Biomass hard carbon material, preparation method thereof and sodium ion battery

Abstract

The invention belongs to the technical field of battery materials, and particularly relates to a biomass hard carbon material, a preparation method thereof and a sodium ion battery. The preparation method of the biomass hard carbon material comprises the following steps: mixing biomass with a composite acid solution, carrying out a first reaction, heating, and carrying out a second reaction to obtain esterified and modified biomass; carrying out gas phase modification treatment on the esterified and modified biomass in a mixed gas atmosphere to obtain a gas phase modified product; immersing the gas-phase modified product into an ethanol solution of tetraethoxysilane, mixing and reacting to obtain a silicon dioxide coating, mixing the silicon dioxide coating with magnesium powder, heating and pre-carbonizing under protective gas, and pickling to obtain a coated pre-carbonized product; and carrying out two-stage gradient sintering on the coated pre-carbonized product, mixing the obtained material with coated asphalt, and carrying out heating coating treatment to prepare the biomass hard carbon material. The biomass hard carbon material has high specific capacity and excellent 1C rate capability.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of battery materials, and particularly relates to a biomass hard carbon material, a preparation method thereof, and a sodium ion battery. BACKGROUND

[0002] With the growing demand for renewable energy storage worldwide, sodium ion batteries have attracted much attention due to their abundant raw materials and low cost. Among various anode materials, biomass hard carbon (such as coconut shell, straw or lignin-based carbon) has shown excellent Na + insertion / extraction performance due to its unique hierarchical pore structure and expanded interlayer spacing, and has become the mainstream choice for current commercial applications. Such materials not only effectively buffer the volume strain during charging and discharging, but also have the advantages of wide raw material sources and strong sustainability.

[0003] However, in the prior art, biomass hard carbon still has two major defects: the first defect is that the actual specific capacity is generally lower than 350 mAh / g; the second defect is that the excellent cycle performance data of biomass hard carbon reported in the current literature are mostly based on 0.1C test conditions. The performance under such low rate has a significant gap with the actual application scenario. When the current density is increased to 1C, the rate performance of biomass hard carbon decreases significantly (for example, the capacity retention rate is lower than 80% after 50 cycles at 1C), which restricts the application of biomass hard carbon materials and sodium ion batteries.

[0004] Therefore, there is an urgent need to provide a new hard carbon material with high specific capacity and good rate performance. SUMMARY

[0005] The present application aims to at least solve one of the technical problems existing in the prior art. To this end, the present application provides a biomass hard carbon material, a preparation method thereof, and a sodium ion battery. The biomass hard carbon material prepared by the preparation method has high specific capacity and further has good rate performance. The biomass hard carbon material is used as an anode in a sodium ion battery, which helps to promote the application of sodium ion batteries.

[0006] The first aspect of the present application provides a preparation method of a biomass hard carbon material.

[0007] The preparation method of the biomass hard carbon material comprises the following steps:

[0008] (1) mixing biomass and a composite acid solution to perform a first reaction, then heating to perform a second reaction, to obtain esterified modified biomass;

[0009] (2) placing the esterified modified biomass prepared in step (1) in a mixed gas atmosphere to perform gas phase modification treatment, to obtain a gas phase modification product;

[0010] (3) immersing the gas-phase modified product obtained in step (2) into an ethyl alcohol solution of tetraethyl orthosilicate, mixing and reacting to obtain a silica coating, mixing the silica coating with magnesium powder, pre-carbonizing under a protective gas, acid washing to obtain a coated pre-carbonized product;

[0011] (4) performing two-stage gradient sintering on the coated pre-carbonized product obtained in step (3), mixing the obtained sintered product with coated pitch, and then performing coating treatment under a protective gas atmosphere to obtain the biomass hard carbon material;

[0012] The acids in the composite acid solution include citric acid, phosphoric acid, and oxalic acid;

[0013] The mixed gas includes oxygen and ammonia.

