A porous carbon, a method for preparing the same and an application thereof in lithium-sulfur batteries

Abstract

The present application relates to the technical field of lithium-sulfur batteries, in particular to a kind of porous carbon and its preparation method and application in lithium-sulfur battery.The porous carbon has micropore, mesopore and macropore, and presents continuous intercommunication honeycomb structure, and there is connecting window between pore wall, the pore volume ratio of the pore with pore diameter <5nm is 50%~75%, the pore volume ratio of the pore with pore diameter of 5~55nm is 10%~30%, and the pore volume ratio of the pore with pore diameter >55nm is 10%~30%.The porous carbon obtained by using sodium polyacrylate and sucrose as carbon source and soft template, potassium chloride as hard template, through liquid phase mixing, vacuum freeze drying, high-temperature carbonization and then using alkali etching, has three-dimensional intercommunication multi-level pore structure, and pore size distribution is reasonable, can effectively solve the problems of limited sulfur loading in existing lithium-sulfur battery positive material, structure collapse caused by volume expansion and cycle life attenuation caused by polysulfide shuttle effect, and improve the electrochemical performance of lithium-sulfur battery.

Description

Technical Field

[0001] This invention relates to the field of lithium-sulfur battery technology, and more specifically, to a porous carbon, its preparation method, and its application in lithium-sulfur batteries. Background Technology

[0002] Lithium-sulfur batteries, due to their high theoretical specific capacity and high energy density, are considered strong contenders for next-generation high-energy storage systems. However, their actual industrialization has long been hampered by a series of key technical challenges arising from the inherent properties of sulfur and its reaction intermediates. On one hand, the soluble polysulfides (Li₂S₂) generated during charging and discharging... n Sulfur (4≤n≤8) is prone to migration in the electrolyte, and traditional commercial polyolefin separators cannot effectively block its diffusion, causing a serious "shuttle effect" that leads to irreversible loss of active material sulfur, rapid capacity decay and coulombic efficiency reduction. On the other hand, the volume expansion rate between sulfur and the final discharge product Li2S is as high as 80%. Repeated volume expansion and contraction will destroy the integrity of the electrode structure, causing the conductive network to collapse and the active material to fall off, which seriously affects the cycle stability.

[0003] To overcome the aforementioned technical bottlenecks, electrode material and separator modification have become research hotspots. Among them, porous carbon materials, due to their high specific surface area, good conductivity, and tunable pore structure, are widely used as cathode carriers or separator modification materials for lithium-sulfur batteries. However, existing porous carbon materials often struggle to balance specific surface area, polysulfide confinement capability, transport performance, and structural stability, failing to simultaneously meet the multiple requirements of high active site exposure, effective polysulfide confinement, rapid ion / electron transport, and volume expansion buffering. Furthermore, conventional preparation methods are insufficient to obtain a three-dimensionally interconnected, wide-distribution hierarchical pore structure with suitable pore size gradation, making it difficult to effectively improve the electrochemical performance of lithium-sulfur batteries.

[0004] Therefore, developing a porous carbon with controllable pore size distribution and capable of achieving three-dimensional interconnected multi-level pore synergistic construction, and its preparation method, to fully utilize the multiple functions of porous carbon in lithium-sulfur batteries, is of great significance to the development of lithium-sulfur batteries.

[0005] In view of this, the present invention is hereby proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a porous carbon, its preparation method, and its application in lithium-sulfur batteries. The porous carbon provided by this invention has a three-dimensional interconnected hierarchical porous structure, which can provide high specific surface area and active sites, effectively confine polysulfides and ensure ion transport, while also ensuring sulfur loading and providing buffer space for volume expansion. This effectively solves the problems of limited sulfur loading, structural collapse due to volume expansion, and cycle life degradation caused by polysulfide shuttle effect in existing lithium-sulfur battery cathode materials.

[0007] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: A porous carbon having micropores, mesopores and macropores, and having a continuous and interconnected honeycomb structure with connecting windows between the pore walls, wherein the pore volume ratio of pores with a diameter < 5 nm is 50%~75%, the pore volume ratio of pores with a diameter of 5~55 nm is 10%~30%, and the pore volume ratio of pores with a diameter > 55 nm is 10%~30%.

[0008] Preferably, the cumulative pore volume of the porous carbon is 0.08~0.3 cm³. 3 g -1 Furthermore, the pore size distribution curve for pores with diameters ranging from 5 to 100 nm has a dv / dw ratio < 0.003 cm. 3 g -1 nm -1 .

