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

Abstract

The application provides a biomass-based hard carbon negative material and a preparation method and application thereof, and belongs to the field of sodium ion batteries. The preparation method of the biomass-based hard carbon negative material comprises the following steps: after pretreatment, natural bamboo fibers are soaked in an acid solution, cleaned by deionized water, and then high-temperature carbonization is carried out in a tube furnace under normal pressure, high temperature and inert atmosphere, and after crushing and screening, a high-purity biomass-based hard carbon negative material is obtained; meanwhile, a copolymer obtained by the common reaction of acrylic acid, 3-amino butene amide and methyl 6-heptene acid ester is prepared as a binder, and a sodium ion battery is assembled; the sodium ion battery prepared by the preparation method has high initial coulomb efficiency and cycle stability, and the natural bamboo fibers used as the precursor of the hard carbon negative material have the advantages of rich source and low cost.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of sodium ion batteries, and particularly relates to a biomass-based hard carbon negative electrode material and a preparation method and application thereof. BACKGROUND

[0002] Sodium, as a homologous element of lithium, has similar chemical properties as lithium, and is abundant in reserves and low in price. Sodium ion batteries are considered to be a possible substitute for lithium ion batteries, especially in the field of large-scale energy storage. The negative electrode material is the key to the development of sodium ion batteries, among which carbon-based materials are the most commonly used. Conventional graphite materials cannot be applied to sodium ion batteries due to their small interlayer spacing and low sodium storage capacity. Amorphous carbon materials have higher sodium storage capacity due to their larger interlayer spacing, and are an ideal negative electrode material for sodium ion batteries.

[0003] At present, hard carbon precursors are derived from resin-based, starch-based and biomass-based materials. Patent No. CN117476908A discloses the preparation of a phenolic resin-based hard carbon composite negative electrode material. Although resin-based precursors have the characteristics of high purity and high carbon content, their preparation cost is high, the process is complex, which is not conducive to large-scale commercial application, and cannot improve the cost advantage of sodium ion batteries. Patent No. CN118851143A discloses a preparation method of a starch-based hard carbon negative electrode material. Starch-based hard carbon has the advantages of wide source and easy-to-control structure, but its preparation efficiency is low, the precursor cost is relatively high, and the first coulombic efficiency is low, which is not conducive to large-scale commercial application and the exertion of low-cost advantage. Therefore, selecting a low-cost, green and environmentally friendly precursor with optimal comprehensive performance is the key to reducing the price of hard carbon materials and the cost of sodium ion batteries.

[0004] Biomass-based materials have abundant raw material sources, low cost, environmental friendliness and unique microstructure, and have become a good carbon source for preparing high-performance sodium ion battery negative electrode materials. However, the current biomass-based precursors face problems such as poor material consistency, uncontrollable material purity and insufficient chemical modification. Therefore, the biomass material needs to be purified to reduce the impurity content in the biomass material and improve the overall battery performance. Patent No. CN118270767B discloses a preparation method of a biomass hard carbon material, which uses an alcohol solvent to modify and purify the biomass. This method not only has high cost, but also dissolves the amorphous components inside the biomass precursor, easily forming ordered graphite structures, which ultimately affects the first coulombic efficiency and reversible capacity of the battery.

[0005] Meanwhile, the interface bonding force between the binder commonly used in the preparation of sodium ion battery electrode and the biomass-based hard carbon particles is weak, and the binder itself has poor flexibility. During the battery cycle process, the electrode will swell due to the embedding and de-embedding of sodium ions, at which time the binder is prone to cracking and falling off, resulting in the separation of hard carbon particles from the current collector, the destruction of the sodium ion transmission channel, and the repeated rupture and reconstruction of the unstable interface, which aggravates the capacity decay and is difficult to meet the requirements of long-term energy storage applications on battery life.

[0006] Therefore, how to improve the biomass purification effect and prepare a high-performance negative electrode material has become a key challenge in the field. SUMMARY

[0007] The present application aims to solve the problems of incomplete impurity removal, low first cycle coulombic efficiency, poor cycle stability of the existing biomass-based hard carbon negative electrode material, and weak interface bonding and insufficient flexibility of the supporting binder and hard carbon, to obtain a biomass-based hard carbon negative electrode material with high reversible capacity, high first efficiency and long cycle life, and meet the application requirements of sodium ion batteries in large-scale energy storage and other fields.

[0008] The technical scheme adopted by the present application to achieve the above-mentioned purpose is:

[0009] A biomass-based hard carbon negative electrode material, the negative electrode material is biomass-based hard carbon particles, and the carbon source of the biomass-based hard carbon particles is natural bamboo fiber.

[0010] Preferably, the specific surface area of the biomass-based hard carbon particles is less than 15m 2 ·g -1 .

[0011] Preferably, in the biomass-based hard carbon negative electrode material, the mass fraction of aluminum is less than 0.05%, the mass fraction of calcium is less than 0.07%, the mass fraction of chromium is less than 0.001%, the mass fraction of potassium is less than 0.03%, the mass fraction of sodium is less than 0.05%, the mass fraction of nickel is less than 0.001%, and the mass fraction of iron is less than 0.09%.

[0012] Preferably, the particle size of the biomass-based hard carbon particles is 1-50μm.

[0013] Preferably, the biomass-based hard carbon particles are obtained by carbonizing the pretreated bamboo fiber after purification.

[0014] Preferably, the purification includes soaking in an acid solution.

[0015] Preferably, the solute of the acid solution includes one of sulfuric acid, hydrochloric acid, acetic acid, phosphoric acid and nitric acid.

[0016] Preferably, the concentration of the acid solution is 1-10mol / L.

[0017] Preferably, the purifying comprises a hydrofluoric acid solution soak.

[0018] Preferably, the concentration of the hydrofluoric acid solution is 1-10 mol / L.

