Ternary positive electrode material with gradient coating, preparation method and solid-state battery

Abstract

The invention relates to a ternary positive electrode material with a gradient coating, a preparation method and a solid-state battery. The ternary positive electrode material comprises an inner core, a gradient ion conductor layer coated on the surface of the inner core and a surface modification layer coated on the surface of the gradient ion conductor layer, the gradient ion conductor layer comprises an inner layer, a middle layer and an outer layer along a direction far away from the inner core; the inner layer is a lithium-rich layer, and the outer layer is a lithium-poor layer. By arranging the gradient ion conductor layer, gradient reduction of a lithium ion transmission energy barrier from an inner core to the solid electrolyte is realized, the inner layer has a cubic phase structure and can be kept stable at high temperature, and the phase change problem of a traditional coating layer is avoided; in the circulation process, aluminum elements in the gradient ion conductor layer are segregated towards an interface, a Li-Al-O interface buffer layer is formed in situ, and side reactions are further inhibited; the surface modification layer and the internal gradient ion conductor layer cooperatively construct a double-conductive network, the ion conductivity is improved, and the preparation method is simple and easy to implement.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of battery materials, and relates to a ternary positive electrode material, in particular to a ternary positive electrode material with a gradient coating, a preparation method and a solid-state battery. BACKGROUND

[0002] With the increasing requirements of electric vehicles and large-scale energy storage systems on energy density and safety, all-solid-state lithium batteries (ASSLBs) have become a research hotspot for the next generation of energy storage technology due to their high safety and potential high energy density. However, the commercial application of ASSLBs still faces many technical challenges, especially the key problems in the positive electrode / solid-state electrolyte interface have been difficult to overcome, such as serious interface side reactions, poor interface contact, poor high-temperature stability, and blocked ion transport.

[0003] To solve the above technical problems, the prior art usually adopts modification methods such as surface coating and element doping. For example, the PVDF-based composite lithium metal halide oxide polymer electrolyte proposed in CN120497421A improves the ion conductivity and cycle stability by mixing polymers and lithium metal halide to prepare an electrolyte film. However, this method has limited improvement on the interface stability under high voltage (>4.5V) conditions. The Ga-doped LLZO (Li 6.25 Ga 0.25 La3Zr2O 12 ) ceramic filler and polymer composite hybrid solid-state electrolyte reported in Journal of Energy Storage achieves an ion conductivity of 6.62×10⁻ 4 S / cm at 30°C. However, this technology is insufficient in protecting the positive electrode material itself; the construction of a Li 1.2 Al 0.1 Ta 1.9 PO8 (LATPO) nano-coating on the surface of LiCoO2 reported in Interdisciplinary Materials enables the battery to achieve a capacity retention rate of 85.4% for 500 cycles under high voltage of 4.6V, however, this technology has not been applied to ultra-high nickel ternary materials with higher capacity. The sulfur / carbon composite material with gradient modification of phosphorus polysulfide in CN119029141A improves the sulfur content and specific capacity through gradient distribution design, but this technology is mainly aimed at lithium-sulfur battery systems and is difficult to be directly applied to oxide positive electrode materials.

[0004] Therefore, it is necessary to provide a ternary positive electrode material with a gradient coating for ternary positive electrode materials, a preparation method and a solid-state battery. SUMMARY

[0005] In view of the deficiencies of the prior art, the present application aims to provide a ternary positive electrode material with a gradient coating, a preparation method and a solid-state battery, which significantly improves the stability of the interface between the ternary positive electrode material and the solid-state electrolyte by constructing a Li-Al-Ta-P-O (LATPO) -based ion conductor coating with a gradient composition, and improves the ion conductivity and structural stability of the ternary positive electrode material.

[0006] To achieve the object of the present application, the following technical solutions are adopted:

[0007] In a first aspect, the present application provides a ternary positive electrode material with a gradient coating, which comprises a core, a gradient ion conductor layer coated on the surface of the core, and a surface modification layer coated on the surface of the gradient ion conductor layer.

[0008] In a first aspect, the present application provides a ternary positive electrode material with a gradient coating, which comprises a core, a gradient ion conductor layer coated on the surface of the core, and a surface modification layer coated on the surface of the gradient ion conductor layer. 1+a Al b Ta c P d O4, the chemical formula of the intermediate layer is LiAl b Ta c P d O4, and the chemical formula of the outer layer is Li 1- a Al b Ta c P d O4, wherein 0.1≤a≤0.3, 0.05≤b≤0.15, 1.85≤c≤1.95, and 0.9≤d≤1.1.

[0009] The surface modification layer is a composite layer composed of a fluorine-containing polymer and a lithium salt.

[0010] The gradient ion conductor layer is provided to achieve a gradient reduction of the lithium ion transmission energy barrier from the core to the solid-state electrolyte. The inner layer has a cubic phase structure and can remain stable at high temperatures, avoiding the phase change problem of traditional coating layers. During the cycle process, the aluminum and tantalum elements in the gradient ion conductor layer will segregate to the interface, forming an in-situ Li-Al-Ta-P-O interface buffer layer, which further suppresses the side reaction. The surface modification layer and the internal gradient ion conductor layer cooperatively construct a double-conductive network, improving the ion conductivity, and the preparation method is simple and easy to implement.

