Positive electrode active material for lithium secondary battery comprising vulcanized coating layer and method for preparing same

Abstract

A positive electrode active material for a lithium secondary battery including a vulcanized coating layer, the positive electrode active material including a core portion containing a lithium transition metal oxide and a vulcanized coating layer for enhancing lithium ion conductivity and cycle life, and a method of preparing the same. The coating may include a sulfur component and may be configured to have a bilayer structure in which a first layer of an alkali metal oxide is applied on the core portion and a second vulcanized layer is disposed on the first layer. A method for preparing the positive electrode active material includes forming a coating using vapor deposition or heat treatment of a mixture of the core portion and a sulfur precursor, thereby producing a uniform vulcanized thin layer. This structure mitigates side reactions with sulfide-based solid electrolytes, thereby obtaining improved coulombic efficiency, rate capability and stability.

Description

Technical Field

[0001] This disclosure relates to positive electrode active materials for lithium secondary batteries comprising a sulfide coating and methods for preparing the same. Background Technology

[0002] All-solid-state secondary batteries using inorganic solid electrolytes are currently the most promising next-generation secondary battery systems. In particular, sulfide-based solid electrolytes, exhibiting high lithium-ion conductivity and excellent adhesion to electrode materials, are nearing commercialization.

[0003] However, sulfide-based solid electrolytes can induce undesirable reactions with the oxide-based positive electrode active material, leading to the formation of by-products at the interface. These by-products increase electrical resistance and are a major cause of electrochemical degradation in all-solid-state secondary batteries.

[0004] To address this issue, one proposed method involves coating the surface of the positive electrode active material with a material stable in sulfide-based solid electrolytes. Common examples of such coating materials include oxides such as Li₂ZrO₃, LiNbO₃, and LiTaO₃, as well as phosphates such as Li₃PO₄. These coating materials can significantly reduce side reactions between the positive electrode active material and the sulfide-based solid electrolyte. However, because the coating material is also an oxide similar to the positive electrode active material, its compatibility with sulfide-based solid electrolytes is limited, making long-term stable contact challenging.

[0005] To address the contact issue between the coating material and the sulfide-based solid electrolyte, another proposed method is to apply the sulfide-based solid electrolyte separately to the coating material. This is done by dissolving the sulfide-based solid electrolyte in an organic solvent to form a liquid, and then applying it to the coating material. However, this method has the following drawbacks: the lithium-ion conductivity of the sulfide-based solid electrolyte decreases significantly during the dissolution process in the organic solvent, causing the additional coating to act as a new resistive layer. Furthermore, forming the sulfide-based solid electrolyte into a liquid and applying it uniformly is challenging. Summary of the Invention

[0006] Therefore, one object of this disclosure is to provide a positive electrode active material for lithium secondary batteries and a method for preparing the same, the positive electrode active material for lithium secondary batteries comprising a sulfide coating that can reduce side reactions between the positive electrode active material and the sulfide-based solid electrolyte.

[0007] Another object of this disclosure is to provide a positive electrode active material for lithium secondary batteries and a method for preparing the same, the positive electrode active material comprising a sulfide coating that is highly compatible with sulfide-based solid electrolytes and can be used stably for a long time.

[0008] The purpose of this disclosure is not limited to the foregoing. The purpose of this disclosure will become clear from the following description, and will be achieved by the apparatus and combinations thereof described in this invention.

[0009] One embodiment of this disclosure provides a positive electrode active material for a lithium secondary battery, comprising a core portion containing a lithium transition metal oxide and a coating disposed on the core portion, wherein the coating may include a sulfur component.

[0010] The coating may include a first layer disposed on the core portion and comprising an alkali metal oxide, and a second layer disposed on the first layer and comprising a sulfur component.

[0011] Alkali metal oxides may include compounds represented by the following chemical formula 1.

[0012] [Chemical Formula 1]

[0013] Li a M b O (a+c) / 2

[0014] In chemical formula 1, M may include at least one of the following groups: niobium (Nb), boron (B), phosphorus (P), tungsten (W), titanium (Ti), tantalum (Ta), tin (Sn), zirconium (Zr), and combinations thereof; a and b may satisfy 1≤a≤10 and 1≤b≤10; and c may be the oxidation number of M.

[0015] The sulfur component can include cations and sulfur-containing anions.

[0016] Anions may include those selected from S - HS - SO - SO2 - SO3 - and at least one group consisting of combinations thereof.