[0019] The natural bamboo fiber still has various metal impurities and inorganic non-metal impurities after pretreatment. The acid solution is used for soaking to react with the metal impurities to generate soluble metal salts, which are completely separated from the bamboo fiber particles through subsequent filtration and water washing. Then, the soaking of the hydrofluoric acid is used to remove the silicon-based components that cannot be removed by acid washing, so as to achieve comprehensive removal of impurities. The deep removal of impurities can reduce the oxidation-reduction side reactions of impurities and electrolyte in the battery charging and discharging process from the root, avoid the invalid consumption of reversible sodium ions, eliminate the surface defects and disordered pores occupied by impurities, regulate the specific surface area of the material, reduce the excessive contact between the electrode and the electrolyte, and inhibit the excessive growth and damage of the solid electrolyte interface film.

[0020] A negative electrode sheet includes a biomass-based hard carbon negative electrode material.

[0021] Preferably, the negative electrode sheet includes a binder.

[0022] Preferably, the binder includes a terpolymer.

[0023] Preferably, the reaction monomers of the terpolymer include acrylic acid, 3-aminobutene amide, and methyl 6-heptenoate.

[0024] Preferably, the mass ratio of the acrylic acid to the 3-aminobutene amide is 5:1-5.

[0025] Preferably, the mass ratio of the acrylic acid to the methyl 6-heptenoate is 5:2-4.

[0026] The 3-aminobutene amide and the methyl 6-heptenoate jointly optimize the performance of the binder, build a close interface bonding network, enhance the interface compatibility of the binder with the hard carbon active material and the copper foil current collector, reduce the interface peeling phenomenon caused by the sodium ion insertion and extraction in the charging and discharging process, and reduce the transmission resistance of the sodium ion at the interface; improve the chain extensibility of the copolymer molecule, when the electrode swells in the cycle process due to the sodium ion insertion, the flexible segment can buffer the internal stress generated by the volume change through its own deformation, avoid the cracks or structural damage of the electrode, protect the integrity of the sodium ion transmission channel, and reduce the repeated rupture and reconstruction of the solid electrolyte interface film caused by the electrode structure degradation, reduce the irreversible sodium ion consumption, and improve the reversible specific capacity and cycle stability of the sodium ion battery.

[0027] A sodium ion battery includes a biomass-based hard carbon negative electrode material, or a negative electrode sheet.

[0028] Preferably, the sodium ion battery has a capacity retention of greater than 93% after 80 cycles at a current density of 0.1C.

[0029] More preferably, the binder comprises a quaternary copolymer, the reaction monomers of the quaternary copolymer comprise vinyl trifluoroacetate, and the mass ratio of acrylic acid to vinyl trifluoroacetate is 5:1-4. Vinyl trifluoroacetate can adjust the surface charge distribution of the copolymer molecular chain, on the one hand, can form weak interaction with the polar groups on the surface of the biomass-based hard carbon particles, further enhance the interface binding tightness of the binder and the hard carbon active material, and reduce the generation of interface gap in the charging and discharging process; participate in the formation of the solid electrolyte interface film, promote the generation of fluorine-containing compounds, improve the compactness and chemical stability of the solid electrolyte interface film, effectively inhibit the continuous decomposition of the electrolyte and the irreversible consumption of sodium ions, at the same time, reduce the ion transmission resistance of the solid electrolyte interface film, accelerate the sodium ion intercalation / deintercalation kinetics, and guarantee the integrity of the sodium ion transmission channel.

[0030] The application further provides a preparation method of pretreated bamboo fiber particles, comprising the following steps:

[0031] The natural bamboo fibers are placed into a tube furnace, heated to a holding temperature at a heating rate of 1-10℃ / min under the protection of an inert atmosphere, held for 1-5h, cooled to room temperature, and then powdered at 10000-40000rpm for 1-10min to obtain pretreated bamboo fiber particles.

[0032] Preferably, the length of the natural bamboo fibers is 1-3cm.

[0033] Preferably, the inert atmosphere is a nitrogen atmosphere or an argon atmosphere.

[0034] Preferably, the flow rate of the inert atmosphere is 10-100sccm.

[0035] More preferably, the flow rate of the inert atmosphere is 30-80sccm.

[0036] Preferably, the holding temperature is 300-1000℃.

[0037] More preferably, the holding temperature is 300-900℃.

[0038] Preferably, the particle size of the pretreated bamboo fiber particles is 1-50μm.

[0039] The application further provides a preparation method of purified carbon particles, comprising the following steps:

[0040] The pretreated bamboo fiber particles are added into a 1-10 mol / L acid solution, and stirred and soaked at 30-100 ℃ for 8-24 h. After suction filtration, the purified bamboo fiber particles are washed with deionized water until the pH of the washing liquid is 7.0. The purified bamboo fiber particles are added into a 1-10 mol / L hydrofluoric acid solution, and stirred and soaked at 30-100 ℃ for 8-24 h. After suction filtration, the purified carbon particles are washed with deionized water until the pH of the washing liquid is 6.9-7.1, and vacuum dried at 60-120 ℃ for 12-24 h to obtain the purified carbon particles.

[0041] Preferably, the solute of the acid solution comprises one of sulfuric acid, hydrochloric acid, acetic acid, phosphoric acid and nitric acid.

[0042] Preferably, the mass-volume ratio of the pretreated bamboo fiber particles to the sulfuric acid solution is 3 g: 20-100 mL.

[0043] Preferably, the mass-volume ratio of the purified bamboo fiber particles to the hydrofluoric acid solution is 3 g: 20-100 mL.

[0044] The application also provides a preparation method of biomass-based hard carbon particles, comprising:

[0045] The purified carbon particles are placed into a tube furnace, and heated to a first holding temperature at a heating rate of 1-10 ℃ / min under the protection of an inert atmosphere, and held for 1-5 h. The temperature is then increased to a second holding temperature at a heating rate of 1-10 ℃ / min, and held for 1-5 h. After cooling to room temperature, the particles are ground and sieved to obtain the biomass-based hard carbon particles.

[0046] Preferably, the inert atmosphere is a nitrogen atmosphere or an argon atmosphere.

