Method for charging and discharging lithium secondary battery

Abstract

The present invention relates to a method for charging and discharging a lithium secondary battery, the method comprising: a step for manufacturing a lithium secondary battery containing a negative electrode containing a silicon-based active material; a first cycle step of repeating N1 times of a first cycle on the lithium secondary battery, the first cycle being discharging under a condition of 3.0 V to 3.5 V after charging; and a second cycle step of performing N2 times of second cycles on the lithium secondary battery, the second cycles being complete discharge after charging, where N1 / N2 is greater than 25 and less than 100.

Description

Technical Field

[0001] The invention relates to a method for charging and discharging a lithium secondary battery.

[0002] This application claims priority to and the benefit of Korean Patent Application No. 10-2022-0175978 filed in the Korean Intellectual Property Office on December 15, 2022, and Korean Patent Application No. 10-2023-0180458 filed in the Korean Intellectual Property Office on December 13, 2023, the entire contents of which are incorporated herein by reference.

[0003] The invention relates to a method for charging and discharging a lithium secondary battery. Background Art

[0004] Recently, along with the rapid popularization of electronic devices such as mobile phones, laptop computers and electric vehicles using batteries, the demand for small, lightweight, relatively high-capacity secondary batteries has increased rapidly. In particular, lithium secondary batteries have become a driving power source for portable devices that has attracted much attention due to their light weight and high energy density. Therefore, research and development work to improve the performance of lithium secondary batteries is being actively carried out.

[0005] Typically, a lithium secondary battery comprises a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, an electrolyte, an organic solvent, etc. In addition, the positive electrode and the negative electrode may be formed with active material layers each containing a positive electrode active material and a negative electrode active material on a current collector. Typically, for the positive electrode, lithium-containing metal oxides such as LiCoO2 and LiMn2O4 are used as positive electrode active materials, and for the negative electrode, carbon-based active materials or silicon-based active materials that do not contain lithium are used as negative electrode active materials.

[0006] Among negative electrode active materials, silicon-based active materials have attracted much attention due to their higher capacity and excellent high-speed charging characteristics than carbon-based active materials. However, the disadvantages of silicon-based active materials are that the volume expansion / contraction caused by charging and discharging is high, resulting in a decrease in life characteristics, and the irreversible capacity is high, resulting in low initial efficiency.

[0007] In addition, in the case of a secondary battery containing a silicon-based active material, cracks and volume changes in the surface portion are large. In particular, when a method of transferring lithium to the negative electrode as one of the pre-lithiation methods is used, the degradation of the surface portion of the negative electrode becomes more serious. The problem this brings is that the degradation of the surface portion is accelerated more quickly when evaluating the life, resulting in a sudden drop. In addition, when a secondary battery containing a silicon-based active material is charged and discharged with a limited cycle, the degradation is accelerated due to uneven use of the negative electrode.

[0008] Therefore, there is a need to develop a secondary battery capable of improving life characteristics while achieving high capacity and energy density of silicon-based active materials. Summary of the invention

[0009] Technical issues

[0010] The present invention relates to a method for charging and discharging a lithium secondary battery, and provides a method for charging and discharging a lithium secondary battery capable of improving durability by adjusting a cycle driving method during charging and discharging to prevent degradation of a lithium secondary battery containing a silicon-based active material.

[0011] Technical Solution

[0012] An exemplary embodiment of the present invention provides a method for charging and discharging a lithium secondary battery, the method comprising:

[0013] a step of manufacturing a lithium secondary battery having a negative electrode including a silicon-based active material;

[0014] First cycle step: repeating the first cycle N1 times on the lithium secondary battery, wherein the first cycle is discharging at 3.0 V to 3.5 V after charging; and

[0015] The second cycle step: performing N2 second cycles on the lithium secondary battery, wherein the second cycle is a complete discharge after charging,

[0016] Where N1 / N2 is greater than 25 and less than 100.

[0017] Beneficial Effects

[0018] According to an exemplary embodiment of the present invention, a method for charging and discharging a lithium secondary battery eliminates an irreversible phase generated in real time by adjusting the number of times to reach SOC 0% (full discharge) during a cycle in an optimal manner, thereby controlling degradation in a negative electrode surface portion, thereby improving the durability of the negative electrode.

[0019] Therefore, a lithium secondary battery to which the method of charging and discharging a lithium secondary battery according to an exemplary embodiment of the present invention is applied is improved in discharge capacity, initial efficiency, resistance properties, and / or lifespan characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a graph showing cycle characteristics of secondary batteries of Example 1-1 and Comparative Examples 1-1 and 1-2 to which the method of charging and discharging a lithium secondary battery according to an exemplary embodiment of the present invention is applied.

[0021] Figure 2 is a graph showing capacity characteristics of secondary batteries of Examples 1-2 and Comparative Examples 1-3 to which the method of charging and discharging a lithium secondary battery according to an exemplary embodiment of the present invention is applied.

[0022] Figure 3 is a graph showing discharge resistance (Rdis) values ​​with respect to time in Examples 1-2 and Comparative Examples 1-3 to which a method of charging and discharging a lithium secondary battery according to an exemplary embodiment of the present invention is applied. DETAILED DESCRIPTION

[0023] The present specification will be described in more detail below.

[0024] In the present specification, unless explicitly described to the contrary, when a part is referred to as “comprising” a certain component, this means that the part may further include another component, not excluding the other component.

[0025] Throughout this specification, when a component is referred to as being 'on' another component, the component may be in direct contact with the other component, or an intervening component may also be present.

[0026] It should be understood that the terms or words used throughout this specification should not be interpreted as limited to their ordinary meanings or dictionary meanings, but should be interpreted as having meanings and concepts consistent with the technical ideas of the present invention based on the concepts that the inventor can appropriately define the words or terms in order to explain the principles of the present invention in the best manner.

[0027] As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise.

[0028] In the present specification, the crystallinity of the structure contained in the negative electrode active material can be confirmed by X-ray diffraction analysis, and the X-ray diffraction analysis can be performed by using an X-ray diffraction (XRD) analyzer (product name: D4-endavor, manufacturer: Bruker), or by appropriately adopting an apparatus used in the art in addition to the above-mentioned apparatus.

[0029] In the present specification, the presence or absence of elements and the content of the elements in the negative electrode active material may be confirmed by ICP analysis, and the ICP analysis may be performed using an inductively coupled plasma atomic emission spectrometer (ICPAES, Perkin-Elmer 7300).

[0030] In this specification, the average particle size (D 50 ) can be defined as the particle size corresponding to 50% cumulative volume in the particle size distribution curve (particle size distribution curve). Average particle size (D 50 ) can be measured using, for example, the laser diffraction method. In the laser diffraction method, particle sizes from the submicron range to several millimeters can generally be measured, and results with high reproducibility and high resolution can be obtained.

[0031] Hereinafter, preferred embodiments of the present invention will be described in detail. However, it should be understood that the embodiments of the present invention can be modified in various forms, and the scope of the present invention is not limited to the embodiments described below.

[0032] <Steps for Manufacturing Lithium Secondary Battery>

[0033] An exemplary embodiment of the present invention provides a method for charging and discharging a lithium secondary battery, the method comprising:

[0034] a step of manufacturing a lithium secondary battery having a negative electrode including a silicon-based active material;

[0035] First cycle step: repeating the first cycle N1 times on the lithium secondary battery, wherein the first cycle is discharging at 3.0 V to 3.5 V after charging; and

[0036] The second cycle step: performing N2 second cycles on the lithium secondary battery, wherein the second cycle is a complete discharge after charging,

[0037] Where N1 / N2 is greater than 25 and less than 100.

