A method for preparing lithium iron phosphate by liquid phase method of nitrogen-doped phenolic resin

Abstract

The application provides a method for preparing lithium iron phosphate by liquid-phase nitrogen-doped phenolic resin and relates to the technical field of lithium ion battery cathode materials, and solves the technical problems of waste lithium source, difficulty in element doping and poor doping effect in the preparation of lithium iron phosphate by the existing liquid-phase method. The application comprises the following steps: adding a surfactant, nitrogen-doped phenolic resin and a boron source into desalted water to dissolve and mix to obtain a dopant solution; adding the surfactant, a phosphorus salt, an iron salt and a lithium salt into desalted water to dissolve and mix to obtain a precursor solution; mixing the precursor solution with the dopant solution to obtain a precursor product through high-temperature and high-pressure reaction. The application improves the doping efficiency and uniformity through liquid-phase nitrogen doping and boron doping, improves the electrochemical performance of lithium iron phosphate through the synergistic effect of nitrogen doping and boron doping, and improves the lithium ion transfer rate while improving the lithium utilization rate by adding the surfactant to form lithium iron phosphate with three-dimensional porous morphology.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery cathode material technology, and more specifically, to a method for preparing lithium iron phosphate using a liquid-phase nitrogen-doped phenolic resin. Background Technology

[0002] Since 2009, the country has emphasized the development of the new energy industry, and various electric vehicles have become the main application areas for lithium iron phosphate (LFP). Although olivine-type LFP has a stable structure, is safe, and has good cycle performance, it has inherent drawbacks such as low electronic conductivity, low lithium-ion diffusion coefficient, and low tap density. Therefore, LFP needs to be doped. Compared with simple carbon coating, nitrogen (N) doping carbon coating can enhance the electronic conductivity of materials such as carbon, graphene, and carbon nanotubes. N-doped modified LFP materials have the potential to be widely used as high-specific-capacity electrode materials. Currently, most manufacturers mainly use carbon sources such as sucrose, glucose, and PEG for coating, but carbon coating can be uneven or defective. Usually, nitrogen doping is used to improve the carbon layer to further improve the carbon coating defects. During calcination, inert nitrogen gas is used for protection to prevent ferrous ions from oxidizing to ferric ions and to prevent the loss of residual carbon.

[0003] For example, patent CN105261741A discloses a method for preparing lithium iron phosphate using phenolic resin. Combining the solution-gel method, water-soluble phenolic resin is used as a solvent and dispersant for the reactants, resulting in more uniform mixing of lithium salts, iron salts, phosphates, and additives at the molecular level. This is more thorough than traditional dry mixing and eliminates the need for grinding. Furthermore, its high carbon content allows it to act as a carbon source for coating, and it reacts with oxygen in the atmosphere during sintering, preventing iron oxidation. While phenolic resin serves as both a solvent and dispersant and a carbon source for coating, ensuring more uniform mixing of materials during lithium iron phosphate preparation, its industrial application in liquid phase doping, compared to solid-phase methods, is prone to lithium source waste. Moreover, to eliminate Li / Fe atom mixing and further improve the capacity of lithium iron phosphate, further calcination is still required, which is no different from direct calcination in solid-phase methods. Therefore, the liquid phase method is not suitable for industrial production of lithium iron phosphate. Additionally, the liquid phase preparation process makes elemental doping of lithium iron phosphate difficult, resulting in poor doping effects. Summary of the Invention

[0004] This invention provides a method for preparing lithium iron phosphate using a liquid-phase method with nitrogen-doped phenolic resin, which solves the technical problems of existing liquid-phase methods for preparing lithium iron phosphate, such as wasting lithium source, difficulty in element doping, and poor doping effect.

[0005] The embodiments of the present invention are achieved through the following technical solutions:

[0006] A method for preparing lithium iron phosphate using nitrogen-doped phenolic resin via liquid phase includes the following steps:

[0007] S1: Preparation of nitrogen-doped phenolic resin: formaldehyde and phenol are mixed, a catalyst is added, the temperature is raised to 80-90℃, melamine is added and reacted for 5-15 minutes, and then vacuum dried to obtain nitrogen-doped phenolic resin.

[0008] S2: Preparation of dopant solution: Add surfactant, nitrogen-doped phenolic resin and boron source to desalinated water to dissolve and mix to obtain dopant solution;

[0009] S3: Preparation of precursor solution: Add surfactant, phosphate salt, iron salt and lithium salt to demineralized water and dissolve and mix to obtain precursor solution;

[0010] S4: Preparation of precursor product: The precursor solution is mixed with the dopant solution and reacted under high temperature and high pressure to obtain the precursor product. The precursor product is washed, dried and pulverized.

