JP2007220585A - Negative electrode for non-aqueous secondary battery, and non-aqueous secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a non-aqueous electrolyte secondary battery with high capacity and superior high load discharge characteristics by using a compound negative electrode active material in which carbon nano-fibers are adhered to active material nucleuses capable of storage and release of at least lithium ion, desirable for high capacity of the negative electrode. <P>SOLUTION: In the non-aqueous electrolyte secondary battery negative electrode 1, the negative electrode mixture layer 12 has a compound negative electrode active material having more adhesion amount of carbon nano-fibers distributed on the side near the current collector 10 and has the compound negative electrode active material having less adhesion amount of carbon nano-fibers distributed on the far side from the current collector 10, that is, on the surface 12A side facing the positive electrode. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a negative electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery, and more specifically, a non-aqueous electrolyte secondary battery including a large-capacity negative electrode for a non-aqueous electrolyte secondary battery without impairing charge / discharge characteristics. It relates to batteries.

  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, the carbon material normally used as the negative electrode active material of the non-aqueous electrolyte secondary battery has a ratio of the volume in the charged state to the volume in the discharged state of 1.1 or less, whereas the theoretical capacity density is larger than that of the carbon material and is 833 mAh. For these materials above / cm ³ , a large volume change of 1.2 or more occurs. This large 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”) are not 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 a plurality of carbon fibers are bonded to an active material particle nucleus containing a metal or metalloid capable of forming a lithium alloy to form a composite particle, the active material capacity and high-load charge / discharge depending on the amount of the carbon fiber. The characteristics are very different. That is, when the amount of carbon fiber is large relative to the active material particles, the conductivity can be ensured more reliably and the high rate charge / discharge characteristics are excellent. However, since the occupation ratio of the active material particles decreases, the capacity per unit weight (or per unit volume) decreases. Conversely, when the amount of carbon fiber is small, the capacity can be increased, but the high load charge / discharge characteristics are slightly inferior. Therefore, in order to realize comprehensively excellent characteristics, there are many restrictions on the synthesis of the active material.

  An object of the present invention is to solve the above-mentioned problems, and to provide a negative electrode for a non-aqueous electrolyte secondary battery that realizes a non-aqueous electrolyte secondary battery having comprehensively excellent characteristics by a simple method. To do.

  In order to achieve the above object, the negative electrode for a non-electrolyte secondary battery according to the present invention is a composite in which a carbon nanofiber (hereinafter referred to as CNF) is attached to a current collector and an active material nucleus capable of occluding and releasing at least lithium ions. And a negative electrode mixture layer provided on the current collector, including a negative electrode active material. The ratio A / B of the volume A in the charged state to the volume B in the discharged state of the active material nucleus is 1.2 or more. In the negative electrode mixture layer, the composite negative electrode active material with a large amount of CNF attached is distributed on the side closer to the current collector, and the composite negative electrode active material with a small amount of CNF attached is distributed on the side far from the current collector. . When the amount of CNF attached to the active material nucleus is large, the active material is excellent in high-load discharge and cycle characteristics. On the other hand, since the amount of active material nuclei per volume in the negative electrode mixture layer is reduced, the charge / discharge capacity as a battery is reduced. Conversely, if the amount of CNF deposited is small, the charge / discharge capacity increases, but high-load discharge and cycle characteristics deteriorate. In the structure of this invention, a high capacity | capacitance and the outstanding high load discharge characteristic are realizable by arrange | positioning effectively the composite negative electrode active material from which these performance differs in a negative mix layer.

  If the negative electrode for nonaqueous electrolyte secondary batteries of this invention is used, the nonaqueous electrolyte secondary battery which can implement | achieve a high capacity | capacitance and the outstanding high load discharge characteristic by a simple method can be provided.

A first invention of the present invention includes a current collector and a negative electrode mixture layer provided on the current collector, including a composite negative electrode active material in which CNF is attached to an active material core capable of occluding and releasing lithium ions. Is a negative electrode for a non-aqueous electrolyte secondary battery. The ratio A / B of the volume A in the charged state to the volume B in the discharged state of the active material nucleus is 1.2 or more, and its theoretical capacity density is larger than that of the carbon material and exceeds 833 mAh / cm ³ . In this negative electrode mixture layer, a composite negative electrode active material with a large amount of CNF attached is distributed on the side close to the current collector, and a composite negative electrode active material with a small amount of CNF attached is distributed on the side far from the current collector. . In this configuration, the composite negative electrode active materials having different performances are functionally arranged in the negative electrode mixture layer, whereby high capacity and excellent high load discharge characteristics can be realized.

