WO2022264499A1

WO2022264499A1 - Bipolar electrode and power storage device

Info

Publication number: WO2022264499A1
Application number: PCT/JP2022/005495
Authority: WO
Inventors: 佑介杉山; 隆弘杉岡; まどか大谷; 修一安田
Original assignee: 株式会社豊田自動織機
Priority date: 2021-06-15
Filing date: 2022-02-10
Publication date: 2022-12-22
Also published as: JP2022190786A

Abstract

This bipolar electrode (10) comprises a bipolar current collector (20), a positive electrode active material layer (30) provided on a surface of a positive electrode current collector (21), and a negative electrode active material layer (40) provided on a surface of a negative electrode current collector (22). The bipolar current collector (20) is provided with: the positive electrode current collector (21), which has an aluminum layer (21a); and the negative electrode current collector (22), which has a copper layer (22a). An oxide film (23) containing magnesium oxide is provided to the portion of the surface of the negative electrode current collector (22) where the negative electrode active material layer (40) is attached.

Description

Bipolar electrode and power storage device

The present disclosure relates to bipolar electrodes and power storage devices.

In recent years, research has been conducted on power storage devices using bipolar electrodes. The bipolar electrode includes a bipolar current collector having a positive electrode current collector and a negative electrode current collector, a positive electrode active material layer provided on the surface of the positive electrode current collector, and a negative electrode active material provided on the surface of the negative electrode current collector. a layer; Power storage devices using bipolar electrodes are expected to be superior to other power storage devices in terms of improvement in volumetric energy density and output.

Patent Document 1 discloses a bipolar electrode in which a clad material obtained by rolling aluminum foil with a thickness of 20 μm and copper foil with a thickness of 10 μm is used as a bipolar current collector. In the case of the bipolar current collector of Patent Document 1, an aluminum layer formed by rolling an aluminum foil serves as a positive electrode collector, and a copper layer formed by rolling a copper foil serves as a negative electrode collector.

JP-A-08-007926

The present inventors produced a bipolar electrode in which an oxide film containing magnesium oxide was provided on a portion of the surface of a bipolar current collector having an aluminum layer and a copper layer, where the negative electrode active material layer was provided. Then, when the battery characteristics of the power storage device using the manufactured bipolar electrode were measured, it was found that the capacity retention rate of the power storage device was improved.

A bipolar electrode according to an aspect of the present disclosure includes a bipolar current collector including a positive electrode current collector having an aluminum layer and a negative electrode current collector having a copper layer, and a positive electrode active material provided on the surface of the positive electrode current collector. and a negative electrode active material layer provided on the surface of the negative electrode current collector. An oxide film containing magnesium oxide is provided on a portion of the surface of the negative electrode current collector to which the negative electrode active material layer is adhered.

Another aspect of the present disclosure provides a power storage device including the bipolar electrode.

Sectional drawing of a bipolar electrode. Sectional drawing of an electrical storage apparatus.

An embodiment of the present disclosure will be described below with reference to the drawings.
(bipolar electrode)
The bipolar electrode of this embodiment is used as an electrode of a power storage device. The power storage device is, for example, a secondary battery such as a nickel-hydrogen secondary battery or a lithium-ion secondary battery. Also, the power storage device may be an electric double layer capacitor. In this embodiment, a bipolar electrode used for a lithium ion secondary battery will be described.

As shown in FIG. 1, the bipolar electrode 10 includes a bipolar current collector 20, a positive electrode active material layer 30 provided on a first main surface 20a of the bipolar current collector 20, and a second main surface 20a of the bipolar current collector 20. and a negative electrode active material layer 40 provided on the surface 20b.

<Bipolar current collector>
In the bipolar current collector 20, a sheet-like positive electrode current collector 21 forming a first main surface 20a and a sheet-like negative electrode current collector 22 forming a second main surface 20b are joined integrally in the thickness direction. It is a laminate formed by Bipolar current collector 20 is a chemically inert electrical conductor for continuing current flow through positive electrode active material layer 30 and negative electrode active material layer 40 during discharge or charge of the power storage device.

The positive electrode current collector 21 has an aluminum layer 21a containing aluminum as a main component as an essential component. The aluminum layer 21a may be a layer made of aluminum alone or a layer made of an aluminum alloy. Examples of aluminum alloys include Al--Mn alloys, Al--Mg alloys and Al--Mg--Si alloys. The content of aluminum in the aluminum layer 21a is, for example, 50% by mass or more, preferably 70% by mass or more. The form of the aluminum layer 21a is, for example, foil, sheet, or film. The thickness of the aluminum layer 21a is, for example, 10 μm or more and 100 μm or less.

In addition, the positive electrode current collector 21 may be a single body composed only of the aluminum layer 21a, or may be a composite body including portions other than the aluminum layer 21a. FIG. 1 shows, as an example, the positive electrode current collector 21 composed only of the aluminum layer 21a.

