Manufacturing method of lithium secondary batteries

Abstract

The present invention provides a method for manufacturing a lithium secondary battery using a perlithium manganese oxide, which can improve the voltage drop that may be induced during high-temperature storage by performing activation under certain conditions.

Description

[Technical Field]

[0001] [Cross-reference of related applications] This application claims priority rights based on Korean Patent Application No. 10-2023-0118663 dated September 6, 2023, and Korean Patent Application No. 10-2024-0119900 dated September 4, 2024, and all content disclosed in the documents of said Korean Patent Applications is incorporated herein by reference.

[0002] The present invention relates to a method for manufacturing a lithium secondary battery, and more specifically, to a method for improving the voltage drop induced during the activation stage of a lithium secondary battery containing a perlithium manganese oxide. [Background technology]

[0003] Lithium-ion batteries, since their commercialization in 1991, are energy storage media that have been applied in a wide range of fields. As the market for products incorporating lithium-ion batteries expands, research to increase the energy density of lithium-ion batteries is actively underway, and one of the most noteworthy methods is the development of cathode active materials with compositions that can utilize even more lithium than existing ones.

[0004] As a cathode active material that can utilize more lithium, perlithium-based transition metal oxides have been developed that have a layered phase structure and a molar ratio of lithium to the transition metal of more than 1. Such perlithium-based transition metal oxides can achieve high capacity by simultaneously utilizing not only the cation redox of the transition metal but also the anion redox reaction using oxygen within the cathode structure. A representative perlithium-based transition metal oxide that is currently being actively researched is a perlithium-manganese oxide in which the molar ratio of lithium to the transition metal is more than 1 and the manganese content of the total transition metal is 50 mol% or more. The aforementioned perlithium-manganese oxide has a structure in which a layered phase structure LiMO2 (where M is the transition metal) and a rock salt phase structure Li2MnO3 are mixed, and high capacity can be achieved by activating Li2MnO3 through an activation process at a high voltage of 4.4V or higher.

[0005] However, in such a high-voltage activation process, if the activation of the positive electrode active material is not performed properly, a problem may arise in which a large change in the crystal structure within the perlithium manganese-based oxide is induced during high-temperature storage, further inducing a large voltage drop. [Overview of the project] [Problems that the invention aims to solve]

[0006] The present invention provides a method for manufacturing a lithium secondary battery using a perlithium manganese oxide that can improve the voltage drop that may be induced during high-temperature storage by performing activation under certain conditions. [Means for solving the problem]

[0007] In one aspect of the invention, a positive electrode containing a lithium-rich manganese-based oxide in which the manganese content in the total metal excluding lithium exceeds 50 mol% and the ratio of the number of moles of lithium to the number of moles of the total metal excluding lithium (Li / Me) exceeds 1, a negative electrode, and a battery cell including an electrolyte are prepared, and the battery cell is charged and discharged at least once or more to be activated. In the activation step, the charging is performed in a constant current-constant voltage mode, and the charging is terminated when the charging current rate reaches 0.01C to 0.04C. A method for manufacturing a lithium secondary battery is provided in which the charging current rate is 0.8C to 1.2C in a region where the state of charge (SOC) of the battery cell is 3% to 60%.

[0008] The charging cut-off voltage can be 4.5V to 4.7V.

[0009] In the activation step, the charging may include a first charging step of performing constant current charging at a first current rate, a second charging step of performing constant current charging at a second current rate different from the first current rate, and a third charging step of performing constant voltage charging after performing constant current charging at a third current rate different from the first current rate.

[0010] At this time, the first current rate, the second current rate, and the third current rate can each independently be 0.1C to 1.2C.

[0011] The first charging step is performed until the SOC reaches 0% to 3%, the second charging step is performed until the SOC reaches 3% to 60%, and the third charging step can be performed until the SOC reaches 60% to 100%. At this time, the second current rate can be 0.8C to 1.2C. Also, the first current rate can be 0.1C to 0.3C, and the third current rate can be 0.3C to 1.0C.

[0012] In the activation step, the discharging can be performed at a current rate of 0.3C to 0.8C until it reaches 2.0V to 3.0V, for example, until it reaches 2.0V.

[0013] The aforementioned perlithium manganese oxide can be represented by the following chemical formula 1. [Chemical formula 1] Li a Ni b Co c Mn d M e O2 In the above chemical formula 1, 1.0 <a、0≦b≦0.5、0≦c≦0.1、0.5≦d≦1.0、0≦e≦0.2であり、 M is at least one element selected from the group consisting of Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr, and Zr.

[0014] In the aforementioned chemical formula 1, it is possible that 1.2 ≤ a ≤ 1.5, 0.1 ≤ b ≤ 0.4, 0 ≤ c ≤ 0.05, 0.5 ≤ d ≤ 0.8, and 0 ≤ e ≤ 0.1. [Effects of the Invention]

[0015] In the lithium secondary battery manufacturing method described above, the activation charging step is performed in constant current-constant voltage mode, and charging is terminated when the charging current rate reaches 0.01C to 0.04C. The charging current rate can be set to 0.8C to 1.2C in the region where the State of Charge (SOC) of the battery cell is 3 to 60%.

