Biomass hard carbon negative electrode material and acid-free preparation method and application thereof

Abstract

This invention discloses a method for preparing biomass hard carbon anode materials without acid washing, comprising the following steps: using pure water as a solvent, placing biomass particles in a Soxhlet extractor, continuously circulating and extracting the biomass particles under reflux conditions for 2-5 hours; after extraction, removing the solid and drying it to obtain water-extracted purified biomass; heating the obtained water-extracted purified biomass to 300-550°C at a heating rate of 2-10°C / min, holding it at this temperature for 2-5 hours, then naturally cooling it to room temperature, and then washing and drying it with pure water; heating the obtained pre-oxidized carbonized product to 1100-1550°C at a heating rate of 2-10°C / min, holding it at this temperature for 3-6 hours, and then naturally cooling it to room temperature; washing the obtained carbonized product with pure water until the filtrate is neutral, and then drying it. Compared with the prior art, this method solves the problems of serious wastewater pollution, significant safety hazards, high costs, long production cycles, and the easy introduction of oxygen-containing defects that damage electrochemical performance.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, specifically to a biomass hard carbon anode material and its acid-free preparation method and application. Background Technology

[0002] Sodium-ion batteries (SIBs) possess significant resource advantages in large-scale energy storage due to the abundance, widespread distribution, and low cost of sodium resources (approximately 2.36% of the Earth's crust). Hard carbon (HC) is currently the most mature and commercially promising anode material for sodium-ion batteries, exhibiting a large interlayer spacing (d...). 002 With a diameter of ≥0.37 nm, abundant micropores, and a highly disordered structure, it can accommodate the insertion and filling of a large number of sodium ions, achieving high reversible capacity and high first coulombic efficiency.

[0003] Biomass-derived hard carbon is currently the main commercial preparation route for hard carbon anodes due to its wide availability of raw materials (agricultural and forestry wastes such as bamboo, coconut shells, rice husks, and corn cobs), high carbonization yield, and tunable structure. However, biomass raw materials naturally contain a large amount of inorganic minerals (ash), including alkali metal salts (chlorides, sulfates, and carbonates of K and Na), alkaline earth metal compounds (Ca and Mg), and siliceous components (SiO2, especially in rice husks and wheat straw, where the content can reach 15-25 wt%). If these impurities remain in the final hard carbon, it will lead to:

[0004] The number of irreversible side reactions increases in the first cycle, and the Coulomb efficiency decreases.

[0005] Catalytic graphitization disrupts the disordered hard carbon structure;

[0006] Increased electrode polarization reduces cycle stability.

[0007] To remove the aforementioned impurities, existing technologies generally employ an acid washing process. Examples of disclosed typical patents and prior art include:

[0008] (1) CN107500263B discloses a method for preparing hard carbon materials from waste rice husks. The steps include: soaking the rice husks in hydrochloric acid solution to remove metal ions, then soaking them in hydrofluoric acid solution to remove SiO2, vacuum drying, and then carbonizing them at high temperature (1000-1400℃) under an argon atmosphere to prepare rice husk-based hard carbon. This method uses highly toxic acidic reagents such as hydrofluoric acid, posing extremely high environmental and safety risks.

[0009] (2) CN118183705A discloses an acid pretreatment method for preparing high-efficiency, high-specific-capacity biomass hard carbon. After cutting and washing the biomass raw material, it is immersed in a mixed solution containing inorganic acids such as HCl and HNO3 for 12-24 h for acid washing, followed by high-temperature carbonization treatment. The resulting hard carbon is used as the negative electrode of sodium-ion batteries, and the initial coulombic efficiency can reach 84-87%. Although this method effectively removes metal impurities, the generation of a large amount of waste acid increases the treatment burden and environmental risks.

[0010] (3) CN107500263A discloses a method for preparing rice husk-derived hard carbon, the resulting material, and its applications. Rice husks are washed sequentially with hydrochloric acid (6 mol / L, stirred for 6–8 h) and hydrofluoric acid (10%, stirred overnight) to remove metal ions and SiO2. After vacuum drying, the carbon is carbonized at 1100–1500 °C for 1–2 h under an argon atmosphere at a rate of 2–5 °C / min. This method uses double acid washing with hydrochloric acid and hydrofluoric acid, achieving a reversible capacity of 346 mAh / g (100 cycles) after carbonization at 1300 °C. However, the initial coulombic efficiency is only about 66%, and the use of hydrofluoric acid poses serious safety hazards and environmental risks.

