Biomass-based hard carbon and preparation method and application thereof

Abstract

The application relates to the technical field of carbon materials, and particularly discloses a biomass-based hard carbon as well as a preparation method and application thereof. The biomass-based hard carbon is prepared from the following raw materials in parts by mass: 100 parts of dry natural biomass powder, 1-8 parts of a composite nitrate, and 0.5-2 parts of a polyhydroxy phenolic crosslinking agent; wherein the composite nitrate comprises a rare earth metal nitrate and a transition metal nitrate. The rare earth-transition metal nitrate is coupled with the polyhydroxy phenolic crosslinking agent to inhibit carbon loss from the source, and the carbon yield of the prepared biomass hard carbon can reach 48-52%, which is much higher than that of a traditional process. The hard carbon interlayer spacing is adapted to sodium ion storage, the electrochemical performance is excellent, and the biomass-based hard carbon can be widely applied to the fields of sodium ion battery negative electrodes, super capacitors, adsorption energy storage and the like.

Description

Technical Field

[0001] This invention relates to the field of carbon materials technology, and in particular to a biomass-based hard carbon, its preparation method, and its applications. Background Technology

[0002] Biomass is an abundant, green, renewable, and low-cost natural carbon source. With its wide availability and environmental friendliness, it has become one of the core raw materials for preparing hard carbon materials. Biomass-based hard carbon possesses a disordered carbon structure, a unique closed-pore structure, suitable interlayer spacing, and good electrochemical stability. It is a key anode material for energy storage devices such as sodium-ion batteries and supercapacitors, and also has broad application prospects in wastewater treatment, gas adsorption, and catalyst support.

[0003] Currently, traditional biomass hard char production processes mainly include direct high-temperature carbonization, acid-base pretreatment carbonization, single metal salt activation, hydrothermal pretreatment, and pre-oxidation modification. Existing technologies suffer from several core defects: First, during biomass pyrolysis, cellulose, hemicellulose, and lignin are prone to thermal decomposition, resulting in the rapid release of large amounts of small carbon molecules and volatile organic compounds. Conventional processes yield only 20%–25% carbon, leading to extremely low carbon source utilization and severe waste of biomass resources. Second, traditional one-step high-temperature carbonization processes have a single heating rate, with intense volatilization of small molecules at low temperatures and a tendency for the carbon skeleton to collapse and develop excessive pores at high temperatures, resulting in low closed-pore ratios, poor structural stability, and limited energy storage performance. Third, acid-base pretreatment processes are highly corrosive and cause severe wastewater pollution, while single metal salt activation modification suffers from low catalytic efficiency, poor carbon fixation, and high energy consumption. Fourth, existing high-carbon-yield technologies largely rely on high-temperature, high-pressure, and solvent-based reactions, requiring sophisticated equipment and complex processes, making industrial-scale production difficult. Summary of the Invention

[0004] Based on this, the purpose of this invention is to provide a biomass-based hard carbon, its preparation method and application. Through an original low-temperature solid-phase catalytic cross-linking free radical locking mechanism, combined with precise gradient carbonization control, the carbon source utilization rate is greatly improved, and high-closed-pore, low-ash, high-performance biomass hard carbon materials are prepared.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: This invention provides a biomass-based hard carbon, characterized in that it is prepared from the following raw materials in parts by weight: 100 parts of dried natural biomass powder, 1-8 parts of composite nitrate, and 0.5-2 parts of polyhydroxyphenolic crosslinking agent; wherein the composite nitrate includes rare earth metal nitrate and transition metal nitrate.

[0006] As a further improvement to the above-described scheme of the present invention, the composite nitrate comprises lanthanum nitrate, nickel nitrate, and zinc nitrate, wherein the mass ratio of lanthanum nitrate, nickel nitrate, and zinc nitrate is (1~3):(0.5~1):(2~5). And / or, the polyhydroxyphenolic crosslinking agent is selected from one of gallic acid, tannic acid, pyrogallol, proanthocyanidins, and tea polyphenols.

