In-situ carbon-coated multi-cation-doped lithium manganese iron phosphate and preparation method thereof

Abstract

The application discloses in-situ carbon-coated multi-element cation-doped lithium manganese iron phosphate and a preparation method thereof. x Fe 1‑x‑a‑b‑c A a B b C c -MOF, wherein 0 A, B and C are respectively one of doped metal elements Mg, Ti, V, Co, Ni, Cr, Zn, Cu, Zr and Nb, and A, B and C are different from each other; then, the MnFePO4-MOF is taken as a precursor, mixed with a lithium source and a phosphorus source, and then subjected to grinding treatment, spray drying and calcination treatment in an inert atmosphere to obtain the in-situ carbon-coated multi-element cation-doped lithium manganese iron phosphate. The application can realize atomic-level uniform distribution of iron, manganese metal elements and three kinds of doped cations in the lithium manganese iron phosphate, fully play a synergistic effect of the doped metal cations, improve the structural stability of the material, and further optimize the electrochemical performance of the lithium manganese iron phosphate.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery electrode materials, and more specifically relates to an in-situ carbon-coated multi-cation-doped lithium manganese iron phosphate and a preparation method thereof. Background Art

[0002] In recent years, lithium iron phosphate (LiFePO4, LFP) has been increasing its market share in the lithium-ion battery cathode material market due to its advantages such as high safety performance, low cost and long cycle life. However, lithium iron phosphate has a low operating voltage (3.4V), which makes the energy density of lithium iron phosphate batteries (580Wh / kg) close to the limit. Although lithium manganese phosphate (LiMnPO4, LMP) cathode materials with the same olivine structure have a higher operating voltage (4.1V) and better energy density (701Wh / kg), the Mn in LMP is too low. 3+ The induced Jahn-Teller effect greatly affects the structural stability of the material, and its poor Li + The diffusion coefficient and conductivity hinder its industrial application as a high energy density lithium ion battery cathode material. x Mn 1-x PO4, 0<x<1), which has a theoretical mass specific capacity close to that of LFP, and also has a specific capacity corresponding to Mn at 4.1V. 3+ / Mn 2+ The high voltage platform generated by the redox couple and Fe 3 + / Fe 2+ The voltage platform generated by the redox couple increases the working voltage and energy density of the material, making it a promising new type of lithium-ion battery cathode material. However, as the proportion of manganese in the lattice increases, the conductivity of the material is greatly reduced, and the Mn 3+ The Jahn-Teller effect and manganese dissolution problems caused by this have become increasingly prominent, resulting in poor cycling stability and rate performance of the material.

[0003] Carbon coating and doping modification methods are used to solve the above problems. Among them, the carbon coating strategy can effectively improve the conductivity of phosphate system cathode materials. Metal cation doping can optimize the conductivity and ion diffusion capacity of the material from the inside of the lattice at the atomic level, improve the structural stability, and inhibit the Mn 3+The Jahn-Teller effect and manganese dissolution induced by the reaction can improve the cycle stability and rate performance of lithium manganese iron phosphate, while suppressing the manganese dissolution rate of the positive electrode material. Common doping elements in positive electrode materials include: Mg, Ti, V, Co, Ni, Cr, Zn, Cu, Zr, Nb, etc. The modification effect of doping with a single metal element on the positive electrode material is limited. The use of two or more metals to simultaneously dope the positive electrode material can make up for the limitations of single doping element modification. However, the current traditional method for preparing cation-doped lithium manganese iron phosphate is mainly based on the solid-phase method, in which the iron source, manganese source, lithium source, phosphorus source, carbon source and dopant are mixed and sintered. Its preparation method easily makes it difficult for multiple doping elements to be evenly distributed in the material, and it is easy to form an inactive layer on the surface of the material, which in turn causes the lithium manganese iron phosphate positive electrode material to have poor conductivity and structural stability, and poor performance in terms of capacity retention, rate performance, compaction density, etc. Summary of the Invention

[0004] In order to solve the above-mentioned defects of the prior art, the primary purpose of the present invention is to provide a method for preparing in-situ carbon-coated multi-cation-doped lithium manganese iron phosphate, thereby obtaining high-density lithium manganese iron phosphate while improving its conductivity.

[0005] The above-mentioned purpose of the present invention is achieved through the following technical solutions:

[0006] A method for preparing in-situ carbon-coated multi-cation-doped lithium manganese iron phosphate comprises the following steps:

[0007] S1. In an inert atmosphere, dissolving an iron source, a manganese source, a doping metal salt A, a doping metal salt B, and a doping metal salt C in a solvent to obtain a solution I;

[0008] S2. In an inert atmosphere, uniformly mix an organic ligand that forms a metal-organic framework, a surfactant, and a solvent to obtain solution II;

[0009] S3. After mixing solution I and solution II, an alkali solution is added to adjust the pH value of the solution to 7-9, and the mixture is reacted in an inert atmosphere to obtain a mixed slurry, followed by solid-liquid separation and washing to obtain a metal-organic framework precursor;

[0010] S4. The metal-organic framework precursor obtained in step S3 is mixed with a lithium source and a phosphorus source, ground, spray-dried, and calcined under an inert atmosphere to obtain in situ carbon-coated multi-cation-doped lithium manganese iron phosphate;

[0011] The general formula of the metal-organic framework precursor is Mn x Fe 1-x-a-b-c A a B b C c-MOF, wherein 0<x<1, 0<a<0.04, 0<b<0.04, 0<c<0.04, a+b+c=0.05; A, B, and C are respectively one of the doped metal elements Mg, Ti, V, Co, Ni, Cr, Zn, Cu, Zr, and Nb, and A, B, and C are different from each other.

