Positive electrode active material and lithium secondary battery including the same

Abstract

The present invention relates to a positive electrode active material and a lithium secondary battery using a positive electrode including the same, and more particularly, to a positive electrode active material and a lithium secondary battery using a positive electrode including the same, in which the pore area and shape of a lithium complex oxide contained in the positive electrode active material are controlled by adjusting the ratio of ammonia and caustic soda in a coprecipitation reaction of a precursor for synthesizing the positive electrode active material, to improve the electrochemical characteristics and stability.

Description

Technical Field

[0001] This invention relates to positive electrode active materials and lithium secondary batteries using a positive electrode comprising the above-mentioned positive electrode active materials. More specifically, this invention relates to a positive electrode active material for improving electrochemical characteristics and stability by controlling the pore area and shape of the lithium composite oxide contained in the above-mentioned positive electrode active material by adjusting the ratio of ammonia and caustic soda in the co-precipitation reaction of the precursor for synthesizing the positive electrode active material. Background Technology

[0002] Batteries use materials capable of electrochemical reactions at the positive and negative electrodes to store electrical energy. As a representative example of such batteries, there are lithium-ion secondary batteries that store electrical energy through the difference in chemical potential during the insertion / extraction of lithium ions at the positive and negative electrodes.

[0003] The aforementioned lithium secondary battery is prepared by using materials capable of reversible lithium-ion insertion / extraction as positive and negative electrode active materials, and filling the space between the positive and negative electrodes with organic or polymer electrolytes.

[0004] Lithium composite oxides are used as positive electrode active materials in lithium secondary batteries. Examples of such composite oxides include LiCoO2, LiMn2O4, LiNiO2, and LiMnO2.

[0005] Among the aforementioned positive electrode active materials, LiCoO2 is the most widely used due to its excellent lifetime characteristics and charge / discharge efficiency. However, due to the limited resources of cobalt used as a raw material, its price is expensive, thus limiting its price competitiveness.

[0006] Lithium manganese oxides such as LiMnO2 and LiMn2O4 have the advantages of excellent thermal stability and low price, but they suffer from small capacity and poor high-temperature performance. In addition, LiNiO2-type cathode active materials exhibit high discharge capacity battery characteristics, but they are difficult to synthesize due to the problem of cation mixing between lithium and transition metals, resulting in significant problems in rate performance.

[0007] Furthermore, depending on the severity of this cation mixing, a large amount of lithium byproducts are generated. Most of these byproducts consist of compounds of LiOH and Li₂CO₃. Therefore, they can lead to gelation during the preparation of the cathode slurry, or become a source of gas generation due to repeated charge-discharge cycles after electrode fabrication. In addition, residual Li₂CO₃ increases monomer swelling, thus reducing battery life and causing battery expansion.

[0008] On the other hand, the lithium composite oxide constituting the positive electrode active material can contain micropores. Because of these pores, the electrolyte can pass through, thereby allowing the electrochemical properties of the lithium composite oxide to be utilized. However, if the lithium composite oxide has too many pores (commonly referred to as the porosity index measured from cross-sectional scanning electron microscopy (SEM) images), the possibility of side reactions between the lithium composite oxide and the electrolyte increases, and therefore stability may decrease.

[0009] Therefore, previous attempts have attempted to balance the electrochemical properties and stability of the aforementioned lithium composite oxides by controlling their porosity. However, since the pore shapes observed in the cross-sectional SEM images of these lithium composite oxides vary, controlling only the porosity can only improve the stability of the lithium composite oxides, which has limitations. Summary of the Invention

[0010] Technical issues

[0011] In order to solve the various problems of existing positive electrode active materials for lithium secondary batteries, the present invention aims to provide a positive electrode active material with improved electrochemical properties and stability.

[0012] In particular, the applicant confirms that by adjusting the ratio of ammonia to caustic soda used in the co-precipitation reaction of the precursor of the lithium composite oxide constituting the above-mentioned positive electrode active material, the pore area and shape in the lithium composite oxide can be controlled. As described above, not only can the porosity or average diameter of the pores in the above-mentioned lithium composite oxide be simply controlled, but the pore area and pore shape can also be more actively controlled, thereby further improving the electrochemical properties and stability of the above-mentioned positive electrode active material.

[0013] Therefore, the object of the present invention is to provide a positive electrode active material in which the pore area and shape of the lithium composite oxide contained in the above-mentioned positive electrode active material are controlled by adjusting the ratio of ammonia and caustic soda used in the co-precipitation reaction for synthesizing the precursor constituting the positive electrode active material, so as to improve the electrochemical properties and stability.

[0014] Furthermore, the present invention aims to provide a positive electrode comprising a positive electrode active material as defined herein.

[0015] Furthermore, the present invention aims to provide a lithium secondary battery using a positive electrode as defined herein.

[0016] Solution to the problem

[0017] According to one aspect of the present invention, a positive electrode active material is provided, the positive electrode active material comprising a lithium composite oxide represented by the following chemical formula 1 and comprising a lithium composite oxide capable of intercalating or deintercalating lithium ions.

[0018] [Chemical Formula 1]

[0019] Li w Ni 1-(x+y+z) Co x M1 y M2 z O 2+α

[0020] (Where M1 is selected from at least one of Mn and Al, and M2 is selected from at least one of Mn, P, Sr, Ba, B, Ti, Zr, Al, Hf, Ta, Mg, V, Zn, Si, Y, Sn, Ge, Nb, W and Cu. M1 and M2 are distinct elements, 0.5≤w≤1.5, 0≤x≤0.50, 0≤y≤0.20, 0≤z≤0.20, 0≤α≤0.02).

[0021] At this point, the average value of the ratio (b / a) of the major axis length (b) of the pores to the minor axis length (a) of the pores observed from the cross-sectional scanning electron microscope (SEM) image of the lithium composite oxide can be 1 to 3.

