Lithium secondary battery manufacturing method

Abstract

The present invention relates to a method for manufacturing a lithium secondary battery, comprising: preparing a battery cell including a cathode, an anode, and an electrolyte, the cathode including a perlithium manganese-based oxide in which the manganese content in all metals excluding lithium is more than 50 mol % and the ratio of the number of moles of lithium to the number of moles of all metals excluding lithium (Li / Me) is more than 1; and activating the battery cell by charging and discharging at least once, the activation step including charging the battery cell at a C-rate of 0.6 to 1.0 C up to an SOC of 5 to 60, and charging at a C-rate of 0.3 to 0.6 C in the range of SOC of 60 to 100, followed by discharging once.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0183696, filed December 23, 2022, and all contents disclosed in the documents of that Korean patent application 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 including a perlithium manganese-based oxide that can improve voltage drop that occurs during repeated charge and discharge. [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, research into increasing the energy density of lithium secondary batteries is actively underway, and one of the most notable methods is the development of positive electrode active materials with a composition that allows for the use of more lithium than currently available.

[0004] As a positive electrode active material that can utilize more lithium, perlithium-based transition metal oxides with a layered structure and a lithium to transition metal molar ratio of more than 1 have been developed. Lithium secondary batteries that use such perlithium-based transition metal oxides typically achieve high capacity by undergoing an activation process at a high voltage of 4.4 V or higher. However, this high-voltage activation process can induce collapse of the positive electrode crystal structure due to oxygen desorption and cation mixing in the crystal structure of the perlithium-based transition metal oxide, resulting in significant degradation of the discharge energy of the lithium secondary battery and voltage sagging during repeated charge and discharge. [Prior art documents] [Patent documents]

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] The present invention is for solving such problems, and aims to provide a method for manufacturing a lithium secondary battery including a lithium-rich manganese-based oxide in which a voltage drop phenomenon occurring during repeated charge and discharge is improved.

Means for Solving the Problems

[0007] In one aspect, the present invention provides a method for manufacturing a lithium secondary battery, including the steps of preparing a battery cell including a positive electrode, a negative electrode, and an electrolyte, the positive electrode including a lithium-rich 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 at least once. At this time, the activation step may include a charging step of charging the battery cell at a C-rate of 0.6 to 1.0 C until the state of charge (SOC) reaches 5 to 60 and then charging at a C-rate of 0.3 C to 0.6 C in the SOC range of 60 to 100, followed by a discharging step of discharging once.

[0008] The lithium-rich manganese-based oxide may be represented by the following Chemical Formula 1. [Chemical Formula 1] Li a Ni b Co c Mn d M e O2 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 the group consisting of Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr, and Zr.

[0009] The charging step may be performed in constant current mode (CC mode) or constant current-constant voltage mode (CCCV mode).

[0010] The charge cut-off voltage of the charging stage may be 4.5V to 4.6V. The activation step may include discharging at a C-rate of 0.3C to 0.7C until 2.0V is reached. [Effects of the Invention]

[0011] In the method for manufacturing a lithium secondary battery according to the present invention, by charging at a C-rate of 0.3C to 0.6C in a part of the activation charge stage, i.e., in the section above SOC60, activation of Li2MnO3 (monoclinic phase) in the crystalline structure of the positive electrode active material is suppressed, thereby suppressing the voltage drop that occurs during repeated charge and discharge, and reducing gas generation. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a graph showing the results of evaluating voltage drops after charging and discharging of a lithium secondary battery according to Experimental Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0013] The terms and words used in this specification and claims should not be interpreted in a limited way to their ordinary or dictionary meanings, but should be interpreted in a way that is consistent with the technical idea of ​​the present invention, in accordance with the principle that the inventor can appropriately define the concept of the term in order to best explain his or her invention.

[0014] In the present invention, "primary particles" refers to particle units that do not appear to have grain boundaries when observed under a scanning electron microscope at a magnification of 5,000 to 20,000. "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. In the present invention, the "secondary particles" are particles formed by agglomeration of a plurality of primary particles.

