Anode for lithium secondary battery, method for manufacturing anode for lithium secondary battery, and lithium secondary battery including anode

Abstract

The present application relates to a negative electrode for a lithium secondary battery, a method for producing a lithium secondary battery, and a lithium secondary battery.

Description

[Technical Field]

[0001] This application claims the benefit of the filing date of Korean Patent Application No. 10-2021-0172282, filed with the Korean Intellectual Property Office on December 3, 2021, the entire contents of which are incorporated herein by reference.

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

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

[0004] Currently, a typical example of an electrochemical element that utilizes such electrochemical energy is a secondary battery, and the range of its use is tending to expand more and more.

[0005] As technological development and demand for mobile devices increase, the demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which have high energy density and voltage, long cycle life, and low self-discharge rate, have been commercialized and widely used. In addition, active research is being conducted on methods for manufacturing high-density electrodes with higher energy density per unit volume as electrodes for such high-capacity lithium secondary batteries.

[0006] Generally, a secondary battery is composed of a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode contains a negative electrode active material that inserts and extracts lithium ions from the positive electrode. The negative electrode active material may be silicon-based particles with a high discharge capacity.

[0007] Generally, carbon materials such as graphite are used for the negative electrode of lithium secondary batteries, and the theoretical capacity density of carbon is 372 mAh / g (833 mAh / cm3 ) Therefore, in order to improve the energy density of the anode, silicon (Si), tin (Sn), and their oxides and alloys, which are alloyed with lithium, are being considered as anode materials. Among these, silicon-based materials have attracted attention due to their low cost and high capacity (4200mAh / g).

[0008] However, silicon undergoes volume changes (shrinkage or expansion) during the lithium ion insertion / extraction process, which reduces its mechanical stability and results in impaired cycle performance. Therefore, there is a need to develop a material that has structural stability and can be used as an active material in electrochemical devices, ensuring excellent stability and cycle performance.

[0009] Furthermore, the use of silicon-based negative electrode active materials poses the problem of high initial irreversible capacity. During the charge-discharge reaction of a lithium secondary battery, lithium is released from the positive electrode and inserted into the negative electrode during charging, and then released from the negative electrode and returned to the positive electrode during discharge. However, with silicon-based negative electrode active materials, volume changes and surface side reactions are so severe that much of the lithium inserted into the negative electrode during initial charging cannot return to the positive electrode, resulting in a high initial irreversible capacity. This high initial irreversible capacity leads to a rapid decrease in battery capacity and cycle life.

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

[0011] Conventional physicochemical methods must be carried out at high temperatures, which poses the risk of fire and explosion due to environmental factors. Conventional electrochemical methods cannot uniformly control the initial irreversible capacity, which increases 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, research is needed into processes and materials that are safer and more efficient when prelithiating a negative electrode and that can uniformly prelithiate lithium metal within the negative electrode active material layer. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-080971 Summary of the Invention [Problem to be solved by the invention]

[0015] We have researched a method for transferring lithium metal to the top of the negative electrode active material layer more safely and efficiently in transfer-based prelithiation, and have found that if a buffer layer with a specific composition, content, and thickness is included on the top of the negative electrode active material layer, direct contact with lithium metal can be prevented and the prelithiation rate can be adjusted as desired.

[0016] Accordingly, the present application relates to a negative electrode for a lithium secondary battery, a method for producing a negative electrode for a lithium secondary battery, and a lithium secondary battery including the negative electrode. [Means for solving the problem]

[0017] One embodiment of the present specification provides a negative electrode for a lithium secondary battery, comprising: a negative electrode current collector layer; a negative electrode active material layer formed on one or both sides of the negative electrode current collector layer, the negative electrode active material layer comprising a negative electrode active material layer composition; and a buffer layer provided on the surface of the negative electrode active material layer opposite the surface facing the negative electrode current collector layer, the buffer layer comprising a buffer layer composition; wherein the buffer layer has a thickness of 0.1 μm to 2 μm, and the buffer layer composition comprises at least one material selected from the group consisting of acrylic polymers and binders; and a conductive material, and the conductive material is present in an amount of 1 part by weight to 10 parts by weight per 100 parts by weight of the buffer layer composition.

[0018] In yet another embodiment, there is provided a method for manufacturing a negative electrode for a lithium secondary battery, the method comprising: 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; coating a buffer layer composition on a surface of the negative electrode active material layer opposite to the surface facing the negative electrode current collector layer to form a negative electrode for a lithium secondary battery including a buffer layer; transferring lithium metal to the surface of the buffer layer opposite to the surface facing the negative electrode active material layer; and prelithiating the negative electrode active material layer, wherein the buffer layer composition comprises at least one material selected from the group consisting of an acrylic polymer and a binder; and a conductive material, and the conductive material is in an amount of 1 part by weight to 10 parts by weight per 100 parts by weight of the buffer layer composition.

[0019] Finally, there is provided a lithium secondary battery including: a positive electrode; a negative electrode for a lithium secondary battery according to the present application; a separator disposed between the positive electrode and the negative electrode; and an electrolyte. [Effects of the Invention]

[0020] According to one embodiment of the present invention, a negative electrode for a lithium secondary battery includes a buffer layer having a thickness of 0.1 μm to 2 μm on top of a negative electrode active material layer. Specifically, the buffer layer includes at least one material selected from the group consisting of an acrylic polymer and a binder; and a conductive material, the conductive material being present in an amount of 1 part by weight to 10 parts by weight based on 100 parts by weight of the buffer layer composition. This prevents direct contact with the reactive silicon-based active material even when lithium metal is transferred to the top of the negative electrode active material layer, thereby suppressing a rapid reaction. This allows for the rate of prelithiation to be controlled, thereby enabling uniform prelithiation of lithium within the negative electrode active material layer.

[0021] As described above, the presence of a buffer layer on the negative electrode for a lithium secondary battery controls the prelithiation rate, reduces by-products with lithium due to violent reactions on the surface of the negative electrode active material layer, and reduces loss of lithium metal. Furthermore, the presence of a buffer layer allows the substrate layer to be quickly removed even with very weak linear pressure after lithium metal is transferred to the negative electrode active material layer, and easily releases reaction heat, suppressing the generation of by-products on the surface of the negative electrode active material layer.

[0022] Furthermore, after the pre-lithiation process in the buffer layer on the upper part of the negative electrode for a lithium secondary battery, one or more materials selected from the group consisting of acrylic polymers and binders dissolve and disappear when the electrolyte is injected, leaving only the conductive material, which later helps to form a conductive network that connects the active material when the negative electrode active material expands or contracts.

