A modified lithium iron phosphate, its preparation method and application

Abstract

The present disclosure provides a modified lithium iron phosphate, a preparation method thereof, and an application thereof. The preparation method includes the following steps: (1) mixing a zirconium salt, a tungstate, and a solvent to obtain a mixed salt solution, adjusting the pH, adding lithium iron phosphate, and performing a heating reaction; (2) after solid-liquid separation of the slurry obtained from the heating reaction, performing a one-step sintering treatment on the obtained solid to obtain a first-sintered material; (3) mixing an organic carbon source, a lithium source, and the first-sintered material, and performing a two-step sintering treatment to obtain the modified lithium iron phosphate. The present disclosure uses a material with a negative thermal expansion effect as a layer of coating, and then performs a conductive substance coating. During the secondary coating process, pores appear due to the negative thermal expansion effect, which is beneficial to the embedding of organic substances, improves the binding tightness between the double-layer coatings, and the pores generated during the negative thermal expansion process are beneficial to the entry of lithium ions, and can make tungsten oxide and zirconium oxide generated by decomposition better generate lithium tungstate and lithium zirconate.

Description

Technical Field

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

[0002] With the development of science and technology and the improvement of people's living standards, the requirements for battery energy density are getting higher and higher. Lithium-ion batteries have been applied in fields such as electric vehicles and portable devices due to their advantages of high energy density, high voltage, environmental friendliness, and no memory effect. Among them, lithium iron phosphate batteries have been widely applied due to their characteristics of high safety and high cycle stability.

[0003] The theoretical specific capacity of the lithium iron phosphate cathode is 170 mAh / g, but in actual application, it can only exert about 75% of the theoretical capacity and is difficult to withstand large current charge and discharge. In the current modification work on lithium iron phosphate, a relatively common method is to coat the material with carbon materials, which can significantly improve the electronic conductivity of the material. However, if only a single modification method is used, it is often difficult to achieve an ideal effect. For example, if only the surface of the material is coated with carbon, the disadvantage of slow ion diffusion rate cannot be well improved.

[0004] CN106848222A discloses a preparation method of a lithium iron phosphate / double carbon layer coated composite material, including the following steps: Step 1: Prepare iron source, phosphorus source, and lithium source with a certain molar ratio, add a reducing agent and dissolve, then add mesoporous carbon, and perform ultrasonic stirring; Step 2: Transfer the mixed solution prepared in Step 1 to a reaction kettle for hydrothermal reaction, cool to room temperature after the reaction, and wash and drum dry the reaction product to obtain the composite material; Step 3: Place the composite material in a tubular furnace, perform the first high-temperature calcination, cool and take out, wash and dry; Step 4: Dissolve an additive in absolute ethanol, add the composite material prepared in Step 3, and prepare a second mixed solution after stirring. Place the second mixed solution in an oven to dry, completely evaporate the solvent, and place the obtained product under the protection of an inert gas for the second high-temperature calcination, cool, wash, and dry to obtain a secondary granulated lithium iron phosphate / double carbon layer coated composite material.

[0005] CN114335481A discloses a lithium iron phosphate coated in-situ with a conductive and ion-conductive double layer. The preparation method thereof 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 element; 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. The lithium iron phosphate matrix material includes a second lithium source compound, a phosphorus source compound and an iron source compound; subjecting the lithium iron phosphate precursor coated with a fast ion conductor to a carbon source coating reaction to obtain the lithium iron phosphate coated in-situ with a conductive and ion-conductive double layer.

[0006] In the above solution, it is difficult for the coating layers of the double-layer coating to be tightly combined, which greatly reduces the structural stability of the material and further reduces its cycling stability. Therefore, there is an urgent need to find a more efficient composite method to improve the electrochemical performance of the lithium iron phosphate cathode material while increasing the tightness of the coating layer combination, which can further enable the lithium iron phosphate material to better exert its energy density and ensure its long-term cycling stability. Summary of the Invention

[0007] The following is an overview of the subject matter described in detail in this article. This overview is not intended to limit the scope of protection of the claims.

[0008] The purpose of the present disclosure is to provide a modified lithium iron phosphate, its preparation method and application. The present disclosure uses a material with a negative thermal expansion effect as one layer of coating, and then conducts a conductive substance coating. During the secondary coating process, pores appear due to the negative thermal expansion effect, which is beneficial to the embedding of organic substances, enabling the double-layer coating to be tightly combined, and the coating layer to be tightly combined with the core, and ensuring the cycling performance of the material. Moreover, the pores generated during the negative thermal expansion process are beneficial to the entry of lithium ions, enabling tungsten oxide and zirconium oxide generated by decomposition to better form lithium tungstate and lithium zirconate.

