Preparation method of coated multi-doped lithium iron phosphate positive electrode material and lithium battery

Abstract

The application belongs to the technical field of lithium batteries, and provides a preparation method of a coated multi-doped lithium iron phosphate positive electrode material and a lithium battery. The method comprises the following steps: adding a first lithium source, a first iron source, a first phosphorus source, a first carbon source, hydrogen boride and a first dispersing agent into a solvent to obtain a first mixed solution; performing first sintering to obtain a first precursor mixture and grinding to a first preset particle size to obtain a first precursor; adding a second lithium source, a second iron source, a second phosphorus source, a second carbon source and a second dispersing agent into a solvent to obtain a second mixed solution, performing second sintering to obtain a second precursor mixture and grinding to a second preset particle size to obtain a second precursor; and performing third sintering on the first precursor, the second precursor and a third carbon source to obtain a lithium iron phosphate positive electrode material, forming a boron-titanium-nitrogen tri-doped and double-layer carbon-coated lithium iron phosphate positive electrode material, improving the conductivity of the lithium iron phosphate positive electrode material and improving the performance of the lithium iron phosphate positive electrode material.

Description

Technical Field

[0001] This application belongs to the field of lithium battery technology, and particularly relates to a method for preparing a multi-doped lithium iron phosphate cathode material and a lithium battery thereof. Background Technology

[0002] The lithium iron phosphate electrode material has an olivine structure. Since Li+ can only be transported along a one-dimensional channel, the diffusion coefficient of lithium ions is low, the conductivity is poor, and the rate performance is low (i.e., the specific capacity is low).

[0003] Currently, commonly used modification methods for lithium iron phosphate (LFP) include elemental doping, crystal nanostructuring, and surface coating. However, a single modification method is insufficient to meet the demands of various applications, falling short of the theoretical specific capacity of 170 mAh / g for LFP electrode materials. Furthermore, single modification methods result in low compaction density of LFP electrode materials, failing to meet the requirements of specific applications, and falling short of the 3.6 g / cm³ required for LFP electrode materials. 3 There is still a gap between the theoretical compaction density and the actual density.

[0004] Existing technologies have limitations in terms of the electrochemical performance of lithium iron phosphate and the compaction density, which cannot meet the requirements. Summary of the Invention

[0005] This application provides a method for preparing a multi-doped lithium iron phosphate cathode material and a lithium battery, aiming to solve, to some extent, the problem of how to improve the electrochemical performance of lithium iron phosphate while also increasing the compaction density to meet the requirements.

[0006] In a first aspect, embodiments of this application provide a method for preparing a lithium iron phosphate cathode material, comprising:

[0007] S1, the first lithium source, the first iron source, the first phosphorus source, the first carbon source, hydrogen boroide and the first dispersant are added to the solvent in a preset ratio to react and obtain the first mixture;

[0008] S2, after drying the first mixture, perform a first sintering in an inert gas atmosphere to obtain a first precursor mixture, and grind the first precursor mixture to a first preset particle size to obtain a first precursor;

[0009] S3, the second lithium source, the second iron source, the second phosphorus source, the second carbon source and the second dispersant are added to the solvent to react and obtain the second mixture. After the second mixture is dried, it is sintered in an inert gas atmosphere to obtain the second precursor mixture. The second precursor mixture is ground to the second preset particle size to obtain the second precursor.

[0010] S4, after mixing and drying the first precursor, the second precursor and the third carbon source, a third sintering is carried out in an inert gas atmosphere to obtain a coated multi-doped lithium iron phosphate cathode material.

[0011] The second carbon source is amino acid chelated titanium, and the second preset particle size is smaller than the first preset particle size.

[0012] This application provides a method for preparing lithium iron phosphate cathode material. Compared with the prior art, the method first performs boron doping and a first carbon coating through hydrogen boride to form a first precursor with a larger particle size, where boron is doped into a first carbon coating layer close to the lithium iron phosphate substrate. Boron doping increases the hole carrier concentration of the coated carbon layer and improves the conductivity of the lithium iron phosphate cathode material. Then, titanium nitride doping and a second carbon coating are performed to form a second precursor with a smaller particle size, where titanium nitride is doped into a second carbon coating layer close to the lithium iron phosphate substrate. Since amino acid chelated titanium is a complex, it can better coat the lithium iron phosphate substrate, thereby improving the coating integrity in the second carbon coating. The large-particle-size first precursor, the small-particle-size second precursor, and the third carbon source are then mixed and sintered. When the size and ratio of the large-particle first precursor and the small-particle second precursor meet the requirements of graded packing, they, together with the third carbon source, form an ultra-high pressure-density lithium iron phosphate cathode material with boron, titanium, and nitrogen triple doping and a two-layer carbon coating. The second carbon coating, covering the first and second precursors respectively, further improves the conductivity of the lithium iron phosphate cathode material. The triple doping of boron, titanium, and nitrogen produces a synergistic effect, improves structural stability, and modifies the interface, thereby further enhancing the performance of the lithium iron phosphate cathode material. The types of the first and second iron sources, the first and second phosphorus sources, and the first and second lithium sources can be the same or different, depending on the specific application requirements.