[0016] This allows for an increase in the activation ratio of Li2MnO3, the rock salt phase within the crystal structure of the positive electrode active material, during high-temperature storage, thereby effectively improving voltage drop. [Brief explanation of the drawing]

[0017] [Figure 1] This graph shows the high-temperature storage characteristics of lithium secondary batteries activated by the methods of Examples 1 and 2, and Comparative Examples 1 and 2. [Modes for carrying out the invention]

[0018] The advantages and features of the invention, and the methods for achieving them, will become clearer with reference to the embodiments described below in detail, along with the accompanying drawings. However, the invention is not limited to the embodiments disclosed below and can be realized in a variety of different forms. These embodiments are merely provided to complete the disclosure of the invention and to fully inform those ordinary skill in the art of which the invention pertains of the scope of the invention, and the invention is defined only by the scope of the claims.

[0019] Unless otherwise defined, all terms used herein (including technical and scientific terms) should be used in a sense that is commonly understood by a person of ordinary skill in the art to which the invention pertains. Furthermore, terms defined in commonly used dictionaries should not be interpreted ideally or excessively unless otherwise clearly defined.

[0020] The terms used herein are for illustrative purposes only and are not intended to limit the invention. In this specification, singular nouns include plural nouns unless otherwise specified in the text. The terms “comprises” and / or “comprising” as used in this specification do not preclude the presence or addition of one or more other components in addition to those mentioned.

[0021] In this specification, when a part includes a component, this does not exclude other components, unless otherwise stated, but rather means that it may include other components.

[0022] In this specification, "A and / or B" means A or B, or A and B.

[0023] In this specification, "primary particle" refers to a particle unit in which no grain boundaries are visible when observed using a scanning electron microscope at a field of view of 5,000x to 20,000x. "Average particle size of primary particles" refers to the arithmetic mean calculated after measuring the particle sizes of primary particles observed in scanning electron microscope images. In this specification, "secondary particle" refers to a particle formed by the aggregation of multiple primary particles.

[0024] In this specification, "average particle size D 50 " refers to the particle size at the 50% reference level of the volume cumulative particle size distribution of the measured particle powder (e.g., positive electrode active material powder, negative electrode active material powder, etc.). The average particle size D 50 This can be measured using the laser diffraction method. For example, the powder of the particles to be measured is dispersed in a dispersion medium, then introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT 3000), and irradiated with ultrasound at approximately 28 kHz with an output of 60 W. After obtaining a volume-cumulative particle size distribution graph, the particle size corresponding to 50% of the volume-cumulative amount can be determined.

[0025] In this specification, "SOC X%" means a state in which the percentage of the capacity charged in a battery cell relative to the discharge capacity that appears when the battery cell is discharged from 4.6V to 2.0V (i.e., (charged capacity / discharge capacity of the battery cell in the voltage range of 4.6V to 2.0V) × 100) is X%.

[0026] The inventors of this invention conducted extensive research to improve the voltage drop that can be induced during high-temperature storage of lithium secondary batteries using perlithium manganese oxides. As a result, they discovered that this can be improved by performing an activation process under specific charging conditions during the manufacturing of lithium secondary batteries, and thus completed the invention. The following describes a method for manufacturing a lithium secondary battery according to an embodiment of the invention.

[0027] A method for manufacturing a lithium secondary battery according to one embodiment includes the steps of: (1) preparing a battery cell containing a positive electrode, a negative electrode, and an electrolyte containing a perlithium manganese oxide in which the manganese content in the total metal excluding lithium exceeds 50 mol%, and the ratio of the number of moles of lithium to the number of moles of the total metal excluding lithium (Li / Me) exceeds 1; and (2) activating the battery cell by charging and discharging it at least once. In the activation stage, the charging is performed in constant current-constant voltage mode, and the charging is terminated when the charging current rate reaches 0.01C to 0.04C. In the region where the State of Charge (SOC) of the battery cell is 3 to 60%, the charging current rate is 0.8C to 1.2C.

[0028] (1) Steps to prepare the battery cells First, prepare a battery cell containing a positive electrode, a negative electrode, and an electrolyte. The battery cell can be manufactured, for example, by forming an electrode assembly including a positive electrode and a negative electrode, then housing the electrode assembly in a battery case, and finally injecting an electrolyte to seal the battery case. In this case, the electrode assembly may include a separator membrane between the positive electrode and the negative electrode. The following sections will provide a more detailed explanation of each component of such a battery cell.

[0029] positive electrode On the other hand, in the battery cell, the positive electrode contains a perlithium manganese oxide as a positive electrode active material in which the manganese content in the total metal excluding lithium exceeds 50 mol%, and the ratio of the number of moles of lithium to the number of moles of the total metal excluding lithium (Li / Me) exceeds 1. Specifically, the positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on at least one surface of the positive electrode current collector, and the positive electrode active material layer contains a perlithium manganese oxide in which the ratio of the number of moles of lithium to the number of moles of the total metal excluding lithium (Li / Me) exceeds 1.

[0030] In the case of a lithium-rich manganese-based oxide containing an excess of lithium, it has a structure in which a layered phase (LiM'O2) and a rock-salt phase (Li2MnO3) coexist. However, during the initial activation process, the rock-salt phase activates while generating excess lithium ions, enabling the realization of a high capacity.