[0023] That is, the negative electrode for a lithium secondary battery according to the present invention is mainly characterized in that it includes a buffer layer having a specific composition and thickness on the top of the negative electrode active material layer, so that prelithiation during the prelithiation step by transfer can be more efficient and uniform across the entire negative electrode active material layer without loss of lithium. [Brief explanation of the drawings]

[0024] [Figure 1]1A to 1C are diagrams illustrating a process for transferring lithium metal to a negative electrode for a lithium secondary battery according to one embodiment of the present application. [Figure 2] FIG. 1 is a diagram showing a stack structure of a lithium secondary battery according to an embodiment of the present application. DETAILED DESCRIPTION OF THE INVENTION

[0025] Before describing the present invention, some terms will first be defined.

[0026] 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.

[0027] In this specification, "p to q" means a range of "not less than p and not more than q."

[0028] 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 (77 K) using a BELSORP-mini II manufactured by BEL Japan Co., Ltd. That is, in the present application, the BET specific surface area may mean the specific surface area measured by the above-mentioned measurement method.

[0029] In this specification, "Dn" refers to the average particle size, or the particle size at the n% point in the cumulative particle number distribution by particle size. That is, D50 is the particle size at the 50% point in the cumulative particle number distribution by particle size, D90 is the particle size at the 90% point in the cumulative particle number distribution by particle size, and D10 is the particle size at the 10% point in the cumulative particle number distribution by particle size. Meanwhile, the average particle size can be measured using the 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 analyzer (e.g., Microtrac S3500). The particle size distribution is calculated by measuring the difference in diffraction patterns due to particle size when the particles pass through a laser beam.

[0030] 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.

[0031] It is understood that the term "polymer" is used in the broad sense herein to include copolymers unless "homopolymer" is specifically stated.

[0032] In this specification, the weight average molecular weight (Mw) and number average molecular weight (Mn) are polystyrene-equivalent molecular weights measured by gel permeation chromatography (GPC) using commercially available monodisperse polystyrene polymers (standard samples) with various degrees of polymerization that are commercially available for molecular weight measurement as standard substances. In this specification, molecular weight means weight average molecular weight unless otherwise specified.

[0033] The present invention will now be described in detail with reference to the accompanying drawings so that those skilled in the art can easily implement the present invention. However, the present invention may be embodied in various different forms and is not limited to the following description.

[0034] One embodiment of the present specification provides a negative electrode for a lithium secondary battery, comprising: a negative electrode current collector layer; a negative electrode active material layer formed on one or both sides of the negative electrode current collector layer, the negative electrode active material layer comprising a negative electrode active material layer composition; and a buffer layer provided on the surface of the negative electrode active material layer opposite the surface facing the negative electrode current collector layer, the buffer layer comprising a buffer layer composition; wherein the buffer layer has a thickness of 0.1 μm to 2 μm, and the buffer layer composition comprises at least one material selected from the group consisting of acrylic polymers and binders; and a conductive material, and the conductive material is present in an amount of 1 part by weight to 10 parts by weight per 100 parts by weight of the buffer layer composition.

[0035] The negative electrode for a lithium secondary battery according to the present invention is mainly characterized in that it comprises a buffer layer having a specific composition and thickness on top of the negative electrode active material layer, so that prelithiation during the prelithiation step can be carried out more efficiently, without loss of lithium, and uniformly throughout the negative electrode active material layer.

[0036] The negative electrode for a lithium secondary battery of the present invention will be specifically described below.

[0037] 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. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, aluminum-cadmium alloy, etc., can be used. Furthermore, the bonding strength of the negative electrode active material can be strengthened by forming fine irregularities on the surface, and the negative electrode current collector layer can be used in various forms such as a film, sheet, foil, net, porous material, foam, nonwoven fabric, etc.

[0038] 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.

[0039] However, the thickness may vary depending on the type and application of the negative electrode used, and is not limited thereto.

[0040] In one embodiment of the present application, the negative electrode current collector layer includes a negative electrode active material layer formed on one or both surfaces thereof, the negative electrode active material layer comprising a negative electrode active material layer composition.

[0041] In one embodiment of the present application, the negative electrode active material layer composition may include a silicon-based active material; a negative electrode conductive material; and a negative electrode binder.

[0042] In one embodiment of the present application, the silicon-based active material is SiOx (x = 0), SiO x It may contain at least one selected from the group consisting of (0 < x < 2), SiC, metal impurities, and Si alloys.

[0043] In one embodiment of the present application, the silicon-based active material is SiO x (x = 0) and SiO x It may contain one or more selected from the group consisting of (0 < x < 2), and based on 100 parts by weight of the silicon-based active material, the SiO x (x = 0) may be contained in an amount of 70 parts by weight or more.

[0044] In another embodiment, based on 100 parts by weight of the silicon-based active material, the SiO x (x = 0) may be contained in an amount of 70 parts by weight or more, preferably 80 parts by weight or more, more preferably 90 parts by weight or more, and may be contained in an amount of 100 parts by weight or less, preferably 99 parts by weight or less, more preferably 95 parts by weight or less.

[0045] In one embodiment of the present application, the silicon-based active material may contain metal impurities. At this time, the metal impurities are common metal substances contained in the silicon-based active material, and the content thereof may be 0.1 part by weight or less based on 100 parts by weight of the silicon-based active material.

[0046] In one embodiment of the present application, particularly, pure silicon (Si) can be used as the silicon-based active material. Using pure silicon (Si) as the silicon-based active material means that, as described above, when based on a total of 100 parts by weight of the silicon-based active material, it may mean containing pure Si particles (SiO x (x = 0)) within the above range.

[0047] Silicon-based active materials have significantly higher capacity than conventional graphite-based active materials, and efforts to use them have been increasing. However, due to their high volume expansion during charge and discharge, they have only been used in small amounts, such as by mixing them with graphite-based active materials. Despite these advantages, however, they suffer from a high initial irreversible capacity, resulting in reduced lifespan. To address this issue, pre-lithiation processes, in which the anode is pre-lithiated, are being developed. However, when the content of silicon-based active materials is increased to maximize anode capacity, problems such as particle cracking of the silicon-based active material on the surface occur due to their high reactivity with lithium metal.

[0048] Therefore, in the present invention, a high-content silicon-based active material is used as the negative electrode active material to improve capacity performance, but a buffer layer having a specific thickness and composition is introduced on the negative electrode active material layer to solve the above-mentioned problems. This is a main object of the present invention.

[0049] Meanwhile, 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 this range, the specific surface area of ​​the particles falls within an appropriate range, and the viscosity of the negative electrode slurry is formed within an appropriate range. This allows for smooth dispersion of the particles constituting the negative electrode slurry. Furthermore, when the size of the silicon-based active material is equal to or greater than the lower limit, the composite of the conductive material and binder in the negative electrode slurry has an excellent contact area between the silicon particles and the conductive material, increasing the likelihood of maintaining a conductive network, and thereby increasing capacity retention. On the other hand, when the average particle size is within this range, excessively large silicon particles are excluded, resulting in a smooth negative electrode surface, thereby preventing non-uniform current density during charge and discharge.