[0013] This application also provides a method for preparing lithium manganese iron phosphate cathode material as described in the first aspect, which improves the electrochemical performance of lithium manganese iron phosphate cathode material in many ways.

[0014] Secondly, embodiments of this application provide a lithium battery, the lithium battery including a positive electrode sheet, the positive electrode sheet being made of lithium iron phosphate positive electrode material as described in any one of the first aspects, or the positive electrode sheet being made of lithium manganese iron phosphate positive electrode material as described in the first aspect.

[0015] The lithium battery of this application, using the lithium iron phosphate cathode material or lithium manganese iron phosphate cathode sheet prepared by the above-mentioned preparation method, greatly improves the conductivity and compaction density of the lithium iron phosphate or lithium manganese iron phosphate cathode material, thereby enabling the lithium battery to have higher energy density and cycle performance. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a schematic flowchart of a method for preparing lithium iron phosphate cathode material according to an embodiment of this application. Detailed Implementation

[0018] To make the technical problems, technical solutions, and beneficial effects of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0019] In this application, the term "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0020] In this application, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can both mean: a, b, c, a~b (i.e., a and b), a~c, b~c, or a~b~c, where a, b, and c can be single or multiple.

[0021] The terms "first" and "second" are used only to describe the purpose and to distinguish objects, such as substances, from one another, and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. For example, without departing from the scope of the provisions of this application, "first XX" may also be referred to as "second XX," and similarly, "second XX" may also be referred to as "first XX." Thus, a feature defined with "first" or "second" may explicitly or implicitly include one or more of that feature.

[0022] The terminology used in the embodiments of this application is for the purpose of describing particular implementations only and is not intended to be limiting of this application. The singular forms “a,” “the,” and “the” used in the implementations of this application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0023] It should be understood that in the various embodiments of this application, the sequence number of each process does not imply the order of execution. Some or all steps may be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the implementation regulations of this application.

[0024] The weights of the relevant components mentioned in the embodiments of this application can refer not only to the specific content of each component, but also to the proportional relationship between the weights of the components. Therefore, any scaling up or down of the content of the relevant components according to the embodiments of this application is within the scope disclosed in the embodiments of this application. Specifically, the mass described in the embodiments of this application can be a mass unit known in the chemical industry, such as μg, mg, g, or kg.

[0025] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0026] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application are available on the market or can be prepared by existing methods.

[0027] The lithium iron phosphate electrode material has an olivine structure. Since Li+ can only be transported along a one-dimensional channel, the diffusion coefficient of lithium ions is low, the conductivity is poor, and the rate performance is low (i.e., the specific capacity is low).

[0028] Currently, commonly used modification methods for lithium iron phosphate (LFP) include elemental doping, crystal nanostructuring, and surface coating. However, a single modification method is insufficient to meet the demands of various applications, falling short of the theoretical specific capacity of 170 mAh / g for LFP electrode materials. Furthermore, single modification methods result in low compaction density of LFP electrode materials, failing to meet the requirements of specific applications, and falling short of the 3.6 g / cm³ required for LFP electrode materials. 3 There is still a gap between the theoretical compaction density and the actual density.

[0029] To address the aforementioned issues to some extent, such as Figure 1 As shown, the first aspect of this application provides a method for preparing a lithium iron phosphate cathode material, comprising:

[0030] S1, the first lithium source, the first iron source, the first phosphorus source, the first carbon source, hydrogen boroide and the first dispersant are added to the solvent in a preset ratio to react and obtain the first mixture;

[0031] S2, after drying the first mixture, perform a first sintering in an inert gas atmosphere to obtain a first precursor mixture, and grind the first precursor mixture to a first preset particle size to obtain a first precursor;

[0032] S3, the second lithium source, the second iron source, the second phosphorus source, the second carbon source and the second dispersant are added to the solvent to react and obtain the second mixture. After drying the second mixture, the second sintering is carried out in an inert gas atmosphere to obtain the second precursor mixture. The second precursor mixture is ground to the second preset particle size to obtain the second precursor.

