Electrode for lithium ion secondary battery and lithium ion secondary battery

Abstract

The problem to be solved by the present invention is to provide an electrode for a lithium ion secondary battery and a lithium ion secondary battery using the electrode for a lithium ion secondary battery, wherein the electrode for a lithium ion secondary battery is an electrode in which an electrode composite material is filled in a metal porous body, and the electrolyte penetration is excellent and the ion diffusivity is improved. In order to solve the above problems, the present invention provides an electrode for a lithium ion secondary battery, comprising a current collector composed of a metal porous body, and an electrode layer obtained by filling the current collector with an electrode composite material containing at least an electrode active material, wherein the porosity of the current collector in the electrode layer in the middle region in the thickness direction is less than the porosity in the two surface regions in the thickness direction, the middle region is filled with a first electrode active material, and the two surface regions are filled with a second electrode active material having a larger particle size than the first electrode active material.

Description

Technical Field

[0001] The present invention relates to an electrode for a lithium ion secondary battery and a lithium ion secondary battery using the electrode for a lithium ion secondary battery. Background Art

[0002] Conventionally, lithium ion secondary batteries have been widely used as secondary batteries with high energy density. A lithium ion secondary battery has a structure in which a separator is present between a positive electrode and a negative electrode and the battery is filled with a liquid electrolyte (electrolyte).

[0003] Such lithium ion secondary batteries have various requirements depending on the application. For example, when used in automobiles, the volume energy density is required to be further improved. For this purpose, a method of increasing the packing density of the electrode active material can be mentioned.

[0004] As a method for increasing the packing density of the electrode active material, a method of using a metal porous body such as a foamed metal as a collector constituting the positive electrode layer and the negative electrode layer is proposed (for example, refer to Patent Document 1). The metal porous body has a mesh structure and a large surface area. By filling the inside of the mesh structure with an electrode composite material containing an electrode active material, the amount of active material per unit area of ​​the electrode layer can be increased.

[0005] On the other hand, a structure of an electrode formed by filling an electrode composite material into a metal porous body is also disclosed, which contains two electrode active materials with different particle sizes in the same electrode to achieve high capacity and excellent cycle characteristics (for example, refer to Patent Document 2).

[0006] [Prior art literature]

[0007] (Patent Document)

[0008] Patent Document 1: Japanese Patent Application Laid-Open No. 7-099058

[0009] Patent Document 2: Japanese Patent Application Publication No. 2012-033280 Summary of the invention

[0010] [Problems to be solved by the invention]

[0011] Compared with the coated electrode using metal foil as the current collector, the electrode using the metal porous body described in Patent Document 1 as the current collector can produce an electrode with a higher weight per unit area, but the film thickness becomes thicker. Therefore, the moving distance of electrons and lithium ions becomes longer, the ion diffusion resistance increases, and the rate characteristics decrease.

[0012] In addition, if the film thickness increases, the permeability of the electrolyte decreases, and the electrolyte penetration into the electrode becomes insufficient. Therefore, the supply of anions and cations is insufficient, the internal resistance of the formed lithium ion secondary battery cell increases, and the output-input characteristics (output density) of the battery decrease.

[0013] The present invention is completed in view of the above-mentioned problems, and its purpose is to provide an electrode for a lithium ion secondary battery and a lithium ion secondary battery using the electrode for a lithium ion secondary battery. The electrode for a lithium ion secondary battery is an electrode filled with an electrode composite material in a metal porous body, has excellent electrolyte penetration and improved ion diffusion.

[0014] [Technical means to solve the problem]

[0015] The inventors of the present invention have conducted in-depth research to solve the above-mentioned problems. Furthermore, it has been found that in an electrode layer of an electrode for a lithium-ion secondary battery using a collector composed of a metal porous body, by changing the particle size of the electrode active material in the thickness direction of the electrode layer and also changing the porosity of the collector, the above-mentioned problems can be solved, thereby completing the present invention. Specifically, the present invention provides the following contents.

[0016] (1) An electrode for a lithium ion secondary battery, comprising a current collector composed of a metal porous body and an electrode layer obtained by filling the current collector with an electrode composite material containing at least an electrode active material, wherein:

[0017] The porosity of the collector in the electrode layer in the middle region in the thickness direction is smaller than the porosity in both surface regions in the thickness direction.

[0018] The intermediate region is filled with a first electrode active material, and the both surface regions are filled with a second electrode active material having a larger particle size than the first electrode active material.

[0019] According to the invention of (1), the porosity in the thickness direction of the collector is configured as large / small / large in the order of surface area / middle area / surface area (back area), and the collector is filled with electrode active materials of different particle sizes in a large / small / large manner. Thus, the ion conduction path starting from the two surface areas can be ensured, and the electrolyte can reliably penetrate into the middle area.

[0020] (2) The lithium ion secondary battery electrode according to (1), wherein a packing density of the electrode active material in the intermediate region is greater than a packing density of the electrode active material in the both surface regions.

