Lithium secondary battery containing perlithium manganese oxide and method for producing same

Abstract

The present invention relates to a lithium secondary battery including a positive electrode, a negative electrode, and an electrolyte, the lithium secondary battery including a perlithium manganese-based oxide in which the manganese content in the total metals excluding lithium exceeds 50 mol % and the ratio of the number of moles of lithium to the number of moles of the total metals excluding lithium (Li / Me) exceeds 1, the lithium secondary battery being charged to 4.6 V at 0.1 C and then discharged to 2.0 V at 0.1 C, and a method for manufacturing the same, in which the discharge curve area in a voltage range of 2.0 to 3.5 V is 35% or less when the total discharge curve area is 100% in a dQ / dV graph obtained by differentiating a graph of voltage V and battery discharge capacity Q measured while charging the lithium secondary battery to 4.6 V at 0.1 C and then discharging the lithium secondary battery to 2.0 V at 0.1 C.

Description

[Technical Field]

[0001] Cross-Citation of Related Applications This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0183648 dated December 23, 2022, and all contents disclosed in the documents of this Korean patent application are incorporated herein by reference.

[0002] The present invention relates to a lithium secondary battery including a lithium-rich manganese-based oxide and a method for manufacturing the same, and more particularly to a lithium secondary battery including a lithium-rich manganese-based oxide having improved resistance characteristics, particularly improved positive electrode resistance characteristics, and a method for manufacturing the same. [Background technology]

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

[0004] As a positive electrode active material that can utilize more lithium, perlithium-based transition metal oxides, which have a layered structure and a lithium to transition metal molar ratio of more than 1, have been developed. Lithium secondary batteries that use such perlithium-based transition metal oxides typically achieve high capacity by undergoing an activation process at a high voltage of 4.4 V or higher. However, performing such a high-voltage activation process can cause oxygen desorption and cation mixing in the crystalline structure of the perlithium-based transition metal oxide, which can significantly increase the resistance of the positive electrode. Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention is for solving the above problems, and aims to provide a lithium secondary battery including a hyper-lithium manganese oxide having improved resistance characteristics and a method for manufacturing the same.

Means for Solving the Problems

[0006] On one aspect, the present invention provides a lithium secondary battery including a positive electrode, a negative electrode, a separator, and an electrolyte, which contains a hyper-lithium manganese oxide in which the content of manganese in the total metal excluding lithium exceeds 50 mol%, and the ratio (Li / Me) of the number of moles of lithium to the number of moles of the total metal excluding lithium exceeds 1. After charging the lithium secondary battery to 4.6 V at 0.1 C, when the total discharge curve area is regarded as 100% in the dQ / dV graph obtained by differentiating the graph of the measured voltage V and the battery discharge capacity Q while discharging to 2.0 V at 0.1 C, the discharge curve area in the voltage range of 2.0 to 3.5 V is 35% or less, preferably 20% to 35%.

[0007] The hyper-lithium manganese oxide may be represented by the following Chemical Formula 1. [Chemical Formula 1] Li a Ni b Co c Mn d M e O2 In the above Chemical Formula 1, M contains at least one or more selected from the group consisting of Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr, and Zr, and 1 < a, 0 ≦ b ≦ 0.5, 0 ≦ c ≦ 0.1, 0.5 ≦ d < 1.0, 0 ≦ e ≦ 0.2. Preferably, 1.1 ≦ a ≦ 1.5, 0.1 ≦ b ≦ 0.4, 0 ≦ c ≦ 0.05, 0.5 ≦ d ≦ 0.80, 0 ≦ e ≦ 0.1 may also be satisfied.

[0008] In another aspect, the present invention provides a method for manufacturing a lithium secondary battery, the method comprising: preparing a battery cell including a cathode, an anode, and an electrolyte, the cathode including a perlithium manganese-based oxide, in which the manganese content in all metals excluding lithium exceeds 50 mol% and the ratio of the number of moles of lithium to the number of moles of all metals excluding lithium (Li / Me) exceeds 1; and activating the battery cell by charging and discharging at least one time, the activation step including a first charging step of charging the battery cell at a C-rate of less than 0.5 C; and a second charging step of charging the battery cell at a C-rate of 0.5 C or more.

[0009] Preferably, the first charging step may be performed until the SOC of the battery cell reaches 10 to 40, and the second charging step may be performed after the first charging step until the SOC of the battery cell reaches 90 to 150.

[0010] The first and second charging steps 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 (CC-CV mode).

[0011] The charge cut-off voltage in the second charging step may be 4.4V or higher, for example, 4.4V to 4.8V or 4.5V to 4.8V.

[0012] Preferably, in the present invention, the charging and discharging in the activation step may be performed in a voltage range of 2.0V to 4.6V.