[0017] The cation may include at least one selected from the group consisting of lithium (Li), nickel (Ni), cobalt (Co), manganese (Mn), and combinations thereof.

[0018] The cation may include at least one selected from the group consisting of lithium (Li), nickel (Ni), cobalt (Co), manganese (Mn), niobium (Nb), boron (B), phosphorus (P), tungsten (W), titanium (Ti), tantalum (Ta), tin (Sn), zirconium (Zr), and combinations thereof.

[0019] The coating may further include at least one selected from the group consisting of Li2CO3, LiOH, and combinations thereof.

[0020] The positive electrode active material can be shown to have a peak at a binding energy of about 165 eV to 168 eV in the S2p XPS spectrum obtained by X-ray photoelectron spectroscopy (XPS).

[0021] Another embodiment of this disclosure provides a positive electrode active material for a lithium secondary battery. The positive electrode active material includes: a core portion comprising a lithium transition metal oxide; and a first layer disposed on the core, the first layer comprising a lithium transition metal oxide. a M b O (a+c) / 2 The alkali metal oxide represents M, wherein M is at least one element selected from the group consisting of niobium (Nb), boron (B), phosphorus (P), tungsten (W), titanium (Ti), tantalum (Ta), tin (Sn), zirconium (Zr), and combinations thereof, and wherein a and b satisfy 1 ≤ a ≤ 10 and 1 ≤ b ≤ 10, and c is the oxidation number of M; and a second layer disposed on the first layer, the second layer comprising a sulfur component, wherein the sulfur component in the second layer comprises elements selected from S - HS - SO - SO 2- SO 3- Anions of the group consisting of lithium (Li), nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), aluminum (Al), and combinations thereof, and cations of lithium (Li), nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), aluminum (Al), and combinations thereof.

[0022] Another embodiment of this disclosure provides a method for preparing a positive electrode active material for a lithium secondary battery, including preparing a core comprising a lithium transition metal oxide and forming a coating on the core.

[0023] Forming a coating may include preparing a mixture of a precursor comprising a core portion and a sulfur component, and forming a coating on the core portion by heat-treating the mixture.

[0024] Forming a coating may include obtaining an intermediate by forming a first layer comprising an alkali metal oxide on a core portion, preparing a mixture comprising the intermediate and a sulfur component, and forming a second layer disposed on the first layer and comprising a sulfur component by heat-treating the mixture.

[0025] The precursors for the sulfur component may include those selected from sulfur, M x S y M x SO4 (where x is an integer from 1 to 3, y is an integer from 1 to 10, and M is at least one metal selected from the group consisting of Li, Na, K, Mg, Ca, and combinations thereof) and combinations thereof.

[0026] Forming a coating may include using vapor deposition to vulcanize the core portion, thereby forming a uniform thin vulcanized layer.

[0027] Based on 100 parts by weight of the core, the mixture may include about 0.01 parts by weight to 2 parts by weight of the sulfur component precursor.

[0028] As discussed, the method and system appropriately include the use of a controller or processor.

[0029] The terms “core” and “core portion” are used interchangeably in this document. Attached Figure Description

[0030] The foregoing 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, which are given hereinafter by way of illustration only and therefore do not limit this disclosure, and wherein:

[0031] Figure 1 A lithium secondary battery according to the present disclosure is shown;

[0032] Figure 2 A positive electrode active material according to a first embodiment of the present disclosure is shown;

[0033] Figure 3 A positive electrode active material according to a second embodiment of the present disclosure is shown;

[0034] Figure 4 The initial discharge curves of all-solid-state batteries including the positive electrode active materials according to Example 1 and Comparative Example 1 are shown.

[0035] Figure 5 The rate characteristics of an all-solid-state battery including the positive electrode active material according to Example 1 and Comparative Example 1 are shown;

[0036] Figure 6 The evaluation results of the lifetime of all-solid-state batteries including the positive electrode active materials according to Example 1 and Comparative Example 1 are shown;

[0037] Figure 7 The Nyquist curve of the positive electrode active material according to Example 1 is shown;

[0038] Figure 8 The Nyquist curve of the positive electrode active material according to Comparative Example 1 is shown;

[0039] Figure 9 Initial discharge curves of all-solid-state batteries comprising the positive electrode active materials according to Examples 1 and 2 and Comparative Examples 1 and 2 are shown.

