KR20140099715A - Binder for secondary battery, cathode and secondary battery comprising the same - Google Patents

Abstract

The present invention relates to a binder for a secondary battery, includ.ing a first polymer and a second polymer having a higher weight-average molecular weight than the first polymer; to a cathode; and to a secondary battery including the binder. By using a mixed binder, the homogeneity of electrodes can be achieved while improving the dispersion of a conductive agent to enhance conductivity of the electrodes and to enhance the rate and cycle properties of the battery.

Description

TECHNICAL FIELD [0001] The present invention relates to a binder for a secondary battery, a positive electrode and a secondary battery including the binder, a cathode,

The present invention relates to a binder for a secondary battery comprising a heterogeneous binder having a different molecular weight, and a positive electrode and a secondary battery including the binder.

Due to the rapid increase in the use of fossil fuels, the demand for the use of alternative energy or clean energy is increasing. As a part of this, the most active field of research is electric power generation and storage.

At present, a typical example of an electrochemical device utilizing such electrochemical energy is a secondary battery, and the use area thereof is gradually increasing.

The secondary battery typically includes an anode, a separator, and a cathode. The anode and the cathode are formed by casting the active material composition on the respective current collectors to form an active material layer. The separator is positioned between the obtained anode and cathode, Wind it up using heat or pressure to assemble the battery.

The active material composition includes each electrode active material and a conductive agent, and a binder is used to increase the contact area between the electrode active material and the conductive agent to improve the output characteristics of the battery.

Conventionally, various methods of using a conductive polymer such as polyacetylene or polyaniline as a binder have been proposed in order to improve the conductivity and reduce the internal resistance of the battery.

However, in spite of these various technical proposals, the conductive polymer generally shows poor solubility in an organic solvent. Therefore, there is a problem that it is difficult to apply the conductive polymer directly to the process of manufacturing an electrode by mixing it with an organic solvent. In addition, as the degree of polymerization of the conductive polymer increases, the electric conductivity improves. When the degree of polymerization is low, desired conductivity improvement can not be exhibited. On the other hand, when the molecular weight is increased, the dispersibility with respect to the solvent is lowered. Due to many problems, a conductive polymer as a binder exhibiting a desired level of physical properties has not yet been developed.

On the other hand, conventionally, a conductive agent such as graphite or carbon black is added to an electrode mixture of a lithium secondary battery in order to improve the conductivity of the electrode. However, when the conductive agent is included in the negative electrode material mixture, such a conductive agent can not penetrate into the inside of the binder which is an insulator, so that there is a limit to improve the electric conductivity. Furthermore, since the carbon-based conductive agent has a low affinity for the polar solvent, the dispersibility is deteriorated and the conductive agent is unevenly distributed, causing a voltage drop, and blocking smooth migration of lithium ions, resulting in deterioration of the rate characteristic of the anode . In addition, the mixing process of the conductive agent becomes longer, so that there is a large restriction in the process, and the lithium ions are prevented from migrating and the deposition of lithium metal is promoted at the cathode while the battery repeats the charge / discharge cycle.

Disclosure of Invention Technical Problem [8] The present invention provides a binder capable of improving the dispersibility of a conductive agent, the uniformity of an electrode, and the conductivity thereof, thereby improving the electrode characteristics and cycle characteristics, and a positive electrode and a secondary battery containing the same.

In order to solve the above problems, the present invention provides a binder comprising a first polymer and a second polymer having a larger weight average molecular weight than the first polymer.

The present invention also provides a positive electrode comprising a positive electrode active material, a conductive agent and the binder.

The present invention provides a secondary battery including the positive electrode, the negative electrode, and the separator interposed between the positive electrode and the negative electrode.

The present invention not only achieves uniformity of electrodes by using a heterogeneous binder but also improves the conductivity of the electrode and improves the rate and cycle characteristics of the battery by making the network and dispersibility of the conductive agent excellent.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments of the invention and, together with the description of the invention, It should not be construed as limited.
1 is a SEM photograph of an electrode surface according to Experimental Example 1 of the present invention.
2 is a cross-sectional SEM photograph of an electrode according to Experimental Example 2 of the present invention.
3 is a graph showing the rate characteristics of a secondary battery according to Experimental Example 4 of the present invention.

