Method for manufacturing lithium secondary battery

Abstract

The present invention provides a method for manufacturing a lithium secondary battery, the method comprising the steps of: preparing a battery cell including a positive electrode, a negative electrode, and an electrolyte, the positive electrode containing an overlithiated manganese-based oxide in which the content of manganese among all metals excluding lithium exceeds 50 mol % and the ratio of the number of moles of lithium to the number of moles of all metals excluding lithium (Li / Me) exceeds 1; and activating the battery cell by charging and discharging the battery cell at least once, wherein in the activation step, the charging is performed in a constant current-constant voltage mode, the charging is terminated when the charging current speed reaches 0.01 C to 0.04 C, and the charging current speed is 0.8 C to 1.2 C in a region where the state of charge (SOC) of the battery cell is 3 to 60%.

Description

Method for manufacturing lithium secondary batteries

[0001] Cross-citation with related application(s)

[0002] This application claims the benefit of priority to Korean Patent Application No. 10-2023-0118663, filed September 6, 2023, and Korean Patent Application No. 10-2024-0119900, filed September 4, 2024, the entire contents of which are incorporated herein by reference.

[0003] The present invention relates to a method for manufacturing a lithium secondary battery, and more specifically, to a method for manufacturing a lithium secondary battery capable of improving a voltage drop caused in an activation step of a lithium secondary battery including a lithium manganese oxide.

[0004] Lithium secondary batteries have been used as an energy storage medium in a variety of fields since their commercialization in 1991. As the market for products equipped with lithium secondary batteries expands, research is actively underway to increase their energy density. One of the most notable approaches is the development of cathode active materials with a composition that allows for greater lithium availability than previously possible.

[0005] As positive electrode active materials that can utilize more lithium, perlithium-based transition metal oxides with a layered structure and a molar ratio of lithium to transition metal exceeding 1 are being developed. These perlithium-based transition metal oxides can realize high capacities because they utilize not only the cation redox of the transition metal but also the anion redox reaction using oxygen within the positive electrode structure to realize capacity. A representative perlithium-based transition metal oxide that is currently being actively researched is perlithium manganese-based oxides that have a molar ratio of lithium to transition metal exceeding 1 and a manganese content of 50 mol% or more among the total transition metal. The above lithium manganese oxide has a mixed structure of LiMO2 (where M is a transition metal) with a layered structure and Li2MnO3 with a rock salt structure, and by performing an activation process at a high voltage of 4.4 V or higher, Li2MnO3 is activated, thereby enabling high capacity to be realized.

[0006] However, in this high-voltage activation process, if the activation of the positive electrode active material is not performed properly, a problem may occur in which the voltage drop is further increased due to a significant change in the crystal structure within the lithium manganese oxide during high-temperature storage.

[0007] The present invention provides a method for manufacturing a lithium secondary battery using a lithium manganese oxide, which can improve voltage drop that may occur during high-temperature storage by performing activation under certain conditions.

[0008] In one aspect of the invention, a method for producing a lithium-ion battery comprises the steps of preparing a battery cell comprising a positive electrode, a negative electrode, and an electrolyte, wherein the content of manganese among all metals excluding lithium exceeds 50 mol% and the ratio of the number of moles of lithium to the number of moles of all metals excluding lithium (Li / Me) exceeds 1; and a step of activating the battery cell by charging and discharging it at least once.

[0009] In the above activation step, the charging is performed in a constant current-constant voltage mode, and the charging is terminated when the charging current rate reaches 0.01 C to 0.04 C, and a method for manufacturing a lithium secondary battery is provided in which the charging current rate is 0.8 C to 1.2 C in a region where the state of charge (SOC) of the battery cell is 3 to 60%.

[0010] The above charging end voltage can be 4.5 V to 4.7 V.

[0011] In the above activating step, the charging may include a first charging step performing constant current charging at a first current rate, a second charging step performing constant current charging at a second current rate different from the first current rate, and a third charging step performing constant current charging at a third current rate different from the first current rate and then constant voltage charging. At this time, the first current rate, the second current rate, and the third current rate may each independently be 0.1 C to 1.2 C.

[0012] The first charging step may be performed from SOC 0 to 3%, the second charging step may be performed from SOC 3 to 60%, and the third charging step may be performed from SOC 60 to 100%. At this time, the second current rate may be 0.8C to 1.2C. In addition, the first current rate may be 0.1C to 0.3C, and the third current rate may be 0.3C to 1.0C.

[0013] In the above activating step, the discharge can be performed at a current rate of 0.3 C to 0.8 C until it reaches 2.0 V to 3.0 V, for example, 2.0 V.

[0014] The above lithium manganese oxide may be represented by the following chemical formula 1.