[0011] In some embodiments, the chemical formula of the core is LiNi x Co y Mn zO2, where 0.9 ≤ x < 1.0, 0 < y ≤ 0.05, 0 < z ≤ 0.05, and x + y + z = 1; preferably x = 0.92, y = 0.04 and z = 0.04, and this preferred core has a layered structure of α-NaFeO2 type.

[0012] In some embodiments, the median particle size D50 of the core is 3 μm to 15 μm, for example, it can be 3 μm, 5 μm, 8 μm, 10 μm, 12 μm or 15 μm, but not limited to the listed values, and the other unlisted values within the numerical range are equally applicable.

[0013] In some embodiments, the specific surface area of the core is 3 m 2 / g to 25 m 2 / g, for example, it can be 3 m 2 / g, 5 m 2 / g, 10 m 2 / g, 15 m 2 / g, 20 m 2 / g or 25 m 2 / g, but not limited to the listed values, and the other unlisted values within the numerical range are equally applicable.

[0014] In some embodiments, the thickness of the inner layer is 2 nm to 5 nm, for example, it can be 2 nm, 3 nm, 4 nm or 5 nm, but not limited to the listed values, and the other unlisted values within the numerical range are equally applicable.

[0015] In some embodiments, the thickness of the intermediate layer is 3 nm to 7 nm, for example, it can be 3 nm, 4 nm, 5 nm, 6 nm or 7 nm, but not limited to the listed values, and the other unlisted values within the numerical range are equally applicable.

[0016] In some embodiments, the thickness of the outer layer is 1 nm to 3 nm, for example, it can be 1 nm, 2 nm or 3 nm, but not limited to the listed values, and the other unlisted values within the numerical range are equally applicable.

[0017] The chemical formula of the inner layer is Li 1+a Al b Ta c P d O4, 0.1 ≤ a ≤ 0.3, 0.05 ≤ b ≤ 0.15, 1.85 ≤ c ≤ 1.95, 0.9 ≤ d ≤ 1.1, and a + b = 0.2, with a thickness of 2 nm to 5 nm. Under these conditions, the lithium ion conductivity of the inner layer is 5 × 10 -4 S / cm to 8 × 10 -4 S / cm.

[0018] The chemical formula of the intermediate layer is LiAlb Ta c P d O4, 0.05≤b≤0.15, 1.85≤c≤1.95, 0.9≤d≤1.1, with a thickness of 3nm~7nm, under these conditions, the lithium-ion conductivity of the intermediate layer is 3×10⁻⁶. -4 S / cm ~5×10 -4 S / cm.

[0019] The outer layer has the chemical formula Li. 1+a Al b Ta c P d O4, 0.1≤a≤0.3, 0.05≤b≤0.15, 1.85≤c≤1.95, 0.9≤d≤1.1, thickness of 1nm~3nm, under these conditions, the lithium-ion conductivity of the inner layer is ≤1×10⁻⁶. -4 S / cm.

[0020] In some embodiments, the fluoropolymer includes polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP).

[0021] In some embodiments, the lithium salt comprises lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0022] In some embodiments, the thickness of the surface modification layer is 10nm to 30nm, for example, it can be 10nm, 15nm, 20nm, 25nm or 30nm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0023] Secondly, the present invention provides a method for preparing a ternary cathode material with a gradient coating, the method comprising the following steps:

[0024] (1) Dissolve lithium source, aluminum source, tantalum source and phosphorus source in alcohol-water mixed solvent, add citric acid as complexing agent, and stir under heating conditions to form precursor solution;

[0025] (2) In a protective atmosphere, the core is mixed with the precursor solution to obtain a mixture;

[0026] (3) The mixture is subjected to gradient heat treatment, and then surface modification and drying are performed to obtain the ternary cathode material with gradient coating as described in the first aspect.

[0027] In some embodiments, the lithium source in step (1) includes lithium hydroxide and / or lithium carbonate.

[0028] In some embodiments, the aluminum source in step (1) includes aluminum nitrate.

[0029] In some embodiments, the tantalum source in step (1) includes Ta(OC2H5)5.

[0030] In some embodiments, the phosphorus source in step (1) includes any one or a combination of at least two of ammonium phosphate, ammonium monohydrogen phosphate, or ammonium dihydrogen phosphate. Typical but non-limiting combinations include a combination of ammonium phosphate and ammonium monohydrogen phosphate, a combination of ammonium monohydrogen phosphate and ammonium dihydrogen phosphate, a combination of ammonium phosphate and ammonium dihydrogen phosphate, or a combination of ammonium phosphate, ammonium monohydrogen phosphate, and ammonium dihydrogen phosphate.

[0031] This invention does not limit the specific ratio of lithium source, aluminum source, tantalum source and phosphorus source, as long as a ternary cathode material with gradient coating that meets the process requirements can be obtained.

[0032] In some embodiments, the volume ratio of ethanol to water in the alcohol-water mixed solvent in step (1) is 2:1 to 5:1, for example, it can be 2:1, 3:1, 4:1 or 5:1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0033] In some embodiments, the molar amount of citric acid in step (1) is 0.5 to 3 times the total number of moles of metal in the precursor solution, for example, it can be 0.5 times, 1 time, 1.5 times, 2 times, 2.5 times or 3 times, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0034] In some embodiments, the heating conditions in step (1) are 35°C to 75°C for 2 hours to 8 hours.

