Positive electrode active material, method for preparing the same, and rechargeable lithium battery including the same

Abstract

Disclosed are a method of preparing a positive electrode active material for a rechargeable lithium battery, a positive electrode active material for a rechargeable lithium battery, and a rechargeable lithium battery, the method including preparing a first aqueous solution including composite particles and a cobalt salt, the composite particles including a lithium-nickel-based composite oxide and being in the form of secondary particles in which a plurality of primary particles are aggregated, and the first aqueous solution having a pH of 6 to 8; adding a basic substance to the first aqueous solution to obtain a second aqueous solution in which a pre-positive electrode active material including the composite particles and a cobalt coating layer formed on the surface thereof is obtained; and heat-treating the pre-positive electrode active material to obtain a final positive electrode active material.

Description

Technical Field

[0001] A positive electrode active material for rechargeable lithium batteries, a method for preparing the material, and a rechargeable lithium battery including the material are disclosed. Background Technology

[0002] Portable information devices such as mobile phones, laptops, and smartphones, as well as electric vehicles, already use rechargeable lithium batteries with high energy density and portability as their power source. Recently, research has been actively underway to use rechargeable lithium batteries with high energy density as a power source or energy storage source for hybrid or electric vehicles.

[0003] Various cathode active materials have been studied to realize rechargeable lithium batteries for these applications. Among them, lithium nickel oxides, lithium nickel manganese cobalt composite oxides, lithium nickel cobalt aluminum composite oxides, and lithium cobalt oxides are mainly used as cathode active materials. However, these cathode active materials suffer from structural collapse or cracking during repeated charging and discharging, resulting in long-term cycle life degradation and increased resistance in rechargeable lithium batteries, thus failing to exhibit satisfactory capacity characteristics. Therefore, there is a need to develop novel cathode active materials that ensure long-term cycle life characteristics and achieve high capacity and high energy density. Summary of the Invention

[0004] A positive electrode active material for a rechargeable lithium battery, a method for preparing the same, and a rechargeable lithium battery including the positive electrode active material are provided. The positive electrode active material achieves high capacity while exhibiting improved cycle life characteristics and suppressing side reactions with the electrolyte solution.

[0005] In an embodiment, the positive electrode active material for a rechargeable lithium battery includes a first positive electrode active material and a second positive electrode active material. The first positive electrode active material includes a lithium-nickel composite oxide, which has a nickel content of greater than or equal to 90 mol% based on the total amount of metals other than lithium. The first positive electrode active material includes secondary particles and a cobalt coating on the surface of the secondary particles. Multiple primary particles are aggregated in the secondary particles, and the secondary particles have a particle size of 10 μm to 25 μm. The second positive electrode active material includes a lithium-nickel composite oxide, which has a nickel content of greater than or equal to 90 mol% based on the total amount of metals other than lithium. The second positive electrode active material includes secondary particles and a cobalt coating on the surface of the secondary particles. Multiple primary particles are aggregated in the secondary particles, and the secondary particles have a particle size of 1 μm to 9 μm. The ratio (X / Y) of the cobalt content (X) on the surface of the secondary particles of the first positive electrode active material to the cobalt content (Y) on the surface of the secondary particles of the second positive electrode active material is greater than 1.

[0006] In another embodiment, a method for preparing a positive electrode active material for a rechargeable lithium battery includes preparing a first aqueous solution comprising composite particles and a cobalt salt, the composite particles comprising lithium-nickel composite oxides and in the form of secondary particles, wherein a plurality of primary particles are aggregated in the secondary particles, and the first aqueous solution having a pH of 6 to 8; adding an alkaline substance to the first aqueous solution to obtain a second aqueous solution, wherein a pre-positive electrode active material comprising composite particles and a cobalt coating formed on its surface is obtained in the second aqueous solution; and heat-treating the pre-positive electrode active material to obtain a final positive electrode active material.

[0007] In another embodiment, a rechargeable lithium battery is provided, comprising a positive electrode containing a positive electrode active material, a negative electrode, and an electrolyte.

[0008] The positive electrode active material for rechargeable lithium batteries prepared according to the embodiments and the rechargeable lithium batteries including the positive electrode active material can exhibit suppressed side reactions, improved cycle life characteristics and safety, while achieving high capacity and high energy density. Attached Figure Description

[0009] Figure 1 A schematic diagram illustrating a rechargeable lithium battery according to an embodiment.

[0010] Figure 2 Scanning electron microscope image of the positive electrode active material in the rinsing step of Comparative Example 1.

[0011] Figure 3 A scanning electron microscope image of the pre-positive electrode active material of Comparative Example 1.

[0012] Figure 4 and Figure 5 A scanning electron microscope image of the final positive electrode active material of Comparative Example 1.

[0013] Figure 6 This is a scanning electron microscope image of the positive electrode active material in the rinsing step of Example 1.

[0014] Figure 7 This is a scanning electron microscope image of the pre-positive electrode active material in Example 1.

[0015] Figure 8 and Figure 9 This is a scanning electron microscope image of the final positive electrode active material of Example 1.

[0016] Figure 10 The graph shows the zeta potential of the positive electrode active materials of Comparative Examples 1 and 2 measured by ESA analysis.

[0017] Figure 11A graph showing the room temperature cycle life characteristics of the battery cells manufactured in Comparative Example 1 and Example 1.

[0018] Figure 12 A graph showing the high-temperature cycle life characteristics of the battery cells manufactured in Comparative Example 1 and Example 1.

[0019] Figure 13 The image shown is an energy dispersive X-ray spectroscopy (EDS) image of the pre-positive electrode active material in Example 1.

[0020] Figure 14 This is an energy dispersive X-ray spectroscopy (EDS) image of the final positive electrode active material of Example 1.

[0021] Figure 15 The image shows an energy dispersive X-ray spectroscopy (EDS) image of the pre-positive active material of Comparative Example 1.

[0022] Figure 16 Energy dispersive X-ray spectroscopy (EDS) image of the final positive electrode active material of Comparative Example 1.

[0023] <Symbol Description>

[0024] 100: Rechargeable lithium battery; 112: Negative electrode

[0025] 113: Separator 114: Positive electrode

[0026] 120: Battery casing; 140: Sealing component Detailed Implementation

[0027] Specific implementation methods will be described in detail below so that those skilled in the art can readily implement them. However, this disclosure may be embodied in many different forms and is not to be construed as limited to the exemplary implementation methods set forth herein.

[0028] The terminology used herein is for descriptive purposes only and is not intended to limit this disclosure. Unless the context clearly indicates otherwise, singular expressions include plural expressions.

[0029] As used in this article, "combinations of" refers to mixtures of components, laminates, composites, copolymers, alloys, blends, and reaction products, etc.

[0030] It should be understood that terms such as “comprising,” “including,” or “having” in this document are intended to indicate the presence of a feature, quantity, step, element, or combination thereof, but do not preclude the possibility of the presence or addition of one or more other features, quantities, steps, elements, or combinations thereof.

[0031] In the accompanying drawings, for clarity, the thicknesses of layers, films, panels, areas, etc., are enlarged, and throughout the specification, the same reference numerals denote the same elements. It will be understood that when an element such as a layer, film, area, or substrate is referred to as being "on" another element, it can be directly on the other element, or an intervening element may be present. Conversely, when an element is referred to as being "directly on" another element, no intervening element is present.

[0032] In addition, the term "layer" in this article includes not only shapes that form across the entire surface when viewed from a plan view, but also shapes that form on a portion of the surface.

[0033] Additionally, the term "particle size" described in this disclosure may refer to the average particle size. The average particle size can be measured using methods well known to those skilled in the art, for example, by a particle size analyzer, or by transmission electron microscopy (TEM) or scanning electron microscopy (SEM) photographs. Optionally, the average particle size value can be obtained by measuring using dynamic light scattering methods, performing data analysis, counting the number of particles in each particle size range, and thereby calculating. Unless otherwise defined, the average particle size (D50) refers to the diameter of particles that accumulate to 50% of the total volume in the particle size distribution. Furthermore, the average particle size can be measured by optical microscopy photographs in such a way that 50 particles are randomly selected in an SEM image, parallel lines are drawn to clamp the projected image of the particles, and the maximum distance between the parallel lines for each particle is measured to determine its particle size. The particle size distribution is then obtained based on the measured particle size, where the D50 of particles having a cumulative volume of 50% is considered the average particle size.

