METHOD FOR PRE-LITHIATING NEGATIVE ELECTRODE FOR LITHIUM SECONDARY BATTERY, ... AND LITHIUM SECONDARY BATTERY COMPRISING NEGATIVE ELECTRODE

Abstract

The present application relates to a method for prelithiation of a negative electrode for a lithium secondary battery, a negative electrode for a lithium secondary battery, and a lithium secondary battery including the negative electrode.

Description

[Technical field]

[0001] This application claims the benefit of the filing date of Korean Patent Application No. 10-2021-0185204, filed with the Korean Intellectual Property Office on December 22, 2021, the entire contents of which are incorporated herein by reference.

[0002] The present application relates to a method for prelithiation of a negative electrode for a lithium secondary battery, a negative electrode for a lithium secondary battery, and a lithium secondary battery including the negative electrode. [Background technology]

[0003] The rapid increase in the use of fossil fuels has led to an increased demand for alternative and clean energy, and one of the most actively researched fields in this area is power generation and storage using electrochemical reactions.

[0004] At present, secondary batteries are a typical example of electrochemical elements that utilize such electrochemical energy, and the range of their use is tending to expand more and more.

[0005] As technological development and demand for mobile devices increases, the demand for secondary batteries as energy sources is rapidly increasing. Among such secondary batteries, lithium secondary batteries, which have high energy density and voltage, long cycle life, and low self-discharge rate, have been commercialized and are widely used. In addition, research into a method for manufacturing a high-density electrode with a higher energy density per unit volume as an electrode for such high-capacity lithium secondary batteries is being actively conducted.

[0006] In general, a secondary battery is composed of a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode includes a negative electrode active material that inserts and removes lithium ions from the positive electrode. Silicon-based particles having a large discharge capacity can be used as the negative electrode active material.

[0007] Generally, carbon materials such as graphite are used for the negative electrode of lithium secondary batteries, but the theoretical capacity density of carbon is 372 mAh / g (833 mAh / cm 3 ). Therefore, in order to improve the energy density of the negative electrode, silicon (Si), tin (Sn), and their oxides and alloys, which are alloyed with lithium, are being considered as negative electrode materials. Among them, silicon-based materials have attracted attention due to their low cost and high capacity (4200mAh / g).

[0008] However, silicon has a problem that the mechanical stability is reduced due to volume change (shrinkage or expansion) during the process of lithium ion insertion / extraction, which impairs cycle characteristics. Therefore, there is a need to develop a material that has structural stability and is therefore stable when used as an active material for electrochemical devices and can ensure cycle characteristics.

[0009] In addition, when a silicon-based negative electrode active material is used, the problem of a large initial irreversible capacity occurs. In the charge / discharge reaction of a lithium secondary battery, lithium released from the positive electrode is inserted into the negative electrode during charging, and is removed from the negative electrode during discharging and returns to the positive electrode. However, in the case of a silicon-based negative electrode active material, volume change and surface side reactions are intense, and most of the lithium inserted into the negative electrode during initial charging cannot return to the positive electrode, resulting in a problem of a large initial irreversible capacity. When the initial irreversible capacity is large, the problem of a rapid decrease in battery capacity and cycles occurs.

[0010] In order to solve the above problems, a method of prelithiating a silicon negative electrode containing a silicon-based negative electrode active material is known. Known prelithiating methods include a method of preparing an electrode after lithiating the negative electrode by a physical / chemical method such as electrolytic plating, lithium metal transfer, or lithium metal deposition, and a method of electrochemically prelithiating the negative electrode.

[0011] Conventional physicochemical methods have the risk of fire and explosion due to the environmental factors that must be addressed at high temperatures, and conventional electrochemical methods have the problem of being unable to uniformly control the initial irreversible capacity, resulting in increased production costs.

[0012] In particular, in the lithium metal transfer process, it is difficult to transfer lithium metal safely and easily, and even if it is transferred, the highly reactive lithium metal immediately begins to react with the negative electrode active material, causing problems such as particle cracking on the surface of the negative electrode active material layer.

[0013] Therefore, there is a need for research into processes and materials for prelithiating a negative electrode that are safer and more efficient and that can uniformly prelithiate lithium within the negative electrode active material layer. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] JP 2009-080971 A Summary of the Invention [Problem to be solved by the invention]

[0015] As a result of research into the above problem, it was found that in the pre-lithiation process of the transfer process, if a layer containing a specific metal is further laminated on top of the negative electrode active material layer, the pre-lithiation speed can be controlled and side reactions on the negative electrode surface can be prevented.

[0016] The present application therefore relates to a method for prelithiation of a negative electrode for a lithium secondary battery, a negative electrode for a lithium secondary battery, and a lithium secondary battery comprising the negative electrode. [Means for solving the problem]

[0017] One embodiment of the present specification provides a method for pre-lithiation of a negative electrode for a lithium secondary battery, the method including the steps of: forming a negative electrode current collector layer and a negative electrode active material layer on one or both sides of the negative electrode current collector layer; forming a metal layer containing silver (Ag) on ​​a surface of the negative electrode active material layer opposite to a surface in contact with the negative electrode current collector layer; and transferring lithium metal to a surface of the metal layer opposite to a surface in contact with the negative electrode active material layer, wherein a thickness of the metal layer is 10 nm or more and 2 μm or less.

[0018] In yet another embodiment, the present invention provides a negative electrode for a lithium secondary battery, comprising: a negative electrode current collector layer; a negative electrode active material layer including a negative electrode active material layer composition formed on one or both sides of the negative electrode current collector layer; and a metal layer including silver (Ag) provided on a side of the negative electrode active material layer opposite to a side in contact with the negative electrode current collector layer, wherein the thickness of the metal layer is 10 nm or more and 2 μm or less.

[0019] In yet another embodiment, there is provided a lithium secondary battery including: a positive electrode; a negative electrode for a lithium secondary battery that has been prelithiated by the method for prelithiation of a negative electrode for a lithium secondary battery of the present application; a separator disposed between the positive electrode and the negative electrode; and an electrolyte. Effect of the Invention

[0020] A pre-lithiation method for a negative electrode for a lithium secondary battery according to one embodiment of the present invention is characterized in that a metal layer containing silver (Ag) having a thickness of 10 nm to 2 μm is formed on the top of a negative electrode active material layer, and lithium metal can be easily transferred to the top of the negative electrode active material layer during the pre-lithiation process, particularly the transfer process.

[0021] Specifically, by including a metal layer having a thickness within the above range, the lithium metal can be easily transferred from the transfer laminate, and the lithium metal transferred to the upper part of the negative electrode active material layer can be prevented from coming into direct contact with the negative electrode active material. This prevents particle cracking on the surface of the negative electrode active material layer due to rapid prelithiation, and allows lithium to be prelithiated more easily and uniformly inside the negative electrode active material layer.

[0022] Therefore, in the case of a negative electrode for a lithium secondary battery manufactured by the above-mentioned pre-lithiation method for a negative electrode for a lithium secondary battery, by having a metal layer containing silver on the upper part of the negative electrode active material layer, it has a feature that lithium can be distributed more uniformly inside the negative electrode active material layer, specifically, lithium metal can be distributed uniformly within an internal 30% range based on the surface of the negative electrode active material layer. [Brief description of the drawings]

[0023] [Figure 1] FIG. 2 is a diagram showing a method for pre-lithiating a negative electrode for a lithium secondary battery according to one embodiment of the present application. [Diagram 2] FIG. 1 is a diagram showing a stacked structure of a lithium secondary battery according to an embodiment of the present application. [Diagram 3] FIG. 2 is a diagram showing an SEM cross-sectional shape of a surface portion of a negative electrode according to Comparative Example 1. [Figure 4] FIG. 2 is a diagram showing an SEM cross-sectional shape of a surface portion of a negative electrode according to Example 1. [Diagram 5] FIG. 2 is a diagram showing a surface portion of the negative electrode during lithium metal transfer of the negative electrode according to Comparative Example 1. [Figure 6] FIG. 2 is a diagram showing a surface portion of the negative electrode during lithium metal transfer of the negative electrode according to Example 1. [Figure 7] FIG. 13 is a diagram showing a surface portion of a negative electrode during lithium metal transfer of the negative electrode according to Comparative Example 4. [Figure 8] FIG. 13 is a diagram showing a surface portion of the negative electrode during lithium metal transfer in the negative electrode according to Comparative Example 5. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Before describing the present invention, some terms will first be defined. In this specification, when a part is said to "comprise" a certain component, this means that it may further include other components, rather than excluding other components, unless specifically stated to the contrary.

