Composite positive electrode active material, method for preparing same, positive electrode layer comprising same, and lithium secondary battery

Abstract

Provided are a composite positive electrode active material, a method for preparing the same, a positive electrode layer comprising the same, and a lithium secondary battery. The composite positive electrode active material includes a core portion of a lithium transition metal compound and a shell portion of a sulfide-based solid electrolyte having a cohesion index of about 37 to 46. The shell constitutes about 2% to 10% by weight of the composite material, having a thickness of about 50 nm to 500 nm and a planar density of about 0.05 mg / cm < 2 > to 0.3 mg / cm < 2 > as determined by X-ray fluorescence spectrometry. The method of preparation includes coating the core with a sulfide-based solid electrolyte by controlled mixing, stirring, and heat treatment, ensuring uniform and consistent coating quality. The composite material enhances the performance of the lithium secondary battery by improving the stability, ionic conductivity and overall electrochemical performance of the positive electrode.

Description

Technical Field

[0001] This disclosure relates to a composite cathode active material for lithium-ion secondary batteries with excellent coating quality and a method for preparing the same. The material comprises a core of a lithium transition metal compound and a shell of a sulfide-based solid electrolyte, optimized for stability and conductivity. The disclosed method involves precise mixing, stirring, and heat treatment to ensure uniform coating, enhancing battery performance and efficiency for applications in electric vehicles and portable electronic devices. Background Technology

[0002] Rechargeable and rechargeable batteries are widely used in a variety of applications, ranging from small electronic devices such as mobile phones and laptops to large transportation vehicles such as hybrid and electric vehicles. With increasing demand for these applications, there is a growing need to develop rechargeable batteries with enhanced stability and higher energy density.

[0003] Most conventional secondary batteries are based on organic solvents (organic liquid electrolytes), which limits their ability to improve stability and energy density.

[0004] Meanwhile, all-solid-state batteries using inorganic solid electrolytes have attracted considerable attention due to their safety and simplicity. By eliminating organic solvents, these batteries offer a safer alternative, allowing for the more direct production of batteries with enhanced stability and performance.

[0005] All-solid-state batteries using sulfide-based solid electrolytes may experience degraded battery characteristics due to interfacial reactions between the sulfide-based solid electrolyte and the positive electrode active material. To address this issue, the surface of the positive electrode active material is typically coated with a stabilizing material. However, most existing research focuses primarily on the reactivity of the positive electrode active material and the sulfide-based solid electrolyte, often neglecting the crucial factors of resistance and reaction rate between them, which significantly affect the actual performance and operation of the battery.

[0006] The information disclosed in this background section is intended only to enhance the understanding of the background technology of this disclosure, and therefore may contain information that does not constitute prior art known to a person skilled in the art in this country. Summary of the Invention

[0007] This disclosure aims to solve the aforementioned problems related to the prior art, and one object of this disclosure is to provide a composite positive electrode active material for lithium secondary batteries and a method for preparing the same, which expands the channel for lithium ions from the positive electrode active material to the sulfide-based solid electrolyte to reduce the resistance between the two materials.

[0008] Another object of this disclosure is to provide a composite positive electrode active material for lithium secondary batteries and a method for preparing the same, wherein the composite positive electrode active material has a uniform coating by coating the positive electrode active material with a sulfide-based solid electrolyte having weak cohesive force to prevent aggregation of the sulfide-based solid electrolyte.

[0009] Another objective of this disclosure is to provide new analytical parameters that enable three-dimensional identification and analysis of coatings containing sulfide-based solid electrolytes based on X-ray fluorescence analysis.

[0010] The purposes of this disclosure are not limited to those described above. Other purposes of this disclosure will become apparent from the following description.

[0011] In one aspect, this disclosure provides a composite positive electrode active material for lithium secondary batteries, the composite positive electrode active material comprising a core containing a lithium transition metal compound and a shell containing a sulfide-based solid electrolyte, wherein the sulfide-based solid electrolyte has a cohesion index of not less than about 37 and less than about 46.

[0012] The composite positive electrode active material may comprise 90% to 98% by weight of a core and 2% to 10% by weight of a shell. In other words, the core constitutes about 90% to 98% by weight of the composite active material, and the shell constitutes about 2% to 10% by weight of the composite positive electrode active material.

[0013] The thickness of the shell, which can be determined by irradiating the composite cathode active material with X-rays via X-ray fluorescence spectroscopy (XRF) and measuring the intensity of the X-rays in response to the sulfur element emitted from the composite cathode active material, can be from about 50 nm to 500 nm.

[0014] The thickness of the shell can be determined by irradiating multiple measurement points of the composite positive electrode active material with X-rays and measuring the intensity of the X-rays originating from sulfur.

[0015] The planar density of the shell, determined by irradiating the composite cathode active material with X-rays via X-ray fluorescence spectroscopy (XRF) and measuring the intensity of the X-rays in response to the sulfur emitted from the composite cathode active material, can be approximately 0.05 mg / cm³. 2 Up to 0.3 mg / cm 2 .

[0016] The planar density of the shell can be determined by irradiating multiple measurement points of the composite positive electrode active material with X-rays and measuring the intensity of the X-rays originating from sulfur.

[0017] In another aspect, this disclosure provides a method for preparing a composite positive electrode active material, the method comprising preparing a sulfide-based solid electrolyte and coating a lithium transition metal compound with the sulfide-based solid electrolyte to obtain a composite positive electrode active material comprising a core containing a lithium transition metal compound and a shell containing a sulfide-based solid electrolyte, wherein the sulfide-based solid electrolyte has a cohesive index of not less than about 37 and less than about 46.

