Preparation method of high electrical performance lithium iron phosphate precursor

Abstract

The application relates to a preparation method of high-electricity-performance lithium iron phosphate precursor, which comprises the following steps: pretreating a titanium white powder by-product ferrous sulfate heptahydrate to prepare a ferrous solution; stirring and mixing an oxidizing agent, a phosphorus salt and a dopant to obtain a solid-liquid uniform mixed phosphate; reacting the ferrous solution and the mixed phosphate to synthesize amorphous sheet-shaped iron phosphate; filtering and washing the synthesized amorphous iron phosphate, then adding water and phosphoric acid to rapidly beat the amorphous iron phosphate to obtain a slurry; high-temperature heating the slurry, then adding nanoscale iron phosphate dihydrate seeds, and obtaining the high-electricity-performance lithium iron phosphate precursor after reaction. The method quantitatively retains titanium and magnesium ions in the titanium white powder by-product in situ doping, uniformly and stably dopes the magnesium and titanium ions in the process of synthesizing the iron phosphate, and adds the seeds, so that the crystal structure growth of the iron phosphate and the improvement of the crystal form are promoted, the prepared iron phosphate has uniform morphology and good repeatability, and the lithium iron phosphate positive electrode material prepared from the iron phosphate has high electric performance.

Description

TECHNICAL FIELD

[0001] The application relates to a preparation method of a high-electricity-performance lithium iron phosphate precursor, in particular to a preparation method of a lithium iron phosphate anode material precursor for a battery. BACKGROUND

[0002] Lithium iron phosphate (LiFePO4) is an important anode material for lithium ion batteries, which has the advantages of high energy density, long cycle life, low cost and environmental friendliness, and is widely used in electric vehicles, energy storage systems and other fields.

[0003] In recent years, with policy adjustment and market demand, the new energy vehicle and energy storage market has developed rapidly, thus driving the demand of the battery market. Lithium iron phosphate batteries are favored by the market due to their advantages, and the capacity layout of lithium iron phosphate batteries has also experienced explosive growth. There are various synthesis methods for lithium iron phosphate, but the solid-phase synthesis-carbon thermal reduction method has become the preferred route for lithium iron phosphate enterprises due to its relatively simple process, high maturity and other advantages. Most enterprises in the market choose the solid-phase method to prepare lithium iron phosphate. Although the solid-phase method for preparing lithium iron phosphate is simple and easy to implement, and can be produced on a large scale, the traditional solid-phase method still has problems such as long production time, low product conductivity, slow ion transmission rate, and difficult control of product consistency.

[0004] Iron phosphate is the main raw material for preparing lithium iron phosphate by the solid-phase method. With the increase of iron phosphate capacity and market competition, iron phosphate production mainly uses low-cost raw materials. Ferrous sulfate, a by-product of titanium dioxide, is chosen by many iron phosphate production enterprises as the iron source due to its low price and large market stock. However, ferrous sulfate from titanium dioxide by-products has many impurity elements, and high-valence metal elements are not effectively utilized.

[0005] Metal element doping technology has been widely used in the new energy field, especially in the preparation process of lithium iron phosphate. By introducing metal ions Ti 4+ , the compaction and conductivity of the product can be effectively improved. Appropriate Mg 2+ doping can occupy the iron site of the material and create a small amount of material lattice defects to effectively improve the electrochemical performance. However, there is little research on metal ion doping of iron phosphate in the prior art, and there is a lack of multi-ion doping test research in the preparation process of iron phosphate. Usually, high-purity iron phosphate is used to prepare lithium iron phosphate, which greatly increases the cost of iron phosphate and lithium iron materials. In addition, the particle size difference in the existing preparation process of iron phosphate is large, and there is a lack of technology to control the particle size of iron phosphate to achieve better consistency. In particular, the above problems faced in the preparation of iron phosphate using ferrous sulfate from titanium dioxide by-products as the iron source have not been well solved. SUMMARY

[0006] The technical problem solved by the present application is to make up for the deficiencies of the prior art, and provide a preparation method of high electrical performance lithium iron phosphate precursor.