[0034] Positive electrode active material and its preparation method

[0035] Generally, high-nickel cathode active materials with high nickel content have the advantage of achieving high capacity. However, because the layered structure on their surface collapses with repeated charging and discharging, they have the disadvantage of reduced cycle life or safety due to side reactions with the electrolyte solution. To address this drawback, attempts have been made to coat cobalt using a wet coating method to form a cobalt-rich layer on the surface of the cathode active material, thereby controlling side reactions and improving cycle life performance.

[0036] However, in conventional wet coating methods, the high nickel content in the positive electrode active material results in cobalt compounds not being uniformly coated on the surface, but rather in clumps and unevenly. This cobalt coating acts as a resistor, thus reducing capacity and charging / discharging efficiency. The traditional wet coating method involves adding the positive electrode active material to an aqueous solution containing an alkaline material (such as NaOH), rinsing, and then slowly adding cobalt salts. Here, the initial pH during rinsing is approximately 13.4, but decreases to 7.4 as the cobalt salts are added.

[0037] Conversely, one implementation uses a coating method in which cobalt salt is first added to distilled water, a positive electrode active material is added to rinse it, and then an alkaline substance such as NaOH is slowly added to increase the pH from 7.4 to 13.4.

[0038] Specifically, in an embodiment, a method for preparing a positive electrode active material for a rechargeable lithium battery includes: preparing a first aqueous solution comprising composite particles and a cobalt salt, the composite particles comprising a lithium-nickel composite oxide and in the form of secondary particles (in which multiple primary particles are aggregated), wherein the first aqueous solution has a pH of about 6 to about 8; adding an alkaline substance to the first aqueous solution to obtain a second aqueous solution (e.g., adding the alkaline substance dropwise to the first aqueous solution, or adding the alkaline substance dropwise to the first aqueous solution), obtaining a pre-positive electrode active material comprising composite particles and a cobalt coating formed on its surface in the second aqueous solution; and heat-treating the pre-positive electrode active material to obtain a final positive electrode active material.

[0039] The above method can be used to obtain a positive electrode active material with a high density of cobalt compound coated very uniformly on the surface, and a positive electrode active material with a cobalt compound uniformly coated inside the secondary particles of the positive electrode active material (i.e., on the surface of the primary particles (grain boundaries) inside the secondary particles). The amount of cobalt compound coated inside the secondary particles can be greater than the amount of cobalt compound in conventional positive electrode active materials.

[0040] In particular, it is more difficult to obtain a uniform cobalt coating when the positive electrode active material has a very high nickel concentration, for example, when the nickel content is greater than or equal to about 80 mol% or greater than or equal to about 90 mol% based on the total amount of metals other than lithium in the positive electrode active material. However, according to the preparation method according to the embodiment, even when the nickel content is greater than or equal to about 90 mol%, a very uniform high-density coating can be obtained.

[0041] This positive electrode active material has high initial discharge capacity, high initial charge / discharge efficiency, and excellent cycle life characteristics, while achieving very high capacity.

[0042] In a method for producing a positive electrode active material, the composite particles include a lithium-nickel-based composite oxide, and the lithium-nickel-based composite oxide may also be referred to as a nickel-based positive electrode active material, and may be represented by, for example, Chemical Formula 1.

[0043] [Chemical Formula 1]

[0044] Li a1 Ni x1 M 1 y1 M 2 1-x1-y1 O2

[0045] In Chemical Formula 1, 0.9≤a1≤1.8, 0.3≤x1≤1, 0≤y1≤0.7, and M 1 and M 2 are each independently selected from the group consisting of Al, B, Ce, Co, Cr, F, Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr and combinations thereof.

[0046] In Chemical Formula 1, 0.3≤x1≤1 and 0≤y1≤0.7, or 0.4≤x1≤1 and 0≤y1≤0.6, or 0.5≤x1≤1 and 0≤y1≤0.5, or 0.6≤x1≤1 and 0≤y1≤0.4, or 0.7≤x1≤1 and 0≤y1≤0.3, or 0.8≤x1≤1 and 0≤y1≤0.2, or 0.9≤x1≤1 and 0≤y1≤0.1.

[0047] For example, the lithium-nickel-based composite oxide may be represented by Chemical Formula 2.

[0048] [Chemical Formula 2]

[0049] Li a2 Ni x2 Co y2 M 3 1-x2-y2 O2

[0050] In Chemical Formula 2, 0.9≤a2≤1.8, 0.3≤x2<1, 0<y2≤0.7, and M 3 is selected from the group consisting of Al, B, Ce, Cr, F, Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr and combinations thereof.

[0051] In Chemical Formula 2, 0.3≤x2<1 and 0<y2≤0.7, or 0.4≤x2<1 and 0<y2≤0.6, or 0.5≤x2<1 and 0<y2≤0.5, or 0.6≤x2<1 and 0<y2≤0.4, or 0.7≤x2<1 and 0<y2≤0.3, or 0.8≤x2<1 and 0<y2≤0.2, or 0.9≤x2<1 and 0<y2≤0.1.

[0052] For example, the lithium-nickel composite oxide can be represented by Chemical Formula 3.

[0053] [Chemical Formula 3]

[0054] Li a3 Ni x3 Co y3 Al z3 M 4 1-x3-y3-z3 O₂

[0055] In Chemical Formula 3, 0.9≤a3≤1.8, 0.3≤x3<1, 0<y3<0.7, 0<z3<0.7, and M 4 is selected from B, Ce, Cr, F, Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr and combinations thereof.

[0056] In Chemical Formula 3, 0.3≤x3<1, 0<y3<0.7, and 0<z3<0.7, or 0.4≤x3<1, 0<y3<0.6, and 0<z3<0.6, or 0.5≤x3<1, 0<y3<0.5, and 0<z3<0.5, or 0.6≤x3<1, 0<y3<0.4, and 0<z3<0.4, or 0.7≤x3<1, 0<y3<0.3, and 0<z3<0.3, or 0.8≤x3<1, 0<y3<0.2, and 0<z3<0.2, or 0.9≤x3<1, 0<y3<0.1, and 0<z3<0.1.

[0057] The average particle diameter of the composite particles may be, for example, from about 1 μm to about 30 μm, for example, from about 2 μm to about 25 μm. For example, the composite particles may be a mixture of large particles and small particles, which will be described later.

[0058] In the preparation method, the cobalt salt may include, for example, cobalt sulfate, cobalt nitrate, cobalt carbonate or a combination thereof.

[0059] The first aqueous solution may be prepared by, for example, adding a cobalt salt to distilled water, then adding the composite particles thereto. Further, preparation of the first aqueous solution may include mixing the composite particles and the cobalt salt. The composite particles can be washed in the first aqueous solution.

[0060] The alkaline substance may be, for example, NaOH, NH4OH, KOH, or a combination thereof, or may be in the form of an alkaline aqueous solution including such a substance. The second aqueous solution prepared by adding the alkaline substance to the first aqueous solution may have a final pH in the range of about 12 to about 14, for example, about 13 to about 14. The second aqueous solution can be obtained by adding (e.g., dropwise) the alkaline substance to the first aqueous solution. In this case, the composite particles can be coated with cobalt. It is understood that the composite particles can be rinsed in the aqueous solution while being coated with cobalt.

[0061] The pre-positive electrode active material obtained from the second aqueous solution is in a state where a cobalt compound is coated on the surface of the composite particles. This pre-positive electrode active material can be a dried product obtained by drying the composite particles (coated with the cobalt compound) obtained from the second aqueous solution. On the surface of the pre-positive electrode active material, the cobalt content (A) can range from about 56 atomic% to about 90 atomic% based on the total amount of metals other than lithium, for example, about 58 atomic% to about 90 atomic%, about 60 atomic% to about 90 atomic%, or about 70 atomic% to about 90 atomic%.

[0062] After heat treatment, the final positive electrode active material is obtained as the final product of the pre-positive electrode active material. Through this heat treatment, some cobalt compounds coated on the surface of the composite particles are understood to penetrate into the secondary particles of the positive electrode active material.

[0063] On the surface of the final positive electrode active material, the cobalt content (B) can range from about 15 atomic% to about 55 atomic% based on the total amount of transition metals other than lithium, for example, about 20 atomic% to about 52 atomic%, about 25 atomic% to about 50 atomic%, about 30 atomic% to about 45 atomic%, or about 34 atomic% to about 38 atomic%.