[0033] S4. After mixing and drying the first precursor, the second precursor and the third carbon source, a third sintering is carried out in an inert gas atmosphere to obtain a coated multi-doped lithium iron phosphate cathode material.

[0034] The second carbon source is amino acid chelated titanium, and the second preset particle size is smaller than the first preset particle size.

[0035] This application provides a method for preparing lithium iron phosphate cathode material. Compared with existing technologies, this method involves adding a first lithium source, a first iron source, a first phosphorus source, boronide nanosheets or boronide nanofilms, a first carbon source, and a first dispersant together to a solvent for reaction. This allows the first carbon source to uniformly cover the surface of the first precursor, resulting in a better first carbon coating during the first sintering process. This improves the integrity of the first carbon coating layer on the surface of the lithium iron phosphate substrate, further enhancing the electronic conductivity of lithium iron phosphate. This forms a first precursor with a larger particle size, doped with boron near the lithium iron phosphate substrate. Boron doping increases the hole carrier concentration in the first carbon coating layer, improving the conductivity of the lithium iron phosphate cathode material. Then, titanium nitride doping and a second carbon coating are performed to form a second precursor with a smaller particle size, doped with titanium nitride near the lithium iron phosphate substrate. Since amino acid-chelated titanium forms a complex, it better coats the lithium iron phosphate substrate, thus improving the coating integrity during the second carbon coating. Finally, the larger particles are... A first precursor with a large particle size, a second precursor with a small particle size, and a third carbon source are mixed and sintered. When the size and ratio of the large-particle first precursor and the small-particle second precursor meet the requirements of graded packing, together with the third carbon source, they form a lithium iron phosphate cathode material with ultra-high compaction density, featuring boron, titanium, and nitrogen triple doping and a two-layer carbon coating. Boron doping also increases the number of lithium-ion transport active sites, reduces the cell parameters, and improves ionic conductivity. Therefore, while increasing the powder compaction density of the lithium iron phosphate cathode material, boron doping also improves the electrochemical activity and charge / discharge specific capacity of the lithium iron phosphate cathode material, thus improving its electrochemical performance in multiple ways. The second carbon coating, covering the first and second precursors respectively, further improves the conductivity of the lithium iron phosphate cathode material. In this way, the triple doping of boron, titanium, and nitrogen can produce a synergistic effect, improve structural stability, and modify the interface, further improving the performance of the lithium iron phosphate cathode material. The types of the first and second iron sources, the first and second phosphorus sources, and the first and second lithium sources can be the same or different, depending on the specific application requirements. Experimental verification shows that the lithium iron phosphate cathode material prepared in this application achieves a first-charge specific capacity of 162.8 mAh / g for lithium batteries at a 0.1C rate, and the cathode sheet compaction density of the lithium iron phosphate cathode material prepared in this application reaches 2.905 g / cm³. 3 .

[0036] In some embodiments, step S1 includes: mixing a first iron source, a first phosphorus source, and a first lithium source to obtain a first mixed solution; mixing the first mixed solution with hydrogen boride to obtain a second mixed solution; and mixing a first carbon source, a first dispersant, and the second mixed solution to obtain a first mixed liquid. The steps of this embodiment form a first mixed liquid, first forming lithium iron phosphate, then distributing the lithium iron phosphate onto hydrogen boride, and then uniformly coating the lithium iron phosphate with the first carbon source on the hydrogen boride using the first dispersant. This improves the effect of boron doping and the first carbon coating, thereby improving the electrochemical performance of the coated multi-doped lithium iron phosphate cathode material.

[0037] In some embodiments, step S2 includes: drying the first mixture and then performing a first sintering in an inert gas atmosphere to obtain a first precursor mixture; irradiating the first precursor mixture with ultraviolet light; and grinding the irradiated first precursor mixture to a first preset particle size to obtain a first precursor. In this embodiment, the first precursor is irradiated with ultraviolet light, causing hydrogen gas in the hydrogen boride nanosheets or hydrogen boride nanofilms to volatilize, thereby forming a loose porous structure of the first precursor on the hydrogen boride nanosheets or hydrogen boride nanofilms. After grinding to the first preset particle size to obtain the first precursor, when the first precursor is sintered a third time, the third carbon source will uniformly cover the lithium iron phosphate precursor after the hydrogen gas has volatilized, improving the coating integrity of the lithium iron phosphate by the third carbon source. While releasing hydrogen, the hydrogen boride is doped with boron ions. Boron doping increases the hole carrier concentration of the coated carbon layer, further improving the conductivity of the carbon layer, and thus improving the electrochemical performance of the lithium iron phosphate cathode material.

