Lithium secondary battery manufacturing method

Abstract

The present invention relates to a method for manufacturing a lithium secondary battery, comprising the steps of preparing a battery cell including a positive electrode, a negative electrode, and an electrolyte, the positive electrode containing a lithium-excess manganese-based oxide in which the content of manganese in the total metal excluding lithium exceeds 50 mol% and the ratio (Li / Me) of the number of moles of lithium to the number of moles of the total metal excluding lithium exceeds 1; and charging and discharging the battery cell at least once to activate it, wherein the activation step includes ending charging when the ratio (Y / X) of the charging capacity (mAh / g) (Y) of the secondary battery to the estimated capacity (mAh / g) (X) of the positive electrode active material at the point where all lithium ions contained in the lithium-excess manganese-based oxide form a Li / Li dumbbell structure corresponds to 1.10 < Y / X ≦ 1.13.

Description

[Technical Field]

[0001] [CROSS-REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of priority based on Korean Patent Application No. 10-2023-0024513 filed February 23, 2023 and Korean Patent Application No. 10-2024-0026192 filed February 23, 2024, and all contents disclosed in the documents of said Korean patent applications are incorporated herein by reference.

[0002] 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 that can suppress generation of oxidizing gas induced during activation of a lithium secondary battery including a lithium-excess manganese-based oxide. [Background technology]

[0003] Lithium secondary batteries are energy storage media that have been used in a variety of fields since their commercialization in 1991. As the market for products equipped with lithium secondary batteries expands, active research is being conducted to increase the energy density of lithium secondary batteries, and one of the most notable methods is the development of positive electrode active materials with a composition that allows for the use of a larger amount of lithium than currently available.

[0004] As a positive electrode active material that can utilize a larger amount of lithium, lithium-rich transition metal oxides have been developed that have a layered structure and a lithium to transition metal molar ratio of more than 1. In lithium secondary batteries that use such lithium-rich transition metal oxides, high capacity can be achieved by performing an activation process at a high voltage of generally 4.4 V or higher.

[0005] However, performing the activation process at such high voltages can induce oxygen desorption and cation mixing in the crystalline structure of lithium-rich transition metal oxides, significantly increasing the resistance of the positive electrode. Specifically, during the high-voltage activation process, lithium ions are desorbed, changing the positive electrode structure, and oxygen redox reactions cause the generation / desorption of oxygen radicals, which can then react with the electrolyte to generate large amounts of oxidizing gases such as CO, CO2, and O2. Additionally, the electrolyte can decompose under high-voltage activation conditions, generating large amounts of oxidizing gases.

[0006] When a large amount of oxidizing gas is generated, the internal pressure of the cell increases, causing a short circuit, or it becomes difficult to separate from the jig, which leads to lithium plating due to the gas trap, resulting in a decrease in stability as well as charge / discharge capacity.

[0007] Therefore, there is a need to develop a method for suppressing the generation of oxidizing gas during the high voltage activation process. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Korean Patent Publication No. 2022-0068016 [Non-patent literature]

[0009] [Non-Patent Document 1] Electrochemistry Communications 6(2004)1045-1050 Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention is for solving the above problems, and an object thereof is to provide a method for manufacturing a lithium secondary battery including a lithium-excess manganese-based oxide capable of suppressing the generation of oxidation gas induced in a high voltage activation stage.

Means for Solving the Problems

[0011] On one side, the present invention preparing a battery cell including a positive electrode, a negative electrode, and an electrolyte, the positive electrode including a lithium-excess manganese-based oxide in which the content of manganese in the total metal excluding lithium exceeds 50 mol%, and the ratio (Li / Me) of the number of moles of lithium to the number of moles of the total metal excluding lithium exceeds 1; and activating the battery cell by charging and discharging it at least once, wherein the activating step includes a step of ending charging when the ratio (Y / X) of the charging capacity (mAh / g) (Y) of the secondary battery to the estimated capacity (mAh / g) (X) of the positive electrode active material at the point where all lithium ions contained in the lithium-excess manganese-based oxide form a Li / Li dumbbell structure corresponds to 1.10 < Y / X ≦ 1.13, and provides a method for manufacturing a lithium secondary battery.

[0012] On the other hand, the estimated capacity (C) (mAh / g) of the positive electrode active material can be calculated by the following formula 1.

[0013] [Formula 1] C (mAh / g) = Q × (i / 100)

[0014] In the above formula 1, Q is the theoretical capacity (mAh / g) of the lithium-excess manganese-based oxide, i is the percentage (%) of the molar ratio of lithium ions that have moved until only lithium ions forming a Li / Li dumbbell structure exist, with respect to the total molar ratio of lithium ions contained in the lithium-excess manganese-based oxide, and can be defined by the following formula 2 ([Equation 1]).

[0015] [Formula 2]

number

[0016] In the formula 2, the Li molar ratio is the total molar ratio of lithium contained in the lithium-excess manganese-based oxide.

[0017] Also, the activation step may include charging at 0.3C to 1.0C up to 4.5V to 4.6V.

[0018] The charging step may be performed in a constant current (CC) mode or a constant current-constant voltage (CCCV) mode, and the cutoff current during constant voltage charging may be 0.05C to 0.15C. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a graph showing the evaluation results of the amount of gas generated during the activation step in Experimental Example 1. [Figure 2] 10 is a graph showing the evaluation results of the initial resistance according to Experimental Example 2. [Figure 3] 10 is a graph showing the results of capacity retention rate in a high-temperature cycle evaluation according to Experimental Example 3. [Figure 4] 1 is a diagram illustrating the structural change of a lithium-excess manganese-based oxide and the generation of an oxidation gas as charging proceeds in an activation stage. [Figure 5] 1 is a graph showing the rate of change in lithium concentration (Li site concenetration) depending on the lithium site of a Lix(Ni0.5Mn0.5)O2 compound. DETAILED DESCRIPTION OF THE INVENTION

[0020] The terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted as meanings and concepts that correspond to the technical ideas of the present invention based on the principle that an inventor can appropriately define the concepts of terms in order to best explain his or her invention.

[0021] In the present invention, "primary particles" refers to particle units that do not appear to have grain boundaries when observed at a magnification of 5,000 to 20,000 using a scanning electron microscope. "Average particle size of primary particles" refers to the arithmetic mean value calculated after measuring the particle sizes of primary particles observed in a scanning electron microscope image.

[0022] In the present invention, the "secondary particles" are particles formed by agglomeration of a plurality of primary particles.