[0035] The heating temperature is 35℃~75℃, for example, it can be 35℃, 40℃, 45℃, 50℃, 55℃, 60℃, 65℃, 70℃ or 75℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0036] The heating time is 2h to 8h, for example, it can be 2h, 4h, 5h, 6h or 8h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0037] In some embodiments, the solid content of the precursor solution in step (1) is 5wt% to 20wt%, for example, it can be 5wt%, 8wt%, 10wt%, 12wt%, 15wt%, 18wt% or 20wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, preferably 8wt% to 12wt%.

[0038] In some embodiments, the protective atmosphere gas in step (2) includes nitrogen.

[0039] In some embodiments, the mass ratio of the kernel to the precursor solution in step (2) is 13:1 to 20:1, for example, it can be 13:1, 15:1, 16:1, 18:1 or 20:1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0040] In some embodiments, the mixing method in step (2) includes ball milling.

[0041] Optionally, the ball-to-material ratio of the ball mill can be 515:1, the rotation speed can be 150rpm~800rpm, and the time can be 0.5h~12h.

[0042] In some embodiments, the gradient heat treatment in step (3) is carried out in an oxygen atmosphere.

[0043] In some embodiments, the gradient heat treatment includes a first heat treatment, a second heat treatment, and a third heat treatment performed sequentially;

[0044] The first heat treatment includes heating to 150°C to 350°C at a rate of 2°C / min to 8°C / min and holding at that temperature for 2h to 4h.

[0045] The heating rate for the first heat treatment is 2℃ / min to 8℃ / min, for example, it can be 2℃ / min, 4℃ / min, 5℃ / min, 6℃ / min or 8℃ / min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0046] The temperature of the first heat treatment is 150℃~350℃, for example, it can be 150℃, 200℃, 250℃, 300℃ or 350℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0047] The holding time for the first heat treatment is 2h to 4h, for example, it can be 2h, 2.5h, 3h, 3.5h or 4h, but it is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0048] The second heat treatment includes heating to 450℃~650℃ at a rate of 0.5℃ / min~1℃ / min and holding at that temperature for 3h~6h.

[0049] The heating rate for the second heat treatment is 0.5℃ / min to 1℃ / min, for example, it can be 0.5℃ / min, 0.6℃ / min, 0.8℃ / min, 0.9℃ / min or 1℃ / min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0050] The temperature for the second heat treatment is 450℃~650℃, for example, it can be 450℃, 500℃, 550℃, 600℃ or 650℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0051] The holding time for the second heat treatment is 3h to 6h, for example, it can be 3h, 4h, 5h or 6h, but it is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0052] The third heat treatment includes heating to 700℃~950℃ at a rate of 0.1℃ / min~0.5℃ / min and holding at that temperature for 6h~18h.

[0053] The heating rate for the third heat treatment is 0.1℃ / min to 0.5℃ / min, for example, it can be 0.1℃ / min, 0.2℃ / min, 0.3℃ / min, 0.4℃ / min or 0.5℃ / min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0054] The temperature for the third heat treatment is 700℃~950℃, for example, it can be 700℃, 750℃, 800℃, 850℃, 900℃ or 950℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0055] The holding time for the third heat treatment is 6h to 18h, for example, it can be 6h, 8h, 10h, 12h, 15h, 16h or 18h, but it is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0056] Under the gradient heat treatment conditions provided by this invention, a gradient ion conductor layer can be obtained, which meets the following conditions:

[0057] The chemical formula of the inner layer is Li. 1+a Al b Ta c P d Given O4, 0.1≤a≤0.3, 0.05≤b≤0.15, 1.85≤c≤1.95, 0.9≤d≤1.1, and a+b=0.2, with a thickness of 2nm~5nm, under these conditions, the lithium-ion conductivity of the inner layer is 5×10⁻⁶. -4 S / cm ~ 8×10 -4 S / cm.

[0058] The chemical formula of the intermediate layer is LiAl. b Ta c P dO4, 0.05≤b≤0.15, 1.85≤c≤1.95, 0.9≤d≤1.1, thickness 3nm~7nm, under these conditions, the lithium-ion conductivity of the intermediate layer is 3×10⁻⁶. -4 S / cm ~5×10 -4 S / cm.

[0059] The outer layer has the chemical formula Li. 1+a Al b Ta c P d O4, 0.1≤a≤0.3, 0.05≤b≤0.15, 1.85≤c≤1.95, 0.9≤d≤1.1, thickness of 1nm~3nm, under these conditions, the lithium-ion conductivity of the inner layer is ≤1×10⁻⁶. -4 S / cm.

[0060] In some embodiments, the surface modification in step (3) includes: after gradient heat treatment, the material is dispersed together with fluoropolymer, lithium salt and succinic acid in N-methylpyrrolidone, and a surface modification layer is formed by spray pyrolysis.

[0061] In some embodiments, the mass of the fluoropolymer is 2wt% to 10wt% of the material after gradient heat treatment, for example, it can be 2wt%, 4wt%, 5wt%, 6wt%, 8wt% or 10wt%, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0062] In some embodiments, the mass of the lithium salt is 5 wt% to 20 wt% of the fluoropolymer, for example, it can be 5 wt%, 8 wt%, 10 wt%, 12 wt%, 15 wt%, 18 wt% or 20 wt%, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0063] In some embodiments, the mass of succinic anion is 5 wt% to 30 wt% of the fluoropolymer, for example, it can be 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt% or 30 wt%, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0064] In some embodiments, the drying temperature in step (3) is 80°C to 150°C and the time is 6h to 18h.