Examples of the composite include a multilayer structure having an aluminum layer 21a on the surface that serves as the first main surface 20a, and a base material coated with an aluminum film corresponding to the aluminum layer 21a. Examples of materials that form portions other than the aluminum layer 21a include metal materials, conductive resin materials, and conductive inorganic materials. Examples of the metal material include titanium and stainless steel (eg, SUS304, SUS316, SUS301, SUS304, etc. defined in JIS G 4305:2015). Examples of the conductive resin material include a resin obtained by adding a conductive filler to a conductive polymer material or a non-conductive polymer material as necessary.

The negative electrode current collector 22 has, as an essential component, a copper layer 22a containing copper as a main component. The copper layer 22a may be a layer made of copper alone or a layer made of a copper alloy. Examples of copper alloys include nickel silver, cupronickel, and beryllium copper. The content of copper in the copper layer 22a is, for example, 50% by mass or more, preferably 70% by mass or more. The form of the copper layer 22a is, for example, a plated layer or foil. The thickness of the copper layer 22a is, for example, 8 μm or more and 20 μm or less. The thickness of the copper layer 22a in the case of being a plated layer is, for example, 1 μm or more and 10 μm or less, preferably 3 μm or more and 8 μm or less.

Further, the negative electrode current collector 22 may be a single body composed only of the copper layer 22a, or may be a multilayer structure including layers other than the copper layer 22a. Examples of the other layer include a nickel layer containing nickel as a main component. The other layers may be provided between the copper layer 22a and the positive electrode current collector 21, or may be provided closer to the second main surface 20b than the copper layer 22a. FIG. 1 shows, as an example, a negative electrode current collector 22 having a two-layer structure including a copper layer 22a and a nickel layer 22b laminated on the surface of the copper layer 22a and forming the second main surface 20b. .

The nickel layer 22b may be a layer made of nickel alone or a layer made of a nickel alloy. Nickel alloys include, for example, Hastelloy, Nichrome, Monel, Sunplatinum, and Permalloy. The content of nickel in the nickel layer 22b is, for example, 50% by mass or more, preferably 70% by mass or more. The form of the nickel layer 22b is, for example, a plated layer or foil. The thickness of the nickel layer 22b is, for example, 8 μm or more and 20 μm or less. Moreover, the thickness of the nickel layer 22b in the case of being a plated layer is, for example, 1 μm or more and 10 μm or less.

As the bipolar current collector 20, for example, a clad metal or a plated aluminum foil that has undergone a predetermined plating treatment can be used. Examples of the clad metal include clad metal obtained by rolling aluminum foil and copper foil, and clad metal obtained by rolling aluminum foil, copper foil and nickel foil. Examples of the plated aluminum foil include a copper-plated aluminum foil obtained by plating one side of an aluminum foil with copper, and a copper-nickel-plated aluminum foil obtained by sequentially plating one side of an aluminum foil with copper and nickel.

An oxide film 23 is provided on the surface of the negative electrode current collector 22 that is the second main surface 20 b of the bipolar current collector 20 . Details of the oxide film 23 will be described later.
<Positive electrode active material layer>
The positive electrode active material layer 30 is integrally adhered to the first main surface 20a of the bipolar current collector 20 . The positive electrode active material layer 30 contains a positive electrode active material capable of intercalating and deintercalating charge carriers such as lithium ions. Examples of positive electrode active materials include polyanionic compounds such as olivine-type lithium iron phosphate (LiFePO ₄ ), lithium composite metal oxides having a layered rock salt structure, and metal oxides having a spinel structure. A positive electrode active material that can be used as a positive electrode active material for a power storage device such as a lithium ion secondary battery is employed.

The positive electrode active material layer 30 contains a conductive aid, a binder, an electrolyte (polymer matrix, ion conductive polymer, liquid electrolyte, etc.) for increasing electrical conductivity, and an electrolyte support for increasing ion conductivity, if necessary. It may contain other components such as salts (lithium salts). The types and compounding ratios of other components contained in the positive electrode active material layer 30 are not particularly limited.

Conductive aids include, for example, acetylene black, carbon black, and graphite.
Examples of binders include fluorine-containing resins such as polyvinylidene fluoride, polytetrafluoroethylene, and fluororubber, thermoplastic resins such as polypropylene and polyethylene, imide resins such as polyimide and polyamideimide, alkoxysilyl group-containing resins, Examples include acrylic resins such as poly(meth)acrylic acid, styrene-butadiene rubber, carboxymethylcellulose, alginates such as sodium alginate and ammonium alginate, water-soluble cellulose ester crosslinked products, and starch-acrylic acid graft polymers. These binders may be used singly or in combination. Water, N-methyl-2-pyrrolidone, and the like are used as the solvent or dispersion medium, for example.

The thickness of the positive electrode active material layer 30 is, for example, 2 to 150 μm.
Examples of the method for forming the positive electrode active material layer 30 on the first main surface 20a of the bipolar current collector 20 include known methods such as roll coating.