[0013] The lithium secondary battery manufactured through the activation process may be charged to 4.6 V at 0.1 C and then discharged to 2.0 V at 0.1 C. In a dQ / dV graph obtained by differentiating a graph of voltage V vs. battery discharge capacity Q measured while differentiating the graph, the area of ​​the entire discharge curve may be 100% or less, and preferably 20% to 35% in the voltage range of 2.0 to 3.5 V. [Effects of the Invention]

[0014] The lithium secondary battery according to the present invention includes a perlithium manganese-based oxide in which the manganese content of all metals excluding lithium exceeds 50 mol % and the ratio of the number of moles of lithium to the number of moles of all metals excluding lithium (Li / Me) exceeds 1. The perlithium manganese-based oxide can be operated at a relatively high voltage compared to lithium nickel-based oxides and has excellent capacity characteristics.

[0015] In addition, the lithium secondary battery of the present invention suppresses oxygen desorption and cation mixing during the activation process, and reduces the capacity development that occurs when Mn ions mixed with cations are reduced. As a result, when the total discharge curve area in a dQ / dV graph is 100%, the discharge curve area in the voltage range of 2.0 to 3.5 V is less than 35%, preferably 20% to 35%. The lithium secondary battery of the present invention, which has the above-described discharge behavior, exhibits excellent resistance and output characteristics due to a small increase in positive electrode resistance during charge and discharge.

[0016] In the method for manufacturing a lithium secondary battery according to the present invention, by charging at a high C-rate of 0.5 C or more during a portion of the activation charge stage, activation of the Li2MnO3 (monoclinic) phase in the crystalline structure of the positive electrode active material can be reduced, thereby reducing the cell and positive electrode resistance.

[0017] In addition, since the manufacturing method according to the present invention includes a second charging step in which charging is performed at a higher C-rate than conventional methods, the total time required for the activation process is reduced, thereby improving the production rate of lithium secondary batteries.

[0018] In addition, the manufacturing method according to the present invention performs a first charging step in which charging is performed at a low C-rate in the early stage of activation, thereby enabling sufficient formation of an SEI film on the electrode in the early stage of activation. If charging is performed at a high C-rate in the early stage of activation, the SEI film may be formed unstably on the electrode, resulting in a decrease in battery life characteristics. [Brief explanation of the drawings]

[0019] [Figure 1] 2 is a graph showing dQ / dV of the lithium secondary batteries manufactured in Examples 1 to 3 and Comparative Example 1. [Figure 2] 1 is a graph showing the positive electrode resistance of lithium secondary batteries manufactured in Examples 1 to 3 and Comparative Example 1, measured by EIS analysis. [Figure 3] 1 is a graph showing the positive electrode resistance of lithium secondary batteries manufactured in Examples 1 to 3 and Comparative Example 1, measured by DCIR analysis. DETAILED DESCRIPTION OF THE INVENTION

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

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

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

[0023] In the present invention, the "average particle size D 50 " means the particle size at 50% of the volume cumulative particle size distribution of the particle powder to be measured (e.g., positive electrode active material powder, negative electrode active material powder, etc.). 50can be measured using the laser diffraction method. For example, the powder of the particles to be measured is dispersed in a dispersion medium, introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT 3000), and irradiated with ultrasonic waves of about 28 kHz at an output of 60 W. A volume cumulative particle size distribution graph is then obtained, and the particle size corresponding to 50% of the volume cumulative amount is determined.

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

[0025] The present inventors have conducted extensive research to improve the resistance and output characteristics of lithium secondary batteries using perlithium manganese-based oxides. As a result, they have found that by performing an activation process under specific charging conditions during the manufacture of a lithium secondary battery, it is possible to minimize cation mixing and oxygen desorption, thereby improving the positive electrode and cell resistance characteristics, and have completed the present invention.

[0026] The present invention will be specifically described below.

[0027] <Lithium secondary battery> The lithium secondary battery according to the present invention includes a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes a perlithium manganese-based oxide in which the manganese content of all metals excluding lithium exceeds 50 mol% and the ratio of the number of moles of lithium to the number of moles of all metals excluding lithium (Li / Me) exceeds 1.

[0028] In addition, the lithium secondary battery according to the present invention has a discharge curve area in the voltage range of 2.0V to 3.5V of 35% or less, preferably 20% to 35%, when the total discharge curve area is 100% in a dQ / dV graph obtained by differentiating a graph of voltage V vs. battery discharge capacity Q measured while charging to 4.6V at 0.1C and then discharging to 2.0V at 0.1C.

[0029] During discharge of a lithium secondary battery, the peak that appears below 3.5 V is known to be due to the oxygen redox reaction and the redox reaction of cation-mixed Mn ions. In other words, a larger discharge curve area below 3.5 V indicates an increase in oxygen redox reaction and cation mixing. On the other hand, an increase in oxygen redox reaction and cation mixing significantly increases positive electrode resistance and reduces output characteristics. Therefore, in the present invention, the activation conditions during the fabrication of a lithium secondary battery are adjusted so that the discharge curve area in the 2.0 V to 3.5 V voltage range on the dQ / dV graph is 35% or less of the total discharge curve area, thereby suppressing the increase in positive electrode resistance due to cation mixing. When the discharge curve area in the 2.0 V to 3.5 V voltage range on the dQ / dV graph is 35% or less of the total discharge curve area, the resistance of the positive electrode and cell is reduced, resulting in excellent output characteristics. However, if the discharge curve area in the 2.0 V to 3.5 V voltage range is too small, capacity characteristics may be degraded. Therefore, it is more preferable that the area of ​​the discharge curve appearing in the voltage range of 2.0V to 3.5V in the dQ / dV graph is about 20% to 35% of the total discharge curve area.