[0065] The drying temperature is 80℃~150℃, for example, it can be 80℃, 100℃, 120℃, 140℃ or 150℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0066] The drying time is 6h to 18h, for example, it can be 6h, 8h, 10h, 12h, 15h or 18h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0067] Under the above process conditions, a surface modification layer with a thickness of 10nm~30nm can be obtained.

[0068] As a preferred embodiment of the preparation method provided in the second aspect, the preparation method includes the following steps:

[0069] (1) Dissolve lithium source, aluminum nitrate, Ta(OC2H5)5 and phosphorus source in alcohol-water mixed solvent, and add citric acid as complexing agent. Stir under heating conditions of 35℃~75℃ and 2h~8h to form a precursor solution with a solid content of 8wt%~12wt%.

[0070] The lithium source includes lithium hydroxide and / or lithium carbonate; the phosphorus source includes any one or a combination of at least two of ammonium phosphate, ammonium monohydrogen phosphate, or ammonium dihydrogen phosphate.

[0071] The volume ratio of ethanol to water in the alcohol-water mixed solvent is 2:1 to 5:1;

[0072] The molar amount of citric acid is 0.5 to 3 times the total molar amount of metal in the precursor solution;

[0073] (2) In a nitrogen atmosphere, the kernel and the precursor solution are ball-milled to obtain a mixture;

[0074] The mass ratio of the core to the precursor solution is 13:1 to 20:1;

[0075] The ball-to-material ratio of the ball mill is 515:1, the rotation speed is 150 rpm to 800 rpm, and the time is 0.5 h to 12 h.

[0076] (3) The mixture is subjected to gradient heat treatment in an oxygen atmosphere, then surface modification is performed, and it is dried at 80℃~150℃ for 6h~18h to obtain a ternary cathode material with a gradient coating.

[0077] The gradient heat treatment includes a first heat treatment, a second heat treatment, and a third heat treatment performed sequentially. The first heat treatment includes heating to 150℃~350℃ at a rate of 2℃ / min~8℃ / min and holding at that temperature for 2h~4h. The second heat treatment includes heating to 450℃~650℃ at a rate of 0.5℃ / min~1℃ / min and holding at that temperature for 3h~6h. The third heat treatment includes heating to 700℃~950℃ at a rate of 0.1℃ / min~0.5℃ / min and holding at that temperature for 6h~18h.

[0078] The surface modification includes: the material after gradient heat treatment is dispersed together with a fluoropolymer, a lithium salt and succinic anion in N-methylpyrrolidone, and a surface modification layer is formed by spray pyrolysis; the mass of the fluoropolymer is 2wt% to 10wt% of the material after gradient heat treatment; the mass of the lithium salt is 5wt% to 20wt% of the fluoropolymer; and the mass of the succinic anion is 5wt% to 30wt% of the fluoropolymer.

[0079] Thirdly, the present invention provides a solid-state battery, the solid-state battery comprising the ternary cathode material with a gradient coating as described in the first aspect, or the ternary cathode material with a gradient coating prepared by the preparation method described in the second aspect.

[0080] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0081] Compared with the prior art, the present invention has the following beneficial effects:

[0082] This invention achieves a gradient reduction in the lithium-ion transport barrier from the core to the solid electrolyte by setting a gradient ion conductor layer. The inner layer has a cubic phase structure, which can remain stable at high temperatures and avoid the phase transition problem that occurs in traditional coating layers. During cycling, aluminum and tantalum elements in the gradient ion conductor layer will agglomerate towards the interface, forming a Li-Al-Ta-PO interface buffer layer in situ, further suppressing side reactions. The surface modification layer and the internal gradient ion conductor layer work together to construct a dual conductive network, which improves the ionic conductivity, and the preparation method is simple and easy to implement. Detailed Implementation

[0083] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0084] The "range" disclosed in this invention can be defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the specific range. This type of range definition can include or exclude endpoints; any endpoint can be independently included or excluded, and they can be arbitrarily combined, meaning any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60~120 and 80~110 are listed for specific parameters, it is understood that ranges of 60~110 and 80~120 are also expected. Furthermore, if minimum range values ​​1 and 2 are listed, and maximum range values ​​3, 4, and 5 are also listed, then the following ranges are all expected: 1~3, 1~4, 1~5, 2~3, 2~4, and 2~5. In this invention, unless otherwise stated, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0~5" indicates that all real numbers between "0" and "5" have been listed in this article; "0~5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is described as an integer ≥2, it is equivalent to listing integers such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc. For instance, when a parameter is described as an integer selected from "2~10", it is equivalent to listing the integers 2, 3, 4, 5, 6, 7, 8, 9, and 10.

[0085] In this invention, "a combination of at least two" refers to a quantity greater than or equal to two, unless otherwise specified. For example, "any combination of one or at least two" means one or more or more items. It can be understood that when referring to "a combination of at least two," it refers to any suitable combination of multiple items, that is, a combination of "at least two" items carried out in a manner that does not conflict with and enables the implementation of this invention.

[0086] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0087] The term "embodiment" as used in this invention means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment or implementation of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described in this invention can be combined with other embodiments.

