JP6948586B2 - Power storage element - Google Patents

Description

The present invention relates to a power storage element such as a nickel metal hydride storage battery.

Conventionally, a nickel-metal hydride storage battery containing a positive electrode having nickel hydroxide as an active material, a negative electrode having a hydrogen storage alloy, and an electrolytic solution is known (for example, Patent Document 1).

In the battery described in Patent Document 1, the hydrogen storage alloy of the negative electrode contains a misch metal containing at least 60 to 90 wt% La, Ni, Co, Mn, and an atomic ratio of 0.015 to 0 with respect to the misch metal. It is a rare earth hydrogen storage alloy containing Mg in the range of 1 as a constituent element. The rare earth hydrogen storage alloy is surface-treated. In addition, the electrolytic solution contains a zinc compound.

Japanese Unexamined Patent Publication No. 2001-291510

In the battery described in Patent Document 1, the capacity may decrease or the internal resistance may increase after repeated charging and discharging.
An object of the present embodiment is to provide a power storage element in which both a decrease in capacity and an increase in internal resistance after repeated charging and discharging are suppressed.

The power storage element of the present embodiment includes a separator having fibers having a thickness of 4 μm or less and an electrolytic solution containing zinc. With the above configuration, both the decrease in capacitance and the increase in internal resistance after repeated charging and discharging are suppressed.

According to the present embodiment, it is possible to provide a power storage element in which both the capacity decrease and the internal resistance increase after repeated charging and discharging are suppressed.

FIG. 1 is a cross-sectional view of a power storage element (nickel-metal hydride storage battery) according to the present embodiment.

Hereinafter, an embodiment of the power storage element according to the present invention will be described with reference to FIG. The power storage element includes a secondary battery. In the present embodiment, a rechargeable secondary battery will be described as an example of the power storage element. The name of each component (each component) of the present embodiment is that of the present embodiment, and may be different from the name of each component (each component) in the background technology.

The power storage element 1 of the present embodiment is an alkaline storage battery containing an alkaline electrolytic solution. More specifically, the power storage element 1 is a nickel-metal hydride storage battery that has a nickel compound as an active material for a positive electrode and uses a hydrogen storage alloy as an active material for a negative electrode. This type of power storage element 1 supplies electrical energy. The power storage element 1 is used alone or in a plurality. Specifically, the power storage element 1 is used alone when the required output and the required voltage are small. On the other hand, when at least one of the required output and the required voltage is large, the power storage element 1 is used in a power storage device (module) in combination with another power storage element 1. In the power storage device, the power storage element 1 used in the power storage device supplies electrical energy.

As shown in FIG. 1, the power storage element 1 includes an electrode group 2 including a positive electrode 21, a negative electrode 22, and a separator 23, and a case 3 accommodating the electrode group 2. Further, the power storage element 1 includes, in addition to the electrode group 2 and the case 3, an electrolytic solution and a connecting member 5 and the like for conducting the positive electrode 21 of the electrode group 2 and a part of the case 3 inside the case.

The electrode group 2 is formed by winding a laminate in which the positive electrode 21 and the negative electrode 22 are laminated with each other insulated by the separator 23.

The positive electrode 21 has a current collector plate and a mixture adhering to the current collector plate. The current collector plate is, for example, a metal plate such as nickel foam. The mixture contains, for example, a particulate active material, a conductive agent, and the like. In the present embodiment, the positive electrode 21 is formed by applying a mixture to a foamed nickel plate as a current collector plate. The thickness of the positive electrode 21 is usually 300 μm or more and 1200 μm or less.

The current collector plate of the positive electrode 21 of the present embodiment is, for example, a foam plate made of nickel. The current collector plate is strip-shaped.

The positive electrode mixture contains a particulate active material (active material particles), a particulate conductive agent, and a binder. The mass of the mixture per unit area of the positive electrode is usually 100 mg / cm ² or more and 500 mg / cm ² or less.

The positive electrode mixture contains particles containing nickel hydroxide as active material particles. Since a part of nickel hydroxide changes to nickel oxyhydroxide with charging, the above particles may contain nickel oxyhydroxide. The particle size of the particles is usually 5 μm or more and 15 μm or less. The positive electrode mixture usually contains 95% by mass or more and 99% by mass or less of the active material.

