JPH11214035A - Battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the performance of a battery such as capacity, while improving adherence of at least one of a positive electrode or a negative electrode to a separator by mainly composing the active material existing in a surface of at least one of the positive and the negative electrodes in contact with a separator of a material, having a grain diameter smaller than that of the active material inside of the electrode. SOLUTION: In a positive electrode 4 having a positive electrode collector 1, a first positive electrode layer 2 and a second positive electrode layer 3, a mean grain diameter of the active material included in the second positive electrode layer 3 in contact with a gel-like electrolyte layer 8 as a separator is made smaller than the mean grain diameter of the active material included in the first positive electrode layer 2. Grain diameter of the active material is computed as an isolated grain diameter in the case of the isolated grains, and as a secondary grain diameter in the case of a coagulated body. The more as the mean grain diameter is reduced, the smaller the space and height of projecting parts and recessed parts formed in the surface of the second positive electrode layer 3 become, and the surface area of the second positive electrode layer 3 becomes large, so that the adherence with the gel-like electrolyte layer 8 is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a positive electrode, a negative electrode,
And a separator disposed between the positive and negative electrodes.

[0002]

2. Description of the Related Art In recent years, with the development of electronic equipment, there has been a demand for the development of a non-aqueous electrolyte secondary battery that is small, lightweight, has a high energy density, and can be repeatedly charged and discharged. As such a secondary battery, a nickel hydrogen secondary battery, a lithium ion secondary battery, and the like are known. In general, a metal case such as stainless steel or iron is used for the exterior of these secondary batteries. In addition, the shape of the battery is mostly cylindrical or square. However, if the metal case is used for the exterior, the weight energy density of the battery naturally decreases because the exterior metal case is heavy. In order to reduce the weight of such a battery, it is necessary to reduce the weight of the outer case.

[0003] Under such circumstances, a polymer secondary battery using a polymer for the positive electrode, the negative electrode, and the electrolyte layer has been proposed. As shown in FIG. 2, the polymer secondary battery stores lithium ions in a current collector 11 such as an aluminum foil.
A positive electrode 13 in which a positive electrode layer 12 containing a releasable active material, a non-aqueous electrolytic solution and a polymer holding the electrolytic solution is laminated, and an active material capable of occluding and releasing lithium ions in a current collector 14 such as a copper foil A gel electrolyte layer 17 containing a non-aqueous electrolyte and a polymer holding the electrolyte is interposed between a non-aqueous electrolyte and a negative electrode 16 on which a negative electrode layer 15 containing the polymer holding the electrolyte is laminated. It has a structure. Since this polymer secondary battery integrates the positive electrode, the gel electrolyte layer, and the negative electrode, the function as a battery can be exhibited in a thin sheet state. In addition, since the non-aqueous electrolyte is held in the polymer, no leakage occurs and the exterior can be simplified. For this reason, a multilayer film such as a laminate film can be used as an exterior, and is thin, lightweight,
A battery with high energy density can be realized.

[0004]

However, in this polymer secondary battery, as the charge / discharge cycle progresses, the positive electrode layer peels off from the gel electrolyte layer and the discharge capacity decreases, and the capacity retention rate during the cycle decreases. There is a problem that it becomes shorter.
The integration of the positive electrode, gel electrolyte layer and negative electrode has been
It is performed by heating and pressing. Therefore, in order to increase the adhesion between the positive electrode layer and the gel electrolyte layer, a method of increasing the pressure during pressing or increasing the pressing temperature may be considered.

[0005] However, when the pressing pressure or the pressing temperature is increased, polymers (for example, vinylidene fluoride (VdF) and hexafluoropropylene) which are contained in the positive and negative electrode layers and the electrolyte layer and have a function of retaining the nonaqueous electrolytic solution are provided. (H
FP) is altered or destroyed, so that the adhesion between the positive and negative electrode layers and the electrolyte layer is reduced, and not only the capacity retention rate during cycling but also the discharge capacity is reduced.

