Positive electrode active material

Abstract

To provide a positive electrode active material in which the structural stability of a lithium composite oxide constituting the positive electrode active material is improved, and a lithium secondary battery including the same.SOLUTION: A positive electrode active material comprising a lithium-based composite oxide having a single-crystal structure, wherein a cation mixed layer is present on a surface of the lithium-based composite oxide, and a ratio (d1 / x1) of a thickness (d1) of the cation mixed layer to an average particle size (x1) of the primary particles is more than 0.00008 and less than 0.00052.SELECTED DRAWING: Figure 9

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material in which the structural stability of the lithium composite oxide constituting the positive electrode active material is improved, and to a lithium secondary battery including the same. [Background technology]

[0002] Compared with other rechargeable battery systems, lithium secondary batteries have advantages such as high operating voltage, light weight, small size, no memory effect, low self-discharge rate, long cycle life, high energy density, etc., and are therefore widely used in mobile phones, notebook computers, tablet computers and other mobile terminals.

[0003] In addition, in recent years, electric vehicles have been rapidly developed under the promotion of the government and automobile manufacturers from the viewpoint of environmental protection, and lithium secondary batteries are considered to be an ideal power source for next-generation electric vehicles due to their excellent performance.

[0004] Lithium-based composite oxides are used as the positive electrode active material for such lithium secondary batteries, and among these, lithium-cobalt composite oxide (LiCoO2) is mainly used because of its high working voltage and excellent capacity characteristics. However, LiCoO2 has poor high-temperature stability due to the destabilization of its crystal structure caused by delithiation, and is expensive, so its use as a power source in fields requiring large-capacity battery systems, such as electric vehicles, is limited.

[0005] Lithium-manganese composite oxides (LiMnO2 or LiMn2O4, etc.), lithium-iron phosphate (LiFePO4, etc.), and lithium-nickel composite oxides (LiNiO2, etc.) have been developed as alternative materials to LiCoO2. Here, lithium-nickel composite oxides have been actively researched and developed because they have a high reversible capacity of about 200 mAh / g, which allows for the realization of large-capacity batteries.

[0006] However, LiNiO2 has poorer high-temperature stability than LiCoO2, and if an internal short circuit occurs due to external pressure while the battery is being charged, the positive electrode active material may decompose or a side reaction may occur at the interface or surface of the electrolyte and the positive electrode active material, resulting in the battery exploding or catching fire.

[0007] Therefore, there is a need to develop a positive electrode active material that can improve the poor high-temperature stability of LiNiO2 while maintaining the excellent reversible capacity of LiNiO2.

[0008] Meanwhile, lithium secondary batteries can be classified into can-type secondary batteries in which an electrode assembly is housed in a metal can, and pouch-type secondary batteries in which an electrode assembly is housed in a pouch made of a sheet such as an aluminum laminate, depending on the shape of the battery case.

[0009] Pouch-type secondary batteries have the advantages of being lightweight, having little possibility of electrolyte leakage, and being able to realize the same amount of secondary battery with a relatively small volume and mass. However, there is a risk of explosion if the internal pressure of the battery case increases suddenly, and one of the important challenges is to ensure stability by controlling gas generation, which is the main cause of an increase in internal pressure of the battery case.

[0010] For example, when an overcurrent exceeding the limit flows through a secondary battery, the internal temperature of the battery rises rapidly, causing a decomposition reaction of the electrolyte and generating gas. Gas may also be generated by a side reaction between the electrolyte and the interface and surface of the positive electrode active material. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Korean Patent Publication No. 10-2017-0048208 Summary of the Invention [Problem to be solved by the invention]

[0012] An object of the present invention is to provide a positive electrode active material that can improve the low thermal stability or structural stability of LiNiO2 while maintaining the excellent reversible capacity, and a lithium secondary battery including the same.

[0013] Another object of the present invention is to provide a cathode active material that can prevent battery expansion due to gas generation in a secondary battery by reducing the possibility of side reactions occurring at the interface and surface between an electrolyte and a cathode active material, and a lithium secondary battery including the same.

[0014] Another object of the present invention is to provide a positive electrode active material in which the surfaces of primary particles are covered with a cation mixed layer, thereby improving the structural stability of the lithium composite oxide, and a lithium secondary battery including the same.

[0015] The objects of the present invention are not limited to the objects mentioned above, and other objects and advantages of the present invention not mentioned above can be understood from the following description and will become more clearly understood from the examples of the present invention. Furthermore, it can be easily seen that the objects and advantages of the present invention can be realized by the means and combinations thereof as claimed. [Means for solving the problem]

[0016] According to one aspect of the present invention, there is provided a positive electrode active material including a lithium-based composite oxide having a single crystal structure.

[0017] The positive electrode active material may include a cation mixed layer present on the surface of the lithium-based composite oxide, and the thickness (d1) of the cation mixed layer may be in the range of more than 0.0008 and less than 0.0052 with respect to the average particle size (x1) of the primary particles.

[0018] According to another aspect of the present invention, there is provided a positive electrode for a lithium secondary battery, which includes the positive electrode active material.

[0019] According to yet another aspect of the present invention, there is provided a lithium secondary battery including the positive electrode. [Effects of the Invention]

[0020] The surfaces and grain boundaries of the lithium-based composite oxide and / or secondary particles formed by aggregation of at least two of the lithium-based composite oxides are regions where side reactions between the positive electrode active material and the electrolyte can occur.

[0021] According to the present invention, the lithium-based composite oxide, which is the primary particle constituting the secondary particle, has a single crystal structure, so that it is possible to reduce side reactions between the positive electrode active material and the electrolyte.

[0022] That is, by reducing the surface area and grain boundary surface of the lithium-based composite oxide and / or the secondary particles, it is possible to improve the high-temperature stability and storage stability of the positive electrode active material. As not only the high-temperature stability but also the storage stability of the positive electrode active material is improved, it is possible to reduce gas generation caused by the positive electrode active material during operation and / or storage of the positive electrode active material.

[0023] Furthermore, according to the present invention, the cation mixed layer present on the surface of the lithium composite oxide constituting the positive electrode active material that satisfies the grain boundary density condition defined in the present application can improve the structural stability of the lithium composite oxide, and therefore the positive electrode active material.

[0024] The specific effects of the present invention will be described together with the above-mentioned effects, while explaining specific matters for carrying out the invention below. [Brief explanation of the drawings]

