Positive active material for lithium secondary battery and lithium secondary battery comprising the same

Abstract

The present invention relates to a positive electrode active material for a lithium secondary battery and a lithium secondary battery including the same. The positive electrode active material is an aggregate of secondary particles having a grain boundary density of 0.5 or less and secondary particles having a grain boundary density of more than 0.5, the secondary particles having a grain boundary density of more than 0.5 include second agglomerates composed of 3 to 6 primary particles and third agglomerates composed of 7 to 10 primary particles, and a ratio of the second agglomerates is 20% or more and 70% or less among the plurality of secondary particles constituting the positive electrode active material: Grain boundary density = (number of interfaces between primary particles in a secondary particle / number of primary particles constituting the secondary particle) A lithium-based composite oxide is represented by the following Chemical Formula 1: [Chemical Formula 1] Li u Ni 1‑(v+w+x+y) Co v M1 w M2 x M3 y O z .

Description

[0001] This application is a divisional application of application number 201910843166.2, filed on September 6, 2019, entitled "Positive electrode active material for lithium secondary batteries and lithium secondary batteries including the same". Technical Field

[0002] This invention relates to a positive electrode active material for lithium secondary batteries and a lithium secondary battery including the same. Background Technology

[0003] Compared to 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, and high energy density. They are currently widely used in mobile phones, laptops, tablets, and other mobile terminals.

[0004] Furthermore, in recent years, driven by environmental protection considerations, electric vehicles have developed rapidly under the impetus of governments and automakers around the world, and lithium secondary batteries have become the ideal power source for the next generation of electric vehicles due to their excellent performance.

[0005] Lithium-based composite oxides have been used as positive electrode active materials for lithium secondary batteries, with lithium cobalt composite oxide (LiCoO2) being the most commonly used among these oxides due to its high operating voltage and excellent capacity characteristics. However, the use of LiCoO2 as a power source for applications requiring high-capacity battery systems, such as electric vehicles, is limited by its very poor high-temperature stability caused by the unstable crystal structure resulting from lithium intercalation and deintercalation, and its high cost.

[0006] Lithium manganese composite oxides (LiMnO2 or LiMn2O4), lithium iron phosphate (LiFePO4, etc.), or lithium nickel composite oxides (LiNiO2, etc.) have been developed as alternatives to LiCoO2. Among these materials, lithium nickel oxide has been studied and developed more actively, as it can realize high-capacity batteries due to its high reversible capacity of approximately 200 mAh / g.

[0007] However, the limitation of LiNiO2 is that it has worse high-temperature stability than LiCoO2, and when an internal short circuit occurs during charging due to external pressure, the positive electrode active material itself decomposes or side reactions occur between the electrolyte and the positive electrode active material interface and surface, which can cause the battery to crack and catch fire.

[0008] Therefore, there is a need to develop positive electrode active materials that can improve the low-temperature stability of LiNiO2 while maintaining its excellent reversible capacity.

[0009] On the other hand, lithium secondary batteries can be classified into can-type secondary batteries and pouch-type secondary batteries according to the shape of the battery casing. The can-type secondary battery has an electrode assembly housed in a metal can, while the pouch-type secondary battery has an electrode assembly housed in a pouch made of a sheet material such as an aluminum laminate.

[0010] Pouch-type rechargeable batteries are lightweight and have a low probability of electrolyte leakage, thus having the advantage of being able to achieve the same amount of rechargeable batteries in a relatively small volume and mass. However, if the pressure inside the battery casing increases sharply, there is a risk of explosion. Therefore, ensuring stability by controlling the generation of gas, which is the main cause of the pressure rise inside the battery casing, is one of the important issues.

[0011] For example, when an overcharge exceeding the limit flows into a secondary battery, the internal temperature of the battery rises rapidly. This may cause the electrolyte to decompose and produce gas, but it may also cause gas due to side reactions between the electrolyte and the interface and surface of the positive electrode active material. Summary of the Invention

[0012] Technical issues

[0013] The purpose of this invention is to provide a positive electrode active material for lithium secondary batteries that can improve the low thermal stability of LiNiO2 and maintain the excellent reversible capacity of LiNiO2, and a lithium secondary battery including the same.

[0014] Furthermore, the present invention aims to provide a positive electrode active material for a lithium secondary battery, and a lithium secondary battery including the same, which can prevent battery swelling caused by gas generation in a secondary battery by reducing the possibility of side reactions occurring between the interface and surface of the electrolyte and the positive electrode active material.

[0015] The objectives of this invention are not limited to those stated above. Other objectives and advantages of this invention not mentioned above can be understood through the following description and through embodiments of this invention. Furthermore, it will be readily understood that the objectives and advantages of this invention can be achieved by the means and combinations thereof described in the claims.

[0016] Solution to the problem

[0017] According to one aspect of the present invention, a positive electrode active material is provided, which is a positive electrode active material comprising a plurality of secondary particles composed of primary particles, wherein the primary particles are lithium-based composite oxides having lithium-ion diffusion paths of equal orientation in the long axis direction.

[0018] Among them, the characteristics of the positive electrode active material are as follows:

[0019] The aforementioned positive electrode active material is an aggregate of secondary particles with a grain boundary density of less than 0.5 and secondary particles with a grain boundary density of greater than 0.5.

[0020] The aforementioned secondary particles with a grain boundary density greater than 0.5 include second aggregates composed of 3 to 6 primary particles and third aggregates composed of 7 to 10 primary particles.

[0021] In the plurality of secondary particles constituting the above-mentioned positive electrode active material, the proportion of the second aggregate is more than 20% and less than 70%:

[0022] Grain boundary density = (Number of interfaces between primary particles in a secondary particle / Number of primary particles constituting a secondary particle)

[0023] The above-mentioned lithium-based composite oxides are represented by the following chemical formula 1:

[0024] [Chemical Formula 1]

[0025] Li u Ni 1-(v+w+x+y) Co v M1 w M2 x M3 y O z

[0026] Where M1 is Mn or Al, and M2 and M3 are each independently selected from Al, Ba, B, Ce, Cr, Mg, Mn, Mo, Na, K, P, Sr, Ti, W, and Zr, and are distinct elements from each other.

[0027] 0.95≤u≤1.05, 0≤v≤0.20, 0≤w≤0.20, 0≤x≤0.05, 0≤y≤0.05, 1.50≤z≤2.1.

[0028] In addition, among the multiple secondary particles constituting the above-mentioned positive electrode active material, the proportion of secondary particles with a grain boundary density of 0.5 or less is more than 30%.

[0029] Grain boundary density = (number of interfaces between primary particles in a secondary particle / number of primary particles constituting a secondary particle).

[0030] Furthermore, according to another aspect of the present invention, a positive electrode for a lithium secondary battery comprising the above-described positive electrode active material is provided.

[0031] Furthermore, according to another aspect of the present invention, a lithium secondary battery comprising the above-described positive electrode is provided.

[0032] The effects of the invention

[0033] The specific surface area and grain boundary surface of secondary particles contained in the positive electrode active material are regions where side reactions occur at the interface and surface between the positive electrode active material and the electrolyte. By reducing the specific surface area and grain boundary surface of secondary particles, the high-temperature stability of the positive electrode active material can be improved and the gas generation caused by the positive electrode active material can be reduced.

[0034] As described above, the positive electrode active material for lithium secondary batteries according to the present invention comprises a plurality of secondary particles composed of primary particles, wherein the primary particles are lithium-based composite oxides having a single-crystal structure. That is, the primary particles constituting the secondary particles are formed by single crystals, thereby reducing the specific surface area and grain boundary surface of the secondary particles.

