Charging and discharging methods for lithium secondary batteries

Abstract

The present invention relates to a method for charging and discharging a lithium secondary battery.

Description

Technical Field

[0001] The present invention relates to a method for charging and discharging a lithium secondary battery. This application claims the benefit of the filing dates of Korean Patent Application No. 10-2022-0175978, filed with the Korean Intellectual Property Office on December 15, 2022, and Korean Patent Application No. 10-2023-0180458, filed with the Korean Intellectual Property Office on December 13, 2023, and all of the content disclosed in the documents of the Korean patent applications is incorporated herein.

Background Art

[0002] Recently, with the rapid spread of electronic devices using batteries, such as mobile phones, notebook computers, and electric vehicles, the demand for secondary batteries that are small, lightweight, and relatively high-capacity has been rapidly increasing. In particular, lithium secondary batteries are lightweight and have a high energy density, and have been in the spotlight as a driving power source for portable devices. Therefore, research and development efforts for improving the performance of lithium secondary batteries have been actively carried out.

[0003] Generally, a lithium secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, an electrolyte, an organic solvent, and the like. Also, an active material layer containing a positive electrode active material and a negative electrode active material can be formed on a current collector for the positive electrode and the negative electrode. Generally, lithium-containing metal oxides such as LiCoO2 and LiMn2O4 are used as the positive electrode active material for the positive electrode, and carbon-based active materials and silicon-based active materials that do not contain lithium are used as the negative electrode active material for the negative electrode.

[0004] In the case of a silicon-based active material among negative electrode active materials, it is noted for having a higher capacity and excellent high-speed charging characteristics compared to carbon-based active materials. However, silicon-based active materials have a large degree of volume expansion / contraction due to charge and discharge, resulting in a problem of deterioration of life characteristics, and have a large irreversible capacity, so there is a disadvantage of low initial efficiency.

[0005] Furthermore, in the case of secondary batteries containing silicon-based active materials, surface cracking and volume changes are significant. In particular, when a method of transferring lithium to the negative electrode is adopted as one of the pre-lithification methods, the deterioration of the negative electrode surface becomes more severe. This poses a problem when evaluating the lifespan, as it accelerates surface deterioration and induces a sudden drop. In addition, when secondary batteries containing silicon-based active materials are charged and discharged at limited cycles, the use of the negative electrode is uneven, accelerating deterioration.

[0006] Therefore, there is a need to develop secondary batteries that can achieve high capacity and energy density of silicon-based active materials while also improving their lifespan characteristics. [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] The present invention relates to a charging and discharging method for lithium secondary batteries, and more particularly to a charging and discharging method for lithium secondary batteries containing silicon-based active materials, which can improve durability by adjusting the cycle drive method during charging and discharging in order to prevent the degradation of the lithium secondary battery. [Means for solving the problem]

[0008] One embodiment of the present invention provides a method for charging and discharging a lithium secondary battery, comprising the steps of: manufacturing a lithium secondary battery having a negative electrode containing a silicon-based active material; a first cycle step of repeating a first cycle of discharging the lithium secondary battery under conditions of 3.0V to 3.5V after charging N1 times; and a second cycle step of performing a second cycle of complete discharging the lithium secondary battery N2 times after charging, wherein N1 / N2 is greater than 25 and less than 100. [Effects of the Invention]

[0009] A charging and discharging method for a lithium secondary battery according to one embodiment of the present invention can control the degeneration of the negative electrode surface and improve the durability of the negative electrode by optimally adjusting the number of times the SOC reaches 0% (complete discharge) during the cycle, thereby eliminating the real-time generation of an irreversible phase. Therefore, a lithium secondary battery to which the charging and discharging method for lithium secondary batteries according to one embodiment of the present invention is applied exhibits the effect of improved discharge capacity, initial efficiency, resistance performance, and / or life characteristics. [Brief explanation of the drawing]

[0010] [Figure 1] This graph shows the cycle characteristics of secondary batteries in Example 1-1, Comparative Examples 1-1 and 1-2, to which the lithium secondary battery charging and discharging method according to the present invention is applied. [Figure 2] This graph shows the capacity characteristics of secondary batteries in Examples 1-2 and Comparative Examples 1-3, to which the lithium secondary battery charging and discharging method according to the present invention is applied. [Figure 3] This graph shows the discharge resistance (Rdis) values ​​over time for Examples 1-2 and Comparative Examples 1-3, which apply the charging and discharging method for lithium secondary batteries according to the present invention. [Modes for carrying out the invention]

[0011] The following provides further details about this specification. In this specification, when a part "includes" a component, this means, unless otherwise stated, that it may include other components rather than excluding them.

[0012] In this specification, when one member is said to be located "on" another member, this includes not only cases where one member is in contact with another member, but also cases where another member exists between the two members.

[0013] Terms and words used herein should not be interpreted in a manner limited to their ordinary or dictionary meanings, but rather in a manner and concept appropriate to the technical idea of ​​the present invention, in accordance with the principle that inventors may appropriately define the concepts of terms in order to best describe their invention.

[0014] In this specification, singular expressions of terms include plural expressions unless the context clearly indicates otherwise.

[0015] In this specification, the crystallinity of the structure contained in the negative electrode active material can be confirmed by X-ray diffraction analysis. X-ray diffraction analysis may be performed using an X-ray diffraction (XRD) analyzer (product name: D4-endeavor, manufacturer: bruker), and other instruments used in this industry may be appropriately adopted.

[0016] In this specification, the presence and content of elements in the negative electrode active material can be confirmed by ICP analysis, which may be performed using an inductively coupled plasma atomic emission spectrometer (ICPAES, Perkin-Elmer 7300).

[0017] In this specification, the average particle size (D 50 The average particle size (D) can be defined as the particle size that corresponds to 50% of the cumulative volume in the particle size distribution curve (graph curve of the particle size distribution diagram). 50 The particle size can be measured, for example, using the laser diffraction method. This laser diffraction method can typically measure particle sizes from the submicron region to about several millimeters, and yields highly reproducible and high-resolution results.

[0018] Preferred embodiments of the present invention will be described in detail below. However, embodiments of the present invention may be modified in various forms, and the scope of the present invention is not limited to the embodiments described below.

[0019] <The manufacturing process of lithium-ion rechargeable batteries> One embodiment of the present invention provides a method for charging and discharging a lithium secondary battery, comprising the steps of: manufacturing a lithium secondary battery having a negative electrode containing a silicon-based active material; a first cycle step of repeating a first cycle of discharging the lithium secondary battery under conditions of 3.0V to 3.5V after charging N1 times; and a second cycle step of performing a second cycle of complete discharging the lithium secondary battery N2 times after charging, wherein N1 / N2 is greater than 25 and less than 100.

[0020] Generally, secondary batteries containing silicon-based active materials exhibit significant surface cracking and volume changes. In particular, when employing a pre-lithiation method that involves transferring lithium to the negative electrode, the degradation of the negative electrode surface becomes even more severe. This poses a problem when evaluating battery life, as it accelerates surface degradation and induces sudden drop.

