Modified lithium iron phosphate, preparation method therefor and use thereof

Abstract

A modified lithium iron phosphate, a preparation method therefor and a use thereof. The preparation method comprises the following steps: (1) mixing a zirconium salt, a tungstate and a solvent, to obtain a mixed salt solution, adjusting the pH, then adding lithium iron phosphate and performing a heating reaction; (2) separating a slurry obtained by the heating reaction into a solid and a liquid, and subjecting the obtained solid to a one-step sintering treatment, to obtain a first sintered material; (3) mixing an organic carbon source, a lithium source and the first sintered material, and performing a second sintering treatment, to obtain a modified lithium iron phosphate. A material having a negative thermal expansion effect is used as a coating layer, which is then coated with an electrically conductive material, causing pores to appear during a second coating process due to the negative thermal expansion effect, which is conducive to the embedding of organic matter and improves bonding tightness between the two layers of coatings. The pores generated during the negative thermal expansion process are conducive to the entry of lithium ions, which can better generate lithium tungstate and lithium zirconate from the tungsten oxide and zirconium oxide generated by decomposition.

Description

A modified lithium iron phosphate and its preparation method and application Technical Field

[0001] The present disclosure belongs to the technical field of lithium-ion batteries and relates to a modified lithium iron phosphate and a preparation method and application thereof. Background Art

[0002] With the advancement of science and technology and the improvement of people's living standards, the demand for battery energy density is increasing. Lithium-ion batteries, due to their high energy density, high voltage, environmental friendliness, and lack of memory effect, have been widely used in electric vehicles, portable devices, and other fields. Among them, lithium iron phosphate batteries, with their high safety and high cycle stability, have been widely used.

[0003] The theoretical gram capacity of lithium iron phosphate cathodes is 170 mAh / g, but in practice, they only utilize about 75% of that theoretical capacity, making them difficult to withstand high current charge and discharge. Currently, a common modification method for lithium iron phosphate is to coat the material with carbon materials, which can significantly improve the material's electronic conductivity. However, relying solely on a single modification method often struggles to achieve ideal results. For example, carbon coating the surface of the material alone does not effectively address the disadvantage of slow ion diffusion.

[0004] CN106848222A discloses a method for preparing a lithium iron phosphate / double carbon layer coated composite material, comprising the following steps: step 1: preparing an iron source, a phosphorus source, and a lithium source in a certain molar ratio, adding a reducing agent and dissolving them, then adding mesoporous carbon, and ultrasonically stirring; step 2: transferring the mixed solution obtained in step 1 to a reactor, performing a hydrothermal reaction, cooling to room temperature after the reaction, washing the reaction product, and air-drying to obtain a composite material; step 3: placing the composite material in a tube furnace, performing a first high-temperature calcination, cooling, removing the composite material, washing, and drying; step 4: dissolving an additive in anhydrous ethanol, adding the composite material obtained in step 3, and stirring to prepare a second mixed solution, placing the second mixed solution in an oven to dry, completely evaporating the solvent, and placing the obtained product under the protection of an inert gas for a second high-temperature calcination, cooling, washing, and drying to obtain a secondary granulated lithium iron phosphate / double carbon layer coated composite material.

[0005] CN114335481A discloses a lithium iron phosphate with an in-situ conductive and ion-conducting double layer coating. The preparation method includes: mixing a fast ion conductor coating source compound, a first lithium source compound, a chelating agent and water to form a colloidal solution, wherein the fast ion conductor coating source compound does not contain lithium; mixing a lithium iron phosphate matrix material with the colloidal solution and then performing a hydrothermal synthesis reaction to obtain a lithium iron phosphate precursor coated with a fast ion conductor, wherein the lithium iron phosphate matrix material includes a second lithium source compound, a phosphorus source compound and an iron source compound; and reacting the fast ion conductor coated lithium iron phosphate precursor with a carbon source coating to obtain a lithium iron phosphate with an in-situ conductive and ion-conducting double layer coating.

