Lithium secondary battery comprising lithium-rich manganese-based oxide and method for manufacturing same

Abstract

The present invention relates to a lithium secondary battery and a method for manufacturing the same, the lithium secondary battery comprising: a positive electrode, a negative electrode, a separator, and an electrolyte, in which the positive electrode comprises a lithium-rich manganese-based oxide in which the content of manganese in all metals other than lithium is greater than 50 mol%; and the ratio of the number of moles of lithium to the number of moles of all metals other than lithium (Li / Me) is greater than 1, where in a dQ / dV map obtained by differentiating a map of a battery discharge capacity Q and a voltage V measured while charging the lithium secondary battery at 0.1 C to 4.6 V and then discharging the lithium secondary battery at 0.1 C to 2.0 V, the total discharge curve area is defined as 100%, and the ratio of the number of moles of lithium to the number of moles of all metals other than lithium (Li / Me) is greater than 1. The discharge curve area in the voltage range of 2.0 to 3.5 V is 35% or less.

Description

Technical Field

[0001] CROSS REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of Korean Patent Application No. 10-2022-0183648 filed in the Korean Intellectual Property Office on December 23, 2022, the disclosure of which is incorporated herein by reference in its entirety.

[0003] The present invention relates to a lithium secondary battery containing a lithium-rich manganese-based oxide and a method for manufacturing the same, and more particularly, to a lithium secondary battery containing a lithium-rich manganese-based oxide with improved resistance characteristics, especially positive electrode resistance characteristics, and a method for manufacturing the same. Background Art

[0004] Since its commercialization in 1991, lithium secondary batteries have become energy storage media used in various fields. As the market for products equipped with lithium secondary batteries expands, research on improving the energy density of lithium secondary batteries has been actively carried out. One of the most popular methods is to develop a positive electrode active material with a composition that can utilize a larger amount of lithium than before.

[0005] As a positive electrode active material capable of utilizing a larger amount of lithium, a lithium-rich transition metal oxide having a layered structure and a molar ratio of lithium to transition metal greater than 1 has been developed. Such a lithium-rich transition metal oxide can achieve a high capacity by performing an activation step at a high voltage of 4.4 V or more. However, when such a high-voltage activation step is performed, oxygen separation and cation mixing are induced in the crystal structure of the lithium-rich transition metal oxide, resulting in a problem of a significant increase in positive electrode resistance. Summary of the invention

[0006] Technical issues

[0007] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a lithium secondary battery including a lithium-rich manganese-based oxide having improved resistance characteristics, especially positive electrode resistance characteristics, and a method for manufacturing the same.

[0008] Technical Solution

[0009] According to one aspect of the present invention, there is provided a lithium secondary battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode contains a lithium-rich manganese-based oxide, in which the content of manganese among all metals other than lithium is greater than 50 mol%, and the ratio of the number of moles of lithium to the number of moles of all metals other than lithium (Li / Me) is greater than 1, and in the dQ / dV graph obtained by differentiating the graph of the voltage V measured while charging the lithium secondary battery at 0.1C to 4.6 V and then discharging it at 0.1C to 2.0 V with respect to the battery discharge capacity Q, when the total discharge curve area is defined as 100%, the discharge curve area in the voltage range of 2.0 to 3.5 V is 35% or less, preferably 20% to 35%.

[0010] The lithium-rich manganese-based oxide can be represented by the following Chemical Formula 1.

[0011] [Chemical Formula 1]

[0012] Li a Ni b Co c Mn d M e O2

[0013] Wherein in Chemical Formula 1, M contains 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, 1 < a, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.1, 0.5 ≤ d < 1.0 and 0 ≤ e ≤ 0.2. Preferably, 1.1 ≤ a ≤ 1.5, 0.1 ≤ b ≤ 0.4, 0 ≤ c ≤ 0.05, 0.5 ≤ d ≤ 0.80 and 0 ≤ e ≤ 0.1.

[0014] According to another aspect of the present invention, there is provided a method for manufacturing a lithium secondary battery, the method comprising the steps of: preparing a battery cell comprising a positive electrode, a negative electrode, and an electrolyte, the positive electrode containing a lithium-rich manganese-based oxide, in which the content of manganese among all metals other than lithium is greater than 50 mol%, and the ratio of the number of moles of lithium to the number of moles of all metals other than lithium (Li / Me) is greater than 1; and subjecting the battery cell to at least one charge and discharge to activate the battery cell, wherein the activation step includes a first charging step of charging the battery cell at a C-rate less than 0.5C and a second charging step of charging the battery cell at a C-rate of 0.5C or more.

[0015] Preferably, the first charging step can be carried out until the SOC of the battery cell reaches 10 to 40, and the second charging step can be carried out after the first charging step until the SOC of the battery cell reaches 90 to 150.

[0016] The first charging step and the second charging step may be performed in a constant current mode (CC mode), or the first charging step may be performed in a constant current mode (CC mode) and the second charging step may be performed in a constant current-constant voltage mode (CCCV mode).

[0017] The charging termination voltage of the second charging step may be 4.4 V or more, for example, 4.4 V to 4.8 V, or 4.5 V to 4.8 V.

[0018] Preferably, in the present invention, the charge and discharge in the activation step may be performed within a voltage range of 2.0 V to 4.6 V.

[0019] In the lithium secondary battery manufactured through the activation step as described above, in the dQ / dV graph obtained by differentiating a graph of the voltage V measured while the lithium secondary battery is charged to 4.6 V at 0.1C and then discharged to 2.0 V at 0.1C and the battery discharge capacity Q, when the total discharge curve area is defined as 100%, the discharge curve area in the voltage range of 2.0 to 3.5 V may be less than 35%, preferably 20% to 35%.

