JP7243381B2 - Electrodes and non-aqueous electrolyte secondary batteries - Google Patents

Description

The present invention relates to electrodes and non-aqueous electrolyte secondary batteries.

Non-aqueous electrolyte secondary batteries are also widely used as power sources for mobile devices such as mobile phones and laptop computers, and hybrid cars. Along with the development of these fields, it is required to improve various performances of non-aqueous electrolyte secondary batteries.

One of the performances is the high energy density of the non-aqueous electrolyte secondary battery. The energy density of a non-aqueous electrolyte secondary battery is greatly affected by the electrodes of the non-aqueous electrolyte secondary battery. In order to obtain an electrode excellent in energy density, various studies such as material development of active material particles and surface treatment are being conducted (for example, Patent Document 1).

JP 2017-84674 A

In order to achieve high energy density, it is necessary to increase the density of the electrode active material layer, and it is required to reduce the voids in the electrode active material layer. On the other hand, the voids in the electrode active material layer hold the electrolytic solution after impregnation with the electrolytic solution, and are also places where lithium ions in the electrolytic solution move during charging and discharging.

The voids in the positive electrode active material layer currently in practical use are relatively large voids on the order of several hundreds of nanometers to μm between the composite portion of the positive electrode active material, the conductive aid and the binder, and the composite of the conductive aid and the binder. It can be roughly classified into minute voids on the order of several tens of nanometers formed in the part. Among them, the former relatively large voids have a great influence on the uniformity of permeation of the electrolytic solution into the positive electrode active material layer in the liquid injection process when manufacturing the battery, depending on the state of distribution thereof. In addition, since the distribution of the voids corresponds to the distribution of the electrolytic solution after the injection, the non-uniform distribution of the electrolytic solution in the positive electrode active material layer makes the use of the electrode active material extremely non-uniform during charging and discharging. It causes deterioration of the cycle characteristics of the electrolyte secondary battery.

Patent Document 1 discloses a first composite oxide containing Li, Ni, Co, Mn and W as a surface-modified positive electrode active material for a lithium ion battery having good battery output characteristics and cycle characteristics. A positive electrode active material for a lithium ion battery is disclosed in which a second composite oxide composed of a composite oxide of Li and Al or Nb is present on the particle surface. However, even if the active material particles designed in this way are used, the structure in the active material layer may not be appropriate, and the assumed characteristics may not be sufficiently exhibited, and in particular, the cycle characteristics may deteriorate. .

The present invention has been made in view of the above problems, and an object of the present invention is to provide an electrode for use in a non-aqueous electrolyte secondary battery having excellent cycle characteristics.

As a result of intensive studies, the present inventors have found that the distribution of voids from the surface of the active material layer to the current collector has a great effect on the deterioration of the cycle characteristics of non-aqueous electrolyte secondary batteries.

That is, in order to solve the above problems, the following means are provided.

The electrode according to the first aspect has a current collector and an active material layer on at least a part of the main surface of the current collector, the active material layer comprising active material particles, a conductive aid, A ratio of the total area S2 of the voids to the total area S1 of the active material layer in the cross section of the active material layer in the direction parallel to the main surface of the current collector, which has a binder and voids. is AR(S1/S2), the minimum value m of AR satisfies 2.5%≦m, and the maximum value M of AR satisfies M≦30%.

In the electrode according to the above aspect, the difference between the AR maximum value M and the AR minimum value m may be M−m<20.

In the positive electrode according to the above aspect, the active material particles are lithium composite oxides, and the lithium composite oxides are represented by the general formula Li _x M1 _y M2 _1-y O ₂ , where M1 is Ni , Co and Mn, and M2 is the group consisting of Al, Fe, Ti, Cr, Mg, Cu, Ga, Zn, Sn, B, V, Ca and Sr. is at least one metal element selected from, x may satisfy 0.05≦x≦1.2, and y may satisfy 0.3≦y≦1.0.

In the positive electrode according to the above aspect, the active material particles may have different compositions or composition ratios of elements constituting the central portion and the outer peripheral portion.

In the positive electrode according to the above aspect, the active material particles may have a coating layer on their surfaces.

The positive electrode according to the above aspect may further include an undercoat layer between the positive electrode current collector and the positive electrode active material layer.

A non-aqueous electrolyte secondary battery according to a second aspect includes the electrode according to the above aspect, an electrode facing the electrode, and a separator interposed therebetween.

When the electrode according to the above aspect is used in a non-aqueous electrolyte secondary battery, a non-aqueous electrolyte secondary battery with excellent cycle characteristics can be obtained.

