Preparation and purification process of biomass hard carbon material for sodium ion battery negative electrode and application

Abstract

The present disclosure relates to the technical field of sodium ion battery negative electrode materials, in particular to a preparation and purification process of a biomass hard carbon material for sodium ion battery negative electrodes and application. The preparation and purification process comprises the following steps: providing biomass carbonization material, preparing a first mixed acid solution comprising nitric acid, hydrochloric acid and hydrofluoric acid, and performing first acid washing; then preparing a second mixed acid solution comprising organic acid, chelating agent and surfactant; performing second acid washing; then performing a ball milling process together with the second mixed acid solution; and finally calcining under a nitrogen atmosphere to obtain a biomass hard carbon material for sodium ion battery negative electrodes.

Description

Technical Field

[0001] This disclosure relates to the field of sodium-ion battery anode material technology, specifically to a preparation and purification process and application of biomass hard carbon material for sodium-ion battery anodes. Background Technology

[0002] Biomass hard carbon materials are considered an important development direction for sodium-ion battery anode materials due to their wide availability, low cost, and controllable structure. Agricultural waste such as coconut shells, rice husks, and straw, after high-temperature carbonization, form hard carbon materials with an amorphous carbon structure. Their unique microporous structure and suitable interlayer spacing facilitate the insertion and extraction of sodium ions. However, biomass raw materials absorb various metal elements from the soil through their roots during growth, and inevitably come into contact with iron equipment during carbonization, resulting in the final carbonized material containing various impurities.

[0003] Coconut shell carbonization is a carbon-rich solid product obtained from the high-temperature pyrolysis of coconut shells, and it is an important precursor for sodium-ion battery anode materials. As the outer shell of a palm fruit, coconut shell has a unique three-dimensional honeycomb structure, thick and dense walls, and high lignin and cellulose content. After carbonization, it forms a biomass hard carbon material with a moderate specific surface area, well-developed pore structure, and excellent mechanical strength. Compared to agricultural waste such as rice husks and straw, coconut shell carbonization has better batch stability and an abundant supply, thus becoming one of the important raw materials for commercial sodium-ion battery anode materials.

[0004] However, coconut shell carbonization has a prominent quality issue: its iron content is significantly higher than other biomass hard carbon precursors. Typical coconut shell carbonization has an iron content of 3000-6000 ppm, while raw materials such as rice husks, bamboo, and nut shells typically have an iron content below 200 ppm, and woody raw materials even below 50 ppm. This characteristic stems from the unique growth habits and ecological environment of coconut trees. From a plant physiology perspective, coconut trees, as typical coastal plants, have roots that are constantly immersed in iron-rich coastal soils or volcanic ash-rich soils. These soils themselves can have an iron content of over 2%, far higher than soils in inland agricultural areas. Coconut tree roots acidify the rhizosphere soil by secreting organic acids, actively absorbing iron via transport proteins and transporting it long distances to the above-ground parts. Iron in coconut shells mainly exists in the form of functional iron and stored iron, including iron-containing proteins such as cytochromes, ferredoxins, and catalase, as well as stored iron encapsulated by ferritin. This organically coordinated iron is tightly bound to cell wall polysaccharides and lignin, exhibiting high stability.

[0005] Iron impurities have a significant negative impact on the performance of sodium-ion batteries: iron has multiple variable valence states within the battery's operating potential range, and its redox reactions are accompanied by volume changes and continuous electrolyte catalytic decomposition, leading to a decrease in initial coulombic efficiency, cycle capacity decay, and increased interfacial impedance. Furthermore, coconut shell carbonization also contains elements such as aluminum, calcium, and magnesium. Although these elements are relatively electrochemically inert, excessive presence can occupy sodium storage active sites and block ion transport channels. Additionally, the silicon content in coconut shell carbonization can reach 10,000-30,000 ppm. Silicon in plants is mainly deposited in cell walls and intercellular spaces, and its negative impact on electrochemistry is relatively small.