[0020] Beneficial Effects

[0021] The lithium secondary battery according to the present invention comprises a lithium-rich manganese-based oxide, wherein the content of manganese in all metals other than lithium is greater than 50 mol %, and the ratio of the mole number of lithium to the mole number of all metals other than lithium (Li / Me) is greater than 1. The lithium-rich manganese-based oxide can be driven at a relatively higher voltage than the lithium nickel-based oxide, and therefore has excellent capacity characteristics.

[0022] In addition, in the lithium secondary battery according to the present invention, oxygen separation and cation mixing are suppressed during activation, and capacity performance hardly occurs during reduction of Mn ions mixed with cations, so that when the total discharge curve area in the dQ / dV diagram is defined as 100%, the discharge curve area in the voltage range of 2.0 to 3.5 V is as small as 35% or less, preferably 20% to 35%. The lithium secondary battery of the present invention having the above-mentioned discharge behavior has a small increase in positive electrode resistance during charge and discharge, thereby having excellent resistance characteristics and output characteristics.

[0023] According to the method for manufacturing a lithium secondary battery of the present invention, charging is performed at a high C rate of 0.5C or more in a part of the activation charging step, thereby reducing the activation of the Li2MnO3 (monoclinic) phase in the crystal structure of the positive electrode active material, thereby reducing the resistance of the battery and the positive electrode.

[0024] Furthermore, since the manufacturing method according to the present invention includes the second charging step of performing charging at a higher C rate than before, the total time required for the activation step is shortened, thereby being able to obtain the effect of improving the manufacturing speed of the lithium secondary battery.

[0025] In addition, the manufacturing method according to the present invention performs the first charging step of charging at a low C rate at the initial stage of activation, thereby ensuring that the SEI film is fully formed on the electrode at the initial stage of activation. If charging is performed at a high C rate at the initial stage of activation, the SEI film formed on the electrode is unstable, which may lead to deterioration of the battery life characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 dQ / dV graphs of the lithium secondary batteries prepared in Examples 1 to 3 and Comparative Example 1.

[0027] Figure 2 It is a graph showing the positive electrode resistance of the lithium secondary batteries manufactured in Examples 1 to 3 and Comparative Example 1 measured by EIS analysis.

[0028] Figure 3 : is a graph showing the positive electrode resistance of the lithium secondary batteries manufactured in Examples 1 to 3 and Comparative Example 1 measured by DCIR analysis. DETAILED DESCRIPTION

[0029] The terms or words used in the specification and the appended claims should not be understood as limited to the ordinary or dictionary meanings, but should be interpreted in the present invention with meanings and concepts consistent with the technical idea of ​​the present invention, based on the principle that the inventor can appropriately define the concept of the term to appropriately describe his own invention in the best manner.

[0030] In the present invention, "primary particles" refer to particle units whose grain boundaries are not visible when observed under a field of view of 5000 to 20000 times using a scanning electron microscope. "Average particle size of primary particles" refers to the arithmetic mean value calculated after measuring the particle size of primary particles observed in a scanning electron microscope image.

[0031] In the present invention, "secondary particles" refer to particles formed by aggregation of a plurality of primary particles.

[0032] In the present invention, the average particle size D 50 It refers to the particle size when the cumulative volume of powder particles (such as positive electrode active material, negative electrode active material, etc.) reaches 50% in the particle size distribution curve. 50It can be measured using a laser diffraction method. It can be measured by a method comprising the following operations: dispersing a powder of the particles to be measured in a dispersion medium; introducing it into a commercially available laser diffraction particle size measuring device (e.g., Microtrac MT 3000); irradiating with an ultrasonic wave of about 28 kHz with an output power of 60 W to obtain a volume cumulative particle size distribution; and determining the particle size corresponding to 50% of the cumulative volume.

[0033] In the present invention, "SOC X" refers to a state in which the percentage of the charge capacity of a battery cell relative to the discharge capacity exhibited when the battery cell is discharged from 4.6 V to 2.0 V is X.

[0034] The inventors have conducted repeated studies on improving the resistance and output characteristics of lithium secondary batteries using lithium-rich manganese oxides. As a result, they found that when manufacturing lithium secondary batteries, an activation step is performed under specific charging conditions, thereby minimizing cation mixing and oxygen separation and improving the resistance characteristics of the positive electrode and the battery, thereby completing the present invention.

[0035] The present invention will be described in detail below.

[0036] <Lithium Secondary Battery>

[0037] The lithium secondary battery according to the present invention comprises a positive electrode, a negative electrode and an electrolyte, wherein the positive electrode comprises a lithium-rich manganese oxide, the manganese content in all metals other than lithium in the lithium-rich manganese oxide is greater than 50 mol %, and the ratio of the molar number of lithium to the molar number of all metals other than lithium (Li / Me) is greater than 1.

[0038] In the lithium secondary battery according to the present invention, in a dQ / dV graph obtained by differentiating a graph of the voltage V measured while the battery is charged to 4.6 V at 0.1C and then discharged to 2.0 V at 0.1C and the battery discharge capacity Q, when the total discharge curve area is defined as 100%, the discharge curve area in the voltage range of 2.0 to 3.5 V is less than 35%, preferably 20% to 35%.