[0023] In the present invention, "average particle size D 50 " refers to the particle size at 50% of the volume cumulative particle size distribution of the particle powder to be measured (e.g., positive electrode active material powder, negative electrode active material powder, etc.). The average particle size D50 can be measured using a laser diffraction method. For example, the powder of particles to be measured is dispersed in a dispersion medium, introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac's MT3000), and irradiated with ultrasonic waves of about 28 kHz at an output of 60 W. A volume cumulative particle size distribution graph is then obtained, and the particle size corresponding to 50% of the volume cumulative amount is determined.

[0024] In addition, in the present invention, "SOC X" means a state in which the percentage of the capacity charged to the battery cell is X based on the discharge capacity that appears when the battery cell is discharged from 4.6V to 2.0V.

[0025] As a result of repeated studies to suppress the generation of oxidation gas during the activation process of a lithium secondary battery using a lithium-excess manganese-based oxide, the inventors have found that by adjusting the end point of charging, it is possible to suppress the generation of oxidation gas during the high-voltage activation process, and thus completed the present invention.

[0026] Hereinafter, the present invention will be specifically described.

[0027] <Method for manufacturing a lithium secondary battery> The method for manufacturing a lithium secondary battery according to the present invention includes: preparing a battery cell including a positive electrode, a negative electrode, and an electrolyte, the positive electrode containing a lithium-excess manganese-based oxide in which the content of manganese in the total metal excluding lithium exceeds 50 mol% and the ratio (Li / Me) of the number of moles of lithium to the number of moles of the total metal excluding lithium exceeds 1; and activating the battery cell by charging and discharging it at least once, where the activating step includes ending the charging when the ratio (Y / X) of the charging capacity (mAh / g) (Y) of the secondary battery to the estimated capacity (mAh / g) (X) of the positive electrode active material at the point where all lithium ions contained in the lithium-excess manganese-based oxide form a Li / Li dumbbell structure corresponds to 1.10 < Y / X ≤ 1.13.

[0028] (1) Step of preparing a battery cell First, a battery cell including a positive electrode, a negative electrode, and an electrolyte is prepared.

[0029] The battery cell can be manufactured, for example, by forming an electrode assembly including a positive electrode and a negative electrode, then housing the electrode assembly in a battery case, and then injecting an electrolytic solution and sealing the battery case. At this time, the electrode assembly can include a separator between the positive electrode and the negative electrode.

[0030] Hereinafter, each component of the battery cell of the present invention will be described in more detail.

[0031] Positive electrode The positive electrode according to the present invention contains a lithium-excess manganese-based oxide in which the content of manganese in the total metal excluding lithium as a positive electrode active material exceeds 50 mol%, and the ratio (Li / Me) of the number of moles of lithium to the number of moles of the total metal excluding lithium exceeds 1. Specifically, the positive electrode of 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, and the positive electrode active material layer contains a lithium-excess manganese-based oxide in which the ratio (Li / Me) of the number of moles of lithium to the number of moles of the total metal excluding lithium exceeds 1.

[0032] In the case of a lithium-excess manganese-based oxide containing excess lithium, it has a structure in which a layered structure (LiM’O2) and a rock-salt structure (Li2MnO3) are mixed. However, in the initial activation process, the rock-salt structure is activated to generate excess lithium ions, so a high capacity can be realized.

[0033] Preferably, the lithium-excess manganese-based oxide can be represented by Chemical Formula 1.

[0034] [Chemical Formula 1] Li a Ni b Co c Mn d M e O2

[0035] In Chemical Formula 1, M can be one or more selected from the group consisting of Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr, and Zr.

[0036] On the other hand, a is the molar ratio of Li in the lithium-excess manganese-based oxide, and 1 < a, 1.1 ≤ a ≤ 1.5, or 1.1 ≤ a ≤ 1.3 can be satisfied.

[0037] The b is the molar ratio of Ni in the lithium-excess manganese-based oxide, and 0 ≤ b ≤ 0.5, 0.1 ≤ b ≤ 0.4, or 0.2 ≤ b ≤ 0.4 can be satisfied. [[ID=​​The c is the molar ratio of Co in the lithium-excess manganese-based oxide and may be 0≦c≦0.1, 0≦c≦0.08, or 0≦c≦0.05. If c exceeds 0.1, it may be difficult to ensure high capacity, and gas generation and deterioration of the positive electrode active material may become severe, resulting in reduced life characteristics.

[0039] The d is the molar ratio of Mn in the lithium-excess manganese-based oxide, and may be 0.50≦d<1.0, 0.50≦d≦0.80, or 0.50≦d≦0.70. If d is less than 0.5, the proportion of rock-salt structure becomes too small, resulting in insignificant irreversible compensation and capacity improvement effects of the negative electrode.

[0040] The e is the molar ratio of the doping element M in the lithium-excess manganese-based oxide, and may be 0≦e≦0.2, 0≦e≦0.1, or 0≦e≦0.05. If the content of the doping element is too high, it may adversely affect the capacity of the active material.

[0041] Meanwhile, in the lithium-excess manganese-based oxide represented by [Chemical Formula 1], the molar ratio of Li to the total molar number of metal elements excluding Li (Li / Me) can be 1.2 to 1.5, 1.25 to 1.5, or 1.25 to 1.4. When the Li / Me ratio is within this range, excellent rate characteristics and capacity characteristics are exhibited. If the Li / Me ratio is too high, electrical conductivity may decrease and the rock salt-like (Li2MnO3) structure may increase, resulting in an accelerated degradation rate. If the Li / Me ratio is too low, the effect of improving energy density may be negligible.

[0042] Meanwhile, the composition of the lithium-excess manganese-based oxide can be expressed by the following [Chemical Formula 2].

[0043] [Chemical formula 2] (X)Li2MnO3·(1-X)Li[Ni 1-y-z-w Mn y Co z M w ]O2

[0044] In the formula 2, M is a metal ion and may be one or more selected from the group consisting of Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr, and Zr.

[0045] The X represents the ratio of the Li2MnO3 phase in the lithium-excess manganese-based oxide, and may be 0.2≦X≦0.5, 0.25≦X≦0.5, or 0.25≦X≦0.4. When the ratio of the Li2MnO3 phase in the lithium-excess manganese-based oxide satisfies this range, high capacity characteristics can be achieved.

[0046] The y is the molar ratio of Mn in the LiM′O 2 layer, and can be 0.4≦y<1, 0.4≦y≦0.8, or 0.4≦y≦0.7.

