KR102335314B1 - Negative electrode for lithium secondary battery, and lithium secondary battery comprising the same - Google Patents

Abstract

The present invention is a negative electrode current collector; a first anode active material layer formed on the anode current collector; and a second anode active material layer formed on the first anode active material layer, wherein the second anode active material layer has a faster lithium diffusion rate than the first anode active material layer, thereby improving lithium diffusivity and excellent prelithiation It relates to an anode for a lithium secondary battery, which can exhibit efficiency, and a lithium secondary battery including the same.

Description

Anode for a lithium secondary battery, and a lithium secondary battery comprising the same

The present invention relates to a negative electrode for a lithium secondary battery, a lithium secondary battery including the same, and a method for manufacturing the same, and more particularly, lithium, which has a high lithium diffusion rate formed on the surface of the negative electrode active material layer, which can be effectively lithiated It relates to a negative electrode for a secondary battery, and a lithium secondary battery including the negative electrode.

As technology development and demand for mobile devices increase, the demand for secondary batteries as an energy source is rapidly increasing. Among such secondary batteries, lithium secondary batteries exhibiting high energy density and operating potential, long cycle life, and low self-discharge rate. Batteries have been commercialized and widely used.

In addition, as interest in environmental issues has increased in recent years, electric vehicles (EVs) and hybrid electric vehicles (HEVs) that can replace vehicles using fossil fuels such as gasoline vehicles and diesel vehicles, which are one of the main causes of air pollution, have been developed. There is a lot of research going on. Although nickel-metal hydride (Ni-MH) secondary batteries are mainly used as power sources for electric vehicles (EVs) and hybrid electric vehicles (HEVs), lithium secondary batteries with high energy density, high discharge voltage and output stability are used. Research is being actively conducted, and some have been commercialized.

A lithium secondary battery has a structure in which a non-aqueous electrolyte containing lithium salt is impregnated in an electrode assembly with a porous separator interposed between a positive electrode and a negative electrode, each of which is coated with an active material on an electrode current collector.

The lithium secondary battery is charged and discharged while repeating the process in which lithium ions of the positive electrode active material of the positive electrode are intercalated into the negative electrode active material of the negative electrode and deintercalated.

Theoretically, lithium insertion and desorption reactions into the negative active material are completely reversible, but in practice, more lithium than the theoretical capacity of the negative active material is consumed, and only a part of it is recovered during discharge. Accordingly, after the second cycle, a smaller amount of lithium ions are inserted during charging, but most of the inserted lithium ions are desorbed during discharging. As described above, the difference in capacity that appears in the first charge and discharge reaction is called irreversible capacity loss. In a commercial lithium secondary battery, lithium ions are supplied from the positive electrode and lithium is not present in the negative electrode. It is important to minimize

It is known that this initial irreversible capacity loss is mostly due to an electrolyte decomposition reaction on the surface of the anode active material, and SEI (Solid Electrolyte Interface) on the surface of the anode active material by an electrochemical reaction through the electrolyte decomposition. this is formed Since many lithium ions are consumed in the formation of the SEI film, there is a problem of causing irreversible capacity loss, but the SEI film formed at the beginning of charging prevents the reaction between lithium ions and the negative electrode or other materials during charging and discharging, and ion tunnel (Ion Tunnel). ), it functions to pass only lithium ions, suppressing multiple electrolyte decomposition reactions, and contributing to the improvement of cycle characteristics of lithium secondary batteries.

Therefore, there is a need for a method for improving the initial irreversibility caused by the formation of the SEI film, etc., and as one of the methods, pre-lithiation is performed before manufacturing a lithium secondary battery to undergo a side reaction that occurs during the first charge. how to do it As such, in the case of performing pre-lithiation, when charging and discharging of the actually manufactured secondary battery is performed, the first cycle proceeds in a state in which irreversibility is reduced by that much, so that the initial irreversibility can be reduced. There is this.

As the pre-lithiation method, for example, after depositing lithium on the surface of the negative electrode, assembling the battery and injecting the electrolyte, lithiation is performed in the wetting process, and the negative electrode and lithium are wetted by immersing the negative electrode in the electrolyte. direct contact with the However, when a material having a high energy density is used as the anode active material, the diffusion rate of lithium is very slow, and it takes a long time compared to the conventional process in which total lithiation is not performed, resulting in poor battery production efficiency.

An object of the present invention is to provide an anode for a lithium secondary battery having improved lithium diffusivity and improved total lithiation efficiency.

Another object of the present invention to be solved is to provide a lithium secondary battery including the negative electrode for the lithium secondary battery.

In order to solve the above problems, the present invention

negative electrode current collector; a first anode active material layer formed on the anode current collector; and a second anode active material layer formed on the first anode active material layer, wherein the first anode active material layer is 5.0×10 ^-11 cm ² /s to 5.0×10 ^-9 cm ² /s at 50% SOC a negative active material having a lithium ion diffusion coefficient of s, wherein the second negative active material layer has a lithium ion diffusion coefficient of 5.0×10 ^-9 cm ² /s to 5.0×10 ^-7 cm ² /s at 50% SOC It provides a negative electrode for a lithium secondary battery, including an anode active material.