[0031] In a specific example, the lithium-rich manganese-based oxide can be represented by Chemical Formula 1. [Chemical Formula 1] [[ID=P8]]Li a Ni b Co c Mn d M e O2 In Chemical Formula 1, M can be at least one or more selected from the group consisting of Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr and Zr.

[0032] On the other hand, a is the molar ratio of Li in the lithium-rich manganese-based oxide, and 1.0 < a, 1.1 ≤ a ≤ 1.5, 1.2 ≤ a ≤ 1.5, or 1.1 ≤ a ≤ 1.3 can be satisfied. When a satisfies such a range, the irreversible capacity of the negative electrode can be sufficiently compensated, and high-capacity characteristics can be realized.

[0033] The b is the molar ratio of Ni in the lithium-rich manganese-based oxide, and 0 ≤ b ≤ 0.5, 0.1 ≤ b ≤ 0.4, or 0.2 ≤ b ≤ 0.4 can be satisfied.

[0034] The c is the molar ratio of Co in the lithium-rich manganese-based oxide, and 0 ≤ c ≤ 0.1, 0 ≤ c ≤ 0.08, or 0 ≤ c ≤ 0.05 can be satisfied. When c exceeds 0.1, it is difficult to ensure a high capacity, and there is a risk that gas generation and degradation of the positive electrode active material will deepen and the life characteristics will deteriorate.

[0035] The aforementioned d is the molar ratio of Mn in the perlithium manganese oxide, and can be 0.5 ≤ d < 1.0, 0.5 ≤ d ≤ 0.8, or 0.5 ≤ d ≤ 0.7. If d is less than 0.5, the proportion of the rock salt phase becomes too low, resulting in minimal compensation for the irreversible negative electrode capacity and capacity improvement.

[0036] The aforementioned e is the molar ratio of the doping element M in the perlithium manganese oxide, and can be 0 ≤ e ≤ 0.2, 0 ≤ e ≤ 0.1, or 0 ≤ e ≤ 0.05. Excessive doping element content may adversely affect the active material capacity.

[0037] On the other hand, in the perlithium manganese oxide represented by chemical formula 1, the molar ratio of Li to the total number of moles of metal elements excluding Li (Li / Me) can be 1.2 to 1.5, 1.25 to 1.5, or 1.25 to 1.4. When the Li / Me ratio satisfies the above range, excellent rate characteristics and capacity characteristics are exhibited. If the Li / Me ratio is too high, the electrical conductivity decreases, the rock salt phase (Li2MnO3) increases, and the degradation rate may accelerate. If it is too low, the effect of improving energy density is minimal.

[0038] On the other hand, the composition of the perlithium manganese oxide can also be represented by the following chemical formula 2. [Chemical formula 2] X Li2MnO3·(1-X)Li[Ni 1-y-z-w Mn y Co z M w ]O2

[0039] In the above chemical formula 2, M can be at least one selected from the group consisting of metal ions Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr, and Zr.

[0040] The aforementioned X represents the ratio of the rock salt phase (Li2MnO3) within the perlithium manganese oxide, and can be 0.2 ≤ X ≤ 0.5, 0.25 ≤ X ≤ 0.5, or 0.25 ≤ X ≤ 0.4. When the ratio of the Li2MnO3 phase within the perlithium manganese oxide satisfies the above range, the irreversible capacity of the silicon-based anode active material can be sufficiently compensated, and high capacity characteristics can be achieved.

[0041] The aforementioned y is the molar ratio of Mn in the LiM'O2 layer phase, and can be 0.4 ≤ y < 1, 0.4 ≤ y ≤ 0.8, or 0.4 ≤ y ≤ 0.7.

[0042] The aforementioned z is the molar ratio of Co in the LiM'O2 layer phase and can be 0 ≤ z ≤ 0.1, 0 ≤ z ≤ 0.08, or 0 ≤ z ≤ 0.05. If z exceeds 0.1, gas generation and degradation of the positive electrode active material may deepen, potentially reducing the lifetime characteristics.

[0043] The aforementioned w is the molar ratio of the doping element M in the LiM'O2 layer phase, and can be 0 ≤ w ≤ 0.2, 0 ≤ w ≤ 0.1, or 0 ≤ w ≤ 0.05.

[0044] On the other hand, the positive electrode active material may, if necessary, further include a coating layer on the surface of the perlithium manganese oxide. When the positive electrode active material includes a coating layer, the coating layer suppresses contact between the perlithium manganese oxide and the electrolyte, reducing electrolyte side reactions, thereby improving the lifespan characteristics.

[0045] The aforementioned coating layer contains coating element M 1 It may include the coating element M 1 The coating element M can be, for example, at least one selected from the group consisting of Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr, and Zr, preferably Al, Co, Nb, W, and combinations thereof, and more preferably Al, Co, and combinations thereof. 1It can contain two or more types, for example, Al and Co.

[0046] The aforementioned coating element exists in oxide form within the coating layer, i.e., M 1 It can exist within Oz (1 ≤ z ≤ 4).

