Method of driving a lithium secondary battery

Abstract

The present invention relates to a method of driving a lithium secondary battery, which is capable of improving the life characteristics of a lithium secondary battery containing a lithium-rich manganese-rich oxide as a positive electrode active material, and also capable of suppressing a voltage drop during driving. The method of the present invention applies a lower minimum discharge voltage at predetermined intervals during a charge-discharge cycle while performing the charge-discharge cycle, thereby effectively expressing additional capacity through the redox reaction of the lithium-rich manganese-rich oxide.

Description

Technical Field

[0001] Cross-references to related applications

[0002] This application claims priority to Korean Patent Application No. 10-2024-0094403, filed on July 17, 2024, the disclosure of which is incorporated herein by reference in its entirety.

[0003] This invention relates to a method for driving a lithium secondary battery, which can improve the lifespan characteristics of a lithium secondary battery containing lithium-rich and manganese-rich oxides as positive electrode active materials, and can also suppress voltage drop during driving. Background Technology

[0004] Recently, as the application fields of lithium secondary batteries have rapidly expanded not only to power sources for electronic devices such as power, electronics, communications, and computers, but also to power storage and supply for large-area devices such as automobiles and power storage systems, the demand for secondary batteries with high capacity, high output, and high stability is increasing.

[0005] Lithium-ion rechargeable batteries typically consist of a positive electrode containing a positive active material, a negative electrode containing a negative active material, an electrolyte serving as the lithium-ion transport medium, and a separator. In this case, lithium transition metal oxides, such as lithium cobalt oxide, lithium nickel oxide, and lithium nickel cobalt manganese composite oxide, are primarily used as the positive active material.

[0006] However, in order to reduce the unit price of cathode active materials and improve the energy density and capacity of lithium secondary batteries, lithium-rich manganese-rich oxides have recently attracted much attention as a next-generation cathode active material. Lithium-rich manganese-rich oxides can achieve relatively low unit manufacturing costs by increasing the content of relatively inexpensive and abundant manganese (Mn) and reducing the content of cobalt (Co).

[0007] Furthermore, lithium-rich manganese oxides possess a structure that combines layered lithium metal oxides with rock-salt-type lithium manganese oxides (Li₂MnO₃). Therefore, during the activation process and / or charge-discharge cycling, the lithium manganese oxides are further activated, enabling manganese redox reactions to contribute to capacity achievement. Thus, lithium-rich manganese oxides represent a next-generation cathode active material with high capacity and low unit manufacturing cost, attracting increasing attention and research.

[0008] However, the typical drive voltage for lithium-ion secondary batteries containing lithium-rich and manganese-rich oxides is known to be 3.0 to 4.3 V. Nevertheless, the additional capacity cannot be adequately achieved through manganese redox reactions at the typical drive voltage. Consequently, if the lithium-ion secondary battery is continuously driven at the typical drive voltage, there is a drawback that the additional capacity cannot be adequately achieved, and the capacity retention and lifetime characteristics of the lithium-ion secondary battery are reduced.

[0009] Conversely, if a lithium secondary battery is continuously driven at varying voltages to achieve additional capacity through manganese redox reactions, this leads to structural changes and degradation of the positive electrode active material, resulting in a voltage drop. This reduces the average discharge voltage and ultimately affects the long-life characteristics of the lithium secondary battery.

[0010] Because of these issues, there is a continuous need for driving methods that can improve the lifespan characteristics of lithium secondary batteries containing lithium-rich and manganese-rich oxides and also suppress voltage drop. Summary of the Invention

[0011] Technical issues

[0012] One object of the present invention is to provide a method for driving a lithium secondary battery, which can improve the lifetime characteristics and capacity retention of a lithium secondary battery containing lithium-rich and manganese-rich oxides as positive electrode active materials, and can also suppress voltage drop during driving.

[0013] Technical solution

[0014] According to one aspect of the present invention, a method for driving a lithium secondary battery is provided, the lithium secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode comprises a positive electrode active material of lithium metal oxide, the method comprising: The first driving step of performing a charge / discharge cycle on the lithium secondary battery between a first minimum discharge voltage and a first maximum charge voltage; and The second driving step involves performing a charge / discharge cycle on the lithium secondary battery between the second minimum discharge voltage and the second maximum charge voltage. Based on the exclusion of all lithium metal content, the lithium metal oxide contains more than 50 mol% and less than 100 mol% manganese, and The second minimum discharge voltage is lower than the first minimum discharge voltage, and the second drive step is performed at fixed intervals while the first drive step is repeated.

[0015] Beneficial effects

[0016] According to the method for driving a lithium secondary battery of the present invention, the charge-discharge cycle is performed at fixed intervals with a low minimum discharge voltage applied, thereby enabling additional capacity to be effectively achieved through the manganese redox reaction.