[0030] Each component of the lithium secondary battery according to the present invention will now be described in more detail.

[0031] positive electrode The positive electrode according to the present invention includes, as a positive electrode active material, a perlithium manganese-based oxide in which the manganese content of all metals excluding lithium exceeds 50 mol % and the ratio of the number of moles of lithium to the number of moles of all metals excluding lithium (Li / Me) exceeds 1. Specifically, the positive electrode according to the present invention includes a positive electrode current collector and a positive electrode active material layer formed on at least one surface of the positive electrode current collector, and the positive electrode active material layer includes a perlithium manganese-based oxide in which the ratio of the number of moles of lithium to the number of moles of all metals excluding lithium (Li / Me) exceeds 1.

[0032] In the case of a lithium-rich manganese-based oxide containing excess lithium, it has a structure in which a layered phase (LiM’O2) and a rock salt phase (Li2MnO3) coexist. During the initial activation process, while the rock salt phase is activated, an excessive amount of lithium ions are generated, enabling the realization of a high capacity.

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

[0034] [Chemical Formula 1] Li a Ni b Co c Mn d M e O2 In Chemical Formula 1 above, M may be at least one selected from the group consisting of Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr, and Zr.

[0035] On the other hand, a is the molar ratio of Li in the lithium-rich manganese-based oxide, and it may be 1 < a, 1.1 ≤ a ≤ 1.5, or 1.1 ≤ a ≤ 1.3. When a satisfies the above range, the irreversible capacity of the silicon-based negative electrode active material can be sufficiently compensated, and high capacity characteristics can be realized.

[0036] The b is the molar ratio of Ni in the lithium-rich manganese-based oxide, and it may be 0 ≤ b ≤ .5, 0.1 ≤ b ≤ 0.4, or 0.2 ≤ b ≤ 0.4.

[0037] The c is the molar ratio of Co in the lithium-rich manganese-based oxide, and it may be 0 ≤ c ≤ 0.1, 0 ≤ c ≤ 0.08, or 0 ≤ c ≤ 0.05. When c exceeds 0.1, it is difficult to ensure a high capacity, and gas generation and degradation of the positive electrode active material may deepen, resulting in a decrease in the life characteristics.

[0038] The d is the molar ratio of Mn in the perlithium manganese-based oxide and may be 0.5≦d<1.0, 0.50≦d≦0.80, or 0.50≦d≦0.70. If d is less than 0.5, the ratio of the rock salt phase is too small, resulting in little negative electrode irreversible compensation and capacity improvement effects.

[0039] The e is the molar ratio of the doping element M in the perlithium manganese-based oxide, and may be 0≦e≦0.2, 0≦e≦0.1, or 0≦e≦0.05. If the content of the doping element is too high, it may adversely affect the capacity of the active material.

[0040] Meanwhile, in the perlithium manganese-based oxide represented by [Chemical Formula 1], the molar ratio of Li to the total molar number of metal elements excluding Li (Li / Me) may be 1.2 to 1.5, 1.25 to 1.5, or 1.25 to 1.4. When the Li / Me ratio satisfies this range, excellent rate characteristics and capacity characteristics are demonstrated. If the Li / Me ratio is too high, electrical conductivity may decrease and the rock salt phase (Li2MnO3) may increase, resulting in an accelerated degradation rate. If the Li / Me ratio is too low, the effect of improving energy density may be minimal.

[0041] Meanwhile, the composition of the perlithium manganese oxide can also be expressed by the following [Chemical Formula 2]. [Chemical formula 2] X Li2MnO3·(1-X)Li[Ni 1-y-z-w Mn y Co z M w ]O2 In the above [Chemical Formula 2], M may be at least one selected from the group consisting of metal ions Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr, and Zr.

[0042] The X represents the ratio of the Li2MnO3 phase in the perlithium manganese-based oxide, and may be 0.2≦X≦0.5, 0.25≦X≦0.5, or 0.25≦X≦0.4. When the ratio of the Li2MnO3 phase in the perlithium manganese-based oxide satisfies this range, the irreversible capacity of the silicon-based negative electrode active material can be sufficiently compensated for, and high capacity characteristics can be achieved.

[0043] The y is the molar ratio of Mn in the LiM'O2 layered phase, and may be 0.4≦y<1, 0.4≦y≦0.8, or 0.4≦y≦0.7.

[0044] The z is the molar ratio of Co in the LiM'O2 layered phase and may be 0≦z≦0.1, 0≦z≦0.08, or 0≦z≦0.05. If z exceeds 0.1, gas generation and degradation of the positive electrode active material may be intensified, resulting in reduced life characteristics.

