CN114735672B - Boron-nitrogen co-doped hard carbon material and preparation method thereof - Google Patents
Boron-nitrogen co-doped hard carbon material and preparation method thereof Download PDFInfo
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Abstract
In order to solve the technical problems that in the prior art, a hard carbon material has too rich pore structure and larger crystal layer spacing, is unfavorable for the storage of lithium/sodium ions, and the capacity of a lithium/sodium ion battery platform prepared by taking the hard carbon material as a negative electrode material is low, the application provides a boron-nitrogen co-doped hard carbon material and a preparation method thereof, wherein the preparation method comprises the following steps: obtaining trisodium citrate, urotropine and a boron-containing compound, uniformly mixing, and performing carbonization reaction in a protective atmosphere to obtain a composite hard carbon material; and taking out the obtained composite hard carbon material, respectively cleaning by using hydrochloric acid and ethanol, and drying after cleaning to obtain the internal loose porous boron-nitrogen co-doped hard carbon material.
Description
Technical Field
The application relates to the technical field of hard carbon material preparation, in particular to a boron-nitrogen co-doped hard carbon material and a preparation method thereof.
Background
Graphite negative electrode materials have reached a limit in specific capacity, cannot meet the continuous high-current discharge capability required for large-scale power batteries, and so on, and therefore people have also begun to direct their eyes toward non-graphite materials, such as hard carbon and other non-carbon materials. Hard carbon materials, which are amorphous carbon, generally refer to carbon that is difficult to completely graphitize at temperatures above 2800 ℃ and whose disordered structure is difficult to eliminate at high temperatures. The current hard carbon preparation technology is mainly realized by medium-temperature pyrolysis and high-temperature pyrolysis of hard carbon precursors (comprising macromolecular lipids, hydrocarbon, biomass materials and the like) with rich oxygen-containing functional groups in an inert gas atmosphere. Compared with graphite cathode materials, the interlayer spacing of the hard carbon is larger, which is more beneficial to the intercalation and deintercalation of metal ions. Unlike graphite anode materials, the storage mechanism of metals in hard carbon is mainly adsorption-desorption in the carbon layer, intercalation-deintercalation between graphite crystallites, and filling of pores formed by interdigitation between graphite crystallites. Therefore, as a negative electrode material, the surface morphology, the structural composition and the defect degree of the hard carbon material have great influence on the performance of the energy storage device.
The hard carbon material prepared by the existing hard carbon material preparation technology is too rich in pore structure, larger in crystal layer spacing, favorable for diffusion of metal ions, but unfavorable for storage of lithium/sodium ions, so that when the existing hard carbon material is applied to a lithium/sodium ion battery anode material, the discharge capacity of the battery is low.
Disclosure of Invention
Aiming at the technical problems that the existing hard carbon material is too rich in pore structure, large in crystal layer spacing and unfavorable for the storage of lithium/sodium ions, and the discharge capacity of a battery prepared by taking the hard carbon material as a negative electrode material is low, the application provides the boron-nitrogen co-doped hard carbon material and the preparation method thereof.
In order to solve the technical problems, in one aspect, the application provides a preparation method of a boron-nitrogen co-doped hard carbon material, which comprises the following steps:
1) Obtaining trisodium citrate, urotropine and a boron-containing compound, uniformly mixing, and performing carbonization reaction in a reducing atmosphere to obtain a composite hard carbon material;
2) And (3) taking out the composite hard carbon material obtained in the step (1), cleaning the composite hard carbon material by using acid liquor, and eluting sodium borate byproducts in the composite hard carbon material to obtain the boron-nitrogen co-doped hard carbon material with loose and porous inside.
Preferably, the mass ratio of the trisodium citrate to the urotropine to the boron-containing compound is (0.5-1.5): 1: (0.1-1.5).
Preferably, the mass ratio of the trisodium citrate to the urotropine to the boron-containing compound is 1:1: (0.5-1).
Preferably, the trisodium citrate comprises anhydrous trisodium citrate or disodium citrate dihydrate; the boron-containing compound includes at least one of boron oxide, boric acid, or tetraphenylboric acid.
Preferably, the protective atmosphere includes at least one of a reducing gas, an inert gas, and nitrogen.
Preferably, the reducing gas comprises hydrogen or ammonia;
the inert gas comprises at least one of helium, argon, neon and krypton;
the volume ratio of the hydrogen to the inert gas is (5-10): (90-95).
Preferably, in the step 1), the temperature rising rate of the carbonization reaction is 2-5 ℃/min; the reaction temperature of the carbonization reaction is 700-900 ℃, and the reaction time of the carbonization reaction is 1-3 h.
Preferably, the acid liquid comprises hydrochloric acid, and the concentration of the hydrochloric acid is 0.1mol/L-0.2mol/L; the washing times by hydrochloric acid are 1-3 times.
Preferably, in the step 2), the washing with the acid solution is followed by washing with absolute ethyl alcohol, and the washing with absolute ethyl alcohol is performed for 2-5 times.
On the other hand, the application provides a boron-nitrogen co-doped hard carbon material, which comprises the boron-nitrogen co-doped hard carbon material prepared by the preparation method of the boron-nitrogen co-doped hard carbon material.
