KR20220061641A - Negative electrode and method of pre-lithiating a negative electrode - Google Patents

Abstract

The present invention relates to a negative electrode and a method for prelithiation of the negative electrode, wherein the negative electrode includes: a negative electrode current collector; a negative electrode active material layer formed on at least one surface of the negative electrode current collector; and a buffer layer formed on the negative electrode active material layer and having a structure in which conductive particles are dispersed in a porous polymer matrix.

Description

Electrolithiation method of negative electrode and negative electrode

The present invention relates to a negative electrode and a method for prelithiation of the negative electrode, and more particularly, to an anode including a buffer layer and a method for prelithiation of the negative electrode.

Recently, rechargeable batteries capable of charging and discharging have been widely used as energy sources for wireless mobile devices. In addition, the secondary battery is attracting attention as an energy source for electric vehicles, hybrid electric vehicles, etc., which have been proposed as a way to solve air pollution of conventional gasoline vehicles and diesel vehicles using fossil fuels. Accordingly, the types of applications using secondary batteries are diversifying due to the advantages of secondary batteries, and it is expected that secondary batteries will be applied to more fields and products in the future than now.

These secondary batteries are sometimes classified into lithium ion batteries, lithium ion polymer batteries, lithium polymer batteries, etc. depending on the composition of the electrode and electrolyte. is increasing In general, secondary batteries include a cylindrical battery and a prismatic battery in which an electrode assembly is embedded in a cylindrical or prismatic metal can, and a pouch-type battery in which the electrode assembly is embedded in a pouch-type case of an aluminum laminate sheet, depending on the shape of the battery case. The electrode assembly built into the battery case consists of a positive electrode, a negative electrode, and a separator structure interposed between the positive electrode and the negative electrode, and is a power generating element capable of charging and discharging. It is classified into a jelly-roll type wound with a separator interposed therebetween, and a stack type in which a plurality of positive and negative electrodes of a predetermined size are sequentially stacked with a separator interposed therebetween.

In the case of such a negative electrode, a passivation film such as a solid electrolyte interface layer (SEI layer) is formed on the surface of the negative electrode during initial charging, and the passivation film prevents the organic solvent from being inserted into the negative electrode and decomposes the organic solvent. Therefore, the stability of the anode structure and the reversibility of the anode are improved, and use as a cathode is possible. However, since the formation reaction of the passivation film is an irreversible reaction, there is a problem of reducing the capacity of the battery by causing consumption of lithium ions. There is a problem that occurs.

In particular, when a silicon-based material is used as an anode active material, the silicon-based material has a problem in that the initial irreversibility is large due to the formation of an SEI layer and Li ₂ O due to oxygen in the active material during charging.

Accordingly, a method of forming a passivation film on the surface of the negative electrode in advance by pre-lithiation by a method of inserting lithium into the negative electrode or the like, preventing capacity reduction and improving cycle life is being developed.

1 is a schematic diagram showing a conventional method of lithiation of a negative electrode.

Referring to FIG. 1 , the negative electrode 10 has a structure in which the negative electrode active material layer 12 is formed on the negative electrode current collector 11 . Conventionally, a method of physically contacting a lithium metal foil 13 on the anode active material layer 12 for prelithiation is used, and in some cases, the anode 10 to which the lithium metal foil 13 is in contact is immersed in an electrolyte solution. process could be carried out. Thereafter, the negative electrode 10 was laminated with the positive electrode 16 with the separator 15 interposed therebetween to prepare an electrode assembly 20 . The positive electrode 16 has a structure in which the positive electrode active material layer 18 is formed on the positive electrode current collector 17 .

However, in this case, the lithium ions 14 diffused from the lithium metal foil 13 react rapidly from the surface side of the anode active material layer 12 , which causes the anode active material layer 12 to have a concentration of lithium ions in the thickness direction. becomes non-uniform, and the active material is detached from the surface or cracks (A) are formed due to the rapid volume expansion of the surface portion may occur. In addition, the more rapidly lithium ions react, the higher the rate of production of side reactants such as oxide formation of lithium ions.

This lowers the conductivity and initial charge/discharge efficiency of the negative electrode, and as a result, reduces the cycle characteristics of the secondary battery.

Korean Patent Publication No. 10-2018-0127044

The present invention has been devised to solve the above problems, by preventing the diffusion rate of lithium ions from excessively increasing during the prelithiation process, thereby preventing physical damage to the anode, reducing capacity and conductivity, and reducing the capacity and conductivity of the anode. An object of the present invention is to provide an anode and a method for prelithiation of the anode, which can improve the initial charge/discharge efficiency of the secondary battery and prevent deterioration of the performance of the secondary battery.

The negative electrode according to the present invention includes a negative electrode current collector; a negative active material layer formed on at least one surface of the negative electrode current collector; and a buffer layer formed on the anode active material layer and having a structure in which conductive particles are dispersed in a porous polymer matrix; includes

In this case, the conductive particles in the buffer layer may be included in an amount of 1 to 40% by weight based on the total weight of the buffer layer.

In a specific example, the porous polymer matrix may be one or more selected from the group consisting of polyvinylidene fluoride (PVDF), polyethylene (PE), polypropylene (PP), polyacrylic acid (PAA), and polyester. there is.

In a specific example, the conductive particles are one or more carbon-based materials selected from the group consisting of graphite, carbon nanotubes (CNT), amorphous carbon, graphene, and carbon black.

The buffer layer has a specific resistance of 5 ohm/cm ² to 5,000,000 ohm/cm ² in a thickness direction.

In a specific example, the thickness of the buffer layer is 0.5 to 15 μm.

The present invention also provides a method for pre-lithiation of a negative electrode, wherein the method for pre-lithiation of a negative electrode according to the present invention comprises the steps of sequentially forming a negative electrode active material layer and a buffer layer on at least one surface of a negative electrode current collector to prepare a negative electrode ; and prelithiating the negative electrode, wherein the buffer layer has a structure in which conductive particles are dispersed in a porous polymer matrix.