[0009] Preferably, the porous carbon has a BET specific surface area of ​​200-400 m². 2 g -1 .

[0010] A method for preparing porous carbon according to any one of the foregoing embodiments includes the following steps: S1. Dissolve sodium polyacrylate and sucrose in water and mix with KCl, then freeze-dry under vacuum to obtain a precursor dry gel; S2. The precursor dry gel is pretreated and carbonized under an inert atmosphere to obtain a primary carbonized product containing a salt template; S3. The primary carbonization product is washed to remove the KCl template and dried to obtain a porous carbon intermediate; S4. The porous carbon intermediate is mixed with an alkali, including KOH and / or NaOH, and then heated and activated under an inert atmosphere. After washing and drying, the porous carbon is obtained.

[0011] Preferably, in step S1, the mass ratio of sodium polyacrylate to sucrose is 1:1 to 9:1.

[0012] Preferably, in step S1, the mass ratio of sodium polyacrylate to KCl is 1:1 to 1:7.

[0013] Preferably, in step S1, the molecular weight of the sodium polyacrylate is 4 million to 5 million.

[0014] Preferably, in step S1, the temperature of the vacuum freeze drying is -40 to -80°C, and the drying time is 24 to 48 hours.

[0015] Preferably, in step S2, the temperature of the heating pretreatment is 400~500℃ and the pretreatment time is 0.5~1.5h; the temperature of the carbonization is 700~900℃ and the holding time is 2~4h.

[0016] Preferably, in step S4, the mass ratio of the porous carbon intermediate to the alkali is 1:2 to 1:6.

[0017] Preferably, in step S4, the temperature of the heating activation treatment is 700~900℃, and the holding time is 1~2h.

[0018] A lithium-sulfur battery cathode material comprising porous carbon and sulfur supported on the porous carbon; The porous carbon is the porous carbon described in any one of the foregoing embodiments or the porous carbon prepared by any one of the foregoing embodiments.

[0019] A lithium-sulfur battery, comprising the lithium-sulfur battery cathode material described in the foregoing embodiments.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The porous carbon provided by the present invention has a continuous and interconnected honeycomb structure, and there are a large number of connection windows between the pore walls, forming a three-dimensional interconnected multi-level pore structure. The pore size distribution range is wide and the pore volume ratio distribution is reasonable. While effectively confining polysulfides and ensuring ion transport, it also has a high specific surface area, active sites and sulfur loading, and provides an isotropic buffer space for sulfur volume expansion. It can effectively solve the problems of limited sulfur loading, structural collapse caused by volume expansion and cycle life decay caused by polysulfide shuttle effect in existing lithium-sulfur battery cathode materials, and improve the cycle performance, rate performance and capacity of lithium-sulfur batteries.

[0021] (2) The method of the present invention uses sodium polyacrylate (PAAS) and sucrose as carbon sources and soft templates, and potassium chloride as a hard template to prepare porous carbon intermediates. Then, porous carbon for lithium-sulfur batteries is obtained by etching with alkali (KOH and / or NaOH). Through the dynamic synergistic process of salt template crystallization and dual carbon source phase separation, and the synergistic effect of KOH etching to open up pore channels and expand pores, a three-dimensional interconnected multi-level pore structure with micropores, mesopores and macropores is formed. The pore size distribution (the pore volume ratio distribution of pores with different pore sizes) is reasonable. Micropores and small mesopores provide high specific surface area and active sites. Mesopores realize the effective confinement of polysulfides and ensure ion transport. Macropores accommodate high sulfur loading and provide buffer space for volume expansion. Thus, a balance is achieved between specific surface area, polysulfide confinement ability, transport performance and structural stability, thereby improving the electrochemical performance of lithium-sulfur batteries. Meanwhile, sucrose, as a small molecule carbon source, fills and cross-links the gaps in the long-chain backbone of PAAS, preventing the backbone from collapsing when removing the KCl template or etching with KOH, thus improving the mechanical strength and tap density of the material. This reinforced structure enables the carbon precursor to withstand the subsequent severe chemical etching by alkali, avoiding the collapse of the carbon backbone during the etching process. Attached Figure Description