[0025] In this specification, "p to q" means a range of "not less than p and not more than q". In this specification, the "specific surface area" is measured by the BET method, and specifically, is calculated from the amount of nitrogen gas adsorption at liquid nitrogen temperature (77K) using a BELSORP-mino II manufactured by BEL Japan Co., Ltd. That is, in this application, the BET specific surface area may mean the specific surface area measured by the above-mentioned measurement method.

[0026] In this specification, "Dn" means particle size distribution, and refers to the particle size at the n% point of the cumulative particle number distribution by particle size. That is, D50 is the particle size (average particle size) at the 50% point of the cumulative particle number distribution by particle size, D90 is the particle size at the 90% point of the cumulative particle number distribution by particle size, and D10 is the particle size at the 10% point of the cumulative particle number distribution by particle size. Meanwhile, particle size distribution can be measured using a laser diffraction method. Specifically, the powder to be measured is dispersed in a dispersion medium, and then introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac S3500), and the difference in the diffraction pattern due to the particle size when the particles pass through a laser beam is measured to calculate the particle size distribution.

[0027] In this specification, when a polymer contains a certain monomer as a monomer unit, it means that the monomer is involved in a polymerization reaction and is contained as a repeating unit in the polymer. In this specification, when a polymer contains a monomer, this is interpreted as the same as when a polymer contains a monomer as a monomer unit.

[0028] It is understood that in this specification, the term "polymer" is used in a broad sense including copolymers, unless specifically stated as a "homopolymer."

[0029] In this specification, the weight average molecular weight (Mw) and the number average molecular weight (Mn) are polystyrene-equivalent molecular weights measured by gel permeation chromatography (GPC) using monodisperse polystyrene polymers (standard samples) of various polymerization degrees that are commercially available for molecular weight measurement as standard substances. In this specification, the molecular weight means the weight average molecular weight unless otherwise specified.

[0030] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in detail with reference to the accompanying drawings in order to enable those skilled in the art to easily carry out the present invention. However, the present invention may be embodied in various different forms and is not limited to the following description.

[0031] One embodiment of the present specification provides a method for prelithiation of a negative electrode for a lithium secondary battery, the method including the steps of: forming a negative electrode current collector layer and a negative electrode active material layer on one or both sides of the negative electrode current collector layer; forming a metal layer containing silver (Ag) on ​​a surface opposite to a surface of the negative electrode active material layer that contacts the negative electrode current collector layer; and transferring lithium metal to a surface of the metal layer that contacts the negative electrode active material layer, the metal layer having a thickness of 10 nm or more and 2 μm or less.

[0032] A method for pre-lithiation of a negative electrode for a lithium secondary battery according to one embodiment of the present invention is characterized in that a metal layer containing silver (Ag) having a thickness of 10 nm to 2 μm is formed on the top of a negative electrode active material layer, and lithium metal can be easily transferred to the top of the negative electrode active material layer during the pre-lithiation process, particularly the transfer process, and the lithium metal can be uniformly distributed inside the negative electrode active material layer.

[0033] Hereinafter, the method for pre-lithiation of a negative electrode for a lithium secondary battery according to the present invention will be described in detail. In one embodiment of the present application, a step of forming a negative electrode current collector layer and a negative electrode active material layer on one or both sides of the negative electrode current collector layer is provided.

[0034] The above steps are processes for stacking lithium secondary batteries, and the step of forming a negative electrode active material layer on one or both sides of a negative electrode current collector layer includes coating one or both sides of the negative electrode current collector layer with a negative electrode slurry including a negative electrode active material layer composition, and the negative electrode active material layer composition includes at least one selected from the group consisting of a silicon-based active material; a negative electrode conductive material; and a negative electrode binder.

[0035] In one embodiment of the present application, the negative electrode current collector layer generally has a thickness of 1 μm to 100 μm. Such a negative electrode current collector layer is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, baked carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. can be used. In addition, the binding force of the negative electrode active material can be strengthened by forming fine irregularities on the surface, and it can be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0036] In one embodiment of the present application, the negative electrode current collector layer may have a thickness of 1 μm or more and 100 μm or less, and the negative electrode active material layer may have a thickness of 20 μm or more and 500 μm or less.

[0037] However, the thickness can be changed in various ways depending on the type and application of the negative electrode used, and is not limited thereto.

[0038] In one embodiment of the present application, the negative electrode slurry may include: a negative electrode active material layer composition; and a slurry solvent.

[0039] In one embodiment of the present application, the solid content of the negative electrode slurry may be 5% or more and 40% or less.

[0040] In another embodiment, the solid content of the negative electrode slurry may satisfy the range of 5% or more and 40% or less, preferably 7% or more and 35% or less, and more preferably 10% or more and 30% or less.

[0041] The solid content of the negative electrode slurry can mean the content of the negative electrode active material layer composition contained in the negative electrode slurry, and can mean the content of the negative electrode active material composition based on 100 parts by weight of the negative electrode slurry.

[0042] When the solid content of the negative electrode slurry satisfies the above range, the viscosity during the formation of the negative electrode active material layer is appropriate, and it has the characteristic that the particle aggregation phenomenon of the negative electrode active material layer composition can be minimized and the negative electrode active material layer can be efficiently formed.

[0043] In one embodiment of the present application, the slurry solvent is not limited as long as it can dissolve and disperse the negative electrode active material layer composition. Specifically, distilled water or NMP can be used.

[0044] The negative electrode according to one embodiment of the present application can be formed by coating and drying the negative electrode slurry on the negative electrode current collector layer.

[0045] Through the drying stage, the slurry solvent in the negative electrode slurry can be dried.

[0046] In one embodiment of the present application, the negative electrode active material layer composition may include at least one selected from the group consisting of silicon-based active materials, negative electrode conductive materials, and negative electrode binders.

[0047] In one embodiment of the present application, the silicon-based active material may include at least one selected from the group consisting of SiOx (x = 0), SiOx (0 <x <2), SiC, and Si alloys.

[0048] In one embodiment of the present application, the silicon-based active material includes one or more selected from the group consisting of SiOx (x = 0) and SiOx (0 < x ≤ 2), and may contain 70 parts by weight or more of the SiOx (x = 0) based on 100 parts by weight of the silicon-based active material.

[0049] In another embodiment, the silicon-based active material may contain 70 parts by weight or more, preferably 80 parts by weight or more, more preferably 90 parts by weight or more of the SiOx (x = 0) based on 100 parts by weight of the silicon-based active material, and may contain 100 parts by weight or less, preferably 99 parts by weight or less, more preferably 95 parts by weight or less.

[0050] In one embodiment of the present application, the silicon-based active material may contain metal impurities. The metal impurities are impurities that can be contained in silicon, and the content thereof may satisfy the range of 0.1 part by weight or less based on 100 parts by weight of the total silicon-based active material.

[0051] In one embodiment of the present application, in particular, pure silicon (Si) can be used as the silicon-based active material. Using pure silicon (Si) as the silicon-based active material may mean that, as described above, when the total silicon-based active material is based on 100 parts by weight, it contains pure Si particles (SiOx (x = 0)) that are not combined with other particles or elements within the above range.

[0052] In the charge-discharge reaction of a lithium secondary battery, lithium released from the positive electrode during charging is inserted into the negative electrode, and during discharging, it desorbs from the negative electrode and returns to the positive electrode again. However, in the case of a silicon-based negative electrode active material, volume change and surface side reactions are intense, and a large amount of the lithium inserted into the negative electrode during the initial charging cannot return to the positive electrode again, thus causing a problem that the initial irreversible capacity increases. When the initial irreversible capacity increases, problems such as a sharp decrease in battery capacity and cycles occur.

