JP2020167000A - Electrode and nonaqueous electrolyte secondary battery - Google Patents

Abstract

To provide an electrode to be used in a nonaqueous electrolyte secondary battery superior in cycle characteristic.SOLUTION: An electrode comprises: a current collector; and an active material layer on at least part of a principal face of the current collector. The active material layer has active material particles, a conductive assistant, a binder, and a gap. When in a section of the active material layer in a direction in parallel with the principal face of the current collector, the rate of a total area S1 of the active material layer in the section to a total area S2 of the gap is AR(S1/S2), the minimum value m of the AR satisfies 2.5%≤m, and the maximum value M of the AR meets M≤30%.SELECTED DRAWING: Figure 6

Description

The present invention relates to an electrode and a non-aqueous electrolyte secondary battery.

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

One of its performances is to increase the energy density of non-aqueous electrolyte secondary batteries. The energy density of the non-aqueous electrolyte secondary battery is greatly affected by the electrodes of the non-aqueous electrolyte secondary battery. In order to obtain an electrode having excellent energy density, various studies such as material development of active material particles and surface treatment have been carried out (for example, Patent Document 1).

JP-A-2017-846774

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 the electrolytic solution is impregnated, 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 hundred nm to μm formed between the composite portion of the positive electrode active material, the conductive auxiliary agent and the binder, and the composite of the conductive auxiliary agent and the binder. It can be roughly divided into minute voids on the order of several tens of nm 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 at the time of manufacturing the battery, depending on the distribution state. Further, since the distribution of the voids is the distribution of the electrolytic solution after injection, the non-uniform distribution of the electrolytic solution in the positive electrode active material layer causes the utilization of the electrode active material during charging and discharging to be significantly uneven, and is non-water. It causes deterioration of the cycle characteristics of the electrolytic solution secondary battery.

Patent Document 1 describes a first composite oxide containing Li, Ni, Co, Mn and W as a positive electrode active material for a surface-modified lithium ion battery having good battery output characteristics and cycle characteristics. A positive electrode active material for a lithium ion battery in which a second composite oxide composed of a composite oxide of Li and Al or Nb is present on the particle surface is disclosed. 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 expected characteristics may not be sufficiently exhibited, and the cycle characteristics may be particularly deteriorated. ..

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

As a result of diligent studies, the present inventors have found that the deterioration of the cycle characteristics of the non-aqueous electrolyte secondary battery is greatly affected by the distribution of voids from the surface of the active material layer to the current collector.

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, and the active material layer includes active material particles, a conductive auxiliary agent, and the like. 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 having a binder and voids in a direction parallel to the main surface of the current collector. When is AR (S1 / S2), the minimum value m of the AR is 2.5% ≦ m, and the maximum value M of the AR is M ≦ 30%.

In the electrode according to the above aspect, the difference between the maximum value M of AR and the minimum AR value m may be Mm <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 in the general formula. , Co and Mn, at least one metal element selected from the group, M2 is a group consisting of Al, Fe, Ti, Cr, Mg, Cu, Ga, Zn, Sn, B, V, Ca and Sr. It is at least one metal element selected from, and 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 the elements composed of the central portion and the outer peripheral portion.

In the positive electrode according to the above aspect, the active material particles may be provided with a coating layer on the surface.

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.

The non-aqueous electrolyte secondary battery according to the second aspect includes an electrode according to the above aspect, an electrode facing the electrode, and a separator disposed between them.

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

It is a schematic diagram of the non-aqueous electrolyte secondary battery which concerns on this embodiment. It is a three-dimensional image of a part of the positive electrode. It is an image which cut the three-dimensional image of the positive electrode active material shown in FIG. 2 in the plane orthogonal to the stacking direction. It is an image of a cut surface obtained by cutting a three-dimensional image of the positive electrode active material shown in FIG. 2 on a surface orthogonal to the stacking direction. It is an image obtained by extracting a void portion from the cross-sectional image of FIG. 3 and making it monochromatic. It is a figure which showed the relationship between the ratio of the void part in the cross-sectional image, and the distance from a current collector. It is a figure which showed the relationship between the ratio of the void part in the cross-sectional image, and the distance from a current collector. This is an example of a large void distribution. In the figure, m is the minimum minimum value and M is the maximum maximum value.

