JP7035702B2 - Lithium ion secondary battery - Google Patents

Description

The present invention relates to a lithium ion secondary battery.

In recent years, electronic devices such as mobile phones and personal computers have been rapidly miniaturized and cordless, and there is an increasing demand for a secondary battery that is compact, lightweight, and has a high energy density as a power source for driving them. Further, under such a situation, a lithium ion secondary battery having a large charge / discharge capacity and a high energy density is attracting attention.

The lithium ion secondary battery generally includes an electrode body and a non-aqueous electrolytic solution, and can be roughly classified into two types, a laminated type battery and a wound type battery, according to the shape of the electrode body. Of these, the electrode body of the winding type battery is manufactured by winding a long sheet-shaped electrode and a long sheet-shaped separator together into a flat shape, so that it is continuous from one roll. It has the advantage of being able to be manufactured and having excellent productivity.

However, it is also known that the wound type battery has a flat surface portion and a curved surface portion on the electrode body due to its structure, and a peculiar problem caused by this curved surface portion occurs. For example, regarding the problem that a minute short circuit occurs in the manufacturing process due to the non-uniformity of stress in the curved surface portion, in Patent Document 1, by providing a surplus separator region in the outermost peripheral portion of the wound body, stress relaxation in the curved surface portion and charge are provided. Methods for improving carrier balance and solving the above problems have been disclosed.

JP-A-2017-10878

However, various characteristics are still not satisfied by the method of the prior art, and it is particularly required to suppress the precipitation of metallic lithium during repeated charging and discharging on the curved surface portion.

The present invention has been made in view of the above-mentioned problems of the prior art, and an object of the present invention is to provide a lithium ion secondary battery in which metal lithium precipitation is suppressed during repeated charging and discharging on a curved surface portion.

In order to solve the above problems, the lithium ion secondary battery according to the present invention includes a wound body in which a positive electrode and a negative electrode are wound via a separator, and an exterior body for accommodating the wound body. The wound body includes a plurality of flat surface portions and a curved surface portion connecting the flat surface portions, and the arithmetic average roughness of the separator in the curved surface portion is Ra ₁ , and the arithmetic average roughness of the separator in the flat surface portion is determined. A lithium ion secondary battery characterized in that the relationship of Ra ₁ > Ra ₂ is satisfied when Ra ₂ is set.

According to this, the curved surface portion of the wound body tends to have a gap at the time of manufacturing, and the charging / discharging tends to be non-uniform due to the gap between the plates. By using the separator according to the present invention, deviation during fabrication of the wound body is suppressed, and the surface area of the separator on the curved surface is increased, so that the amount of local electrolytic solution retained is significantly improved. By promptly supplying lithium ions, charging and discharging proceed uniformly, and metal lithium precipitation during repeated charging and discharging on the curved surface portion is suppressed.

Further, in the lithium ion secondary battery according to the present invention, it is preferable that the arithmetic average roughness Ra ₁ of the separator on the curved surface portion is 0.40 μm or more.

According to this, it is suitable as the arithmetic mean roughness of the separator on the curved surface portion, and the precipitation of metallic lithium during repeated charging and discharging on the curved surface portion is suppressed.

Further, in the lithium ion secondary battery according to the present invention, it is preferable that the separator in the curved surface portion has a surface coat layer containing a cellulose derivative on the main surface in contact with the negative electrode.

According to the present invention, there is provided a lithium ion secondary battery in which metal lithium precipitation is suppressed during repeated charging and discharging on a curved surface portion.

It is a schematic cross-sectional view of the lithium ion secondary battery which concerns on this embodiment. It is a figure which developed the winding body in the lithium ion secondary battery which concerns on this embodiment. It is a schematic cross-sectional view which enlarged the end part of the winding body in the lithium ion secondary battery which concerns on this embodiment.

Hereinafter, the present embodiment will be described in detail with reference to the drawings as appropriate. The drawings used in the following description may be enlarged for convenience in order to make the features of the present invention easy to understand, and the dimensional ratios of each component may differ from the actual ones. be. The materials, dimensions, etc. exemplified in the following description are examples, and the present invention is not limited thereto, and the present invention can be appropriately modified without changing the gist thereof.

