CN112408351A - Preparation method of high-compaction iron phosphate and lithium iron phosphate - Google Patents
Preparation method of high-compaction iron phosphate and lithium iron phosphate Download PDFInfo
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Abstract
The invention discloses a preparation method of high-compaction iron phosphate and lithium iron phosphate, and belongs to the field of preparation of lithium battery anode materials. Aiming at the problems of poor compaction effect and unstable performance of the existing iron phosphate, the invention provides a preparation method of high-compaction iron phosphate, which comprises the following steps of S1: preparing ferric iron source slurry, and dividing the ferric iron source slurry into two parts; s2: adding a mixed solution of phosphoric acid and liquid alkali into one part of the mixture and reacting; s3: when the mixed slurry in the step S2 is white, adding the other part in the step S1 into the mixed slurry, and reacting; s4: and finally, washing, filtering, drying and roasting to obtain the high-compaction iron phosphate. The method starts from the reaction essence in the iron phosphate generation process, increases the compactness of the secondary particles formed by stacking the primary iron phosphate particles, widens the distribution state of the particle size of the secondary iron phosphate, and obtains the high-compaction and stable-performance iron phosphate product. Meanwhile, the lithium iron titanate prepared by using the iron phosphate prepared by the method as a raw material has higher compaction density.
Description
Technical Field
The invention belongs to the field of preparation of lithium battery positive electrode materials, and particularly relates to a preparation method of high-compaction iron phosphate and lithium iron phosphate.
Background
In recent years, with the increasing influence of fossil energy on the global environment, new energy batteries represented by clean energy are becoming the first choice in the fields of passenger vehicles, buses and energy storage. At present, the anode material of the power battery is mainly lithium iron phosphate (LFP), ternary (NCA/NCM) material and lithium manganate. Although the performance advantages of each material are obvious, the respective defects of the materials are not ignored, wherein the ternary material is widely applied due to the ultrahigh capacity and rate performance, but the preparation cost is higher and the safety performance is poor; the lithium manganate has poor cycle performance and short service life; the lithium iron phosphate is widely applied in recent years due to low preparation cost and high safety performance, but the low gram-volume density and the low energy density of the lithium iron phosphate are important factors for restricting the development of the lithium iron phosphate, and the compaction density of the lithium iron phosphate on the market is mostly 2.2-2.4g/cm3On the other hand, iron phosphate, which is a raw material of lithium iron phosphate, has a direct influence on the compaction density of lithium iron phosphate. At present, the mainstream method for preparing high-compaction battery-grade iron phosphate is mainly a mechanical mixing method, but the universal compaction effect is not obvious, and the final lithium iron phosphate product has unstable performance.
For example, chinese patent application No. CN201810997760.2, published as 2019, 1, 11, discloses a high-compaction-density lithium iron phosphate and a preparation method thereof, where the lithium iron phosphate positive electrode material includes large lithium iron phosphate particles and small lithium iron phosphate particles, the small lithium iron phosphate particles are filled in gaps between the large lithium iron phosphate particles, and the small lithium iron phosphate particles are spherical. The preparation method comprises the following steps: sintering the lithium iron phosphate precursor in a protective gas atmosphere, wherein the sintering is three-stage sintering, the sintering temperature of the three-stage sintering is sequentially increased, and the lithium iron phosphate anode material is obtained by cooling after the three-stage sintering is finished; wherein the shape of the lithium iron phosphate precursor comprises a sphere. The disadvantages of the patent are that: solid lithium iron phosphate is mixed by adopting a physical mechanical method, so that the mixing is not uniform, small particles cannot be effectively filled, the compaction lifting effect is not obvious, in addition, the particle diameters of the precursor iron phosphate of the lithium iron phosphate are different inevitably, the preparation processes are different, and the iron phosphate prepared by two different process parameters has certain uncertain influence on the stability of the product after being mixed and used after being prepared into the lithium iron phosphate.