The application has the beneficial effects that:
1. the preparation method of the boron-nitrogen co-doped hard carbon material provided by the application can synthesize the hard carbon material by a one-step method, has simple preparation process and low cost, and is suitable for industrial mass production;
2. compared with boron atoms or single-atom doping of nitrogen atoms, boron-nitrogen diatomic co-doping can effectively improve the electron and ion conductivity of the hard carbon material; meanwhile, compared with the double-atom doping of other sulfur atoms, phosphorus atoms and the like, the boron atoms, carbon atoms and nitrogen atoms are adjacent, and the co-doping of the boron atoms and the nitrogen atoms can cooperatively improve the electropositivity of the boron atoms and the electronegativity of the nitrogen atoms, so that the electrostatic adsorption capacity of the hard carbon material on ions is effectively improved;
3. according to the application, the sodium borate salt byproducts generated in the sodium borate salt byproducts are eluted through acid liquid treatment in the carbonization process, so that the hard carbon material has a rich loose porous structure, and rich defect sites are provided, so that the prepared boron-nitrogen co-doped hard carbon material has a loose surface structure and a porous inner space, and can be used as an electrochemical storage device, the adsorption and storage capacity of the hard carbon material to metal ions/metal compounds and the like can be improved, and the boron-nitrogen co-doped hard carbon material can be widely applied to negative electrodes of lithium/sodium batteries, sulfur carriers of lithium-sulfur batteries or diaphragm coating modified materials for lithium-sulfur batteries and the like, and is used for improving the discharge capacity and rate performance of the batteries.
Drawings
The accompanying drawings are included to provide a further understanding of the application, and are incorporated in and constitute a part of this specification, illustrate the application and together with the description serve to explain, without limitation, the application.
FIG. 1 is a scanning electron microscope image of a boron-nitrogen co-doped hard carbon material 1 prepared in comparative example 1;
FIG. 2 is a scanning electron microscope image of the boron-nitrogen co-doped hard carbon material 2 prepared in example 1;
FIG. 3 is a scanning electron microscope image of the boron-nitrogen co-doped hard carbon material 3 prepared in example 2;
FIG. 4 is a scanning electron microscope image of the boron-nitrogen co-doped hard carbon material 4 prepared in example 3;
FIG. 5 is a Raman diagram of the boron-nitrogen co-doped hard carbon materials prepared in comparative example 1 and example 1;
FIG. 6 is an XRD pattern before cleaning of the composite hard carbon materials prepared in examples 1 to 3 and comparative example 1;
FIG. 7 is an XRD pattern of the composite hard carbon materials prepared in examples 1 to 3 and comparative example 1 after washing;
FIG. 8 is a scanning electron microscope image of the boron-nitrogen co-doped hard carbon material 5 prepared in example 4;
FIG. 9 is a scanning electron microscope image of the boron-nitrogen co-doped hard carbon material 6 prepared in example 5;
FIG. 10 is an XRD pattern of the composite hard carbon materials prepared in examples 1, 4 and 5 before cleaning;
fig. 11 is an XRD pattern after cleaning of the composite hard carbon materials prepared in examples 1, 4, and 5.
Detailed Description
In order to make the technical problems, technical schemes and beneficial effects solved by the application more clear, the application is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the application.
The application provides a preparation method of a boron-nitrogen co-doped hard carbon material, which comprises the following steps:
1) Obtaining trisodium citrate, urotropine and a boron-containing compound, uniformly mixing, and performing carbonization reaction in a protective atmosphere to obtain a composite hard carbon material;
2) And (3) taking out the composite hard carbon material obtained in the step (1), cleaning the composite hard carbon material by using acid liquor, and eluting sodium borate byproducts in the composite hard carbon material to obtain the boron-nitrogen co-doped hard carbon material with loose and porous inside.
Boron doping belongs to p-type doping, and can effectively reduce the Fermi level of a hard carbon material and enhance the intercalation-deintercalation capability of metal ions; the nitrogen doping can further improve the conductivity of the hard carbon material; compared with monoatomic doping, boron atom and nitrogen atom co-doping not only maintains the lifting effect of the hard carbon materials, but also further enhances the ion adsorption capacity through synergistic effect, thereby being beneficial to improving the discharge capacity of the hard carbon materials as the negative electrode of the lithium/sodium ion battery and the inhibiting effect of the hard carbon materials as the sulfur carrier of the lithium-sulfur battery/the diaphragm coating material for the lithium-sulfur battery on polysulfide shuttle effect.