[0047] The coating layer can be formed by methods such as dry coating, wet coating, chemical vapor deposition (CVD), physical vapor deposition (PVD), and atomic layer deposition (ALD). Among these, formation by atomic layer deposition is preferred because it allows for the formation of a large coating layer area.

[0048] The area of ​​the coating layer can be 10% to 100%, 30% to 100%, or 50% to 100% of the total surface area of ​​the perlithium manganese oxide particles. When the area of ​​the coating layer meets the above range, it exhibits excellent improvement in lifespan characteristics.

[0049] On the other hand, the positive electrode active material can be in the form of secondary particles in which a plurality of primary particles are aggregated, and the average particle size D of the secondary particles is 50 The diameter can be 2 μm to 10 μm, or 2 μm to 8 μm, or 4 μm to 8 μm. D of the positive electrode active material 50 When the above range is met, an excellent electrode density can be achieved, and the degradation of capacity and rate characteristics can be minimized.

[0050] Furthermore, the positive electrode active material has a BET specific surface area of ​​1 m². 2 / g~10m 2 / g, 3m 2 / g~8m 2 / g or 4m 2 / g~6m 2 It can be / g. If the BET specific surface area of ​​the positive electrode active material is too low, the reaction area with the electrolyte will be insufficient, making it difficult to achieve sufficient capacity. If the specific surface area is too high, moisture absorption will be rapid, accelerating side reactions with the electrolyte and making it difficult to ensure lifespan characteristics.

[0051] On the other hand, the perlithium manganese oxide can be produced by mixing a transition metal precursor with a lithium raw material and then calcining it.

[0052] Examples of the lithium raw material include lithium-containing carbonates (e.g., lithium carbonate), hydrates (e.g., lithium hydroxide hydrate (LiOH·H2O)), hydroxides (e.g., lithium hydroxide), nitrates (e.g., lithium nitrate (LiNO3)), chlorides (e.g., lithium chloride (LiCl)), etc.), and one of these alone or a mixture of two or more can be used.

[0053] On the other hand, the transition metal precursor can be in the form of a hydroxide, oxide, or carbonate. Using a carbonate precursor is more preferable because it allows for the production of a positive electrode active material with a relatively high specific surface area.

[0054] The transition metal precursor can be produced through a coprecipitation process. For example, the transition metal precursor can be produced by dissolving each transition metal-containing raw material in a solvent to produce a metal solution, then mixing the metal solution, an ammonium cation complex-forming agent, and a basic compound, and then proceeding with a coprecipitation reaction. Furthermore, an oxidizing agent or oxygen gas may be added during the coprecipitation reaction as needed.

[0055] In this case, the transition metal-containing raw material can be an acetate, carbonate, nitrate, sulfate, halogen compound, sulfide, etc. of each transition metal. Specifically, the transition metal-containing raw material can be NiO, NiCO3·2Ni(OH)2·4H2O, NiC2O2·2H2O, Ni(NO3)2·6H2O, NiSO4, NiSO4·6H2O, Mn2O3, MnO2, Mn3O4, MnCO3, Mn(NO3)2, MnSO4·H2O, manganese acetate, manganese halide, cobalt sulfate, cobalt nitrate, cobalt carbonate, cobalt oxide, cobalt acetate, cobalt halide, etc.

[0056] The ammonium cation complex-forming agent can be at least one selected from the group consisting of NH4OH, (NH4)2SO4, NH4NO3, NH4Cl, CH3COONH4, and NH4CO3.

[0057] The basic compound can be at least one selected from the group consisting of NaOH, Na2CO3, KOH, and Ca(OH)2. The form of the precursor can change depending on the type of basic compound used. For example, when NaOH is used as the basic compound, a hydroxide-form precursor can be obtained, and when Na2CO3 is used as the basic compound, a carbonate-form precursor can be obtained. Furthermore, when used together with an oxidizing agent, an oxide-form precursor can be obtained.

[0058] On the other hand, the transition metal precursor and the lithium raw material can be mixed in an amount such that the overall transition metal (Ni+Co+Mn):Li molar ratio is 1:1.05 to 1:2, preferably 1:1.1 to 1:1.8, and more preferably 1:1.25 to 1:1.8.

[0059] On the other hand, the firing can be carried out at a temperature of 600°C to 1000°C or 700°C to 950°C, and the firing time can be 5 to 30 hours or 5 to 20 hours. The firing atmosphere can be an air atmosphere or an oxygen atmosphere, for example, an atmosphere containing 20% ​​to 100% by volume of oxygen.

[0060] On the other hand, the positive electrode active material layer may further contain a conductive material and a binder in addition to the positive electrode active material.

[0061] Examples of the conductive material include spherical or flake graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, single-walled carbon nanotubes, and multi-walled carbon nanotubes; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. One of these alone or a mixture of two or more can be used. The conductive material may be included in an amount of 0.1% to 20% by weight, 1% to 20% by weight, or 1% to 10% by weight based on the total weight of the positive electrode active material layer.

[0062] Examples of the binder include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one of these alone or a mixture of two or more can be used. The binder may be included in amounts of 1% to 20% by weight, 2% to 20% by weight, or 2% to 10% by weight based on the total weight of the positive electrode active material layer.