[0050] 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 m 2 / g~150.0m 2 / g, more preferably 0.1m2 / g~100.0m 2 / g, particularly preferably 0.2m 2 / g~80.0m 2 / g, most preferably 0.2m 2 / g~18.0m 2 / g. The BET specific surface area is determined in accordance with DIN 66131 (using nitrogen).

[0051] In one embodiment of the present application, the silicon-based active material can be, for example, in crystalline or amorphous form, and is preferably not porous. The silicon particles are preferably spherical or shard-like particles. Alternatively, but less preferably, the silicon particles may also have a fibrous structure or be in the form of a silicon-containing film or coating.

[0052] In one embodiment of the present application, the silicon-based active material may be present in an amount of 60 parts by weight or more based on 100 parts by weight of the negative electrode active material layer composition.

[0053] 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.

[0054] The negative electrode active material layer composition according to the present application uses a conductive material and a binder that can suppress the volume expansion rate during charge and discharge even when a silicon-based active material with extremely high capacity is used within the above range, and therefore does not deteriorate the performance of the negative electrode even when the range is included, and has excellent output characteristics during charge and discharge.

[0055] In one embodiment of the present application, the silicon-based active material may have a non-spherical shape, and the circularity thereof 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.

[0056] In this application, the circularity is determined by the following formula 1, where A is the area and P is the perimeter.

[0057] [Formula 1] 4πA / P 2

[0058] While graphite-based compounds have traditionally been used exclusively as anode active materials, attempts to incorporate silicon-based compounds into batteries to increase capacity have been increasing in recent years as demand for high-capacity batteries has grown. However, silicon-based compounds have limitations, such as their rapid volume expansion during charge / discharge processes, damaging the conductive pathways formed within the anode active material layer and reducing battery performance.

[0059] Thus, in one embodiment of the present application, the negative electrode conductive material may include at least one selected from the group consisting of dot-like conductive material, linear conductive material, and sheet-like conductive material.

[0060] In one embodiment of the present application, the dot-like conductive material may be used to improve the conductivity of the negative electrode and is preferably conductive without inducing chemical changes. Specifically, the 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 preferably includes carbon black, which realizes high conductivity and excellent dispersibility.

[0061] In one embodiment of the present application, the point-like 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 60m2 / g or less.

[0062] In one embodiment of the present application, the particle size of the dotted conductive material may be 10 nm to 100 nm, preferably 20 nm to 90 nm, and more preferably 20 nm to 60 nm.

[0063] In one embodiment of the present application, the conductive material may include a planar conductive material.

[0064] The planar conductive material increases the surface contact between silicon particles in the negative electrode to improve conductivity, and at the same time, prevents the conductive path from being broken due to volume expansion. The planar conductive material may be referred to as a plate-type conductive material or a bulk-type conductive material.

[0065] 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.

[0066] 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. When the average particle size satisfies this range, the sufficient particle size facilitates dispersion without excessively increasing the viscosity of the negative electrode slurry. Therefore, when dispersion is performed using the same equipment and time, the dispersion effect is excellent.

[0067] In one embodiment of the present application, the sheet conductive material provides a negative electrode 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.

[0068] 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.

[0069] In one embodiment of the present application, the planar conductive material may be a high-specific surface area planar conductive material or a low-specific surface area planar conductive material without any restrictions. However, the planar conductive material of 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 planar conductive material that does not cause dispersion problems.

[0070] In one embodiment of the present application, the sheet conductive material has a BET specific surface area of ​​1 m 2 / g or more.

[0071] In another embodiment, the sheet conductive material has a BET specific surface area of ​​1 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.

[0072] 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.

[0073] In another embodiment, the sheet conductive material is a sheet conductive material with a low specific surface area, and has a BET specific surface area of ​​1 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.

[0074] Other conductive materials include linear conductive materials such as carbon nanotubes. The carbon nanotubes may be bundled carbon nanotubes. The bundled carbon nanotubes may include multiple carbon nanotube units. Specifically, unless otherwise specified, the term "bundle type" refers to a secondary shape in which multiple carbon nanotube units are arranged side by side or entangled with their longitudinal axes aligned in substantially the same direction, forming a bundle or rope-like shape. The carbon nanotube units have cylindrical graphite sheets with nanosized diameters and an sp2 bonding structure. Depending on the winding angle and structure of the graphite sheets, the carbon nanotubes may exhibit conductive or semiconductive properties. Compared to entangled carbon nanotubes, the bundled carbon nanotubes can be more uniformly dispersed during negative electrode fabrication, smoothly forming a conductive network within the negative electrode, and improving the conductivity of the negative electrode.

[0075] 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.

[0076] In another embodiment, the negative electrode conductive material may contain 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 10 parts by weight or more and 20 parts by weight or less, based on 100 parts by weight of the negative electrode active material layer composition.

[0077] The negative electrode conductive material according to the present application has a completely different structure from the conductive material used in the positive electrode. That is, the negative electrode conductive material according to the present application serves to secure the contact point between the silicon-based active materials, which undergo a large volume expansion of the electrode upon charge and discharge, while the positive electrode conductive material serves to provide some conductivity while acting as a buffer during rolling, and is completely different in structure and role from the negative electrode conductive material of the present invention.

[0078] Furthermore, 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 conductive materials applied to graphite-based active materials. That is, conductive materials used in electrodes having graphite-based active materials simply have smaller particles than the active material, and therefore have the properties of improving output characteristics and imparting some conductivity, and their structure and role are completely different from those of negative electrode conductive materials applied together with silicon-based active materials as in the present invention.

[0079] In one embodiment of the present application, the plate-like conductive material used as the negative electrode conductive material has a structure and function 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 the 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.

[0080] Meanwhile, the plate-like 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 rather a material to ensure a planar conductive path within the negative electrode active material layer.

[0081] That is, in this 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 to ensure a conductive path rather than to store or release 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 lithium ions transferred from the positive electrode.

[0082] 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 dotted or spherical shape and is used as a material that stores or releases lithium.

[0083] 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.

[0084] The negative electrode binder according to one embodiment of the present application plays a role in holding down the active material and conductive material to prevent twisting and structural deformation of the negative electrode structure when the volume expansion and relaxation of the silicon-based active material occurs. As long as the binder fulfills this role, any common binder can be used, and specifically, a water-based binder can be used, and more specifically, a PAM-based binder can be used.

[0085] In one embodiment of the present application, there is provided a negative electrode for a lithium secondary battery, comprising: a buffer layer including a buffer layer composition provided on a surface of the negative electrode active material layer opposite to a surface facing the negative electrode current collector layer.

