JP2019207797A - Secondary battery - Google Patents

Abstract

To perform an excellent secondary battery performance for a negative electrode comprising an anode activator containing a silicon-based material and a graphite, and a conductive agent.SOLUTION: A secondary battery is formed by including an anode activator (a silicon-based material and graphite), a conductive agent, a negative electrode with a binder, and a fluoroethylene carbonate as an electrolyte, and the negative electrode is achieved by adopting a polyvinyl pyrrolidone (PVP) or a negative electrode additive agent consisting of the polyvinyl pyrrolidone as a main component.SELECTED DRAWING: None

Description

Field of Invention

  The present invention relates to a secondary battery, and particularly to a secondary battery having a characteristic negative electrode.

  In recent years, with the development and popularization of mobile tools and electric motors, a high-capacity energy source has been demanded, and a typical example is a secondary battery. In particular, lithium ion secondary batteries have a high average operating voltage and high capacity, so they are rapidly used not only for mobile and stationary personal computers (PCs) but also for vehicle applications (for example, automobiles, motorcycles, buses, etc.). Is progressing.

  In recent years, secondary batteries using a negative electrode active material in which graphite is mixed with a silicon-based material such as silicon monoxide (SiO) have been proposed. (Patent Document 1: Japanese Patent Application Laid-Open No. 2014-67581, Patent Document 2: Japanese Translation of PCT International Publication No. 2015-503836, Patent Document 3: Japanese Patent Application Laid-Open No. 2015-165482). In particular, in order to further increase the cruising distance of electric vehicles in the future, it will be necessary to increase the proportion of silicon-based materials mixed with graphite.

  However, the expansion rate during charging of a silicon-based material exhibits various values depending on the component of the silicon-based material, the crystal structure, etc., but is several times higher than that of graphite. Cracks occur, pulverization occurs, and the conductive path is lost. As a result, the utilization rate of the active material is reduced every time charging and discharging are repeated, causing capacity deterioration.

  In order to suppress the generation and micronization of cracks, it is possible to make the silicon-based material amorphous and suppress the cracking of the particles. On the other hand, during charging, the volume and surface area of the particles can be reduced by alloying with lithium. It is recognized that it will definitely grow.

  In addition, in order to improve the high capacity and life characteristics of the secondary battery and to maintain the uniformity of the thickness of the charge / discharge electrode, uniform mixing of the negative electrode active material (especially silicon-based material and graphite) and the conductive agent is performed. is required.

  In response to such demands, styrene butadiene rubber (SBR) is used as an aqueous binder, or a thickener such as carboxymethyl cellulose (CMC) is used in combination, and the electrode active material and the conductive agent are combined together. When the mixture was kneaded, sufficient dispersibility was not obtained as compared with mixing using a solvent system. In addition, the obtained negative electrode has very poor impregnation (wetability) and liquid retention with an electrolyte, and a secondary battery using this negative electrode cannot fully exhibit its electrochemical performance. It was. In particular, in a secondary battery using a silicon-based material that has a lower conductivity than graphite as an electrode active material, the initial few cycles cannot fully utilize its characteristics, and as a result The capacity of the secondary battery decreased, or the capacity (deviation) varied among the plurality of secondary batteries.

  Further, it is known that a lithium secondary battery forms a solid electrolyte interface (SEI) film on the surface of the negative electrode during the first charge / discharge. In particular, when a silicon-based material is used as the negative electrode active material, it has been proposed to use fluoroethylene carbonate (FEC) as an electrolytic solution in order to form a stable SEI film (Patent Document 4: Japanese Patent Laid-Open No. 2004-208867). 2006-86058, Patent Document 5: JP 2013-69503 A). In the case of graphite, if a SEI film is formed during the initial charge, side reactions with the electrolyte hardly occur during subsequent charging, and charging and discharging can be stably repeated. However, when a silicon-based material is used as the negative electrode active material, a phenomenon occurs in which the SEI film is destroyed due to an increase in cracks and surface area associated with expansion during charging, and a new surface of the silicon-based material is exposed. Furthermore, when charge / discharge progresses, silicon contained in the silicon-based material is separated from the silicon-based material and appears more on the surface of the active material particles.

  For this reason, even when an electrolytic solution to which FEC is added is used, the silicon and the electrolytic solution are reacted and FEC is consumed for each charge on the new surface generated by repeated charging and discharging, so that a large amount of silicon-based material is contained. In the electrode, it is necessary to increase the FEC content in proportion to the silicon content.

  However, since FEC has a higher viscosity than general-purpose electrolyte solvents such as ethylene carbonate (EC), diethylene carbonate (DEC), and propylene carbonate (PC), when the content of FEC increases, electrolyte impregnation ( The wettability) and the liquid retaining property are lowered, the initial capacity of the secondary battery is not exhibited as designed, the battery capacity is lowered, and the battery capacity is varied (deviation).

  In addition, when a mixture of a silicon-based material and graphite is used as the negative electrode active material, since graphite is a hydrophobic material, the impregnation property (wetting property) and liquid retention of the electrolyte solution are further reduced. . Furthermore, in order to impart conductivity to the silicon-based material, it has been proposed to coat the surface with carbon (Patent Document 6: Japanese Patent Application Laid-Open No. 2015-210959). In this case, the surface of the silicon-based material also becomes hydrophobic. As a result, the impregnation property (wetting property) and the liquid retention property of the electrolytic solution containing FEC are further reduced.

