Positive electrode slurry for lithium secondary battery and method for preparing positive electrode for lithium secondary battery using same

Abstract

The present invention relates to: a positive electrode slurry for a lithium secondary battery, the positive electrode slurry comprising: a positive electrode active material comprising a lithium nickel-based oxide having a composition in which the amount of nickel in all metals other than lithium is 50 mol% or more; and is in the form of at least one of a single particle or a quasi-single particle that is a secondary particle in which 30 or less primary particles are agglomerated; a lithium borate compound; a binder; a conductive material; and a solvent; a positive electrode for a lithium secondary battery including a positive electrode active material layer formed by coating at least one surface of a positive electrode current collector with a positive electrode slurry; a method for preparing a positive electrode; and a lithium secondary battery including the positive electrode.

Description

Technical Field

[0001] This application claims the benefit of priority of Korean Patent Application No. 10-2023-0009079, filed on January 20, 2023, the disclosure of which is incorporated herein by reference.

[0002] The present invention relates to a positive electrode paste for a lithium secondary battery that can help improve performance in a high-temperature and high-voltage operating environment while using a positive electrode active material in the form of single particles or quasi-single particles without performing a washing process and a boron coating process during the synthesis process, a method for preparing a positive electrode for a lithium secondary battery using the same, a positive electrode for a lithium secondary battery prepared by the preparation method, and a lithium secondary battery including the positive electrode. Background Art

[0003] A lithium secondary battery generally includes a positive electrode, a negative electrode, a separator, and an electrolyte, and the positive electrode and the negative electrode each include an active material capable of intercalating and deintercalating lithium ions.

[0004] Lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2 or LiMn2O4), or a lithium iron phosphate compound (LiFePO4) has been used as a positive electrode active material for a lithium secondary battery. Among them, lithium cobalt oxide has the advantages of a high operating voltage and excellent capacity characteristics, but it is difficult to be commercially applied in large-capacity batteries due to the high price and unstable supply of cobalt as a raw material. Because of the poor structural stability of lithium nickel oxide, it is difficult to achieve sufficient life characteristics. Lithium manganese oxide has excellent stability, but has a problem of poor capacity characteristics. Therefore, in order to compensate for the problems of lithium transition metal oxides containing only Ni, Co, or Mn, lithium composite transition metal oxides containing two or more transition metals have been developed, and among them, lithium nickel cobalt manganese oxide containing Ni, Co, and Mn has been widely used in the field of electric vehicle batteries.

[0005] Conventional lithium nickel cobalt manganese oxide is generally in the form of spherical secondary particles in which dozens to hundreds of primary particles are aggregated. However, for lithium nickel cobalt manganese oxide in the form of secondary particles in which many primary particles are aggregated as described above, there are problems that particle breakage may occur (wherein, during the preparation of the positive electrode, the primary particles are detached during the rolling process) and cracks may appear in the particles during the charge and discharge process. In the case where particle breakage or cracking occurs in the positive electrode active material, since the contact area with the electrolyte increases, the deterioration of the active material and the generation of gas due to side reactions with the electrolyte increase, and as a result, there is a problem of reduced life characteristics.

[0006] In addition, the demand for high-output and high-capacity batteries such as batteries for electric vehicles has been increasing recently. Therefore, the nickel content in the positive electrode active material has a tendency to gradually increase. When the nickel content in the positive electrode active material increases, the initial capacity characteristics are improved. However, if charge and discharge are repeated, the structure of the positive electrode active material collapses. As a result, there is a problem that the deterioration rate of the positive electrode active material increases, deteriorating the battery life characteristics and reducing the battery safety.

[0007] To solve the above problems, a technique has been proposed for preparing a positive electrode active material in the form of single particles rather than secondary particles by increasing the sintering temperature during the preparation of lithium nickel cobalt manganese oxide. For the positive electrode active material in the form of single particles, since the contact area with the electrolyte is smaller than that of the conventional positive electrode active material in the form of secondary particles, the side reaction with the electrolyte is less, and since the particle strength is excellent, there is less particle breakage during electrode preparation. Therefore, when using the positive electrode active material in the form of single particles, there are advantages of excellent gas generation and life characteristics.

[0008] However, for the conventional positive electrode active material in the form of single particles, since there are fewer interfaces between primary particles that serve as the migration path of lithium ions in the particles, the lithium mobility is low, and since it is prepared at a relatively high sintering temperature, there are excessive lithium by-products on its surface. When there are excessive lithium by-products on the surface of the positive electrode active material, due to the side reaction between the lithium by-products and the electrolyte during high-temperature storage, the gas generation amount increases. Washing to remove the lithium by-products can reduce the gas generation amount, but there is a problem of an increase in resistance due to the destruction of the surface structure of the positive electrode active material during washing. Summary of the Invention

[0009] Technical Problem

[0010] The present invention aims to provide a positive electrode paste that can contribute to improving the cycle characteristics under high-temperature and high-voltage operating conditions, a positive electrode using the same, a method for preparing a positive electrode, and a lithium secondary battery including the positive electrode.

[0011] Technical Solution

[0012] According to one embodiment, the present invention provides a positive electrode paste for a lithium secondary battery, the positive electrode paste for a lithium secondary battery comprising:

[0013] a positive electrode active material, the positive electrode active material comprising a lithium nickel-based oxide having a composition in which the amount of nickel among all metals except lithium is 50 mol% or more, and being in at least one form of single particles or quasi-single particles which are secondary particles in which 30 or fewer primary particles are aggregated;

[0014] Lithium borate compounds;

[0015] Binder;

[0016] Conductive material; and

[0017] Solvent.

