JP2007227139A - Anode for nonaqueous electrolyte secondary battery, its manufacturing method, and nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an anode for nonaqueous electrolyte secondary batter in which an active substance nucleus having a high capacity density is used and moreover, a cycle characteristics is improved as expected by controlling a gas generation caused by a carbon fiber of a conductivity source, and a high capacity and a long service life are realized. <P>SOLUTION: The anode of a nonaqueous electrolyte secondary battery is composed of a mixture layer containing an active substance nucleus which can charge/discharge lithium ions and a composite anode active substance made of a carbon nano fiber (CNF) deposited on a surface of the above active substance nucleus and a CNF, whose impurities of the surface are removed, is used. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a high-capacity nonaqueous electrolyte secondary battery, and more particularly to a technique for improving cycle characteristics.

  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. Larger volume changes of 1.2 or more occur with these materials above / cm ³ . Due to this large volume change, the active material particles are pulverized, and as a result, the conductivity between the active material particles is lowered. Therefore, sufficient charge / discharge cycle characteristics (hereinafter referred to as “cycle characteristics”) are not obtained.

Thus, it has been proposed to combine a plurality of carbon fibers into composite particles by using particles containing a metal or semimetal capable of forming a lithium alloy as an active material nucleus. 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 nucleus changes (for example, Patent Document 1).
JP 2004-349056 A

  However, if carbon fibers shown in Patent Document 1 are only bonded to the active material core, the reaction area is reduced due to the presence of a large amount of gas generated between charge and discharge between the positive electrode and the negative electrode, and the battery capacity is rather reduced. May decrease.

  The present invention solves the above-mentioned problems, and improves cycle characteristics as expected by using an active material core having a high capacity density and suppressing the generation of gas derived from carbon fiber as a conductive source. Then, it aims at providing the negative electrode for nonaqueous electrolyte secondary batteries which implement | achieves a high capacity | capacitance and a long life nonaqueous electrolyte secondary battery.

  In order to solve the above problems, a negative electrode for a non-aqueous electrolyte secondary battery according to the present invention includes an active material nucleus capable of charging and discharging lithium ions, and a carbon nanofiber (CNF) attached to the surface of the active material nucleus. It is characterized by using a mixture layer containing a composite negative electrode active material comprising: and removing CNF surface impurities.

CNF is a useful material with high conductivity among carbon fibers, but when it is synthesized while adhering to the surface of the active material core, impurities such as moisture and low molecular weight organic matter are mixed slightly.
Adsorption / deposition may occur on F. Depending on the synthesis conditions, oxygen atoms and hydrogen atoms contained in the reaction atmosphere and the precursor may be present as impurities with a CHO group, a COOH group, and a CH ₃ group at the end of CNF. These impurities generate hydrogen gas, methane gas, carbon dioxide gas, and the like by reacting with the electrolytic solution during charging and discharging, and reducing the reaction area by being present between the positive electrode and the negative electrode as described above.

  As a result of intensive studies, the present inventors have found the mechanism of this problem, and have found that gas generation associated with charging / discharging is suppressed by removing impurities on CNF in advance.

  If the negative electrode for nonaqueous electrolyte secondary batteries of this invention is used, the nonaqueous electrolyte secondary battery which can implement | achieve high capacity | capacitance and practical cycling characteristics can be provided.

1st invention has the mixture layer containing the composite negative electrode active material which consists of the active material nucleus which can be charged / discharged lithium ion, and CNF adhering to the surface of this active material nucleus, The surface of CNF The present invention relates to a negative electrode for a non-aqueous electrolyte secondary battery, characterized in that an impurity is removed. CNF is a useful material with high conductivity among carbon fibers, but when it is synthesized while adhering it to the surface of the active material core, a slight amount of impurities such as moisture and low molecular weight organic substances are mixed and deposited on the CNF.・ It may be adsorbed. Depending on the synthesis conditions, oxygen atoms and hydrogen atoms contained in the reaction atmosphere and the precursor may be present as impurities with a CHO group, a COOH group, and a CH ₃ group at the end of CNF. These impurities generate hydrogen gas, methane gas, carbon dioxide gas, and the like by reacting with the electrolytic solution during charging and discharging, and reducing the reaction area by being present between the positive electrode and the negative electrode as described above. The present invention has discovered the mechanism of this problem, and by removing impurities on the CNF in advance, gas generation accompanying charge / discharge can be suppressed.