In addition, the present invention in order to solve the other problems,

It provides a lithium secondary battery including the negative electrode for the lithium secondary battery.

The negative electrode for a lithium secondary battery according to the present invention has an anode active material layer having a high diffusion rate of lithium formed on the surface, so that the diffusion rate of lithium to the negative electrode active material layer is fast, so that it can exhibit improved lithium diffusivity and excellent pre-lithiation efficiency. have.

1 is a photograph of the electrode surface for each time period for determining the degree of prelithiation when the negative electrode prepared in Example 4 is immersed in an electrolyte.
2 is a photograph of the electrode surface for each time period for determining the degree of prelithiation when the negative electrode prepared in Comparative Example 2 is immersed in an electrolyte.

Hereinafter, the present invention will be described in more detail to help the understanding of the present invention.

The terms or words used in the present specification and claims should not be construed as being limited to their ordinary or dictionary meanings, and the inventor may properly define the concept of the term in order to best describe his invention. Based on the principle that there is, it should be interpreted as meaning and concept consistent with the technical idea of the present invention.

A negative electrode for a lithium secondary battery according to the present invention includes a negative electrode current collector; a first anode active material layer formed on the anode current collector; and a second anode active material layer formed on the first anode active material layer, wherein the first anode active material layer is 5.0×10 ^-11 cm ² /s to 5.0×10 ^-9 cm ² /s at 50% SOC a negative active material having a lithium ion diffusion coefficient of s, wherein the second negative active material layer has a lithium ion diffusion coefficient of 5.0×10 ^-9 cm ² /s to 5.0×10 ^-7 cm ² /s at 50% SOC The eggplant includes an anode active material.

The first anode active material layer is specifically 7.5×10 ^-11 cm ² /s to 2.5×10 ^-9 cm ² /s, more specifically 1.0×10 ^-10 cm ² /s to 1.0×10 ^-9 cm ² /s may include an anode active material having a lithium ion diffusion coefficient of s, and the second anode active material layer is specifically 7.5×10 ^-9 cm ² /s to 2.5×10 ^-7 cm ² /s, more specifically 1.0× A negative active material having a lithium ion diffusion coefficient of ^{10 -8} cm ² /s to 1.0×10 ^-7 cm ^{2 /s may be included.}

When the lithium ion diffusion coefficient of the first anode active material layer is within the above range, an appropriate energy density may be exhibited and excellent charge/discharge efficiency may be exhibited. In addition, when the lithium ion diffusion coefficient of the second anode active material layer is within the above range, lithium ions may be rapidly received during prelithiation while exhibiting an appropriate energy density. As described above, since the second anode active material layer has a higher lithium ion diffusion coefficient than that of a conventional active material, it can quickly and effectively occlude lithium ions, solve irreversible capacity and achieve rapid lithiation, and the first anode Efficient prelithiation can be achieved prior to the active material layer.

The second anode active material layer positioned on the surface of the anode may include an anode active material having a higher lithium ion diffusion coefficient than that of the first anode active material layer, wherein the second anode active material layer is the first anode active material layer Since it can exhibit a relatively fast lithium ion diffusion rate compared to the The lithiation can be performed more effectively, and the time required for all-lithiation can be shortened. Part of the lithium diffused inside the second anode active material layer resolves the irreversibility of the anode active material included in the second anode active material layer, and a portion is charged, and over time, the lithium received in the second anode active material layer is It diffuses into the first anode active material layer to resolve irreversibility of the anode active material included in the first anode active material layer.

In addition, the second anode active material layer may have a smaller porosity than the first anode active material layer in order to exhibit a faster lithium diffusion rate than the first anode active material layer. Since the second anode active material layer has a smaller porosity than the first anode active material layer, the contact area between the lithium metal and the active material layer is relatively wide during prelithiation, thereby exhibiting an excellent lithium diffusion rate.

The first anode active material layer may have a porosity of 25% to 35%, specifically, a porosity of 28% to 33%, more specifically, a porosity of 30% to 32%, and the second anode active material layer may have a porosity of 20% It may have a porosity of % to 30%, specifically, it may have a porosity of 22% to 28%, more specifically, a porosity of 23% to 25%. In addition, the porosity of the second anode active material layer may have a smaller value than that of the first anode active material layer.

Since the second anode active material layer has a smaller porosity value than that of the first anode active material layer, the contact area between the anode active material and lithium metal included in the second anode active material layer is relatively wide during prelithiation, resulting in better diffusibility can be achieved. When the porosity of the first anode active material layer and the second anode active material layer is equal to or greater than the lower limit of the above range, the ionic liquid diffusion performance is not impaired, so that lithium ions can be more appropriately diffused during charging and discharging of the battery. . On the other hand, when the porosity of the first anode active material layer is less than the upper limit of the range, an appropriate energy density may be exhibited, and when the porosity of the second anode active material is less than the upper limit of the range, a better lithium diffusion rate during prelithiation can indicate

In the negative electrode for a lithium secondary battery according to an example of the present invention, the first negative active material layer and the second negative active material layer may each include a negative active material having a tap density in a predetermined range so as to have a porosity within the above range. . For example, the first negative active material may include a negative active material having a tap density of 0.7 g/cc to 1.2 g/cc, specifically 0.8 g/cc to 1.1 g/cc, and the second negative active material is 1.2 g It may include a negative active material having a tap density of /cc to 1.4 g/cc, specifically 1.25 g/cc to 1.35 g/cc.