[0006] The double-layer coating described in the above solution is difficult to tightly bond, which greatly reduces the structural stability of the material and thus its cycling stability. Therefore, there is an urgent need to find a more efficient composite method to improve the electrochemical performance of lithium iron phosphate positive electrode materials while increasing the tightness of the coating layer bonding, so that the lithium iron phosphate material can better utilize the material's energy density and ensure its long-term cycling stability.

[0007] Summary of the Invention

[0008] The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

[0009] The present disclosure aims to provide a modified lithium iron phosphate, its preparation method, and application. This disclosure utilizes a material exhibiting a negative thermal expansion effect as a coating layer, followed by a conductive coating. This secondary coating process creates pores due to the negative thermal expansion effect, facilitating the embedding of organic matter. This allows for a tight bond between the two coating layers, and a tight bond between the coating layer and the core, ensuring the material's cycling performance. Furthermore, the pores created during the negative thermal expansion process facilitate the ingress of lithium ions, allowing the decomposed tungsten oxide and zirconium oxide to be more effectively converted into lithium tungstate and lithium zirconate.

[0010] To achieve this purpose, the present disclosure adopts the following technical solutions:

[0011] In a first aspect, the present disclosure provides a method for preparing modified lithium iron phosphate, the preparation method comprising the following steps:

[0012] (1) zirconium salt, tungstate and solvent are mixed to obtain a mixed salt solution, lithium iron phosphate is added after adjusting the pH, and heating reaction is carried out;

[0013] (2) after solid-liquid separation of the slurry obtained by the heating reaction, the obtained solid is subjected to a one-step sintering treatment to obtain a single-fired material;

[0014] (3) Mixing an organic carbon source, a lithium source and the first-sintered material, and subjecting the mixture to a two-step sintering process to obtain the modified lithium iron phosphate.

[0015] The present invention pre-compounds zirconium tungstate on the surface of lithium iron phosphate. The zirconium tungstate gradually shrinks thermally during the secondary sintering process and then decomposes into tungsten oxide and zirconium oxide. The tungsten oxide and zirconium oxide react with a lithium source to generate lithium tungstate and lithium zirconate, which can replenish lithium during the cycle and replenish the lattice lithium precipitated during the solution coating process, thereby increasing the gram capacity of the material. The organic matter present in the thermal shrinkage gaps of the zirconium tungstate can generate a bridging effect with the zirconium oxide produced after decomposition, thereby effectively alleviating the loss of gram capacity and reducing the side reaction between the material and the electrolyte.

[0016] In one embodiment, the zirconium salt in step (1) comprises zirconyl nitrate.

[0017] In one embodiment, the tungstate salt comprises ammonium tungstate.

[0018] In one embodiment, the molar ratio of the zirconium element in the zirconium salt to the tungstate in the tungstate is 1:(1.8-2.2), for example, 1:1.8, 1:1.9, 1:2, 1:2.1 or 1:2.2.

[0019] In one embodiment, the concentration of the mixed salt solution is 0.1 to 0.5 mol / L, for example, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L or 0.5 mol / L.

[0020] In one embodiment, the mixing stirring time is 2 to 5 hours, for example, 2 hours, 2.5 hours, 3 hours, 4 hours or 5 hours.

[0021] In one embodiment, the pH in step (1) is 4 to 6, for example, 4, 4.5, 5, 5.5 or 6.

[0022] In one embodiment, the solid-to-liquid ratio of the lithium iron phosphate and the mixed salt solution is 0.01 to 0.1 g / mL, for example, 0.01 g / mL, 0.02 g / mL, 0.05 g / mL, 0.08 g / mL or 0.1 g / mL.

[0023] In one embodiment, the ultrasonic treatment is performed for 20 to 60 minutes after the lithium iron phosphate is added, for example, 20 minutes, 30 minutes, 40 minutes, 50 minutes or 60 minutes.

[0024] In one embodiment, the temperature of the heating reaction in step (1) is 120-180°C, for example, 120°C, 130°C, 150°C, 160°C or 180°C.

[0025] In one embodiment, the heating reaction time is 6 to 12 hours, for example, 6 hours, 7 hours, 8 hours, 10 hours or 12 hours.

[0026] In one embodiment, the temperature of the one-step sintering treatment in step (2) is 500-600°C, for example, 500°C, 520°C, 550°C, 580°C or 600°C.