  Therefore, as the content of FEC increases, it becomes difficult to sufficiently retain the electrolyte in the electrode and the electrode active material, particularly in the initial stage of charging, and the design capacity may not be sufficiently developed. Also, if the components of the electrolyte solution in the electrode are biased, particularly the FEC distribution, the capacity deterioration due to repeated charging tends to occur from the portion where the SEI film is not sufficiently formed by FEC on the surface of the silicon-based material. . From this, after manufacturing the electrode and assembling the secondary battery, in order to improve the electrolyte impregnation property (wetting property) and liquid retention, a very long aging time is required. Cost and manufacturing time, resulting in lack of economy and productivity.

  Moreover, when manufacturing the negative electrode containing a silicon-type substance, graphite, and a electrically conductive agent, it is desirable to disperse | distribute a silicon-type substance uniformly and to make the expansion coefficient of the whole electrode uniform. Further, since the electronic conductivity of the silicon-based material is very low, it is necessary to improve the dispersibility of the conductive agent as compared with the case where only graphite is used as the negative electrode active material. Furthermore, it is preferable that the conductive agent is selectively attached to the silicon-based material as much as possible.

  Therefore, there is still a negative electrode including a silicon-based active material, a graphite active material, and a conductive agent, and a secondary battery adopting FEC as an electrolyte, which is effective for exhibiting excellent negative electrode characteristics and electrolyte characteristics. Development of a secondary battery equipped with an additive for negative electrode is required.

JP 2014-67581 A JP-T-2015-503836 JP 2015-165482 A JP 2006-086058 A JP 2013-0669503 A Japanese Patent Application Laid-Open No. 2015-210959

  The present invention provides a negative electrode comprising polyvinyl pyrrolidone (PVP) as a negative electrode additive in a secondary battery adopting a silicon-based material and graphite as a negative electrode active material and FEC as an electrolyte. Gives excellent hydrophilicity to the negative electrode, improves the impregnation (wetability) and liquid retention of the electrolyte (containing FEC) to the negative electrode, and uniformly disperses the silicon-based active material and the conductive agent in the negative electrode. As a result, the utilization factor of the negative electrode active material becomes 100%, and the desired battery capacity designed can be obtained reliably during initial charge / discharge, thus reducing the variation (deviation) in battery capacity between multiple secondary batteries. I got the knowledge to be. In the present invention, since the PVP exhibits an excellent effect as a dispersant in the negative electrode, a plurality of negative electrode active materials and a conductive agent are uniformly dispersed, so that a change in the electrode thickness due to charge / discharge can be reduced. We obtained the knowledge that it is uniform regardless of the position in the electrode, and the secondary battery is less likely to be distorted and current is concentrated, and the life characteristics can be improved.

[One embodiment of the present invention]
One aspect of the present invention is as follows.
[1] A secondary battery,
A positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolyte;
The negative electrode comprises a negative electrode additive, a negative electrode active material, a conductive agent, and a binder,
The negative electrode additive is polyvinyl pyrrolidone, or comprises polyvinyl pyrrolidone as a main component,
The negative electrode additive is a hydrophilic agent, a dispersant, and an electrolyte impregnation (acceleration) agent or an electrolyte retention agent,
A secondary battery in which the electrolyte is fluoroethylene carbonate or contains fluoroethylene carbonate as a main component.
[2] The secondary battery according to [1], wherein the content of the negative electrode additive is 0% by mass and 5.0% by mass or less based on the total mass of the negative electrode.
[3] The silicon-based material is silicon powder, silicon alloy, silicon oxide (SiOx [x = 1 to 4]), amorphous silicon powder, silicon nanofiber, silicon nanowire; silicon-based material, graphite, carbon [1] or [2], which is a composite with a carbon material selected from nanotubes (CNT) or graphene; one or a mixture of two or more selected from the group consisting of the above silicon-based materials doped with lithium The secondary battery as described.
[4] The secondary content according to any one of [1] to [3], wherein the content of the silicon-based material is 2% by mass and 50% by mass or less based on the total mass of the negative electrode. battery.
[5] The secondary battery according to any one of [1] to [4], wherein an average particle size (MV) of the silicon-based material is 0.1 μm or more and 15 μm or less.
[6] The secondary battery according to any one of [1] to [5], wherein the graphite is natural graphite, artificial graphite, or a mixture thereof.
[7] The secondary content according to any one of [1] to [6], wherein a content of the graphite is 0% by mass and 98% by mass or less based on the total mass of the secondary battery. battery.
[8] The secondary battery according to any one of [1] to [7], wherein the graphite has an average particle size (MV) of 3 μm to 30 μm.
[9] The average particle diameter (MV) ratio (graphite / silicon-based material) between the graphite and the silicon-based material is at least twice or more, according to any one of [1] to [8]. Secondary battery.
[10] The secondary battery according to any one of [1] to [9], wherein the conductive agent is carbon fiber or metal fiber.
[11] The secondary battery according to any one of [1] to [10], wherein the content of the conductive agent is greater than 0% by mass and equal to or less than 5% by mass based on the total mass of the negative electrode. .