The second invention is characterized in that, in the first invention, CNF has high crystallinity. Not only does the conductivity increase due to the high crystallization of CNF, but the functional group on the surface of CNF disappears, so that impurities in the battery manufacturing process are less likely to adhere, so that gas generation associated with charge / discharge can be further suppressed. The high crystallinity in the second invention can be expressed by the following definition. That is, in Raman spectroscopy, the ratio R of the intensity of E _{1 g} spectrum observed in the vicinity of 1360 cm ^-1 to the intensity of the E '' _{2 g} spectrum observed near _{^{1580cm -1 (I 1580) (I}} 1360) (= I 1360 / I ₁₅₈₀ ) is 0.80 or less, it can be judged that CNF has high crystallinity. The spectrum of E _1g is not observed in perfect graphite, but reflects disorder of the crystal structure.

A third invention is characterized in that, in the first invention, 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. An active material nucleus in which a large volume change of 1.2 or more has a theoretical capacity density exceeding 833 mAh / cm ³ is preferable because a large capacity can be obtained as a composite negative electrode active material.

  A fourth invention is characterized in that, in the third invention, the active material nucleus contains silicon. Among the active material nuclei having a large theoretical capacity density, those containing silicon are preferable because the capacity density is particularly high.

A fifth invention is characterized in that, in the fourth invention, the active material nucleus is silicon oxide particles represented by SiO _x (0.05 <x <1.95). Silicon oxide particles whose active material nuclei are represented by SiO _x (0.05 <x <1.95) are preferable because they have a large theoretical capacity, are relatively inexpensive, and have high electrochemical stability.

  A sixth invention is characterized in that, in the first invention, CNF is attached to the active material nucleus by at least one kind of catalytic element selected from the group consisting of Cu, Fe, Co, Ni, Mo and Mn. And By using these elements as catalysts, CNF can be stably grown on the surface of the active material nucleus, and the grown CNF and the active material nucleus can be reliably adhered.

  The seventh invention relates to a non-aqueous electrolyte secondary battery using the negative electrode of claims 1 to 6. By using the negative electrode of claims 1 to 6, a non-aqueous electrolyte secondary battery having high capacity and high cycle characteristics can be obtained.

  According to an eighth aspect of the present invention, there is provided a first step of attaching CNF to the surface of an active material nucleus capable of occluding and releasing lithium ions, and a second step of improving CNF crystallinity while eliminating impurities attached to CNF at a high temperature. And a process for producing a negative electrode for a non-aqueous electrolyte secondary battery.

  Impurities adhering to the CNF surface disappear by a reduction reaction or the like when exposed to a high temperature (700 to 1500 ° C.) in an inert gas. However, when exposed to such a high temperature in the presence of the CNF raw material, the diameter of the CNF becomes thick, so the flexibility of the composite negative electrode active material is impaired, and the composite negative electrode active material expands and contracts with the cycle. In this case, the conductivity between the particles cannot be kept high. Therefore, by adding a further heat treatment step (second step) after the step of attaching CNF to the surface of the particles (first step), the impurities on the CNF surface disappear while keeping the composite negative electrode active material flexible. be able to. As a further effect of this second step, the functional group at the end of the graphene layer of CNF disappears and the crystallinity of CNF increases. This not only increases the conductivity of CNF, but also makes it difficult for impurities in the battery manufacturing process to adhere to the CNF surface, so that the cycle characteristics can be significantly improved.

  The first step is preferably performed at 400 to 650 ° C., and the second step is preferably performed at 600 to 1800 ° C. (the optimum range is 700 to 1500 ° C.). If the temperature of the first step is less than 400 ° C., CNF does not grow or grows too slowly and the productivity is impaired, and if it exceeds 650 ° C., decomposition of the raw material gas is promoted and CNF is hardly generated. Further, if the temperature in the second step is less than 600 ° C., impurities on the CNF surface cannot be sufficiently removed, and if it exceeds 1800 ° C., the adhesion between the active material nucleus and CNF is lowered. The second step is preferably performed in an inert gas atmosphere such as helium.

  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.

  FIG. 1 is a cross-sectional view showing an example of the nonaqueous electrolyte secondary battery of the present invention. FIG. 2 is a schematic view of the composite negative electrode active material 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 a case 6 using a gasket 4 and a lid 5 together with a 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. Polyvinylidene fluoride (PVDF), polytetrafluoroethylene, polyethylene, polypropylene, aramid resin, polyamide, polyimide, polyamideimide, polyacrylonitrile, polyacrylic acid, polyacrylic acid methyl ester, 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 (CMC) or 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. 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, a non-aqueous 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 membrane 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 electrolyte 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 non-aqueous electrolyte 3 include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAlCl ₄ , LiSbF ₆ , LiSCN, LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ), LiN (C ₂ F ₅ SO _{2) 2, LiAsF 6, LiB} 10 Cl 10, lower aliphatic lithium carboxylate, LiF, LiCl, LiBr, LiI, chloroborane lithium, bis (1,2-benzene diolate (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-olate-1-benzenesulfonic acid-O, O ′) borate salts such as lithium borate, lithium tetraphenylborate, etc. Salts used in batteries can be applied.

