Method for manufacturing lithium secondary battery

Abstract

The present invention relates to a method for manufacturing a lithium secondary battery, the method comprising the steps of: preparing a battery cell comprising a positive electrode, a negative electrode, and an electrolyte, in which the positive electrode comprises a lithium-rich manganese-based oxide, the manganese content being greater than 50 mol% with respect to all metals other than lithium, and the electrolyte content being greater than 50 mol% with respect to all metals other than lithium; and the ratio of the number of moles of lithium to the number of moles of all metals other than lithium (Li / Me) is greater than 1; and charging and discharging the battery cell at least once or more to activate the battery, in which the ratio (N / P ratio) of the discharge capacity per unit area of the negative electrode to the discharge capacity per unit area of the positive electrode of the battery cell when charged at 2.0 V to 4.3 V is 1.05 to 1.15, and in which the charging in the activation step is performed from SOC 60 to SOC 65.

Description

Technical Field

[0001] Cross-reference to related applications

[0002] This application claims the priority benefits of Korean Patent Application No. 10-2022-0183739, filed with the Korean Intellectual Property Office on December 23, 2022, and Korean Patent Application No. 10-2023-0187697, filed with the Korean Intellectual Property Office on December 20, 2023, the entire disclosures of which are incorporated herein by reference in their entirety.

[0003] The present invention relates to a method for manufacturing a lithium secondary battery, and more particularly, to a method for manufacturing a lithium secondary battery including a lithium-rich manganese-based oxide, the method being capable of appropriately controlling activation voltage conditions and improving overdesign of a negative electrode. Background Art

[0004] Lithium secondary batteries have been used as energy storage media in various fields since their commercialization in 1991. As the market for products equipped with lithium secondary batteries has expanded, research on increasing the energy density of lithium secondary batteries has been actively conducted. One of the most attention-grabbing methods is to develop a positive electrode active material having a composition that can utilize a larger amount of lithium than before.

[0005] As a positive electrode active material that can utilize a larger amount of lithium, a lithium-rich transition metal oxide having a layered structure and a molar ratio of lithium to transition metal greater than 1 has been developed. This lithium-rich transition metal oxide can achieve a high capacity through an activation step at a high voltage of 4.4 V or higher.

[0006] On the other hand, because the lithium-rich transition metal oxide has a high initial irreversibility, a negative electrode design with a running capacity higher than the actual running capacity is required to adapt to the high positive electrode capacity during the high voltage activation process and prevent lithium precipitation. However, if the negative electrode is overdesigned in this way, there is a problem of an increase in the volume and weight of the secondary battery.

[0007] Therefore, in order to miniaturize and lighten lithium secondary batteries used as power supply devices for electric vehicles, portable electronic devices, etc., it is highly necessary to develop new technologies that can solve the increase in the volume and weight of lithium secondary batteries.

[0008] [Prior Art Documents]

[0009] [Patent Documents]

[0010] Japanese Unexamined Patent Publication No. 2017-130557 Summary of the Invention

[0011] Technical problem

[0012] Accordingly, the present invention aims to solve the above problems, and an object of the present invention is to provide a method for manufacturing a lithium secondary battery including a lithium-rich manganese-based oxide, the method being capable of appropriately controlling activation voltage conditions and improving overdesign of a negative electrode.

[0013] Technical solution

[0014] According to one aspect of the present invention, there is provided a method for manufacturing a lithium secondary battery, the method including the steps of:

[0015] Preparing a battery cell including a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode includes a lithium-rich manganese-based oxide, wherein the content of manganese is greater than 50 mol% with respect to all metals other than lithium, and the molar ratio of lithium to all metals other than lithium (Li / Me) is greater than 1; and

[0016] Charging and discharging the battery cell at least once to activate the battery,

[0017] wherein when charging at 2.0 V to 4.3 V, the ratio of the discharge capacity per unit area of the negative electrode of the battery cell to the discharge capacity per unit area of the positive electrode (N / P ratio) is 1.05 to 1.15, and

[0018] wherein in the activation step, charging is performed from SOC 60 to SOC 65.

[0019] The lithium-rich manganese-based oxide may be represented by Chemical Formula 1 below.

[0020] [Chemical Formula 1]

[0021] Li a Ni b Co c Mn d M e O2

[0022] wherein, in Chemical Formula 1,

[0023] 1 < a, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.1, 0.5 ≤ d < 1.0, 0 ≤ e ≤ 0.2, and

[0024] M is at least one selected from Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr, and Zr.

[0025] The charging step may be performed from SOC 60 to SOC 65 at a rate of 0.3 C to 1.0 C.

[0026] The charging step may include:

[0027] A first charging step of charging at a rate of 0.1 C to 0.3 C within the range of SOC 0 to SOC 5, and

[0028] A second charging step of charging from SOC 60 to SOC 65 at a rate of 0.3 C to 1.0 C after the first charging step.

[0029] The charging step can be carried out in a constant current mode (CC mode) or a constant current constant voltage mode (CCCV mode).