[0047] Preferably, the flow rate of the inert atmosphere is 10-100 sccm.

[0048] More preferably, the flow rate of the inert atmosphere is 30-80 sccm.

[0049] Preferably, the first holding temperature is 300-1000 ℃.

[0050] More preferably, the first holding temperature is 300-900 ℃.

[0051] Preferably, the second holding temperature is 1000-1600 ℃.

[0052] More preferably, the second holding temperature is 1200-1500 ℃.

[0053] Preferably, the grinding speed is 10,000-40,000 rpm.

[0054] More preferably, the grinding speed is 20,000-30,000 rpm.

[0055] Preferably, the powdering time is 1-10 min.

[0056] More preferably, the powdering time is 5-8 min.

[0057] Preferably, the particle size of the biomass-based hard carbon particles is 1-50 μm.

[0058] More preferably, the particle size of the biomass-based hard carbon particles is 5-25 μm.

[0059] The application also provides a preparation method of the copolymer, comprising:

[0060] The acrylic acid is dispersed in deionized water, stirred for 5-15 min to obtain an acrylic acid solution; under the condition of ice water bath and stirring, 35-45 wt% sodium hydroxide solution is added into the acrylic acid solution, after the solution temperature returns to room temperature, the reaction monomer and the initiator are added, nitrogen is introduced for 10-20 min, and the reaction is carried out at 65-75℃ and under stirring for 20-30 h; after filtration, the copolymer is washed with deionized water and dried at 55-65℃ for 2-5 h.

[0061] Preferably, in the acrylic acid solution, the mass-volume ratio of the acrylic acid to the deionized water is 1 g:50-100 mL.

[0062] Preferably, the mass of the acrylic acid solution is measured by the mass of the acrylic acid therein, and the mass of the sodium hydroxide solution is measured by the mass of the sodium hydroxide therein, and the mass ratio of the acrylic acid to the sodium hydroxide is 9:2-5.

[0063] Preferably, the reaction monomer comprises 3-aminobutene amide and methyl 6-heptenoate.

[0064] Preferably, the mass ratio of the acrylic acid to the 3-aminobutene amide is 5:1-5.

[0065] Preferably, the mass ratio of the acrylic acid to the methyl 6-heptenoate is 5:2-4.

[0066] Preferably, the initiator comprises azobisdimethylamid hydrogen chloride.

[0067] Preferably, the mass ratio of the acrylic acid to the azobisdimethylamid hydrogen chloride is 5:0.05-0.15.

[0068] More preferably, the reaction monomer further comprises vinyl trifluoroacetate.

[0069] More preferably, the mass ratio of the acrylic acid to the vinyl trifluoroacetate is 5:1-4.

[0070] The application also provides a preparation method of the negative electrode sheet, comprising:

[0071] The active material, the binder and the conductive agent are dispersed in deionized water, and dispersed for 5-24 h at 3000-5000 rpm to obtain a negative electrode slurry; the negative electrode slurry is coated on a copper foil current collector, vacuum dried at 50-120℃ for 10-48 h, cooled to room temperature, and cut to obtain a negative electrode sheet.

[0072] Preferably, the active material is biomass-based hard carbon particles.

[0073] Preferably, the binder is sodium alginate or sodium carboxymethyl cellulose.

[0074] More preferably, the binder is a copolymer.

[0075] Preferably, the conductive agent is conductive carbon black.

[0076] Preferably, the mass-volume ratio of the binder to deionized water is 1 g:100-150 mL.

[0077] Preferably, the mass ratio of the active material to the binder is 1-10:1.

[0078] Preferably, the mass ratio of the active material to the conductive agent is 1-10:1.

[0079] Preferably, the thickness of the copper foil current collector is 10-30 μm.

[0080] Preferably, the diameter of the negative electrode sheet is 14-16 mm.

[0081] The application also provides a preparation method of a sodium ion battery, comprising:

[0082] Ethylene carbonate and diethyl carbonate are uniformly mixed, sodium hexafluorophosphate is added, and mixed uniformly to obtain an electrolyte; in an argon glove box, 8-12 μL of the electrolyte is added dropwise in the center of a CR2025 positive electrode shell, and a negative electrode sheet is placed in the center of the positive electrode shell; 8-12 μL of the electrolyte is added dropwise, a glass fiber separator is placed in the center of the electrode sheet, 10-15 μL of the electrolyte is added dropwise, a sodium sheet and a stainless steel gasket are placed in the center of the separator, a spring is placed in the center of the gasket, and a negative electrode shell is covered; packaged under a pressure of 400-600 Pa for 2-5 s, and kept at a constant temperature of 33-37℃ for 40-50 h to obtain a sodium ion battery.

[0083] Preferably, in the electrolyte, the volume ratio of ethylene carbonate to diethyl carbonate is 0.5-2:1.

[0084] Preferably, the molar-volume ratio of sodium hexafluorophosphate to ethylene carbonate is 1-3 mmol:1 mL.

[0085] Preferably, the diameter of the glass fiber separator is 15-17 mm.

[0086] Preferably, the thickness of the sodium sheet is 0.8-1.2mm, and the diameter is 13-15mm.

[0087] Preferably, the thickness of the stainless steel gasket is 0.3-0.7mm, and the diameter is 14-16mm.

[0088] Preferably, the thickness of the elastic sheet is 0.2-0.4mm, and the diameter is 14-16mm.

[0089] The present application has the following beneficial effects: the impurity content of the hard carbon particles is low, the specific surface area is adapted to the sodium storage requirement, and the side reaction is effectively reduced; the copolymer binder takes into account the interfacial bonding strength and flexibility, and optimizes the stability of the solid electrolyte interface film; and finally, the first cycle coulombic efficiency and reversible specific capacity of the sodium ion battery are improved, and the cycle capacity retention rate is also improved. Therefore, the present application is a biomass-based hard carbon negative material which is renewable in raw materials, controllable in process, and has performance and cost advantages, and a preparation method and application thereof. BRIEF DESCRIPTION OF DRAWINGS

[0090] Figure 1 The specific surface area test results of the biomass-based hard carbon particles are shown in the figure.