Meanwhile, the second anode active material layer may include an anode active material having a lower crystallinity than that of the first anode active material layer. In the negative electrode for a lithium secondary battery according to the present invention, when the second negative electrode active material layer located close to the surface of the negative electrode includes a negative electrode active material having a low crystallinity, diffusion of lithium is smooth during prelithiation, so that the entire negative electrode is can increase the lithium diffusion rate.

Meanwhile, the second anode active material layer may include an active material having a lower capacity than that of the first anode active material layer. Therefore, in order to increase only the lithium diffusion rate without reducing the capacity of the entire anode more than necessary, it is necessary to adjust the thickness of the second anode active material layer at an appropriate ratio. Therefore, in the negative electrode for a lithium secondary battery according to an example of the present invention, the thickness of the second negative electrode active material layer may be 5% to 30% based on the thickness of the first negative electrode active material layer, specifically 10% to 20% may be, and more specifically, may be 10% to 15%. When the thickness of the second anode active material layer satisfies the above range, the diffusion rate of lithium throughout the anode may be increased, and the anode may have an appropriate capacity.

In the negative electrode for a lithium secondary battery according to the present invention, a plurality of negative electrode active material layers are formed on the surface of the negative electrode current collector, and the second negative electrode active material layer located on the surface layer exhibits a relatively fast lithium diffusion rate. This may be effectively diffused into the anode active material layer. Therefore, in all cases of prelithiation of the method of depositing lithium on the negative electrode and immersing it in the electrolyte and the method of direct contacting the negative electrode and lithium after immersion in the electrolyte, in all cases of prelithiation, lithium rapidly diffuses into the negative electrode active material layer and lithium may be included in the negative electrode.

In one example of the present invention, the negative electrode for a lithium secondary battery may include a lithium metal layer for all-lithiation. That is, the negative electrode for a lithium secondary battery according to the present invention may further include a lithium metal layer formed on the second negative electrode active material layer.

The lithium metal layer may be formed by physical vapor deposition (PVD) or chemical vapor deposition (PVD) including, for example, sputtering, E-Beam, evaporation or thermal evaporation ( It may be deposited and formed by chemical vapor deposition (CVD).

When an electrode assembly is prepared using the negative electrode for a lithium secondary battery and an electrolyte is injected therein, the lithium metal layer supplies lithium metal by diffusion to the second negative electrode active material layer, and through this, the irreversible capacity of the negative electrode An amount of lithium metal capable of compensating for this may be included in the active material layers of the negative electrode.

On the other hand, the negative electrode for a lithium secondary battery according to an example of the present invention may further include a third negative electrode active material layer formed on the second negative electrode active material layer. The third anode active material layer may include an anode active material having a lithium ion diffusion coefficient of ^{5.0×10 −7} cm ² /s to 5.0×10 ⁻⁶ cm ^{2 /s at 50% SOC, specifically 5.0×10 A} negative active material having a lithium ion diffusion coefficient of -7 cm ² /s to 1.0×10 ^-6 cm ^{2 /s may be included.}

The negative active material included in the third negative active material layer may have a faster lithium diffusion rate than the negative active material included in the second negative active material layer. Accordingly, the third anode active material layer is formed on the second anode active material layer to more easily accept lithium compared to the second anode active material layer, thereby further improving the prelithiation efficiency and speed of the anode. .

In addition, the third anode active material layer may have a smaller porosity than the second anode active material layer, and the third anode active material layer may include an anode active material having a lower crystallinity than that of the second anode active material layer. .

That is, in the negative electrode for a lithium secondary battery according to an example of the present invention, the lithium ion diffusion coefficient of the negative electrode active material included in each negative electrode active material layer may increase sequentially from the negative electrode current collector toward the surface of the negative electrode. In addition, the negative electrode for a lithium secondary battery according to an example of the present invention may include negative electrode active material layers whose porosity is sequentially decreased, and also includes a negative electrode active material having a lower crystallinity sequentially from the negative electrode current collector toward the surface of the negative electrode. It may include anode active material layers.

In the negative electrode for a lithium secondary battery according to the present invention, a plurality of negative electrode active material layers are formed on the surface of the negative electrode current collector, and the negative electrode active material layer located on the outermost layer exhibits the relatively fastest lithium diffusion rate. This may be more effectively diffused into the anode active material layer. Therefore, in all cases of pre-lithiation of the method of depositing lithium on the negative electrode and immersing it in the electrolyte and the pre-lithiation of the method of directly contacting the negative electrode and lithium after immersion in the electrolyte, lithium moves more rapidly into the negative electrode active material layer By diffusion, lithium can be incorporated into the negative electrode quickly and effectively.