[0025] [Figure 1] 2A and 2B are schematic diagrams illustrating cross sections of secondary particles that may be included in positive electrode active materials according to various embodiments of the present invention; [Figure 2] 2A and 2B are schematic diagrams illustrating cross sections of secondary particles that may be included in positive electrode active materials according to various embodiments of the present invention; [Figure 3] 2A and 2B are schematic diagrams illustrating cross sections of secondary particles that may be included in positive electrode active materials according to various embodiments of the present invention; [Figure 4] 1 is a cross-sectional SEM photograph of a positive electrode active material according to Example 1, showing the measurement results of the density of grain boundaries of secondary particles contained in the positive electrode active material. [Figure 5] 1 is a cross-sectional SEM photograph of a positive electrode active material according to Example 2, showing the measurement results of the density of grain boundaries of secondary particles contained in the positive electrode active material. [Figure 6] 1 is a cross-sectional SEM photograph of a positive electrode active material according to Comparative Example 1, showing the measurement results of the density of grain boundaries of secondary particles contained in the positive electrode active material. [Figure 7] 1 is an SEM photograph of the positive electrode active material according to Example 1. [Figure 8] 1 is an SEM photograph of the positive electrode active material according to Example 3. [Figure 9] 1 is an SEM photograph of the positive electrode active material according to Example 4. [Figure 10] 1 is an SEM photograph of the positive electrode active material according to Example 5. [Figure 11] 10 is a SEM photograph of the positive electrode active material according to Example 7. [Figure 12] 1 is an SEM photograph of the positive electrode active material according to Example 11. [Figure 13] 1 is an SEM photograph of the positive electrode active material according to Comparative Example 1. [Figure 14] 10 is an SEM photograph of the positive electrode active material according to Comparative Example 2. [Figure 15] 1 is a TEM photograph of the positive electrode active material according to Example 1. [Figure 16] 1 is a TEM photograph of the positive electrode active material according to Example 3. [Figure 17] 1 is a TEM photograph of the positive electrode active material according to Example 4. [Figure 18] 1 is a TEM photograph of the positive electrode active material according to Example 5. [Figure 19] 1 is a TEM photograph of the positive electrode active material according to Example 7. [Figure 20] 1 is a TEM photograph of the positive electrode active material according to Example 11. [Figure 21] 1 is a TEM photograph of the positive electrode active material according to Comparative Example 1. [Figure 22] 1 is a TEM photograph of the positive electrode active material according to Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0026] In order to more readily understand the present invention, certain terms are defined herein for convenience. Unless otherwise defined herein, scientific and technical terms used herein shall have the meanings commonly understood by those of ordinary skill in the art. Furthermore, unless otherwise required by context, singular terms shall be understood to include their plural forms, and plural terms shall be understood to include their singular forms.

[0027] Explanation of terms The term "lithium-based composite oxide" used herein refers to an oxide capable of absorbing and releasing lithium ions, and includes lithium and a metal element. In particular, the lithium-based composite oxide used herein may be a lithium-nickel-based composite oxide including lithium and nickel. Furthermore, the term "lithium-based composite oxide" used herein may be defined as primary particles constituting a positive electrode active material.

[0028] The term "single crystal" used herein means a crystal that does not contain grains or grain boundaries within the particle. Furthermore, the term "primary particle" used herein means a particle that exists independently without forming an aggregate. Accordingly, a "lithium-based composite oxide having a single crystal structure" refers to a primary particle made of a lithium-based composite oxide that is composed of multiple grains or does not contain grain boundaries within the primary particle.

[0029] The term "secondary particle" used herein refers to a primary particle of the lithium-based composite oxide described above, or a particle formed by agglomeration of at least two primary particles. In this case, a secondary particle formed by a single primary particle and / or a secondary particle formed by agglomeration of at least two primary particles may coexist within the positive electrode active material. When at least two primary particles are agglomerated to form a secondary particle, a grain boundary or grain boundary surface formed at the interface between the two primary particles exists within the secondary particle.

[0030] The term "grain boundary density" as used herein refers to the number of grain boundaries or grain boundary surfaces formed due to the presence of at least two primary particles within a secondary particle; the greater the number of primary particles within a secondary particle, the greater the density of the grain boundaries; and the fewer the number of primary particles within a secondary particle, the lower the density of the grain boundaries.

[0031] In the present application, the density of grain boundaries can be calculated by the following formula:

[0032] Grain boundary density = (number of boundaries between primary particles within a secondary particle / number of primary particles that make up a secondary particle)

[0033] If the number of primary particles present in the secondary particle is 1, the density of the grain boundaries calculated by the above formula is 0, and if the number of primary particles present in the secondary particle is 3 or more, the density of the grain boundaries calculated by the above formula exceeds 0.5.

[0034] Meanwhile, the secondary particles herein may be particles consisting of a single primary particle having a single crystal structure, but are not necessarily limited thereto. That is, the secondary particles herein may be understood as particles consisting of a single single crystal particle or particles formed by agglomeration of at least two single crystal particles. The definition of such secondary particles may be more clearly explained by the definition of the positive electrode active material described below.

[0035] The term "positive electrode active material" used in the present application is a broad concept that includes the above-mentioned secondary particles, and although a single secondary particle itself may be a positive electrode active material, in the present application, an aggregate of multiple secondary particles having the same and / or different grain boundary densities can be defined as a positive electrode active material.

[0036] Therefore, in the definition of the positive electrode active material described below, the explanation of the secondary particles having the properties of an aggregate of multiple secondary particles and the explanation of the secondary particles constituting the positive electrode active material and the primary particles constituting the secondary particles need to be understood separately.

[0037] The positive electrode active material and the lithium secondary battery including the same according to the present invention will be described in more detail below.

[0038] positive electrode active material The surface area of ​​the secondary particles contained in the positive electrode active material and the grain boundary defined by the primary particles present within the secondary particles are regions where side reactions can occur at the interface and between the positive electrode active material and the electrolyte. Here, the grain boundary defined by the primary particles present within the secondary particles can be understood as, for example, the interface between two adjacent primary particles.

[0039] In this case, by reducing the surface area of ​​the secondary particles and the grain boundary surface defined by the primary particles within the secondary particles, it is possible to improve the thermal stability of the positive electrode active material and prevent or mitigate problems caused by the instability of the positive electrode active material (e.g., deterioration of storage stability such as gas generation).

[0040] A positive electrode active material for a lithium secondary battery according to an embodiment of the present invention includes secondary particles made of primary particles that are lithium-based composite oxides having a single crystal structure.

[0041] In particular, since the primary particles constituting the secondary particles have a single crystal structure, it is possible to reduce the surface area of ​​the primary particles within the secondary particles and the grain boundary surfaces partitioned by the primary particles present within the secondary particles. Additionally, the primary particles constituting the secondary particles have lithium ion diffusion paths that are uniform in the longitudinal direction, thereby concentrating lithium ions in the secondary particles in one direction instead of diffusing them in multiple directions, thereby improving the conductivity of lithium ions in the secondary particles.

[0042] 1 to 3 are schematic cross-sectional views of secondary particles that may be included in positive electrode active materials according to various embodiments of the present invention.

[0043] 1 to 3, the secondary particles 110, 120, and 130 included in the positive electrode active material are formed by agglomeration of 1 to 10 primary particles 111, 121, and 131, respectively, and therefore have a relatively small surface area compared to secondary particles formed by agglomeration of tens to hundreds of primary particles. This reduces the surface area where side reactions with the electrolyte occur. Furthermore, since the number of primary particles forming the secondary particles is small, the density of the grain boundaries is reduced, thereby reducing side reactions at the grain interfaces within the secondary particles.

[0044] Furthermore, according to the present invention, the primary particles constituting the plurality of secondary particles contained in the positive electrode active material can have a high probability of having lithium ion diffusion paths that are uniform in the longitudinal direction. As such, as the proportion of lithium ion diffusion paths that are uniform in the longitudinal direction within the secondary particles increases, the lithium ion conductivity and electronic conductivity of the positive electrode active material can be improved.

[0045] In an embodiment, the positive electrode active material according to the present invention may be an aggregate of secondary particles having the same or different grain boundary densities.

[0046] In the present application, the density of grain boundaries can be calculated by the following formula:

[0047] Grain boundary density = (number of boundaries between primary particles within a secondary particle / number of primary particles that make up a secondary particle)

[0048] Secondary particles with different grain boundary densities may have different physical and chemical properties. Physical properties that may change due to differences in grain boundary densities include, for example, differences in surface area of ​​the secondary particles before and after pressing, and chemical properties that may change due to differences in surface and / or interface of the secondary particles and the electrolyte.

[0049] Specifically, in the positive electrode active material according to the present invention, the ratio of secondary particles having a grain boundary density of 0.5 or less among a plurality of secondary particles constituting the positive electrode active material may be 30% or more, preferably 50% or more, and more preferably 70% or more.