The preparation method of the boron-nitrogen co-doped hard carbon material provided by the application can synthesize the hard carbon material by a one-step method, has a simple preparation process and low cost, and is suitable for industrial mass production. Compared with the single-atom doping of boron atoms or nitrogen atoms, the boron atoms and nitrogen atoms double-atom co-doping can effectively improve the electron and ion conductivity of the hard carbon material; meanwhile, compared with the double-atom doping of other sulfur atoms, phosphorus atoms and the like, the boron atoms, carbon atoms and nitrogen atoms are adjacent, and the co-doping of the boron atoms and the nitrogen atoms can cooperatively improve the electropositivity of the boron atoms and the electronegativity of the nitrogen atoms, so that the electrostatic adsorption capacity of the hard carbon material on ions is effectively improved. According to the application, the sodium borate salt byproducts generated in the sodium borate salt byproducts are eluted through acid liquid treatment in the carbonization process, so that the hard carbon material has a rich loose porous structure, and rich defect sites are provided, so that the prepared boron-nitrogen co-doped hard carbon material has a loose surface structure and a porous inner space, and can be used as an electrochemical storage device, the adsorption and storage capacity of the hard carbon material to metal ions/metal compounds and the like can be improved, and the boron-nitrogen co-doped hard carbon material can be widely applied to negative electrodes of lithium/sodium batteries, diaphragm coating modified materials for lithium-sulfur batteries and the like, and is used for improving the discharge capacity and rate performance of the batteries.
In some embodiments, the trisodium citrate, urotropine, boron-containing compound mass ratio is (0.5-1.5): 1: (0.1-1.5).
In some embodiments of the application, the mass ratio of trisodium citrate to urotropine is 1:1, and the mass ratio of trisodium citrate to urotropine to the mass ratio of the boron-containing compound is 1:1:0.5-1:1: when the temperature is between 1.5, the proper amount of boron and nitrogen co-doping can obviously reduce the interlayer spacing of the hard carbon material, promote the closed pore degree of the internal pore structure of the material and be beneficial to increasing the discharge capacity of the lithium/sodium ion battery; meanwhile, the graphitization degree of the hard carbon material is improved due to boron-nitrogen doping to a certain extent, the short-range order in the hard carbon crystal structure is increased, and the electron and ion conduction capacity of the hard carbon material is improved. The application explores the influence of boron element doping content on the spacing between hard carbon layers and the defect degree of hard carbon, realizes the adjustment of the surface pore structure of the hard carbon material, the spacing between carbon material layers and the graphitization degree, and can provide positive and beneficial reference for the preparation of the hard carbon material obtained under other doping conditions with different boron and nitrogen contents. In some embodiments, the application also discusses trisodium citrate, urotropine, and boron-containing compound in a mass ratio of (0.5-1.5): 1:0.5, the influence of trisodium citrate with different proportions on the structure of the hard carbon material is added, the trisodium citrate influences the graphitization degree of the hard carbon material, and a reference is provided for improving the ionic or electronic conductivity of the hard carbon material.
Trisodium citrate, urotropine and boron-containing compound in the mass ratio of (0.5-1.5): 1: when the range of (0.1-1.5), ammonia generated by decomposition of urotropine in the carbonization process causes the surface of the material to generate a loose and porous structure; with the addition of boron element, ammonia (NH) generated by decomposition of urotropine 3 ) Can replace the-ONa group in the trisodium citrate to form-NH 2 . The addition of the boron-containing compound assists ammonia gas generated by decomposition of urotropine to replace-ONa groups in trisodium citrate to form-NH 2 The opening degree of the pore structure in the hard carbon material is further reduced, and more closed structures are formed, so that the storage of the boron-nitrogen co-doped hard carbon material on metal ions/metal compounds is facilitated, and the discharge capacity and the rate capability of the battery are improved. Meanwhile, during the doping process, part of boron-containing compounds can form sodium borate and other byproducts with generated-ONa groups; and the byproducts such as sodium borate and the like are cleaned later, so that the porous structure on the surface of the boron-nitrogen co-doped hard carbon material is increased, the defect sites are increased, and the removal-embedding of metal ions/metal compounds is facilitated.
Trisodium citrate, urotropine and boron-containing compound in a mass ratio exceeding (0.5-1.5): 1: in the range of (0.1-1.5), the content of byproducts such as sodium borate salts and the like in the prepared boron-nitrogen co-doped hard carbon material is increased, boron and nitrogen are doped seriously and excessively, so that the carbonization reaction of trisodium citrate is obviously influenced, the formation of a hard carbon short-range ordered structure is not facilitated, and the electronic and ion conduction capacity of the hard carbon material is further reduced; in addition, in the hard carbon material, the content of boron element is too high, the interlayer spacing of the hard carbon material is reduced, and excessive boron doping can influence the embedding of the hard carbon material into metal ions/metal compounds, so that the capacity of the battery is reduced.
In some preferred embodiments, the trisodium citrate, urotropine, and boron-containing compound are in a mass ratio of 1:1: (0.5-1).
The mass ratio of trisodium citrate to urotropine to the boron-containing compound is 1:1: in the preferable range of (0.5-1), the prepared boron-nitrogen co-doped hard carbon material has better electronic/ionic conductivity, is applied to a lithium/sodium ion battery cathode, a lithium-sulfur battery sulfur carrier or a diaphragm coating modified material for a lithium-sulfur battery and the like, and can effectively improve the discharge capacity and the rate capability of the battery.
Further, the mass ratio of trisodium citrate to urotropine to the boron-containing compound can be 1:1:0.1 or 1:1:0.5 or 1:1:0.8 or 1:1:1.2; it may also be 0.7:1:0.5 or 1.2:1:0.5.