[0022] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0023] Figure 1 The XRD patterns of the porous carbon frameworks prepared in Examples 1-3 of this invention; Figure 2 SEM images of the porous carbon frameworks prepared in Examples 1-3 of this invention (where a is Example 1, b is Example 2, and c is Example 3). Figure 3 SEM images of the porous carbon frameworks prepared in Comparative Examples 1-4 of this invention (where a is Comparative Example 1, b is Comparative Example 2, c is Comparative Example 3, and d is Comparative Example 4). Figure 4 The images show the nitrogen adsorption-desorption curves of the porous carbon frameworks prepared in Examples 1-3 of this invention. Figure 5 The images are pore size distribution curves obtained by DFT model calculation of the porous carbon frameworks prepared in Examples 1-3 of this invention; Figure 6 The images show the cumulative pore volume of the porous carbon frameworks prepared in Examples 1-3 of this invention, calculated using a DFT model. Figure 7 The rate performance of batteries assembled using the porous carbon framework prepared in Examples 1-3 of this invention as the positive electrode material carrier; Figure 8 The cycle performance of batteries assembled using the porous carbon framework prepared in Examples 1-3 of this invention as the cathode material carrier is shown at 0.5C. Figure 9 The rate performance of batteries assembled using the porous carbon framework prepared in Comparative Examples 1-4 of this invention as the positive electrode material carrier is shown. Figure 10 The cycling performance at 0.5C is shown for batteries assembled using the porous carbon framework prepared in Comparative Examples 1-4 of this invention as the cathode material carrier. Detailed Implementation

[0024] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the 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. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0025] The first aspect of the present invention provides a porous carbon having micropores, mesopores, and macropores, and exhibiting a continuous and interconnected honeycomb structure with connecting windows between the pore walls. The pore volume ratio of pores with a diameter < 5 nm is 50% to 75%, for example, it can be any single value or a range of any two values ​​from 50%, 55%, 60%, 65%, 70%, and 75%; the pore volume ratio of pores with a diameter of 5 to 55 nm is 10% to 30%, for example, it can be any single value or a range of any two values ​​from 10%, 14%, 20%, 26%, and 30%; and the mesopores are continuously varying without any particular size dominating; the pore volume ratio of pores with a diameter > 55 nm is 10% to 30%, for example, it can be any single value or a range of any two values ​​from 10%, 15%, 20%, 24%, and 30%.

[0026] The porous carbon provided by this invention has a three-dimensional interconnected hierarchical pore structure. Micropores and small mesopores of <5nm can provide high specific surface area, increase reactive sites, and help improve initial capacity. Pores of 5~55nm provide an ideal space for confining polysulfides, effectively suppressing the shuttle effect, while providing good ion transport channels and improving the kinetic performance of electrochemical reactions. Macropores of >55nm can accommodate more sulfur active materials and provide buffer space for sulfur volume expansion, maintaining the stability of the electrode structure. The synergistic effect of the hierarchical pores can effectively solve the problems of limited sulfur loading, structural collapse caused by volume expansion, and cycle life decay caused by polysulfide shuttle effect in existing lithium-sulfur battery cathode materials, thereby improving the electrochemical performance of lithium-sulfur batteries.

[0027] However, if the proportion of micropores is too high, on the one hand, polysulfides have difficulty entering the pores and tend to accumulate on the surface of the carbon material, dissolving upon contact with the electrolyte and triggering a shuttle effect; on the other hand, the strong capillary action of micropores leads to excessive electrolyte consumption, reducing energy density. If the proportion of micropores is too low, the specific surface area decreases, reducing active sites and hindering the improvement of initial capacity. If the proportion of macropores is too high, the electron transport distance increases, causing some sulfur to move away from the conductive carbon wall, easily forming "dead sulfur," making it difficult to fully utilize the capacity; if the proportion of macropores is too low, the loading of active sulfur is limited and it is not conducive to volume expansion buffering. The porous carbon of this invention has a reasonable pore size distribution, which can avoid the above defects and has a better overall effect.

[0028] In some specific embodiments of the present invention, the cumulative pore volume of the porous carbon is 0.08~0.3 cm³. 3 g -1 For example, it can be 0.08cm 3 g -1 0.098cm 3 g -1 0.1cm 3 g -1 0.135cm 3 g -1 0.2cm 3 g -1 0.234cm 3 g -1 0.3cm 3 g -1 The value at any single point or the range of values ​​formed by any two points in the curve; the dv / dw of the pore size distribution curve for pores with a diameter of 5~100nm is <0.003cm. 3 g -1 nm -1This characteristic indicates that the material does not have sharp pore size peaks in this scale range, but rather exhibits a smooth and continuous broad spectrum distribution. This non-specific broad distribution is beneficial for the uniform loading of sulfur and the isotropic buffering of volume expansion in a wide scale range.