[0091] Figure 2 The first cycle coulombic efficiency test results of the sodium ion battery are shown in the figure.

[0092] Figure 3 The cycle performance test results of the sodium ion battery are shown in the figure. DETAILED DESCRIPTION

[0093] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0094] The concepts involved in the present application will be described below with reference to the drawings. It should be pointed out here that the following descriptions of the concepts are only to make the content of the present application easier to understand, and do not limit the scope of protection of the present application; meanwhile, the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The present application will be described in detail below with reference to the drawings and in combination with the embodiments.

[0095] Embodiment 1:

[0096] Preparation of pretreated bamboo fiber particles: natural bamboo fibers were placed in a tube furnace, heated to 600℃ at a heating rate of 5℃ / min under the protection of an inert atmosphere, and kept for 3h. After cooling to room temperature, the fibers were ground into powder at 30000rpm for 6min to obtain pretreated bamboo fiber particles. The length of the natural bamboo fibers was 2cm; the inert atmosphere was a nitrogen atmosphere, and the nitrogen flow rate was 50sccm.

[0097] Preparation of purified carbon particles: the pretreated bamboo fiber particles were added to a 5mol / L sulfuric acid solution and stirred at 80℃ for 16h. After suction filtration, the particles were washed with deionized water until the pH of the washing liquid reached 7.0. The purified bamboo fiber particles were then added to a 5mol / L hydrofluoric acid solution and stirred at 80℃ for 16h. After suction filtration, the particles were washed with deionized water until the pH of the washing liquid reached 7.0, and vacuum dried at 100℃ for 18h to obtain purified carbon particles. The mass-volume ratio of the pretreated bamboo fiber particles to the sulfuric acid solution was 3g:50mL, and the mass-volume ratio of the purified bamboo fiber particles to the hydrofluoric acid solution was 3g:50mL.

[0098] Preparation of biomass-based hard carbon particles: the purified carbon particles were placed in a tube furnace, heated to 600℃ at a heating rate of 5℃ / min under the protection of an inert atmosphere, and kept for 3h. The temperature was then increased to 1300℃ at a heating rate of 5℃ / min, and kept for 3h. After cooling to room temperature, the particles were ground into powder at 25000rpm for 6min, and sieved through a 20μm sieve to obtain biomass-based hard carbon particles. The inert atmosphere was a nitrogen atmosphere, and the nitrogen flow rate was 50sccm.

[0099] Preparation of negative electrode sheet: the active material, binder, and conductive agent were dispersed in deionized water at 4000rpm for 10h to obtain a negative electrode slurry. The negative electrode slurry was coated on a copper foil current collector, vacuum dried at 80℃ for 20h, and cooled to room temperature before being cut to obtain a negative electrode sheet. The active material was biomass-based hard carbon particles, the binder was sodium alginate, and the conductive agent was conductive carbon black. The mass-volume ratio of the binder to deionized water was 1g:117mL, the mass ratio of the active material to the binder was 8:1, and the mass ratio of the active material to the conductive agent was 8:1. The thickness of the copper foil current collector was 20μm, and the diameter of the negative electrode sheet was 15mm.

[0100] Preparation of sodium-ion battery: ethylene carbonate and diethyl carbonate were uniformly mixed, sodium hexafluorophosphate was added, and the mixture was uniformly mixed to obtain an electrolyte; in an argon glove box, 10 μL of the electrolyte was added to the center of a CR2025 positive electrode shell, and a negative electrode sheet was placed in the center of the positive electrode shell; 10 μL of the electrolyte was added, and a glass fiber separator was placed in the center of the sheet, 12 μL of the electrolyte was added, and a sodium sheet and a stainless steel gasket were placed in the center of the separator, a spring was placed in the center of the gasket, and a negative electrode shell was covered; the package was sealed under a pressure of 500 Pa for 3 s, and the temperature was kept at 35°C for 48 h to obtain a sodium-ion battery. In the electrolyte, the volume ratio of ethylene carbonate to diethyl carbonate was 1:1, and the molar volume ratio of sodium hexafluorophosphate to ethylene carbonate was 2 mmol:1 mL; the diameter of the glass fiber separator was 16 mm; the thickness of the sodium sheet was 1 mm, and the diameter was 14 mm; the thickness of the stainless steel gasket was 0.5 mm, and the diameter was 15 mm; the thickness of the spring was 0.3 mm, and the diameter was 15 mm.

[0101] Example 2: Compared with Example 1, the difference is only in the preparation of the negative electrode sheet.

[0102] Preparation of copolymer: acrylic acid was dispersed in deionized water, stirred for 10 min to obtain an acrylic acid solution; under the condition of ice water bath and stirring, 40 wt% sodium hydroxide solution was added to the acrylic acid solution, and after the solution temperature returned to room temperature, 3-aminobutene amide, methyl 6-heptenoate and azobisdimethylaminoformamide hydrochloride were added, nitrogen was introduced for 15 min, and the reaction was carried out at 70°C and under stirring for 24 h. After filtration, deionized water was used for washing, and drying was carried out at 60°C for 4 h to obtain the copolymer. In the acrylic acid solution, the mass-volume ratio of acrylic acid to deionized water was 1 g:60 mL; the mass of the acrylic acid solution was measured based on the mass of acrylic acid therein, and the mass of 40 wt% sodium hydroxide solution was measured based on the mass of sodium hydroxide therein, and the mass ratio of acrylic acid to sodium hydroxide was 9:4; the mass ratio of acrylic acid to 3-aminobutene amide was 5:2, the mass ratio of acrylic acid to methyl 6-heptenoate was 5:3, and the mass ratio of acrylic acid to azobisdimethylaminoformamide hydrochloride was 5:0.1.