In some embodiments, the trisodium citrate comprises anhydrous trisodium citrate or trisodium citrate dihydrate; the boron-containing compound includes at least one of boron oxide, boric acid, or tetraphenylboric acid.
And (3) taking trisodium citrate as a carbon source, urotropine as a nitrogen source and a boron-containing compound as a boron source, and performing carbonization reaction to obtain the boron-nitrogen co-doped hard carbon material, wherein the boron-containing compound is in a powder form, and mixing the three materials by grinding.
In some embodiments, preferably, the protective atmosphere comprises at least one of a reducing gas, an inert gas, and nitrogen.
Preferably, the reducing gas comprises hydrogen or ammonia;
the inert gas comprises at least one of helium, argon, neon and krypton;
the volume ratio of the hydrogen to the inert gas is (5-10): (90-95).
When reactants trisodium citrate, urotropine and boron-containing compounds carry out carbonization reaction, the carbonization reaction is carried out under the atmosphere of reducing gas, and meanwhile, the generated hard carbon material is prevented from being oxidized under the action of inert gas or nitrogen or ammonia. Ammonia gas can be used as a reducing gas in carbonization reaction and can also be used as a protective gas to prevent the oxidation of hard carbon materials. When hydrogen is a reducing gas, the hydrogen needs to be mixed with inert gas or ammonia gas to prevent explosion.
In some embodiments, in the step 1), the temperature rising rate of the carbonization reaction is 2-5 ℃/min; the reaction temperature of the carbonization reaction is 700-900 ℃, and the reaction time of the carbonization reaction is 1-3 h.
The carbonization reaction temperature can be 700 ℃, 750 ℃, 780 ℃, 800 ℃, 820 ℃, 850 ℃, 860 ℃, 890 ℃, 900 ℃, and trisodium citrate and urotropine and boron-containing compound can be subjected to the carbonization reaction within 700 ℃ to 900 ℃; the carbonization reaction time can be 1h, 1.5h, 2h, 2.5h, 3h. The heating rate of the carbonization reaction is 2-5 ℃/min, and the slow heating rate is beneficial to the polymerization carbonization reaction of trisodium citrate, urotropine and boron-containing compounds.
In some embodiments, the acid solution comprises hydrochloric acid, the concentration of which is 0.1mol/L to 0.2mol/L; the washing times by hydrochloric acid are 1-3 times.
In some embodiments, the step 2) is performed with an acid wash followed by an absolute ethanol wash; the washing times by using absolute ethyl alcohol are 2-5 times.
The sodium borate byproducts react with hydrochloric acid, and are cleaned by hydrochloric acid, and are mainly used for removing sodium borate byproducts. And then washing with absolute ethyl alcohol to remove impurities adhered to the boron-nitrogen co-doped hard carbon material, such as sodium chloride adhered to the surface of the hard carbon material and other washed products.
On the other hand, the application provides a boron-nitrogen co-doped hard carbon material, which comprises the boron-nitrogen co-doped hard carbon material prepared by the preparation method of the boron-nitrogen co-doped hard carbon material.
The boron-nitrogen co-doped hard carbon material prepared by the preparation method of the boron-nitrogen co-doped hard carbon material is used as an electrochemical storage device, and can be more beneficial to improving the adsorption and storage capacity of the boron-nitrogen co-doped hard carbon material to metal ions/metal compounds and the like; the method is applied to the battery fields of lithium/sodium ion battery cathodes, lithium-sulfur battery sulfur carriers or diaphragm coating modified materials for lithium-sulfur batteries and the like, and can effectively improve the discharge capacity and the rate capability of the battery.
The technical scheme of the present application will be described in further detail by the following specific embodiments, but the scope of the present application should not be construed as being limited to the following examples. Various substitutions and alterations are also within the scope of this disclosure, as will be apparent to those of ordinary skill in the art and by routine experimentation, without departing from the spirit and scope of the application as defined by the foregoing description.
Example 1
1) According to the mass ratio of 1:1:0.5, weighing trisodium citrate dihydrate, urotropine and boron oxide powder, grinding in a mortar in a fume hood, and uniformly mixing;
2) Loading the mixed powder into a ark, placing into a tube furnace, and adding Ar/H of 5% hydrogen 2 Heating to 800 ℃ at a heating rate of 3 ℃/min under a mixed atmosphere, and performing carbonization reaction for 2 hours to obtain a composite hard carbon material;
3) And 3) after natural cooling, taking out the composite hard carbon material prepared in the step 2) from the tubular furnace, cleaning 3 times by using hydrochloric acid solution with the concentration of 0.1mol/L, and then cleaning 2 times by using absolute ethyl alcohol. After the cleaning is completed, the sample is placed in a blast drying oven and dried at 60 ℃ to obtain the boron-nitrogen co-doped hard carbon material 2 (the number is HC-0.5) with a porous microstructure.