The positive electrode mixture contains, for example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), ethylene-propylene-dienter polymer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), and the like as binders. do. The positive electrode mixture usually contains a binder in an amount of 0.1% by mass or more and 0.8% by mass or less. The positive electrode mixture may contain a cellulose derivative such as methyl cellulose or a polysaccharide such as xanthan gum as a thickener for the mixture during production.

The positive electrode mixture contains a carbonaceous material, a metal material, or the like as a conductive agent. Examples of the carbonaceous material include graphite such as natural graphite and artificial graphite, carbon black such as Ketjen black (registered trademark) and acetylene black, carbon whisker, carbon fiber, and vapor-grown carbon fiber. Examples of the metal material include nickel, gold, cobalt compounds and the like. The positive electrode mixture usually contains 0% by mass or more and 5% by mass or less of the conductive agent. The conductive agent is usually in the form of particles.

The negative electrode 22 has a current collector plate and a mixture adhering to the current collector plate. The current collector plate is, for example, a metal plate (perforated steel plate or the like) having a plurality of holes in the thickness direction. The mixture contains, for example, a particulate hydrogen storage alloy, a conductive agent, and the like. In the present embodiment, the negative electrode 22 is formed by applying a mixture to a perforated steel plate as a current collector plate. The thickness of the negative electrode 22 is usually 200 μm or more and 800 μm or less.

The current collector plate of the negative electrode 22 of the present embodiment is, for example, a nickel-plated perforated steel plate.

The mixture of the negative electrode contains a particulate hydrogen storage alloy, a particulate conductive agent, and a binder. The mass of the negative electrode mixture per unit area is usually 100 mg / cm ² or more and 500 mg / cm ² or less.

The negative electrode mixture contains, for example, particles of a hydrogen storage alloy containing a rare earth element. The hydrogen storage alloy is, for example, an alloy such as AB ₅ type (rare earth-Ni type), AB _3.0-3.8 type (rare earth-Mg-Ni type) or AB ₂ type (Laves phase). As the hydrogen storage alloy, for example, an alloy having a composition of _{Mm 1.0} Ni _4.0 Co _0.7 Al _0.3 Mn _{0.3 is adopted.} Mm is a misch metal [lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), etc.] which is a mixture of rare earth elements. The particle size of the hydrogen storage alloy is usually 30 μm or more and 60 μm or less. The negative electrode mixture usually contains 95% by mass or more and 99% by mass or less of a hydrogen storage alloy.

The particles of the hydrogen storage alloy are preferably those treated with an alkaline aqueous solution. As the alkaline aqueous solution (treatment solution), it is preferable to use any one of potassium hydroxide (KOH), sodium hydroxide (NaOH), lithium hydroxide (LiOH) or a mixed solution. The concentration of the alkaline aqueous solution is preferably 7 mol / L or more and 12 mol / L or less, the temperature in the treatment is preferably 80 ° C. or more and less than the boiling point, and the treatment time is 1 hour or more and 4 hours or less. preferable. In particular, when the hydrogen storage alloy is AB _3.0-3.8 type (rare earth-Mg-Ni-Al type) and does not contain Co, which is easily corroded by alkali, an alkaline aqueous solution containing KOH has a concentration of 10 mol / L or more. It is preferable that the treatment time is 1 hour or more at a temperature of 110 ° C. or higher using the alkaline aqueous solution of. Under such treatment conditions, a dense Ni layer can be formed on the particle surface of the hydrogen storage alloy, whereby the corrosion resistance of the hydrogen storage alloy can be improved and the cycle life can be improved.

The negative electrode mixture contains the same binder as the positive electrode mixture binder described above. The negative electrode mixture usually contains a binder of 0.1% by mass or more and 2.0% by mass or less. The negative electrode mixture may contain a cellulose derivative such as methyl cellulose or a polysaccharide such as xanthan gum as a thickener of the mixture at the time of production. The negative electrode mixture usually contains a thickener in an amount of 0.01% by mass or more and 0.5% by mass or less.

The negative electrode mixture contains the same conductive agent as the positive electrode mixture conductive agent described above. The negative electrode mixture usually contains 0% by mass or more and 5% by mass or less of the conductive agent. The conductive agent is usually in the form of particles.