[0064] The ratio (B / A) of the cobalt content (B) based on the total amount of transition metals other than lithium on the surface of the final positive electrode active material to the cobalt content (A) based on the total amount of transition metals other than lithium on the surface of the pre-positive electrode active material can be in the range of about 0.3 to about 0.49, for example, about 0.35 to about 0.49, or about 0.40 to about 0.49.

[0065] Based on the total amount of cobalt compound coated on the positive electrode active material (corresponding to A), the amount of cobalt compound remaining on the surface of the positive electrode active material (corresponding to B) is understood to be in the range of about 30 atomic% to about 49 atomic%, while the remaining amount of cobalt compound penetrating into the interior of the positive electrode active material (corresponding to AB) is in the range of about 51 atomic% to about 70 atomic%.

[0066] In this way, when the cobalt content (A) on the surface of the pre-positive active material, the cobalt content (B) on the surface of the final positive active material, and their ratio (B / A) meet the specified range, the positive active material can achieve high capacity with almost no structural collapse as it is optimized for repeated charging and discharging because the cobalt content on the surface and inside of the positive active material is optimized. Therefore, rechargeable lithium batteries including this material can achieve high initial discharge capacity, high charge / discharge efficiency, excellent cycle life characteristics, high capacity, and high energy density.

[0067] According to conventional cobalt coating methods, the ratio of B to A (B / A) is greater than or equal to about 0.59. In contrast, when the positive electrode active material is prepared according to the method described in the embodiment, the ratio of B to A (B / A) can be in the range of about 0.3 to about 0.49. Even with repeated charging and discharging, the positive electrode active material exhibits almost no structural collapse and also demonstrates excellent cycle life characteristics while ensuring safety.

[0068] Specifically, the final positive electrode active material may include secondary particles (in which multiple primary particles are aggregated), and the final positive electrode active material may include a first coating portion, which is a cobalt coating on the surface of the secondary particles, and may further include a second coating portion, which is a cobalt coating on the surface of the primary particles inside the secondary particles.

[0069] The second coating portion exists within the secondary particles, not on the surface, and is coated along the interface between the primary and secondary particles within the secondary particles; therefore, it can be represented as being coated at the grain boundaries. Here, the interior of the secondary particles refers to the entire interior excluding the surface, for example, it can refer to the entire interior starting at a depth of approximately 2 μm from the outer surface. According to an embodiment, the cobalt compound can be coated across the entire grain boundary within the secondary particles, regardless of the depth from the surface.

[0070] The ratio (C / (C+D)) of the cobalt content (C) of the first coating portion to the sum of the cobalt content (C) of the first coating portion and the cobalt content (D) of the second coating portion in the final positive electrode active material can be in the range of about 0.30 to about 0.49, for example, about 0.35 to about 0.49, or about 0.40 to about 0.49. When the ratio (C / (C+D)) of the cobalt content (C) of the first coating portion to the sum of the cobalt content (C) of the first coating portion and the cobalt content (D) of the second coating portion in the final positive electrode active material is within this range, the positive electrode active material can achieve high capacity with almost no structural collapse with repeated charging and discharging. Therefore, rechargeable lithium batteries including it can achieve high initial discharge capacity and charge and discharge efficiency, and excellent cycle life characteristics, while achieving high capacity and high energy density.

[0071] In the final positive electrode active material, the first coating portion refers to a cobalt coating present on the surface of the positive electrode active material, and its thickness can be from about 1 nm to about 500 nm, for example, from about 1 nm to about 400 nm, or from about 50 nm to about 300 nm. The second coating portion refers to a cobalt coating present at the grain boundaries inside the positive electrode active material, and its thickness can be in the range of from about 1 nm to about 250 nm, from about 1 nm to about 200 nm, from about 5 nm to about 150 nm, or from about 10 nm to about 100 nm. In the positive electrode active material according to the embodiment, the first coating portion and the second coating portion can be coated very uniformly, while having the thickness ranges described above. The first coating portion and the second coating portion enable the positive electrode active material to achieve a long cycle life, without structural collapse even after repeated charging and discharging, and can effectively suppress side reactions with the electrolyte solution.

[0072] The final positive electrode active material may be a positive electrode active material including lithium nickel composite oxides, and may include the aforementioned compounds represented by chemical formula 1, chemical formula 2 or chemical formula 3, having a particle size (e.g., average particle size) of about 1 μm to about 30 μm or about 2 μm to about 25 μm, and the final positive electrode active material may be in the form of a mixture of large and small particles, which will be described later.

[0073] Meanwhile, in embodiments, a mixture of large and small particles can be used as composite particles. For example, the composite particles can be a mixture of large particles with a particle size (e.g., average particle size) of about 10 μm to about 25 μm and small particles with a particle size (e.g., average particle size) of about 1 μm to about 9 μm. Accordingly, the final positive electrode active material can be a mixture of a first positive electrode active material with a particle size of about 10 μm to about 25 μm and a second positive electrode active material with a particle size of 1 μm to 9 μm.

[0074] When large and small particles are mixed in this way, according to conventional cobalt wet coating methods, for the pre-positive electrode active material dried before heat treatment, the large particles have a higher cobalt to transition metal ratio than the small particles. However, in the final positive electrode active material after coating is completed through heat treatment, the small particles have a higher cobalt to transition metal ratio than the large particles.

[0075] Conversely, according to the preparation method of the embodiment, when large particles are mixed with small particles, the cobalt content on the surface of the large particles is also higher than the cobalt content on the surface of the small particles in the initial positive electrode active material. However, in the final positive electrode active material, unlike conventional methods, the cobalt content on the surface of the large particles is much higher than the cobalt content on the surface of the small particles. That is, the ratio (X / Y) of the cobalt content X based on the total amount of transition metals other than lithium on the surface of the first positive electrode active material to the cobalt content Y based on the total amount of transition metals other than lithium on the surface of the second positive electrode active material can be greater than about 1. Rechargeable lithium batteries including positive electrode active materials that meet these requirements exhibit high initial discharge capacity and high charge and discharge efficiency, while also exhibiting excellent cycle life characteristics and achieving high capacity and high energy density.

[0076] The cobalt content X of the total amount of transition metals other than lithium on the surface of the secondary particles based on the first positive electrode active material can be in the range of about 25 atomic% to about 60 atomic%, for example about 30 atomic% to about 55 atomic%, or about 35 atomic% to about 50 atomic%.

[0077] In addition, the cobalt content Y of the total amount of transition metals other than lithium on the surface of the secondary particles based on the second positive electrode active material can be in the range of about 15 atomic% to about 45 atomic%, for example about 20 atomic% to about 43 atomic%, or about 25 atomic% to about 40 atomic%.

[0078] On the entire surface of the positive electrode active material, which includes the first positive electrode active material and the second positive electrode active material, the cobalt content, based on the total amount of transition metals other than lithium, can range from about 20 atomic% to about 55 atomic%, for example, from about 25 atomic% to about 53 atomic%, or from about 30 atomic% to about 50 atomic%.

[0079] When the cobalt content on each surface of the first and second positive electrode active materials is equal to the cobalt content on the entire surface of the positive electrode active material including them, the positive electrode active materials can exhibit excellent charge and discharge efficiency, cycle life characteristics, and high capacity and high energy density.

[0080] In the composite particles, the large particles can have a particle size of about 10 μm to about 25 μm, for example, about 10 μm to about 20 μm, or about 12 μm to about 20 μm. The small particles can have a particle size of about 1 μm to about 9 μm, for example, about 1 μm to about 8 μm, or about 2 μm to about 6 μm. When the large and small particles of the composite particles have the above-mentioned particle size ranges, the positive electrode active material and the rechargeable lithium battery prepared therefrom can achieve very high energy density, while exhibiting excellent charge and discharge efficiency and cycle life characteristics.

[0081] When the composite particles are a mixture of large and small particles, the composite particles can be a mixture of about 50 wt% to about 90 wt% large particles and about 10 wt% to about 50 wt% small particles, for example, a mixture of about 60 wt% to about 90 wt% large particles and about 10 wt% to about 40 wt% small particles. In this case, the final positive electrode active material can exhibit very high energy density, while achieving high charge / discharge efficiency and cycle life characteristics.

