Lithium iron phosphate with high compaction density and preparation method thereof

Abstract

The invention belongs to the technical field of lithium battery positive electrode materials, and provides high-compaction-density lithium iron phosphate and a preparation method thereof.The preparation method comprises the following steps that 1, pretreated polyvinyl alcohol is added into deionized water, the temperature is increased to 80-85 DEG C, stirring is conducted, the pH value is adjusted to 10.5-11.0, epichlorohydrin is added for a reaction, aniline is added for a continuous reaction, cooling is conducted, and a product is obtained; adjusting the pH value to 1-2, adding ammonium persulfate for reaction, and dialyzing to remove impurities to obtain modified polyvinyl alcohol; step 2, adding a lithium source, an iron source and a phosphorus source into deionized water for mixing, and performing ball milling and drying to obtain a lithium iron phosphate precursor; step 3, putting the lithium iron phosphate precursor into a tubular furnace, adding an inorganic carbon source, heating for reaction, cooling and crushing to obtain primary sintered lithium iron phosphate; and step 4, mixing the primarily sintered lithium iron phosphate and the modified polyvinyl alcohol, placing the mixture in a tubular furnace, carrying out heating reaction, cooling and crushing to obtain the lithium iron phosphate.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium battery positive electrode materials, and particularly relates to lithium iron phosphate with high compaction density and a preparation method thereof. Background Art

[0002] Lithium iron phosphate (LiFePO4) is one of the mainstream cathode materials for lithium-ion batteries on the market. Compared to other cathode materials, such as lithium cobalt oxide and ternary cathode materials, LiFePO4 boasts advantages such as low cost, non-toxicity, and environmental friendliness. Furthermore, it offers excellent safety and cycling stability, and holds great promise for application in the power and energy storage sectors.

[0003] However, the current battery system composed of lithium iron phosphate has a low energy density, resulting in a short driving range, which limits its application in power batteries. In order to improve the energy density of lithium iron phosphate batteries, it is necessary to increase the capacity, voltage platform or compaction density of lithium iron phosphate. The compaction density of lithium iron phosphate directly affects the energy density and volume energy density of the battery. The compaction density is not only affected by the microscopic morphology of the material, but is also closely related to the particle size and gradation of the particles. Therefore, improving its compaction density is the key to improving battery performance. Generally speaking, the smaller the particle size of the material, the greater the bulk density. However, for lithium iron phosphate, the smaller the particle size, the larger the specific surface area, which makes processing more difficult. Secondary granulation is usually required to reduce the specific surface area. In addition, when slightly larger particles of equal diameter are stacked, there will be a large number of gaps between the spheres. If there are no suitable small particles to fill these gaps, the bulk density will be very low.

[0004] Therefore, it is of great significance to develop a lithium iron phosphate with high compaction density, simple preparation method and good performance and its preparation method. Summary of the Invention

[0005] The object of the present invention is to provide a lithium iron phosphate with a high compaction density and a preparation method thereof, so as to solve the technical problem of low energy density of lithium iron phosphate positive electrode materials in the prior art.

[0006] The purpose of the present invention can be achieved through the following technical solutions: A first aspect of the present invention provides a method for preparing lithium iron phosphate with high compaction density, comprising the following steps: Step 1, adding polyvinyl alcohol, urea and dicyandiamide to dimethyl sulfoxide, raising the temperature to 95-100° C., stirring until completely dissolved, adding phytic acid solution, continuing to raise the temperature to 105-110° C., continuing stirring for 3-5 hours, precipitating, filtering and washing, and vacuum drying to obtain pretreated polyvinyl alcohol; Add the pretreated polyvinyl alcohol to deionized water, heat to 80-85°C, stir until completely dissolved, adjust the pH value to 10.5-11.0, add epichlorohydrin and react for 1.0-1.5 hours, add aniline and continue to react for 1.5-2 hours, cool to 0°C, adjust the pH value to 1-2, add ammonium persulfate and react for 24-28 hours, dialyze to remove impurities to obtain modified polyvinyl alcohol; Step 2: adding a lithium source, an iron source, and a phosphorus source into deionized water, mixing the mixture, and ball-milling and drying the mixture to obtain a lithium iron phosphate precursor; Step 3: placing the lithium iron phosphate precursor in a tubular furnace under a nitrogen atmosphere, adding an inorganic carbon source, heating the precursor to react, cooling the precursor, and crushing the precursor to obtain primary sintered lithium iron phosphate; Step 4: Mix the primary sintered lithium iron phosphate and modified polyvinyl alcohol, place them in a tubular furnace with a nitrogen protective atmosphere, heat them for reaction, cool them, and crush them to obtain lithium iron phosphate.

