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 in which the content of manganese in all metals other than lithium is greater than 50 mol%; 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 to activate the battery, in which the activating step includes charging the battery cell from SOC 5 to SOC 60 at a C rate of 0.6-1.0 C, charging the battery cell at a C rate of 0.3-0.6 C in a range of SOC 60 to SOC 100, and then discharging the battery cell once.

Description

Technical Field

[0001] Cross - Reference to Related Applications

[0002] This application claims the priority benefit of Korean Patent Application No. 10-2022-0183696, filed with the Korean Intellectual Property Office on December 23, 2022, the disclosure of which is incorporated herein by reference in its entirety.

[0003] The present invention relates to a method of manufacturing a lithium secondary battery, and more particularly, to a method of manufacturing a lithium secondary battery containing a lithium-rich manganese-based oxide, which can improve a voltage drop generated during repeated charging and discharging. Background Art

[0004] Lithium secondary batteries are energy storage media, and since they were commercialized in 1991, they have been applied in various fields. As the market for products equipped with lithium secondary batteries has expanded, research has been actively conducted to increase the energy density of lithium secondary batteries. One of the most focused-on 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. Such a lithium-rich transition metal oxide can achieve a high capacity through an activation step at a high voltage of 4.4 V or more. However, when performing the high-voltage activation process in this way, due to oxygen detachment, cation mixing, etc. in the crystal structure of the lithium-rich transition metal oxide, the crystal structure of the positive electrode collapses, which may cause a great deterioration in the discharge energy of the lithium secondary battery or cause a problem of voltage drop (voltage reduction) during repeated charging and discharging.

[0006] [Prior Art Documents]

[0007] [Patent Documents]

[0008] Japanese Unexamined Patent Publication No. 5810320 Summary of the Invention

[0009] Technical Problem

[0010] The present invention aims to solve the above problems, and thus, an object of the present invention is to provide a method of manufacturing a lithium secondary battery containing a lithium-rich manganese-based oxide, which can improve a voltage drop generated during repeated charging and discharging.

[0011] Technical Solution

[0012] According to one aspect of the present invention, there is provided a method of manufacturing a lithium secondary battery, the method including the following steps:

[0013] Prepare a battery cell including a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode includes a lithium-rich manganese-based oxide, in which the content of manganese among all metals other than lithium is greater than 50 mol %, and the ratio (Li / Me) of the number of moles of lithium to the number of moles of all metals other than lithium is greater than 1; and

[0014] Charge and discharge the battery cell at least once to activate the battery,

[0015] wherein the activation step includes: charging the battery cell from SOC 5 to SOC 60 at a C-rate of 0.6 to 1.0 C, charging at a C-rate of 0.3 C to 0.6 C in the range of SOC 60 to SOC 100, and then performing one discharge.

[0016] The lithium-rich manganese-based oxide can be represented by Chemical Formula 1 below.

[0017] [Chemical Formula 1]

[0018] Li a Ni b Co c Mn d M e O2

[0019] wherein in Chemical Formula 1,

[0020] 1 < a, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.1, 0.5 ≤ d < 1.0, 0 ≤ e ≤ 0.2, and

[0021] M is at least one selected from Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr, and Zr.

[0022] The charging step can be performed in a constant current mode (CC mode) or a constant current-constant voltage mode (CCCV mode).

[0023] The charging termination voltage in the charging step can be 4.5 V to 4.6 V.

[0024] The activation step can include discharging at a C-rate of 0.3 C to 0.7 C until reaching 2.0 V.

[0025] Advantageous Effects

[0026] According to the method for manufacturing a lithium secondary battery of the present invention, in a part of the activation charging step, that is, charging is performed at a C-rate of 0.3 C to 0.6 C in the range above SOC 60, so that activation of the Li2MnO3 (monoclinic) phase in the crystal structure of the positive electrode active material can be suppressed, thereby suppressing the voltage drop phenomenon generated during repeated charging and discharging, and also reducing gas generation. Description of the Drawings

[0027] Figure 1 It is a figure showing the result of evaluating the voltage drop after charging / discharging of a lithium secondary battery according to Experimental Example 1. Detailed Description of the Invention

[0028] The terms or words used in this specification and the appended claims should not be construed as limited to their ordinary or dictionary meanings, and the present invention should be interpreted as having meanings and concepts consistent with the technical gist of the present invention based on the principle that the inventors can appropriately define the concepts of the terms so as to appropriately describe their invention in the best way.

