JP2007165079A - Negative electrode for nonaqueous electrolyte secondary battery and nonaqueous electrolyte secondary battery using it - Google Patents

Negative electrode for nonaqueous electrolyte secondary battery and nonaqueous electrolyte secondary battery using it Download PDF

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JP2007165079A
JP2007165079A JP2005358755A JP2005358755A JP2007165079A JP 2007165079 A JP2007165079 A JP 2007165079A JP 2005358755 A JP2005358755 A JP 2005358755A JP 2005358755 A JP2005358755 A JP 2005358755A JP 2007165079 A JP2007165079 A JP 2007165079A
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negative electrode
silicon
containing particles
electrolyte secondary
secondary battery
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Kaoru Inoue
Kokukiyo Kashiwagi
Takayuki Shirane
薫 井上
克巨 柏木
隆行 白根
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Matsushita Electric Ind Co Ltd
松下電器産業株式会社
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    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02EREDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00Enabling technologies or technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10Energy storage
    • Y02E60/12Battery technologies with an indirect contribution to GHG emissions mitigation
    • Y02E60/122Lithium-ion batteries

Abstract

<P>PROBLEM TO BE SOLVED: To provide a negative electrode having excellent cycle characteristics by suppressing increase in impedance of the whole negative electrode using silicon-containing particles capable of charging and discharging at least lithium ions as an active material, and to provide a battery using the negative electrode. <P>SOLUTION: The negative electrode for a nonaqueous electrolyte secondary battery has a composite negative active material 13 comprising silicon-containing particles 11 capable of absorbing/releasing at least lithium ions and carbon nano-fibers 12; and a mix layer containing an expansion/contraction buffer material 14 having conductivity. The carbon nano-fibers 12 are bonded to the surface of the silicon-containing particles 11. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a negative electrode for a non-aqueous electrolyte secondary battery, and more particularly to a technique for improving charge / discharge cycle characteristics of a negative electrode using silicon-containing particles that store and release lithium ions at high density as an active material.

  As electronic devices become more portable and cordless, expectations for non-aqueous electrolyte secondary batteries that are small and lightweight and have high energy density are increasing. Currently, carbon materials such as graphite are put into practical use as negative electrode active materials for non-aqueous electrolyte secondary batteries. However, its theoretical capacity density is 372 mAh / g. Therefore, silicon (Si), tin (Sn), germanium (Ge), and oxides and alloys thereof, which are alloyed with lithium, are being studied in order to further increase the energy density of nonaqueous electrolyte secondary batteries. . The theoretical capacity density of these negative electrode active material materials is larger than that of carbon materials. In particular, silicon-containing particles such as Si particles and silicon oxide particles are widely studied because they are inexpensive.

  However, when these materials are used as the negative electrode active material and the charge / discharge cycle is repeated, the volume of the active material particles changes due to charge / discharge. This volume change makes the active material particles fine, and as a result, the conductivity between the active material particles decreases. Therefore, sufficient charge / discharge cycle characteristics (hereinafter referred to as “cycle characteristics”) cannot be obtained.

Therefore, it has been proposed to combine a plurality of carbon fibers into a composite particle by using active material particles containing a metal or a semimetal capable of forming a lithium alloy as a core. In this configuration, it has been reported that conductivity is ensured and cycle characteristics can be maintained even if the volume of the active material particles changes (for example, Patent Document 1).
JP 2004-349056 A

  However, when the negative electrode mixture layer is composed only of composite negative electrode active material particles each having a core made of silicon-containing particles and a plurality of carbon fibers having tips attached to the surface thereof, the silicon-containing particles can be charged and discharged over a long period of time. The carbon fiber comes off or the carbon fiber is cut due to significant expansion and contraction. Moreover, since the expansion of the silicon-containing particles during charging is large, the negative electrode mixture layer is unlikely to return to its original shape even when discharged. As a result, the negative electrode mixture layer irreversibly deforms and expands. Due to such a phenomenon, the conductivity in the negative electrode mixture layer is lowered and the impedance of the whole negative electrode is increased. That is, if charging / discharging is repeated over a long period of time, the function as an electrode cannot be maintained, and battery characteristics including discharge capacity are deteriorated.

  This phenomenon is more conspicuous in the case of a cylindrical or rectangular battery that is formed by winding a thin electrode plate that is longer than a coin-type battery. That is, in the coin type battery, the deformation direction of the negative electrode is mainly in the thickness direction, whereas in the wound type battery, the deformation occurs in an arbitrary direction of the negative electrode plane, and wrinkles or peeling occurs in an arbitrary place. Therefore, current collection failure locally occurs. As a result, the deterioration of the battery characteristics accompanying the charge / discharge cycle is remarkable.

  The present invention uses a composite negative electrode active material in which carbon nanofibers (CNF), which are carbon fibers, are added to silicon-containing particles, and utilizes the characteristics of this composite negative electrode active material to construct a structure with excellent cycle characteristics, An object of the present invention is to provide a negative electrode for a non-aqueous electrolyte secondary battery having excellent cycle characteristics by suppressing an increase in impedance of the entire negative electrode over a long period of time and a non-aqueous electrolyte secondary battery using the same.

  In order to solve the above problems, the negative electrode for a non-aqueous electrolyte secondary battery of the present invention comprises at least silicon-containing particles capable of occluding and releasing lithium ions, and carbon nanofibers attached (fixed) to the surfaces of the silicon-containing particles ( And a composite negative electrode active material composed of CNF) and an expansion / contraction buffer material having conductivity. In this configuration, a network of electronic bonds is formed between the silicon-containing particles having poor conductivity via CNF and the expansion / contraction buffer material. In addition, the expansion and contraction buffer material is deformed during charging, thereby relaxing expansion of the mixture layer due to expansion of the silicon-containing particles. In addition, the silicon-containing particles contract during discharge, but the expansion / contraction buffer material entangled with CNF is pulled and spreads, so that a decrease in conductivity during discharge is also suppressed.