[0007] As a further embodiment of the present invention, the usage ratio of pretreated polyvinyl alcohol, deionized water, epichlorohydrin, aniline and ammonium persulfate is 9.8-10.2 g:100 mL:0.5-1.0 g:3.0-3.2 g:2.4-3.0 g.

[0008] As a further embodiment of the present invention, the usage ratio of polyvinyl alcohol, urea, dicyandiamide, dimethyl sulfoxide and phytic acid solution is 3.0-3.2 g: 1.78-1.80 g: 0.70-0.72 g: 100 mL: 5.0-5.5 mL; the phytic acid solution is 50 wt%.

[0009] As a further embodiment of the present invention, the molar ratio of the lithium source, the iron source and the phosphorus source in step 2 is 1:1:1; the lithium source in step 2 is one of lithium carbonate, lithium hydroxide, lithium sulfate and lithium nitrate; the iron source is one of ferrous sulfate, ferrous nitrate and ferrous chloride; and the phosphorus source is one of ammonium dihydrogen phosphate, sodium dihydrogen phosphate and potassium dihydrogen phosphate.

[0010] As a further embodiment of the present invention, the amount of the inorganic carbon source used in step 3 is 2-5wt% of the lithium iron phosphate precursor; the inorganic carbon source is at least one of graphene and carbon nanotubes. During high-temperature calcination, the inorganic carbon source can directly contact the lithium iron phosphate to form a conductive network, significantly improving the material's conductivity. Furthermore, during the sintering process, it can inhibit excessive growth and agglomeration of lithium iron phosphate particles, thereby maintaining the material's uniformity and small particle size, helping to reduce the length of the lithium ion diffusion path and improve the battery's charge and discharge performance.

[0011] As a further solution of the present invention, in step 3, the temperature is raised to 660-680° C. and kept for reaction for 12-14 hours; the heating rate is 8-10° C. / min.

[0012] As a further solution of the present invention, in step 4, the temperature is raised to 700-730° C. and kept for reaction for 15-18 hours; the heating rate is 16-20° C. / min.

[0013] As a further solution of the present invention, the amount of modified polyvinyl alcohol used in step 4 is 2 to 5 wt% of the primary sintered lithium iron phosphate.

[0014] The second aspect of the present invention provides a lithium iron phosphate with a high compaction density obtained by the preparation method described in the first aspect.

[0015] Beneficial effects of the present invention: The present invention provides a method for preparing lithium iron phosphate with high compaction density. By improving the carbon source and sintering process, an inorganic carbon source is used as a primary coating carbon source, modified polyvinyl alcohol is used as a secondary coating carbon source, and a method of combining secondary sintering is used to synthesize a lithium iron phosphate cathode material with high compaction density. Specifically, the method comprises the following steps: first, a lithium iron phosphate precursor is prepared, and a primary sintering is performed with an inorganic carbon source. The introduction of the inorganic carbon source can significantly improve the conductivity of the lithium iron phosphate material, but its coating uniformity is poor, resulting in uneven particle size. Although this non-uniform coating is conducive to improving the compaction density, small particles are prone to agglomeration. The generation of agglomeration reduces the effective contact area between the positive electrode material and the electrolyte, thereby causing poor electrochemical performance. For this reason, the present invention further adds modified polyvinyl alcohol for secondary sintering. The secondary sintering can cause secondary growth of the grains, and the film-like coating carbon layer formed can have a lower specific surface area while ensuring uniform coating, reducing the agglomeration of small particles, thereby improving the compaction density of the lithium iron phosphate material. At the same time, modified polyvinyl alcohol, as a secondary added carbon source, can be well coated on the surface of the lithium iron phosphate material, thereby effectively inhibiting the side reaction between the material and the electrolyte, improving the cycle performance, and finally preparing a lithium iron phosphate material with a high compaction density.