[0011] S5: The pulverized precursor product is calcined at 650℃~800℃ for 4h~6h under nitrogen protection to obtain lithium iron phosphate composite material.

[0012] In this technical solution, phosphate, iron, and lithium salts exist in liquid form during the preparation process. In the precursor solution, surfactants are added, and the lithium salts adsorb sufficient surfactants, reducing waste and loss during the reaction process and increasing the system's homogeneity during lithium iron phosphate formation. Part of the nitrogen-containing phenolic resin in the liquid phase embeds into the lithium iron phosphate lattice, improving the lithium iron phosphate structure from within and increasing electronic conductivity. Nitrogen doping of the lithium iron phosphate composite material through nitrogen-doped phenolic resin enhances nitrogen doping efficiency and uniformity. Simultaneously, a boron source is added during the hydrothermal process for boron doping. The synergistic effect of nitrogen and boron doping allows the nitrogen to enter the lattice, replacing phosphorus sites and improving the electrochemical performance of lithium iron phosphate. During the hydrothermal preparation of the lithium iron phosphate composite material, the addition of surfactants forms a three-dimensional porous lithium iron phosphate structure, providing a rapid transport channel for electrons and ions, increasing the specific surface area, reducing wall thickness, promoting charge transfer, and improving the lithium-ion transfer rate.

[0013] Preferably, in step S1, the amount of melamine added is 2.5 wt% to 3.5 wt%, based on a total formaldehyde and phenol content of 100 wt%.

[0014] Preferably, in step S2, the boron source is at least one of boric acid, boron oxide, and borate, and the boron-nitrogen ratio of the boron source to the melamine is 3.8–4.2:1; the amount of surfactant added is 5 wt%–7 wt% based on 100 wt% of the dopant solution. The amount of nitrogen-doped phenolic resin added is 8 wt%–12 wt%, with the remainder being the boron source and demineralized water.

[0015] Preferably, in step S3, the molar ratio of the phosphate salt, iron salt, and lithium salt is 0.965–0.98:1:1.01–1.04; and the amount of surfactant added is 10 wt%–15 wt% based on 100 wt% of the precursor solution.

[0016] Preferably, in steps S2 and S3, the surfactant is sodium dodecyl sulfate.

[0017] Preferably, in step S1, the molar ratio of formaldehyde to phenol is 1.0 to 1.2:1.

[0018] Preferably, in step S1, the catalyst is at least one of acetic acid, acetic anhydride, and dodecylbenzenesulfonic acid.

[0019] Preferably, in step S1, after adding the catalyst, the temperature is first raised to 80-90°C, then melamine is added, the reaction is carried out for 8-12 minutes, and finally the mixture is dried under vacuum at 100°C to obtain nitrogen-doped phenolic resin.

[0020] Preferably, in step S4, the ratio of the amount of precursor solution to the amount of dopant solution added is 1.9 to 2.1:1.

[0021] Preferably, in step S4, the precursor solution and the dopant solution are reacted in a high-pressure reactor at 180°C and a rotation speed of 400 r / min for 3 hours to obtain the precursor product.

[0022] Using this technical solution, in the hydrothermal method, the size of lithium iron phosphate particles increases with the increase of hydrothermal temperature. At 180℃, lithium iron phosphate particles have the best morphology, with a particle size of 1μm to 2μm.

[0023] The technical solutions of the embodiments of the present invention have at least the following advantages and beneficial effects:

[0024] 1. In the preparation process of this invention, phosphate salts, iron salts and lithium salts all exist in liquid form. During the formation of lithium iron phosphate, some nitrogen-containing phenolic resin in the liquid phase will penetrate into the intercrystalline voids, thereby improving the structure of lithium iron phosphate from the inside and increasing the electronic conductivity.

[0025] 2. This invention uses nitrogen-doped phenolic resin to dope lithium iron phosphate composite materials, thereby improving the nitrogen doping efficiency and uniformity. At the same time as nitrogen doping, a boron source is added during the hydrothermal process to dope boron. The synergistic effect of nitrogen doping and boron doping allows the nitrogen to enter the lattice and replace the phosphorus position, thereby improving the electrochemical performance of lithium iron phosphate.

[0026] 3. In the hydrothermal preparation of lithium iron phosphate composite material of the present invention, a surfactant is added to form a three-dimensional porous lithium iron phosphate, which provides a channel for rapid transport of electrons and ions, and also increases the specific surface area, reduces the wall thickness, promotes charge transfer and improves the lithium ion transfer rate. Attached Figure Description

[0027] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 This is a schematic diagram of the process flow for preparing lithium iron phosphate using a liquid-phase nitrogen-doped phenolic resin according to Embodiment 1 of the present invention. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments.

