Method for manufacturing a negative electrode for a lithium secondary battery, a negative electrode manufactured by the method, and a lithium secondary battery including the negative electrode

Abstract

The present invention relates to a method for manufacturing a negative electrode for a lithium secondary battery, the method comprising the step of forming a negative electrode by transferring a lithium metal layer to at least one surface of a negative electrode active material layer including a negative electrode active material, a negative electrode conductive material, and a negative electrode binder, wherein the maximum temperature on the surface of the negative electrode after transfer of the lithium metal layer is 35°C or less, a negative electrode manufactured by the method, and a lithium secondary battery including the negative electrode, and the risk of fire can be reduced.

Description

[Technical Field]

[0001] The present invention claims the benefit of the filing date of Korean Patent Application No. 10-2023-0136902 filed with the Korean Intellectual Property Office on October 13, 2023, and Korean Patent Application No. 10-2024-0137285 filed with the Korean Intellectual Property Office on October 10, 2024, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a method for producing a negative electrode for a lithium secondary battery, a negative electrode produced thereby, 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 in this area 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] With technological development and increasing demand for mobile devices, the demand for secondary batteries as an energy source 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, as electrodes for such high-capacity lithium secondary batteries, active research is being conducted on methods for manufacturing high-density electrodes with higher energy density per unit volume, and there is a trend toward increasingly higher loading to improve energy density.

[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, but the theoretical capacity density of carbon is 372 mAh / g (833 mAh / cm 3 ) Therefore, in order to improve the energy density of the 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 are attracting attention due to their low cost and high capacity (4200mAh / g).

[0008] However, silicon suffers from 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 is stable enough to be used as an active material in electrochemical devices, ensuring good 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 lithiating a silicon negative electrode by a physical / chemical method such as electroplating, lithium metal transfer, or lithium metal vapor deposition, followed by fabrication of an electrode, and a method of electrochemically prelithiating a negative electrode. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Korean Patent Publication No. 10-2023-0114731 Summary of the Invention [Problem to be solved by the invention]

[0012] The present invention provides a method for producing a negative electrode for a lithium secondary battery, which can reduce the heat generated during prelithiation and significantly reduce the risk of fire due to heat accumulation, a negative electrode produced thereby, and a lithium secondary battery including the negative electrode. [Means for solving the problem]

[0013] One embodiment of the present invention provides a method for forming a negative electrode, the method comprising: transferring a lithium metal layer onto at least one surface of a negative electrode active material layer including a negative electrode active material; a negative electrode conductive material; and a negative electrode binder; The present invention provides a method for producing a negative electrode for a lithium secondary battery, wherein the maximum temperature on the surface of the negative electrode after the lithium metal layer is transferred is 35° C. or less.

[0014] One embodiment of the present specification relates to a positive electrode for a lithium secondary battery; a negative electrode for a lithium secondary battery produced by the aforementioned production method; a separator disposed between the positive electrode for the lithium secondary battery and the negative electrode for the lithium secondary battery; and electrolyte The present invention provides a lithium secondary battery comprising:

[0015] One embodiment herein is a lithium secondary battery comprising: a positive electrode; a prelithiated negative electrode; a separator; and an electrolyte, the prelithiated negative electrode includes a negative electrode current collector layer and a negative electrode active material layer provided on at least one surface of the negative electrode current collector layer, The negative electrode active material layer provides a lithium secondary battery having a surface hardness of 65 MPa or less.

[0016] Another embodiment of the present specification provides a battery module or a battery pack including the above-described lithium secondary battery.

[0017] Finally, one embodiment of the present specification provides a battery pack including the above-described battery module. [Effects of the Invention]

[0018] A method for manufacturing a negative electrode for a lithium secondary battery according to an embodiment of the present specification can manufacture a negative electrode for a lithium secondary battery having an extremely low surface temperature, thereby preventing heat from accumulating inside the negative electrode during a subsequent winding process, preventing the temperature from rising to an ignition point, and suppressing ignition of the negative electrode.

[0019] A method for manufacturing a negative electrode for a lithium secondary battery according to an embodiment of the present specification can manufacture a negative electrode for a lithium secondary battery having reduced surface hardness by satisfying certain compositional conditions. As a result, pressure can be evenly transmitted during the pressing step of lithium transfer, thereby significantly reducing the surface temperature of the pre-lithiated negative electrode. Thereafter, heat accumulation inside the negative electrode during the winding process can be prevented, preventing the temperature from rising to the ignition point and suppressing the ignition phenomenon of the negative electrode. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a flowchart illustrating a method for producing a negative electrode for a lithium secondary battery according to an embodiment of the present specification. [Figure 2] 1 is a diagram showing a laminated structure of a negative electrode for a lithium secondary battery manufactured according to an embodiment of the present invention. [Figure 3] 1 is a diagram showing a stack structure of a lithium secondary battery manufactured according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0021] Before describing the present invention, some terms will first be defined.

[0022] In this specification, when a part is said to "comprise" a certain component, this does not mean that it may further include other components, unless specifically stated to the contrary, but rather that it does not exclude other components.

[0023] In this specification, "p to q" means a range of "not less than p and not more than q."

[0024] 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. In other words, in this specification, the BET specific surface area can mean the specific surface area measured by the above measurement method.

[0025] In this specification, "surface hardness" refers to a value measured using a micro-indenter, specifically, an NX10 from Park System Co., Ltd., where the surface of an object is measured using a diamond indenter at a constant indentation speed of 3000 nm / min to an indentation depth of 5 μm in a displacement-controlled manner. In other words, in this specification, surface hardness refers to the surface hardness measured using this measurement method.

[0026] In this specification, "Dn" refers to particle size distribution, and refers to the particle size at the n% point in the cumulative particle number distribution by particle size. That is, D50 is the particle size (average 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 difference in diffraction patterns due to particle size when the particles pass through a laser beam is measured, and the particle size distribution is calculated.

[0027] The average particle size can be measured using a Microtrac device (manufacturer: Microtrac model name: S3500) with water and Triton-X100 dispersant. Specifically, the average particle size of the positive electrode active material can be measured at a refractive index of 1.5 to 1.7, and the average particle size of the negative electrode active material can be measured at a refractive index of 1.97 or 2.42. For example, the particles can be dispersed in a dispersion medium, introduced into a commercially available laser diffraction particle size analyzer, and irradiated with ultrasonic waves of approximately 28 kHz at an output of 60 W. A volume cumulative particle size distribution graph can then be obtained, and the particle size corresponding to 50% of the cumulative volume can be determined.

[0028] In one embodiment of the present specification, the particle size or particle size may refer to the average diameter or representative diameter of each particle constituting the metal powder.

[0029] The terms and words used in this specification should not be interpreted in a limited way to their ordinary or dictionary meanings, but should be interpreted in a way that is consistent with the technical idea of ​​the present invention, based on the principle that the inventor can appropriately define the concept of the term in order to best explain his / her invention.

[0030] As used herein, the singular expression of a term includes the plural expression unless the context clearly indicates otherwise.

[0031] Preferred embodiments of the present invention will be described in detail below. However, the embodiments of the present invention can be modified in various ways, and the scope of the present invention is not limited to the embodiments described below.

