Positive electrode, preparation method thereof and lithium secondary battery comprising positive electrode

Abstract

The invention provides a positive electrode, a preparation method thereof and a lithium secondary battery comprising the positive electrode. The positive electrode includes a positive electrode mixture layer including a single-particle-type positive electrode active material having a volume cumulative particle size distribution satisfying formula 1, the single-particle-type positive electrode active material having an average particle size of 4.0 [mu] m to 10.5 [mu] m; [Formula 1] 0.6 < = SL / (SR + SL) < = 0.8; sR is an area obtained by integrating a region on the right side of the particle size of the maximum peak in a logarithmic scale volume-accumulated particle size distribution curve of the single-particle positive electrode active material contained in the positive electrode, and SL is an area obtained by integrating a region on the right side of the particle size of the maximum peak in the logarithmic scale volume-accumulated particle size distribution curve of the single-particle positive electrode active material contained in the positive electrode. And an area for integrating the left region of the particle diameter of the maximum peak. The positive electrode adopts a single-particle positive electrode active material, so that relatively high electrode density can be obtained. The invention also provides a preparation method of the positive electrode and a lithium secondary battery comprising the positive electrode.

Description

Technical Field

[0001] This invention relates to a positive electrode, a method for preparing the same, and a lithium secondary battery containing the positive electrode, belonging to the field of lithium battery technology.

[0002] This invention claims priority to Korean Patent Application No. 10-2024-0085822, filed in Korea on June 28, 2024, the disclosure of which is incorporated herein by reference. Background Technology

[0003] To achieve higher energy density, lithium-ion batteries often employ high-nickel cathode active materials. Due to their high capacity, high-nickel cathode materials are considered an ideal choice for developing high-energy-density batteries. However, increasing the nickel content in the material also leads to problems such as decreased thermal stability, increased side reactions during electrochemical reactions, resulting in reduced battery life and increased gas generation.

[0004] To address the aforementioned issues, increasing energy density no longer relies solely on increasing nickel content, but rather on improving roll compaction density. Simultaneously, to improve thermal stability and reduce lifetime degradation and gas generation caused by side reactions during electrochemical processes, single-crystal cathode active materials with minimized interfaces can be employed. However, compared to secondary particle cathode materials, single-crystal cathode active materials suffer from longer lithium-ion diffusion paths; furthermore, their output performance is relatively poor due to factors such as surface rock salt structures induced by transition metal reduction. Therefore, using single-crystal cathode active materials alone presents certain challenges in practical applications, especially with commercially available high-nickel single-crystal materials (with a D50 particle size of less than 5 μm, classifying them as small particles). Due to their larger specific surface area, lower roll compaction and tap densities, certain risks are also encountered in slurry preparation and other process stages.

[0005] In summary, using high-nickel single-crystal active materials alone still faces many challenges. Currently, they are mostly used in bimodal hybrid systems in the form of small particles to overcome their shortcomings in practical preparation and application. However, when using a bimodal hybrid system composed of small-diameter single-crystal materials and large-diameter secondary particles, the generation of microcracks in the large-diameter secondary particles will gradually intensify with the increase of charge-discharge cycles, resulting in more internal interfaces and more side reactions. This adversely affects long-term lifetime characteristics and the effect of suppressing gas generation. Summary of the Invention

[0006] To solve the above-mentioned technical problems, the purpose of this invention is to provide a positive electrode with a high electrode density.

[0007] To achieve the above objectives, the present invention provides a positive electrode comprising a positive electrode mixture layer, wherein the positive electrode mixture layer comprises a single-particle positive electrode active material having a volume cumulative particle size distribution satisfying the following formula 1, wherein the average particle size of the single-particle positive electrode active material is 4.0 μm to 10.5 μm;

[0008] [Formula 1]

[0009] 0.6≤S L / (S R +S L ≤0.8

[0010] In Equation 1, the S R S refers to the area to the right of the maximum peak particle size in the logarithmically scaled cumulative particle size distribution curve of the single-particle positive electrode active material contained within the positive electrode. L It refers to the area of ​​the region to the left of the maximum peak particle size in the logarithmic scale volume cumulative particle size distribution curve of the single-particle positive electrode active material contained in the positive electrode.

[0011] According to a specific embodiment of the present invention, preferably, the volume cumulative particle size distribution of the single-particle positive electrode active material contained in the positive electrode mixture layer is S L / (S R +S L The range of the value is greater than or equal to 0.62 and less than or equal to 0.80, more preferably greater than or equal to 0.64 and less than or equal to 0.78, further preferably greater than or equal to 0.64 and less than or equal to 0.74, and even more preferably greater than or equal to 0.66 and less than or equal to 0.70.

[0012] According to a specific embodiment of the present invention, preferably, the single-particle positive electrode active material comprises 1 to 40 nodules, more preferably 1 to 30 nodules, even more preferably 1 to 25 nodules, and even more preferably 1 to 20 nodules.

[0013] According to a specific embodiment of the present invention, preferably, the average particle size of the agglomerates is 1 μm to 10 μm, more preferably 2 μm to 8 μm, and even more preferably 3 μm to 7 μm.

[0014] According to a specific embodiment of the present invention, preferably, the average particle size of the single-particle positive electrode active material is 4.2 μm to 8.5 μm, more preferably 4.6 μm to 8.0 μm, and even more preferably 4.6 μm to 7.0 μm.

[0015] According to a specific embodiment of the present invention, preferably, the single-particle positive electrode active material has a single-peak particle size distribution.

[0016] According to a specific embodiment of the present invention, preferably, the single-particle positive electrode active material comprises: a lithium nickel oxide in which the nickel content is 60 mol% or more in all metals other than lithium, more preferably 70 mol% or more, further preferably 80 mol% or more, and even more preferably 90 mol% or more.

[0017] According to a specific embodiment of the present invention, preferably, the single-particle positive electrode active material comprises a lithium-nickel oxide as shown in Formula 1:

[0018] [Chemical Formula 1]

[0019] Li 1+x [Ni a Co b M 1 c M 2 d O2

[0020] In chemical formula 1, M 1 Selected from one or a combination of two of Mn and Al, M 2 It is selected from one or more of the groups consisting of Zr, Y, B, V, W, Mg, Ce, Hf, Ta, La, Ti, Sr, Ba, F, P and S, and -0.2≤x≤0.2, 0.60≤a<1, 0<b<0.40, 0<c<0.40, 0≤d≤0.2.

[0021] According to a specific embodiment of the present invention, preferably, the single-particle positive electrode active material further comprises: a coating layer formed on the surface of the lithium nickel oxide, wherein the coating layer is selected from one or more elements selected from the group consisting of Co, Al, W, Ti, Mg, Zr, Y, Ba, Ca, Sr, Ta, Nb, P, B and Mo.

[0022] According to a specific embodiment of the present invention, preferably, the electrode density of the positive electrode is 3.55 g / cm³. 3 The above, more preferably 3.58 g / cm³ 3 The above is further preferred to be 3.60 g / cm³. 3 above.

[0023] According to a specific embodiment of the present invention, preferably, the content of the single-particle positive electrode active material in the positive electrode mixture layer is 50 wt% or more.

[0024] According to a specific embodiment of the present invention, preferably, the content of the single-particle positive electrode active material in the positive electrode mixture layer is 90 wt% or more.

[0025] According to a specific embodiment of the present invention, preferably, in the positive electrode active material used for the positive electrode compound layer, the content of the single-particle positive electrode active material is approximately 100 wt%.

