A lithium-rich lithium ferrite composite material and its preparation method

Abstract

This invention discloses a lithium-rich lithium ferrite composite material and its preparation method. First, inexpensive straw is used as a carbon source and reacts with an iron source to generate an Fe / C precursor with Fe-O-C bonds. Then, this precursor is mixed with a lithium source and sintered to obtain an LFO / C composite. Finally, the surface is coated with a nitrogen-containing organic polymer to obtain the lithium-rich lithium ferrite composite material. The lithium-rich lithium ferrite composite material obtained by this invention has the characteristics of low production cost, excellent conductivity, and good stability in air, which is beneficial for improving the electrochemical performance of lithium-ion batteries.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a lithium-rich lithium iron phosphate composite material and its preparation method. Background Technology

[0002] In recent years, with the rapid development of lithium battery technology, factors limiting the capacity of lithium battery cathode materials have gradually emerged. One of the main influencing factors is that during the first charge of a lithium battery, a large amount of active Li-O2 is consumed on the negative electrode surface during the formation of a solid electrolyte interphase (SEI) film. + This reduces the battery capacity of lithium batteries.

[0003] Therefore, to reduce the negative impacts during SEI film formation, the concept of lithium replenishment additives was proposed. Currently, commonly used lithium replenishment methods are divided into positive electrode lithium replenishment and negative electrode lithium replenishment. Positive electrode lithium replenishment materials have received widespread attention due to their advantages such as high potential, good compatibility with existing lithium battery materials, and ease of use and operation. Among them, lithium-rich iron oxide (LFO) has become a hot topic due to its excellent lithium replenishment effect and low preparation cost; however, some of its shortcomings have also been exposed:

[0004] (1) Poor stability: LFO material has high activity and is difficult to exist stably in air. It is prone to react with CO2 and H2O and deteriorates, resulting in its capacity not being able to be fully utilized and its processing performance deteriorating; (2) Poor conductivity: LFO material requires a high voltage to completely delithiate. In response to the above problems, some scholars have proposed a scheme of coating the surface of LFO. However, problems such as uneven coating, increased particle size after coating, complicated coating steps, and high cost still limit the application of LFO lithium supplementation additives. Summary of the Invention

[0005] Based on the technical problems existing in the background art, the present invention proposes a low-cost lithium iron ferrite composite material with excellent conductivity and good stability in air, and its preparation method.

[0006] The present invention proposes a method for preparing lithium-rich lithium ferrite composite material, comprising the following steps:

[0007] S1. Under an inert atmosphere, the straw powder is calcined and pyrolyzed at 400-600℃ for 30-90 minutes, then alkali washed, washed, and dried to obtain pretreated straw charcoal powder.

[0008] S2. The pretreated straw char powder is mixed evenly with an iron source and calcined at 350-500°C under an inert atmosphere to obtain the Fe / C precursor.

[0009] S3. The Fe / C precursor is mixed evenly with a lithium source, and then sintered at 650-800°C under an inert atmosphere. The mixture is then pulverized to obtain the LFO / C composite.

[0010] S4. The LFO / C composite is mixed with a nitrogen-containing organic polymer solution with a mass fraction of 30-50 wt%, and then reacted by heating in a water bath. After drying and pulverizing, a lithium-rich lithium iron phosphate composite material is obtained.

[0011] This invention obtains pretreated straw charcoal powder through pyrolysis and alkali washing of straw powder. The straw powder used as raw material has high porosity and a large specific surface area after pyrolysis, allowing it to immobilize a significant amount of Li ions. The alkali washing step increases the specific surface area and surface functional group content of the charcoal while preventing acidic residues and avoiding lithium ion consumption during secondary roasting. The pretreated straw charcoal powder is then reacted with an iron source to synthesize an LFO / C composite containing Fe-OC bonds. The carbon exposed on the surface of the LFO / C composite acts as a "base layer," allowing the nitrogen-containing organic polymer to be more uniformly coated on its surface, thus effectively isolating it from H2O and CO2. This results in a lithium-rich lithium iron ferrite composite material with excellent stability in air, and the nitrogen doping further enhances the material's electronic conductivity. Furthermore, the lithium-rich lithium iron ferrite composite material obtained by this method contains a large number of Fe-OC bonds, which are uniformly distributed throughout the interior and surface of the composite material, further improving its electrical conductivity.