[0029] In the present invention, a "primary particle" refers to a particle unit in which no grain boundaries are seen when observed with a scanning electron microscope at a magnification of 5000 to 20000 times. The "average particle size of primary particles" refers to the arithmetic mean calculated by measuring the particle sizes of the primary particles observed in a scanning electron microscope image.

[0030] In the present invention, a "secondary particle" is a particle formed by aggregation of a plurality of primary particles.

[0031] In the present invention, "average particle size D 50 " refers to the particle size at which the cumulative volume of powder particles (e.g., positive electrode active material, negative electrode active material, etc.) in the particle size distribution curve of the particles reaches 50%. D 50 It can be measured by the laser diffraction method. It can be measured by the following method, which includes: 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 (e.g., Microtrac MT 3000), irradiating it with ultrasonic waves at an output power of 60 W at about 28 kHz to obtain a volume cumulative particle size distribution, and determining the particle size corresponding to 50% cumulative volume.

[0032] In addition, in the present invention, "SOC X" refers to a state in which the percentage of the charge capacity in the battery cell with respect to the discharge capacity exhibited when the battery cell is discharged from 4.6 V to 2.0 V is X.

[0033] In order to improve the voltage drop during the charging for activation of a lithium secondary battery using a lithium-rich manganese-based oxide, the present inventors conducted repeated studies and found that by performing the activation process under specific charging conditions when manufacturing a lithium secondary battery, cation mixing and oxygen release can be minimized and the activation of the Li2MnO3 (monoclinic) phase in the crystal structure of the positive electrode active material can be suppressed, preventing the voltage drop during battery charging and discharging, and thus completed the present invention.

[0034] Now, the present invention will be described in detail.

[0035] The method for manufacturing a lithium secondary battery according to the present invention is characterized in that the method includes:

[0036] 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, in which the content of manganese among all metals other than lithium is greater than 50 mol%, and the ratio (Li / Me) of the number of moles of lithium to the number of moles of all metals other than lithium is greater than 1; and

[0037] a step of charging and discharging the battery cell at least once to activate the battery,

[0038] wherein the activation step includes: charging the battery cell from SOC 5 to SOC 60 at a C-rate of 0.6 to 1.0 C, charging at a C-rate of 0.3 C to 0.6 C in the SOC 60 to SOC 100 range, and then performing a single discharge step.

[0039] (1) Step of preparing the battery cell

[0040] First, a battery cell including a positive electrode, a negative electrode, and an electrolyte is prepared.

[0041] The battery cell can be prepared, for example, by the following steps: forming an electrode assembly including a positive electrode and a negative electrode, placing 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.

[0042] Each component of the battery cell according to the present invention will be described in more detail below.

[0043] Positive Electrode

[0044] The positive electrode according to the present invention includes a lithium-rich manganese-based oxide, in which the content of manganese among all metals other than lithium is greater than 50 mol%, and the ratio (Li / Me) of the number of moles of lithium to the number of moles of all metals other than lithium 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, wherein the positive electrode active material layer includes a lithium-rich manganese-based oxide, in which the ratio (Li / Me) of the number of moles of lithium to the number of moles of all metals other than lithium is greater than 1.

[0045] When the lithium-rich manganese-based oxide contains an excessive amount of 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, while the rock salt phase is activated, an excessive amount of lithium ions are generated, thereby enabling high capacity.

[0046] Preferably, the lithium-rich manganese-based oxide can be represented by the following Chemical Formula 1.

[0047] [Chemical Formula 1]

[0048] Li a Ni b Co c Mn d M e O2

[0049] Among them, in Chemical Formula 1, M can be at least one selected from Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr, and Zr.

[0050] 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.

[0051] 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.

[0052] 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 is greater than 0.1, it is difficult to ensure high capacity, and the generation of gas and the deterioration of the positive electrode active material become aggravated, and the life characteristics may deteriorate.