[0039] The peak that appears below 3.5 V when the lithium secondary battery is discharged is known to be a peak that appears due to the redox reaction of oxygen and the redox reaction of Mn ions mixed with cations. In other words, this means that the larger the discharge curve area that appears below 3.5 V, the more the redox reaction of oxygen and the mixing of cations increase. On the other hand, when the redox reaction of oxygen and the mixing of cations increase, the positive electrode resistance will increase significantly and the output characteristics will deteriorate. Therefore, in the present invention, the activation conditions in the manufacturing process of the lithium secondary battery are adjusted so that the discharge curve area that appears in the dQ / dV diagram within the voltage range of 2.0 V to 3.5 V is less than 35% of the total discharge curve area, so that the increase in positive electrode resistance caused by cation mixing can be suppressed. When the discharge curve area that appears in the dQ / dV diagram within the voltage range of 2.0 V to 3.5 V meets less than 35% of the total discharge curve area, the resistance of the positive electrode and the battery is reduced, and excellent output characteristics can be achieved. However, if the discharge curve area within the voltage range of 2.0 V to 3.5 V is too small, the capacity characteristics may deteriorate. Therefore, it is more preferred that the discharge curve area appearing in the voltage range of 2.0 V to 3.5 V in the dQ / dV graph is about 20% to 35% of the total discharge curve area.

[0040] The various components of the lithium secondary battery according to the present invention are described in more detail below.

[0041] positive electrode

[0042] The positive electrode according to the present invention comprises a lithium-rich manganese-based oxide, wherein the content of manganese in all metals other than lithium is greater than 50 mol %, and the ratio of the number of moles of lithium to the number of moles of all metals other than lithium (Li / Me) is greater than 1. Specifically, the positive electrode according to the present invention comprises a positive electrode collector and a positive electrode active material layer formed on at least one surface of the positive electrode collector, wherein the positive electrode active material layer comprises a lithium-rich manganese-based oxide, wherein the ratio of the number of moles of lithium to the number of moles of all metals other than lithium (Li / Me) is greater than 1.

[0043] In the case of lithium-rich manganese oxides (with excess lithium content), they have a structure in which a layered phase (LiM'O2) is mixed with a rock salt phase (Li2MnO3). During the initial activation process, the rock salt phase is activated and generates excess lithium ions, thereby achieving high capacity.

[0044] Preferably, the lithium-rich manganese-based oxide may be represented by the following Chemical Formula 1.

[0045] [Chemical formula 1]

[0046] Li a Ni b Co cMn d M e O2

[0047] Among them, in Chemical Formula 1, M can 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.

[0048] On the other hand, a is the molar ratio of Li in the lithium-rich manganese-based oxide, and can be 1 < a, 1.1 ≤ a ≤ 1.5, or 1.1 ≤ a ≤ 1.3. If a is within the above range, the irreversible capacity of the silicon-based negative electrode active material can be sufficiently compensated, and high-capacity characteristics can be achieved.

[0049] b is the molar ratio of Ni in the lithium-rich manganese-based oxide, and can be 0 ≤ b ≤ 0.5, 0.1 ≤ b ≤ 0.4, or 0.2 ≤ b ≤ 0.4.

[0050] c is the molar ratio of Co in the lithium-rich manganese-based oxide, and can be 0 ≤ c ≤ 0.1, 0 ≤ c ≤ 0.08, or 0 ≤ c ≤ 0.05. If c exceeds 0.1, it is difficult to ensure high capacity, and gas generation and deterioration of the positive electrode active material are aggravated, and the life characteristics may deteriorate.

[0051] d is the molar ratio of Mn in the lithium-rich manganese-based oxide, and can be 0.5 ≤ d < 1.0, 0.50 ≤ d ≤ 0.80, or 0.50 ≤ d ≤ 0.70. If d is less than 0.5, the proportion of the rock salt phase is too small, so the effect of compensating for the negative electrode irreversibility and improving the capacity is not obvious.

[0052] e is the molar ratio of the doping element M in the lithium-rich manganese-based oxide, and can be 0 ≤ e ≤ 0.2, 0 ≤ e ≤ 0.1, or 0 ≤ e ≤ 0.05. If the content of the doping element is too high, it may have an adverse effect on the capacity of the active material.

[0053] On the other hand, in the lithium-rich manganese-based oxide represented by [Chemical Formula 1], the ratio of the number of moles of Li to the number of moles of all metal elements other than Li (Li / Me) can be 1.2 to 1.5, 1.25 to 1.5, or 1.25 to 1.4. When the Li / Me ratio satisfies the above range, the rate performance and capacity characteristics can be excellently exhibited. If the Li / Me ratio is too high, the conductivity may decrease and the rock salt phase (Li2MnO3) may increase, which may lead to accelerated deterioration. If the Li / Me ratio is too low, the effect of improving the energy density is small.

[0054] On the other hand, the composition of the lithium-rich manganese-based oxide can be represented by the following [Chemical Formula 2].

[0055] [Chemical Formula 2]

[0056] X Li2MnO3·(1-X)Li[Ni 1-y-z-w Mn y Co z M w ]O2

[0057] In [Chemical Formula 2], M may be at least one metal ion selected from the group consisting of Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr, and Zr.

[0058] X refers to the ratio of the Li2MnO3 phase in the lithium-rich manganese oxide, and can be 0.2≤X≤0.5, 0.25≤X≤0.5 or 0.25≤X≤0.4. When the ratio of the Li2MnO3 phase in the lithium-rich manganese oxide satisfies the above range, the irreversible capacity of the silicon-based negative electrode active material can be fully compensated, and high capacity characteristics can be achieved.