[0045] The w is the molar ratio of the doping element M in the LiM'O2 layered phase, and may be 0≦w≦0.2, 0≦w≦0.1, or 0≦w≦0.05.

[0046] Meanwhile, the positive electrode active material according to the present invention may further include a coating layer on the surface of the perlithium manganese-based oxide, if necessary. When the positive electrode active material includes a coating layer, the coating layer may prevent contact between the perlithium manganese-based oxide and the electrolyte, thereby reducing electrolyte side reactions and improving life characteristics.

[0047] The coating layer is made of a coating element M 1 The coating element M 1 For example, the coating element M may be at least one selected from the group consisting of Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr, and Zr, and may be preferably Al, Co, Nb, W, and combinations thereof, and more preferably Al, Co, and combinations thereof. 1may contain two or more kinds, for example, Al and Co.

[0048] The coating elements are in the oxide form in the coating layer, i.e., M 1 It can exist as Oz (1≦z≦4).

[0049] The coating layer can be formed by dry coating, wet coating, chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD), etc. Among these, atomic layer deposition is preferred because it allows the coating layer to be formed over a large area.

[0050] The area where the coating layer is formed may be 10 to 100%, preferably 30 to 100%, and more preferably 50 to 100% of the total surface area of ​​the perlithium manganese-based oxide particles. When the area where the coating layer is formed satisfies the above range, the effect of improving the life characteristics is excellent.

[0051] Meanwhile, the cathode active material according to the present invention may be in the form of secondary particles formed by agglomeration of a plurality of primary particles, and the average particle diameter D 50 The D of the positive electrode active material may be 2 μm to 10 μm, preferably 2 μm to 8 μm, and more preferably 4 μm to 8 μm. 50 When the above range is satisfied, excellent electrode density can be achieved and the deterioration of capacity and rate characteristics can be minimized.

[0052] The positive electrode active material has a BET specific surface area of ​​1 m 2 / g~10m 2 / g, 3-8m 2 / g or 4~6m 2 If the BET specific surface area of ​​the positive electrode active material is too low, the reaction area with the electrolyte is insufficient, making it difficult to achieve sufficient capacity, whereas if the specific surface area is too high, moisture absorption is rapid, accelerating side reactions with the electrolyte and making it difficult to ensure life characteristics.

[0053] Meanwhile, the perlithium manganese-based oxide can be prepared by mixing a transition metal precursor and a lithium source material and then calcining the mixture.

[0054] Examples of the lithium source material include lithium-containing carbonates (e.g., lithium carbonate, etc.), hydrates (e.g., lithium hydroxide hydrate (LiOH·H2O)), hydroxides (e.g., lithium hydroxide, etc.), nitrates (e.g., lithium nitrate (LiNO3), etc.), and chlorides (e.g., lithium chloride (LiCl)), and any of these may be used alone or in combination.

[0055] Meanwhile, the transition metal precursor may be in the form of a hydroxide, oxide, or carbonate. When a carbonate precursor is used, it is more preferable in that a positive electrode active material having a relatively high specific surface area can be prepared.

[0056] The transition metal precursor can be prepared through a co-precipitation process. For example, the transition metal precursor can be prepared by dissolving each transition metal-containing raw material in a solvent to prepare a metal solution, and then mixing the metal solution, an ammonium cation complexing agent, and a basic compound, followed by a co-precipitation reaction. If necessary, an oxidizing agent or oxygen gas can be further added during the co-precipitation reaction.

[0057] In this case, the transition metal-containing raw material may be acetate, carbonate, nitrate, sulfate, halide, sulfide, etc. of each transition metal. Specifically, the transition metal-containing raw material may be NiO, NiCO3·2Ni(OH)2·4H2O, NiC2O2·2H2O, Ni(NO3)2·6H2O, NiSO4, NiSO4·6H2O, Mn2O3, MnO2, Mn3O4MnCO3, Mn(NO3)2, MnSO4·H2O, manganese acetate, manganese halide, cobalt oxide, cobalt carbonate, cobalt sulfate, cobalt acetate, cobalt halide, etc.

[0058] The ammonium cation complexing agent may be at least one selected from the group consisting of NH4OH, (NH4)2SO4, NH4NO3, NH4Cl, CH3COONH4, and NH4CO3.

[0059] The basic compound may be at least one selected from the group consisting of NaOH, Na2CO3, KOH, and Ca(OH)2. The form of the precursor may vary depending on the type of basic compound used. For example, when NaOH is used as the basic compound, a hydroxide-form precursor can be obtained, and when Na2CO3 is used as the basic compound, a carbonate-form precursor can be obtained. Furthermore, when a basic compound and an oxidizing agent are used together, an oxide-form precursor can be obtained.

[0060] Meanwhile, the transition metal precursor and the lithium source material may be mixed in amounts such that the total transition metal (Ni+Co+Mn):Li molar ratio 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.