[0035] Furthermore, according to the present invention, the positive electrode active material comprises secondary particles with different grain boundary densities, and the secondary particles contained in the positive electrode active material are formed by agglomerating 1 to 10 primary particles, thereby having a relatively small specific surface area compared to secondary particles formed by agglomerating tens to hundreds of primary particles, thus reducing the surface area for side reactions with the electrolyte. In addition, since the number of primary particles forming secondary particles is small, the grain boundary density is reduced, thus also reducing side reactions at the grain boundary surfaces of the secondary particles.

[0036] Furthermore, according to the present invention, since the secondary particles contained in the positive electrode active material are formed by agglomerating 1 to 10 primary particles, the probability that the primary particles in the secondary particles have equidirectional lithium-ion diffusion paths in the long axis direction may be high. As described above, as the ratio of equidirectional lithium-ion diffusion paths in the long axis direction in the secondary particles increases, the lithium-ion conductivity and electronic conductivity can be improved by the positive electrode active material. Attached Figure Description

[0037] Figure 1 The cross-section of an existing positive electrode active material is schematically shown.

[0038] Figures 2 to 4 The diagram schematically illustrates a cross-section of a secondary particle that may be contained in the positive electrode active material according to the invention.

[0039] Figure 5 and Figure 6 The lithium-ion diffusion path is shown for the positive electrode active material (secondary particles) in the single crystal state.

[0040] Figure 7 and Figure 8 The lithium-ion diffusion pathway is shown in the positive electrode active material (secondary particles) formed by the aggregation of multiple primary particles.

[0041] Figure 9A cross-sectional SEM image of the positive electrode active material prepared according to Example 1 is shown, that is, the grain boundary density of the secondary particles included in the positive electrode active material is shown.

[0042] Figure 10 A cross-sectional SEM image of the positive electrode active material prepared according to Example 2 is shown, that is, the grain boundary density of the secondary particles included in the positive electrode active material is shown.

[0043] Figure 11 A cross-sectional SEM image of the positive electrode active material prepared according to Comparative Example 1 is shown, that is, the grain boundary density of the secondary particles included in the positive electrode active material is shown.

[0044] Figure 12 and Figure 13 SEM images of positive electrode active materials prepared according to some embodiments of the present invention are shown.

[0045] Figure 14 A graph is shown to illustrate the Co content measured by EP-EDX analysis of secondary particles constituting the positive electrode active material according to some embodiments of the present invention.

[0046] Figure 15 A graph is shown to illustrate the results of the cycle-based lifetime characteristic evaluation of secondary batteries using the positive electrode active materials prepared according to Example 2 and Comparative Example 1.

[0047] Figure 16 and Figure 17 The Nyquist plot is shown to illustrate the impedance measurement results of secondary batteries using positive electrode active materials prepared according to the examples and comparative examples before and after high-temperature storage.

[0048] Figure 18 A graph showing the volume change based on storage days for secondary batteries prepared using the positive electrode active materials prepared according to Example 2 and Comparative Example 1.

[0049] Figure 19 A graph is shown to illustrate the thermogravimetric analysis results of secondary batteries prepared using the positive electrode active materials prepared according to Example 2 and Comparative Example 1. Detailed Implementation

[0050] To facilitate understanding of this invention, specific terms have been appropriately defined herein. Unless otherwise defined herein, scientific and technical terms used in this invention have the meanings commonly understood by one of ordinary skill in the art to which this invention pertains. Furthermore, unless the context otherwise requires, singular terms shall include plural forms and plural terms shall include singular forms.

[0051] As used herein, the term "lithium-based composite oxide" refers to an oxide capable of adsorbing and releasing lithium ions, and includes lithium and metal elements. In particular, the lithium-based composite oxide used herein may be a lithium-nickel composite oxide that includes lithium and nickel.

[0052] As used herein, the term "single crystal" refers to a crystal in a state where grain boundaries are not present within the particle, and the term "primary particle" as used herein refers to a particle that exists alone and does not form an aggregate. Therefore, "a primary particle as a lithium-based composite oxide having a single crystal structure" refers to a particle in a state where grain boundaries are not present in primary particles made of lithium-based composite oxides.

[0053] As used herein, the term "secondary particle" refers to a particle formed by the agglomeration of the aforementioned primary particles, which are lithium-based composite oxides. Thus, when at least two primary particles agglomerate to form a secondary particle, a grain boundary or grain boundary surface is present within the secondary particle, formed by the interface between the two primary particles.

[0054] As used herein, “grain boundary density” refers to the number of grain boundaries or grain boundary surfaces formed by the presence of at least two primary particles in a secondary particle. The greater the number of primary particles in a secondary particle, the greater the grain boundary density, and the less the number of primary particles in a secondary particle, the smaller the grain boundary density.

[0055] In this paper, the grain boundary density can be calculated using the following formula.

[0056] Grain boundary density = (number of interfaces between primary particles in a secondary particle / number of primary particles constituting a secondary particle).

[0057] On the other hand, in this paper, secondary particles also include particles composed of a single primary particle. A secondary particle composed of a single primary particle should be understood as a primary particle that exists as a single particle even after post-processing such as heat treatment to condense at least two primary particles into a secondary particle. This definition of secondary particles can be more accurately illustrated by the definition of positive electrode active materials described below.

[0058] As used herein, the term "positive electrode active material" is a broad concept that includes the aforementioned secondary particles. While a single secondary particle can be a positive electrode active material, in this document, not only can an aggregate of multiple secondary particles with the same grain boundary density be defined as a positive electrode active material, but also an aggregate of multiple secondary particles with different grain boundary densities can be defined as a positive electrode active material. The definition of positive electrode active material in this document will be described in more detail below.

[0059] The following describes in further detail the positive electrode active material for lithium secondary batteries according to the present invention and the lithium secondary battery including the same.

[0060] The specific surface area and grain boundary surface of secondary particles contained in the positive electrode active material are regions where side reactions occur at the interface and surface between the positive electrode active material and the electrolyte. By reducing the specific surface area and grain boundary surface of secondary particles, the high-temperature stability of the positive electrode active material can be improved and the gas generation caused by the positive electrode active material can be reduced.

[0061] According to an embodiment of the present invention, the positive electrode active material for a lithium secondary battery comprises a plurality of secondary particles composed of primary particles, wherein the primary particles are lithium-based composite oxides having a single-crystal structure. The primary particles constituting the secondary particles are formed from single crystals, thereby reducing the specific surface area and grain boundary surface of the secondary particles. The primary particles constituting the secondary particles have equidirectional lithium-ion diffusion paths along their long axis, thereby improving the conductivity of lithium ions by concentrating lithium ions in one direction rather than diffusing lithium ions in multiple directions.

[0062] Figure 1 The cross-section of an existing positive electrode active material is schematically shown. Figures 2 to 4 The diagram schematically illustrates a cross-section of a secondary particle that may be included in a positive electrode active material according to various embodiments of the invention.

[0063] like Figures 2 to 4 The secondary particles 110, 120, and 130 shown in the positive electrode active material are formed by agglomerating 1 to 10 primary particles 111, 121, and 131, thereby interacting with... Figure 1 Compared to the secondary particles 11 formed by agglomerating tens to hundreds of primary particles 10, the secondary particles 11 have a relatively small specific surface area, thus reducing the surface area for side reactions with the electrolyte. Furthermore, the smaller number of primary particles forming the secondary particles leads to a lower grain boundary density, which also reduces side reactions at the grain boundary surfaces of the secondary particles.

