Bismuth-based alloy composite electrode material based on biomass char and preparation method thereof

Abstract

The application relates to the technical field of electrode materials, in particular to a bismuth-based alloy composite electrode material based on biomass charcoal and a preparation method thereof, which comprises the following steps: after pretreatment of waste biomass materials, KOH is used for activation and pore making, and porous biomass charcoal is obtained after acid pickling and drying; the porous biomass charcoal, basic bismuth carbonate, tin oxalate, copper oxalate and magnesium powder are mixed to obtain mixed powder, anhydrous ethanol is added to the mixed powder for grinding to a paste, and a precursor mixture is obtained after vacuum drying; the precursor mixture is subjected to a magnesium thermal reduction reaction in an argon atmosphere, the reaction product is cooled to room temperature, and acid pickling and drying are carried out to obtain the bismuth-based alloy composite electrode material based on biomass charcoal. The application solves the problems of large volume expansion and poor conductive stability of the existing bismuth-based alloy electrode, and realizes resource utilization of waste biomass.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of electrode materials, in particular to a bismuth-based alloy composite electrode material based on biomass charcoal and a preparation method thereof. BACKGROUND

[0002] In the field of energy storage devices such as lithium ion batteries and sodium ion batteries, the performance of electrode materials directly determines the energy density, cycle life and rate performance of the batteries. Among the current mainstream negative electrode materials, graphite materials have a low theoretical capacity (372 mAh / g) and are difficult to meet the demand for high energy density; although silicon-based and tin-based materials have high theoretical capacity, the volume expansion rate during the charging and discharging process is extremely high (up to 400% for silicon-based materials and up to 260% for tin-based materials), which easily leads to the collapse of the electrode structure and poor cycle stability.

[0003] Bismuth-based materials have become a potential negative electrode candidate material because of the high theoretical lithium storage capacity (386 mAh / g) and the lower volume expansion rate (about 130%) than silicon-based / tin-based materials, but there are still some problems: (1) the volume change of pure bismuth particles during charging and discharging will still cause repeated rupture-repair of the SEI film, resulting in loss of active substances and increase of impedance; (2) the electronic conductivity of bismuth (8.6 S / m) is low, and particle agglomeration easily occurs in the traditional preparation process, resulting in poor ion / electron transmission efficiency at high rates. In addition, the annual output of agricultural and domestic waste exceeds 1 billion tons, most of which is incinerated or landfilled, causing resource waste and environmental pollution. How to convert waste biomass into high-value electrode materials and solve the performance problems of bismuth-based materials has become an important research direction in the field of electrode materials. SUMMARY

[0004] The application aims to provide a bismuth-based alloy composite electrode material based on biomass charcoal and a preparation method thereof, which solves the problems of large volume expansion and poor conductivity stability of existing bismuth-based alloy electrodes and realizes the resource utilization of waste biomass.

[0005] To achieve the above-mentioned purpose, the application provides a preparation method of a bismuth-based alloy composite electrode material based on biomass charcoal, which comprises the following steps:

[0006] S1, after pretreating the waste biomass material, using KOH to activate and form pores, and after acid washing and drying, obtaining a porous biomass charcoal;

[0007] S2, mixing the porous biomass charcoal of S1, bismuth subcarbonate, tin oxalate, copper oxalate and magnesium powder to obtain a mixed powder, adding anhydrous ethanol to the mixed powder and grinding to a paste, and after vacuum drying, obtaining a precursor mixture;

[0008] S3, the precursor mixture of S2 is subjected to magnesium reduction reaction under argon atmosphere, and the reaction product obtained after being cooled to room temperature is subjected to acid washing and drying to obtain a biomass charcoal-based bismuth-based alloy composite electrode material.

[0009] Preferably, in S1, the waste biomass material comprises one or more of rice husk, wheat straw, corn straw, kapok, willow catkin, dandelion fluff, bagasse, coffee grounds.

[0010] Preferably, in S1, the pretreatment is to wash the waste biomass material and then vacuum dry at 70-90℃ for 8-16h, crush through a 100 mesh sieve to obtain biomass powder; then acetone is added to the biomass powder for washing to remove soluble impurities, and vacuum drying at 60℃ for 8h.