  According to a second aspect of the present invention, in the first aspect, the negative electrode mixture layer is provided on the side close to the current collector, the first layer containing the composite negative electrode active material having a large amount of CNF attached, and the current collector This is a negative electrode for a nonaqueous electrolyte secondary battery including a second layer that is provided on the far side and includes a composite negative electrode active material with a small amount of CNF attached. This configuration is an example of the negative electrode for a non-aqueous electrolyte secondary battery according to the first invention, and the above configuration can be obtained by a simple method.

  3rd invention of this invention is a negative electrode for nonaqueous electrolyte secondary batteries which made the 1st layer thicker than the 2nd layer in 2nd invention. By configuring the first layer containing the composite negative electrode active material having a large amount of CNF to be thick, a nonaqueous electrolyte secondary battery suitable for higher load discharge can be obtained.

  4th invention of this invention is a negative electrode for nonaqueous electrolyte secondary batteries which made the 2nd layer thicker than the 1st layer in 2nd invention. A non-aqueous electrolyte secondary battery with higher capacity can be obtained by forming the second layer containing the composite negative electrode active material with a small amount of CNF adhesion.

  The fifth invention of the present invention is a negative electrode for a non-aqueous electrolyte secondary battery in which the active material nucleus is silicon-containing particles in the first invention. Silicon-containing particles are typical examples of active material nuclei and have a high capacity and are relatively inexpensive.

  A sixth invention of the present invention is the negative electrode for a nonaqueous electrolyte secondary battery according to the fifth invention, wherein the silicon-containing particles are silicon oxide and the ratio of oxygen to silicon is 0.3 or more and 1.3 or less. . By setting it as this composition range, the volume change by charging / discharging can be suppressed, ensuring active material capacity | capacitance.

  7th invention of this invention is a 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 contents described below as long as it is based on the basic characteristics described in this specification.

(Embodiment 1)
FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to Embodiment 1 of the present invention. FIG. 2 is a schematic view of a composite negative electrode active material contained in the negative electrode mixture layer of the nonaqueous electrolyte secondary battery shown in FIG. This coin-type battery includes a negative electrode 1, a positive electrode 2 that faces the negative electrode 1 and reduces lithium ions during discharge, and a nonaqueous electrolyte 3 that is interposed between the negative electrode 1 and the positive electrode 2 and conducts lithium ions. . The negative electrode 1 and the positive electrode 2 are accommodated in the case 6 using the gasket 4 and the lid 5 together with the nonaqueous electrolyte 3. The positive electrode 2 includes a current collector 7 and a positive electrode mixture layer 8 including a positive electrode active material. The negative electrode 1 has a current collector 10 and a negative electrode mixture layer 12 provided on the surface thereof.

The positive electrode mixture layer 8 includes a lithium-containing composite oxide such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O 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 positive electrode mixture layer 8 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 is 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 7 used for the positive electrode 2, aluminum (Al), carbon, conductive resin, or the like can be used. Further, any of these materials may be surface-treated with carbon or the like.

  As the non-aqueous electrolyte 3, 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 at least an electrolyte solution, a separator (not shown) such as a nonwoven fabric or a microporous film made of polyethylene, polypropylene, aramid resin, amideimide, polyphenylene sulfide, polyimide, etc. is used between the positive electrode 2 and the negative electrode 1. It is preferable to impregnate the solution. Further, the inside or the surface of the separator may contain a heat resistant filler such as alumina, magnesia, silica, titania. Apart from the separator, a heat-resistant layer composed of these fillers and a binder similar to that used for the electrode may be provided.

The material of the nonaqueous electrolyte 3 is selected based on the redox potential of the active material. Solutes preferably used for the nonaqueous electrolyte 3 include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAlCl ₄ , LiSbF ₆ , LiSCN, LiCF ₃ SO ₃ , LiN (CF ₃ CO ₂ ), LiN (CF ₃ SO ₂ ). ₂ , LiAsF ₆ , LiB ₁₀ Cl ₁₀ , lower aliphatic lithium carboxylate, LiF, LiCl, LiBr, LiI, chloroborane lithium, 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 (5- fluoro-2-oleate-1-benzenesulfonic acid -O, O ') borate borate salts such as _{lithium, (CF 3 SO 2) 2} NL _{, LiN (CF 3 SO 2)} (C 4 F 9 SO 2), can be applied salts used in _{(C 2 F 5 SO 2)} 2 NLi, lithium tetraphenyl borate, etc., generally lithium battery.