[0021] According to the invention of (2), the effect of (1) can be further enhanced by setting the packing density of the electrode active material in the middle region>the both surface regions.

[0022] (3) A lithium ion secondary battery comprising a positive electrode, a negative electrode, and a separator between the positive electrode and the negative electrode, wherein:

[0023] At least one of the positive electrode and the negative electrode is the electrode for a lithium ion secondary battery according to (1) or (2).

[0024] According to the invention of (3), a lithium ion secondary battery capable of achieving the effects of (1) and (2) can be obtained.

[0025] (4) A method for producing an electrode for a lithium ion secondary battery, comprising:

[0026] The first step is to form a current collector composed of a porous metal body, wherein the porosity in the middle region in the thickness direction of the current collector is smaller than the porosity in both surface regions; and

[0027] In the second step, the middle region of the current collector is filled with an electrode composite material including a first electrode active material, and both surface regions of the current collector are filled with an electrode composite material including a second electrode active material having a larger particle size than the first electrode active material.

[0028] According to the invention of the manufacturing method of (4), a lithium ion secondary battery capable of achieving the effects of (1) to (3) can be obtained.

[0029] (5) The method for producing an electrode for a lithium ion secondary battery according to (4), wherein the electrode composite material containing the first electrode active material and the second electrode active material is applied and filled from the two surface regions of the collector.

[0030] According to the invention of the manufacturing method of (5), when the electrode composite material containing the first electrode active material and the second electrode active material is coated and filled from the two surface area sides respectively, the change in the porosity of the collector in the thickness direction exerts a filtering effect, and the first electrode active material with a relatively small particle size is filled in the middle area, and the second electrode active material with a relatively large particle size is filled in the two surface areas.

[0031] (Effects of the Invention)

[0032] According to the present invention, there is provided an electrode for a lithium ion secondary battery and a lithium ion secondary battery using the electrode for a lithium ion secondary battery. The electrode for a lithium ion secondary battery is an electrode in which an electrode composite material is filled in a metal porous body, has excellent electrolyte permeability and improved ion diffusivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1This is a schematic exploded perspective view showing one embodiment of the electrode for a lithium ion secondary battery of the present invention.

[0034] Figure 2 This is a conceptual diagram showing one embodiment of the electrode for a lithium ion secondary battery of the present invention.

[0035] Figure 3 This is a conceptual diagram showing an example of a method for producing an electrode for a lithium ion secondary battery of the present invention.

[0036] Figure 4 This is a graph of battery cell resistance measured for initial characteristics in Examples.

[0037] Figure 5 This is a graph showing the capacity maintenance rate of the initial characteristics measured in Examples.

[0038] Figure 6 This is a graph showing the capacity retention rate after durability measurement in Examples.

[0039] Figure 7 This is a graph showing the resistance change rate after durability measurement in Examples. DETAILED DESCRIPTION

[0040] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. The content of the present invention is not limited to the description of the following embodiments. The electrode for lithium-ion secondary battery of the present invention can be applied to the positive electrode, the negative electrode, or both in the lithium-ion secondary battery. In addition, in the following embodiments, a lithium-ion battery in which the electrolyte is a liquid is used as an example for description, but the present invention is not limited to this and can also be applied to a secondary battery with a solid electrolyte. In addition, it can also be applied to batteries other than lithium-ion batteries.

[0041] <Overall structure of lithium-ion secondary battery>

[0042] like Figure 1 As shown, in the lithium-ion secondary battery 10, the positive electrode layer 21 and the negative electrode layer 31 as electrodes for the lithium-ion secondary battery are stacked with a separator 41 between them. An electrolyte not shown is arranged between the layers to form the lithium-ion secondary battery 10. The positive electrode tab 22 extends from the positive electrode layer 21 for current collection, and the negative electrode tab 32 extends from the negative electrode layer 31 for current collection. The positive electrode layer 21 constitutes the positive electrode in the present invention, and the negative electrode layer 31 constitutes the negative electrode in the present invention. The structure of the electrode for the lithium-ion secondary battery of the present invention is not particularly limited, and can be a stacked type or a wound type.

[0043] For the positive electrode and the negative electrode, two materials that can constitute electrodes are selected, and the charge and discharge potentials of the two compounds are compared. The one that shows a higher potential is used for the positive electrode, and the one that shows a lower potential is used for the negative electrode. Any battery can be constructed. The positive electrode / electrolyte / negative electrode is used as a single cell unit, and a lithium-ion secondary battery is constructed by stacking any number of them.