[0011] More preferably, the acetone washing is to add acetone to the biomass powder at a solid-liquid ratio (1-2): 10g / mL, ultrasonic for 10-15min, then magnetic stirring for 25-35min, and finally centrifugation at a speed of 3000rpm for 5min, repeated for 3 times.

[0012] Preferably, in S1, the KOH activation for pore making is to mix and grind the pretreated biomass material with KOH uniformly, then heat to 400-450℃ at a rate of 3-5℃ / min under argon atmosphere, keep for 2h, then heat to 800-850℃ at a rate of 3-5℃ / min, keep for 2h, and naturally cool to room temperature; the mass ratio of the pretreated biomass material to KOH is 1:1-3.

[0013] Preferably, in S2, the mass ratio of the porous biomass charcoal, bismuth subcarbonate, tin oxalate, copper oxalate and magnesium powder is 1:(0.9-1.1):(0.5-0.6):(0.05-0.08):(0.5-0.6).

[0014] Preferably, in S2, the solid-liquid ratio of the mixed powder to anhydrous ethanol is 1:(6-8)g / mL.

[0015] Preferably, in S2, the temperature for vacuum drying is 50-60℃, and the drying time is 6-8h.

[0016] Preferably, in S3, the magnesium reduction reaction condition is to heat to 480-520℃ at a rate of 4-6℃ / min, keep for 2h.

[0017] Preferably, in S1 and S3, the acid washing and drying are both to add 1mol / L hydrochloric acid, stir at room temperature for 4h, wash with ultrapure water until neutral, and finally freeze-dry at -50℃ for 12h.

[0018] The present invention also provides a bismuth-based alloy composite electrode material based on biomass char, which is prepared by the above-mentioned method for preparing a bismuth-based alloy composite electrode material based on biomass char.

[0019] Therefore, the present invention employs the above-mentioned bismuth-based alloy composite electrode material based on biomass carbon and its preparation method, which has the following beneficial effects:

[0020] (1) In this invention, waste biomass materials are activated with KOH to form a multi-level structure, which not only provides nanoscale confinement space for alloy particles and inhibits their agglomeration, but also acts as an expansion buffer layer to absorb volume changes. At the same time, the three-dimensional carbon skeleton improves the conductivity of the material, and the Bi in the nanoscale alloy particles... 57 Sn 43 The volume expansion rate of the eutectic phase is significantly lower than that of pure Bi. The Cu6Sn5 phase not only further inhibits expansion but also catalyzes the uniform formation of the SEI film, reducing irreversible lithium consumption. The nanoscale alloy particles shorten the Li... + Diffusion path, improving high-rate performance.

[0021] (2) This invention realizes the resource utilization of waste biomass materials, reduces incineration pollution, has no toxic reagent emissions during the preparation process, and has low preparation cost. By adjusting the ratio of waste biomass materials to bismuth-based alloy components, it can be adapted to different energy storage scenarios such as lithium-ion batteries and sodium-ion batteries. The process is compatible with existing electrode production lines and is easy to scale up for mass production.

[0022] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0023] Figure 1 This is a SEM image of the porous biochar and the bismuth-based alloy composite electrode material based on biochar from Embodiment 1 of the present invention. Figure 1 In the image, 'a' is a SEM image of the porous biochar from Example 1. Figure 1 b in the figure is the SEM image of the bismuth-based alloy composite electrode material based on biochar in Example 1. Detailed Implementation

[0024] The present invention will be further described below with reference to the accompanying drawings and embodiments. Unless otherwise defined, the technical or scientific terms used in this invention should be understood in their ordinary sense by those skilled in the art. The features mentioned above or in the specific examples mentioned in this invention can be combined arbitrarily, and these specific embodiments are only used to illustrate the invention and are not intended to limit the scope of the invention.

[0025] This invention provides a method for preparing a bismuth-based alloy composite electrode material based on biochar, comprising the following steps:

[0026] S1, after the waste biomass material is pretreated, KOH is used for activating pore, and after acid washing and drying, the porous biomass charcoal is obtained; the porous biomass charcoal prepared from the waste biomass material has the advantages of low cost, high specific surface area and porous structure, and can provide volume expansion buffer space and three-dimensional conductive network.