[0050] For example, if a secondary particle consists of a single primary particle with a single crystal structure, the density of the grain boundaries will be 0 (number of boundary surfaces between primary particles within the secondary particle = 0 / number of primary particles making up the secondary particle = 1), and if the secondary particle consists of two primary particles with a single crystal structure, the density of the grain boundaries will be 0.5.

[0051] In other words, the smaller the number of boundaries between primary particles in a secondary particle, the lower the grain boundary density. In this case, as the number of primary particles constituting a secondary particle increases, the number of boundaries between primary particles in the secondary particle also increases. Therefore, in order for the grain boundary density to be 0.5 or less, the secondary particle must consist of a single primary particle with a single crystal structure or two primary particles with a single crystal structure.

[0052] For example, the positive electrode active material according to the present invention may include secondary particles consisting of one to two primary particles (hereinafter, for convenience, referred to as "first aggregates"), secondary particles consisting of three to six primary particles (hereinafter, for convenience, referred to as "second aggregates"), and secondary particles consisting of seven to ten primary particles (hereinafter, for convenience, referred to as "third aggregates").

[0053] The first aggregate 110 shown in FIG. 1 has one or two primary particles 111 that form secondary particles, and since the number of primary particles that form secondary particles is smaller than that of the second and third aggregates, it has a relatively small surface area, which makes it possible to reduce the area where side reactions with the electrolyte occur.

[0054] Furthermore, the number of grain boundary surfaces b formed by primary particles 111 in first aggregates 110 is 1 or less. This can reduce the probability of side reactions with the electrolyte occurring at the grain boundary surfaces compared to when the number of grain boundary surfaces is 2 or more.

[0055] Such primary aggregates preferably account for 30% or more of the aggregate of secondary particles that constitute the positive electrode active material. Here, the percentage of primary aggregates in the aggregate of secondary particles refers to the percentage of primary aggregates out of the total number of secondary particles that constitute the aggregate.

[0056] When the ratio of the first aggregates to the aggregate of secondary particles constituting the positive electrode active material is less than 30%, the average surface area and grain boundary density of the secondary particles constituting the positive electrode active material increase as the ratio of the first aggregates to the aggregate decreases, which increases the possibility of side reactions between the positive electrode active material and the electrolyte, which can cause deterioration in the high-temperature stability and storage properties of the positive electrode active material.

[0057] On the other hand, if the firing temperature is increased, it is possible to increase the ratio of primary aggregates among the aggregates of secondary particles that constitute the positive electrode active material. However, if the firing temperature is too high, the possibility of deterioration of the positive electrode active material may increase.

[0058] Furthermore, the second aggregates 120 shown in FIG. 2 have 3 to 6 primary particles 121 that form the secondary particles, and such second aggregates 120 may be present in an amount of 70% or less, preferably 20% to 70% of the aggregate of secondary particles that constitute the positive electrode active material.

[0059] Furthermore, the third aggregate 130 shown in FIG. 3 has 7 to 10 primary particles 131 that form the secondary particles, and it is preferable that such third aggregates 130 exist in an amount of 10% or less of the aggregate of secondary particles that constitute the positive electrode active material.

[0060] In this case, the positive electrode active material may also include secondary particles formed by aggregating a larger number of primary particles than the third aggregates (secondary particles each consisting of 7 to 10 primary particles). In this case, too, the ratio of the third aggregates and secondary particles formed by aggregating a larger number of primary particles than the third aggregates to the aggregate of secondary particles is preferably 10% or less.

[0061] When the ratio of the first aggregates is 70% and the ratio of the second aggregates is reduced to less than 20%, the ratio of the second aggregates becomes smaller compared to the third aggregates, resulting in an increase in the average surface area and grain boundary density of the secondary particles constituting the positive electrode active material, which can act as a cause of a decrease in the high-temperature stability and storage properties of the positive electrode active material.

[0062] On the other hand, if the ratio of second aggregates among the aggregates of secondary particles constituting the positive electrode active material exceeds 30%, the ratio of second aggregates becomes relatively larger than that of first aggregates, and similarly, the average surface area and density of grain boundaries of the secondary particles constituting the positive electrode active material may increase.

[0063] The average particle size of the primary particles of the lithium-based composite oxide having a single crystal structure may be preferably 1.0 μm to 5.0 μm. By having the average particle size of the primary particles of the lithium-based composite oxide having a single crystal structure within the range of 1.0 μm to 5.0 μm, the optimal density of the positive electrode active material formed of the lithium-based composite oxide can be realized, and at the same time, the effect of improving the structural stability due to the cation mixed layer described below can be maximized.

[0064] The average particle size of the secondary particles may vary depending on the number of aggregated primary particles, but may be 1.0 μm to 20.0 μm. Preferably, the secondary particles may include small secondary particles and large secondary particles, with the small secondary particles having an average particle size of 1.0 μm to 5.0 μm and the large secondary particles having an average particle size of 10.0 μm to 20.0 μm. Thus, the average particle size of the aggregate of secondary particles, which is a mixture of the small and large secondary particles, may be 3.0 μm to 18.0 μm.

[0065] The cathode active material according to the present invention may be an aggregate of secondary particles having the same or different grain boundary densities, and may also be an aggregate of a mixture of small and large secondary particles having different average particle sizes. The characteristics of the aggregate of secondary particles may improve the structural stability of the cathode active material, thereby improving the high-temperature stability and storage stability of a lithium secondary battery using the cathode active material.

[0066] The lithium-based composite oxide constituting the positive electrode active material according to the present invention can be represented by the following chemical formula 1.

[0067] [Chemical formula 1] Li a Ni 1-(b+c+d+e) Co b M1 c M2 d M3 e O f

[0068] Here, M1 is Mn or Al, M2 and M3 are each independently selected from Al, Ba, B, Ce, Cr, Mg, Mn, Mo, Na, K, P, Sr, Ti, W, Nb, and Zr, and M1 to M3 are metals different from one another (0.90≦a≦1.05, 0≦b≦0.20, 0≦c≦0.20, 0≦d≦0.05, 0≦e≦0.05, 1.0≦f≦2.0).

[0069] In this case, M3 is a metal different from M1 and may be added as a flux when preparing the lithium-based composite oxide represented by Chemical Formula 1. Such a flux may lower the melting point and promote particle growth, thereby inducing the formation of a cation mixed layer, which will be described later, at an appropriate ratio.

[0070] In addition, the positive electrode active material according to the present invention may include a cation mixed layer present on the surface of the lithium-based composite oxide represented by Chemical Formula 1.

[0071] Specifically, the positive electrode active material may include primary particles that are the lithium-based composite oxide having a single crystal structure and secondary particles that are aggregations of the primary particles, and the cation mixed layer may be present on the surface of at least one selected from the primary particles and the secondary particles.

[0072] In this case, the cation mixed layer may include a composite oxide represented by the following Chemical Formula 2, which is an oxide different from the lithium-based composite oxide represented by the above Chemical Formula 1.

[0073] [Chemical formula 2] Li g Ni 1-(h+i+j+k) Co h M4 i M5 j M6 k O l

[0074] Here, M4 is Mn or Al, M5 and M6 are each independently selected from Al, Ba, B, Ce, Cr, Mg, Mn, Mo, Na, K, P, Sr, Ti, W, Nb, and Zr, and M4 to M6 are metals different from one another (0.0≦g≦1.05, 0≦h≦0.20, 0≦i≦0.20, 0≦j≦0.05, 0≦k≦0.05, 1.0≦l≦2.0).

[0075] The presence of the cation mixed layer containing the composite oxide represented by Chemical Formula 2 on the surface of the lithium-based composite oxide can improve the structural stability of the lithium-based composite oxide, which is a primary particle.

