JP6539029B2 - Method of manufacturing lithium ion secondary battery - Google Patents

Description

  The present invention relates to a method of manufacturing a lithium ion secondary battery.

  In order to reduce the irreversible capacity of the lithium ion secondary battery and to improve the cycle characteristics such as the capacity retention rate after repeated charge and discharge, it is conventionally practiced to pre-dope lithium in the negative electrode active material (for example, Patent Document 1). However, the lithium ion secondary battery in which charge and discharge are repeated is required to further improve cycle characteristics.

International Publication No. 2013/180083

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method of manufacturing a lithium ion secondary battery having excellent cycle characteristics.

  When the present inventors examined improvement of the lithium pre-doped negative electrode active material layer, the capacity retention rate of the secondary battery provided with the negative electrode active material layer was determined by performing discharge treatment in advance before the initial charge. It has been found that the invention can be improved to a level, and the present invention has been completed. The present invention discloses the following means.

(1) A method of manufacturing a lithium ion secondary battery comprising a negative electrode having a negative electrode active material layer pre-doped with lithium, a positive electrode, and an electrolyte, wherein the electrolyte is disposed between the negative electrode and the positive electrode A method of manufacturing a lithium ion secondary battery, comprising performing discharge processing before initial charge after assembly so as to be able to conduct to an external circuit.
(2) The method for producing a lithium ion secondary battery according to (1), wherein the negative electrode active material layer contains a metal oxide capable of storing lithium ions, a binder and a conductive agent.
(3) The method for producing a lithium ion secondary battery according to (1) or (2), characterized in that the negative electrode and the positive electrode are connected to an external circuit and discharged to a constant voltage.
(4) The negative electrode and the positive electrode are connected to an external circuit, and discharged until the cell voltage falls in the range of 0 to 3.0 V. Any one of the above (1) to (3) The manufacturing method of the lithium ion secondary battery as described.
(5) A liquid in which metal lithium or lithium ions can diffuse after the pre-doping method forms a negative electrode active material layer containing a metal oxide capable of storing lithium ions, a binder and a conductive agent on the current collector The method for producing a lithium ion secondary battery according to any one of the above (1) to (4), which is a method of bringing the negative electrode active material layer into contact with metallic lithium via

  According to the present invention, a lithium ion secondary battery having an excellent capacity retention rate can be manufactured.

It is a cross-sectional schematic diagram which shows an example of a lithium ion secondary battery.

«Method for manufacturing lithium ion secondary battery»
A method of manufacturing a lithium ion secondary battery according to a first embodiment of the present invention is a method of manufacturing a lithium ion secondary battery including a negative electrode having a negative electrode active material layer pre-doped with lithium, a positive electrode, and an electrolyte. In the manufacturing method, the electrolyte is disposed between the negative electrode and the positive electrode and assembled so as to be conductive (that is, conductive) to the external circuit and then subjected to a discharge treatment before the initial charge. Here, "initial charging" means connecting the assembled lithium ion secondary battery to an external circuit and charging for the first time.

<Negative electrode>
The negative electrode used in the assembly has the negative electrode active material layer disposed on a current collector. The method for forming the negative electrode is not particularly limited, and a method for manufacturing a negative electrode in the production of a known lithium ion secondary battery can be applied.

  The material of the current collector that constitutes the negative electrode is not particularly limited, and examples thereof include metals such as copper, aluminum, titanium, nickel and stainless steel. The shape of the current collector is not particularly limited, and is preferably in the form of a sheet. The thickness of the sheet-like current collector is not particularly limited, and may be, for example, 5 to 20 μm.

The method for forming the negative electrode active material layer is not particularly limited. For example, after applying a negative electrode material containing the negative electrode active material on the current collector to a thickness of about 10 to 100 μm, the solvent contained in the negative electrode material is removed by drying. Thus, the negative electrode active material layer can be formed on the current collector. The negative electrode active material layer preferably contains a metal oxide capable of absorbing and desorbing lithium ions, a binder, and a conductive additive. The metal oxide is preferably a metal oxide that can be alloyed with lithium. The types and combinations of these metal oxides, binders and conductive assistants are not particularly limited, and materials that constitute a known negative electrode active material layer of a lithium ion secondary battery can be applied.
The metal oxide capable of absorbing and desorbing lithium ions may be pre-doped with lithium in advance. The method for pre-doping is not particularly limited, and known methods are applied.

The method for applying the negative electrode material is not particularly limited, and examples thereof include methods using a bar coater, a gravure coater, etc., and known methods such as a doctor blade method and a dipping method.
The method for drying the applied negative electrode material is not particularly limited, and examples thereof include a method of drying for several minutes to several hours at a temperature of about 40 to 150 ° C. under normal pressure or reduced pressure.

  The negative electrode active material layer may be formed directly on the current collector, or may be formed on another substrate and then moved onto the current collector to be pressed onto the current collector. The negative electrode active material layer formed directly on the current collector may be pressure bonded.

Lithium is pre-doped in the negative electrode active material layer in order to reduce the irreversible capacity.
The method for pre-doping lithium into the negative electrode active material layer is not particularly limited, and a known method is applied. As a specific example, there is a method in which a negative electrode active material layer is brought into contact with metallic lithium via a liquid in which metallic lithium or lithium ions can be diffused. According to this pre-doping method, lithium can be pre-doped in a wide range of the negative electrode active material layer. As said liquid, the electrolyte solution which comprises a well-known lithium ion secondary battery, or the organic solvent which comprises this electrolyte solution is mentioned, for example. The metal lithium to be brought into contact with the negative electrode active material layer is preferably a sheet or foil metal lithium. It is preferable to carry out pre-doping of lithium using metallic lithium instead of lithium ion. After completion of the pre-doping process, it is preferable to remove the metallic lithium in contact with the negative electrode active material layer before assembling the battery.