1 is a schematic diagram of a non-aqueous electrolyte secondary battery according to an embodiment; FIG. A three-dimensional image of a portion of the positive electrode. It is the image which cut|disconnected the three-dimensional image of the positive electrode active material shown in FIG. 2 in the plane orthogonal to the lamination direction. FIG. 3 is an image of a cross section obtained by cutting the three-dimensional image of the positive electrode active material shown in FIG. 2 along a plane orthogonal to the stacking direction. It is a monochromatic image obtained by extracting void portions from the cross-sectional image of FIG. FIG. 4 is a diagram showing the relationship between the ratio of void portions in a cross-sectional image and the distance from the current collector. FIG. 4 is a diagram showing the relationship between the ratio of void portions in a cross-sectional image and the distance from the current collector. A large example is the distribution of voids. In the figure, m is the minimum minimum value and M is the maximum maximum value.

Hereinafter, this embodiment will be described in detail with appropriate reference to the drawings. Electrodes of non-aqueous electrolyte secondary batteries include a positive electrode and a negative electrode, but the following description will focus on the positive electrode. In the drawings used in the following description, there are cases where characteristic portions are enlarged for the sake of convenience in order to make it easier to understand the features of the present invention, and the dimensional ratios and the like of each component may differ from the actual ones. be. The materials, dimensions, and the like exemplified in the following description are examples, and the present invention is not limited to them, and can be implemented with appropriate modifications without changing the gist of the invention.

[Non-aqueous electrolyte secondary battery]
FIG. 1 is a schematic diagram of a non-aqueous electrolyte secondary battery according to this embodiment. A non-aqueous electrolyte secondary battery 100 shown in FIG. 1 includes a power generation element 40 and an exterior body 50 . The exterior body 50 covers the periphery of the power generation element 40 . The power generation element 40 is connected to the outside by a pair of connected terminals 60 and 62 . Also, although not shown, the power generation element 40 and the electrolytic solution are accommodated in the exterior body 50 .

(power generation element)
The power generation element 40 includes a positive electrode 20 , a negative electrode 30 and a separator 10 . The separator 10 is arranged between the positive electrode 20 and the negative electrode 30 .

<Separator>
The separator 10 may be formed of an electrically insulating porous structure. , polyacrylonitrile, polyamide, polyethylene and polypropylene.

Separator 10 may be a solid electrolyte instead of being formed from an electrically insulating porous structure.

A known solid electrolyte can be used. For example, _{a polymer solid electrolyte obtained by dissolving an alkali metal salt in a polyethylene oxide polymer, Li1.3Al0.3Ti1.7(PO4)3 ( Nasicon type )} , _{Li1.07Al0 . 69Ti1.46 ( PO4} ) ₃ ( _glass ceramics) _{, Li0.34La0.51TiO2.94} (perovskite _type ), _Li7La3Zr2O12 ( _{garnet type} ), _{Li2.9PO 3.3} N _0.46 (amorphous, LIPON), 50Li ₄ SiO 4.50Li ₂ BO ₃ (glass) _, 90Li ₃ BO _{3.10Li 2} SO ₄ (glass ceramics) oxide-based solid electrolytes, Li _3.25 Ge _0.25 P _0.75 S ₄ (crystal), Li ₁₀ GeP ₂ S ₁₂ (crystal, LGPS), Li ₆ PS ₅ Cl (crystal, aldirodite type), Li _9.54 Si _1.74 P _{1.44 S11.7C10.3 ( crystal} ) _{, Li3.25P0.95S4} (glass ceramics) _{, Li7P3S11} (glass ceramics) _{, 70Li2S.30P2S5 ( glass} ), _{30Li2S.26B2S3.44LiI ( glass ) , 50Li2S.17P2S5.33LiBH4 (} glass _{) , 63Li2S.36SiS2.Li3PO4 (} glass) _{, 57Li2S.38SiS2} • Sulfide-based solid electrolytes such as 5Li ₄ SiO ₄ (glass) can be mentioned.

<Positive electrode>
The positive electrode 20 has a plate-like (film-like) positive electrode current collector 22 and a positive electrode active material layer 24 . The positive electrode active material layer 24 is formed on at least one surface of the positive electrode current collector 22 .

[Positive collector]
The positive electrode current collector 22 may be a conductive plate material, and for example, a thin metal plate such as aluminum foil, copper foil, nickel foil, or the like can be used.

[Positive electrode active material layer]
The active material particles 24A used in the positive electrode active material layer 24 can reversibly cause occlusion and release of ions, desorption and insertion (intercalation) of ions, or doping and dedoping of ions and counter anions. possible electrode active materials can be used. As the ions, for example, lithium ions, sodium ions, magnesium ions, etc. can be used, and it is particularly preferable to use lithium ions.

For example, in the case of lithium ion secondary batteries, lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganate (LiMnO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), and general formula: LiNi _x Co _y Mn _z M _a O ₂ (x+y+z+a=1, 0≦x<1, 0≦y<1, 0≦z<1, 0≦a<1, M is Al, Mg, Nb, Ti, Cu, Zn , one or more elements selected from Cr), lithium vanadium compound (LiV ₂ O ₅ ), olivine-type LiMPO ₄ (where M is Co, Ni, Mn, Fe, Mg, represents _{one or more elements selected from Nb, Ti, Al, and Zr, or VO), lithium titanate (Li4Ti5O12 ) , LiNixCoyAlzO2 ( 0.9 <} x+y+z<1. 1) and the like, polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene, and the like can be used as the active material particles 24A.