[0047] The z is the molar ratio of Co in the LiM'O2 layer and may be 0≦z≦0.1, 0≦z≦0.08, or 0≦z≦0.05. If z exceeds 0.1, gas generation and degradation of the positive electrode active material may become severe, resulting in reduced life characteristics.

[0048] The 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.

[0049] Meanwhile, the positive electrode active material according to the present invention may further include a coating layer on the surface of the lithium-excess manganese-based oxide, if necessary. When the positive electrode active material includes a coating layer, the coating layer prevents contact between the lithium-excess manganese-based oxide and the electrolyte, thereby reducing electrolyte side reactions and improving life characteristics.

[0050] The coating layer is made of a coating element M 1 The coating element M 1For example, the coating element M may be one or more selected from the group consisting of Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr, and Zr, and may be preferably Al, Co, Nb, W, or a combination thereof, and more preferably Al, Co, or a combination thereof. 1 may contain two or more kinds, for example, Al and Co.

[0051] The coating elements are in the oxide form in the coating layer, i.e., M 1 O z It can exist in the range (1≦z≦4).

[0052] The coating layer can be formed by dry coating, wet coating, chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD), etc. Among these, atomic layer deposition is preferred because it allows the coating layer to be formed over a large area.

[0053] The area where the coating layer is formed may be 10 to 100%, preferably 30 to 100%, more preferably 50 to 100% of the total surface area of ​​the lithium-excess manganese-based oxide particle. When the area where the coating layer is formed satisfies the above range, an excellent effect of improving life characteristics can be achieved.

[0054] Meanwhile, the cathode active material according to the present invention may be in the form of secondary particles formed by agglomeration of a plurality of primary particles, and the average particle diameter D of the secondary particles may be 50 The D of the positive electrode active material can be 2 μm to 10 μm, preferably 2 μm to 8 μm, and more preferably 4 μm to 8 μm. 50 When satisfies the above range, excellent electrode density can be achieved and the decrease in capacity and rate characteristics can be minimized.

[0055] The positive electrode active material has a BET specific surface area of ​​1 m 2 / g~10m 2 / g, 3-8m 2 / g or 4~6m 2 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, whereas if the specific surface area is too high, moisture absorption is rapid, accelerating side reactions with the electrolyte and making it difficult to ensure life characteristics.

[0056] Meanwhile, the lithium-excess manganese-based oxide may be prepared by mixing a transition metal precursor and a lithium source material and then calcining the mixture.

[0057] Examples of the lithium source material include lithium-containing carbonates (e.g., lithium carbonate, etc.), hydrates (e.g., lithium hydroxide hydrate (LiOH·H2O)), hydroxides (e.g., lithium hydroxide, etc.), nitrates (e.g., lithium nitrate (LiNO3), etc.), and chlorides (e.g., lithium chloride (LiCl)), and any of these may be used alone or in combination.

[0058] Meanwhile, the transition metal precursor may be in the form of a hydroxide, oxide, or carbonate. When a carbonate precursor is used, it is more preferable in that a positive electrode active material having a relatively high specific surface area can be prepared.

[0059] The transition metal precursor may be prepared through a co-precipitation process. For example, the transition metal precursor may be prepared by dissolving each transition metal-containing raw material in a solvent to prepare a metal solution, and then mixing the metal solution, an ammonium cation complexing agent, and a basic compound, followed by co-precipitation. If necessary, an oxidizing agent or oxygen gas may be added during the co-precipitation reaction.

[0060] At this time, the transition metal-containing raw material may be acetate, carbonate, nitrate, sulfate, halide, sulfide, etc. of each transition metal. Specifically, the transition metal-containing raw material may be NiO, NiCO3·2Ni(OH)2·4H2O, NiC2O2·2H2O, Ni(NO3)2·6H2O, NiSO4, NiSO4·6H2O, Mn2O3, MnO2, Mn3O 4、It can be MnCO3, Mn(NO3)2, MnSO4·H2O, manganese acetate, manganese halides, etc.

[0061] The ammonium cation complexing agent can be at least one selected from the group consisting of NH4OH, (NH4)2SO4, NH4NO3, NH4Cl, CH3COONH4, and NH4CO3.

[0062] The basic compound may be at least one selected from the group consisting of NaOH, Na2CO3, KOH, and Ca(OH)2. The form of the precursor may vary depending on the type of basic compound used. For example, when NaOH is used as the basic compound, a hydroxide-form precursor can be obtained, and when Na2CO3 is used as the basic compound, a carbonate-form precursor can be obtained. Furthermore, when a basic compound and an oxidizing agent are used together, an oxide-form precursor can be obtained.

[0063] Meanwhile, the transition metal precursor and the lithium source material may be mixed in amounts such that the molar ratio of 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.

[0064] The firing may be performed at a temperature of 600 to 1000°C or 700 to 950°C for a period of 5 to 30 hours or 5 to 20 hours. The firing atmosphere may be air or oxygen, for example, an atmosphere containing 20 to 100% by volume of oxygen.

[0065] Meanwhile, the positive electrode active material layer may further include a conductive material and a binder in addition to the positive electrode active material.

[0066] Examples of the conductive material include spherical or flake graphite; carbonaceous materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, single-walled carbon nanotubes, and multi-walled carbon nanotubes; metal powder or metal fiber such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive polymers such as polyphenylene derivatives. These materials may be used alone or in combination. The conductive material may be included in an amount of 0.1 to 20 wt %, 1 to 20 wt %, or 1 to 10 wt %, based on the total weight of the positive electrode active material layer.

[0067] The binder is a component that improves adhesion between positive electrode active material particles and between the positive electrode active material and the current collector. Examples include fluororesin-based binders including polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE); rubber-based 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; polyalcohol-based binders including polyvinyl alcohol; polyolefin-based binders including polyethylene and polypropylene; polyimide-based binders; polyester-based binders; and silane-based binders. The binder may be present in an amount of 1 to 20 wt %, 2 to 20 wt %, or 2 to 10 wt % based on the total weight of the positive electrode active material layer.

[0068] negative electrode The negative electrode may include, for example, a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. The negative electrode active material layer may optionally include a binder and a conductive material in addition to the negative electrode active material.