[0007] As a further improvement to the above-mentioned solution of the present invention, the dried natural biomass powder is selected from at least one of dried bamboo powder, dried wood chips, dried straw powder, dried coconut shell powder, and dried walnut shell powder, and the moisture content of the dried natural biomass powder is ≤5wt%.

[0008] As a further improvement to the above-mentioned scheme of the present invention, the biomass-based hard carbon has a closed-pore rate of ≥60%, an ash content of ≤1%, and a hard carbon interlayer spacing d. 002 The wavelength range is 0.38~0.40 nm.

[0009] The present invention also provides a method for preparing biomass-based hard carbon as described above, which includes the following steps: S1. Crush, sieve, and dry the natural biomass raw materials to obtain dried natural biomass powder; S2. Mix the dried natural biomass powder, composite nitrate, and polyhydroxyphenolic crosslinking agent in proportion to obtain a precursor mixture; S3. The precursor mixture is placed under normal pressure, low oxygen or air atmosphere to carry out solid-phase catalytic crosslinking reaction to obtain carbon-rich precursor; S4. The carbon-rich precursor is placed in an inert atmosphere, subjected to gradient carbonization, and naturally cooled to obtain a primary hard carbon material. S5. The primary hard carbon material is acid-washed, water-washed, and dried to obtain biomass-based hard carbon.

[0010] This invention innovatively employs a rare-earth-transition-metal nitrate coupled with a polyhydroxyphenolic crosslinking agent for low-temperature solid-phase catalytic crosslinking technology. Under solvent-free, ambient-pressure, and low-temperature conditions, it induces intermolecular crosslinking, aromatization, and free radical stabilization reactions in biomass cellulose, hemicellulose, and lignin, pre-locking the carbon precursor framework and suppressing the volatilization loss of small molecules during pyrolysis from the source. Combined with a three-stage gradient inert atmosphere precise carbonization process, deoxidation, framework shaping, and deep carbonization are completed step-by-step, ultimately yielding high-quality hard carbon materials with high carbon yield, high closed-porosity, and low ash content. This invention eliminates the need for high-temperature pre-oxidation, avoids acid and alkali-corroded solvents, and allows for catalyst recyclability and reuse. The process is green, low-cost, and easily scaled up for industrial applications.

[0011] As a further improvement to the above-mentioned solution of the present invention, in step S1, the sieving is performed through an 80-200 mesh sieve, and the drying is performed at 100-110°C for 1.5-2.5 hours.

[0012] As a further improvement to the above-mentioned solution of the present invention, in step S2, the mixing is carried out by dry ball milling, the rotation speed of the dry ball milling is 300~500 r / min, and the time is 30~60 min; the whole process is solvent-free and operated at room temperature and pressure.

[0013] As a further improvement to the above-mentioned scheme of the present invention, in step S3, the temperature of the solid-phase catalytic crosslinking reaction is 180~220℃ and the time is 2~4h; the crosslinking aromatization and free radical stabilization reaction of biomass macromolecules are induced to solidify the carbon skeleton. And / or, in step S4, the gradient carbonization is: First stage: First, heat to 300-400℃ at a heating rate of 1-2℃ / min and hold for 30-60min; complete the gentle deoxygenation and removal of volatile small molecule impurities; Second stage: Heat to 600-700℃ at a heating rate of 3-5℃ / min and hold for 20-40min; complete the deep aromatization of the biomass skeleton and the initial closed-cell formation; The third stage involves heating the carbon to 900-1000℃ at a rate of 8-12℃ / min and holding it at that temperature for 15-30 minutes to complete the hard carbon structure shaping, microstructure regulation, and suppress skeleton collapse.

[0014] In step S4, the inert atmosphere is high-purity nitrogen or high-purity argon, and the gas flow rate is 50~100mL / min.

[0015] As a further improvement to the above-mentioned scheme of the present invention, in step S5, the pickling uses dilute hydrochloric acid with a concentration of 0.5~1 mol / L, the pickling temperature is 75~85℃, and the pickling time is 1~2 hours; the water washing uses deionized water to bring the pH value of the washing solution to 6.8~7.2; the drying temperature is 100~110℃, and the time is 1~2 hours. Residual metal impurities and ash are removed through pickling and water washing.