Hereinafter, the present invention will be described in detail in order to facilitate understanding of the present invention.

The terms and words used in the present specification and claims should not be construed as limited to ordinary or dictionary terms and the inventor may appropriately define the concept of the term in order to best describe its invention It should be construed as meaning and concept consistent with the technical idea of the present invention.

A binder according to an embodiment of the present invention includes a mixture of a first polymer and a second polymer having a weight average molecular weight larger than that of the first polymer. Such a binder can be used for producing positive and negative electrodes, but can preferably be used for positive electrodes.

Generally, the binder not only provides adhesion between the electrode collector and the active material or between the active material and the active material in coating the active material on the surface of the electrode collector, but also inhibits the volume expansion due to charging and discharging of the battery, .

Therefore, preferable characteristics required as a binder are not only excellent adhesion but also chemical stability, electrical stability, nonflammability, good electrolyte impregnability, and high dispersibility and crystallinity. In addition, when the dispersing ability and crystallinity are high, the rate characteristic and the cycle characteristic of the battery can be improved.

In one embodiment of the present invention, the first polymer may have a weight average molecular weight (Mw) of 200,000 to 450,000, preferably 250,000 to 400,000, and a number average molecular weight (Mn) of 80,000 to 150,000. The second polymer may have a weight average molecular weight (Mw) of 500,000 to 1,000,000, preferably 550,000 to 900,000, and a number average molecular weight (Mn) of 200,000 to 400,000.

The present invention can further improve the dispersibility of the conductive agent by using the first polymer having a low molecular weight in the above range and by using the second polymer having a weight average molecular weight larger than that of the first polymer, Not only affects the formation, but also the same or similar binding force can be obtained even if a small amount of binder is used. By using a heterogeneous binder having a different molecular weight in the above range, the uniformity of the electrode can be ensured and the network and dispersibility of the conductive agent can be improved.

Here, the mixing ratio of the first polymer and the second polymer may be 40 to 60 parts by weight: 40 to 60 parts by weight. If the content of the first polymer exceeds the above range, the specific resistance of the electrode may increase due to the excessive amount of the first polymer. If the content of the first polymer is less than the above range, the resistance of the electrode There may arise a problem of increasing the number.

If the content of the second polymer exceeds the above range, the conductivity of the electrode can be improved. However, it may be difficult to manufacture and coat the slurry by increasing the viscosity of the positive electrode slurry. If the content of the second polymer is less than the above range, the electrode resistance may increase due to the electrical insulation of the binder.

The first polymer according to an embodiment of the present invention can be used without limitation as long as it has a weight average molecular weight or a number average molecular weight within the above range. For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyolefin-based and polyacrylate, or a mixture of two or more thereof. Commercially available ones can also be used. Commercially available polyvinylidene fluoride can be used, for example, KF1100 (Kureha Chemicals, Ltd.).

The second polymer according to an embodiment of the present invention may be any polymer having a weight average molecular weight or a number average molecular weight within the above range without limitation. For example, vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co (HFP), polyvinylidene fluoride, polytetrafluoroethylene (PTEF), styrene butadiene rubber (SBR), and polyacrylate, or a mixture of two or more thereof, Can be used. For example, commercially available polyvinylidene fluoride is Solef 6020 (Solvay).

The present invention also provides a secondary battery comprising a positive electrode including the binder, a negative electrode, and a separator interposed between the positive electrode and the negative electrode.

The anode for a secondary battery according to an embodiment of the present invention may include a cathode active material, a conductive agent, and a binder. The cathode for a secondary battery may include a negative electrode active material, a conductive agent, and a binder, ≪ / RTI >

The positive electrode is prepared, for example, by applying a mixture of a positive electrode active material, a conductive agent, a binder and the like on the positive electrode current collector, and drying the mixture. The negative electrode is formed by applying a mixture of an anode active material, Followed by drying.

