A method for preparing lithium iron phosphate

Abstract

The application relates to the technical field of lithium ion batteries, in particular to a preparation method of lithium iron phosphate. The preparation method of the lithium iron phosphate comprises the following steps: reacting a mixed solution containing ferrous phosphate and ferrous biphosphate, and performing first grinding to obtain slurry; and sequentially performing second grinding, spray drying, calcination and crushing on a mixture of the slurry, a lithium source, a carbon source and a dopant to obtain the lithium iron phosphate. The two ferrous salts of the ferrous phosphate and the ferrous biphosphate are adopted, and superfine grinding can be realized under the dual action of physical grinding and chemical reaction; the generation of strong reducing gas is avoided, and the content of magnetic substances in the product is reduced. The lithium iron phosphate prepared by adopting the preparation method has the characteristics of small primary particle size, high rate performance, low powder resistivity and excellent low-temperature performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a preparation method of lithium iron phosphate. BACKGROUND

[0002] The lithium iron phosphate material is widely used in energy storage, power and other fields, and can also be applied to start-stop power supply and other fields. The lithium iron phosphate is used in start-stop power supply, which has the advantages of good cycle performance and high energy density.

[0003] The intrinsic conductivity of the lithium iron phosphate material is relatively poor, and the primary particle size thereof needs to be reduced, that is, the rate performance and low temperature performance thereof can be improved.

[0004] At present, the preparation method of high-rate lithium iron phosphate material includes a solid phase method process, and the particle size of material sanding is controlled to realize the improvement of the rate performance of lithium iron phosphate. However, the smaller the sanding particle size of the conventional precursor is, the longer the grinding time is, and the grinding particle size is limited by the diameter of the zirconium ball, which belongs to pure mechanical grinding, the cost is high, and the average value of the primary particle size of the final lithium iron phosphate is large. Moreover, the existing high-rate lithium iron phosphate material has the disadvantages of low product purity and high magnetic substance content.

[0005] Therefore, the present application is proposed. SUMMARY

[0006] The present application provides a preparation method of lithium iron phosphate, which realizes superfine grinding, reduces the content of magnetic substances in the product, and has the characteristics of small primary particle size, high rate performance, low powder resistivity and excellent low temperature performance.

[0007] In order to achieve the above-mentioned purpose of the present application, the following technical scheme is adopted:

[0008] The present application provides a preparation method of lithium iron phosphate, which includes the following steps:

[0009] A mixed solution containing ferrous phosphate and ferrous dihydrogen phosphate is reacted and first ground to obtain a slurry;

[0010] The mixture of the slurry, a lithium source, a carbon source and a dopant is sequentially subjected to second grinding, spray drying, calcination and crushing to obtain the lithium iron phosphate.

[0011] Compared with the prior art, the present application has the following advantages:

[0012] (1) The present application realizes superfine grinding by using ferrous phosphate and ferrous dihydrogen phosphate under the dual action of physical grinding and chemical reaction, and the required time is short, so that the small particle size precursor can be quickly obtained.

[0013] (2) The present application can effectively avoid consuming extra carbon to reduce ferric iron by using two ferrous salts as raw materials, thereby saving cost and avoiding the generation of carbon monoxide gas and other strong reducing atmospheres in the redox reaction, so as to avoid the generation of ferrophosphorus, elemental iron and the like due to the reaction of the strong reducing atmosphere, and a product with lower content of magnetic substances can be obtained. BRIEF DESCRIPTION OF DRAWINGS

[0014] In order to more clearly illustrate the technical solutions in the specific embodiments or prior art of the present application, the drawings needed in the specific embodiments or prior art description will be briefly introduced below. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0015] Figure 1 SEM image of ferrous phosphate of Example 1 of the present application.

[0016] Figure 2 SEM image of the spray material of Example 1 of the present application.

[0017] Figure 3 SEM image of lithium manganese iron phosphate of Example 2 of the present application.

[0018] Figure 4 SEM image of lithium manganese iron phosphate of Comparative Example 1 of the present application.

[0019] Figure 5 Charge-discharge curve of lithium manganese iron phosphate of Example 2 of the present application. DETAILED DESCRIPTION

[0020] The technical solutions of the present application will be described clearly and completely below in combination with the drawings and specific embodiments, but those skilled in the art will understand that the following described embodiments are part of the embodiments of the present application, not all the embodiments, and are only used to illustrate the present application, and should not be regarded as limiting the scope of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application. The specific conditions are not specified in the embodiments, and are carried out according to the conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments used are not specified by the manufacturer, and are conventional products that can be purchased on the market.