Among the materials exemplified above, the active material particles 24A are preferably materials represented by the general formula Li _x M1 _y M2 _1-y O ₂ . In the general formula, M1 is at least one metal element selected from the group consisting of Ni, Co and Mn, and M2 is Al, Fe, Ti, Cr, Mg, Cu, Ga, Zn, Sn, B, V , Ca and Sr, x satisfies 0.05≦x≦1.2, and y satisfies 0.3≦y≦1.0. The material can achieve high energy density.

It is preferable that the active material particles 24A have different compositions or composition ratios of elements between the central portion and the outer peripheral portion. Different performances are required for the outer peripheral portion and the central portion, which are in direct contact with the electrolytic solution. For example, by using a material with excellent reactivity in the outer peripheral portion and using a high-capacity material in the central portion, it is possible to obtain active material particles 24A with excellent charge/discharge characteristics and high capacity.

The active material particles 24A preferably have a coating layer on their surfaces. As the coating layer, for example, a Zr fluoro complex, graphene, or the like can be used. By forming the coating layer with a material different from that of the active material particles 24A, it is possible to achieve the performance required for each portion as described above.

The positive electrode active material layer 24 may contain a conductive aid, a binder, and a solid electrolyte in addition to the active material particles 24A. Examples of conductive aids include carbon powders such as carbon blacks, carbon nanotubes, carbon materials, metal fine powders such as copper, nickel, stainless steel and iron, mixtures of carbon materials and metal fine powders, and conductive oxides such as ITO. can be used. Among these, carbon materials such as carbon black are preferable. If sufficient conductivity can be secured only with the active material, the conductive aid may not be included.

A known binder can be used. For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoro Fluororesins such as ethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), and polyvinyl fluoride (PVF).

In addition to the above, binders such as vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based fluororubber), vinylidene fluoride-pentafluoropropylene fluororubber (VDF-PFP fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene fluororubber (VDF-PFP-TFE fluororubber), vinylidene fluoride Vinylidene fluoride-based fluorine such as Ride-perfluoromethyl vinyl ether-tetrafluoroethylene-based fluororubber (VDF-PFMVE-TFE-based fluororubber), vinylidene fluoride-chlorotrifluoroethylene-based fluororubber (VDF-CTFE-based fluororubber) Rubber may be used.

As the solid electrolyte, the same one as mentioned for the separator can be used.

The density of the positive electrode active material layer 24 is preferably 2.9 g/cm ³ or higher, more preferably 3.2 g/cm ³ or higher, and even more preferably 3.5 g/cm ³ or higher. The high density of the positive electrode active material layer 24 allows the nonaqueous electrolyte secondary battery 100 to have a high energy density.

An undercoat layer is preferably provided between the positive electrode current collector 22 and the positive electrode active material layer 24 . A mixture of a conductive material and a binder can be used for the undercoat layer. The undercoat layer improves adhesion with the positive electrode active material layer 24 and suppresses peeling of the positive electrode active material layer.

[analysis method]
FIG. 2 is a three-dimensional image of part of the positive electrode. The three-dimensional image shown in FIG. 2 is a combination of a total of 200 cross-sectional images in the stacking direction measured using an FIB (Focused Ion Beam)-SEM (scanning electron microscope), and a three-dimensional image using image analysis software ImageJ. I got it by restoring it. 200 FIB-SEM images were taken under the conditions of an acceleration voltage of 30 kV, a current value of 4 nA, a processing width of 80 μm, a processing depth of 50 μm, and a depth pitch of 0.25 μm. The resolution of the cross-sectional image was 380×320 in the case of FIG.

FIG. 3 is an example of an image obtained by cutting the three-dimensional image of the positive electrode active material layer shown in FIG. 2 along a plane orthogonal to the stacking direction. Using image analysis software ImageJ, 380 cross-sectional images were reconstructed from the electrode surface toward the current collector according to 380 pixels in the vertical direction. FIG. 3 is an image cut at the 150th sheet from the surface.

FIG. 4 is a cross-sectional image of the 150th sheet from the surface. As shown in FIG. 4, the positive electrode active material layer 24 has active material particles 24A. Further, as shown in FIG. 4, portions other than the positive electrode active material particles 24A include a first region 24B with low contrast and a second region 24C with even lower contrast. The first region 24B is composed of a conductive aid and a binder, and the second region 24C is composed of voids. However, since it is not possible to accurately distinguish between the conductive additive and binder portion and the void portion only by the contrast, the accuracy can be improved by comparing the cross-sectional images before and after.

FIG. 5 is a monochromatic image obtained by extracting the cut gap portion from FIG. 4 . In this image, the void percentage was 4.3%.

