A lotus root hard carbon, a lotus root hard carbon / silicon composite material, its preparation method and application

Abstract

This invention relates to the field of lithium-ion battery anode materials, and particularly to a lotus root hard carbon, a lotus root hard carbon / silicon composite material, its preparation method, and its application. The preparation method of the lotus root hard carbon includes the following steps: (S.1) pre-treating the lotus root after slicing to oxidize the polyphenols in the lotus root; (S.2) placing the pre-treated lotus root in a solution containing acid and a structural stabilizer to dissolve the soluble substances in the lotus root; (S.3) drying and high-temperature carbonizing the treated lotus root to obtain crude lotus root hard carbon; (S.4) removing impurities from the crude lotus root hard carbon to obtain the lotus root hard carbon. The hard carbon / silicon composite material obtained by this invention has excellent characteristics of high capacity, high rate capability, low expansion, and high cycle life.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery anode materials, and in particular to a lotus root hard carbon, a lotus root hard carbon / silicon composite material, its preparation method and application. Background Technology

[0002] In recent years, as portable electronic products have increasingly demanded larger battery sizes, there is an urgent need to further improve battery capacity and energy density. Graphite materials with a theoretical specific capacity of only 372 mAh / g can no longer meet market needs, while silicon materials with a theoretical specific capacity as high as 4200 mAh / g have become the mainstream development trend for small electronic products.

[0003] Unlike the intercalation reaction of graphite, silicon materials undergoing lithium insertion / extraction in an alloying form experience significant volume expansion (~300%) during charge and discharge. The particles are highly susceptible to pulverization and loss of electrical contact with the current collector, leading to continuous particle deactivation during cycling and a sharp decline in cycle performance. To mitigate negative electrode expansion, secondary particles are often formed by granulation using graphite, silicon, and organic carbon to suppress this problem.

[0004] For example, patent CN112310363A describes a series of operations, including dispersing, granulating, molding, cold / static pressing, heat treatment, pulverizing, and carbon coating of graphite, silicon, and organic carbon precursors, to prepare graphite / silicon composite materials.

[0005] However, this preparation method has many drawbacks. Due to the two-order-of-magnitude difference in particle size between nano-silicon and graphite, it is difficult to disperse them uniformly, resulting in different expansion amplitudes in different regions, which is very limited in suppressing electrode expansion. In graphite materials where lithium ion intercalation / deintercalation occurs through intercalation reaction, the ion intercalation / deintercalation is slow, which inhibits the development of the composite material's kinetic properties. Furthermore, this preparation method is complex and has high processing costs. Summary of the Invention

[0006] The present invention aims to overcome the shortcomings of existing lithium-ion battery anode materials, such as low theoretical specific capacity and huge volume expansion during charging and discharging. It provides a lotus root hard carbon, a lotus root hard carbon / silicon composite material, its preparation method and application to overcome the above-mentioned deficiencies.

[0007] To achieve the above-mentioned objectives, the present invention is implemented through the following technical solution:

[0008] In a first aspect, the present invention provides a method for preparing lotus root hard carbon, comprising the following steps:

[0009] (S.1) Cut the lotus root and pre-treat it to oxidize the polyphenols in the lotus root;

[0010] (S.2) The pretreated lotus root is placed in a solution containing acid and structural stabilizer, so that the soluble substances in the lotus root are dissolved.

[0011] (S.3) The lotus root processed in step (S.2) is dried and carbonized at high temperature to obtain crude lotus root hard carbon.

[0012] (S.4) Remove impurities from the crude lotus root hard carbon to obtain the lotus root hard carbon.

[0013] The hard carbon in this application uses lotus root as raw material. Compared with other biomass, lotus root has more micropores and fibrous structure. This structure may help to form specific pore structure and morphology during the carbonization process, thereby giving hard carbon good physical properties, such as porosity and specific surface area.

[0014] However, fresh lotus root contains a significant amount of soluble substances (such as sugars and organic acids). The presence of these soluble substances may introduce impurities during the subsequent carbonization process, affecting the purity and quality of the hard carbon. Furthermore, these soluble substances can interfere with the carbonization reaction, causing damage to the hard carbon's structure through decomposition or phase transformation, thereby reducing its structural stability and lifespan, ultimately impacting the uniformity and efficiency of carbonization. Therefore, this application incorporates a pretreatment step before lotus root carbonization to remove soluble substances, resulting in a purer hard carbon product with a more ideal structure and performance, making the carbonization process more controllable and stable.