[0027] In one embodiment, the one-step sintering treatment takes 3 to 5 hours, for example, 3 hours, 3.5 hours, 4 hours, 4.5 hours or 5 hours.

[0028] In one embodiment, the organic carbon source in step (3) comprises any one of polyvinyl alcohol, polyvinyl butyral, petroleum asphalt, coal asphalt, building asphalt, sucrose, glucose or starch, or a combination of at least two thereof.

[0029] In one embodiment, the organic carbon source is dissolved in a solvent.

[0030] In one embodiment, the mass of the organic carbon source is 2-10% of the mass of the calcined material, for example, 2%, 4%, 6%, 8% or 10%.

[0031] In one embodiment, the solvent includes any one of ethanol, deionized water, acetone, kerosene, ethanol, carbon tetrachloride or benzene, or a combination of at least two thereof.

[0032] In one embodiment, the lithium source comprises lithium hydroxide and / or lithium carbonate.

[0033] In one embodiment, the molar ratio of the lithium element in the lithium source to the zirconium element in the zirconium salt is (2.05-2.2):1, for example: 2.05:1, 2.08:1, 2.1:1, 2.15:1 or 2.2:1.

[0034] In one embodiment, the atmosphere of the two-step sintering treatment in step (3) includes any one of nitrogen, argon or carbon dioxide, or a combination of at least two of them.

[0035] In one embodiment, the two-step sintering process includes a first-stage sintering and a second-stage sintering.

[0036] In one embodiment, the temperature of the first stage sintering is 400-600°C, for example, 400°C, 450°C, 500°C, 550°C or 600°C.

[0037] In one embodiment, the sintering time is 2 to 5 hours, for example, 2 hours, 2.5 hours, 3 hours, 4 hours or 5 hours.

[0038] In one embodiment, the temperature of the second stage sintering is 600-850°C, for example, 600°C, 650°C, 700°C, 800°C or 850°C.

[0039] In one embodiment, the second-stage sintering time is 5 to 12 hours, for example, 5 hours, 8 hours, 9 hours, 10 hours or 12 hours.

[0040] In a second aspect, the present disclosure provides a modified lithium iron phosphate, which is prepared by the method described in the first aspect.

[0041] In a third aspect, the present disclosure provides a positive electrode plate, wherein the positive electrode plate comprises the modified lithium iron phosphate as described in the second aspect.

[0042] In a fourth aspect, the present disclosure provides a lithium-ion battery, comprising the positive electrode sheet as described in the third aspect.

[0043] Compared with the prior art, the present disclosure has the following beneficial effects:

[0044] (1) The present disclosure utilizes a material with a negative thermal expansion effect as a coating layer, followed by a conductive coating. Pores are formed during the secondary coating process due to the negative thermal expansion effect, which facilitates the embedding of organic matter, allowing for a tight bond between the two coating layers and a tight bond between the coating layer and the core, thereby ensuring the material's cycling performance. Furthermore, the pores generated during the negative thermal expansion process facilitate the entry of lithium ions, allowing the decomposed tungsten oxide and zirconium oxide to better generate lithium tungstate and lithium zirconate.

[0045] (2) The 0.1C discharge specific capacity of the battery made of lithium iron phosphate disclosed in the present invention can reach more than 164.69 mAh / g, and the capacity retention rate after 100 cycles can reach more than 99.65%.

[0046] Still other aspects will become apparent upon reading and understanding the detailed description. DETAILED DESCRIPTION

[0047] The technical solution of the present disclosure is further described below through specific implementation methods. Those skilled in the art should understand that the embodiments are only used to help understand the present disclosure and should not be regarded as specific limitations of the present disclosure.

[0048] The lithium iron phosphate used in the embodiments and comparative examples of the present disclosure was prepared by the following method:

[0049] The ferrophosphorus precursor was mixed with lithium carbonate, and calcined at 450° C. for 6 h and then at 720° C. for 8 h in a nitrogen atmosphere to obtain lithium iron phosphate.

