Modified starch suitable for carbon coating of lithium battery materials and preparation method and application thereof

Abstract

The present invention discloses a modified starch suitable for carbon coating of lithium battery materials and its preparation method and application, which belongs to the field of new materials industry. The preparation method of the modified starch is: starch is mixed with water to obtain starch slurry; LiOH and Li2CO3 are mixed to obtain a composite system; the starch slurry is adjusted to alkalinity using the composite system, and then an etherifying agent is added, and the first stage etherification reaction is carried out at a temperature of 40-60°C; the temperature is then raised to 65-80°C, and the second stage etherification reaction is carried out; after the reaction is completed, the resulting product is separated to obtain modified starch. The modified starch preparation method provided by the present invention has a simple process, reduces subsequent neutralization and washing steps, and avoids the introduction of sodium and potassium ions that have an impact on electrical performance. The obtained product is suitable for the carbon coating process of battery materials, can effectively improve the polarization of battery materials and increase the compaction density of materials, thereby improving battery performance.

Description

Technical Field

[0001] The present invention belongs to the field of new materials 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 enhancing 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, the solubility and viscosity of starch can be improved, thereby improving the coating performance of starch. 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 high pH in the initial stage of the reaction will lead to excessive hydrolysis of starch, and during the reaction, OH groups will cause the starch to be degraded. - The sudden drop causes the reaction to stagnate, making it difficult to control the reaction rate. The traditional etherified starch preparation process uses sodium sulfate and other neutralization agents. These alkaline catalysts and the neutralization reaction process easily lead to sodium and potassium elements remaining in the final starch product. When sodium and potassium enter the battery material system, they occupy lithium active sites, resulting in a decrease in reversible capacity and the formation of an unstable solid electrolyte interface film in the electrolyte, which leads to poor cycling performance of the material.

[0004] Patent CN114231577 discloses a method for preparing porous etherified starch, using sodium sulfate and dilute alkali solution as acid-base regulators to prepare starch. Sodium ions are introduced during the process, and subsequent neutralization and washing are required. However, simple acid-base neutralization cannot remove the sodium and potassium elements in the product. Removing sodium and potassium elements through a deionization process increases the complexity and cost of production. Summary of the Invention

[0005] In response to the deficiencies in the prior art, the present invention aims to provide a modified starch suitable for carbon coating of lithium battery materials, as well as a preparation method and application thereof. The present invention solves the problem of excessive sodium and potassium content in common etherified starches on the market by using a LiOH-Li2CO3 composite system instead of common single substances such as sodium hydroxide or potassium hydroxide. At the same time, a two-stage temperature control program is used to achieve precise control of the degree of substitution of starch. The prepared 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 a lithium battery material with excellent performance.

[0006] The technical solution adopted in the present invention is:

[0007] The first object of the present invention is to provide a method for preparing carbon-coated modified starch suitable for lithium battery materials, comprising the following steps:

[0008] 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.

[0009] LiOH and Li2CO3 are mixed to obtain a LiOH-Li2CO3 composite system; specifically, LiOH and Li2CO3 can be mixed and dissolved in water to obtain a LiOH-Li2CO3 composite system;

[0010] The starch slurry is adjusted to alkaline using a LiOH-Li2CO3 composite system, preferably with a pH of 10-14. An etherifying agent is then added to carry out the first stage etherification reaction at 40-60°C for 5-12 hours. The temperature is then raised to 65-80°C for the second stage etherification reaction for 5-12 hours. Compared to the alkaline environment provided by a single strong base such as LiOH or NaOH, the LiOH-Li2CO3 composite system can maintain a long-lasting and continuous high OH content. - concentration, the principle is: LiOH provides high concentration OH during the reaction - , Li2CO3 is used as a reserve alkali source, in OH - Replenish when consumed, and high pH conditions will inhibit CO3 2- The hydrolysis and loss of the PH value are prolonged to maintain the high pH value, keep the reaction controllable and stable, and use two-stage temperature control during the reaction process. The first stage is 40-60℃ to promote the substitution reaction, and the substitution degree is controlled at a lower temperature to avoid side reactions caused by excessive temperature, such as starch hydrolysis; the second stage is 65-80℃ to promote CO3 2- Hydrolysis, continuous replenishment of OH -, and accelerate the etherification process until the reaction stops, achieving precise control of the degree of 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.

[0011] After the reaction is completed, the obtained product is centrifugally dried until the moisture content is less than 8 wt%, and sieved to obtain modified starch.

[0012] As a preferred technical solution, the molar ratio of LiOH to Li2CO3 in the composite system is (3-6):1. If it is less than 3:1, the excess lithium carbonate may lead to a low pH, preventing the catalytic reaction from occurring in the early stages of the reaction. If it is greater than 6:1, the amount of lithium carbonate may be too little to function as a reserve base.

[0013] 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. Furthermore, preferably, the etherifying agent is added in an amount of 5%-10% by weight of the starch.