[0086] As described above, the buffer layer has the characteristics of being able to suppress a rapid reaction by preventing direct contact with the reactive silicon-based active material even when lithium metal is transferred onto the top of the negative electrode active material layer, and being able to uniformly pre-lithiate lithium in the negative electrode active material layer by controlling the rate of pre-lithiation.

[0087] In one embodiment of the present application, the thickness of the buffer layer can be in the range of 0.1 μm to 2 μm.

[0088] In another embodiment, the thickness of the buffer layer may be in the range of 0.1 μm or more and 2 μm or less, preferably 0.2 μm or more and 1.5 μm or less, and more preferably 0.5 μm or more and 1 μm or less.

[0089] The buffer layer according to the present application, having a thickness within the above range, is characterized by controlling the prelithiation rate within an appropriate range, suppressing the generation of by-products, and achieving uniform prelithiation within the negative electrode active material layer. That is, if the buffer layer thickness exceeds the above range, the prelithiation rate decreases sharply, lithium metal reacts with air, generating by-products and loss, and the buffer layer remains until the battery is assembled, increasing the battery resistance and reducing initial efficiency. Also, if the buffer layer thickness is below the above range, the buffer layer is too thin, causing the prelithiation rate to increase sharply, resulting in lithium loss and the generation of by-products, and particle cracking of the silicon-based active material on the surface of the negative electrode active material layer.

[0090] In one embodiment of the present application, the buffer layer composition may include at least one material selected from the group consisting of an acrylic polymer and a binder; and a conductive material.

[0091] In one embodiment of the present application, there is provided a negative electrode for a lithium secondary battery, wherein the acrylic polymer comprises at least one selected from the group consisting of polyethylene (PE); polypropylene (PP); polyacrylic acid (PAA); and polyester.

[0092] In particular, in one embodiment of the present application, the buffer layer composition is characterized by including a specific content of a conductive material.

[0093] In the buffer layer according to the present application, after the pre-lithiation process, one or more materials selected from the group consisting of acrylic polymers and binders dissolve and disappear when an electrolyte solution is injected, leaving only the conductive material, which later helps to form a conductive network that connects the active material when the negative electrode active material expands or contracts.

[0094] In conclusion, the one or more materials selected from the group consisting of the acrylic polymer and the binder smoothly adjust the rate of the pre-lithiation process and suppress the generation of by-products. If the material remains on the top of the negative electrode afterwards, it acts as a resistor. However, the material contains a material that has the property of dissolving in the electrolyte, and the conductive material still remains on the top of the negative electrode, which helps improve the conductive network.

[0095] In one embodiment of the present application, there is provided a negative electrode for a lithium secondary battery, in which the conductive material is 1 part by weight or more and 10 parts by weight or less based on 100 parts by weight of the buffer layer composition.

[0096] In another embodiment, the conductive material may comprise 1 part by weight or more and 10 parts by weight or less, preferably 3 parts by weight or more and 7 parts by weight or less, and more preferably 3 parts by weight or more and 5 parts by weight or less, based on 100 parts by weight of the buffer layer composition.

[0097] In one embodiment of the present application, there is provided a negative electrode for a lithium secondary battery, in which the one or more materials selected from the group consisting of an acrylic polymer and a binder are present in an amount of 90 parts by weight or more and 99 parts by weight or less, based on 100 parts by weight of the buffer layer composition.

[0098] In another embodiment, the one or more substances selected from the group consisting of acrylic polymers and binders may be contained in an amount of 90 parts by weight or more and 99 parts by weight or less, preferably 93 parts by weight or more and 97 parts by weight or less, and more preferably 95 parts by weight or more and 97 parts by weight or less, based on 100 parts by weight of the buffer layer composition.

[0099] As described above, the inclusion of the conductive material in the buffer layer composition in the above amount allows for easy control of the pre-lithiation rate, and even after pre-lithiation, the buffer layer remains on the top of the negative electrode active material layer during battery assembly, thereby providing the characteristics of strengthening the conductive network without reducing the resistance characteristics.

[0100] That is, the conductive material content in this range not only has the advantage of being able to adjust the rate of prelithiation, but also remains in the battery itself after prelithiation to achieve the above-mentioned effects. If the conductive material content exceeds this range, the density of the buffer layer increases significantly, and the conductive material with small particle size clogs the pores on the electrode surface, resulting in a very slow prelithiation rate, which leads to lithium loss. On the other hand, if the conductive material content is below this range, the prelithiation rate becomes very fast, the generation of by-products increases, and particle cracking of the active material becomes severe, resulting in significant lithium loss and a low battery capacity retention rate.

[0101] In one embodiment of the present application, the conductive material contained in the buffer layer can be any of the negative electrode conductive materials described above without limitation, and specifically, carbon black; SWCNT; or MWCNT may be used.

[0102] In one embodiment of the present application, there is provided a negative electrode for a lithium secondary battery, wherein the binder contained in the buffer layer composition includes a binder copolymer containing a monomer containing a fluoro group, and the monomer includes a perfluoroolefin.

[0103] In one embodiment of the present application, the binder copolymer may include at least one various copolymer selected from the group consisting of 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, and polyacrylic acid.

[0104] In this case, one embodiment of the present application is characterized in that the binder copolymer contains a monomer containing a fluoro group.

[0105] In one embodiment of the present application, the binder copolymer containing a monomer containing a fluoro group may mean that the monomer unit containing a fluoro group may be contained in the copolymer as a monomer unit in the form of random, alternating, or block.

[0106] In one embodiment of the present application, the fluoro-group-containing monomer may include C2-C8 fluoroolefins or perfluoroolefins such as tetrafluoroethylene (TFE), hexafluoropropylene (HFP), pentafluoropropylene, and hexafluoroisobutylene.

[0107] Specifically, in one embodiment of the present application, the fluoro group-containing monomer may be hexafluoropropylene (HFP).

[0108] The binder copolymer according to the present application may contain 5 to 20 parts by weight of the monomer based on 100 parts by weight of the binder copolymer.

[0109] In another embodiment, the binder copolymer may contain 5 parts by weight or more and 20 parts by weight or less, preferably 8 parts by weight or more and 15 parts by weight or less of the monomer, based on 100 parts by weight of the binder copolymer.

[0110] The content of the monomer based on the binder copolymer may refer to the content of the monomer based on the total binder copolymer formed by the reaction of two or more monomers with each other.

[0111] The binder according to the present application has the above-described composition, which allows the rate of prelithiation of lithium metal to be appropriate during the subsequent prelithiation, and has the characteristics of being able to suppress side reactions and prevent cracking of the negative electrode active material particles.

[0112] In one embodiment of the present application, the binder copolymer may be polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP).