[0021] In a first aspect, the present application provides a preparation method of lithium iron phosphate, comprising the following steps:

[0022] Step S1, a mixed solution containing ferrous phosphate and ferrous dihydrogen phosphate is reacted and first ground to obtain a slurry;

[0023] Step S2, the mixture of the slurry, the lithium source, the carbon source and the dopant is sequentially subjected to second grinding, spray drying, calcination and crushing to obtain the lithium iron phosphate.

[0024] In the conventional iron phosphate process, the iron phosphate with two crystal waters is calcined to obtain the iron phosphate without crystal water, so that the crystal grain of the obtained anhydrous iron phosphate is relatively large, and it is difficult to grind the particle size to below 100 nm in the grinding process. According to practical experience, the time consumed for grinding the particle size from 10 μm to 500 nm is only about 1 / 2 of the time consumed for grinding the particle size from 500 nm to 300 nm, and the time consumed for grinding the particle size from 300 nm to 100 nm is 3-6 times of the time consumed for grinding the particle size from 500 nm to 300 nm, and it is difficult to grind to below 100 nm by using a conventional sand mill.

[0025] In the present application, the ferrous phosphate and the ferrous phosphate dihydrogen are mixed and ground, and under the dual action of physical grinding and chemical reaction, the ultrafine grinding is realized, and the small particle size precursor is quickly obtained. In the grinding process, the chemical reaction of generating ferrous phosphate monohydrate is generated, and the physical grinding is realized, that is, the grinding is carried out while the specific surface area of the material is increased, so that the progress and reaction speed of the chemical reaction are further improved.

[0026] In the present application, the two kinds of ferrous salts are used as raw materials, so that the consumption of additional carbon for reducing the trivalent iron is effectively avoided, the cost is saved, and the strong reducing atmosphere such as carbon monoxide gas generated by the oxidation-reduction reaction is avoided, so that the generation of phosphide iron (FeP, Fe2P, etc.), elemental iron and other substances is avoided, and thus the product with low content of magnetic substances can be obtained.

[0027] In a preferred embodiment, in step S1, the molar ratio of the ferrous phosphate to the ferrous phosphate dihydrogen in the first grinding is (0.9-1.1):(0.9-1.1).

[0028] In a preferred embodiment, in step S1, the temperature of the first grinding is 20-30℃, and the time of the first grinding is 2-4.5 h.

[0029] In a preferred embodiment, in step S1, the particle size of the solid particles in the slurry is 20-40 nm; typically but not limitedly, for example, the particle size of the solid particles in the slurry can be 20 nm, 25 nm, 29 nm, 30 nm, 35 nm, 40 nm or a range value formed by any two of them.

[0030] In the present application, the two kinds of ferrous salts are mixed and ground, and under the dual action of physical grinding and chemical reaction, the small particle size particles with a particle size of 20-40 nm can be quickly obtained.

[0031] In a preferred embodiment, the solid content of the slurry in step S1 is 35-40 wt%.

[0032] In a preferred embodiment, the method for preparing ferrous phosphate in step S1 comprises:

[0033] The mixed solution containing ferrous acetate, ascorbic acid and phosphoric acid is spray dried to obtain ferrous phosphate.

[0034] In a preferred embodiment, the particle size of the ferrous phosphate is 5-25 μm; typically but not limitedly, for example, the particle size of the ferrous phosphate can be 5 μm, 10 μm, 15 μm, 20 μm, 25 μm or a range value formed by any two of them.

[0035] In a preferred embodiment, in the method for preparing ferrous phosphate, the molar ratio of ferrous ions to phosphate in the mixed solution is 3:(2.01-2.08); the molar ratio of ferrous ions to ascorbic acid is (1.5-2):(0.01-0.05). Preferably, the mixed solution is obtained by mixing a ferrous acetate solution with a concentration of 1.5-2 mol / L, an ascorbic acid solution with a concentration of 0.01-0.05 mol / L and a phosphoric acid solution with a concentration of 8-10 mol / L.

[0036] The ferrous phosphate prepared by the above method has the advantages of small primary particle size, small BET and high reactivity.