An object of the present invention is to provide a battery having improved performance such as capacity by improving the adhesion between at least one of a positive electrode and a negative electrode and a separator.

[0007]

A battery according to the present invention comprises a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, wherein at least one electrode of the positive electrode and the negative electrode is provided. The active material existing on the surface in contact with the separator is mainly characterized in that the particle size is smaller than the active material inside the electrode.

A battery according to the present invention comprises a positive electrode containing an active material, a non-aqueous electrolyte and a polymer holding this electrolyte, a negative electrode containing an active material, a non-aqueous electrolyte and a polymer holding this electrolyte, A gel electrolyte layer disposed between the positive electrode and the negative electrode and containing a non-aqueous electrolyte and a polymer holding the electrolyte; and the gel electrolyte layer of at least the positive electrode among the positive electrode and the negative electrode The active material that is present on the surface in contact with is mainly characterized by having a smaller particle size than the active material inside the positive electrode.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A battery (for example, a polymer secondary battery) according to the present invention will be described below with reference to FIG. FIG. 1 is a partially cutaway perspective view showing a power generating element of a battery according to the present invention.

As shown in FIG. 1, a sheet-like power generating element includes a positive electrode current collector 1, a first positive electrode layer 2 laminated on the positive electrode current collector 1, and a first positive electrode layer 2 laminated on the first positive electrode layer 2. A positive electrode 4 having a second positive electrode layer 3; a negative electrode 7 having a negative electrode current collector 5 and a negative electrode layer 6 laminated on the negative electrode current collector 5; and the second positive electrode layer 3 and the negative electrode layer 6 And a gel electrolyte layer 8 as a separator interposed therebetween.

Leads are attached to the positive and negative electrode current collectors of such a power generating element, respectively, and are covered with a multilayer film having a gas barrier property and having a heat-sealing resin disposed on the inner surface. The polymer secondary battery can be obtained by sealing by attaching.

As the positive electrode, the negative electrode and the gel electrolyte layer of the secondary battery, for example, those described below can be used. (Positive Electrode) The positive electrode 4 includes the positive electrode current collector 1 and the current collector 1
A first positive electrode layer 2 containing a positive electrode active material, a non-aqueous electrolyte and a polymer holding the electrolyte, and a first positive electrode layer 2 A second positive electrode layer 3 containing a polymer that holds the liquid. The average particle size of the active material contained in the second positive electrode layer 3 in contact with the gel electrolyte layer 8 is smaller than the average particle size of the active material contained in the first positive electrode layer 2. However, the particle diameter of the active material is calculated as an isolated particle diameter in the case of isolated particles, and as a secondary particle diameter in the case of an aggregate.

Examples of the positive electrode active material include various oxides (eg, lithium manganese composite oxide such as LiMn ₂ O ₄ , manganese dioxide, lithium-containing nickel oxide such as LiNiO _2, and lithium-containing cobalt oxide such as LiCoO ₂ , for example). Particles mainly composed of oxides, lithium-containing nickel cobalt oxides, amorphous vanadium pentoxide containing lithium, etc., and particles mainly composed of chalcogen compounds (eg, titanium disulfide, molybdenum disulfide, etc.) Can be mentioned. Above all, it is preferable to use particles containing lithium-manganese composite oxide as a main component, particles containing lithium-containing cobalt oxide as a main component, and particles containing lithium-containing nickel oxide as a main component.

The average particle size of the active material contained in the first positive electrode layer is preferably in the range of 3.0 to 5.0 μm. By setting the average particle size in such a range, a high discharge capacity and a long life can be secured. A more preferable range of the average particle size is 3.0 to 3.5 μm.

