JP2018032602A - Method of producing negative electrode material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve a further improvement in function of a negative electrode material containing a silicon material.SOLUTION: A method of producing a negative electrode material includes a processing step of subjecting a silicon material to carbon coating and lithium doping, the silicon material having a structure where a plurality of plate-like silicon bodies are stacked in a thickness direction.SELECTED DRAWING: None

Description

  The present invention relates to a method for producing a negative electrode material.

  As a negative electrode active material for a lithium ion secondary battery, a negative electrode active material containing Si having a high lithium ion storage capacity is known. For example, Patent Literature 1 and Patent Literature 2 describe lithium ion secondary batteries in which the negative electrode active material is silicon. Patent Literature 3 and Patent Literature 4 describe lithium ion secondary batteries in which the negative electrode active material is SiO.

  For the purpose of further improving the battery performance of the lithium ion secondary battery, a search for a new negative electrode active material containing Si is in progress. As a new negative electrode active material containing Si, Patent Document 5 discloses a silicon material having a structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction. Patent Document 5 further describes that a carbon-silicon composite was manufactured by coating the silicon material with carbon, and that the composite can be used as a negative electrode active material of a lithium ion secondary battery. Has been.

JP 2014-203595 A Japanese Patent Laying-Open No. 2015-57767 JP 2015-185509 A JP-A-2015-179625 International Publication No. 2015/114692

  By the way, in recent years, a lithium ion secondary battery having better battery performance has been demanded. As for the negative electrode active material for a lithium ion secondary battery, various functions are desired to be improved in order to contribute to the improvement of the battery performance of the lithium ion secondary battery. Similarly, further improvement of the function is desired for the silicon material disclosed in Patent Document 5.

  This invention is made | formed in view of this situation, and aims at aiming at the further improvement of a function about the negative electrode material containing a silicon material.

The method for producing the negative electrode material of the present invention is as follows.
This is a method for producing a negative electrode material, which includes a treatment step of carbon coating and lithium doping a silicon material having a structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction.

  According to the method for producing a negative electrode material of the present invention, the function of the negative electrode material containing a silicon material can be improved.

3 is a SEM image of a negative electrode material of Example 2. It is an EDS analysis result for Si in the negative electrode material of Example 2. It is an EDS analysis result for O in the negative electrode material of Example 2. It is an EDS analysis result for C in the negative electrode material of Example 2.

  The best mode for carrying out the present invention will be described below. Unless otherwise specified, the numerical range “x to y” described in this specification includes the lower limit x and the upper limit y. The numerical range can be configured by arbitrarily combining these upper limit value and lower limit value and the numerical values listed in the examples. Furthermore, numerical values arbitrarily selected from these numerical ranges can be used as new upper and lower numerical values.

The negative electrode material manufactured by the negative electrode material manufacturing method of the present invention is obtained by carbon coating and lithium doping on the silicon material disclosed in Patent Document 5 described above.
Therefore, the manufacturing method of the negative electrode material of this invention has the process process which carries out carbon coating and lithium dope to the said silicon material. Hereinafter, if necessary, the production method of the negative electrode material of the present invention is simply abbreviated as the production method of the present invention, and the negative electrode material produced by the production method of the present invention is referred to as the negative electrode material of the present invention. Furthermore, the negative electrode material of the present invention has a carbon-containing portion formed by the above carbon coating. Hereinafter, this portion is referred to as a carbon coat layer.

The silicon material disclosed in Patent Document 5 (hereinafter simply referred to as “silicon material”) will be described in detail. For example, the silicon material may include a step of reacting CaSi ₂ with an acid to synthesize a layered silicon compound containing polysilane as a main component, and a step of heating the layered silicon compound at 300 ° C. or higher to release hydrogen. It is manufactured after that.

The method for producing a silicon material described in Patent Document 5 is represented as follows by using an ideal reaction formula when hydrogen chloride is used as the acid.
3CaSi ₂ + 6HCl → Si ₆ H ₆ + 3CaCl ₂
Si ₆ H ₆ → 6Si + 3H ₂ ↑

However, in the upper reaction for synthesizing Si ₆ H ₆ which is polysilane, water is usually used as a reaction solvent from the viewpoint of removing by-products and impurities. Since Si ₆ H ₆ can react with water, in the step of synthesizing the layered silicon compound including the upper reaction, the layered silicon compound is hardly manufactured as containing only Si ₆ H ₆ , and the layered The silicon compound is represented by Si ₆ H _s (OH) _t X _u (X is an element or group derived from an acid anion, s + t + u = 6, 0 <s <6, 0 <t <6, 0 <u <6). Manufactured as a product. In the above chemical formula, inevitable impurities such as Ca that can remain are not taken into consideration. A silicon material obtained by heating the layered silicon compound also contains an element derived from an anion of oxygen or acid.