[0017] Furthermore, by applying a relatively high minimum discharge voltage during the remaining charge and discharge cycles, voltage drop can be suppressed during the operation of the lithium secondary battery.

[0018] Therefore, based on the above driving method, without changing the material of the positive electrode active material itself or the battery structure, the capacity retention and life characteristics of lithium secondary batteries can be further improved, and voltage drop can also be suppressed. Attached Figure Description

[0019] Figure 1 This is a graph showing the results of evaluating capacity retention by accumulating energy for the lithium secondary batteries of the embodiments and comparative examples.

[0020] Figure 2 This is a graph showing the evaluation results of the voltage drop of the lithium secondary batteries of the embodiments and comparative examples. Detailed Implementation

[0021] A method for driving a lithium secondary battery according to one embodiment of the present invention is a method for driving a lithium secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode comprises a positive electrode active material of lithium metal oxide, the method comprising: The first driving step of performing a charge / discharge cycle on the lithium secondary battery between a first minimum discharge voltage and a first maximum charge voltage; and The second driving step involves performing a charge / discharge cycle on the lithium secondary battery between the second minimum discharge voltage and the second maximum charge voltage. Based on the exclusion of all lithium metal content, the lithium metal oxide contains more than 50 mol% and less than 100 mol% manganese, and The second minimum discharge voltage is lower than the first minimum discharge voltage, and the second drive step is performed at fixed intervals while the first drive step is repeated.

[0022] In one implementation of the driving method, a lithium secondary battery containing lithium-rich and manganese-rich oxide is driven by a method in which a second driving step is performed with a lower minimum discharge voltage at a fixed cycle interval of the first driving step.

[0023] In this second driving step, additional capacity is effectively achieved through the manganese redox reaction by charging / discharging under conditions where the second minimum discharge voltage in the relevant section is controlled at a low level, such as 2.0 to 2.6 V or 2.1 to 2.5 V, and the capacity retention of the lithium secondary battery can be further enhanced.

[0024] However, if such low minimum discharge voltages and second drive steps are continuously applied, irreversible structural changes and degradation of lithium-rich manganese-rich oxides may occur, leading to a voltage drop, in which the average discharge voltage of the lithium secondary battery continuously decreases. Because of this voltage drop, the overall energy density, considering both capacity and voltage, may continue to decline, resulting in potentially reduced lifetime characteristics of the lithium secondary battery.

[0025] In contrast, in one implementation of the driving method, the second driving step is performed at fixed intervals for each specific cycle, thereby relatively suppressing structural changes and degradation of lithium-rich and manganese-rich oxides, and consequently suppressing the reduction in voltage drop and lifetime characteristics.

[0026] Therefore, according to one embodiment of the driving method, the capacity retention and lifespan characteristics of lithium secondary batteries containing lithium-rich and manganese-rich oxides can be further improved, and voltage drop and other problems can also be suppressed.

[0027] The method for driving a lithium secondary battery according to one embodiment of the present invention will now be described in more detail.

[0028] In the method for driving a lithium secondary battery according to the above embodiments, the lithium secondary battery contains a lithium metal oxide (such as lithium-rich manganese-rich oxide) as the positive electrode active material, wherein the manganese content is 50 mol% or more and less than 100 mol% of the total metal content excluding lithium. More specifically, the lithium secondary battery may contain a positive electrode, a negative electrode, and an electrolyte containing such a positive electrode active material.

[0029] In one specific embodiment, the lithium metal oxide used as a lithium-rich manganese-rich oxide can be represented by the following chemical formula 1.

[0030] [Chemical Formula 1]

[0031] Li a [Mn 1-b-c Ni b M c ] 2-a O2

[0032] In chemical formula 1, M is selected from at least one of Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr, and Zr, and a, b, and c are the atomic fractions of each independent element, where 1 <a,0≤b≤0.5,0≤c≤0.5,0<b+c≤0.5。

[0033] Specifically, a is the molar ratio of Li in the lithium-rich and manganese-rich oxide, which can be 1 < a, 1.1 ≤ a ≤ 1.5 or 1.1 ≤ a ≤ 1.3. When a satisfies the above range, high capacity can be achieved.

[0034] b is the molar ratio of Ni in the lithium-rich and manganese-rich oxide, which can be 0 ≤ b ≤ 0.5, 0.1 ≤ b ≤ 0.4 or 0.2 ≤ b ≤ 0.4.

[0035] c is the molar ratio of the doping element M in the lithium-rich and manganese-rich oxide, which can be 0 ≤ c ≤ 0.5, 0 ≤ c ≤ 0.3 or 0 ≤ c ≤ 0.1. For example, the doping element M can be Co, and if the content of the doping element is too high, this may not only have an adverse effect on the capacity of the active material, but also may exacerbate the generation of gas and the deterioration of the positive electrode active material due to the increase in the oxygen-redox reaction, which may lead to a reduction in the life characteristics.