[0018] According to another embodiment, the present invention provides a positive electrode for a lithium secondary battery, the positive electrode for a lithium secondary battery comprising a positive electrode active material layer formed by coating at least one surface of a positive electrode current collector with a positive electrode paste. [[ID=1y]]

[0019] According to another embodiment, the present invention provides a positive electrode for a lithium secondary battery, the positive electrode for a lithium secondary battery comprising:

[0020] A positive electrode active material comprising a lithium nickel-based oxide having a composition in which the amount of nickel among all metals other than lithium is 50 mol% or more, and being in at least one form of single particles or quasi-single particles in which secondary particles are agglomerated with 30 or less primary particles; and

[0021] Lithium borate compounds.

[0022] According to another embodiment, the present invention provides a method for preparing a positive electrode for a lithium secondary battery, the method comprising the following steps:

[0023] Preparing a positive electrode paste by mixing: a positive electrode active material comprising a lithium nickel-based oxide having a composition in which the amount of nickel among all metals other than lithium is 50 mol% or more, and being in at least one form of single particles or quasi-single particles in which secondary particles are agglomerated with 30 or less primary particles; lithium borate compounds; a binder; a conductive material; and a solvent, and

[0024] Forming a positive electrode active material layer by coating at least one surface of a positive electrode current collector with the positive electrode paste.

[0025] According to another embodiment, the present invention provides a lithium secondary battery, the lithium secondary battery comprising:

[0026] A positive electrode;

[0027] A negative electrode comprising a negative electrode active material;

[0028] A separator disposed between the positive electrode and the negative electrode; and

[0029] An electrolyte.

[0030] Beneficial effects

[0031] When preparing a positive electrode by using the positive electrode paste according to the present invention, without performing a boron coating and a washing process during the synthesis process of the lithium nickel-based oxide, the surface of the particles can be stabilized and the capacity development ratio can be increased. Ultimately, the effect of improving the performance of the lithium secondary battery at high temperature and high voltage can be achieved.

[0032] According to the conventional general process for preparing a positive electrode active material, after sintering, the residual lithium present on the surface of the lithium transition metal oxide particles is removed through a washing process. In this case, while the surface of the particles reacts with water, damage or collapse of the layered structure may occur, which may lead to a decrease in the capacity and rate performance of the battery and an increase in resistance at high temperature.

[0033] To prevent damage caused by this washing process, a method of forming a boron coating on the surface of the particles is mainly used. In this case, a high-temperature sintering process is required. Since boron grows in a plate shape, the hardness of the particles increases, which may increase the difficulty during subsequent rolling. In addition, due to its amorphous nature, the boron coating may cause an increase in the surface resistance of the particles. Particularly, for the positive electrode active material in the form of single particles, since the lithium mobility is relatively lower compared to the positive electrode active material in the form of secondary particles, not only is the initial resistance high, but the surface resistance may also be further increased because if the sintering temperature is increased to form single particles, it is likely to turn into a rock salt phase, which is an electrochemically inert phase.

[0034] Therefore, in the present invention, a lithium borate compound is introduced into the positive electrode paste, and it is confirmed that during the process of preparing the positive electrode active material, the effect of boron coating can be achieved without introducing a boron coating process and a washing process. That is, without experiencing the problems of an increase in surface resistance caused by the boron coating and surface damage caused by the washing process, effects such as stabilization of the particle surface, suppression of side reactions, and improvement of capacity can be achieved.

[0035] In addition, during the electrolyte injection and battery activation process, while extracting the lithium borate compound dispersed in the positive electrode by dissolving the lithium borate compound in the electrolyte, pores are formed in the positive electrode. Since the pores thus ensured are impregnated with the electrolyte, not only can the residual amount of the electrolyte be increased, but also, because the lithium borate compound dissolved in the electrolyte replaces the LiPF6 salt to ensure additional lithium ions, it can also contribute to improving the battery life by suppressing the decomposition of the electrolyte. Brief Description of the Drawings

[0036] Figure 1 is a graph showing the results of the room temperature life evaluation of the battery using the positive electrodes of the examples and the comparative examples.

[0037] Figure 2It is a graph showing the high-temperature life evaluation results of batteries using the positive electrodes of the examples and comparative examples. Detailed Description of the Invention

[0038] Each component of the present invention will be described in more detail below.

[0039] In the present invention, the term "primary particle" refers to a particle unit that does not have grain boundaries in appearance when observed using a scanning electron microscope with a field of view of 5,000 times to 20,000 times, and the term "secondary particle" refers to a particle formed by the aggregation of multiple primary particles.

[0040] The term "average particle size of primary particles" in the present invention refers to the arithmetic average of the particle sizes calculated after measuring the particle sizes of at least 20 primary particles observed in a scanning electron microscope image. In this case, the particle size refers to the diameter of the longest axis of the primary particle.

[0041] The term "secondary particle" in the present invention refers to a particle formed by the aggregation of multiple primary particles. In the present invention, a secondary particle in which 30 or fewer primary particles are aggregated is called a quasi-single particle, thereby distinguishing it from a conventional secondary particle formed by the aggregation of dozens to hundreds of primary particles.

[0042] The term "D50" in the present invention refers to the particle size corresponding to 50% of the cumulative volume in the volume-based cumulative particle size distribution of the corresponding powder, which can be measured using the laser diffraction method. For example, after dispersing the positive electrode active material powder in a dispersion medium, the dispersion medium is introduced into a commercially available laser diffraction particle size analyzer (e.g., S-3500 of Microtrac), and then irradiated with ultrasonic waves at about 28 kHz with an output of 60 W to obtain a volume-based cumulative particle size distribution diagram, and D50 can be measured by the following method: in the obtained volume-based cumulative particle size distribution diagram, obtain the particle size at 50% of the cumulative volume.

[0043] Positive electrode paste

[0044] The positive electrode slurry according to the present invention contains: a positive electrode active material; a lithium borate compound; a binder; a conductive material; and a solvent, wherein the positive electrode active material is a lithium nickel-based oxide, the lithium nickel-based oxide has a composition in which the amount of nickel in all metals except lithium is 50 mol% or more, and is in at least one form of single particles or quasi-single particles as secondary particles in which 30 or fewer primary particles are aggregated.