[0038] Generally, in the case of a secondary battery containing a silicon-based active material, cracks and volume changes in the surface portion are large. In particular, when a method of transferring lithium to the negative electrode as one of the pre-lithiation methods is used, the degradation in the surface portion of the negative electrode becomes more serious. This brings about the problem of accelerating the degradation in the surface portion more quickly when evaluating the life, resulting in a sudden drop.

[0039] In addition, when a secondary battery including a silicon-based active material is charged and discharged, if the charge and discharge are performed in a limited cycle in which partial discharge (3.0 V to 3.5 V) is performed after charge, irreversible LixSiy (x, y are real numbers) is generated, and therefore, at the end of discharge, the capacity cannot be properly exhibited within the use range of silicon. As a result, the resistance of the negative electrode increases, and a phenomenon of temporary capacity degradation / recovery (saw-like) is repeated.

[0040] On the other hand, in the present invention, when the lithium secondary battery is charged and discharged, a limited cycle in which partial discharge (3.0 V to 3.5 V) is performed after charging, for example, a first cycle in which discharge is performed under the condition of 3.0 V to 3.5 V after charging is repeated a predetermined number of times, and then a cycle in which full discharge (SOC 0%) is performed after charging, for example, a second cycle in which full discharge is performed after charging is performed a predetermined number of times. As a result, the first cycle is repeated to eliminate the irreversible phase generated in real time in the negative electrode, thereby controlling the deterioration in the surface portion of the negative electrode, thereby improving the durability of the negative electrode.

[0041] Specifically, the first cycle is repeated N1 times, and the second cycle is repeated N2 times. In this case, the number of times of each cycle is adjusted such that N1 / N2 is greater than 25 and less than 100.

[0042] The second cycle, for example, a full discharge (SOC 0%) cycle, induces de-lithiation / lithiation within the usable range of silicon at the end of discharge to reduce the resistance of the negative electrode, thereby reducing the degree of temporary capacity degradation by eliminating the irreversible phase generated in the first cycle.

[0043] Therefore, a lithium secondary battery applying the method for charging and discharging a lithium secondary battery according to an exemplary embodiment of the present invention is improved in terms of discharge capacity, initial efficiency, resistance performance, and / or life characteristics.

[0044] The method for charging and discharging a lithium secondary battery according to an exemplary embodiment of the present invention includes the step of manufacturing a lithium secondary battery including a negative electrode containing a silicon-based active material.

[0045] The negative electrode contains a silicon-based active material, and by optimally adjusting the number of times of reaching SOC 0% during the cycle, real-time generation of an irreversible phase in the negative electrode is eliminated, and degradation in the surface portion of the negative electrode caused by volume expansion of the silicon-based active material due to charge and discharge is controlled, thereby improving the durability of the negative electrode, and preferably, high capacity and energy density of the silicon-based active material can be exhibited.

[0046] In an exemplary embodiment of the present invention, the step of manufacturing a lithium secondary battery including the negative electrode containing the silicon-based active material may include the step of forming a negative electrode active material layer on one surface or both surfaces of a negative electrode current collector layer.

[0047] The step of forming a negative electrode active material layer on one surface or both surfaces of a negative electrode current collector layer includes the step of coating a negative electrode paste containing a negative electrode active material layer composition on one surface or both surfaces of the negative electrode current collector layer, and the negative electrode active material layer composition may include one or more selected from the group consisting of a negative electrode active material, a negative electrode conductive material, and a negative electrode binder.

[0048] In an exemplary embodiment of the present invention, the negative electrode active material may include a silicon-based active material, and the silicon-based active material may include one or more selected from the group consisting of SiO x (x = 0), SiO x (0 < x < 2), SiC, and Si alloy.

[0049] In an exemplary embodiment of the present invention, the silicon-based active material may include a compound represented by the following Chemical Formula 1.

[0050] [Chemical Formula 1]

[0051] SiO x (0 ≤ x < 2)

[0052] In the above Chemical Formula 1, when x = 2, that is, in the case of SiO2, since SiO2 does not react with lithium ions and thus cannot store lithium, x is preferably within the above range.

[0053] Specifically, the silicon-based active material may include silicon (Si). Generally, the advantage of Si is that its capacity is about 2.5 to 3 times higher than that of silicon oxides (such as SiO x (0 < x < 2)). However, the volume expansion / contraction of Si caused by charge / discharge is much larger than that of silicon oxides, so Si is not easily commercialized. However, in the case of the present invention, by optimally adjusting the number of times of reaching SOC 0% during the secondary battery cycle, the real-time generation of irreversible phases in the negative electrode can be eliminated, thereby effectively solving the problem of deterioration of life characteristics. In addition, more preferably, the advantages of high capacity, high energy density, and high efficiency characteristics of Si can be achieved.

[0054] In an exemplary embodiment of the present invention, the silicon-based active material may include one or more selected from the group consisting of SiO x (x = 0) and SiO x (0 < x < 2). Specifically, based on 100 parts by weight of the silicon-based active material, the silicon-based active material may contain 70 parts by weight or more of SiO x (x = 0). More specifically, based on 100 parts by weight of the silicon-based active material, the silicon-based active material may contain 80 parts by weight or more, 90 parts by weight or more, or 95 parts by weight or more of SiO x (x = 0). In another exemplary embodiment, the silicon-based active material may be composed of SiO x (x = 0).

[0055] In an exemplary embodiment of the present invention, the average particle diameter (D 50) can be 5 μm to 10 μm, specifically 5.5 μm to 8 μm, and more specifically 6 μm to 7 μm. When the average particle size is within the above range, the specific surface area of ​​the particles is within a suitable range, so that the viscosity of the formed negative electrode slurry is within an appropriate range. As a result, the particles constituting the negative electrode slurry are smoothly dispersed. In addition, when the value of the size of the silicon-based active material is equal to or greater than the lower limit of the range, the composite formed by the conductive material and the binder in the negative electrode slurry causes the contact surface between the silicon particles and the conductive material to be extremely good, so that the possibility of maintaining a conductive network is increased, resulting in an increase in the capacity retention rate. Note that when the average particle size satisfies the above range, excessively large silicon particles are excluded, so that the surface of the formed negative electrode is smooth. Therefore, the phenomenon of uneven current density during charge and discharge can be prevented.

[0056] In an exemplary embodiment of the present invention, 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 to 150.0 m 2 / g, more preferably 0.1 m 2 / g to 100.0 m 2 / g, particularly preferably 0.2 m 2 / g to 80.0 m 2 / g, and most preferably 0.2 m 2 / g to 18.0 m 2 The BET specific surface area is measured according to DIN 66131 (using nitrogen).

[0057] In an exemplary embodiment of the present invention, the silicon-based active material may be present, for example, in a crystalline or amorphous form, and is preferably not porous. The silicon particles are preferably spherical or fragmentary particles. Alternatively, but less preferably, the silicon particles may also have a fibrous structure or be present in the form of a silicon-containing film or coating.

[0058] In an exemplary embodiment of the present invention, the thickness of the negative electrode collector layer may generally be 1 μm to 100 μm. There is no particular limitation on such a negative electrode collector layer as long as it has high conductivity and does not cause chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel each surface-treated with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. may be used. In addition, the negative electrode collector layer may have fine concave-convex formed on the surface to enhance the binding force of the negative electrode active material, and may be used in various forms, such as a film, a sheet, a foil, a net, a porous body, a foam, or a non-woven fabric.