[0018] The preparation of sulfide-based solid electrolytes may include preparing starting materials, reacting the starting materials to obtain intermediate materials, and heat-treating the intermediate materials to obtain sulfide-based solid electrolytes.

[0019] Sulfide-based solid electrolytes can have an average particle size (D50) of about 2 μm or less.

[0020] The preparation of sulfide-based solid electrolytes may include heat-treating intermediate materials at temperatures above about 400°C and below about 500°C.

[0021] The preparation of composite positive electrode active materials may include mixing a sulfide-based solid electrolyte with a lithium transition metal compound at a first rate to obtain a mixture, stirring the mixture at a second rate higher than the first rate to disperse the mixture, and stirring the dispersed mixture at a third rate higher than the second rate to coat the lithium transition metal compound with the sulfide-based solid electrolyte.

[0022] The third speed can be from approximately 2,000 rpm to 4,000 rpm.

[0023] Lithium transition metal compounds can be coated with sulfide-based solid electrolytes by stirring the dispersed mixture at a third rate for a period of time longer than about 10 minutes and not longer than about 30 minutes.

[0024] Other aspects and preferred embodiments of this disclosure are discussed below.

[0025] Also provided is a composite positive electrode active material for lithium secondary batteries, comprising: a core containing a lithium transition metal compound; and a shell containing a sulfide-based solid electrolyte. The sulfide-based solid electrolyte has a cohesion index of about 40 to 45. The core constitutes about 90% to 98% by weight of the composite positive electrode active material; and the shell constitutes about 2% to 10% by weight of the composite positive electrode active material.

[0026] The shell thickness can range from approximately 50 nm to 500 nm, and the shell planar density can be approximately 0.05 mg / cm³. 2 Up to 0.3 mg / cm 2 .

[0027] As discussed, the method and system appropriately include the use of a controller or processor.

[0028] It also provides a positive electrode layer for lithium secondary batteries, which includes the aforementioned composite positive electrode active material.

[0029] A lithium secondary battery including the aforementioned positive electrode layer.

[0030] In another embodiment, a vehicle including the apparatus disclosed herein is provided. Attached Figure Description

[0031] The above and other features of this disclosure will now be described in detail with reference to certain exemplary embodiments of this disclosure illustrated in the accompanying drawings, in which the drawings are given by way of illustration only and therefore do not limit the disclosure, and wherein:

[0032] Figure 1 A lithium secondary battery according to the present disclosure is shown;

[0033] Figure 2 The positive electrode layer according to this disclosure is shown;

[0034] Figure 3 A composite positive electrode active material according to a first embodiment of the present disclosure is shown;

[0035] Figure 4 A composite positive electrode active material according to a second embodiment of the present disclosure is shown;

[0036] Figure 5 An exemplary apparatus for measuring the flow angle is shown;

[0037] Figure 6 A reference diagram is shown to illustrate the method used to measure the flow angle;

[0038] Figure 7 A reference diagram is shown to illustrate the X-ray fluorescence spectroscopy performed in this disclosure;

[0039] Figure 8A The results of scanning electron microscopy of the composite positive electrode active material according to Example 1 are shown;

[0040] Figure 8B The results of energy-dispersive X-ray spectroscopy of the composite positive electrode active material according to Example 1 are shown;

[0041] Figure 9A The results of scanning electron microscopy of the composite positive electrode active material according to Comparative Example 1 are shown;

[0042] Figure 9B The results of energy-dispersive X-ray spectroscopy of the composite positive electrode active material according to Comparative Example 1 are shown;

[0043] Figure 10A The results of scanning electron microscopy of the composite positive electrode active material according to Comparative Example 2 are shown;

[0044] Figure 10B The results of energy-dispersive X-ray spectroscopy of the composite positive electrode active material according to Comparative Example 2 are shown.

[0045] Figure 11A The results of scanning electron microscopy of the composite positive electrode active material according to Comparative Example 3 are shown;

[0046] Figure 11B The results of energy-dispersive X-ray spectroscopy of the composite positive electrode active material according to Comparative Example 3 are shown;

[0047] Figure 12 The measurement results of capacity retention of lithium secondary batteries using composite positive electrode active materials including Example 2, Example 3, Comparative Example 4 and Comparative Example 5 are shown.

[0048] Figure 13 The results of X-ray fluorescence spectroscopy for the composite positive electrode active material according to Example 3 are shown;

[0049] Figure 14 The results of X-ray fluorescence spectroscopy for the composite positive electrode active material according to Example 4 are shown;

[0050] Figure 15 The results of X-ray fluorescence spectroscopy for the composite positive electrode active material according to Example 6 are shown;

[0051] Figure 16 The results of X-ray fluorescence spectroscopy for the composite positive electrode active material according to Comparative Example 6 are shown.

[0052] Figure 17 The thickness and planar density of the shell portion of the composite positive electrode active material according to Examples 3 to 6 are shown; and

[0053] Figure 18 The capacity retention of lithium secondary batteries comprising composite positive electrode active materials according to Examples 3 to 6 and Comparative Example 6 is shown. Detailed Implementation

[0054] The above-described objects, as well as other objects, features, and advantages, will become clearer from the following preferred embodiments, with reference to the accompanying drawings. However, this disclosure is not limited to these embodiments and may be embodied in different forms. The embodiments are provided merely to offer a comprehensive and complete understanding of the disclosure and to fully inform those skilled in the art of the technical concepts of this disclosure.