[0030] The activation step can include discharging at a C rate of 0.3 C to 0.7 C until reaching 2.0 V. Detailed implementation mode

[0031] The terms or words used in this specification and the appended claims should not be construed as limited to the ordinary meaning or dictionary meaning, but should be interpreted based on the principle that the inventor can appropriately define the terms so as to best and appropriately elaborate the inventor's own invention, according to the meaning and concept consistent with the technical gist of the present invention.

[0032] In the present invention, a "primary particle" refers to a particle unit in which no grain boundary can be regarded as existing when observed with a scanning electron microscope at a magnification of 5000 to 20000. The "average particle size of primary particles" refers to the arithmetic mean calculated after measuring the particle size of the primary particles observed in the scanning electron microscope image.

[0033] In the present invention, a "secondary particle" is a particle formed by the aggregation of a plurality of primary particles.

[0034] In the present invention, the average particle size D 50 means the particle size at which the cumulative volume of powder particles (for example, positive electrode active material, negative electrode active material, etc.) reaches 50% in the particle size distribution curve of the particles. D 50 can be measured using the laser diffraction method. It can be measured by a method including the following steps: dispersing the powder of the particles to be measured in a dispersion medium, introducing it into a commercially available laser diffraction particle size measuring device (such as Microtrac MT 3000), irradiating it with ultrasonic waves of about 28 kHz with an output power of 60 W to obtain a volume cumulative particle size distribution, and determining the particle size corresponding to 50% cumulative volume.

[0035] In addition, in the present invention, "SOC X" means a state in which the charging capacity in the battery cell is X% based on the discharge capacity that appears when the battery cell is discharged from 4.6 V to 2.0 V.

[0036] The present inventors conducted repeated studies to improve the problem of increased volume and weight of a lithium secondary battery using a lithium-rich manganese-based oxide. As a result, it was found that when manufacturing a lithium secondary battery, an activation step is performed under specific charging conditions, so that the negative electrode loading amount can be set to the amount required for the actual driving voltage, thereby solving the problem of increased volume and weight of the lithium secondary battery and completing the present invention.

[0037] Now, a method for manufacturing a lithium secondary battery according to the present invention will be described.

[0038] The method for manufacturing a lithium secondary battery according to the present invention includes:

[0039] A step of preparing a battery cell including a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode includes a lithium-rich manganese-based oxide, wherein the content of manganese is greater than 50 mol% with respect to all metals other than lithium, and the molar ratio of lithium to all metals other than lithium (Li / Me) is greater than 1; and

[0040] A step of charging and discharging the battery cell at least once to activate the battery,

[0041] wherein when charging at 2.0 V to 4.3 V, the ratio of the discharge capacity per unit area of the negative electrode to the discharge capacity per unit area of the positive electrode (N / P ratio) of the battery cell is 1.05 to 1.15, and

[0042] wherein in the activation step, charging is performed from SOC 60 to SOC 65.

[0043] (1) Step of preparing the battery cell

[0044] First, a battery cell including a positive electrode, a negative electrode, and an electrolyte is prepared.

[0045] For example, the battery cell can be prepared by the following steps: forming an electrode assembly including a positive electrode and a negative electrode, accommodating the electrode assembly in a battery case, and then injecting an electrolyte to seal the battery case. At this time, the electrode assembly may include a separator between the positive electrode and the negative electrode.

[0046] Each component of the battery cell according to the present invention will be described in more detail below.

[0047] Positive electrode

[0048] The positive electrode according to the present invention contains a lithium-rich manganese-based oxide, in which the content of manganese is greater than 50 mol% relative to all metals except lithium, and the molar ratio of lithium to all metals except lithium (Li / Me) is greater than 1. Specifically, the positive electrode according to the present invention 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, where the positive electrode active material layer contains a lithium-rich manganese-based oxide, and the molar ratio of lithium to all metals except lithium (Li / Me) is greater than 1.

[0049] When the lithium-rich manganese-based oxide contains too much lithium, it has a structure in which a layered phase (LiM'O2) and a rock salt phase (Li2MnO3) are mixed. During the initial activation process, the rock salt phase is activated and generates an excessive amount of lithium ions, thereby achieving a high capacity.

[0050] Preferably, the lithium-rich manganese-based oxide can be represented by the following Chemical Formula 1.

[0051] [Chemical Formula 1]

[0052] Li a Ni b Co c Mn d M e O2

[0053] Wherein, in Chemical Formula 1,

[0054] 1 < a, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.1, 0.5 ≤ d < 1.0, 0 ≤ e ≤ 0.2, and

[0055] M is at least one selected from Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr, and Zr.

[0056] On the other hand, a is the molar ratio of Li in the lithium-rich manganese-based oxide, and can be 1 < a, 1.1 ≤ a ≤ 1.5, or 1.1 ≤ a ≤ 1.3.

[0057] b is the molar ratio of Ni in the lithium-rich manganese-based oxide, and can be 0 ≤ b ≤ 0.5, 0.1 ≤ b ≤ 0.4, or 0.2 ≤ b ≤ 0.4.