[0088] Those skilled in the art will understand that the order in which the steps are written in the methods of the various embodiments does not imply a strict execution order. The detailed execution order of each step should be determined by its function and possible internal logic. Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, but are preferably performed sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the method may also include step (c), meaning that step (c) can be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0089] In this invention, open-ended technical features or solutions described using terms such as "comprising" do not exclude additional members beyond those listed unless otherwise specified. They can be considered as providing both closed-ended features or solutions comprised of the listed members and open-ended features or solutions that include additional members beyond the listed members. For example, A includes a1, a2, and a3. Unless otherwise specified, it may also include other members or exclude additional members. This can be considered as providing both technical features or solutions where "A is composed of a1, a2, and a3" or "A is selected from a1, a2, and a3," and technical features or solutions where "A includes not only a1, a2, and a3, but also other members."

[0090] In this invention, unless otherwise specified, the features or solutions corresponding to "and / or" include any one of two or more of the related listed items, as well as any and all combinations of the related listed items. These arbitrary and all combinations include any two related listed items, any more related listed items, or a combination of all related listed items. For example, "A and / or B" represents a group consisting of A, B, and "a combination of A and B". "Containing A and / or B" can mean "containing A, containing B, and containing A and B", or "containing A, containing B, or containing A and B", and can be appropriately understood according to the context.

[0091] In this invention, the terms "first aspect," "second aspect," "third aspect," "fourth aspect," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly indicating the importance or quantity of the indicated technical features. Moreover, "first," "second," "third," "fourth," etc., serve only as a non-exhaustive enumeration and should be understood not to constitute a closed limitation on the quantity.

[0092] In this invention, "optional" means that something is optional, that is, it refers to any one of the two parallel solutions of "having" or "not having". If there are multiple "optional" options in a technical solution, unless otherwise specified, and there are no contradictions or mutual constraints, then each "optional" option is independent.

[0093] In this invention, "room temperature" generally refers to 4℃~35℃, and can refer to 20℃±5℃. In some embodiments of this invention, room temperature refers to 20℃~30℃.

[0094] Example 1

[0095] This embodiment provides a method for preparing a ternary cathode material with a gradient coating, comprising the following steps:

[0096] (1) Lithium carbonate, aluminum nitrate, Ta(OC2H5)5 and ammonium phosphate were dissolved in an alcohol-water mixed solvent, and citric acid was added as a complexing agent. The mixture was stirred at a temperature of 50°C for 5 hours to form a precursor solution with a solid content of 10 wt%.

[0097] The volume ratio of ethanol to water in the alcohol-water mixed solvent is 3:1;

[0098] The molar amount of citric acid is twice the total number of moles of metal in the precursor solution;

[0099] (2) In a nitrogen atmosphere, the core (LiNi) 0.92 Co 0.04 Mn 0.04 O2, D50 is 10μm, specific surface area is 15m² 2 / g) was ball-milled with the precursor solution to obtain a mixture;

[0100] The mass ratio of the core to the precursor solution is 15:1;

[0101] The ball-to-material ratio of the ball mill was 515:1, the rotation speed was 400 rpm, and the time was 4 hours.

[0102] (3) The mixture is subjected to gradient heat treatment in an oxygen atmosphere, then surface modification is performed, and it is dried at 120°C for 12 hours to obtain a ternary cathode material with a gradient coating.

[0103] The gradient heat treatment includes a first heat treatment, a second heat treatment, and a third heat treatment performed sequentially; the first heat treatment includes heating to 250°C at a rate of 5°C / min and holding at that temperature for 3 hours; the second heat treatment includes heating to 550°C at a rate of 0.8°C / min and holding at that temperature for 5 hours; the third heat treatment includes heating to 850°C at a rate of 0.3°C / min and holding at that temperature for 12 hours.

[0104] The surface modification includes: the material after gradient heat treatment is co-dispersed with PVDF-HFP (Kynar Flex LBG), LiTFSI and succinate in N-methylpyrrolidone, and a surface modification layer is formed by spray pyrolysis; the mass of the fluoropolymer is 6 wt% of the material after gradient heat treatment; the mass of the lithium salt is 12 wt% of the fluoropolymer; and the mass of the succinate is 20 wt% of the fluoropolymer.

[0105] The ternary cathode material with a gradient coating obtained in this embodiment includes a core, a gradient ion conductor layer coated on the surface of the core, and a surface modification layer coated on the surface of the gradient ion conductor layer; along the direction away from the core, the gradient ion conductor layer includes an inner layer, a middle layer, and an outer layer; the chemical formula of the inner layer is Li. 1.15 Al 0.05 Ta 1.9 PO4, the chemical formula of the middle layer is LiAl 0.05 Ta 1.9 PO4, the outer chemical formula is Li 0.85 Al 0.05 Ta 1.9 PO4; the inner layer is 4nm thick, the middle layer is 5nm thick, the outer layer is 2nm thick, and the surface modification layer is 20nm thick.

[0106] Example 2

[0107] This embodiment provides a method for preparing a ternary cathode material with a gradient coating, comprising the following steps:

[0108] (1) Lithium carbonate, aluminum nitrate, Ta(OC2H5)5 and ammonium phosphate were dissolved in an alcohol-water mixed solvent, and citric acid was added as a complexing agent. The mixture was stirred at a temperature of 35°C for 8 hours to form a precursor solution with a solid content of 8 wt%.

