Composite positive electrode material, preparation method thereof and secondary battery

Abstract

The invention relates to a composite positive electrode material, a preparation method thereof and a secondary battery, the composite positive electrode material comprises a manganese-containing positive electrode active material, a rare earth metal element and a sulfur element, and the sulfur element and a manganese element in the manganese-containing positive electrode active material are bonded to form an S-Mn bond. Wherein after the rare earth metal element is doped, crystal lattices of the manganese-containing positive electrode active material can be stabilized, distortion can be inhibited, a nanoscale composite interface layer can be constructed in situ, side reactions are reduced, ion conduction is improved, interface impedance is reduced, and the sulfur element can inhibit dissolution of manganese ions through a chemical bond anchoring effect; under the synergistic effect, the rate capability and the cycle life of the manganese-containing positive electrode active material are improved.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to a composite cathode material and its preparation method, and a secondary battery. Background Technology

[0002] Manganese-containing cathode materials refer to cathode materials containing manganese, such as lithium iron manganese phosphate, lithium manganese oxide, and lithium nickel cobalt manganese oxide. Manganese-containing cathode materials offer significant advantages. Taking lithium iron manganese phosphate as an example, it combines the high safety of lithium iron phosphate with the high voltage characteristics of lithium manganese phosphate, exhibiting advantages such as environmental friendliness, high energy density, and good cycle stability, making it suitable for electric vehicles and energy storage systems.

[0003] However, manganese-containing cathode materials exhibit a significant Jahn-Teller effect. The dissolution of manganese ions leads to instability in the material's crystal lattice structure, resulting in decreased cycle performance. Furthermore, the dissolved manganese may react with the electrolyte to generate harmful byproducts, further deteriorating its electrical performance. Therefore, suppressing Jahn-Teller distortion and reducing manganese ion dissolution has become crucial. Summary of the Invention

[0004] The purpose of this invention is to provide a composite cathode material and its preparation method, as well as a secondary battery. This composite cathode material can reduce manganese ion dissolution and structural distortion.

[0005] To achieve the objectives of this invention, the following technical solution is provided:

[0006] In a first aspect, the present invention provides a composite cathode material, the composite cathode material comprising a manganese-containing cathode active material, rare earth metal elements and sulfur elements, wherein the sulfur elements are bonded to the manganese elements in the manganese-containing cathode active material to form S-Mn bonds.

[0007] In some embodiments, the manganese-containing positive electrode active material includes lithium manganese iron phosphate, and the rare earth metal element includes one or more of La, Ce, Pr, Y, Nd, Eu, Tb, Sm, Gd, and Er.

[0008] In some embodiments, the composite cathode material further includes a carbon material, at least a portion of which coats the manganese-containing cathode active material, and at least a portion of which the sulfur element bonds with the carbon element in the carbon material to form SC bonds.

[0009] In some embodiments, the composite cathode material further includes silicon, and at least a portion of the silicon is doped into the manganese-containing cathode active material.

[0010] In some embodiments, the content of the rare earth metal element in the composite cathode material is 100ppm to 800ppm.

[0011] In some embodiments, the sulfur content in the composite cathode material is 200 ppm to 500 ppm.

[0012] In some embodiments, the silicon content in the composite cathode material is 100 ppm to 400 ppm.

[0013] Secondly, the present invention provides a method for preparing a composite cathode material, the method comprising: surface modification of a rare earth metal oxide to obtain a hydroxylated rare earth metal oxide; mixing the hydroxylated rare earth metal oxide with a mercapto-containing organic compound to obtain a mercapto-modified rare earth metal oxide; mixing a lithium source, a manganese source, an iron source, a phosphorus source and a mercapto-modified rare earth metal oxide and sintering them to obtain a composite cathode material; wherein the composite cathode material comprises a manganese-containing cathode active material, a rare earth metal element and a sulfur element, wherein the sulfur element bonds with the manganese element in the manganese-containing cathode active material to form an S-Mn bond.

[0014] In some embodiments, the surface modification of rare earth metal oxides to obtain hydroxylated rare earth metal oxides includes: dispersing rare earth metal oxides in an acidic or alkaline solution, subjecting them to ultrasonic treatment, and then washing them to obtain the hydroxylated rare earth metal oxides.

[0015] In some embodiments, the step of mixing the hydroxylated rare earth metal oxide and the thiol-containing organic compound to obtain the thiol-modified rare earth metal oxide includes: adding the hydroxylated rare earth metal oxide and the thiol-containing organic compound to a first mixed solvent to obtain a mixture; reacting and separating the mixture under an inert atmosphere to obtain a preproduct; alternately washing the preproduct with a nonpolar solvent and a polar solvent, and drying it to obtain the thiol-modified rare earth metal oxide; wherein the first mixed solvent includes the nonpolar solvent and the polar solvent, and the thiol-containing organic compound includes a thiol silane coupling agent.

[0016] In some embodiments, the mercaptosilane coupling agent includes one or more of (3-mercaptopropyl)trimethoxysilane, γ-mercaptopropyltriethoxysilane, mercaptosilane-PEG derivatives, bis(triethoxysilylpropyl)disulfide, and capped mercaptosilanes.

[0017] In some embodiments, the step of mixing and sintering the lithium source, manganese source, iron source, phosphorus source, and the thiol-modified rare earth metal oxide to obtain a composite cathode material includes: dissolving the iron source, manganese source, and complexing agent in a second mixed solvent, adjusting the pH value and evaporation temperature to remove the second mixed solvent to obtain a gel product, and subjecting the gel product to heat treatment and calcination to obtain an iron-manganese oxide compound; and mixing and sintering the lithium source, the phosphorus source, the iron-manganese oxide compound, the thiol-modified rare earth metal oxide, and a carbon source to obtain the composite cathode material.

[0018] In some embodiments, the complexing agent includes one or more of citric acid, ammonium citrate, maleic acid, oxalic acid, ammonium oxalate, ascorbic acid, ethanolamine, diethanolamine, and triethanolamine.

[0019] In some embodiments, the molar ratio of metal ions to the complexing agent is 1:(1.5 to 1.8), and the molar amount of the metal ions is the sum of the molar amounts of the iron source and the manganese source.

[0020] In some embodiments, the pH value of the second mixed solvent is 3.5 to 4.0.

[0021] In some embodiments, the evaporation temperature is 90°C to 110°C.

[0022] In some embodiments, the heat treatment temperature is 200°C to 300°C.

[0023] In some embodiments, the calcination temperature is 600°C to 800°C.

[0024] In some embodiments, the rare earth metal oxide is 2% to 5% of the molar amount of the manganese-containing positive electrode active material.

[0025] In some embodiments, the hydroxylated rare earth metal oxide is 1% to 4% of the molar amount of the manganese-containing positive electrode active material.

[0026] In some embodiments, the thiol-modified rare earth metal oxide is 1% to 3% of the molar amount of the manganese-containing positive electrode active material.

[0027] Thirdly, the present invention provides a secondary battery, the secondary battery comprising the composite positive electrode material as described in the first aspect, or the secondary battery comprising the composite positive electrode material prepared by the method for preparing the composite positive electrode material as described in the second aspect.

[0028] The composite cathode material provided by this invention introduces sulfur to form S-Mn bonds with manganese-containing cathode active materials. Sulfur can be used for reduction and to maintain manganese ions in a stable divalent state, improving compatibility while significantly reducing the presence of trivalent manganese ions and manganese dissolution, thereby suppressing structural distortions in manganese-containing cathode active materials caused by the Jahn-Teller effect. Simultaneously, by introducing rare earth metal elements, rare earth metals are used to replace Li... + or Fe 2+ / Mn 2+ The site can form strong covalent bonds in the manganese-containing positive electrode active material with olivine structure, thereby significantly improving the lattice energy, increasing the dissociation energy of Mn-O bonds in the electrolyte, further stabilizing the lattice structure, suppressing distortion, and enhancing the stability of the composite positive electrode material, thus improving the energy density and electrochemical performance of the composite positive electrode material. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0030] Figure 1 This is a method for preparing a composite cathode material in one embodiment;

[0031] Figure 2 This is a method for preparing a manganese-containing positive electrode active material in one embodiment. Detailed Implementation

[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used in this invention includes any and all combinations of one or more of the associated listed items.

[0034] It should be noted that the "range" disclosed in this invention is 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. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, 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 ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are 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 "0~5" have been listed in this article; "0~5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0035] All steps of the present invention can be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates 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.

[0036] The following detailed description of some embodiments of the present invention is provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0037] This invention provides a composite cathode material, which includes a manganese-containing cathode active material, rare earth metal elements, and sulfur elements, wherein the sulfur elements are bonded to the manganese elements in the manganese-containing cathode active material to form S-Mn bonds.

