Preparation method and application of lithium pyrophosphate

Abstract

The present invention relates to the technical field of preparation of lithium pyrophosphate, and specifically provides a preparation method and application of lithium pyrophosphate, wherein pyrophosphoric acid is added dropwise to a lithium hydroxide aqueous solution, and the addition is stopped when the pH reaches 6.5-7.5 to obtain a slurry 1; the slurry 1 is subjected to solid-liquid separation, and the obtained solid product is dried and ground to obtain a lithium pyrophosphate product. The present invention uses lithium hydroxide monohydrate and pyrophosphoric acid as raw materials, adopts a liquid phase method to simultaneously control the dropping rate of pyrophosphoric acid to obtain lithium pyrophosphate, and the method has high yield and purity, and the particles have good appearance, uniform particle size distribution, low impurity content, low cost, and are convenient for industrial production and application. The obtained high-purity lithium pyrophosphate helps to improve the comprehensive performance of the battery such as impedance, conductivity, and cycle life, and therefore has broad application prospects in the field of batteries.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium pyrophosphate preparation, and in particular to a preparation method and application of lithium pyrophosphate. Background Art

[0002] Lithium-ion batteries (LIBs) are widely used in electronics, communications tools, hybrid vehicles, and other devices, serving as the primary energy storage device for many devices. Currently, research on anode materials and electrolytes for lithium-ion batteries is relatively mature, making the study of cathode materials crucial for improving their performance. Polyanionic cathode materials, with their high operating voltage and relatively stable crystal structure, have become a current research hotspot.

[0003] Polyanionic cathode materials mainly include phosphates, silicates, sulfates, borates, and titanates. Phosphates can be divided into orthophosphates, pyrophosphates, and metaphosphates. Lithium pyrophosphate, as a new type of polyanionic cathode material, has a three-dimensional network structure and exhibits good structural stability. Compared with other phosphate cathode materials, lithium pyrophosphate cathode materials provide lithium ions with a two-dimensional tunnel structure that allows for free movement, giving them better electrochemical properties. They can be used to coat lithium iron phosphate or ternary materials to improve the material's conductivity, reduce resistance, and minimize power loss. In addition, lithium pyrophosphate can also be used as an additive for lithium batteries. Its excellent flame retardant properties can improve the safety performance of lithium batteries. Therefore, lithium pyrophosphate has potential market demand.

[0004] Currently, there are few reports on methods for preparing lithium pyrophosphate. Chinese patent CN103441269 discloses a method for preparing a lithium pyrophosphate / carbon-coated lithium iron phosphate composite material. This method utilizes a calcination process, whereby the added phosphorus source and excess lithium oxide are sintered to form lithium pyrophosphate during a secondary sintering process. This method suffers from low conversion rates and purity. Therefore, there is a need for an improved method for preparing lithium pyrophosphate to address these issues. Summary of the Invention

[0005] The present invention aims to provide a preparation method and application of lithium pyrophosphate. The method uses lithium hydroxide monohydrate and pyrophosphoric acid as raw materials, adopts a liquid phase method and simultaneously controls the dropping speed of the pyrophosphoric acid to obtain lithium pyrophosphate. The method has high yield and purity, and the particles have good appearance, uniform particle size distribution, low impurity content, low cost, and are convenient for industrial production and application.

[0006] To achieve the above-mentioned object, the present invention provides a method for preparing lithium pyrophosphate, comprising: adding pyrophosphoric acid dropwise to a lithium hydroxide aqueous solution, and stopping the addition when the pH reaches 6.5-7.5 to obtain a slurry 1; performing solid-liquid separation on the slurry 1, and drying and grinding the obtained solid product to obtain a lithium pyrophosphate product.

[0007] The reaction mechanism of lithium hydroxide and pyrophosphoric acid is: 4LiOH + H4P2O7 → Li4P2O7 + 4H2O. By adding pyrophosphoric acid to the lithium hydroxide aqueous solution, the lithium hydroxide is in excess relative to the pyrophosphoric acid during the reaction. The added pyrophosphoric acid quickly reacts with the lithium hydroxide to form pyrophosphoric acid, preventing the pyrophosphoric acid from hydrolyzing into phosphoric acid, which would affect the yield and purity of the lithium pyrophosphate. When the pH reaches 6.5-7.5, indicating that the lithium hydroxide has essentially reacted, the addition of pyrophosphoric acid is stopped at this point to ensure product purity.