  Further, the organic solvent for dissolving the salt includes ethylene carbonate (EC), propylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate (DMC), diethyl carbonate, ethyl methyl carbonate (EMC), dipropyl carbonate, methyl formate, acetic acid. Methyl, methyl propionate, ethyl propionate, dimethoxymethane, γ-butyrolactone, γ-valerolactone, 1,2-diethoxyethane, 1,2-dimethoxyethane, ethoxymethoxyethane, trimethoxymethane, tetrahydrofuran, 2-methyl Tetrahydrofuran derivatives such as tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, dioxolane derivatives such as 4-methyl-1,3-dioxolane, formamide, aceto Toamide, dimethylformamide, acetonitrile, propylnitrile, nitromethane, ethyl monoglyme, phosphoric acid triester, acetate ester, propionate ester, sulfolane, 3-methylsulfolane, 1,3-dimethyl-2-imidazolidinone, 3-methyl 2-Oxazolidinone, propylene carbonate derivatives, ethyl ether, diethyl ether, 1,3-propane sultone, anisole, mixtures of one or more such as fluorobenzene, and the like, commonly used in lithium batteries Applicable.

  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, 0-terphenyl, m-terphenyl may be included.

In addition, the nonaqueous electrolyte 3 may be formed by mixing one or more polymer materials such as polyethylene oxide, polypropylene oxide, polyphosphazene, polyaziridine, polyethylene sulfide, polyvinyl alcohol, polyvinylidene fluoride, polyhexafluoropropylene, and the like. A solute may be mixed and 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.

  As shown in FIG. 2, the negative electrode mixture layer 12 includes a composite negative electrode active material 34 in which carbon nanofibers (CNF) 36 are attached to the surface of an active material core 35 capable of absorbing and releasing lithium ions. The CNF 36 adheres to or adheres to the surface of the active material core 35, and resistance to current collection is reduced in the battery, and high electron conductivity is maintained.

  The negative electrode mixture layer 12 further contains a binder. Examples of the binder include polyvinylidene fluoride (PVDF), polytetrafluoroethylene, polyethylene, polypropylene, aramid resin, polyamide, polyimide, polyamideimide, polyacrylonitrile, polyacrylic acid, polyacrylic acid methyl ester, and polyacrylic acid. Ethyl ester, polyacrylic acid hexyl ester, polymethacrylic acid, polymethacrylic acid methyl ester, polymethacrylic acid ethyl ester, polymethacrylic acid hexyl ester, polyvinyl acetate, polyvinyl pyrrolidone, polyether, polyether sulfone, hexafluoropolypropylene, Styrene butadiene rubber, carboxymethyl cellulose and the like can be used. 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.

  If necessary, natural graphite such as flake graphite, graphite such as artificial graphite and expanded graphite, carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black and thermal black, carbon fiber Conductive agents such as conductive fibers such as metal fibers, metal powders such as copper and nickel, and organic conductive materials such as polyphenylene derivatives may be mixed in the negative electrode mixture layer 12.

  The current collector 10 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.

Next, the composite negative electrode active material 34 will be described in detail. As the active material core 35, a material that can occlude and release a large amount of lithium ions, such as silicon (Si) or tin (Sn), can be used. With such a material, the effect of the present invention can be exhibited with any of a simple substance, an alloy, a compound, a solid solution, and a composite active material including a silicon-containing material and a tin-containing material. That is, as a silicon-containing material, Si, SiO _x (0.05 <x <1.95), or any of these, B, Mg, Ni, Ti, Mo, Co, Ca, Cr, Cu, Fe, Mn An alloy, a compound, a solid solution, or the like in which a part of Si is substituted with at least one element selected from the group consisting of Nb, Ta, V, W, Zn, C, N, and Sn can be used. As the tin-containing material, Ni ₂ Sn ₄ , Mg ₂ Sn, SnO _x (0 <x <2), SnO ₂ , SnSiO ₃ , LiSnO, or the like can be applied.

These materials may constitute the active material nucleus 35 alone, or the active material nucleus 35 may be composed of a plurality of types of materials. Examples of constituting the active material nucleus 35 with the above-mentioned plural kinds of materials include a compound containing Si, oxygen and nitrogen, and a composite of a plurality of compounds containing Si and oxygen and having different constituent ratios of Si and oxygen. Can be mentioned. Among these, SiO _x (0.3 ≦ x ≦ 1.3) is preferable because it has a large discharge capacity density and an expansion coefficient lower than that of Si.

  The CNF 36 is formed by growing using a catalyst element (not shown) supported on the surface of the active material nucleus 35 as a nucleus. As the catalyst element, at least one selected from the group consisting of copper (Cu), iron (Fe), cobalt (Co), nickel (Ni), molybdenum (Mo) and manganese (Mn) can be used. Promote growth. As described above, the CNF 36 is attached to the surface of the active material nucleus 35, so that good charge / discharge characteristics can be expected. Further, the presence of the catalytic element has a strong binding force to the active material core 35, and the durability of the negative electrode 1 against the rolling load when the negative electrode mixture layer 12 is applied on the current collector 10 can be improved.

