WO2018133429A1 - Method for modifying current collector, current collector, and energy storage device - Google Patents

Abstract

A method for modifying a current collector, a current collector, and an energy storage device, for use in solving the problem in the art that there are only a small number of contact points between a current collector and an electrode. The method comprises: growing, on a base of the current collector, a carbon nano tube array which is perpendicular to the base of the current collector; and growing a graphene layer, which is perpendicular to the carbon nano tube array, on the base of the current collector which comprises the carbon nano tube array.

Description

Method for modifying current collector, current collector and energy storage device

The present application claims priority to the Chinese Patent Application, filed on Jan. 23, 2017, with the application number of 201710058842.6, entitled "A method for modifying a current collector, a current collector and an energy storage device", The entire contents are incorporated herein by reference.

Technical field

The present application relates to the field of microelectronics, and in particular, to a method, a current collector and an energy storage device for modifying a current collector.

Background technique

In an energy storage device such as a battery or a supercapacitor, the current collector functions to conduct the conduction of the electric charge stored in the electrode. However, since the surface roughness of the electrode material is large and the surface of the current collector is relatively smooth, there is only a small amount of contact point between the current collector material and the electrode material, thereby forming a shrinkage diffusion resistance at the interface between the two, hindering the charge transfer. And collecting to reduce the power density of the energy storage device.

In the prior art, a three-dimensional foam-like current collector, such as nickel foam, is generally used, although the contact points of the current collector and the electrode can be increased to some extent, and the contact resistance is lowered, but between the current collector of the above structure and the electrode The number of contact points is still small, resulting in the power density of the energy storage device being difficult to meet the requirements.

Summary of the invention

The present application provides a method for modifying a current collector, a current collector, and an energy storage device for solving the problem of a small number of contact points between the current collector and the electrode in the prior art.

In a first aspect, the present application provides a method for modifying a current collector, first, growing an array of carbon nanotubes perpendicular to a current collector substrate on a current collector substrate, and then, collecting a current collector having the carbon nanotube array A graphene layer perpendicular to the carbon nanotube array is grown on the substrate, and a composite structure of the carbon nanotube and the graphene layer is formed on the current collector substrate, the composite structure can improve the surface roughness of the current collector, and increase the current collector and the electrode The contact point between the contacts reduces the contact resistance between the current collector and the electrode, thereby increasing the power density of the energy storage device.

In an alternative design of the first aspect, the carbon nanotubes perpendicular to the current collector substrate are grown on the current collector substrate by a chemical vapor deposition (CVD) process, a solid phase pyrolysis process, or a sputtering process. Array.

In an alternative design of the first aspect, a carbon nanotube array perpendicular to the current collector substrate is grown on the current collector substrate using a CVD process. First, catalyst particles are deposited on a current collector substrate. The catalyst particles are metals, metal compounds or alloys, acting as an active center for carbon source decomposition and a nucleation center and energy carrier for carbon nanotube growth. Then, the current collector substrate is heated, and a carbon source gas (such as ethylene) is used as a reaction gas by a CVD process, and the carbon nanotube array is grown on the current collector substrate under the catalytic action of the catalyst particles.

In an optional design of the first aspect, after the graphene layer is grown, the catalyst particles deposited on the current collector substrate are removed according to a wet etching process or a dry etching process to avoid residues. The catalyst particles adversely affect the conductivity of the current collector.

In an alternative design of the first aspect, after growing the carbon nanotube array perpendicular to the current collector substrate on the current collector substrate, the carbon source gas (eg, methane) is used in the set having the carbon nanotube array by a CVD process A graphene layer is grown on the surface of the fluid substrate, wherein the carbon nanotubes in the array of carbon nanotubes pass vertically through the graphene layer.

In a second aspect, the present application provides a current collector comprising: a current collector substrate disposed on the current collector substrate An array of carbon nanotubes on and perpendicular to the current collector substrate, and a graphene layer disposed on the array of carbon nanotubes and perpendicular to the array of carbon nanotubes. In the solution, the current collecting substrate has a composite structure of carbon nanotubes and a graphene layer, and the composite structure can improve the surface roughness of the current collector, increase the contact point between the current collector and the electrode, and thereby reduce the current collector and The contact resistance between the electrodes increases the power density of the energy storage device.