[0103] Preparation of negative electrode sheet: active material, binder and conductive agent were dispersed in deionized water, and dispersed at 4000 rpm for 10 h to obtain a negative electrode slurry; the negative electrode slurry was coated on a copper foil current collector, vacuum dried at 80°C for 20 h, cooled to room temperature, and cut to obtain a negative electrode sheet. The active material was biomass-based hard carbon particles, the binder was a copolymer, and the conductive agent was conductive carbon black; the mass-volume ratio of the binder to deionized water was 1 g:117 mL, the mass ratio of the active material to the binder was 8:1, and the mass ratio of the active material to the conductive agent was 8:1; the thickness of the copper foil current collector was 20 μm, and the diameter of the negative electrode sheet was 15 mm.

[0104] Example 3: The difference between this example and Example 2 is only in the preparation of the copolymer.

[0105] Preparation of the copolymer: Acrylic acid was dispersed in deionized water, stirred for 10 min to obtain an acrylic acid solution; under the condition of ice water bath and stirring, 40wt% sodium hydroxide solution was added into the acrylic acid solution, after the solution temperature returned to room temperature, 3-aminobutyric amide, methyl 6-heptenoate, and azobisdimethylvaleric acid hydrochloride were added, nitrogen was introduced for 15 min, and the reaction was carried out at 70°C for 24 h under stirring. After filtration, deionized water was used for washing, and drying was carried out at 60°C for 4 h to obtain the copolymer. In the acrylic acid solution, the mass-volume ratio of acrylic acid to deionized water was 1g:60mL; the mass of the acrylic acid solution was measured based on the mass of acrylic acid therein, the mass of 40wt% sodium hydroxide solution was measured based on the mass of sodium hydroxide therein, and the mass ratio of acrylic acid to sodium hydroxide was 9:4; the mass ratio of acrylic acid to 3-aminobutyric amide was 5:3, the mass ratio of acrylic acid to methyl 6-heptenoate was 5:3, and the mass ratio of acrylic acid to azobisdimethylvaleric acid hydrochloride was 5:0.1.

[0106] Example 4: The difference between this example and Example 2 is only in the preparation of the copolymer.

[0107] Preparation of the copolymer: Acrylic acid was dispersed in deionized water, stirred for 10 min to obtain an acrylic acid solution; under the condition of ice water bath and stirring, 40wt% sodium hydroxide solution was added into the acrylic acid solution, after the solution temperature returned to room temperature, 3-aminobutyric amide, methyl 6-heptenoate, and azobisdimethylvaleric acid hydrochloride were added, nitrogen was introduced for 15 min, and the reaction was carried out at 70°C for 24 h under stirring. After filtration, deionized water was used for washing, and drying was carried out at 60°C for 4 h to obtain the copolymer. In the acrylic acid solution, the mass-volume ratio of acrylic acid to deionized water was 1g:60mL; the mass of the acrylic acid solution was measured based on the mass of acrylic acid therein, the mass of 40wt% sodium hydroxide solution was measured based on the mass of sodium hydroxide therein, and the mass ratio of acrylic acid to sodium hydroxide was 9:4; the mass ratio of acrylic acid to 3-aminobutyric amide was 5:2, the mass ratio of acrylic acid to methyl 6-heptenoate was 5:3, and the mass ratio of acrylic acid to azobisdimethylvaleric acid hydrochloride was 5:0.1.

[0108] Example 5: The difference between this example and Example 2 is only in the preparation of the copolymer.

[0109] Preparation of the copolymer: acrylic acid was dispersed in deionized water, stirred for 10 min to obtain an acrylic acid solution; under the condition of ice water bath and stirring, 40 wt% sodium hydroxide solution was added into the acrylic acid solution, after the solution temperature returned to room temperature, 3-aminobutene amide, methyl 6-heptenoate, vinyl trifluoroacetate and azobisdimethylaminoformamide hydrochloride were added, nitrogen was introduced for 15 min, the reaction was carried out at 70°C and under stirring for 24 h, after filtration, deionized water was used for washing, and drying was carried out at 60°C for 4 h to obtain the copolymer. The mass / volume ratio of acrylic acid to deionized water in the acrylic acid solution was 1 g:60 mL; the mass of the acrylic acid solution was measured based on the mass of acrylic acid therein, the mass of 40 wt% sodium hydroxide solution was measured based on the mass of sodium hydroxide therein, and the mass ratio of acrylic acid to sodium hydroxide was 9:4; the mass ratio of acrylic acid to 3-aminobutene amide was 5:2, the mass ratio of acrylic acid to methyl 6-heptenoate was 5:3, the mass ratio of acrylic acid to vinyl trifluoroacetate was 5:3, and the mass ratio of acrylic acid to azobisdimethylaminoformamide hydrochloride was 5:0.1.

[0110] Comparative Example 1: Compared with Example 1, the only difference is the preparation of the biomass-based hard carbon particles.

[0111] Preparation of the pretreated bamboo fiber particles: natural bamboo fibers were placed into a tube furnace, heated to 600°C at a heating rate of 5°C / min under the protection of an inert atmosphere, and kept for 3 h, and then cooled to room temperature, and then powdered at 30,000 rpm for 6 min to obtain pretreated bamboo fiber particles. The length of the natural bamboo fibers was 2 cm; the inert atmosphere was a nitrogen atmosphere, and the nitrogen flow rate was 50 sccm.

[0112] Preparation of the biomass-based hard carbon particles: the pretreated bamboo fiber particles were placed into a tube furnace, heated to 600°C at a heating rate of 5°C / min under the protection of an inert atmosphere, and kept for 3 h, and then heated to 1300°C at a heating rate of 5°C / min, kept for 3 h, and then cooled to room temperature, and then powdered at 25,000 rpm for 6 min, and then sieved through a 20 μm sieve to obtain biomass-based hard carbon particles. The inert atmosphere was a nitrogen atmosphere, and the nitrogen flow rate was 50 sccm.

[0113] Comparative Example 2: Compared with Example 2, the only difference is that 3-aminobutene amide is not used in the preparation of the copolymer.

[0114] Comparative Example 3: Compared with Example 1, the only difference is that methyl 6-heptenoate is not used in the preparation of the copolymer.