[0061] The firing may be carried out at a temperature of 600 to 1000°C or 700 to 950°C for a period of 5 to 30 hours or 5 to 20 hours. The firing atmosphere may be air or oxygen, for example, an atmosphere containing 20 to 100% by volume of oxygen.

[0062] Meanwhile, the positive electrode active material layer may further include a conductive material and a binder in addition to the positive electrode active material.

[0063] Examples of the conductive material include spherical or flaky graphite; carbonaceous materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, single-walled carbon nanotubes, and multi-walled carbon nanotubes; metal powder or metal fiber such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive polymers such as polyphenylene derivatives, and the like. These materials may be used alone or in combination. The conductive material may be included in an amount of 0.1 to 20 wt %, 1 to 20 wt %, or 1 to 10 wt %, based on the total weight of the positive electrode active material layer.

[0064] Examples of the binder include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof, and these may be used alone or in combination. The binder may be included in an amount of 1 to 20 wt %, 2 to 20 wt %, or 2 to 10 wt %, based on the total weight of the positive electrode active material layer.

[0065] negative electrode The negative electrode may include, for example, a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. The negative electrode active material layer may optionally include a binder and a conductive material in addition to the negative electrode active material.

[0066] The negative electrode current collector may be any material that does not induce chemical changes in the battery and has high conductivity, and examples of such materials include copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surfaces treated with carbon, nickel, titanium, silver, or aluminum-cadmium alloys. The negative electrode current collector typically has a thickness of 3 to 500 μm, and, like the positive electrode current collector, the current collector surface may be provided with fine irregularities to enhance the binding strength of the negative electrode active material. The negative electrode current collector may be in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0067] The negative electrode active material may be a compound capable of reversible intercalation and deintercalation of lithium. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, and Al alloys; and SiO. β (0<β<2), metal oxides that can be doped and dedoped with lithium, such as SnO2, vanadium oxide, and lithium vanadium oxide; or composites containing the above metallic compounds and carbonaceous materials, such as Si-C composites or Sn-C composites, and any one or a mixture of two or more of these can be used.

[0068] In addition, a thin film of metallic lithium may be used as the negative electrode active material. Carbon materials can be either low-crystalline or high-crystalline. Typical low-crystalline carbons include soft carbon and hard carbon, while typical high-crystalline carbons include amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite, kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and high-temperature-fired carbons such as petroleum or coal tar pitch-derived cokes.

[0069] The conductive material is used to impart conductivity to the electrode and can be any material that does not cause chemical changes in the resulting battery and has electronic conductivity. Specific examples include graphite, such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, and carbon nanotubes; metal powders or fibers, such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; and conductive polymers, such as polyphenylene derivatives. These materials may be used alone or in combination. The conductive material may typically be present in an amount of 1 to 30 wt %, preferably 1 to 20 wt %, and more preferably 1 to 10 wt %, based on the total weight of the negative electrode active material layer.

[0070] The binder improves adhesion between negative electrode active material particles and between the negative electrode active material and the negative electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These may be used alone or in combination. The binder may be present in an amount of 1 to 30 wt %, preferably 1 to 20 wt %, and more preferably 1 to 10 wt %, based on the total weight of the negative electrode active material layer.

[0071] For example, the negative electrode active material layer may be prepared by coating a negative electrode slurry containing a negative electrode active material, and optionally a binder and a conductive material, on a negative electrode current collector and drying the coating. Alternatively, the negative electrode slurry may be cast on a separate support, and then peeled off from the support to form a film, which may be laminated on the negative electrode current collector.

[0072] Separation membrane The lithium secondary battery according to the present invention may further include a separator interposed between the positive and negative electrodes. The separator separates the negative and positive electrodes and provides a path for lithium ions to move. Any separator commonly used in lithium secondary batteries may be used without any particular limitations. A separator with low resistance to electrolyte ion movement and excellent electrolyte humidification capacity is particularly preferred. Specifically, porous polymer films, such as those made from polyolefin-based polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminates of two or more layers thereof, may be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made from high-melting-point glass fibers or polyethylene terephthalate fibers, may also be used. Separators coated with ceramic components or polymeric materials to ensure heat resistance or mechanical strength may also be used, and may be used in either a single-layer or multi-layer structure.

[0073] electrolyte The electrolyte is not particularly limited and may be any of various electrolytes that can be used in lithium secondary batteries, such as organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, or combinations thereof.

[0074] For example, the electrolyte can include an organic solvent and a lithium salt.

[0075] The organic solvent may be any solvent capable of acting as a medium for the movement of ions involved in the electrochemical reaction of the battery. Specific examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether and tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate. Examples of solvents that can be used include carbonate solvents such as PC (carbonate); alcohol solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (where R is a hydrocarbon group having 2 to 20 carbon atoms and having a linear, branched, or cyclic structure, which may contain a double-bonded aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; and sulfolanes.