[0082] High-nickel bimodal cathode active materials

[0083] In an embodiment, a positive electrode active material for a rechargeable lithium-ion battery is provided, comprising a first positive electrode active material and a second positive electrode active material. The first positive electrode active material comprises a lithium-nickel composite oxide having a nickel content greater than or equal to about 90 mol% based on the total amount of metals other than lithium. The first positive electrode active material includes secondary particles (in which a plurality of primary particles are aggregated), and the first positive electrode active material includes a cobalt coating on the surface of the secondary particles, and the secondary particles have a particle size of about 10 μm to about 25 μm. The second positive electrode active material comprises a lithium-nickel composite oxide having a nickel content greater than or equal to about 90 mol% based on the total amount of metals other than lithium. The second positive electrode active material includes secondary particles (in which a plurality of primary particles are aggregated), and the second positive electrode active material includes a cobalt coating on the surface of the secondary particles, and the secondary particles have a particle size of about 1 μm to about 9 μm.

[0084] In this paper, the ratio (X, atomic %) of the total cobalt content (X, atomic %) of the total amount of transition metals other than lithium on the surface of the secondary particles based on the first positive electrode active material to the total cobalt content (Y, atomic %) of the total amount of transition metals other than lithium on the surface of the secondary particles based on the second positive electrode active material is greater than 1.

[0085] This positive electrode active material for rechargeable lithium batteries achieves very high capacity and very high energy density, while exhibiting high initial discharge capacity and charge / discharge efficiency as well as excellent cycle life characteristics.

[0086] The X / Y ratio can be, for example, about 1.01 to about 10, about 1.01 to about 8, about 1.01 to about 6, about 1.01 to about 4, about 1.01 to about 3, about 1.01 to about 2, or about 1.1 to about 2.

[0087] The cobalt content X of the total amount of transition metals other than lithium on the surface of the secondary particles of the first positive electrode active material, the cobalt content Y of the total amount of transition metals other than lithium on the surface of the secondary particles of the second positive electrode active material, and the cobalt content of the total amount of transition metals other than lithium on the entire surface of the positive electrode active material are the same as those described in the above method for preparing the positive electrode active material.

[0088] The first positive electrode active material can be of a large particle type, and the particle size of the secondary particles can be in the range of about 10 μm to about 25 μm, for example, about 10 μm to about 20 μm, or about 12 μm to about 20 μm. The second positive electrode active material can be of a small particle type, and the particle size of the secondary particles can be in the range of about 1 μm to about 9 μm, for example, about 1 μm to about 8 μm, or about 2 μm to about 6 μm. When the particle sizes of the first and second positive electrode active materials are within their respective ranges, the positive electrode active materials and the rechargeable lithium batteries including them can achieve very high energy densities while exhibiting excellent charge / discharge efficiency and cycle life characteristics.

[0089] In this document, each of the first positive electrode active material and the second positive electrode active material may include a lithium-nickel composite oxide, which has a nickel content of greater than or equal to about 90 mol% based on the total amount of metals other than lithium, and such lithium-nickel composite oxide may be represented by any one of chemical formula 11, chemical formula 12 and chemical formula 13.

[0090] [Chemical Formula 11]

[0091] Li a11 Ni x11 M 11 y11 M 12 1-x11-y11 O2

[0092] In chemical formula 11, 0.9 ≤ a11 ≤ 1.8, 0.9 ≤ x11 ≤ 1, 0 ≤ y11 ≤ 0.1, and M 11 and M 12 Each is independently selected from Al, B, Ce, Co, Cr, F, Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr and their combinations.

[0093] [Chemical Formula 12]

[0094] Li a12 Ni x12 Co y12 M 13 1-x12-y12 O2

[0095] In Chemical Formula 12, 0.9≤a12≤1.8, 0.9≤x12<1, 0<y12≤0.1, and M 13 is selected from Al, B, Ce, Cr, F, Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr and combinations thereof.

[0096] [Chemical Formula 13]

[0097] Li a13 Ni x13 Co y13 Al z13 M 14 1-x13-y13-z13 O2

[0098] In Chemical Formula 13, 0.9≤a13≤1.8, 0.9≤x13<1, 0<y13<0.1, 0<z13<0.1, and M 14 is selected from B, Ce, Cr, F, Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr and combinations thereof.

[0099] In addition, each of the first positive electrode active material and the second positive electrode active material includes a first coating portion, which is a cobalt coating on the surface of secondary particles, and further includes a second coating portion, which is a cobalt coating on the surface of primary particles inside the secondary particles. The detailed description of the first coating portion and the second coating portion is the same as that described above.

[0100] In the first positive electrode active material, the ratio of the cobalt content (C1) of the first coating portion to the sum of the cobalt content (C1) of the first coating portion and the cobalt content (D1) of the second coating portion (C1 / (C1+D1)) may range from about 0.20 to about 0.49, for example from about 0.25 to about 0.49, from about 0.30 to about 0.49, or from about 0.40 to about 0.49.

[0101] In addition, in the second positive electrode active material, the ratio of the cobalt content (C2) of the first coating portion to the sum of the cobalt content (C2) of the first coating portion and the cobalt content (D2) of the second coating portion (C2 / (C2+D2)) may range from about 0.20 to about 0.49, for example from about 0.25 to about 0.49, from about 0.30 to about 0.49, or from about 0.40 to about 0.49.

[0102] When the first coating portion and the second coating portion have the above-mentioned cobalt content ratio, the positive electrode active material experiences almost no structural collapse with repeated charging and discharging, and achieves high capacity. Therefore, the rechargeable lithium battery including it can achieve high initial discharge capacity and charge / discharge efficiency, as well as excellent cycle life characteristics, and achieve high capacity and high energy density.

[0103] Based on the total amount of the first positive electrode active material and the second positive electrode active material, the content of the first positive electrode active material can be from about 50 wt% to about 90 wt%, while the content of the second positive electrode active material can be from about 10 wt% to about 50 wt%. For example, the content of the first positive electrode active material can be from about 60 wt% to about 90 wt%, while the content of the second positive electrode active material can be from about 10 wt% to about 40 wt%. When the first positive electrode active material and the second positive electrode active material are included within the above-mentioned content range, the positive electrode active material and the rechargeable lithium battery including it can exhibit very high energy density, as well as high charge and discharge efficiency and cycle life characteristics.

[0104] positive electrode

[0105] The positive electrode for a rechargeable lithium battery may include a current collector and a layer of positive electrode active material on the current collector. The positive electrode active material layer may include the aforementioned positive electrode active material, and may further include a binder and / or a conductive material.

[0106] The binder improves the bonding properties between the positive electrode active material particles and the bonding properties between the positive electrode active material particles and the current collector. Examples of binders include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylic styrene-butadiene rubber, epoxy resin, and nylon, but are not limited to these.

[0107] Based on the total weight of the positive electrode active material layer, the content of binder in the positive electrode active material layer can be from about 1 wt% to about 5 wt%.

[0108] Conductive materials are included to provide electrode conductivity. Any conductive material can be used as a conductive material unless it causes a chemical change. Examples of conductive materials may include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, and carbon fibers; metallic materials such as metal powders or metal fibers, including copper, nickel, aluminum, and silver; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0109] Based on the total weight of the positive electrode active material layer, the content of conductive material in the positive electrode active material layer can be from about 1 wt% to about 5 wt%.

[0110] Aluminum foil can be used as a current collector, but is not limited thereto.

[0111] negative electrode

[0112] A negative electrode for a rechargeable lithium battery includes a current collector and a negative active material layer on the current collector. The negative active material layer may include a negative active material, and may further include a binder and / or a conductive material.

[0113] The negative active material may include a material that reversibly intercalates / deintercalates lithium ions, lithium metal, a lithium metal alloy, a material capable of doping / dedoping lithium, or a transition metal oxide.

[0114] The material that reversibly intercalates / deintercalates lithium ions may include, for example, crystalline carbon, amorphous carbon, or a combination thereof as a carbon-based negative active material. The crystalline carbon may be amorphous, or natural graphite or artificial graphite in the shape of platelets, sheets, spheres or fibers. The amorphous carbon may be soft carbon, hard carbon, mesophase pitch carbonization products, calcined coke, and the like.

[0115] The lithium metal alloy includes an alloy of lithium and an element selected from the group consisting of Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al and Sn.