[0040] Figure 10The rate characteristics of all-solid-state batteries comprising the positive electrode active materials according to Examples 1 and 2 and Comparative Examples 1 and 2 are shown.

[0041] Figure 11 The evaluation results of the lifetime of all-solid-state batteries including the positive electrode active materials according to Examples 1 and 2 and Comparative Examples 1 and 2 are shown;

[0042] Figure 12 A cross section of the positive electrode active material according to Comparative Example 1 is shown, analyzed using transmission electron microscopy (TEM).

[0043] Figure 13 A cross-section of the positive electrode active material according to Example 1 is shown using TEM analysis;

[0044] Figure 14 A cross-section of the positive electrode active material according to Comparative Example 2 is shown using TEM analysis.

[0045] Figure 15 A cross-section of the positive electrode active material according to Example 2 is shown using TEM analysis;

[0046] Figure 16 The results of X-ray photoelectron spectroscopy (XPS) of the positive electrode active material are shown after the all-solid-state battery comprising the positive electrode active material according to Comparative Example 1 has been disassembled before charging and discharging.

[0047] Figure 17 XPS results of the positive electrode active material in an all-solid-state battery including the positive electrode active material according to Comparative Example 1 after 300 charge and discharge cycles are shown.

[0048] Figure 18 XPS results of the positive electrode active material in an all-solid-state battery including the positive electrode active material according to Example 1 after 300 charge and discharge cycles are shown.

[0049] Figure 19 XPS results of the positive electrode active material in an all-solid-state battery including the positive electrode active material according to Comparative Example 2 after 300 charge and discharge cycles are shown.

[0050] Figure 20 XPS results of the positive electrode active material in an all-solid-state battery including the positive electrode active material according to Example 2 after 300 charge and discharge cycles are shown.

[0051] Figure 21 XPS results for the positive electrode active material of Comparative Example 1 are shown at different etching times.

[0052] Figure 22XPS results for the positive electrode active material according to Example 1 are shown at different etching times;

[0053] Figure 23 XPS results for the positive electrode active material of Comparative Example 2 at different etching times are shown; and

[0054] Figure 24 XPS results for the positive electrode active material according to Example 2 are shown at different etching times. Detailed Implementation

[0055] The above and other objects, features, and advantages of this disclosure will become more apparent from the following preferred embodiments, taken in conjunction with the accompanying drawings. However, this disclosure is not limited to the embodiments disclosed herein and may be modified in different forms. These embodiments are provided to fully explain this disclosure and to fully convey the spirit of this disclosure to those skilled in the art.

[0056] 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,” “machine,” “device,” 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.

[0057] Although exemplary embodiments are described as using multiple units to perform exemplary processes, it should be understood that exemplary processes can also be performed using 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.

[0058] 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).

[0059] Unless otherwise specified or obvious from the context, as used herein, the term “about” should be understood as being within the normal tolerance range in the field, such as within 2 standard deviations of the mean. “About” can be understood as being within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the stated value. Unless the context otherwise clarifies, all numerical values ​​provided herein are modified by the term “about”.

[0060] Throughout the accompanying drawings, the same reference numerals will indicate the same or similar elements. For clarity of this disclosure, the dimensions of the structures are depicted as larger than their actual dimensions. It will be understood that although terms such as “first” and “second” may be used herein to describe various elements, these elements are not limited by these terms. These terms are used only to distinguish one element from another. For example, a “first” element discussed below may be referred to as a “second” element without departing from the scope of this disclosure. Similarly, a “second” element may also be referred to as a “first” element. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise.

[0061] It will be further understood that when the terms "comprising," "including," "having," etc., are used in this specification, they specify the presence of the stated feature, integral, step, operation, element, component, or combination thereof, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components, or combinations thereof. Furthermore, it will be understood that when an element such as a layer, film, region, or sheet is referred to as being "on" another element, it may be directly on the other element, or there may be intermediate elements therein. Similarly, when an element such as a layer, film, region, or sheet is referred to as being "under" another element, it may be directly under the other element, or there may be intermediate elements therein.

[0062] Unless otherwise specified, all numerical values, specifications, and / or representations used herein to indicate the amounts of components, reaction conditions, polymer compositions, and mixtures should be considered approximate, including various uncertainties affecting the measurement, which inherently occur in obtaining these values, and therefore should in all cases be understood to be modified by the term "about". Furthermore, when numerical ranges are disclosed in this specification, the range is continuous and includes all values ​​from the minimum to the maximum of the range, unless otherwise specified. Additionally, when such ranges involve integer values, all integers from the minimum to the maximum are included, unless otherwise specified.