FIG. 6 is a cross-sectional image corresponding to the number of pixels in the vertical direction. FIG. 4 is a diagram showing the relationship of distances from the body; Since the interface between the current collector and the positive electrode active material layer as shown in FIG. 5 is not flat, the portion where the positive electrode active material layer begins to appear was set as the starting point (distance=0). In the figure, m is the minimum minimum value and M is the maximum maximum value. In this example, the distribution of voids is small.

FIG. 7 is a diagram showing the relationship between the ratio of void portions in a cross-sectional image and the distance from the current collector, similar to FIG. 6. In this example, the distribution of voids is large.

The positive electrode active material layer 24 according to the present embodiment does not significantly decrease or increase the ratio of voids in each cutting. Specifically, when AR is the ratio of the total area S2 of the voids to the total area S1 of the active material, the conductive aid, the binder, and the voids in the cross section in the direction parallel to the main surface of the current collector, When the vertical axis is the length from the surface of the active material layer to the contact surface with the current collector, and the horizontal axis is the AR value at each length, the AR value is 2.5% ≤ 2.5%. AR≦30%.

If the AR value is too low, the penetration of the electrolytic solution into the positive electrode active material layer is insufficient in the injection process, increasing the non-uniformity of charge/discharge reaction. When the non-uniformity of the charge/discharge reaction increases, a portion of excessive reaction is generated, and the cycle characteristics are degraded. On the other hand, in a layer with an excessively high AR value, the ratio of active material decreases, increasing non-uniformity of charge/discharge reactions in the positive electrode active material layer, and degrading cycle characteristics.

In contrast, in the positive electrode active material layer 24 according to the present embodiment, the active material particles 24A exhibit the expected performance. In addition, since non-uniformity between the electrolyte and the active material particles 24A in the positive electrode active material layer 24 is eliminated, the cycle characteristics of the non-aqueous electrolyte secondary battery 100 can be improved.

Among the peaks on the horizontal axis, it is preferable that the difference Mm between the maximum value M and the minimum minimum value m satisfies Mm<20.

If the difference in Mm is too large, the concentration gradient of the electrolyte during charge/discharge in the stacking direction of the positive electrode active material layer becomes large, the charge/discharge reaction becomes non-uniform, and the cycle characteristics deteriorate. By distributing the voids at an appropriate ratio, the non-uniform charge/discharge reaction in the positive electrode active material layer during charge/discharge is alleviated, and the cycle characteristics of the non-aqueous electrolyte secondary battery 100 can be improved. can.

<Negative Electrode>
The negative electrode 30 has a plate-like (film-like) negative electrode current collector 32 and a negative electrode active material layer 34 . The negative electrode active material layer 34 is formed on at least one surface of the negative electrode current collector 32 .

[Negative electrode current collector]
The negative electrode current collector 32 may be a conductive plate material, and the same material as the positive electrode current collector 22 can be used.

[Negative electrode active material layer]
The negative electrode active material layer 34 contains a negative electrode active material. Moreover, a conductive material, a binder, and a solid electrolyte may be included as necessary.

The negative electrode active material may be any compound that can occlude and release ions, and negative electrode active materials used in known non-aqueous electrolyte secondary batteries can be used. Examples of negative electrode active materials include alkali or alkaline earth metals such as metallic lithium, graphite capable of absorbing and releasing ions (natural graphite, artificial graphite), carbon nanotubes, non-graphitizable carbon, easily graphitizable carbon, low temperature carbon materials such as calcined carbon; metals such as aluminum, silicon, tin, etc. that can combine with metals such as lithium; SiO _x (0<x<2); compounds, lithium titanate (Li ₄ Ti ₅ O ₁₂ ), and the like.

The negative electrode active material layer may include only the current collector without containing the negative electrode active material. In this case, during charging, an alkali or alkaline earth metal such as metallic lithium is deposited to form a negative electrode active material.

The same conductive material and binder as those used for the positive electrode 20 can be used. In addition to the binders listed for the positive electrode, the binder used for the negative electrode may be, for example, cellulose, styrene/butadiene rubber, ethylene/propylene rubber, polyimide resin, polyamideimide resin, acrylic resin, or the like.

As the solid electrolyte, the same one as mentioned for the separator can be used.

[analysis method]
The analysis method similar to that for the positive electrode can also be used for the negative electrode.

(Terminal)
Terminals 60 and 62 are connected to positive electrode 20 and negative electrode 30, respectively. A terminal 60 connected to the positive electrode 20 is a positive terminal, and a terminal 62 connected to the negative electrode 30 is a negative terminal. Terminals 60 and 62 are responsible for electrical connection with the outside. Terminals 60, 62 are made of a conductive material such as aluminum, nickel, or copper. The connection method may be welding or screwing. Terminals 60, 62 are preferably protected with insulating tape to prevent short circuits.

(Exterior body)
The exterior body 50 seals the power generation element 40 and the electrolytic solution therein. The exterior body 50 is not particularly limited as long as it can prevent leakage of the electrolytic solution to the outside and entry of moisture into the non-aqueous electrolyte secondary battery 100 from the outside.