[0011] (4) CN119551660A discloses a method for preparing sodium-ion battery anode materials that balances high capacity and high first-cycle efficiency. Biomass precursors (bamboo, gourd shells, walnut shells, etc.) are purified by acid washing (1–10 mol / L hydrochloric acid / sulfuric acid / nitric acid / hydrofluoric acid, 1–12 h), then pre-oxidized in air or oxygen atmosphere (100–500℃, 0.5–10 h), and finally carbonized at high temperature in an inert atmosphere (1000–1600℃, 1–5 h) to obtain hard carbon materials. The first-cycle coulombic efficiency is ≥90%, and the first-cycle reversible specific capacity is ≥360 mAh / g. Although this method employs a pre-oxidation strategy, acid washing is still required as a pre-purification step, thus failing to achieve a truly acid-free preparation.

[0012] The aforementioned documents all disclose that pickling requires strong inorganic acids (HCl, HF, HNO3, H2SO4, etc.), and share the following common drawbacks:

[0013] Pickling waste liquid contains a large number of metal ions and acid radicals, which are costly to treat and can cause serious pollution to water bodies and soil if the discharge does not meet the standards.

[0014] The use of highly toxic acids such as HF poses significant safety hazards and requires high standards for both operators and equipment.

[0015] The pickling process increases additional equipment investment (acid-resistant tanks, waste liquid treatment facilities) and operating steps, extends the production cycle, and increases the cost of hard carbon production by approximately 15% to 30%.

[0016] Strong acid treatment may introduce excessive oxygen-containing defects onto the surface of carbon materials, which to some extent affects the material's first-cycle coulombic efficiency and cycling stability. Summary of the Invention

[0017] To address the shortcomings of existing technologies, the present invention aims to provide a biomass hard carbon anode material and its acid-free preparation method and application, thereby solving the problems of serious wastewater pollution, significant safety hazards, high costs, long production cycles, and easy introduction of oxygen-containing defects that damage electrochemical performance in existing technologies that use inorganic strong acid washing.

[0018] To achieve the above objectives, the first aspect of the present invention adopts the following technical solution: a method for preparing biomass hard carbon anode material without acid washing, comprising the following steps:

[0019] Soxhlet water extraction procedure: Using pure water as a solvent, place the biomass particles in a Soxhlet extractor.

[0020] Biomass particles were continuously circulated and extracted under reflux conditions for 2–5 hours. After extraction, the solid was removed and dried to obtain water-extracted purified biomass.

[0021] Pre-oxidation step: In an air atmosphere, the obtained water-extracted purified biomass is heated to 300-550℃ at a heating rate of 2-10℃ / min, kept at this temperature for 2-5 hours, and then naturally cooled to room temperature. It is then washed with pure water and dried to obtain the pre-oxidized carbonized product.

[0022] High-temperature carbonization step: Under inert gas protection, the pre-oxidized carbonized product is heated to 1100-1550℃ at a heating rate of 2-10℃ / min, held at that temperature for 3-6 h, and then naturally cooled to room temperature to obtain the carbonized product.

[0023] Post-processing steps: The obtained carbonized product is washed with pure water until the filtrate is neutral, and then dried to obtain biomass hard carbon anode material.

[0024] The Soxhlet extraction method is a commonly used solid-liquid continuous reflux extraction technique in the field of chemical analysis. It utilizes a cyclic process of solvent evaporation-condensation-reflux to achieve efficient and thorough extraction of soluble components from solid samples. Using pure water as the solvent for Soxhlet extraction allows for efficient desalination of biomass raw materials under mild conditions (near the boiling point of water), and its removal effect on water-soluble inorganic salts (KCl, K₂SO₄, NaCl, etc.) is far superior to that of a single soaking and washing process.

[0025] Furthermore, it also includes a crushing and grading step:

[0026] The obtained biomass hard carbon anode material is ball-milled or air-jet pulverized to make the D50 particle size 5-20 μm.

[0027] Furthermore, in the Soxhlet water extraction step, the biomass particles have a particle size of 100–500 μm and are obtained by crushing the raw material.

[0028] The biomass raw materials are selected from one or more of the following: bamboo, coconut shell, rice husk, camellia shell, wheat straw, sawdust, corn cob, sugarcane bagasse, walnut shell, and peanut shell.

[0029] Furthermore, the pure water is deionized water or ultrapure water, and its conductivity is not higher than 2 μS / cm;

[0030] Furthermore, in the Soxhlet water extraction step, the loading amount of biomass particles in the siphon tube of the Soxhlet extractor is 50–200 g / L.

