Charge and Discharge Method of Lithium Secondary Battery

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 contents disclosed in the documents of the Korean patent applications are 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, have a high energy density, and are in the spotlight as a driving power source for portable devices. Therefore, research and development efforts for improving the performance of lithium secondary batteries are actively underway.

[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, respectively. 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] Among the negative electrode active materials, in the case of silicon-based active materials, they are 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 they have the disadvantage of low initial efficiency.

[0005] In the case of a secondary battery containing a silicon-based active material, cracks and volume changes on the surface portion are large. In particular, when adopting a method of transferring lithium to the negative electrode as one of the pre-lithiation methods, the deterioration of the surface portion of the negative electrode becomes more serious. However, when evaluating the lifespan, there is a problem that the deterioration of the surface portion is accelerated earlier, inducing a sudden drop. In addition, when a secondary battery containing a silicon-based active material is charged and discharged in a limited cycle, the use of the negative electrode is uneven and the deterioration is accelerated.

[0006] Therefore, there is a current situation where there is a demand for the development of a secondary battery that can improve the lifespan characteristics while embodying the high capacity and energy density of the silicon-based active material.

Summary of the Invention

Problems to be Solved by the Invention

[0007] The present invention relates to a charge and discharge method of a lithium secondary battery. In order to prevent the degradation of a lithium secondary battery containing a silicon-based active material, by adjusting the cycle driving method during charging and discharging, it relates to a charge and discharge method of a lithium secondary battery whose durability can be improved.

Means for Solving the Problems

[0008] One embodiment of the present invention includes 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 3.0 V to 3.5 V after charging N1 times; and a second cycle step of performing a second cycle of fully discharging the lithium secondary battery N2 times after charging, where N1 / N2 is greater than 25 and less than 100, and provides a charge and discharge method of a lithium secondary battery.

Effects of the Invention

[0009] The charge-discharge method of a lithium secondary battery according to an embodiment of the present invention can eliminate the generation of an irreversible phase in real time by optimally adjusting the number of times of reaching SOC 0% (fully discharged) during a cycle, control the degradation of the surface portion of the negative electrode, and improve the durability of the negative electrode. Therefore, a lithium secondary battery to which the charge-discharge method of a lithium secondary battery according to an embodiment of the present invention is applied has an effect that the discharge capacity, initial efficiency, resistance performance, and / or life characteristics are improved.

Brief Description of Drawings

[0010]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0011] Hereinafter, the present specification will be described in more detail. In the present specification, when a certain part "includes" a certain component, this means that, unless otherwise stated, it does not exclude other components, but may further include other components.

[0012] In the present specification, when a certain member is located "on" another member, this includes not only the case where a certain member is in contact with another member, but also the case where there is another member between the two members.

[0013] The terms and words used in this specification should not be construed as being limited to their ordinary or dictionary meanings. Instead, in accordance with the principle that the inventor can appropriately define the concept of a term in order to best explain his or her invention, they should be construed in a meaning and concept suitable for the technical idea of the present invention.

[0014] The singular forms of the terms used in this specification include the plural forms unless the context clearly dictates 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. The X-ray diffraction analysis may be performed using an X-ray diffraction (XRD) analyzer (product name: D4-endeavor, manufacturer: bruker), and other devices used in the art may be appropriately adopted in addition to the said device.

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

[0017] In this specification, the average particle size (D 50 ) can be defined as the particle size corresponding to 50% of the volume cumulative amount in the particle size distribution curve (graph curve of the particle size distribution diagram). The average particle size (D 50 ) can be measured, for example, using the laser diffraction method. The laser diffraction method can usually measure particle sizes in the submicron region to about several mm, and results with high reproducibility and high resolution can be obtained.

[0018] Hereinafter, preferred embodiments of the present invention will be described in detail. However, the 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 stage of manufacturing a lithium secondary battery> One embodiment of the present invention includes 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 3.0 V to 3.5 V after charging N1 times; and a second cycle step of performing a second cycle of fully discharging the lithium secondary battery N2 times after charging, where N1 / N2 is greater than 25 and less than 100, and provides a method for charging and discharging a lithium secondary battery.

[0020] Generally, in the case of a secondary battery containing a silicon-based active material, cracks and volume changes on the surface portion are large. In particular, when adopting a method of transferring lithium to the negative electrode as one of the pre-lithiation methods, the deterioration of the negative electrode surface portion becomes more serious. However, when evaluating the lifespan, there is a problem that the degradation of the surface portion is accelerated earlier, inducing a sudden drop.

[0021] Also, when charging and discharging a secondary battery containing a silicon-based active material, when performing charging and discharging with a limited cycle of partially discharging (3.0 V to 3.5 V) after charging, irreversible LixSiy (x, y are real numbers) is generated, and capacity manifestation is not properly performed 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 is temporarily decreased and recovered repeatedly appears.

[0022] On the other hand, the present invention can eliminate the generation of an irreversible phase in the negative electrode in real time by repeating the first cycle, control the deterioration of the negative electrode surface portion, and improve the durability of the negative electrode by adjusting, during charging and discharging of a lithium secondary battery, a limited cycle of partially discharging (3.0 V to 3.5 V) after charging, for example, repeating a first cycle of discharging under conditions of 3.0 V to 3.5 V after charging a predetermined number of times, and then performing a cycle of fully discharging (SOC 0%) after charging, for example, performing a second cycle of fully discharging after charging a predetermined number of times.