[0026] The present invention also provides a method for preparing the above-mentioned positive electrode, which includes:

[0027] A cathode slurry was prepared using single-particle positive electrode active materials with an average particle size of 6.0-12.0 μm and a degree of monocrystallization of 0.10-0.60 as raw materials.

[0028] A positive electrode slurry is coated onto a positive electrode current collector, and then dried and calendered to form a positive electrode mixture layer, thus preparing the positive electrode.

[0029] According to a specific embodiment of the present invention, preferably, the linear pressing pressure of the calendering is 2.0-8.0 tons / cm.

[0030] According to a specific embodiment of the present invention, preferably, the single-particle positive electrode active material used as a raw material has a single-peak particle size distribution.

[0031] The present invention also provides a lithium secondary battery comprising the above-mentioned positive electrode provided by the present invention;

[0032] Negative electrode; and

[0033] Electrolytes.

[0034] The positive electrode provided by this invention uses a single-particle positive electrode active material, which can achieve high electrode density and good output performance. The lithium secondary battery containing this positive electrode can achieve high energy density. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the logarithmic scale volumetric cumulative particle size distribution curve involved in the present invention.

[0036] Figure 2 The logarithmic scale volume cumulative particle size distribution curves of the positive electrode active material in the positive electrode mixture layer of the examples and comparative examples are shown. Detailed Implementation

[0037] In order to provide a clearer understanding of the technical features, objectives and beneficial effects of the present invention, the technical solution of the present invention will now be described in detail below, but it should not be construed as limiting the scope of implementation of the present invention.

[0038] It should be noted that, unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0039] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0040] It should be understood that the terms “comprising,” “including,” and / or “containing” as used herein specify the presence of the stated features, integers, steps, components, or combinations thereof, but do not exclude the presence or addition of one or more other features, integers, steps, components, or combinations thereof.

[0041] In this invention, a "single particle" refers to a particle composed of 1 to 40 clumps; a composite particle composed of fewer than 40 clumps (i.e., 2 to 40 clumps) may also be called a "pseudo-single particle" or "quasi-single particle".

[0042] In this invention, a "nodule" is a particle unit that constitutes a single particle. When observed using a scanning electron microscope (SEM) at a magnification range of 5000 to 20000, a nodule can be a single crystal without visible grain boundaries, or a polycrystalline material without visible grain boundaries. The average particle diameter of the nodule can be calculated using the arithmetic mean of the particle diameters of each nodule measured by the scanning electron microscope (SEM). A "nodule" can also be referred to as a "primary particle."

[0043] In this invention, a "secondary particle" refers to a particle formed by the aggregation of multiple, for example, dozens to hundreds, of primary particles. Specifically, a secondary particle can be an aggregate of more than 40 primary particles.

[0044] In this invention, "particle" includes any one or all of single particles, pseudo-single particles, clumps (i.e., primary particles) and secondary particles.

[0045] In this invention, the "logarithmic-scale cumulative volumetric particle size distribution curve" refers to a cumulative volumetric particle size distribution curve with the logarithm of particle size (in μm) as the abscissa, which can be measured using laser diffraction. For example, after dispersing the positive electrode active material in a dispersion medium, the resulting mixture is introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT 3000), and then irradiated with ultrasound at approximately 28 kHz at an output power of 60 W to obtain a cumulative volumetric particle size distribution curve with the logarithm of particle size (in μm) as the abscissa.

[0046] “D 50 "" refers to the particle size that constitutes 50% of the total volumetric particle size distribution of the positive electrode active material (powder).50 Laser diffraction can be used for measurement. For example, after obtaining the logarithmically scaled volumetric cumulative particle size distribution curve, the average particle size D is obtained by obtaining the particle size corresponding to 50% of the cumulative volume. 50 .

[0047] “D mean "" refers to the average particle size of the nodules as measured using an electron backscatter diffraction (EBSD) spectrometer. The EBSD analysis was performed as follows: electrodes were fabricated using the positive electrode active material powder being measured. The electrodes were cut into cross-sections by ion milling (HITACHI IM-500, accelerating voltage: 6kV) prior to the calendering process, and the cross-sections were measured using an FE-SEM apparatus (JEOL JSM-7900F). In this case, measurements were performed at a scale of approximately 400 ± 10 primary particles under conditions of an accelerating voltage of 15kV and a WD of 15mm.

[0048] "Degree of monocrystallization" refers to the average particle size (D) of the nodules or primary particles that make up the cathode active material powder. mean ) and the average particle size (D) of the positive electrode active material powder 50 The ratio of (D) mean / D 50 This can also be referred to as "degree of monocrystalline".

[0049] Here, the positive electrode and the lithium secondary battery containing it may each include at least one technical feature and / or technical configuration described below, which may be combined in various ways.

[0050] positive electrode

[0051] A first aspect of the present invention provides a positive electrode comprising a positive electrode mixture layer, the positive electrode mixture layer comprising a single-particle positive electrode active material having a volume cumulative particle size distribution satisfying the following formula 1, wherein the average particle size of the single-particle positive electrode active material is 4.0 μm to 10.5 μm;

[0052] [Formula 1]

[0053] 0.6≤S L / (S R +S L ≤0.8

[0054] In Equation 1, the S R S refers to the area to the right of the maximum peak particle size in the logarithmically scaled cumulative particle size distribution curve of the single-particle positive electrode active material contained within the positive electrode. LIt refers to the area of ​​the region to the left of the maximum peak particle size in the logarithmic scale volume cumulative particle size distribution curve of the single-particle positive electrode active material contained in the positive electrode.

[0055] Wherein, the volumetric cumulative particle size distribution shown in Equation 1 is the volumetric cumulative particle size distribution of the single-particle positive electrode active material in the positive electrode mixture layer, that is, the volumetric cumulative particle size distribution after calendering. The average particle size of the single-particle positive electrode active material in the positive electrode mixture layer of the present invention is 4.0 μm to 10.5 μm (that is, the average particle size after calendering), and the volumetric cumulative particle size distribution of the single-particle positive electrode active material after calendering satisfies Equation 1.

[0056] In Equation 1, the region to the right of the maximum peak particle size in the logarithmic scale volumetric cumulative particle size distribution curve can be determined as follows: In the logarithmic scale volumetric cumulative particle size distribution curve, draw a straight line perpendicular to the horizontal axis at the particle size of the maximum peak. The portion to the right of this line is the region to the right of the maximum peak particle size; simultaneously, the portion to the left of this line is the region to the left of the maximum peak particle size. Figure 1 As shown.

[0057] In the positive electrode, when small-diameter single-particle positive electrode active material and large-diameter secondary-particle positive electrode active material are mixed, the formation of microcracks in the large-diameter secondary-particle positive electrode active material intensifies with increasing charge-discharge cycles, thereby affecting the long-term lifespan characteristics of the battery and its gas generation suppression effect. For the positive electrode of this invention, its positive electrode additive layer contains an average particle size (D... 50 The positive electrode active material has a particle size of 4.0 μm to 10.5 μm and a volume cumulative particle size distribution that satisfies Equation 1. This positive electrode active material has a single-peak particle size distribution that shows a single peak in the volume cumulative particle size distribution curve. It can obtain an electrode density similar to that of a bimodal positive electrode active material, thereby improving the output performance of the prepared lithium secondary battery. At the same time, it can also avoid the defects of microcracks in the positive electrode active material that affect the long-term life characteristics of the battery and the effect of suppressing gas generation.