[0015] However, in the process of removing soluble substances from lotus root, the applicant found that both acid dissolution and alkaline solutions can remove soluble substances. However, when using alkaline solutions, substances such as hemicellulose in lotus root are also easily dissolved and removed, resulting in poor structural stability of the final lotus root. It is easy to break or even melt and form muddy products during transfer and handling, thus failing to maintain its porous properties.

[0016] Therefore, in this application, the inventors use acid soaking to selectively dissolve the soluble substances, while retaining the insoluble substances such as cellulose and hemicellulose in situ, thus helping to maintain its porous structure. To further maintain these porous structures, the inventors also added a certain amount of structural stabilizers to the solution. These structural stabilizers are mainly water-soluble polymers, which can provide some support to the lotus root skeleton after the organic matter has dissolved, thereby greatly improving the structural stability of the lotus root and preventing the collapse of the porous structure.

[0017] Furthermore, the applicant discovered that fresh lotus root contains a large amount of polyphenols, which have high reducing properties and can react with oxygen to form black quinones. The applicant found that the transformation of polyphenols into quinones in lotus root has a significant impact on subsequent carbonization steps. During the carbonization process, high temperatures cause the organic matter in the lotus root to decompose and transform, forming a hard carbon structure. Quinones possess certain chemical activity and may participate in a series of reactions at high temperatures, affecting the formation and properties of hard carbon. Specifically, firstly, the presence of quinones allows them to undergo cross-linking and polymerization reactions with other organic matter, thus contributing to increased structural stability and mechanical strength of hard carbon materials. Secondly, the presence of quinones may affect the pore structure of hard carbon materials. They may participate in the pore formation process or react on the pore walls, thus affecting the size, distribution, and connectivity of the pores. Finally, the functional groups in quinones may provide active sites on the surface of hard carbon materials, which is highly beneficial for applications such as energy storage and adsorption. Therefore, this application aims to regulate the structure and properties of hard carbon materials to a certain extent by oxidizing polyphenols into quinones before carbonization.

[0018] In summary, this invention has made targeted selections and improvements to the types of raw materials for hard carbon materials, the processing methods of raw materials, and the transformation of specific substances in the raw materials. This effectively removes components that may interfere with the carbonization process and affect the quality of the final product, resulting in hard carbon materials with higher purity and better structural stability.

[0019] Preferably, the pretreatment in step (S.1) includes soaking the lotus root in an alkaline solution with a pH of 8.5-10;

[0020] The preprocessing time in step (S.1) is greater than or equal to 6 hours.

[0021] In step (S.1), soaking is performed in an alkaline solution with a pH of 8.5-10. This is because an alkaline solution within this pH range can react with soluble substances in the lotus root to a certain extent, dissolving and removing them, thus reducing impurities in the subsequent carbonization process. Simultaneously, it can maintain the original porous microstructure of the lotus root, creating favorable conditions for the subsequent carbonization process and helping to form a more ideal hard carbon structure. Furthermore, the alkaline solution environment helps promote the oxidation reaction of polyphenols in the lotus root, thereby contributing to achieving the desired oxidation effect.

[0022] Preferably, the pH value of the acid solution in step (S.2) is 2-3.

[0023] Preferably, the structural stabilizer is one or more of polyvinyl alcohol, polyethylene glycol, acrylamide, carboxymethyl cellulose, sodium polyacrylate, and sodium alginate.

[0024] Preferably, the concentration of the structural stabilizer is 0.5-10 g / 100 ml.

[0025] Preferably, the high-temperature carbonization temperature range in step (S.2) is 500℃-900℃.

[0026] In this application, the high-temperature carbonization temperature range is 500℃-900℃. This is because within this temperature range, the organic matter in lotus root can be fully decomposed and transformed, forming a carbon structure with a certain degree of crystallinity and stability. Furthermore, a suitable temperature helps to control the pore size and distribution of hard carbon. Lower temperatures may result in underdeveloped pores, while excessively high temperatures may cause excessive pore shrinkage or merging. The 500℃-900℃ range is conducive to forming a suitable pore structure to meet the pore characteristics required for different applications. Simultaneously, this temperature range allows for sufficient carbonization of the material while avoiding excessive embrittlement or structural collapse caused by excessively high temperatures, thus maintaining a certain mechanical strength and contributing to the formation of hard carbon with good electrical conductivity, resulting in better charge-discharge performance in applications such as batteries.