[0059] In an exemplary embodiment of the present invention, the thickness of the negative electrode current collector layer may be 1 μm or more and 100 μm or less, and the thickness of the negative electrode active material layer may be 20 μm or more and 500 μm or less. However, the thickness may be variously modified according to the type and purpose of the negative electrode used, and is not limited thereto.

[0060] In an exemplary embodiment of the present invention, the step of forming a negative electrode active material layer on one surface or both surfaces of the negative electrode collector layer may include the step of coating a negative electrode slurry containing a negative electrode active material layer composition on one surface or both surfaces of the negative electrode collector layer, wherein the negative electrode active material layer composition may contain one or more selected from the group consisting of a negative electrode active material, a negative electrode conductive material and a negative electrode binder.

[0061] In an exemplary embodiment of the present invention, the negative electrode slurry may include a negative electrode active material layer composition and a slurry solvent.

[0062] In an exemplary embodiment of the present invention, the solid content of the negative electrode slurry may satisfy a range of 5% or more and 40% or less.

[0063] In another exemplary embodiment, the solid content of the negative electrode slurry may satisfy a range of 5% to 40% both inclusive, preferably 7% to 35% both inclusive, and more preferably 10% to 30% both inclusive.

[0064] 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 layer composition based on 100 parts by weight of the negative electrode slurry.

[0065] When the solid content of the negative electrode slurry satisfies the above range, the viscosity is appropriate during the formation of the negative electrode active material layer, so that particle aggregation of the negative electrode active material layer composition is minimized, thereby effectively forming the negative electrode active material layer.

[0066] In an exemplary embodiment of the present invention, the slurry solvent is not limited thereto as long as it can dissolve the negative electrode active material layer composition, but specifically, distilled water may be used.

[0067] The negative electrode according to an exemplary embodiment of the present invention may be formed by coating the negative electrode slurry on the negative electrode collector layer and drying it.

[0068] Through the drying step, the slurry solvent in the negative electrode slurry may be dried.

[0069] In an exemplary embodiment of the present invention, the negative electrode active material layer composition may include one or more selected from the group consisting of a negative electrode active material, a negative electrode conductive material, and a negative electrode binder.

[0070] In an exemplary embodiment of the present invention, a silicon-based active material may be used as the negative electrode active material, or both a silicon-based active material and a carbon-based active material may be used. In this case, a lithium secondary battery having further improved overall performance such as cycle life characteristics may be manufactured.

[0071] In another exemplary embodiment, based on 100 parts by weight of the silicon-based active material, SiO x The content of (x=0) may be 70 parts by weight or more, preferably 80 parts by weight or more, more preferably 90 parts by weight or more, and 100 parts by weight or less, preferably 99 parts by weight or less, more preferably 95 parts by weight or less.

[0072] In an exemplary embodiment of the present invention, for the silicon-based active material, in particular, pure silicon (Si) may be used as the silicon-based active material. Using pure silicon (Si) as the silicon-based active material may mean that, based on 100 parts by weight of the total silicon-based active material as described above, pure Si particles (SiO 2 ) not combined with other particles or elements are included within the above range. x (x=0)). More specifically, the silicon-based active material may be composed of Si.

[0073] In an exemplary embodiment of the present invention, the negative electrode may include a negative electrode collector layer and a negative electrode active material layer, and the content of the silicon-based active material may be 60 parts by weight or more based on 100 parts by weight of the negative electrode active material layer.

[0074] In an exemplary embodiment of the present invention, the content of the silicon-based active material may be 60 parts by weight or more based on 100 parts by weight of the negative electrode active material layer composition.

[0075] In another exemplary embodiment, based on 100 parts by weight of the negative electrode active material layer composition, the content of the silicon-based active material may be 60 parts by weight or more, preferably 65 parts by weight or more, more preferably 70 parts by weight or more, and 95 parts by weight or less, preferably 90 parts by weight or less, more preferably 80 parts by weight or less.

[0076] The negative electrode active material layer composition according to the present invention includes both a negative electrode conductive material and a negative electrode binder capable of controlling the volume expansion rate during charge and discharge even when a significantly high capacity silicon-based active material is used within the above range. Therefore, even when the silicon-based active material is included within the above range, the negative electrode active material layer composition does not deteriorate the performance of the negative electrode, and has excellent output characteristics during charge and discharge.

[0077] In an exemplary embodiment of the present invention, the silicon-based active material may have a non-spherical shape, and the sphericity (roundness) 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.

[0078] In the present invention, sphericity (roundness) is determined by the following formula A, where A is the area and P is the boundary line.

[0079] [Formula A]

[0080] 4πA / P 2

[0081] In the related art, only graphite compounds are usually used as negative electrode active materials. However, in recent years, due to the increasing demand for high-capacity batteries, attempts to mix and use silicon-based compounds in order to increase capacity are increasing. However, the limitation of the silicon-based compounds is that the volume expands rapidly during charging and discharging, resulting in the destruction of the conductive path formed in the negative electrode active material layer, thereby deteriorating the performance of the battery. Therefore, the type of negative electrode conductive material used with the silicon-based active material is important.

[0082] In an exemplary embodiment of the present invention, the negative electrode conductive material may include one or more selected from the group consisting of a point-shaped conductive material, a planar conductive material, and a line-shaped conductive material.

[0083] In an exemplary embodiment of the present invention, the dot-shaped conductive material refers to a conductive material that can be used to improve the conductivity of the negative electrode, and has conductivity without causing chemical changes. Specifically, the dot-shaped conductive material can be one or more substances 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, preferably including carbon black in terms of high conductivity and excellent dispersibility.

[0084] In an exemplary embodiment of the present invention, the BET specific surface area of ​​the dot-shaped conductive material may be 40 m 2 / g and above 70 m 2 / g or less, preferably 45 m2 / g and above 65 m 2 / g or less, more preferably 50 m 2 / g and above 60 m 2 / g or less.

[0085] In an exemplary embodiment of the present invention, the particle size of the dot-shaped conductive material may be 10 nm to 100 nm, preferably 20 nm to 90 nm, and more preferably 40 nm to 60 nm.

[0086] In an exemplary embodiment of the present invention, the negative electrode conductive material may include a planar conductive material.

[0087] The planar conductive material can be used to improve conductivity by increasing surface contact between silicon particles in the negative electrode while suppressing disconnection of a conductive path due to volume expansion, and is used as a concept including a bulk conductive material or a plate conductive material.

[0088] In an exemplary embodiment of the present invention, the planar conductive material may include at least one selected from the group consisting of plate-like graphite, graphene, graphene oxide, and graphite flakes, and may preferably be plate-like graphite.

[0089] In an exemplary embodiment of the present invention, the average particle size (D 50 ) may be 2 μm to 7 μm, specifically 3 μm to 6 μm, more specifically 4 μm to 5 μm. When the above range is met, sufficient particle size leads to easy dispersion without causing excessive increase in viscosity of the negative electrode slurry. Therefore, when the same equipment and time are used for dispersion, the dispersion effect is excellent.

[0090] In an exemplary embodiment of the present invention, D of the planar conductive material 10 It can be 0.5 μm or more and 1.5 μm or less. 50 It can be 2.5 μm or more and 3.5 μm or less. 90 The thickness may be 7.0 μm or more and 15.0 μm or less.

[0091] In an exemplary embodiment of the present invention, as the planar conductive material, a planar conductive material having a high specific surface area and a high BET specific surface area or a planar conductive material having a low specific surface area may be used.