[0050] Example 1

[0051] This embodiment provides a modified lithium iron phosphate, and the preparation method of the modified lithium iron phosphate is as follows:

[0052] (1) preparing an aqueous solution of zirconium oxynitrate and ammonium tungstate at a concentration of 0.2 mol / L, mixing the two aqueous solutions in a molar ratio of zirconium ion: tungsten ion = 1:2, stirring for 3 h, then adding an acid solution to adjust the pH to 5, transferring the mixed solution to a reactor, adding lithium iron phosphate powder at a solid-liquid ratio of 0.01 g / mL, ultrasonically dispersing for 20 min, and then heating at 120° C. for 12 h for heating reaction to obtain a primary coated lithium iron phosphate slurry;

[0053] (2) filtering and washing the slurry obtained by the heating reaction, and then sintering it at 500°C for 5 hours under vacuum to obtain a sintered material;

[0054] (3) A glucose aqueous solution is uniformly mixed with lithium hydroxide and a calcined material, wherein the amount of glucose is 2% of the mass of the calcined material. After the mixture is uniformly mixed, it is sintered at 450° C. for 3 hours under a nitrogen atmosphere, and then roasted at 800° C. for 6 hours. After roasting, the mixture is cooled to obtain the modified lithium iron phosphate. The lithium source is lithium hydroxide, and the molar ratio of lithium ions to zirconium in the lithium source is 2.08:1.

[0055] Example 2

[0056] This embodiment provides a modified lithium iron phosphate, and the preparation method of the modified lithium iron phosphate is as follows:

[0057] (1) preparing an aqueous solution of zirconium oxynitrate and ammonium tungstate at a concentration of 0.1 mol / L, mixing the two aqueous solutions in a molar ratio of zirconium ion:tungsten ion = 1:2, stirring for 3 h, then adding an acid solution to adjust the pH to 4, transferring the mixed solution to a reactor, adding lithium iron phosphate powder at a solid-liquid ratio of 0.05 g / ml, ultrasonically dispersing for 30 min, and then heating at 120° C. for 12 h for heating reaction to obtain a primary coated lithium iron phosphate slurry;

[0058] (2) filtering and washing the slurry obtained by the heating reaction, and then sintering it at 550°C for 4 hours under vacuum to obtain a sintered material;

[0059] (3) Evenly mixing a starch aqueous solution with lithium hydroxide and a calcined material, wherein the starch content is 5% of the mass of the calcined material, sintering the mixture at 400° C. for 5 hours under a nitrogen atmosphere, and then calcining at 700° C. for 10 hours. After calcination, the mixture is cooled to obtain the modified lithium iron phosphate. The lithium source is lithium hydroxide, and the molar ratio of lithium ions to zirconium in the lithium source is 2.15:1.

[0060] Example 3

[0061] This embodiment provides a modified lithium iron phosphate, and the preparation method of the modified lithium iron phosphate is as follows:

[0062] (1) preparing an aqueous solution of zirconyl nitrate and ammonium tungstate at a concentration of 0.5 mol / L, mixing the two aqueous solutions in a molar ratio of zirconium ion:tungsten ion = 1:2, stirring for 5 h, then adding an acid solution to adjust the pH to 6, transferring the mixed solution to a reactor, adding lithium iron phosphate powder at a solid-liquid ratio of 0.1 g / ml, ultrasonically dispersing for 60 min, and then heating at 180° C. for 6 h to react, to obtain a primary coated lithium iron phosphate slurry;

[0063] (2) filtering and washing the slurry obtained by the heating reaction, and then sintering it at 600°C for 3 hours under vacuum to obtain a sintered material;

[0064] (3) The starch aqueous solution is uniformly mixed with lithium hydroxide and a calcined material, wherein the starch content is 10% of the mass of the calcined material. The mixture is uniformly mixed and then sintered at 600° C. for 2 hours under a nitrogen atmosphere, and then roasted at 850° C. for 5 hours. After roasting, the mixture is cooled to obtain the modified lithium iron phosphate. The lithium source is lithium hydroxide, and the molar ratio of lithium ions to zirconium in the lithium source is 2.2:1.

[0065] Example 4

[0066] The only difference between this embodiment and embodiment 1 is that the concentration of the mixed salt solution in step (1) is 0.05 mol / L, and the other conditions and parameters are exactly the same as those in embodiment 1.

