JPH1140161A - Paste type positive electrode and alkaline secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a paste-type positive electrode with high performance, which has superior holding power of a depolarizing mix for cell containing nickel hydroxide, and can reduce the falling out amount of the depolarizing mix for cell during manufacture, or when producing a group of spiral electrodes. SOLUTION: In a paste-type positive electrode, equipped with a conductive base plate and a depolarizing mix for cell which contains nickel hydroxide and binder and is held on the aforementioned conductive base plate, the depolarizing mix for cell is divided into a least 3 layers, and binder on the outside layer of the depolarizing mix for cell contains one or more kinds of polymers selected from fluororesin and rubber latex, and binder arranged in an area between the two outside layers of the depolarizing mix for cell does not contain the polymer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a paste-type positive electrode containing nickel hydroxide and an alkaline secondary battery.

[0002]

2. Description of the Related Art A cylindrical nickel-metal hydride secondary battery, which is an example of an alkaline secondary battery, has an electrode group formed by interposing a separator between a nickel positive electrode and a hydrogen storage alloy negative electrode together with an alkaline electrolyte. Has a structure housed inside. Heretofore, the nickel positive electrode has been manufactured by a so-called sintering method in which a sintered substrate prepared by molding and sintering carbonyl nickel powder is impregnated with a nickel salt solution and then chemically formed. However, the sintered nickel electrode requires a very complicated operation when manufacturing the sintered substrate or impregnating the sintered substrate with the solution. .

[0003] For this reason, in the secondary battery, a paste-type nickel positive electrode is widely used. The paste-type nickel positive electrode is made into a paste by dispersing nickel hydroxide powder as an active material using a binder,
After the paste is filled in a conductive substrate made of, for example, a three-dimensional network porous body, the paste is dried, rolled, and then cut into a desired size.

As the binder, conventionally, for example, carboxymethylcellulose, polytetrafluoroethylene,
Polyacrylate, polyvinyl alcohol and the like are used. Since the binder is non-conductive, it does not contribute to the electrochemical reaction, but rather becomes a factor inhibiting the reaction. For this reason, it is not preferable to add a large amount. However, when the paste containing no binder is filled in a conductive substrate, dried, and then rolled to produce a paste-type positive electrode, there is a problem that nickel hydroxide powder is likely to fall off the positive electrode. In particular, when an electrode group is produced by spirally winding the positive electrode and the negative electrode produced by such a method with a separator interposed therebetween, cracks occur in the positive electrode, and the mixture is mixed from the conductive substrate. It was easily released, and the amount of nickel hydroxide powder dropped off was extremely large.

[0005]

DISCLOSURE OF THE INVENTION The present invention is excellent in holding power of a mixture containing nickel hydroxide, and can reduce the amount of the mixture falling off during manufacturing or during the preparation of a spiral electrode group. It is an object of the present invention to provide a high performance paste type positive electrode and an alkaline secondary battery.

[0006]

According to the present invention, there is provided a paste-type positive electrode comprising a conductive substrate, and a mixture containing nickel hydroxide and a binder held on the conductive substrate. The agent is divided into at least three layers, and the binder of the outer mixture layer contains at least one polymer selected from a fluororesin and a rubber-based latex.
A paste-type positive electrode is provided, wherein the binder in the mixture region disposed between the two outer mixture layers does not contain the polymer.

Further, according to the present invention, a paste-type positive electrode comprising a conductive substrate, and a mixture containing nickel hydroxide and a binder held on the conductive substrate; a negative electrode; an alkaline electrolyte; In the alkaline secondary battery, the mixture of the positive electrode is divided into at least three layers, and the binder of the outer mixture layer includes at least one polymer selected from a fluorine-based resin and a rubber-based latex. An alkaline secondary battery is provided, wherein the binder in the mixture region disposed between the two outer mixture layers does not contain the polymer.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a paste type positive electrode according to the present invention will be described in detail. The positive electrode includes, for example, a conductive substrate, and a mixture held on the conductive substrate and including nickel hydroxide and a binder, wherein the mixture has a three-layer structure, and an outer mixture layer The binder of the (outermost mixture layer) contains at least one polymer selected from a fluororesin and a rubber-based latex, respectively, and as a mixture region arranged between the two outermost mixture layers. , The binder of the intermediate mixture layer may not contain the polymer.