[0008] As a further improvement of the present invention, the pyrophosphoric acid is added dropwise for 30-60 minutes. By controlling the addition speed, the reaction process can be regulated to ensure the formation of lithium pyrophosphate and reduce the generation of by-products.

[0009] As a further improvement of the present invention, the lithium hydroxide aqueous solution is prepared by mixing lithium hydroxide monohydrate and water in a mass ratio of 1:(5-6). Since lithium pyrophosphate has a certain solubility in water, the amount of water should not be excessive, thereby reducing product waste and simplifying product recovery.

[0010] As a further improvement of the present invention, the solid-liquid separation includes: filtering the slurry 1 to obtain mother liquor 1 and lithium pyrophosphate wet material; adding a polyol to the lithium pyrophosphate wet material, stirring and dissolving for 30-60 minutes to obtain slurry 2; filtering the slurry 2 to obtain lithium pyrophosphate and mother liquor 2; the polyol is one or more of glycerol, 1,2-butanediol and 1,2-propylene glycol, preferably 1,2-propylene glycol, which has a moderate boiling point and is convenient for controlling its content by drying, so that a small amount of remaining 1,2-propylene glycol is coated on the surface of the product. The present invention further purifies the lithium pyrophosphate wet material by adding a polyol, which can avoid product agglomeration during drying and facilitates grinding to obtain a product with small and uniform particle size.

[0011] As a further improvement of the present invention, the added amount of the polyol is 4-5 times the mass of the lithium pyrophosphate wet material.

[0012] As a further improvement of the present invention, the drying temperature is 70-80°C and the drying time is 2.5-3 hours. During the drying process at 70-80°C, after 1-1.5 hours of drying, the moisture content is approximately 0.1%; after further drying for 2.5-3 hours, the moisture content further decreases; the 1,2-propylene glycol content is approximately 0.1%. If the drying time is too short, the 1,2-propylene glycol content is too high, making subsequent use inconvenient. If the drying time is too long, the 1,2-propylene glycol content is insufficient, making packaging impossible. Therefore, the optimal drying time is controlled to 2.5-3 hours. During use, the 1,2-propylene glycol can be further removed by high temperature.

[0013] As a further improvement of the present invention, the mother liquor 1 is used to prepare the next batch of lithium hydroxide aqueous solution. The mother liquor 1 is essentially a saturated solution of lithium pyrophosphate. When reused, the solubility of lithium pyrophosphate in water can be reduced. That is, during repeated uses, the unrecovered lithium pyrophosphate is always the lithium pyrophosphate in the first mother liquor 1, thereby reducing the waste of lithium pyrophosphate.

[0014] As a further improvement of the present invention, the polyol is heated to 101-103° C. to remove water and then reused.

[0015] As a further improvement of the present invention, the following steps are included:

[0016] S1, lithium hydroxide monohydrate and water are mixed in a mass ratio of 1:5-6, and stirred and dissolved until clear to obtain a lithium hydroxide aqueous solution;

[0017] S2. Add pyrophosphoric acid dropwise to the lithium hydroxide aqueous solution, and stop adding when the pH reaches 6.5-7.5 to obtain slurry 1;

[0018] S3, filtering the slurry 1 to obtain a mother liquor 1 and a lithium pyrophosphate wet material 1;

[0019] S4, adding polyol to the lithium pyrophosphate wet material 1, stirring and dissolving for 30-60 minutes to obtain slurry 2;

[0020] S5, filtering the slurry 2 to obtain a lithium pyrophosphate wet material 2 and a mother liquor 2, drying and grinding the lithium pyrophosphate wet material 2 to obtain a lithium pyrophosphate product;

[0021] S6. Return the mother liquor 1 to step S1 to replace part of the water for the preparation of the lithium hydroxide aqueous solution, and repeat steps S2 to S5. During the repeated process, the mother liquor 2 is heated to remove water, and after cooling, returns to step S4 to replace part of the polyol for repeated use.