In the electrode group 2 of the present embodiment, the positive electrode 21 and the negative electrode 22 configured as described above are wound in a state of being insulated by the separator 23. That is, in the electrode group 2 of the present embodiment, the laminated body of the positive electrode 21, the negative electrode 22, and the separator 23 is wound. The separator 23 is a member having an insulating property. The separator 23 is arranged between the positive electrode 21 and the negative electrode 22. As a result, in the electrode group 2 (specifically, the laminated body), the positive electrode 21 and the negative electrode 22 are insulated from each other. Further, the separator 23 holds the electrolytic solution in the case 3.

The separator 23 is strip-shaped. The separator 23 is arranged between the positive electrode 21 and the negative electrode 22 in order to prevent a short circuit between the positive electrode 21 and the negative electrode 22.

The separator 23 has at least fibers having a thickness of 4 μm or less (hereinafter, also referred to as ultrafine fibers). The thickness of the ultrafine fibers is usually 1 μm or more. The thickness of the ultrafine fibers may be 2 μm or more. Further, the thickness of the ultrafine fibers may be 3 μm or less.
Since the separator 23 contains relatively fine ultrafine fibers, the number of contacts between the fibers increases as the fiber thickness is thin, and the separator 23 can sufficiently hold the electrolytic solution. Since the separator 23 can hold the electrolytic solution more sufficiently, it is considered that the decrease in capacity and the increase in internal resistance are suppressed even after repeated charging and discharging.

The separator 23 may have fibers having a thickness of 4 μm or less (ultrafine fibers) and fibers having a thickness of 10 μm or more (hereinafter, also referred to as ordinary fibers). Usually, the thickness of the fiber is preferably 20 μm or less. Usually, the thickness of the fiber may be 10 μm or more and 25 μm or less.

The thickness of each fiber in the separator 23 is calculated by observing the separator 23 with a scanning electron microscope or an optical microscope and calculating the thickness of each fiber from the magnification of the observation image based on the thickness measured in the observation image. It is determined by doing. The thickness of each fiber is determined by measuring the thickness of five randomly selected points other than the tip of one fiber and averaging them.

The mass ratio of the ultrafine fibers to the normal fibers in the separator 23 may be 5% or more and 40 % or less.

The separator 23 is porous. The separator 23 is, for example, a woven fabric or a non-woven fabric. Examples of the material of the separator 23 include polymer compounds, glass, and ceramics. Examples of the polymer compound include polyesters such as polyacrylonitrile (PAN), polyamide (PA) and polyethylene terephthalate (PET), and polyolefins (PO) such as polypropylene (PP) and polyethylene (PE).

The case 3 has a case main body 31 having a circular opening and a disk-shaped lid portion 32 that closes (closes) the opening of the case main body 31. The case 3 further has a gasket 33 arranged between the opening peripheral edge of the case body 31 and the peripheral edge of the lid 32. The case 3 accommodates the electrode group 2 and the electrolytic solution in the internal space. The case 3 is formed of a metal that is resistant to the electrolytic solution. Examples of the material of the case 3 include iron-based metal materials such as iron and steel. The case 3 is formed of, for example, a metal plate whose surface is nickel-plated or zinc-plated.

The gasket 33 is formed of, for example, an insulating synthetic resin. The case 3 is formed by joining the opening peripheral edge of the case body 31 and the peripheral edge of the disk-shaped lid 32 in a state of being overlapped with each other via the gasket 33. The gasket 33 electrically insulates the case body 31 and the lid 32. Further, the case 3 has an internal space defined by the case main body 31 and the lid portion 32. In the present embodiment, the peripheral edge of the opening of the case body 31 and the peripheral edge of the lid 32 are joined by caulking.

The case body 31 has a cylindrical shape (that is, a bottomed cylindrical shape) in which the end portion in the direction opposite to the opening direction is closed. The case main body 31 has a hollow cylindrical side wall portion and a disk-shaped bottom portion that closes one opening of the side wall portion (the opening on the opposite side of the opening of the case main body 31). Inside the case body 31, the electrode group 2 is housed so that the winding axis direction of the electrode group 2 and the cylindrical axis direction of the side wall portion are in the same direction. The bottom of the case body 31 functions as a negative electrode terminal by electrically connecting the negative electrode 22.

