High-temperature-resistant low-iron-dissolution lithium iron phosphate material as well as preparation method and application thereof

Abstract

The invention discloses a high-temperature-resistant low-iron-dissolution lithium iron phosphate material as well as a preparation method and application thereof. The preparation method comprises the following steps: mixing an inorganic carbon source with uniform slurry containing iron phosphate, lithium salt and titanium salt, and carrying out thermal evaporation to obtain a primary carbon-coated precursor; fully contacting the primary carbon-coated precursor with a mixed solution containing an organic carbon source and fluoride, and carbonizing to obtain a secondary carbon-coated body with a fluorine-doped carbon layer; and sintering the secondary carbon coated body step by step in a protective atmosphere to obtain the double-carbon coated doped lithium iron phosphate material, namely the high-temperature-resistant low-iron-dissolution lithium iron phosphate material. The high-temperature-resistant low-iron-dissolution lithium iron phosphate material disclosed by the invention has the graphene layer and the fluorine-doped carbon layer, so that high-temperature iron dissolution can be synergistically inhibited, and the interface stability and the electronic conductivity are improved; and titanium-fluorine anion and cation doping optimizes charge transfer, realizes balance of conductivity and structural stability, and can be widely applied to positive electrode materials or lithium ion batteries.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium battery materials, and in particular relates to a high-temperature resistant low-iron elution lithium iron phosphate material and a preparation method and application thereof. Background Art

[0002] As a core technology to promote energy transformation, lithium-ion batteries are widely used in new energy vehicles, smart grids and consumer electronics. Although traditional ternary cathode materials (such as NCM and NCA) have high energy density, they are prone to metal ion dissolution and structural collapse at high temperatures, leading to the risk of thermal runaway. Lithium iron phosphate (LiFePO4) has become the mainstream choice for power batteries due to its high stability, low cost and environmentally friendly properties of its olivine structure. However, its intrinsic conductivity is low (10 -9 S / cm) and the problem of iron ion dissolution at high temperature still restrict its application in extreme working conditions.

[0003] Lithium iron phosphate cathode materials have attracted much attention due to their excellent thermal safety and cycle performance, but their electronic conductivity is extremely low and they need to rely on carbon coating and element doping to improve their electrochemical activity. In addition, under high temperature (≥60℃) conditions, the Fe 2+ Easily oxidized to Fe 3 + and dissolve, reacting with the electrolyte to form byproducts, accelerating capacity decay. Existing technologies often improve performance through single carbon coating or doping, but it is difficult to achieve both improved conductivity and suppressed iron dissolution. Innovative synergistic modification solutions are urgently needed.

[0004] In the prior art, CN 115566160 A proposed the use of an organic-inorganic dual carbon source mixture and bimetallic doping, combined with a staged sintering process to homogenize the material particles, and the capacity retention rate reached 99.5% after 500 cycles. CN 116409773A directly synthesized a dihydrogen ferric phosphate precursor using a solid-phase iron source, simplified the process flow, and achieved a capacity retention rate of >80% after 1900 cycles. CN 119447490A improved the overall battery life by modifying the negative electrode material (such as silica coating and carbon layer gridding), but did not directly solve the problem of positive electrode iron dissolution. CN 119822346A proposed a sodium-magnesium co-doping and cuprous sulfide / graphene oxide double-layer coating strategy to inhibit iron dissolution through a multi-step composite process, but the process was complicated and costly. CN 119370820A introduced P2O5 to absorb carbonized water vapor to optimize the uniformity of the carbon coating, but did not involve element doping, and the conductivity improvement was limited. CN 119660702A forms a core-shell structure through secondary coating of a colloidal carbon source and titanium doping. Although it reduces the iron dissolution rate, titanium doping may cause lattice distortion and affect the uniformity of the material. CN 119176536A uses plasma treatment and double ball milling liquid for synergistic modification, but the energy consumption of multi-step ball milling and heat treatment is high. CN 119370822A enhances the cycle performance through ZIF-8 assisted porous structure design and rare earth coating, but rare earth oxide coating significantly increases the cost of raw materials. CN 119683590A proposes a carbon ball template solid phase method process optimization, and the compaction density reaches 2.35g / cm 3 , but it does not solve the problem of high-temperature iron dissolution. CN 119349545A uses a two-step catalytic graphitization process, which reduces the flocculent carbon content, but requires hydrogen protection sintering, and the process safety requirements are strict. CN 119683597A increases the compaction density to 2.4g / cm through particle size classification and carbon layer gradient coating. 3 , but the improvement in rate performance is limited. CN 119750534 A developed a low-temperature synthesis process for lithium iron phosphate nanomaterials, but the material cycle stability is insufficient. CN 119581522 A improved rate performance through high-energy ball milling and spherical granulation technology, but the high-temperature capacity decay rate is relatively fast. CN 119943913 A proposed an in-situ titanium-doped sheet lithium iron phosphate / carbon composite material, and the conductivity was increased to 10 -4 S / cm, but the iron dissolution rate is unknown. CN 119542386 A uses a lithium iron phosphate / graphene composite electrode, but fails to address the issue of interfacial side reactions at high temperatures. The aforementioned patent rarely mentions slowing iron dissolution at high temperatures, which can directly affect the stability of the material structure. Existing lithium batteries operate in harsh environments, and extreme conditions can significantly accelerate crystal structure destruction, increase iron dissolution rates, and accelerate performance degradation.