The scanning electron microscope image of the boron-nitrogen co-doped hard carbon material 2 (with the number of HC-0.5) synthesized from FIG. 2, namely example 1, can be seen: compared with the boron nitrogen co-doped hard carbon material 1 (with the number of HC-0) prepared in the comparative example 1, the thickness of the graphite sheets forming the material in the example 1 is slightly increased (about 80 nm), and because the thickness of the graphite sheets is increased, the opening degree of the pore structure of the material in the example 1 is obviously reduced, more closed pore structures are formed, and further the deintercalation and adsorption of the boron nitrogen co-doped hard carbon material 2 to metal ions/metal compounds are facilitated, and the discharge capacity and the rate capability of the battery are improved.
Example 2
1) According to the mass ratio of 1:1:1, weighing trisodium citrate dihydrate, urotropine and boric acid powder, grinding the trisodium citrate dihydrate, urotropine and boric acid powder in a mortar in a fume hood, and uniformly mixing the ground trisodium citrate dihydrate, urotropine and boric acid powder;
2) Loading the mixed powder into a ark, placing into a tube furnace, and adding Ar/H of 5% hydrogen 2 Heating to 800 ℃ at a heating rate of 3 ℃/min under a mixed atmosphere, and performing carbonization reaction for 2 hours to obtain a composite hard carbon material;
3) And 3) taking out the carbon-based composite material prepared in the step 2) from the tubular furnace after natural cooling, cleaning 3 times by using hydrochloric acid solution with the concentration of 0.1mol/L, and then cleaning 2 times by using absolute ethyl alcohol. After the cleaning is completed, the sample is placed in a blast drying oven and dried at 60 ℃ to obtain the boron-nitrogen co-doped hard carbon material 3 (the number is HC-1).
The scanning electron microscope image of the boron nitrogen co-doped hard carbon 3 (numbered HC-1) synthesized from example 2 described in FIG. 3 can be seen: the hard carbon material is also composed of graphite-like flakes, and as the boron content increases, the flake thickness increases to about 120 nm. Compared with the trumpet-shaped opening shown in the HC-0 material shown in FIG. 1 corresponding to the comparative example 1, the hard carbon material of example 2 has a cross-linked network structure inside, and the opening is straight-through type instead of trumpet-shaped, and the material with the number HC-1 has an increased closed pore degree and a more porous structure compared with the material with the number HC-0, and is more beneficial to improving the storage capacity of metal ions/metal compounds when being applied to an electrochemical energy storage device, thereby further improving the discharge capacity and the rate capability of the battery.
Example 3
1) According to the mass ratio of 1:1:1.5, anhydrous trisodium citrate, urotropine and tetraphenylboric acid powder are weighed and ground and mixed uniformly in a mortar in a fume hood;
2) Filling the mixed powder into a square boat, placing the square boat into a tube furnace, heating to 900 ℃ at a heating rate of 5 ℃/min under an ammonia atmosphere, and performing carbonization reaction for 2 hours to obtain a carbon-based composite material;
3) And 3) after natural cooling, taking out the composite hard carbon material prepared in the step 2) from the tubular furnace, cleaning 3 times by using hydrochloric acid solution with the concentration of 0.2mol/L, and then cleaning 3 times by using absolute ethyl alcohol. After the cleaning is completed, the sample is placed in a blast drying oven and dried at 60 ℃ to obtain the boron-nitrogen co-doped hard carbon material 4 (the number is HC-1.5) with hard texture.
The scanning electron microscope image of the boron nitrogen co-doped hard carbon 4 (numbered HC-1.5) synthesized from example 3 described in fig. 4 can be seen: the hard carbon material consisted essentially of graphite-like sheets, which had a significantly reduced pore structure and a portion of the stack formation compared to the materials obtained in comparative example 1, example 1 and example 2. The pore structure affects the deintercalation and storage of metal ions, the number of pore structures of the boron-nitrogen co-doped hard carbon material obtained in the embodiment 3 is obviously reduced, and the inventor obtains through a great amount of experiments that when the mass content of the boron-containing compound is greater than that of the embodiment 3, the prepared boron-nitrogen doped hard carbon material pore structure is not beneficial to the adsorption of lithium/sodium ions and polysulfide, so that the application effect of the hard carbon material in lithium/sodium ion batteries and lithium/sulfur batteries is limited, and the discharge capacity and the multiplying power performance of the batteries are not beneficial to improvement.
Example 4:
1) According to the mass ratio of 0.5:1:0.5, weighing trisodium citrate dihydrate, urotropine and boron oxide powder, grinding in a mortar in a fume hood, and uniformly mixing;
2) Loading the mixed powder into a ark, placing into a tube furnace, and adding Ar/H of 5% hydrogen 2 In the mixed atmosphere, at 3 ℃ for min -1 Heating to 800 ℃ at a heating rate, performing carbonization reaction, and annealing for 2 hours to obtain the composite hard carbon material;
3) And 3) after natural cooling, taking out the composite hard carbon material prepared in the step 2) from the tubular furnace, cleaning 3 times by using hydrochloric acid solution with the concentration of 0.1mol/L, and then cleaning 3 times by using absolute ethyl alcohol. After the cleaning is completed, the sample is placed in a blast drying oven and dried at 60 ℃ to obtain the boron-nitrogen co-doped hard carbon material 5 (number NM-0.5).