[0038] In some embodiments, amino acid chelated titanium is a complex formed by the chelation of amino acids and titanium ions; step S3 includes S31, S32, S33, and S34.

[0039] S31, the first precursor, the second lithium source, the second iron source, the second phosphorus source, the second carbon source, and the second dispersant are mixed in a solvent and reacted to obtain a second mixture.

[0040] S32, the pH value of the second mixture is adjusted to a preset alkaline pH value using an alkaline solution to obtain a third mixture. The preset alkaline pH value is 8–10.

[0041] S33, after drying the third mixture, a second sintering is carried out in an inert gas atmosphere to obtain a second precursor mixture.

[0042] S34, the second precursor mixture is ground to a second preset particle size to obtain the second precursor.

[0043] In this embodiment, the structure of amino acid chelated titanium is changed by adjusting the pH value of the first precursor mixture, thereby decomposing or separating the chelate from titanium ions to achieve titanium doping. Then, nitrogen doping and carbon coating are achieved by second sintering. Finally, the precursor is ground to a second preset particle size to obtain a second precursor doped with titanium nitrogen and coated with a carbon layer.

[0044] In some embodiments, the mass ratio of hydrogen boride to the first precursor mixture is 1:(10000~100000);

[0045] And / or, in the lithium iron phosphate cathode material, the mass ratio of titanium and nitrogen to the second precursor mixture is 1:(10000~100000);

[0046] And / or, the molar ratio of the first phosphorus source, the first iron source, and the first lithium source, and the molar ratio of the second phosphorus source, the second iron source, and the second lithium source are all (0.90~1.2):(0.95~1.15):(0.90~1.20).

[0047] This embodiment sets a range for the mass ratio of hydrogen boride to the first precursor mixture to facilitate boron doping during carbon coating. This avoids excessive or insufficient boron doping in the first carbon coating layer of the first precursor. Within this embodiment, the boron doping level in the carbon coating layer better improves the conductivity of the lithium iron phosphate material. A boron doping level below this range results in a low hole carrier concentration in the carbon coating layer, leading to a minimal improvement in conductivity. Conversely, a boron doping level above this range leads to excessive defects in the carbon coating layer, disrupting its continuity and hindering electrochemical performance improvement. This embodiment also sets a mass ratio for titanium, nitrogen, and the lithium iron phosphate cathode material. Controlling the titanium and nitrogen doping levels within this range improves the performance of the lithium iron phosphate cathode material, preventing excessive or insufficient doping. Furthermore, this embodiment sets a molar ratio for the phosphorus, iron, and lithium sources. This range allows for better reaction, preventing incomplete reactions that could affect the pH value of the cathode material.

[0048] In some embodiments, the mass ratio of the first carbon source to the first precursor mixture in step S1 is (0.01 to 0.04):1;

[0049] And / or, the mass ratio of the first carbon source to the first iron source and the mass ratio of the second carbon source to the second iron source are both (0.03 to 0.08):1;

[0050] And / or, in step S3, the mass ratio of the second carbon source to the second precursor mixture is (0.04~0.10):1;

[0051] And / or, the mass ratio of the first dispersant to the first mixture and the mass ratio of the second dispersant to the second mixture are both (0.005~0.02):1;

[0052] And / or, the carbon content of the lithium iron phosphate cathode material is less than or equal to 0.5%.

[0053] In the lithium iron phosphate cathode material of this embodiment, if the carbon content of the first carbon source and the second carbon source is lower than the mass ratio range of this embodiment, the carbon layer coated on the surface of the lithium iron phosphate matrix will be discontinuous, resulting in poor conductivity and electrochemical performance. If the carbon content of the first carbon source and the second carbon source is higher than the mass ratio range of this embodiment, the coated carbon layer will be thicker and more dense, increasing the resistance to lithium-ion migration and reducing electrochemical performance. The carbon content of the first carbon source and the second carbon source within the mass ratio range of this embodiment can improve the integrity of the carbon layer coating and improve the electrochemical performance of the lithium iron phosphate cathode material. The mass ratio of the dispersant to the mixture allows for better uniform dispersion of each component in the solvent, which is deionized water. If the carbon content in the lithium iron phosphate cathode material is greater than 0.5%, the carbon layer coating the lithium iron phosphate will be thicker and more dense, increasing the resistance to lithium-ion migration and reducing electrochemical performance.