[0058] In a specific embodiment of the present invention, the volume cumulative particle size distribution curve of the single-particle positive electrode active material in the positive electrode mixture layer exhibits a negative skewness characteristic.

[0059] In a specific embodiment of the present invention, the volume cumulative particle size distribution of the single-particle positive electrode active material contained in the positive electrode mixture layer is S L / (S R +S LA value greater than or equal to 0.6 and less than or equal to 0.8, with an average particle size of 4.0 μm to 10.5 μm, can provide a suitable lithium-ion diffusion path, which helps the efficient migration of lithium ions, thereby achieving excellent rate performance and overall conductivity. It can also achieve better stacking and compaction performance, which can improve the compaction density of the cathode, thereby helping to increase the content of cathode active material per unit volume and improve the energy density of lithium secondary batteries. It can also improve the thermal stability of cathode materials and reduce the lifespan decay and gas generation caused by side reactions during the electrochemical reaction process.

[0060] When the S of the single-particle positive electrode active material L / (S R +S L When the value is less than 0.6, it indicates that there is almost no particle breakage or a low degree of breakage during the electrode rolling process. The positive electrode binder layer contains a relatively large number of large-sized single-particle positive electrode active materials, resulting in a low compaction density and consequently a low electrode density. Furthermore, the lithium-ion diffusion path of these large single-particle positive electrode active materials is longer, leading to higher diffusion resistance. This increases the battery's internal resistance, hindering lithium-ion migration and consequently affecting the rate performance and conductivity of the lithium-ion secondary battery.

[0061] When the S of the single-particle positive electrode active material L / (S R +S L When the value is greater than 0.8, it indicates that excessive particle breakage occurred during the electrode rolling process, and there are relatively more small-sized single-particle positive electrode active materials in the positive electrode mixture layer. These single-particle positive electrode active materials have a large specific surface area and a large contact area with the electrolyte, which can easily lead to an increase in side reactions, resulting in problems such as lifespan decay and increased gas generation.

[0062] When the average particle size of the single-particle positive electrode active material is less than 4.0 μm, there are many small-sized single-particle positive electrode active materials in the positive electrode mixture layer. These single-particle positive electrode active materials have a relatively low compaction density, resulting in a relatively low electrode density. Moreover, these single-particle positive electrode active materials have a large specific surface area and a large contact area with the electrolyte, which easily leads to an increase in side reactions, resulting in problems such as lifespan degradation and increased gas generation.

[0063] When the average particle size of the single-particle positive electrode active material is greater than 10.5 μm, there are many large-sized single-particle positive electrode active materials in the positive electrode mixture layer obtained after calendering. The lithium-ion diffusion path inside the large-sized single-particle positive electrode active material is longer and has higher diffusion resistance, which is not conducive to the migration of lithium ions, and thus affects the rate performance and conductivity of lithium secondary batteries.

[0064] Specifically, the S-value of the volumetric cumulative particle size distribution of the single-particle positive electrode active material contained in the positive electrode mixture layer. L / (S R +S L The range of the value is preferably greater than or equal to 0.62 and less than or equal to 0.80, more preferably greater than or equal to 0.64 and less than or equal to 0.78, further preferably greater than or equal to 0.64 and less than or equal to 0.74, and even more preferably greater than or equal to 0.66 and less than or equal to 0.70.

[0065] Specifically, the average particle size (average particle size after calendering, D) of the single-particle positive electrode active material contained in the positive electrode mixture layer. 50 The preferred size is 4.2 μm to 8.5 μm, more preferably 4.6 μm to 8.0 μm, and even more preferably 4.6 μm to 7.0 μm.

[0066] Lithium nickel-based oxides existing in the form of secondary particles tend to aggregate from 41 to hundreds of primary particles, resulting in a large contact area with the electrolyte. This often leads to side reactions with the electrolyte, generating gas. Especially under high temperature and / or high voltage conditions, the amount of gas generated further increases, causing rapid battery degradation. In contrast, the single-particle positive electrode active material included in the positive electrode mixture layer of this invention has particles formed from fewer agglomerates. This results in a smaller internal interface within the particles and a smaller contact area with the electrolyte, thus reducing side reactions with the electrolyte and significantly decreasing gas generation. Therefore, when using single-particle lithium nickel-based oxides as the positive electrode active material, excellent service life can be achieved even under high voltage and high temperature conditions.

[0067] In a specific embodiment of the present invention, the single-particle lithium nickel-based oxide used as the positive electrode active material may contain fewer than 40 nodules, that is, the single-particle positive electrode active material contains 1 to 40 nodules, preferably 1 to 30 nodules, more preferably 1 to 25 nodules, and even more preferably 1 to 20 nodules. When the lithium nickel-based oxide is composed of more than 40 nodules, the probability of particle breakage during electrode manufacturing will increase significantly. At the same time, since the nodules will undergo volume expansion / contraction during charging and discharging, the probability of internal cracking will also increase, thereby reducing high-temperature service life and high-temperature storage performance.

[0068] In a specific embodiment of the present invention, the average particle size of the agglomerates can be between 1 μm and 10 μm, preferably between 2 μm and 8 μm, and more preferably between 3 μm and 7 μm. When the average particle size of the agglomerates is controlled within the above range, the degree of particle breakage during electrode manufacturing is appropriate, and the increase in resistance can be suppressed more effectively. In the present invention, the average particle size of the agglomerates represents the arithmetic mean of the agglomerate particle size measured in SEM images obtained by analyzing the positive electrode active material powder using a scanning electron microscope.

[0069] In a specific embodiment of the present invention, the lithium nickel-based oxide may have the following composition: a lithium nickel oxide in which the nickel content in all metals other than lithium is 60 mol% or more. When the nickel content in the lithium nickel-based oxide meets the above range, excellent capacity characteristics can be achieved.

[0070] In some embodiments, the lithium nickel-based oxide has the composition shown in Formula 1:

[0071] [Chemical Formula 1]

[0072] Li 1+x [Ni a Co b M 1 c M 2 d O2

[0073] In chemical formula 1, M 1 Selected from one or a combination of two of Mn and Al, preferably Mn or a combination of Mn and Al; M 2 The lithium nickel-based oxide of the present invention may optionally contain element M. It is selected from one or more elements grouped together with Zr, Y, B, V, W, Mg, Ce, Hf, Ta, La, Ti, Sr, Ba, F, P, and S. 2 When included in appropriate amounts, it can promote grain growth and / or improve crystal structure stability during sintering.

[0074] 1+x represents the molar ratio of lithium in lithium nickel-based oxides, where x can satisfy -0.2≤x≤0.2, -0.15≤x≤0.15, -0.1≤x≤0.1, or -0.05≤x≤0.05, etc. When the molar ratio of lithium satisfies the above ranges, a stable layered crystal structure can be formed.

[0075] a represents the molar ratio of nickel among all metals other than lithium in the lithium nickel-based oxide. Among them, a can satisfy 0.60 ≤ a ≤ 1.0, 0.7 ≤ a < 1.0, 0.8 ≤ a < 1.0, or 0.9 ≤ a < 1.0, etc. When the molar ratio of nickel satisfies the above range, excellent capacity characteristics can be achieved. In particular, if the molar ratio of nickel is 0.9 or more (i.e., 0.9 ≤ a < 1.0), more excellent capacity characteristics can be achieved.