[0012] Preferably, the straw powder is at least one of corn straw powder, rice straw powder, rapeseed straw powder, and wheat straw powder.

[0013] Preferably, the particle size of the straw powder is 20-100 mesh.

[0014] After the pyrolysis of S1, a process of grinding the resulting material into a uniform form may also be included.

[0015] Preferably, in step S1, the alkaline washing includes: adding the material obtained after pyrolysis to an inorganic alkaline solution with a concentration of 4-8 mol / L at a solid-liquid ratio of 1:10-20 and stirring for 60-300 min. In this invention, by controlling the concentration and amount of LiOH solution and the washing time within a certain range, the adsorption and reactivity of the obtained pretreated straw charcoal powder can be further improved, which helps to improve the uniformity of Fe-OC bond distribution in the material and better improve the conductivity of the material.

[0016] Preferably, the inorganic base is at least one selected from LiOH, KOH, and NaOH.

[0017] After alkaline washing in S1, conventional solid-liquid separation processes are also included, such as centrifugation and filtration.

[0018] Preferably, the washing in S1 is performed using deionized water. The purpose of washing is to clean the cations on the surface of the material and avoid interfering with the purity of the material. Preferably, the washing conditions in S1 include: adding deionized water and stirring to wash, repeating the washing 2 to 5 times, and the solid-liquid ratio of each repeated washing is 1:10 to 20.

[0019] After washing in S1, conventional solid-liquid separation processes are also included, such as centrifugation and filtration.

[0020] In this invention, the centrifugation conditions are preferably 4000-12000 rpm for 3-5 min.

[0021] Preferably, in S2, the iron source is at least one of Fe(NO3)2, FeSO4, FeSO4·7H2O, Fe2O3, and FeCl3.

[0022] Preferably, in S2, the specific method for uniformly mixing the pretreated straw charcoal powder with the iron source includes: ball milling the pretreated straw charcoal powder and the iron source for 30 to 90 minutes under the conditions of a ball-to-material ratio of 5 to 10:1 and a rotation speed of 300 to 750 rpm.

[0023] Preferably, in S2, the calcination time is 180–300 min.

[0024] Preferably, in S2, the heating rate of the calcination is 1 to 5 °C / min.

[0025] Preferably, in S3, the molar ratio of Fe in the Fe / C precursor to Li in the lithium source is 1:5-6; the lithium source is at least one selected from LiOH, LiOH·H2O, Li2CO3, and Li2O. In this invention, by controlling the Fe / Li ratio within a certain range, the purity of the material can be improved. This is because an excessively low Fe / Li ratio will result in excessively high residual alkali, while an excessively high ratio will lead to an increase in byproducts, thereby reducing the purity of the material.

[0026] Preferably, in S3, the specific method for uniformly mixing the Fe / C precursor and the lithium source includes: ball milling the Fe / C precursor and the lithium source for 30 to 90 minutes at a ball-to-material ratio of 5 to 10:1 and a rotation speed of 300 to 750 rpm.

[0027] Preferably, in S3, the sintering time is 180–600 min.

[0028] Preferably, in S3, the heating rate of the sintering is 3-5 °C / min.

[0029] Preferably, the particle size D50 of the pulverized LFO / C composite is 8-15 μm. In this invention, by controlling the particle size of the LFO / C composite within a suitable range, the consistency of the material can be improved, capacity fluctuations caused by large particles can be avoided, thereby enhancing the electrochemical stability of the material.

[0030] Preferably, in step S4, the LFO / C composite is mixed with a nitrogen-containing organic polymer solution at a solid-liquid ratio of 1:10-30; the nitrogen-containing organic polymer is at least one of polyethyleneimine and melamine; and the solvent of the nitrogen-containing organic polymer solution is methanol. In this invention, by controlling the type of nitrogen-containing organic polymer and the amount of nitrogen-containing organic polymer solution, a denser coating layer can be formed, which is beneficial for improving the air stability of the material.