The binder may be used in an amount of 0.1 to 10 parts by weight, preferably 1 to 4 parts by weight based on 100 parts by weight of the positive electrode active material according to an embodiment of the present invention.

If the binder is used excessively, the electrical resistance of the battery electrode plate is increased, and the content of the electrode active material is relatively lowered to cause a decrease in the output of the battery. When the binder is contained in an excessively small amount, contact between the electrode plate active material and the conductive agent is insufficient And the output of the battery can be lowered.

Meanwhile, the cathode active material may include a manganese-based spinel active material, a lithium metal oxide, or a mixture thereof. Further, the lithium metal oxide may be selected from the group consisting of lithium-cobalt oxide, lithium-manganese oxide, lithium-nickel-manganese oxide, lithium-manganese-cobalt oxide and lithium-nickel-manganese-cobalt oxide And more specifically LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li (Ni _a Co _b Mn _c ) O ₂ (where 0 <a <1, 0 <b < c <1, a + b + c = 1), in _{LiNi 1-Y Co Y O 2} , LiCo 1-Y Mn Y O 2, LiNi 1-Y Mn Y O 2 ( here, 0≤Y <1), _{Li (Ni a Co b Mn c} ) O 4 (0 <a <2, 0 <b <2, 0 <c <2, a + b + c = 2), LiMn 2-z Ni z O 4, LiMn 2 _-z Co _z O ₄ (where 0 <Z <2).

The conductive agent is a component for further improving the conductivity of the electrode active material and may be added in an amount of 0.01 to 30% by weight based on 100 parts by weight of the electrode active material. Such a conductive agent is not particularly limited as long as it has electrical conductivity without causing a chemical change in the battery, and examples thereof include graphite such as natural graphite and artificial graphite; Carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Carbon fibers such as carbon nanotubes and fullerene; conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskers such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

The current collector in the electrode is a portion where electrons move in the electrochemical reaction of the active material, and the positive electrode collector and the negative electrode collector exist depending on the type of the electrode.

The cathode current collector is generally made to have a thickness of 3 to 500 mu m. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical change in the battery, and may be formed of a material such as stainless steel, aluminum, nickel, titanium, sintered carbon, or a surface of aluminum or stainless steel Treated with carbon, nickel, titanium, silver, or the like may be used.

The negative electrode collector is generally made to have a thickness of 3 to 500 mu m. Such an anode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery, and may be formed of a material such as copper, stainless steel, aluminum, nickel, titanium, fired carbon, surface of copper or stainless steel Nickel, titanium or silver, an aluminum-cadmium alloy, or the like may be used.

These current collectors may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam or a nonwoven fabric, or the like, by forming fine irregularities on the surface of the current collectors to enhance the binding force of the electrode active material.

On the other hand, as the negative electrode active material, carbon-based negative electrode active materials such as crystalline carbon, amorphous carbon or carbon composite may be used alone or in combination of two or more.

The binder for a negative electrode used in an embodiment of the present invention may be prepared by using a heterogeneous binder having a molecular weight different from that of the positive electrode or by using a binder such as vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP) One or more selected from the group consisting of polyvinylidenefluoride, polyacrylonitrile and polymethylmethacrylate, styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC) and polyacrylate A mixture of two or more kinds of binder polymers may be used.

According to an embodiment of the present invention, the mixture (electrode mixture) of the electrode active material, the conductive agent and the binder may further include at least one other additive selected from the group consisting of a viscosity adjusting agent and a filler.

The viscosity adjusting agent may be added up to 30% by weight based on the total weight of the electrode mixture, so as to control the viscosity of the electrode mixture so that the mixing process of the electrode mixture and the coating process on the collector may be easy. Examples of such viscosity modifiers include carboxymethylcellulose or polyvinylidene fluoride, but are not limited thereto.