[0012] The present invention synthesizes a ternary metal cation-doped manganese-iron-based metal-organic framework Mn by a solvent thermal method x Fe 1-x-a-b-c A a B b C c -MOF, and then used it as a precursor to react with lithium source and phosphorus source to synthesize high-performance lithium manganese iron phosphate. Since the metal-organic framework is a three-dimensional skeleton with periodic atomic distribution, Mn x Fe 1-x-a-b-c A a B b C c -The elements in MOF are evenly distributed, and the lithium manganese iron phosphate prepared with it as a precursor can achieve atomic-level uniform distribution of iron, manganese metal elements and three doped metal cations, which is conducive to fully exerting the synergistic effect between the three doped metal elements, improving the structural stability of the material, and thus optimizing the electrochemical performance of lithium manganese iron phosphate.

[0013] Furthermore, the general formula of the metal-organic framework precursor is Mn x Fe 1-x-a-b-c A a B b C c -MOF, wherein 0<x<1, 0<a<0.04, 0<b<0.04, 0<c<0.04, a+b+c=0.05; A, B, and C are respectively one of the doped metal elements Mg, Ti, V, Co, Zn, and Nb, and A, B, and C are different from each other.

[0014] Furthermore, the organic ligand in step S2 is one or more of 4-aminobenzoic acid, 2-aminoterephthalic acid, melamine, imidazole, triazole, benzimidazole, ethylenediamine, ethylenediaminetetraacetic acid, triethylamine, nitrogen tetroxide, 2,2'-bipyridine, 2,4'-bipyridine, 1,10-phenanthroline, 2,6-pyridinedicarboxylic acid, and triethylenediamine.

[0015] The organic ligand in the present invention is a nitrogen-containing organic ligand, which is used to prepare a nitrogen-containing metal-organic framework. In the subsequent stage sintering process, the nitrogen-containing metal-organic framework can serve as a nitrogen source and a carbon source to generate an in-situ N-doped carbon layer on the phosphate surface. The in-situ non-metallic doping formed by the highly electronegative nitrogen atoms can induce an electron-deficient structure in the carbon skeleton, thereby optimizing the electronic structure and enhancing the conductivity of the material, which is beneficial to the Li + Combined with N atoms to optimize lithium storage performance.

[0016] Furthermore, in step S2, the ratio of the amount of the organic ligand to the total amount of the iron salt, the manganese salt and the metal cation doped metal cation salt is (0.5-5): (0.5-5).

[0017] Furthermore, the surfactant in step S2 is one or more of cetyltrimethylammonium bromide, sodium dodecylbenzenesulfonate, sodium lauryl sulfonate, polyvinyl pyrrolidone, sodium lauryl sulfate, lithium dodecylbenzenesulfonate, and polyethylene glycol.

[0018] Furthermore, the molar ratio of the surfactant to the organic ligand in step S2 is (0.1-0.6): (1-15).

[0019] Furthermore, the solvent in step S2 is dimethyldiformamide and / or anhydrous ethanol and / or ultrapure water. Preferably, the solvent is a mixture of dimethyldiformamide, anhydrous ethanol and ultrapure water in a volume ratio of 1:1:1.

[0020] Furthermore, the inert atmosphere in step S2 is nitrogen and / or argon.

[0021] Furthermore, in step S4, the molar ratio of the lithium source, the phosphorus source and the metal source in the metal-organic framework precursor is Li:P:(Fe+Mn+A+B+C)=0.95-1.15:0.95-1.15:1.

[0022] Furthermore, in step S4, the lithium source is one or more of lithium carbonate, lithium hydroxide, lithium nitrate, lithium silicate, lithium sulfate, lithium phosphate, lithium orthosilicate, lithium permanganate, lithium metaphosphate, lithium fluoride, lithium bromide, lithium oxide, lithium nitride, lithium sulfide, lithium oxalate, lithium formate, lithium octanoate, lithium citrate, lithium salicylate, lithium trifluoroacetate, lithium acetoacetate, lithium difluorophosphate, lithium hexafluorophosphate, lithium benzoate, lithium pyruvate, lithium acetate, lithium methoxide, and lithium ethoxide.

[0023] Furthermore, the phosphorus source in step S4 is one or more of ammonium dihydrogen phosphate, ammonium phosphate, sodium pyrophosphate, ammonium hypophosphite, ammonium polyphosphate, ammonium hexafluorophosphate, lithium dihydrogen phosphate, lithium phosphate, diammonium hydrogen phosphate, sodium phosphite, sodium metaphosphate, sodium tripolyphosphate, and sodium hexafluorophosphate.

[0024] Further, the calcination treatment in step S4 is low-temperature sintering at a temperature increasing rate of 0.5-20℃ / min to 350-550℃, and then high-temperature sintering at a temperature of 650-950℃.

[0025] Further, the time of the low-temperature sintering is 0.5-10h.

[0026] Further, the time of the high-temperature sintering is 5-15h.

[0027] In some embodiments of the present application, after the heat preservation sintering in step S4, a gas flow crushing and sieving process is performed to obtain the in-situ carbon-coated multi-cation-doped lithium manganese iron phosphate.

[0028] Further, the inert atmosphere in step S4 is nitrogen and / or argon, and further, the oxygen content is controlled to be less than 10ppm.