[0109] The volume ratio of ethanol to water in the alcohol-water mixed solvent is 2:1;

[0110] The molar amount of citric acid is 0.5 times the total molar amount of metal in the precursor solution;

[0111] (2) In a nitrogen atmosphere, the core (LiNi) 0.92 Co 0.04 Mn 0.04 O2, D50 is 3μm, specific surface area is 3m² 2 / g) was ball-milled with the precursor solution to obtain a mixture;

[0112] The mass ratio of the core to the precursor solution is 13:1;

[0113] The ball-to-material ratio of the ball mill was 515:1, the rotation speed was 400 rpm, and the time was 4 hours.

[0114] (3) The mixture is subjected to gradient heat treatment in an oxygen atmosphere, then surface modification is performed, and it is dried at 80°C for 18 hours to obtain a ternary cathode material with a gradient coating.

[0115] The gradient heat treatment includes a first heat treatment, a second heat treatment, and a third heat treatment performed sequentially; the first heat treatment includes heating to 150°C at a rate of 2°C / min and holding at that temperature for 4 hours; the second heat treatment includes heating to 450°C at a rate of 0.5°C / min and holding at that temperature for 6 hours; the third heat treatment includes heating to 700°C at a rate of 0.1°C / min and holding at that temperature for 18 hours.

[0116] The surface modification includes: the material after gradient heat treatment is co-dispersed with PVDF-HFP (Kynar Flex LBG), LiTFSI and succinate in N-methylpyrrolidone, and a surface modification layer is formed by spray pyrolysis; the mass of the fluoropolymer is 5 wt% of the material after gradient heat treatment; the mass of the lithium salt is 5 wt% of the fluoropolymer; and the mass of the succinate is 5 wt% of the fluoropolymer.

[0117] The ternary cathode material with a gradient coating obtained in this embodiment includes a core, a gradient ion conductor layer coated on the surface of the core, and a surface modification layer coated on the surface of the gradient ion conductor layer; along the direction away from the core, the gradient ion conductor layer includes an inner layer, a middle layer, and an outer layer; the chemical formula of the inner layer is Li. 1.1 Al 0.1 Ta 1.85 P 0.9 O4, the chemical formula of the intermediate layer is LiAl 0.1 Ta 1.85 P 0.9 O4, the outer chemical formula of which is Li 0.9 Al 0.1 Ta 1.85 P 0.9 O4; the inner layer is 2nm thick, the middle layer is 3nm thick, the outer layer is 1nm thick, and the surface modification layer is 10nm thick.

[0118] Example 3

[0119] This embodiment provides a method for preparing a ternary cathode material with a gradient coating, comprising the following steps:

[0120] (1) Lithium carbonate, aluminum nitrate, Ta(OC2H5)5 and ammonium phosphate were dissolved in an alcohol-water mixed solvent, and citric acid was added as a complexing agent. The mixture was stirred at a temperature of 75°C for 2 hours to form a precursor solution with a solid content of 12 wt%.

[0121] The volume ratio of ethanol to water in the alcohol-water mixed solvent is 5:1;

[0122] The molar amount of citric acid is three times the total molar amount of metal in the precursor solution;

[0123] (2) In a nitrogen atmosphere, the core (LiNi) 0.92 Co 0.04 Mn 0.04 O2, D50 is 15μm, specific surface area is 25m² 2 / g) was ball-milled with the precursor solution to obtain a mixture;

[0124] The mass ratio of the core to the precursor solution is 20:1;

[0125] The ball-to-material ratio of the ball mill was 515:1, the rotation speed was 400 rpm, and the time was 4 hours.

[0126] (3) The mixture is subjected to gradient heat treatment in an oxygen atmosphere, then surface modification is performed, and it is dried at 150°C for 6 hours to obtain a ternary cathode material with a gradient coating.

[0127] The gradient heat treatment includes a first heat treatment, a second heat treatment, and a third heat treatment performed sequentially; the first heat treatment includes heating to 350°C at a rate of 8°C / min and holding at that temperature for 2 hours; the second heat treatment includes heating to 650°C at a rate of 1°C / min and holding at that temperature for 3 hours; the third heat treatment includes heating to 950°C at a rate of 0.5°C / min and holding at that temperature for 6 hours.

[0128] The surface modification includes: the material after gradient heat treatment is co-dispersed with PVDF-HFP (Kynar Flex LBG), LiTFSI and succinate in N-methylpyrrolidone, and a surface modification layer is formed by spray pyrolysis; the mass of the fluoropolymer is 10 wt% of the material after gradient heat treatment; the mass of the lithium salt is 2 wt% of the fluoropolymer; and the mass of the succinate is 30 wt% of the fluoropolymer.

[0129] The ternary cathode material with a gradient coating obtained in this embodiment includes a core, a gradient ion conductor layer coated on the surface of the core, and a surface modification layer coated on the surface of the gradient ion conductor layer; along the direction away from the core, the gradient ion conductor layer includes an inner layer, a middle layer, and an outer layer; the chemical formula of the inner layer is Li. 1.1 Al 0.1 Ta 1.95 P 1.1 O4, the chemical formula of the intermediate layer is LiAl 0.1 Ta 1.95 P 1.1O4, the outer chemical formula of which is Li 0.9 Al 0.1 Ta 1.95 P 1.1 O4; the inner layer is 5nm thick, the middle layer is 7nm thick, the outer layer is 3nm thick, and the surface modification layer is 30nm thick.

[0130] Example 4

[0131] This embodiment provides a method for preparing a ternary cathode material with a gradient coating. Except for the absence of a first heat treatment, the method is the same as that in Example 1.