[0055] As used herein, the term "all-solid-state battery" refers to a rechargeable secondary battery that includes a solid electrolyte, which may include other electrolytic components for transporting ions between the electrodes of the battery.

[0056] Throughout the description of the accompanying drawings, the same reference numerals denote the same elements. In the drawings, the dimensions of the structures may be enlarged for clarity. It will be understood that although the terms “first,” “second,” etc., may be used herein to describe various elements, these elements should not be construed as being limited by these terms, which are used only to distinguish one element from another. For example, within the scope defined by this disclosure, a “first” element may be referred to as a “second” element, and similarly, a “second” element may be referred to as a “first” element. Unless the context clearly indicates otherwise, the singular form is intended to include the plural form as well.

[0057] It should also be understood that when the terms "comprising" and / or "having" are used in this specification, they specify the presence of the stated features, integers, steps, operations, elements, components, or combinations thereof, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, or combinations thereof. Furthermore, it will be understood that when an element such as a layer, film, region, or substrate is referred to as being "on" another element, the element may be directly on the other element, or there may be intermediate elements present. It should also be understood that when an element such as a layer, film, region, or substrate is referred to as being "below" another element, the element may be directly below the other element, or there may be intermediate elements present.

[0058] It should be understood that the term "vehicle" or "of a vehicle" or other similar terms as used herein include motor vehicles in a broad sense, such as passenger cars, including sport utility vehicles (SUVs), buses, trucks, various commercial vehicles, watercraft (including various boats and vessels), aircraft, etc., and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles, and other alternative fuel vehicles (e.g., fuels derived from resources other than petroleum). As mentioned herein, a hybrid vehicle is a vehicle with two or more power sources, such as a gasoline-powered and an electric vehicle.

[0059] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit this disclosure. As used herein, unless the context clearly indicates otherwise, the singular forms “a,” “an,” and “the” are intended to also include the plural forms. These terms are intended only to distinguish one component from another, and these terms do not limit the nature, order, or sequence of the constituent components. As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items. Furthermore, the terms “unit,” “uniter,” “unitor,” and “module” described in the specification mean a unit for performing at least one function and operation, and can be implemented by hardware components or software components and combinations thereof.

[0060] Although the exemplary implementation is described as using multiple units to perform the exemplary process, it should be understood that the exemplary process can also be performed by one or more modules. Furthermore, it should be understood that the term controller / control unit refers to a hardware device that includes a memory and a processor and is specifically programmed to perform the processes described herein. The memory is configured to store modules, and the processor is specifically configured to execute said modules to perform one or more processes further described below.

[0061] Furthermore, the control logic of this disclosure may be embodied in a non-volatile computer-readable medium containing executable program instructions that can be executed by a processor, controller, or the like. Examples of computer-readable media include, but are not limited to, ROM, RAM, optical disc (CD)-ROM, magnetic tape, floppy disk, flash drive, smart card, and optical data storage devices. The computer-readable medium may also be distributed across a network-coupled computer system, enabling it to be stored and executed in a distributed manner, for example, via a telematics server or a controller area network (CAN).

[0062] Unless otherwise specified or obvious from the context, the term “about” as used herein shall be understood to mean within the normal tolerances in the field, such as within 2 standard deviations of the mean. “About” can be understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the stated value. All numerical values ​​provided herein are modified by the term “about” unless the context otherwise clarifies.

[0063] Unless the context clearly indicates otherwise, all numerical values, numbers, and / or expressions representing amounts of components, reaction conditions, polymer compositions, and mixtures used in this specification are approximate values that reflect the various uncertainties inherent in the measurements made to obtain these numbers, etc. For this reason, it should be understood that in all cases, the term "about" should be understood to modify all numerical values, numbers, and / or expressions. In addition, when numerical ranges are disclosed in the specification, these ranges are continuous and include all numerical values from the minimum value to the maximum value, including the maximum value within each range, unless otherwise defined. In addition, when the range refers to integers, it includes all integers from the minimum value to the maximum value, including the maximum value within the range, unless otherwise defined.

[0064] Figure 1 A lithium secondary battery according to the present disclosure is shown. The lithium secondary battery may include a positive electrode layer 10, a negative electrode layer 20, and a solid electrolyte layer 30 disposed between the positive electrode layer 10 and the negative electrode layer 20.

[0065] Figure 2 The positive electrode layer 10 according to the present disclosure is shown. The positive electrode layer 10 may include a composite positive electrode active material 11 and a positive electrode material 12. The positive electrode material 12 may include a solid electrolyte, a binder, a conductive material, a dispersant, etc.

[0066] Figure 3 The composite positive electrode active material 11 according to the first embodiment of the present disclosure is shown. The composite positive electrode active material 11 may include a core portion 110 and a shell portion 111 coating the outer surface of the core portion 110.

[0067] The shell portion 111 may cover about 10% or more, about 20% or more, about 30% or more, about 40% or more, about 50% or more, about 60% or more, about 70% or more, about 80% or more, about 90% or more, or about 99% or more of the surface of the core portion 110.

[0068] The core portion 110 may contain a lithium transition metal compound capable of intercalating and deintercalating lithium ions.