[0022] Furthermore, when the average particle size (D50) of the aforementioned lithium composite oxide is d, the average value of the major axis length (b) of the pores observed from the cross-sectional SEM image of the aforementioned lithium composite oxide can be less than 0.15d. That is, preferably, the average value of the major axis length (b) of the aforementioned pores is less than 15% of the average particle size (D50) of the aforementioned lithium composite oxide. Here, the average particle size (D50) of the aforementioned lithium composite oxide refers to the average particle size (D50) of the lithium composite oxide as secondary particles.

[0023] Furthermore, at least a portion of the surface of the aforementioned lithium composite oxide may also include an alloy oxide represented by the following chemical formula 2.

[0024] [Chemical Formula 2]

[0025] Li a M3 b O c

[0026] (Where M3 is selected from at least one of Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, Ce, V, Ba, Ta, Sn, Hf, Gd, and Nd, 0≤a≤10, 0 <b≤8,2≤c≤13)。

[0027] Furthermore, according to another aspect of the present invention, a positive electrode comprising the above-described positive electrode active material is provided.

[0028] Furthermore, according to another aspect of the present invention, a lithium secondary battery using the above-described positive electrode is provided.

[0029] Invention Effects

[0030] According to the present invention, by adjusting the ratio of ammonia and caustic soda used in the co-precipitation reaction of the precursor of the lithium composite oxide constituting the above-mentioned positive electrode active material, the above-mentioned lithium composite oxide with controlled pore area and pore shape can be obtained. Furthermore, by using the positive electrode active material comprising the above-mentioned lithium composite oxide with controlled pore area and pore shape as described above, various electrochemical characteristics such as capacity characteristics, lifetime characteristics, and charge / discharge efficiency characteristics, which are important indicators for evaluating the performance of lithium secondary batteries, can be improved.

[0031] Furthermore, as described above, when the porosity of the aforementioned lithium composite oxide is controlled simultaneously with the pore area and pore shape, a greater synergistic effect of the aforementioned electrochemical properties can be expected.

[0032] On the other hand, in the methods for controlling the porosity of the lithium composite oxide described in the prior art, stability can be improved by suppressing the side reactions between the lithium composite oxide and the electrolyte, but there are shortcomings in improving the particle strength of the lithium composite oxide or suppressing cracks that occur during charging and discharging.

[0033] However, as described in this invention, when the pore area and pore shape are controlled by adjusting the ratio of ammonia and caustic soda used in the co-precipitation reaction, the particle strength and crack suppression ability of the aforementioned lithium composite oxide can be improved. Similarly, when the pore area, pore shape, and porosity of the aforementioned lithium composite oxide are controlled simultaneously, the aforementioned synergistic effect on the stability of the lithium composite oxide can be expected. Detailed Implementation

[0034] The positive electrode active material according to the present invention, the positive electrode including the above-described positive electrode active material, and the lithium secondary battery using the above-described positive electrode will be described in more detail below.

[0035] Positive electrode active material

[0036] According to one aspect of the present invention, a positive electrode active material comprising a lithium composite oxide capable of lithium ion intercalation / deintercalation is provided.

[0037] The aforementioned lithium composite oxide can be in the form of single-crystal or polycrystalline oxide particles, but is preferably in the form of polycrystalline particles. Polycrystalline lithium composite oxide refers to a condensate containing primary particles and secondary particles formed by agglomerating multiple of the aforementioned primary particles.

[0038] The aforementioned primary particle refers to a single grain (or crystallite), while secondary particles refer to aggregates formed by the aggregation of multiple primary particles. Pores and / or grain boundaries may exist between the primary particles constituting the secondary particles.

[0039] For example, the primary particles can be spaced apart from adjacent primary particles within the secondary particles to form internal pores. Furthermore, the primary particles can form surfaces existing within the secondary particles by contacting these internal pores, rather than forming grain boundaries by contacting adjacent primary particles.

[0040] On the other hand, the surface of the primary particle that exists on the outermost surface of the secondary particle is exposed to the external air, forming the surface of the secondary particle.

[0041] When the average particle size (D50) of the primary particles is in the range of 0.1 μm to 5 μm, preferably in the range of 0.1 μm to 3 μm, the optimal density of the positive electrode prepared using the positive electrode active material according to various embodiments of the present invention can be achieved. Furthermore, the average particle size (D50) of the secondary particles varies depending on the number of primary particles aggregated, but can be from 5 μm to 20 μm.

[0042] The aforementioned primary particles and / or secondary particles may have rod-shaped, elliptical, and / or irregular shapes.

[0043] The aforementioned lithium composite oxide is represented by the following chemical formula 1.

[0044] [Chemical Formula 1]

[0045] Li w Ni 1-(x+y+z) Co x M1 y M2 z O 2+α

[0046] (Where M1 is selected from at least one of Mn and Al, and M2 is selected from at least one of Mn, P, Sr, Ba, B, Ti, Zr, Al, Hf, Ta, Mg, V, Zn, Si, Y, Sn, Ge, Nb, W, and Cu.)

[0047] M1 and M2 are distinct elements, with the following conditions: 0.5 ≤ w ≤ 1.5, 0 ≤ x ≤ 0.50, 0 ≤ y ≤ 0.20, 0 ≤ z ≤ 0.20, and 0 ≤ α ≤ 0.02.

[0048] In this case, the lithium composite oxide described above can be a lithium composite oxide having a layered crystal structure containing at least nickel and cobalt. Furthermore, the lithium composite oxide described above is preferably a high-nickel type lithium composite oxide in which x+y+z in the above chemical formula 1 is 0.40 or less, preferably 0.20 or less.

[0049] In one embodiment, the positive electrode active material according to the present invention, by including a lithium composite oxide that controls the pore area and pore shape and using a positive electrode active material including the above-mentioned lithium composite oxide, can improve various electrochemical characteristics such as capacity characteristics, lifetime characteristics, charge-discharge efficiency characteristics, etc., which are important indicators for evaluating the performance of lithium secondary batteries.