  As another method of pre-doping lithium, there is a method of pre-doping by mixing metallic lithium or a lithium alloy in an active material slurry (a composition of a material forming a negative electrode active material layer). When this method is applied, the battery is usually assembled without removing the used lithium metal or lithium alloy from the negative electrode active material layer.

  The amount of lithium pre-doped into the negative electrode is preferably 1 to 4 times the molar amount, and 2 to 4 times the molar amount with respect to the metal oxide in the negative electrode active material layer, eg silicon oxide. Is more preferable, and a molar amount of 3 to 4 times is particularly preferable.

Lithium pre-doped to the negative electrode can irreversibly react with the metal oxide in the negative electrode active material layer. For example, when silicon oxide is used as the metal oxide, it is presumed that lithium pre-doping produces lithium silicate (Li ₄ SiO ₄ ). In the lithium ion secondary battery provided with the pre-doped negative electrode, the above reaction with silicon oxide in the negative electrode active material layer is suppressed when lithium is absorbed in the negative electrode in the initial charging step, Discharge of lithium during discharge is not impeded, and a decrease in discharge capacity can be suppressed.

  The suitable material (negative electrode material) which comprises a negative electrode, and a manufacturing method are explained in full detail later.

<Positive electrode>
The configuration of the positive electrode used for the assembly is not particularly limited, and the positive electrode in a known lithium ion secondary battery can be applied. As the above configuration, for example, a positive electrode active material layer formed using a positive electrode material formed by using a positive electrode active material, a binder resin, a solvent, and, as necessary, a conductive auxiliary agent is provided on a current collector. Can be illustrated. The thickness of the positive electrode active material layer is not particularly limited, and may be, for example, about 10 to 100 μm.

  The formation method of the said positive electrode in particular is not restrict | limited, The manufacturing method of the positive electrode in manufacture of a well-known lithium ion secondary battery is applicable. For example, the positive electrode active material layer can be formed by the same method as in the case of the negative electrode active material layer except that the positive electrode material is used instead of the negative electrode material.

A well-known metal lithium compound can be illustrated as a positive electrode active material. For example, lithium cobaltate, lithium nickelate, lithium manganate, olivine-type lithium phosphate and the like can be exemplified. As a suitable conductive support agent in a positive electrode, carbon-type materials, such as a graphite and acetylene black, are mentioned, for example.
The positive electrode active material and the conductive additive may be used singly or in combination of two or more. When two or more positive electrode active materials or a conductive auxiliary agent are used in combination, the combination and ratio thereof may be appropriately selected depending on the purpose.

  The binder resin, the solvent and the current collector in the positive electrode may be all similar to the binder resin, the solvent and the current collector in the negative electrode.

  The content of each component of the positive electrode active material, the binder resin, the solvent, and the conductive additive with respect to the total mass of each component blended in the positive electrode material is the total mass of each component blended in the negative electrode material, The content of each component of the negative electrode active material, the binder resin, the solvent, and the conductive additive may be the same as or different from each other.

<Electrolyte>
The electrolyte used for the assembly is not particularly limited, and electrolytes used for known lithium ion secondary batteries can be applied. The electrolyte disposed between the negative electrode and the positive electrode may be a liquid electrolyte solution, or may be a gel-like or solid electrolyte.

  The method for preparing the electrolyte solution is not particularly limited, and any known method for preparing the electrolyte solution in the production of a lithium ion secondary battery can be applied. A gel electrolyte can be prepared by containing a known matrix polymer in the electrolytic solution. In addition, the solid electrolyte can be prepared by drying and removing the solvent constituting the electrolyte solution or gel electrolyte.

  As an electrolyte solution, the electrolyte solution used by the conventional lithium ion secondary battery can be used. For example, an electrolytic solution containing lithium carboxylate as an electrolyte is preferable. An electrolytic solution containing a boron trifluoride complex in addition to the lithium carboxylate is also suitable. Specific examples of lithium carboxylate and boron trifluoride complex will be described in detail later.

<Assembly>
In the present embodiment, the method of assembling the battery element is particularly limited in such a manner that the electrolyte is disposed between the positive electrode and the negative electrode having the negative electrode active material layer in which lithium is pre-doped and the positive electrode. Not, the assembly method in a known lithium ion secondary battery can be applied. Here, “to be able to conduct to an external circuit” is to electrically connect the negative electrode and the positive electrode through the external circuit. The battery element obtained by this assembly may or may not be housed in the battery case (casing).

  The lithium ion secondary battery can be manufactured according to a known method, for example, in a glove box or under a dry air atmosphere. For example, the electrode surfaces of the negative electrode and the positive electrode are made to face each other, and a laminate formed by sandwiching the separator between both electrodes is put in a resin bag, and after the electrolyte is injected, the drawers connected to each electrode A so-called laminate cell can be manufactured by sealing in a state where the wiring is exposed to the outside. Instead of the above-mentioned electrolyte solution, a gel electrolyte or a solid electrolyte may be arranged to be in contact with the electrode surfaces of the negative electrode and the positive electrode.

  The shape of the lithium ion secondary battery manufactured in the present embodiment is not particularly limited, and examples thereof include cylindrical, square, coin and sheet types.

  FIG. 1 is a schematic cross-sectional view showing an example of an embodiment of a lithium ion secondary battery obtained by the manufacturing method of the present embodiment. In the lithium ion secondary battery 1, a sheet-like negative electrode 13 and a positive electrode 11 are stacked via a separator 12. The electrolyte is impregnated in the separator 12. The electrolyte may be in the form of an electrolytic solution, a gel electrolyte or a solid electrolyte. Here, a configuration in which one negative electrode 13 and one positive electrode 11 are stacked is shown, but as another configuration, for example, a laminate in which the negative electrode 13, the separator 12 and the positive electrode 11 are stacked in this order is In addition, a plurality of layers may be repeatedly stacked via the separator 12. The material of the separator 12 is not particularly limited, and examples thereof include porous polymer membranes, nonwoven fabrics, glass fibers, and the like.