[0015] In the present invention, the "average particle size D 50 " means the particle size at 50% of the volume cumulative particle size distribution of the particle powder to be measured (for example, positive electrode active material powder, negative electrode active material powder, etc.). 50 can be measured using the laser diffraction method. For example, powder of the particles to be measured is dispersed in a dispersion medium, and then introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT 3000). Ultrasound of approximately 28 kHz is irradiated at an output of 60 W, and a volume cumulative particle size distribution graph is obtained. Then, the particle size corresponding to 50% of the volume cumulative amount is determined.

[0016] In addition, in the present invention, "SOC X" means that 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.

[0017] The present inventors have conducted extensive research into improving the voltage drop during the charging stage for activation of lithium secondary batteries using perlithium manganese-based oxides. As a result, they have discovered that by performing the activation process under specific charging conditions during the manufacture of a lithium secondary battery, it is possible to minimize cation mixing and oxygen desorption, suppress activation of Li2MnO3 (monoclinic phase) in the crystalline structure of the positive electrode active material, and prevent voltage drop during battery charge and discharge, thereby completing the present invention.

[0018] The method for producing a lithium secondary battery according to the present invention will now be described. The method for manufacturing a lithium secondary battery according to the present invention includes the steps of: preparing a battery cell including a cathode, an anode, and an electrolyte, the cathode including a perlithium manganese-based oxide in which the manganese content in all metals excluding lithium is more than 50 mol % and the ratio of the number of moles of lithium to the number of moles of all metals excluding lithium (Li / Me) is more than 1; activating the battery cell by charging and discharging at least one time; At this time, the activating step includes charging the battery cell at a C-rate of 0.3C to 0.6C in a secondary battery state of charge (SOC) range of 60 to 100.

[0019] (1) Preparing the battery cells First, a battery cell including a positive electrode, a negative electrode, and an electrolyte is prepared. The battery cell may be manufactured by, for example, forming an electrode assembly including a positive electrode and a negative electrode, housing the electrode assembly in a battery case, injecting an electrolyte, and sealing the battery case. In this case, the electrode assembly may include a separator between the positive electrode and the negative electrode. Each component of the battery cell of the present invention will be described in more detail below.

[0020] positive electrode The positive electrode according to the present invention includes, as a positive electrode active material, a perlithium manganese-based oxide in which the manganese content of all metals excluding lithium is greater than 50 mol % and the ratio of the number of moles of lithium to the number of moles of all metals excluding lithium (Li / Me) is greater than 1. Specifically, the positive electrode according to the present invention includes a positive electrode current collector and a positive electrode active material layer formed on at least one surface of the positive electrode current collector, and the positive electrode active material layer includes a perlithium manganese-based oxide in which the ratio of the number of moles of lithium to the number of moles of all metals excluding lithium (Li / Me) is greater than 1.

[0021] In the case of perlithium manganese oxides containing excess lithium, they have a structure in which the layered phase (LiM'O2) and the rock salt phase (Li2MnO3) are mixed. During the initial activation process, the rock salt phase is activated and generates excess lithium ions, enabling high capacity to be achieved.

[0022] Preferably, the perlithium manganese oxide may be represented by Chemical Formula 1. [Chemical formula 1] Li a Ni b Coc Mn d M e O2 In Chemical Formula 1, M may be at least one selected from the group consisting of Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr, and Zr.

[0023] On the other hand, a is the molar ratio of Li in the perlithium manganese oxide, and is 1 <a、1.1≦a≦1.5、または1.1≦a≦1.3であってもよい。

[0024] The b is the molar ratio of Ni in the perlithium manganese oxide, and may be 0≦b≦0.5, 0.1≦b≦0.4, or 0.2≦b≦0.4.

[0025] The "c" is the molar ratio of Co in the perlithium manganese 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.

[0026] The d is the molar ratio of Mn in the perlithium manganese oxide, and may 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 ratio of the rock salt phase becomes too small, resulting in little irreversible compensation and capacity improvement effect of the negative electrode.