[0016] This invention also provides an application of biomass-based hard carbon as a negative electrode material for sodium-ion batteries, as described above.

[0017] Compared with the prior art, the present invention has the following beneficial effects: This invention employs rare earth-transition metal nitrate coupled with a polyhydroxyphenol crosslinking agent to simultaneously achieve a triple effect of free radical initiation, crosslinking catalysis, and free radical stabilization under solvent-free conditions. Rare earth ions can reduce the activation energy of biomass macromolecules and induce the breaking and recombination of cellulose and lignin molecular chains; transition metal ions act as active sites to catalyze intermolecular etherification, dehydration, and aromatization reactions; and the polyhydroxyphenol crosslinking agent can capture carbon free radicals generated by pyrolysis, inhibit the volatilization of small molecule fragments, and firmly lock free carbon in the framework, forming a dense and stable carbon-rich precursor in advance, thus solving the carbon loss problem at its source.

[0018] This invention features precise structural control through gradient carbonization: abandoning the traditional one-step high-temperature carbonization mode, it adopts a three-stage graded heating and heat preservation process. The low-temperature stage involves gentle deoxidation and impurity removal, the medium-temperature stage involves deep aromatization and shaping, and the high-temperature stage involves rapid structural solidification. This avoids the violent volatilization of small molecules at low temperatures and prevents the collapse of the carbon skeleton and the destruction of closed pores caused by long-term high-temperature carbonization, thus achieving a balance between high carbon yield and excellent microstructure.

[0019] This invention induces intermolecular crosslinking, aromatization, and free radical stabilization reactions in biomass cellulose, hemicellulose, and lignin through a low-temperature solid-phase crosslinking reaction under solvent-free, normal-pressure, and low-temperature conditions. This locks in the carbon precursor skeleton in advance, inhibiting the volatilization and loss of small carbon molecules from the source. The carbon yield of biomass hard char is increased from the traditional 20%~25% to 48%~52%, and the carbon source utilization rate is increased by more than 2 times. This greatly reduces the cost of raw materials and realizes the efficient resource utilization of biomass. It does not require high-temperature pre-oxidation, does not use acid or alkali strongly corrosive solvents, and the catalyst can be recycled and reused. The process is green, low-cost, and easy to scale up industrially.

[0020] The gradient carbonization process of this invention precisely controls the microstructure of hard carbon, resulting in a hard carbon with a closed-pore rate of ≥60%, which is much higher than that of traditional materials (30%~40%). The interlayer spacing is 0.38~0.40nm, which is perfectly adapted to sodium ion insertion and extraction. The initial reversible capacity can reach 320~370mAh / g, the initial efficiency is 85%~88%, and the cycle stability is excellent. It can be widely used in sodium-ion battery anodes, supercapacitors, adsorption energy storage and other fields. Attached Figure Description

[0021] Figure 1 This is a transmission electron microscope (TEM) image of the hard carbon anode material prepared in Example 1. Detailed Implementation

[0022] To facilitate understanding of the present invention, a more comprehensive description will be given below with reference to specific embodiments. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of the present invention.

[0023] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0024] Example 1 This embodiment proposes a method for preparing biomass-based hard carbon, which includes the following steps: S1. Raw material pretreatment: Bamboo powder is selected as raw material, crushed and passed through a 100-mesh sieve, dried at 105℃ for 2 hours, cooled and used for later use. The moisture content of the dried bamboo powder is 4.2wt%.

[0025] S2. Ingredient mixing: Take 100g of dried bamboo powder, 4.5g of ternary composite nitrate (2g of lanthanum nitrate hexahydrate, 0.8g of nickel nitrate hexahydrate, and 1.7g of zinc nitrate hexahydrate), and 1g of gallic acid. Dry ball mill at 400r / min for 45min and mix evenly to obtain a precursor mixture.

[0026] S3. Low-temperature solid-phase catalytic crosslinking reaction: Under normal pressure air atmosphere, the precursor mixture is kept at 200℃ for 3h to complete free radical crosslinking and lock, and obtain a dense carbon-rich precursor.