[0027] Secondly, the present invention also provides lotus root hard carbon, which is prepared by the method described above.

[0028] Thirdly, the present invention also provides a lotus root hard carbon / silicon composite material, which is obtained by combining lotus root hard carbon and nano-silicon as described above.

[0029] Silicon possesses an extremely high theoretical specific capacity, which can significantly improve the lithium storage capacity of composite materials, thereby increasing the energy density of batteries. However, during charge and discharge, silicon undergoes significant volume expansion and contraction. The lotus root hard carbon / silicon composite material provided by this invention utilizes the porous structure of lotus root carbonized material to provide buffer space for silicon, reducing the stress caused by volume changes and thus improving the cycle stability of the material. Simultaneously, the three-dimensional structure of hard carbon in the composite material can provide support and constraint for silicon particles, preventing silicon particle agglomeration and maintaining the integrity of the electrode structure.

[0030] Furthermore, this porous carbon material itself possesses high electrical conductivity, serving as an electronic and ion conductor network for electrodes. When combined with silicon, it can mitigate the poor conductivity of silicon, improving electron transport efficiency and thus enhancing the battery's rate performance. Additionally, the specific capacity of this lotus root hard carbon material (790 mAh / g @ 700℃ high-temperature carbonization) is higher than that of graphite, further increasing battery capacity. Ultimately, the hard carbon / silicon composite material obtained by this invention exhibits excellent characteristics of high capacity, high rate capability, low expansion, and high cycle life.

[0031] Compared to using silicon materials alone, this application achieves better performance by incorporating a certain proportion of lotus root hard carbon. Furthermore, lotus root hard carbon and nano-silicon have complementary properties, and their combination can exert a synergistic effect, resulting in a composite material exhibiting superior overall electrochemical performance compared to either material alone.

[0032] Preferably, the mass ratio of lotus root hard carbon to nano-silicon is 4:1 to 1:1.

[0033] Fourthly, the present invention also provides a method for preparing the lotus root hard carbon / silicon composite material as described above, wherein lotus root hard carbon and nano-silicon are mixed, ethanol and zirconium balls are added, the composite material is ball-milled to make the composite material uniformly mixed, and dried to obtain the lotus root hard carbon / silicon composite material.

[0034] Preferably, the mass ratio of the zirconium spheres to the lotus root hard carbon and nano-silicon mixed material is 1:5-15.

[0035] Fifthly, the present invention also provides the application of the lotus root hard carbon or lotus root hard carbon / silicon composite material as described above in lithium-ion batteries.

[0036] In summary, this application has the following beneficial effects:

[0037] To address the problems of high expansion rate, poor rate performance, and battery degradation after short-term cycling associated with silicon anodes, this invention provides a novel anode material preparation method that offers significant improvements compared to traditional graphite / silicon composites. The resulting hard carbon / silicon composite material exhibits excellent properties such as high capacity, high rate performance, low expansion, and long cycle life. Detailed Implementation

[0038] The present invention will be further described below with reference to specific embodiments. Those skilled in the art will be able to implement the present invention based on these descriptions. Furthermore, the embodiments of the present invention described below are generally only some, not all, of the embodiments of the present invention. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0039] Example 1

[0040] Preparation of hard carbon material from lotus root: Lotus root slices were soaked in a sodium carbonate solution with a pH of 9 for 24 hours, with a mass ratio of lotus root to sodium carbonate solution of 2:1. After soaking, the lotus root slices changed from white to black. Then, the lotus root strips were removed and soaked in a hydrochloric acid solution with a pH of 2.5, with a mass ratio of lotus root to hydrochloric acid solution of 2:1. The hydrochloric acid solution also contained 5 g / 100 ml of polyvinyl alcohol. After soaking for 24 hours, the lotus root strips were removed and dried at 110℃ for 12 hours. Then, they were carbonized at 700℃ for 2 hours in a quartz tube under a N2 atmosphere. After carbonization, the strips were washed with 1 mol / L HCl solution to remove residual potassium salts and impurities, then washed with deionized water and ethanol until neutral, and finally dried at 80℃.