[0058] c is the molar ratio of Co in the lithium-rich manganese-based oxide, and can be 0 ≤ c ≤ 0.1, 0 ≤ c ≤ 0.08, or 0 ≤ c ≤ 0.05. If c exceeds 0.1, it is difficult to ensure a high capacity, and gas generation and deterioration of the positive electrode active material become aggravated, and the life characteristics may deteriorate.

[0059] d is the molar ratio of Mn in the lithium-rich manganese-based oxide, and can be 0.5 ≤ d < 1.0, 0.50 ≤ d ≤ 0.80 or 0.50 ≤ d ≤ 0.70. If d is less than 0.5, the proportion of the rock salt phase becomes too small, so the effect of improving the capacity is slight.

[0060] e is the molar ratio of the doping element M in the lithium-rich manganese-based oxide, and can 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 an adverse effect on the capacity of the active material.

[0061] On the other hand, in the lithium-rich manganese-based oxide represented by [Chemical Formula 1], the ratio of the number of moles of Li to the number of moles of all metal elements other than 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 performance and capacity characteristics can be exhibited. If the Li / Me ratio is too high, the conductivity may decrease, and the rock salt phase (Li2MnO3) may increase, which may increase the degradation rate. If the Li / Me ratio is too low, the effect of improving the energy density is slight.

[0062] On the other hand, the composition of the lithium-rich manganese-based oxide can be represented by the following [Chemical Formula 2].

[0063] [Chemical Formula 2]

[0064] XLi2MnO3·(1 - X)Li[Ni 1-y-z-w Mn y Co z M w O2

[0065] Among them, in [Chemical Formula 2], M can be at least one metal ion selected from Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr and Zr.

[0066] X refers to the ratio of the Li2MnO3 phase in the lithium-rich manganese-based 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 in the lithium-rich manganese-based oxide satisfies the above range, high capacity characteristics can be achieved.

[0067] y is the molar ratio of Mn in the LiM'O2 layer, and can be 0.4 ≤ y < 1, 0.4 ≤ y ≤ 0.8 or 0.4 ≤ y ≤ 0.7.

[0068] z is the molar ratio of Co in the LiM'O2 layer, and can be 0 ≤ z ≤ 0.1, 0 ≤ z ≤ 0.08, or 0 ≤ z ≤ 0.05. If z is greater than 0.1, gas generation and deterioration of the positive electrode active material may become aggravated, and the life characteristics may deteriorate.

[0069] w is the molar ratio of the doping element M in the LiM'O2 layer, and can be 0 ≤ w ≤ 0.2, 0 ≤ w ≤ 0.1, or 0 ≤ w ≤ 0.05.

[0070] On the other hand, if necessary, the positive electrode active material according to the present invention may further include a coating on the surface of the lithium-rich manganese-based oxide. When the positive electrode active material includes a coating, the contact between the lithium-rich manganese-based oxide and the electrolyte is inhibited by the coating, and side reactions of the electrolyte are reduced, thereby being able to obtain the effect of improving life characteristics.

[0071] The coating may include a coating element M 1 , the coating element M 1 may be at least one selected from, for example, Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr, and Zr, preferably Al, Co, Nb, W, and combinations thereof, more preferably Al, Co, and combinations thereof. The coating element M 1 may include two or more types, such as Al and Co.

[0072] The coating element may exist in the coating in the form of an oxide, i.e., M 1 Oz (1 ≤ z ≤ 4).

[0073] The coating 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 them, it is preferably formed by atomic layer deposition because a coating with a wider area can be formed.

[0074] Based on the total surface area of the lithium-rich manganese-based oxide particles, the formation area of the coating can be 10% to 100%, preferably 30% to 100%, more preferably 50% to 100%. When the formation area of the coating satisfies the above range, the effect of improving life characteristics is excellent.

[0075] On the other hand, the positive electrode active material according to the present invention may be in the form of secondary particles in which a plurality of primary particles are agglomerated, and the average particle size D of the secondary particles 50 can be 2 μm to 10 μm, preferably 2 μm to 8 μm, more preferably 4 μm to 8 μm. When the D of the positive electrode active material 50 satisfies the above range, the electrode density can be excellently achieved, and the deterioration of the capacity and rate characteristics can be minimized.

[0076] In addition, the BET specific surface area of the positive electrode active material may be 1 m 2 / g to 10 m 2 / g, 3 to 8 m 2 / g or 4 to 6 m 2 / g. If the BET specific surface area of the positive electrode active material is too small, the reaction area with the electrolyte is insufficient, making it difficult to achieve sufficient capacity. If the specific surface area is too high, water absorption is rapid and side reactions with the electrolyte are accelerated, making it difficult to ensure life characteristics.

[0077] On the other hand, a lithium-rich manganese-based oxide can be produced by mixing a transition metal precursor and a lithium raw material and then firing the mixture.

[0078] As the lithium raw material, for example, lithium-containing carbonates (such as lithium carbonate, etc.), hydrates (such as lithium hydroxide hydrate (LiOH·H2O), etc.), hydroxides (such as lithium hydroxide, etc.), nitrates (such as lithium nitrate (LiNO3), etc.), chlorides (such as lithium chloride (LiCl), etc.) can be used, and one kind alone or a mixture of two or more kinds can be used.

