KR102796388B1 - Positive active material for rechargeable lithium battery, preparing method thereof and rechargeable lithium battery including the same - Google Patents

Abstract

The present invention relates to a cathode active material for a lithium secondary battery, a method for producing the same, and a lithium secondary battery comprising the same, the first cathode active material comprising a lithium nickel-based composite oxide, the second cathode active material comprising a lithium nickel-based composite oxide, the second cathode active material comprising a single crystal, and a cobalt coating portion positioned on the surface of the single crystal, wherein the second cathode active material has a rough surface having roughness and a flat surface without roughness.

Description

POSITIVE ACTIVE MATERIAL FOR RECHARGEABLE LITHIUM BATTERY, PREPARING METHOD THEREOF AND RECHARGEABLE LITHIUM BATTERY INCLUDING THE SAME

The present invention relates to a cathode active material for a lithium secondary battery, a method for producing the same, and a lithium secondary battery including the same.

Lithium secondary batteries, which have high energy density and are easy to carry, are mainly used as power sources for mobile information terminals such as mobile phones, laptops, and smart phones. Recently, research is actively being conducted to use lithium secondary batteries with high energy density as power sources for driving hybrid or electric vehicles or as power storage power sources.

In order to implement lithium secondary batteries suitable for these purposes, various cathode active materials are being studied. Among them, lithium nickel oxide, lithium nickel manganese cobalt composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium cobalt oxide are mainly used as cathode active materials. However, these cathode active materials have problems in that their structure collapses or cracks occur as charge and discharge are repeated, which reduces the long-term life of the lithium secondary battery and increases resistance, failing to exhibit satisfactory capacity characteristics. Therefore, the development of a new cathode active material that can secure long-term life characteristics while implementing high capacity and high energy density is required.

Provided are a cathode active material for a lithium secondary battery having improved life characteristics and safety while implementing high capacity, a method for producing the same, and a lithium secondary battery including the same.

In one embodiment, a cathode active material for a lithium secondary battery is provided, comprising: a first cathode active material including a lithium nickel-based composite oxide, the first cathode active material including a secondary particle in which a plurality of primary particles are aggregated and a cobalt coating portion positioned on the surface of the secondary particle; and a second cathode active material including a lithium nickel-based composite oxide, the second cathode active material including a single crystal and a cobalt coating portion positioned on the surface of the single crystal, wherein the second cathode active material has a rough surface having roughness and a flat surface without roughness.

In another embodiment, a method for producing a cathode active material for a lithium secondary battery is provided, including mixing a first nickel-based hydroxide and a lithium raw material and performing a first heat treatment to prepare a first nickel-based oxide; mixing a second nickel-based hydroxide and a lithium raw material and performing a second heat treatment to prepare a second nickel-based oxide; and mixing the first nickel-based oxide, the second nickel-based oxide, and the cobalt raw material and performing a third heat treatment.

In another embodiment, a lithium secondary battery is provided, which includes a positive electrode and a negative electrode including the positive electrode active material, and an electrolyte.

A cathode active material for a lithium secondary battery manufactured according to one embodiment of the present invention and a lithium secondary battery including the same can exhibit excellent charge/discharge efficiency, life characteristics, and safety while implementing high capacity and high energy density.

Figure 1 is a schematic diagram illustrating a lithium secondary battery according to one embodiment.
Figure 2 is a scanning electron microscope photograph of the second positive electrode active material of Example 1.
Figure 3 is a scanning electron microscope photograph of the second positive electrode active material before the third heat treatment after cobalt coating in Example 2.
Figure 4 is a scanning electron microscope photograph of the final second positive electrode active material in Example 2.
Figure 5 is a scanning electron microscope photograph of the second positive electrode active material of Comparative Example 1.
Figure 6 is a scanning electron microscope photograph of the second positive electrode active material of Comparative Example 2.
Figure 7 is a photograph mapping cobalt elements using SEM-EDS (Scanning Electron Microscope - Energy Dispersive Spectroscopy) analysis of the second positive electrode active material according to Example 1.
Figure 8 is a photograph mapping cobalt elements using SEM-EDS analysis of the second positive electrode active material according to Comparative Example 1.

Hereinafter, specific implementation examples will be described in detail so that those with ordinary skill in the art can easily implement the invention. However, the present invention may be implemented in various different forms and is not limited to the implementation examples described herein.

The terminology used herein is for the purpose of describing exemplary embodiments only and is not intended to limit the invention. The singular expression includes the plural expression unless the context clearly indicates otherwise.

Here, “combination of these” means a mixture, laminate, composite, copolymer, alloy, blend, reaction product, etc. of the components.

It should be understood that the terms "include," "comprising," or "having" herein are intended to specify the presence of a feature, number, step, component, or combination thereof, but do not exclude in advance the possibility of the presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

In order to clearly express various layers and regions in the drawings, the thickness is enlarged and shown, and the same drawing symbols are used for similar parts throughout the specification. When a part such as a layer, film, region, or plate is said to be "over" or "on" another part, this includes not only the case where it is "directly over" the other part, but also the case where there is another part in between. Conversely, when a part is said to be "directly over" another part, it means that there is no other part in between.

In addition, the term “layer” here includes not only a shape formed on the entire surface when observed in a plan view, but also a shape formed on a portion of the surface.

In addition, the average particle size can be measured by a method widely known to those skilled in the art, for example, by measuring with a particle size analyzer, or by measuring with a transmission electron microscope photograph or a scanning electron microscope photograph. Alternatively, the average particle size value can be obtained by measuring using a dynamic light scattering method, performing data analysis, counting the number of particles for each particle size range, and calculating from the counted number. Unless otherwise defined, the average particle size can mean the diameter of particles (D50) having a cumulative volume of 50% by volume in the particle size distribution.

Bipolar active material

In one embodiment, a first cathode active material including a lithium nickel-based composite oxide, the first cathode active material having a secondary particle form in which a plurality of primary particles are aggregated and a cobalt coating portion positioned on a surface of the secondary particle, and a second cathode active material including a lithium nickel-based composite oxide, the second cathode active material having a single crystal form and a cobalt coating portion positioned on a surface of the single crystal, wherein the second cathode active material has a rough surface having roughness and a flat surface without roughness, are provided. Such a cathode active material can exhibit improved cycle characteristics while implementing high capacity and high energy density.

First positive electrode active material

The first cathode active material is in a polycrystal form and includes secondary particles in which at least two or more primary particles are aggregated.

According to one embodiment, a first cathode active material includes a cobalt coating portion positioned on the surface of the secondary particle. The cobalt coating portion may be positioned on the entirety or at least a portion of the surface of the secondary particle. Since the first cathode active material is coated with cobalt, structural collapse due to repeated charge and discharge can be effectively suppressed, thereby improving room temperature and high temperature life characteristics.

The above cobalt coating portion includes a cobalt-containing compound. The cobalt-containing compound may be, for example, cobalt oxide, cobalt hydroxide, cobalt carbonate, a composite thereof, a mixture thereof, or the like, and may further include lithium and/or nickel, or the like. For example, the cobalt-containing compound may be lithium cobalt oxide, or the like.

The content of the cobalt coating portion in the first positive electrode active material can be 0.01 mol% to 7 mol% with respect to the entire first positive electrode active material, for example, 0.01 mol% to 6 mol%, 0.1 mol% to 5 mol%, 0.5 mol% to 4 mol%, 1 mol% to 5 mol%, or 2 mol% to 5 mol%, and further can be 0.01 at% to 7 at%, 0.1 at% to 5 at%, or 0.5 at% to 4 at%. In this case, a lithium secondary battery including the first positive electrode active material can implement excellent room temperature and high temperature cycle characteristics.

The thickness of the cobalt coating portion in the first positive electrode active material varies depending on the firing temperature during coating, and cobalt may penetrate into the active material to coat and/or dope depending on the firing temperature. Accordingly, the thickness of the cobalt coating layer may be, for example, 1 nm to 2 ㎛, 1 nm to 1.5 ㎛, 1 nm to 1 ㎛, 1 nm to 900 nm, 1 nm to 700 nm, 1 nm to 500 nm, 1 nm to 300 nm, 5 nm to 100 nm, or 5 nm to 50 nm. In this case, a lithium secondary battery including the first positive electrode active material may exhibit excellent room temperature and high temperature cycle characteristics.

