Carbon-coated modified starch suitable for lithium battery material as well as preparation method and application of carbon-coated modified starch

Abstract

The invention discloses carbon-coated modified starch suitable for a lithium battery material as well as a preparation method and application of the carbon-coated modified starch, and belongs to the field of new material industry. The preparation method of the modified starch comprises the following steps: mixing starch with water to obtain starch slurry; liOH and Li2CO3 are mixed to obtain a composite system; adjusting the starch slurry to be alkaline by using a composite system, then adding an etherifying agent, and carrying out first-stage etherification reaction at the temperature of 40-60 DEG C; then raising the temperature to 65-80 DEG C, and carrying out a second-stage etherification reaction; and after the reaction is finished, separating the obtained product to obtain the modified starch. The preparation method of the modified starch is simple in process, subsequent neutralization and washing steps are reduced, introduction of sodium and potassium ions influencing electrical properties is avoided, the prepared product is suitable for a carbon coating process of a battery material, polarization of the battery material can be effectively improved, the compaction density of the material is improved, and therefore the battery performance is improved.

Description

Technical Field

[0001] The present invention belongs to the field of new material industry, and specifically relates to a modified starch suitable for carbon coating of lithium battery materials, and a preparation method and application thereof. Background Art

[0002] Battery materials are the core component of battery manufacturing, and their performance directly affects the overall performance of the battery. Lithium iron phosphate positive materials have the advantages of good safety performance, long cycle life and lower price compared to ternary materials. Carbon coating is a common performance improvement method in the production of lithium iron phosphate positive materials. By coating the surface of the material with a carbon layer, the conductivity and stability of the material can be effectively improved, thereby improving the performance of the material.

[0003] Starch is a natural polysaccharide with good biocompatibility and biodegradability. It can be used in the field of carbon coating of battery materials. Among the starch modification methods, etherification is a very common method to improve starch solubility and viscosity. By introducing hydrophilic ether groups, starch solubility and viscosity can be improved, thereby improving starch coating performance. However, the traditional starch etherification process usually uses a single substance such as sodium hydroxide, potassium hydroxide, and lithium hydroxide to provide an alkaline environment. During the reaction, the starch will be over-hydrolyzed due to the high pH at the beginning of the reaction, and during the reaction, OH will be used to hydrolyze the starch. - The sudden drop causes the reaction to stagnate, and it is difficult to control the degree of reaction. In the traditional preparation process of etherified starch, sodium sulfate and other substances are used for neutralization reaction. These alkaline catalysts and neutralization reaction processes easily lead to sodium and potassium elements remaining in the final starch product. When sodium and potassium elements enter the battery material system, they will occupy the lithium active sites, resulting in a decrease in reversible capacity and forming an unstable solid electrolyte interface film in the electrolyte, resulting in poor cycle performance of the material.

[0004] Patent CN114231577 discloses a method for preparing porous etherified starch, in which sodium sulfate and dilute alkali solution are used as acid-base regulators to prepare starch. Sodium ions are introduced during the process, and neutralization and washing are required subsequently. Simple acid-base neutralization cannot remove the sodium and potassium elements in the product, and removing the sodium and potassium elements through a deionization process will increase the complexity and cost of production. Summary of the invention

[0005] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a modified starch suitable for carbon coating of lithium battery materials and a preparation method and application thereof. 2 CO 3The composite system replaces common single substances such as sodium hydroxide or potassium hydroxide, solving the problem of excessive sodium and potassium content in common etherified starch on the market. At the same time, it cooperates with the two-stage temperature control program to achieve precise control of the degree of starch substitution. The obtained etherified starch has good water solubility and coating properties. The branched structure of the etherified starch enables it to form a uniform coating layer on the surface of lithium iron phosphate particles, thereby preparing lithium battery materials with excellent performance.

[0006] The technical solution adopted by the present invention is: The first object of the present invention is to provide a method for preparing a carbon-coated modified starch suitable for lithium battery materials, comprising the following steps: Starch is mixed with water to obtain a starch slurry with a mass fraction of 25-50 wt%; preferably, the starch is at least one of corn starch, wheat starch, potato starch, and tapioca starch.