[0116] The material capable of doping / dedoping lithium may be a Si-based negative active material or a Sn-based negative active material. The Si-based negative active material may include silicon, a silicon-carbon composite, SiO x (0<x<2), a Si-Q alloy (wherein Q is an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a Group 15 element, a Group 16 element, a transition metal, a rare earth element, or a combination thereof, but is not Si), and the Sn-based negative active material may include Sn, SnO2, a Sn-R alloy (wherein R is an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a Group 15 element, a Group 16 element, a transition metal, a rare earth element, or a combination thereof, but is not Sn). At least one of these materials may be mixed with SiO2. Element Q and R may be selected from Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Tl, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof.

[0117] The silicon-carbon composite material may, for example, include a core comprising crystalline carbon and silicon particles, and an amorphous carbon coating disposed on the surface of the core. The crystalline carbon may be artificial graphite, natural graphite, or a combination thereof. The amorphous carbon precursor may be coal tar pitch, mesophase pitch, petroleum tar pitch, coal oil, petroleum heavy oil, or polymer resins such as phenolic resin, furan resin, or polyimide resin. In this case, the silicon content may be from about 10 wt% to about 50 wt% based on the total weight of the silicon-carbon composite material. Additionally, the crystalline carbon content may be from about 10 wt% to about 70 wt% based on the total weight of the silicon-carbon composite material, and the amorphous carbon content may be from about 20 wt% to about 40 wt% based on the total weight of the silicon-carbon composite material. Furthermore, the thickness of the amorphous carbon coating may be from about 5 nm to about 100 nm. The average particle size (D50) of the silicon particles may be from about 10 nm to about 20 μm. The average particle size (D50) of the silicon particles may preferably be from about 10 nm to about 200 nm. Silicon particles can exist in an oxidized form, and in this case, the atomic ratio of Si:O, indicating the degree of oxidation, in the silicon particles can be from about 99:1 to about 33:67. The silicon particles can be SiO₂. x Particles, and in this case, SiO x The range of x can be greater than about 0 and less than about 2. In this specification, unless otherwise specified, the average particle size (D50) indicates the diameter of particles that constitute about 50% of the cumulative volume in the particle size distribution.

[0118] Si-based or Sn-based anode active materials can be mixed with carbon-based anode active materials. When mixing and using Si-based or Sn-based anode active materials and carbon-based anode active materials, the mixing ratio can be from about 1:99 to about 90:10 by weight.

[0119] In the negative electrode active material layer, the content of the negative electrode active material can be from about 95 wt% to about 99 wt%, based on the total weight of the negative electrode active material layer.

[0120] In an embodiment, the negative electrode active material layer further includes a binder and optionally further includes a conductive material. Based on the total weight of the negative electrode active material layer, the binder content in the negative electrode active material layer can be from about 1 wt% to about 5 wt%. Alternatively, when further including a conductive material, the negative electrode active material layer may include about 90 wt% to about 98 wt% of negative electrode active material, about 1 wt% to about 5 wt% of binder, and about 1 wt% to about 5 wt% of conductive material.

[0121] The binder is used to ensure good adhesion between particles of the negative electrode active material, and also to adhere the negative electrode active material to the current collector. The binder may be a water-insoluble binder, a water-soluble binder, or a combination thereof.

[0122] Examples of water-insoluble binders include polyvinyl chloride, carboxylated polyvinyl chloride, fluorinated polyethylene, ethylene oxide-containing polymers, ethylene-propylene copolymers, polystyrene, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyimide, or combinations thereof.

[0123] Water-soluble adhesives may include rubber adhesives or polymeric resin adhesives. Rubber adhesives may be selected from styrene-butadiene rubber, acrylic styrene-butadiene rubber, acrylonitrile-butadiene rubber, acrylic rubber, butyl rubber, fluororubber, and combinations thereof. Polymeric resin adhesives may be selected from polyethylene oxide, polyvinylpyrrolidone, polyepoxychlorohydrin, polyphosphazene, polyacrylonitrile, ethylene-propylene-diene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, polyester resin, acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol, and combinations thereof.

[0124] When a water-soluble binder is used as a negative electrode binder, it may further include a cellulose-based compound capable of imparting viscosity as a thickener. As a cellulose-based compound, one or more of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, and their alkali metal salts may be mixed and used. As an alkali metal, Na, K, or Li may be used. Based on 100 parts by weight of the negative electrode active material, the amount of thickener used may be from about 0.1 parts by weight to about 3 parts by weight.

[0125] Conductive materials are included to provide electrode conductivity. Any conductive material can be used as a conductive material unless it causes a chemical change. Examples of conductive materials include carbon-based materials such as natural graphite, synthetic graphite, carbon black, acetylene black, Ketjen black, and carbon fibers; metallic materials such as metal powders or metal fibers, including copper, nickel, aluminum, and silver; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0126] The current collector may include one selected from copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, polymer substrate coated with conductive metal, and combinations thereof.

[0127] Rechargeable lithium batteries

[0128] Another embodiment provides a rechargeable lithium battery, including a positive electrode, a negative electrode, a separator between the positive and negative electrodes, and an electrolyte. Here, the aforementioned electrodes can be a positive electrode and a negative electrode.

[0129] Figure 1 A schematic diagram illustrating a rechargeable lithium battery according to an embodiment. (Reference) Figure 1According to an embodiment of the present disclosure, a rechargeable lithium battery 100 includes a battery cell, the battery cell including a positive electrode 114; a negative electrode 112 facing the positive electrode 114; a separator 113 between the positive electrode 114 and the negative electrode 112; an electrolyte for a rechargeable lithium battery impregnating the positive electrode 114, the negative electrode 112 and the separator 113; a battery housing 120 housing the battery cell; and a sealing member 140 sealing the battery housing 120.

[0130] Electrolytes include non-aqueous organic solvents and lithium salts.

[0131] Non-aqueous organic solvents are used as media for transporting ions involved in the electrochemical reactions of a battery. Non-aqueous organic solvents can be carbonates, esters, ethers, ketones, alcohols, or proton-inert solvents. Examples of carbonate solvents include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), and butyl carbonate (BC). Examples of ester solvents include methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, decanolactone, valerate, mevalonolactone, and caprolactone. Ether solvents can be dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, dimethoxyethane, 2-methyltetrahydrofuran, and tetrahydrofuran, and ketone solvents can be cyclohexanone, etc. In addition, alcohol solvents can be ethanol, isopropyl alcohol, etc., and proton-inert solvents can be nitriles, such as R-CN (where R is a C2 to C20 straight-chain, branched or cyclic hydrocarbon group and may include double bonds, aromatic rings or ether bonds); amides, such as dimethylformamide; dioxolane, such as 1,3-dioxolane; and sulfolane, etc.

[0132] Non-aqueous organic solvents can be used alone or in mixtures. When non-aqueous organic solvents are used in mixtures, the mixture ratio can be controlled according to the desired battery performance.

[0133] Alternatively, in the case of carbonate solvents, a mixture of cyclic and linear carbonates can be used. In this case, the electrolyte solution exhibits excellent performance when the cyclic and linear carbonates are mixed in a volume ratio of about 1:1 to about 1:9.

[0134] In addition to carbonate solvents, non-aqueous organic solvents may further include aromatic hydrocarbon organic solvents. In this case, carbonate solvents and aromatic hydrocarbon organic solvents can be mixed in a volume ratio of about 1:1 to about 30:1.

[0135] Aromatic hydrocarbon organic solvents can be aromatic hydrocarbon compounds represented by chemical formula I.

[0136] [Chemical Formula I]

[0137]

[0138] In chemical formula I, R 4 To R 9 The same or different, and selected from hydrogen, halogen, C1 to C10 alkyl, C1 to C10 haloalkyl and combinations thereof.

[0139] Specific examples of aromatic hydrocarbon organic solvents may be selected from benzene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene, chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, iodobenzene, 1,2-diiodobenzene, 1,3-diiodobenzene, 1,4-diiodobenzene, 1,2,3-triiodobenzene, 1,2,4-triiodobenzene, toluene, and fluorotoluene. 2,3-Difluorotoluene, 2,4-Difluorotoluene, 2,5-Difluorotoluene, 2,3,4-Trifluorotoluene, 2,3,5-Trifluorotoluene, Chlorotoluene, 2,3-Dichlorotoluene, 2,4-Dichlorotoluene, 2,5-Dichlorotoluene, 2,3,4-Trichlorotoluene, 2,3,5-Trichlorotoluene, Iodotoluene, 2,3-Diiodotoluene, 2,4-Diiodotoluene, 2,5-Diiodotoluene, 2,3,4-Triiodotoluene, 2,3,5-Triiodotoluene, Xylene and combinations thereof.