  The present invention further relates to a non-aqueous electrolyte secondary battery using a negative electrode containing the composite negative electrode active material described above.

  According to the present invention, a non-aqueous electrolyte secondary battery having a high capacity and excellent cycle characteristics can be provided.

  According to a first aspect of the present invention, there is provided a composite negative electrode active material comprising silicon-containing particles capable of charging / discharging at least lithium ions, and carbon nanofibers (CNF) attached (fixed) to the surfaces of the silicon-containing particles, and conductivity. It is a negative electrode for nonaqueous electrolyte secondary batteries which has the mixture layer containing the expansion-contraction buffer material which has. In this configuration, not only one end of CNF is fixed on the surface of the silicon-containing particles, but also a conductor expansion / contraction buffer material exists between the particles of the composite negative electrode active material. That is, one end of the conductive CNF is fixed to the surface of the silicon-containing particles, and the other end is connected to the expansion / contraction buffer material. As a result, a conductive network is formed between the silicon-containing particles having poor conductivity via the CNF and the expansion / contraction buffer material. Then, the expansion / shrinkage buffering material interposed between the composite negative electrode active materials is deformed during charging, thereby relaxing the expansion of the mixture layer due to the expansion of the silicon-containing particles. Further, since the CNF fixed to the surface of the silicon-containing particles is appropriately entangled with the expansion / shrinkage buffer material, the expansion / shrinkage buffer material tends to exist uniformly between the composite active materials. Can be suppressed. In addition, the silicon-containing particles contract during discharge, but the expansion / contraction buffer material entangled with CNF is pulled and spreads, so that a decrease in conductivity during discharge is also suppressed. Thus, excellent cycle characteristics can be obtained by maintaining a conductive network between silicon-containing particles which are active material nuclei.

  A second invention is a negative electrode for a non-aqueous electrolyte secondary battery in which the expansion / contraction buffer material is carbon black (CB) having a structure structure in the first invention. A CB having a structure structure is inexpensive and has high conductivity and expansion / shrinkage buffering action, and therefore well meets the gist of the present invention.

  A third invention is a negative electrode for a non-aqueous electrolyte secondary battery in which the amount of CB added in the second invention is 5 to 30 parts by weight with respect to 100 parts by weight of the composite negative electrode active material. When the amount is less than 5 parts by weight, the stress of expansion and contraction cannot be absorbed, electrode plate expansion increases, and accordingly, current collection performance decreases due to repeated charge and discharge, resulting in poor cycle characteristics. When the amount is more than 30 parts by weight, the proportion of silicon-containing particles becomes low, and the capacity merit is lost as compared with a negative electrode using graphite as a negative electrode active material.

A fourth invention is a negative electrode for a non-aqueous electrolyte secondary battery in which the silicon-containing particles in the first invention are silicon oxide particles represented by SiO x (0.05 <x <1.95). Such a material has a large discharge capacity density and an expansion coefficient during charging smaller than that of Si alone.

  5th invention is the nonaqueous electrolyte secondary battery comprised using either of the said negative electrodes for nonaqueous electrolyte secondary batteries.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The present invention is not limited to the following contents as long as it is based on the basic features described in this specification.

(Embodiment 1)
FIG. 1A is a cross-sectional view showing the configuration of the nonaqueous electrolyte secondary battery according to Embodiment 1 of the present invention, and FIG. 1B is a partially enlarged view of FIG. FIG. 2 is a schematic diagram showing the internal structure of the negative electrode mixture layer using the present invention, particularly the state around the composite negative electrode active material particles.

  The power generation element of this battery is configured by winding a long and thin positive electrode 2 as well as a long and thin negative electrode 1 through a separator 3. The case 4 contains the power generation element configured as described above and a non-aqueous electrolyte (not shown) impregnated in the power generation element. The sealing plate 5 seals the opening of the case 4. The lead 4C of the negative electrode 1 is connected to the case 4 and also serves as a negative electrode terminal. A lead 2C of the positive electrode 2 is connected to a metal portion insulated from the case 4 of the sealing plate 5 to constitute a positive electrode terminal.

  As shown in FIG. 1B, the negative electrode 1 is composed of a current collector 1A and a mixture layer 1B provided on both surfaces of the current collector 1A. One end of the lead 1C is connected to the current collector 1A. The positive electrode 2 includes a current collector 2A and a mixture layer 2B provided on both surfaces of the current collector 2A. One end of the lead 2C is connected to the current collector 2A.

  As shown in FIG. 2, the mixture layer 1 </ b> B includes a composite negative electrode active material 13 and an expansion / contraction buffer material 14. The composite negative electrode active material 13 includes at least silicon-containing particles 11 capable of occluding and releasing lithium ions, and carbon nanofibers (CNF) 12 attached to the surface thereof. There are no particular limitations on the particle shape or type of the silicon-containing particles 11 and the magnitude of expansion and contraction.

The silicon-containing particles 11 include Si, SiO x (0.05 <x <1.95), or any of these materials including B, Mg, Ni, Ti, Mo, Co, Ca, Cr, Cu, Fe, An alloy or compound containing at least Si or a solid solution in which a part of Si is substituted with at least one element selected from Mn, Nb, Ta, V, W, Zn, C, N, and Sn can be applied. . These may constitute the silicon-containing particles 11 alone, or a plurality of types may constitute the silicon-containing particles 11 at the same time. Examples in which a plurality of types simultaneously constitute the silicon-containing particles 11 include a compound containing Si, oxygen and nitrogen, and a composite of a plurality of compounds containing Si and oxygen and having different ratios of Si and oxygen. . As described above, the silicon-containing particles 11 include at least one selected from the group consisting of a simple substance of Si, an alloy containing Si, and a compound containing Si. Among these, SiO x (0.05 <x <1.95) is preferable because the discharge capacity density is large and the expansion rate during charging is smaller than that of Si alone.