[0076] b represents the molar ratio of cobalt among all metals other than lithium in the lithium nickel-based oxide. Among them, b can satisfy 0 < b < 0.40, 0 < b < 0.30, or 0 < b < 0.20, etc. When the molar ratio of cobalt satisfies the above range, good resistance characteristics and output characteristics can be achieved.

[0077] c represents the molar ratio of element M among all metals other than lithium in the lithium nickel-based oxide 1 where c can satisfy 0 < c < 0.40, 0 < c < 0.30, or 0 < c < 0.20, etc. When the molar ratio of element M 1 satisfies the above range, the positive electrode active material can obtain excellent structural stability.

[0078] d represents the molar ratio of element M among all metals other than lithium in the lithium nickel-based oxide 2 where d can satisfy 0 ≤ d ≤ 0.2, 0 ≤ d ≤ 0.15, or 0 ≤ d ≤ 0.1.

[0079] Specifically, the above lithium nickel-based oxide can have the following composition: a lithium nickel-based oxide in which the nickel content in all metals other than lithium is preferably 70 mol% or more, more preferably 80 mol% or more, and further preferably 90 mol% or more.

[0080] In a specific embodiment of the present invention, the single-particle type positive electrode active material further includes: a coating layer formed on the surface of the lithium nickel-based oxide.

[0081] In some embodiments, the material of the coating layer is preferably selected from one or more elements in the group consisting of Co, Al, W, Ti, Mg, Zr, Y, Ba, Ca, Sr, Ta, Nb, P, B, and Mo.

[0082] The coating layer preferably contains Co (cobalt). A cobalt-containing coating layer can suppress the degradation of the positive electrode active material during battery charging and discharging, thereby improving high-temperature lifespan characteristics. Based on 100 moles of lithium nickel-based oxide, the cobalt content in the coating layer can be from 1 mol% to 5 mol%, preferably from 1 mol% to 4 mol%, more preferably from 1 mol% to 3 mol%. If the cobalt content in the coating layer is less than 1 mol% (based on 100 moles of lithium nickel-based oxide), the high-temperature lifespan will be poor due to insufficient cobalt coating; if the cobalt content in the coating layer is greater than 5 mol%, the nickel content in all transition metals will be reduced, which may lead to a decrease in charging capacity. Furthermore, in addition to containing Co, the coating layer may further contain one or more of the following: Al, W, Ti, Mg, Zr, Y, Ba, Ca, Sr, Ta, Nb, P, B, and Mo. Preferably, the coating layer contains Co and B, which further enhances the capacity improvement effect.

[0083] In some embodiments, the shape of the coating layer includes dot-like and film-like regions. In other words, the coating layer includes dot-like regions and film-like regions.

[0084] In a specific embodiment of the present invention, the electrode density of the positive electrode is 3.55 g / cm³. 3 The above describes the positive electrode of this invention, which has a high electrode density, enabling an increase in the energy density of lithium-ion batteries. This allows for greater capacity within a given volume, while also contributing to improved structural stability and manufacturing efficiency of lithium-ion batteries. When the electrode density of the positive electrode is below 3.55 g / cm³... 3 This will lead to a decrease in the energy density of lithium secondary batteries, limited capacity, and increased manufacturing costs, while also posing risks to consistency and structural stability. Specifically, the electrode density of the positive electrode is preferably 3.58 g / cm³. 3 The above, more preferably 3.60 g / cm³ 3 The above, for example, can be 3.58-3.80 g / cm³. 3 3.58-3.75 g / cm³ 3 3.58-3.72 g / cm³ 3 3.60-3.80 g / cm³ 3 3.60-3.75 g / cm³ 3 3.60-3.72 g / cm³ 3 Or 3.60-3.69 g / cm³ 3 .

[0085] In a specific embodiment of the present invention, in the positive electrode active material used in the positive electrode compound layer, the content of the single-particle positive electrode active material is 50 wt% or more, preferably 60 wt% or more, 70 wt% or more, 80 wt% or more, 90 wt% or more, 95 wt% or more, 97.5 wt% or more, 98 wt% or more, or 99 wt% or more; more preferably, the content of the single-particle positive electrode active material is essentially 100 wt%. "Essentially 100 wt%" means that in the positive electrode compound layer, the positive electrode active material is essentially all single-particle positive electrode active material, but this does not exclude various unmentioned arbitrary components that do not have a substantial impact on the present invention, such as secondary particles introduced due to raw materials or processes.

[0086] In addition to the positive electrode mixture layer, the positive electrode of the present invention may also include a positive electrode current collector.

[0087] In some embodiments, the positive electrode current collector may comprise a highly conductive metal, provided that it is conductive without causing adverse chemical changes in the battery, and that the positive electrode additive layer can easily adhere to the positive electrode current collector; there are no particular limitations. Specifically, the positive electrode current collector may be made of stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc. Furthermore, the thickness of the positive electrode current collector can typically be from 3 μm to 500 μm, and minute irregularities may be formed on the surface of the current collector to improve the adhesion of the positive electrode active material. In addition, the positive electrode current collector can be used in various shapes, such as films, sheets, foils, meshes, porous bodies, foams, or nonwoven fabrics.

[0088] In some embodiments, in addition to the aforementioned single-particle positive electrode active material, the positive electrode compound layer may selectively include a conductive agent and / or a binder, if necessary. In this case, the content of the positive electrode active material may account for 80% to 99% of the total weight of the positive electrode compound layer, preferably 90% to 98%.

[0089] The conductive agent provides conductivity to the electrode. Any conductive agent can be used without particular limitation, as long as it has suitable electronic conductivity and does not cause adverse chemical changes in the battery. Specific examples of conductive agents may include: graphite, such as natural or artificial graphite; carbon materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black, and carbon fibers; metal powders or metal fibers, such as copper, nickel, aluminum, and silver; conductive tubes, such as carbon nanotubes; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or conductive polymers, such as polyphenylene derivatives; any one or a mixture of two or more of the above substances may be used. Based on the total weight of the positive electrode binder layer, the content of the conductive agent can be from 0.01% to 10%, preferably from 0.1% to 9%, more preferably from 0.1% to 5%.

[0090] The adhesive serves to improve the adhesion between the positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples of adhesives may include: polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, polymethyl methacrylate, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, polymers in which hydrogen is substituted by Li, Na, or Ca, or various copolymers of the above polymers, and any one or a mixture of two or more of the above substances may be used. Based on the total weight of the positive electrode binder layer, the adhesive content may be 1% to 30%, preferably 1% to 20%, more preferably 1% to 10%.

[0091] In a specific embodiment of the present invention, the preparation process of the positive electrode includes the following steps:

[0092] A cathode slurry was prepared using single-particle positive electrode active materials with an average particle size of 6.0-12.0 μm and a degree of monocrystallization of 0.10-0.60 as raw materials.

[0093] A positive electrode slurry is coated onto the positive electrode current collector, and then dried and calendered to form a positive electrode mixture layer, thereby preparing the positive electrode.

[0094] This invention, by appropriately adjusting the average particle size, particle size distribution, and degree of monocrystalline formation of the single-particle positive electrode active material used as raw material in the manufacture of the positive electrode additive layer, and in conjunction with the calendering process conditions, results in a positive electrode additive layer containing S with a volume cumulative particle size distribution. L / (S R +S LThe positive electrode active material has a particle value of ≥0.6 and ≤0.8, and an average particle size of 4.0μm to 10.5μm, thereby improving the electrode density of the positive electrode.