[0054] In some embodiments, the first preset particle size of the first precursor particles is 0.6 μm. <D501≦1.5μm;

[0055] And / or, the second preset particle size of the second precursor particles is 0.05μm≦D502≦0.3μm;

[0056] And / or, the mass ratio of the first precursor to the second precursor is (0.67 to 9.0):1.

[0057] In this embodiment, since the particle size of the first precursor particles is larger than that of the second precursor particles, and the particle size of the second precursor particles is smaller than that of the first precursor particles, the smaller-sized second precursor particles can fully fill the gaps between the larger-sized first precursor particles. Therefore, the two form a graded packing density, thereby achieving high compaction density.

[0058] In some embodiments, the first sintering parameters are: heating rate of 1℃ / min to 10℃ / min, sintering temperature of 500℃ to 700℃, and isothermal time during sintering of 2h to 8h.

[0059] And / or, the second sintering parameters are: heating rate of 2℃ / min~8℃ / min, sintering temperature of 700℃~800℃, and isothermal time during sintering of 4h~8h;

[0060] And / or, the parameters for the third sintering are: heating rate of 2℃ / min to 5℃ / min, sintering temperature of 700℃ to 850℃, and sintering time of 3h to 10h.

[0061] Sintering according to the first, second, and third sintering parameters provided in this embodiment can effectively carry out the reduction reaction, the first carbon coating of large and small particles, the graded stacking of mixed large and small particles, and the second carbon coating of large and small particles, thereby improving the carbon coating integrity and compaction density of the lithium iron phosphate cathode material, and thus improving the electrochemical performance of the lithium iron phosphate cathode material.

[0062] In some embodiments, step S4 may further be: mixing and drying the manganese source, the first precursor, the second precursor and the third carbon source, and then performing a third sintering in an inert gas atmosphere to obtain a coated multi-doped lithium manganese iron phosphate cathode material.

[0063] In this embodiment, a manganese source, a large-particle-size first precursor, a small-particle-size second precursor, and a third carbon source are mixed and subjected to a third sintering. The manganese source can be uniformly distributed in the structure of the first and second precursors, so that manganese ions can be uniformly doped into the first and second precursors during the third sintering. Furthermore, the third carbon source performs a second carbon coating on the first and second precursors, thereby improving the conductivity of the lithium manganese iron phosphate cathode material and thus improving its electrochemical performance.

[0064] In some embodiments, the lithium source includes at least one of lithium hydroxide, lithium oxalate, lithium carbonate, lithium acetate, lithium phosphate, lithium dihydrogen phosphate, dilithium hydrogen phosphate, lithium chloride, and lithium nitrate; and / or, the phosphorus source includes at least one of ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, ferric phosphate, lithium phosphate, and lithium dihydrogen phosphate; and / or, the iron source includes at least one of ferrous oxalate, ferrous acetate, ferrous chloride, ferrous sulfate, ferric oxide, ferric phosphate, ferric nitrate, ferric chloride, ferric sulfate, ferric hydroxide, ferric citrate, and ferric acetate; and / or, the manganese source is one or more combinations of manganese dihydrogen phosphate, manganese tetroxide, manganese dioxide, manganese oxide, and manganese carbonate; and / or, the inert gas includes any one of nitrogen, argon, or a nitrogen-argon mixture. And / or, the dispersant includes at least one selected from citric acid, succinic acid, tripropyl phosphate, dibutyl phosphate, polyamide-based silicone oil, methylcyclosiloxane, sodium dodecylbenzenesulfonate, and sodium octylbenzenesulfonate; and / or, the first carbon source and the third carbon source are at least one selected from glucose, sucrose, starch, citric acid, amino acids, polyethylene glycol, and polyvinylidene fluoride, respectively. This embodiment specifies the types of lithium source, carbon source, phosphorus source, manganese source, dispersant, and inert gas, providing more choices and facilitating selection based on specific application requirements. Furthermore, the diversification of sources reduces production costs. The first carbon source and the third carbon source can be the same or different, selected according to specific application requirements; this embodiment does not impose any restrictions.

[0065] The second aspect of this application provides a lithium battery, which includes a positive electrode sheet. The positive electrode sheet is made of lithium iron phosphate positive electrode material as described in any one of the first aspects, or the positive electrode sheet is made of lithium manganese iron phosphate positive electrode material as described in the first aspect.

[0066] The lithium battery of this application includes a positive electrode sheet made of lithium iron phosphate positive electrode material with high compaction density coated with boron, titanium, and nitrogen triple doping carbon or lithium manganese iron phosphate positive electrode material with high compaction density coated with boron, titanium, and nitrogen triple doping carbon. Due to the greatly improved conductivity and compaction density of the positive electrode material, the lithium battery has higher energy density and cycle performance.