[0032] A method for manufacturing a negative electrode for a lithium secondary battery according to one embodiment of the present specification includes forming a negative electrode by transferring a lithium metal layer onto at least one surface of a negative electrode active material layer including a negative electrode active material; a negative electrode conductive material; and a negative electrode binder, and after the lithium metal layer is transferred, the maximum temperature on the surface of the negative electrode is 35°C or less.

[0033] Previously, during direct contact prelithiation, it was necessary to minimize the heat generated in the electrodes and the resulting risk of fire. In particular, prelithiation of anodes using silicon-based active materials had the problem of heat accumulation in the electrodes due to the rapid exothermic reaction between lithium and the active material after lamination, greatly increasing the risk of fire.

[0034] Therefore, a method for manufacturing a negative electrode for a lithium secondary battery according to one embodiment of the present specification is characterized in that the surface temperature of the negative electrode after pre-lithiation is prevented from rising above 35°C, thereby significantly reducing the risk of fire due to heat accumulation associated with winding the electrode after pre-lithiation, and is characterized in that the surface hardness of the negative electrode active material layer is reduced to a certain level or below by adjusting the composition of the negative electrode active material layer in order to reduce the surface temperature of the negative electrode.

[0035] A method for manufacturing a negative electrode for a lithium secondary battery according to one embodiment of the present specification includes transferring a lithium metal layer onto at least one surface of the negative electrode active material layer to form a negative electrode.

[0036] Specifically, in the method for manufacturing a negative electrode for a lithium secondary battery according to one embodiment of the present specification, the step of forming a negative electrode by transferring a lithium metal layer to at least one surface of the negative electrode active material layer may include the steps of: preparing a transfer laminate including a lithium metal layer and a substrate layer; contacting the transfer laminate with at least one surface of the negative electrode active material layer so that the lithium metal layer faces the surface; and separating the substrate layer from the lithium metal layer.

[0037] The method for manufacturing a negative electrode for a lithium secondary battery according to the present specification involves prelithiation of at least one surface of a negative electrode active material layer in order to solve the irreversibility problem of silicon-based electrodes and improve Coulombic efficiency, and is characterized by the use of a transfer method for prelithiation, which has a lower degree of cell degradation during battery operation and a faster reaction rate than prelithiation using a stabilized lithium metal powder (SLMP) method or an electrochemical method.

[0038] However, the prelithiation reaction is an exothermic reaction, and the transfer-type prelithiation, in particular, has a fast reaction rate, which can cause the maximum temperature of the negative electrode surface to rise to about 40°C or more during prelithiation, resulting in heat accumulation and the risk of ignition. To address this issue, the present invention reduces the surface hardness of the negative electrode active material layer by adjusting the composition of the negative electrode active material layer, thereby preventing the surface temperature of the negative electrode, including the negative electrode active material layer and the negative electrode current collector layer, from rising above 35°C after prelithiation.

[0039] That is, in the negative electrode having the lithium metal layer transferred onto at least one surface of the negative electrode active material layer according to one embodiment of the present specification, the maximum temperature of the surface onto which the lithium metal layer is transferred may be 35° C. or less.

[0040] If the surface temperature of the negative electrode is 35° C. or less, heat is not released to a certain area during the winding process of the negative electrode, and condensation can be prevented, preventing the temperature from rising to the ignition point.

[0041] In the method for manufacturing a negative electrode for a lithium secondary battery according to one embodiment of the present specification, the transfer laminate includes a lithium metal layer and a substrate layer. In this case, the lithium metal layer is a layer containing lithium metal for prelithiating at least one surface of the negative electrode active material layer, and may be, but is not limited to, a commonly used Li metal foil.

[0042] In one embodiment of the present specification, the thickness of the lithium metal layer may be 0.1 μm or more and 15 μm or less, specifically 1 μm or more and 13 μm or less, and more specifically 1 μm or more and 10 μm or less.

[0043] In one embodiment of the present specification, when the thickness of the lithium metal layer satisfies the above range, pre-lithiation can be performed to a degree that can sufficiently compensate for irreversible capacity, and the possibility of incomplete transfer due to difficulty in peeling of the lithium metal layer can be reduced.

[0044] In addition, in one embodiment of the present specification, the substrate layer may be any layer that can withstand process conditions such as high temperatures in the step of depositing the lithium metal layer and that can prevent a problem of reverse peeling, in which the deposited lithium metal layer is transferred onto the substrate layer, during a winding process for transferring the deposited lithium metal layer.

[0045] Specifically, in one embodiment of the present specification, the base layer may be one or more selected from the group consisting of polyethylene terephthalate (PET), polyimide (PI), poly(methylmethacrylate) (PMMA), polypropylene, polyethylene, and polycarbonate.

[0046] In one embodiment of the present specification, the thickness of the base layer may be 1 μm or more and 300 μm or less, specifically 5 μm or more and 200 μm or less, and more specifically 10 μm or more and 100 μm or less.

[0047] When the thickness of the substrate layer satisfies the above range, the lithium metal can be efficiently transferred to the negative electrode active material layer side, and reverse transfer can be prevented.

[0048] In one embodiment of the present specification, a deposition method for depositing the lithium metal layer on the substrate layer may be selected from physical vapor deposition (PVD) and chemical vapor deposition (CVD). Among the physical vapor deposition methods, thermal evaporation may be mainly used, but is not limited thereto, and various deposition methods used in the art may be used.

[0049] In the method for producing a negative electrode for a lithium secondary battery according to one embodiment of the present specification, the transfer laminate may further include a release layer between the lithium metal layer and the substrate layer.

[0050] In one embodiment of the present specification, the release layer may be one or more selected from the group consisting of polycarbonate (PC), polydimethylsiloxane (PDMS), polymethylhydrosiloxane (PMHS), polyimide (PI), and polymethylmethacrylate (PMMA).

[0051] In one embodiment of the present specification, the thickness of the release layer may be 0.2 μm or more and 3 μm or less, specifically 0.5 μm or more and 1 μm or less.

[0052] In one embodiment of the present specification, when the thickness of the release layer satisfies the above range, a sufficient release force can be ensured, and the release layer located on the surface after the lithium metal layer is transferred does not function to prevent heat release, so that the formation of by-products can be prevented from being accelerated.

[0053] In one embodiment of the present specification, 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. Various coating methods known in the art for forming a coating layer may be used.

[0054] In one embodiment of the present specification, the step of contacting the transfer laminate with at least one surface of the negative electrode active material layer so that the lithium metal layer faces the negative electrode active material layer is a step of transferring the negative electrode active material layer to the lithium metal.

[0055] Specifically, in one embodiment of the present specification, the step of contacting the transfer laminate with at least one surface of the negative electrode active material layer so that the lithium metal layer faces the negative electrode active material layer may further include a step of applying pressure, which has the effect of more actively pre-lithiating the negative electrode active material layer by transfer, thereby enabling the negative electrode to be formed thin despite having a high energy density.

[0056] In this case, the transfer laminate is positioned so that the lithium metal layer contacts one or both sides of the negative electrode active material layer, and then a load of 10 kgf to 500 kgf is applied to the transfer laminate through roll pressing, and a subsequent step of removing the base layer or the release layer and the base layer may be included.