The lid portion 32 is a disk-shaped member that closes the opening of the case body 31. The central portion of the lid portion 32 projects outward. The protruding portion functions as a positive electrode terminal by being electrically connected to the positive electrode 21 via an elastic connecting member 5.

The lid portion 32 has a safety valve 321 capable of discharging the gas in the case 3 to the outside. The safety valve 321 discharges gas from the inside of the case 3 to the outside when the internal pressure of the case 3 rises to a predetermined pressure.

The electrolytic solution is an alkaline (for example, pH 14 to 15) aqueous solution. The electrolyte is obtained by dissolving the electrolyte in water. The electrolyte is an alkali metal hydroxide such as potassium hydroxide or sodium hydroxide. The electrolytic solution of the present embodiment is prepared by dissolving potassium hydroxide, sodium hydroxide or the like in water so that the concentration of the alkali metal is 6 to 9M.

The electrolytic solution contains zinc. Specifically, the electrolytic solution contains zinc dissolved in the electrolytic solution. The dissolved zinc is usually in an ionic state (Zn ²⁺ , [Zn (OH) ₄ ] ^2-, etc.). For example, the electrolytic solution is prepared by dissolving zinc oxide, zinc hydroxide, or the like.
Since the electrolytic solution contains zinc, it is possible to prevent the β-oxynickel hydroxide of the positive electrode 21 from being changed to γ-oxynickel hydroxide. When β-oxynickel hydroxide is converted to a γ-form, the swelling of the positive electrode active material becomes relatively large, and the capacity may decrease or the internal resistance may increase after repeated charging and discharging. On the other hand, zinc in the electrolytic solution suppresses the conversion of β-oxynickel hydroxide to γ-form, so that the capacity decreases and the internal resistance increases after repeated charging and discharging. Can be suppressed.
Further, since the electrolytic solution contains zinc, it is possible to prevent the hydrogen storage alloy of the negative electrode 22 from corroding. This also makes it possible to prevent the capacitance from decreasing and the internal resistance from increasing after repeated charging and discharging.

The electrolytic solution may contain zinc of 0.05 mol / L or more and 1.0 mol / L or less in terms of Zn. The Zn conversion is to convert the zinc content in the electrolytic solution into the amount of zinc element (Zn).

Next, a method of manufacturing the power storage element 1 of the above embodiment will be described.

In the method for manufacturing the power storage element 1, first, the positive electrode 21 is manufactured by applying a mixture containing an active material to a current collector plate such as a nickel foam plate. Further, the negative electrode 22 is manufactured by applying a mixture containing an active material to a current collector plate such as a perforated steel plate. Next, the positive electrode 21, the separator 23, and the negative electrode 22 are superposed to form the electrode group 2. Subsequently, the electrode group 2 is put into the case 3, and the electrolytic solution is put into the case 3 to assemble the power storage element 1.

In the production of the positive electrode 21, the current collector plate is coated and filled with a mixture containing an active material. The mixture usually further contains a rare earth oxide such as Y or Yb, a binder and a solvent (water). By changing the coating amount of the mixture, the mass of the mixture per unit area can be adjusted. As a method of applying the mixture, a general method is adopted. Then, the water in the mixture is volatilized by the drying treatment.

In the production of the negative electrode 22, a mixture containing an active material is applied to the current collector plate. The mixture usually further contains a conductive agent, a binder and a solvent (water). By changing the coating amount of the mixture, the mass of the mixture per unit area can be adjusted. As a method of applying the mixture, a general method is adopted. Then, the water in the mixture is volatilized by the drying treatment. As the separator 23, a commercially available one can be used.

In the formation of the electrode group 2, the electrode group 2 is formed by winding a laminate in which the separator 23 is sandwiched between the positive electrode 21 and the negative electrode 22. Specifically, the positive electrode 21, the separator 23, and the negative electrode 22 are superposed to form a laminated body. The laminate is wound to form the electrode group 2.

The electrolytic solution can be prepared, for example, by dissolving potassium hydroxide, sodium hydroxide or the like in water, and further dissolving a zinc compound such as zinc oxide. The composition of the electrolytic solution can be confirmed by analysis using ICP or an ion chromatograph.