[0076] The lithium salt can be used without any particular limitation as long as it is a compound that can provide lithium ions used in lithium secondary batteries. Specifically, the anion of the lithium salt can be F - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4, CF3CF2SO3 - , (CF3SO2)2N- , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - , and (CF3CF2SO2)2N - The lithium salt may be at least one selected from the group consisting of LiPF6, LiN(FSO2)2, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, etc. The concentration of the lithium salt is preferably within a range of 0.1 to 5.0M.

[0077] In addition to the above components, the electrolyte may further contain additives for purposes such as improving battery life, suppressing battery capacity loss, and improving battery discharge capacity. Examples of additives include, but are not limited to, haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphate, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, and aluminum trichloride, which may be used alone or in combination. The additives may be present in an amount of 0.1 to 10 wt %, preferably 0.1 to 5 wt %, based on the total weight of the electrolyte.

[0078] <Method of manufacturing lithium secondary batteries> Next, a method for producing a lithium secondary battery according to the present invention will be described.

[0079] The method for manufacturing a lithium secondary battery according to the present invention includes: (1) preparing a battery cell including a positive electrode, a negative electrode, and an electrolyte; and (2) activating the battery cell, wherein the activation step includes a first charging step of charging the battery cell at a C-rate of less than 0.5 C; and a second charging step of charging the battery cell at a C-rate of 0.5 C or more.

[0080] (1) Preparing the battery cells First, a battery cell including a positive electrode, a negative electrode, and an electrolyte is prepared.

[0081] The battery cell may be manufactured by, for example, forming an electrode assembly including a positive electrode and a negative electrode, housing the electrode assembly in a battery case, injecting an electrolyte, and sealing the battery case. In this case, the electrode assembly may include a separator between the positive electrode and the negative electrode.

[0082] The positive electrode, negative electrode, separator, and electrolyte are the same as those described above, and therefore detailed description thereof will be omitted.

[0083] Meanwhile, the electrode assembly may be an electrode assembly of various types well known in the art, for example, a jelly roll type, a stack type, a stack and lamination type, or a stack and folding type, and the type is not particularly limited.

[0084] The jelly roll type electrode assembly can be manufactured by interposing a sheet-shaped separator between a sheet-shaped positive electrode and a sheet-shaped negative electrode, and then winding the electrodes in one direction.

[0085] The stacked electrode assembly can be manufactured by cutting a positive electrode, a separator, and a negative electrode into a desired shape, and then stacking the cut positive electrode / separator / negative electrode in order.

[0086] The stack and lamination type electrode assembly may be manufactured by stacking a positive electrode, a separator, and a negative electrode to form a plurality of unit cells, stacking the unit cells with a separator interposed therebetween, and laminating the stacked unit cells by a method such as heating.

[0087] The stack-and-fold type electrode assembly may be manufactured by stacking a positive electrode, a separator, and a negative electrode to form a plurality of unit cells, arranging the unit cells on one or both sides of a long folding separator, and then winding up the folding separator.

[0088] Meanwhile, the battery case is not particularly limited and may be any of various battery cases known in the art, such as a cylindrical battery case, a square battery case, or a pouch-type battery case.

[0089] (2) Activation stage Next, an activation step is performed in which the battery cell is charged and discharged at least once to electrically activate the battery. The activation step is a process of charging and discharging the battery cell to impart electrical characteristics and forming a solid electrolyte interphase (SEI) film on the electrodes to stabilize the battery.

[0090] In the present invention, the charging in the activation process is performed in two or more stages with different rate limiting steps, thereby forming a robust SEI film on the electrode and simultaneously minimizing the occurrence of oxygen desorption and cation mixing during the activation process.

[0091] Specifically, the charging includes a first charging stage in which the battery cells are charged at a C-rate less than 0.5C; and a second charging stage in which the battery cells are charged at a C-rate equal to or greater than 0.5C.

[0092] The first charging step is performed at a rate (C-rate) of less than 0.5 C, preferably 0.05 C or more but less than 0.5 C. If the current rate during the first charging step is fast, at 0.5 C or more, an SEI film may be formed unstably on the electrode surface. If the SEI film is formed unstably on the electrode surface, it may be easily decomposed during battery operation, causing rapid deterioration of the electrode and significantly reducing the lifespan characteristics.

[0093] Meanwhile, the first charging step is preferably performed from an SOC of 0 until the SOC of the battery cell reaches 10 to 40. When the charge 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.

[0094] Meanwhile, the first charging step can be performed in two or more stages with different rate limits, if necessary. For example, the first charging step can include a 1-1 charging step in which the battery cell is charged at a C-rate of 0.1 to less than 0.3 C, and a 1-2 charging step in which the battery cell is charged at a C-rate higher than the C-rate of the 1-1 charging step but less than 0.5 C. In this case, the 1-1 charging step is performed from SOC 0 to SOC 3 to 5, and the 1-2 charging step is performed from SOC 3 to 5 to SOC 10 to 40. When the first charging step is performed in two stages as described above, the current rate at the initial stage of SEI film formation can be maintained lower, thereby forming a stronger and denser SEI film.

