Sulfur-nitrogen double-doped porous hard carbon as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of negative electrode materials, and particularly relates to sulfur-nitrogen double-doped porous hard carbon as well as a preparation method and application thereof. Compared with a traditional carbon material, the sulfur-nitrogen double-doped porous hard carbon provided by the invention has richer mesopores and more energy storage sites provided by sulfur-nitrogen heteroatoms, and when the sulfur-nitrogen double-doped porous hard carbon is used as a negative electrode material, the sodium storage capability and rate capability of a sodium ion battery can be obviously improved.

Description

Technical Field

[0001] The invention belongs to the technical field of negative electrode materials, and specifically relates to sulfur-nitrogen dual-doped porous hard carbon and a preparation method and application thereof. Background Art

[0002] Secondary batteries are a widely used chemical "power source" in the world. They have the advantages of stable voltage, safety and reliability, low price, wide application range, rich raw materials and high recycling rate. They are the hot spot of international research and development, and are also the key link of major applications such as new generation information communication, electric vehicles, energy storage power stations and energy Internet. On the one hand, science and technology are constantly developing and the quality of human life is constantly improving. On the other hand, oil resources are facing a crisis and the earth's ecological environment is deteriorating. Under this dual social background, the urgent demand of the market has led to the emergence of new secondary batteries. Among them, sodium-ion batteries have developed rapidly in China, and enterprises in its industrial chain have sprung up like mushrooms after rain.

[0003] At present, the negative electrode materials of sodium ion batteries include hard carbon, soft carbon, titanium-based materials, alloy materials, organic compounds and other systems. Among them, the hard carbon negative electrode is amorphous carbon, which has the characteristics of high disorder, high layer spacing, excellent isotropy, etc. Hard carbon as the negative electrode of sodium ion batteries has the characteristics of good rate, excellent low temperature and high safety.

[0004] Hard carbon includes coal-based hard carbon, bio-based hard carbon and resin-based hard carbon. Among them, bio-based hard carbon has attracted extensive attention from researchers due to its wide source of precursors, low cost and rich and diverse structures. Hard carbon energy storage is mainly based on the adsorption-intercalation-pore filling mechanism, in which the ion diffusion kinetics of the pores are poor, making it difficult to achieve both high energy storage sites and high rate performance. Therefore, how to balance high capacity and high rate performance is the top priority for hard carbon to move towards large-scale application.

[0005] The main technical means to improve the capacity and rate performance of hard carbon include heteroatom doping, pore creation and improved sintering process. Among them, heteroatom doping is due to the high sintering temperature of hard carbon, and the chemical bonds between heteroatoms and carbon atoms are easily broken, resulting in low doping efficiency and high heteroatom loss rate, which seriously limits the doping effect. Summary of the invention

[0006] The purpose of the present invention is to provide a sulfur-nitrogen dual-doped porous hard carbon and a preparation method and application thereof. The sulfur-nitrogen dual-doped porous hard carbon provided by the present invention has both high capacity and high rate performance.

[0007] In order to achieve the above object, the present invention provides the following technical solutions:

[0008] The present invention provides a method for preparing sulfur-nitrogen dual-doped porous hard carbon, comprising the following steps:

[0009] The biomass raw material, water, sulfur-nitrogen doping agent and activator are mixed, freeze-dried and activated to obtain sulfur-nitrogen dual-doped porous carbon; the sulfur-nitrogen doping agent includes one or more of ammonium sulfonate, ammonium benzene sulfonate and ammonium dodecylbenzene sulfonate;

[0010] The sulfur-nitrogen dual-doped porous carbon and a carbon coating agent are mixed and sintered to obtain the sulfur-nitrogen dual-doped porous hard carbon.

[0011] Preferably, the biomass raw material includes one or more of lignin, cellulose, humic acid, chitin and chitosan; and the mass ratio of the biomass raw material to water is 1:5-100.

