Biomass carbon / sulfur composite positive electrode material and preparation method, lithium-sulfur battery and electrode sheet

Abstract

The application relates to the technical field of lithium-sulfur battery electrode materials, and particularly discloses a biomass carbon / sulfur composite positive electrode material, a preparation method thereof, a lithium-sulfur battery and an electrode sheet. Nitrogen and oxygen double-doped porous carbon is prepared by taking agricultural waste biomass as a carbon source, through hydrothermal carbonization and KOH activation, the formation of a hollow structure is realized by regulating the hydrothermal temperature and time, and the sulfur element is loaded by adopting a melting-diffusion method. The specific surface area of the obtained composite material is 850-1500 m 2 / g, the sulfur content is controlled in a range of 65-75 wt%, and the micropore volume ratio is not less than 30%. The material is mixed with conductive carbon black and a binder to prepare a positive electrode sheet, and the discharge specific capacity retention rate of the positive electrode sheet can reach more than 68% after being cycled for 200 times under a current density of 0.5 C, and the average coulombic efficiency is higher than 98.5%. The application has wide raw material sources, a simple process and application value in the field of electrochemical energy storage.

Description

Technical Field

[0001] This invention relates to the field of lithium secondary battery electrode materials technology, specifically to a biomass carbon / sulfur composite cathode material and its preparation method, a lithium-sulfur battery and electrode sheet. Background Technology

[0002] Lithium-sulfur batteries are considered an important development direction for the next generation of high-energy-density battery systems due to their advantages such as high theoretical energy density (2600 Wh / kg), low raw material cost, and environmental friendliness. However, this system faces several key bottlenecks in practical applications: first, the electronic insulation properties of elemental sulfur and the discharge product Li2S lead to low utilization of active materials; second, the volume expansion of sulfur (approximately 80%) during charging and discharging causes damage to the electrode structure; and third, the intermediate product lithium polysulfide is easily soluble in the electrolyte and migrates between the positive and negative electrodes, causing loss of active materials and corrosion of the lithium negative electrode.

[0003] Carbon materials are widely used as sulfur carriers due to their excellent electrical conductivity, tunable structure, and high chemical stability. The high specific surface area and abundant pores of porous carbon can achieve nanoscale confinement of sulfur, improving conductivity and suppressing volume expansion. Doping with heteroatoms (N, O, S, etc.) can introduce polar sites on the carbon framework surface, enhancing the chemisorption of polysulfides and mitigating the shuttle effect.

[0004] Biomass-based carbon materials have attracted research attention in recent years due to their advantages such as renewable raw materials, low cost, and the presence of nitrogen / oxygen elements. For example, porous carbon prepared from coconut shells and rice husks has been used in supercapacitors and batteries. However, existing biomass carbon / sulfur composites have the following problems: First, pore structure control is difficult; too many micropores limit sulfur loading, while too many mesopores result in poor confinement. Second, the amount and configuration of heteroatom doping are difficult to control, and some methods require the introduction of additional nitrogen / sulfur chemicals, increasing cost and process complexity. Third, there are significant discrepancies in reported electrochemical performance; some literature shows high specific capacity data at low sulfur loadings (<1 mg / cm³). 2 The electrolyte content is obtained under conditions of high electrolyte usage, but there is a significant gap between this and actual applications.

[0005] Chinese patent CN104052668A discloses a method for preparing porous carbon / sulfur composite materials using coconut shells as raw materials, employing ZnCl2 activation, but the resulting material has a low specific surface area (<600 m²). 2 The sulfur content is only about 50%. CN105226104A reported nitrogen-doped biomass carbon / sulfur materials, but melamine was used as an external nitrogen source and the pore structure distribution was not optimized, so the cycle stability still needs to be improved.

[0006] Therefore, developing a biomass carbon / sulfur composite material with simple processing, widely available raw materials, and stable electrochemical performance is of great significance for promoting the practical application of lithium-sulfur batteries. Summary of the Invention

[0007] The present invention aims to at least solve one of the technical problems existing in the prior art, and provides a biomass carbon / sulfur composite cathode material and its preparation method, a lithium-sulfur battery and electrode sheet.

