Lithium iron phosphate positive electrode material, and preparation method therefor and use thereof

Abstract

A lithium iron phosphate positive electrode material, and a preparation method therefor and a use thereof, relating to the technical field of electrode materials. The preparation method comprises: mixing iron phosphate, a lithium source, and a carbon source and then sintering same to obtain the lithium iron phosphate positive electrode material. The carbon source comprises liquid polyacrylonitrile; and the sintering temperature ranges from 750°C to 810°C and is not 750°C. According to the method, by introducing liquid polyacrylonitrile as a carbon source, iron phosphide can be generated at a relatively low sintering temperature, so that an iron phosphide-containing lithium iron phosphate positive electrode material is prepared. Because iron phosphide has relatively high conductivity, the use amount of the carbon source can be reduced to a certain extent so as to reduce the content of amorphous carbon in the lithium iron phosphate material, such that the compaction density of the lithium iron phosphate material is improved, thereby improving the electrochemical specific capacity of the lithium iron phosphate material and the energy density of batteries. Additionally, the preparation method is simple, is easy to operate, achieves excellent performance, and is suitable for large-scale mass production.

Description

A lithium iron phosphate positive electrode material and its preparation method and application

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on June 26, 2023, with application number 202310754083.2 and invention name “A lithium iron phosphate positive electrode material, its preparation method and application”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present invention relates to the technical field of electrode materials, and in particular to a lithium iron phosphate positive electrode material and a preparation method and application thereof. Background Art

[0003] In recent years, the electric vehicle industry has experienced rapid growth. Ternary power batteries, due to their high energy density, have captured a significant share of the power battery market. However, ternary cathode materials have issues such as oxygen release during phase transitions and thermal instability. Consequently, these cathodes are associated with significant safety concerns. While lithium iron phosphate cathodes have lower energy density than ternary cathodes, they offer superior safety and cycle stability. This has led to renewed development of lithium iron phosphate cathodes in recent years. However, the inherently poor electrical conductivity of lithium iron phosphate prevents its full capacity from being fully utilized, resulting in incomplete capacity release, low energy density, and poor cycle performance. Therefore, to improve the energy density and cycle life of lithium iron phosphate batteries, the electrical conductivity of the lithium iron phosphate material must first be increased.

[0004] The current industrial method usually uses carbon coating to improve the conductivity of lithium iron phosphate, generally using glucose or sucrose as the carbon source. Studies have shown that when the temperature reaches the melting point of glucose or sucrose, they will adhere to the surface of the lithium iron phosphate precursor. During high-temperature sintering, the carbon source inhibits the grain growth of lithium iron phosphate and forms a carbon coating layer. This method can effectively improve the conductivity of lithium iron phosphate and is conducive to the infiltration of the electrolyte. However, after excessive amorphous carbon is compounded with lithium iron phosphate, the compaction density will be greatly reduced, resulting in a decrease in the energy density of the battery.

[0005] Summary of the Invention

[0006] The purpose of the present invention is to provide a lithium iron phosphate positive electrode material and its preparation method and application. The lithium iron phosphate positive electrode material prepared by the preparation method of the present invention has high compaction density and high specific capacity.

[0007] In order to achieve the above-mentioned purpose of the invention, the present invention provides the following technical solutions: The present invention provides a method for preparing a lithium iron phosphate positive electrode material, comprising the following steps: mixing iron phosphate, a lithium source and a carbon source, and sintering to obtain the lithium iron phosphate positive electrode material; the carbon source includes liquid polyacrylonitrile; the sintering temperature is 750-810°C, and is not 750°C.

[0008] Preferably, the lithium source includes lithium carbonate and / or lithium hydroxide; the molar ratio of the iron in the iron phosphate, the phosphorus in the iron phosphate, and the lithium in the lithium source is 1:1:(1-1.05).

[0009] Preferably, the carbon source further comprises glucose and / or tannic acid.

[0010] Preferably, the mass of the carbon source accounts for 5-15% of the total mass of the iron phosphate and the lithium source.

[0011] Preferably, the mass of the liquid polyacrylonitrile is 2-12% of the total mass of the iron phosphate and the lithium source.