[0044] [Electrolytes]

[0045] The electrolyte is a liquid electrolyte solution obtained by dissolving an electrolyte in a non-aqueous solvent. The electrolyte dissolved in the non-aqueous solvent is not particularly limited, and examples thereof include LiPF 6 , LiBF 4 、LiC1O 4 、LiN(SO 2 CF 3 )、LiN(SO 2 C 2 F 5 ) 2 、LiCF 3 SO 3 ,LiC 4 F 9 SO 3 、LiC(SO 2 CF 3 ) 3 、LiF、LiCl、LiI、Li 2 S. Li 3 N.Li 3 P.Li 10 G 2 S 12 (LGPS), Li 3 PS 4 , Li 6 PS 5 Cl, Li 7 P 2 S 8 I. Li x PO y N z (x=2y+3z-5、LiPON)、Li 7 La 3 Zr 2 O 12 (LLZO), Li 3x La 2 / 3- x TiO 3 (LLTO), Li 1+x Al x Ti 2-x (PO 4 ) 3(0≤x≤1, LATP), Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 (LAGP), Li 1+x+ y Al x Ti 2-x Si y P 3-y O 12 , Li 1+x+y Al x (Ti,Ge) 2-x Si y P 3-y O 12 , Li 4-2x Zn x GeO 4 (LISICON), etc. These may be used alone or in combination of two or more.

[0046] The nonaqueous solvent contained in the electrolyte solution is not particularly limited, and examples thereof include aprotic solvents such as carbonates, esters, ethers, nitriles, sulfones, and lactones. Specifically, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), tetrahydrofuran (THF), 2-methyltetrahydrofuran, dioxane, 1,3-dioxolane, diethylene glycol dimethyl ether, ethylene glycol dimethyl ether, acetonitrile (AN), propionitrile, nitromethane, N,N-dimethylformamide (DMF), dimethyl sulfoxide, sulfolane and γ-butyrolactone. The above-mentioned can be used alone or in combination of two or more.

[0047] [Separator]

[0048] The lithium ion secondary battery of the present invention may include a separator when using a liquid electrolyte. The separator is located between the positive electrode and the negative electrode. Its material and thickness are not particularly limited, and a known separator such as polyethylene and polypropylene that can be used for lithium ion secondary batteries can be used.

[0049] The solid electrolyte used in the solid electrolyte layer of the solid battery is not particularly limited, and examples thereof include sulfide-based solid electrolyte materials, oxide-based solid electrolyte materials, nitride-based solid electrolyte materials, and halide-based solid electrolyte materials. Examples of sulfide-based solid electrolyte materials include LPS-based halogens (Cl, Br, I), Li 2 SP 2 S 5 , Li 2 SP 2 S 5 -LiI, etc. It should be noted that the above-mentioned "Li 2 SP 2 S 5 " refers to the use of Li 2 S and P 2 S 5 The sulfide-based solid electrolyte material is a raw material composition of the sulfide-based solid electrolyte material, and the same applies to other records. As oxide-based solid electrolyte materials, for example, if it is a lithium-ion battery, there can be listed: sodium superion conductor (NASICON) type oxides, garnet type oxides, perovskite type oxides, etc. As NASICON type oxides, there can be listed, for example: oxides containing Li, Al, Ti, P and O (such as Li 1.5 Al 0.5 Ti 1.5 (PO 4 ) 3 As garnet-type oxides, for example, oxides containing Li, La, Zr, and O (such as Li 7 La 3 Zr 2 O 12 As perovskite-type oxides, for example, oxides containing Li, La, Ti, and O (e.g., LiLaTiO 3 ).

[0050] <Structure of electrode layer>

[0051] Next, the electrode layer which is a characteristic of the present invention will be described. Figure 2 As shown in the cross-sectional schematic diagram of , the positive electrode layer 21 and the negative electrode layer 31 respectively have planar current collectors 25 and 35 composed of a metal porous body, and the metal porous body has pores (connecting pores) that are continuous with each other. The pores of the current collectors 25 and 35 are filled and arranged with a positive electrode composite material 27 containing a positive electrode active material 26 and a negative electrode composite material 37 containing a negative electrode active material 36. It should be noted that in Figure 2In FIG. 1 , an example of the positive electrode layer 21 is shown, and the negative electrode layer 31 has the same structure, so only the reference numerals are given in parentheses. In the figure, the direction of D is the thickness direction.

[0052] [Current Collector]

[0053] The current collectors 25 and 35 use a metal porous body made of metal, i.e., a current collector. Examples thereof include mesh, woven fabric, nonwoven fabric, embossed body, punched body, porous metal, foamed body, etc., and preferably use a foamed metal. Among them, a foamed metal having a three-dimensional network structure with continuous pores is preferably used, for example, CELMET (registered trademark) (manufactured by Sumitomo Electric Industries, Ltd.) and the like can be used.

[0054] The metal porous body has a mesh structure and a large surface area. By using the metal porous body composed of metal as a current collector, the electrode composite material containing the electrode active material can be filled inside the mesh structure, thereby increasing the amount of active material per unit area of ​​the electrode layer, and as a result, the volume energy density of the lithium ion secondary battery can be improved.