[0016] Polyvinyl alcohol has excellent film-forming properties. When used as a carbon source, its film-like coated carbon layer can ensure uniform coating on the surface of the material while having a low specific surface area, thereby effectively increasing the compaction density of the lithium iron phosphate material. However, the electrical conductivity of polyvinyl alcohol itself is poor. Therefore, in the present invention, phytic acid and polyaniline are used to improve polyvinyl alcohol to improve the electrical conductivity of polyvinyl alcohol. The present invention first reacts polyvinyl alcohol with phytic acid, and utilizes the strong hydrogen bond between C-OH on the polyvinyl alcohol molecular chain and P=O on the phytic acid molecule to prepare pretreated polyvinyl alcohol; then uses epichlorohydrin to graft polyaniline onto the pretreated polyvinyl alcohol, effectively improving the electrical conductivity of polyvinyl alcohol. In addition, the modified polyvinyl alcohol contains P and N elements, of which P has a higher electron-donating ability, thereby promoting electronic conductivity and improving bonding stability. At the same time, the high specific surface area generated by the PC bond formed after the P-doped carbon layer helps to improve the graphitization level of the carbon layer, thereby producing excellent electrochemical performance. N can effectively inhibit the growth and agglomeration of lithium iron phosphate nanoparticles, thereby improving cycle stability.

[0017] The preparation method provided by the present invention has a simple process and strong operability, can effectively improve the compaction density of the lithium iron phosphate positive electrode material, and further effectively improve the energy density of the lithium iron phosphate positive electrode material, thereby broadening the application range of the lithium iron phosphate material. DETAILED DESCRIPTION

[0018] The following will be combined with the embodiments to clearly and completely describe the technical solutions of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0019] Obviously, the following descriptions are only some examples or embodiments of the present application. For those of ordinary skill in the art, the present application can also be applied to other similar scenarios without paying any creative work. In addition, it can also be understood that although the efforts made in this development process may be complex and lengthy, for those of ordinary skill in the art related to the contents disclosed in this application, some changes in design, manufacturing or production based on the technical contents disclosed in this application are just conventional technical means and should not be understood as the contents disclosed in this application being insufficient. However, there are cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the following description is provided for those skilled in the art to fully understand the present application and is not intended to limit the subject matter recorded in the claims.

[0020] Example 1 This embodiment provides a method for preparing lithium iron phosphate with high compaction density, comprising the following steps: Step 1: Add 3.0 g of polyvinyl alcohol, 1.78 g of urea and 0.70 g of dicyandiamide to 100 mL of dimethyl sulfoxide, raise the temperature to 100° C., stir until completely dissolved, add 5.0 mL of phytic acid solution (50 wt %), continue to heat to 110° C., continue stirring for 3-5 hours, precipitate, filter and wash, and vacuum dry to obtain pretreated polyvinyl alcohol; 9.8 g of pretreated polyvinyl alcohol was added to 100 mL of deionized water, heated to 80°C, stirred until completely dissolved, the pH value was adjusted to 11.0, 0.5 g of epichlorohydrin was added and reacted for 1.0 h, 3.0 g of aniline was added and the reaction was continued for 1.5 h, cooled to 0°C, the pH value was adjusted to 1, 2.4 g of ammonium persulfate was added and reacted for 24 h, and the modified polyvinyl alcohol was obtained after dialysis to remove impurities; Step 2: lithium carbonate, ferrous sulfate, and ammonium dihydrogen phosphate are added to deionized water in a molar ratio of 1:1:1, mixed, and ball-milled and dried to obtain a lithium iron phosphate precursor; Step 3: Place the lithium iron phosphate precursor in a tube furnace under a nitrogen atmosphere, add graphene (2 wt% of the lithium iron phosphate precursor), heat to 660°C at 8°C / min, hold for 14 hours, cool naturally to room temperature, and crush to obtain primary sintered lithium iron phosphate; Step 4: Mix the once-sintered lithium iron phosphate and modified polyvinyl alcohol (2 wt% of the once-sintered lithium iron phosphate), place them in a tubular furnace with a nitrogen atmosphere, heat them to 700°C at 16°C / min, keep them warm for 18 hours, cool them naturally to room temperature, and crush them to obtain lithium iron phosphate.