In this case, the conductive particles in the buffer layer may be included in an amount of 1 to 40% by weight based on the total weight of the buffer layer.

Specifically, in the step of manufacturing the negative electrode, the buffer layer is formed by coating a dispersion in which a polymer compound and conductive particles are dispersed on the negative electrode active material layer.

Specifically, the prelithiation of the negative electrode includes: forming a lithium metal layer on the buffer layer; manufacturing a secondary battery by accommodating the electrode assembly manufactured by stacking the negative electrode with the positive electrode with a separator interposed therebetween, and injecting an electrolyte into the battery case; and aging the secondary battery.

Specifically, the lithium metal layer is formed by transferring the lithium metal film formed on one surface of the release film.

In this case, the secondary battery may be aged in a pressurized state.

In a specific example, the porous polymer matrix may be one or more selected from the group consisting of polyvinylidene fluoride (PVDF), polyethylene (PE), polypropylene (PP), polyacrylic acid (PAA), and polyester. there is.

In a specific example, the conductive particles are one or more carbon-based materials selected from the group consisting of graphite, carbon nanotubes (CNT), amorphous carbon, graphene, and carbon black.

The buffer layer has a specific resistance of 5 ohm/cm ² to 5,000,000 ohm/cm ² in a thickness direction.

In a specific example, the thickness of the buffer layer is 0.5 to 15 μm.

In the present invention, by forming a buffer layer exhibiting electrical conductivity on the anode active material layer, transferring lithium metal, and then manufacturing a battery, the pre-lithiation process of the anode can proceed in the battery without exposure to external air. In addition, the transferred lithium ions are electrically connected to the negative electrode through the buffer layer to form a potential difference, and the lithium ions pass through the buffer layer and drift to the negative electrode. In this process, lithium ions can be inserted into the negative electrode, and the diffusion rate of lithium ions in the negative electrode can be controlled.

Accordingly, side reactions of lithium ions can be suppressed, and the negative electrode can be uniformly prelithiated in the thickness direction, thereby minimizing physical damage to the negative electrode, and reducing the capacity and conductivity of the negative electrode. As a result, it is possible to prevent the performance of the secondary battery from being deteriorated.

1 is a schematic diagram showing a conventional method of lithiation of a negative electrode.
2 is a schematic diagram showing the structure of the negative electrode according to the present invention.
3 is a flowchart showing the sequence of a method for manufacturing a negative electrode according to the present invention.
4 is a schematic diagram showing a method for prelithiation of an anode according to the present invention.
5 and 6 are schematic diagrams showing a process of forming a lithium metal layer in the method of prelithiation of the negative electrode according to the present invention.

Hereinafter, the present invention will be described in detail. Prior to this, the terms or words used in the present specification and claims should not be construed as being limited to conventional or dictionary meanings, and the inventor should properly understand the concept of the term in order to best describe his invention. It should be interpreted as meaning and concept consistent with the technical idea of the present invention based on the principle that it can be defined in

In the present application, terms such as "comprise" or "have" are intended to designate that a feature, number, step, operation, component, part, or a combination thereof described in the specification exists, but one or more other features It is to be understood that it does not preclude the possibility of the presence or addition of numbers, steps, operations, components, parts, or combinations thereof. Also, when a part of a layer, film, region, plate, etc. is said to be “on” another part, it includes not only the case where the other part is “directly on” but also the case where there is another part in between. Conversely, when a part of a layer, film, region, plate, etc. is said to be “under” another part, it includes not only cases where it is “directly under” another part, but also cases where another part is in between. In addition, in the present application, “on” may include the case of being disposed not only on the upper part but also on the lower part.

Hereinafter, the present invention will be described in detail.

2 is a schematic diagram showing the structure of the negative electrode according to the present invention.

Referring to FIG. 2 , the negative electrode 100 according to the present invention includes a negative electrode current collector 110 ; a negative active material layer 120 formed on at least one surface of the negative electrode current collector 110; and a buffer layer 130 formed on the anode active material layer 120 and having a structure in which conductive particles are dispersed in a porous polymer matrix; includes

As described above, in the prior art, lithium metal was physically brought into contact with the anode active material layer during prelithiation. In this case, lithium ions diffusing from the lithium metal foil react rapidly from the surface side of the anode active material layer. As a result, the concentration of lithium ions in the anode active material layer becomes non-uniform in the thickness direction, and the active material is detached from the surface or cracks may be formed due to the rapid volume expansion of the surface portion. This reduces the capacity and conductivity of the negative electrode, and deteriorates the performance of the secondary battery.

In the present invention, by forming a buffer layer exhibiting electrical conductivity on the anode active material layer, transferring lithium metal, and then manufacturing a battery, the pre-lithiation process of the anode can proceed in the battery without exposure to external air. In addition, since the transferred lithium ions pass through the buffer layer and drift to the negative electrode, lithium ions can be inserted into the negative electrode during this process, and the diffusion rate of lithium ions in the negative electrode can be controlled.

Accordingly, side reactions of lithium ions can be suppressed, and the negative electrode can be uniformly prelithiated in the thickness direction, thereby minimizing physical damage to the negative electrode, and reducing the capacity and conductivity of the negative electrode. As a result, it is possible to prevent the performance of the secondary battery from being deteriorated.

Hereinafter, the negative electrode and the method for prelithiation of the negative electrode according to the present invention will be described in detail.

<Cathode>

Referring to FIG. 2 , the negative electrode 100 according to the present invention has a structure in which the negative electrode current collector 110 , the negative electrode active material layer 120 , and the buffer layer 130 are sequentially stacked.

In the case of the negative electrode current collector 110, it is generally made to have a thickness of 3 ~ 500㎛. Such a negative current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery. For example, the surface of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel. Carbon, nickel, titanium, a surface-treated material such as silver, aluminum-cadmium alloy, etc. may be used. In addition, like the positive electrode current collector, the bonding strength of the negative electrode active material may be strengthened by forming fine irregularities on the surface, and may be used in various forms such as a film, sheet, foil, net, porous body, foam, non-woven body, and the like.