[0027] S4. Gradient carbonization: The carbon-rich precursor is placed in a tube furnace, and nitrogen gas is introduced into the tube furnace at a flow rate of 80 mL / min. Starting from room temperature, the temperature is increased to 350℃ at a heating rate of 1.5℃ / min and held for 45 min; then the temperature is increased to 650℃ at a heating rate of 4℃ / min and held for 30 min; then the temperature is increased to 950℃ at a heating rate of 10℃ / min and held for 20 min. The furnace is then cooled to obtain the primary hard carbon material.

[0028] S5. Post-treatment: Add the primary hard carbon material to 0.8 mol / L dilute hydrochloric acid, acid wash at 80℃ for 1.5 h, then wash with water until neutral, and dry at 105℃ for 1.5 h to obtain biomass-based hard carbon material.

[0029] Figure 1 The image shown is a transmission electron microscope (TEM) image of the biomass-based hard carbon prepared in this embodiment. It can be seen that the prepared hard carbon anode material has a high closed-pore ratio and a large interlayer spacing.

[0030] Example 2 This embodiment proposes a method for preparing biomass-based hard carbon, which includes the following steps: S1. Raw material pretreatment: Coconut shell powder is selected as raw material, crushed and passed through a 120-mesh sieve, dried at 105℃ for 2 hours, cooled and used for later use. The moisture content of the dried coconut shell powder is 3.3wt%.

[0031] S2. Ingredient mixing: Take 100g of dried coconut shell powder, 3.8g of ternary composite nitrate (1.5g of lanthanum nitrate hexahydrate, 0.6g of nickel nitrate hexahydrate, and 1.7g of zinc nitrate hexahydrate), and 1.2g of tannic acid, and dry ball mill them at 350r / min for 50min until they are mixed evenly to obtain a precursor mixture.

[0032] S3. Low-temperature solid-phase catalytic crosslinking reaction: Under normal pressure and low oxygen atmosphere (oxygen content 5% vol), the precursor mixture is kept at 190℃ for 3.5 h to complete free radical crosslinking and lock, and obtain a dense carbon-rich precursor.

[0033] S4. Gradient carbonization: The carbon-rich precursor is placed in a tube furnace, and nitrogen gas is introduced into the tube furnace at a flow rate of 70 mL / min. Starting from room temperature, the temperature is increased to 320℃ at a heating rate of 1℃ / min and held for 60 min; then the temperature is increased to 680℃ at a heating rate of 5℃ / min and held for 25 min; then the temperature is increased to 980℃ at a heating rate of 12℃ / min and held for 18 min. The furnace is then cooled to obtain the primary hard carbon material.

[0034] S5. Post-treatment: Add the primary hard carbon material to 1 mol / L dilute hydrochloric acid, acid wash at 80℃ for 1 h, then wash with water until neutral, and dry at 105℃ for 1.5 h to obtain biomass-based hard carbon material.

[0035] Example 3 This embodiment proposes a method for preparing biomass-based hard carbon, which includes the following steps: S1. Raw material pretreatment: Wood chips are selected as raw material, crushed and passed through an 80-mesh sieve, dried at 105℃ for 2 hours, cooled and stored for later use. The moisture content of the dried wood chips obtained is 3.7wt%.

[0036] S2. Ingredient mixing: Take 100g of dry wood chips, 7g of ternary composite nitrate (3g of lanthanum nitrate hexahydrate, 1g of nickel nitrate hexahydrate, and 3g of zinc nitrate hexahydrate), and 1.8g of gallic acid. Dry ball mill at 450r / min for 35min and mix evenly to obtain a precursor mixture.

[0037] S3. Low-temperature solid-phase catalytic crosslinking reaction: Under normal pressure air atmosphere, the precursor mixture is kept at 210℃ for 2.5h to complete free radical crosslinking and lock, and obtain a dense carbon-rich precursor.

[0038] S4. Gradient carbonization: The carbon-rich precursor is placed in a tube furnace, and nitrogen gas is introduced into the tube furnace at a flow rate of 90 mL / min. Starting from room temperature, the temperature is increased to 380℃ at a heating rate of 2℃ / min and held for 30 min; then the temperature is increased to 620℃ at a heating rate of 3℃ / min and held for 40 min; then the temperature is increased to 920℃ at a heating rate of 9℃ / min and held for 25 min. The furnace is then cooled to obtain the primary hard carbon material.