[0023] Specifically, the first cycle is repeatedly executed N1 times, and the second cycle is executed N2 times. At this time, the number of executions of each cycle is adjusted so that N1 / N2 is greater than 25 and less than 100.

[0024] The second cycle, for example, by inducing delithiation / lithiation in the silicon usage range at the discharge end by a full discharge (SOC 0%) cycle to reduce the resistance of the negative electrode, can eliminate the irreversible phase generated by the first cycle and alleviate the degree to which the capacity is temporarily reduced.

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

[0026] The charge / discharge method of the lithium secondary battery of the present invention includes a step of manufacturing a lithium secondary battery having a negative electrode containing a silicon-based active material.

[0027] The negative electrode contains a silicon-based active material. By optimally adjusting the number of times of reaching SOC 0% during the cycle, the generation of an irreversible phase in the negative electrode in real time is eliminated. Instead, the degradation of the surface portion of the negative electrode generated by the volume expansion due to the charge / discharge of the silicon-based active material can be controlled, the durability of the negative electrode can be improved, and the high capacity and energy density of the silicon-based active material can be suitably exhibited.

[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 a step of forming a negative electrode active material layer on one or both surfaces of a 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 / contraction degree due to charge and 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 the 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 contain 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 contain 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, 5.5 μm to 8 μm, and more specifically, 6 μm to 7 μm. When the average particle size is within the above range, the specific surface area of the particles is within an appropriate range, and the viscosity of the negative electrode slurry is formed within an appropriate range. As a result, the dispersion of the particles constituting the negative electrode slurry becomes smooth. In addition, when 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 negative electrode surface is smoothly formed, thereby preventing the non-uniform current density phenomenon 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 2It is / g. The BET specific surface area is measured according to DIN 66131 (using nitrogen).

[0036] In one embodiment of the present application, the silicon-based active material may exist, for example, in crystalline or amorphous form, and preferably is not porous. The silicon particles are preferably spherical or multi-piece particles. Alternatively, the silicon particles may have a fibrous structure or may exist in the form of a thin film or coating containing silicon, but this is not so 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 a chemical change in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, those surface-treated with carbon, nickel, titanium, silver, etc. on the surface of copper or stainless steel, aluminum-cadmium alloy, etc. may be used. Also, fine irregularities may be formed on the surface to strengthen the binding force of the negative electrode active material, and it may be used in various forms such as a film, sheet, foil, net, porous body, foam, non-woven fabric body, etc.

[0038] In one embodiment of the present invention, the thickness of the negative electrode current collector layer is 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 diversely depending on the type and use of the negative electrode used, and is not limited thereto.

[0039] In one embodiment of the present invention, the step of forming the negative electrode active material layer on one or both sides of the 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 the negative electrode active material layer composition, 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.

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

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

[0042] In yet another embodiment, the content of the solid component of the negative electrode slurry can satisfy the range of 5% or more and 40% or less, preferably 7% or more and 35% or less, and more preferably 10% or more and 30% or less.

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

[0044] When the content of the solid component of the negative electrode slurry satisfies the above range, during the formation of the negative electrode active material layer, the viscosity is appropriate, the aggregation phenomenon of the particles of the negative electrode active material layer composition is minimized, and the negative electrode active material layer can be efficiently formed.

[0045] In one embodiment of the present invention, the slurry solvent is not limited as long as it can dissolve the above-mentioned negative electrode active material layer composition, and specifically, distilled water may be used.

[0046] The 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 slurry solvent in the negative electrode slurry can be dried by the drying step.

[0047] In one 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.

[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 various performances 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 contain 70 parts by weight or more, preferably 80 parts by weight or more, more preferably 90 parts by weight or more, and may contain 100 parts by weight or less, preferably 99 parts by weight or less, more preferably 95 parts by weight or less.

[0050] In one embodiment of the present invention, the silicon-based active material may be, in particular, pure silicon (Si) used as the silicon-based active material. 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 can mean containing pure Si particles (SiOx (x = 0)) not bonded to other particles or elements within the above range. 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 contained 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 contained 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 contained in an amount of 60 parts by weight or more, preferably 65 parts by weight or more, more preferably 70 parts by weight or more, and may be contained in an amount of 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 the present application contains together a silicon-based active material with a significantly high capacity, a negative electrode conductive material, and a negative electrode binder that can control the volume expansion rate during the charge and discharge process even when used within the above range, and has the characteristic of not degrading the performance of the negative electrode even when contained within the above range and having excellent output characteristics during charging and discharging.

[0055] In one embodiment of the present application, the silicon-based active material may have a non-spherical form, and its 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 the present application, the sphericity is determined by the following formula A, where A is the area and P is the boundary line. [Formula A] 4πA / P 2

[0057] Conventionally, it has been common to use only graphite-based compounds as the negative electrode active material. However, recently, as the demand for high-capacity batteries has increased, in order to increase the capacity, attempts have been increasing to use a silicon-based compound in combination. However, in the case of a silicon-based compound, there is a limit in that the volume rapidly expands during the charge / discharge process, cutting off the conductive path formed in the negative electrode active material layer and actually degrading the performance of the battery. Therefore, the type of negative electrode conductive material used together with the silicon-based active material is important.