[0009] To achieve this purpose, the present disclosure adopts the following technical solutions:

[0010] In the first aspect, the present disclosure provides a preparation method of a modified lithium iron phosphate. The preparation method includes the following steps:

[0011] (1) Mix a zirconium salt, a tungstate and a solvent to obtain a mixed salt solution. After adjusting the pH, add lithium iron phosphate and carry out a heating reaction;

[0012] (2) After solid-liquid separation of the slurry obtained from the heating reaction, perform a one-step sintering treatment on the obtained solid to obtain a first-sintered material;

[0013] (3) Mix an organic carbon source, a lithium source and the first-sintered material, and perform a two-step sintering treatment to obtain the modified lithium iron phosphate.

[0014] In the present disclosure, zirconium tungstate is pre-compounded on the surface of lithium iron phosphate. During the secondary sintering process, zirconium tungstate gradually undergoes thermal shrinkage and then decomposes into tungsten oxide and zirconium oxide. Tungsten oxide and zirconium oxide react with the lithium source to generate lithium tungstate and lithium zirconate, which can achieve the effect of lithium compensation during the cycling process and can supplement the lattice lithium precipitated during the solution coating process, thereby increasing the specific capacity of the material. The organic matter present in the thermal shrinkage gaps of zirconium tungstate can produce a bridging effect with the zirconium oxide produced after decomposition, which can well alleviate the loss of specific capacity and can well reduce the side reactions between the material and the electrolyte.

[0015] In one embodiment, the zirconium salt in step (1) includes zirconyl nitrate.

[0016] In one embodiment, the tungstate includes ammonium tungstate.

[0017] In one embodiment, the molar ratio of zirconium element in the zirconium salt to tungstate radical 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, etc.

[0018] In one embodiment, the concentration of the mixed salt solution is 0.1 - 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, etc.

[0019] In one embodiment, the stirring time for the mixing is 2 - 5 h, for example: 2 h, 2.5 h, 3 h, 4 h or 5 h, etc.

[0020] In one embodiment, the pH in step (1) is 4 - 6, for example: 4, 4.5, 5, 5.5 or 6, etc.

[0021] In one embodiment, the solid-liquid ratio of the lithium iron phosphate and the mixed salt solution is 0.01 - 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, etc.

[0022] In one embodiment, after adding the lithium iron phosphate, ultrasonic treatment is carried out for 20 - 60 min, for example: 20 min, 30 min, 40 min, 50 min or 60 min, etc.

[0023] 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, etc.

[0024] In one embodiment, the time of the heating reaction is 6 to 12 h, for example: 6 h, 7 h, 8 h, 10 h or 12 h, etc.

[0025] In one embodiment, the temperature of the one-step sintering treatment in step (2) is 500 to 600 °C, for example: 500 °C, 520 °C, 550 °C, 580 °C or 600 °C, etc.

[0026] In one embodiment, the time of the one-step sintering treatment is 3 to 5 h, for example: 3 h, 3.5 h, 4 h, 4.5 h or 5 h, etc.

[0027] In one embodiment, the organic carbon source in step (3) includes any one or a combination of at least two of polyvinyl alcohol, polyvinyl butyral, petroleum pitch, coal pitch, building asphalt, sucrose, glucose or starch.

[0028] In one embodiment, the organic carbon source is dissolved in a solvent.

[0029] In one embodiment, the mass of the organic carbon source is 2 to 10% of the mass of the first-fired material, for example: 2%, 4%, 6%, 8% or 10%, etc.

[0030] In one embodiment, the solvent includes any one or a combination of at least two of ethanol, deionized water, acetone, kerosene, ethanol, carbon tetrachloride or benzene.

[0031] In one embodiment, the lithium source includes lithium hydroxide and / or lithium carbonate.

[0032] In one embodiment, the molar ratio of lithium element in the lithium source to zirconium element in the zirconium salt is (2.05 to 2.2):1, for example: 2.05:1, 2.08:1, 2.1:1, 2.15:1 or 2.2:1, etc.

[0033] In one embodiment, the atmosphere of the two-step sintering treatment in step (3) includes any one or a combination of at least two of nitrogen, argon or carbon dioxide.

[0034] In one embodiment, the two-step sintering treatment includes one-stage sintering and two-stage sintering.

[0035] In one embodiment, the temperature of the one-stage sintering is 400 to 600 °C, for example: 400 °C, 450 °C, 500 °C, 550 °C or 600 °C, etc.