The filler is an auxiliary component for suppressing the expansion of the electrode. The filler is not particularly limited as long as it is a fibrous material without causing a chemical change in the battery, and examples thereof include olefin polymers such as polyethylene and polypropylene; Fibrous materials such as glass fibers or carbon fibers are used.

The separator may be a porous polymer film such as a porous polymer film made of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer Or may be a laminate of two or more kinds. In addition, nonwoven fabrics made of conventional porous nonwoven fabrics, for example, glass fibers having a high melting point, polyethylene terephthalate fibers, or the like can be used, but the present invention is not limited thereto.

The secondary battery generally comprises a non-aqueous electrolyte solution containing a lithium salt in addition to the electrode and the separator.

The lithium-containing non-aqueous electrolyte is composed of a non-aqueous electrolyte and a lithium salt.

In the electrolyte solution used in the embodiment of the present invention, the lithium salt that can be included as the electrolyte may be any of those conventionally used for the electrolyte for the secondary battery. For example, the anion of the lithium salt may include F ^- , Cl ^{- , Br -, I -, NO} 3 -, N (CN) 2 -, BF 4 -, ClO 4 -, PF 6 -, (CF 3) 2 PF 4 -, (CF 3) 3 PF 3 -, (CF ^{_{3) 4 PF 2 -, (}} CF 3) 5 PF -, (CF 3) 6 P -, CF 3 SO 3 -, CF 3 CF 2 SO 3 -, (CF 3 SO 2) 2 N -, (FSO 2 _{^{) 2 N -, CF 3 CF}} 2 (CF 3) 2 CO -, (CF 3 SO 2) 2 CH -, (SF 5) 3 C -, (CF 3 SO 2) 3 C -, CF 3 (CF 2 ) ₇ SO ₃ ^- , CF ₃ CO ₂ ^- , CH ₃ CO ₂ ^- , SCN ^- and (CF ₃ CF ₂ SO ₂ ) ₂ N ^- .

Examples of the organic solvent included in the electrolytic solution include propylene carbonate (PC), ethylene carbonate (EC), ethylene carbonate (EC), and the like. ), Diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane , Vinylene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite, and tetrahydrofuran, or a mixture of two or more thereof. In particular, ethylene carbonate and propylene carbonate, which are cyclic carbonates in the carbonate-based organic solvent, can be preferably used because they have high permittivity as a high-viscosity organic solvent and dissociate the lithium salt in the electrolyte well. In this cyclic carbonate, dimethyl carbonate and diethyl When a low viscosity, low dielectric constant linear carbonate such as carbonate is mixed in an appropriate ratio, an electrolyte having a high electric conductivity can be prepared, and thus it can be more preferably used.

Alternatively, the electrolytic solution stored in accordance with the present invention may further include an additive such as an overcharge inhibitor or the like contained in an ordinary electrolytic solution.

The secondary battery according to the present invention can be applied to electric vehicles, hybrid electric vehicles (HEV), fuel cell vehicles (Fuel), and the like which require long cycle characteristics and high rate characteristics, Cell Electric Vehicle (FCEV), and a transportation device such as a battery scooter.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail with reference to examples. However, the embodiments according to the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments of the present invention are provided to enable those skilled in the art to more fully understand the present invention.

Example

EXAMPLES The present invention will be further illustrated by the following examples and experimental examples, but the present invention is not limited by these examples and experimental examples.

Example  One

&Lt; Preparation of positive electrode &

Solef 6020 (Solvay) having a weight average molecular weight (Mw) of 550,000 and a number average molecular weight (Mn) of 130,000 and KF1100 (Kureha Chemicals, Ltd.) having a weight average molecular weight (Mw) of 260,000 and a number average molecular weight (Mn) of 80,000. ) In a mixing ratio of 5: 5 was dissolved in 50 parts by weight of N-methylpyrrolidone (NMP). Then, 96 parts by weight of LiCoO ₂ as a positive electrode active material and 2 parts by weight of acetylene black as a conductive agent were added, Slurry. The obtained positive electrode slurry was coated on an aluminum current collector, followed by drying and rolling to prepare a positive electrode.