[0007] LiOH and Li 2 CO 3 Mix to obtain LiOH-Li 2 CO 3 Composite system; specifically, LiOH and Li 2 CO 3 Mix and dissolve in water to obtain LiOH-Li 2 CO 3 Composite system; Using LiOH-Li 2 CO 3 The composite system adjusts the starch slurry to an alkaline state, preferably a pH of 10-14, and then adds an etherifying agent to carry out the first-stage etherification reaction at 40-60°C for 5-12 hours; then the temperature is raised to 65-80°C for the second-stage etherification reaction for 5-12 hours. Compared with the alkaline environment provided by a single strong base such as LiOH or NaOH, LiOH-Li 2 CO 3 The composite system can maintain a long-lasting and continuous high OH - concentration, the principle is: LiOH provides high concentration OH during the reaction - , Li 2 CO 3 As a reserve alkali source, in OH - When consumed, it is replenished, and high pH conditions inhibit CO 3 2- The hydrolysis and loss of PH can be prolonged to maintain the high pH, ​​keep the reaction controllable and stable, and use two-stage temperature control during the reaction. The first stage is 40-60℃ to promote the substitution reaction, control the degree of substitution at a lower temperature, and avoid side reactions caused by excessive temperature, such as starch hydrolysis; the second stage is 65-80℃ to promote CO 32- Hydrolysis, continuous replenishment of OH - , and accelerate the etherification process until the reaction stops, achieving precise control of the degree of substitution. The etherified starch obtained has good water solubility and coating properties. The branched structure of the etherified starch enables it to form a uniform coating layer on the surface of the lithium iron phosphate particles.

[0008] After the reaction is completed, the obtained product is centrifugally dried to a moisture content of less than 8 wt%, and sieved to obtain modified starch.

[0009] As a preferred technical solution, LiOH and Li 2 CO 3 The molar ratio is (3-6): 1. When it is less than 3:1, the pH may be too low due to excessive lithium carbonate, and the catalytic reaction cannot be achieved in the early stage of the reaction; when it is greater than 6:1, the amount of lithium carbonate may be too little to serve as a reserve base.

[0010] As a preferred technical solution, the etherifying agent is at least one of ethylene oxide, propylene oxide, N-(2,3-epoxypropyl)diethylamine, N-(2,3-epoxychloropropyl)trimethylammonium chloride, and 3-chloro-2-hydroxypropyl-trimethylammonium chloride. Further preferably, the etherifying agent is added in an amount of 5%-10% of the mass of the starch.

[0011] The second object of the present invention is to provide a modified starch, wherein the modified starch is prepared by the preparation method described in the first object.

[0012] The third object of the present invention is to provide the modified starch as described in the second object as a carbon source for use in carbon coating of lithium battery materials.

[0013] Compared with the prior art, the present invention has the following advantages: (1) In the process of preparing modified etherified starch, the present invention reduces the subsequent neutralization and washing steps, and avoids the introduction of sodium and potassium ions that affect the electrical properties of lithium batteries. The obtained product is suitable for the carbon coating process of battery materials, which can effectively improve the polarization of battery materials and increase the compaction density of materials. In addition, the amount of wastewater discharged in the production process can be significantly reduced, which is energy-saving and environmentally friendly and can reduce the cost of wastewater treatment.

[0014] (2) The traditional etherified starch preparation process uses sodium hydroxide or potassium hydroxide to provide an alkaline environment, resulting in a high sodium and potassium content in the finished product. The entry of sodium and potassium into the battery material will cause the reversible capacity of the battery material to decrease and affect the cycle performance of the battery material. This technical solution uses LiOH-Li 2 CO 3The composite system is used to prepare etherified starch, which avoids the deterioration of the battery material electrode performance caused by the introduction of sodium and potassium elements. 2 CO 3 The compound can ensure a high pH during the reaction, making the reaction process controllable, and will not cause the reaction to stagnate due to a rapid drop in pH, or cause starch hydrolysis due to an excessively high pH at the beginning of the reaction.

[0015] (3) The etherified starch prepared by this technology has good water solubility and dispersibility. When applied to lithium iron phosphate battery materials, it can form a uniform carbon coating layer. At the same time, the residual active lithium sites can form Li+ with the surface of lithium iron phosphate during the carbon coating sintering process. 3 PO 4 The conductive network can effectively reduce the electrode interface impedance, effectively improve the material polarization and increase the compaction density.

[0016] (4) Compared with the use of LiOH alone, LiOH-Li 2 CO 3 The composite use reduces the excessive use of strong alkali, and the total lithium usage is reduced by 40-50%. At the same time, it simplifies the production process and reduces the acid-base neutralization and washing process, thereby reducing the complexity and cost of the production process. It is easy to process and low-priced, the reaction conditions are relatively mild, the preparation energy consumption and cost are low, and it is suitable for large-scale industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 The state of charge-voltage curve of batteries assembled from different materials. DETAILED DESCRIPTION

[0018] The present invention is further described below in conjunction with the embodiments so that those skilled in the art can better understand the present invention and implement it, but the embodiments are not intended to limit the present invention.