[0045] In one embodiment of the present invention, the lithium borate compound can be lithium tetraborate (Li2B4O7). Since the reactivity of lithium tetraborate with moisture is lower than that of other lithium salt compounds such as LiPF6, LiBF4, LiN(SO2CF2CF3)2, LiNO3, and LiB(C2O4)2 with moisture, it is advantageous in preventing electrolyte decomposition reactions, current collector corrosion, and positive electrode transition metal dissolution caused by side reactions with moisture.

[0046] Based on 100 parts by weight of the lithium nickel-based oxide, the content of the lithium borate compound can be from 0.005 parts by weight to 0.5 parts by weight, preferably from 0.005 parts by weight to 0.1 parts by weight, and more preferably from 0.007 parts by weight to 0.05 parts by weight. When the content of the lithium borate compound is 0.005 parts by weight or more based on 100 parts by weight of the lithium nickel-based compound, the effects caused by the addition of the lithium borate compound can be fully realized, and it is desirable to contain 0.5 parts by weight or less of the lithium nickel-based compound in terms of preventing hardening due to boron and making rolling difficult.

[0047] The amount of nickel among all the metals other than lithium in the lithium nickel-based oxide can be 55 mol% or more, preferably 60 mol% or more.

[0048] Specifically, the lithium nickel-based oxide can have the composition of Chemical Formula 1 below.

[0049] [Chemical Formula 1]

[0050] Li 1+x (Ni a Co b M 1 c M 2 d )O2

[0051] In Chemical Formula 1,

[0052] M 1 is manganese (Mn), aluminum (Al), or a combination thereof,

[0053] M 2 is at least one selected from the following: tungsten (W), copper (Cu), iron (Fe), vanadium (V), chromium (Cr), titanium (Ti), zirconium (Zr), zinc (Zn), aluminum (Al), indium (In), tantalum (Ta), yttrium (Y), lanthanum (La), strontium (Sr), gallium (Ga), scandium (Sc), gadolinium (Gd), samarium (Sm), calcium (Ca), cerium (Ce), niobium (Nb), magnesium (Mg), boron (B), and molybdenum (Mo), and

[0054] 1+x, a, b, c, and d are the atomic fractions of the respective elements,

[0055] where -0.2 ≤ x ≤ 0.2, 0.50 ≤ a < 1, 0 < b < 0.50, 0 < c < 0.50, 0 ≤ d ≤ 0.10 and a + b + c + d = 1.

[0056] 1 + x represents the molar ratio of lithium in the lithium nickel-based oxide, where x can satisfy -0.1 ≤ x ≤ 0.2 or 0 ≤ x ≤ 0.2. When the molar ratio of lithium satisfies the above range, the crystal structure of the lithium nickel-based oxide can be formed stably.

[0057] a represents the molar ratio of nickel in all metals except lithium in the lithium nickel-based oxide, where a can satisfy 0.50 < a < 1, 0.55 ≤ a < 1 or 0.60 ≤ a < 1. When the molar ratio of nickel satisfies the above range, high energy density can be exhibited, thereby achieving high capacity and operating stably at high voltage.

[0058] b represents the molar ratio of cobalt in all metals except lithium in the lithium nickel-based oxide, where b can satisfy 0 < b ≤ 0.40, 0 < b ≤ 0.25 or 0 < b ≤ 0.15. When the molar ratio of cobalt satisfies the above range, good resistance characteristics and output characteristics can be achieved.

[0059] c represents the molar ratio of M in all metals except lithium in the lithium nickel-based oxide 1 where c can satisfy 0 < c ≤ 0.40, 0 < c ≤ 0.35 or 0 < c ≤ 0.30. For example, when M 1 is Mn, when the molar ratio of Mn satisfies the above range, the structural stability of the positive electrode active material is excellent.

[0060] d represents the molar ratio of element M in all metals except lithium in the lithium nickel-based oxide 2 where d can satisfy 0 ≤ d ≤ 0.08, 0 ≤ d ≤ 0.05 or 0 ≤ d ≤ 0.03.

[0061] Preferably, M 1 in Chemical Formula 1 can be Mn. That is, the lithium nickel-based oxide can be a lithium nickel cobalt manganese-based oxide.

[0062] Lithium nickel-based oxides may have a coating containing at least one element of Al and W on the surface of the particles, preferably a coating containing Al and W on the surface of the particles. In this case, since Al and W can react with the lithium by-products remaining on the surface to form LiAlO2, LiWO3, etc., the residual amount of lithium by-products can be reduced, and thus the coating has the effect of reducing the amount of gas generated. The coating elements may exist in the form of oxides in the coating and can be formed by methods such as dry coating, wet coating, chemical vapor deposition (CVD), physical vapor deposition (PVD), and atomic layer deposition (ALD). Among them, it is preferred to form the coating by atomic layer deposition because a coating with a large area can be formed.

[0063] Based on the total surface area of the lithium nickel-based oxide particles, the formation area of the coating can be in the range of 10% to 100%, preferably 30% to 100%, more preferably 50% to 100%. When the formation area of the coating satisfies the above range, the effects of reducing the amount of gas generated and improving the life characteristics are excellent.

[0064] When the coating contains Al, based on the total weight of the lithium nickel-based oxide, the amount of Al in the coating can be 5 wt% or less, preferably in the range of 0.1 wt% to 5 wt%, more preferably in the range of 0.5 wt% to 1 wt%. When the coating contains W, based on the total weight of the lithium nickel-based oxide, the amount of W in the coating can be 5 wt% or less, preferably in the range of 0.1 wt% to 5 wt%, more preferably in the range of 0.5 wt% to 1 wt%.