[0036] 1 - b - c is the molar ratio of Mn in the lithium-rich and manganese-rich oxide, which can be 0.50 ≤ 1 - b - c < 1.0, 0.50 ≤ 1 - b - c ≤ 0.80 or 0.50 ≤ 1 - b - c ≤ 0.70. When 1 - b - c is less than 0.5, that is, when b + c exceeds 0.5, the proportion of the rock salt phase becomes too low, so that the irreversible compensation of the negative electrode and the capacity improvement effect may be small.

[0037] In the case of the lithium-rich and manganese-rich oxide, it can have a structure in which a layered-structured lithium metal oxide and a rock salt-structured lithium manganese oxide (Li2MnO3) are mixed. Among them, the rock salt-structured lithium manganese oxide is activated at a high voltage of, for example, 4.6 V or more to generate excessive ions, which causes a manganese redox reaction at the second minimum discharge voltage in the second driving step, thereby contributing to the achievement of additional capacity.

[0038] Therefore, the driving method of one embodiment may further include a step of activating the lithium secondary battery after the preparation of the lithium secondary battery and before the following first driving step. This activation step can be carried out at a voltage of 4.6 V or more, or at a voltage of 4.6 V to 4.9 V. This activation step can further improve the capacity retention rate and life characteristics of the lithium secondary battery. In a more specific embodiment, the activation can be carried out, for example, by charging at 0.1C and 45 °C until a voltage of 4.6 V or more, and then discharging at 0.1C until 2.0 V, but it is not limited thereto.

[0039] On the other hand, in lithium-rich and manganese-rich oxides represented by Formula 1, the molar ratio of Li to the total number of moles of metal elements excluding Li (Li / Me) can be 1.2 to 1.5, 1.25 to 1.5, or 1.30 to 1.45. When the Li / Me ratio meets the above range, excellent rate performance and capacity characteristics are exhibited. If the Li / Me ratio is too high, the conductivity decreases and the rock salt phase (Li₂MnO₃) increases, which may accelerate the degradation rate. If the Li / Me ratio is too low, the improvement in energy density is weak.

[0040] On the other hand, the composition of lithium-rich and manganese-rich oxides can 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 In chemical formula 2, M is selected from at least one of Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr, and Zr. Where 0.2≤X≤0.5, 0.4≤y<1, 0≤z≤0.1, and 0≤w≤0.2.

[0041] X refers to the proportion of the Li2MnO3 phase (rock salt structure compound phase) in lithium-rich and manganese-rich oxides, where 0.2≤X≤0.5, 0.25≤X≤0.5, or 0.25≤X≤0.4. When the proportion of the Li2MnO3 phase in lithium-rich and manganese-rich oxides meets the above range, high capacity characteristics can be achieved.

[0042] y can be LiM'O2 (M'=[Ni 1-y-z Mn y M z The molar ratio of Mn in the layered compound phase, wherein 0.4≤y<1, 0.4≤y≤0.8 or 0.4≤y≤0.7.

[0043] z can be LiM'O2 (M'=[Ni 1-y-z Mn y M z The molar ratio of dopant element M in a layered compound phase, wherein 0≤z≤0.5, 0≤z≤0.3 or 0≤z≤0.1.

[0044] On the other hand, the positive electrode active material may optionally be further included in a coating on the surface of the lithium-rich manganese-rich oxide. When the positive electrode active material includes a coating, the contact between the lithium-rich manganese-rich oxide and the electrolyte is suppressed by the coating, thereby reducing electrolyte side reactions and improving lifetime characteristics.

[0045] The coating may include coating element M 1 Coating element M 1 It can be at least one selected from, for example, Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr, and Zr. Preferably, it can be Al, Co, Nb, W, or a combination thereof; more preferably, it can be Al, Co, or a combination thereof. Coating element M 1 It can contain more than two types, such as Al and Co.

[0046] The coating element can be an oxide, i.e., M 1 O z The form (1≤z≤4) exists in the coating.

[0047] The coating can be formed using methods such as dry coating, wet coating, chemical vapor deposition (CVD), physical vapor deposition (PVD), and atomic layer deposition (ALD). Among these methods, atomic layer deposition is preferred because it allows for the formation of coatings over large areas.

[0048] Furthermore, based on the total surface area of ​​the lithium-rich and manganese-rich oxide particles, the area in which the coating is formed can be in the range of 10% to 100%, 30% to 100%, or 50% to 100%. When the area in which the coating is formed meets the above ranges, the improvement in lifetime characteristics is excellent.