[0067] To enable those skilled in the art to clearly understand the above-described implementation details and operations, and to demonstrate the significant improvement in the performance of the high-compact lithium iron phosphate cathode material and lithium battery of the embodiments of this application, the following examples illustrate the above technical solutions.

[0068] Example 1

[0069] 1) Add 1.04 parts of ammonium phosphate, 1 part of ferrous oxalate, 1.04 parts of lithium oxalate, 3% of glucose (the first carbon source relative to the mass of ferrous oxalate), 1 mg of hydrogen borate (the first mixture in a mass-volume ratio of 1 mg:10 mL relative to the first mixture), and 0.5% of citric acid (the first dispersant relative to the first mixture) to a deionized aqueous solvent and mix them to obtain the first mixture.

[0070] 2) The first mixture is dried and sintered in a nitrogen atmosphere. The heating rate during sintering is 1℃ / min, the sintering temperature is 500℃, and the isothermal time during sintering is 2h to obtain the first precursor mixture. The first precursor mixture is ground to a first preset particle size of 0.6μm to obtain the first precursor.

[0071] 3) 1.04 parts by molar amount of ammonium phosphate, 1 part by molar amount of ferrous oxalate, 1.04 parts by molar amount of lithium oxalate, 3% by molar amount of the second carbon source amino acid chelated titanium relative to the mass of ferrous oxalate, and 0.5% by molar amount of the second dispersant methylcyclosiloxane are added to a deionized aqueous solvent for mixing and reaction to obtain a second mixture. After drying the second mixture, a second sintering is carried out in a nitrogen atmosphere. The heating rate during the sintering process is 2℃ / min, the sintering temperature is 700℃, and the isothermal time during sintering is 4h to obtain a second precursor mixture. The second precursor mixture is then ground to a second preset particle size of 0.05μm to obtain the second precursor.

[0072] 4) After mixing and drying the first precursor, the second precursor and the third carbon source starch, the third sintering is carried out in a nitrogen atmosphere. The heating rate of the sintering process is 2℃ / min, the sintering temperature is 700℃, and the isothermal time during sintering is 3h, so as to obtain the coated multi-doped lithium iron phosphate cathode material.

[0073] Example 2: The difference from Example 1 is that the values ​​of each parameter are adjusted to the center value.

[0074] 1) Add 1.04 parts of ammonium phosphate, 1 part of ferrous oxalate, 1.04 parts of lithium oxalate, 5.5% of glucose (the first carbon source relative to the mass of ferrous oxalate), 1 mg of borohydride (the first mixture is in a mass-volume ratio of 1 mg:50 mL relative to the first mixture), and 1.75% of citric acid (the first dispersant relative to the first mixture) to a deionized aqueous solvent and mix them to obtain the first mixture.

[0075] 2) The first mixture is dried and sintered in a nitrogen atmosphere. The heating rate during sintering is 5℃ / min, the sintering temperature is 600℃, and the isothermal time during sintering is 5h to obtain the first precursor mixture. The first precursor mixture is ground to a first preset particle size of 1.05μm to obtain the first precursor.

[0076] 3) 1.04 parts by molar amount of ammonium phosphate, 1 part by molar amount of ferrous oxalate, 1.04 parts by molar amount of lithium oxalate, 5.5% by molar amount of the second carbon source amino acid chelated titanium relative to the mass of ferrous oxalate, and 0.5% by molar amount of the second dispersant methylcyclosiloxane were added to a deionized aqueous solvent for mixing and reaction to obtain a second mixture. After drying the second mixture, a second sintering was carried out in a nitrogen atmosphere. The heating rate during the sintering process was 5℃ / min, the sintering temperature was 750℃, and the isothermal time during sintering was 6h to obtain a second precursor mixture. The second precursor mixture was then ground to a second preset particle size of 0.175μm to obtain the second precursor.

[0077] 4) After mixing and drying the first precursor, the second precursor and the third carbon source starch, the third sintering is carried out in a nitrogen atmosphere. The heating rate of the sintering process is 3.5℃ / min, the sintering temperature is 775℃, and the isothermal time during sintering is 6.5h, to obtain the coated multi-doped lithium iron phosphate cathode material.

[0078] Example 3: The difference from Example 1 is that the values ​​of each parameter are adjusted to the upper limit.

[0079] 1) Add 1.04 parts of ammonium phosphate, 1 part of ferrous oxalate, 1.04 parts of lithium oxalate, 8% of glucose (the first carbon source relative to the mass of ferrous oxalate), 1 mg of hydrogen borate (the first mixture is in a mass-volume ratio of 1 mg:100 mL relative to the first mixture), and 2.0% of citric acid (the first dispersant relative to the first mixture) to a deionized aqueous solvent and mix them to obtain the first mixture.