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

[0059] In one embodiment of the present invention, the dot-shaped conductive material can be used to improve the conductivity of the negative electrode, and means a conductive material that does not induce a chemical change and has conductivity. Specifically, the dot-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. Preferably, it may contain carbon black in terms of realizing high conductivity and excellent dispersibility.

[0060] In one embodiment of the present invention, the dot-shaped conductive material has a BET specific surface area of 40 m 2 / g or more and 70 m 2 / g or less, preferably 45 m 2 / g or more and 65 m 2 / g or less, more preferably 50 m 2 / g or more and 60 m 2 / g or less.

[0061] In one 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, 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 planar conductive material can play a role in increasing the surface contact between silicon particles in the negative electrode to improve conductivity, and at the same time suppressing the interruption of the conductive path due to volume expansion. It is used in the concept including a bulk-type conductive material or a plate-like conductive material.

[0063] In one 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 preferably may be plate-like graphite.

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

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

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

[0067] In one embodiment of the present invention, as the planar conductive material, a high specific surface area planar conductive material; or a low specific surface area planar conductive material may be used without limitation. However, in particular, 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 low specific surface area planar conductive material that does not cause problems in dispersion.

[0068] In one embodiment of the present invention, the planar conductive material may have a BET specific surface area of 5 m 2 / g or more.

[0069] In another embodiment, the planar conductive material may have a BET specific surface area of 5 m 2 / g or more and 500 m 2 / g or less, preferably 5 m 2 / g or more and 300 m 2 / g or less, more preferably 5 m 2 / g or more and 250 m 2 / g or less.

[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 m 2 / g or more and 500 m 2 / g or less, preferably 80 m 2 / g or more and 300 m 2 / g or less, more preferably 100 m 2 / g or more and 300 m 2 / g or less can be satisfied.

[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 5 m 2 / g or more and 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 or more and 25 m 2 / g or less can be satisfied.

[0072] In addition, as the conductive material, there may be a linear conductive material such as a carbon nanotube. The carbon nanotube may be a bundled carbon nanotube. The bundled carbon nanotube may include a plurality of carbon nanotube units. Specifically, here, 'bundle type' refers to a secondary shape in which a plurality of carbon nanotube units are arranged in parallel with substantially the same orientation of the longitudinal axis of the carbon nanotube unit or are twisted into a bundle or rope shape, unless otherwise mentioned. The carbon nanotube unit has a cylinder shape with a nanosize diameter and a sp2 bonding structure. At this time, depending on the winding angle and structure of the graphite sheet, the characteristics of a conductor or a semiconductor can be exhibited. The bundled carbon nanotube can be uniformly dispersed during the manufacture of the negative electrode compared to the entangled type carbon nanotube, and the conductive network in the negative electrode can be smoothly formed, and the conductivity of the negative electrode can be improved.

[0073] In one embodiment of the present invention, 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.

[0074] In another embodiment, the negative electrode conductive material may contain 10 parts by weight or more and 40 parts by weight or less, preferably 10 parts by weight or more and 30 parts by weight or less, and more preferably 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.

[0075] In the case of the negative electrode conductive material according to the present application, it has a completely different configuration from the conductive material applied to the positive electrode. That is, in the case of the negative electrode conductive material according to the present invention, it plays a role of controlling the contact points between silicon-based active materials with a very large volume expansion of the electrode during charging and discharging. The positive electrode conductive material plays a role of imparting partial conductivity while acting as a buffer with a buffering effect when rolled, and the configuration and role of the negative electrode conductive material of the present application are completely different.

[0076] In addition, the negative electrode conductive material according to the present invention is applied to silicon-based active materials and has a completely different configuration from the conductive material applied to graphite-based active materials. That is, the conductive material used in an electrode having a graphite-based active material simply has smaller particles than the active material, and thus has the characteristics of improving output characteristics and imparting partial conductivity, and is completely different in configuration and role from the negative electrode conductive material 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 above-mentioned negative electrode conductive material generally has a structure and role different from those of the carbon-based active material 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 means a material processed into a spherical or dot-like form in order to facilitate the storage and release of lithium ions.

[0078] On the one hand, the sheet-like conductive material used as the negative electrode conductive material is a substance having a sheet or plate form, and can be represented by plate-like graphite. That is, it is a substance contained to maintain a conductive path within the negative electrode active material layer, and means a substance for ensuring a conductive path in a planar form inside the negative electrode active material layer that does not play a role in the storage and release of lithium.

[0079] That is, in the present invention, the fact that plate-like graphite is used as the conductive material means that it has been processed into a sheet-like or plate-like form and is used as a substance for ensuring a conductive path without playing a role in the storage or release of lithium. At this time, the negative electrode active material contained together has high capacity characteristics for the storage and release of lithium and plays a role of being able to store and release all lithium ions transmitted from the positive electrode.

[0080] On the other hand, in the present application, the fact that a carbon-based active material is used as the active material means that it has been processed into a dot-like or spherical form and is used as a substance for playing a role in storing or releasing lithium.

[0081] That is, in one embodiment of the present invention, artificial graphite or natural graphite, which is a carbon-based active material, can satisfy the range of a BET specific surface area of 0.1 m 2 / g or more and 4.5 m 2 / g or less. Also, the plate-like graphite, which is a sheet-like conductive material, may be sheet-like and have a BET specific surface area of 5 m 2 / g or more.