[0041] Preparation of composite material: Lotus root hard carbon material and nano-silicon were mixed at a mass ratio of 1:1, and ethanol and zirconium balls were added. The mass ratio of zirconium balls to lotus root hard carbon and nano-silicon mixed material was 1:10. The mixture was ball-milled for 4 hours to make the composite material uniformly mixed. The mixture was then dried at 80℃ to obtain lotus root hard carbon / silicon composite material.

[0042] Example 2

[0043] Preparation of hard carbon material from lotus root: Lotus root slices were soaked in a sodium carbonate solution with a pH of 9 for 24 hours, with a mass ratio of lotus root to sodium carbonate solution of 2:1. After soaking, the lotus root slices changed from white to black. Then, the lotus root strips were removed and soaked in a hydrochloric acid solution with a pH of 2.5, with a mass ratio of lotus root to hydrochloric acid solution of 2:1. The hydrochloric acid solution also contained 5 g / 100 ml of polyvinyl alcohol. After soaking for 24 hours, the lotus root strips were removed and dried at 110℃ for 12 hours. Then, they were carbonized at 500℃ for 4 hours in a quartz tube under a N2 atmosphere. After carbonization, the strips were washed with 1 mol / L HCl solution to remove residual potassium salts and impurities, then washed with deionized water and ethanol until neutral, and finally dried at 80℃.

[0044] Preparation of composite material: Lotus root hard carbon material and nano-silicon were mixed at a mass ratio of 1:1, and ethanol and zirconium balls were added. The mass ratio of zirconium balls to lotus root hard carbon and nano-silicon mixed material was 1:5. The mixture was ball-milled for 4 hours to make the composite material uniformly mixed. The mixture was then dried at 80℃ to obtain lotus root hard carbon / silicon composite material.

[0045] Example 3

[0046] Preparation of hard carbon material from lotus root: Lotus root slices were soaked in a sodium carbonate solution with a pH of 9 for 24 hours, with a mass ratio of lotus root to sodium carbonate solution of 2:1. After soaking, the lotus root slices changed from white to black. Then, the lotus root strips were removed and soaked in a hydrochloric acid solution with a pH of 2.5, with a mass ratio of lotus root to hydrochloric acid solution of 2:1. The hydrochloric acid solution also contained 5 g / 100 ml of polyvinyl alcohol. After soaking for 24 hours, the lotus root strips were removed and dried at 110℃ for 12 hours. Then, they were carbonized at 600℃ for 3 hours in a quartz tube under a N2 atmosphere. After carbonization, the strips were washed with 1 mol / L HCl solution to remove residual potassium salts and impurities, then washed with deionized water and ethanol until neutral, and finally dried at 80℃.

[0047] Preparation of composite material: Lotus root hard carbon material and nano-silicon were mixed at a mass ratio of 1:1, and ethanol and zirconium balls were added. The mass ratio of zirconium balls to lotus root hard carbon and nano-silicon mixed material was 1:10. The mixture was ball-milled for 4 hours to make the composite material uniformly mixed. The mixture was then dried at 80℃ to obtain lotus root hard carbon / silicon composite material.

[0048] Example 4

[0049] Preparation of hard carbon material from lotus root: Lotus root slices were soaked in a sodium carbonate solution with a pH of 9 for 24 hours, with a mass ratio of lotus root to sodium carbonate solution of 2:1. After soaking, the lotus root slices changed from white to black. Then, the lotus root strips were removed and soaked in a hydrochloric acid solution with a pH of 2.5, with a mass ratio of lotus root to hydrochloric acid solution of 2:1. The hydrochloric acid solution also contained 5 g / 100 ml of polyvinyl alcohol. After soaking for 24 hours, the lotus root strips were removed and dried at 110℃ for 12 hours. Then, they were carbonized at 800℃ for 2 hours in a quartz tube under a N2 atmosphere. After carbonization, the strips were washed with 1 mol / L HCl solution to remove residual potassium salts and impurities, then washed with deionized water and ethanol until neutral, and finally dried at 80℃.

[0050] Preparation of composite material: Lotus root hard carbon material and nano-silicon were mixed at a mass ratio of 1:1, and ethanol and zirconium balls were added. The mass ratio of zirconium balls to lotus root hard carbon and nano-silicon mixed material was 1:10. The mixture was ball-milled for 4 hours to make the composite material uniformly mixed. The mixture was then dried at 80℃ to obtain lotus root hard carbon / silicon composite material.