[0069] The negative electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. can be used. In addition, the negative electrode current collector typically has a thickness of 3 to 500 μm. As with the positive electrode current collector, the current collector surface can be formed with fine irregularities to strengthen the binding strength of the negative electrode active material. The negative electrode current collector can be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0070] The negative electrode active material may be a compound capable of reversible intercalation and deintercalation of lithium. Specific examples thereof include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, and Al alloys; and SiO β (0<β<2), metal oxides that can be doped and dedoped with lithium, such as SnO2, vanadium oxide, and lithium vanadium oxide; or composites containing the above metallic compounds and carbonaceous materials, such as Si-C composites or Sn-C composites, and any one or a mixture of two or more of these can be used.

[0071] A thin film of metallic lithium can also be used as the negative electrode active material. Carbon materials can be either low-crystalline or high-crystalline. Examples of low-crystalline carbon include soft carbon and hard carbon. Examples of high-crystalline carbon include amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite, kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and high-temperature-fired carbon such as petroleum or coal tar pitch-derived cokes.

[0072] The conductive material is used to impart conductivity to the electrode. Any material with electronic conductivity that does not undergo chemical changes in the resulting battery can be used without any particular limitations. Specific examples include graphite, such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, and carbon nanotubes; metal powders or fibers, such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; and conductive polymers, such as polyphenylene derivatives. These materials can be used alone or in combination. The conductive material is typically present in an amount of 1 to 30 wt %, preferably 1 to 20 wt %, and more preferably 1 to 10 wt %, based on the total weight of the negative electrode active material layer.

[0073] The binder functions to improve adhesion between negative electrode active material particles and between the negative electrode active material and the negative electrode current collector. Specific examples include fluororesin-based binders including polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE); rubber-based 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; polyalcohol-based binders including polyvinyl alcohol; polyolefin-based binders including polyethylene and polypropylene; polyimide-based binders; polyester-based binders; and silane-based binders. These binders can be used alone or in combination. The binder can be present in an amount of 1 to 30 wt %, preferably 1 to 20 wt %, and more preferably 1 to 10 wt %, based on the total weight of the negative electrode active material layer.

[0074] For example, the negative electrode active material layer may be prepared by coating a negative electrode slurry containing a negative electrode active material, and optionally a binder and a conductive material, on a negative electrode current collector and drying the coating, or by casting the negative electrode slurry on a separate support, peeling the negative electrode slurry from the support, and laminating the resulting film on the negative electrode current collector.

[0075] Separation membrane The lithium secondary battery according to the present invention may further include a separator interposed between the positive and negative electrodes. The separator separates the negative and positive electrodes and provides a path for lithium ions to move. Any separator commonly used in lithium secondary batteries may be used without any particular restrictions. A separator with low resistance to electrolyte ion movement and excellent electrolyte humidification is particularly preferred. Specifically, porous polymer films, such as those made of polyolefin-based polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminates of two or more layers thereof, may be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may also be used. Separators coated with ceramic components or polymeric materials to ensure heat resistance or mechanical strength may also be used, and may be used in either a single-layer or multi-layer structure.

[0076] electrolyte The electrolyte is not particularly limited, and may be any of 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.

[0077] For example, the electrolyte can include an organic solvent and a lithium salt.

[0078] The organic solvent may be any solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specific examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether and tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate. Examples of solvents that can be used include carbonate solvents such as PC (carbonate); alcohol solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (R is a hydrocarbon group having 2 to 20 carbon atoms and having a linear, branched, or cyclic structure, which may contain a double-bonded aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; and sulfolanes.

[0079] The lithium salt can be used without any particular limitation as long as it is a compound that can provide lithium ions used in lithium secondary batteries. Specifically, the anion of the lithium salt can be F - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 -, CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - The lithium salt may be one or more selected from the group consisting of LiPF6, LiN(FSO2)2, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, etc. The concentration of the lithium salt used is preferably within a range of 0.1 to 5.0M.

[0080] In addition to the above components, the electrolyte may further include an additive for the purpose of improving the battery life characteristics, suppressing a decrease in battery capacity, improving the battery discharge capacity, etc. For example, the additive may include at least one selected from the group consisting of halogen-substituted or unsubstituted carbonate-based compounds, sulfate-based compounds, sultone-based compounds, borate-based compounds, benzene-based compounds, amine-based compounds, silane-based compounds, and lithium salt-based compounds. Specifically, the additive may be at least one selected from the group consisting of vinylene carbonate, vinylethylene carbonate, fluoroethylene carbonate (FEC), ethylene sulfate (ESA), trimethylene sulfate (TMS), methyl trimethylene sulfate (MTMS), 1,3-propane sultone (1,3-PS), 1,3-propene sultone, 1,4-butane sultone, lithium oxalyl difluoroborate (LiODFB), lithium bis(oxalato)borate (LiBOB), fluorobenzene, triethanolamine, ethylenediamine, tetravinylsilane, lithium difluorophosphate (LiPOF, LiDFP), and LiBF. The additive may be included in an amount of 0.1 to 10 wt %, preferably 0.1 to 5 wt %, based on the total weight of the electrolyte.

[0081] Meanwhile, the electrode assembly may be an electrode assembly of various types known in the art, such as a jelly roll type, a stack type, a stack and lamination type, or a stack and folding type, but the type is not particularly limited.

[0082] The jelly roll type electrode assembly may be manufactured by interposing a sheet-shaped separator between a sheet-shaped positive electrode and a sheet-shaped negative electrode, and then winding the electrodes in one direction.

[0083] The stacked electrode assembly may be manufactured by cutting a positive electrode, a separator, and a negative electrode into a desired shape, and then stacking the cut positive electrode / separator / negative electrode in order.

[0084] The stack-and-lamination type electrode assembly may be manufactured by stacking a positive electrode, a separator, and a negative electrode to form a plurality of unit cells, stacking the unit cells with a separator interposed therebetween, and then laminating the unit cells by a method such as heating.

[0085] The stack-and-folding type electrode assembly may be manufactured by stacking a positive electrode, a separator, and a negative electrode to manufacture a plurality of unit cells, arranging the plurality of unit cells on one or both sides of a long folding separator, and then winding up the folding separator.

[0086] Meanwhile, the electric case may be any of various battery cases known in the art, such as a cylindrical battery case, a square battery case, or a pouch-type battery case, but the type is not particularly limited.

[0087] (2) Activation stage Next, an activation step is performed in which the battery cell is charged and discharged at least once to electrically activate the battery, which is a process of charging and discharging the battery cell to impart electrical characteristics and forming a solid electrolyte interphase (SEI) film on the electrodes to stabilize the battery.

