JPH10188993A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve a utilization factor of an active material in a positive electrode so as to provide a secondary battery having high capacity by including a conductive agent, which is formed by including a predetermined quantity of expanded graphite or crushed material of the compression molding of the expanded graphite, in a positive electrode of a non-aqueous electrolyte secondary battery using lithium included composite oxide for positive electrode active material. SOLUTION: A non-aqueous electrolyte secondary battery is formed of a positive electrode, in which lithium-containing manganese composite oxide is used as an active material and in which conductive agent, and binder agent are included and conductive polymer is included at need, an electrolyte layer of high molecular solid, which includes the non-aqueous electrolyte, and a negative electrode, which can store and release lithium. At this stage, as a conductive agent to be included in the positive electrode, crushed material of expanded graphite at 5wt.% or less is used. As the expanded graphite crushed material, an expanded graphite crushed material having mean particle diameter of 1-30μm, specific surface area of 10m<2> /g or more and true density of 2.00g/cm<3> is desirable from the view point of dispersebility, film-forming and adhesiveness, and an expanded graphite crushed material having a crystal layer interval of d002 of 0.337nm or less and crystallite size in the c-axis direction Lc of 10nm or more is desirable from viewpoint of conductivity.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a non-aqueous electrolyte secondary battery.

[0002]

2. Description of the Related Art In recent years, there has been remarkable progress in downsizing, thinning, and lightening of electronic devices. Particularly in the OA field, the size and weight of electronic devices have been reduced from desktop type to laptop type and notebook type. In addition, new small electronic devices such as electronic notebooks and electronic still cameras have emerged,
Furthermore, in addition to miniaturization of conventional hard disks and floppy disks, development of a new memory medium, a memory card, is also underway. In the wave of the miniaturization, thinning, and weight reduction of such electronic devices, there is a demand for higher performance of secondary batteries that support these electric powers. Under such demands, development of lithium secondary batteries as high energy density batteries replacing lead storage batteries and nickel cadmium batteries has been rapidly advanced.

When a lithium metal is used as an electrode as a negative electrode active material for these, a high electromotive force is obtained, light weight and high density are easily obtained, but dendrite is generated by charging and discharging, and this decomposes an electrolytic solution. In addition, when the dendrite grows, it reaches the positive electrode and causes a short circuit in the battery. Thus, when a lithium alloy is used as the negative electrode, such a problem is alleviated, but a capacity sufficient as a secondary battery cannot be obtained. For this reason, it has been proposed to use a highly safe carbon material capable of inserting and extracting lithium as the negative electrode active material, and much research has been made to date.
For example, Japanese Patent Application Laid-Open No. 2-66656 discloses a negative electrode active material.
It has been proposed to use a conductive carbon material obtained by burning furfuryl resin at 1100 ° C. Also, Japanese Unexamined Patent Publication No.
No. 77515 discloses that a conductive carbon material obtained by heat-treating an aromatic polyimide at a temperature of 2000 ° C. or more in an inert atmosphere is used as a negative electrode active material. The use of graphitized spheroidal carbon as a negative electrode active material is disclosed. Further, Japanese Patent Application Laid-Open No. 61-77275 discloses a secondary battery in which an insulating or semiconductive carbon material having a polyacene structure obtained by heat-treating a phenolic polymer is used for an electrode.

On the other hand, TiS ₂ , M
There are transition metal oxides such as oS ₂ , Co ₂ S ₆ , V ₂ O ₅ , MnO ₂ , and CoO ₂ , or transition metal chalcogen compounds, and many examples using inorganic materials as active materials have been studied. Furthermore, recently, it is represented by a composite oxide having a layered structure represented by LiMO ₂ , such as lithium cobalt oxide, lithium nickel oxide, or LiM ₂ O _4, which has an operating voltage of 4 V for increasing energy. Lithium composite oxides having a spinel structure have been proposed (JP-B-63-59507, JP-B-8-21431). These lithium composite oxides are synthesized by firing at a high temperature using carbonates, hydroxides, nitrates and the like as starting materials. These active materials, in a solvent in which the binder is dissolved,
Mixed and dispersed with a conductive agent to impart conductivity,
An electrode is produced by coating and drying on a current collector. Here, as the conductive agent, natural graphite, artificial graphite, and the like have been generally used so far. However, if these contents are set to obtain sufficient conductivity, the content of the active material is reduced. It is disadvantageous for increasing the capacity, while reducing the content of the conductive agent,
Insufficient conductivity was obtained, the utilization rate of the positive electrode active material was low,
After all, the capacity has dropped.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a high capacity nonaqueous electrolyte secondary battery by increasing the utilization of an active material in a positive electrode.