[0049] On the other hand, the aforementioned positive electrode active material can take the form of secondary particles in which multiple primary particles are aggregated, and the average particle size D of the secondary particles is... 50 The diameter can be from 2 μm to 10 μm, preferably from 2 μm to 8 μm, and more preferably from 4 μm to 8 μm. When the D of the positive electrode active material... 50 When the above range is met, excellent electrode density can be achieved, and the reduction in capacity and rate characteristics can be minimized.

[0050] Furthermore, the BET specific surface area of ​​the positive electrode active material can be 0.1 m². 2 / g to 10 m 2 / g, specifically 0.1 m 2 / g to 5 m 2 / g, more specifically 0.1 m 2 / g to 1 m 2 / g. If the BET specific surface area of ​​the positive electrode active material is too low, the reaction area with the electrolyte is insufficient, making it difficult to achieve sufficient capacity. If the specific surface area is too high, water absorption is rapid, and side reactions with the electrolyte are accelerated, making it difficult to ensure lifetime characteristics.

[0051] On the other hand, lithium-rich and manganese-rich oxides can be prepared by mixing transition metal precursors with lithium feedstocks and then calcining them. However, since this preparation method can be based on typical preparation procedures and conditions known to be used for lithium-rich and manganese-rich oxides, further description will be omitted.

[0052] In addition to using lithium-rich and manganese-rich oxides as the positive electrode active material, the lithium secondary battery employing one embodiment of the driving method can be based on the typical configuration of a lithium secondary battery. For example, a lithium secondary battery can be manufactured by inserting a separator or electrolyte membrane between a positive electrode containing a positive electrode active material and a negative electrode containing a negative electrode active material, then stacking and drying them to prepare an electrode assembly, inserting the electrode assembly into a case, and then optionally injecting electrolyte and sealing the case. The lithium secondary battery can be cylindrical, prismatic, coin-shaped, or pouch-shaped.

[0053] The positive and negative electrodes can be manufactured by coating an active material layer forming composition containing electrode active materials onto a current collector and then drying the coating.

[0054] In addition to the positive electrode active material containing lithium-rich and manganese-rich oxides, the composition for forming the positive electrode active material layer may optionally further include binders, conductive materials, fillers, etc., as needed. In addition to the positive electrode active material containing lithium-rich and manganese-rich oxides, the composition for forming the positive electrode active material layer may optionally further include binders, conductive materials, fillers, etc., as needed. In addition to the negative electrode active material, the composition for forming the negative electrode active material layer may optionally further include binders, conductive materials, fillers, etc., as needed.

[0055] There are no particular restrictions on the current collector, as long as it has high conductivity and will not cause chemical changes in the battery. Examples of materials that can be used include: copper; stainless steel; aluminum; nickel; titanium; calcined carbon; or copper or stainless steel with surface treatments such as carbon, nickel, titanium, or silver; and aluminum-cadmium alloys. Furthermore, current collectors can typically have a thickness ranging from 3 μm to 500 μm. Additionally, current collectors can have fine irregularities formed on their surface to enhance the adhesion of the active material. For example, current collectors can be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven structures.

[0056] In addition to the above-mentioned lithium-rich and manganese-rich oxides, the positive electrode active material may further include typical positive electrode active materials. For example, it may further include any one or more positive electrode active materials selected from LCO (LiCoO2), LNO (LiNiO2), LFP (LiFePO4), and NCM (Li[Ni p Co q Mn r1 O2 (where 0 < p < 1, 0 < q < 1, 0 < r1 < 1, and p + q + r1 = 1). Preferably, it may include a lithium-rich and manganese-rich oxide of at least 70% by weight or more based on the total weight of all positive electrode active materials, and may be composed only of the lithium-rich and manganese-rich oxide.

[0057] Based on the total weight of the positive electrode active material layer, the content of the positive electrode active material may be 80% to 99% by weight.

[0058] In a specific embodiment, as the negative electrode active material, the negative electrode may include at least one selected from the following: carbon-based materials; silicon-based materials; metals or alloys of lithium and the metals; metal composite oxides; materials that can be doped and undoped with lithium; lithium metal; and transition metal oxides. Preferably, it may include carbon-based materials, silicon-based materials, or a mixture thereof.

[0059] As the carbon-based material, any carbon-based negative electrode active material commonly used in lithium secondary batteries may be used without particular limitation. As typical examples thereof, crystalline carbon, amorphous carbon, or both may be used. Examples of crystalline carbon may include graphite, such as natural graphite or artificial graphite that is irregular, planar, flaky, spherical, or fibrous, and examples of amorphous carbon include soft carbon (low-temperature calcined carbon), hard carbon, mesophase pitch carbide, and calcined coke.

[0060] The silicon-based material is at least one selected from Si, SiO x (0 < x < 2) and Si-Y alloys (where Y is an element selected from alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Si), preferably SiO. The capacity of the silicon-based negative electrode active material is nearly 10 times higher than that of graphite, thereby enabling the reduction of the mass loading (mg·cm -2 ), thus improving the fast charging performance of the battery.