[0021] Furthermore, when charging and discharging secondary batteries containing silicon-based active materials, if the charging and discharging is performed using a limited cycle in which a portion is discharged after charging (3.0V to 3.5V), an irreversible LixSiy (where x and y are real numbers) is generated, preventing proper capacity development within the silicon usage range at the end of discharge. As a result, the resistance of the negative electrode increases, and a phenomenon (saw pattern) in which the capacity temporarily decreases and then recovers repeatedly occurs.

[0022] On the other hand, the present invention adjusts the charging and discharging of a lithium secondary battery by performing a limiting cycle in which a partial discharge (3.0V to 3.5V) occurs after charging, for example, by repeating a first cycle in which a discharge is performed under 3.0V to 3.5V conditions a predetermined number of times after charging, and then performing a cycle in which a complete discharge (SOC 0%) occurs after charging, for example, by performing a second cycle in which a complete discharge is performed after charging a predetermined number of times. This eliminates the generation of an irreversible phase in the negative electrode in real time due to the repeated execution of the first cycle, thereby controlling the deterioration of the negative electrode surface and improving the durability of the negative electrode.

[0023] Specifically, the first cycle is repeated N1 times, and the second cycle is executed N2 times, adjusting the number of executions for each cycle so that N1 / N2 is greater than 25 and less than 100.

[0024] The second cycle, for example, a complete discharge (SOC 0%) cycle, induces delithiation / lithiation in the silicon usage range at the discharge end, thereby reducing the resistance of the negative electrode. This eliminates the irreversible phase generated by the first cycle and mitigates the degree to which the capacity is temporarily reduced.

[0025] Therefore, a lithium secondary battery to which a charging and discharging method for a lithium secondary battery is applied according to one embodiment of the present invention has the effect of improving discharge capacity, initial efficiency, resistance performance and / or life characteristics.

[0026] The charging and discharging method for a lithium secondary battery of the present invention includes the step of manufacturing a lithium secondary battery having a negative electrode containing a silicon-based active material.

[0027] The anode contains a silicon-based active material, and by optimally adjusting the number of cycles to reach SOC 0%, the generation of an irreversible phase within the anode in real time is eliminated. Conversely, the degeneration of the anode surface caused by volume expansion due to charging and discharging of the silicon-based active material is controlled, thereby improving the durability of the anode and allowing the high capacity and energy density of the silicon-based active material to be effectively utilized.

[0028] In one embodiment of the present invention, the step of manufacturing a lithium secondary battery having a negative electrode containing the silicon-based active material may include the step of forming a negative electrode active material layer on one or both sides of the negative electrode current collector layer.

[0029] The step of forming a negative electrode active material layer on one or both surfaces of the negative electrode current collector layer includes coating a negative electrode slurry containing a negative electrode active material layer composition on one 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.

[0030] In one embodiment of the present invention, 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 SiOx (x = 0), SiOx (0 < x < 2), SiC, and Si alloys.

[0031] In one embodiment of the present invention, the silicon-based active material may include a compound represented by the following Chemical Formula 1. [Chemical Formula 1] SiOx (0 ≤ x < 2) In the Chemical Formula 1, when x = 2, that is, in the case of SiO2, since it does not react with lithium ions and cannot store lithium, x is preferably within the above range.

[0032] Specifically, the silicon-based active material may include silicon (Si). Conventionally, Si is advantageous in that its capacity is about 2.5 to 3 times higher than that of silicon oxide (for example, SiOx (0 < x < 2)), but the volume expansion / shrinkage degree due to charge / discharge of Si is much larger than that of silicon oxide, so commercialization is more difficult. However, in the case of the present invention, by optimally adjusting the number of times of reaching SOC0% during the cycle of the secondary battery, the generation of an irreversible phase in the negative electrode in real time is eliminated, and the problem of deterioration of life characteristics can be effectively eliminated, and the advantages of high capacity, high energy density, and high efficiency characteristics of Si can be more preferably realized.

[0033] In one embodiment of the present invention, the silicon-based active material includes one or more selected from the group consisting of SiOx (x = 0) and SiOx (0 < x < 2). Specifically, the silicon-based active material may include 70 parts by weight or more of the SiOx (x = 0) based on 100 parts by weight of the silicon-based active material. More specifically, the silicon-based active material may include 80 parts by weight or more, 90 parts by weight or more, or 95 parts by weight or more of the SiOx (x = 0) based on 100 parts by weight of the silicon-based active material. In another embodiment, the silicon-based active material may consist of SiOx (x = 0).

[0034] In one embodiment of the present invention, the average particle size (D 50 ) of the silicon-based active material may be 5 μm to 10 μm, specifically, it may be 5.5 μm to 8 μm, and more specifically, it may be 6 μm to 7 μm. When the average particle size is within the above range, the specific surface area of the particles is within an appropriate range, and the viscosity of the negative electrode slurry is formed within an appropriate range. As a result, the dispersion of the particles constituting the negative electrode slurry becomes smooth. In addition, since the size of the silicon-based active material has a value equal to or greater than the lower limit value of the above range, the contact area between the silicon particles and the conductive material by the composite composed of the conductive material and the binder in the negative electrode slurry is excellent, and the possibility of maintaining the conductive network is increased, and the capacity retention rate is increased. On the other hand, when the average particle size satisfies the above range, silicon particles that are too large are excluded, and the surface of the negative electrode is smoothly formed, thereby preventing the non-uniform phenomenon of the current density during charge and discharge.

[0035] In one embodiment of the present invention, the silicon-based active material has a specific 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, most preferably 0.2 m 2 / g to 18.0 m 2The value is / g. The BET specific surface area (using nitrogen) is measured according to DIN 66131.

[0036] In one embodiment of this application, the silicon-based active material may exist, for example, in a crystalline or amorphous form, and is preferably non-porous. The silicon particles are preferably spherical or multi-piece particles. Alternatively, the silicon particles may have a fibrous structure, or exist in the form of a silicon-containing thin film or coating, but these are less preferred.

[0037] In one embodiment of the present invention, 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 has high conductivity without inducing chemical changes in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., or aluminum-cadmium alloy may be used. Furthermore, fine irregularities may be formed on the surface to strengthen the bonding force of the negative electrode active material, and it may be used in various forms such as film, sheet, foil, net, porous body, foam, nonwoven fabric.

[0038] In one 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 vary depending on the type and application of the negative electrode used, and is not limited thereto.

[0039] In one embodiment of the present invention, the step of forming a negative electrode active material layer on one or both sides of a negative electrode current collector layer includes the step of coating one or both sides of the negative electrode current collector layer with a negative electrode slurry containing a negative electrode active material layer composition, 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.

[0040] In one embodiment of the present invention, the negative electrode slurry may contain a negative electrode active material layer composition and a slurry solvent.

