JP5929183B2 - Electrode, lithium secondary battery, and electrode manufacturing method - Google Patents

Description

  The present invention relates to an electrode, a lithium secondary battery, and a method for manufacturing the electrode.

  Conventionally, an electrode including an electrode active material, a conductive material, a binder, and an additive has been proposed (see, for example, Patent Document 1). In this electrode, the outer surface of the layer is controlled from the back surface of the layer (current collector side) by controlling the compression rolling conditions at the time of production, the content of the solid substance in the slurry, the composition in the slurry, the drying temperature after forming the composite layer, etc. It is said that the capacity density and the electrochemical utilization rate can be further increased by reducing the porosity in the direction toward the surface.

JP 2008-508672 A

  However, in the electrode of Patent Document 1 described above, the porosity of the composite layer containing the active material is controlled to improve the battery performance. However, it is not sufficient yet, for example, the improvement of the discharge capacity or the charge / discharge It has been demanded to further improve the charge / discharge characteristics such as the capacity retention rate in the cycle.

  This invention is made | formed in view of such a subject, and it aims at providing the manufacturing method of the electrode which can improve charge / discharge characteristic more, a lithium secondary battery, and an electrode.

  As a result of earnest research to achieve the above-described object, the present inventors can improve the charge / discharge characteristics by controlling the specific surface area of the active material contained in the composite layer on the current collector side and the surface layer side. The inventors have found what can be done and have completed the present invention.

  That is, the present invention is an electrode for use in a battery, and includes the current collector and the current collector that includes the active material in a tendency that the specific surface area of the active material is larger on the current collector side than on the surface layer side. And an electrode mixture layer formed thereon.

  A lithium secondary battery of the present invention includes one or more of the positive electrode and the negative electrode described above, and an ion conductive medium that is in contact with the electrode and conducts lithium ions.

  The method for producing an electrode of the present invention is a method for producing an electrode used in a battery, and the active material is formed on the current collector so that the specific surface area of the active material is larger on the current collector side than on the surface layer side. Including a composite material layer forming step.

  The electrode, lithium secondary battery, and electrode manufacturing method of the present invention can further improve charge / discharge characteristics. The reason why such an effect is obtained is presumed as follows. For example, by increasing the reaction specific surface area in the deep part (current collector side) of the electrode that is difficult to react, it contributes to charge / discharge up to the active material arranged in the thick part on the current collector side. It is presumed that the charge / discharge characteristics can be further improved, such as improvement of capacity and improvement of discharge capacity maintenance ratio after charge / discharge cycle.

1 is a cross-sectional view illustrating a schematic configuration of a lithium secondary battery 10.

  The electrode of the present invention is an electrode used in a battery, and the active material is included on the current collector in a tendency that the specific surface area of the active material is larger on the current collector and the current collector side than on the surface layer side. And a formed composite material layer. “The specific surface area tends to increase” means that the specific surface area increases from the surface of the composite layer to the current collector-side surface formed on the current collector. In addition, it is intended to allow a region where the specific surface area does not change or a region where the specific surface area is small to partially exist from the surface layer surface of the composite material layer to the surface on the current collector side. The electrode of the present invention is not particularly limited as long as it is an electrode of an electricity storage device, but is preferably an electrode of an alkaline secondary battery. The alkali may be, for example, lithium, sodium, or potassium, and lithium is more preferable. Below, the electrode and manufacturing method which are mainly used for a lithium secondary battery are demonstrated.

  Current collectors used in the electrodes of the present invention include aluminum, titanium, stainless steel, nickel, iron, calcined carbon, conductive polymer, conductive glass, etc., as well as improved adhesion, conductivity and oxidation resistance. For the purpose, an aluminum or copper surface treated with carbon, nickel, titanium or silver can be used. For these, the surface can be oxidized. These can be used as a current collector for a positive electrode. The current collector for the negative electrode includes copper, nickel, stainless steel, titanium, aluminum, calcined carbon, conductive polymer, conductive glass, Al-Cd alloy, etc., as well as adhesion, conductivity, and reduction resistance. For the purpose of improving the properties, for example, a surface of copper or the like treated with carbon, nickel, titanium, silver or the like can be used. Examples of the shape of the current collector include foil, film, sheet, net, punched or expanded, lath, porous, foam, and formed fiber group. The thickness of the current collector is, for example, 1 to 500 μm.