[0029] Further, the grinding treatment in step S4 is wet ball milling, specifically, the metal-organic framework precursor, lithium source, phosphorus source obtained in step S3 are mixed with ethanol and ultrapure water to prepare a slurry, and the slurry is ground in a sand mill to a slurry particle size of 0.28μm≤D 50 ≤0.3μm. The volume ratio of ethanol and ultrapure water is 1:1, and the solid content of the slurry is 30-40%.

[0030] Further, the spray drying in step S4 is specifically sending the ground slurry into a spray drying system, and controlling the system settings to have an inlet air temperature of 210-250℃ and an outlet air temperature of 110-120℃, and adjusting the feed amount according to the particle size of the spray-dried discharge and the outlet air temperature to obtain a spray powder. Further, the particle size of the spray-dried discharge is 22μm≤D 50 ≤25μm.

[0031] Further, the reaction temperature in step S3 is 80-120℃.

[0032] Further, the reaction time in step S3 is 4-30h.

[0033] Further, the alkali solution in step S3 is one or more of NaOH solution, KOH solution, and ammonia water.

[0034] Further, the solution used for washing in step S3 is a mixed solution of dimethylformamide, anhydrous ethanol, and ultrapure water (V:V:V=1:1:1).

[0035] Furthermore, the doping metal salt A, the doping metal salt B, and the doping metal salt C in step S1 are respectively one of any soluble magnesium salt, titanium salt, vanadium salt, cobalt salt, nickel salt, chromium salt, zinc salt, copper salt, zirconium salt, and niobium salt known in the art, and the metal cations of the doping metal salt A, the doping metal salt B, and the doping metal salt C are different from each other.

[0036] The iron source and manganese source used in the present invention can be any one or more iron salts and manganese salts known in the art for preparing positive electrode active materials.

[0037] Furthermore, the iron source in step S1 is one or more of ferric chloride, ferrous sulfate, ferrous chloride, ferric nitrate, ferric acetate, ammonium ferric sulfate, ferric citrate, and ferrous oxalate.

[0038] Furthermore, the manganese source in step S1 is one or more of manganese chloride, manganous sulfate, manganese nitrate, manganese carbonate, manganese acetate, and manganese oxalate.

[0039] Furthermore, the solvent in step S1 is dimethyldiformamide and / or anhydrous ethanol, preferably, the solvent is a mixture of dimethyldiformamide and anhydrous ethanol in a volume ratio of 1:1. Furthermore, the inert atmosphere in step S1 is nitrogen and / or argon.

[0040] Another object of the present invention is to provide an in-situ carbon-coated multi-cation-doped lithium manganese iron phosphate prepared by the above preparation method.

[0041] Furthermore, the compaction density of the in-situ carbon-coated multi-cation-doped lithium manganese iron phosphate is ≥2.46 g / cm 3 , powder resistance ≤10.42Ω·cm.

[0042] Another object of the present invention is to provide a positive electrode sheet comprising the above-mentioned in-situ carbon-coated multi-cation-doped lithium manganese iron phosphate.

[0043] Furthermore, the positive electrode sheet also includes a conductive current collector, a binder, a conductive agent, etc. In some embodiments of the present invention, the conductive current collector can be any conductive current collector for a positive electrode known in the art, such as aluminum foil or carbon-coated aluminum foil; the binder can be any binder known in the art, such as one or more of sodium hydroxycellulose, polyvinylidene fluoride, and styrene-butadiene rubber; and the conductive agent can be any conductive agent known in the art, such as one or more of acetylene black, graphene, and carbon nanotubes.

[0044] Another object of the present invention is to provide a battery, comprising the above-mentioned positive electrode sheet.

[0045] Further, the battery further comprises a negative electrode sheet, a separator between the positive electrode sheet and the negative electrode sheet, and an electrolyte.

[0046] Compared with the prior art, the present application has the following beneficial effects:

[0047] The present application realizes the co-doping modification of three metal cations by introducing three types of metal cations into the preparation of manganese iron-based metal-organic frameworks as precursors to prepare manganese iron lithium phosphate, so that the manganese iron lithium phosphate has excellent electrochemical performance as a positive electrode material of a lithium ion battery. The manganese iron-based precursor doped with three metal cations can simultaneously form in-situ occupation doping of three types of metal cations in the manganese iron lithium phosphate crystal lattice, and the three metal cations form an atomic-level uniform distribution in the manganese iron lithium phosphate, which can fully exert the synergistic effect between the selected doped metal elements, improve the structural stability of the material, and improve the rate and cycle stability performance of the material. In addition, the metal-organic framework precursor prepared by selecting a nitrogen-containing organic ligand can form a uniform N-doped carbon coating in-situ after high-temperature sintering, which can further enhance the conductivity of the material. The preparation method of the present application has strong matching with the existing industrial lithium iron phosphate production process equipment, and has industrial application prospects.

[0048] The in-situ carbon-coated multi-cation-doped manganese iron lithium phosphate prepared by the present application has high conductivity and compaction density, and when used as a positive electrode material of a lithium ion battery, the first discharge efficiency, rate performance and cycle performance are greatly improved. BRIEF DESCRIPTION OF DRAWINGS

[0049] Figure 1 The scanning electron microscope image of the in-situ carbon-coated multi-cation-doped manganese iron lithium phosphate of Example 1 of the present application. DETAILED DESCRIPTION

[0050] The content of the present application will be further described below in combination with specific embodiments, but the embodiments do not limit the present application in any form. Unless otherwise specified, the technical means used in the embodiments are conventional means familiar to those skilled in the art. Unless otherwise specified, the reagents, methods and equipment used in the present embodiments are conventional reagents, methods and equipment in the technical field.