[0015] [Chemical Formula 1]

[0016] Li aNi b Co c Mn d M e O2

[0017] In the above chemical formula 1, 1.0 <a, 0≤b≤0.5, 0≤c≤0.1, 0.5≤d≤1.0, 0≤e≤0.2이고,

[0018] 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. In the chemical formula 1, 1.2≤a≤1.5, 0.1≤b≤0.4, 0≤c≤0.05, 0.5≤d≤0.8, 0≤e≤0.1 may be satisfied.

[0019] In the above method for manufacturing a lithium secondary battery, the activation charging step is performed in a constant current-constant voltage mode, charging is terminated when the charging current rate reaches 0.01 C to 0.04 C, and the charging current rate can be performed at 0.8 C to 1.2 C in a region where the state of charge (SOC) of the battery cell is 3 to 60%.

[0020] Accordingly, the activation ratio of Li2MnO3, which is a rock salt phase within the crystal structure of the positive electrode active material, was increased during high-temperature storage, effectively improving the voltage drop.

[0021] Figure 1 is a graph showing the high-temperature storage characteristics of lithium secondary batteries activated by the methods of Examples 1 and 2 and Comparative Examples 1 and 2.

[0022] The advantages and features of the invention, and the methods for achieving them, will become clearer with reference to the embodiments described in detail below, along with the accompanying drawings. However, the invention is not limited to the embodiments disclosed below and may be implemented in various different forms. These embodiments are provided solely to ensure complete disclosure of the invention and to fully inform those skilled in the art of the invention of the scope of the invention. The invention is defined solely by the scope of the claims.

[0023] Unless otherwise defined, all terms (including technical and scientific terms) used herein may be used in their common sense to those 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 explicitly and specifically defined otherwise.

[0024] The terminology used herein is for the purpose of describing embodiments and is not intended to limit the invention. In this specification, singular forms also include plural forms, unless specifically stated otherwise. As used herein, the terms "comprises" and / or "comprising" do not exclude the presence or addition of one or more other components in addition to the components mentioned.

[0025] In this specification, when it is said that a part includes a certain component, this does not mean that other components are excluded, but rather that other components may be included, unless specifically stated otherwise.

[0026] In this specification, the description of “A and / or B” means A, or B, or A and B.

[0027] In this specification, "primary particle" means a particle unit that has no apparent grain boundary when observed under a magnification of 5,000 to 20,000 times using a scanning electron microscope. "Average particle diameter of primary particles" means the arithmetic mean value calculated after measuring the particle diameters of primary particles observed in a scanning electron microscope image.

[0028] In this specification, a “secondary particle” is a particle formed by the agglomeration of a plurality of primary particles.

[0029] In this specification, "average particle diameter D 50 " means the particle size based on 50% of the volume cumulative particle size distribution of the target particle powder (e.g., positive electrode active material powder, negative electrode active material powder, etc.). The average particle diameter D 50 can be measured using the laser diffraction method. For example, after dispersing the powder of the particles to be measured in a dispersion medium, it can be measured by introducing it into a commercially available laser diffraction particle size measuring device (e.g., Microtrac MT 3000), irradiating it with ultrasonic waves of approximately 28 kHz at an output of 60 W, obtaining a volume cumulative particle size distribution graph, and then calculating the particle size corresponding to 50% of the volume cumulative amount.

[0030] In this specification, “SOC X%” means a state in which the percentage of the capacity charged in the battery cell to the discharge capacity that appears when the battery cell is discharged from 4.6 V to 2.0 V (i.e., (charged capacity / discharge capacity of the battery cell in the voltage range of 4.6 V to 2.0 V) 100) is X%.

[0031]

[0032] The inventors of the present invention have conducted repeated research to improve the voltage drop that may occur during high-temperature storage of a lithium secondary battery using a lithium manganese oxide, and have found that this can be improved by performing an activation process under specific charging conditions during the manufacture of a lithium secondary battery, thereby completing the invention.

[0033] Hereinafter, a method for manufacturing a lithium secondary battery according to an embodiment of the invention will be described.

[0034] A method for manufacturing a lithium secondary battery according to one embodiment comprises: (1) preparing a battery cell including a positive electrode, a negative electrode, and an electrolyte including a lithium manganese oxide in which the content of manganese among all metals excluding lithium exceeds 50 mol% and the ratio of the number of moles of lithium to the number of moles of all metals excluding lithium (Li / Me) exceeds 1; and (2) activating the battery cell by charging and discharging it at least once.

[0035] In the above activation step, the charging is performed in a constant current-constant voltage mode, and the charging is terminated when the charging current rate reaches 0.01 C to 0.04 C, and in a region where the state of charge (SOC) of the battery cell is 3 to 60%, the charging current rate becomes 0.8 C to 1.2 C.

[0036] (1) Step for preparing battery cells

[0037] First, a battery cell containing a positive electrode, a negative electrode, and an electrolyte is prepared.