[0012] Preferably, the mass of the glucose is 3-12% of the total mass of the iron phosphate and the lithium source; the mass of the tannic acid is 1-3% of the total mass of the iron phosphate and the lithium source.

[0013] Preferably, the particle size of the mixture obtained by mixing is ≤300 nm; and the sintering time is 3 to 24 hours.

[0014] The present invention also provides a lithium iron phosphate positive electrode material prepared by the preparation method described in the above technical solution, wherein the lithium iron phosphate positive electrode material contains carbon and iron phosphide.

[0015] Preferably, the carbon content in the lithium iron phosphate positive electrode material is 0.5-2.5 wt %; the iron phosphide content in the lithium iron phosphate positive electrode material is 0.2-2 wt %.

[0016] The present invention also provides the use of the lithium iron phosphate positive electrode material described in the above technical solution in a lithium ion battery.

[0017] The present invention provides a method for preparing a lithium iron phosphate positive electrode material, comprising the following steps: mixing iron phosphate, a lithium source, and a carbon source, and then sintering to obtain the lithium iron phosphate positive electrode material; the carbon source includes liquid polyacrylonitrile; and the sintering temperature is 750-810°C, but not 750°C. The present invention introduces liquid polyacrylonitrile as a carbon source, which can produce iron phosphide at a lower sintering temperature, thereby preparing a lithium iron phosphate positive electrode material containing iron phosphide; because iron phosphide has a high electrical conductivity, the amount of carbon source used can be reduced to a certain extent, thereby reducing the content of amorphous carbon in the lithium iron phosphate material, thereby increasing the compaction density of the lithium iron phosphate material and thereby increasing its electrochemical specific capacity. Furthermore, the preparation method of the present invention is simple, easy to operate, and has excellent performance, making it suitable for large-scale mass production. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG1 is an XRD diagram of the lithium iron phosphate positive electrode material described in Example 3.

[0019] FIG2 is an XRD diagram of the lithium iron phosphate positive electrode material described in Comparative Example 1.

[0020] FIG3 is an XRD diagram of the lithium iron phosphate positive electrode material described in Comparative Example 2.

[0021] FIG4 is a SEM image of the lithium iron phosphate positive electrode material described in Example 3.

[0022] FIG5 is a cycling stability curve of the lithium-ion CR2032 button battery described in Example 3 at a current density of 1C.

[0023] FIG6 is a diagram showing the charge and discharge voltage of the first cycle of the lithium-ion CR2032 button battery described in Example 3. DETAILED DESCRIPTION

[0024] The present invention provides a method for preparing a lithium iron phosphate positive electrode material, comprising the following steps: mixing iron phosphate, a lithium source and a carbon source, and then sintering to obtain the lithium iron phosphate positive electrode material; the carbon source comprises liquid polyacrylonitrile; and the sintering temperature is 750-810°C, but not 750°C.

[0025] In the present invention, unless otherwise specified, all preparation raw materials are commercially available products well known to those skilled in the art.

[0026] In the present invention, the ferric phosphate serves as both an iron source and a phosphorus source.

[0027] In the present invention, the lithium source preferably includes lithium carbonate and / or lithium hydroxide. When the lithium source is lithium carbonate and lithium hydroxide, the present invention does not have any special restrictions on the amount of the lithium carbonate and lithium hydroxide, and they can be mixed in any ratio.

[0028] In the present invention, the carbon source includes liquid polyacrylonitrile. In the present invention, the carbon source also preferably includes glucose and / or tannic acid. When the carbon source is two or more of the above-mentioned specific options, the present invention has no special restrictions on the ratio of the above-mentioned specific substances.

[0029] In the present invention, the molar ratio of the iron in the ferric phosphate, the phosphorus in the ferric phosphate, and the lithium in the lithium source is preferably 1:1:(1-1.05).

[0030] In the present invention, the mass of the carbon source preferably accounts for 5 to 15% of the total mass of the iron phosphate and the lithium source, and more preferably 8 to 12%.

[0031] In the present invention, the mass of the liquid polyacrylonitrile is preferably 2-12% of the total mass of the iron phosphate and the lithium source, more preferably 4-10%, and most preferably 6-8%.