The composite layer of the electrode of the present invention may include an active material and a binder. Alternatively, the composite layer of the electrode of the present invention may include an active material and a binder, and may further include a conductive material. The composite layer may be formed such that the specific surface area ratio B / A, which is the ratio of the specific surface area B of the active material on the current collector side to the specific surface area A of the active material on the surface layer side, is 1.25 or more. . That is, the specific surface area of the active material is preferably 1.25 times or more on the current collector side as compared with the surface layer side. In this way, for example, the charge / discharge characteristics such as the discharge capacity and the capacity retention rate in the charge / discharge cycle can be further improved. This specific surface area ratio B / A is more preferably 1.50 or more, further preferably 2.00 or more, and is preferably 6.00 or less, taking into consideration the formation strength of the composite material layer, and the like, and 4.00 or less. It is more preferable that Also, the composite layer of the electrode of the present invention has a specific surface area ratio increase rate (B / A−) obtained by subtracting 1 from the ratio of the specific surface area B of the active material on the current collector side to the specific surface area A of the active material on the surface layer side. It is preferable that 1) is formed with 0.25 or more. In this way, for example, the charge / discharge characteristics such as the discharge capacity and the capacity retention rate in the charge / discharge cycle can be further improved. The specific surface area ratio increase rate (B / A-1) is more preferably 0.50 or more, and further preferably 1.00 or more. Further, the specific surface area ratio increase rate (B / A-1) is preferably 5.00 or less, and more preferably 3.00 or less in consideration of the formation strength of the composite material layer. The specific surface area of the active material may be calculated from the average particle diameter, assuming that the powder of the active material is a sphere, for example. At this time, the average particle diameter of the active material is determined by observing the raw material powder on the surface layer side and the raw material powder on the current collector side with an electron microscope (SEM). 300 are selected, and the diameter is measured and averaged to obtain the average particle diameter of each active material. Further, in the electrode mixture layer, the surface from which the current collector and the mixture layer were peeled off was observed with an electron microscope (SEM), 300 active material particles included in this observation image were selected, and the diameter was selected. The value measured and averaged is taken as the average particle diameter of the active material on the current collector side. Further, the surface of the surface layer is observed with an SEM, 300 active material particles included in the observed image are selected, and the diameter is measured and averaged to obtain the average particle size of the active material on the surface layer side. At this time, the specific surface area may be obtained in consideration of the observed particle shape, for example, surface irregularities. Or the specific surface area of an active material is good also as the value which measured the active material, for example using nitrogen gas by the BET specific surface area measurement by gas adsorption. The specific surface area of the active material on the surface layer side is preferably in the range of, for example, 10 m ² / g or more and 2500 m ² / g or less.

  The composite layer of the electrode of the present invention is preferably formed with a film thickness (thickness of the composite layer) of 20 μm or more. If the film thickness is 20 μm or more, the battery capacity can be further increased. In addition, when the film thickness is 20 μm or more, ions hardly reach the active material on the current collector side, and the charge / discharge reaction is difficult to be uniform, and the significance of applying the present invention is high. That is, the effect of further improving the charge / discharge characteristics becomes more remarkable. The film thickness is preferably thicker, for example, preferably 25 μm or more, and more preferably 50 μm or more. The film thickness of the composite material layer is preferably 1000 μm or less in consideration of the binding property with the current collector and the like.

The composite layer of the electrode of the present invention has a specific surface area ratio increase rate (B / A-1) obtained by subtracting 1 from the ratio of the specific surface area B of the active material on the current collector side to the specific surface area A of the active material on the surface layer side. The specific surface area ratio increase rate {(B / A-1) / T} per unit thickness divided by the thickness T (μm) of the composite material layer is 0.010 (μm ⁻¹ ) or more. preferable. In this way, for example, the charge / discharge characteristics such as the discharge capacity and the capacity retention rate in the charge / discharge cycle can be further improved. The specific surface area ratio increase rate per unit thickness is more preferably 0.020 (μm ⁻¹ ) or more, and further preferably 0.030 (μm ⁻¹ ) or more. The specific surface area ratio increase rate per unit thickness is preferably 0.100 or less in consideration of the formation strength of the composite layer.

  The composite layer of the electrode of the present invention is preferably formed on the current collector by electrostatic coating using a raw material (also referred to as a composite material) containing at least a binder and an active material. Electrostatic coating refers to a technique in which a material mixture is attached to a current collector by electrostatic force, and then the binder is melted by heating to fix the material to the current collector. In this electrostatic coating, the specific surface area of the active material, the added amount of the binder and the conductive material, etc. are changed, and the first layer, the second layer, and the mixed material powder whose characteristics are changed are sequentially applied. It is possible, and the manufacturing process of the electrode in which the specific surface area of the active material is inclined can be simplified. Further, for example, a drying process required when performing wet processing can be omitted, and the electrode manufacturing process can be further simplified. Moreover, charge / discharge characteristics can be further improved.