[0038] In specific embodiments, manganese-containing positive electrode active materials include lithium manganese oxide (LiMn2O4, LMO) and lithium nickel cobalt manganese oxide (LiNi). x Co y Mn 1-x O2, NCM), lithium manganese iron phosphate (LiMn) x Fe 1-x PO4, LMFP), spinel lithium nickel manganese oxide (LiNi)0.5 Mn 1.5 One or more of O4, LNMO.

[0039] In a specific embodiment, the manganese-containing positive electrode active material includes lithium manganese iron phosphate, and the chemical formula of the composite positive electrode material includes Li a Mn b Fe 1-b-c M c PO4S d Where M is a rare earth metal element, 1.00≤a≤1.08, 0.2≤b<0.7, 0.01≤c<0.105, 0.012≤d<0.125.

[0040] In a specific embodiment, rare earth metal elements and sulfur are doping elements in the composite cathode material. Both are at least partially doped into the manganese-containing cathode active material. The sulfur element bonds with the manganese element in the manganese-containing cathode active material, forming S-Mn bonds. These S-Mn bonds are used to immobilize manganese ions in the manganese-containing cathode active material, reducing manganese ion dissolution. It should be noted that bonding refers to the strong interaction between two or more adjacent atoms, where atoms are linked together to form a molecule.

[0041] In specific embodiments, rare earth metal elements include one or more selected from La, Ce, Pr, Y, Nd, Eu, Tb, Sm, Gd, and Er. These rare earth metal elements are used to replace Li. + or Fe 2+ / Mn 2+ At specific sites, rare earth metal elements form covalent bonds in manganese-containing cathode active materials, which can significantly enhance the lattice energy of manganese-containing cathode active materials and increase the dissociation energy of Mn-O bonds in the electrolyte.

[0042] In a specific embodiment, the rare earth metal element in the composite cathode material is derived from rare earth metal compounds, specifically rare earth metal oxides, including one or more of lanthanum oxide (La2O3), neodymium oxide (Nd2O3), samarium oxide (Sm2O3), europium oxide (EuO / Eu2O3), cerium oxide (CeO2), ytterbium oxide (Yb2O3), and gadolinium (Gd2O3).

[0043] In specific embodiments, at least a portion of the sulfur element is derived from rare earth metal compounds, such as rare earth metal sulfides (lanthanum sulfate). The sulfur element can also be derived from modified rare earth metal oxides, such as rare earth metal oxides containing mercapto groups or rare earth metal oxides containing sulfonic acid groups.

[0044] The composite cathode material provided by this invention introduces sulfur to form S-Mn bonds with manganese-containing cathode active materials. Sulfur can be used for reduction and to maintain manganese ions in a stable divalent state, improving compatibility while significantly reducing the presence of trivalent manganese ions and manganese dissolution, thereby suppressing structural distortions in manganese-containing cathode active materials caused by the Jahn-Teller effect. Simultaneously, by introducing rare earth metal elements, rare earth metals are used to replace Li... + or Fe 2+ / Mn 2+ The site can form strong covalent bonds in the manganese-containing positive electrode active material with olivine structure, thereby significantly improving the lattice energy, increasing the dissociation energy of Mn-O bonds in the electrolyte, further stabilizing the lattice structure, suppressing distortion, and enhancing the stability of the composite positive electrode material, thus improving the energy density and electrochemical performance of the composite positive electrode material.

[0045] In some embodiments, the composite cathode material further includes carbon material, at least a portion of which coats the manganese-containing cathode active material, and at least a portion of the sulfur element bonds with the carbon element in the carbon material to form SC bonds. This invention introduces sulfur into the composite cathode material, and the sulfur element can form SC bonds with the carbon material, thereby creating a three-dimensional conductive network structure with SC bonds on the outside of the manganese-containing cathode active material. This three-dimensional conductive network structure enhances electron conductivity, giving the carbon coating layer stronger electron conductivity, thus improving the rate performance of the composite cathode material.

[0046] In a specific embodiment, carbon material is coated on the outer surface of the manganese-containing positive electrode active material to form a carbon coating layer. The carbon material can be used to improve the conductivity of the composite positive electrode material. In this way, the carbon material can bond with some sulfur elements to form SC bonds, and the carbon coating layer is a three-dimensional conductive network structure with SC bonds.

[0047] It should be noted that the composite cathode material includes both S-Mn bonds and SC bonds. Some sulfur elements can independently bond with manganese elements to form S-Mn bonds, while other sulfur elements can independently bond with carbon elements to form SC bonds. Alternatively, a Mn-SC chemical structure can be formed in the composite cathode material, which can improve the bonding strength between carbon materials and manganese-containing cathode active materials, thereby improving electronic and ion conduction capabilities.

[0048] In some embodiments, the composite cathode material further includes a composite interface layer located on the outside of the composite cathode material. The composite interface layer comprises Li-MO, where M is a rare earth metal element, and is a nanoscale film. This invention, by introducing rare earth metal elements, enables the in-situ construction of a nanoscale composite interface layer outside the manganese-containing cathode active material. This reduces side reactions while improving ion conduction and lowering interfacial impedance. The prepared composite cathode material exhibits excellent rate performance and cycle life.

[0049] In a specific embodiment, rare earth metal elements react with the electrolyte during the first charge-discharge process to form a nanoscale composite interface layer. This composite interface layer coats the outermost layer of the composite cathode material and exhibits high ionic conductivity and low interfacial impedance. Optionally, the composite interface layer can be located between the carbon coating layer and the manganese-containing cathode active material.

[0050] In some embodiments, the composite cathode material further includes silicon, with at least a portion of the silicon doped into the manganese-containing cathode active material. This invention stabilizes the anion redox reaction by doping silicon into the manganese-containing cathode active material. Silicon has tetrahedral sites, enabling it to form strong covalent bonds with oxygen. Simultaneously, the addition of silicon lowers the valence state of manganese, maintaining manganese ions in a stable divalent state, thereby reducing the presence of trivalent manganese ions and manganese dissolution, and mitigating the Jahn-Teller effect.

[0051] In a specific embodiment, rare earth metal elements, sulfur elements, and silicon elements are provided from the same material, and the raw materials for preparing the composite cathode material include thiol-modified rare earth metal oxides; the thiol-modified rare earth metal oxides are prepared by a thiol silane coupling agent and rare earth metal oxides. Thus, by mixing the thiol-modified rare earth metal oxides with the raw materials of the manganese-containing cathode active material, a composite cathode material containing rare earth metal elements, sulfur elements, and silicon elements can be prepared.

[0052] In some embodiments, the content of rare earth metal elements in the composite cathode material is 100ppm to 800ppm. Optionally, the content of rare earth metal elements in the composite cathode material can be 100ppm, 200ppm, 300ppm, 400ppm, 500ppm, 600ppm, 700ppm, or 800ppm.

[0053] Maintaining the above-mentioned content range for rare earth metal elements in the composite cathode material ensures reduced manganese ion dissolution and guarantees the capacity and structural stability of the composite cathode material. When the doping amount of rare earth metal elements is too low, the rare earth metal ions (elements) replace the Li... + or Fe 2+ / Mn 2+With fewer sites, it is difficult to suppress distortion; when the doping amount of rare earth metal elements is too high, rare earth metal ions (elements) replace sites and introduce local stress, which will destroy the stability of the olivine structure of the manganese-containing positive electrode active material, and will form an excessively thick composite interface layer that increases impedance.

[0054] In some embodiments, the sulfur content in the composite cathode material is 200 ppm to 500 ppm. Optionally, the sulfur content in the composite cathode material can be 200 ppm, 250 ppm, 300 ppm, 350 ppm, 400 ppm, 450 ppm, or 500 ppm.

[0055] Maintaining the sulfur content within the aforementioned range in the composite cathode material ensures reduced manganese ion dissolution, minimizes side reactions, and stabilizes the crystal structure. When the sulfur doping level is too low, the S-Mn bonds between sulfur and manganese decrease, leading to increased trivalent manganese ion dissolution. Conversely, excessive sulfur doping results in disulfide bonds (SS) forming between sulfur atoms. These disulfide bonds cannot anchor divalent manganese ions and can also trigger electrolyte decomposition and lithium ion consumption, ultimately disrupting the crystal structure of the manganese-containing cathode active material.

[0056] In some embodiments, the silicon content in the composite cathode material is 100ppm to 400ppm. Optionally, the silicon content in the composite cathode material can be 100ppm, 150ppm, 200ppm, 250ppm, 300ppm, 350ppm, or 400ppm.