Example 5:
1) According to the mass ratio of 1.5:1:0.5, weighing trisodium citrate dihydrate, urotropine and boric acid powder, grinding in a mortar in a fume hood, and uniformly mixing;
2) Loading the mixed powder into a ark, placing into a tube furnace, and adding Ar/H of 5% hydrogen 2 Heating to 800 ℃ at a heating rate of 3 ℃/min under a mixed atmosphere, performing carbonization reaction, and annealing for 2 hours to obtain a composite hard carbon material;
3) And 3) after natural cooling, taking out the composite hard carbon material prepared in the step 2) from the tubular furnace, cleaning 3 times by using hydrochloric acid solution with the concentration of 0.2mol/L, and then cleaning 5 times by using absolute ethyl alcohol. After the cleaning is completed, the sample is placed in a blast drying oven and dried at 60 ℃ to obtain the boron-nitrogen co-doped hard carbon material 6 (NM-1.5).
In examples 1, 4 and 5, the influence on the hard carbon material was examined by adjusting the amount ratio of trisodium citrate to the amount ratio of urotropine and the boron-containing compound under the same condition. Wherein FIG. 2 is a scanning electron microscope image of boron-nitrogen co-doped hard carbon material 5 (No. HC-0.5) prepared in example 1,
FIG. 8 is a scanning electron microscope image of a boron-nitrogen co-doped hard carbon material 5 (No. NM-0.5) prepared in example 4,
fig. 9 is a scanning electron microscope image of a boron-nitrogen co-doped hard carbon material 6 (No. NM-1.5) prepared in example 5, and as shown in fig. 2, 8 and 9, the morphology of the hard carbon material obtained in fig. 8 and 9 is similar to that of stacking of flake graphite, and the surface pore structure of the hard carbon material obtained in fig. 8 and 9 is less than that of the hard carbon material obtained in fig. 2. By comparison, the proportion of trisodium citrate is adjusted, so that the influence on improving the pore structure of the hard carbon material is small.
Comparative example 1
1) According to the mass ratio of 1:1:0, weighing anhydrous trisodium citrate, urotropine and boron oxide powder, grinding the anhydrous trisodium citrate, urotropine and boron oxide powder in a mortar in a fume hood, and uniformly mixing the materials;
2) Loading the above mixed powder into a ark, placing into a tube furnace, and adding Ar/H containing 8% hydrogen 2 Heating to 700 ℃ at a heating rate of 2 ℃/min under a mixed atmosphere, and performing carbonization reaction for 2 hours to obtain a carbon-based composite material;
3) After natural cooling, the sample was taken out of the tube furnace, washed 3 times with hydrochloric acid solution having a concentration of 0.1mol/L, and then washed 2 times with absolute ethanol. After the cleaning is completed, the sample is placed in a blast drying oven and dried at 60 ℃ to obtain the boron-nitrogen co-doped hard carbon material 1 (the number is HC-0) with a porous microstructure.
The scanning electron microscope image of the boron-nitrogen co-doped hard carbon material 1 (number: HC-0) synthesized from FIG. 1, comparative example 1, can be seen: the hard carbon material is composed of two-dimensional graphite sheets, and the thickness of the graphite sheets forming a pore structure is about 50 nm. The internal structure is mainly composed of a series of horn-shaped opening pore structures with large and small sizes, which is beneficial to the diffusion and shuttling of metal ions or polysulfides, but the internal less closed pore structure is likely to be unfavorable to the storage of metal ions/metal compounds, so that the discharge capacity and the rate capability of the battery are unfavorable to be improved when the boron-nitrogen co-doped hard carbon material 1 is applied to a lithium/sodium ion battery anode material, a sulfur carrier of a lithium-sulfur battery or a diaphragm coating modified material for the lithium-sulfur battery.
FIG. 5 is a Raman diagram of the final products obtained in example 1 and comparative example 1, at 1354cm -1 And 1590cm -1 The D peak and the G peak respectively represent the disorder degree and the graphitization degree of the carbon material, I G /I D The larger the value, the higher the graphitization degree of the carbon material. As can be seen from the calculation, comparative example 1, which was obtained without boron-nitrogen co-doping, has I G /I D A value of 1.02, and I of example 1 obtained after co-doping with boron and nitrogen G /I D The value is 1.08, and the result shows that the boron-nitrogen co-doping is more beneficial to improving the short-range order of the hard carbon material, improving the graphitization degree of the hard carbon material and enhancing the ion/electron conductivity, thereby being beneficial to increasing the storage capacity of metal ions/metal compounds.