&Lt; Preparation of Secondary Battery >

SBR as a binder and CMC as a thickener were mixed at 96 wt%, 3 wt%, and 1 wt%, respectively, and dispersed in water to prepare an anode slurry. The anode slurry was mixed with a copper current collector Coated, dried and rolled to prepare a negative electrode.

Ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 30:70, and LiPF ₆ was added to the solvent to prepare a 1 M LiPF ₆ nonaqueous electrolyte solution.

After the porous polyethylene separator having the inorganic particles introduced between the anode and the cathode thus prepared was interposed, the electrolyte solution was injected to prepare a lithium secondary battery.

Comparative Example  One

A positive electrode was prepared in the same manner as in Example 1 except that KF 7208 (Kureha Chemicals, Ltd.) having a weight average molecular weight (Mw) of 1,100,000 and a number average molecular weight (Mn) of 490,000 was used alone as a binder Thereby preparing a lithium secondary battery.

Comparative Example  2

A positive electrode was produced in the same manner as in Example 1 except that KF 1100 (Kureha Chemicals, Ltd.) having a weight average molecular weight (Mw) of 260,000 and a number average molecular weight (Mn) of 80,000 was used alone as a binder Thereby preparing a lithium secondary battery.

Comparative Example  3

The procedure of Example 1 was repeated except that Solef 6020 (Solvay) having a weight average molecular weight (Mw) of 550,000 and a number average molecular weight (Mn) of 120,000 was used alone as a binder in the production of the positive electrode, .

Experimental Example  One

&Lt; Electrode surface shape (SEM) >

The surface of the electrode for a lithium secondary battery manufactured in Example 1 and Comparative Examples 1 to 3 was photographed by a scanning electron microscope (SEM) and is shown in Fig.

In FIG. 1, the top photograph is a SEM image at a ratio of 200 times generated as a signal source by the secondary battery, which is generated by transmitting an electron beam of 5 KeV to the electrode surface, and the bottom image is an image at a ratio of 1,000 times. The black portions of the top and bottom photographs are the conductive agents.

As can be seen from Fig. 1, in Comparative Examples 1 to 3 in which the high molecular weight, low molecular weight and medium molecular weight binders were used singly, it was confirmed that the conductive agent was not aggregated or evenly dispersed on the electrode surface. On the other hand, it can be seen that the electrode surface of Example 1 using a heterogeneous binder of low molecular weight and medium molecular weight is uniformly distributed throughout without aggregation. That is, by using a heterogeneous binder having a different molecular weight, it can be seen that the dispersibility on the electrode surface is excellent.

Experimental Example  2

&Lt; Electrode cross-sectional shape (SEM) >

SEM photographs of the electrodes for lithium secondary batteries manufactured in Example 1 and Comparative Examples 1 to 3 were shown in FIG.

As can be seen from FIG. 2, Comparative Example 1 using a binder having a high molecular weight had poor dispersibility and Comparative Example 2 using a binder having a low molecular weight showed good electrode surface dispersion, As shown in FIG. Also, in Comparative Example 3 using a binder having an intermediate molecular weight, it is seen that the localization of the conductive agent to the electrode surface is somewhat alleviated, but the dispersibility and uniformity are lowered than in Example 1.

In the case of Example 1, it can be confirmed that the conductive agent is evenly distributed over the entire surface of the electrode. This shows that the network formation of the conductive agent is improved and the uniformity of the electrode is excellent.

Experimental Example  3

&Lt; Electrode resistivity measurement >

The electrode resistivities of the lithium secondary batteries prepared in Example 1 and Comparative Examples 1 to 3 were measured using a four point probe and a sheet resistance. Specifically, electrical resistivity measurements were made with four probes of equal spacing and probes were used with probes arranged in a line at 1 mm intervals. The resistance was measured using current and voltage with four probes. The results are shown in Table 1 below.