[0039] S5. Post-treatment: Add the primary hard carbon material to 0.6 mol / L dilute hydrochloric acid, acid wash at 80℃ for 2 h, then wash with water until neutral, and dry at 105℃ for 1.5 h to obtain biomass-based hard carbon material.

[0040] Comparative Example 1 This comparative example presents a method for preparing biomass-based hard carbon, which includes the following steps: S1. Carbonization: Place 100g of bamboo powder (the same bamboo powder raw material as in step S1 of Example 1) in a tube furnace, introduce nitrogen into the tube furnace at a flow rate of 80mL / min, start from room temperature, heat to 950℃ at a heating rate of 1.5℃ / min and hold for 2h, cool with the furnace to obtain primary hard carbon material.

[0041] S2. Post-treatment: The primary hard carbon material is added to 0.8 mol / L dilute hydrochloric acid, acid-washed at 80℃ for 1.5 h, then washed with water until neutral, and dried at 105℃ for 1.5 h to obtain biomass-based hard carbon material.

[0042] Comparative Example 2 This comparative example presents a method for preparing biomass-based hard carbon, which includes the following steps: S1. Raw material pretreatment: Bamboo powder is selected as raw material, crushed and passed through a 100-mesh sieve, dried at 105℃ for 2 hours, cooled and used for later use. The moisture content of the dried bamboo powder is 4.2wt%.

[0043] S2. Ingredient mixing: Take 100g of dried bamboo powder and 4.5g of zinc nitrate hexahydrate, and dry ball mill them at 400r / min for 45min. Mix them evenly to obtain the precursor mixture.

[0044] S3. Low-temperature solid-phase catalytic crosslinking reaction: Under normal pressure air atmosphere, the precursor mixture is kept at 200℃ for 3h to complete free radical crosslinking and lock, and obtain a dense carbon-rich precursor.

[0045] S4. Gradient carbonization: The carbon-rich precursor is placed in a tube furnace, and nitrogen gas is introduced into the tube furnace at a flow rate of 80 mL / min. Starting from room temperature, the temperature is increased to 350℃ at a heating rate of 1.5℃ / min and held for 45 min; then the temperature is increased to 650℃ at a heating rate of 4℃ / min and held for 30 min; then the temperature is increased to 950℃ at a heating rate of 10℃ / min and held for 20 min. The furnace is then cooled to obtain the primary hard carbon material.

[0046] S5. Post-treatment: Add the primary hard carbon material to 0.8 mol / L dilute hydrochloric acid, acid wash at 80℃ for 1.5 h, then wash with water until neutral, and dry at 105℃ for 1.5 h to obtain biomass-based hard carbon material.

[0047] Comparative Example 3 This comparative example presents a method for preparing biomass-based hard carbon, which includes the following steps: S1. Raw material pretreatment: Bamboo powder is selected as raw material, crushed and passed through a 100-mesh sieve, dried at 105℃ for 2 hours, cooled and used for later use. The moisture content of the dried bamboo powder is 4.2wt%.

[0048] S2. Ingredient mixing: Take 100g of dried bamboo powder and 4.5g of ternary composite nitrate (2g of lanthanum nitrate hexahydrate, 0.8g of nickel nitrate hexahydrate, and 1.7g of zinc nitrate hexahydrate), and dry ball mill at 400r / min for 45min to mix evenly to obtain a precursor mixture.

[0049] S3. Low-temperature solid-phase catalytic crosslinking reaction: Under normal pressure air atmosphere, the precursor mixture is kept at 200℃ for 3h to complete free radical crosslinking and lock, and obtain a dense carbon-rich precursor.

[0050] S4. Gradient carbonization: The carbon-rich precursor is placed in a tube furnace, and nitrogen gas is introduced into the tube furnace at a flow rate of 80 mL / min. Starting from room temperature, the temperature is increased to 350℃ at a heating rate of 1.5℃ / min and held for 45 min; then the temperature is increased to 650℃ at a heating rate of 4℃ / min and held for 30 min; then the temperature is increased to 950℃ at a heating rate of 10℃ / min and held for 20 min. The furnace is then cooled to obtain the primary hard carbon material.