[0050] The pore area, pore shape, and porosity of the aforementioned lithium composite oxides can be determined from cross-sectional SEM images of the lithium composite oxides.

[0051] Specifically, when the average particle size (D50) of the aforementioned lithium composite oxide is d, the average value of the major axis length (b) of the pores observed from the cross-sectional SEM image of the aforementioned lithium composite oxide can be less than 0.15d, preferably less than 0.137d. That is, preferably, the average value of the major axis length (b) of the aforementioned pores is less than 15% of the average particle size (D50) of the aforementioned lithium composite oxide. Here, the average particle size (D50) of the aforementioned lithium composite oxide refers to the average particle size (D50) of the lithium composite oxide as secondary particles.

[0052] In addition, while satisfying the condition that the average value of the long axis length (b) of the pores in the lithium composite oxide is less than 15% of the average particle size (D50) of the lithium composite oxide, the average value of the ratio (b / a) of the long axis length (b) of the pores to the short axis length (a) of the pores is adjusted to 1 to 3.

[0053] In this case, the shape of the pores may include a near-spherical shape in which the major axis length (b) and minor axis length (a) appear to be the same (where the average ratio (b / a) of the major axis length (b) and minor axis length (a) of the pores is 1) or a rod shape in which the major axis length (b) is greater than the minor axis length (a) (where the average ratio (b / a) of the major axis length (b) and minor axis length (a) of the pores is 3).

[0054] On the other hand, when the average value of the long axis length (b) of the aforementioned pores is greater than 15% of the average particle size (D50) of the aforementioned lithium composite oxide, the pore size is excessively large when the average ratio (b / a) of the long axis length (b) to the short axis length (a) of the aforementioned pores is in the range of 1 to 3. This not only negatively impacts the particle strength of the aforementioned lithium composite oxide, but also increases the likelihood of cracking during the charging and discharging process of a lithium secondary battery. Furthermore, the excessively large pore size in the aforementioned lithium composite oxide leads to an increase in the amount of electrolyte impregnated within the pores, potentially causing side reactions.

[0055] Furthermore, the proportion of pores in the total porosity observed from the cross-sectional SEM image of the lithium composite oxide, in which the ratio (b / a) of the major axis length (b) of the pores to the minor axis length (a) of the pores is greater than 3, is preferably less than 50%.

[0056] When the ratio of pores with a major axis length (b) to a minor axis length (a) greater than 3 (b / a) exceeds 50% in the total porosity observed from the cross-sectional SEM image of the aforementioned lithium composite oxide, it is not only detrimental to the particle strength of the lithium composite oxide, but also may cause cracks in the lithium composite oxide during the charging and discharging process of a lithium secondary battery. Furthermore, because the pore size in the aforementioned lithium composite oxide is too large, the amount of electrolyte impregnated in the pores increases, potentially causing side reactions.

[0057] Furthermore, in the aforementioned lithium composite oxide, the average value of the major axis length (b) of the pores is (0.x)d to (0.y)d, and the average value of the ratio (b / a) of the major axis length (b) to the minor axis length (a) of the pores satisfies a range of 1 to 3. Preferably, the average area of ​​the pores observed from the cross-sectional SEM image of the aforementioned lithium composite oxide is 0.02 μm. 2 Up to 1.5μm 2 .

[0058] Furthermore, the occupancy rate of the aforementioned pores in the cross-section of the aforementioned lithium composite oxide, as observed from the cross-sectional SEM image of the aforementioned lithium composite oxide, can be from 0.3% to 3.5%.

[0059] When the average value of the long axis length (b) of the pores in the above-mentioned lithium composite oxide is less than 15% of the average particle size (D50) of the above-mentioned lithium composite oxide, preferably less than 13.7%, and the average value of the ratio (b / a) of the long axis length (b) of the above-mentioned pores to the short axis length (a) of the above-mentioned pores is in the range of 1 to 3, and the occupancy of the above-mentioned pores (also known as porosity) in the cross section of the above-mentioned lithium composite oxide is 0.3% to 3.5%, a higher synergistic effect of electrochemical properties can be expected compared with positive electrode active materials in which only the porosity of the above-mentioned lithium composite oxide is in the above-mentioned range.

[0060] Furthermore, the surface of the aforementioned lithium composite oxide contained in the positive electrode active material is a region where side reactions may occur with the electrolyte during the charging, discharging, and / or storage of the lithium secondary battery. The larger the surface area of ​​the positive electrode active material (e.g., expressed by indicators such as BET specific surface area), the greater the possibility of side reactions. Therefore, the crystal structure within the surface of the positive electrode active material may undergo a phase transformation (e.g., layered structure → rock salt structure). As described above, the phase transformation of the crystal structure within the surface of the positive electrode active material has been identified as one of the reasons for the reduced lifespan characteristics of lithium secondary batteries.

[0061] As proposed in this invention, when the ratio of ammonia to caustic soda used in the co-precipitation reaction of the precursor for synthesizing the lithium composite oxide is adjusted in order to control the pore area and pore shape of the lithium composite oxide, the pore area and pore shape of the synthesized lithium composite oxide are controlled, and the BET specific surface area of ​​the synthesized lithium composite oxide can be adjusted to 0.2 m². 2 / g to 2.0m 2 Within the range of / g.

[0062] In another embodiment, the positive electrode active material may further include a coating covering at least a portion of the surface of the lithium composite oxide.

[0063] In this case, the coating may include an alloy oxide represented by the following chemical formula 2. That is, the coating may be defined as a region containing an alloy oxide represented by the following chemical formula 2.