[0063] negative electrode Next, the negative electrode may include, for example, a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector. The negative electrode active material layer may selectively include a binder and a conductive material together with the negative electrode active material.

[0064] The negative electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloy can be used. The negative electrode current collector can also have a thickness of 3 μm to 500 μm, and, similar to the positive electrode current collector, fine irregularities can be formed on the surface of the current collector to strengthen the bonding force of the negative electrode active material. For example, it can be used in various forms such as film, sheet, foil, net, porous material, foam, and nonwoven fabric.

[0065] As the negative electrode active material, compounds capable of reversible intercalation and deintercalation of lithium can be used. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; and SiO2. β Examples include (0<β<2), SnO2, vanadium oxide, lithium vanadium oxide, and other metal oxides that can be doped and dedoped with lithium; or composites containing the metallic compound and carbonaceous material, such as Si-C composites or Sn-C composites, and any one or more mixtures of these can be used.

[0066] Furthermore, a metallic lithium thin film can also be used as the negative electrode active material. In addition, all types of carbon materials, including low-crystalline carbon and high-crystalline carbon, can be used. Typical low-crystalline carbons include soft carbon and hard carbon, while typical high-crystalline carbons include amorphous, plate-like, flake-like, spherical or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch-derived cokes.

[0067] The conductive material is used to impart conductivity to the electrodes and can be used in any battery without particular limitations as long as it does not undergo chemical changes and has electronic conductivity. 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 powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. One of these can be used alone or a mixture of two or more. The conductive material can usually be included in an amount of 1% to 30% by weight, preferably 1% to 20% by weight, and more preferably 1% to 10% by weight, relative to the total weight of the negative electrode active material layer.

[0068] The binder plays a role in improving adhesion between negative electrode active material particles and adhesion between the negative electrode active material and the negative electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, of which one or more can be used. The binder may be included in an amount of 1% to 30% by weight, preferably 1% to 20% by weight, and more preferably 1% to 10% by weight, based on the total weight of the negative electrode active material layer.

[0069] The negative electrode active material layer can also be manufactured, for example, by applying a negative electrode slurry containing a negative electrode active material and selectively a binder and conductive material onto a negative electrode current collector and drying it, or by casting the negative electrode slurry onto a separate support, peeling it off the support, and laminating the resulting film onto the negative electrode current collector.

[0070] separation membrane Next, the separation membrane separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. It can be used without particular limitations as long as it is the type of separation membrane normally used in lithium secondary batteries, and is particularly preferred if it has low resistance to ion movement of the electrolyte and excellent electrolyte moisture absorption capacity. Specifically, porous polymer films, such as porous polymer films made from polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or laminated structures of two or more layers thereof can be used. In addition, ordinary porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. Furthermore, to ensure heat resistance or mechanical strength, coated separation membranes containing ceramic components or polymeric substances can be used, and can be selectively used in single-layer or multi-layer structures.

[0071] electrolyte Next, the electrolyte can be a variety of electrolytes usable in lithium secondary batteries, such as organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, or combinations thereof, and the type is not particularly limited.

[0072] For example, the electrolyte may include an organic solvent and a lithium salt. The organic solvent can be used without particular limitations, as long as it can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the organic solvents include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (propylene Carbonate solvents such as carbonate (PC); alcoholic solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (where R is a hydrocarbon group with 2 to 20 carbon atoms in a linear, branched, or cyclic structure, and can include a double-bonded aromatic ring or ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes can be used.

[0073] The lithium salt can be used without particular limitations as long as it is a compound that can provide lithium ions for use in lithium secondary batteries. Specifically, as the anion of the lithium salt, F - Cl - , Br - , I - NO3 - , N(CN)2 - BF4 - CF3CF2SO3- (CF3SO2)2N - , (FSO2)2N - CF3CF2(CF3)2CO - (CF3SO2) 2CH - (SF5)3C - (CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - CH3CO2 - SCN - and (CF3CF2SO2)2N - The lithium salt can be at least one selected from the group consisting of LiPF6, LiN(FSO2)2, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2) 2. LiCl, LiI, or LiB(C2O4)2 can be used. The concentration of the lithium salt is preferably within the range of 0.1 M to 5.0 M.

[0074] Furthermore, the electrolyte may contain additives in addition to the components mentioned above, for the purpose of improving the battery's lifespan characteristics, suppressing the decrease in battery capacity, and improving the battery's discharge capacity. For example, the additives may include, but are not limited to, haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphate, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexamethyl phosphate triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, either alone or in combination. The additives may be included in an amount of 0.1% to 10% by weight, preferably 0.1% to 5% by weight, relative to the total weight of the electrolyte.

[0075] On the other hand, the electrode assembly can be a variety of electrode assemblies well known in the art, such as a jelly-roll type, a stack type, a stack-and-lamination type, or a stack-and-folding type electrode assembly, and its form is not particularly limited.

[0076] A jelly-roll type electrode assembly can be manufactured by interposing a sheet-shaped separation membrane between a sheet-shaped positive electrode and a sheet-shaped negative electrode, and then winding the assembly in one direction.