[0061] As the metal or the alloy of lithium and the metal, a metal or an alloy of lithium and the metal selected from the following may be used: Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn.

[0062] As a metal composite oxide, at least one of the following substances can be used: PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, Bi2O5, Li x Fe2O3 (0≤x≤1), Li x WO2(0≤x≤1) and Sn x Me 1-x Me' y O z (Where Me represents Mn, Fe, Pb, and Ge; Me' represents Al, B, P, Si, elements in groups I, II, and III of the periodic table, and halogens; 0) <x≤1;1≤y≤3;1≤z≤8)。

[0063] Materials that can be doped and dedoped with lithium may include: Sn, SnO2, Sn-Y (where Y is an element selected from alkali metals, alkaline earth metals, group 13 elements, group 14 elements, transition metals, rare earth elements and combinations thereof, and is not Sn), and at least one of these substances can be mixed with SiO2 and used.

[0064] In Si-Y and Sn-Y, element Y can be selected from: Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db(𬭊), 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.

[0065] Examples of transition metal oxides may include lithium-containing titanium oxide (LTO), vanadium oxides, lithium vanadium oxides, etc.

[0066] Based on the total weight of solids in the negative electrode slurry, the content of the negative electrode active material can be from 80% to 99% by weight.

[0067] The adhesive is a component that assists in the bonding between the conductive material, the active material, and the current collector, and is added in an amount from 0.1% to 10% by weight based on the total weight of the active material layer. Examples of adhesives may include: polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, nitrile rubber, fluororubber, and various copolymers thereof.

[0068] Conductive materials are components used to further improve the conductivity of active materials, and their addition amount based on the total weight of the active material layer can be less than 10% by weight, specifically less than 5% by weight. There are no particular restrictions on such conductive materials, as long as they are conductive and do not cause chemical changes in the battery. Examples of such materials include: graphite such as natural or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermally cracked carbon black; conductive fibers such as carbon fibers or metal fibers; fluorocarbons; metal powders such as aluminum and nickel powders; conductive whiskers such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides such as titanium oxides; and conductive materials such as polyphenylene derivatives.

[0069] On the other hand, lithium secondary batteries can be provided by forming an electrolyte layer (or an electrolyte-containing layer) between the positive and negative electrodes without the need for a separate separator, but may further include inserting a separator between the positive and negative electrodes. Such a separator is designed to separate the negative and positive electrodes and provide a path for lithium ions to move, and any separator can be used without particular limitation, as long as it is a separator commonly used in lithium secondary batteries. In particular, separators with high electrolyte retention capacity and low resistance to ion movement in the electrolyte are preferred. Specifically, as separators, porous polymer membranes can be used, such as porous polymer membranes prepared using polyolefin polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers; or laminated structures having two or more layers thereof. Furthermore, typical porous nonwoven fabrics can be used, such as nonwoven fabrics formed from high-melting-point glass fibers, polyethylene terephthalate fibers, etc. In addition, diaphragms coated with ceramic components or polymer materials can be used to ensure heat resistance or mechanical strength, and can be selectively used in single-layer or multi-layer structures.

[0070] In addition, electrolytes may include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, etc. (all of these electrolytes can be used to manufacture lithium secondary batteries), but are not limited to these.

[0071] In one specific embodiment, the electrolyte may comprise an organic solvent and a lithium salt.

[0072] Any organic solvent can be used without particular limitation, as long as it can serve as a medium through which ions participating in the battery electrochemical reaction can move. Specifically, the following organic solvents can be used: ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethanol and isopropanol; nitriles such as R-CN (where R is a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms and may contain double bonds, aromatic rings, or ether bonds); amides such as dimethylformamide; dioxolane such as 1,3-dioxolane; sulfolane, etc. Among these solvents, carbonate solvents are preferred, and mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and high dielectric constant that can improve the charge / discharge performance of the battery and low viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate) are even more preferred.

[0073] Lithium salts can be used without particular restrictions, as long as they can provide the lithium ions used in lithium secondary batteries. Specifically, the anion of the lithium salt can be at least one selected from the following substances: F - Cl - ,Br - I - NO3 - N(CN)2 - BF4 - CF3CF2SO3 - (CF3SO2)2N - (FSO2)2N - CF3CF2(CF3)2CO - (CF3SO2)2CH - (SF5)3C - (CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - CH3CO2 - SCN - and (CF3CF2SO2)2N -As lithium salts, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, and LiB(C2O4)2 can be used. Lithium salts can be used in concentrations ranging from 0.1 M to 2.0 M. When the concentration of the lithium salt is within this range, the electrolyte exhibits suitable conductivity and viscosity, thus demonstrating excellent electrolyte performance, and lithium ions can move efficiently.