[0115] Test Example 1: Element content test of the biomass-based hard carbon particles.

[0116] Test sample: biomass-based hard carbon particles prepared in Example 1 and Comparative Example 1.

[0117] Test method: 0.1 g of biomass-based hard carbon particles was placed in a polytetrafluoroethylene digestion tank, and high-purity HNO3 and HF were added, and the temperature was raised to 120 DEG C in 5 min, kept for 5 min, then raised to 180 DEG C in 5 min, kept for 15 min, and digested, and after digestion, the acid was chased to a volume of 1-2 mL at 150 DEG C electric hot plate, and after cooling, deionized water with a resistivity of 18.2 M Omega. cm was used to dilute to 25 mL of a polytetrafluoroethylene volumetric flask, and a reagent blank experiment was performed; 1000 ug / mL of Al, Ca, Cr, K, Na, Ni, Fe single-element standard stock solution was used to prepare a series of concentration standard solutions, and the standard curve was drawn under the parameters of radio frequency power 1200 W, atomizer flow 1.0 L / min, axial observation, and integration time 2 s using ICP-OES instrument; the Al, Ca, Cr, K, Na, Ni, Fe element concentrations in the test sample solution were determined, and the mass fraction of each element was calculated according to the formula w (%) = C x V x m -1 x 100%, wherein w is the mass fraction of the element, C is the concentration of the element in the test sample solution, V is the constant volume, and m is the mass of the biomass-based hard carbon particles.

[0118] The element content test results of the biomass-based hard carbon particles prepared by the application are shown in Table 1.

[0119] Table 1 Element content test results of biomass-based hard carbon particles

[0120]

[0121] The impurity elements of the biomass-based hard carbon particles prepared in Example 1 are less than those of Comparative Example 1, and the Al content is significantly reduced; acid leaching converts metal impurities into soluble salts, and hydrofluoric acid removes silicon-based impurities, avoiding the problems of high cost and incomplete impurity removal in existing alcohol purification; and the reduction of impurity content can improve the specific capacity and energy density of the material, reduce the occurrence of side reactions, and thus improve the overall electrochemical performance of the sodium ion battery anode material.

[0122] Test Example 2: Specific surface area test of biomass-based hard carbon particles.

[0123] Test sample: biomass-based hard carbon particles prepared in Example 1 and Comparative Example 1.

[0124] Test method: 0.1 g of biomass-based hard carbon particles was placed in the sample tube of the BET analyzer; the pretreatment temperature was set to 120 DEG C, the vacuum degree was greater than or equal to 0.09 MPa, and the degassing treatment was carried out for 4 h to remove the residual moisture and impurity gas in the sample; after the pretreatment was completed, the sample tube was placed in a liquid nitrogen environment, and the nitrogen adsorption-desorption isotherm of the sample was measured in the range of relative pressure (P / P0) 0.01~1.0; the Brunauer-Emmett-Teller model was used to analyze the adsorption isotherm data, and the specific surface area of the pretreated bamboo fiber particles and the purified carbon particles was calculated.

[0125] The specific surface area test results of the biomass-based hard carbon particles prepared in the embodiment 1 are shown in the following table 1. Figure 1 The biomass-based hard carbon particles prepared in the embodiment 1 show smaller specific surface area, which is because the purification process not only removes metal and silicon-based impurities, but also eliminates the disordered pores occupied by impurities, and optimizes the microstructure of the material; the residual impurities and silicon-based components in the comparative example 1 form more open pores, resulting in larger specific surface area; smaller specific surface area can reduce the excessive contact of hard carbon with electrolyte, reduce the side reaction in the battery, and is beneficial to improve the first circle coulomb efficiency of the sodium ion battery.

[0126] Test example 3: elongation at break test of the binder.

[0127] Test sample: the binder prepared in the embodiment 1-5 and the comparative example 2-3.

[0128] Test method: the binder prepared in the embodiment 1-5 and the comparative example 2-3 was prepared into a 3wt% aqueous solution, poured into a polytetrafluoroethylene rectangular mold, and placed at room temperature until preliminary drying, then vacuum dried at 80 DEG C for 2 h, cut into a dumbbell-shaped standard sample with a size of 50mm*4mm, and the universal material testing machine was used to test the sample at a tensile rate of 5mm / min in an environment of 25±2 DEG C, relative humidity 45±5%, the load-displacement curve was recorded in real time until the sample was broken, and the elongation at break was calculated according to the formula E(%)=L1 / L0*100%, wherein E is the elongation at break, L1 is the elongation at break, and L0 is the initial length.

[0129] The elongation at break test results of the binder prepared in the embodiment are shown in the following table 2.

[0130] Table 2: Elongation at break test results of the binder

[0131]

[0132] Example 1 uses traditional sodium alginate as the binder, which cannot be optimized in terms of flexibility through molecular structure design, so its elongation at break is the lowest among all test samples; in Example 2, a copolymer is used as the binder, the reaction monomers of the copolymer include acrylic acid, 3-aminobutyric amide and methyl 6-heptenoate, which endows the copolymer with a certain flexibility, so that the elongation at break is significantly better than that of Example 1; Example 3 increases the amount of 3-aminobutyric amide, which further enhances the extensibility of the molecular chain segment while ensuring the bonding strength, and the elongation at break is better than that of Example 2; in Example 4, the reaction monomers of the copolymer further include vinyl trifluoroacetate, which can reduce the aggregation of rigid chain segments of the copolymer, improve the movement ability of the molecular chain segment, further optimize the flexibility of the binder, and the elongation at break is higher than that of Example 3; Example 5 increases the amount of vinyl trifluoroacetate, and the degree of improvement of the flexibility of the molecular chain segment is more significant, so the elongation at break is the best among all examples; Comparative Example 2 does not add 3-aminobutyric amide, and Comparative Example 3 lacks methyl 6-heptenoate, which loses the buffering effect of the flexible ester segment provided by the monomer, and the overall rigidity of the copolymer is significantly enhanced, and the mechanical toughness is further reduced.