[0055] In addition, since the electrode composite material is easily fixed, the electrode composite material layer can be thickened without thickening the coating slurry of the electrode composite material. In addition, the amount of binder composed of an organic polymer compound required for thickening can be reduced.

[0056] Therefore, compared with conventional electrodes using metal foil as current collectors, the electrode composite material layer can be thickened, resulting in an increase in the capacity per unit area of ​​the electrode, thereby achieving a higher capacity lithium ion secondary battery.

[0057] The current collectors 25 and 35 are continuous in the thickness direction in this embodiment, but have a surface region including at least two surfaces and an intermediate region sandwiched by two surface regions in the thickness direction. Specifically, in this embodiment, the intermediate regions 25B and 35B, the surface regions 25A and 35A, and the surface regions (back regions) 25C and 35C of the current collectors 25 and 35 have different porosities. It should be noted that the thickness direction refers to the out-of-plane direction of the planar current collector. That is, the current collector is formed into a three-layer structure of surface region 25A / intermediate region 25B / surface region (back region) 25C or surface region 35A / intermediate region 35B / surface region (back region) 35C, and its porosity is surface region>intermediate region. It should be noted that the intermediate regions 25B and 35B are arranged in the approximate center of the thickness direction.

[0058] In the present invention, the two surface regions and the intermediate region may be a single continuous current collector as described above, or may be a current collector in which a plurality of current collectors in each region are joined.

[0059] By making the porosity of the middle region of the collector different from that of the two surface regions, a filtering effect is exerted when the electrode composite material containing at least an electrode active material is filled into the pores of the collector. The electrode active material particles with large particle size remain in the two surface regions, and the electrode active material particles with small particle size are easily filled into the middle region of the collector.

[0060] The intermediate regions 25B and 35B are preferably arranged to be 20% or more and 80% or less of a thickness D of an electrode layer described later.

[0061] The overall average porosity of the metal porous body is preferably 90 to 99%. The average porosity of the metal porous body is within this range, thereby increasing the filling amount of the electrode composite material and improving the energy density of the battery. Specifically, if the average porosity exceeds 99%, the mechanical strength of the metal porous body is significantly reduced, and it is easy to break due to the change in the electrode volume caused by charging and discharging. On the contrary, if it is less than 90%, not only the filling amount of the electrode composite material decreases, but also the ion conductivity of the electrode decreases, and it is not easy to obtain sufficient input-output characteristics. From these viewpoints, the more preferred average porosity is 93 to 98%. It should be noted that since the collector of the present invention has a porosity difference between the surface area and the middle area, the average porosity is the porosity of the collector as a whole constituting the electrode layer. It should be noted that the above porosity is the (pore space volume) / (total volume of the metal porous body) of the metal porous body in the state of the metal porous body before the electrode layer is formed, and the volume and mass are measured and calculated using the ratio to the true density of the metal.

[0062] From the viewpoint of reliably obtaining the filtering effect, the porosity of the porous metal body in the intermediate regions 25B and 35B is preferably 93% to 95%, and the porosity of the surface regions 25A, 35A, 25C, and 35C is preferably 95% to 98%.

[0063] The average pore size of the metal porous body in the electrode layer is preferably less than 500 μm. By making the average pore size of the metal porous body within this range, the distance between the metal skeleton and the negative electrode active material 13 filled in the interior of the metal porous body is stabilized, and the electronic conductivity is improved, thereby suppressing the increase in the internal resistance of the battery. In addition, even if the volume change caused by charging and discharging occurs, the shedding of the electrode composite material can be suppressed. It should be noted that the above-mentioned average pore size is the value of the median particle size (d50) measured by mercury porosimetry.

[0064] The specific surface area of ​​the porous metal body is preferably 1000 to 10000 m 2 / m 3This is 2 to 10 times the specific surface area of ​​conventional collector foils. By making the specific surface area of ​​the porous metal body within this range, the contact between the electrode composite material and the collector 11 is improved, thereby suppressing the increase in the internal resistance of the battery. A more preferred specific surface area is 4000 to 7000 m 2 / m 3 .

[0065] Examples of the metal of the metal porous body include nickel, aluminum, stainless steel, titanium, copper, silver, nickel-chromium alloy, etc. Among them, foamed aluminum is preferred as the current collector constituting the positive electrode, and foamed copper or foamed stainless steel is preferred as the current collector constituting the negative electrode.

[0066] [Electrode layer]

[0067] The electrode layer of the lithium ion secondary battery electrode of the present embodiment is formed by filling a current collector, which is a metal porous body made of metal, with an electrode composite material.

[0068] The thickness of the electrode layer is not particularly limited, but the electrode for lithium ion secondary battery of the present invention can form a thick electrode layer because a metal porous body composed of metal is used as a current collector. As a result, the amount of active material per unit area of ​​the electrode layer increases, and a battery with a high energy density can be obtained.

[0069] The thickness D of the electrode layer of the lithium ion secondary battery electrode of the present invention is, for example, 200 to 500 μm.