In an optional design of the first aspect or the second aspect, the thickness of the graphene layer perpendicular to the carbon nanotube array on the current collector substrate is 1 to 50 μm, for example, the thickness of the graphene layer is 5 μm, 10 μm. 20 μm, 30 μm, 40 μm or 50 μm. The graphene layer of the above thickness enables the composite structure of the carbon nanotube and the graphene layer to have good electrical conductivity, thermal conductivity, and large structural strength.

In an optional design of the first aspect or the second aspect, the mass ratio of the graphene layer to the carbon nanotube array on the current collector substrate is in the range of 10-15:1, for example, graphene layer and carbon nanometer. The mass ratio of the tube array is 10:1, 11:1, 12:1, 13:1, 14:1 or 15:1. The above mass ratio of the graphene layer to the carbon nanotubes makes the composite structure of the carbon nanotubes and the graphene layer have good electrical conductivity, thermal conductivity, and large structural strength.

In an alternative design of the first aspect or the second aspect, the array of carbon nanotubes grown on the current collector substrate comprises at least one of single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes.

In an optional design of the first aspect or the second aspect, the current collector substrate is any one of a carbon fiber substrate, a graphene substrate, a graphene intercalation compound substrate, a carbon nanotube substrate, a metal substrate, and an alloy substrate. .

In a third aspect, the present application provides an energy storage device including a first electrode, a second electrode, an electrolyte, a first current collector, and a second current collector; wherein the electrolyte is disposed on the first electrode and Between the two electrodes, charge is transferred between the first electrode and the second electrode through the electrolyte; the first current collector is disposed on the first electrode for deriving the charge on the first electrode; and the second current collector is disposed in the second On the electrode, for deriving the charge on the second electrode; the first current collector and/or the second current collector having the structure of the current collector according to the second aspect. Since the current collecting device of the energy storage device has a composite structure of carbon nanotubes and a graphene layer, the composite structure can improve the surface roughness of the current collector, increase the contact point between the current collector and the electrode, and further reduce the set. The contact resistance between the fluid and the electrode increases the power density of the energy storage device.

DRAWINGS

1 is a schematic flow chart of a method for modifying a current collector according to the present application;

2a-2b are schematic views of a process of modifying a current collector in the present application;

3 is a schematic structural view of an energy storage device provided by the application.

detailed description

In order to make the objects, technical solutions and advantages of the present application more clear, the present application will be further described in detail below with reference to the accompanying drawings.

The plurality referred to in the present application means two or more. In addition, it should be understood that in the description of the present application, the terms "first", "second" and the like are used only to distinguish the purpose of description, and are not to be understood as indicating or implying relative importance, nor as an indication. Or suggest the order. The term “and/or” in the present application is merely an association relationship describing an associated object, indicating that there may be three relationships, for example, A and/or B, which may indicate that A exists separately, and A and B exist simultaneously. There is a case of B.

Chemical vapor deposition (CVD) refers to introducing a vapor containing a gaseous reactant or a liquid reactant constituting an element of a target film and other gases required for the reaction into the reaction chamber to cause a chemical reaction on the surface of the substrate. The process of forming a film. Depending on the activation energy of the reaction, CVD can be further divided into microwave plasma assisted CVD (MWCVD), plasma enhanced chemical vapor deposition (PECVD), and the like.

Electron beam evaporation refers to a method in which a material is directly heated by evaporation of an electron beam under vacuum to vaporize an evaporation material and transport it to a substrate to form a film or a particle on a substrate.

FIG. 1 is a schematic diagram of a method for modifying a current collector according to the present application, comprising the following steps:

Step 401, growing a carbon nanotube array 20 perpendicular to the current collector substrate 10 on the current collector substrate 10.

2a shows a carbon nanotube (CNT) array 20 on a current collector substrate 10. The current collector substrate 10 may be a carbon fiber substrate (carbon fiber cloth, carbon fiber paper, carbon fiber film, etc.), a graphene substrate (graphene paper, graphene sponge, graphene foam, etc.), a graphene intercalation compound substrate, a carbon nanotube substrate, Any of a metal substrate, an alloy substrate, and the like. The material of the metal substrate may be any one of copper Cu, aluminum Al, nickel Ni, silver Ag, platinum Pt, gold Au, etc., and the metal substrate may be metal fiber cloth, metal mesh, metal film, foam metal, etc. The alloy substrate may be an alloy fiber cloth, an alloy mesh, an alloy film, or a foam alloy.