[0079] On the other hand, the transition metal precursor can be in the form of a hydroxide, oxide or carbonate. When using a precursor in the form of a carbonate, it is more preferred because a positive electrode active material with a relatively high specific surface area can be produced.

[0080] The transition metal precursor can be prepared by a coprecipitation process. For example, the transition metal precursor can be prepared by the following steps: dissolving each transition metal-containing raw material in a solvent to prepare a metal solution, then mixing the metal solution, an ammonium cation complex former and a basic compound, and carrying out a coprecipitation reaction. In addition, if necessary, an oxidizing agent or oxygen can also be added during the coprecipitation reaction.

[0081] At this time, the transition metal-containing raw material can be acetates, carbonates, nitrates, sulfates, halides, sulfides, 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, etc.

[0082] The ammonium cation complex former can be at least one selected from NH4OH, (NH4)2SO4, NH4NO3, NH4Cl, CH3COONH4 and (NH4)2CO3.

[0083] The alkaline compound can be at least one selected from NaOH, Na2CO3, KOH, and Ca(OH)2. The form of the precursor can vary according to the type of alkaline compound used. For example, when using NaOH as the alkaline compound, a precursor in the form of a hydroxide can be obtained, while when using Na2CO3 as the alkaline compound, a precursor in the form of a carbonate can be obtained. In addition, when the alkaline compound is used together with an oxidizing agent, a precursor in the form of an oxide can be obtained.

[0084] On the other hand, the transition metal precursor and the lithium raw material can be mixed in such amounts that the molar ratio of the total transition metals (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.

[0085] 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 hours to 30 hours or 5 hours to 20 hours. In addition, the firing atmosphere can be an air atmosphere or an oxygen atmosphere, for example, an atmosphere containing 20 to 100% by volume of oxygen.

[0086] On the other hand, in addition to the positive electrode active material, the positive electrode active material layer can further contain a conductive material and a binder.

[0087] Examples of the conductive material include spherical or flaky graphite; carbonaceous materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black, carbon fiber, single-walled carbon nanotube, and multi-walled carbon nanotube; 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, etc. Among them, one kind alone or a mixture of two or more kinds can be used. Based on the total weight of the positive electrode active material layer, the content of the conductive material can be 0.1 to 20% by weight, 1 to 20% by weight, or 1 to 10% by weight.

[0088] In addition, the binder is a component that helps the adhesion between the positive electrode active materials and the adhesion force between the positive electrode active materials and the current collector. Examples thereof include: fluororesin binders, including polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE); rubber-like binders, including styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber, and styrene-isoprene rubber; cellulose-based binders, including carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, and regenerated cellulose; polyol binders, including polyvinyl alcohol; polyolefin binders, including polyethylene and polypropylene; polyimide binders; polyester binders; silane binders, etc. Based on the total weight of the positive electrode active material layer, the content of the binder can be 1 to 20% by weight, 2 to 20% by weight, or 2 to 10% by weight.

[0089] Negative electrode

[0090] 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 optionally contain a binder, a conductive material, and the negative electrode active material.

[0091] There is no particular limitation on the negative electrode current collector as long as it has high conductivity and does not cause chemical changes in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, or copper or stainless steel with a surface treated with carbon, nickel, titanium, silver, etc., aluminum cadmium alloy, etc. can be used. In addition, the negative electrode current collector usually may have a thickness of 3 μm to 500 μm. In addition, similar to the positive electrode current collector, the negative electrode current collector may have fine irregularities formed on its surface to enhance the adhesion of the negative electrode active material. For example, the negative electrode current collector can be used in different forms, such as a film, sheet, foil, net, porous body, foam, and non-woven fabric structure.

[0092] As the negative electrode active material, a compound capable of reversibly inserting and extracting lithium can be used. Specific examples of the negative electrode active material may include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, or amorphous carbon; (semi) metallic materials capable of forming an alloy with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, or Al alloy; metal oxides capable of doping and undoping lithium, such as SiO β (0 < β < 2), SnO2, vanadium oxides, and lithium vanadium oxides; composites containing (semi) metallic materials and carbonaceous materials, such as Si-C composites or Sn-C composites, and any one of them alone or a mixture of two or more thereof can be used.

[0093] In addition, a thin film of lithium metal can be used as the negative electrode active material. In addition, low-crystalline carbon and high-crystalline carbon can be used as the carbon materials. Typical examples of the low-crystalline carbon include soft carbon and hard carbon. Typical examples of the high-crystalline carbon include natural graphite or artificial graphite, which is irregular, plate-shaped, flake-shaped, spherical or fibrous, condensed graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesophase carbon microbead, high-temperature sintered carbon such as mesophase pitch and coke derived from petroleum or coal tar pitch.