[0059] y is a molar ratio of Mn in the LiM'O2 phase, and may be 0.4≤y<1, 0.4≤y≤0.8, or 0.4≤y≤0.7.

[0060] z is a molar ratio of Co in the LiM'O2 phase and may be 0≤z≤0.1, 0≤z≤0.08, or 0≤z≤0.05. If z is greater than 0.1, gas generation and degradation of the positive active material may be aggravated, and life characteristics may be deteriorated.

[0061] w is a molar ratio of the doping element M in the LiM'O2 phase, and may be 0≤w≤0.2, 0≤w≤0.1, or 0≤w≤0.05.

[0062] On the other hand, the positive electrode active material according to the present invention may further include a coating on the surface of the lithium-rich manganese oxide as required. When the positive electrode active material includes the coating, the contact between the lithium-rich manganese oxide and the electrolyte is suppressed by the coating, and the side reaction of the electrolyte is reduced, thereby being able to obtain the effect of improving the life characteristics.

[0063] The coating may contain a coating element M 1 , and the coating element M 1 It can 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 a combination thereof, more preferably Al, Co and a combination thereof. 1 Two or more kinds may be contained, for example, Al and Co.

[0064] The coating element may be in the form of an oxide, i.e., M 1Oz (1≤z≤4) exists in the coating.

[0065] 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, it is preferably formed by atomic layer deposition because a larger area of ​​the coating layer can be formed.

[0066] Based on the total surface area of ​​the lithium-rich manganese oxide particles, the coating layer may be formed in an area of ​​10% to 100%, preferably 30% to 100%, and more preferably 50% to 100%. When the coating layer is formed in an area within the above range, the effect of improving the life characteristics is excellent.

[0067] On the other hand, 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 size D of the secondary particles is 50 The thickness 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 achieved, and degradation of capacity and rate characteristics can be minimized.

[0068] In addition, the BET specific surface area of ​​the positive electrode active material can be 1 m 2 / g~10 m 2 / g, 3~8 m 2 / g or 4-6 m 2 / g. If the BET specific surface area of ​​the positive electrode active material is too low, the reaction area with the electrolyte is insufficient, making it difficult to obtain sufficient capacity. If the specific surface area is too high, moisture absorption becomes faster and the side reaction with the electrolyte is accelerated, making it difficult to ensure life characteristics.

[0069] On the other hand, the lithium-rich manganese-based oxide can be prepared by mixing a transition metal precursor with a lithium raw material and then firing the mixture.

[0070] As lithium raw materials, for example, lithium-containing carbonates (such as lithium carbonate, etc.), hydrates (such as lithium hydroxide hydrate (LiOH·H2O), etc.), hydroxides (such as lithium hydroxide, etc.), nitrates (such as lithium nitrate (LiNO3)), etc.), chlorides (such as lithium chloride (LiCl)), etc. can be used, among which one kind can be used alone or a mixture of two or more kinds can be used.

[0071] On the other hand, the transition metal precursor may be in the form of a hydroxide, an oxide or a carbonate. When a precursor in the form of a carbonate is used, it is more preferred because a positive electrode active material having a relatively high specific surface area can be produced.

[0072] The transition metal precursor can be prepared by a coprecipitation method. 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, then mixing the metal solution, an ammonium cation complex forming agent and an alkaline compound, and performing a coprecipitation reaction. In addition, an oxidant or oxygen can also be added during the coprecipitation reaction as required.

[0073] At this time, the raw material containing transition metals can be acetates, carbonates, nitrates, sulfates, halides, sulfides, etc. of each transition metal. Specifically, the raw material containing transition metals can be NiO, NiCO3·2Ni(OH)2·4H2O, NiC2O2·2H2O, Ni(NO3)2·6H2O, NiSO4, NiSO4·6H2O, Mn2O3, MnO2, Mn3O4, MnCO3, Mn(NO3)2, MnSO4·H2O, manganese acetate, manganese halide, cobalt oxide, cobalt carbonate, cobalt acetate, cobalt halide, etc.

[0074] The ammonium cation complex forming agent may be at least one selected from the group consisting of NH4OH, (NH4)2SO4, NH4NO3, NH4Cl, CH3COONH4 and NH4CO3.

[0075] The alkaline compound can be at least one selected from the group consisting of NaOH, Na2CO3, KOH and Ca(OH)2. The form of the precursor can vary with the type of alkaline compound used. For example, when NaOH is used as the alkaline compound, a precursor in the form of a hydroxide can be obtained, and when Na2CO3 is used as the alkaline compound, a precursor in the form of a carbonate can be obtained. In addition, when the alkaline compound is used together with an oxidant, a precursor in the form of an oxide can be obtained.

[0076] On the other hand, the transition metal precursor and the lithium raw material are mixed in such an amount that the molar ratio of total transition metal (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.

[0077] On the other hand, firing can be performed at a temperature of 600° C. to 1000° C. or 700° C. to 950° C., and the firing time can be 5 hours to 30 hours or 5 hours to 20 hours. In addition, the firing atmosphere can be an air atmosphere or an oxygen atmosphere, for example, an atmosphere with an oxygen content of 20 to 100% by volume.

[0078] On the other hand, the positive electrode active material layer may further contain a conductive material and a binder in addition to the positive electrode active material.

[0079] Examples of the conductive material include: spherical or flaky graphite; carbon 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 powders or metal fibers such as copper, nickel, aluminum and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. Among them, a single one or a mixture of two or more can be used. Based on the total weight of the positive electrode active material layer, the content of the conductive material can be 0.1 to 20% by weight, 1 to 20% by weight or 1 to 10% by weight.