  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 (VC), 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, trifluoro Additives such as propylene carbonate, 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. The composite negative electrode active material 34 has at least an active material nucleus 35 capable of occluding and releasing lithium ions and a CNF 36 attached to the surface thereof. 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 (not shown). Examples of the binder include PVDF, polytetrafluoroethylene, polyethylene, polypropylene, aramid resin, polyamide, polyimide, polyamideimide, polyacrylonitrile, polyacrylic acid, polyacrylic acid methyl ester, polyacrylic acid ethyl ester, and polyacrylic. Acid hexyl ester, polymethacrylic acid, polymethacrylic acid methyl ester, polymethacrylic acid ethyl ester, polymethacrylic acid hexyl ester, polyvinyl acetate, polyvinylpyrrolidone, polyether, polyethersulfone, hexafluoropolypropylene, styrene butadiene rubber, CMC Etc. 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.

  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 nucleus 35, a material that can occlude and release a large amount of lithium ions at a lower potential than the positive electrode active material contained in the positive electrode mixture layer 8, such as silicon (Si) or tin (Sn), is used. be able to. 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. In particular, a silicon-containing material is preferable because it has a large capacity density and is inexpensive. 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.

  The CNF 36 is formed by growing a catalyst element (not shown) supported on the surface of the active material nucleus 35 as a nucleus. As the catalytic 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. , Promoting the growth of the second CNF 36. 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 desirable 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 Si element, Sn element and the like electrochemically react 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, an example of a method for producing 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 three 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 carbon-containing gas and hydrogen gas at 400 to 650 ° C. (corresponding to the first step in the eighth invention).

  (C) A step of firing the active material core 35 to which the CNF 36 is adhered at 700 to 1500 ° C. in an inert gas atmosphere (corresponding to the second step in the eighth invention).

After step (c), the composite negative electrode active material 34 is further heated to 100 ° C. or higher and 400 ° C. in the atmosphere.
In the following, the catalyst element may be oxidized by heat treatment. 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, and a combination of Si element or Sn element with the catalytic element Examples include a step of synthesizing alloy particles. However, step (a) is not limited to the above. There is also a method of obtaining the composite negative electrode active material 34 without going through this step.

  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 400 to 650 ° 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 lower than 400 ° 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 650 ° 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 600 to 1800 ° C. (more preferably 700 to 1500 ° C.) in an inert gas atmosphere. By doing so, impurities on the surface of the CNF 36 can be eliminated while the composite negative electrode active material 34 is flexible, and further, the functional group at the end of the graphene layer of the CNF 36 disappears, and the crystallinity of the CNF 36 increases. . The increase in crystallinity of CNF36 not only increases the conductivity of CNF36 but also makes it difficult for impurities in the battery manufacturing process to adhere to the surface of CNF36, so that the cycle characteristics can be significantly improved. If such a firing process is not performed or if the firing temperature is less than 600 ° C., impurities on the surface of the CNF 36 cannot be sufficiently removed, and if it exceeds 1800 ° C., the adhesion between the active material nucleus 35 and the CNF 36 Decreases.

  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.

Note that it is preferable to crush the fired active material core 35 to which the CNF 36 that has passed through the step (c) is attached because a composite negative electrode active material 34 with good filling properties is obtained. 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, specific examples in each embodiment of the present invention will be described. In this example, a cylindrical nonaqueous electrolyte secondary battery was configured instead of the coin type shown in FIG.

Example 1
(1) Production of positive electrode 3 kg of lithium cobalt oxide was stirred with a double-arm kneader together with 1 kg of PVDF # 1320 (NMP solution having a solid content of 12%) manufactured by Kureha Chemical Co., Ltd., 90 g of acetylene black and an appropriate amount of NMP. A positive electrode paste was prepared. This paste is intermittently applied to and dried on a current collector 7 made of aluminum having a thickness of 15 μm to form a positive electrode mixture layer 8 and then rolled so that the active material density of the positive electrode mixture layer 8 is 3.5 g / ml. After that, the positive electrode 2 was obtained by cutting into predetermined dimensions.