For example, as shown in FIG. 1, a metal laminate film obtained by coating a metal foil with polymer films from both sides may be used as the exterior body 50 . The exterior body 50 shown in FIG. 1 has a metal foil 52 and a resin layer 54 laminated on each surface of the metal foil 52 .

For example, aluminum foil can be used as the metal foil 52 . A polymer film such as polypropylene can be used for the resin layer 54 . The material forming the resin layer 54 may be different between the inner side and the outer side. For example, a polymer with a high melting point such as polyethylene terephthalate (PET) or polyamide (PA) is used as the outer material, and polyethylene (PE) or polypropylene (PP) is used as the inner polymer film material. be able to.

(Electrolyte)
The electrolytic solution is enclosed in the exterior body 50 and impregnates the power generation element 40 .

As the electrolyte, an electrolyte solution containing a lithium salt or the like (electrolyte aqueous solution, electrolyte solution using an organic solvent) can be used. However, since the electrolytic aqueous solution has a low electrochemical decomposition voltage, the withstand voltage during charging is limited to a low value. Therefore, an electrolytic solution (non-aqueous electrolytic solution) using an organic solvent is preferable.

The non-aqueous electrolytic solution has an electrolyte dissolved in a non-aqueous solvent, and may contain a cyclic carbonate and a chain carbonate as the non-aqueous solvent.

As the cyclic carbonate, one that can solvate the electrolyte can be used. For example, ethylene carbonate (EC), propylene carbonate (PC) and butylene carbonate (BC) can be used. The cyclic carbonate preferably contains at least PC.

Chain carbonates can reduce the viscosity of cyclic carbonates. Examples include diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). In addition, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, etc. may be mixed and used.

The volume ratio of the cyclic carbonate to the chain carbonate in the non-aqueous solvent is preferably 1:9 to 1:1.

A metal salt can be used as the electrolyte. For _{example , LiPF6} , LiClO4, _{LiBF4 , LiCF3SO3} , _{LiCF3CF2SO3 ,} LiC _{( CF3SO2 ) 3} , LiN( _{CF3SO2 ) 2} , LiN( _{CF3CF2SO2 ) 2} , LiN _{( CF3SO2 )} ( _{C4F9SO2 )} , LiN( _CF3CF2CO ) ₂ , LiBOB and the _like . In addition, these lithium salts may be used individually by 1 type, and may use 2 or more types together. In particular, from the viewpoint of the degree of ionization, it is preferable to contain LiPF ₆ as the electrolyte.

When dissolving LiPF ₆ in a non-aqueous solvent, it is preferable to adjust the concentration of the electrolyte in the non-aqueous electrolyte to 0.5 to 2.0 mol/L. When the concentration of the electrolyte is 0.5 mol/L or more, a sufficient lithium ion concentration in the non-aqueous electrolyte can be ensured, and a sufficient capacity can be easily obtained during charging and discharging. In addition, by suppressing the concentration of the electrolyte to within 2.0 mol/L, the increase in the viscosity of the non-aqueous electrolyte can be suppressed, the mobility of lithium ions can be sufficiently secured, and sufficient capacity can be obtained during charging and discharging. easier.

When LiPF ₆ is mixed with other electrolytes, it is preferable to adjust the lithium ion concentration in the non-aqueous electrolyte to 0.5 to 2.0 mol/L, and the lithium ion concentration from LiPF ₆ is 50 mol% of that. It is more preferable to include the above.

An ionic liquid that dissolves an alkali metal salt may be used instead of the non-aqueous solvent.

Examples of ionic liquids include cations such as ammonium cations such as imidazolium salts and pyridinium salts and phosphonium cations, and anions such as bromide ions and halogen anions such as triflate, boron anions such as tetraphenylborate, hexa Examples include those using phosphorus-based anions such as fluorophosphate.

Instead of impregnating the separator, positive electrode, and negative electrode with an electrolytic solution, a solid electrolyte may be used instead of the separator and added to the positive electrode and negative electrode to form a solid electrolyte battery.

As described above, the embodiments of the present invention have been described in detail with reference to the drawings. , substitutions, and other modifications are possible.

Hereinafter, the present invention will be described in further detail using examples and comparative examples based on the above embodiments.

[Example 1]
(Preparation of positive electrode composite particles)
First, a "raw material liquid" (3% by weight of acetylene black and 2% by weight of polyvinylidene fluoride) was prepared by dispersing acetylene black in a solution of PVDF dissolved in N,N-dimethylformamide {(DMF): solvent}. .

The positive electrode composite particles 1 are prepared by generating a current of air in a container of a fluidized bed granulator, and adding LiNi _0.85 Co _0.10 Al _0.05 O ₂ powder having an average particle size of 3 μm as an active material. This was made into a fluidized bed. Next, the raw material liquid was sprayed onto the powder of the active material formed into a fluidized bed, and the solution was adhered to the surface of the powder. The composition of the composite particles was 95.0% by weight of the active material, 3.0% by weight of the conductive aid, and 2.0% by weight of the binder. By keeping the temperature of the atmosphere in which the powder was placed during the spraying constant, the N,N-dimethylformamide was removed from the surface of the powder almost simultaneously with the spraying. Acetylene black and polyvinylidene fluoride were adhered to the surface of the powder in this manner to obtain positive electrode composite particles 1 (average particle diameter: 15 μm).