[0080] In addition, the binder may include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVdF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene rubber (EPDM rubber), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber and various copolymers thereof. Among them, a single one or a mixture of two or more can be used. Based on the total weight of the positive active material layer, the content of the binder may be 1 to 20% by weight, 2 to 20% by weight or 2 to 10% by weight.

[0081] negative electrode

[0082] The negative electrode may include, for example, a negative electrode current collector and a negative electrode active material layer on the negative electrode current collector. The negative electrode active material layer may optionally include a binder and a conductive material in addition to the negative electrode active material.

[0083] The negative electrode current collector is not particularly limited as long as it has high conductivity and does not cause chemical changes in the battery, for example, it can be used: copper, stainless steel, aluminum, nickel, titanium, sintered carbon; or copper or stainless steel whose surface is treated with carbon, nickel, titanium, silver, etc.; aluminum-cadmium alloy, etc. In addition, the negative electrode current collector can generally have a thickness of 3 μm to 500 μm. In addition, similar to the positive electrode current collector, the negative electrode current collector may have fine concave-convex formed on its surface to enhance the adhesion of the negative electrode active material. For example, the negative electrode current collector can be used in various forms such as films, sheets, foils, nets, porous bodies, foams, and non-woven fabric structures.

[0084] As the negative electrode active material, a compound capable of reversibly inserting and deinserting lithium can be used. Specific examples of the negative electrode active material may include: carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber or amorphous carbon; metal compounds capable of forming an alloy with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy or Al alloy; metal oxides capable of doping and dedoping lithium such as SiO β (0<β<2), SnO2, vanadium oxide and lithium vanadium oxide; a composite material containing the metal compound and a carbonaceous material such as a Si-C composite material or a Sn-C composite material, and any one of them alone or a mixture of two or more thereof can be used.

[0085] In addition, as the negative electrode active material, a metallic lithium film can be used. In addition, as a carbon material, both low-crystalline carbon and high-crystalline carbon can be used. Typical examples of low-crystalline carbon include soft carbon and hard carbon. Typical examples of high-crystalline carbon include amorphous, plate-like, flaky, spherical or fibrous natural graphite or artificial graphite, floating graphite, pyrolytic carbon, mesophase pitch-like carbon fibers, mesophase carbon microspheres, high-temperature sintered carbon such as mesophase pitch and coke derived from petroleum or coal tar pitch.

[0086] The conductive material is used to impart conductivity to the electrode, and can be used without particular limitation as long as the conductive material has electronic conductivity and does not cause chemical changes in the constructed battery. Specific examples thereof include: graphite such as natural graphite and artificial graphite; carbon materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber and carbon nanotubes; metal powders or metal fibers such as copper, nickel, aluminum and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and any one thereof alone or a mixture of two or more thereof can be used. Based on the total weight of the negative electrode active material layer, the content of the conductive material can generally be 1 to 30% by weight, preferably 1 to 20% by weight, and more preferably 1 to 10% by weight.

[0087] The binder serves to improve the adhesion between the negative electrode active material particles and the adhesion between the negative electrode active material and the negative electrode collector. Its specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene rubber (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber or its various copolymers, and any one thereof alone or a mixture of two or more thereof can be used. Based on the gross weight of the negative electrode active material layer, the content of the binder can be 1 to 30% by weight, preferably 1 to 20% by weight, and more preferably 1 to 10% by weight.

[0088] The negative electrode active material layer may be prepared by coating a negative electrode slurry containing a negative electrode active material and optionally a binder and a conductive material onto a negative electrode collector and drying the coated slurry, or may be prepared by casting the negative electrode slurry onto a separate support and then laminating a film peeled off from the support onto a current collector.

[0089] Diaphragm

[0090] The lithium secondary battery according to the present invention may also include a separator inserted between the positive electrode and the negative electrode. The separator separates the negative electrode and the positive electrode from each other and provides a mobile channel for lithium ions. The separator can be used without particular limitation as long as it is used as a separator in a typical lithium secondary battery. In particular, it is preferred that the separator has low resistance to ion migration of the electrolyte and has excellent ability to impregnate the electrolyte. Specifically, the separator may be: a porous polymer film, such as a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer; or a laminated structure of two or more layers thereof. In addition, a typical porous non-woven fabric can be used, such as a non-woven fabric made of glass fiber with a high melting point, polyethylene terephthalate fiber, etc. In addition, in order to ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer material can be used, and can be optionally used as a single layer or multilayer structure.

[0091] Electrolytes

[0092] The electrolyte used herein may include various electrolytes that can be used in lithium secondary batteries, such as organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, or combinations thereof, and there is no particular limitation on the type thereof.

[0093] For example, the electrolyte may include an organic solvent and a lithium salt.

[0094] The organic solvent can be used without any particular limitation, as long as it is used as a medium through which ions participating in the electrochemical reaction of the battery can migrate. Specific examples of organic solvents may include: ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone or ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene or fluorobenzene; carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC) or propylene carbonate (PC); alcohol solvents such as ethanol or isopropanol; nitrile such as R-CN (wherein R is a linear, branched or cyclic hydrocarbon group having 2 to 20 carbon atoms, and may contain a double bond aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolanes; cyclopentane; and the like.