[0069] The lithium transition metal oxide may include any common electrolyte used in the technical field to which the present disclosure pertains. For example, the lithium transition metal oxide may include LiNi x1 Co x2 Mn x3 O2 (0.65 ≤ x1 ≤ 0.85, 0.05 < x2 < 0.25, 0.03 < x3 < 0.2, and x1 + x2 + x3 = 1).

[0070] The core 110 can be in the form of secondary particles, in which primary particles containing lithium transition metal oxides are aggregated. When a cross-section of the core 110 is observed using an instrument such as a scanning electron microscope (SEM), the primary particles can refer to the smallest particle unit that is divided into lumps. Primary particles can be formed as single particles or multiple particles. Secondary particles can refer to a structure in which multiple primary particles are aggregated. The shape of the secondary particles is not particularly limited and can be, for example, spherical or elliptical.

[0071] The average particle size (D50) of the core 110 is not particularly limited, and can be, for example, from 1 μm to 20 μm. The average particle size (D50) of the core 110 can be measured using a commercially available laser diffraction scattering particle size analyzer, such as a Microtrac particle size analyzer. Alternatively, 200 particles can be randomly extracted from an electron microscope and their average particle size can be calculated.

[0072] The shell portion 111 may contain a sulfide-based solid electrolyte.

[0073] Sulfide-based solid electrolytes can include any common electrolyte used in the art to which this disclosure pertains. For example, sulfide-based solid electrolytes can include Li₂S-P₂S₅, Li₂S-P₂S₅-LiI, Li₂S-P₂S₅-LiCl, Li₂S-P₂S₅-LiBr, Li₂S-P₂S₅-Li₂O, Li₂S-P₂S₅-Li₂O-LiI, Li₂S-SiS₂, Li₂S-SiS₂-LiI, Li₂S-SiS₂-LiBr, Li₂S-SiS₂-LiCl, Li₂S-SiS₂-B₂S₃-LiI, Li₂S-SiS₂-P₂S₅-LiI, Li₂S-B₂S₃, and Li₂S-P₂S₅-Z. m S n (where m and n are positive numbers and Z is Ge, Zn, or Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, Li2S-SiS2-Li x MO y (where x and y are positive numbers, and M is P, Si, Ge, B, Al, Ga, or In), Li 10 GeP2S 12 Furthermore, preferably, the sulfide-based solid electrolyte may include a sulfide-based solid electrolyte having a sulfide-germanium sulfide crystal structure. The sulfide-based solid electrolyte having a sulfide-germanium sulfide crystal structure may include at least one selected from the group consisting of: Li 7-y PS 6-y Ha y (Where Ha includes Cl, Br, or I, and y satisfies 0 < y ≤ 2), Li 7-z PS6-z (Ha1 1-b Ha2 b ) z (Ha1 and Ha2 are distinct from each other and each independently includes Cl, Br or I, and b and z satisfy 0 < b < 1 and 0 < z ≤ 2 respectively) and their combinations.

[0074] Figure 4 A composite positive electrode active material 11' according to a second embodiment of the present disclosure is shown. The composite positive electrode active material 11' may include a core portion 110' and a shell portion 111' coated on the outer surface of the core portion 110'.

[0075] The core portion 110' may include a central portion 110a containing a lithium transition metal compound and a peripheral portion 110b coating the surface of the central portion. The central portion 110a is the same as the core portion 110 of the first embodiment, and therefore its detailed description will be omitted below.

[0076] The peripheral portion 110b may contain an inorganic compound. The inorganic compound may include at least one selected from the group consisting of LiNbO3, Li2ZrO3, Li3PO4, Li2SiO3, and combinations thereof.

[0077] The shell portion 111' is the same as the shell portion 111 in the first embodiment, and therefore its detailed description will be omitted below.

[0078] The present disclosure is characterized by the use of a compound having lower cohesive strength than that of a conventional sulfide-based solid electrolyte constituting the shell 111. Conventional sulfide-based solid electrolytes have strong interparticle cohesive strength and therefore form aggregates during the production process of the shell 111. Consequently, the shell 111 is not properly formed, and the electrochemical properties deteriorate due to the increased resistance caused by the aggregates in the positive electrode layer 10.

[0079] The present disclosure is characterized in that the shell portion 111 is formed using a sulfide-based solid electrolyte having a cohesive index of not less than about 37 and less than about 46, or about 40 to 45.

[0080] The cohesive index is the degree of cohesion between powder particles during powder flow. As the cohesive index decreases, dispersibility increases.

[0081] The cohesive index can be calculated using the flow angle and the change in the interface between air and powder rotating at a predetermined rate in a drum.

[0082] The flow angle is a parameter that describes the flowability of a powder. The term "flowability" refers to the ability of a powder to flow freely and uniformly in the form of individual particles. As the flow angle decreases, the attraction between particles decreases and the flowability of the powder increases.

[0083] Figure 5 An exemplary apparatus for measuring the flow angle is shown. First, a predetermined amount of powder 80 is injected into a cylindrical drum 90. The drum 90 rotates at a constant rate, and as the drum 90 rotates, the layer of powder 80 is pulled upwards. Then, an avalanche occurs when the balance between the attraction between the powder particles 80 and gravity is lost. The avalanches that periodically appear in the rotating drum 90 are continuously imaged using a digital camera (800×800 pixels). The images are analyzed to measure the flow angle. Figure 6 As shown, when an avalanche occurs, the angle between the slope of the powder layer 80 and the ground can be defined as the flow angle. The device used to measure the flow angle can be, for example, GranuDruM... TM Powder rheometer, and the results can be used with GranuTools. TM The software performs the analysis.