[0063] Figure 1A lithium secondary battery according to the present disclosure is shown. The lithium secondary battery may include a all-solid-state battery. The lithium secondary battery may include a positive electrode 10, a negative electrode 20, and a solid electrolyte layer 30 interposed between the positive electrode 10 and the negative electrode 20.

[0064] The positive electrode 10 may include a positive electrode active material, a sulfide-based solid electrolyte, a binder, a conductive material, etc.

[0065] Figure 2 A positive electrode active material 100 according to a first embodiment of the present disclosure is shown. The positive electrode active material 100 may include a core 110 containing a lithium transition metal oxide and a coating 120 covering the core 110.

[0066] The coating 120 according to the present disclosure is sulfided and is in the form of a structure close to a sulfide. The positive electrode active material according to the present disclosure has a gradually changing chemical potential of lithium ions (Li + ) from the core 110 to the external sulfide-based solid electrolyte, thereby preventing a rapid change in the concentration of lithium ions (Li + ). Briefly, a non-uniform lithium ion (Li + ) concentration layer may not be formed in the positive electrode 10 according to the present disclosure. Therefore, lithium ions can effectively move at the interface between the positive electrode active material 100 and the sulfide-based solid electrolyte. This can result in an increase in Coulomb efficiency and an improvement in high-rate characteristics, enabling fast charging and discharging and improving cycle characteristics.

[0067] The core 110 may include a lithium transition metal oxide that intercalates and deintercalates lithium.

[0068] The lithium transition metal oxide may include any material common in the art 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), LiFePO4, LiNiCoAlO2, LiCoO2, LiMn2O4, LiFe 1-y Mn y O2 (0 < y < 1), etc.

[0069] Core 110 can be in the form of secondary particles, in which primary particles of lithium transition metal oxide aggregate. Here, the term "primary particle" can refer to the smallest particle unit characterized as a one lump when a cross-section of core 110 is observed using a device such as a scanning electron microscope (SEM). Primary particles can consist of a single microparticle (grain) or multiple microparticles. Moreover, the term "secondary particle" can refer to a structural body formed by the aggregation of multiple primary particles. The shape of the secondary particles is not particularly limited and can be, for example, spherical or elliptical.

[0070] The average particle size (D) of core 110 50 There are no particular limitations, and the particle size can range from, for example, about 1 μm to 20 μm. A commercially available laser diffraction scattering particle size analyzer, such as the Microtrac particle size analyzer, can be used to measure the average particle size (D) of core 110. 50 Alternatively, 200 particles can be randomly extracted from an electron micrograph and their average particle size can be calculated.

[0071] The vulcanized coating 120 may include a sulfur component. The method of vulcanizing the coating 120 will be described below.

[0072] The sulfur component may include cations and sulfur-containing anions. The sulfur component may also include compounds in which the anions and cations are ionically bonded.

[0073] Anions may include those selected from S - HS - SO - SO2 - SO3 - and at least one group consisting of combinations thereof.

[0074] The cation may include at least one selected from the group consisting of lithium (Li), nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), aluminum (Al), and combinations thereof. The cation may be a cation derived from an element of core 110.

[0075] The coating 120 may further comprise at least one selected from the group consisting of Li₂CO₃, LiOH, and combinations thereof. These may be byproducts formed during the preparation of the coating 120. Therefore, based on the total weight of the positive electrode active material 100, its amount can be very small and may be about 100 ppm or less.

[0076] Figure 3A positive electrode active material 100' according to a second embodiment of the present disclosure is shown. The positive electrode active material 100' may include a core 110' and a coating 120' covering the core 110'. The coating 120' may include a first layer 121 disposed on the core 110' and comprising an alkali metal oxide and a second layer 122 disposed on the first layer 121 and comprising a sulfur component.

[0077] Since the core 110' is the same as that in the first embodiment described above, its description will be omitted below.

[0078] The alkali metal oxides of the first layer 121 may include compounds represented by the following chemical formula 1.

[0079] [Chemical Formula 1]

[0080] Li a M b O (a+c) / 2

[0081] In chemical formula 1, M may include at least one of the following groups: niobium (Nb), boron (B), phosphorus (P), tungsten (W), titanium (Ti), tantalum (Ta), tin (Sn), zirconium (Zr), and combinations thereof; a and b may satisfy 1≤a≤10 and 1≤b≤10; and c may be the oxidation number of M.