[0076] More specifically, the composite oxide contained in the cation mixed layer can be represented by the following Chemical Formula 3.

[0077] [Chemical formula 3] Li m Ni n O l (0.0≦m≦0.5, 0.0 <n≦1、1.0≦l≦2.0)。

[0078] The complex oxide contained in the cation mixed layer may have a crystal structure selected from a layered structure, a rock salt structure, and a spinel structure, or may have a crystal structure in which complex oxides having at least one crystal structure selected from a layered structure, a rock salt structure, and a spinel structure are mixed. In this case, however, it is preferable that the complex oxide contained in the cation mixed layer has a crystal structure different from that of the lithium-based complex oxide represented by Chemical Formula 1.

[0079] The thickness of the cation mixed layer present on the surface of the lithium-based composite oxide, which is the primary particle constituting the positive electrode active material, may be 0.24 nm to 12.03 nm, and the ratio of the thickness of the cation mixed layer to the average particle size of the primary particles may be more than 0.0008 and less than 0.0052.

[0080] When the ratio of the thickness of the cation mixed layer to the average particle size of the primary particles is 0.0008 or less, the effect of the cation mixed layer in improving the structural stability of the lithium-based composite oxide may be insufficient.On the other hand, when the ratio of the thickness of the cation mixed layer to the average particle size of the primary particles is 0.0052 or more, the thickness of the cation mixed layer becomes too large, which may result in a deterioration in the electrical properties of the lithium-based composite oxide.

[0081] The cation mixed layer may also be present on the surface of the secondary particles, and the ratio (d2 / x2) of the thickness (d2) of the cation mixed layer to the average particle size (x2) of the secondary particles may be greater than 0.00014 and less than 0.00281.

[0082] When the ratio of the thickness of the cation mixed layer to the average particle size of the secondary particles is 0.00014 or less, the effect of the cation mixed layer in improving the structural stability of the secondary particles may be insufficient, whereas when the ratio of the thickness of the cation mixed layer to the average particle size of the secondary particles is 0.00281 or more, the thickness of the cation mixed layer becomes too large, which may result in a deterioration in the electrical properties of the secondary particles.

[0083] On the other hand, the thickness of the cation mixed layer present on the surface of the secondary particles may be thicker than the thickness of the cation mixed layer present on the surface of the primary particles, and in this case, it is possible to improve the electrical properties of the positive electrode active material without reducing the structural stability of the positive electrode active material composed of the primary particles and secondary particles formed by aggregation of the primary particles.

[0084] In this case, the ratio (d1 / d2) of the thickness (d1) of the cation mixed layer present on the surface of the primary particle to the thickness (d2) of the cation mixed layer present on the surface of the secondary particle is preferably more than 0.53 and less than 1.0.

[0085] In this way, when the cation mixed layer is present on the surface of each of the primary particles and the secondary particles, the thickness of the cation mixed layer present on the surface of the secondary particles is formed to be thicker than the thickness of the cation mixed layer present on the surface of the primary particles, thereby minimizing deterioration in the electrical properties of the positive electrode active material including the primary particles and the secondary particles and improving the effect of structural stability induced by the cation mixed layer.

[0086] The cathode active materials according to various embodiments of the present invention may exhibit improved electrochemical properties and structural stability by satisfying the above-described predetermined ranges for the ratio (d1 / x1) of the thickness (d1) of the cation mixed layer to the average particle size (x1) of the primary particles, the ratio (d2 / x2) of the thickness (d2) of the cation mixed layer to the average particle size (x2) of the secondary particles, and the ratio (d1 / d2) of the thickness (d1) of the cation mixed layer present on the surface of the primary particles to the thickness (d2) of the cation mixed layer present on the surface of the secondary particles.

[0087] Additionally, the Ni occupancy rate in the Li3a site obtained from Rietveld analysis by X-ray diffraction of the secondary particles may be more than 0.53%, preferably 1.0% or more, more preferably 1.3% or more. Also, the Ni occupancy rate in the Li3a site of the secondary particles is preferably less than 6.44%. 3+ is a thermodynamically stable Ni alloy in a high temperature range. 2+ There is a tendency to exist in this state, and in this case, Ni 2+ (0.69Å) is Li + (0.76 Å), it is possible for the Li atoms to occupy the 3a site.

[0088] Here, in the positive electrode active material according to the present invention, the Ni occupancy rate in the 3a site of the secondary particles is 0.9% or more, and it can be inferred that the cation mixed layer is effectively present on the surface of the secondary particles.

[0089] As described above, the positive electrode active material according to the present invention may have improved structural stability due to the cation mixed layer, thereby improving the thermal stability and storage stability of the positive electrode active material.

[0090] When manufacturing a positive electrode for a lithium secondary battery using a positive electrode active material, a slurry containing the positive electrode active material is applied to a positive electrode current collector, followed by drying and rolling (pressing). In particular, rolling under high pressure conditions can produce a positive electrode with high energy density. In this process, as the rolling density increases, positive electrode active materials with low particle strength can break down, resulting in a loss of desired electrical properties. However, positive electrode active materials with high particle strength can maintain performance without particle breakage even under high rolling conditions.

[0091] In particular, when particle collapse occurs under rolling conditions, the proportion of secondary particles in a relatively small particle size distribution range increases, but the positive electrode active material according to the present invention exhibits a change in particle size distribution before and after pressing of only 13% when pressed at 4.5 tons, and a change in particle size distribution before and after pressing of only 26% when pressed at 6 tons. As a result, the positive electrode active material according to the present invention can maintain performance while minimizing particle collapse even under high rolling conditions.

[0092] Lithium secondary battery According to another aspect of the present invention, a positive electrode may be provided, including a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector. Here, the positive electrode active material layer may include a positive electrode active material according to various embodiments of the present invention. Therefore, since the positive electrode active material is the same as that described above, detailed description thereof will be omitted for convenience, and only the remaining components not described above will be described below.

[0093] The positive electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. can be used. The positive electrode current collector typically has a thickness of 3 to 500 μm, and fine irregularities can be formed on the surface of the current collector to enhance the adhesive strength of the positive electrode active material. It can be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0094] The positive electrode active material layer may be prepared by coating a positive electrode slurry composition containing the positive electrode active material, a conductive material, and optionally a binder, on the positive electrode current collector.

[0095] In this case, the positive electrode active material may be contained in an amount of 80 to 99 wt%, more specifically, 85 to 98.5 wt%, based on the total weight of the positive electrode active material layer. When contained in the above content range, excellent capacity characteristics can be exhibited, but the amount is not necessarily limited thereto.

[0096] The conductive material is used to impart conductivity to the electrode. Any conductive material can be used without particular limitations as long as it does not cause chemical changes in the resulting battery and has electronic conductivity. Specific examples include graphite, such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powder or metal fiber, such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; and conductive polymers, such as polyphenylene derivatives. These materials may be used alone or in combination. The conductive material may be present in an amount of 0.1 to 15 wt % based on the total weight of the positive electrode active material layer.

[0097] The binder improves adhesion between positive electrode active material particles and between the positive electrode active material and the current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. One or more of these may be used alone or in combination. The binder may be included in an amount of 0.1 to 15 wt% of the total weight of the positive electrode active material layer.

[0098] The positive electrode may be manufactured by a conventional method for manufacturing a positive electrode, except for using the positive electrode active material described above. Specifically, the positive electrode may be manufactured by dissolving or dispersing the positive electrode active material described above and, optionally, a binder and a conductive material in a solvent to prepare a positive electrode slurry composition, which is then coated on a positive electrode current collector, followed by drying and rolling.