[0133] Test Example 4: Capacity and initial coulombic efficiency test of sodium ion battery.

[0134] Test sample: sodium ion batteries prepared in each example and comparative example.

[0135] Test method: The pole piece with a diameter of 15 mm was cut by a punching machine, the mass was weighed by a precision balance, the aluminum foil current collector with the same size was cut by a punching machine and the mass was weighed, the mass of the hard carbon active material in the negative pole piece was calculated according to the formula, m(g)=(m1-m0)×0.8, wherein m is the mass of the hard carbon active material in the negative pole piece, m1 is the mass of the negative pole piece, and m0 is the mass of the aluminum foil current collector; 1 cycle of constant current charge-discharge test was carried out under the test voltage range of 0.001-2.5V and the current density of 0.1C by using CT-4008 battery performance tester, the first cycle charge specific capacity and discharge specific capacity were recorded, and the initial coulombic efficiency was calculated according to the formula, η(%)=(C 1+ / C 1- )×100%, wherein η is the initial coulombic efficiency, C 1+ is the first cycle charge specific capacity, and C 1- is the first cycle discharge specific capacity; the reversible specific capacity was calculated according to the formula C(mAh / g)=C 1- / m.

[0136] The test results of the initial coulombic efficiency of the sodium ion battery prepared in the application are as follows: Figure 2As shown, the first coulombic efficiency of the sodium ion battery prepared from the hard carbon material of Example 1 of the present application is higher than 85%, and the first coulombic efficiency of Comparative Example 1 is 75%, so that the hard carbon negative electrode material obtained by using the technical solution in the present application has higher capacity and first efficiency.

[0137] The capacity test results of the sodium ion battery prepared in the present application are shown in Table 3.

[0138] Table 3 Capacity test results of sodium ion battery

[0139]

[0140] Example 1 uses traditional sodium alginate as the binder, and the interface bonding force of the hard carbon active material and the copper foil current collector is weak. Moreover, the sodium ion transmission channel inside the electrode cannot be optimized, and an unstable solid electrolyte interface film is easily formed during the charging and discharging process, resulting in the irreversible consumption of part of the sodium ions. Therefore, the reversible specific capacity is the lowest among all the test samples. Example 2 uses a copolymer composed of acrylic acid, 3-aminobutene amide and methyl 6-heptenoate as the binder. The amino group of 3-aminobutene amide can form a hydrogen bond with the hydroxyl group on the surface of the hard carbon, enhancing the interface bonding stability. The flexible ester group of methyl 6-heptenoate can reduce the structural damage caused by internal stress of the electrode, improving the sodium ion transmission environment. The reversible specific capacity is significantly better than that of Example 1. Example 3 increases the amount of 3-aminobutene amide based on Example 2. More amino groups can construct a more dense interface hydrogen bond network, further reducing the transmission resistance of sodium ions at the interface, while improving the stability of the solid electrolyte interface film and reducing the irreversible capacity loss. Therefore, the reversible specific capacity is better than that of Example 2. Example 4 adds vinyl trifluoroacetate as a monomer to the copolymer, which promotes sodium ion transmission. The fluorinated group of this monomer can adjust the charge distribution inside the electrode, accelerate the migration rate of sodium ions, and further optimize the density of the solid electrolyte interface film, reducing the irreversible insertion of sodium ions. Therefore, the reversible specific capacity is higher than that of Example 3. Example 5 further increases the amount of vinyl trifluoroacetate, making the optimization effect of the sodium ion transmission channel more significant, and the stability of the solid electrolyte interface film is further enhanced. Therefore, the reversible specific capacity is the best among all examples. Comparative Example 1 uses unrefined hard carbon particles that contain more metal impurities. These impurities will react with the electrolyte during the charging and discharging process, consuming a large amount of reversible sodium ions, and easily forming a thicker and unstable solid electrolyte interface film on the surface, further increasing the irreversible capacity loss. Therefore, the reversible specific capacity is the lowest among all test samples. Comparative Example 2 does not add 3-aminobutene amide to the copolymer, and the interface hydrogen bond network is missing. The hard carbon and the binder are prone to peeling, so the reversible specific capacity is lower than that of Example 2. Comparative Example 3 lacks methyl 6-heptenoate, and the rigidity of the copolymer is enhanced. The electrode is prone to cracking due to volume expansion during the charging and discharging process, which destroys the sodium ion transmission path. The reversible specific capacity is also lower than that of Example 2.

[0141] Test Example 5: Cycle performance test of sodium ion battery.

[0142] Test sample: sodium ion batteries prepared by Example 1 and Comparative Example 1.

[0143] Test method: CT-4008 battery performance tester was used to carry out 80 cycles of constant current charge-discharge test under the test voltage range of 0.001-2.5V, the current density of 0.1C, the discharge process was constant current discharge from 2.5V to 0.01V, and the charging process was constant current charging from 0.01V to 2.5V; the discharge specific capacity of each cycle was recorded, the discharge specific capacity of the 2nd cycle was taken as a reference, the capacity retention rate of the 80th cycle was calculated, and the formula was: R (%)=(C c / C dn / C d2 )×100%, wherein R c is the capacity retention rate, C dn is the discharge specific capacity of the nth cycle, and C d2 is the discharge specific capacity of the 2nd cycle.

[0144] The cycle performance test results of the sodium ion battery prepared in the application are shown in Table 4. Figure 3 The cycle performance test results of the sodium ion battery prepared in the application are shown in Table 4.