[0082] Specific examples of alkali metal oxides can include LiNbO3, LiBO2, Li3PO4, etc.

[0083] Since the second layer 122 is the same as the coating 120 according to the first embodiment described above, its description will be omitted below. However, when the second layer 122 containing the sulfur component is formed on the first layer 121, the cation of the sulfur component may be element M derived from the first layer 121, in addition to lithium (Li), nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), or aluminum (Al) derived from the core 110'. For example, the cation may include at least one selected from the group consisting of lithium (Li), nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), aluminum (Al), niobium (Nb), boron (B), phosphorus (P), tungsten (W), titanium (Ti), tantalum (Ta), tin (Sn), zirconium (Zr), and combinations thereof.

[0084] The method for preparing the positive electrode active material according to this disclosure may include preparing a core portion comprising a lithium transition metal oxide and forming a coating on the core portion.

[0085] In the method for preparing the positive electrode active material 100 according to the first embodiment described above, forming a coating may include preparing a mixture containing a precursor and a core comprising a sulfur component, and forming a coating covering the core by heat-treating the mixture.

[0086] In the method for preparing the positive electrode active material 100' according to the second embodiment described above, forming the coating may include obtaining an intermediate by forming a first layer comprising an alkali metal oxide on the core portion, preparing a mixture comprising a precursor and the intermediate comprising a sulfur component, and forming a second layer disposed on the first layer and comprising a sulfur component by heat treating the mixture.

[0087] In this disclosure, a coating or second layer can be formed by vulcanizing a core or intermediate using vapor deposition. After mixing a sulfur component precursor with the core or intermediate, the sulfur component precursor can be evaporated by heat treatment to form sulfur gas, which can then react with the surface of the core or intermediate. Therefore, the surface or outer portion of the core or intermediate can be vulcanized. In this disclosure using vapor deposition, a very uniform thin coating can be formed.

[0088] The thickness of the coating can be adjusted by the amount of sulfur precursors and the heat treatment temperature and time.

[0089] The precursors for the sulfur component may include those selected from sulfur, M x S y M x SO4 and combinations thereof constitute at least one group. In M x S y and M x In SO4, x can be an integer from 1 to 3, y can be an integer from 1 to 10, and M can be at least one metal selected from the group consisting of Li, Na, K, Mg, Ca, and combinations thereof.

[0090] Based on 100 parts by weight of the core, the mixture may include approximately 0.01 to 2 parts by weight of a sulfur component precursor. If the amount of the sulfur component precursor is less than 0.01 parts by weight, it will be difficult to form a sufficient coating. On the other hand, if the amount of the sulfur component precursor exceeds 2 parts by weight, the coating may become thicker and the lithium-ion conductivity may be reduced.

[0091] There are no particular restrictions on the method, temperature, and time of heat treatment, and heat treatment can be carried out using any equipment under any conditions, as long as the precursor of elemental sulfur is sufficiently evaporated to form a coating.

[0092] A better understanding of this disclosure can be obtained through the following examples. These examples are merely illustrative and should not be construed as limiting the scope of this disclosure.

[0093] Example 1

[0094] Fine sulfur powder was obtained by pulverizing solid sulfur (purity: 99.998%). The sulfur powder and core were mixed at approximately 30 Hz for about 3 minutes using a mixing mill (Hz mixer) to obtain a mixture. NCM 811 (LiNi) was used as the core. 0.8 Mn 0.1 Co 0.1 O2). Use 100 parts by weight of core and 0.1 parts by weight of sulfur powder.

[0095] The mixture is placed in a sealed tube under an argon atmosphere and heat-treated at about 300°C for about 4 hours to vulcanize the surface of the core, thereby obtaining a positive electrode active material comprising the core and a coating covering the core and including a sulfur component.

[0096] Comparative Example 1

[0097] The core from Example 1 was used as the positive electrode active material.

[0098] Figure 4 The initial discharge curves of all-solid-state batteries comprising the positive electrode active materials according to Example 1 and Comparative Example 1 are shown. Figure 5 The rate characteristics of all-solid-state batteries comprising the positive electrode active materials according to Example 1 and Comparative Example 1 are shown. Referring to these, Example 1 and Comparative Example 1 have similar initial capacities, but Example 1 exhibits superior rate characteristics.