[0008] In a first aspect, embodiments of the present invention provide a biomass carbon / sulfur composite cathode material modified with polar sites, comprising the following components: Biomass-based porous carbon framework with a specific surface area of ​​850-1500 m² 2 / g, pore volume 0.8-1.5 cm³ 3 / g, containing a two-level pore structure of micropores and mesopores, of which micropores account for 30-50% of the total pore volume and mesopores account for 40-60%; Polar heteroatom doping sites include pyridine-type nitrogen, pyrrole-type nitrogen, and carboxyl and hydroxyl oxygen-containing functional groups; wherein the nitrogen content is 3.0-6.5 wt% and the oxygen content is 4.0-8.0 wt%. Active sulfur, with a content of 65-75 wt%, is distributed in micropores and small mesopores.

[0009] Optionally, the raw material for the biomass-based porous carbon skeleton is one or more of coconut shells, rice husks, cotton, straw, and walnut shells.

[0010] Optionally, the composite cathode material is granular or sheet-like, with a particle size distribution of 1-50 μm, and has no obvious hollow structure or has a partial internal cavity structure.

[0011] Secondly, embodiments of the present invention provide a method for preparing the composite cathode material described above, comprising the following steps: (1) Raw material pretreatment: The biomass raw materials are washed with water and acid to remove ash, dried and then crushed through an 80-120 mesh sieve; (2) Hydrothermal carbonization: The treated biomass powder is mixed with water at a solid-liquid ratio of 1:8-1:15, placed in a reaction vessel, and reacted at 160-200℃ for 8-16 h. The product is washed with water and ethanol and then dried at 50-70℃. (3) KOH activation: Grind and mix hydrothermal carbon and KOH solid at a mass ratio of 1:3-1:5, heat to 750-850℃ at 3-5℃ / min under N2 atmosphere, keep at temperature for 1-3h, cool with furnace, wash with water until neutral, and dry at 90-110℃. (4) Sulfur loading: The activated carbon material is mixed with sublimed sulfur at a mass ratio of 1:3-1:4, ground, and then placed in a closed reactor. The mixture is heated at 145-160℃ for 12-20 h and then naturally cooled to obtain the final product.

[0012] Optionally, the hydrothermal reaction temperature in step (2) is 180℃ and the reaction time is 12 h.

[0013] Optionally, in step (3), the mass ratio of hydrothermal carbon to KOH is 1:4, the activation temperature is 800℃, and the activation time is 2 h.

[0014] Thirdly, embodiments of the present invention provide a positive electrode sheet for a lithium-sulfur battery, comprising the following components by mass percentage: The composite cathode material described above contains 80-85%... Conductive agent Super P 10-15%, Adhesive LA133 or PVDF 5-8%, The coating was applied to aluminum foil current collectors and dried under vacuum at 80℃ for 12 h, resulting in an areal density of 2.0-4.0 mg / cm³. 2 Sulfur surface loading 1.5-3.0 mg / cm³ 2 .

[0015] Fourthly, embodiments of the present invention provide a lithium-sulfur battery, wherein the positive electrode is the electrode sheet described above, the negative electrode is a lithium metal sheet, the separator is a Celgard 2325 polypropylene membrane, and the electrolyte is 1.0 M lithium bis(trifluoromethanesulfonyl)imide dissolved in a mixed solvent of 1,3-dioxolane / ethylene glycol dimethyl ether in a volume ratio of 1:1 to 1:3, with 0.5-1.0 wt% LiNO3 added as an additive.

[0016] Optionally, the first discharge specific capacity at 0.2 C rate is 950-1150 mAh / g, and the specific capacity retention rate after 150-200 cycles at 0.5 C rate is 65-75%.