[0067] Example 5

[0068] The only difference between this embodiment and embodiment 1 is that the concentration of the mixed salt solution in step (1) is 0.8 mol / L, and the other conditions and parameters are exactly the same as those in embodiment 1.

[0069] Example 6

[0070] The only difference between this embodiment and embodiment 1 is that the temperature of the sintering treatment in step (2) is 400° C., and the other conditions and parameters are exactly the same as those in embodiment 1.

[0071] Example 7

[0072] The only difference between this embodiment and embodiment 1 is that the temperature of the sintering treatment in step (2) is 700° C., and the other conditions and parameters are exactly the same as those in embodiment 1.

[0073] Comparative Example 1

[0074] The only difference between this comparative example and Example 1 is that zirconium salt and tungstate are not added, and other conditions and parameters are exactly the same as those in Example 1.

[0075] Comparative Example 2

[0076] The only difference between this comparative example and Example 1 is that no lithium source is added in step (3), and the other conditions and parameters are exactly the same as those in Example 1.

[0077] Comparative Example 3

[0078] The only difference between this comparative example and Example 1 is that no organic carbon source is added in step (3), and the other conditions and parameters are exactly the same as those in Example 1.

[0079] Performance testing:

[0080] The lithium iron phosphate prepared in the examples and comparative examples, the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 90:5:5 with N-methylpyrrolidone (NMP) as the solvent, and then coated on aluminum foil. After drying, the mixture was rolled to form a simulated battery positive electrode. The negative electrode was a metal lithium sheet, the separator was Celgard 2400, and the electrolyte was 1 mol / L LiPF6 / DMC+DEC (volume ratio was 1:1), forming a CR2025 type simulated battery. The charge and discharge voltage range was 2.9 to 3.7 V, and the obtained lithium-ion battery positive electrode material and lithium iron phosphate electrochemical performance data were shown in Table 1:

[0081] Table 1

[0082] As can be seen from Table 1, from Examples 1-3, the 0.1C discharge specific capacity of the battery made of lithium iron phosphate disclosed in the present disclosure can reach more than 164.69 mAh / g, and the 100-cycle capacity retention rate can reach more than 99.65%.

[0083] By comparing Example 1 with Examples 4-5, it can be seen that in the preparation process of the modified lithium iron phosphate described in the present disclosure, the concentration of the mixed salt solution of zirconium salt and tungstate will affect its performance. The concentration of the mixed salt solution is controlled at 0.1-0.5 mol / L, and the performance of the modified iron phosphate obtained is better. If the concentration of the mixed salt solution is too high, the zirconium tungstate layer generated on the surface will be thicker under the same reaction time, and an excessively thick zirconium tungstate layer will reduce the capacity of the material. If the concentration of the mixed salt solution is too low, the zirconium tungstate layer generated on the surface will be thinner under the same reaction time, and an excessively thin zirconium tungstate layer will make it difficult to fully connect the carbon coating layer with the lithium iron phosphate core, affecting the cycle stability of the material.

[0084] By comparing Example 1 with Examples 6-7, it can be seen that in the preparation process of the modified lithium iron phosphate disclosed in the present invention, the temperature of the sintering treatment in step (2) will affect its performance. The temperature of the sintering treatment in step (2) is controlled at 500-600°C, and the performance of the modified iron phosphate obtained is better. If the temperature of the sintering treatment in step (2) is too high, zirconium tungstate may also be decomposed, but it will cause energy waste. If the temperature of the sintering treatment in step (2) is too low, zirconium tungstate cannot be decomposed into tungsten oxide and zirconium oxide. Due to its thermal contraction and cold expansion properties, cracks will appear on the surface of the particles after the reaction is completed, affecting the coating tightness, thereby reducing the material cycle stability.

[0085] From the comparison between Example 1 and Comparative Examples 1-2, it can be seen that the present invention pre-compounds zirconium tungstate on the surface of lithium iron phosphate. The zirconium tungstate gradually shrinks thermally during the secondary sintering process and then decomposes into tungsten oxide and zirconium oxide. The tungsten oxide and zirconium oxide react with the lithium source to generate lithium tungstate and lithium zirconate, which can replenish lithium during the cycle and replenish the lattice lithium precipitated during the solution coating process, thereby increasing the material's gram capacity.