The silicon material has a structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction. For efficient insertion and desorption reaction of charge carriers such as lithium ions, the plate-like silicon body preferably has a thickness in the range of 10 nm to 100 nm, more preferably in the range of 20 nm to 50 nm. . The length in the longitudinal direction of the plate-like silicon body is preferably within a range of 0.1 μm to 50 μm. The plate-like silicon body preferably has (length in the longitudinal direction) / (thickness) in the range of 2 to 1000. The laminated structure of the plate-like silicon body can be confirmed by observation with a scanning electron microscope or the like. This laminated structure is considered to be a remnant of the Si layer in the raw material CaSi ₂ .

  The silicon material preferably includes amorphous silicon and / or silicon crystallites. In particular, the above plate-like silicon body is preferably in a state where amorphous silicon is used as a matrix and silicon crystallites are scattered in the matrix. The size of the silicon crystallite is preferably in the range of 0.5 nm to 300 nm, more preferably in the range of 1 nm to 100 nm, still more preferably in the range of 1 nm to 50 nm, and particularly preferably in the range of 1 nm to 10 nm. The size of the silicon crystallite is calculated from the Scherrer equation using X-ray diffraction measurement on the silicon material and using the half-value width of the diffraction peak of the Si (111) plane of the obtained X-ray diffraction chart. .

  The abundance and size of the plate-like silicon body, amorphous silicon and silicon crystallite contained in the silicon material mainly depend on the heating temperature and the heating time. The heating temperature is preferably in the range of 350 ° C to 950 ° C, more preferably in the range of 400 ° C to 900 ° C.

  The average particle diameter of the silicon material or the negative electrode material of the present invention when measured with a general laser diffraction particle size distribution measuring device is preferably in the range of 0.6 to 30 μm, more preferably in the range of 1 to 20 μm. The range of 2 to 10 μm is more preferable, and the range of 3 to 8 μm is particularly preferable. In the present specification, the average particle diameter means a 50% cumulative diameter (D50).

  The negative electrode material of the present invention is produced by further doping the above silicon material with carbon coating and lithium doping. In the manufacturing method of the present invention, the step of carbon coating and lithium doping on a silicon material is called a processing step. Among these, the carbon coating process is called a coating process.

  A silicon material is inferior in electroconductivity compared with common negative electrode active materials, such as graphite. Therefore, when a silicon material is used as the negative electrode active material, it is considered good to improve the conductivity of the whole negative electrode material by using a conductive material such as a conductive aid in combination with the silicon material. In addition, the conductivity of the negative electrode material can be further improved by integrating the conductive material with the silicon material and bringing the silicon material and the conductive material into close contact with each other. Specifically, if the silicon material is carbon-coated by a known method such as chemical vapor deposition, a carbon coat layer can be easily and uniformly formed on the surface of the silicon material.

  If the silicon material is further doped with lithium in addition to the carbon coating described above, the irreversible capacity of the silicon material is canceled, so that there is an advantage that the initial efficiency of the lithium ion secondary battery is improved. Furthermore, if the silicon material as the negative electrode active material is lithium-doped, the charge carrier can be efficiently supplied to the negative electrode active material because the negative electrode active material and the charge carrier are present in close proximity.

  Thus, according to the manufacturing method of the present invention, the function as a conductive agent can be imparted to the negative electrode material by coating the silicon material with carbon. In addition, the lithium material can be provided with a function of supplying lithium ions to the silicon material by doping the silicon material with lithium. That is, according to the manufacturing method of the present invention including such processing steps, the function of the negative electrode material can be improved.

  In the production method of the present invention, lithium doping and carbon coating on the silicon material in the treatment step may be performed by any method. In the production method of the present invention, a mechanical milling method or a molten salt method can be adopted as a lithium doping method. In the production method of the present invention, the step of lithium doping by the mechanical milling method is called a mechanical milling doping step, and the step of lithium doping by the molten salt method is called a molten salt doping step.

  Mechanical milling is a method in which silicon material and lithium source are chemically reacted by mechanically mixing silicon material and lithium source and applying external force such as impact, tension, friction, compression, and shear. Point to. Equipment for mechanically mixing silicon materials and lithium sources in the mechanical milling method includes rolling mills, vibration mills, planetary mills, rocking mills, horizontal mills, attritor mills, jet mills, crushers, and homogenizers. , Fluidizer, paint shaker, mixer and the like. In addition, mechanofusion (surface melting) is also included in the mechanical milling method in the present invention. The magnitude of external force, load time, temperature, and the like in the mechanical milling method may be based on a regular method.

  The molten salt method refers to a method in which a silicon material and a lithium source in a molten state are brought into contact with each other to chemically react the silicon material and the lithium source. In this case, it is necessary to dope lithium in a state where the silicon material is not melted and the lithium source is melted. The conditions of the molten salt method may also be based on a conventional method.