As another example, chinese patent application No. CN201910256555.5, published as 2019, 5, and 21, discloses a method for preparing iron phosphate for high-purity high-compaction lithium iron phosphate, which includes the following steps: the method comprises the steps of using ferrous sulfate as a titanium dioxide byproduct as a raw material, basically removing impurity elements in the ferrous sulfate through the reverse precipitation of sulfide and ferric hydroxide, preparing ferric phosphate from a ferric hydroxide filter cake to obtain ultra-fine ferric phosphate slurry, mixing the superfine ferric phosphate slurry and the ferric phosphate with the slurry granularity distribution of 3.0-16 mu m in a pressure container according to a certain proportion, stirring, aging, performing suction filtration and washing to obtain ferric phosphate dihydrate, and performing spray granulation and dehydration to obtain the anhydrous ferric phosphate. The disadvantages of the patent are that: the physical mechanical mixing is completed in the solution, although the uniformity degree of mixing with different particle sizes is increased to a certain extent, the improvement space for increasing the compaction density of the lithium iron phosphate is limited by simply mixing the two types of iron phosphate with different particle sizes, and the difference of the other two types of iron phosphate preparation processes also brings some uncertain influences on the stability of the product.
Disclosure of Invention
1. Problems to be solved
Aiming at the problems of poor compaction effect and unstable performance of the iron phosphate prepared by adopting a mechanical mixing method in the prior art, the invention provides a preparation method of high-compaction iron phosphate and lithium iron phosphate. The preparation method starts with the reaction essence in the iron phosphate generation process, increases the compactness of the primary iron phosphate particles stacked into the secondary particles, and widens the distribution state of the secondary iron phosphate particle size, thereby achieving the purpose of synthesizing high-compaction battery-grade iron phosphate by a one-step liquid-phase chemical method, and ensuring that the finally obtained iron phosphate has high compaction density and stable performance. Meanwhile, the iron phosphate prepared by the method is used as a raw material, and the large and small lithium iron phosphate particles are tightly embedded, so that the compaction density of the whole lithium iron phosphate is improved.
2. Technical scheme
In order to solve the above problems, the present invention adopts the following technical solutions.
A preparation method of high-compaction iron phosphate comprises the following steps:
s1: preparing ferric iron source slurry, and dividing the ferric iron source slurry into two parts;
s2: adding a mixed solution of phosphoric acid and liquid alkali into one part of the ferric iron source slurry to form mixed slurry, and reacting;
s3: when the mixed slurry in the step S2 is white, adding the other part of the ferric iron source slurry in the step S1 into the mixed slurry, and reacting to obtain final slurry;
s4: and washing, filtering, drying and roasting the final slurry to obtain the high-compaction iron phosphate.
Further, the ferric iron source slurry in step S1 is obtained by dissolving and oxidizing ferrous sulfate as a titanium white byproduct.
Furthermore, the molar concentration of iron ions in the ferric iron source slurry is 0.8-1.5mol/L, and the ferrous sulfate is oxidized by hydrogen peroxide.
Furthermore, the mass fraction of the hydrogen peroxide is 20-50%, and the mass ratio of the hydrogen peroxide to the iron ions is (1.1-1.5): 1.
furthermore, the mass ratio of the ferric iron source slurry in the step S2 to the total ferric iron source slurry in the step S1 is (0.5-0.8): 1.
Furthermore, the reaction temperature in the step S2 and the step S3 is 60-90 ℃, and the reaction time is 2-5 h.
Furthermore, in the step S2, the concentration of phosphoric acid in the mixed solution of phosphoric acid and liquid alkali is 4-8 mol/L, and the mass ratio of the phosphoric acid to the iron ions in the ferric iron source slurry is (1.5-2.2): 1.
furthermore, the pH value of the reaction process in the step S2 is controlled to be 0.6-1.1, and the pH value of the reaction process in the step S3 is controlled to be 0.8-1.3.
Furthermore, in the step S4, the roasting temperature is 400-700 ℃, and the roasting time is 1-3 h.
The lithium iron phosphate is prepared by adopting the high-compaction iron phosphate prepared by the preparation method of the high-compaction iron phosphate as a raw material.