[0064] Furthermore, according to the present invention, the probability that the primary particles constituting the plurality of secondary particles contained in the positive electrode active material have equidirectional lithium-ion diffusion paths in the long axis direction is likely to be high. As described above, as the ratio of equidirectional lithium-ion diffusion paths in the secondary particles in the long axis direction increases, the lithium-ion conductivity and electronic conductivity can be improved by the positive electrode active material.

[0065] Figure 5 and Figure 6 The lithium-ion diffusion path is shown in the positive electrode active material (secondary particles) in the single crystal state. Figure 7 and Figure 8 The lithium-ion diffusion pathway is shown in the positive electrode active material (secondary particles) formed by the aggregation of multiple primary particles.

[0066] Reference Figure 5 and Figure 6 It can be confirmed that all lithium-ion diffusion paths at any point (A to D) in the positive electrode active material (secondary particles) are equidirectional along their long axis. In other words, since lithium ions in the positive electrode active material (secondary particles) can diffuse in a concentrated manner in one direction rather than in multiple directions, the conductivity of lithium ions can be improved through the positive electrode active material (secondary particles).

[0067] On the other hand, refer to Figure 7 and Figure 8 It can be confirmed that the positive electrode active material (secondary particles) is formed by the aggregation of multiple primary particles, and the lithium-ion diffusion paths at any point (① to ⑤) in the left-hand primary particles are inconsistent with the lithium-ion diffusion paths at any point (① to ⑤) in the right-hand primary particles. In this case, as shown in the figure... Figure 5 and Figure 7 Compared to the positive electrode active material shown, the diffusion ability of lithium ions is poor, therefore the conductivity of lithium ions caused by the positive electrode active material (secondary particles) is inevitably low.

[0068] Therefore, preferably, the proportion of primary particles with equidirectional lithium-ion diffusion paths in the long axis direction among the secondary particles included in the positive electrode active material according to the present invention is 30% or more, more preferably 70% or more. As described above, as the proportion of equidirectional lithium-ion diffusion paths in the long axis direction among the secondary particles increases, the lithium-ion conductivity in the positive electrode active material can be improved.

[0069] In one embodiment, the positive electrode active material according to the present invention can be an aggregate of multiple secondary particles with different grain boundary densities.

[0070] In this paper, the grain boundary density can be calculated using the following formula.

[0071] Grain boundary density = (number of interfaces between primary particles in a secondary particle / number of primary particles constituting a secondary particle).

[0072] Secondary particles with different grain boundary densities can have different physical and chemical properties. Examples of physical properties that vary with different grain boundary densities include differences in specific surface area before / after pressing the secondary particles, and examples of chemical properties include differences in the ratio of side reactions between the surface and / or interface of the secondary particles and the electrolyte.

[0073] For example, such as Figure 1 The secondary particle 10 shown will be combined with, as Figures 2 to 4The secondary particles 110, 120, and 130 shown are formed by the aggregation of a greater number of primary particles 11, therefore the grain boundary density formed by primary particles 11 is higher than that shown. Figures 2 to 4 The grain boundary densities of secondary particles 110, 120, and 130 are shown. Furthermore, it can be confirmed that... Figures 2 to 4 The number of grain boundary surfaces b formed by primary particles 111, 121, and 131 in the secondary particles 110, 120, and 130 shown is significantly less than that in the example shown. Figure 1 The number of grain boundary surfaces of the secondary particles 10 is shown. Generally speaking, the grain boundary surfaces formed by primary particles in secondary particles are regions where side reactions can occur with the electrolyte. The lower the grain boundary density of secondary particles or the fewer the number of grain boundary surfaces, the lower the possibility of side reactions occurring with the electrolyte.

[0074] Specifically, the positive electrode active material according to the present invention is characterized in that the proportion of secondary particles having a grain boundary density of 0.5 or less among the plurality of secondary particles constituting the positive electrode active material is 30% or more, preferably 50% or more, more preferably 70% or more.

[0075] For example, when a secondary particle is composed of a single primary particle with a single-crystal structure, the grain boundary density is 0 (number of boundary surfaces between primary particles in the secondary particle = 0 / number of primary particles constituting the secondary particle = 1). When a secondary particle is composed of two primary particles with a single-crystal structure, the grain boundary density is 0.5. In other words, the fewer the number of boundary surfaces between primary particles in a secondary particle, the smaller the grain boundary density value becomes. Specifically, as the number of primary particles constituting a secondary particle increases, the number of boundary surfaces between primary particles in the secondary particle also increases. Therefore, to achieve a grain boundary density below 0.5, the secondary particle should be composed of either a single primary particle with a single-crystal structure or two primary particles with a single-crystal structure.

[0076] The average particle size of the primary particles of the lithium-based composite oxide with a single-crystal structure can be from 0.01 μm to 50 μm, preferably from 0.01 μm to 20 μm. When the average particle size of the primary particles of the lithium-based composite oxide with a single-crystal structure is in the range of 0.01 μm to 20 μm, the optimal density of the cathode prepared using the cathode active material can be achieved.

[0077] Furthermore, the average particle size of the secondary particles can vary depending on the number of primary particles condensed, but can range from 0.01 μm to 50 μm.

[0078] More specifically, the positive electrode active material may include secondary particles consisting of 1 to 2 primary particles (hereinafter referred to as "first condensate" for convenience), secondary particles consisting of 3 to 6 primary particles (hereinafter referred to as "second condensate" for convenience), and secondary particles consisting of 7 to 10 primary particles (hereinafter referred to as "third condensate" for convenience).

[0079] like Figure 2 The number of primary particles 111 forming secondary particles in the first aggregate 110 shown is one or two. Compared with the second and third aggregates, the number of primary particles forming secondary particles is smaller, thus having a relatively small specific surface area, thereby reducing the area where side reactions with the electrolyte can occur.

[0080] Furthermore, in the first aggregate 110, the number of grain boundary surfaces b formed by primary particles 111 may be one or less. Therefore, compared to the case where the number of grain boundary surfaces is two or more, the probability of side reactions occurring between the grain boundary surfaces and the electrolyte may be reduced.

[0081] Furthermore, if a secondary particle consists of one primary particle, the lithium-ion diffusion path at any point within the secondary particle is identical. Even if the secondary particle consists of two primary particles, the likelihood that the lithium-ion diffusion path at any point is equidirectional along its long axis may increase compared to the case where the secondary particle consists of three or more primary particles.

[0082] In other words, the fewer primary particles that make up secondary particles, the higher the probability that the secondary particles have an isotropic lithium-ion diffusion path in the long axis direction. Thus, the lithium-ion conductivity and electronic conductivity can be improved through positive electrode active materials.

[0083] In the aggregates of secondary particles constituting the positive electrode active material, the content of this first condensate is preferably 30% or more. The percentage (%) of the first condensate present in the aggregates of secondary particles represents the percentage (%) of the number of first condensates out of the total number of secondary particles constituting the aggregates.

[0084] If the proportion of the first aggregate in the aggregates of secondary particles constituting the positive electrode active material is less than 30%, the proportion of the first aggregate in the aggregates decreases, thereby increasing the average specific surface area and grain boundary density of the secondary particles constituting the positive electrode active material. This increases the likelihood of side reactions between the positive electrode active material and the electrolyte, which may contribute to reduced high-temperature stability and storage performance of the positive electrode active material.