[0012] Preferably, the mass ratio of the biomass raw material to the sulfur-nitrogen dopant is 1:0.1-5.

[0013] Preferably, the activator is an inorganic salt; the inorganic salt includes one or more of sodium sulfate, potassium sulfate, sodium chloride, potassium chloride, calcium chloride, magnesium chloride and zinc chloride; the mass ratio of the biomass raw material to the activator is 1:0.5-6.

[0014] Preferably, the activation adopts a two-stage activation; the two-stage activation includes a first stage activation and a second stage activation; the temperature of the first stage activation is 350-550°C, and the time is 1-6 hours; the temperature of the second stage activation is 650-950°C, and the time is 1-6 hours.

[0015] Preferably, the carbon coating agent comprises asphalt and / or phenolic resin, and the mass of the carbon coating agent is 1 to 20% of the mass of the sulfur-nitrogen dual-doped porous carbon.

[0016] Preferably, the sintering temperature is 1000-1600° C., and the holding time is 2-5 hours.

[0017] The present invention also provides sulfur-nitrogen dual-doped porous hard carbon prepared by the preparation method described in the above technical solution, wherein the pore volume of the sulfur-nitrogen dual-doped porous hard carbon is 0.05-0.4 cm 3 / g, the mesopore volume accounts for 25% to 65%, and the specific surface area is 5 to 50m 2 / g.

[0018] Preferably, the mass content of N element in the sulfur-nitrogen dual-doped porous hard carbon is 2-7.2%, and the mass content of S element is 1-1.8%.

[0019] The present invention also provides the use of the sulfur-nitrogen dual-doped porous hard carbon described in the above technical solution as a negative electrode material in a battery or a capacitor.

[0020] The present invention provides a method for preparing sulfur-nitrogen dual-doped porous hard carbon, comprising the following steps: mixing biomass raw materials, water, sulfur-nitrogen dopants and activators, freeze-drying and activating to obtain sulfur-nitrogen dual-doped porous carbon; the sulfur-nitrogen dopants include one or more of ammonium sulfonate, ammonium benzenesulfonate and ammonium dodecylbenzenesulfonate; sintering and coating the sulfur-nitrogen dual-doped porous carbon to obtain the sulfur-nitrogen dual-doped porous hard carbon. The sulfur-nitrogen dopants provided by the present invention are amphiphilic, can combine with functional groups on biomass raw materials in aqueous solution, orientedly arrange to form micelles, improve doping efficiency, and can act as a soft template in activation, working together with the activator to further improve porosity and mesopore content.

[0021] The present invention also provides sulfur-nitrogen dual-doped porous hard carbon prepared by the preparation method described in the above technical solution. Compared with traditional carbon materials, the sulfur-nitrogen dual-doped porous hard carbon prepared by the present invention has richer mesopores and more energy storage sites provided by sulfur and nitrogen heteroatoms. The results of the embodiments show that under the condition of a charge and discharge cut-off voltage of 2.0 to 0V, the first reversible capacity of the sulfur-nitrogen dual-doped porous hard carbon provided by the present invention is above 367mAh / g, and the first efficiency is above 86.4%; under room temperature conditions, the 5C capacity retention rate of the sulfur-nitrogen dual-doped porous hard carbon provided by the present invention is greater than 80%. It can be seen that the sulfur-nitrogen dual-doped porous hard carbon prepared by the present invention can have both high capacity and high rate performance.

[0022] The present invention also provides the use of the sulfur-nitrogen dual-doped porous hard carbon described in the above technical solution as a negative electrode material in a battery or capacitor. Compared with traditional carbon materials, the sulfur-nitrogen dual-doped porous hard carbon provided by the present invention has more abundant mesopores and more energy storage sites provided by N and S heteroatoms. When used as a negative electrode material in a battery or capacitor, it can significantly improve the sodium storage capacity and rate performance of the battery or capacitor. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a nitrogen adsorption-desorption curve of the sulfur-nitrogen dual-doped porous hard carbon prepared in Example 1;