<Discharge treatment>
Before initial charging of the battery element assembled so as to be conductive to an external circuit, discharge processing is performed. Since lithium is pre-doped in advance in the negative electrode active material layer, lithium ion or lithium is present in a considerable amount. Therefore, it is possible to discharge before the initial charge. As a specific example of the discharge treatment, for example, there is a method of discharging by electrically connecting a negative electrode and a positive electrode through an external circuit.

  The configuration of the external circuit is not particularly limited as long as the negative electrode and the positive electrode can be electrically connected, and examples thereof include a voltmeter, an ammeter, and an electric circuit having a predetermined electric resistance.

In the present embodiment, the negative electrode and the positive electrode of the battery element are connected via an external circuit, and a substance such as an electrolyte in contact with the negative electrode drops to a cell voltage which is not substantially reduced. It is preferable to discharge until the electrochemical reaction at the interface of the liquid) substantially stops.
By discharging to the voltage, the cycle characteristics of the battery element can be more reliably improved.

In the present embodiment, the negative electrode and the positive electrode of the battery element are connected via an external circuit, preferably in the range of 0 to 3.0 V, more preferably in the range of 0.2 to 2.8 V, still more preferably 0.5 to 2.5 V It is preferable to discharge until the cell voltage falls in the range of 1.5 to 2.0 V, most preferably in the range of 1.5 to 2.0 V.
By discharging until the cell voltage falls in the above range, the cycle characteristics of the battery element can be more reliably improved.

In the present embodiment, the negative electrode and the positive electrode of the battery element are connected via an external circuit, and a current corresponding to the range of 0.05 C to 500 C is preferably used with respect to the theoretical capacity of the battery. It is preferable to discharge the current corresponding to the range of 2C to 300C, more preferably the current corresponding to the range of 1C to 200C.
By discharging in the above range, the cycle characteristics of the battery element can be more reliably improved.
Here, the theoretical capacity of the battery is calculated as the calculated value of the theoretical capacity of the positive electrode and the calculated value of the theoretical capacity of the negative electrode, and the smaller value is taken as the theoretical capacity. The theoretical capacity of each electrode can be determined as the product of the mass (g) of the active material on the electrode and the theoretical capacity (mAh / g) of the active material.

  The processing time to discharge can not be determined indiscriminately because it depends on the electrical resistance of the external circuit, but can be, for example, several seconds to several hours.

  By performing initial charging of the battery element by a known method after the discharge treatment, a lithium ion secondary battery which can be used similarly to the conventional product and has improved cycle characteristics can be obtained.

  Although the mechanism by which the cycle characteristics of the lithium ion secondary battery are improved by performing discharge treatment before the initial charge has not been elucidated, the SEI film (solid electrolyte interface) formed on the surface of the negative electrode active material layer with which the electrolyte contacts However, it is assumed that it is denser and more stable than before. That is, since the excess lithium is released from the negative electrode active material layer among the pre-doped lithium by performing the discharge treatment, the deposition of the excess lithium on the surface of the negative electrode active material layer at the time of initial charge is reduced. It is estimated that an excellent SEI film is formed on the surface.

  An example of the improvement of the cycle characteristics is the improvement of at least one of the capacity retention rate and the capacity appearance rate.

“Capacity development rate” means the ratio (%) of discharge capacity at the first cycle to rated capacity, and the formula “{[discharge capacity at the first cycle (mAh)] / [rated capacity (mAh)]} × 100 ".
“Capacity maintenance rate” means a ratio (%) of discharge capacity in a specific number of cycles to discharge capacity in the first cycle when charge and discharge cycles are repeated, and the formula “{[specific cycle number Discharge capacity (mAh)] / [first cycle discharge capacity (mAh)]} × 100) ”. Here, the “specific number of cycles” is preferably 50 cycles or more.

  The capacity retention rate when the charge and discharge cycle of the lithium ion secondary battery manufactured according to this embodiment is repeated is high, showing a capacity retention rate of 98% or more in the 100th cycle, and 85% or more in the 200th cycle Can indicate the capacity retention rate of

<Suitable Material and Manufacturing Method of Negative Electrode>
Hereinafter, preferable materials and methods in the method of manufacturing a lithium ion secondary battery of the present embodiment will be illustrated in more detail.

(Drying temperature of negative electrode material)
By implementing the drying temperature of the negative electrode material applied on the current collector in the range of 30 to 140 ° C., the cycle characteristics of the secondary battery can be improved. As the mechanism presumed here, the water remaining in the negative electrode material can be removed by setting the temperature to 30 ° C. or higher, and the dehydration reaction of the carboxyl group of the binder resin in the negative electrode material is obtained by drying at 140 ° C. or lower It is considered that the cycle characteristics are improved because the binding property with the metal oxide surface is improved by the increase in the interaction point between the surface hydroxyl group of the metal oxide and the carboxyl group because the dehydration condensation reaction is suppressed. .

  In consideration of the above mechanism, the drying temperature of the negative electrode material is preferably 35 to 140 ° C, more preferably 50 to 120 ° C, and still more preferably 80 to 100 ° C.

(Drying time of negative electrode material)
The drying time of the negative electrode material coated on the current collector is preferably 4 hours to 30 hours, more preferably 5 hours to 20 hours, and still more preferably 6 hours to 12 hours in the temperature range of 30 to 140 ° C.
By performing the drying process for 4 to 6 hours or more, for example, the negative electrode material coated to a thickness of about 18 to 35 μm can be sufficiently dried and fixed on the current collector.
By performing the drying treatment for 12 to 30 hours or less, the dehydration reaction and the dehydration condensation reaction of the carboxyl group of the binder resin in the negative electrode material can be suppressed.