Hereinafter, the present embodiment will be described in detail with reference to the drawings as appropriate. The electrodes of the non-aqueous electrolyte secondary battery have a positive electrode and a negative electrode, but the positive electrode will be mainly described below. In the drawings used in the following description, in order to make the features of the present invention easy to understand, the featured portions may be enlarged and shown, and the dimensional ratios and the like of each component may differ from the actual ones. is there. The materials, dimensions, etc. exemplified in the following description are examples, and the present invention is not limited thereto, and the present invention can be appropriately modified without changing the gist thereof.

[Non-aqueous electrolyte secondary battery]
FIG. 1 is a schematic view of a non-aqueous electrolyte secondary battery according to the present embodiment. The 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. Although not shown, the electrolytic solution is housed in the exterior body 50 together with the power generation element 40.

(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 from an electrically insulating porous structure, for example, a monolayer of a film made of polyolefin such as polyethylene or polypropylene, a laminate, a stretched film of a mixture of the above resins, or cellulose or polyester. , Polyacrylonitrile, polyamide, polyethylene and fiber non-woven fabrics made of at least one constituent material selected from the group consisting of polypropylene.

The separator 10 may be a solid electrolyte instead of the one formed from the electrically insulating porous structure.

As the solid electrolyte, known ones can be used. For example, a polymer solid electrolyte in which an alkali metal salt is dissolved in a polyethylene oxide-based polymer, Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ (Nashikon type), Li _1.07 Al _{0. 69} Ti _1.46 (PO ₄ ) ₃ (glass ceramics), Li _0.34 La _0.5 1Ti _O2.94 (perovskite type), Li ₇ La ₃ Zr ₂ O ₁₂ (garnet type), Li _2.9 PO _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 ceramic) such oxide-based solid _{electrolyte, 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, algyrodite type), Li _9.54 Si _1.74 P _1.44 S1 _1.7 C _l0.3 (crystal), Li _3.25 P _0.95 S ₄ (glass ceramics), Li ₇ P ₃ S ₁₁ (glass ceramics), 70Li ₂ S / 30P ₂ S ₅ (glass), _{30Li 2 S · 26B 2 S 3} · 44LiI ( _{glass), 50Li 2 S · 17P 2} S 5 · 33LiBH 4 ( _{glass), 63Li 2 S · 36SiS 2} · Li 3 PO 4 ( _glass), 57Li ₂ S · 38SiS 2 A sulfide-based solid electrolyte such as 5Li ₄ SiO ₄ (glass) can be mentioned.

<Positive electrode>
The positive electrode 20 has a plate-shaped (film-shaped) 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 current collector]
The positive electrode current collector 22 may be a conductive plate material, and for example, a thin metal plate such as an aluminum foil, a copper foil, or a nickel foil can be used.

[Positive electrode active material layer]
The active material particles 24A used in the positive electrode active material layer 24 can reversibly proceed with 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 ion, for example, lithium ion, sodium ion, magnesium ion and the like can be used, and it is particularly preferable to use lithium ion.

For example, in the case of a lithium ion secondary battery, lithium cobalt oxide (LiCoO _2), lithium nickelate (LiNiO _2), lithium manganate (LiMnO _2), lithium manganese spinel (LiMn ₂ O _4), and the 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, nb, shows Ti, Al, one or more elements or VO selected from Zr), lithium titanate _{(Li 4 Ti 5 O 12)} , LiNi x Co y Al z O 2 (0.9 <x + y + z <1. Composite metal oxides such as 1), polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene and the like can be used as the active material particles 24A.

Among those exemplified above, the active material particle 24A is preferably a material 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 and V. , Ca and Sr, at least one metal element selected from the group, 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 the elements constituting the central portion and the outer peripheral portion. The required performance differs between the outer peripheral portion and the central portion, which are in direct contact with the electrolytic solution. For example, by using a material having excellent reactivity in the outer peripheral portion and a high-capacity material in the central portion, it is possible to obtain active material particles 24A having excellent charge / discharge characteristics and high capacity.