[0070] 〔Electrode composite materials〕

[0071] The electrode composite material constituting the electrode layer of the present invention contains at least an electrode active material. The electrode composite material applicable to the present invention may contain other components as long as it contains an electrode active material as an essential component. As other components, there is no particular limitation, as long as they are components that can be used when making a lithium ion secondary battery. Examples include solid electrolytes, conductive additives, binders, etc.

[0072] (Positive electrode composite material)

[0073] The positive electrode composite material constituting the positive electrode layer contains at least a positive electrode active material, and may also contain other components such as a solid electrolyte, a conductive additive, and a binder. The positive electrode active material is not particularly limited as long as it is a material that can embed and de-embed lithium ions, and examples thereof include: LiCoO 2 、Li(Ni 5 / 10 Co 2 / 10 Mn 3 / 10 ) 2 、Li(Ni 6 / 10 Co2 / 10 Mn 2 / 10 ) 2 、Li(Ni 8 / 10 Co 1 / 10 Mn 1 / 10 ) 2 、Li(Ni 0.8 Co 0.15 Al 0.05 ) 2 、Li(Ni 1 / 6 Co 4 / 6 Mn 1 / 6 ) 2 、Li(Ni 1 / 3 Co 1 / 3 Mn 1 / 3 ) 2 、LiCoO 4 、LiMn 2 O 4 、LiNiO 2 、LiFePO 4 , lithium sulfide, sulfur, etc.

[0074] (Negative electrode composite material)

[0075] The negative electrode composite material constituting the negative electrode layer contains at least a negative electrode active material, and may also contain other components such as a solid electrolyte, a conductive aid, and a binder. The negative electrode active material is not particularly limited as long as it can embed and de-embed lithium ions, and examples thereof include: metallic lithium, lithium alloys, metal oxides, metal sulfides, metal nitrides, Si, SiO, and carbon materials such as artificial graphite, natural graphite, hard carbon, and soft carbon.

[0076] (Other ingredients)

[0077] The electrode composite material may contain other components other than the electrode active material at will. As other components, there is no particular limitation, as long as they are components that can be used when making lithium-ion secondary batteries. Examples include: conductive aids, binders, etc. As conductive aids for the positive electrode, acetylene black, etc. can be exemplified, and as binders for the positive electrode, polyvinylidene fluoride, etc. can be exemplified. As binders for the negative electrode, sodium carboxymethyl cellulose, styrene-butadiene rubber, sodium polyacrylate, etc. can be exemplified.

[0078] (Particle size of electrode active material)

[0079] The middle regions 25B and 35B are filled with first electrode active materials 26a and 36a, and the both surface regions 25A, 35A, 25C and 35C are filled with second electrode active materials 26a and 36a. The particle size of the second electrode active material is larger than that of the first electrode active material.

[0080] Specifically, the particle size of the first electrode active material 26a and 36a preferably has a median particle size (D50) of 3 μm or more and less than 7 μm, and the particle size of the second electrode active material 26b and 36b preferably has a median particle size (D50) of 7 μm or more and 15 μm or less. Thus, the ion conduction path starting from the two surface regions can be ensured, and the electrolyte can reliably penetrate into the middle region.

[0081] (Packing density of electrode active material)

[0082] In the electrode layers 21 and 31, the packing density of the electrode active material in the middle region is preferably greater than that in the surface region. Specifically, in the positive electrode, the packing density of the electrode active material in the middle region is preferably 2.8 to 3.8 g / cm 3 The packing density of the electrode active material in the surface area is preferably 2.0 to 2.8 g / cm 3 In the negative electrode, the packing density of the electrode active material in the middle region is preferably 1.0 to 2.0 g / cm 3 The packing density of the electrode active material in the surface area is preferably 0.5 to 2.0 g / cm 3 .

[0083] <Method for producing electrode layer>

[0084] (First step)

[0085] In the first step, a planar collector 25, 35 composed of a metal porous body is formed, wherein the porosity in the middle region in the thickness direction of the collector is smaller than the porosity in the two surface regions. This step only requires pre-fabrication of collectors having different porosities in the middle region and the surface region, and joining them into a layered stack.

[0086] (Second step)

[0087] In the second step, the electrode composite material including the first electrode active material is filled in the middle region of the current collector, and the electrode composite material including the second electrode active material having a larger particle size than the first electrode active material is filled in both surface regions of the current collector.

[0088] like Figure 3 As shown, the electrode composite material 70 containing the first electrode active material and the second electrode active material is applied and filled from both surface areas of the collector. Figure 3In one example, an electrode layer can be formed by slurrying the positive electrode composite material 27 and the negative electrode composite material 37, and then using die coaters 50 and 60 to squeeze the slurry with plungers 50a and 60a of die coaters 50 and 60 and discharge it from a die head, thereby coating the electrode composite material into a surface from both sides of the collector, and filling the mesh structure of the collector with the slurry containing the electrode composite material.