[0082] In one embodiment of the present application, the negative electrode binder may include at least one selected from the group consisting of polyvinylidene fluoride - hexafluoropropylene copolymer (PVDF - co - HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene - propylene - diene monomer (EPDM), sulfonated EPDM, styrene - butadiene rubber (SBR), fluororubber, poly acrylic acid, and substances in which hydrogen thereof is substituted with Li, Na, Ca, etc., and may also include various copolymers thereof.

[0083] The negative electrode binder according to one embodiment of the present application plays a role in controlling the negative electrode active material and the negative electrode conductive material in order to prevent the twisting and structural deformation of the negative electrode structure during the volume expansion and relaxation of the silicon - based active material. If the above role is satisfied, any general negative electrode binder can be applied. 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, based on 100 parts by weight of the negative electrode active material layer composition, it may include 30 parts by weight or less of the negative electrode binder, preferably 25 parts by weight or less, more preferably 20 parts by weight or less, and may also include 1 part by weight or more, 3 parts by weight or more.

[0085] In one embodiment of the present invention, the negative electrode may be pre - lithiated. 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 a step of pre - lithiating the negative electrode containing the silicon - based active material.

[0086] Specifically, the step of pre-lithiating the negative electrode containing the silicon-based active material may include: forming a negative electrode active material layer on one or both surfaces 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 onto the negative electrode active material layer; and removing the base film. This relates to a method for pre-lithiating an electrode for a lithium secondary battery using a lithium metal transfer process.

[0087] In the charge-discharge reaction of a lithium secondary battery, during charging, lithium released from the positive electrode is inserted into the negative electrode, and during discharging, it desorbs from the negative electrode and returns to the positive electrode again. However, in the case of a silicon-based negative electrode active material, volume change and surface side reactions are serious, and among the lithium inserted into the negative electrode during the initial charging, the amount that returns to the positive electrode again is small, resulting in a problem of a large initial irreversible capacity. When the initial irreversible capacity increases, problems such as a sharp decrease in battery capacity and cycles occur.

[0088] When adopting the charge-discharge method of the secondary battery according to the present invention, 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 a secondary battery with a pre-lithiated negative electrode, the durability of the battery can be improved more effectively.

[0089] Generally, the pre-lithiation process may be performed by chemically or physically pre-lithiating a lithium metal layer on the electrode. Specifically, it may be performed by a lithium metal transfer process, lithium metal powder vapor deposition, an electro / chemical process, or a lithium metal vapor deposition process. The pre-lithiation process according to the present invention may include a lithium metal transfer process.

[0090] In the case of the lithium metal layer transfer process, it has the characteristic that the highly reactive lithium metal can be transferred more stably onto the negative electrode active material layer. At this time, a process capable of easily transferring the lithium metal layer from the transfer laminate onto the negative electrode active material layer is required.

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

[0092] In one embodiment of the present invention, as the vapor deposition method for vapor-depositing the lithium metal layer on the base film, it may be selected from among vacuum evaporation deposition, chemical vapor deposition, CVD (chemical vapor deposition), and physical vapor deposition, but is not limited thereto, and various vapor deposition methods used in the art may be used.

[0093] At this time, a load of 5 kgf / cm 2 to 500 kgf / cm 2 may be applied to the electrode for a lithium secondary battery on which the transfer laminate is laminated, and a lamination process may be performed by roll pressing. Thereafter, a step of removing the base film is included.

[0094] In one embodiment of the present invention, the base film may be used without limitation as long as it can withstand process conditions such as a high temperature in the step of vapor-depositing lithium metal and can prevent the problem of reverse peeling in which the lithium metal is transferred onto the base film in the winding process for transferring the vapor-deposited lithium metal.

[0095] Specifically, in one embodiment of the present invention, the base film may be at least one selected from the group consisting of polyethylene terephthalate (PET), polyimide (PI), poly(methylmethacrylate) (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 may satisfy the range of 5 μm or more and 200 μm or less, or 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 may satisfy 3 μm or more and 10 μm or less.

[0098] By satisfying the above ranges for the thicknesses of the base film and the lithium metal, the transfer of the lithium metal to the negative electrode active material layer side can be efficiently performed, and the characteristic of preventing reverse transfer can be achieved.

[0099] In one embodiment of the present invention, in order to improve the peelability of the lithium metal layer, ensure the transferability to the electrode active material layer, and serve as a protective layer after the transfer of the lithium metal, the surface of the base film of the transfer laminate in contact with the lithium metal layer may include a transfer force improvement layer.

[0100] That is, the base film may have a transfer force improving layer formed on at least one surface. Specifically, it may have a transfer force improving layer formed on one surface or both surfaces. In the winding process for transferring the lithium metal layer vapor-deposited by the transfer force improving layer to the electrode, the problem of reverse peeling where the lithium metal layer is transferred onto the base film can be prevented, and after the lithium metal layer is transferred onto the electrode active material layer, the base film can be easily separated.

[0101] The transfer force improving layer may contain one or more selected from the group consisting of a silicon-modified polyester in which a silicon chain is graft-bonded to a polyester main chain, an acrylic resin, Si, melamine, and fluorine.

[0102] In one embodiment of the present invention, the transfer force improving layer may contain poly(methyl methacrylate) (PMMA).