[0014] The second object of the present invention is to provide a modified starch, which is prepared by the preparation method described in the first object.

[0015] 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.

[0016] Compared with the prior art, the present invention has the following advantages:

[0017] (1) The present invention reduces the subsequent neutralization and washing steps during the preparation of modified etherified starch, while also avoiding the introduction of sodium and potassium ions that could affect the electrical performance of lithium batteries. The resulting product is suitable for carbon coating of battery materials, effectively improving the polarization of battery materials and increasing the compaction density of the materials. Furthermore, the present invention significantly reduces wastewater discharge during the production process, saving energy and protecting the environment, and lowering wastewater treatment costs.

[0018] (2) The traditional etherified starch preparation process uses sodium hydroxide or potassium hydroxide to provide an alkaline environment, resulting in a high content of sodium and potassium elements in the finished product. The entry of sodium and potassium elements 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 a LiOH-Li2CO3 composite system to prepare etherified starch, avoiding the deterioration of the battery material electrode performance caused by the introduction of sodium and potassium elements. At the same time, the LiOH-Li2CO3 composite can ensure a high pH during the reaction process, making the reaction process controllable and preventing the reaction from stagnating due to a rapid drop in pH or starch hydrolysis due to an excessively high pH at the initial stage of the reaction.

[0019] (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 a Li3PO4 conductive network with the lithium iron phosphate surface during the carbon coating sintering process, which can effectively reduce the electrode interface impedance, effectively improve the material polarization and increase the compaction density.

[0020] (4) Compared with the use of LiOH alone, the combined use of LiOH-Li2CO3 reduces the excessive use of strong bases, reducing the total lithium usage 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, with relatively mild reaction conditions, low energy consumption and low cost, making it suitable for large-scale industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The state of charge-voltage curve of batteries assembled with different materials. DETAILED DESCRIPTION

[0022] The present invention will be further described below with reference to 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.

[0023] In addition, unless otherwise specified, the preparation processes in the following examples are all conventional means in the prior art in this field, and therefore, they are not described in detail; the raw materials used in the following examples are all commercially available products.

[0024] Example 1

[0025] A method for preparing carbon-coated modified starch suitable for lithium battery materials comprises the following steps:

[0026] 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%.

[0027] Step 2: Adjust the corn starch to a starch slurry with a concentration of 40 wt%; add a LiOH-Li2CO3 composite system with a molar ratio of 6:1 to adjust the pH to 11.5±0.2, then add propylene oxide at 6 wt% of the mass of the starch, mix well, and place the mixture in a heatable reaction vessel and stir continuously.

[0028] 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.

[0029] 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 future use.

[0030] Example 2

[0031] A method for preparing carbon-coated modified starch suitable for lithium battery materials comprises the following steps:

[0032] 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%.

[0033] Step 2: Adjust the corn starch to a starch slurry with a concentration of 40 wt%; add a LiOH-Li2CO3 composite system with a molar ratio of 4:1 to adjust the pH to 11.5±0.2, then add 6 wt% of N-(2,3-epoxypropyl)diethylamine based on the mass of the starch, mix well, and place the mixture in a heatable reaction vessel with continuous stirring.

[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 and then centrifuged to dry to obtain modified starch, which is then stored for future use.

[0036] Example 3

[0037] A method for preparing carbon-coated modified starch suitable for lithium battery materials comprises the following steps:

[0038] 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%.

[0039] Step 2: Adjust the corn starch to a starch slurry with a concentration of 40 wt%; add a LiOH-Li2CO3 composite system with a molar ratio of 5:1 to adjust the pH to 11.5±0.2, then add 6 wt% of N-(2,3-epoxychloropropyl)trimethylammonium chloride based on the mass of the starch, mix well, and place the mixture in a heatable reaction vessel and stir continuously.

[0040] 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.

[0041] 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 future use.

[0042] Comparative Example 1

[0043] 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:

[0044] Step 1: Prepare an appropriate amount of corn starch with a moisture content of 10-12%, and prepare lithium hydroxide with a battery grade specification and a purity greater than 99%.

[0045] 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 propylene oxide at 6 wt% of the mass of the starch, mix well, and place the mixture in a heatable reaction vessel and stir continuously.

[0046] 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.

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

[0048] Comparative Example 2

[0049] 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:

[0050] Step 1: Prepare an appropriate amount of corn starch with a moisture content of 10-12%, and prepare lithium carbonate with a battery grade purity greater than 99%.

[0051] 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 propylene oxide at 6 wt% of the mass of the starch, mix well, and place the mixture in a heatable reaction vessel and stir continuously.

[0052] 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.

[0053] Step 4: After the reaction is completed, the mixture is cooled to room temperature, centrifuged and dried, and stored for later use.

[0054] Comparative Example 3

[0055] 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:

[0056] 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%.