3. Advantageous effects
Compared with the prior art, the invention has the beneficial effects that:
(1) starting from the reaction essence in the generation process of the iron phosphate, the compactness of the secondary particles formed by stacking the primary iron phosphate particles is increased, and the distribution state of the particle size of the secondary iron phosphate is widened, so that the high-compaction battery-grade iron phosphate is synthesized by a one-step liquid-phase chemical method; adding the oxidized ferric ions into an excessive phosphoric acid solution to generate primarily compact high-compaction ferric phosphate by reasonably regulating the pH value of the solution, then continuously adding a proper amount of the solution into the generated ferric phosphate slurry to obtain the ferric ions, increasing the pH value of the solution after the ferric ions are added, continuously reacting the quickly added ferric ions with phosphate ions in the solution, continuously growing a part of the ferric phosphate serving as an attachment point, enabling the particles to be more compact, enabling the particle size of the whole particle to be increased to a certain extent, generating new primary ferric phosphate crystals by the other part to form secondary ferric phosphate crystals, enabling the particle size of the whole particle to be smaller, and facilitating the embedding of large and small particles together in the subsequent lithium iron phosphate tabletting process so as to improve the compaction density; compared with a high-compaction lithium iron phosphate prepared by a conventional mechanical mixing method, the method is simpler in process, the iron phosphate crystal structure is tighter, the particle size distribution is wider, and the compacted density of the iron phosphate is higher;
(2) the titanium dioxide is obtained by dissolving and oxidizing the titanium dioxide byproduct ferrous sulfate, and the titanium dioxide byproduct ferrous sulfate is suitable for being used as an iron source due to low impurity content, can be utilized, and saves the production cost; meanwhile, the concentration of iron ions in the ferric iron source slurry in the step S2 is controlled within a certain numerical range, and the iron ion concentration is too low, so that the large-scale production is not facilitated, and the subsequent high-compaction iron phosphate is not facilitated to be formed; if the concentration of the iron ions is too high, the iron ions are difficult to dissolve in the solution, and the formation of iron phosphate is not facilitated; the mass fraction of the oxidant is controlled, so that the formation of high-compaction iron phosphate is further ensured;
(3) according to the invention, the molar quantity of the ferric iron source slurry in the step S2 is controlled to be 50-80% of the total molar quantity of the ferric iron source slurry, and the use amount of the ferric iron source slurry participating in the reaction for the first time is too large, so that the phosphorus-iron ratio in the whole step S2 is too small, and the formation of high-compaction iron phosphate is not facilitated; if the usage amount of the ferric iron source slurry participating in the reaction for the first time is too small, the usage amount of the alkali required in the step S2 is greatly increased, so that the reaction cost is increased, and meanwhile, since too many ferric iron sources are present in the ferric iron source slurry participating in the reaction for the second time, it is difficult to completely participate in the reaction, so that the utilization rate of the ferric ions is greatly reduced, and the performance of the formed ferric phosphate is poor.
(4) The reaction temperature and the reaction time in the step S2 and the step S3 are controlled, the reaction temperature is too low to ensure that the energy required by the reaction cannot be normally carried out, and the reaction temperature is too high to supply energy in the actual industrialized production process, so that the reaction cost is increased; for the reaction time, controlling the reaction time within the period of reaction time to ensure that the secondary ferric iron source slurry can fully react so as to finally form high-compaction ferric phosphate;
(5) the method controls the concentration of the phosphoric acid within a certain range, avoids the influence of too low concentration of the phosphoric acid on the production efficiency, is not beneficial to the preparation of high-compaction iron phosphate, and simultaneously avoids the problems that the reaction of mixing the phosphoric acid and sodium hydroxide is too violent due to too high concentration of the phosphoric acid, and a large amount of heat is suddenly released in the reaction, so that the accurate control of the reaction temperature is not beneficial; and controlling the reaction pH in steps S2 and S3 to further promote the formation of high compaction iron phosphate;
(6) according to the invention, the roasting temperature and the roasting time are accurately controlled, so that the condition that the secondary particles of the iron phosphate are too large due to overhigh roasting temperature and the multiplying power of the finally formed lithium iron phosphate is reduced is avoided, and the mechanical processing in the process of preparing the lithium iron phosphate is not facilitated; the combined water in the iron phosphate can not be completely removed due to the excessively low roasting temperature, so that the prepared lithium iron material has a hole phenomenon, and the performance of the finally formed product is poor;
drawings
FIG. 1 is an electron micrograph of dehydrated iron phosphate according to example 1;
FIG. 2 is an electron micrograph of dehydrated iron phosphate in comparative example 2.
Detailed Description
The invention is further described with reference to specific embodiments and the accompanying drawings.