[0051] Example 5

[0052] Preparation of hard carbon material from lotus root: Lotus root slices were soaked in a sodium carbonate solution with a pH of 9 for 24 hours, with a mass ratio of lotus root to sodium carbonate solution of 2:1. After soaking, the lotus root slices changed from white to black. Then, the lotus root strips were removed and soaked in a hydrochloric acid solution with a pH of 2.5, with a mass ratio of lotus root to hydrochloric acid solution of 2:1. The hydrochloric acid solution also contained 5 g / 100 ml of polyvinyl alcohol. After soaking for 24 hours, the lotus root strips were removed and dried at 110℃ for 12 hours. Then, they were carbonized at 900℃ for 2 hours in a quartz tube under a N2 atmosphere. After carbonization, the strips were washed with 1 mol / L HCl solution to remove residual potassium salts and impurities, then washed with deionized water and ethanol until neutral, and finally dried at 80℃.

[0053] Preparation of composite material: Lotus root hard carbon material and nano-silicon were mixed at a mass ratio of 1:1, and ethanol and zirconium balls were added. The mass ratio of zirconium balls to lotus root hard carbon and nano-silicon mixed material was 1:10. The mixture was ball-milled for 4 hours to make the composite material uniformly mixed. The mixture was then dried at 80℃ to obtain lotus root hard carbon / silicon composite material.

[0054] Example 6

[0055] Preparation of hard carbon material from lotus root: Lotus root slices were soaked in a KOH solution with a pH of 8.5 for 24 hours, with a mass ratio of lotus root to KOH solution of 2:1. After soaking, the lotus root slices changed from white to black. Then, the lotus root strips were removed and soaked in a hydrochloric acid solution with a pH of 2, with a mass ratio of lotus root to hydrochloric acid solution of 2:1. The hydrochloric acid solution also contained 0.5 g / 100 ml of carboxymethyl cellulose. After soaking for 24 hours, the lotus root strips were removed and dried at 110℃ for 12 hours. Then, they were carbonized at 700℃ for 2 hours in a quartz tube under a N2 atmosphere. After carbonization, the strips were washed with 1 mol / L HCl solution to remove residual potassium salts and impurities, then washed with deionized water and ethanol until neutral, and finally dried at 80℃.

[0056] Preparation of composite material: Lotus root hard carbon material and nano-silicon were mixed at a mass ratio of 4:1, and ethanol and zirconium balls were added. The mass ratio of zirconium balls to lotus root hard carbon and nano-silicon mixed material was 1:10. The mixture was ball-milled for 4 hours to make the composite material uniformly mixed. The mixture was then dried at 80℃ to obtain lotus root hard carbon / silicon composite material.

[0057] Example 7

[0058] Preparation of hard carbon material from lotus root: Lotus root slices were soaked in a 3 mol / L KOH solution for 24 hours, with a lotus root to KOH solution mass ratio of 2:1. After soaking, the lotus root slices changed from white to black. Then, the lotus root slices were removed and soaked in a hydrochloric acid solution with a pH of 3, with a lotus root to hydrochloric acid solution mass ratio of 2:1. The hydrochloric acid solution also contained 10 g / 100 ml of polyethylene glycol. After soaking for 24 hours, the lotus root slices were removed and dried at 110℃ for 12 hours. Then, they were carbonized at 700℃ for 2 hours in a quartz tube under a N2 atmosphere. After carbonization, the slices were washed with a 1 mol / L HCl solution to remove residual potassium salts and impurities, then washed with deionized water and ethanol until neutral, and finally dried at 80℃.

[0059] Preparation of composite material: Lotus root hard carbon material and nano-silicon were mixed at a mass ratio of 1.5:1, and ethanol and zirconium balls were added. The mass ratio of zirconium balls to lotus root hard carbon and nano-silicon mixed material was 1:10. The mixture was ball-milled for 4 hours to make the composite material uniformly mixed. The mixture was dried at 80℃ to obtain lotus root hard carbon / silicon composite material.