[0024] Figure 2 TEM image of sulfur-nitrogen dual-doped porous hard carbon prepared in Example 1;

[0025] Figure 3 This is a graph showing the first charge and discharge curve of a sodium button cell assembled using the sulfur-nitrogen dual-doped porous hard carbon prepared in Example 1 as the negative electrode material;

[0026] Figure 4 This is a rate curve of 1 to 5C of a sodium battery assembled using the sulfur-nitrogen dual-doped porous hard carbon prepared in Examples 1 to 6 and Comparative Examples 1 to 4 as the negative electrode material. DETAILED DESCRIPTION

[0027] The present invention provides a method for preparing sulfur-nitrogen dual-doped porous hard carbon, comprising the following steps:

[0028] The biomass raw material, water, sulfur-nitrogen doping agent and activator are mixed, freeze-dried and activated to obtain sulfur-nitrogen dual-doped porous carbon; the sulfur-nitrogen doping agent includes one or more of ammonium sulfonate, ammonium benzene sulfonate and ammonium dodecylbenzene sulfonate;

[0029] The sulfur-nitrogen dual-doped porous carbon and a carbon coating agent are mixed and sintered to obtain the sulfur-nitrogen dual-doped porous hard carbon.

[0030] In the present invention, unless otherwise specified, all preparation raw materials are commercially available products well known to those skilled in the art.

[0031] The invention mixes biomass raw materials, water, sulfur-nitrogen doping agents and activating agents, and freeze-dries and activates them to obtain sulfur-nitrogen dual-doped porous carbon; the sulfur-nitrogen doping agents include one or more of ammonium sulfonate, ammonium benzene sulfonate and ammonium dodecylbenzene sulfonate.

[0032] In the present invention, the biomass raw material preferably includes one or more of lignin, cellulose, humic acid, chitin and chitosan, and more preferably humic acid.

[0033] In the present invention, the water preferably includes pure water and / or ultrapure water. In the present invention, the mass ratio of the biomass raw material to water is preferably 1:5-100, and more preferably 1:20-50. In the present invention, the mass ratio of the biomass raw material to water should not be too low or too high. By controlling the mass ratio of the biomass raw material to water, the micelle concentration and salt concentration in the aqueous solution can be regulated to ensure that the biomass raw material and the sulfur and nitrogen dopant can be fully mixed, thereby ensuring the pore-forming effect and the doping effect.

[0034] In the present invention, the mass ratio of the biomass raw material to the sulfur and nitrogen doping agent is preferably 1: 0.1 to 5, and more preferably 1: 0.5 to 3. In the present invention, the mass ratio of the biomass raw material to the sulfur and nitrogen doping agent should not be too low or too high. By accurately controlling the mass ratio of the biomass raw material to the sulfur and nitrogen doping agent, the doping effect can be ensured while taking into account the use efficiency of the sulfur and nitrogen doping agent, thus avoiding waste of resources and better meeting the needs of production.

[0035] In the present invention, the sulfur-nitrogen dopant can provide N and S elements. In sodium-ion batteries, N elements can provide more sodium ion storage sites, while S elements can increase the proportion of energy storage in the slope section, thereby improving the rate performance. At the same time, the sulfur-nitrogen dopant of the present invention is amphiphilic and can combine with the functional groups on the biomass raw materials in an aqueous solution, form micelles in a directional arrangement, improve the doping efficiency, and act as a soft template in activation, working together with the activator to further improve the porosity and mesopore content.

[0036] In the present invention, the activator is preferably an inorganic salt, and further preferably includes one or more of sodium sulfate, potassium sulfate, sodium chloride, potassium chloride, calcium chloride, magnesium chloride and zinc chloride. In the present invention, the activator has weak corrosion to the equipment and little pollution. After activation, the porous carbon has developed pores and high mesopore content, and can provide more sodium ion storage sites and wider sodium ion transmission channels. In the present invention, the mass ratio of the biomass raw material and the activator is preferably 1:0.5~6, and further preferably 1:2~4. In the present invention, by precisely adjusting the mass ratio of the biomass raw material and the activator, the number and size of the porous carbon pores can be controlled to ensure that the pore structure is more suitable for the needs of high-capacity and high-rate hard carbon.