[0064] [Chemical Formula 2]

[0065] Li a M3 b O c

[0066] (Where M3 is selected from at least one of Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, Ce, V, Ba, Ta, Sn, Hf, Gd, and Nd, 0≤a≤10, 0 <b≤8,2≤c≤13)。

[0067] Furthermore, the coating can have different types of alloy oxides coexisting in one layer or different types of alloy oxides represented by the above chemical formula 2 coexisting in different layers.

[0068] The alloy oxide represented by the above chemical formula 2 can be in a state of physical and / or chemical bonding to the above lithium composite oxide. Furthermore, the above alloy oxide can exist in a state of forming a solid solution with the above lithium composite oxide.

[0069] The aforementioned alloy oxide can be an oxide formed by complexing lithium with an element represented by M3, or an oxide of M3. For example, the aforementioned oxide can be Li. a W b O c Li a Zr b O c Li a Ti b O c Li a Ni b O c Li a B b O c W b O c Zr b O c Ti b O c Or B b O c However, the examples above are only for ease of understanding, and the oxides defined herein are not limited to the examples above.

[0070] In another embodiment, the aforementioned alloy oxide may be an oxide formed by complexing lithium with at least two elements represented by M3, or further includes an oxide formed by complexing lithium with at least two elements represented by M3. The oxide formed by complexing lithium with at least two elements represented by M3 may be Li a (W / Ti) b O c Li a (W / Zr) b O c Li a (W / Ti / Zr) b O c Li a (W / Ti / B) b O c And so on, but not limited to these.

[0071] The aforementioned alloy oxide can exhibit a concentration gradient that decreases from the surface of the secondary particle towards its center. Therefore, the concentration of the aforementioned alloy oxide can decrease from the outermost surface of the secondary particle towards its center.

[0072] As described above, the alloy oxide exhibits a concentration gradient that decreases from the surface of the secondary particles towards the center, thereby further reducing residual lithium on the surface of the lithium composite oxide. Furthermore, the alloy oxide can prevent a decrease in crystallinity in the inner surface region of the lithium composite oxide. In addition, the alloy oxide can prevent the overall structural collapse of the positive electrode active material during electrochemical reactions.

[0073] In addition, the coating may include a first coating and a second coating, wherein the first coating contains at least one alloy oxide represented by the above chemical formula 2, and the second coating contains at least one alloy oxide represented by the above chemical formula 2 and an oxide different from the oxide contained in the first coating.

[0074] Lithium secondary batteries

[0075] According to another aspect of the present invention, a positive electrode can be provided comprising a positive current collector and a positive active material layer formed on the positive current collector. The positive active material layer may comprise the positive active material according to various embodiments of the present invention. Therefore, the positive active material is the same as described above, and for convenience, its specific description will be omitted hereafter; only the remaining undescribed components will be described.

[0076] There are no particular limitations on the aforementioned positive electrode current collector, as long as it does not induce chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel surfaces treated with carbon, nickel, titanium, silver, etc., can be used. Furthermore, the aforementioned positive electrode current collector typically has a thickness of 3μm to 500μm, and fine irregularities can be formed on the surface of the current collector to improve the adhesion of the positive electrode active material. For example, it can be used in various forms such as thin films, sheets, foils, meshes, porous bodies, foams, and nonwovens.

[0077] The aforementioned positive electrode active material layer can be prepared by coating the aforementioned positive electrode current collector with a positive electrode slurry composition comprising the aforementioned positive electrode active material, a conductive material, and, if desired, a binder.

[0078] In this case, the content of the positive electrode active material relative to the total weight of the positive electrode active material layer can be from 80% to 99% by weight, more specifically from 85% to 98.5% by weight. When the positive electrode active material is included in the above content range, excellent capacity characteristics can be exhibited, but it is not limited thereto.

[0079] The aforementioned conductive materials are used to impart conductivity to the electrodes. In the constructed battery, they can be used without restriction as long as they do not cause chemical changes and possess electronic conductivity. Specific examples include graphite such as natural or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; 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; or conductive polymers such as polyphenylene derivatives. One or more mixtures of these materials may be used. The total weight of the positive electrode active material layer may range from 0.1% to 15% by weight of the aforementioned conductive materials.

[0080] The aforementioned binder enhances the adhesion between multiple positive electrode active material particles and the bonding force between the positive electrode active material and the 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, tetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated ethylene propylene diene monomer (EPDM), styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof; one or more mixtures thereof may be used. The binder may comprise 0.1% to 15% by weight of the aforementioned binder relative to the total weight of the positive electrode active material layer.

[0081] In addition to utilizing the aforementioned positive electrode active material, the positive electrode can be prepared according to conventional positive electrode preparation methods. Specifically, it can be prepared by coating a positive electrode slurry composition onto a positive electrode current collector and then drying and calendering it. The positive electrode slurry composition is prepared by dissolving or dispersing the aforementioned positive electrode active material in a solvent and selectively dissolving or dispersing the binder and conductive material in the solvent.

[0082] The solvents mentioned above can be solvents commonly used in this technical field, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and one or a mixture of two or more of them can be used. Considering the coating thickness and preparation yield of the slurry, the amount of the solvent used should be sufficient to dissolve or disperse the positive electrode active material, conductive material, and binder, and to have a viscosity that exhibits excellent thickness uniformity when coated for the preparation of the positive electrode.

[0083] Furthermore, in another embodiment, the positive electrode can also be prepared by casting the positive electrode slurry composition onto a separate support, and then laminating the thin film obtained by peeling off the support onto the positive electrode current collector.

[0084] Furthermore, according to another aspect of the present invention, an electrochemical device including the aforementioned positive electrode can be provided. Specifically, the aforementioned electrochemical device can be a battery, a capacitor, etc., and more specifically, it can be a lithium secondary battery.