[0099] Figure 6 The evaluation results of the lifetime of all-solid-state batteries including the positive electrode active materials according to Example 1 and Comparative Example 1 are shown. Compared with Comparative Example 1, Example 1 exhibits extremely superior lifetime characteristics.

[0100] Figure 7 The Nyquist curve of the positive electrode active material according to Example 1 is shown. Figure 8 The Nyquist curves for the positive electrode active material according to Comparative Example 1 are shown. These are results from an all-solid-state battery comprising the corresponding positive electrode active material after 300 charge-discharge cycles. Referring to these, the impedance portion of Example 1 is much smaller. This means that Example 1 has low impedance to lithium-ion movement.

[0101] Example 2

[0102] A coating solution was obtained by dissolving lithium ethoxide and polyphosphate in an alcohol solvent. A core was then added to this coating solution. NCM 811 (LiNi) was used as the core. 0.8 Mn 0.1 Co 0.1Using approximately 3 g of core and approximately 30 ml of alcohol solvent, and based on 100 parts by weight of core, approximately 0.15 parts by weight of lithium ethoxide and polyphosphate were used. The solvent was then evaporated at approximately 70°C with stirring. After the remaining solvent was completely evaporated in a vacuum oven, a heat treatment was performed at approximately 400°C for approximately 1 hour to obtain an intermediate in which the core was coated with Li3PO4.

[0103] Fine sulfur powder was obtained by pulverizing solid sulfur (purity: 99.998%). A mixture was obtained by mixing the sulfur powder and intermediates at approximately 30 Hz for about 3 minutes using a mixing mill (Hz mixer). Based on 100 parts by weight of core, 0.1 parts by weight of sulfur powder was used.

[0104] The mixture is placed in a sealed tube under an argon atmosphere and heat-treated at about 300°C for about 4 hours to vulcanize the surface of the core, thereby obtaining a positive electrode active material comprising the core and a coating covering the core and including a sulfur component.

[0105] Comparative Example 2

[0106] The intermediate from Example 2 was used as the positive electrode active material.

[0107] Figure 9 Initial discharge curves of all-solid-state batteries comprising positive electrode active materials according to Examples 1 and 2 and Comparative Examples 1 and 2 are shown. Figure 10 Rate characteristics of all-solid-state batteries comprising the positive electrode active materials according to Examples 1 and 2 and Comparative Examples 1 and 2 are shown. Referring to these, Example 2 exhibits superior initial capacity and rate characteristics compared to Comparative Example 2.

[0108] Figure 11 The evaluation results of the lifetime of all-solid-state batteries including the positive electrode active materials according to Examples 1 and 2 and Comparative Examples 1 and 2 are shown. Compared with Comparative Example 2, Examples 1 and 2 exhibited very superior lifetime characteristics.

[0109] Figure 12 The cross section of the positive electrode active material according to Comparative Example 1 is shown, analyzed using transmission electron microscopy (TEM). Figure 13 A cross section of the positive electrode active material according to Example 1 is shown using TEM analysis. Figure 14 A cross section of the positive electrode active material according to Comparative Example 2 is shown using TEM analysis. Figure 15 Cross-sections of the positive electrode active material according to Example 2 are shown, analyzed using TEM. These are results from an all-solid-state battery comprising the corresponding positive electrode active material after 300 charge-discharge cycles. (Refer to...) Figure 12 A side reaction layer with a thickness of approximately 25 nm was observed on the surface of Comparative Example 1, but reference... Figure 13In Example 1, the thickness of the side reaction layer was reduced to approximately 15 nm. Meanwhile, referring to... Figure 14 and Figure 15 The side reaction layer of Example 2 has a thickness of about 5 nm, which is thinner than the side reaction layer of Comparative Example 2.

[0110] Figure 16 The results of X-ray photoelectron spectroscopy (XPS) of the positive electrode active material are shown after the all-solid-state battery, which includes the positive electrode active material according to Comparative Example 1, was removed before charging and discharging.

[0111] Figure 17 XPS results of the positive electrode active material in an all-solid-state battery including the positive electrode active material according to Comparative Example 1 after 300 charge and discharge cycles are shown. Figure 18 XPS results of the positive electrode active material in an all-solid-state battery including the positive electrode active material according to Example 1 after 300 charge and discharge cycles are shown. Figure 19 XPS results of the positive electrode active material in an all-solid-state battery including the positive electrode active material according to Comparative Example 2 after 300 charge and discharge cycles are shown. Figure 20 XPS results for the positive electrode active material of an all-solid-state battery comprising the positive electrode active material according to Example 2 after 300 charge-discharge cycles are shown. The respective results are S2p XPS spectra.