[0037] In a preferred embodiment, in the method for preparing ferrous phosphate, the spray drying obtains ferrous phosphate precipitate and waste gas; hot nitrogen gas with a temperature of 300-350°C is used as the heat source during the spray drying; the waste gas is condensed to recover an acetic acid solution, which is reacted with iron to obtain ferrous acetate, which is then used to prepare ferrous phosphate; the gas after condensing and recovering acetic acid and water is returned to the spray drying as the heat source.

[0038] In a preferred embodiment, the method for preparing ferrous phosphate in step S1 comprises: reacting iron and a phosphoric acid solution at 40-60°C until the pH of the system is 2.2-2.5, then stopping the reaction, and filtering to obtain a ferrous phosphate solution.

[0039] In a preferred embodiment, the first grinding in step S1 comprises: mixing the ferrous phosphate and the ferrous phosphate solution, then adding pure water to slurry, the solid content of the slurry after slurring is 35 wt%-40 wt%, then grinding in a grinding machine to obtain a material, the particle size of the solid particles in the material is 20-40 nm.

[0040] In a preferred embodiment, in step S2, the second grinding comprises: continuing the grinding of the slurry obtained in step S1 in the grinding machine by adding a lithium source, a carbon source and a dopant.

[0041] The sand mill is filled with zirconium balls, the content of zirconium oxide in the zirconium balls is 95wt%, the content of yttrium oxide is 4.5wt%, the diameter of the zirconium balls is 0.25-0.3μm, and the filling volume of the zirconium balls is 75%-95% of the effective volume of the inner cavity of the sand mill.

[0042] In a preferred embodiment, the total time of the first grinding in step S1 and the second grinding in step S2 is 3-5h; preferably, the time of the second grinding in step S2 is 0.5-1h.

[0043] Using the above grinding device, it takes 20-25h to grind the iron phosphate and ferrous phosphate with a particle size of about 15μm to 150nm, and it takes 45-50h to grind to 139nm, and the particle size does not decrease but increases with further grinding. However, according to the present application, it only takes 3-5h to grind the ferrous phosphate and ferrous biphosphate with a particle size of 5-25μm to a solid particle size of 20-40nm, which significantly reduces the grinding time and overcomes the problem that it is difficult to grind the solid particle size to below 100nm.

[0044] In a preferred embodiment, in step S2, the lithium source comprises at least one of lithium acetate, lithium gluconate and lithium citrate.

[0045] In a preferred embodiment, in step S2, the carbon source comprises at least one of glucose, polyethylene glycol and sucrose.

[0046] In a preferred embodiment, in step S2, the dopant comprises nano-titanium dioxide and / or nano-magnesium dioxide.

[0047] The titanium and magnesium in the nano-titanium dioxide and nano-magnesium dioxide play the role of ion doping. The titanium can realize the crystal defects of the lithium iron phosphate, because its valence is different from that of the ferrous ion and its ionic radius is also different from that of the ferrous ion, so it will cause defects of the lithium iron phosphate, thereby improving the lithium ion conductivity and the rate performance. The magnesium ion can replace the ferrous ion into the crystal lattice, because it is not a variable valence metal ion, so it can stabilize the structure of the material and improve the cycle performance.

[0048] In a preferred embodiment, in step S2, the molar ratio of lithium to iron in the lithium iron phosphate is (1.02-1.03):1.

[0049] In a preferred embodiment, in step S2, the content of carbon element in the lithium iron phosphate is 1.5wt%-1.8wt%; typically but not limitedly, for example, the content of carbon element in the lithium iron phosphate can be 1.5wt%, 1.6wt%, 1.7wt%, 1.8wt% or a range value formed by any two of them.

[0050] In a preferred embodiment, in step S2, the content of titanium element in the lithium iron phosphate is 0.6wt%-0.9wt%; typically but not limitedly, for example, the content of titanium element in the lithium iron phosphate can be 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt% or a range value formed by any two of them.

[0051] In a preferred embodiment, in step S2, the content of magnesium element in the lithium iron phosphate is 0.15wt%-0.35wt%; typically but not limitedly, for example, the content of magnesium element in the lithium iron phosphate can be 0.15wt%, 0.2wt%, 0.25wt%, 0.3wt%, 0.35wt% or a range value formed by any two of them.

[0052] In a preferred embodiment, in step S2, after the spray drying, the spray material obtained has a particle size of 10-25μm and a free water content of ≤0.5wt%.