[0005] In short, while there are numerous existing lithium iron phosphate modification methods, most of them focus on improving a single aspect of performance, such as rate capability or long-cycle performance. However, they are ineffective in inhibiting high-temperature iron dissolution, leading to structural collapse and capacity degradation. Furthermore, the complexity of the modification process poses a significant cost challenge. Summary of the Invention

[0006] The main purpose of the present invention is to provide a high temperature resistant low iron elution lithium iron phosphate material and its preparation method and application, so as to overcome the shortcomings of the prior art.

[0007] To achieve the aforementioned object of the invention, the technical solutions adopted by the present invention include:

[0008] One aspect of the present invention provides a method for preparing a high-temperature resistant, low-iron leaching lithium iron phosphate material, which comprises: mixing an inorganic carbon source with a uniform slurry comprising iron phosphate, a lithium salt, and a titanium salt, and subjecting the mixture to thermal evaporation to obtain a primary carbon-coated precursor; fully contacting the primary carbon-coated precursor with a mixed liquid comprising an organic carbon source and a fluoride, and carbonizing the mixture to obtain a secondary carbon-coated body having a fluorine-doped carbon layer; and sintering the secondary carbon-coated body in a protective atmosphere in steps to obtain a double-carbon-coated, doped lithium iron phosphate material, i.e., a high-temperature resistant, low-iron leaching lithium iron phosphate material.

[0009] Another aspect of the present invention provides a high-temperature resistant low-iron leaching lithium iron phosphate material prepared by the preparation method.

[0010] Another aspect of the present invention provides the use of high temperature resistant low iron dissolution lithium iron phosphate material in the preparation of positive electrode materials.

[0011] Another aspect of the present invention further provides a positive electrode material, which includes the high-temperature resistant and low-iron elution lithium iron phosphate material.

[0012] Another aspect of the present invention provides a lithium-ion button battery, comprising a positive electrode, a negative electrode, an electrolyte, and a separator between the positive electrode and the negative electrode, wherein the positive electrode comprises the positive electrode material.

[0013] Compared with the prior art, the present invention has at least the following beneficial effects:

[0014] (1) The preparation method provided by the present invention is different from the existing technology that relies on single carbon coating or chemical doping to inhibit iron dissolution. Instead, it adopts a graphene-fluorocarbon double-layer coating structure design. Through the dual mechanism of graphene physical barrier and fluorocarbon layer chemical bonding, it synergistically reduces the dissolution risk from the two aspects of migration path blocking and lattice anchoring, inhibits the high-temperature iron dissolution of lithium iron phosphate, and at the same time improves the interface stability and electronic conductivity, significantly improving the material stability in high-temperature environments.

[0015] (2) The preparation method provided by the present invention adopts a titanium-fluorine doping process. Through the dual doping design of titanium with high ionic conductivity and anionic fluorine with high electronegativity, the lattice charge transfer path is optimized, forming a continuous conductive network to improve conductivity, and reducing lattice distortion through the adaptability of the doping elements, taking into account high conductivity and structural integrity, and achieving a balance between conductivity and structural stability.

[0016] (3) The preparation method provided by the present invention is a one-step process, which integrates the carbon coating, element doping and sintering processes into a single continuous step, breaking through the limitations of traditional multi-step separation, reducing equipment investment and energy consumption, improving process safety and economy, and significantly reducing energy consumption and production costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments recorded in the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0018] Figure 1 This is a product morphology diagram of the lithium iron phosphate material according to Example 1 of the present invention;

[0019] Figure 2 Graph showing the iron dissolution values ​​of the lithium iron phosphate materials of Examples 1-3 of the present invention and Comparative Example 1 after being immersed in a lithium hexafluorophosphate electrolyte at a high temperature of 60° C. for 30 days;

[0020] Figure 3 1 is a graph showing the capacity retention rate of lithium-ion button batteries assembled in Application Examples 1-4 of the present invention during normal temperature cycling;

[0021] Figure 4 1 is a graph showing the high-temperature and long-cycle capacity retention of lithium-ion button batteries assembled in Application Examples 1-4 of the present invention;

[0022] Figure 5 This is a comparison of XRD patterns of the lithium iron phosphate material of Comparative Example 1 of the present invention before and after immersion in a lithium hexafluorophosphate electrolyte at 60° C. for 30 days. DETAILED DESCRIPTION

[0023] In view of the problems existing in the above-mentioned prior art, the inventors of the present invention have conducted extensive and in-depth research and provided a high-temperature resistant, low-iron dissolution lithium iron phosphate material and its preparation method and application. The material mainly adopts a synergistic strategy of secondary carbon source coating (graphene + fluorocarbon layer) and element doping to inhibit iron dissolution through the dual mechanism of physical barrier and chemical bonding, while optimizing the lattice conductivity, breaking through the bottleneck of existing technology and significantly reducing the high-temperature iron dissolution rate.

[0024] The technical solution, its implementation process and principles are further explained below.

[0025] The present invention provides a preparation method for a high-temperature resistant and low-iron dissolution lithium iron phosphate material, which mainly comprises the following steps: using iron phosphate and lithium salt as raw materials, mixing them according to the Li / Fe molar ratio, adding a titanium source, and wet ball milling to form a uniform slurry; subsequently adding a graphene dispersion and spray drying to achieve a primary graphene coating; then immersing the primary carbon coating precursor in a mixed solution of an organic carbon source and a fluorine source, and carbonizing at a low temperature to form a secondary fluorine-doped carbon layer; finally, performing segmented sintering under a protective atmosphere to promote the embedding of doped atoms into the lattice and strengthen the interface bonding of the carbon layer, thereby obtaining a lithium iron phosphate positive electrode material with both high conductivity and resistance to high-temperature iron dissolution.