In assembling the power storage element 1, the electrode group 2 is put into the case main body 31 of the case 3, and the electrolytic solution is put into the case main body 31. By caulking, the opening of the case body 31 is closed with the lid portion 32.

The power storage element 1 of the present embodiment configured as described above includes a separator 23 having fibers having a thickness of 4 μm or less, and an electrolytic solution containing zinc. With such a configuration, both a decrease in capacitance and an increase in internal resistance after repeated charging and discharging are suppressed.

The power storage element of the present invention is not limited to the above embodiment, and it goes without saying that various modifications can be made without departing from the gist of the present invention. For example, the configuration of one embodiment can be added to the configuration of another embodiment, and a part of the configuration of one embodiment can be replaced with the configuration of another embodiment. In addition, some of the configurations of certain embodiments can be deleted.

In the above embodiment, the power storage element 1 including the electrode group 2 in which the laminated body is wound has been described in detail, but the power storage element of the present invention may include the laminated body that is not wound. Specifically, the power storage element may include an electrode group in which a positive electrode, a separator, a negative electrode, and a separator each formed in a rectangular shape are stacked a plurality of times in this order. The shape of the power storage element 1 is not particularly limited, and is, for example, a cylindrical shape, a rectangular parallelepiped shape, a sheet shape, or the like.

In the above embodiment, the case where the power storage element 1 is used as a chargeable / dischargeable alkaline storage battery (for example, a nickel hydrogen storage battery) has been described, but the type and size (capacity) of the power storage element 1 are arbitrary. Further, in the above-described embodiment, the nickel-metal hydride storage battery has been described as an example of the power storage element 1, but the present invention is not limited to this.

The power storage element 1 (for example, a battery) may be used in a power storage device (a battery module when the power storage element is a battery). The power storage device includes at least two power storage elements 1 and a bus bar member that electrically connects two (different) power storage elements 1 to each other. In this case, the technique of the present invention may be applied to at least one power storage element.

An alkaline storage battery (nickel-metal hydride storage battery) was manufactured as shown below.

(Example 1)
(1) Preparation of positive electrode The surface of nickel hydroxide particles containing 3% by mass of zinc and 0.06% by mass of cobalt in a solid solution state is coated with 5% by mass of cobalt hydroxide, and further. A material subjected to air oxidation treatment at 110 ° C. for 1 hour using an 18M aqueous sodium hydroxide solution was used as the positive electrode active material. A paste was prepared by mixing an aqueous solution in which a thickener (carboxymethyl cellulose) was dissolved, an active material, and 1% by mass of itterbium oxide with respect to the active material to prepare a paste having a substrate surface density of 320 g / m ² . Nickel foam was filled with paste and dried. Then, a positive electrode plate of 2400 mAh was produced by pressing to a predetermined thickness (0.63 mm).

(2) Preparation of negative electrode A hydrogen storage alloy powder (La _0.62 Y _0.13 Mg _0.15 Ni _3.45 Al _0.15 composition) crushed so that the average particle size D50 is 60 μm is prepared at 110 ° C. The mixture was vigorously stirred in a 10 M potassium hydroxide aqueous solution for 2 hours. Then, washing with water was repeated until the pH reached 10. As a result of measuring the treated alloy powder with a vibrating sample magnetometer, the magnetization was 0.7 Am ² / kg. A paste was prepared by mixing 100 parts by mass of the dried powder with an aqueous solution in which a thickener (methyl cellulose) was dissolved, and further adding 1 part by mass of a binder (styrene butadiene rubber). The prepared paste was applied to both sides of a perforated steel plate (opening ratio 50%) having a thickness of 35 μm, dried, and then pressed to a thickness of 0.26 mm to prepare a negative electrode plate having a thickness of 3100 mAh.

(3) Separator As the separator, a non-woven fabric having ^{a basis weight of 40 g / m 2} , which had been subjected to a sulfonization treatment, was used. The separator contained PP fibers (ultrafine fibers) having a thickness of 2 to 3 μm and PP / PE fibers (normal fibers) having a thickness of 15 to 20 μm. The mass ratio of the ultrafine fibers to the normal fibers was 30%.

(4) Preparation of electrolytic solution As the electrolytic solution, the one prepared by the following method was used. KOH, NaOH, and LiOH were added to water as a solvent, and an electrolytic solution was prepared so that the concentration was K / Na / Li = 2.5 / 5 / 0.5 mol / L. Further, ZnO was added and dissolved so as to have a concentration of 0.5 mol / L, and the electrolytic solution was adjusted.