[0065] When measuring the weight of the lithium compounds remaining on the surface of the particles, the lithium nickel-based oxide may contain more Li2CO3 than LiOH. Considering that Li2CO3 is easier to remove by surface washing than LiOH, since the presence of more Li2CO3 than LiOH means that no washing process is introduced, there is an effect of reducing the initial resistance.

[0066] Specifically, based on the total weight of the lithium nickel-based oxide, the amount of Li2CO3 can be in the range of 0.1 wt% to 0.5 wt%, and based on the total weight of the lithium nickel-based oxide, the amount of LiOH can be in the range of 0.01 wt% to 0.05 wt%. The amounts of LiOH and Li2CO3 can be confirmed by pH titration of the lithium nickel-based oxide.

[0067] The D of the lithium nickel-based oxide 50 can be 2 μm to 8 μm, preferably 2 μm to 5 μm, more preferably 3 μm to 4 μm.

[0068] Based on the total weight of the solids in the positive electrode paste, the content of the positive electrode active material can be 80% to 99% by weight, specifically 90% to 99% by weight. In this case, since the energy density decreases when the amount of the positive electrode active material is less than 80% by weight, the capacity may be reduced.

[0069] The binder can be at least one selected from the following substances: polyvinylidene fluoride (PVDF), polyvinylidene 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 or various copolymers thereof, and preferably can be polyvinylidene fluoride (PVDF).

[0070] Based on the total weight of the solids in the positive electrode paste, the content of the binder can be 0.5% to 2.5% by weight, preferably 0.5% to 2% by weight, and more preferably 1% to 2% by weight. Since when the amount of the binder is within the above range, the adhesion to the current collector and the binding between particles are sufficiently ensured, the positive electrode durability can be improved while maintaining a low initial resistance.

[0071] The conductive material can be at least one selected from the following substances: graphite, such as natural graphite or artificial graphite; carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal crack carbon black; carbonaceous materials, such as carbon fiber and carbon nanotube; powders or fibers of metals such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; and conductive polymers, such as polyphenylene derivatives, and preferably can be carbon black.

[0072] Based on the total weight of the solids in the positive electrode paste, the content of the conductive material can be 0.5% to 2.5% by weight, preferably 0.5% to 2% by weight, and more preferably 1% to 2% by weight. When the amount of the conductive material is within the above range, it is expected that the dead volume can be reduced while maintaining the conductivity between the active materials.

[0073] The solvent of the positive electrode paste can be an organic solvent, such as NMP (N - methyl - 2 - pyrrolidone), and can be used in an amount that gives the positive electrode paste a desired viscosity. For example, based on the total weight of the positive electrode paste, the content of the solids in the positive electrode paste can be 40% to 90% by weight, preferably 50% to 85% by weight, and more preferably 60% to 70% by weight.

[0074] Positive electrode and its preparation method

[0075] The positive electrode according to the present invention includes a positive electrode active material layer formed by coating at least one surface of a positive electrode current collector with a positive electrode paste.

[0076] There is no particular limitation on the positive electrode current collector as long as it has conductivity and does not cause chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, fired carbon can be used; or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc. In addition, the thickness of the positive electrode current collector can generally be 3 μm to 500 μm, and fine irregularities can be formed on the surface of the positive electrode current collector to improve the adhesion of the positive electrode active material. For example, the positive electrode current collector can be used in various shapes such as film, sheet, foil, net, porous body, foam body, non-woven fabric body, etc.

[0077] The porosity of the positive electrode active material layer can be 19% to 25%, preferably 20% to 24%, more preferably 21% to 23%. Such porosity can be exhibited while the lithium borate compound is dissolved in the electrolyte as described above. Herein, the porosity refers to the value calculated by the following [Mathematical Formula 1].

[0078] [Mathematical Formula 1]

[0079] Porosity of positive electrode active material layer (%) = ((true density of positive electrode active material - positive electrode density) / true density of positive electrode active material) × 100

[0080] In Mathematical Formula 1, the positive electrode density is the value calculated by the following Mathematical Formula 2.

[0081] [Mathematical Formula 2]

[0082] Positive electrode density = (weight of positive electrode - weight of positive electrode current collector) / (a × b)

[0083] In Mathematical Formula 2, a is the area of the positive electrode, and b is the value obtained by subtracting the thickness of the positive electrode current collector from the thickness of the positive electrode.

[0084] The method for preparing a positive electrode according to the present invention includes the following steps:

[0085] Prepare a positive electrode paste by mixing the following substances: a positive electrode active material, the positive electrode active material containing a lithium nickel-based oxide having a composition in which the amount of nickel among all metals other than lithium is 50 mol% or more, and being in at least one form of single particles or quasi-single particles in which 30 or less primary particles are aggregated as secondary particles; a lithium borate compound; a binder; a conductive material; and a solvent, and

[0086] Form a positive electrode active material layer by coating at least one surface of a positive electrode current collector with the positive electrode paste.

[0087] The method for preparing a positive electrode according to an embodiment of the present invention further includes a step of preparing a lithium nickel-based oxide by sintering a mixture of a nickel transition metal precursor having a nickel content of 50 mol% or more and a lithium raw material, wherein the step of preparing the lithium nickel-based oxide may not include a washing step after sintering. As described above, since the surface properties of the lithium nickel-based oxide deteriorate during the washing process, resulting in an increase in resistance, it is desirable to introduce the positive electrode paste according to the present invention without including a washing step.

[0088] In this case, the nickel transition metal precursor can be used by purchasing a commercially available precursor, or can be prepared according to a method for preparing a precursor known in the art.

[0089] Preferably, the nickel transition metal precursor can be a nickel transition metal hydroxide in which the amount of nickel among all transition metals is 50 mol% or more, and more preferably can be a nickel transition metal hydroxide in which the amount of nickel is 55 mol% or more or 60 mol% or more. When the amount of nickel in the nickel transition metal precursor satisfies the above range, high-capacity characteristics can be achieved.