[0057] In one embodiment of the present specification, the step of removing the substrate layer or the release layer and the substrate layer may be performed after the lithium metal layer and the negative electrode active material layer are brought into contact with each other and transferred by pressure.

[0058] In the method for manufacturing a negative electrode for a lithium secondary battery according to one embodiment of the present specification, prelithiation may proceed from the moment the lithium metal layer comes into contact with the negative electrode active material layer, or from the step of applying pressure, or from the step of removing the base material layer or the release layer and the base material layer.

[0059] FIG. 1 is a flowchart showing a method for manufacturing a negative electrode for a lithium secondary battery according to an embodiment of the present specification. Specifically, (a) of FIG. 1 shows a method for manufacturing a negative electrode for a lithium secondary battery, including a step (S1) of forming a negative electrode by transferring a lithium metal layer onto one surface of a negative electrode active material layer including a negative electrode active material, a negative electrode conductive material, and a negative electrode binder.

[0060] (b) of FIG. 1 shows a method for manufacturing a negative electrode for a lithium secondary battery, including a step (S10) of preparing a transfer laminate including a lithium metal layer and a base material layer, a step (S11) of bringing the transfer laminate into contact with one surface of the negative electrode active material layer such that the lithium metal layer faces the surface, and a step (S12) of separating the base material layer from the lithium metal layer. At this time, the step of providing a negative electrode active material layer on at least one surface of the negative electrode current collector layer according to an embodiment of the present specification (not shown) may be performed before step S10, may be performed simultaneously with S10, or may be performed between steps S10 and S11, but there is no limitation thereto.

[0061] The negative electrode manufactured by the manufacturing method according to an embodiment of the present specification described above means a pre-lithiated negative electrode.

[0062] The method for manufacturing a negative electrode for a lithium secondary battery according to an embodiment of the present specification further includes a step of forming a negative electrode active material layer including a negative electrode active material, a negative electrode conductive material, and a negative electrode binder on at least one surface of the negative electrode current collector layer.

[0063] In an embodiment of the present specification, the negative electrode active material layer includes a negative electrode active material, a negative electrode conductive material, and a negative electrode binder. The negative electrode active material includes a silicon-based active material, and the silicon-based active material may include one or more selected from the group consisting of Si, SiOx (0 < x < 2), Si / C, and Si alloys.

[0064] In an embodiment of the present specification, the silicon-based active material may include one or more selected from the group consisting of Si and SiOx (0 < x < 2).

[0065] In one embodiment of this specification, the negative electrode active material may use pure silicon (Si) as the silicon-based active material. Using pure silicon (Si) as the negative electrode active material means that, as described above, when the total amount of the negative electrode active material is based on 100 parts by weight, pure Si particles not bonded to other particles or elements may be included in an amount of 60 parts by weight or more, specifically 65 parts by weight or more, more specifically 69 parts by weight or more, based on 100 parts by weight of the silicon-based active material, and may be included in a range of 95 parts by weight or less, specifically 90 parts by weight or less, more specifically 85 parts by weight or less.

[0066] In one embodiment of this specification, the negative electrode active material may use SiOx (0 < x < 2) as the silicon-based active material, and the SiOx (0 < x < 2) corresponds to an amorphous phase matrix within the silicon-based active material. The SiOx (0 < x < 2) may be in a form that partially contains Si and SiO2, and the Si may form a phase. That is, the x corresponds to the number ratio of O to Si contained in the SiOx (0 < x < 2).

[0067] The silicon-based active material can be formed by heating and vaporizing a mixed powder of Si powder and SiO2 powder, and then depositing the vaporized mixed gas. Specifically, the mixed powder of Si powder and SiO2 powder can be heat-treated at 1400°C to 1800°C or 1400°C to 1600°C under vacuum.

[0068] In one embodiment of this specification, based on 100 parts by weight of the positive electrode active material layer, the SiOx (0 < x < 2) may be included in an amount of 40 parts by weight or more, specifically 50 parts by weight or more, more specifically 60 parts by weight or more, and may be included in a range of 100 parts by weight or less, specifically 90 parts by weight or less, more specifically 80 parts by weight or less. When the silicon-based active material contains SiOx (0 < x < 2) within the above range, the discharge capacity of the lithium secondary battery can be improved.

[0069] In one embodiment of the present specification, the negative electrode active material may contain metal impurities.

[0070] The metal impurities are impurities that can be contained in silicon, and the content thereof may be in the range of 0.1 parts by weight or less based on 100 parts by weight of the negative electrode active material layer.

[0071] On the other hand, the average particle size (D50) of the silicon-based active material in this specification may be 5 μm to 10 μm, specifically 5.5 μm to 8 μm, and more specifically 6 μm to 7 μm.

[0072] When the average particle size is within the above range, the specific surface area of ​​the particles is within an appropriate range, and the viscosity of the negative electrode slurry is formed within an appropriate range. This allows 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 above lower limit range, the contact area between the silicon-based active material particles and the conductive material is improved by the composite of the conductive material and the binder in the negative electrode slurry, increasing the likelihood of maintaining the conductive network and improving the capacity retention rate. On the other hand, when the average particle size is within the above range, excessively large silicon-based active material particles are excluded, resulting in a smooth negative electrode surface, thereby preventing non-uniform current density during charge and discharge.

[0073] In one embodiment of the present specification, 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 0.1m 2 / 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).

[0074] In one embodiment of the present specification, 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 can also have a fibrous structure or exist in the form of a silicon-containing film or coating.

[0075] In one embodiment of the present specification, the silicon-based active material may be 40 parts by weight or more based on a total of 100 parts by weight of the negative electrode active material layer.

[0076] In one embodiment of the present specification, the silicon-based active material may be included in an amount of 40 parts by weight or more, specifically 50 parts by weight or more, more specifically 60 parts by weight or more, based on a total of 100 parts by weight of the negative electrode active material layer, and may be included in an amount of 95 parts by weight or less, specifically 90 parts by weight or less, more specifically 80 parts by weight or less.

[0077] In 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, in the case of silicon-based negative electrode active materials, volume changes and surface side reactions are severe, so much of the lithium inserted into the negative electrode during initial charging cannot return to the positive electrode, resulting in a problem of large initial irreversible capacity. This large initial irreversible capacity leads to a rapid decrease in battery capacity and cycle life.

[0078] In order to solve the above-mentioned problems, the present invention relates to a process of pre-lithiating the negative electrode of a lithium secondary battery to solve the problem of initial irreversible capacity. Specifically, the present invention relates to a process of pre-treating the negative electrode prior to the pre-lithiating process so that lithium metal is easily transferred from the transfer laminate during the lithium transfer process and lithium in the negative electrode active material layer is uniformly pre-lithiated.

[0079] In this regard, the negative electrode active material layer according to one embodiment of the present specification uses a silicon-based active material having an extremely high capacity within the above range, while using a specific amount of negative electrode conductive material to have low surface hardness. This allows pressure to be evenly transmitted during the pressure application process during lithium transfer, and prevents heat generated during pre-lithiation from condensing in a certain portion of the negative electrode, thereby reducing the surface temperature of the lithium secondary battery.