[0017] Compared with existing technologies, the biomass carbon / sulfur composite cathode material, its preparation method, lithium-sulfur battery, and electrode sheet proposed in this invention have the following beneficial effects: (1) Raw materials and costs: Agricultural waste such as coconut shells, rice husks, and cotton are used. The raw materials are widely available and inexpensive (<5 yuan / kg). There is no need to add nitrogen-containing chemicals. Nitrogen doping is achieved by utilizing the protein in the biomass itself, which simplifies the process. However, it should be noted that the composition of different batches of raw materials (such as place of origin, season, and storage conditions) will lead to fluctuations in product performance. In actual production, raw material screening standards need to be established.

[0018] (2) Process Feasibility: Both hydrothermal carbonization and KOH activation are conventional chemical processes with low equipment requirements and are easy to scale up. However, the corrosion of the hydrothermal reactor and the strong alkalinity and high temperature operation during KOH activation require attention to safety precautions. The water washing step after activation generates wastewater containing KCl, which needs to be treated before discharge.

[0019] (3) Electrochemical performance: The first-cycle discharge specific capacity of the material of this invention at a 0.2 C rate is typically 950-1150 mAh / g, lower than the high values ​​reported in some literature (>1300 mAh / g). This is mainly because a sulfur loading (>1.5 mg / cm³) closer to practical applications is used. 2 It features a relatively low electrolyte content (E / S ratio of approximately 10-15). After 200 cycles at 0.5 C, it retains 65-75% of its specific capacity and has a coulombic efficiency of 98-99.5%, exhibiting above-average cycle stability. However, it should be noted that capacity decay is still significant after long-term cycling (>500 cycles), which is a common problem for lithium-sulfur batteries. Detailed Implementation

[0020] To enable those skilled in the art to better understand the technical solutions of the present invention, the present invention will be further described in detail below with reference to specific embodiments. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the described embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Unless otherwise specifically stated, the technical or scientific terms used in the embodiments of this invention should be understood in their ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains. The terms "comprising" or "including," as used in the embodiments of this invention, do not limit the shapes, numbers, steps, actions, operations, components, elements, and / or groups thereof mentioned, nor do they exclude the appearance or addition of one or more other different shapes, numbers, steps, actions, operations, components, elements, and / or groups thereof, or the inclusion of these.

[0022] In the description of the embodiments of the present invention, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the present invention. In the embodiments of the present invention, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in a suitable manner in any one or more embodiments or examples. Furthermore, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in the embodiments of the present invention, as well as the features of different embodiments or examples.

[0023] The following will describe in detail exemplary embodiments according to the present invention. Obviously, the described embodiments are merely some embodiments of the present invention, and not all embodiments of the present invention. It should be understood that the present invention is not limited to the exemplary embodiments described herein.

[0024] In a first aspect, embodiments of the present invention provide a biomass carbon / sulfur composite cathode material modified with polar sites, comprising: Biomass-based porous carbon framework with a specific surface area of ​​850-1500 m² 2 / g, pore volume 0.8-1.5 cm³ 3 / g contains a two-level pore structure of micropores (<2nm) and mesopores (2-50 nm), with micropores accounting for 30-50% of the total pore volume, mesopores accounting for 40-60%, and macropores (>50 nm) accounting for <15%. This pore distribution is beneficial for sulfur loading (micropores confine small sulfur molecules, while mesopores accommodate S8 cyclic molecules) and electrolyte wetting.

[0025] Polar heteroatom doping sites originate from the biomass feedstock itself and functional group transformations during hydrothermal and activation processes. These include pyridine nitrogen (binding energy 398.0-398.5 eV in XPS N1s spectrum), pyrrole nitrogen (399.5-400.5 eV), and oxygen-containing functional groups such as carboxyl groups (-COOH, 288.5-289.0 eV) and hydroxyl groups (-OH, 532.5-533.5 eV). The nitrogen content is 3.0-6.5 wt%, and the oxygen content is 4.0-8.0 wt%. These polar sites can be chemisorbed (mainly by Li) through chemical adsorption. + The coordination effect of the substance and the hydrogen bonding effect with polysulfide anions slow down the dissolution of polysulfides, but cannot completely suppress the shuttle effect. It needs to be used in conjunction with electrolyte additives.