[0095] In some embodiments, calendering can be performed by rolling, for example using a rolling unit comprising two or more pairs of rollers, to prepare a positive electrode in sheet form.

[0096] The rolling equipment used in the calendering process can include a rolling unit, which can be a pair of oppositely arranged rollers, and multiple such roller pairs can be arranged continuously within the rolling unit. When multiple rollers are arranged continuously, the temperature and speed ratio (speed ratio of the roller pair) of each roller can be the same or different. Furthermore, the rolling process can be repeated within the rolling unit until the desired particle size distribution of the single-particle positive electrode active material is obtained, i.e., the volumetric cumulative particle size distribution satisfying Equation 1 is achieved. During the rolling process, samples can be taken at any time to test the particle size distribution and average particle size to confirm the particle size distribution of the single-particle positive electrode active material.

[0097] In some embodiments, the linear pressing pressure of the calender is preferably 2.0-8.0 tons / cm, for example, it can be 3.0-5.0 tons / cm, 3.0-4.0 tons / cm, 3.2-3.8 tons / cm, or 3.3-3.7 tons / cm.

[0098] In some embodiments, the average particle size of the single-particle positive electrode active material used as raw material (before calendering) is preferably 6.0 μm to 12.0 μm, more preferably 6.2 μm to 10.0 μm, and even more preferably 6.3 μm to 8.0 μm.

[0099] In some embodiments, the degree of monocrystallization of the single-particle positive electrode active material used as raw material is 0.10-0.60. Using a single-particle positive electrode active material with the above-mentioned degree of monocrystallization as raw material, the single-particle positive electrode active material in the positive electrode mixture layer obtained after calendering can have a smaller internal particle interface and a smaller contact area with the electrolyte. This reduces the occurrence of side reactions, lowers the lifespan degradation and gas generation caused by side reactions, and thus achieves excellent lifespan even under high voltage and high temperature conditions. When the degree of monocrystallization is higher than 0.60, the diffusion path of lithium ions is relatively longer, and the reduction of transition metals will lead to the formation of rock salt structures on the surface, resulting in a deterioration of the battery's output performance. Moreover, since single-crystallized particles are generally regular polyhedra or spheres, the stacking methods are relatively limited, and it is difficult for the particles to be tightly packed. Therefore, if the degree of monocrystallization of the single-particle positive electrode active material is high, it will lead to a decrease in the compaction density of the positive electrode sheet, a decrease in specific capacity, and thus affect the volumetric energy density of the battery. When the degree of monocrystallization is below 0.10, the single-particle positive electrode active material contains a large number of polycrystalline particles. These polycrystalline particles have numerous grain boundaries, resulting in high charge transfer resistance between the grain boundaries, which is detrimental to the electrochemical performance and thermal stability of lithium-ion batteries. Specifically, the degree of monocrystallization of the single-particle positive electrode active material used as raw material is 0.20-0.50, 0.30-0.50, 0.30-0.40, or 0.32-0.37.

[0100] In some embodiments, the single-particle positive electrode active material as a raw material has a single-peak particle size distribution, that is, the single-particle positive electrode active material as a raw material has a single-peak particle size distribution that shows a single peak in the volume cumulative particle size distribution curve.

[0101] In some embodiments, the above-mentioned positive electrode slurry is prepared by dissolving or dispersing a single-particle positive electrode active material as a raw material in a solvent. If necessary, binders and / or conductive agents may also be selectively dissolved or dispersed in the solvent, and further selectively, dispersants may be dissolved or dispersed in the solvent.

[0102] The solvent can be any solvent commonly used in the art, specifically including dimethyl sulfoxide (DMSO), isopropanol, N-methylpyrrolidone (NMP), dimethylformamide (DMF), acetone, and water, etc., and any one or a mixture of two or more of the above substances can be used. Considering the coating thickness and preparation yield of the cathode slurry, if the solvent can dissolve or disperse the cathode active material, conductive agent, binder, and / or dispersant, and enables the cathode slurry to have a viscosity that exhibits excellent thickness uniformity when used to prepare the cathode, then the amount of solvent used is sufficient.

[0103] In some embodiments, the positive electrode slurry can be coated onto the positive electrode current collector using a roller coating method.

[0104] Lithium secondary batteries

[0105] A second aspect of the present invention provides a lithium secondary battery comprising the positive electrode, negative electrode, and electrolyte described above. Specifically, the lithium secondary battery of the present invention may include a positive electrode, a negative electrode disposed facing the positive electrode, an electrolyte, and / or a separator disposed between the positive and negative electrodes, wherein the positive electrode is the same as described above. Furthermore, the lithium secondary battery may also include a battery container housing an electrode assembly composed of the positive electrode, negative electrode, and / or separator, and a sealing member for sealing the battery container.

[0106] In a lithium secondary battery, the negative electrode includes a negative electrode current collector and a negative electrode additive layer (or negative electrode active material layer) disposed on the negative electrode current collector.

[0107] There are no particular limitations on the negative electrode current collector, as long as it has high conductivity and will not cause adverse chemical changes in the battery. Materials such as copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, or silver, and aluminum-cadmium alloys can be used. Furthermore, the thickness of the negative electrode current collector can typically range from 3 μm to 500 μm, and similar to the positive electrode current collector, micro-protrusions can be formed on the surface of the current collector to improve the adhesion of the negative electrode active material. In addition, the negative electrode current collector can be used in various shapes, such as films, sheets, foils, meshes, porous bodies, foams, or nonwoven fabrics.

[0108] In addition to the negative electrode active material, the negative electrode binder layer may optionally include a binder and / or a conductive agent.

[0109] As negative electrode active materials, compounds capable of reversibly inserting and deintercalating lithium can be used. Specific examples of negative electrode active materials can include: carbonaceous materials, such as artificial graphite, natural graphite, graphitized carbon fibers, and amorphous carbon; metal compounds that can be alloyed with lithium, such as silicon (Si), aluminum (Al), tin (Sn), lead (Pb), zinc (Zn), bismuth (Bi), indium (In), magnesium (Mg), gallium (Ga), cadmium (Cd), Si alloys, Sn alloys, or Al alloys; and metal oxides that can be doped with lithium and de-doped with lithium, such as SiO₂. β(0<β<2), SnO2, vanadium oxide, and lithium vanadium oxide; or composites comprising metal compounds and carbonaceous materials, such as Si-C composites or Sn-C composites, and any one or a mixture of two or more of the above substances may be used. Furthermore, lithium metal films can be used as negative electrode active materials. Additionally, both low-crystallinity carbon and high-crystallinity carbon can be used as carbonaceous materials. Specific examples of low-crystallinity carbon can include soft carbon and hard carbon, while specific examples of high-crystallinity carbon can include irregular, planar, sheet-like, spherical, or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fibers, mesophase carbon microspheres, mesophase pitch, and high-temperature sintered carbon (e.g., coke derived from petroleum or coal tar pitch).

[0110] Based on the total weight of the negative electrode mixture layer, the content of the negative electrode active material can be 80% to 99%, 82% to 99%, or 84% to 99%.

[0111] The adhesive serves to improve the adhesion between the negative electrode active material particles and the adhesion between the negative electrode active material and the negative electrode current collector. Specific examples of adhesives may include: polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), nitrile rubber, fluororubber, or various copolymers of the above polymers, and any one or a mixture of two or more of the above substances may be used. Based on the total weight of the negative electrode adhesive layer, the adhesive content can be from 0.1% to 10%.