[0030] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0031] Example 1

[0032] A method for preparing lithium iron phosphate using nitrogen-doped phenolic resin via liquid phase includes the following steps:

[0033] S1: Formaldehyde and phenol are mixed in a molar ratio of 1:1, a catalyst is added, the temperature is raised to 85°C, and 3 wt% melamine is added based on a total amount of formaldehyde and phenol of 100 wt%. After reacting for 10 min, the mixture is dried under vacuum at 100°C to obtain nitrogen-doped phenolic resin.

[0034] S2: Iron phosphate and lithium carbonate are added to a high-pressure reactor at a molar ratio of 1:1.03 for reaction. An appropriate amount of demineralized distilled water is added. Based on a precursor solution of 100 wt%, 5 wt% sodium dodecyl sulfate is added and mixed to obtain the precursor solution.

[0035] With the dopant solution being 100 wt%, the boron-nitrogen ratio in the system is 4:1, and the nitrogen-doped phenolic resin accounts for 9 wt% of the system. The amounts of boric acid and nitrogen-doped phenolic resin are calculated. The nitrogen-doped phenolic resin and boric acid are added to the demineralized water, and then 10 wt% sodium dodecyl sulfate is added and mixed to obtain the dopant solution.

[0036] The precursor solution and the dopant solution were mixed at a volume ratio of 2:1 and added to a high-pressure reactor. The reactor speed was adjusted to 400 r / min and the reaction was carried out at 180°C for 3 h to obtain the precursor product.

[0037] S3: Wash the precursor product with distilled water, dry the washed product at 120℃ for 12h, and pulverize the dried product.

[0038] S4: The dried precursor product was placed in a graphite crucible and calcined at 700°C for 6 hours under high-purity nitrogen protection to obtain lithium iron phosphate composite material.

[0039] Example 2

[0040] The only difference between this embodiment and Embodiment 1 is that, in step S1, the amount of melamine added is 3.5 wt%.

[0041] Example 3

[0042] The only difference between this embodiment and Embodiment 1 is that, in step S1, the molar ratio of formaldehyde to phenol is 1.2:1, and the amount of melamine added is 2.5 wt%.

[0043] Example 4

[0044] The only difference between this embodiment and Embodiment 1 is that, in step S1, the molar ratio of formaldehyde to phenol is 1.2:1, and the amount of melamine added is 3.5 wt%.

[0045] Example 5

[0046] The only difference between this embodiment and Embodiment 1 is that, in step S2, the amount of sodium dodecyl sulfate added to the precursor solution is 7 wt%, and the amount of sodium dodecyl sulfate added to the dopant solution is 15 wt%.

[0047] Example 6

[0048] The only difference between this embodiment and Embodiment 1 is that, in step S2, the amount of nitrogen-doped phenolic resin added is 12 wt%.

[0049] Example 7

[0050] The only difference between this embodiment and Embodiment 1 is that, in step S2, the boron-nitrogen ratio of the added boric acid to the nitrogen-doped phenolic resin is 3.8:1.

[0051] Comparative Example 1

[0052] The only difference between this comparative example and Example 1 is that, in step S1, nitrogen doping is achieved directly by sintering with nitrogen gas.

[0053] Comparative Example 2

[0054] The only difference between this comparative example and Example 1 is that no surfactant was added during step S2, in the process of preparing the precursor solution and the dopant solution.

[0055] Comparative Example 3

[0056] The only difference between this comparative example and Example 1 is that boric acid was not added during the preparation of the precursor product in step S2.

[0057] Experimental Example

[0058] The lithium iron phosphate composite materials prepared in Examples 1-7 and Comparative Examples 1-3 were used as positive electrode materials and assembled into 2032 coin cells for electrochemical performance testing. The lithium metal sheet served as the negative electrode and reference electrode, the Celgard 2400 polypropylene microporous membrane as the separator, and 1 mol / LLIPF6 / EC+DMC (1:1 volume ratio) as the electrolyte. First, the lithium iron phosphate composite materials prepared in Examples 1-7 and Comparative Examples 1-3, the conductive agent Super P, and the binder PVDF were weighed at a ratio of 80:12:8 (wt%). After stirring the three components evenly, an appropriate amount of 1-methyl-2-pyrrolidone (NMP) was added and thoroughly mixed to form a paste. This paste was then uniformly coated onto aluminum foil and dried in a forced-air drying oven at 80°C for 6 hours. The dried material was then cut into round pieces. Finally, the prepared positive electrode pieces were assembled into coin cells in an argon-filled glove box and allowed to stand for 12 hours.