[0026] Active sulfur, with a content of 65-75 wt%, is mainly distributed in micropores and small mesopores (<10 nm) channels, and exists in the form of S8 molecules. During the melt loading process, some sulfur may form CS bonds with carbon (the content is usually <0.5 wt%, and its contribution to capacity is limited).

[0027] Optionally, the biomass raw material is one or more selected from coconut shells, rice husks, cotton, straw, and walnut shells. Different raw materials have different ash contents and cellulose / lignin ratios, which will affect the yield, pore structure, and heteroatom content of the final carbon material. Coconut shells and walnut shells have high lignin content and good carbonization yields; rice husks and straw have high silicon content, requiring enhanced acid washing pretreatment; cotton has high cellulose purity but low nitrogen content.

[0028] The material appears as a black powder with irregular particle morphology and a particle size distribution of 1-50 μm, depending on the raw material and the degree of grinding. During the hydrothermal carbonization process, some particles may form preliminary cavity or fissure structures, but these are not strictly regular hollow structures, depending on the type of raw material and the hydrothermal conditions.

[0029] Secondly, the present invention provides a method for preparing the above-mentioned composite material, comprising: (1) Raw material pretreatment: The collected biomass raw materials are washed with deionized water to remove mud and sand, and then stirred with 1-2 M hydrochloric acid or nitric acid at 60℃ for 2-4 h to remove metal ions and ash (especially SiO2 in rice husks and straw). After filtration and washing with water until neutral, the raw materials are dried at 80-120℃, pulverized, and sieved through an 80-120 mesh sieve for later use. This step is very important for ensuring the purity and electrochemical stability of the carbon materials. Excessive ash content will lead to an increase in the irreversible capacity in the first cycle.

[0030] (2) Hydrothermal carbonization: Pretreated biomass powder is mixed with deionized water at a solid-liquid ratio of 1:8-1:15 (g:mL), ultrasonically dispersed for 30 min, and then transferred to a PTFE-lined stainless steel reactor. The filling degree is controlled below 70%, and the reaction is carried out at 160-200℃ for 8-16 h. During the hydrothermal process, hemicellulose and cellulose in the biomass undergo hydrolysis, dehydration, and condensation, while proteins decompose to produce nitrogen-containing functional groups, forming oxygen- and nitrogen-rich hydrothermal carbon microspheres or irregular particles. If the temperature is too low (<160℃) or the time is too short (<8 h), carbonization will be incomplete; if the temperature is too high (>200℃) or the time is too long (>16 h), the carbon skeleton will undergo excessive condensation, which is not conducive to subsequent activation and pore formation. The preferred conditions are a reaction at 180℃ for 12 h.

[0031] After the reaction is complete, the product is naturally cooled to room temperature. It is then washed three times with deionized water and once with ethanol to remove soluble impurities, and dried at 60°C. The hydrothermal carbon yield is typically 30-45 wt% (relative to dry basis feedstock), with a carbon content of approximately 60-70 wt%, an H / C atomic ratio of approximately 0.8-1.2, an O / C atomic ratio of approximately 0.3-0.5, and contains a relatively large number of oxygen-containing functional groups.

[0032] (3) KOH activation: Grind and mix hydrothermal carbon and solid KOH in an agate mortar at a mass ratio of 1:3-1:5 for 30 min, transfer to a corundum boat, place in a tube furnace, and heat to 750-850℃ at 3-5℃ / min under a nitrogen or argon atmosphere (flow rate 100-200 mL / min), maintain the temperature for 1-3 h, and then cool naturally to room temperature.

[0033] The activation mechanism mainly involves the reaction of KOH with carbon at high temperature: 6KOH + 2C → 2K + 3H2 + 2K2CO3. The generated K2CO3 further decomposes (K2CO3 → K2O + CO2), and CO2 and H2O also play a physical activation role at high temperature. The intercalation of metallic potassium can expand the carbon interlayer spacing, while salts such as K2CO3 act as templates occupying space, leaving pores after water washing. The KOH / C ratio is a key factor affecting the pore structure: activation is insufficient when the ratio is below 1:3, resulting in a specific surface area <800 m². 2 / g; a ratio higher than 1:5 leads to over-activation, excessive micropore development causing pore wall collapse, resulting in a low yield (<15%), and potentially creating too many large pores that reduce the sulfur confinement effect. Activation temperature and time also need to be matched; below 750℃, the reaction kinetics are slow, while above 850℃, micropores transform into mesopores accompanied by a sharp increase in burn-off rate. The preferred conditions are KOH / C = 1:4, activation at 800℃ for 2 h.