[0027] S2, the porous biomass charcoal of S1, bismuth subcarbonate, tin oxalate, copper oxalate and magnesium powder are mixed to obtain mixed powder, anhydrous ethanol is added to the mixed powder and grinded to paste, and the precursor mixture is obtained after vacuum drying;

[0028] S3, the precursor mixture of S2 is subjected to magnesium thermal reduction reaction under argon atmosphere, and after the reaction product is cooled to room temperature, acid washing and drying are carried out to obtain the bismuth-based alloy composite electrode material based on biomass charcoal.

[0029] In the application, bismuth subcarbonate ((BiO)2CO3), tin oxalate (C2O4Sn) and copper oxalate (CuC2O4) are pyrolyzed to generate Bi2O3, SnO2 and CuO, which are reduced to metal elements with Mg, and the reduced Bi and Sn elements are eutectic at 500 DEG C to form Bi 57 Sn 43 eutectic phase, and the Sn in the Cu and Bi 57 Sn 43 eutectic phase is subjected to intermetallic compound reaction to generate Cu6Sn5 phase in situ, to obtain BiSn / Cu6Sn5 alloy, the alloy particles are limited in the pore of the biomass charcoal, the particle size is controlled to be 5-20nm, the ion diffusion path can be shortened, the rate performance can be improved, and the carbon oxides (CO2 and CO) released in the pyrolysis process can react with magnesium powder to generate a graphene-like carbon layer, which can be directly coated on the surface of the BiSn / Cu6Sn5 alloy to form a dense coating layer, which can effectively block the electrolyte erosion and inhibit the alloy particle agglomeration.

[0030] Preferably, in S1, the waste biomass material includes one or more of rice husk, wheat straw, corn straw, kapok, willow catkin, dandelion fluff, sugarcane residue and coffee residue.

[0031] Preferably, the pretreatment in S1 is to wash the waste biomass material, then vacuum dry at 70-90℃ for 8-16h, crush through a 100 mesh sieve to obtain biomass powder; then add acetone to the biomass powder to remove soluble impurities, and vacuum dry at 60℃ for 8h. The vacuum drying in S1 of the present application avoids the shrinkage of biomass fibers caused by high temperature drying, and the temperature range of 70-90℃ can efficiently remove free water while retaining bound water to maintain fiber elasticity. Sieving through a 100 mesh sieve controls the uniformity of the particle size of the biomass powder, ensuring sufficient solid-solid contact when mixed with KOH later, and avoiding uneven local activation.

[0032] More preferably, the acetone washing is to add acetone to the biomass powder at a solid-liquid ratio of (1-2):10g / mL, ultrasonic for 10-15min, then magnetic stirring for 25-35min, and finally centrifugation at 3000rpm for 5min, repeating the washing 3 times. In the present application, acetone as a polar organic solvent can dissolve the soluble impurities in the biomass, ultrasonic agitation can strengthen the desorption of impurities, and centrifugal separation can avoid the residue of impurities; repeating 3 times can improve the impurity removal rate to more than 95%.

[0033] Preferably, the KOH activation and pore formation in S1 is to mix and grind the pretreated biomass material with KOH uniformly, then heat to 400-450℃ at 3-5℃ / min under argon atmosphere, keep for 2h, then heat to 800-850℃ at 3-5℃ / min, keep for 2h, and cool to room temperature naturally; the mass ratio of the pretreated biomass material to KOH is 1:1-3.

[0034] In the present application, the KOH activation and pore formation is in the temperature range of 400-450℃, KOH reacts with hydroxyl (-OH) and carboxyl (-COOH) in the biomass to generate intermediate products such as K2CO3 and K2O, and at the same time, cellulose begins to pyrolyze to form a preliminary carbon skeleton, and this stage is mainly for constructing micropores. In the range of 800-850℃, K2CO3 / K2O further reacts with the carbon skeleton to generate gases such as CO and CO2, which etch the carbon skeleton during the escape process, forming mesopores-macropores (2-50nm), and at the same time, K vapor can insert between the carbon layers to improve the degree of graphitization.

[0035] Preferably, in S2, the mass ratio of the porous biomass carbon, bismuth subcarbonate, tin oxalate, copper oxalate and magnesium powder is 1:(0.9-1.1):(0.5-0.6):(0.05-0.08):(0.5-0.6).