[0007] To achieve the above object, the technical scheme adopted by the present application is as follows: a preparation method of high electrical performance lithium iron phosphate precursor, comprising the following steps:

[0008] (1) Dissolve the titanium dioxide by-product by adding water, add a special concentration and proportion of a reducing agent to reduce impurities, retain a certain amount of doping elements, and then prepare a ferrous sulfate solution containing metal elements Ti and Mg with a specific content after precise filtration and separation;

[0009] (2) Mix hydrogen peroxide, phosphorus salt and water to obtain a homogeneous mixed phosphate salt;

[0010] (3) Mix the ferrous sulfate solution of step (1) with the mixed phosphate salt to prepare amorphous sheet-shaped iron phosphate;

[0011] (4) Filter and wash the synthesized amorphous iron phosphate, add water and phosphoric acid to make a slurry, move the homogeneous slurry into a high-temperature reaction kettle, increase the stirring speed for mixing, then add nano-sized iron phosphate dihydrate seed crystals, react for 2-5 hours, and then filter and wash to obtain qualified iron phosphate dihydrate;

[0012] (5) Dry and sinter the qualified iron phosphate dihydrate to obtain the high electrical performance lithium iron phosphate precursor.

[0013] The titanium dioxide by-product is ferrous sulfate heptahydrate produced by a sulfuric acid process in a titanium dioxide factory, and the content of the titanium dioxide by-product ferrous sulfate heptahydrate is more than 85%, the content of Ti is 1500-5000 ppm, and the content of Mg is 4000-6000 ppm. In some embodiments, the finer physicochemical properties of the titanium dioxide by-product are as follows:

[0014]

[0015] The reducing agent is phosphoric acid, ammonium hydrogen fluoride and ammonium sulfide, the mass concentration of phosphoric acid is 40%, ammonium hydrogen fluoride and ammonium sulfide are high-purity reagents, the content is greater than 99.5%, the mass ratio of ammonium hydrogen fluoride and ammonium sulfide is 1:6, the mass ratio of hydrogen fluoride and titanium dioxide by-product is 1:1000, the mass ratio of phosphoric acid and titanium dioxide by-product is 8:1000, the reaction temperature is 50-80℃, the stirring speed is 500rmp, and the stirring time is 20min. After the reaction, the impurities are separated by a precision filter, and the filter screen is less than 10μm.

[0016] The production of titanium dioxide in China mainly adopts the sulfuric acid method process, which uses ilmenite and acid to dissolve, separates the soluble titanium sulfate from the solid impurities, hydrolyzes the titanium sulfate to form an insoluble hydrolysis product or metatitanic acid, removes water by calcination, and generates dry titanium dioxide. Producing 1t of titanium dioxide by the sulfuric acid method requires 3-4t of ferrous sulfate heptahydrate (Gong Jiazhu-Titanium Dioxide Production Process Technology Progress). The composition of the titanium dioxide by-product is relatively fixed, and specific concentrations and proportions of reducing agents are used in the application to reduce the element content in the titanium dioxide by-product, and the content of Ti and Mg ions is directionally and quantitatively retained. The ions are relatively uniform and stable, realizing the uniform mixing of raw materials at the atomic level. Compared with the prior art, the application shortens the diffusion path of doped atoms in the phosphoric acid iron crystallization process, can efficiently and stably dope the phosphoric acid iron Fe site, thereby improving the lithium electrochemical performance of phosphoric acid iron, and the phosphoric acid iron doped with two ions has a synergistic effect on the charge-discharge performance of phosphoric acid lithium, and has the characteristics of high electrical performance.

[0017] The content of hydrogen peroxide is 30-35%, and the phosphorus salt is any one or a mixture of two of phosphoric acid and ammonia, industrial diammonium phosphate and phosphoric acid, and industrial monammonium phosphate and ammonia.

[0018] The mass ratio of hydrogen peroxide and ferrous sulfate is 1:1-1:3, and the phosphorus content of the mixed phosphorus salt is 5%-9%.

[0019] The Fe content of the ferrous sulfate solution ranges from 100 to 200 g / kg.