  The CNF 12 adheres to the silicon-containing particles 11 on the surface of the silicon-containing particles 11 at the start of growth. That is, the CNF 12 is directly attached to the surface of the silicon-containing particles 11 without using a binder made of resin. Further, depending on the growth mode, CNF 12 may be chemically bonded to the surface of silicon-containing particles 11 at least at one end where the growth starts. Therefore, resistance to current collection is reduced in the battery, and high electron conductivity is ensured. Therefore, good charge / discharge characteristics can be expected. Further, when CNF 12 is bonded to the silicon-containing particles 11 by a catalytic element (not shown), the CNF 12 is not easily detached from the silicon-containing particles 11. Therefore, the tolerance of the negative electrode 1 with respect to the rolling load is improved.

  Until the growth of CNF 12 is completed, it is desirable that the catalytic element is present in a metallic state in the surface layer portion of the silicon-containing particles 11 in order for the catalytic element to exhibit good catalytic action. The catalyst element is desirably present in a state of metal particles having a particle diameter of 1 nm to 1000 nm, for example. On the other hand, after the growth of CNF 12 is finished, it is desirable to oxidize the metal particles made of the catalyst element.

  The fiber length of CNF12 is preferably 1 nm to 1 mm, and more preferably 500 nm to 100 μm. When the fiber length of CNF12 is less than 1 nm, the effect of increasing the conductivity of the electrode becomes too small, and when the fiber length exceeds 1 mm, the active material density and capacity of the electrode tend to decrease. Although the form of CNF 12 is not particularly limited, it is desirable that the CNF 12 is composed of at least one selected from the group consisting of tubular carbon, accordion carbon, plate carbon, and herringbone carbon. The CNF 12 may take a catalytic element into itself during the growth process. The fiber diameter of CNF12 is preferably 1 nm to 1000 nm, and more preferably 50 nm to 300 nm.

  The catalytic element provides an active point for growing CNF 12 in the metallic state. That is, when the silicon-containing particles 11 whose catalytic elements are exposed in a metallic state are introduced into a high-temperature atmosphere containing the source gas of CNF 12, the growth of CNF 12 proceeds. If no catalytic element is present on the surface of the active material particles, the CNF 12 does not grow.

  The method of providing the metal particles comprising the catalytic element on the surface of the silicon-containing particles 11 is not particularly limited, but for example, a method of supporting the metal particles on the surface of the particles capable of occluding and releasing lithium ions is suitable.

  When the metal particles are supported by the above method, it is conceivable to mix the solid metal particles with the silicon-containing particles 11, but a method of immersing the silicon-containing particles 11 in a solution of a metal compound that is a raw material of the metal particles is available. Is preferred. When the solvent is removed from the silicon-containing particles 11 immersed in the solution and heat-treated as necessary, the metal is uniformly and highly dispersed on the surface, and is made of a catalyst element having a particle size of 1 nm to 1000 nm, preferably 10 nm to 100 nm. It is possible to obtain silicon-containing particles 11 carrying the particles.

  When the particle size of the metal particles composed of the catalyst element is less than 1 nm, it is very difficult to generate the metal particles. When the particle size exceeds 1000 nm, the size of the metal particles becomes extremely non-uniform, making it difficult to grow CNF12. Or an electrode having excellent conductivity may not be obtained. Therefore, it is desirable that the particle size of the metal particles made of the catalyst element is 1 nm or more and 1000 nm or less.

  Examples of the metal compound for obtaining the solution include nickel nitrate, cobalt nitrate, iron nitrate, copper nitrate, manganese nitrate, hexaammonium hexamolybdate tetrahydrate and the like. A suitable solvent may be selected from water, an organic solvent, and a mixture of water and an organic solvent in consideration of the solubility of the compound and compatibility with the electrochemically active phase. As the organic solvent, for example, ethanol, isopropyl alcohol, toluene, benzene, hexane, tetrahydrofuran and the like can be used.

  On the other hand, alloy particles containing a catalyst element can be synthesized and used as the silicon-containing particles 11. In this case, an alloy of Si and a catalytic element is synthesized by a normal alloy manufacturing method. Since the Si element electrochemically reacts with lithium to form an alloy, an electrochemically active phase is formed. On the other hand, at least a part of the metal phase composed of the catalytic element is exposed on the surface of the alloy particles in the form of particles having a particle diameter of 10 nm to 100 nm, for example.

  The metal particles or metal phase composed of the catalytic element is preferably 0.01% by weight to 10% by weight of the silicon-containing particles 11, and more preferably 1% by weight to 3% by weight. If the content of the metal particles or the metal phase is too small, it takes a long time to grow the CNF 12, which may reduce the production efficiency. On the other hand, if the content of the metal particles or metal phase composed of the catalyst element is too large, the CNF 12 having a non-uniform and thick fiber diameter grows due to the aggregation of the catalyst element, so the conductivity and active material density in the mixture layer 1B Leading to a decline. In addition, the proportion of the electrochemically active phase is relatively reduced, making it difficult to make the composite negative electrode active material 13 into a high-capacity electrode material.

  Next, the manufacturing method of the composite negative electrode active material 13 comprised from the silicon-containing particle | grains 11 and CNF12 is described. This manufacturing method includes the following four steps.

  (A) At least one catalyst element selected from the group consisting of Cu, Fe, Co, Ni, Mo, and Mn that promotes the growth of CNF 12 in at least the surface layer portion of the silicon-containing particles 11 capable of occluding and releasing lithium. Providing a step.