[0085] Specifically, the aforementioned lithium secondary battery may include a positive electrode, a negative electrode disposed opposite to the positive electrode, and a separator and electrolyte disposed between the positive electrode and the negative electrode. The positive electrode is the same as described above; therefore, for convenience, a detailed description is omitted. The remaining components not previously described will be explained below.

[0086] The aforementioned lithium secondary battery may optionally include: a battery container for housing the electrode assembly comprising the positive electrode, the negative electrode, and the separation membrane; and a sealing component for sealing the battery container.

[0087] The aforementioned negative electrode may include a negative current collector and a layer of negative active material located on the aforementioned negative current collector.

[0088] There are no particular limitations on the aforementioned negative electrode current collector, as long as it does not induce chemical changes in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel surfaces treated with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloys can be used. Furthermore, the aforementioned negative electrode current collector can typically have a thickness of 3μm to 500μm. Similar to the positive electrode current collector, the bonding force of the negative electrode active material can be enhanced by forming fine irregularities on the surface of the current collector. For example, it can be used in various forms such as thin films, sheets, foils, meshes, porous bodies, foams, and nonwovens.

[0089] The aforementioned negative electrode active material layer can be prepared by coating the aforementioned negative electrode current collector with a negative electrode slurry composition comprising the aforementioned negative electrode active material and conductive material, and selectively including a binder as needed.

[0090] As the aforementioned negative electrode active material, compounds capable of reversible lithium insertion and extraction can be used. Specific examples include carbon materials such as artificial graphite, natural graphite, graphitized carbon fibers, and amorphous carbon; and metal compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; such as SiO₂. βMetal oxides capable of being doped and dedoped with lithium, such as (0 < β < 2), SnO2, vanadium oxide, and lithium vanadium oxide; or composites containing the above-mentioned metal compounds and carbon materials, such as Si-C composites or Sn-C composites, may be used, and one or more mixtures thereof may be used. Furthermore, lithium metal films may also be used as the above-mentioned negative electrode active material. Moreover, both low-crystalline carbon and high-crystalline carbon can be used as carbon materials. Representative low-crystalline carbons are soft carbon and hard carbon, while representative high-crystalline carbons are amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite, condensed graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitch, and high-temperature calcined carbons such as petroleum or coal tar pitch-derived cokes.

[0091] Based on the total weight of the negative electrode active material layer, it may contain 80% to 99% of the aforementioned negative electrode active material.

[0092] The aforementioned binder is a component that facilitates the bonding between the conductive material, the active material, and the current collector. Typically, 0.1% to 10% by weight of the binder can be added based on the total weight of the negative electrode active material layer. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, nitrile rubber, fluororubber, and various copolymers thereof.

[0093] The aforementioned conductive material is a component used to further improve the conductivity of the negative electrode active material. Based on the total weight of the negative electrode active material layer, up to 10% by weight of the aforementioned conductive material can be added, preferably up to 5% by weight. There are no particular limitations on this conductive material as long as it does not induce chemical changes in the corresponding battery and possesses conductivity. For example, graphite such as natural graphite or artificial graphite can be used; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber or metal fiber; metal powders such as fluorinated carbon, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0094] In one embodiment, the aforementioned negative electrode active material layer can be prepared by coating a negative electrode slurry composition onto a negative electrode current collector and drying it. The negative electrode slurry composition is prepared by dissolving or dispersing the negative electrode active material in a solvent and selectively dissolving or dispersing the binder and conductive material in the solvent. Alternatively, the negative electrode slurry composition can be cast onto a separate support, and then a thin film layer obtained by peeling off the support can be pressed onto the negative electrode current collector.

[0095] Furthermore, in another embodiment, the aforementioned negative electrode active material layer can also be prepared by coating a negative electrode slurry composition onto a negative electrode current collector and drying it. The aforementioned negative electrode slurry composition is prepared by dissolving or dispersing the negative electrode active material in a solvent and selectively dissolving or dispersing the binder and conductive material in the solvent. Alternatively, the aforementioned negative electrode slurry composition can be cast onto a separate support, and then a thin film layer obtained by peeling off the support can be pressed onto the negative electrode current collector.

[0096] On the other hand, in the aforementioned lithium secondary battery, the separator membrane is used to separate the negative electrode and the positive electrode and provide a channel for the movement of lithium ions. Any separator membrane commonly used in lithium secondary batteries can be used without limitation. In particular, it is preferable to have low impedance and excellent electrolyte moisture-holding capacity for electrolyte ion movement. Specifically, porous polymer films can be used, for example, porous polymer films prepared using polyolefin polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminated structures of two or more layers thereof. Furthermore, conventional porous nonwoven fabrics can also be used, for example, nonwoven fabrics made of high-melting-point glass fibers, polyethylene terephthalate fibers, etc. Moreover, to ensure heat resistance or mechanical strength, separator membranes coated with ceramic components and polymeric substances can also be used, selectively in single-layer or multi-layer structures.

[0097] Furthermore, examples of electrolytes used in this invention include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes, but they are not limited to these.

[0098] Specifically, the electrolyte may contain organic solvents and lithium salts.

[0099] As the aforementioned organic solvents, organic solvents that can act as a medium for the movement of ions participating in the electrochemical reactions of the battery can be used without limitation. Specifically, as the aforementioned organic solvents, ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone can be used; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; dimethyl carbonate (DMC), diethyl carbonate (DEC), methylethyl carbonate (MEC), ethylmethyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate can be used. Carbonate solvents such as carbonate (PC); alcohol solvents such as ethanol and isopropanol; nitriles such as R-CN (where r is a straight-chain, branched, or cyclic hydrocarbon group with 2 to 20 carbon atoms, which may contain double bonds, aromatic rings, or ether bonds); amides such as dimethylformamide; dioxanes such as 1,3-dioxane; or sulfolane, etc. Among these, carbonate solvents are preferred, and more preferably, a mixture of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate, etc.) with high ionic conductivity and high dielectric constant, and low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate, etc.) that can improve the charge-discharge performance of the battery. In this case, when the cyclic carbonate and the linear carbonate are mixed in a volume ratio of about 1:1 to about 1:9, excellent electrolyte performance can be observed.