[0140] The electrolyte solution may further include vinylene carbonate or ethylene carbonate-like compounds represented by chemical formula II as additives to improve the cycle life of the battery.

[0141] [Chemical Formula II]

[0142]

[0143] In chemical formula II, R 10 and R 11 The same or different, and selected from hydrogen, halogen, cyano, nitro and fluorinated C1 to C5 alkyl groups, as long as R 10 and R 11 At least one of them is a halogenated, cyano, nitro, or fluorinated C1 to C5 alkyl group, and R 10 and R 11 They are not both hydrogen.

[0144] Examples of ethylene carbonate compounds include difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate, or fluoroethylene carbonate. Additives for improving cycle life may be used in appropriate amounts within suitable limits.

[0145] Lithium salts dissolved in non-aqueous organic solvents supply lithium ions in the battery, ensuring the basic operation of the rechargeable lithium battery and improving the transport of lithium ions between the positive and negative electrodes.

[0146] Examples of lithium salts include at least one supported salt selected from the following: LiPF6, LiBF4, LiSbF6, LiAsF6, LiN(SO2C2F5)2, Li(CF3SO2)2N, LiN(SO3C2F5)2, Li(FSO2)2N (lithium bis(fluorosulfonyl)imide):LiFSI), LiC4F9SO3, LiClO4, LiAlO2, LiAlCl4, LiPO2F2, LiN(C x F 2x+1 SO2)(C y F 2y+1 SO2 (where x and y are natural numbers, for example, integers from 1 to 20), lithium difluoro(bis(oxalate)phosphate), LiCl, LiI, LiB(C2O4)2 (lithium bis(oxalate)borate, LiBOB) and lithium difluoro(oxalate)borate (LiDFOB).

[0147] Lithium salts can be used in concentration ranges from about 0.1 M to about 2.0 M. When lithium salts are included in the above concentration range, the electrolyte can exhibit excellent performance and lithium-ion mobility due to optimal electrolyte conductivity and viscosity.

[0148] Separator 113 separates the positive electrode 114 and the negative electrode 112 and provides a transport channel for lithium ions. It can be any commonly used separator in lithium-ion batteries. In other words, it can have low ion migration resistance and excellent electrolyte solution impregnation properties. For example, the separator can be selected from glass fiber, polyester, polyethylene, polypropylene, polytetrafluoroethylene, or combinations thereof. It can be in the form of non-woven or woven fabric. For example, in lithium-ion batteries, polyolefin polymer separators such as polyethylene separators and polypropylene separators are mainly used. To ensure heat resistance or mechanical strength, coated separators including ceramic components or polymer materials can be used. Optionally, it can have a single-layer structure or a multi-layer structure.

[0149] Depending on the presence of a separator and the type of electrolyte used, rechargeable lithium batteries can be classified into lithium-ion batteries, lithium-ion polymer batteries, and lithium polymer batteries. Rechargeable lithium batteries can have various shapes and sizes, including cylindrical, prismatic, coin-shaped, or pouch-shaped batteries, and can be thin-film batteries or quite large in size. The structures and manufacturing methods of lithium-ion batteries related to this disclosure are well known in the art.

[0150] The rechargeable lithium battery according to the embodiments can be used in electric vehicles (EVs) and hybrid electric vehicles such as plug-in hybrid electric vehicles (PHEVs) because it achieves high capacity and has excellent storage stability, cycle life characteristics and high rate characteristics at high temperatures.

[0151] Embodiments and comparative examples of this disclosure are described below. However, it should be understood that these embodiments are for illustrative purposes and should not be construed as limiting this disclosure.

[0152] Comparative Example 1

[0153] NaOH aqueous solution was added to distilled water to adjust the pH to 13.4. Then, LiNi with a particle size of approximately 17 μm was added. 0.91 Co 0.05 Al 0.04 Large O2 particles and LiNi with a particle size of approximately 3 μm 0.91 Co 0.05 Al 0.04 O2 particles were added at a weight ratio of 8:2, and rinsing was initiated. The resulting mixture was stirred, and an aqueous solution of cobalt sulfate was added at a predetermined ratio for 1 hour. Subsequently, the product was dried at approximately 120°C for 10 hours to obtain a cobalt-coated pre-positive electrode active material. This pre-positive electrode active material was heat-treated at 700°C to obtain the final positive electrode active material. During this heat treatment of the pre-positive electrode active material, a portion of the coated cobalt compound permeated into the secondary particles of the positive electrode active material. The final positive electrode active material comprises a first positive electrode active material with a particle size of approximately 17 μm to 18 μm and a second positive electrode active material with a particle size of approximately 3 μm to 4 μm.

[0154] Example 1

[0155] A cobalt sulfate aqueous solution was added to distilled water. At this point, the pH was 7.4. Subsequently, LiNi with a particle size of approximately 17 μm was added at a weight ratio of 8:2. 0.91 Co 0.05 Al 0.04 Large O2 particles and LiNi with a particle size of approximately 3 μm 0.91 Co 0.05 Al 0.04 O2 particles were used to begin rinsing. Then, while stirring, an aqueous NaOH solution was added at a predetermined ratio for 1 hour. The final pH was 13.4. Subsequently, the result was dried at approximately 120°C for 10 hours to obtain a cobalt-coated pre-positive electrode active material. This pre-positive electrode active material was then heat-treated at 700°C to obtain the final positive electrode active material. The final positive electrode active material comprises a first positive electrode active material with a particle size of approximately 17 μm to 18 μm and a second positive electrode active material with a particle size of approximately 3 μm to 4 μm.

[0156] Evaluation Example 1: Comparison of Coating Results

[0157] Figures 2 to 5 The image shown is a scanning electron microscope image of the positive electrode active material according to Comparative Example 1. Figure 2 Display the positive electrode active material during the rinsing step. Figure 3 The positive electrode active material is displayed before, and Figure 4 and Figure 5 Photograph showing the final positive electrode active material. (Reference) Figures 2 to 5 As can be seen, the final active material of Comparative Example 1 is unevenly coated with a plate-shaped cobalt compound.

[0158] Figures 6 to 9 The image shows a scanning electron microscope image of the positive electrode active material according to Example 1. Figure 6 The positive electrode active material in the rinsing step is shown. Figure 7 The pre-positive electrode active material is shown, and Figure 8 and Figure 9 A photograph of the final positive electrode active material is shown. (Reference) Figures 6 to 9 Example 1 shows that the cobalt compound is uniformly deposited on the surface of the positive electrode active material from the rinsing step, and the pre-positive electrode active material and the final positive electrode active material are also uniformly coated with needle-shaped cobalt compound.

[0159] This difference between Comparative Example 1 and Example 1 can be explained by the shape of the cobalt compound according to pH. When an aqueous solution of cobalt sulfate is added to distilled water without the positive electrode active material, a wavy pattern first appears (pH 7.5 to 10) as the pH increases to 7.5, 9, 10, 11, 12, 13, etc., but a needle-like shape appears at approximately pH 10 to 12, and a hexagonal plate shape appears at approximately pH 12 to 13. Because cobalt coating in Comparative Example 1 begins at pH 13.4, plate-like cobalt (β-type) can be easily produced. Conversely, because cobalt coating in Example 1 begins at pH 7.4, wavy cobalt (α-type) can be easily produced; therefore, α-type cobalt can be uniformly coated on the surface of the positive electrode active material.

[0160] On the other hand, when the positive electrode active material was coated with cobalt using the method of Comparative Example 1, based on the total amount of transition metals other than lithium, the positive electrode active material contained less than 90 mol% of nickel, for example, 88 mol% (hereinafter referred to as Comparative Example 2), and the cobalt was coated relatively uniformly. However, Comparative Example 1, which contained 90 mol% or more of nickel, exhibited a very non-uniform coating. To analyze this result, the positive electrode active materials of Comparative Example 1 and Comparative Example 2 were analyzed by ESA (electroacoustic amplitude) to measure the zeta potential, and the results are as follows: Figure 10 As shown. (Refer to...) Figure 10The two positive electrode active materials of Comparative Example 1 (Ni91) and Comparative Example 2 (Ni88) exhibited different potential behaviors on the surface depending on the pH. Therefore, for positive electrode active materials containing 90 mol% or more nickel, Comparative Example 1 and Example 1 exhibited significantly different coating quality due to the fine adjustment of pH.