[0080] In one embodiment of the present specification, the surface hardness of the negative electrode active material layer may be 65 MPa or less. In this case, the surface hardness refers to the surface hardness of a portion of the surface of the negative electrode active material layer to which the lithium metal layer is transferred, and is measured using a microindenter. For example, the surface may be measured by indenting a diamond indenter into the surface of an object to an indentation depth of 5 μm at a constant indentation speed of 3000 nm / min.

[0081] In one embodiment of the present specification, the surface hardness of the negative electrode active material layer may be 65 MPa or less, specifically 10 MPa or more and 65 MPa or less, more specifically 20 MPa or more and 63.5 MPa or less.

[0082] The negative electrode active material layer according to an embodiment of the present specification has a surface hardness within the above range, thereby enabling pressure to be evenly transferred during the pressure application process during lithium transfer and preventing heat generated during pre-lithiation from condensing in a certain portion of the negative electrode, thereby reducing the surface temperature of the negative electrode.

[0083] 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 recently been increasing in response to growing demand for high-capacity batteries. 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 adversely reducing battery performance. Therefore, the type of anode conductive material used together with the silicon-based active material is important.

[0084] Thus, in one embodiment of the present specification, the negative electrode conductive material may include one or more selected from the group consisting of a sheet conductive material, a linear conductive material, and a point-like conductive material, and specifically, may include one or more selected from the group consisting of a sheet conductive material and a linear conductive material.

[0085] In one embodiment of the present specification, the negative electrode conductive material may be 13 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.

[0086] Specifically, in the one embodiment, the negative electrode conductive material may be contained in an amount of 13 parts by weight or more and 40 parts by weight or less, specifically 15 parts by weight or more and 35 parts by weight or less, and more specifically 16 parts by weight or more and 30 parts by weight or less, based on 100 parts by weight of the negative electrode active material layer.

[0087] When the content of the negative electrode conductive material satisfies the above range, the rollability of the electrode is improved and the heat generation level during pre-lithiation progresses is reduced without impairing the conductive path formed in the negative electrode active material layer.

[0088] In one embodiment of the present specification, the planar 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.

[0089] Meanwhile, the planar conductive material used as the negative electrode conductive material is a material having a planar or plate-like shape and can be expressed as plate-like graphite. That is, it is a material contained in the negative electrode active material layer to maintain a conductive path, and does not play a role in storing and releasing lithium, but rather a material that ensures a planar conductive path within the negative electrode active material layer.

[0090] That is, in one embodiment of the present specification, the use of plate-like graphite as a conductive material means that the graphite is processed into a planar or plate-like shape and is used as a material for securing a conductive path rather than for storing or releasing lithium. In this case, the negative electrode active material included therein has high capacity characteristics for storing and releasing lithium, and therefore plays a role in storing and releasing all lithium ions transferred from the positive electrode.

[0091] Meanwhile, in one embodiment of the present specification, the term "a carbon-based active material is used 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.

[0092] In one embodiment of the present specification, the negative electrode conductive material may include a sheet conductive material.

[0093] In this specification, the term "planar conductive material" refers to a conductive material with a two-dimensional (2D) structure in which atoms have a thickness of a single atomic layer or two or more atomic layers and form a planar crystalline structure. The planar conductive material is a material that ensures a planar conductive path within the negative electrode active material layer and can also play a role in preventing the conductive path from being broken due to volume expansion.

[0094] In one embodiment of the present specification, the planar 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.

[0095] In one embodiment of the present specification, 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 3 μm to 5 μm. When the average particle size satisfies this range, the sufficient particle size facilitates dispersion without causing an excessive increase in the viscosity of the negative electrode slurry. Therefore, when dispersing using the same equipment and time, the dispersion effect is excellent.

[0096] In one embodiment of the present specification, the sheet conductive material may have a D10 of 0.5 μm or more and 1.7 μm or less, a D50 of 2.5 μm or more and 3.5 μm or less, and a D90 of 6.8 μm or more and 15.0 μm or less.

[0097] In one embodiment of the present specification, 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.

[0098] In one embodiment of the present specification, 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, since the planar conductive material according to the present specification may have some effect on the performance of the electrode due to dispersion, it may be desirable to use a planar conductive material with a low specific surface area that does not cause dispersion problems.

[0099] In one embodiment of the present specification, the planar conductive material has a BET specific surface area of ​​5 m 2 / g or more.

[0100] In another embodiment, the sheet conductive material has a BET specific surface area of ​​5 m 2 / g or more 500m 2 / g or less, preferably 5m 2 / g or more 300m 2 / g or less, more preferably 5m 2 / g or more 250m 2 / g or less.

[0101] 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.

[0102] In another embodiment, the sheet conductive material is a low specific surface area sheet conductive material, and has a BET specific surface area of ​​5 m 2 / g or more 40m 2 / g or less, preferably 5m 2 / g or more 30m 2 / g or less, more preferably 5m 2 / g or more 25m 2 / g or less.

[0103] In one embodiment of the present specification, the negative electrode conductive material may include a sheet conductive material, and the sheet conductive material may be included in an amount of 15 parts by weight or more based on 100 parts by weight of the negative electrode active material layer.

[0104] In one embodiment of the present specification, the planar conductive material may be contained in an amount of 15 parts by weight or more and 40 parts by weight or less, specifically 16 parts by weight or more and 35 parts by weight or less, and more specifically 20 parts by weight or more and 30 parts by weight or less, based on 100 parts by weight of the negative electrode active material layer.

[0105] When the sheet conductive material satisfies the above range, the surface hardness of the negative electrode active material layer can be reduced to an appropriate level to make it soft while ensuring a conductive path, and pressure can be evenly transmitted during the pressure application step when transferring lithium to the soft negative electrode active material layer, preventing heat condensation in a certain area during the pre-lithiation step. At the same time, the negative electrode active material can be contained in an appropriate content, preventing the electrode from becoming excessively thick.

[0106] In one embodiment of the present specification, the negative electrode conductive material may include a linear conductive material.

[0107] In this specification, the term "linear conductive material" refers to a conductive material with a one-dimensional (1D) structure having a nanometer-sized diameter and a high aspect ratio, or a conductive material with a fibrous structure such as a cylindrical or tubular structure. An example of the linear conductive material is a carbon nanotube, which may be a bundle-type carbon nanotube. The bundle-type carbon nanotube may include a plurality of carbon nanotube units. Specifically, unless otherwise specified, the term "bundle-type" refers to a secondary shape of a bundle or rope in which a plurality of carbon nanotube monomers are arranged side by side or intertwined with the longitudinal axes of the carbon nanotube monomers in substantially the same orientation. The carbon nanotube units have graphite sheets in the form of cylinders with nanometer-sized diameters, and the sp 2 The bundled carbon nanotubes have a bonded structure. Depending on the angle and structure of the graphite plane wrapping, they can 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.

[0108] In one embodiment of the present specification, the negative electrode conductive material may preferably include, but is not limited to, one or more selected from the group consisting of platelet graphite, single-walled carbon nanotubes (SWCNTs), and multi-walled carbon nanotubes (MWCNTs).

[0109] In one embodiment of the present specification, the linear conductive material has a BET specific surface area of ​​100 m 2 / g or more 10,000m 2 / g or less, preferably 500m 2 / g or more 5,000m 2 / g or less, more preferably 1,000m 2 / g or more 1,500m 2 / g or less.