  (B) A step of growing CNF 12 on the surface of the silicon-containing particles 11 in an atmosphere containing a carbon-containing gas and a hydrogen gas.

  (C) A step of firing the silicon-containing particles 11 to which the CNF 12 is adhered in an inert gas atmosphere at 400 ° C. or higher and 1600 ° C. or lower.

(D) CNF12 the step of adjusting the tap density was crushed silicon-containing particles 11 adhering to the following 0.42 g / cm 3 or more 0.91 g / cm 3.

  After step (c), the composite negative electrode active material 13 may be further heat-treated at 100 ° C. or higher and 400 ° C. or lower in the atmosphere to oxidize the catalytic element. If the heat treatment is performed at 100 ° C. or more and 400 ° C. or less, it is possible to oxidize only the catalytic element without oxidizing CNF 12.

  As the step (a), a step of supporting metal particles comprising a catalytic element on the surface of the silicon-containing particles 11, a step of reducing the surface of the silicon-containing particles 11 containing the catalytic element, and an alloy particle of Si element and catalytic element Examples include a step of synthesis. However, step (a) is not limited to the above.

  Next, conditions for growing CNF 12 on the surface of the silicon-containing particles 11 in step (b) will be described. When silicon-containing particles 11 having a catalytic element at least in the surface layer portion are introduced into a high-temperature atmosphere containing a source gas of CNF 12, the growth of CNF 12 proceeds. For example, the silicon-containing particles 11 are put into a ceramic reaction vessel and heated up to a high temperature of 100 ° C. to 1000 ° C., preferably 300 ° C. to 600 ° C. in an inert gas or a gas having a reducing power. Thereafter, carbon-containing gas and hydrogen gas, which are source gases of CNF 12, are introduced into the reaction vessel. If the temperature in the reaction vessel is less than 100 ° C., the growth of CNF 12 does not occur or the growth is too slow and the productivity is impaired. On the other hand, when the temperature in the reaction vessel exceeds 1000 ° C., decomposition of the raw material gas is promoted and CNF 12 is difficult to grow.

  As the source gas, a mixed gas of carbon-containing gas and hydrogen gas is suitable. As the carbon-containing gas, methane, ethane, ethylene, butane, carbon monoxide, or the like can be used. The molar ratio (volume ratio) of the carbon-containing gas in the mixed gas is preferably 20% to 80%. When the catalytic element in the metallic state is not exposed on the surface of the silicon-containing particles 11, the reduction of the catalytic element and the growth of the CNF 12 can proceed in parallel by controlling the ratio of hydrogen gas to a large extent. it can. When terminating the growth of CNF 12, the mixed gas of carbon-containing gas and hydrogen gas is replaced with an inert gas, and the inside of the reaction vessel is cooled to room temperature.

  Subsequently, in step (c), the silicon-containing particles 11 to which CNF 12 is adhered are fired at 400 ° C. or higher and 1600 ° C. or lower in an inert gas atmosphere. By doing in this way, since the irreversible reaction of the electrolyte and CNF12 which advance at the time of the initial charge of a battery is suppressed, and the outstanding charging / discharging efficiency can be obtained, it is preferable. If such a firing process is not performed, or if the firing temperature is less than 400 ° C., the above irreversible reaction may not be suppressed, and the charge / discharge efficiency of the battery may decrease. Further, when the firing temperature exceeds 1600 ° C., the electrochemically active phase of the silicon-containing particles 11 reacts with the CNF 12 to inactivate the active phase, or the electrochemically active phase is reduced to cause a decrease in capacity. There are things to do. For example, when the electrochemically active phase of the silicon-containing particles 11 is Si, Si and CNF 12 react with each other to generate inactive silicon carbide, causing a reduction in charge / discharge capacity of the battery. When the silicon-containing particles 11 are Si, the firing temperature is particularly preferably 1000 ° C. or higher and 1600 ° C. or lower. Note that the crystallinity of the CNF 12 can be increased depending on the growth conditions. Thus, when the crystallinity of CNF12 is high, since the irreversible reaction of electrolyte and CNF12 is also suppressed, step (c) is not essential.

  The composite negative electrode active material 13 after firing in an inert gas further oxidizes at least a part (for example, the surface) of metal particles or a metal phase composed of a catalytic element in the atmosphere at 100 ° C. or higher and 400 ° C. or lower. It is preferable to heat-treat with. If the heat treatment temperature is less than 100 ° C., it is difficult to oxidize the metal, and if it exceeds 400 ° C., the grown CNF 12 may burn.

In step (d), the fired silicon-containing particles 11 to which CNF 12 is adhered are crushed. By doing in this way, since the composite negative electrode active material 13 with favorable filling property is obtained, it is preferable. However, it disintegrated rather tap density also is 0.42 g / cm 3 or more, it is not always necessary to crushing in the case of 0.91 g / cm 3 or less. That is, when silicon-containing particles with good filling properties are used as a raw material, it may not be necessary to crush.

  The expansion / shrinkage buffer material 14 has conductivity, and the material is not particularly limited as long as it has a function of absorbing stress due to expansion / contraction of the silicon-containing particles 11. For example, a composite material in which a conductive substance such as copper is fixed to the surface of an elastic particle such as a resin is applicable. In particular, it is preferable to use carbon black (CB) having a structure structure. A CB having a structure structure is inexpensive and has high conductivity and expansion / shrinkage buffering action, and therefore well meets the gist of the present invention. Examples of CB include furnace black, lamp black, and ketjen black, and acetylene black (AB) is more preferably used from the viewpoint of having a high structure structure and high conductivity. In these structure structures, the primary particles in which the particles are agglomerated are in a fused state or a state of agglomerated aggregate called agglomerate, and an aggregate of fine fibers in which ultrafine fibers are entangled by van der Waals force is formed. In addition, as a method for obtaining a CB structure structure, a solvent in which an appropriate dispersant (such as a soluble binder, a thickener, or a surface active agent) is added using the fact that CB has high lyophobic properties. And a method of simultaneously mixing and dispersing CB and other negative electrode materials. Here, in order not to destroy the structure structure of CB, the contaminants are not in a funicular state during dispersion (a state in which liquid between particles is continuously present in a network shape and bubbles are isolated and interspersed therebetween). It is preferable to do this.