A preparation method of high-compaction iron phosphate comprises the following steps:
s1: preparing ferric iron source slurry, and dividing the ferric iron source slurry into two parts;
specifically, the ferric iron source slurry is obtained by dissolving and oxidizing a titanium white byproduct ferrous sulfate, is suitable for serving as an iron source due to low impurity content in the titanium white byproduct ferrous sulfate, can utilize the titanium white byproduct, and saves the production cost; meanwhile, ferrous sulfate is oxidized by hydrogen peroxide to obtain ferric iron source slurry, and hydrogen peroxide is used as an oxidant, so that the source is wide and the oxidation effect is good. The molar concentration of iron ions in the ferric iron source slurry is controlled within the range of 0.8-1.5mol/L, and if the concentration of the iron ions is too low, the scale production is not facilitated, and the subsequent high-compaction iron phosphate is not formed; if the concentration of the iron ions is too high, the iron ions are difficult to dissolve in the solution, and the formation of iron phosphate is not facilitated. Preferably, the ferrous sulfate is oxidized by hydrogen peroxide with the mass fraction of 20-50%, and the mass ratio of the hydrogen peroxide to the ferric ion is (1.1-1.5): 1. the concentration of hydrogen peroxide is too dilute, so that the concentration of the whole ferrous iron is too dilute, the industrialization and the preparation of high-compaction iron phosphate are not facilitated, and the convenience is provided for the subsequent reaction.
S2: adding a mixed solution of phosphoric acid and liquid alkali into one part of ferric source slurry to form mixed slurry, and reacting, wherein in the step, the oxidized ferric ions form initial compact high-compaction ferric phosphate in the mixed solution of phosphoric acid and liquid alkali;
in order to further ensure the performance stability of the formation of the iron phosphate, the mass ratio of the ferric iron source slurry in the step to the total ferric iron source slurry in the step S1 is (0.5-0.8): 1, so if the molar amount of the ferric iron source slurry in the step is too large, the phosphorus-iron ratio in the whole step S2 is too small, and the formation of the high-compaction iron phosphate in the step is very unfavorable; if the molar quantity of the ferric iron source slurry in the step is too small, the alkali consumption required in the step is greatly increased, so that the reaction cost is increased, and if the molar quantity of the ferric iron source slurry in the step is too small, more residual ferric iron source slurry is used in the subsequent steps, so that the ferric ions in the ferric iron source slurry are difficult to completely participate in the reaction, the utilization rate of the ferric ions is greatly reduced, and the performance of the finally prepared ferric phosphate product is poor. And simultaneously, further controlling the concentration of phosphoric acid in the mixed solution of phosphoric acid and liquid alkali to be 4-8 mol/L, wherein the mass ratio of the substances of phosphoric acid to the substances of iron ions in the ferric iron source slurry is (1.5-2.2): 1, the production efficiency is prevented from being influenced by too low phosphoric acid concentration, the phosphoric acid reacts too violently due to too high phosphoric acid concentration, and the reaction is overheated and is not favorable for accurate control of temperature. And the pH value of the reaction process in the step is controlled to be 0.6-1.1, so that the normal synthesis of the iron phosphate is ensured.
S3: when the mixed slurry in the step S2 is white, adding the other part of the ferric iron source slurry in the step S1 into the mixed slurry, and reacting to obtain final slurry; in the step, ferric ions are continuously added on the basis of the preliminarily compact high-compaction ferric phosphate, the pH value of the solution after the ferric ions are added is increased, the rapidly added ferric ions and phosphate ions in the solution continuously react, one part of the solution continuously grows by taking the generated ferric phosphate as an attachment point, the particles are more compact, the particle size of the whole particle is increased to a certain extent, and the other part of the solution generates new primary ferric phosphate crystals to form secondary ferric phosphate crystals; the formed secondary iron phosphate particles have compact structure, wider particle size distribution and high compaction density.
It is explained that, in the step S1, the other part of the ferric source slurry is uniformly added to the mixed slurry, and the adding time is controlled to be 0.2-0.8 h, so as to ensure that the ferric ions fully react and prepare the highly compacted ferric phosphate. Meanwhile, in the step S2 and the step S3, the reaction temperature is controlled to be 60-90 ℃, the reaction time is 2-5 hours, and for the reaction temperature, the energy required by the reaction is difficult to ensure if the reaction temperature is too low, so that the reaction cannot be carried out; if the temperature is too high, energy supply is difficult in the actual industrialization process, and the production cost is increased to a certain extent. For the reaction time, the reaction is controlled within the range, so that iron ions can fully react, the iron ions and phosphoric acid can be fully reacted to form iron phosphate, and the finally formed iron phosphate is guaranteed to have high compaction property and stable performance. And in step S3, the pH is controlled to 0.8-1.3 during the reaction process, since iron phosphate is generated in step S2, the pH in the solution is higher due to the continued reaction of iron phosphate in step S3, and the particle size range of the generated iron phosphate is larger and the compaction is higher in order to accelerate the reaction within the pH range.