[0113] In one embodiment of the present application, there is provided a negative electrode for a lithium secondary battery, in which the binder is present in an amount of 90 parts by weight or more and 99 parts by weight or less, based on 100 parts by weight of the buffer layer composition.

[0114] In another embodiment, the binder may comprise 90 parts by weight or more and 99 parts by weight or less, preferably 93 parts by weight or more and 97 parts by weight or less, and more preferably 95 parts by weight or more and 97 parts by weight or less, based on 100 parts by weight of the buffer layer composition.

[0115] In one embodiment of the present application, there is provided a method for manufacturing a negative electrode for a lithium secondary battery, the method comprising 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; coating a buffer layer composition on a surface of the negative electrode active material layer opposite to the surface facing the negative electrode current collector layer to form a negative electrode for a lithium secondary battery including a buffer layer; transferring lithium metal to the surface of the buffer layer opposite to the surface facing the negative electrode active material layer; and prelithiating the negative electrode active material layer, wherein the buffer layer composition comprises one or more materials selected from the group consisting of acrylic polymers and binders; and a conductive material, and the conductive material is present in an amount of 1 part by weight to 10 parts by weight, based on 100 parts by weight of the buffer layer composition.

[0116] 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 surfaces of the negative electrode current collector layer is provided.

[0117] The step is a process of stacking lithium secondary batteries, and the step of forming a negative electrode current collector layer and a negative electrode active material layer on one or both surfaces of the negative electrode current collector layer may include coating one or both surfaces 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 may include at least one selected from the group consisting of a silicon-based active material; a negative electrode conductive material; and a negative electrode binder.

[0118] In this case, the above-mentioned contents regarding the silicon-based active material, the negative electrode conductive material, and the negative electrode binder can be applied.

[0119] In one embodiment of the present application, the negative electrode slurry may include a negative electrode active material layer composition; and a slurry solvent.

[0120] In one embodiment of the present application, the solid content of the negative electrode slurry may be 5% or more and 40% or less.

[0121] In another embodiment, the solid content of the negative electrode slurry may be in the range of 5% to 40%, preferably 7% to 35%, and more preferably 10% to 30%.

[0122] The solid content of the negative electrode slurry may refer to the content of the negative electrode active material layer composition contained in the negative electrode slurry, and may refer to the content of the negative electrode active material composition based on 100 parts by weight of the negative electrode slurry.

[0123] 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 particle aggregation of the negative electrode active material layer composition is minimized, thereby enabling the negative electrode active material layer to be efficiently formed.

[0124] In one embodiment of the present application, the slurry solvent is not limited as long as it can dissolve the negative electrode active material layer composition, and specifically, acetone; distilled water; or NMP can be used.

[0125] The negative electrode according to one embodiment of the present application can be formed by coating the negative electrode slurry on a negative electrode current collector layer and drying it.

[0126] The drying step may be performed to dry out the slurry solvent in the negative electrode slurry, and the method may further include rolling the negative electrode.

[0127] In one embodiment of the present application, a buffer layer composition may be coated on a surface of the negative electrode active material layer opposite to a surface facing the negative electrode current collector layer to form a negative electrode for a lithium secondary battery including a buffer layer.

[0128] The step of forming a negative electrode for a lithium secondary battery including a buffer layer by coating the buffer layer composition may include coating one surface of the negative electrode active material layer with a buffer layer composition slurry.

[0129] The buffer layer composition can be applied to the above-mentioned composition.

[0130] In one embodiment of the present application, the buffer layer composition slurry may include: a buffer layer composition; and a slurry solvent.

[0131] In this case, the above description can be applied to the slurry solvent.

[0132] In one embodiment of the present application, the solid content of the buffer layer composition slurry may be 5% or more and 40% or less.

[0133] In another embodiment, the solid content of the buffer layer composition slurry may be in the range of 5% to 40%, preferably 7% to 35%, and more preferably 10% to 30%.

[0134] The solid content of the buffer layer composition slurry may refer to the content of the buffer layer composition contained in the buffer layer composition slurry, and may refer to the content of the buffer layer composition based on 100 parts by weight of the buffer layer composition slurry.

[0135] In one embodiment of the present application, the conductive material in the buffer layer composition slurry may occupy a solid content of 0.5% to 10% of the solid content.

[0136] When the solid content of the buffer layer composition slurry satisfies the above range, the particles of the buffer layer are uniformly distributed without aggregation, thereby ensuring a route for lithium metal during prelithiation, thereby enabling smooth control of the prelithiation rate.

[0137] The buffer layer according to one embodiment of the present application may be formed by coating the buffer layer composition slurry on the negative electrode active material layer and drying it.

[0138] In one embodiment of the present application, the drying step may include a step of vacuum drying at a temperature of 100° C. or higher for 5 to 15 hours.

[0139] The buffer layer may be coated using a coating method commonly used in the art, for example, 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.

[0140] In one embodiment of the present application, the method includes transferring lithium metal to a surface of the buffer layer opposite to the surface facing the negative electrode active material layer.

[0141] Generally, the prelithiation process is a chemical or physical process of prelithiating lithium metal onto a negative electrode, and specifically can be carried out by a lithium metal transfer process, a lithium metal powder deposition process, an electrochemical process, or a lithium metal deposition process. The prelithiation process according to the present application may include a lithium metal transfer process.

[0142] The lithium metal transfer process has the advantage that highly reactive lithium metal can be transferred more stably to the top of the negative electrode active material layer. At this time, a process is required that can easily transfer lithium metal from the transfer laminate to the top of the negative electrode active material layer. By forming the buffer layer on the top of the negative electrode active material layer according to the present application, the efficiency of the transfer process can also be improved.

[0143] In one embodiment of the present application, there is provided a method for producing a negative electrode for a lithium secondary battery, wherein the step of transferring lithium metal to the surface of the buffer layer opposite to the surface facing 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 buffer layer so that the surface of the lithium metal opposite to the surface facing the substrate layer is in contact with the surface of the buffer layer opposite to the surface facing the negative electrode active material layer; and removing the substrate layer.

[0144] In one embodiment of the present application, the deposition method for depositing the lithium metal on the substrate layer may be selected from, but is not limited to, vacuum deposition (evaporation deposition), chemical vapor deposition (CVD), and physical vapor deposition (physical vapor deposition), and various deposition methods used in the art may be used.

[0145] 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 the steps of preparing a transfer laminate 100 including a substrate layer 10 and lithium metal 20, forming a negative electrode active material layer 30 on a negative electrode current collector layer 40, and laminating the resulting negative electrode 200 for a lithium secondary battery in which a buffer layer 35 is formed on the negative electrode active material layer 30 so that the buffer layer 35 contacts the lithium metal 20, removing the substrate layer 10, and transferring only the lithium metal 20 onto the buffer layer 35.