[0053] 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 effects of negative electrode irreversible compensation and capacity improvement are slight.

[0054] 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.

[0055] On the other hand, in the lithium-rich manganese-based oxide represented by [Chemical Formula 1], the ratio of the molar number of Li to the molar number 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, the rate performance and capacity characteristics can be excellently exhibited. When the Li / Me ratio is too high, the conductivity may decrease, and the electrochemically inert rock salt phase (Li2MnO3) may increase, which may increase the degradation rate. When the Li / Me ratio is too low, the effect of improving the energy density is slight.

[0056] On the other hand, the composition of the lithium-rich manganese-based oxide can be represented by the following [Chemical Formula 2].

[0057] [Chemical Formula 2]

[0058] X Li2MnO3·(1-X)Li[Ni 1-y-z-w Mn y Co z M w O2

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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. When z is greater than 0.1, the generation of gas and the deterioration of the positive electrode active material may become aggravated, and the life characteristics may deteriorate.

[0063] 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.

[0064] On the other hand, 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 as needed. 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 the side reaction of the electrolyte is reduced, thereby achieving the effect of improving the life characteristics.

[0065] The coating may include the coating element M 1 , and the coating element M 1 can be, for example, at least one selected from 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 can include two or more kinds, such as Al and Co.

[0066] The coating element may exist in the coating in the form of an oxide, i.e., M 1 O z (1 ≤ z ≤ 4).

[0067] 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 in which a coating with a larger area can be formed.

[0068] 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%, and more preferably 50% to 100%. When the formation area of the coating satisfies the above range, the effect of improving the life characteristics is excellent.

[0069] On the other hand, the positive electrode active material according to the present invention can be in the form of secondary particles in which a plurality of primary particles are agglomerated, and the average particle size D 50 of the secondary particles can be 2 μm to 10 μm, preferably 2 μm to 8 μm, and more preferably 4 μm to 8 μm. When the D 50 of the positive electrode active material satisfies the above range, the electrode density can be excellently achieved, and the degradation of the capacity and rate characteristics can be minimized.

[0070] In addition, the BET specific surface area of the positive electrode active material can 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 low, the reaction area with the electrolyte is insufficient, making it difficult to achieve sufficient capacity. If the specific surface area is too high, water adsorption is rapid, and the side reaction with the electrolyte is accelerated, making it difficult to ensure the life characteristics.

[0071] On the other hand, the lithium-rich manganese-based oxide can be manufactured by mixing a transition metal precursor and a lithium raw material and then firing the mixture.

[0072] 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 among them, a single kind or a mixture of two or more kinds can be used.

[0073] On the other hand, the transition metal precursor may be in the form of a hydroxide, oxide or carbonate. When using a precursor in the form of a carbonate, it is more preferable to be able to manufacture a positive electrode active material having a relatively high specific surface area.

[0074] The transition metal precursor can be prepared by a coprecipitation process. For example, the transition metal precursor can be prepared by dissolving raw materials each containing a transition metal in a solvent to prepare a metal solution, then mixing the metal solution, an ammonium cation complexing agent and a basic compound, and carrying out a coprecipitation reaction. In addition, an oxidizing agent or oxygen can be added during the coprecipitation reaction as needed.

[0075] At this time, the raw materials containing transition metals can be acetates, carbonates, nitrates, sulfates, halides, sulfides, etc. of the respective transition metals. Specifically, the raw materials containing transition metals 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.

[0076] The ammonium cation complexing agent can be at least one selected from NH4OH, (NH4)2SO4, NH4NO3, NH4Cl, CH3COONH4 and (NH4)2CO3.

[0077] The basic compound can be at least one selected from NaOH, Na2CO3, KOH and Ca(OH)2. The form of the precursor can vary depending on the type of basic compound used. For example, when using NaOH as the basic compound, a precursor in the form of a hydroxide can be obtained, and when using Na2CO3 as the basic compound, a precursor in the form of a carbonate can be obtained. In addition, when the basic compound is used together with an oxidizing agent, a precursor in the form of an oxide can be obtained.