[0051] Example 1

[0052] 1. NH2-Mn 0.6 Fe 0.35 Ti 0.01 Co 0.01 Mg 0.03 Synthesis of -MOF

[0053] First, a mixed solution of 6 L of dimethyldiformamide and 6 L of anhydrous ethanol was added to a high-efficiency sealed synthesis reactor. 10 mol of 2-aminoterephthalic acid and 0.5 mol of hexadecyltrimethylammonium bromide were slowly added to the reactor, and nitrogen protective gas was introduced. The mixture was stirred at 600 rpm at 100°C for 20 minutes to prepare the bottom liquid in the reactor.

[0054] Secondly, under a nitrogen atmosphere, 6 mol of manganese chloride tetrahydrate, 3.5 mol of ferric chloride hexahydrate, 0.1 mol of ammonium titanium oxalate, 0.1 mol of cobalt acetate, and 0.3 mol of magnesium chloride were slowly added to a mixed solution of 6 L of dimethyldiformamide, 6 L of anhydrous ethanol, and 6 L of ultrapure water, and stirred at room temperature until completely dissolved to prepare a uniform salt solution I of manganese, iron, and doped cations. The above salt solution I was slowly pumped into the reactor using a peristaltic pump. After the salt solution I was completely pumped into the reactor, a 2 mol / L NaOH solution was slowly pumped into the reaction system using a peristaltic pump to control the pH value to 7.5, and the mixture was stirred thoroughly for 30 minutes. The above system was then reacted at 100 ° C for 12 hours to obtain a mixed slurry II.

[0055] After the reactor was cooled to room temperature, the mixed slurry II was filtered to separate the solid and liquid, and washed three times with a mixed solution of dimethyldiformamide, anhydrous ethanol and ultrapure water (V:V:V=1:1:1) to obtain NH2-Mn 0.6 Fe 0.35 Ti 0.01 Co 0.01 Mg 0.03 -MOF precursor.

[0056] 2. Synthesis of in-situ carbon-coated multi-cation-doped lithium manganese iron phosphate cathode materials

[0057] Lithium dihydrogen phosphate, lithium carbonate, NH2-Mn and NH4-Mn were weighed according to the molar ratio of Li:P:(Fe+Mn+Ti+Co+Mg)=1.05:1:1. 0.6 Fe 0.35 Ti 0.01 Co 0.01 Mg 0.03 -MOF precursor, and dispersed in a mixed solution of ethanol and ultrapure water (V:V = 1:1) to prepare a slurry with a solid content of 35%, and the mixed slurry was fully ground in a sand mill to a slurry particle size of 0.28 μm ≤ D 50 When the particle size is less than 0.3μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet air temperature of 220℃ and an outlet air temperature of 110℃. According to the outlet air temperature and the particle size of the material (22μm≤D 50The spray powder was placed in a graphite crucible and placed in a nitrogen atmosphere box furnace (oxygen content less than 10 ppm). The temperature was first increased to 500°C at a rate of 3°C / min for 6 hours, and then increased to 700°C for 9 hours.

[0058] After the nitrogen atmosphere box furnace is naturally cooled to room temperature, the sintered powder is subjected to air flow pulverization and then sieved using a sieve to obtain the in-situ carbon-coated multi-cation doped lithium manganese iron phosphate positive electrode material.

[0059] Example 2

[0060] 1.NH2-Mn 0.7 Fe 0.25 Nb 0.015 Mg 0.015 V 0.02 -MOF synthesis

[0061] First, a mixed solution of 6 L of dimethyldiformamide and 6 L of anhydrous ethanol was added to a high-efficiency sealed synthesis reactor. 10 mol of 2-aminoterephthalic acid and 0.5 mol of hexadecyltrimethylammonium bromide were slowly added to the reactor, and nitrogen protective gas was introduced. The mixture was stirred at 650 rpm at 100°C for 20 minutes to prepare the bottom liquid in the reactor.

[0062] Secondly, under a nitrogen atmosphere, 7 mol of manganese chloride tetrahydrate, 2.5 mol of ferric chloride hexahydrate, 0.15 mol of niobium oxalate, 0.15 mol of magnesium chloride, and 0.2 mol of vanadium oxalate were slowly added to a mixed solution of 6 L of dimethyldiformamide, 6 L of anhydrous ethanol, and 6 L of ultrapure water, and stirred at room temperature until completely dissolved to prepare a uniform salt solution I of manganese, iron, and doped cations. The above salt solution I was slowly pumped into the reactor using a peristaltic pump. After the salt solution I was completely pumped into the reactor, a 2 mol / L NaOH solution was slowly pumped into the reaction system using a peristaltic pump to control the pH value to 7.8, and the mixture was stirred thoroughly for 30 minutes. The above system was then reacted at 100 ° C for 12 hours to obtain a mixed slurry II.

[0063] After the reactor was cooled to room temperature, the mixed slurry II was filtered to separate the solid and liquid, and washed three times with a mixed solution of dimethyldiformamide, anhydrous ethanol and ultrapure water (V:V:V=1:1:1) to obtain NH2-Mn 0.7 Fe 0.25 Nb 0.015 Mg 0.015 V 0.02 -MOF precursor.