[0088] Specifically, the higher the charging voltage, the longer the charging time and depth of charge (capacity), and the greater the reaction degree of Li2MnO3 during activation, which increases the amount of activated gas (oxidizing gas). Therefore, in the present invention, the end point of charge during the activation charging stage is adjusted to control the activation degree of the Li2MnO3 (monoclinic) phase, thereby suppressing the generation of gases such as oxidizing gas.

[0089] Meanwhile, the activation charging step may include a first charging step of charging at SOC5 or less.

[0090] Specifically, the first charging step can be performed at a C-rate of 0.1C to 0.3C at SOC0 to SOC5.

[0091] When the charging rate in the first charging satisfies the above range, the current rate at the initial stage of SEI film formation can be maintained lower, so that a more robust and dense SEI film can be formed, and a robust and dense SEI film is formed on the electrode surface, realizing excellent life characteristics. When the C rate of the current is as high as 0.3C or more in the first charging stage, the SEI film may be unstably formed on the electrode surface. When the SEI film is unstably formed on the electrode surface, the SEI film is easily decomposed during battery operation, resulting in rapid degradation of the electrode, and thus the life characteristics may be significantly reduced.

[0092] On the other hand, the first charging stage can be performed in a constant current mode (CC mode).

[0093] In addition, in the method of the present invention, when the ratio (Y / X) of the charging capacity (mAh / g) (Y) of the secondary battery to the estimated capacity (mAh / g) (X) of the positive electrode active material in terms of all lithium ions contained in the lithium-excess manganese-based oxide forming the Li / Li dumbbell structure during the activation charging satisfies 1.10 < Y / X ≦ 1.13, specifically when 1.11 ≦ Y / X ≦ 1.12, the method can include a second charging stage for ending the charging.

[0094] Figure 4 illustrates the structural changes of a lithium-excess manganese-based oxide as charging progresses during the activation stage. In the case of the lithium-excess manganese-based oxide represented by Formula 1, nickel begins to oxidize first during the initial second charging stage. Subsequently, lithium ions contained within the lattice of oxygen atoms in the first, second, and third metal oxide layers (MO layers) adjacent to each other migrate from octahedral positions to tetrahedral positions between the first and second MO layers and to tetrahedral positions between the second and third MO layers. As a result, some lithium ions begin to form a dumbbell Li / Li structure within the Mn honeycomb pattern. At this time, a first plateau may appear around 3.x V. Furthermore, during charging, a second flattening section appears from the point of 4.4 V (point (1) in Figure 4) where all nickel is oxidized to the point where all lithium ions exist only in Li / Li dumbbell structures. In other words, as shown in Figure 5, all lithium ions migrate to the tetrahedral positions between the first and second MO layers and the tetrahedral positions between the second and third MO layers, and the lithium ion concentration at the MO layer cross section and the octahedral positions becomes zero. This point is called the "Only Li / Li dumbbell point" and is shown in Figure 2(b) of "Electrochemistry Communications 6 (2004) 1045-1050" (Non-Patent Document 1)). Meanwhile, during high-voltage charging of 4.4V or more, in-plane migration of transition metal ions occurs in earnest, and oxidizing gases such as CO / CO2 containing oxygen gradually begin to evolve due to side reactions between the electrolyte and lattice oxygen elements.That is, even after all lithium ions exist in the Li / Li dumbbell structure and the activation charging continues to proceed, when lithium ions are desorbed and removed in the Li / Li dumbbell structure, the structural change of the transition metal in the positive electrode suddenly becomes intense, and bulk O2 is formed due to the influence of vacancy clustering in the transition metal layer from which lithium has been desorbed, resulting in a sharp increase in the generation of oxidation gases such as CO / CO2 containing oxygen (the point (3) in FIG. 4).

[0095] In the method of the present invention, a second charging stage is included in which the activation charging is terminated when the ratio (Y / X) of the charging capacity (mAh / g) (Y) of the secondary battery to the estimated capacity (mAh / g) (X) of the positive electrode active material at the point where all lithium ions contained in the lithium-excess manganese-based oxide form the Li / Li dumbbell structure satisfies 1.10 < Y / X ≦ 1.13, and a method is provided that can suppress and reduce the generation of oxidation gases.

[0096] By terminating the second charging stage in the range where the ratio (Y / X) of the charging capacity (Y) of the secondary battery to the estimated capacity (X) of the positive electrode active material exceeds 1.10, the amount of lithium available through the O / Mn redox reaction can be increased, and a sufficient charging capacity can be ensured. Also, by terminating the second charging stage in the range where the ratio (Y / X) of the charging capacity (Y) of the secondary battery to the estimated capacity (X) of the positive electrode active material is 1.13 or less, overcharging can be prevented, the reduction of oxidation gas generation and the sudden phase transition change in the positive electrode can be minimized, and the effect of improving the initial resistance of the cell can be obtained. In particular, by terminating the charging when the ratio (Y / X) of the charging capacity (Y) of the secondary battery to the estimated capacity (X) of the positive electrode active material is 1.13, a sufficient activation process can be performed up to the vicinity of the Only Li / Li dumbbell point, so that an additional activation process does not occur again in the subsequent battery charge and discharge process, and gas generation in the subsequent charge and discharge process can be prevented.

[0097] Meanwhile, the estimated capacity (C) (mAh / g) of the positive electrode active material can be estimated and calculated using a calculation formula such as Equation 1 below.

[0098] [Formula 1] C (mAh / g) = Q × (i / 100)

[0099] In the above formula 1, Q is the theoretical capacity (mAh / g) of the lithium-excess manganese oxide, i is the percentage (%) of the molar ratio of lithium ions that have migrated until only lithium ions that form a Li / Li dumbbell structure are present relative to the total molar ratio of lithium ions contained in the lithium-excess manganese oxide.

[0100] Specifically, the theoretical capacity of the lithium-excess manganese-based oxide can be defined by a general formula such as the following formula 3 ([Equation 2]).

[0101] [Formula 3]

number

[0102] In the above formula 3, F is the Faraday constant, 96485.3321 (C / mol e - ) and MW is the weight average molecular weight (g / mol) of the lithium-excess manganese oxide.

[0103] In the above formula, 3600 C (coulomb) is a constant that defines the amount of charge (Ah) that moves when a current of 1 ampere (A) flows for 1 hour (h).

[0104] Meanwhile, the i can be defined by the following equation 2 ([Equation 3]).