[0099] The solvent may be a solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and may be used alone or in combination. The amount of solvent used is sufficient to dissolve or disperse the cathode active material, conductive material, and binder, taking into consideration the coating thickness of the slurry and the production yield, and to provide a viscosity that allows excellent thickness uniformity during subsequent coating for the production of a cathode.

[0100] In another embodiment, the positive electrode may be manufactured by casting the positive electrode slurry composition onto a separate support, peeling it off from the support, and laminating the resulting film onto a positive electrode current collector.

[0101] According to yet another aspect of the present invention, there is provided an electrochemical device including the above-described positive electrode. The electrochemical device may be specifically a battery, a capacitor, or the like, and more specifically a lithium secondary battery.

[0102] The lithium secondary battery may specifically include a positive electrode, a negative electrode facing the positive electrode, and a separator and an electrolyte interposed between the positive electrode and the negative electrode. Here, since the positive electrode is the same as that described above, detailed description thereof will be omitted for convenience, and only the remaining components not described above will be described in detail below.

[0103] The lithium secondary battery may optionally further include a battery container that houses an electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery container.

[0104] The negative electrode may include a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector.

[0105] The negative electrode current collector is not particularly limited as long as it has high conductivity and does not induce chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, aluminum-cadmium alloy, etc., can be used. The negative electrode current collector typically has a thickness of 3 μm to 500 μm. As with the positive electrode current collector, the surface of the current collector can be formed with fine irregularities to strengthen the binding force of the negative electrode active material. The negative electrode current collector can be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0106] The negative electrode active material layer may be prepared by coating a negative electrode slurry composition containing the negative electrode active material, a conductive material, and optionally a binder, on the negative electrode current collector.

[0107] The negative electrode active material may be a compound capable of reversible intercalation and deintercalation of lithium. Specific examples thereof include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, and Al alloys; and SiO β Examples of the negative electrode active material include metal oxides capable of doping and dedoping lithium, such as SnO2, vanadium oxide, and lithium vanadium oxide (0<β<2); or composites containing the metallic compounds and carbonaceous materials, such as Si-C composites and Sn-C composites. A mixture of two or more of these may be used. A thin film of metallic lithium may also be used as the negative electrode active material. Both low-crystalline carbon and high-crystalline carbon may be used as the carbon material. Typical low-crystalline carbons are soft carbon and hard carbon, while typical high-crystalline carbons are amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and high-temperature-fired carbons such as petroleum or coal tar pitch-derived cokes.

[0108] The negative electrode active material may be included in an amount of 80 to 99 wt % based on the total weight of the negative electrode active material layer.

[0109] The binder is a component that aids in bonding between the conductive material, active material, and current collector, and is typically added in an amount of 0.1 to 10 wt% based on the total weight of the negative electrode active material layer. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, and various copolymers thereof.

[0110] The conductive material is a component for further improving the conductivity of the negative electrode active material and may be added in an amount of 10 wt% or less, preferably 5 wt% or less, based on the total weight of the negative electrode active material layer. The conductive material is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. Examples of the conductive material include graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0111] In one embodiment, the negative electrode active material layer may be prepared by coating a negative electrode slurry composition, which is prepared by dissolving or dispersing a negative electrode active material, and optionally a binder and a conductive material, in a solvent, on a negative electrode current collector and drying the coating. Alternatively, the negative electrode slurry composition may be cast on a separate support, peeled from the support, and the resulting film may be laminated on the negative electrode current collector.

[0112] In another embodiment, the negative electrode active material layer may be prepared by coating a negative electrode slurry composition prepared by dissolving or dispersing a negative electrode active material, and optionally a binder and a conductive material, in a solvent on a negative electrode current collector and drying the coating; or by casting the negative electrode slurry composition on a separate support, peeling it off from the support, and laminating the resulting film on the negative electrode current collector.

[0113] Meanwhile, in the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator commonly used in lithium secondary batteries can be used without any particular limitations. It is particularly preferable that the separator has low resistance to electrolyte ion movement and excellent electrolyte impregnation capacity. Specifically, porous polymer films, such as those made of polyolefin-based polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminate structures of two or more layers thereof can be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. Furthermore, to ensure heat resistance or mechanical strength, separators coated with ceramic components or polymer materials can also be used, and they can be used in either a single-layer or multi-layer structure.

[0114] Furthermore, examples of the electrolyte used in the present invention include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the production of lithium secondary batteries, but are not limited to these.

[0115] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0116] The organic solvent may be any organic solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specific examples of the organic solvent that can be used include ester-based solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether-based solvents such as dibutyl ether and tetrahydrofuran; ketone-based solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol-based solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (R is a hydrocarbon group having a linear, branched, or cyclic structure and having 2 to 20 carbon atoms, which may contain a double-bonded aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; and sulfolanes. Among these, carbonate-based solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and a high dielectric constant, which can improve the charge / discharge performance of a battery, and a low-viscosity linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred. In this case, the cyclic carbonate and the chain carbonate are mixed in a volume ratio of about 1:1 to about 1:9, which can result in excellent electrolyte performance.

[0117] The lithium salt may be any compound capable of providing lithium ions used in lithium secondary batteries without any particular limitations. Specifically, examples of the lithium salt include LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, and LiB(C2O4)2. The lithium salt is preferably used at a concentration in the range of 0.1 to 2.0 M. When the lithium salt concentration is within this range, the electrolyte has appropriate conductivity and viscosity, resulting in excellent electrolyte performance and efficient lithium ion migration.

[0118] In addition to the electrolyte components, the electrolyte may further contain one or more additives, such as haloalkylene carbonate compounds (e.g., difluoroethylene carbonate), pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purposes of improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity. In this case, the additives may be contained in an amount of 0.1 to 5 wt % based on the total weight of the electrolyte.

[0119] As described above, the lithium secondary battery including the cathode active material according to the present invention stably exhibits excellent discharge capacity, output characteristics, and life characteristics, and is therefore useful in portable devices such as mobile phones, notebook computers, and digital cameras, and in electric vehicles such as hybrid electric vehicles (HEVs).

[0120] The shape of the lithium secondary battery according to the present invention is not particularly limited, and may be a cylindrical shape using a can, a prismatic shape, a pouch shape, a coin shape, etc. Furthermore, the lithium secondary battery may be used as a battery cell used as a power source for a small device, and may also be preferably used as a unit battery for a medium- to large-sized battery module including a plurality of battery cells.

[0121] According to yet another aspect of the present invention, there may be provided a battery module including the lithium secondary battery as a unit cell and / or a battery pack including the same.

[0122] The battery module or the battery pack is used as a power source for one or more medium- to large-sized devices, including power tools; electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); and power storage systems.

[0123] In particular, in the case of a pouch-type lithium secondary battery using a cathode including the cathode active material according to various embodiments of the present invention, there is little possibility of a side reaction occurring between the cathode active material and the electrolyte, which has the advantage of improving stability during storage and / or operation and reducing gas generation.

[0124] Therefore, in the case of a lithium secondary battery using a positive electrode including the positive electrode active material according to various embodiments of the present invention, for example, after charging at 0.2 C to 4.25 V and storing at 60° C. for 14 days, the volume of the lithium secondary battery increases by 0.3 cm 3 Reliability and stability can be ensured by:

[0125] The present invention will be described in more detail below through examples. However, these examples are merely for the purpose of illustrating the present invention, and it should be understood that the scope of the present invention is not limited to these examples.

[0126] Manufacturing example: Manufacturing of positive electrode active material Example 1 NiCoAl(OH)2 (d(50)=3.0μm) prepared by coprecipitation reaction and Li2CO3 and LiOH (Li / M ratio=1.05±0.05) as lithium compounds were weighed and mixed, and then the first heat treatment was performed at 800℃ for 12 hours to obtain Li 1.0 Ni 0.9 Co 0.078 Al 0.022 A lithium-based composite oxide with the composition of O2 was produced.