The average particle diameter of the first positive electrode active material, i.e., the average particle diameter of the secondary particles, may be 7 ㎛ to 25 ㎛. For example, it may be 9 ㎛ to 25 ㎛, 10 ㎛ to 25 ㎛, 15 ㎛ to 25 ㎛, or 10 ㎛ to 20 ㎛. The average particle diameter of the secondary particles of the first positive electrode active material may be equal to or larger than the average particle diameter of the second positive electrode active material as a single crystal to be described later. The positive electrode active material according to one embodiment may be a mixed form of the first positive electrode active material which is polycrystalline and has a large particle size and the second positive electrode active material which is single crystal and has small particles, and thus the composite density can be improved and high capacity and high energy density can be implemented.

The first cathode active material is a nickel-based cathode active material and includes a lithium nickel-based composite oxide (or the first nickel-based oxide). The content of nickel in the lithium nickel-based composite oxide may be 30 mol% or more, based on the total amount of transition metals excluding lithium, and may be, for example, 40 mol% or more, 50 mol% or more, 60 mol% or more, 70 mol% or more, 80 mol% or more, or 90 mol% or more, and 99.9 mol% or less, or 99 mol% or less. For example, the content of nickel in the lithium nickel-based composite oxide may be higher than the content of each of other metals, such as cobalt, manganese, and aluminum. When the content of nickel satisfies the above range, the cathode active material may exhibit excellent battery performance while implementing a high capacity.

The first cathode active material may specifically include a lithium nickel-based composite oxide represented by the chemical formula 1 below.

[Chemical Formula 1]

Li _a1 Ni _x1 M ¹ _y1 M ² _1-x1-y1 O ₂

In the 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 Al, B, Ba, Ca, Ce, Co, Cr, Cu, F, Fe, Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr, and combinations thereof.

In the above chemical formula 1, 0.4≤x1≤1 and 0≤y1≤0.6 may be satisfied, 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.

The first cathode active material may include, for example, a lithium nickel-based composite oxide represented by the chemical formula 2 below.

[Chemical formula 2]

Li _a2 Ni _x2 Co _y2 M ³ _1-x2-y2 O ₂

In the above chemical formula 2, 0.9≤a2≤1.8, 0.3≤x2<1, 0<y2≤0.7, and M ³ is selected from Al, B, Ba, Ca, Ce, Cr, Cu, F, Fe, Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr, and combinations thereof.

In the above chemical formula 2, 0.3≤x2≤0.99 and 0.01≤y2≤0.7 may be satisfied, or 0.4≤x2≤0.99 and 0.01≤y2≤0.6, or 0.5≤x2≤0.99 and 0.01≤y2≤0.5, or 0.6≤x2≤0.99 and 0.01≤y2≤0.4, or 0.7≤x2≤0.99 and 0.01≤y2≤0.3, or 0.8≤x2≤0.99 and 0.01≤y2≤0.2, or 0.9≤x2≤0.99 and 0.01≤y2≤0.1.

The first positive electrode active material may include, for example, a compound of chemical formula 3 below.

[Chemical Formula 3]

Li _a3 Ni _x3 Co _y3 M ⁴ _z3 M ⁵ _1-x3-y3-z3 O ₂

In the chemical formula 3, 0.9≤a3≤1.8, 0.3≤x3≤0.98, 0.01≤y3≤0.69, 0.01≤z3≤0.69, ^M4 is selected from Al, Mn, and combinations thereof, and ^M5 is selected from B, Ba, Ca, Ce, Cr, Cu, F, Fe, Mg, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr, and combinations thereof.

In the above chemical formula 3, 0.4≤x3≤0.98, 0.01≤y3≤0.59, and 0.01≤z3≤0.59, or 0.5≤x3≤0.98, 0.01≤y3≤0.49, and 0.01≤z3≤0.49, or 0.6≤x3≤0.98, 0.01≤y3≤0.39, and 0.01≤z3≤0.39, or 0.7≤x3≤0.98, 0.01≤y3≤0.29, and 0.01≤z3≤0.29, or 0.8≤x3≤0.98, 0.01≤y3≤0.19, and 0.01≤z3≤0.19, or 0.9≤x3≤0.98, 0.01≤y3≤0.09, and 0.01≤z3≤0.09.

Second positive electrode active material

The second cathode active material is in the form of a single crystal, which means that it exists alone without a grain boundary within the particle and is composed of a single particle, and may mean a monolithic structure or a single-body structure or non-agglomerated particles in which the particles do not aggregate with each other in terms of morphology, and may also be expressed as a single crystal particle. The cathode active material according to one embodiment can exhibit improved life characteristics while implementing high capacity and high energy density by including the second cathode active material.

The shape of the second positive electrode active material is not particularly limited, and may have various shapes such as a polyhedral shape, a sphere shape, an oval shape, a plate shape, a rod shape, and an irregular shape. The second positive electrode active material of a single crystal according to one embodiment may have a polyhedral structure having two or more faces.

The second cathode active material according to one embodiment includes a cobalt coating portion located on the surface of the single crystal. Since the second cathode active material is coated with cobalt, the collapse of the structure due to repeated charge and discharge can be effectively suppressed, thereby improving the room temperature and high temperature life characteristics.

The above cobalt coating portion includes a cobalt-containing compound. The cobalt-containing compound may be, for example, cobalt oxide, cobalt hydroxide, cobalt carbonate, a composite thereof, a mixture thereof, or the like, and may further include lithium and/or nickel, or the like. For example, the cobalt-containing compound may be lithium cobalt oxide, or the like.

In the method for manufacturing a cathode active material according to the embodiment described below, cobalt coating is not performed separately on the first cathode active material and the second cathode active material, but the first nickel-based oxide, the second nickel-based oxide, and the cobalt raw material are mixed together, and then a third heat treatment is performed to perform simultaneous coating (or simultaneous firing). According to this, it is confirmed that cobalt is effectively coated only on a part of the crystal surface of the single crystal second cathode active material, and that unevenness occurs in this part. Accordingly, one single crystal particle of the second cathode active material includes both an uneven surface having a specific shape of unevenness and a flat or smooth flat surface. It is understood that cobalt is effectively coated only on the crystal surface where lithium ions are easily introduced and extracted, and unevenness occurs. The surface roughness of the second cathode active material increases on a specific surface, and the specific surface area of the second cathode active material and the entire cathode active material including it increases. A lithium secondary battery including such a cathode active material can have improved initial discharge capacity, charge/discharge efficiency, and life characteristics.

The irregularities in the above-mentioned uneven surface may be linear or irregular, etc. In addition, the above-mentioned uneven surface may be expressed as a shape in which a cobalt-containing compound is attached in a linear or irregular shape, etc., or a shape in which a cobalt-containing compound covers the surface of a single crystal in an uneven shape. This shape is distinguished from the existing island-shaped coating.

A single crystal particle of the second positive electrode active material may include a rough surface having high surface roughness and a flat surface having low surface roughness. That is, the rough surface in the second positive electrode active material has high surface roughness. The surface roughness may be measured using a surface roughness measuring device, such as an optical profiler, using a photograph taken of an object using an atomic force microscope (AFM).

Among surface roughness, maximum roughness (R _max ; peak to peak height; maximum roughness depth) can mean the vertical distance between the highest peak and the deepest peak (lowest valley) within the reference length of the roughness cross-section curve (roughness profile). Average roughness (R _a ; average roughness), also called centerline average roughness, can mean the arithmetic mean of the absolute values of the ordinate (length from the center line to the peak) within the reference length of the roughness profile. Root mean square roughness (R _q ; root mean square roughness) can mean the root mean square (rms) of the ordinate values within the reference length of the roughness profile. For such surface roughness, refer to the parameter definitions and measurement methods defined in KS B 0601 or ISO 4287/1.

The rough surface of the second positive electrode active material may have a maximum roughness (R _max ; peak to peak height) of 18 nm or more, for example, 20 nm or more, and may be from 18 nm to 100 nm, from 18 nm to 80 nm, from 19 nm to 60 nm, or from 20 nm to 40 nm. In this case, the positive electrode active material for a lithium secondary battery including the second positive electrode active material may exhibit high energy density and high capacity and may implement excellent charge/discharge efficiency and cycle life characteristics.