  As the lithium source used for lithium doping, lithium alone or various lithium compounds may be used. As the lithium compound, those that do not bring many elements other than lithium into the negative electrode, specifically those that easily vaporize constituent elements other than lithium, are preferred, such as lithium hydroxide, lithium hydride, lithium oxide, lithium chloride, etc. Preferably used. Other examples of the lithium source include lithium oxoacids such as lithium acetate, lithium carbonate, and lithium nitrate.

When the molten salt method is selected as the lithium doping method, it is necessary to dope lithium in a state where the silicon material is not melted and the lithium source is melted as described above. Therefore, in this case, it is preferable to use a lithium source having a melting point lower than that of the silicon material. Specifically, the melting point of the lithium source is preferably less than 1000 ° C., more preferably 900 ° C. or less, and further preferably 750 ° C. or less. If the temperature exceeds 900 ° C., SiO contained in the silicon material may be disproportionated into Si and SiO ₂ and the silicon material may be altered. Therefore, the molten salt doping step is preferably performed at 900 ° C. or lower. The melting points of the above lithium sources are all 750 ° C. or lower. For reference, the melting point of Si is 1414 ° C., and the melting point of SiO ₂ is 1650 ± 75 ° C.

  In the molten salt method, the molten lithium source and the silicon material may be brought into contact with each other by heating the mixture of the lithium source and the silicon material to a temperature not lower than the melting point of the lithium source and lower than the melting point of the silicon material. . Alternatively, the molten lithium source may be brought into contact with the silicon material by adding the silicon material to the molten lithium source.

  In any case, the reaction is preferably performed in an inert gas such as argon gas or helium gas, that is, in an inert atmosphere.

  The amount of lithium doped into the silicon material is not particularly limited, but is preferably equal to or more than the amount corresponding to the irreversible capacity. In consideration of the capacity of the negative electrode, the amount of lithium with respect to the amount of silicon material should not be excessive. In order to dope more lithium into the silicon material, the amount of lithium with respect to the amount of silicon material should not be too small. Taking these into consideration, the molar ratio Li / Si of lithium atoms to Si atoms is preferably 0.01 or more and 0.7 or less, and more preferably 0.02 or more and 0.6 or less.

  As the carbon coating method, in addition to the above-described chemical vapor deposition (CVD), a physical vapor deposition method such as a vacuum vapor deposition method, a sputtering method, an ion plating method, or the like can be employed. Other methods can also be adopted. The carbon coat layer may be formed only on a part of the surface of the silicon material, may be formed on the entire surface of the silicon material, or may partially enter the silicon material.

  The material of the carbon coat layer, that is, the carbon source, preferably contains carbon and does not bring many elements other than carbon into the negative electrode, and the state thereof is not particularly limited such as solid or gaseous. Examples of the carbon source include carbon simple substance, hydrocarbon gas, and organic polymer. When the CVD method is adopted as the carbon coating method, it is preferable to select a hydrocarbon gas such as methane, ethane, propane, or butane from the viewpoint of work efficiency.

  The thickness of the carbon coat layer is not particularly limited, but a thicker one is preferable in consideration of durability and conductivity of the negative electrode material. On the other hand, considering the capacity maintenance and ion conductivity of the negative electrode material, it is preferable that the carbon coat layer is thin. This is because when the amount of the carbon coat layer in the negative electrode material is excessive, the amount of the silicon material is relatively reduced, so that the capacity of the negative electrode material is reduced and lithium ions are difficult to pass. Taking these into consideration, the thickness of the carbon coat layer is preferably 1 to 100 nm, more preferably 10 to 50 nm, and even more preferably 5 to 50 nm.

  The carbon coat layer may contain only carbon or may contain other elements. For example, the carbon coat layer may contain at least one metal atom selected from transition metals. Since the metal atom improves the conductivity in the carbon coat layer, the lithium ion conductivity in the negative electrode is improved. As a metal atom selected from transition metals, Cu, Fe, Ni and the like are preferable, and Cu is particularly preferable. The metal atom content in the carbon coat layer is preferably in the range of 0.1 to 10% by mass.

  Either lithium doping or carbon coating may be performed first, or both may be performed simultaneously. However, as described later, it is preferable to first perform carbon coating and then perform lithium doping by a molten salt method.

  By the way, when lithium doping is performed on a silicon material by a mechanical milling method, a relatively large external force acts on the silicon material. Accordingly, when a silicon material previously coated with carbon is subjected to lithium doping by a mechanical milling method, the carbon coating layer may be deteriorated due to external force. For this reason, when the mechanical milling method is selected as the lithium doping method, carbon coating is preferably performed after lithium doping.