[0053] In the present invention, in order to solve the above-mentioned problems, the negative electrode of the lithium secondary battery is prelithiated to solve the initial irreversible capacity problem. Specifically, the prelithiation process relates to a process in which lithium metal is easily transferred from the transfer laminate during the lithium transfer process, and lithium in the negative electrode active material layer can be uniformly prelithiated.

[0054] In addition, in the case of the present invention, while only a silicon-based active material is used as the negative electrode active material to improve capacity performance, the problems of maintaining the conductive path due to volume expansion and maintaining the bond between the conductive material, binder, and active material are solved by using a binder that meets specific conditions and a conductive material composite bonded to the binder.

[0055] On the other hand, the average particle size (D50) of the silicon-based active material of the present invention may be 5 μm to 10 μm, specifically 5.5 μm to 8 μm, and more specifically 6 μm to 7 μm. When the average particle size is within the above range, the specific surface area of ​​the particles is within an appropriate range, and the viscosity of the negative electrode slurry is formed within an appropriate range. This allows smooth dispersion of the particles constituting the negative electrode slurry. In addition, when the size of the silicon-based active material is equal to or greater than the above lower limit range, the contact area between the silicon particles and the conductive material is excellent due to the composite of the conductive material and the binder in the negative electrode slurry, and the conductive network is more likely to be maintained, thereby increasing the capacity retention rate. On the other hand, when the average particle size satisfies the above range, silicon particles that are too large are excluded, and the surface of the negative electrode is formed smoothly, thereby preventing the current density non-uniformity phenomenon during charging and discharging.

[0056] In one embodiment of the present application, the silicon-based active material generally has a characteristic BET specific surface area. The BET specific surface area of ​​the silicon-based active material is preferably 0.01 to 150.0 m 2 / g, more preferably 0.1 to 100.0 m 2 / g, particularly preferably 0.2 to 80.0 m 2 / g, most preferably 0.2 to 18.0 m 2 / g. The BET specific surface area is determined in accordance with DIN 66131 (using nitrogen).

[0057] In one embodiment of the present application, silicon-based active material can be, for example, in crystalline or amorphous form, and is preferably not porous.Silicon particles are preferably spherical or shard particles.Alternatively, but less preferably, silicon particles can also have a fibrous structure or be in the form of silicon-containing film or coating.

[0058] In one embodiment of the present application, the silicon-based active material may be 60 parts by weight or more based on 100 parts by weight of the negative electrode active material layer composition.

[0059] In yet another embodiment, the silicon-based active material may be contained in an amount of 60 parts by weight or more, preferably 65 parts by weight or more, and more preferably 70 parts by weight or more, based on 100 parts by weight of the negative electrode active material layer composition, and may be contained in an amount of 95 parts by weight or less, preferably 90 parts by weight or less, and more preferably 80 parts by weight or less.

[0060] The negative electrode composition according to the present application uses a specific conductive material and binder that can suppress the volume expansion rate during charging and discharging, even when the above range is included, and has the characteristic of excellent output characteristics during charging and discharging without deteriorating the performance of the negative electrode.

[0061] In one embodiment of the present application, the silicon-based active material may have a non-spherical morphology, and the sphericity is, for example, 0.9 or less, for example, 0.7 to 0.9, for example, 0.8 to 0.9, for example, 0.85 to 0.9.

[0062] In this application, the circularity is determined by the following formula 1-1, where A is the area and P is the perimeter. [Formula 1-1] 4πA / P 2

[0063] In the past, it was common to use only graphite-based compounds as negative electrode active materials, but in recent years, as the demand for high-capacity batteries has increased, there have been increasing attempts to mix and use silicon-based compounds to increase capacity. However, silicon-based compounds have a limitation in that their volume expands rapidly during the charge / discharge process, damaging the conductive path formed in the negative electrode active material layer and actually reducing the performance of the battery. Therefore, the type of negative electrode conductive material used together with the silicon-based active material is important.

[0064] In one embodiment of the present application, the negative electrode conductive material may include at least one selected from the group consisting of a dot-like conductive material, a sheet-like conductive material, and a linear conductive material.

[0065] In one embodiment of the present application, the dot-like conductive material refers to a dot-like or spherical conductive material that can be used to improve the conductivity of the negative electrode and has conductivity without inducing a chemical change. Specifically, the dot-like conductive material may be at least one selected from the group consisting of natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, conductive fiber, fluorocarbon, aluminum powder, nickel powder, zinc oxide, potassium titanate, titanium oxide, and polyphenylene derivatives, and may preferably include carbon black in that it realizes high conductivity and has excellent dispersibility.

[0066] In one embodiment of the present application, the dot-shaped conductive material has a BET specific surface area of ​​40 m 2 / g or more 70m 2 / g or less, preferably 45m 2 / g or more 65m 2 / g or less, more preferably 50m 2 / g or more 60m 2 / g or less.

[0067] In one embodiment of the present application, the particle diameter of the dotted conductive material may be 10 nm to 100 nm, preferably 20 nm to 90 nm, and more preferably 40 nm to 60 nm.

[0068] In one embodiment of the present application, the negative electrode conductive material may include a sheet conductive material. The planar conductive material can improve conductivity by increasing surface contact between silicon particles in the negative electrode, and at the same time, can suppress the disconnection of the conductive path due to volume expansion, and is used as a concept including a bulk conductive material or a plate-shaped conductive material.

[0069] In one embodiment of the present application, the sheet conductive material may include at least one selected from the group consisting of platelet graphite, graphene, graphene oxide, and graphite flakes, and may be preferably platelet graphite.

[0070] In one embodiment of the present application, the average particle size (D50) of the sheet conductive material may be 2 μm to 7 μm, specifically 3 μm to 6 μm, and more specifically 4 μm to 5 μm. If the above range is satisfied, the sufficient particle size makes it easy to disperse the negative electrode slurry without causing an excessive increase in viscosity. Therefore, when dispersing using the same device and time, the dispersion effect is excellent.

[0071] In one embodiment of the present application, the sheet conductive material provides a negative electrode active material layer composition having a D10 of 0.5 μm or more and 1.5 μm or less, a D50 of 2.5 μm or more and 3.5 μm or less, and a D90 of 7.0 μm or more and 15.0 μm or less.

[0072] In one embodiment of the present application, the sheet conductive material may be a sheet conductive material having a high BET specific surface area; or a sheet conductive material having a low specific surface area.

[0073] In one embodiment of the present application, the sheet conductive material may be a high-specific surface area sheet conductive material or a low-specific surface area sheet conductive material without any restrictions. However, the sheet conductive material in the present application may be affected to some extent by dispersion in terms of electrode performance, and it may be particularly preferable to use a low-specific surface area sheet conductive material that does not cause dispersion problems.

[0074] In one embodiment of the present application, the sheet conductive material has a BET specific surface area of ​​5 m 2 / g or more.

[0075] In another embodiment, the sheet conductive material has a BET specific surface area of ​​5 m 2 / g or more 500m 2 / g or less, preferably 5m 2 / g or more 300m 2 / g or less, more preferably 5m 2 / g or more 250m 2 / g or less.

[0076] In another embodiment, the sheet conductive material is a high specific surface area sheet conductive material having a BET specific surface area of ​​50 m 2 / g or more 500m 2 / g or less, preferably 80m 2 / g or more 300m 2 / g or less, more preferably 100m 2 / g or more 300m 2 / g or less.

[0077] In another embodiment, the sheet conductive material is a sheet conductive material having a low specific surface area and a BET specific surface area of ​​5 m 2 / g or more 40m 2 / g or less, preferably 5m 2 / g or more 30m 2 / g or less, more preferably 5m 2 / g or more 25m 2 / g or less.

[0078] Other examples of the negative electrode conductive material include linear conductive materials such as carbon nanotubes. The carbon nanotubes may be bundled carbon nanotubes. The bundled carbon nanotubes may include a plurality of carbon nanotube units. Specifically, unless otherwise specified, the term "bundle type" refers to a secondary shape in the form of a bundle or rope in which a plurality of carbon nanotube units are arranged in a line with the longitudinal axes of the carbon nanotube units substantially aligned in the same direction or are entangled. The carbon nanotube unit has a graphite sheet having a cylindrical shape with a nano-sized diameter and an sp2 bond structure. In this case, the graphite sheet may exhibit conductive or semiconductive properties depending on the angle and structure at which the graphite sheet is wound. The bundled carbon nanotubes can be uniformly dispersed during the manufacture of the negative electrode compared to entangled type carbon nanotubes, and the conductivity of the negative electrode is improved by smoothly forming a conductive network in the negative electrode.