[0006]

Means for Solving the Problems To achieve the above object, if the conductive agent for the positive electrode contains expanded graphite or a pulverized product of expanded graphite or a compression molded product of expanded graphite, the content is reduced. However, the maximum utilization rate of the positive electrode active material is maximized, and a high-capacity non-aqueous electrolyte secondary battery is found to be obtained.Based on this finding, the present invention has been achieved, and the above technical problem has been solved. Could be solved. Hereinafter, the non-aqueous electrolyte secondary battery of the present invention will be described.

[0007] 1. Conductive agent As described above, the conductive agent used in the present invention is configured to include expanded graphite or a pulverized product of expanded graphite or a compression molded product of expanded graphite. As the expanded graphite, those obtained by a known method can be used, and the kind thereof is not particularly limited. For example, natural graphite, quiche graphite, highly crystallized graphite such as pyrolytic graphite is mixed with concentrated sulfuric acid and nitric acid. Perform a chemical treatment of immersing in a strong oxidizing solution such as a mixed acid of concentrated sulfuric acid and potassium permanganate, a mixed acid of concentrated sulfuric acid and hydrogen peroxide, an oxidation treatment such as an electrolytic treatment, and generate a graphite-sulfuric acid intercalation compound. Washed with water, dried, and rapidly heated to expand the graphite crystal in the C-axis direction.

The above-mentioned expanded graphite or a compressed product of the expanded graphite can be obtained by pulverizing the expanded graphite or the compression-molded product of the expanded graphite. When the expanded graphite is pulverized, it may be pulverized as it is. Compression molding into an arbitrary shape such as a sheet, a block, a ring, or the like, has better crushing efficiency than crushing as it is. The pulverization can be performed by a known mechanical pulverization method. In view of the dispersibility of the coating material, film forming property, adhesiveness of the coating film, etc., these average particle diameters are 1 to 30 μm,
Specific surface area is 10 m ² / g or more, true density is 2.00 g / c
It is preferably at least m ³ . Further, from the viewpoint of conductivity, the distance d ₀₀₂ between crystal layers is 0.337 nm or less, and C
It is preferable that the size Lc of the crystallite in the axial direction is 10 nm or more. These can be used alone as a conductive agent,
It may be used by mixing with ordinary natural graphite and artificial graphite. The conductive agent of the present invention is generally used in an amount of 5% by weight or less based on the total weight of the positive electrode.

[0009] 2. Positive Electrode Active Material and Positive Electrode Hereinafter, the configuration of the battery used in the present invention will be described. The positive electrode active material used in the battery of the present invention is TiS ₂ ,
Transition metal oxides such as MoS ₂ , Co ₂ S ₆ , V ₂ O ₅ , MnO ₂ , and CoO ₂ , transition metal chalcogen compounds, and complexes thereof with Li (Li composite oxide; LiCoO ₂ ,
LiNiO ₂ , LiFeO ₂ , LiMn ₂ O ₄ , and C
o, LiCoXO ₂ , LiNiXO ₂ , LiFeXO ₂ , Li in which a part of Ni, Fe, Mn is replaced by another element X
Inorganic active materials such as Mn ₂ XO ₄ ). Among them, Li-containing composite oxides, especially Li-containing Mn composite oxides,
As described above, it has good compatibility with the conductive agent of the present invention, and shows extremely good conductivity in a small amount.For example, carbonate, hydroxide, nitrate or the like as a starting material, by firing at a high temperature. Synthesized.

[0010] These inorganic positive electrode active materials may contain other active materials such as conductive polymers. For example, LiMn
Inorganic active materials such as ₂ O ₄ and LiMn _(2-a) X _a O ₄ alone have poor conductivity and do not have self-molding properties, so that it is necessary to add a large amount of a conductive agent and a binder. On the other hand, conductive polymer materials such as polyacetylene, polypyrrole, and polyaniline have advantages such as light weight and workability, but have a drawback of low energy density per volume. Therefore, as a method of compensating for the disadvantages of the inorganic active material and the conductive polymer, composite electrodes of the inorganic active material and the conductive polymer have been proposed (JP-A-63-102162, JP-A-63-314763, JP-A-63-314763, 298067, JP-A-4-322257, JP-A-6-68866, JP-A-6-68866
-318452). When the conductive agent of the present invention is used for a composite electrode of an inorganic active material and a conductive polymer as described above, a very good coating film can be formed and is extremely effective.