[0038] The above battery cell can be manufactured, for example, by forming an electrode assembly including a positive electrode and a negative electrode, housing the electrode assembly in a battery case, injecting an electrolyte, and sealing the battery case. At this time, the electrode assembly can include a separator between the positive electrode and the negative electrode.

[0039] Below, each component of these battery cells is described in more detail.

[0040] anode

[0041] Meanwhile, in the battery cell, the positive electrode includes a lithium manganese-based oxide having a manganese content exceeding 50 mol% among all metals excluding lithium as a positive electrode active material and a ratio of the number of moles of lithium to the number of moles of all metals excluding lithium (Li / Me) exceeding 1. Specifically, the positive electrode includes a positive electrode current collector, 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 includes a lithium manganese-based oxide having a ratio of the number of moles of lithium to the number of moles of all metals excluding lithium (Li / Me) exceeding 1.

[0042] In the case of lithium manganese oxide containing an excess of lithium, it has a structure in which a layered phase (LiM'O2) and a rock salt phase (Li2MnO3) are mixed, and when the rock salt phase is activated during the initial activation process, an excess amount of lithium ions are generated, enabling high capacity to be realized.

[0043] In a specific example, the lithium manganese oxide may be represented by chemical formula 1.

[0044] [Chemical Formula 1]

[0045] Li a Ni b Co c Mn d M e O2

[0046] In the above chemical formula 1, M may be 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.

[0047] Meanwhile, a is the molar ratio of Li in the lithium manganese oxide, which is 1.0. <a, 1.1≤a≤1.5, 1.2≤a≤1.5, 또는 1.1≤a≤1.3일 수 있다. a가 이러한 범위를 충족하여 음극의 비가역 용량을 충분히 보상할 수 있고, 고용량 특성을 구현할 수 있다.

[0048] The above b is the molar ratio of Ni in the lithium manganese oxide, and may be 0≤b≤0.5, 0.1≤b≤0.4, or 0.2≤b≤0.4.

[0049] The above c is the molar ratio of Co in the lithium manganese oxide, and may be 0≤c≤0.1, 0≤c≤0.08, or 0≤c≤0.05. When c exceeds 0.1, it is difficult to secure high capacity, and gas generation and degradation of the positive electrode active material may be aggravated, resulting in a deterioration in the life characteristics.

[0050] The above d is the molar ratio of Mn in the lithium manganese oxide, and may be 0.5≤d<1.0, 0.5≤d≤0.8, or 0.5≤d≤0.7. When d is less than 0.5, the ratio of the rock salt phase is too small, so the negative electrode irreversible capacity compensation and capacity improvement effects are minimal.

[0051] The above e is the molar ratio of the doping element M in the lithium manganese oxide, and may be 0≤e≤0.2, 0≤e≤0.1, or 0≤e≤0.05. If the content of the doping element is too high, it may have a negative effect on the capacity of the active material.

[0052] Meanwhile, in the lithium manganese oxide represented by the above chemical formula 1, the molar ratio of Li to the molar ratio of all metal elements excluding Li (Li / Me) may 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, the rate characteristics and capacity characteristics are excellent. If the Li / Me ratio is too high, the electrical conductivity may decrease and the rock salt phase (Li2MnO3) may increase, which may accelerate the degradation rate, and if it is too low, the effect of improving the energy density is minimal.

[0053] Meanwhile, the composition of the above lithium manganese oxide may be represented by the following chemical formula 2.

[0054] [Chemical Formula 2]

[0055] X Li2MnO3·(1-X)Li[Ni 1-y-z-w Mn y Coz M w ]O2

[0056] In the above chemical formula 2, M may 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.

[0057] The above X represents the ratio of the rock salt phase (Li2MnO3) in the lithium manganese oxide, and may be 0.2≤X≤0.5, 0.25≤X≤0.5, or 0.25≤X≤0.4. When the ratio of the Li2MnO3 phase in the lithium manganese oxide satisfies the above range, the irreversible capacity of the silicon-based negative electrode active material can be sufficiently compensated, and high-capacity characteristics can be implemented.

[0058] The above y is the molar ratio of Mn on the LiM'O2 layer, and may be 0.4≤y<1, 0.4≤y≤0.8, or 0.4≤y≤0.7.

[0059] The above z is the molar ratio of Co on the LiM'O2 layer, and may be 0≤z≤0.1, 0≤z≤0.08, or 0≤z≤0.05. When z exceeds 0.1, gas generation and degradation of the positive electrode active material may be aggravated, resulting in a deterioration in the life characteristics.

[0060] The above w is the molar ratio of the doping element M on the LiM'O2 layer, and may be 0≤w≤0.2, 0≤w≤0.1, or 0≤w≤0.05.