S4: and washing, filtering, drying and roasting the final slurry to obtain the high-compaction iron phosphate.
Specifically, the roasting temperature in the step is controlled to be 400-700 ℃, the roasting time is controlled to be 1-3 h, the condition that the secondary particles of the iron phosphate are too large due to overhigh roasting temperature is avoided, the multiplying power of the finally formed lithium iron phosphate is reduced, and the mechanical processing in the process of preparing the lithium iron phosphate is not facilitated; the combined water in the iron phosphate can not be completely removed due to the excessively low roasting temperature, so that the prepared lithium iron material has poor product performance due to the occurrence of a hole phenomenon.
According to the invention, excessive phosphoric acid solution is added into the oxidized ferric iron ions to generate initial compact high-compaction ferric phosphate, then a proper amount of ferric iron ions are continuously and slowly added into the generated ferric phosphate slurry, the ferric iron ions and phosphate ions in the solution continuously react, a part of the ferric phosphate which is generated is used as an attachment point to continue growing, the particles are more compact, and the particle size of the whole particles is increased to a certain extent; and a new primary iron phosphate crystal is generated at the other part of the lithium iron phosphate tabletting material to form a secondary iron phosphate crystal, so that the particle size of the whole particle is small, and the subsequent embedding of large and small particles in the lithium iron phosphate tabletting process is facilitated, thereby improving the compaction density. The concentration of impurity ions in the whole reaction filtrate is low, and the prepared secondary iron phosphate particles have compact structure, wider particle size distribution and high compaction density. Different from the original method of preparing the iron phosphate by adopting a mechanical mixing method, the method starts from the reaction essence in the iron phosphate generation process, and the high-compaction iron phosphate is directly synthesized by chemical reaction in a liquid phase, so that the prepared iron phosphate has a tighter crystal structure, wider particle size distribution, simple process, high lithium titanate compaction density and stable performance.
The lithium iron phosphate is prepared by adopting the high-compaction iron phosphate prepared by the preparation method of the high-compaction iron phosphate as a raw material. The prepared lithium iron titanate has higher compaction density and stable product performance.
Example 1
A preparation method of high-compaction iron phosphate comprises the following steps:
s1: dissolving 3mol of ferrous sulfate crystal serving as a titanium white byproduct in 1L of water, adding 340ml of 30 mass percent hydrogen peroxide solution for full oxidation, adding water to fix the volume to 2L of ferric iron source slurry, and dividing the 2L of ferric iron source slurry into two parts, wherein one part of the ferric iron source slurry is 1.6L, and the other part is 0.4L;
s2: heating 1.6L of ferric iron source slurry obtained in the step S1 in a water bath kettle at 85 ℃ for reaction, adding a mixed solution of phosphoric acid and sodium hydroxide, adjusting the pH value of the solution to 0.8, and reacting the mixed slurry, wherein the molar weight of the phosphoric acid in the mixed solution is 5.28mol, and the volume of the phosphoric acid is 0.66L;
s3: when the mixed slurry in the step S2 is white, adding the residual 0.4L of ferric iron source slurry in the step S1 into the mixed slurry at a constant speed within 48min, controlling the pH of the solution to be 1.0, and continuously reacting for 5h to form the final slurry;
s4: and washing, filtering and drying the final slurry, and roasting at 600 ℃ for 2h to obtain the high-compaction battery-grade iron phosphate.