(2) Production of Negative Electrode As active material core 35, silicon oxide having an O / Si ratio (ratio of oxygen to silicon) pulverized to a particle size of 10 μm or less was 1.01 in terms of molar ratio.

  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 device so that the particle size is 1 nm to 1 nm in a uniform and highly dispersed state on the surface layer of the silicon oxide particles. 1000 nm iron nitrate was supported.

  Next, the active material nucleus 35 carrying iron nitrate was put into a ceramic reaction vessel and heated to 500 ° C. in the presence of helium gas (first step in the eighth invention). 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. The reaction time at this time was 15 minutes. As a result, iron nitrate was reduced and CNF 36 was grown on the surface of the active material core 35.

  Next, the mixed gas was replaced with helium gas again and held at 600 ° C. for 1 hour (second step in the eighth invention), and then the reaction vessel was cooled to room temperature, and the composite negative electrode active material 34 was Obtained.

As a result of observing the obtained composite negative electrode active material 34 with a scanning electron microscope (SEM), plate-like CNF 36 having a fiber diameter of about 50 nm and an average fiber length of 15 μm was observed on the surface of silicon oxide. Further, iron nitrate supported on silicon oxide was reduced to particulate metal having a particle size of about 100 nm as a result of confirmation by SEM (Hitachi scanning electron microscope apparatus S-4500). The weight ratio of CNF36 to silicon oxide was determined from the change in weight before and after the CNF36 was deposited, in a manner that ignores the weight of iron nitrate. As a result, the weight ratio of silicon oxide: CNF36 was 76:24. Furthermore, in order to confirm the crystallinity of CNF36, HORIBA
As a result of measurement using LabRAM HR-800 manufactured by JOBIN YVON, the R value of CNF36 was 0.80. In addition, the ratio of carbon atoms in CNF 36 was determined by the amount of carbon dioxide collected by combustion, and as a result, it was 87%.

  3 kg of the composite negative electrode active material 34 thus prepared was weighed and mixed with 750 g of a styrene-butadiene copolymer rubber particle binder BM-400B (solid content 40%) manufactured by Nippon Zeon Co., Ltd. The mixture was stirred in a combined machine to prepare a negative electrode paste. This paste was intermittently applied and dried on a current collector 10 made of copper having a thickness of 20 μm to form a negative electrode mixture layer 12, and then cut into predetermined dimensions to obtain a negative electrode 1. At this time, the active material density of the negative electrode mixture layer 12 was 1.0 g / ml.

(3) Production of non-aqueous electrolyte secondary battery The positive electrode 2 and the negative electrode 1 are overlapped with each other via a separator (Hypore: product name, manufactured by Asahi Kasei Co., Ltd.) made of a polyethylene microporous film having a thickness of 20 μm. And it cut | disconnected by predetermined length, and produced the electrode group.

Subsequently, an electrode group was inserted into a cylindrical battery case (18 mm in diameter, 65 mm in height, 17.85 mm in inner diameter) in which iron was nickel-plated, and then LiPF ₆ was added to the EC / DMC / EMC mixed solvent at 1 M. 5.0 g of an electrolytic solution in which 3% of VC was dissolved was added and sealed. The separator became non-aqueous electrolyte 3 by including this electrolytic solution. The non-aqueous electrolyte secondary battery (theoretical capacity 2800 mAh) produced in this way is referred to as Example 1.

(Examples 2 to 5)
Compared to Example 1, the temperature of the second step in the eighth invention was 600 ° C. to 700 ° C. (Example 2, the weight ratio of silicon oxide: CNF36 was 77:23, and the R value of the Raman spectrum was 0.00). 73, the ratio of carbon atoms in CNF36 is 91%), 1000 ° C. (Example 3, the weight ratio of silicon oxide: CNF36 is 79:21, the R value of Raman spectrum is 0.66, the ratio of carbon atoms in CNF36) 96%), 1500 ° C. (Example 4, weight ratio of silicon oxide: CNF36 is 80:20, R value of Raman spectrum is 0.55, the ratio of carbon atoms in CNF36 is 98%) and 1800 ° C. (implementation) Example 5, except that the weight ratio of silicon oxide: CNF36 was changed to 80:20, the R value of the Raman spectrum was 0.51, and the ratio of carbon atoms in CNF36 was 99%),施例 was prepared in the same manner as in the non-aqueous electrolyte secondary battery 1. Let this be Examples 2-5.