[0079] In one embodiment of the present application, the linear conductive material may be a SWCNT (single-walled carbon nanotube). In one embodiment of the present application, there is provided a negative electrode for a lithium secondary battery, wherein the negative electrode conductive material includes: a sheet conductive material; and a linear conductive material.

[0080] When the above-mentioned dot-shaped conductive materials are not used, the output characteristics of the secondary battery of the present invention can be improved by including linear and sheet-shaped conductive materials as the negative electrode conductive materials.

[0081] In one embodiment of the present application, the negative electrode conductive material may be 10 parts by weight or more and 40 parts by weight or less based on 100 parts by weight of the negative electrode active material layer composition.

[0082] In another embodiment, the negative electrode conductive material may be 10 parts by weight or more and 40 parts by weight or less, preferably 10 parts by weight or more and 30 parts by weight or less, and more preferably 15 parts by weight or more and 25 parts by weight or less, based on 100 parts by weight of the negative electrode active material layer composition.

[0083] In one embodiment of the present application, the negative electrode conductive material may include, based on 100 parts by weight of the negative electrode conductive material, 90 parts by weight or more and 99.9 parts by weight or less of the sheet conductive material; and 0.1 parts by weight or more and 10 parts by weight or less of the linear conductive material.

[0084] In another embodiment, the negative electrode may contain 90 to 99.9 parts by weight, preferably 93 to 99.9 parts by weight, and more preferably 95 to 99.9 parts by weight of the sheet conductive material, based on 100 parts by weight of the negative electrode conductive material.

[0085] In another embodiment, the negative electrode may contain 0.1 to 10 parts by weight, preferably 0.1 to 7 parts by weight, and more preferably 0.1 to 5 parts by weight of the linear conductive material, based on 100 parts by weight of the negative electrode conductive material.

[0086] In one embodiment of the present application, the negative electrode conductive material may include a linear conductive material and a sheet conductive material, and the ratio of the linear conductive material:sheet conductive material may satisfy 0.01:1 to 0.1:1.

[0087] In one embodiment of the present application, the ratio of the linear conductive material:sheet conductive material may satisfy 0.1:1.

[0088] In one embodiment of the present application, the negative electrode conductive material includes a linear conductive material and a planar conductive material, each of which satisfies the above-mentioned composition and ratio, and thus has the characteristics of having no significant effect on the life characteristics of existing lithium secondary batteries, increasing the number of points at which charging and discharging are possible, and providing excellent output characteristics at a high C-rate.

[0089] The negative electrode conductive material according to the present application has a completely different structure from the positive electrode conductive material applied to the positive electrode. That is, the negative electrode conductive material according to the present application serves to capture the contact between the silicon-based active materials, which have a very large volume expansion of the electrodes due to charging and discharging, while the positive electrode conductive material serves to provide some conductivity while acting as a buffer when rolled, and has a completely different structure and role from the negative electrode conductive material of the present invention.

[0090] In addition, the negative electrode conductive material according to the present application is applied to a silicon-based active material, and has a completely different structure from a conductive material applied to a graphite-based active material. That is, a conductive material used in an electrode having a graphite-based active material has a property of improving output characteristics and imparting some electrical conductivity simply because it has smaller particles than the active material, and its structure and role are completely different from a negative electrode conductive material applied together with a silicon-based active material as in the present invention.

[0091] In one embodiment of the present application, the planar conductive material used as the negative electrode conductive material has a structure and role different from that of a carbon-based active material generally used as a negative electrode active material. Specifically, the carbon-based active material used as a negative electrode active material may be artificial graphite or natural graphite, and refers to a material that is processed into a spherical or dot-like shape to facilitate the storage and release of lithium ions.

[0092] Meanwhile, the planar conductive material used as the negative electrode conductive material is a material having a planar or plate-like shape and can be expressed as plate-like graphite. That is, it is a material contained in the negative electrode active material layer to maintain a conductive path, and does not play a role in storing and releasing lithium, but means a material for securing a planar conductive path inside the negative electrode active material layer.

[0093] That is, in the present application, the use of plate-like graphite as a conductive material means that it is processed into a planar or plate-like shape and used as a material that ensures a conductive path, rather than storing or releasing lithium. In this case, the negative electrode active material contained therein has high capacity characteristics for storing and releasing lithium, and plays a role in storing and releasing all the lithium ions transferred from the positive electrode.

[0094] Meanwhile, in the present application, the use of a carbon-based active material as an active material means that the carbon-based active material is processed into a dot-like or spherical shape and is used as a material that serves to store or release lithium.

[0095] That is, in one embodiment of the present application, the carbon-based active material, artificial graphite or natural graphite, is in a dot-like form and has a BET specific surface area of ​​0.1 m 2 / g or more 4.5m 2 / g or less. The plate-shaped graphite, which is a planar conductive material, may have a planar BET specific surface area of ​​5 m 2 / g or more.

[0096] In one embodiment of the present application, the negative electrode binder may include at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, 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 substances in which hydrogen is substituted with Li, Na, Ca, or the like, or may include various copolymers thereof.

[0097] The binder according to one embodiment of the present application plays a role in suppressing the negative electrode active material and the negative electrode conductive material in order to prevent twisting and structural deformation of the negative electrode structure during volume expansion and relaxation of the silicon-based active material. As long as the binder fulfills this role, all common negative electrode binders can be applied, specifically, a water-based binder can be used, and more specifically, a PAM-based binder can be used.

[0098] In one embodiment of the present application, the amount of the negative electrode binder is 30 parts by weight or less, preferably 25 parts by weight or less, and more preferably 20 parts by weight or less, based on 100 parts by weight of the negative electrode active material layer composition, and may be 5 parts by weight or more, or 10 parts by weight or more.

[0099] In one embodiment of the present application, the method may include the step of: forming a metal layer containing silver (Ag) on ​​a surface of the negative electrode active material layer opposite to a surface in contact with the negative electrode current collector layer.

[0100] In the method for pre-lithiation of a negative electrode for a lithium secondary battery according to the present application, the inclusion of the above step facilitates the subsequent transfer of lithium metal from the transfer laminate, prevents the lithium metal transferred to the top of the negative electrode active material layer from coming into direct contact with the negative electrode active material, and prevents particle cracking on the surface of the negative electrode active material layer due to rapid pre-lithiation, thereby enabling lithium to be pre-lithiated more easily and uniformly inside the negative electrode active material layer.

[0101] In particular, silver (Ag) contained in the metal layer of the present application can form a bond with the silicon-based active material on the surface of the negative electrode active material layer to stabilize the silicon-based active material, and when lithium metal is later transferred and pre-lithiated into the negative electrode active material layer, a bond is formed by the reaction between lithium and silver, thereby increasing the efficiency of the pre-lithiation process.

[0102] In one embodiment of the present application, the metal layer may have a thickness of 10 nm or more and 2 μm or less.

[0103] In another embodiment, the thickness of the metal layer may satisfy the range of 10 nm to 2 μm, preferably 10 nm to 1 μm, and more preferably 10 nm to 800 nm.

[0104] When the thickness of the metal layer satisfies the above range, excellent resistance characteristics can be obtained within a range in which there is no problem with lithium ion exchange between the positive electrode and the negative electrode during subsequent assembly of a lithium secondary battery. Furthermore, when the thickness is within the above range, the pre-lithiation rate of lithium metal can be stably controlled and uniform transfer into the negative electrode active material layer is possible.

[0105] Specifically, if the thickness of the metal layer exceeds the upper limit, lithium ions are not transferred by lithium metal later, making prelithiation difficult, and if the thickness of the metal layer is below the lower limit, it is difficult to cover the entire area of ​​the negative electrode.