[0009] Such a positive electrode is produced, for example, by the method described below. First, a first paste is prepared by kneading a binder containing at least one polymer selected from a fluorine-based resin and a rubber-based latex, particles mainly composed of nickel hydroxide, and a conductive assistant together with water. . On the other hand, the second paste is prepared by kneading the binder, the particles containing nickel hydroxide as a main component and the conductive additive together with water. After filling the conductive paste with the second paste, the first paste
The paste having the above-described configuration can be manufactured by applying the paste described above, drying and applying pressure molding.

First, the first paste constituting the outermost mixture layer will be described. Examples of the particles containing nickel hydroxide as a main component include nickel hydroxide particles, and nickel hydroxide particles in which zinc and / or cobalt are coprecipitated with metallic nickel. The latter positive electrode containing nickel hydroxide particles can further improve the charging efficiency in a high temperature state.

From the viewpoint of improving the charge / discharge efficiency of the alkaline secondary battery, the nickel hydroxide has a peak half-value width of the (101) plane determined by an X-ray powder diffraction method of 0.8 ° / 2θ (Cu
-Kα) or more. A more preferable range of the peak half width is 0.9 to 1.0 °.

The particles mainly composed of nickel hydroxide have an average particle diameter of 5 to 30 μm and a tap density of 1.8 g / g.
cm ³ or more and a specific surface area of 1 to ²⁰ m ² / g.

It is preferable that the particles mainly composed of nickel hydroxide have a spherical shape or a shape similar thereto. Examples of the conductive assistant include metal cobalt, cobalt hydroxide (Co (OH) ₂ ), and cobalt monoxide (CoO). Among them, cobalt hydroxide and cobalt monoxide are preferred. However, this conductive agent is allowed to contain trace amounts of dicobalt trioxide and tricobalt tetroxide. Further, the conductive agent may be present in the form of particles in the positive electrode, or may be present in the form of a layer on the surface of the particles containing nickel hydroxide as a main component.

[0014] As the binder, a binder composed of one or more polymers selected from a fluororesin and a rubber latex, or a binder composed of the above-mentioned polymer and another combined polymer can be used. Examples of the fluorine-based resin include polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene, polyvinylidene, and hexafluoroethylene. In addition, polytetrafluoroethylene is
It can be used in the form of a dispersion. on the other hand,
Examples of the rubber latex include styrene butadiene rubber (SBR) latex, acrylonitrile butadiene rubber (NBR) latex, ethylene propylene diene monomer (EPDM) latex, styrene / butadiene / MMA, and the like. Among them, those containing polytetrafluoroethylene are preferable.

Examples of the concomitant polymer include carboxymethylcellulose (CMC), methylcellulose (MC), hydroxymethylcellulose (HPMC),
Polyacrylate (eg, sodium polyacrylate (SPA)), polyvinyl alcohol (PVA), polyethylene oxide, a copolymer of vinyl alcohol with a monomer having at least one COOX group (where X is hydrogen, alkali metal And at least one element selected from hydrophilic polymers (e.g., one or more elements selected from alkaline earth metals). Among the aforementioned types, carboxymethylcellulose (CM
C) is preferred.

It is preferable that the compounding amount of the binder is in the range of 0.1% by weight to 10% by weight based on the particles containing nickel hydroxide as a main component. The thickness of the outermost mixture layer is 0.1% with respect to the thickness of the intermediate mixture layer, respectively.
% To 50%. This is due to the following reasons. When the thickness of each of the outermost mixture layers is less than 0.1% of the thickness of the intermediate mixture layer, it is possible to prevent the mixture powder from falling off during the preparation of the positive electrode and the spiral electrode group. It can be difficult. On the other hand, if the thickness of each of the outermost mixture layers exceeds 50% of the thickness of the intermediate mixture layer, it is possible to more effectively prevent the mixture powder from falling off as described above. The discharge capacity and cycle life may be reduced.

Hereinafter, the second paste constituting the intermediate mixture layer will be described. Examples of the particles mainly composed of nickel hydroxide and the conductive additive include those similar to those described in the first paste.