  Next, the manufacturing method of the negative electrode 1 is demonstrated. After the expansion / shrinkage buffer material 14 is mixed with the composite negative electrode active material 13 composed of the silicon-containing particles 11 having the CNF 12 attached to the surface as described above, the binder and the solvent are further mixed. At this time, a mixture slurry is prepared in a paste state by mixing so as not to become a phencular state. As the binder and the solvent, for example, polyvinylidene fluoride (PVDF) and N-methyl-2-pyrrolidone (NMP), or an emulsion of polytetrafluoroethylene and water can be used. Other binders include polyethylene, polypropylene, aramid resin, polyamide, polyimide, polyamideimide, polyacrylonitrile, polyacrylic acid, polyacrylic acid methyl ester, polyacrylic acid ethyl ester, polyacrylic acid hexyl ester, poly Methacrylic acid, polymethacrylic acid methyl ester, polymethacrylic acid ethyl ester, polymethacrylic acid hexyl ester, polyvinyl acetate, polyvinylpyrrolidone, polyether, polyethersulfone, hexafluoropolypropylene, styrene butadiene rubber, carboxymethylcellulose, etc. can be used It is. Two types selected from tetrafluoroethylene, hexafluoroethylene, hexafluoropropylene, perfluoroalkyl vinyl ether, vinylidene fluoride, chlorotrifluoroethylene, ethylene, propylene, pentafluoropropylene, fluoromethyl vinyl ether, acrylic acid, and hexadiene A copolymer of the above materials may be used.

  The obtained slurry is applied to both surfaces of the current collector 1A using a doctor blade and dried to form a mixture layer 1B on the current collector 1A. Then, it rolls and adjusts the thickness of the mixture layer 1B. The completed strip-shaped negative electrode continuum is punched or cut to a predetermined size. Then, a nickel or copper lead 1C is connected to the exposed portion of the current collector 1A by welding or the like to complete the negative electrode 1.

  The current collector 1A can be made of a metal foil such as stainless steel, nickel, copper, or titanium, or a thin film of carbon or conductive resin. Further, surface treatment may be performed with carbon, nickel, titanium or the like.

  If necessary, natural graphite such as flake graphite, graphite such as artificial graphite and expanded graphite, conductive fibers such as carbon fiber and metal fiber, metal powders such as copper and nickel, and organic such as polyphenylene derivatives A conductive agent such as a conductive material may be mixed into the mixture layer 1B. In that case, it is preferable to attach CNF12 to these conductive agent particles.

Next, the positive electrode 2 will be described. The mixture layer 2B includes a lithium-containing composite oxide such as LiCoO 2 , LiNiO 2 , Li 2 MnO 4 , or a mixture or composite compound thereof as a positive electrode active material. In addition to the above, as the positive electrode active material, olivine type lithium phosphate represented by the general formula of LiMPO 4 (M = V, Fe, Ni, Mn), Li 2 MPO 4 F (M = V, Fe, Ni, Mn) ) Lithium fluorophosphate represented by the general formula can also be used. Further, a part of these lithium-containing compounds may be substituted with a different element. Surface treatment may be performed with a metal oxide, lithium oxide, a conductive agent, or the like, or the surface may be subjected to a hydrophobic treatment.

  The mixture layer 2B further includes a conductive agent and a binder. As the conductive agent, natural graphite and artificial graphite graphite, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black and other carbon black, conductive fibers such as carbon fiber and metal fiber, Metal powders such as carbon fluoride and aluminum, conductive whiskers such as zinc oxide and potassium titanate, conductive metal oxides such as titanium oxide, and organic conductive materials such as phenylene derivatives can be used.

  Moreover, as a binder, the thing similar to what was used for the negative electrode 1 can be used. That is, PVDF, polytetrafluoroethylene, polyethylene, polypropylene, aramid resin, polyamide, polyimide, polyamideimide, polyacrylonitrile, polyacrylic acid, polyacrylic acid methyl ester, polyacrylic acid ethyl ester, polyacrylic acid hexyl ester, poly Methacrylic acid, polymethacrylic acid methyl ester, polymethacrylic acid ethyl ester, polymethacrylic acid hexyl ester, polyvinyl acetate, polyvinylpyrrolidone, polyether, polyethersulfone, hexafluoropolypropylene, styrene butadiene rubber, carboxymethylcellulose, etc. can be used It is. Two types selected from tetrafluoroethylene, hexafluoroethylene, hexafluoropropylene, perfluoroalkyl vinyl ether, vinylidene fluoride, chlorotrifluoroethylene, ethylene, propylene, pentafluoropropylene, fluoromethyl vinyl ether, acrylic acid, and hexadiene A copolymer of the above materials may be used. Two or more selected from these may be mixed and used.

  As the current collector 2A and the lead 2C used for the positive electrode 2, stainless steel, aluminum (Al), titanium, carbon, conductive resin, or the like can be used. Further, any of these materials may be surface-treated with carbon, nickel, titanium or the like.

  As the non-aqueous electrolyte, an electrolyte solution in which a solute is dissolved in an organic solvent, or a so-called polymer electrolyte layer containing these and non-fluidized with a polymer can be applied. When using an electrolyte solution at least, a separator 3 such as a nonwoven fabric or a microporous membrane made of polyethylene, polypropylene, aramid resin, amideimide, polyphenylene sulfide, polyimide, etc. is used between the positive electrode 2 and the negative electrode 1 and impregnated with the solution. Preferably. Further, the inside or the surface of the separator 3 may contain a heat-resistant filler such as alumina, magnesia, silica, and titania. Apart from the separator 3, a heat-resistant layer composed of these fillers and a binder similar to that used for the electrode may be provided.