The active material contained in the composite layer may be, for example, a positive electrode active material or a negative electrode active material used for a lithium secondary battery. For example, as the active material, a sulfide containing a transition metal element or an oxide containing lithium and a transition metal element can be used as the positive electrode active material. Specifically, transition metal sulfides such as TiS ₂ , TiS ₃ , MoS ₃ , FeS ₂ , Li _(1-x) MnO ₂ (0 ≦ x ≦ 1, the same shall apply hereinafter), Li _(1-x) Mn ₂ Lithium manganese composite oxide such as O ₄ , Lithium cobalt composite oxide such as Li _(1-x) CoO ₂ , Lithium nickel composite oxide such as Li _(1-x) NiO ₂ , Lithium vanadium such as LiV ₂ O ₃ A composite oxide, a transition metal oxide such as V ₂ O _5, or the like can be used. Of these, lithium transition metal composite oxides such as LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , and LiV ₂ O ₃ are preferable. As the active material particles, a carbonaceous material capable of inserting and extracting lithium ions, a titanium composite oxide such as Li ₄ Ti ₅ O ₁₂ , a conductive polymer, and the like can be used as the negative electrode active material. Of these, carbonaceous materials are preferable from the viewpoint of safety. The carbonaceous material is not particularly limited, and examples thereof include cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, and carbon fibers. Of these, graphites such as artificial graphite and natural graphite have an operating potential close to that of metallic lithium, and can be charged and discharged at a high operating voltage. When lithium salt is used as the supporting salt, self-discharge is suppressed. In addition, it is preferable because the irreversible capacity during charging can be reduced. The active material particles preferably have an average particle size of 0.5 μm or more and 20 μm or less because the treatment can be easily performed on the current collector, and more preferably 1 μm or more and 10 μm or less. The average particle diameter of the raw material powder is an average value obtained by observing the raw material powder by SEM, selecting 300 active material particles included in the observed image, and measuring the diameter.

  The binder contained in the composite material layer plays a role of securing the active material and the conductive material. For example, a fluorine-containing resin such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride, and fluorine rubber, or polypropylene. , Thermoplastic resins such as polyethylene, ethylene-propylene-dienemer (EPDM), sulfonated EPDM, natural butyl rubber (NBR) and the like can be used alone or as a mixture of two or more. In addition, an aqueous dispersion of cellulose or styrene butadiene rubber (SBR), which is an aqueous binder, can also be used. The conductive material included in the composite layer is not particularly limited as long as it is an electron conductive material that does not adversely affect the battery performance of the electrode. For example, graphite such as natural graphite (scale-like graphite, scale-like graphite) or artificial graphite , Acetylene black, carbon black, ketjen black, carbon whisker, needle coke, carbon fiber, metal (copper, nickel, aluminum, silver, gold, etc.) or a mixture of two or more can be used. . Of these, carbon black and acetylene black are preferable as the conductive material from the viewpoint of electron conductivity.

  The electrode of the present invention can be used for, for example, a lithium secondary battery. The lithium secondary battery of the present invention includes a current collector and a composite formed on the current collector so as to have a larger specific surface area of the active material on the current collector side than on the surface side. It is good also as what is provided with the electrode provided with the material layer, and the ion conduction medium which contact | connects an electrode and conducts lithium ion. In the lithium secondary battery of the present invention, at least one of the positive electrode and the negative electrode may be the electrode of the present invention described above, and both the positive electrode and the negative electrode may be the electrode of the present invention. As the negative electrode active material, an inorganic compound such as lithium, a lithium alloy, or a tin compound may be used.

  As the ion conduction medium of the lithium secondary battery of the present invention, a non-aqueous electrolyte solution containing a supporting salt, a non-aqueous gel electrolyte solution, or the like can be used. Examples of the solvent for the nonaqueous electrolytic solution include carbonates, esters, ethers, nitriles, furans, sulfolanes and dioxolanes, and these can be used alone or in combination. Specifically, as carbonates, cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, chloroethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl-n-butyl carbonate, methyl-t -Chain carbonates such as butyl carbonate, di-i-propyl carbonate, t-butyl-i-propyl carbonate, cyclic esters such as γ-butyllactone and γ-valerolactone, methyl formate, methyl acetate, ethyl acetate, Chain esters such as methyl butyrate, ethers such as dimethoxyethane, ethoxymethoxyethane, and diethoxyethane; nitriles such as acetonitrile and benzonitrile; Examples include furans such as lan, methyltetrahydrofuran, sulfolanes such as sulfolane and tetramethylsulfolane, and dioxolanes such as 1,3-dioxolane and methyldioxolane. Among these, the combination of cyclic carbonates and chain carbonates is preferable. According to this combination, not only the cycle characteristics representing the battery characteristics in repeated charge and discharge are excellent, but also the viscosity of the electrolyte, the electric capacity of the obtained battery, the battery output, etc. should be balanced. it can. The cyclic carbonates are considered to have a relatively high relative dielectric constant and increase the dielectric constant of the electrolytic solution, and the chain carbonates are considered to suppress the viscosity of the electrolytic solution.