[0019] In addition, unless otherwise specified, the preparation processes in the following examples are all conventional means in the prior art in the art, and therefore, they are not described in detail; the raw materials used in the following examples are all commercially available products.

[0020] Example 1 A method for preparing carbon-coated modified starch suitable for lithium battery materials comprises the following steps: Step 1: Prepare an appropriate amount of corn starch with a moisture content of 10-12%, and prepare lithium hydroxide and lithium carbonate with battery grade specifications and a purity greater than 99%.

[0021] Step 2: Adjust corn starch to a starch slurry with a concentration of 40 wt%; add LiOH-Li2 CO 3 The pH of the composite system was adjusted to 11.5±0.2, and then 6 wt% of propylene oxide based on the mass of starch was added. After mixing evenly, the mixture was placed in a heatable reaction container and stirred continuously.

[0022] Step 3: Gradually heat the mixture to 50°C and keep the temperature constant. During this process, stir the mixture continuously to allow it to react fully. The reaction time is 10 hours. Then heat the mixture to 70°C and stir continuously. The reaction time is 5 hours.

[0023] Step 4: After the reaction is completed, the mixture is cooled to room temperature and then centrifuged to dry to obtain modified starch, which is then stored for later use.

[0024] Example 2 A method for preparing carbon-coated modified starch suitable for lithium battery materials comprises the following steps: Step 1: Prepare an appropriate amount of corn starch with a moisture content of 10-12%, and prepare lithium hydroxide and lithium carbonate with battery grade specifications and a purity greater than 99%.

[0025] Step 2: Adjust corn starch to a starch slurry with a concentration of 40 wt%; add LiOH-Li 2 CO 3 The pH of the composite system was adjusted to 11.5±0.2, and then 6 wt% of N-(2,3-epoxypropyl)diethylamine based on the mass of starch was added. After mixing evenly, the mixture was placed in a heatable reaction container and stirred continuously.

[0026] Step 3: Gradually heat the mixture to 50°C and keep the temperature constant. During this process, stir the mixture continuously to allow it to react fully. The reaction time is 10 hours. Then heat the mixture to 70°C and stir continuously. The reaction time is 5 hours.

[0027] Step 4: After the reaction is completed, the mixture is cooled to room temperature and then centrifuged to dry to obtain modified starch, which is then stored for later use.

[0028] Example 3 A method for preparing carbon-coated modified starch suitable for lithium battery materials comprises the following steps: Step 1: Prepare an appropriate amount of corn starch with a moisture content of 10-12%, and prepare lithium hydroxide and lithium carbonate with battery grade specifications and a purity greater than 99%.

[0029] Step 2: Adjust corn starch to a starch slurry with a concentration of 40 wt%; add LiOH-Li 2 CO 3The pH of the composite system was adjusted to 11.5±0.2, and then 6 wt% of N-(2,3-epichloropropyl)trimethylammonium chloride based on the mass of starch was added. After mixing evenly, the mixture was placed in a heatable reaction container and stirred continuously.

[0030] Step 3: Gradually heat the mixture to 50°C and keep the temperature constant. During this process, stir the mixture continuously to allow it to react fully. The reaction time is 10 hours. Then heat the mixture to 70°C and stir continuously. The reaction time is 5 hours.

[0031] Step 4: After the reaction is completed, the mixture is cooled to room temperature and then centrifuged to dry to obtain modified starch, which is then stored for later use.

[0032] Comparative Example 1 The difference between this comparative example and Example 1 is that only lithium hydroxide is used to prepare the modified starch, and the method is as follows: Step 1: Prepare an appropriate amount of corn starch with a moisture content of 10-12%, and prepare lithium hydroxide with battery grade and purity greater than 99%.

[0033] Step 2: Adjust the corn starch to a starch slurry with a concentration of 40 wt%, add sodium hydroxide to adjust the solution pH to 11.5±0.2, then add 6 wt % of propylene oxide based on the mass of starch, mix well, put the mixture into a heatable reaction vessel and stir continuously.

[0034] Step 3: Gradually heat the mixture to 50°C and keep the temperature constant. During this process, stir the mixture continuously to allow it to react fully. The reaction time is 10 hours. Then heat the mixture to 70°C and stir continuously. The reaction time is 5 hours.

[0035] Step 4: After the reaction is completed, the mixture is cooled to room temperature, then centrifuged and dried, and stored for later use.

[0036] Comparative Example 2 The difference between this comparative example and Example 1 is that only lithium carbonate is used to prepare the modified starch, and the method is as follows: Step 1: Prepare an appropriate amount of corn starch with a moisture content of 10-12%, and prepare lithium carbonate with battery grade and purity greater than 99%.