When the molten salt method is selected as the lithium doping method, the carbon coat layer is hardly deteriorated by the above-described external force. Considering this point, when the molten salt method is adopted as the lithium doping method, either carbon coating or lithium doping may be performed first. However, as described later, when carbon coating is performed on a lithium-doped silicon material using a SiO ₂ container such as a quartz tube, lithium doped in the silicon material reacts with SiO ₂ constituting the container. There is a risk that. In this case, an electrically inactive reaction product may be generated, which may cause a decrease in the capacity of the negative electrode. For this reason, carbon coating is preferably performed before lithium doping.

  Although not included in the manufacturing method of the present invention, the same treatment process may be applied to the Si-containing negative electrode active material other than the silicon material. Also in this case, the negative electrode material whose function is improved than before can be obtained by the carbon coat layer and lithium doping. In this case as well, the functionality of the negative electrode material can be further improved by performing lithium doping by a molten salt method after carbon coating.

  The negative electrode material of the present invention can be used for a negative electrode for a lithium ion secondary battery. Hereinafter, the lithium ion secondary battery including the negative electrode material of the present invention is referred to as the lithium ion secondary battery of the present invention as necessary. In addition, if necessary, the negative electrode and the positive electrode are collectively referred to as an electrode, the negative electrode active material and the positive electrode active material are collectively referred to as an active material, and the negative electrode active material layer and the positive electrode active material layer are comprehensively included. Called the active material layer.

  The lithium ion secondary battery of the present invention includes a positive electrode, an electrolytic solution, and a separator in addition to the negative electrode including the negative electrode material of the present invention.

  The negative electrode has a current collector and a negative electrode active material layer bound to the surface of the current collector.

  The current collector refers to a chemically inert electronic conductor that keeps a current flowing through an electrode during discharge or charging of a lithium ion secondary battery. As the current collector, at least one selected from silver, copper, gold, aluminum, tungsten, cobalt, zinc, nickel, iron, platinum, tin, indium, titanium, ruthenium, tantalum, chromium, molybdenum, and stainless steel, etc. Metal materials can be exemplified. The current collector may be covered with a known protective layer. What collected the surface of the electrical power collector by the well-known method may be used as an electrical power collector.

  The current collector can take the form of a foil, a sheet, a film, a linear shape, a rod shape, a mesh, or the like. Therefore, for example, a metal foil such as a copper foil, a nickel foil, an aluminum foil, and a stainless steel foil can be suitably used as the current collector. When the current collector is in the form of foil, sheet or film, the thickness is preferably in the range of 1 μm to 100 μm.

  The negative electrode active material layer includes the above-described negative electrode material, and contains a silicon material contained in the negative electrode material as a negative electrode active material. The lithium ion secondary battery of this invention may contain only a silicon material as a negative electrode active material, and may use well-known negative electrode active materials, such as graphite, with a silicon material.

  A negative electrode active material layer contains a conductive support agent and / or a binder as needed besides a negative electrode material.

  The conductive assistant is added to increase the conductivity of the electrode. Therefore, the conductive auxiliary agent may be added arbitrarily when the electrode conductivity is insufficient, and may not be added when the electrode conductivity is sufficiently excellent. The conductive auxiliary agent may be any chemically inert electronic high conductor such as carbon black, graphite, vapor grown carbon fiber (VGCF), and various metal particles. Illustrated. Examples of carbon black include acetylene black, ketjen black (registered trademark), furnace black, and channel black. These conductive assistants can be added to the active material layer alone or in combination of two or more.

  The blending ratio of the conductive additive in the active material layer is preferably a mass ratio of active material: conductive additive = 1: 0.005 to 1: 0.5, and 1: 0.01 to 1: 0. .2 is more preferable, and 1: 0.03 to 1: 0.1 is even more preferable. This is because if the amount of the conductive auxiliary is too small, an efficient conductive path cannot be formed, and if the amount of the conductive auxiliary is too large, the moldability of the active material layer is deteriorated and the energy density of the electrode is lowered.

  The binder plays a role of securing an active material or a conductive auxiliary agent to the surface of the current collector and maintaining a conductive network in the electrode. Examples of the binder include fluorine-containing resins such as polyvinylidene fluoride, polytetrafluoroethylene, and fluororubber, thermoplastic resins such as polypropylene and polyethylene, imide resins such as polyimide and polyamideimide, alkoxysilyl group-containing resins, poly ( Examples thereof include acrylic resins such as (meth) acrylic acid, styrene-butadiene rubber (SBR), and carboxymethyl cellulose. These binders may be used singly or in plural.

  In addition, as the binder, a polymer having a hydrophilic group such as a carboxyl group may be employed. The lithium ion secondary battery of the present invention comprising a polymer having a hydrophilic group as a binder can maintain the capacity more suitably. Examples of the polymer having a hydrophilic group include carboxyl group-containing polymers such as polyacrylic acid and polymethacrylic acid. As the binder, a cross-linked polymer obtained by cross-linking these carboxyl group-containing polymers with diamine, and a mixture or a reaction product of a carboxyl group-containing polymer and a polyamideimide are preferable. This type of binder can be produced and used by the method disclosed in WO2016 / 063882.