The average particle size of the active material contained in the second positive electrode layer is preferably in the range of 1.0 to 3.0 μm. As the average particle size decreases, the interval and height of the irregularities formed on the surface of the second positive electrode layer decrease,
Since the surface area of the second positive electrode layer is increased, the adhesion to the gel electrolyte layer is improved. When the average particle size exceeds 3.0 μm, the surface area of the second positive electrode layer is not so large because unevenness formed on the surface of the second positive electrode layer is not so large. It becomes difficult to improve the adhesion to the layer, and the discharge capacity and the capacity retention during cycling may be reduced. Further, since the height of the unevenness is increased, the active material present on the surface of the second positive electrode layer may penetrate the gel electrolyte layer to cause an internal short circuit. on the other hand,
If the average particle size is less than 1.0 μm, the performance of the polymer secondary battery may be impaired. A more preferable range of the average particle size is 2.5 to 2.7 μm.

The non-aqueous electrolyte is prepared by dissolving an electrolyte in a non-aqueous solvent. As the non-aqueous solvent,
Ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DE
C), ethyl methyl carbonate (EMC), γ-butyrolactone (γ-BL), sulfolane, acetonitrile, 1,2-dimethoxyethane, 1,3-dimethoxypropane, dimethyl ether, tetrahydrofuran (TH
F) and 2-methyltetrahydrofuran. The non-aqueous solvents may be used alone or as a mixture of two or more.

Examples of the electrolyte include lithium perchlorate (LiClO ₄ ) and lithium hexafluorophosphate (L
iPF ₆ ), lithium borotetrafluoride (LiBF ₄ ), lithium arsenic hexafluoride (LiAsF ₆ ), lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ), lithium bistrifluoromethylsulfonylimide [LiN
(CF ₃ SO ₃ ) ₂ ].

The amount of the electrolyte dissolved in the non-aqueous solvent is desirably 0.2 mol / l to 2 mol / l. Examples of the polymer having the property of retaining a non-aqueous electrolyte include a polyethylene oxide derivative, a polypropylene oxide derivative, a polymer containing the derivative, a copolymer of vinylidene fluoride (VdF) and hexafluoropropylene (HFP), and the like. Can be used. The copolymerization ratio of the HFP depends on the method of synthesizing the copolymer, but is usually at most about 20% by weight.

The positive electrode may include a conductive material from the viewpoint of improving conductivity. Examples of the conductive material include artificial graphite, carbon black (eg, acetylene black), nickel powder, and the like.

The current collector may be, for example, a metal thin film such as an aluminum foil, an aluminum expanded metal,
It can be formed from a porous metal plate such as an aluminum mesh or an aluminum punched metal.

(Negative Electrode) The negative electrode 7 is formed from a negative electrode active material, a non-aqueous electrolyte, and a negative electrode layer 6 containing a polymer holding the electrolyte supported on a current collector 5.

Examples of the negative electrode active material include carbonaceous materials that occlude and release lithium ions. Such carbonaceous materials include, for example, those obtained by firing organic polymer compounds (eg, phenolic resin, polyacrylonitrile, cellulose, etc.), those obtained by firing coke and mesophase pitch, and those made by artificial graphite. And carbonaceous materials represented by natural graphite and the like. Among them, 500
It is preferable to use a carbonaceous material obtained by calcining the mesophase pitch at a temperature of from ℃ to 3,000 ℃ under normal pressure or reduced pressure.

The form of the carbonaceous material that stores and releases lithium ions can be particles or fibers. As the non-aqueous electrolyte and the polymer having the property of retaining the non-aqueous electrolyte, those similar to those described for the positive electrode described above are used.

The negative electrode is allowed to contain conductive materials such as artificial graphite, natural graphite, carbon black, acetylene black, Ketjen black, nickel powder, polyphenylene derivatives, and fillers such as olefin polymers and carbon fibers. I do.

The current collector can be formed of, for example, a metal thin film such as a copper foil, a porous metal plate such as a copper expanded metal, a copper mesh, or a copper punched metal.