[0074] To improve battery life characteristics, suppress battery capacity reduction, and improve battery discharge capacity, in addition to the electrolyte components, at least one additive can be added to the electrolyte, such as: alkylene carbonate halide compounds like difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, (condensed) glycol dimethyl ethers, hexamethylphosphoric triamine, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted alkyl ketones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride. In this case, the additive content can be from 0.1 to 5% by weight, based on the total weight of the electrolyte.

[0075] On the other hand, in one embodiment of the driving method, a first driving step is performed, wherein the lithium secondary battery is charged / discharged between a first minimum discharge voltage and a first maximum charging voltage. In this first driving step, the first minimum discharge voltage may be 2.8 to 3.2 V or 2.9 to 3.1 V, and the first maximum charging voltage may be 4.1 to 4.5 V or 4.2 to 4.4 V.

[0076] In this first driving step, the main contribution of the metal cation redox reaction contained in the layered compound is to achieve capacity, and it enables normal charging / discharging and driving of the lithium secondary battery while suppressing structural changes or degradation of lithium-rich and manganese-rich oxides. However, at the first minimum discharge voltage for the first driving step, the manganese redox reaction mainly caused by the rock salt structure compound and the additional capacity expression caused by these reactions are difficult to achieve. Therefore, if only the first driving step is continued, the capacity retention of the lithium secondary battery may decrease. On the other hand, if the first minimum discharge voltage becomes too high, the capacity expression caused by the metal cation redox reaction of the layered compound may not be properly achieved.

[0077] In one implementation of the driving method, a second driving step is performed at fixed intervals while a first driving step is repeated, wherein a lower second minimum discharge voltage is applied. For example, this second driving step may involve a charge / discharge cycle between a second minimum discharge voltage, which is lower than the first minimum discharge voltage, and a second maximum charging voltage.

[0078] In a more specific embodiment, the second minimum discharge voltage can be 2.0 to 2.6 V or 2.1 to 2.5 V, and the second maximum charging voltage can be 4.1 to 4.5 V or 4.2 to 4.4 V. Therefore, by applying a lower second minimum discharge voltage in the second driving step, the manganese redox reaction induced by the rock salt structure compound and the resulting additional capacity expression can be effectively achieved.

[0079] However, if a lower second minimum discharge voltage is continuously applied or the minimum discharge voltage is continuously reduced in order to achieve this additional capacity, structural changes or degradation of lithium-rich manganese-rich oxides will continue to occur, which leads to a voltage drop in the lithium secondary battery and ultimately a significant reduction in long-term life characteristics.

[0080] Thus, for example, in one embodiment of the driving method, the first driving step is performed 2 to 15 times, 2 to 14 times, or 3 to 13 times, and then the second driving step is performed regularly 1 to 2 times. This suppresses structural changes or degradation of lithium-rich and manganese-rich oxides while effectively expressing capacity through manganese redox reactions. As a result, the capacity retention and lifetime characteristics of lithium secondary batteries containing lithium-rich and manganese-rich oxides can be improved, and voltage drop is also significantly suppressed.

[0081] On the other hand, in one embodiment of the driving method, the charging in the first and second driving steps can be performed at a rate of 0.05C to 2C, 0.1C to 1.5C, or 0.2C to 1.2C. Furthermore, the discharging in the first and second driving steps can be performed at a rate of 0.05C to 3C, 0.1C to 2C, or 0.2C to 1.2C.

[0082] Furthermore, in the driving method, charging can be performed using the CC-CV (constant current-constant voltage) method, and discharging can be performed using the CC (constant current) method.

[0083] Preferred embodiments are provided below to aid in understanding the invention; however, these embodiments are for illustrative purposes only, and it will be apparent to those skilled in the art that various changes and modifications can be made within the scope of the invention and its technical concept, and such changes and modifications also fall within the scope of the appended claims.

[0084] [Preparation Example]

[0085] The composition of the positive electrode active material is Li 1.16 Ni 0.305 Co 0.004 Mn 0.531 Lithium- and manganese-rich oxides of O2, carbon nanotubes as conductive materials, polyvinylidene fluoride as a binder, and NBR as a dispersant were added to N-methyl-2-pyrrolidone (NMP) as a solvent in a weight ratio of 97.46:0.62:1.70:0.22 to prepare a positive electrode slurry. The positive electrode slurry was coated onto a 12 μm thick positive electrode current collector (Al film), dried, and then rolled to manufacture the positive electrode.

[0086] A negative electrode slurry was prepared by adding graphite and SiO (mixed at a weight ratio of 90:10) as the negative electrode active material, carbon black as the conductive material, SBR as the binder, and CMC as the thickener to water as the solvent in a weight ratio of 95.6:1.0:2.3:1.1. The negative electrode slurry was coated onto an 8 μm thick copper (Cu) film serving as the negative electrode current collector, dried, and then rolled to manufacture the negative electrode.