On the other hand, the negative electrode active material layer 120 may be formed by drying and rolling after the negative electrode slurry including the negative electrode active material is applied on the current collector. Here, the negative active material may further include a binder and a conductive material.

The negative active material may include at least one selected from the group consisting of a carbon-based active material and a silicon-based active material.

The silicon-based active material may impart excellent capacity characteristics to the negative electrode or secondary battery of the present invention, and may include a compound represented by SiO _x (0≤x<2). In the case of SiO ₂ , since lithium cannot be stored because it does not react with lithium ions, x is preferably within the above range, and more preferably, the silicon-based oxide may be SiO. The average particle diameter (D ₅₀ ) of the silicon-based oxide may be 1 to 30 µm, preferably 3 to 15 µm, in terms of reducing side reactions with the electrolyte while ensuring structural stability during charging and discharging. The average particle diameter (D ₅₀ ) may be measured using, for example, a laser diffraction method.

The carbon-based active material may impart excellent cycle characteristics or battery life performance to the anode or secondary battery for a secondary battery of the present invention. Specifically, the carbon-based active material may include at least one selected from the group consisting of artificial graphite, natural graphite, hard carbon, soft carbon, carbon black, acetylene black, Ketjen black, super P, graphene, and fibrous carbon. and preferably at least one selected from the group consisting of artificial graphite and natural graphite. The average particle diameter (D ₅₀ ) of the carbon-based oxide may be 10 to 30 µm, preferably 15 to 25 µm, in terms of structural stability during charging and discharging and reducing side reactions with the electrolyte.

Specifically, the negative active material may use both the silicon-based active material and the carbon-based active material in terms of simultaneously improving capacity characteristics and cycle characteristics, and specifically, the negative active material is the carbon-based active material and the silicon-based active material 50:50 to 95:5 weight ratio, preferably 60:40 to 80:20 weight ratio.

The conductive material is typically added in an amount of 1 to 30% by weight based on the total weight of the mixture including the positive active material. Such a conductive material is not particularly limited as long as it has conductivity without causing a chemical change in the battery. For example, graphite such as natural graphite or artificial graphite; carbon black, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black; conductive fibers such as carbon fibers and metal fibers; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskeys such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives may be used.

The binder is a component that assists in bonding between the active material and the conductive material and bonding to the current collector, and is typically added in an amount of 1 to 30% by weight based on the total weight of the mixture including the positive electrode active material. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, poly propylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butyrene rubber, fluororubber, various copolymers, and the like.

On the other hand, the buffer layer 130 is positioned between the lithium metal and the anode active material layer during prelithiation to provide a passage for lithium ions to move, and to prevent the lithium ions from rapidly diffusing into the anode active material layer. .

To this end, the buffer layer 130 may have a structure in which conductive particles are dispersed in a porous polymer matrix. That is, lithium ions and electrolyte can move through the pores in the porous polymer matrix, and by electrons moving through the conductive particles in the polymer matrix, the transferred lithium can pass through the buffer layer and continuously move to the negative electrode.

In order to provide appropriate conductivity to the buffer layer 130 , the conductive particles in the buffer layer 130 may be included in an amount of 1 to 40% by weight based on the total weight of the buffer layer 130 . Specifically, the conductive particles in the buffer layer 130 may be included in 5 to 40% by weight, more specifically 10 to 40% by weight, even more specifically 20 to 30% by weight based on the total weight of the buffer layer 130 . there is. When the content of the conductive particles is within the above range, the conductive particles may be effectively dispersed in the buffer layer 130 and the buffer layer 130 may exhibit appropriate electrical conductivity. When the content of conductive particles in the buffer layer 130 is less than the above range, the conductivity of the buffer layer is reduced and the transfer from the transferred lithium to the negative electrode is slowed, so prelithiation may not be completed within the time limit. If it is exceeded, it may be difficult to control the insertion rate of lithium ions into the negative electrode.

Specifically, the porous polymer matrix may be, for example, one or more selected from the group consisting of polyvinylidene fluoride (PVDF), polyethylene (PE), polypropylene (PP), polyacrylic acid (PAA), and polyester. can That is, since the porous polymer matrix is composed of a polymer compound exhibiting binding force, pores are formed therein to provide a passage for lithium ions and electrolyte to move, and at the same time, it can be attached to the anode active material layer without the use of a separate binder compound. However, the polymer compound constituting the porous polymer matrix is not limited to the above range, and any compound capable of forming pores therein while exhibiting binding force may be used.

On the other hand, the conductive particles may use the same or similar type of conductive material that can be used in the negative electrode slurry, for example, the conductive particles include graphite, carbon nanotubes (CNT), amorphous carbon, graphene and carbon black. It may be one or two or more carbon-based materials selected from the group.

As such, by dispersing conductive particles having a content in a specific range in the porous polymer matrix, the buffer layer 130 may exhibit appropriate electrical conductivity. Specifically, the specific resistance of the buffer layer 130 in the thickness direction may be 5 ohm/cm ² to 5,000,000 ohm/cm ² , specifically 7 ohm/cm ² to 31,000 ohm/cm ² , and more specifically may be 14 to 24 ohm/cm ² . When the resistivity is less than 5 ohm/cm ² , the electrical conductivity of the negative electrode is low, so that the electrolithiation is not sufficiently performed, and then the performance of the negative electrode may be deteriorated. In addition, when the resistivity exceeds 5,000,000 ohm/cm ² , the total lithiation rate may be excessively increased.

On the other hand, the buffer layer 130 is composed of polymer compound particles bound to each other, receives lithium ions from lithium metal in the state of being impregnated with an electrolyte, and diffuses lithium ions into the negative electrode to perform prelithiation of the negative electrode. do. In this case, the diffusion rate of lithium ions may be limited by the pore structure in the buffer layer.