(Content of solvent in negative electrode)
One of the criteria for ending the drying treatment is that the content of the solvent in the negative electrode material is sufficiently reduced. That is, when the negative electrode material contains a solvent, the amount of the solvent contained in the negative electrode obtained after drying is preferably 100 ppm or less, more preferably 50 ppm or less, by removing the solvent by drying treatment. More preferably, it is 10 ppm or less. By sufficiently removing the solvent contained in the negative electrode, it is possible to remove the adverse effect of the solvent on the battery performance.

(Binder resin)
In the manufacturing method of the present embodiment, the binder resin constituting the negative electrode material coated on the current collector is preferably a known binder resin having a carboxyl group as a functional group. Specific examples thereof include polyacrylic acid, polymethacrylic acid, lithium polyacrylate, lithium polymethacrylate, carboxymethylcellulose, sodium carboxymethylcellulose and the like.

  Poly (meth) acrylic acid is included from the viewpoint of improving both the structural strength of the negative electrode active material layer made of the negative electrode material coated on the current collector and the cycle characteristics of the secondary battery provided with the negative electrode active material layer. It is preferred to use a binder resin. It is preferable that content of poly (meth) acrylic acid contained in the said binder resin is 50-100 mass% with respect to the total mass of binder resin. In this embodiment, commercially available poly (meth) acrylic acid may be used, or a polymer obtained by polymerizing acrylic acid, methacrylic acid, acrylic acid ester, methacrylic acid ester or the like as a raw material monomer by a known method may be used. . The weight average molecular weight Mw of the poly (meth) acrylic acid is not particularly limited, but from the viewpoint of enhancing the binding property, a Mw of about 10,000 to 500,000 is preferable in GPC measurement using a polystyrene standard substance.

  Here, the term "poly (meth) acrylic acid" is used to mean "a polymer containing at least one of acrylic acid and methacrylic acid as a monomer", and "(meth) acrylic acid" is an acrylic. It is used in the meaning of "at least one of acid and methacrylic acid". The poly (meth) acrylic acid may contain at least one of acrylic acid ester and methacrylic acid ester as a monomer copolymerized with acrylic acid or methacrylic acid. The poly (meth) acrylic acid preferably contains at least one of acrylic acid and methacrylic acid at a monomer composition ratio of 50% or more.

  In the production method of the present embodiment, the hydrogen atom of the carboxyl group of the binder resin may be unsubstituted, or at least a part of the hydrogen atom may be substituted. As a substance which substitutes the said hydrogen atom, monovalent | monohydric or more cation is mentioned, for example. Specifically, alkali metal ions and alkaline earth metal ions can be mentioned. In the production method of the present embodiment, lithium ions can be exemplified as suitable ions for replacing a part of the hydrogen atoms. The poly (meth) acrylate lithium described above is a binder resin in which at least a part of hydrogen atoms of carboxyl groups of poly (meth) acrylic acid is substituted by lithium ions.

When at least a part of the carboxyl group of the binder resin is substituted by lithium ion, the substitution rate of the hydrogen atom is preferably 10 to 50%, more preferably 20 to 40%, and 25 to 35% More preferable. Here, the substitution rate of the hydrogen atom by lithium ion is a numerical value obtained by measurement by acid / base titration of poly (meth) acrylic acid.
When the substitution rate is 10 to 50%, cycle characteristics of the lithium ion secondary battery can be improved. As a mechanism of improvement of this cycle characteristic, since the interaction of carboxyl groups is suppressed by the substitution, it is thought that aggregation of poly (meta) acrylic acid is suppressed and dispersibility improves.

  The method of adjusting the substitution rate to 10 to 50% is not particularly limited, and can be adjusted, for example, by mixing poly (meth) acrylic acid and lithium poly (meth) acrylate.

Lithium polyacrylate can be obtained, for example, by the following method.
In an aqueous solution, polyacrylic acid (manufactured by Aldrich, model number: 416002-500ML) and lithium hydroxide (manufactured by Wako Pure Chemical Industries, Ltd., model number: 169-18591) are mixed and reacted at a molar ratio of 100: 20. Thereby, it is possible to prepare lithium polyacrylate in which 20 mol% of all the acid groups have become a lithium salt. As for lithium polyacrylate used by this embodiment, it is preferable that 10-50 mol% of all the acid groups (all carboxyl groups) become a lithium salt.

  In the manufacturing method of the present embodiment, one binder resin may be used alone, or two or more binder resins may be used in combination.

  In the manufacturing method of the present embodiment, the content of the binder resin having a carboxyl group is preferably 10 to 100 mass%, more preferably 30 to 90 mass%, with respect to the total mass of the binder resin constituting the negative electrode material. 50-80 mass% is still more preferable. By using a binder resin having a carboxyl group at a content of 50% or more by mass, an advantage of improving the affinity to a silicon-based active material is obtained.

  As a binder resin which does not have a carboxyl group, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, styrene butadiene rubber, polyvinyl alcohol, polyethylene oxide, polyethylene glycol, polyacrylonitrile, polyimide etc. can be illustrated, for example. .

(Metal oxides capable of storing lithium ions)
Preferred examples include silicon oxide.
(Silicon oxide)
The silicon oxide used in the manufacturing method of this embodiment, the formula _"(wherein, z is a number of either 0.5 to 1.5.) SiO z" silicon oxide represented by Exemplified it can. Here, when silicon oxide is seen as "SiO" units, this SiO is amorphous SiO, or the circumference of Si of nanoclusters so that the molar ratio of Si: SiO ₂ becomes about 1: 1. And SiO ₂ is a composite of Si and SiO ₂ . It is speculated that SiO ₂ has a buffer action on expansion and contraction of Si during charge and discharge.