[0014] The preparation method of the application uses biomass as raw material, through the synergistic effect of composite acid esterification modification and gas phase oxidation-ammoniation, silicon-carbon core-shell constrained volume strain, to construct a biomass hard carbon material with multi-level pore structure and nitrogen and oxygen co-doping: the carboxyl / carbonyl functional groups generated by the esterification reaction in step (1) provide surface adsorption sites, and the synergistic effect of oxalic acid and phosphoric acid forms more phosphate ester crosslinking network on the surface of biomass, and generates phosphorus-doped hard carbon with expanded interlayer spacing during subsequent carbonization, and the two-stage holding reaction avoids local excessive esterification, improves the uniformity of the raw material, and reduces the defects during subsequent carbonization; in step (2), nitrogen doping enhances the electrical conductivity, and the ·NH2 free radicals generated by the decomposition of ammonia are embedded in the carbon layer, which can realize in-situ nitrogen doping and form nitrogen-containing functional groups to improve the electronic conductivity; in step (3), the SiO2 coating layer is converted into nano-silicon to form a core-shell structure (Si@C) constrained by carbon, which buffers the volume expansion of silicon and avoids direct exposure of silicon particles, and cooperates with hard carbon to form a continuous conductive network, and the core-shell interface provides a fast ion migration channel, which is beneficial to improve the rate performance of the finally obtained biomass hard carbon material; in step (3), although the silicon dioxide (SiO2) is coated on the gas phase modification product (the main component of the gas phase modification product is biomass) in the initial stage, but during the subsequent co-heating process with magnesium powder, two synchronous and fundamental changes occur: first, the surface SiO2 coating layer is broken and decomposed into discrete nano-silicon (Si) particles by magnesium thermal reduction reaction; at the same time, the biomass body wrapped inside undergoes pre-carbonization, and is not simply carbonized, but is converted into a continuous and strong three-dimensional amorphous carbon matrix. This newly born carbon matrix is like a "sponge body", not only itself shaped, but also captures, embeds and wraps the newly generated nano-silicon particles outside it. Therefore, the final structure is a carbon matrix as a continuous matrix, coated with nano-silicon particles as discrete phase, thereby forming a stable Si@C structure. After the two-stage gradient sintering in step (4) is completed, the obtained material is mixed with coated pitch, and then heated and coated under a protective gas atmosphere. This further coating treatment realizes double carbon layer coating of biomass-based and pitch-based, improves the coating effect, avoids Si exposure, and thus more effectively suppresses Si expansion. The two-stage gradient sintering in step (4) is beneficial to form a hierarchical pore structure, which improves the sodium ion diffusion rate. Through the above four steps, the biomass hard carbon material forms an integrated system of hierarchical pores (accelerate Na + diffusion), conductive network (N / P co-doping) and buffer interface (Si@C), thereby realizing high specific capacity and excellent 1C rate performance at the same time. The hard carbon material prepared by the application has high specific capacity and good rate performance, and even when cyclic charging and discharging is carried out at a 1C current density, it still maintains good capacity retention rate, solving the technical problem of rapid capacity decay of the biomass hard carbon material in the prior art when cyclic charging and discharging is carried out at a 1C current density.

[0015] The second aspect of the present application provides a biomass hard carbon material.

[0016] A biomass hard carbon material prepared by the above preparation method.

[0017] The third aspect of the present application provides an application of the biomass hard carbon material.

[0018] A battery comprising a negative electrode, wherein the negative electrode comprises the biomass hard carbon material.

[0019] In some embodiments of the present application, the battery comprises a sodium ion battery.

[0020] Compared with the prior art, the present application has the following beneficial effects:

[0021] (1) The preparation method of the present application uses biomass as raw material, and through the synergistic effect of complex acid esterification modification, gas phase oxidation-ammoniation, and silicon-carbon core-shell constraint volume strain, a biomass hard carbon material with multi-level pore structure and nitrogen and oxygen co-doping is constructed, so that the biomass hard carbon material forms an integrated system of hierarchical pores (accelerate Na + diffusion), conductive network (N / P co-doping), and buffer interface (Si@C), thereby realizing high specific capacity and excellent 1C rate performance at the same time.

[0022] (2) In step (2) of the preparation method of the present application, ammonium vanadate is also added during the gas phase modification treatment to perform vanadium doping treatment. Vanadium doping expands the carbon layer spacing, provides more unobstructed Na + embedding channels, reduces the diffusion energy barrier, and the V-N-C bond formed between the carbon layers forms a cross-network similar to graphene, which reduces the volume expansion rate control. Vanadium atoms act as "nanosoldering points" to inhibit carbon layer peeling through strong covalent bonds, thereby also improving the specific capacity and rate performance at 1C of the finally prepared biomass hard carbon material. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 SEM (scanning electron microscope) image of the biomass hard carbon material prepared for Example 1 of the present application;

[0024] Figure 2 Cycle performance graph of the button cell assembled by the biomass hard carbon material prepared for Example 1 of the present application at 1C. DETAILED DESCRIPTION

[0025] In order to make those skilled in the art more clearly understand the technical solutions of the present application, the following examples are given for illustration. It should be pointed out that the following examples do not constitute a limitation on the scope of protection required by the present application.

[0026] The first aspect of the present application provides a preparation method of a biomass hard carbon material.

[0027] A preparation method of a biomass hard carbon material, comprising the following steps:

[0028] (1) mixing biomass with a composite acid solution to perform a first reaction, then heating to perform a second reaction, to obtain esterification-modified biomass;

[0029] (2) placing the esterification-modified biomass obtained in step (1) in a mixed gas atmosphere to perform gas-phase modification treatment, to obtain a gas-phase modification product;

[0030] (3) immersing the gas-phase modification product obtained in step (2) in an ethyl silicate ethanol solution, mixing and reacting, then separating, to obtain a silicon dioxide coating, then mixing the silicon dioxide coating with magnesium powder, heating under a protective gas to perform pre-carbonization, and performing acid washing, to obtain a coated pre-carbonization product;

[0031] (4) performing two-stage gradient sintering on the coated pre-carbonization product obtained in step (3), mixing the obtained material with coated pitch, and then heating and coating under a protective gas atmosphere, to obtain the biomass hard carbon material;

[0032] The acids in the composite acid solution include citric acid, phosphoric acid and oxalic acid;

[0033] The mixed gas includes oxygen and ammonia.