  The blending ratio of the binder in the active material layer is preferably a mass ratio of active material: binder = 1: 0.001 to 1: 0.3, and 1: 0.005 to 1: 0. .2 is more preferable, and 1: 0.01 to 1: 0.15 is still more preferable. This is because when the amount of the binder is too small, the moldability of the electrode is lowered, and when the amount of the binder is too large, the energy density of the electrode is lowered.

  In order to form an active material layer on the surface of the current collector, a current collecting method such as a roll coating method, a die coating method, a dip coating method, a doctor blade method, a spray coating method, or a curtain coating method can be used. An active material may be applied to the surface of the body. Specifically, an active material, a solvent, and, if necessary, a binder and / or a conductive aid are mixed to prepare a slurry. Examples of the solvent include N-methyl-2-pyrrolidone, methanol, methyl isobutyl ketone, and water. The slurry is applied to the surface of the current collector and then dried. In order to increase the electrode density, the dried product may be compressed.

  The positive electrode has a current collector and a positive electrode active material layer bound to the surface of the current collector. Further, the positive electrode active material layer includes a positive electrode active material and, if necessary, a conductive additive and / or a binder. The current collector, conductive additive and binder are the same as those of the negative electrode.

As the positive electrode active material, the general formula of the layered rock salt _{structure: Li a Ni b Co c Mn} d D e O f (0.2 ≦ a ≦ 2, b + c + d + e = 1,0 ≦ e <1, D is W, Mo, Re, Pd, Ba, Cr, B, Sb, Sr, Pb, Ga, Al, Nb, Mg, Ta, Ti, La, Zr, Cu, Ca, Ir, Hf, Rh, Fe, Ge, Zn, Ru, At least one element selected from Sc, Sn, In, Y, Bi, S, Si, Na, K, P, and V, 1.7 ≦ f ≦ 3), a lithium mixed metal oxide represented by Li ₂ MnO ₃ can be mentioned. Further, as a positive electrode active material, a solid solution composed of a spinel such as LiMn ₂ O ₄ and Li ₂ Mn ₂ O ₄ and a mixture of a spinel and a layered compound, LiMPO ₄ , LiMVO _4, or Li ₂ MSiO ₄ (M in the formula) Are selected from at least one of Co, Ni, Mn, and Fe). Furthermore, as the positive electrode active material, tavorite compound (the M a transition metal) LiMPO ₄ F, such as LiFePO ₄ F represented by, Limbo ₃ such LiFeBO ₃ (M is a transition metal) include borate-based compound represented by be able to. Any metal oxide used as the positive electrode active material may have the above composition formula as a basic composition, and a metal element contained in the basic composition may be substituted with another metal element. Further, as a positive electrode active material, a positive electrode active material that does not contain lithium ions that contribute to charge / discharge, for example, sulfur alone, a compound in which sulfur and carbon are combined, a metal sulfide such as TiS ₂ , V ₂ O ₅ , MnO ₂ and other oxides, polyaniline and anthraquinone, compounds containing these aromatics in the chemical structure, conjugated materials such as conjugated diacetate-based organic substances, and other known materials can also be used. Further, a compound having a stable radical such as nitroxide, nitronyl nitroxide, galvinoxyl, phenoxyl, etc. may be adopted as the positive electrode active material. When using a positive electrode active material that does not contain lithium, in addition to the above-described lithium doping of the negative electrode material, the positive electrode may be further doped with lithium by a known method.

From the viewpoint of excellent and high capacity, and durability, as the positive electrode active material, the general formula of the layered rock salt _{structure: Li a Ni b Co c Mn} d D e O f (0.2 ≦ a ≦ 2, b + c + d + e = 1,0 ≦ e <1, D is W, Mo, Re, Pd, Ba, Cr, B, Sb, Sr, Pb, Ga, Al, Nb, Mg, Ta, Ti, La, Zr, Cu, Ca, Ir, Hf, At least one element selected from Rh, Fe, Ge, Zn, Ru, Sc, Sn, In, Y, Bi, S, Si, Na, K, P, V, 1.7 ≦ f ≦ 3) It is preferable to employ a lithium composite metal oxide.

  In the above general formula, the values of b, c, and d are not particularly limited as long as the above conditions are satisfied, but those in which 0 <b <1, 0 <c <1, 0 <d <1 are satisfied. And at least one of b, c, and d is in the range of 10/100 <b <90/100, 10/100 <c <90/100, 5/100 <d <70/100. More preferably, the ranges are 20/100 <b <80/100, 12/100 <c <70/100, 10/100 <d <60/100, 30/100 <b <70/100, More preferably, the ranges are 15/100 <c <50/100 and 12/100 <d <50/100.