  The silicon oxide is preferably in the form of powder, more preferably in the form of particles, for example, the average particle diameter is preferably 30 μm or less, more preferably 20 μm or less, and 10 μm or less. Are particularly preferred, and most preferably 2.0 μm or less. In addition, depending on the mixing ratio of silicon oxide and binder, it is most preferable that the average particle diameter is 1.0 μm or less. By using such finely powdered silicon oxide, the effect of the combined use of the particulate conductive aid and the fibrous conductive aid described later can be obtained more remarkably.

The average particle size of the silicon oxide can be obtained, for example, by measuring the particle size of about 100 particles of any silicon oxide using an electron microscope and calculating the average value thereof.
The average particle diameter of silicon oxide can be adjusted to a desired value by grinding, for example, by a known method using a ball mill or the like.

  In the negative electrode material, the content of the metal oxide is preferably 40 to 89% by mass, and more preferably 60 to 80% by mass, based on the total mass of the metal oxide, the conductive additive, and the binder resin. Is more preferred. The discharge capacity of the lithium ion secondary battery is further improved when the content of the metal oxide is 40% by mass or more, and the negative electrode structure is more stable when the content of the metal oxide is 89% by mass or less Is obtained.

(Mass ratio of metal oxide in binder material to binder resin)
It is preferable that it is 1-10, and, as for mass ratio (metal oxide / binder resin) of the metal oxide in the negative electrode material used in the manufacturing method of this embodiment, and binder resin, it is more preferable that it is 2-10. And 3 to 8 are more preferable, and 4 to 7 is particularly preferable.

  The metal oxide and the other negative electrode active material can be sufficiently fixed on the current collector as the range of 1 to 10 is closer to the particularly preferable range of 4 to 7, and the content ratio of the metal oxide is increased. High energy density can be obtained. In addition, the rate at which the surface of the metal oxide is covered by the binder resin can be increased. By increasing the ratio, the binder resin such as polyacrylic acid functions as a surface protective film of the metal oxide to prevent excessive reaction between the metal oxide and the electrolyte, thereby further improving the cycle characteristics of the lithium ion secondary battery. It can be improved. For example, it is preferable that 50% or more of the surface is covered, and more preferably 70% or more of the surface is covered.

(Conduction agent)
Examples of suitable conductive aids include particulate conductive aids and fibrous conductive aids.
(Particulate conductive auxiliary agent)
The particulate conductive aid is not particularly limited as long as it is a particulate one that functions as a conductive aid, but preferred are carbon blacks such as acetylene black and ketjen black; graphite (graphite); fullerene etc. It can be illustrated.

  The average particle diameter of the particulate conductive aid is preferably 10 nm to 100 nm, and more preferably 15 nm to 60 nm. Moreover, it is preferable that the particulate conductive support agent has a structure in which the particles are connected to each other, such that the particles are connected in a beaded manner. When the particulate conductive support agent having such an average particle size or structure is used, the effect by combined use with the fibrous conductive support agent can be more remarkably obtained.

  The average particle size of the particulate conductive additive is determined by the same method as the above-mentioned average particle size of silicon oxide. Moreover, the method of adjusting the average particle diameter of general microparticles | fine-particles can be applied to the method of adjusting the average particle diameter of a particulate-form conductive support agent.

  The particulate conductive support has a structure in which particles are connected to one another in the negative electrode active material layer described later, and the negative electrode is enhanced by increasing the contact area with at least one of the metal oxide and the fibrous conductive support described later. It is presumed that it contributes to the improvement of the conductivity of the

  In the manufacturing method of the present embodiment, the particulate conductive assistant may be used singly or in combination of two or more, and in the case of using two or more in combination, the combination and ratio thereof May be adjusted appropriately according to the purpose.

  In the negative electrode material, the content of the particulate conductive aid is 3 to 30% by mass with respect to the total mass of the metal oxide, the particulate conductive aid, a fibrous conductive aid described later, and a binder resin. Is preferable, and 5 to 20% by mass is more preferable. When the content of the particulate conductive aid is 3% by mass or more, the effect by using the particulate conductive aid is more remarkably obtained, and the content of the particulate conductive aid is 30% by mass or less The effect by combined use with a fibrous conductive support agent is acquired more notably as it is.

(Fibrous conductive auxiliary agent)
The fibrous conductive aid is not particularly limited as long as it is a fibrous one that functions as a conductive aid, and preferred examples thereof include carbon nanotubes and carbon nanohorns.

  The fibrous conductive aid preferably has an average fiber diameter of 5 nm to 300 nm, and more preferably 10 nm to 150 nm. Moreover, it is preferable that an average fiber length is 0.1 micrometer-30 micrometers, and, as for a fibrous conductive support agent, it is more preferable that it is 0.5 micrometer-20 micrometers. By using such a fibrous conductive aid, the effect by the combined use with the particulate conductive aid can be more remarkably obtained.

  The average fiber diameter of the fibrous conductive support agent can be obtained, for example, by measuring the fiber diameter of about 100 arbitrary fibrous conductive support agents using an electron microscope and calculating the average value thereof. Similarly, the average fiber length of the fibrous conductive aid can be obtained, for example, by measuring the fiber length for about 100 arbitrary fibrous conductive additives using an electron microscope and calculating the average value thereof.

  The fibrous conductive support agent contributes to the structural stabilization of the negative electrode active material layer by forming a network structure preferably in the entire negative electrode active material layer in the negative electrode active material layer described later, and also as a negative electrode active material layer. It is presumed that a conductive network is formed therein to contribute to the improvement of the conductivity.

  In the manufacturing method of the present embodiment, the fibrous conductive auxiliary may be used singly or in combination of two or more, and in the case of using two or more in combination, the combination and ratio thereof May be adjusted appropriately according to the purpose.