[0034] The preparation method provided by the present application uses biomass as a raw material, and through the synergistic effect of composite acid esterification modification and gas-phase oxidation-ammoniation, and silicon-carbon core-shell volume strain constraint, a biomass hard carbon material with a multi-level pore structure and nitrogen and oxygen co-doping is constructed: the carboxyl / carbonyl functional groups generated by the esterification reaction in step (1) provide surface adsorption sites, and the synergistic effect of oxalic acid and phosphoric acid forms more phosphate ester crosslinking networks on the surface of the biomass, and the phosphorus-doped hard carbon with an expanded interlayer spacing is generated during subsequent carbonization, and the two-stage holding reaction avoids local excessive esterification, improves the uniformity of the raw material, and reduces defects during subsequent carbonization; nitrogen doping in step (2) enhances the electrical conductivity, and the ·NH2 free radicals generated by the decomposition of ammonia are embedded in the carbon layer, which can realize in-situ nitrogen doping, form nitrogen-containing functional groups, and improve the electronic conductivity; when the SiO2 coating layer in step (3) is converted into nano-silicon, a carbon-constrained core-shell structure (Si@C) is formed, which buffers the volume expansion of silicon and avoids direct exposure of silicon particles, and cooperates with the hard carbon to form a continuous conductive network, and the core-shell interface provides a rapid ion migration channel, which is beneficial to improving the rate performance of the finally obtained biomass hard carbon material; the two-stage gradient sintering in step (4) is beneficial to forming a hierarchical pore structure and improving the sodium ion diffusion rate. Through the above four steps, the biomass hard carbon material forms a hierarchical pore structure (accelerates Na +The integrated system of the diffusion, the conductive network (N / P co-doping) and the buffer interface (Si@C) is integrated, so that high specific capacity and excellent 1C rate performance are realized at the same time. The prepared hard carbon material has high specific capacity and good rate performance, and even when the cyclic charging and discharging is carried out at a 1C current density, the capacity retention rate is still good, and the technical problem of rapid capacity attenuation of the biomass hard carbon material in the prior art when the cyclic charging and discharging is carried out at a 1C current density is solved.

[0035] In some embodiments of the present application, in step (1), the biomass is subjected to drying treatment at 105-110 DEG C for 3-5 hours.

[0036] In some embodiments of the present application, in step (1), the biomass includes at least one of corn stalks, soybean stalks and coconut shells.

[0037] In some embodiments of the present application, in step (1), the biomass has a D 50 of 70-250 mu m. For example, 70 mu m, 100 mu m, 150 mu m, 200 mu m, 250 mu m.

[0038] In some embodiments of the present application, in step (1), the mass ratio of citric acid to phosphoric acid, oxalic acid in the composite acid solution is 2-3: (0.8-1.5): 1. For example, 2-3:1:1.

[0039] In some embodiments of the present application, in step (1), the mass concentration of the composite acid solution is 12-16%. For example, 12%, 13%, 14%, 15%, 16%.

[0040] In some embodiments of the present application, in step (1), the mass ratio of the biomass to the composite acid in the composite acid solution is 85: (10-15). It can be 85:10, 85:11, 85:12, 85:13, 85:15.

[0041] In some embodiments of the present application, in step (1), the temperature of the first reaction is 95-105 DEG C, and the reaction time is 0.8-1.2 hours.

[0042] In some embodiments of the present application, in step (1), the temperature of the second reaction is 140-150 DEG C, and the reaction time is 1.5-2.5 hours.

[0043] Oxalic acid, phosphoric acid and citric acid synergistically form more phosphate crosslinking networks on the surface of biomass, and generate phosphorus-doped hard carbon with expanded interlayer spacing during subsequent carbonization. The two-stage holding reaction of step (1) avoids local excessive esterification, improves the uniformity of raw materials, and reduces defects in subsequent carbonization.

[0044] In some embodiments of the present application, after the second reaction in step (1), a solid-liquid separation and drying are further included, the drying temperature is 100-110°C, and the drying time is 2-5 hours.

[0045] In some embodiments of the present application, in step (2), the mixed gas further includes an inert gas. For example, nitrogen.

[0046] In some embodiments of the present application, in step (2), the mixed gas includes 6-8% oxygen and 6-9% ammonia by volume fraction. For example, the mixed gas includes 7-8% oxygen and 8-9% ammonia by volume fraction.

[0047] In some embodiments of the present application, in step (2), the temperature of the gas phase modification treatment is 215-225°C, and the holding time at the temperature is 3-6 hours.

[0048] Suitable oxygen and ammonia contents can reduce the oxidation intensity and reduce the micropore collapse caused by excessive oxidation. The relatively low sintering temperature and the relatively long sintering time are conducive to retaining more oxygen-containing functional groups, increasing the pyridine nitrogen / pyrrole nitrogen content, and improving the electronic conductivity and Na + adsorption sites.

[0049] In some embodiments of the present application, in step (3), the mass ratio of the gas phase modification product to the ethanol solution of tetraethyl orthosilicate is 1: (8-12).

[0050] In some embodiments of the present application, in step (3), the temperature of the mixed reaction is 70-80°C, and the time of the mixed reaction is 3-5 hours.

[0051] In some embodiments of the present application, in step (3), the mass ratio of the silica coating to the magnesium powder is 60: (46-50), for example, 60: (48-50).

[0052] In some embodiments of the present application, in step (3), the mixing of the silica coating and the magnesium powder includes ball milling.

[0053] In some embodiments of the present application, in step (3), the acid pickling is performed using 0.5-1 mol / L hydrochloric acid.

[0054] In some embodiments of the present application, in step (3), the protective gas includes nitrogen or a noble gas. The noble gas may, for example, be argon, helium, or krypton.

[0055] In some embodiments of the present application, in step (3), the pre-carbonization process is to increase the temperature from room temperature to 650-700°C at a rate of 2-4°C / min, and then to keep the temperature for 2-3 hours.