[0106] In one embodiment of the present application, if the metal layer is not included and lithium metal is transferred, a reaction between the lithium metal and the silicon-based active material on the surface of the negative electrode active material layer may occur suddenly, and the pre-lithiation process may be completed within a few minutes, resulting in an uneven surface and cracking of silicon-based particles. It is a main object of the present invention to solve the above problems by including a metal layer that satisfies the above thickness and composition.

[0107] In one embodiment of the present application, there is provided a pre-lithiation method for a negative electrode for a lithium secondary battery, wherein the step of forming a metal layer containing silver (Ag) on ​​a surface of the negative electrode active material layer opposite to a surface in contact with the negative electrode current collector layer includes a step of depositing the silver (Ag) on ​​the negative electrode active material layer by physical vapor deposition (PVD).

[0108] Specifically, the physical vapor deposition method may refer to a process in which a solid material is heated and evaporated in a high vacuum, and the evaporated material is solidified on a substrate or material to be surface-treated to form a thin film, and may specifically include a sputtering process, an e-beam process, or a thermal evaporator process.

[0109] After forming the silver-containing metal layer by the above-mentioned process, a step of vacuum drying at a temperature of 100° C. to 150° C. for 5 hours to 10 hours may be included.

[0110] Through the above-mentioned process, a metal layer containing silver can be formed on the negative electrode active material layer.

[0111] In one embodiment of the present application, the method may include the step of: transferring lithium metal to a surface of the metal layer opposite to a surface in contact with the negative electrode active material layer.

[0112] In general, the prelithiation process is a process of chemically or physically prelithiating lithium metal to a negative electrode, and specifically, it may proceed through a lithium metal transfer process, a lithium metal powder deposition process, an electro / chemical process, or a lithium metal deposition process. The prelithiation process according to the present application may include a lithium metal transfer process.

[0113] The lithium metal transfer process has the characteristic that highly reactive lithium metal can be transferred more stably to the upper part of the negative electrode active material layer. At this time, a process is required that can easily transfer the lithium metal from the transfer laminate to the upper part of the negative electrode active material layer, and the efficiency of the transfer process can be improved by forming the metal layer on the upper part of the negative electrode active material layer according to the present application.

[0114] In one embodiment of the present application, there is provided a pre-lithiation method for a negative electrode for a lithium secondary battery, wherein the step of transferring lithium metal to the surface of the metal layer opposite to the surface in contact with the negative electrode active material layer includes the steps of: preparing a transfer laminate including a substrate layer and lithium metal provided on the substrate layer; laminating the transfer laminate on the metal layer such that the surface of the lithium metal opposite to the surface in contact with the substrate layer is in contact with the surface of the metal layer opposite to the surface in contact with the negative electrode active material layer; and removing the substrate layer.

[0115] In one embodiment of the present application, a deposition method for depositing the lithium metal on the substrate layer may be selected from among evaporation deposition, chemical vapor deposition (CVD), and physical vapor deposition, but is not limited thereto, and various deposition methods used in the art may be used.

[0116] 1 is a diagram showing a pre-lithiation method for a negative electrode for a lithium secondary battery according to one embodiment of the present application. Specifically, the process shows a step of preparing a transfer laminate 100 including a base layer 10 and lithium metal 20, forming a negative electrode active material layer 30 on a negative electrode current collector layer 40, laminating the negative electrode for a lithium secondary battery 200 in which a metal layer 35 is formed on the negative electrode active material layer 30 so that the metal layer 35 and the lithium metal 20 are in contact with each other, and then removing the base layer 10 to transfer only the lithium metal 20 onto the metal layer 35.

[0117] At this time, the negative electrode for lithium secondary battery on which the transfer laminate is laminated can be subjected to a transfer process through roll pressing with a load of 100 kgf to 800 kgf. After this, a process of removing the base layer is included, and at the time of removal, a bond between silver (Ag) and lithium can be formed by including the metal layer according to the present application, and further, direct contact with the silicon-based active material is prevented, so that the transfer of lithium metal can be easily performed.

[0118] The reason for this is that in the case where there is no metal layer as in the conventional case, lithium metal comes into direct contact with the top of the negative electrode active material layer, and a large number of by-products such as Li nitrides are generated, which makes it difficult to transfer to the negative electrode active material layer, resulting in detachment of lithium metal. However, by including a metal layer satisfying the thickness and composition according to the present application, the above problem is solved.

[0119] In one embodiment of the present application, the base layer can be used without limitation as long as it can withstand process conditions such as high temperatures in the step of depositing lithium metal and has characteristics that can prevent a problem of reverse peeling in which lithium metal is transferred onto the base layer during a winding process for transferring the deposited lithium metal.

[0120] Specifically, in one embodiment of the present application, the base layer may be at least one selected from the group consisting of polyethylene terephthalate (PET), polyimide (PI), poly(methylmethacrylate) (PMMA), polypropylene, polyethylene, and polycarbonate.

[0121] In one embodiment of the present application, the thickness of the base layer may be in the range of 1 μm or more and 300 μm or less, 5 μm or more and 200 μm or less, or 10 μm or more and 100 μm or less.

[0122] In one embodiment of the present application, the thickness of the lithium metal may be 1 μm or more and 10 μm or less, and preferably 3 μm or more and 10 μm or less.

[0123] When the thicknesses of the substrate layer and the lithium metal satisfy the above ranges, the lithium metal can be efficiently transferred to the negative electrode active material layer, and reverse transfer can be prevented.

[0124] In one embodiment of the present application, there is provided a method for pre-lithiation of a negative electrode for a lithium secondary battery, which further includes a release layer on a surface of the transfer laminate in contact with the base layer and the lithium metal, in order to improve the peelability of the lithium metal and ensure transferability to the negative electrode active material layer.

[0125] That is, the base layer may have a release layer formed on at least one surface thereof, or may have release layers formed on both surfaces thereof. The release layer can prevent a reverse peeling problem in which the deposited lithium metal is transferred onto the base layer during a winding process for transferring the deposited lithium metal to the negative electrode, and can also easily separate the base layer after the lithium metal is transferred onto the negative electrode active material layer.

[0126] The release layer may contain at least one selected from the group consisting of silicon-modified polyester in which a silicon chain is graft-bonded to a polyester main chain, Si, melamine, and fluorine.

[0127] In one embodiment of the present application, the release layer may be formed by a coating method, for example, the coating method may be a method selected from the group consisting of dip coating, spray coating, spin coating, die coating, gravure coating, micro-gravure coating, comma coating, and roll coating, but is not limited thereto, and various coating methods used in the art to form coating layers may be used.

[0128] In one embodiment of the present application, a pre-lithiation process may be performed from the step of stacking and transferring the lithium metal onto the metal layer, which may be expressed as a pre-lithiation reaction due to the high reactivity of lithium metal before the activation process.

[0129] In one embodiment of the present application, a step of activating the transferred lithium metal may be included.

[0130] As described above, due to the high reactivity of lithium metal, a reaction of lithium metal may proceed at the moment when the metal layer comes into contact with the upper part of the negative electrode active material layer, which is a reaction prior to the activation step. Thereafter, a step of removing the substrate layer and activating the lithium to pre-lithiate it may be defined as the activation step.

[0131] The present application provides a pre-lithiation method for a negative electrode for a lithium secondary battery, comprising: transferring lithium metal to a surface of the metal layer opposite to a surface in contact with the negative electrode active material layer; and then activating the lithium metal, wherein the activation reaction occurs within 30 minutes to 3 hours under conditions of 25° C. and 1 atm.

[0132] The activation step is a step of setting conditions for diffusing lithium metal into the negative electrode active material layer, and whether or not prelithiation is completed can be determined by whether or not lithium on the upper part of the metal layer is completely removed.

[0133] In one embodiment of the present application, the activation reaction time may be 30 minutes to 3 hours, preferably 1 hour to 2 hours. That is, unlike the conventional method, the metal layer according to the present application can increase the reaction time between lithium metal and the negative electrode active material layer, thereby enabling uniform lithium transfer to the entire negative electrode active material layer.

[0134] In one embodiment of the present application, there is provided a negative electrode for a lithium secondary battery, comprising: a negative electrode current collector layer; a negative electrode active material layer including a negative electrode active material layer composition formed on one or both sides of the negative electrode current collector layer; and a metal layer including silver (Ag) provided on a side of the negative electrode active material layer opposite to a side in contact with the negative electrode current collector layer; wherein the metal layer has a thickness of 10 nm or more and 2 μm or less.