  About a, e, and f, what is necessary is just a numerical value within the range prescribed | regulated by the said general formula, Preferably 0.5 <= a <= 1.5, 0 <= e <0.2, 1.8 <= f <= 2 0.5, more preferably 0.8 ≦ a ≦ 1.3, 0 ≦ e <0.1, 1.9 ≦ f ≦ 2.1, respectively.

  The electrolytic solution includes a nonaqueous solvent and an electrolyte dissolved in the nonaqueous solvent.

  As the non-aqueous solvent, cyclic esters, chain esters, ethers and the like can be used. Examples of cyclic esters include ethylene carbonate, propylene carbonate, butylene carbonate, gamma butyrolactone, vinylene carbonate, 2-methyl-gamma butyrolactone, acetyl-gamma butyrolactone, and gamma valerolactone. Examples of chain esters include dimethyl carbonate, diethyl carbonate, dibutyl carbonate, dipropyl carbonate, ethyl methyl carbonate, propionic acid alkyl ester, malonic acid dialkyl ester, and acetic acid alkyl ester. Examples of ethers include tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, and 1,2-dibutoxyethane. As the non-aqueous solvent, a compound in which part or all of hydrogen in the chemical structure of the specific solvent is substituted with fluorine may be employed.

Examples of the electrolyte include lithium salts such as LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , and LiN (CF ₃ SO ₂ ) ₂ .

As an electrolytic solution, 0.5 mol / L to 1.7 mol of a lithium salt such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO _{3 in} a nonaqueous solvent such as ethylene carbonate, dimethyl carbonate, propylene carbonate, and diethyl carbonate. A solution dissolved at a concentration of about / L can be exemplified.

  The separator separates the positive electrode and the negative electrode and allows lithium ions to pass while preventing a short circuit due to contact between the two electrodes. As separators, natural resins such as polytetrafluoroethylene, polypropylene, polyethylene, polyimide, polyamide, polyaramid (Aromatic polymer), polyester, polyacrylonitrile and other polysaccharides, cellulose, amylose and other polysaccharides, fibroin, keratin, lignin and suberin Examples thereof include porous bodies, nonwoven fabrics, and woven fabrics using one or more electrically insulating materials such as polymers and ceramics. The separator may have a multilayer structure.

  Next, the manufacturing method of a lithium ion secondary battery is demonstrated.

  A separator is sandwiched between a positive electrode and a negative electrode to form an electrode body. The electrode body may be any of a stacked type in which a positive electrode, a separator, and a negative electrode are stacked, or a wound type in which a stacked positive electrode, a separator, and a negative electrode are provided. After connecting the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal connected to the outside using a current collecting lead or the like, an electrolyte is added to the electrode body to form a lithium ion secondary battery. Good. Moreover, the lithium ion secondary battery of this invention should just perform charging / discharging in the voltage range suitable for the kind of active material contained in an electrode.

  The shape of the lithium ion secondary battery of the present invention is not particularly limited, and various shapes such as a cylindrical shape, a square shape, a coin shape, and a laminate shape can be adopted.

  The lithium ion secondary battery of the present invention may be mounted on a vehicle. The vehicle may be a vehicle that uses electric energy from a lithium ion secondary battery for all or a part of its power source, and may be, for example, an electric vehicle or a hybrid vehicle. When a lithium ion secondary battery is mounted on a vehicle, a plurality of lithium ion secondary batteries may be connected in series to form an assembled battery. Examples of devices equipped with lithium ion secondary batteries include various home appliances driven by batteries such as personal computers and portable communication devices, office devices, and industrial devices in addition to vehicles. Furthermore, the lithium ion secondary battery of the present invention includes wind power generation, solar power generation, hydroelectric power generation and other power system power storage devices and power smoothing devices, power supplies for ships and / or auxiliary power supply sources, aircraft, Power supply for spacecraft and / or auxiliary equipment, auxiliary power supply for vehicles that do not use electricity as a power source, power supply for mobile home robots, power supply for system backup, power supply for uninterruptible power supply, You may use for the electrical storage apparatus which stores temporarily the electric power required for charge in the charging station for electric vehicles.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to the said embodiment. The present invention can be implemented in various forms without departing from the gist of the present invention, with modifications and improvements that can be made by those skilled in the art.

  Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. In addition, this invention is not limited by these Examples.

Example 1
(Manufacture of negative electrode materials)
(Silicon material manufacturing process)
CaSi ₂ was added to an aqueous 36% by mass HCl solution in an ice bath and stirred under an argon gas atmosphere. The reaction solution was filtered, the residue was washed with distilled water and acetone, and further dried under reduced pressure to separate the layered silicon compound containing polysilane. The layered silicon compound was heated at 900 ° C. for 1 hour under an argon gas atmosphere to obtain a silicon material. This silicon material was pulverized with a jet mill NJ-30 (Aisin Nano Technologies Co., Ltd.) to obtain a particulate silicon material.