[0037] In the present invention, the mixing is preferably carried out by stirring in a water bath; the mixing is preferably carried out by stirring in a water bath and comprises:

[0038] The biomass raw material, water and an activating agent are first stirred to obtain a first mixture;

[0039] The sulfur-nitrogen dopant is added into the first mixture, and a second stirring is performed to obtain a second mixture.

[0040] In the present invention, the temperature of the first stirring is preferably 25-100°C, and the time is preferably 2-6 hours; the first stirring at the above temperature and time can better disperse the components to form a stable and uniform dispersion. In the present invention, the temperature of the second stirring is preferably 50-80°C, and the time is preferably 2-4 hours. In the present invention, by precisely controlling the temperature and time of the mixing water bath stirring, it is possible to ensure that the biomass raw material is fully directional combined with the sulfur and nitrogen dopant in the aqueous solution to form micelles, and to ensure that the activator is fully dissolved and fully impregnated with the biomass raw material.

[0041] In the present invention, the freeze-drying step preferably includes drying the second mixture; the drying temperature is preferably 80-150°C, and the drying time is preferably 2-8 hours. In the present invention, after drying, the moisture content of the second mixture is preferably less than 30%. In the present invention, if the drying temperature is too high or the drying time is too long, the material may be oxidized, resulting in irreversible effects.

[0042] In the present invention, the freeze drying temperature is preferably -50°C, the air pressure is preferably less than 1 Pa, and the time is preferably 48 hours. In the present invention, the freeze drying can remove excess water while uniformly dispersing the sulfur-nitrogen dopant and the activator in the biomass raw material precursor.

[0043] In the present invention, the activation is preferably performed under a nitrogen atmosphere.

[0044] In the present invention, the activation preferably adopts a two-stage activation; the two-stage activation preferably includes a first stage activation and a second stage activation in sequence; the temperature of the first stage activation is preferably 350-550°C, more preferably 400-500°C; the heating rate is preferably 2°C / min; the time is preferably 1-6h, more preferably 2-4h. In the present invention, the first stage activation includes processes such as dehydration, condensation, polymerization and aromatization, during which the activator acts as a template, occupies the original space of carbon atoms, and forms more carbon microcrystal defects and free radicals. In the present invention, in the first stage activation, when the activation temperature is heated to 150°C-200°C, the sulfur-nitrogen dopant begins to decompose, forming partial pores while accompanied by the combination of heteroatoms with the above-mentioned free radicals, thereby improving the utilization rate of the sulfur-nitrogen dopant.

[0045] In the present invention, the temperature of the second stage activation is preferably 650-950°C, more preferably 700-900°C; the heating rate is preferably 2°C / min; the time is preferably 1-6h, more preferably 2-4h. In the present invention, the second stage activation is accompanied by the processes of pore generation, pore expansion, collapse, and closure, which can expand a large number of micropores into mesopores, widen the ion transmission channel, and improve the kinetics and rate performance.

[0046] After the activation, the present invention also preferably includes naturally cooling and purifying the activated material; the purification preferably includes washing with water and drying in sequence; the number of washings is preferably 5 to 10 times; after washing, the washing liquid of the sulfur-nitrogen dual-doped porous carbon is preferably neutral; the drying temperature is preferably 100°C, and the drying time is preferably 6 hours.

[0047] After obtaining the sulfur-nitrogen dual-doped porous carbon, the present invention mixes the sulfur-nitrogen dual-doped porous carbon with a carbon coating agent and sinters them to obtain the sulfur-nitrogen dual-doped porous hard carbon.

