 a of the electrode 11, the separator 19, and the main body portion 16 a of the counter electrode 16 so that the main body portion 16 a and the active material layer 13 face each other with the separator 19 interposed therebetween. .

The laminate film that encloses and seals the electrode plate group 10 has a bag shape in which four sides are hermetically sealed. From one side of the four sides of the laminate film, a part of the tabs 15 and 17 of both poles extends outward for electrical connection with the outside. Further, in the laminate film, a nonaqueous electrolyte in which LiPF ₆ is dissolved at a concentration of 1 mol in a mixed solvent in which ethylene carbonate and diethyl carbonate are mixed at 1: 1 (volume ratio) is enclosed.

  The laminate cell was obtained by placing the electrode plate group 10 in a bag-like laminate film sealed on three sides, injecting the nonaqueous electrolyte, and then sealing the remaining one side. Regardless of which electrode was used, the electrode 11 became a positive electrode and the counter electrode 16 became a negative electrode by using Li foil as a counter electrode.

<Measurement of resistance value>
One of the electrodes # C0 to # C3 and # 01 was provided as a positive electrode, and the DC resistance values of five types of laminate cells produced by the above procedure were measured. The resistance value is measured by regulating the current capacity to 300 mAh / g, repeatedly charging and discharging under the conditions of a charge end voltage of 1 V and a discharge end voltage of 0 V, and the direct current of the laminate cell 10 seconds after the start of the fifth cycle discharge. The resistance value was measured. The current density was 0.2C in the first cycle, 0.5C in the second cycle, 0.2C in the third cycle, 1C in the fourth cycle, and 0.2C in the fifth cycle. The results are shown in Table 3.

  In the laminate cell using # 01 as the positive electrode, the resistance value after charging / discharging was smaller than in the laminate cell using # C1 to # C3 as the positive electrode.

<Cycle characteristics>
In Table 3, the charge current capacities of two types of laminate cells having the above-described electrode # C0 or # 01 having a small resistance value after charging / discharging as a positive electrode were measured. Two laminate cells were prepared for each. In the capacity measurement, the current capacity was regulated to 800 mAh / g, charge / discharge was repeated under the conditions of a charge end voltage of 1 V and a discharge end voltage of 0 V, and the current capacity during charging in each cycle was measured. Charging / discharging shows the current capacity at the time of charging with respect to the number of cycles performed at 0.2C up to the 1st to 5th cycles, 0.5C up to the 6th to 10th cycles, and 1.0C up to the 11th to 15th cycles. 9 shows.

  Regarding the electrode # C0, the capacity reduction after the 6th cycle considered that the Si powder was pulverized was remarkable. However, in the laminate cell including the electrode # 01, the capacity reduction was suppressed even after the sixth cycle. In addition, two measurements were performed for each of the same electrodes, but the effect of suppressing the decrease in capacity was obtained with good reproducibility with electrode # 01.

Claims (8)

Carbon powder containing fluffy aggregated particles formed by entanglement of an active material containing silicon-containing powder composed of silicon and / or silicon compound and carbon fibers having an average fiber diameter of 5 nm to 100 nm and an average fiber length of 1 μm to 6 μm An electrode mixture preparation step of obtaining an electrode mixture by mixing a conductive additive comprising a binder and a binder;
An application step of applying the electrode mixture to a current collector;
The carbon fiber is partially agglomerated and comprises the active material, the conductive additive, the active material, and the binder that binds the conductive additive. An electrode for a non-aqueous secondary battery.   The electrode for a non-aqueous secondary battery according to claim 1, wherein an average particle diameter of the aggregated particles mixed in the electrode mixture preparation step is 1 μm or more and 1000 μm or less.   The electrode for a non-aqueous secondary battery according to claim 1 or 2, wherein an aspect ratio (average fiber length / average fiber diameter) of the carbon fiber is 10 or more and 1200 or less.   The electrode for a non-aqueous secondary battery according to any one of claims 1 to 3, wherein an average particle diameter of the Si-containing powder mixed in the electrode mixture preparation step is 0.01 µm or more and 10 µm or less. Carbon powder containing fluffy aggregated particles formed by entanglement of an active material containing silicon-containing powder composed of silicon and / or silicon compound and carbon fibers having an average fiber diameter of 5 nm to 100 nm and an average fiber length of 1 μm to 6 μm An electrode mixture preparation step of obtaining an electrode mixture by mixing a conductive additive comprising a binder and a binder;
An application step of applying the electrode mixture to a current collector;
A non-aqueous secondary battery comprising the active material, the conductive additive, the active material, and the binder that binds the conductive additive, wherein the carbon fibers are partially aggregated A method for producing an electrode for a non-aqueous secondary battery, characterized in that an electrode is obtained.   The method for producing an electrode for a non-aqueous secondary battery according to claim 5, wherein an average particle size of the aggregated particles mixed in the electrode mixture preparation step is 1 μm or more and 1000 μm or less.   The method for producing an electrode for a non-aqueous secondary battery according to claim 5 or 6, wherein an aspect ratio (average fiber length / average fiber diameter) of the carbon fiber is 10 or more and 1200 or less.   The method for producing an electrode for a non-aqueous secondary battery according to claim 5, wherein an average particle diameter of the Si-containing powder mixed in the electrode mixture preparation step is 0.01 μm or more and 10 μm or less. .