The active material particles 24A preferably have a coating layer on the surface. As the coating layer, for example, a Zrfluoro complex, graphene or the like can be used. By producing the coating layer with a material different from that of the active material particles 24A, the performance required for each part can be realized as described above.

The positive electrode active material layer 24 may contain a conductive auxiliary agent, a binder, and a solid electrolyte in addition to the active material particles 24A. As the conductive auxiliary agent, for example, carbon powder such as carbon black, carbon nanotube, carbon material, metal fine powder such as copper, nickel, stainless steel, iron, a mixture of carbon material and metal fine powder, conductive oxide such as ITO, etc. Can be used. Among these, a carbon material such as carbon black is preferable. If sufficient conductivity can be ensured only by the active material, it is not necessary to contain the conductive additive.

As the binder, 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 Fluororesin such as ethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinylidene fluoride (PVF) can be mentioned.

In addition to the above, as binders, for example, vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based) Fluororesin), Vinylidene Fluoride-Pentafluoropropylene Fluororesin (VDF-PFP Fluororubber), Vinylidene Fluoride-Pentafluoropropylene-Tetrafluoroethylene Fluororesin (VDF-PFP-TFE Fluororesin), Vinylidene Fluoro Vinylidene fluoride-based fluoropolymers such as Ride-Perfluoromethyl Vinyl Ether-Tetrafluoroethylene Fluororesin (VDF-PFMVE-TFE Fluororesin) and Vinylidene Fluoride-Chlorotrifluoroethylene Fluororesin (VDF-CTFE Fluororesin) Rubber may be used.

As the solid electrolyte, the same ones as those mentioned in the separator can be used.

The density of the positive electrode active material layer 24 is preferably 2.9 g / cm ³ or more, more preferably 3.2 g / cm ³ or more, and even more preferably 3.5 g / cm ³ or more. Since the density of the positive electrode active material layer 24 is high, the energy density of the non-aqueous electrolyte secondary battery 100 is increased.

It is preferable to provide an undercoat layer between the positive electrode current collector 22 and the positive electrode active material layer 24. As the undercoat layer, a mixture of a conductive material and a binder can be used. The undercoat layer improves the adhesion with the positive electrode active material layer 24 and suppresses the peeling of the positive electrode active material layer.

[analysis method]
FIG. 2 is a three-dimensional image of a 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 a FIB (Focused Ion Beam) -SEM (scanning electron microscope), and is tertiary using the image analysis software ImageJ. I got it by converting 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 a three-dimensional image of the positive electrode active material layer shown in FIG. 2 at a plane orthogonal to the stacking direction. Using the image analysis software ImageJ, 380 cross-sectional images were reconstructed from the electrode surface toward the current collector according to the number of pixels in the vertical direction of 380. FIG. 3 is an image cut at the 150th portion 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, as a portion other than the positive electrode active material particles 24A, there is a first region 24B having a low contrast and a second region 24C having a lower contrast. The first region 24B is composed of a conductive auxiliary agent and a binder, and the second region 24C is composed of voids. However, since the conductive auxiliary agent, the binder portion and the void portion cannot be accurately separated only by the contrast, the accuracy can be improved by separating the conductive auxiliary agent, the binder portion and the void portion while comparing with the front and rear cross-sectional images.

FIG. 5 is an image obtained by extracting a portion of the separated voids and monochromaticizing the image with respect to FIG. In this image, the ratio of voids was 4.3%.

In FIG. 6, with respect to the cross-sectional image corresponding to the number of pixels in the vertical direction, the gap portion is extracted and monochromatic as shown in FIG. 5, the gap ratio is obtained, and the gap portion ratio and current collection in the cross-sectional image are obtained. It is a figure which showed the relationship of the distance 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 is 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 the void portion in the cross-sectional image and the distance from the current collector as in FIG. 6, but in this example, the distribution of the voids is large.