[0060] Comparative Example 1

[0061] Preparation of hard carbon material from lotus root: Lotus root slices were soaked in a sodium carbonate solution with a pH of 9 for 24 hours, with a mass ratio of lotus root to sodium carbonate solution of 2:1. After soaking, the lotus root slices changed from white to black. Then, the lotus root strips were removed and soaked in a hydrochloric acid solution with a pH of 2.5, with a mass ratio of lotus root to hydrochloric acid solution of 2:1. The hydrochloric acid solution also contained 5 g / 100 ml of polyvinyl alcohol. After soaking for 24 hours, the lotus root strips were removed and dried at 110℃ for 12 hours. Then, they were carbonized at 400℃ for 4 hours in a quartz tube under a N2 atmosphere. After carbonization, the strips were washed with 1 mol / L HCl solution to remove residual potassium salts and impurities, then washed with deionized water and ethanol until neutral, and finally dried at 80℃.

[0062] Preparation of composite material: Lotus root hard carbon material and nano-silicon were mixed at a mass ratio of 1:1, and ethanol and zirconium balls were added. The mass ratio of zirconium balls to lotus root hard carbon and nano-silicon mixed material was 1:10. The mixture was ball-milled for 4 hours to make the composite material uniformly mixed. The mixture was then dried at 80℃ to obtain lotus root hard carbon / silicon composite material.

[0063] Comparative Example 2

[0064] Preparation of hard carbon material from lotus root: Lotus root slices were soaked in a sodium carbonate solution with a pH of 9 for 24 hours, with a mass ratio of lotus root to sodium carbonate solution of 2:1. After soaking, the lotus root slices changed from white to black. Then, the lotus root strips were removed and soaked in a hydrochloric acid solution with a pH of 2.5, with a mass ratio of lotus root to hydrochloric acid solution of 2:1. The hydrochloric acid solution also contained 5 g / 100 ml of polyvinyl alcohol. After soaking for 24 hours, the lotus root strips were removed and dried at 110℃ for 12 hours. Then, they were carbonized at 1000℃ for 2 hours in a quartz tube under a N2 atmosphere. After carbonization, the strips were washed with 1 mol / L HCl solution to remove residual potassium salts and impurities, then washed with deionized water and ethanol until neutral, and finally dried at 80℃.

[0065] Preparation of composite material: Lotus root hard carbon material and nano-silicon were mixed at a mass ratio of 1:1, and ethanol and zirconium balls were added. The mass ratio of zirconium balls to lotus root hard carbon and nano-silicon mixed material was 1:10. The mixture was ball-milled for 4 hours to make the composite material uniformly mixed. The mixture was then dried at 80℃ to obtain lotus root hard carbon / silicon composite material.

[0066] Comparative Example 3

[0067] Preparation of hard carbon material from lotus root: Lotus root slices were soaked in a sodium carbonate solution with a pH of 9 for 24 hours, with a mass ratio of lotus root to sodium carbonate solution of 2:1. After soaking, the lotus root slices changed from white to black. Then, the lotus root strips were removed and soaked in a hydrochloric acid solution with a pH of 2.5, with a mass ratio of lotus root to hydrochloric acid solution of 2:1. The hydrochloric acid solution also contained 5 g / 100 ml of polyvinyl alcohol. After soaking for 24 hours, the lotus root strips were removed and dried at 110℃ for 12 hours. Then, they were carbonized at 700℃ for 4 hours in a quartz tube under a N2 atmosphere. After carbonization, the strips were washed with 1 mol / L HCl solution to remove residual potassium salts and impurities, then washed with deionized water and ethanol until neutral, and finally dried at 80℃.

[0068] Preparation of composite material: Lotus root hard carbon material and nano-silicon were mixed at a mass ratio of 1:1.5, and ethanol and zirconium balls were added. The mass ratio of zirconium balls to lotus root hard carbon and nano-silicon mixed material was 1:10. The mixture was ball-milled for 4 hours to make the composite material uniformly mixed. The mixture was dried at 80℃ to obtain lotus root hard carbon / silicon composite material.