The above-described uneven surface of the second positive electrode active material may have an average roughness (R _a ) of 1.9 nm or more, for example, 2.0 nm or more, and may be from 1.9 nm to 10 nm, from 1.9 nm to 8.0 nm, from 1.9 nm to 6.0 nm, from 1.9 nm to 5.0 nm, or from 2.0 nm to 10 nm. In this case, the positive electrode active material for a lithium secondary battery including the second positive electrode active material can exhibit high energy density and high capacity and implement excellent charge/discharge efficiency and cycle life characteristics.

The above-described uneven surface of the second positive electrode active material may have a root mean square roughness (R _q ) of 2.3 nm or more, for example, 2.4 nm or more, and may be from 2.3 nm to 10 nm, from 2.3 nm to 8 nm, from 2.3 nm to 6 nm, from 2.3 nm to 5 nm, or from 2.4 nm to 10 nm. In this case, the positive electrode active material for a lithium secondary battery including the second positive electrode active material can exhibit high energy density and high capacity and implement excellent charge/discharge efficiency and cycle life characteristics.

On the other hand, the flat surface of the second positive electrode active material may exhibit lower surface roughness than the uneven surface. For example, the flat surface of the second positive electrode active material may have a maximum roughness (R _max ) of 15 nm or less, for example, 0.1 nm to 14 nm, or 1 nm to 13 nm. In addition, the flat surface of the second positive electrode active material may have an average roughness (R _a ) of 1.8 nm or less, for example, 0.1 nm to 1.8 nm, or 0.5 nm to 1.7 nm. The flat surface of the second positive electrode active material may have a root mean square roughness (R _q ) of 2.2 nm or less, for example, 0.1 nm to 2.2 nm, or 0.5 nm to 2.1 nm.

In this way, the second positive electrode active material including both a rough surface and a flat surface can have a stable structure without structural collapse according to the charge and discharge of the battery while implementing high capacity and high energy density, and can exhibit excellent life characteristics.

The content of the cobalt coating portion in the second positive electrode active material can be 0.01 mol% to 10 mol% with respect to the entire second positive electrode active material, for example, 0.05 mol% to 9 mol%, 0.1 mol% to 8 mol%, 0.5 mol% to 7 mol%, or 1 mol% to 6 mol%, and further can be 0.01 at% to 10 at%, 0.1 at% to 8 at%, or 0.5 at% to 6 at%. In this case, a lithium secondary battery including the second positive electrode active material can implement excellent room temperature and high temperature cycle characteristics.

The ratio of the uneven surface to the total surface area of the second positive electrode active material may be approximately 40% to 80%, for example, 45% to 80%, or 50% to 75%. In addition, the ratio of the flat surface to the total surface area of the second positive electrode active material may be approximately 20% to 60%, for example, 20% to 55%, or 25% to 50%. By including the uneven surface and the flat surface in these ratios, the second positive electrode active material can implement high capacity and at the same time exhibit high cycle characteristics.

The thickness of the cobalt coating portion in the second positive electrode active material may be about 1 nm to 2 ㎛, for example, 1 nm to 1 ㎛, 1 nm to 900 nm, 1 nm to 700 nm, 1 nm to 500 nm, 1 nm to 300 nm, 5 nm to 100 nm, or 5 nm to 50 nm. In this case, a lithium secondary battery including the second positive electrode active material may exhibit excellent room temperature and high temperature cycle life characteristics.

Meanwhile, the second cathode active material may have planes having different cobalt contents. That is, the second cathode active material may have both a plane having a high cobalt content and a plane having a low cobalt content in one single crystal. For example, the cobalt content in the plane having a high cobalt content may be 30 at% to 70 at% or 35 at% to 65 at% with respect to the entire transition metal excluding lithium. The cobalt content in the plane having a low cobalt content may be 1 at% to 30 at% or 3 at% to 25 at% with respect to the entire transition metal excluding lithium. In addition, the proportion of the plane having a high cobalt content in one single crystal may be 20% to 50%, and the proportion of the plane having a low cobalt content may be 50% to 80%. Such a second cathode active material can improve the capacity characteristics and life characteristics of the battery.

The average particle size of the second positive electrode active material, i.e., the average particle size of a single crystal, may be 0.05 µm to 10 µm, for example, 0.1 µm to 8 µm, 0.1 µm to 7 µm, 0.1 µm to 6 µm, 0.1 µm to 10 µm, and specifically, 1 µm to 5 µm. The particle size of the second positive electrode active material may be the same as or smaller than that of the first positive electrode active material, and thus, the density of the positive electrode active material may be further increased.

The BET specific surface area of the entire positive electrode active material including the first positive electrode active material and the second positive electrode active material may be from 0.2 m ² /g to 0.6 m ² /g, for example, from 0.3 m ² /g to about 0.5 m ² /g, or from 0.3 m ² /g to about 0.4 m ² /g. In this case, the positive electrode active material can realize excellent charge/discharge efficiency and cycle life characteristics.

The second cathode active material includes a lithium nickel-based composite oxide (or a second nickel-based oxide) as a nickel-based active material. The content of nickel in the lithium nickel-based composite oxide may be 30 mol% or more, based on the total amount of transition metals excluding lithium, for example, 40 mol% or more, 50 mol% or more, 60 mol% or more, 70 mol% or more, 80 mol% or more, or 90 mol% or more, and 99.9 mol% or less, or 99 mol% or less. For example, the content of nickel in the lithium nickel-based composite oxide may be higher than the content of each of other metals, such as cobalt, manganese, and aluminum. When the content of nickel satisfies the above range, the cathode active material may exhibit excellent battery performance while implementing a high capacity.

The second positive electrode active material may include, for example, a lithium nickel-based composite oxide represented by the following chemical formula 11.

[Chemical Formula 11]

Li _a11 Ni _x11 M ¹¹ _y11 M ¹² _1-x11-y11 O ₂

In the chemical formula 11, 0.9≤a11≤1.8, 0.3≤x11≤1, 0≤y11≤0.7, and M ¹¹ and M ¹² are each independently selected from Al, B, Ba, Ca, Ce, Co, Cr, Cu, F, Fe, Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr, and combinations thereof.

In the above chemical formula 11, 0.4≤x11≤1 and 0≤y11≤0.6 may be satisfied, or 0.5≤x11≤1 and 0≤y11≤0.5, or 0.6≤x11≤1 and 0≤y11≤0.4, or 0.7≤x11≤1 and 0≤y11≤0.3, or 0.8≤x11≤1 and 0≤y11≤0.2, or 0.9≤x11≤1 and 0≤y11≤0.1.

The second positive electrode active material may include, for example, a lithium nickel-based composite oxide represented by the following chemical formula 12.

[Chemical Formula 12]

Li _a12 Ni _x12 Co _y12 M ¹³ _1-x12-y12 O ₂

In the chemical formula 12, 0.9≤a12≤1.8, 0.3≤x12<1, 0<y12≤0.7, and M ¹³ is selected from Al, B, Ba, Ca, Ce, Cr, Cu, F, Fe, Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr, and combinations thereof.

In the above chemical formula 12, 0.3≤x12≤0.99 and 0.01≤y12≤0.7 may be satisfied, or 0.4≤x12≤0.99 and 0.01≤y12≤0.6, or 0.5≤x12≤0.99 and 0.01≤y12≤0.5, or 0.6≤x12≤0.99 and 0.01≤y12≤0.4, or 0.7≤x12≤0.99 and 0.01≤y12≤0.3, or 0.8≤x12≤0.99 and 0.01≤y12≤0.2, or 0.9≤x12≤0.99 and 0.01≤y12≤0.1.

The above second positive electrode active material may include, as a specific example, a lithium nickel cobalt composite oxide represented by the following chemical formula 13.

[Chemical Formula 13]

Li _a13 Ni _x13 Co _y13 M ¹⁴ _z13 M ¹⁵ _{1-x13-y13-z13} O ₂

In the chemical formula 13, 0.9≤a13≤1.8, 0.3≤x13≤0.98, 0.01≤y13≤0.69, 0.01≤z13≤0.69, M ¹⁴ is selected from Al, Mn, and combinations thereof, and M ¹⁵ is selected from B, Ba, Ca, Ce, Cr, Cu, F, Fe, Mg, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr, and combinations thereof.