[0031] Furthermore, in the pre-oxidation step, the pre-oxidation is carried out in a tube furnace or a muffle furnace, and the air flow rate introduced into the tube furnace or muffle furnace is 50-250 mL / min.

[0032] Furthermore, in the pre-oxidation step, the pre-oxidation adopts a staged heating method; the staged heating method is as follows: first, the temperature is raised to 200-280℃ at a rate of 2-5℃ / min and held for 0.5-1 h, and then the temperature is raised to 350-500℃ at a rate of 2-5℃ / min and held for 2-4 h.

[0033] Furthermore, in the pre-oxidation step, the pure water washing is:

[0034] The product obtained after cooling to room temperature was mixed with pure water at a solid-liquid ratio of 20-50 mL of pure water per g of product. The mixture was stirred at 50-90°C for 30-60 min and then filtered. The operation was repeated 2-4 times until the conductivity of the filtrate dropped to 50 μS / cm.

[0035] Furthermore, in the high-temperature carbonization step, the inert gas is nitrogen or argon, and its flow rate is 50–300 mL / min.

[0036] The second aspect of the present invention adopts the following technical solution: a biomass hard carbon anode material, which is prepared by the acid-free preparation method of the biomass hard carbon anode material described in the first aspect of the present invention.

[0037] The third aspect of the present invention adopts the following technical solution: a sodium-ion battery, wherein the negative electrode of the sodium-ion battery comprises a biomass hard carbon negative electrode material as described in the second aspect of the present invention.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] 1. This invention replaces acid washing with the synergistic effect of Soxhlet water extraction and air atmosphere pre-oxidation, completely eliminating the use of inorganic strong acids such as HCl, HF, and H2SO4, and fundamentally solving the safety risks associated with waste acid treatment and HF.

[0040] 2. This invention uses Soxhlet water extraction for efficient desalination: Compared with traditional one-time water immersion, the continuous circulation reflux mode of Soxhlet extraction allows biomass to be in continuous contact with fresh pure water, and the removal rate of water-soluble alkali metal salts can reach more than 95%, which is far superior to ordinary soaking and washing (removal rate of about 60-80%).

[0041] 3. This invention employs air atmosphere pre-oxidation for multiple benefits: air atmosphere pre-oxidation not only achieves the initial carbonization of organic components, but also transforms metal impurities into more easily removed forms (such as soluble metal oxides / carbonates) through oxidation, and can inhibit graphitization through aerobic cross-linking, which is conducive to the formation of hard carbon structures with high-level spacing, thus comprehensively improving the sodium storage performance of the material.

[0042] 4. The product prepared by this invention has excellent performance: the interlayer spacing d of the prepared biomass hard carbon anode material is high. 0002 The wavelength range is 0.375–0.420 nm, the ash content is ≤0.3 wt%, and the specific surface area is ≤8 m². 2 / g; and its reversible capacity at 0.1 A / g is not less than 320mAh / g, and its initial coulombic efficiency is not less than 85%; at the same time, its capacity retention rate after 500 cycles at a current density of 1 A / g is not less than 90%; it has excellent cycle stability, and its performance indicators are superior to or equivalent to similar products using traditional acid washing processes.

[0043] 5. The process of this invention is green and environmentally friendly, and the cost is controllable: it eliminates the waste liquid treatment cost of the pickling process, removes the special investment in acid-resistant equipment, and can reduce the process cost by about 15-25%, which is in line with the direction of green manufacturing and carbon neutrality and development. Detailed Implementation

[0044] The present invention will be further described in detail below through specific embodiments:

[0045] Existing methods for preparing biomass hard carbon materials involve pickling with strong inorganic acids, which suffers from severe wastewater pollution, significant safety hazards, high costs, long production cycles, and the potential for introducing oxygen-containing defects that impair electrochemical performance. To address these issues, this invention proposes an acid-free preparation method for biomass hard carbon anode materials, comprising the following steps:

[0046] 1. Soxhlet water extraction procedure: The biomass raw material is pulverized into particles with a diameter of 100–500 μm. Using pure water (deionized water or ultrapure water, conductivity ≤2 μS / cm) as the solvent, the biomass particles are placed in a Soxhlet extractor and continuously circulated for extraction under reflux conditions for 2–5 hours. Soxhlet extraction, through continuous solvent circulation and reflux, ensures that the biomass raw material remains in constant contact with fresh solvent, thus facilitating the extraction of water-soluble inorganic salts (K+). + Na + Cl - SO4 2- The removal efficiency of water-extracted purified biomass (such as glutaraldehyde, saturates, and saturates) and low molecular weight organic matter is far higher than that of traditional static soaking. After extraction, the solid is removed, dried, and the purified biomass is obtained.