[0048] In the present invention, the mixing and sintering of the sulfur-nitrogen dual-doped porous carbon and the carbon coating agent are preferably carried out simultaneously; the carbon coating agent preferably includes asphalt and / or resin; the mass of the carbon coating agent is preferably 1-20% of the mass of the sulfur-nitrogen dual-doped porous carbon.

[0049] In the present invention, the sintering temperature is preferably 1000-1600°C, more preferably 1100-1400°C; the holding time is preferably 2-5h, more preferably 2-4h. In the present invention, the starting temperature of the sintering is preferably room temperature; the heating rate to the sintering temperature is preferably 2-8°C / min; the sintering is preferably carried out under an inert protective atmosphere, and the inert protective atmosphere preferably includes one or more of nitrogen, argon and helium; the oxygen content of the inert protective atmosphere is preferably less than 100ppm. In the present invention, during the sintering process, when the temperature is 200-300°C, the carbon coating agent reaches the softening point, and can evenly wrap the N atoms and S atoms and the pores, reduce the specific surface area, reduce surface defects, and greatly reduce the loss rate of N atoms and S atoms, thereby increasing the content of N and S elements in hard carbon.

[0050] After the sintering is completed, the present invention also preferably includes naturally cooling the obtained material to room temperature.

[0051] The present invention also provides sulfur-nitrogen dual-doped porous hard carbon prepared by the preparation method described in the above technical solution, wherein the pore volume of the sulfur-nitrogen dual-doped porous hard carbon is 0.05-0.4 cm 3 / g, the mesopore volume accounts for 25% to 65%, and the specific surface area is 5 to 50m 2 / g.

[0052] In the present invention, the mass content of the N element in the sulfur-nitrogen dual-doped porous hard carbon is preferably 2-7.2%, and the mass content of the S element is preferably 1-1.8%.

[0053] The present invention also provides the use of the sulfur-nitrogen dual-doped porous hard carbon described in the above technical solution as a negative electrode material in a battery or a capacitor.

[0054] In order to further illustrate the present invention, a sulfur-nitrogen dual-doped porous hard carbon and its preparation method and application provided by the present invention are described in detail below in combination with the accompanying drawings and examples, but they should not be construed as limiting the scope of protection of the present invention.

[0055] Example 1

[0056] 10 g of humic acid was added to 500 g of water, and 15 g of sodium chloride and 15 g of potassium chloride were added, and the mixture was stirred for 4 hours at 80° C. to obtain a first mixture. 10 g of ammonium sulfonate was added to the evenly dispersed first mixture, and the mixture was stirred in a water bath at 80° C. for 4 hours to obtain a second mixture.

[0057] After the uniformly dispersed second mixture is dried to a moisture content of 30%, freeze drying is performed for 48 hours, wherein the freeze drying pressure is less than 1Pa and the temperature is -50°C. The material obtained after freeze drying is placed in a tubular furnace, nitrogen is introduced, and the temperature is raised from room temperature to 500°C at a heating rate of 2°C / min, and kept warm for 2 hours to perform the first stage of activation. Then the temperature is raised from 500°C to 850°C at a heating rate of 2°C / min, and kept warm for 2 hours to perform the second stage of activation. After natural cooling, the material is discharged, and the obtained material is washed and filtered using a Buchner funnel, and the filtrate is repeated 5 to 10 times until the filtrate is clear and the pH is neutral, and then placed in a blast oven and dried at 100°C for 6 hours to obtain purified sulfur-nitrogen dual-doped porous carbon.

[0058] Add 0.5 g of asphalt to the purified sulfur-nitrogen dual-doped porous carbon and mix evenly, place it in a tubular furnace, introduce nitrogen, and heat it from room temperature to 1350°C at a heating rate of 2°C / min. Keep it warm for 4 hours for sintering and coating. After natural cooling, discharge the material to obtain sulfur-nitrogen dual-doped porous hard carbon.

[0059] Example 2

[0060] The other conditions in Example 1 were kept unchanged, except that humic acid was replaced with lignin to prepare sulfur-nitrogen dual-doped porous hard carbon.