Example 2
Substantially as in example 1, a method of preparing high compacted iron phosphate comprises the steps of:
s1: dissolving 3mol of ferrous sulfate crystal serving as a titanium white byproduct in 1L of water, adding 600ml of hydrogen peroxide solution with the mass fraction of 20% for full oxidation, adding water to fix the volume to 3L of ferric iron source slurry, and dividing the 3L of ferric iron source slurry into two parts, wherein one part of the ferric iron source slurry is 1.8L, and the other part is 1.2L;
s2: heating 1.8L of ferric iron source slurry obtained in the step S1 in a water bath kettle at 80 ℃ for reaction, adding a mixed solution of phosphoric acid and sodium hydroxide, wherein the molar weight and the volume of the phosphoric acid in the mixed solution are 3.6mol and 0.6L, adjusting the pH value of the solution to 1.0, and reacting the mixed slurry;
s3: when the mixed slurry in the step S2 is white, adding the 1.2L of the ferric iron source slurry left in the step S1 into the mixed slurry at a constant speed within 24min, controlling the pH value of the solution to be 1.0, and continuously reacting for 3h to form the final slurry;
s4: and washing, filtering and drying the final slurry, and roasting at 500 ℃ for 2.5h to obtain the high-compaction battery grade iron phosphate.
Example 3
Substantially as in example 1, a method of preparing high compacted iron phosphate comprises the steps of:
s1: dissolving 3mol of ferrous sulfate crystal serving as a titanium white byproduct in 1L of water, adding 390ml of a 30 mass percent hydrogen peroxide solution for full oxidation, adding water to fix the volume to 3L of ferric iron source slurry, and dividing the 3L of ferric iron source slurry into two parts, wherein one part of the ferric iron source slurry is 2.1L, and the other part is 0.9L;
s2: heating 2.1L of the ferric iron source slurry obtained in the step S1 in a water bath kettle at 85 ℃ for reaction, adding a mixed solution of phosphoric acid and sodium hydroxide, wherein the molar weight of the phosphoric acid in the mixed solution is 3.57mol, the volume of the phosphoric acid is 0.714L, adjusting the pH value of the solution to 0.9, and reacting the mixed slurry;
s3: when the mixed slurry in the step S2 is white, adding the residual 0.9L of ferric iron source slurry in the step S1 into the mixed slurry at a constant speed within 30min, controlling the pH of the solution to be 1.2, and continuously reacting for 3.5h to form the final slurry;
s4: and washing, filtering and drying the final slurry, and roasting at 700 ℃ for 2h to obtain the high-compaction battery grade iron phosphate.
Example 4
Substantially as in example 1, a method of preparing high compacted iron phosphate comprises the steps of:
s1: dissolving 3mol of ferrous sulfate crystal serving as a titanium white byproduct in 1L of water, adding 228ml of hydrogen peroxide solution with the mass fraction of 50% for full oxidation, adding water to fix the volume to 3.4L of ferric iron source slurry, and dividing the 3.4L of ferric iron source slurry into two parts, wherein one part of the ferric iron source slurry is 2.38L, and the other part is 1.02L;
s2: heating 2.38L of the ferric iron source slurry obtained in the step S1 in a water bath kettle at 75 ℃ for reaction, adding a mixed solution of phosphoric acid and sodium hydroxide, adjusting the pH value of the solution to 0.9, and reacting the mixed slurry, wherein the molar weight of the phosphoric acid in the mixed solution is 4.00mol, and the volume of the phosphoric acid is 0.8L;
s3: when the mixed slurry in the step S2 is white, adding the 1.02L of ferric iron source slurry remained in the step S1 into the mixed slurry at a constant speed within 30min, controlling the pH of the solution to be 1.1, and continuously reacting for 4h to form the final slurry;
s4: and washing, filtering and drying the final slurry, and roasting at 650 ℃ for 3h to obtain the high-compaction battery grade iron phosphate.
Example 5
Substantially as in example 1, a method of preparing high compacted iron phosphate comprises the steps of:
s1: dissolving 3mol of ferrous sulfate crystal serving as a titanium white byproduct in 1L of water, adding 420ml of 30 mass percent hydrogen peroxide solution for full oxidation, adding water to fix the volume to 3.4L of ferric iron source slurry, and dividing the 3L of ferric iron source slurry into two parts, wherein one part of the ferric iron source slurry is 2.38L, and the other part is 1.02L;
s2: heating 2.38L of the ferric iron source slurry obtained in the step S1 in a water bath kettle at 75 ℃ for reaction, adding a mixed solution of phosphoric acid and sodium hydroxide, wherein the molar weight and the volume of the phosphoric acid in the mixed solution are 3.36mol and 0.84L, adjusting the pH value of the solution to 1.0, and reacting the mixed slurry;
s3: when the mixed slurry in the step S2 is white, adding the 1.02L of ferric iron source slurry remained in the step S1 into the mixed slurry at a constant speed within 35min, controlling the pH value of the solution to be 1.2, and continuously reacting for 4h to form the final slurry;
s4: and washing, filtering and drying the final slurry, and roasting at 650 ℃ for 2h to obtain the high-compaction battery grade iron phosphate.