[0161] Evaluation Example 2: SEM-EDS Analysis

[0162] Scanning electron microscope (SEM) images of the pre-positive electrode active material and the final positive electrode active material of Comparative Example 1 and Example 1, respectively, were obtained by energy-dispersive X-ray spectroscopy (EDS) analysis, and the results are shown in... Figures 13 to 16 In. Figures 13 to 16 In the middle, light gray indicates cobalt. Figure 13 This is a photograph of the front positive electrode active material of Example 1, and Figure 14 This is a photograph of the final positive electrode active material in Example 1. Figure 15 This is a photograph of the pre-positive electrode active material of Comparative Example 1, and Figure 16 Photograph of the final positive electrode active material of Comparative Example 1.

[0163] Compare Figure 13 and Figure 15 In Example 1, the cobalt compound on the surface of the pre-positive electrode active material was coated with a higher density and a more uniform coating than that on the surface of Comparative Example 1. Furthermore, the comparative... Figure 14 and Figure 16 Compared to Comparative Example 1, Example 1 exhibits even greater coating of cobalt positive electrode active material inside the secondary particles, that is, greater coating of cobalt active material on the surface of the primary particles inside the secondary particles in the final positive electrode active material after heat treatment.

[0164] Additionally, refer to Figure 13 and Figure 14 In the final positive electrode active material of Example 1, the cobalt coating portion (grain boundary coating) on ​​the surface of the primary particles inside the secondary particles has a thickness of 1 nm to 10 nm (about 5 nm).

[0165] Evaluation Example 3: Analysis of Cobalt Content

[0166] The pre-positive electrode active material and final positive electrode active material of Comparative Example 1 and the pre-positive electrode active material and final positive electrode active material of Example 1 were subjected to EP-EDS (energy-dispersive X-ray spectroscopy) analysis to measure the respective contents of nickel, cobalt, etc. on the surface, and the cobalt content (atomic %) and the total amount of nickel and cobalt on each surface were calculated. The results are shown in Table 1.

[0167] (Table 1)

[0168]

[0169]

[0170] First, as shown at the top of Table 1, for all the positive electrode active materials containing a mixture of large and small particles, specifically, for the pre-positive electrode active material in a dry state after cobalt coating, Example 1 exhibits a higher cobalt content than Comparative Example 1. This is because in the pre-positive electrode active material of Example 1, cobalt is coated more uniformly at a higher density. Furthermore, for the final positive electrode active material obtained by heat-treating the pre-positive electrode active material, Example 1 exhibits a lower cobalt content than Comparative Example 1. This is because, when heat-treated under the same conditions, Example 1 exhibits a higher cobalt compound coating rate on the surface (grain boundaries) of the primary particles within the secondary particles, i.e., a higher coating ability for cobalt compounds to penetrate into the secondary particles.

[0171] Specifically, the ratio (B / A) of the cobalt content (B) of the transition metal on the surface of the final positive electrode active material to the cobalt content (A) of the transition metal on the surface of the pre-positive electrode active material was 44.2% in Example 1 and 62% in Comparative Example 1. In Example 1, 44.2% of the total cobalt content remained on the surface of the positive electrode active material, while 55.8% of the remaining cobalt content permeated into the positive electrode active material. However, in Comparative Example 1, 62% of the total cobalt content remained on the surface of the positive electrode active material, while 38% of the remaining cobalt content permeated into the positive electrode active material. Therefore, the cobalt content permeating into the positive electrode active material in Example 1 (i.e., the cobalt content coated at the grain boundaries) was greater than the cobalt content permeating into the positive electrode active material in Comparative Example 1.

[0172] In this text, (B) corresponds to the cobalt content (C) of the first coating portion in the final positive electrode active material, and (AB) obtained by subtracting (B) from (A) corresponds to the cobalt content (D) of the second coating portion infiltrated into the secondary particles of the final positive electrode active material. Therefore, in the total positive electrode active material in which large and small particles are mixed, Example 1 exhibits a C / (C+D) ratio of 44.2% and a D / (C+D) ratio of 55.8%.

[0173] Furthermore, for the first positive electrode active material composed of large particles as shown in Table 1, Example 1 also exhibited a (B / A) ratio of 43.2%, while Comparative Example 1 exhibited a (B / A) ratio of 59.3%, which was lower than that of Example 1. In other words, for the first positive electrode active material, the positive electrode active material in Example 1 also had a higher content of cobalt coated inside compared to Comparative Example 1. The first positive electrode active material of Example 1 exhibited a C1 / (C1+D1) ratio of 43.2% and a D1 / (C1+D1) ratio of 56.8%.

[0174] For the small-particle second positive electrode active material, Example 1 exhibits a (B / A) ratio of 46.4%, which is smaller than that of Comparative Example 1 (74.5%). In other words, the second positive electrode active material of Example 1 is also internally coated with a greater cobalt content than that of Comparative Example 1. The second positive electrode active material of Example 1 exhibits a C2 / (C2+D2) ratio of 46.4% and a D2 / (C2+D2) ratio of 53.6%.

[0175] Furthermore, referring to the bottom of Table 4, in Example 1, the surface of the first positive electrode active material (large particles) was coated with more cobalt than the surface of the second positive electrode active material (small particles). Specifically, the ratio (X / Y) of the cobalt content based on the total amount of transition metals on the surface of the first positive electrode active material (X, 37.18) to the cobalt content based on the total amount of transition metals on the surface of the second positive electrode active material (Y, 28.04) was 1.33, which is greater than 1.

[0176] Conversely, when a conventional wet coating method was used as in Comparative Example 1, more cobalt was coated on the surface of the second positive electrode active material than on the surface of the first positive electrode active material. Specifically, the ratio (X / Y) of the cobalt content based on the total amount of transition metals on the surface of the first positive electrode active material (X, 43.21) to the cobalt content based on the total amount of transition metals on the surface of the second positive electrode active material (Y, 45.78) was calculated to be 0.94.

[0177] Evaluation Example 4: Comparison of charge-discharge capacity and efficiency, and room temperature cycle life characteristics of half-cells

[0178] Each coin half-cell was manufactured using the positive electrode active material prepared in Comparative Example 1 and Example 1. Specifically, a positive electrode active material slurry was prepared by mixing 95 wt% of the positive electrode active material, 3 wt% of polyvinylidene fluoride binder, and 2 wt% of Ketjen Black conductive material in an N-methylpyrrolidone solvent. The positive electrode active material slurry was applied to an aluminum current collector, dried, and pressed to manufacture the positive electrode. The coin half-cell was manufactured by placing a separator with a polyethylene-polypropylene multilayer structure between the manufactured positive electrode and the lithium metal counter electrode, and injecting an electrolyte solution in which 1.0 M LiPF6 lithium salt was added to a solvent in which ethylene carbonate and diethyl carbonate were mixed in a 50:50 volume ratio.

[0179] Each coin half-cell from Comparative Example 1 and Example 1 was charged at a constant current of 0.2C to an upper limit voltage of 4.3V until it dropped to 0.05C at a constant voltage, which was set as the cutoff condition. It was then discharged at 25°C to a discharge cutoff voltage of 3.0V at 0.2C, and the discharge capacity was measured relative to the initial discharge capacity. In this paper, the ratio of discharge capacity to charge capacity is calculated as efficiency, and the results are shown in Table 2. Furthermore, 30 charge / discharge cycles were repeated at 25°C to evaluate capacity retention, i.e., room temperature cycle life characteristics, and the results are shown in Table 2.

[0180] (Table 2)

[0181]

[0182] Referring to Table 2, compared with Comparative Example 1, Example 1 exhibits higher charging and discharging capacity, higher charging and discharging efficiency, and improved cycle life characteristics at room temperature.

[0183] Evaluation Example 5: Comparison of room temperature and high temperature cycle life characteristics of full cells

[0184] The positive electrode active materials prepared in Comparative Example 1 and Example 1 were used to manufacture each full cell. After preparing the positive electrode in the same manner as in Evaluation Example 4, a negative electrode active material slurry was prepared by mixing 97.3 wt% of graphite, 0.5 wt% of Denca Black, 0.9 wt% of carboxymethyl cellulose, and 1.3 wt% of styrene-butadiene rubber as the negative electrode active material in an aqueous solvent. The negative electrode active material slurry was coated onto copper foil, and then dried and compressed to manufacture the negative electrode.