[0100] The lithium salts described above can be any compound capable of providing lithium ions for use in lithium secondary batteries, without limitation. Specifically, the lithium salts can be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2, etc. Preferably, the concentration of the lithium salts is used in the range of 0.1M to 2.0M. When the concentration of the lithium salt is within the above range, the electrolyte has appropriate conductivity and viscosity, thus exhibiting excellent electrolyte performance and enabling efficient movement of lithium ions.

[0101] In addition to the electrolyte components described above, for the purpose of improving battery life characteristics, suppressing battery capacity reduction, and increasing battery discharge capacity, the electrolyte may also contain one or more additives such as halogenated alkylene carbonate compounds like difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glycol dimethyl ether, triammonium hexaphosphate, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolides, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxyethanol, or aluminum trichloride. In this case, the electrolyte may contain 0.1% to 5% by weight of the aforementioned additives relative to its total weight.

[0102] As described above, lithium secondary batteries containing the positive electrode active material of the present invention stably exhibit excellent discharge capacity, output characteristics, and lifespan characteristics. Therefore, they can be used in portable devices such as mobile phones, laptops, and digital cameras, as well as in electric vehicles such as hybrid electric vehicles (HEVs).

[0103] The lithium secondary battery according to the present invention is not particularly limited in shape and can be cylindrical, prismatic, pouch-shaped, or coin-shaped, etc. Furthermore, preferably, the lithium secondary battery can be used not only as a battery cell for powering small devices, but also as a unit battery in medium or large battery modules comprising multiple battery cells.

[0104] According to another aspect of the invention, a battery module comprising the aforementioned lithium secondary battery as a single unit and / or a battery pack comprising the same can be provided.

[0105] The aforementioned battery module or battery pack can be used as a power tool; an electric vehicle, including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or a power source for one or more medium or large-sized devices in an energy storage system.

[0106] The invention will be described in more detail below by way of examples. However, these examples are intended to illustrate the invention only, and the scope of the invention should not be construed as being limited to these examples.

[0107] Preparation Example 1. Preparation of Positive Electrode Active Material

[0108] (1) Example 1

[0109] Spherical Ni was synthesized using the co-precipitation method. 0.91 Co 0.08 Mn 0.01 (OH)2 hydroxide precursor.

[0110] Specifically, in a 90L reactor, NaOH is added to a 2.0M composite transition metal sulfuric acid aqueous solution prepared by mixing nickel sulfate, cobalt sulfate and manganese sulfate in a molar ratio of 91:8:1, such that the concentration of NaOH is 1.8M based on the transition metal concentration in the composite transition metal sulfuric acid aqueous solution. NH4OH is also added to the composite transition metal sulfuric acid aqueous solution, such that the concentration of NH4OH is 0.8M based on the transition metal concentration in the composite transition metal sulfuric acid aqueous solution.

[0111] The pH was maintained at 11.5 in the reactor, and the reactor temperature was maintained at 60°C. N2 was added to the reactor as an inert gas to prevent the oxidation of the prepared precursor. After the synthesis was stirred, the precursor was washed and dehydrated using a filter press (F / P) to obtain Ni. 0.91 Co 0.08 Mn 0.01 (OH)2 hydroxide precursor.

[0112] Subsequently, LiOH (Li / (Ni+Co+Mn) molar ratio = 1.01) was mixed into the synthesized precursor, and the temperature was raised to 800°C at a rate of 2°C per minute in a sintering furnace while maintaining an O2 atmosphere, and heat-treated for 10 hours to obtain lithium composite oxide.

[0113] Then, distilled water was added to the above lithium composite oxide, and the mixture was washed for 1 hour. The washed lithium composite oxide was then filtered and dried to obtain the positive electrode active material.

[0114] (2) Example 2

[0115] Except for the addition of NaOH to the above-mentioned composite transition metal sulfuric acid aqueous solution so that the concentration of NaOH is 1.6M based on the transition metal concentration and the addition of NH4OH to the above-mentioned composite transition metal sulfuric acid aqueous solution so that the concentration of NH4OH is 0.4M based on the transition metal concentration, the positive electrode active material was prepared in the same manner as in Example 1.

[0116] (3) Example 3

[0117] Except for the addition of NaOH to the above-mentioned composite transition metal sulfuric acid aqueous solution so that the concentration of NaOH is 2.2M based on the transition metal concentration and the addition of NH4OH to the above-mentioned composite transition metal sulfuric acid aqueous solution so that the concentration of NH4OH is 1.2M based on the transition metal concentration, the positive electrode active material was prepared in the same manner as in Example 1.

[0118] (4) Example 4

[0119] Spherical Ni was synthesized using the co-precipitation method. 0.91 Co 0.08 Mn 0.01 (OH)2 hydroxide precursor.

[0120] Specifically, in a 90L reactor, NaOH is added to a 1.5M composite transition metal sulfuric acid aqueous solution prepared by mixing nickel sulfate, cobalt sulfate and manganese sulfate in a molar ratio of 91:8:1, such that the concentration of NaOH is 1.6M based on the transition metal concentration in the composite transition metal sulfuric acid aqueous solution. NH4OH is also added to the composite transition metal sulfuric acid aqueous solution, such that the concentration of NH4OH is 0.4M based on the transition metal concentration in the composite transition metal sulfuric acid aqueous solution.

[0121] The pH was maintained at 11.5 in the reactor, and the reactor temperature was maintained at 60°C. N2 was added as an inert gas to prevent the oxidation of the prepared precursor. After the synthesis was stirred, the precursor was washed and dehydrated using a filter press (F / P) to obtain Ni. 0.91 Co 0.08 Mn 0.01 (OH)2 hydroxide precursor.