When the third anode active material layer is additionally formed on the second anode active material layer, the ratio of the first anode active material layer to the total thickness of the anode active material layer needs to be maintained so as not to affect the total capacity of the anode. Therefore, when the third anode active material layer is formed on the second anode active material layer, the sum of the thicknesses of the second anode active material layer and the third anode active material layer is the third anode active material layer is not formed. It is preferable to be limited to the thickness range of the second negative active material layer of the negative electrode. Accordingly, the sum of the thicknesses of the second anode active material layer and the third anode active material layer may be 5% to 30%, specifically 10% to 20%, based on the thickness of the first anode active material layer, and more Specifically, it may be 10% to 15%. When the sum of the thicknesses of the second anode active material layer and the third anode active material layer satisfies the above range, the diffusion rate of lithium throughout the anode may be increased, and the anode may have an appropriate capacity.

When the negative electrode for a lithium secondary battery according to an example of the present invention has the third negative electrode active material layer, the negative electrode for the lithium secondary battery may further include a lithium metal layer formed on the third negative electrode active material layer. The lithium metal layer, when the electrolyte is injected into the electrode assembly using the negative electrode for the lithium secondary battery, the lithium metal is supplied to the third negative electrode active material layer by diffusion, and the lithium metal is the second negative electrode Since it sequentially moves to the active material layer, an amount of lithium metal capable of compensating for the irreversible capacity of the negative electrode by prelithiation can be included in the active material layers of the negative electrode.

The first anode active material layer is an anode active material with high crystalline carbon, Si, silicon oxide particles (SiO _x , 0<x≤2), Si-metal alloy, and Si and silicon oxide particles (SiO _x , 0<x≤2) ) may include at least one selected from the group consisting of alloys, and the highly crystalline carbon includes natural graphite, kish graphite, pyrolytic carbon, and liquid crystal pitch based carbon (mesophase pitch based carbon). fiber), carbon microbeads, mesophase pitches, and petroleum or coal tar pitch derived cokes that have undergone a heat treatment process of over 3,000 ° C. carbon is mentioned. The highly crystalline carbon means a carbon structure having a graphene (graphite layer) interplanar distance in the range of 0.3354 nm to 0.3370 nm, and specifically, it may be a graphite-based active material having a graphene interplanar distance within 0.3600 nm.

The second anode active material layer is an anode active material, such as soft carbon, hard carbon, or low crystalline carbon such as carbon microspheres, meso-phase low crystalline carbon, and non-graphitizable carbon or graphite. It may include at least one selected from the group consisting of amorphous carbon such as pyrogenic carbon. Examples of the low-crystalline carbon include soft carbon calcined at 1500°C to 2,500°C, or carbon microspheres. The low crystalline carbon means a carbon structure having a graphene (graphite layer) interplanar distance of 0.3370 nm to 0.3600 nm, or including an amorphous structure. The amorphous structure refers to a structure having a graphene interplanar distance of 0.3600 nm or more, and the amorphous carbon refers to a carbon structure having a graphene interplanar distance of 0.3600 nm or more.

On the other hand, when the negative electrode for a lithium secondary battery according to an example of the present invention includes the third negative electrode active material layer, the third negative electrode active material layer has low crystalline carbon and amorphous carbon as the negative electrode active material, similarly to the second negative electrode active material layer. It may include one or more selected from the group consisting of, and the negative active material included in the third negative active material layer is a negative active material included in the second negative active material layer. Appropriate types that can satisfy the above conditions are selected. can

The third anode active material layer may include an anode active material having a lower crystallinity than that of the second anode active material layer, and specifically may include amorphous carbon. On the other hand, when the negative electrode for a lithium secondary battery according to an example of the present invention includes the third negative electrode active material layer, the second negative electrode active material layer may include low crystalline carbon.

As described above, the negative electrode for a lithium secondary battery of the present invention exhibits excellent all-lithiation efficiency, and can be used in a lithium secondary battery in which all-lithiation is performed. Accordingly, the negative electrode for a lithium secondary battery according to an example of the present invention may include lithium metal diffused by pre-lithiation.

The negative electrode may be manufactured by a conventional method known in the art. For example, each negative electrode slurry is prepared by mixing and stirring additives such as a negative active material, a binder, and a conductive material included in the first negative electrode layer and the second negative electrode layer, and then, the negative electrode slurry of the first negative electrode layer is applied to the current collector. After coating and drying, the negative electrode slurry of the second negative electrode layer may be applied on the first negative electrode layer, dried, and then compressed.

Examples of the solvent for forming the negative electrode include organic solvents such as NMP (N-methyl pyrrolidone), DMF (dimethyl formamide), acetone, and dimethyl acetamide or water, and these solvents are used alone or in two or more types. can be mixed and used. The amount of the solvent used is sufficient as long as it is capable of dissolving and dispersing the negative electrode active material, the binder, and the conductive material in consideration of the application thickness of the slurry and the production yield.