[0047] 2. Pre-oxidation step: The water-purified biomass obtained from the Soxhlet water extraction step is heated to 300-550℃ in air at a heating rate of 2-10℃ / min, kept at this temperature for 2-5 hours, and then naturally cooled to room temperature.

[0048] Pre-oxidation in an air atmosphere has multiple functions:

[0049] Aerobic pyrolysis causes oxidative cracking and cross-linking of organic macromolecules such as cellulose, hemicellulose, and lignin in biomass, forming a pre-carbonized carbon skeleton rich in oxidized functional groups. This suppresses the tendency to graphitize during subsequent high-temperature carbonization and is beneficial for generating a high-level inter-space (d) structure. 002 Hard carbon structure (>0.38nm);

[0050] The residual metal impurities (K, Na, Ca, Mg, etc.) are oxidized into more water-soluble metal oxides or carbonates, making them easier to remove in subsequent water washing steps.

[0051] Air oxidation is used to dissociate the organic-inorganic hybrid structure in SiO2-containing biomass (such as rice husks), exposing SiO2 particles for easy separation by water washing. After cooling following pre-oxidation, the solid product is washed 2-4 times with pure water to remove water-soluble salts generated during oxidation, and then dried to obtain the pre-oxidized carbonized product.

[0052] 3. High-Temperature Carbonization Step: The pre-oxidized carbonization product obtained from the pre-oxidation step is heated to 1100–1550°C at a heating rate of 2–10°C / min under inert gas (N2 or Ar) protection, held at this temperature for 3–6 hours, and then naturally cooled to room temperature. High-temperature carbonization is carried out in an inert atmosphere, which further orders the carbon framework, forming a hard carbon microcrystalline structure with suitable interlayer spacing. Simultaneously, the high temperature (>1100°C) partially decomposes and volatilizes residual alkaline earth metal oxides (CaO, MgO, etc.), further purifying the carbon material. No acidic medium needs to be introduced during the high-temperature carbonization stage.

[0053] 4. Post-processing steps: The carbonized product obtained from the high-temperature carbonization step is repeatedly washed with pure water until the filtrate is neutral (pH=6.5~7.5), and then dried to obtain the biomass hard carbon anode material.

[0054] It can be further ball-milled or air-jet pulverized to make the D50 particle size 5-20μm.

[0055] The above preparation method uses pure water as the cleaning solvent throughout the entire process and does not use any inorganic strong acids such as HCl, HF, H2SO4, or HNO3 for acid washing.

[0056] The biomass raw materials are selected from one or more of bamboo, coconut shell, rice husk, camellia shell, wheat straw, sawdust, corn cob, sugarcane bagasse, walnut shell, and peanut shell.

[0057] The pre-oxidation temperature is 350–500℃, the heating rate is 2–5℃ / min, and the holding time is 2–4 h. For biomass raw materials with high SiO2 content (such as rice husks and wheat straw), the pre-oxidation temperature is preferably 450–500℃; for low ash biomass (such as coconut shells), the pre-oxidation temperature can be appropriately reduced to 350–420℃.

[0058] The high-temperature carbonization temperature is 1200–1450℃, the holding time is 3–5 h, and the inert gas flow rate is 100–300 mL / min.

[0059] The biomass hard carbon anode material prepared according to the above method has the following properties:

[0060] Surface spacing d 002 The wavelength range is 0.375–0.420 nm; the ash content is not higher than 0.3 wt%; and the specific surface area is not higher than 8 m². 2 / g; reversible discharge specific capacity at a current density of 0.1 A / g is not less than 320mAh / g; initial coulombic efficiency is not less than 85%; capacity retention after 500 cycles at a current density of 1A / g is not less than 90%.

[0061] The following specific examples illustrate this. All reagents (including pure water) used in the following examples are of analytical grade. Unless otherwise specified, pure water refers to ultrapure water with a conductivity ≤1 μS / cm; the Soxhlet extraction apparatus used is a standard glass Soxhlet extractor.