As the binder, one not containing one or more polymers selected from the above-mentioned fluororesins and rubber latexes, for example, one or two or more hydrophilic polymers can be used. Examples of the hydrophilic polymer include the same polymers as those described in the outermost layer described above.

The compounding amount of the binder is preferably in the range of 0.1% by weight to 10% by weight based on the particles containing nickel hydroxide as a main component. As the conductive substrate on which the two outermost mixture layers and the intermediate mixture layer are held, for example, those having a two-dimensional structure such as punched metal, expanded metal, wire mesh, nickel sponge-like substrate, nickel fiber And a composite substrate obtained by combining the two-dimensional structure and the three-dimensional structure described above. Among them, a substrate having a three-dimensional structure is optimal.

An example of an alkaline secondary battery incorporating the paste-type positive electrode described above will be described in detail with reference to FIG. An electrode group 5 produced by spirally winding the above-mentioned paste-type positive electrode 2 and negative electrode 4 with a separator 3 interposed therebetween is accommodated in a bottomed cylindrical container 1. The negative electrode 4 is arranged at the outermost periphery of the electrode group 5 and is in electrical contact with the container 1. The alkaline electrolyte is contained in the container 1. A circular first sealing plate 7 having a hole 6 in the center is arranged at the upper opening of the container 1. The ring-shaped insulating gasket 8 is disposed between the peripheral edge of the sealing plate 7 and the inner surface of the upper opening of the container 1, and the sealing plate is formed on the container 1 by caulking to reduce the diameter of the upper opening inward. 7
Are hermetically fixed via the gasket 8. One end of the positive electrode lead 9 is connected to the positive electrode 2, and the other end is connected to the lower surface of the sealing plate 7. The positive electrode terminal 10 having a hat shape is attached on the sealing plate 7 so as to cover the hole 6. A rubber safety valve 11 is disposed so as to close the hole 6 in a space surrounded by the sealing plate 7 and the positive electrode terminal 10. A circular holding plate 12 made of an insulating material having a hole in the center is arranged on the positive electrode terminal 10 such that a protrusion of the positive electrode terminal 10 projects from the hole of the holding plate 12. The outer tube 13 covers the periphery of the holding plate 12, the side surface of the container 1, and the periphery of the bottom of the container 1.

Next, the negative electrode 4, the separator 3, and the alkaline electrolyte will be described in detail. 1) Negative Electrode The negative electrode 4 is, for example, a hydrogen storage alloy negative electrode including hydrogen storage alloy particles that store and release hydrogen. Such a negative electrode has a structure in which a mixture having a composition including the hydrogen storage alloy powder, a conductive agent, and a binder is fixed to a conductive substrate serving as a current collector.

Examples of the hydrogen storage alloy compounded in the mixture of the negative electrode 1 include LaNi ₅ , MmNi ₅ (Mm;
Misch metal), LmNi ₅ (Lm; lanthanum-enriched misch metal), and a part of Ni of these alloys is A
l, Mn, Co, Ti, Cu, Zn, Zr, Cr, a multi-element system substituted with elements such as B, or TiNi
And TiFe-based ones. In particular, the general formula _{LmNi w Co x Mn y Al z} ( atomic ratio w, x,
(The total value of y and z is 5.00 ≦ w + x + y + z ≦ 5.5.) A hydrogen storage alloy having a composition represented by the following formula: It is.

Examples of the conductive agent include carbon black and graphite. Examples of the binder include polyacrylates such as sodium polyacrylate and potassium polyacrylate, fluororesins such as polytetrafluoroethylene, and carboxymethyl cellulose.

Examples of the conductive substrate include those having a two-dimensional structure such as punched metal, expanded metal, and wire mesh, and those having a three-dimensional structure such as a nickel sponge-like substrate, a nickel fiber-like substrate, and a nickel felt-like substrate. And a composite substrate obtained by combining the two-dimensional structure and the three-dimensional structure described above. 2) Separator Examples of the separator 3 include a polyolefin nonwoven fabric such as polypropylene and polyethylene, a nylon nonwoven fabric, and a mixture of these fibers. Further, those subjected to a hydrophilic treatment as necessary can be applied. In particular, a polypropylene nonwoven fabric whose fiber surface has been hydrophilized is suitable as the separator 3. 3) Alkaline Electrolyte Examples of the alkaline electrolyte include a mixed aqueous solution of sodium hydroxide and lithium hydroxide, a mixed aqueous solution of potassium hydroxide and lithium hydroxide, or a mixed aqueous solution of sodium hydroxide, potassium hydroxide, and lithium hydroxide. Can be used.