[0146] In one embodiment of the present application, the laminating step is performed under a temperature condition of 60°C to 80°C and 20 kgf / cm 2 ~60kgf / cm 2 The present invention provides a method for producing a negative electrode for a lithium secondary battery, which comprises laminating the negative electrode under a pressure condition of 1000 kJ / cm 2 .

[0147] The lamination can be performed by a transfer process using roll pressing. A subsequent process of removing the substrate layer is included. The inclusion of the buffer layer according to the present application during removal prevents direct contact with the silicon-based active material, facilitating the transfer of lithium metal. Furthermore, the presence of the buffer layer allows for easy removal of the substrate layer with only weak linear pressure, facilitates control of the prelithiation rate, facilitates the release of heat generated during prelithiation, and suppresses the generation of by-products.

[0148] The reason for this is that, in the case where there is no buffer layer as in the past, lithium metal comes into direct contact with the top of the negative electrode active material layer, generating a large number of by-products such as Li nitride, which makes it difficult to transfer to the negative electrode active material layer. As a result, pre-lithiation including the substrate layer causes desorption of lithium metal. However, the present invention solves this problem by including a buffer layer that satisfies the thickness and composition requirements.

[0149] In one embodiment of the present application, the substrate 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 substrate layer, during a winding process for transferring the deposited lithium metal.

[0150] 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.

[0151] In one embodiment of the present application, the thickness of the substrate 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.

[0152] 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.

[0153] 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.

[0154] In one embodiment of the present application, in order to improve the releasability of the lithium metal and ensure its transferability to the negative electrode active material layer, the transfer laminate may further include a release layer on the surface where the base material layer and the lithium metal come into contact.

[0155] That is, the substrate 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 the problem of reverse peeling, in which the deposited lithium metal is transferred onto the substrate layer during a winding process for transferring the lithium metal to the negative electrode, and can also facilitate the separation of the substrate layer after the lithium metal is transferred onto the negative electrode active material layer.

[0156] The release layer may contain at least one selected from the group consisting of silicon-modified polyester in which silicon chains are graft-bonded to a polyester main chain, Si, melamine, and fluorine.

[0157] 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.

[0158] In one embodiment of the present application, even after the step of depositing and transferring the lithium metal onto the metal layer, the buffer layer may prevent direct contact with the negative electrode active material layer, thereby delaying the progress of the pre-lithiation process.

[0159] In one embodiment of the present application, there is provided a method for manufacturing a negative electrode for a lithium secondary battery, wherein the step of prelithiating the negative electrode active material layer is performed within 30 minutes to 20 hours after transferring lithium metal.

[0160] That is, when the negative electrode active material layer is pre-lithiated, the pre-lithiation completion time satisfies the above range, and the inclusion of a buffer layer controls the pre-lithiation rate, thereby allowing the pre-lithiation process to proceed more smoothly. The pre-lithiation completion time can be calculated by measuring the time from the time of lithium metal transfer until the transferred lithium metal is no longer observed on the surface of the negative electrode active material layer.

[0161] 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 according to the present application; a separator disposed between the positive electrode and the negative electrode; and an electrolyte.

[0162] 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.

[0163] The positive electrode current collector in the positive electrode is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. can be used. The positive electrode current collector typically has a thickness of 3 μm to 500 μm, and fine irregularities can be formed on the surface of the current collector to enhance the adhesive strength of the positive electrode active material. It may be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0164] 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) or lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; a lithium iron oxide such as LiFe3O4; 1+c1 Mn 2-c1 O4 (0≦c1≦0.33), lithium manganese oxides such as LiMnO3, LiMn2O3, and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, and Cu2V2O7; chemical formula LiNi 1-c2 M c2 O2 (where M is at least one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, and 0.01≦c2≦0.6 is satisfied); 2-c3 M c3Examples of suitable lithium manganese composite oxides include, but are not limited to, lithium manganese composite oxides represented by Li2Mn3MO8 (where 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 (where 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 substituted with an alkaline earth metal ion. The positive electrode may be Li metal.

[0165] 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 may include single particles or secondary particles, and the single particles may have an average particle size (D50) of 1 μm or more.

[0166] 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.

[0167] The single particles can have excellent particle strength even when they are formed with a small particle size (D50) of 1 μm or more and 12 μm or less. For example, the single particles can have a strength of 650 kgf / cm 2 When 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 alleviated, thereby improving the life characteristics of the battery.

[0168] The single particles may be prepared by mixing a transition metal precursor and a lithium source material and calcining the mixture. The secondary particles may be prepared by a method different from that for the single particles, and the composition thereof may be the same as or different from that of the single particles.

[0169] The method for forming the single particles is not particularly limited, but they can generally 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.

[0170] For example, the calcination is performed at a temperature that allows the formation of single particles. To achieve this, the calcination must be performed at a temperature higher than that used for the production of secondary particles. For example, when the precursor composition is the same, the calcination temperature should be about 30°C to 100°C higher than that used for the production of secondary particles. The calcination temperature for the formation of the single particles may vary depending on the metal composition of the precursor. For example, when a high-nickel (Ni) NCM-based lithium composite transition metal oxide having a nickel (Ni) content of 80 mol% or more is to be formed as single particles, the calcination temperature may be about 700°C to 1000°C, preferably about 800°C to 950°C. When the calcination temperature satisfies the above range, a positive electrode active material containing single particles with excellent electrochemical properties can be produced. If the calcination temperature is lower than 790°C, a positive electrode active material containing a lithium composite transition metal compound in the form of secondary particles may be produced. However, if the calcination temperature exceeds 950°C, excessive calcination may occur, resulting in an improper formation of a layered crystal structure and reduced electrochemical properties.

[0171] 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 quasi-single particle form that is an agglomeration of 30 or less primary particles.

[0172] Specifically, the single particle in the present invention may be in the form of a single particle consisting of one primary particle or a similar-single particle that 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.

[0173] 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.

[0174] In the present invention, the single particle may be in the form of a single particle consisting of one primary particle or a similar-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.

[0175] 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 can be distinguished from the concept of single particles, which includes one primary particle, one single particle, or a similar-single particle form that is an agglomeration of 30 or less primary particles.

[0176] The particle diameter (D50) of the secondary particles may be 1 μm to 20 μm, 2 μm to 17 μm, preferably 3 μm to 15 μm. The specific surface area (BET) of the secondary particles may be 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.

[0177] In a further embodiment of the present application, the secondary particles are aggregates of primary particles, and the average particle size (D50) of the primary particles is 0.5 μm to 3 μm. Specifically, the secondary particles may be in the form of aggregates of several hundred primary particles, and the average particle size (D50) of the primary particles may be 0.6 μm to 2.8 μm, 0.8 μm to 2.5 μm, or 0.8 μm to 1.5 μm.