[0061] Meanwhile, the positive electrode active material may further include a coating layer on the surface of the lithium manganese oxide, if necessary. When the positive electrode active material includes a coating layer, the contact between the lithium manganese oxide and the electrolyte is suppressed by the coating layer, thereby reducing electrolyte side reactions, and thereby improving the life characteristics.

[0062] The above coating layer comprises a coating element M 1 may include the coating element M 1For example, the coating element M may be 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. 1 It may contain two or more kinds, for example, it may contain Al and Co.

[0063] The above coating element is in the form of oxide within the coating layer, i.e., M 1 It can exist as Oz(1≤z≤4).

[0064] The above coating layer can be formed through methods such as dry coating, wet coating, chemical vapor deposition (CVD), physical vapor deposition (PVD), and atomic layer deposition (ALD). Among these, formation through atomic layer deposition is preferable because it can form a large coating layer area.

[0065] The formation area of ​​the above coating layer may be 10% to 100%, or 30% to 100%, or 50% to 100% based on the total surface area of ​​the lithium manganese oxide particles. When the formation area of ​​the coating layer satisfies the above range, the effect of improving the life characteristics is excellent.

[0066] Meanwhile, the positive electrode active material may be in the form of secondary particles in which a plurality of primary particles are aggregated, and the average particle diameter of the secondary particles is D 50 This may be 2㎛ to 10㎛, or 2㎛ to 8㎛, or 4㎛ to 8㎛. D of the positive electrode active material 50 When this above range is satisfied, the electrode density can be implemented excellently, and the deterioration of capacity and rate characteristics can be minimized.

[0067] In addition, the above-mentioned positive electrode active material has a BET specific surface area of ​​1 m 2 / g to 10m 2 / g, 3m 2 / g to 8m 2 / g or 4m 2 / g to 6m 2 / g. If the BET surface area of ​​the positive electrode active material is too low, the reaction area with the electrolyte is insufficient, making it difficult to achieve sufficient capacity. If the surface area is too high, moisture absorption is rapid, and side reactions with the electrolyte are accelerated, making it difficult to secure life characteristics.

[0068] Meanwhile, the above lithium manganese oxide can be manufactured by mixing a transition metal precursor and a lithium raw material and then calcining them.

[0069] Examples of the lithium raw material include lithium-containing carbonates (e.g., lithium carbonate, etc.), hydrates (e.g., lithium hydroxide hydrate (LiOH·H2O)), hydroxides (e.g., lithium hydroxide, etc.), nitrates (e.g., lithium nitrate (LiNO3)), chlorides (e.g., lithium chloride (LiCl)), etc., and one of these may be used alone or a mixture of two or more may be used.

[0070] Meanwhile, the transition metal precursor may be in the form of a hydroxide, oxide, or carbonate. Using a precursor in the form of a carbonate is more preferable because it allows for the production of a cathode active material with a relatively high specific surface area.

[0071] The above transition metal precursor can be prepared through a co-precipitation process. For example, the above transition metal precursor can be prepared by dissolving each transition metal-containing raw material in a solvent to prepare a metal solution, then mixing the metal solution, an ammonium cation complex forming agent, and a basic compound, and then performing a co-precipitation reaction. Additionally, an oxidizing agent or oxygen gas can be additionally added during the co-precipitation reaction, if necessary.

[0072] At this time, the transition metal-containing raw material may be an acetate, carbonate, nitrate, sulfate, halide, sulfide, etc. of each transition metal. Specifically, the transition metal-containing raw material may be NiO, NiCO3·2Ni(OH)2·4H2O, NiC2O2·2H2O, Ni(NO3)2·6H2O, NiSO4, NiSO4·6H2O, Mn2O3, MnO2, Mn3O4MnCO3, Mn(NO3)2, MnSO4·H2O, manganese acetate, manganese halide, cobalt sulfate, cobalt nitrate, cobalt carbonate, cobalt oxide, cobalt acetate, cobalt halide, etc.

[0073] The above ammonium cation complex forming agent may be at least one selected from the group consisting of NH4OH, (NH4)2SO4, NH4NO3, NH4Cl, CH3COONH4, and NH4CO3.

[0074] The basic compound may be at least one selected from the group consisting of NaOH, Na2CO3, KOH, and Ca(OH)2. The form of the precursor may vary depending on the type of basic compound used. For example, when NaOH is used as the basic compound, a precursor in the form of a hydroxide may be obtained, and when Na2CO3 is used as the basic compound, a precursor in the form of a carbonate may be obtained. In addition, when a basic compound and an oxidizing agent are used together, a precursor in the form of an oxide may be obtained.

[0075] Meanwhile, the transition metal precursor and the lithium raw material may be mixed in an amount such that the molar ratio of the total transition metal (Ni+Co+Mn):Li 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.