[0057] Maintaining the silicon content within the aforementioned range in the composite cathode material ensures reduced manganese ion dissolution and guarantees the material's capacity. Insufficient silicon doping leads to an increase in trivalent manganese ions and consequently, increased manganese ion dissolution. Conversely, excessive silicon doping results in a decrease in the capacity of the composite cathode material because silicon does not participate in the charge-discharge reaction and forms excessive silicon-oxygen bonds, crowding out lithium ion sites.

[0058] In a specific embodiment, the S-Mn and SC bonds in the above embodiments can be characterized by Raman spectroscopy. The peak intensity of the S-Mn bond in the Raman spectrum is 400 cm⁻¹. -1 ~500cm -1 The peak intensity of the SC bond is 600 cm⁻¹. -1 ~700cm -1 Furthermore, Raman spectroscopy and mass spectrometry (MS) can be coupled. Since the rare earth metal elements are introduced in relatively small amounts, qualitative analysis of valence state and bond length can be performed simultaneously using ICP detection coupled with X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS).

[0059] This invention provides a method for preparing a composite cathode material; please refer to [the relevant documentation]. Figure 1 The preparation method is used to prepare the composite cathode material as described in the above embodiments, and the preparation method specifically includes the following steps:

[0060] Step S100: Surface modification of rare earth metal oxides is performed to obtain hydroxylated rare earth metal oxides.

[0061] Step S200: The hydroxylated rare earth metal oxide and the thiol-containing organic compound are mixed to obtain the thiol-modified rare earth metal oxide.

[0062] In step S300, lithium source, manganese source, iron source, phosphorus source and mercapto-modified rare earth metal oxide are mixed and sintered to obtain composite cathode material.

[0063] The method for preparing composite cathode materials provided by the present invention involves first preparing thiol-modified rare earth metal oxides, and then mixing and sintering the thiol-modified rare earth metal oxides with raw materials containing manganese cathode active materials to obtain composite cathode materials. This method can simultaneously introduce rare earth metal elements and sulfur elements into the composite cathode materials, while reducing the introduction of other impurities and improving the purity of the composite cathode materials.

[0064] In a specific embodiment, in step S100, the rare earth metal oxide includes one or more of the following: lanthanum oxide (La2O3), neodymium oxide (Nd2O3), samarium oxide (Sm2O3), europium oxide (EuO / Eu2O3), cerium oxide (CeO2), ytterbium oxide (Yb2O3), and gadolinium (Gd2O3).

[0065] In a specific embodiment, step S100 involves surface modification of rare earth metal oxides using methods including, but not limited to, one or more of acid-base treatment, hydrothermal treatment, and plasma treatment. It should be noted that surface modification of rare earth metal oxides can yield surface-hydroxylated rare earth metal oxides, meaning that hydroxyl groups (-OH) are grafted onto the surface of the rare earth metal oxides. Hydroxylation modification of rare earth metal oxides facilitates subsequent grafting of thiol-containing organic compounds.

[0066] In a specific embodiment, in step S200, the thiol-containing organic compound includes one or more of the following: thiol silane coupling agents, thiol phosphates and their derivatives, thiol acetates and their derivatives, dithiocarbamates, and thiol benzothiazoles and their derivatives. Optionally, the thiol-containing organic compound may specifically include (3-mercaptopropyl)trimethoxysilane, titanium mercaptoacetate, 2-mercaptoethyl phosphate, and thiol ethyl dimethyl dithiocarbamate, etc.

[0067] It should be noted that thiol-containing organic compounds refer to organic compounds containing thiol (-SH) functional groups. By mixing hydroxylated rare earth metal oxides with thiol-containing organic compounds, thiol groups can be modified on rare earth metal oxides, thereby enabling rare earth metal oxides to carry abundant sulfur elements.

[0068] In a specific embodiment, in step S300, the lithium source includes one or more of lithium hydroxide, lithium carbonate, lithium nitrate, lithium sulfate, lithium chloride, lithium phosphate, and lithium acetate. The manganese source includes one or more of manganese acetate, manganese nitrate, manganese sulfate, manganese carbonate, and manganese chloride. The iron source includes one or more of ferric acetate, ferric nitrate, ferric sulfate, ferric carbonate, and ferric chloride. The phosphorus source includes one or more of sodium phosphate, calcium phosphate, sodium dihydrogen phosphate, and ammonium dihydrogen phosphate.

[0069] In some embodiments, step S100 involves surface modification of the rare earth metal oxide to obtain hydroxylated rare earth metal oxide, including: dispersing the rare earth metal oxide in an acidic or alkaline solution, and washing it after ultrasonic treatment to obtain hydroxylated rare earth metal oxide.

[0070] In a specific embodiment, in step S100, the rare earth metal oxide used for surface modification is 2% to 5% of the molar amount of the manganese-containing positive electrode active material. Optionally, the rare earth metal oxide is 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, or 5% of the molar amount of the manganese-containing positive electrode active material. For example, the amount of rare earth metal oxide can be 0.05 mol to 0.50 mol.

[0071] In a specific embodiment, in step S100, the acidic solution can be hydrogen peroxide or a strong acid mixture, wherein the volume fraction of the hydrogen peroxide or strong acid mixture can be 5% to 10%. Optionally, the volume fraction of the hydrogen peroxide or strong acid mixture can be 5%, 6%, 7%, 8%, 9%, or 10%. The strong acid can include one or more of sulfuric acid, phosphoric acid, nitric acid, and perchloric acid.

[0072] In a specific embodiment, in step S100, the temperature of the ultrasonic treatment is 40℃~60℃. Optionally, the temperature of the ultrasonic treatment can be 40℃, 42℃, 44℃, 46℃, 48℃, 50℃, 52℃, 54℃, 56℃, 58℃, or 60℃.

[0073] In a specific embodiment, in step S100, the ultrasonic rate is 28kHz to 40kHz. Optionally, the ultrasonic rate can be 28kHz, 30kHz, 32kHz, 34kHz, 36kHz, 38kHz, or 40kHz.

[0074] In a specific embodiment, the ultrasound time in step S100 is 1 hour to 3 hours. Optionally, the ultrasound time can be 1 hour, 1.2 hours, 1.4 hours, 1.6 hours, 1.7 hours, 2 hours, 2.2 hours, 2.4 hours, 2.6 hours, 2.8 hours, or 3 hours.

[0075] In a specific embodiment, in step S100, the detergent used for washing may include ethanol, and the number of washing cycles is 3 to 5. Optionally, the number of washing cycles can be 3, 4, or 5.

[0076] In a specific embodiment, in step S100, the drying temperature after washing is 90℃~110℃. Optionally, the drying temperature after washing can be 90℃, 92℃, 94℃, 96℃, 98℃, 100℃, 102℃, 104℃, 106℃, 108℃, or 110℃.

[0077] In a specific embodiment, the drying time in step S100 is 8h to 12h. Optionally, the drying time can be 8h, 8.5h, 9h, 9.5h, 10h, 10.5h, 11h, 11.5h, or 12h.

[0078] In some embodiments, step S200 involves mixing a hydroxylated rare earth metal oxide with a thiol-containing organic compound to obtain a thiol-modified rare earth metal oxide. This includes: adding the hydroxylated rare earth metal oxide and the thiol-containing organic compound to a first mixed solvent to obtain a mixture; reacting and separating the mixture under an inert atmosphere to obtain a preproduct; washing the preproduct alternately with a nonpolar solvent and a polar solvent, and drying it to obtain a thiol-modified rare earth metal oxide; wherein the first mixed solvent includes a nonpolar solvent and a polar solvent, and the thiol-containing organic compound includes a thiol silane coupling agent.

[0079] In a specific embodiment, in step S200, the amount of hydroxylated rare earth metal oxide used is 1% to 4% of the molar amount of the manganese-containing positive electrode active material. Optionally, the amount of hydroxylated rare earth metal oxide used is 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, or 4% of the molar amount of the manganese-containing positive electrode active material. For example, the amount of hydroxylated rare earth metal oxide used can be 0.05 mol to 0.30 mol.

[0080] In a specific embodiment, in step S200, the mercaptosilane coupling agent includes one or more of (3-mercaptopropyl)trimethoxysilane, γ-mercaptopropyltriethoxysilane, mercaptosilane-PEG derivative, bis(triethoxysilylpropyl)disulfide, and capped mercaptosilane.

[0081] In a specific embodiment, in step S200, the nonpolar solvent includes one or more of toluene, benzene, cyclohexane, n-hexane, ethyl acetate, THF tetrahydrofuran, and dichloromethane. The polar solvent includes one or more of ethanol, methanol, isopropanol, tert-butanol, and ethylene glycol monomethyl ether.