The nonaqueous electrolyte material is selected based on the redox potential of the active material. Solutes preferably used for the nonaqueous electrolyte include LiPF 6 , LiBF 4 , LiClO 4 , LiAlCl 4 , LiSbF 6 , LiSCN, LiCF 3 SO 3 , LiCF 3 CO 2 , Li (CF 3 SO 2 ) 2 , LiAsF 6. , LiB 10 Cl 10 , lower aliphatic lithium carboxylate, LiF, LiCl, LiBr, LiI, lithium chloroborane, bis (1,2-benzenediolate (2-)-O, O ′) lithium borate, bis (2 , 3-Naphthalenedioleate (2-)-O, O ') lithium borate, bis (2,2'-biphenyldiolate (2-)-O, O') lithium borate, bis (2,2'-biphenyl) Dioleate (2-)-O, O ′) lithium borate, bis (5-fluoro-2-olate-1-benzenesulfonic acid-O, O ′) Borates such as lithium borate, (CF 3 SO 2 ) 2 NLi, LiN (CF 3 SO 2 ) (C 4 F 9 SO 2 ), (C 2 F 5 SO 2 ) 2 NLi, lithium tetraphenylborate, etc. Generally, salts used in lithium batteries can be applied.

  Further, the organic solvent for dissolving the salt includes ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dipropyl carbonate, methyl formate, methyl acetate, methyl propionate, ethyl propionate. , Tetrahydrofuran derivatives such as dimethoxymethane, γ-butyrolactone, γ-valerolactone, 1,2-diethoxyethane, 1,2-dimethoxyethane, ethoxymethoxyethane, trimethoxymethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, Dioxolane derivatives such as 1,3-dioxolane and 4-methyl-1,3-dioxolane, formamide, acetamide, dimethylformamide , Acetonitrile, propylnitrile, nitromethane, ethyl monoglyme, phosphoric acid triester, acetic acid ester, propionic acid ester, sulfolane, 3-methylsulfolane, 1,3-dimethyl-2-imidazolidinone, 3-methyl-2- Solvents such as those commonly used in lithium batteries, such as oxazolidinone, propylene carbonate derivatives, ethyl ether, diethyl ether, 1,3-propane sultone, anisole, one or more mixtures of fluorobenzene, etc., can be applied.

  Furthermore, vinylene carbonate, cyclohexyl benzene, biphenyl, diphenyl ether, vinyl ethylene carbonate, divinyl ethylene carbonate, phenyl ethylene carbonate, diallyl carbonate, fluoroethylene carbonate, catechol carbonate, vinyl acetate, ethylene sulfite, propane sultone, trifluoropropylene carbonate, Additives such as dibenisofuran, 2,4-difluoroanisole, o-terphenyl, m-terphenyl may be included.

The non-aqueous electrolyte is composed of one or more kinds of polymer materials such as polyethylene oxide, polypropylene oxide, polyphosphazene, polyaziridine, polyethylene sulfide, polyvinyl alcohol, polyvinylidene fluoride, polyhexafluoropropylene, and the like. May be used as a solid electrolyte. Moreover, you may mix with the said organic solvent and use it in a gel form. Further, lithium nitride, lithium halide, lithium oxyacid salt, Li 4 SiO 4, Li 4 SiO 4 -LiI-LiOH, Li 3 PO 4 -Li 4 SiO 4, Li 2 SiS 3, Li 3 PO 4 -Li Inorganic materials such as 2 S—SiS 2 and phosphorus sulfide compounds may be used as the solid electrolyte.

  The case 4 is made of a metal such as iron, nickel-plated iron, or aluminum. The sealing plate 5 includes an insulating member 5A for insulating from the case 4 and a metal portion 5B that functions as a positive terminal. When the case 4 is caulked and the sealing plate 5 is fixed as shown in FIG. 1A, the insulating member 5 </ b> A is a gasket that is compressed by a part of the case 4. The gasket is made of a resin material such as hard polypropylene. In the case of sealing with a hermetic seal, the insulating member 5A is made of an inorganic material such as glass. An explosion-proof mechanism that operates when the battery internal pressure rises may be incorporated in the sealing plate 5.

  Next, the structural change in the mixture layer 1B accompanying charging / discharging is demonstrated using FIG. FIG. 2A is a schematic diagram showing a state around the composite negative electrode active material 13 during charging, and FIG. 2B is a schematic diagram showing a state during the discharging.

  In the mixture layer 1B, a composite negative electrode active material 13 composed of silicon-containing particles 11 and CNF 12 and a conductive expansion / contraction buffer material 14 are included. One end of the CNF 12 is attached to the surface of the silicon-containing particles 11. The expansion / contraction buffer material 14 is interposed between the particles of the composite negative electrode active material 13. That is, one end of the conductive CNF 12 is fixed to the surface of the silicon-containing particle 11, and the other end is connected to the expansion / contraction buffer material 14. As a result, a conductive network is formed between the silicon-containing particles 11 having poor conductivity via the expansion / contraction buffer material 14.

  As shown in FIG. 2A, when charged, the silicon-containing particles 11 occlude lithium ions. Along with this, the silicon-containing particles 11 expand. The expansion and contraction cushioning material 14 absorbs the stress due to the expansion. Thereby, the expansion of the mixture layer 1B due to the expansion of the silicon-containing particles 11 is alleviated. Further, since the CNF 12 attached to the surface of the silicon-containing particles 11 is appropriately entangled with the expansion / contraction buffer material 14, the expansion / contraction buffer material 14 easily exists between the composite negative electrode active materials 13. Aggregation of the buffer material 14 is physically suppressed.

