Secondary battery manufacturing method

Abstract

The method for manufacturing a battery according to the present invention can reduce the number of charge / discharge cycles during the activation process and shorten the time required for the battery manufacturing process by adding a CV section. Furthermore, the method for manufacturing a battery can improve the electrochemical performance of the battery, such as improving battery capacity, by converting the remaining LiPS that was not completely phase-changed due to overvoltage to its final form to the maximum extent possible, reducing overvoltage, and uniformly distributing the LiPS around the positive electrode.

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a secondary battery, and more particularly to a method for initial discharge in a battery activation process.

[0002] This application claims priority based on Korean Patent Application No. 10-2022-0155730, filed on November 18, 2022, the entire contents of which are incorporated herein by reference in their entirety in the specification and drawings thereof. [Background technology]

[0003] Interest in energy storage technology has been growing in recent years. As its applications expand to include energy sources in mobile phones, camcorders, notebook PCs, and even electric vehicles, research and development of electrochemical devices such as secondary batteries has been thriving. Electrochemical devices are the field that has attracted the most attention in this area, with the development of rechargeable secondary batteries being a particular focus of attention. Recently, in the development of such batteries, research and development into new electrode and battery designs has been progressing in order to improve capacity density and specific energy.

[0004] Among these secondary batteries, lithium-sulfur batteries have been attracting attention as high-energy density secondary batteries due to their high energy density, which is achieved by using a lightweight lithium anode and a sulfur cathode with a high discharge capacity.

[0005] In a lithium-sulfur battery, sulfur serves as the starting material for the reaction during discharge, with S8 in a cyclic structure being the starting material. As the discharge progresses, it goes through stages of lithium polysulfide (LiPS: Li2S8, Li2S6, Li2S5, Li2S4), which has a linear structure of continuous reduction reactions, dissolves in the electrolyte, and moves from the positive electrode to the negative electrode, and is sequentially reduced to lower monomeric polysulfides. Such negatively charged polysulfides dissolve in a liquid organic solvent and move to the negative electrode due to the chemical potential and the concentration gradient between the positive and negative electrodes. Eventually, Li2S (S8 + 16Li → 8Li2S), an insoluble substance in the reduced state, is produced. During charging, the "shuttle" mechanism occurs by returning to the original substance S8 through an oxidation reaction in the reverse order of the above sequence. In a lithium-sulfur battery with such mechanism characteristics, the activation process before the life cycle is a core process that can suppress further resistance variables during the driving of life evaluation by evenly distributing LiPS in the electrolyte sufficiently uniformly around the positive electrode. In the above process, the shuttling of lithium polysulfide (LiPS) affects the overall performance degradation of the cell, and in particular, can have a profound impact on the degradation of the negative electrode. During the discharge process, sulfur is reduced to lithium polysulfide (lithium-polysulfides, Li2Sn, where 4 < n < 8), and the dissolution and diffusion of this intermediate product into the electrolyte cause problems. Lithium polysulfide causes a decrease in capacity due to the shuttle effect on the surface of the negative electrode, which is made of lithium metal, and part of it is reduced to lithium sulfide (Li2S2, Li2S), which is a non-conductor, interfering with the movement of active material loss and lithium ions and reducing the rate characteristics. Such side reactions lead to a decrease in Coulombic efficiency and destabilization of the negative electrode, reducing the overall performance of the battery. Therefore, it is necessary to establish a new activation process for lithium-sulfur batteries and apply it to battery manufacturing. SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[0006] The present invention aims to provide a method for manufacturing a lithium-sulfur battery to solve the above-mentioned problems, and in particular, a method for manufacturing a battery that includes a novel battery activation step.

[0007] Other objects and advantages of the present invention can be realized by the means and methods and combinations thereof recited in the claims. [Means for solving the problem]

[0008] According to a first aspect of the present invention, there is provided a method for manufacturing a lithium-sulfur battery, the method comprising: (S10) manufacturing a reserve battery including an electrode assembly and an electrolyte; (S20) an activation step of activating the backup battery; The electrode assembly includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, and the positive electrode includes a positive electrode active material containing sulfur, In step S10, the electrode assembly is immersed in the electrolyte. Step S20 includes discharging the backup battery, which is performed in the order of constant current discharge and constant voltage discharge.

[0009] According to a second aspect of the present invention, in the first aspect, in step S20, the constant current discharge is continued until a preset discharge cut-off voltage is reached, and the discharge cut-off voltage is set in the range of 1.3V to 1.8V.

[0010] According to a third aspect of the present invention, in the second aspect, in step S20, the constant current discharge is performed at a C rate set in the range of 0.05 to 2 until a preset discharge cut-off voltage is reached.

[0011] According to a fourth aspect of the present invention, in the second or third aspect, the constant voltage discharge is performed at a preset discharge cut-off voltage until the remaining current of the battery reaches a point where it is 0 mAh.

[0012] According to a fifth aspect of the present invention, in any one of the first to fourth aspects, the backup battery is not charged or discharged before step S20 is performed, and discharging is first applied to the backup battery in step S20.

[0013] According to a sixth aspect of the present invention, in any one of the second to fifth aspects, before step S20 is performed, the backup battery is not charged or discharged, and step S20 consists of only discharging the backup battery once.

[0014] According to a seventh aspect of the present invention, in any one of the first to sixth aspects, the discharge is performed less than six times in step S20.

[0015] According to an eighth aspect of the present invention, in any one of the first to eighth aspects, in step S20, the constant current discharge is performed at a C rate in the range of 0.05 to 2.00.

[0016] According to a ninth aspect of the present invention, in any one of the first to eighth aspects, in step S20, discharging is performed in a constant current manner up to a preset discharge cut-off voltage.

[0017] According to a tenth aspect of the present invention, in any one of the first to ninth aspects, in step S20, discharging is performed at a C rate in the range of 0.50 to 1.00.