The binder may be used to bind the negative active material particles to maintain the molded body, and is not particularly limited as long as it is a conventional binder used in preparing a slurry for the negative electrode active material, for example, polyvinyl alcohol, carboxymethyl cellulose, hydroxy non-aqueous binder. Propylene cellulose, diacetylene cellulose, polyvinyl chloride, polyvinylpyrrolidone, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyethylene or polypropylene, etc. can be used, and acrylic as an aqueous binder Any one selected from the group consisting of lonitrile-butadiene rubber, styrene-butadiene rubber, and acrylic rubber, or a mixture of two or more thereof may be used. Aqueous binders are economical and eco-friendly compared to non-aqueous binders, are harmless to workers' health, and have a superior binding effect compared to non-aqueous binders. Preferably, a styrene-butadiene rubber may be used.

The binder may be included in an amount of 10% by weight or less in the total weight of the slurry for the negative electrode active material, and specifically, it may be included in an amount of 0.1 to 10% by weight. If the content of the binder is less than 0.1% by weight, the effect of using the binder is insignificant and is not preferable. not.

The conductive material is not particularly limited as long as it has conductivity without causing a chemical change in the battery. Examples of the conductive material include graphite such as natural graphite or artificial graphite; carbon black such as 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 whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive materials such as polyphenylene derivatives. The conductive material may be used in an amount of 1 wt% to 9 wt% based on the total weight of the slurry for the negative electrode active material.

The negative electrode current collector used for the negative electrode according to an embodiment of the present invention may have a thickness of 3 μm to 500 μm. The negative electrode current collector is not particularly limited as long as it has conductivity without causing a chemical change in the battery, for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, carbon on the surface of copper or stainless steel; Nickel, titanium, silver, etc. surface-treated, aluminum-cadmium alloy, etc. may be used. In addition, 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 films, sheets, foils, nets, porous bodies, foams, and nonwovens.

The present invention also provides a lithium secondary battery including the negative electrode and the positive electrode for the lithium secondary battery, and a separator interposed between the negative electrode and the positive electrode.

The positive electrode may be manufactured by a conventional method known in the art. For example, a positive electrode can be manufactured by mixing and stirring the positive electrode active material with a solvent and, if necessary, a binder, a conductive material, and a dispersant, then applying (coating) it to a current collector made of a metal material, compressing it, and drying it. have.

The current collector of the metallic material is a metal with high conductivity, and is a metal to which the slurry of the positive electrode active material can be easily adhered. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or a surface treated with carbon, nickel, titanium, silver, etc. on the surface of aluminum or stainless steel may be used. In addition, by forming fine irregularities on the surface of the current collector, the adhesive force of the positive electrode active material may be increased. The current collector may be used in various forms, such as a film, sheet, foil, net, porous body, foam, non-woven body, and the like, and may have a thickness of 3 μm to 500 μm.

The positive active material is, for example _{, a layered compound such as lithium cobalt oxide [Li x} CoO ₂ (0.5<x<1.3)], lithium nickel oxide [Li _x NiO ₂ (0.5<x<1.3)] or an additional transition metal substituted with compound; Lithium manganese oxide, such as formula Li _{1 + x} Mn _{2 - x} O ₄ (where x is 0 to 0.33), LiMnO ₃ , LiMn ₂ O ₃ , or [Li _x MnO ₂ (0.5<x<1.3)]; lithium copper oxide (Li ₂ CuO ₂ ); vanadium oxides such as LiV ₃ O ₈ , LiFe ₃ O ₄ , V ₂ O ₅ , or Cu ₂ V ₂ O _{7 ;} Ni site-type lithium nickel oxide represented by the formula LiNi _{1 - x} M _x O ₂ (wherein, M=Co, Mn, Al, Cu, Fe, Mg, B or Ga, and x=0.01 to 0.3); Formula LiMn _{2 - x} M _x O ₂ (wherein M=Co, Ni, Fe, Cr, Zn or Ta and x=0.01 to 0.1) or Li ₂ Mn ₃ MO ₈ (where M=Fe, Co, lithium manganese composite oxide represented by Ni, Cu or Zn; _{LiMn 2} O _{4 in} which a part of Li in the formula is substituted with an alkaline earth metal ion; disulfide compounds; Fe ₂ (MoO ₄ ) _{3 and the} like.

Examples of the solvent for forming the positive electrode include organic solvents such as NMP (N-methyl pyrrolidone), DMF (dimethyl formamide), acetone, and dimethyl acetamide or water, and these solvents are used alone or in two or more types. can be mixed and used. The amount of the solvent used is sufficient as long as it is capable of dissolving and dispersing the positive electrode active material, the binder, and the conductive material in consideration of the application thickness of the slurry and the production yield.

As the binder, polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidenefluoride, polyacrylonitrile, polymethylmethacrylate, Polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), Sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, polyacrylic acid, and a polymer in which hydrogen is substituted with Li, Na or Ca, or the like; or Various types of binder polymers such as various copolymers may be used.

The 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 acetylene black, Ketjen black, channel black, farness black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes; metal powders such as fluorocarbon, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives may be used. The conductive material may be used in an amount of 1 wt% to 20 wt% based on the total weight of the positive electrode slurry.

The dispersant may be an aqueous dispersant or an organic dispersant such as N-methyl-2-pyrrolidone.