  In order for the catalytic element to exhibit good catalytic action until the growth of the CNF 36 is completed, it is desirable that the catalytic element exists in a metal state in the surface layer portion of the active material nucleus 35. 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 36 is completed, it is desirable to oxidize the metal particles made of the catalyst element.

  The fiber length of CNF36 is preferably 1 nm to 1 mm, and more preferably 500 nm to 100 μm. When the fiber length of the CNF 36 is less than 1 nm, the effect of increasing the conductivity of the electrode is too small, and when the fiber length exceeds 1 mm, the active material density and capacity tend to decrease.

  Although the form of CNF 36 is not particularly limited, it is preferable that the CNF 36 is made of at least one selected from the group consisting of tubular carbon, accordion carbon, plate carbon, and herringbone carbon. The CNF 36 may take the catalytic element into itself during the growth process. The fiber diameter of CNF36 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 36 in the metallic state. That is, when the active material nucleus 35 having the catalytic element exposed on the surface in a metallic state is introduced into a high temperature atmosphere containing the source gas of the CNF 36, the growth of the CNF 36 proceeds. When no catalytic element is present on the surface of the active material particles, the CNF 36 does not grow.

  A method of providing the metal particles made of the catalytic element on the surface of the active material nucleus 35 is not particularly limited. For example, it is conceivable to mix solid metal particles with the active material nucleus 35. Further, a method of immersing the active material core 35 in a solution of a metal compound that is a raw material for the metal particles is preferable. When the solvent is removed from the active material core 35 after being immersed in the solution, and heat treatment is performed as necessary, the metal is uniformly and highly dispersed on the surface and is made of a catalyst element having a particle diameter of 1 nm to 1000 nm, preferably 10 nm to 100 nm. It is possible to obtain the active material core 35 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 and it is difficult to grow CNF36. Or an electrode having excellent conductivity may not be obtained. Therefore, the particle size of the metal particles made of the catalyst element is desirably 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 suitability for 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 catalytic element can be synthesized and used as the active material core 35. In this case, an alloy of Si, Sn and the like and a catalytic element is synthesized by a normal alloy manufacturing method. Since elements such as Si and Sn react electrochemically 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 active material core 35, 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 36, which may reduce the production efficiency. On the other hand, if the content of the metal particles or metal phase comprising the catalyst element is too large, the CNF 36 having a non-uniform and large fiber diameter grows due to the aggregation of the catalyst element, so that the conductivity and active material density in the mixture layer are reduced. 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 34 a high-capacity electrode material.

  Next, a manufacturing method of the composite negative electrode active material 34 composed of the active material nucleus 35 and the CNF 36 will be 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 36 on at least the surface layer portion of the active material core 35 capable of occluding and releasing lithium Providing a step.

  (B) A step of growing CNF 36 on the surface of the active material nucleus 35 in an atmosphere containing a carbon-containing gas and a hydrogen gas.

  (C) A step of firing the active material core 35 to which the CNF 36 is adhered in an inert gas atmosphere at 400 ° C. or higher and 1600 ° C. or lower.

(D) CNF36 is the tap density 0.42 g / cm ³ or more and then disintegrated an active material core 35 attached, the step of adjusting the 0.91 g / cm ³ or less.

  After step (c), the composite negative electrode active material 34 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 higher and 400 ° C. or lower, it is possible to oxidize only the catalytic element without oxidizing CNF 36.

  As the step (a), a step of supporting metal particles made of a catalytic element on the surface of the active material nucleus 35, a step of reducing the surface of the active material nucleus 35 containing the catalytic element, an element such as Si, Sn, and the catalytic element, And a step of synthesizing the alloy particles. However, step (a) is not limited to the above.

  Next, conditions for growing CNF 36 on the surface of the active material nucleus 35 in step (b) will be described. When the active material nucleus 35 having the catalytic element at least in the surface layer portion is introduced into a high temperature atmosphere containing the source gas of the CNF 36, the growth of the CNF 36 proceeds. For example, the active material nucleus 35 is put into a ceramic reaction vessel, and the temperature is raised to 100 ° C. to 1000 ° C., preferably 300 ° C. to 600 ° C., in an inert gas or a gas having a reducing power. Thereafter, a carbon-containing gas and hydrogen gas, which are source gases of CNF 36, are introduced into the reaction vessel. If the temperature in the reaction vessel is less than 100 ° C., the growth of CNF 36 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 36 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 metal state is not exposed on the surface of the active material nucleus 35, the reduction of the catalytic element and the growth of the CNF 36 can be performed in parallel by controlling the ratio of the hydrogen gas more. it can. When terminating the growth of the CNF 36, the mixed gas of the carbon-containing gas and the 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 active material nucleus 35 to which the CNF 36 is attached is 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 CNF36 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. When the firing temperature exceeds 1600 ° C., the electrochemically active phase of the active material core 35 reacts with the CNF 36 to deactivate 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 active material nucleus 35 is Si, Si and CNF 36 react to generate inactive silicon carbide, causing a reduction in charge / discharge capacity of the battery. When the active material nucleus 35 is Si, the firing temperature is particularly preferably 1000 ° C. or higher and 1600 ° C. or lower. Note that the crystallinity of the CNF 36 can be increased depending on the growth conditions. Thus, when the crystallinity of CNF36 is high, since the irreversible reaction of electrolyte and CNF36 is also suppressed, step (c) is not essential.