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

[0104] In one embodiment of the present invention, the step of transferring the lithium metal layer onto 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 onto the upper part of the negative electrode active material layer may include a step of laminating the transfer laminate onto the electrode active material layer such that the surface of the lithium metal layer opposite to the surface in contact with the transfer force improving layer contacts the surface of the electrode active material layer opposite to the surface in contact with the electrode current collector layer.

[0106] At this time, the laminating step may be performed under temperature conditions of 20°C to 90°C and a pressure condition of 5 kgf / cm 2 to 500 kgf / cm 2 .

[0107] In one embodiment of the present invention, the laminating step can satisfy a pressure condition of 5 kgf / cm 2 to 500 kgf / cm 2 , preferably, a pressure condition of 10 kgf / cm 2 to 150 kgf / cm 2 .

[0108] However, the temperature condition may be omitted in the laminating step. In particular, when the laminating step satisfies the above pressure condition, the prelithiation rate of the lithium metal can be smoothly adjusted, thereby suppressing the generation of a large amount of oxides and nitrides in the transfer process. Further, by satisfying the above pressure range, the lithium metal is smoothly carried out on the upper part of the electrode active material layer and no reverse transfer problem occurs.

[0109] After the laminating step, prelithiation of the reactive lithium metal and the electrode active material layer may be performed, or the reaction may not be performed and prelithiation may be performed during battery assembly.

[0110] In one embodiment of the present invention, the step of prelithiating the negative electrode may further include a step of prelithiating the negative electrode active material layer after removing the base material film, and the step of prelithiating the negative electrode active material layer may prelithiate the negative electrode 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 transfer force improvement layer and the lithium metal are transferred together onto the upper part of the electrode active material layer. At this time, the transfer force improvement layer can serve as a protective layer that can prevent the reaction of the highly reactive lithium metal in the air.

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

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

[0114] The pre-lithiation step is a step of setting conditions for the diffusion of lithium metal into the negative electrode active material layer. Whether the pre-lithiation is completed can be determined by whether 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 including a positive electrode, a negative electrode, a separator provided between the positive electrode and the negative electrode, and an electrolyte.

[0116] At this time, when the negative electrode is a pre-lithiated negative electrode, the transfer force improvement layer used during pre-lithiation may be removed, thereby preventing an unnecessary increase in resistance without remaining on the upper part of the electrode. That is, the transfer force improvement layer may be used to improve the transfer force and also protect the lithium metal layer before pre-lithiation, and may be removed before injecting the electrolyte.

[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 surfaces 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, fired carbon, or those obtained by surface-treating the surface of aluminum or stainless steel with carbon, nickel, titanium, silver, etc. may be used. Further, the positive electrode current collector layer may usually have a thickness of 3 μm to 500 μm, and fine irregularities may be formed on the surface of the positive electrode current collector layer to enhance the adhesion of the positive electrode active material. For example, it may be used in various forms such as films, sheets, foils, nets, porous bodies, foams, non-woven fabrics, etc.

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

[0120] In one embodiment of the present invention, the step of forming the positive electrode active material layer on one or both sides of the 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 the positive electrode active material layer composition, and 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.

[0121] In one embodiment of the present invention, the content for the positive electrode slurry may be the same as the content for the negative electrode slurry described above, and there is only a difference in that it is a positive electrode.

[0122] In one 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.

[0123] The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material is a layered compound such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; lithium iron oxide such as LiFe3O4; 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 oxides represented by Mc2O2 (where M is at least any one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, and satisfies 0.01≦c2≦0.5); chemical formula LiMn 2-c3 M c3 O2 (where M is at least any one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, and satisfies 0.01≦c3≦0.1) or lithium manganese composite oxides represented by Li2Mn3MO8 (where M is at least any one selected from the group consisting of Fe, Co, Ni, Cu, and Zn). Examples include, but are not limited to, LiMn2O4 in which part of the Li in the chemical formula is substituted with alkaline earth metal ions. 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] In addition, the positive electrode may contain a sacrificial positive electrode material, and the types thereof may be used without limitation as those used in the art.

[0126] At this time, the positive electrode conductive material is used to impart conductivity to the electrode, and in the battery to be configured, it can be used without particular limitation as long as it has electron conductivity without causing a chemical change. Specific examples include graphite such as natural graphite and artificial graphite; carbon-based substances 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. Among these, one kind alone or a mixture of two or more kinds may be used.

[0127] In addition, the positive electrode binder plays a role in improving the adhesion between the positive electrode active material particles and the adhesive force 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, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene - propylene - diene polymer (EPDM), sulfonated EPDM, styrene - butadiene rubber (SBR), fluororubber, or various copolymers thereof. Among these, one kind alone or a mixture of two or more kinds may be used.

[0128] In addition, the contents of the positive electrode active material, the positive electrode conductive material, and the positive electrode binder included in the positive electrode active material layer composition may be applied in the same way as the content applied to the negative electrode active material layer composition described above.

[0129] In one embodiment of the present invention, examples of the electrolytic solution include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries. Specifically, the electrolyte may contain a non-aqueous organic solvent and a metal salt.

[0130] Examples of the non-aqueous organic solvent include aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triphosphate ester, trimethoxymethane, dioxolane derivative, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ether, methyl propionate, and ethyl propionate.