High molecular weight
bookbinder
(Mw: 1,100,000) Low molecular weight
bookbinder
(Mw: 260,000) Medium molecular weight
bookbinder
(Mw: 550,000) Heterogeneous binder
(Mw: 550,000 and
260,000) Comparative Example 1 Comparative Example 2 Comparative Example 3 Example 1 Electrode resistivity (? Cm) 1.945x10 ^-3 0.691x10 ^-9 0.586x10 ^-3 0.329x10 ^-3

As can be seen from Table 1, the electrical resistivity of Example 1 using the first polymer of the present invention and a heterogeneous binder of a second polymer having a weight average molecular weight larger than that of the first polymer was high, Is significantly lower than that of Comparative Examples 1 to 3 in which the binder having the respective molecular weights alone is used.

It can be seen that the rate characteristic and the cycle characteristic can be improved by this excellent conductivity.

Experimental Example  4

<Measurement of rate characteristics>

The lithium secondary batteries prepared in Example 1 and Comparative Examples 1 to 3 were charged at a constant current of 0.5 C at 23 캜 until the voltage reached 4.3 V and then charged at a constant voltage of 4.3 V to complete the charging when the charging current reached 66Ma . Thereafter, the battery was allowed to stand for 10 minutes, discharged at a constant current of 0.2 C until it reached 3.0 V, and discharge capacity (%) of the battery was measured by discharging at 0.5 C, 1.0 C, 1.5 C and 2.0 C under the same charging conditions. The results are shown in Fig.

In Fig. 3, C represents the charging / discharging current speed, C-Rate, of the battery expressed by A (Ampere), which is usually expressed in proportion to the battery capacity. That is, 1C of the lithium secondary batteries manufactured in Example 1 and Comparative Examples 1 to 3 means a current of 1320 mA.

Referring to FIG. 3, the capacities of Example 1 and Comparative Examples 1 and 3 were similar to 0.5C and 1.0C, but the capacities of Comparative Examples 1 to 3, particularly Comparative Example 2, decreased drastically from 1.0C. On the other hand, in Example 1 using a different kind of binder, there was little change in capacity as compared with Comparative Examples 1 to 3.

Claims (11)

1. A binder comprising a first polymer and a second polymer having a weight average molecular weight greater than that of the first polymer.
The method according to claim 1,
Wherein the first polymer has a weight average molecular weight (Mw) of 200,000 to 450,000.
The method according to claim 1,
Wherein the second polymer has a weight average molecular weight (Mw) of 500,000 to 1,000,000.
The method according to claim 1,
Wherein the first polymer is at least one selected from the group consisting of polyvinylidene fluoride, polytetrafluoroethylene (PTEF), styrene butadiene rubber (SBR), polyolefin and polyacrylate, or a mixture of two or more thereof .
The method according to claim 1,
The second polymer may be selected from the group consisting of vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polytetrafluoroethylene (PTEF), styrene butadiene rubber (SBR) Or a mixture of two or more thereof.
The method according to claim 1,
Wherein the mixing ratio of the first polymer to the second polymer is 40-60 parts by weight: 40-60 parts by weight.
A positive electrode comprising a positive electrode active material, a conductive agent, and a binder according to claim 1.
8. The method of claim 7,
Wherein the content of the binder is 0.1 to 10 parts by weight based on 100 parts by weight of the positive electrode active material.
8. The method of claim 7,
Wherein the cathode active material is a manganese-based spinel active material, a lithium metal oxide, or a mixture thereof.
10. The method of claim 9,
The lithium metal oxide is selected from the group consisting of lithium-cobalt oxide, lithium-manganese oxide, lithium-nickel-manganese oxide, lithium-manganese-cobalt oxide and lithium-nickel-manganese-cobalt oxide The anode.
A secondary battery comprising: a positive electrode; a negative electrode; and a separator interposed between the positive electrode and the negative electrode, wherein the positive electrode is the positive electrode according to claim 7.

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