  The composite negative electrode active material 34 after firing in an inert gas further oxidizes at least a part (for example, the surface) of the metal particles or metal phase comprising the 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 36 may burn.

In step (d), the fired active material core 35 to which the CNF 36 is attached is crushed. By doing in this way, since the composite negative electrode active material 34 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 an active material nucleus with good filling properties is used as a raw material, it may not be necessary to crush.

  Next, the configuration of the negative electrode mixture layer 12 will be described. FIG. 3 is an enlarged cross-sectional view of the negative electrode 1. In the negative electrode mixture layer 12 in the present embodiment, the composite negative electrode active material 34 having a large amount of CNF 36 attached is distributed on the side close to the current collector 10, and the amount of CNF 36 attached is small on the surface 12 A side facing the positive electrode 2. The composite negative electrode active material 34 is distributed. That is, the composite negative electrode active material 34 located far from the current collector 10 has a small amount of CNF 36 attached thereto.

  The amount of CNF 36 attached to the active material nucleus 35 can be changed by changing the production conditions of the CNF 36, such as changing the growth time of the CNF 36 or the amount of catalyst element attached.

  When the CNF 36 is adhered to the active material nucleus 35, the influence on the battery characteristics is greatly different depending on the ratio. When the amount of the CNF 36 attached to the active material nucleus 35 is large, the active material is excellent in high-load discharge and cycle characteristics. On the other hand, since the amount of the active material nucleus 35 per volume is reduced, the charge / discharge capacity as a battery is reduced. Conversely, if the amount of CNF 36 attached is small, the charge / discharge capacity increases, but high-load discharge and cycle characteristics deteriorate.

  In the present embodiment, high capacity and excellent high load discharge characteristics can be realized by effectively arranging the composite negative electrode active materials 34 having different performances in the negative electrode mixture layer 12. The characteristics can be adjusted according to the required performance. That is, an active material having a high adhesion amount of CNF 36 excellent in high-load discharge characteristics is distributed in the vicinity of the current collector 10, and the adhesion amount of CNF 36 is small for the purpose of securing a charge / discharge capacity on the surface 12 A side facing the positive electrode 2. Distribute the active material.

  In order to make the negative electrode mixture layer 12 have such a configuration, for example, several types of pastes containing composite negative electrode active materials 34 with different amounts of CNFs 36 are prepared. Then, a paste containing the composite negative electrode active material 34 having the largest amount of CNF 36 attached is sprayed onto the current collector 10 by a spray method. After drying, a paste containing the composite negative electrode active material 34 with a smaller amount of CNF 36 attached thereto is sprayed and dried thereon. Thereafter, the paste is sprayed in descending order of the amount of CNF 36 attached. In this way, the negative electrode mixture layer 12 in which the adhesion amount of the CNF 36 continuously changes with respect to the active material nucleus 35 is obtained.

  In addition to the above, the negative electrode mixture layer 12 may be formed in a configuration as shown in the cross-sectional view of FIG. In this configuration, the first layer 12B including the composite negative electrode active material 34 with a large amount of CNF 36 attached is provided on the side close to the current collector 10, and the surface 12A side facing the positive electrode 2 (the side far from the current collector 10). The second layer 12 </ b> C containing the composite negative electrode active material 34 with a small amount of CNF 36 attached is provided. Even if it does in this way, the same effect as the above is acquired. Such a negative electrode mixture layer 12 is formed as follows. First, a paste containing the composite negative electrode active material 34 with a large amount of CNF 36 attached is applied by a doctor blade method or the like to form the first layer 12B. Next, a paste containing a composite negative electrode active material 34 with a small amount of CNF 36 attached is applied on the first layer 12B by a doctor blade method or the like to form the second layer 12C. This method is relatively simple. At this time, it is more preferable to form the second layer 12C before the first layer 12B is completely dried. By doing in this way, it becomes difficult for the 1st layer 12B and the 2nd layer 12C to exfoliate.

  Furthermore, as shown in the cross-sectional view of FIG. 5, when the surface of the first layer 12B is provided with irregularities and the second layer 12C is provided thereon, the bond between the first layer 12B and the second layer 12C is strengthened. Since it becomes difficult to peel, it is preferable.