[0056] In some embodiments of the present application, in step (4), the two-stage gradient sintering process is to keep the temperature at 1100-1200°C for 0.7-1.4 hours, and then to keep the temperature at 1450-1500°C for 0.7-1.2 hours.

[0057] In some embodiments of the present application, in step (4), the two-stage gradient sintering process is to increase the temperature from room temperature to 1100-1200°C at a rate of 5-8°C / min, to keep the temperature at 1100-1200°C for 0.7-1.4 hours, to increase the temperature to 1450-1500°C at a rate of 2-3°C / min, and to keep the temperature at 1450-1500°C for 0.7-1.2 hours.

[0058] The control of the temperature increasing rate in the two-stage gradient sintering process can promote the gradual removal of oxygen-containing functional groups from the carbon skeleton of the biomass, avoid micro-cracks caused by violent decomposition, expand the carbon layer spacing to 0.34 nm at 1100-1200°C, and provide sufficient channels for sodium ion intercalation; the smaller temperature increasing rate of 2-3°C / min at 1450-1500°C can slow down the stacking rate of carbon layers, allow volatile gases to escape in an orderly manner, form uniformly distributed nano-closed pores in the carbon matrix, and make the carbon layers partially arrange in the form of graphite microcrystals at 1450-1500°C, so as to maintain the diffusion channels for sodium ions and inhibit the co-intercalation of electrolyte through the thickening of the closed pore wall, thereby improving the cycle stability of the biomass hard carbon material.

[0059] In some embodiments of the present application, in step (2), the gas-phase modification process further includes a vanadium doping treatment.

[0060] The vanadium doping can expand the carbon layer spacing, provide more unobstructed Na + intercalation channels, reduce the diffusion energy barrier, form a cross-network of V-N-C bonds between carbon layers to reduce the volume expansion rate, and act as "nanosoldering points" to inhibit the peeling of carbon layers through strong covalent bonds, thereby improving the specific capacity and rate performance at 1C of the final biomass hard carbon material.

[0061] In some embodiments of the present application, in step (2), the gas phase modification treatment comprises: mixing and reacting the esterification modified biomass obtained in step (1) with an ammonium vanadate solution, solid-liquid separation and drying the solid, then placing it in a mixed gas atmosphere, and performing gas phase modification treatment to obtain a gas phase modified product. In the preparation process of the gas phase modified product, since the ammonium vanadate solution is introduced, it includes vanadium doping treatment.

[0062] In some embodiments of the present application, in the ammonium vanadate solution, the mass-volume ratio of the ammonium vanadate to the solvent is 12.5 g: (180-220) mL, for example, 12.5 g: 200 mL.

[0063] In some embodiments of the present application, the mass ratio of the esterification modified biomass to the ammonium vanadate solution is 1: (10-15).

[0064] In some embodiments of the present application, the solvent is deionized water.

[0065] In some embodiments of the present application, each of steps (1)-(4) can independently include a crushing or pulverizing process to control the particle size, wherein the crushing or pulverizing can use conventional methods in the art, which are not limited here.

[0066] In some embodiments of the present application, in step (1), the esterification modified biomass is subjected to a crushing or pulverizing process to regulate the particle size, for example, it can be crushed to a particle size D 50 of 10-100 μm.

[0067] In some embodiments of the present application, in step (2), the material after the gas phase modification treatment is subjected to a crushing or pulverizing process to regulate the particle size, for example, it can be crushed to a particle size D 50 of 10-50 μm.

[0068] In some embodiments of the present application, in step (3), the material after the pre-carbonization is subjected to a crushing or pulverizing process to regulate the particle size, for example, it can be crushed to a particle size D 50 of 5-20 μm.

[0069] In some embodiments of the present application, after step (3) of pre-carbonization and / or step (4) of two-stage gradient sintering, crushing or pulverizing can be performed to regulate the particle size of the biomass hard carbon material. Specifically, the particle size D 505-20 μm, specifically 5-9 μm, 9-15 μm, 15-20 μm, etc. In some embodiments, the material after pre-carbonization in step (3) is crushed or pulverized to a D 50 5-20 μm, and then the product after two-stage gradient sintering in step (4) is crushed or pulverized to a D 50 5-20 μm.

[0070] In some embodiments of the present application, in step (4), the coating pitch has a softening point of 220-280 °C.

[0071] In some embodiments of the present application, the weight of the coating pitch is 5%-11% of the weight of the material after carbonization.

[0072] In some embodiments of the present application, before the heating and coating process, a crushing process is further included, and the material after carbonization is mixed with the coating pitch to form a material which is crushed to a D 50 3-20 μm, specifically 3-8 μm, 8-10 μm, 10-15 μm, 15-20 μm, etc.

[0073] In some embodiments of the present application, the temperature of the heating and coating process is 1200-1700 °C, and the time is 1-3 hours.

[0074] In some embodiments of the present application, the protective atmosphere includes an atmosphere formed by one or more of nitrogen and inert gas. In the present application, the inert gas can include one or more of helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn).

[0075] The second aspect of the present application provides a biomass hard carbon material prepared by the above preparation method.

[0076] The third aspect of the present application provides an application of a biomass hard carbon material.

[0077] A sodium ion battery includes a negative electrode, and the negative electrode includes the above biomass hard carbon material.

[0078] The raw materials, reagents or devices used in the following examples, unless otherwise specified, can be obtained from conventional commercial channels or can be obtained by existing known methods.