[0094] The conductive material is used to impart conductivity to the electrode, and the conductive material can be used without particular limitation as long as it has conductivity and does not cause a chemical change in the battery to be configured. Specific examples thereof include graphite such as natural graphite and artificial graphite; carbonaceous materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black, carbon fiber and carbon nanotube; 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 any one of them or a mixture of two or more thereof can be used. Based on the total weight of the negative electrode active material layer, the content of the conductive material is usually 1 to 30% by weight, preferably 1 to 20% by weight, more preferably 1 to 10% by weight.

[0095] The binder serves to improve the adhesion between the negative electrode active material particles and the adhesion between the negative electrode active material and the negative electrode current collector. Specific examples thereof include: fluororesin binders including polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE); rubber binders including styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber and styrene-isoprene rubber; cellulose binders including carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose and regenerated cellulose; polyol binders including polyvinyl alcohol; polyolefin binders including polyethylene and polypropylene; polyimide binders; polyester binders; silane binders, etc., and any one of them or a mixture of two or more thereof can be used. Based on the total weight of the negative electrode active material layer, the content of the binder can be 1 to 30% by weight, preferably 1 to 20% by weight, more preferably 1 to 10% by weight.

[0096] The negative electrode active material layer can be prepared by coating a negative electrode slurry containing the negative electrode active material and optionally a binder and a conductive material onto the negative electrode current collector and drying the coated slurry, or can be prepared by casting the negative electrode slurry on a separate support and then laminating the film peeled from the support on the current collector.

[0097] On the other hand, when charging at 2.0 V to 4.3 V, the ratio (N / P ratio) of the discharge capacity per unit area of the negative electrode to the discharge capacity per unit area of the positive electrode of the battery cell of the present invention can be 1.05 to 1.15. When the ratio (N / P ratio) of the discharge capacity per unit area of the negative electrode to the discharge capacity per unit area of the positive electrode is within the above range, the negative electrode capacity capable of accommodating the high positive electrode capacity exhibited in the activation stage can be ensured, thereby ensuring the stability of the secondary battery.

[0098] Separator

[0099] The lithium secondary battery according to the present invention may further include a separator disposed between the positive electrode and the negative electrode. The separator separates the negative electrode and the positive electrode from each other and provides a migration path for lithium ions. The separator can be used without particular limitation as long as it is used as a separator in a typical lithium secondary battery. In particular, it is preferable that the separator has a low resistance to ion migration of the electrolyte and has an excellent ability to impregnate the electrolyte. Specifically, the separator can be a porous polymer membrane, such as a porous polymer membrane made of polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof. In addition, typical porous non-woven fabrics, such as non-woven fabrics made of high melting point glass fibers, polyethylene terephthalate fibers, etc., can be used. In addition, in order to ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer material can be used, and it can be used as a single layer or a multi-layer structure optionally.

[0100] Electrolyte

[0101] The electrolyte used herein may include various electrolytes that can be used 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 their types are not particularly limited.

[0102] For example, the electrolyte may contain an organic solvent and a lithium salt.

[0103] The use of an organic solvent may not be subject to any particular limitation as long as it serves as a medium through which ions participating in the electrochemical reaction of the battery can migrate. Specific examples of the organic solvent may include: ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone or ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene or fluorobenzene; carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC) or propylene carbonate (PC); alcohol solvents such as ethanol or isopropanol; nitriles such as R-CN (where R is a linear, branched or cyclic hydrocarbon group having 2 to 20 carbon atoms and may contain a double bond, an aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolene; sulfolane and the like.

[0104] The use of a lithium salt may not be subject to any particular limitation as long as it is a compound capable of providing lithium ions for a lithium secondary battery. Specifically, the anion of the lithium salt may be selected from 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 - and at least one of them. The lithium salts that can be used in this article may be LiPF6, LiN(FSO2)2, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, etc. The concentration of the lithium salt is preferably used in the range of 0.1 to 5.0 M.

[0105] In order to improve the life characteristics of the battery, suppress the reduction of the battery capacity, improve the discharge capacity of the battery, etc., in addition to the above electrolyte components, the electrolyte may further contain additives. For example, as additives, halogenated ethylene carbonate compounds such as difluoroethylene carbonate may be used alone or in combination; pyridine; triethyl phosphite; triethanolamine; cyclic ethers; ethylenediamine; (poly)ethylene glycol dimethyl ethers; hexamethylphosphoric triamide; nitrobenzene derivatives; sulfur; quinone imine dyes; N-substituted oxazolidinone; N,N-substituted imidazoline; ethylene glycol dialkyl ether; ammonium salts; pyrrole; 2-methoxyethanol; aluminum trichloride, etc., but not limited thereto. Based on the total weight of the electrolyte, the content of the additive may be 0.1 to 10% by weight, preferably 0.1 to 5% by weight.

[0106] On the other hand, the electrode assembly may be various forms of electrode assemblies well-known in the art, such as a jelly-roll type, a stacked type, a stacked laminated type or a stacked folded type electrode assembly, and its form is not particularly limited.

[0107] A jelly-roll type electrode assembly can be produced by inserting a sheet-like separator between a sheet-like positive electrode and a sheet-like negative electrode and then winding it in one direction.