[0112] Reference Figure 16 Before charging and discharging, it is represented as PS4 3- The S2p peak observed in sulfide-based solid electrolytes appears clearly. Furthermore, metal sulfides, P-[S]... n The -P and SS peaks are relatively weak.

[0113] Reference Figure 17 In Comparative Example 1, a large peak of metal sulfide appeared after charging and discharging. Here, the metal sulfide is produced through an interfacial reaction between the positive electrode active material and the sulfide-based solid electrolyte, thus the above results indicate that Comparative Example 1 has many by-reaction products.

[0114] Reference Figures 18 to 20 The peak intensities of the metal sulfides were relatively low in all Examples 1 and 2, as well as Comparative Example 2. This means that side reactions can be significantly mitigated through core surface treatment.

[0115] Specifically, refer to Figure 18 and Figure 20 Compared with Comparative Examples 1 and 2, Examples 1 and 2 showed large peaks at binding energies of 165 eV to 168 eV in the XPS spectra of S2p obtained by X-ray photoelectron spectroscopy (XPS).

[0116] Figure 21 XPS results for the positive electrode active material of Comparative Example 1 at different etching times are shown. Figure 22 XPS results for the positive electrode active material according to Example 1 at different etching times are shown. Referring to these, the S2p peak is clearly visible in the positive electrode active material of Example 1, in which a coating containing a sulfur component is formed by sulfurizing the surface of the core.

[0117] Figure 23 XPS results for the positive electrode active material of Comparative Example 2 at different etching times are shown. Figure 24 XPS results for the positive electrode active material according to Example 2 at different etching times are shown. Similarly, the S2p peak is clearly observed in the positive electrode active material of Example 2 in which a second layer containing sulfur components is formed.

[0118] Applying an oxide coating to conventional cathode active materials used in lithium-ion batteries mitigates side reactions at the interface with sulfide-based solid electrolytes to some extent; however, this coating is an oxide, therefore its lithium-ion (Li) content remains relatively low. + The chemical potential of lithium ions is not as high as that of sulfides, which makes it impossible to fundamentally prevent the formation of a non-uniform lithium ion concentration layer.

[0119] On the other hand, in the positive electrode active material according to this disclosure, the sulfidated coating containing sulfur components can prevent rapid changes in the lithium-ion concentration in the positive electrode, thereby controlling the formation of an uneven lithium-ion concentration layer as much as possible. Therefore, lithium ions can move effectively at the interface between the positive electrode active material and the sulfide-based solid electrolyte, which can lead to increased coulombic efficiency and improved high-rate characteristics.

[0120] As is apparent from the above description, according to this disclosure, a positive electrode active material for lithium secondary batteries and a method for preparing the same are provided. The positive electrode active material for lithium secondary batteries includes a sulfide coating capable of mitigating side reactions between the positive electrode active material and a sulfide-based solid electrolyte.

[0121] According to this disclosure, a positive electrode active material for lithium secondary batteries and a method for preparing the same are provided. The positive electrode active material includes a sulfide coating that has excellent compatibility with sulfide-based solid electrolytes and can be used stably for a long time.

[0122] The effects of this disclosure are not limited to those described above. It should be understood that the effects of this disclosure include all effects that can be inferred from the description herein.

[0123] The embodiments of this disclosure have been described in detail above, but the scope of this disclosure is not limited to the above embodiments, and various modifications and improvements made by those skilled in the art using the basic concepts of this disclosure as defined in the appended claims are also within the scope of this disclosure.

Claims

1. A positive electrode active material for lithium secondary batteries, the positive electrode active material comprising: The core portion contains lithium transition metal oxide; and A coating disposed on the core portion The coating contains a sulfur component.

2. The positive electrode active material according to claim 1, wherein the coating comprises: A first layer comprising an alkali metal oxide is disposed on the core portion; and A second layer is disposed on the first layer and contains sulfur components.

3. The positive electrode active material according to claim 2, wherein the alkali metal oxide comprises a compound represented by the following chemical formula 1: [Chemical Formula 1] Li a M b O (a+c) / 2 In chemical formula 1, M includes at least one of the group consisting of niobium (Nb), boron (B), phosphorus (P), tungsten (W), titanium (Ti), tantalum (Ta), tin (Sn), zirconium (Zr), and combinations thereof, where a and b satisfy 1≤a≤10 and 1≤b≤10, and c is the oxidation number of M.