  When the negative electrode mixture layer 12 is formed in the configuration shown in FIGS. 4 and 5 and the negative electrode 1 is rolled after drying, rearrangement of the composite negative electrode active material 34 occurs in the negative electrode mixture layer 12, and the CNF 36 is simulated. Thus, the negative electrode mixture layer 12 in which the amount of the adhering material is continuously changed is obtained. 4 and 5, the negative electrode mixture layer 12 is formed in two layers. However, as the distance from the current collector 10 increases, the composite negative electrode active material 34 having a small amount of CNF 36 attached decreases so as to have three or more layers. You may form with a layer.

  Next, specific examples in the embodiment of the present invention will be described. In the following examples, for the purpose of clarifying the effect of the negative electrode 1, a coin-type secondary battery similar to that in FIG. 1 was fabricated and evaluated using metal lithium instead of the positive electrode 2.

(1) Production of Negative Electrode As active material core 35 capable of occluding and releasing lithium ions, sample 1 has an O / Si ratio (ratio of oxygen to silicon) pulverized to a particle size of 10 μm or less at a molar ratio of 1.01. Some silicon oxide was used.

  Further, in order to bind the catalyst element to the surface layer portion of the silicon oxide particles, a solution in which 1 g of iron nitrate nonahydrate (special grade) was dissolved in 100 g of ion-exchanged water was used. The molar ratio of the silicon oxide particles was measured according to a gravimetric analysis method based on JIS Z2613. The mixture of the silicon oxide particles and the iron nitrate solution is stirred for 1 hour, and then the water is removed by an evaporator, so that the particle size is 1 nm to 1000 nm in a uniform and highly dispersed state on the surface layer of the silicon oxide particles. Iron nitrate was supported.

  Next, the active material nucleus 35 carrying iron nitrate was put into a ceramic reaction vessel, and the temperature was raised to 500 ° C. in the presence of helium gas. Thereafter, the helium gas was replaced with a mixed gas of 50% by volume of hydrogen gas and 50% by volume of carbon monoxide gas, and maintained at 500 ° C. for 1 hour. Thereby, iron nitrate was reduced, and a plate-like CNF 36 having a fiber diameter of about 80 nm and a fiber length of about 50 μm was grown on the surface of the active material core 35.

  Next, the mixed gas was replaced with helium gas again, and the reaction vessel was cooled to room temperature. The amount of CNF36 grown was 25 parts by weight per 100 parts by weight of silicon oxide. In this way, a composite negative electrode active material 34 was obtained.

  Next, for preparing a paste, 10 parts by weight of a 1% polyacrylic acid aqueous solution having an average molecular weight of 150,000 as a binder and 100 parts by weight of the composite negative electrode active material 34 as a binder, 10 parts by weight of the type-modified styrene-butadiene copolymer was mixed, and 200 parts by weight of distilled water was further added and mixed and dispersed to prepare a negative electrode mixture paste A.

  Further, in the growth process of CNF 36, a composite negative electrode active material 34 having a growth time of 30 minutes was prepared, and a negative electrode mixture paste B was prepared in the same manner as described above. Further, a composite negative electrode active material 34 with a growth time of 2 hours was prepared, and a negative electrode mixture paste C was prepared in the same manner as described above. The amount of CNF 36 in the negative electrode mixture paste B and the negative electrode mixture paste C was 15 parts by weight and 35 parts by weight, respectively, per 100 parts by weight of silicon oxide.

  Using these negative electrode mixture pastes, the current collector 10 made of Cu foil having a thickness of 14 μm is coated and dried by the doctor blade method so that the total thickness after drying (including the copper foil) becomes 100 μm. A negative electrode mixture layer 12 was formed. Thereafter, the negative electrode 1 was obtained by punching into a circle having a diameter of 12.5 mm.

  In Samples 1 to 3, the negative electrode mixture paste A was applied first, and then dried slightly before applying the negative electrode mixture paste B. That is, the first layer 12B was formed from the negative electrode mixture paste A, and the second layer 12C was formed from the negative electrode mixture paste B.

  In sample 1, the thicknesses of the first layer 12B and the second layer 12C were each 50 μm. In sample 2, the thickness of the first layer 12B was 30 μm, and the thickness of the second layer 12B was 70 μm. In Sample 3, the thickness of the first layer 12B was 10 μm, and the thickness of the second layer 12B was 90 μm.

  In Samples 4 to 6, the negative electrode mixture paste C was applied first, and then slightly dried, and then the negative electrode mixture paste A was applied. That is, the first layer 12B was formed from the negative electrode mixture paste C, and the second layer 12C was formed from the negative electrode mixture paste A.