The negative electrode has a structure in which one type of negative electrode layer is supported on a current collector as described above, and also includes the negative electrode current collector, the first negative electrode layer, and the One having a structure in which two negative electrode layers are stacked in this order can be used.

When the form of the active material contained in the negative electrode is particles, the average particle size of the active material contained in the first negative electrode layer is set to 10 to 20 μm, and the second negative electrode layer in contact with the gel electrolyte is formed. The average particle size of the active material contained in
Is preferable.

When the form of the active material contained in the negative electrode is a fiber, the length of the fiber is regarded as the particle size, and
It is preferable that the average length of the active material included in the negative electrode layer is 30 to 40 μm, and the average length of the active material included in the second negative electrode layer in contact with the gel electrolyte is 20 to 30 μm.

(Gel Electrolyte Layer) The electrolyte layer 8 contains a non-aqueous electrolyte and a polymer holding the electrolyte. As the non-aqueous electrolyte and the polymer having the property of retaining the non-aqueous electrolyte, those similar to those described for the positive electrode described above are used.

The electrolyte layer may contain an inorganic filler such as silicon oxide powder from the viewpoint of further improving the strength. In FIG. 1 described above, the positive electrode is formed by laminating the first and second positive electrode layers on one side of the positive electrode current collector. However, a porous metal plate is used as the current collector, and the positive electrode layers are supported on both sides. You may let it. In this case, the first metal sheet is provided on both sides of the porous metal plate.
Stacking positive electrode layers, stacking a second positive electrode layer on each first positive electrode layer,
The positive electrode can be configured. Similarly, in the case of a negative electrode, a negative electrode layer can be supported on both surfaces of a porous metal plate as a current collector.

As described above, the battery according to the present invention comprises:
A positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, and an active material present on a surface of at least one of the positive electrode and the negative electrode that is in contact with the separator has a particle size. Mainly those smaller than the active material inside the electrode. According to such a battery, the adhesiveness between the positive electrode and the separator, or the adhesiveness between the negative electrode and the separator, or the adhesiveness between both the positive and negative electrodes and the separator can be increased, so that the performance is improved. Can be. Further, when the separator is made thinner to reduce the thickness of the battery, the active material of one electrode can be prevented from penetrating through the separator, so that an internal short circuit due to the reduction in thickness can be avoided. In particular, a positive electrode containing an active material, a non-aqueous electrolyte and a polymer holding this electrolyte, a negative electrode containing an active material, a non-aqueous electrolyte and a polymer holding this electrolyte, a non-aqueous electrolyte and this electrolyte In a polymer secondary battery comprising a gel electrolyte layer containing a polymer that retains, the active material present on at least the surface of the positive electrode and the negative electrode that is in contact with the gel electrolyte layer of the positive electrode has a particle size of the positive electrode By mainly using a material smaller than the inner active material, the positive electrode layer can be prevented from peeling from the gel electrolyte layer as the charge / discharge cycle progresses. Can be improved.

The ratio of the active material having a smaller particle size in the active material on the surface of the electrode in contact with the separator is preferably 10% or more as compared with the inside of the electrode. A more preferred range is 15% or more.

[0033]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments according to the present invention will be described below in detail with reference to the drawings. (Example) <Preparation of Positive Electrode> (Preparation of First Positive Electrode Layer) First, lithium hydroxide / monohydrate (LiOH · H ₂ O) and manganese dioxide (MnO ₂ ) were used at a molar ratio of Li to Mn of 1. Mix 5: 1
After dehydrating the mixture at a temperature of 110 degrees for 2 hours,
This was heated at 800 ° C. for 2 hours and 20 minutes to produce lithium manganese composite oxide particles (active material) having a composition formula of LiMn ₂ O ₄ and an average particle size of 3.4 μm. 56% by weight of the lithium manganese composite oxide particles,
Vinylidene fluoride-hexafluoropropylene (V
17% by weight of a copolymer of dF-HFP), 5% by weight of carbon black, and dibutyl phthalate (DB) as a plasticizer.
P) A paste was prepared by mixing 22% by weight in acetone. The obtained paste is applied on a polyethylene terephthalate film (PET film) to form a sheet,
A first positive electrode sheet having a thickness of 150 μm and not impregnated with a non-aqueous electrolyte was prepared.