The growth of the carbon nanotube array 20 perpendicular to the current collector substrate 10 on the current collector substrate 10 can be accomplished by a variety of processes including, but not limited to, CVD processes, solid phase pyrolysis processes, and sputtering processes.

Taking the CVD process as an example, first, catalyst particles are deposited on the current collector substrate 10, and the catalyst particles may be any one of a metal, a metal compound or an alloy. The role of the catalyst particles includes the active center of decomposition of the carbon source and the nucleation centers and energy carriers of the growth of the carbon nanotubes. Then, the current collector substrate 10 is heated, and a carbon source gas (such as a hydrocarbon gas) is used as a reaction gas by a CVD process, and the carbon nanotube array 20 is grown on the current collector substrate 10 under the catalytic action of the catalyst particles.

The carbon nanotube array 20 prepared by the CVD process will be described in detail below in conjunction with a specific implementation.

Embodiment 1, firstly, depositing metal catalyst particles on a current collector substrate by an electron beam evaporation process, the specific process is: heating the metal catalyst at 500-1000 ° C for 1 to 30 min in a N 2 /H 2 plasma to make a metal catalyst The surface of the current collector substrate is evaporated and deposited in the form of nanoparticles, and the thickness of the metal catalyst particles is from 1 to 100 nm.

Then, the temperature of the current collector substrate is maintained at 500 to 1000 ° C and the pressure of the reaction chamber of MWCVD is 10 to 100 Torr, and ethylene C ₂ H _{4 is introduced} into the reaction chamber at a flow rate of 10 to 100 standard milliliters per minute (standard -state cubic centimeter per minute, sccm), the ethylene is introduced for a period of 1 to 10 minutes, so that the arrayed carbon nanotubes are vertically grown on the surface of the current collector substrate.

In Embodiment 2, the current collector substrate 10 is an aluminum foil. First, nickel Ni particles are deposited on the aluminum foil substrate by an electron beam evaporation process, and Ni particles are used as a catalyst. The specific process is: in the N2/H2 plasma, at 800 ° C. Ni was heated for 5 min to evaporate Ni and deposit the surface of the aluminum foil substrate in the form of nanoparticles having a thickness of 10 nm.

Then, maintaining the aluminum foil substrate temperature at 800 ° C and the MWCVD reaction chamber pressure is 30 Torr, introducing ethylene C ₂ H ₄ into the reaction chamber at a flow rate of 14 sccm, and introducing ethylene into the electrode for 1 min, so that the arrayed carbon nanotubes are vertical. Grown on the surface of the aluminum foil substrate.

In Embodiment 3, the current collector substrate 10 is a foamed nickel substrate. First, iron Fe particles are deposited on the foam nickel substrate by an electron beam evaporation process, and Fe particles are used as a catalyst. The specific process is: in the N2/H2 plasma, at 1000. The Fe was heated at ° C for 2 min to evaporate the Fe and deposit the surface of the foamed nickel substrate in the form of nanoparticles having a thickness of 20 nm.

Then, maintaining the foam nickel substrate temperature at 700 ° C and the MWCVD reaction chamber pressure is 50 Torr, introducing ethylene C ₂ H ₄ into the reaction chamber at a flow rate of 20 sccm, and introducing ethylene into the electrode for 2 min, so that the arrayed carbon nanotubes are vertical. Growing on the surface of a foamed nickel substrate.

Step 402, growing a graphene layer 30 perpendicular to the carbon nanotube array 20 on the current collector substrate 10 having the carbon nanotube array 20.

Figure 2b shows a graphene layer 30 perpendicular to the carbon nanotube array 20. The growth of the graphene layer 30 perpendicular to the carbon nanotube array 20 on the current collector substrate 10 can be achieved by various processes including, but not limited to, a CVD process, an epitaxial growth process, and a solid carbon source catalytic process.

Taking a CVD process as an example, a carbon source gas (such as a hydrocarbon gas) is introduced to the reaction chamber, and then the gaseous carbon source is pyrolyzed on the surface of the current collector substrate 10 in the reaction chamber, and the current collector substrate 10 is grown after a certain time. Graphene layer 30.