[0082] In a specific embodiment, the first mixed solvent is made using a non-polar solvent and a polar solvent because, in the reaction of grafting thiol groups onto rare earth metal oxides, mixing the non-polar and polar solvents can balance the polarity of the reaction system and the solubility / reactivity of the silane coupling agent. The non-polar solvent can effectively dissolve the hydrophobic thiol silane coupling agent (such as MPTMS), preventing it from prematurely hydrolyzing and condensing due to a polar environment (such as pure ethanol). The non-polar solvent may be too hydrophobic, making it difficult for the thiol silane coupling agent to approach the hydrophilic oxide surface. The polar solvent can slightly swell the hydroxyl groups on the surface of the rare earth metal oxide, enhancing the contact efficiency between the thiol silane coupling agent molecules and the surface -OH groups.

[0083] In a specific embodiment, the first mixed solvent can be made from anhydrous toluene and ethanol. Using a toluene-ethanol mixed solvent achieves an optimal balance between stability (for mercaptosilane coupling agents), wettability (for rare earth metal oxides), and reaction controllability. The advantages of both solvents are good solubility, low toxicity, low volatility, and suitable boiling points that are compatible with the reaction temperature range, thus not affecting the stability of the mercaptosilane coupling agent or causing side reactions.

[0084] In a specific embodiment, in step S200, the volume ratio of the non-polar solvent to the polar solvent is (8-12):1. Optionally, the volume ratio of the non-polar solvent to the polar solvent can be 8:1, 8.5:1, 9:1, 9.5:1, 10:1, 10.5:1, 11:1, 11.5:1, or 12:1. For example, the volume of the non-polar solvent and the polar solvent used can be 20 ml to 40 ml.

[0085] In a specific embodiment, step S200, the method of reacting and separating the mixture under an inert atmosphere, may include: refluxing the mixture under an inert atmosphere, and centrifuging the product after the reflux reaction to obtain a pre-product. The inert atmosphere includes one or more of nitrogen, helium, or argon.

[0086] In a specific embodiment, in step S200, the reflux reaction temperature is 70℃~90℃. Optionally, the reflux reaction temperature can be 70℃, 72℃, 74℃, 76℃, 78℃, 80℃, 82℃, 84℃, 86℃, 88℃, or 90℃.

[0087] In a specific embodiment, the reflux reaction time in step S200 is 6h to 12h. Optionally, the reflux reaction time can be 6h, 7h, 8h, 9h, 10h, 11h, or 12h.

[0088] In a specific embodiment, during step S200, the centrifuge speed is 2000 rpm to 4000 rpm during centrifugation. Optionally, the centrifuge speed can be 2000 rpm, 2500 rpm, 3000 rpm, 3500 rpm, or 4000 rpm.

[0089] In a specific embodiment, alternating washing of the preproduct with non-polar solvents and polar solvents means washing the preproduct with a non-polar solvent alone, then washing it with a polar solvent, and then washing it with a non-polar solvent again, and so on, in an alternating manner.

[0090] In specific embodiments, the main purpose of alternating between non-polar and polar solvents is to efficiently remove unreacted mercaptosilane coupling agents, byproducts, and physical adsorbates, while avoiding incomplete cleaning caused by using only one solvent. Non-polar solvents can effectively dissolve hydrophobic mercaptosilane coupling agents and their self-polymerization products (such as siloxane oligomers), preventing them from re-adsorbing onto the material surface. Polar solvents can dissolve silanols (Si-OH) generated from the partial hydrolysis of silanes or small amounts of water-soluble byproducts (such as HCl).

[0091] In a specific embodiment, in step S200, the number of alternating washes is 3 to 5. Optionally, the number of alternating washes can be 3, 4, or 5.

[0092] In a specific embodiment, in step S200, the drying temperature after alternating washing is 60℃~80℃. Optionally, the drying temperature can be 60℃, 62℃, 64℃, 66℃, 68℃, 70℃, 72℃, 74℃, 76℃, 78℃, or 80℃.

[0093] In a specific embodiment, in step S200, the drying time after alternating washing is 8h to 12h. Optionally, the drying time can be 8h, 8.5h, 9h, 9.5h, 10h, 10.5h, 11h, 11.5h, or 12h.

[0094] For some implementation methods, please refer to Figure 2 In step S300, lithium source, manganese source, iron source, phosphorus source and mercapto-modified rare earth metal oxide are mixed and sintered to obtain a composite cathode material, including the following steps:

[0095] In step S310, the iron source, manganese source and complexing agent are dissolved in a second mixed solvent. The pH value and evaporation temperature are adjusted to remove the second mixed solvent to obtain a gel product. The gel product is then subjected to heat treatment and calcination to obtain an iron-manganese oxide compound.

[0096] In step S320, lithium source, phosphorus source, iron-manganese oxide, mercapto-modified rare earth metal oxide and carbon source are mixed and sintered to obtain composite cathode material.

[0097] It should be noted that this embodiment uses the sol-gel method to prepare iron-manganese oxides. This method is simple and produces products with high purity and good uniformity. Furthermore, the sol-gel method for preparing iron-manganese composite oxides can achieve atomic-level uniform mixing of manganese and iron and suppress charge-discharge segregation. In other embodiments, methods for preparing iron-manganese oxides include co-precipitation, hydrothermal methods, mechanical mixing, pyrolysis, and redox methods.

[0098] In a specific embodiment, in step S310, the complexing agent includes one or more of citric acid, ammonium citrate, maleic acid, oxalic acid, ammonium oxalate, ascorbic acid, ethanolamine, diethanolamine, and triethanolamine.

[0099] In a specific embodiment, in step S310, the molar ratio of metal ions to complexing agent is 1:(1.5~1.8), and the molar amount of metal ions is the sum of the molar amounts of the iron source and the manganese source. Optionally, the molar ratio of metal ions to complexing agent can be 1:1.5, 1:1.6, 1:1.7, or 1:1.8.

[0100] In a specific embodiment, in step S310, the second mixed solvent may include deionized water and ethanol. The volume ratio of deionized water to ethanol is (1-3):1. Optionally, the volume ratio of deionized water to ethanol may be 1:1, 1.5:1, 2:1, 2.5:1, or 3:1.

[0101] In a specific embodiment, in step S310, the iron source, manganese source, and complexing agent are dissolved in a second mixed solvent and stirred at a certain temperature. The stirring temperature is 40℃~60℃. Optionally, the stirring temperature can be 40℃, 42℃, 44℃, 46℃, 48℃, 50℃, 52℃, 54℃, 56℃, 58℃, or 60℃.

[0102] In a specific embodiment, in step S310, the pH adjuster includes one or more of ammonia, sodium bicarbonate, and sodium hydroxide.

[0103] In a specific embodiment, in step S310, the pH value of the second mixed solvent is 3.5 to 4.0. Optionally, the pH value of the second mixed solvent can be 3.5, 3.6, 3.7, 3.8, 3.9, or 4.0.

[0104] In a specific embodiment, in step S310, the evaporation temperature is 90℃~110℃. Optionally, the evaporation temperature can be 90℃, 92℃, 94℃, 96℃, 98℃, 100℃, 102℃, 104℃, 106℃, 108℃, or 110℃.

[0105] In a specific embodiment, in step S310, the temperature for heat treatment of the gel product is 200℃~300℃. Optionally, the heat treatment temperature can be 200℃, 210℃, 220℃, 230℃, 240℃, 250℃, 260℃, 270℃, 280℃, 290℃, or 300℃.

[0106] In a specific embodiment, the heat treatment time in step S310 is 2h to 4h. Optionally, the heat treatment time can be 2h, 2.2h, 2.4h, 2.6h, 2.8h, 3h, 3.2h, 3.4h, 3.6h, 3.8h, or 4h.

[0107] In a specific embodiment, in step S310, the calcination temperature after heat treatment is 600℃~800℃. Optionally, the calcination temperature can be 600℃, 620℃, 640℃, 660℃, 680℃, 700℃, 720℃, 740℃, 760℃, 780℃, or 800℃.

[0108] In a specific embodiment, the calcination time in step S310 is 4h to 6h. Optionally, the calcination time can be 4h, 4.2h, 4.4h, 4.6h, 4.8h, 5h, 5.2h, 5.4h, 5.6h, 5.8h, or 6h.

[0109] In a specific embodiment, in step S310, the heating rate of calcination is 2℃ / min to 5℃ / min. Optionally, the heating rate of calcination can be 2℃ / min, 2.5℃ / min, 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min, or 5℃ / min.

[0110] In a specific embodiment, in step S320, the thiol-modified rare earth metal oxide is 1% to 3% of the molar amount of the manganese-containing positive electrode active material. Optionally, the thiol-modified rare earth metal oxide is 1%, 1.5%, 2%, 2.5%, or 3% of the molar amount of the manganese-containing positive electrode active material.