[0006] Chinese patent document CN109592681A discloses an industrial-scale stepwise purification method for preparing high-purity activated carbon from coconut shell supercapacitors. This method involves separating impurities by introducing oxides to react with mixed impurities in the system to generate a eutectic phase, removing surface impurity phases through surface polishing, eliminating acid-soluble impurities in the inner layer or crystal lattice through acid washing, and removing functional group impurities formed on the carbon structure through a reduction reaction, thus significantly reducing the impurity content. However, this application requires the introduction of oxides and multiple high-temperature treatments. Therefore, there is an urgent need to develop a simple preparation and purification process for coconut shell hard carbon materials used in sodium-ion battery anodes. Summary of the Invention

[0007] The purpose of this disclosure is to provide a preparation and purification process and application of biomass hard carbon material for sodium-ion battery anodes, in order to address the shortcomings in related technologies.

[0008] According to a first aspect of the present disclosure, a preparation and purification process for a biomass hard carbon material for a sodium-ion battery negative electrode is provided, wherein the preparation and purification process uses biomass carbonized material as raw material; the preparation and purification process includes the following steps:

[0009] Step 1: Provide the biomass carbonized material and prepare a first mixed acid solution; add the washed biomass carbonized material to the first mixed acid solution, heat to 60℃-70℃, and acid wash for 8-20 hours; then filter, wash, and dry to obtain the biomass carbonized material after the first acid washing; wherein, the first mixed acid solution contains nitric acid, hydrochloric acid, and hydrofluoric acid;

[0010] Step 2: Prepare a second mixed acid solution; the second mixed acid solution contains organic acid, chelating agent and surfactant; add the biomass carbonized material obtained in Step 1 to the second mixed acid solution, heat to 40℃-50℃, and acid wash for 4-10 hours; then filter, wash and dry to obtain the biomass carbonized material after the second acid washing;

[0011] Step 3: The biomass carbonized material obtained in Step 2 is ball-milled together with the second mixed acid solution; then the mixture obtained from ball milling is calcined under a nitrogen atmosphere to obtain the biomass hard carbon material for the sodium-ion battery negative electrode.

[0012] In one aspect of this disclosure, the biomass hard carbon material is selected from coconut shell carbonized material, rice husk carbonized material, straw carbonized material, sludge-based biocharized material, nut shell carbonized material, or palm kernel shell carbonized material. Preferably, the preparation and purification process provided in this disclosure uses coconut shell carbonized material as raw material.

[0013] In one aspect of this disclosure, the first mixed acid solution is prepared by the following steps:

[0014] Step 1-a: Place 95 parts by weight of deionized water in a plastic or polytetrafluoroethylene container;

[0015] Step 2-a: Slowly add 25-35 parts by weight of 37% concentrated hydrochloric acid while stirring.

[0016] Step 3-a: After the solution temperature drops to room temperature, slowly add 14-45 parts by weight of 68% concentrated nitric acid while stirring.

[0017] Step 4-a: After the solution temperature drops to room temperature, slowly add 5-25 parts by weight of 40% hydrofluoric acid; stir while adding, and then cool to room temperature to obtain the first mixed acid solution.

[0018] In one aspect of this disclosure, preferably, the first mixed acid solution is prepared by the following steps:

[0019] Step 1-a: Place 95 parts by weight of deionized water in a plastic or polytetrafluoroethylene container;

[0020] Step 2-a: Slowly add 30 parts by weight of 37% concentrated hydrochloric acid while stirring.

[0021] Step 3-a: After the solution temperature drops to room temperature, slowly add 25 parts by weight of 68% concentrated nitric acid while stirring.

[0022] Step 4-a: After the solution temperature drops to room temperature, slowly add 10 parts by weight of 40% hydrofluoric acid; stir while adding, and then cool to room temperature to obtain the first mixed acid solution.

[0023] In one aspect of this disclosure, in the second mixed acid solution, the organic acid is selected from at least one of oxalic acid, citric acid, or tartaric acid; the chelating agent is selected from ethylenediaminetetraacetic acid or diethylenetriaminepentaacetic acid; the surfactant is selected from sodium dodecylbenzenesulfonate, fatty alcohol polyoxyethylene ether, or fluorocarbon surfactant; and, based on the total mass of the second mixed acid solution, the mass percentage of the organic acid is selected from 3% to 8%; the mass percentage of the chelating agent is selected from 0.5% to 2.5%; and the mass percentage of the surfactant is selected from 0.05% to 0.2%.

[0024] In one aspect of this disclosure, the second mixed acid solution further includes a reducing agent and a penetration enhancer.