  In sample 4, the thicknesses of the first layer 12B and the second layer 12C were set to 50 μm, respectively. In Sample 5, the thickness of the first layer 12B was 30 μm, and the thickness of the second layer 12B was 70 μm. In Sample 6, the thickness of the first layer 12B was 10 μm, and the thickness of the second layer 12B was 90 μm.

  In Samples 7 and 8, silicon oxide having an O / Si ratio of 0.3 and 1.3 in terms of molar ratio was used as the active material nucleus 35, respectively. Otherwise, the negative electrode 1 was produced in the same manner as in the sample 3.

  For comparison, the following samples were prepared. In Samples C1, C2, and C3, the negative electrode mixture layer 12 was prepared using only the negative electrode mixture paste A, the negative electrode mixture paste B, and the negative electrode mixture paste C, respectively. In Sample C4, silicon oxide having an O / Si ratio of 0.1 in terms of molar ratio was used as the active material nucleus 35. Otherwise, the negative electrode 1 was produced in the same manner as in the sample 3.

  In Samples 7, 8, and C4, since the O / Si ratio in the active material nucleus 35 is different, the adhesion amount of CNF 36 is slightly different from that in Sample 3. In Sample 7, the amount of CNF 36 in negative electrode mixture paste A and negative electrode mixture paste B was 29 parts by weight and 16 parts by weight, respectively, per 100 parts by weight of silicon oxide. In sample 8, the amount of CNF 36 in negative electrode mixture paste A and negative electrode mixture paste B was 27 parts by weight and 18 parts by weight, respectively, per 100 parts by weight of silicon oxide. In sample C4, the amounts of CNF 36 in negative electrode mixture paste A and negative electrode mixture paste B were 24 parts by weight and 15 parts by weight, respectively, per 100 parts by weight of silicon oxide.

(2) Production and Evaluation of Battery The negative electrode 1 produced as described above was inserted into a case 6 having a diameter of 20 mm and a thickness of 1.6 mm. On top of that, lithium metal was placed through a separator having a thickness of 20 μm, and then a few drops of the electrolyte solution as the non-aqueous electrolyte 3 were injected and sealed to prepare a battery having a theoretical capacity of about 5 mAh. The electrolytic solution was prepared by dissolving 1.0 mol / dm ³ of LiPF ₆ in a mixed solvent of EC: DMC: EMC = 2: 3: 3 (volume ratio).

  Each battery thus produced was discharged to 0 V with a constant current of 0.5 mA, and then charged to 1 V with a constant current of 0.5 mA. This operation was repeated three times. This third charge capacity was defined as a low load capacity. Thereafter, the battery was discharged to 0 V with a constant current of 0.5 mA, and then charged to 1 V with a constant current of 2.5 mA. This charge capacity was defined as a high load capacity. In this embodiment, the negative electrode 1 having a potential higher than that of metallic lithium is combined with metallic lithium to form a battery. Therefore, the negative electrode 1 releases lithium ions by charging, and the negative electrode 1 absorbs lithium ions by discharging. . That is, it is the reverse of the case of a normal battery.

  First, specifications and evaluation results of samples 1 to 6 and samples C1 to C3 are shown in (Table 1).

  When comparing the sample C1 and the samples 1 to 3, the high load capacity is larger in the sample C1, but the low load capacity is larger in the samples 1 to 3. This is because the amount of the active material nucleus 35 is increased due to the small amount of the CNF 36 attached to the second layer 12C, and the low load capacity is increased. Comparing sample C2 and samples 1 to 3, sample C2 has a larger low load capacity, but samples 1 to 3 have a higher high load capacity. This is because the first layer 12B reduces the overvoltage (voltage rise) during high load charging and increases the high load capacity. This tendency is the same in the comparison between the samples C1 and C3 and the samples 4 to 6.

  The capacity at low load discharge is almost proportional to the amount of active material filled in the battery. On the other hand, the capacity in high-load discharge is determined by two parameters: the amount of active material filled in the battery and the ease of discharging the active material. In the comparison between the sample C2 and the samples 1 to 3 and the comparison between the sample C1 and the samples 4 to 6, the influence of the latter is largely reflected as indicated by the capacity retention ratio in the high load discharge.