[0062] Example 1: Preparation of biomass hard carbon anode material (numbered HC-B-1) using bamboo as raw material

[0063] (1) Soxhlet water extraction: Fresh, dried bamboo (3-5 years old, moisture content ≤8%) was crushed into particles with a diameter of 150-300 μm. 20 g of bamboo powder was weighed and placed in the filter paper tube of a standard Soxhlet extractor (loading capacity approximately 80 g / L), and 600 mL of ultrapure water was added to the distillation flask. The bamboo powder was extracted in the Soxhlet extractor by reflux heating in a water bath, with the pure water continuously circulating and refluxed to extract the bamboo powder. The reflux rate was controlled at approximately 3-5 times / h, and the extraction was carried out continuously for 3 h. After the extraction was completed, the bamboo powder in the filter paper tube was removed and dried in a forced-air dryer at 80℃ for 12 h to obtain water-extracted purified bamboo powder. The conductivity of the water extract was measured to be 1120 μS / cm (the conductivity of the original bamboo powder leachate was approximately 1850 μS / cm), the desalination rate was approximately 67%, and the ash content of the bamboo powder decreased from 3.2 wt% to 1.8 wt%.

[0064] (2) Air atmosphere pre-oxidation: The water-extracted purified bamboo powder was placed in a tube furnace, and air was introduced (flow rate 200 mL / min). The temperature was raised to 400℃ at a rate of 3℃ / min and held for 3 h. The mixture was then allowed to cool naturally to room temperature. After cooling, the pre-oxidized product was mixed with ultrapure water at a ratio of 1:30 (g / mL), and the mixture was stirred and washed at 70℃ for 40 min. The mixture was then filtered. The washing was repeated three times until the conductivity of the filtrate was <30 μS / cm. The filtrate was then dried at 100℃ for 12 h to obtain the pre-oxidized carbonized product (denoted as BPC-400, yield approximately 36 wt%).

[0065] (3) High temperature carbonization: Place BPC-400 in a tube furnace, introduce argon gas for protection (Ar flow rate is 150 mL / min), raise the temperature to 1300℃ at 3℃ / min, hold for 4 h, and cool naturally to room temperature (cooling for about 2 h).

[0066] (4) Post-treatment: The carbonization product was mixed with ultrapure water at a ratio of 1:30 (g / mL), stirred and washed at 80℃ for 50 min, and filtered; the water washing was repeated 3 times until the pH of the filtrate was 6.8~7.2, dried at 120℃ for 10 h, and ball-milled to D50=10±2μm to obtain the acid-free bamboo-based hard carbon anode material HC-B-1.

[0067] Performance testing: XRD measured d 002 =0.394nm; Raman ID / IG = 1.08; BET specific surface area = 4.2 m² 2 / g; Ash content = 0.15 wt%.

[0068] Example 2: Preparation of biomass hard carbon anode material (numbered HC-C-2) using coconut shell as raw material

[0069] (1) Soxhlet water extraction: Take coconut shells (inner pericarp of coconut, moisture content ≤5%) and crush them into particles with a diameter of 200-400 μm. Weigh 20 g of coconut shell powder and place it in the filter paper tube of a Soxhlet extractor (loading capacity about 67 g / L). Add 500 mL of ultrapure water to the distillation flask and extract continuously for 2.5 h under reflux conditions (reflux rate about 4 times / h). Take out the solid and dry it at 80℃ for 12 h. The ash content of the coconut shell powder decreased from 2.1 wt% to 0.9 wt%, and the desalination rate was about 58%.

[0070] (2) Pre-oxidation in air atmosphere: The water-extracted purified coconut shell powder was heated to 380℃ at a rate of 5℃ / min in air atmosphere (air flow rate of 200mL / min), kept at this temperature for 2.5 h, and then allowed to cool naturally. After cooling, it was washed three times with ultrapure water at 60℃ (40min each time) until the conductivity of the filtrate was <20μS / cm, and then dried. (Coconut shell has a low ash content and SiO2 content <2 wt%, so the pre-oxidation temperature is lower than that of rice husk; the yield of the pre-oxidized product is about 38 wt%.)

[0071] (3) High-temperature carbonization: The pre-oxidized coconut shell carbon was heated to 1400℃ at 3℃ / min under N2 protection (150mL / min), kept at the temperature for 3.5 h, and then cooled naturally.

[0072] (4) Post-treatment: The carbonized product was repeatedly washed with ultrapure water until pH=7.0, dried at 120℃ for 10 h, and ball-milled to D50=11±2μm to obtain acid-free coconut shell-based hard carbon HC-C-2.

[0073] Performance testing: d 002 =0.381nm; BET=3.1 m 2 / g; Ash content = 0.18 wt%. Initial discharge capacity at 0.1 A / g is 369 mAh / g, reversible capacity is 322 mAh / g, ICE = 87.3%; capacity retention after 500 cycles at 1 A / g is 94.1%.