[0132] Example 5

[0133] This embodiment provides a method for preparing a ternary cathode material with a gradient coating. Except for the absence of a second heat treatment, the method is the same as in Example 1.

[0134] Comparative Example 1

[0135] This comparative example provides a method for preparing a ternary cathode material with a gradient coating. Except for the absence of an aluminum source, the method is the same as in Example 1.

[0136] Comparative Example 2

[0137] This comparative example provides a method for preparing a ternary cathode material with a gradient coating. Except for the absence of a tantalum source, the method is the same as in Example 1.

[0138] Comparative Example 3

[0139] This comparative example provides a method for preparing a ternary cathode material. Except for the fact that only the third heat treatment was performed, and the first and second heat treatments were not performed, the rest is the same as in Example 1.

[0140] Performance Characterization

[0141] The ternary cathode material and sulfide solid electrolyte (Li7La3Zr2O) provided in the above embodiments and comparative examples were respectively compared. 12 -Li2S-P2S5 and conductive agent (Super P) are mixed in a mass ratio of 8:1.5:0.5 and cold-pressed into a 12mm diameter positive electrode sheet under a pressure of 150MPa (ternary positive electrode material loading is 4mg / cm³). 2In an argon glove box, a solid-state battery was obtained by stacking a Li-In alloy as the counter electrode, a positive electrode sheet and a solid electrolyte (500nm thick 70Li2S-30P2S5) in a battery case, and applying a pressure of 0.5MPa to fix it. A charge-discharge test system was used to activate the battery twice in the range of 3V~4.6V at a rate of 0.1C, and then cycled 500 times at a rate of 0.5C. The discharge capacity of the first and 500th cycles was recorded. The percentage of the discharge capacity of the 500th cycle to the discharge capacity of the first cycle was taken as the room temperature cycle capacity retention rate.

[0142] In addition, the assembled solid-state battery was placed in a 60°C constant temperature chamber for 4 hours and cycled 300 times at a 0.5C rate in the range of 3V~4.6V. The discharge capacity of the 1st and 300th cycles was recorded. The percentage of the discharge capacity of the 300th cycle to the discharge capacity of the 1st cycle was taken as the high-temperature cycle capacity retention rate.

[0143] The electrochemical impedance spectroscopy (EIS) of the solid-state battery was tested using an electrochemical workstation at a frequency band of 10 GHz. -2 The EIS spectra of the solid-state battery after activation were measured at Hz and with an amplitude of 5mV. After one cycle and 200 cycles, the interfacial impedance between the positive electrode and the solid electrolyte was extracted by equivalent circuit fitting. The percentage increase in impedance after 200 cycles relative to the impedance after one cycle was taken as the interfacial impedance growth rate.

[0144] The characterization results are shown in Table 1.

[0145] Table 1

[0146]

[0147] As can be seen from Examples 1 to 3 in Table 1, the ternary cathode material with gradient coating provided by the present invention exhibits excellent electrochemical performance, not only with high initial discharge specific capacity, but also outstanding capacity retention rate at room temperature and high temperature cycles, and low interface impedance growth rate.

[0148] A comparison of Examples 1 with Examples 4 and 5 shows that the first and second heat treatments are crucial for improving material performance. Without either the first or second heat treatment, the initial discharge specific capacity and room temperature cycling capacity retention decrease, while the interfacial impedance growth rate increases. This is because the first heat treatment promotes the formation of a lithium-rich inner cubic phase structure, which helps ensure high-temperature stability; the second heat treatment promotes the uniform distribution of elements in the gradient ion conductor layer.

[0149] A comparison of Example 1 and Comparative Example 1 shows that the addition of an aluminum source is beneficial for improving the material's cycling performance and suppressing the increase in interfacial impedance. Without an aluminum source, the material's initial discharge specific capacity is only 169.2 mAh / g, the high-temperature cycling capacity retention is only 64.5%, and the interfacial impedance growth rate reaches 49.3%. This is because the aluminum source can segregate towards the interface during cycling, forming a Li-Al-O interfacial buffer layer in situ. Without an aluminum source, this buffer layer cannot be formed, and the exacerbated side reactions lead to capacity decay and increased impedance.

[0150] A comparison of Example 1 and Comparative Example 2 shows that the introduction of a tantalum source is indispensable for optimizing the electrochemical performance of the material. Without a tantalum source, the initial discharge specific capacity and cycle capacity retention of the material both decreased further, and the interface impedance growth rate reached as high as 59.5%. This is because the tantalum source, together with the surface modification layer and the gradient ion conductor layer, synergistically constructs a dual conductive network, improving ion conductivity. The lack of a tantalum source leads to an incomplete conductive network, reduced ion transport efficiency, and difficulty in effectively suppressing side reactions.

[0151] A comparison of Example 1 and Comparative Example 3 shows that the third heat treatment alone cannot meet the material performance requirements. With only the third heat treatment, the material's initial discharge specific capacity is only 159.5 mAh / g, the room temperature cycling capacity retention is less than 50%, and the interface impedance growth rate is as high as 118.5%. This is because the three heat treatments need to work synergistically: the first and second heat treatments ensure the structural integrity and elemental gradient distribution of the gradient ion conductor layer, while the third heat treatment optimizes the bonding force between the surface modification layer and the gradient layer. A single heat treatment step cannot form a stable gradient coating structure and interface state.