(Comparative example)
Compared to Example 1, the high temperature holding under helium gas performed after growing CNF 36 on the surface of the active material nucleus 35 was not performed (the weight ratio of silicon oxide: CNF 36 was 75:25, the R value of the Raman spectrum) Was 0.86, and the ratio of carbon atoms in CNF 36 was 85%). A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1. This is a comparative example.

For each battery described above, charging / discharging was repeated 300 times under the following conditions. The ratio of the discharge capacity at the 300th cycle to the discharge capacity at the second cycle is shown in Table 1 as a measure of the cycle characteristics.
Constant current charging: Charging current value 1960mA / end-of-charge voltage 4.2V
Constant voltage charging: Charging voltage value 4.2V / end-of-charge current 100mA
Constant current discharge: discharge current 2800 mA / discharge end voltage 2.5 V

In the comparative example using the composite negative electrode active material 34 in which impurities are attached to the surface of the CNF 36 and the crystallinity of the CNF 36 remains low without performing the second step in the eighth invention, the CNF 36 is attached to the active material nucleus 35. In spite of increasing the conductivity, the cycle characteristics as expected were not achieved. The reason for this is that impurities adhering to the surface of CNF 36 during charge / discharge react with the electrolyte to produce hydrogen gas, methane gas, carbon dioxide gas, etc., and the reaction area is interposed between the positive electrode 2 and the negative electrode 1. It is thought that it decreased gradually.

  In contrast to this comparative example, each example in which impurities attached to the surface of the CNF 36 through the second step in the eighth invention were removed and the CNF 36 was highly crystallized showed excellent cycle characteristics across the board. However, in Example 2 where the temperature in the second step was 600 ° C. and Example 5 where the temperature was 1800 ° C., the cycle characteristics slightly decreased as compared with Examples 2 to 4. The reason for this is that in Example 1, the impurities on the surface of the CNF 36 could not be sufficiently removed, and the crystallinity of the CNF 36 was insufficient and some functional groups remained, so that the composite negative electrode active material 34 was produced. It is thought that CNF36 took in a little impurity after that. On the other hand, in Example 5, since the temperature was slightly high, it is considered that the adhesion between the active material nucleus 35 and the CNF 36 was slightly lowered. From this result, it is understood that the second step in the eighth invention is more preferably performed at 700 to 1500 ° C.

  In this embodiment, the case where iron (Fe) is used as a catalytic element is shown, but even when copper (Cu), cobalt (Co), nickel (Ni), molybdenum (Mo) and manganese (Mn) are used. It goes without saying that CNF can be grown as well.

  The present invention uses a composite negative electrode active material in which CNF from which impurities are removed is attached to an active material core in which the ratio of the volume in the charged state to the volume in the discharged state is 1.2 or more, capable of occluding and releasing lithium ions Thus, a negative electrode mixture layer is formed. With such a configuration, a nonaqueous electrolyte secondary battery having practical cycle characteristics can be obtained. That is, the present invention contributes to improving the cycle characteristics of a nonaqueous electrolyte secondary battery using a composite negative electrode active material in which CNF is attached to an active material nucleus having a higher capacity density than that of a carbon material. 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 which shows an example of the nonaqueous electrolyte secondary battery of this invention Schematic of the composite negative electrode active material of the present invention

Explanation of symbols

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

Claims (8)

An active material core capable of occluding and releasing lithium ions;
A negative electrode for a non-aqueous electrolyte secondary battery having a mixture layer containing a composite negative electrode active material composed of carbon nanofibers attached to the surface of the active material nucleus,
A negative electrode for a non-aqueous electrolyte secondary battery using the carbon nanofiber from which impurities on the surface are removed. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the carbon nanofiber has high crystallinity. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein a ratio of a volume in a charged state to a volume in a discharged state of the active material nucleus is 1.2 or more. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 3, wherein the active material nucleus contains silicon. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 4, wherein the active material nucleus is silicon oxide particles represented by SiO _x (0.05 <x <1.95). 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the carbon nanofibers are attached to the active material core by at least one catalytic element selected from the group consisting of Cu, Fe, Co, Ni, Mo, and Mn. Negative electrode. A nonaqueous electrolyte secondary battery using the negative electrode according to claim 1. A first step of attaching carbon nanofibers to the surface of an active material nucleus capable of occluding and releasing lithium ions;
A method for producing a negative electrode for a non-aqueous electrolyte secondary battery, comprising: a second step of increasing the crystallinity of the carbon nanofiber while eliminating impurities adhering to the carbon nanofiber at high temperature.