[0041] In one embodiment of the present invention, the solid content of the negative electrode slurry can satisfy a value of 5% or more and 40% or less.

[0042] In another embodiment, the solid content of the negative electrode slurry can be in the range of 5% to 40%, preferably 7% to 35%, and more preferably 10% to 30%.

[0043] The solid content of the negative electrode slurry can refer to the content of the negative electrode active material layer composition contained in the negative electrode slurry, and can refer to the content of the negative electrode active material layer composition based on 100 parts by weight of the negative electrode slurry.

[0044] When the solid content of the negative electrode slurry satisfies the aforementioned range, the viscosity is appropriate during the formation of the negative electrode active material layer, minimizing the aggregation phenomenon of particles in the negative electrode active material layer composition, and enabling efficient formation of the negative electrode active material layer.

[0045] In one embodiment of the present invention, the slurry solvent is not limited to that which can dissolve the aforementioned negative electrode active material layer composition, but specifically, distilled water may be used.

[0046] A negative electrode according to one embodiment of the present invention can be formed by coating and drying the negative electrode slurry on a negative electrode current collector layer. The aforementioned drying step can dry the slurry solvent in the negative electrode slurry.

[0047] In one embodiment of the present invention, 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.

[0048] In one embodiment of the present invention, a silicon-based active material may be used as the negative electrode active material, or a silicon-based active material and a carbon-based active material may be used together. In this case, a lithium secondary battery with improved performance in various aspects, such as cycle life characteristics, can be manufactured.

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

[0050] In one embodiment of the present invention, the silicon-based active material may be pure silicon (Si) in particular. Using pure silicon (Si) as the silicon-based active material means, as described above, that when based on 100 parts by weight of the entire silicon-based active material, it contains pure Si particles (SiOx(x=0)) that are not bonded with other particles or elements within the range described above. More specifically, the silicon-based active material may consist of Si.

[0051] In one embodiment of the present invention, the negative electrode includes a negative electrode current collector layer and a negative electrode active material layer, and the silicon-based active material may be present in an amount of 60 parts by weight or more, based on 100 parts by weight of the negative electrode active material layer.

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

[0053] In another embodiment, the silicon-based active material may be present in amounts of 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, based on 100 parts by weight of the negative electrode active material layer composition.

[0054] The negative electrode active material layer composition according to this application includes a negative electrode conductive material and a negative electrode binder that together can control the volume expansion rate during the charge-discharge process even when a silicon-based active material with significantly high capacity is used within the aforementioned range, and has the characteristic of not degrading the performance of the negative electrode even when included within the aforementioned range, and having excellent output characteristics during charging and discharging.

[0055] In one embodiment of this application, the silicon-based active material may have a non-spherical shape, and its degree of sphericity is, for example, 0.9 or less, for example, 0.7 to 0.9, for example, 0.8 to 0.9, for example, 0.85 to 0.9.

[0056] In this application, the circularity is determined by the following formula A, where A is the area and P is the boundary line. [Formula A] 4πA / P 2

[0057] Traditionally, graphite-based compounds were commonly used as the negative electrode active material. However, with the increasing demand for high-capacity batteries, there has been a growing trend to mix in silicon-based compounds to increase capacity. However, silicon-based compounds have a limitation: they rapidly expand in volume during the charge / discharge process, disrupting the conductive pathways formed within the negative electrode active material layer and actually degrading battery performance. Therefore, the type of negative electrode conductive material used in combination with the silicon-based active material is crucial.

[0058] In one embodiment of the present invention, the negative electrode conductive material may include one or more selected from the group consisting of point conductive materials, planar conductive materials, and linear conductive materials.

[0059] In one embodiment of the present invention, the point-shaped conductive material can be used to improve conductivity in the negative electrode and means a conductive material that does not induce chemical changes and is conductive. Specifically, the point-shaped 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 contains carbon black in terms of embodying high conductivity and having excellent dispersibility.

[0060] In one embodiment of the present invention, the point-shaped conductive material has a BET specific surface area of ​​40 m². 2 / g or more 70m 2 It may be less than or equal to / g, preferably 45m 2 / g or more 65m 2 / g or less, more comfortably, 50m 2 / g or more 60m 2 It may be less than / g.

[0061] In one embodiment of the present invention, the particle size of the dot-like conductive material may be 10 nm to 100 nm, preferably 20 nm to 90 nm, and more preferably 40 nm to 60 nm.

[0062] In one embodiment of the present invention, the negative electrode conductive material may include a planar conductive material. The aforementioned planar conductive material can improve conductivity by increasing surface contact between silicon particles within the negative electrode, while simultaneously suppressing the disruption of the conductive path due to volume expansion. It is used in concepts that include bulk-type conductive materials or plate-type conductive materials.

[0063] In one embodiment of the present invention, the planar conductive material may include at least one selected from the group consisting of plate graphite, graphene, graphene oxide, and graphite flakes, and preferably plate graphite.

[0064] In one embodiment of this application, the average particle size (D) of the planar conductive material 50 The particle size may be 2 μm to 7 μm, more specifically 3 μm to 6 μm, and more specifically 4 μm to 5 μm. When the above range is met, the particle size is sufficient, making dispersion easy while preventing the viscosity of the negative electrode slurry from increasing too much. Therefore, the dispersion effect is superior when dispersed using the same equipment and time.

[0065] In one embodiment of the present invention, the planar conductive material is D 10 The size is 0.5 μm or more and 1.5 μm or less, D 50 The size is between 2.5 μm and 3.5 μm, and D 90 The particle size may be between 7.0 μm and 15.0 μm.

[0066] In one embodiment of the present invention, the planar conductive material may be a high specific surface area planar conductive material with a high BET specific surface area, or a low specific surface area planar conductive material.

[0067] In one embodiment of the present invention, the planar conductive material may be any planar conductive material with a high specific surface area or a planar conductive material with a low specific surface area without limitation. However, since the planar conductive material according to the present invention may be affected to some extent by dispersion in terms of electrode performance, it is particularly preferable to use a planar conductive material with a low specific surface area that does not cause dispersion problems.

[0068] In one embodiment of the present invention, the planar conductive material has a BET specific surface area of ​​5 m². 2 It may be more than / g.

[0069] In another embodiment, the planar conductive material has a BET specific surface area of ​​5 m². 2 / g or more 500m 2 It may be less than / g, preferably 5m 2 / g or more 300m 2 / g or less, more preferably 5m 2 / g or more 250m 2 It may be less than / g.

[0070] In another embodiment, the planar conductive material is a planar conductive material with a high specific surface area, and the BET specific surface area is 50 m². 2 / g or more 500m 2 / g or less, preferably 80m 2 / g or more 300m 2 Less than / g, more comfortably, 100m 2 / g or more 300m 2 It can satisfy the range of / g or less.

[0071] In another embodiment, the planar conductive material is a planar conductive material with a low specific surface area, and the BET specific surface area is 5m². 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 It can satisfy the range of / g or less.