[0022] In some embodiments, Figure 1 As shown, the preparation method of lithium pyrophosphate includes the following steps:

[0023] 1) 2 mol of lithium hydroxide monohydrate and pure water were stirred and dissolved in a mass ratio of 1:5-6 until clear to obtain solution 1.

[0024] 2) Pyrophosphoric acid was added dropwise to solution 1. The addition was stopped when the pH reached 6.5-7.5. A white precipitate was generated to obtain slurry 1 (or pyrophosphoric acid was added dropwise at a molar ratio of lithium hydroxide monohydrate to pyrophosphoric acid of 3.8-4.2:1).

[0025] 3) Filter the slurry 1 to obtain mother liquor 1 and lithium pyrophosphate wet material 1. The mother liquor 1 is returned to step 1 to replace part of the pure water for dissolving lithium hydroxide monohydrate. The solubility of lithium pyrophosphate in water is 6g / 100ml at room temperature. According to the solubility, it is a water-soluble substance and insoluble in 1,2-propylene glycol. Therefore, the mother liquor 1 is actually a saturated solution of lithium pyrophosphate. Reusing it to step 1 can reduce the dissolution of subsequent batches of lithium pyrophosphate in water, reduce product waste, and improve the stability of the lithium pyrophosphate reaction to a certain extent.

[0026] 4) Add 1,2-propylene glycol in an amount 4-5 times the mass of the lithium pyrophosphate wet material 1 and stir to dissolve for 30-60 minutes to obtain slurry 2. Since water and 1,2-propylene glycol are miscible, while lithium pyrophosphate is insoluble in 1,2-propylene glycol, the addition of 1,2-propylene glycol can remove water from the lithium pyrophosphate wet material 1, thereby minimizing the water content before drying, thereby preventing agglomeration during drying, and facilitating grinding to obtain a lithium pyrophosphate product with small and uniform particle size.

[0027] 5) Slurry 2 is filtered to obtain lithium pyrophosphate 2 and mother liquor 2. The lithium pyrophosphate 2 is dried at 70-80°C for 2.5-3 hours and ground to obtain a lithium pyrophosphate product with uniform particles. Mother liquor 2 is heated to 101-103°C, cooled to room temperature, and then returned to step 4 to be used again to dissolve lithium pyrophosphate wet material 1. This repeated use saves raw materials, reduces costs, and ensures the yield of lithium pyrophosphate.

[0028] The present invention also provides lithium pyrophosphate, which is prepared by any of the above preparation methods.

[0029] As a further improvement of the present invention, the purity of the lithium pyrophosphate is ≥98%, preferably ≥99.8%, the water content is <0.1%, preferably <0.01%, the 1,2-propylene glycol content is 0.05-0.1%, and the particle size is 3-15 microns.

[0030] The present invention also provides an application of the lithium pyrophosphate described above, wherein the lithium pyrophosphate is used for the preparation of ionic solid electrolytes, lithium iron phosphate material coating modifiers, ternary material coating modifiers, positive and negative electrode material conductive agents, positive electrode material lithium supplement agents, negative electrode material pre-lithiation agents, positive or negative electrode material inorganic conductive adhesives or solid electrolyte interface agents.

[0031] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0032] 1. The present invention uses lithium hydroxide monohydrate and pyrophosphoric acid as raw materials, adopts a liquid phase method and simultaneously controls the dripping speed of pyrophosphoric acid to obtain lithium pyrophosphate. Compared with the calcination process, the present invention has a high conversion rate, a single yield of 85.05%, high product purity, low production energy consumption, and is convenient for industrial production.

[0033] 2. The present invention adopts 1,2-propylene glycol for washing. After the water is dried, a small amount of 1,2-propylene glycol is wrapped on the surface of the product to prevent the product from absorbing moisture and agglomerating, and a stable and uniform small particle product can be obtained.