[0085] On the other hand, increasing the firing temperature can increase the proportion of the first aggregate in the aggregates of secondary particles constituting the positive electrode active material. However, when the firing temperature is too high, the possibility of degradation of the positive electrode active material actually increases.

[0086] And, as Figure 3 The number of primary particles 121 forming secondary particles in the second aggregate 120 shown is 3 to 6. In the aggregate of secondary particles constituting the positive electrode active material, the proportion of such second aggregate 120 can be 70% or less, preferably 20% or more and 70% or less.

[0087] And, as Figure 4 The number of primary particles 131 forming secondary particles in the third aggregate 130 shown is 7 to 10. In the aggregate of secondary particles constituting the positive electrode active material, the ratio of such third aggregate 130 is preferably 10% or less.

[0088] In this case, the positive electrode active material may also include secondary particles formed by agglomerating a greater number of primary particles than the third aggregate (which consists of 7 to 10 primary particles constituting secondary particles). In this case, the ratio of the third aggregate to the secondary particles formed by agglomerating a greater number of primary particles than the third aggregate is preferably 10% or less within the aggregate of secondary particles.

[0089] When the proportion of the first aggregate is 70% and the proportion of the second aggregate is reduced to less than 20%, the proportion of the second aggregate decreases relative to the third aggregate. Therefore, the average specific surface area and grain boundary density of the secondary particles constituting the positive electrode active material increase, which may be the reason for the reduction in the high-temperature stability and storage properties of the positive electrode active material.

[0090] On the other hand, when the proportion of the second aggregate in the aggregate of secondary particles constituting the positive electrode active material is greater than 30%, the proportion of the second aggregate is relatively increased compared to the first aggregate. Therefore, similarly, the average specific surface area and grain boundary density of the secondary battery constituting the positive electrode active material are increased.

[0091] As described above, the positive electrode active material according to the present invention includes a first aggregate, a second aggregate, and a third aggregate, and the ratio of the first aggregate, the second aggregate, and the third aggregate can be adjusted to make the average ratio of primary particles with equidirectional lithium-ion diffusion paths in the long axis direction of the secondary particles constituting the positive electrode active material 50% or more.

[0092] The lithium-based composite oxide that forms the positive electrode active material according to the present invention can be represented by the following chemical formula 1.

[0093] Li u Ni1-(v+w+x+y) Co v M1 w M2 x M3 y O z

[0094] Among them, M1 is Mn or Al, and M2 and M3 are each independently selected from Al, Ba, B, Ce, Cr, Mg, Mn, Mo, Na, K, P, Sr, Ti, W and Zr, and are different metals from each other.

[0095] (0.95≤u≤1.05, 0≤v≤0.20, 0≤w≤0.20, 0≤x≤0.05, 0≤y≤0.05, 1.50≤z≤2.1)

[0096] As described above, the specific surface area and grain boundary surface of the secondary particles contained in the positive electrode active material according to the present invention are regions where side reactions occur at the interface and surface between the positive electrode active material and the electrolyte. According to the present invention, by reducing the specific surface area and grain boundary surface of the secondary particles contained in the positive electrode active material, the high-temperature stability of the positive electrode active material can be improved and the gas generation caused by the positive electrode active material can be reduced.

[0097] In particular, the positive electrode active material according to the present invention comprises secondary particles formed by 1 to 10 primary particles having equidirectional lithium-ion diffusion paths in the long axis direction, and the positive electrode active material can be an aggregate of secondary particles with different grain boundary densities.

[0098] The secondary particle aggregates include a first aggregate consisting of 1 to 2 primary particles, a second aggregate consisting of 3 to 6 primary particles, and a third aggregate consisting of 7 to 10 primary particles. The ratio of the first, second, and third aggregates is adjusted by considering the average specific surface area of ​​the cathode active material, the grain boundary surface, and the lithium-ion diffusion path, thereby improving the stability and electrical properties of the cathode active material.

[0099] For example, in the case of positive electrode active materials according to various embodiments of the present invention, the grain boundary density is coordinated according to the ratio of the first condensate, the second condensate, and the third condensate, thereby stabilizing the crystal structure. In particular, by preventing the collapse of the crystal structure of the positive electrode active material under relatively high temperature conditions, the thermal stability of the positive electrode active material can be improved.

[0100] The positive electrode BET specific surface area formed by the aggregate of secondary particles as described above can be 0.1 m². 2 / g to 1.5m 2 / g.

[0101] When the proportion of the second aggregate in an aggregate of multiple secondary particles contained in the positive electrode active material is greater than 30% and the proportion of the first aggregate is less than 30%, the proportion of the first aggregate in the aggregate decreases, leading to an increase in the average specific surface area of ​​the secondary particles contained in the positive electrode active material. At this point, the BET specific surface area can be greater than 1.5 m². 2 / g.

[0102] Furthermore, when the proportion of the third aggregate in the positive electrode active material is greater than 10%, the average specific surface area of ​​the secondary particles contained in the positive electrode active material increases. At this point, the BET specific surface area can be greater than 1.5 m². 2 / g.

[0103] The d(10) change rate of the particle size distribution of the secondary particles contained in the positive electrode active material according to the present invention before and after 2.5-ton pressing can be less than 5%, the d(10) change rate of the particle size distribution before and after 4.5-ton pressing can be less than 15%, and the d(10) change rate of the particle size distribution before and after 6.0-ton pressing can be less than 30%.

[0104] Furthermore, the d(50) change rate of the particle size distribution of the secondary particles contained in the positive electrode active material according to the present invention before and after 2.5-ton pressing can be less than 3%, the d(50) change rate of the particle size distribution before and after 4.5-ton pressing can be less than 20%, and the d(50) change rate of the particle size distribution before and after 6.0-ton pressing can be less than 30%.

[0105] Furthermore, the d(90) change rate of the particle size distribution of the secondary particles contained in the positive electrode active material according to the present invention before and after 2.5-ton pressing can be less than 5%, the d(90) change rate of the particle size distribution before and after 4.5-ton pressing can be less than 15%, and the d(90) change rate of the particle size distribution before and after 6.0-ton pressing can be less than 20%.

[0106] In the preparation of positive electrodes for lithium-ion batteries using positive electrode active materials, a slurry containing the positive electrode active material is coated onto a positive electrode current collector, followed by drying and calendering (pressing). In particular, a positive electrode with high energy density can be prepared by calendering under high pressure. However, with increasing calendering density, in the case of positive electrode active materials with low particle strength, the desired electrical performance cannot be expected due to particle collapse. In the case of positive electrode active materials with high particle strength, the particles do not collapse even under high calendering conditions, thus maintaining performance.

[0107] In particular, when particles collapse under calendering conditions, the proportion of secondary particles, including those within a relatively small particle size distribution range, increases. In this paper, the strength of the particles is represented by the rate of change of particle size distribution d(10), d(50), and d(90) before and after calendering.

[0108] The positive electrode active material according to the present invention exhibits a particle size distribution change rate of only 13% before and after pressing at 4.5 tons, and a particle size distribution change rate of only 26% before and after pressing at 6 tons. Therefore, even under high-pressure calendering conditions, the positive electrode active material according to the present invention can maintain its performance and minimize particle collapse.

[0109] Additionally, the positive electrode active material according to various embodiments of the present invention may include a coating that covers at least a portion of the surface of the primary particles (e.g., the interface between the primary particles) and / or secondary particles formed by agglomeration of the primary particles.

[0110] For example, the coating may be present in a manner that covers at least a portion of the exposed surface of the primary particles. In particular, the coating may be present in a manner that covers at least a portion of the exposed surface of the primary particles that are present on the outermost periphery of the secondary particles.