In the above chemical formula 13, 0.4≤x13≤0.98, 0.01≤y13≤0.59, and 0.01≤z13≤0.59 may be satisfied, or 0.5≤x13≤0.98, 0.01≤y13≤0.49, and 0.01≤z13≤0.49, or 0.6≤x13≤0.98, 0.01≤y13≤0.39, and 0.01≤z13≤0.39, or 0.7≤x13≤0.98, 0.01≤y13≤0.29, and 0.01≤z13≤0.29, or 0.8≤x13≤0.98, 0.01≤y13≤0.19, and 0.01≤z13≤0.19, or It can be 0.9≤x13≤0.98, 0.01≤y13≤0.09, and 0.01≤z13≤0.09.

In a cathode active material according to one embodiment, the first cathode active material may be included in an amount of 50 wt% to 90 wt%, and the second cathode active material may be included in an amount of 10 wt% to 50 wt%, with respect to the total amount of the first cathode active material and the second cathode active material. The first cathode active material may be included in an amount of, for example, 60 wt% to 90 wt%, or 70 wt% to 90 wt%, and the second cathode active material may be included in an amount of 10 wt% to 40 wt%, or 10 wt% to 30 wt%. When the content ratio of the first cathode active material and the second cathode active material is as such, the cathode active material including them can implement a high capacity, have an improved composite density, and exhibit a high energy density.

Method for manufacturing positive electrode active material

In one embodiment, a method for producing a cathode active material for a lithium secondary battery is provided, including mixing a first nickel-based hydroxide and a lithium raw material and performing a first heat treatment to prepare a first nickel-based oxide, mixing a second nickel-based hydroxide and a lithium raw material and performing a second heat treatment to prepare a second nickel-based oxide, and mixing the first nickel-based oxide, the second nickel-based oxide, and the cobalt raw material and performing a third heat treatment to obtain a final cathode active material including a first cathode active material and a second cathode active material.

Here, the first nickel-based oxide and the first cathode active material are in the form of secondary particles formed by agglomeration of a plurality of primary particles, and the second nickel-based oxide and the second cathode active material are in the form of single crystals. The first cathode active material may be a material in which cobalt is coated on the surface of the first nickel-based oxide, and the second cathode active material may be a material in which cobalt is coated on the surface of the second nickel-based oxide.

In one embodiment, instead of performing cobalt coating on the first cathode active material and the second cathode active material separately, cobalt coating is performed simultaneously while mixing the first nickel-based oxide and the second nickel-based oxide. Accordingly, the cobalt-coated single crystal second cathode active material has both an uneven surface and a smooth flat surface, and thus has high surface roughness and a specific surface area. A cathode active material for a lithium secondary battery including such a second cathode active material exhibits a high specific surface area and can implement excellent capacity characteristics and life characteristics.

The first nickel-based hydroxide and the second nickel-based hydroxide are precursors of the positive electrode active material, and may each independently be nickel hydroxide or a nickel-based composite hydroxide including an element other than nickel, a nickel-metal composite hydroxide including a metal other than nickel, or a nickel-transition metal composite hydroxide including a transition metal other than nickel.

For example, the first nickel-based hydroxide and the second nickel-based hydroxide may each be independently represented by the chemical formula 21 below.

[Chemical Formula 21]

Ni _x21 M ²¹ _y21 M ²² _1-x21-y21 (OH) ₂

In the chemical formula 21, 0.3≤x21≤1, 0≤y21≤0.7, and M ²¹ and M ²² are each independently selected from Al, B, Ba, Ca, Ce, Co, Cr, Cu, F, Fe, Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr, and combinations thereof.

The particle size of the first nickel-based hydroxide can be from 10 μm to 30 μm, for example, from 10 μm to 25 μm, from 15 μm to 25 μm, or from 10 μm to 20 μm. The particle size of the second nickel-based hydroxide can be from 1 μm to 9 μm, for example, from 2 μm to 9 μm, from 2 μm to 8 μm, or from 3 μm to 7 μm.

The above lithium raw material may be, for example, Li ₂ CO ₃ , LiOH, a hydrate thereof, or a combination thereof as a lithium source for the cathode active material.

When mixing the first nickel-based hydroxide and the lithium raw material, the ratio of the number of moles of lithium in the lithium raw material to the number of moles of metal included in the first nickel-based hydroxide may be, for example, 0.8 or more, 0.85 or more, 0.9 or more, 0.95 or more, or 1.0 or more, and may be 1.8 or less, 1.5 or less, 1.2 or less, 1.1 or less, or 1.05 or less.

The first heat treatment may be performed in an oxidizing gas atmosphere, for example, in an oxygen atmosphere or an air atmosphere. In addition, the first heat treatment may be performed at, for example, 600° C. to 900° C. or 600° C. to 800° C., and may be performed for, for example, 5 to 20 hours or 5 to 15 hours. The second nickel-based oxide obtained through the first heat treatment may also be referred to as a first lithium nickel-based oxide.

When mixing the second nickel-based hydroxide and the lithium raw material, the ratio of the number of moles of lithium in the lithium raw material to the number of moles of metal included in the second nickel-based hydroxide may be, for example, 0.8 or more, 0.85 or more, 0.9 or more, 0.95 or more, or 1.0 or more, and may be 1.8 or less, 1.5 or less, 1.2 or less, 1.1 or less, or 1.05 or less.

The above second heat treatment can also be performed in an oxidizing gas atmosphere, for example, in an oxygen atmosphere or an air atmosphere. In addition, the second heat treatment can be performed at, for example, 800° C. to 1100° C., or 900° C. to 1000° C., and can be performed for, for example, 5 to 20 hours or 5 to 15 hours. The second nickel-based oxide obtained through the second heat treatment can also be expressed as a second lithium nickel-based oxide.

The second nickel-based oxide is in the form of a single crystal, and can be obtained in the form of a single crystal by controlling the conditions of the second heat treatment, such as temperature and time, or can be obtained in the form of a single crystal through various conditions when synthesizing the second nickel-based hydroxide by the coprecipitation method.

The above method for producing a positive electrode active material for a lithium secondary battery may further include mixing a second nickel-based hydroxide and a lithium raw material, performing a second heat treatment, and then crushing the obtained product, through which a single-crystal second nickel-based oxide may be obtained. The crushing may be performed using various crushing devices such as a jet mill. Here, crushing the obtained product is a process of obtaining a single-crystal active material, and is distinguished from the crushing of general active materials.

In the process of mixing the first nickel-based oxide and the second nickel-based oxide, the weight ratio of the first nickel-based oxide and the second nickel-based oxide may be 9:1 to 5:5, for example, 9:1 to 6:4, or 8:2 to 7:3. When the mixing ratio of the first nickel-based oxide and the second nickel-based oxide satisfies the above-mentioned range, the manufactured positive electrode active material may exhibit high capacity and high energy density and may exhibit high plate density.

Next, cobalt coating is performed by mixing the first nickel-based oxide, the second nickel-based oxide, and the cobalt raw material, and performing a third heat treatment. The cobalt coating can be performed in a dry or wet manner. For example, dry coating can be performed by mixing the first nickel-based oxide, the second nickel-based oxide, and the cobalt raw material without a solvent, and then performing a third heat treatment. Alternatively, the first nickel-based oxide and the second nickel-based oxide can be added to a solvent such as distilled water, mixed and washed, and the cobalt raw material can be dropped to perform wet coating, and then a third heat treatment can be performed.

The above cobalt raw material may be, for example, cobalt sulfate, cobalt oxide, cobalt nitrate, etc.

The above cobalt raw material may be mixed so that when the content of metals other than lithium in the entire positive electrode active material is 100 mol parts, the cobalt contained in the cobalt raw material is 0.01 mol parts to 7 mol parts, or 0.01 mol parts to 5 mol parts, or 0.1 mol parts to 4 mol parts, etc.

The third heat treatment can be performed in an oxidizing gas atmosphere, such as an oxygen or air atmosphere. In addition, the third heat treatment can be performed at, for example, 650° C. to 900° C. or 650° C. to 800° C. The third heat treatment time varies depending on the heat treatment temperature, etc., but is performed for, for example, 5 to 30 hours or 10 to 24 hours.