The paste-type positive electrode according to the present invention described in detail above is a paste-type positive electrode including a conductive substrate, and a mixture containing nickel hydroxide and a binder held on the conductive substrate, The mixture is divided into at least three layers, and the binder of the outer mixture layer contains at least one polymer selected from a fluororesin and a rubber-based latex, respectively. Is characterized in that the binder in the mixture region arranged in the above does not contain the polymer. In such a positive electrode, since the mixture near the surface is firmly held by the binder containing the polymer on the conductive substrate, the mixture drops off from the positive electrode during the positive electrode manufacturing process or during the preparation of the spiral electrode group. Can be suppressed. As a result, a decrease in the positive electrode capacity can be suppressed, and an internal short circuit caused by the dropped mixture coming into contact with the negative electrode can be prevented. Although the polymer (for example, polytetrafluoroethylene) can enhance the bonding force between the mixture and the conductive substrate, it is insulative, so if contained in a large amount, conduction between the nickel hydroxide and the conductive substrate can occur. Deteriorates. In the positive electrode, since the mixture region disposed between the outer mixture layers does not contain the polymer, deterioration of conduction between nickel hydroxide and the conductive substrate due to the polymer can be avoided. . Therefore, such a positive electrode, a negative electrode and an alkaline secondary battery provided with an alkaline electrolyte, while maintaining excellent battery characteristics such as discharge capacity, operating voltage, cycle life, internal pressure characteristics and positive electrode swelling ratio, The internal short-circuit occurrence rate at the time of manufacturing can be reduced.

By setting the thickness of each of the outer mixture layers in the range of 0.1% to 50% of the thickness of the mixture region, a higher performance paste-type positive electrode and alkaline secondary battery can be obtained. It can be provided in yield.

[0027]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below in detail. (Example) <Preparation of paste-type positive electrode> 2% by weight of zinc and 2% by weight of cobalt were contained in terms of metal, and the average particle size was 18%.
Dispersion (specific gravity: 1.90 parts by weight) of cobalt monoxide and polytetrafluoroethylene (PTFE) as a binder was added to 90 parts by weight of spherical nickel hydroxide powder of μm.
5, a solid content of 60% by weight) and 2.5 parts by weight of a solid content and 0.2 parts by weight of a copolymer of sodium acrylate (SPA) -vinyl alcohol (PVA) were added. To this, 60 parts by weight of pure water was added and kneaded to prepare a first paste. On the other hand, the nickel hydroxide powder 90
10 parts by weight of cobalt monoxide and 0.2 parts by weight of a copolymer of sodium acrylate (SPA) -vinyl alcohol (PVA) as a binder were added to 60 parts by weight of pure water. And kneaded to prepare a second paste.

Next, after applying and filling the second paste into a nickel sintered fiber substrate as a conductive substrate,
Further, the first paste is applied to both surfaces and dried,
Rolling by roller press, capacity is 1500mA
Thus, a paste-type nickel positive electrode having a thickness of 0.6 mm was manufactured. The positive electrode has a mixture layer (outermost mixture layer) containing polytetrafluoroethylene as a binder on both surfaces, and a binder of an intermediate mixture layer existing between these mixture layers. Has a configuration not containing polytetrafluoroethylene. The thickness of each of the outermost mixture layers was equivalent to 2% of the thickness of the intermediate mixture layer. <Preparation of Paste Type Negative Electrode> LmNi _4.0 Co _0.4 Mn _0.3 Al is obtained by high frequency melting using commercially available lanthanum-enriched misch metal Lm, Ni, Co, Mn and Al.
_A hydrogen storage alloy having a composition of _0.3 was prepared. The hydrogen storage alloy was mechanically pulverized and passed through a sieve of 200 mesh or less. To 100 parts by weight of the obtained alloy powder, 0.5 parts by weight of sodium polyacrylate, 0.12 parts by weight of carboxymethyl cellulose (CMC), and a dispersion of polytetrafluoroethylene (specific gravity 1.5, solid content 60% by weight) Was added with 2.5 parts by weight in terms of solid content and 1.0 part by weight of carbon black as a conductive agent, and kneaded with 50 parts by weight of water to prepare a paste.
The paste was applied to a punched metal as a conductive substrate, dried, and then rolled by a roller press to produce a paste-type hydrogen storage alloy negative electrode.