[0026] As one aspect of the technical solution of the present invention, a method for preparing a high-temperature resistant low-iron dissolution lithium iron phosphate material is provided, comprising:

[0027] An inorganic carbon source is mixed with a uniform slurry containing iron phosphate, lithium salt, and titanium salt, and subjected to thermal evaporation to obtain a primary carbon coating precursor;

[0028] Fully contacting the primary carbon coating precursor with a mixed solution containing an organic carbon source and a fluoride, and obtaining a secondary carbon coating body having a fluorine-doped carbon layer after carbonization;

[0029] The secondary carbon coating body is sintered step by step in a protective atmosphere to obtain a double-carbon-coated doped lithium iron phosphate material, that is, a high-temperature resistant low-iron elution lithium iron phosphate material.

[0030] In some embodiments, the preparation method of the high-temperature resistant low-iron leaching lithium iron phosphate material specifically includes: wet-grinding the iron phosphate, lithium salt, and titanium salt to obtain the uniform slurry, and then adding the dispersion of the inorganic carbon source to the uniform slurry, and obtaining the primary carbon-coated precursor through thermal evaporation and dry grinding.

[0031] In the above embodiment, the inorganic carbon source and the doped titanium salt are evenly coated on the surface of the particles, which is beneficial for the penetration of inorganic ions into the lattice during high-temperature sintering, and the carbon is evenly coated on the surface of the particles, which not only improves the conductivity of the positive electrode material, but also reduces the direct exposure of the positive electrode material to the electrolyte, slowing down the corrosion of the particles by the dissociation products of the electrolyte.

[0032] Furthermore, the molar ratio of lithium element to iron element in the iron phosphate and lithium salt is 1.02-1.05:1.

[0033] Furthermore, the molar ratio of the titanium salt to the iron phosphate is 0.001-0.005:1.

[0034] Furthermore, the mass ratio of the inorganic carbon source to the uniform slurry is 0.01-0.05:1, and the carbon content in the dispersion is 1-2 wt%.

[0035] Furthermore, the thermal evaporation temperature is 60-120° C., and the time is 1-2 hours.

[0036] Furthermore, the wet grinding requires the addition of anhydrous ethanol, with a liquid-to-solid volume ratio of the anhydrous ethanol to the total amount of the iron phosphate, lithium salt, and titanium salt being 0.8 to 1.2:1, the wet grinding speed being 400 to 600 rpm, and the time being 6 to 10 hours. The wet grinding is performed using a planetary ball mill with zirconia balls.

[0037] Furthermore, the dry grinding speed is 200-400 rpm, and the dry grinding time is 1-2 hours. In the present invention, since wet grinding alone can cause partial stratification during the dry stage, where the low-density, fine-particle carbon powder tends to settle on the upper surface, the subsequent dry grinding can achieve more uniform mixing of the raw materials and reduce the uneven stratification caused by sedimentation of low-density materials during the evaporation process.

[0038] Furthermore, the inorganic carbon source includes one or more combinations of graphene and graphene oxide.

[0039] Furthermore, the lithium salt includes one or more combinations of lithium carbonate and lithium hydroxide.

[0040] Furthermore, the titanium salt includes any one of tetrabutyl titanate, titanium tetrachloride, and titanyl sulfate, or a combination of two or more thereof.

[0041] In some embodiments, the preparation method of the high-temperature resistant low-iron leaching lithium iron phosphate material specifically includes: immersing the primary carbon-coated precursor in a mixed solution containing an organic carbon source and a fluoride, forming a dense fluorine-doped carbon layer after carbonization, and obtaining the secondary carbon-coated organic carbon source.

[0042] In the above embodiment, fluoride is added, wherein fluoride ions can enter the crystal lattice to replace part of the oxygen ions, thereby improving the electrical conductivity, and can also participate in the formation of the CEI film on the positive electrode surface, thereby improving the lithium ion transmission efficiency.

[0043] Furthermore, the organic carbon source includes one or more combinations of glucose, sucrose, polyethylene glycol, chitosan, polyvinyl alcohol, and biomass carbon.

[0044] Furthermore, the fluoride includes a combination of one or more of polyvinylidene fluoride, ammonium fluoride, and lithium fluoride.

[0045] Furthermore, the molar ratio of carbon to fluorine in the mixed solution of the organic carbon source and the fluoride is 1:1 to 5:1.

[0046] Furthermore, the carbonization temperature is 350-450° C., and the carbonization time is 2-4 hours.

[0047] Furthermore, the carbon content in the secondary carbon coating is 1.0-1.5 wt%.

[0048] In some embodiments, the preparation method of the high-temperature resistant low-iron leaching lithium iron phosphate material specifically includes: in a protective atmosphere, heating the secondary carbon coating to a first sintering temperature at a first heating rate, performing a first sintering, and then heating to a second sintering temperature at a second heating rate, performing a second sintering to obtain the high-temperature resistant low-iron leaching lithium iron phosphate material.

[0049] Furthermore, the protective atmosphere includes an inert gas atmosphere, or a mixed atmosphere of a reducing gas and an inert gas.