Thereafter, the heat treatment is terminated and the lithium secondary battery cathode active material according to the above-described embodiment is cooled to room temperature, thereby manufacturing the cathode active material. The manufactured cathode active material is a mixture of a first cathode active material including secondary particles formed by agglomeration of primary particles and a second cathode active material in the form of a single crystal, and each of the first cathode active material and the second cathode active material is coated with cobalt, and the second cathode active material includes both an uneven surface and a flat surface.

anode

A cathode for a lithium secondary battery may include a current collector and a cathode active material layer positioned on the current collector. The cathode active material layer includes a cathode active material and may further include a binder and/or a conductive material.

The above binder serves to attach the positive electrode active material particles well to each other and also to attach the positive electrode active material well to the current collector, and representative examples thereof include, but are not limited to, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, nylon, etc.

The content of the binder in the above positive electrode active material layer may be approximately 1 wt% to 5 wt% with respect to the total weight of the positive electrode active material layer.

The conductive material is used to provide conductivity to the electrode, and any material that does not cause a chemical change in the battery to be formed and is electronically conductive can be used. Examples of such conductive materials include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fibers, and carbon nanotubes; metal-based materials containing copper, nickel, aluminum, silver, etc. and in the form of metal powder or metal fibers; conductive polymers such as polyphenylene derivatives; or conductive materials including mixtures thereof.

The content of the conductive material in the above positive electrode active material layer may be 1 wt% to 5 wt% with respect to the total weight of the positive electrode active material layer.

Aluminum foil may be used as the above positive electrode collector, but is not limited thereto.

cathode

A negative electrode for a lithium secondary battery includes a current collector and a negative electrode active material layer positioned on the current collector. The negative electrode active material layer includes a negative electrode active material and may further include a binder and/or a conductive material.

The above negative active material includes a material capable of reversibly intercalating/deintercalating lithium ions, lithium metal, an alloy of lithium metal, a material capable of doping and dedoping lithium, or a transition metal oxide.

The material capable of reversibly intercalating/deintercalating the lithium ions may include a carbon-based negative electrode active material, for example, crystalline carbon, amorphous carbon, or a combination thereof. Examples of the crystalline carbon include graphite such as natural graphite or artificial graphite in an amorphous, plate-like, flake-like, spherical, or fibrous form, and examples of the amorphous carbon include soft carbon or hard carbon, mesophase pitch carbide, calcined coke, and the like.

As the above lithium metal alloy, an alloy of lithium and a metal 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 can be used.

As the material capable of doping and dedoping the lithium, a Si-based negative electrode active material or a Sn-based negative electrode active material can be used. As the Si-based negative electrode active material, silicon, a silicon-carbon composite, SiO _x (0 < x < 2), a Si-Q alloy (wherein Q is an element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, Group 15 elements, Group 16 elements, transition metals, rare earth elements and combinations thereof, but is not Si), and as the Sn-based negative electrode active material, Sn, SnO ₂ , a Sn-R alloy (wherein R is an element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, Group 15 elements, Group 16 elements, transition metals, rare earth elements and combinations thereof, but is not Sn), and the like. In addition, at least one of these and SiO ₂ can be mixed and used. The above elements Q and R may be selected from the group consisting of 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.

The above silicon-carbon composite may be, for example, a silicon-carbon composite including a core including crystalline carbon and silicon particles and an amorphous carbon coating layer positioned on a surface of the core. The crystalline carbon may be artificial graphite, natural graphite, or a combination thereof. As the amorphous carbon precursor, coal pitch, mesophase pitch, petroleum pitch, coal oil, petroleum heavy oil, or a polymer resin such as a phenol resin, a furan resin, or a polyimide resin may be used. At this time, the content of silicon may be 10 wt% to 50 wt% with respect to the total weight of the silicon-carbon composite. In addition, the content of the crystalline carbon may be 10 wt% to 70 wt% with respect to the total weight of the silicon-carbon composite, and the content of the amorphous carbon may be 20 wt% to 40 wt% with respect to the total weight of the silicon-carbon composite. In addition, the thickness of the amorphous carbon coating layer may be 5 nm to 100 nm. The average particle diameter (D50) of the above silicon particles may be 10 nm to 20 μm. The average particle diameter (D50) of the above silicon particles may be preferably 10 nm to 200 nm. The silicon particles may exist in an oxidized form, and atomic content ratio of Si:O in the silicon particles indicating the degree of oxidation may be 99:1 to 33:67. The silicon particles may be SiO _x particles, and at this time, x in SiO _x may be greater than 0 and less than 2. In the present specification, unless otherwise defined, the average particle diameter (D50) means the diameter of particles having a cumulative volume of 50 volume% in a particle size distribution.

The above Si-based negative electrode active material or Sn-based negative electrode active material can be used in a mixture with a carbon-based negative electrode active material. When the Si-based negative electrode active material or Sn-based negative electrode active material and the carbon-based negative electrode active material are used in a mixture, the mixing ratio can be 1:99 to 90:10 in weight ratio.

The content of the negative active material in the above negative active material layer may be 95 wt% to 99 wt% with respect to the total weight of the negative active material layer.

In one embodiment, the negative electrode active material layer further includes a binder and may optionally further include a conductive material. The content of the binder in the negative electrode active material layer may be 1 wt% to 5 wt% with respect to the total weight of the negative electrode active material layer. In addition, when the negative electrode active material layer further includes a conductive material, the negative electrode active material layer may include 90 wt% to 98 wt% of the negative electrode active material, 1 wt% to 5 wt% of the binder, and 1 wt% to 5 wt% of the conductive material.

The above binder serves to adhere the negative active material particles well to each other and also to adhere the negative active material well to the current collector. The binder may be an insoluble binder, a water-soluble binder, or a combination thereof.

The above-mentioned insoluble binders may include polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, ethylene propylene copolymers, polystyrene, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyimide or combinations thereof.

The above water-soluble binder may be a rubber-based binder or a polymer resin binder. The rubber-based binder may be selected from styrene-butadiene rubber, acrylated styrene-butadiene rubber, acrylonitrile-butadiene rubber, acrylic rubber, butyl rubber, fluororubber, and combinations thereof. The polymer resin binder may be selected from polyethylene oxide, polyvinylpyrrolidone, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, ethylene propylene diene copolymer, polyvinyl pyridine, chlorosulfonated polyethylene, latex, polyester resin, acrylic resin, phenol resin, epoxy resin, polyvinyl alcohol, and combinations thereof.

When a water-soluble binder is used as the negative electrode binder, a cellulose-based compound capable of imparting viscosity may be further included. As the cellulose-based compound, one or more of carboxymethyl cellulose, hydroxypropylmethyl cellulose, methyl cellulose, or an alkali metal salt thereof may be mixed and used. As the alkali metal, Na, K or Li may be used. The amount of such thickener used may be 0.1 to 3 parts by weight based on 100 parts by weight of the negative electrode active material.

The conductive material is used to provide conductivity to the electrode, and any material that does not cause a chemical change in the battery to be formed and is electronically conductive can be used. Examples of such conductive materials include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fibers, and carbon nanotubes; metal-based materials including copper, nickel, aluminum, and silver in the form of metal powder or metal fibers; conductive polymers such as polyphenylene derivatives; or conductive materials including mixtures thereof.

The negative electrode current collector may be selected from the group consisting of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and combinations thereof.

Lithium secondary battery

Another embodiment provides a lithium secondary battery including a cathode, an anode, a separator positioned between the cathode and the anode, and an electrolyte.

FIG. 1 is a schematic diagram illustrating a lithium secondary battery according to an embodiment. Referring to FIG. 1, a lithium secondary battery (100) according to an embodiment includes a battery cell including a cathode (114), an anode (112) positioned opposite to the cathode (114), a separator (113) positioned between the cathode (114) and the anode (112), and an electrolyte for a lithium secondary battery that impregnates the cathode (114), the anode (112), and the separator (113), a battery container (120) containing the battery cell, and a sealing member (140) that seals the battery container (120).

The above electrolyte comprises a non-aqueous organic solvent and a lithium salt.

The above non-aqueous organic solvent acts as a medium through which ions involved in the electrochemical reaction of the battery can move. As the non-aqueous organic solvent, a carbonate-based, ester-based, ether-based, ketone-based, alcohol-based, or aprotic solvent can be used. As the carbonate-based solvent, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and the like can be used. Examples of the ester solvent that can be used include methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, decanolide, valerolactone, mevalonolactone, caprolactone, etc. Examples of the ether solvent that can be used include dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, etc. Examples of the ketone solvent that can be used include cyclohexanone, etc. In addition, as the alcohol solvent, ethyl alcohol, isopropyl alcohol, etc. can be used, and as the aprotic solvent, nitriles such as R-CN (wherein, R is a C2 to C20 linear, branched, or cyclic hydrocarbon group and may include a double bond, an aromatic ring, or an ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, and sulfolanes can be used.