[0152] This invention achieves a gradient reduction in the lithium-ion transport barrier from the core to the solid electrolyte by setting a gradient ion conductor layer. The inner layer has a cubic phase structure, which can remain stable at high temperatures and avoid the phase transition problem that occurs in traditional coating layers. During cycling, aluminum and tantalum elements in the gradient ion conductor layer will agglomerate towards the interface, forming a Li-Al-Ta-PO interface buffer layer in situ, further suppressing side reactions. The surface modification layer and the internal gradient ion conductor layer work together to construct a dual conductive network, which improves the ionic conductivity, and the preparation method is simple and easy to implement.

[0153] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A ternary cathode material with a gradient coating, characterized in that, The ternary cathode material with gradient coating includes a core, a gradient ion conductor layer coated on the surface of the core, and a surface modification layer coated on the surface of the gradient ion conductor layer. Along a direction away from the core, the gradient ion conductor layer comprises an inner layer, an intermediate layer, and an outer layer; the chemical formula of the inner layer is Li. 1+a Al b Ta c P d O4, the chemical formula of the intermediate layer is LiAl b Ta c P d O4, the outer chemical formula of which is Li 1-a Al b Ta c P d O4, where 0.1≤a≤0.3, 0.05≤b≤0.15, 1.85≤c≤1.95, and 0.9≤d≤1.1; The surface modification layer is a composite layer composed of a fluoropolymer and a lithium salt.

2. The ternary cathode material according to claim 1, characterized in that, The chemical formula of the core is LiNi x Co y Mn z O2, where 0.9 ≤ x < 1.0, 0 < y ≤ 0.05, 0 < z ≤ 0.05, and x + y + z = 1; And / or, the median particle size D50 of the kernel is 3μm~15μm; And / or, the specific surface area of ​​the core is 3m². 2 / g~25m 2 / g.

3. The ternary cathode material according to claim 1 or 2, characterized in that, The thickness of the inner layer is 2nm~5nm; And / or, the thickness of the intermediate layer is 3nm~7nm; And / or, the thickness of the outer layer is 1nm~3nm.

4. The ternary cathode material according to any one of claims 1 to 3, characterized in that, The fluoropolymer includes PVDF-HFP; And / or, the lithium salt includes LiTFSI; And / or, the thickness of the surface modification layer is 10nm~30nm.

5. A method for preparing a ternary cathode material with a gradient coating, characterized in that, The preparation method includes the following steps: (1) Dissolve lithium source, aluminum source, tantalum source and phosphorus source in alcohol-water mixed solvent, add citric acid as complexing agent, and stir under heating conditions to form precursor solution; (2) In a protective atmosphere, the core is mixed with the precursor solution to obtain a mixture; (3) The mixture is subjected to gradient heat treatment, and then surface modification and drying are performed to obtain the ternary cathode material with gradient coating as described in any one of claims 1 to 4.

6. The preparation method according to claim 5, characterized in that, The lithium source in step (1) includes lithium hydroxide and / or lithium carbonate; And / or, the aluminum source in step (1) includes aluminum nitrate; And / or, the tantalum source in step (1) includes Ta(OC2H5)5; And / or, the phosphorus source in step (1) includes any one or a combination of at least two of ammonium phosphate, ammonium monohydrogen phosphate or ammonium dihydrogen phosphate; And / or, the volume ratio of ethanol to water in the alcohol-water mixed solvent in step (1) is 2:1 to 5:1; And / or, the molar amount of citric acid in step (1) is 0.5 to 3 times the total molar amount of metal in the precursor solution; And / or, the heating conditions in step (1) are 35℃~75℃ for 2h~8h; And / or, the solid content of the precursor solution in step (1) is 5wt%~20wt%.

7. The preparation method according to claim 5 or 6, characterized in that, The protective atmosphere gas used in step (2) includes nitrogen; And / or, the mass ratio of the kernel to the precursor solution in step (2) is 13:1 to 20:1; And / or, the mixing method described in step (2) includes ball milling.

8. The preparation method according to any one of claims 5 to 7, characterized in that, The gradient heat treatment in step (3) is carried out in an oxygen atmosphere; And / or, the gradient heat treatment includes a first heat treatment, a second heat treatment and a third heat treatment performed sequentially; The first heat treatment includes heating to 150°C to 350°C at a rate of 2°C / min to 8°C / min and holding at that temperature for 2h to 4h. The second heat treatment includes heating to 450℃~650℃ at a rate of 0.5℃ / min~1℃ / min and holding at that temperature for 3h~6h; The third heat treatment includes heating to 700℃~950℃ at a rate of 0.1℃ / min~0.5℃ / min and holding at that temperature for 6h~18h.

9. The preparation method according to any one of claims 5 to 8, characterized in that, The surface modification in step (3) includes: after gradient heat treatment, the material is dispersed together with fluoropolymer, lithium salt and succinic acid in N-methylpyrrolidone, and a surface modification layer is formed by spray pyrolysis; And / or, the mass of the fluoropolymer is 2wt% to 10wt% of the material after gradient heat treatment; And / or, the lithium salt is 5wt% to 20wt% of the fluoropolymer; And / or, 5 wt% to 30 wt% of the fluoropolymer; And / or, the drying temperature in step (3) is 80℃~150℃ and the time is 6h~18h.

10. A solid-state battery, characterized in that, The solid-state battery includes the ternary cathode material with a gradient coating as described in any one of claims 1 to 4, or the ternary cathode material with a gradient coating prepared by the preparation method described in any one of claims 5 to 9.

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