Fig. 6 is an XRD pattern after synthesis of samples of examples 1, 2, 3 and comparative example 1, before acid washing and water washing. As can be seen from FIG. 6, when the boron-containing compound content in the three mixture of trisodium citrate, urotropine and boron-containing compound is 0, the hard carbon material formed contains a small amount of sodium carbonate (Na 2 CO 3 ) Meanwhile, in the carbonization process, ammonia generated by decomposition of urotropine can also enable the surface of the material to form a loose and porous structure (shown in figure 1). When the mass ratio of the mixture of trisodium citrate, urotropine and the boron-containing compound is 1:1:0.5, ammonia (NH) generated by decomposition of urotropine is generated along with the addition of boron element 3 ) Will replace the medium-ONa group to form-NH 2 The method comprises the steps of carrying out a first treatment on the surface of the Meanwhile, during the doping process, part of the boron-containing compound can form a byproduct of sodium metaborate (NaBO) with-ONa groups generated by trisodium citrate 2 ) Is a compound of (a). With an increase in the content of the boron-containing compound for doping,when the mass ratio of the trisodium citrate to the urotropine to the boron-containing compound is 1:1:1, the by-product is converted into sodium tetraborate (Na 2 B 4 O 7 ) (characteristic peak at 19.2 ℃) but lower peak intensity, which shows that the boron-containing compound fully participates in the reaction at the proportion, the boron doping content of the obtained product is obviously improved, and the carbon content of the product is correspondingly improved. When the mass ratio of the trisodium citrate to the urotropine to the boron-containing compound is 1:1:1.5, the byproduct Na 2 B 4 O 7 The peak intensity of (2) is obvious, which shows that the serious excessive boron doping has significantly affected the carbonization of trisodium citrate, and the obtained product has a hand feeling similar to borax and a harder texture.
Fig. 7 is an XRD pattern of the final product after synthesis of the samples of examples 1, 2, 3 and comparative example 1. Because of by-product Na 2 CO 3 、NaBO 2 And Na (Na) 2 B 4 O 7 Since the amorphous carbon diffraction peaks at the positions of 26.7 ° and 41.8 ° are present in examples 1 and 2, respectively, and since the amount of the boron-containing compound substance added is slightly larger in example 3, the by-product produced affects the test result, and the values of the amorphous carbon diffraction peaks cannot be accurately obtained.
The results of the calculation of the interplanar spacing of the hard carbon materials obtained in comparative example 1, example 1 and example 2 are respectively 0.378nm, 0.336nm and 0.330nm, which show that the increase of the pore spacing of the hard carbon layers and the increase of the pore structure are accompanied by the increase of boron-nitrogen co-doping, and are helpful for improving the platform capacity of lithium/sodium ion storage when the boron-nitrogen co-doped hard carbon materials are applied to a negative electrode. Meanwhile, as can be seen from fig. 7, the (002) diffraction peaks of examples 1 and 2 in the XRD patterns are significantly shifted to higher angles than the (002) diffraction peak of comparative example 1, and it is demonstrated from the side that the interlayer spacing of the hard carbon material becomes smaller with the addition of boron element, thereby facilitating the formation of more short-range ordered structures and increasing the electron and ion conductivity of the hard carbon material. The (002) diffraction peak angles of comparative examples 1 and 2 show that the interlayer spacing of the hard carbon material becomes smaller as the boron content increases, indicating that excessive boron doping affects the intercalation of the hard carbon material into metal ions.
According to the preparation method of the boron-nitrogen co-doped hard carbon provided by the application, as shown in the examples 1-3 and the comparative example 1, a proper amount of boron-nitrogen co-doping (trisodium citrate: urotropine: boron-containing compound=1:1:0.5) can enrich the internal pore structure of the hard carbon material, obviously reduce the interlayer spacing of the hard carbon material and improve the closed pore degree of the material. When the hard carbon material is used as a battery anode material or a diaphragm coating modified material, the deintercalation and storage of metal ions/metal compounds can be further promoted, so that the discharge capacity of the battery can be increased, and the capacity platform and the rate capability of the battery can be improved; meanwhile, the boron-nitrogen co-doping to a certain extent also improves the graphitization degree of the hard carbon material, increases the short-range order in the hard carbon crystal structure, and improves the electron and ion conductivity of the material.
Fig. 10 is an XRD pattern before the composite hard carbon materials prepared in examples 1, 4 and 5 are subjected to acid washing and water washing. As can be seen from fig. 10, when the content of trisodium citrate added as a carbon source is small, the mass ratio of the three mixture of trisodium citrate, urotropine, and boron-containing compound is 0.5 as in example 4: 1: at 0.5, the resulting composite hard carbon material product has a portion of the Na formed by trisodium citrate and the boron-containing compound, in addition to the hard carbon characteristic peak (at 26.5 ℃) 2 B 4 O 7 By-product (characteristic peak at 18.9 °). Example 4 in comparison with example 1 (the mass ratio of the three mixtures of trisodium citrate, urotropine and boron-containing compound is 1:1:0.5), na is preferentially formed in the product in addition to the hard carbon material 2 B 4 O 7 The formation of by-products consumes more trisodium citrate and thus affects the carbon content of the hard carbon material. When the content of the trisodium citrate added is moderate (example 1: the mass ratio of the trisodium citrate to the urotropine to the boron-containing compound is 1:1:0.5), the byproduct is NaBO 2 Indicating a significant reduction in the carbon source consumed; when trisodium citrate is added in an excessive amount (example 5: the mass ratio of trisodium citrate to urotropine to boron-containing compound is 1.5:1:0.5), more by-products are produced, except NaBO 2 Besides, sodium diborate (Na 4 B 2 O 5 ) And the carbonization process of the material is significantly affected.