[0064] 2. Synthesis of in-situ carbon-coated multi-cation-doped lithium manganese iron phosphate cathode materials

[0065] Lithium dihydrogen phosphate, lithium carbonate, NH2-Mn and NH4-Mn were weighed according to the molar ratio of Li:P:(Fe+Mn+Nb+Mg+V)=1.05:1:1. 0.7 Fe 0.25 Nb 0.015 Mg 0.015 V 0.02 -MOF precursor, and dispersed in a mixed solution of ethanol and ultrapure water (V:V = 1:1) to prepare a slurry with a solid content of 38%, and the mixed slurry was fully ground in a sand mill to a slurry particle size of 0.28 μm ≤ D 50 When the particle size is less than 0.3μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet air temperature of 230℃ and an outlet air temperature of 115℃. According to the outlet air temperature and the particle size of the material (22μm≤D 50 The spray powder was placed in a graphite crucible and placed in a nitrogen atmosphere box furnace (oxygen content less than 10 ppm). The temperature was first increased to 500°C at a rate of 2.3°C / min for 6 hours, and then increased to 680°C for 9 hours.

[0066] After the nitrogen atmosphere box furnace is naturally cooled to room temperature, the sintered powder is subjected to air flow pulverization and then sieved using a sieve to obtain the in-situ carbon-coated multi-cation doped lithium manganese iron phosphate positive electrode material.

[0067] Example 3

[0068] 1.NH2-Mn 0.75 Fe 0.2 Nb 0.015 Zn 0.01 V 0.025 -MOF synthesis

[0069] First, a mixed solution of 6 L of dimethyldiformamide and 6 L of anhydrous ethanol was added to a high-efficiency sealed synthesis reactor. 10 mol of 2-aminoterephthalic acid and 0.5 mol of hexadecyltrimethylammonium bromide were slowly added to the reactor, and nitrogen protective gas was introduced. The mixture was stirred at 750 rpm at 100°C for 20 minutes to prepare the bottom liquid in the reactor.

[0070] Secondly, under a nitrogen atmosphere, 7.5 mol of manganese chloride tetrahydrate, 2 mol of ferric chloride hexahydrate, 0.15 mol of niobium oxalate, 0.1 mol of zinc acetate, and 0.25 mol of vanadium oxalate were slowly added to a mixed solution of 6 L of dimethyldiformamide, 6 L of anhydrous ethanol, and 6 L of ultrapure water, and stirred at room temperature until completely dissolved to prepare a uniform salt solution I of manganese, iron, and doped cations. The above salt solution I was slowly pumped into the reactor using a peristaltic pump. After the salt solution I was completely pumped into the reactor, a 2 mol / L NaOH solution was slowly pumped into the reaction system using a peristaltic pump to control the pH value to 7, and the mixture was stirred thoroughly for 30 minutes. The above system was then reacted at 108 ° C for 12 hours to obtain a mixed slurry II.

[0071] After the reactor was cooled to room temperature, the mixed slurry II was filtered to separate the solid and liquid, and washed three times with a mixed solution of dimethyldiformamide, anhydrous ethanol and ultrapure water (V:V:V=1:1:1) to obtain NH2-Mn 0.75 Fe 0.2 Nb 0.015 Zn 0.01 V 0.025 -MOF precursor.

[0072] 2. Synthesis of in-situ carbon-coated multi-cation-doped lithium manganese iron phosphate cathode materials

[0073] Lithium dihydrogen phosphate, lithium carbonate, NH2-Mn and NH4-Mn were weighed according to the molar ratio of Li:P:(Fe+Mn+Nb+Zn+V)=1.05:1:1. 0.75 Fe 0.2 Nb 0.015 Zn 0.01 V 0.025 -MOF precursor, and dispersed in a mixed solution of ethanol and ultrapure water (V:V = 1:1) to prepare a slurry with a solid content of 38%, and the mixed slurry was fully ground in a sand mill to a slurry particle size of 0.28 μm ≤ D 50 When the particle size is less than 0.3μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet air temperature of 240℃ and an outlet air temperature of 110℃. According to the outlet air temperature and the particle size of the discharge (22μm≤D 50 The spray powder was placed in a graphite crucible and placed in a nitrogen atmosphere box furnace (oxygen content less than 10 ppm). The temperature was first increased to 500°C at a rate of 2.3°C / min for 6 hours, and then increased to 680°C for 9 hours.

[0074] After the nitrogen atmosphere box furnace is naturally cooled to room temperature, the sintered powder is subjected to air flow pulverization and then sieved using a sieve to obtain the in-situ carbon-coated multi-cation doped lithium manganese iron phosphate positive electrode material.

[0075] Comparative Example 1

[0076] The difference between Comparative Example 1 and Example 1 is that the prepared lithium manganese iron phosphate material does not contain doped metal elements, and specifically includes the following steps:

[0077] 1.NH2-Mn 0.6 Fe 0.4 -MOF synthesis

[0078] First, a mixed solution of 6 L of dimethyldiformamide and 6 L of anhydrous ethanol was added to a high-efficiency sealed synthesis reactor. 10 mol of 2-aminoterephthalic acid and 0.5 mol of hexadecyltrimethylammonium bromide were slowly added to the reactor, and nitrogen protective gas was introduced. The mixture was stirred at 600 rpm at 100°C for 20 minutes to prepare the bottom liquid in the reactor.

[0079] Next, under a nitrogen atmosphere, 6 mol of manganese chloride tetrahydrate and 4 mol of ferric chloride hexahydrate were slowly added to a mixed solution of 6 L of dimethyldiformamide, 6 L of anhydrous ethanol, and 6 L of ultrapure water, and stirred at room temperature until completely dissolved to prepare manganese and iron salt solution I. The above salt solution I was slowly pumped into the reactor using a peristaltic pump. After the salt solution I was completely pumped into the reactor, a 2 mol / L NaOH solution was slowly pumped into the reaction system using a peristaltic pump to control the pH value to 7.5, and the mixture was stirred thoroughly for 30 minutes. The above system was then reacted at 100°C for 12 hours to obtain mixed slurry II.