[0076] Meanwhile, the firing may be performed at a temperature of 600°C to 1000°C or 700°C to 950°C, and the firing time may be 5 hours to 30 hours or 5 hours to 20 hours. In addition, the firing atmosphere may be an air atmosphere or an oxygen atmosphere, and for example, may be an atmosphere containing 20% ​​to 100% by volume of oxygen.

[0077] Meanwhile, the positive electrode active material layer may further include a conductive material and a binder in addition to the positive electrode active material.

[0078] Examples of the conductive material include spherical or flaky graphite; carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, summer black, carbon fibers, single-walled carbon nanotubes, multi-walled carbon nanotubes, and the like; 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, and one of these may be used alone or a mixture of two or more thereof. The conductive material may be included in an amount of 0.1 wt% to 20 wt%, 1 wt% to 20 wt%, or 1 wt% to 10 wt% based on the total weight of the positive electrode active material layer.

[0079] In addition, the binder may include, for example, polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated-EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one of these may be used alone or a mixture of two or more thereof. The binder may be included in an amount of 1 wt% to 20 wt%, 2 wt% to 20 wt%, or 2 wt% to 10 wt% based on the total weight of the positive electrode active material layer.

[0080] cathode

[0081] Next, the negative electrode may include, for example, a negative electrode current collector and a negative electrode active material layer positioned on the negative electrode current collector. The negative electrode active material layer may optionally include a binder and a conductive material together with the negative electrode active material.

[0082] The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. can be used. In addition, the negative electrode current collector can typically have a thickness of 3 ㎛ to 500 ㎛, and like the positive electrode current collector, fine unevenness can be formed on the surface of the current collector to strengthen the bonding strength of the negative electrode active material. For example, it can be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a non-woven fabric, etc.

[0083] As the negative electrode active material, a compound capable of reversible intercalation and deintercalation of lithium may 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 alloy, Sn alloy, or Al alloy; SiO β (0 <β< 2), metal oxides capable of doping and dedoping lithium, such as SnO2, vanadium oxide, and lithium vanadium oxide; or composites containing the above metallic compounds and carbonaceous materials, such as Si-C composites or Sn-C composites, and any one or a mixture of two or more of these may be used.

[0084] In addition, a metallic lithium thin film may be used as the negative active material. In addition, both low-crystalline carbon and high-crystalline carbon may be used as the carbon material. Representative examples of low-crystalline carbon include soft carbon and hard carbon, and representative examples of high-crystalline carbon include amorphous, plate-like, flaky, spherical or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch derived cokes.

[0085] The conductive material is used to provide conductivity to the electrode, and in the battery to be formed, as long as it does not cause a chemical change and has electronic conductivity, it can be used without any particular limitation. 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 nanotube; metal powder or metal fiber such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and the like. One type alone or a mixture of two or more types of these may be used. The conductive material may typically be included 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.

[0086] The above binder serves to improve adhesion between negative electrode active material particles and adhesion between the negative electrode active material and the negative electrode current collector. Specific examples thereof include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one of these may be used alone or a mixture of two or more thereof may be used. The above binder may be included 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.

[0087] The above-described negative electrode active material layer may be manufactured by, for example, applying a negative electrode slurry containing a negative electrode active material and optionally a binder and a conductive material onto a negative electrode current collector and drying it, or by casting the negative electrode slurry onto a separate support and then laminating the resulting film onto a negative electrode current collector by peeling it off from the support.

[0088] membrane

[0089] Next, the separator separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. Any separator commonly used in lithium secondary batteries can be used without any particular restrictions, and in particular, one having low resistance to ion movement of the electrolyte and excellent electrolyte moisture retention capacity is preferable. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof, may be used. In addition, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of high-melting-point glass fiber, polyethylene terephthalate fiber, etc. may be used. In addition, a coated separator containing a ceramic component or a polymer material to secure heat resistance or mechanical strength may be used, and may optionally be used in a single-layer or multi-layer structure.

[0090] electrolyte

[0091] Next, the electrolyte may be any of various electrolytes usable in a lithium secondary battery, for example, an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, a molten inorganic electrolyte, or a combination thereof, and the type thereof is not particularly limited.

[0092] For example, the electrolyte may include an organic solvent and a lithium salt.

[0093] The organic solvent may be used without any particular limitation as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the organic solvent may 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; Carbonate solvents such as dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (where R represents a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms and may include a double-bonded aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes can be used.

[0094] The above lithium salt can be used without any special limitation as long as it is a compound that can provide lithium ions used in a lithium secondary battery. Specifically, the anion of the above lithium salt is 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 - At least one selected from the group consisting of, and the lithium salt is LiPF6, LiN(FSO2)2, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAl04, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2) 2. LiCl, LiI, or LiB(C2O4)2 can be used. It is preferable to use the lithium salt at a concentration in the range of 0.1M to 5.0M.