[0108] A stacked type electrode assembly can be produced by cutting the positive electrode, the separator and the negative electrode into desired shapes and then sequentially stacking the cut positive electrode / separator / negative electrode.

[0109] A stacked laminated type electrode assembly can be produced by the following method: stacking the positive electrode, the separator and the negative electrode to produce a plurality of unit cells, stacking the plurality of unit cells and the separator inserted therebetween, and then laminating them by a method such as heating.

[0110] A stacked folded type electrode assembly can be produced by the following method: stacking the positive electrode, the separator and the negative electrode to produce a plurality of unit cells, arranging the plurality of unit cells on one surface or both surfaces of a long folded separator, and then winding the folded separator.

[0111] On the other hand, as the battery case, various battery cases known in the art can be used, such as a cylindrical battery case, a square prismatic battery case or a pouch type battery case, etc., and its type is not particularly limited.

[0112] (2) Activation step

[0113] Next, a step of charging and discharging the battery cell at least once or more to electroactivate the battery is performed. The activation step is a step of charging and discharging the battery cell to endow electrical characteristics and form a SEI (solid electrolyte interface) film on the electrode to stabilize the battery.

[0114] In the present invention, in the step of preparing a battery cell, a step of controlling the ratio (N / P ratio) of the discharge capacity per unit area of the negative electrode to the discharge capacity per unit area of the positive electrode to 1.05 to 1.15 and charging the battery cell from SOC 60 to SOC 65 is performed, so that the negative electrode loading amount required for actual operation can be set, and the problem of increasing the volume and weight of the secondary battery due to unnecessary use of the negative electrode can be solved.

[0115] More specifically, it is more desirable to control the ratio (N / P ratio) of the discharge capacity per unit area of the negative electrode to the discharge capacity per unit area of the positive electrode to 1.05 to 1.10. Within this range, over-design of the negative electrode can be prevented while maximizing the capacity of the positive electrode.

[0116] The charging step can be carried out at a rate of 0.3 C to 1.0 C from SOC 60 to SOC 65. Specifically, the charging step can include a first charging step of charging the battery cell to below SOC 5, and a second charging step of charging the battery cell from SOC 60 to SOC 65 at a rate of 0.3 C to 1.0 C after the first charging step.

[0117] Specifically, the first charging step can be carried out at a C-rate of below 0.3 C, preferably 0.1 C to 0.3 C, within the range of SOC 0 to SOC 5.

[0118] When the charging capacity in the first charging step satisfies the above range, a firm and dense SEI film is formed on the electrode surface, thereby achieving excellent life characteristics.

[0119] In addition, if the current rate in the first charging stage is faster than 0.3 C, the SEI film may be formed unstably on the electrode surface. If the SEI film is formed unstably on the electrode surface, the SEI film is likely to decompose during battery operation, resulting in rapid deterioration of the electrode, which may significantly reduce the life characteristics.

[0120] On the other hand, the first charging step can be carried out in a constant current mode (CC mode).

[0121] Next, the activation step can include a second charging step of charging from SOC 60 to SOC 65 starting from the time point when the first charging step is completed.

[0122] Specifically, the charging step can be carried out at a C-rate of 0.3 C to 1.0 C from SOC 60 to SOC 65.

[0123] If the charging capacity in the second charging stage satisfies the above range, it is possible to suppress the activation capacity that is excessively high compared to the driving capacity, and obtain an actual driving charge-discharge capacity at the same level as the activation charge-discharge capacity, so that there is no longer a need for the excessive design of the negative electrode required for the activation process.

[0124] On the other hand, if the second charging step proceeds to an SOC less than 60, Li2MnO3 is not activated at all, so the characteristics of a battery using a lithium-rich manganese-based oxide, which attempts to achieve a high capacity through the Li2MnO3 activation reaction, cannot be realized. In addition, if the second charging step proceeds to a range exceeding SOC 65, the positive electrode capacity is excessively high compared to the designed standard positive electrode driving capacity, so the difference between the designed standard N / P ratio and the driving voltage standard N / P ratio increases, resulting in the problem that a thicker negative electrode design than actually required is needed.

[0125] On the other hand, the second charging step can be carried out in a constant current mode (CC mode) or a constant current constant voltage mode (CCCV mode).

[0126] When the second charging step is carried out in the constant current constant voltage mode (CCCV mode), CC charging is carried out while providing a C rate of 0.3 C to 1.0 C when charging from SOC 60 to SOC 65, and when reaching SOC 60 - SOC 65, CV charging can be carried out with the charging C rate gradually decreasing to about 0.05 C.

[0127] Next, the activation step discharges the battery cell charged through the first charging step and the second charging step. At this time, the discharge can be carried out at a C rate of 0.3 C to 0.7 C. When the discharge rate satisfies the above range, the activation time can be appropriately controlled, and the discharge capacity characteristics within the desired range can be achieved.

[0128] On the other hand, the discharge can be carried out in the constant current mode (CC mode).

[0129] On the other hand, the discharge termination voltage can be 2.0 V to 3.0 V, specifically 2.0 V.