[0079] The corn stalks or soybean stalks used in the following examples are dried at 105 °C for 4 hours.

[0080] Example 1

[0081] A preparation method of a biomass hard carbon material, comprising the following steps:

[0082] (1) composite acid esterification modification: corn straw (D 50 =100 μm±5 μm) is mixed with a composite acid solution (the mass ratio of citric acid to phosphoric acid to oxalic acid in the composite acid solution is 2:1:1, and the mass concentration of the composite acid solution is 16%), and the mass ratio of the corn straw to the composite acid in the composite acid solution is 85:15, the mixture is reacted at 100 ℃ for 1 hour, then the temperature is increased to 150 ℃ at a rate of 5 ℃ / min and reacted for 2 hours, then filtered, and dried at 105 ℃ for 3 hours to obtain an esterification modified biomass;

[0083] (2) gas phase oxidation-ammoniation treatment: the esterification modified biomass prepared in step (1) is placed in a tube furnace, a mixed gas (the mixed gas contains 8% oxygen, 8% ammonia and 84% nitrogen by volume fraction) is flowed into the tube furnace at a rate of 45 mL / min, sintered at 220 ℃ for 4 hours, and then cooled to room temperature in a nitrogen atmosphere to obtain a gas phase modified product;

[0084] (3) the gas phase modified product obtained in step (2) is immersed in an ethyl silicate ethanol solution (the mass concentration of ethyl silicate in the ethyl silicate ethanol solution is 5%), the mass ratio of the gas phase modified product to the ethyl silicate ethanol solution is 1:10, stirred at 80 ℃ for 4 hours, then filtered, and dried at 60 ℃ for 6 hours to obtain a silica coating, then the silica coating is mixed with magnesium powder at a mass ratio of 60:48, ball milled (ball-to-material ratio is 10:1, rotation speed is 300 rpm) under argon protection for 2 hours, heated to 650 ℃ at a rate of 5 ℃ / min under an argon atmosphere (argon flow is 50 mL / min), and kept at 650 ℃ for 2 hours, and then naturally cooled to room temperature, the obtained solid product is subjected to acid washing with 1 mol / L hydrochloric acid at a mass ratio of 1:15, oscillated at room temperature for 1 hour, then washed to neutral with deionized water, and vacuum dried at 60 ℃ for 12 hours to obtain a coated pre-carbonized product;

[0085] (4) gradient sintering process: the coated pre-carbonized product obtained in step (3) is subjected to two-stage sintering under an argon atmosphere, specifically, heated to 1150 ℃ at a rate of 5 ℃ / min, kept at 1150 ℃ for 1 hour, then heated to 1450 ℃ at a rate of 2 ℃ / min, kept at 1450 ℃ for 1 hour, mixed with coating pitch (the coating pitch is petroleum pitch with a softening point of 220 ℃, and the weight of the coating pitch is 10% of the material after gradient sintering), and then the mixture is placed in a tube furnace and heated to 1300 ℃ under inert gas (argon) protection for 1 h to obtain the biomass hard carbon material.

[0086] Figure 1 The SEM (scanning electron microscope) image of the biomass hard carbon material prepared in Example 1 is shown in the following figure. The prepared straw hard carbon material is irregular particles.

[0087] Example 2

[0088] A preparation method of a biomass hard carbon material, comprising the following steps:

[0089] (1) Complex acid esterification modification: corn straw (D 50 =100 μm±5 μm) is mixed with a complex acid solution (the mass ratio of citric acid to phosphoric acid and oxalic acid in the complex acid solution is 2:1.2:1, and the mass concentration of the complex acid solution is 14%), and the mass ratio of the corn straw to the complex acid in the complex acid solution is 85:13, and then the mixture is reacted at 100°C for 1 hour, and then the temperature is increased to 145°C at a rate of 5°C / min and reacted for 2 hours, filtered, and dried at 105°C for 3 hours to obtain an esterification modified biomass;

[0090] (2) Gas phase oxidation-ammoniation treatment: the esterification modified biomass prepared in step (1) is placed in a tube furnace, and a mixed gas (the mixed gas contains 7% oxygen, 7% ammonia and 86% nitrogen by volume fraction) is flowed into the tube furnace at a rate of 40 mL / min, and then sintered at 215°C for 5 hours, and then cooled to room temperature in a nitrogen atmosphere to obtain a gas phase modified product;

[0091] (3) The gas phase modified product obtained in step (2) is immersed in an ethyl silicate ethanol solution (the mass concentration of ethyl silicate in the ethyl silicate ethanol solution is 4%), and the mass ratio of the gas phase modified product to the ethyl silicate ethanol solution is 1:12, and then stirred at 70°C for 5 hours, and then filtered and dried at 65°C for 6 hours to obtain a silica coating, and then the silica coating is mixed with magnesium powder at a mass ratio of 60:49, and then ball milled (ball-to-material ratio is 10:1, and the rotation speed is 300 rpm) under argon protection for 2 hours, and then heated to 680°C at a rate of 5°C / min under an argon atmosphere (the flow rate of argon is 50 mL / min), and then maintained at 680°C for 2 hours, and then naturally cooled to room temperature, and then the obtained solid product is subjected to acid washing with 1 mol / L hydrochloric acid at a mass ratio of 1:12, and then oscillated at room temperature for 1.5 hours, and then washed with deionized water until neutral, and then vacuum dried at 80°C for 10 hours to obtain a coated pre-carbonized product;

[0092] (4) Gradient sintering process: the coated pre-carbonization product obtained in step (3) is subjected to two-stage sintering under an argon atmosphere, specifically, the temperature is raised to 1200°C at a rate of 5°C / min, then the temperature is kept at 1200°C for 1 hour, then the temperature is raised to 1500°C at a rate of 2°C / min, then the temperature is kept at 1500°C for 1 hour, the obtained material is mixed with coated pitch (the coated pitch is petroleum pitch with a softening point of 220°C, the weight of the coated pitch is 11% of the material after gradient sintering), then the mixture is placed in a tube furnace and heated to 1300°C under inert gas (argon) protection for 1 hour of coating treatment, to obtain the biomass hard carbon material.