[0095] In addition, the electrolyte may further include additives in addition to the above components for the purpose of improving the life characteristics of the battery, suppressing the decrease in battery capacity, and improving the discharge capacity of the battery. For example, the additives may include haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethylphosphite, triethanolamine, cyclic ethers, ethylene diamine, n-glyme, hexamethylphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, which may be used alone or in combination, but are not limited thereto. The additives may be included 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.

[0096] Meanwhile, the electrode assembly may be of various types 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 type is not particularly limited.

[0097] A jelly-roll type electrode assembly can be manufactured by a method of interposing a sheet-shaped separator between a sheet-shaped positive electrode and a sheet-shaped negative electrode and then winding the separator in one direction.

[0098] A stacked electrode assembly can be manufactured by cutting the anode, separator, and cathode into a desired shape and then sequentially stacking the cut anode / separator / cathode.

[0099] A stack and lamination type electrode assembly can be manufactured by a method of manufacturing a plurality of unit cells by stacking a positive electrode, a separator, and a negative electrode, stacking the plurality of unit cells with a separator interposed therebetween, and then laminating them through a method such as heating.

[0100] A stack-and-fold electrode assembly can be manufactured by stacking a positive electrode, a separator, and a negative electrode to manufacture a plurality of unit cells, arranging the plurality of unit cells on one or both sides of a long folding separator, and then winding the folding separator.

[0101] Meanwhile, as the above electric case, various battery cases known in the art, such as a cylindrical battery case, a square battery case, or a pouch-type battery case, can be used, and the type thereof is not particularly limited.

[0102] (2) Activation phase

[0103] Next, a step of electrically activating the battery by charging and discharging the battery cell at least once is performed. The activation step is a process of charging and discharging the battery cell to impart electrical characteristics and forming a solid electrolyte interphase (SEI) film on the electrode to stabilize the battery.

[0104] In an embodiment of the invention, by controlling the activation depth for Li2MnO3(monoclinic) in some sections of the activation charge step, the rate of change of the Li2MnO3(monoclinic) phase can be increased, i.e., activation can be increased, and thus the voltage drop that may occur during high-temperature storage can be effectively improved.

[0105] First, the charging may include a first charging step of charging at a state of charge (SOC) of 3% or less.

[0106] The first charging step may be performed at a first current rate of 0.3 C or less, or 0.1 C to 0.3 C. If the current rate in the first charging step is faster than 0.3 C, an SEI film may be formed unstably on the electrode surface. If the SEI film is formed unstably on the electrode surface, the SEI film may be easily decomposed during battery operation, causing rapid deterioration of the electrode, which may significantly deteriorate the life characteristics.

[0107] The above first charging step is preferably performed at a SOC of 0 to 3%. When the state of charge in the first charging step satisfies the above range, a solid and dense SEI film is formed on the electrode surface, thereby realizing excellent life characteristics.

[0108] Next, the charging may include a second charging step of charging at a second current rate of 0.8 C to 1.2 C, preferably 0.9 C to 1.1 C, at a state of charge (SOC) of 3 to 60%. This second current rate may be different from the first current rate of the first charging step, and more specifically, may be a current rate greater than the first current rate.

[0109] When the second charging step is performed at 0.8C to 1.2C, the activation of the Li2MnO3 (monoclinic) phase within the crystal structure of the positive electrode active material is increased, thereby suppressing oxygen desorption and cation mixing during high-temperature storage, and reducing the phase change rate of the additional positive electrode active material, thereby improving the voltage drop that may occur during high-temperature storage. In addition, by performing the second charging step at a relatively fast C-rate, the time required for the activation process is reduced, thereby shortening the battery production time.

[0110] In addition, when the charge capacity in the second charging step satisfies the above range, the incomplete formation of the SEI film can be suppressed, and the occurrence of oxygen desorption and cation mixing during the activation process can be suppressed, thereby minimizing the increase in anode resistance.

[0111] Next, the charging may include a third charging step of charging in a constant current mode at a third current rate different from the first current rate, for example, up to a charge termination voltage, at a state of charge (SOC) of 60 to 100%, and then charging in a constant voltage mode.

[0112] By performing charging in a constant current-constant voltage mode in this way, the non-activated Li2MnO3 (monoclinic) phase can be minimized, thereby preventing abnormal behavior from occurring during battery operation.

[0113] In the third charging step, the constant current charging may proceed at a third current rate that is the same as the second current rate of the second charging step, but this third current rate may be different from the second current rate, and in a more specific example, the third current rate may be adjusted to be smaller than the second current rate. This third current rate may be 0.3C to 1.0C, or 0.3C to 0.6C, or 0.8C to 1.0C.

[0114] By performing a constant current charge in the C-rate range as described above, the activation depth of the Li2MnO3 (monoclinic) phase within the crystal structure of the cathode active material can be controlled, thereby more effectively suppressing voltage sagging during high-temperature storage.