[0036] In the present application, when bismuth subcarbonate, tin oxalate and copper oxalate are mixed with porous biomass carbon, their organic functional groups (such as -CO3 2- , -C2O4 2-It can form weak interactions (such as hydrogen bonds, coordination bonds) with the C-O, hydroxyl and other functional groups on the surface of the biomass charcoal, so that the metal ions are uniformly dispersed in the pores of the porous biomass charcoal; during subsequent magnesium reduction, metal oxides (Bi2O3, SnO2, CuO) are generated in situ and are immediately reduced, avoiding the settlement and agglomeration caused by the density difference when traditional Bi2O3 and other inorganic oxides are directly mixed, and ensuring uniform dispersion of BiSn / Cu6Sn5 alloy particles.

[0037] Preferably, in S2, the solid-liquid ratio of the mixed powder to anhydrous ethanol is 1: (6-8) g / mL.

[0038] Preferably, in S2, the temperature of vacuum drying is 50-60℃, and the drying time is 6-8h.

[0039] Preferably, in S3, the magnesium reduction reaction conditions are: heating at a rate of 4-6℃ / min to 480-520℃, and holding for 2h. In the present application, slow heating can avoid local overheating leading to alloy particle agglomeration, and at the same time ensure uniform reduction reaction, avoiding Mg residue caused by excessive local magnesium powder.

[0040] Preferably, in S1 and S3, the acid pickling and drying are both adding 1 mol / L hydrochloric acid, stirring at room temperature for 4h, washing with ultrapure water until neutral, and finally freeze-drying at-50℃ for 12h.

[0041] The present application also provides a kind of above-mentioned biomass charcoal-based bismuth-based alloy composite electrode material, which is prepared by the preparation method of the biomass charcoal-based bismuth-based alloy composite electrode material.

[0042] In the bismuth-based alloy composite electrode material of the present application, Bi provides high capacity, Sn suppresses expansion, Cu regulates SEI film, and biomass charcoal enhances conductivity and support, forming a complementary system of capacity-stability-conductivity, the hierarchical pores of biomass charcoal provide nano-confined space for alloy particles to suppress volume expansion, and build a three-dimensional conductive network to enhance electron transport, and nano-alloy particles maximize the number of active sites, balancing high capacity and high rate.

[0043] Example 1

[0044] The present application provides a biomass charcoal-based bismuth-based alloy composite electrode material, which is prepared by the following method, comprising:

[0045] S1, the waste biomass material (rice husk, wheat straw, corn straw mixed according to the mass ratio 1:1:1) is washed and dried at 70 DEG C under vacuum for 8h, crushed through 100 mesh sieve, and then acetone is added to the biomass powder according to the solid-liquid ratio 1:10 g / mL, ultrasonic treatment is carried out for 10 min, magnetic stirring is carried out for 25 min, finally centrifugation is carried out at 3000 rpm for 5 min, and the washing is repeated for 3 times to remove soluble impurities, and finally vacuum drying is carried out at 60 DEG C for 8h.

[0046] The pretreated biomass material is mixed with KOH according to the mass ratio 1:1, and then grinded uniformly, and then heated to 400 DEG C at 3 DEG C / min under argon atmosphere, and then heated to 800 DEG C at 3 DEG C / min, and then naturally cooled to room temperature, and then hydrochloric acid with a concentration of 1 mol / L and a solid-liquid ratio of 1:10 g / mL is added, and stirred at room temperature for 4h, and then washed with ultrapure water until neutral, and finally freeze-dried at-50 DEG C for 12h to obtain the porous biomass charcoal.

[0047] S2, the porous biomass charcoal of S1, bismuth subcarbonate, tin oxalate, copper oxalate and magnesium powder are mixed according to the mass ratio 1:0.9:0.5:0.05:0.5 to obtain a mixed powder, anhydrous ethanol is added to the mixed powder according to the solid-liquid ratio 1:6 g / mL, and grinded into a paste, and then vacuum drying is carried out at 50 DEG C for 6-8h to obtain a precursor mixture.

[0048] S3, the precursor mixture of S2 is subjected to magnesium thermal reduction reaction under argon atmosphere, and the magnesium thermal reduction reaction conditions are as follows: heating to 480 DEG C at a rate of 4 DEG C / min, and then keeping for 2h, and then the obtained reaction product is cooled to room temperature, and then hydrochloric acid with a concentration of 1 mol / L and a solid-liquid ratio of 1:10 g / mL is added, and stirred at room temperature for 4h, and then washed with ultrapure water until neutral, and finally freeze-dried at-50 DEG C for 12h to obtain the bismuth-based alloy composite electrode material based on biomass charcoal.