[0059] Constant current charge-discharge testing is an important experimental method for studying the electrochemical performance of cathode materials. It is used to detect the specific capacity and cycle performance of lithium secondary battery electrode materials. The equipment used for battery charge-discharge testing was a NEWARE BTS (5V, 50mA) control and testing system, with a test voltage range of 2.5-4.0V. The equipment used for cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) was a CHI660D electrochemical workstation, with a CV test voltage range of 2.5-4.2V and an EIS test frequency range of 10mHz-1 MHz. All tests were conducted at room temperature (25℃). The test results are shown in Table 1.

[0060] Table 1. Data and properties related to the preparation process of electrode materials obtained in Examples 1-7 and comparative examples.

[0061]

[0062]

[0063] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing lithium iron phosphate using a liquid-phase nitrogen-doped phenolic resin, characterized in that: The steps include the following: S1: Preparation of nitrogen-doped phenolic resin: formaldehyde and phenol are mixed, a catalyst is added, the temperature is raised to 80-90℃, melamine is added and reacted for 5-15 minutes, and then vacuum dried to obtain nitrogen-doped phenolic resin. S2: Preparation of dopant solution: Add surfactant, nitrogen-doped phenolic resin and boron source to desalinated water to dissolve and mix to obtain dopant solution; S3: Preparation of precursor solution: Add surfactant, phosphate salt, iron salt and lithium salt to demineralized water and dissolve and mix to obtain precursor solution; S4: Preparation of precursor product: The precursor solution is mixed with the dopant solution, and the reaction is carried out to obtain the precursor product. The precursor product is washed, dried and pulverized. The ratio of the amount of precursor solution to the amount of dopant solution added is 1.9 to 2.1:1; S5: The pulverized precursor product is calcined at 650℃~800℃ for 4h~6h under nitrogen protection to obtain lithium iron phosphate composite material.

2. The method for preparing lithium iron phosphate using a liquid-phase nitrogen-doped phenolic resin according to claim 1, characterized in that: In step S1, based on a total formaldehyde and phenol content of 100 wt%, the amount of melamine added is 2.5 wt% to 3.5 wt%.

3. The method for preparing lithium iron phosphate using a liquid-phase nitrogen-doped phenolic resin according to claim 2, characterized in that: In step S2, the boron source is at least one of boric acid, boron oxide, and borate, and the boron-nitrogen ratio of the boron source to the melamine is 3.8 to 4.2:1; based on a dopant solution of 100 wt%, the amount of surfactant added is 5 wt% to 7 wt%, the amount of nitrogen-doped phenolic resin added is 8 wt% to 12 wt%, and the balance is boron source and demineralized water.

4. The method for preparing lithium iron phosphate using a liquid-phase nitrogen-doped phenolic resin according to claim 3, characterized in that: In step S3, the molar ratio of the phosphate salt, iron salt, and lithium salt is 0.965–0.98:1:1.01–1.04; and the amount of surfactant added is 10 wt%–15 wt% based on 100 wt% of the precursor solution.

5. The method for preparing lithium iron phosphate using a liquid-phase nitrogen-doped phenolic resin according to claim 4, characterized in that: In steps S2 and S3, the surfactant is sodium dodecyl sulfate.

6. A method for preparing lithium iron phosphate using a liquid-phase nitrogen-doped phenolic resin according to any one of claims 1-5, characterized in that: In step S1, the molar ratio of formaldehyde to phenol is 1.0 to 1.2:

1.

7. A method for preparing lithium iron phosphate using a liquid-phase nitrogen-doped phenolic resin according to any one of claims 1-5, characterized in that: In step S1, the catalyst is at least one of acetic acid, acetic anhydride, and dodecylbenzenesulfonic acid.

8. A method for preparing lithium iron phosphate using a liquid-phase nitrogen-doped phenolic resin according to any one of claims 1-5, characterized in that: In step S1, after adding the catalyst, the temperature is raised to 80-90°C, then melamine is added, the reaction is carried out for 8-12 minutes, and finally the mixture is dried under vacuum at 100°C to obtain nitrogen-doped phenolic resin.

9. A method for preparing lithium iron phosphate using a liquid-phase nitrogen-doped phenolic resin according to any one of claims 1-5, characterized in that: In step S4, the precursor solution and the dopant solution are reacted in a high-pressure reactor at 180°C and a rotation speed of 400 r / min for 3 hours to obtain the precursor product.

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