[0111] Therefore, the coating can exist as a layer that continuously or discontinuously coats the surface of the primary particles and / or the secondary particles formed by agglomeration of the primary particles. When the coating exists discontinuously, it can exist in the form of islands.

[0112] The coatings described above can help improve the physical and electrochemical properties of the positive electrode active material.

[0113] Furthermore, the coating can exist in the form of a solid solution in which there is no boundary between the primary particles and / or the secondary particles formed by the aggregation of the primary particles.

[0114] The coating may include at least one oxide represented by the following chemical formula 2. That is, the coating may be defined as the region in which the oxide represented by the following chemical formula 2 is present.

[0115] [Chemical Formula 2]

[0116] Li a A b O c

[0117] (Where A is selected from at least one of Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, W, Ce, V, Ba, Ta, Sn, Hf, Ce, Gd, and Nd.)

[0118] (And 0≤a≤6, 0≤b≤4, 2≤c≤8)

[0119] Furthermore, the coating can be in the form of different types of oxides existing simultaneously in one layer, or it can be in the form of different types of oxides represented by the above chemical formula 2 existing in different layers.

[0120] The oxide represented by the above chemical formula 2 can be in a state of physical and / or chemical bonding with the primary particle represented by the above chemical formula 1. Furthermore, the oxide can exist in a state where it forms a solid solution with the primary particle represented by the above chemical formula 1.

[0121] The positive electrode active material according to this embodiment includes a coating that covers at least a portion of the surface of the primary particles (e.g., the interface between the primary particles) and / or the secondary particles formed by the aggregation of the primary particles, thereby improving structural stability. When the positive electrode active material is used in a lithium secondary battery, its high-temperature storage stability and lifetime characteristics can be improved. Furthermore, the oxide reduces residual lithium in the positive electrode active material and serves as a pathway for lithium ions, thereby improving the efficiency characteristics of the lithium secondary battery.

[0122] Furthermore, depending on the circumstances, the aforementioned oxides may exist not only at the interfaces between the primary particles and at least in a portion of the surface of the secondary particles, but also in the internal voids formed within the secondary particles.

[0123] The aforementioned oxides are oxides formed by combining lithium with an element represented by A, or oxides of A. Examples of such oxides include Li. a W b O c Li a Zr b O c Li a Ti b O c Li a Ni b O c Li a Ba b O c Li a B b Oc W b O c Zr b O c Ti b O c Or Ba b O c However, the examples above are only for ease of understanding, and the oxides defined herein are not limited to the examples above.

[0124] In another embodiment, the oxide described above can be an oxide formed by combining lithium with at least two elements represented by A, or further includes an oxide formed by combining lithium with at least two elements represented by A. An example of an oxide formed by combining lithium with at least two elements represented by A is Li. a (W / Ti) b O c Li a (W / Zr) b O c Li a (W / Ti / Zr) b O c Li a (W / Ti / Ba) b O c and Li a (W / Ti / B) b O c However, the present invention is not limited thereto.

[0125] The oxide may have a concentration gradient that decreases from the surface of the secondary particle toward the center of the secondary particle. Therefore, the concentration of the oxide can decrease from the outermost surface of the secondary particle toward the center of the secondary particle.

[0126] As described above, the oxide has a concentration gradient that decreases from the surface of the secondary particles toward the center of the secondary particles, thereby preventing side reactions caused by unreacted residual lithium by effectively reducing the residual lithium present on the surface of the positive electrode active material. Furthermore, the oxide also prevents a decrease in crystallinity in the inner surface region of the positive electrode active material. In addition, the oxide can prevent the overall structural collapse of the positive electrode active material during electrochemical reactions.

[0127] In addition, the coating may include a first oxide layer and a second oxide layer, wherein the first oxide layer contains at least one oxide represented by the above chemical formula 2, and the second oxide layer contains at least one oxide represented by the above chemical formula 2 but contains an oxide different from the oxide contained in the first oxide layer.

[0128] For example, the first oxide layer may be present in such a way that it covers at least a portion of the exposed surface of the primary particle that is present on the outermost periphery of the secondary particle, and the second oxide layer may be present in such a way that it covers at least a portion of the exposed surface of the primary particle that is not covered by the first oxide layer and the surface of the first oxide layer.

[0129] According to another aspect of the present invention, a positive electrode and a secondary battery comprising the above-described positive electrode active material are provided.

[0130] The positive electrode includes a positive current collector and a positive active material layer located on the positive current collector, wherein, according to one aspect of the present invention, the positive active material is present in the positive active material layer.

[0131] There are no particular restrictions on the positive electrode current collector, as long as it has high conductivity without causing chemical changes in the battery. Examples of such materials include stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc. Furthermore, the positive electrode current collector can typically have a thickness from 3 μm to 500 μm, or the adhesion of the positive electrode active material can be increased by forming tiny irregularities on the surface of the current collector. Such positive electrode current collectors can take various forms, such as films, sheets, boxes, meshes, porous materials, foams, and nonwoven fabrics.

[0132] Furthermore, the positive electrode active material layer can be a layer comprising the aforementioned positive electrode active material, conductive material, and adhesive.

[0133] The aforementioned conductive material is used to impart conductivity to the electrode. There are no limitations on the conductive material, as long as it is conductive and does not cause a chemical change in the positive electrode active material. Non-limiting examples of conductive materials include graphite such as natural or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking black, and carbon fibers; metal powders or fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides such as titanium oxides; or conductive polymers such as polyphenylene derivatives. Typically, the content of the conductive material can range from 1% to 30% by weight, based on the total weight of the positive electrode active material layer.

[0134] Furthermore, the adhesive serves to adhere the positive electrode active material particles to each other and enhance the adhesion between the positive electrode active material and the current collector. Non-limiting examples of adhesives include: polyvinylidene fluoride (PVDF), PVDF-co-HFP copolymer, polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof. Typically, the adhesive content can range from 1% to 30% by weight relative to the total weight of the positive electrode active material layer.

[0135] In addition to using the aforementioned positive electrode active material, the positive electrode of one embodiment of the present invention can be prepared using conventional positive electrode preparation methods for lithium secondary batteries. Specifically, the positive electrode can be prepared by coating a slurry for forming the positive electrode active material layer onto a positive electrode current collector, and then drying and calendering the resulting material. The slurry includes the aforementioned positive electrode active material as well as selective binders and conductive materials. According to another example, the positive electrode can also be prepared by casting the aforementioned slurry for forming the positive electrode active material layer onto a separate carrier, and then pressing the film layer obtained by peeling the positive electrode active material layer from the carrier onto a positive electrode current collector.

[0136] According to another aspect of the present invention, an electrochemical device comprising the aforementioned positive electrode is provided. Specifically, the electrochemical device may be a battery, a capacitor, etc., and more specifically, it may be a lithium secondary battery.

[0137] A lithium secondary battery includes a positive electrode, a negative electrode positioned opposite the positive electrode, a separator and an electrolyte (electrolyte) located between the positive and negative electrodes. Furthermore, a lithium secondary battery may include a battery container (casing) housing an electrode assembly including the positive electrode, negative electrode, and separator, and a sealing member for sealing the battery container.

[0138] Based on the shape of the battery container (shell), lithium secondary batteries can be divided into can-type lithium secondary batteries and pouch-type lithium secondary batteries. The can-type lithium secondary battery has an electrode assembly housed in a metal can, while the pouch-type lithium secondary battery has an electrode assembly housed in a pouch made of a sheet material such as an aluminum laminate.