[0051] S5. Post-treatment: Add the primary hard carbon material to 0.8 mol / L dilute hydrochloric acid, acid wash at 80℃ for 1.5 h, then wash with water until neutral, and dry at 105℃ for 1.5 h to obtain biomass-based hard carbon material.

[0052] Comparative Example 4 This comparative example presents a method for preparing biomass-based hard carbon, which includes the following steps: S1. Raw material pretreatment: Bamboo powder is selected as raw material, crushed and passed through a 100-mesh sieve, dried at 105℃ for 2 hours, cooled and used for later use. The moisture content of the dried bamboo powder is 4.2wt%.

[0053] S2. Ingredient mixing: Take 100g of dried bamboo powder, 2g of lanthanum nitrate hexahydrate, 0.8g of nickel nitrate hexahydrate, and 1g of gallic acid, and dry ball mill them at 400r / min for 45min until they are mixed evenly to obtain a precursor mixture.

[0054] S3. Low-temperature solid-phase catalytic crosslinking reaction: Under normal pressure air atmosphere, the precursor mixture is kept at 200℃ for 3h to complete free radical crosslinking and lock, and obtain a dense carbon-rich precursor.

[0055] S4. Gradient carbonization: The carbon-rich precursor is placed in a tube furnace, and nitrogen gas is introduced into the tube furnace at a flow rate of 80 mL / min. Starting from room temperature, the temperature is increased to 350℃ at a heating rate of 1.5℃ / min and held for 45 min; then the temperature is increased to 650℃ at a heating rate of 4℃ / min and held for 30 min; then the temperature is increased to 950℃ at a heating rate of 10℃ / min and held for 20 min. The furnace is then cooled to obtain the primary hard carbon material.

[0056] S5. Post-treatment: Add the primary hard carbon material to 0.8 mol / L dilute hydrochloric acid, acid wash at 80℃ for 1.5 h, then wash with water until neutral, and dry at 105℃ for 1.5 h to obtain biomass-based hard carbon material.

[0057] Comparative Example 5 This comparative example presents a method for preparing biomass-based hard carbon, which includes the following steps: S1. Raw material pretreatment: Bamboo powder is selected as raw material, crushed and passed through a 100-mesh sieve, dried at 105℃ for 2 hours, cooled and used for later use. The moisture content of the dried bamboo powder is 4.2wt%.

[0058] S2. Ingredient mixing: Take 100g of dried bamboo powder, 4.5g of ternary composite nitrate (2g of lanthanum nitrate hexahydrate, 0.8g of nickel nitrate hexahydrate, and 1.7g of zinc nitrate hexahydrate), and 1g of gallic acid. Dry ball mill at 400r / min for 45min and mix evenly to obtain a precursor mixture.

[0059] S3. Gradient carbonization: The precursor mixture is placed in a tube furnace, and nitrogen gas is introduced into the tube furnace at a flow rate of 80 mL / min. Starting from room temperature, the temperature is increased to 350℃ at a heating rate of 1.5℃ / min and held for 45 min; then the temperature is increased to 650℃ at a heating rate of 4℃ / min and held for 30 min; then the temperature is increased to 950℃ at a heating rate of 10℃ / min and held for 20 min. The furnace is then cooled to obtain primary hard carbon material.

[0060] S4. Post-treatment: Add the primary hard carbon material to 0.8 mol / L dilute hydrochloric acid, acid wash at 80℃ for 1.5 h, then wash with water until neutral, and dry at 105℃ for 1.5 h to obtain biomass-based hard carbon material.

[0061] Comparative Example 6 This comparative example presents a method for preparing biomass-based hard carbon, which includes the following steps: S1. Raw material pretreatment: Bamboo powder is selected as raw material, crushed and passed through a 100-mesh sieve, dried at 105℃ for 2 hours, cooled and used for later use. The moisture content of the dried bamboo powder is 4.2wt%.