4. The positive electrode active material according to claim 1, wherein the sulfur component comprises sulfur-containing anions and cations.

5. The positive electrode active material according to claim 4, wherein the anion comprises S-selected... - HS - SO - SO2 - SO3 - and at least one group consisting of combinations thereof.

6. The positive electrode active material according to claim 4, wherein the cation comprises at least one selected from the group consisting of lithium (Li), nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), aluminum (Al), and combinations thereof.

7. The positive electrode active material according to claim 2, wherein the sulfur component comprises an anion containing sulfur; and a cation, and the cation comprises at least one selected from the group consisting of lithium (Li), nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), aluminum (Al), niobium (Nb), boron (B), phosphorus (P), tungsten (W), titanium (Ti), tantalum (Ta), tin (Sn), zirconium (Zr), and combinations thereof.

8. The positive electrode active material according to claim 1, wherein the positive electrode active material shows a peak at a binding energy of 165 eV to 168 eV in the S2p XPS spectrum obtained by X-ray photoelectron spectroscopy (XPS).

9. A positive electrode active material for lithium secondary batteries, said positive electrode active material comprising: The core portion contains lithium transition metal oxide; A first layer disposed on the core portion, the first layer comprising Li a M b O (a+c) / 2 The alkali metal oxide represents a metal containing M, wherein M is at least one element selected from the group consisting of niobium (Nb), boron (B), phosphorus (P), tungsten (W), titanium (Ti), tantalum (Ta), tin (Sn), zirconium (Zr), and combinations thereof, and wherein a and b satisfy 1 ≤ a ≤ 10 and 1 ≤ b ≤ 10, and c is the oxidation number of M; and A second layer comprising a sulfur component is disposed on the first layer, wherein the sulfur component in the second layer comprises sulfur components selected from S. - HS - SO - SO 2- SO 3- Anions of the group consisting of lithium (Li), nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), aluminum (Al), and combinations thereof, and cations of lithium (Li), nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), aluminum (Al), and combinations thereof.

10. A method for preparing a positive electrode active material for lithium secondary batteries, the method comprising: Prepare a core portion containing lithium transition metal oxide; and A coating is formed on the core portion. The coating contains a sulfur component.

11. The method of claim 10, wherein forming the coating comprises: Prepare a mixture comprising a sulfur-containing precursor and the core portion; and A coating is formed on the core portion by heat treatment of the mixture.

12. The method of claim 10, wherein forming the coating comprises: An intermediate is obtained by forming a first layer comprising an alkali metal oxide on the core portion; Prepare a mixture comprising a sulfur component, a precursor, and the intermediate; and A second layer containing the sulfur component is formed by heat treatment of the mixture, which is disposed on the first layer.

13. The method of claim 11, wherein the precursor of the sulfur component comprises selected from sulfur, M x S y M x SO4 and combinations thereof, wherein x is an integer from 1 to 3, y is an integer from 1 to 10, and M is at least one metal selected from the group consisting of Li, Na, K, Mg, Ca and combinations thereof.

14. The method of claim 11, wherein the mixture comprises 0.01 to 2 parts by weight of the sulfur component precursor based on 100 parts by weight of the core portion.

15. The method of claim 10, wherein the sulfur component comprises sulfur-containing anions and cations.

16. The method of claim 15, wherein the anion comprises S... - HS - SO - SO2 - SO3 - and at least one group consisting of combinations thereof.

17. The method of claim 15, wherein the cation comprises at least one selected from the group consisting of lithium (Li), nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), aluminum (Al), and combinations thereof.

18. The method of claim 12, wherein the sulfur component comprises a sulfur-containing anion; and a cation, and the cation comprises at least one selected from the group consisting of lithium (Li), nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), aluminum (Al), niobium (Nb), boron (B), phosphorus (P), tungsten (W), titanium (Ti), tantalum (Ta), tin (Sn), zirconium (Zr), and combinations thereof.

19. The method of claim 10, wherein forming the coating comprises using a vapor deposition method to vulcanize the core portion, thereby forming a uniform vulcanized thin layer.

20. The method of claim 10, wherein the positive electrode active material shows a peak at a binding energy of 165 eV to 168 eV in the S2p XPS spectrum obtained by X-ray photoelectron spectroscopy (XPS).

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