<Lithium-ion secondary battery>
FIG. 1 is a schematic cross-sectional view of a lithium ion secondary battery according to the present embodiment. As shown in FIG. 1, the lithium ion secondary battery 100 according to the present embodiment includes a power generation element 1 and an exterior body 2. The power generation element 1 has a positive electrode 10, a negative electrode 20, and a separator 30. The power generation element 1 shown in FIG. 1 is a wound body in which a positive electrode 10 and a negative electrode 20 are arranged so as to face each other with the separator 30 interposed therebetween and wound. Each of the positive electrode 10 and the negative electrode 20 is provided with terminals 15 and 25 for electrical connection with the outside.

FIG. 2 is a developed view of a wound body in the lithium ion secondary battery according to the present embodiment. The positive electrode 10 is a plate-shaped positive electrode current collector 12 provided with a positive electrode active material layer 14. The negative electrode 20 is a plate-shaped negative electrode current collector 22 provided with a negative electrode active material layer 24. Further, an insulating tape 40 is attached to a part of the positive electrode 10 and the negative electrode 20. The insulating tape 40 prevents short circuits of the terminals 15 and 25 and suppresses the active material layer from peeling from the current collector.

FIG. 3 is an enlarged schematic cross-sectional view of an end portion of a wound body in the lithium ion secondary battery according to the present embodiment. The winding body end portion has a flat surface portion _RP and a curved surface portion _RC connecting the flat surface portions _RP .

<Positive electrode>
The positive electrode 10 has a positive electrode current collector 12 and a positive electrode active material layer 14 provided on both sides of the positive electrode current collector 12.

(Positive current collector)
The positive electrode current collector 12 may be any conductive plate material, and for example, a thin metal plate (metal foil) such as aluminum or an alloy thereof or stainless steel can be used.

(Positive electrode active material layer)
The positive electrode active material layer 14 is mainly composed of a positive electrode active material, a positive electrode binder, and a positive electrode conductive auxiliary agent.

(Positive electrode active material)
As the positive electrode active material, the storage and release of lithium ions, the desorption and insertion (intercalation) of lithium ions, or the doping and dedoping of the counter anion of the lithium ions (for example, PF _6- ⁾ are reversibly performed. A known positive electrode active material can be used without particular limitation as long as it can be advanced. Examples of the positive electrode active material include lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganate (LiMnO ₂ ), lithium nickel manganate, and Li (Ni _{x Coy} Mn _{z Ma} ) O. ₂ (x + y + z + a = 1, 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ z ≦ 1, 0 ≦ a ≦ 1, and M is selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr. Li _a M _b (PO ₄ ) _c (1 ≦ a ≦ 4, 1 ≦ b ≦ 2, 1 ≦ c ≦ 3, where M is Fe, V, Co. , Mn, Ni, at least one selected from VO), a polyanion olivine type positive electrode, and the like.

(Binder for positive electrode)
The positive electrode binder binds the positive electrode active materials to each other, and also bonds the positive electrode active material layer 14 and the positive electrode current collector 12. The binder may be any one capable of the above-mentioned binding, for example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroelene-. Perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF) ) And other fluororesins, vinylidene fluoride-hexafluoropropylene (VDF-HFP), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene (VDF-HFP-TFE), vinylidene fluoride-chlorotrifluoroethylene (VDF-). Using a vinylidene fluoride-based fluororesin such as CTFE), a non-fluororesin such as styrene / butadiene rubber (SBR), ethylene / propylene rubber (EPR), polyamide (PA), polyimide (PI), and polyamideimide (PAI). May be good.

Further, an electron conductive conductive polymer or an ion conductive conductive polymer may be used as the binder. Examples of the electron-conducting conductive polymer include polyacetylene, polythiophene, polyaniline and the like. Examples of the ionic conductive polymer include a composite of a polyether polymer compound such as polyethylene oxide and polypropylene oxide and a lithium salt such as LiClO ₄ , LiBF ₄ , and LiPF ₆ . Be done.