The supporting salt contained in the lithium secondary battery of the present invention is, for example, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , Examples include LiSbF ₆ , LiSiF ₆ , LiAlF ₄ , LiSCN, LiClO ₄ , LiCl, LiF, LiBr, LiI, and LiAlCl ₄ . Among these, from the group consisting of inorganic salts such as LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , and organic salts such as LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) _3. It is preferable from the viewpoint of electrical characteristics to use a combination of one or two or more selected salts. The supporting salt preferably has a concentration in the non-aqueous electrolyte of 0.1 mol / L or more and 5 mol / L or less, and more preferably 0.5 mol / L or more and 2 mol / L or less. If the concentration of the supporting salt is 0.1 mol / L or more, a sufficient current density can be obtained, and if it is 5 mol / L or less, the electrolytic solution can be made more stable. Moreover, you may add flame retardants, such as a phosphorus type and a halogen type, to this non-aqueous electrolyte.

  Further, instead of the liquid ion conducting medium, a solid ion conducting polymer may be used as the ion conducting medium. As the ion conductive polymer, for example, a polymer gel composed of a polymer such as acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate, vinyl pyrrolidone, and polyvinylidene fluoride and a supporting salt can be used. Further, an ion conductive polymer and a non-aqueous electrolyte can be used in combination. In addition to the ion conductive polymer, an inorganic solid electrolyte, a mixed material of an organic polymer electrolyte and an inorganic solid electrolyte, an inorganic solid powder bound by an organic binder, or the like can be used as the ion conductive medium.

  The lithium secondary battery of the present invention may include a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as it has a composition that can withstand the range of use of the lithium secondary battery. For example, a polymer nonwoven fabric such as a polypropylene nonwoven fabric or a polyphenylene sulfide nonwoven fabric, or a thin fine olefin resin such as polyethylene or polypropylene is used. A porous membrane is mentioned. These may be used alone or in combination.

  The shape of the lithium secondary battery of the present invention is not particularly limited, and examples thereof include a coin type, a button type, a sheet type, a laminated type, a cylindrical type, a flat type, and a square type. Moreover, you may apply to the large sized thing used for an electric vehicle etc. FIG. 1 is a schematic diagram showing an example of a lithium secondary battery 10 of the present invention. The lithium secondary battery 10 includes a positive electrode sheet 13 in which a positive electrode mixture layer 12 is formed on a current collector 11, a negative electrode sheet 18 in which a negative electrode mixture layer 17 is formed on the surface of the current collector 14, a positive electrode sheet 13, A separator 19 provided between the negative electrode sheet 18 and an ion conductive medium 20 (nonaqueous electrolyte solution) that fills between the positive electrode sheet 13 and the negative electrode sheet 18 are provided. In this lithium secondary battery 10, the separator 19 is sandwiched between the positive electrode sheet 13 and the negative electrode sheet 18, and these are wound and inserted into the cylindrical case 22, and the positive electrode terminal 24 and the negative electrode sheet 18 connected to the positive electrode sheet 13. And a negative electrode terminal 26 connected to each other. The positive electrode mixture layer 12 of the lithium secondary battery 10 includes a surface layer-side mixture layer 12 a on the separator 19 side and a current collector-side mixture layer 12 b that contacts the current collector 11. The collector-side composite material layer 12b is formed so that the specific surface area of the positive electrode active material is larger than that of the layer 12a. The negative electrode mixture layer 17 of the lithium secondary battery 10 includes a surface layer-side mixture layer 17 a on the separator 19 side and a current collector-side mixture layer 17 b that contacts the current collector 11. The current collector side composite material layer 17b is formed so that the specific surface area of the negative electrode active material is larger than that of the layer 17a.

  Next, the manufacturing method of the electrode of this invention is demonstrated. The method for producing an electrode of the present invention includes a composite material production step of producing a powdery composite powder by mixing at least a binder and an active material, and an active material on the current collector side as compared with the surface layer side. And a mixture layer forming step of forming an active material on the current collector so as to have a large specific surface area. The method for producing an electrode of the present invention may be a method for producing a positive electrode using a positive electrode active material as an active material, or a method for producing a negative electrode using a negative electrode active material as active material particles. In this manufacturing method, the electrode is preferably an electrode of an alkaline secondary battery. The alkali of the alkaline secondary battery may be, for example, lithium, sodium, or potassium, and among these, lithium is more preferable. Below, the manufacturing method of a lithium secondary battery is mainly demonstrated.

[Composite production process]
In this step, at least a binder and active material particles are mixed to produce a powdery composite powder. The composite powder may further contain a conductive material. For example, when the active material has conductivity, the composite powder is composed only of the binder, and when the active material has low conductivity or no conductivity, the composite powder is bound. A material and a conductive material may be included. In the composite powder, the active material may be 30% by volume or more and 90% by volume or less of the whole. In the composite powder, the binder may be 1% by volume or more and 20% by volume or less of the whole. In the composite powder, the conductive material may be 5% by volume or more and 60% by volume or less of the whole.