[0034] The activated product was stirred with 1 M dilute hydrochloric acid for 2 h to remove potassium salt (generating KCl), then washed with deionized water until the filtrate pH was neutral (approximately 5-6 washes), and dried at 100 °C for 12 h to obtain black activated porous carbon powder. The yield is typically 15-25 wt% (relative to hydrothermal carbon). BET analysis shows a specific surface area of ​​850-1500 m². 2 / g, micropore volume 0.3-0.6 cm³ 3 / g, mesoporous pore volume 0.4-0.8cm 3 / g.

[0035] Elemental analysis showed a nitrogen content of 3.0-6.5 wt% and an oxygen content of 4.0-8.0 wt%. Nitrogen primarily originates from proteins in the raw materials; some amino acids decompose during the hydrothermal process, and some nitrogen escapes as NH3 and HCN during high-temperature activation. The remaining nitrogen is embedded in the carbon skeleton as pyridine, pyrrole, and graphitic nitrogen. Oxygen originates from oxygen-containing functional groups in the hydrothermal carbon and carbonyl and hydroxyl groups adsorbed on the surface after activation.

[0036] (4) Sulfur Loading: Activated porous carbon and sublimed sulfur are ground and mixed in an agate mortar at a mass ratio of 1:3-1:4 (corresponding to a sulfur content of 60-75 wt%, with an actual sulfur content of approximately 65-75 wt% after considering loading efficiency) for 30 min. The mixture is then transferred to a sealed reactor (such as a sealed glass tube or a PTFE-lined reactor) and placed in a forced-air oven or tube furnace. The mixture is heated at 155℃ (the melting point of sulfur is approximately 115℃; 155℃ ensures sufficient melting and good fluidity) for 12-20 h. The molten sulfur enters the pores of the carbon material through capillary action and solidifies upon cooling. Excess sulfur adsorbed on the surface can be removed by heating the product at 200℃ for 1-2 h under an argon atmosphere or by washing with CS2. However, this step is usually omitted to avoid the toxicity of CS2 and the complexity of the treatment; the surface sulfur content is minimized only by controlling the carbon-sulfur ratio and melting time.

[0037] Thirdly, the present invention provides a positive electrode sheet for a lithium-sulfur battery, which is composed of the following components by mass percentage: The composite material comprises 80-85% conductive agent Super P, 10-15% binder, and 5-8% binder LA133 (water-based acrylic binder) or PVDF (polyvinylidene fluoride, soluble in NMP). The slurry solid content is controlled at 30-40%, and after coating, drying, and pressing, the areal density is 2.0-4.0 mg / cm³. 2 The corresponding sulfur surface loading is 1.5-3.0 mg / cm³. 2 The compacted density is 0.8-1.2 g / cm³. 3 The electrode fabrication process is compatible with conventional lithium-ion battery cathode processes and requires no special equipment.

[0038] Fourthly, the present invention provides a lithium-sulfur battery comprising: Positive electrode: The above-mentioned coated electrode sheet, a circular sheet with a diameter of 12-16 mm; Negative electrode: Lithium metal sheet (thickness 0.5-1.0 mm, diameter 14-16 mm), gently scrape off the surface oxide layer with a scraper before use; Membrane: Celgard 2325 or 2400 polypropylene microporous membrane (25 μm thick), or membrane modified with Al2O3 coating; Electrolyte: 1.0 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved in a mixed solvent of 1,3-dioxolane (DOL) / ethylene glycol dimethyl ether (DME) (volume ratio 1:1), with 1.0 wt% LiNO3 added as an additive. The electrolyte dosage is usually 15-25 μL / mg sulfur (E / S ratio, i.e., the mass ratio of electrolyte to sulfur).