[0074] Example 3: Preparation of biomass hard carbon anode material (numbered HC-R-3) using rice husk as raw material

[0075] (The high SiO2 content of rice husks (approximately 18–22 wt%) is a key example for verifying the applicability of the process of this invention to high-silicon biomass.)

[0076] (1) Soxhlet water extraction: Take agricultural rice husks (moisture content ≤10%) and crush them to a particle size of 100-250 μm. Weigh 20 g of rice husk powder and place it in the filter paper tube of a Soxhlet extractor (loading capacity approximately 100 g / L). Add 800 mL of ultrapure water to a distillation flask and reflux for 4 h (reflux rate approximately 3 times / h). Dry the extract. Soluble salts (K+) in rice husks... + Na+ The removal rate was approximately 72%, and the ash content decreased from 19.5 wt% to 17.8 wt% (Note: SiO2 is an inorganic substance that is insoluble in water. Water extraction cannot remove SiO2 and requires subsequent steps).

[0077] (2) Pre-oxidation in air atmosphere: The water-extracted purified rice husk powder was heated to 480℃ at 3℃ / min in air atmosphere (200mL / min), held at this temperature for 4 h, and then naturally cooled. High-temperature air oxidation can dissociate the organic-inorganic hybrid bonds of SiO2 in the carbon skeleton of rice husk, and at the same time oxidize and transform some residual alkali metal salts, exposing washable particles. After cooling, it was washed 4 times with ultrapure water at 80℃ (50 min each time) until the conductivity was <40μS / cm, and then dried (the yield of the pre-oxidized product was about 34 wt%).

[0078] (3) High-temperature carbonization: The pre-oxidized rice husk carbon was heated to 1300℃ at 3℃ / min under Ar protection (200mL / min), held for 5 h, and then cooled naturally. (Note: SiO2 in rice husk will melt at high temperatures (>1414℃), so the carbonization temperature is controlled at 1300℃ to avoid SiO2 melting and embedding into the carbon skeleton; in fact, during the high-temperature carbonization stage, C and SiO2 will undergo a partial reduction reaction: SiO2 + C → SiO + CO↑, and the generated SiO gas escapes, which helps to reduce the Si content.)

[0079] (4) Post-processing: The carbonized product was washed multiple times with ultrapure water until pH=7, dried at 120℃ for 10 h, and ball-milled to D50=8±2μm to obtain HC-R-3.

[0080] Performance testing: d 002 =0.404nm; BET=5.8 m 2 / g; Ash content = 0.28 wt% (SiO2 residue < 0.1 wt%). Initial discharge capacity at 0.1 A / g is 402 mAh / g, reversible capacity is 325 mAh / g, ICE = 80.8% (because rice husk has a high silicon content, it requires more sufficient pre-oxidation conditions to remove SiO2. The initial efficiency in this example is relatively low, but it still meets the requirements for industrial applications); Note: If it is necessary to further improve the initial efficiency of rice husk hard carbon, a NaOH solution (5 wt%) can be added between steps (2) and (4) to soak at 60°C for 2 h to specifically dissolve residual SiO2 (SiO2 + 2NaOH → Na2SiO3 + H2O). The ash content can be further reduced to below 0.1 wt% without acid washing, and the initial efficiency can be increased to above 85% (see Example 4).

[0081] Example 4: Rice husk hard carbon alkali-assisted water washing process (numbered HC-R-4, rice husk high silica improvement scheme)

[0082] Based on Example 3, after the water washing in step (2) and before the high-temperature carbonization in step (3), an alkali-assisted silicon dissolution step is added: the pre-oxidized rice husk carbon after pre-oxidation and water washing is added to a 5 wt% NaOH solution, stirred and soaked at 60°C for 2 h, filtered, washed with ultrapure water until neutral, and dried. The remaining steps are the same as in Example 3.

[0083] Performance testing: d 002 =0.397nm; BET=5.2 m 2 / g; Ash content = 0.10 wt% (SiO2 content < 0.05 wt%). Reversible capacity at 0.1 A / g = 335 mAh / g, ICE = 85.9%; Capacity retention after 500 cycles at 1 A / g = 91.8%. This example demonstrates that for high-silica biomass, high-performance acid-free hard carbon can be prepared by using only alkaline solution (NaOH) to assist in the dissolution of SiO2, in addition to inorganic strong acids.

[0084] Example 5: Preparation of biomass hard carbon anode material (numbered HC-T-5) using camellia oleifera shells as raw material

[0085] (Camellia oleifera shell is the shell of the fruit after the seeds are removed. It is a major agricultural and forestry waste in the southern camellia oleifera producing areas. It is rich in lignin and cellulose, and the ash content is mainly K and Ca. The SiO2 content is low (about 3-5 wt%). It is one of the ideal biomass raw materials for preparing hard carbon anodes.)