[0080] 2) The first mixture is dried and sintered in a nitrogen atmosphere. The heating rate during sintering is 10℃ / min, the sintering temperature is 700℃, and the isothermal time during sintering is 8h to obtain the first precursor mixture. The first precursor mixture is ground to a first preset particle size of 1.5μm to obtain the first precursor.

[0081] 3) 1.04 parts by molar amount of ammonium phosphate, 1 part by molar amount of ferrous oxalate, 1.04 parts by molar amount of lithium oxalate, 8% by molar amount of the second carbon source amino acid chelated titanium relative to the mass of ferrous oxalate, and 2.0% by molar amount of the second dispersant methylcyclosiloxane are added to a deionized aqueous solvent for mixing and reaction to obtain a second mixture. After drying the second mixture, a second sintering is carried out in a nitrogen atmosphere. The heating rate during the sintering process is 8℃ / min, the sintering temperature is 800℃, and the isothermal time during sintering is 8h to obtain a second precursor mixture. The second precursor mixture is then ground to a second preset particle size of 0.3μm to obtain the second precursor.

[0082] 4) After mixing and drying the first precursor, the second precursor and the third carbon source starch, the third sintering is carried out in a nitrogen atmosphere. The heating rate of the sintering process is 5℃ / min, the sintering temperature is 850℃, and the isothermal time during sintering is 10h, to obtain the coated multi-doped lithium iron phosphate cathode material.

[0083] Example 4: The difference from Example 3 is that a manganese source was added in step 4) to obtain lithium iron manganese phosphate cathode material.

[0084] 4) After mixing and drying the manganese source, the first precursor, the second precursor and the third carbon source starch, a third sintering was carried out in a nitrogen atmosphere. The heating rate of the sintering process was 3.5℃ / min, the sintering temperature was 775℃, and the isothermal time during sintering was 6.5h, thus obtaining a multi-doped lithium iron phosphate cathode material.

[0085] Comparative Example 1: Compared to Example 3, no hydrogen boroide and amino acid chelated titanium were added, and all other conditions were the same, resulting in a coated doped comparative lithium iron phosphate cathode material.

[0086] Comparative Example 2: Compared to Example 4, no hydrogen boroide and amino acid chelated titanium were added, and all other conditions were the same, resulting in a coated doped comparative lithium manganese iron phosphate cathode material.

[0087] The lithium iron phosphate cathode materials obtained in Examples 1 to 4 and Comparative Examples 1 and 2 were reassembled into coin cells to test the first discharge specific capacity at 0.1C rate and the first discharge specific capacity at 1C rate. The comparative data are shown in Table 1.

[0088] Table 1 shows the first charge specific capacity, first discharge specific capacity, and first discharge specific capacity at 0.1C rate for the batteries corresponding to the lithium iron phosphate cathode materials obtained in Examples 1 to 4 and Comparative Examples 1 and 2, respectively.

[0089]

[0090] As can be seen from Table 1, Example 3, which uses lithium iron phosphate cathode material with boron doping and titanium nitride doping and has the maximum particle size parameter, has the highest compaction density. The prior art Comparative Example 1, which does not use boron doping and titanium nitride doping, has the lowest charge-discharge specific capacity and compaction density.

[0091] As can be seen from Table 1, the charge-discharge specific capacity and compaction density of the lithium manganese iron phosphate cathode material of Example 4, which is doped with hydrogen boride and titanium nitrogen and has the maximum particle size parameter, are higher than those of the prior art Comparative Example 2, which is not doped with hydrogen boride and titanium nitrogen.

[0092] In the above embodiments, the descriptions of each embodiment have different focuses. For parts that are not described in detail or recorded in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0093] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application, and should all be included within the protection scope of this application.