(5) Arrangement of Electrodes Group in Case Using the above positive electrode, the above negative electrode, the above electrolytic solution, the separator, and the case, a battery was manufactured by a general method.
First, a sheet-like material formed by arranging and laminating a separator between the positive electrode and the negative electrode was wound around. Next, the wound electrode group was placed in a hollow cylindrical case body in which iron was nickel-plated. Subsequently, the positive electrode was electrically connected to the lid of the case. On the other hand, the negative electrode was brought into contact with the side surface of the case (electric tank can) to energize. Further, the above electrolytic solution was placed in a 2.3 g case. Finally, a lid portion was attached to the case body and crimped to seal the case, and a battery of 2,400 mAh was produced.

(Comparative Example 1)
The battery was manufactured in the same manner as in Example 1 except that the battery was manufactured without adding zinc oxide to the electrolytic solution.

(Comparative Example 2)
The battery was manufactured in the same manner as in Example 1 except that the battery was manufactured using a separator having only ordinary fibers.

(Comparative Example 3)
The battery was manufactured in the same manner as in Example 1 except that the battery was manufactured without adding zinc oxide to the electrolytic solution and the battery was manufactured using a separator having only normal fibers.

(Initialization)
The batteries of Examples and Comparative Examples were initialized according to the following procedure. Charging was carried out at 20 ° C. under the condition of 0.1 ItA (240 mA) for 12 hours, then discharging was carried out under the condition of 0.2 ItA (480 mA) until the voltage reached 1 V, and this was repeated for 2 cycles. Then, it was stored at 40 ° C. for 48 hours. Then, it is charged at 20 ° C. under the condition of 0.1 ItA (240 mA) for 16 hours, paused for 1 hour, discharged under the condition of 0.2 ItA (480 mA) until it reaches 1 V, and this is repeated for 3 cycles to initialize. Was finished.

(Capacity measurement and internal resistance after 100 cycle test)
The battery was charged at −dV = 5 mV at 0.5 ItA (1200 mA) at 20 ° C., paused for 0.5 hours, discharged under the condition of 1 ItA (2400 mA) until it reached 1 V, and this was repeated for 100 cycles. Then, the discharge was carried out under the condition of 0.2 ItA (480 mA) until it became 1 V.
Then, the battery was charged at 20 ° C. under the condition of 0.1 ItA (240 mA) for 16 hours, paused for 1 hour, discharged to 1 V under the condition of 0.2 ItA (480 mA), and the discharge capacity was measured. Further, the internal resistance at 1 kHz was measured at 20 ° C. using a contact resistance tester of the battery in the discharged state.

(Cycle life)
At 20 ° C., charge −dV = 5 mV at 0.5 ItA (1200 mA), rest for 0.5 hours, and discharge at 1 ItA (2400 mA) until it reaches 1 V, which is 60% of the discharge capacity in the first cycle. The number of cycles at that time was defined as the cycle life.

As can be seen from Table 1, in the battery of the example, both the decrease in capacity and the increase in internal resistance after repeated charging and discharging were remarkably suppressed. In addition, the cycle life of the batteries of the examples was significantly longer than the cycle life of the batteries of the comparative examples.

1: Power storage element (nickel-metal hydride storage battery),
2: Electrode group,
21: Positive electrode, 22: Negative electrode, 23: Separator,
3: Case, 31: Case body, 32: Cover.

Claims (2)

Positive electrode and negative electrode,
A separator arranged between the positive electrode and the negative electrode,
Containing with an electrolytic solution containing zinc,
The positive electrode contains particles containing nickel oxyhydroxide as active material particles.
The negative electrode contains particles of a hydrogen storage alloy containing a rare earth element as active material particles.
The separator is a power storage element having only polypropylene fibers having a thickness of 1 μm or more and 4 μm or less and polyolefin fibers having a thickness of 15 μm or more and 20 μm or less as fibers. The power storage element according to claim 1, wherein the separator has polypropylene fibers having a thickness of 2 μm or more and 3 μm or less as the polypropylene fibers having a thickness of 1 μm or more and 4 μm or less.

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