[0027] The e is the molar ratio of the doping element M in the perlithium 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.

[0028] Meanwhile, in the perlithium 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) may be 1.2 to 1.5, 1.25 to 1.5, or 1.25 to 1.4. When the Li / Me ratio satisfies this range, excellent rate characteristics and capacity characteristics are achieved. If the Li / Me ratio is too high, electrical conductivity decreases and the electrochemically inactive rock salt phase (Li2MnO3) increases, resulting in a rapid degradation rate. If the Li / Me ratio is too low, the effect of improving energy density is minimal.

[0029] Meanwhile, the composition of the perlithium manganese-based oxide may be represented by the following Chemical Formula 2. [Chemical formula 2] XLi2MnO3·(1-X)Li[Ni 1-y-z-w Mn y Co z M w ]O2

[0030] In Chemical Formula 2, M may be at least one selected from the group consisting of metal ions Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr, and Zr.

[0031] The X represents the ratio of the Li2MnO3 phase in the perlithium 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 perlithium manganese-based oxide satisfies this range, high capacity characteristics can be achieved.

[0032] The y is the molar ratio of Mn on the LiM'O2 layer, and may be 0.4≦y<1, 0.4≦y≦0.8, or 0.4≦y≦0.7.

[0033] The z is the molar ratio of Co on the LiM'O2 layer and may be 0≦z≦0.1, 0≦z≦0.08, or 0≦z≦0.05. If z exceeds 0.1, gas generation and deterioration of the positive electrode active material may become severe, resulting in reduced life characteristics.

[0034] The w is the molar ratio of the doping element M on the LiM'O2 layer, and may be 0≦w≦0.2, 0≦w≦0.1, or 0≦w≦0.05.

[0035] Meanwhile, the positive electrode active material according to the present invention may further include a coating layer on the surface of the perlithium manganese-based oxide, if necessary. When the positive electrode active material includes a coating layer, the coating layer inhibits contact between the perlithium manganese-based oxide and the electrolyte, thereby reducing electrolyte side reactions and improving life characteristics.

[0036] The coating layer is made of a coating element M 1 The coating element M 1 may be, for example, at least one selected from the group consisting of Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr and Zr, preferably Al, Co, Nb, W and combinations thereof, more preferably Al, Co and combinations thereof. 1 may contain two or more kinds, and may contain, for example, Al and Co.

[0037] The coating elements are in the oxide form in the coating layer, i.e., M 1 It may exist in Oz (1≦z≦4).

[0038] 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 them, atomic layer deposition is preferred because it allows the coating layer to be formed over a large area.

[0039] The area of ​​the coating layer may be 10% to 100%, preferably 30% to 100%, and more preferably 50% to 100% of the total surface area of ​​the perlithium manganese-based oxide particles. When the area of ​​the coating layer satisfies the above range, an excellent effect of improving life characteristics is achieved.

[0040] Meanwhile, the positive electrode 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 50 The D of the positive electrode active material may be 2 μm to 10 μm, preferably 2 μm to 8 μm, and more preferably 4 μm to 8 μm. 50 When the above range is satisfied, excellent electrode density can be realized and the deterioration of capacity and rate characteristics can be minimized.

[0041] 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 long life characteristics.

[0042] Meanwhile, the perlithium manganese-based oxide can be prepared by mixing a transition metal precursor and a lithium source material and then calcining the mixture.

[0043] 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 these can be used alone or in combination of two or more.

[0044] Meanwhile, the transition metal precursor may be in the form of a hydroxide, oxide, or carbonate. When a precursor in the form of a carbonate is used, it is more preferable in that a cathode active material having a relatively large specific surface area can be prepared.

[0045] The transition metal precursor can be prepared by a co-precipitation process. For example, the transition metal precursor can be prepared by dissolving each transition metal-containing raw material in a solvent to prepare a metal solution, and then mixing the metal solution with an ammonium cation complexing agent and a basic compound, followed by a co-precipitation reaction. If necessary, an oxidizing agent or oxygen gas can be added during the co-precipitation reaction.