[0036] In one embodiment, the time of the one-stage sintering is 2 to 5 h, for example: 2 h, 2.5 h, 3 h, 4 h or 5 h, etc.

[0037] In one embodiment, the temperature of the two-stage sintering is 600 to 850 °C, for example: 600 °C, 650 °C, 700 °C, 800 °C, or 850 °C, etc.

[0038] In one embodiment, the time of the two-stage sintering is 5 to 12 h, for example: 5 h, 8 h, 9 h, 10 h, or 12 h, etc.

[0039] In a second aspect, the present disclosure provides a modified lithium iron phosphate, which is prepared by the method as described in the first aspect.

[0040] In a third aspect, the present disclosure provides a positive electrode sheet, which comprises the modified lithium iron phosphate as described in the second aspect.

[0041] In a fourth aspect, the present disclosure provides a lithium-ion battery, which comprises the positive electrode sheet as described in the third aspect.

[0042] Compared with the prior art, the present disclosure has the following beneficial effects:

[0043] (1) The present disclosure uses a material with a negative thermal expansion effect as a layer of coating, and then conducts a coating of a conductive substance. During the secondary coating process, pores appear due to the negative thermal expansion effect, which is beneficial to the embedding of organic substances, enables the double-layer coatings to be tightly combined, and the coating layer and the core to be tightly combined, and can ensure the cycling performance of the material. Moreover, the pores generated during the negative thermal expansion process are beneficial to the entry of lithium ions, and can better generate lithium tungstate and lithium zirconate from the decomposed tungsten oxide and zirconium oxide.

[0044] (2) The discharge specific capacity of the battery prepared from the lithium iron phosphate of the present disclosure can reach more than 164.69 mAh / g at 0.1C, and the capacity retention rate after 100 cycles can reach more than 99.65%.

[0045] Other aspects can be understood after reading and understanding the detailed description. Specific Embodiments

[0046] The technical solutions of the present disclosure will be further described below through specific embodiments. Those skilled in the art should understand that the embodiments are only for helping to understand the present disclosure and should not be regarded as specific limitations on the present disclosure.

[0047] The lithium iron phosphate used in the examples and comparative examples of the present disclosure was all prepared by the following method:

[0048] Mix the phosphorus-iron precursor with lithium carbonate, and first calcine at 450 °C for 6 h under a nitrogen atmosphere, and then calcine at 720 °C for 8 h to obtain lithium iron phosphate.

[0049] Example 1

[0050] This embodiment provides a modified lithium iron phosphate, and the preparation method of the modified lithium iron phosphate is as follows:

[0051] (1) Prepare aqueous solutions of zirconyl nitrate and ammonium tungstate with a concentration of 0.2 mol / L. Mix the two aqueous solutions according to the molar ratio of zirconium ions:tungsten ions = 1:2, stir for 3 h, then add an acid solution to adjust the pH to 5. Transfer the mixed solution to a reaction kettle, add lithium iron phosphate powder, with a solid-liquid ratio of 0.01 g / mL, ultrasonically disperse for 20 min, and then heat at 120 °C for 12 h for a heating reaction to obtain a once-coated lithium iron phosphate slurry;

[0052] (2) After filtering and washing the slurry obtained from the heating reaction, sinter it at 500 °C under vacuum for 5 h to obtain a first-fired material;

[0053] (3) Mix the aqueous glucose solution, lithium hydroxide, and the first-fired material evenly. The dosage of glucose is 2% of the mass of the first-fired material. After mixing the above mixture evenly, sinter it at 450 °C for 3 h under a nitrogen atmosphere, then calcine it at 800 °C for 6 h, and cool it after calcination 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.

[0054] Example 2

[0055] This embodiment provides a modified lithium iron phosphate, and the preparation method of the modified lithium iron phosphate is as follows:

[0056] (1) Prepare aqueous solutions of zirconyl nitrate and ammonium tungstate with a concentration of 0.1 mol / L. Mix the two aqueous solutions according to the molar ratio of zirconium ions:tungsten ions = 1:2, stir for 3 h, then add an acid solution to adjust the pH to 4. Transfer the mixed solution to a reaction kettle, add lithium iron phosphate powder, with a solid-liquid ratio of 0.05 g / ml, ultrasonically disperse for 30 min, and then heat at 120 °C for 12 h for a heating reaction to obtain a once-coated lithium iron phosphate slurry;

[0057] (2) After filtering and washing the slurry obtained from the heating reaction, sinter it at 550 °C under vacuum for 4 h to obtain a first-fired material;

[0058] (3) Mix the aqueous starch solution, lithium hydroxide, and the first-fired material evenly. The dosage of starch is 5% of the mass of the first-fired material. After mixing the above mixture evenly, sinter it at 400 °C for 5 h under a nitrogen atmosphere, then calcine it at 700 °C for 10 h, and cool it after calcination 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.