(Conductive aid for positive electrode)
The conductive auxiliary agent for the positive electrode is not particularly limited as long as it improves the conductivity of the positive electrode active material layer 14, and a known conductive auxiliary agent can be used. Examples thereof include carbon-based materials such as graphite and carbon black, metals such as copper, nickel, stainless steel and iron, and conductive oxides such as indium tin oxide (ITO). Further, if sufficient conductivity can be ensured only by the positive electrode active material, the positive electrode active material layer 14 may not contain the conductive auxiliary agent.

<Negative electrode>
The negative electrode 20 has a negative electrode current collector 22 and a negative electrode active material layer 24 provided on both sides of the negative electrode current collector 22.

(Negative electrode current collector)
The negative electrode current collector 22 may be a conductive plate material, and for example, a thin metal plate (metal leaf) such as copper can be used.

(Negative electrode active material layer)
The negative electrode active material layer 24 is mainly composed of a negative electrode active material, a negative electrode binder, and a negative electrode conductive auxiliary agent.

(Negative electrode active material)
As the negative electrode active material, occlusion and release of lithium ions, desorption and insertion (intercalation) of lithium ions, or doping and dedoping of the counter anion of the lithium ions (for example, PF _6- ⁾ are reversibly performed. A known negative electrode active material can be used without particular limitation as long as it can be advanced. Examples of the negative electrode active material include carbon materials such as graphite, hard carbon and soft carbon, metals capable of forming alloys with lithium such as aluminum, silicon and tin, and amorphous substances such as silicon oxide and tin oxide. Examples thereof include oxides, lithium titanate (Li ₄ Ti ₅ O ₁₂ ), and the like.

(Binder for negative electrode)
The binder for the negative electrode is not particularly limited, and the same binder as the binder for the positive electrode described above can be used.

(Conductive aid for negative electrode)
The conductive auxiliary agent for the negative electrode is not particularly limited, and the same conductive auxiliary agent for the positive electrode described above can be used.

<Separator>
The separator 30 has an electrically insulating porous structure. Examples of the separator include a monolayer or laminate of polyolefin such as polyethylene and polypropylene, and a fibrous nonwoven fabric made of cellulose, polyester, polyacrylonitrile and the like.

Further, the surface of the separator may be coated with an inorganic compound or an organic compound. As the inorganic compound, an insulating metal oxide such as alumina or boehmite can be used, and as the organic compound, polyvinylidene fluoride (PVDF), polyethylene oxide (PEO) or the like can be used.

The separator according to the present embodiment will be described in more detail below.

The separator according to the present embodiment is Ra ₁ > Ra ₂ when the arithmetic average roughness of the separator in the curved surface portion _RC is Ra ₁ and the arithmetic average roughness of the separator in the flat surface portion R _P is Ra ₂ .

According to this, the curved surface portion of the wound body tends to have a gap at the time of manufacturing, and the charging / discharging tends to be non-uniform due to the gap between the plates. By using the separator according to the present embodiment, the displacement during the production of the wound body is suppressed, and the surface area of the separator on the curved surface is increased, so that the amount of local electrolytic solution retained is greatly improved. By promptly supplying lithium ions, charging and discharging proceed uniformly, and metal lithium precipitation during repeated charging and discharging on the curved surface portion is suppressed.

Further, the separator according to the present embodiment preferably has an arithmetic average roughness Ra ₁ of the separator on the curved surface portion of 0.40 μm or more.

According to this, it is suitable as the arithmetic mean roughness of the separator on the curved surface portion, and the precipitation of metallic lithium during repeated charging and discharging on the curved surface portion is suppressed.

As a method of adjusting the arithmetic mean roughness to the above-mentioned predetermined range, for example, a method of applying a protective layer composed of an inorganic filler and a polymer resin can be mentioned. In particular, by setting the particle size and particle shape of the inorganic filler to be appropriate, a separator having an arbitrary arithmetic mean roughness can be obtained. The inorganic filler is not particularly limited, but alumina, boehmite and the like can be used, and the polymer resin is not particularly limited, but general materials such as PVDF, PTFE and polyethylene oxide (PEO) are used. Can be done.

Further, the separator according to the present embodiment preferably has a surface coat layer containing a cellulose derivative on the main surface of the curved surface portion where the separator is in contact with the negative electrode.