[0062] S2. Ingredient mixing: Take 100g of dried bamboo powder, 4.5g of ternary composite nitrate (2g of lanthanum nitrate hexahydrate, 0.8g of nickel nitrate hexahydrate, and 1.7g of zinc nitrate hexahydrate), and 1g of gallic acid. Dry ball mill at 400r / min for 45min and mix evenly to obtain a precursor mixture.

[0063] S3. Low-temperature solid-phase catalytic crosslinking reaction: Under normal pressure air atmosphere, the precursor mixture is kept at 200℃ for 3h to complete free radical crosslinking and lock, and obtain a dense carbon-rich precursor.

[0064] S4. Carbonization: The carbon-rich precursor is placed in a tube furnace, and nitrogen gas is introduced into the tube furnace at a flow rate of 80 mL / min. Starting from room temperature, the temperature is increased to 950℃ at a rate of 1.5℃ / min and held for 2 hours. The furnace is then cooled to obtain primary hard carbon material.

[0065] S5. Post-treatment: Add the primary hard carbon material to 0.8 mol / L dilute hydrochloric acid, acid wash at 80℃ for 1.5 h, then wash with water until neutral, and dry at 105℃ for 1.5 h to obtain biomass-based hard carbon material.

[0066] Test case The biomass-based hard char obtained in Examples 1-3 and Comparative Examples 1-6 were subjected to the following performance tests: (1) Carbon production rate: (mass of final biomass-based hard carbon / mass of original biomass) × 100%.

[0067] (2) Ash content: determined according to GB / T 12496.3-1999.

[0068] (3) Electrochemical performance testing: Biomass-based hard carbon, conductive carbon black, and polyvinylidene fluoride were mixed in a mass ratio of 80:10:10 and then added to N-methylpyrrolidone. The mixture was then slurried, coated onto copper foil, vacuum dried, and cut into 12mm diameter electrode sheets. Using a sodium metal sheet as the counter electrode, 1M NaClO4 / EC+DEC (volume ratio 1:1) as the electrolyte, and Whatman GF / D glass fiber as the separator, CR2032 coin cells were assembled in an argon glove box. Constant current charge-discharge tests were performed on the coin cells within a voltage range of 0.01-2.5V.

[0069] The test results are shown in Table 1.

[0070] Table 1 Performance Test Results

[0071] According to the results in Table 1: In Examples 1-3 of this invention, the carbon production rate was consistently between 48% and 52%, all exceeding 45%; while in Comparative Example 1, the carbon production rate of traditional direct carbonization was only 22.3%; in Comparative Example 2, the carbon production rate of single zinc salt modification was only 30.5%; and in Comparative Example 3, after removing the polyhydroxyphenol crosslinking agent, the carbon production rate dropped to 39.2%. This demonstrates that the synergistic effect of the ternary La-Ni-Zn composite metal system, the polyhydroxyphenol free radical stabilizer, and the low-temperature solid-phase crosslinking can inhibit the volatilization of small molecules from biomass pyrolysis at the source, significantly reducing carbon loss; single metal modification and conventional pre-oxidation have extremely weak carbon-locking capabilities, and the phenolic free radical capturing component is the core key to improving the carbon fixation rate.

[0072] The biomass-based hard carbon prepared in Examples 1-3 of this invention all have a closed-pore rate of ≥60%; while the conventional direct carbonization process in Comparative Example 1 has a closed-pore rate as low as 35.6%, with its carbon skeleton prone to collapse and an excessively high proportion of open pores. The low-temperature free radical crosslinking of this invention can construct a rigid and dense three-dimensional carbon network. Combined with gradient segmented carbonization, it avoids high-temperature skeleton collapse, directionally increases the proportion of closed sodium storage channels in the hard carbon, and optimizes the unique microstructure of the hard carbon. Simultaneously, the metal catalytic precursor of this invention, combined with a gradient thermal decontamination and synergistic acid washing purification system, effectively removes native biomass ash and residual metal impurities, improving the purity of the carbon material.