(Processing process)
(Coating process)
Particulate silicon material obtained in the silicon material manufacturing process is put into a rotary kiln type reactor, and is kept under a condition of 880 ° C. and residence time of 3 hours under aeration of a propane-argon mixed gas (propane 20%, argon 80%). Thermal CVD was performed. The furnace core tube of the reactor is made of quartz and is disposed in the horizontal direction, and the rotation speed is 1 rpm. A baffle plate is disposed on the inner peripheral wall of the core tube, and the contents accumulated on the baffle plate with rotation are configured to fall from the baffle plate when reaching a predetermined height. With this configuration, the contents in the reactor are agitated. After 3 hours, the reactor was cooled to 25 ° C. over 12 hours under a stream of argon, and sieved to obtain a carbon-coated silicon material.

(Molten salt dope process)
A mixture of 75 parts by mass of the carbon-coated silicon material obtained in the coating step and 25 parts by mass of lithium hydroxide monohydrate was heated at 850 ° C. in an inert atmosphere. Since the temperature at this time was higher than the melting point of lithium hydroxide and lower than the melting point of the carbon-coated silicon material, the lithium hydroxide melted and the carbon-coated silicon material did not melt. Therefore, the carbon-coated silicon material was in contact with the molten lithium hydroxide, and lithium derived from lithium hydroxide was doped into the carbon-coated silicon material.

  The negative electrode material of Example 1 which was carbon-coated and lithium-doped was obtained by the above treatment steps. This negative electrode material was placed in 1M HCl and subjected to ultrasonic cleaning for 1 hour. Thereafter, it was washed with water, filtered and dried, and used for the production of the following lithium ion secondary battery.

(Manufacture of lithium ion secondary batteries)
72.5 parts by mass of a carbon-coated silicon material as a negative electrode material, 13.5 parts by mass of acetylene black as a conductive additive, 14 parts by mass of a mixture of polyacrylic acid and 4,4′-diaminodiphenylmethane as a binder, and an appropriate amount N-methyl-2-pyrrolidone was mixed to prepare a slurry. A copper foil was prepared as a negative electrode current collector. The slurry was applied in a film form on the surface of the copper foil using a doctor blade. The copper foil to which the slurry was applied was dried to remove N-methyl-2-pyrrolidone, and then the current collector and the negative electrode active material layer were firmly adhered and bonded by a roll press machine. This was heated and dried at 200 ° C. under reduced pressure to produce a negative electrode. The mixture of polyacrylic acid and 4,4′-diaminodiphenylmethane used as the binder undergoes a dehydration reaction by drying at 200 ° C., and polyacrylic acid is converted into 4,4′-diaminodiphenylmethane. It changes to a crosslinked polymer.

  A lithium ion secondary battery (half cell) was produced using the negative electrode produced by the above procedure as an evaluation electrode. The counter electrode was a metal lithium foil (thickness 500 μm).

The counter electrode was cut to φ13 mm and the evaluation electrode was cut to φ11 mm, and a separator (Hoechst Celanese glass filter and Celgard “Celgard 2400”) was interposed between the two electrodes to form an electrode body. This electrode body was accommodated in a battery case (CR2032-type coin battery member, manufactured by Hosen Co., Ltd.). In the battery case, a nonaqueous electrolyte solution in which LiPF ₆ was dissolved at a concentration of 1M was injected into a mixed solvent in which ethylene carbonate and diethyl carbonate were mixed at a ratio of 1: 1 (volume ratio), and the battery case was hermetically sealed. The lithium ion secondary battery of Example 1 was manufactured.

(Example 2)
Example 2 was carried out in the same manner as in Example 1 except that lithium doping was performed by a mechanical milling dope process instead of the molten salt dope process in the treatment process and that the coating process was performed after the mechanical milling dope process. A negative electrode material and a lithium ion secondary battery were manufactured. Below, the mechanical milling dope process in the manufacturing method of the negative electrode material of Example 2 is demonstrated.

(Mechanical milling dope process)
95 parts by mass of particulate silicon material obtained by the same process as the silicon material production process of Example 1 and 5 parts by mass of lithium foil are placed in a zirconia pot and mechanically mixed at 350 rpm for 20 minutes in a ball mill. Thus, a lithium-doped silicon material in which lithium was doped into the silicon material was obtained.

  After the mechanical milling dope step, a carbon coat layer was formed on the lithium-doped silicon material by the same coating step as in Example 1 to obtain the negative electrode material of Example 2.

(Comparative Example 1)
The negative electrode material and the lithium ion secondary battery of Comparative Example 1 were manufactured in the same manner as in Example 1, except that the negative electrode material manufacturing method of Comparative Example 1 was provided with only a coating process and no lithium doping was performed. Manufactured.