[0072] Other 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 contain multiple carbon nanotube units. Specifically, here, "bundle type" refers to a secondary shape in which multiple carbon nanotube units are arranged in parallel with substantially the same orientation along their longitudinal axes, or are twisted together to form a bundle or rope. The carbon nanotube units have a graphite sheet that is cylindrical with a nanoscale diameter and has an sp2 bond structure. In this case, the properties of a conductor or semiconductor can be observed depending on the angle and structure in which the graphite sheet is wound. Compared to entangled type carbon nanotubes, the bundle-type carbon nanotubes can be uniformly dispersed during the manufacture of the negative electrode, smoothly forming a conductive network within the negative electrode and improving the conductivity of the negative electrode.

[0073] In one embodiment of the present invention, the negative electrode conductive material may be 10 to 40 parts by weight, based on 100 parts by weight of the negative electrode active material layer composition.

[0074] In another embodiment, the negative electrode conductive material may contain 10 to 40 parts by weight, preferably 10 to 30 parts by weight, and more preferably 15 to 25 parts by weight, based on 100 parts by weight of the negative electrode active material layer composition.

[0075] The negative electrode conductive material of this application has a completely different configuration from the conductive material applied to the positive electrode. That is, the negative electrode conductive material according to the present invention plays a role in controlling the contact points between silicon-based active materials, which experience very large volume expansion during charging and discharging, while the positive electrode conductive material acts as a buffer during rolling, while also imparting some conductivity. Its configuration and role are completely different from the negative electrode conductive material of the present invention.

[0076] Furthermore, the negative electrode conductive material according to the present invention is applied to silicon-based active materials and has a completely different structure from conductive materials applied to graphite-based active materials. That is, conductive materials used in electrodes with graphite-based active materials simply have smaller particles compared to the active material, and thus have the properties of improving output characteristics and imparting some conductivity. This is completely different in structure and role from negative electrode conductive materials applied together with silicon-based active materials, as in the present invention.

[0077] In one embodiment of the present invention, the planar conductive material used as the negative electrode conductive material described above has a structure and role different from that of carbon-based active materials generally used as negative electrode active materials. 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 point-like form to facilitate the storage and release of lithium ions.

[0078] On the other hand, planar conductive materials used as negative electrode conductive materials are substances having a planar or plate-like form, and can be represented as plate-like graphite. In other words, they are substances included to maintain conductive pathways within the negative electrode active material layer, and do not play a role in lithium storage and release; they are substances that secure conductive pathways in a planar form within the negative electrode active material layer.

[0079] In other words, in this invention, the use of plate-shaped graphite as a conductive material means that it was not processed into a planar or plate shape to play a role in lithium storage or release, but rather used as a material to secure a conductive path. In this case, the negative electrode active material included together has high capacity characteristics for lithium storage and release, and plays a role in storing and releasing all lithium ions transmitted from the positive electrode.

[0080] On the other hand, in this application, the use of a carbon-based active material as the active material means that it was processed into a point-like or spherical shape and used as a substance that plays a role in storing or releasing lithium.

[0081] In other words, in one embodiment of the present invention, the carbon-based active material, artificial graphite or natural graphite, has a BET specific surface area of ​​0.1 m². 2 / g or more 4.5m 2 It can satisfy the range of less than / g. Also, plate-shaped graphite, which is a planar conductive material, is planar and has a BET specific surface area of ​​5m². 2 It may be more than / g.

[0082] In one embodiment of this application, the negative electrode binder may contain at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and substances in which the hydrogen atoms of these substances are substituted with Li, Na, or Ca, or may contain a variety of copolymers thereof.

[0083] The negative electrode binder according to one embodiment of this application plays a role in controlling the negative electrode active material and negative electrode conductive material in order to prevent twisting and structural deformation of the negative electrode structure during volume expansion and relaxation of the silicon-based active material. Any general negative electrode binder can be applied as long as it fulfills the above role. Specifically, an aqueous binder may be used, and more specifically, a PAM-based binder may be used.

[0084] In one embodiment of the present invention, the negative electrode binder may be included in a quantity of 30 parts by weight or less, preferably 25 parts by weight or less, more preferably 20 parts by weight or less, based on 100 parts by weight of the negative electrode active material layer composition, and may also be included in a quantity of 1 part by weight or more, or 3 parts by weight or more.

[0085] In one embodiment of the present invention, the negative electrode may be pre-lithified. In one embodiment of the present invention, the step of manufacturing a lithium secondary battery having a negative electrode containing the silicon-based active material may further include the step of pre-lithifying the negative electrode containing the silicon-based active material.

[0086] Specifically, the step of pre-lithifying the negative electrode containing the silicon-based active material may include the steps of: forming a negative electrode active material layer on one or both sides of the negative electrode current collector layer; preparing a transfer laminate in which a base film and a lithium metal layer are sequentially laminated; transferring the lithium metal layer to the upper part of the negative electrode active material layer; and removing the base film. This relates to a method for pre-lithifying electrodes for lithium secondary batteries using a lithium metal transfer process.

[0087] In the charge-discharge reaction of lithium secondary batteries, lithium released from the positive electrode is inserted into the negative electrode during charging, and desorbed from the negative electrode and returned to the positive electrode during discharging. However, in the case of silicon-based negative electrode active materials, volume changes and surface side reactions are serious, resulting in a small amount of lithium inserted into the negative electrode during initial charging returning to the positive electrode, leading to a problem of a large initial irreversible capacity. A large initial irreversible capacity leads to a rapid decrease in battery capacity and cycle life.

[0088] When the charging and discharging method for secondary batteries according to the present invention is adopted, the irreversible phase of the lithium secondary battery is efficiently removed, so the deterioration of the negative electrode can be controlled. Therefore, as described above, when applied to secondary batteries in which the negative electrode has been pre-lithified, the durability of the battery can be improved more effectively.

[0089] Generally, the pre-lithiation process involves chemically or physically pre-lithifying a lithium metal layer onto an electrode, and may be specifically carried out by a lithium metal transfer process, lithium metal powder deposition, electrochemical process, or lithium metal deposition process. The pre-lithiation process according to the present invention may include a lithium metal transfer process.

[0090] In the lithium metal layer transfer process, the highly reactive lithium metal can be transferred more stably to the top of the electrode active material layer. In this process, a step is needed to easily transfer the lithium metal layer from the transfer laminate to the top of the electrode active material layer.

[0091] In one embodiment of the present invention, the step of pre-lithifying the negative electrode includes the steps of: preparing a transfer laminate in which a base film and a lithium metal layer are sequentially laminated; transferring the lithium metal layer to the upper part of the electrode active material layer; and removing the base film.

[0092] In one embodiment of the present invention, the deposition method for depositing the lithium metal layer onto the substrate film can be selected from among vacuum deposition, chemical vapor deposition, chemical vapor deposition (CVD), and physical vapor deposition, but is not limited thereto, and various deposition methods used in the industry may be used.