[0049] Example 2

[0050] The application provides a bismuth-based alloy composite electrode material based on biomass charcoal, which is prepared by the following method, comprising the following steps:

[0051] S1, the waste biomass material (cotton, willow catkin, dandelion fluff mixed according to the mass ratio 1:1:1) is washed and dried at 80 DEG C under vacuum for 12h, crushed through 100 mesh sieve, and then acetone is added to the biomass powder according to the solid-liquid ratio 2:10 g / mL, ultrasonic treatment is carried out for 12 min, magnetic stirring is carried out for 30 min, finally centrifugation is carried out at 3000 rpm for 5 min, and the washing is repeated for 3 times to remove soluble impurities, and finally vacuum drying is carried out at 60 DEG C for 8h.

[0052] The pretreated biomass material is mixed with KOH at a mass ratio of 1:2, and then uniformly ground, and then heated to 420 DEG C at a rate of 4 DEG C / min under an argon atmosphere, and then kept for 2 hours, and then heated to 820 DEG C at a rate of 4 DEG C / min, and then kept for 2 hours, and then naturally cooled to room temperature, and then hydrochloric acid with a concentration of 1 mol / L and a solid-liquid ratio of 1:10 g / mL is added, and then stirred at room temperature for 4 hours, and then washed with ultrapure water until neutral, and finally freeze-dried at-50 DEG C for 12 hours to obtain the porous biomass charcoal.

[0053] S2, the porous biomass charcoal, bismuth subcarbonate, tin oxalate, copper oxalate and magnesium powder in S1 are mixed at a mass ratio of 1:1:0.55:0.06:0.55 to obtain a mixed powder, and then anhydrous ethanol is added to the mixed powder at a solid-liquid ratio of 1:7 g / mL to grind into a paste, and then a precursor mixture is obtained after vacuum drying at 50 DEG C for 7 hours.

[0054] S3, the precursor mixture in S2 is subjected to a magnesium thermal reduction reaction under an argon atmosphere, and the magnesium thermal reduction reaction conditions are as follows: heated to 500 DEG C at a rate of 5 DEG C / min, and then kept for 2 hours, and then the obtained reaction product is cooled to room temperature, and then hydrochloric acid with a concentration of 1 mol / L and a solid-liquid ratio of 1:10 g / mL is added, and then stirred at room temperature for 4 hours, and then washed with ultrapure water until neutral, and finally freeze-dried at-50 DEG C for 12 hours to obtain the bismuth-based alloy composite electrode material based on biomass charcoal.

[0055] Example 3

[0056] The application provides a bismuth-based alloy composite electrode material based on biomass charcoal, which is prepared by the following method, comprising the following steps:

[0057] S1, the waste biomass material (sugarcane residue and coffee residue mixed at a mass ratio of 1:1) is washed and then vacuum dried at 90 DEG C for 16 hours, and then crushed through a 100-mesh sieve to obtain biomass powder; then acetone is added to the biomass powder at a solid-liquid ratio of 2:10 g / mL, and then ultrasonic treatment is performed for 15 minutes, and then magnetic stirring is performed for 35 minutes, and finally centrifugation is performed at a speed of 3000 rpm for 5 minutes, and the washing is repeated for 3 times to remove soluble impurities, and then vacuum drying is performed at 60 DEG C for 8 hours.

[0058] The pretreated biomass material is mixed with KOH at a mass ratio of 1:3, and then uniformly ground, and then heated to 450 DEG C at a rate of 5 DEG C / min under an argon atmosphere, and then kept for 2 hours, and then heated to 850 DEG C at a rate of 5 DEG C / min, and then kept for 2 hours, and then naturally cooled to room temperature; and then hydrochloric acid with a concentration of 1 mol / L and a solid-liquid ratio of 1:10 g / mL is added, and then stirred at room temperature for 4 hours, and then washed with ultrapure water until neutral, and finally freeze-dried at-50 DEG C for 12 hours to obtain the porous biomass charcoal.