(Evaluation example 1)
The lithium ion secondary batteries of Example 1, Example 2 and Comparative Example 1 were charged at a constant current of 0.2 mA under the condition of a temperature of 25 ° C. until the voltage with respect to the counter electrode of the evaluation electrode became 0.01V. Then, initial charging / discharging was performed by a method of discharging until the voltage of the counter electrode with respect to the counter electrode reached 1V.

  A value obtained by dividing the initial discharge capacity by the initial charge capacity was calculated as the initial efficiency, and the results are shown in Table 1. In Evaluation Example 1, charging the evaluation electrode with Li is referred to as charging, and discharging Li from the evaluation electrode is referred to as discharging.

  The lithium ion secondary batteries of Example 1 and Example 2 were superior to the lithium ion secondary battery of Comparative Example 1 in initial efficiency. From this result, it was confirmed that the negative electrode material of the present invention is suitable as a negative electrode material for a lithium ion secondary battery.

  The initial charge capacity of the lithium ion secondary battery of Example 1 was 1200 mAh / g, which was a very high value compared to the initial charge capacity of 1030 mAh / g of the lithium ion secondary battery of Example 2. From this result, it can be said that it is preferable to perform the molten salt dope process after the coating process in the production method of the present invention. The reason is discussed below.

(Evaluation example 2)
The negative electrode material of Example 2 was subjected to elemental analysis by energy dispersive X-ray spectrometry (EDS: Energy Dispersive X-ray spectroscopy). The SEM image of the negative electrode material of Example 2 is shown in FIG. 1, and the EDS analysis results of the negative electrode material of Example 2 are shown in FIGS. 2 to 4 are the results of elemental analysis of the negative electrode material of Example 2 at the same position as FIG. Specifically, FIG. 2 shows an analysis result regarding Si, and a light-colored portion in the drawing is a portion where Si is present. FIG. 3 shows an analysis result regarding O, and a light-colored portion in the drawing is a portion where O exists. FIG. 4 shows an analysis result regarding C, and a light-colored portion in the drawing is a portion where C exists.

As shown in FIGS. 2 to 4, in the negative electrode material of Example 2 in which carbon material was further coated on lithium-doped silicon material by a mechanical milling method, a large amount of C was present on the surface of the negative electrode material, and the inside of the negative electrode material was substantially Si. It consists of In addition, a large amount of O is present together with Si on the inner side of C constituting the surface of the negative electrode material. For this reason, the surface of the negative electrode material of Example 2 is composed of a carbon coat layer, the interior is composed of Si, and a layer composed of Si and O is formed between the internal Si and the external carbon coat layer. It is thought that. The layer composed of Si and O is presumed to be Li ₂ SiO ₃ .

When manufacturing the negative electrode material of Example 2, the coating process was performed after the mechanical milling dope process. The lithium-doped silicon material was exposed to the atmosphere when placed in a quartz tube. Therefore, it is considered that the lithium-doped silicon material is oxidized by oxygen in the atmosphere, which may contribute to the capacity reduction of the lithium ion secondary battery of Example 2. In addition, since the coating process is performed at a high temperature close to 900 ° C., SiO ₂ derived from the quartz tube reacts with lithium contained in the lithium-doped silicon material and moves to the lithium-doped silicon material side to produce Li ₂ SiO _3. The probability is high.

Since Li ₂ SiO ₃ does not function as a negative electrode active material, the lithium ion secondary battery of Example 2 containing a large amount of Li ₂ SiO ₃ exhibits a lower initial charge capacity than the lithium ion secondary battery of Example 1. It is thought. From this result, it can be said that it is preferable to perform the molten salt dope process after the coating process in the production method of the present invention.

In addition, in the manufacturing method of Example 1, since the molten salt dope process was performed after the coating process, lithium does not exist in the vicinity of the silicon material in the coating process, and SiO ₂ derived from the quartz tube described above and Reaction with lithium contained in the lithium-doped silicon material did not occur. For this reason, it is considered that the content of Li ₂ SiO ₃ in the negative electrode material of Example 1 is small. As a result, the lithium ion secondary battery of Example 1 is considered to be excellent in initial capacity.

Claims (3)

  A method for producing a negative electrode material, comprising a treatment step of carbon coating and lithium doping on a silicon material having a structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction.   2. The method for producing a negative electrode material according to claim 1, wherein in the treatment step, the silicon material is lithium-doped by a molten salt method in which the silicon material is brought into contact with a molten lithium source. The processing step includes
A coating step of carbon coating the silicon material;
A molten salt doping step of lithium-doping the silicon material carbon-coated by a molten salt method in which the carbon-coated silicon material is brought into contact with a molten lithium source after the coating step. The manufacturing method of the negative electrode material of Claim 2.