[0061] Example 3

[0062] The other conditions in Example 1 were kept unchanged, except that the activator was replaced with 30 g of sodium sulfate to prepare sulfur-nitrogen dual-doped porous hard carbon.

[0063] Example 4

[0064] The other conditions in Example 1 were kept unchanged, except that the amount of sulfur-nitrogen doping agent ammonium sulfonate was increased to 40 g, and the amounts of activating agents sodium chloride and potassium chloride were increased to 25 g, respectively, to prepare sulfur-nitrogen dual-doped porous hard carbon.

[0065] Example 5

[0066] The other conditions in Example 1 were kept unchanged, except that the amount of sulfur-nitrogen doping agent ammonium sulfonate was reduced to 2 g, and the amounts of activators sodium chloride and potassium chloride were reduced to 5 g, respectively, to prepare sulfur-nitrogen dual-doped porous hard carbon.

[0067] Example 6

[0068] The other conditions in Example 1 were kept unchanged, except that the sulfur-nitrogen doping agent was changed to ammonium dodecylbenzenesulfonate, the activating agent was changed to 15 g of sodium sulfate and 15 g of potassium sulfate, and the coating agent was changed to phenolic resin to prepare sulfur-nitrogen dual-doped porous hard carbon.

[0069] Comparative Example 1

[0070] The other conditions in Example 1 were kept unchanged, except that no activator was added to prepare sulfur-nitrogen dual-doped porous hard carbon.

[0071] Comparative Example 2

[0072] The other conditions in Example 1 were kept unchanged, except that the amount of sulfur and nitrogen dopants added was reduced to 0.5 g, and sulfur and nitrogen dual-doped porous hard carbon was prepared.

[0073] Comparative Example 3

[0074] The other conditions in Example 1 remain unchanged, except that the activator is 1 g of sodium chloride and 1 g of potassium chloride.

[0075] Comparative Example 4

[0076] The other conditions in Example 1 were kept unchanged, except that no coating agent was added during sintering to prepare sulfur-nitrogen dual-doped porous hard carbon.

[0077] Performance Testing

[0078] (1) The performance of the sulfur-nitrogen dual-doped porous hard carbons prepared in Examples 1 to 6 and Comparative Examples 1 to 4 was characterized. The test results are shown in Table 1.

[0079] Table 1 Performance characteristics of sulfur-nitrogen dual-doped porous hard carbons prepared in Examples 1 to 6 and Comparative Examples 1 to 4

[0080]

[0081] (2) Pore size distribution test of sulfur-nitrogen dual-doped porous hard carbon prepared in Example 1

[0082] The pore size distribution of sulfur-nitrogen dual-doped porous hard carbon prepared in Example 1 was tested using American Quantachrome Autosorb IQ3. Figure 1 This is the nitrogen adsorption-desorption curve of the sulfur-nitrogen dual-doped porous hard carbon prepared in Example 1.

[0083] Depend on Figure 1 It can be seen that: when the relative pressure P / P0 is less than 0.01, it rises rapidly, the adsorption amount of N2 increases vertically, and the sulfur-nitrogen dual-doped porous hard carbon contains a certain amount of micropores; when the relative pressure P / P0 is in the range of 0.50-1.0, the sulfur-nitrogen dual-doped porous hard carbon has obvious hysteresis loops and tails, indicating that the sulfur-nitrogen dual-doped porous hard carbon prepared in Example 1 contains a large number of large mesopores and macropores; when the relative pressure is in the range of 0.4<P / P0<0.6, there is an obvious hysteresis loop, and when P / P0 is greater than 0.9, there is an obvious tail, indicating that the pore size distribution of the sulfur-nitrogen dual-doped porous hard carbon is mainly micropores, and it contains a certain amount of mesopores and macropores.