[0135] The components contained in the negative electrode for a lithium secondary battery are the same as those described above. In one embodiment of the present application, the negative electrode for a lithium secondary battery may be pre-lithiated through the above-mentioned pre-lithiation process, and a negative electrode in which the pre-lithiation is completed may be referred to as a pre-lithiated negative electrode for a lithium secondary battery.

[0136] In one embodiment of the present application, there is provided a lithium secondary battery including: a positive electrode; a negative electrode for a lithium secondary battery that has been prelithiated by the method for prelithiating a negative electrode for a lithium secondary battery of the present application; a separator provided between the positive electrode and the negative electrode; and an electrolyte.

[0137] 2 is a diagram showing a laminated structure of a lithium secondary battery according to an embodiment of the present application. Specifically, a lithium secondary battery anode 200 including an anode active material layer 30 on one side of an anode current collector layer 40 can be seen, and a lithium secondary battery cathode 300 including a cathode active material layer 70 on one side of a cathode current collector layer 60 can be seen, and the lithium secondary battery anode 200 and the lithium secondary battery cathode 100 are laminated with a separator 50 sandwiched therebetween.

[0138] The negative electrode for a lithium secondary battery according to the present application is characterized in that a metal layer containing silver is included on the top of the negative electrode active material layer and pre-lithiation is carried out as described above.

[0139] At this time, the pre-lithiated anode can be compared through SEM photographs, and more specifically, the degree of particle cracking from the SEM photographs can be expressed as a ratio through image analysis.

[0140] Image analysis can be performed by selecting particles on the negative electrode surface using the ImageJ program, and calculating the area after pre-lithiation based on 100 of the area of ​​the existing particles on the negative electrode surface before pre-lithiation.

[0141] In one embodiment of the present application, the image analysis of the negative electrode according to the present application can satisfy the following formula 1:

[0142] [Formula 1] 90≦(A / B)x100(%)≦100

[0143] In the above formula 1, A means the ImageJ area ratio of the surface of the negative electrode active material layer after pre-lithiation of the negative electrode including the metal layer, B means the ImageJ area ratio of the negative electrode active material layer surface before prelithiation of the negative electrode including the metal layer.

[0144] In one embodiment of the present application, the silver-containing metal layer provided on the negative electrode current collector layer may be dissolved in the electrolyte and removed after prelithiation, or may remain on the negative electrode as is. However, even if it remains on the negative electrode, it satisfies the thickness range of the present application and does not cause problems such as electrode resistance.

[0145] A secondary battery according to an embodiment of the present specification may include, in particular, the negative electrode for a lithium secondary battery described above. Specifically, the secondary battery may include a negative electrode, a positive electrode, a separator and an electrolyte interposed between the positive electrode and the negative electrode, and the negative electrode is the same as the negative electrode described above. The negative electrode has been described above, so a detailed description thereof will be omitted.

[0146] The positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and including the positive electrode active material.

[0147] In the positive electrode, the positive electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and has conductivity, and for example, stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. can be used. In addition, the positive electrode current collector can usually have a thickness of 3 to 500 μm, and fine irregularities can be formed on the collector surface to increase the adhesive strength of the positive electrode active material. For example, it may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0148] The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material may be a layered compound such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; a lithium iron oxide such as LiFe3O4; 1+c1 Mn2-c1 O4 (0≦c1≦0.33), lithium manganese oxides such as LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, Cu2V2O7; chemical formula LiNi 1-c2 M c2 Ni-site type lithium nickel oxide represented by the chemical formula LiMnO2 (wherein M is at least one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B and Ga, and satisfies 0.01≦c2≦0.6); 2-c3 M c3 Examples of the lithium manganese composite oxide include, but are not limited to, lithium manganese composite oxide represented by Li2Mn3MO8 (wherein M is at least one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, and satisfies 0.01≦c3≦0.6) or Li2Mn3MO8 (wherein M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn); and LiMn2O4 in which part of the Li in the chemical formula is replaced with an alkaline earth metal ion. The positive electrode may be lithium metal (Li-metal).

[0149] In one embodiment of the present application, the positive electrode active material includes a lithium transition metal composite compound including nickel (Ni), cobalt (Co), and manganese (Mn), and the lithium transition metal composite compound includes single particles or secondary particles, and the average particle size (D50) of the single particles may be 1 μm or more.

[0150] For example, the average particle size (D50) of the single particles may be 1 μm or more and 12 μm or less, 1 μm or more and 8 μm or less, 1 μm or more and 6 μm or less, more than 1 μm and 12 μm or less, more than 1 μm and 8 μm or less, or more than 1 μm and 6 μm or less.

[0151] The single particle can have excellent particle strength even if it is formed with a small particle size having an average particle size (D50) of 1 μm or more and 12 μm or less. For example, the single particle can have a strength of 650 kgf / cm 2When the particle is rolled with a force of 650 kgf / cm, the particle strength is 100 MPa to 300 MPa. 2 Even if the electrode is rolled with a strong force, the phenomenon of an increase in fine particles in the electrode due to particle cracking is mitigated, thereby improving the life characteristics of the battery.

[0152] The single particles may be prepared by mixing a transition metal precursor and a lithium source material and sintering the mixture. The secondary particles may be prepared by a method different from that of the single particles, and the composition of the secondary particles may be the same as or different from that of the single particles.

[0153] The method for forming the single particles is not particularly limited, but generally, the single particles can be formed by over-firing at an elevated firing temperature, and can be prepared by using additives such as grain growth promoters that are useful for over-firing, or by changing the starting material.

[0154] For example, the sintering is performed at a temperature at which single particles can be formed. In order to form the single particles, the sintering should be performed at a higher temperature than that at which the secondary particles are produced. For example, when the precursor composition is the same, the sintering should be performed at a temperature about 30°C to 100°C higher than that at which the secondary particles are produced. The sintering temperature for forming the single particles may vary depending on the metal composition in the precursor. For example, when a high-content nickel (High-Ni) NCM-based lithium transition metal composite oxide having a nickel (Ni) content of 80 mol% or more is to be formed as a single particle, the sintering temperature may be about 700°C to 1000°C, preferably about 800°C to 950°C. When the sintering temperature satisfies the above range, a positive electrode active material including single particles having excellent electrochemical properties can be produced. When the sintering temperature is less than 790°C, a positive electrode active material including a lithium transition metal composite compound in the form of secondary particles may be produced, and when the sintering temperature exceeds 950°C, the sintering may occur excessively, the layered crystal structure may not be formed properly, and the electrochemical properties may be deteriorated.

[0155] In this specification, the term "single particle" is used to distinguish it from conventional secondary particles formed by agglomeration of tens to hundreds of primary particles, and is a concept that includes a single particle consisting of one primary particle and a pseudo-single particle form that is an agglomeration of 30 or less primary particles.

[0156] Specifically, the single particle in the present invention may be in the form of a single particle consisting of one primary particle or a pseudo-single particle which is an aggregate of 30 or less primary particles, and the secondary particle may be in the form of an aggregate of several hundred primary particles.

[0157] In one embodiment of the present application, the lithium transition metal composite compound serving as the positive electrode active material further includes secondary particles, and the average particle size (D50) of the single particles is smaller than the average particle size (D50) of the secondary particles.

[0158] In the present invention, a single particle may be in the form of a single particle consisting of one primary particle or a quasi-single particle which is an aggregate of 30 or less primary particles, and a secondary particle may be in the form of an aggregate of several hundred primary particles.

[0159] The lithium transition metal composite compound may further include secondary particles. The secondary particles refer to a form formed by agglomeration of primary particles, and may be distinguished from the concept of a single particle including one primary particle, one single particle, or a pseudo-single particle form that is an agglomeration of 30 or less primary particles.

[0160] The particle diameter (D50) of the secondary particles may be 1 μm to 20 μm, 2 μm to 17 μm, and preferably 3 μm to 15 μm. The specific surface area (BET) of the secondary particles is less than 0.05 m 2 / g~10m 2 / g, preferably 0.1m 2 / g~1m 2 / g, and more preferably 0.3m 2 / g~0.8m 2 / g.