[0025] In one aspect of this disclosure, the reducing agent comprises ascorbic acid or sodium sulfite; the penetration enhancer is selected from dimethyl sulfoxide or N-methylpyrrolidone; and, based on the total mass of the second mixed acid solution, the mass percentage of the reducing agent is selected from 0.25% to 0.75%; the mass percentage of the penetration enhancer is selected from 1% to 5%.

[0026] In one aspect of this disclosure, in step 1, the mass of the first mixed acid solution is 2-4 times the mass of the biomass carbonized material; in step 2, the mass of the second mixed acid solution is 3-7 times the mass of the biomass carbonized material obtained in step 1.

[0027] In one aspect of the embodiments of this disclosure, in step 3, the rotational speed of the ball milling process is selected from 150-250 rpm, the ball-to-material ratio is selected from 10:1 to 15:1, the ball milling time is selected from 30-60 min, and the calcination temperature under nitrogen atmosphere is selected from 450℃-500℃.

[0028] According to a second aspect of the present disclosure, a biomass hard carbon material is provided, which is prepared from coconut shell carbonized material through the aforementioned preparation and purification process.

[0029] According to a third aspect of the present disclosure, the application of the aforementioned biomass hard carbon material in the preparation of a sodium-ion battery anode is provided.

[0030] The technical solutions provided by the embodiments of this disclosure may include the following beneficial effects:

[0031] As can be seen from the above embodiments, this disclosure provides a simple process for preparing and purifying coconut shell carbonized materials that significantly reduces iron content while also significantly reducing other metallic impurities, thus meeting the requirements for its use as a biomass hard carbon anode material for sodium-ion batteries.

[0032] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and are not intended to limit this disclosure. Detailed Implementation

[0033] Exemplary embodiments will now be described in detail. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this disclosure. Rather, they are merely examples of apparatuses and methods consistent with some aspects of this disclosure as detailed in the appended claims.

[0034] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions of this application will be clearly and completely described below in conjunction with embodiments. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. The embodiments described herein are illustrative in nature and are used to provide a basic understanding of this application. The embodiments of this application should not be construed as limiting this application.

[0035] For the sake of brevity, this article only discloses a few specific numerical ranges. However, any lower limit can be combined with any upper limit to form an unspecified range; and any lower limit can be combined with other lower limits to form an unspecified range, just as any upper limit can be combined with any other upper limit to form an unspecified range. Furthermore, each individually disclosed point or single value can itself serve as a lower or upper limit and be combined with any other point or single value or with other lower or upper limits to form an unspecified range.

[0036] In this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0037] In this description, unless otherwise stated, "above" and "below" include the stated number.

[0038] Unless otherwise stated, the terms used in this disclosure have their common meanings as commonly understood by those skilled in the art. Unless otherwise stated, the values ​​of the parameters mentioned in this disclosure can be measured using various measurement methods commonly used in the art (e.g., they can be tested according to the methods given in the embodiments of this disclosure).

[0039] The term "about" is used to describe and indicate small variations. When used in conjunction with an event or situation, the term may refer to examples in which the event or situation occurred precisely or in examples in which the event or situation occurred very approximately. For example, when used in conjunction with numerical values, the term may refer to a range of variation less than or equal to ±10% of the numerical value, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%. Additionally, quantities, ratios, and other numerical values ​​are sometimes presented in range format herein. It should be understood that such range format is for convenience and brevity and should be interpreted flexibly to include not only numerical values ​​explicitly specified as range limits but also all individual numerical values ​​or subranges covered within the range, as if each numerical value and subrange were explicitly specified.

[0040] The list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms can mean any combination of the listed items. For example, if items A and B are listed, then the phrase "at least one of A and B" means only A; only B; or A and B. In another instance, if items A, B, and C are listed, then the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single component or multiple components. Item B may contain a single component or multiple components. Item C may contain a single component or multiple components.

[0041] The present disclosure will be further described below by way of specific embodiments. Unless otherwise specified, all chemical reagents used in the embodiments of the present disclosure are obtained through conventional commercial means. Unless otherwise specified, all contents mentioned below are mass contents. Unless otherwise specified, it is understood that the process is carried out at room temperature.

[0042] Example

[0043] The coconut shell carbonized materials used in the examples and comparative examples were all from the same batch and were mixed evenly before being used as raw materials.