[0095] Meanwhile, the first charging step can be performed in a constant current mode (CC mode).

[0096] The second charging step is then performed at a C-rate of 0.5 C or higher, preferably 0.5 C to 2 C. When the second charging step is performed at a C-rate of 0.5 C or higher, activation of the Li2MnO3 (monoclinic) phase in the crystalline structure of the positive electrode active material is reduced, thereby suppressing oxygen desorption and cation mixing during the activation process. Furthermore, performing the second charging step at a relatively fast C-rate reduces the time required for the activation process, thereby shortening battery production time.

[0097] Meanwhile, the second charging step is preferably performed after the first charging step, i.e., until the SOC of the battery cell reaches from 10 to 40 to 90 to 150. When the charge capacity in the second charging step satisfies the above range, the incomplete formation of the SEI film can be suppressed, and oxygen desorption and cation mixing generation during the activation process can be reduced, thereby minimizing an increase in the positive electrode resistance.

[0098] The second charging step can be performed in a constant current mode (CC mode) or a constant current-constant voltage mode (CC-CV mode).

[0099] In the second charging step, the end-of-charge 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 end-of-charge voltage in the second charging step satisfies the above range, the perlithium manganese-based oxide is activated, thereby achieving high capacity characteristics.

[0100] Next, the battery cell charged through the first and second charging steps is discharged. At this time, the discharge can be performed at a C-rate of 0.1 C to 1 C, preferably 0.3 C to 1 C. When the discharge rate satisfies this range, the activation process time can be shortened.

[0101] On the other hand, the discharge can be carried out in a constant current mode (CC mode).

[0102] On the other hand, the discharge cut-off voltage may be 2.0V to 3.0V.

[0103] Preferably, in the present invention, the charging and discharging in the activation step can be carried out in a voltage range of 2.0V to 4.6V.

[0104] Meanwhile, the activation process is preferably carried out at a temperature of 25°C to 55°C, and more preferably 30°C to 45°C. When the activation process is carried out within this temperature range, activation is smoothly carried out even at a relatively high C-rate. If the activation temperature is too low, lithium ion mobility may decrease, preventing smooth activation, while if the activation temperature is too high, it may cause the elution of transition metals such as manganese.

[0105] Furthermore, the activation process may be performed under pressurized conditions, if necessary. The pressurization may be performed by mounting the battery cell in a jig and applying pressure to the battery cell through the jig. Performing the activation process under pressurized conditions has the advantage of facilitating the release of gases generated during the activation process.

[0106] Meanwhile, although not essential, if necessary, the activation step may further include an aging step, which is performed to allow the electrolyte to be uniformly impregnated into the electrode assembly and stabilize the battery, and may be performed before charging, during charging, and / or after discharging, and may be performed one or more times.

[0107] The aging step can be carried out at a temperature of, for example, 20° C. to 60° C., preferably 20° C. to 50° C., and more preferably 30° C. to 50° C. When aging is carried out at the above temperatures, electrolyte impregnation and lithium mobility are improved, and activation proceeds more smoothly.

[0108] The lithium secondary battery manufactured through the activation process may have a discharge curve area of ​​35% or less, preferably 20% to 35%, in the voltage range of 2.0 to 3.5 V, when the total discharge curve area is 100% in a dQ / dV graph obtained by differentiating a graph of voltage V vs. battery discharge capacity Q measured while charging to 4.6 V at 0.1 C and then discharging to 2.0 V at 0.1 C.

[0109] The present invention will be described in more detail below through specific examples. [Example]

[0110] Example 1 The positive electrode active material, conductive material, and PVDF binder were mixed in N-methylpyrrolidone in a weight ratio of 96:1.5:2.5 to prepare a positive electrode slurry. 0.595 Co 0.005 Mn 0.4 ]O2 was used as the cathode material, and carbon black was used as the conductive material. The cathode slurry was applied to an aluminum current collector sheet, dried, and then rolled to prepare a cathode.

[0111] Anode active material, conductive material, and binder were mixed in water in a weight ratio of 97:1:2 to prepare anode slurry. Graphite was used as the anode active material, carbon black as the conductive material, and a mixture of SBR (styrene-butadiene) and carboxymethyl cellulose (CMC) was used as the binder. The anode slurry was applied to a copper current collector sheet, dried, and then rolled to prepare anodes.

[0112] An electrode assembly was fabricated by interposing a separator between the cathode and anode fabricated as described above, and the electrode assembly was inserted into a battery case, followed by injecting an electrolyte solution to fabricate a battery cell.

[0113] The battery cell was charged at 45°C in a constant current mode at 0.2C to SOC 3 (1-1 charging stage), and then charged at a constant current mode at 0.3C to SOC 30 (1-2 charging stage). Thereafter, it was charged at a constant current mode at 0.5C to 4.6V (2nd charging stage), and then discharged at a constant current of 0.5C to 2.0V to perform an activation process, thereby producing a lithium secondary battery.