<Negative electrode active material layer>
The negative electrode active material layer 40 is integrally adhered to the second main surface 20b of the bipolar current collector 20 . The negative electrode active material layer 40 contains a negative electrode active material capable of intercalating and deintercalating charge carriers such as lithium ions. The negative electrode active material is not particularly limited as long as it is a simple substance, alloy, or compound that can occlude and release charge carriers such as lithium ions. Examples of negative electrode active materials include Li, carbon, metal compounds, elements that can be alloyed with lithium, and compounds thereof. Examples of carbon include natural graphite, artificial graphite, hard carbon (non-graphitizable carbon), and soft carbon (easily graphitizable carbon). Examples of artificial graphite include highly oriented graphite and mesocarbon microbeads. Elements that can be alloyed with lithium include, for example, silicon (silicon) and tin.

The negative electrode active material layer 40 contains a conductive aid, a binder, an electrolyte (polymer matrix, ion-conductive polymer, liquid electrolyte, etc.) for increasing electrical conductivity, and an electrolyte support for increasing ion conductivity, if necessary. Other components such as salts (lithium salts) may be contained. The types and compounding ratios of other components contained in the positive electrode active material layer 30 are not particularly limited.

The thickness of the negative electrode active material layer 40 is, for example, 2 to 150 μm.
Examples of the method for forming the negative electrode active material layer 40 on the second main surface 20b of the bipolar current collector 20 include known methods such as roll coating.

<Oxide film>
An oxide film 23 is provided on the surface of the negative electrode current collector 22 , which is the second main surface 20 b of the bipolar current collector 20 . The oxide film 23 is provided on at least part of the portion of the second main surface 20b to which the negative electrode active material layer 40 is adhered. In addition, the oxide film 23 is preferably provided on the entire portion of the second main surface 20b to which the negative electrode active material layer 40 is adhered, and more preferably provided on the entire second main surface 20b. . FIG. 1 shows, as an example, the case where the oxide film 23 is provided over the entire second main surface 20b.

The thickness of the oxide film 23 is, for example, 10 μm or more, preferably 30 μm or more. Moreover, the thickness of the oxide film 23 is, for example, 500 μm or less, preferably 200 μm or less.

The oxide film 23 is a film containing magnesium oxide. The content ratio of magnesium oxide in the oxide film 23 is the mass ratio of magnesium based on the detected value of each component measured by fluorescent X-ray analysis of the portion of the second main surface 20b where the oxide film 23 is provided. Defined.

The content of magnesium in the oxide film 23 is, for example, 1% by mass or more, preferably 1.5% by mass or more. The content of magnesium in the oxide film 23 is, for example, 30% by mass or less.

It is assumed that the second main surface 20b of the bipolar current collector 20 is formed of the copper layer 22a. In this case, the content of magnesium in the oxide film 23 is, for example, 1 part by mass or more, preferably 1.5 parts by mass or more, as a relative value with the mass of copper based on the detected value being 100 parts by mass, More preferably, it is 2 parts by mass or more. Moreover, the content ratio of magnesium in the oxide film 23 in this case is, for example, 30 parts by mass or less as the relative value.

Further, as shown in FIG. 1, the negative electrode current collector 22 of the bipolar current collector 20 has a two-layer structure of a copper layer 22a and a nickel layer 22b, and the second main surface 20b is formed of the nickel layer 22b. do. In this case, the content of magnesium in the oxide film 23 is, for example, 1 part by mass or more, preferably 1.5 parts by mass or more, as a relative value with the total mass of copper and nickel based on the detected value being 100 parts by mass. and more preferably 2 parts by mass or more. Moreover, the content ratio of magnesium in the oxide film 23 in this case is, for example, 30 parts by mass or less as the relative value.

Next, an example of chemical conversion treatment for forming the oxide film 23 on the second main surface 20b of the bipolar current collector 20 will be described.
First, a chemical conversion treatment solution to be used for chemical conversion treatment is prepared. The chemical conversion treatment liquid is obtained by subjecting a mixed liquid of a magnesium salt and a solvent to heat treatment, and then filtering the precipitate. In addition, it is preferable to further mix one or both of a basic compound and a Lewis acid into the mixed solution. In this case, oxide film 23 can be formed more efficiently.

Examples of the magnesium salts include magnesium acetate, magnesium formate, magnesium carbonate, and magnesium ethoxide. One of these may be used alone, or two or more may be used in combination. The magnesium salt contained in the mixed solution may be of one of the above specific examples, or may be a combination of two or more. The concentration of the magnesium salt in the mixed solution is, for example, 0.1 g/ml or more and 5 g/ml or less, preferably 0.5 g/ml or more and 3 g/ml or less.

As the solvent, a solvent capable of dissolving the mixed magnesium salt is appropriately selected and used. Examples of the solvent include water, ethanol, and acetic acid.
Examples of the basic compound include sodium hydroxide, lithium hydroxide, potassium hydroxide and sodium acetate. The basic compound contained in the liquid mixture may be one of the above specific examples, or may be a combination of two or more. The concentration of the basic compound in the mixture is, for example, 0.01 g/ml or more and 0.5 g/ml or less, preferably 0.05 g/ml or more and 0.1 g/ml or less.