  According to the present invention, the negative electrode active material and the surface of the conductive agent are made hydrophilic by containing PVP having excellent hydrophilicity, impregnation property, and liquid retention imparting effect in the negative electrode, and the electrolyte for the negative electrode (particularly, Impregnation property (wetting property) and liquid retention property of FEC are improved. Moreover, PVP imparts an effect of uniformly dispersing the negative electrode active material, the conductive agent, and the like in the negative electrode. As a result, all the negative electrode active materials are utilized at the time of initial charge and discharge, and the initial capacity calculated and set stoichiometrically can be sufficiently expressed, and between a plurality of secondary batteries (cells). Deviations can be eliminated. In addition, according to the present invention, it is possible to suppress dendrite precipitation caused by current concentration due to uneven distribution of the negative electrode active material, and the presence or absence of a local SEI film and pressure film formation due to a difference in electrolyte concentration in the negative electrode. The life characteristics of the secondary battery can be improved at a very high level.

[Secondary battery]
The secondary battery according to the present invention is characterized in that, in the negative electrode, in particular, a negative electrode additive, a negative electrode active material, a conductive agent, and a binder are provided.
(Additive for negative electrode)
The additive for negative electrode is an essential constituent material of the negative electrode. Preferably, the negative electrode additive is contained in a negative electrode comprising a negative electrode active material (silicon-based material and graphite) and a conductive agent. In the present invention, the secondary battery in which the electrolyte (liquid) contains fluoroethylene carbonate (FEC) is preferably used as an additive for the negative electrode.

  The additive for a negative electrode imparts hydrophilicity to a negative electrode component, disperses the negative electrode component, and impregnation (promoting) property (wetting property) of an electrolyte (particularly, an electrolytic solution containing FEC) with respect to the negative electrode It becomes possible to improve liquid retention. That is, the negative electrode additive imparts hydrophilicity to the negative electrode component, disperses the negative electrode component, or imparts a function or action of electrolyte impregnation (acceleration) (wetability) and liquid retention to the negative electrode. is there.

  Therefore, the negative electrode additive more specifically imparts hydrophilicity imparting agent, dispersibility imparting agent for the negative electrode component (constituent material), and electrolyte impregnation (acceleration) to the negative electrode to the secondary battery negative electrode. Agent or liquid retention agent. That is, the negative electrode additive is a hydrophilic agent, a dispersant, an electrolyte impregnation (acceleration) agent, or an electrolyte liquid retention agent that is imparted to the negative electrode.

(Polyvinylpyrrolidone: “PVP”)
The additive for negative electrodes is polyvinyl pyrrolidone (PVP) or comprises polyvinyl pyrrolidone (PVP) as a main component. PVP has an effect of hydrophilizing the surface of a substance (material), and has high compatibility with a resin or the like, so that it is used as a dispersant for battery constituent components. Moreover, PVP can improve the wettability of the negative electrode with respect to the electrolyte solution with high viscosity, and the dispersibility of the silicon-based substance, the conductive agent, and the binder in the negative electrode material. PVP is a nonionic water-soluble polymer, is water-soluble, and has high lower solvent solubility, hygroscopicity, film formability, and salt resistance. Furthermore, PVP can be used also when using a water-based binder and a solvent-based binder because of its water solubility and solvent solubility. However, in the present invention, PVP is used as an additive for the negative electrode that imparts the above functions and functions, and is not used for binder applications. In this sense, PVP is distinguished from binder applications. The

(Preparation of additive for negative electrode)
The negative electrode additive may be PVP itself. Further, in consideration of the ease of production of the negative electrode, it may be prepared as an aqueous solution mixed and dissolved with water. Moreover, since it can melt | dissolve in solvents (solvents), such as N-methyl-2-pyrrolidone (NMP) and ethanol other than water, you may prepare as a solution dissolved in the solvent. Or you may prepare as a dispersion liquid form which mixed PVP with the solvent and disperse | distributed to the solvent. Alternatively, it may be prepared as an additive in which PVP is mixed in a powder form and a thickener aqueous solution such as carboxymethyl cellulose (CMC) is added and kneaded.

  The content of the negative electrode additive (PVP) is 0% by mass over 5.0% by mass, preferably 0.1% by mass or more, and 3.0% by mass based on the total mass of the negative electrode. Or less, more preferably 0.3% by mass or more and 2.0% by mass or less.

When the content of the negative electrode additive (PVP) is in the above numerical range, that is, when the content is not more than the upper limit value, the ratio of the amount of the negative electrode active material in the battery becomes sufficient, and a desired design battery capacity is realized. In addition, although the negative electrode additive (PVP) is a non-conductor, it is possible to keep the internal resistance of the battery low. In addition, when the content of the negative electrode additive (PVP) exceeds the lower limit value, the silicon-based material is sufficiently dispersed and segregation within the electrode is difficult, and the wettability or liquid retention of the electrolyte is sufficient. Can be demonstrated.
Therefore, when the content of the negative electrode additive (PVP) is in the above numerical range, it is possible to exert the most excellent effects on battery capacity, inter-battery capacity deviation, life characteristics, and electrode expansion coefficient. .

(Negative electrode active material)
The secondary battery includes at least a silicon-based material and graphite as a negative electrode active material.