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

In the layer where the AR value is too low, the permeation of the electrolytic solution into the positive electrode active material layer in the liquid injection step is insufficient, which increases the non-uniformity of the charge / discharge reaction. When the non-uniformity of the charge / discharge reaction increases, an overreaction portion is formed and the cycle characteristics deteriorate. On the other hand, in the layer where the AR value is too high, the active material ratio decreases, the non-uniformity of the charge / discharge reaction in the positive electrode active material layer increases, and the cycle characteristics deteriorate.

On the other hand, in the positive electrode active material layer 24 according to the present embodiment, the active material particles 24A exhibit the expected performance. Further, since the non-uniformity between the electrolytic solution and the active material particles 24A in the positive electrode active material layer 24 is eliminated, the cycle characteristics of the non-aqueous electrolytic solution secondary battery 100 can be improved.

Of the peaks on the horizontal axis, the difference Mm between the maximum value M and the minimum minimum value m is preferably Mm <20.

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

<Negative electrode>
The negative electrode 30 has a plate-shaped (film-shaped) 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 any conductive plate material, and the same one 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. Further, if necessary, a conductive material, a binder, and a solid electrolyte may be contained.

The negative electrode active material may be any compound that can occlude and release ions, and a negative electrode active material used in a known non-aqueous electrolyte secondary battery can be used. Examples of the negative electrode active material include alkali or alkaline earth metals such as metallic lithium, graphite capable of storing and releasing ions (natural graphite, artificial graphite), carbon nanotubes, non-graphitizable carbon, easily graphitized carbon, and low temperature. Amorphous, mainly composed of carbon materials such as calcined carbon, metals that can be combined with metals such as lithium such as aluminum, silicon, and tin, and oxides such as SiO _x (0 <x <2) and tin dioxide. Examples thereof include particles containing a compound, lithium titanate (Li ₄ Ti ₅ O ₁₂ ) and the like.

The negative electrode active material layer may not contain the negative electrode active material and may be only a current collector. In this case, during charging, an alkali or alkaline earth metal such as metallic lithium is precipitated to form a negative electrode active material.

As the conductive material and the binder, the same ones as those of the positive electrode 20 can be used. As the binder used for the negative electrode, for example, cellulose, styrene / butadiene rubber, ethylene / propylene rubber, polyimide resin, polyamide-imide resin, acrylic resin and the like may be used in addition to those listed for the positive electrode.

As the solid electrolyte, the same ones as those mentioned in the separator can be used.

[analysis method]
For the negative electrode, the same analysis method as for the positive electrode can be used.

(Terminal)
The terminals 60 and 62 are connected to the positive electrode 20 and the negative electrode 30, respectively. The terminal 60 connected to the positive electrode 20 is a positive electrode terminal, and the terminal 62 connected to the negative electrode 30 is a negative electrode terminal. The terminals 60 and 62 are responsible for electrical connection with the outside. The terminals 60 and 62 are formed of a conductive material such as aluminum, nickel, or copper. The connection method may be welding or screwing. It is preferable to protect the terminals 60 and 62 with insulating tape in order to prevent a short circuit.

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

For example, as shown in FIG. 1, a metal laminate film in which a metal foil is coated from both sides with a polymer film 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.

As the metal foil 52, for example, an aluminum foil can be used. A polymer film such as polypropylene can be used for the resin layer 54. The material constituting the resin layer 54 may be different between the inside and the outside. For example, a polymer having a high melting point, for example, polyethylene terephthalate (PET), polyamide (PA), etc. is used as the outer material, and polyethylene (PE), polypropylene (PP), etc. are used as the material of the inner polymer film. be able to.

(Electrolytic solution)
The electrolytic solution is sealed in the exterior body 50 and impregnated in the power generation element 40.

As the electrolyte solution, 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 electrolyte aqueous solution has a low electrochemical decomposition voltage, the withstand voltage during charging is low and limited. Therefore, it is preferable that the electrolyte solution (non-aqueous electrolyte solution) uses an organic solvent.