  On the other hand, as shown in FIG. 2 (b), the silicon-containing particles 11 contract during discharge, but the expansion / contraction buffer material 14 entangled with the CNF 12 is pulled and spread, so that a decrease in conductivity during discharge is also suppressed. . Thus, the conductive network between the silicon-containing particles 11 which are active material nuclei is maintained. Further, the expansion / contraction stress of the mixture layer 1B is relieved and deformation is suppressed. Thereby, peeling of the mixture layer 1B and the current collector 1A is also suppressed. As a result, excellent cycle characteristics can be obtained.

  The addition amount of the expansion / contraction buffer material 14 affects the cycle characteristics and capacity. That is, when the amount is less than 5 parts by weight, the absorption of the expansion and contraction stress becomes insufficient. On the other hand, when it exceeds 30 parts by weight, the proportion of the silicon-containing particles 11 in the mixture layer 1B is reduced, and the capacity merit is lowered. This is because the expansion / contraction buffer material 14 itself does not have a function of occluding (charging) lithium ions. Accordingly, the addition amount of the expansion / contraction buffer material 14 is preferably 5 parts by weight or more and 30 parts by weight or less per 100 parts by weight of the silicon-containing particles 11.

The features and effects of the present invention will be described below using specific experiments and results. First, preparation of the positive electrode 2 will be described. 85 parts by weight of LiNi 0.8 Co 0.2 O 2 powder as a positive electrode active material, 10 parts by weight of acetylene black as a conductive agent, and 5 parts by weight of PVDF as a solid content as a binder were mixed and dehydrated. A slurry-like positive electrode mixture slurry was prepared by dispersing in N-methyl-2-pyrrolidone (NMP). This slurry is applied to both sides of a current collector 2A made of an Al foil having a thickness of 20 μm, dried and rolled to form a mixture layer 2B, cut to a predetermined size, and then made of Al on the exposed portion of the current collector 2A. The lead 2C was attached by welding or ultrasonic welding to produce the positive electrode 2.

  Next, preparation of the negative electrode 1 will be described. In Sample 1, SiO was used as the silicon-containing particles 11. SiO was pulverized in advance to a particle size of 10 μm or less, and 100 parts by weight of this and 1 part by weight of nickel (II) nitrate hexahydrate were mixed using ion-exchanged water as a solvent. After stirring this for 1 hour, the solvent was removed with an evaporator and dried to support nickel nitrate on the SiO particle surfaces. As a result of analyzing the particles by SEM, it was confirmed that nickel nitrate (II) was deposited on the surface of the silicon-containing particles.

  SiO loaded with nickel nitrate is put into a ceramic reaction vessel, heated to 550 ° C. in helium gas, then replaced with a mixed gas of 50% hydrogen gas and 50% ethylene and held at 550 ° C. for 1 hour. Then, nickel nitrate (II) was reduced, and CNF 12 having tips attached to the surface of the silicon-containing particles 11 was grown. Then, CNF12 was heat-processed by hold | maintaining at 700 degreeC in mixed gas for 1 hour, and the composite negative electrode active material 13 was obtained.

  As a result of analyzing the obtained composite negative electrode active material 13 by SEM, it was confirmed that an infinite number of CNFs 12 having a fiber diameter of about 80 nm grew on the surface of the SiO particles. The weight of the grown CNF 12 was 25 parts by weight with respect to 100 parts by weight of SiO.

  Next, 100 parts by weight of the composite negative electrode active material 13 and 20 parts by weight of acetylene black having a structure structure as the expansion / contraction buffer material 14 were dry mixed. Furthermore, 3 parts by weight of carboxymethylcellulose as a thickener, 10 parts by weight of polystyrene butadiene as a binder are added in solid form, and mixed so as not to become a phencular state while adding ion-exchanged water. The slurry was adjusted. This slurry is applied to both surfaces of a current collector 1A made of a copper foil having a thickness of 15 μm, dried and rolled to form a mixture layer 1B, cut to a predetermined size, and then made of nickel on the exposed portion of the current collector 1A. The lead 1C was attached by ultrasonic welding, and the negative electrode 1 was produced.

  Using the positive electrode 2 and the negative electrode 1 produced as described above, a nonaqueous electrolyte secondary battery was produced as follows. That is, a polyethylene microporous film having a thickness of 20 μm and a porosity of about 40% was sandwiched between the positive electrode 2 and the negative electrode 1 as a separator 3 and wound to form a power generation element. The power generation element was housed in a case 4 having a diameter of 18 mm and a height of 65 mm, the lead 1C was welded to the bottom of the case 4, and the lead 2C was welded to the metal portion 5B of the sealing plate 5.

Next, 6 g of an electrolytic solution in which LiPF 6 was dissolved at a concentration of 1.0 mol / dm 3 was injected into a mixed solvent (weight ratio 3: 7) of ethylene carbonate (EC) and ethyl methyl carbonate (EMC), and the pressure was reduced. The electrolyte was allowed to penetrate into the power generation element. Finally, the sealing plate 5 was mechanically caulked to seal the case 4 to produce a cylindrical nonaqueous electrolyte secondary battery. The battery thus manufactured is charged to 4.1 V with a constant current of 0.7 CmA, charged and discharged twice to 2.5 V with a constant current of 0.7 CmA, and further stored for 7 days in a 45 ° C. environment. did. The battery was pre-evaluated as described above.

In Samples 2 and 3, the negative electrode 1 was produced in the same manner as in Sample 1 except that SiO 0.06 and SiO 1.94 were used as the silicon-containing particles 11, respectively, and a non-aqueous electrolyte secondary battery was manufactured using the negative electrode 1. Produced.