In this case, the porosity of the buffer layer 130 may be calculated by Equation 1 below, and the porosity of the buffer layer may be 5 to 40%, specifically 10 to 35%, and more specifically 15 to 30%. .

[Equation 1]

Porosity (%) = (true density - density of buffer layer) / true density × 100

When the porosity of the buffer layer 130 is less than the above range, the diffusion rate of lithium ions may be excessively reduced, and the impregnability of the negative electrode to the electrolyte may decrease. On the other hand, when the porosity of the buffer layer exceeds the above range, the effect of preventing direct contact between the transferred lithium metal layer and the negative electrode is reduced, making it difficult to achieve the object of the present invention.

In addition, the average diameter of the pores formed in the buffer layer 130 may be in the range of 0.1 to 10㎛, specifically, 0.1 to 2㎛. In this case, the average diameter of the pores may be determined from the pore volume and the pore area obtained from the pore size distribution measured by the mercury intrusion method.

In addition, the thickness of the buffer layer 130 may be 0.5 to 15㎛, specifically 1 to 10㎛, may be more specifically 2 to 5㎛. When the thickness of the buffer layer 130 is within the above range, the diffusion rate of lithium ions may be appropriately adjusted. When the thickness of the buffer layer 130 is less than 0.5 μm, the effect of preventing direct contact between the transferred lithium metal layer and the negative electrode may be insufficient. When the thickness of the buffer layer 130 exceeds 15 μm, the diffusion rate of lithium ions is excessively reduced, so that the total lithiation within the time limit may be insufficient, and the resistance of the battery may be increased due to the buffer layer.

Meanwhile, referring to FIG. 2 , the negative electrode 100 according to the present invention may further include a lithium metal layer 140 formed on the buffer layer 130 . The lithium metal layer 140 is for prelithiation of the negative electrode 100 . In the present invention, a lithium metal layer 140 is formed on the buffer layer 130 , a secondary battery is manufactured using the negative electrode 100 , and then a prelithiation process can be performed in the secondary battery. Through this, the process of prelithiation of the negative electrode by a separate physical method or an electrochemical method can be omitted. In the present invention, pre-lithiation of the negative electrode may be performed in the process of aging the secondary battery after the secondary battery is manufactured, as will be described later.

The area of the lithium metal layer 140 may be large enough to cover all of the anode active material layer. In addition, the thickness of the lithium metal layer may be determined according to a target degree of total lithiation, and may be, for example, 1 to 50 μm. The lithium metal layer may be formed by transferring a lithium metal film onto the anode active material layer as described below.

<Method for all-lithiation of cathode>

3 is a flowchart showing the sequence of a method for manufacturing a negative electrode according to the present invention, and FIG. 4 is a schematic diagram showing a method for prelithiation of the negative electrode according to the present invention.

3 and 4, in the method of prelithiation of the negative electrode according to the present invention, the negative electrode is formed by sequentially forming the negative electrode active material layer 120 and the buffer layer 130 on at least one surface of the negative electrode current collector 110. manufacturing step (S1); and prelithiating the negative electrode (S2). In this case, the buffer layer has a structure in which conductive particles are dispersed in a porous polymer matrix.

According to the present invention, by forming the buffer layer on the negative electrode active material layer, it is possible to prevent excessive increase in the diffusion rate of lithium ions due to the electrical conductivity and pore structure characteristics of the buffer layer during prelithiation of the negative electrode.

Accordingly, the negative electrode can be uniformly all-lithiated in the thickness direction, thereby reducing the side reaction of lithium. In addition, it is possible to minimize the physical damage of the negative electrode, it is possible to alleviate the reduction in capacity and conductivity of the negative electrode. As a result, it is possible to prevent the performance of the secondary battery from being deteriorated.

Hereinafter, a method for prelithiation of the negative electrode 100 according to the present invention will be described in detail.

First, the anode active material layer 120 is formed on at least one surface of the anode current collector 110 . The negative electrode active material layer 120 is formed by applying a negative electrode slurry containing the negative electrode active material to at least one surface of the negative electrode current collector 110, drying and rolling. In this case, the description of the negative electrode active material and the negative electrode slurry is the same as described above.

Next, the buffer layer 130 is formed on the anode active material layer 120 . The buffer layer 130 is formed by coating a dispersion in which a polymer compound and conductive particles are dispersed on the anode active material layer 120 . Accordingly, the polymer compound constitutes a porous polymer matrix in the buffer layer. At this time, the coating method may be applied by any one or more selected from the group consisting of a spray coating method, an inkjet printing method, a slit coating method, a die coating method, a spin coating method, and a sputtering coating method, but is not limited thereto. In addition, the coated buffer layer may be dried for a predetermined time.

The porous polymer matrix may be one or a combination of two or more selected from the group consisting of polyvinylidene fluoride (PVDF), polyethylene (PE), polypropylene (PP), polyacrylic acid (PAA), and polyester.

In addition, the conductive particles may be one or more carbon-based materials selected from the group consisting of graphite, carbon nanotubes (CNT), amorphous carbon, graphene, and carbon black.

In addition, an organic solvent such as tetrahydrofuran (THF), N-methyl pyrrolidone (NMP) or hexane may be used as a dispersion medium for preparing the dispersion, but is not limited thereto.

At this time, the content of the conductive particles in the buffer layer may be 1 to 40% by weight based on the total weight of the buffer layer, specifically 5 to 40% by weight, more specifically 20 to 30% by weight as described above. there is. Therefore, when preparing the dispersion, the weight ratio of the polymer compound to the conductive particles may be 2.5:1 to 100:1.

In addition, in the dispersion for preparing the buffer layer, the content of the solid content may be 1 to 40% by weight, specifically 5 to 40% by weight based on the weight of the dispersion. When the solid content is within the above range, pores at an appropriate level may be formed in the buffer layer.