[0086] (1) Soxhlet water extraction: Camellia oleifera shells (from Chongqing, Hunan, and Jiangxi, with a moisture content ≤8%) were collected, residual tea seeds and impurities were removed, and the powder was pulverized to a particle size of 150–300 μm. 25 g of Camellia oleifera shell powder was weighed and placed in the filter paper tube of a Soxhlet extractor (loading capacity approximately 125 g / L). 600 mL of ultrapure water was added to the distillation flask, and extraction was carried out continuously for 3 h under reflux conditions (reflux rate approximately 3–4 times / h). The solid was then removed and dried at 80 °C for 12 h. The ash content of the Camellia oleifera shell powder decreased from 4.8 wt% to 2.1 wt%, and the desalination rate was approximately 56%.

[0087] (2) Pre-oxidation in air atmosphere: The purified camellia oleifera shell powder was heated to 420℃ at a rate of 3℃ / min in air atmosphere (air flow rate of 200mL / min), kept at this temperature for 3 h, and then allowed to cool naturally. After cooling, it was washed three times with ultrapure water at 70℃ (45 min each time) until the conductivity of the filtrate was <25μS / cm, and then dried. (Camellia oleifera shell has a high lignin content, and the pre-oxidation temperature is appropriate to promote the oxidative cross-linking of lignin; the yield of the pre-oxidation product is about 35 wt%.)

[0088] (3) High-temperature carbonization: The pre-oxidized camellia shell carbon was heated to 1350℃ at 3℃ / min under Ar protection (150mL / min), kept at the temperature for 4 h, and then cooled naturally.

[0089] (4) Post-processing: The carbonized product was repeatedly washed with ultrapure water until pH=6.8~7.2, dried at 120℃ for 10 h, and ball-milled to D50=9±2μm to obtain acid-free Camellia oleifera shell-based hard carbon HC-T-5.

[0090] Performance testing: d 002 =0.389nm; BET=3.8 m 2 / g; Ash content = 0.22 wt%. Initial discharge capacity at 0.1 A / g is 395 mAh / g, reversible capacity is 352 mAh / g, ICE = 89.0%; capacity retention after 500 cycles at 1 A / g is 93.5%. This embodiment demonstrates that camellia shells, as agricultural and forestry waste, can be processed using the acid-free washing process of this invention to prepare hard carbon anode materials with excellent electrochemical performance, fully reflecting the wide applicability of this invention to different types of biomass.

[0091] Comparative example: Traditional HCl pickling process (designated HC-Acid)

[0092] Comparative hard carbon was prepared using bamboo as raw material according to a typical acid washing process in the prior art, referring to the preparation process of Example 1 in CN119551660A:

[0093] Bamboo powder is pulverized to the same particle size (150-300μm);

[0094] Prepare a 3 mol / L hydrochloric acid solution, add bamboo powder and stir for 6 h, wash with distilled water until neutral, and dry.

[0095] Pre-oxidation treatment was carried out in air at a temperature of 5℃ / min to 250℃ for 2 h.

[0096] Under N2 protection, the temperature was increased to 1300℃ at 5℃ / min for 3 h for high-temperature carbonization, cooled, and ball-milled to D50=10±2μm to obtain traditional acid-washed bamboo-based hard carbon HC-Acid.

[0097] Performance Comparison and Analysis

[0098] The above samples were characterized and their electrochemical performance was tested in accordance with national standards and industry practices:

[0099] Ash content was determined according to GB / T 212 standard (calcination at 815℃); d 002The BET specific surface area was calculated from the XRD (002) peak by the Bragg equation; the N2 adsorption-desorption curve was calculated; the electrochemical tests were performed in a sodium metal half-cell (CR2032, HC:SuperP:CMC=8:1:1, electrolyte 1mol / L NaClO4 / EC+DEC containing 5 vol% FEC, voltage 0.005~2.5 V).