According to this, by having a surface coat layer containing a cellulose derivative on the main surface of the curved surface portion in contact with the negative electrode of the separator, the interfacial resistance between the separator and the negative electrode is reduced, and the metal during repeated charging and discharging on the curved surface portion. Lithium precipitation is more suppressed.

<Electrolytic solution>
The electrolytic solution according to the present invention is mainly composed of a solvent and an electrolyte.

(solvent)
As the solvent, a solvent generally used for a lithium ion secondary battery can be mixed and used at an arbitrary ratio. For example, cyclic carbonate compounds such as ethylene carbonate (EC), propylene carbonate (PC) and butylene carbonate, chain carbonate compounds such as diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC), γ-butyrolactone. Examples thereof include cyclic ester compounds such as (GBL), and chain ester compounds such as propyl propionate (PrP), ethyl propionate (PrE), and ethyl acetate.

(Electrolytes)
The electrolyte is not particularly limited as long as it is a lithium salt used as an electrolyte for a lithium ion secondary battery, and is, for example, an inorganic acid anion salt such as LiPF ₆ , LiBF ₄ , lithium bisoxalate boronate, LiCF ₃ SO ₃ , and the like. Organic acid anionic salts such as (CF ₃ SO ₂ ) ₂ NLi and (FSO ₂ ) ₂ NLi can be used.

Although the preferred embodiment of the present invention has been described above, the present invention is not limited to the above embodiment.

Hereinafter, the present invention will be described in more detail based on Examples and Comparative Examples, but the present invention is not limited to the following Examples.

[Example 1]
(Preparation of positive electrode)
Li (Ni _0.80 Co _0.15 Al _0.05 ) O ₂ 85 parts by mass, acetylene black 5 parts by mass, PVDF 10 parts by mass are dispersed in N-methyl-2-pyrrolidone (NMP) to form a positive electrode active material layer. The slurry for was adjusted. This slurry was applied to one surface of an aluminum metal foil having a thickness of 20 μm so that the amount of the positive electrode active material applied was 9.0 mg / cm ² , and dried at 100 ° C. to form a positive electrode active material layer. Then, it was pressure-molded by a roller press to prepare a positive electrode.

(Manufacturing of negative electrode)
90 parts by mass of scaly artificial graphite, 5 parts by mass of acetylene black, and 5 parts by mass of PVDF were dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a slurry for forming a negative electrode active material layer. The slurry was applied to one surface of a copper foil having a thickness of 20 μm so that the amount of the negative electrode active material applied was 6.0 mg / cm ² , and dried at 100 ° C. to form a negative electrode active material layer. Then, it was pressure-molded by a roller press to prepare a negative electrode.

(Making a separator)
As a separator, a polyethylene porous film having a thickness of 16 μm and a porosity of 60% was prepared. A slurry in which 95 parts by mass of α-Al ₂ O ₃ (average particle size 1.0 μm) as an inorganic filler and 5 parts by mass of PVDF as a polymer resin are dispersed in water is applied to the separator and vacuum dried at 60 ° C. Formed a protective layer with. Then, using a hydraulic heating press, the region corresponding to the flat surface portion was pressure-molded at a temperature of 60 ° C. and a pressure of 400 kPa, and the region corresponding to the curved surface portion was pressure-molded at a temperature of 40 ° C. and a pressure of 100 kPa.

A solution prepared by dissolving 1 wt% of carboxymethyl cellulose in water: methyl ethyl ketone (MEK) = 70: 30 (vol%) is applied to the separator prepared above so that the coating amount is 0.1 mg / cm ² on the surface. A coat layer was formed.

(Measurement of arithmetic mean roughness)
Observe the surface of the separator produced above with a laser microscope (VK-9710 manufactured by KEYENCE CORPORATION), and use analysis software (VK-Analyzer manufactured by KEYENCE Software Co., Ltd.) under the conditions according to JIS B0601: 1994. The arithmetic average surface roughness Ra ₁ of the separator on the curved surface portion and the arithmetic average roughness Ra ₂ of the separator on the flat surface portion were obtained. The results are shown in Table 1.