[0131] In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are organic solvents with high viscosity, high dielectric constant, and good dissociation of lithium salts, so they can be preferably used. By mixing linear carbonates with low viscosity and low dielectric constant such as dimethyl carbonate and diethyl carbonate with such cyclic carbonates in an appropriate ratio, an electrolyte with high electrical conductivity can be produced, so it can be more preferably used.

[0132] A lithium salt may be used as the metal salt. The lithium salt is a substance that is easily dissolved in the non-aqueous electrolyte solution. For example, as the anion of the lithium salt, F - , Cl - , I - ,, NO3 -, N(CN)2 - , BF4 - , ClO4 - , PF6 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - One or more selected from the group consisting of may be used.

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

[0134] <Charge - discharge stage of lithium secondary battery> The charge and discharge method of a lithium secondary battery according to an embodiment of the present invention includes: a first cycle stage in which a first cycle of discharging the lithium secondary battery under the condition of 3.0V to 3.5V after charging is repeated N1 times; and a second cycle stage in which a second cycle of completely discharging the lithium secondary battery after charging is performed N2 times, where N1 / N2 may be greater than 25 and less than 100.

[0135] Specifically, the secondary battery can be driven by the electrochemical charge and discharge. According to the charge and discharge method of the secondary battery of the present invention, by using the negative electrode containing the silicon-based active material, the initial efficiency and life performance of the secondary battery can be improved. Also, by optimally adjusting the number of times of reaching SOC0% (complete discharge) during the cycles of the aforementioned secondary battery, while preventing performance degradation due to volume expansion of the silicon-based active material and collapse of the electrode structure, the generation of irreversible phases in the negative electrode can be controlled, the deterioration of the negative electrode surface can be controlled, and the life performance of the secondary battery can be improved to a remarkable level.

[0136] Therefore, a lithium secondary battery to which the charge and discharge method of a lithium secondary battery according to an embodiment of the present invention is applied has the effect that the discharge capacity, initial efficiency, resistance performance, and / or life characteristics are improved.

[0137] In this specification, "SOC" can mean the remaining capacity (State of Charge) of an electrode (positive electrode or negative electrode) or a secondary battery. The SOC after charging may be the SOC at the time when the electrochemical charge is completed during the driving of the secondary battery, and the SOC after discharge may be the SOC at the time when the electrochemical discharge is completed during the driving of the secondary battery. That is, SOC100% after charging means that the remaining capacity at the time when the charging of the secondary battery is completed is 100%, and in this case, it means a fully charged state. SOC65% after discharge means that the remaining capacity at the time when the discharge of the secondary battery is completed is 65%. SOC0% after discharge means that the remaining capacity at the time when the discharge of the secondary battery is completed is 0%, and in this case, it means a fully discharged state.

[0138] In one embodiment of the present invention, for the lithium secondary battery, the first cycle of discharging under the conditions of 3.0 V to 3.5 V after charging may be repeated N1 times. Then, the second cycle of fully discharging after charging may be performed N2 times. At this time, N1 and N2 may each be an integer of 1 or more.

[0139] In one example, when N1 is 25 and N2 is 1, after repeating the first cycle (restricted cycle) 25 times, the second cycle (full discharge cycle) can be performed once.

[0140] In one embodiment of the present invention, the first cycle stage and the second cycle stage may be repeatedly executed. In one example, when N1 is 25 and N2 is 1, after repeating the first cycle (restricted cycle) 25 times, the second cycle (full discharge cycle) may be executed once, and then, after repeating the first cycle 25 more times, the stage of performing the second cycle (full discharge cycle) once may be repeatedly executed.

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

[0142] In one example, the discharging may be performed under constant current conditions up to a specific SOC at 0.5C, and may be performed so as to be cut-off at the discharging voltage according to the specific SOC.

[0143] At this time, the charge and discharge conditions and the C rate are not limited to those described above, and a method used in the art may be appropriately applied.

[0144] In one embodiment of the present invention, the charging may be performed at SOC 95% to SOC 100%. For example, the charging may be 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 SOC50% to SOC90%. Specifically, the voltage at the end of the discharge may be 3.0V to 3.5V, and more specifically, it may be 3.25V (SOC65%).

[0146] In one example, in the second cycle, the full discharge is performed until SOC0%, and specifically, the voltage at the end of the discharge may be 2.5V.

[0147] The first cycle and the second cycle may be performed under conditions each having a rest time of 25°C and 20 minutes.

[0148] For the lithium secondary battery, when performing N2 full charge / discharge cycles after N1 limited cycles, the formation of the irreversible phase can be effectively controlled. At this time, the irreversible phase in the negative electrode decreases only when the voltage under the full discharge (SOC0%) condition (for example, 2.5V) is reached, and the deterioration of the surface part can be easily controlled. If the full discharge condition cannot be reached, there is a problem that it is not easy to control the irreversible phase.

[0149] Also, when the full charge / discharge cycle is performed excessively, the number of full charge / discharge cycles in the overall cycle increases, and a large amount of Si at the lower part of the negative electrode active material layer is used, so there is a problem that the volume expansion of the negative electrode becomes serious and the durability of the negative electrode decreases.

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

[0151] When the N1 / N2 satisfies the above-mentioned range, the generation of the irreversible phase in the negative electrode can be controlled in real time, the deterioration of the negative electrode surface part can be controlled, and the life performance of the secondary battery can be improved to a remarkable level.