[0050] Furthermore, the reducing gas includes hydrogen.

[0051] Furthermore, the inert gas includes one or more of nitrogen and argon.

[0052] Furthermore, the volume ratio of the reducing gas in the mixed atmosphere is 1 to 5%.

[0053] Furthermore, the volume ratio of the reducing gas in the mixed atmosphere is 3%.

[0054] Furthermore, the first heating rate is 3-5°C / min.

[0055] Furthermore, the first sintering temperature is 450-600° C., and the first sintering time is 4-6 hours.

[0056] Furthermore, the second heating rate is 1-3°C / min.

[0057] Furthermore, the second sintering temperature is 720-750° C., and the second sintering time is 8-12 hours.

[0058] In the above embodiment, the first sintering temperature is in the low temperature range, which can remove volatile impurities in the raw materials, make the reaction more stable, and reduce the defects caused by direct high-temperature reaction; and the two sinterings can regulate the product grains to a certain extent, make the obtained grains more uniform, and promote the initial solid-phase reaction of lithium.

[0059] Among them, in some more specific embodiments, the steps of the preparation method of the high temperature resistant low iron dissolution lithium iron phosphate material are as follows:

[0060] Using iron phosphate, lithium carbonate and titanium salt as raw materials, according to the proportion, wet ball milling is performed to obtain a prefabricated material, and then a certain proportion of graphene is added and low-temperature thermal evaporation is performed to obtain a primary carbon coating material.

[0061] The primary carbon coating material is immersed in a mixed solution of organic carbon and fluoride for secondary carbon coating, and then carbonized at low temperature in a tube furnace to form a dense fluorine-doped carbon layer;

[0062] The material was then sintered in steps at 750°C (controlling the sintering atmosphere and heating rate) to produce a dual-carbon-coated, doped lithium iron phosphate material. Particle size classification and performance testing were then conducted to verify its high-temperature cycling stability and iron dissolution inhibition.

[0063] As another aspect of the technical solution of the present invention, it provides a high-temperature resistant low-iron leaching lithium iron phosphate material prepared by the preparation method.

[0064] In some embodiments, the high temperature resistant low iron dissolution lithium iron phosphate material has a composite carbon coating layer, and the composite coating layer is a first carbon coating layer and a second fluorine-doped carbon layer sequentially coated on the surface of the lithium iron phosphate.

[0065] In some embodiments, the high temperature resistant low iron leaching lithium iron phosphate material has a carbon content of 1.0 to 1.5%, a titanium content of 0.1 to 0.5%, and a fluorine content of 0.2 to 1.0%.

[0066] In some embodiments, the high temperature resistant low iron dissolution lithium iron phosphate material has an iron dissolution rate below 0.065% at a temperature above 60°C.

[0067] As another aspect of the technical solution of the present invention, it provides the use of high-temperature resistant and low-iron leaching lithium iron phosphate materials in the preparation of positive electrode materials.

[0068] As another aspect of the technical solution of the present invention, a positive electrode material is also provided, and the positive electrode material includes the high-temperature resistant and low-iron elution lithium iron phosphate material.

[0069] As another aspect of the technical solution of the present invention, it also provides a lithium-ion button battery, including a positive electrode, a negative electrode, an electrolyte, and a separator between the positive electrode and the negative electrode, wherein the positive electrode includes the positive electrode material.

[0070] In some embodiments, the lithium-ion button battery has a 0.1C first charge capacity of 161.6 to 165.2 mAh / g, a 0.1C first discharge capacity of 161.2 to 164.5 mAh / g, and a first discharge efficiency of 99.20 to 99.81%.

[0071] In summary, the present invention proposes a synergistic strategy of carbon double-layer coating and element doping to prepare a high-temperature resistant and low-iron dissolution lithium iron phosphate material. The double-carbon coating strategy provides a physical barrier to block the corrosion of the electrolyte and the oxidation reaction of divalent iron, and the high-temperature iron dissolution rate of the material is significantly reduced. Moreover, by optimizing the coating and doping process, the coating and doping are completed simultaneously in one-step pyrolysis, which reduces energy consumption and is more economical than the traditional multi-step process. The preparation method provided by the present invention effectively overcomes the technical bottlenecks of high-temperature iron dissolution and insufficient conductivity, and provides a core positive electrode material for high-stability lithium iron phosphate batteries.

[0072] In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with embodiment. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. Those skilled in the art will make modifications or equivalent substitutions based on understanding the technical solution of the present invention, without departing from the spirit and scope of the technical solution of the present invention, and all should be encompassed within the protection scope of the present invention.

[0073] Where specific experimental procedures or conditions are not specified in the examples, the experiments were carried out according to conventional experimental procedures or conditions described in literature in the field. Reagents or instruments used without manufacturer specified are commercially available. Commercially available options for other raw materials and instruments not mentioned are conventional and do not relate to the core technical means of the present invention.

[0074] Example 1

[0075] (1) Raw material ratio and wet ball milling: Iron phosphate and lithium carbonate were mixed at a ratio of Li:Fe = 1.03:1, tetrabutyl titanate (0.3 wt%) and anhydrous ethanol (liquid-to-solid volume ratio of 1:1) were added, and the mixture was ball milled in a planetary ball mill (zirconia grinding balls, speed of 400 rpm) for 8 hours to form a uniform prefabricated slurry;

[0076] (2) Primary carbon coating and drying: Graphene dispersion (mass ratio of 0.03:1, carbon content of 1.5 wt%) was added to the slurry, the solvent was evaporated in an oven at 60 °C, and dry ball milling (400 rpm, 1.5 h) was performed to obtain a primary carbon coating material;

[0077] (3) Secondary carbon coating and low-temperature carbonization: The primary coating material was immersed in a glucose-polyvinylidene fluoride mixture (carbon to fluorine molar ratio 3:1) and carbonized at 400 °C in a tube furnace for 2 h to form a fluorine-doped carbon layer (total carbon content 1.2 wt%).