[0029] In some specific embodiments of the present invention, the BET specific surface area of ​​porous carbon is 200~400 m². 2 g -1 For example, it can be 200m 2 g -1 250m 2 g -1 300m 2 g -1 350m 2 g -1 400m 2 g -1 The range of values ​​consisting of any one point or any two points in the range.

[0030] A second aspect of the present invention provides a method for preparing porous carbon according to any one of the foregoing embodiments, comprising the following steps: S1. Dissolve sodium polyacrylate and sucrose in water and mix with KCl, then freeze-dry under vacuum to obtain a precursor dry gel; S2. The precursor dry gel is pretreated and carbonized under an inert atmosphere to obtain a primary carbonized product containing a salt template. S3. The primary carbonization product is washed to remove the KCl template and dried to obtain a porous carbon intermediate; S4. The porous carbon intermediate is mixed with an alkali and heated for activation under an inert atmosphere. After washing and drying, porous carbon is obtained. The alkali used includes KOH and / or NaOH.

[0031] This invention uses sodium polyacrylate and sucrose as carbon sources and soft templates, and potassium chloride as an inorganic salt hard template. Porous carbon materials are obtained through liquid-phase mixing, vacuum freeze-drying, high-temperature carbonization, and then alkaline etching. It provides a porous carbon framework preparation technology with controllable pore size, low cost, and excellent performance. Through the dynamic synergistic process of salt template crystallization and dual carbon source phase separation, as well as the synergistic effect of KOH etching to open and expand pore channels, a three-dimensional interconnected hierarchical porous structure with micropores, mesopores, and macropores is formed. The pore size distribution is reasonable, effectively solving the problems of limited sulfur loading, structural collapse due to volume expansion, and cycle life degradation caused by polysulfide shuttle effects in existing lithium-sulfur battery cathode materials.

[0032] This invention utilizes the synergistic pore-forming effect of dual carbon sources and a potassium chloride hard template to achieve a hierarchical effect between macropores and micropores. Sucrose, as a small-molecule carbon source, fills and cross-links the gaps in the long-chain backbone of PAAS, preventing backbone collapse during KCl template removal or KOH etching, thus improving the material's mechanical strength and tap density. This reinforced structure allows the carbon precursor to withstand the subsequent severe chemical etching by KOH, avoiding backbone collapse during pore expansion and successfully achieving precise control of the pore radial diameter within the range of 50–100 nm. Furthermore, sodium polyacrylate contains negatively charged -COO... - And K + Positively charged, they attract each other in water (electrostatic chelation). This "ion coupling" strategy ensures that KCl crystal nuclei are uniformly distributed in the polymer matrix during subsequent drying and carbonization. The in-situ framework support of KCl crystals during high-temperature carbonization prevents pore collapse caused by polymer carbonization shrinkage, reserving sufficient mechanical buffer space for the volume expansion of active sulfur during charge and discharge. KOH etching opens up the originally closed micropores and expands the diameter of the originally smaller micropores and mesopores to the 50-100 nm range, broadening the pore distribution. When used in lithium-sulfur batteries, it can effectively improve the electrochemical performance of lithium-sulfur batteries.

[0033] Furthermore, the traditional silica hard template method requires the use of highly corrosive hydrofluoric acid to etch and remove the template in the post-processing stage. This is not only dangerous to operate, but also generates fluorine-containing waste liquid that imposes a serious burden on the environment, making it difficult to meet the needs of large-scale green chemistry production. The method of this invention only requires water washing or acid washing to completely remove the KCl template and residual alkali in the post-processing stage, without generating any toxic or harmful byproducts.

[0034] In some specific embodiments of the present invention, the mass ratio of sodium polyacrylate to sucrose in step S1 is 1:1 to 9:1. For example, it can be any one value or a range of any two values ​​from 1:1, 3:1, 5:1, 7:1, to 9:1.

[0035] In some specific embodiments of the present invention, the mass ratio of sodium polyacrylate to KCl in step S1 is 1:1 to 1:7. For example, it can be any one value or a range of any two values ​​from 1:1, 1:3, 1:5, 1:7.