[0092] In an exemplary embodiment of the present invention, for the planar conductive material, a planar conductive material with a high specific surface area or a planar conductive material with a low specific surface area can be used without limitation. However, in particular, the electrode performance of the planar conductive material according to the present invention may be affected by the dispersion effect to a certain extent, so it is particularly preferred to use a planar conductive material with a low specific surface area that does not cause problems in dispersion.

[0093] In an exemplary embodiment of the present invention, the BET specific surface area of ​​the planar conductive material may be 5 m 2 / g or above.

[0094] In another exemplary embodiment, the BET specific surface area of ​​the planar conductive material may be 5 m 2 / g and above 500 m 2 / g or less, preferably 5 m 2 / g and above 300 m 2 / g or less, more preferably 5 m 2 / g and above 250 m 2 / g or less.

[0095] In another exemplary embodiment, the planar conductive material is a planar conductive material with a high specific surface area, and the BET specific surface area may satisfy 50 m 2 / g and above 500 m 2 / g or less, preferably 80 m 2 / g and above 300 m 2 / g or less, more preferably 100 m 2 / g and above 300 m 2 / g or less.

[0096] In another exemplary embodiment, the planar conductive material is a planar conductive material with a low specific surface area, and the BET specific surface area may satisfy 5 m 2 / g and above 40 m 2 / g or less, preferably 5 m 2 / g or more and 30 m 2 / g or less, more preferably 5 m 2 / g and above 25 m 2 / g or less.

[0097] Other negative electrode conductive materials may include linear conductive materials such as carbon nanotubes. The carbon nanotubes may be bundle-type carbon nanotubes. The bundle-type carbon nanotubes may include a plurality of carbon nanotube units. Specifically, unless otherwise specified, the term "bundle-type" herein refers to a secondary shape in which a plurality of carbon nanotube units are arranged side by side in a certain direction so that the longitudinal axes of the carbon nanotube units are substantially the same or are wound bundle-like or rope-like. The carbon nanotube unit comprises a graphite sheet having a cylindrical shape with a nanometer diameter and having an sp2 bonding structure. In this case, the characteristics of a conductor or a semiconductor may be exhibited according to the rolling angle and structure of the graphite sheet. Compared with wound carbon nanotubes, the bundle-type carbon nanotubes may be more evenly dispersed during the manufacture of the negative electrode, and may more smoothly form a conductive network in the negative electrode, thereby improving the conductivity of the negative electrode.

[0098] In an exemplary embodiment of the present invention, the content of 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.

[0099] In another exemplary embodiment, the content of the negative electrode conductive material may be 10 parts by weight or more and 40 parts by weight or less, preferably 10 parts by weight or more and 30 parts by weight or less, and more preferably 15 parts by weight or more and 25 parts by weight or less, based on 100 parts by weight of the negative electrode active material layer composition.

[0100] The negative electrode conductive material according to the present invention has a completely different constitution from the conductive material applied to the positive electrode. That is, the negative electrode conductive material according to the present invention is used to maintain contact between the silicon-based active material whose electrode volume expands greatly due to charge and discharge, while the positive electrode conductive material is used to provide a certain conductivity while playing a buffering role during rolling, and is completely different from the negative electrode conductive material of the present invention in terms of constitution and function.

[0101] In addition, the negative electrode conductive material according to the present invention is applied to the silicon-based active material, and its composition is completely different from that of the conductive material applied to the graphite-based active material. That is, since the conductive material used for the electrode with the graphite-based active material has only smaller particles than the active material, the conductive material has the characteristics of improving the output characteristics and imparting a certain conductivity, and is completely different from the negative electrode conductive material applied together with the silicon-based active material in the present invention in terms of composition and function.

[0102] In an exemplary embodiment of the present invention, the structure and function of the planar conductive material used as the above-mentioned negative electrode conductive material are different from the structure and function of the carbon-based active material generally used as the 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 processed into a spherical or dot-like shape and used to promote the storage and release of lithium ions.

[0103] On the other hand, the planar conductive material used as the negative electrode conductive material is a material having a plane or plate shape, and can be represented by plate-like graphite. That is, the planar conductive material is a material contained in order to maintain a conductive path in the negative electrode active material layer, and means a material for ensuring a conductive path in a plane shape within the negative electrode active material layer rather than playing a role in storing and releasing lithium.

[0104] That is, in the present invention, using plate-like graphite as a conductive material means that graphite is processed into a plane or plate-like shape and used as a material that ensures a conductive path rather than a material that plays a role in storing or releasing lithium. In this case, the negative electrode active material contained together has high capacity characteristics in terms of storing and releasing lithium, and is used to store and release all lithium ions transferred from the positive electrode.

[0105] On the other hand, in the present invention, using a carbon-based active material as an active material means that the carbon-based active material is processed into a dot-like or spherical shape and used as a material for storing or releasing lithium.

[0106] That is, in one exemplary embodiment of the present invention, the BET specific surface area of ​​artificial graphite or natural graphite as a carbon-based active material may satisfy 0.1 m 2 / g and above 4.5 m 2 In addition, plate-like graphite as a planar conductive material has a planar shape, and its BET specific surface area can be 5 m 2 / g or above.

[0107] In an exemplary embodiment of the present invention, 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, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene propylene diene monomer rubber (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, polyacrylic acid, and the above materials in which hydrogen is replaced by Li, Na, Ca, etc., and may also include various copolymers thereof.

[0108] The binder according to an exemplary embodiment of the present invention is used to hold the negative electrode active material and the negative electrode conductive material to prevent distortion and structural deformation of the negative electrode structure when the volume of the silicon-based active material expands and relaxes. When such a role is met, all general negative electrode binders can be applied. Specifically, an aqueous binder can be used, and more specifically, a PAM-based binder can be used.

[0109] In an exemplary embodiment of the present invention, the content of the negative electrode binder may be 30 parts by weight or less, preferably 25 parts by weight or less, more preferably 20 parts by weight or less and 1 part by weight or more or 3 parts by weight or more, based on 100 parts by weight of the negative electrode active material layer composition.

[0110] In an exemplary embodiment of the present invention, the negative electrode may be pre-lithiated.

[0111] In an exemplary embodiment of the present invention, the step of manufacturing a lithium secondary battery including a negative electrode including the silicon-based active material may further include a step of pre-lithiating the negative electrode including the silicon-based active material.

[0112] Specifically, the step of pre-lithiation of the negative electrode comprising the silicon-based active material may include:

[0113] forming a negative electrode active material layer on one surface or both surfaces of the negative electrode current collector layer;

[0114] preparing a transfer stack in which a substrate film and a lithium metal layer are sequentially stacked;

[0115] transferring the lithium metal layer onto the negative electrode active material layer; and

[0116] The substrate film is removed.

[0117] This relates to a method for pre-lithiation of an electrode for a lithium secondary battery using a lithium metal transfer process.

[0118] In the charge and discharge reaction of the lithium secondary battery, the lithium released from the positive electrode is embedded in the negative electrode during charging, and is deintercalated from the negative electrode during discharge and returns to the positive electrode again. In the case of silicon-based negative electrode active materials, the volume change and surface side reactions are serious, so that a large amount of lithium embedded in the negative electrode during the initial charge will not return to the positive electrode again, thereby increasing the initial irreversible capacity. When the initial irreversible capacity increases, the problem of rapid reduction in battery capacity and cycle occurs.

[0119] When the method for charging and discharging a secondary battery according to the present invention is used, the irreversible phase of the lithium secondary battery is effectively removed, so the degradation of the negative electrode can be controlled. Therefore, when the method is applied to a secondary battery in which the negative electrode is pre-lithiated as described above, the durability of the battery can be more effectively improved.