[0021] The lithium iron phosphate with high compaction density is prepared through the above steps.

[0022] Example 2 The only difference from Example 1 is that step 1 is different: Step 1: Add 3.2 g of polyvinyl alcohol, 1.80 g of urea and 0.72 g of dicyandiamide to 100 mL of dimethyl sulfoxide, raise the temperature to 100 ° C, stir until completely dissolved, add 5.5 mL of phytic acid solution (50 wt%), continue to heat to 110 ° C, continue stirring for 3-5 hours, precipitate, filter and wash, and vacuum dry to obtain pretreated polyvinyl alcohol; 9.8 g of pretreated polyvinyl alcohol was added to 100 mL of deionized water, heated to 80°C, stirred until completely dissolved, the pH value was adjusted to 11.0, 0.5 g of epichlorohydrin was added and reacted for 1.0 h, 3.0 g of aniline was added and the reaction was continued for 1.5 h, cooled to 0°C, the pH value was adjusted to 1, 2.4 g of ammonium persulfate was added and reacted for 24 h, and the modified polyvinyl alcohol was obtained after dialysis to remove impurities; Example 3 The only difference from Example 1 is that step 1 is different: Step 1: Add 3.0 g of polyvinyl alcohol, 1.78 g of urea and 0.70 g of dicyandiamide to 100 mL of dimethyl sulfoxide, raise the temperature to 100° C., stir until completely dissolved, add 5.0 mL of phytic acid solution (50 wt %), continue to heat to 110° C., continue stirring for 3-5 hours, precipitate, filter and wash, and vacuum dry to obtain pretreated polyvinyl alcohol; 10.2 g of pretreated polyvinyl alcohol was added to 100 mL of deionized water, heated to 80°C, stirred until completely dissolved, the pH value was adjusted to 11.0, 1.0 g of epichlorohydrin was added and reacted for 1.0 h, 3.2 g of aniline was added and the reaction was continued for 1.5 h, cooled to 0°C, the pH value was adjusted to 1, 3.0 g of ammonium persulfate was added and reacted for 24 h, and the modified polyvinyl alcohol was obtained after dialysis to remove impurities; Example 4 The only difference from Example 1 is that step 1 is different: Step 1, add 3.0g polyvinyl alcohol, 1.79g urea and 0.71g dicyandiamide to 100mL dimethyl sulfoxide, raise the temperature to 100°C, stir until completely dissolved, add 5.3mL phytic acid solution (50wt%), continue to raise the temperature to 110°C, continue stirring for 3-5h, precipitate, filter and wash, and vacuum dry to obtain pretreated polyvinyl alcohol; 10.0 g of pretreated polyvinyl alcohol was added to 100 mL of deionized water, heated to 80°C, stirred until completely dissolved, the pH value was adjusted to 11.0, 0.75 g of epichlorohydrin was added and reacted for 1.0 h, 3.1 g of aniline was added and the reaction was continued for 1.5 h, cooled to 0°C, the pH value was adjusted to 1, 2.7 g of ammonium persulfate was added and reacted for 24 h, and the modified polyvinyl alcohol was obtained after dialysis to remove impurities; Example 5 The only difference from Example 1 is step 3: Step 3: Place the lithium iron phosphate precursor in a tubular furnace with a nitrogen protective atmosphere, add graphene (4.5 wt% of the lithium iron phosphate precursor), heat to 660°C at 8°C / min, keep warm for 14 hours, cool to room temperature by self-heating, and crush to obtain primary sintered lithium iron phosphate.

[0023] Example 6 The only difference from Example 1 is step 3: Step 3: Place the lithium iron phosphate precursor in a tubular furnace with a nitrogen protective atmosphere, add graphene (2 wt% of the lithium iron phosphate precursor), heat to 680°C at 10°C / min, keep warm for 12 hours, cool to room temperature by self-heating, and crush to obtain primary sintered lithium iron phosphate.