[0059] Example 3

[0060] This embodiment provides a modified lithium iron phosphate, and the preparation method of the modified lithium iron phosphate is as follows:

[0061] (1) Prepare an aqueous solution of zirconyl nitrate and ammonium tungstate with a concentration of 0.5 mol / L. Mix the two aqueous solutions according to the molar ratio of zirconium ions:tungsten ions = 1:2, stir for 5 h, then add an acid solution to adjust the pH to 6. Transfer the mixed solution to a reaction kettle, add lithium iron phosphate powder with a solid-liquid ratio of 0.1 g / ml, ultrasonically disperse for 60 min, and then heat and react at 180 °C for 6 h to obtain a once-coated lithium iron phosphate slurry;

[0062] (2) After filtering and washing the slurry obtained from the heat reaction, sinter at 600 °C under vacuum for 3 h to obtain a first-fired material;

[0063] (3) Mix the aqueous starch solution, lithium hydroxide and the first-fired material evenly. The amount of starch used is 10% of the mass of the first-fired material. After mixing the above mixture evenly, sinter at 600 °C for 2 h under a nitrogen atmosphere, then calcine at 850 °C for 5 h, and cool after calcination 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.

[0064] Example 4

[0065] The difference between this example and Example 1 is only that the concentration of the mixed salt solution in step (1) is 0.05 mol / L, and other conditions and parameters are exactly the same as those in Example 1.

[0066] Example 5

[0067] The difference between this example and Example 1 is only that the concentration of the mixed salt solution in step (1) is 0.8 mol / L, and other conditions and parameters are exactly the same as those in Example 1.

[0068] Example 6

[0069] The difference between this example and Example 1 is only that the sintering temperature in step (2) is 400 °C, and other conditions and parameters are exactly the same as those in Example 1.

[0070] Example 7

[0071] The difference between this example and Example 1 is only that the sintering temperature in step (2) is 700 °C, and other conditions and parameters are exactly the same as those in Example 1.

[0072] Comparative Example 1

[0073] The difference between this comparative example and Example 1 is only that zirconium salt and tungstate are not added, and other conditions and parameters are exactly the same as those in Example 1.

[0074] Comparative Example 2

[0075] The difference between this comparative example and Example 1 is only that no lithium source is added in step (3), and other conditions and parameters are exactly the same as those in Example 1.

[0076] Comparative Example 3

[0077] The difference between this comparative example and Example 1 is only that no organic carbon source is added in step (3), and other conditions and parameters are exactly the same as those in Example 1.

[0078] Performance test:

[0079] The lithium iron phosphate, conductive agent acetylene black, and binder polyvinylidene fluoride PVDF prepared in the examples and comparative examples were mixed evenly at a mass ratio of 90:5:5 using N-methylpyrrolidone NMP as a solvent, coated on aluminum foil, dried, and rolled to be used as the positive electrode of a simulated battery. The negative electrode was a lithium metal sheet, the separator was Celgard 2400, and the electrolyte was 1 mol / L LiPF6 / DMC + DEC (volume ratio 1:1) to form a CR2025-type simulated battery. The charge-discharge voltage range was 2.9 - 3.7 V, and the electrochemical performance data of the lithium iron phosphate, the positive electrode material of the lithium-ion battery, were obtained. The test results are shown in Table 1:

[0080] Table 1

[0081]

[0082]

[0083] It can be seen from Table 1 that from Examples 1 - 3, the 0.1C discharge specific capacity of the battery prepared from the lithium iron phosphate described in the present disclosure can reach above 164.69 mAh / g, and the 100-cycle capacity retention rate can reach above 99.65%.

[0084] From the comparison between Example 1 and Examples 4 - 5, it can be obtained 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. Controlling the concentration of the mixed salt solution at 0.1 - 0.5 mol / L results in better performance of the prepared modified iron phosphate. If the concentration of the mixed salt solution is too high, the thicker the zirconium tungstate layer formed on the surface under the same reaction time, and the too thick zirconium tungstate layer will reduce the capacity of the material. If the concentration of the mixed salt solution is too low, the thinner the zirconium tungstate layer formed on the surface under the same reaction time, and the too thin zirconium tungstate layer is difficult to fully connect the carbon coating layer and the lithium iron phosphate core, affecting the cycle stability of the material.