[0044] Example 1:

[0045] Example 1 includes the following steps:

[0046] Preparation of the first mixed acid solution: Place 95 parts by weight of deionized water in a plastic container, then add 30 parts by weight of 37% concentrated hydrochloric acid while stirring; after the addition is complete, wait for the solution temperature to drop to room temperature, then slowly add 25 parts by weight of 68% concentrated nitric acid while stirring; after the addition is complete, wait for the solution temperature to drop to room temperature, then slowly add 5 parts by weight of 40% hydrofluoric acid while stirring; after the addition is complete, wait for the solution temperature to drop to room temperature to obtain the first mixed acid solution.

[0047] Preparation of the second mixed acid solution: Oxalic acid, ethylenediaminetetraacetic acid and sodium dodecylbenzenesulfonate were added to deionized water in sequence to obtain the second mixed acid solution; wherein the mass percentages of oxalic acid, ethylenediaminetetraacetic acid and sodium dodecylbenzenesulfonate were 5%, 0.2% and 0.12% respectively.

[0048] Coconut shell carbonized material was provided and washed three times each with deionized water and ethanol, and then dried to constant weight in an oven at 60°C. The coconut shell carbonized material was added to a first mixed acid solution with a mass of 3 times its weight, heated to 70°C, and acid-washed for 12 hours. The acid washing process was carried out in a polytetrafluoroethylene reactor equipped with a tail gas treatment system, and stirred with a polytetrafluoroethylene agitator at a stirring speed of 300 rpm. After acid washing, the material was filtered, repeatedly washed with deionized water until neutral, and dried to constant weight in an oven at 60°C to obtain the biomass carbonized material after the first acid washing.

[0049] The biomass carbonized material after the first acid wash was added to a second mixed acid solution with a mass of 5 times, heated to 45°C, and acid washed for 6 hours. Then, it was filtered, repeatedly washed with deionized water until neutral, and dried in an oven at 60°C to constant weight to obtain the biomass carbonized material after the second acid wash.

[0050] The biomass carbonized material after the second acid washing was added to a ball mill jar, followed by 0.75 times the mass of the second mixed acid solution, and then silicon nitride grinding balls were added for ball milling. The ball-to-material ratio was 12:1, the ball milling speed was 200 rpm, and the ball milling time was 45 min. After ball milling, the material was washed and dried, and then added to a tube furnace. Under a nitrogen atmosphere, the temperature was increased from room temperature to 475°C at a rate of 5°C / min and held for 2.5 h. Then, it was naturally cooled to room temperature to obtain the biomass hard carbon material of this embodiment.

[0051] Example 2:

[0052] Example 2 includes the following steps:

[0053] Preparation of the first mixed acid solution: Place 95 parts by weight of deionized water in a plastic container, then add 30 parts by weight of 37% concentrated hydrochloric acid while stirring; after the addition is complete, wait for the solution temperature to drop to room temperature, then slowly add 25 parts by weight of 68% concentrated nitric acid while stirring; after the addition is complete, wait for the solution temperature to drop to room temperature, then slowly add 10 parts by weight of 40% hydrofluoric acid while stirring; after the addition is complete, wait for the solution temperature to drop to room temperature to obtain the first mixed acid solution.

[0054] Preparation of the second mixed acid solution: Oxalic acid, ethylenediaminetetraacetic acid and sodium dodecylbenzenesulfonate were added to deionized water in sequence to obtain the second mixed acid solution; wherein the mass percentages of oxalic acid, ethylenediaminetetraacetic acid and sodium dodecylbenzenesulfonate were 5%, 0.2% and 0.12% respectively.

[0055] Coconut shell carbonized material was provided and washed three times each with deionized water and ethanol, and then dried to constant weight in an oven at 60°C. The coconut shell carbonized material was added to a first mixed acid solution with a mass of 3 times its weight, heated to 70°C, and acid-washed for 12 hours. The acid washing process was carried out in a polytetrafluoroethylene reactor equipped with a tail gas treatment system, and stirred with a polytetrafluoroethylene agitator at a stirring speed of 300 rpm. After acid washing, the material was filtered, repeatedly washed with deionized water until neutral, and dried to constant weight in an oven at 60°C to obtain the biomass carbonized material after the first acid washing.