  In sample 4, SiO was used as the silicon-containing particles 11, and graphite was mixed with SiO in a weight ratio of 9: 1. CNF12 was grown on this mixed powder in the same manner as in sample 1. A negative electrode 1 was produced in the same manner as in Sample 1 except that the composite negative electrode active material 13 prepared in this way was used, and a nonaqueous electrolyte secondary battery was produced using it.

  In Samples 5 to 8, the negative electrode 1 was produced in the same manner as in the sample 1 except that the addition amount of the expansion / contraction buffer material 14 was changed, and a nonaqueous electrolyte secondary battery was produced using the negative electrode 1. The amount of acetylene black added to each sample is 3, 5, 30, and 40 parts by weight per 100 parts by weight of SiO, which is the silicon-containing particles 11, respectively.

  Moreover, the negative electrode 1 was produced like the sample 1 except not having added the acetylene black which is the expansion-contraction buffer material 14 as a comparative sample, and the nonaqueous electrolyte secondary battery was produced using it.

  Next, a method for evaluating the produced sample battery will be described. The battery was charged to 4.1 V with a constant current of 0.7 CmA, held at 4.2 V, charged until the current decreased to 0.05 CmA, and discharged to 2.5 V with a constant current of 0.5 CmA. The discharge capacity at this time was defined as the initial discharge capacity, and the discharge capacity ratio was calculated based on the design capacity of Sample 1. Here, 0.05 CmA means a current value obtained by dividing the battery design capacity by 20 hours.

  Next, 50 cycles of charging and discharging were performed under the same conditions as in the initial discharge capacity evaluation, and discharging to 2.5 V at a constant current of 1 CmA. The capacity retention rate was calculated by dividing the discharge capacity at the 50th cycle by the discharge capacity at the first cycle at this time, and used as an index of the cycle characteristics. Furthermore, the battery after charge / discharge cycle evaluation was disassembled, and the expansion coefficient of the mixture layer 1B was measured microscopically. The specifications and evaluation results of each sample are shown in (Table 1).

  As apparent from (Table 1), the cycle characteristics of Samples 1 to 8 are improved compared to the comparative sample. As is apparent from the measurement result of the expansion coefficient, the expansion of the mixture layer 1B is suppressed by the addition of the expansion / shrinkage buffer material 14, and the conductive network in the mixture layer 1B is maintained. It is done.

In addition, even when Samples 1 to 3 are compared, there is no significant difference in capacity and cycle characteristics. When the silicon-containing particles 11 are made of SiO x , this material can be used regardless of the material of 0.05 <x <1.95. It turns out that the effect of invention is acquired.

  Compared to sample 1, sample 4 has a reduced discharge capacity by the proportion of graphite with a lower capacity density added, but it can be seen that cycle characteristics are improved as in sample 1.

  Comparing Samples 1 and 5 to 8 reveals that there is a deep relationship between the amount of expansion / shrinkage buffer 14 added and the cycle characteristics. That is, in the sample 5 in which the addition amount of the expansion / contraction buffer material 14 is 3 parts by weight, the expansion rate of the mixture layer 1B is slightly large, and therefore the cycle characteristics are slightly deteriorated. On the other hand, in the sample 8 in which the added amount of the expansion / contraction buffer material 14 is 40 parts by weight, the cycle characteristics are good, but the capacity is slightly small. From this result, the addition amount of the expansion / contraction buffer material 14 is more preferably 5 parts by weight or more and 30 parts by weight or less with respect to 100 parts by weight of the silicon-containing particles 11.

  As described above, the winding type battery using the thin and long positive and negative electrodes has been described as an example, but the same effect can be obtained even when applied to a coin type battery. In the case of a coin-type battery, the current collector 1A is not necessarily required, and the mixture layer 1B may be provided directly on the inner surface of a metal case such as iron that also serves as an external terminal or nickel-plated iron. Further, the powder binder, the composite negative electrode active material 13 and the expansion / shrinkage buffer material 14 may be mixed and used by pressing the mixture without using a wet process as in the case of the mixture paste.

  The negative electrode for a non-aqueous electrolyte secondary battery according to the present invention can provide a non-aqueous electrolyte secondary battery with significantly improved cycle characteristics while realizing a high capacity. Therefore, it contributes to higher energy density of lithium batteries, which will increase in the future.

(A) Sectional drawing which shows the structure of the nonaqueous electrolyte secondary battery in Embodiment 1 of this invention (b) The elements on larger scale (A) Schematic diagram showing a state during charging around the composite negative electrode active material particles inside the negative electrode mixture layer using the present invention (b) Schematic diagram showing a state during discharge around the composite negative electrode active material particles

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Negative electrode 1A Current collector 1B Mixture layer 1C Lead 2 Positive electrode 2A Current collector 2B Mixture layer 2C Lead 3 Separator 4 Case 5 Sealing plate 11 Silicon-containing particle 12 Carbon nanofiber (CNF)
13 Composite negative electrode active material 14 Expansion / shrinkage buffer material

Claims (5)

  1. A composite negative electrode active material comprising at least silicon-containing particles capable of occluding and releasing lithium ions, and carbon nanofibers attached to the surface of the silicon-containing particles;
    A negative electrode for a non-aqueous electrolyte secondary battery, comprising a mixture layer including a conductive expansion / shrinkage buffer material.
  2. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the expansion / contraction buffer material is carbon black having a structure structure.
  3. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 2, wherein the addition amount of the carbon black is 5 parts by weight or more and 30 parts by weight or less per 100 parts by weight of the composite negative electrode active material.
  4. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the silicon-containing particles contain SiO x (0.05 <x <1.95).
  5. The nonaqueous electrolyte secondary battery provided with the negative electrode for nonaqueous electrolyte secondary batteries of any one of Claims 1-4.
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