[0122] Subsequently, LiOH (Li / (Ni+Co+Mn) molar ratio = 1.01) and 0.5 mol% Zr were mixed in the synthesized precursor, and the temperature was raised to 700°C at a rate of 2°C per minute in a sintering furnace while maintaining an O2 atmosphere, and heat-treated for 10 hours to obtain lithium composite oxide.

[0123] Then, distilled water was added to the above lithium composite oxide, and the mixture was washed for 1 hour. The washed lithium composite oxide was then filtered and dried to obtain the positive electrode active material.

[0124] (5) Example 5

[0125] Except for mixing 1 mol% Al2O3, 0.25 mol% TiO2 and 0.05 mol% ZrO2 relative to the lithium composite oxide before washing with water, and performing an additional heat treatment for 10 hours at a rate of 2°C per minute to 680°C while maintaining an O2 atmosphere, the positive electrode active material was prepared in the same manner as in Example 1.

[0126] (6) Example 6

[0127] In addition to using spherical Ni synthesized by coprecipitation method 0.80 Co 0.10 Mn 0.10 Apart from the (OH)2 hydroxide precursor, the other positive electrode active materials were prepared in the same manner as in Example 1.

[0128] (7) Comparative Example 1

[0129] Except for the addition of NaOH to the above-mentioned composite transition metal sulfuric acid aqueous solution so that the concentration of NaOH is 1.2M based on the transition metal concentration and the addition of NH4OH to the above-mentioned composite transition metal sulfuric acid aqueous solution so that the concentration of NH4OH is 0.3M based on the transition metal concentration, the positive electrode active material was prepared in the same manner as in Example 1.

[0130] (8) Comparative Example 2

[0131] Except for the addition of NaOH to the above-mentioned composite transition metal sulfuric acid aqueous solution so that the concentration of NaOH is 2.5M based on the transition metal concentration and the addition of NH4OH to the above-mentioned composite transition metal sulfuric acid aqueous solution so that the concentration of NH4OH is 1.5M based on the transition metal concentration, the positive electrode active material was prepared in the same manner as in Example 1.

[0132] (9) Comparative Example 3

[0133] Except for the addition of NaOH to the above-mentioned composite transition metal sulfuric acid aqueous solution so that the concentration of NaOH is 1.8M based on the transition metal concentration and the addition of NH4OH to the above-mentioned composite transition metal sulfuric acid aqueous solution so that the concentration of NH4OH is 1.5M based on the transition metal concentration, the positive electrode active material was prepared in the same manner as in Example 1.

[0134] (10) Comparative Example 4

[0135] Except for the addition of NaOH to the above-mentioned composite transition metal sulfuric acid aqueous solution so that the concentration of NaOH is 2.5M based on the transition metal concentration and the addition of NH4OH to the above-mentioned composite transition metal sulfuric acid aqueous solution so that the concentration of NH4OH is 0.4M based on the transition metal concentration, the positive electrode active material was prepared in the same manner as in Example 1.

[0136] (11) Comparative Example 5

[0137] Except for the addition of NaOH to the above-mentioned composite transition metal sulfuric acid aqueous solution so that the concentration of NaOH is 1.2M based on the transition metal concentration and the addition of NH4OH to the above-mentioned composite transition metal sulfuric acid aqueous solution so that the concentration of NH4OH is 1.5M based on the transition metal concentration, the positive electrode active material was prepared in the same manner as in Example 1.

[0138] Preparation Example 2. Preparation of Lithium Secondary Batteries

[0139] A positive electrode slurry was prepared by dispersing 92% by weight of each of the positive electrode active materials prepared according to Preparation Example 1, 4% by weight of artificial graphite, and 4% by weight of PVDF binder in 30g of N-methyl-2-pyrrolidone (NMP). The above positive electrode slurry was uniformly coated on an aluminum film with a thickness of 15μm and vacuum dried at 135°C to prepare a positive electrode for lithium secondary batteries.

[0140] In contrast to the above positive electrode, lithium foil was used as the counter electrode, a porous polyethylene membrane (Celgard 2300, thickness: 25 μm) was used as the separation membrane, and a button cell was prepared using an electrolyte containing ethylene carbonate and ethyl methyl carbonate in a volume ratio of 3:7 and LiPF6 at a concentration of 1.15 M.

[0141] Experimental Example 1. SEM Analysis of Positive Electrode Active Material

[0142] A cross-sectional SEM image of the positive electrode active material prepared according to Preparation Example 1 was taken to confirm the porosity characteristics in the cross-section of the lithium composite oxide contained in the positive electrode active material.

[0143] Specifically, cross-sectional SEM images were obtained after cross-sectional processing of the lithium composite oxide contained in the above-mentioned positive electrode active material using a focused ion beam (FIB). From these cross-sectional SEM images, the average particle size (D50), average long axis length (b), average short axis length (a), average b / a ratio, average pore area, and pore occupancy of the lithium composite oxide were determined. The results are shown in Table 1 below.

[0144] Table 1

[0145]

[0146] The symbol in parentheses for the major axis length indicates the value relative to D50(d).

[0147] Table 2

[0148]

[0149] The symbol in parentheses for the major axis length indicates the value relative to D50.

[0150] Experimental Example 2. Particle Strength Measurement of Positive Electrode Active Material

[0151] When preparing a positive electrode for a lithium-ion secondary battery using positive electrode active materials, the process involves coating a slurry containing the positive electrode active material onto the positive electrode current collector, drying, and then calendering (pressing). During this process, calendering under high pressure can cause the particles of the positive electrode active material coated on the positive electrode current collector to disintegrate, potentially reducing the performance of the positive electrode active material.