[0059] S2, the porous biomass charcoal, bismuth subcarbonate, tin oxalate, copper oxalate and magnesium powder of S1 were mixed in a mass ratio of 1:1.1:0.6:0.08:0.6 to obtain a mixed powder, anhydrous ethanol was added to the mixed powder in a solid-liquid ratio of 1:8 g / mL to grind into a paste, and the precursor mixture was obtained after vacuum drying at 50°C for 6-8h.

[0060] S3, the precursor mixture of S2 was subjected to a magnesium thermal reduction reaction under an argon atmosphere, the magnesium thermal reduction reaction conditions were: heating to 520°C at a rate of 6°C / min, holding for 2h, the obtained reaction product was cooled to room temperature, hydrochloric acid with a concentration of 1 mol / L was added in a solid-liquid ratio of 1:10 g / mL, stirring at room temperature for 6h, then washing with ultrapure water until neutral, and finally freeze-drying at-50°C for 12h to obtain a bismuth-based alloy composite electrode material based on biomass charcoal.

[0061] Comparative Example 1

[0062] Comparative Example 1

[0063] Comparative Example 2

[0064] Comparative Example 2

[0065] Comparative Example 3

[0066] Comparative Example 3

[0067] Performance Test

[0068] The electrode material prepared in Example 1 was detected by an electron scanning microscope (SEM), and the results are shown in Figure 1 The biomass charcoal after KOH activation and pore formation showed a rich porous structure, the alloy particles grew in situ in the pores of the porous biomass charcoal after the porous biomass charcoal was compounded with the bismuth-based alloy, which provided nano-confined space for the alloy particles, inhibited volume expansion, avoided electrode structure rupture, and the continuous carbon skeleton connected the dispersed alloy particles into a conductive whole, reducing the electron transmission resistance.

[0069] The average particle size of the electrode materials prepared in Example 2 and Comparative Examples 1-3 was determined by transmission electron microscopy (TEM) by counting 50 alloy particles, and the specific surface area (BET) of the electrode materials was determined by liquid nitrogen adsorption method, and the results are shown in Table 1.

[0070] The electrode materials prepared in Example 2 and Comparative Examples 1-3 were mixed with carbon black and sodium alginate at a mass ratio of 8:1:1, and then added with ultrapure water and ground for 30 minutes to prepare a slurry; the slurry was coated on a foam nickel disc with a diameter of 16 mm and a thickness of 2 mm, vacuum dried at 80°C for 12 hours, and then pressed into a circular electrode tablet. The above tablet was used as a working electrode, and a lithium metal sheet (counter electrode), a Celgard 2400 separator, and a 1 mol / L LiPF6 electrolyte (EC: DMC = 1:1 by volume) were assembled into a CR2032 button cell, and then the button cell was subjected to charge-discharge test at a rate of 1C and a voltage of 1.0 V. The volume expansion rate was calculated by the volume change of the powder tablet before and after charge-discharge, and the volume expansion rate (%) = [(volume after discharge - volume before discharge) / volume before discharge] x 100%, and the results are shown in Table 1. Then, the electrochemical performance at different rates was tested in the voltage range of 0.01-3.0 V, and the results are shown in Table 2.

[0071] Table 1 Physical properties of different electrode materials

[0072]

[0073] As shown in Table 1, the alloy particle size of Example 2 is the smallest, the porous biomass carbon has abundant multi-level pores (micropores-mesopores-macropores), and in the S3 magnesium thermal reduction process, the Bi-Sn-Cu alloy particles are limited to grow in the pores, avoiding agglomeration; and the weak interaction between the hydroxyl groups and C-O functional groups on the surface of the porous biomass carbon and metal ions further promotes the dispersion of the particles. The particle size of Comparative Example 1 is the largest, there is no confined space of carbon material, and the alloy particles are prone to melt and agglomerate during magnesium thermal reduction, forming micron-sized large particles, which leads to a prolonged ion diffusion path. The particle size of Comparative Example 2 is larger than that of Example 2, the commercial activated carbon is mainly microporous, and the pore volume is small, and the confining ability of the alloy particles is weaker than the multi-level pore structure of the self-made biomass carbon, and part of the particles grow and agglomerate outside the pores.