[0087] The electrolyte was prepared by mixing ethylene carbonate (EC): ethyl methyl carbonate (EMC) in a volume ratio of 30:70 and then dissolving LiPF6 to a concentration of 1.0 M.

[0088] An electrode assembly is prepared by sequentially stacking a positive electrode, a polyolefin porous membrane coated with inorganic particles (Al2O3), and a negative electrode. The assembled electrode assembly is then placed into a pouch cell and injected with the prepared non-aqueous electrolyte to manufacture a lithium secondary battery.

[0089] [Example]

[0090] Example 1.

[0091] The lithium secondary battery manufactured in the preparation example was connected to a PNE charger and charged at 45°C at a rate of 0.1C to 100% SOC (4.6 V) for approximately 10 hours, and then discharged at 45°C at a rate of 0.1C to 0% SOC (2.0 V) for approximately 10 hours to perform an activation (formation) process. The battery was then aged at 60°C for 15 hours, followed by a degassing process.

[0092] For a degassed lithium secondary battery, the process of performing 9 charge and discharge cycles under the first charge / discharge condition and then 1 charge and discharge cycle under the second charge / discharge condition will be repeated 10 times, for a total of 100 charge and discharge cycles.

[0093] - First charge / discharge: Charge at 0.33C under CC-CV (constant current-constant voltage) conditions at 45°C until 4.25 V, and discharge at 0.33C under CC conditions until 3.0 V.

[0094] - Second charge / discharge: Charge at 0.33C under CC-CV (constant current-constant voltage) conditions at 45°C until 4.3 V, and discharge at 0.33C under CC conditions until 2.5 V.

[0095] Example 2.

[0096] The lithium secondary battery manufactured in the preparation example was connected to a PNE charger and charged at 45°C at a rate of 0.1C to 100% SOC (4.6 V) for approximately 10 hours, and then discharged at 45°C at a rate of 0.1C to 0% SOC (2.0 V) for approximately 10 hours to perform an activation (formation) process. The battery was then aged at 60°C for 15 hours, followed by a degassing process.

[0097] For a degassed lithium secondary battery, the process of charging and discharging 4 times under the first charge / discharge condition and then charging and discharging once under the second charge / discharge condition will be repeated 20 times, for a total of 100 charge and discharge cycles.

[0098] - First charge / discharge: Charge at 0.33C under CC-CV (constant current-constant voltage) conditions at 45°C until 4.3 V, and discharge at 0.33C under CC conditions until 3.0 V.

[0099] - Second charge / discharge: Charge at 0.33C under CC-CV (constant current-constant voltage) conditions at 45°C until 4.3 V, and discharge at 0.33C under CC conditions until 2.5 V.

[0100] Example 3.

[0101] The lithium secondary battery manufactured in the preparation example was connected to a PNE charger and charged at 45°C at a rate of 0.1C to 100% SOC (4.6 V) for approximately 10 hours, and then discharged at 45°C at a rate of 0.1C to 0% SOC (2.0 V) for approximately 10 hours to perform an activation (formation) process. The battery was then aged at 60°C for 15 hours, followed by a degassing process.

[0102] For a degassed lithium secondary battery, the process of charging and discharging once under the first charge / discharge condition and then charging and discharging once under the second charge / discharge condition will be repeated 50 times, for a total of 100 charge and discharge cycles.

[0103] - First charge / discharge: Charge at 0.33C under CC-CV (constant current-constant voltage) conditions at 45°C until 4.3 V, and discharge at 0.33C under CC conditions until 3.0 V.

[0104] - Second charge / discharge: Charge at 0.33C under CC-CV (constant current-constant voltage) conditions at 45°C until 4.3 V, and discharge at 0.33C under CC conditions until 2.5 V.

[0105] Comparative Example 1.

[0106] The lithium secondary battery manufactured in the preparation example was connected to a PNE charger and charged at 0.1C at 45°C to 100% SOC (4.6 V) for approximately 10 hours, and then discharged at 0.1C at 45°C to 0% SOC (2.0 V) for an activation (formation) process. The battery was then aged at 60°C for 15 hours, followed by a degassing process.

[0107] For the degassed lithium secondary battery, a total of 100 charge and discharge cycles are performed under the following first charge / discharge conditions.

[0108] - First charge / discharge: Charge at 0.33C under CC-CV (constant current-constant voltage) conditions at 45°C until 4.3 V, and discharge at 0.33C under CC conditions until 3.0 V.

[0109] Compare Example 2.