Examples of the Lewis acid include boric acid, acetic acid, and calcium chloride. The Lewis acid contained in the mixed solution may be one of the above specific examples, or may be a combination of two or more. The concentration of the Lewis acid in the mixed solution is, for example, 0.1 g/ml or more and 5 g/ml or less, preferably 0.5 g/ml or more and 3 g/ml or less.

The heating temperature in the heat treatment is, for example, 20°C or higher and 60°C or lower, preferably 30°C or higher and 50°C or lower. The heating time in the heat treatment is, for example, 0.5 minutes or more and 10 minutes or less, preferably 1 minute or more and 5 minutes or less.

Next, using the resulting chemical conversion treatment solution, a chemical conversion treatment is performed to form an oxide film 23 on the second main surface 20 b of the bipolar current collector 20 . The second main surface 20b of the bipolar current collector 20 to be subjected to chemical conversion treatment may be a metal surface, a surface on which an oxide film such as an anodized film is formed, or an alkaline It may be a treated surface.

In the chemical conversion treatment, first, the second main surface 20b of the bipolar current collector 20 is subjected to the first heat treatment while the chemical conversion treatment liquid is adhered to the second main surface 20b. Examples of methods for applying the chemical conversion treatment solution include spray treatment and immersion treatment. The heating temperature in the first heat treatment is, for example, 20° C. or higher and 60° C. or lower, preferably 30° C. or higher and 50° C. or lower. The heating time in the first heat treatment is, for example, 0.5 minutes or more and 10 minutes or less, preferably 1 minute or more and 5 minutes or less.

After that, the chemical conversion treatment solution remaining on the second main surface 20b of the bipolar current collector 20 is removed by washing with water or the like, and a second heat treatment is performed to form an oxide film 23 containing magnesium oxide. The heating temperature in the second heat treatment is, for example, 20° C. or higher and 60° C. or lower, preferably 30° C. or higher and 50° C. or lower. The heating time in the second heat treatment is, for example, 0.5 minutes or more and 10 minutes or less, preferably 1 minute or more and 5 minutes or less.

<Lithium ion secondary battery>
Next, an example of a lithium ion secondary battery using the above bipolar electrode 10 will be described.

As shown in FIG. 2, the lithium ion secondary battery 50 includes a plurality of bipolar electrodes 10, a positive electrode 51, a negative electrode 52, a separator 53, spacers 54, and an electrolyte 55.

The plurality of bipolar electrodes 10 are stacked such that the positive electrode active material layer 30 and the negative electrode active material layer 40 face each other with the separator 53 interposed therebetween.
The positive electrode 51 includes a current collector 51a and a positive electrode active material layer 51b provided on one surface of the current collector 51a. The positive electrode 51 is arranged so that the positive electrode active material layer 51b faces the negative electrode active material layer 40 located at one end in the stacking direction of the stacked bipolar electrodes 10 with the separator 53 interposed therebetween. It is

The negative electrode 52 includes a current collector 52a and a negative electrode active material layer 52b provided on one surface of the current collector 52a. The negative electrode 52 is arranged so that the negative electrode active material layer 52b faces the positive electrode active material layer 30 located at the end of the stacked bipolar electrodes 10 on the other side in the stacking direction with the separator 53 interposed therebetween. It is

The spacers 54 are provided between the bipolar electrodes 10 adjacent in the stacking direction, between the bipolar electrodes 10 adjacent in the stacking direction and the positive electrode 51, and between the bipolar electrodes 10 and the negative electrode 52 adjacent in the stacking direction. are arranged so as to surround each active material layer between. Also, the spacer 54 is adhered to the bipolar current collector 20 of the bipolar electrode 10 , the current collector 51 a of the positive electrode 51 , and the current collector 52 a of the negative electrode 52 .

A closed space surrounded by a bipolar current collector 20, a positive electrode 51, a negative electrode 52, and a spacer 54 is formed inside the lithium ion secondary battery 50. A separator 53 and an electrolyte 55 are housed in this sealed space. The lithium ion secondary battery 50 is charged and discharged through terminals connected to the current collector 51a of the positive electrode 51 and the current collector 52a of the negative electrode 52, respectively.

Next, the operation and effects of this embodiment will be described.
(1) The bipolar electrode 10 includes a bipolar current collector 20, a positive electrode active material layer 30 provided on the surface of the positive electrode current collector 21, and a negative electrode active material layer 40 provided on the surface of the negative electrode current collector 22. Prepare. The bipolar current collector 20 comprises a positive electrode current collector 21 having an aluminum layer 21a and a negative electrode current collector 22 having a copper layer 22a. An oxide film 23 containing magnesium oxide is provided on a portion of the surface of the negative electrode current collector 22 to which the negative electrode active material layer 40 is adhered.