<Silicon-based substances (materials)>
The silicon-based substance (material) is, for example, silicon powder, silicon alloy, silicon oxide (SiOx [x = 1 to 4]), amorphous silicon powder, silicon nanofiber, silicon nanowire; the silicon-based substance, graphite, A composite with a carbon material selected from carbon nanotubes (CNT) or graphene; may be one or a mixture of two or more selected from the group consisting of the silicon-based materials doped with lithium in advance.

  The content of the silicon-based material is not particularly defined, but considering the balance between capacity and expansion coefficient, it is more than 2% by mass and less than 50% by mass, preferably 2% by mass or more, based on the total mass of the negative electrode. It may be 30% by mass or less.

  In the present invention, the silicon-based material may be in the form of particles. In this case, the average particle size of the silicon-based material is 0.1 μm or more and 15 μm or less, preferably 0.5 μm or more and 10 μm or less.

  By making the average particle size of the silicon-based material within the above numerical range, that is, since the silicon-based material has a higher expansion rate due to charging than graphite, the particle size (particle diameter) of the silicon-based material is below the above upper limit value. In the negative electrode, it can be arranged in the gap between the graphite, the electrode thickness when the secondary battery is expanded can be made uniform, or the particle size of the silicon-based material (particle By setting the (diameter) to be equal to or more than the above lower limit value, it is possible to prevent the specific surface area from expanding and to significantly suppress the side reaction with the electrolyte.

<graphite>
The graphite (graphite) may be natural graphite, artificial graphite, or a mixture thereof. Natural graphite is generally softer than artificial graphite and silicon-based materials, and is well adapted to silicon-based materials by kneading, and some graphite can cleave to play a role as a conductive agent. Since the expansion coefficient of artificial graphite itself is lower than that of natural graphite, an electrode using artificial graphite has an effect that the expansion coefficient of the entire electrode is also reduced.

  The graphite content is 0% by mass over 98% by mass, preferably 50% by mass or more and 70% by mass or less, based on the total mass of the negative electrode.

  In the present invention, the graphite may be in the form of particles, and the average particle size of the graphite is 3 μm or more and 30 μm or less, preferably 5 μm or more and 20 μm or less. In the present invention, preferably, the average particle diameter of graphite is smaller than the average particle diameter of the silicon-based material.

  According to a preferred embodiment, the ratio of the average particle diameter of graphite and silicon-based material (graphite / silicon-based material) is at least 2 times, preferably 3 times or more. Since the silicon-based material has a higher expansion coefficient due to charging than graphite, the ratio of the average particle diameter of graphite to the silicon-based material is set to the above value, that is, the particle size (particle diameter) of the silicon-based material is set to graphite. By making the particle size (particle diameter) as small as possible, it becomes possible to arrange the silicon-based material in the voids between the graphite in the negative electrode, and to make the electrode thickness uniform when the secondary battery expands. it can.

  Therefore, when the average particle size of the silicon-based material, the average particle size of graphite, and the ratio of the average particle size of graphite and silicon-based material (graphite / silicon-based material) are within the above numerical range, the negative electrode due to partial expansion It is possible to significantly prevent the active material from falling off and the formation of cracks in the negative electrode. Even when charging and discharging are repeated, the utilization rate of the negative electrode active material does not decrease, and the capacity deterioration of the secondary battery is a high level. Can be suppressed.

  In the present invention, the average particle diameter is a volume average diameter (MV), and can be measured using a volume standard (volume distribution) for a particulate material. For example, microtrack (laser diffraction / scattering method) The volume distribution can be measured using a method and apparatus such as), and can be calculated using analysis software. In the present invention, not only the average particle diameters of silicon-based materials and graphite but also the average particle diameters of other substances are measured in the same manner and determined as numerical values.

(Conductive agent)
The conductive agent can impart battery characteristics equivalent to those of the graphite electrode in a negative electrode active material using a silicon-based material. The conductive agent is preferably one that does not induce a chemical change in the secondary battery. For example, graphite such as natural graphite or artificial graphite; carbon black, acetylene black, ketjen black (trade name), graphene, carbon nanotube, carbon nano Carbon black such as fiber, channel black, furnace black, lamp black and thermal black; conductive fiber such as carbon fiber and metal fiber; metal powder such as fluorocarbon, aluminum and nickel; conductive whisker such as zinc oxide and potassium titanate One or a mixture of two or more selected from the group consisting of conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives, etc., preferably conductive fibers. In the case where particles containing silicon atoms are used as the negative electrode active material, the conductive fiber maintains a conductive path between the negative electrode active materials or between the negative electrode active material and the current collector even by expansion and contraction due to charge and discharge. It is particularly preferable because it has a structure that does not easily fall off the substance.

  The content of the conductive agent is 0% by mass and 5% by mass or less, preferably 0.5% by mass or more, and 2% by mass or less based on the total mass of the negative electrode.

(binder)
The negative electrode contains a binder as an essential component. The binder is a component that adheres the negative electrode active material and the conductive agent, or promotes bonding to the electrode current collector. In the present invention, polyvinylpyrrolidone (PVP) is used as an essential component as an additive for a negative electrode, but as already mentioned, it is not used as a binder.