  In the composite material production process, as the active material, the binder, and the conductive material, the positive electrode active material, the negative electrode active material, the binder, and the conductive material used in the lithium secondary battery described in the electrode of the present invention are used. Can do. In the composite material production step, the active material particles, the binder, and further the conductive material may be dispersed and mixed in a solvent, and then dried and crushed to produce a composite powder. In this way, the active material particles and the secondary material can be mixed more uniformly. Examples of the solvent include organic solvents such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, N, N-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran. it can. Further, a dispersant, a thickener, or the like may be added to water, and the active material particles may be slurried and mixed with a latex such as SBR.

[Composite layer forming step]
In this step, the active material is formed on the current collector so that the specific surface area of the active material is larger on the current collector side than on the surface layer side. As a method for increasing the specific surface area of the active material on the current collector side as compared with the surface layer side, for example, an active material having a plurality of specific surface areas is prepared, and an active material having a large specific surface area is arranged on the current collector. Two or more layers may be formed by forming a single layer and then forming an active material having a smaller specific surface area as a second layer on the first layer. The adjustment of the specific surface area of the active material may be performed by, for example, the particle size of the active material, or the surface characteristics of the active material particles may be changed, for example, the particle surface may be dimpled or cracks may be formed on the particle surface. It is good also as what to do. This composite material layer forming step may be performed by wet processing in which a solvent is added to the composite powder such as electrodeposition coating, or may be performed by dry processing using the composite powder as it is, such as electrostatic coating. Among these, the dry process can simplify the drying process and is preferable from the viewpoint of manufacturing energy. For example, in the composite material layer forming step, the active material may be formed on the current collector by electrostatic coating using the composite material. This electrostatic coating is more preferable because the control of the composite layer is easy, such as increasing the specific surface area of the composite layer on the current collector side and increasing the specific surface area of the composite layer on the surface side. Moreover, in electrostatic coating, omission of a drying process, segregation of a raw material component, etc. can be suppressed. The control of the specific surface area of the composite material layer changes, for example, the specific surface area and particle size of the active material particles, the added amount of the binder and the conductive material, and the characteristics of the first layer and the second layer are changed. This can be realized by coating the mixed powder. When electrostatic coating is performed, the mixture powder is atomized by using the repulsion of the charged mixture powder itself to spray the mixture powder in the form of a mist. For example, a method of applying an electric charge to the material powder from an external electrode by corona discharge may be used. Electrodeposition coating refers to a method in which a current collector is immersed in an electrolytic solution to be used as a cathode or an anode, and a composite powder is electrodeposited and formed on the current collector through direct current electricity.

  In the composite material layer forming step, after forming the composite material powder on the current collector, if necessary, a compression process for compressing with a press or the like to increase the electrode density, or a heat treatment to fix the composite material powder. You may go. For example, it is good also as what performs a compression process and a heat processing simultaneously with the heated roll press. The compression treatment or heat treatment is preferably performed so that no internal stress (strain) remains in the binder. Moreover, it is preferable to perform a compression process so that a moderate space | gap may exist. The heat treatment for fixing the composite powder is preferably performed at a temperature equal to or higher than the glass transition point Tg of the binder, and more preferably performed at a temperature equal to or higher than the softening point of the binder. It is preferable to perform the heat treatment at a temperature equal to or higher than the glass transition point Tg because internal stress (strain) hardly remains in the binder, and the active material particles can be more firmly fixed on the current collector.

In the composite layer forming step, the specific surface area ratio increase rate (B / A-1) obtained by subtracting the value 1 from the ratio of the specific surface area B of the active material on the current collector side to the specific surface area A of the active material on the surface layer side is 0. It is preferable to form the composite material layer so as to be 25 or more. As described above, the specific surface area ratio increase rate is preferably 0.50 or more, and more preferably 1.00 or more. The specific surface area ratio increase rate (B / A-1) is preferably 5.00 or less, and more preferably 3.00 or less. In the composite layer forming step, the specific surface area ratio increase rate (B / A-1) obtained by subtracting the value 1 from the ratio of the specific surface area B of the active material on the current collector side to the specific surface area A of the active material on the surface side. The composite layer is formed so that the specific surface area ratio increase rate {(B / A-1) / T} per unit thickness divided by the thickness T (μm) of the composite layer is 0.010 or more. It is preferable. As described above, the specific surface area ratio increase rate per unit thickness is more preferably 0.020 (μm ⁻¹ ) or more, and further preferably 0.030 (μm ⁻¹ ) or more. The specific surface area ratio increase rate per unit thickness is preferably 0.100 or less. In the composite material layer forming step, the composite material layer is preferably formed so that the film thickness (the thickness of the composite material layer) is 20 μm or more. The film thickness is preferably thicker, for example, preferably 25 μm or more, and more preferably 50 μm or more. Moreover, it is preferable that the film thickness of a composite material layer shall be 1000 micrometers or less.