By applying the bipolar electrode 10 having the above configuration to a power storage device such as the lithium ion secondary battery 50, the capacity retention rate of the power storage device is improved. Moreover, a decrease in the peel strength between the bipolar current collector 20 and the negative electrode active material layer 40 after repeated charging and discharging is suppressed. It can be inferred that each of the above effects is obtained as follows.

In the case of the bipolar electrode 10 configured as described above, the negative electrode active material layer 40 is bonded to the oxide film 23 provided on the surface of the negative electrode current collector 22 . More specifically, they are bound via magnesium derived from magnesium oxide contained in the oxide film 23 . The bonding state of the negative electrode active material layer 40 via magnesium is more resistant to reduction than the bonding state in which the negative electrode active material layer 40 is directly bonded to copper forming the copper layer 22a or nickel forming the nickel layer 22b. is high. This suppresses decomposition of the bond between the negative electrode active material layer 40 and the bipolar current collector 20 due to reaction with charge carriers such as lithium ions. As a result, a decrease in peel strength between the bipolar current collector 20 and the negative electrode active material layer 40 after repeated charging and discharging is suppressed. By maintaining the adhesion between the bipolar current collector 20 and the negative electrode active material layer 40, the capacity retention rate of the power storage device is improved.

(2) The content of magnesium measured by fluorescent X-ray analysis of the portion of the surface of the negative electrode current collector 22 where the oxide film 23 is provided is 1% by mass or more.
According to the above configuration, the effect of improving the capacity retention rate of the power storage device can be significantly obtained.

(3) The oxide film 23 is provided on the surface of the copper layer 22a. The content of magnesium measured by fluorescent X-ray analysis targeting the portion of the surface of copper layer 22a where oxide film 23 is provided is 1 mass with respect to 100 parts by mass of copper measured by fluorescent X-ray analysis. Department or above.

Alternatively, the negative electrode current collector 22 includes a copper layer 22a and a nickel layer 22b laminated on the surface of the copper layer 22a. The oxide film 23 is provided on the surface of the nickel layer 22b. The content of magnesium measured by fluorescent X-ray analysis targeting the portion of the surface of nickel layer 22b where oxide film 23 is provided is 100 parts by mass of the total mass of copper and nickel measured by fluorescent X-ray analysis. 1 part by mass or more. According to each of the above configurations, the effect of improving the capacity retention rate of the power storage device can be significantly obtained.

(4) The copper layer 22a is a plated layer. The negative electrode active material layer 40 contains a negative electrode active material capable of intercalating and deintercalating lithium ions.
When the copper layer 22a is a plated layer, pinholes are easily formed in the copper layer 22a because the copper layer 22a is formed thin. If there is a pinhole in the copper layer 22a, lithium ions passing through the pinhole react with the aluminum layer 21a, corroding the aluminum layer 21a. When the aluminum layer 21a corrodes, the voltage does not drop to the reaction potential of the negative electrode active material, and the operation of the power storage device becomes unstable at the reaction potential of the negative electrode active material. Moreover, there is a possibility that the corroded portion of the aluminum layer 21a may reach the opposite surface of the aluminum layer 21a and the bipolar current collector 20 may be perforated.

According to the above configuration, if a pinhole exists in the thin copper layer 22a formed by copper plating or the like, the oxide film 23 is also formed inside the pinhole. Magnesium oxide contained in the oxide film 23 has an activation energy smaller than the formation energy of lithium aluminum, which is a corrosion product, and is stable against a reduction reaction by lithium. Corrosion of aluminum layer 21a is suppressed by oxide film 23 located in the pinhole acting as a lithium corrosion-resistant film. As a result, deterioration in stability of operation of the power storage device at the reaction potential of the negative electrode active material is suppressed.

Examples that further embody the above embodiment will be described below.
(Preparation of bipolar current collector A)
A test piece for plating treatment was obtained by attaching an aluminum foil having a thickness of 15 μm to a SUS plate using a masking tape so that an aluminum surface of 70 mm×70 mm was exposed.

The exposed aluminum surface of the test piece was degreased by treating it with an aqueous solution of Top Alclean 101 from Okuno Pharmaceutical Co., Ltd. at 60°C for 5 minutes. After degreasing, the test piece was washed with water and etched by treating it with an aqueous solution of Top Alsoft 108 manufactured by Okuno Pharmaceutical Co., Ltd. at 55° C. for 1 minute. After the etching, the test piece was washed with water and treated with Top Desmut N-20 from Okuno Pharmaceutical Co., Ltd. for 1 minute at room temperature to remove the smut.

Next, the test piece was washed with water and treated with an aqueous solution of Okuno Pharmaceutical Substar ZN-291 at room temperature for 1 minute to replace the surface with zinc. The test piece after substitution was washed with water, and the zinc was stripped off once with an aqueous nitric acid solution, and then treated with an aqueous solution of Substar Zn-291 at room temperature for 1 minute to perform zinc substitution again.