[0080] After the reactor was cooled to room temperature, the mixed slurry II was filtered to separate the solid and liquid, and washed three times with a mixed solution of dimethyldiformamide, anhydrous ethanol and ultrapure water (V:V:V=1:1:1) to obtain NH2-Mn 0.6 Fe 0.4 -MOF precursor.

[0081] 2. Synthesis of in-situ carbon-coated lithium manganese iron phosphate cathode material

[0082] According to the molar ratio of Li:P:(Fe+Mn)=1.05:1:1, lithium dihydrogen phosphate, lithium carbonate, NH2-Mn 0.6 Fe 0.4 -MOF precursor, and dispersed in a mixed solution of ethanol and ultrapure water (V:V = 1:1) to prepare a slurry with a solid content of 35%, and the mixed slurry was fully ground in a sand mill to a slurry particle size of 0.28 μm ≤ D 50 When the particle size is less than 0.3μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet air temperature of 220℃ and an outlet air temperature of 110℃. According to the outlet air temperature and the particle size of the material (22μm≤D 50The spray powder was placed in a graphite crucible and placed in a nitrogen atmosphere box furnace (oxygen content less than 10 ppm). The temperature was first increased to 500°C at a rate of 3°C / min for 6 hours, and then increased to 700°C for 9 hours.

[0083] After the nitrogen atmosphere box furnace is naturally cooled to room temperature, the sintered powder is subjected to air flow pulverization and then sieved using a sieve to obtain the in-situ carbon-coated lithium manganese iron phosphate positive electrode material.

[0084] Comparative Example 2

[0085] The difference between Comparative Example 2 and Example 2 is that the prepared lithium manganese iron phosphate material does not contain doped metal elements, and specifically includes the following steps:

[0086] 1.NH2-Mn 0.7 Fe 0.3 -MOF synthesis

[0087] First, a mixed solution of 6 L of dimethyldiformamide and 6 L of anhydrous ethanol was added to a high-efficiency sealed synthesis reactor. 10 mol of 2-aminoterephthalic acid and 0.5 mol of hexadecyltrimethylammonium bromide were slowly added to the reactor, and nitrogen protective gas was introduced. The mixture was stirred at 650 rpm at 100°C for 20 minutes to prepare the bottom liquid in the reactor.

[0088] Next, under a nitrogen atmosphere, 7 mol of manganese chloride tetrahydrate and 3 mol of ferric chloride hexahydrate were slowly added to a mixed solution of 6 L of dimethyldiformamide, 6 L of anhydrous ethanol, and 6 L of ultrapure water, and stirred at room temperature until completely dissolved to prepare manganese and iron salt solution I. The above salt solution I was slowly pumped into the reactor using a peristaltic pump. After the salt solution I was completely pumped into the reactor, a 2 mol / L NaOH solution was slowly pumped into the reaction system using a peristaltic pump to control the pH value to 7.8, and the mixture was stirred thoroughly for 30 minutes. The above system was then reacted at 100°C for 12 hours to obtain mixed slurry II.

[0089] After the reactor was cooled to room temperature, the mixed slurry II was filtered to separate the solid and liquid, and washed three times with a mixed solution of dimethyldiformamide, anhydrous ethanol and ultrapure water (V:V:V=1:1:1) to obtain NH2-Mn 0.7 Fe 0.3 -MOF precursor.

[0090] 2. Synthesis of in-situ carbon-coated lithium manganese iron phosphate cathode material

[0091] According to the molar ratio of Li:P:(Fe+Mn)=1.05:1:1, lithium dihydrogen phosphate, lithium carbonate, NH2-Mn 0.7 Fe0.3 -MOF precursor, and dispersed in a mixed solution of ethanol and ultrapure water (V:V = 1:1) to prepare a slurry with a solid content of 38%, and the mixed slurry was fully ground in a sand mill to a slurry particle size of 0.28 μm ≤ D 50 When the particle size is less than 0.3μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet air temperature of 230℃ and an outlet air temperature of 115℃. According to the outlet air temperature and the particle size of the material (22μm≤D 50 The spray powder was placed in a graphite crucible and placed in a nitrogen atmosphere box furnace (oxygen content less than 10 ppm). The temperature was first increased to 500°C at a rate of 2.3°C / min for 6 hours, and then increased to 680°C for 9 hours.

[0092] After the nitrogen atmosphere box furnace is naturally cooled to room temperature, the sintered powder is subjected to air flow pulverization and then sieved using a sieve to obtain the in-situ carbon-coated lithium manganese iron phosphate positive electrode material.

[0093] Comparative Example 3

[0094] The difference between Comparative Example 3 and Example 3 is that the prepared lithium manganese iron phosphate material does not contain doped metal elements, and specifically includes the following steps:

[0095] 1.NH2-Mn 0.75 Fe 0.25 -MOF synthesis

[0096] First, a mixed solution of 6 L of dimethyldiformamide and 6 L of anhydrous ethanol was added to a high-efficiency sealed synthesis reactor. 10 mol of 2-aminoterephthalic acid and 0.5 mol of hexadecyltrimethylammonium bromide were slowly added to the reactor, and nitrogen protective gas was introduced. The mixture was stirred at 750 rpm at 100°C for 20 minutes to prepare the bottom liquid in the reactor.