The positive electrode composite particles 2 are produced by generating a current of air in a container of a fluidized bed granulator, and adding LiNi _0.85 Co _0.10 Al _0.05 O ₂ powder having an average particle size of 15 μm as an active material. This was made into a fluidized bed. Next, the above-described raw material solution diluted to two times was sprayed onto the fluidized bed of the active material powder, and the solution was adhered to the surface of the powder. The composition of the composite particles was 97.5% by weight of the active material, 1.5% by weight of the conductive aid, and 1.0% by weight of the binder. By keeping the temperature of the atmosphere in which the powder was placed during the spraying constant, the N,N-dimethylformamide was removed from the surface of the powder almost simultaneously with the spraying. Acetylene black and polyvinylidene fluoride were adhered to the surface of the powder in this manner to obtain positive electrode composite particles 2 (average particle diameter: 18 μm).

(Preparation of slurry)
As a positive electrode slurry, 65.8 parts by weight of positive electrode composite particles 1, 28.2 parts by weight of positive electrode composite particles 2, 1.1 parts by weight of acetylene black (conductive auxiliary agent), and 0.7 parts by weight of polyfluoride Vinylidene (PVDF: binder) and N-methyl-2-pyrrolidone (solvent) were mixed and dispersed to prepare a pasty positive electrode slurry.

(Preparation of positive electrode)
The resulting positive electrode slurry was evenly coated on both sides of a 20 μm thick aluminum foil using a comma roll coater so that the coating amount of the active material particles was 12.5 mg/cm ² . Next, N-methyl-2-pyrrolidone was dried in a drying oven at 110° C. under an air atmosphere. Then, the obtained active material layer was pressed by a roll press machine, and the lower layer was pressure-bonded to both surfaces of the positive electrode current collector. The average composition of the active material layer was 94% by weight of active material, 3.6% by weight of conductive aid, 2.4% by weight of binder, and the density was 3.4 g/cm ³ .

(Preparation of negative electrode)
94 wt% lithium-ion battery grade graphite (negative electrode active material), 2 wt% acetylene black (conductive aid), 4 wt% PVDF (binder), and N-methyl-2-pyrrolidone (solvent). were mixed and dispersed to prepare a pasty negative electrode slurry. The negative electrode slurry was applied to both sides of an electrolytic copper foil having a thickness of 10 μm. For the coating amount, the amount of lithium ions received during charging was taken into consideration so as to balance the coating amount of the active material particles of the positive electrode. After coating, the coating was dried at 100° C., the solvent was removed, and pressure molding was performed using a roll press.

(Production of cells)
The produced negative electrode and positive electrode were punched into a predetermined shape, and laminated alternately with a polypropylene separator having a thickness of 16 μm interposed therebetween. The negative electrode terminal made of nickel was attached to the projecting end of the copper foil on which the negative electrode active material layer was not provided in the negative electrode of the laminate. The positive electrode terminal made of aluminum was attached to the projecting end portion of the aluminum foil on which the positive electrode active material layer was not provided in the positive electrode of the laminate. The negative and positive terminals were attached by an ultrasonic welder.

An opening was formed by inserting the laminate into an outer package made of an aluminum laminate film and heat-sealing the laminate except for one peripheral portion. The exterior contains a solvent containing EC, EMC, and DEC in a volume ratio of 3:5:2, and a non-aqueous electrolyte containing 1.5 M (mol/L) of LiPF ₆ as a lithium salt. and injected. Then, the remaining one portion was heat-sealed while the pressure was reduced by a vacuum sealer, and a lithium ion secondary battery according to Example 1 was produced.

(Structural analysis and evaluation of batteries)
After the initial characteristics evaluation, the fabricated lithium ion battery was subjected to cycle characteristics evaluation and structural analysis of the positive electrode. Cycle characteristics were evaluated by a 1C/1C charge/discharge cycle.

(Measurement of charge-discharge cycle characteristics)
Cycle characteristics were measured using a secondary battery charge/discharge tester. The voltage range was from 4.3V to 3.0V. Only the initial charging was performed at 0.2C constant current charging, and discharging was performed at 1C. Charging was performed at constant current and constant voltage. The battery was charged at a current value of 1.0 C, and after reaching 4.3 V, charging was terminated when the current value reached 5% of the 1 C current value. After that, the cycle characteristics were measured under the condition of discharging at a current value of 1.0C.