[0084] The rotational speed of roller 90 is not particularly limited; for example, roller 90 can rotate from about 1 rpm to 70 rpm.

[0085] In addition, the digital camera can image the powder 80 at intervals of approximately 500ms to 1,000ms.

[0086] The average value of the interface between powder 80 and air (excluding powder 80) within roller 90 is calculated from the imaging results, and then the standard deviation (σ(x)) of this average value is calculated in pixel units. The cohesion index is obtained by summing all values ​​calculated in pixel units, calculating the average value, and indexing the result. Specifically, the standard deviation and cohesion index can be calculated according to the following equation, and more details can be found in Powder Technology, 224 (2012) 19-27.

[0087]

[0088] N y (x) is the number of y-axis coordinates corresponding to the x-axis of the average interface.

[0089] σ(x) is the standard deviation of the x-axis coordinate.

[0090] It is the x-axis coordinate of the average interface.

[0091] y i (x) is the y-axis coordinate corresponding to the x-axis of the average interface.

[0092] N is the number of images.

[0093] n iThis refers to the number of pixels at the interface between the positive electrode active material and air. The term "cohesion index" is mentioned here to refer to a specific material value as given above, i.e., through... Figure 5 The process described above and the equations above determine the cohesion index of the sulfide-based solid electrolyte. When the sulfide-based solid electrolyte according to this disclosure is injected into a drum 90 rotating at about 10 rpm and imaged 50 times at 1,000 ms intervals using a digital camera, the cohesion index of the sulfide-based solid electrolyte, calculated using the average value based on the flow angle and deviation acting as the interface between air and the sulfide-based solid electrolyte, can be no less than about 37 and less than about 46, or about 40 to 45. When the cohesion index of the sulfide-based solid electrolyte is less than 37, the sulfide-based solid electrolyte does not form a shell 111 and remains as an aggregate, which may increase the grain boundary resistance in the positive electrode layer 10. To achieve a cohesion index of 46 or greater for the sulfide-based solid electrolyte, the heat treatment temperature must be high, as described later. Therefore, the crystallinity of the sulfide-based solid electrolyte increases, which may lead to increased resistance and reduced efficiency in the positive electrode layer 10.

[0094] The method for preparing a sulfide-based solid electrolyte with a low cohesion index and a composite positive electrode active material comprising the present disclosure will now be described in detail.

[0095] This method may include preparing a sulfide-based solid electrolyte and coating a lithium transition metal compound with the sulfide-based solid electrolyte to obtain a composite positive electrode active material.

[0096] The preparation of sulfide-based solid electrolytes may include preparing starting materials, reacting the starting materials to obtain intermediate materials, and heat-treating the intermediate materials to obtain sulfide-based solid electrolytes.

[0097] There are no particular restrictions on the type of starting material, and the starting material can be, for example, a lithium source, a phosphorus source, a halogen compound, etc.

[0098] Lithium sources can include Li₂S, Li₂S₂, Li₂S₄, Li₂S₈, elemental lithium, etc. Phosphorus sources can include P₂S₃, P₂S₅, elemental phosphorus, etc. Halogen compounds can include LiCl, LiBr, LiI, etc. Starting materials can further include compounds of elemental sulfur and substituted or doped elements.

[0099] The content of each starting material can be appropriately adjusted to suit the desired composition of the sulfide-based solid electrolyte.

[0100] There are no particular limitations on the method used to react the starting material. Intermediate materials can be obtained by reacting the starting material using dry and / or wet methods. For example, a dry method involves injecting the starting material and a ball into a container and applying energy to the starting material while rotating it. A wet method involves injecting the starting material and a polar solvent capable of dissolving the starting material, such as tetrahydrofuran or acetonitrile, into a container and applying energy to the starting material by stirring. When preparing intermediate materials by a wet method, further drying can be performed to remove residual polar solvents prior to heat treatment.

[0101] This disclosure is characterized by heat-treating intermediate materials at specific temperatures to obtain sulfide-based solid electrolytes with low cohesion index. Heat treatment can be a process of transforming intermediate materials into sulfide-based solid electrolytes with a complete crystal structure. Preparation of sulfide-based solid electrolytes can include heat-treating the intermediate materials at temperatures above about 400°C and below about 500°C. When the heat treatment temperature is 400°C or lower, a suitable crystal structure for the sulfide-based solid electrolyte may not form, and lithium-ion conductivity may decrease; and when the heat treatment temperature is above 500°C, the cohesion index of the sulfide-based solid electrolyte may increase.

[0102] The steps for preparing the sulfide-based solid electrolyte may further include grinding the sulfide-based solid electrolyte obtained above. Therefore, a sulfide-based solid electrolyte with an average particle size (D50) of about 2 μm or less can be obtained. When the average particle size (D50) of the sulfide-based solid electrolyte exceeds 2 μm, the shell portion 111 can be formed non-uniformly using the sulfide-based solid electrolyte.

[0103] The preparation of the composite positive electrode active material includes mixing a sulfide-based solid electrolyte with a lithium transition metal compound at a first rate to obtain a mixture, stirring the mixture at a second rate higher than the first rate to disperse it, and stirring the dispersed mixture at a third rate higher than the second rate to coat the lithium transition metal compound with the sulfide-based solid electrolyte.