In addition, as described above, the buffer layer may have a specific resistance in the thickness direction of 5 ohm/cm ² to 5,000,000 ohm/cm ² , specifically 7 ohm/cm ² to 31,000 ohm/cm ² , and more specifically may be 14 to 24 ohm/cm ² .

In addition, the thickness of the buffer layer may be 0.5 to 15 μm, specifically 1 to 10 μm, and more specifically 2 to 5 μm, as described above.

When the negative electrode 100 is manufactured, the step of prelithiating the negative electrode 100 is performed. The step of prelithiating the negative electrode includes aging the negative electrode 100 in a state in which it is impregnated with an electrolyte. Through the impregnation, the volume of the anode expands to a certain level, and the distance between the anode active material particles is appropriately increased so that the electrolyte can penetrate between the particles, and thus lithium ions can better penetrate into the anode.

More specifically, referring to FIG. 4 , the prelithiation of the negative electrode 100 may include forming a lithium metal layer 140 on the buffer layer 130 ; The process of manufacturing a secondary battery by accommodating the electrode assembly 400 manufactured by stacking the negative electrode 100 with the positive electrode 300 with the separator 200 interposed therebetween in a battery case (not shown) and injecting an electrolyte; and aging the secondary battery.

In this case, the lithium metal layer 140 may be formed by transferring the lithium metal film 140a formed on one surface of the release film 142 .

5 and 6 are schematic diagrams showing a process of forming a lithium metal layer in the method of prelithiation of the negative electrode according to the present invention.

5 and 6 , a lithium metal film 140a may be formed on one surface of the release film 142 . The lithium metal film 140a may be formed by vapor deposition or vacuum deposition of lithium metal on a release film. For example, referring to FIG. 5 , lithium metal is deposited on one surface of the release film 142 while the release film 142 is transported by a roll to form a lithium metal film 140a. The release film 142 on which the lithium metal film 140a is formed is wound on a separate roll, and then unwound for transfer when necessary.

The release film 142 may include at least one selected from the group consisting of polyethylene terephthalate, triacetyl cellulose, polyacrylate and polyimide, and is preferably wound by having excellent heat resistance and excellent flexibility. / In terms of ease of unwinding, polyethylene terephthalate may be included.

The thickness of the release film 142 may be 20 μm to 100 μm, specifically 25 μm to 75 μm, and when it is in the above range, vapor deposition of lithium can be smoothly performed by having sufficient heat resistance, and excellent flexibility It is preferable for winding / unwinding by providing.

When the lithium metal film 140a is formed on one surface of the release film 142 , a process of transferring the lithium metal film 140a onto the buffer layer 140 is performed. The process may be performed by a roll-to-roll.

Referring to FIG. 6 , the negative electrode 100 on which the buffer layer 130 is formed is wound on a separate unwinding roll 500 , and is unwound from the unwinding roll 500 when the transfer starts. The release film 142 on which the lithium metal film 140a is formed is also wound on a separate roll (not shown) and is unwound from it when the transfer starts. Subsequently, the lithium metal film 140a is transferred onto the buffer layer 130 by pressing the release film 142 and the negative electrode 100 using a pair of pressure rolls 700 to form the lithium metal layer 140 . there is. At this time, the size of the pressing force may be appropriately designed by a person skilled in the art. The negative electrode 100 on which the lithium metal layer 140 is formed is wound on a rewinding roll 600 , and in this process, the release film 142 may be removed.

The negative electrode 100 manufactured in this way is assembled in the form of a secondary battery. Referring to FIG. 4 , in the secondary battery, the electrode assembly 400 manufactured with the separator 200 interposed between the negative electrode 100 and the positive electrode 300 is accommodated in a battery case (not shown), and electrolyte is injected. It can be manufactured by The positive electrode 300 has a structure in which a positive electrode active material layer 320 is formed by applying a positive electrode slurry including a positive electrode active material on a positive electrode current collector 310 .

In the present invention, the positive electrode current collector 310 is generally made to have a thickness of 3 to 500 μm. The positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical change in the battery, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel. Carbon, nickel, titanium, silver, etc. surface-treated on the surface of the can be used. The current collector may increase the adhesion of the positive electrode active material by forming fine irregularities on the surface thereof, and various forms such as a film, sheet, foil, net, porous body, foam body, and non-woven body are possible.

In the present invention, the positive active material is a material capable of causing an electrochemical reaction, as a lithium transition metal oxide, containing two or more transition metals, for example, lithium cobalt oxide (LiCoO ₂ ) substituted with one or more transition metals. ), layered compounds such as lithium nickel oxide (LiNiO ₂ ); lithium manganese oxide substituted with one or more transition metals; Formula LiNi _1-y M _y O ₂ (wherein M = Co, Mn, Al, Cu, Fe, Mg, B, Cr, Zn or Ga and includes at least one of the above elements, 0.01≤y≤0.7) Lithium nickel-based oxide represented by; Li _1+z Ni _1/3 Co _1/3 Mn _1/3 O ₂ , Li _1+z Ni _0.4 Mn _0.4 Co _0.2 O ₂ , etc. Li _1+z Ni _b Mn _c Co _{1-(b+c+d )} M _d O _(2-e) A _e (where -0.5≤z≤0.5, 0.1≤b≤0.8, 0.1≤c≤0.8, 0≤d≤0.2, 0≤e≤0.2, b+c+d <1, M = Al, Mg, Cr, Ti, Si or Y, and A = F, P or Cl) a lithium nickel cobalt manganese composite oxide; Formula Li _1+x M _1-y M' _y PO _4-z X _z where M = transition metal, preferably Fe, Mn, Co or Ni, M' = Al, Mg or Ti, X = F, S, or N, and -0.5≤x≤+0.5, 0≤y≤0.5, 0≤z≤0.1 olivine-based lithium metal phosphate represented by), but is not limited thereto.

In addition, the positive electrode slurry further includes a conductive material and a binder in addition to the positive electrode active material, and the contents thereof are the same as described above.