[0120] Typically, the pre-lithiation process chemically or physically pre-lithiates the lithium metal layer into the electrode, specifically, it can be performed by a lithium metal transfer process, lithium metal powder deposition, an electrochemical process or a lithium metal deposition process, and the pre-lithiation process according to the present invention may include a lithium metal transfer process.

[0121] The lithium metal layer transfer process has the characteristic of being able to more stably transfer highly reactive lithium metal to the upper portion of the electrode active material layer. In this case, a process is required that can easily transfer the lithium metal layer from the transfer stack to the upper portion of the electrode active material layer.

[0122] In an exemplary embodiment of the present invention, the step of pre-lithiating the negative electrode includes: preparing a transfer stack in which a substrate film and a lithium metal layer are sequentially stacked; transferring the lithium metal layer to an upper portion of the electrode active material layer; and removing the substrate film.

[0123] In an exemplary embodiment of the present invention, a deposition method of depositing the lithium metal layer onto the substrate film may be selected from evaporation deposition, chemical vapor deposition (CVD), and physical vapor deposition, but is not limited thereto, and various deposition methods used in the art may be used.

[0124] In this case, the lamination process can be performed in the following manner: 2 Up to 500kgf / cm 2 The electrode for lithium secondary battery is rolled with a load of 10000. Thereafter, the process of removing the substrate film is included.

[0125] In an exemplary embodiment of the present invention, any substrate film may be used without restriction as long as it has the following characteristics: it can withstand process conditions such as high temperature in the step of depositing lithium metal and can prevent the reverse peeling problem of lithium metal transferred to the substrate film during the winding process for transferring the deposited lithium metal.

[0126] Specifically, in an exemplary embodiment of the present invention, the substrate film may be one or more selected from the group consisting of polyethylene terephthalate (PET), polyimide (PI), poly(methyl methacrylate) (PMMA), polypropylene, polyethylene, and polycarbonate.

[0127] In an exemplary embodiment of the present invention, the thickness of the base film may be 1 μm or more and 300 μm or less, and may satisfy the ranges of 5 μm or more and 200 μm or less, and 10 μm or more and 100 μm or less.

[0128] In an exemplary embodiment of the present invention, the thickness of the lithium metal may be 1 μm or more and 10 μm or less, and preferably may satisfy the range of 3 μm or more and 10 μm or less.

[0129] When the thicknesses of the base film and the lithium metal satisfy the above range, the lithium metal may be effectively transferred to the negative electrode active material layer, and reverse transfer may be prevented.

[0130] In an exemplary embodiment of the present invention, a transfer force enhancing layer may be included on the surface of the transfer stack in contact with the substrate film and the lithium metal layer to improve the strippability of the lithium metal layer, ensure the transferability to the electrode active material layer, and serve as a protective layer after the lithium metal layer is transferred.

[0131] That is, the substrate film may have a transfer force enhancing layer formed on at least one surface thereof. Specifically, the transfer force enhancing layer may be formed on one surface or both surfaces. The transfer force enhancing layer makes it possible to prevent the reverse peeling problem of the lithium metal layer being transferred to the substrate film during the winding process of transferring the deposited lithium metal layer to the electrode, and also makes it possible to easily separate the substrate film after the lithium metal layer is transferred to the electrode active material layer.

[0132] The transfer force enhancing layer may include one or more substances selected from the group consisting of silicon-modified polyester in which a silicon chain is grafted to a polyester main chain, acrylic resin, silicon, melamine, and fluorine.

[0133] In an exemplary embodiment of the present invention, the transfer force enhancing layer may include polymethyl methacrylate (PMMA).

[0134] In an exemplary embodiment of the present invention, the transfer force enhancing 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 roller coating, but is not limited thereto. That is, various coating methods that can be used to form a coating layer in the art may be used.

[0135] In an exemplary embodiment of the present invention, the step of transferring the lithium metal layer to the upper portion of the negative electrode active material layer may include laminating the transfer laminate on the negative electrode active material layer.

[0136] Specifically, the step of transferring the lithium metal layer to the upper part of the negative electrode active material layer may include stacking the transfer stack onto the electrode active material layer so that the surface of the lithium metal layer opposite to the surface in contact with the transfer force enhancing layer contacts the surface of the electrode active material layer opposite to the surface in contact with the electrode collector layer.

[0137] In this case, the lamination can be carried out under the temperature conditions of 20°C to 90°C and 5 kgf / cm 2 Up to 500 kgf / cm 2 under pressurized conditions.

[0138] In an exemplary embodiment of the present invention, the stacking may satisfy 5 kgf / cm 2 Up to 500 kgf / cm 2 , preferably 10 kgf / cm 2 Up to 150 kgf / cm 2 pressurized conditions.

[0139] However, the temperature condition in the stacking can be omitted. In particular, when the stacking satisfies the above-mentioned pressurization condition, the pre-lithiation rate of the lithium metal can be smoothly controlled, so that the generation of a large amount of oxides and nitrides during the transfer process can be suppressed. In addition, since the pressurization range is met, the lithium metal is smoothly transferred to the upper part of the electrode active material layer, so that the problem of reverse transfer does not occur.

[0140] After the lamination, pre-lithiation of the highly reactive lithium metal and the electrode active material layer may be performed, or may not be reacted, and pre-lithiation may be performed during battery assembly.

[0141] In an exemplary embodiment of the present invention, the step of pre-lithiation of the negative electrode may also include the step of pre-lithiation of the negative electrode active material layer after removing the substrate film, and the step of pre-lithiation of the negative electrode active material layer may be pre-lithiation of the negative electrode within 30 minutes to 7 days after transferring the lithium metal.

[0142] In an exemplary embodiment of the present invention, the transfer force enhancing layer and the lithium metal are transferred to the upper portion of the electrode active material layer together, rather than transferring only the lithium metal layer. In this case, the transfer force enhancing layer can also serve as a protective layer capable of preventing highly reactive lithium metal from reacting in the air.

[0143] In an exemplary embodiment of the present invention, the step of pre-lithiating the negative electrode may include pre-lithiating the negative electrode active material layer.

[0144] The step of pre-lithiating the negative electrode active material layer may be pre-lithiating the negative electrode active material layer under conditions of 25° C. and 1 atm within 30 minutes to 24 hours.

[0145] The pre-lithiation step is a step of setting conditions for lithium metal to diffuse into the negative electrode active material layer, wherein whether the pre-lithiation is completed can be determined based on whether the lithium on the upper portion of the negative electrode active material layer has completely disappeared.

[0146] In an exemplary embodiment of the present invention, the lithium secondary battery manufactured by the above method may be a lithium secondary battery including a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte.

[0147] In this case, when the negative electrode is a pre-lithiated negative electrode, the transfer force enhancement layer used during pre-lithiation can be removed so that it does not remain on the upper portion of the electrode, thereby preventing unnecessary resistance increase. That is, the transfer force enhancement layer improves the transfer force, can be used to protect the lithium metal layer before pre-lithiation, and can be removed before injecting the electrolyte.

[0148] In an exemplary embodiment of the present invention, the step of manufacturing a lithium secondary battery including the negative electrode including the silicon-based active material may further include forming a positive electrode active material layer on one surface or both surfaces of a positive electrode collector layer.