[0039] Battery assembly was performed in an argon-filled glove box (water and oxygen content <1 ppm), using CR2032 or CR2025 button cell cases. Electrochemical tests were conducted after a 12-hour settling period.

[0040] The present invention will be described in detail below with reference to several embodiments.

[0041] Example 1: Coconut shell-based carbon / sulfur composite material (CS-S) Coconut shell fragments were washed with water, stirred in 1 M HCl at 60℃ for 3 h to remove ash, washed with water until neutral, dried at 120℃, and then pulverized through a 100-mesh sieve. 10 g of coconut shell powder was mixed with 100 mL of deionized water, sonicated for 30 min, and then transferred to a 200 mL hydrothermal reactor. The mixture was reacted at 180℃ for 12 h. The product was washed three times with water and once with ethanol, and then dried at 60℃ to obtain hydrothermal carbon (yield 38 wt%).

[0042] Hydrothermal carbon and KOH were ground and mixed at a mass ratio of 1:4. The mixture was heated to 800℃ at 3℃ / min under a N2 atmosphere (150 mL / min) and held at that temperature for 2 h, followed by natural cooling. The product was then acid-washed with 1 M HCl for 2 h, washed with water until neutral, and dried at 100℃ for 12 h to obtain activated porous carbon (yield 20 wt%, relative to hydrothermal carbon).

[0043] Activated carbon specific surface area 1180 m² 2 / g, micropore volume 0.45 cm³ 3 / g, mesoporous pore volume 0.52 cm³ 3 / g, nitrogen content 5.2 wt%, oxygen content 6.5 wt%.

[0044] Activated carbon and sublimed sulfur were mixed at a mass ratio of 27:73, ground, sealed, and heated at 155℃ for 16 h to obtain the CS-S composite material. The actual sulfur content was determined to be 70.5 wt% by thermogravimetric analysis (TGA, Ar atmosphere, heated to 600℃).

[0045] A slurry was prepared according to the composite material:Super P:LA133 mass ratio of 82:12:6, coated onto aluminum foil, vacuum dried at 80℃ for 12 h, and cut into 12 mm diameter discs with an areal density of 3.2 mg / cm³. 2 Sulfur loading 2.25 mg / cm³ 2 .

[0046] A CR2032 battery was assembled using lithium foil as the negative electrode, Celgard 2400 as the separator, and 1 M LiTFSI / (DOL:DME=1:1) + 1wt% LiNO3 as the electrolyte with an electrolyte volume of 50 μL (E / S ratio of approximately 11).

[0047] Electrochemical testing: With a charge / discharge voltage window of 1.7-2.8 V, at 0.2 C (1 C = 1675 mA / g sulfur), the initial discharge specific capacity was 1085 mAh / g, and the charging specific capacity was 1050 mAh / g, with an initial efficiency of 96.8%. After 200 cycles at 0.5 C, the discharge specific capacity was 735 mAh / g, with a retention rate (relative to 998 mAh / g in the second cycle) of 73.6%, and an average coulombic efficiency of 98.9%.

[0048] Rate performance: The discharge specific capacity at 0.1 C, 0.2 C, 0.5 C, 1 C and 2 C rates are 1150, 1085, 950, 820 and 680 mAh / g, respectively, and the capacity after restoring to 0.2 C is 1020 mAh / g.

[0049] Example 2: Rice husk-based carbon / sulfur composite material (RH-S) Rice husks have a high silicon content (approximately 15-20 wt% SiO2), so pretreatment needs to be strengthened: After washing the rice husks with water, stir them with 2 M HCl at 80℃ for 4 hours, filter and wash them with water, then stir them with 10 wt% HF at room temperature for 2 hours to remove silicon (HF is highly corrosive and must be operated in a fume hood with protective equipment worn), wash them with water until neutral, dry them, and pulverize them.

[0050] Hydrothermal conditions: reaction at 190℃ for 10 h. Activation conditions: KOH / C = 1:3.5, activation at 780℃ for 2.5 h. Other steps are the same as in Example 1.