[0056] The biomass carbonized material after the first acid wash was added to a second mixed acid solution with a mass of 5 times, heated to 45°C, and acid washed for 6 hours. Then, it was filtered, repeatedly washed with deionized water until neutral, and dried in an oven at 60°C to constant weight to obtain the biomass carbonized material after the second acid wash.

[0057] The biomass carbonized material after the second acid washing was added to a ball mill jar, followed by 0.75 times the mass of the second mixed acid solution, and then silicon nitride grinding balls were added for ball milling. The ball-to-material ratio was 12:1, the ball milling speed was 200 rpm, and the ball milling time was 45 min. After ball milling, the material was washed and dried, and then added to a tube furnace. Under a nitrogen atmosphere, the temperature was increased from room temperature to 475°C at a rate of 5°C / min and held for 2.5 h. Then, it was naturally cooled to room temperature to obtain the biomass hard carbon material of this embodiment.

[0058] Example 3:

[0059] The steps of Example 3 are basically the same as those of Example 2, except that 15 parts by weight of 40% hydrofluoric acid were used in Example 3 when preparing the first mixed acid solution.

[0060] Example 4:

[0061] The steps in Example 4 are basically the same as those in Example 2, except that 20 parts by weight of 40% hydrofluoric acid were used in Example 3 when preparing the first mixed acid solution.

[0062] Example 5:

[0063] The steps in Example 5 are basically the same as those in Example 2, except that 25 parts by weight of 40% hydrofluoric acid were used in Example 3 when preparing the first mixed acid solution.

[0064] Example 6:

[0065] Example 6 includes the following steps:

[0066] Preparation of the first mixed acid solution: Place 95 parts by weight of deionized water in a plastic container, then add 30 parts by weight of 37% concentrated hydrochloric acid while stirring; after the addition is complete, wait for the solution temperature to drop to room temperature, then slowly add 25 parts by weight of 68% concentrated nitric acid while stirring; after the addition is complete, wait for the solution temperature to drop to room temperature, then slowly add 10 parts by weight of 40% hydrofluoric acid while stirring; after the addition is complete, wait for the solution temperature to drop to room temperature to obtain the first mixed acid solution.

[0067] Preparation of the second mixed acid solution: Oxalic acid, ethylenediaminetetraacetic acid, sodium dodecylbenzenesulfonate and ascorbic acid are added to deionized water in sequence to obtain the second mixed acid solution; wherein the mass percentages of oxalic acid, ethylenediaminetetraacetic acid, sodium dodecylbenzenesulfonate and ascorbic acid are 5%, 0.2%, 0.12% and 0.5% respectively.

[0068] Coconut shell carbonized material was provided and washed three times each with deionized water and ethanol, and then dried to constant weight in an oven at 60°C. The coconut shell carbonized material was added to a first mixed acid solution with a mass of 3 times its weight, heated to 70°C, and acid-washed for 12 hours. The acid washing process was carried out in a polytetrafluoroethylene reactor equipped with a tail gas treatment system, and stirred with a polytetrafluoroethylene agitator at a stirring speed of 300 rpm. After acid washing, the material was filtered, repeatedly washed with deionized water until neutral, and dried to constant weight in an oven at 60°C to obtain the biomass carbonized material after the first acid washing.

[0069] The biomass carbonized material after the first acid wash was added to a second mixed acid solution with a mass of 5 times, heated to 45°C, and acid washed for 6 hours. Then, it was filtered, repeatedly washed with deionized water until neutral, and dried in an oven at 60°C to constant weight to obtain the biomass carbonized material after the second acid wash.

[0070] The biomass carbonized material after the second acid washing was added to a ball mill jar, followed by 0.75 times the mass of the second mixed acid solution, and then silicon nitride grinding balls were added for ball milling. The ball-to-material ratio was 12:1, the ball milling speed was 200 rpm, and the ball milling time was 45 min. After ball milling, the material was washed and dried, and then added to a tube furnace. Under a nitrogen atmosphere, the temperature was increased from room temperature to 475°C at a rate of 5°C / min and held for 2.5 h. Then, it was naturally cooled to room temperature to obtain the biomass hard carbon material of this embodiment.

[0071] The main difference between Example 6 and Example 2 is that the reducing agent ascorbic acid was added in Example 6.