[0011] Even in the case of such a composite with a conductive polymer, the conductive agent of the present invention can form a very good coating film and is extremely effective. When these active materials do not contain the conductive polymer, in a solvent in which the binder is dissolved, the conductive agent of the present invention is added, mixed and dispersed, coated on a current collector and dried to form an electrode. Is prepared. Examples of the binder include Teflon, polyethylene, nitrile rubber, polybutadiene, butyl rubber, polystyrene, styrene / butadiene rubber, nitrocellulose, cyanoethylcellulose, polyacrylonitrile, polyvinyl fluoride, polyvinylidene fluoride, polychloroprene, and polyvinylpyridine. These may be used alone, or may be used by enhancing the resistance to an electrolytic solution by mixing or further copolymerization. Accordingly, the present invention is not limited to the above-mentioned ones unless it dissolves or reacts in the electrolytic solution. The content of these binders is preferably 1 to 20% by weight.

3. Negative electrode active material and negative electrode As the negative electrode material used in the non-aqueous electrolyte secondary battery of the present invention, lithium metal, Pb, Bi, an alloy of Li and a low melting point metal such as Sn, a lithium alloy such as a Li-Al alloy, A carbon material or the like is used. Among these, a carbon material is most preferable. Examples of this are synthetic polymers such as phenol and polyimide and natural polymers at 400 to 800 ° C.
An insulating or semiconductive carbon body obtained by firing in a reducing atmosphere, coal, pitch, a synthetic polymer, or a conductive polymer obtained by firing in a reducing atmosphere at 800 to 1300 ° C., Coke, pitch, synthetic polymers, those obtained by firing natural polymers at a temperature of 2000 ° C. or more in a reducing atmosphere, and graphite-based carbon bodies such as natural graphite, but are not limited thereto. These may be used alone or as a mixture of two or more. The carbon body is formed into a sheet by a wet papermaking method using a carbon body and the same binder as in the case of the positive electrode, or by a coating method using a coating material in which an appropriate binder is mixed with a carbon material.

The positive / negative electrode can be manufactured by supporting the current collector on a current collector by a method such as application, adhesion, and pressure bonding as required.

4. Positive and negative electrode current collector As the positive electrode current collector used in the present invention, for example, stainless steel, gold, platinum, nickel, aluminum, molybdenum,
Examples include a metal sheet such as titanium, a metal foil, a metal net, a punching metal, an expanded metal, a net made of metal-plated fiber, metal-deposited wire, metal-containing synthetic fiber, and a nonwoven fabric. Among them, electrical conductivity, chemical stability,
It is particularly preferable to use aluminum or stainless steel in consideration of electrochemical stability, economy, workability, and the like. Aluminum is more preferred because of its light weight and electrochemical stability.

Further, the surface of the positive electrode current collector layer and / or the negative electrode current collector layer used in the present invention is preferably roughened. By performing the surface roughening, the contact area of the active material layer is increased and the adhesiveness is improved, which has the effect of lowering the impedance as a battery. Further, in the preparation of an electrode using a coating solution, the adhesion between the active material and the current collector can be greatly improved by performing a surface roughening treatment. Roughening treatments include polishing with emery paper, blasting, chemical or electrochemical etching,
Thereby, the current collector can be roughened. In particular, in the case of stainless steel, it is preferable to use blasted aluminum, and in the case of aluminum, it is preferable to use etched aluminum. Since aluminum is a soft metal, it is difficult to perform effective surface roughening by blasting,
This causes deformation of the aluminum itself. On the other hand, the etching treatment can effectively roughen the surface on the order of micrometer without reducing the deformation of aluminum and its strength itself, and is the most preferable method for roughening aluminum. is there.