[0037] Step 2: Adjust the corn starch to a starch slurry with a concentration of 40 wt%, add lithium carbonate to adjust the solution pH to 11.5±0.2, then add 6 wt % of propylene oxide based on the mass of starch, mix well, put the mixture into a heatable reaction vessel and stir continuously.

[0038] Step 3: Gradually heat the mixture to 50°C and keep the temperature constant. During this process, stir the mixture continuously to allow it to react fully. The reaction time is 10 hours. Then heat the mixture to 70°C and stir continuously. The reaction time is 5 hours.

[0039] Step 4: After the reaction is completed, the mixture is cooled to room temperature, then centrifuged and dried, and stored for later use.

[0040] Comparative Example 3 The difference between this comparative example and Example 1 is that: in step 3, two-stage temperature control is not performed, and the temperature is always maintained at 50° C. for 15 hours. The method is as follows: Step 1: Prepare an appropriate amount of corn starch with a moisture content of 10-12%, and prepare lithium hydroxide and lithium carbonate with battery grade specifications and a purity greater than 99%.

[0041] Step 2: Adjust corn starch to a starch slurry with a concentration of 40 wt%, and add LiOH-Li 2 CO 3 The pH of the composite system was adjusted to 11.5±0.2, and then 6 wt% of propylene oxide based on the mass of starch was added. After mixing evenly, the mixture was placed in a heatable reaction container and stirred continuously.

[0042] Step 3: Gradually heat the mixture to 50°C and keep the temperature constant. During this process, stir the mixture continuously to allow it to react fully. The reaction time is 15 hours.

[0043] Step 4: After the reaction is completed, the mixture is cooled to room temperature, then centrifuged and dried, and stored for later use.

[0044] Comparative Example 4 The difference between this comparative example and Example 1 is that: in step 3, two-stage temperature control is not performed, and the temperature is always maintained at 70° C. for 15 hours. The method is as follows: Step 1: Prepare an appropriate amount of corn starch with a moisture content of 10-12%, and prepare lithium hydroxide and lithium carbonate with battery grade specifications and a purity greater than 99%.

[0045] Step 2: Adjust corn starch to a starch slurry with a concentration of 40 wt%, and add LiOH-Li 2 CO 3 The pH of the composite system was adjusted to 11.5±0.2, and then 6 wt% of propylene oxide based on the mass of starch was added. After mixing evenly, the mixture was placed in a heatable reaction container and stirred continuously.

[0046] Step 3: Gradually heat the mixture to 70°C and keep the temperature constant. During this process, stir the mixture continuously to allow it to react fully. The reaction time is 15 hours.

[0047] Step 4: After the reaction is completed, the mixture is cooled to room temperature, then centrifuged and dried, and stored for later use.

[0048] Application Examples The modified starch prepared in the above-mentioned embodiments and comparative examples was used as a carbon source and applied to the carbon coating preparation process of lithium iron phosphate to prepare battery materials. The method is as follows: iron phosphate, lithium carbonate and carbon source are mixed evenly in deionized water in a mass ratio of 40:10:1, stirred for 2 hours, and then ground, sintered and classified to obtain a battery material coated with a carbon layer. The sintering temperature is 783±3°C.

[0049] Performance testing and result analysis The battery material prepared by using the modified starch prepared in each embodiment and comparative example as the carbon source is the positive electrode material. The positive electrode material, conductive graphite, and polytetrafluoroethylene are evenly mixed in a mass ratio of 7:2:1, and N-methylpyrrolidone solution is added dropwise for mixing and stirring. After forming a slurry, it is coated on a PE film and vacuum dried to obtain a positive electrode sheet. A lithium metal sheet is selected as the negative electrode material to form a button battery for measuring its electrical performance.

[0050] The properties of the modified starches prepared in Example 1 and Comparative Examples 1 to 4, as well as the performance indicators of the battery materials and positive electrode sheets prepared from these modified starches, are compared, as shown in Table 1: Table 1