[0114] Example 2 A lithium secondary battery was manufactured in the same manner as in Example 1, except that charging was performed at 1.0 C in the second charging step.

[0115] Example 3 A lithium secondary battery was manufactured in the same manner as in Example 1, except that charging was performed at 1.5 C in the second charging step.

[0116] Comparative Example 1 A lithium secondary battery was manufactured in the same manner as in Example 1, except that the second charging step was performed in a constant current-constant voltage mode (0.05C CV-cut) at 0.3C.

[0117] Experimental Example 1 The secondary batteries prepared in the examples and comparative examples were charged to 4.60 V at 25°C in a 0.1 C constant current-constant voltage mode (CC-CV mode, CV mode cut-off condition: 0.05 C) and then discharged to 2.0 V at a constant current of 0.1 C to measure a voltage-discharge capacity graph, which was then differentiated to obtain a dQ / dV graph. The measured dQ / dV graph is shown in Figure 1. The percentage of the discharge curve area in the 2 V to 3.5 V range relative to the total discharge curve area (i.e., the discharge curve area in the 2.0 V to 4.6 V range) is shown in Table 1 below.

[0118] [Table 1]

[0119] As shown in Table 1 and FIG. 1, the lithium secondary batteries of Examples 1 to 3, which were manufactured by performing the second charge at a rate of 0.5 C or more in the activation process, showed a discharge curve area in the 2 V to 3.5 V region of the total discharge curve area of ​​35% or less, while the lithium secondary battery of Comparative Example 1, which was performed by performing the second charge at a rate of 0.3 C, showed a discharge curve area in the 2 V to 3.5 V region of more than 40%.

[0120] Experimental Example 2 A three-electrode cell (reference electrode: LTO) was prepared using each of the positive electrodes prepared in Examples 1 to 3 and Comparative Example 1 as a working electrode, and the positive electrode resistance was measured by EIS (Electrochemical Impedance Spectroscopy) analysis and DCIR (Direct Current Internal Resistance) analysis of the three-electrode cell.

[0121] The EIS analysis results are shown in Figure 2, and the DCIR analysis results are shown in Figure 3.

[0122] From FIG. 2, it can be seen that the lithium secondary batteries of Examples 1 to 3 have lower impedance values ​​than the lithium secondary battery of Comparative Example 1, and the frequency (Hz) at which impedance appears is also increased, indicating that the kinetics of the resistance response are also improved.

[0123] 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, the positive electrode comprising a perlithium manganese-based oxide in which the manganese content in all metals excluding lithium exceeds 50 mol% and the ratio of the number of moles of lithium to the number of moles of all metals excluding lithium (Li / Me) exceeds 1; The lithium secondary battery is charged to 4.6 V at 0.1 C and then discharged to 2.0 V at 0.1 C. The graph of voltage V vs. battery discharge capacity Q is then differentiated to obtain a dQ / dV graph. When the total discharge curve area is 100%, the discharge curve area in the voltage range of 2.0 to 3.5 V is 35% or less.

2. 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% or less.

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

4. 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. Preparing a battery cell including a cathode, an anode, and an electrolyte, the cathode including a perlithium manganese-based oxide in which the manganese content in all metals excluding lithium exceeds 50 mol % and the ratio of the number of moles of lithium to the number of moles of all metals excluding lithium (Li / Me) exceeds 1; and activating the battery cell by charging and discharging the battery cell at least once; The activation step includes a first charging step of charging the battery cell at a C-rate of less than 0.5 C, and a second charging step of charging the battery cell at a C-rate of 0.5 C or more.

6. 6. The method for manufacturing a lithium secondary battery according to claim 5, wherein the perlithium manganese-based oxide is represented by the following Chemical Formula 1: [Chemical formula 1] Li ａ Ni ｂ Co ｃ Mn ｄ M ｅ O ２ In the above Chemical Formula 1, 1<a, 0≦b≦0.5, 0≦c≦0.1, 0.5≦d<1.0, 0≦e≦0.2, and M is at least one selected from the group consisting of Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr, and Zr.

7. 7. The method of 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 after the first charging step until the SOC of the battery cell reaches 90 to 150.

8. The method of claim 5, wherein the first charging step and the second charging step are performed in a constant current (CC) mode.

9. 6. The method of 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 (CC-CV mode).

10. 6. The method for manufacturing a lithium secondary battery according to claim 5, wherein an end-of-charge voltage in the second charging stage is 4.4 V or more.

11. 6. The method for producing a lithium secondary battery according to claim 5, wherein the charging and discharging are carried out in a voltage range of 2.0 V to 4.6 V.

12. 6. The method for manufacturing a lithium secondary battery according to claim 5, wherein the lithium secondary battery is charged to 4.6 V at 0.1 C and then discharged to 2.0 V at 0.1 C, and the graph of voltage V vs. battery discharge capacity Q is differentiated to obtain a dQ / dV graph. When the total discharge curve area is 100%, the discharge curve area in a voltage range of 2.0 to 3.5 V is 35% or less.

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