[0078] (4) Stepwise sintering and crystallization: In a 3% argon-hydrogen mixed atmosphere, the temperature was first raised to 600°C at 5°C / min and kept for 5 hours, and then raised to 750°C at 3°C / min and kept for 10 hours to complete titanium doping and carbon layer graphitization to obtain lithium iron phosphate material. The product morphology is shown in the figure. Figure 1 shown.

[0079] Example 2

[0080] (1) Raw material ratio and wet ball milling: Iron phosphate and lithium carbonate were mixed at a ratio of Li:Fe = 1.05:1, titanyl sulfate (0.5 wt%) and anhydrous ethanol (liquid-to-solid volume ratio of 0.8:1) were added, and the mixture was ball milled in a planetary ball mill (zirconia grinding balls, 400 rpm) for 10 h to form a uniform prefabricated slurry;

[0081] (2) Primary carbon coating and drying: Graphene dispersion (mass ratio of 0.03:1, carbon content of 2 wt%) was added to the slurry, the solvent was evaporated in an oven at 60 °C, and dry ball milling (400 rpm, 2 h) was performed to obtain the primary carbon coating material;

[0082] (3) Secondary carbon coating and low-temperature carbonization: The primary coating material was immersed in a polyethylene glycol-chitosan-ammonium fluoride mixture (carbon to fluorine molar ratio 1:1) and carbonized at 450 °C in a tube furnace for 2 h to form a fluorine-doped carbon layer (total carbon content 1.5 wt%).

[0083] (4) Step-by-step sintering and crystallization: In a 3% nitrogen-hydrogen mixed atmosphere, the temperature was first raised to 600°C at a rate of 5°C / min and kept at that temperature for 6 hours, and then raised to 750°C at a rate of 3°C / min and kept at that temperature for 8 hours to complete titanium doping and carbon layer graphitization to obtain lithium iron phosphate material.

[0084] Example 3

[0085] (1) Raw material ratio and wet ball milling: Iron phosphate and lithium carbonate were mixed at a ratio of Li:Fe = 1.02:1, titanium tetrachloride (0.3 wt%) and anhydrous ethanol (liquid-to-solid volume ratio of 1:1) were added, and the mixture was ball milled in a planetary ball mill (zirconia grinding balls, speed of 400 rpm) for 6 h to form a uniform prefabricated slurry;

[0086] (2) Primary carbon coating and drying: Graphene dispersion (mass ratio of 0.05:1, carbon content of 1 wt%) was added to the slurry, the solvent was evaporated in an oven at 60 °C, and then dry ball milled (400 rpm, 1 h) to obtain the primary carbon coating material;

[0087] (3) Secondary carbon coating and low-temperature carbonization: The primary coating material was immersed in a glucose-chitosan-lithium fluoride mixture (carbon to fluorine molar ratio 2:1) and carbonized at 350 °C in a tube furnace for 2 h to form a fluorine-doped carbon layer (total carbon content 1.3 wt%).

[0088] (4) Step-by-step sintering and crystallization: In an argon atmosphere, the temperature was first raised to 600°C at a rate of 5°C / min and kept at that temperature for 4 hours, and then raised to 750°C at a rate of 3°C / min and kept at that temperature for 12 hours to complete titanium doping and carbon layer graphitization to obtain lithium iron phosphate material.

[0089] Example 4

[0090] (1) Raw material ratio and wet ball milling: Iron phosphate and lithium carbonate were mixed at a ratio of Li:Fe = 1.02:1, titanium tetrachloride (0.1 wt%) and anhydrous ethanol (liquid-to-solid volume ratio of 1.2:1) were added, and the mixture was ball milled in a planetary ball mill (zirconia grinding balls, speed of 400 rpm) for 6 hours to form a uniform prefabricated slurry;

[0091] (2) Primary carbon coating and drying: Graphene dispersion (mass ratio of 0.05:1, carbon content of 1 wt%) was added to the slurry, the solvent was evaporated in an oven at 60 °C, and then dry ball milled (400 rpm, 1 h) to obtain the primary carbon coating material;

[0092] (3) Secondary carbon coating and low-temperature carbonization: The primary coating material was immersed in a glucose-chitosan-lithium fluoride mixture (carbon to fluorine molar ratio 5:1) and carbonized at 350 °C in a tube furnace for 2 h to form a fluorine-doped carbon layer (total carbon content 1.0 wt%);

[0093] (4) Step-by-step sintering and crystallization: In an argon atmosphere, the temperature was first raised to 550°C at a rate of 5°C / min and kept for 6 hours, and then raised to 720°C at a rate of 1°C / min and kept for 12 hours to complete titanium doping and carbon layer graphitization to obtain lithium iron phosphate material.