[0111] In a specific embodiment, in step S320, the carbon source includes one or more of sucrose, glucose, fructose, cellulose, starch, citric acid, tannic acid, graphene, and polyvinylpyrrolidone.

[0112] In a specific embodiment, in step S320, the molar ratio of the lithium source to the iron-manganese oxide compound is (1-1.2):1. Optionally, the molar ratio of the lithium source to the iron-manganese oxide compound can be 1:1, 1.1:1, or 1.2:1.

[0113] In a specific embodiment, in step S320, the molar ratio of manganese source, iron source, and phosphorus source is 1:(1.5-2.3):(2.5-3.3), wherein the sum of the molar numbers of manganese source and iron source is equal to the molar number of phosphorus source. Optionally, the molar ratio of manganese source, iron source, and phosphorus source can be 1:1.5:(2.5-3.3), 1:1.7:(2.5-3.3), 1:1.9:(2.5-3.3), 1:2.1:(2.5-3.3), 1:2.3:(2.5-3.3), 1:(1.5-2.3):2.5, 1:(1.5-2.3):2.7, 1:(1.5-2.3):2.9, 1:(1.5-2.3):3.1, or 1:(1.5-2.3):3.3.

[0114] In a specific embodiment, in step S320, the molar ratio of the mercapto-modified rare earth oxide to the manganese source is (0.05–0.15):1. Optionally, the molar ratio of the mercapto-modified rare earth oxide to the manganese source can be 0.05:1, 0.1:1, or 0.15:1.

[0115] In a specific embodiment, in step S320, the molar ratio of carbon source to iron-manganese oxide is (0.5-0.8):1. Optionally, the molar ratio of carbon source to iron-manganese oxide can be 0.5:1, 0.6:1, 0.7:1, or 0.8:1.

[0116] In a specific embodiment, in step S320, the sintering atmosphere is an inert atmosphere, which includes one or more of nitrogen, helium, or argon.

[0117] In a specific embodiment, in step S320, the sintering process adopts a three-stage sintering method, wherein the sintering temperature of the first stage is 200℃~350℃. Optionally, the sintering temperature of the first stage can be 200℃, 220℃, 240℃, 260℃, 280℃, 300℃, 320℃, 340℃, or 350℃.

[0118] In a specific embodiment, in step S320, the sintering time of the first stage is 4h to 5h. Optionally, the sintering time of the first stage can be 4h, 4.1h, 4.2h, 4.3h, 4.4h, 4.5h, 4.6h, 4.7h, 4.8h, 4.9h, or 5h.

[0119] In a specific embodiment, in step S320, the sintering temperature of the second stage is 500℃~650℃. Optionally, the sintering temperature of the second stage can be 500℃, 520℃, 540℃, 560℃, 580℃, 600℃, 620℃, 640℃, or 650℃.

[0120] In a specific embodiment, in step S320, the sintering time of the second stage is 8h to 10h. Optionally, the sintering time of the second stage can be 8h, 8.2h, 8.4h, 8.6h, 8.8h, 9h, 9.2h, 9.4h, 9.6h, 9.8h, or 10h.

[0121] In a specific embodiment, in step S320, the sintering temperature of the third stage is 700℃~850℃. Optionally, the sintering temperature of the third stage can be 700℃, 720℃, 740℃, 760℃, 780℃, 800℃, 820℃, 840℃, or 850℃.

[0122] In a specific embodiment, in step S320, the sintering time of the third stage is 2h to 4h. Optionally, the sintering time of the third stage can be 2h, 2.2h, 2.4h, 2.6h, 2.8h, 3h, 3.2h, 3.4h, 3.6h, 3.8h, or 4h.

[0123] In a specific embodiment, in step S320, the heating rate of the three-stage sintering is 2℃ / min to 5℃ / min. Optionally, the heating rate of the three-stage sintering can be 2℃ / min, 3℃ / min, 4℃ / min, or 5℃ / min.

[0124] In a specific embodiment, manganese iron oxide is first prepared, and then the manganese iron oxide is mixed with lithium source and phosphorus source and sintered. Compared with the traditional solid-state method, this method can optimize the ratio of manganese iron oxide, so that the manganese iron elements are evenly distributed, which is conducive to suppressing the dissolution of manganese ions. Moreover, the particle size of the obtained composite cathode material is in the nanometer range, which has higher compaction.

[0125] In other embodiments, lithium source, phosphorus source, iron source, manganese source, mercapto-modified rare earth metal oxide and carbon source can be directly mixed and composite cathode materials can be prepared by solid-state method, co-precipitation method, hydrothermal method, spray drying method, etc.

[0126] The present invention also provides a positive electrode sheet, which includes the positive electrode material provided in the above embodiments, or the positive electrode sheet includes the positive electrode material prepared by the preparation method of the positive electrode material provided in the above embodiments. In a specific embodiment, the positive electrode sheet includes a positive current collector and a positive electrode film layer located on at least one surface of the positive current collector, the positive electrode film layer including the positive electrode material obtained by the above preparation method or the above-described positive electrode material. Optionally, the positive current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0127] In one embodiment, the positive electrode film layer may optionally include a positive electrode conductive agent. The present invention does not particularly limit the type of positive electrode conductive agent; as an example, the positive electrode conductive agent includes at least one selected from superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene oxide, and carbon nanofibers. In some embodiments, the mass percentage of the positive electrode conductive agent is ≤5% based on the total mass of the positive electrode film layer.

[0128] In one embodiment, the positive electrode film layer may optionally include a positive electrode binder. The present invention does not particularly limit the type of positive electrode binder; as an example, the positive electrode binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride, tetrafluoroethylene, propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resins. In some embodiments, the mass percentage of the positive electrode binder is ≤5% based on the total mass of the positive electrode film layer.

[0129] In one embodiment, the positive current collector may be a metal foil or a composite current collector. As an example of a metal foil, aluminum foil may be used. The composite current collector may include a polymeric material substrate and a metal material layer formed on at least one surface of the polymeric material substrate. As an example, the metal material may include at least one of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy. As an example, the polymeric material substrate may include at least one of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0130] In one embodiment, the positive electrode film layer is typically formed by coating a positive electrode slurry onto a positive electrode current collector, followed by drying and cold pressing. The positive electrode slurry is typically formed by dispersing the positive electrode material, optional conductive agent, optional binder, and any other components in a solvent and stirring until homogeneous. The solvent may be N-methylpyrrolidone (NMP), but is not limited thereto.

[0131] The present invention also provides a secondary battery, which includes a positive electrode, a negative electrode, an electrolyte, etc., wherein the positive electrode is the positive electrode sheet provided in the above embodiments; or, the secondary battery includes the positive electrode material provided in the above embodiments; or the secondary battery includes the positive electrode material obtained by the above preparation method.

[0132] The technical solution of the present invention will be described in detail below through specific embodiments.

[0133] Example 1

[0134] This embodiment provides a composite cathode material, which includes lithium manganese iron phosphate, rare earth metal element (samarium), sulfur element, silicon element, and carbon material.

[0135] This embodiment also provides a method for preparing a composite cathode material:

[0136] 1) 0.2 mol samarium oxide (Sm2O3) was dispersed in 30 mL of 5% hydrogen peroxide solution, ultrasonicated at 40 °C and 30 kHz for 2 h, washed three times with ethanol, and then vacuum dried in an oven at 90 °C for 8 h to obtain surface hydroxylated rare earth metal oxides.

[0137] 2) 0.1 mol of hydroxylated rare earth metal oxide was dispersed in 20 mL of a mixed solution of anhydrous toluene and ethanol at a volume ratio of 8:1. 0.02 mol of (3-mercaptopropyl)trimethoxysilane was added, and the mixture was refluxed at 80 °C for 10 h under a nitrogen atmosphere. The preproduct was then obtained by centrifugation at 2000 rpm. The unreacted silane was removed by washing the product three times with toluene and ethanol alternately. The washed product was then placed in a 60 °C oven and vacuum dried for 10 h to obtain mercapto-modified rare earth metal oxide.

[0138] 3) Dissolve 1 mol ferric sulfate, 0.5 mol manganese sulfate and 2.25 mol citric acid in a 1:1 volume ratio of deionized water and ethanol at 60°C with stirring. Adjust the pH of the solution to 3.5-3.8 with ammonia. Then, evaporate the adjusted solution at 100°C to form a gel. Heat treat at 200°C for 3 hours to remove organic matter. Finally, calcine at 600°C in air for 6 hours to obtain iron-manganese oxides. The heating rate is 5°C / min.