[0084] (3) Electrochemical performance test of sulfur-nitrogen dual-doped porous hard carbon prepared in Examples 1 to 6 and Comparative Examples 1 to 4

[0085] The sulfur-nitrogen dual-doped porous hard carbons prepared in Examples 1 to 6 and Comparative Examples 1 to 4 were used as negative electrode materials to test their electrochemical properties.

[0086] The negative electrode materials are assembled into button cells, which are composed of commercial negative electrode shells, positive electrode shells, separators, sodium sheets, nickel foam, pole pieces and electrolytes; the conductive agent is acetylene black, the binders are CMC and SBR, and the solvent is deionized water; the electrolyte is composed of three parts: sodium salt, solvent and additives, among which the sodium salt is sodium hexafluorophosphate, the solvent is ethylene carbonate (EC), and the additive is dimethyl carbonate (DMC); the separator thickness is 30 μm, the current collector is aluminum foil (thickness is 12 μm), and a metal sodium sheet is used as the counter electrode.

[0087] The preparation method of sodium button cell comprises the following steps: mixing hard carbon negative electrode material, conductive carbon black and binder in deionized water in a mass ratio of 96:1:3, homogenizing, controlling the solid content to 55%, coating on aluminum foil current collector, and then vacuum baking at 100°C for 8h, pressing and forming, and the compacted density of the pole piece is 0.9g / cm 3 The negative electrode sheet was prepared by punching; the button half-cell was assembled in a glove box filled with argon, the counter electrode was a metal sodium sheet, the diaphragm used was PE, and the electrolyte was 1 mol / L NaPF6 EC / DMC (Vol1:1).

[0088] The button cell obtained was subjected to charge and discharge tests (the button cell test equipment was the LAND battery test system of Wuhan Landian Electronics Co., Ltd.), the counter electrode was a metal sodium sheet, the test process was 0.1C DC to 0V, 0.05C DC to 0V, 0.01C DC to 0V, 0V CV 20uA, Rest 10min, 0.1C CC to 2V, and the button cell capacity retention rate test conditions were: 2C rate discharge (sodium insertion), 0.5C rate charge (sodium removal), 50 cycles were completed, and the capacity retention rate was the discharge capacity of the 50th week divided by the discharge capacity of the first week. The first reversible capacity and efficiency test results of the sulfur-nitrogen dual-doped porous hard carbon are shown in Table 2;

[0089] Table 2 Electrochemical properties of sulfur-nitrogen dual-doped porous hard carbons prepared in Examples 1 to 6 and Comparative Examples 1 to 4

[0090]

[0091] It can be seen from Table 2 that: since no activator was added in Comparative Example 1 and Comparative Example 3 or the amount of activator added was too low, the sulfur-nitrogen doping efficiency was too low, almost no mesopores were contained, the ion transfer rate was limited, and the rate performance was poor; since the amount of sulfur-nitrogen dopant added in Comparative Example 2 was only 0.5 g, the amount of doped elements was insufficient, and its rate performance and gram capacity were slightly improved; since no coating agent was added in Comparative Example 4, the doped elements were not protected, a large amount of doped elements were lost at high temperatures, and the mesopores, macropore structures and surface defects could not be repaired, resulting in low reversible capacity and initial efficiency.

[0092] Figure 2 This is a TEM image of sulfur-nitrogen dual-doped porous hard carbon prepared in Example 1. Figure 2 It can be seen that the sulfur-nitrogen dual-doped porous hard carbon prepared in Example 1 shows the morphology of amorphous carbon under high-magnification TEM, and the carbon microcrystals are all short-range ordered worm-like structures, which surround abundant pores. There are abundant "bright" mesopores on the surface of hard carbon, which provide channels for the rapid transmission of sodium ions, thereby improving the rate performance of hard carbon.

[0093] Figure 3 This is the first charge and discharge curve of the sodium button cell assembled using the sulfur-nitrogen dual-doped porous hard carbon prepared in Example 1 as the negative electrode material. Figure 3 It can be seen that under the condition of charge and discharge cut-off voltage of 2.0~0V, the first reversible capacity of sulfur-nitrogen dual-doped porous hard carbon is above 367mAh / g, and the first efficiency is above 86.4%.