[0161] In a further embodiment of the present application, the secondary particles are aggregates of primary particles, and the primary particles have an average particle size (D50) of 0.5 μm to 3 μm. Specifically, the secondary particles may be in the form of aggregates of several hundred primary particles, and the primary particles may have an average particle size (D50) of 0.6 μm to 2.8 μm, 0.8 μm to 2.5 μm, or 0.8 μm to 1.5 μm.

[0162] When the average particle size (D50) of the primary particles satisfies the above range, a single particle positive electrode active material having excellent electrochemical properties can be formed. If the average particle size (D50) of the primary particles is too small, the number of agglomerates of the primary particles forming the lithium nickel oxide particles increases, and the effect of suppressing particle cracking during rolling decreases. If the average particle size (D50) of the primary particles is too large, the lithium diffusion path inside the primary particles becomes long, which increases resistance and may decrease output characteristics.

[0163] According to a further embodiment of the present application, the average particle size (D50) of the single particles is smaller than the average particle size (D50) of the secondary particles, so that the single particles can have excellent particle strength even when formed with a small particle size, and thus the increase in fine particles in the electrode due to particle cracking can be mitigated, thereby improving the life characteristics of the battery.

[0164] In one embodiment of the present application, the average particle size (D50) of the single particles is smaller than the average particle size (D50) of the secondary particles by 1 μm to 18 μm.

[0165] For example, the average particle size (D50) of the single particles may be 1 μm to 16 μm smaller, 1.5 μm to 15 μm smaller, or 2 μm to 14 μm smaller than the average particle size (D50) of the secondary particles.

[0166] When the average particle size (D50) of the single particles is smaller than the average particle size (D50) of the secondary particles, for example, when the above range is satisfied, the single particles can have excellent particle strength even if they are formed with a small particle size, and thus the phenomenon of an increase in fine particles in the electrode due to particle cracking is mitigated, resulting in an improvement in the life characteristics and energy density of the battery.

[0167] According to a further embodiment of the present application, the single particles are contained in an amount of 15 parts by weight to 100 parts by weight relative to 100 parts by weight of the positive electrode active material. The single particles may be contained in an amount of 20 parts by weight to 100 parts by weight, or 30 parts by weight to 100 parts by weight relative to 100 parts by weight of the positive electrode active material.

[0168] For example, the single particles may be included in an amount of 15 parts by weight or more, 20 parts by weight or more, 25 parts by weight or more, 30 parts by weight or more, 35 parts by weight or more, 40 parts by weight or more, or 45 parts by weight or more relative to 100 parts by weight of the positive electrode active material. The single particles may be included in an amount of 100 parts by weight or less relative to 100 parts by weight of the positive electrode active material.

[0169] When the single particles are contained in the above range, excellent battery characteristics can be exhibited in combination with the above-mentioned negative electrode material. In particular, when the single particles are 15 parts by weight or more, the increase in fine particles in the electrode due to particle cracking during the rolling process after the electrode is manufactured is mitigated, thereby improving the life characteristics of the battery.

[0170] In one embodiment of the present application, the lithium composite transition metal compound may further include secondary particles, and the secondary particles may be 85 parts by weight or less relative to 100 parts by weight of the positive electrode active material. The secondary particles may be 80 parts by weight or less, 75 parts by weight or less, or 70 parts by weight or less relative to 100 parts by weight of the positive electrode active material. The secondary particles may be 0 parts by weight or more relative to 100 parts by weight of the positive electrode active material.

[0171] When the above range is satisfied, the above-mentioned effect due to the presence of the single particle positive electrode active material can be maximized. When the secondary particle positive electrode active material is included, the component may be the same component as the above-mentioned single particle positive electrode active material or a different component, and may mean a form in which the single particle form is aggregated.

[0172] In one embodiment of the present application, the positive electrode active material in 100 parts by weight of the positive electrode active material layer may be contained in an amount of 80 parts by weight or more and 99.9 parts by weight or less, preferably 90 parts by weight or more and 99.9 parts by weight or less, more preferably 95 parts by weight or more and 99.9 parts by weight or less, and still more preferably 98 parts by weight or more and 99.9 parts by weight or less.

[0173] The positive electrode active material layer may contain a positive electrode conductive material and a positive electrode binder in addition to the above-mentioned positive electrode active material.

[0174] At this time, the positive electrode conductive material is used to impart conductivity to the electrode, and can be used without any particular limitation as long as it has electronic conductivity without causing chemical changes in the battery that is constructed.Specific examples include graphite such as natural graphite and artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powder or metal fiber such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and the like, and one or more of these may be used alone or in mixture.

[0175] The positive electrode binder plays a role of improving the adhesion between the positive electrode active material particles and the adhesive strength between the positive electrode active material and the positive electrode current collector.Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof, and one or more of these may be used alone or in combination.

[0176] The separator separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. Any separator that is generally used in secondary batteries can be used without any particular restrictions. In particular, a separator that has low resistance to ion movement of the electrolyte and excellent electrolyte humidification ability is preferred. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof may be used. In addition, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of high-melting glass fiber, polyethylene terephthalate fiber, etc., may be used. In addition, a coated separator containing a ceramic component or a polymeric substance may be used to ensure heat resistance or mechanical strength, and may be used selectively in a single layer or multilayer structure.

[0177] Examples of the electrolyte include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in manufacturing lithium secondary batteries, but are not limited to these. Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0178] Examples of the non-aqueous organic solvent include aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0179] In particular, ethylene carbonate and propylene carbonate, which are cyclic carbonates among the carbonate-based organic solvents, are preferably used since they are high-viscosity organic solvents with high dielectric constants and can dissociate lithium salts well. When such cyclic carbonates are mixed with linear carbonates having low viscosity and low dielectric constants, such as dimethyl carbonate and diethyl carbonate, in an appropriate ratio, an electrolyte having high electrical conductivity can be produced, and therefore such a mixture can be more preferably used.

[0180] The metal salt may be a lithium salt, which is a substance that is easily dissolved in the non-aqueous electrolyte. For example, the anion of the lithium salt may be F - , Cl - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , PF6 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 -, (CF3)5PF - , (CF3)6P - , CF3SO3 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - One or more selected from the group consisting of:

[0181] In addition to the electrolyte components, the electrolyte may further contain one or more additives such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, and aluminum trichloride for the purposes of improving the life characteristics of the battery, suppressing a decrease in battery capacity, and improving the discharge capacity of the battery.

[0182] According to an embodiment of the present invention, there is provided a battery module including the secondary battery as a unit cell, and a battery pack including the same. The battery module and the battery pack include a secondary battery having high capacity, high rate-limiting characteristics, and high cycle characteristics, and therefore can be used as a power source for a medium- to large-sized device selected from the group consisting of an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, and a power storage system. EXAMPLES

[0183] Below, preferred examples are presented to aid in understanding the present invention. However, these examples are intended to illustrate the present description, and it will be apparent to those skilled in the art that various changes and modifications are possible within the scope of the scope and technical ideas of the present description. Naturally, such changes and modifications fall within the scope of the appended claims.

[0184] <Example> <Production of Transfer Laminate> A transfer laminate was produced by depositing lithium metal at a level of 6 to 7 μm using the PVD method on top of a PET substrate coated with a release layer (PET manufactured by IONE Film with a release layer coated on one side, thickness 20 μm to 50 μm).

[0185] <Production of negative electrodes> A negative electrode slurry was prepared by adding Si (average particle size (D50): 3.5 μm) as a silicon-based active material, Denka Black as a conductive material, SBR as a binder, and CMC as a thickener in a weight ratio of 80:15.8:3:1.2 to distilled water as a solvent for forming a negative electrode slurry (solid content concentration: 25 wt%).

[0186] The mixing method was as follows: the conductive material, binder, thickener and water were dispersed using a homomixer at 2500 rpm for 30 minutes, and then the active material was added and dispersed at 2500 rpm for 30 minutes to prepare a slurry.