  In the negative electrode material, the content of the fibrous conductive aid is preferably 1 to 25% by mass with respect to the total mass of the metal oxide, the particulate conductive aid, the fibrous conductive aid, and the binder resin. And 2 to 15% by mass. The effect by using a fibrous conductive support is acquired more notably as content of the said fibrous conductive support is 1 mass% or more, and content of the said fibrous conductive support is 25 mass% or less The effect by the combination use with a particulate-form conductive support agent is acquired more notably as it is.

  In the negative electrode material used in the manufacturing method of the present embodiment, the mass ratio (compounding mass ratio) of the compounding amount of "particulate conductive auxiliary: fibrous conductive auxiliary" is 90:10 to 30:70. Is preferable, and 80:20 to 40:60 is more preferable. When the compounding mass ratio of the particulate conductive aid and the fibrous conductive aid is in such a range, the effect by the combined use of the particulate conductive aid and the fibrous conductive aid can be more remarkably obtained.

(Other ingredients)
The above-mentioned negative electrode material may be further blended with other components which do not correspond to these, in addition to the silicon oxide, the particulate conductive aid, the fibrous conductive aid and the binder.
The other components are not particularly limited, and can be arbitrarily selected according to the purpose. For example, as a preferable component, a solvent for dissolving or dispersing the compounding component (silicon oxide, particulate conductive aid, fibrous conductive aid, binder) can be exemplified.
The negative electrode material into which the solvent is blended is preferably a liquid composition having fluidity at the time of use.

The said solvent can be arbitrarily selected according to the kind of said compounding component. For example, as a preferable solvent, water and an organic solvent can be exemplified.
Preferred examples of the organic solvent include alcohols such as methanol, ethanol, 1-propanol and 2-propanol; linear or cyclic amides such as N-methylpyrrolidone and N, N-dimethylformamide; and ketones such as acetone .
The solvents may be used alone or in combination of two or more. When two or more are used in combination, the combination and ratio thereof may be appropriately selected depending on the purpose.

  The compounding quantity of the said solvent in negative electrode material is not specifically limited, What is necessary is just to adjust suitably according to the objective. For example, when a negative electrode material, which is a liquid composition containing a solvent, is coated and dried to form a negative electrode active material layer described later, the liquid composition has a viscosity suitable for coating. The amount of the solvent may be adjusted. Specifically, in the negative electrode material, the solvent is blended so that the ratio of the total amount of the blending components other than the solvent is preferably 5 to 60 mass%, more preferably 10 to 35 mass% with respect to the total amount of the blending components. You should adjust the amount.

  When the component other than the solvent (other solid component) is blended as the other component, in the negative electrode material, the ratio of the blending amount of the other solid component to the total amount of the blended component other than the solvent is 10% by mass It is preferable that it is the following and it is more preferable that it is 5 mass% or less.

  The negative electrode material is particularly preferably a negative electrode material in which the particulate conductive support agent is carbon black and the fibrous conductive support agent is a carbon nanotube.

The said negative electrode material can be manufactured by mix | blending the said silicon oxide, a particulate-form conductive support agent, a fibrous conductive support agent, binder resin, and the other component as needed.
At the time of blending each component, each component is preferably added and sufficiently mixed by various means. Each component may be mixed while being added separately one by one or all components may be added at once and then mixed, and it is preferable that the components be uniformly mixed.

  When the solvent is blended as the other component, the solvent is at least partially selected from the group consisting of the silicon oxide, the particulate conductive aid, the fibrous conductive aid, the binder, and other solid components. It may be mixed in advance with one or more of them as a solution or dispersion of the selected component. The solvent used to prepare the solution or dispersion may be the total amount of the solvent to be blended.

The mixing method of each component is not particularly limited, and for example, known methods using a stirrer, a stirring blade, a ball mill, a stirrer, an ultrasonic disperser, an ultrasonic homogenizer, a revolution mixer, etc. can be applied. Also, plural kinds of methods may be combined and mixed.
The mixing conditions such as the mixing temperature and the mixing time may be appropriately set according to various methods. In general, the mixing temperature is preferably 10 to 50 ° C., and more preferably 15 to 35 ° C. The mixing time is preferably about 3 to 40 minutes, and more preferably about 5 to 20 minutes.

  The composition obtained by mixing the respective components may be used as it is as a negative electrode material, or some operation may be performed on the obtained composition, for example, by removing a part of the blended solvent by distillation or the like. A material obtained by adding it may be used as a negative electrode material.

(Suitable electrolyte)
Specific examples of lithium carboxylates mentioned as preferable examples of the electrolyte include lithium salts of monovalent carboxylic acids such as lithium formate and lithium acetate; and divalent carboxylic acids such as lithium oxalate and lithium malonate. Lithium salts; lithium salts of monovalent carboxylic acids having a hydroxyl group such as lithium lactate and the like can be exemplified. Among these, lithium formate, lithium acetate, lithium oxalate and lithium succinate are more preferable, and lithium oxalate is particularly preferable.
The lithium carboxylate may be used alone or in combination of two or more. In the case of using two or more kinds in combination, the combination and ratio thereof may be appropriately selected depending on the purpose.

It can also be exemplified to use boron trifluoride or boron trifluoride complex capable of complex formation reaction with lithium carboxylate and used as lithium salt together with lithium carboxylate.
As preferred boron trifluoride complexes, trifluorides of boron trifluoride dimethyl ether complex (BF ₃ · O (CH ₃ ) ₂ ), boron trifluoride diethyl ether complex, boron trifluoride di n-butyl ether complex, etc. Boron alkyl ether complexes; boron trifluoride alcohol complexes such as boron trifluoride methanol complex (BF ₃ · HOCH ₃ ), boron trifluoride propanol complex, boron trifluoride phenol complex and the like can be exemplified.