[0036] In some specific embodiments of the present invention, the molecular weight of sodium polyacrylate in step S1 is 4 million to 5 million. By synergistically controlling the molecular weight of sodium polyacrylate and the type of inorganic salt template agent, the pore structure of porous carbon can be controlled. When the molecular weight of sodium polyacrylate is 4 million to 5 million, KCl can be used as an inorganic salt template agent to obtain the pore structure required by the present invention, while NaCl is difficult to obtain an ideal pore structure.

[0037] In some specific embodiments of the present invention, the temperature of vacuum freeze drying in step S1 is -40 to -80°C, for example, it can be any one value or a range of any two values ​​among -40°C, -50°C, -60°C, -70°C, and -80°C; the vacuum freeze drying time is 24 to 48 hours, for example, it can be any one value or a range of any two values ​​among 24 hours, 30 hours, 36 hours, and 48 hours.

[0038] In some specific embodiments of the present invention, the heating pretreatment temperature in step S2 is 400~500℃, for example, it can be any one value or a range of any two values ​​among 400℃, 420℃, 450℃, 480℃, and 500℃; the pretreatment time is 0.5~1.5h, for example, it can be any one value or a range of any two values ​​among 0.5h, 0.8h, 1h, 1.2h, and 1.5h; the carbonization temperature is 700~900℃, for example, it can be 7 The temperature can be any one value or a range of any two values ​​from 00℃, 750℃, 800℃, 850℃, and 900℃; the holding time can be 2~4h, for example, any one value or a range of any two values ​​from 2h, 2.5h, 3h, 3.5h, and 4h; the heating rate of the pretreatment and carbonization process can be 5~8℃ / min, for example, any one value or a range of any two values ​​from 5℃ / min, 6℃ / min, 7℃ / min, and 8℃ / min.

[0039] In some specific embodiments of the present invention, in step S2, the inert gas flow rate is 40~60 mL / min (for example, it can be any one value or a range of any two values ​​among 40 mL / min, 45 mL / min, 50 mL / min, 55 mL / min, and 60 mL / min), and the gas is continuously circulated for 30~60 min (for example, it can be any one value or a range of any two values ​​among 30 min, 40 min, 50 min, and 60 min). Then, the temperature is raised, and after the temperature is raised to the pretreatment temperature, it is kept at that temperature. After the temperature is kept at that temperature, the temperature is raised to the carbonization temperature and kept at that temperature for carbonization. The inert gas is continuously circulated throughout the entire process.

[0040] In some specific embodiments of the present invention, the mass ratio of porous carbon intermediate to alkali in step S4 is 1:2 to 1:6. For example, it can be any one value or any two values ​​from 1:2, 1:3, 1:4, 1:5, 1:6.

[0041] In some specific embodiments of the present invention, the temperature of the heating activation treatment in step S4 is 700~900℃, for example, it can be any one value or a range of any two values ​​among 700℃, 750℃, 800℃, 850℃, and 900℃; the heat preservation time is 1~2h, for example, it can be any one value or a range of any two values ​​among 1h, 1.2h, 1.5h, 1.8h, and 2h.

[0042] In some specific embodiments of the present invention, the washing in step S4 includes acid washing and / or water washing until the pH of the filtrate is neutral, and the drying time is 12-24 hours.

[0043] A third aspect of the present invention provides a lithium-sulfur battery cathode material comprising porous carbon and sulfur supported on the porous carbon; The porous carbon is the porous carbon described in any one of the foregoing embodiments or the porous carbon prepared by any one of the foregoing embodiments.

[0044] As an example, methods for preparing cathode materials for lithium-sulfur batteries include: Porous carbon and sublimed sulfur were placed in a hydrothermal reactor for melt diffusion for 12-15 hours. After complete diffusion, the resulting product was dispersed in NMP with conductive carbon black and binder PVDF to form a uniformly mixed slurry. The slurry was then coated onto aluminum foil using a coating machine. After vacuum drying and the NMP volatilization, lithium-sulfur battery cathode material was obtained.

[0045] In some specific embodiments of the present invention, the mass ratio of porous carbon, conductive carbon black (SuperP), and PVDF is 8~7:2~1:1~0.5; as an example, the amounts can be 80~70mg, 20~10mg, and 10~5mg respectively, and the amount of NMP solvent used is 8~12mL.