(Preparation of Second Positive Electrode Layer) The average particle size is 2.5 μm.
Except for m, lithium manganese composite oxide particles (active material) similar to those described in the first positive electrode layer described above were produced. A paste is prepared from the obtained particles in the same manner as described in the first positive electrode layer, and the obtained paste is applied to a PET film, formed into a sheet, and formed into a sheet having a thickness of 50%.
A second positive electrode sheet not impregnated with a non-aqueous electrolyte solution having a thickness of μm was prepared.

Aluminum foil as a positive electrode current collector,
The positive electrode sheet and the second positive electrode sheet were stacked in this order, and integrated using a double-roll laminator to produce a non-aqueous electrolyte-unimpregnated positive electrode.

<Preparation of Negative Electrode> As an active material, 58% by weight of mesophase pitch carbon fiber, 17% by weight of a VdF-HFP copolymer, and 25% by weight of DBP were mixed in acetone to prepare a paste. Put the obtained paste into PET
The resultant was applied on a film and formed into a sheet to prepare an electrolyte-impregnated negative electrode sheet having a thickness of 180 μm. A negative electrode sheet was stacked on a copper foil as a negative electrode current collector and integrated using a double roll laminator to produce a non-aqueous electrolyte-unimpregnated negative electrode.

<Preparation of Gel Electrolyte Layer> Silicon Oxide Powder 3
3.3% by weight, 22.2% by weight of a copolymer of VdF-HFP, and 44.5% by weight of DBP were mixed in acetone to form a paste. The obtained paste was applied on a PET film, formed into a sheet, and an electrolyte-impregnated electrolyte layer having a thickness of 85 μm was prepared.

<Preparation of Non-Aqueous Electrolyte> LiPF ₆ as an electrolyte was mixed with ethylene carbonate (EC) and dimethyl carbonate (DMC) in a non-aqueous solvent mixed at a volume ratio of 2: 1 at a concentration of 1 mol / mol. 1 to prepare a non-aqueous electrolyte.

<Preparation of Unit Cell> The positive electrode, the negative electrode, and the gel electrolyte layer were laminated so that the gel electrolyte layer was disposed between the second positive electrode layer and the negative electrode layer. It was integrated as shown in FIG. 1 using a roll laminator. The obtained laminate was immersed in methanol, and the DBP in the laminate was extracted with methanol, removed, and dried. Next, positive and negative leads were connected to the positive and negative electrodes, respectively, to obtain a sheet-like power generating element.

<Assembly of Battery> A multilayer film in which a polyethylene terephthalate film, an aluminum foil, a polyethylene film and an ionomer film were laminated in this order was prepared as an exterior material. The power generating element was covered with this multilayer film so that the positive and negative electrode leads extended from the film, and the opening of the film was sealed by heat fusion except for one opening. The power generating element was impregnated with the non-aqueous electrolyte by injecting the non-aqueous electrolyte having the above composition from the remaining openings. Next, the opening was sealed by heat fusion to produce a polymer secondary battery having a theoretical capacity of 230 mAh. (Comparative Example) <Preparation of Positive Electrode> (Preparation of Positive Electrode Layer) A lithium manganese composite oxide particle (active material) similar to that described in the first positive electrode layer described above was used to explain the first positive electrode layer described above. A paste is prepared in the same manner, and the obtained paste is applied to a PET film.
The sheet was formed into a positive electrode sheet having a thickness of 200 μm and not impregnated with a non-aqueous electrolyte.