[0074] The specific surface area of Example 2 is the highest, which is due to the multi-level pore structure constructed by KOH activation, which not only provides a high specific surface area, but also provides sufficient space for electrolyte infiltration and Li + storage. The specific surface area of Comparative Example 1 is extremely low, which is only contributed by the gap between the alloy particles, and there is no support of the porous structure of the carbon material, resulting in a significant reduction in the number of active sites. The specific surface area of Comparative Example 2 is lower than that of Example 2, although the commercial activated carbon has a certain specific surface area, it lacks the mesoporous-macroporous structure of the self-made biomass carbon, and the surface functional groups are less, and the binding ability with the alloy particles is weak, and part of the pores are blocked by the agglomerated particles.

[0075] Example 2 has the lowest expansion rate, and the porous biochar can absorb the volume expansion of the alloy during charge and discharge, while the three-dimensional carbon skeleton limits the excessive expansion of the particles, avoiding the rupture of the electrode structure. Comparative Example 1 has the highest expansion rate, and without the buffering of the carbon material, the alloy particles are squeezed when they expand, leading to the cracking of the electrode tablet and the rapid increase in volume. Comparative Example 2 has a higher expansion rate than Example 2, and the microporous structure of the commercial activated carbon has limited buffering capacity, and the interface bonding force with the alloy particles is weak, which is prone to interface separation during the expansion process, aggravating the volume change.

[0076] Table 2 Cycle performance of different electrode materials at 1C rate

[0077]

[0078] As can be seen from Table 2, the initial specific discharge capacity and the initial coulombic efficiency of Example 2 are better than those of Comparative Examples 1-3, because the high specific surface area of the electrode material of Example 2 provides sufficient active sites, making the lithium storage capacity of Bi close to the theoretical value; at the same time, Cu catalyzing the decomposition of the electrolyte to form a dense film layer, promotes the uniform generation of the SEI film, and reduces the irreversible lithium consumption in the first cycle. While the large particles of alloy in Comparative Example 1 lead to insufficient exposure of active sites, and the carbon material has no support for the conductive network, the electronic transmission resistance is large; the SEI film is unevenly distributed due to particle agglomeration, and the irreversible capacity increases. The inert surface of the commercial activated carbon in Comparative Example 2 makes it unable to regulate the formation of the SEI film like the porous biochar in Example 2 through functional groups, and the stability of the film layer is poor, resulting in an increase in irreversible lithium loss.

[0079] The capacity retention rate of Example 2 after 100 cycles is also better than that of Comparative Examples 1-3, and the hierarchical porous structure of the porous biochar in the electrode material of Example 2 buffers the expansion, the Sn / Cu alloying inhibits the lattice deformation of Bi and the carbon layer generated by pyrolysis blocks the electrolyte corrosion, which cooperatively maintains the stability of the electrode structure. The absence of carbon support in Comparative Example 1 leads to the shedding of alloy particles after charge and discharge, and the collapse of the electrode structure, resulting in rapid capacity decay. Comparative Example 3 lacks Sn / Cu alloying, and the pure Bi particles have a high expansion rate, and the SEI film is repeatedly broken and repaired after multiple cycles, leading to the loss of active material and the increase in impedance.

[0080] Table 3 Discharge capacity of different electrode materials at different rates

[0081]

[0082] From Table 3, the discharge capacity of Example 2 at each rate is better than Comparative Examples 1-3. The multi-level pore structure of the porous biomass charcoal in Example 2 supports and cooperates with the nano BiSn / Cu6Sn5 alloy to build an efficient ion and electron transmission network, realizing excellent capacity retention from low rate to high rate. It is proved that the nano alloy and multi-level pore cooperate to improve the transmission efficiency of ions and electrons. Comparative Example 1 has no nano confinement effect of carbon material, and only relies on the contact of alloy particles to form a conductive path. During the charging and discharging process, the volume expansion of the alloy leads to the separation of the particles, and the electron transmission resistance increases sharply. The capacity at a rate of more than 2C is greatly attenuated. The commercial activated carbon used in Comparative Example 2 is mainly microporous, and the content of functional groups (hydroxyl, carboxyl) on the surface of the commercial activated carbon is low. The interaction with the BiSn / Cu6Sn5 alloy is weak, and the alloy particles are easy to fall off from the surface of the carbon during the cycle process, leading to the loss of active sites. Comparative Example 3 only forms pure Bi particles without Cu6Sn5 phase to catalyze the formation of SEI film. The SEI film is easy to break and reconfigure at high rate, consuming a large amount of Li + and electrolyte, leading to severe capacity attenuation.