[0020] The mixing reaction temperature in step (2) is 40-50℃, the stirring speed is 300-1000 rpm, the reaction time is 30-60 min, and the pH of the reaction process is 1-3.

[0021] In step (3), the stirring speed is 500-1500 rpm, the reaction temperature is 80-100℃, the size of the added ferrous phosphate dihydrate seed is 200-500 nm, and the amount of the added nanoscale ferrous phosphate dihydrate is 5%-15% of the total mass of the ferrous sulfate.

[0022] In step (4), the sintering temperature is 550-700℃, and the sintering time is 1-4h.

[0023] In the present application, the water for dissolution and the pure water described throughout are water with an electrical conductivity < 5 μS / cm.

[0024] Advantages of the present application:

[0025] 1. The lithium iron phosphate doping step is advanced to the precursor preparation stage, which can directly in-situ double ion doping in the front end, simplifying the lithium iron phosphate production process and reducing the lithium iron phosphate production operation steps.

[0026] 2. The specific proportion of the impurity reducing agent is used to quantitatively retain Ti and Mg ions in the titanium dioxide by-product, realize the uniform mixing of the doping ions at the atomic level in the raw material, shorten the diffusion path of the doping atoms in the synthesis of phosphorus iron, realize the efficient and stable doping of phosphorus iron Fe, fully utilize the metal Ti and Mg ions in the titanium dioxide by-product, and effectively reduce the production cost and synthesis difficulty of the lithium iron phosphate in the back end.

[0027] 3. The crystal seeds are added to optimize the particle size of the phosphorus iron, and ensure the consistency of the product. By quantitatively controlling the particle size distribution and quantity of the crystal seeds, the synthesis process of the phosphorus iron is accelerated, the problem of uneven nucleation in the nucleation process of the phosphorus iron is solved, and the particle size distribution and morphology of the phosphorus iron are optimized.

[0028] 4. Compared with the prior art, the phosphorus iron prepared by this method has high stability and good repeatability, the double ion doped phosphorus iron has a synergistic effect on the charge and discharge performance of the lithium iron phosphate, and the lithium iron phosphate synthesized by the phosphorus iron prepared by the method has high electrical performance. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 SEM image of the phosphorus iron synthesized in Example 3, wherein the left image is an SEM image at 200 nm, and the right image is an SEM image at 300 nm. DETAILED DESCRIPTION

[0030] The present application will be further described in detail below in combination with the drawings and specific examples.

[0031] Example 1

[0032] 1-1 Preparation of ferrous source solution: 950.0 g of titanium dioxide by-product ferrous sulfate heptahydrate and 2300.0 g of pure water (conductivity < 5 μS / cm) were weighed using an electronic balance and added to a reaction kettle, and fully dissolved. The solution was tested by ICP, and the results are shown in Table 1. 8 g of 40% high-purity phosphoric acid was weighed and added to the reaction kettle. The high-speed stirring was started at 500 rpm, and the reaction was carried out for ten minutes. Then 0.96 g of ammonium fluoride and 5.76 g of ammonium sulfide were weighed and added to the reaction kettle. The temperature was raised to 60°C, and the titanium reduction reaction was carried out for 20 min. After the reaction was completed, the impurities were separated by a precision filter. The solution was tested by ICP, and the results are shown in Table 2.

[0033] 1-2 Preparation of mixed phosphorus salt: a) 344 g of 85% phosphoric acid, 384 g of 22.7% ammonia water, and 410 g of pure water were stirred and mixed as a phosphorus salt. b) 246.5 g of 30% hydrogen peroxide was weighed and added to the phosphorus salt for high-speed stirring.

[0034] 1-3 Pre-synthesis of iron phosphate: The ferrous sulfate solution in step 1.1 was added to the reaction kettle, and the water bath was heated to 40°C with a stirring speed of 800 rpm. The mixed phosphorus salt in step 1-2 was pumped into the reaction kettle by a high-pressure peristaltic pump for synthesis reaction. The pre-synthesized iron phosphate was filtered and washed, and the washing water pH was greater than 4.