Although the organic solvent which comprises the said electrolyte solution is not specifically limited, For example, ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, vinylene carbonate, (gamma) -butyrolactone, tetrahydrofuran, dimethoxyethane, methyl formate, It is preferably at least one selected from the group consisting of methyl acetate, methyl propionate, acetonitrile and sulfolane.
An organic solvent may be used individually by 1 type, and may use 2 or more types together. In the case of using two or more kinds in combination, the combination and ratio thereof may be appropriately selected depending on the purpose.

  The compounding quantity of the organic solvent in electrolyte solution is not specifically limited, For example, what is necessary is just to adjust suitably according to the kind of electrolyte. Usually, the blending amount is adjusted so that the concentration of lithium atom (Li) is preferably 0.2 to 3.0 mol / kg, more preferably 0.4 to 2.0 mol / kg. preferable.

  In addition, an electrolytic solution in which a known lithium salt such as lithium hexafluorophosphate, lithium tetrafluoride lithium, lithium bisfluorosulfonyl imide, and lithium bis (trifluoromethanesulfonyl) imide is dissolved in an organic solvent can be exemplified as the electrolyte. .

The lithium ion secondary battery manufactured according to the present embodiment has a negative electrode active material layer formed using a negative electrode material in which silicon oxide, particulate conductive support agent, fibrous conductive support agent and binder resin are blended. It is preferable to have a negative electrode pre-doped with lithium.
When the lithium ion secondary battery uses a particulate conductive support agent and a fibrous conductive support agent in combination and further uses a lithium-predoped negative electrode, the capacity expression ratio and charge and discharge are repeatedly performed. Both the capacity retention rate is high and the charge and discharge characteristics are excellent.

  Hereinafter, the present invention will be described in more detail by way of specific examples. However, the present invention is not limited at all to the examples shown below.

(1) Raw material used The raw material used by the present Example is shown below.
・ Conduction agent Acetylene black ("HS-100" manufactured by Denki Kagaku Kogyo Co., Ltd., average particle diameter 48 nm)
Carbon nanotube ("NT-7" manufactured by Hodogaya Chemical Co., Ltd., average fiber diameter 65 nm, average fiber length 6 μm or more)
-Binder Styrene-Butadiene resin (hereinafter abbreviated as "SBR") (manufactured by JSR Corporation)
Polyacrylic acid (manufactured by Aldrich)
Lithium hydroxide (made by Wako Pure Chemical Industries, Ltd.)
-Carboxylic acid lithium salt lithium oxalate (manufactured by Alfa)
· Boron trifluoride complex Boron trifluoride diethyl etherate complex (BF ₃ · O (C ₂ H ₅ ) ₂ ) (manufactured by Aldrich)
-Organic solvent ethylene carbonate (hereinafter abbreviated as "EC") (Kishida Chemical Co., Ltd.)
Propylene carbonate (hereinafter abbreviated as "PC") (Kishida Chemical Co., Ltd.)
-Solvent Dimethyl carbonate (hereinafter abbreviated as "DMC") (Kishida Chemical Co., Ltd.)

<Synthesis of organic acid lithium salt-boron trifluoride complex>
Synthesis Example 1 (Synthesis of lithium oxalate-boron trifluoride complex)
Lithium oxalate (22.3 g, 223 mmol) was weighed into a round bottom flask and suspended in 200 mL DMC. To this was slowly added dropwise boron trifluoride diethyl etherate (63.3 g, 446 mmol) at 23 ° C., followed by stirring for 24 hours at room temperature (23 ° C.), the reaction liquid became transparent and insoluble matter was not found It confirmed that it became a homogeneous solution. Then, the solvent and impurities were distilled off from the reaction solution using a rotary evaporator. Thereafter, the precipitated white solid was dried at 50 ° C. to obtain white powder of lithium oxalate-boron trifluoride complex ((COOLi) ₂ · (BF ₃ ) ₂ ) (yield 96.5) %).

<Manufacturing of lithium ion secondary battery>
Example 1
(Manufacture of negative electrode material)
Silicon monoxide (SiO, average particle diameter 1.0 μm, 69 parts by mass), acetylene black (10 parts by mass), carbon nanotubes (6 parts by mass), lithium polyacrylate (20 mol% of all acid groups with lithium salt) (10 parts by mass, sometimes abbreviated as “PAALi” below) and SBR (5 parts by mass) in a reagent bottle, and distilled water is further added thereto to adjust the concentration, and The concentration adjusted using a mixer was mixed at 2000 rpm for 3 minutes. Next, this mixture was subjected to dispersion treatment for 10 minutes using an ultrasonic homogenizer, and again, this dispersion was mixed for 3 minutes at 2000 rpm using a revolution mixer to obtain a negative electrode material. All the operations up to here were performed at 25 ° C. Each blending component at this time and the ratio (mass%) of each blending component to the total amount of the blending components are shown in Table 1.

(Manufacture of electrolyte)
A mixed solvent of EC and PC (EC: PC = 30: 70 (volume ratio)) is weighed in a sample bottle as an organic solvent, lithium oxalate-boron trifluoride complex (1.82 g) is added thereto, An electrolytic solution was obtained by mixing at 23 ° C. such that the concentration of lithium atoms in the lithium oxalate-boron trifluoride complex was 1.0 mol / kg.

(Manufacture of negative electrode)
The resulting negative electrode material is coated on both sides of a copper foil of 18 μm thickness using a bar coater, dried on a hot plate at 60 ° C., and pressed with 1 kN using a roll press machine. A 25 μm-thick negative electrode active material layer was formed on a copper foil as a current collector to obtain a negative electrode precursor. In addition, the electrodes were dried under vacuum at 50 ° C. for 6 hours prior to fabrication of the lithium battery to fully remove residual moisture.

The electrolytic solution obtained above is dropped on the negative electrode active material layer of the obtained negative electrode precursor so that the dropping amount is 50 μL / cm ^2, and a lithium foil of 200 μm thickness is overlapped on the dropping surface. In this state, the negative electrode precursor was pre-doped with lithium by standing for 24 hours to obtain a negative electrode. The lithium foil was removed from the negative electrode after pre-doping.