[0105] [Formula 2]

number

[0106] In the above formula 2, The Li molar ratio means the total molar ratio of lithium contained in the lithium-excess manganese-based oxide.

[0107] Specifically, the Li molar ratio means "a" in the lithium-excess manganese-based oxide represented by the above chemical formula 1 (Li a Ni b Co c Mn d M e O2). For example, in the lithium-excess manganese-based oxide of Li 1.16 Ni 0.305 Co 0.004 Mn 0.531 O2, the Li molar ratio can be defined as 1.16. In this case, i is 72.5%, and the ratio of the number of moles of lithium to the total number of moles of metals excluding lithium (Li / Me) can be 1.38.

[0108] On the other hand, the charging capacity (Y) of the secondary battery can be the measured capacity obtained by visually monitoring the display of the detector in real time while the battery cell is being charged.

[0109] On the other hand, the second charging stage can be carried out at a C-rate of 0.3C to 1.0C, and if necessary, it can be carried out in two or more stages with different C-rates. For example, the second charging stage can include a second-1 charging stage of charging the battery cell at a C-rate of 0.5C to 1.0C and a second-2 charging stage of charging at a C-rate lower than the C-rate of the second-1 charging stage. At this time, the second-1 charging stage is carried out until the SOC reaches 60, and the second-2 charging stage is carried out until the ratio (Y / X) of the charging capacity (mAh / g) (Y) of the secondary battery to the estimated capacity (mAh / g) (X) of the positive electrode active material at the point where all lithium ions contained in the lithium-excess manganese-based oxide form a Li / Li dumbbell structure from SOC60 corresponds to 1.10 < Y / X ≦ 1.13. When the second charging stage is carried out in two stages as described above, the activation charging time can be efficiently shortened.

[0110] Meanwhile, the charge cut-off voltage of the second charging stage may be 4.5V to 4.6V, specifically 4.55V to 4.6V.

[0111] The second charging step can be performed in a constant current mode (CC mode) or a constant current-constant voltage mode (CCCV mode).

[0112] If the second charging step is performed in a constant current-constant voltage (CCCV) mode, CC charging is performed until the end-of-charge voltage is reached, and once the end-of-charge voltage is reached, CV charging may be performed in which the charge C-rate is gradually decreased from about 0.05C to about 0.15C.

[0113] Next, in the activation step, the battery cell charged through the first and second charging steps is discharged. At this time, the discharge can be performed at a C-rate of 0.3 C to 0.7 C. When the discharge rate satisfies this range, the activation time can be appropriately controlled, and a desired range of discharge capacity characteristics can be achieved.

[0114] Alternatively, the discharge may be performed in a constant current mode (CC mode).

[0115] Meanwhile, the discharge cut-off voltage may be 2.0V to 3.0V, specifically 2.0V.

[0116] Meanwhile, the activation process is preferably carried out at a temperature of 25° C. to 70° C., more preferably 40° C. to 50° C. When the activation process is carried out within this temperature range, a high capacity can be achieved through proper activation of Li2MnO3.

[0117] The activation process may be performed under pressurized conditions, if necessary. The pressurization may be performed by mounting the battery cell in a jig and applying pressure to the battery cell through the jig. Performing the activation process under pressurized conditions has advantages in that electrolyte impregnation is improved and gas generated during the activation process can be easily discharged.

[0118] Meanwhile, although not required, the activation step may further include an aging step, if necessary, to allow the electrolyte to be uniformly impregnated into the electrode assembly and stabilize the battery. The aging step may be performed before, during, and / or after charging, and may be performed one or more times.

[0119] The aging step may 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 such a temperature, electrolyte impregnation and lithium mobility are improved, and activation can be carried out more smoothly.

[0120] <Lithium secondary battery> The present invention also includes a lithium secondary battery manufactured by the method for manufacturing a lithium secondary battery.

[0121] The lithium secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes a lithium-excess manganese-based oxide in which the manganese content in all metals excluding lithium exceeds 50 mol % and the ratio of the number of moles of lithium to the number of moles of all metals excluding lithium (Li / Me) exceeds 1.

[0122] The present invention will be described in more detail below through specific examples.

[0123] [Example] Example 1 (battery cell manufacturing) The positive electrode active material, conductive material, and PVDF binder were mixed in a weight ratio of 97:1:2 in N-methylpyrrolidone to prepare a positive electrode slurry. 1.38 [Ni 0.363 Co 0.005 Mn 0.632 ]O2 (Li / Me=1.38) was used, and carbon nanotubes (CNTs) were used as the conductive material. The positive electrode slurry was applied to an aluminum current collector sheet, dried, and then rolled to prepare a positive electrode.

[0124] Anode active material, conductive material, and binder were mixed in water at a weight ratio of 96:1:3 to prepare anode slurry. Graphite was used as the anode active material, carbon black was used as the conductive material, and SBR and CMC were mixed at a weight ratio of 2:1 to prepare a binder. The anode slurry was applied to a copper current collector sheet, dried, and then rolled to prepare anodes.

[0125] An electrode assembly was fabricated by interposing a separator between the cathode and anode fabricated as described above, and the electrode assembly was inserted into a battery case, followed by injecting an electrolyte solution to fabricate a battery cell.

[0126] (Activation stage) The battery cell was pre-aged for two days, and then first charged at 45°C in a constant current mode at 0.2C until the SOC reached 3. Secondly, it was charged at a constant current-constant voltage mode at 0.33C (0.05C CV cutoff) until the charge capacity (Y) of the secondary battery reached 287mAh / g (end-of-charge voltage: 4.5V). An activation process was then performed in which the battery was discharged to 2.0V at a constant current of 0.5C to produce a lithium secondary battery.

[0127] The charging process was performed by connecting the fabricated cell to a charger / discharger, and the continuously changing charge capacity value during charging was visually monitored on the detector display. Charging was terminated when the desired secondary battery charge capacity was obtained. Meanwhile, the ratio (Y / X) of the charge capacity (mAh / g) (Y) of the secondary battery to the estimated capacity (mAh / g) (X) of the positive electrode active material at the point where all lithium ions contained in the lithium-excess manganese-based oxide form a Li / Li dumbbell structure is shown in Table 1 below.