[0018] According to an eleventh aspect of the present invention, in any one of the first to tenth aspects, step S20 is performed in an environment where the reserve battery is pressurized.

[0019] According to a twelfth aspect of the present invention, in the eleventh aspect, the pressure applied is equal to or higher than atmospheric pressure.

[0020] According to a thirteenth aspect of the present invention, in any one of the first to twelfth aspects, step S20 is carried out at a temperature of 15°C to 40°C.

[0021] According to a 14th aspect of the present invention, in any one of the 1st to 13th aspects, after step S10 is performed and before step S20 is performed, an aging step of maintaining the spare battery in an atmosphere of 15°C to 40°C is further performed.

[0022] According to a 15th aspect of the present invention, in any one of the 1st to 14th aspects, a charging process is performed during the discharging process in step S20, and the charging is performed in the order of constant current charging and constant voltage charging. [Effects of the Invention]

[0023] The battery manufacturing method according to the present invention can reduce the number of charge / discharge cycles during the battery activation process and shorten the time required for the battery manufacturing process by adding a CV section. Furthermore, the battery manufacturing method can improve the electrochemical performance of the battery, such as improving the battery capacity, by converting the remaining LiPS that was not completely phase-changed due to overvoltage to its final form to the maximum extent possible, thereby reducing the overvoltage and uniformly distributing the LiPS around the positive electrode.

[0024] The following drawings attached to this specification illustrate preferred embodiments of the present invention and, together with the detailed description of the invention, serve to further understand the technical concept of the present invention, so the present invention should not be interpreted as being limited to the matters depicted in the drawings. Note that the shape, size, scale, or ratio of elements in the drawings attached to this specification may be exaggerated to emphasize a clearer description. [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 10 is a graph showing a comparison of high-rate discharge characteristics of batteries according to an example and a comparative example. [Figure 2] FIG. 10 is a graph showing a comparison of coulombic efficiency characteristics of batteries according to an example and a comparative example. [Figure 3]FIG. 10 is a graph showing a comparison of power density for each SOC range of batteries according to an embodiment and a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0026] Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. Prior to this, the terms and words used in this specification and claims should not be construed as being limited to their ordinary or dictionary meanings, but should be construed as meanings and concepts corresponding to the technical concept of the present invention, in accordance with the principle that the inventor himself can appropriately define the concept of terms in order to best explain the invention. Therefore, it should be understood that the embodiments described in this specification and the configurations shown in the drawings are merely the most preferred embodiment of the present invention and do not represent the entire technical concept of the present invention, and therefore, various equivalents and modifications that can be substituted therefor may exist at the time of filing this application.

[0027] Throughout this specification, when a part is described as "including," "having," or "comprising" a certain element, it does not mean that it excludes other elements, but that it may further include other elements, unless otherwise specified.

[0028] The terms "about," "substantially," and the like used throughout this specification are used to mean a numerical value or a approximation thereof when inherent manufacturing and material tolerances are present in the stated meaning, and are used to prevent unscrupulous infringers from unfairly using disclosures in which precise or absolute numerical values ​​are recited to aid in the understanding of this application.

[0029] Throughout this specification, the phrase "A and / or B" means "A or B, or all of them."

[0030] Certain terminology used in the following detailed description is for convenience only and is not intended to be limiting. The words "right," "left," "top," and "bottom" designate directions in the drawings to which reference is made. The words "inward" and "outward" designate directions toward or away from the geometric center of the designated device, system, and components thereof, respectively. "Front," "rear," "upper," "lower," and related words and phrases designate locations and orientations in the drawings to which reference is made and are not intended to be limiting. Such terms include the example words above, derivatives thereof, and words of similar import.

[0031] <Secondary battery manufacturing method> The present invention relates to a method for manufacturing a secondary battery, and more particularly to a method for manufacturing a lithium-sulfur secondary battery.

[0032] Specifically, a method for manufacturing a lithium-sulfur secondary battery according to the present invention includes fabricating a spare battery including an electrode assembly and an electrolyte, and activating the spare battery. The method for manufacturing the battery may include a degassing step for removing gas generated during the activation step. The electrode assembly includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode. The positive electrode includes a positive electrode active material including sulfur. Prior to the activation step, the electrode assembly is impregnated with the electrolyte. The activation step of the method for manufacturing the battery according to the present invention may include discharging the spare battery, and the discharging may be performed in the order of a constant current (CC) discharge and a constant voltage (CV) discharge.

[0033] As mentioned above, by performing the CC-CV discharge step during the activation process, the remaining LiPS that did not undergo a complete phase change due to overvoltage can be converted to its final form to the maximum extent possible. This increases the capacity and ensures excellent lifespan performance compared to conventional lithium-sulfur batteries. In addition, the process time is shortened and stability is improved.

[0034] Next, each step of the method for manufacturing a lithium-sulfur battery according to the present invention will be described in detail.

[0035] <S10: Manufacture of Reserve Battery> First, a reserve battery including an electrode assembly and an electrolyte is manufactured (S10).

[0036] In the present specification, the reserve battery means a battery in a state where the formation process has not been completed. In the present invention, the formation process may mean a battery shipping preparation process including an activation and degassing process after an electrolytic solution is injected.

[0037] The electrode assembly includes a negative electrode, a positive electrode, and a separator disposed between the negative electrode and the positive electrode. Details regarding the negative electrode, the positive electrode, and the separator will be described later.

[0038] The method for manufacturing a secondary battery according to the present invention includes a step of impregnating the electrode assembly with an electrolytic solution. After the electrode assembly is impregnated with the electrolytic solution, it can be activated by discharge described later. The electrolytic solution may include an organic solvent and a lithium salt. Details regarding the organic solvent and the lithium salt will be described later.