[0053] In a preferred embodiment, in step S2, the calcining comprises sequentially performing a first holding treatment, a second holding treatment and a cooling treatment; wherein the second holding treatment is performed in an atmosphere containing propyl alcohol.

[0054] During the second holding treatment, the gas containing propyl alcohol can effectively perform secondary carbon coating. Compared with the method of adding organic carbon source for thermal decomposition to perform carbon coating, this method can perform carbon deposition on the surface of the lithium iron phosphate which has not been coated with carbon, perfect the carbon coating layer, further reduce the powder resistivity of the product and improve the rate performance.

[0055] The content of carbon in the lithium iron phosphate of the present application is determined by the carbon source and propyl alcohol. Reducing the content of carbon corresponding to the carbon source will increase the cost due to the low utilization rate of propyl alcohol; increasing the content of carbon corresponding to the carbon source will reduce the carbon produced by gas phase deposition, thereby reducing the uniformity of carbon coating and affecting the rate performance.

[0056] In a preferred embodiment, in the atmosphere containing propanol, the volume percentage of propanol gas is 0.5% to 1% in step S2; typically but not limitedly, for example, the volume percentage of propanol gas in the atmosphere containing propanol can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1% or a range value formed by any two of them. Preferably, the atmosphere containing propanol comprises nitrogen and propanol gas.

[0057] If the content of propanol gas in the atmosphere containing propanol is too high, it will cause the increase of carbon content and the thickness of carbon coating layer, thereby affecting the conductive transmission distance of lithium ions and affecting the electrical performance, and increasing the cost.

[0058] In a preferred embodiment, the first holding treatment comprises: holding at 720-750℃ for 5-7h; preferably, the first holding treatment is carried out under a nitrogen atmosphere.

[0059] In a preferred embodiment, the second holding treatment comprises: holding at 720-750℃ for 2-3h.

[0060] In a preferred embodiment, the cooling treatment comprises: cooling at a rate of 3-5℃ / h under a nitrogen atmosphere.

[0061] In a preferred embodiment, during the calcination process, the first holding treatment further comprises a temperature rising treatment; preferably, the temperature rising treatment comprises: rising the temperature at a rate of 2.5-3.5℃ / h under a nitrogen atmosphere.

[0062] During the temperature rising treatment, the first holding treatment, the second holding treatment and the cooling treatment, the volume flow ratio of nitrogen is 1:(0.3-0.4):(0.1-0.2):(0.2-0.3); the furnace pressure of the first holding treatment is 10-20Pa, the relative humidity in the furnace of the first holding section is ≤3%, the relative humidity of the second holding treatment is ≤5%, an air inlet is provided during the temperature rising treatment and the cooling treatment, the air inlet is communicated with an air extractor to extract the exhaust gas.

[0063] In a preferred embodiment, in step S2, the crushing comprises: crushing the calcined material obtained after calcination to a particle size of 1-2.5μm. Preferably, the crushing comprises using a fluidized bed jet mill.

[0064] In a preferred embodiment, in step S2, after crushing, it further comprises screening, mixing, iron removal and vacuum packaging. Preferably, the screening is carried out by using an ultrasonic vibrating screen, the mixing is carried out by using a spiral belt mixer, the iron removal is carried out by using an electromagnetic iron remover, and the vacuum packaging is carried out in a constant temperature and humidity room.

[0065] In a preferred embodiment, the lithium iron phosphate prepared by the above method has a primary particle size of 20-40 nm, a compaction density of ≥2 g / mL, a powder resistivity of ≤5 Ω·cm at 10 MPa, a discharge capacity of ≥150 mAh / g at 10 C, and a capacity retention rate of ≥90% at 0.5 C and -20°C. The compaction density is tested at a pressure of 3 T.

[0066] The embodiments of the present application will be described in detail below with reference to the examples, but those skilled in the art will understand that the following examples are only used to illustrate the present application and should not be regarded as limiting the scope of the present application. The specific conditions not specified in the examples are carried out according to the conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments used are not specified by the manufacturer, and are all conventional products that can be obtained commercially.

[0067] Example 1

[0068] The method for preparing ferrous phosphate provided in this example comprises the following steps:

[0069] The scrap iron sheet is reacted with a phosphoric acid solution (concentration of 3 mol / L) at 50°C until the pH of the system is 2.34, and the reaction is stopped. After filtration, a ferrous phosphate solution is obtained.