[0083] Therefore, the present application adopts the above-mentioned one kind of biomass charcoal-based bismuth-based alloy composite electrode material and its preparation method, using waste biomass as raw material, preparing multi-level pore biomass charcoal by KOH activation, and then generating BiSn / Cu6Sn5 alloy in situ by magnesium hot reduction, forming a synergistic system of carbon support and alloying, which not only solves the pain points of large volume expansion and poor conductivity of bismuth-based materials, but also realizes the high-value resourceization of agricultural waste, and has the advantages of low cost, green environmental protection and easy mass production.

[0084] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application but not to limit it. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or equivalently replace the technical solutions of the present application, and these modifications or equivalent replacements cannot make the modified technical solutions deviate from the spirit and scope of the technical solutions of the present application.

Claims

1. A method for preparing a biomass-char-based bismuth-based alloy composite electrode material, characterized by: The method comprises the following steps: S1, after pretreatment of the waste biomass material, KOH is used to activate pore formation, and after acid washing and drying, a porous biomass charcoal with a multi-level pore structure is obtained; The waste biomass material comprises one or more of rice husk, wheat straw, corn straw, kapok, willow catkin, dandelion fluff, sugarcane residue, and coffee residue; The KOH activation pore formation is as follows: the pretreated biomass material is uniformly mixed and ground with KOH, then heated to 400-450 DEG C at a rate of 3-5 DEG C / min under an argon atmosphere, and then heated to 800-850 DEG C at a rate of 3-5 DEG C / min for 2h, and then naturally cooled to room temperature; the mass ratio of the pretreated biomass material to KOH is 1:1-3; S2, the porous biomass charcoal, bismuth subcarbonate, tin oxalate, copper oxalate and magnesium powder of S1 are mixed to obtain a mixed powder, anhydrous ethanol is added to the mixed powder and ground into a paste, and then vacuum dried to obtain a precursor mixture; the mass ratio of the porous biomass charcoal, bismuth subcarbonate, tin oxalate, copper oxalate and magnesium powder is 1:(0.9-1.1):(0.5-0.6):(0.05-0.08):(0.5-0.6); S3, the precursor mixture of S2 is subjected to a magnesium reduction reaction under an argon atmosphere, and the reaction product obtained after cooling to room temperature is subjected to acid washing and drying to obtain a bismuth-based alloy composite electrode material based on biomass charcoal.

2. The method of claim 1, wherein the biomass-based charcoal is prepared by the following steps: (1) collecting biomass; (2) drying the biomass; (3) carbonizing the biomass; (4) grinding the carbonized biomass; and (5) activating the carbonized biomass. In S1, the waste biomass material is washed and then vacuum dried at 70-90 DEG C for 8-16h, crushed through a 100 mesh sieve to obtain a biomass powder; then acetone is added to the biomass powder for washing to remove soluble impurities, and vacuum dried at 60 DEG C for 8h.

3. The method for preparing a bismuth-based alloy composite electrode material based on biochar according to claim 1, characterized in that: In S2, the solid-liquid ratio of the mixed powder to anhydrous ethanol is 1:(6-8)g / mL.

4. The method for preparing a bismuth-based alloy composite electrode material based on biomass char according to claim 1, characterized in that: In S2, the temperature for vacuum drying is 50-60 DEG C, and the drying time is 6-8h.

5. The method for preparing a bismuth-based alloy composite electrode material based on biomass char according to claim 1, characterized in that: In S3, the magnesium reduction reaction conditions are as follows: heating to 480-520 DEG C at a rate of 4-6 DEG C / min, and then holding for 2h.

6. The method for preparing a bismuth-based alloy composite electrode material based on biochar according to claim 1, characterized in that: In S1 and S3, the acid washing and drying are both as follows: adding 1mol / L hydrochloric acid, stirring at room temperature for 4-6h, then washing with ultrapure water until neutral, and finally freeze-drying at-50 DEG C for 12h.

7. A biomass-char-based bismuth-based alloy composite electrode material, characterized by: A bismuth-based alloy composite electrode material based on biomass charcoal is prepared by the method of any one of claims 1-6.

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