[0139] In particular, in the case of pouch-type lithium secondary batteries using positive electrode materials including the positive electrode active materials of various embodiments of the present invention, since the possibility of side reactions between the positive electrode active material and the electrolyte is low, it has the advantages of improving stability during storage and / or operation and reducing gas generation.

[0140] Therefore, in the case of a lithium secondary battery using a positive electrode comprising the positive electrode active material of various embodiments of the present invention, for example, after being charged to 4.25V at 0.2C and then stored at 60°C for 14 days, the volume increase of the lithium secondary battery is 0.3 cm. 3 This ensures reliability and stability.

[0141] The invention will be described in more detail below with reference to embodiments. However, these embodiments are for illustrative purposes only and the scope of the invention should not be construed as limited by these embodiments.

[0142] Preparation of positive electrode active materials

[0143] Example 1

[0144] The NiCo(OH)2 precursor prepared by co-precipitation reaction, Li2CO3 and LiOH (Li / M ratio = 1.05 ± 0.05) as lithium compounds, and Al were weighed and mixed to form LiNi. 0.9 Co 0.078 Al 0.022 The mixture was then subjected to a first heat treatment at 800±50℃ to prepare a positive electrode active material for lithium secondary batteries. The prepared positive electrode active material was then immersed in distilled water and washed while maintaining the temperature. After dehydration, the mixture was dried under a vacuum atmosphere at 150℃. Subsequently, the dried positive electrode active material underwent a second heat treatment at 700±50℃.

[0145] Example 2

[0146] Except for the first heat treatment temperature of 850±50°C in Example 1, the positive electrode active material was prepared in the same manner as in Example 1.

[0147] Example 3

[0148] Except for the further mixing of the Ba-containing precursor (Ba(OH)2) during the co-precipitation reaction in Example 1, so that the Ba content in the final prepared positive electrode active material is 0.3 mol%, the positive electrode active material was prepared in the same manner as in Example 1.

[0149] Example 4

[0150] Except for the further mixing of the Zr-containing precursor (Zr(OH)4) during the co-precipitation reaction in Example 1 so that the Zr content in the final prepared positive electrode active material is 0.1 mol%, the positive electrode active material was prepared in the same manner as in Example 1.

[0151] Example 5

[0152] The positive electrode active material obtained in Example 1 was mixed with a Co-containing raw material (Co3O4) using a mixer. The Co-containing raw material (Co3O4) was mixed such that the content of the Co-containing raw material (Co3O4) was 3.0% by weight relative to the total weight of the positive electrode active material. Subsequently, the mixture was kept in an O2 atmosphere in the same firing furnace and the temperature was increased by 2°C per minute, and held at a heat treatment temperature of 700°C for 8 hours, followed by natural cooling.

[0153] Example 6

[0154] Except for replacing the Co-containing raw material (Co3O4) with a W-containing raw material (WO3), the positive electrode active material was prepared in the same manner as in Example 5. The W-containing raw material (WO3) was mixed such that its content relative to the total weight of the positive electrode active material was 0.3% by weight.

[0155] Example 7

[0156] Except for replacing the Co-containing raw material (Co3O4) with a Ti-containing raw material (TiO2), the positive electrode active material was prepared in the same manner as in Example 5. The Ti-containing raw material (TiO2) was mixed such that its content relative to the total weight of the positive electrode active material was 0.1% by weight.

[0157] Example 8

[0158] Except for replacing the Co-containing raw material (Co3O4) with a Zr-containing raw material (ZrO2), the positive electrode active material was prepared in the same manner as in Example 5. The Zr-containing raw material (ZrO2) was mixed such that its content relative to the total weight of the positive electrode active material was 0.1% by weight.

[0159] Example 9

[0160] Except for replacing the Co-containing raw material (Co3O4) with an Al-containing raw material (Al2O3), the positive electrode active material was prepared in the same manner as in Example 5. The Al-containing raw material (Al2O3) was mixed such that its content relative to the total weight of the positive electrode active material was 0.5% by weight.

[0161] Comparative Example 1

[0162] Except for the first heat treatment temperature of 700±50°C in Example 1, the positive electrode active material was prepared in the same manner as in Example 1.

[0163] Comparative Example 2

[0164] Except for the further mixing of a Ba-containing precursor (Ba(OH)2) during the co-precipitation reaction in Comparative Example 1, so that the Ba content in the final prepared positive electrode active material is 0.3 mol%, the positive electrode active material was prepared in the same manner as in Comparative Example 1.

[0165] Comparative Example 3

[0166] Except for the further mixing of the Zr-containing precursor (Zr(OH)4) during the co-precipitation reaction in Example 1 so that the Zr content in the final prepared positive electrode active material is 0.1 mol%, the positive electrode active material was prepared in the same manner as in Comparative Example 1.

[0167] Comparative Example 4

[0168] The positive electrode active material obtained in Comparative Example 1 was mixed with a Co-containing raw material (Co3O4) using a mixer. The Co-containing raw material (Co3O4) was mixed such that the content of the Co-containing raw material (Co3O4) was 3.0% by weight relative to the total weight of the positive electrode active material. Subsequently, the mixture was kept in an O2 atmosphere in the same firing furnace and the temperature was increased by 2°C per minute, and then held at a heat treatment temperature of 700°C for 8 hours, followed by natural cooling.

[0169] Comparative Example 5

[0170] Except for replacing the Co-containing raw material (Co3O4) with a W-containing raw material (WO3), the positive electrode active material was prepared in the same manner as in Comparative Example 4. The W-containing raw material (WO3) was mixed such that its content relative to the total weight of the positive electrode active material was 0.3% by weight.

[0171] Comparative Example 6

[0172] Except for replacing the Co-containing raw material (Co3O4) with a Ti-containing raw material (TiO2), the positive electrode active material was prepared in the same manner as in Comparative Example 4. The Ti-containing raw material (TiO2) was mixed such that its content relative to the total weight of the positive electrode active material was 0.1% by weight.

[0173] Comparative Example 7

[0174] Except for replacing the Co-containing raw material (Co3O4) with a Zr-containing raw material (ZrO2), the positive electrode active material was prepared in the same manner as in Comparative Example 4. The Zr-containing raw material (ZrO2) was mixed such that its content relative to the total weight of the positive electrode active material was 0.1% by weight.

[0175] Comparative Example 8

[0176] Except for replacing the Co-containing raw material (Co3O4) with an Al-containing raw material (Al2O3), the positive electrode active material was prepared in the same manner as in Comparative Example 4. The Al-containing raw material (Al2O3) was mixed such that its content relative to the total weight of the positive electrode active material was 0.5% by weight.

[0177] Experimental Example 1. Structure of Positive Electrode Active Material

[0178] Figure 9 and Figure 10 Cross-sectional SEM images of the aggregates of secondary particles constituting the positive electrode active materials prepared according to Examples 1 and 2, and of the primary particles constituting the secondary particles, were taken to confirm their composition. Figure 11 To show a cross-sectional SEM image of the positive electrode active material prepared according to Comparative Example 1, that is, to show the grain boundary density of the secondary particles included in the positive electrode active material.

[0179] Reference Figures 9 to 11 It can be confirmed that, compared with the positive electrode active material prepared according to Comparative Example 1, the proportion of secondary particles with a grain boundary density of 0.5 or less among the multiple secondary particles constituting the positive electrode active material is greater.