[0039] The impregnation sufficiently wets the electrode assembly with the electrolytic solution so that activation by charging and discharging of the electrode assembly can be performed more smoothly. The impregnation of the electrolytic solution can be performed, for example, by housing the electrode assembly in a battery case, injecting the electrolytic solution into the battery case, and then maintaining it at a predetermined temperature for a predetermined time. For example, after injection of the electrolytic solution, it can be maintained for 12 hours to 100 hours, more preferably 24 hours to 75 hours, so that the electrolytic solution penetrates into the inside of the electrode assembly (aging step).

[0040] The impregnation of the electrode assembly may be performed at a temperature of 15°C to 30°C, more specifically 20°C to 27°C. When within the above range, the impregnability of the electrode assembly can be improved, and it is desirable because it can prevent the formation of dendrites in the negative electrode due to an excessively high temperature and the resulting risk of low voltage generation.

[0041] Meanwhile, in the present invention, the electrode assembly can be manufactured by a method known in the art, and the form of the electrode assembly is not particularly limited. For example, it may be a wound type, a stacked type, or a stacked / folded type.

[0042] The winding-type electrode assembly is manufactured by coating a current collector with an electrode active material, drying and washing the coated current collector, cutting the current collector into a band shape with a desired width and length, and spirally winding the resulting band with a separator sandwiched between the cathode and anode.

[0043] The stacked electrode assembly has a structure in which a plurality of positive and negative electrode unit cells are sequentially stacked, and has the advantage of easily obtaining a rectangular shape. However, it has the disadvantage of being complicated in its manufacturing process and of being susceptible to short circuits due to the electrodes becoming detached when subjected to an impact.

[0044] Therefore, an electrode assembly with an advanced structure that is a combination of the winding type and the stacking type is an electrode assembly in which full cells or bi-cells of a certain unit size are folded using a long, continuous separator film. This electrode assembly has a combined structure of the existing folding type and stacking type, and is called a "stacked / folding type electrode assembly."

[0045] The "full cell" refers to a unit cell having a positive electrode / separator / negative electrode unit structure, with a positive electrode and a negative electrode located on either side of the cell. Examples of such full cells include the most basic positive electrode / separator / negative electrode cell and a positive electrode / separator / negative electrode / separator / positive electrode / separator / negative electrode cell. To construct an electrochemical cell using such full cells, multiple full cells are stacked such that the positive electrode and negative electrode face each other with a separator interposed therebetween.

[0046] The "bicell" is a unit cell in which the same electrodes are located on both sides of the cell, such as the unit structure of anode / separator / cathode / separator / anode and the unit structure of cathode / separator / anode / separator / cathode. In order to construct an electrochemical cell using such a bicell, a bicell with an anode / separator / cathode / separator / anode structure (anode bicell) and a bicell with a cathode / separator / anode / separator / cathode structure (cathode bicell) are stacked so as to face each other with a separation film interposed therebetween. In some cases, more stacked bicells may be used.

[0047] On the other hand, in one embodiment of the present invention, after the impregnation step of the electrolytic solution and before the activation step, a degassing step for removing gas generated inside the battery can be performed.

[0048] <S20: Activation of the preliminary battery> An activation step is performed on the obtained preliminary battery. On the other hand, in the present specification, the term "activation" means a process of supplying a predetermined amount of electricity to an electrode assembly or a battery cell that does not have electrical characteristics to cause the anode and the cathode to have electrical characteristics. By the activation step, reactions such as formation of the SEI film, partial charging of the battery capacity, and lithiation of the cathode when using a carbon-based negative electrode active material can occur.

[0049] On the other hand, in the present invention, the preliminary battery is in a state where charging and discharging have not been performed after impregnation with the electrolytic solution and before the activation step. More preferably, in the present invention, in the activation step, discharging is first applied to the preliminary battery among charging and discharging.

[0050] In the present invention, the activation step can be performed by simply discharging the preliminary battery prepared in the step S10 once.

[0051] Alternatively, the activation step can repeat charging and discharging after the discharging step is first applied. At this time, it is desirable that the activation step is performed with the number of discharges less than 6 times.

[0052] In a more preferred embodiment of the present invention, the discharge is performed in the order of a constant current (CC) discharge and a constant voltage (CV) discharge. If discharge is performed two or more times in the activation step, at least one discharge is performed in the order of a constant current discharge and a constant voltage discharge. In this case, it is preferable that the first discharge is performed in the order of a constant current discharge and a constant voltage discharge. In another embodiment, all discharges performed in the activation step may be performed in the order of a constant current discharge and a constant voltage discharge.

[0053] If constant voltage discharge is performed first, not only will a momentary overcurrent occur in the lithium-sulfur battery, damaging the cell, but the amount of unreacted active material will be considerable. Therefore, in the present invention, it is preferable to perform constant current discharge followed by constant voltage discharge.

[0054] The constant current discharge is continued until a preset discharge cut-off voltage is reached, and the discharge cut-off voltage can be set within a range of 1.3V to 1.8V.

[0055] In step S20, the constant current discharge may be performed at a C rate set within a range of 0.05C to 2.00C or 0.50C to 1.00C until the predetermined discharge cut-off voltage is reached.

[0056] The constant voltage discharge may be performed at a predetermined discharge end voltage until a charge end current is reached. In a specific embodiment, the discharge may be terminated when the charge end current reaches a point where the remaining current of the battery is 0 mAh.

[0057] In another embodiment, the constant voltage mode may be performed for about 20 minutes or more, about 1 hour or more, about 2 hours or more, or about 4 hours or more at a discharge cut-off voltage of 1.3 V to 1.8 V. Meanwhile, the constant voltage discharge time is preferably within a range that does not cause over-discharge and does not exceed 6 hours.

[0058] In this way, the battery activation process begins with discharge, and when the discharge is performed in the CC-CV mode, the remaining inactive LiPS that was not completely phase-changed due to overvoltage is activated, thereby increasing the battery capacity. Furthermore, the activation process is simplified by discharging in this manner, which reduces the process time compared to the conventional activation process performed in a charge-discharge sequence. Furthermore, similar life characteristics can be achieved compared to batteries manufactured using the conventional activation process.