[0031] Preferably, in step S4, the water bath heating temperature is 50–65°C, and the time is 30–60 min. In this invention, by controlling the water bath heating conditions within a certain range, the uniformity of the nitrogen-containing polymer coating and the density of the formed coating layer can be improved, which is beneficial to better improving the air stability and conductivity of the material.

[0032] Preferably, in step S4, the reaction is carried out under stirring at a speed of 200–300 rpm.

[0033] After the reaction in S4 is completed, conventional solid-liquid separation processes are also included, such as centrifugation and filtration.

[0034] Preferably, in S1 and S4, the drying step includes drying at 60-120°C for 120-150 min.

[0035] Preferably, the particle size of the pulverized lithium iron ferrite composite material is 400-600 mesh. In this invention, by controlling the particle size of the lithium iron ferrite composite material within a suitable range, the consistency of the material can be improved, capacity fluctuations caused by large particles can be avoided, thereby enhancing the electrochemical performance stability of the material; furthermore, controlling the particle size to be smaller can also effectively reduce polarization and improve Li... + The transmission rate is increased, further enhancing the rate performance of the material.

[0036] In this invention, the inert atmosphere refers to an atmosphere formed by a non-reactive gas, which can be nitrogen or an inert gas such as argon or helium.

[0037] A lithium-rich lithium iron ferrite composite material is obtained by the preparation method described above.

[0038] The beneficial effects of this invention are as follows:

[0039] This invention first uses inexpensive straw as a carbon source, reacting it with an iron source to generate a Fe / C precursor with Fe-OC bonds. Then, this precursor is mixed with a lithium source and sintered to obtain an LFO / C composite. Finally, a nitrogen-containing organic polymer is used for surface coating to obtain a lithium-rich lithium ferrite composite material. On one hand, this invention first synthesizes an LFO / C composite containing Fe-OC bonds by reacting pretreated straw char powder obtained from straw powder pyrolysis and alkali washing with an iron source. The carbon exposed on the surface of the LFO / C composite acts as a "base layer," allowing the nitrogen-containing organic polymer to more uniformly coat its surface, thereby achieving sufficient isolation between H2O and CO2. This results in a lithium-rich lithium ferrite composite material with excellent stability in air, and the nitrogen doping further enhances the material's electronic conductivity. On the other hand, the lithium-rich lithium ferrite composite material obtained by this method has a large number of Fe-OC bonds, which are uniformly distributed inside and on the surface of the composite material, further improving its electrical conductivity. In summary, the lithium-rich lithium iron ferrite composite material obtained by this invention has the characteristics of low production cost, excellent conductivity, and good stability in air, and its rate performance is good. + It has high transmission efficiency and no particle agglomeration or overgrowth during the preparation process, which can significantly improve the energy density of lithium-ion batteries without significantly affecting their cycle performance, thus improving the electrochemical performance of lithium-ion batteries. Detailed Implementation

[0040] The technical solution of the present invention will now be described in detail through specific embodiments.

[0041] Example 1

[0042] Preparation of lithium-rich lithium iron phosphate composite materials:

[0043] S1. The corn straw powder with a particle size of 40 mesh is first calcined in a muffle furnace at 500℃ for 30 min under a nitrogen atmosphere. Then, the obtained material is ground in a mortar for 5 min. It is then added to a 5 mol / L LiOH solution at a solid-liquid ratio of 1:15 and stirred at 400 rpm for 90 min. After filtration, the filter residue is added to deionized water at a solid-liquid ratio of 1:15 and stirred and washed for 10 min. The washing is repeated 3 times. Then, it is centrifuged at 8000 rpm for 3 min. The precipitate is dried at 60℃ for 120 min to obtain pretreated straw charcoal powder.

[0044] S2. Place 20g of pretreated straw charcoal powder and 100g of Fe2O3 in a ball mill jar, and ball mill at 400rpm for 60min under a ball-to-material ratio of 5:1. Then, calcine the mixture at 400℃ under a nitrogen atmosphere with a heating rate of 3℃ / min for 180min to obtain the Fe / C precursor.