[0115] The third charging step performs constant voltage charging after constant current charging under the third current rate, and the constant voltage charging can be terminated when the charging current rate reaches 0.01 C to 0.04 C, preferably 0.01 C to 0.035 C, and more preferably 0.01 C to 0.03 C. By terminating the third charging step when the charging current rate is reached, damage to the passive film formed on the electrode surface can be effectively prevented, thereby preventing collapse of the crystal structure of the positive electrode. Activation of the Li2MnO3(monoclinic) phase refers to a change in the phase that can exhibit high-capacity characteristics of the positive electrode active material. When the activation depth of the Li2MnO3(monoclinic) phase increases, additional phase changes can be reduced during high-temperature storage, which can reduce the voltage enhancement effect during high-temperature storage.

[0116] In the third charging step, the constant current mode charging end voltage may be 4.5 V to 4.7 V, specifically 4.6 V. When the charging end voltage of the third charging step satisfies the above range, the lithium manganese oxide is activated, thereby realizing high capacity characteristics.

[0117] Next, the activation step may include a step of discharging the battery cell charged through the first charging step to the third charging step.

[0118] At this time, the discharge can be performed at a current rate of 0.3 C to 0.8 C. When the discharge rate satisfies the above range, the activation time can be appropriately controlled, and discharge capacity characteristics within the desired range can be implemented.

[0119] Meanwhile, the above discharge can be performed in constant current mode (CC mode).

[0120] Meanwhile, the discharge termination voltage may be 2.0 V to 3.0 V, specifically 2.0 V.

[0121] Meanwhile, the above activation process is preferably performed at a temperature of 25°C to 70°C, preferably 40°C to 50°C. When the activation process is performed within the above temperature range, a high-capacity implementation effect can be achieved through appropriate activation of Li2MnO3.

[0122] Additionally, the activation process may be performed under pressurized conditions, if necessary. This pressurization can be performed by mounting the battery cell on a jig and then applying pressure to the battery cell through the jig. Performing the activation process under pressurized conditions offers the advantages of improved electrolyte impregnation and facilitating the discharge of gases generated during the activation process.

[0123] Meanwhile, although not essential, the activating step may further include an aging step, if necessary. The aging step is intended to ensure that the electrolyte is uniformly impregnated into the electrode assembly and to stabilize the battery. The aging step may be performed before charging, during charging, and / or after discharging, and may be performed more than once.

[0124] The above aging step may be performed at a temperature of, for example, 20°C to 60°C, 20°C to 50°C, and preferably 30°C to 50°C. When aging is performed at the above temperature, electrolyte impregnation and lithium mobility are improved, allowing activation to be performed more smoothly.

[0125]

[0126] Hereinafter, embodiments of the invention will be described in detail so that those skilled in the art can easily implement the invention. However, the invention may be implemented in various different forms and is not limited to the embodiments described herein.

[0127]

[0128] Examples and Comparative Examples

[0129] Example 1

[0130] (Lithium secondary battery manufacturing)

[0131] A cathode slurry was prepared by mixing the cathode active material: conductive material: PVDF binder in a weight ratio of 97:1:2 in N-methylpyrrolidone. At this time, Li was used as the cathode 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 onto an aluminum current collector sheet, dried, and then rolled to manufacture a positive electrode.

[0132] A negative electrode slurry was prepared by mixing a negative electrode active material, a conductive material, and a binder in a weight ratio of 96:1:3 in water. At this time, graphite was used as the negative electrode active material, carbon black was used 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 onto a copper current collector sheet, dried, and then rolled to prepare a negative electrode.

[0133] An electrode assembly was manufactured by interposing a separator between the positive and negative electrodes manufactured as described above, and the electrode assembly was inserted into a battery case and an electrolyte was injected to manufacture a lithium secondary battery.

[0134]

[0135] (Activation phase)

[0136] After pre-aging the lithium secondary battery for 2 days, it was charged in a constant current-constant voltage mode at 45°C, charging at a current rate of 0.2C to SOC 3% (first charging stage), charging at a current rate of 1.0C to SOC 60% (second charging stage), constant current charging at a current rate of 0.4C to 4.6 V, then changing to constant voltage mode and charging was performed, and charging was terminated when the charging current rate reached 0.02 C (third charging stage). Then, the charged lithium secondary battery was discharged to 2.0 V at a constant current of 0.6C.

[0137]

[0138] Example 2

[0139] (Activation phase)

[0140] The lithium secondary battery manufactured in Example 1 was pre-aged for 2 days, then charged at a current rate of 0.2 C to an SOC of 3% at 45°C, and then charged at a current rate of 1.0 C to an SOC of 100% and 4.6 V, and then changed to constant voltage mode to perform charging, and charging was terminated when the charging current rate reached 0.02 C. Then, the charged lithium secondary battery was activated by discharging to 2.0 V at a constant current of 0.6 C.