A copper plating bath was prepared by making copper sulfate 150 g/L, sulfuric acid 150 g/L, and hydrochloric acid 80 ppm, and adding Top Lucina SF Base WR, Top Lucina SF-B, and Top Lucina SF Leveler manufactured by Okuno Seiyaku Co., Ltd. as additives. A test piece was immersed in the prepared plating bath, and copper plating was formed on the surface of the test piece using phosphorous copper as an anode at room temperature at a current density of 2 A/dm ² for 2 minutes. A bipolar current collector A was obtained by washing the plated test piece with water and applying an antirust treatment using Okuno Seiyaku Co., Ltd. Top Rinse.

(Test example 1)
After adding 2 g of magnesium ethoxide and 1.05 g of acetic acid to 9 ml of water and performing heat treatment at 60° C. for 2 hours, a chemical conversion treatment liquid A was obtained by filtering the precipitate. A first heat treatment was performed at 60° C. for 5 minutes while the bipolar current collector A was immersed in the chemical conversion treatment solution A. After the first heat treatment, the bipolar current collector A was washed with water and subjected to a second heat treatment at 120° C. for 1 hour to obtain a bipolar current collector of Test Example 1.

(Test example 2)
After adding 2 g of magnesium acetate, 50 mg of sodium hydroxide and 77 mg of boric acid to 8 ml of water and performing heat treatment at 60° C. for 2 hours, a chemical conversion treatment liquid B was obtained by filtering the precipitate. A first heat treatment was performed at 60° C. for 5 minutes while the bipolar current collector A was immersed in the chemical conversion treatment solution B. After the first heat treatment, the bipolar current collector A was washed with water and subjected to a second heat treatment at 120° C. for 1 hour to obtain a bipolar current collector of Test Example 2.

(Test example 3)
After adding 2 g of magnesium acetate and 50 mg of calcium chloride to 8 ml of water and performing heat treatment at 60° C. for 2 hours, a chemical conversion treatment liquid C was obtained by filtering the precipitate. A first heat treatment was performed at 60° C. for 5 minutes while the bipolar current collector A was immersed in the chemical conversion treatment solution C. The bipolar current collector A of Test Example 3 was obtained by washing the bipolar current collector A after the first heat treatment with water and performing the second heat treatment at 120° C. for 1 hour.

(Test example 4)
After adding 2 g of magnesium acetate to 8 ml of water and performing heat treatment at 60° C. for 2 hours, a chemical conversion treatment liquid D was obtained by filtering the precipitate. Further, an alkaline treatment liquid was obtained by adding 100 mg of sodium hypophosphite to 10 ml of a 3% by mass sodium hydroxide aqueous solution.

The bipolar current collector A was impregnated with the alkaline treatment liquid and heat-treated at 50°C for 2 minutes to obtain an alkaline-treated bipolar current collector A. This bipolar current collector A was immersed in the chemical conversion treatment solution D and heat-treated at 60° C. for 2 minutes to obtain a bipolar current collector of Test Example 4.

(Test Example 5)
A bipolar current collector A that was not subjected to chemical conversion treatment was used as Test Example 5.
(Fluorescent X-ray analysis and measurement of resistance)
Each mass of magnesium (Mg) and copper (Cu) was measured by performing fluorescent X-ray analysis on the chemically treated portions of the bipolar current collectors of Test Examples 1 to 5. Then, the content ratio of magnesium to 100 parts by mass of copper was calculated. In addition, the resistance of the chemically treated surfaces of the bipolar current collectors of Test Examples 1 to 5 was measured using a four-probe method. Those results are shown in Table 1. In addition, "n.d." in the component ratio column of Table 1 indicates non-detection.

From the results of the fluorescent X-ray analysis, it can be confirmed that an oxide film containing magnesium oxide was formed in Test Examples 1 to 4 in which chemical conversion treatment was performed. By varying the composition of the chemical conversion treatment solution, the content of magnesium oxide in the oxide film could be adjusted. Moreover, the resistances of Test Examples 1 to 4 in which an oxide film was formed were comparable to Test Example 5 in which an oxide film was not formed. Based on these results, it is considered that there is no or very little decrease in electrical conductivity due to the provision of the oxide film.

(Preparation of bipolar current collector B)
A test piece for plating treatment was obtained by attaching an aluminum foil having a thickness of 15 μm to a SUS plate using a masking tape so that an aluminum surface of 70 mm×70 mm was exposed.

A copper plating bath was prepared by preparing a bath containing 150 g/l of copper sulfate, 150 g/l of sulfuric acid, and 80 ppm of hydrochloric acid, and adding Top Lucina SF Base WR, Top Lucina SF-B, and Top Lucina SF Leveler manufactured by Okuno Seiyaku Co., Ltd. as additives. A test piece was immersed in the prepared plating bath, and copper plating was formed on the surface of the test piece using phosphorous copper as an anode at room temperature at a current density of 2 A/dm ² for 5 minutes. A bipolar current collector B was obtained by washing the plated test piece with water and applying an anticorrosion treatment using Okuno Seiyaku Co., Ltd. Top Rinse.