[0130] On the other hand, the activation step is preferably carried out under temperature conditions of 25°C to 70°C, more preferably 40°C to 50°C. When the activation step is carried out within the above temperature range, the effect of achieving a high capacity can be obtained through the appropriate activation of Li2MnO3.

[0131] In addition, if necessary, the activation step can be carried out under pressurized conditions. Pressurization can be carried out by installing the battery cell in a fixture and then applying pressure to the battery cell via the fixture. When the activation step is carried out under pressurized conditions, the advantage is that the gas generated during the activation step can be easily discharged.

[0132] On the other hand, although this is not essential in nature, if necessary, the activation step may further include an aging step. The aging step allows the electrolyte to be uniformly impregnated into the electrode assembly and stabilizes the battery, and can be carried out before charging, during charging, and / or after discharging, and can be carried out more than once.

[0133] The aging step can be carried out at a temperature of, for example, 20°C to 60°C, 20°C to 50°C, preferably 30°C to 50°C. When aging is carried out at the above temperatures, the impregnation property of the electrolyte and the mobility of lithium are improved, enabling activation to proceed more smoothly.

[0134] Hereinafter, the present invention will be described in more detail with reference to specific examples.

[0135] Example 1

[0136] (Manufacture of a single battery)

[0137] The positive electrode active material: conductive material: PVDF binder was mixed in N-methylpyrrolidone at a weight ratio of 97:1:2 to prepare a positive electrode paste. At this time, Li 1.16 Ni 0.31 Mn 0.53 O2 was used as the positive electrode active material, and carbon nanotubes (CNT) were used as the conductive material. The positive electrode paste was coated on an aluminum current collector sheet, dried, and then calendered to manufacture the positive electrode.

[0138] The negative electrode active material: conductive material: binder was mixed in water at a weight ratio of 96:1:3 to prepare a negative electrode paste. At this time, graphite was used as the negative electrode active material, carbon black was used as the conductive material, and SBR and CMC with a weight ratio of 2:1 were used as the binder. The negative electrode paste was coated on a copper current collector sheet, dried, and then calendered to manufacture the negative electrode (loading: 7.5 mg / cm 2 ).

[0139] At this time, the ratio of the discharge capacity per unit area of the negative electrode to the discharge capacity per unit area of the positive electrode (N / P ratio) was set to 1.10.

[0140] A separator was inserted between the positive electrode and the negative electrode manufactured as described above to produce an electrode assembly, and the electrode assembly was inserted into a battery case and then an electrolyte was injected to manufacture a single battery.

[0141] (Activation step)

[0142] The battery cells were pre-aged for 2 days, then charged to SOC 3 at 45 °C in a constant current mode at 0.2 C (first charging stage), charged to SOC 60 at 0.3 C in a constant current mode, then charged at a constant voltage to 0.05 C (second charging stage), and then discharged at a constant current of 0.5 C to 2.0 V. The activation step was carried out in this way to fabricate the lithium secondary battery.

[0143] Example 2

[0144] The battery cells fabricated as in Example 1 were pre-aged for 2 days, then charged to SOC 60 at 45 °C in a constant current mode at 0.3 C, then charged at a constant voltage to 0.05 C (second charging step), and discharged at a constant current of 0.5 C to 2.0 V. The activation step was carried out in this way to fabricate the lithium secondary battery.

[0145] Comparative Example 1

[0146] (Fabrication of battery cells)

[0147] The positive electrode active material: conductive material: PVDF binder were mixed at a weight ratio of 97:1:2 in N-methylpyrrolidone to prepare the positive electrode slurry. At this time, Li 1.16 Ni 0.31 Mn 0.53 O2 was used as the positive electrode active material, and carbon nanotubes (CNT) were used as the conductive material. The positive electrode slurry was coated on an aluminum current collector sheet, dried, and then calendered to fabricate the positive electrode.

[0148] The negative electrode active material: conductive material: binder were mixed at a weight ratio of 96:1:3 in water to prepare the negative electrode slurry. At this time, graphite was used as the negative electrode active material, carbon black was used as the conductive material, and SBR and CMC with a weight ratio of 2:1 were used as the binder. The negative electrode slurry was coated on a copper current collector sheet, dried, and then calendered to fabricate the negative electrode (loading: 13.0 mg / cm 2 ).

[0149] At this time, the ratio of the discharge capacity per unit area of the negative electrode to the discharge capacity per unit area of the positive electrode (N / P ratio) was set to 1.49.

[0150] A separator was inserted between the positive electrode and the negative electrode fabricated as above to produce an electrode assembly, and the electrode assembly was inserted into a battery case, and then an electrolyte was injected to fabricate the battery cells.

[0151] (Activation step)

[0152] The battery cells were pre-aged for 2 days, and then charged to SOC 3 at 45 °C in a constant current mode of 0.2 C (the first charging stage), charged to SOC 60 in a constant current mode of 0.3 C, and then charged at a constant voltage to 0.05 C (the second charging stage), and then discharged at a constant current of 0.5 C to 2.0 V. The activation step was carried out in this way to fabricate the lithium secondary battery.