[0187] The negative electrode current collector was a copper current collector (thickness: 8 μm) on both sides of which the negative electrode slurry was applied at 85 mg / 25 cm. 2 The coated layer was rolled and dried in a vacuum oven at 130° C. for 10 hours to form a negative electrode active material layer (thickness: 33 μm), which was used as a negative electrode (negative electrode thickness: 41 μm, negative electrode porosity: 40.0%).

[0188] Thereafter, a deposition layer having the composition shown in Table 1 below was deposited on the negative active material layer to a thickness range shown in Table 1 below by thermal evaporation.

[0189] Then, in order to transfer the transfer laminate to the negative electrode active material layer, the lithium metal of the transfer laminate was placed on the top of the negative electrode active material layer and roll pressed with a load of 200 kgf / cm.The temperature was 80° C., and immediately after lamination, the PET layer of the transfer laminate was removed to pre-lithiate the negative electrode.

[0190] [Table 1]

[0191] <Manufacturing lithium secondary batteries> A bi-cell type battery was fabricated using the negative electrode (pre-lithiation process) prepared in Table 1 and NCM as the counter electrode. The electrolyte used in this battery was ethylene carbonate (FEC) / ethyl methyl carbonate (EMC) = 3 / 7 (volume ratio) with 1M LiPF6 dissolved therein.

[0192] FIG. 3 is a diagram showing the SEM cross-sectional shape of the surface portion of the negative electrode according to Comparative Example 1, and FIG. 4 is a diagram showing the SEM cross-sectional shape of the surface portion of the negative electrode according to Example 1.

[0193] Specifically, as can be seen from Figures 3 and 4, it can be seen that during pre-lithiation of lithium metal, the degree of cracking of silicon-based particles occurs at the portion where the lithium metal and the surface of the negative electrode active material layer come into contact with each other. Figure 3 shows the case where there is no silver-containing metal layer, and it can be seen that the degree of particle cracking in the surface portion occurs more than when there is a silver-containing metal layer.

[0194] This is because the presence of the silver-containing metal layer reduces the reaction with the silicon-based active material, reduces particle cracking of the silicon-based active material, and minimizes reaction by-products.

[0195] FIG. 5 is a diagram showing the surface portion of the negative electrode when lithium metal is transferred to the negative electrode of Comparative Example 1, and FIG. 6 is a diagram showing the surface portion of the negative electrode when lithium metal is transferred to the negative electrode of Example 1.

[0196] As can be seen from Fig. 5, when the negative electrode active material layer surface does not have a metal layer containing silver, it is difficult to remove the base layer during lithium metal transfer, and therefore the lithium metal is not easily transferred, resulting in electrode detachment. In contrast, as can be seen from Fig. 6, when the negative electrode includes a metal layer containing silver according to the present application, it was confirmed that the transfer of lithium metal also occurs easily.

[0197] Comparative Examples 2 and 3 correspond to cases in which the negative electrode active material layer has a silver-containing metal layer on its surface, but the thickness of the metal layer is greater than the upper limit and less than the lower limit. Specifically, in the case of Comparative Example 2, the thickness of the metal layer is small, and particle cracking still occurs on the surface as in Comparative Example 1. In the case of Comparative Example 3, the thickness of the metal layer is large, and prelithiation does not occur easily.

[0198] Fig. 7 is a diagram showing a surface portion of a negative electrode during lithium metal transfer of a negative electrode according to Comparative Example 4. Fig. 8 is a diagram showing a surface portion of a negative electrode during lithium metal transfer of a negative electrode according to Comparative Example 5. Comparative Examples 4 and 5 each contain an inorganic layer other than silver (Ag) as the metal layer, and as with Comparative Example 3, it can be seen that lithium metal ions cannot pass through smoothly, pre-lithiation does not easily occur, and the substrate layer cannot easily be removed during lithium metal transfer, making it difficult to transfer lithium metal and causing electrode detachment. [Explanation of symbols]

[0199] 10...Base material layer 20 Lithium metal 30...Negative electrode active material layer 35 Silver-containing metal layer 40 Negative electrode current collector layer 50...Separation membrane 60 Positive electrode current collector layer 70...Cathode active material layer 100 ··Transfer laminate 200 ··Negative electrode for lithium secondary batteries 300 ··Positive electrode for lithium secondary batteries

Claims

1. forming a negative electrode current collector layer and a negative electrode active material layer on one or both sides of the negative electrode current collector layer; forming a metal layer including silver (Ag) on ​​a surface of the negative electrode active material layer opposite to a surface of the negative electrode active material layer that contacts the negative electrode current collector layer; and transferring lithium metal to a surface of the metal layer opposite to a surface of the metal layer that contacts the negative electrode active material layer; A method for prelithiating a negative electrode for a lithium secondary battery, comprising: The method for pre-lithiation of a negative electrode for a lithium secondary battery, wherein the thickness of the metal layer is 10 nm or more and 2 μm or less.

2. forming a negative electrode current collector layer and a negative electrode active material layer on one or both sides of the negative electrode current collector layer includes coating one or both sides of the negative electrode current collector layer with a negative electrode slurry including a negative electrode active material layer composition; 2. The method for pre-lithiation of a negative electrode for a lithium secondary battery according to claim 1, wherein the negative electrode active material layer composition comprises at least one selected from the group consisting of a silicon-based active material, a negative electrode conductive material, and a negative electrode binder.

3. 2. The method of claim 1, wherein the step of forming a metal layer including silver (Ag) on ​​a surface of the negative electrode active material layer opposite to a surface in contact with the negative electrode current collector layer comprises depositing the silver (Ag) on ​​the negative electrode active material layer via a physical vapor deposition (PVD) method.

4. transferring lithium metal to a surface of the metal layer opposite to a surface of the metal layer that contacts the negative electrode active material layer; and after the transferring step, activating the lithium metal; 2. The method of claim 1, wherein the activation reaction of the lithium metal occurs at 25° C. and 1 atm within 30 minutes to 3 hours.

5. The step of transferring lithium metal to a surface of the metal layer opposite to the surface in contact with the negative electrode active material layer includes: Providing a transfer laminate including a substrate layer and lithium metal provided on the substrate layer; laminating the transfer laminate on the metal layer such that a surface of the lithium metal opposite to a surface in contact with the base layer is in contact with a surface of the metal layer opposite to a surface in contact with the negative electrode active material layer; removing the substrate layer; The method for pre-lithiation of a negative electrode for a lithium secondary battery according to claim 1 , comprising:

6. The method for pre-lithiation of a negative electrode for a lithium secondary battery according to claim 5 , further comprising a release layer on a surface of the transfer laminate in contact with the base layer and the lithium metal.

7. 3. The method for pre-lithiation of a negative electrode for a lithium secondary battery according to claim 2, wherein the silicon-based active material includes at least one selected from the group consisting of SiOx (x=0), SiOx (0<x<2), SiC, and a Si alloy.

8. 3. The method of claim 2, wherein the silicon-based active material comprises at least one selected from the group consisting of SiOx (x=0) and SiOx (0<x<2), and the silicon-based active material comprises 70 parts by weight or more of the SiOx (x=0) based on 100 parts by weight of the silicon-based active material.

9. 2. The method for pre-lithiation of a negative electrode for a lithium secondary battery according to claim 1, wherein the thickness of the lithium metal is 1 μm or more and 10 μm or less.

10. A negative electrode current collector layer, a negative electrode active material layer including a negative electrode active material layer composition formed on one or both surfaces of the negative electrode current collector layer; and a metal layer containing silver (Ag) provided on a surface of the negative electrode active material layer opposite to a surface in contact with the negative electrode current collector layer; Including, The thickness of the metal layer is 10 nm or more and 2 μm or less, The negative electrode for a lithium secondary battery, wherein lithium is distributed inside the negative electrode active material layer.

11. The thickness of the negative electrode current collector layer is 1 μm or more and 100 μm or less, 11. The negative electrode for a lithium secondary battery according to claim 10, wherein the negative electrode active material layer has a thickness of 20 μm or more and 500 μm or less.

12. Positive electrode; A negative electrode for a lithium secondary battery prelithiated by the method for prelithiation of a negative electrode for a lithium secondary battery according to any one of claims 1 to 9; A separator disposed between the positive electrode and the negative electrode; and Electrolyte; A lithium secondary battery comprising:

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