[0077] A stacked electrode assembly can be manufactured by cutting the positive electrode, separator membrane, and negative electrode into the desired shapes, and then sequentially stacking the cut positive electrode / separator membrane / negative electrode.

[0078] A stack-and-lamination electrode assembly can be manufactured by stacking a positive electrode, a separator membrane, and a negative electrode to produce multiple unit cells, stacking the multiple unit cells with the separator membrane in between, and then laminating them through methods such as heating.

[0079] A stack-and-fold electrode assembly can be manufactured by stacking a positive electrode, a separator membrane, and a negative electrode to produce multiple unit cells, arranging the multiple unit cells on one or both sides of a long folding separator membrane, and then winding up the folding separator membrane.

[0080] On the other hand, as the electrical case, a variety of battery cases known in the art, such as cylindrical battery cases, rectangular battery cases, or pouch-type battery cases, can be used, and there are no particular limitations on the type.

[0081] (2) Activation stage Next, the battery cells are charged and discharged at least once to electrically activate the battery. This activation step involves charging and discharging the battery cells to impart electrical characteristics and stabilizing the battery by forming an SEI (Solid Electrolyte Interphase) film on the electrodes.

[0082] In embodiments of the invention, by adjusting the activation depth for Li2MnO3(monoclinic) in a portion of the activation charging stage, the rate of change of the Li2MnO3(monoclinic) phase can be increased, i.e., activation can be increased, thereby effectively improving the voltage drop that may be induced during high-temperature storage.

[0083] First, the charging may include a first charging stage in which the State of Charge (SOC) is 3% or less.

[0084] The first charging stage can be performed at a first current rate of 0.3C or less, or between 0.1C and 0.3C. If the current rate is faster than 0.3C in the first charging stage, an SEI film may be formed unstably on the electrode surface. If an SEI film is formed unstably on the electrode surface, the SEI film may easily decompose during battery operation, causing rapid degradation of the electrode, which can significantly reduce its lifespan.

[0085] The first charging stage is preferably performed at a State of Charge (SOC) of 0-3%. When the charge state in the first charging stage satisfies the above range, a solid and dense SEI film is formed on the electrode surface, enabling excellent lifespan characteristics.

[0086] Next, the charging may include a second charging stage in which the device is charged at a state of charge (SOC) of 3-60% at a second current rate of 0.8C-1.2C, preferably 0.9C-1.1C. Such a second current rate may be different from the first current rate of the first charging stage, and more specifically, it may be a current rate greater than the first current rate.

[0087] When the second charging stage is performed at 0.8C to 1.2C, the activation of the Li2MnO3 (monoclinic) phase in the positive electrode active material crystal structure is increased, suppressing oxygen desorption and cation mixing during high-temperature storage. This reduces the additional phase change ratio of the positive electrode active material, thereby improving the voltage drop that may be induced during high-temperature storage. Furthermore, performing the second charging stage at a relatively fast C-rate reduces the time required for the activation process, thereby shortening the battery production time.

[0088] Furthermore, when the charging capacity in the second charging stage satisfies the above range, it is possible to suppress the incomplete formation of the SEI film, and to suppress the occurrence of oxygen desorption and cation mixing during the activation process, thereby minimizing the increase in positive electrode resistance.

[0089] Next, the charging may include a third charging stage in which, at a state of charge (SOC) of 60-100%, the device is charged in constant current mode at a third current rate different from the first current rate until it reaches the charging termination voltage, and then charged in constant voltage mode.

[0090] By charging in this constant current-constant voltage mode, the unactivated Li2MnO3 (monoclinic) phase can be minimized, thereby preventing abnormal operation during battery operation.

[0091] In the third charging stage, the constant current charging may proceed at the same third current rate as the second current rate in the second charging stage, but such a third current rate may be different from the second current rate, and in a more specific example, the third current rate can be adjusted to be lower than the second current rate. Such a third current rate can be 0.3C to 1.0C, or 0.3C to 0.6C, or 0.8C to 1.0C.

[0092] By performing constant current charging within this C-rate range, the activation depth of the Li2MnO3 (monoclinic) phase in the positive electrode active material crystal structure can be adjusted, thereby more effectively suppressing voltage sagging during high-temperature storage.

[0093] The third charging stage proceeds with constant voltage charging after constant current charging at the third current rate, and such constant voltage charging can be terminated when the charging current rate reaches 0.01C to 0.04C, preferably 0.01C to 0.035C, and more preferably 0.01C to 0.03C. By terminating the third charging stage when the charging current rate is reached, damage to the passive film formed on the electrode surface can be effectively prevented, and the collapse of the positive electrode crystal structure can be prevented. Activation of the Li2MnO3(monoclinic) phase means a phase change that allows the positive electrode active material to exhibit high capacity characteristics. Increasing the activation depth of the Li2MnO3(monoclinic) phase reduces additional phase changes during high-temperature storage, thereby reducing the voltage drop effect during high-temperature storage.

[0094] In the third charging stage, the constant current mode charging termination voltage can be 4.5V to 4.7V, specifically 4.6V. When the charging termination voltage in the third charging stage satisfies the above range, the perlithium manganese oxide is activated, enabling high capacity characteristics to be achieved.