[0032] In the present invention, the mass of the glucose is preferably 3-12% of the total mass of the iron phosphate and the lithium source, more preferably 5-10%, and most preferably 6-8%.

[0033] In the present invention, the mass of the tannic acid is preferably 1 to 3% of the total mass of the iron phosphate and the lithium source, more preferably 1.5 to 2.5%, and most preferably 1.8 to 2.2%.

[0034] In the present invention, the mixing method is preferably sand milling; the sand milling is preferably wet sand milling; the sand milling medium used in the wet sand milling is preferably one or more of water, ethanol, and methanol; when the sand milling medium is two or more of the above-mentioned specific selections, the present invention does not have any specific restrictions on the ratio of the above-mentioned specific substances, and they can be mixed in any ratio. The present invention does not have any specific restrictions on the amount of the sand milling medium used, and an amount familiar to those skilled in the art can be used. The present invention does not have any specific restrictions on the conditions for the sand milling, and conditions familiar to those skilled in the art can be used.

[0035] Before the sintering, the method further includes: spray drying the mixture obtained by mixing; the spray drying temperature is preferably 100-300°C, more preferably 150-250°C, and most preferably 180-220°C; the spray drying time is preferably 1-6h, more preferably 2-5h, and most preferably 2-3h.

[0036] In the present invention, the particle size of the mixture obtained after the mixing is preferably ≤300 nm.

[0037] In the present invention, the sintering is preferably performed in a reducing atmosphere or a protective atmosphere; the reducing atmosphere is preferably an argon-hydrogen mixture, wherein the volume ratio of argon to hydrogen in the argon-hydrogen mixture is preferably 95:5; the protective atmosphere is preferably argon. In the present invention, the sintering temperature is 750-810°C, preferably 750-800°C, and more preferably 770-790°C; the sintering time is preferably 3-24 hours, more preferably 5-20 hours, and most preferably 10-15 hours.

[0038] The present invention also provides a lithium iron phosphate positive electrode material prepared by the preparation method described in the above technical solution, wherein the lithium iron phosphate positive electrode material contains carbon and iron phosphide.

[0039] In the present invention, the carbon content in the lithium iron phosphate positive electrode material is preferably 0.5-2.5wt%, more preferably 0.8-2.2wt%, and more preferably 1.0-1.8wt%; the iron phosphide content in the lithium iron phosphate positive electrode material is preferably 0.2-2wt%, more preferably 0.4-1.6wt%, and most preferably 0.6-1.2wt%.

[0040] In the present invention, the particle size of the lithium iron phosphate positive electrode material is preferably 300 to 800 nm.

[0041] The present invention also provides the use of the lithium iron phosphate cathode material described in the above technical solution in a lithium ion battery. The present invention does not have any special restrictions on the method of the application, and the method well known to those skilled in the art can be used.

[0042] The lithium iron phosphate positive electrode material provided by the present invention, its preparation method and application are described in detail below with reference to the embodiments, but they should not be understood as limiting the scope of protection of the present invention.

[0043] Example 1

[0044] 1.4 kg of iron phosphate, 0.349 kg of lithium carbonate, 87 g of glucose, 122 g of liquid polyacrylonitrile and 4.1 L of deionized water were mixed, transferred to a sand mill and sand-milled for 3 h, spray-dried at 220 ° C for 2 h, and sintered at 800 ° C for 10 h under an argon atmosphere to obtain lithium iron phosphate positive electrode material (carbon content of about 1.34 wt%, iron phosphide content of about 0.5 wt%, particle size of 350 nm).

[0045] The lithium iron phosphate positive electrode material, conductive carbon black and polyvinylidene fluoride were mixed in a mass ratio of 100:3.5:3, and N-methylpyrrolidone was used as a dispersant. After magnetic stirring for 8 hours, the obtained slurry was coated on aluminum foil, dried at 70°C for 12 hours, and vacuum dried at 80°C for 12 hours to obtain the positive electrode sheet of the battery.

[0046] In a glove box with an argon atmosphere, a lithium-ion CR2032 button cell was assembled. The counter electrode was a metal lithium sheet, the lithium hexafluorophosphate electrolyte was the electrolyte of the lithium-ion CR2032 button cell, and the battery separator was Celgard 2400.