[0178] When the average particle size (D50) of the primary particles satisfies the above range, a single-particle positive electrode active material with 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-based oxide particles increases, reducing the effect of suppressing particle cracking during rolling. If the average particle size (D50) of the primary particles is too large, the lithium diffusion path within the primary particles becomes longer, increasing resistance and potentially reducing output characteristics.

[0179] 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, thereby mitigating the phenomenon of an increase in fine particles in the electrode due to particle cracking, thereby improving the life characteristics of the battery.

[0180] 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.

[0181] 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.

[0182] 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 though they are formed with a small particle size, thereby mitigating the phenomenon of an increase in fine particles in the electrode due to particle cracking, and thereby improving the battery life characteristics and energy density.

[0183] According to a further embodiment of the present application, the single particles are contained in an amount of 15 to 100 parts by weight relative to 100 parts by weight of the positive electrode active material, or may be contained in an amount of 20 to 100 parts by weight, or 30 to 100 parts by weight relative to 100 parts by weight of the positive electrode active material.

[0184] 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.

[0185] 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 contained in an amount of 15 parts by weight or more, the increase in fine particles in the electrode due to particle cracking during the rolling process after electrode fabrication can be mitigated, thereby improving the battery life characteristics.

[0186] 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.

[0187] When the above range is satisfied, the above-described effects 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 components thereof may be the same as or different from those exemplified for the single particle positive electrode active material, and may refer to an aggregated form of single particles.

[0188] In one embodiment of the present application, the positive electrode active material 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 even more preferably 98 parts by weight or more and 99.9 parts by weight or less, in 100 parts by weight of the positive electrode active material layer.

[0189] 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.

[0190] In this case, 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 undergoing 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; and conductive polymers such as polyphenylene derivatives, and the like, and one or more of these may be used alone or in combination.

[0191] The positive electrode binder improves adhesion between positive electrode active material particles and between the positive electrode active material and the positive electrode current collector. Specific examples of the positive electrode binder 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 monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These binders may be used singly or in combination.

[0192] The separator separates the negative electrode and positive electrode and provides a path for lithium ions to move. Any separator commonly used in secondary batteries can be used without particular limitations. In particular, a separator that exhibits low resistance to electrolyte ion migration and excellent electrolyte humidification capacity is preferred. Specifically, a porous polymer film, such as a porous polymer film made from a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof, can be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made from high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. To ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material can also be used, and it can be used in either a single-layer or multi-layer structure.

[0193] 2 is a diagram showing a laminated structure of a lithium secondary battery according to one 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 lithium secondary battery cathode 100 are stacked with a separator 50 sandwiched between them.

[0194] At this time, the buffer layer used in the pre-lithiation may be partially removed depending on the electrolyte used, resulting in a small amount being contained on the top of the negative electrode. The buffer layer is not shown in Figure 2.

[0195] In one embodiment of the present application, the electrolytic solution may be an organic liquid electrolytic solution, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolytic solution, a solid inorganic electrolytic solution, or a molten inorganic electrolytic solution that can be used in manufacturing a lithium secondary battery, but is not limited to these.

[0196] Specifically, the electrolyte may contain a non-aqueous organic solvent and a metal salt.

[0197] Examples of the non-aqueous organic solvent that may be used 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, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0198] In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are preferably used because 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 prepared, and therefore, these cyclic carbonates are more preferably used.

[0199] The metal salt may be a lithium salt, which is a substance that is easily dissolved in the non-aqueous electrolyte solution. 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:

[0200] In addition to the components of the electrolyte solution, the electrolyte solution 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, or aluminum trichloride, for the purpose of improving the life characteristics of the battery, suppressing a decrease in battery capacity, and improving the discharge capacity of the battery.

[0201] According to one embodiment of the present invention, there is provided a battery module including the lithium 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 characteristics, and high cycle characteristics, and can be used as a power source for medium- to large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

[0202] Below, preferred examples are presented to help understand the present invention, but these examples are for the purpose of illustrating 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, and it is natural that such changes and modifications fall within the scope of the appended claims.

[0203] <Example> <Production of Transfer Laminate> A transfer laminate was produced by depositing lithium metal at a thickness of 6 μm on the top of the PET substrate using the PVD method on top of the release layer coated PET (PET with release layer coating on one side, I-One film, thickness 20 μm to 50 μm).

[0204] In this case, the thickness of the PET in the transfer laminate was 25 μm, the release layer was coated at a thickness of 0.5 μm, and lithium metal was vapor-deposited at a thickness of 6 μm on top of the release layer.

[0205] <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 concentration: 25 wt%).

[0206] 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.

[0207] The negative electrode current collector was a copper current collector (thickness: 8 μm) and the negative electrode slurry was applied to both sides of the copper current collector at a rate of 85 mg / 25 cm. 2 The coated layer was roll pressed 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%).

[0208] Thereafter, the buffer layer composition slurry was bar coated on the negative electrode active material layer and dried to form a buffer layer having a thickness of 0.5 μm to 2 μm.

[0209] Thereafter, in order to transfer the transfer laminate to the negative electrode active material layer, the lithium metal of the transfer laminate was placed on top of the negative electrode active material layer, and then pressure of 40 kgf / cm 2 The temperature was 80° C. Immediately after lamination, the PET layer of the transfer laminate was removed, and the negative electrode was prelithiated.

[0210] In preparing the negative electrode, the composition of the buffer layer and the thickness of the buffer layer were adjusted as shown in Table 1 below.

[0211] [Table 1]

[0212] <Manufacturing lithium secondary batteries> A bi-cell battery was fabricated using the negative electrode (pre-lithiation process) fabricated in Table 1 above and NCM as the counter electrode. The electrolyte used in this battery was ethylene carbonate (EC) / ethyl methyl carbonate (EMC) = 3 / 7 (volume ratio) in which 1M LiPF6 was dissolved.

[0213] <Experimental Example> <1. Initial Coulombic efficiency charge / discharge experiment> The cycle characteristics of the bi-cell battery fabricated above were confirmed using an electrochemical charge-discharge machine. Up to the third cycle, the voltage was 4.2 V (vs. Li / Li) at a current density of 0.1 C. + ) Charging 2.5V (vs.Li / Li + From the fourth cycle, the cells were charged and discharged at a current density of 0.5 C under the same voltage conditions.