[0112] The role of the conductive agent is to further improve the conductivity of the negative electrode active material. Any conductive agent can be used without particular restrictions, as long as it has suitable electronic conductivity and does not cause adverse chemical changes in the battery. Specific examples of conductive agents can include: graphite, such as natural or artificial graphite; carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermally cracked carbon black; conductive tubes, such as carbon nanotubes; conductive fibers, such as carbon fibers and metal fibers; fluorinated carbon; metal powders, such as aluminum powder and nickel powder; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; conductive polymers, such as polyphenylene derivatives; and any one or a mixture of two or more of the above substances can be used. Based on the total weight of the negative electrode binder layer, the content of the conductive agent can be 1% to 30%, 1% to 20%, or 1% to 10%.

[0113] The negative electrode composite layer can be prepared by coating a negative electrode slurry composition onto a negative electrode current collector and then drying and calendering the coated negative electrode current collector, or by casting the negative electrode slurry composition onto a separate carrier and then pressing the film layer peeled off from the carrier onto the negative electrode current collector. The negative electrode slurry composition is prepared by dissolving or dispersing the aforementioned negative electrode active material in a solvent, and may also selectively dissolve or disperse binders and / or conductive agents in the solvent.

[0114] In lithium-ion batteries, the separator serves to separate the negative and positive electrodes and provide a path for lithium ions to move. Any separator can be used without particular limitation, as long as it is commonly used in lithium-ion batteries. Specifically, separators with high wettability to the electrolyte and low resistance to the movement of electrolyte ions can be used. Porous polymer membranes, such as those made from polyolefin polymers (e.g., ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers) or their two- or more-layered structures, can be used. Alternatively, conventional porous nonwoven fabrics, such as those formed from high-melting-point glass fibers or polyethylene terephthalate fibers, can be used. Furthermore, separators with coatings containing ceramic components or polymer materials can be used to ensure heat resistance or mechanical strength, and such coated separators with single-layer or multi-layer structures can be selectively used.

[0115] Furthermore, the electrolyte used in this invention may include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, or molten inorganic electrolytes that can be used to prepare lithium secondary batteries, but this invention is not limited thereto.

[0116] Specifically, the electrolyte of the present invention may contain an organic solvent and a lithium salt.

[0117] This invention does not impose any particular limitations on organic solvents, as long as they can serve as a medium through which ions participating in the electrochemical reaction of the battery can move. Specifically, as organic solvents, the following can be used: ester solvents, such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents, such as dibutyl ether and tetrahydrofuran; ketone solvents, such as cyclohexanone; aromatic hydrocarbon solvents, such as benzene and fluorobenzene; carbonate solvents, such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), and vinylene carbonate (VC); alcohol solvents, such as ethanol and isopropanol; nitrile solvents, such as R-CN (where R is a straight-chain, branched, or cyclic C2-C20 hydrocarbon group, which may include double-bonded aromatic rings or ether bonds); amide solvents, such as dimethylformamide; dioxolane, such as 1,3-dioxolane; sulfone solvents, etc.; and any one or a mixture of two or more of the above substances can be used. Among these solvents, carbonate solvents are preferred, and more preferably cyclic carbonates (e.g., ethylene carbonate and / or propylene carbonate) with high ionic conductivity and high dielectric constant (which can improve the charge and discharge performance of the battery) and mixtures of low-viscosity linear carbonates (e.g., one or more of ethyl methyl carbonate, dimethyl carbonate and diethyl carbonate).

[0118] This invention does not impose any particular limitations on lithium salts, as long as they are compounds capable of providing lithium ions for use in lithium secondary batteries. Specifically, the anion in the lithium salt can be selected from F... - Cl - ,Br - I - NO3 - N(CN)2 - BF4 - CF3CF2SO3 - (CF3SO2)2N - (FSO2)2N - CF3CF2(CF3)2CO - (CF3SO2)2CH - (SF5)3C - (CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - CH3CO2 - SCN - and (CF3CF2SO2)2N -At least one of the following groups. More specifically, as a lithium salt, one or more of the following can be used: 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 can be used in a concentration range of 0.1 M (mol / L) to 4.0 M, preferably 0.5 M to 3.0 M. By controlling the concentration of the lithium salt within the above range, the electrolyte can have suitable conductivity and viscosity, thus obtaining excellent electrolyte performance, and lithium ions can move efficiently.

[0119] To improve battery life characteristics, suppress battery capacity reduction, and increase battery discharge capacity, additives can be further included in the electrolyte. For example, additives may include one or more of the following: halogenated alkyl carbonate compounds such as ethylene difluorocarbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glycol ethers, hexamethylphosphoric triamine, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolides, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxyethanol, and aluminum trichloride, but the invention is not limited thereto. The content of the additive can be from 0.1% to 10% based on the total weight of the electrolyte.

[0120] As described above, since lithium secondary batteries including the positive electrode active material of the present invention have improved degassing performance and lifespan characteristics, and thus stably exhibit excellent discharge capacity, output characteristics and capacity retention, the lithium secondary batteries of the present invention are suitable for portable devices such as mobile phones, laptops, and digital cameras, as well as electric vehicles such as hybrid electric vehicles (HEVs).

[0121] Therefore, according to another specific embodiment of the present invention, the present invention provides a battery module including the lithium secondary battery as a unit cell and a battery pack including the battery module.

[0122] The battery module or battery pack can serve as a power source for at least one medium to large-sized device, including: power tools; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); and power storage systems.

[0123] The present invention will be specifically described below through examples and comparative examples, but the present invention is not limited to these examples. Of course, various modifications can be made within the scope of the essential points of the present invention.

[0124] Example 1

[0125] This embodiment provides a positive electrode, which is prepared through the following steps:

[0126] (1) Single-particle positive electrode active material powder (LiNi) with an average particle size of 6.5 μm, a single crystallization degree of 0.33, and an SL / (SR+SL) value of 0.5 determined by logarithmic-scale volume cumulative distribution plot was obtained. 0.8 Co 0.1 Mn 0.1 O2);

[0127] The above-mentioned single-particle positive electrode active material powder, conductive agent (carbon black, Super P) and binder (polyvinylidene fluoride, PVDF) were mixed in N-methylpyrrolidone at a weight ratio of 96:2:2 to prepare a positive electrode slurry.

[0128] (2) The positive electrode slurry is coated on both sides of the aluminum current collector (aluminum foil with a thickness of 15μm), dried at 130℃, and then rolled at a linear pressure of 3.5 tons / cm (by roller pressing) to form a positive electrode mixture layer, thereby manufacturing the positive electrode.

[0129] Example 2

[0130] This embodiment provides a positive electrode, wherein, in addition to using a single-particle positive electrode active material powder (LiNi) with an average particle size of 7.2 μm, a single crystallinity of 0.36, and an SL / (SR+SL) value of 0.5 determined by logarithmic-scale volume cumulative distribution plot, it utilizes a single-particle positive electrode active material powder. 0.8 Co 0.1 Mn 0.1 Except for O2), everything else is the same as in Example 1.

[0131] Comparative Example 1

[0132] This comparative example provides a positive electrode in which a single-particle positive electrode active material powder (LiNi) with an average particle size of 4.0 μm, a degree of monocrystallization of 0.5, and an SL / (SR+SL) value of 0.5 as determined by logarithmic-scale volume accumulation distribution plot is used. 0.8 Co 0.1 Mn 0.1 Except for O2), everything else is the same as in Example 1.