[0070] The parameters of the ferrous phosphate solution prepared in this example are shown in Table 1.

[0071] Table 1

[0072] Fe element content (g / L) P content (g / L) Fe 3+ (g / L)]]> pH Water insoluble content (g / L) 83.19 46.98 0.05 2.41 0.0046 Mn content (g / L) Al content (g / L) Mg content (g / L) Ca content (g / L) Na content (g / L) 0.13 0.09 0.07 0.02 0.01

[0073] The method for preparing ferrous phosphate provided in this example comprises the following steps:

[0074] To the aqueous ferrous acetate solution with a concentration of 1.8 mol / L, ascorbic acid is added to make the concentration of ascorbic acid 0.03 mol / L, and then Fe 2+ and PO4 3- are mixed at a molar ratio of 3:2.04, and a phosphoric acid solution with a concentration of 9 mol / L is added. After mixing, spray drying is carried out to obtain ferrous phosphate with a particle size of 12.5 μm and waste gas. During the spray drying process, nitrogen gas is used as a heat source at 300-350°C. A dust collection bag is used to separate the ferrous phosphate from the waste gas. The waste gas is collected and led out by an induced draft fan. The exhaust gas is condensed and recovered by a cooling coil to obtain an acetic acid solution (3.55 mol / L). Additional acetic acid is added to return the dissolved scrap iron sheet to obtain a ferrous acetate solution, which is used to prepare ferrous phosphate. The gas after condensation and recovery of acetic acid and water is nitrogen gas, which can be returned to the spray drying process as a heat source.

[0075] The parameters of the ferrous phosphate prepared in this example are shown in Table 2.

[0076] Table 2

[0077] Fe element content (wt%) P content (wt%) Fe and P molar ratio S content (ppm) Mn content (wt%) 34.17 12.94 1.47：1 35 0.03 Al content (wt%) Ca content (wt%) Mg content (wt%) Na content (wt%) BET(m 2 / g) <!-- 5 -->]]> 0.04 0.01 0.04 0.004 37.9 Loose bulk density (g / L) D10 (pm) D50 (pm) D90 (pm) Fe 3+ (ppm)]]> 0.67 4.5 17.5 35.7 256

[0078] The crystallization water of ferrous phosphate is 7.3, which is caused by the loss of part of the crystallization water due to high temperature in the spray drying process.

[0079] The preparation method of lithium iron phosphate provided by the embodiment comprises the following steps:

[0080] S1, the ferrous phosphate and the ferrous dihydrogen phosphate solution are mixed according to a molar ratio of 1:1, pure water is added to slurry, and then the mixture is put into a sand mill for grinding for 170 min to obtain a slurry in a paste state, the solid content of the slurry is 38 wt%, and the particle size (D50) of the solid particles in the slurry is 38 nm.

[0081] S2, lithium acetate, glucose, PEG2000, nano titanium dioxide and nano magnesium oxide are added to the above material, and the grinding is continued for 40 min to obtain a ground material. The molar ratio of lithium acetate to Fe element in the material is 1.025:1, the mass ratio of glucose to PEG2000 is 1:1, the content of carbon element in the final lithium iron phosphate is 1.63 wt%, the addition of nano titanium dioxide makes the content of titanium element in the lithium iron phosphate be 0.75 wt%, and the addition of nano magnesium oxide makes the content of magnesium element in the lithium iron phosphate be 0.25 wt%.

[0082] The ground material is put into a spray dryer for spray drying to obtain a spray material; the spray material is spherical, the particle size (D50) is 21 μm, the free water content is 0.37 wt%, and the tap density is 1.21 g / mL; in the spray drying process, nitrogen is used as a heat source, the inlet air temperature is 250-300℃, the outlet air temperature is 85-90℃, and the rotation speed of the atomizing wheel is 15000 r / min.

[0083] The spray material is calcined in a roller furnace under an inert atmosphere to obtain a calcined material; the calcination process is as follows: the temperature is raised to 735℃ at a rate of 3℃ / h, the temperature is kept for 9 h, and then the temperature is lowered to below 100℃ at a rate of 4℃ / h, the furnace pressure is 15 Pa, and the volume flow ratio of nitrogen during the processes of temperature rising, temperature keeping and temperature lowering is 1:0.35:0.25.