[0139] 4) 0.025 mol of mercapto-modified rare earth metal oxide and 0.25 mol of iron-manganese oxide were mixed with 0.3 mol of lithium sulfate, 0.75 mol of sodium phosphate and 0.13 mol of glucose. The mixture was sintered at 350 °C for 4 h in a nitrogen atmosphere, then at 550 °C for 8 h, and finally at 750 °C for 3 h to obtain a gray-black composite cathode material. The heating rate was 5 °C / min.

[0140] Example 2

[0141] This embodiment provides a composite cathode material. The difference between this embodiment and Embodiment 1 is that the content of rare earth metal element (samarium) is increased.

[0142] The method for preparing composite cathode materials provided in this embodiment is as follows:

[0143] 1) 0.3 mol samarium oxide (Sm2O3) was dispersed in 30 mL of 6% hydrogen peroxide solution, ultrasonicated at 50 °C and 32 kHz for 2 h, washed three times with ethanol, and then vacuum dried in an oven at 100 °C for 10 h to obtain surface hydroxylated rare earth metal oxides.

[0144] 2) 0.15 mol of hydroxylated rare earth metal oxide was dispersed in 20 mL of a mixed solution of anhydrous toluene and ethanol at a volume ratio of 10:1. 0.02 mol of (3-mercaptopropyl)trimethoxysilane was added, and the mixture was refluxed at 85 °C for 10 h under a nitrogen atmosphere. The preproduct was then obtained by centrifugation at 2000 rpm. The unreacted silane was removed by washing the product three times with toluene and ethanol alternately. The washed product was then placed in a 65 °C oven and vacuum dried for 10 h to obtain mercapto-modified rare earth metal oxide.

[0145] 3) Dissolve 1 mol ferric sulfate, 0.5 mol manganese sulfate and 2.25 mol citric acid in a 1:1 volume ratio of deionized water and ethanol at 60°C with stirring. Adjust the pH of the solution to 3.5-3.8 with ammonia. Then, evaporate the adjusted solution at 100°C to form a gel. Heat treat at 200°C for 3 hours to remove organic matter. Finally, calcine at 650°C in air for 6 hours to obtain iron-manganese oxides. The heating rate is 4°C / min.

[0146] 4) 0.0375 mol of mercapto-modified rare earth metal oxide and 0.25 mol of iron-manganese oxide were mixed with 0.3 mol of lithium sulfate, 0.75 mol of sodium phosphate and 0.15 mol of glucose. The mixture was sintered at 300℃ for 5 h in a nitrogen atmosphere, then at 500℃ for 8 h, and finally at 800℃ for 3 h to obtain a gray-black lithium iron manganese phosphate cathode material. The heating rate was 4℃ / min.

[0147] Example 3:

[0148] This embodiment provides a composite cathode material. The difference between this embodiment and Embodiment 2 is that the content of rare earth metal element (samarium) is increased, and the content of sulfur and silicon elements is increased.

[0149] The method for preparing composite cathode materials provided in this embodiment is as follows:

[0150] 1) 0.4 mol of samarium oxide (Sm2O3) was dispersed in 40 mL of 8% hydrogen peroxide solution. After ultrasonic treatment at 60 °C and 35 kHz for 3 h, it was washed three times with ethanol and then vacuum dried in an oven at 100 °C for 10 h to obtain surface hydroxylated rare earth oxides.

[0151] 2) 0.2 mol of hydroxylated rare earth metal oxide was dispersed in 30 mL of a mixed solution of anhydrous toluene and ethanol at a volume ratio of 12:1. 0.03 mol of (3-mercaptopropyl)trimethoxysilane was added, and the mixture was refluxed at 85 °C for 12 h under a nitrogen atmosphere. The preproduct was then obtained by centrifugation at 2500 rpm. The unreacted silane was removed by washing the product three times with toluene and ethanol alternately. The washed product was then placed in a 70 °C oven and vacuum dried for 12 h to obtain mercapto-modified rare earth metal oxide.

[0152] 3) Dissolve 0.75 mol ferric sulfate, 0.5 mol manganese sulfate and 1.95 mol citric acid in a mixed solution of deionized water and ethanol with a volume ratio of 2:1 at 60°C with stirring. Adjust the pH of the solution to 3.8-4.0 with ammonia. Then, evaporate the adjusted solution at 110°C to form a gel. Heat treat at 250°C for 3 hours to remove organic matter. Finally, calcine at 700°C in air for 6 hours to obtain iron-manganese oxides. The heating rate is 5°C / min.

[0153] 4) 0.0375 mol of mercapto-modified rare earth metal oxide and 0.3 mol of iron-manganese oxide were mixed with 0.33 mol of lithium sulfate, 0.75 mol of sodium phosphate and 0.15 mol of glucose. The mixture was sintered at 350 °C for 5 h in a nitrogen atmosphere, then at 600 °C for 10 h, and finally at 850 °C for 4 h to obtain a gray-black lithium iron manganese phosphate cathode material. The heating rate was 4 °C / min.

[0154] Example 4:

[0155] This embodiment provides a composite cathode material. The difference between this embodiment and Embodiment 3 is that the rare earth metal elements include samarium and neodymium, and the contents of sulfur and silicon are increased.

[0156] The method for preparing composite cathode materials provided in this embodiment is as follows:

[0157] 1) 0.2 mol samarium oxide (Sm2O3) and 0.2 mol neodymium oxide (Nd2O3) were dispersed in 40 mL of 10% hydrogen peroxide solution. After ultrasonic treatment at 60 °C and 40 kHz for 3 h, the mixture was washed three times with ethanol and then vacuum dried in an oven at 110 °C for 10 h to obtain surface-hydroxylated rare earth metal oxides.

[0158] 2) 0.3 mol of hydroxylated rare earth metal oxide was dispersed in 30 mL of a mixed solution of anhydrous toluene and ethanol at a volume ratio of 10:1. 0.04 mol of (3-mercaptopropyl)trimethoxysilane was added, and the mixture was refluxed at 85 °C for 12 h under a nitrogen atmosphere. The preproduct was then obtained by centrifugation at 3000 rpm. The unreacted silane was removed by washing the product three times with toluene and ethanol alternately. The washed product was then placed in an oven at 80 °C and vacuum dried for 12 h to obtain mercapto-modified rare earth metal oxide.

[0159] 3) Dissolve 1 mol ferric sulfate, 0.5 mol manganese sulfate and 2.3 mol citric acid in a 1:1 volume ratio of deionized water and ethanol at 60°C with stirring. Adjust the pH of the solution to 3.8-4.0 with ammonia. Then, evaporate the adjusted solution at 110°C to form a gel. Heat treat at 300°C for 4 hours to remove organic matter. Finally, calcine at 800°C in air for 5 hours to obtain iron-manganese oxides. The heating rate is 4°C / min.

[0160] 4) 0.045 mol of mercapto-modified rare earth metal oxide and 0.3 mol of iron-manganese oxide were mixed with 0.36 mol of lithium sulfate, 0.90 mol of sodium phosphate and 0.24 mol of glucose. The mixture was sintered at 350 °C for 5 h in a nitrogen atmosphere, then at 600 °C for 10 h, and finally at 750 °C for 4 h to obtain a gray-black lithium iron manganese phosphate cathode material. The heating rate was 4 °C / min.

[0161] Example 5

[0162] This embodiment provides a composite cathode material. The difference between this embodiment and Embodiment 1 is that the content of rare earth metal element (samarium) is too low.

[0163] The method for preparing composite cathode materials provided in this embodiment is as follows:

[0164] 1) 0.04 mol samarium oxide (Sm2O3) was dispersed in 30 mL of 5% hydrogen peroxide solution, ultrasonicated at 40 °C and 30 kHz for 2 h, washed three times with ethanol, and then vacuum dried in an oven at 90 °C for 8 h to obtain surface hydroxylated rare earth metal oxides.

[0165] 2) 0.02 mol of hydroxylated rare earth metal oxide was dispersed in 20 mL of a mixed solution of anhydrous toluene and ethanol with a volume ratio of 8:1. 0.02 mol of (3-mercaptopropyl)trimethoxysilane was added, and the mixture was refluxed at 80 °C for 10 h under a nitrogen atmosphere. The preproduct was then obtained by centrifugation at 2000 rpm. The unreacted silane was removed by washing the product three times with toluene and ethanol alternately. The washed product was then placed in a 60 °C oven and vacuum dried for 10 h to obtain mercapto-modified rare earth metal oxide.

[0166] 3) Dissolve 1 mol ferric sulfate, 0.5 mol manganese sulfate and 2.25 mol citric acid in a 1:1 volume ratio of deionized water and ethanol at 60°C with stirring. Adjust the pH of the solution to 3.5-3.8 with ammonia. Then, evaporate the adjusted solution at 100°C to form a gel. Heat treat at 200°C for 3 hours to remove organic matter. Finally, calcine at 600°C in air for 6 hours to obtain iron-manganese oxides. The heating rate is 5°C / min.