[0024] Example 7 The only difference from Example 1 is step 4: Step 4: Mix the once-sintered lithium iron phosphate and modified polyvinyl alcohol (4.8 wt% of the once-sintered lithium iron phosphate), place the mixture in a tubular furnace with a nitrogen atmosphere, heat the mixture to 700°C at a rate of 16°C / min, keep the temperature for 18 hours, cool the mixture naturally to room temperature, and crush the mixture to obtain lithium iron phosphate.

[0025] Example 8 The only difference from Example 1 is step 4: Step 4: Mix the once-sintered lithium iron phosphate and modified polyvinyl alcohol (2 wt% of the once-sintered lithium iron phosphate), place them in a tubular furnace with a nitrogen protective atmosphere, heat them to 730°C at 20°C / min, keep them warm for 15 hours, cool them naturally to room temperature, and crush them to obtain lithium iron phosphate.

[0026] Comparative Example 1 The only difference compared to Example 1 is that graphene is not added in step 3: Step 3: Place the lithium iron phosphate precursor in a tubular furnace with a nitrogen protective atmosphere, heat it to 660°C at 8°C / min, keep it warm for 14 hours, cool it naturally to room temperature, and crush it to obtain primary sintered lithium iron phosphate.

[0027] Comparative Example 2 The only difference compared with Example 1 is that modified polyvinyl alcohol is not added in step 4: Step 4: Place the primary sintered lithium iron phosphate in a tubular furnace with a nitrogen protective atmosphere, heat it to 700°C at 16°C / min, keep it warm for 18 hours, cool it naturally to room temperature, and crush it to obtain lithium iron phosphate.

[0028] Comparative Example 3 The only difference compared with Example 1 is that the modified polyvinyl alcohol obtained in step 1 is replaced by polyvinyl alcohol: Step 1, lithium carbonate, ferrous sulfate and ammonium dihydrogen phosphate are added to deionized water in a molar ratio of 1:1:1, mixed, and ball-milled and dried to obtain a lithium iron phosphate precursor; Step 2: Place the lithium iron phosphate precursor in a tube furnace with a nitrogen atmosphere, add graphene (2 wt% of the lithium iron phosphate precursor), heat to 660°C at 8°C / min, hold for 14 hours, cool naturally to room temperature, and crush to obtain primary sintered lithium iron phosphate; Step 3: Mix the once-sintered lithium iron phosphate and polyvinyl alcohol (2 wt% of the once-sintered lithium iron phosphate), place the mixture in a tubular furnace with a nitrogen atmosphere, heat the mixture to 700°C at a rate of 16°C / min, keep the temperature for 18 hours, cool the mixture naturally to room temperature, and crush the mixture to obtain lithium iron phosphate.

[0029] Comparative Example 4 The only difference from Example 1 is that step 1 is different: Step 1, add 2.8g polyvinyl alcohol, 1.78g urea and 0.70g dicyandiamide to 100mL dimethyl sulfoxide, raise the temperature to 100°C, stir until completely dissolved, add 5.0mL phytic acid solution (50wt%), continue to raise the temperature to 110°C, continue stirring for 3-5h, precipitate, filter and wash, and vacuum dry to obtain pretreated polyvinyl alcohol; 9.8 g of pretreated polyvinyl alcohol was added to 100 mL of deionized water, heated to 80°C, stirred until completely dissolved, the pH value was adjusted to 11.0, 0.5 g of epichlorohydrin was added and reacted for 1.0 h, 3.0 g of aniline was added and the reaction was continued for 1.5 h, cooled to 0°C, the pH value was adjusted to 1, 2.4 g of ammonium persulfate was added and reacted for 24 h, and modified polyvinyl alcohol was obtained after dialysis to remove impurities.