[0085] Comparing Example 1 with Examples 6-7, it can be seen that during the preparation process of the modified lithium iron phosphate described in the present disclosure, the temperature of the sintering treatment in step (2) affects its performance. Controlling the temperature of the sintering treatment in step (2) at 500-600 °C results in better performance of the prepared modified iron phosphate. If the temperature of the sintering treatment in step (2) is too high, zirconium tungstate can be decomposed, but it will cause waste of energy. 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 property of thermal contraction and cold expansion, cracks will appear on the surface of the particles after the reaction, affecting the coating tightness and thus reducing the cycle stability of the material.

[0086] Comparing Example 1 with Comparative Examples 1-2, it can be seen that in the present disclosure, zirconium tungstate is pre-compounded on the surface of lithium iron phosphate. During the secondary sintering process, zirconium tungstate gradually thermally shrinks and then decomposes into tungsten oxide and zirconium oxide. Tungsten oxide and zirconium oxide react with the lithium source to generate lithium tungstate and lithium zirconate, which can play a lithium supplementing effect during the cycling process and can also supplement the lattice lithium precipitated during the solution coating process, improving the specific capacity of the material.

[0087] Comparing Example 1 with Comparative Example 3, it can be seen that in the present disclosure, an organic carbon source is added. The organic carbon source present in the thermal contraction gaps of zirconium tungstate can produce a bridging effect with the zirconium oxide produced after decomposition, well alleviating the loss of specific capacity and well reducing the side reactions between the material and the electrolyte.

Claims

1. A preparation method of modified lithium iron phosphate, comprising the following steps: (1) Mix a zirconium salt, a tungstate and a solvent to obtain a mixed salt solution. After adjusting the pH, add lithium iron phosphate and carry out a heating reaction; (2) After solid-liquid separation of the slurry obtained from the heating reaction, perform a one-step sintering treatment on the obtained solid to obtain a first-sintered material; (3) Mix an organic carbon source, a lithium source and the first-sintered material, and perform a two-step sintering treatment to obtain the modified lithium iron phosphate; The concentration of the mixed salt solution is 0.1 - 0.5 mol / L.

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, wherein, The tungstate includes ammonium tungstate.

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

5. The preparation method according to claim 1, wherein, The stirring time for the mixing is 2 - 5 h.

6. The preparation method according to claim 1, wherein, The pH in step (1) is 4 - 6.

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

8. The preparation method according to claim 1, wherein, After adding the lithium iron phosphate, perform ultrasonic treatment for 20 - 60 min.

9. The preparation method according to claim 1, wherein, The temperature of the heating reaction in step (1) is 120 - 180 °C.

10. The preparation method according to claim 9, wherein, The time of the heating reaction is 6 - 12 h.

11. The preparation method according to claim 1, wherein, The temperature of the one-step sintering treatment in step (2) is 500 - 600 °C.

12. The preparation method according to claim 11, wherein, The time of the one-step sintering treatment is 3 - 5 h.

13. The preparation method according to claim 1, wherein, The organic carbon source in step (3) includes any one or a combination of at least two of polyvinyl alcohol, polyvinyl butyral, petroleum asphalt, coal tar pitch, building asphalt, sucrose, glucose or starch.

14. The preparation method according to claim 13, wherein, The organic carbon source is dissolved in a solvent.

15. The preparation method according to claim 13, wherein, The mass of the organic carbon source is 2 - 10% of the mass of the first-sintered material.

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

17. The preparation method according to claim 1, wherein, The lithium source includes lithium hydroxide and / or lithium carbonate.

18. The preparation method according to claim 17, wherein, The molar ratio of lithium element in the lithium source to zirconium element in the zirconium salt is (2.05 - 2.2):

1.

19. The preparation method according to claim 1, wherein, The atmosphere of the two-step sintering treatment in step (3) includes any one or a combination of at least two of nitrogen, argon or carbon dioxide.

20. The preparation method according to claim 19, wherein, The two-step sintering treatment includes a first-stage sintering and a second-stage sintering.

21. The preparation method according to claim 20, wherein, The temperature of the first-stage sintering is 400 - 600 °C.

22. The preparation method according to claim 20, wherein, The time of the first-stage sintering is 2 - 5 h.

23. The preparation method according to claim 20, wherein, The temperature of the second-stage sintering is 600 - 850 °C.

24. The preparation method according to claim 20, wherein, The time of the second-stage sintering is 5 - 12 h.

25. A modified lithium iron phosphate obtained by the method according to any one of claims 1 - 24.

26. A positive electrode sheet comprising the modified lithium iron phosphate according to claim 25.

27. A lithium ion battery comprising the positive electrode sheet according to claim 26.

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