[0110] In addition, in one embodiment of the present specification, the aspect ratio of the linear conductive material may be 500 or more, preferably 1,000 or more, more preferably 10,000 or more, and may be 1,000,000 or less, preferably 100,000 or less.

[0111] In one embodiment of the present specification, when the linear conductive material satisfies the above-mentioned ranges of BET specific surface area and aspect ratio, deterioration of the conductive path due to volume change of the negative electrode active material is prevented, and there is an effect of maintaining or improving the life performance.

[0112] In one embodiment of the present specification, the negative electrode conductive material may further include dot-like conductive material.

[0113] The dot-like conductive material may be used to improve the conductivity of the negative electrode and may be a conductive material that exhibits conductivity without undergoing chemical changes. Specifically, the dot-like conductive material may be at least one selected from the group consisting of natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, conductive fiber, fluorocarbon, aluminum powder, nickel powder, zinc oxide, potassium titanate, titanium oxide, and polyphenylene derivatives, and preferably includes carbon black, which realizes high conductivity and has excellent dispersibility.

[0114] In one embodiment of the present specification, 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 60m 2 / g or less.

[0115] In one embodiment of the present specification, 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.

[0116] The negative electrode conductive material according to the present specification 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 specification serves to secure the contact points between the silicon-based active materials, which undergo a very large volume expansion during charging and discharging of the electrode, and has a completely different structure and role from the positive electrode conductive material, which serves as a buffer during rolling and also provides partial conductivity.

[0117] Furthermore, the negative electrode conductive material according to the present invention is applied to a silicon-based negative electrode 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 are completely different in structure and role from negative electrode conductive materials applied together with silicon-based negative electrode active materials as in the present invention.

[0118] In one embodiment of the present specification, 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, polyacrylamide, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, 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.

[0119] The negative electrode binder according to one embodiment of the present specification fulfills the role of capturing the active material and conductive material to prevent distortion and structural deformation of the negative electrode structure during volume expansion and relaxation of the silicon-based active material. Any common binder can be used as long as it fulfills this role, and specifically, the binder may include one or more binders selected from the group consisting of polyacrylamide (PAM) and styrene-butadiene rubber.

[0120] In one embodiment of the present specification, the negative electrode active material layer may contain 1 part by weight or more and 30 parts by weight or less, specifically 3 parts by weight or more and 20 parts by weight or less, more specifically 5 parts by weight or more and 10 parts by weight or less, based on a total of 100 parts by weight of the negative electrode active material layer.

[0121] In one embodiment of the present invention, the weight average molecular weight of the binder may be 100,000 g / mol or more and 1,500,000 g / mol or less.

[0122] When the weight-average molecular weight of the negative electrode binder satisfies the above range, the binder has excellent mechanical strength and strong intermolecular interaction, resulting in excellent electrode binding strength. Furthermore, when the weight-average molecular weight satisfies the above range, the binder viscosity can be selected within an appropriate range, and when a negative electrode is manufactured using the binder, the coating properties of the electrode can be further improved.

[0123] A negative electrode according to one embodiment of the present specification may include the above-described negative electrode active material; a negative electrode conductive material; and a negative electrode binder.

[0124] Specifically, the negative electrode may include a negative electrode current collector layer and a negative electrode active material layer provided on at least one surface of the negative electrode current collector layer. The negative electrode active material layer may include the negative electrode active material. Furthermore, the negative electrode active material layer may further include a thickener.

[0125] 2 is a diagram showing a laminated structure of a negative electrode for a lithium secondary battery according to one embodiment of the present specification. Specifically, a negative electrode for a lithium secondary battery 100 including a negative electrode active material layer 20 on one surface of a negative electrode current collector layer 10 can be seen. While FIG. 2 shows the negative electrode active material layer formed on one surface of the negative electrode current collector layer, it may also be formed on both surfaces of the negative electrode current collector layer.

[0126] In one embodiment of the present specification, the negative electrode current collector layer may generally have a thickness of 1 μm to 100 μm. Such a negative electrode current collector layer is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, or an aluminum-cadmium alloy may be used. Furthermore, the surface may be provided with fine irregularities to strengthen the binding force of the negative electrode active material, and the negative electrode current collector layer may be used in various forms such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0127] In one embodiment of the present specification, the thickness of the negative electrode current collector layer may be 1 μm to 100 μm, and the thickness of the negative electrode may be 20 μm to 500 μm, but the thickness may vary depending on the type and application of the negative electrode used and is not limited thereto.

[0128] The negative electrode active material layer may be formed by coating at least one surface of the negative electrode current collector layer with a negative electrode slurry including a negative electrode active material, a negative electrode binder, a negative electrode conductive material, and / or a thickener. This means that the negative electrode active material layer may be formed by applying the negative electrode slurry to at least one surface of the negative electrode current collector layer, drying, and rolling.

[0129] In one embodiment of the present specification, the negative electrode slurry may include a negative electrode active material layer composition including the negative electrode active material, a negative electrode binder, a negative electrode conductive material, and / or a thickener; and a slurry solvent.

[0130] In one embodiment of the present specification, the solid content of the negative electrode slurry may be 5% or more and 40% or less.

[0131] 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%.

[0132] 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.

[0133] 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 the agglomeration of particles of the negative electrode active material layer composition is minimized, thereby enabling the negative electrode active material layer to be efficiently formed.

[0134] In one embodiment of the present specification, the solvent may include those known to those skilled in the art. For example, the solvent may be water (e.g., distilled water) or NMP (N-methyl-2-pyrrolidone).

[0135] The negative electrode according to one embodiment of the present specification may be formed by coating one or both surfaces of a negative electrode current collector layer with the negative electrode slurry and drying the coating. The drying step may evaporate a slurry solvent from the negative electrode slurry.

[0136] In one embodiment of the present specification, at least one surface of the negative electrode active material layer may be pre-lithiated. That is, in one embodiment of the present specification, one surface of the negative electrode active material layer may be pre-lithiated, or both surfaces of the negative electrode active material layer may be pre-lithiated.

[0137] Lithium secondary batteries begin to lose lithium during the first charge after battery fabrication, and continue to lose lithium during subsequent charge-discharge cycles and high-temperature storage, resulting in battery degradation. Pre-lithiation refers to the process of adding additional lithium to a battery before it is operated to compensate for this lithium loss. In an anode for a lithium secondary battery according to one embodiment of the present specification, at least one surface of the anode active material layer is pre-lithiated, thereby increasing the battery's capacity and improving its lifespan.

[0138] In one embodiment of the present specification, the pre-lithiation may be performed by a lithium electroplating process; a lithium metal transfer process; a lithium metal vapor deposition process; or a stabilized lithium metal powder (SLMP) coating process. Specifically, the pre-lithiation may be performed by a method of injecting a compound containing an excess amount of lithium as an additive during battery fabrication; a process of laminating a lithium metal thin film on the surface of an electrode active material to prepare an electrode assembly, and then injecting an electrolyte solution to cause lithium to diffuse into the negative electrode active material, thereby pre-lithiating the negative electrode; a process of directly contacting lithium metal with an electrode under dry room conditions, thereby causing a spontaneous pre-lithiation reaction; or a process of adding lithium, in the form of stabilized metal powder, to the electrode in an amount equivalent to that consumed during the first charge.