(Preparation of bipolar current collector C)
The steps up to the step of forming the copper plating were performed in the same manner as in the preparation of the bipolar current collector B described above, and then the test piece was washed with water. The test piece was immersed in a plating bath prepared to have 250 g/l nickel sulfate, 50 g/l nickel chloride, and 50 g/l boric acid, and the test piece was immersed in a current density of 1 A/dm ² at room temperature for 2 minutes. Nickel plating was formed on the surface. A bipolar current collector C was obtained by washing the plated test piece with water.

(Test Example 6 and Test Example 7)
With the bipolar current collector B immersed in the chemical conversion treatment solution B, a first heat treatment was performed at 60° C. for 5 minutes. The bipolar current collector B after the first heat treatment was washed with water and subjected to a second heat treatment at 120° C. for 1 hour to obtain a bipolar current collector of Test Example 6. A bipolar current collector B that was not subjected to chemical conversion treatment was used as Test Example 7.

(Test Example 8 and Test Example 9)
With the bipolar current collector C immersed in the chemical conversion treatment solution B, the first heat treatment was performed at 60° C. for 5 minutes. The bipolar current collector C after the first heat treatment was washed with water and subjected to a second heat treatment at 120° C. for 1 hour to obtain a bipolar current collector of Test Example 8. A bipolar current collector C that was not subjected to chemical conversion treatment was used as Test Example 9.

(Fluorescent X-ray analysis)
By performing fluorescent X-ray analysis on the chemically treated portions of the bipolar current collectors of Test Examples 6 to 9, each mass of magnesium (Mg), copper (Cu), and nickel (Ni) was determined. It was measured. Then, the content ratio of magnesium to 100 parts by mass of the total mass of copper and nickel was calculated. Those results are shown in Table 2.

(Measurement of capacity retention rate and peel strength)
A negative electrode slurry was prepared by mixing 95 parts by mass of graphite, 2.5 parts by mass of carboxymethyl cellulose, and 2.5 parts by mass of styrene-butadiene rubber with water. Next, test pieces of 40 mm×40 mm were cut out from the bipolar current collectors of Test Examples 6-9. A negative electrode active material layer was formed by applying the negative electrode slurry to a center area of 15 mm×15 mm on the plated side of the cut test piece and drying it. The negative electrode slurry was applied so as to have a basis weight of 5 mg/mm ² after drying. The test piece on which the negative electrode active material layer was formed was pressed with a linear pressure of 1.5 N/mm, and then heat-treated at 120° C. for 6 hours to obtain an electrode sheet for evaluation. In addition, below, the surface of the electrode sheet on which the negative electrode active material layer is provided is referred to as the negative electrode side surface.

On the negative electrode side surface of the electrode sheet, a square frame-shaped acid-modified polypropylene sheet covering 10 mm from the end of each side of the electrode sheet and protruding 5 mm from each side was stacked and heat-sealed by heating. A SUS foil cut into a size of 50 mm × 50 mm was overlapped on the positive electrode side of the electrode sheet and heated, so that the portion of the acid-modified polypropylene sheet protruding 5 mm from each side of the electrode sheet was heated to the SUS foil. Welded. As a result, a test electrode was obtained in which the aluminum surface of the electrode sheet was treated so as not to come into contact with the electrolytic solution.

A half cell was fabricated with the obtained test electrode as the negative electrode and the metallic lithium foil as the positive electrode. A separator made of polyethylene was used as the separator. As the electrolyte, a non-aqueous electrolyte was used in which lithium hexafluorophosphate was dissolved to a concentration of 1M in a mixed solvent of ethylene carbonate and diethyl carbonate at a volume ratio of 1:1. The resulting half-cell was discharged at a DC current of 0.2 mA until the voltage of the negative electrode with respect to the positive electrode reached 0.01 V. Ten minutes after the discharge, the positive electrode at the negative electrode was discharged at a DC current of 0.2 mA. Charging was carried out until the voltage to the electrodes reached 0.5V. The above discharge and charge were repeated for 30 cycles. Then, the capacity retention rate was calculated as the ratio of the charge capacity after 30 cycles to the charge capacity after the first charge being 100.

Next, the test electrode was taken out from the half cell after 30 cycles of discharging and charging, cut into strips with a width of 10 mm, and peeled off to obtain a measurement sample for the peel test. The peel strength of the measurement sample was measured by performing a 90-degree peel test by fixing the negative electrode active material layer of the test sample with a double-sided tape and peeling off the current collector portion. Table 2 shows the results of the obtained capacity retention rate and peel strength.

From the results of the fluorescent X-ray analysis, it was found that an oxide film containing magnesium oxide was formed in Test Examples 6 and 8 in which chemical conversion treatment was performed, and Test Examples 7 and 7 in which chemical conversion treatment was not performed. 9, it can be confirmed that no oxide film is formed. Then, in Test Examples 6 and 8, in which an oxide film was formed, the capacity retention ratio and the peel strength increased as compared with Test Examples 7 and 9, in which no oxide film was formed. This result indicates that the provision of the oxide film improves the peel strength between the negative electrode active material and the negative electrode current collector, thereby improving the capacity retention rate. In addition, from the above results, it can be seen that the effect of improving the capacity retention rate and the peel strength can be obtained both when an oxide film is formed on the surface of the copper plating and when the oxide film is formed on the surface of the nickel plating. I understand.