The non-aqueous electrolyte 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 capable of solvating an electrolyte can be used. For example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) and the like can be used. The cyclic carbonate preferably contains at least PC.

The chain carbonate can reduce the viscosity of the cyclic carbonate. For example, diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) can be mentioned. In addition, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane and the like may be mixed and used.

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

As the electrolyte, a metal salt can be used. For example, LiPF ₆ , LiClO ₄ , LiBF ₄ , LiCF ₃ SO ₃ , LiCF ₃ CF ₂ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ CF ₂ SO ₂ ) _2. Lithium salts such as LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiN (CF ₃ CF ₂ CO) ₂ , LiBOB can be used. One of these lithium salts may be used alone, or two or more thereof may be used in combination. 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 solution to 0.5 to 2.0 mol / L. When the concentration of the electrolyte is 0.5 mol / L or more, the lithium ion concentration of the non-aqueous electrolyte solution can be sufficiently secured, and a sufficient capacity can be easily obtained during charging / discharging. Further, by suppressing the concentration of the electrolyte to 2.0 mol / L or less, it is possible to suppress an increase in the viscosity of the non-aqueous electrolyte solution, to sufficiently secure the mobility of lithium ions, and to obtain a sufficient capacity during charging and discharging. It will be easier.

Even when LiPF ₆ is mixed with other electrolytes, it is preferable to adjust the lithium ion concentration in the non-aqueous electrolyte solution to 0.5 to 2.0 mol / L, and the lithium ion concentration from LiPF ₆ is 50 mol% thereof. 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.

The ionic liquid includes, for example, ammonium cations such as imidazolium salts and pyridinium salts and phosphonium cations as cations, halogen anions such as bromide ions and trifurates, boron anions such as tetraphenylborate, and hexagons as anions. Examples thereof include those using a phosphorus anion such as fluorophosphate.

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

Although the embodiments of the present invention have been described in detail with reference to the drawings, the configurations and combinations thereof in the respective embodiments are examples, and the configurations are added or omitted within a range not deviating from the gist of the present invention. , Replacement, and other changes are possible.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples based on the above-described embodiment.

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

The positive electrode composite particle 1 generates an air flow consisting of air in the container of the fluidized bed granulator, and LiNi _0.85 Co _0.10 Al _0.05 O ₂ powder having an average particle size of 3 μm is charged as an active material. This was fluidized. Next, the above raw material solution was sprayed onto the fluidized bed active material powder, and the solution was adhered to the powder surface. The composition of the composite particles was 95.0% by weight of the active material, 3.0% by weight of the conductive additive, and 2.0% by weight of the binder. By keeping the temperature in the atmosphere in which the powder was placed constant at the time of this spraying, N, N-dimethylformamide was removed from the surface of the powder almost at the same time as the spraying. In this way, acetylene black and polyvinylidene fluoride were brought into close contact with the powder surface to obtain positive electrode composite particles 1 (average particle diameter: 15 μm).

The positive electrode composite particles 2 generate an air flow consisting of air in the container of the fluidized bed granulator, and LiNi _0.85 Co _0.10 Al _0.05 O ₂ powder having an average particle size of 15 μm is charged as an active material. This was fluidized. Next, the raw material solution diluted twice as described above was sprayed on the powder of the active material in a fluidized bed, 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 additive, and 1.0% by weight of the binder. By keeping the temperature in the atmosphere in which the powder was placed constant at the time of this spraying, N, N-dimethylformamide was removed from the surface of the powder almost at the same time as the spraying. In this way, acetylene black and polyvinylidene fluoride were brought into close contact with the powder surface to obtain positive electrode composite particles 2 (average particle diameter: 18 μm).

(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 aid), and 0.7 parts by weight of polyvinylidene fluoride. Vinylidene (PVDF: binder) and N-methyl-2-pyrrolidone (solvent) were mixed and dispersed to prepare a paste-like positive electrode slurry.