  Examples of the binder include water-based and solvent-based binders. Examples of the water-based binder include styrene butadiene rubber (SBR), polyacrylic acid, polyimide, polyvinylidene fluoride, polyacrylonitrile, polyvinylidene fluoride-hexafluoropropylene copolymer, and polyfluoride. Vinylidene fluoride, polyvinylidene fluoride-trichloroethylene, polyvinylidene fluoride-chlorotrifluoroethylene, polymethyl methacrylate, polyacrylonitrile, polyvinyl acetate, ethylene vinyl acetate copolymer, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate propio Cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl skull , Pullulan, carboxymethyl cellulose (CMC), acrylonitrile-styrene-butadiene copolymer, alkyl-modified carboxyl group-containing copolymer, and a copolymer of vinyl ester and ethylenically unsaturated carboxylic acid ester One or a mixture of two or more is preferably mentioned.

  The binder content is, based on the total mass of the negative electrode, more than 0% by mass and 15% by mass or less, preferably 1% by mass or more and may be 10% by mass or less. It may be 5% by mass or more and 5% by mass or less.

(Thickener)
The negative electrode may contain a thickener as an optional component. The thickener is useful as a component that maintains the dispersibility of the negative electrode active material and the conductive agent and promotes bonding to the electrode current collector. In particular, it plays the role of adjusting the negative electrode slurry composition to an appropriate viscosity and preventing sedimentation of solids. Examples of thickeners include cellulose compounds such as carboxymethylcellulose, methylcellulose and hydroxypropylcellulose, ammonium salts and alkali metal salts of the above cellulose compounds, poly (meth) acrylic acid and poly (meth) acrylic acid such as modified poly (meth) acrylic acid. Carboxylic acids, alkali metal salts of the above polycarboxylic acids, polyvinyl alcohols, modified polyvinyl alcohols and polyvinyl alcohol-based (co) polymers such as ethylene-vinyl alcohol copolymers, and (meth) acrylic acid, maleic acid or fumaric acid One or a mixture of two or more selected from the group consisting of a water-soluble polymer such as a saponification product of a copolymer of a saturated carboxylic acid and a vinyl ester, and an anionic (meth) acrylic polymer thickener is preferred.

  The content of the thickener may be 0% by mass and 5% by mass or less based on the total mass of the negative electrode.

(filler)
The negative electrode may contain a filler as an optional component. The filler is a component that suppresses the expansion of the negative electrode, is selectively used, does not induce a chemical change in the secondary battery, and a fibrous material is preferably used. Specific examples of the filler preferably include one or a mixture of two or more selected from the group consisting of olefin polymers such as polyethylene and polypropylene; and fibrous materials such as glass fibers and carbon fibers.

(Negative electrode composition and production method thereof)
The negative electrode composition is prepared by preparing silicon-based material and graphite as a negative electrode active material, polyvinylpyrrolidone (PVP), a conductive agent, and a binder as negative electrode additives as essential components. A sticky agent, a filler, an optional additive, and water or a solvent are prepared, and these substances are mixed and preferably kneaded and manufactured (prepared).

According to the preferable aspect of this invention, the composition for negative electrodes can be manufactured (prepared) with the following procedures.
First, it is preferable to mix the silicon-based material and an additive for negative electrode (PVP, PVP aqueous solution, PVP solvent solution or the like). Thereby, it becomes possible to disperse the silicon-based material in the negative electrode and to sufficiently form the SEI film by FEC on the particle surface of the silicon-based material.
At this time, by adding a thickener aqueous solution such as CMC or a very small amount of binder solution, PVP can be more effectively adhered to the surface of the silicon-based material.
Next, it is preferable to add a conductive agent and disperse the conductive agent so that the conductive agent adheres to a silicon-based material having lower conductivity than graphite.
Thereafter, graphite and an appropriate amount of thickener, if necessary, a binder is added, and by sufficiently kneading so that the solid content (non-volatile component) is 60% or more, dispersibility of the silicon substance further increases, Hydrophilicity can also be imparted to the surface of the graphite material.
Finally, if necessary, the viscosity may be adjusted by adding water and a solvent so that the viscosity is easy to apply.
The “kneading (kneading)” can be performed using a general mixer or a mixer having shearing properties, and can be performed by adjusting the rotation speed, temperature, or pressure.

(Negative electrode)
In the present invention, the negative electrode can be constituted by applying the above-described negative electrode component (negative electrode composition) to the negative electrode current collector.

<Negative electrode current collector>
The negative electrode current collector is not particularly limited as long as it does not induce a chemical change in the secondary battery and has high conductivity. For example, the surface treatment of copper, stainless steel, aluminum, nickel, titanium, sintered carbon, aluminum or stainless steel with one or a mixture of two or more selected from the group consisting of carbon, nickel, titanium, and silver Used. The negative electrode current collector can increase the adhesion of the negative electrode material by forming fine irregularities on the surface, and is selected from the group consisting of a film, a sheet, a wheel, a net, a porous body, a foam, and a nonwoven fabric body Various forms such as one kind or a mixture of two or more kinds are possible. The thickness of the negative electrode current collector may be about 3 μm to 50 μm.

(Production method of negative electrode)
In general, the negative electrode can be obtained by forming a negative electrode component on a negative electrode current collector, or applying and drying the negative electrode composition according to the present invention.