[0100] Table 1. Comparison of structural characterization and electrochemical performance between the embodiments and comparative examples

[0101]

[0102] As shown in Table 1, the biomass hard carbon anode materials prepared in Examples 1-5 of this invention all have reversible specific capacities exceeding 320 mAh / g, and initial coulombic efficiencies, except for Example 3 (80.8%, due to high SiO2 residue), are all no less than 85%. Example 4, by adding an alkali-assisted step, can further increase the initial coulombic efficiency of rice husk hard carbon to 85.9%, and the cycle stability is all above 90%. The reversible capacity of Example 5 (352 mAh / g), initial coulombic efficiency (89.0%), and cycle retention rate (93.5%) further demonstrate the broad applicability of this invention to different ash characteristics and different biomass types. Compared with the comparative example HC-Acid, which uses a traditional 3 mol / L hydrochloric acid washing process, the embodiments of the present invention achieve comparable or even better electrochemical performance (HC-B-1 has a reversible capacity that is about 6.3% higher than the comparative example, and HC-T-5 has an initial coulombic efficiency that is 3.1% higher than the comparative example). No inorganic strong acid was used throughout the process, which fully verifies the effectiveness, feasibility and superiority of the technical solution of the present invention.

[0103] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing a biomass hard carbon anode material without acid washing, characterized in that, Includes the following steps: Soxhlet water extraction procedure: Using pure water as a solvent, place the biomass particles in a Soxhlet extractor. Biomass particles were continuously circulated and extracted under reflux conditions for 2–5 hours. After extraction, the solid was removed and dried to obtain water-extracted purified biomass. Pre-oxidation step: In an air atmosphere, the obtained water-extracted purified biomass is heated to 300-550℃ at a heating rate of 2-10℃ / min, kept at this temperature for 2-5 hours, and then naturally cooled to room temperature. It is then washed with pure water and dried to obtain the pre-oxidized carbonized product. High-temperature carbonization step: Under inert gas protection, the pre-oxidized carbonized product is heated to 1100-1550℃ at a heating rate of 2-10℃ / min, held at that temperature for 3-6 h, and then naturally cooled to room temperature to obtain the carbonized product. Post-processing steps: The obtained carbonized product is washed with pure water until the filtrate is neutral, and then dried to obtain biomass hard carbon anode material.

2. The method for preparing a biomass hard carbon anode material without acid washing according to claim 1, characterized in that, It also includes a crushing and grading step: The obtained biomass hard carbon anode material is ball-milled or air-jet pulverized to make the D50 particle size 5-20 μm.

3. A method for preparing a biomass hard carbon anode material without acid washing according to claim 1 or 2, characterized in that, In the Soxhlet water extraction step, the biomass particles have a particle size of 100-500 μm and are obtained by crushing the raw material. The biomass raw materials are selected from one or more of the following: bamboo, coconut shell, rice husk, camellia shell, wheat straw, sawdust, corn cob, sugarcane bagasse, walnut shell, and peanut shell.

4. A method for preparing a biomass hard carbon anode material without acid washing according to claim 1 or 2, characterized in that, The pure water is deionized water or ultrapure water, and its conductivity is not higher than 2 μS / cm; Furthermore, in the Soxhlet water extraction step, the loading amount of biomass particles in the siphon tube of the Soxhlet extractor is 50-200 g / L.

5. A method for preparing a biomass hard carbon anode material without acid washing according to claim 1 or 2, characterized in that, In the pre-oxidation step, the pre-oxidation is carried out in a tube furnace or muffle furnace, and the air flow rate introduced into the tube furnace or muffle furnace is 50-250 mL / min.

6. A method for preparing a biomass hard carbon anode material without acid washing according to claim 1 or 2, characterized in that, In the pre-oxidation step, the pre-oxidation adopts a staged heating method; the staged heating method is as follows: first, the temperature is raised to 200-280℃ at 2-5℃ / min and held for 0.5-1 h, and then the temperature is raised to 350-500℃ at 2-5℃ / min and held for 2-4 h.

7. A method for preparing a biomass hard carbon anode material without acid washing according to claim 1 or 2, characterized in that, In the pre-oxidation step, the pure water rinsing is: The product obtained after cooling to room temperature was mixed with pure water at a solid-liquid ratio of 20-50 mL of pure water per g of product. The mixture was stirred at 50-90°C for 30-60 min and then filtered. The operation was repeated 2-4 times until the conductivity of the filtrate dropped to 50 μS / cm.

8. A method for preparing a biomass hard carbon anode material without acid washing according to claim 1 or 2, characterized in that, In the high-temperature carbonization step, the inert gas is nitrogen or argon, and its flow rate is 50-300 mL / min.

9. A biomass hard carbon anode material, characterized in that, It is prepared by any one of the acid-free preparation methods of biomass hard carbon anode material according to any one of claims 1-8.

10. A sodium-ion battery, characterized in that, The negative electrode of the sodium-ion battery comprises a biomass hard carbon negative electrode material as described in claim 9.

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