5. Non-aqueous electrolyte The electrolyte salt constituting the non-aqueous electrolyte is LiCl.
O ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiBr,
LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (C
F ₃ SO ₂ ) _{3 and the} like are not particularly limited. The electrolyte concentration varies depending on the electrode and electrolyte used, but is preferably 0.1 to 10 mol / l. And as the solvent constituting the non-aqueous electrolyte, for example,
Tetrahydrofuran, 2-methyltetrahydrofuran,
1,4-dioxane, ethers such as dimethoxyethane, dimethylformamide, amides such as dimethylacetamide, acetonitrile, nitriles such as benzonitrile, sulfur compounds such as dimethylsulfoxysulfolane, dimethyl carbonate, diethyl carbonate,
Methyl ethyl carbonate, chain carbonates such as methyl isopropyl carbonate, ethylene carbonate, propylene carbonate, cyclic carbonates such as butylene carbonate and the like, but are not limited thereto, and these alone , 2
More than one kind may be mixed and used.

In the present invention, there is also a great effect when a solid polymer electrolyte is used. As a polymer matrix such as polyethylene oxide, polypropylene oxide, polyvinylidene fluoride, and polyacrylonitrile, a composite in which an electrolyte salt is dissolved, Alternatively, a crosslinked gel containing a solvent, a low molecular weight polyethylene oxide, a polymer solid electrolyte in which an ion dissociating group such as a crown ether is grafted to the polymer main chain,
A gel-like polymer solid electrolyte in which the above-mentioned electrolytic solution is contained in a high-molecular-weight polymer is exemplified.

In the battery of the present invention, a separator can be used. As the separator, it is preferable to use a separator having a low resistance to the ion transfer of the electrolyte solution and having an excellent solution retention. Examples of such a separator include a glass fiber, a filter, a nonwoven fabric filter made of a polymer fiber such as polyester, Teflon, polyflon, and polypropylene, and a nonwoven fabric filter in which the glass fiber is mixed with the polymer fiber.

[0019]

【Example】

Example 1 Li ₂ CO ₃ and CoCO ₃ were mixed at a molar ratio of 1.5 / 1 and calcined in air at 900 ° C. for 5 hours to obtain L
iCoO ₂ was synthesized. In addition, 50 g of natural graphite
The mixture was stirred in 500 g of concentrated sulfuric acid at 500% by weight, and 25% by weight of hydrogen peroxide was added to obtain a graphite intercalation compound. Then, this was washed with water, dried and heated to 800 ° C. to produce expanded graphite expanded 200 times. This expanded graphite was compression-molded into a sheet shape using a roll, and pulverized to an average particle size of 7.5 μm with a jet mill to prepare a pulverized product of a compression-molded product of expanded graphite. Its d ₀₀₂ is 0.336 nm, Lc is 42 nm, specific surface area is 20 m ² / g, and true density is 2.25 g / g.
cm ³ .

4 parts by weight of polyvinylidene fluoride (PVDF) are dissolved in 67 parts by weight of N-methylpyrrolidone, 89 parts by weight of the above LiCoO ₂ and 7 parts by weight of a pulverized product of the above-mentioned expanded graphite are added to a roll mill. The mixture was mixed and dispersed under an inert atmosphere by a method to prepare a coating for a positive electrode. Using a doctor blade in the atmosphere,
This was coated on a 1 foil, dried at 130 ° C. for 20 minutes, and roll-pressed to produce a 50 μm-thick positive electrode. The positive electrode (Φ20 mm) prepared as described above, a Li metal as a counter electrode, a polypropylene porous membrane as a separator, and L
Using 2.0 mol / l of a solution of iPF ₆ in ethylene carbonate / dimethyl carbonate (5/5, volume ratio), a coin cell was prepared, and a charge / discharge test was performed. The charge / discharge test was performed using a Hokuto Denko HJ-201B charge / discharge measurement device at a current of 1.5 mA until the battery voltage reached 4.2 V. The battery was discharged to a voltage of 3.0 V, the discharge capacity was measured, and the discharge capacity density (mAh / cm ³ ) was obtained.

Example 2 LiOH, Ni (OH) ₂ and Co (OH) ₂ were added in a ratio of 1.0 /
Mix at a molar ratio of 0.9 / 0.1 and in air at 800 ° C for 2 hours.
This was the same as Example 1 except that LiNi _0.9 Co _0.1 O ₂ was synthesized by baking for 4 hours, and this was used as the positive electrode active material.