[0095] Next, the activation step may include a step of discharging the battery cells that have been charged through the first to third charging steps.

[0096] In this case, the discharge can be performed at a current rate of 0.3C to 0.8C. When the discharge rate satisfies the above range, the activation time can be appropriately controlled, and a desired range of discharge capacity characteristics can be achieved.

[0097] On the other hand, the discharge can be performed in constant current mode (CC mode). On the other hand, the discharge termination voltage can be 2.0V to 3.0V, specifically 2.0V.

[0098] On the other hand, the activation step is preferably carried out at a temperature of 25°C to 70°C, more preferably 40°C to 50°C. When the activation step is carried out within this temperature range, the effect of achieving high capacity through appropriate activation of Li2MnO3 can be obtained.

[0099] Furthermore, the activation process may be carried out under pressurized conditions as needed. This pressurization can be performed by mounting the battery cells in a jig and then applying pressure to the battery cells through the jig. Performing the activation process under pressurized conditions has the advantages of improved electrolyte impregnation and easier gas discharge during the activation process.

[0100] On the other hand, although not essential, the activation step may further include an aging step as needed. The aging step is for the purpose of stabilizing the battery by allowing the electrolyte to be uniformly impregnated into the electrode assembly, and can be performed before charging, during charging, and / or after discharging, and may be performed one or more times.

[0101] The aging step can be carried out at temperatures such as 20°C to 60°C, 20°C to 50°C, and preferably 30°C to 50°C. When aging is carried out at the above temperatures, electrolyte impregnation and lithium mobility are improved, allowing for smoother activation.

[0102] The embodiments of the invention will be described in detail below so that they can be easily implemented by a person with ordinary skill in the art to which the invention pertains. However, the invention can be realized in a variety of different forms and is not limited to the embodiments described herein.

[0103] Examples and Comparative Examples Example 1 (Manufacturing of lithium-ion batteries) A positive electrode slurry was prepared by mixing positive electrode active material, conductive material, and PVDF binder in a weight ratio of 97:1:2 in N-methylpyrrolidone. At this time, Li was used as the positive electrode active material. 1.38 [Ni 0.363 Co 0.005 Mn 0.642 O2 was used, and carbon nanotubes (CNTs) were used as the conductive material. The positive electrode slurry was applied to an aluminum current collector sheet, dried, and then rolled to produce the positive electrode.

[0104] A negative electrode slurry was prepared by mixing a negative electrode active material, conductive material, and binder in water in a weight ratio of 96:1:3. Graphite was used as the negative electrode active material, carbon black as the conductive material, and SBR and CMC were mixed in a weight ratio of 2:1 as the binder. The negative electrode slurry was applied to a copper current collector sheet, dried, and then rolled to produce the negative electrode.

[0105] An electrode assembly was manufactured by interposing a separator membrane between the positive and negative electrodes produced as described above. After inserting the electrode assembly into a battery case, an electrolyte solution was injected to manufacture a lithium secondary battery.

[0106] (Activation stage) After pre-aging the lithium secondary battery for two days, it was charged at 45°C in constant current-constant voltage mode. The first charging stage involved charging at a current rate of 0.2C until the SOC reached 3%, the second charging stage involved charging at a current rate of 1.0C until the SOC reached 60%, and then constant current charging at a current rate of 0.4C until the voltage reached 4.6V. After that, the charging was switched to constant voltage mode, and the charging was terminated when the charging current rate reached 0.02C (third charging stage). Subsequently, the charged lithium secondary battery was discharged to 2.0V with a constant current of 0.6C.

[0107] Example 2 (Activation stage) The lithium secondary battery manufactured in Example 1 was pre-aged for two days, then charged at 45°C at a current rate of 0.2C until the State of Charge (SOC) reached 3%, and then charged at a current rate of 1.0C until the SOC reached 100% and the voltage reached 4.6V. After that, the charging mode was changed to constant voltage mode and charging was stopped when the charging current rate reached 0.02C. Subsequently, the charged lithium secondary battery was activated by discharging it to 2.0V with a constant current of 0.6C.

[0108] Comparative Example 1 In the third charging stage, the lithium secondary battery manufactured in Example 1 was charged to 4.6V at a current rate of 0.4C, then switched to constant voltage mode for charging, and the charging was terminated when the charging current rate reached 0.05C. The lithium secondary battery was activated in the same manner as in Example 1.

[0109] Comparative Example 2 In the third charging stage, the lithium secondary battery manufactured in Example 1 was charged to 4.6V at a current rate of 0.4C, then switched to constant voltage mode for charging, and the charging was terminated when the charging current rate reached 0.15C. The lithium secondary battery was activated in the same manner as in Example 1.

[0110] Experimental Example 1: Li 2 MnO 3 Evaluation of the activation ratio After calculating the Li2MnO3 activation ratio (%) for lithium secondary batteries activated by the methods of Examples 1 and 2, and Comparative Examples 1 and 2, the Li2MnO3 activation ratio (%) of each was expressed as a percentage, using the Li2MnO3 activation ratio (%) of Comparative Example 1 as the baseline, and is shown in Table 1 below. At this time, the Li2MnO3 activation ratio (%) of each lithium secondary battery can be calculated by dividing the charge capacity of the cell obtained by charging from 4.3V to the charge termination voltage by the charge capacity of the cell when charging from 2.0V to 4.6V.