[0046] In this case, 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, Mn3O4, MnCO3, Mn(NO3)2, MnSO4·H2O, manganese acetate, manganese halide, etc.

[0047] The ammonium cation complexing agent may be at least one selected from the group consisting of NH4OH, (NH4)2SO4, NH4NO3, NH4Cl, CH3COONH4, and NH4CO3.

[0048] 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 precursor can be obtained, and when Na2CO3 is used as the basic compound, a carbonate precursor can be obtained. Furthermore, when a basic compound and an oxidizing agent are used together, an oxide precursor can be obtained.

[0049] 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.

[0050] The firing may be performed at a temperature of 600 to 1000°C or 700 to 950°C for a time of 5 to 30 hours or 5 to 20 hours. The firing atmosphere may be air or an oxygen atmosphere, for example, an atmosphere containing 20 to 100% by volume of oxygen.

[0051] Meanwhile, the positive electrode active material layer may further include a conductive material and a binder in addition to the positive electrode active material.

[0052] 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, or silver; conductive whiskers such as zinc oxide or 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% by weight to 20% by weight, 1% by weight to 20% by weight, or 1% by weight to 10% by weight, based on the total weight of the positive electrode active material layer.

[0053] 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 included in an amount of 1 wt % to 20 wt %, 2 wt % to 20 wt %, or 2 wt % to 10 wt % based on the total weight of the positive electrode active material layer.

[0054] 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 include a negative electrode active material, and optionally a binder and a conductive material.

[0055] The negative electrode current collector may be any material that has high conductivity and does not induce chemical changes in the battery, and examples of such materials include copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surfaces treated with carbon, nickel, titanium, silver, or aluminum-cadmium alloys. The negative electrode current collector typically has a thickness of 3 μm to 500 μm. As with the positive electrode current collector, the surface of the current collector may be provided with fine irregularities to enhance the binding strength of the negative electrode active material. The negative electrode current collector may be available in a variety of forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0056] Examples of the negative electrode active material include lithium metal, carbonaceous materials capable of reversibly intercalating / deintercalating lithium ions, metals or alloys of these metals with lithium, materials capable of doping and dedoping lithium, and composites containing the metallic compounds and carbonaceous materials, such as Si-C composites or Sn-C composites. Any one or a mixture of two or more of these can be used.

[0057] The carbonaceous material capable of reversibly intercalating / deintercalating lithium ions can be any carbonaceous negative electrode active material commonly used in lithium ion secondary batteries. Representative examples include crystalline carbon, amorphous carbon, or a combination of these. Examples of crystalline carbon include graphite, such as amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite. Examples of amorphous carbon include soft carbon (low-temperature calcined carbon), hard carbon, mesophase pitch carbide, and calcined coke. Examples of metals or alloys of these metals with lithium include metals selected from the group consisting of Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn, or alloys of these metals with lithium. Examples of materials capable of doping and dedoping lithium include Si, SiO x(0 < x < 2), Si-Y alloy (where Y is an element selected from the group consisting of 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, Sn-Y (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Sn), etc. may be mentioned, and at least one of these and SiO2 may be mixed and used. As the element Y, 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 can be selected from the group consisting of them.

[0058] Further, a thin film of metallic lithium may be used as the negative electrode active material. Furthermore, as the carbon material, all of low-crystalline carbon and highly crystalline carbon can be used. Representative examples of low-crystalline carbon are soft carbon and hard carbon, and representative examples of highly crystalline carbon are amorphous, plate-like, flaky, spherical or fibrous natural graphite or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch-derived cokes.

[0059] The conductive material is used to impart conductivity to the electrode and can be any material that exhibits electronic conductivity without causing chemical changes in the resulting battery. 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 powder or fiber, such as copper, nickel, aluminum, or silver; conductive whiskers, such as zinc oxide or 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 typically be present in an amount of 1 to 30% by weight, preferably 1 to 20% by weight, and more preferably 1 to 10% by weight, based on the total weight of the negative electrode active material layer.