(Making a wound body)
The positive electrode, the negative electrode, and the separator prepared above were wound around one end side as an axis to prepare a wound body. Here, the wound body was prepared so that the surface coat layer of the separator was arranged on the main surface in contact with the negative electrode.

(Preparation of electrolyte)
The mixture was mixed so that the volume ratio was EC: DEC = 30: 70, and LiPF ₆ was dissolved therein so as to have a concentration of 1.0 mol / L to prepare an electrolytic solution.

(Manufacturing of lithium-ion secondary battery for evaluation)
The wound body manufactured above is housed in the outer shell of the aluminum laminated film in which the accommodation space is formed, the electrolytic solution prepared above is injected, and vacuum sealing is performed to prepare a lithium ion secondary battery for evaluation. did.

(Battery)
The evaluation lithium ion secondary battery produced above is charged with a constant current at a charging rate of 0.2 C in a constant temperature bath at 25 ° C using a charge / discharge test device (manufactured by Hokuto Denko Co., Ltd.), and the battery voltage is 4.2 V. After charging until the battery voltage reached 2.8 V, the battery was discharged at a constant current discharge rate of 0.2 C until the battery voltage became 2.8 V. Here, X (C) indicates a current value at which charging ends in 1 / X time when constant current charging is performed at 25 ° C.

(Confirmation of lithium precipitation after the cycle)
The evaluation lithium ion secondary battery converted into a battery above is charged in a constant temperature bath at 25 ° C. with a constant current charge at a charging rate of 2.0 C until the battery voltage reaches 4.2 V, and then the discharge rate is 1. The battery was discharged with a constant current discharge of 0.0 C until the battery voltage became 2.8 V. The charge / discharge pattern was set to 1 cycle, and 100 cycles of charge / discharge were performed. Regarding the battery after 100 cycles, when the battery was disassembled in the glove box in an inert atmosphere and the negative electrode curved surface portion was confirmed, lithium precipitation was not confirmed.

[Example 2]
(Making a separator)
The separator of Example 2 was prepared in the same manner as in Example 1 except that α-Al ₂ O ₃ (average particle size 3.0 μm) was used as the inorganic filler.

(Manufacturing of lithium-ion secondary battery for evaluation)
A lithium secondary battery for evaluation of Example 2 was produced by the same method as in Example 1 except that the separator produced above was used.

[Example 3]
(Making a separator)
The separator of Example 3 was prepared in the same manner as in Example 1 except that boehmite (average particle size 4.0 μm) was used as the inorganic filler.

(Manufacturing of lithium-ion secondary battery for evaluation)
A lithium secondary battery for evaluation of Example 3 was prepared by the same method as in Example 1 except that the separator prepared above was used.

[Example 4]
(Making a separator)
In the production of the separator, the separator of Example 4 was produced in the same manner as in Example 1 except that the region corresponding to the curved surface portion was pressure-molded at a temperature of 40 ° C. and a pressure of 200 kPa.

(Manufacturing of lithium-ion secondary battery for evaluation)
The evaluation lithium secondary battery of Example 4 was produced by the same method as in Example 1 except that the separator produced above was used.

[Example 5]
(Making a separator)
In the preparation of the separator, examples except that 1 wt% of methyl cellulose was dissolved in water: MEK = 50: 50 (vol%) and applied so that the coating amount was 0.1 mg / cm ² to form a surface coat layer. The separator of Example 5 was prepared in the same manner as in 1.

(Manufacturing of lithium-ion secondary battery for evaluation)
The evaluation lithium secondary battery of Example 5 was produced by the same method as in Example 1 except that the separator produced above was used.

[Example 6]
(Making a separator)
In the preparation of the separator, examples except that 1 wt% of ethyl cellulose was dissolved in water: MEK = 50: 50 (vol%) and applied so that the coating amount was 0.1 mg / cm ² to form a surface coat layer. The separator of Example 6 was prepared in the same manner as in 1.

(Manufacturing of lithium-ion secondary battery for evaluation)
The evaluation lithium secondary battery of Example 6 was produced by the same method as in Example 1 except that the separator produced above was used.