[0167] 4) 0.015 mol of mercapto-modified rare earth metal oxide and 0.25 mol of iron-manganese oxide were mixed with 0.3 mol of lithium sulfate, 0.75 mol of sodium phosphate and 0.13 mol of glucose. The mixture was sintered at 350 °C for 4 h in a nitrogen atmosphere, then at 550 °C for 8 h, and finally at 750 °C for 3 h to obtain a gray-black lithium iron manganese phosphate cathode material. The heating rate was 5 °C / min.

[0168] Example 6

[0169] This embodiment provides a composite cathode material. The difference between this embodiment and Embodiment 1 is that the content of rare earth metal element (samarium) is excessive.

[0170] The method for preparing composite cathode materials provided in this embodiment is as follows:

[0171] 1) 0.6 mol of samarium oxide (Sm2O3) was dispersed in 30 mL of 5% hydrogen peroxide solution, ultrasonicated at 40 °C and 30 kHz for 2 h, washed three times with ethanol, and then vacuum dried in an oven at 90 °C for 8 h to obtain surface-hydroxylated rare earth metal oxides.

[0172] 2) 0.4 mol of hydroxylated rare earth metal oxide was dispersed in 20 mL of a mixed solution of anhydrous toluene and ethanol at a volume ratio of 8:1. 0.05 mol of (3-mercaptopropyl)trimethoxysilane was added, and the mixture was refluxed at 80 °C for 10 h under a nitrogen atmosphere. The preproduct was then obtained by centrifugation at 2000 rpm. The unreacted silane was removed by washing the product three times with toluene and ethanol alternately. The washed product was then placed in a 60 °C oven and vacuum dried for 10 h to obtain mercapto-modified rare earth metal oxide.

[0173] 3) Dissolve 1 mol ferric sulfate, 0.5 mol manganese sulfate and 2.25 mol citric acid in a 1:1 volume ratio of deionized water and ethanol at 60°C with stirring. Adjust the pH of the solution to 3.5-3.8 with ammonia. Then, evaporate the adjusted solution at 100°C to form a gel. Heat treat at 200°C for 3 hours to remove organic matter. Finally, calcine at 600°C in air for 6 hours to obtain iron-manganese oxides. The heating rate is 5°C / min.

[0174] 4) 0.025 mol of mercapto-modified rare earth metal oxide and 0.25 mol of iron-manganese oxide were mixed with 0.3 mol of lithium sulfate, 0.75 mol of sodium phosphate and 0.13 mol of glucose. The mixture was sintered at 350 °C for 4 h in a nitrogen atmosphere, then at 550 °C for 8 h, and finally at 750 °C for 3 h to obtain a gray-black lithium iron manganese phosphate cathode material. The heating rate was 5 °C / min.

[0175] Comparative Example 1

[0176] This embodiment provides a composite cathode material. The difference between this comparative example and Example 1 is that it does not include rare earth metal elements, sulfur elements, and silicon elements.

[0177] The method for preparing the composite cathode material provided in this comparative example is as follows:

[0178] 1) Dissolve 1 mol ferric sulfate, 0.5 mol manganese sulfate and 2.25 mol citric acid in a 1:1 volume ratio of deionized water and ethanol at 60°C with stirring. Adjust the pH of the solution to 3.5-3.8 with ammonia. Then, evaporate the adjusted solution at 100°C to form a gel. Heat treat at 200°C for 3 hours to remove organic matter. Finally, calcine at 600°C in air for 6 hours to obtain iron-manganese oxides. The heating rate is 5°C / min.

[0179] 2) 0.25 mol of iron-manganese oxide compound was mixed with 0.3 mol of lithium sulfate, 0.75 mol of sodium phosphate and 0.13 mol of glucose. The mixture was sintered at 350 °C for 4 h in a nitrogen atmosphere, then sintered at 550 °C for 8 h, and finally sintered at 750 °C for 3 h to obtain a gray-black lithium iron manganese phosphate cathode material. The heating rate was 5 °C / min.

[0180] Comparative Example 2

[0181] This embodiment provides a composite cathode material. The difference between this comparative example and Example 1 is that it does not include rare earth metal elements.

[0182] The method for preparing the composite cathode material provided in this comparative example is as follows:

[0183] 1) Dissolve 1 mol ferric sulfate, 0.5 mol manganese sulfate and 2.25 mol citric acid in a 1:1 volume ratio of deionized water and ethanol at 60°C with stirring. Adjust the pH of the solution to 3.5-3.8 with ammonia. Then, evaporate the adjusted solution at 100°C to form a gel. Heat treat at 200°C for 3 hours to remove organic matter. Finally, calcine at 600°C in air for 6 hours to obtain iron-manganese oxides. The heating rate is 5°C / min.

[0184] 2) 0.025 mol (3-mercaptopropyl)trimethoxysilane and 0.25 mol iron-manganese oxide were mixed with 0.3 mol lithium sulfate, 0.75 mol sodium phosphate and 0.13 mol glucose. The mixture was sintered at 350 °C for 4 h in a nitrogen atmosphere, then at 550 °C for 8 h, and finally at 750 °C for 3 h to obtain a gray-black lithium iron manganese phosphate cathode material. The heating rate was 5 °C / min.

[0185] Comparative Example 3

[0186] This embodiment provides a composite cathode material. The difference between this comparative example and Example 1 is that it does not include sulfur and silicon.

[0187] The method for preparing the composite cathode material provided in this comparative example is as follows:

[0188] 1) Dissolve 1 mol ferric sulfate, 0.5 mol manganese sulfate and 2.25 mol citric acid in a 1:1 volume ratio of deionized water and ethanol at 60°C with stirring. Adjust the pH of the solution to 3.5-3.8 with ammonia. Then, evaporate the adjusted solution at 100°C to form a gel. Heat treat at 200°C for 3 hours to remove organic matter. Finally, calcine at 600°C in air for 6 hours to obtain iron-manganese oxides. The heating rate is 5°C / min.

[0189] 2) 0.025 mol samarium oxide (Sm2O3) and 0.25 mol iron-manganese oxide were mixed with 0.3 mol lithium sulfate, 0.75 mol sodium phosphate and 0.13 mol glucose. The mixture was sintered at 350 °C for 4 h in a nitrogen atmosphere, then at 550 °C for 8 h, and finally at 750 °C for 3 h to obtain a gray-black lithium iron manganese phosphate cathode material. The heating rate was 5 °C / min.

[0190] Test case

[0191] Battery fabrication:

[0192] The composite cathode materials provided in Examples 1-6 and Comparative Examples 1-3 were used to prepare cathode sheets. The preparation method of the cathode sheet included: mixing the composite cathode material, polyvinylidene fluoride, and carbon black in a mass ratio of 96.5%, 2.2%, and 1.3%, respectively, and mixing them with a certain amount of N-methylpyrrolidone and stirring thoroughly to obtain a cathode slurry. The cathode slurry was then dried on the surface of the cathode current collector at 120°C using a coating machine to form a film, and subsequently rolled in a roller press to obtain the cathode sheet.

[0193] The positive electrode sheets provided in Examples 1-6 and Comparative Examples 1-3 were used to prepare secondary batteries. The preparation method of the secondary battery included: cutting the positive electrode sheets into pieces, fixing the positive electrode sheets onto a positive electrode shell, and transferring the fixed positive electrode shell into an oven for drying. The prepared positive electrode shell, gasket, spring sheet, separator, electrolyte, negative electrode sheet, and negative electrode shell were placed in a glove box for assembly. The assembled coin cell was placed in the center of a tablet press with the negative electrode facing up and the positive electrode facing down for tablet pressing. The battery assembly was completed after tablet pressing. The electrolyte can be any electrolyte suitable for lithium-ion batteries in the industry; no restriction is placed on the type of electrolyte.

[0194] The composite cathode materials and secondary batteries provided in Examples 1-6 and Comparative Examples 1-3 were tested under the following methods and conditions:

[0195] 1) ICP testing: The content of the composite cathode material was tested using an ICP spectrometer (inductively coupled plasma spectrometer). The test results are shown in Table 1.

[0196] 2) Metal leaching test: The composite cathode material was digested in aqua regia to obtain a sample, which was then tested using ICP-MS. If the concentration of the sample solution exceeded the range of the working curve, it was diluted. The dilution standard was based on the preparation method of the stock standard solution. The test results are shown in Table 1.

[0197] 3) Charge-discharge test: The electrochemical performance was tested on an electrochemical test cabinet at room temperature (25℃), using constant current at 0.1C, 1C, and 3C. The test results are shown in Table 2.