[0104] For example, a mixture can be obtained by injecting a sulfide-based solid electrolyte and a lithium transition metal compound into a container such as a mill and mixing them at a rate of about 300 rpm to 700 rpm or about 500 rpm. The mixture can then be stirred at a rate of about 1,200 rpm to 1,800 rpm or about 1,500 rpm to increase dispersibility. Finally, the lithium transition metal compound can be coated with the sulfide-based solid electrolyte while the dispersed mixture is stirred at about 2,000 rpm to 4,000 rpm or about 3,000 rpm for a period longer than about 10 minutes and shorter than about 30 minutes.

[0105] When the mixing time at the third rate is less than 10 minutes, the coverage of shell 111 may decrease and the desired performance improvement may not be achieved.

[0106] The composite positive electrode active material 11 according to this disclosure may include about 90% to 98% by weight of a core portion 110 and about 2% to 10% by weight of a shell portion 111. When the content of the shell portion 111 is less than 2% by weight, the shell portion 111 may be too thin to achieve the desired effect of introducing the shell portion 111, and when the content exceeds 10% by weight, the shell portion 111 may be formed unevenly.

[0107] The thickness of the shell 111 can be from about 50 nm to 500 nm. The thickness of the shell 111 is measured by irradiating the composite positive electrode active material 11 with X-rays via X-ray fluorescence spectroscopy (XRF) and measuring the intensity of the X-rays in response to the X-rays originating from sulfur atoms released from the composite positive electrode active material 11. Figure 7 A reference diagram is shown to illustrate the X-ray fluorescence spectroscopy performed in this disclosure. (See figure) Figure 7 As shown, sample A can be divided into multiple measurement portions B, and X-rays can be irradiated to measurement point C of measurement portion B. The thickness of shell 111 can be calculated by measuring the intensity of X-rays originating from sulfur atoms released from measurement point C. Furthermore, the planar density of shell 111 can be approximately 0.05 mg / cm³. 2 Up to 0.3 mg / cm 2 The planar density can be measured and calculated in the same manner as the thickness. According to this disclosure, the 3D shape of the composite positive electrode active material 11 can be seen because, as described above, X-rays are irradiated to multiple measurement points C, and the intensity of the X-rays emitted from the measurement points C is measured.

[0108] The present disclosure will be described in more detail below with reference to the following embodiments. However, these embodiments are provided for a better understanding only and should not be construed as limiting the scope of the present disclosure.

[0109] Example 1 and Comparative Examples 1 to 3

[0110] Starting materials such as Li₂S and P₂S₅ were prepared in equal amounts and injected into a container and stirred to induce a reaction to obtain intermediate materials. The intermediate materials were heat-treated under the temperature conditions shown in Table 1 below to obtain sulfide-based solid electrolytes according to Example 1 and Comparative Examples 1 to 3. The cohesion index and average particle size (D50) of each sulfide-based solid electrolyte are shown in Table 1.

[0111] Each sulfide-based solid electrolyte was mixed with a lithium transition metal oxide at a rate of approximately 500 rpm to obtain a mixture. The mixture was stirred and dispersed at a rate of approximately 1,500 rpm, and the dispersed mixture was stirred at approximately 3,000 rpm for 20 minutes to form a composite positive electrode active material comprising a core and a shell. Each composite positive electrode active material comprises approximately 2% by weight of a shell.

[0112] Lithium-ion batteries were fabricated using various composite cathode active materials, and the initial efficiency, initial charge capacity, and discharge capacity of the lithium-ion batteries were measured and are shown in Table 1 below.

[0113] Table 1

[0114]

[0115] As can be seen from Table 1, the sulfide-based solid electrolyte of Example 1, heat-treated at a temperature above 400°C and below 500°C, has a cohesion index of not less than 37 and less than 46 and an average particle size (D50) of 2 μm. The initial efficiency, first charge capacity, and discharge capacity of the lithium secondary battery according to Example 1 are all higher than those of Comparative Examples 1 to 3. Comparative Examples 1 and 2 have a cohesion index of 46 or greater and a first charge capacity similar to Example 1, but have low initial efficiency and discharge capacity. In particular, Comparative Example 2 exhibits increased crystallinity of the sulfide-based solid electrolyte due to the high heat treatment temperature, resulting in high resistance in the electrode and slightly lower initial efficiency. Comparative Example 3 has a cohesion index of less than 37 and the highest heat treatment temperature. Therefore, the sulfide-based solid electrolyte of Comparative Example 3 has excessively high crystallinity, making it difficult to form a shell, and the remaining sulfide-based solid electrolyte remains as aggregates, resulting in high grain boundary resistance and poor electrochemical performance.

[0116] Figure 8A The results of scanning electron microscopy of the composite positive electrode active material according to Example 1 are shown. Figure 8B The results of energy-dispersive X-ray spectroscopy for the composite positive electrode active material according to Example 1 are shown. In the composite positive electrode active material according to Example 1, the sulfide-based solid electrolyte does not aggregate and forms a uniform shell.

[0117] Figure 9A The results of scanning electron microscopy of the composite positive electrode active material according to Comparative Example 1 are shown. Figure 9B The results of energy-dispersive X-ray spectroscopy of the composite positive electrode active material according to Comparative Example 1 are shown. Figure 10A The results of scanning electron microscopy of the composite positive electrode active material according to Comparative Example 2 are shown. Figure 10BThe results of energy-dispersive X-ray spectroscopy for the composite cathode active material according to Comparative Example 2 are shown. In the composite cathode active materials according to Comparative Examples 1 and 2, the sulfide-based solid electrolyte does not aggregate, but due to the high crystallinity, particle size control and shell formation are difficult.