[0057] Step 2: Adjust the corn starch to a starch slurry with a concentration of 40 wt%, add a LiOH-Li2CO3 composite system with a molar ratio of 6:1 to adjust the pH to 11.5±0.2, then add 6 wt% of propylene oxide based on the mass of starch, mix well, and place the mixture in a heatable reaction vessel and stir continuously.

[0058] Step 3: Gradually heat the mixture to 50°C and maintain the temperature constant. During this process, continuously stir the mixture to allow it to react fully. The reaction time is 15 hours.

[0059] Step 4: After the reaction is completed, the mixture is cooled to room temperature, then centrifuged and dried, and stored for future use.

[0060] Comparative Example 4

[0061] The difference between this comparative example and Example 1 is that: in step 3, no two-stage temperature control is performed, and the temperature is always maintained at 70°C for 15 hours. The method is as follows:

[0062] 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%.

[0063] Step 2: Adjust the corn starch to a starch slurry with a concentration of 40 wt%, add a LiOH-Li2CO3 composite system with a molar ratio of 6:1 to adjust the pH to 11.5±0.2, then add 6 wt% of propylene oxide based on the mass of starch, mix well, and place the mixture in a heatable reaction vessel and stir continuously.

[0064] Step 3: Gradually heat the mixture to 70°C and maintain the temperature constant. During this process, continuously stir the mixture to allow it to react fully. The reaction time is 15 hours.

[0065] Step 4: After the reaction is completed, the mixture is cooled to room temperature, then centrifuged and dried, and stored for future use.

[0066] Application Examples

[0067] The modified starch prepared in the above 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 was as follows: iron phosphate, lithium carbonate, and carbon source were 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 was 783±3°C.

[0068] Performance testing and result analysis

[0069] The battery materials prepared using the modified starches prepared in each of the Examples and Comparative Examples as the carbon source served as the positive electrode materials. The positive electrode material, conductive graphite, and polytetrafluoroethylene were uniformly mixed in a mass ratio of 7:2:1. N-methylpyrrolidone solution was added dropwise and stirred to form a slurry. The slurry was then coated onto a PE film and vacuum-dried to obtain the positive electrode sheet. A lithium metal sheet was used as the negative electrode material to construct a button cell battery for measuring its electrical performance.

[0070] 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 sheet compaction density prepared from these modified starches, are compared as shown in Table 1:

[0071] Table 1

[0072]

[0073] 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 beginning of the reaction. 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 - Due to insufficient supply, substitution is concentrated in the easily reactive sites. Due to insufficient substitution, the coating layer has poor adhesion and forms 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 electrode compaction density of Example 1 is the highest, reaching above 2.63g / cc, while that of Comparative Example 1 can only reach 2.57g / cc. Due to the poor adhesion of the carbon layer, the compaction density of Comparative Example 2 can only reach 2.53g / cc. Compared with Example 1, the starch prepared in Comparative Example 3 does not have a heating stage, so the reaction degree is insufficient, the substitution degree is low, and the water solubility is poorer than that of Example 1. Therefore, the compaction density of the prepared battery material is also low. Due to the use of high temperature at the beginning of the reaction in the preparation process of Comparative Example 4 and the heat release of lithium hydroxide, the reaction temperature is difficult to control. If the temperature is too high at the beginning of the reaction, the starch substitution degree 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.

[0074] 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 reduces the discharge voltage platform and deteriorates the polarization improvement performance; 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 increases the interfacial impedance; the starch prepared in Comparative Example 3 does not have a heating stage compared to Example 1, resulting in a lower degree of substitution, lower water solubility compared to Example 1, and relatively poor coating performance, and the polarization improvement effect is also lower; Comparative Example 4 is similar to Comparative Example 2 and will lead to aggravated polarization.

[0075] The above embodiments are merely preferred embodiments of the present invention and are intended to be illustrative rather than restrictive of the present invention. Those skilled in the art will appreciate that many changes, modifications, and even equivalents may be made to the embodiments within the spirit and scope of the claims, all of which 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; LiOH and Li2CO3 are mixed to obtain a composite system; Use a composite system to adjust the starch slurry to alkalinity, then add an etherifying agent and conduct 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; The molar ratio of LiOH to Li2CO3 in the composite system is (3-6):1; the time for the first-stage etherification reaction is 5-12 hours, and the time for the second-stage etherification reaction is 5-12 hours.

2. The method for preparing carbon-coated modified starch suitable for 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 carbon-coated modified starch suitable for 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 carbon-coated modified starch suitable for 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.

5. The method for preparing carbon-coated modified starch suitable for lithium battery materials according to claim 4, characterized in that: The amount of the etherifying agent added is 5%-10% of the mass of the starch.

6. A modified starch, characterized in that: The modified starch is prepared by the preparation method according to any one of claims 1 to 5.

7. The modified starch according to claim 6 is used as a carbon source for carbon coating of lithium battery materials.

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