The separator is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength is used. The pore diameter of the separator is generally 0.01 to 10 μm, and the thickness is generally 5 to 300 μm. As such a separation membrane, For example, olefin polymers, such as chemical-resistant and hydrophobic polypropylene; A sheet or non-woven fabric made of glass fiber or polyethylene is used. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as a separator.

Meanwhile, the battery case is not particularly limited as long as it is used as an exterior material for battery packaging, and a cylindrical, prismatic, or pouch type may be used, but in detail, a pouch type battery case may be used. The pouch-type battery case is typically made of an aluminum laminate sheet, and may include an inner sealant layer for sealing, a metal layer preventing material penetration, and an outer resin layer forming the outermost layer of the case. Hereinafter, detailed description of the battery case will be omitted since it is known to those skilled in the art.

Meanwhile, the electrolyte may include a lithium salt and an organic solvent.

Specifically, the lithium salt is LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF ₆ , LiSbF ₆ , LiAlCl ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, (CF ₃ SO ₂ ) ₂ NLi, (FSO ₂ ) ₂ NLi, lithium chloroborane, lithium lower aliphatic carboxylate, lithium 4 phenyl borate, or one or more of these.

For example, the organic solvent may include a carbonate-based solvent, an ester-based solvent, or two or more thereof. Examples of the non-aqueous solvent include propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane , diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethylmethyl carbonate (EMC), gamma butyrolactone (g-butyrolactone), ethyl propionate, methyl bropionate and the like may be used alone or in combination of two or more, but is not limited thereto.

When the secondary battery is manufactured, all-lithiation of the negative electrode is performed by aging the secondary battery. Since the buffer layer has electrical conductivity due to the conductive particles inside, a short circuit is formed between the lithium metal layer 140 and the anode active material layer 120 due to the conductivity of the buffer layer, and accordingly, in the aging process of the secondary battery Lithium ions 141 in the lithium metal layer 140 may pass through the buffer layer 130 due to electrolyte impregnation and a potential difference between the lithium metal layer 130 and the negative electrode active material layer 120 to be injected into the negative electrode. At this time, since the diffusion rate of the lithium ions 141 is controlled by the buffer layer 130 , it is possible to prevent the lithium ions 141 from reacting rapidly on the surface of the negative electrode, and thus the surface of the negative electrode active material layer 120 . It can prevent expansion or cracking caused by it. In addition, as the diffusion rate of the lithium ions 141 is controlled, the lithium ions 141 may be uniformly introduced into the anode active material layer 120 .

In addition, in a state in which the anode is separately immersed in the electrolyte, the anode with the lithium metal layer may be aged to prelithiate, but by prelithiating the anode with the lithium metal layer as described above in the assembled state as a secondary battery, the anode is physically separated Alternatively, the prelithiation process by the electrochemical method may be omitted, and thus the time and cost required for the process may be reduced.

In this case, the secondary battery may be aged in a pressurized state. The movement of lithium ions can be more effectively promoted through pressurization, and the volume change of the electrode can be suppressed during the prelithiation process. The pressing force may be appropriately designed by a person skilled in the art, for example, a force of 10 to 1,000 kgf, specifically, 30 to 100 kgf per unit area may be applied. If the pressing force is less than 10 kgf, the pressing force is low, so it cannot cope with the thickness change of the negative electrode during the prelithiation process, so a void may occur between the separator and the negative electrode. can occur

Meanwhile, the aging time may be appropriately designed by a person skilled in the art, for example, may be 30 minutes to 96 hours, and specifically may be 48 hours to 72 hours.

As described above, in the present invention, by forming a buffer layer having electrical conductivity on the anode active material layer, the pre-lithiation process of the anode can proceed in the battery without being exposed to the outside air, and lithium ions in the process of passing through the buffer layer It is possible to prevent the diffusion rate from increasing excessively.

Accordingly, side reactions of lithium ions are suppressed, and the negative electrode can be uniformly prelithiated in the thickness direction, thereby minimizing physical damage to the negative electrode. In addition, it is possible to alleviate the reduction in the capacity and conductivity of the cathode. As a result, it is possible to prevent the performance of the secondary battery from being deteriorated.

Hereinafter, examples will be given to help the understanding of the present invention be described in detail. However, the embodiments according to the present invention may be modified in various other forms, and the scope of the present invention should not be construed as being limited to the following examples. The embodiments of the present invention are provided to more completely explain the present invention to those of ordinary skill in the art.

Example 1

<Production of cathode>

As an anode active material, a mixture of 85% by weight of graphite and 15% by weight of SiO was prepared. 94.5 wt% of the anode active material, 1.3 wt% of Denka Black as a conductive material, 3.0 wt% of SBR as a binder, and 1.2 wt% of CMC as a thickener were added to water to prepare a negative electrode slurry.

A negative electrode active material layer was formed on one surface of a copper negative electrode current collector by coating the negative electrode slurry to a thickness of 80 μm on one surface of a copper current collector (thickness: 8 μm), drying and rolling in a vacuum oven at 130° C.

In addition, a dispersion solution in which PVDF and multi-wall CNTs (MWCNTs) were dispersed was prepared for the formation of a buffer layer. The dispersion was prepared by adding 5% by weight of PVDF (model name: KF 9700) and 0.25% by weight of MWCNT to an NMP solvent. The dispersion was mixed for 1 hour at 2000 RPM using a Homodisper disperser. The dispersion was coated on the surface of the negative active material layer to a thickness of 5 μm using a micro-gravure coater and dried to form a buffer layer. That is, the content of conductive particles (MWCNT) in the buffer layer was 5% by weight based on the total weight of the buffer layer.

Then, lithium metal was deposited on one surface of the PET release film to a thickness of 6 μm using a vacuum deposition method. This was pressurized together with the negative electrode using a roll press equipment (Calendering machine, CIS CLP-1015), and the lithium metal layer was transferred onto the buffer layer.