[0139] In one embodiment of the present specification, the pre-lithiation may be performed by a lithium metal transfer process. When the pre-lithiation is performed by a lithium metal transfer process, lithium loss can be minimized, and the reaction rate is fast, making it suitable for mass production.

[0140] In this specification, whether or not a negative electrode has been pre-lithiated can be confirmed by checking whether the total weight of the prepared negative electrode increases due to the weight of lithium inserted into the negative electrode active material layer compared to before pre-lithiation. In this case, the weight increase due to lithium inserted into the negative electrode active material layer due to pre-lithiation may be about 5% or more. The weight of the negative electrode and lithium can be measured in a glove box or glove compartment filled with argon (Ar) gas, using a scale capable of measuring from mg to μg units.

[0141] Alternatively, whether or not the negative electrode is prelithiated can be indirectly confirmed by determining whether the charge-discharge efficiency of the final battery in the first charge-discharge cycle is increased to 85% or more. The charge-discharge efficiency can be confirmed by measuring the charge-discharge capacity of the battery and then calculating it according to the following Equation A-1:

[0142] [Formula A-1] Charge / discharge efficiency = (discharge capacity / charge capacity) x 100 (%)

[0143] Alternatively, in this specification, whether or not a negative electrode has been pre-lithiated can also be confirmed by a color change in the negative electrode active material layer. The negative electrode active material layer before pre-lithiation is black, whereas the carbon-based negative electrode active material layer after pre-lithiation exhibits a surface color change to gold, and the silicon-based negative electrode active material layer exhibits a surface color change to green or purple, etc. This color change is due to the generation of nitrogen oxides due to a side reaction.

[0144] The description of the pre-lithiation step by the transfer method described above can be applied to the description of the pre-lithiation step.

[0145] A lithium secondary battery according to an embodiment of the present specification may include a positive electrode for a lithium secondary battery, a negative electrode for a lithium secondary battery manufactured by the above-described manufacturing method, a separator disposed between the positive electrode for the lithium secondary battery and the negative electrode for the lithium secondary battery, and an electrolyte, and the negative electrode for the lithium secondary battery is the same as the negative electrode for the lithium secondary battery described above. Since the negative electrode for a lithium secondary battery has been described above, detailed description thereof will be omitted.

[0146] 3 is a diagram showing a laminated structure of a lithium secondary battery according to an embodiment of the present disclosure. Specifically, a lithium secondary battery anode 100 including an anode active material layer 20 on one side of an anode current collector layer 10 can be seen, and a lithium secondary battery cathode 200 including a cathode active material layer 40 on one side of a cathode current collector layer 50 can be seen, and the lithium secondary battery anode 100 and the lithium secondary battery cathode 200 are stacked with a separator 30 sandwiched between them. In this case, the lithium secondary battery anode may be manufactured by a manufacturing method according to an embodiment of the present disclosure.

[0147] A lithium secondary battery according to another embodiment of the present specification is a lithium secondary battery including a positive electrode; a pre-lithiated negative electrode; a separator; and an electrolyte, wherein the pre-lithiated negative electrode includes a negative electrode current collector layer and a negative electrode active material layer provided on at least one surface of the negative electrode current collector layer, and the negative electrode active material layer may have a surface hardness of 65 MPa or less.

[0148] In this case, the pre-lithiated negative electrode is the same as the negative electrode formed by transferring a lithium metal layer onto at least one surface of the above-described negative electrode active material layer, and the same details as those of the above-described negative electrode active material layer, lithium metal layer, etc. can be applied.

[0149] In this specification, when the surface hardness of the negative electrode active material layer satisfies 65 MPa or less, pressure is evenly transmitted during the pressure application process during lithium transfer, preventing condensation of heat generated during pre-lithiation in a certain part of the negative electrode, thereby reducing the surface temperature of the lithium secondary battery; specifically, the surface hardness may be 60 MPa or less.

[0150] The range of the surface hardness of the negative electrode active material layer can be satisfied by adjusting the content of the negative electrode conductive material, specifically, by adjusting the content of the sheet conductive material.

[0151] Specifically, in another embodiment of the present specification, the negative electrode active material layer includes a negative electrode active material, a negative electrode conductive material, and a negative electrode binder, the negative electrode conductive material includes one or more selected from the group consisting of a planar conductive material and a linear conductive material, and the negative electrode conductive material may be included in an amount of 13 parts by weight or more based on 100 parts by weight of the negative electrode active material layer.

[0152] More specifically, in another embodiment of the present specification, the negative electrode active material layer includes a negative electrode active material, a negative electrode conductive material, and a negative electrode binder, the negative electrode conductive material includes a sheet-like conductive material, and the sheet-like conductive material may be included in an amount of 15 parts by weight or more based on 100 parts by weight of the negative electrode active material layer.

[0153] In this specification, the planar conductive material may be contained in an amount of 15 parts by weight or more and 40 parts by weight or less, specifically 20 parts by weight or more and 35 parts by weight or less, and preferably 20.91 parts by weight or more and 30 parts by weight or less, based on 100 parts by weight of the negative electrode active material layer.

[0154] When the sheet conductive material satisfies the above range, the surface hardness of the negative electrode active material layer can be reduced to an appropriate level to make it soft while ensuring a conductive path, and pressure can be evenly transmitted during the pressurizing step of transferring lithium to the soft negative electrode active material layer, preventing heat condensation in a certain area during the pre-lithiation step. At the same time, the negative electrode active material can be contained in an appropriate content, preventing the electrode from becoming excessively thick.

[0155] The positive electrode may include a positive electrode current collector layer and a positive electrode active material layer formed on at least one surface of the positive electrode current collector layer and containing the positive electrode active material.

[0156] The positive electrode current collector layer in the positive electrode is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. 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 layer may typically have a thickness of 3 μm to 500 μm, and fine irregularities can be formed on the surface of the current collector layer to enhance adhesion of the positive electrode active material. For example, the positive electrode current collector layer may be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0157] The positive electrode active material according to one embodiment of the present specification may include one or more selected from the group consisting of lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium iron oxide, lithium aluminum oxide, and lithium composite oxides combining these.

[0158] 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 Ni-site type lithium nickel oxide represented by the chemical formula LiMnO2 (wherein M is at least one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, and 0.01≦c2≦0.5 is satisfied); 2-c3 M c3 Examples of the lithium manganese composite oxide 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≦0.1) or Li2Mn3MO8 (where M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn); or 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.

[0159] 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.

[0160] In this case, the positive electrode conductive material is used to impart conductivity to the electrode, and can be any material that does not cause chemical changes in the battery and has electronic conductivity. 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. These materials can be used alone or in combination.

[0161] 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 include polyvinylidene fluoride (PVdF), vinylidene fluoride-hexafluoropropylene copolymer (PVdF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These may be used alone or in combination.

[0162] The solvent used in the positive electrode composition slurry may be a solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and may be used alone or in combination. The amount of solvent used is sufficient to dissolve or disperse the active material, conductive material, and binder, and to provide a viscosity that allows excellent thickness uniformity during subsequent application to produce positive and negative electrodes, taking into account the coating thickness and production yield of the slurry. Alternatively, the positive and negative electrodes may be fabricated by casting the active material layer-forming composition on a separate support, peeling it off from the support, and laminating the resulting film on a current collector.