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

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

(Creation of cell)
The prepared negative electrode and positive electrode were punched into a predetermined shape, and alternately laminated via a polypropylene separator having a thickness of 16 μm, and three negative electrodes and two positive electrodes were laminated to prepare a laminated body. The nickel negative electrode terminal was attached to the protruding end of the copper foil in which the negative electrode active material layer was not provided in the negative electrode of the laminated body. Further, the positive electrode terminal made of aluminum was attached to the protruding end portion of the aluminum foil not provided with the positive electrode active material layer in the positive electrode of the laminated body. The negative electrode terminal and the positive electrode terminal were attached by an ultrasonic welder.

An opening was formed by inserting the laminate into the exterior of the aluminum laminate film and heat-sealing it except for one peripheral portion. Inside the exterior, a non-aqueous electrolytic solution containing a solvent containing EC, EMC and DEC in a volume ratio of 3: 5: 2 and 1.5 M (mol / L) of LiPF ₆ as a lithium salt. And injected. Then, the remaining one place was sealed with a heat seal while reducing the pressure with a vacuum sealer to prepare a lithium ion secondary battery according to Example 1.

(Battery structural analysis and evaluation)
After the initial characteristic evaluation, the produced lithium ion battery was subjected to cycle characteristic evaluation and structural analysis of the positive electrode. The cycle characteristics were evaluated in a 1C / 1C charge / discharge cycle.

(Measurement of charge / discharge cycle characteristics)
The cycle characteristics were performed using a secondary battery charge / discharge test device. The voltage range was from 4.3V to 3.0V. Only the first charge was performed by 0.2C constant current charging, and the discharge was performed at 1C. Charging was performed with a constant current and a constant voltage. It was charged with 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. Then, the cycle characteristics were measured under the condition of discharging at a current value of 1.0 C.

The charge / discharge cycle characteristics were evaluated as the capacity retention rate (%). The capacity retention rate (%) is the ratio of the discharge capacity to the initial discharge capacity in the number of cycles, with the discharge capacity of the first cycle as the initial discharge capacity. The capacity retention rate (%) is expressed by the following mathematical formula (1).
Capacity retention rate (%) = ("Discharge capacity in each cycle number" / "Discharge capacity in the first cycle") x 100 ... (1)

Note that 1C is a current value at which a battery cell having a capacity of a nominal capacity value is charged with a constant current or discharged with a constant current, and charging / discharging is completed in exactly one hour. The higher the capacity retention rate, the better the 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 the charge / discharge cycle characteristics.

(Structural analysis of positive electrode)
In the structural analysis of the positive electrode, the lithium ion battery in a completely discharged state after charging and discharging was disassembled in a glove box under a dry Ar atmosphere, and the positive electrode was separated. Then, it was sufficiently washed with DMC (dimethyl carbonate) and then vacuum dried. After cutting out the dried positive electrode to a predetermined size, 200 cross sections in the stacking direction were photographed using FIB-SEM (Versa3D manufactured by FEI). The FIB-SEM image was taken under the conditions of an accelerating 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 by the image analysis software ImageJ, and the ratio AR of the conductive aid and the binder portion in the cross-sectional image and the distance from the current collector are shown in the cross-sectional image. The minimum minimum value m, the maximum maximum value M, and the value of M-m 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 the positive electrode active material layer of the lithium ion secondary battery were produced by a spray-drying fluidized bed granulator. Here, the 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. Further, the positive electrode composite particles 2 were produced in the same manner as in Example 1.

The positive electrode composite particles 1 had an average particle diameter of 25 μm. Further, positive electrode composite particles 1 (average particle diameter: 25 μm) were obtained. The positive electrode composite particles 2 had an average particle diameter of 18 μm, which was the same as in Example 1.

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

(Preparation of positive electrode)
The obtained positive electrode slurry was uniformly applied to both sides of an aluminum foil having a thickness of 20 μm using a comma roll coater so that the amount of active material particles applied was 12.5 mg / cm2. Next, N-methyl-2-pyrrolidone was dried in an air atmosphere at 110 ° C. in a drying oven. Then, the obtained active material layer was pressed by a roll press machine, and the lower layer was pressed against both sides of the positive electrode current collector. The average composition of the active material layer was 94% by weight of the active material, 3.6% by weight of the conductive additive, 2.4% by weight of the 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, 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 are mixed. 92% by weight of the substance 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. The positive electrode slurry of the above was prepared.