[Example 7]
(Making a separator)
In the preparation of the separator, 1 wt% of hydroxyethyl cellulose was dissolved in water: MEK = 70: 30 (vol%) and applied so that the coating amount was 0.1 mg / cm ² to form a surface coat layer. The separator of Example 7 was prepared in the same manner as in Example 1.

(Manufacturing of lithium-ion secondary battery for evaluation)
The evaluation lithium secondary battery of Example 7 was produced by the same method as in Example 1 except that the separator produced above was used.

[Example 8]
(Making a separator)
In the production of the separator, the separator of Example 8 was produced by the same method as in Example 1 except that the protective layer was intermittently applied only to the region corresponding to the curved surface portion of the separator.

(Manufacturing of lithium-ion secondary battery for evaluation)
The evaluation lithium secondary battery of Example 8 was produced by the same method as in Example 1 except that the separator produced above was used.

[Comparative Example 1]
(Manufacturing of lithium-ion secondary battery for evaluation)
A lithium secondary battery for evaluation of Comparative Example 1 was produced in the same manner as in Example 1 except that an untreated separator that did not form a protective layer and a surface coat layer was used.

[Comparative Example 2]
(Manufacturing of lithium-ion secondary battery for evaluation)
A lithium secondary battery for evaluation of Comparative Example 2 was produced by the same method as in Example 1 except that a separator having only a surface coat layer formed was used.

(Measurement of arithmetic mean roughness)
For the separators produced in Examples 2 to 8 and Comparative Examples 1 and 2, the arithmetic average surface roughness Ra ₁ of the separator on the curved surface portion and the arithmetic average roughness Ra ₂ of the separator on the flat surface portion were obtained in the same manner as in Example 1. I asked. The results are shown in Table 1.

(Confirmation of lithium precipitation after the cycle)
For the evaluation lithium ion secondary batteries produced in Examples 2 to 8 and Comparative Examples 1 and 2, lithium precipitation after the cycle was confirmed by the same method as in Example 1. The results are shown in Table 1. In the table, "◎" indicates no lithium precipitation, "○" indicates a slight lithium precipitation (1% or less with respect to the total area of the negative electrode curved surface), and "△" indicates lithium. Precipitation is confirmed (5% or less with respect to the total area of the negative electrode curved surface), and "x" is a state with a large amount of lithium precipitation confirmed (10% or less with respect to the total area of the negative electrode curved surface). Is shown.

In each of Examples 1 to 8, lithium precipitation in the negative electrode curved surface portion during repeated charging and discharging can be improved as compared with Comparative Examples 1 and 2 in which the arithmetic mean roughness of the separator in the curved surface portion and the flat surface portion was not optimized. confirmed.

From the results of Examples 1 to 4, it was confirmed that when carboxymethyl cellulose was provided as the surface coat layer, the lithium precipitation on the curved surface portion of the negative electrode during repeated charging and discharging was further improved.

INDUSTRIAL APPLICABILITY According to the present invention, a lithium ion secondary battery in which metal lithium precipitation is suppressed during repeated charging and discharging on a curved surface portion is provided.

1 ... Power generation element, 2 ... Exterior body, 10 ... Positive electrode, 12 ... Positive electrode current collector, 14 ... Positive electrode active material layer, 15 ... Terminal, 20 ... Negative electrode, 22 ... Negative electrode current collector, 24 ... Negative electrode active material layer, 25 ... Terminal, 30 ... Separator, 40 ... Insulation tape, 100 ... Lithium ion secondary battery, _RP ... Flat part, _RC ... Curved part

Claims (2)

A wound body in which the positive electrode and the negative electrode are wound via a separator, and
With an exterior body for storing the winding body,
The wound body includes a plurality of flat surface portions and a curved surface portion connecting the flat surface portions.
When the arithmetic average roughness of the separator on the curved surface portion is Ra ₁ and the arithmetic average roughness of the separator on the flat surface portion is Ra ₂ , Ra ₁ is 0.40 μm or more, and the relationship of Ra ₁ > Ra ₂ is established. A lithium-ion secondary battery characterized by filling. The lithium ion secondary battery according to claim 1 or 2, wherein the separator on the curved surface portion has a surface coat layer containing a cellulose derivative on a main surface in contact with the negative electrode.

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