The graphene layer 30 prepared by the CVD process will be described in detail below in conjunction with a specific implementation.

In the implementation mode a, after the carbon nanotube array 20 is grown, argon Ar gas is introduced into the reaction chamber at a flow rate of 10 to 200 sccm, and after heating the reaction chamber to 500 to 1000 ° C, methane CH ₄ reaction gas is introduced, and 10 is maintained. ～60 min, the graphene nanosheets are horizontally coated on the surface of the current collector substrate 10 in a manner of penetrating the carbon nanotube array to form a graphene layer 30.

Embodiment b, after the end of the growth of the carbon nanotube array 20, argon Ar gas is introduced into the reaction chamber at a flow rate of 150 sccm, and after heating the reaction chamber to 950 ° C, a methane CH ₄ reaction gas is introduced and held for 30 minutes to make graphene. The nanosheet is horizontally coated on the surface of the current collector substrate 10 in a manner of penetrating the carbon nanotube array to form a graphene layer 30.

In the embodiment c, after the growth of the carbon nanotube array 20 is completed, argon Ar gas is introduced into the reaction chamber at a flow rate of 200 sccm, and after heating the reaction chamber to 1050 ° C, the CH ₄ reaction gas is introduced and held for 20 minutes, so that the graphene nanosheets are obtained. The graphene layer 30 is formed by horizontally coating the surface of the current collector substrate 10 in such a manner as to penetrate the carbon nanotube array.

Through the above scheme, a composite structure of the carbon nanotube array 20 and the graphene layer 30 is formed on the current collector substrate 10, and the composite structure can improve the surface roughness of the current collector and increase the contact point between the current collector substrate 10 and the electrode. Further, the contact resistance between the current collector substrate 10 and the electrode is reduced, and the power density of the energy storage device is increased. Moreover, the composite structure of the carbon nanotube array 20 and the graphene layer 30 described above can also improve the adhesion strength of the electrode material on the current collector and improve the stability of the electrode structure. Furthermore, since both the carbon nanotube array 20 and the graphene layer 30 have excellent electrical conductivity, the composite structure of the carbon nanotube array 20 and the graphene layer 30 can provide an efficient and rapid electron transport network for the electrode, further reducing the electrode and The contact resistance of the current collector improves the electrode rate performance. In addition, since both the carbon nanotube array 20 and the graphene layer 30 have excellent thermal conductivity, the composite structure of the carbon nanotube array 20 and the graphene layer 30 facilitates the rapid transfer of the internal heat of the energy storage device along the three-dimensional direction. In addition, the capacity attenuation and safety problems caused by the temperature rise of the energy storage device are effectively alleviated.

As an alternative, if the catalyst particles are deposited on the current collector substrate 10 when the carbon nanotube array 20 is prepared, the catalyst on the current collector substrate 10 is removed by an etching process after the completion of the preparation of the graphene layer 30. Particles.

The etching process can be a dry etching process, such as removing catalyst particles by plasma bombardment. The etching process can also be a wet etching process. Several possible implementations of the wet etching process are described below.

First, after step 402, the current collector substrate 10 is cooled, and the current collector substrate 10 on which the reaction product is deposited is immersed in an aqueous solution of 1 to 10 moles (M) of HCl, and heat treated at 60 to 100 ° C for 5 to 24 hours (h). Then, it is further immersed in an aqueous solution of 1 to 10 moles of HF at 60 to 100 ° C for 5 to 24 hours to remove metal or alloy catalyst particles on the current collector substrate 10.

Secondly, after step 402, the current collector substrate 10 is cooled, and the current collector substrate 10 on which the reaction product is deposited is immersed in 6 mol aqueous HCl solution, heat treated at 80 ° C for 12 h, and then immersed in 6 mol HF aqueous solution at 80 ° C for heating. After treatment for 12 h, the metal or alloy catalyst particles on the current collector substrate 10 are removed.

Third, after step 402, the current collector substrate 10 is cooled, and the current collector substrate 10 on which the reaction product is deposited is 10 The metal or alloy catalyst particles on the current collector substrate 10 were removed by immersing in a 3 molar (M) aqueous solution of HCl, heat treatment at 80 ° C for 24 hours, and immersing in a 3 molar aqueous solution of HF at 80 ° C for 24 hours.