[0095] The lithium salt may be used without any particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, the anion of the lithium salt may be at least one selected from the group consisting of: - , Cl - Br - ,I - 、NO3 - 、N(CN)2 - 、BF4 - CF3CF2SO3 - 、(CF3SO2)2N - 、(FSO2)2N - CF3CF2(CF3)2CO - 、(CF3SO2)2CH - 、(SF5)3C - 、(CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - 、CH3CO2 - 、SCN - and (CF3CF2SO2)2N -, and the lithium salt that can be used herein can be LiPF6, LiN(FSO2)2, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, etc. The concentration of the lithium salt can be preferably used in the range of 0.1 to 5.0M.

[0096] In order to improve the life characteristics of the battery, inhibit the decrease of the battery capacity, increase the discharge capacity of the battery, etc., the electrolyte may further contain additives on the basis of containing the above electrolyte components. For example, as additives, the following can be used alone or in combination: halogenated alkylene carbonate compounds such as difluoroethylene carbonate; pyridine; triethyl phosphite; triethanolamine; cyclic ether; ethylenediamine; n-glyme dimethyl ether; hexamethylphosphoric triamide; nitrobenzene derivatives; sulfur; quinone imine dyes; N-substituted Oxazolidinone; N, N-substituted imidazolidine; ethylene glycol dialkyl ether; ammonium salt; pyrrole; 2-methoxyethanol; aluminum chloride, etc., but not limited thereto. Based on the total weight of the electrolyte, the content of the additive may be 0.1 to 10 wt%, preferably 0.1 to 5 wt%.

[0097] <Method for Manufacturing Lithium Secondary Battery>

[0098] Next, a method for manufacturing a lithium secondary battery according to the present invention will be described in detail below.

[0099] The method for manufacturing a lithium secondary battery according to the present invention comprises: (1) preparing a battery cell comprising a positive electrode, a negative electrode and a separator; and (2) activating the battery cell, wherein the activation step comprises a first charging step of charging the battery cell at a C rate less than 0.5C and a second charging step of charging the battery cell at a C rate greater than 0.5C.

[0100] (1) Steps for preparing battery cells

[0101] First, a battery cell including a positive electrode, a negative electrode, and an electrolyte is prepared.

[0102] The battery cell can be prepared, for example, by forming an electrode assembly including a positive electrode and a negative electrode, installing the electrode assembly in a battery case, and then injecting an electrolyte to seal the battery case. At this time, the electrode assembly may include a separator located between the positive electrode and the negative electrode.

[0103] The positive electrode, the negative electrode, the separator and the electrolyte are the same as those described above, and thus a detailed description thereof is omitted.

[0104] On the other hand, the electrode assembly may be various forms of electrode assemblies well known in the art, such as a jelly roll type, a stack type, a stack laminate type, or a stack folding type electrode assembly, and its form is not particularly limited.

[0105] The jelly-roll type electrode assembly can be manufactured by inserting a sheet-shaped separator between a sheet-shaped positive electrode and a sheet-shaped negative electrode and then winding them in one direction.

[0106] The stacked type electrode assembly can be manufactured by cutting a positive electrode, a separator, and a negative electrode into desired shapes and then sequentially stacking the cut positive electrode / separator / negative electrode.

[0107] The stacked laminate type electrode assembly can be manufactured by stacking a positive electrode, a separator, and a negative electrode to manufacture a plurality of unit cells, stacking the plurality of unit cells with the separator interposed therebetween, and then laminating them by a method such as heating.

[0108] The stacked folding type electrode assembly can be manufactured by stacking a positive electrode, a separator, and a negative electrode to manufacture a plurality of unit cells, arranging the plurality of unit cells on one surface or both surfaces of a folding separator having a long length, and then winding the folding separator.

[0109] On the other hand, as the battery case, various battery cases known in the art, such as a cylindrical battery case, a square battery case, a pouch-type battery case, etc., can be used, and the type thereof is not particularly limited.

[0110] (2) Activation step

[0111] Next, a step of charging and discharging the battery cell at least once to electrically activate the battery is performed. The activation step is a step of charging and discharging the battery cell to impart electrical properties and form a SEI (solid electrolyte interface) film on the electrode to stabilize the battery.

[0112] In the present invention, charging in the activation step is divided into two or more steps and performed at different C rates, thereby forming a strong SEI film on the electrode while maximally suppressing oxygen desorption and cation mixing during the activation step.

[0113] The charging step includes: a first charging step of charging the battery cell at a C rate less than 0.5C; and a second charging step of charging the battery cell at a C rate greater than 0.5C.

[0114] The first charging step is performed at a C rate of less than 0.5 C, preferably 0.05 C or more and less than 0.5 C. If the current rate of the first charging step is 0.5 C or more, the SEI film may be unstably formed on the electrode surface. If the SEI film is unstably formed on the electrode surface, the SEI film is easily decomposed during battery operation, resulting in rapid degradation of the electrode, which may significantly reduce the life characteristics.

[0115] On the other hand, the first charging step is preferably performed from SOC 0 of the battery cell until SOC reaches 10 to 40. When the charging capacity in the first charging step satisfies the above range, a strong and dense SEI film is formed on the electrode surface, thereby achieving excellent life characteristics.

[0116] On the other hand, if necessary, the first charging step may be performed in two or more steps at different C rates. For example, the first charging step may include: 1-1 a charging step of charging the battery cell at a C rate of 0.1C to less than 0.3C; and 1-2 a charging step of charging the battery cell at a C rate higher than the C rate of the 1-1 charging step and less than 0.5C. At this time, the 1-1 charging step may be performed from SOC 0 to SOC 3 to 5, and the 1-2 charging step may be performed from SOC 3 to 5 to SOC 10 to 40. When the first charging step is performed in two steps as described above, the current rate in the initial stage of SEI film formation can be kept low, thereby forming a stronger and denser SEI film.