<Half-cell manufacturing and pre-lithiation>

A half cell was prepared for the prepared negative electrode. Specifically, a lithium metal counter electrode is prepared, the negative electrode porous polyethylene separator (Toray's B09PA1) is interposed between the negative electrode and the lithium metal counter electrode, stacked, and stored in a pouch-type battery case, followed by injecting electrolyte and sealing the secondary battery was prepared. The electrolyte is dissolved in an organic solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3: ₇ to a concentration of 1.2M, and vinylene carbonate (VC) as an additive by 1 weight % added was used.

The secondary battery was aged for 72 hours in a pressurized state at 100 kgf per unit area so that all lithium ions in the lithium metal layer could be injected into the negative electrode.

Example 2

By adding 5 wt% of PVDF (model name: KF 9700) and 0.5 wt% of MWCNT to the NMP solvent as a dispersion for forming the buffer layer, the content of conductive particles (MWCNT) in the buffer layer was 10 wt% with respect to the total weight of the buffer layer Except for that, an anode was prepared in the same manner as in Example 1, and prelithiation was performed.

Example 3

By adding 5 wt% of PVDF (model name: KF 9700) and 1.0 wt% of MWCNT to the NMP solvent as a dispersion for forming the buffer layer, the content of conductive particles (MWCNT) in the buffer layer was set to 20 wt% with respect to the total weight of the buffer layer Except for that, an anode was prepared in the same manner as in Example 1, and prelithiation was performed.

Example 4

By adding 5 wt% of PVDF (model name: KF 9700) and 1.5 wt% of MWCNT to the NMP solvent as a dispersion for the formation of the buffer layer, the content of conductive particles (MWCNT) in the buffer layer was set to 30 wt% with respect to the total weight of the buffer layer Except for that, an anode was prepared in the same manner as in Example 1, and prelithiation was performed.

Example 5

By adding 5 wt% of PVDF (model name: KF 9700) and 2.0 wt% of MWCNT to the NMP solvent as a dispersion for forming the buffer layer, the content of conductive particles (MWCNT) in the buffer layer was set to 40 wt% with respect to the total weight of the buffer layer Except for that, an anode was prepared in the same manner as in Example 1, and prelithiation was performed.

Comparative Example 1

A negative electrode was prepared in the same manner as in Example 1, except that a lithium metal layer was directly formed on the negative electrode active material layer without forming a buffer layer, and prelithiation was performed.

Comparative Example 2

By adding 5 wt% of PVDF (model name: KF 9700) and 0.025 wt% of MWCNT to the NMP solvent as a dispersion for forming the buffer layer, the content of conductive particles (MWCNT) in the buffer layer was 0.5 wt% with respect to the total weight of the buffer layer Except for that, an anode was prepared in the same manner as in Example 1, and prelithiation was performed.

Comparative Example 3

By adding 5 wt% of PVDF (model name: KF 9700) and 2.5 wt% of MWCNT to the NMP solvent as a dispersion for forming the buffer layer, the content of conductive particles (MWCNT) in the buffer layer was set to 50 wt% with respect to the total weight of the buffer layer Except for that, an anode was prepared in the same manner as in Example 1, and prelithiation was performed.

Comparative Example 4

A negative electrode was prepared in the same manner as in Example 3, and prelithiation was carried out in the same manner as in Example 3, except that the dispersion solution for forming the buffer layer was coated on the surface of the negative electrode active material to a thickness of 0.2 μm and dried to form the buffer layer.

Experimental example

<Measurement of lithium doping amount of cathode>

For each of the half cells according to the embodiment and comparative example, after fastening the cell with a pressure of 50 kpa, it is fully charged with a constant current (CC) of 0.1C, left for 60 minutes, and then the constant current (CC) of 0.1C becomes 3.0V discharged until From this, the discharge profile was obtained and the discharge capacity (capacity 1) was calculated.

In addition, the discharge capacity (capacity 2) was calculated in the same manner after preparing a half cell in the same manner as in Examples and Comparative Examples, except that a buffer layer was not formed on the cathode and prelithiation was not performed. From the difference between the capacity 1 and the capacity 2, the lithium doping amount of the half cells according to Examples and Comparative Examples was calculated as shown in the following formula. The results are shown in Table 1.

Lithium doping amount (Lithiation dosage) (%) = (Capacity 1 - Volume 2) / Capacity 2 × 100

<Initial Coulomb Efficiency Measurement>

The half-cells according to Examples and Comparative Examples were respectively charged and discharged at 0.1C, and an initial charge capacity and an initial discharge capacity were respectively calculated therefrom. The results are shown in Table 1.

Initial Coulombic Efficiency (%) = {(Initial Discharge Capacity)/(Initial Charge Capacity)} × 100

<Capacity retention rate>

The half-cell, on which the initial coulombic efficiency measurement was completed, was fully charged with a constant current (CC) of 0.2 C, left for 60 minutes, and then discharged at a constant current (CC) of 0.2 C until 3.0V to calculate the discharge capacity. This was defined as the initial dose. Then, after repeating the process of charging and discharging the half-cell at 0.2 C 49 times, the ratio of the initial capacity to the discharge capacity of 50 times was calculated and this was used as the capacity retention rate. The results are shown in Table 1.

<Buffer layer resistivity evaluation>

The specific resistances of the buffer layers used in Examples 1 to 6 and Comparative Example 2 were measured. Specifically, the dispersions used in Examples 1 to 6 and Comparative Example 2 were coated on a copper current collector to form a buffer layer. And after pressing it at 75 kPa in the thickness direction, the sheet resistance was measured. The sheet resistance was calculated by measuring the resistance using a resistance measuring device (HIOKI 3244-60), and dividing it by the cross-sectional area. The results are shown in Table 1.