[0035] 1-4 Aging synthesis of iron phosphate: 47.5 g of nano-sized iron phosphate dihydrate with a particle size of 200 nm, 2886 g of pure water, and 14.98 g of 85% phosphoric acid were weighed and added to the reaction kettle. The stirring speed was 1000 rpm, the aging temperature was 92°C, and the aging time was 2h. The aged material was filtered and washed, and the washing water pH was greater than 3.5.

[0036] 1-5 Drying and sintering to obtain iron phosphate: The aged material was first dried at 105°C for 12 h to remove free water, and then sintered using an integrated dynamic sintering furnace at a sintering temperature of 600°C for 3 h to obtain anhydrous iron phosphate. The anhydrous iron phosphate was used as a precursor to synthesize a positive electrode material lithium iron phosphate. The assembled button cell (half cell) was tested for charge and discharge, and the results are shown in Table 4.

[0037] Example 2

[0038] 2-1 Preparation of ferrous source solution: 878.4 g of 99.5% high-purity ferrous sulfate heptahydrate and 2300.0 g of pure water (conductivity < 5 μS / cm) were weighed using an electronic balance and added to a reaction kettle, and fully dissolved. The solution was tested by ICP, and the results are shown in Table 3.

[0039] 2-2 Preparation of mixed phosphorus salt: The same as step 1-2 in Example 1.

[0040] 2-3 Pre-synthesis of iron phosphate: the same as step 1-3 in Example 1.

[0041] 2-4 Aging synthesis of iron phosphate: the same as step 1-4 in Example 1.

[0042] 2-5 Dry sintering to obtain iron phosphate: the aged material is first dried at 105 ℃ for 12 h to remove free water, and then sintered in an integrated dynamic sintering furnace, with a sintering temperature of 600 ℃ and a sintering time of 3 h to obtain anhydrous iron phosphate. Lithium iron phosphate is synthesized using the iron phosphate as a precursor, and a coin cell (half cell) is assembled and subjected to charge and discharge tests. The results are shown in Table 4.

[0043] Example 3:

[0044] 3-1 Preparation of ferrous source solution: titanium white by-product ferrous sulfate heptahydrate 950.0 g and pure water 2300.0 g (conductivity < 5 μS / cm) are weighed using an electronic balance and added to a reaction kettle for thorough dissolution. High-purity phosphoric acid is diluted to a concentration of 40 %, and 8 g of 40 % high-purity phosphoric acid is weighed and added to the reaction kettle. High-speed stirring is started at 500 rpm for ten minutes, then ammonium fluoride 0.96 g and ammonium sulfide 5.76 g are weighed and added to the reaction kettle. The temperature is raised to 60 ℃, and the titanium reduction reaction is carried out for 20 min. After the reaction is completed, impurities are separated by a precision filter.

[0045] 3-2 Preparation of mixed phosphorus salt: a) 344 g of 85 % phosphoric acid, 384 g of 22.7 % ammonia water, and 410 g of pure water are weighed and stirred to mix as a phosphorus salt. b) 246.5 g of 30 % hydrogen peroxide is weighed and added to the phosphorus salt for high-speed stirring.

[0046] 3-3 Pre-synthesis of iron phosphate: the ferrous sulfate solution in step 3-1 is added to the reaction kettle, and water bath heating is carried out to 40 ℃ with a stirring speed of 800 rpm. The mixed phosphorus salt in step 3-2 is pumped into the reaction kettle through a high-pressure peristaltic pump for synthesis reaction. After 30 min, the pre-synthesized iron phosphate is filtered and washed, and the washing water pH is greater than 4.

[0047] 3-4 Aging synthesis of iron phosphate: the pre-synthesized iron phosphate after filtration and washing, pure water, and 14.98 g of 85 % phosphoric acid are added to the reaction kettle, with a stirring speed of 1000 rpm, an aging temperature of 92 ℃, and an aging time of 2 h. The aged material is filtered and washed, and the washing water pH is greater than 3.5.