[0152] On the one hand, when N1 / N2 is 25 or less, the number of complete charge / discharge cycles in the overall cycle increases, and the volume expansion of the negative electrode becomes severe, which has the problem of accelerating the deterioration of the negative electrode. When N1 / N2 is 100 or more, there is a problem that the irreversible phase is not eliminated and the use of the negative electrode becomes non-uniform.

[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, the complete charge / discharge cycle is excessively performed, and a large amount of the lower end of Si in the negative electrode active material layer is used, resulting in severe volume expansion of the negative electrode and a problem of reduced durability of the negative electrode. When N1 is above the above range, there is a problem that the irreversible phase is not eliminated in real time and the use of the negative electrode becomes non-uniform.

[0154] In one embodiment of the present invention, N2 may be 1 or more and 3 or less. Specifically, N2 may be 1. When N2 exceeds the above range, the complete charge / discharge cycle is excessively performed, and a large amount of Si at the lower part of the negative electrode active material layer is used, resulting in severe volume expansion of the negative electrode and a problem of reduced durability of the negative electrode.

[0155] The electrochemical charging of the secondary battery may be performed by employing an electrochemical charge / discharge device. Specifically, as the electrochemical charge / discharge device, WOCS3000s (manufactured by Won A Tech Co., Ltd.) may be employed.

[0156] Moreover, one embodiment of the present invention provides a battery system including the secondary battery. Specifically, the charge / discharge method of the secondary battery described above can be implemented 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 composed of one secondary battery or a secondary battery module which is an aggregate of a plurality of secondary batteries. The battery system may include a control unit together with the secondary battery.

[0158] The control unit can set the SOC and driving voltage range of the secondary battery during the electrochemical charging and discharging of the secondary battery. As a result, the electrochemical charging and discharging of the secondary battery can be performed within the electrochemical charge-discharge range and SOC of the secondary battery set by the control unit, and the charge / discharge SOC and the number of times can be adjusted.

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

[0160] The present invention also provides a battery pack including the aforementioned battery system. In addition to the aforementioned secondary battery and control unit, the battery pack may further include known configurations in the art, such as a BMS (Battery Management System), a cooling system, etc.

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

Example

[0162] Hereinafter, in order to specifically describe this specification, examples will be given and described in detail. However, the examples according to this specification may be deformed into 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 more fully explain this specification to those with average knowledge in the industry.

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

[0164] On one side of a copper current collector (thickness: 15 μm) as the negative electrode current collector, the negative electrode slurry was coated at a loading amount of 120 mg / 25 cm 2 and 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 made into a negative electrode (negative electrode before pre-lithiation).

[0165] 2) Pre-lithiation of negative electrode Lithium metal having a thickness of 6.2 μm was transferred to the negative electrode using a rolling (Roll press) equipment.

[0166] Specifically, electrochemical charging was performed at a current density of 1.1 mA / cm 2 so as to be 10% of the charge capacity of the negative electrode before performing the pre-lithiation.

[0167] 3) Production of positive electrode As the positive electrode active material, Li[Ni 0.8 Co 0.1 Mn 0.1O2, carbon black (product name: Super C65, manufacturer: Timcal) as the conductive material, and polyvinylidene fluoride (PVdF) as the binder were added to N-methyl-2-pyrrolidone (NMP) as the solvent for forming the positive electrode slurry at a weight ratio of 97:1.5:1.5 to produce a positive electrode slurry (solid content concentration: 78 wt%).

[0168] The positive electrode slurry was coated on both sides of an aluminum current collector (thickness: 12 μm) as the positive electrode current collector at a loading of 620 mg / 25 cm 2 and rolled (roll press), and dried in a vacuum oven at 130 °C for 10 hours to form a positive electrode active material layer, thereby producing a positive electrode.

[0169] 4) Fabrication of secondary battery Two of the pre-lithiated negative electrodes were respectively disposed on both sides of the positive electrode fabricated above, a polypropylene separator was interposed between the positive electrode and the negative electrode, and an electrolyte was injected to fabricate a secondary battery in the form of a bi-cell. The electrolyte used was a mixture of fluoroethylene carbonate (FEC) and diethyl carbonate (DEC) at a volume ratio of 30:70, with vinylene carbonate added at 3 wt% based on the total weight of the electrolyte, and LiPF6 added at a concentration of 1 M as the lithium salt.

[0170] Production Example 2 A lithium secondary battery was fabricated in the same manner as in Production Example 1, except that the negative electrode was not pre-lithiated.

[0171] <Experimental Example 1: Cycle performance evaluation> The lithium secondary batteries fabricated in Production Examples 1 and 2 were charged and discharged to the set SOC in Table 1 below under driving conditions of 4.2 V - 2.5 V and cycle conditions of 4.2 V - 3.2 V for N1 cycles, and then N2 complete discharge cycles were performed to drive the secondary batteries.