[0089] In this case, there are methods of filling the electrode composite material from both the arbitrary surface and the surface opposite to the current collector at once, and methods of filling the electrode composite material on each of the arbitrary surface and the surface opposite to the current collector. Figure 3 As shown, a method of filling the electrode composite material at once from both the arbitrary surface of the current collector and the surface opposite thereto is preferred.

[0090] The electrode composite material can be filled with an electrode composite material containing both a first electrode active material and a second electrode active material. That is, as long as the electrode composite material containing electrode active material particles having multiple peaks in the particle size distribution is filled. By filling the collector after the first step, a filtering effect caused by the difference in the porosity of the collector is generated, and the electrode composite material containing the second electrode active material with a relatively large particle size is filled in the two surface areas of the collector, and the first electrode active material with a relatively small particle size is filled in the middle area.

[0091] It should be noted that, without being limited to this, the electrode layer of the present invention can also be obtained by the following method: filling the electrode composite material containing the first electrode active material into the collector constituting the middle area, and filling the electrode composite material containing the second electrode active material into the collector constituting the two surface areas, and then connecting the two collectors.

[0092] It should be noted that the method of filling the electrode composite material is not limited to the die coating method, and a dipping method of impregnating the electrode composite material may also be used.

[0093] <Method for producing lithium-ion secondary battery>

[0094] The method for manufacturing the lithium ion secondary battery of the present invention using the above electrode layer is not particularly limited, and a common method in the art can be applied. Figure 1 As shown, the electrode layers are joined to each other through the electrolyte, thereby obtaining the lithium ion secondary battery electrode of this embodiment. The method of joining the electrode layers to each other can apply the usual method in the technical field. For example, the collector filled with the electrode composite material is dried and then pressed to obtain the lithium ion secondary battery electrode. The density of the electrode composite material can be increased by pressing and can be adjusted to a desired density.

[0095] According to the lithium ion secondary battery electrode of the present invention and the lithium ion secondary battery using the lithium ion secondary battery electrode, even if the film thickness of the electrode layer is thick, the electrolyte can penetrate into the central area in the thickness direction. Moreover, the movement distance of the ions in the electrode can be shortened, so the increase in ion diffusion resistance can be suppressed, and as a result, the durability such as rate characteristics can be improved. In particular, even when high loads such as rapid charging and discharging are applied, ions can be quickly supplied, so it can contribute to the improvement of durability under high load environments.

[0096] Furthermore, even if the electrode layer is thick, it is possible to suppress the shortage of electron supply, thereby suppressing the increase of electronic resistance and improving the output characteristics of the lithium ion secondary battery.

[0097] [Example]

[0098] Hereinafter, the present invention will be described in more detail based on examples, but the present invention is not limited thereto.

[0099] <Example 1>

[0100] [Production of positive electrode]

[0101] (Positive electrode collector)

[0102] As the positive electrode collector in the middle region, a foamed aluminum with a thickness of 0.5 mm and a porosity of 95% was prepared. As the positive electrode collector in the surface region, a foamed aluminum with a thickness of 0.5 mm and a porosity of 97% was prepared. The positive electrode collector in the surface region was sandwiched by the current collector in the middle region and the positive electrode collector in the surface region was joined by roller pressing with a linear pressure of 0.1 ton / cm.

[0103] (Preparation of positive electrode composite material slurry)

[0104] As the positive electrode active material in the middle region, LiNi with a median particle size (D50) of 5 μm was prepared. 0.5 Co 0.2 Mn 0.3 O 2 As the positive electrode active material in the surface area, LiNi with a median particle size (D50) of 12 μm was prepared. 0.5 C0 0.2 Mn 0.3 O 2. After mixing 47 mass % of a positive electrode active material with D50 = 5 μm, 47 mass % of a positive electrode active material with D50 = 12 μm, 4 mass % of acetylene black as a conductive aid, and 2 mass % of polyvinylidene fluoride (PVDF) as a binder, the obtained mixture is dispersed in an appropriate amount of N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode composite material slurry.

[0105] (Filling of positive electrode composite material)

[0106] Use a plunger die coater to coat the positive electrode composite material slurry at a coating amount of 100 mg / cm 2 After being coated on the positive electrode collector in a manner, it was dried at 120°C for 12 hours under vacuum conditions. Subsequently, the positive electrode collector filled with the positive electrode composite material was roll-pressed at a pressure of 15 tons to produce a positive electrode. The unit area weight of the electrode composite material constituting the obtained positive electrode was 100 mg / cm 2 , the average density is 3.4g / cm 3 The positive electrode was punched into a size of 3 cm × 4 cm before use.