[0069] Comparative Example 4

[0070] Preparation of hard carbon material from lotus root: Lotus root slices were soaked in a sodium carbonate solution with a pH of 9 for 24 hours, with a mass ratio of lotus root to sodium carbonate solution of 2:1. After soaking, the lotus root slices changed from white to black. Then, the lotus root strips were removed and soaked in a hydrochloric acid solution with a pH of 2.5, with a mass ratio of lotus root to hydrochloric acid solution of 2:1. The hydrochloric acid solution also contained 5 g / 100 ml of polyvinyl alcohol. After soaking for 24 hours, the lotus root strips were removed and dried at 110℃ for 12 hours. Then, they were carbonized at 700℃ for 4 hours in a quartz tube under a N2 atmosphere. After carbonization, the strips were washed with 1 mol / L HCl solution to remove residual potassium salts and impurities, then washed with deionized water and ethanol until neutral, and finally dried at 80℃.

[0071] Preparation of composite material: Lotus root hard carbon material and nano-silicon were mixed at a mass ratio of 1:4, and ethanol and zirconium balls were added. The mass ratio of zirconium balls to lotus root hard carbon and nano-silicon mixed material was 1:10. The mixture was ball-milled for 4 hours to make the composite material uniformly mixed. The mixture was dried at 80℃ to obtain lotus root hard carbon / silicon composite material.

[0072] Comparative Example 5

[0073] Preparation of hard carbon material from lotus root: Lotus root slices were soaked in a 3 mol / L KOH solution for 24 h, with a lotus root to KOH solution mass ratio of 2:1. After soaking, the lotus root slices changed from white to a black muddy state. The black lotus root mud was removed and dried at 110℃ for 12 h. Then, it was carbonized at 700℃ for 4 h in a quartz tube under a N2 atmosphere. After carbonization, it was washed with a 1 mol / L HCl solution to remove residual potassium salts and impurities, then washed with deionized water and ethanol until neutral, and finally dried at 80℃.

[0074] Preparation of composite material: Lotus root hard carbon material and nano-silicon were mixed at a mass ratio of 1:1, and ethanol and zirconium balls were added. The mass ratio of zirconium balls to lotus root hard carbon and nano-silicon mixed material was 1:10. The mixture was ball-milled for 4 hours to make the composite material uniformly mixed. The mixture was then dried at 80℃ to obtain lotus root hard carbon / silicon composite material.

[0075] Comparative Example 6

[0076] Preparation of hard carbon material from lotus root: Lotus root slices were soaked in a sodium carbonate solution with a pH of 9 for 24 hours, with a mass ratio of lotus root to sodium carbonate solution of 2:1. After soaking, the lotus root slices changed from white to black. Then, the lotus root strips were removed and soaked in a hydrochloric acid solution with a pH of 2.5, with a mass ratio of lotus root to hydrochloric acid solution of 2:1, for 24 hours. After soaking, the lotus root strips were removed and dried at 110℃ for 12 hours. Then, they were carbonized at 700℃ for 2 hours in a quartz tube under a N2 atmosphere. After carbonization, the strips were washed with 1 mol / L HCl solution to remove residual potassium salts and impurities, then washed with deionized water and ethanol until neutral, and finally dried at 80℃.

[0077] Preparation of composite material: Lotus root hard carbon material and nano-silicon were mixed at a mass ratio of 1:1, and ethanol and zirconium balls were added. The mass ratio of zirconium balls to lotus root hard carbon and nano-silicon mixed material was 1:10. The mixture was ball-milled for 4 hours to make the composite material uniformly mixed. The mixture was then dried at 80℃ to obtain lotus root hard carbon / silicon composite material.

[0078] Comparative Example 7

[0079] Comparative Example 7 is a pure silicon anode.

[0080] Electrodes were prepared using the negative electrode materials obtained in Examples 1-7 and Comparative Examples 1-7.

[0081] Electrode preparation: Finally, the obtained lotus root hard carbon / silicon composite material was applied to the negative electrode to assemble a coin cell. The lotus root hard carbon / silicon composite material (SP:CMC:SBR) was added to an agate mortar at a mass ratio of 96:1:1:2, and a suitable amount of deionized water solvent was added. The mixture was then thoroughly ground until homogeneous. Finally, the mixture was uniformly coated onto the surface of a copper foil (i.e., the negative electrode current collector) and vacuum dried at 60°C for 12 hours. The dried electrode was then rolled and cut into 12mm diameter discs. A lithium foil was used as the counter electrode to determine the battery's specific capacity, rate performance, cycle performance, and electrode expansion. The specific capacity performance study mainly tests the specific capacity after three weeks of activation at 25℃ and 0.2C. The rate performance study mainly tests the discharge capacity retention rate under constant current discharge at 25℃ and 2C. The cycle performance study mainly tests the capacity retention rate after 300 cycles at 25℃ and 0.2C. The study on the expansion of the electrode after cycling mainly involves disassembling the battery after 300 cycles and measuring the ratio of the thickness difference of the electrode after activation and after 300 cycles to the thickness before cycling using SEM to calculate the volume expansion rate.