[0060] The binder serves 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, which may be used alone or in combination. The binder may be contained in an amount of 1 to 30% by weight, preferably 1 to 20% by weight, and more preferably 1 to 10% by weight, relative to the total weight of the negative electrode active material layer.

[0061] For example, the negative electrode active material layer may be manufactured 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.

[0062] 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 limitations. It is particularly preferable for the separator to have low resistance to electrolyte ion movement and excellent electrolyte humidification capability. 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. Furthermore, a separator coated with a ceramic component or polymer material to ensure heat resistance or mechanical strength may be used, and it may be used in a single-layer or multi-layer structure.

[0063] 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 polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, or combinations thereof. For example, the electrolyte can include an organic solvent and a lithium salt.

[0064] 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. Specifically, 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.

[0065] The lithium salt can be any compound that can provide lithium ions used in lithium secondary batteries without any particular limitations. 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 at least one selected from the group consisting of: LiPF6, LiN(FSO2)2, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, etc. The concentration of the lithium salt is preferably within the range of 0.1M to 5.0M.

[0066] In addition to the above components, the electrolyte may further include additives for the purposes of improving the battery's life characteristics, suppressing a decrease in battery capacity, improving the battery's 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 present in an amount of 0.1 to 10% by weight, preferably 0.1 to 5% by weight, based on the total weight of the electrolyte.

[0067] Meanwhile, the electrode assembly may be an electrode assembly of various types well known in the art, for example, a jelly roll type, a stack type, a stack and lamination type, or a stack and folding type, and the type is not particularly limited.

[0068] The jelly roll type electrode assembly can be manufactured by interposing a sheet-shaped separator between a sheet-shaped positive electrode and a sheet-shaped negative electrode, and then winding the electrode in one direction.

[0069] The stacked electrode assembly can 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.

[0070] 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 laminating the stacked unit cells by a method such as heating.

[0071] The stack-and-fold type electrode assembly may be manufactured by stacking a positive electrode, a separator, and a negative electrode to form a plurality of unit cells, arranging the unit cells on one or both sides of a long folding separator, and then winding up the folding separator.

[0072] 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, and the type is not particularly limited.

[0073] (2) Activation stage Next, the battery cell is electrically activated by charging and discharging it at least once, which is a process of charging and discharging the battery cell to impart electrical properties and forming a solid electrolyte interphase (SEI) film on the electrodes to stabilize the battery.

[0074] In the present invention, the voltage drop that occurs during repeated charge and discharge can be improved by adjusting the charging current during a portion of the activation charge stage to control the activation level of Li2MnO3 (monoclinic) after activation. Specifically, by adjusting the charging current to a C-rate of 0.6 C or less during a portion of the activation charge stage and increasing the reaction time, the Li2MnO3 (monoclinic) structure changes to LiMO2 (M = Ni, Co, Mn, etc.) with a rhombohedral structure, and these form a mixed solid solution phase compound, allowing activation to proceed more stably, thereby improving the voltage drop that occurs during repeated charge and discharge.

[0075] First, the charging may include a first charging stage in which charging is performed up to SOC5. The first charging step may be performed at a rate (C-rate) of 0.3 C or less, preferably 0.1 C to 0.3 C. If the current rate in the first charging step is fast, such as 0.3 C or more, an SEI film may be formed unstably on the surface of the electrode. If the SEI film is formed unstably on the surface of the electrode, the SEI film may be easily decomposed during battery operation, causing rapid deterioration of the electrode and resulting in a significant decrease in lifespan characteristics.

[0076] The first charging step is preferably performed from SOC0 to SOC5. If the charge capacity in the first charging step satisfies the above range, a strong and dense SEI film is formed on the surface of the electrode, thereby achieving excellent life characteristics.

[0077] Meanwhile, the first charging step may be performed in a constant current mode (CC mode). Next, the charging may include a second charging stage of charging from SOC5 to SOC100. At this time, the second charging stage may be performed in one stage or in two or more stages with different rates, as necessary.

[0078] When the second charging step is performed in one step, the second charging step may be performed at a C-rate of 0.3C or more, preferably 0.3C to 0.6C.