[0093] Example 3

[0094] A method for preparing a biomass hard carbon material, comprising the following steps:

[0095] (1) Composite acid esterification modification: corn straw (D 50 =100μm±5μm) is mixed with a composite acid solution (the mass ratio of citric acid to phosphoric acid to oxalic acid in the composite acid solution is 2:1:1, and the mass concentration of the composite acid solution is 16%), and the mass ratio of corn straw to composite acid in the composite acid solution is 85:15, the mixture is kept at 100°C for 1 hour, then the temperature is raised to 150°C at a rate of 5°C / min and kept for 2 hours, filtered, and dried at 105°C for 3 hours to obtain esterification-modified biomass;

[0096] (2) Gas phase oxidation-ammoniation-vanadium doping treatment: the esterification-modified biomass obtained in step (1) is mixed with an ammonium vanadate solution at a weight ratio of 1:10 at 80°C for 2 hours, then filtered, dried at 105°C for 3 hours, and then placed in a tube furnace, a mixed gas (the mixed gas contains 8% oxygen, 8% ammonia and 84% nitrogen by volume fraction) is flowed into the tube furnace at a rate of 45mL / min, and sintered at 220°C for 4 hours, then cooled to room temperature under a nitrogen atmosphere to obtain a gas phase modified product;

[0097] (3) The gas phase modified product obtained in step (2) is immersed in an ethyl alcohol solution of tetraethyl orthosilicate (the mass concentration of the tetraethyl orthosilicate in the ethyl alcohol solution is 5%), the mass ratio of the gas phase modified product to the ethyl alcohol solution of tetraethyl orthosilicate is 1:10, stirring is carried out at 80℃ for 4 hours, then filtering, drying at 60℃ for 6 hours, obtaining a silica coating, then mixing the silica coating with magnesium powder according to a mass ratio of 60:48, ball milling (the ball-to-material ratio is 10:1, the rotation speed is 300 rpm) under the protection of argon for 2 hours, heating to 650℃ at a rate of 5℃ / min under the atmosphere of argon (the flow rate of argon is 50 mL / min), and keeping the temperature at 650℃ for 2 hours, then naturally cooling to room temperature, obtaining a solid product, then washing the solid product with 1 mol / L hydrochloric acid, the mass ratio of the solid product to the hydrochloric acid is 1:15, oscillating at room temperature for 1 hour, then washing to neutral with deionized water, and vacuum drying at 60℃ for 12 hours, obtaining a coated pre-carbonized product;

[0098] (4) Gradient sintering process: the coated pre-carbonized product obtained in step (3) is sintered in two stages under the atmosphere of argon, specifically heating to 1150℃ at a rate of 5℃ / min, then keeping the temperature at 1150℃ for 1 hour, then heating to 1450℃ at a rate of 2℃ / min, then keeping the temperature at 1450℃ for 1 hour, obtaining a material, then mixing the material with coating pitch (the coating pitch is petroleum pitch with a softening point of 220℃, and the weight of the coating pitch is 10% of the weight of the sintered material), then placing the mixture into a tube furnace, heating to 1300℃ under the protection of inert gas (argon) for 1 hour, and obtaining a biomass hard carbon material.

[0099] Example 4

[0100] A method for preparing a biomass hard carbon material, comprising the following steps:

[0101] (1) Complex acid esterification modification: corn straw (D 50 =100μm±5μm) is mixed with a complex acid solution (the mass ratio of citric acid to phosphoric acid to oxalic acid in the complex acid solution is 2:1:1, and the mass concentration of the complex acid solution is 16%), and the mass ratio of the corn straw to the complex acid in the complex acid solution is 85:15, then the mixture is kept at 100℃ for 1 hour, then heating to 150℃ at a rate of 5℃ / min and keeping the temperature at 150℃ for 2 hours, then filtering, and drying at 105℃ for 3 hours, obtaining an esterification modified biomass;

[0102] (2) Gas phase oxidation-ammoniation treatment: the esterification modified biomass prepared in step (1) was placed in a tube furnace, and a mixed gas (the mixed gas was composed of 10% oxygen, 10% ammonia and 80% nitrogen by volume fraction) was flowed into the tube furnace at a rate of 45 mL / min, sintered at 220℃ for 4 hours, and then cooled to room temperature under nitrogen atmosphere to obtain a gas phase modified product;