[0094] Example 5

[0095] (1) Raw material ratio and wet ball milling: Iron phosphate and lithium carbonate were mixed at a ratio of Li:Fe = 1.02:1, titanium tetrachloride (0.1 wt%) and anhydrous ethanol (liquid-to-solid volume ratio of 1:1) were added, and the mixture was ball milled in a planetary ball mill (zirconia grinding balls, speed of 600 rpm) for 4 h to form a uniform prefabricated slurry;

[0096] (2) Primary carbon coating and drying: Graphene dispersion (mass ratio of 0.04:1, carbon content of 1 wt%) was added to the slurry, the solvent was evaporated in an oven at 120 °C, and then dry ball milled (200 rpm, 2 h) to obtain the primary carbon coating material;

[0097] (3) Secondary carbon coating and low-temperature carbonization: The primary coating material was immersed in a mixture of polyvinyl alcohol-chitosan-ammonium fluoride (carbon to fluorine molar ratio 4:1) and carbonized at 400 °C in a tube furnace for 2 h to form a fluorine-doped carbon layer (total carbon content 1.2 wt%).

[0098] (4) Step-by-step sintering and crystallization: In an argon atmosphere, the temperature was first raised to 450°C at a rate of 4°C / min and kept at that temperature for 6 hours, and then raised to 750°C at a rate of 2°C / min and kept at that temperature for 8 hours to complete titanium doping and carbon layer graphitization to obtain lithium iron phosphate material.

[0099] Example 6

[0100] (1) Raw material ratio and wet ball milling: Iron phosphate and lithium carbonate were mixed at a ratio of Li:Fe = 1.05:1, titanyl sulfate (0.3 wt%) and anhydrous ethanol (liquid-to-solid volume ratio of 1:1) were added, and the mixture was ball milled in a planetary ball mill (zirconia grinding balls, 500 rpm) for 5 h to form a uniform prefabricated slurry;

[0101] (2) Primary carbon coating and drying: Reduced graphene oxide dispersion (mass ratio of 0.05:1, carbon content of 1.0 wt%) was added to the slurry, and the solvent was evaporated in an oven at 80°C, followed by dry ball milling (400 rpm, 1 h) to obtain the primary carbon coating material;

[0102] (3) Secondary carbon coating and low-temperature carbonization: The primary coating material was immersed in a mixture of polyethylene glycol-chitosan-lithium fluoride-ammonium fluoride (carbon to fluorine molar ratio 1:1) and carbonized at 400 °C in a tube furnace for 2 h to form a fluorine-doped carbon layer (total carbon content 1.5 wt%);

[0103] (4) Step-by-step sintering and crystallization: In an argon atmosphere, the temperature was first raised to 600°C at a rate of 3°C / min and kept at that temperature for 4 hours, and then raised to 750°C at a rate of 2°C / min and kept at that temperature for 10 hours to complete titanium doping and carbon layer graphitization to obtain lithium iron phosphate material.

[0104] Example 7

[0105] (1) Raw material ratio and wet ball milling: Iron phosphate and lithium carbonate were mixed at a ratio of Li:Fe = 1.05:1, titanium tetrachloride (0.3 wt%) and anhydrous ethanol (liquid-to-solid volume ratio of 1:1) were added, and the mixture was ball milled in a planetary ball mill (zirconia grinding balls, speed of 400 rpm) for 4 hours to form a uniform prefabricated slurry;

[0106] (2) Primary carbon coating and drying: Reduced graphene oxide dispersion (mass ratio of 0.02:1, carbon content of 1.0 wt%) was added to the slurry, and the solvent was evaporated in an oven at 80 °C, followed by dry ball milling (300 rpm, 2 h) to obtain the primary carbon coating material;

[0107] (3) Secondary carbon coating and low-temperature carbonization: The primary coating material was immersed in a glucose-biochar-polyvinylidene fluoride mixture (carbon to fluorine molar ratio 3:1) and carbonized at 350 °C in a tube furnace for 2 h to form a fluorine-doped carbon layer (total carbon content 1.0 wt%).

[0108] (4) Step-by-step sintering and crystallization: In an argon atmosphere, the temperature was first raised to 500°C at a rate of 5°C / min and kept at that temperature for 6 hours, and then raised to 750°C at a rate of 1°C / min and kept at that temperature for 8 hours to complete titanium doping and carbon layer graphitization to obtain lithium iron phosphate material.

[0109] Application Example 1

[0110] Iron dissolution rate: Weigh 2g of lithium iron phosphate material powder prepared in Example 1 and add it to 10ml of lithium hexafluorophosphate electrolyte. Then, immerse it in a sealed container at high temperature of 60℃ for 30 days. Detect the iron ion concentration in the electrolyte and calculate the iron dissolution rate. The results are as follows: Figure 2 The iron dissolution rate is 0.019%.

[0111] Performance test: The lithium iron phosphate material prepared in Example 1 was ground and sieved, and then slurried and coated with SuperP and PVDF at a ratio of 90:5:5. The button cell was assembled and the cycle performance was tested. The results are shown in Table 1 and Figure 3 As shown, the 0.1C first charge capacity is 165.2mAh / g, the 0.1C first discharge capacity is 164.5mAh / g, the first discharge efficiency is 99.58%, the 3.3V platform discharge capacity is 147.9mAh / g, and the capacity retention rate after 500 cycles at room temperature is 95.5%.

[0112] Application Example 2

[0113] Iron dissolution rate: Weigh 2g of lithium iron phosphate material powder prepared in Example 2 and add it to 10ml of lithium hexafluorophosphate electrolyte. Then, immerse it in a sealed container at high temperature of 60℃ for 30 days. Detect the iron ion concentration in the electrolyte and calculate the iron dissolution rate. The results are as follows: Figure 2 The iron dissolution rate is 0.037%.