[0034] 3. In response to the continuous improvement of lithium battery performance requirements, the present invention provides a method for preparing lithium pyrophosphate. The obtained lithium pyrophosphate helps to improve the comprehensive performance of the battery, such as impedance, conductivity, and cycle life, and therefore has broad application prospects in the battery field. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0036] Figure 1 This is a flow chart for preparing lithium pyrophosphate provided in Example 1 of the present invention. DETAILED DESCRIPTION

[0037] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention are described clearly and completely below. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0038] Example 1

[0039] like Figure 1 As shown, 2 mol of lithium hydroxide monohydrate and 420 g of pure water were stirred and dissolved in a mass ratio of 1:5 until clear, yielding Solution 1. Pyrophosphoric acid was added dropwise to Solution 1 at a constant rate over 30 minutes. Addition was stopped when the pH reached 7, and a white precipitate formed, yielding Slurry 1. Slurry 1 was filtered to yield Mother Liquor 1 and Lithium Pyrophosphate Wet Material 1. The Lithium Pyrophosphate Mother Liquor was returned to the previous step to replace part of the pure water to dissolve the lithium hydroxide monohydrate. Five times the mass of Lithium Pyrophosphate Wet Material 1 was added to 1,2-propylene glycol, stirred and dissolved for 30 minutes, yielding Slurry 2. Slurry 2 was filtered to yield Lithium Pyrophosphate Wet Material 2 and Mother Liquor 2. Lithium Pyrophosphate Wet Material 2 was dried at 70°C for 2.5 hours and ground to yield 85.9 g of uniformly sized product. Testing confirmed the product to be lithium pyrophosphate. Mother Liquor 2 was heated to 101°C, cooled to room temperature, and then returned to the previous step to dissolve Lithium Pyrophosphate Wet Material 1.

[0040] Example 2

[0041] 2 mol of lithium hydroxide monohydrate and 504 g of pure water were stirred and dissolved in a mass ratio of 1:6 until clear to obtain solution 1. Pyrophosphoric acid was added dropwise to solution 1 at a uniform rate for 40 minutes. When the pH reached 6.5, the addition was stopped and a white precipitate was formed, obtaining slurry 1. Slurry 1 was filtered to obtain mother liquor 1 and lithium pyrophosphate wet material 1. The lithium pyrophosphate mother liquor was returned to the previous step to replace part of the pure water to dissolve lithium hydroxide monohydrate. 4.5 times the mass of lithium pyrophosphate wet material 1 was added to 1,2-propylene glycol and stirred and dissolved for 60 minutes to obtain slurry 2. Slurry 2 was filtered to obtain lithium pyrophosphate wet material 2 and mother liquor 2. The lithium pyrophosphate wet material 2 was dried at 80°C for 2.5 hours and ground to obtain 83.1 g of uniformly granulated product. Mother liquor 2 was heated to 103°C, cooled to room temperature, and then returned to the previous step to dissolve lithium pyrophosphate wet material 1.

[0042] Example 3

[0043] 2 mol of lithium hydroxide monohydrate and 420 g of pure water were stirred and dissolved in a mass ratio of 1:5 until clear to obtain solution 1. Pyrophosphoric acid was added dropwise to solution 1 at a uniform rate for 45 minutes. When the pH reached 7.5, the addition was stopped and a white precipitate was generated to obtain slurry 1. Slurry 1 was filtered to obtain mother liquor 1 and lithium pyrophosphate wet material 1. The lithium pyrophosphate mother liquor was returned to the previous step to replace part of the pure water to dissolve lithium hydroxide monohydrate. 1,2-propylene glycol was added to lithium pyrophosphate wet material 1 in an amount of 4 times its mass and stirred to dissolve for 45 minutes to obtain slurry 2. Slurry 2 was filtered to obtain lithium pyrophosphate wet material 2 and mother liquor 2. The lithium pyrophosphate wet material 2 was dried at 75°C for 3 hours and ground to obtain 86.3 g of product with uniform particles. The mother liquor 2 was heated to 102°C, cooled to room temperature, and then returned to the previous step to dissolve lithium pyrophosphate wet material 1.