[0117] On the other hand, the first charging step may be performed in a constant current mode (CC mode).

[0118] Next, the second charging step can be performed at a C rate of 0.5C or more, preferably 0.5C to 2C. When the second charging step is performed at a C rate of 0.5C or more, the activation of the Li2MnO3 (monoclinic) phase in the crystal structure of the positive electrode active material is reduced, which makes it possible to suppress the oxygen separation and cation mixing that occur during the activation step. In addition, by performing the second charging step at a relatively fast C rate, the time required for the activation step is reduced, and the manufacturing time of the battery can be shortened.

[0119] On the other hand, the second charging step is preferably performed after the first charging step, that is, the SOC of the battery cell is from 10 to 40 until it reaches 90 to 150. When the charging capacity of the second charging step meets the above range, it is possible to suppress the incomplete formation of the SEI film, reduce the occurrence of oxygen separation and cation mixing during the activation step, and minimize the increase in positive electrode resistance.

[0120] The second charging step may be performed in a constant current mode (CC mode) or a constant current-constant voltage mode (CCCV mode).

[0121] In the second charging step, the charge termination voltage may be 4.4 V or more, for example, 4.4 V to 4.8 V or 4.5 V to 4.7 V. When the charge termination voltage of the second charging step satisfies the above range, the lithium-rich manganese-based oxide is activated, thereby achieving high capacity characteristics.

[0122] Next, the battery cell charged in the first charging step and the second charging step is discharged. At this time, the discharge can be performed at a C rate of 0.1C to 1C, preferably 0.3 to 1C. When the discharge rate satisfies the above range, the activation process time can be shortened.

[0123] On the other hand, the discharge can be performed in a constant current mode (CC mode).

[0124] On the other hand, the discharge termination voltage may be 2.0 V to 3.0 V.

[0125] Preferably, in the present invention, the charge and discharge in the activation step may be performed within a voltage range of 2.0 V to 4.6 V.

[0126] On the other hand, the activation step is preferably performed at a temperature of 25°C to 55°C, preferably 30°C to 45°C. When the activation step is performed within the above temperature range, the activation can be smoothly performed even at a relatively high C rate. If the activation temperature is too low, the lithium ion mobility may be reduced, and the activation may not be smoothly performed. If the activation temperature is too high, the dissolution of transition metals such as manganese may be caused.

[0127] In addition, if necessary, the activation step may be performed under pressurized conditions. Pressurization can be performed by mounting the battery cell on a jig and then applying pressure to the battery cell through the jig. When the activation step is performed under pressurized conditions, it has an advantage that the gas generated during the activation step can be easily discharged.

[0128] On the other hand, although this is not essential, if necessary, the activation step may further include an aging step. The aging step allows the electrolyte to be uniformly impregnated into the electrode assembly and stabilizes the battery, and may be performed before charging, during charging, and / or after discharging, and may be performed one or more times.

[0129] The aging step may be performed at a temperature of, for example, 20° C. to 60° C., 20° C. to 50° C., and preferably 30° C. to 50° C. When aging is performed at the above temperature, electrolyte impregnation and lithium mobility can be improved, thereby enabling smoother activation.

[0130] In the lithium secondary battery prepared by the above-mentioned activation step, in the dQ / dV graph obtained by differentiating the voltage V measured when the lithium secondary battery is charged to 4.6V at 0.1C and then discharged to 2.0V at 0.1C and the battery discharge capacity Q, when the total discharge curve area is defined as 100%, the discharge curve area in the voltage range of 2.0 to 3.5 V can be less than 35%, preferably 20% to 35%.

[0131] The present invention will be described in more detail below with reference to specific embodiments.

[0132] Example 1

[0133] The positive electrode active material: conductive material: PVDF binder were mixed in N-methylpyrrolidone at a weight ratio of 96:1.5:2.5 to prepare a positive electrode slurry. 0.595 Co 0.005 Mn 0.4 ]O2 is used as a positive electrode active material, and carbon black is used as a conductive material. The positive electrode slurry is coated on an aluminum current collector sheet, dried, and then rolled to manufacture a positive electrode.

[0134] The negative electrode active material: conductive material: binder are mixed in water at a weight ratio of 97:1:2 to prepare a negative electrode slurry. At this time, graphite is used as the negative electrode active material, carbon black is used as the conductive material, and a mixture of SBR (styrene butadiene rubber) and carboxymethyl cellulose (CMC) is used as a binder. The negative electrode slurry is coated on a copper collector sheet, dried, and then rolled to manufacture a negative electrode.

[0135] The separator is inserted between the positive electrode and the negative electrode manufactured as above to manufacture an electrode assembly, and the electrode assembly is inserted into a battery case, and then an electrolyte is injected, thereby manufacturing a battery cell.

[0136] The activation process is performed by the following operations: charging the battery cell to SOC 3 at 0.2C and 45°C in a constant current mode (1-1 charging step); charging to SOC 30 at 0.3C in a constant current mode (1-2 charging step); charging to 4.6 V at 0.5C in a constant current mode (second charging step); and discharging to 2.0 V at 0.5C in a constant current mode, thereby manufacturing a lithium secondary battery.

[0137] Example 2

[0138] A lithium secondary battery was manufactured in the same manner as in Example 1, except that in the second charging step, charging was performed at 1.0 C.

[0139] Example 3

[0140] A lithium secondary battery was manufactured in the same manner as in Example 1, except that in the second charging step, charging was performed at 1.5 C.