[0078] On the other hand, the transition metal precursor and the lithium raw material can be mixed in the following amounts: such that the molar ratio of all transition metals (Ni + Co + Mn):Li is 1:1.05 to 1:2, preferably 1:1.1 to 1:1.8, more preferably 1:1.25 to 1:1.8.

[0079] 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.

[0080] On the other hand, in addition to the positive electrode active material, the positive electrode active material layer may further contain a conductive material and a binder.

[0081] 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 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 and the like. Among them, a single kind 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.

[0082] In addition, the binder is a component that assists the adhesion between the positive electrode active materials and the adhesion force between the positive electrode active material and the current collector, and examples thereof include: fluororesin binders including polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE); rubbery 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 and the like. 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.

[0083] Negative Electrode

[0084] 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 contain a negative electrode active material and optionally a binder and a conductive material.

[0085] 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. 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 can 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 various forms such as films, sheets, foils, meshes, porous bodies, foams and non-woven fabric structures.

[0086] The negative electrode active material may include lithium metal, a carbon material capable of reversibly intercalating / deintercalating lithium ions, a metal or an alloy of lithium and these metals, a material capable of doping and undoping lithium, or a composite containing a (semi) metallic material and a carbonaceous material, such as a Si-C composite or a Sn-C composite, and any one of them alone or a mixture of two or more of them may be used.

[0087] As the carbon material capable of reversibly intercalating / deintercalating lithium ions, any carbon material may be used without particular limitation as long as it is a carbon-based negative electrode active material commonly used in lithium ion secondary batteries, and as typical examples, crystalline carbon, amorphous carbon, or both of them may be used. Examples of crystalline carbon may be graphite, such as natural graphite or artificial graphite in irregular, planar, flaky, spherical or fibrous forms, and examples of amorphous carbon may be soft carbon (low-temperature fired carbon) or hard carbon, mesophase pitch carbide, and fired coke. In addition, as the metal or the alloy of lithium and these metals, a metal selected from Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn or an alloy of lithium and these metals may be used. In addition, the material capable of doping and undoping lithium may include Si, SiO x (0 < x ≤ 2), a Si-Y alloy (where Y is an element selected from alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof and is not Si), Sn, SnO2, and Sn-Y (where Y is an element selected from alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof and is not Sn), and a mixture of SiO2 and at least one of them may also be used. The element Y may be selected from the following: Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof.

[0088] In addition, a thin film of metallic lithium may be used as the negative electrode active material. In addition, both low-crystalline carbon and high-crystalline carbon may be used as the carbon material. Typical examples of low-crystalline carbon include soft carbon and hard carbon. Typical examples of high-crystalline carbon include natural graphite or artificial graphite in irregular, planar, flaky, spherical or fibrous forms, condensed graphite, pyrolytic carbon, mesophase pitch-based carbon fibers, mesophase carbon microspheres, high-temperature sintered carbon such as mesophase pitch, and coke derived from petroleum or coal tar pitch.

[0089] A 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 electron conductivity and does not cause chemical changes in the battery to be formed. 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 alone 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.

[0090] The binder serves to improve the adhesion between the negative electrode active material particles and the adhesion force 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); rubbery 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 alone 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.

[0091] The negative electrode active material layer can be prepared, for example, 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 by casting the negative electrode slurry onto a separate support and then laminating the film peeled from the support on the current collector.

[0092] Separator

[0093] 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 conventional lithium secondary battery. In particular, a separator having a low resistance to ion migration of the electrolyte and excellent ability to impregnate the electrolyte is preferred. Specifically, the separator may be a porous polymer film, such as a porous polymer film 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. Further, 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 optionally be used as a single layer or a multi-layer structure.

[0094] Electrolyte

[0095] 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 there is no particular limitation on their types.

[0096] For example, the electrolyte may include an organic solvent and a lithium salt.

[0097] The organic solvent can be used without 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 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 double bonds, aromatic rings, or ether bonds); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; sulfolane, etc.

[0098] The lithium salt can be used without any particular limitation as long as it is a compound capable of providing lithium ions used in lithium secondary batteries. Specifically, the anion of the lithium salt may be at least one selected from the following substances: 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 the lithium salts that can be used in this text can 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 can preferably be used in the range of 0.1 to 5.0 M.