(Manufacture of positive electrode)
Nickel, cobalt, lithium manganate (Ni: Co: Mn = 1: 1: 1, LiNMC) (93 parts by mass), polyvinylidene fluoride (PVDF) (3 parts by mass), and carbon black as a conductive agent 4 parts by mass) were mixed to prepare a positive electrode mixture, which was dispersed in N-methylpyrrolidone (NMP) to prepare a positive electrode material (slurry). Next, this positive electrode material is coated on both sides of a 15 μm thick aluminum foil using a bar coater, dried under reduced pressure at 100 ° C., 0.1 MPa, 10 hours, and then roll pressed to obtain a current collector. A positive electrode active material layer having a thickness of 60 μm was formed on the aluminum foil.

(Production of battery element)
The negative electrode and the positive electrode obtained above were punched into a disk shape having a diameter of 16 mm. Moreover, using what consists of glass fibers as a separator, this was pierced in disk shape with a diameter of 17 mm. The disk-shaped positive electrode, the separator and the negative electrode are stacked in this order in a battery container made of SUS (CR2032) in this order, the electrolytic solution obtained above is impregnated into the separator, the negative electrode and the positive electrode, and further on the negative electrode A coin-made cell (theoretical capacity 4 mAh) was manufactured as a lithium ion secondary battery by placing a plate (thickness 1.2 mm, diameter 16 mm) and placing a lid.

(Discharge treatment)
The positive and negative electrodes of the coin cell were connected with a 0.1 ohm resistor and discharged until the initial voltage 3.3V dropped to 1.7V.

Example 2
At the time of preparation of the negative electrode material, except that the additive vinylene carbonate (VC) was blended at 5% by mass with respect to the total mass of the negative electrode material, and the amounts of other components were changed as described in Table 1, A lithium ion secondary battery was manufactured in the same manner as in Example 1. Moreover, after performing discharge processing similarly to Example 1, initial stage charge was performed and cycle characteristics were evaluated.

Comparative Example 1
A lithium ion secondary battery was manufactured in the same manner as in Example 1 except that the initial charge was performed without the discharge treatment.

Comparative Example 2
A lithium ion secondary battery was manufactured in the same manner as in Example 2, except that the initial charge was performed without performing the discharge treatment.

<Evaluation of charge and discharge characteristics of lithium ion secondary battery>
With respect to the lithium ion secondary batteries obtained in the above Examples and Comparative Examples, constant current constant voltage charging at 0.2C was performed at 25 ° C until the current value converges to 0.1C with the upper limit voltage 4.2V. Thereafter, a constant current discharge of 0.2 C was performed up to 2.5V. Next, the charge and discharge cycle was repeated twice in the same manner with the charge and discharge current set to 1 C to stabilize the state of the lithium ion secondary battery. Next, charge and discharge cycles are repeated in the same manner with 1 C as the charge and discharge current, and the capacity expression ratio ({[discharge capacity in the first cycle (mAh)] / [rated capacity (mAh)]} × 100) (% And the capacity retention rate at 100 cycles ({[discharge capacity at 100 cycles (mAh)] / [discharge capacity at first cycle (mAh)]} × 100) (%) were calculated. Similarly, the capacity retention rate at the 200th cycle was calculated. The results are shown in Table 2.

  As shown in the above results, in Examples 1 and 2 in which the discharge treatment was performed before the initial charge, the capacity retention rate was clearly improved.

  The present invention is applicable in the field of lithium ion secondary batteries.

DESCRIPTION OF SYMBOLS 1 ... Lithium ion secondary battery, 11 ... Positive electrode, 12 ... Separator and electrolyte, 13 ... Negative electrode, 14 ... Housing | casing (case), 15 ... Insulating gasket, 16 ... Cap

Claims (7)

A method for producing a lithium ion secondary battery, comprising: a negative electrode having a negative electrode active material layer pre-doped with lithium; a positive electrode; and an electrolyte,
The electrolyte comprises lithium carboxylate and
The electrolyte is disposed between the negative electrode and the positive electrode and assembled so as to be able to conduct to the external circuit, and then the negative electrode and the positive electrode are connected to the external circuit, and 1.5-2 before the initial charge. . A method of manufacturing a lithium ion secondary battery, comprising performing discharge treatment until the cell voltage falls within the range of 0 V.   The method for producing a lithium ion secondary battery according to claim 1, wherein the electrolyte further comprises boron trifluoride or a boron trifluoride complex.   The electrolyte comprises ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, vinylene carbonate, γ-butyrolactone, tetrahydrofuran, dimethoxyethane, methyl formate, methyl acetate, methyl propionate, acetonitrile and sulfolane The method for producing a lithium ion secondary battery according to claim 1 or 2, comprising one or more organic solvents selected from the group.   The method for producing a lithium ion secondary battery according to any one of claims 1 to 3, wherein the concentration of lithium atoms in the electrolyte is 0.2 to 3.0 mol / kg.   The method for manufacturing a lithium ion secondary battery according to any one of claims 1 to 4, wherein the negative electrode active material layer contains a metal oxide capable of storing lithium ions, a binder, and a conductive support agent. .   The lithium ion secondary battery according to any one of claims 1 to 5, wherein the negative electrode and the positive electrode are connected to an external circuit and discharged until the electrochemical reaction at the negative electrode is substantially stopped. Manufacturing method. The method of pre-doping forms the negative electrode active material layer containing a metal oxide capable of storing lithium ions, a binder and a conductive agent on a current collector, and then through a liquid capable of diffusing metal lithium or lithium ions. The method of manufacturing a lithium ion secondary battery according to any one of claims 1 to 6 , wherein the negative electrode active material layer is in contact with metallic lithium.

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