[0090] For example, the precursor can be prepared by a coprecipitation reaction while adding an aqueous transition metal solution, an ammonium cation complexing agent, and an alkaline compound to a reactor and stirring.

[0091] The aqueous transition metal solution can be prepared by dissolving a transition metal-containing raw material in a solvent such as water. For example, it can be prepared by dissolving a nickel-containing raw material, a cobalt-containing raw material, and a raw material containing M 1 in water.

[0092] The transition metal-containing raw material can be an acetate, carbonate, nitrate, sulfate, halide, sulfide, or oxide of a transition metal.

[0093] Specifically, the nickel-containing raw material can be, for example, NiO, NiCO3·2Ni(OH)2·4H2O, NiC2O2·2H2O, Ni(NO3)2·6H2O, NiSO4, NiSO4·6H2O, nickel halide, or a combination thereof.

[0094] The cobalt-containing raw material can be, for example, CoSO4, Co(OCOCH3)2·4H2O, Co(NO3)2·6H2O, CoSO4·7H2O, or a combination thereof.

[0095] As an example, when M 1 is manganese, the raw material containing M 1 can be Mn2O3, MnO2, Mn3O4, MnCO3, Mn(NO3)2, MnSO4·H2O, manganese acetate, manganese halide, or a combination thereof.

[0096] In this case, the addition amounts of the raw materials containing transition metals can be determined by considering the molar ratio of the transition metals in the positive electrode active material to be finally prepared.

[0097] The ammonium cation complexing agent may include at least one compound selected from NH4OH, (NH4)2SO4, NH4NO3, NH4Cl, CH3COONH4, and (NH4)2CO3, and may be added to the reactor in the form of a solution in which the above compound is dissolved in a solvent. In this case, water or a mixture of water and an organic solvent that can be uniformly mixed with water (specifically, alcohols, etc.) can be used as the solvent.

[0098] The basic compound may be at least one compound selected from NaOH, KOH, and Ca(OH)2, and may be added to the reactor in the form of a solution in which the above compound is dissolved in a solvent. In this case, water or a mixture of water and an organic solvent that can be uniformly mixed with water (specifically, alcohols, etc.) can be used as the solvent.

[0099] If the transition metal aqueous solution, the ammonium cation complexing agent, and the basic compound are added to the reactor and stirred as described above, precursor particles in the form of transition metal hydroxides are formed, and at the same time, the transition metals in the transition metal aqueous solution co-precipitate.

[0100] In this case, the addition amounts of the transition metal aqueous solution, the ammonium cation complexing agent, and the basic compound are such that the pH of the reaction solution is within the desired range.

[0101] If the precursor particles are formed by the above method, the precursor is obtained by separating the particles from the reaction solution. For example, after separating the precursor from the reaction solution by filtering the reaction solution, the separated precursor can be washed and dried to obtain the precursor. In this case, processes such as grinding and / or classification can be carried out as needed.

[0102] The nickel transition metal precursor thus prepared is mixed with a lithium raw material and then sintered to prepare a lithium nickel-based oxide. In this case, as needed, the raw materials containing M 2 metals can be mixed together and sintered.

[0103] As the lithium raw material, lithium-containing sulfates, nitrates, acetates, carbonates, oxalates, citrates, halides, hydroxides, or hydroxyoxides can be used. For example, Li2CO3, LiNO3, LiNO2, LiOH, LiOH·H2O, LiH, LiF, LiCl, LiBr, LiI, CH3COOLi, Li2O, Li2SO4, Li3C6H5O7, or a mixture thereof can be used.

[0104] A lithium raw material and a nickel transition metal precursor can be mixed such that the molar ratio of lithium (Li) to all metals in the precursor is in the range of 1:1 to 1.2:1, preferably 1:1 to 1.1:1. When the mixing ratio of the lithium raw material to the metals in the precursor satisfies the above range, since the layered crystal structure of the lithium nickel-based oxide develops well, a positive electrode active material having excellent capacity characteristics and structural stability can be prepared.

[0105] Sintering is carried out at a temperature at which single particles or quasi-single particles can be formed. In order to form single particles or quasi-single particles, sintering must be carried out at a temperature higher than the temperature during the preparation of a conventional lithium composite transition metal oxide in the form of secondary particles. For example, when the precursor composition is the same, sintering must be carried out at a temperature about 30 °C to 100 °C higher than the temperature during the preparation of a conventional lithium composite transition metal oxide in the form of secondary particles.

[0106] In one embodiment of the present invention, sintering in the step of preparing the lithium nickel-based oxide can be carried out at 700 °C to 1050 °C, preferably at 750 °C to 1000 °C, more preferably at 800 °C to 900 °C.

[0107] When the sintering temperature satisfies the above range, a positive electrode active material in the form of single particles or quasi-single particles having excellent electrochemical performance can be prepared. Specifically, it is desirable that the sintering temperature is 700 °C or higher to prevent the formation of a positive electrode active material in the form of secondary particles. However, since excessive sintering occurs, a layered crystal structure is not properly formed, resulting in deterioration of the electrochemical performance. Therefore, it is desirable that the sintering temperature is 1,050 °C or lower.

[0108] In addition, sintering can be carried out in an oxygen atmosphere for 5 hours to 35 hours. In the present specification, the oxygen atmosphere means an atmosphere containing sufficient oxygen for sintering other than the air atmosphere. In particular, it is desirable to carry out sintering in an atmosphere having a higher oxygen partial pressure than the oxygen partial pressure in the air atmosphere.

[0109] In addition, the method for preparing a positive electrode according to an embodiment of the present invention may not include a step of coating boron on the lithium nickel-based oxide between the step of preparing the lithium nickel-based oxide and the step of preparing the positive electrode paste. As described above, since the boron coating increases the surface resistance, it is desirable to form the positive electrode paste according to the present invention without including a separate boron coating formation step.