Comparative example 1
As a comparative example to example 1, a method for preparing high compacted iron phosphate comprising the steps of:
s1: dissolving 3mol of ferrous sulfate crystal serving as a titanium white byproduct in 1L of water, adding 340ml of 30 mass percent hydrogen peroxide solution for full oxidation, adding water to fix the volume to 2L of ferric iron source slurry, and dividing the 2L of ferric iron source slurry into two parts, wherein one part of the ferric iron source slurry is 1.6L, and the other part is 0.4L;
s2: heating 1.6L of ferric iron source slurry obtained in the step S1 in a water bath kettle at 85 ℃ for reaction, adding a mixed solution of phosphoric acid and sodium hydroxide, wherein the molar weight and the volume of the phosphoric acid in the mixed solution are 3.12mol and 0.66L, adjusting the pH value of the solution to 0.8, and reacting the mixed slurry;
s3: when the mixed slurry in the step S2 is white, adding the residual 0.4L of ferric iron source slurry in the step S1 into the mixed slurry at a constant speed within 12min, controlling the pH of the solution to be 1.0, and continuously reacting for 5h to form the final slurry;
s4: and washing, filtering and drying the final slurry, and roasting at 600 ℃ for 2h to obtain the high-compaction battery-grade iron phosphate.
Compared with example 1, comparative example 1 reduces the ratio of phosphoric acid and iron source in step S2, so that the excess degree of phosphate with respect to iron ions during the reaction in step S2 decreases, the free energy of phosphate-bound iron ions decreases, the reaction rate decreases, the growth rate of primary particles becomes slow, the driving force of the entire reaction decreases, the driving force of primary particles to pile up into secondary particles in a limited space decreases, the degree of compaction of the secondary particles is weaker, and the overall compaction of the particles decreases.
Comparative example 2
As a comparative example to example 1, a method for preparing high compacted iron phosphate comprising the steps of:
s1: taking 2.4mol of ferrous sulfate crystal as a titanium white byproduct, adding 0.8L of water for dissolving, adding 272ml of hydrogen peroxide solution with the mass fraction of 30% for full oxidation, and adding water to fix the volume to 1.6L of ferric iron source slurry;
s2: heating 1.6L of ferric iron source slurry obtained in the step S1 in a water bath kettle at 85 ℃ for reaction, adding a mixed solution of phosphoric acid and sodium hydroxide, wherein the molar weight and the volume of the phosphoric acid in the mixed solution are 2.88mol and 0.66L, adjusting the pH value of the solution to 0.9, and reacting the mixed slurry;
s3: and (4) after the mixed slurry in the step S2 is washed, filtered and dried, roasting for 2 hours at 600 ℃ to obtain the high-compaction battery-grade iron phosphate.
As shown in fig. 1 and 2, the iron phosphate synthesized in one step continues to react without adding an iron source in the subsequent step, so that the utilization rate of phosphate ions in the solution is reduced, the reaction cost is increased, and the formed iron phosphate has a low compacted density.
Comparative example 3
As a comparative example to example 1, a method for preparing high compacted iron phosphate comprising the steps of:
s1: dissolving 3mol of ferrous sulfate crystal serving as a titanium white byproduct in 1L of water, adding 340ml of 30 mass percent hydrogen peroxide solution for full oxidation, adding water to fix the volume to 2L of ferric iron source slurry, and dividing the 2L of ferric iron source slurry into two parts, wherein one part of the ferric iron source slurry is 1.6L, and the other part is 0.4L;
s2: heating 1.6L of ferric iron source slurry obtained in the step S1 in a water bath kettle at 85 ℃ for reaction, adding a mixed solution of phosphoric acid and sodium hydroxide, adjusting the pH value of the solution to 0.8, and reacting the mixed slurry, wherein the molar weight of the phosphoric acid in the mixed solution is 5.28mol, and the volume of the phosphoric acid is 0.66L;
s3: when the mixed slurry in the step S2 is white, adding the residual 0.4L of ferric iron source slurry in the step S1 into the mixed slurry at a constant speed within 12min, controlling the pH of the solution to be 0.6, and continuously reacting for 5h to form the final slurry;
s4: and washing, filtering and drying the final slurry, and roasting at 600 ℃ for 2h to obtain the high-compaction battery-grade iron phosphate.