[0073] Electrochemical performance data comparison: Examples 1-3 showed an initial reversible capacity of 350~370mAh / g and an initial coulombic efficiency of 85.1%~87.2%; Comparative Examples 1-6 showed a significant decrease in reversible capacity and initial coulombic efficiency. In Comparative Example 1, the initial reversible capacity of the traditional process was only 265mAh / g and the initial coulombic efficiency was only 74.7%, further confirming that the optimized hard carbon-specific microstructure process of this invention can improve the electrochemical performance of sodium electrode anodes.

[0074] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0075] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A biomass-based hard carbon, characterized in that, It is prepared from the following raw materials in parts by weight: 100 parts of dried natural biomass powder, 1-8 parts of composite nitrate, and 0.5-2 parts of polyhydroxyphenol crosslinking agent; wherein the composite nitrate includes rare earth metal nitrate and transition metal nitrate.

2. The biomass-based hard carbon according to claim 1, characterized in that, The composite nitrate comprises lanthanum nitrate, nickel nitrate, and zinc nitrate, wherein the mass ratio of lanthanum nitrate, nickel nitrate, and zinc nitrate is (1~3):(0.5~1):(2~5). And / or, the polyhydroxyphenolic crosslinking agent is selected from one of gallic acid, tannic acid, pyrogallol, proanthocyanidins, and tea polyphenols.

3. The biomass-based hard carbon according to claim 1, characterized in that, The dried natural biomass powder is selected from at least one of dried bamboo powder, dried wood chips, dried straw powder, dried coconut shell powder, and dried walnut shell powder, and the moisture content of the dried natural biomass powder is ≤5wt%.

4. The biomass-based hard carbon according to claim 1, characterized in that, The biomass-based hard carbon has a closed-pore rate of ≥60%, an ash content of ≤1%, and a hard carbon interlayer spacing d. 002 The wavelength range is 0.38~0.40 nm.

5. A method for preparing biomass-based hard carbon as described in any one of claims 1-4, characterized in that, It includes the following steps: S1. Crush, sieve, and dry the natural biomass raw materials to obtain dried natural biomass powder; S2. Mix the dried natural biomass powder, composite nitrate, and polyhydroxyphenolic crosslinking agent in proportion to obtain a precursor mixture; S3. The precursor mixture is placed under normal pressure, low oxygen or air atmosphere to carry out solid-phase catalytic crosslinking reaction to obtain carbon-rich precursor; S4. The carbon-rich precursor is placed in an inert atmosphere, subjected to gradient carbonization, and naturally cooled to obtain a primary hard carbon material. S5. The primary hard carbon material is acid-washed, water-washed, and dried to obtain biomass-based hard carbon.

6. The method for preparing biomass-based hard carbon according to claim 5, characterized in that, In step S1, the sieving is done through an 80-200 mesh sieve, and the drying is done at 100-110°C for 1.5-2.5 hours.

7. The method for preparing biomass-based hard carbon according to claim 5, characterized in that, In step S2, the mixing is carried out by dry ball milling, and the rotation speed of the dry ball mill is 300~500 r / min and the time is 30~60 min.

8. The method for preparing biomass-based hard carbon according to claim 5, characterized in that, In step S3, the solid-phase catalytic crosslinking reaction is carried out at a temperature of 180~220℃ for 2~4 hours. And / or, in step S4, the gradient carbonization is as follows: first, the temperature is increased to 300-400℃ at a heating rate of 1-2℃ / min and held for 30-60min; then, the temperature is increased to 600-700℃ at a heating rate of 3-5℃ / min and held for 20-40min; then, the temperature is increased to 900-1000℃ at a heating rate of 8-12℃ / min and held for 15-30min.

9. The method for preparing biomass-based hard carbon according to claim 5, characterized in that, In step S5, the pickling uses dilute hydrochloric acid with a concentration of 0.5~1mol / L, the pickling temperature is 75~85℃, and the pickling time is 1~2h; the water washing uses deionized water until the pH value of the washing solution is 6.8~7.2; the drying temperature is 100~110℃ and the time is 1~2h.

10. An application of biomass-based hard carbon as described in any one of claims 1-4 as a negative electrode material for sodium-ion batteries.

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