[0114] Performance test: The lithium iron phosphate material prepared in Example 2 was ground and sieved, and then slurried and coated with SuperP and PVDF at a ratio of 90:5:5. The button cell was assembled and the cycle performance was tested. The results are shown in Table 1 and Figure 3 The 0.1C first charge capacity is 162.7mAh / g, the 0.1C first discharge capacity is 161.4mAh / g, the first discharge efficiency is 99.20%, the 3.3V platform discharge capacity is 149.20mAh / g, and the capacity retention rate after 500 cycles at room temperature is 94.9%.

[0115] Application Example 3

[0116] Iron dissolution rate: Weigh 2g of lithium iron phosphate material powder prepared in Example 3 and add it to 10ml of lithium hexafluorophosphate electrolyte. Then, immerse it in a sealed container at high temperature of 60℃ for 30 days. Detect the iron ion concentration in the electrolyte and calculate the iron dissolution rate. The results are as follows: Figure 2The iron dissolution rate is 0.065%.

[0117] Performance test: The lithium iron phosphate material prepared in Example 3 was ground and sieved, and then slurried and coated with SuperP and PVDF at a ratio of 90:5:5. The button cell was assembled and the cycle performance was tested. The results are shown in Table 1 and Figure 3 The 0.1C first charge capacity is 161.6 mAh / g, the 0.1C first discharge capacity is 161.3 mAh / g, the first discharge efficiency is 99.81%, the 3.3V platform discharge capacity is 149.10 mAh / g, and the capacity retention rate after 500 cycles at room temperature is 96.9%.

[0118] Comparative Example 1

[0119] The high temperature resistant and low iron dissolution lithium iron phosphate material provided in this comparative example is an iron phosphate material that is only once coated with carbon and doped with titanium:

[0120] (1) Raw material ratio and wet ball milling: Iron phosphate and lithium carbonate were mixed at a ratio of Li:Fe = 1.05:1, titanyl sulfate (0.5 wt%) in anhydrous ethanol (liquid-to-solid volume ratio of 1:1) was added, and the mixture was ball milled in a planetary ball mill (zirconia grinding balls, 400 rpm) for 4 h to form a uniform prefabricated slurry;

[0121] (2) Glucose was added to the slurry at a ratio of 1.2 wt % and ball milling was continued for 4 hours. The solvent was evaporated in an oven at 80° C. to obtain a carbon-coated material;

[0122] (3) low-temperature carbonization at 350°C for 2 h in a tube furnace to form a carbon layer (total carbon content 1.2 wt%);

[0123] (4) Step-by-step sintering and crystallization: In an argon atmosphere, the temperature was first raised to 600°C at a rate of 5°C / min and kept at that temperature for 6 hours, and then raised to 750°C at a rate of 3°C / min and kept at that temperature for 8 hours to obtain lithium iron phosphate material.

[0124] The comparative material was immersed in an electrolyte at 60° C. for 30 days, and button batteries were assembled to test the cycle performance.

[0125] Application Example 4

[0126] Iron dissolution rate: Weigh 2g of lithium iron phosphate material powder prepared in Comparative Example 1 and add it to 10ml of lithium hexafluorophosphate electrolyte. Then, immerse it in a sealed container at high temperature of 60℃ for 30 days. Detect the iron ion concentration in the electrolyte and calculate the iron dissolution rate. The results are as follows: Figure 2 The iron dissolution rate was 0.26%.

[0127] Performance test: The lithium iron phosphate material prepared in Comparative Example 1 was ground and sieved, and then slurried and coated with SuperP and PVDF at a ratio of 90:5:5. The button cell was assembled and the cycle performance was tested. The results are shown in Table 1 and Figure 3 The 0.1C first charge capacity is 160.8mAh / g, the 0.1C first discharge capacity is 156.5mAh / g, the first discharge efficiency is 97.33%, the 3.3V platform discharge capacity is 146.5mAh / g, and the capacity retention rate after 400 cycles at room temperature is 44%.

[0128] Table 1 Sample cycle capacity and coulombic efficiency

[0129]

[0130] The various aspects, embodiments, features and examples of the present invention should be considered as illustrative in all respects and are not intended to limit the present invention, the scope of which is defined solely by the claims. Other embodiments, modifications and uses will be apparent to those skilled in the art without departing from the spirit and scope of the invention as claimed.

[0131] In addition, the inventors of this case also referred to the aforementioned embodiments and conducted experiments using other raw materials, process operations, and process conditions described in this specification, and obtained relatively ideal results.

[0132] Although the present invention has been described with reference to illustrative embodiments, it will be understood by those skilled in the art that various other changes, omissions, and / or additions may be made and that substantial equivalents may be substituted for the elements of the embodiments without departing from the spirit and scope of the present invention. Additionally, many modifications may be made to adapt specific circumstances or materials to the teachings of the present invention without departing from the scope of the present invention. Therefore, it is not intended herein to limit the present invention to the disclosed specific embodiments for carrying out the present invention, but rather to include all embodiments within the scope of the appended claims. Furthermore, unless specifically stated, any use of the terms first, second, etc. does not indicate any order or importance, but rather uses the terms first, second, etc. to distinguish one element from another.