[0185] A full cell is fabricated by sequentially stacking a fabricated positive electrode, a separator with a polyethylene-polypropylene multilayer structure, and a fabricated negative electrode, and then injecting an electrolyte solution in which 1.0 M LiPF6 lithium salt is added to a solvent in which ethylene carbonate and diethyl carbonate are mixed in a 50:50 volume ratio.

[0186] Each full cell according to Example 1 and Comparative Example 1 was charged to 4.3V in constant current mode at a rate of 0.7C at room temperature (25°C) and high temperature (45°C), and then cut off at a rate of 0.1C in constant voltage mode at 4.3V. The battery was then discharged to 3.0V at a rate of 1C, and this charge-discharge cycle was repeated 50 times. The room temperature cycle life characteristics evaluation results are as follows: Figure 11 As shown, the high-temperature cycling life characteristic evaluation results are as follows: Figure 12 As shown, the capacity retention rate for the 50th cycle is provided in Table 3.

[0187] (Table 3)

[0188]

[0189] refer to Figure 11 and Figure 12 And as shown in Table 3, compared with Comparative Example 1, the battery of Example 1 exhibits superior room temperature and high temperature cycle life characteristics.

[0190] Although this disclosure has been described in conjunction with exemplary embodiments now regarded as practice, it should be understood that the invention is not limited to the disclosed embodiments. Rather, it is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

1. A positive electrode active material for a rechargeable lithium battery, comprising: The first positive electrode active material comprises a lithium-nickel composite oxide, wherein, based on the total amount of metals other than lithium, the lithium-nickel composite oxide has a nickel content greater than or equal to 90 mol%, wherein the first positive electrode active material comprises secondary particles and includes a cobalt coating on the surface of the secondary particles, wherein multiple primary particles are aggregated within the secondary particles, and the secondary particles have a particle size of 10 μm to 25 μm. The second positive electrode active material comprises a lithium-nickel composite oxide, wherein the lithium-nickel composite oxide has a nickel content greater than or equal to 90 mol% based on the total amount of metals other than lithium, wherein the second positive electrode active material comprises secondary particles and includes a cobalt coating on the surface of the secondary particles, wherein multiple primary particles are aggregated in the secondary particles, and the secondary particles have a particle size of 1 μm to 9 μm. The ratio X / Y of the cobalt content X, which is the total amount of transition metals other than lithium on the surface of the secondary particles based on the first positive electrode active material, to the cobalt content Y, which is the total amount of transition metals other than lithium on the surface of the secondary particles based on the second positive electrode active material, is between 1.01 and 10. Based on the total amount of transition metals other than lithium on the surface of the secondary particles of the first positive electrode active material, the cobalt content X is in the range of 25 atomic% to 60 atomic%. Based on the total amount of transition metals other than lithium on the surface of the secondary particles of the second positive electrode active material, the cobalt content Y is in the range of 15 atomic% to 45 atomic%. In the first positive electrode active material, the cobalt coating on the surface of the secondary particles is referred to as the first coating portion. The first positive electrode active material further includes a second coating portion, which is the cobalt coating on the surface of the primary particles within the secondary particles. In the second positive electrode active material, the cobalt coating on the surface of the secondary particles is referred to as the first coating portion, and the second positive electrode active material further includes a second coating portion, which is the cobalt coating on the surface of the primary particles inside the secondary particles.

2. The positive electrode active material of claim 1, wherein the cobalt content is in the range of 20 atomic% to 55 atomic% on the entire surface of the positive electrode active material comprising the first positive electrode active material and the second positive electrode active material, based on the total amount of transition metals other than lithium.

3. The positive electrode active material as described in claim 1, wherein: The ratio of the cobalt content C1 of the first coating portion to the sum of the cobalt content C1 of the first coating portion and the cobalt content D1 of the second coating portion, C1 / (C1+D1), is in the range of 0.20 to 0.

49.

4. The positive electrode active material as described in claim 1, wherein: The ratio of the cobalt content C2 of the first coating portion to the sum of the cobalt content C2 of the first coating portion and the cobalt content D2 of the second coating portion, C2 / (C2+D2), is in the range of 0.20 to 0.

49.

5. The positive electrode active material as claimed in claim 3 or claim 4, wherein the second coating portion has a thickness of 1 nm to 250 nm.

6. The positive electrode active material as claimed in claim 1, wherein, based on the total amount of the first positive electrode active material and the second positive electrode active material, the content of the first positive electrode active material is 50 wt% to 90 wt%, and the content of the second positive electrode active material is 10 wt% to 50 wt%.

7. A method for preparing a positive electrode active material for a rechargeable lithium battery as described in any one of claims 1 to 6, comprising: A first aqueous solution is prepared, comprising composite particles and a cobalt salt, wherein the composite particles comprise a lithium-nickel composite oxide and are in the form of secondary particles, wherein multiple primary particles are aggregated within the secondary particles, and the first aqueous solution has a pH of 6 to 8. An alkaline substance is added to the first aqueous solution to obtain a second aqueous solution. The pre-positive electrode active material comprising the composite particles and a cobalt coating formed on their surface is obtained in the second aqueous solution, and the second aqueous solution has a pH of 12 to 14. The pre-positive active material is heat-treated to obtain the final positive active material.

8. The method of claim 7, wherein on the surface of the pre-positive active material, the cobalt content A is in the range of 56 atomic% to 90 atomic% based on the total amount of transition metals other than lithium.

9. The method of claim 7, wherein on the surface of the final positive electrode active material, the cobalt content B is in the range of 15 atomic% to 55 atomic% based on the total amount of transition metals other than lithium.

10. The method of claim 7, wherein the ratio B / A, based on the total amount of transition metals other than lithium on the surface of the final positive electrode active material, to the cobalt content A, based on the total amount of transition metals other than lithium on the surface of the pre-positive electrode active material, is in the range of 0.3 to 0.

49.

11. The method of claim 7, wherein: The final positive electrode active material includes secondary particles, in which multiple primary particles are aggregated. The final positive electrode active material includes a first coating portion and further includes a second coating portion. The first coating portion is a cobalt coating on the surface of the secondary particles, and the second coating portion is a cobalt coating on the surface of the primary particles within the secondary particles. In the final positive electrode active material, the ratio of the cobalt content C of the first coating portion to the sum of the cobalt content C of the first coating portion and the cobalt content D of the second coating portion, C / (C+D), is in the range of 0.30 to 0.

49.

12. The method of claim 7, wherein in the lithium-nickel composite oxide, the nickel content is greater than or equal to 90 mol based on the total amount of metals other than lithium.

13. The method of claim 7, wherein: The composite particles are a mixture of large particles with a particle size of 10 μm to 25 μm and small particles with a particle size of 1 μm to 9 μm. The final positive electrode active material is a mixture of a first positive electrode active material with a particle size of 10 μm to 25 μm and a second positive electrode active material with a particle size of 1 μm to 9 μm.

14. The method of claim 13, wherein the ratio X / Y, which is the cobalt content X based on the total amount of transition metals other than lithium on the surface of the first positive electrode active material and the cobalt content Y based on the total amount of transition metals other than lithium on the surface of the second positive electrode active material, is greater than 1.

15. The method of claim 13, wherein: Based on the total amount of transition metals other than lithium on the surface of the secondary particles of the first positive electrode active material, the cobalt content X is in the range of 25 atomic% to 60 atomic% and The cobalt content Y of the total amount of transition metals other than lithium on the surface of the secondary particles based on the second positive electrode active material is in the range of 15 atomic% to 45 atomic%.

16. The method of claim 13, wherein the cobalt content is in the range of 20 atomic% to 55 atomic% on the entire surface of the positive electrode active material comprising the first positive electrode active material and the second positive electrode active material, based on the total amount of transition metals other than lithium.

17. The method of claim 13, wherein the composite particles are a mixture of 50 wt% to 90 wt% of the large particles and 10 wt% to 50 wt% of the small particles.

18. A rechargeable lithium battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode comprises a positive electrode active material according to any one of claims 1 to 6 or a positive electrode active material prepared by any one of claims 7 to 17.

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