(Electrolytes)
<Fluoroethylene carbonate (FEC)>
In the secondary battery of the present invention, the (non-aqueous) electrolyte is fluoroethylene carbonate or contains fluoroethylene carbonate as a main component. By containing fluoroethylene carbonate (FEC), a good SEI film is formed in the process of alloying the silicon-based material and lithium in the active material, and stable charge / discharge is performed. can get.

FEC has the above-mentioned excellent characteristics, but on the other hand, it has a higher viscosity than a general carbonate-based electrolyte and is difficult to impregnate an electrode. Therefore, in the negative electrode (slurry) composition, by adding polyvinylpyrrolidone (PVP) which is an additive for the negative electrode as an essential component, the impregnation property (wetting property) and liquid retention of the negative electrode are improved. Thus, the original design capacity can be achieved in the first charge / discharge. In addition, since silicon-based materials have several orders of magnitude lower conductivity than graphite and other carbon-based negative electrode active materials, the capacity of silicon-based materials is difficult to be exhibited when no conductive agent is in contact with the silicon-based material. Since PVP also has a function as a dispersant, the dispersibility of the silicon-based material and the conductive agent can be promoted, and sufficient conductivity can be imparted to the silicon-based material.
Therefore, PVP has both the function of promoting the impregnation property (wetting property) and liquid retention of the fluoroethylene carbonate (FEC) -containing electrolyte with respect to the negative electrode and the function of promoting the dispersion of the silicon-based material and the conductive agent. It is a thing.

  The content of fluoroethylene carbonate (FEC) is more than 0.1% by mass and not more than 50% by mass, preferably not less than 0.5% by mass and not more than 30% by mass, based on the total mass of the electrolyte. Good. Even if the content of fluoroethylene carbonate (FEC) is within the above numerical range, the silicon-based material expands due to repeated charge and discharge, and the SEI film formed at the initial stage of charging is broken to expose the silicon-based material. Since sufficient FEC can be supplied, a new SEI film is formed, and alloying of lithium and a silicon-based material proceeds stably, and good life characteristics can be obtained. That is, since the FEC content is not less than the above lower limit value, the FEC is not depleted in the early stage of charge / discharge, and as a result, the capacity deterioration can be suppressed significantly. Is set to be not more than the above upper limit value, it is possible to suppress the expansion of the battery due to gas generation at a high level particularly at high temperatures.

<Other electrolytes>
In the present invention, FEC may be the main component and other electrolytes may be contained. Other electrolytes can include, for example, cyclic carbonates and / or chain carbonates. Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (GBL), fluoroethylene carbonate (FEC), and the like. Examples of chain carbonates are preferably one or a mixture of two or more selected from the group consisting of diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and methyl propyl carbonate (MPC). However, it is not limited to this, and γ-butyl lactone (γ-BL), tetrahydrofuran (THF), acetonitrile and derivatives thereof, and one or a mixture of two or more selected from the group consisting of ionic liquids, etc. It may be used.

<Electrolyte additive>
In the present invention, one or a mixture of two or more selected from the group consisting of vinylene carbonate, biphenyl, propane sultone, and diphenyl disulfide may be added as an additive. The nonaqueous electrolyte contains a lithium salt together with a carbonate compound, and specific examples are selected from the group consisting of LiClO ₄ , LiCF ₃ SO ₃ , LiPF ₆ , LiBF ₄ , LiAsF ₆ and LiN (CF ₃ SO ₂ ) _2. Although the 1 type, or 2 or more types of mixture made are preferable, it is not limited to this.

(Positive electrode)
The positive electrode can be formed by forming a positive electrode composition in which a positive electrode active material, a conductive agent, a binder or a thickener, and a filler are further added on a positive electrode current collector. The positive electrode current collector, the conductive agent, the binder, the filler, and the like may be the same as those described for the negative electrode. The thickness of the positive electrode current collector is about 3 μm to 50 μm.

<Positive electrode active material>
As the positive electrode active material, a lithium-containing transition metal oxide can be desirably used. For example, Li _x CoO ₂ (0.5 <x <1.3), Li _x NiO ₂ (0.5 <x <1.3), Li _x MnO ₂ (0.5 <x <1.3), Li _x Mn ₂ O ₄ (0.5 <x <1.3), Li _x (Ni _a Co _b Mn _c ) O ₂ (0.5 <X <1.3, 0 <a <1, 0 <b <1, 0 <c <1, a + b + c = 1), Li _x Ni _1-y Co _y O ₂ (0.5 <x <1.3 , 0 <y <1), Li x Co 1-y Mn y O 2 (0.5 <x <1.3,0 ≦ y <1), Li x Ni 1-y Mn y O 2 (0.5 <X <1.3, 0 ≦ y <1), Li _x (Ni _a Co _b Mn _c ) O ₄ (0.5 <x <1.3, 0 <a <2, 0 <b <2, 0 <c <2, a + b + c = 2), Li x Mn 2-z Ni z O 4 (0.5 <x <1.3 0 <z <2), Li x Mn 2-z Co z O 4 (0.5 <x <1.3,0 <z <2), Li x CoPO 4 (0.5 <x <1.3) And a mixture of two or more selected from the group consisting of Li _x FePO ₄ (0.5 <x <1.3) can be used.