[0180] As described above, the higher the proportion of secondary particles with a grain boundary density of 0.5 or less among the multiple secondary particles constituting the positive electrode active material, the more the side reactions between the electrolyte and the grain boundary surface of the secondary particles can be reduced. Moreover, as the ratio of lithium-ion diffusion paths in the same direction along the long axis in the secondary particles increases, the lithium-ion conductivity and electronic conductivity can be improved through the positive electrode active material.

[0181] The results of calculating the grain boundary density of secondary particles contained in the positive electrode active materials prepared according to the examples and comparative examples are shown in Table 1 below.

[0182] Table 1

[0183]

[0184]

[0185] Figure 12 and Figure 13 SEM images of the positive electrode active materials prepared according to Examples 1 and 5 of the present invention are shown. For comparison... Figure 12 and Figure 13 Then it can be confirmed that... Figure 13 The surface of the positive electrode active material prepared according to Example 5 is coated with Co.

[0186] Furthermore, referring to the graphs showing the Co content determined by EP-EDX analysis of secondary particles constituting the positive electrode active materials according to Examples 1 and 5 respectively, Figure 14 It can be confirmed that the concentration change of the positive electrode active material prepared according to Example 5 is greater from the surface to the center of the secondary particles compared to Example 1. Therefore, it can be confirmed that the surface of the positive electrode active material prepared according to Example 5 is coated with Co.

[0187] Experimental Example 2. Strength Measurement of Positive Electrode Active Materials

[0188] When preparing a positive electrode for a lithium-ion secondary battery using positive electrode active materials, a slurry containing the positive electrode active material is coated onto a positive electrode current collector, followed by drying and calendering (pressing). During calendering under high pressure, the particles of the positive electrode active material coated onto the current collector may collapse, potentially degrading the performance of the positive electrode active material.

[0189] In this experimental example, in order to confirm the strength change of the positive electrode active material based on the composition of the aggregate of multiple secondary particles contained in the positive electrode active material, positive electrode active materials prepared according to Example 2 and Comparative Example 1 were prepared respectively, and then the particle size distribution d(10), d(50) and d(90) change rates were measured before pressing, after 2.5 tons pressing, after 4.5 tons pressing and after 6 tons pressing.

[0190] The results of the determination of the change rates of particle size distribution d(10), d(50) and d(90) after compression under various pressure conditions are shown in Tables 2 to 4 below.

[0191] Table 2

[0192]

[0193] Table 3

[0194]

[0195] Table 4

[0196]

[0197] Referring to Tables 2 to 4, the change rates of the particle size distribution d(10), d(50), and d(90) of the secondary particles included in the positive electrode active material prepared according to Example 2 before and after pressing are smaller than those of the positive electrode active material prepared according to Comparative Example 1.

[0198] Experimental Example 3. Determination of Electrochemical Properties of Positive Electrode Active Materials

[0199] (1) Preparation of lithium secondary batteries

[0200] To confirm whether the lithium secondary battery exhibits electrochemical characteristics at the same or similar level as existing positive electrode active materials when prepared using the positive electrode active material prepared according to the embodiments of the present invention, in this experimental example, a lithium secondary battery suitable for the positive electrode was prepared using the positive electrode active material prepared according to the embodiments and comparative examples.

[0201] A slurry was prepared by mixing the prepared positive electrode active material, super-P as a conductive agent, and polyvinylidene fluoride (PVdF) as a binder in a weight ratio of 92:4:4. The slurry was uniformly coated onto an aluminum foil with a thickness of 15 μm and vacuum dried at 135 °C to prepare a positive electrode for lithium secondary batteries.

[0202] A button cell was prepared using the above-mentioned positive electrode and lithium foil as the relative electrode, a porous polyethylene membrane (Celgard 2300, thickness: 25 μm) as the separator, and a liquid electrolyte of LiPF6 at a concentration of 1.15 M dissolved in a solvent in which ethylene carbonate and methyl ethyl carbonate were mixed in a volume ratio of 3:7, according to a generally known manufacturing process.

[0203] (2) Evaluation of the capacity and lifespan characteristics of lithium secondary batteries

[0204] The lithium secondary batteries prepared by the above method were subjected to charge-discharge experiments using an electrochemical analyzer (Toyo, Toscat-3100) at 25°C with a voltage range of 3.0V to 4.25V and a discharge rate of 0.5C to 4.0C. The initial charge capacity, initial discharge capacity, initial reversible efficiency characteristics, and rate characteristics were determined.

[0205] Furthermore, at a temperature of 25°C and a driving voltage range of 3.0V to 4.25V, under 1C / 1C conditions, the lithium secondary battery prepared by the above method was subjected to 50 charge-discharge cycles, and then the ratio of the discharge capacity to the initial capacity after the 50th cycle (cycle capacity retention) was measured.

[0206] The measured battery capacity and lifespan characteristics are shown in Tables 5 and 6 below. Figure 15 .

[0207] Table 5

[0208]

[0209]

[0210] Table 6

[0211]

[0212] Referring to Tables 5 and 6, although the initial capacity and reversible efficiency of lithium secondary batteries including positive electrodes using positive electrode active materials prepared according to the examples and comparative examples are at similar levels, in the case of lithium secondary batteries including positive electrodes using positive electrode active materials prepared according to the examples, the rate performance and lifetime performance can be confirmed to be higher than those of the comparative examples in charge-discharge test results at applicable discharge rates of 0.5C to 4.0C.

[0213] (3) Impedance changes of lithium secondary batteries based on high-temperature storage

[0214] For the lithium secondary batteries prepared by the above method (Example 1, Example 2, Comparative Example 1 and Comparative Example 2), the impedance at 25°C before and after high-temperature storage was measured using a PARSTAT multichannel (PMC) rack (METEK) according to the 2-probe method (amplitude ±10mV, frequency range 0.01Hz to 10kHz). A storage condition of 60°C for 168 hours was used as the high-temperature storage condition. The impedance measurement results are as follows: Figure 16 (before high-temperature storage) and Figure 17 (After high-temperature storage) as shown.

[0215] exist Figure 16 and Figure 17 In the Nyquist plot shown, the interface resistance of the electrode is determined by the position and size of the semicircle, and the difference between the left and right x-intercepts of the semicircle represents the interface resistance at the electrode.

[0216] Reference Figure 16 and Figure 17 The lithium secondary batteries prepared using the positive electrode active materials prepared according to Examples 1 and 2 exhibited lower interface resistance before and after high-temperature storage compared to Comparative Examples 1 and 2.

[0217] Experimental Example 4. Stability Evaluation of Positive Electrode Active Materials and Secondary Batteries

[0218] (1) Preparation of lithium secondary batteries

[0219] To confirm whether the lithium secondary battery exhibits electrochemical characteristics at the same or similar level as existing positive electrode active materials when prepared using the positive electrode active material prepared according to the embodiments of the present invention, in this experimental example, a lithium secondary battery suitable for the positive electrode was prepared using the positive electrode active material prepared according to Example 2 and Comparative Example 1.

[0220] Specifically, a slurry was prepared by mixing the prepared positive electrode active material, super-P as a conductive agent, and polyvinylidene fluoride (PVdF) as a binder in a weight ratio of 92:4:4. The slurry was then uniformly coated onto an aluminum foil with a thickness of 15 μm and vacuum dried at 135 °C to prepare a positive electrode for lithium secondary batteries.