Claims

1. A method for preparing a high temperature resistant low iron elution lithium iron phosphate material, characterized in that: include: An inorganic carbon source is mixed with a uniform slurry containing iron phosphate, lithium salt, and titanium salt, and subjected to thermal evaporation to obtain a primary carbon coating precursor; Fully contacting the primary carbon coating precursor with a mixed solution containing an organic carbon source and a fluoride, and obtaining a secondary carbon coating body having a fluorine-doped carbon layer after carbonization; The secondary carbon coating body is sintered step by step in a protective atmosphere to obtain a double-carbon-coated doped lithium iron phosphate material, that is, a high-temperature resistant low-iron elution lithium iron phosphate material.

2. The preparation method according to claim 1, characterized in that Specifically include: Wet-grinding the iron phosphate, lithium salt, and titanium salt to obtain the uniform slurry, then adding the dispersion of the inorganic carbon source to the uniform slurry, and performing thermal evaporation and dry grinding to obtain the primary carbon coating precursor; and / or, the molar ratio of lithium to iron in the iron phosphate and lithium salt is 1.02 to 1.05:1; and / or, the molar ratio of the titanium salt to the iron phosphate is 0.001 to 0.005:1; Preferably, the mass ratio of the inorganic carbon source to the uniform slurry is 0.01 to 0.05:1, and the carbon content in the dispersion is 1 to 2 wt%; Preferably, the thermal evaporation temperature is 60-100°C and the time is 1-2 hours; Preferably, the wet grinding requires the addition of anhydrous ethanol, the liquid-solid volume ratio of the anhydrous ethanol to the total amount of the iron phosphate, lithium salt, and titanium salt is 0.8-1.2:1, the rotation speed of the wet grinding is 400-600 rpm, and the time is 6-10 hours; Preferably, the dry grinding speed is 200-400 rpm, and the dry grinding time is 1-2 hours; And / or, the inorganic carbon source includes one or more of graphene and graphene oxide; And / or, the lithium salt includes one or more combinations of lithium carbonate and lithium hydroxide; And / or, the titanium salt includes any one of tetrabutyl titanate, titanium tetrachloride, and titanyl sulfate, or a combination of two or more thereof.

3. The preparation method according to claim 1, characterized in that Specifically include: Immersing the primary carbon coating precursor in a mixed solution containing an organic carbon source and a fluoride, and forming a dense fluorine-doped carbon layer after carbonization to obtain the secondary carbon coating body; And / or, the organic carbon source includes one or more of glucose, sucrose, polyethylene glycol, chitosan, polyvinyl alcohol, and biomass carbon; And / or, the fluoride includes one or more combinations of polyvinylidene fluoride, ammonium fluoride, and lithium fluoride; and / or, the molar ratio of carbon to fluorine in the mixture of the organic carbon source and the fluoride is 1:1 to 5:1; And / or, the carbonization temperature is 350-450° C., and the carbonization time is 2-4 hours; Preferably, the carbon content in the secondary carbon coating is 1.0-1.5 wt%.

4. The preparation method according to claim 1, characterized in that Specifically include: In a protective atmosphere, heating the secondary carbon coating to a first sintering temperature at a first heating rate, performing a first sintering, and then heating the secondary carbon coating to a second sintering temperature at a second heating rate, performing a second sintering, to obtain the high-temperature resistant low-iron eluted lithium iron phosphate material; Preferably, the protective atmosphere comprises an inert gas atmosphere, or a mixed atmosphere of a reducing gas and an inert gas; Particularly preferably, the reducing gas comprises hydrogen; Particularly preferably, the inert gas includes one or more of nitrogen and argon; Particularly preferably, the volume ratio of the reducing gas in the mixed atmosphere is 1-5%; Preferably, the first heating rate is 3-5°C / min; Preferably, the first sintering temperature is 450-600° C., and the first sintering time is 4-6 hours; Preferably, the second heating rate is 1-3°C / min; Preferably, the second sintering temperature is 720-750° C., and the second sintering time is 8-12 hours.

5. A high temperature resistant and low iron elution lithium iron phosphate material prepared by the preparation method according to any one of claims 1 to 4.

6. The high temperature resistant low iron elution lithium iron phosphate material according to claim 5, characterized in that: The high temperature resistant low iron dissolution lithium iron phosphate material has a composite carbon coating layer, wherein the composite coating layer is a first carbon coating layer and a second fluorine-doped carbon layer sequentially coated on the surface of the lithium iron phosphate; And / or, the carbon content of the high temperature resistant low iron elution lithium iron phosphate material is 1.0-1.5%, the titanium content is 0.1-0.5%, and the fluorine content is 0.2-1.0%; And / or, the high temperature resistant low iron dissolution lithium iron phosphate material has an iron dissolution rate below 0.065% at a temperature above 60°C.

7. Use of the high temperature resistant low iron elution lithium iron phosphate material according to claim 5 or 6 in the preparation of positive electrode materials.

8. A positive electrode material, characterized in that It includes the high temperature resistant low iron elution lithium iron phosphate material according to claim 5 or 6.

9. A lithium-ion button battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator between the positive electrode and the negative electrode, characterized in that: The positive electrode comprises the positive electrode material according to claim 8.

10. The lithium-ion button battery according to claim 9, wherein: The lithium ion button battery has a 0.1C first charge capacity of 161.6 to 165.2 mAh / g, a 0.1C first discharge capacity of 161.2 to 164.5 mAh / g, and a first discharge efficiency of 99.20 to 99.81%.

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