[0214] <2. Lithium loss calculation experiment> When measuring the initial efficiency, the Li loss was measured as the difference in the first charge capacity value. Lithium loss (%) = 1 - {("non-lithiated electrode charge capacity" - "lithiated electrode charge capacity") / theoretical capacity of lithium used during prelithiation}

[0215] <3.100-cycle charge / discharge experiment> The cycle characteristics of the bi-cell battery prepared above were confirmed using an electrochemical charge / discharge machine. For 100 cycles, the battery was charged at 1C and discharged at 0.5C between 4.2V and 2.5V. The capacity retention rate was calculated based on the discharge capacity. Capacity retention rate (%) = (100th discharge capacity / 1st discharge capacity) × 100

[0216] <4. Initial cell resistance measurement experiment> The bicell was charged at 4.2 V at 0.33 C, then discharged at 2.5 V and 0.33 C until half of the discharge capacity was reached. At this time, a 30-second pulse current of 2.5 C was applied.

[0217] The resistance value was calculated by dividing the voltage difference before and after the application of the current by the current.

[0218] The evaluation results for the above experimental examples are shown in Table 2 below.

[0219] [Table 2]

[0220] As can be seen from Table 2, Examples 1 to 5 of the present application include a specific buffer layer, which prevents direct contact with the highly reactive silicon-based active material, thereby suppressing rapid reactions even when lithium metal is transferred to the top of the negative electrode active material layer. This prevents direct contact with the highly reactive silicon-based active material, thereby suppressing rapid reactions. This allows the rate of prelithiation to be adjusted, thereby enabling uniform prelithiation of lithium within the negative electrode active material layer. As described above, the presence of a buffer layer on the top of the negative electrode for a lithium secondary battery controls the rate of prelithiation, reducing by-products with lithium due to violent reactions on the surface of the negative electrode active material layer and reducing loss of lithium metal. Furthermore, the presence of a buffer layer allows the substrate layer to be quickly removed with very little linear pressure after lithium metal is transferred to the negative electrode active material layer, and easily releases reaction heat, thereby suppressing by-products on the surface of the negative electrode active material layer.

[0221] In Comparative Example 1, the content of the conductive material in the buffer layer exceeded the upper limit. It was confirmed that the increase in the content of the conductive material reduced the pre-lithiation rate, which in turn led to a side reaction with Li metal in the air, resulting in poor capacity retention.

[0222] In Comparative Examples 2 and 6, the thickness of the buffer layer was below the lower limit of the present application or was not provided at all. It was confirmed that the thinning of the buffer layer or the absence of the buffer layer resulted in a very fast pre-lithiation rate (increased pre-lithiation rate), which in turn resulted in cracks on the surface of the electrode active material and increased lithium loss.

[0223] In Comparative Examples 3 and 4, the thickness of the buffer layer exceeded the upper limit of the present application, resulting in a significantly reduced pre-lithiation rate. This resulted in a longer exposure time to air, which increased side reactions, resulting in significant lithium loss and poor capacity retention.

[0224] Finally, in Comparative Example 5, which does not contain a conductive material, the prelithiation rate is partially controlled, preventing lithium loss to some extent, but as confirmed by charge-discharge experiments, the capacity retention rate after 100 cycles is poor. This is due to the breakdown of the conductive network, which is caused by the absence of a conductive material on the anode, unlike in the other examples.

[0225] In conclusion, it was confirmed that when a buffer layer of a specific composition and thickness according to the present application is provided, the reaction rate is appropriately adjusted during pre-lithiation, a conductive network is subsequently formed, and the performance of the battery containing the same is improved. [Explanation of symbols]

[0226] 10...Base material layer 20 Lithium metal 30...Negative electrode active material layer 35 Buffer 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 ···Anode for lithium secondary battery 300 ···Positive electrode for lithium secondary battery

Claims

1. 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 buffer layer including a buffer layer composition provided on a surface of the negative electrode active material layer opposite to the surface facing the negative electrode current collector layer; A negative electrode for a lithium secondary battery, comprising: the thickness of the buffer layer is 0.1 μm or more and 2 μm or less; the buffer layer composition includes polyethylene and a conductive material; the polyethylene is in an amount of 90 parts by weight or more and 99 parts by weight or less based on 100 parts by weight of the buffer layer composition; The conductive material is present in an amount of 1 part by weight to 10 parts by weight based on 100 parts by weight of the buffer layer composition.

2. the negative electrode active material layer composition includes a silicon-based active material, a negative electrode conductive material, and a negative electrode binder; The silicon-based active material is SiO ｘ (x=0), SiO ｘ 2. The negative electrode for a lithium secondary battery according to claim 1, comprising at least one selected from the group consisting of (0<x<2), SiC, metal impurities, and Si alloys.

3. The silicon-based active material is SiO ｘ (x=0) and SiO ｘ (0<x<2), and based on 100 parts by weight of the silicon-based active material, the SiO ｘ 3. The negative electrode for a lithium secondary battery according to claim 2, comprising 70 parts by weight or more of (x=0).

4. the thickness of the negative electrode current collector layer is 1 μm or more and 100 μm or less, 2. The negative electrode for a lithium secondary battery according to claim 1, wherein the thickness of the negative electrode active material layer is 20 μm or more and 500 μm or less.

5. 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 negative electrode for a lithium secondary battery including a buffer layer by coating a buffer layer composition on a surface of the negative electrode active material layer opposite to the surface facing the negative electrode current collector layer; transferring lithium metal to a surface of the buffer layer opposite to the surface facing the negative electrode active material layer; and prelithiating the negative electrode active material layer; A method for producing a negative electrode for a lithium secondary battery, comprising: the buffer layer composition includes polyethylene and a conductive material; the polyethylene is in an amount of 90 parts by weight or more and 99 parts by weight or less based on 100 parts by weight of the buffer layer composition; The conductive material is present in an amount of 1 part by weight to 10 parts by weight based on 100 parts by weight of the buffer layer composition.

6. The step of transferring lithium metal to the surface of the buffer layer opposite to the surface facing 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 onto the buffer layer so that a surface of the lithium metal opposite to a surface facing the substrate layer is in contact with a surface of the buffer layer opposite to a surface facing the negative electrode active material layer; and removing the substrate layer; The method for producing a negative electrode for a lithium secondary battery according to claim 5 , comprising:

7. The lamination step is carried out under a temperature condition of 60°C to 80°C and a pressure of 20 kgf / cm ２ ~60 kgf / cm ２ 7. The method for producing a negative electrode for a lithium secondary battery according to claim 6, wherein the lamination is carried out under a pressure condition of 1000 MPa.

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

9. 9. The method for producing a negative electrode for a lithium secondary battery according to claim 5, wherein the lithium metal has a thickness of 1 μm or more and 10 μm or less.

10. 6. The method of claim 5, wherein the prelithiation of the negative electrode active material layer is performed within 30 minutes to 20 hours after transferring the lithium metal.

11. positive electrode, The negative electrode for a lithium secondary battery according to claim 1 . a separator provided between the positive electrode and the negative electrode; and electrolyte, A lithium secondary battery comprising:

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