The above non-aqueous organic solvents can be used alone or in combination of one or more, and when using one or more in combination, the mixing ratio can be appropriately adjusted depending on the desired battery performance, which is widely understood by those engaged in the relevant field.

In addition, in the case of the above carbonate solvent, a cyclic carbonate and a chain carbonate can be used in combination. In this case, when the cyclic carbonate and the chain carbonate are mixed and used in a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte can be excellent.

The above non-aqueous organic solvent may further include an aromatic hydrocarbon-based organic solvent in addition to the carbonate-based solvent. In this case, the carbonate-based solvent and the aromatic hydrocarbon-based organic solvent may be mixed in a volume ratio of about 1:1 to about 30:1.

As the above aromatic hydrocarbon solvent, an aromatic hydrocarbon compound of the following chemical formula I can be used.

[Chemical Formula I]

In the above chemical formula I, R ⁴ to R ⁹ are the same as or different from each other and are selected from the group consisting of hydrogen, halogen, an alkyl group having 1 to 10 carbon atoms, a haloalkyl group, and combinations thereof.

Specific examples of the above aromatic hydrocarbon solvents include 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, 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.

The above electrolyte may further contain vinylene carbonate or an ethylene carbonate compound of the following chemical formula II as a life-promoting additive to improve the battery life.

[Chemical Formula II]

In the above chemical formula II, R ¹⁰ and R ¹¹ are the same as or different from each other and are selected from the group consisting of hydrogen, a halogen group, a cyano group, a nitro group and a fluorinated alkyl group having 1 to 5 carbon atoms, and at least one of R ¹⁰ and R ¹¹ is selected from the group consisting of a halogen group, a cyano group, a nitro group and a fluorinated alkyl group having 1 to 5 carbon atoms, provided that both R ¹⁰ and R ¹¹ are not hydrogen.

Representative examples of the above ethylene carbonate compounds include difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate, or fluoroethylene carbonate. When such life-promoting additives are further used, the amount used can be appropriately adjusted.

The above lithium salt is a substance that is dissolved in a non-aqueous organic solvent and acts as a source of lithium ions within the battery, enabling the basic operation of a lithium secondary battery and promoting the movement of lithium ions between the positive and negative electrodes.

Representative examples of lithium salts include LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiN(SO ₂ C ₂ F ₅ ) ₂ , Li(CF ₃ SO ₂ ) ₂ N, LiN(SO ₃ C ₂ F ₅ ) ₂ , Li(FSO ₂ ) ₂ N (lithium bis(fluorosulfonyl)imide: LiFSI), LiC ₄ F ₉ SO ₃ , LiClO ₄ , LiAlO ₂ , LiAlCl ₄ , LiPO ₂ F ₂ , LiN(C _x F _2x+1 SO ₂ )(C _y F _2y+1 SO ₂ ) (wherein, x and y are natural numbers, for example, integers from 1 to 20), lithium difluorobisoxalatophosphate (lithium The present invention relates to a lithium ion battery, which may be a lithium ion battery, and _{more than one selected} from _the group consisting of lithium bis(oxalato)borate (LiBOB), lithium difluoro(bisoxalato)phosphate (LiCl), lithium I, lithium B(C2O4)2 (lithium bis(oxalato)borate): LiBOB), and lithium difluoro(oxalato)borate (LiDFOB).

It is recommended that the concentration of the lithium salt be used within the range of 0.1 M to 2.0 M. When the concentration of the lithium salt is within the above range, the electrolyte has appropriate conductivity and viscosity, so that it can exhibit excellent electrolyte performance and lithium ions can move effectively.

The separator (113) is also called a separator, and separates the positive electrode (114) and the negative electrode (112) and provides a passage for lithium ions to move. Any separator commonly used in lithium ion batteries can be used. That is, anything having low resistance to ion movement of the electrolyte and excellent electrolyte moisture retention capacity can be used. For example, it can be selected from glass fiber, polyester, Teflon, polyethylene, polypropylene, polytetrafluoroethylene, or a combination thereof, and may be in the form of a non-woven fabric or a woven fabric. For example, a polyolefin polymer separator such as polyethylene or polypropylene is mainly used in lithium ion batteries, and a coated separator containing a ceramic component or a polymer material can also be used to secure heat resistance or mechanical strength, and can optionally be used in a single-layer or multi-layer structure.

Lithium secondary batteries can be classified into lithium ion batteries, lithium ion polymer batteries, and lithium polymer batteries depending on the type of separator and electrolyte used, and can be classified into cylindrical, square, coin, and pouch types depending on the shape, and can be divided into bulk type and thin film type depending on the size. The structure and manufacturing method of these batteries are widely known in this field, so a detailed description is omitted.

A lithium secondary battery according to an embodiment of the present invention has high capacity and excellent storage stability at high temperatures, life characteristics, and high-rate characteristics, and thus can be used in electric vehicles (EVs), hybrid vehicles such as plug-in hybrid electric vehicles (PHEVs), and can also be used in portable electronic devices.

Hereinafter, examples and comparative examples of the present invention will be described. The following examples are only examples of the present invention, and the present invention is not limited to the following examples.

Example 1

1. Preparation of first nickel-based oxide in the form of secondary particles

A first nickel-based hydroxide (Ni _0.95 Co _0.04 Mn _0.01 (OH) ₂ ) obtained by a coprecipitation method is mixed with LiOH such that the molar ratio of lithium to the total amount of transition metals becomes 1.04, and the mixture is subjected to a first heat treatment at about 750° C. for 15 hours in an oxygen atmosphere, thereby obtaining a first nickel-based oxide (LiNi _0.95 Co _0.04 Mn _0.01 O ₂ ). The obtained first nickel-based oxide has an average particle size of approximately 13.8 μm, and is in the form of secondary particles in which primary particles are aggregated.

2. Preparation of second nickel-based oxide in single crystal form

[Co-precipitation process]

Nickel sulfate (NiSO ₄ 6H ₂ O), cobalt sulfate (CoSO ₄ 7H ₂ O), and manganese sulfate (MnSO ₄ H ₂ O) are dissolved in distilled water as a solvent to prepare a mixed solution. In order to form a complex, a diluted ammonia aqueous solution (NH ₄ OH) and sodium hydroxide (NaOH) are prepared as a precipitant. After that, the metal raw material mixture solution, ammonia aqueous solution, and sodium hydroxide are each introduced into the reactor. Next, the reaction is carried out for about 20 hours while stirring. Thereafter, the slurry solution inside the reactor is filtered and washed with high-purity distilled water, and dried for 24 hours to obtain a second nickel-based hydroxide (Ni _0.94 Co _0.05 Mn _0.01 (OH) ₂ ) powder. The obtained second nickel hydroxide powder has an average particle size of about 4.0 ㎛ and a specific surface area measured by the BET measurement method of about 15 m ² /g.

[Oxidation process]

The obtained second nickel-based hydroxide and LiOH are mixed so as to satisfy Li/(Ni+Co+Mn) = 1.05, placed into a kiln, and a second heat treatment is performed at 910° C. for 8 hours in an oxygen atmosphere. Thereafter, the obtained product is pulverized for about 30 minutes to separate/disperse into a plurality of second nickel-based oxides having a single crystal structure. The average particle size of the obtained second nickel-based oxide (LiNi _0.94 Co _0.05 Mn _0.01 O ₂ ) having a single crystal structure is about 3.7 μm.

3. Cobalt coating and preparation of final cathode active material

The first nickel-based oxide and the second nickel-based oxide are mixed in a weight ratio of 7:3, and the weight ratio of the mixture to water is 1:1, followed by washing in a stirrer and drying at 150°C. To this, 5 mol parts of lithium hydroxide and 2.5 mol parts of cobalt oxide are additionally mixed with respect to 100 mol parts of the transition metal of the nickel-based oxide, and the mixture is placed in a sintering furnace, followed by a third heat treatment at approximately 710°C for 15 hours in an oxygen atmosphere. Thereafter, the sintering furnace is cooled to room temperature to obtain a final cathode active material in which the first cathode active material and the second cathode active material are mixed.