[0099] In addition, in order to improve the life characteristics of the battery, suppress the decrease in battery capacity, improve the discharge capacity of the battery, etc., in addition to the above electrolyte components, the electrolyte can further contain additives. For example, the additive can be at least one selected from the following substances: vinylene carbonate, ethylene vinyl carbonate, fluoroethylene carbonate (FEC), ethylene sulfite (Esa), trimethylenesulfate (TMS), methyltrimethylenesulfate (MTMS), 1,3 - propane sultone (1,3 - PS), 1,3 - propene sultone, 1,4 - butane sultone, lithium oxydifluoroborate (LiODFB), lithium bis(oxalato)borate (LiBOB), fluorobenzene, triethanolamine, ethylenediamine, tetravinylsilane, lithium difluorophosphate (LiPO2F2, LiDFP), and LiBF4, but not limited thereto. Based on the total weight of the electrolyte, the content of the additive can be 0.1 to 10% by weight, preferably 0.1 to 5% by weight.

[0100] On the other hand, the electrode assembly can be various forms of electrode assemblies known in the art. For example, it can be a jelly - roll type, a stacked type, a stacked and laminated type, or a stacked and folded type electrode assembly, and there is no particular limitation on its form.

[0101] The wound electrode assembly can be manufactured by inserting a sheet-shaped separator between a sheet-shaped positive electrode and a sheet-shaped negative electrode, and then winding in one direction.

[0102] The stacked electrode assembly can be manufactured by cutting the positive electrode, separator, and negative electrode into desired shapes, and then sequentially stacking the cut positive electrode / separator / negative electrode.

[0103] The stacked and laminated electrode assembly can be manufactured by the following method: stacking the positive electrode, separator, and negative electrode to manufacture a plurality of unit cells, stacking the plurality of unit cells with the separator interposed therebetween, and then laminating them by a method such as heating.

[0104] The stacked and folded electrode assembly can be manufactured by the following method: stacking the positive electrode, separator, and negative electrode to manufacture 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.

[0105] 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 prismatic battery case, or a pouch-shaped battery case, etc., and there is no particular limitation on their types.

[0106] (2) Activation step

[0107] Next, a step of charging and discharging the battery cell at least once to electroactivate the battery is performed. The activation step is a step of charging and discharging the battery cell to impart electrical performance and form a SEI (solid electrolyte interface) film on the electrode to stabilize the battery.

[0108] In the present invention, the charging current in a part of the activation charging step is adjusted, and the activation degree of Li2MnO3 (monoclinic) after activation is adjusted, so that the voltage drop generated during repeated charging and discharging can be improved. Specifically, by adjusting the charging current in a part of the interval to a C rate of 0.6C or less, the Li2MnO3 (monoclinic) structure becomes LiMO2 (M = Ni, Co, Mn, etc.) having a rhombohedral structure, and a compound of a solid solution phase in which these structures are mixed is formed, and activation can be performed more stably, which can lead to an improvement in the voltage drop generated during repeated charging and discharging.

[0109] First, the charging may include a first charging step of charging until SOC 5.

[0110] The first charging step can be carried out at a C-rate of less than 0.3C, preferably 0.1C to 0.3C. When the charging rate during the first charging is faster than 0.3C, the SEI film may be formed unstably on the electrode surface. When the SEI film is formed unstably on the electrode surface, the SEI film is liable to decompose during battery operation, leading to rapid deterioration of the electrode, which may result in a significant decline in life characteristics.

[0111] The first charging step is preferably carried out from SOC 0 to SOC 5, and 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, and excellent life characteristics can be achieved.

[0112] On the other hand, the first charging step can be carried out in a constant current mode (CC mode).

[0113] Next, the charging may include a second charging step of charging from SOC 5 to SOC 100. At this time, if necessary, the second charging step can be carried out in one step or in two or more steps with different rates.

[0114] If the second charging step is carried out in one step, the second charging step can be carried out at a C-rate of 0.3C or more, preferably 0.3C to 0.6C.

[0115] If the second charging step is carried out at a C-rate of 0.3C to 0.6C, the charging time increases, but the activation stability of Li2MnO3 (monoclinic) can be increased instead. At this time, if the second charging step is carried out at a relatively faster C-rate than the first charging step, the time required for the activation process is shortened, and thus the manufacturing time of the battery can be shortened.