[0127] The lithium-based composite oxide prepared above was washed by adding it to distilled water and maintaining the temperature. After dehydration, it was dried in a vacuum atmosphere at 150°C. The dried lithium-based composite oxide was then subjected to a second heat treatment at 700°C for 12 hours to obtain a positive electrode active material.

[0128] Example 2 A positive electrode active material was prepared in the same manner as in Example 1, except that the first heat treatment temperature was 850°C.

[0129] Example 3 A positive electrode active material was prepared in the same manner as in Example 1, except that the first heat treatment temperature was 900°C.

[0130] Example 4 A positive electrode active material was prepared in the same manner as in Example 1, except that the first heat treatment temperature was 730°C.

[0131] Example 5 A positive electrode active material was prepared in the same manner as in Example 1, except that the first heat treatment temperature was 750°C.

[0132] Example 6 A positive electrode active material was prepared in the same manner as in Example 1, except that 0.25 mol % of a Ba-containing compound (Ba(OH)2) was additionally mixed before the first heat treatment and then the first heat treatment was performed at 770°C.

[0133] Example 7 A positive electrode active material was prepared in the same manner as in Example 1, except that 0.3 mol % of a Ba-containing compound (Ba(OH)2) was additionally mixed before the first heat treatment and then the first heat treatment was performed at 770°C.

[0134] Example 8 A positive electrode active material was prepared in the same manner as in Example 1, except that 0.4 mol % of a Ba-containing compound (Ba(OH)2) was additionally mixed before the first heat treatment and then the first heat treatment was performed at 770°C.

[0135] Example 9 A positive electrode active material was prepared in the same manner as in Example 1, except that 0.3 mol % of a Sr-containing compound (Sr(OH)2) was additionally mixed before the first heat treatment and then the first heat treatment was performed at 770°C.

[0136] Example 10 A positive electrode active material was prepared in the same manner as in Example 1, except that 0.05 mol % of a Zr-containing compound (ZrO2) was additionally mixed before the first heat treatment and then the first heat treatment was performed at 770°C.

[0137] Example 11 A positive electrode active material was prepared in the same manner as in Example 1, except that 0.1 mol % of a Zr-containing compound (ZrO2) was additionally mixed before the first heat treatment and then the first heat treatment was performed at 770°C.

[0138] Example 12 A positive electrode active material was prepared in the same manner as in Example 1, except that 0.2 mol % of a Zr-containing compound (ZrO2) was additionally mixed before the first heat treatment and then the first heat treatment was performed at 770°C.

[0139] Example 13 NiCoMn(OH)2 (d(50) = 3.0 μm) prepared by coprecipitation reaction, Li2CO3 and LiOH (Li / M ratio = 1.05 ± 0.05) as lithium compounds, and 2.0 mol% of KCl were weighed and mixed, and then the first heat treatment was performed at 770 °C for 12 hours to obtain Li 1.0 Ni 0.9 Co 0.078 Mn 0.022 A lithium-based composite oxide having a composition of O2 was prepared. The prepared lithium-based composite oxide was placed in distilled water and washed while maintaining the temperature. After dehydration, it was dried in a vacuum atmosphere at 150°C. The dried lithium-based composite oxide was then subjected to a second heat treatment at 700°C for 12 hours to obtain a positive electrode active material.

[0140] Example 14 A positive electrode active material was prepared in the same manner as in Example 1, except that 1.0 mol % of NH4H2PO4 was additionally mixed instead of 2.0 mol % of KCl before the first heat treatment, and the mixture was subjected to the first heat treatment at 770°C.

[0141] Example 15 A positive electrode active material was prepared in the same manner as in Example 1, except that 2.0 mol % of NaCl was additionally mixed instead of 2.0 mol % of KCl before the first heat treatment, and the mixture was subjected to the first heat treatment at 770°C.

[0142] Comparative Example 1 A positive electrode active material was prepared in the same manner as in Example 1, except that the first heat treatment temperature was 650°C.

[0143] Comparative Example 2 A positive electrode active material was prepared in the same manner as in Example 1, except that the first heat treatment temperature was 990°C.

[0144] Experimental Example 1: Structural analysis of positive electrode active material (1) SEM photo analysis of positive electrode active material 4 to 6 are cross-sectional SEM photographs of the positive electrode active materials according to Example 1, Example 2, and Comparative Example 1, respectively, and each shows the measurement results of the density of the grain boundaries of the secondary particles contained in the positive electrode active material.

[0145] 4 to 6, it can be seen that the positive electrode active materials according to Examples 1 and 2 have a higher ratio of secondary particles having a grain boundary density of 0.5 or less among the multiple secondary particles constituting the positive electrode active material than the positive electrode active material according to Comparative Example 1.

[0146] Thus, the greater the proportion of secondary particles having a grain boundary density of 0.5 or less among the multiple secondary particles constituting the positive electrode active material, the more effectively side reactions can be reduced between the electrolyte and the grain boundaries within the secondary particles. Furthermore, the greater the proportion of lithium ion diffusion paths that are isotropic in the longitudinal direction within the secondary particles, the more effectively the lithium ion conductivity and electron conductivity of the positive electrode active material can be improved.

[0147] Table 1 below shows the measurement results for the density fraction of the grain boundaries of secondary particles contained in the positive electrode active materials prepared according to Preparation Examples.

[0148] [Table 1]

[0149] 7 to 12 are SEM photographs of the positive electrode active materials according to Examples 1, 3, 4, 5, 7, and 11, respectively, and FIGS. 13 and 14 are SEM photographs of the positive electrode active materials according to Comparative Examples 1 and 2, respectively.

[0150] First, referring to FIGS. 7 to 10, it can be seen that as the first heat treatment temperature increases, the density of the grain boundaries of the secondary particles decreases.

[0151] 11 and 12, which show SEM photographs of the cathode active materials according to Examples 7 and 11, respectively, it can be seen that the addition of flux during the preparation of the lithium-based composite oxide does not significantly reduce the proportion of secondary particles having a grain boundary density of 0.5 or less among the multiple secondary particles constituting the cathode active material. Furthermore, it can be seen that although the first heat treatment temperature (770°C) in Examples 7 and 11 is lower than the first heat treatment temperature (800°C) in Example 1, the addition of flux promotes particle growth, resulting in a grain boundary density fraction similar to that of Example 1.

[0152] 13, which shows an SEM image of the cathode active material according to Comparative Example 1, it can be seen that, unlike the cathode active materials according to the Examples, the cathode active material includes secondary particles with a relatively high density of crystal grain boundaries.On the other hand, referring to FIG. 14, it can be seen that the cathode active material according to Comparative Example 2 includes secondary particles with a similar level of density of crystal grain boundaries to the Examples.

[0153] (2) TEM photo analysis of the positive electrode active material 15 to 20 are TEM photographs of the positive electrode active materials according to Examples 1, 3, 4, 5, 7, and 11, respectively, and FIGS. 21 and 22 are TEM photographs of the positive electrode active materials according to Comparative Examples 1 and 2, respectively.

[0154] 15 to 18, it can be seen that the cation mixed layer (rock salt structure) has a different crystal structure from the bulk (layered structure) portion. Also, it can be seen that the thickness of the cation mixed layer tends to increase as the ratio of secondary particles having a grain boundary density of 0.5 or less among the multiple secondary particles constituting the positive electrode active material increases.