In addition, as the separator, a conventional porous polymer film conventionally used as a separator, such as an ethylene homopolymer, a propylene homopolymer, an ethylene-butene copolymer, an ethylene-hexene copolymer, and an ethylene-methacrylate copolymer such as a polyolefin-based polymer The porous polymer film prepared by ? can be used alone or by laminating them, or a conventional porous nonwoven fabric, such as a nonwoven fabric made of high melting point glass fiber, polyethylene terephthalate fiber, etc., can be used, but is not limited thereto.

Lithium salts that may be included as the electrolyte used in the present invention may be used without limitation, those commonly used in electrolytes for lithium secondary batteries, for example, as an anion of the lithium salt ^{, F -} , Cl ^- , Br ^- , I ^- , NO ₃ ^- , N(CN) ₂ ^- , BF ₄ ^- , ClO ₄ ^- , PF ₆ ^- , (CF ₃ ) ₂ PF ₄ ^- , (CF ₃ ) ₃ PF ₃ ^- , (CF ₃ ) ₄ PF ₂ ^- , (CF ₃ ) ₅ PF ^- , (CF ₃ ) ₆ P ^- , CF ₃ SO ₃ ^- , CF ₃ CF ₂ SO ₃ ^- , (CF ₃ SO ₂ ) ₂ N ^- , (FSO ₂ ) ₂ N ^- , CF ₃ CF ₂ (CF ₃ ) ₂ CO ^- , (CF ₃ SO ₂ ) ₂ CH ^- , (SF ₅ ) ₃ C ^- , (CF ₃ SO ₂ ) ₃ C ^- , CF ₃ (CF ₂ ) ₇ SO ₃ ^- , CF ₃ CO ₂ ^- , CH ₃ CO ₂ ^- , SCN ^- and (CF ₃ CF ₂ SO ₂ ) ₂ N ^- may be any one selected from the group consisting of.

In the electrolyte used in the present invention, as an organic solvent included in the electrolyte, those commonly used in electrolytes for secondary batteries may be used without limitation, and representatively, propylene carbonate (PC), ethylene carbonate (EC) ), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), methylpropyl carbonate, dipropyl carbonate, dimethyl sulfuroxide, acetonitrile, dimethoxyethane, diethoxyethane , vinylene carbonate, sulfolane, gamma-butyrolactone, any one selected from the group consisting of propylene sulfite and tetrahydrofuran, or a mixture of two or more of these may be used representatively. Specifically, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are highly viscous organic solvents and have a high dielectric constant and thus well dissociate lithium salts in the electrolyte. When a low-viscosity, low-dielectric constant linear carbonate such as ethyl carbonate is mixed in an appropriate ratio, an electrolyte having high electrical conductivity can be prepared, and thus it can be used more preferably. In particular, the lithium secondary battery including the negative active material for lithium secondary batteries of the present invention has excellent propylene carbonate resistance including graphite having an alkali carbonate layer formed on the surface, so that the lithium secondary battery can exhibit excellent low temperature performance. Preferably, it may include the propylene carbonate.

Optionally, the electrolyte stored according to the present invention may further include additives such as an overcharge inhibitor included in a conventional electrolyte.

The external shape of the lithium secondary battery of the present invention is not particularly limited, but may be a cylindrical shape, a prismatic shape, a pouch type, or a coin type using a can.

The lithium secondary battery according to the present invention can be used not only in a battery cell used as a power source for a small device, but can also be preferably used as a unit cell in a medium or large battery module including a plurality of battery cells.

Preferred examples of the medium-large device include, but are not limited to, an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, and a power storage system.

Example

Hereinafter, examples and experimental examples will be described in more detail to describe the present invention in detail, but the present invention is not limited by these examples and experimental examples. 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 embodiments described in detail below. 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>

94% by weight of natural graphite having a tap density of 1.0 g/cc and a lithium ion diffusion coefficient of 5.8×10 ^-10 cm ² /s, 1% by weight of carbon black (conductive agent) and 3% by weight of SBR (binder), and CMC ( Thickener) 2% by weight was added to water to prepare a first negative electrode slurry.

94 wt% of hard carbon having an average particle diameter (D ₅₀ ) of 6 μm, a tap density of 1.3 g/cc, and a lithium ion diffusion coefficient of 4.2×10 ^-8 cm ² /s, 1 wt% of carbon black (conductive agent), and SBR 3 wt% (binder) and 2 wt% CMC (thickener) were added to water to prepare a second negative electrode slurry.

The prepared first negative electrode slurry was coated on one surface of a copper current collector to a thickness of 85 μm, and dried to form a first negative electrode active material layer. After the first anode active material layer was formed on the current collector, the prepared second anode active material layer was coated on the first anode active material layer to a thickness of 15 μm, and dried to form a second anode active material layer. This was rolled to prepare a negative electrode.

In the prepared negative electrode, it was confirmed that the porosity of the first negative active material layer was 30%, and the porosity of the second negative active material layer was 25%.

Example 2

A negative electrode was manufactured in the same manner as in Example 1, except that the first negative electrode slurry was coated to a thickness of 85 µm and the second negative electrode slurry to a thickness of 5 µm.