[0107] [Production of negative electrode]

[0108] (Preparation of negative electrode composite material slurry)

[0109] 96.5 mass % of natural graphite, 1 mass % of carbon black as a conductive aid, 1.5 mass % of styrene butadiene rubber (SBR) as a binder, and 1 mass % of sodium carboxymethyl cellulose (CMC) as a thickener were mixed, and the resulting mixture was dispersed in an appropriate amount of distilled water to prepare a negative electrode composite material slurry.

[0110] (Formation of negative electrode composite material layer)

[0111] As a negative electrode current collector, a copper foil having a thickness of 8 μm was prepared.

[0112] Use a die coater to coat the negative electrode composite material slurry at a coating amount of 45 mg / cm 2 After being coated on the current collector in a manner, it was dried at 120°C for 12 hours under vacuum conditions. Subsequently, the current collector with the negative electrode composite material layer was roll-pressed at a pressure of 10 tons to produce a negative electrode. The unit area weight of the electrode composite material layer constituting the obtained negative electrode was 45 mg / cm 2 , density is 1.5g / cm 3The negative electrode was punched into a size of 3 cm × 4 cm before use.

[0113] [Manufacturing of lithium-ion secondary batteries]

[0114] As a separator, a microporous membrane made of a three-layer laminate of polypropylene / polyethylene / polypropylene with a thickness of 25 μm was prepared and punched into a size of 3 cm×4 cm before use.

[0115] After the aluminum laminate for secondary batteries is processed into a bag shape by heat sealing, a laminated body in which a separator is arranged between a positive electrode and a negative electrode is inserted into the processed product to produce a laminated battery cell.

[0116] As the electrolyte, prepare 1.2 mol of LiPF dissolved in a solvent. 6 The obtained solution is a mixture of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate in a volume ratio of 3:4:3. The electrolyte is injected into the laminated battery cell to produce a lithium ion secondary battery.

[0117] <Example 2>

[0118] The positive electrode active material in the intermediate region was prepared in the same manner as in Example 1, except that the median particle size (D50) was 3 μm and the median particle size (D50) was 10 μm as the positive electrode active material in the surface region.

[0119] <Comparative Example 1>

[0120] The preparation was carried out in the same manner as in Example 1 except that only a positive electrode active material having a median particle size (D50) of 10 μm was used and the positive electrode active material content of the composite material slurry was set to 94% by mass.

[0121] <Evaluation of initial characteristics of lithium-ion secondary batteries>

[0122] The lithium ion secondary batteries of Examples 1 and 2 and Comparative Example 1 were subjected to the following evaluation of initial characteristics.

[0123] [Initial discharge capacity]

[0124] After the lithium ion secondary battery was placed at the measurement temperature (25°C) for 3 hours, constant current charging was performed at 0.33C until 4.2V, and then constant voltage charging was performed at 4.2V for 5 hours. Next, after the lithium ion secondary battery was placed for 30 minutes, it was discharged at a discharge rate of 0.33C until 2.5V, and the discharge capacity was measured. The obtained discharge capacity was set as the initial discharge capacity.

[0125] [Initial Cell Resistance]

[0126] After measuring the initial discharge capacity, the lithium-ion secondary battery is adjusted to a charge level (SOC (State of Charge)) of 50%. Next, the current value is set to 0.2C, and the discharge is performed for 10 seconds, and the voltage is measured 10 seconds after the discharge ends. Next, after the lithium-ion secondary battery is left for 10 minutes, supplementary charging is performed to restore the SOC to 50%, and the lithium-ion secondary battery is left for 10 minutes. Next, the above operations are performed at each C rate of 0.5C, 1C, 1.5C, 2C, and 2.5C, and a plot is made with the horizontal axis being the current value and the vertical axis being the voltage. The slope of the approximate straight line obtained from the plot is set as the initial battery cell resistance of the lithium-ion secondary battery. The results are shown in Figure 4 .like Figure 4 As shown, the cell resistance ratio of Examples 1 and 2 is lower than that of Comparative Example 1. In particular, in Examples 1 and 2, the ion diffusion resistance is suppressed compared to Comparative Example 1, and the effect of the present invention is understood.

[0127] [C rate characteristics]

[0128] After the initial discharge capacity was measured, the lithium ion secondary battery was placed at the measurement temperature (25°C) for 3 hours, and then charged at a constant current of 0.33C to 4.2V, and then charged at a constant voltage of 4.2V for 5 hours. Then, after the lithium ion secondary battery was placed for 30 minutes, it was discharged at a discharge rate (C rate) of 0.5C to 2.5V, and the initial discharge capacity was measured.

[0129] The above operation was performed at each C rate of 0.33C, 1C, 1.5C, 2C, and 2.5C, and the initial discharge capacity at each C rate was converted to the capacity retention rate when the initial discharge capacity at 0.33C was set to 100%, which was defined as the C rate characteristic. The results are shown in Figure 5 .like Figure 5 As shown, it can be understood that the volume maintenance rate of Examples 1 and 2 can be maintained higher than that of Comparative Example 1.