[0030] Comparative Example 5 The only difference from Example 1 is that step 1 is different: Step 1: Add 3.0 g of polyvinyl alcohol, 1.78 g of urea and 0.70 g of dicyandiamide to 100 mL of dimethyl sulfoxide, raise the temperature to 100° C., stir until completely dissolved, add 5.0 mL of phytic acid solution (50 wt %), continue to heat to 110° C., continue stirring for 3-5 hours, precipitate, filter and wash, and vacuum dry to obtain pretreated polyvinyl alcohol; 9.5 g of pretreated polyvinyl alcohol was added to 100 mL of deionized water, heated to 80°C, stirred until completely dissolved, the pH value was adjusted to 11.0, 0.5 g of epichlorohydrin was added and reacted for 1.0 h, 3.0 g of aniline was added and the reaction was continued for 1.5 h, cooled to 0°C, the pH value was adjusted to 1, 2.4 g of ammonium persulfate was added and reacted for 24 h, and modified polyvinyl alcohol was obtained after dialysis to remove impurities.

[0031] Comparative Example 6 The only difference from Example 1 is step 2: Step 2: lithium carbonate, ferrous sulfate and ammonium dihydrogen phosphate are added to deionized water in a molar ratio of 1.1:1:1, mixed, and ball-milled and dried to obtain a lithium iron phosphate precursor.

[0032] Comparative Example 7 The only difference from Example 1 is step 3: Step 3: Place the lithium iron phosphate precursor in a tubular furnace with a nitrogen protective atmosphere, add graphene (1 wt% of the lithium iron phosphate precursor), heat to 660°C at 8°C / min, keep warm for 14 hours, cool naturally to room temperature, and crush to obtain primary sintered lithium iron phosphate.

[0033] Comparative Example 8 The only difference from Example 1 is step 3: Step 3: Place the lithium iron phosphate precursor in a tubular furnace under a nitrogen atmosphere, add graphene (2 wt% of the lithium iron phosphate precursor), heat to 630°C at 8°C / min, hold for 14 hours, cool naturally to room temperature, and crush to obtain primary sintered lithium iron phosphate; Comparative Example 9 The only difference from Example 1 is step 4: Step 4: Mix the once-sintered lithium iron phosphate and modified polyvinyl alcohol (1 wt% of the once-sintered lithium iron phosphate), place them in a tubular furnace with a nitrogen atmosphere, heat them to 700°C at 16°C / min, keep them warm for 18 hours, cool them naturally to room temperature, and crush them to obtain lithium iron phosphate.

[0034] Comparative Example 10 The only difference from Example 1 is step 4: Step 4: Mix the once-sintered lithium iron phosphate and modified polyvinyl alcohol (2 wt% of the once-sintered lithium iron phosphate), place them in a tubular furnace with a nitrogen atmosphere, heat them to 680°C at a rate of 16°C / min, keep them warm for 18 hours, cool them naturally to room temperature, and crush them to obtain lithium iron phosphate.

[0035] The following application performance tests were performed on Examples 1 to 8 and Comparative Examples 1 to 10: The lithium iron phosphate obtained in the Examples and Comparative Examples was prepared into lithium-ion batteries. Specifically, 800 g of the lithium iron phosphate obtained in the Examples or Comparative Examples, 100 g of acetylene black (a conductive agent), and 100 g of polyvinylidene fluoride (PVDF) (a binder) were added to 800 g of N-methylpyrrolidone solution (NMP solution) and stirred in a vacuum mixer for 2 hours to prepare a positive electrode slurry. The slurry was evenly coated on aluminum foil, dried in a vacuum drying oven at 120°C for 12 hours, and then punched into 14 mm diameter discs to serve as the positive electrode sheets. The positive electrode sheets, negative electrode sheet (a 14.5 mm diameter lithium metal sheet), separator (Celgard 2400 microporous polypropylene membrane), and electrolyte (1 mol / L LiPF6 / EC+DMC (volume ratio 1:1)) were assembled into CR2025 button-type lithium-ion batteries in a hydrogen-filled glove box. The electrochemical performance of the resulting lithium-ion batteries was tested; the test results are shown in Table 1. Table 1

[0036] It can be seen from Table 1 that compared with the lithium iron phosphate prepared in Comparative Examples 1-10, the lithium iron phosphate prepared in Examples 1-8 of the present invention has a higher compaction density, a larger first discharge specific capacity, and better battery cycle performance. It can be seen that the present invention provides a method for preparing lithium iron phosphate with a high compaction density, and the prepared product has excellent compaction density and electrochemical properties, and the preparation method is simple in process and highly operable, which broadens the application range of lithium iron phosphate materials.