[0046] The porous carbon framework material prepared in this invention is coated onto an aluminum foil current collector to prepare a lithium-sulfur battery cathode, and its core advantages are as follows: 1. By utilizing the in-situ framework support of KCl crystals during the high-temperature carbonization process, the collapse of pores caused by polymer carbonization shrinkage is prevented, and sufficient mechanical buffer space is reserved for the volume expansion of active sulfur during charging and discharging, which significantly improves the integrity of the electrode structure. 2. During carbonization, sucrose molecules melt and penetrate into the molecular chains of sodium polyacrylate (PAAS), forming a dense carbon-filled phase after carbonization, effectively increasing the wall thickness and mechanical strength of the carbon skeleton. This reinforced structure enables the carbon precursor to withstand the severe chemical etching of subsequent alkalis (KOH and / or NaOH), avoiding skeleton collapse during pore expansion. This successfully achieves precise control of the pore radial diameter within the range of 50-100 nm, broadening the pore size distribution of porous carbon. While suppressing polysulfide shuttle, it increases the sulfur loading and provides an isotropic buffer space for sulfur volume expansion, solving the problems of limited sulfur loading, structural collapse due to volume expansion, and cycle life degradation caused by polysulfide shuttle effect in existing lithium-sulfur battery cathode materials. 3. By utilizing an oxygen-rich carbon framework derived from sodium polyacrylate and sucrose to achieve chemical adsorption of polysulfides, combined with the physical confinement of hierarchical pores, the shuttle effect was effectively suppressed. The prepared lithium-sulfur battery achieved capacities of 1182 mAh g at 0.1C, 0.2C, 0.5C, 1C, and 2C. -1 998.2 mAh g -1 887.7mAh g -1 796.4mAh g -1 and 751.1mAh g -1 This demonstrates that the lithium-sulfur battery can maintain a high specific capacity at high current densities.

[0047] A fourth aspect of the present invention provides a lithium-sulfur battery, comprising the lithium-sulfur battery cathode material described in the foregoing embodiments.

[0048] The following detailed description of some embodiments of the present invention is provided in conjunction with specific examples. Unless otherwise specified, all raw materials used in the examples are commercially available. The molecular weight of sodium polyacrylate used in the examples and comparative examples is 4 million to 5 million.

[0049] Example 1 S1. Accurately weigh 0.5g sodium polyacrylate, 0.5g sucrose, and 1.5g potassium chloride in a mass ratio of 1:1:3, and add them together to 50mL of deionized water. Place the mixture on a magnetic stirrer and stir at room temperature for 12h to ensure that the sodium polyacrylate and sucrose are completely dissolved and the potassium chloride is uniformly dispersed to form a homogeneous and stable mixed solution. Pour the above mixed solution into a petri dish and place it in a vacuum freeze dryer. Freeze-dry at -40℃ and 1Pa for 24h to completely remove moisture and obtain a dry solid precursor (precursor dry gel). S2. The dried precursor was ground into powder and placed in a ceramic crucible in a tube furnace. Argon gas was introduced as a protective gas to purge the air from the furnace (gas flow rate 50 mL / min, for 30 min). The temperature was increased to 400℃ at a rate of 5℃ / min and held for 1 h. Then, the temperature was increased to 800℃ at a rate of 5℃ / min and held for 2 h for carbonization. After calcination, the product was allowed to cool naturally to room temperature to obtain a black solid product (a primary carbonized product containing a salt template). S3. Transfer the calcined product to a beaker, add 100 mL of deionized water, wash and dry for 24 h to obtain a porous carbon intermediate; S4. The obtained porous carbon intermediate is mixed with KOH solid at a mass ratio of 1:3, ground evenly in a mortar, and then placed in a tube furnace again and calcined at 800℃ for 1h in Ar gas; the activated product is acid washed until pH=7, then filtered, and the filtered filter cake is placed in a vacuum drying oven and dried at 60℃ for 12h. After grinding, the porous carbon material is obtained.

[0050] Example 2 Example 2 is similar to Example 1, except that the amount of potassium chloride used in step S1 is 2.5g, the mass ratio of sodium polyacrylate, sucrose and potassium chloride is 1:1:5, and all other conditions are the same as in Example 1.

[0051] Example 3 Example 3 is similar to Example 1, except that the amount of potassium chloride used in step S1 is 3.5g, the mass ratio of sodium polyacrylate, sucrose and potassium chloride is 1:1:7, and all other conditions are the same as in Example 1.