An aluminum foil as a positive electrode current collector and a positive electrode sheet were stacked in this order, and integrated using a double roll laminator to prepare a positive electrode not impregnated with a nonaqueous electrolyte.

The obtained positive electrode, a negative electrode similar to the example, and a gel electrolyte layer similar to the example were laminated so that the gel electrolyte layer was disposed between the positive electrode layer and the negative electrode layer.
These were integrated using a double roll laminator. After removing DBP in the obtained laminate in the same manner as in the example, positive and negative electrode leads were connected to the positive and negative electrodes, respectively, to obtain a sheet-like power generating element.

<Assembly of Battery> The power generating element is covered with a multilayer film similar to that of the embodiment so that the positive and negative electrode leads extend from the film, and the opening of the film is sealed by heat sealing except for one opening. Stopped. The power generating element was impregnated with the non-aqueous electrolyte by injecting the non-aqueous electrolyte having the above composition from the remaining openings. Next, the opening was sealed by heat fusion to produce a polymer secondary battery having a theoretical capacity of 230 mAh.

The obtained secondary batteries of Examples and Comparative Examples were charged and discharged at a constant current of 40 mA, 4.2 V, and a constant current and constant voltage of 10 hours, and then discharged at a current of 40 mA to 2.7 V. The cycle was repeated, and the discharge capacity at the first cycle and the 100th cycle of each battery was measured. The results are shown in Table 1 below.

Table 1 Discharge capacity at 1st cycle Discharge capacity at 100th cycle Example 220 mAh 180 mAh Comparative example 200 mAh 130 mAh As is clear from Table 1, the secondary batteries of Examples have initial discharge capacity and capacity at 100 cycles. It can be seen that the maintenance ratio is higher than the comparative example.

Further, each of the secondary batteries of Examples and Comparative Examples was 10
A semi image of the cross section was observed before and after the above-described charge / discharge cycle for each piece. Prior to the charge / discharge cycle, there was no defect such as peeling in the secondary batteries of Examples and Comparative Examples. After 100 charge / discharge cycles, the positive electrode layer was peeled from the gel electrolyte layer in two out of ten gels in the example, and the positive electrode layer was peeled from the gel electrolyte in eight out of ten gels in the comparative example. I confirmed that. From this fact, the positive electrode layer is separated from the gel electrolyte layer during the charge / discharge cycle by making the particle size of the active material present on the surface of the positive electrode layer in contact with the gel electrolyte smaller than that inside as in the example. It can be seen that the discharge capacity and the capacity retention rate during the cycle can be improved.

[0047]

As described in detail above, according to the present invention, the adhesion between at least one of the positive electrode and the negative electrode and the separator is improved, and a battery having excellent performance can be provided.

[Brief description of the drawings]

FIG. 1 is a partially cutaway perspective view showing a power generating element of a battery according to the present invention.

FIG. 2 is a partially cutaway perspective view showing a power generation element of a conventional battery.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Positive electrode collector, 2 ... 1st positive electrode layer, 3 ... 2nd positive electrode layer, 4 ... Positive electrode, 5 ... Negative electrode current collector, 6 ... Negative electrode layer, 7 ... Negative electrode, 8 ... Gel electrolyte layer.

Claims (2)

[Claims] 1. An active material, comprising: a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode; and an active material present on a surface of at least one of the positive electrode and the negative electrode in contact with the separator. Is a battery mainly composed of a material having a particle size smaller than that of an active material inside an electrode. 2. A positive electrode containing an active material, a non-aqueous electrolyte and a polymer holding the electrolyte, a negative electrode containing an active material, a non-aqueous electrolyte and a polymer holding the electrolyte, the positive electrode and the negative electrode And a gel electrolyte layer containing a non-aqueous electrolyte and a polymer holding the electrolyte, wherein at least a surface of the positive electrode and the negative electrode is in contact with the gel electrolyte layer of the positive electrode. A battery mainly comprising an active material having a smaller particle diameter than the active material inside the positive electrode.