[0037] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "includes," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus.

[0038] It should be understood that in the various embodiments of the present application, the size of the serial number of each process does not mean the order of execution. Some or all steps can 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.

[0039] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0040] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution, and all technical features and optional technical features of the present application can be combined with each other to form a new technical solution.

[0041] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing lithium iron phosphate with high compaction density, characterized in that: The following steps are involved: Step 1, adding polyvinyl alcohol, urea and dicyandiamide to dimethyl sulfoxide, raising the temperature to 95-100° C., stirring until completely dissolved, adding phytic acid solution, continuing to raise the temperature to 105-110° C., continuing stirring for 3-5 hours, precipitating, filtering and washing, and vacuum drying to obtain pretreated polyvinyl alcohol; Add the pretreated polyvinyl alcohol to deionized water, heat to 80-85°C, stir, adjust the pH value to 10.5-11.0, add epichlorohydrin to react, add aniline to continue the reaction, cool, adjust the pH value to 1-2, add ammonium persulfate to react, and dialyze to remove impurities to obtain modified polyvinyl alcohol; Step 2: adding a lithium source, an iron source, and a phosphorus source into deionized water, mixing the mixture, and ball-milling and drying the mixture to obtain a lithium iron phosphate precursor; Step 3: placing the lithium iron phosphate precursor in a tube furnace, adding an inorganic carbon source, heating the reaction, cooling, and crushing to obtain primary sintered lithium iron phosphate; Step 4: Mix the primary sintered lithium iron phosphate and modified polyvinyl alcohol, place them in a tube furnace, heat them for reaction, cool them, and crush them to obtain lithium iron phosphate.

2. The method for preparing lithium iron phosphate with high compaction density according to claim 1, characterized in that: The dosage ratio of the pretreated polyvinyl alcohol, deionized water, epichlorohydrin, aniline and ammonium persulfate is 9.8-10.2 g: 100 mL: 0.5-1.0 g: 3.0-3.2 g: 2.4-3.0 g.

3. The method for preparing lithium iron phosphate with high compaction density according to claim 1, characterized in that: The dosage ratio of polyvinyl alcohol, urea, dicyandiamide, dimethyl sulfoxide and phytic acid solution is 3.0-3.2 g: 1.78-1.80 g: 0.70-0.72 g: 100 mL: 5.0-5.5 mL; the phytic acid solution is 50 wt%.

4. The method for preparing lithium iron phosphate with high compaction density according to claim 1, characterized in that: The molar ratio of the lithium source, iron source and phosphorus source in step 2 is 1:1:1; the lithium source in step 2 is one of lithium carbonate, lithium hydroxide, lithium sulfate and lithium nitrate; the iron source is one of ferrous sulfate, ferrous nitrate and ferrous chloride; and the phosphorus source is one of ammonium dihydrogen phosphate, sodium dihydrogen phosphate and potassium dihydrogen phosphate.

5. The method for preparing lithium iron phosphate with high compaction density according to claim 1, characterized in that: In step 3, the amount of the inorganic carbon source is 2 to 5 wt % of the lithium iron phosphate precursor; the inorganic carbon source is at least one of graphene and carbon nanotubes.

6. The method for preparing lithium iron phosphate with high compaction density according to claim 1, characterized in that: In step 3, the temperature is raised to 660-680° C. and kept for reaction for 12-14 hours; the heating rate is 8-10° C. / min.

7. The method for preparing lithium iron phosphate with high compaction density according to claim 1, characterized in that: In step 4, the temperature is raised to 700-730° C. and kept for reaction for 15-18 hours; the heating rate is 16-20° C. / min.

8. The method for preparing lithium iron phosphate with high compaction density according to claim 1, characterized in that: The amount of modified polyvinyl alcohol used in step 4 is 2 to 5 wt% of the primary sintered lithium iron phosphate.

9. A lithium iron phosphate with a high compaction density, characterized in that: The lithium iron phosphate is prepared by the preparation method of high compaction density lithium iron phosphate according to any one of claims 1 to 8.

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