EXAMPLE 3 LiMn ₂ O ₄ was synthesized by mixing Li ₂ CO ₃ and Mn ₂ O ₃ at a モ ル molar ratio and firing at 850 ° C. for 10 hours in air. Further, the same expanded graphite as in Example 1 was compression-molded into a sheet by a roll, and the average particle size was 1.1 μm by a jet mill.
m to obtain a pulverized product of a compression molded product of expanded graphite. Its d ₀₀₂ is 0.336 nm and Lc is 55 n
m, specific surface area was 32 m ² / g, and true density was 2.26 g / cm ³ . 3 parts by weight of polyvinylidene fluoride (PVDF) are dissolved in 67 parts by weight of N-methylpyrrolidone,
94 parts by weight of CoO ₂ and 3 parts by weight of a pulverized product of the above-mentioned expanded graphite compression-molded product were added and mixed and dispersed under an inert atmosphere by a roll mill method to prepare a coating for a positive electrode. This was applied on a 20 μm Al foil in the atmosphere using a doctor blade, dried at 130 ° C. for 20 minutes, and roll-pressed to produce a 50 μm-thick positive electrode.

Hereinafter, a battery was prepared and evaluated in the same manner as in Example 1. Example 4 As a conductive agent, in Example 3, the average particle size of the pulverized product of the compression molded product of expanded graphite was 4.6 μm, and d ₀₀₂ was 0.33.
The same thing was used except that 6 nm, Lc was 48 nm, the specific surface area was 27 m ² / g, and the true density was 2.25 g / cm ³ .

Example 5 In Example 3, the average particle diameter of the pulverized product of the expanded graphite was 16.0 μm, and d ₀₀₂ was 0.33.
The same thing was used except that 6 nm, Lc was 41 nm, the specific surface area was 24 m ² / g, and the true density was 2.23 g / cm ³ .

Example 6 The average particle size of the pulverized product of the compression molded product of expanded graphite in Example 3 was 27.0 μm and d ₀₀₂ was 0.33 in Example 3 as a conductive agent.
The same thing was used except that 7 nm, Lc was 15 nm, the specific surface area was 16 m ² / g, and the true density was 2.09 g / cm ³ .

Example 7 Li ₂ CO ₃ , Mn ₂ O ₃ , Fe ₂ O
₃ were mixed in a molar ratio of 1 / 1.8 / 0.2 and calcined in air at 750 ° C for 8 hours to synthesize LiMn _1.8 Fe _0.2 O ₄ . Hereinafter, a battery was fabricated in the same manner as in Example 5, except that this was used as the positive electrode active material.

Example 8 7 parts by weight of polyaniline were dissolved in 67 parts by weight of N-methylpyrrolidone, and 90 parts by weight of V ₂ O ₅ and 3 parts by weight of a pulverized product of the above-mentioned expanded graphite compression molded product were added. At
The mixture was mixed and dispersed in an inert atmosphere to prepare a coating for a positive electrode. Using a doctor blade in the atmosphere,
Coated on a 0 μm Al foil, dried at 120 ° C. for 20 minutes,
Roll pressing was performed to produce a positive electrode having a thickness of 50 μm. The positive electrode (Φ20 mm) prepared as described above and Li
As a metal and a porous polypropylene membrane as a separator, an electrolyte solution of LiPF ₆ in ethylene carbonate / dimethyl carbonate (5/5, volume ratio) 2.0 mol / l
Was used to produce a coin cell, and a charge / discharge test was performed.
The charge / discharge test was performed using a Hokuto Denko HJ-201B charge / discharge measurement device at a current of 0.5 mA until the battery voltage reached 3.7 V. After a pause of 10 minutes, a current of 0.5 mA was used.
The battery was discharged to a voltage of 2.5 V, the discharge capacity was measured, and the discharge capacity density (mAh / cm ³ ) was determined.

Example 9 An electrolyte was prepared by mixing 20 parts by weight of LiPF ₆ and 70 parts by weight of ethylene carbonate / dimethyl carbonate (5/5 by volume). 12.8 parts by weight of polyoxyethylene acrylate, 0.2 parts by weight of trimethylolpropane triacrylate, and 0.02 parts by weight of benzoin isopropyl ether were added thereto, mixed and dissolved to prepare a photopolymerizable solution. In addition, polyvinylidene fluoride (PVD)
F) 3 parts by weight were dissolved in 65 parts by weight of N-methylpyrrolidone, 32 parts by weight of a coke calcined at 2500 ° C. were added, and mixed and dispersed under an inert atmosphere by a roll mill method to prepare a negative electrode coating material. Prepared. This was applied on a 20 μm copper foil in the atmosphere using a doctor blade,
After drying at 0 ° C. for 20 minutes, a negative electrode having a thickness of 85 μm was prepared. Then, the positive electrode and the negative electrode prepared in Example 5 were cut into 50 × 80 mm, the photopolymerizable solution was infiltrated, and irradiated with a high-pressure mercury lamp to solidify the electrolytic solution. By stacking these, applying pressure evenly to the power generation element part,
After heat sealing three sides, the remaining one side was sealed under reduced pressure to produce a battery. The charge and discharge test was made by Hokuto Denko HJ-201B
Using a charge / discharge measuring device, charge / discharge was performed at a battery voltage of 3.0 to 4.2 V at a current of 5 mA, and the discharge capacity was shown in Table 2.