[0059] Meanwhile, in one embodiment of the present invention, step S20 may be performed in an environment in which the backup battery is pressurized. The pressurization may be performed at a pressure equal to or greater than atmospheric pressure. The term "atmospheric pressure" refers to a pressure in a state in which no separate pressurizing or decompression device is applied. For example, it may refer to a pressure of approximately 1 atm, which is a normal atmospheric condition. The pressurization is performed to perform a safety process, and specifically, the impregnated electrode assembly may be clamped between two pressure jigs or pressure plates and pressurized.

[0060] The pressure range may be, for example, 1 atm to 15 atm, more specifically, 4 atm to 10 atm. Pressurization within this pressure range is desirable because it prevents volume expansion and deformation of the negative electrode, allowing lithium to be stably and uniformly inserted into the negative electrode. Meanwhile, the activation process may be performed at a temperature between 15°C and 40°C, or between 20°C and 30°C. Within this range, lithium mobility is improved, preventing localized insertion of lithium into the electrode and enabling uniform charging.

[0061] Meanwhile, in the present invention, a charging process may be performed during the discharging process in S20. When the charging process is performed, the charging may be performed in the order of constant current charging and constant voltage charging.

[0062] The charging can be performed by charging the electrode assembly such that the remaining capacity SOC of the electrode assembly becomes 40% or more. Specifically, during charging, in terms of preventing excessive volume expansion of the negative electrode active material (for example, a silicon-based active material), deformation of the negative electrode, formation of an overvoltage of the negative electrode due to overcharging, and the phenomenon in which lithium is reduced and deposited due to the potential drop of the negative electrode, the charging can be performed by charging the electrode assembly such that the remaining capacity of the electrode assembly becomes 60% to 90%, more preferably 70% to 85%.

[0063] <S30: Degassing> In one embodiment of the present invention, after performing the activation step, a degassing process for removing gas remaining in the battery can be performed.

[0064] Gas can be generated as a by-product due to the formation of the SEI layer and the formation of irreversible capacity after the activation of the battery. Therefore, a step of removing the by-product is performed for the use of the activated secondary battery, and the secondary battery can be manufactured in a form that can be used by the gas removal step. As the step of removing the gas, a method commonly used in the field of secondary batteries can be performed without limitation. For example, the step of removing the gas can be performed by opening a part of the case in which the electrode assembly is housed to remove the gas and then sealing it.

[0065] <Electrode assembly> In one embodiment of the present invention, the electrode assembly may include a negative electrode, a positive electrode, and a separator interposed between the negative electrode and the positive electrode.

[0066] The electrode assembly is placed in a battery case together with an electrolytic solution, and the case is sealed. In one embodiment of the present invention, the battery case is made of a metal material and can be a rectangular case or a cylindrical case.

[0067] The electrode assembly may include a negative electrode, a positive electrode, and a separator, and may be stacked with the separator interposed between the negative and positive electrodes. The method for manufacturing the electrode assembly is not particularly limited and may be performed by a known method. The electrode assembly may also be formed without particular limitation as long as it includes a negative electrode, a positive electrode, and a separator interposed between the negative and positive electrodes. For example, the electrode assembly may have a jelly roll type, a stacked type, or a stacked / folded type structure.

[0068] <Negative electrode> In a specific embodiment of the present invention, the negative electrode may include a current collector and a negative electrode active material layer formed on a surface of the current collector. According to a specific embodiment of the present invention, the negative electrode active material layer includes a negative electrode active material, and may further include a binder and a conductive material, as necessary.

[0069] In one embodiment of the present invention, the negative electrode active material is lithium metal; Li x Fe2O3(0≦x≦1), Li x WO2(0≦x≦1), Sn x Me 1-x Me' y O z(Me: Mn, Fe, Pb, Ge; Me': Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, halogen; 0 < x ≤ 1; 1 ≤ y ≤ 3; 1 ≤ z ≤ 8), etc. metal composite oxides; lithium alloys in which lithium and a different metal are alloyed; silicon-based alloys; tin-based alloys; metal oxides such as SnO, SnO2, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4 and Bi2O5; conductive polymers such as polyacetylene; Li-Co-Ni-based materials; titanium oxides; lithium titanium oxides; it may contain one or a mixture of two or more selected from the group consisting of carbon-based materials. In the lithium alloy, the different metal may contain one or more selected from Al and Mg. The carbon-based material may be one or more selected from the group consisting of crystalline artificial graphite, crystalline natural graphite, amorphous hard carbon, low-crystalline soft carbon, carbon black, acetylene black, graphene and fibrous carbon. Desirably, in the present invention, the negative electrode active material may be lithium metal or a lithium alloy.

[0070] In one embodiment of the present invention, the negative electrode active material may be lithium metal or an alloy of lithium and a different metal. At this time, the negative electrode active material layer is prepared in the form of a thin film containing the metal and can be bonded to a current collector to form a negative electrode. The bonding may be performed by pressurization, and the current collector and the thin film may be heated to a temperature above room temperature during pressurization. Also, the bonding may be performed by applying a roll press method.

[0071] In one embodiment of the present invention, the negative electrode current collector may be a negative electrode current collector commonly used in the art. Examples include copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or an aluminum-cadmium alloy. The negative electrode current collector typically has a thickness of 3 to 500 μm. As with the positive electrode current collector, the current collector may have a surface with fine irregularities to enhance the binding strength of the negative electrode active material. The negative electrode current collector may be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric. For example, when a metal thin film current collector is used, the current collector may have a thickness of 1 μm to 20 μm.

[0072] <Positive electrode> The lithium ion secondary battery is a lithium-sulfur battery. The positive electrode according to the present invention includes a current collector and a positive electrode active material layer disposed on at least one surface of the current collector.