[0048] 3-5 Dry sintering to obtain iron phosphate: the aged material is first dried at 105 °C for 12 h to remove free water, and then sintered using an integrated dynamic sintering furnace, with a sintering temperature of 600 °C and a sintering time of 3 h, to obtain anhydrous iron phosphate. Lithium iron phosphate, a positive electrode material, is synthesized using iron phosphate as a precursor, and a coin cell (half cell) is assembled and subjected to charge and discharge tests. The results are shown in Table 4.

[0049] Example 4:

[0050] 4-1 Preparation of ferrous source solution: 811.1 g of 99.5 % high-purity ferrous sulfate heptahydrate, 8.65 g of titanium dioxide, and 2300.0 g of pure water (conductivity < 5 μS / cm) are weighed using an electronic balance and added to a reaction kettle, and thoroughly dissolved.

[0051] 4-2 Preparation of mixed phosphorus salt: a) 344 g of 85 % phosphoric acid, 384 g of 22.7 % ammonia water, and 410 g of pure water are weighed and stirred to mix as a phosphorus salt. b) 246.5 g of 30 % hydrogen peroxide is weighed and added to the phosphorus salt for high-speed stirring.

[0052] 4-3 Pre-synthesis of iron phosphate: the ferrous sulfate solution in step 4-1 is added to the reaction kettle, and heated to 40 °C in a water bath with a stirring speed of 1000 rpm. The mixed phosphorus salt in 4-2 is pumped into the reaction kettle by a high-pressure peristaltic pump for synthesis reaction. After 30 min, the pre-synthesized iron phosphate is filtered and washed, with the washing water having a pH greater than 4.

[0053] 4-4 Aging synthesis of iron phosphate: 47.5 g of 200 nm nano-sized iron phosphate dihydrate, 2886 g of pure water, and 14.98 g of 85 % phosphoric acid are weighed and added to the reaction kettle, and stirred at high speed for 10 min. Then, 47.5 g of 200 nm seed iron phosphate dihydrate is weighed and added to the reaction kettle, with a stirring speed of 1000 rpm, an aging temperature of 92 °C, and an aging time of 2 h. The aged material is filtered and washed, with the washing water having a pH greater than 3.5.

[0054] 4-5 Dry sintering to obtain iron phosphate: the aged material is first dried at 105 °C for 12 h to remove free water, and then sintered using an integrated dynamic sintering furnace, with a sintering temperature of 600 °C and a sintering time of 3 h, to obtain anhydrous iron phosphate. Lithium iron phosphate, a positive electrode material, is synthesized using iron phosphate as a precursor, and a coin cell (half cell) is assembled and subjected to charge and discharge tests. The results are shown in Table 4.

[0055] Example 5:

[0056] 5-1 Preparation of ferrous source solution: 811.1 g of 99.5 % high-purity ferrous sulfate heptahydrate, 15 g of high-purity magnesium oxide, and 2300.0 g of pure water (conductivity < 5 μS / cm) were weighed using an electronic balance and added to a reaction kettle for complete dissolution.

[0057] 5-2 Preparation of mixed phosphorus salt: the same as step 4-2 in Example 4.

[0058] 5-3 Pre-synthesis of iron phosphate: the same as step 4-3 in Example 4.

[0059] 5-4 Aging of synthesized iron phosphate: 47.5 g of nano-sized iron phosphate dihydrate with a particle size of 200 nm, 2886 g of pure water, and 14.98 g of 85 % phosphoric acid were weighed and added to a reaction kettle, the stirring speed was 1000 rpm, the aging temperature was 92 ℃, and the aging time was 2 h. The aged material was filtered and washed, and the washing water pH was greater than 3.5.

[0060] 5-5 Drying and sintering to obtain iron phosphate: the aged material was first dried at 105 ℃ for 12 h to remove free water, and then sintered using an integrated dynamic sintering furnace, the sintering temperature was 600 ℃, and the sintering time was 3 h to obtain anhydrous iron phosphate. Lithium iron phosphate was synthesized using iron phosphate as a precursor, and a coin cell (half cell) was assembled and subjected to charge and discharge tests, and the results are shown in Table 4.