Through the above implementation, the catalyst particles used for preparing the carbon nanotube array 20 on the current collector substrate 10 are removed to prevent the residual catalyst particles from adversely affecting the conductivity of the current collector.

As an alternative, the graphene layer 30 prepared in step 402 has a thickness of 1 to 50 μm. For example, the graphene layer 30 has a thickness of 5 μm, 10 μm, 20 μm, 30 μm, 40 μm or 50 μm. The graphene layer 30 of the above thickness allows the composite structure of the carbon nanotube array 20 and the graphene layer 30 to have good electrical conductivity, thermal conductivity, and large structural strength.

As an alternative manner, the mass ratio of the graphene layer 30 to the carbon nanotube array 20 is in the range of 10 to 15:1. For example, the mass ratio of the graphene layer 30 to the carbon nanotube array 20 may be 10:1. , 11:1, 12:1, 13:1, 14:1 or 15:1. The above mass ratio of the graphene layer 30 to the carbon nanotube array 20 allows the composite structure of the carbon nanotube array 20 and the graphene layer 30 to have good electrical conductivity, thermal conductivity, and large structural strength.

As an alternative, all the carbon nanotubes in the carbon nanotube array 20 prepared in step 401 may be single-walled carbon nanotubes, double-walled carbon nanotubes or multi-walled carbon nanotubes, and multi-walled carbon nanotubes The carbon nanotubes have a number of layers greater than 2.

As an alternative manner, the number of layers of different carbon nanotubes in the carbon nanotube array 20 may be different. For example, a part of the carbon nanotubes in the carbon nanotube array 20 is a single-walled carbon nanotube, and another part of the carbon nanometer. The tube is a double-walled carbon nanotube, and a part of the carbon nanotube may also be a multi-walled carbon nanotube. The carbon nanotube array 20 includes at least one of single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes.

With continued reference to FIG. 2b, the present application further provides a current collector 100 comprising: a current collector substrate 10, a carbon nanotube array 20 disposed on the current collector substrate 10 and perpendicular to the current collector substrate 10, and an array of carbon nanotubes disposed 20 is perpendicular to the graphene layer 30 of the carbon nanotube array 20.

The current collector 100 of the embodiment of the present application can be prepared by the method of modifying the current collector described in FIG. 1, and the specific implementation of the current collector 100 will not be described in detail herein.

In the above solution, the current collector substrate 10 has a composite structure of the carbon nanotube array 20 and the graphene layer 30, which can improve the surface roughness of the current collector and increase the contact point between the current collector substrate 10 and the electrode. Further, the contact resistance between the current collector substrate 10 and the electrode is reduced, and the power density of the energy storage device is increased. Moreover, the composite structure of the carbon nanotube array 20 and the graphene layer 30 described above can also improve the adhesion strength of the electrode material on the current collector and improve the stability of the electrode structure. Furthermore, since both the carbon nanotube array 20 and the graphene layer 30 have excellent electrical conductivity, the composite structure of the carbon nanotube array 20 and the graphene layer 30 can provide an efficient and rapid electron transport network for the electrode, further reducing the electrode and The contact resistance of the current collector improves the electrode rate performance. In addition, since both the carbon nanotube array 20 and the graphene layer 30 have excellent thermal conductivity, the composite structure of the carbon nanotube array 20 and the graphene layer 30 facilitates the rapid transfer of the internal heat of the energy storage device along the three-dimensional direction. In addition, the capacity attenuation and safety problems caused by the temperature rise of the energy storage device are effectively alleviated.

Referring to FIG. 3, the present application further provides an energy storage device 50, which may be a battery, such as a lithium battery, or a super capacitor. The energy storage device 50 includes a first electrode 51, a second electrode 52, an electrolyte 53, a first current collector 54, and a second current collector 55;

Wherein, the electrolyte 53 is disposed between the first electrode 51 and the second electrode 52, and the charge is transferred between the first electrode 51 and the second electrode 52 through the electrolyte 53;

The first current collector 54 is disposed on the first electrode 51 for deriving the electric charge on the first electrode 51; the second current collector 55 is disposed on the second electrode 52 for deriving the charge on the second electrode 52; the first current collector 54 and/or the second current collector 55 have the structure of the current collector 100 shown in Fig. 2b.