[0073] The current collector may be any current collector commonly used in the art, as long as it is electrically conductive and used as a current collecting component. For example, the positive electrode current collector may be stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like. The positive electrode current collector typically has a thickness of 3 μm to 500 μm, and fine irregularities may be formed on the surface of the positive electrode current collector to enhance adhesion of the positive electrode active material. The positive electrode current collector may be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0074] In one embodiment of the present invention, the positive electrode active material layer includes a positive electrode active material, a conductive material, and a binder material, and may include 80 wt% to 97 wt% of the positive electrode active material, 2 wt% to 10 wt% of the conductive material, and 2 wt% to 10 wt% of the binder, based on the total weight of the positive electrode active material, the conductive material, and the binder.

[0075] In the present invention, the positive electrode active material includes a sulfur-carbon composite. Preferably, the positive electrode active material includes 80 wt% or more, more preferably 90 wt% or more, of the sulfur-carbon composite relative to 100 wt% of the positive electrode active material. More preferably, the positive electrode active material may consist solely of the sulfur-carbon composite. Also, the sulfur content is preferably 70 wt% or more relative to 100 wt% of the sulfur-carbon composite.

[0076] In one embodiment of the present invention, the sulfur-carbon composite may be a composite formed by simply mixing sulfur and a carbon material, or may be in the form of a core-shell coating or support. The core-shell coating may involve either sulfur or a carbon material coating the other material. For example, the surface of the carbon material may be surrounded by sulfur, or vice versa. The carbon material may have a porous structure with pores inside and on the surface of the body, and in particular, the internal pores may be filled with sulfur. The sulfur-carbon composite may be in any form as long as it satisfies the content ratio of the sulfur-based compound to the carbon material described below, and is not limited to the present invention. In the present invention, the sulfur content is preferably 70 wt % or more of the total weight of the cathode active material.

[0077] The sulfur alone does not have electrical conductivity, and therefore is used in combination with a carbon material.

[0078] In one embodiment of the present invention, the sulfur is inorganic sulfur (S), Li2S n (n≧1), disulfide compounds such as 2,5-dimercapto-1,3,4-thiadiazole, 1,3,5-trithiocyanuric acid, etc., organic compounds, and carbon-sulfur polymers (C2S x ) n , x=2.5 to 50, n≧2). Preferably, inorganic sulfur (S8) may be included.

[0079] The carbon material has a porous structure including a plurality of irregular pores on the surface and inside, and serves as a support that provides a framework for uniform and stable immobilization of sulfur, thereby compensating for the low electrical conductivity of sulfur and facilitating electrochemical reactions. In particular, the carbon material that serves as a sulfur support in the sulfur-carbon composite has a large BET specific surface area and an appropriate particle size D 50 When the catalyst has a certain size, it can have a high sulfur loading but a low irreversible capacity, thereby increasing the energy density and improving the utilization rate of sulfur during electrochemical reactions.

[0080] In one embodiment of the present invention, the BET specific surface area of ​​the carbon material is at least 100 m 2 / g or more, up to 3,000m 2 In addition, or independently, the carbon material may have a primary particle diameter D 50 can be 1 μm to 50 μm.

[0081] The BET specific surface area and particle size D of the carbon material 50 When the content of sulfur in the carbon material satisfies the above range, sulfur can be uniformly dispersed on the inside and outside surfaces of the carbon material, while the irreversible capacity can be reduced and the electrochemical reactivity of sulfur can be increased. In addition, the use of a carbon material improves the electrochemical reactivity, stability, and electrical conductivity of the sulfur-carbon composite, thereby improving the capacity and life characteristics of the lithium-sulfur battery and ensuring optimal charge-discharge performance even when sulfur loss or volume change occurs during charge-discharge.

[0082] The particle size D of the primary particles 50 If the particle size exceeds 50 μm, it is difficult for lithium ions to move into the particle due to limitations on mass transport, making it difficult to efficiently use sulfur located at the carbon center. 50 If the particle size is less than 1 μm, it is difficult to increase the solid content because a large amount of solvent is required in the process of producing the electrode slurry, and sufficient pores between the particles are not secured, resulting in a decrease in output.

[0083] In the sulfur-carbon composite of the present invention, the carbon material used as the sulfur support can usually be produced by carbonizing various carbon precursors.

[0084] In one embodiment of the present invention, the pores of the carbon material may have a diameter of 0.5 nm to 200 nm based on the longest diameter. The carbon material may be in the form of a sphere, rod, needle, plate, tube, or bulk, and is not particularly limited as long as it is generally usable in lithium-sulfur batteries.

[0085] The carbon material may be any porous and conductive carbon-based material commonly used in the art, such as graphite, graphene, carbon black (e.g., denka black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black), carbon nanotubes (CNTs) (e.g., single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs), carbon fibers (e.g., graphite nanofibers (GNFs), carbon nanofibers (CNFs), and activated carbon fibers (ACFs), graphite (e.g., natural graphite, artificial graphite, and expanded graphite), carbon nanoribbons, carbon nanobelts, carbon nanorods, and activated carbon.

[0086] In the sulfur-carbon composite according to the present invention, the sulfur is located on at least one of the interior and exterior surfaces of the pores of the carbon material. In this case, the sulfur may be present on less than 100%, preferably 1% to 95%, more preferably 60% to 90% of the total interior and exterior surfaces of the carbon material. When the sulfur is present on the surface of the carbon material within this range, it can achieve the maximum effect in terms of electron transfer area and electrolyte wettability. Specifically, within this range, the sulfur is impregnated thinly and uniformly on the surface of the carbon material, thereby increasing the electron transfer contact area during charge and discharge. If the sulfur is located on 100% of the total surface area of ​​the carbon material, the carbon material is completely covered with sulfur, resulting in poor electrolyte wettability and reduced contact with the conductive material contained in the electrode, making it difficult for electrons to be transferred and unable to participate in the reaction.