[0045] S3. Place 10.63g of Fe / C precursor and 10g of lithium oxide in a ball mill jar (the molar ratio of Fe in the Fe / C precursor to Li in the lithium oxide is 1:5.35), and ball mill at 400rpm for 30min under a ball-to-material ratio of 5:1. Then place it in a muffle furnace and sinter at 650℃ under a nitrogen atmosphere at a heating rate of 3℃ / min for 300min. Then pulverize it to a D50 of 8-15μm to obtain the LFO / C composite.

[0046] S4. The LFO / C composite was mixed with a methanol solution of 40 wt% polyethyleneimine at a solid-liquid ratio of 1:15. The mixture was then heated in a water bath at 200 rpm with stirring for 30 min at a temperature of 65 ℃. After the reaction was completed, the mixture was centrifuged at 8000 rpm for 3 min. The precipitate was dried at 60 ℃ for 120 min and then ground until the D50 was 3-10 μm to obtain the lithium-rich lithium iron ferrite composite material.

[0047] Example 2

[0048] The only difference between Example 2 and Example 1 is that S1 is different, as follows: S1, rapeseed straw powder with a particle size of 40 mesh is first calcined in a muffle furnace at 550°C for 60 min under a nitrogen atmosphere. Then, the obtained material is ground in a mortar for 5 min, and added to a 5 mol / L LiOH solution at a solid-liquid ratio of 1:17. The mixture is stirred at 400 rpm for 90 min, then filtered. The filter residue is added to deionized water at a solid-liquid ratio of 1:15 and stirred and washed for 10 min. The washing is repeated 5 times. Then, the mixture is centrifuged at 8000 rpm for 3 min. The precipitate is dried at 60°C for 120 min to obtain pretreated straw charcoal powder.

[0049] The remaining steps are the same as S2, S3, and S4 in Example 1.

[0050] Example 3

[0051] The only difference between Example 3 and Example 1 is that S2 is different, as follows: S2, 10g of pretreated straw charcoal powder and 100g of FeCl3 are placed in a ball mill jar and ball-milled at 400rpm for 60min under a ball-to-material ratio of 5:1. Then, under a nitrogen atmosphere, the temperature is raised to 400℃ at a heating rate of 3℃ / min for calcination for 180min to obtain the Fe / C precursor.

[0052] The remaining steps are the same as S1, S3, and S4 in Example 1.

[0053] Example 4

[0054] The only difference between Example 4 and Example 1 is that S3 is different, as follows: S3, 10.17g of Fe / C precursor and 10g of lithium oxide (the molar ratio of Fe in Fe / C precursor to Li in lithium oxide is 1:5.17) are placed in a ball mill jar and ball-milled at 400rpm for 30min at a ball-to-material ratio of 5:1. Then, the mixture is placed in a muffle furnace and sintered at 650℃ under a nitrogen atmosphere at a heating rate of 3℃ / min for 300min. The mixture is then pulverized to a D50 of 8-15μm to obtain the LFO / C composite.

[0055] The remaining steps are the same as S1, S2, and S4 in Example 1.

[0056] Example 5

[0057] The only difference between Example 5 and Example 1 is that S4 is different, as follows: S4, the LFO / C composite and a methanol solution of 40wt% melamine are mixed at a solid-liquid ratio of 1:15, and then the reaction is carried out by stirring at 200 rpm in a water bath. The water bath temperature is 50℃ and the time is 30 min. After the reaction is completed, the mixture is centrifuged at 8000 rpm for 3 min, the precipitate is dried at 60℃ for 120 min, and then ground and pulverized to a D50 of 3-10 μm to obtain a lithium iron ferrite composite material.

[0058] The remaining steps are the same as S1, S2, and S3 in Example 1.

[0059] Example 6

[0060] The only difference between Example 6 and Example 1 is that S4 is different, as follows: S4, the LFO / C composite and a methanol solution of polyethyleneimine with a mass fraction of 30 wt% are mixed at a solid-liquid ratio of 1:15, and then the reaction is carried out by stirring at 200 rpm in a water bath. The water bath temperature is 60℃ and the time is 60 min. After the reaction is completed, the mixture is centrifuged at 8000 rpm for 3 min, the precipitate is dried at 60℃ for 120 min, and then ground and pulverized to a D50 of 3-10 μm to obtain a lithium iron ferrite composite material.