(Test Example 10 and Test Example 11)
Using a focused ion beam device, the bipolar current collector B was gradually etched over a 10 μm square range of the copper-plated portion. By stopping the etching when the underlying aluminum foil was observed, a bipolar current collector with pinholes in which artificial pinholes were formed in the copper-plated portion was obtained.

A first heat treatment was performed at 60°C for 5 minutes while the bipolar current collector with pinholes was immersed in the chemical conversion treatment solution B. The bipolar current collector B after the first heat treatment was washed with water and subjected to a second heat treatment at 120° C. for 1 hour to obtain a bipolar current collector of Test Example 10. Further, Test Example 11 was a bipolar current collector with pinholes that was not subjected to chemical conversion treatment.

(Fluorescent X-ray analysis)
Each mass of magnesium (Mg) and copper (Cu) was measured by performing fluorescent X-ray analysis on the chemically treated portions of the bipolar current collectors of Test Examples 10 and 11. Then, the content ratio of magnesium to 100 parts by mass of copper was calculated. Those results are shown in Table 3.

(Electrochemical test)
A negative electrode (evaluation electrode) formed by cutting the bipolar current collectors of Test Examples 10 and 11 into a circle with a diameter of 11 mm, and a positive electrode formed by cutting a metal lithium foil with a thickness of 500 μm into a circle with a diameter of 13 mm. A separator was sandwiched between them to form an electrode battery. A half cell for an electrochemical test was obtained by housing the electrode body battery in a battery case, injecting a non-aqueous electrolyte, and sealing the battery case. As a separator, a glass filter manufactured by Hoechst Celanese was used. As the nonaqueous electrolyte, a nonaqueous electrolyte obtained by dissolving lithium hexafluorophosphate to a concentration of 1M in a mixed solvent of ethylene carbonate and diethyl carbonate at a volume ratio of 1:1 was used.

The obtained half-cell for electrochemical test was swept to 0 V as the counter electrode lithium potential at a current of 0.1 mA. After the voltage reached 0 V, a current of 0.1 mA and a voltage of 0 V were applied by CC-CV for 3 hours. Table 3 shows the voltage reached during the sweep, the holding time during which the voltage of 0 V was held, and the CC-CV voltage after 3 hours.

From the results of the fluorescent X-ray analysis, it was confirmed that an oxide film containing magnesium oxide was formed in Test Example 10, and that an oxide film was not formed in Test Example 11.

In Test Example 11 in which no oxide film was formed, although the final voltage reached 0 V during sweeping, the voltage could not be maintained at 0 V after 90 minutes, and the CC-CV voltage after 3 hours was 0.0 V. It was 11V. From this result, in the case of Test Example 11, after reaching 0 V, lithium ions reached the aluminum foil through pinholes in the copper plating after a while, and the lithium ions corroded the aluminum foil. On the other hand, in Test Example 10 in which an oxide film was formed, the state of 0 V was maintained during the measurement time of 3 hours. From this result, it can be seen that the provision of an oxide film containing magnesium oxide has the effect of suppressing corrosion of the aluminum foil caused by pinholes in the copper plating.

Claims

a bipolar current collector comprising a positive electrode current collector having an aluminum layer and a negative electrode current collector having a copper layer;
a positive electrode active material layer provided on the surface of the positive electrode current collector;
and a negative electrode active material layer provided on the surface of the negative electrode current collector,
A bipolar electrode, wherein an oxide film containing magnesium oxide is provided on a portion of the surface of the negative electrode current collector to which the negative electrode active material layer is adhered.
2. The bipolar electrode according to claim 1, wherein the content of magnesium measured by fluorescent X-ray analysis of the portion of the surface of the negative electrode current collector provided with the oxide film is 1% by mass or more. .
The oxide film is provided on the surface of the copper layer,
The content of magnesium measured by fluorescent X-ray analysis targeting the portion of the surface of the copper layer where the oxide film is provided is 1 per 100 parts by mass of copper measured by the fluorescent X-ray analysis. 3. The bipolar electrode according to claim 1 or 2, which is greater than or equal to parts by mass.
The negative electrode current collector includes the copper layer and a nickel layer laminated on the surface of the copper layer,
The oxide film is provided on the surface of the nickel layer,
The content of magnesium measured by fluorescent X-ray analysis targeting the portion of the surface of the nickel layer provided with the oxide film is the total mass of copper and nickel measured by the fluorescent X-ray analysis 100 mass 3. The bipolar electrode according to claim 1 or 2, which is 1 part by mass or more per part.
The copper layer is a plated layer,
The bipolar electrode according to any one of claims 1 to 4, wherein the negative electrode active material layer contains a negative electrode active material capable of intercalating and deintercalating lithium ions.
A power storage device comprising the bipolar electrode according to any one of claims 1 to 5.