Note that the charge-discharge cycle characteristics were evaluated as a capacity retention rate (%). The capacity retention rate (%) is the ratio of the discharge capacity at each number of cycles to the initial discharge capacity, with the discharge capacity at the first cycle as the initial discharge capacity. The capacity retention rate (%) is represented by the following formula (1).
Capacity retention rate (%) = (“discharge capacity at each number of cycles”/“discharge capacity at first cycle”) × 100 (1)

Note that 1C is a current value at which charging/discharging is completed in just one hour when a battery cell having a nominal capacity value is charged or discharged at a constant current. A higher capacity retention rate means better charge-discharge cycle characteristics.

The lithium ion secondary battery produced in Example 1 was repeatedly charged and discharged under the above conditions, and the capacity retention rate after 500 cycles was evaluated as charge/discharge cycle characteristics.

(Structure analysis of positive electrode)
Structural analysis of the positive electrode was carried out by disassembling a fully discharged lithium ion battery after charging and discharging in a glove box under a dry Ar atmosphere to separate the positive electrode. After thoroughly washing with DMC (dimethyl carbonate), it was dried in a vacuum. After cutting the dried positive electrode into a predetermined size, 200 cross-sections in the stacking direction were photographed using an FIB-SEM (Versa3D manufactured by FEI). The FIB-SEM image was taken under the conditions of an acceleration voltage of 30 kV, a current value of 4 nA, a processing width of 80 μm, a processing depth of 50 μm, and a depth pitch of 0.25 μm.

Then, the captured image is three-dimensionalized with image analysis software ImageJ, and from the diagram showing the relationship between the ratio AR of the conductive aid and the binder portion in the cross-sectional image and the distance from the current collector, AR The values of the minimum minimum value m, the maximum maximum value M, and Mm were obtained.

[Example 2]
(Preparation of positive electrode composite particles)
In the same manner as in Example 1, positive electrode composite particles 1 and 2 used for forming a positive electrode active material layer of a lithium ion secondary battery were produced using a spray-drying fluidized bed granulator. Here, positive electrode composite particles 1 were produced in the same manner as in Example 1, except that a LiNi _0.85 Co _0.10 Al _0.05 O ₂ positive electrode (active material) having an average particle size of 5 μm was used. In addition, positive electrode composite particles 2 were produced in the same manner as in Example 1.

Positive electrode composite particles 1 having an average particle size of 25 μm were obtained. Moreover, positive electrode composite particles 1 (average particle size: 25 μm) were obtained. Positive electrode composite particles 2 having an average particle diameter of 18 μm, which is the same as that of Example 1, were obtained.

(Preparation of slurry)
As the positive electrode slurry, 65.8 parts by weight of positive electrode composite particles 1, 28.2 parts by weight of positive electrode composite particles 2, 1.1 parts by weight of acetylene black (conductive assistant), and 0.7 parts by weight of polyfluoride. Vinylidene chloride (PVDF: binder) and N-methyl-2-pyrrolidone (solvent) were mixed and dispersed to prepare a pasty positive electrode slurry.

(Preparation of positive electrode)
The resulting positive electrode slurry was evenly coated on both sides of a 20 μm thick aluminum foil using a comma roll coater so that the coating amount of the active material particles was 12.5 mg/cm 2 . Next, N-methyl-2-pyrrolidone was dried in a drying oven at 110° C. under an air atmosphere. Then, the obtained active material layer was pressed by a roll press machine, and the lower layer was pressure-bonded to both surfaces of the positive electrode current collector. The average composition of the active material layer was 94% by weight of active material, 3.6% by weight of conductive aid, 2.4% by weight of binder, and the density was 3.3 g/cm3.

Preparation of the negative electrode, preparation of the cell, evaluation of the cycle characteristics of the battery, and structural analysis of the positive electrode were carried out in the same manner as in Example 1.

[Example 3]
(Preparation of slurry)
First, as a positive electrode slurry for the lower layer, an active material was prepared by mixing 70% by weight of LiNi _0.85 Co _0.10 Al _0.05 O ₂ (active material particles) having an average particle size of 3 μm and 30% by weight of active material particles having an average particle size of 15 μm. 92% by weight of material particles, 4.8% by weight of acetylene black (conductive aid), 3.2% by weight of PVDF (binder), and N-methyl-2-pyrrolidone (solvent) are mixed and dispersed to form a paste. was prepared.

Further, as the positive electrode slurry for the upper layer, 70% by weight of LiNi _0.85 Co _0.10 Al _0.05 O ₂ (active material particles) having an average particle size of 3 μm and 30% by weight of active material particles having an average particle size of 15 μm were mixed. 97% by weight of substance particles, 2.0% by weight of acetylene black (conductive aid), 1.0% by weight of PVDF (binder), and N-methyl-2-pyrrolidone (solvent) are mixed and dispersed to form a paste. was prepared.

(Preparation of positive electrode)
The lower layer positive electrode slurry thus obtained was evenly coated on both sides of a 20 μm thick aluminum foil using a comma roll coater so that the coating amount of the active material particles was 6.2 mg/cm ² . Next, N-methyl-2-pyrrolidone was dried in a drying oven at 110° C. under an air atmosphere.