[0087] <Production of sulfur-carbon composite> In one embodiment of the present invention, the sulfur-carbon composite is obtained by the following production method.

[0088] The method for producing the sulfur-carbon composite according to the present invention is not particularly limited and may be a method commonly known in the art, including a composite method comprising (S1) mixing a carbon material with sulfur and (S2) composite formation.

[0089] The mixing in step S1 is intended to enhance the degree of mixing of sulfur and the carbon material, and may be carried out using a stirrer commonly used in the art. In this regard, the mixing time and speed may be selectively adjusted depending on the content and conditions of the raw materials.

[0090] The compounding method in step S2 is not particularly limited in the present invention, and may be a method commonly used in the art. For example, a method commonly used in the art, such as dry compounding or wet compounding such as spray coating, may be used. For example, a method may be used in which the mixture of sulfur and carbon material obtained after mixing is heat-treated so that the molten sulfur is uniformly coated on the inner and outer surfaces of the carbon material. Meanwhile, in one embodiment of the present invention, a process of pulverizing the mixture of sulfur and carbon material by a method such as ball milling is performed before the heat treatment. In one embodiment of the present invention, the heat treatment is performed at a temperature of 120°C to 160°C for about 20 minutes to 24 hours, and a heating device such as an oven may be used.

[0091] The sulfur-carbon composite prepared by the above-described method has a high specific surface area, a high sulfur loading, and a structure that improves sulfur utilization. This not only improves the electrochemical reactivity of sulfur but also improves the accessibility and contactability of the electrolyte, thereby improving the capacity and life characteristics of lithium-sulfur batteries.

[0092] In one embodiment of the present invention, the cathode active material may consist solely of the sulfur-carbon composite, or may further contain one or more additives selected from the group consisting of a transition metal element, a Group IIIA element, a Group IVA element, sulfur compounds of these elements, and alloys of these elements with sulfur.

[0093] In one embodiment of the present invention, the positive electrode active material layer may include a lithium transition metal composite oxide represented by the following Chemical Formula 1:

[0094] [Chemical formula 1] Li a Ni b Co c M 1 d M 2 e O2 In the above formula 1, M 1is Mn, Al or a combination thereof, and preferably may be Mn or Mn and Al.

[0095] Said M 2 is at least one selected from the group consisting of Zr, W, Y, Ba, Ca, Ti, Mg, Ta and Nb, preferably at least one selected from the group consisting of Zr, Y, Mg and Ti, more preferably Zr, Y or a combination thereof. 2 Although the elements are not essential, when contained in an appropriate amount, they can play a role in promoting particle growth during firing or improving the stability of the crystal structure.

[0096] Conductive material The conductive material is used to impart conductivity to the negative electrode. Any conductive material can be used without particular limitations, as long as it is conductive without inducing chemical changes in the resulting battery. 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, carbon fiber, and carbon nanotubes; metal powder or fiber, such as copper, nickel, aluminum, or silver; conductive whiskers, such as zinc oxide or potassium titanate; conductive metal oxides, such as titanium oxide; and conductive polymers, such as polyphenylene derivatives. These materials may be used alone or in combination. The conductive material is typically present in an amount of 1 wt% to 30 wt%, preferably 1 wt% to 20 wt%, and more preferably 1 wt% to 10 wt%, based on the total weight of the negative electrode active material layer.

[0097] Binder material The binder material serves to improve adhesion between positive electrode active material particles and between the positive electrode active material and the positive electrode current collector. Specific examples of the binder material include polyvinyllidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These may be used alone or in combination.

[0098] Separation membrane The separator is disposed in the electrode assembly between the negative electrode and the positive electrode. The separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator commonly used as a separator in lithium secondary batteries can be used without any particular limitations. Specifically, the separator may be a porous polymer film, such as a porous polymer film made of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof. Alternatively, a conventional porous nonwoven fabric, such as a nonwoven fabric made of high-melting-point glass fiber or polyethylene terephthalate fiber, may be used. To ensure heat resistance or mechanical strength, a separator may be coated with a coating layer containing a ceramic component or a polymer material.

[0099] battery Another aspect of the present invention relates to a secondary battery including the electrode assembly. The secondary battery includes a battery case containing both the electrode assembly and an electrolyte, and the battery case may be of any suitable type commonly used in the art, such as a pouch type or a metal can type, without any particular limitation. The shape of the battery is not particularly limited, and may be various shapes such as a cylindrical shape, a stacked type, or a coin type.

[0100] electrolyte In the present invention, the electrolyte may include an organic solvent and a lithium salt.

[0101] organic solvents The organic solvent serves as a medium for the migration of ions involved in the electrochemical reaction of the battery. As the organic solvent, any organic solvent commonly used in lithium secondary battery electrolytes may be used without limitation. For example, ethers, esters, amides, linear carbonates, cyclic carbonates, etc. may be used alone or in combination. Among these, ether-based compounds may be used as a representative example.

[0102] The ether-based compound may include an acyclic ether or a cyclic ether.

[0103] For example, the acyclic ether may be at least one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, dimethoxyethane, diethoxyethane, ethylene glycol ethyl methyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methyl ethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methyl ethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol methyl ethyl ether, polyethylene glycol dimethyl ether, polyethylene glycol diethyl ether, and polyethylene glycol methyl ethyl ether.