Example 3

A negative electrode was manufactured in the same manner as in Example 1, except that the first negative electrode slurry was coated to a thickness of 70 μm and the second negative electrode slurry to a thickness of 30 μm.

Comparative Example 1

<Production of cathode>

94% by weight of natural graphite having a tap density of 1.0 g/cc and a lithium ion diffusion coefficient of 5.8×10 ^-10 cm ² /s, 1% by weight of carbon black (conductive agent) and 3% by weight of SBR (binder), and CMC ( After preparing a negative electrode slurry by adding 2 wt% of a thickener) to water, the prepared negative electrode slurry was coated on one surface of a copper current collector to a thickness of 100 μm, dried and rolled to prepare a negative electrode.

Examples 4 to 6

<Manufacture of a cathode on which a lithium metal layer is deposited>

A lithium metal layer was formed on the second negative active material layer of the negative electrode prepared in Examples 1 to 3 through physical vapor deposition. For deposition, lithium metal as a raw material is put into a deposition equipment (thermal evaporator, manufacturer), and lithium metal is deposited on the second anode active material layer with a thickness of 5 μm through thermal evaporation. A lithium metal layer containing the was formed.

Comparative Example 2

<Manufacture of a cathode on which a lithium metal layer is deposited>

A lithium metal layer was formed in the same manner except that the negative electrode prepared in Comparative Example 1 was used instead of the negative electrode prepared in Example 1.

Experimental Example 1

_{1M LiPF 6} was dissolved in a solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed in a volume ratio of 30:70, respectively, on the negative electrode on which the lithium metal layer prepared in Example 4 and Comparative Example 2 was deposited. The electrolyte solution was added and soaked, and the change was observed for 10 minutes, and the results are shown in FIGS. 1 and 2 , respectively.

Referring to FIG. 1 , in the negative electrode of Example 4, it can be seen that the lithium metal layer formed on the uppermost surface of the negative electrode disappeared in about 5 minutes. On the other hand, referring to FIG. 2 , in the negative electrode of Comparative Example 2, it can be confirmed that the lithium metal layer formed on the uppermost surface of the negative electrode disappears to a similar extent to the negative electrode of Example 4 only after about 10 minutes. The disappeared lithium metal layer was due to the diffusion of lithium metal into the second anode active material layer by pre-lithiation, thereby confirming that the negative electrode of Example 4 was more rapidly lithiated than the negative electrode of Comparative Example 2.

Experimental Example 2

<Measurement time required for total lithiation>

_{1M LiPF 6} was added to a solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed in a volume ratio of 30:70, respectively, on the negative electrode on which the lithium metal layer prepared in Examples 4 to 6 and Comparative Example 2 was deposited. The dissolved electrolyte solution was added and soaked, and the time until prelithiation was completed by observing the change with time was measured. The results are shown in Table 1 below.

Experimental Example 3

<Production of anode>

_{Li(Co 0.2} Ni _0.6 Mn _0.2 )O ₂ 94 wt% as a cathode active material, carbon black 3 wt% as a conductive material, PVdF 3 wt% as a binder, and N-methyl-2 pyrrolidone (NMP) as a solvent was added to prepare a positive electrode mixture slurry. The positive electrode mixture slurry was applied to an aluminum (Al) thin film, which is a positive electrode current collector, having a thickness of about 20 μm, and dried to prepare a positive electrode, and then roll press was performed.

<Manufacture of lithium secondary battery>

After interposing a polyethylene porous film having a thickness of 12 μm between the negative electrode on which the lithium metal layer prepared in Examples 4 to 6 and Comparative Example 2 was deposited and the positive electrode prepared above, ethylene carbonate (EC) and di _{An electrolyte in which 1M LiPF 6} was dissolved was injected into a solvent in which ethyl carbonate (DEC) was mixed in a volume ratio of 30:70, followed by prelithiation for 30 minutes to prepare coin-type secondary batteries, respectively.

<Evaluation of life characteristics>

Each of the prepared secondary batteries was charged at 1 C up to 4.2V 0.05C cut-off at 25° C. under constant current/constant voltage (CC/CV) conditions, and then at 1 C up to 2.5 V under constant current (CC) conditions. C was discharged, and the discharge capacity was measured. This was repeated for 1 to 100 cycles.

The energy density was expressed as Wh/L, which represents the battery capacity per 1 L of volume when the anode active material was coated, and the discharge capacity after 100 cycles was divided by the discharge capacity of 1 cycle to show the lifespan characteristics. The results are shown in Table 1 below.

<Evaluation of fast charging performance>

A fast charging test was performed on each of the manufactured secondary batteries using an electrochemical charger/discharger. The activation process was performed prior to the rapid charge test, and specifically, the activation process was performed by charging and discharging 3 cycles of a voltage section of 1.5 V to 0.005 V at a current density of 0.1 C-rate.

A rapid charge test was performed on the battery that had completed the activation process in this way, and specifically, rapid charge was performed for 12 minutes at a current density of 2.9 C-rate, and based on the voltage graph obtained at this time, the charge depth ( SOC) was measured. The SOC when lithium precipitation occurred is shown in Table 1 below.