[0061] The remaining steps are the same as S1, S2, and S3 in Example 1.

[0062] Comparative Example 1

[0063] Preparation of lithium-rich lithium iron phosphate composite materials:

[0064] S1. Place 20g of conductive carbon black and 100g of Fe2O3 in a ball mill jar, and ball mill at 400rpm for 60min at a ball-to-material ratio of 5:1. Then, calcine the mixture at 400℃ under a nitrogen atmosphere with a heating rate of 3℃ / min for 180min to obtain the Fe / C precursor.

[0065] S2. Place 10.63g of Fe / C precursor and 10g of lithium oxide in a ball mill jar (the molar ratio of Fe in the Fe / C precursor to Li in the lithium oxide is 1:5.35), and ball mill at 400rpm for 30min under a ball-to-material ratio of 5:1. Then place it in a muffle furnace and sinter at 650℃ under a nitrogen atmosphere at a heating rate of 3℃ / min for 300min. Then pulverize it to a D50 of 8-15μm to obtain the LFO / C composite.

[0066] S3. The LFO / C composite was mixed with a methanol solution of 40 wt% polyethyleneimine at a solid-liquid ratio of 1:15. The mixture was then heated in a water bath at 200 rpm with stirring for 30 min at a temperature of 65 ℃. After the reaction was completed, the mixture was centrifuged at 8000 rpm for 3 min. The precipitate was dried at 60 ℃ for 120 min and then ground until the D50 was 3-10 μm to obtain the lithium-rich lithium iron ferrite composite material.

[0067] Comparative Example 2

[0068] The only difference between Comparative Example 2 and Example 1 is that in S1, corn stalk powder with a particle size of 40 mesh is first calcined in a muffle furnace at 700°C for 120 min under a nitrogen atmosphere, while the rest of the operation remains unchanged.

[0069] Comparative Example 3

[0070] The only difference between Comparative Example 3 and Example 1 is that the washing step is omitted in S1. Specifically, in S1, corn straw powder with a particle size of 40 mesh is first calcined in a muffle furnace at 500°C for 30 min under a nitrogen atmosphere. Then, the obtained material is ground in a mortar for 5 min and added to a 5 mol / L LiOH solution at a solid-liquid ratio of 1:15. The mixture is stirred at 400 rpm for 90 min, then filtered. The filter residue is dried at 60°C for 120 min to obtain pretreated straw charcoal powder. The remaining operations remain unchanged.

[0071] Comparative Example 4

[0072] The only difference between Comparative Example 4 and Example 1 is that in S2, 50g of pretreated straw charcoal powder and 100g of Fe2O3 are placed in a ball mill jar, while the rest of the operation remains the same.

[0073] Comparative Example 5

[0074] The only difference between Comparative Example 5 and Example 1 is that in S2, the temperature was increased to 700°C at a heating rate of 3°C / min under a nitrogen atmosphere and calcined for 180 min, while the other operations remained unchanged.

[0075] Comparative Example 6

[0076] The only difference between Comparative Example 6 and Example 1 is that in S4, the mass fraction of the methanol solution of polyethyleneimine is 10 wt%, and the rest of the operation remains the same.

[0077] Comparative Example 7

[0078] The only difference between Comparative Example 7 and Example 1 is that in S4, the mass fraction of the methanol solution of polyethyleneimine is 70 wt%, and the rest of the operation remains the same.

[0079] Test case

[0080] The performance of the lithium-rich lithium iron ferrite composite materials prepared in Examples 1-6 and Comparative Examples 1-7 was tested. The specific methods are as follows: The obtained materials were mixed into a slurry with a mass ratio of lithium-rich lithium iron ferrite: conductive agent: binder = 8:1:1 to form a positive electrode sheet, and lithium metal was used as the negative electrode to form a coin cell. The test voltage range was 2.0-4.0V. The electrochemical performance test results under air exposure time during different coin cell preparation processes are shown in Table 1.