[0149] In an exemplary embodiment of the present invention, there is no particular limitation on the positive electrode collector layer, as long as it has conductivity and does not cause chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, aluminum or stainless steel each surface-treated with carbon, nickel, titanium, silver, etc., etc. can be used. In addition, the thickness of the positive electrode collector layer can generally be 3 μm to 500 μm, and the surface of the positive electrode collector layer can be formed with fine concave-convex to enhance the adhesion of the positive electrode active material. For example, the positive electrode collector can be used in different forms, such as films, sheets, foils, nets, porous bodies, foams, and non-woven fabrics.

[0150] The thickness of the positive electrode current collector layer may be variously modified according to the type and purpose of the negative electrode used, and is not limited thereto.

[0151] In an exemplary embodiment of the present invention, the step of forming a positive electrode active material layer on one surface or both surfaces of the positive electrode collector layer may include the step of coating a positive electrode slurry containing a positive electrode active material layer composition on one surface or both surfaces of the positive electrode collector layer, wherein the positive electrode active material layer composition may contain one or more selected from the group consisting of a positive electrode active material, a positive electrode conductive material and a positive electrode binder.

[0152] In an exemplary embodiment of the present invention, the above description of the negative electrode slurry may also be applied to the positive electrode slurry in addition to the positive electrode.

[0153] In an exemplary embodiment of the present invention, the positive electrode active material layer composition may include one or more selected from the group consisting of a positive electrode active material, a positive electrode conductive material, and a positive electrode binder.

[0154] 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) and lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; lithium iron oxide, such as LiFe3O4; lithium manganese oxide, such as Li 1+c1 Mn 2-c1 O4 (0≤c1≤0.33), LiMnO3, LiMn2O3 and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides, such as LiV3O8, V2O5 and Cu2V2O7; 1-c2 M c2 Ni-site lithium nickel oxide represented by the chemical formula LiMn 2-c3 M c3 Lithium manganese composite oxide represented by Li2Mn3MO8 (wherein M is at least one selected from the group consisting of Co, Ni, Fe, Cr, Zn and Ta, and satisfies 0.01≤c3≤0.1) or Li2Mn3MO8 (wherein M is at least one selected from the group consisting of Fe, Co, Ni, Cu and Zn); LiMn2O4 in which part of Li in the chemical formula is replaced by alkaline earth metal ions, etc., but is not limited thereto. The positive electrode may be Li metal.

[0155] Specifically, the positive electrode active material may include one or more selected from the group consisting of Ni, Co, Mn, LTO, LFP, RuO2, Nb2O5, Mn3O4, Fe2O3, and Co3O4.

[0156] In addition, the positive electrode may include a sacrificial positive electrode material, and regarding the type thereof, a sacrificial positive electrode material used in the art may be used without limitation.

[0157] In this case, the positive electrode conductive material is used to impart conductivity to the electrode, and can be used without particular limitation, as long as the positive electrode conductive material has electronic conductivity without causing chemical changes in the battery to be constituted. Specific examples may include graphite, such as natural graphite and artificial graphite; carbon-based materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powder or metal fiber, such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; or conductive polymers, such as polyphenylene derivatives, and the like, and any one of them or a mixture of two or more thereof may be used.

[0158] In addition, the positive electrode binder is used to improve the bonding between the positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode collector. Specific examples may include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene rubber (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, and any one or a mixture of two or more thereof may be used.

[0159] In addition, the above description of the negative electrode active material layer composition can also be applied to the contents of the positive electrode active material, the positive electrode conductive material and the positive electrode binder contained in the positive electrode active material layer composition.

[0160] In an exemplary embodiment of the present invention, the electrolyte may be, but is not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, etc., which can be used to manufacture lithium secondary batteries.

[0161] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0162] As the nonaqueous organic solvent, for example, an aprotic organic solvent such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate or ethyl propionate can be used.

[0163] Especially, in carbonate-based organic solvents, ethylene carbonate and propylene carbonate as cyclic carbonate are high viscosity organic solvents, and because their dielectric constant is high, lithium salts are well dissociated, so they can be preferably used. When the cyclic carbonate is mixed with low viscosity and low dielectric constant linear carbonate such as dimethyl carbonate and diethyl carbonate in a suitable ratio and used, the electrolyte with high conductivity can be manufactured, so such combination can be more preferred.

[0164] As the metal salt, a lithium salt can be used. The lithium salt is a material that is easily soluble in a non-aqueous electrolyte. For example, a lithium salt selected from the group consisting of 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 species of the group consisting of are used as anions of the lithium salt.

[0165] In order to improve the life characteristics of the battery, inhibit the decline in battery capacity, improve the discharge capacity of the battery, etc., in addition to containing the above-mentioned electrolyte components, the electrolyte may also contain one or more additives, such as halogenated alkyl carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, (condensed) glycol dimethyl ethers, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol or aluminum chloride.

[0166] <Steps for Charging and Discharging a Lithium Secondary Battery>

[0167] A method for charging and discharging a lithium secondary battery according to an exemplary embodiment of the present invention may include:

[0168] First cycle step: repeating the first cycle N1 times on the lithium secondary battery, wherein the first cycle is discharging at 3.0 V to 3.5 V after charging; and

[0169] The second cycle step: performing N2 second cycles on the lithium secondary battery, wherein the second cycle is a complete discharge after charging, and

[0170] N1 / N2 may be greater than 25 and less than 100.

[0171] Specifically, the secondary battery can be driven by electrochemical charging and electrochemical discharging. According to the method for charging and discharging a secondary battery of the present invention, the initial efficiency and life performance of the secondary battery can be improved by using a negative electrode containing the silicon-based active material, and at the same time, by adjusting the number of times the SOC 0% (full discharge) is reached during the cycle of the secondary battery in an optimal manner, the generation of irreversible phases in the negative electrode can be controlled, and the degradation in the surface portion of the negative electrode can be controlled, while preventing performance degradation and electrode structure collapse caused by volume expansion of the silicon-based active material, thereby significantly improving the life performance of the secondary battery.

[0172] Therefore, a lithium secondary battery to which the method of charging and discharging a lithium secondary battery according to an exemplary embodiment of the present invention is applied is improved in discharge capacity, initial efficiency, resistance properties, and / or lifespan characteristics.

[0173] In this specification, "SOC" may refer to the remaining capacity (state of charge) of an electrode (positive or negative electrode) or a secondary battery. The SOC after charging may be the SOC when electrochemical charging is completed when the secondary battery is driven, and the SOC after discharging may be the SOC when electrochemical discharging is completed when the secondary battery is driven. That is, SOC 100% after charging means that the remaining capacity of the secondary battery is 100% when charging is completed, in this case, it means a fully charged state. SOC 65% after discharging may mean that the remaining capacity of the secondary battery is 65% when discharging is completed. SOC 0% after discharging means that the remaining capacity of the secondary battery is 0% when charging is completed, in this case, it means a fully discharged state.

[0174] In an exemplary embodiment of the present invention, a first cycle of discharging the lithium secondary battery at 3.0 V to 3.5 V after charging may be repeated N1 times. Subsequently, a second cycle of fully discharging after charging may be repeated N2 times. In this case, N1 and N2 may each be an integer greater than 1.

[0175] In one example, when N1 is 25 and N2 is 1, the first cycle (limited cycle) may be repeated 25 times, and then the second cycle (full discharge cycle) may be performed once.

[0176] In an exemplary embodiment of the present invention, the first cycle step and the second cycle step can be repeated. In one example, when N1 is 25 and N2 is 1, the following steps can be repeated: repeating the first cycle (limited cycle) 25 times, performing the second cycle (full discharge cycle) once, repeating the first cycle 25 times again, and then performing the second cycle (full discharge cycle) once.