[0097] Next, under a nitrogen atmosphere, 7.5 mol of manganese chloride tetrahydrate and 2.5 mol of ferric chloride hexahydrate were slowly added to a mixed solution of 6 L of dimethyldiformamide, 6 L of anhydrous ethanol, and 6 L of ultrapure water, and stirred at room temperature until completely dissolved to prepare manganese and iron salt solution I. The above salt solution I was slowly pumped into the reactor using a peristaltic pump. After the salt solution I was completely pumped into the reactor, a 2 mol / L NaOH solution was slowly pumped into the reaction system using a peristaltic pump to control the pH value to 7, and the mixture was stirred thoroughly for 30 minutes. The above system was then reacted at 108°C for 12 hours to obtain mixed slurry II.

[0098] After the reactor was cooled to room temperature, the mixed slurry II was filtered to separate the solid and liquid, and washed three times with a mixed solution of dimethyldiformamide, anhydrous ethanol and ultrapure water (V:V:V=1:1:1) to obtain NH2-Mn 0.7.5 Fe 0.25 -MOF precursor.

[0099] 2. Synthesis of in-situ carbon-coated lithium manganese iron phosphate cathode material

[0100] Lithium dihydrogen phosphate, lithium carbonate, NH2-Mn 0.7.5 Fe 0.25 -MOF precursor, and dispersed in a mixed solution of ethanol and ultrapure water (V:V = 1:1) to prepare a slurry with a solid content of 38%, and the mixed slurry was fully ground in a sand mill to a slurry particle size of 0.28 μm ≤ D 50 When the particle size is less than 0.3μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet air temperature of 240℃ and an outlet air temperature of 110℃. According to the outlet air temperature and the particle size of the discharge (22μm≤D 50 The spray powder was placed in a graphite crucible and placed in a nitrogen atmosphere box furnace (oxygen content less than 10 ppm). The temperature was first increased to 500°C at a rate of 2.3°C / min for 6 hours, and then increased to 680°C for 9 hours.

[0101] After the nitrogen atmosphere box furnace is naturally cooled to room temperature, the sintered powder is subjected to air flow pulverization and then sieved using a sieve to obtain the in-situ carbon-coated lithium manganese iron phosphate positive electrode material.

[0102] Comparative Example 4

[0103] 1. Synthesis of carbon-coated multi-cation-doped lithium manganese iron phosphate cathode materials

[0104] In Comparative Example 4, the precursor NH2-Mn in Example 1 does not need to be synthesized. 0.6 Fe 0.35 Ti 0.01 Co 0.01 Mg 0.03-MOF, directly using lithium carbonate as the lithium source, lithium dihydrogen phosphate as the phosphorus source, manganese chloride tetrahydrate as the manganese source, ferric chloride hexahydrate as the iron source and glucose as the carbon source, ammonium oxytitanium oxalate, cobalt acetate and magnesium chloride as dopants, lithium carbonate, lithium dihydrogen phosphate, manganese chloride tetrahydrate, ferric chloride hexahydrate, ammonium oxytitanium oxalate, cobalt acetate and magnesium chloride were weighed according to the same molar ratio as in Example 1 and mixed with ethanol and ultrapure water (V:V=1:1), and at the same time, glucose with the same mass as the organic ligand in Example 1 was added to the above mixed slurry, and the same parameters as in Example 1 were controlled to carry out grinding, spraying, sintering, crushing and sieving processes to obtain a carbon-coated multi-cation doped lithium manganese iron phosphate positive electrode material.

[0105] Performance Testing

[0106] Particle size test: The particle size of the lithium manganese iron phosphate composite materials obtained in the embodiment and the comparative example was tested using a Malvern laser particle size analyzer. The results are shown in Table 1.

[0107] Powder resistance test: The powder resistance of the lithium manganese iron phosphate composite materials obtained in the embodiment and the comparative example was tested by an ST2742B powder resistivity tester. The measured results are shown in Table 1.

[0108] Electrochemical Performance Testing: The lithium manganese iron phosphate composite materials provided in the above examples and comparative examples were mixed with polyvinylidene fluoride and acetylene black in a mass ratio of 80:8:12. N-methylpyrrolidone was added to a solid content of 30% and the mixture was homogenized. The mixture was then coated, dried, and roller-pressed to produce a positive electrode sheet. A polypropylene film was used as a separator, a lithium sheet was used as a negative electrode, and 1 mol / L LiPF6 (DEC:EC = 1:1 Vol%) was used as an electrolyte. The positive electrode, separator, and negative electrode were assembled in sequence to produce a lithium-ion battery. The lithium-ion battery was subjected to electrochemical performance testing under the following test conditions: charge and discharge tests were conducted at room temperature of 25°C at rates of 0.1C and 1C, respectively. The results are shown in Table 1.

[0109] Compaction density test: The lithium manganese iron phosphate powder samples prepared in the examples and comparative examples were compacted using a powder compaction density meter with a pressure of 5 T, and their volumes and masses were measured to calculate the compaction density.

[0110] Manganese dissolution test: The inductively coupled plasma test method was used to detect the dissolution amount of Mn in the electrolyte of the batteries assembled with the lithium manganese iron phosphate materials obtained in Examples 1 to 3 and Comparative Examples 1 to 4 after 100 cycles at a current density of 1C.

[0111] The in-situ carbon-coated multi-cation-doped lithium manganese iron phosphate prepared in Example 1 was characterized by scanning electron microscopy, and the results were as follows: Figure 1 As shown. Figure 1It can be seen that the in-situ carbon-coated multi-cation-doped lithium manganese iron phosphate prepared in Example 1 is in the form of nanoscale particles after airflow crushing, and the particle size is uniform, which is beneficial to shortening the migration path of lithium ions during the charge and discharge process and increasing the electrochemical reaction active sites, thereby improving the comprehensive electrochemical performance of the material.