[0082] The test results are shown in Table 1 below:

[0083] Table 1

[0084]

[0085] Comparing Example 1 with Comparative Example 7, it can be seen that the introduction of lotus root hard carbon material can significantly improve the rate performance, cycle performance, and active expansion of the battery. This is mainly due to the conductive electron and ion network and buffer cavity provided by the lotus root hard carbon material. Comparing Example 1 with Examples 2-5 and Comparative Example 1 / 2, it can be seen that carbonization temperature affects the electrical properties of the material. This is mainly because carbonization temperature changes the material's structure, with the optimal electrical performance observed at a carbonization temperature of 700℃. Comparing Example 1 with Examples 6 / 7 and Comparative Example 3 / 4, it can be seen that the mass ratio of lotus root hard carbon material to nano-silicon material affects battery performance. As the proportion of nano-silicon material increases, the battery capacity increases, but the rate performance and cycle performance decrease, and the electrode expansion increases. When the proportion of nano-silicon in the composite material exceeds 60%, the battery cycle performance drops sharply, even drastically. When the proportion of nano-silicon in the composite material exceeds 50%, the differences in battery cycle performance, rate capability, and volume expansion are relatively small. Considering the characteristic that the higher the proportion of nano-silicon in the composite material, the higher the capacity utilization, it was finally determined that the optimal battery performance is achieved when the mass ratio of lotus root hard carbon material to nano-silicon is 1:1. Compared with Comparative Example 5, in Example 1, soaking in a high-concentration potassium hydroxide solution caused a significant decrease in the performance of the lotus root hard carbon prepared from it due to the destruction of the porous structure. In Comparative Example 6, the absence of a structure stabilizer resulted in the loss of some porous structure, leading to a certain degree of performance degradation.

[0086] The specific embodiments described herein are merely illustrative of the spirit of the invention. Those skilled in the art to which this invention pertains may make various modifications or additions to the described specific embodiments or use similar methods to substitute them, without departing from the spirit of the invention or exceeding the scope defined by the appended claims.

Claims

1. A method for preparing lotus root hard carbon, characterized in that, Comprising the following steps: (S.1) Cutting and pretreating lotus root, the pretreatment comprising soaking lotus root in an alkaline solution with pH at 8.5-10 for more than or equal to 6h, so that polyphenols in lotus root are oxidized; (S.2) Placing the pretreated lotus root in a solution containing acid and structure stabilizer, the pH of the acid solution being 2-3, the concentration of the structure stabilizer being 0.5-10g / 100ml, so that soluble substances in lotus root are dissolved out; (S.3) Drying and high-temperature carbonizing the lotus root treated in step (S.2), the high-temperature carbonizing temperature being 500-900℃, to obtain crude lotus root hard carbon; (S.4) Removing impurities in the crude lotus root hard carbon to obtain the lotus root hard carbon.

2. The method according to claim 1, wherein the structure stabilizer is a combination of one or more of polyvinyl alcohol, polyethylene glycol, acrylamide, carboxymethyl cellulose, sodium polyacrylate and sodium alginate.

3. A lotus root hard carbon prepared by the method of any one of claims 1-2.

3. A lotus root hard carbon, characterized by, 4. A lotus root hard carbon / silicon composite material, wherein the material is prepared by combining the lotus root hard carbon of claim 3 with nano-silicon.

5. A method for preparing the lotus root hard carbon / silicon composite material of claim 4, wherein the material is prepared by mixing lotus root hard carbon and nano-silicon, adding ethanol and zirconium balls, ball-milling to mix the composite material uniformly, and drying.

6. The method of claim 5, wherein the mass ratio of the zirconium balls to the mixture of lotus root hard carbon and nano-silicon is 1:5-15.

7. Use of the lotus root hard carbon of claim 3 or the lotus root hard carbon / silicon composite material of claim 4 in lithium ion batteries. ​ ​ ​ ​ ​

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