[0177] In one example, the charging may be performed under 1 C and constant current / constant voltage (CC / CV) conditions.

[0178] In one example, the discharge may be performed under a constant current condition of 0.5 C to a specific SOC, and may be performed in a manner of being cut off at a discharge voltage according to the specific SOC.

[0179] In this case, the charge and discharge conditions and the C rate are not limited to those described above, and methods used in the art may be appropriately applied.

[0180] In an exemplary embodiment of the present invention, the charging may be performed at SOC 95% to SOC 100%. For example, the charging may be in a fully charged condition, and the voltage may be 4.2 V when the charging is finished.

[0181] In an exemplary embodiment of the present invention, in the first cycle, the discharge may be performed to SOC 50% to SOC 90%. Specifically, the voltage at the end of the discharge may be 3.0 V to 3.5 V, more specifically 3.25 V (SOC 65%).

[0182] In one example, in the second cycle, full discharge is performed to SOC 0%, specifically, the voltage may be 2.5 V at the end of discharge.

[0183] The first cycle and the second cycle may be performed under the conditions of 25° C. and a rest time of 20 minutes.

[0184] When the lithium secondary battery is subjected to N2 full charge and discharge cycles after N1 limited cycles, the formation of the irreversible phase can be effectively controlled. In this case, the irreversible phase in the negative electrode is reduced only when the voltage (e.g., 2.5 V) of the full discharge (SOC 0%) condition is reached, and the degradation in the surface portion can be easily controlled. If the full discharge condition is not reached, the irreversible phase cannot be easily controlled.

[0185] Furthermore, if full charge and discharge cycles are excessively performed, the number of full charge and discharge cycles in the entire cycle increases, and a large amount of Si at the bottom of the negative electrode active material layer is used, which aggravates the volume expansion of the negative electrode and deteriorates the durability of the negative electrode.

[0186] In an exemplary embodiment of the present invention, N1 / N2 may be greater than 25 and less than 100. Specifically, N1 / N2 may be greater than 30 and less than 80, or greater than 40 and less than 60. More specifically, N1 / N2 may be 50.

[0187] When N1 / N2 satisfies the above range, generation of an irreversible phase in the negative electrode can be controlled in real time, and degradation in a surface portion of the negative electrode can be controlled, thereby significantly improving life performance of the secondary battery.

[0188] On the other hand, if N1 / N2 is below 25, the number of complete charge and discharge cycles in the entire cycle increases, which aggravates the volume expansion of the negative electrode and accelerates the degradation of the negative electrode. If N1 / N2 is above 100, the irreversible phase is not eliminated and the negative electrode is used unevenly.

[0189] In an exemplary embodiment of the present invention, N1 may be greater than 25 and less than 100. Specifically, N1 may be greater than 30 and less than 80, or greater than 40 and less than 60. More specifically, N1 may be 50. If N1 is lower than the range, the full charge and discharge cycle is excessively performed, and a large amount of Si at the bottom of the negative electrode active material layer is used, which aggravates the volume expansion of the negative electrode and deteriorates the durability of the negative electrode. If N1 is higher than the range, the irreversible phase is not eliminated in real time, and the negative electrode is unevenly used.

[0190] In an exemplary embodiment of the present invention, N2 may be greater than or equal to 1 and less than or equal to 3. Specifically, N2 may be 1. If N2 exceeds the range, the full charge and discharge cycle is excessively performed, and a large amount of Si at the bottom of the negative electrode active material layer is used, which aggravates the volume expansion of the negative electrode and deteriorates the durability of the negative electrode.

[0191] The electrochemical charging of the secondary battery can be performed using an electrochemical charging and discharging device. Specifically, WOCS3000s (manufactured by Yuantai Corporation) can be used as the electrochemical charging and discharging device.

[0192] In addition, an exemplary embodiment of the present invention provides a battery system, which includes the secondary battery. Specifically, the above-mentioned method for charging and discharging the secondary battery can be implemented by using the battery system.

[0193] For example, the secondary battery may be included in a battery system in the form of a secondary battery unit composed of a single secondary battery or a secondary battery module which is an assembly of a plurality of secondary batteries.

[0194] The battery system may include a control unit and the secondary battery.

[0195] The control unit may set the SOC of the secondary battery during electrochemical charging and electrochemical discharging of the secondary battery, a driving voltage range, etc. Therefore, the electrochemical charging and electrochemical discharging of the secondary battery may be performed with the electrochemical charging and discharging range of the secondary battery and the SOC of the secondary battery set by the control unit, and the SOC and the number of charge and discharge times may be adjusted.

[0196] The control unit is not particularly limited as long as it can control the driving voltage range during electrochemical charging and electrochemical discharging of the secondary battery, for example, it can be an electrochemical charging and discharging device. Specifically, the control unit can be built into a battery management system (BMS) included in the battery pack.

[0197] In addition, the present invention provides a battery pack comprising the above battery system.

[0198] In addition to the above-mentioned secondary battery and control unit, the battery pack may further include components known in the art, such as a battery management system (BMS) and a cooling system.

[0199] The battery system or battery pack can be used in the field of portable devices such as mobile phones, laptop computers and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs). The battery system or battery pack can be advantageously applied to power sources requiring high output and large capacity, such as electric vehicles, hybrid electric vehicles and power storage devices.

[0200] Invention Mode

[0201] Embodiments will be described in detail below so as to specifically describe this specification. However, embodiments according to this specification may be modified in other forms, and the scope of the present application is not to be construed as being limited to the following embodiments. The embodiments of the present application are provided to fully explain this specification to those skilled in the art.

[0202] <Examples and Comparative Examples>

[0203] Production Example 1

[0204] 1) Manufacturing of negative electrode

[0205] A silicon-based active material Si (average particle size (D50): 5 μm) as a negative electrode active material, carbon black (product name: Super C65, manufacturer: Temeco Corporation) as a conductive material, and polyacrylic acid as a binder were added to distilled water as a negative electrode slurry forming solvent at a weight ratio of 70:20:10 to prepare a negative electrode slurry (solid concentration: 20 wt %).

[0206] The negative electrode slurry was prepared at 120 mg / 25 cm 2 A loading amount of was coated on one surface of a copper current collector (thickness: 15 μm) serving as a negative electrode current collector, which was then rolled and dried in a vacuum oven at 130° C. for 10 hours to form a negative electrode active material layer, thereby preparing a negative electrode (negative electrode before pre-lithiation).

[0207] 2) Pre-lithiation of negative electrode

[0208] Lithium metal with a thickness of 6.2 μm was transferred to the negative electrode using a calendaring device.

[0209] Specifically, at 1.1 mA / cm 2 The negative electrode is electrochemically charged at a current density of 1000 Å to 10% of the charge capacity of the negative electrode before pre-lithiation.

[0210] 3) Manufacturing of positive electrode

[0211] As the positive electrode active material, Li[Ni 0.8 Co 0.1 Mn 0.1 ]O2, carbon black (product name: Super C65, manufacturer: Temeco) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were added to N-methyl-2-pyrrolidone (NMP) as a solvent for forming a positive electrode slurry in a weight ratio of 97:1.5:1.5 to produce a positive electrode slurry (solid concentration: 78 wt %).

[0212] The positive electrode slurry was prepared at 620 mg / 25 cm 2 A loading amount of 100 wt % was coated on both surfaces of an aluminum current collector (thickness: 12 μm) serving as a positive electrode current collector, which was then rolled and dried in a vacuum oven at 130° C. for 10 hours to form a positive electrode active material layer, thereby manufacturing a positive electrode.