[0084] The calcined material is crushed to a particle size (D50) of 1.54 μm by using a fluidized bed jet mill, and then is sequentially screened by using an ultrasonic vibration screen, mixed by using a screw belt mixer, de-ironed by using an electromagnetic iron remover, and vacuum packaged to obtain the lithium iron phosphate.

[0085] Example 2

[0086] The preparation method of lithium iron phosphate in this example refers to that in Example 1, except that in step S2, the calcination process is as follows: first temperature maintaining treatment is performed at 735℃ for 6h at a rate of 3℃ / h under nitrogen atmosphere, then first temperature maintaining treatment is performed at 735℃ for 3h under the mixed gas atmosphere of nitrogen and propanol with a volume ratio of 92:0.8, and finally the temperature is decreased to below 100℃ at a rate of 4℃ / h under nitrogen atmosphere; the furnace pressure during the first temperature maintaining treatment is 15Pa, and the volume flow ratio of nitrogen during the processes of temperature increasing, first temperature maintaining treatment, second temperature maintaining treatment and temperature decreasing is 1:0.35:0.15:0.25.

[0087] The parameters of lithium iron phosphate prepared in this example are shown in Table 3.

[0088] Table 3

[0089]

[0090]

[0091] Example 3

[0092] The preparation method of lithium iron phosphate in this example refers to that in Example 2, except that in step S1, the molar ratio of ferrous phosphate and ferrous dihydrogen phosphate is 1:0.5.

[0093] Example 4

[0094] The preparation method of lithium iron phosphate in this example refers to that in Example 2, except that in step S2, the volume fraction of propanol in the mixed gas during the second temperature maintaining treatment is 2%.

[0095] Example 5

[0096] The preparation method of lithium iron phosphate in this example refers to that in Example 2, except that in step S2, glucose and PEG2000 are added to finally make the content of carbon element in lithium iron phosphate 1.5wt%, nano titanium dioxide is added to make the content of titanium element in lithium iron phosphate 0.06wt%, and nano magnesium oxide is added to make the content of magnesium element in lithium iron phosphate 0.15wt%.

[0097] Comparative Example 1

[0098] The preparation method of lithium iron phosphate in this comparative example 1 refers to that in Example 2, except that the particle size of iron phosphate is about 15μm (BET is 9.13m 2The mixture was prepared by adding water to a solid content of 38 wt% (g), followed by the addition of lithium acetate, glucose, PEG2000, nano-titanium dioxide, and nano-magnesium oxide. The mixture was then ground for 48 hours to obtain a grinding material with a solid particle size of 139 nm. The Fe, P, C, Ti, and Mg contents in the lithium iron phosphate prepared in this comparative example were the same as those in Example 2.

[0099] Experimental Example 1

[0100] Scanning electron microscopy (SEM) tests were performed on the ferrous phosphate and spray-dried powder in Example 1, the lithium iron phosphate in Example 2, and the lithium iron phosphate in Comparative Example 1. The results are as follows: Figure 1 , Figure 2 , Figure 3 and Figure 4 As shown.

[0101] from Figure 1 It can be seen that the ferrous phosphate particles are spherical.

[0102] from Figure 2 It can be seen that the sprayed material particles are spherical and have good flowability.

[0103] from Figure 3 It can be seen that the primary particle size of the lithium iron phosphate prepared in Example 2 is very small, ranging from 20 to 40 nm.

[0104] from Figure 4 It can be seen that the primary particle size of lithium iron phosphate prepared in Comparative Example 1 is 120-150 nm, which is much larger than that of lithium iron phosphate prepared in Example 2.

[0105] The atmosphere in the middle of the first heat preservation treatment during the calcination process of Example 2 and Comparative Example 1 was measured, and the results are shown in Table 4.

[0106] Table 4

[0107] CO volume concentration (ppm) [H2 volume concentration (ppm)] CO2volume concentration (ppm) Example 2 27.8 12.5 41.6 Comparative Example 1 297.9 78.4 647.6

[0108] As can be seen from Table 4, the concentration of strong reducing gas produced during the calcination process of Comparative Example 1 is much higher than that of Example 2.

[0109] The performance of lithium iron phosphate prepared in Examples 1-5 and Comparative Example 1 was tested, and the results are recorded in Table 5.

[0110] Among them, the powder resistivity test: a four-needle probe method was used, and the test pressure was 10MPa.