[0072] Example 7:

[0073] Example 7 includes the following steps:

[0074] Preparation of the first mixed acid solution: Place 95 parts by weight of deionized water in a plastic container, then add 30 parts by weight of 37% concentrated hydrochloric acid while stirring; after the addition is complete, wait for the solution temperature to drop to room temperature, then slowly add 25 parts by weight of 68% concentrated nitric acid while stirring; after the addition is complete, wait for the solution temperature to drop to room temperature, then slowly add 10 parts by weight of 40% hydrofluoric acid while stirring; after the addition is complete, wait for the solution temperature to drop to room temperature to obtain the first mixed acid solution.

[0075] Preparation of the second mixed acid solution: Oxalic acid, ethylenediaminetetraacetic acid, sodium dodecylbenzenesulfonate, ascorbic acid, and N-methylpyrrolidone were added sequentially to deionized water to obtain the second mixed acid solution; wherein the mass percentages of oxalic acid, ethylenediaminetetraacetic acid, sodium dodecylbenzenesulfonate, ascorbic acid, and N-methylpyrrolidone were 5%, 0.2%, 0.12%, 0.5%, and 4.5%, respectively.

[0076] Coconut shell carbonized material was provided and washed three times each with deionized water and ethanol, and then dried to constant weight in an oven at 60°C. The coconut shell carbonized material was added to a first mixed acid solution with a mass of 3 times its weight, heated to 70°C, and acid-washed for 12 hours. The acid washing process was carried out in a polytetrafluoroethylene reactor equipped with a tail gas treatment system, and stirred with a polytetrafluoroethylene agitator at a stirring speed of 300 rpm. After acid washing, the material was filtered, repeatedly washed with deionized water until neutral, and dried to constant weight in an oven at 60°C to obtain the biomass carbonized material after the first acid washing.

[0077] The biomass carbonized material after the first acid wash was added to a second mixed acid solution with a mass of 5 times, heated to 45°C, and acid washed for 6 hours. Then, it was filtered, repeatedly washed with deionized water until neutral, and dried in an oven at 60°C to constant weight to obtain the biomass carbonized material after the second acid wash.

[0078] The biomass carbonized material after the second acid washing was added to a ball mill jar, followed by 0.75 times the mass of the second mixed acid solution, and then silicon nitride grinding balls were added for ball milling. The ball-to-material ratio was 12:1, the ball milling speed was 200 rpm, and the ball milling time was 45 min. After ball milling, the material was washed and dried, and then added to a tube furnace. Under a nitrogen atmosphere, the temperature was increased from room temperature to 475°C at a rate of 5°C / min and held for 2.5 h. Then, it was naturally cooled to room temperature to obtain the biomass hard carbon material of this embodiment.

[0079] The main difference between Example 7 and Example 6 is that the penetration enhancer N-methylpyrrolidone was also added in Example 7.

[0080] Comparative Example 1:

[0081] The steps of Comparative Example 1 are basically the same as those of Example 1, except that hydrofluoric acid is not used in the preparation of the first mixed acid solution in Comparative Example 1.

[0082] Comparative Example 2:

[0083] Comparative Example 2 includes the following steps:

[0084] Preparation of the first mixed acid solution: Place 95 parts by weight of deionized water in a plastic container, then add 30 parts by weight of 37% concentrated hydrochloric acid while stirring; after the addition is complete, wait for the solution temperature to drop to room temperature, then slowly add 25 parts by weight of 68% concentrated nitric acid while stirring; after the addition is complete, wait for the solution temperature to drop to room temperature, then slowly add 10 parts by weight of 40% hydrofluoric acid while stirring; after the addition is complete, wait for the solution temperature to drop to room temperature to obtain the first mixed acid solution.

[0085] Coconut shell carbonized material was provided and washed three times each with deionized water and ethanol, and then dried to constant weight in an oven at 60°C. The coconut shell carbonized material was added to a first mixed acid solution with a mass of 3 times its weight, heated to 70°C, and acid-washed for 18 hours. The acid washing process was carried out in a polytetrafluoroethylene reactor equipped with a tail gas treatment system, and stirred with a polytetrafluoroethylene agitator at a stirring speed of 300 rpm. After acid washing, the material was filtered, repeatedly washed with deionized water until neutral, and dried to constant weight in an oven at 60°C to obtain biomass carbonized material after the first acid washing. The biomass carbonized material after the first acid washing was added to a tube furnace and heated from room temperature to 475°C at a rate of 5°C / min under a nitrogen atmosphere, held at that temperature for 2.5 hours, and then naturally cooled to room temperature to obtain the biomass hard carbon material of this comparative example.