[0052] Comparative Example 1 Comparative Example 1 is similar to Example 2, except that no sucrose was added in step S1, and all other conditions were the same as in Example 2.

[0053] Comparative Example 2 Comparative Example 2 is similar to Example 2, except that in step S1, vacuum freeze drying is replaced with vacuum drying. The mixed solution is placed in a vacuum dryer and vacuum dried at 60°C and 100Pa for 24 hours. All other conditions are the same as in Example 2.

[0054] Comparative Example 3 Comparative Example 3 is similar to Example 2, except that step S4 was not performed, and all other conditions are the same as in Example 2.

[0055] Comparative Example 4 Comparative Example 4 is similar to Example 2, except that 0.5g of sucrose is replaced with 0.5g of polyvinyl alcohol, and all other conditions are the same as in Example 2.

[0056] Test case (a) Product characterization XRD tests were performed on Examples 1, 2, and 3, as follows: Figure 1 As shown, no characteristic diffraction peaks of potassium chloride or other impurities were detected in the spectrum, indicating that pure amorphous carbon can be obtained through the above steps.

[0057] The SEM test results of Examples 1-3 are as follows: Figure 2 As shown in the figure, a three-dimensional interconnected porous structure is presented, with interconnected channels forming a continuous pore network. There are no obvious agglomerations or closed pores, ensuring the continuity of mass transport. Large cavities (such as dark areas) are clearly visible in the figure, which can quickly accommodate electrolyte and reduce the macroscopic resistance to ion diffusion.

[0058] like Figure 3 As shown in Figure a, the sample of Comparative Example 1 without sucrose as a carbon source could not form a continuous carbon skeleton, and the carbon skeleton collapsed significantly. Only scattered carbon-based fragments could be observed, with no effective connection structure between the fragments. In some areas, there was even agglomeration and stacking, and the pore structure almost completely disappeared.

[0059] like Figure 3 As shown in Figure b, in Comparative Example 2, no vacuum freeze-drying was used. Due to the different crystallization points of the salt template and the mixed carbon source, the sample was agglomerated in blocks, the pores were squeezed closed, the surface had almost no porous structure, only the pores stacked between the blocks, there was no loose network morphology, the boundaries between the particles were blurred, they were melted and stuck together, the surface was rough and formed a dense block, and there was no dispersed skeletal structure.

[0060] like Figure 3 As shown in Figure c, Comparative Example 3, without KOH etching, shows that the carbon inside is not interconnected and contains closed and blind pores, forming a blocky accumulation.

[0061] like Figure 3 As shown in Figure d, Comparative Example 4 uses a mixed carbon source composed of polyvinyl alcohol and sodium polyacrylate to prepare porous carbon. During the carbonization process, polyvinyl alcohol undergoes a physical melting stage. Unlike sucrose, this stage is prone to melt adhesion and has high melt viscosity, making it difficult to achieve uniform composite with the sodium polyacrylate matrix and easily causing local agglomeration. At the same time, the pyrolysis process of polyvinyl alcohol is slow and the gas release is uneven, leading to pore collapse and a significant decrease in specific surface area. This results in a thick skeleton, preventing a large number of internal carbon atoms from contacting the external interface, resulting in low utilization of active sites and unsatisfactory gravimetric specific capacity.

[0062] Nitrogen adsorption-desorption tests were performed on the carbon materials prepared in Examples 1, 2, and 3. Figure 4 As shown, Example 2 was performed at a low relative pressure (p / p 0<0.4) The adsorption capacity increases rapidly (microporous / mesoporous adsorption), and the adsorption capacity increases rapidly in the high-pressure region (p / p 0 >0.8) The curve rises sharply (mesoporous and macroporous characteristics); compared with Example 2, the adsorption amount of Examples 1 and 3 is low and the curve is flat, indicating that they have relatively fewer pores and relatively lower specific surface area.

[0063] The aperture distributions of Examples 1, 2, and 3 were calculated using DFT as follows: Figure 5 As shown, the porous carbon prepared by this invention does not have sharp pore size peaks in the 5-100 nm range, and the pore size exhibits a non-specific wide distribution characteristic, which is beneficial to the uniform loading of sulfur and the isotropic buffering of volume expansion over a wide scale range.