[0061] The test results are as follows:

[0062] Table 1: ICP test results:

[0063]

[0064] Table 2: ICP test results:

[0065]

[0066] Table 3: ICP test results:

[0067]

[0068] Table 4: Electrical performance test results:

[0069]

[0070] The above examples are only preferred technical solutions of the present application, and should not be regarded as limiting the present application. The examples in the present application and the features in the examples can be arbitrarily combined with each other without conflict. The protection scope of the present application should be based on the technical solutions recited in the claims, including equivalent replacement solutions of the technical features recited in the claims. That is, equivalent replacement improvements within this scope are also within the protection scope of the present application.

Claims

1. A method for preparing a high electrical performance lithium iron phosphate precursor, characterized in that, It comprises the following steps: (1) The titanium dioxide by-product is dissolved by adding water, impurity is removed, and then a ferrous sulfate solution containing a certain amount of metal elements Ti and Mg is prepared after separation, a certain mass and concentration of phosphoric acid is added in the process of removing impurities, then a certain proportion of ammonium hydrogen fluoride and ammonium sulfide is added, the reaction temperature is 50-80℃, the stirring speed is 500-1000 rpm, and the stirring time is 10-20 min, then the mixture is filtered and separated, the mass concentration of the phosphoric acid is 40-60%, the mass ratio of the specific concentration of the phosphoric acid to the titanium dioxide by-product is 1-10:1000, the ammonium hydrogen fluoride and the ammonium sulfide are high-purity reagents, and the mass ratio of the ammonium hydrogen fluoride to the ammonium sulfide is 1:2-8; (2) The hydrogen peroxide, the phosphorus salt and the pure water are stirred and mixed to form a uniform mixed phosphorus salt; (3) The ferrous sulfate solution in step (1) is mixed with the mixed phosphorus salt to prepare amorphous sheet-shaped iron phosphate; (4) The synthesized amorphous iron phosphate is filtered and washed, pure water and phosphoric acid are added for beating, the uniform slurry is moved into a high-temperature reaction kettle, the stirring speed is increased for mixing, then the nanoscale iron phosphate seed crystal is added, the reaction is carried out for 2-5 hours, and then the mixture is filtered and washed to obtain qualified iron phosphate dihydrate; (5) The qualified iron phosphate dihydrate is dried and sintered to obtain a high-electricity-performance lithium iron phosphate precursor.

2. The method for preparing the high-electrical-performance lithium iron phosphate precursor according to claim 1, characterized in that: The titanium dioxide by-product is ferrous sulfate heptahydrate produced by a sulfuric acid process in a titanium dioxide factory, the content of the titanium dioxide by-product ferrous sulfate heptahydrate is more than 85%, the content of Ti is 1500-5000 ppm, and the content of Mg is 4000-6000 ppm.

3. The method for preparing the high-electrical-performance lithium iron phosphate precursor according to claim 1, characterized in that: The content of the hydrogen peroxide in step (2) is 30-35%, the phosphorus salt is any one or two of phosphoric acid and ammonia water, industrial diammonium and phosphoric acid, and industrial monammonium and ammonia water.

4. The method of claim 1, wherein the method further comprises: The mass ratio of the hydrogen peroxide to the ferrous sulfate is 1:1-1:3, and the content of phosphorus in the mixed phosphorus salt is 5%-9%. ​ 5. The method of claim 1, wherein the method further comprises: In step (3), the mixing reaction temperature is 40-50℃, the stirring speed is 300-1000 rpm, the reaction time is 30-60 min, and the pH in the reaction process is 1-3. ​ 6. The method of claim 1, wherein the method further comprises: In step (4), the stirring speed is 500-1500 rpm, the reaction temperature is 80-100℃, the size of the iron phosphate dihydrate seed crystal is 200-500 nm, and the nanoscale iron phosphate dihydrate accounts for 5%-15% of the total mass of the ferrous sulfate. ​ 7. The method of claim 1, wherein the method further comprises: In step (5), the sintering temperature is 550-700℃, and the sintering time is 1-4 h. ​

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