[0130] <Evaluation of post-life characteristics of lithium-ion secondary batteries>

[0131] The lithium ion secondary batteries of Example 1 and Comparative Examples 1 and 2 were subjected to the following evaluation of post-endurance characteristics.

[0132] [Discharge capacity after endurance]

[0133] In a constant temperature chamber at 45°C, the lithium-ion secondary battery is charged at a constant current of 0.6C until it reaches 4.2V, and then charged at a constant voltage of 4.2V for 5 hours or charged until the current value reaches 0.1C. Then, after the lithium-ion secondary battery is placed for 30 minutes, the following operation is repeated 200 times: constant current discharge is performed at a discharge rate of 0.6C until it reaches 2.5V, and then placed for 30 minutes. Then, in a constant temperature chamber at 25°C, the lithium-ion secondary battery is placed for 24 hours in a state after being discharged to 2.5V, and the post-durable discharge capacity is measured in the same manner as the initial discharge capacity. This operation is repeated every 200 cycles, and the post-durable discharge capacity is measured until 600 cycles are performed.

[0134] [Battery cell resistance after endurance]

[0135] After 600 cycles in the measurement of the discharge capacity after endurance were completed, the charge level (SOC (State of Charge)) was adjusted to 50%, and the cell resistance after endurance was determined in the same manner as the initial cell resistance.

[0136] [Capacity maintenance rate]

[0137] The ratio of the discharge capacity after endurance for every 200 cycles to the initial discharge capacity was calculated and defined as the capacity retention rate in each cycle. Figure 6 .like Figure 6 As shown, it can be understood that the volume maintenance rate of Examples 1 and 2 can be maintained higher than that of Comparative Example 1.

[0138] [Resistance change rate]

[0139] The ratio of the battery cell resistance after endurance to the initial battery cell resistance was calculated and defined as the resistance change rate. Figure 7 .like Figure 7 As shown, it can be understood that the resistance change rate of Examples 1 and 2 can be maintained higher than that of Comparative Example 1.

[0140] Reference numerals

[0141] 10: Electrodes for lithium-ion secondary batteries

[0142] 21: Positive electrode layer (electrode layer)

[0143] 22: Positive electrode tab

[0144] 25: Current collector (positive electrode)

[0145] 25A: Surface area

[0146] 25B: Middle area

[0147] 25C: Surface area

[0148] 26: Positive electrode active material

[0149] 26a: first electrode active material

[0150] 26b: Second electrode active material

[0151] 27: Positive electrode composite materials

[0152] 31: Negative electrode layer (electrode layer)

[0153] 32: Negative electrode tab

[0154] 35: Current collector (negative electrode)

[0155] 35A: Surface area

[0156] 35B: Middle area

[0157] 35C: Surface area

[0158] 36: Negative electrode active material

[0159] 36a: first electrode active material

[0160] 36b: Second electrode active material

[0161] 37: Negative electrode composite materials

[0162] 41: Separator membrane

[0163] 50, 60: Die coating machine

[0164] 50a, 60a: plunger

Claims

1. An electrode for a lithium ion secondary battery, comprising a current collector composed of a metal porous body, and an electrode layer obtained by filling the current collector with an electrode composite material containing at least an electrode active material, in, The porosity of the collector in the electrode layer in the middle region in the thickness direction is smaller than the porosity in both surface regions in the thickness direction. The middle region is filled with a first electrode active material, and the two surface regions are filled with a second electrode active material having a larger particle size than the first electrode active material. The particle size of the first electrode active material is a median particle size (D50) of 3 μm or more and less than 5 μm, and the particle size of the second electrode active material is a median particle size (D50) of 10 μm or more and 12 μm or less, The packing density of the electrode active material in the intermediate region is greater than the packing density of the electrode active material in the two surface regions.

2. A lithium ion secondary battery comprising a positive electrode, a negative electrode, and a separator or a solid electrolyte layer located between the positive electrode and the negative electrode, in, At least one of the positive electrode and the negative electrode is the electrode for a lithium ion secondary battery according to claim 1.

3. A method for producing an electrode for a lithium-ion secondary battery, comprising: The first step is to form a current collector composed of a porous metal body, wherein the porosity in the middle region in the thickness direction of the current collector is smaller than the porosity in both surface regions; and In the second step, the middle region of the current collector is filled with an electrode composite material containing a first electrode active material, and both surface regions of the current collector are filled with an electrode composite material containing a second electrode active material having a larger particle size than the first electrode active material. The particle size of the first electrode active material is a median particle size (D50) of 3 μm or more and less than 5 μm, and the particle size of the second electrode active material is a median particle size (D50) of 10 μm or more and 12 μm or less, The packing density of the electrode active material in the intermediate region is greater than the packing density of the electrode active material in the two surface regions.

4. The method for producing an electrode for a lithium ion secondary battery according to claim 3, in, The electrode composite material containing the first electrode active material and the second electrode active material is applied and filled from the two surface regions of the current collector.

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