[0079] If the second charging step is performed at a C-rate of 0.3 C to 0.6 C, the charging time increases but the activation stability of Li2MnO3 (monoclinic) can be improved. In this case, if the second charging step is performed at a C-rate that is relatively faster than the first charging step, the time required for the activation process decreases, thereby shortening the battery production time.

[0080] The second charging step performed in the first step is preferably performed up to SOC5 to SOC100. When the charging capacity in the second charging step satisfies the above range, the incomplete formation of the SEI film can be suppressed, and oxygen desorption and cation mixing during the activation process can be suppressed, thereby minimizing an increase in the resistance of the positive electrode and achieving a high capacity.

[0081] In addition, when the second charging step is performed in two or more steps, it may include a 2-1 charging step performed at a C-rate of 0.3C to 1.0C and a 2-2 charging step performed at a C-rate of 0.6C or less, specifically 0.3C to 0.6C.

[0082] At this time, the 2-1 charging step may be performed from SOC5 to SOC60, and the 2-2 charging step may be performed from SOC60 to SOC100.

[0083] In the second charging step, the battery is first charged at a C-rate of 0.3C to 1.0C (2-1 charging step) and then charged at a C-rate of 0.3C to 0.6C (2-2 charging step). This shortens the charging time and increases the activation stability of Li2MnO3 (monoclinic).

[0084] Meanwhile, the second charging step may be performed in a constant current mode (CC mode) or a constant current-constant voltage mode (CCCV mode).

[0085] If the second charging step is performed in a constant current-constant voltage (CCCV) mode, CC charging may be performed by supplying a C-rate of 0.3C to 0.6C until the end-of-charge voltage is reached, and once the end-of-charge voltage is reached, CV charging may be performed by gradually decreasing the charging C-rate to about 0.05C.

[0086] Meanwhile, in the second charging step, the end-of-charge voltage may be 4.5 V to 4.6 V, specifically 4.6 V. When the end-of-charge voltage in the second charging step satisfies this range, the perlithium manganese-based oxide is activated, thereby achieving high capacity characteristics.

[0087] Next, the activation step involves discharging the battery cell charged in the first and second charging steps. At this time, the discharging may 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.

[0088] Alternatively, the discharge may be performed in a constant current mode (CC mode). On the other hand, the discharge cut-off voltage may be 2.0V to 3.0V, specifically 2.0V.

[0089] Meanwhile, the activation step is preferably carried out at a temperature of 30° C. to 75° C., more preferably 40° C. to 45° C. When the activation step is carried out within this temperature range, an appropriate activation effect of the Li2MnO3 (monoclinic) phase can be obtained.

[0090] 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 then applying pressure to the battery cell through the jig. Performing the activation process under pressurized conditions has the advantages of improving electrolyte impregnation and facilitating the release of gas generated during the activation process.

[0091] Meanwhile, if necessary, but not necessarily, the activation step may further include an aging step, which is intended to allow the electrolyte to uniformly impregnate the electrode assembly and stabilize the battery, and may be performed before charging, during charging, and / or after discharging, or may be performed one or more times.

[0092] 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. The present invention will be described in more detail below through specific examples.

[0093] [Example] Comparative 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 0.642 ]O2 was used as the cathode material, and carbon nanotubes (CNTs) were used as the conductive material. The cathode slurry was applied to an aluminum current collector sheet, dried, and then rolled to produce a cathode.

[0094] 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.

[0095] A separator was interposed between the positive electrode and negative electrode to prepare an electrode assembly. The electrode assembly was then inserted into a battery case, and an electrolyte was injected to prepare a battery cell.

[0096] (Activation stage) The battery cell was pre-aged for 2 days, and then charged at 45°C in a constant current mode at 0.2C to SOC3 (first charging step), charged at a constant current-constant voltage mode at 0.3C (0.05C CV cut-off) to SOC100 (charge end voltage: 4.6V) (second charging step), and then discharged at a constant current of 0.6C to 2.0V, thereby producing an activation process to prepare a lithium secondary battery.