[0221] A button cell was prepared using the above-mentioned positive electrode and lithium foil as the relative electrode, a porous polyethylene membrane (Celgard 2300, thickness: 25 μm) as the separator, and a liquid electrolyte of LiPF6 at a concentration of 1.15 M dissolved in a solvent in which ethylene carbonate and methyl ethyl carbonate were mixed in a volume ratio of 3:7, according to a generally known manufacturing process.

[0222] (2) Measurement of gas generation in lithium secondary batteries

[0223] The lithium secondary battery prepared by the above method was charged to 4.25V at a constant current of 0.2C and stored at 60°C for 14 days to measure the volume change of the lithium secondary battery due to gas generation. The volume change measurement results are shown in... Figure 18 middle.

[0224] Reference Figure 18 It can be confirmed that the amount of gas generated in the lithium secondary battery prepared using the positive electrode active material prepared according to Example 2 is reduced compared with that in Comparative Example 1.

[0225] When an overcharge exceeding the limit flows through a lithium secondary battery, the internal temperature of the battery rises sharply, leading to a decomposition reaction of the electrolyte, which may result in gas. However, in this embodiment, the condition of an overcharge flowing through the lithium secondary battery is not present. Therefore, it can be predicted that gas will occur due to side reactions between the electrolyte and the cross-section and surface of the positive electrode active material.

[0226] Considering this, it can be seen that the positive electrode active material prepared according to Example 2, which has a relatively high ratio of secondary particles with smaller grain boundary density compared to Comparative Example 1, not only has a small specific surface area, but also has a narrower cross-section and surface area of ​​secondary particles, resulting in a reduced side reaction rate with the electrolyte and thus a reduced amount of gas generated.

[0227] (3) Thermal stability evaluation

[0228] To evaluate the thermal stability of the positive electrode active materials prepared according to Example 2 and Comparative Example 1, the weight loss was measured by raising the temperature from 25°C to 350°C at a rate of 10°C / min using a thermogravimetric analyzer (TA Instruments, Q20) under an Ar atmosphere at atmospheric pressure. The measurement results are shown in Table 19 below.

[0229] Reference Figure 19 It can be confirmed that in the case of the positive electrode active material prepared according to Comparative Example 1, weight loss occurs at a temperature below about 220°C, but in the case of the positive electrode active material prepared according to Example 2, weight loss occurs at a temperature above about 230°C.

[0230] In view of this, it can be confirmed that the high-temperature stability of the positive electrode active material prepared according to Example 2 is relatively high compared with Comparative Example 1.

[0231] Furthermore, after charging the positive electrodes comprising the positive electrode active materials prepared according to Example 2 and Comparative Example 1 to 4.3V, 4.4V, and 4.6V respectively, the temperature was increased from 25°C to 350°C at a heating rate of 10°C / min under an Ar atmosphere at atmospheric pressure, thereby measuring the weight loss. The measurement results are shown in Table 7 below.

[0232] Table 7

[0233]

[0234]

[0235] Referring to the results in Table 7 above, it can be confirmed that in the thermogravimetric analysis performed after charging the positive electrodes containing the positive electrode active materials prepared according to Example 2 and Comparative Example 1 to 4.3V and 4.4V respectively, the difference in weight loss between Example 2 and Comparative Example 1 was not significant up to 200°C. However, it can be confirmed that at temperatures above 200°C, the weight loss of Example 2 was smaller than that of Comparative Example 1.

[0236] On the other hand, it can be confirmed that in the thermogravimetric analysis performed after the positive electrode containing the positive electrode active material prepared according to Example 2 and Comparative Example 1 was charged to 4.6V, the thermal stability of Example 2 was better than that of Comparative Example 1 under most temperature conditions.

[0237] This result is due to the difference in thermal stability of the lattice structure of the positive electrode active materials prepared according to the examples and comparative examples. In particular, when the lattice structure of the positive electrode active material collapses under relatively high temperature conditions, O2 is released from the positive electrode active material. However, in the case of the positive electrode active materials according to the various embodiments of the present invention, the thermal stability of the lattice structure can be improved due to the coordination of grain boundary density.

[0238] The embodiments of the present invention have been described above, but the scope of the present invention is not limited to the embodiments described herein. Those skilled in the art can easily propose other embodiments by adding, modifying, deleting, or adding elements within the same scope of the present invention, and these fall within the scope of the present invention.

Claims

1. A positive electrode active material for lithium secondary batteries, comprising a plurality of secondary particles composed of primary particles, wherein the primary particles are lithium-based composite oxides having a single-crystal structure, characterized in that... The aforementioned positive electrode active material is an aggregate of secondary particles with a grain boundary density of less than 0.5 and secondary particles with a grain boundary density of greater than 0.

5. The aforementioned secondary particles with a grain boundary density greater than 0.5 include second aggregates composed of 3 to 6 primary particles and third aggregates composed of 7 to 10 primary particles. In the plurality of secondary particles constituting the above-mentioned positive electrode active material, the proportion of the second aggregate is more than 20% and less than 70%. Grain boundary density = (Number of interfaces between primary particles in a secondary particle / Number of primary particles constituting a secondary particle) The above-mentioned lithium-based composite oxides are represented by the following chemical formula 1: [Chemical Formula 1] Li u Ni 1-(v+w+x+y) Co v M1 w M2 x M3 y O z in, M1 is either Mn or Al, while M2 and M3 are each independently selected from Al, Ba, B, Ce, Cr, Mg, Mn, Mo, Na, K, P, Sr, Ti, W, and Zr, and are distinct elements from each other. 0.95≤u≤1.05, 0≤v≤0.20, 0≤w≤0.20, 0≤x≤0.05, 0≤y≤0.05, 1.50≤z≤2.

1.

2. The positive electrode active material for lithium secondary batteries according to claim 1, characterized in that, The proportion of primary particles constituting the aforementioned secondary particles that have lithium-ion diffusion paths oriented in the same direction as the long axis of the aforementioned secondary particles is 30% or more.

3. The positive electrode active material for lithium secondary batteries according to claim 1, characterized in that, The primary particles constituting the aforementioned secondary particles have lithium-ion diffusion paths oriented in the same direction as the long axis of the aforementioned secondary particles.

4. The positive electrode active material for lithium secondary batteries according to claim 1, characterized in that, The proportion of the third aggregate among the multiple secondary particles constituting the above-mentioned positive electrode active material is less than 10%, and the BET specific surface area of ​​the above-mentioned positive electrode active material is 1.5 m². 2 / g or less.

5. The positive electrode active material for lithium secondary batteries according to claim 1, characterized in that, The BET specific surface area of ​​the above-mentioned positive electrode active material is 0.1 m². 2 / g or more and 1.5m 2 / g or less.

6. The positive electrode active material for lithium secondary batteries according to claim 1, characterized in that, The average particle size of the aforementioned primary particles ranges from 0.01 μm to 20 μm.

7. The positive electrode active material for lithium secondary batteries according to claim 1, characterized in that, The average particle size of the aforementioned secondary particles ranges from 0.01 μm to 50 μm.

8. The positive electrode active material for lithium secondary batteries according to claim 1, characterized in that, The aforementioned positive electrode active material for lithium secondary batteries also includes a coating covering at least a portion of the surface of the aforementioned secondary particles. The coating described above comprises at least one oxide represented by the following chemical formula 2: [Chemical Formula 2] Li a A b O c Wherein, A is selected from at least one of Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, W, Ce, V, Ba, Ta, Sn, Hf, Gd, and Nd. 0≤a≤6, 0≤b≤4, 2≤c≤8.

9. A positive electrode for a lithium secondary battery, characterized in that, The positive electrode active material includes any one of claims 1 to 8.

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

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