[0155] 19 and 20, which show TEM images of the cathode active materials according to Examples 7 and 11, respectively, it can be seen that the addition of flux during the preparation of the lithium-based composite oxide resulted in an increase in the thickness of the cation mixed layer, despite the fact that the first heat treatment temperature was lower than that of Example 1.

[0156] 20, which shows a TEM image of the cathode active material according to Comparative Example 1, it can be seen that the measured thickness of the cation mixed layer is excessively thin, unlike the cathode active materials according to the Examples. On the other hand, referring to FIG. 22, it can be seen that an excessively thick cation mixed layer was formed on the surface of the cathode active material according to Comparative Example 2. Such an excessively thick cation mixed layer may actually degrade the electrical properties of the cathode active material.

[0157] Table 2 below shows the average particle diameters of the primary particles and secondary particles contained in the positive electrode active materials prepared according to the Preparation Examples, and the ratio of the thickness of the cation mixed layer formed on the surface of the primary particles and the secondary particles to the average particle diameter of the primary particles and the secondary particles.

[0158] [Table 2]

[0159] Referring to Table 2, the positive electrode active materials according to Examples 1 to 15 have a ratio (d1 / x1) of the thickness (d1) of the cation mixed layer to the average particle size (x1) of the primary particles of more than 0.0008 and less than 0.0052, a ratio (d2 / x2) of the thickness (d2) of the cation mixed layer to the average particle size (x2) of the secondary particles of more than 0.00014 and less than 0.00281, and a ratio (d1 / d2) of the thickness (d1) of the cation mixed layer present on the surface of the primary particles to the thickness (d2) of the cation mixed layer present on the surface of the secondary particles of more than 0.53 and less than 1.0.

[0160] The positive electrode active material according to the embodiment of the present invention can exhibit improved electrochemical properties and stability, as described below, by satisfying the above-mentioned predetermined ranges for the ratio (d1 / x1) of the thickness (d1) of the cation mixed layer to the average particle size (x1) of the primary particles, the ratio (d2 / x2) of the thickness (d2) of the cation mixed layer to the average particle size (x2) of the secondary particles, and the ratio (d1 / d2) of the thickness (d1) of the cation mixed layer present on the surface of the primary particles to the thickness (d2) of the cation mixed layer present on the surface of the secondary particles.

[0161] Furthermore, when comparing the cathode active materials of Example 1 and Examples 6 to 15, it can be seen that the cathode active materials of Examples 6 to 15, which used flux during the preparation of the cathode active material, had increased ratios (d1 / x1) of the thickness (d1) of the cation mixed layer to the average particle size (x1) of the primary particles and ratios (d2 / x2) of the thickness (d2) of the cation mixed layer to the average particle size (x2) of the secondary particles, compared to Example 1, which did not use flux.

[0162] (3) XRD analysis of positive electrode active material Through XRD analysis of the positive electrode active material manufactured according to the manufacturing example, the Ni occupancy rate (Ni Li occupancy) was measured.

[0163] The XRD analysis was performed using Cu-Kα radiation (λ = 1.5406 Å) in the Bragg-Brentano parafocusing geometry (θ-2θ scan) range of 10-120° (2θ) at a scan interval of 0.01° / min. The results are shown in Table 3.

[0164] [Table 3]

[0165] Referring to Table 3, it can be seen that the occupancy rate of Ni inserted into the Li3a site of the positive electrode active material shows a similar tendency to the thickness (d1, d2) of the cation mixed layer shown in Table 2 above.

[0166] The occupancy of Ni inserted into the Li3a site of the cathode active materials according to Examples 1 to 15 falls within the range of more than 0.53% and less than 6.44%. The cathode active materials according to various embodiments of the present invention have a cation mixed layer of appropriate thickness on the surfaces of the primary particles and secondary particles constituting the cathode active material, and as a result, exhibit an appropriate level of Ni occupancy in the Li3a site, which can improve the electrochemical properties and structural stability, which will be described later, compared to the comparative examples.

[0167] Experimental Example 2: Measurement of the electrochemical properties of the positive electrode active material (1) Manufacture of lithium secondary batteries In order to confirm whether a lithium secondary battery manufactured using the cathode active materials according to the embodiments of the present invention can exhibit electrochemical properties equivalent to or similar to those of conventional cathode active materials, lithium secondary batteries including cathodes manufactured using the cathode active materials according to the examples and comparative examples were prepared in this experimental example.

[0168] The prepared positive electrode active material, Super-P as a conductive agent, and polyvinylidene fluoride (PVdF) as a binder were mixed in a weight ratio of 92:4:4 to prepare a slurry. The slurry was uniformly coated on an aluminum foil with a thickness of 15 μm and dried in a vacuum at 135°C to prepare a positive electrode for a lithium secondary battery.

[0169] A coin battery was fabricated using the positive electrode and lithium wheel as counter electrodes, a porous polyethylene film (Celgard 2300, thickness: 25 μm) as a separator, and a liquid electrolyte in which LiPF6 was dissolved at a concentration of 1.15 M in a solvent in which ethylene carbonate and ethyl methyl carbonate were mixed in a volume ratio of 3:7, according to a commonly known manufacturing process.

[0170] (2) Evaluation of the battery capacity and life characteristics of lithium secondary batteries The lithium secondary battery prepared by the above method was subjected to a charge-discharge experiment using an electrochemical analyzer (Toyo, Toscat-3100) at 25°C, a voltage range of 3.0V to 4.25V, and a discharge rate of 0.5C to 4.0C, to measure the initial charge capacity, initial discharge capacity, initial reversible efficiency, and rate characteristics.

[0171] In addition, the lithium secondary battery prepared by the above method was charged and discharged 50 times at 1C / 1C within a driving voltage range of 3.0V to 4.25V at a temperature of 25°C, and the ratio of the discharge capacity at the 50th cycle to the initial capacity (cycle capacity retention) was measured.

[0172] The measured battery capacity and life characteristics are shown in Tables 4 and 5 below.

[0173] [Table 4]

[0174] [Table 5]

[0175] Referring to Table 4, it can be seen that the initial capacity and reversible efficiency are relatively excellent when the cathode active materials of Examples 1 to 15 are used. In addition, when comparing the cathode active materials of Examples 1 and 7, it can be seen that the cathode active material of Example 7, which used flux during preparation of the cathode active material, showed slight improvements in terms of initial capacity and reversible efficiency compared to Example 1, which did not use flux.

[0176] Meanwhile, in the case of the cathode active material according to Comparative Example 2, the initial reversible efficiency was the lowest compared to the other Examples and Comparative Examples, and it was confirmed that it is not suitable for application as a cathode active material for practical lithium secondary batteries.

[0177] Furthermore, referring to Table 5, it can be seen that when the positive electrode active material according to the Example is used, the rate characteristics and life characteristics are generally higher than those of the Comparative Example in the charge-discharge experiments at a discharge rate of 0.5 C to 4.0 C. Furthermore, when comparing the positive electrode active materials according to Example 1 and Example 7, it can be seen that the positive electrode active material according to Example 7, which uses flux in the preparation of the positive electrode active material, has further improved life characteristics compared to Example 1, which does not use flux.

[0178] Meanwhile, in the case of the cathode active material according to Comparative Example 1, the lifespan characteristics were the lowest compared to the other Examples and Comparative Examples, and it was confirmed that it is not suitable for application as a cathode active material for a lithium secondary battery.

[0179] Experimental Example 3: Evaluation of the stability of positive electrode active materials and secondary batteries (1) Evaluation of thermal stability The weight loss of the positive electrode active materials according to the examples and comparative examples was measured at a heating rate of 10°C / min from 25°C to 350°C under an Ar atmosphere at atmospheric pressure using a thermogravimetric analyzer (TA Instruments, Q20) to evaluate the thermal stability. The measurement results are shown in Table 6 below.