[0086] Comparative Example 2 did not use the second mixed acid solution for pickling, but instead extended the pickling time of the first mixed acid solution.

[0087] Comparative Example 3:

[0088] The steps of Comparative Example 3 are basically the same as those of Example 7, except that Comparative Example 3 does not include the ball milling step, but directly roasts the biomass carbonized material after the second acid washing under a nitrogen atmosphere.

[0089] Element content test:

[0090] The metallic impurity composition of coconut shell carbonized raw materials, samples from Examples 1-7, and Comparative Examples 1-4 was analyzed using a PerkinElmer Optima 8000 inductively coupled plasma optical emission spectrometer (ICP-OES). The results are shown in Table 1.

[0091] Table 1

[0092]

[0093] Comparing Examples 1-5 and Comparative Example 1, it can be seen that the removal rate of iron and aluminum elements is significantly improved after adding hydrofluoric acid to the first mixed solution. While the reaction rate between hydrofluoric acid and iron is not fast, the inorganic impurity composition of coconut shell carbonized material has certain unique characteristics. Its silicon content is high, although lower than that of gramineous materials such as rice husks, it is significantly higher than that of woody materials. Furthermore, silicon is tightly bound to the carbon matrix in the form of highly dispersed phytoliths and silicates. More importantly, during high-temperature carbonization, silicon reacts with elements such as iron and aluminum to form complex mineral phases such as iron silicates and aluminum silicates. These inert phases are encapsulated between the carbon layers, becoming a physical barrier hindering the deep removal of iron and aluminum. Therefore, the introduction of hydrofluoric acid can disrupt the silicate framework, releasing the internally encapsulated iron and aluminum impurities, thereby allowing these impurities to be removed through interaction with strong acids (hydrochloric acid, nitric acid).

[0094] Comparing Examples 1-5 and Comparative Example 2, it can be seen that simply extending the pickling time of the first mixed acid solution is significantly less effective than using the first mixed acid solution and the second mixed acid solution separately. This is because the first mixed acid solution, as a strong acid, can consume the surface-accessible metals of the coconut shell carbonized material. However, the coconut shell carbonized material itself has a large number of interlayer structures, micropores, and especially closed pores, which makes it easy for exposed metal ions to form a high concentration in the interlayer structures, micropores, and especially closed pores. During the washing process, these ions are easily hydrolyzed and precipitated, making them impossible to remove. In contrast, the second mixed acid solution can form soluble complexes with metal ions, thereby releasing them from the pores and then washing them, further reducing the removal rate of metal impurities.

[0095] Comparing Examples 2, 6, and 7, it can be seen that the introduction of the reducing agent ascorbic acid can significantly improve the iron removal rate, because it can remove Fe... 3+ Reduced to Fe 2+ Fe 2+ -Oxalic acid complexes have significantly higher solubility than Fe.3+ -Oxalic acid can promote elution; it can also be seen that the penetration enhancer N-methylpyrrolidone can significantly improve the removal rate of most metal heteroatoms. This is because, as a strongly polar aprotic solvent, it can easily penetrate into carbon micropores and interlayer, carrying acid to contact the metal ions remaining in the carbon micropores and interlayer, thus promoting elution.

[0096] Comparing Example 7 and Comparative Example 3, it can be seen that the ball milling process can cut the carbon matrix that encapsulates impurities through the impact, shearing, and friction during ball milling, exposing the metal ions in the deeper layers. At the same time, the second mixed acid solution complexes with the metal ions, preventing them from being encapsulated again.

[0097] Other embodiments of this disclosure will readily occur to those skilled in the art upon consideration of the specification and practice of the disclosure herein. This disclosure is intended to cover any variations, uses, or adaptations of this disclosure that follow the general principles of this disclosure and include common knowledge or customary techniques in the art not disclosed herein.