[0110] In the step of forming the positive electrode active material layer, the loading amount of the positive electrode paste can be from 14 mg / cm 2 to 24 mg / cm 2 , preferably from 16 mg / cm 2 to 22 mg / cm 2 , more preferably from 18 mg / cm 2 to 20 mg / cm2 within the range.

[0111] The method for preparing a positive electrode for a lithium secondary battery according to an embodiment of the present invention may further include a step of calendering a laminate of a positive electrode current collector and a positive electrode active material layer. After cutting the laminate, calendering can be performed by placing the cut laminate between two calendering rollers and pressing it by adjusting the distance between the calendering rollers.

[0112] Each component of the method for preparing a positive electrode for a lithium secondary battery according to an embodiment of the present invention can refer to the description of each component of the above positive electrode paste.

[0113] Lithium secondary battery

[0114] Next, a lithium secondary battery according to the present invention will be described.

[0115] The lithium secondary battery of the present invention includes:

[0116] the above positive electrode for a lithium secondary battery;

[0117] a negative electrode including a negative electrode active material;

[0118] a separator disposed between the positive electrode and the negative electrode; and

[0119] an electrolyte.

[0120] In an embodiment of the present invention, the operating voltage of the lithium secondary battery can be 4.3 V or more, particularly 4.3 V to 4.5 V, and more particularly 4.35 V to 4.45 V. Since the contact with the electrolyte is reduced, side reactions such as metal dissolution are inhibited. Because the positive electrode according to the present invention includes a boron (B) coating and exhibits excellent characteristics at high voltages, the lithium secondary battery containing it can operate at high voltages.

[0121] The lithium secondary battery may also optionally include a battery container for accommodating the positive electrode, the negative electrode, and the separator, and a sealing member for sealing the battery container.

[0122] In the lithium secondary battery, the negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector.

[0123] There is no particular limitation on the negative electrode current collector as long as it has high electrical conductivity and does not cause chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, fired carbon can be used; copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloy. In addition, the thickness of the negative electrode current collector can generally be 3 μm to 500 μm, and similar to the positive electrode current collector, fine irregularities can be formed on the surface of the current collector to improve the adhesion of the negative electrode active material. For example, the negative electrode current collector can be used in various shapes such as film, sheet, foil, net, porous body, foam, non-woven fabric body, etc.

[0124] In addition to the negative electrode active material, the negative electrode active material layer may optionally contain a binder and a conductive material.

[0125] Compounds capable of reversibly inserting and extracting lithium can be used as the negative electrode active material. Specific examples of the negative electrode active material can be carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; (semi) metallic materials that can form alloys with lithium such as silicon (Si), aluminum (Al), tin (Sn), lead (Pb), zinc (Zn), bismuth (Bi), indium (In), magnesium (Mg), gallium (Ga), cadmium (Cd), Si alloy, Sn alloy or Al alloy; (semi) metal oxides that can be doped and de-doped with lithium such as SiO β (0 < β < 2), SnO2, vanadium oxides and lithium vanadium oxides; or composite materials containing (semi) metallic materials and carbonaceous materials such as Si-C composite materials or Sn-C composite materials, and any one of them or a mixture of two or more of them can be used.

[0126] In addition, a thin film of metallic lithium can be used as the negative electrode active material. In addition, both low-crystalline carbon and high-crystalline carbon can be used as carbon materials. Typical examples of low-crystalline carbon can be soft carbon and hard carbon, and typical examples of high-crystalline carbon can be irregular, planar, sheet-like, spherical or fibrous natural graphite or artificial graphite, condensed graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesophase carbon microbead, mesophase pitch, and high-temperature sintered carbon such as coke derived from petroleum or coal tar pitch.

[0127] The conductive material of the negative electrode active material layer is used to provide conductivity to the electrode. Any conductive material can be used without particular limitation as long as it has appropriate electron conductivity without causing adverse chemical changes in the battery. Specific examples of the conductive material can be graphite, such as natural graphite or artificial graphite; carbonaceous materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black, carbon fiber, and carbon nanotube; powders or fibers of metals such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or conductive polymers, such as polyphenylene derivatives, and any one of them or a mixture of two or more of them 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 wt% to 30 wt%, preferably 1 wt% to 20 wt%, and more preferably 1 wt% to 10 wt%.

[0128] The binder of the negative electrode active material layer improves the adhesion between the negative electrode active material particles and the adhesion between the negative electrode active material and the negative electrode current collector. Specific examples of the binder can be polyvinylidene fluoride (PVDF), polyvinylidene fluoride - hexafluoropropylene copolymer (PVDF - co - HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene - propylene - diene rubber (EPDM rubber), sulfonated EPDM, styrene - butadiene rubber (SBR), fluororubber, or various copolymers thereof, and any one of them or a mixture of two or more of them can be used. Based on the total weight of the negative electrode active material layer, the content of the binder can be 1 wt% to 30 wt%, preferably 1 wt% to 20 wt%, and more preferably 1 wt% to 10 wt%.

[0129] For example, the negative electrode active material layer can be prepared by coating a negative electrode slurry containing the negative electrode active material and optionally a binder and a conductive material on the negative electrode current collector and drying the coated negative electrode current collector, or can be prepared by casting the negative electrode slurry on a separate carrier and then laminating the film separated from the carrier on the negative electrode current collector.

[0130] In a lithium secondary battery, a separator separates the negative electrode and the positive electrode and provides a path for the movement of lithium ions. Any separator can be used as this separator without particular limitation as long as it is commonly used in lithium secondary batteries. In particular, a separator with a high moisture retention capacity for the electrolyte and a low resistance to the movement of electrolyte ions can be used. Specifically, a porous polymer membrane can be used, such as a porous polymer membrane prepared from polyolefin polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or a laminated structure having two or more layers thereof. In addition, typical porous non-woven fabrics can be used, such as non-woven fabrics formed from high melting point glass fibers or polyethylene terephthalate fibers. In addition, a coated separator containing a ceramic component or a polymer material can be used to ensure heat resistance or mechanical strength, and a separator having a single-layer or multi-layer structure can be optionally used.