[0047] The assembled lithium-ion CR2032 button battery was charged and discharged on the Blue Dot test system. At a current density of 0.1C, the authorized charge and discharge specific capacities of the lithium-ion CR2032 button battery were 160.8mAh·g -1 and 156mAh·g -1 ;Good cycle stability.

[0048] Example 2

[0049] 1.4 kg of iron phosphate, 0.349 kg of lithium carbonate, 122 g of glucose, 157 g of liquid polyacrylonitrile and 4.1 L of deionized water were mixed, transferred to a sand mill and sand-milled for 3 hours, spray-dried at 220 ° C for 2 hours, and then sintered at 800 ° C for 10 hours under an argon atmosphere to obtain lithium iron phosphate positive electrode material (carbon content of about 2.05 wt%, iron phosphide content of about 0.7 wt%, particle size of about 350 nm).

[0050] The lithium iron phosphate positive electrode material, conductive carbon black and polyvinylidene fluoride were mixed in a mass ratio of 100:3.5:3, and N-methylpyrrolidone was used as a dispersant. After magnetic stirring for 8 hours, the obtained slurry was coated on aluminum foil, dried at 70°C for 12 hours, and vacuum dried at 80°C for 12 hours to obtain the positive electrode sheet of the battery.

[0051] In a glove box with an argon atmosphere, a lithium-ion CR2032 button cell was assembled. The counter electrode was a metal lithium sheet, the lithium hexafluorophosphate electrolyte was the electrolyte of the lithium-ion CR2032 button cell, and the battery separator was Celgard 2400.

[0052] The assembled lithium-ion CR2032 button battery was charged and discharged on the Blue Dot test system. At a current density of 0.1C, the authorized charge and discharge specific capacities of the lithium-ion CR2032 button battery were 154.7mAh·g -1 and 150.1mAh·g -1 ;Good cycle stability.

[0053] Example 3

[0054] 1.4 kg of iron phosphate, 0.349 kg of lithium carbonate, 72 g of glucose, 105 g of liquid polyacrylonitrile and 4.1 L of deionized water were mixed, transferred to a sand mill and sand-milled for 3 h, spray-dried at 220 ° C for 2 h, and sintered at 800 ° C for 10 h under an argon atmosphere to obtain lithium iron phosphate positive electrode material (carbon content of about 1.1 wt%, iron phosphide content of about 0.4 wt%, particle size of about 350 nm).

[0055] FIG1 is an XRD diagram of the lithium iron phosphate positive electrode material. As shown in FIG1 , the lithium iron phosphate positive electrode material prepared according to the conditions of Example 1 has a carbon content of about 1.34 wt % and contains about 0.5 wt % of iron phosphide.

[0056] Figure 4 is a SEM image of the lithium iron phosphate positive electrode material. As shown in Figure 4, the lithium iron phosphate positive electrode material prepared according to the conditions of Example 1 has relatively uniform particles, with a particle size of less than 500 nm. The lithium iron phosphate positive electrode material, conductive carbon black, and polyvinylidene fluoride were mixed in a mass ratio of 100:3.5:3, and N-methylpyrrolidone was used as a dispersant. After magnetic stirring for 8 hours, the resulting slurry was coated on aluminum foil, dried at 70°C for 12 hours, and then vacuum-dried at 80°C for 12 hours to obtain the positive electrode sheet of the battery.

[0057] In a glove box with an argon atmosphere, a lithium-ion CR2032 button cell was assembled. The counter electrode was a metal lithium sheet, the lithium hexafluorophosphate electrolyte was the electrolyte of the lithium-ion CR2032 button cell, and the battery separator was Celgard 2400.

[0058] The assembled lithium-ion CR2032 button battery was charged and discharged on the Blue Dot test system. At a current density of 0.1C, the first cycle charge and discharge specific capacity of the lithium-ion CR2032 button battery was 159.8mAh·g -1 and 153.9 mAh g -1 ; The cycle stability is good, and the capacity retention rate is 93.4% after 100 cycles at 1C current density, as shown in Figure 5.