[0104] For example, the cyclic ethers include 1,3-dioxolane, 4,5-dimethyl-dioxolane, 4,5-diethyl-dioxolane, 4-methyl-1,3-dioxolane, 4-ethyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, 2,5-dimethoxytetrahydrofuran, 2-ethoxytetrahydrofuran, 2-methyl-1,3-dioxolane, 2-vinyl-1,3-dioxolane, 2,2-dimethyl-1,3-dioxolane, 2-methoxy-1,3-dioxolane, 2-ethyl-2-methyl-1,3-dioxolane, tetrahydropyran, 1,4-dioxane, 1,2-dimethoxybenzene, 1,3-dimethoxybenzene, 1,4-dimethoxybenzene, and isosorbide dimethyl ether. The thiophene may be at least one selected from the group consisting of, but not limited to, ether, furan, 2-methylfuran, 3-methylfuran, 2-ethylfuran, 2-butylfuran, 2,3-dimethylfuran, 2,4-dimethylfuran, 2,5-dimethylfuran, pyran, 2-methylpyran, 3-methylpyran, 4-methylpyran, benzofuran, 2-(2-nitrovinyl)furan, thiophene, 2-methylthiophene, 2-ethylthiophene, 2-propylthiophene, 2-butylthiophene, 2,3-dimethylthiophene, 2,4-dimethylthiophene, and 2,5-dimethylthiophene.

[0105] Examples of the ester of the organic solvent include, but are not limited to, any one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, ε-caprolactone, and mixtures of two or more thereof.

[0106] Specific examples of the chain carbonate compound include, but are not limited to, any one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate, and ethyl propyl carbonate, or a mixture of two or more thereof.

[0107] Specific examples of the cyclic carbonate compound include ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, vinylethylene carbonate, and halides thereof, or mixtures of two or more thereof. Examples of such halides include, but are not limited to, fluoroethylene carbonate (FEC).

[0108] lithium salts The lithium salt is a compound capable of providing lithium ions in the electrolyte. Examples of such lithium salts include LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3CO2, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiCH3SO3, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiC4BO8, LiCl, LiBr, and LiB 10 Cl 10 , LiI, or LiB(C2O4)2 may be used. In the present invention, in order to increase the utilization potential of sulfur and realize a high-capacity, high-voltage battery, the lithium salt preferably includes Li-TFSI. More preferably, the lithium salt may include LiN(CF3SO2)2 (Li-TFSI) in an amount of 80 wt% or more, 90 wt% or more, or 100% based on 100 wt% of the total lithium salt.

[0109] The concentration of the lithium salt is in the range of 0.1 M to 2.0 M, preferably 0.5 M to 1 M, and more preferably 0.5 to 0.75 M. When the lithium salt concentration is within this range, the electrolyte has appropriate conductivity and viscosity, thereby exhibiting excellent electrolyte performance and allowing effective lithium ion mobility. When the lithium salt concentration is below this range, it is difficult to ensure ionic conductivity suitable for battery operation. When the lithium salt concentration exceeds this range, the viscosity of the electrolyte increases, reducing lithium ion mobility or increasing the decomposition reaction of the lithium salt itself, which can degrade battery performance.

[0110] In one specific embodiment of the present invention, in an electrolyte including a first solvent, a second solvent, and a lithium salt, the molar ratio of the lithium salt to the second solvent to the first solvent may be 1:0.5 to 3:4.1 to 15. In addition, in one embodiment of the present invention, the molar ratio of the lithium salt to the second solvent to the first solvent may be 1:2:4 to 13, or 1:3:3 to 10, or 1:4:5 to 10. For example, the electrolyte included in the lithium-sulfur battery of the present invention may include a first solvent including a fluorinated ether compound at a higher content ratio than the second solvent including a glyme-based compound.

[0111] Other additives In addition to the electrolyte components, the electrolyte may further contain additives for purposes such as improving battery life characteristics, suppressing battery capacity loss, and improving battery discharge capacity. For example, additives may include nitric acid compounds, nitrous acid compounds, 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, and aluminum trichloride, which may be used alone or in combination. The additives may be present in an amount of 0.1 wt % to 10 wt %, preferably 0.1 wt % to 5 wt %, based on the total weight of the electrolyte.

[0112] In a specific embodiment of the present invention, the electrolyte may include a nitrate compound and / or a nitrite compound as an additive. The nitrate compound / nitrite compound forms a stable coating on the lithium metal electrode, which serves as the negative electrode, thereby improving charge / discharge efficiency. Examples of such nitric acid or nitrite compounds include, but are not limited to, inorganic nitric acid or nitrite compounds such as lithium nitrate (LiNO), potassium nitrate (KNO), cesium nitrate (CsNO), barium nitrate (Ba(NO)), ammonium nitrate (NHNO), lithium nitrite (LiNO), potassium nitrite (KNO), cesium nitrite (CsNO), and ammonium nitrite (NHNO); organic nitric acid or nitrite compounds such as methyl nitrate, dialkylimidazolium nitrate, guanidine nitrate, imidazolium nitrate, pyridinium nitrate, ethyl nitrate, propyl nitrate, butyl nitrate, pentyl nitrate, and octyl nitrate; and organic nitro compounds such as nitromethane, nitropropane, nitrobutane, nitrobenzene, dinitrobenzene, nitropyridine, dinitropyridine, nitrotoluene, and dinitrotoluene, and combinations thereof. In a preferred embodiment of the present invention, the additive may include lithium nitrate.

[0113] The present invention will be described in detail below with reference to examples. However, the examples according to the present invention can be modified in various other forms, and the scope of the present invention is not limited to the following examples. The examples of the present invention are provided to more completely explain the present invention to those skilled in the art.

[0114] <Example> (Manufacture of spare batteries) Carbon nanotubes and sulfur were homogeneously mixed and placed in an oven at 155°C for 30 minutes to prepare a sulfur-carbon composite. The sulfur content of the 100 wt% sulfur-carbon composite was 75 wt%. 90 wt% of the prepared sulfur-carbon composite, 5 wt% Denka black, and 5 wt% of a binder (styrene butadiene rubber / carboxymethyl cellulose, 7:3 weight ratio) were mixed to prepare a slurry for preparing a positive electrode. The solid concentration in the slurry was 25 wt%. The slurry was applied to both sides of a 20 μm-thick aluminum current collector, dried at 50°C for 12 hours, and pressed using a roll press to prepare a positive electrode. The loading of the positive electrode active material (relative to the electrode area) was 2.5-2.7 mAh / cm. 2 The porosity was 70 to 83 vol%.