[0145] Table 4 Cycle performance test results of sodium ion battery

[0146]

[0147] The hard carbon material obtained by the technical scheme in the application in Example 1 still has a capacity retention rate of more than 93% after nearly 80 cycles, which shows that the hard carbon has good stability and can greatly improve the service life of the battery in the application of sodium-ion batteries; the copolymer formed by copolymerization of acrylic acid, 3-aminobutene amide and methyl 6-heptenoate is used as the binder in Example 2, the amino group of 3-aminobutene amide can form stable hydrogen bonds with the hydroxyl groups on the surface of the hard carbon, enhancing the interface bonding stability, the flexible ester groups of methyl 6-heptenoate can buffer the stress generated by the volume change of the electrode, reducing the damage of the electrode structure, at the same time, the copolymer can regulate the composition and compactness of the solid electrolyte interface film, reducing the capacity attenuation in the cycle process, compared with the sodium-ion battery composed of sodium alginate as the binder in Example 1, the capacity retention rate is improved; the amount of 3-aminobutene amide is increased in Example 3 based on Example 2, more amino groups can construct a denser interface hydrogen bond network, further improving the anti-cycle stability of the overall structure of the electrode, at the same time, reducing the transmission resistance and irreversible loss of sodium ions at the interface, so the capacity retention rate is slightly higher than that of Example 2; vinyl trifluoroacetate is added as a reaction monomer in the copolymer of Example 4, the fluorinated group contained therein can optimize the ion conductivity and chemical stability of the solid electrolyte interface film, reduce the side reactions of the solid electrolyte interface film in the cycle process, at the same time, the fluorinated group can adjust the charge distribution inside the electrode, reducing the kinetic resistance of sodium ion intercalation / deintercalation, so the capacity retention rate is further improved compared with Example 3; the amount of vinyl trifluoroacetate is further increased in Example 5, the stabilizing effect of the fluorinated group on the solid electrolyte interface film and the adjusting effect on the charge distribution are more significant, the electrode still maintains good structural integrity and ion transmission efficiency after 80 cycles, so the capacity retention rate is the best among all examples; the hard carbon particles used in Comparative Example 1 are not purified, the metal impurities such as Al, Na and Fe remaining in the particles will continuously react with the electrolyte in the cycle process, not only consuming the reversible sodium ions, but also damaging the stability of the solid electrolyte interface film, leading to accelerated degradation of the electrode structure, so the capacity retention rate is the lowest among all test samples; the copolymer does not contain 3-aminobutene amide in Comparative Example 2, the hydrogen bond effect of the amino group with the hydroxyl groups on the surface of the hard carbon is missing, the electrode is prone to peeling of the hard carbon particles and the binder in the cycle process, the solid electrolyte interface film is also frequently damaged due to the unstable interface, the capacity decay rate is accelerated, so the capacity retention rate is lower than that of Example 2; the copolymer in Comparative Example 3 does not contain methyl 6-heptenoate, the overall rigidity of the copolymer is significantly enhanced, which cannot buffer the volume expansion stress in the cycle process of the electrode, leading to obvious cracks in the electrode, damaging the sodium ion transmission channel, at the same time, new side reaction sites are easily generated at the cracks, accelerating the capacity decay, so the capacity retention rate is also lower than that of Example 2.

[0148] The above-described embodiments and / or implementations are merely intended to illustrate the preferred embodiments and / or implementations of the present technology, and are not intended to limit the embodiments of the present technology in any form, and any person skilled in the art can make some changes or modifications as other equivalent embodiments without departing from the scope of the technology disclosed in the present disclosure, but should be considered as the same technology or embodiments as the present disclosure.

[0149] The principles and implementations of the present application are described herein using specific examples. The above description of the embodiments is only intended to help understand the method and core idea of the present application. The above description is only the preferred embodiments of the present application. It should be noted that due to the limited nature of the language, there are objectively infinite specific structures. For those skilled in the art, without departing from the principles of the present application, some improvements, refinements or changes can be made, and the above technical features can be combined in an appropriate manner. These improvements, refinements, changes or combinations, or the direct application of the inventive concept and technical solution to other occasions without improvement, shall be considered as the protection scope of the present application.

Claims

1. A negative electrode sheet, characterized in that: The negative electrode comprises a biomass-based hard carbon negative electrode material and a binder. The negative electrode material is biomass-based hard carbon particles, and the carbon source of the biomass-based hard carbon particles is natural bamboo fiber. The specific surface area of ​​the biomass-based hard carbon particles is less than 15 m². 2 ·g -1 The biomass-based hard carbon particles have a particle size of 1-50 μm; the binder comprises a quaternary copolymer, and the reactive monomers of the quaternary copolymer include acrylic acid, 3-aminobutenamide, methyl 6-heptenoate and vinyl trifluoroacetate.

2. The negative electrode sheet according to claim 1, characterized in that: In the biomass-based hard carbon anode material, the mass fraction of aluminum is less than 0.05%, the mass fraction of calcium is less than 0.07%, the mass fraction of chromium is less than 0.001%, the mass fraction of potassium is less than 0.03%, the mass fraction of sodium is less than 0.05%, the mass fraction of nickel is less than 0.001%, and the mass fraction of iron is less than 0.09%.

3. The negative electrode sheet according to claim 1, characterized in that: The biomass-based hard carbon particles are obtained by carbonizing pretreated bamboo fiber after purification. The purification process includes adding the pretreated bamboo fiber to a 1-10 mol / L acid solution, stirring and soaking at 30-100℃ for 8-24 hours, filtering, and washing with deionized water to obtain purified bamboo fiber particles; adding the purified bamboo fiber particles to a 1-10 mol / L hydrofluoric acid solution, stirring and soaking at 30-100℃ for 8-24 hours, filtering, washing with deionized water, and vacuum drying at 60-120℃ for 12-24 hours; the solute in the acid solution includes one of sulfuric acid, hydrochloric acid, acetic acid, phosphoric acid, and nitric acid.

4. The negative electrode sheet according to claim 1, characterized in that: The mass ratio of acrylic acid to 3-aminobutenamide is 5:1-5, the mass ratio of acrylic acid to methyl 6-heptenoate is 5:2-4, and the mass ratio of acrylic acid to vinyl trifluoroacetate is 5:1-4.

5. A sodium-ion battery, comprising the negative electrode sheet as described in any one of claims 1-4.

6. A sodium-ion battery according to claim 5, characterized in that: The sodium-ion battery retains more than 93% of its capacity after 80 cycles at a current density of 0.1C.

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