[0051] From the data in Table 1, it can be seen that in Example 1, a composite catalyst system is used, lithium hydroxide quickly starts the hydroxyl substitution at the C6 position in starch, and lithium carbonate slowly releases OH - The hydroxyl substitution at the C2 / C3 position is promoted to form a uniform branched structure, so a more uniform thin film carbon layer is formed during the carbon coating process, and the carbon coating is more uniform; Comparative Example 1 uses a single lithium hydroxide catalyst, the pH is high at the initial stage of the reaction, and excessive alkalinity will lead to random substitution, increase the rigidity of the molecular chain, and the starch has poor water solubility. It is easy to crack during carbonization and form an island structure on the surface of the material; Comparative Example 2 uses only lithium carbonate, OH -Insufficient supply, substitution is concentrated in the reactive sites, insufficient substitution, poor adhesion of the coating layer, and local accumulation on the surface of the material. At the same time, the uniformity of the carbon layer will affect the compaction density of the material. Therefore, the compaction density of the pole piece of Example 1 is the highest, which can reach more than 2.63g / cc, while the comparative example 1 can only reach 2.57g / cc. Due to the poor adhesion of the carbon layer, the compaction density of the comparative example 2 can only reach 2.53g / cc; the starch prepared in the comparative example 3 does not have a heating stage compared to the embodiment 1, so the degree of reaction is not enough, the degree of substitution is low, and the water solubility is poor compared to the embodiment 1, so the compaction density of the prepared battery material is also low; the preparation process of the comparative example 4 uses high temperature at the beginning of the reaction, and the heat release of lithium hydroxide makes it difficult to control the reaction temperature. If the temperature is too high at the beginning of the reaction, the degree of substitution of the starch will be significantly reduced, and it will be degraded after high temperature etherification, resulting in a decrease in viscosity and poor coating performance, which will affect the compaction performance of the material.

[0052] Figure 1 The state of charge-voltage curve of batteries assembled from different materials can reflect the polarization phenomenon of battery materials. It can be seen from the figure that the modified starch prepared in Example 1 has the best improvement effect on battery polarization due to the formation of a uniform carbon layer; the modified starch prepared in Comparative Example 1 has an island-like accumulation of the carbon layer, which will reduce the discharge voltage platform and the polarization improvement performance will also deteriorate; the modified starch prepared in Comparative Example 2 has poor carbon layer adhesion, resulting in aggravated polarization, and the local area without carbon layer coating reacts with the electrolyte, which will increase the interface impedance; the starch prepared in Comparative Example 3 has no heating stage compared to Example 1, which will result in a lower degree of substitution, lower water solubility than Example 1, and relatively poor coating performance, and the polarization improvement effect will become lower; Comparative Example 4 is similar to Comparative Example 2, which will lead to aggravated polarization.

[0053] The above embodiments are only preferred embodiments of the present invention and are only illustrative, not restrictive, for the present invention. Those skilled in the art understand that many changes, modifications, and even equivalences may be made to the embodiments within the spirit and scope defined by the claims of the present invention, but all of these will fall within the scope of protection of the present invention.

Claims

1. A method for preparing carbon-coated modified starch suitable for lithium battery materials, characterized in that: The following steps are involved: Mixing starch with water to obtain starch slurry; The composite system is obtained by mixing LiOH and Li2CO3; The starch slurry is adjusted to alkaline using a composite system, and then an etherifying agent is added to carry out the first stage etherification reaction at a temperature of 40-60°C; Then the temperature is raised to 65-80°C to carry out the second stage etherification reaction; after the reaction is completed, the obtained product is separated to obtain modified starch.

2. The method for preparing the modified starch suitable for carbon coating of lithium battery materials according to claim 1, characterized in that: The concentration of the starch slurry is 25-50 wt %.

3. The method for preparing the modified starch suitable for carbon coating of lithium battery materials according to claim 1, characterized in that: The starch is at least one of corn starch, wheat starch, potato starch and tapioca starch.

4. The method for preparing the modified starch suitable for carbon coating of lithium battery materials according to claim 1, characterized in that: The molar ratio of LiOH to Li2CO3 in the composite system is (3-6):

1.

5. The method for preparing the modified starch suitable for carbon coating of lithium battery materials according to claim 1, characterized in that: The etherifying agent is at least one of ethylene oxide, propylene oxide, N-(2,3-epoxypropyl)diethylamine, N-(2,3-epoxychloropropyl)trimethylammonium chloride, and 3-chloro-2-hydroxypropyl-trimethylammonium chloride.

6. The method for preparing the modified starch suitable for carbon coating of lithium battery materials according to claim 5, characterized in that: The amount of the etherifying agent added is 5%-10% of the mass of the starch.

7. The method for preparing the modified starch suitable for carbon coating of lithium battery materials according to claim 1, characterized in that: The time of the first stage etherification reaction is 5-12 hours, and the time of the second stage etherification reaction is 5-12 hours.

8. A modified starch, characterized in that: The modified starch is prepared by the preparation method according to any one of claims 1 to 7.

9. The modified starch as claimed in claim 8 is used as a carbon source for carbon coating of lithium battery materials.

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