[0044] Example 4

[0045] Compared with Example 1, the difference is that the pyrophosphoric acid was added for 60 min. Other steps are the same as those in Example 1 and will not be repeated here.

[0046] Comparative Example 1

[0047] Compared with Example 1, the difference is that the required amount of pyrophosphoric acid is directly added to the lithium hydroxide aqueous solution at one time. Other steps are the same as those in Example 1 and will not be repeated here.

[0048] Comparative Example 2

[0049] Compared with Example 1, the difference is that no 1,2-propylene glycol treatment is used, that is, the lithium pyrophosphate wet material 1 is directly dried and ground to obtain the lithium pyrophosphate product. Other steps are the same as those in Example 1 and will not be repeated here.

[0050] The finished product content is shown in Table 1 below. As can be seen, in terms of product main content and particle size effect, Example 4 has the highest purity and uniform particle size distribution. When pyrophosphoric acid is added all at once in Comparative Example 1, the purity of the lithium pyrophosphate decreases, the particle size distribution is uneven, and there is agglomeration. Comparative Example 2 has a slightly lower purity, a significantly lower particle size uniformity, and severe agglomeration.

[0051] Table 1 Test results of lithium pyrophosphate content obtained in Examples and Comparative Examples

[0052]

[0053] The unit of each impurity content is ppm, and the method for detecting ion content is XRF full analysis method.

[0054] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A method for preparing lithium pyrophosphate, characterized in that: include: Pyrophosphoric acid was added dropwise to the lithium hydroxide aqueous solution, and the addition was stopped when the pH reached 6.5-7.5 to obtain slurry 1; The slurry 1 is subjected to solid-liquid separation, and the obtained solid product is dried and ground to obtain a lithium pyrophosphate product; The pyrophosphoric acid is added for 30-60 minutes. The solid-liquid separation comprises: filtering the slurry 1 to obtain a mother liquor 1 and a lithium pyrophosphate wet material 1; Add polyol to the lithium pyrophosphate wet material 1, stir and dissolve for 30-60 minutes to obtain slurry 2; filter the slurry 2 to obtain lithium pyrophosphate wet material 2 and mother liquor 2; the polyol is one or more of glycerol, 1,2-butanediol and 1,2-propylene glycol.

2. The method for preparing lithium pyrophosphate according to claim 1, wherein The lithium hydroxide aqueous solution is prepared by mixing lithium hydroxide monohydrate and water in a mass ratio of 1:(5-6).

3. The method for preparing lithium pyrophosphate according to claim 1, wherein The amount of the polyol added is 4-5 times the mass of the lithium pyrophosphate wet material 1; And / or, the drying temperature is 70-80° C. and the drying time is 2.5-3 hours.

4. The method for preparing lithium pyrophosphate according to claim 1, wherein The mother liquor 1 is used to prepare the next batch of lithium hydroxide aqueous solution; And / or, the polyol is heated to 101-103° C. to remove water and then reused.

5. The method for preparing lithium pyrophosphate according to any one of claims 1 to 4, characterized in that: The following steps are involved: S1, lithium hydroxide monohydrate and water are mixed in a mass ratio of 1:5-6, and stirred and dissolved until clear to obtain a lithium hydroxide aqueous solution; S2. Add pyrophosphoric acid dropwise to the lithium hydroxide aqueous solution, and stop adding when the pH reaches 6.5-7.5 to obtain slurry 1; S3, filtering the slurry 1 to obtain a mother liquor 1 and a lithium pyrophosphate wet material 1; S4, adding polyol to the lithium pyrophosphate wet material 1, stirring and dissolving for 30-60 minutes to obtain slurry 2; S5, filtering the slurry 2 to obtain a lithium pyrophosphate wet material 2 and a mother liquor 2, drying and grinding the lithium pyrophosphate wet material 2 to obtain a lithium pyrophosphate product; S6. Return the mother liquor 1 to step S1 to replace part of the water for the preparation of the lithium hydroxide aqueous solution, and repeat steps S2 to S5. During the repeated process, the mother liquor 2 is heated to remove water, and after cooling, returns to step S4 to replace part of the polyol for repeated use.

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