[0093] In this case, the electrode for the lithium secondary battery on which the transfer laminate is stacked is subjected to a pressure of 5 kgf / cm². 2 ~500 kgf / cm² 2 The lamination process may be carried out by rolling (roll pressing) with the application of a load. This is followed by a step of removing the base film.

[0094] In one embodiment of the present invention, the base film may be used without limitation, as long as it has the characteristics of being able to withstand process conditions such as high temperatures during the lithium metal deposition stage and preventing the reverse peeling problem in which the lithium metal is transferred onto the base film during the winding process for transferring the deposited lithium metal.

[0095] Specifically, in one embodiment of the present invention, the base 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.

[0096] In one embodiment of the present invention, the thickness of the base film may be 1 μm or more and 300 μm or less, and can satisfy the range of 5 μm or more and 200 μm or less, and 10 μm or more and 100 μm or less.

[0097] In one embodiment of the present invention, the thickness of the lithium metal may be 1 μm or more and 10 μm or less, and preferably 3 μm or more and 10 μm or less.

[0098] By ensuring that the thickness of the base film and the lithium metal meets the specified range, the lithium metal can be efficiently transferred to the negative electrode active material layer and reverse transfer can be prevented.

[0099] In one embodiment of the present invention, a transfer strength improving layer may be included on the surface of the transfer laminate in contact with the substrate film and the lithium metal layer in order to improve the peelability of the lithium metal layer, ensure transferability to the electrode active material layer, and serve as a protective layer after the transfer of lithium metal.

[0100] In other words, the base film may have a transfer-enhancing layer formed on at least one surface. Specifically, the transfer-enhancing layer may be formed on one or both surfaces. The transfer-enhancing layer prevents the reverse delamination problem in which the lithium metal layer is transferred onto the base film during the winding process for transferring the deposited lithium metal layer to the electrode, and also allows for easy separation of the base film after the lithium metal layer has been transferred onto the electrode active material layer.

[0101] The transfer strength improving layer may contain one or more selected from the group consisting of silicon-modified polyester in which silicon chains are grafted onto a polyester main chain, acrylic resin, Si, melamine, and fluorine.

[0102] In one embodiment of the present invention, the transferability-enhancing layer may contain poly(methyl methacrylate) (PMMA).

[0103] In one embodiment of the present invention, the transfer strength improving layer may be formed by a coating method, for example, the coating method may be selected from the group consisting of dip coating, spray coating, spin coating, die coating, gravure coating, micro-gravure coating, comma coating, and roll coating, but is not limited thereto, and a variety of coating methods that can be used in the industry to form a coating layer may be employed.

[0104] In one embodiment of the present invention, the step of transferring the lithium metal layer to the upper part of the negative electrode active material layer may include the step of laminating the transfer laminate onto the negative electrode active material layer.

[0105] Specifically, the step of transferring the lithium metal layer to the upper part of the negative electrode active material layer may include the step of laminating the transfer laminate onto the electrode active material layer such that the opposite side of the lithium metal layer that contacts the transfer power improving layer contacts the opposite side of the electrode active material layer that contacts the electrode current collector layer.

[0106] In this process, the lamination step is performed under temperature conditions of 20°C to 90°C and a load of 5 kgf / cm². 2 ~500 kgf / cm² 2 Lamination may be performed under these pressure conditions.

[0107] In one embodiment of the present invention, the lamination step is 5 kgf / cm 2 ~500 kgf / cm² 2 The pressurization conditions are preferably 10 kgf / cm². 2 ~150 kgf / cm² 2 The pressurization conditions can be met.

[0108] However, temperature conditions may be omitted during the lamination stage. In particular, if the lamination stage satisfies the aforementioned pressurization conditions, the pre-lithiation rate of the lithium metal can be smoothly adjusted, thereby suppressing the generation of large amounts of oxides and nitrides during the transfer process. Furthermore, by satisfying the aforementioned pressurization range, the lithium metal is smoothly transferred to the upper part of the electrode active material layer, and the problem of reverse transfer does not occur.

[0109] The reaction may be omitted and the pre-lithification of the electrode active material layer with a highly reactive lithium metal may be performed after the lamination step, or the reaction may be omitted and the pre-lithification may be performed during battery assembly.

[0110] In one embodiment of the present invention, the step of pre-lithifying the negative electrode may further include a step of pre-lithifying the negative electrode active material layer after the step of removing the substrate film, and the step of pre-lithifying the negative electrode active material layer may be performed within 30 minutes to 7 days after transferring the lithium metal.

[0111] In one embodiment of the present invention, instead of transferring the lithium metal layer alone, the transferability-enhancing layer and the lithium metal are transferred together to the upper part of the electrode active material layer. In this case, the transferability-enhancing layer can act as a protective layer that can prevent the highly reactive lithium metal from reacting in air.

[0112] In one embodiment of the present invention, the step of pre-lithifying the negative electrode may include the step of pre-lithifying the negative electrode active material layer.

[0113] The step of pre-lithifying the negative electrode active material layer may be performed within 30 minutes to 24 hours under conditions of 25°C and 1 atm.

[0114] The aforementioned pre-lithiation step is a step in which conditions are set for the diffusion of lithium metal into the negative electrode active material layer. Whether or not pre-lithiation is complete can be determined by whether or not the lithium on the upper part of the negative electrode active material layer has completely disappeared.

[0115] In one embodiment of the present invention, the lithium secondary battery manufactured by the above method may be a lithium secondary battery comprising a positive electrode, a negative electrode, a separator membrane provided between the positive electrode and the negative electrode, and an electrolyte.

[0116] In this case, if the negative electrode is a pre-lithified negative electrode, the transfer power improving layer used during pre-lithification may be removed, thereby preventing it from remaining on the top of the electrode and preventing an unnecessary increase in resistance. That is, the transfer power improving layer may be used to improve transfer power and protect the lithium metal layer before pre-lithification, and may be removed before the electrolyte is injected.

[0117] In one embodiment of the present invention, the step of manufacturing a lithium secondary battery having a negative electrode containing the silicon-based active material may further include the step of forming a positive electrode active material layer on one or both sides of the positive electrode current collector layer.

[0118] In one embodiment of the present invention, the positive electrode current collector layer is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc. may be used. The positive electrode current collector layer may also have a thickness of 3 μm to 500 μm, and fine irregularities can be formed on the surface of the positive electrode current collector layer to increase the adhesion strength of the positive electrode active material. For example, it may be used in various forms such as film, sheet, foil, net, porous material, foam, nonwoven fabric.

[0119] The thickness of the positive electrode current collector layer may vary depending on the type and application of the negative electrode used, and is not limited thereto.

[0120] In one embodiment of the present invention, the step of forming a positive electrode active material layer on one or both sides of a positive electrode current collector layer includes the step of coating one or both sides of the positive electrode current collector layer with a positive electrode slurry containing a positive electrode active material layer composition, 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.

[0121] In one embodiment of the present invention, the contents of the positive electrode slurry may be the same as those of the negative electrode slurry described above, with the only difference being that it is a positive electrode.