Further, as a 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 are mixed. 97% by weight of substance particles, 2.0% by weight of acetylene black (conducting auxiliary), 1.0% by weight of PVDF (binder), and N-methyl-2-pyrrolidone (solvent) are mixed and dispersed to form a paste. The positive electrode slurry of the above was prepared.

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

Next, the obtained positive electrode slurry for the upper layer was uniformly applied onto the lower layer using a comma roll coater so that the amount of the active material particles applied was 6.2 mg / cm ² . Next, the N-methyl-2-pyrrolidone solvent was dried in an air atmosphere at 110 ° C. in a drying oven. The obtained active material layer was pressed by a roll press to obtain a positive electrode having a density of 3.3 g / cm ³ . The average composition of the active material layer was 94.5% by weight of the active material, 3.4% by weight of the conductive additive, and 2.1% by weight of the 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)
LiNi _0.85 Co _0.10 Al _0.05 O ₂ (active material particles) with an average particle size of 20 μm 94% by weight, Ketjen black (conductive aid) 3.6% by weight, PVDF (binder) 2.4 By weight% and N-methyl-2-pyrrolidone (solvent) were mixed and dispersed to prepare a paste-like positive electrode slurry.

(Preparation of positive electrode)
The obtained positive electrode slurry was uniformly applied to both sides of an aluminum foil having a thickness of 20 μm using a comma roll coater so that the amount of active material particles applied was 12.5 mg / cm ² . Next, N-methyl-2-pyrrolidone was dried in an air atmosphere at 110 ° C. in a drying oven.

Next, the obtained active material layer was pressed by a roll press to obtain a positive electrode having 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)
LiNi _0.85 Co _0.10 Al _0.05 O ₂ (active material particles) with an average particle size of 20 μm 96% by weight, Ketjen Black (conductive aid) 2.4% by weight, PVDF (binder) 1.6 By weight% and N-methyl-2-pyrrolidone (solvent) were mixed and dispersed to prepare a paste-like positive electrode slurry.

(Preparation of positive electrode)
The obtained positive electrode slurry was uniformly applied to both sides of an aluminum foil having a thickness of 20 μm using a comma roll coater so that the amount of active material particles applied was 12.5 mg / cm ² . Next, N-methyl-2-pyrrolidone was dried in an air atmosphere at 110 ° C. in a drying oven.

Next, the obtained active material layer was pressed by a roll press to obtain a positive electrode having 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 electrode 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, the minimum AR that is the ratio of the total area of the electrode cross section to the total area of the active material, the conductive auxiliary agent, the binder, and the voids. It was confirmed that when the value is 2.5% ≦ m and the maximum value of AR satisfies M ≦ 30%, the cycle characteristics are particularly excellent. Further, it was confirmed that the cycle characteristics were excellent when the difference between the maximum maximum value M and the minimum value m was Mm <20.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide an electrode capable of improving cycle characteristics and a non-aqueous electrolyte secondary battery using the electrode. These are suitably used as a power source for portable electronic devices, and are also used as electric vehicles, home and industrial storage batteries.

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 Exterior body 52 Metal foil 54 Resin layer 60, 62 Terminal 100 Non-aqueous electrolyte secondary battery 100 Non-aqueous electrolyte secondary battery

Claims (3)

It 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 has active material particles, a conductive auxiliary agent, a binder, and voids.
In the cross section of the active material layer in the direction parallel to the main surface of the current collector, the ratio of the total area S2 of the voids to the total area S1 of the active material layer in the cross section was defined as AR (S1 / S2). When
An electrode in which the minimum value m of the AR is 2.5% ≦ m and the maximum value M of the AR is M ≦ 30%. The electrode according to claim 1, wherein the difference between the maximum value M of AR and the minimum AR value m is Mm <20. A non-aqueous electrolyte secondary battery using the electrode according to claim 1 or 2.

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