[0131] In addition, the electrolyte contained in the lithium secondary battery according to the present invention can be an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, or a molten-type inorganic electrolyte, but the present invention is not limited thereto.

[0132] In one embodiment of the present invention, the electrolyte is an organic liquid electrolyte, which can contain an organic solvent and a lithium salt.

[0133] Any organic solvent can be used as this organic solvent without particular limitation as long as it can be used as a medium through which ions participating in the electrochemical reaction of the battery can move. Specifically, as the organic solvent, the following can be used: ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; or carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethanol and isopropanol; nitriles such as R-CN (where R is a linear, branched, or cyclic C2-C20 hydrocarbon group and may contain a double bond, an aromatic ring, or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes. Among these solvents, carbonate solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant and a low-viscosity linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate), which can improve the charge / discharge performance of the battery, is more preferred.

[0134] A lithium salt can be used without particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, as the lithium salt, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2 can be used. The lithium salt can be used in a concentration range of 0.1 M to 5.0 M, preferably 0.1 M to 3.0 M. If the concentration of the lithium salt is included within the above range, excellent electrolyte performance can be obtained and lithium ions can move effectively because the electrolyte can have appropriate conductivity and viscosity.

[0135] To improve the life characteristics of the battery, suppress the reduction of battery capacity, and improve the discharge capacity of the battery, additives can also be included in the electrolyte. For example, as the additives, carbonate compounds, sultone compounds, sulfate compounds, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, (poly)glycol dimethyl ethers, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride can be used alone or in a mixture thereof, but the present invention is not limited thereto. Based on the total weight of the electrolyte, the content of the additive can be 0.1 wt% to 10 wt%, preferably 0.1 wt% to 5 wt%.

[0136] As described above, since the lithium secondary battery including the positive electrode according to the present invention has excellent initial resistance and high-temperature storage characteristics, the lithium secondary battery is suitable for portable devices such as mobile phones, laptop computers, and digital cameras, as well as electric vehicles.

[0137] Therefore, according to another embodiment of the present invention, a battery module including a lithium secondary battery as a unit cell and a battery pack including the battery module are provided.

[0138] The battery module or the battery pack can be used as a power source for at least one of the following medium and large-sized devices: power tools; electric vehicles (EVs); or power storage systems.

[0139] Hereinafter, embodiments of the present invention will be described in detail in a manner that those of ordinary skill in the art to which the present invention pertains can easily implement the present invention.

[0140] Preferred Embodiments

[0141] [Examples and Comparative Examples: Preparation of Positive Electrodes]

[0142] Example 1.

[0143] Mix the Ni 0.60 Co 0.10 Mn 0.30 (OH)₂ transition metal precursor with LiOH·H₂O so that the molar ratio of transition metal (Ni + Co + Mn):Li is 1:1.05. Then, raise the temperature to 850 °C at a heating rate of 5 °C per minute, then hold for 10 hours, and cool to room temperature at a cooling rate of 5 °C per minute to prepare lithium nickel cobalt manganese oxide with the composition Li[Ni 0.60 Co 0.10 Mn 0.30 O₂ and D 50 = 3.5 μm. Thereafter, form a coating containing Al and W on its surface. Observe the prepared lithium nickel cobalt manganese oxide using a scanning electron microscope, and it can be confirmed that it has a single particle form.

[0144] Mix lithium nickel cobalt manganese oxide, carbon black conductive material, and PVDF binder in N-methylpyrrolidone at a weight ratio of 97.3:1.43:1.27, and add 0.01 parts by weight of Li₂B₄O₇ based on 100 parts by weight of lithium nickel cobalt manganese oxide to prepare a positive electrode paste with a solids content of 74% by weight.

[0145] Coat the prepared positive electrode paste onto one surface of a 10-μm-thick aluminum current collector at a loading of 20 mg / cm 2 , and then dry at 130 °C to prepare a positive electrode active material layer.

[0146] After cutting the laminate having the positive electrode active material layer formed on the current collector, place the cut laminate between two calendering rolls and calender at 25 °C. After calendering, the porosity of the positive electrode active material layer obtained by Mathematical Formula 1 is 22%.

[0147] Comparative Example 1.

[0148] Prepare the positive electrode in the same manner as in Example 1, except that Li₂B₄O₇ is not added during the preparation of the positive electrode paste, and mix lithium nickel cobalt manganese oxide, carbon black conductive material, and PVDF binder in N-methylpyrrolidone at a weight ratio of 97.3:1.43:1.27 to prepare a positive electrode paste.

[0149] [Experimental Example 1: Room Temperature Life and High Temperature Life Evaluation]

[0150] (1) Preparation of Battery

[0151] After preparing an electrode assembly by disposing a 15-μm-thick polyethylene-based separator between a lithium metal anode and each of the cathodes prepared in the examples and comparative examples, the electrode assembly was placed in a battery case, an electrolyte solution was injected into the case, the case was sealed, and then an activation step was performed to prepare a half cell.

[0152] An organic solution was prepared by dissolving 1 M LiPF6 in a mixed organic solvent in which ethylene carbonate and ethyl methyl carbonate were mixed at a volume ratio of 3:7, and a mixture of the organic solution and 0.2 wt% vinylene carbonate (VC) was used as the electrolyte solution.

[0153] (2) Room temperature life evaluation

[0154] Each of the batteries prepared above was charged to 4.4 V at a rate of 0.7 C under constant current / constant voltage conditions at 25 °C, then a 0.05 C current cut-off was performed, and discharged at a constant current of 0.5 C to 3.0 V. When the above charge and discharge were performed more than 200 times, the capacity retention rate was measured, and the results are shown in Figure 1 .