[0051] Activated carbon specific surface area 1350 m² 2 / g, high micropore ratio (micropore volume 0.62 cm³) 3 / g, mesoporous pore volume 0.41cm 3 The composite material contains 4.1 wt% nitrogen and 7.2 wt% oxygen. The sulfur content is 68 wt%.

[0052] Battery testing: 1020 mAh / g at 0.2 C initial discharge, 68.2% capacity retention after 200 cycles at 0.5 C. Excessive micropores resulted in slightly poorer high-rate performance (only 720 mAh / g at 1 C), but the cycle stability was comparable to Example 1.

[0053] Example 3: Cotton-based carbon / sulfur composite material (CT-S) Degreased cotton was used as raw material. Hydrothermal conditions: reaction at 170℃ for 14 h (lower temperature to retain more fiber structure). Activation conditions: KOH / C = 1:5, activation at 820℃ for 1.5 h.

[0054] Activated carbon specific surface area 980 m² 2 The nitrogen content was low (2.8 wt%, due to the low protein content of cotton), and the oxygen content was 5.5 wt%. To increase the nitrogen content, NH3 was introduced during the activation process (flow rate 50 mL / min, treatment at 600℃ for 1 h), which increased the nitrogen content to 4.5 wt%, but the specific surface area decreased to 860 m². 2 / g.

[0055] Battery testing: The untreated sample had a first cycle strength of 980 mAh / g at 0.2 C and a retention rate of 65% after 200 cycles; the treated sample had a first cycle strength of 1050 mAh / g and a retention rate of 71% after 200 cycles, showing that nitrogen doping has an improving effect on cycle stability, but the improvement is limited.

[0056] Comparative Example 1: Coconut shells without hydrothermal activation (direct activation with KOH) Coconut shell powder was activated directly with KOH at a 1:4 ratio (800℃ × 2 h) without hydrothermal carbonization. The resulting carbon material had a specific surface area of ​​1650 m². 2 / g, but the micropore ratio is too high (>70%), the pore volume distribution is not conducive to sulfur loading, and the surface functional groups are few (nitrogen content 1.2 wt%, oxygen content 3.5 wt%).

[0057] After sulfur loading, the initial capacity at 0.2 C was 1120 mAh / g, but the cycling performance was poor, with the capacity decaying to below 400 mAh / g after 50 cycles. This indicates that hydrothermal pretreatment is crucial for controlling pore structure and retaining heteroatoms.

[0058] Comparative Example 2: Hydrothermal carbonization without activation (HTC-S) Coconut shell hydrothermal carbon directly loads sulfur without KOH activation. Its specific surface area is only 45 m². 2 The sulfur content is limited by the pore volume (<40 wt%), and the initial capacity at 0.2 C is only 650 mAh / g, with severe polarization. This indicates that pore formation activation is a necessary step to obtain high capacity.

[0059] Comparative Example 3: High Sulfur Loading Test (CT-S-High) Example 3: Material preparation of electrode sheets, increasing areal density to 5.5 mg / cm³ 2 Sulfur loading 3.8 mg / cm³ 2The initial capacity at 0.2 C was 4.1 mAh / cm², but it decreased significantly after 50 cycles, and the capacity retention was only 45% after 200 cycles. This indicates that the current material system is suitable for medium sulfur loading (<3 mg / cm²). 2 For high loads, further optimization of the pore structure or electrode design is required.

[0060] This invention relates to a biomass carbon / sulfur composite cathode material and its preparation method, as well as a lithium-sulfur battery and electrode sheet. The method for preparing nitrogen-oxygen-doped porous carbon / sulfur composite materials using agricultural waste biomass as raw material achieves the control of pore structure and heteroatom doping through the synergistic effect of hydrothermal carbonization and KOH activation. The resulting material exhibits above-average electrochemical performance in lithium-sulfur batteries. The process is simple, the raw materials are readily available, and it is suitable for large-scale production. However, there is still room for performance improvement, such as further controlling the macroporous structure to alleviate volume expansion, introducing catalytic sites to promote polysulfide conversion, and optimizing the electrode structure to increase sulfur loading. These can be considered as directions for future improvements.