The final cathode active material is a mixture of a first cathode active material in the form of secondary particles and a second cathode active material in the form of single crystals, each of which is coated with cobalt.

Fig. 2 is a scanning electron microscope photograph of particles corresponding to the second positive electrode active material in the final positive electrode active material manufactured according to Example 1. Referring to Fig. 2, it can be confirmed that the second positive electrode active material, which is a single crystal, has a rough surface and a flat surface formed on its surface.

4. Manufacturing of the anode

A slurry of positive electrode active material is prepared by mixing 95 wt% of the final positive electrode active material, 3 wt% of a polyvinylidene fluoride binder, and 2 wt% of a carbon nanotube conductive material in an N-methylpyrrolidone solvent. The slurry of positive electrode active material is applied to an aluminum current collector, dried, and then rolled to prepare a positive electrode.

5. Manufacturing of lithium secondary batteries

A coin half-cell is manufactured by using a prepared positive electrode and a lithium metal counter electrode, interposing a polyethylene-polypropylene multilayer separator between them, and injecting an electrolyte solution containing 1.0 M LiPF ₆ lithium salt added to a solvent containing ethylene carbonate and diethyl carbonate in a volume ratio of 50:50.

Example 2

In “3. Cobalt coating and production of final positive electrode active material” of the above Example 1, the positive electrode active material, positive electrode, and battery are produced in the same manner as in Example 1, except that the cobalt coating is performed using a wet method. The cobalt coating process is as follows. A first nickel-based oxide and a second nickel-based oxide are mixed in a weight ratio of 7:3, placed in distilled water, and washed while mixing. To this, cobalt sulfate (CoSO ₄ ) corresponding to 2.5 mol parts per 100 mol parts of the transition metal of the nickel-based oxide is slowly added, thereby performing cobalt coating. In addition, sodium hydroxide (NaOH) is slowly added. Thereafter, the obtained product is dried at 150° C. for 12 hours. Fig. 3 is a scanning electron microscope photograph of the second nickel-based oxide corresponding to a single crystal among the dried materials, from which it can be confirmed that an uneven surface and a flat surface are formed on the surface.

Next, the dried material is placed in a sintering furnace and subjected to a third heat treatment at approximately 710° C. for 15 hours in an oxygen atmosphere. Thereafter, the sintering furnace is cooled to room temperature to obtain a final cathode active material in which the first cathode active material and the second cathode active material are mixed.

Figure 4 is a scanning electron microscope image of a particle corresponding to a single crystal of the second positive electrode active material in the obtained final positive electrode active material. Referring to Figure 4, it can be confirmed that a rough surface and a flat surface are formed on the surface of the second positive electrode active material. The average particle size of the second positive electrode active material is about 4 ㎛.

Comparative Example 1

In “3. Cobalt coating and production of final positive electrode active material” of the above Example 1, a positive electrode active material and a battery are produced in the same manner as in Example 1, except that the first nickel-based oxide and the second nickel-based oxide are not mixed and then cobalt coated, but rather the first nickel-based oxide and the second nickel-based oxide are individually coated with cobalt and then mixed.

The cobalt coating process is as follows. 5 mol parts of lithium hydroxide and 3 mol parts of cobalt oxide are mixed with respect to 100 mol parts of a transition metal in a first nickel-based oxide, and the mixture is placed in a sintering furnace, followed by a third heat treatment at about 700° C. for 15 hours in an oxygen atmosphere, and the sintering furnace is cooled to room temperature to obtain a first cathode active material. In addition, 5 mol parts of lithium hydroxide and 3 mol parts of cobalt oxide are mixed with respect to 100 mol parts of a transition metal in a second nickel-based oxide, and the mixture is placed in a sintering furnace, followed by a third heat treatment at about 850° C. for 15 hours in an oxygen atmosphere, and the sintering furnace is cooled to room temperature to obtain a second cathode active material. The first cathode active material coated with cobalt and the second cathode active material coated with cobalt are mixed in a weight ratio of 7:3 to prepare a final cathode active material according to Comparative Example 1.

Fig. 5 is a scanning electron microscope photograph of the second positive electrode active material manufactured according to Comparative Example 1. Referring to Fig. 5, it can be confirmed that the surface of the second positive electrode active material of Comparative Example 1 is smooth and flat without any unevenness.

Comparative Example 2

A cathode active material and a battery are manufactured in the same manner as in Comparative Example 1, except that the third heat treatment step for the second nickel-based oxide in Comparative Example 1 is performed at about 700° C. for 15 hours in an oxygen atmosphere.

Fig. 6 is a scanning electron microscope photograph of the second positive electrode active material manufactured according to Comparative Example 2. Referring to Fig. 6, it can be confirmed that the surface of the second positive electrode active material of Comparative Example 2 is smooth and flat without forming unevenness.

Evaluation Example 1: Evaluation of Surface Roughness of Second Positive Electrode Active Material

For the positive electrode active materials manufactured in Examples 1 and 2 and Comparative Examples 1 and 2, the surface roughness of the second positive electrode active material was measured using a surface roughness measuring device using an atomic force microscope (DME UHV AFM; scan speed 0.25 ㎛/s, non-contact mode range 250 nm x 250 nm). The results are shown in Table 1 below. The unit of each value in Table 1 is nm.

Example 1 Example 2 Comparative Example 1 Comparative Example 2 Uneven surface Flat surface Uneven surface Flat surface Maximum roughness (R _max ) 35 12.0 23.8 10 6.7 10.1 Average roughness (R _a ) 3.35 1.7 2.0 1.2 0.73 0.99 Root mean square roughness (R _q ) 4.5 2.0 2.5 1.8 0.8 1.3

Evaluation Example 2: Surface Area Evaluation The surface areas of the positive electrode active materials of Examples 1 and 2 and Comparative Examples 1 and 2 were measured, and the results are shown in Table 2 below. The surface areas were measured using physical adsorption and chemical adsorption phenomena and the Brunauer-Emmett-Teller (BET) method. That is, after measuring the weight of the manufactured active material, nitrogen was adsorbed on the surface of the active material, the amount of adsorbed nitrogen gas was measured, and the specific surface area was calculated using the BET calculation formula.

BET specific surface area (m ² /g) Example 1 0.3856 Example 2 0.3652 Comparative Example 1 0.2575 Comparative Example 2 0.2521

Referring to Table 2, it can be confirmed that in the case of Examples 1 and 2, the specific surface area of the entire positive electrode active material including the first positive electrode active material and the second positive electrode active material increased compared to the comparative examples.

Evaluation Example 3: Evaluation of cobalt content on the surface

In the positive electrode active materials manufactured in Example 1 and Comparative Example 1, scanning electron microscope-energy dispersive spectrometry (SEM-EDS) was performed on the surface of the single-crystal second positive electrode active material, and the content ratio (at%) of cobalt to nickel and cobalt on the surface was measured. Fig. 7 is an SEM-EDS analysis photograph of the second positive electrode active material of Example 1, in which the cobalt content was measured at positions 1 to 4, and the results are shown in Table 3 below. Fig. 8 is an SEM-EDS analysis photograph of the second positive electrode active material of Comparative Example 1, in which the cobalt content was measured at positions 5 to 8, and the results are shown in Table 4 below.

location Co/(Ni+Co)(%) Example 1 #1 19.4 #2 60.2 #3 40.7 #4 52.9

　 location Co/(Ni+Co)(%) Comparative Example 1 #5 8.9 #6 15.5 #7 12 #8 8.6

Referring to FIG. 7 and Table 3, it can be seen that in the case of Example 1, there exists a surface with a low cobalt content of about 19.4 at%, and a surface with a high cobalt content of about 40 to 60 at%. Referring to FIG. 8 and Table 4, in the case of Comparative Example 1 in which cobalt coating was individually performed on the first positive electrode active material and the second positive electrode active material, the cobalt content of the surface was approximately in the range of 8 to 16 at%, and it was confirmed that there was almost no difference between the crystal planes. In addition, it can be confirmed that the cobalt coating content of the surface is in a lower range than that of Example 1.