[0163] The separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator typically used in secondary batteries can be used without particular limitations. In particular, a separator that exhibits low resistance to ion migration in the electrolyte and excellent electrolyte humidification is preferred. Specifically, a porous polymer film, such as a porous polymer film made of a polyolefin polymer, such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof, may be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may also be used. To ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material may be used, and may be used in a single-layer or multi-layer structure.

[0164] Examples of the electrolyte include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in manufacturing lithium secondary batteries, but are not limited to these.

[0165] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0166] Examples of the non-aqueous organic solvent that can be used include non-protonic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0167] In particular, among the carbonate-based organic solvents, cyclic carbonates such as ethylene carbonate and propylene carbonate are preferably used because they are high-viscosity organic solvents with high dielectric constants and can effectively dissociate lithium salts. When such cyclic carbonates are mixed with linear carbonates such as dimethyl carbonate and diethyl carbonate, which have low viscosity and low dielectric constants, in an appropriate ratio, an electrolyte solution having high electrical conductivity can be prepared, and these cyclic carbonates are more preferably used.

[0168] The metal salt can 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 can 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:

[0169] In addition to the components of the electrolytic solution, the electrolyte may further contain one or more additives, such as a haloalkylene carbonate compound such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivative, sulfur, quinoneimine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, 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.

[0170] Whether the lithium secondary battery manufactured according to an embodiment of the present specification is manufactured by the manufacturing method of the present specification can be confirmed by analyzing the cross-sectional shape of the electrode using a scanning electron microscope (SEM) or analyzing the composition of the electrode using an energy dispersive spectrometer (EDS).

[0171] One embodiment of the present specification provides a battery module including the lithium secondary battery as a unit cell, and a battery pack including the same.

[0172] Another embodiment of the present specification provides a battery pack including the lithium secondary battery.

[0173] The lithium secondary batteries according to the embodiments of the present specification stably exhibit excellent discharge capacity, output characteristics, and cycle performance, and therefore can be used as power sources for portable devices such as mobile phones, laptops, and digital cameras, as well as 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. For example, the battery modules or battery packs can be used as power sources for one or more medium- to large-sized devices, including power tools; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0174] [Example] Below, preferred examples are shown to help understand the present invention, but the following examples are merely illustrative of the present description, and it will be apparent to those skilled in the art that various changes and modifications are possible within the scope and technical spirit of the present description, and it is natural that such changes and modifications fall within the scope of the appended claims.

[0175] <Production example> Example 1 <Production of negative electrodes> A negative electrode active material layer composition was prepared using a silicon-based active material (average particle size (D50): 6.6 μm), a plate-shaped conductive material A (SFG-6L), a conductive material B (SWCNT), and polyacrylamide and styrene butadiene rubber (SBR) as binders in a weight ratio of 69.71:20.91:0.35:6.24:2.79. Distilled water was added as the solvent for forming the negative electrode slurry to produce a negative electrode slurry (solid concentration: 28 wt%).

[0176] At this time, the plate-shaped conductive material A (SFG-6L) had a BET specific surface area of ​​17 m 2 / g, D10 is 1.7 μm, D50 is 3.5 μm, and D90 is 6.8 μm. The conductive material B (SWCNT) has a BET specific surface area of ​​1,000 to 1,500 m 2 / g and has an aspect ratio of 10,000 or more.

[0177] As a mixing method, the conductive material A, conductive material B, binder, and water were dispersed using a homomixer at 2500 rpm for 30 minutes, and then the negative electrode active material was added and dispersed at 2500 rpm for 30 minutes to prepare a negative electrode slurry.

[0178] The negative electrode current collector layer was formed by applying the negative electrode slurry to both sides of a copper current collector (thickness: 15 μm) at 3.675 mAh / cm 2 The coated layer was rolled and dried in a vacuum oven at 130° C. for 10 hours to form a negative electrode active material layer (thickness: 58 μm) having the surface hardness shown in Table 1 below.

[0179] Lithium metal (thickness: 6.2 μm) was transferred onto the top of the negative electrode active material layer, and 1.28 mAh / cm 2 A negative electrode having the maximum surface temperature shown in Table 1 below was produced (negative electrode thickness: 62 μm).

[0180] Example 2 A negative electrode of Example 2 having a maximum surface temperature shown in Table 1 below was produced in the same manner as in Example 1 above (negative electrode thickness: 62 μm), except that a negative electrode active material layer (thickness: 57 μm) having a surface hardness shown in Table 1 below was formed using a negative electrode slurry (solid content concentration: 28 wt %) produced by adding a silicon-based active material (average particle size (D50): 6.6 μm), plate-shaped conductive material A (SFG-6L), conductive material B (SWCNT), polyacrylamide as a binder, and styrene butadiene rubber (SBR) in a weight ratio of 73.83:16.24:0.37:6.61:2.95 to distilled water as a solvent for forming the negative electrode active material layer composition.

[0181] Comparative Example 1 A negative electrode of Comparative Example 1 having a maximum surface temperature shown in Table 1 below was manufactured in the same manner as in Example 1, except that a negative electrode active material layer composition (thickness: 48 μm) having a surface hardness shown in Table 1 below was formed using a negative electrode slurry (solid content concentration: 28 wt %) prepared by adding a silicon-based active material (average particle size (D50): 6.6 μm), a conductive material B, and a polyacrylamide binder in a weight ratio of 88.5:0.44:11.06 to distilled water as a solvent for forming the negative electrode active material layer composition (solid content concentration: 28 wt %).

[0182] Comparative Example 2 A negative electrode of Comparative Example 2 having a maximum surface temperature shown in Table 1 below was produced in the same manner as in Example 1 above (negative electrode thickness: 58 μm), except that a negative electrode active material layer composition was prepared by mixing a silicon-based active material (average particle size (D50): 6.6 μm), a plate-shaped conductive material A, a conductive material B, and a polyacrylamide binder in a weight ratio of 80:9.6:0.4:10, and adding this to distilled water as a solvent for forming a negative electrode slurry to produce a negative electrode slurry (solid content concentration: 28 wt %).

[0183] Comparative Example 3 A negative electrode of Comparative Example 3 having a maximum surface temperature shown in Table 1 below was manufactured in the same manner as in Example 1 above, except that a negative electrode active material layer (thickness: 50 μm) having a surface hardness shown in Table 1 below was formed using a negative electrode slurry (solid content concentration: 28 wt %) prepared by adding a silicon-based active material (average particle size (D50): 6.6 μm), plate-shaped conductive material A, conductive material B, polyacrylamide binder, and styrene butadiene rubber (SBR) in a weight ratio of 85.14:3.4:0.43:7.63:3.4 to distilled water as a solvent for forming the negative electrode active material layer composition. The negative electrode active material layer was prepared using a negative electrode slurry (solid content concentration: 28 wt %) having a surface hardness shown in Table 1 below.