Lithium doping amount (%) Initial Coulombic Efficiency (%) Capacity retention rate (%) Buffer layer sheet resistance (kohm/cm ² ) Example 1 88 89 89 19-31 Example 2 90 91 90 8-12 Example 3 95 98 93 0.024 Example 4 94 97 93 0.014 Example 5 94 95 90 0.007 Comparative Example 1 84 88 88 - Comparative Example 2 - - - 5,000 to 9,000 Comparative Example 3 86 88 87 0.001 Comparative Example 4 86 88 89 0.005

Referring to Table 1, the lithium doping amount and initial coulombic efficiency were found to be higher than those of Comparative Example 1 in the case of the example in which the negative electrode was prepared by the all-lithiation method using the buffer layer having electrical conductivity as in the present invention. This is because the pre-lithiation process proceeds without exposure to external air, thereby suppressing the generation of by-products by the reaction of the negative electrode active material with lithium and atmospheric components.

Similarly, in the case of the half-cell manufactured according to the example, it was found that the capacity retention rate was higher than that of Comparative Example 1. This is because the lithium ion insertion rate into the negative electrode is controlled during all-lithiation to minimize physical damage to the negative electrode.

In Comparative Example 2, the specific resistance of the buffer layer was found to be very large. Also, in the case of Comparative Example 2, it was confirmed that the prelithiation hardly proceeded and most of the transferred lithium remained on the buffer layer, which was found to be due to the high resistivity of the buffer layer.

In the case of Comparative Example 3, since many conductive particles were included in the buffer layer, the specific resistance of the buffer layer was too low. As a result, it was difficult to control the insertion rate of lithium ions into the negative electrode. As a result, it can be seen that the performance of the battery decreased.

On the other hand, in the case of Comparative Example 4, the thickness of the buffer layer was too thin, and it was difficult to achieve the effect of the present invention of preventing direct contact between the negative electrode and the lithium metal layer, and as a result, it can be seen that the performance of the battery was reduced compared to the Example. there is.

The above description is merely illustrative of the technical spirit of the present invention, and various modifications and variations will be possible without departing from the essential characteristics of the present invention by those skilled in the art to which the present invention pertains. Accordingly, the drawings disclosed in the present invention are for explanation rather than limiting the technical spirit of the present invention, and the scope of the technical spirit of the present invention is not limited by these drawings. The protection scope of the present invention should be construed by the following claims, and all technical ideas within the scope equivalent thereto should be construed as being included in the scope of the present invention.

Meanwhile, in this specification, terms indicating directions such as up, down, left, right, front, and back are used, but these terms are for convenience of explanation only, and may vary depending on the location of the object or the position of the observer. It is self-evident that

10, 100: cathode
11, 110: negative electrode current collector
12, 120: anode active material layer
13, 140: lithium metal layer
14, 141 lithium ion
15, 200: separator
16, 300: positive electrode
17, 310: positive electrode current collector
18, 400: electrode assembly
130: buffer layer
131: conductive particles
140a: lithium metal film
142: release film
500: unwinding roll
600: rewinding roll
700: pressure roll

Claims (17)

negative electrode current collector;
a negative active material layer formed on at least one surface of the negative electrode current collector; and
a buffer layer formed on the anode active material layer and having a structure in which conductive particles are dispersed in a porous polymer matrix; A negative electrode comprising a.
According to claim 1,
The anode in which the conductive particles in the buffer layer are included in an amount of 1 to 40% by weight based on the total weight of the buffer layer.
According to claim 1,
The porous polymer matrix is polyvinylidene fluoride (PVDF), polyethylene (PE), polypropylene (PP), polyacrylic acid (PAA), one or two or more kinds of negative electrode selected from the group comprising polyester.
According to claim 1,
The conductive particles are graphite, carbon nanotubes (CNTs), amorphous carbon, graphene, and a negative electrode which is one or more carbon-based materials selected from the group consisting of carbon black.
According to claim 1,
The buffer layer has a specific resistance in a thickness direction of 5 ohm/cm ² to 5,000,000 ohm/cm ² A cathode.
According to claim 1,
The thickness of the buffer layer is 0.5 to 15㎛ the cathode.
According to claim 1,
The negative electrode further comprising a lithium metal layer formed on the buffer layer.
manufacturing a negative electrode by sequentially forming a negative electrode active material layer and a buffer layer on at least one surface of the negative electrode current collector; and
prelithiating the negative electrode;
The method of prelithiation of the anode in which the buffer layer is a structure in which conductive particles are dispersed in a porous polymer matrix.
9. The method of claim 8,
The method for prelithiation of the negative electrode in which the conductive particles in the buffer layer are included in an amount of 1 to 40% by weight based on the total weight of the buffer layer.
9. The method of claim 8,
In the step of preparing the negative electrode,
The buffer layer is a method of prelithiation of a negative electrode formed by coating a dispersion in which a polymer compound and conductive particles are dispersed on the negative electrode active material layer.
9. The method of claim 8,
The step of prelithiation of the negative electrode,
forming a lithium metal layer on the buffer layer;
manufacturing a secondary battery by accommodating the electrode assembly manufactured by stacking the negative electrode with the positive electrode with a separator interposed therebetween, and injecting an electrolyte solution; and
A method of prelithiation of a negative electrode comprising the step of aging the secondary battery.
9. The method of claim 8,
The lithium metal layer is a pre-lithiation method of a negative electrode formed by transferring a lithium metal film formed on one surface of the release film.
12. The method of claim 11,
The method for prelithiation of the negative electrode in which the secondary battery is aged in a pressurized state.
9. The method of claim 8,
The porous polymer matrix is one or more combinations of polyvinylidene fluoride (PVDF), polyethylene (PE), polypropylene (PP), polyacrylic acid (PAA), and polyester. Lithation method.
9. The method of claim 8,
The method for prelithiation of an anode, wherein the conductive particles are one or more carbon-based materials selected from the group consisting of graphite, carbon nanotubes (CNT), amorphous carbon, graphene, and carbon black.
9. The method of claim 8,
The buffer layer has a specific resistance in a thickness direction of 5 ohm/cm ² to 5,000,000 ohm/cm ² A method of prelithiation of a cathode.
9. The method of claim 8,
The thickness of the buffer layer is 0.5 to 15㎛ prelithiation method of the negative electrode.

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