[0118] Figure 11A The results of scanning electron microscopy of the composite positive electrode active material according to Comparative Example 3 are shown. Figure 11B The results of energy-dispersive X-ray spectroscopy of the composite positive electrode active material according to Comparative Example 3 are shown. It can be seen that the sulfide-based solid electrolyte aggregates in the composite positive electrode active material according to Comparative Example 3.

[0119] Examples 2 and 3 and Comparative Examples 4 and 5

[0120] Each sulfide-based solid electrolyte prepared in Example 1 was mixed with a lithium transition metal oxide at a rate of about 500 rpm to obtain a mixture. The mixture was stirred and dispersed at a rate of about 1,500 rpm, and the dispersed mixture was stirred at about 3,000 rpm for 10 minutes (Comparative Example 4), 20 minutes (Example 2), and 30 minutes (Example 3) to form a composite positive electrode active material, each comprising a core and a shell. Each composite positive electrode active material comprises about 2% by weight of a shell.

[0121] A composite positive electrode active material was prepared on a laboratory scale at the same rate and time as Comparative Example 4, and this was defined as Comparative Example 5.

[0122] Figure 12 The capacity retention of lithium secondary batteries using composite cathode active materials from Examples 2, 3, 4, and 5 is shown. Measurement conditions were approximately 30°C and 2.5V to 4.35V.

[0123] In Comparative Examples 4 and 5, due to the excessively short coating retention time, the core could not be completely covered by the shell, thus only partially forming the core. Compared to Comparative Examples 4 and 5, Examples 2 and 3 exhibit improved battery output characteristics.

[0124] Examples 3 to 6 and Comparative Example 6

[0125] Composite positive electrode active materials were prepared in the same manner as in Example 2, except that the content of the shell was changed to 2% by weight (Example 3), 5% by weight (Example 4), 7% by weight (Example 5), 10% by weight (Example 6), and 20% by weight (Comparative Example 6).

[0126] Figure 13 The results of X-ray fluorescence spectroscopy of the composite positive electrode active material according to Example 3 are shown. Figure 14The results of X-ray fluorescence spectroscopy of the composite positive electrode active material according to Example 4 are shown. Figure 15 The results of X-ray fluorescence spectroscopy of the composite positive electrode active material according to Example 6 are shown. Figure 16 The results of X-ray fluorescence spectroscopy of the composite positive electrode active material according to Comparative Example 6 are shown. In this way, the 3D shape of the composite positive electrode active material can be seen by irradiating the composite positive electrode active material 11 with X-rays and measuring the intensity of the X-rays originating from sulfur atoms released from the composite positive electrode active material.

[0127] from Figure 16 It can be seen that when an excessive amount of sulfide-based solid electrolyte is added, as in Comparative Example 6, the coating is partially uneven.

[0128] Figure 17 The thickness and areal density of the shell portion of the composite positive electrode active materials according to Examples 3 to 6 are shown. The thickness and areal density increase linearly with increasing shell content, indicating that no aggregates are formed and all added sulfide-based solid electrolyte constitutes the shell portion.

[0129] Figure 18 The capacity retention rates of lithium secondary batteries comprising the composite positive electrode active materials according to Examples 3 to 6 and Comparative Example 6 are shown. The results of Examples 3 to 6 show that the discharge capacity and capacity retention rate increase with increasing shell content. However, as in Comparative Example 6, when the shell content exceeds 10% by weight, the sulfide-based solid electrolyte retained without coating forms aggregates, thus increasing grain boundary resistance, reducing the reaction rate in the electrode, which determines the overall reaction rate of the battery, and degrading the rate characteristics.

[0130] According to this disclosure, a composite positive electrode active material for lithium secondary batteries and a method for manufacturing the same can be obtained. The composite positive electrode active material can reduce the resistance between the two materials by expanding the channel of lithium ions from the positive electrode active material to the sulfide-based solid electrolyte.

[0131] According to this disclosure, a composite positive electrode active material for lithium secondary batteries with a uniform coating and a method thereof can be obtained by coating a positive electrode active material with a sulfide-based solid electrolyte having weak cohesive force to prevent aggregation of the sulfide-based solid electrolyte.

[0132] According to this disclosure, new analytical parameters can be obtained using X-ray fluorescence analysis, which can identify and analyze coatings containing sulfide-based solid electrolytes in 3D form.

[0133] According to this disclosure, when synthesizing sulfide-based solid electrolytes, by controlling the temperature of the heat treatment, a sulfide-based solid electrolyte with appropriate cohesion and crystallinity suitable for coating can be obtained.

[0134] According to this disclosure, by increasing the coating holding time during the coating process of coating the positive electrode active material with a sulfide-based solid electrolyte, a composite positive electrode active material for lithium secondary batteries with a uniform coating and a method for manufacturing the same can be obtained.

[0135] According to this disclosure, a composite positive electrode active material and a method for manufacturing the same can be obtained. This composite positive electrode active material has reduced resistance caused by aggregates by utilizing changes in the conditions of the process of coating the positive electrode active material with a sulfide-based solid electrolyte, such as preventing the formation of aggregates by grinding a sulfide-based solid electrolyte that does not form a coating.