In addition to the lithium-containing transition metal oxide, sulfides, selenides, halides, and the like can also be used. Preferably, Li _x CoO ₂ (0.5 <x <1.3) and Li _x (Ni _a Co _b Mn _c ) O ₂ (0.5 <x <1.3, 0 <a <1, 0 < A mixture with b <1, 0 <c <1, a + b + c = 1) can be used as the positive electrode active material. In _{particular, Li x (Ni a Co b} Mn c) O 2 (0.5 <x <1.3,0 <a <1,0 <b <1,0 <c <1, a + b + c = 1) is a high voltage Although it is desirable in that it can exhibit high output characteristics under conditions, it is not limited to these.

(Separator)
The separator is an insulating thin film that is interposed between the positive electrode and the negative electrode and has high ion permeability and mechanical strength. Generally, the separator has a pore diameter of 0.01 to 10 μm and a thickness of 5 to 300 μm. As such a separator, for example, an olefin polymer such as chemically resistant and hydrophobic polypropylene, a sheet or a nonwoven fabric made of polyimide, glass fiber, polyethylene, or the like is used. There may be an oxide layer such as alumina, titania or silica on the surface. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte can also serve as a separator.

[Manufacture of secondary batteries]
The secondary battery according to the present invention is manufactured by inserting a porous separator between a positive electrode and a negative electrode and introducing an electrolyte by a usual method. The secondary battery according to the present invention is used regardless of the outer shape, such as a cylindrical type, a square type, or a pouch type battery. The secondary battery may be a single secondary battery or may be configured as a plurality (in series or parallel) of secondary batteries.

  The content of the present invention will be described using the following examples, but the scope of the present invention should not be construed as being limited to these examples. Further, various aspects of the present invention disclosed in the present specification can be easily implemented by those skilled in the art from this embodiment.

[Example 1]
A secondary battery was manufactured as follows.
(Create negative electrode)
An SiO powder having an average particle diameter of 5 μm and a carbon coating having a thickness of 20 nm on the particle surface and natural graphite having an average particle diameter of 15 μm were mixed at a weight ratio of 10:90 to obtain a negative electrode active material.
Next, 95% by weight of the negative electrode active material, 1.0% by weight of carbon black as a conductive agent, and 1.0% by weight of PVP as a negative electrode additive were mixed. Viscosity that is easy to apply by mixing 1.5% by weight of styrene butadiene rubber (SBR) as a binder and 1.5% by weight of carboxymethylcellulose (CMC) as a thickener, adding pure water and kneading into a slurry. Then, pure water was added to adjust so that a negative electrode (slurry) composition was obtained.
And the prepared slurry composition for negative electrodes was apply | coated to 20-micrometer-thick copper foil so that it might become a thickness of about 100 micrometers, and it dried at 80 degreeC. Furthermore, after vacuum drying at 120 ° C. and pressing, a negative electrode having an electrode density of 1.7 g / cc was prepared by punching into a circle having a diameter of 13 mm.

(Creating positive electrode)
Metal lithium having a thickness of 0.3 mm was used as the positive electrode.

(Preparation of electrolyte)
Fluoroethylene carbonate (FEC) and diethyl carbonate were mixed at a ratio of 3: 7 to prepare an electrolytic solution in which 1 mol of LiPF ₆ was dissolved.

(Manufacture of secondary batteries)
The secondary battery component was assembled to produce a 2016 coin battery.

[Example 2]
In the production of the negative electrode of Example 1, a coin battery was produced in the same manner as in Example 1 except that the amount of PVP was 0.5% by weight.

[Example 3]
In the production of the negative electrode of Example 1, three coin batteries were produced in the same manner as in Example 1 except that the amount of PVP was 2.0% by weight.

[Comparative Example 1]
In Example 1, a coin battery was produced in the same manner as in Example 1 except that PVP was not added to the negative electrode.

[Comparative Example 2]
In Example 1, PVP was not added to the negative electrode, and in adjusting the electrolyte, fluoroethylene carbonate (FEC) was not used, and ethylene carbonate and ethyl methyl carbonate were mixed at a ratio of 3: 7, A coin battery was prepared in the same manner as in Example 1.

[Comparative Example 3]
In Example 1, 1.0% of PVP was added, fluoroethylene carbonate (FEC) was not used, and ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a ratio of 3: 7, A coin battery was prepared in the same manner as in Example 1.

〔Evaluation test〕
The following evaluation tests were performed on the coin batteries in Examples and Comparative Examples. The results were as shown in Table 1 below.

(Evaluation test 1: Evaluation of capacity deviation between cells)
The initial capacity for each of the three manufactured coin batteries was as shown in Table 1. The capacity per unit weight was calculated by dividing the battery capacity by the total weight of SiO, graphite, and conductive agent.

(Evaluation result 1)
In Example 1, a design capacity of 485 mAh / g was obtained even with an electrolyte composition containing 1.0% by weight of PVP and 30% of high-viscosity FEC, and three coin batteries There was no capacity deviation between.
In Example 2, since 0.5% by weight of PVP was added, there was a slight difference (deviation) in the capacity between the three coin batteries, and the capacity itself was also smaller than in Example 1.
In Example 3, an initial capacity comparable to that of Example 1 was obtained, and no inter-battery deviation was observed.