[0094] Figure 4 The 1-5C rate curves of sodium button cells assembled using sulfur-nitrogen dual-doped porous hard carbons prepared in Examples 1-6 and Comparative Examples 1-4 as negative electrode materials. Figure 4 It can be seen that the sulfur-nitrogen dual-doped porous hard carbons prepared in Examples 1 to 6 all have good rate performance. At room temperature, the 5C capacity retention rate is greater than 80%, while the sulfur-nitrogen dual-doped porous hard carbons prepared in Comparative Examples 1 to 4 have poor rate performance, and the capacity retention rate is all below 80%.

[0095] The following conclusions can be drawn from the above data: The sulfur-nitrogen dual-doped porous hard carbon provided by the present invention has a higher gram capacity and higher rate performance than traditional hard carbon materials. When used as a negative electrode material for a secondary battery, it can significantly improve the rate performance of the secondary battery.

[0096] Although the above embodiment describes the present invention in detail, it is only a part of the embodiments of the present invention, not all of the embodiments. Other embodiments can be obtained based on this embodiment without creativity, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for preparing sulfur-nitrogen dual-doped porous hard carbon, characterized in that: The following steps are involved: The biomass raw material, water, sulfur-nitrogen doping agent and activator are mixed, freeze-dried and activated to obtain sulfur-nitrogen dual-doped porous carbon; the sulfur-nitrogen doping agent includes one or more of ammonium sulfonate, ammonium benzene sulfonate and ammonium dodecylbenzene sulfonate; The sulfur-nitrogen dual-doped porous carbon and a carbon coating agent are mixed and sintered to obtain the sulfur-nitrogen dual-doped porous hard carbon.

2. The preparation method according to claim 1, characterized in that: The biomass raw material includes one or more of lignin, cellulose, humic acid, chitin and chitosan; the mass ratio of the biomass raw material to water is 1:5-100.

3. The preparation method according to claim 1, characterized in that: The mass ratio of the biomass raw material to the sulfur-nitrogen dopant is 1:0.1-5.

4. The preparation method according to claim 1, characterized in that: The activator is an inorganic salt; the inorganic salt includes one or more of sodium sulfate, potassium sulfate, sodium chloride, potassium chloride, calcium chloride, magnesium chloride and zinc chloride; the mass ratio of the biomass raw material to the activator is 1:0.5-6.

5. The preparation method according to claim 1, characterized in that: The activation adopts a two-stage activation; the two-stage activation includes sequentially performing a first stage activation and a second stage activation; the temperature of the first stage activation is 350-550° C., and the time is 1-6 hours; the temperature of the second stage activation is 650-950° C., and the time is 1-6 hours.

6. The preparation method according to claim 1, characterized in that: The carbon coating agent comprises asphalt and / or phenolic resin, and the mass of the carbon coating agent is 1-20% of the mass of the sulfur-nitrogen dual-doped porous carbon.

7. The preparation method according to claim 1 or 6, characterized in that: The sintering temperature is 1000-1600° C., and the heat preservation time is 2-5 hours.

8. The sulfur-nitrogen dual-doped porous hard carbon prepared by the preparation method according to any one of claims 1 to 7, characterized in that: The pore volume of the sulfur-nitrogen dual-doped porous hard carbon is 0.05 to 0.4 cm 3 / g, the mesopore volume accounts for 25% to 65%, and the specific surface area is 5 to 50m 2 / g.

9. The sulfur-nitrogen dual-doped porous hard carbon according to claim 8, characterized in that: The mass content of N element in the sulfur-nitrogen dual-doped porous hard carbon is 2-7.2%, and the mass content of S element is 1-1.8%.

10. Use of the sulfur-nitrogen dual-doped porous hard carbon according to claim 8 or 9 as a negative electrode material in a battery or a capacitor.

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