[0141]

[0142] Comparative Example 1

[0143] The lithium secondary battery manufactured in Example 1 was activated in the same manner as in Example 1, except that the lithium secondary battery was charged to 4.6 V at a current rate of 0.4 C in the third charging step, then changed to constant voltage mode to perform charging, and charging was terminated when the charging current rate reached 0.05 C.

[0144]

[0145] Comparative Example 2

[0146] The lithium secondary battery manufactured in Example 1 was activated in the same manner as in Example 1, except that the lithium secondary battery was charged to 4.6 V at a current rate of 0.4 C in the third charging step, then changed to constant voltage mode to perform charging, and charging was terminated when the charging current rate reached 0.15 C.

[0147]

[0148] Experimental Example 1: Evaluation of the activation rate of rock salt (lithium manganese oxide)

[0149] The Li2MnO3 activation ratio (%) of the lithium secondary batteries activated by the methods of Examples 1 and 2 and Comparative Examples 1 to 3 was calculated, and then the Li2MnO3 activation ratio (%) was expressed as a percentage based on the Li2MnO3 activation ratio (%) of Comparative Example 1, 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 4.3 V to the end-of-charge voltage by the charge capacity of the cell when charged from 2.0 V to 4.6 V.

[0150] Li2MnO3Activation Ratio (%)Li2MnO3Activation Ratio (%) based on Comparative Example 1Example 159.75101.20Example 260.23102.02Comparative Example 159.04100Comparative Example 258.3598.83

[0151] As shown in the above [Table 1], in the case of the lithium secondary battery activated by the methods of Examples 1 and 2, it can be confirmed that the activation ratio of Li2MnO3 increased compared to the lithium secondary battery activated by the method of the comparative example.

[0152] Experimental Example 2: Voltage Drop Evaluation

[0153] Lithium secondary batteries activated by the methods of Examples 1 and 2 and Comparative Examples 1 to 3 were each subjected to constant current-constant voltage mode at 0.33 C at 25°C, charged at a charge current rate of 0.05 C until the voltage reached 4.35 V, and then discharged at a constant current of 0.33 C until the voltage reached 2.0 V. After storing the batteries at 60°C for 4 weeks with an SOC of 100, the batteries were again driven at 25°C in a voltage range of 0.33 C, and the decrease rate (%) calculated by calculating the degree of decrease in voltage compared to the initial voltage is shown in [Table 2] and Fig. 1.

[0154] At this time, 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) means the value of the discharge energy for the discharge capacity at 0.33C.

[0155] dOCV(mV)Reduction rate(%)Example 141.61.17Example 238.61.09Comparative example 145.41.27Comparative example 251.51.44

[0156] As shown in the above [Table 2] and Fig. 1, it can be confirmed that the lithium secondary battery activated according to the method of the example did not show a significant increase in voltage drop when stored at high temperatures, whereas the lithium secondary battery activated according to the method of the comparative example showed a significant increase in voltage drop when stored at high temperatures.

[0157] Meanwhile, 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 controlled by controlling the charge capacity for Li2MnO3 in some sections of the charging stage of the activation process. At SOC 60 to 100, when charging in constant current-constant voltage mode (CC-CV mode), the depth of charge decreases as the charge termination current rate increases during constant voltage mode (CV mode) charging, which causes the activation ratio of Li2MnO3 to decrease. Accordingly, during high-temperature storage, the change in the crystal structure within the lithium-based transition metal oxide becomes greater, which further increases the change in the nominal voltage, which may increase the voltage drop.

Claims

1. A step of preparing a battery cell including a positive electrode, a negative electrode, and an electrolyte including a lithium manganese oxide in which the manganese content among all metals excluding lithium exceeds 50 mol% and the ratio of the number of moles of lithium to the number of moles of all metals excluding lithium (Li / Me) exceeds 1; and a step of activating the battery cell by charging and discharging it at least once; In the above 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 having a charging current rate of 0.8C to 1.2C in a range where the state of charge (SOC) of the battery cell is 3 to 60%.

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

3. In claim 1, in the activation step, the charging is a first charging step that performs constant current charging at a first current rate; A second charging step that performs constant current charging at a second current rate different from the first current rate, and A method for manufacturing a lithium secondary battery, comprising a third charging step of performing constant current charging at a third current rate different from the first current rate and then performing constant voltage charging.

4. A 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. In claim 3, A method for manufacturing a lithium secondary battery, wherein the first charging step is performed to an SOC of 0 to 3%, the second charging step is performed to an SOC of 3 to 60%, and the third charging step is performed to an SOC of 60 to 100%.

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

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

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

9. A method for manufacturing a lithium secondary battery according to claim 1, wherein the lithium manganese oxide is represented by the following chemical formula 1. [Chemical Formula 1] Li a Ni b Co c Mr 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≤0.2이고, 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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