[0152] In this experimental example, in order to confirm the strength change of the positive electrode active material based on the composition of the aggregate of multiple secondary particles contained in the positive electrode active material, the positive electrode active materials prepared according to Examples 1 to 3 and Comparative Examples 1, 2 and 5 were dried in a vacuum oven at 60°C for 12 hours, and then a particle (lithium composite oxide) corresponding to D50 was selected to measure the breaking strength (pressure when the particle is broken).

[0153] Table 3 below shows the average value of the above measurements obtained after measuring the destructive strength of each positive electrode active material 10 times.

[0154] Table 3

[0155]

[0156] Referring to the results in Table 3 above, it can be confirmed that the destructive strength of the positive electrode active materials according to Examples 1 to 3 is slightly lower than that of the positive electrode active material according to Comparative Example 1 (porosity of 0.1%), which appears to have no pores in the positive electrode active material. However, compared with the positive electrode active materials according to Comparative Examples 2 and 5, they exhibit increased destructive strength. In particular, it can be confirmed that when comparing the positive electrode active material according to Example 3 and the positive electrode active material according to Comparative Example 5, even with the same porosity, the positive electrode active material according to Example 3 has a higher destructive strength.

[0157] In other words, according to the present invention, it can be confirmed that by adjusting the ratio of ammonia and caustic soda used in the co-precipitation reaction for synthesizing the precursor constituting the positive electrode active material, the pore area and shape of the lithium composite oxide contained in the positive electrode active material can be controlled, which can help improve the stability of the positive electrode active material.

[0158] Experimental Example 3. Evaluation of Capacity and Lifetime Characteristics of Lithium Secondary Batteries

[0159] The lithium secondary battery (button cell) prepared according to Preparation Example 2 was subjected to charge-discharge experiments at 25°C, a voltage range of 3.0V to 4.3V, and a discharge rate of 0.1C to determine its charge capacity and discharge capacity.

[0160] In addition, the same lithium secondary battery was subjected to 50 charge / discharge cycles at 1C / 1C conditions within a driving voltage range of 3.0V to 4.4V at 25°C, and the ratio of the discharge capacity at the 50th cycle to the initial capacity was then measured (capacity retention).

[0161] The results of the above measurements are shown in Table 4 below.

[0162] Table 4

[0163]

[0164] Referring to the results in Table 4 above, it can be confirmed that the electrochemical properties of the above-mentioned positive electrode active material can be improved not only by controlling the porosity of the lithium composite oxide contained in the positive electrode active material, but also by adjusting the ratio of ammonia and caustic soda used in the co-precipitation reaction of the synthesis precursor to control the pore area and shape of the above-mentioned lithium composite oxide.

[0165] While the embodiments of the present invention have been described above, those skilled in the art will understand that various modifications and alterations can be made to the present invention by adding, modifying, deleting, or supplementing the constituent elements without departing from the spirit of the present invention as described in the claims, and these modifications and alterations also fall within the scope of the present invention.

Claims

1. A positive electrode active material, characterized in that, Includes lithium composite oxides represented by the following chemical formula 1 and capable of intercalating or deintercalating lithium ions. [Chemical Formula 1] Li w Ni 1-(x+y+z) Co x M1 y M2 z O 2+α (in, M1 is at least one selected from Mn and Al. M2 is selected from at least one of Mn, P, Sr, Ba, B, Ti, Zr, Al, Hf, Ta, Mg, V, Zn, Si, Y, Sn, Ge, Nb, W, and Cu. M1 and M2 are distinct elements. 0.5≤w≤1.5, 0≤x≤0.50, 0≤y≤0.20, 0≤z≤0.20, 0≤α≤0.02) The average ratio (b / a) of the major axis length (b) to the minor axis length (a) of the pores observed from the cross-sectional scanning electron microscope images of the aforementioned lithium composite oxides is 1 to 3. The porosity of the aforementioned pores in the cross-section of the lithium composite oxide, as observed from cross-sectional scanning electron microscopy images of the aforementioned lithium composite oxide, ranges from 0.3% to 3.5%.

2. The positive electrode active material according to claim 1, characterized in that, When the average particle size (D50) of the above-mentioned lithium composite oxide is d, the average value of the long axis length (b) of the pores observed from the cross-sectional scanning electron microscope image of the above-mentioned lithium composite oxide is less than 0.15d.

3. The positive electrode active material according to claim 1, characterized in that, The proportion of pores in the total porosity observed from the cross-sectional scanning electron microscope images of the above-mentioned lithium composite oxides, in which the ratio (b / a) of the major axis length (b) of the above-mentioned pores to the minor axis length (a) of the above-mentioned pores is greater than 3, is less than 50%.

4. The positive electrode active material according to claim 1, characterized in that, The average pore area observed in the cross-sectional scanning electron microscope images of the aforementioned lithium composite oxide was 0.02 μm. 2 Up to 1.0μm 2 .

5. The positive electrode active material according to claim 1, characterized in that, The average particle size (D50) of the above-mentioned lithium composite oxides is 5 μm to 20 μm.

6. The positive electrode active material according to claim 1, characterized in that, The BET specific surface area of ​​the aforementioned lithium composite oxide is 0.2 m². 2 / g to 2.0m 2 / g.

7. The positive electrode active material according to claim 1, characterized in that, The total content of LiOH and Li2CO3 relative to the total weight of the above positive electrode active materials is less than 1.0% by weight.

8. The positive electrode active material according to claim 1, characterized in that, At least a portion of the surface of the aforementioned lithium composite oxide also includes an alloy oxide represented by the following chemical formula 2: [Chemical Formula 2] Read a M3 b Oh c (in, M3 is selected from at least one of Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, Ce, V, Ba, Ta, Sn, Hf, Gd, and Nd. 0≤a≤10,0 <b≤8,2≤c≤13)。 9. A positive electrode, characterized in that, Includes the positive electrode active material according to any one of claims 1 to 8.

10. A lithium secondary battery, characterized in that, Use the positive electrode as described in claim 9.

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