[0153] Experimental Example 1

[0154] One charge-discharge cycle was carried out, in which the battery cells fabricated in Examples 1 to 2 and Comparative Example 1 were charged to 4.3 V at a constant current and constant voltage mode of 0.33 C (cut-off at 0.05 C) at room temperature (25 °C), and discharged at a constant current of 0.33 C to 2.0 V. The initial charge-discharge capacities were measured and shown in Table 1 below.

[0155]

[0156] Referring to Table 1, it can be seen that in the case of the lithium secondary batteries of Examples 1 and 2 fabricated according to the present invention, even if the negative electrode is not over-designed, the initial charge-discharge capacities comparable to those of the lithium secondary battery of Comparative Example 1 are obtained.

[0157] Experimental Example 2. (Storage characteristics or cycle characteristics are required)

[0158] Two hundred charge-discharge cycles were carried out, in which the battery cells fabricated in Examples 1 and 2 and Comparative Example 1 were charged to 4.3 V at a constant current and constant voltage mode of 0.33 C (cut-off at 0.05 C) at 45 °C, and discharged at a constant current of 0.33 C to 2.0 V. The ratio of the 200th discharge capacity to the initial discharge capacity is shown in Table 2 below as the cycle capacity retention rate.

[0159] In addition, the battery cells of Examples 1 and 2 and Comparative Example 1 were charged to 4.3 V at a constant current and constant voltage mode of 0.33 C (cut-off at 0.05 C) at 25 °C, and then discharged at a constant current of 0.33 C to 2.0 V to measure the initial discharge capacity, and charged again to 4.3 V at a constant current and constant voltage mode of 0.33 C (cut-off at 0.05 C) at 25 °C, stored in a chamber at 60 °C for 8 weeks, and then discharged at a constant current of 0.33 C to 2.0 V. The ratio of the discharge capacity after storage to the discharge capacity before storage was measured, and the results are shown in Table 2 below as the storage capacity retention rate.

[0160]

[0161] Referring to Table 2, it can be confirmed that in the case of the lithium secondary batteries of Examples 1 and 2 manufactured according to the present invention, even if the negative electrode is not over-designed, they exhibit capacity retention and high-temperature storage characteristics comparable to those of the lithium secondary battery of Comparative Example 1, and are quite excellent. It is understood that this is because in the lithium secondary battery of Comparative Example 1, due to the over-design of the negative electrode, side reactions increase.

[0162] Industrial Applicability

[0163] The method for manufacturing a lithium secondary battery according to the present invention can complete charging at SOC 60 to SOC 65 in the activation step, thereby improving the over-design of the negative electrode and solving the problem of increasing the volume and weight of the lithium secondary battery.

Claims

1. A method for manufacturing a lithium secondary battery, the method comprising the following steps: Preparing an electrode assembly including a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode includes a lithium-rich manganese-based oxide, wherein the content of manganese is greater than 50 mol% with respect to all metals other than lithium, and the molar ratio of lithium to all metals other than lithium (Li / Me) is greater than 1; and Charging and discharging the electrode assembly at least once to activate the battery, wherein when charging at 2.0 V to 4.3 V, the ratio of the discharge capacity per unit area of the negative electrode to the discharge capacity per unit area of the positive electrode (N / P ratio) of the electrode assembly is 1.05 to 1.15, and wherein in the activation step, the charging is performed from SOC 60 to SOC 65.

2. The method for manufacturing a lithium secondary battery according to claim 1, wherein: The lithium-rich manganese-based oxide is represented by the following Chemical Formula 1: [Chemical Formula 1] Li a Ni b Co c Mn d M e O2 Wherein, in Chemical Formula 1, 1 < a, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.1, 0.5 ≤ d < 1.0, 0 ≤ e ≤ 0.2, and M is at least one selected from Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr, and Zr.

3. The method for manufacturing a lithium secondary battery according to claim 2, wherein: In Chemical Formula 1, 1.1 ≤ a ≤ 1.5, 0.1 ≤ b ≤ 0.4, 0 ≤ c ≤ 0.05, 0.5 ≤ d ≤ 0.80, 0 ≤ e ≤ 0.

1.

4. The method for manufacturing a lithium secondary battery according to claim 1, wherein: The charging step is performed in a constant current mode (CC mode) or a constant current constant voltage mode (CCCV mode).

5. The method for manufacturing a lithium secondary battery according to claim 1, wherein: The charging step is performed at a rate of 0.3 C to 1.0 C from SOC 60 to SOC 65.

6. The method for manufacturing a lithium secondary battery according to claim 1, wherein: The charging step includes: A first charging step of charging at a rate of 0.1 C to 0.3 C in the range of SOC 0 to SOC 5, and A second charging step of charging at a rate of 0.3 C to 1.0 C from SOC 60 after the first charging step to SOC 65.

7. The method for manufacturing a lithium secondary battery according to claim 6, wherein: The charging step is performed in a constant current mode (CC mode).

8. The method for manufacturing a lithium secondary battery according to claim 1, wherein: The activation step includes discharging at a C rate of 0.3 C to 0.7 C until reaching 2.0 V.

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