[0133] Comparative Example 2

[0134] This comparative example provides a positive electrode in which a single-particle positive electrode active material powder (LiNi) with an average particle size of 7.0 μm, a degree of single crystallization of 0.61, and an SL / (SR+SL) value of 0.5 as determined by logarithmic-scale volume cumulative distribution plot is used. 0.8 Co 0.1 Mn0.1 Except for O2), everything else is the same as in Example 1.

[0135] Comparative Example 3

[0136] This comparative example provides a positive electrode in which a single-particle positive electrode active material powder (LiNi) with an average particle size of 9.0 μm, a degree of single crystallization of 0.04, and an SL / (SR+SL) value of 0.5 as determined by logarithmic-scale volume cumulative distribution plot is used. 0.8 Co 0.1 Mn 0.1 Except for O2), everything else is the same as in Example 1.

[0137] Comparative Example 4

[0138] This comparative example provides a positive electrode in which a single-particle positive electrode active material powder (LiNi) with an average particle size of 4.0 μm, a degree of single crystallization of 0.38, and an SL / (SR+SL) value of 0.5 as determined by logarithmic-scale volume cumulative distribution plot is used. 0.8 Co 0.1 Mn 0.1 Except for O2), everything else is the same as in Example 1.

[0139] Experimental Example 1

[0140] The volumetric cumulative particle size distribution and electrode density of the positive electrode active material in the positive electrode mixture layers prepared in Examples 1-2 and Comparative Examples 1-4 were measured according to the following method, and the SL / (SR+SL) value was calculated using the measured cumulative distribution map (logarithmic scale). The measurement results are shown in Table 1 and Figure 2 As shown.

[0141] (1) Measurement method of volumetric cumulative particle size distribution

[0142] Positive electrode composite powder was obtained by scraping a positive electrode current collector layer from the positive electrode prepared in the examples and comparative examples. Then, the obtained positive electrode composite powder was heat-treated at 600°C for 10 hours to remove the binder and conductive agent, resulting in positive electrode active material powder. The positive electrode active material powder was then introduced into a laser diffraction particle size analyzer (e.g., Microtrac MT 3000) and irradiated with an ultrasonic wave of about 28 kHz at an output power of 60 W to obtain a logarithmic scale volume accumulation distribution map. Subsequently, the SL / (SR+SL) value of the positive electrode active material in the corresponding positive electrode composite layer was calculated using the measured logarithmic scale volume accumulation distribution map.

[0143] (2) Electrode density (g / cm³) 3 Measurement methods

[0144] The positive electrodes prepared in the examples and comparative examples were stamped into circles with a diameter of 1.4 cm to obtain stamped positive electrodes corresponding to each example and comparative example. The weight Wt (in g) and thickness Tt (in cm) of the stamped positive electrode were measured. Then, the weight Wc (in g) and thickness Tc (in cm) of the positive electrode current collector in the above-mentioned stamped positive electrode were measured. Then, Wc was subtracted from Wt to ​​calculate the weight W (in g) of the positive electrode mixture layer, and Tc was subtracted from Tt to calculate the thickness T (in cm) of the positive electrode mixture layer. Subsequently, the electrode density was calculated using the above weight W and thickness T. Electrode density = W / (π×0.7) 2 ×T), unit is g / cm³ 3 .

[0145] Table 1

[0146]

[0147] According to Table 1 and Figure 2 The experimental results shown indicate that:

[0148] The volumetric cumulative particle size distribution (S) of the single-particle positive electrode active material in the positive electrode mixture layer obtained in Examples 1 and 2 after calendering L / (S R +S L The α value is in the range of 0.6-0.8, with a single-peak distribution. Moreover, the average particle size of the single-particle positive electrode active material after calendering is in the range of 4.0μm to 10.5μm. The resulting positive electrode has a high electrode density, and the prepared lithium secondary battery can have a high energy density.

[0149] The volumetric cumulative particle size distribution S of the single-particle positive electrode active material in the positive electrode mixture layer obtained in Comparative Example 1 after calendering L / (S R +S L The low value indicates that the particles are not broken down enough during the calendering process. In addition, the average particle size and compaction density of the single-particle positive electrode active material after calendering are relatively low, which leads to a low electrode density of the obtained positive electrode and a relatively low energy density of the prepared lithium secondary battery.

[0150] The volumetric cumulative particle size distribution S of the single-particle positive electrode active material in the positive electrode mixture layer of Comparative Example 2 after calendering L / (S R +S L A value less than 0.6 indicates that the particles were not sufficiently broken during the rolling process, and a relatively large number of large particles were retained. This results in a relatively low electrode density of the obtained positive electrode, and consequently, a relatively low energy density of the prepared lithium secondary battery.

[0151] The volumetric cumulative particle size distribution S of the single-particle positive electrode active material in the positive electrode mixture layer of Comparative Example 3 after calendering L / (S R +S L The value is greater than 0.8 and has a bimodal distribution, which indicates that the particles were excessively broken during the rolling process, resulting in a relatively large number of small particles and a low compaction density. This leads to a relatively low electrode density of the obtained positive electrode and a relatively low energy density of the prepared lithium secondary battery.

[0152] The volumetric cumulative particle size distribution (S) of the single-particle positive electrode active material in the positive electrode mixture layer of Comparative Example 4 after calendering L / (S R +S L The value is in the range of 0.6-0.8. However, the average particle size of the single-particle positive electrode active material after calendering is relatively low. This indicates that there are many small particles in the obtained positive electrode mixture layer, and the compaction density is relatively low. As a result, the electrode density of the obtained positive electrode is also relatively low, and the energy density of the prepared lithium secondary battery will also be relatively low.

[0153] Experimental Example 2

[0154] Preparation of lithium secondary batteries:

[0155] A polyethylene separator was sandwiched between the positive electrode and the lithium metal electrode prepared in Example 1 to obtain an electrode assembly. The electrode assembly was then installed into a battery case and injected with electrolyte to prepare a half cell.

[0156] The electrolyte is prepared by dissolving 1M LiPF6 in a mixed organic solvent (composed of ethylene carbonate, dimethyl carbonate, and diethyl carbonate) in a volume ratio of 3:4:3.

[0157] Following the above method, the positive electrodes provided in Example 2 and Comparative Examples 1-4 were respectively used to make lithium secondary batteries.

[0158] The initial resistance and gas generation of the above half-cell were measured using the following method. The measurement results are shown in Table 2.

[0159] (1) Initial resistance (Ω):

[0160] The half-cells prepared above were activated under constant current (CC) at 0.1C and then degassed.

[0161] At 25°C, each half-cell was charged using constant current-constant voltage (CC-CV) charging. It was charged to 4.30V with a constant current of 0.33C, and then discharged to 50% SOC with a constant current of 0.33C. After that, a discharge pulse of 1.0C was applied for 10 seconds, and the voltage drop was measured to obtain the initial resistance value.

[0162] (2) Gas generation (μL):

[0163] Each half-cell was charged to 4.3V in constant current-constant voltage mode (charging cutoff condition: 1 / 20C); then, the charged battery was disassembled to obtain 2 positive electrodes and 2 separators. The positive electrodes and separators were alternately stacked at the bottom of the button cell, electrolyte was injected, and the button cell was reassembled; then, the battery was stored at 70°C for 4 weeks, and the amount of gas produced was measured using gas chromatography-mass spectrometry (GC-MS).