  In the electrode, lithium secondary battery, and electrode manufacturing method of the present invention described in detail above, by increasing the reaction specific surface area in the deep part (current collector side) of the electrode that is difficult to react, the film thickness on the current collector side can be increased. Since it contributes to charging / discharging up to the active material arranged in a deep part, for example, the charging / discharging characteristics such as improvement of the discharge capacity and improvement of the discharge capacity maintenance rate after the charge / discharge cycle can be further enhanced. In addition, since the composite material layer is formed on the current collector by electrostatic coating, a large amount of solvent is not required as compared with the case where an electrode is manufactured by applying the paste to the current collector, for example. Thus, the drying process and the process for maintaining the properties of the paste can be omitted, and the electrode can be manufactured by a simpler process. Moreover, the compositional unevenness of the electrode mixture can be suppressed to further improve the charge / discharge characteristics.

  It should be noted that the present invention is not limited to the above-described embodiment, and it goes without saying that the present invention can be implemented in various modes as long as it belongs to the technical scope of the present invention.

Below, the example which produced the lithium secondary battery of this invention concretely is demonstrated as an experiment example. Experimental examples 1 , 2, 4, and 6 correspond to examples, experimental examples 7 and 8 correspond to comparative examples, and experimental examples 3, 5, and 9 to 11 correspond to reference examples .

[Preparation of powdered positive electrode mixture]
Carbon black as a conductive material was mixed at 5% by volume with respect to polyvinylidene fluoride as a binder. This mixture was mixed with a layered structure lithium nickel composite oxide of LiNi _0.8 Co _0.15 Al _0.05 O ₂ having an average particle diameter of 2 to 10 μm at a volume ratio of 35% with N-methyl-2-pyrrolidone. Dispersed. The powder is dried under reduced pressure and crushed, and a conductive binder, which is a mixture of carbon black and polyvinylidene fluoride, is adhered to the surface of lithium nickel composite oxide particles, which are positive electrode active materials, in a thin film or island shape. A positive electrode active material (positive electrode mixture) for coating was obtained. The average particle size was obtained by observing the raw material particles with an electron microscope (SEM), selecting 300 active material particles included in the observed image, measuring the diameter, and averaging.

[Preparation of powdered negative electrode mixture]
A binder obtained by mixing polyvinylidene fluoride as a binder with natural graphite having an average particle diameter of 2 to 10 μm at a volume ratio of 35% was dispersed with N-methyl-2-pyrrolidone. This was dried under reduced pressure and pulverized to obtain a negative electrode active material for coating powder (negative electrode mixture) in which polyvinylidene fluoride was adhered in the form of a thin film or an island on the surface of natural graphite particles as the negative electrode active material.

[Electrode production]
The positive electrode active material for powder coating (positive electrode mixture) and the negative electrode active material for powder coating (negative electrode mixture) are each electrostatically coated on the surface of the current collector using an electrostatic coating apparatus for powder coating. did. The positive electrode mixture was applied to both surfaces of an Al foil having a thickness of 20 μm as a current collector using an electrostatic coating apparatus. The negative electrode mixture was applied to both surfaces of a 10 μm thick Cu foil as a current collector using an electrostatic coating apparatus. The application of the composite material was performed two to three times per side, and the specific surface area of the active material on the current collector side and the surface layer side was controlled by changing the active material particle diameter for each application. Moreover, the coating amount was also changed as appropriate to change the electrode film thickness after pressing. In the electrostatic coating apparatus, -60 kV was applied to charge the active material, and the Al and Cu current collector foils were grounded to generate a potential difference therebetween, so-called powder electrostatic coating was performed. The positive electrode and the negative electrode after electrostatic coating were each densified with a roll press. At this time, the temperature of the roll press is controlled to 120 ° C. so that the binders on the surface of the active material are brought into close contact with each other without any internal strain, and both the positive electrode having an active material density of 2.0 g / cm ³ A negative electrode having a material density of 1.2 g / cm ³ was obtained.

[Battery fabrication]
The above positive electrode and negative electrode were appropriately combined, and rolled with a 25 μm-thick porous polyethylene separator interposed therebetween to produce a roll electrode body. This roll-shaped electrode body was inserted into a 18650 type cylindrical case, impregnated with a non-aqueous electrolyte, and then sealed to produce a cylindrical lithium ion secondary battery. As the non-aqueous electrolyte, a solution obtained by dissolving LiPF ₆ at a concentration of 1M in a mixed solvent in which ethylene carbonate and diethyl carbonate were mixed at 30: 70% by volume was used.

(High current density charge / discharge cycle test)
In the charge / discharge cycle test, charging was performed at a constant current with a current density of 10 mA / cm ^{2 up to} a charge upper limit voltage of 4.1 V under a temperature condition of 20 ° C., and then the discharge lower limit voltage was 3 with a constant current of a current density of 10 mA / cm ^2. Charging / discharging for discharging to 0.0 V is defined as one cycle, and this cycle is performed for a total of 100 cycles. Then, using the equation {discharge capacity after 100 cycles / initial discharge capacity × 100%} as the initial discharge capacity as the discharge capacity before the charge / discharge cycle test, the capacity retention rate (%) after 100 cycles was calculated. . The higher the initial discharge capacity, the higher the output, and the higher the capacity retention rate after 100 cycles, the higher the uniform reactivity.