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

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

[0174] - Capacity retention rate (%) = {(Discharge capacity at the 600th cycle) / (Discharge capacity at the first cycle)} × 100 (Full charge and full discharge capacity ratio (4.2V - 2.5V) before and after 600 cycles) - Point of sudden drop = Number of cycles at which the real-time capacity retention rate reaches 50% <Electrochemical charging and electrochemical discharging conditions> - Charging: Charge at 1C to the charging SOC of the secondary battery in Table 1 in CC / CV mode (cut off at 0.05C) - Discharging: Discharge at 0.5C to the discharging SOC of the secondary battery in Table 1 in CC mode (cut-off at the discharge voltage according to each SOC range)

[0175]

Table 1

[0176] When applying the charge and discharge method of a lithium secondary battery as in the present invention, by optimally adjusting the number of times reaching SOC 0% (fully discharged) during cycling, the generation of irreversible phases in real time is eliminated, the deterioration of the surface portion of the negative electrode is controlled, and the durability of the negative electrode can be improved.

[0177] According to Table 1 above, in the case of Examples 1-1 and 2-1 where a charge-discharge method of repeating the first cycle 50 times and then performing a full discharge cycle once (N1 / N2 satisfies 50) is applied to a lithium secondary battery having a negative electrode containing a silicon-based active material, it was confirmed that the capacity retention rate was excellent and the number of cycles until a sudden drop occurred was large. On the other hand, in the case of Comparative Examples 1-1, 1-2, 2-1, and 2-2 where the first cycle was repeated 25 times and then a full discharge cycle was performed once (N1 / N2 is 25), or the first cycle was repeated 100 times and then a full discharge cycle was performed once (N1 / N2 is 100), it was confirmed that the number of cycles until a sudden drop occurred was small and the deterioration of the lithium secondary battery was accelerated compared to Examples 1-1 and 2-1.

[0178] Therefore, a lithium secondary battery to which the charge-discharge method of one embodiment of the present invention is applied has an effect that the durability of the negative electrode is significantly improved.

[0179] <Experimental Example 2: Evaluation of Capacity and Discharge Resistance Characteristics> <Example 1-2> The charge-discharge method was applied in the same manner as in Example 1-1 of Table 1 above, except that the lithium secondary battery manufactured in Production Example 1 was charged and discharged under cycle conditions of 4.2V - 3.3V.

[0180] <Comparative Example 1-3> Also, the charge-discharge method was applied in the same manner as in Example 1-1 of Table 1 above, except that the lithium secondary battery manufactured in Production Example 1 was not subjected to any full discharge cycles under cycle conditions of 4.2V - 3.3V.

[0181] In the lithium secondary batteries of Example 1-2 and Comparative Example 1-3, a graph showing the capacity characteristics with the progress of the cycle is shown in FIG. 2, and a graph showing the discharge resistance (Rdis) value over time is shown in FIG. 3.

[0182] Figure 2 shows the measurement of the capacity under the conditions of 4.2V - 2.5V every 50 cycles under electrochemical charging and electrochemical discharging conditions at 0.33C / 0.33C. According to this, in the case of Examples 1 - 2, by appropriately adjusting and performing the number of complete discharge cycles, it can be confirmed that the irreversible phase is eliminated and the phenomenon that the capacity is temporarily decreased / recovered as the number of cycles increases is alleviated compared with Comparative Examples 1 - 3.

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

Claims

1. The step of manufacturing a lithium secondary battery having a negative electrode containing a silicon-based active material; For the lithium secondary battery, a first cycle in which discharging is performed under conditions of 3.0 V to 3.5 V after charging is repeated N １ times in a first cycle step; and For the lithium secondary battery, a second cycle in which a full discharge is performed after charging is carried out N ２ times; a second cycle stage comprising N １ / N ２ is a charge-discharge method of a lithium secondary battery that is greater than 25 and less than 100.

2. The above-mentioned N ２ is a charge-discharge method of the lithium secondary battery according to claim 1, where it is 1 or more and 3 or less.

3. The above-mentioned N １ is greater than 25 and less than 100, The above-mentioned N ２ is 1, and it is the charge and discharge method of the lithium secondary battery according to claim 1.

4. The charging and discharging method of the lithium secondary battery according to Claim 1, wherein the first cycle step and the second cycle step are repeatedly executed.

5. The charging and discharging method of the lithium secondary battery according to Claim 1, wherein the silicon-based active material contains one or more selected from the group consisting of SiO x (x = 0), SiO x (0 < x < 2), SiC, and Si alloys.

6. The charging and discharging method of the lithium secondary battery according to Claim 5, wherein based on 100 parts by weight of the silicon-based active material, it contains 70 parts by weight or more of the SiO x (x = 0).

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

8. The charging and discharging method of the lithium secondary battery according to Claim 1, wherein the negative electrode is pre-lithiated.

9. The charging and discharging method of the lithium secondary battery according to any one of Claims 1 to 8, 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-lithiating the negative electrode containing the silicon-based active material.

10. The step of pre-lithiating the negative electrode containing the silicon-based active material includes The step of forming a negative electrode active material layer on one or both sides of the negative electrode current collector layer; The step of preparing a transfer laminate in which a base film and a lithium metal layer are sequentially laminated; The step of transferring the lithium metal layer onto the negative electrode active material layer; and The step of removing the base film; The charging and discharging method of the lithium secondary battery according to Claim 9, comprising.

11. The charging and discharging method of the lithium secondary battery according to Claim 10, wherein the base film has a transfer force improving layer formed on at least one surface.

12. The charging and discharging method of the lithium secondary battery according to Claim 10, including the step of laminating the transfer laminate onto the negative electrode active material layer.

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