[0213] 4) Manufacturing of secondary batteries

[0214] A bi-cell type secondary battery is manufactured by placing two pre-lithiated negative electrodes on both surfaces of the positive electrode prepared above, inserting a polypropylene separator between the positive electrode and the negative electrode, and injecting an electrolyte. The electrolyte is obtained by adding vinylene carbonate in an amount of 3% by weight based on the total weight of the electrolyte to an organic solvent in which fluoroethylene carbonate (FEC) and diethyl carbonate (DEC) are mixed in a volume ratio of 30:70, and adding LiPF6 as a lithium salt to a concentration of 1 M.

[0215] Production Example 2

[0216] A lithium secondary battery was manufactured in the same manner as in Manufacturing Example 1, except that the negative electrode was not pre-lithiated.

[0217] <Experimental Example 1: Evaluation of Cycle Characteristics>

[0218] The lithium secondary batteries manufactured in Manufacturing Examples 1 and 2 were driven and subjected to N1 charge and discharge cycles to the SOC set in Table 1 below under a driving condition of 4.2 V-2.5 V and a cycle condition of 4.2 V-3.2 V, and then subjected to N2 full discharge cycles.

[0219] The capacity retention rate according to the cycles was evaluated using the following formula. Figure 1 Graphs showing the cycle characteristics of Example 1-1 using Production Example 1 (pre-lithiated negative electrode) and Comparative Examples 1-1 and 1-2 are shown.

[0220] In addition, the capacity retention rate at the 600th cycle and the sudden decrease time point are shown in Table 1.

[0221] - Capacity retention rate (%) = {(discharge capacity at the 600th cycle) / (discharge capacity at the first cycle)}×100

[0222] (Full charge-full discharge capacity ratio before and after 600 cycles (4.2 V-2.5 V))

[0223] - Sudden drop time point = the number of cycles when the real-time capacity retention rate reaches 50%

[0224] <Electrochemical charging and electrochemical discharging conditions>

[0225] - Charging: Charge at 1 C in CC / CV mode to the charge SOC of the secondary battery as described in Table 1 below (cut off at 0.05 C)

[0226] - Discharge: Discharged at 0.5 C in CC mode to the discharge SOC of the secondary battery described in Table 1 below (cut off at the discharge voltage according to each SOC range)

[0227]

[0228] When a method for charging and discharging a lithium secondary battery such as the present invention is applied, the irreversible phase generated in real time is eliminated by adjusting the number of times that SOC 0% (full discharge) is reached during the cycle in an optimal manner, thereby controlling the degradation in the surface portion of the negative electrode, thereby improving the durability of the negative electrode.

[0229] According to Table 1 above, in the case of Examples 1-1 and 2-1 in which the charge-discharge method of repeating the first cycle 50 times and then performing the full discharge cycle once (N1 / N2 satisfies 50) is applied to a lithium secondary battery containing a negative electrode containing a silicon-based active material, it can be confirmed that the capacity retention rate is excellent and the number of cycles until a sudden drop occurs is large. On the other hand, in the case of Comparative Examples 1-1, 1-2, 2-1 and 2-2 in which the first cycle is repeated 25 times and then the full discharge cycle is performed once (N1 / N2 is 25) or the first cycle is repeated 100 times and then the full discharge cycle is performed once (N1 / N2 is 100), it can be confirmed that the number of cycles until a sudden drop occurs is small, and the degradation of the lithium secondary battery is accelerated compared to Examples 1-1 and 2-1.

[0230] Therefore, the lithium secondary battery to which the method of charging and discharging the lithium secondary battery according to an exemplary embodiment of the present invention is applied is significantly improved in durability of the negative electrode.

[0231] <Experimental Example 2: Evaluation of Capacity and Discharge Resistance Characteristics>

[0232] <Example 1-2>

[0233] The lithium secondary battery manufactured in Manufacturing Example 1 was subjected to the same charge and discharge method as in Example 1-1 in Table 1 above, but the charge and discharge method was applied under a cycle condition of 4.2 V to 3.3 V.

[0234] <Comparative Examples 1-3>

[0235] In addition, the lithium secondary battery manufactured in Manufacturing Example 1 was subjected to the same charge and discharge method as in Example 1-1 in Table 1 above, but without performing a full discharge cycle under the cycle condition of 4.2 V-3.3 V.

[0236] In the lithium secondary batteries of Examples 1-2 and Comparative Examples 1-3, the graph showing the capacity characteristics according to the progress of the cycle is shown in Figure 2 In the graph showing the discharge resistance (Rdis) value versus time, a graph is shown in Figure 3 middle.

[0237] Figure 2 The capacity measured under the conditions of 4.2 V-2.5 V and 0.33 C / 0.33 C every 50 cycles under the conditions of electrochemical charge and electrochemical discharge is shown. Accordingly, in Example 1-2 in which charge and discharge are performed while the number of full discharge cycles is appropriately adjusted, it can be confirmed that as the number of cycles increases, the irreversible phase is eliminated and the phenomenon of temporary capacity degradation / recovery is alleviated compared to Comparative Example 1-3.

[0238] according to Figure 3 In the case of Example 1-2, it can be confirmed that, compared with Comparative Example 1-3, by performing the complete discharge cycle, delithiation / lithiation is induced within the use range of silicon at the end of discharge, thereby reducing the discharge resistance value of the negative electrode. Figure 3 In the discharge resistance evaluation of , the same battery was evaluated twice.

Claims

1. A method for charging and discharging a lithium secondary battery, the method comprising: a step of manufacturing a lithium secondary battery having a negative electrode including a silicon-based active material; First cycle step: repeating the first cycle N1 times on the lithium secondary battery, wherein the first cycle is discharging at 3.0 V to 3.5 V after charging; as well as The second cycle step: performing N2 second cycles on the lithium secondary battery, wherein the second cycle is a complete discharge after charging, Where N1 / N2 is greater than 25 and less than 100.

2. The method according to claim 1, wherein N2 is 1 or more and 3 or less.

3. The method according to claim 1, wherein N1 is greater than 25 and less than 100, and Where N2 is 1. The method according to claim 1 , wherein the first circulation step and the second circulation step are repeated.

5. The method according to claim 1, wherein the silicon-based active material comprises one or more selected from the group consisting of SiO x (x = 0), SiO x (0 < x < 2), SiC, and Si alloy.

6. The method according to claim 5, wherein based on 100 parts by weight of the silicon-based active material, SiO x The content of (x=0) is 70 parts by weight or more.

7. The method according to claim 1, wherein the negative electrode comprises a negative electrode current collector layer and a negative electrode active material layer, and The content of the silicon-based active material is 60 parts by weight or more based on 100 parts by weight of the negative electrode active material layer. The method of claim 1 , wherein the negative electrode is pre-lithiated.

9. The method according to claim 1, wherein the step of manufacturing a lithium secondary battery having a negative electrode containing a silicon-based active material further comprises: The negative electrode including the silicon-based active material is pre-lithiated.

10. The method according to claim 9, wherein pre-lithiation of the negative electrode comprising the silicon-based active material comprises: forming a negative electrode active material layer on one surface or both surfaces of the negative electrode current collector layer; preparing a transfer stack in which a substrate film and a lithium metal layer are sequentially stacked; transferring the lithium metal layer to an upper portion of the negative electrode active material layer; as well as The substrate film is removed. 11 . The method according to claim 10 , wherein the substrate film comprises a transfer force enhancing layer formed on at least one surface thereof.

12. The method according to claim 10, comprising: The transfer laminate is laminated on the negative electrode active material layer.

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