[0103] (3) The gas phase modified product obtained in step (2) was immersed in an ethyl alcohol solution of tetraethyl orthosilicate (the mass concentration of tetraethyl orthosilicate in the ethyl alcohol solution was 5%), and the mass ratio of the gas phase modified product to the ethyl alcohol solution of tetraethyl orthosilicate was 1:10, stirred at 80℃ for 4 hours, then filtered, and dried at 60℃ for 6 hours to obtain a silica coating product. Then, the silica coating product was mixed with magnesium powder at a mass ratio of 60:48, ball milled (the ball-to-material ratio was 10:1, and the rotation speed was 300 rpm) under argon protection for 2 hours, heated to 650℃ at a rate of 5℃ / min under argon (the flow rate of argon was 50 mL / min) atmosphere, and kept at 650℃ for 2 hours. After natural cooling to room temperature, the obtained solid product was subjected to acid washing with 1 mol / L hydrochloric acid at a mass ratio of the solid product to the hydrochloric acid of 1:15, oscillated at room temperature for 1 hour, then washed to neutral with deionized water, and vacuum dried at 60℃ for 12 hours to obtain a coated pre-carbonization product;

[0104] (4) Gradient sintering process: the coated pre-carbonization product obtained in step (3) was subjected to two-stage sintering under argon atmosphere, specifically heated to 1150℃ at a rate of 5℃ / min, kept at 1150℃ for 2 hours, then heated to 1400℃ at a rate of 5℃ / min, kept at 1400℃ for 1 hour, mixed with coating pitch (the coating pitch was petroleum pitch with a softening point of 220℃, and the weight of the coating pitch was 10% of the material after gradient sintering), and then placed in a tube furnace and heated to 1300℃ under inert gas (argon) protection for 1 hour to prepare a biomass hard carbon.

[0105] Comparative Example 1

[0106] Comparative Example 1 is different from Example 1 only in that equal amount of citric acid is used instead of oxalic acid in step (1), and other processes are the same as those in Example 1.

[0107] Comparative Example 2

[0108] Comparative Example 2 is different from Example 1 only in that equal amount of nitrogen is used instead of ammonia in step (2), i.e., the mixed gas comprises oxygen and nitrogen, and other processes are the same as those in Example 1.

[0109] Comparative Example 3

[0110] The difference between Comparative Example 3 and Example 1 is only that step (4) is different from Example 1, and other processes are the same as Example 1.

[0111] The specific content of step (4) of Comparative Example 3 is as follows:

[0112] The composite pre-carbonization product obtained in step (3) is sintered in two stages under an argon atmosphere, specifically, the temperature is raised to 1150°C at a rate of 5°C / min, and then the temperature is kept at 1150°C for 2 hours, to obtain a biomass hard carbon material.

[0113] Comparative Example 4

[0114] The difference between Comparative Example 4 and Example 1 is only that step (3) is different from Example 1. The content of step (3) of Comparative Example 4 is as follows:

[0115] The gas phase modified product obtained in step (2) is mixed with nano silicon powder (particle size 50 nm) in a mass ratio of 100:3, and is pre-carbonized at 650°C for 2 hours under argon protection, to obtain a pre-carbonization product.

[0116] Product effect test

[0117] The biomass hard carbon material prepared in Examples 1-4 and Comparative Examples 1-4 above is mixed with conductive carbon Super-P and PVDF (polyvinylidene fluoride) in a mass ratio of 92:4:4, and then N-methyl pyrrolidone is used as a solvent, and after being mixed uniformly, it is coated on an aluminum foil and dried in a vacuum oven at 102°C. After rolling, it is cut into 8mm diameter circular electrode sheets, sodium sheets are used as anodes, electrolyte is NaPF6 lipid electrolyte of type NP-001, and the separator is a glass fiber separator, and a button cell is assembled in an argon-filled glove box.

[0118] The charge-discharge performance of the prepared sodium ion battery is tested, the battery test uses a new battery test system, the test method uses constant current constant voltage discharge, constant current charging, and the charge-discharge test is carried out at a current density of 1C, the discharge cutoff voltage is 0V, and the charge cutoff voltage is 2.5V.

[0119] Under the above charge-discharge conditions, 100 cycle stability tests are carried out, and the battery capacity retention rate (the greater the capacity retention rate, the better the 1C rate performance) after 100 cycles is obtained, and the results are shown in Table 1.

[0120] Figure 1 SEM (scanning electron microscope) of the biomass hard carbon material prepared in Example 1 of the application.

[0121] Figure 2The cycle performance chart of the button cell assembled by the biomass hard carbon material prepared in Example 1 at 1C. It can be seen that the biomass hard carbon material prepared in Example 1 can maintain stable capacity under 1C cycle charge-discharge conditions, and has good rate performance. Figure 2 It can be seen that the biomass hard carbon material prepared in Example 1 can maintain stable capacity under 1C cycle charge-discharge conditions, and has good rate performance.

[0122]

[0123] It can be seen from Table 1 that the biomass hard carbon material prepared in Example has good specific capacity and good 1C rate performance.

[0124] Compared with Example 1, Comparative Example 1 lacks oxalic acid, and oxalic acid can provide a strong reducing environment to promote the formation of C-O-P bonds between phosphoric acid and biomass hydroxyl to stabilize the carbon skeleton, and oxalic acid penetrates the micropores of corn straw and cooperates with phosphoric acid to form pores, thereby improving the sodium ion adsorption energy; Comparative Example 1 lacks oxalic acid, and the specific capacity and cycle stability of the biomass hard carbon material prepared in Comparative Example 1 decrease.

[0125] Comparative Example 2 lacks nitrogen doping, which cannot construct pyridine / pyrrole nitrogen active sites, and will also cause the final obtained biomass hard carbon to have more edge defect sites, which will exacerbate the side reaction with the electrolyte, and will cause the specific capacity and cycle stability of the biomass hard carbon material to decrease.