  When samples 1 to 3 are compared, the larger the thickness of the second layer 12C with a small amount of CNF 36 attached, the larger the low load capacity, but the smaller the high load capacity. This is considered to be because the influence of the parameter of the ease of discharge of the active material is largely reflected as indicated by the capacity retention ratio in high-load discharge. On the other hand, when samples 4 to 6 are compared, both the low load capacity and the high load capacity increase as the thickness of the second layer 12C with a small amount of CNF 36 attached increases. This is presumably because the capacity retention rate at high load discharge exceeds 90%, and as a result, the influence of the parameter of the amount of active material filled in the battery is greatly reflected. As described above, if the first layer 12B is thicker than the second layer 12C, a battery suitable for high load discharge is obtained, and if the second layer 12C is thicker than the first layer 12B, a battery suitable for low load discharge is obtained. can get. Depending on the battery performance required in this way, it is possible to obtain optimum characteristics by optimizing the composition ratio of the first layer 12B having a large amount of CNF 36 and the second layer 12C having a small amount of CNF 36. In other words, the second layer 12C is configured with a relatively large amount for an application that requires a certain amount of output while having a certain output, such as an electric power tool. Moreover, when an output is required like a hybrid electric vehicle use, the characteristic suitable for each use can be acquired by comprising many 1st layers 12B.

  Next, the specifications and evaluation results of Samples 3, 7, 8 and Sample C4 are shown in (Table 2).

Although the amount of CNF 36 attached to the active material nucleus 35 in each of the first layer 12B and the second layer 12C is slightly different in each sample, samples 3, 7, and 8 have almost the same high load capacity. . Compared to this, the high load capacity is small in the sample C4. From the above, the O / Si ratio in the active material nucleus 35 is preferably 0.3 or more and 1.3 or less. In addition, although an attempt was made to synthesize SiO _x having a larger O / Si ratio by increasing the firing temperature and firing time, in this experiment, it was not possible to synthesize those having an O / Si ratio exceeding 1.35.

  In this embodiment, the coin-type secondary battery has been described. However, the shape of the battery according to the present invention is not limited to this, and a flat battery, a wound cylindrical battery, a wound type, or a stacked type is used. The present invention can also be applied to a prismatic battery having a structure.

  In the present invention, a composite negative electrode active material having CNF attached to the active material core is used, and in the negative electrode mixture layer containing the composite negative electrode active material, a composite negative electrode active material having a large amount of CNF attached is disposed on the side close to the current collector. The composite negative electrode active material with a small amount of CNF attached is distributed on the side facing the positive electrode. With such a configuration, a low load discharge characteristic and a high load discharge characteristic are realized in a balanced manner. That is, the present invention contributes to improving the characteristics of a non-aqueous electrolyte secondary battery using a composite negative electrode active material in which CNF is attached to an active material nucleus in which the ratio of the volume in the charged state to the volume in the discharged state is 1.2 or more. Therefore, it can contribute to the high energy density of the lithium secondary battery, which is expected to have a great demand in the future.

Sectional drawing of the nonaqueous electrolyte secondary battery by Embodiment 1 of this invention Schematic of composite negative electrode active material in Embodiment 1 of the present invention Sectional drawing of the negative electrode for nonaqueous electrolyte secondary batteries in Embodiment 1 of this invention Sectional drawing which shows the other structure of the negative electrode for nonaqueous electrolyte secondary batteries in Embodiment 1 of this invention Sectional drawing which shows the other structure of the negative electrode for nonaqueous electrolyte secondary batteries in Embodiment 1 of this invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Negative electrode 2 Positive electrode 3 Nonaqueous electrolyte 4 Gasket 5 Lid 6 Case 7, 10 Current collector 8 Positive electrode mixture layer 12 Negative electrode mixture layer 12A Surface 12B First layer 12C Second layer 34 Composite negative electrode active material 35 Active material nucleus 36 Carbon nanofiber (CNF)

Claims (7)

A current collector,
Including a composite negative electrode active material in which carbon nanofibers are attached to an active material nucleus capable of at least occluding and releasing lithium ions, and comprising a negative electrode mixture layer provided on the current collector,
The ratio of the volume in the charged state to the volume in the discharged state of the active material nucleus is 1.2 or more, and in the negative electrode mixture layer, the composite with a large amount of the carbon nanofiber attached on the side close to the current collector A negative electrode for a non-aqueous electrolyte secondary battery in which a negative electrode active material is distributed and a composite negative electrode active material with a small amount of carbon nanofiber attached is distributed on a side far from the current collector. The negative electrode mixture layer is
A first layer including a composite negative electrode active material provided on a side close to the current collector and having a large amount of carbon nanofiber attached;
2. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 1, further comprising: a second layer including a composite negative electrode active material provided on a side far from the current collector and having a small amount of carbon nanofibers attached thereto. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 2, wherein the first layer is configured to be thicker than the second layer. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 2, wherein the second layer is configured to be thicker than the first layer. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the active material nucleus is silicon-containing particles. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 5, wherein the silicon-containing particles are silicon oxide, and the ratio of oxygen to silicon is 0.3 or more and 1.3 or less. The nonaqueous electrolyte secondary battery provided with the negative electrode for nonaqueous electrolyte secondary batteries as described in any one of Claim 1 to 6.

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