Compared with example 1, the method has the advantages that the pH of the solution is low in the reaction process of step S3, and when the pH of the solution is too low, the reaction rate of the whole reaction is reduced seriously, which is very unfavorable for the process of quickly stacking the iron phosphate generated in step S3 into the iron phosphate, so that the iron phosphate prepared in step S3 is more fluffy and adheres to the surface of the iron phosphate prepared in step S2, and the compaction of the whole iron phosphate is not improved, but is reduced, which is contrary to the desired purpose.
In summary, the physical parameters of the iron phosphate and the lithium iron phosphate prepared in five examples and three comparative examples of the present invention are shown in table 1:
TABLE 1 comparison of physical parameters of iron phosphate and lithium iron phosphate prepared in each example and comparative example
As is apparent from table 1, when the iron phosphate is prepared by the method of the present invention, based on the essence of the iron phosphate in the crystal growth process, the iron phosphate prepared by the liquid phase chemical method has a tighter crystal structure and a wider particle size distribution, and has a higher compacted density than lithium iron phosphate prepared by mixing according to the conventional simple physical mechanical method. Therefore, the lithium iron phosphate prepared by taking the iron phosphate as a raw material has higher compaction density within the range of 2.45-2.5, and the current lithium iron phosphate has the highest compaction density of only 2.4, so that the lithium iron phosphate has strong performance stability and higher compaction density.
The examples described herein are merely illustrative of the preferred embodiments of the present invention and do not limit the spirit and scope of the present invention, and various modifications and improvements made to the technical solutions of the present invention by those skilled in the art without departing from the design concept of the present invention shall fall within the protection scope of the present invention.
Claims (10)
1. A preparation method of high-compaction ferric phosphate is characterized by comprising the following steps: the method comprises the following steps:
s1: preparing ferric iron source slurry, and dividing the ferric iron source slurry into two parts;
s2: adding a mixed solution of phosphoric acid and liquid alkali into one part of the ferric iron source slurry to form mixed slurry, and reacting;
s3: when the mixed slurry in the step S2 is white, adding the other part of the ferric iron source slurry in the step S1 into the mixed slurry, and reacting to obtain final slurry;
s4: and washing, filtering, drying and roasting the final slurry to obtain the high-compaction iron phosphate.
2. The method for preparing high compacted iron phosphate according to claim 1, characterized in that: in step S1, the ferric source slurry is obtained by dissolving and oxidizing ferrous sulfate, which is a titanium white byproduct.
3. The method for preparing high compacted iron phosphate according to claim 2, characterized in that: the molar concentration of iron ions in the ferric source slurry is 0.8-1.5mol/L, and the ferrous sulfate is oxidized by hydrogen peroxide.
4. The method for preparing high compacted iron phosphate according to claim 3, characterized in that: the mass fraction of the hydrogen peroxide is 20-50%, and the mass ratio of the hydrogen peroxide to the iron ions is (1.1-1.5): 1.
5. the method for preparing high compacted iron phosphate according to claim 1, characterized in that: the mass ratio of the ferric iron source slurry in the step S2 to the total ferric iron source slurry in the step S1 is (0.5-0.8): 1.
6. The method for preparing high compacted iron phosphate according to claim 1 or 5, characterized in that: the reaction temperature in the step S2 and the step S3 is 60-90 ℃, and the reaction time is 2-5 h.
7. The method of preparing high compacted iron phosphate according to claim 6, wherein: in the step S2, the concentration of phosphoric acid in the mixed liquid of phosphoric acid and liquid alkali is 4-8 mol/L, and the mass ratio of the phosphoric acid to the ferric ions in the ferric iron source slurry is (1.5-2.2): 1.
8. the method for preparing high compacted iron phosphate according to claim 1, characterized in that: the pH value of the reaction process in the step S2 is controlled to be 0.6-1.1, and the pH value of the reaction process in the step S3 is controlled to be 0.8-1.3.
9. The method for preparing high compacted iron phosphate according to claim 1, characterized in that: in the step S4, the roasting temperature is 400-700 ℃, and the roasting time is 1-3 h.
10. A lithium iron phosphate characterized in that: the high compacted iron phosphate produced by the method for producing high compacted iron phosphate according to any one of claims 1 to 9.
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