[Experiment 1]
According to the method described above, the film thickness of the electrode mixture layer as a whole is 20 μm, the number of times of single-sided application is 2, the average particle size of the active material on the current collector side is 4 μm, and the average particle size of the active material on the surface layer side is 5 μm. A positive electrode and a negative electrode were produced under the same conditions, and the produced positive electrode and negative electrode were combined and wound. In electrostatic powder coating, an active material having an average particle size of 4 μm is used for the first formation of the mixture layer on the current collector, and an active material having an average particle size of 5 μm is used for the formation of the second mixture layer. It was. Table 1 shows the production conditions and evaluation results of Experimental Example 1 and Experimental Examples 2 to 11 described later.

[Experimental Examples 2 to 5]
A battery obtained by performing the same process as in Experimental Example 1 except that the film thickness of the entire electrode mixture layer was 40 μm and the average particle size of the active material on the current collector side was 2.5 μm was used as Experimental Example 2. A battery obtained by performing the same process as in Experimental Example 1 except that the total thickness of the electrode mixture layer was 40 μm and the average particle diameter of the active material on the current collector side was 2 μm was defined as Experimental Example 3. Further, the same steps as in Experimental Example 1 were performed except that the total thickness of the electrode mixture layer was 100 μm, the average particle size of the active material on the current collector side was 5 μm, and the average particle size of the active material on the surface layer side was 10 μm. The battery thus obtained was named experimental example 4. The total thickness of the electrode mixture layer is 100 μm, the number of times of single-sided coating is 3 times, the average particle size of the active material on the current collector side is 3 μm, the average particle size of the active material on the middle layer is 5.5 μm, A battery obtained by performing the same process as in Experimental Example 1 except that the average particle size of the active material was 10 μm was set as Experimental Example 5.

[Experimental Examples 6 and 7]
The same steps as in Experimental Example 1 can be performed except that the total thickness of the electrode mixture layer is 15 μm, the average particle size of the active material on the current collector side is 2 μm, and the average particle size of the active material on the surface layer side is 3 μm. The obtained battery was designated as experimental example 6. Further, a battery obtained by performing the same process as in Experimental Example 1 except that the film thickness of the entire composite material layer of the electrode was 15 μm, and the average particle diameter of the active material on the current collector side and the surface layer side was 3 μm was obtained. It was.

[Experimental Examples 8 to 11]
A battery obtained by performing the same process as in Experimental Example 1 except that the average particle diameter of the active material on the current collector side and the surface layer side was set to 5 μm was set as Experimental Example 8. A battery obtained by performing the same process as in Experimental Example 1 except that the average particle size of the active material on the current collector side was 4.5 μm was set as Experimental Example 9. Further, the same steps as those in Experimental Example 1 were performed except that the total thickness of the electrode mixture layer was 40 μm, the average particle size of the active material on the current collector side was 8 μm, and the average particle size of the active material on the surface layer side was 10 μm. The battery thus obtained was named experimental example 10. Further, the same steps as those in Experimental Example 1 were performed except that the total thickness of the electrode mixture layer was 100 μm, the average particle size of the active material on the current collector side was 8 μm, and the average particle size of the active material on the surface layer side was 10 μm. The battery thus obtained was named experimental example 11.

  In Experimental Examples 1 to 4 and 6 to 11, the thickness of each layer of the current collector layer and the surface material layer was approximately ½ of the entire electrode film thickness. In Experimental Example 5, the thickness of each of the current mixture layer, the middle composite layer, and the surface composite layer was about 1/3 of the entire electrode film thickness.

(Results and discussion)
Table 1 shows the measurement results of the electrode film thickness (μm) on one side, the number of coatings per side, the average particle size (μm) of the active material on the current collector side, the middle layer and the surface layer, and the increase in the specific surface area ratio of the active material. The values obtained by dividing the rate, the specific surface area ratio increase rate by the electrode film thickness, the discharge capacity per unit weight of the positive electrode active material (mAh / g), and the capacity retention rate (%) at 100 cycles are shown together. The specific surface area increase rate is calculated by calculating the specific surface area of the active material on the surface layer side and the current collector side from the average particle diameter assuming that the active material is a sphere, and the active material on the current collector side with respect to the specific surface area of the active material on the surface layer side This is a value obtained by subtracting 1 from the ratio of the specific surface area. The specific surface area S (m ² / g) is calculated from the equation S = 6 / (D × ρ) using the average particle diameter D (m) and the density ρ (g / m ³ ) of the active material. be able to.