[0180] In Table 6 below, the onset temperature (on-set) refers to the temperature at which the weight loss (thermal decomposition) peak of the positive electrode active material appears as a result of TGA analysis, and the peak temperature (peak) refers to the temperature at which the weight loss (thermal decomposition) peak reaches its highest point.

[0181] It can be seen that the positive electrode active materials according to the remaining examples, excluding Examples 4, 5, and 15, exhibit a weight loss peak at about 230° C. or higher, whereas the positive electrode active materials according to Comparative Examples 1 and 2 exhibit a weight loss peak at about 225° C. or lower. Here, it can be seen that the positive electrode active materials according to Examples 4, 5, and 15 exhibit a weight loss peak at about 225° C. or higher, and therefore have relatively higher high-temperature stability than the positive electrode active materials according to Comparative Examples 1 and 2.

[0182] Meanwhile, the temperature at which the weight loss peak reaches its highest point is also higher for the positive electrode active materials according to Examples 1 to 15 than for the positive electrode active materials according to Comparative Examples 1 and 2, confirming that they have relatively high high-temperature stability.

[0183] [Table 6]

[0184] (2) Manufacture of lithium secondary batteries In order to confirm whether lithium secondary batteries manufactured using the cathode active materials according to the examples and comparative examples of the present invention can exhibit electrochemical properties equivalent to or similar to those of conventional cathode active materials, lithium secondary batteries were prepared in this experimental example, employing cathodes manufactured using the cathode active materials according to the examples and comparative examples.

[0185] Specifically, the prepared positive electrode active material, Super-P as a conductive agent, and polyvinylidene fluoride (PVdF) as a binder were mixed in a weight ratio of 92:4:4 to prepare a slurry. The slurry was uniformly coated on an aluminum foil with a thickness of 15 μm and dried in a vacuum at 135°C to prepare a positive electrode for a lithium secondary battery.

[0186] A coin battery was fabricated using the positive electrode and lithium wheel as counter electrodes, a porous polyethylene film (Celgard 2300, thickness: 25 μm) as a separator, and a liquid electrolyte in which LiPF6 was dissolved at a concentration of 1.15 M in a solvent in which ethylene carbonate and ethyl methyl carbonate were mixed in a volume ratio of 3:7, according to a commonly known manufacturing process.

[0187] (3) Measurement of gas generation rate of lithium secondary batteries The lithium secondary battery manufactured by the above method was charged to 4.25 V at a constant current of 0.2 C, and then stored at 60°C for 14 days, and the volume change of the lithium secondary battery due to gas generation in the lithium secondary battery was measured. The volume change measurement results are shown in Table 7 below.

[0188] [Table 7]

[0189] Referring to Table 7, it can be seen that the amount of gas generated was reduced in the lithium secondary batteries using the positive electrode active materials according to Examples 1 to 15 and Comparative Example 2 compared to the lithium secondary battery using the positive electrode active material according to Comparative Example 1.

[0190] When an overcharge exceeding the limit flows through a lithium secondary battery, the internal temperature of the battery rises rapidly, which can cause a decomposition reaction of the electrolyte and generate gas. However, in this experimental example, the lithium secondary battery was not subjected to an overcurrent, so it is believed that gas was generated due to a side reaction at the interface and surface of the electrolyte and the positive electrode active material.

[0191] Considering the results in Table 7, it can be seen that the thermal stability and storage stability resulting from the structural stability of the positive electrode active materials according to Examples 1 to 15 are similar to or superior to those of conventional positive electrode active materials.

[0192] Although the embodiments of the present invention have been described above, a person having ordinary knowledge in the art may modify and change the present invention in various ways by adding, changing, deleting or adding components within the scope of the concept of the present invention as set forth in the claims, and this also falls within the scope of the present invention.

Claims

1. A positive electrode active material containing a lithium-based composite oxide having a single crystal structure, a cation mixed layer is present on the surface of the lithium-based composite oxide, a ratio (d1 / x1) of the thickness (d1) of the cation mixed layer to the average particle size (x1) of the primary particles is more than 0.0008 and less than 0.0052.

2. the positive electrode active material includes primary particles that are the lithium-based composite oxide having a single crystal structure and secondary particles that are aggregations of the primary particles, 2. The positive electrode active material according to claim 1, wherein the cation mixed layer is present on the surface of at least one selected from the primary particles and the secondary particles.

3. the cation mixed layer is present on the surface of the secondary particle, 3. The positive electrode active material according to claim 2, wherein a ratio (d2 / x2) of the thickness (d2) of the cation mixed layer to the average particle diameter (x2) of the secondary particles is more than 0.00014 and less than 0.00281.

4. the cation mixed layer is present on the surfaces of the primary particles and the secondary particles, 3. The positive electrode active material according to claim 2, wherein a thickness (d1) of the cation mixed layer present on the surface of the primary particle is smaller than a thickness (d2) of the cation mixed layer present on the surface of the secondary particle.

5. 5. The cathode active material according to claim 4, wherein a ratio (d1 / d2) of a thickness (d1) of the cation mixed layer present on the surface of the primary particle to a thickness (d2) of the cation mixed layer present on the surface of the secondary particle is more than 0.53 and less than 1.

0.

6. 2 . The positive electrode active material according to claim 1 , wherein a ratio of secondary particles having a grain boundary density of 0.5 or less to a plurality of secondary particles constituting the positive electrode active material is 30% or more. Grain boundary density = (number of boundaries between primary particles within a secondary particle / number of primary particles constituting a secondary particle)

7. 2. The positive electrode active material according to claim 1, wherein the lithium-based composite oxide has an average particle size of 1.0 μm to 5.0 μm.

8. 3. The positive electrode active material according to claim 2, wherein the secondary particles have an average particle size of 1.0 μm to 20.0 μm.

9. The positive electrode active material according to claim 1 , wherein the lithium-based composite oxide is represented by the following chemical formula 1: [Chemical formula 1] Li ａ Ni １－（ｂ＋ｃ＋ｄ＋ｅ） Co ｂ M1 ｃ M2 ｄ M3 ｅ O ｆ where M1 is Mn or Al; M2 and M3 are each independently selected from Al, Ba, B, Ce, Cr, Mg, Mn, Mo, Na, K, P, Sr, Ti, W, Nb, and Zr; M1 to M3 are different metals. (0.90≦a≦1.05, 0≦b≦0.20, 0≦c≦0.20, 0≦d≦0.05, 0≦e≦0.05, 1.0≦f≦2.0)

10. The positive electrode active material according to claim 1 , wherein the cation mixed layer comprises a complex oxide represented by the following chemical formula 2: [Chemical formula 2] Li ｇ Ni １－（ｈ＋ｉ＋ｊ＋ｋ） Co ｈ M4 ｉ M5 ｊ M6 ｋ O ｌ where M4 is Mn or Al; M5 and M6 are each independently selected from Al, Ba, B, Ce, Cr, Mg, Mn, Mo, Na, K, P, Sr, Ti, W, Nb, and Zr; M4 to M6 are different metals. (0.0≦g≦1.05, 0≦h≦0.20, 0≦i≦0.20, 0≦j≦0.05, 0≦k≦0.05, 1.0≦l≦2.0)

11. The positive electrode active material according to claim 10 , wherein the composite oxide represented by Chemical Formula 2 has a crystal structure different from that of the lithium-based composite oxide.

12. The cathode active material according to claim 1 , wherein the thickness of the cation mixed layer is more than 0.23 nm and less than 12.04 nm.

13. A positive electrode comprising the positive electrode active material according to any one of claims 1 to 12.

14. A lithium secondary battery comprising the positive electrode according to claim 13.

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