[0111] [Table 1]

[0112] As shown in [Table 1] above, it can be confirmed that the activation ratio of Li2MnO3 increased in the lithium secondary batteries activated by the methods of Examples 1 and 2 compared to the lithium secondary batteries activated by the comparative example method.

[0113] Experimental Example 2: Evaluation of Voltage Drop The lithium secondary batteries activated by the methods of Examples 1 and 2, and Comparative Examples 1 and 2, were each charged in constant current-constant voltage mode at 25°C and 0.33C until the charging current rate reached 0.05C and the voltage reached 4.35V. They were then discharged at a constant current of 0.33C until the voltage reached 2.0V. After storing these batteries at 60°C in a SOC 100 state for 4 weeks, they were again driven at 25°C within a 0.33C voltage range. The percentage decrease in voltage compared to the initial voltage is calculated and shown in Table 2 and Figure 1 below.

[0114] In this case, the voltage sagging is the value obtained by subtracting the nominal voltage after high-temperature storage from the initial nominal voltage, and the nominal voltage (normal voltage) refers to the value of the discharge energy relative to the discharge capacity at 0.33C.

[0115] [Table 2]

[0116] As shown in Table 2 and Figure 1 above, it can be confirmed that the lithium secondary battery activated according to the method of the example did not experience a significant increase in voltage drop during high-temperature storage, whereas the lithium secondary battery activated according to the method of the comparative example showed a significant increase in voltage drop during high-temperature storage.

[0117] On the other hand, it can be confirmed that the lithium secondary battery activated according to the method of Comparative Example 2 has a larger voltage drop than the lithium secondary battery activated according to the method of Comparative Example 1. The activation ratio of the rock salt phase can be adjusted by adjusting the charging capacity for Li2MnO3 in a certain section of the charging stage of the activation process. However, at SOC 60 to 100, while charging in constant current-constant voltage mode (CC-CV mode), the charging depth decreases as the charging termination current rate increases during the constant voltage mode (CV mode) charging, thereby decreasing the activation ratio of Li2MnO3. As a result, the change in crystal structure within the perlithium-based transition metal oxide during high-temperature storage increases even more, and the change in nominal voltage increases even more, which can increase the voltage drop.

Claims

1. The process includes the steps of preparing a battery cell containing a positive electrode, a negative electrode, and an electrolyte containing a perlithium manganese oxide in which the manganese content in the total metal excluding lithium exceeds 50 mol%, and the ratio of the number of moles of lithium to the number of moles of the total metal excluding lithium (Li / Me) exceeds 1; and activating the battery cell by charging and discharging it at least once. In the activation step, the charging is performed in constant current-constant voltage mode, and the charging is terminated when the charging current rate reaches 0.01C to 0.04C. A method for manufacturing a lithium secondary battery, wherein the charging current rate is 0.8C to 1.2C in the region where the State of Charge (SOC) of the battery cell is 3 to 60%.

2. The method for manufacturing a lithium secondary battery according to claim 1, wherein the charging termination voltage is 4.5V to 4.7V.

3. In the activation step, the charging consists of a first charging step in which constant current charging is performed at a first current rate, A second charging stage in which constant current charging is performed at a second current rate different from the first current rate, A method for manufacturing a lithium secondary battery according to claim 1 or 2, comprising a third charging step of performing constant current charging at a third current speed different from the first current speed, followed by constant voltage charging.

4. The method for manufacturing a lithium secondary battery according to claim 3, wherein the first current rate, the second current rate, and the third current rate are each independently 0.1C to 1.2C.

5. A method for manufacturing a lithium secondary battery according to claim 3, wherein the first charging stage is performed until the SOC is 0 to 3%, the second charging stage is performed until the SOC is 3 to 60%, and the third charging stage is performed until the SOC is 60 to 100%.

6. The method for manufacturing a lithium secondary battery according to claim 3, wherein the first current rate is 0.1C to 0.3C.

7. The method for manufacturing a lithium secondary battery according to claim 3, wherein the third current rate is 0.3C to 1.0C.

8. The method for manufacturing a lithium secondary battery according to claim 1, wherein in the activation step, discharge is performed at a current rate of 0.3C to 0.8C until the voltage reaches 2.0V to 3.0V.

9. The method for producing a lithium secondary battery according to claim 1, wherein the perlithium manganese oxide is represented by the following chemical formula 1. [Chemical formula 1] Li ａ Ni ｂ Co ｃ Mn ｄ M ｅ O ２ In the above chemical formula 1, 1.0 < a, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.1, 0.5 ≤ d ≤ 1.0, and 0 ≤ e ≤ 0.

2. M is at least one selected from the group consisting of Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr, and Zr.

10. A method for manufacturing a lithium secondary battery according to claim 9, wherein in the chemical formula 1, 1.2 ≤ a ≤ 1.5, 0.1 ≤ b ≤ 0.4, 0 ≤ c ≤ 0.05, 0.5 ≤ d ≤ 0.8, and 0 ≤ e ≤ 0.1.

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