[0136] According to this disclosure, a lithium secondary battery and a method for manufacturing the same can be obtained. The lithium secondary battery can be rapidly charged by increasing the reaction rate in the positive electrode.

[0137] The effects of this disclosure are not limited to those mentioned above. It should be understood that the effects of this disclosure include all effects that can be inferred from the description of this disclosure.

[0138] This disclosure has been described in detail with reference to embodiments thereof. However, those skilled in the art will understand that changes may be made to these embodiments without departing from the principles and spirit of this disclosure, the scope of which is defined in the appended claims and their equivalents.

Claims

1. A composite positive electrode active material for a lithium secondary battery, the composite positive electrode active material comprising: a core portion containing a lithium transition metal compound; and a shell portion containing a sulfide-based solid electrolyte, wherein the sulfide-based solid electrolyte has a cohesion index of not less than 37 and less than 46. The composite positive electrode active material comprises:

2. The composite cathode active material according to claim 1, wherein 90 to 98% by weight of the core portion; and 2 to 10% by weight of the shell portion.

3. The composite cathode active material of claim 1, wherein, The thickness of the shell portion is determined to be 50 to 500 nm by radiating X-rays to the composite positive electrode active material via X-ray fluorescence spectroscopy and measuring the intensity of X-rays originating from sulfur elements released from the composite positive electrode active material in response to the X-rays.

4. The composite cathode active material of claim 1, wherein, The planar density of the shell portion is determined to be 0.05 mg / cm2to 0.3 mg / cm2by X-ray fluorescence spectroscopy by irradiating X-rays to the composite cathode active material and measuring the intensity of X-rays from the sulfur element released from the composite cathode active material in response to the X-ray source. 2 to 0.3 mg / cm2 2 .

5. The composite cathode active material of claim 1, wherein, The thickness and planar density of the shell portion are determined by radiating X-rays to a plurality of measurement points of the composite positive electrode active material and measuring the intensity of X-rays originating from sulfur elements.

6. A method of producing a composite positive electrode active material, comprising: producing a sulfide-based solid electrolyte; and coating a lithium transition metal compound with the sulfide-based solid electrolyte to obtain a composite positive electrode active material comprising a core portion containing a lithium transition metal compound and a shell portion containing a sulfide-based solid electrolyte, wherein the sulfide-based solid electrolyte has a cohesion index of not less than 37 and less than 46.

7. The method of claim 6, wherein, Producing the sulfide-based solid electrolyte comprises: preparing a starting material; reacting the starting material to obtain an intermediate material; and heat-treating the intermediate material to obtain a sulfide-based solid electrolyte.

8. The method of claim 6, wherein, The sulfide-based solid electrolyte has an average particle diameter D50 of 2 pm or less.

9. The method of claim 7, wherein, Producing the sulfide-based solid electrolyte comprises heat-treating the intermediate material at a temperature higher than 400°C and lower than 500°C.

10. The method of claim 6, wherein, Producing the composite positive electrode active material comprises: mixing the sulfide-based solid electrolyte with a lithium transition metal compound at a first rate to obtain a mixture; stirring the mixture at a second rate higher than the first rate to disperse the mixture; and stirring the dispersed mixture at a third rate higher than the second rate to coat the lithium transition metal compound with the sulfide-based solid electrolyte.

11. The method of claim 10, wherein, The third rate is 2,000 to 4,000 rpm.

12. The method of claim 10, wherein, The lithium transition metal compound is coated with the sulfide-based solid electrolyte by stirring the dispersed mixture at the third rate for a period longer than 10 minutes and not longer than 30 minutes.

13. The method of claim 6, wherein, The composite positive electrode active material comprises: 90 to 98% by weight of the core portion; and 2 to 10% by weight of the shell portion.

14. The method of claim 6, wherein, The thickness of the shell portion is determined to be 50 to 500 nm by radiating X-rays to the composite positive electrode active material via X-ray fluorescence spectroscopy and measuring the intensity of X-rays originating from sulfur elements released from the composite positive electrode active material in response to the X-rays.

15. The method of claim 6, wherein, The planar density of the shell portion is determined to be 0.05 mg / cm2to 0.3 mg / cm2by irradiating X-rays to the composite cathode active material via X-ray fluorescence spectroscopy and measuring the intensity of X-rays from the sulfur element released from the composite cathode active material in response to the X-ray source 2 to 0.3 mg / cm2 2 .

16. The method of claim 6, wherein, The thickness and planar density of the shell portion are determined by radiating X-rays to a plurality of measurement points of the composite positive electrode active material and measuring the intensity of X-rays originating from sulfur elements.

17. A composite positive electrode active material for a lithium secondary battery, the composite positive electrode active material comprising: a core portion containing a lithium transition metal compound; and a shell portion containing a sulfide-based solid electrolyte, wherein the sulfide-based solid electrolyte has a cohesion index of 40 to 45, and wherein the core portion constitutes 90 to 98% by weight of the composite positive electrode active material; and the shell portion constitutes 2 to 10% by weight of the composite positive electrode active material.

18. The composite cathode active material of claim 17, wherein, the thickness of the shell portion is 50 nm to 500 nm, and the planar density of the shell portion is 0.05 mg / cm 2 to 0.3 mg / cm 2 .

19. A positive electrode layer for a lithium secondary battery, comprising the composite positive electrode active material of claim 1.

20. A lithium secondary battery comprising the positive electrode layer of claim 19.

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