[0155] (3) High temperature life evaluation

[0156] Each of the batteries prepared above was charged to 4.4 V at a rate of 0.7 C under constant current / constant voltage conditions at 45 °C, then a 0.05 C current cut-off was performed, and discharged at a constant current of 0.5 C to 3.0 V. When the above charge and discharge were performed 300 times, the capacity retention rate was measured, and the results are shown in Figure 2 .

[0157] According to Figure 1 and Figure 2 , when comparing the cathode of Example 1 in which Li2B4O7 was added during the preparation of the cathode slurry with the cathode of Comparative Example 1 in which Li2B4O7 was not added, it was confirmed that the life improvement effect of the cathode of Example 1 at high temperature was significantly better. Therefore, as described above, in the case of adding Li2B4O7 during the preparation of the cathode slurry, it can be understood that by stabilizing the surface of the particles and suppressing side reactions, the battery life is ultimately improved.

Claims

1. A positive electrode paste for a lithium secondary battery, the positive electrode paste for a lithium secondary battery comprising: A positive electrode active material, the positive electrode active material comprising a lithium nickel-based oxide, the lithium nickel-based oxide having a composition in which the amount of nickel among all metals other than lithium is 50 mol% or more, and being in at least one form of a single particle or a quasi-single particle which is a secondary particle in which 30 or less primary particles are aggregated; A lithium borate compound; A binder; A conductive material; And A solvent.

2. The positive electrode paste for a lithium secondary battery according to claim 1, wherein the lithium borate compound is lithium tetraborate (Li2B4O7).

3. The positive electrode paste for a lithium secondary battery according to claim 1, wherein based on 100 parts by weight of the lithium nickel-based oxide, the content of the lithium borate compound is 0.005 parts by weight to 0.5 parts by weight.

4. The positive electrode paste for a lithium secondary battery according to claim 1, wherein based on the total weight of the solids in the positive electrode paste, the content of the binder is 0.5 wt% to 2.5 wt%.

5. The positive electrode paste for a lithium secondary battery according to claim 1, wherein based on the total weight of the solids in the positive electrode paste, the content of the conductive material is 0.5 wt% to 2.5 wt%.

6. The positive electrode paste for a lithium secondary battery according to claim 1, wherein based on the total weight of the positive electrode paste, the content of the solids in the positive electrode paste is 40 wt% to 90 wt%.

7. The positive electrode paste for a lithium secondary battery according to claim 1, wherein the lithium nickel-based oxide has a composition represented by Chemical Formula 1: [Chemical Formula 1] Li 1+x (Ni a Co b M 1 c M 2 d )O2 Wherein, in Chemical Formula 1, M 1 is Mn, Al or a combination thereof, M 2 is at least one selected from the following substances: W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and 1+x, a, b, c and d are respectively atomic fractions of independent elements, Where -0.2 ≤ x ≤ 0.2, 0.50 ≤ a < 1, 0 < b < 0.50, 0 < c < 0.50, 0 ≤ d ≤ 0.10 and a + b + c + d = 1.

8. The positive electrode paste for a lithium secondary battery according to claim 1, wherein the lithium nickel-based oxide contains a coating comprising at least one element of Al and W on the surface of the particles.

9. The positive electrode paste for a lithium secondary battery according to claim 1, wherein D of the lithium nickel-based oxide 50 is from 2 μm to 8 μm.

10. The positive electrode paste for a lithium secondary battery according to claim 1, wherein when measuring the weight of the lithium compound remaining on the surface of the particles, the lithium nickel-based oxide contains more Li2CO3 than LiOH.

11. A positive electrode for a lithium secondary battery, the positive electrode for a lithium secondary battery comprising a positive electrode active material layer formed by coating at least one surface of a positive electrode current collector with the positive electrode paste according to claim 1.

12. The positive electrode for a lithium secondary battery according to claim 11, wherein the porosity of the positive electrode active material layer is 19% to 25%.

13. A positive electrode for a lithium secondary battery, the positive electrode for a lithium secondary battery comprising: A positive electrode active material, the positive electrode active material comprising a lithium nickel-based oxide, the lithium nickel-based oxide having a composition in which the amount of nickel among all metals other than lithium is 50 mol% or more, and being in at least one form of a single particle or a quasi-single particle which is a secondary particle in which 30 or less primary particles are aggregated, and Lithium borate compound 14. A method for preparing a positive electrode for a lithium secondary battery, the method comprising the following steps: Preparing a positive electrode paste by mixing: a positive electrode active material comprising a lithium nickel-based oxide having a composition in which the amount of nickel among all metals other than lithium is 50 mol% or more, and being in at least one of the forms of a single particle or a quasi-single particle which is a secondary particle in which 30 or less primary particles are aggregated; a lithium borate compound; a binder; a conductive material; and a solvent, and Forming a positive electrode active material layer by coating at least one surface of a positive electrode current collector with the positive electrode paste.

15. The method according to claim 14, the method further comprising the step of preparing the lithium nickel-based oxide by sintering a mixture of a nickel transition metal precursor having a nickel content of 50 mol% or more and a lithium raw material, wherein the step of preparing the lithium nickel-based oxide does not include a washing step after the sintering.

16. The method according to claim 15, wherein the method does not include a step of coating boron on the lithium nickel-based oxide between the step of preparing the lithium nickel-based oxide and the step of preparing the positive electrode paste.

17. A lithium secondary battery, the lithium secondary battery comprising: The positive electrode according to claim 11 or 13; A negative electrode comprising a negative electrode active material; A separator disposed between the positive electrode and the negative electrode; and An electrolyte.

18. The lithium secondary battery according to claim 17, wherein the operating voltage is 4.3 V or more.

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