[0128] Example 2. The battery cell prepared in Example 1 was pre-aged for two days, and then first charged at 45°C in a constant current mode of 0.2C to an SOC of 3, and then charged at a constant current of 1.0C to an SOC of 60. Thereafter, second charged at a constant current-constant voltage mode of 0.4C (0.15C CV cutoff) until the charge capacity (Y) of the lithium secondary battery reached 290mAh / g (end-of-charge voltage: 4.6V), and then discharged at a constant current of 0.5C to 2.0V, thereby preparing a lithium secondary battery.

[0129] Meanwhile, the ratio (Y / X) of the charge capacity (mAh / g) (Y) of the secondary battery to the estimated capacity (mAh / g) (X) of the positive electrode active material at the point where all lithium ions contained in the lithium-excess manganese-based oxide form a Li / Li dumbbell structure is as shown in Table 1 below.

[0130] Comparative Example 1 The battery cell prepared in Example 1 was pre-aged for two days, and then first charged at 45°C in a constant current mode of 0.2C up to SOC3. Secondly charged at a constant current-constant voltage mode of 0.33C (0.05C CV cutoff) until the charge capacity (Y) of the lithium secondary battery reached 297mAh / g (end-of-charge voltage: 4.6V). An activation process was then performed in which the battery cell was discharged to 2.0V at a constant current of 0.5C to prepare a lithium secondary battery.

[0131] Meanwhile, the ratio (Y / X) of the charge capacity (mAh / g) (Y) of the secondary battery to the estimated capacity (mAh / g) (X) of the positive electrode active material at the point where all lithium ions contained in the lithium-excess manganese-based oxide form a Li / Li dumbbell structure is as shown in Table 1 below.

[0132] Comparative Example 2 The battery cell prepared in Example 1 was pre-aged for two days, and then first charged at 45°C in a constant current mode of 0.2C to an SOC of 3, then charged at a constant current of 0.33C to an SOC of 60, and then second charged at a constant current-constant voltage mode of 0.1C (0.05C CV cutoff) until the charge capacity (Y) of the lithium secondary battery reached 293mAh / g (end-of-charge voltage: 4.6V). An activation process was then performed in which the battery cell was discharged at a constant current of 0.5C to 2.0V to prepare a lithium secondary battery.

[0133] Meanwhile, the ratio (Y / X) of the charge capacity (mAh / g) (Y) of the secondary battery to the estimated capacity (mAh / g) (X) of the positive electrode active material at the point where all lithium ions contained in the lithium-excess manganese-based oxide form a Li / Li dumbbell structure is as shown in Table 1 below.

[0134] Comparative Example 3. The battery cell prepared in Example 1 was pre-aged for two days, and then first charged at 45°C in a constant current mode of 0.2C to SOC3, then charged at a constant current of 1.0C to SOC60, and then second charged at a constant current of 0.4C until the charge capacity (Y) of the lithium secondary battery reached 278mAh / g (end-of-charge voltage: 4.6V). An activation process was then performed in which the battery cell was discharged at a constant current of 0.5C to 2.0V to prepare a lithium secondary battery.

[0135] Meanwhile, the ratio (Y / X) of the charge capacity (mAh / g) (Y) of the secondary battery to the estimated capacity (mAh / g) (X) of the positive electrode active material at the point where all lithium ions contained in the lithium-excess manganese-based oxide form a Li / Li dumbbell structure is as shown in Table 1 below.

[0136] Comparative Example 4. The battery cell prepared in Example 1 was pre-aged for two days, and then first charged at 45°C in a constant current mode of 0.2 C to SOC3, and second charged at a constant current of 1.0 C until the charge capacity (Y) of the lithium secondary battery reached 244 mAh / g (cut-off voltage: 4.6 V). An activation process was then performed in which the battery cell was discharged to 2.0 V at a constant current of 0.5 C to prepare a lithium secondary battery.

[0137] Meanwhile, the ratio (Y / X) of the charge capacity (mAh / g) (Y) of the secondary battery to the estimated capacity (mAh / g) (X) of the positive electrode active material at the point where all lithium ions contained in the lithium-excess manganese-based oxide form a Li / Li dumbbell structure is as shown in Table 1 below.

[0138] Comparative Example 5. The battery cell prepared in Example 1 was pre-aged for two days, and then first charged at 45°C in a constant current mode of 0.2C to an SOC of 3, and then charged at a constant current of 1.0C to an SOC of 60. Then, second charged at a constant current-constant voltage mode of 0.4C (0.05C CV cutoff) until the charge capacity (Y) of the lithium secondary battery reached 305mAh / g (end-of-charge voltage: 4.6V), followed by an activation process of discharging at a constant current of 0.5C to 2.0V to prepare a lithium secondary battery.

[0139] Example 3 Li as the positive electrode active material 1.34 [Ni 0.354 Mn 0.646 A battery cell was fabricated in the same manner as in Example 1, except that ]O2 (Li / Me=1.34) was used.

[0140] The fabricated battery cell was pre-aged for two days, and then first charged at 45°C in a constant current mode of 0.2C up to SOC3. Secondly, it was charged at a constant current-constant voltage mode of 0.33C (0.05C CV cutoff) until the charge capacity (Y) of the lithium secondary battery reached 286mAh / g (charge end voltage: 4.6V). An activation process was then performed in which the battery was discharged to 2.0V at a constant current of 0.5C to fabricate a lithium secondary battery.

[0141] Meanwhile, the ratio (Y / X) of the charge capacity (mAh / g) (Y) of the secondary battery to the estimated capacity (mAh / g) (X) of the positive electrode active material at the point where all lithium ions contained in the lithium-excess manganese-based oxide form a Li / Li dumbbell structure is as shown in Table 1 below.

[0142] [Table 1]

[0143] [Experimental Example] Experimental example 1. The amounts of gas generated during activation of the lithium secondary batteries prepared in Examples 1 to 3 and the secondary batteries prepared in Comparative Examples 1 to 5 were measured, and the results are shown in Table 2 below and FIG.

[0144] [Table 2]

[0145] 1, it can be seen that the lithium secondary batteries of Examples 1, 2, and 3 generated less total oxidizing gases, including oxygen gas, than the lithium secondary batteries of Comparative Examples 1, 2, and 5. That is, in the lithium secondary batteries of Comparative Examples 1, 2, and 5, in which the ratio (Y / X) of the charge capacity (mAh / g) (Y) of the secondary battery to the estimated capacity (mAh / g) (X) of the positive electrode active material at the point where all lithium ions form a Li / Li dumbbell structure exceeded 1.13, it can be seen that the total gas generation increased due to the intense desorption of lithium and structural changes of the transition metal in the positive electrode.