  As shown in Table 1, when compared with the same film thickness, when the specific surface area of the active material on the current collector side was larger than that on the surface layer side, the initial discharge capacity and capacity retention ratio were improved. In addition, when the thickness of the composite layer is 20 μm or more, the specific surface area ratio increase rate is 0.25 or more, and the specific surface area ratio increase rate / thickness value is 0.010 or more, the initial discharge capacity is remarkable. As a result, the capacity retention rate was greatly improved. In particular, the film thickness ranges from 20 μm to 100 μm, the specific surface area ratio increase rate ranges from 0.25 to 3.0, and the specific surface area ratio increase rate / film thickness ranges from 0.010 to 0.040. I liked it. The thicker the film thickness, the larger the battery output per unit volume. Therefore, it is considered that the film thickness is preferably 20 μm or more, or 50 μm or more. It was inferred that the improvement of the charge / discharge characteristics was due to the significant contribution to charge / discharge up to the active material disposed in the thick part of the current collector side. It can be said that this is mainly because the reaction specific surface area in the deep part of the electrode which is difficult to react is increased. In this example, the specific surface area on the surface layer and the current collector side is changed by changing the average particle size of the active material. However, the surface shape of the active material is changed to, for example, dimples, or the active material surface is cracked. It was speculated that the same effect can be obtained if the specific surface area is different even if the average particle size is the same.

  In addition, as a method for producing an electrode, a method in which a paste is applied a plurality of times is conceivable instead of a powder electrostatic coating method. However, a drying process is required and the production cost of the electrode increases, which is not practical. . Furthermore, it was confirmed that when the paste of the next layer was applied to the dried composite material layer, not only the flow of the active material but also the binding material and the conductive material were subjected to flow segregation, so that the above effect could not be obtained. . Considering from this example, in the composite layer (active material) on the current collector side where the ion conduction medium is difficult to flow, the specific surface area is increased to increase the reactivity, thereby further increasing the output and the long life. It turned out that an electrode can be produced. Moreover, it turned out that electrode preparation by the electrostatic powder coating method is more suitable.

  DESCRIPTION OF SYMBOLS 10 Lithium secondary battery, 11, 14 Current collector, 12 Positive electrode mixture layer, 12a Surface layer side mixture layer, 12b Current collector side mixture layer, 13 Positive electrode sheet, 17 Negative electrode mixture layer, 17a Surface layer side mixture layer , 17b Current collector side composite material layer, 18 negative electrode sheet, 19 separator, 20 ion conductive medium, 22 cylindrical case, 24 positive electrode terminal, 26 negative electrode terminal.

Claims (6)

An electrode used in a battery,
A current collector,
A mixture layer formed on the current collector including the active material in a tendency that the specific surface area of the active material is larger on the current collector side than on the surface layer side,
The mixture layer, the collector specific surface area ratio subtracting the value 1 from the specific surface area ratio increases of B in the active material of the body side with respect to the specific surface area A of the active material of the surface layer side (B / A-1) is 0 .25 is formed in to 1.0 range, the surface layer the collector specific surface area ratio subtracting the value 1 from the specific surface area ratio increases of B in the active material of the body side with respect to the specific surface area a of the active material (B / A-1) divided by the thickness T (μm) of the composite layer, the specific surface area ratio increase rate per unit thickness {(B / A-1) / T} is 0.010 or more and 0.040 or less. Range of
The active material is any one or more of a lithium manganese composite oxide, a lithium cobalt composite oxide, and a lithium nickel composite oxide.
electrode. The electrode according to claim 1, wherein the composite material layer is formed with a film thickness of 20 μm or more. The electrode according to claim 1 or 2 , wherein the composite layer has a specific surface area ratio increase rate {(B / A-1) / T} per unit thickness of 0.020 or more. The electrode according to any one of claims 1 to 3 , which is a positive electrode;
An ion conducting medium in contact with the electrode and conducting lithium ions;
Rechargeable lithium battery. A method for producing an electrode used in a battery,
A mixture layer forming step of forming an active material on the current collector in a tendency that the specific surface area of the active material is larger on the current collector side than on the surface layer side,
The composite layer containing the active material has a specific surface area ratio increase rate (B / A−) obtained by subtracting the value 1 from the ratio of the specific surface area B of the active material on the current collector side to the specific surface area A of the active material on the surface layer side. 1) is formed in a range of 0.25 to 1.0, the current collector specific surface area the specific surface area ratio obtained by subtracting the value 1 from the ratio of B of the active material body side with respect to the specific surface area a of the active material of the surface layer side Specific surface area ratio increase rate {(B / A-1) / T} per unit thickness obtained by dividing the increase rate (B / A-1) by the thickness T (μm) of the composite material layer is 0.010 or more Formed in a range of 0.040 or less,
The active material uses any one or more of lithium manganese composite oxide, lithium cobalt composite oxide and lithium nickel composite oxide,
Electrode manufacturing method. The method for producing an electrode according to claim 5 , wherein in the composite material layer forming step, an active material is formed on the current collector by electrostatic coating using a raw material including at least a binder and the active material.

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