[0164] Table 2

[0165] Initial resistance (Ω) Gas production amount (μL) Example 1 1.48 52 Example 2 1.51 48 Comparative Example 1 1.48 79 Comparative Example 2 1.72 49 Comparative Example 3 1.45 126 Comparative Example 4 1.43 91

[0166] The experimental results shown in Table 2 indicate that:

[0167] The volumetric cumulative particle size distribution (S) of the single-particle positive electrode active material in the positive electrode mixture layer obtained in Examples 1 and 2 after calendering L / (S R +S L The ) value is in the range of 0.6-0.8 and has a single peak distribution. Moreover, the average particle size of the single-particle positive electrode active material after calendering is in the range of 4.0μm to 10.5μm. The resulting lithium secondary battery has a low initial resistance, thus having good conductivity and output performance. Furthermore, the amount of gas generated is relatively low after long-term storage, resulting in a long lifespan.

[0168] Although the initial resistance of the lithium secondary battery obtained in Comparative Example 1 is relatively low, the average particle size and compaction density of the single-particle positive electrode active material in the positive electrode mixture layer are relatively low after calendering. After long-term storage, the amount of gas generated is relatively high, which indicates that more side reactions occurred during storage, which will lead to a decrease in the life of the lithium secondary battery.

[0169] The volumetric cumulative particle size distribution S of the single-particle positive electrode active material in the positive electrode mixture layer of Comparative Example 2 after calendering L / (S R +S LA value less than 0.6 indicates that the particles were not sufficiently broken during the rolling process, and a relatively large number of large particles were still retained. Although the resulting lithium secondary battery has a low gas production after long-term storage, the initial resistance of the lithium secondary battery is relatively high, which is not conducive to the migration of lithium ions. This will result in low activity and poor output performance of the lithium secondary battery.

[0170] The volumetric cumulative particle size distribution S of the single-particle positive electrode active material in the positive electrode mixture layer of Comparative Example 3 after calendering L / (S R +S L The value is greater than 0.8 and has a bimodal distribution, which indicates that the particles were excessively broken during the rolling process, resulting in a relatively large number of small particles and a low compaction density. Although the initial resistance of the resulting lithium secondary battery is relatively low, the amount of gas generated is very high after long-term storage. This indicates that many side reactions occurred during storage, which will lead to a serious degradation of the lifespan of the lithium secondary battery.

[0171] The volumetric cumulative particle size distribution (S) of the single-particle positive electrode active material in the positive electrode mixture layer of Comparative Example 4 after calendering L / (S R +S L The value is within the range of 0.6-0.8. However, the average particle size of the single-particle positive electrode active material after calendering is relatively low, which indicates that there are many small particles in the obtained positive electrode mixture layer. Although the initial resistance of the obtained lithium secondary battery is relatively low, the amount of gas generated is relatively high after long-term storage. This indicates that many side reactions occurred during storage, which will lead to the degradation of the lithium secondary battery life.

[0172] Based on the above, it can be seen that when the single-particle positive electrode active material in the positive electrode mixture layer obtained after calendering satisfies the "S" of the volume cumulative particle size distribution of the single-particle positive electrode active material... L / (S R +S L When the α value is greater than or equal to 0.6 and less than or equal to 0.8, and the average particle size is within the range of 4.0 μm to 10.5 μm, the obtained positive electrode can have a high electrode density, and the prepared lithium secondary battery has a low initial resistance, good output performance, and can reduce the occurrence of side reactions. It also has a low amount of gas generation during long-term storage and good stability.

Claims

1. A positive electrode comprising a positive electrode mixture layer, the positive electrode mixture layer comprising a single particle type positive electrode active material having a volume cumulative particle size distribution satisfying the following formula 1, the single particle type positive electrode active material having an average particle diameter of 4.0 μm to 10.5 μm; [Formula 1] 0 < x < 1 The single particle type positive electrode active material comprises 1 to 40 agglomerates. 0.6 ≤ S L / (S R +S L ) ≤ 0.8 In formula 1, the S R means an area in a log scale volume cumulative particle size distribution curve, which is integrated with respect to a right side region of a particle diameter of a maximum peak, of the single particle type positive electrode active material contained in the positive electrode L means an area in a log scale volume cumulative particle size distribution curve, which is integrated with respect to a left side region of a particle diameter of a maximum peak, of the single particle type positive electrode active material contained in the positive electrode 2. The positive electrode according to claim 1, wherein The agglomerate has an average particle diameter of 1 μm to 10 μm.

3. The positive electrode according to claim 2, wherein The single particle type positive electrode active material has an average particle diameter of 4.2 μm to 8.5 μm.

4. The positive electrode according to claim 1, wherein The single particle type positive electrode active material has a unimodal particle size distribution.

5. The positive electrode according to claim 1, wherein The single particle type positive electrode active material comprises a lithium nickel-based oxide having a nickel content of 60 mol% or more among all metals other than lithium.

6. The positive electrode according to claim 1, wherein The single particle type positive electrode active material comprises a lithium nickel-based oxide represented by Chemical Formula 1:

7. The positive electrode according to claim 1, wherein [Chemical Formula 1] Li1-xNixO2 The single particle type positive electrode active material further comprises a coating layer formed on a surface of the lithium nickel-based oxide, wherein the coating layer is selected from one or two or more elements selected from the group consisting of Co, Al, W, Ti, Mg, Zr, Y, Ba, Ca, Sr, Ta, Nb, P, B, and Mo. Li 1+x [Ni a Co b M 1 c M 2 d ]O2 wherein, in Chemical Formula 1, M 1 one or a combination of two selected from the group consisting of Mn and Al, M 2 one or two or more selected from the group consisting of Zr, Y, B, V, W, Mg, Ce, Hf, Ta, La, Ti, Sr, Ba, F, P, and S, and -0.2≤x≤0.2, 0.60≤a<1, 0 0.40, 0≤d≤0.

2.

8. The positive electrode according to claim 7, wherein In the positive electrode mixture layer, the single particle type positive electrode active material is contained in an amount of 50 wt% or more.

9. The positive electrode according to claim 1, wherein The electrode density of the positive electrode is 3.55 g / cm 3 The above.

10. The positive electrode according to claim 1, wherein In the positive electrode mixture layer, the single particle type positive electrode active material is contained in an amount of 90 wt% or more.

11. The positive electrode according to claim 1, wherein In the positive electrode active material used for the positive electrode mixture layer, the single particle type positive electrode active material is contained in an amount of substantially 100 wt%.

12. The positive electrode according to claim 1, wherein 13. A method for producing the positive electrode according to any one of claims 1 to 12, comprising: using a single particle type positive electrode active material having an average particle diameter of 6.0 to 12.0 μm and a degree of single crystallization of 0.10 to 0.60 as a raw material, to prepare a positive electrode slurry; coating the positive electrode slurry on a positive electrode current collector, followed by drying and calendering to form a positive electrode mixture layer, thereby producing the positive electrode. The calendering is performed at a linear pressure of 2.0 to 8.0 tons / cm.

14. The method for producing a positive electrode according to claim 13, wherein The single particle type positive electrode active material used as the raw material has a unimodal particle size distribution.

15. The method for producing a positive electrode according to claim 13, wherein 16. A lithium secondary battery comprising the positive electrode according to any one of claims 1 to 12; a negative electrode; and an electrolyte. ​

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