[0086] From the comparison between Example 1 and Comparative Example 3, it can be seen that the organic carbon source added in the present invention, the organic carbon source present in the thermal shrinkage gap of zirconium tungstate, can produce a bridging effect with the zirconium oxide produced after decomposition, which effectively alleviates the loss of gram capacity and can effectively reduce the side reaction between the material and the electrolyte.

Claims

1. A method for preparing modified lithium iron phosphate, comprising the following steps: (1) mixing zirconium salt, tungstate and solvent to obtain a mixed salt solution, adding lithium iron phosphate after adjusting the pH, and heating to react; (2) after solid-liquid separation of the slurry obtained by the heating reaction, the obtained solid is subjected to a one-step sintering treatment to obtain a sintered material; (3) Mixing an organic carbon source, a lithium source and the first-sintered material, and subjecting the mixture to a two-step sintering treatment to obtain the modified lithium iron phosphate.

2. The preparation method according to claim 1, wherein The zirconium salt in step (1) includes zirconyl nitrate.

3. The preparation method according to claim 1 or 2, wherein: The tungstate includes ammonium tungstate.

4. The preparation method according to any one of claims 1 to 3, wherein The molar ratio of the zirconium element in the zirconium salt to the tungstate in the tungstate is 1:(1.8-2.2).

5. The preparation method according to any one of claims 1 to 4, wherein: The concentration of the mixed salt solution is 0.1-0.5 mol / L.

6. The preparation method according to any one of claims 1 to 5, wherein: The mixing time is 2 to 5 hours.

7. The preparation method according to any one of claims 1 to 6, wherein: The pH of step (1) is 4-6.

8. The preparation method according to any one of claims 1 to 7, wherein: The solid-to-liquid ratio of the lithium iron phosphate and the mixed salt solution is 0.01-0.1 g / mL.

9. The preparation method according to any one of claims 1 to 8, wherein: After adding the lithium iron phosphate, ultrasonic treatment is performed for 20 to 60 minutes.

10. The preparation method according to any one of claims 1 to 9, wherein: The temperature of the heating reaction in step (1) is 120-180° C. Optionally, the heating reaction time is 6 to 12 hours.

11. The preparation method according to any one of claims 1 to 10, wherein: The temperature of the one-step sintering treatment in step (2) is 500-600° C. Optionally, the one-step sintering process takes 3 to 5 hours.

12. The preparation method according to any one of claims 1 to 11, wherein: The organic carbon source in step (3) comprises any one of polyvinyl alcohol, polyvinyl butyral, petroleum asphalt, coal asphalt, building asphalt, sucrose, glucose or starch, or a combination of at least two thereof; Optionally, the organic carbon source is dissolved in a solvent; Optionally, the mass of the organic carbon source is 2-10% of the mass of the calcined material.

13. The preparation method according to any one of claims 1 to 12, wherein: The solvent includes any one of ethanol, deionized water, acetone, kerosene, ethanol, carbon tetrachloride or benzene, or a combination of at least two thereof.

14. The preparation method according to any one of claims 1 to 13, wherein: The lithium source includes lithium hydroxide and / or lithium carbonate; Optionally, the molar ratio of the lithium element in the lithium source to the zirconium element in the zirconium salt is (2.05-2.2):

1.

15. The preparation method according to any one of claims 1 to 14, wherein: The atmosphere of the two-step sintering treatment in step (3) includes any one of nitrogen, argon or carbon dioxide, or a combination of at least two of them; Optionally, the two-step sintering process includes a first-stage sintering and a second-stage sintering; Optionally, the temperature of the first stage sintering is 400-600°C; Optionally, the sintering time is 2 to 5 hours; Optionally, the temperature of the second stage sintering is 600-850°C; Optionally, the second-stage sintering time is 5 to 12 hours.

16. A modified lithium iron phosphate prepared by the method according to any one of claims 1 to 15.

17. A positive electrode sheet comprising the modified lithium iron phosphate as claimed in claim 16.

18. A lithium ion battery comprising the positive electrode sheet according to claim 17.

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