  In contrast to the examples, in Comparative Example 1, since no PVP was added, the electrolyte composition containing FEC could only obtain an initial capacity of about 5% lower than the design capacity. From these results, by adding PVP at the time of preparing the negative electrode slurry, the wettability and liquid retention of the active material and the conductive agent are increased, and the electrolytic solution penetrates all the active materials uniformly and quickly, and the active material In addition, since the dispersibility of the conductive agent in the electrode is also improved, the utilization factor of the active material is 100% from the initial charge / discharge, the capacity as designed capacity is obtained, and the deviation between the batteries is improved. Clearly understood.

  In Comparative Example 2 and Comparative Example 3, since a mixed solvent of EC and EMC having a low viscosity is used for the electrolytic solution, it is originally easy to impregnate, there is no capacity deviation between coin cells, and it is possible to design without adding PVP. Capacity was obtained. However, since no FEC was contained, the capacity began to deteriorate from the initial charge / discharge, and the capacity was clearly reduced as compared with Example 1.

(Evaluation Test 2: Lifetime Characteristic Evaluation)
The ratio of the discharge capacity at 50 cycles to the initial discharge capacity for each of the three coin batteries manufactured was as shown in Table 1.

(Evaluation result 2)
In Examples 1 to 3, since 30% of FEC was added to the electrolytic solution, the silicon-based material expands and contracts due to charge and discharge, and the shape, surface area, and volume of the silicon-based material are good. The SEI film is formed, and by adding PVP, high electrolyte retention and a plurality of active materials and conductive agents are evenly dispersed, and the change in electrode thickness associated with charge / discharge does not depend on the site in the electrode. Since it becomes uniform and it is difficult for battery distortion and current concentration to occur, a high cycle capacity retention rate was exhibited.

On the other hand, in Comparative Example 1, since no PVP was added, the initial capacity was low and a deviation was observed. However, since the electrolytic solution to which 30% of FEC was added was used, the capacity retention rate of 90% or more was used. Was obtained. However, the dispersibility of the conductive agent was insufficient, and the cycle capacity retention rate was clearly lower compared to Examples 1 to 3 to which PVP was added.
Moreover, in Comparative Example 2 and Comparative Example 3, since the electrolytic solution compositions did not contain FEC, significant capacity deterioration was shown.

(Comprehensive evaluation)
According to the present invention, in the secondary battery negative electrode, in the secondary battery system in which the silicon-based material and graphite are used as the negative electrode active material, the conductive agent is included, and the electrolyte contains fluoroethylene carbonate (FEC). Occasionally by adding polyvinylpyrrolidone (PVP), PVP hydrophilizes the surface of the active material and the conductive agent, thereby improving the impregnation property and liquid retention of the high viscosity FEC, and the active material and In order to enhance the uniform dispersion of the conductive agent, a theoretically calculated design capacity was developed at the time of the first discharge, and the capacity deviation between the plurality of batteries could be improved.
In addition, by uniformly dispersing multiple active materials and conductive agents, changes in electrode thickness due to charge / discharge are made uniform regardless of the location in the electrode, and battery distortion and current concentration are less likely to occur. A secondary battery with excellent life characteristics could be obtained.

Claims (11)

A secondary battery,
A positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolyte;
The negative electrode comprises a negative electrode additive, a negative electrode active material, a conductive agent, and a binder,
The negative electrode additive is polyvinyl pyrrolidone, or comprises polyvinyl pyrrolidone as a main component,
The negative electrode additive is a hydrophilic agent, a dispersant, and an electrolyte impregnation (acceleration) agent or an electrolyte retention agent,
A secondary battery in which the electrolyte is fluoroethylene carbonate or contains fluoroethylene carbonate as a main component.   The secondary battery according to claim 1, wherein the content of the negative electrode additive is 0% by mass and 5.0% by mass or less based on the total mass of the negative electrode.   The silicon-based material includes silicon powder, silicon alloy, silicon oxide (SiOx [x = 1 to 4]), amorphous silicon powder, silicon nanofiber, silicon nanowire; the silicon-based material, graphite, and carbon nanotube (CNT). Or a composite with a carbon material selected from graphene; the secondary battery according to claim 1, which is one or a mixture of two or more selected from the group consisting of the silicon-based materials doped with lithium .   4. The secondary battery according to claim 1, wherein the content of the silicon-based material is more than 2% by mass and 50% by mass or less based on the total mass of the negative electrode.   5. The secondary battery according to claim 1, wherein an average particle size (MV) of the silicon-based material is 0.1 μm or more and 15 μm or less.   The secondary battery according to any one of claims 1 to 5, wherein the graphite is natural graphite, artificial graphite, or a mixture thereof.   7. The secondary battery according to claim 1, wherein a content of the graphite is 0% by mass and 98% by mass or less based on the total mass of the secondary battery.   The secondary battery according to claim 1, wherein the graphite has an average particle size (MV) of 3 μm or more and 30 μm or less.   The secondary battery according to claim 1, wherein a ratio of an average particle diameter (MV) between the graphite and the silicon-based material (graphite / silicon-based material) is at least twice or more.   The secondary battery according to claim 1, wherein the conductive agent is carbon fiber or metal fiber.   11. The secondary battery according to claim 1, wherein the content of the conductive agent is greater than 0% by mass and 5% by mass or less based on the total mass of the negative electrode.