[0116] The second charging step carried out in one step is preferably carried out from SOC 5 to SOC 100. When the charging capacity in the second charging step satisfies the above range, the incomplete formation of the SEI film can be suppressed, and the oxygen detachment and cation mixing during the activation step can be suppressed, thereby minimizing the increase in the positive electrode resistance and also achieving a high capacity.

[0117] In addition, when the second charging step is carried out in two or more steps, it may include a 2-1 charging step carried out at a C-rate of 0.3C to 1.0C, and a 2-2 charging step carried out at a C-rate of less than 0.6C, specifically 0.3C to 0.6C.

[0118] At this time, the 2-1 charging step is carried out from SOC 5 to SOC 60, and the 2-2 charging step is carried out from SOC 60 to SOC100.

[0119] If the second charging step first charges at a C-rate of 0.3C to 1.0C (2-1 charging step) as described above, and then charges at a C-rate of 0.3C to 0.6C (2-2 charging step), the charging time can be shortened, and the activation stability of Li2MnO3 (monoclinic) can also be increased.

[0120] 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).

[0121] When the second charging step is carried out in the constant current-constant voltage mode (CCCV mode), CC charging is carried out while supplying a C-rate of 0.3C to 0.6C until the charging termination voltage is reached, and when the charging termination voltage is reached, CV charging can be carried out, where the charging C-rate is sequentially reduced to about 0.05C.

[0122] On the other hand, the charging termination voltage in the second charging step can be 4.5V to 4.6V, specifically 4.6V. When the charging termination voltage in the second charging stage satisfies the above range, the lithium-rich manganese-based oxide is activated, and high-capacity characteristics can be achieved.

[0123] Next, the activation step discharges the battery cell charged by the first charging step and the second charging step. At this time, the discharging can be carried out at a C-rate of 0.3C to 0.7C. When the discharging rate satisfies the above range, the activation time can be appropriately controlled, and the discharging capacity characteristics within the desired range can be achieved.

[0124] On the other hand, the discharging can be carried out in the constant current mode (CC mode).

[0125] On the other hand, the discharging termination voltage can be 2.0 V to 3.0 V, specifically 2.0 V.

[0126] On the other hand, the activation step is preferably carried out under temperature conditions of 30°C to 75°C, more preferably 40°C to 45°C. When the activation step is carried out within the above temperature range, the effect of achieving high capacity can be obtained through the appropriate activation of the Li2MnO3 (monoclinic) phase.

[0127] In addition, if necessary, the activation step can be carried out under pressurized conditions. Pressurization can be carried out by installing the battery cell on a fixture and then applying pressure to the battery cell via the fixture. When the activation step is carried out under pressurized conditions, this has the advantage of being able to easily discharge the gas generated during the activation step.

[0128] On the other hand, although not necessary, if desired, the activation step may further include an aging step. The aging step uniformly impregnates the electrolyte into the electrode assembly and stabilizes the battery, and can be performed before charging, during charging, and / or after discharging, and can be performed more than once.

[0129] 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 electrolyte impregnation property and lithium mobility are improved, enabling activation to proceed more smoothly.

[0130] Hereinafter, the present invention will be described in more detail with reference to specific embodiments.

[0131] [Examples]

[0132] Comparative Example 1

[0133] (Manufacture of battery cell)

[0134] The positive electrode active material: conductive material: PVDF binder were mixed at a weight ratio of 97:1:2 in N-methylpyrrolidone to prepare a positive electrode paste. At this time, Li 1.38 [Ni 0.363 Co 0.005 Mn 0.642 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.

[0135] The negative electrode active material: conductive material: binder were mixed at a weight ratio of 96:1:3 in water 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 were mixed at a weight ratio of 2:1 and 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.

[0136] A separator was inserted between the positive electrode and the negative electrode manufactured as above to manufacture an electrode assembly, and the electrode assembly was inserted into a battery case, and then an electrolyte was injected therein to manufacture a battery cell.