[0184] Comparative Example 4 A negative electrode of Comparative Example 4 having a maximum surface temperature shown in Table 1 below was produced in the same manner as in Example 1, except that a negative electrode active material layer (thickness: 54 μm) having a surface hardness shown in Table 1 below was formed using a negative electrode slurry (solid concentration: 28 wt %) produced by adding a silicon-based active material (average particle size (D50): 6.6 μm), plate-shaped conductive material A, conductive material B, polyacrylamide as a binder, and styrene butadiene rubber (SBR) in a weight ratio of 77.85:11.68:0.39:6.97:3.11. The negative electrode was prepared in the same manner as in Example 1, except that a negative electrode active material layer (thickness: 54 μm) having a surface hardness shown in Table 1 below was formed using a negative electrode slurry (solid concentration: 28 wt %) produced by adding a silicon-based active material (average particle size (D50): 6.6 μm), plate-shaped conductive material A, conductive material B, polyacrylamide as a binder, and styrene butadiene rubber (SBR) in a weight ratio of 77.85:11.68:0.39:6.97:3.11.

[0185] In Examples 1 and 2 and Comparative Examples 1 to 4 prepared according to the above Preparation Examples, the surface hardness of the surface portion of each negative electrode active material layer to be transferred to lithium metal was measured using a diamond micro-indenter (NX10 manufactured by Park System) at the same indentation speed of 3000 nm / min to an indentation depth of 5 μm using a displacement control method before transferring to lithium metal. The results are shown in Table 1 below.

[0186] In addition, for the negative electrodes of Examples 1 and 2 and Comparative Examples 1 to 4 manufactured by the above manufacturing examples, a thermocouple was brought into contact with the surface transferred to the lithium metal, and the surface temperature was measured for 1 hour after the completion of pre-lithiation (i.e., after the color of the negative electrode surface changed to green, purple, etc.), and the maximum temperature was recorded in Table 1 below.

[0187] [Table 1]

[0188] <Experimental Example> Whether or not ignition occurred in the lithium secondary batteries including the negative electrodes of Examples 1 and 2 and Comparative Examples 1 to 4 prepared according to the above Preparation Examples is shown in Table 2 below. 'X' means that ignition did not occur, and 'O' means that ignition occurred.

[0189] [Table 2]

[0190] As can be seen from Table 1, in Comparative Examples 1 and 3, which contained very small amounts of negative electrode conductive material, the surface hardness of the negative electrode active material layer was as high as 70 MPa or more. This meant that pressure was not evenly distributed during the pressure application step during lithium transfer. As a result, heat was condensed in certain areas during the pre-lithiation step, causing the maximum surface temperature of the negative electrode to rise significantly to 53°C or more, resulting in the problem of fire.

[0191] In the case of Comparative Examples 2 and 4, which contained approximately 10 parts by weight of sheet conductive material based on a total of 100 parts by weight of the negative electrode active material layer, the surface hardness of the negative electrode active material layer was low at 70 MPa or less, compared to Comparative Examples 1 and 3, which contained a small amount of the negative electrode conductive material itself.As a result, the maximum surface temperature of the negative electrode was measured at approximately 40 to 41°C, and although no fire occurred, there was a high risk of fire in the subsequent winding step, and the winding process could not be carried out.

[0192] In Examples 1 and 2, which contained 15 parts by weight or more of a sheet conductive material based on a total of 100 parts by weight of the negative electrode active material layer, the surface hardness was measured to be lower at 65 MPa or less than in Comparative Examples 1 to 4. As a result, compared to Comparative Examples 1 to 4, which measured a surface hardness greater than 65 MPa, it was confirmed that pressure could be transmitted evenly during the pressure application step during lithium transfer, heat did not condense in a certain area, and the maximum surface temperature of the negative electrode was significantly lowered to 35°C or less. This confirmed that no fire occurred in the lithium secondary battery.

[0193] Therefore, the present invention can significantly reduce the risk of fire due to heat accumulation during the winding process by maintaining the maximum temperature of the negative electrode surface at 35°C or less after the completion of pre-lithiation. This can be achieved by adjusting the composition conditions of the negative electrode active material layer, particularly the content of the planar conductive material, to reduce the surface hardness to 65 MPa or less, thereby evenly transmitting pressure during the pressing step during lithium transfer. [Explanation of symbols]

[0194] 10 Negative electrode current collector layer 20...Negative electrode active material layer 30...Separation membrane 40...Cathode active material layer 50 Positive electrode current collector layer 100 ···Negative electrode for lithium secondary battery 200 ···Positive electrode for lithium secondary battery

Claims

1. forming a negative electrode by transferring a lithium metal layer onto at least one surface of a negative electrode active material layer including a negative electrode active material, a negative electrode conductive material, and a negative electrode binder; the maximum temperature on the surface of the negative electrode after the transfer of the lithium metal layer is 35° C. or less.

2. 10. The method of claim 1, further comprising forming a negative electrode active material layer on at least one surface of the negative electrode current collector layer, the negative electrode active material layer including a negative electrode active material, a negative electrode conductive material, and a negative electrode binder.

3. the negative electrode conductive material in the negative electrode active material layer includes at least one selected from the group consisting of a sheet conductive material and a linear conductive material, The method for manufacturing a negative electrode for a lithium secondary battery according to claim 1 , wherein the negative electrode conductive material is contained in an amount of 13 parts by weight or more based on 100 parts by weight of the negative electrode active material layer.

4. the negative electrode conductive material in the negative electrode active material layer includes a planar conductive material, The method for manufacturing a negative electrode for a lithium secondary battery according to claim 1 , wherein the sheet conductive material is contained in an amount of 15 parts by weight or more based on 100 parts by weight of the negative electrode active material layer.

5. The method for producing a negative electrode for a lithium secondary battery according to claim 1 , wherein the surface hardness of the negative electrode active material layer is 65 MPa or less.

6. the negative electrode active material includes a silicon-based active material, 2. The method for producing a negative electrode for a lithium secondary battery according to claim 1, wherein the silicon-based active material comprises at least one selected from the group consisting of Si, SiOx (0<x<2), Si / C, and a Si alloy.

7. In the step of transferring the lithium metal layer to form a negative electrode, 2. The method for producing a negative electrode for a lithium secondary battery according to claim 1, wherein the lithium metal layer has a thickness of 1 μm or more and 10 μm or less.

8. Positive electrodes for lithium secondary batteries; A negative electrode for a lithium secondary battery produced by the production method according to claim 1; a separator disposed between the positive electrode for the lithium secondary battery and the negative electrode for the lithium secondary battery; and electrolyte A lithium secondary battery comprising:

9. A lithium secondary battery comprising a positive electrode, a prelithiated negative electrode, a separator, and an electrolyte, the prelithiated negative electrode includes a negative electrode current collector layer and a negative electrode active material layer provided on at least one surface of the negative electrode current collector layer, The negative electrode active material layer has a surface hardness of 65 MPa or less.

10. the negative electrode active material layer includes a negative electrode active material, a negative electrode conductive material, and a negative electrode binder, the negative electrode conductive material includes at least one selected from the group consisting of a sheet conductive material and a linear conductive material, The lithium secondary battery according to claim 9 , wherein the negative electrode conductive material is contained in an amount of 13 parts by weight or more based on 100 parts by weight of the negative electrode active material layer.

11. A battery module comprising the lithium secondary battery according to claim 8 or 9.

12. A battery pack comprising the lithium secondary battery according to claim 8 or 9.

13. A battery pack comprising the battery module according to claim 11.

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