[0141] Comparative Example 1

[0142] A lithium secondary battery was manufactured in the same manner as in Example 1, except that in the second charging step, charging was performed at 0.3 C in a constant current-constant voltage mode (0.05 C CV cutoff).

[0143] Experimental Example 1

[0144] The secondary batteries manufactured in the examples and comparative examples were charged to 4.60 V at 25°C at 0.1C in a constant current-constant voltage mode (CC-CV mode, CV mode cutoff condition: 0.05C), and discharged to 2.0 V at a constant current of 0.1C to measure the voltage-discharge capacity graph. The above voltage-capacity graph was differentiated to obtain a dQ / dV graph. The measured dQ / dV graph is shown in Figure 1 In addition, the ratio of the discharge curve area in the 2 V to 3.5 V region to the total discharge curve area (ie, the discharge curve area in the 2.0 to 4.6 V region) is shown in Table 1 below.

[0145]

[0146] As shown in Table 1 and Figure 1 As shown, the lithium secondary batteries of Examples 1 to 3 manufactured by performing a second charge at a rate of 0.5C or more in the activation step show that the discharge curve area in the 2 V to 3.5 V region is less than 35% of the total discharge curve area, while the lithium secondary battery of Comparative Example 1 performing a second charge at a rate of 0.3C shows that the discharge curve area in the 2 V to 3.5 V range is greater than 40%.

[0147] Experimental Example 2

[0148] Using each of the positive electrodes manufactured in Examples 1 to 3 and Comparative Example 1 as a working electrode, a three-electrode battery (reference electrode: LTO) was manufactured and subjected to EIS (electrochemical impedance spectroscopy) analysis and DCIR (direct current internal resistance) analysis to measure the positive electrode resistance.

[0149] The EIS analysis results are shown in Figure 2 The DCIR analysis results are shown in Figure 3 middle.

[0150] from Figure 2 It can be confirmed that the lithium secondary batteries of Examples 1 to 3 have lower impedance values ​​than the lithium secondary battery of Comparative Example 1, and it can be seen that the frequency (Hz) of impedance presentation also increases and the kinetics of resistance response also improves.

[0151] In addition, from Figure 3 It can be seen that the lithium secondary batteries of Examples 1 to 3 have lower internal resistance than the lithium secondary battery of Comparative Example 1.

Claims

1. A lithium secondary battery, comprising: a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode comprises a lithium-rich manganese-based oxide, in which the content of manganese among all metals other than lithium is greater than 50 mol%, and the ratio of the number of moles of lithium to the number of moles of all metals other than lithium (Li / Me) is greater than 1. Wherein in the dQ / dV graph obtained by differentiating the graph of the voltage V and the battery discharge capacity Q measured while charging the lithium secondary battery at 0.1C to 4.6 V and then discharging it at 0.1C to 2.0 V, when the total discharge curve area is defined as 100%, the discharge curve area in the voltage range of 2.0 to 3.5 V is 35% or less.

2. The lithium secondary battery according to claim 1, wherein: The discharge curve area in the voltage range of 2.0 to 3.5 V is 20% to 35%.

3. The lithium secondary battery according to claim 1, wherein: The lithium-rich manganese-based oxide is represented by the following Chemical Formula 1. [Chemical Formula 1] Li a Ni b Co c Mr d M e O2 Wherein 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.

4. The lithium secondary battery according to claim 3, 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.

5. A method for manufacturing a lithium secondary battery, the method comprising the steps of: Preparing a battery cell comprising a positive electrode, a negative electrode, and an electrolyte, the positive electrode comprising a lithium-rich manganese-based oxide, in which the content of manganese among all metals other than lithium is greater than 50 mol%, and the ratio of the number of moles of lithium to the number of moles of all metals other than lithium (Li / Me) is greater than 1; and Performing at least one charge and discharge on the battery cell to activate the battery. Wherein the activation step comprises a first charging step of charging the battery cell at a C-rate less than 0.5C and a second charging step of charging the battery cell at a C-rate of 0.5C or more.

6. The method for manufacturing a lithium secondary battery according to claim 5, wherein: The lithium-rich manganese-based oxide is represented by the following Chemical Formula 1. [Chemical Formula 1] Li a Ni b Co c Mr d M e O2 Wherein 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.

7. The method for manufacturing a lithium secondary battery according to claim 5, wherein: The first charging step is performed until the SOC of the battery cell reaches 10 to 40, and the second charging step is performed until the SOC of the battery cell reaches 90 to 150 after the first charging step.

8. The method for manufacturing a lithium secondary battery according to claim 5, wherein: The first charging step and the second charging step are performed in a constant current mode (CC mode).

9. The method for manufacturing a lithium secondary battery according to claim 5, wherein: The first charging step is performed in a constant current mode (CC mode), and the second charging step is performed in a constant current-constant voltage mode (CCCV mode).

10. The method for manufacturing a lithium secondary battery according to claim 5, wherein: The charge termination voltage in the second charging step is 4.4 V or higher.

11. The method for manufacturing a lithium secondary battery according to claim 5, wherein: The charging and discharging is performed in a voltage range of 2.0 V to 4.6 V.

12. The method for manufacturing a lithium secondary battery according to claim 5, wherein: In the dQ / dV graph obtained by differentiating a graph of voltage V and battery discharge capacity Q measured while the lithium secondary battery is charged to 4.6 V at 0.1C and then discharged to 2.0 V at 0.1C, when the total discharge curve area is defined as 100%, the discharge curve area in the voltage range of 2.0 to 3.5 V is less than 35%.

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