Comparative Example 1 Example 1 was the same as Example 1 except that the positive electrode conductive agent was artificial graphite (KS-6 manufactured by Lonza).

Comparative Example 2 Example 2 was the same as Example 2 except that the positive electrode conductive agent was artificial graphite (Desarco # 9039 manufactured by Superior).

Comparative Example 3 The procedure was the same as in Example 6, except that the positive electrode conductive agent was artificial graphite (KS-6 manufactured by Lonza).

Comparative Example 4 The procedure was the same as in Example 7, except that the positive electrode conductive agent was artificial graphite (Desarco # 9039 manufactured by Superior).

Comparative Example 5 Example 8 was the same as Example 8 except that the positive electrode conductive agent was artificial graphite (KS-6 manufactured by Lonza).

Comparative Example 6 Example 9 was the same as Example 9 except that the positive electrode conductive agent was artificial graphite (KS-6 manufactured by Lonza).

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

【effect】

[Claim 1] A high capacity non-aqueous electrolyte secondary battery is obtained by improving the conductivity of the positive electrode and the dispersibility of the coating material. [Claim 2] In particular, the improvement in the dispersibility of the paint leads to the improvement of the film forming property and the increase in the capacity. [Claim 3] In particular, improvement in the conductivity of the conductive agent has led to an increase in capacity. [Claim 4] The active material content can be increased,
This led to higher capacity. [Claim 5] In particular, the improvement in the dispersibility of the paint leads to the improvement of the film forming property and the increase of the capacity. [Claim 6] It is possible to increase the active material content,
This led to higher capacity. [Claims 7 and 8] The characteristics of the lithium-containing composite oxide can be effectively brought out, leading to an increase in capacity.
[Claim 9] The dispersibility with the conductive polymer is good, and it is possible to effectively bring out the characteristics of the active material, leading to an increase in capacity. [Claim 10] The matching with the polymer solid electrolyte is also excellent, and no decrease in capacity due to the use of the polymer solid electrolyte was observed.

Claims (10)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising at least a positive electrode, an electrolyte layer containing a non-aqueous electrolyte, and a negative electrode capable of inserting and extracting lithium, wherein the positive electrode contains a conductive agent,
A secondary battery, wherein the conductive agent comprises expanded graphite. 2. The expanded graphite is a pulverized product of expanded graphite or a compression molded product of expanded graphite, and has an average particle size of 1 to 30 μm.
The secondary battery according to claim 1, wherein m is m. 3. The inter-crystal distance d _{002 of} expanded graphite or a pulverized product of a compression-molded product of expanded graphite is 0.337 nm or less,
3. The secondary battery according to claim 1, wherein the size Lc of the crystallite in the C-axis direction is 10 nm or more. 4. A specific surface area of the expanded graphite or a pulverized product of a compression-molded product of the expanded graphite is 10 m ² / g or more.
4. The secondary battery according to 2 or 3. 5. The secondary battery according to claim 1, wherein the true density of the expanded graphite or the pulverized product of the expanded molded product of the expanded graphite is 2.00 g / cm ³ or more. 6. The secondary material according to claim 1, wherein the content of the expanded graphite or the pulverized product of the compressed molded product of the expanded graphite in the positive electrode is 5% by weight or less of the total weight of the positive electrode. battery. 7. The secondary battery according to claim 1, wherein the positive electrode active material is a lithium-containing composite oxide. 8. The secondary battery according to claim 7, wherein the lithium-containing composite oxide is a lithium-containing manganese composite oxide. 9. The secondary battery according to claim 1, wherein the positive electrode contains a conductive polymer. 10. The secondary battery according to claim 1, wherein the electrolyte layer is a solid polymer electrolyte layer.