When the first current collector 54 has the structure of the current collector 100, the surface of the first current collector 54 that is in contact with the first electrode 51 has an array of carbon nanotubes perpendicular to the first current collector 54 and perpendicular to the carbon nanotube array. The graphene layer, the composite structure formed by the carbon nanotubes and the graphene layer can increase the surface roughness of the first current collector 54, increase the contact point between the first current collector 54 and the first electrode 51, thereby reducing the first The contact resistance between the current collector 54 and the first electrode 51 increases the power density of the energy storage device 50.

When the first current collector 54 has the structure of the current collector 100, the surface of the second current collector 55 that is in contact with the second electrode 52 has an array of carbon nanotubes perpendicular to the second current collector 55 and perpendicular to the carbon nanotube array. a graphene layer, the composite structure formed by the carbon nanotubes and the graphene layer can increase the surface roughness of the second current collector 55, increase the contact point between the second current collector 55 and the second electrode 52, thereby reducing the The contact resistance between the second current collector 55 and the second electrode 52 increases the power density of the energy storage device 50.

When both the first current collector 54 and the first electrode 51 have the structure of the current collector 100, the power density of the energy storage device 50 can be remarkably improved.

It will be apparent to those skilled in the art that various modifications and changes can be made in the present application without departing from the spirit and scope of the application. Thus, it is intended that the present invention cover the modifications and variations of the present invention.

Claims (11)

A method for modifying a current collector, comprising: Growing an array of carbon nanotubes perpendicular to the current collector substrate on a current collector substrate; A graphene layer perpendicular to the array of carbon nanotubes is grown on the current collector substrate having the array of carbon nanotubes. The method of claim 1 wherein growing an array of carbon nanotubes perpendicular to said current collector substrate on a current collector substrate comprises: The array of carbon nanotubes perpendicular to the current collector substrate is grown on the current collector substrate using a chemical vapor deposition CVD process, a solid phase pyrolysis process, or a sputtering process. The method of claim 1 wherein growing an array of carbon nanotubes perpendicular to said current collector substrate on a current collector substrate comprises: Depositing catalyst particles on the current collector substrate, the catalyst particles being any one of a metal, a metal compound, and an alloy; The current collector substrate is heated, and the carbon source gas is used as a reaction gas by a CVD process, and the carbon nanotube array is grown on the current collector substrate under the catalytic action of the catalyst particles. The method according to claim 3, after the growing the graphene layer, further comprising: Etching removes the catalyst particles on the current collector substrate. The method according to any one of claims 1 to 4, wherein growing the graphene layer perpendicular to the carbon nanotube array on the current collector substrate having the carbon nanotube array comprises: Growing the graphene layer on a surface of the current collector substrate having the carbon nanotube array based on a carbon source gas by a CVD process, wherein carbon nanotubes in the carbon nanotube array vertically pass through the graphene layer . The method according to any one of claims 1 to 5, wherein the graphene layer has a thickness of from 1 to 50 μm. The method according to any one of claims 1 to 6, wherein the mass ratio of the graphene layer to the carbon nanotube array is in the range of 10 to 15:1. The method according to any one of claims 1 to 7, wherein the carbon nanotube array comprises at least one of single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes. The method according to any one of claims 1 to 8, wherein the current collector substrate is a carbon fiber substrate, a graphene substrate, a graphene intercalation compound substrate, a carbon nanotube substrate, a metal substrate, or an alloy substrate. Any one. A current collector, comprising: Current collector substrate An array of carbon nanotubes disposed on the current collector substrate and perpendicular to the current collector substrate; A graphene layer disposed on the array of carbon nanotubes and perpendicular to the array of carbon nanotubes. An energy storage device, comprising: a first electrode, a second electrode, an electrolyte, a first current collector, and a second current collector; Wherein an electrolyte is disposed between the first electrode and the second electrode, and charge is transferred between the first electrode and the second electrode through the electrolyte; the first current collector is disposed at The first electrode is configured to derive a charge on the first electrode; the second current collector is disposed on the second electrode for deriving the second electrode The charge on the first current collector and/or the second current collector has the structure of the current collector according to claim 10.

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