[0081] Table 1

[0082]

[0083] Among them, the air exposure time in different coin cell preparation processes refers to the time that the positive electrode is exposed to air after it has been made.

[0084] As can be seen from the specific data in Table 1, the above-mentioned embodiments of the present invention achieve the following technical effects: Examples 1 to 7 all exhibit superior lithium replenishment effects. Compared with Comparative Examples 1 to 6, the lithium-rich lithium iron ferrite composite material prepared by the present invention has excellent physicochemical properties and significant processing performance advantages.

[0085] Comparing Examples 1-2 with Comparative Example 1, it can be seen that the present invention selects pretreated straw char powder obtained by pyrolysis and alkali washing of straw powder as a carbon source, which is beneficial to improve the coating effect, increase the charging specific capacity of LFO, and enhance its air stability.

[0086] Comparing Example 1 and Comparative Example 2, it can be seen that limiting the temperature and time of carbon source pyrolysis treatment to a specific range is beneficial to significantly improve the purity of LFO synthesis and enhance conductivity.

[0087] Comparing Examples 1-2 and Comparative Example 3, it can be seen that thoroughly cleaning the carbon source after pyrolysis and alkali washing helps to reduce the impact of impurities on the purity of LFO, improve the coating effect, and enhance the stability of LFO in air.

[0088] Comparing Examples 3-4 and Comparative Examples 4-5, it can be seen that limiting the ratio of carbon source to iron source within a specific range is beneficial to improving the purity, electrochemical performance, and coating effect of LFO synthesis.

[0089] Comparing Examples 5-6 and Comparative Examples 6-7, it can be seen that limiting the concentration of the nitrogen-containing organic polymer solution within a specific range is beneficial to improving the coating effect and enhancing air stability.

[0090] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a lithium-rich lithium iron ferrite composite material, characterized in that, Includes the following steps: S1. Under an inert atmosphere, the straw powder is calcined and pyrolyzed at 400-600℃ for 30-90 minutes, then alkali washed, washed, and dried to obtain pretreated straw charcoal powder. S2. The pretreated straw charcoal powder and iron source are mixed evenly at a mass ratio of 1:5 to 10, and calcined at 350 to 500°C under an inert atmosphere to obtain the Fe / C precursor. S3. The Fe / C precursor and the lithium source are mixed evenly at a molar ratio of Fe in the Fe / C precursor to Li in the lithium source of 1:5-6. Then, the mixture is heated to 650-800℃ under an inert atmosphere for sintering and pulverization to obtain lithium-rich lithium iron ore LFO / C composite. S4. The lithium-rich lithium ferrite LFO / C composite is mixed with a nitrogen-containing organic polymer solution with a mass fraction of 30-50 wt%, and then reacted by heating in a water bath. After drying and pulverizing, the lithium-rich lithium ferrite composite material is obtained.

2. The method for preparing lithium-rich lithium iron ferrite composite material according to claim 1, characterized in that, In S1, the alkaline washing includes: adding the material obtained after pyrolysis to an inorganic alkaline solution with a concentration of 4-8 mol / L at a solid-liquid ratio of 1:10-20 and stirring for 60-300 min.

3. The method for preparing lithium-rich lithium iron ferrite composite material according to claim 1, characterized in that, The particle size D50 of the pulverized lithium iron ferrite LFO / C composite is 8-15 μm.

4. The method for preparing the lithium-rich lithium iron ferrite composite material according to claim 1, characterized in that, In step S4, the lithium-rich lithium ferrite LFO / C composite is mixed with a nitrogen-containing organic polymer solution at a solid-liquid ratio of 1:10 to 30; the nitrogen-containing organic polymer is at least one of polyethyleneimine and melamine.

5. The method for preparing lithium-rich lithium iron ferrite composite material according to claim 1, characterized in that, In S4, the water bath heating temperature is 50–65℃, and the time is 30–60 min.

6. The method for preparing lithium-rich lithium iron ferrite composite material according to claim 1, characterized in that, The particle size D50 of the lithium-rich lithium iron ferrite composite material after pulverization is 3-10 μm.

7. A lithium-rich lithium iron ferrite composite material, characterized in that, It is obtained by the preparation method according to any one of claims 1 to 6.

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