Evaluation Example 4: Charge/Discharge Efficiency and Lifetime Characteristics

Each of the coin half-cells of Examples 1 and 2 and Comparative Examples 1 and 2 was charged under constant current (0.2 C) and constant voltage (4.25 V, 0.05 C cut-off) conditions to measure the charge capacity, paused for 10 minutes, and discharged to 3.0 V under constant current (0.2 C) conditions to measure the discharge capacity. The ratio of the discharge capacity to the charge capacity is expressed as efficiency. The results are shown in Table 5 below.

In addition, after the initial charge and discharge, the discharge capacity at 50 cycles was measured by repeating the charge and discharge at 1C at 45°C for 50 cycles, and the ratio (%) of the 50-cycle discharge capacity to the initial discharge capacity is shown in Table 5 as the capacity retention rate, i.e., the life characteristics.

Charging capacity (mAh/g) Discharge capacity (mAh/g) Efficiency (%) 50 cycle capacity retention (%, 45℃) Example 1 238.3 211.6 88.8 95.2 Example 2 238.8 208.5 87.3 96.4 Comparative Example 1 236.2 203.8 86.3 93.5 Comparative Example 2 234.5 200.7 85.6 92.3

Referring to Table 5 above, in the case of Examples 1 and 2, it can be confirmed that the discharge capacity increases, the charge/discharge efficiency improves, and the high-temperature life characteristics are also improved compared to Comparative Examples 1 and 2 in which the first and second positive electrode active materials were individually coated with cobalt and then fired. Both the first and second positive electrode active materials were coated with cobalt, but since cobalt was effectively coated only on some surfaces of the second positive electrode active material, which is a single crystal, changes such as the occurrence of an uneven surface and an increase in the specific surface area occurred, and it can be confirmed that the characteristics such as the initial discharge capacity, charge/discharge efficiency, and lifespan are improved in the case of a battery using such positive electrode active materials.

Although the preferred embodiments have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements made by those skilled in the art using the basic concepts defined in the following claims also fall within the scope of the present invention.

100: Lithium secondary battery 112: Cathode
113: Membrane 114: Anode
120: Battery container 140: Encapsulation member

Claims (20)

A first cathode active material comprising a lithium nickel-based composite oxide, a secondary particle in which a plurality of primary particles are aggregated, and a cobalt coating portion positioned on the surface of the secondary particle, and
A second cathode active material comprising a lithium nickel-based composite oxide, and comprising a single crystal and a cobalt coating portion positioned on the surface of the single crystal,
The second positive electrode active material has a rough surface with irregularities and a flat surface without irregularities.
The above rough surface has a maximum roughness (R _max ; peak to peak height) of 18 nm or more,
The above flat surface is a positive electrode active material for a lithium secondary battery, having a maximum roughness (R _max ) of 15 nm or less. In paragraph 1,
A cathode active material for a lithium secondary battery, wherein the rough surface of the second cathode active material has a maximum roughness (R _max ) of 20 nm or more. In paragraph 1,
A cathode active material for a lithium secondary battery, wherein the uneven surface of the second cathode active material has an average roughness (R _a ; average roughness) of 1.9 nm or more and a root mean square roughness (R _q ; root mean square roughness) of 2.3 nm or more. In paragraph 1,
A cathode active material for a lithium secondary battery, wherein the flat surface of the second cathode active material has a maximum roughness (R _max ) of 14 nm or less. In paragraph 1,
A positive electrode active material for a lithium secondary battery, wherein the flat surface of the second positive electrode active material has an average roughness (R _a ) of 1.8 nm or less and a root mean square roughness (R _q ) of 2.2 nm or less. delete In paragraph 1,
A cathode active material for a lithium secondary battery, comprising a first cathode active material and a second cathode active material, wherein the cathode active material for a lithium secondary battery has a BET specific surface area of 0.2 m ² /g to 0.6 m ² /g. In paragraph 1,
The average particle size of the first cathode active material is 7 ㎛ to 25 ㎛,
A cathode active material for a lithium secondary battery, wherein the average particle size of the second cathode active material is 1 ㎛ to 10 ㎛. In paragraph 1,
A cathode active material for a lithium secondary battery, wherein the first cathode active material is contained in an amount of 50 to 90 wt%, and the second cathode active material is contained in an amount of 10 to 50 wt%, based on the total amount of the first cathode active material and the second cathode active material. In paragraph 1,
The first cathode active material comprises a lithium nickel-based composite oxide represented by the following chemical formula 1:
The second cathode active material is a cathode active material for a lithium secondary battery including a lithium nickel-based composite oxide represented by the following chemical formula 11:
[Chemical Formula 1]
Li _a1 Ni _x1 M ¹ _y1 M ² _1-x1-y1 O ₂
In the 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 Al, B, Ba, Ca, Ce, Co, Cr, Cu, F, Fe, Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr, and combinations thereof.
[Chemical Formula 11]
Li _a11 Ni _x11 M ¹¹ _y11 M ¹² _1-x11-y11 O ₂
In the chemical formula 11, 0.9≤a11≤1.8, 0.3≤x11≤1, 0≤y11≤0.7, and M ¹¹ and M ¹² are each independently selected from Al, B, Ba, Ca, Ce, Co, Cr, Cu, F, Fe, Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr, and combinations thereof. A first nickel-based hydroxide and a lithium raw material are mixed and subjected to a first heat treatment to prepare a first nickel-based oxide in the form of secondary particles in which a plurality of primary particles are aggregated;
A second nickel-based oxide is prepared in the form of a single crystal by mixing a second nickel-based hydroxide and a lithium raw material and performing a second heat treatment; and
A method for producing a cathode active material for a lithium secondary battery according to claim 1, comprising mixing a first nickel-based oxide, a second nickel-based oxide, and a cobalt raw material and performing a third heat treatment. In Article 11,
A method for producing a cathode active material for a lithium secondary battery, wherein the first nickel-based hydroxide and the second nickel-based hydroxide are each independently represented by the chemical formula 21 below:
[Chemical Formula 21]
Ni _x21 M ²¹ _y21 M ²² _1-x21-y21 (OH) ₂
In the chemical formula 21, 0.3≤x21≤1, 0≤y21≤0.7, and M ²¹ and M ²² are each independently selected from Al, B, Ba, Ca, Ce, Co, Cr, Cu, F, Fe, Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr, and combinations thereof. In Article 11,
Mixing the first nickel-based hydroxide and the lithium raw material is such that the ratio of the number of moles of lithium in the lithium raw material to the number of moles of metal included in the first nickel-based hydroxide is 0.9 or more and 1.2 or less.
A method for producing a cathode active material for a lithium secondary battery, wherein the mixing of a second nickel-based hydroxide and a lithium raw material is performed such that the ratio of the number of moles of lithium in the lithium raw material to the number of moles of metal included in the second nickel-based hydroxide is 0.9 or more and 1.2 or less. In Article 11,
A method for producing a cathode active material for a lithium secondary battery, wherein the first heat treatment is performed at a temperature range of 600°C to 900°C for 5 to 20 hours. In Article 11,
A method for producing a cathode active material for a lithium secondary battery, wherein the second heat treatment is performed at a temperature range of 800°C to 1100°C for 5 to 20 hours. In Article 11,
A method for producing a cathode active material for a lithium secondary battery, wherein preparing a second nickel-based oxide comprises mixing a second nickel-based hydroxide and a lithium raw material, performing a second heat treatment, and pulverizing the obtained product to obtain a single crystal second nickel-based oxide. In Article 11,
A method for producing a cathode active material for a lithium secondary battery, wherein mixing a first nickel-based oxide and a second nickel-based oxide is performed such that a weight ratio of the first nickel-based oxide and the second nickel-based oxide is 9:1 to 5:5. In Article 11,
A method for producing a cathode active material for a lithium secondary battery, wherein mixing a first nickel-based oxide, a second nickel-based oxide, and a cobalt compound is performed such that when the total content of metals excluding lithium in the first nickel-based oxide and the second nickel-based oxide is 100 molar parts, cobalt included in the cobalt compound is 0.01 molar parts to 7 molar parts. In Article 11,
A method for producing a cathode active material for a lithium secondary battery, wherein the third heat treatment is performed at a temperature range of 650°C to 900°C for 5 to 30 hours. A lithium secondary battery comprising a positive electrode, a negative electrode, and an electrolyte comprising a positive electrode active material according to any one of claims 1 to 5 and claims 7 to 10.