Claims

1. A preparation and purification process for biomass hard carbon material used as a negative electrode in sodium-ion batteries, wherein the preparation and purification process uses biomass carbonized material as raw material; characterized in that, The preparation and purification process includes the following steps: Step 1: Provide the biomass carbonized material and prepare a first mixed acid solution; add the washed biomass carbonized material to the first mixed acid solution, heat to 60℃-70℃, and acid wash for 8-20 hours; then filter, wash, and dry to obtain the biomass carbonized material after the first acid washing; wherein, the first mixed acid solution contains nitric acid, hydrochloric acid, and hydrofluoric acid; Step 2: Prepare a second mixed acid solution; the second mixed acid solution contains organic acid, chelating agent and surfactant; add the biomass carbonized material obtained in Step 1 to the second mixed acid solution, heat to 40℃-50℃, and acid wash for 4-10 hours; then filter, wash and dry to obtain the biomass carbonized material after the second acid washing; Step 3: The biomass carbonized material obtained in Step 2 is ball-milled together with the second mixed acid solution; then the mixture obtained from ball milling is calcined under a nitrogen atmosphere to obtain the biomass hard carbon material for the sodium-ion battery negative electrode.

2. The preparation and purification process of biomass hard carbon material for sodium-ion battery negative electrode according to claim 1, characterized in that, The biomass hard carbon material is selected from coconut shell carbonized material, rice husk carbonized material, straw carbonized material, sludge-based biocarbonized material, nut shell carbonized material, or palm kernel shell carbonized material.

3. The preparation and purification process of biomass hard carbon material for sodium-ion battery negative electrode according to claim 1, characterized in that, The first mixed acid solution is prepared by the following steps: Step 1-a: Place 95 parts by weight of deionized water in a plastic or polytetrafluoroethylene container; Step 2-a: Slowly add 25-35 parts by weight of 37% concentrated hydrochloric acid while stirring. Step 3-a: After the solution temperature drops to room temperature, slowly add 14-45 parts by weight of 68% concentrated nitric acid while stirring. Step 4-a: After the solution temperature drops to room temperature, slowly add 5-25 parts by weight of 40% hydrofluoric acid; stir while adding, and then cool to room temperature to obtain the first mixed acid solution.

4. The preparation and purification process of biomass hard carbon material for sodium-ion battery negative electrode according to claim 1, characterized in that, In the second mixed acid solution, the organic acid is selected from at least one of oxalic acid, citric acid, or tartaric acid; the chelating agent is selected from ethylenediaminetetraacetic acid or diethylenetriaminepentaacetic acid; the surfactant is selected from sodium dodecylbenzenesulfonate, fatty alcohol polyoxyethylene ether, or fluorocarbon surfactant; and, based on the total mass of the second mixed acid solution, the mass percentage of the organic acid is selected from 3% to 8%; the mass percentage of the chelating agent is selected from 0.5% to 2.5%; and the mass percentage of the surfactant is selected from 0.05% to 0.2%.

5. The preparation and purification process of biomass hard carbon material for sodium-ion battery negative electrode according to claim 4, characterized in that, The second mixed acid solution also includes a reducing agent and a penetration enhancer.

6. The preparation and purification process of biomass hard carbon material for sodium-ion battery negative electrode according to claim 5, characterized in that, The reducing agent includes ascorbic acid or sodium sulfite; the penetration enhancer is selected from dimethyl sulfoxide or N-methylpyrrolidone; and, based on the total mass of the second mixed acid solution, the mass percentage of the reducing agent is selected from 0.25% to 0.75%; the mass percentage of the penetration enhancer is selected from 1% to 5%.

7. The preparation and purification process of biomass hard carbon material for sodium-ion battery negative electrode according to claim 1, characterized in that, In step 1, the mass of the first mixed acid solution is 2-4 times the mass of the biomass carbonized material; in step 2, the mass of the second mixed acid solution is 3-7 times the mass of the biomass carbonized material obtained in step 1.

8. The preparation and purification process of biomass hard carbon material for sodium-ion battery negative electrode according to claim 1, characterized in that, In step 3, the ball milling speed is selected from 150-250 rpm, the ball-to-material ratio is selected from 10:1 to 15:1, and the ball milling time is selected from 30-60 min; the calcination temperature under nitrogen atmosphere is selected from 450℃-500℃.

9. A biomass hard carbon material, characterized in that, The biomass hard carbon material is prepared from coconut shell carbonized material using the preparation and purification process described in any one of claims 3-8.

10. The application of the biomass hard carbon material according to claim 9 in the preparation of sodium-ion battery anodes.

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