An electrode group was manufactured by spirally winding the positive electrode and the negative electrode while interposing a separator made of a nonwoven fabric made of polyolefin subjected to a hydrophilic treatment between them. The obtained electrode group was housed in a nickel-plated steel, bottomed cylindrical container. This is 7
A cylindrical nickel-metal hydride secondary battery having a theoretical capacity of 1500 mAh having the structure shown in FIG. 1 described above was assembled by containing and sealing an alkaline electrolyte composed of N KOH and 1N LiOH. (Comparative Example 1) A cylindrical nickel-metal hydride secondary battery similar to the example was assembled except that a paste type positive electrode described below was used.

That is, after applying and filling the above-mentioned second paste on the same conductive substrate as in the embodiment, the second paste is further applied to both surfaces thereof, dried and rolled by a roller press. As a result, a paste-type nickel positive electrode having the same capacity and thickness as in Example 1 was manufactured. (Comparative Example 2) A cylindrical nickel-metal hydride secondary battery similar to that of the example was assembled except that a paste-type positive electrode described below was used.

That is, after applying and filling the above-mentioned first paste on the same conductive substrate as in the embodiment, the first paste is further applied to both surfaces thereof, dried and rolled by a roller press. As a result, a paste-type nickel positive electrode having the same capacity and thickness as in Example 1 was manufactured.

With respect to the obtained secondary batteries of Examples and Comparative Examples 1 and 2, the amount of the cathode mixture dropped off, the internal short-circuit rate, the battery capacity,
The positive electrode swelling ratio, battery internal pressure, operating voltage, and cycle number were evaluated by the methods described below. (Measurement of dropout amount) This measurement was performed during the production of the paste type positive electrode. In other words, between the time when the paste is applied to the conductive substrate and the time when it is roll-formed by the roller press, the paste is dropped at the joint of each work, the deposited mixture is collected and weighed, and the mixture is divided by the production quantity. Table 1 below shows the dropout amount.
Shown in (Internal Short-Circuit Occurrence Rate) When assembling the secondary batteries of Examples and Comparative Examples 1 and 2, it is checked whether or not an internal short-circuit has occurred at the completion of the spiral electrode group fabrication and at the completion of the sealing step, and the occurrence of the internal short-circuit occurs in each step. The number of internal short circuits thus obtained is totaled, the result is divided by the production quantity, and the result expressed in percentage is defined as the internal short circuit occurrence rate. The results are also shown in Table 1 below. (Battery capacity) The secondary batteries of Examples and Comparative Examples 1 and 2 were allowed to stand at room temperature for 48 hours, and at a current of 0.2 CA for 150 hours.
%, And a charge / discharge cycle of discharging to 1 V with a current of 0.2 CA was repeated twice. Subsequently, after charging 150% with a current of 1 C, the charge / discharge cycle of discharging to 1 V with a current of 1 C is repeated three times, the discharge capacity at the third cycle is measured, and the results are also shown in Table 1 below. Also,
FIG. 2 shows the discharge curve of the third cycle. (Operating Voltage) Based on the discharge curve of FIG. 2 described above, the operating voltage values of the secondary battery of the embodiment and the secondary batteries of Comparative Examples 1 and 2 at each time point were compared. I will write it together. (Internal pressure characteristics) After measuring the battery capacity, half of each type was attached to an internal pressure measuring device, and charged / discharged to 1% at a current of 1 C, and then discharged to 1 V at a current of 1 C, and repeated 15 cycles. Measure the battery internal pressure,
The results are shown in Table 1 below. (Swelling Ratio of Positive Electrode) The remaining secondary battery whose battery capacity was measured was further charged at a current of 1 C by 150%,
A charge / discharge cycle of discharging at 1 C to 1 V was repeated, and a total of 20 charge / discharge cycles were performed. Next, each secondary battery was disassembled and the positive electrode was taken out. Each positive electrode is washed so that the mixture does not fall off, drained, and its thickness is measured with a non-contact measuring instrument. The obtained thickness is divided by the thickness of the positive electrode before being incorporated into the secondary battery, and expressed as a percentage to obtain a swelling ratio. The results are also shown in Table 1 below. (Cycle number) For the secondary batteries of Examples and Comparative Examples 1 and 2, after performing 1C charging for cutting the charging current when the operating voltage drops by 10 mV, a charge / discharge cycle of discharging 1C was repeated at 20 ° C. The cycle life was measured, and the results are shown in Table 1 below. FIG. 3 shows the change in discharge capacity in this charge / discharge cycle.