[0122] In one embodiment of the present invention, 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.

[0123] The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material may be a layered compound such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; lithium iron oxide such as LiFe3O4; or a compound with the chemical formula Li 1+c1 Mn 2-c1Lithium manganese oxides such as O4 (0 ≤ c1 ≤ 0.33), LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, Cu2V2O7; chemical formula LiNi 1-c2 Ni-site type lithium nickel oxide represented as Mc2O2 (where M is at least one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, satisfying 0.01 ≤ c2 ≤ 0.5); chemical formula LiMn 2-c3 M c3 Lithium manganese composite oxides represented as O2 (where M is at least one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, satisfying 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); LiMn2O4 in which part of the Li in the chemical formula is substituted with an alkaline earth metal ion, etc., are examples but are not limited to these. The positive electrode may be metallic lithium (Li-metal).

[0124] Specifically, the positive electrode active material may contain one or more selected from the group consisting of Ni, Co, Mn, LTO, LFP, RuO2, Nb2O5, Mn3O4, Fe2O3, and Co3O4.

[0125] Furthermore, the positive electrode may include a sacrificial positive electrode material, and any type used in this industry is not limited to that which may be used.

[0126] In this case, the positive electrode conductive material is used to impart conductivity to the electrode and can be used without particular limitations as long as it has electronic conductivity in the battery without causing chemical changes. Specific examples include graphite such as natural graphite or 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 powders or metal fibers 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. One of these alone or a mixture of two or more may be used.

[0127] Furthermore, the positive electrode binder plays a role in improving adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one or more of these may be used.

[0128] Furthermore, the content of the positive electrode active material, positive electrode conductive material, and positive electrode binder contained in the positive electrode active material layer composition may be the same as that applied to the negative electrode active material layer composition described above.

[0129] In one embodiment of the present invention, the electrolyte can be an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, or a molten inorganic electrolyte, but is not limited to these. Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0130] As the non-aqueous organic solvent, for example, aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate may be used.

[0131] In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are suitable for use because they are high-viscosity organic solvents with high dielectric constants and readily dissociate lithium salts. Furthermore, by mixing such cyclic carbonates with linear carbonates with low viscosity and low dielectric constant, such as dimethyl carbonate and diethyl carbonate, in appropriate ratios, electrolytes with high electrical conductivity can be produced, making them even more suitable for use.

[0132] The metal salt may be a lithium salt, which is a substance that dissolves easily in the non-aqueous electrolyte, for example, F as the anion of the lithium salt. - 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 - You may use one or more selected from the group consisting of the following:

[0133] In addition to the components of the electrolyte, the electrolyte may further contain one or more additives for the purpose of improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity, such as haloalkylene carbonate compounds like difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexalic acid triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride.

[0134] <Charging and discharging stages of lithium-ion secondary batteries> A charging and discharging method for a lithium secondary battery according to one embodiment of the present invention includes a first cycle stage in which the lithium secondary battery is charged and then discharged under conditions of 3.0V to 3.5V, repeated N1 times; and a second cycle stage in which the lithium secondary battery is charged and then completely discharged, repeated N2 times, wherein N1 / N2 may be greater than 25 and less than 100.

[0135] Specifically, the secondary battery can be driven by the electrochemical charging and electrochemical discharging described above. According to the charging and discharging method for secondary batteries of the present invention, by using a negative electrode containing the silicon-based active material, the initial efficiency and lifespan of the secondary battery can be improved. Furthermore, by optimally adjusting the number of cycles in which the State of Charge (SOC) reaches 0% (complete discharge), it is possible to control the generation of an irreversible phase within the negative electrode while preventing performance degradation due to volume expansion of the silicon-based active material and collapse of the electrode structure, thereby controlling the deterioration of the negative electrode surface and significantly improving the lifespan of the secondary battery.

[0136] Therefore, a lithium secondary battery to which a charging and discharging method for a lithium secondary battery is applied according to one embodiment of the present invention has the effect of improving discharge capacity, initial efficiency, resistance performance and / or life characteristics.

[0137] In this specification, "SOC" may mean 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 at the time when electrochemical charging is completed in the operation of the secondary battery, and the SOC after discharge may be the SOC at the time when electrochemical discharge is completed in the operation of the secondary battery. That is, an SOC of 100% after charging means that the remaining capacity of the secondary battery at the time when charging is completed is 100%, in which case it means a fully charged state. An SOC of 65% after discharge means that the remaining capacity of the secondary battery at the time when discharge is completed is 65%. An SOC of 0% after discharge means that the remaining capacity of the secondary battery at the time when discharge is completed is 0%, in which case it means a fully discharged state.

[0138] In one embodiment of the present invention, the lithium secondary battery may be subjected to a first cycle of charging followed by discharging under conditions of 3.0V to 3.5V, repeated N1 times. Subsequently, a second cycle of charging followed by complete discharge may be performed N2 times. In this case, N1 and N2 may each be integers of 1 or more.

[0139] In one example, if N1 is 25 and N2 is 1, the first cycle (limiting cycle) can be repeated 25 times, followed by one second cycle (complete discharge cycle).

[0140] In one embodiment of the present invention, the first cycle stage and the second cycle stage may be repeated. For example, if N1 is 25 and N2 is 1, the first cycle (limiting cycle) may be repeated 25 times, followed by the second cycle (complete discharge cycle) being performed once, and then the first cycle may be repeated another 25 times, followed by the second cycle (complete discharge cycle) being performed once.

[0141] In one example, the charging may be performed under 1C and constant current / constant voltage (CC / CV) conditions.

[0142] In one example, the discharge may be performed under constant current conditions at 0.5C up to a specific state of charge (SOC), or it may be performed so as to be cut off at the discharge voltage corresponding to the specific SOC.

[0143] In this regard, the charge / discharge conditions and C-rate are not limited to those described above, and any method used in this industry may be appropriately applied.

[0144] In one embodiment of the present invention, the charging may be performed to a state of charge (SOC) of 95% to 100%. For example, the charging may be performed under full charge conditions, and the voltage at the end of charging may be 4.2V.

[0145] In one embodiment of the present invention, in the first cycle, the discharge may be performed from SOC 50% to SOC 90%. Specifically, the voltage at the end of the discharge may be 3.0V to 3.5V, and more specifically, 3.25V (SOC 65%).

[0146] In one example, in the second cycle, complete discharge is performed until SOC 0%, and specifically, the voltage at the end of the discharge may be 2.5V.

[0147] The first and second cycles may be performed under conditions of 25°C and a rest period of 20 minutes, respectively.

[0148] When the aforementioned lithium secondary battery undergoes N2 full charge / discharge cycles after N1 limited cycles, the formation of an irreversible phase can be effectively controlled. However, a problem arises in that the irreversible phase in the negative electrode decreases only when the voltage for complete discharge (SOC 0%) conditions (e.g., 2.5V) is reached, making it easy to control surface degradation. If the complete discharge conditions cannot be reached, controlling the irreversible phase is not easy.