[0059] Figure 6 shows the charge-discharge voltage during the first cycle of the lithium-ion CR2032 button-type battery. This figure shows that the lithium iron phosphate cathode prepared according to Example 1 has a high specific capacity and a low polarization voltage during the first charge-discharge cycle. This indicates that optimizing the carbon content and iron phosphide content plays a key role in improving the electrochemical performance of the lithium iron phosphate cathode.

[0060] Comparative Example 1

[0061] 1.4g of iron phosphate, 0.349kg of lithium carbonate, 209g of glucose, and 4.1L of deionized water were mixed, transferred to a sand mill, and sand-milled for 3 hours. After spray drying at 220°C for 2 hours, the mixture was sintered at 800°C for 10 hours under an argon atmosphere to obtain a lithium iron phosphate cathode material (with a carbon content of approximately 1.31wt%, an iron phosphide content of 0, and a particle size of approximately 350nm). This indicates that lithium iron phosphate cathode materials containing iron phosphide cannot be prepared without the addition of liquid polyacrylonitrile.

[0062] FIG2 is an XRD diagram of the lithium iron phosphate positive electrode material. As shown in FIG2 , the lithium iron phosphate positive electrode material prepared according to the conditions of Comparative Example 1 (without adding liquid acrylonitrile) does not contain iron phosphide.

[0063] Comparative Example 2

[0064] Referring to Example 1, the difference is that the sintering temperature is 750°C, and a lithium iron phosphate positive electrode material (carbon content is about 1.5wt%, iron phosphide content is 0, and particle size is about 360nm) is obtained. This shows that if the sintering temperature is too low, a lithium iron phosphate positive electrode material containing iron phosphide cannot be obtained.

[0065] FIG3 is an XRD diagram of the lithium iron phosphate positive electrode material. As shown in FIG3 , the lithium iron phosphate positive electrode material prepared according to the conditions of Comparative Example 2 (sintering at 750° C.) does not contain iron phosphide.

[0066] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for preparing a lithium iron phosphate positive electrode material, characterized in that: The following steps are involved: After mixing iron phosphate, a lithium source and a carbon source, sintering is performed to obtain the lithium iron phosphate positive electrode material; The carbon source includes liquid polyacrylonitrile; The sintering temperature is 750-810°C, but not 750°C.

2. The preparation method according to claim 1, characterized in that The lithium source includes lithium carbonate and / or lithium hydroxide.

3. The preparation method according to claim 1 or 2, characterized in that: The molar ratio of the iron in the ferric phosphate, the phosphorus in the ferric phosphate, and the lithium in the lithium source is 1:1:(1-1.05).

4. The preparation method according to claim 1, characterized in that: The carbon source also includes glucose and / or tannic acid.

5. The preparation method according to claim 4, characterized in that: The mass of the carbon source accounts for 5-15% of the total mass of the iron phosphate and the lithium source.

6. The preparation method according to claim 5, characterized in that: The mass of the liquid polyacrylonitrile is 2-12% of the total mass of the iron phosphate and the lithium source.

7. The preparation method according to claim 5, characterized in that: The mass of the glucose is 3-12% of the total mass of the iron phosphate and the lithium source.

8. The preparation method according to claim 5, characterized in that: The mass of the tannic acid is 1-3% of the total mass of the iron phosphate and the lithium source.

9. The preparation method according to claim 1, characterized in that: The particle size of the mixture obtained by mixing is ≤300nm.

10. The preparation method according to claim 1 or 9, characterized in that: Before the sintering, the step further includes: spray drying the mixture obtained by mixing; The spray drying temperature is 100-300° C. and the time is 1-6 hours.

11. The preparation method according to claim 1 or 9, characterized in that: The sintering time is 3 to 24 hours.

12. The lithium iron phosphate positive electrode material prepared by the preparation method according to any one of claims 1 to 11, characterized in that: The lithium iron phosphate positive electrode material contains carbon and iron phosphide.

13. The lithium iron phosphate positive electrode material according to claim 12, characterized in that: The carbon content in the lithium iron phosphate positive electrode material is 0.5-2.5wt%; The content of iron phosphide in the lithium iron phosphate positive electrode material is 0.2-2 wt %.

14. Use of the lithium iron phosphate positive electrode material according to claim 12 or 13 in a lithium ion battery.

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