[0115] A lithium thin film with a thickness of 60 μm was prepared as the negative electrode.

[0116] The electrolyte used was a mixture of 1M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 1 wt% lithium nitrate (LiNO3) dissolved in an organic solvent consisting of 1,3-dioxolane and dimethyl ether (DOL:DME = 1:1 (volume ratio)).

[0117] The prepared cathode and anode were positioned facing each other, and a 16 μm thick polyethylene porous film with a porosity of 68 vol% was placed between them as a separator. The resulting mixture was then placed in a pouch, the prepared electrolyte was injected, and the pouch was sealed to prepare a spare battery. The battery was constructed by stacking and folding a total of seven cathodes and eight anodes with the separator sandwiched between them.

[0118] Example The prepared spare battery was placed in a jig and pressurized to approximately 7 atm. It was then discharged in CC-CV mode using a PNE-0506 charger / discharger (manufacturer: PNE solution). Constant current discharge was performed at a C rate of 0.5 C, and after reaching the discharge cutoff voltage of 1.8 V, the constant voltage mode was maintained for approximately 4 hours until the discharge cutoff current reached 0 mAh.

[0119] Comparative Example The prepared spare battery was placed in a jig and pressurized to approximately 7 atm. It was then discharged at a constant current of 0.5 C to 1.7 V using a PNE-0506 charger / discharger (manufacturer: PNE solution), and then charged at a constant current of 0.3 C to 2.5 V, repeating this process six times.

[0120] Lifetime characteristic evaluation The lifespan characteristics of the batteries manufactured after the activation process in each example and comparative example were evaluated using a PNE-0506 charger / discharger (manufacturer: PNE solution). Each battery was charged at a constant current of 0.3 C to 2.5 V and discharged at a constant current of 1 C to 1.7 V, and the charge / discharge cycle was repeated until the discharge capacity reached 80% of the initial discharge capacity (100%).

[0121] Figure 1 shows a comparison of the specific capacity of each battery at 1C discharge, and Figure 2 shows the Coulombic efficiency characteristics. From this, it was confirmed that the specific capacity was similar between the Example and Comparative Example at similar charge / discharge cycles, but the Coulombic efficiency was superior in the Example.

[0122] Figure 3 shows the power density for each SOC (state of charge) range. The power density for each SOC range was measured by decreasing the SOC from 95% to 15% in 10% increments. When each specific SOC (95%, 85%, 75%, 65%, 55%, 45%, 35%, 25%, and 15%) was reached, a 5C pulse current was applied for 10 seconds in the discharge direction. This was converted to a voltage value to calculate the power density (kW / kg). This can serve as an indicator of the degree of overvoltage when a pulse current is applied to each battery. After a 1-minute rest period, the battery was further charged at 0.1C to 100% SOC, and then a 0.3C charging current was applied for 10 seconds. This procedure was repeated when the next specific SOC was reached. For lithium-sulfur batteries, the 75% SOC range is the range where the lowest voltage and highest overvoltage are observed, making it a key range for assessing battery performance. 3, the power density of the example and comparative example is comparable at the SOC 75%, and the example and comparative example are also comparable in other sections. This shows that the battery manufacturing method of the present invention not only ensures a power density comparable to that of the prior art, but also reduces the number of charge / discharge cycles during the activation process compared to the prior art, thereby shortening the time required for the battery manufacturing process and providing excellent process efficiency.

Claims

1. (S10) manufacturing a spare battery including an electrode assembly and an electrolyte; (S20) activating the backup battery; Including, the electrode assembly includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, the positive electrode including a positive electrode active material containing sulfur, In step S10, the electrode assembly is immersed in the electrolyte. The method for manufacturing a lithium-sulfur battery includes discharging the reserve battery in step S20, wherein the discharge is performed in the order of a constant current discharge and a constant voltage discharge.

2. 2. The method of claim 1, wherein in step S20, the constant current discharge is continued until a predetermined end-of-discharge voltage is reached, and the end-of-discharge voltage is set in a range of 1.3 V to 1.8 V.

3. 3. The method of claim 2, wherein in step S20, the constant current discharge is performed at a C rate set in a range of 0.05 to 2 until a predetermined discharge cut-off voltage is reached.

4. 3. The method of claim 2, wherein the constant voltage discharge is performed at a predetermined discharge cut-off voltage until the remaining current of the battery reaches 0 mAh.

5. 5. The method of claim 1, wherein the reserve battery is not charged or discharged before step S20, and wherein discharging is first applied to the reserve battery among charging and discharging in step S20.

6. 3. The method of claim 2, wherein the reserve battery is not charged or discharged before step S20, and step S20 comprises discharging the reserve battery only once.

7. 2. The method of claim 1, wherein the discharging step S20 is performed less than six times.

8. 2. The method of claim 1, wherein in step S20, the constant current discharge is performed at a C rate in the range of 0.05 to 2.

00.

9. 2. The method of claim 1, wherein the discharging step S20 is performed in a constant current manner until a predetermined discharge cutoff voltage is reached.

10. 2. The method of claim 1, wherein the discharging step S20 is performed at a C rate in the range of 0.50 to 1.

00.

11. 2. The method of claim 1, wherein step S20 is performed in an environment where the reserve battery is pressurized.

12. The method for manufacturing a lithium-sulfur battery according to claim 11, wherein the pressure applied is equal to or greater than atmospheric pressure.

13. 2. The method of claim 1, wherein step S20 is performed at a temperature of 15 to 40°C.

14. 2. The method of claim 1, wherein an aging step is further performed after step S10 and before step S20, in which the reserve battery is maintained in an atmosphere of 15°C to 40°C.

15. 2. The method of claim 1, wherein a charging process is performed during a discharging process in step S20, and the charging process is performed in the order of constant current charging and constant voltage charging.

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