[0111] Table 5

[0112]

[0113]

[0114] As can be seen from Table 5, the magnetic substance content of the lithium iron phosphate prepared in Comparative Example 1 is more than 3 times that of the lithium iron phosphate prepared in the present application, and the powder resistivity is much higher than that of the present application.

[0115] The electrochemical performance of the lithium iron phosphate prepared in Examples 1-5 and Comparative Example 1 was tested, and the results are recorded in Table 6. The charge-discharge curve of Example 2 is shown in Figure 1. Figure 5

[0116] The 0.1C initial charge capacity and 0.1C initial discharge capacity were measured using a button cell. The 10C charge-discharge data and the -20°C capacity retention data were measured using a soft package cell. In the cell, the electrolyte was lithium hexafluorophosphate, and the mass ratio of lithium iron phosphate: SP: CNT: PVDF was 91: 3: 1: 5.

[0117] Measurement method: first 0.1C charge-discharge for 10 cycles, then 1C charge-discharge for 10 cycles, and then 10C charge-discharge.

[0118] Table 6

[0119]

[0120] As can be seen from Table 6, the lithium iron phosphate prepared using the preparation method of the present application has the advantages of high rate performance and excellent low temperature performance.

[0121] Although the present application has been illustrated and described with reference to specific embodiments, it should be recognized that the above examples are merely illustrative of the technical solutions of the present application, and are not limiting thereof; it should be understood by those of ordinary skill in the art that the technical solutions described in the above examples can be modified, or some or all of the technical features thereof can be replaced by equivalents, without departing from the spirit and scope of the present application; and such modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present application; therefore, this means that all such replacements and modifications within the scope of the present application are included in the appended claims.​

Claims

1. A method for producing lithium iron phosphate, characterized by, The method comprises the following steps: a mixed solution containing ferrous phosphate and ferrous dihydrogen phosphate is reacted and first ground to obtain a slurry; the slurry, a lithium source, a carbon source and a dopant are sequentially subjected to second grinding, spray drying, calcination and crushing to obtain the lithium iron phosphate; the molar ratio of the ferrous phosphate to the ferrous dihydrogen phosphate is (0.9-1.1):(0.9-1.1); the particle size of the solid particles in the slurry is 20-40 nm.

2. The method of claim 1, wherein the lithium iron phosphate is prepared by the steps of: the first grinding is performed at a temperature of 20-30°C for 2-4.5 h. ​ 3. The method of claim 1, wherein the lithium iron phosphate is prepared by the steps of: the solid content of the slurry is 35-40 wt%. ​ 4. The method of claim 1, wherein the lithium iron phosphate is prepared by the steps of: the preparation method of the ferrous phosphate comprises: ​ a mixed solution containing ferrous acetate, ascorbic acid and phosphoric acid is spray dried to obtain the ferrous phosphate.

5. The method of claim 1, wherein the lithium iron phosphate is prepared by the steps of: the lithium source comprises at least one of lithium acetate, lithium gluconate and lithium citrate; ​ or / and, the carbon source comprises at least one of glucose, polyethylene glycol and sucrose; or / and, the dopant comprises nano-titanium dioxide and / or nano-magnesium dioxide.

6. The method of claim 5, wherein the lithium iron phosphate is prepared by the steps of: in the lithium iron phosphate, the molar ratio of lithium to iron is (1.02-1.03):1; the content of carbon is 1.5 wt%-1.8 wt%; the content of titanium is 0.6 wt%-0.9 wt%; and the content of magnesium is 0.15 wt%-0.35 wt%. ​ 7. The method of claim 1, wherein the lithium iron phosphate is prepared by the steps of: the spray drying obtains a spray material, and the particle size of the spray material is 10-25 μm, and the free water content is ≤0.5 wt%. ​ 8. The method of claim 1, wherein the lithium iron phosphate is prepared by the steps of: the calcination comprises sequentially performing first heat preservation, second heat preservation and cooling treatment; ​ the second heat preservation is performed in an atmosphere containing propanol.

9. The method of claim 8, wherein the lithium iron phosphate is prepared by the steps of: in the atmosphere containing propanol, the volume percentage of propanol gas is 0.5%-1%. ​ 10. The method of claim 8, wherein the lithium iron phosphate is prepared by the steps of: the first heat preservation comprises heat preservation at 720-750°C for 5-7 h; ​ the second heat preservation comprises heat preservation at 720-750°C for 2-3 h.

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