Next, the obtained positive electrode slurry for the upper layer was uniformly coated on the lower layer using a comma roll coater so that the coating amount of the active material particles was 6.2 mg/cm ² . Next, the N-methyl-2-pyrrolidone solvent was dried in a drying oven at 110° C. under an air atmosphere. The obtained active material layer was pressed by a roll press to obtain a positive electrode with a density of 3.3 g/cm ³ . The average composition of the active material layer was 94.5% by weight of active material, 3.4% by weight of conductive aid, and 2.1% by weight of binder.

Preparation of the negative electrode, preparation of the cell, evaluation of the cycle characteristics of the battery, and structural analysis of the positive electrode were carried out in the same manner as in Example 1.
[Comparative Example 1]

(Preparation of slurry)
94% by weight of LiNi _0.85 Co _0.10 Al _0.05 O ₂ (active material particles) having an average particle size of 20 μm, 3.6% by weight of Ketjenblack (conductive aid), and 2.4% by weight of PVDF (binder) % by weight and N-methyl-2-pyrrolidone (solvent) were mixed and dispersed to prepare a pasty positive electrode slurry.

(Preparation of positive electrode)
The resulting positive electrode slurry was evenly coated on both sides of a 20 μm thick aluminum foil using a comma roll coater so that the coating amount of the active material particles was 12.5 mg/cm ² . Next, N-methyl-2-pyrrolidone was dried in a drying oven at 110° C. under an air atmosphere.

Next, the obtained active material layer was pressed by a roll press to obtain a positive electrode with a density of 3.4 g/cm ³ .

Preparation of the negative electrode, preparation of the cell, evaluation of the cycle characteristics of the battery, and structural analysis of the positive electrode were carried out in the same manner as in Example 1.

[Comparative Example 2]

(Preparation of slurry)
96% by weight of LiNi _0.85 Co _0.10 Al _0.05 O ₂ (active material particles) with an average particle size of 20 μm, 2.4% by weight of Ketjenblack (conductive aid), and 1.6% by weight of PVDF (binder) % by weight and N-methyl-2-pyrrolidone (solvent) were mixed and dispersed to prepare a pasty positive electrode slurry.

(Preparation of positive electrode)
The resulting positive electrode slurry was evenly coated on both sides of a 20 μm thick aluminum foil using a comma roll coater so that the coating amount of the active material particles was 12.5 mg/cm ² . Next, N-methyl-2-pyrrolidone was dried in a drying oven at 110° C. under an air atmosphere.

Next, the obtained active material layer was pressed by a roll press to obtain a positive electrode with a density of 3.6 g/cm ³ .

Preparation of the negative electrode, preparation of the cell, evaluation of the cycle characteristics of the battery, and structural analysis of the positive electrode were carried out in the same manner as in Example 1.

Table 1 shows the evaluation results of the cycle characteristics and the structural analysis results of the positive electrodes in Examples 1 to 3 and Comparative Examples 1 and 2.

From the results of Examples 1 to 3 and Comparative Examples 1 and 2, AR, which is the ratio of the total area of the electrode cross section to the total area of the active material, conductive aid, binder, and voids, was minimal. It was confirmed that when the value satisfies 2.5%≦m and the maximum value of AR satisfies M≦30%, the cycle characteristics are particularly excellent. It was also confirmed that when the difference between the maximum maximum value M and the minimum value m is Mm<20, the cycle characteristics are excellent.

ADVANTAGE OF THE INVENTION By this invention, it becomes possible to provide the electrode which can improve a cycling characteristic, and a non-aqueous-electrolyte secondary battery using the same. They are suitable for use as power sources for portable electronic devices, and are also used as electric vehicles and as storage batteries for domestic and industrial use.

10 Separator 20 Positive electrode 22 Positive electrode current collector 24 Positive electrode active material layer 24A Active material particles 24B First region (conductive aid and binder)
24C second region (void)
30 Negative Electrode 32 Negative Electrode Current Collector 34 Negative Electrode Active Material Layer 40 Power Generation Element 50 Package 52 Metal Foil 54 Resin Layers 60, 62 Terminal 100 Non-aqueous Electrolyte Secondary Battery 100 Non-aqueous Electrolyte Secondary Battery

Claims (3)

having a current collector and an active material layer on at least part of the main surface of the current collector;
The active material layer has active material particles, a conductive aid, a binder, and voids,
In the range from the surface of the active material layer to the contact surface with the current collector,
AR( S2/S1 ) is the ratio of the total area S2 of the voids to the total area S1 of the active material layer in the cross section of the active material layer in the direction parallel to the main surface of the current collector. when
The positive electrode, wherein the AR minimum value m satisfies 2.5%≦m, and the AR maximum value M satisfies M≦30%. 2. The positive electrode according to claim 1, wherein the difference between the AR maximum value M and the AR minimum value m is M−m<20. A non-aqueous electrolyte secondary battery using the positive electrode according to claim 1 or 2.

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