[0149] Furthermore, excessive execution of full charge / discharge cycles increases the number of full charge / discharge cycles in the overall cycle, leading to increased use of Si in the lower part of the negative electrode active material layer. This results in severe volume expansion of the negative electrode, which reduces its durability.

[0150] In one embodiment of the present invention, the N1 / N2 ratio may be greater than 25 and less than 100. Specifically, the N1 / N2 ratio may be greater than 30 and less than 80 or greater than 40 and less than 60. More specifically, the N1 / N2 ratio may be 50.

[0151] When the N1 / N2 values ​​satisfy the aforementioned range, the generation of the irreversible phase within the negative electrode can be controlled in real time, thereby controlling the degradation of the negative electrode surface and significantly improving the lifespan performance of the secondary battery.

[0152] On the other hand, if the N1 / N2 ratio is 25 or less, the number of complete charge / discharge cycles in the overall cycle increases, leading to serious volume expansion of the negative electrode and accelerating its deterioration. If the N1 / N2 ratio is 100 or more, the irreversible phase is not eliminated, resulting in uneven use of the negative electrode.

[0153] In one 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. When N1 is below the above range, excessive full charge / discharge cycles occur, using a large amount of the lower Si of the negative electrode active material layer, which leads to serious volume expansion of the negative electrode and a decrease in the durability of the negative electrode. When N1 is greater than the above range, irreversible phases are not resolved in real time, which leads to uneven use of the negative electrode.

[0154] In one embodiment of the present invention, N2 may be between 1 and 3. Specifically, N2 may be 1. If N2 exceeds the above range, excessive full charge / discharge cycles occur, using a large amount of Si in the lower part of the negative electrode active material layer, which leads to serious volume expansion of the negative electrode and a decrease in the durability of the negative electrode.

[0155] The electrochemical charging of the secondary battery may be carried out using an electrochemical charger / discharger. Specifically, the WOCS3000s (manufactured by Won A Tech Co., Ltd.) can be used as the electrochemical charger / discharger.

[0156] Furthermore, one embodiment of the present invention provides a battery system including the aforementioned secondary battery. Specifically, the charging and discharging method of the secondary battery described above can be realized using the battery system.

[0157] For example, the secondary battery may be included in the battery system in the form of a secondary battery cell consisting of one secondary battery or a secondary battery module which is an assembly of multiple secondary batteries. The battery system may include a control unit along with the secondary battery.

[0158] The control unit can set the State of Charge (SOC) and drive voltage range of the secondary battery during electrochemical charging and electrochemical discharging. This allows electrochemical charging and discharging of the secondary battery to be performed within the electrochemical charge / discharge range and SOC set by the control unit, and the charge / discharge SOC and number of cycles can be adjusted.

[0159] The control unit is not particularly limited as long as it can control the drive voltage range during electrochemical charging and electrochemical discharge of the secondary battery, and may, for example, be an electrochemical charger / discharger. Specifically, the control unit may be built into a Battery Management System (BMS) included in the battery pack.

[0160] Furthermore, the present invention provides a battery pack including the aforementioned battery system. In addition to the secondary battery and control unit mentioned above, the battery pack may further include known components in the field, such as a Battery Management System (BMS) and a cooling system.

[0161] The aforementioned battery system or battery pack is useful in portable devices such as mobile phones, laptop computers, and digital cameras, as well as in the field of electric vehicles such as hybrid electric vehicles (HEVs). The aforementioned battery system or battery pack can be preferably applied to power sources that require high output and large capacity, such as electric vehicles, hybrid vehicles, and power storage devices. [Examples]

[0162] The following describes the Specification in detail with reference to examples. However, the examples relating to this Specification may be modified in various different forms, and the scope of this application should not be construed as being limited to the examples described below. The examples of this application are provided to give a more complete explanation of this Specification to a person of average knowledge in the art.

[0163] <Examples and Comparative Examples> Manufacturing Example 1 1) Manufacturing of the negative electrode A negative electrode slurry was prepared by adding a silicon-based active material Si (average particle size (D50): 5 μm) as the negative electrode active material, carbon black (product name: Super C65, manufacturer: Timcal) as the conductive material, and polyacrylic acid as the binder in a weight ratio of 70:20:10 to distilled water as the solvent for forming the negative electrode slurry (solid content concentration: 20% by weight).

[0164] As the negative electrode current collector, a copper current collector (thickness: 15 μm) is used, and the negative electrode slurry is applied to one surface at a rate of 120 mg / 25 cm². 2 The material was coated with the specified amount, rolled (roll press), and dried in a vacuum oven at 130°C for 10 hours to form a negative electrode active material layer, which was then used as the negative electrode (the negative electrode before pre-lithification).

[0165] 2) Pre-lithiumization of the negative electrode A lithium metal with a thickness of 6.2 μm was transferred to the negative electrode using a roll press.

[0166] Specifically, the charge rate should be 1.1 mA / cm², which is 10% of the negative electrode's charge capacity before the pre-lithiation process. 2 Electrochemical charging was performed at the following current density.

[0167] 3) Manufacturing of the positive electrode As the positive electrode active material, Li[Ni 0.8 Co 0.1 Mn 0.1A cathode slurry was prepared by adding O2, carbon black (product name: Super C65, manufacturer: Timcal) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder in a weight ratio of 97:1.5:1.5 to N-methyl-2-pyrrolidone (NMP) as a solvent for cathode slurry formation (solid content concentration: 78% by weight).

[0168] As the positive electrode current collector, 620 mg / 25 cm of the positive electrode slurry was applied to both sides of an aluminum current collector (thickness: 12 μm). 2 The positive electrode was manufactured by coating the supported amount with a coating, rolling it (roll press), and drying it in a vacuum oven at 130°C for 10 hours to form a positive electrode active material layer.

[0169] 4) Manufacturing of secondary batteries A bi-cell secondary battery was manufactured by placing two pre-lithified negative electrodes on both sides of the positive electrode manufactured as described above, interposing a polypropylene separation membrane between the positive and negative electrodes, and injecting an electrolyte. The electrolyte used was an organic solvent mixture of fluoroethylene carbonate (FEC) and diethyl carbonate (DEC) in a volume ratio of 30:70, to which vinylene carbonate was added at 3% by weight relative to the total weight of the electrolyte, and LiPF6 was added as a lithium salt at a concentration of 1M.

[0170] Manufacturing Example 2 A lithium secondary battery was manufactured using a similar method to that in Manufacturing Example 1, except that the negative electrode was not pre-lithified.

[0171] <Experimental Example 1: Evaluation of Cycle Characteristics> The lithium secondary batteries manufactured in Manufacturing Examples 1 and 2 were charged and discharged to the state of charge (SOC) set in Table 1 below, N1 times under driving conditions of 4.2V-2.5V and cycle conditions of 4.2V-3.2V, and then N2 times under complete discharge cycles to power the secondary batteries.