[0061] It is understood that the above embodiments are merely exemplary implementations used to illustrate the principles of the present invention, and the present invention is not limited thereto. For those skilled in the art, various modifications and improvements can be made without departing from the spirit and essence of the present invention, and these modifications and improvements are also considered to be within the scope of protection of the present invention.

Claims

1. A biomass carbon / sulfur composite cathode material modified with polar sites, characterized in that, Includes the following components: Biomass-based porous carbon framework with a specific surface area of ​​850-1500 m² 2 / g, pore volume 0.8-1.5 cm³ 3 / g, containing a two-level pore structure of micropores and mesopores, of which micropores account for 30-50% of the total pore volume and mesopores account for 40-60%; Polar heteroatom doping sites include pyridine-type nitrogen, pyrrole-type nitrogen, and carboxyl and hydroxyl oxygen-containing functional groups; wherein the nitrogen content is 3.0-6.5 wt% and the oxygen content is 4.0-8.0 wt%. Active sulfur, with a content of 65-75 wt%, is distributed in micropores and small mesopores.

2. The composite cathode material according to claim 1, characterized in that, The raw materials for the biomass-based porous carbon skeleton are one or more of coconut shells, rice husks, cotton, straw, and walnut shells.

3. The composite cathode material according to claim 1, characterized in that, The composite cathode material is granular or sheet-like, with a particle size distribution of 1-50 μm, and has no obvious hollow structure or has a partial internal cavity structure.

4. A method for preparing the composite cathode material according to any one of claims 1-3, comprising the following steps: (1) Raw material pretreatment: The biomass raw materials are washed with water and acid to remove ash, dried and then crushed through an 80-120 mesh sieve; (2) Hydrothermal carbonization: The treated biomass powder is mixed with water at a solid-liquid ratio of 1:8-1:15, placed in a reaction vessel, and reacted at 160-200℃ for 8-16 h. The product is washed with water and ethanol and then dried at 50-70℃. (3) KOH activation: Grind and mix hydrothermal carbon and KOH solid at a mass ratio of 1:3-1:5, heat to 750-850℃ at 3-5℃ / min under N2 atmosphere, keep at temperature for 1-3h, cool with furnace, wash with water until neutral, and dry at 90-110℃. (4) Sulfur loading: The activated carbon material is mixed with sublimed sulfur at a mass ratio of 1:3-1:4, ground, and then placed in a closed reactor. The mixture is heated at 145-160℃ for 12-20 h and then naturally cooled to obtain the final product.

5. The preparation method according to claim 4, characterized in that, In step (2), the hydrothermal reaction temperature is 180℃ and the reaction time is 12 h.

6. The preparation method according to claim 4, characterized in that, In step (3), the mass ratio of hydrothermal carbon to KOH is 1:4, the activation temperature is 800℃, and the activation time is 2 h.

7. A positive electrode sheet for a lithium-sulfur battery, characterized in that, Includes the following components by mass percentage: The composite cathode material according to any one of claims 1-3 accounts for 80-85%. Conductive agent Super P 10-15%, Adhesive LA133 or PVDF 5-8%, The coating was applied to aluminum foil current collectors and dried under vacuum at 80℃ for 12 h, resulting in an areal density of 2.0-4.0 mg / cm³. 2 Sulfur surface loading: 1.5-3.0 mg / cm³ 2 .

8. A lithium-sulfur battery, characterized in that, The positive electrode is the electrode sheet as described in claim 7, the negative electrode is a lithium metal sheet, the separator is a Celgard 2325 polypropylene membrane, and the electrolyte is 1.0 M lithium bis(trifluoromethanesulfonyl)imide dissolved in a mixed solvent of 1,3-dioxolane / ethylene glycol dimethyl ether in a volume ratio of 1:1 to 1:3, with 0.5-1.0 wt% LiNO3 added as an additive.

9. The lithium-sulfur battery according to claim 8, characterized in that, The first discharge specific capacity at 0.2 C rate is 950-1150 mAh / g, and the specific capacity retention rate after 150-200 cycles at 0.5 C rate is 65-75%.