[0198] Table 1. Results of ICP testing, metal dissolution testing, and lattice energy calculation for the composite cathode materials of the examples and comparative examples.

[0199]

[0200] Table 2. Test results of the secondary batteries assembled in the examples and comparative examples

[0201]

[0202] As can be seen from the test results of Examples 1-6 and Comparative Example 1 in Table 2, the composite cathode material provided by the present invention has a higher initial discharge specific capacity (the initial discharge specific capacity at 1C is greater than 120mAh / g) and a higher capacity retention rate after 200 cycles (the capacity retention rate after 200 cycles at 1C is greater than 75%) compared with ordinary lithium manganese iron phosphate.

[0203] As can be seen from the test results of Examples 1-6 and Comparative Example 1 in Table 1, the composite cathode material provided by the present invention has less manganese ion dissolution and higher lattice energy, thus exhibiting higher electrochemical performance. Furthermore, it should be noted that the sulfur element in Comparative Example 1 is derived from sulfates (such as manganese sulfate). As shown in Table 1, the sulfur content in Comparative Example 1 is significantly lower than that in Examples 1-6, due to the additional addition of thiol-modified rare earth metal oxides.

[0204] As can be seen from the test results of Examples 1, 2, and 3 in Table 2, the effect of single doping with sulfur or rare earth metal elements is worse than that of simultaneous doping with both elements. Therefore, this invention demonstrates that by simultaneously introducing sulfur and rare earth metal elements, the two elements can work synergistically to suppress manganese ion dissolution.

[0205] As can be seen from the test results of Examples 1-3 in Tables 1 and 2, the rate performance improves with the increase of the amount of mercapto-modified rare earth metal oxide added. This is because, during the charge and discharge process, the rare earth metal elements react with the electrolyte to generate a nanoscale Li-MO composite interface layer, which improves the electronic and ionic conductivity of lithium manganese iron phosphate.

[0206] As can be seen from the test results of Examples 1 and 4 in Tables 1 and 2, the performance of the secondary battery can be further improved by introducing two rare earth metal elements. Table 1 also shows that the amount of manganese ion dissolution is significantly reduced, and the lattice energy is significantly improved, thus the capacity retention rate of the secondary battery is significantly improved after multiple cycles. Therefore, a reasonable combination of rare earth metal elements is beneficial to improving the performance of secondary batteries.

[0207] As can be seen from the test results of Examples 1, 5, and 6 in Tables 1 and 2, both insufficient and excessive addition of thiol-modified rare earth metal oxides affect the performance of the secondary battery, leading to a decrease in rate performance and cycle performance. In Example 5, insufficient addition of thiol-modified rare earth metal oxides resulted in limited introduction of sulfur and rare earth metal elements, failing to suppress manganese ion dissolution. Table 1 also shows that the dissolution of both manganese and iron ions increased in Example 5. In Example 6, excessive addition of thiol-modified rare earth metal oxides formed an excessively thick composite interface layer, increasing impedance, and excessive rare earth ion doping replaced Li. + or Fe 2+ The presence of these sites introduces localized stress, disrupting the stability of the olivine structure. Furthermore, excess -SH groups readily form -disulfide bonds during sintering, losing their support for Mn. 2+ The anchoring effect can even trigger electrolyte decomposition.

[0208] In the description of the embodiments of the present invention, it should be noted that the orientation or positional relationship of the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer" and other indicators are based on the orientation or positional relationship of the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention.

[0209] The above description discloses only one preferred embodiment of the present invention, and should not be construed as limiting the scope of the present invention. Those skilled in the art will understand that all or part of the processes of the above embodiments can be implemented, and equivalent changes made in accordance with the claims of the present invention are still within the scope of the present invention.

Claims

1. A composite cathode material, characterized in that, The composite cathode material includes a manganese-containing cathode active material, rare earth metal elements, and sulfur elements, wherein the sulfur elements are bonded to the manganese elements in the manganese-containing cathode active material to form S-Mn bonds.

2. The composite cathode material according to claim 1, characterized in that, The manganese-containing positive electrode active material includes lithium manganese iron phosphate, and the rare earth metal elements include one or more of La, Ce, Pr, Y, Nd, Eu, Tb, Sm, Gd, and Er.

3. The composite cathode material according to claim 1, characterized in that, The composite cathode material further includes carbon material, at least a portion of which coats the manganese-containing cathode active material, and at least a portion of which sulfur elements are bonded to carbon elements in the carbon material to form SC bonds.

4. The composite cathode material according to claim 1, characterized in that, The composite cathode material also includes silicon, and at least a portion of the silicon is doped into the manganese-containing cathode active material.

5. The composite cathode material according to claim 4, characterized in that, The rare earth metal element content in the composite cathode material is 100ppm to 800ppm; and / or The sulfur content in the composite cathode material is 200 ppm to 500 ppm; and / or The silicon content in the composite cathode material is 100ppm to 400ppm.

6. A method for preparing a composite cathode material, characterized in that, The preparation method includes: Surface modification of rare earth metal oxides yields hydroxylated rare earth metal oxides. The hydroxylated rare earth metal oxide is mixed with a thiol-containing organic compound to obtain a thiol-modified rare earth metal oxide. A composite cathode material is obtained by mixing and sintering lithium source, manganese source, iron source, phosphorus source and mercapto-modified rare earth metal oxide; The composite cathode material includes a manganese-containing cathode active material, rare earth metal elements, and sulfur elements, wherein the sulfur elements are bonded to the manganese elements in the manganese-containing cathode active material to form S-Mn bonds.

7. The preparation method according to claim 6, characterized in that, The surface modification of rare earth metal oxides to obtain hydroxylated rare earth metal oxides includes: Rare earth metal oxides are dispersed in an acidic or alkaline solution, and then washed after ultrasonic treatment to obtain the hydroxylated rare earth metal oxides.

8. The preparation method according to claim 6, characterized in that, The process of mixing hydroxylated rare earth metal oxides with thiol-containing organic compounds to obtain thiol-modified rare earth metal oxides includes: The hydroxylated rare earth metal oxide and the mercapto-containing organic compound are added to a first mixed solvent to obtain a mixture; the mixture is placed under an inert atmosphere for reaction and separation to obtain a preproduct; the preproduct is washed alternately with a non-polar solvent and a polar solvent, and dried to obtain the mercapto-modified rare earth metal oxide. The first mixed solvent includes the nonpolar solvent and the polar solvent, and the mercapto-containing organic compound includes a mercaptosilane coupling agent.

9. The preparation method according to claim 8, characterized in that, The mercaptosilane coupling agent includes one or more of (3-mercaptopropyl)trimethoxysilane, γ-mercaptopropyltriethoxysilane, mercaptosilane-PEG derivative, bis(triethoxysilylpropyl)disulfide, and capped mercaptosilane.

10. The preparation method according to claim 6, characterized in that, The process of mixing and sintering a lithium source, a manganese source, an iron source, a phosphorus source, and a mercapto-modified rare earth metal oxide yields a composite cathode material, comprising: Iron source, manganese source and complexing agent are dissolved in a second mixed solvent. After adjusting the pH value and evaporation temperature to remove the second mixed solvent, a gel product is obtained. The gel product is then subjected to heat treatment and calcination to obtain iron-manganese oxides. The lithium source, the phosphorus source, the iron-manganese oxide, the thiol-modified rare earth metal oxide, and the carbon source are mixed and sintered to obtain the composite cathode material.

11. The preparation method according to claim 10, characterized in that, The complexing agent includes one or more of citric acid, ammonium citrate, maleic acid, oxalic acid, ammonium oxalate, ascorbic acid, ethanolamine, diethanolamine, and triethanolamine; and / or, the molar ratio of the metal ion to the complexing agent is 1:(1.5 to 1.8), and the molar amount of the metal ion is the sum of the molar amounts of the iron source and the manganese source.

12. The preparation method according to claim 10, characterized in that, The pH value of the second mixed solvent is 3.5–4.0; and / or The evaporation temperature is 90℃~110℃; and / or The heat treatment temperature is 200℃~300℃; and / or The calcination temperature is 600℃~800℃.

13. The preparation method according to claim 6, characterized in that, The rare earth metal oxide is 2% to 5% of the molar amount of the manganese-containing positive electrode active material; and / or The hydroxylated rare earth metal oxide is 1% to 4% of the molar amount of the manganese-containing positive electrode active material; and / or The thiol-modified rare earth metal oxide is 1% to 3% of the molar amount of the manganese-containing positive electrode active material.

14. A secondary battery, characterized in that, The secondary battery includes a composite cathode material as described in any one of claims 1-5, or the secondary battery includes a composite cathode material prepared by the method for preparing the composite cathode material as described in any one of claims 6-13.

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