Claims

1. A method for preparing a lithium iron phosphate cathode material, characterized in that, include: S1, the first lithium source, the first iron source, the first phosphorus source, the first carbon source, hydrogen boroide and the first dispersant are added to the solvent in a preset ratio to react and obtain the first mixture; S2, after drying the first mixture, perform a first sintering in an inert gas atmosphere to obtain a first precursor mixture, and grind the first precursor mixture to a first preset particle size to obtain a first precursor; S3, the second lithium source, the second iron source, the second phosphorus source, the second carbon source and the second dispersant are added to the solvent to react and obtain the second mixture. After the second mixture is dried, it is sintered in an inert gas atmosphere to obtain the second precursor mixture. The second precursor mixture is ground to the second preset particle size to obtain the second precursor. S4, after mixing and drying the first precursor, the second precursor and the third carbon source, a third sintering is carried out in an inert gas atmosphere to obtain a coated multi-doped lithium iron phosphate cathode material. Wherein, the second carbon source is amino acid chelated titanium, and the second preset particle size is smaller than the first preset particle size; Among them, hydrogen boride is hydrogen boride nanosheets or hydrogen boride nanofilms; Step S2 includes: After drying the first mixture, a first sintering was performed in an inert gas atmosphere to obtain a first precursor mixture; The first precursor mixture was irradiated with ultraviolet light; The first precursor mixture after irradiation is ground to a first preset particle size to obtain a first precursor. Ultraviolet light irradiation causes hydrogen gas in the hydrogen boride nanosheets or hydrogen boride nanofilms to volatilize, forming a loose porous structure of the first precursor on the hydrogen boride nanosheets or hydrogen boride nanofilms.

2. The preparation method according to claim 1, characterized in that, Step S1 includes: The first iron source, the first phosphorus source, and the first lithium source are mixed and reacted to obtain a first mixed solution; The first mixed solution and the hydrogen boride are mixed to obtain a second mixed solution; The first carbon source, the first dispersant, and the second mixed solution are mixed to obtain a first mixed solution.

3. The preparation method according to claim 1, characterized in that, The amino acid chelated titanium is a complex formed by the chelation of amino acids and titanium ions. Step S3 includes: The first precursor, the second lithium source, the second iron source, the second phosphorus source, the second carbon source, and the second dispersant are mixed in a solvent and reacted to obtain the second mixture. The pH value of the second mixture is adjusted to a preset alkaline pH value using an alkaline solution to obtain a third mixture; After drying the third mixture, a second sintering process is carried out in an inert gas atmosphere to obtain the second precursor mixture; The second precursor mixture is ground to the second preset particle size to obtain the second precursor.

4. The preparation method according to claim 1, characterized in that, The mass ratio of the hydrogen boride to the first precursor mixture is 1:(10000~100000); And / or, the mass ratio of titanium and nitrogen in the lithium iron phosphate cathode material to the second precursor mixture is 1:(10000~100000); And / or, the molar ratio of the first phosphorus source, the first iron source, and the first lithium source and the molar ratio of the second phosphorus source, the second iron source, and the second lithium source are all (0.90~1.2):(0.95~1.15):(0.90~1.20).

5. The preparation method according to claim 1, characterized in that, In step S1, the mass ratio of the first carbon source to the first precursor mixture is (0.01~0.04):1; And / or, the mass ratio of the first carbon source to the first iron source and the mass ratio of the second carbon source to the second iron source are both (0.03~0.08):1; And / or, in step S3, the mass ratio of the second carbon source to the second precursor mixture is (0.04~0.10):1; And / or, the mass ratio of the first dispersant to the first mixture and the mass ratio of the second dispersant to the second mixture are both (0.005~0.02):1; And / or, the carbon content of the lithium iron phosphate cathode material is less than or equal to 0.5%.

6. The preparation method according to claim 1, characterized in that, The first preset particle size of the first precursor particles is 0.6 μm. <D501≦1.5 μm; And / or, the second preset particle size of the second precursor particles is 0.05 μm ≤ D502 ≤ 0.3 μm; And / or, the mass ratio of the first precursor to the second precursor is (0.67~9.0):

1.

7. The preparation method according to any one of claims 1 to 6, characterized in that, The first sintering parameters are: heating rate of 1 ℃ / min~10 ℃ / min, sintering temperature of 500 ℃~700 ℃, and isothermal time of 2 h~8 h during sintering; And / or, the second sintering parameters are: heating rate of 2 ℃ / min~8 ℃ / min, sintering temperature of 700 ℃~800 ℃, and isothermal time of 4 h~8 h during sintering; And / or, the parameters for the third sintering are: heating rate of 2 ℃ / min~5 ℃ / min, sintering temperature of 700 ℃~850 ℃, and sintering time of 3 h~10 h.

8. The preparation method according to claim 1, characterized in that, Step S4 can also be: After mixing and drying the manganese source, the first precursor, the second precursor and the third carbon source, a third sintering is carried out in an inert gas atmosphere to obtain a coated multi-doped lithium manganese iron phosphate cathode material.

9. A lithium battery, characterized in that, The lithium battery includes a positive electrode sheet, which is made of lithium iron phosphate positive electrode material as described in any one of claims 1 to 8.

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