[0097] Example 1 The battery cell prepared in Comparative Example 1 was pre-aged for two days, and then charged at 45°C in a constant current mode at 0.2 C to an SOC of 3 (first charging step), charged at a constant current mode at 1.0 C to an SOC of 60 (second charging step), charged at a constant current mode at 0.4 C to an SOC of 100 (end-of-charge voltage: 4.6 V) (second charging step), and then discharged at a constant current of 0.6 C to 2.0 V. A lithium secondary battery was prepared in the same manner as in Comparative Example 1, except for the activation process.

[0098] Example 2. A lithium secondary battery was fabricated in the same manner as in Comparative Example 1, except that the battery cell fabricated in Comparative Example 1 was pre-aged for two days, and then charged at 45°C in a constant current mode at 0.2 C to an SOC of 3 (first charging step), charged at a constant current mode at 1.0 C to an SOC of 60 (second-first charging step), charged at a constant current mode at 0.4 C (0.05 C CV cut-off) to an SOC of 100 (end-of-charge voltage: 4.6 V) (second-second charging step), and then discharged at a constant current of 0.6 C to 2.0 V, thereby performing an activation process.

[0099] Comparative Example 2 A lithium secondary battery was manufactured in the same manner as in Comparative Example 1, except that the battery cell manufactured in Comparative Example 1 was pre-aged for 2 days, and then activated by charging it at 45°C in a constant current mode at 0.2 C to SOC3, charging it at a constant current mode at 1.0 C to SOC100 (end-of-charge voltage: 4.6 V), and then discharging it at a constant current of 0.6 C to 2.0 V.

[0100] [Experimental Example] Experimental example 1: Voltage drop evaluation The lithium secondary batteries prepared in Examples 1 and 2 and the secondary batteries prepared in Comparative Examples 1 and 2 were charged in a 0.33 C constant current-constant voltage mode (0.05 C CV cut-off) and discharged to 2.0 V at a constant current of 0.6 C, and 100 cycles were performed at high temperature (45°C).

[0101] The voltage after 50 cycles was measured, and the voltage reduction rate was calculated based on the voltage after the first cycle, and the results are shown in Table 1. In addition, the nominal voltage after discharge for each cycle was measured, and ΔV (voltage drop, voltage sagging) was calculated, and the results are shown in Figure 1.

[0102] [Table 1]

[0103] 1, the lithium secondary batteries of Examples 1 and 2 exhibited lower voltage reduction rates (voltage drop amounts) during 50 and 100 charge-discharge cycles than the lithium secondary battery of Comparative Example 2. In addition, although the voltage reduction rate was low in Comparative Example 1, the voltage itself after 50 cycles was found to be low, which was problematic.

Claims

1. preparing a battery cell including a cathode, an anode, and an electrolyte, the cathode including a perlithium manganese-based oxide in which the manganese content in all metals excluding lithium is greater than 50 mol% and the ratio of the number of moles of lithium to the number of moles of all metals excluding lithium (Li / Me) is greater than 1; activating the battery cell by charging and discharging at least one time; The activation step includes a step of charging the battery cell at a C-rate of 0.6 to 1.0 C up to an SOC of 5 to 60, and a step of charging the battery cell at a C-rate of 0.3 to 0.6 C in the range of SOC of 60 to 100, followed by a step of discharging the battery cell once.

2. 2. The method of claim 1, wherein the perlithium 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 Chemical Formula 1, 1<a, 0≦b≦0.5, 0≦c≦0.1, 0.5≦d<1.0, and 0≦e≦0.2, and 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 producing 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. 2. The method of 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. 5. The method for producing a lithium secondary battery according to claim 1, wherein an end-of-charge voltage in the charging stage is 4.5V to 4.6V.

6. 2. The method of claim 1, wherein the activating step further comprises a first charging step of charging at a C-rate of 0.1C to 0.3C from SOC0 to SOC5.

7. 7. The method of claim 6, wherein the first charging step is performed in a constant current (CC) mode.

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

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