As is clear from the comparison of FIG. 10, in the reaction process of trisodium citrate, urotropine and boron-containing compound, na is generated due to too little trisodium citrate 2 B 4 O 7 By-products, when the trisodium citrate content is excessive, are increased, and all influence the carbonization process of the hard carbon material; as shown by the comparison, when the mass ratio of the trisodium citrate to the urotropine to the boron-containing compound is 1:1:0.5, fewer byproducts are generated in the carbonization reaction, and the carbonization process of the hard carbon material is less influenced.
Figure 11 is an XRD pattern of the final product synthesized from the samples of examples 1, 4 and 5. Because of by-product Na 2 B 4 O 7 、NaBO 2 、Na 4 B 2 O 5 Are all soluble in strong acids, so that after washing with hydrochloric acid and absolute ethanol, examples 1, 4 and 5 all present typical peaks of carbon material at 26.7 ° and 41.8 °. As can be seen from the observation of the peak intensities and half-widths of the XRD patterns in fig. 11, the hard carbon material obtained in example 4 has a stronger graphitization degree when the citric acid content is small, and the half-widths of examples 1 and 5 are increased with the increase of the trisodium citrate content, indicating an increase of the amorphous state of the obtained hard carbon material. By further comparing the interplanar spacings of examples 4, 1 and 5, the results obtained were 0.334nm, 0.336nm and 0.335nm, respectively, and the interlayer spacings of the three samples were relatively close, indicating that the trisodium citrate content was mainly responsible for adjusting the graphitization degree of the hard carbon material, with less influence on the interlayer spacing.
As is evident from the comparison of examples 1, 4, 5 above, trisodium citrate: urotropin: when the boron-containing compound is 1:1:0.5, the content of byproducts produced in the carbonization reaction process is small, the influence degree on the carbonization reaction is small, and the graphitization degree of the prepared boron-nitrogen co-doped hard carbon material is high, so that the ionic and electronic conductivity of the material is improved, and the discharge capacity and the multiplying power performance of the battery are improved. The proportion of trisodium citrate is adjusted, the change of the interlayer spacing of the hard carbon material is not influenced, and the method has a certain guiding significance for researching and improving the graphitization degree of the boron-nitrogen co-doped hard carbon material.
The foregoing description of the preferred embodiments of the application is not intended to be limiting, but rather is intended to cover all modifications, equivalents, and alternatives falling within the spirit and principles of the application.
Claims (9)
1. The preparation method of the boron-nitrogen co-doped hard carbon material is characterized by comprising the following steps of:
1) Obtaining trisodium citrate, urotropine and a boron-containing compound, uniformly mixing, and carrying out carbonization reaction in a protective atmosphere at the reaction temperature of 700-900 ℃ to obtain a composite hard carbon material;
2) Taking out the composite hard carbon material obtained in the step 1), washing with acid liquor, and eluting sodium borate byproducts in the composite hard carbon material to obtain the boron-nitrogen co-doped hard carbon material with loose and porous inside;
the mass ratio of the trisodium citrate to the urotropine to the boron-containing compound is (0.5-1.5): 1: (0.1-1.5); the boron-containing compound includes at least one of boron oxide, boric acid, or tetraphenylboric acid.
2. The preparation method of the boron-nitrogen co-doped hard carbon material according to claim 1, wherein the mass ratio of the trisodium citrate to the urotropine to the boron-containing compound is 1:1 (0.5-1).
3. The method for preparing a boron-nitrogen co-doped hard carbon material according to claim 1, wherein the trisodium citrate comprises anhydrous trisodium citrate or disodium citrate dihydrate.
4. The method of claim 1, wherein the protective atmosphere comprises at least one of a reducing gas, an inert gas, and nitrogen.
5. The method for producing a boron-nitrogen co-doped hard carbon material according to claim 4, wherein the reducing gas comprises hydrogen or ammonia;
the inert gas comprises at least one of helium, argon, neon and krypton;
the volume ratio of the hydrogen to the inert gas is (5-10): (90-95).
6. The method for preparing boron-nitrogen co-doped hard carbon material according to claim 1, wherein in the step 1), the heating rate of the carbonization reaction is 2-5 ℃/min; the reaction time of the carbonization reaction is 1-3 h.
7. The method for preparing the boron-nitrogen co-doped hard carbon material according to claim 1, wherein the acid solution comprises hydrochloric acid, and the concentration of the hydrochloric acid is 0.1mol/L-0.2mol/L; the washing times by hydrochloric acid are 1-3 times.
8. The method for preparing boron-nitrogen co-doped hard carbon material according to claim 7, wherein the step 2) is performed by using absolute ethyl alcohol after the acid liquid is used for cleaning, and the number of times of cleaning by using absolute ethyl alcohol is 2-5.
9. The boron-nitrogen co-doped hard carbon material is characterized by comprising the boron-nitrogen co-doped hard carbon material prepared by the preparation method of the boron-nitrogen co-doped hard carbon material according to any one of claims 1-8.
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