[0126] Comparative Example 3 lacks high-temperature treatment at 1450°C, which causes the carbon microcrystal structure of the prepared biomass hard carbon material to develop insufficiently and the sodium storage capacity caused by closed pore defects to collapse, which causes the specific capacity and cycle stability of the finally prepared biomass hard carbon material to decrease significantly.

[0127] Comparative Example 4 does not use the tetraethyl orthosilicate liquid phase coating process, but directly adds nano-silicon, which makes the silicon-carbon interface not firmly combined, and the stability of 1C cycle charge-discharge decreases significantly.

Claims

1. A method for producing a biomass hard carbon material, characterized by, The method comprises the following steps: (1) mixing biomass with a complex acid solution to perform a first reaction, and then heating to perform a second reaction, to obtain esterified modified biomass; (2) placing the esterified modified biomass obtained in step (1) in a mixed gas atmosphere to perform a gas phase modification treatment, to obtain a gas phase modified product; (3) immersing the gas phase modified product obtained in step (2) in an ethyl silicate ethanol solution, mixing and reacting to obtain a silica coating, and then mixing the silica coating with magnesium powder, heating under a protective gas to pre-carbonize, and performing acid washing to obtain a coated pre-carbonized product; (4) performing two-stage gradient sintering on the coated pre-carbonized product obtained in step (3), mixing the obtained sintered material with coated pitch, and then heating and coating under a protective gas atmosphere to obtain the biomass hard carbon material. The acids in the complex acid solution include citric acid, phosphoric acid and oxalic acid; The mixed gas includes oxygen and ammonia.

2. The production method according to claim 1, characterized by, In step (1): The biomass has been dried at 105-110°C for 3-5 hours; and / or, the biomass includes at least one of corn stalks, soybean stalks and coconut shells; and / or the biomass has a D 50 of 70-250 pm; And / or, the mass ratio of citric acid to phosphoric acid and oxalic acid in the complex acid solution is (2-3):(0.8-1.5):1; And / or, the mass concentration of the complex acid solution is 12-16%; And / or, the mass ratio of the biomass to the complex acid in the complex acid solution is 85:(10-15); And / or, the temperature of the first reaction is 95-105°C, and the reaction time is 0.8-1.2 hours; And / or, the temperature of the second reaction is 140-150°C, and the reaction time is 1.5-2.5 hours; And / or, after the second reaction, solid-liquid separation and drying are further included, the drying temperature is 100-110°C, and the drying time is 2-5 hours.

3. The preparation method according to claim 1, characterized in that, In step (2): The mixed gas includes 8-12% oxygen and 12-16% ammonia by volume fraction; And / or, the mixed gas includes an inert gas; And / or, the temperature of the gas phase modification treatment is 215-225°C, and the holding time at the temperature is 3-6 hours.

4. The production method according to claim 1, characterized by, In step (3): The mass ratio of the gas phase modified product to the ethyl silicate ethanol solution is 1:(8-12); And / or, the temperature of the mixing reaction is 70-80°C, and the mixing reaction time is 3-5 hours; And / or, the mass ratio of the silica coating to the magnesium powder is 60:(46-50); And / or, the mixing of the silica coating and the magnesium powder includes ball milling; And / or, the acid washing is performed using 0.5-1 mol / L hydrochloric acid; And / or, the pre-carbonization process is to heat from room temperature to 650-700°C at a rate of 2-4°C / min, and then hold for 2-3 hours; And / or, the protective gas includes one or more of nitrogen and an inert gas.

5. The preparation method according to claim 1, characterized in that, In step (4): The two-stage gradient sintering process is first holding at 1100-1200℃ for 0.7-1.4 hours, and then holding at 1450-1500℃ for 0.7-1.2 hours. And / or, the coating pitch is petroleum pitch with a softening point of 220-280℃. And / or, the weight of the coating pitch is 5%-11% of the weight of the material after the gradient sintering. And / or, the heating coating process also includes a crushing process, the material after the carbonization process is crushed to D 50 is 3-20 μm; And / or, the temperature of the heating coating treatment is 1200-1700℃, and the time is 1-3 hours. And / or, the protective atmosphere includes one or more of nitrogen and inert gas.

6. The production method according to claim 5, wherein In step (4), the two-stage gradient sintering process is first heating from room temperature to 1100-1200℃ at a rate of 5-8℃ / min, holding at 1100-1200℃ for 0.7-1.4 hours, then heating to 1450-1500℃ at a rate of 2-3℃ / min, and holding at 1450-1500℃ for 0.7-1.2 hours.

7. The preparation method according to claim 1, characterized in that, In step (2), the gas phase modification treatment further includes vanadium doping treatment.

8. The preparation method according to claim 7, characterized in that, In step (2), the gas phase modification treatment includes: mixing and reacting the esterification-modified biomass obtained in step (1) with an ammonium vanadate solution, solid-liquid separation and drying the solid, then placing it in a mixed gas atmosphere for gas phase modification treatment to obtain a gas phase modification product.

9. A biomass hard carbon material, characterized by, Prepared by the preparation method of any one of claims 1-8.

10. A sodium-ion battery, characterized in that, A negative electrode comprising the biomass hard carbon material of claim 9 or prepared by the preparation method of any one of claims 1-8. A negative electrode comprising the biomass hard carbon material of claim 9 or prepared by the preparation method of any one of claims 1-8.

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