[0033]

[Table 1]

As is apparent from Table 1, a mixture layer containing a fluorine-based resin was formed on both surfaces, and a mixture region existing between the two mixture layers was provided with a positive electrode having a structure not containing the resin. It can be seen that the secondary batteries of Examples can make all of the discharge capacity, the positive electrode swelling rate, the internal pressure characteristics, the operating voltage and the cycle life excellent, and can reduce the amount of the mixture dropped and the internal short circuit occurrence rate. .

On the other hand, the secondary battery of Comparative Example 1 provided with the positive electrode having a structure in which only the mixture containing no fluorine-based resin was supported on the conductive substrate was larger in the positive electrode than the secondary battery of Example. It can be seen that the swelling ratio is high, the amount of the mixture dropped off is large, and the internal short circuit occurrence ratio is extremely high. In addition, the secondary battery of Comparative Example 2 including the positive electrode having a structure in which only the mixture containing the fluorine-based resin was supported on the conductive substrate had a higher battery capacity than the secondary battery of Example.
It can be seen that the operating voltage and the cycle life are low and the internal pressure of the battery is high.

In the above-described embodiment, an example in which the present invention is applied to a cylindrical alkaline secondary battery is described. However, the present invention can be similarly applied to a rectangular alkaline secondary battery. Further, the electrode group housed in the container of the secondary battery is not limited to the spiral shape, but may be a form in which a plurality of positive electrodes, separators, and negative electrodes are stacked in this order. Further, in the above-described embodiment, the example in which the mixture region of the positive electrode has one layer has been described, but the mixture region may have a laminated structure of two or more layers.

[0037]

As described above in detail, according to the paste-type positive electrode and the alkaline secondary battery of the present invention, a paste-type positive electrode and an alkaline secondary battery having practical characteristics can be provided at a high yield. Has a remarkable effect.

[Brief description of the drawings]

FIG. 1 is a partially cutaway perspective view showing an example of an alkaline secondary battery according to the present invention.

FIG. 2 shows a secondary battery according to an embodiment of the present invention and Comparative Examples 1 and 2.
FIG. 4 is a characteristic diagram showing a relationship between a discharge time and a battery voltage in the secondary battery of FIG.

FIG. 3 shows a secondary battery according to an embodiment of the present invention and Comparative Examples 1 and 2.
FIG. 4 is a characteristic diagram showing a relationship between the number of cycles and the discharge capacity in the secondary battery of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Container, 2 ... Positive electrode, 4 ... Negative electrode, 5 ... Electrode group, 7 ... Sealing plate.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Tsutomu Sato 3-4-1-10 Minamishinagawa, Shinagawa-ku, Tokyo Inside Toshiba Battery Corporation (72) Inventor Tatsu Takahashi 3-4-1-10 Minamishinagawa, Shinagawa-ku, Tokyo Toshiba Battery Corporation

Claims (4)

[Claims] 1. A paste-type positive electrode comprising a conductive substrate and a mixture containing nickel hydroxide and a binder held on the conductive substrate, wherein the mixture is divided into at least three layers. The binder of the outer mixture layer includes at least one polymer selected from a fluororesin and a rubber-based latex, and the binder of the mixture region disposed between the two outer mixture layers. Is a paste-type positive electrode not containing the polymer. 2. The paste-type positive electrode according to claim 1, wherein the binder of each of the outer mixture layers contains polytetrafluoroethylene. 3. The paste-type positive electrode according to claim 1, wherein the thickness of each of the outer mixture layers ranges from 0.1% to 50% of the thickness of the mixture region. 4. An alkaline secondary battery comprising the paste-type positive electrode according to claim 1, a negative electrode, and an alkaline electrolyte.