[0146] Meanwhile, in the lithium secondary batteries of Comparative Examples 3 and 4, the total amount of gases, including oxygen gas, generated was found to be lower than that of the lithium secondary batteries of Examples 1, 2, and 3. This is believed to be a phenomenon caused by an insufficient activation process. The specific reason can be explained through the evaluation results of the capacity retention rate in Experimental Example 3 below.

[0147] Experimental Example 2: Evaluation of initial resistance The lithium secondary batteries prepared in Examples 1, 2, and 3 and the secondary batteries prepared in Comparative Examples 1 to 5 were charged at 25°C under CC-CV (constant current-constant voltage) conditions at a rate of 0.33 C to 4.35 V, and then discharged under CC conditions at a rate of 0.33 C to 2.0 V. This charge-discharge cycle was counted as one cycle, and two cycles of initial charge-discharge were performed.

[0148] The SOC (state of charge) was set to 50% based on the discharge capacity of the second charge / discharge. A discharge pulse was applied at 2.5C for 10 seconds at 50% SOC, and the DC internal resistance was calculated through the voltage drop that occurred. This resistance was set as the initial resistance and is shown in Figure 2 below.

[0149] Referring to FIG. 2, it can be seen that the lithium secondary batteries of Examples 1, 2, and 3 have improved initial resistance compared to the lithium secondary batteries of Comparative Examples 1, 2, and 5.

[0150] Meanwhile, the lithium secondary batteries of Comparative Examples 3 and 4 were found to have lower or similar initial resistances compared to the lithium secondary batteries of Examples 1, 2, and 3. This is believed to be a phenomenon caused by an insufficient activation process. The specific reason can be explained through the evaluation results of the capacity retention rate in Experimental Example 3 below.

[0151] Experimental Example 3: Evaluation of capacity retention rate The lithium secondary batteries prepared in Examples 1, 2, and 3 and the secondary batteries prepared in Comparative Examples 3 and 4 were charged at 25°C at a rate of 0.33 C to 4.35 V under CC-CV (constant current-constant voltage) conditions, and then discharged at a rate of 0.33 C to 2.0 V under CC conditions. This charge-discharge cycle was counted as one cycle, and a total of two initial charge-discharge cycles were performed. The discharge capacity of the second cycle was defined as the initial discharge capacity.

[0152] Next, each lithium secondary battery was charged at a high temperature (45°C) under CC-CV conditions at a rate of 0.33 C to 4.35 V, and discharged under CC conditions at a rate of 0.33 C to 2.0 V. This charge / discharge cycle was counted as one cycle, and 50 cycles were performed.

[0153] The capacity after the first cycle and the capacity after the 50th cycle were substituted into the following formula A to calculate the capacity retention rate, and the results are shown in FIG.

[0154] [Formula A] Capacity retention rate (%) = (discharge capacity after 50th cycle at high temperature / discharge capacity after 1st cycle at high temperature) × 100

[0155] Referring to FIG. 3, it can be seen that the lithium secondary batteries of Examples 1 to 3 were able to achieve a stable capacity retention rate during high-temperature cycles because a sufficient activation process was performed.

[0156] On the other hand, it was confirmed that the capacity retention rate continuously increased during the high-temperature cycling process in the lithium secondary batteries of Comparative Examples 3 and 4. That is, in the lithium secondary batteries of Comparative Examples 3 and 4, the activation process was completed insufficiently in the first activation step, which induced an additional activation process in the subsequent cycling process, resulting in an abnormal phenomenon in which the capacity increased as the number of cycles increased. [Industrial Applicability]

[0157] In the method for manufacturing a lithium secondary battery according to the present invention, the ratio (Y / X) of the charge capacity (mAh / g) (Y) of the secondary battery to the estimated capacity (mAh / g) (X) of the positive electrode active material at the point where all lithium ions contained in the lithium-excess manganese-based oxide form a Li / Li dumbbell structure during activation is 1.10.

Claims

1. Preparing a battery cell including a positive electrode, a negative electrode, and an electrolyte, each of which includes a lithium-rich manganese-based oxide in which the manganese content in all metals excluding lithium exceeds 50 mol % and the ratio of the number of moles of lithium to the number of moles of all metals excluding lithium (Li / Me) exceeds 1; an activation step of activating the battery cell by charging and discharging the battery cell at least once; The activation step includes terminating charging when a ratio (Y / X) of a charge capacity (mAh / g) (Y) of the secondary battery to an estimated capacity (mAh / g) (X) of a positive electrode active material at a point where all lithium ions contained in the lithium-excess manganese-based oxide form a Li / Li dumbbell structure satisfies 1.10<Y / X≦1.

13.

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

3. 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, and 0≦e≦0.

1.

4. The estimated capacity (C) (mAh / g) of the positive electrode active material is calculated using the following formula 1: [Formula 1] C (mAh / g)=Q×(i / 100) In the above formula 1, Q is the theoretical capacity (mAh / g) of the lithium-excess manganese oxide; 2. The method for manufacturing a lithium secondary battery according to claim 1, wherein i is a percentage (%) of the molar ratio of lithium ions that have migrated until only lithium ions forming a Li / Li dumbbell structure are present relative to the total molar ratio of lithium ions contained in the lithium-excess manganese-based oxide.

5. The i is represented by the following formula 2: [Formula 2] [Equation 1] In the above formula 2, 5. The method for producing a lithium secondary battery according to claim 4, wherein the Li molar ratio is the total molar ratio of lithium ions contained in the lithium-excess manganese oxide.

6. 2. The method of claim 1, wherein the charge capacity (Y) of the secondary battery is a capacity measured by real-time monitoring through a display of a detector while the battery cell is being charged.

7. 2. The method of claim 1, wherein the activation step includes a charging step of charging the battery at 0.3C to 1.0C up to 4.5V to 4.6V.

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

9. 9. The method for manufacturing a lithium secondary battery according to claim 8, wherein a cutoff current during constant voltage charging in the constant current-constant voltage (CCCV) mode is 0.05C to 0.15C.

10. 2. The method of claim 1, wherein the activation step further comprises a charging step of charging at a C rate of 0.1C to 0.3C with SOC 0 to SOC 5.

11. The method of claim 10, wherein the charging step is performed in a constant current (CC) mode.

12. 2. The method of claim 1, wherein the activation step comprises discharging at a C rate of 0.3C to 0.7C until the voltage reaches 2.0V.

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