Total lithiation time required per volume
energy density Life characteristics
(After 100 cycles, %) fast charging performance
(Li-Plating SOC%) Example 4 15 minutes 700 Wh/L 88 48 Example 5 20 minutes 710 Wh/L 83 45 Example 6 10 minutes 650 Wh/L 89 50 Comparative Example 2 30 minutes 720 Wh/L 30 28

As can be seen in Table 1, it was found that the negative electrode of Examples 4 to 6 took a shorter total lithiation time than the negative electrode of Comparative Example 2. In addition, the secondary battery including the negative electrode of Examples 4 to 6 was superior to the secondary battery including the negative electrode of Comparative Example 2 in terms of lifespan characteristics and fast charging performance.

In terms of energy density, when the second anode active material layer having a relatively small energy capacity compared to the anode active material included in the first anode active material layer is formed, it has a small value, and the ratio of the second anode active material layer to the total anode active material layer As this value increases, the energy density gradually decreases, but the lifespan characteristics and rapid charging performance are relatively improved. Through this, it was confirmed that it was necessary to adjust the thickness ratio of the first anode active material layer and the thickness ratio of the second anode active material layer to an appropriate ratio by harmonizing appropriate energy density, lifespan characteristics, and fast charging performance.

Claims (18)

negative electrode current collector; a first anode active material layer formed on the anode current collector; and a second anode active material layer formed on the first anode active material layer,
The first anode active material layer includes an anode active material having a lithium ion diffusion coefficient of ^{5.0×10 -11} cm ² /s to 5.0×10 ^-9 cm ^{2 /s at 50% SOC,}
The second anode active material layer includes an anode active material having a lithium ion diffusion coefficient of ^{5.0×10 -9} cm ² /s to 5.0×10 ^-7 cm ^{2 /s at 50% SOC,}
The porosity of the second anode active material layer has a small value compared to the porosity of the first anode active material layer, a negative electrode for a lithium secondary battery.
The method of claim 1,
The first anode active material layer has a porosity of 25% to 35%, a negative electrode for a lithium secondary battery.
The method of claim 1,
The second anode active material layer includes an anode active material having a lower crystallinity than that of the first anode active material layer, a negative electrode for a lithium secondary battery.
The method of claim 1,
The thickness of the second anode active material layer is 5% to 30% based on the thickness of the first anode active material layer, a negative electrode for a lithium secondary battery.
The method of claim 1,
A negative electrode for a lithium secondary battery further comprising a lithium metal layer formed on the second negative electrode active material layer.
The method of claim 1,
Further comprising a third anode active material layer formed on the second anode active material layer, wherein the third anode active material layer is 5.0×10 ^-7 cm ² /s to 5.0×10 ^-6 cm ^{2 at 50% SOC} A negative electrode for a lithium secondary battery comprising a negative active material having a lithium ion diffusion coefficient of /s.
7. The method of claim 6,
The negative electrode active material included in the third negative electrode active material layer has a lithium ion diffusion coefficient at 50% SOC having a larger value than that of the negative electrode active material included in the second negative electrode active material layer, a negative electrode for a lithium secondary battery.
7. The method of claim 6,
The third anode active material layer has a porosity smaller than that of the second anode active material layer within a porosity range of 20% to 30%, a negative electrode for a lithium secondary battery.
7. The method of claim 6,
The third anode active material layer includes an anode active material having a lower crystallinity than that of the second anode active material layer, a negative electrode for a lithium secondary battery.
7. The method of claim 6,
The sum of the thickness of the second anode active material layer and the third anode active material layer is 5% to 30% based on the thickness of the first anode active material layer, a negative electrode for a lithium secondary battery.
7. The method of claim 6,
A negative electrode for a lithium secondary battery further comprising a lithium metal layer formed on the third negative electrode active material layer.
The method of claim 1,
The first anode active material layer is an anode active material with high crystalline carbon, Si, silicon oxide particles (SiO _x , 0<x≤2), Si-metal alloy, and Si and silicon oxide particles (SiO _x , 0<x≤2) ) A negative electrode for a lithium secondary battery comprising at least one selected from the group consisting of an alloy.
The method of claim 1,
The first anode active material layer has a tap density of 0.7 g/cc to 1.2 g/cc and an interlayer distance of 0.3354 nm to 0.3370 nm, comprising a graphite-based active material.
The method of claim 1,
The second anode active material layer comprises at least one selected from the group consisting of low-crystalline carbon and amorphous carbon as an anode active material, a negative electrode for a lithium secondary battery.
The method of claim 1,
The second anode active material layer has a tap density of 1.2 g/cc to 1.4 g/cc, and an interlayer distance comprising a graphite-based active material having a range of 0.3370 nm to 0.3600 nm, a negative electrode for a lithium secondary battery.
The method of claim 1,
The negative electrode for a lithium secondary battery comprises lithium ions diffused by pre-lithiation, a negative electrode for a lithium secondary battery.
A lithium secondary battery comprising the negative electrode for a lithium secondary battery according to claim 1. The method of claim 1,
The second anode active material layer has a porosity of 20% to 30%, a negative electrode for a lithium secondary battery.

Images