[0110] The lithium secondary battery manufactured in the preparation example was connected to a PNE charger and charged at 45°C at a rate of 0.1C to 100% SOC (4.6 V) for approximately 10 hours, and then discharged at 45°C at a rate of 0.1C to 0% SOC (2.0 V) for approximately 10 hours to perform an activation (formation) process. The battery was then aged at 60°C for 15 hours, followed by a degassing process.

[0111] For the degassed lithium secondary battery, a total of 100 charge and discharge cycles were performed under the second charge / discharge conditions described below.

[0112] - Second charge / discharge: Charge at 0.33C under CC-CV (constant current-constant voltage) conditions at 45°C until 4.3 V, and discharge at 0.33C under CC conditions until 2.5 V.

[0113] [Experimental Example: Measurement of Capacity Retention and Voltage Drop]

[0114] While charging and discharging according to the embodiments and comparative examples, the capacity retention rate and voltage drop are measured by measuring the capacity, discharge energy and average discharge voltage value corresponding to each cycle. Figure 1 The results of evaluating capacity retention by accumulating energy are shown for the lithium secondary batteries of the examples and comparative examples. Figure 2 Evaluation results of the voltage drop of the lithium secondary batteries in the examples and comparative examples are shown. For reference, in the driving methods of Example 1 (mix(9+1)), Example 2 (mix(4+1)) and Example 3 (mix(1+1)), the voltage ranges of the first and second charge / discharge conditions are different from each other, resulting in different energy ranges (retention rates) and average discharge voltages (voltage drops) for each charge / discharge condition. Therefore, the data for each charge / discharge condition are displayed separately as data at the upper limit and data at the lower limit.

[0115] refer to Figure 1 and Figure 2 It was confirmed that the lithium secondary batteries using the driving conditions of Examples 1 to 3 exhibited superior capacity retention compared to Comparative Example 1, even while showing a similar voltage drop as Comparative Example 1. In particular, it was confirmed that Example 1 exhibited the highest capacity retention.

[0116] On the other hand, it was confirmed that when the driving conditions of Comparative Example 2 were applied, the capacity retention rate was relatively high (see [reference]). Figure 1 ), but if Figure 2 As shown, a significant voltage drop occurred, and significant structural changes and degradation of the positive electrode active material occurred, resulting in a significant reduction in lifetime characteristics.

Claims

1. A method for driving a lithium secondary battery, the lithium secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode comprises a positive electrode active material of lithium metal oxide. The method includes: The first driving step of performing a charge / discharge cycle on the lithium secondary battery between a first minimum discharge voltage and a first maximum charge voltage; and The second driving step involves performing a charge / discharge cycle on the lithium secondary battery between the second minimum discharge voltage and the second maximum charge voltage. Based on the exclusion of all lithium metal content, the lithium metal oxide contains more than 50 mol% and less than 100 mol% manganese, and Wherein the second minimum discharge voltage is lower than the first minimum discharge voltage, and While repeating the first driving step, the second driving step is performed at fixed intervals.

2. The method for driving a lithium secondary battery according to claim 1, wherein the lithium metal oxide comprises a compound represented by the following chemical formula 1: [Chemical Formula 1] Li a [Mr 1-b-c Ni b M c ] 2-a O2 In chemical formula 1, M is selected from at least one of Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr, and Zr, and a, b, and c are the atomic fractions of each independent element, where 1 <a,0≤b≤0.5,0≤c≤0.5,0<b+c≤0.5。 3. The method for driving a lithium secondary battery according to claim 1, wherein the lithium metal oxide comprises a compound represented by the following chemical formula 2: [Chemical Formula 2] XLi2MnO3·(1-X)Li[Ni 1-y-z-w Mr y Co z M w ]O2 In chemical formula 2, M is selected from at least one of Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr, and Zr. Where 0.2≤X≤0.5, 0.4≤y<1, 0≤z≤0.1, and 0≤w≤0.

2.

4. The method for driving a lithium secondary battery according to claim 3, wherein the lithium metal oxide comprises Li₂MnO₃ having a rock salt structure in a mixed state and Li[Ni] having a layered structure. 1-y-z-w Mn y Co z M w O2.

5. The method for driving a lithium secondary battery according to claim 1, wherein the first minimum discharge voltage is 2.8 V to 3.2 V, and the second minimum discharge voltage is 2.0 V to 2.6 V.

6. The method for driving a lithium secondary battery according to claim 1, wherein the first maximum charging voltage and the second maximum charging voltage are 4.1 V to 4.5 V.

7. The method for driving a lithium secondary battery according to claim 1, wherein after performing the first driving step 1 to 15 times, the second driving step is repeated 1 to 2 times.

8. The method for driving a lithium secondary battery according to claim 1, wherein prior to the first driving step, the method further includes a step of activating (forming) the lithium secondary battery.

9. The method for driving a lithium secondary battery according to claim 8, wherein the activation step is performed at a voltage of 4.6 V or higher.

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