[0137] (Activation step)

[0138] After pre-aging the battery cell for 2 days, the following activation process was carried out to manufacture a lithium secondary battery: charging at a constant current mode at 45°C until SOC 3 (0.2C) (first charging step), charging in a constant current-constant voltage mode (0.05C CV cut-off) until SOC 100 (0.33C) (charging termination voltage: 4.6V) (second charging step), and then discharging at a constant current of 0.6C until 2.0 V.

[0139] Example 1

[0140] After pre - aging the battery cells manufactured in Comparative Example 1 for 2 days, a lithium battery was manufactured in the same manner as in Comparative Example 1, except that the following activation steps were performed: charging at a constant current mode at 45 °C until SOC 3 (0.2C) (the first charging step), charging at a constant current mode until SOC 60 (1.0C) (2 - 1 charging stage), charging at a constant current mode until SOC 100 (0.4C) (charging termination voltage: 4.6V) (2 - 2 charging step), and then discharging at a constant current of 0.6C until 2.0 V.

[0141] Example 2

[0142] After pre - aging the battery cells manufactured in Comparative Example 1 for 2 days, a lithium battery was manufactured in the same manner as in Comparative Example 1, except that the following activation steps were performed: charging at a constant current mode at 45 °C until SOC 3 (0.2C) (the first charging step), charging at a constant current mode until SOC 60 (1.0C) (2 - 1 charging stage), charging in constant current - constant voltage mode (0.05C CV cut - off) until SOC 100 (0.4C) (charging termination voltage: 4.6V) (2 - 2 charging step), and then discharging at a constant current of 0.6C until 2.0 V.

[0143] Comparative Example 2

[0144] After pre - aging the battery cells manufactured in Comparative Example 1 for 2 days, a lithium battery was manufactured in the same manner as in Comparative Example 1, except that the following activation steps were performed: charging at a constant current mode at 45 °C until SOC 3, charging at a constant current mode until SOC 100 (1.0C) (charging termination voltage: 4.6V), and then discharging at a constant current of 0.6C until 2.0 V.

[0145] [Experimental Example]

[0146] Experimental Example 1: Evaluation of Voltage Drop

[0147] The following charge / discharge process was set as one cycle: charging the lithium secondary batteries manufactured in Examples 1 and 2 and the secondary batteries manufactured in Comparative Examples 1 and 2 in a constant current - constant voltage mode of 0.33C (0.05C CV cut - off), and discharging at a constant current of 0.6C until 2.0V; and performing 100 cycles at a high temperature (45 °C).

[0148] At this time, the voltage after 50 cycles was measured, and the voltage drop rate was calculated based on the voltage after the first cycle. The results are shown in Table 1 below. In addition, the nominal voltage after each cycle of discharge was measured, and ΔV (voltage drop) was measured. The results are shown below Figure 1 below.

[0149]

[0150] As shown in Table 1 and Figure 1 as shown, it can be seen that, compared with the lithium secondary battery of Comparative Example 2, in the case of the lithium secondary batteries of Examples 1 and 2, the voltage drop rate (amount of voltage drop) was lower during the charge and discharge processes of 50 cycles and 100 cycles. In addition, it was confirmed that in the case of Comparative Example 1, although the voltage drop rate was low, the voltage itself after 50 cycles was low, which was problematic.

Claims

1. A method of manufacturing a lithium secondary battery, the method comprising the steps of: 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 in which the content of manganese among all metals other than lithium is greater than 50 mol%, and the ratio (Li / Me) of the number of moles of lithium to the number of moles of all metals other than lithium is greater than 1; And Charging and discharging the electrode assembly at least once to activate the battery, wherein the activation step includes: charging the electrode assembly from SOC 5 to SOC 60 at a C-rate of 0.6C to 1.0C, charging in the range of SOC 60 to SOC 100 at a C-rate of 0.3C to 0.6C, and then performing one discharge.

2. The method of 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 of 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 of 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 of manufacturing a lithium secondary battery according to claim 1, wherein the charging termination voltage in the charging step is 4.5 V to 4.6 V.

6. The method for manufacturing a lithium secondary battery according to claim 1, wherein the activation step further comprises: Charging from SOC 0 to SOC 5 at a C-rate of 0.1C to 0.3C.

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

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