[0112] Table 1

[0113]

[0114] According to the results in Table 1, the in-situ carbon-coated multi-cation-doped lithium manganese iron phosphate prepared by the present invention has a high compaction density of 2.46 g / cm 3 The above results show that the powder resistance is ≤10.42Ω·cm, the conductivity is excellent, the first discharge efficiency is high and the 1C discharge specific capacity is high, the rate performance is good, the manganese dissolution amount is below 88ppm, and the material structure stability is excellent, which is conducive to improving the cycle stability performance.

[0115] According to the analysis in Table 1, Example 1 has an initial charge / discharge specific capacity of 160.3 / 154.3 mAh / g at a current density of 0.1C, and an initial discharge efficiency of 96.3%. Under the same conditions, Comparative Example 1 has an initial charge / discharge specific capacity of 149.3 / 139.1 mAh / g, and an initial discharge efficiency of only 93.2%. Compared with the examples, the electrochemical performance of the examples is reduced. The same pattern is observed between Example 2 and Comparative Example 2, and between Example 3 and Comparative Example 3. This is because the doping of three different metal cations in Example 1 is uniformly distributed at the atomic level in the lithium manganese iron phosphate lattice, effectively regulating the electronic structure and structural stability of the lithium manganese iron phosphate, while optimizing the morphology and crystallinity of the nanoparticles, thereby improving the conductivity and charge / discharge capacity of the material.

[0116] It can be seen from Example 1 and Comparative Example 4 that, compared with the traditional mixed solid phase sintering method, the preparation method of the present invention significantly reduces the manganese dissolution amount of the lithium manganese iron phosphate positive electrode material prepared by the present invention, significantly improves the compaction density and conductivity, and significantly improves the electrochemical performance.

[0117] The above embodiments are preferred experimental methods of the present invention, but the implementation methods of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for preparing in-situ carbon-coated multi-cation-doped lithium manganese iron phosphate, characterized in that: The following steps are involved: S1. In an inert atmosphere, dissolving an iron source, a manganese source, a doping metal salt A, a doping metal salt B, and a doping metal salt C in a solvent to obtain a solution I; S2. In an inert atmosphere, uniformly mix an organic ligand that forms a metal-organic framework, a surfactant, and a solvent to obtain solution II; S3. After mixing solution I and solution II, an alkali solution is added to adjust the pH value of the solution to 7-9, and the mixture is reacted in an inert atmosphere to obtain a mixed slurry, followed by solid-liquid separation and washing to obtain a metal-organic framework precursor; S4. The metal-organic framework precursor obtained in step S3 is mixed with a lithium source and a phosphorus source, ground, spray-dried, and calcined under an inert atmosphere to obtain in situ carbon-coated multi-cation-doped lithium manganese iron phosphate; The general formula of the metal-organic framework precursor is Mn x Fe 1-x-a-b-c A a B b C c -MOF, wherein 0<x<1, 0<a<0.04, 0<b<0.04, 0<c<0.04, a+b+c=0.05; A, B, and C are each one of the doped metal elements Mg, Ti, V, Co, Ni, Cr, Zn, Cu, Zr, and Nb, and A, B, and C are different from each other; The organic ligand in step S2 is one or more of 4-aminobenzoic acid, 2-aminoterephthalic acid, melamine, imidazole, triazole, benzimidazole, ethylenediamine, ethylenediaminetetraacetic acid, triethylamine, nitrogen tetroxide, 2,2'-bipyridine, 2,4'-bipyridine, 1,10-phenanthroline, 2,6-pyridinedicarboxylic acid, and triethylenediamine.

2. The preparation method according to claim 1, characterized in that The general formula of the metal-organic framework precursor is Mn x Fe 1-x-a-b-c A a B b C c -MOF, wherein 0<x<1, 0<a<0.04, 0<b<0.04, 0<c<0.04, a+b+c=0.05; A, B, and C are respectively one of the doped metal elements Mg, Ti, V, Co, Zn, and Nb, and A, B, and C are different from each other.

3. The preparation method according to claim 1, characterized in that In step S4, the molar ratio of the lithium source, the phosphorus source and the metal source in the metal-organic framework precursor is Li:P:(Fe+Mn+A+B+C)=0.95-1.15:0.95-1.15:

1.

4. The preparation method according to claim 1, characterized in that The calcination treatment in step S4 is to heat the material to 350-550° C. at a heating rate of 0.5-20° C. / min for low-temperature sintering, and then heat the material to 650-950° C. for high-temperature sintering.

5. The preparation method according to claim 1, characterized in that The reaction temperature in step S3 is 80-120°C.

6. In-situ carbon-coated multi-cation-doped lithium manganese iron phosphate prepared by the method according to any one of claims 1 to 5.

7. The in-situ carbon-coated multi-cation-doped lithium manganese iron phosphate according to claim 6, characterized in that: The compaction density of the in-situ carbon-coated multi-cation-doped lithium manganese iron phosphate is ≥2.46 g / cm 3 , powder resistance ≤10.42Ω·cm.

8. A positive electrode sheet, characterized in that: The positive electrode sheet comprises the in-situ carbon-coated multi-cation-doped lithium manganese iron phosphate as claimed in claim 6 or 7.

9. A lithium-ion battery, characterized in that: The lithium-ion battery comprises the positive electrode sheet according to claim 8.

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