[0172] The capacity retention rate over cycles was evaluated using the following formula. Figure 1 shows graphs illustrating the cycle characteristics of Example 1-1, Comparative Examples 1-1 and 1-2, which used Manufacturing Example 1 (pre-lithified negative electrode).

[0173] Table 1 also shows the capacity retention rate and the point of sudden drop at the 600th cycle.

[0174] -Capacity retention rate (%) = {(Discharge capacity at 600th cycle) / (Discharge capacity at 1st cycle)} × 100 (Full charge / discharge capacity ratio after approximately 600 cycles (4.2V-2.5V)) -Sudden drop point = Number of cycles at which the real-time capacity retention rate reaches 50% <Electrochemical charging and electrochemical discharge conditions> - Charging: Charge the secondary battery in CC / CV mode at 1C until it reaches the State of Charge (SOC) of the secondary battery shown in Table 1 (cut off at 0.05C). - Discharge: Discharge in CC mode at 0.5C until the discharge state of charge (SOC) of the secondary battery shown in Table 1 above (cut-off at the discharge voltage corresponding to each SOC range).

[0175] [Table 1]

[0176] When applying a lithium secondary battery charging and discharging method like the present invention, by optimally adjusting the number of cycles to reach SOC 0% (complete discharge), it is possible to eliminate the real-time generation of irreversible phases, control the degradation of the negative electrode surface, and improve the durability of the negative electrode.

[0177] According to Table 1, in Examples 1-1 and 2-1, where a charge-discharge method (N1 / N2 satisfying 50) was applied to a lithium secondary battery having a silicon-based active material in the negative electrode, in which the first cycle was repeated 50 times followed by one complete discharge cycle, the capacity retention rate was excellent, and it was confirmed that the number of cycles until a sudden drop occurred was large. On the other hand, in Comparative Examples 1-1, 1-2, 2-1, and 2-2, in which the first cycle was repeated 25 times followed by one complete discharge cycle (N1 / N2 being 25), or the first cycle was repeated 100 times followed by one complete discharge cycle (N1 / N2 being 100), the number of cycles until a sudden drop occurred was small, and it was confirmed that the degradation of the lithium secondary battery was accelerated compared to Examples 1-1 and 2-1.

[0178] Therefore, a lithium secondary battery to which a charging and discharging method for a lithium secondary battery is applied according to one embodiment of the present invention has the effect of significantly improving the durability of the negative electrode.

[0179] <Experimental Example 2: Evaluation of Capacity and Discharge Resistance Characteristics> <Examples 1-2> Except for applying the same charge / discharge method as in Example 1-1 of Table 1 to the lithium secondary battery manufactured in Manufacturing Example 1 under cycle conditions of 4.2V-3.3V, the test was carried out using the same charge / discharge method as in Example 1-1 of Table 1.

[0180] <Comparative Example 1-3> Furthermore, the lithium secondary battery manufactured in Manufacturing Example 1 was subjected to the same charge-discharge method as in Example 1-1 of Table 1, except that zero complete discharge cycles were performed on the lithium secondary battery manufactured in Manufacturing Example 1 under cycle conditions of 4.2V-3.3V.

[0181] Figure 2 shows a graph illustrating the capacity characteristics as the cycle progresses for the lithium secondary batteries of Examples 1-2 and Comparative Examples 1-3, and Figure 3 shows a graph illustrating the discharge resistance (Rdis) value as time progresses.

[0182] Figure 2 shows the capacitance measured every 50 cycles under electrochemical charging and electrochemical discharge conditions at 4.2V-2.5V and 0.33C / 0.33C. This confirms that, in Examples 1-2, by appropriately adjusting the number of complete discharge cycles, the irreversible phase is eliminated, mitigating the phenomenon of temporary capacitance decrease / recovery as the number of cycles increases compared to Comparative Examples 1-3.

[0183] As shown in Figure 3, in Examples 1-2, it can be confirmed that by performing a complete discharge cycle, delithiation / lithiation is induced in the silicon usage range at the end of discharge, thereby reducing the discharge resistance value of the negative electrode compared to Comparative Examples 1-3. The discharge resistance evaluation in Figure 3 was performed by evaluating the same cell twice.

Claims

1. The step of manufacturing a lithium secondary battery having a negative electrode containing a silicon-based active material; For the aforementioned lithium secondary battery, a first cycle is performed in which the battery is discharged under conditions of 3.0V to 3.5V after charging. １ The first cycle stage is repeated several times; and After the first cycle stage, a second cycle is performed on the lithium secondary battery, in which it is charged and then completely discharged. ２ The second cycle stage, which is performed multiple times; Includes, In the first cycle described above, discharge is performed from SOC 50% to SOC 90%. N １ / N ２ It is between 25 and 100. The aforementioned N1 is greater than 30 and less than 80. A method for charging and discharging a lithium secondary battery, wherein N2 is between 1 and 3.

2. The aforementioned N １ This is between 40 and 60 years old. The aforementioned N ２ The charging and discharging method for a lithium secondary battery according to claim 1, wherein is 1.

3. A method for charging and discharging a lithium secondary battery according to claim 1, comprising repeatedly performing a set of the first cycle stage and the second cycle stage.

4. The charging and discharging method for a lithium secondary battery according to claim 1, wherein the silicon-based active material comprises one or more selected from the group consisting of SiOx (x=0), SiOx (0<x<2), SiC, and Si alloys.

5. A method for charging and discharging a lithium secondary battery according to claim 4, wherein the silicon-based active material is based on 100 parts by weight and the SiOx (x=0) is 70 parts by weight or more.

6. The negative electrode includes a negative electrode current collector layer and a negative electrode active material layer. The charging and discharging method for a lithium secondary battery according to claim 1, wherein the silicon-based active material is present in an amount of 60 parts by weight or more, based on 100 parts by weight of the negative electrode active material layer.

7. The charging and discharging method for a lithium secondary battery according to claim 1, wherein the negative electrode is pre-lithified.

8. A method for charging and discharging a lithium secondary battery according to any one of claims 1 to 7, wherein the step of manufacturing a lithium secondary battery having a negative electrode containing the silicon-based active material further includes a step of pre-lithifying the negative electrode containing the silicon-based active material.

9. The step of pre-lithifying the negative electrode containing the silicon-based active material is as follows: A step of forming a negative electrode active material layer on one or both sides of the negative electrode current collector layer; A step in which a transfer laminate is prepared in which a base film and a lithium metal layer are sequentially laminated; A step of transferring the lithium metal layer onto the upper part of the negative electrode active material layer; and Steps to remove the aforementioned base film; A method for charging and discharging a lithium secondary battery according to claim 8, including the following:

10. The charging and discharging method for a lithium secondary battery according to claim 9, wherein the base film has a transferability-enhancing layer formed on at least one surface.

11. A method for charging and discharging a lithium secondary battery according to claim 9, comprising the step of laminating the transfer laminate onto the negative electrode active material layer.

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