[0064] Table 1 shows the pore volume, BET specific surface area, and pore volume ratio of different pore sizes of porous carbon in each embodiment and comparative example of the present invention. The pore volume distribution diagrams of the porous carbon prepared in Examples 1, 2, and 3 are shown in Table 1. Figure 6 .

[0065] Table 1

[0066] (ii) Electrochemical performance Porous carbon was used to prepare cathodes and assemble batteries in the examples and comparative examples, respectively, and then electrochemical performance was tested. The battery assembly steps are as follows: (1) Weigh 70mg of porous carbon and 280mg of sublimed sulfur powder respectively, grind them evenly in a mortar, and then put them into a hydrothermal reactor to melt and diffuse at 155℃ for 15h. (2) Grind the product obtained in step (1) with the conductive agent (SuperP), and then add it to 2.8 ml of N-methylpyrrolidone (NMP) with polyvinylidene fluoride (PVDF) at a mass ratio of 7:2:1 to obtain a uniformly dispersed solution. Stir continuously for 1 h and coat the generated stable slurry onto the surface of aluminum foil. (3) The positive electrode sheet with a diameter of 12 mm was obtained by vacuum drying at 60℃ for 12 h and then cut by a cutting machine. Celgard 2300 membrane was used as the separator, lithium metal plate was used as the anode, and 1M lithium bis(trifluoromethanesulfonylimide) (LiTFSI) solution was used as the electrolyte. The solvent was composed of 1,3-dioxane (DOL) and 1,2-dimethoxyethane (DME) in a volume ratio of 1:1. (4) Finally, the positive electrode, electrolyte, separator, lithium sheet, gasket and spring are assembled in the glove box.

[0067] Figure 7The graphs show the cycle performance of lithium-sulfur batteries prepared using porous carbon in Examples 1-3 at different rates. The lithium-sulfur battery prepared in Example 2 achieved capacities of 1182 mAh g⁻¹ at 0.1C, 0.2C, 0.5C, 1C, and 2C. -1 998.2 mAh g -1 887.7mAh g -1 796.4mAh g -1 and 751.1mAh g -1 The specific capacity corresponding to Examples 1 and 3 is lower than that of Example 2. This result confirms that appropriately increasing the number of pores of 5-55 nm and >55 nm within a limited range can improve its rate performance.

[0068] Figure 8 The figures show the long-cycle performance of lithium-sulfur batteries prepared using porous carbon in Examples 1-3 at 0.5C. As can be seen from the figures, at 0.5C, the initial capacity of Example 2 is 1188 mAh g⁻¹. -1 It still retains 887mAh g after 300 cycles. -1 The average capacity decay rate per cycle is only 0.08%, which indicates that the porous carbon framework of the present invention can improve its cycling performance through physical dynamic confinement and efficient electron transport.

[0069] Figure 9 The figures show the cycle performance of lithium-sulfur batteries prepared using porous carbon in Comparative Examples 1-4 at different rates. Comparative Example 1, lacking sucrose support for the carbon framework, failed to form a continuous conductive network, resulting in a significant decrease in rate performance. Comparative Example 2 exhibited blocky agglomeration due to the different crystallization points of the salt template and the mixed carbon source, which prevented effective restriction of polysulfide shuttle. Comparative Example 3, without KOH etching, showed unconnected carbon interiors with closed and blind pores, leading to a decrease in sulfur loading and a significant reduction in energy density. Comparative Example 4 exhibited the worst performance because polyvinyl alcohol melted and adhered, causing pore collapse and a significant decrease in specific surface area. This resulted in a thick framework, preventing many internal carbon atoms from contacting the external interface, leading to low utilization of active sites and a substantial reduction in the contact area with sulfur, severely impacting rate performance.

[0070] Figure 10 The figures show the long-cycle performance of lithium-sulfur batteries prepared using porous carbon from Comparative Examples 1-4 at 0.5C rate. As can be seen from the figures, Comparative Example 1 suddenly failed after 104 cycles. The main reason for this was that non-conductive Li₂S blocked some of the space, and the discontinuous carbon skeleton caused an interruption in the conductive network. The agglomeration of the sample in Comparative Example 2 could not provide effective pore catalysis for sulfide conversion, resulting in a significant performance drop to 419.8 mAh g⁻¹ in the 10-70 cycle range.-1 Furthermore, the decay rate increased significantly; the internal pores of Comparative Example 3 were not fully open, and it could still cycle normally for 200 cycles under short-cycle, low-load conditions, but its capacity was significantly lower than that of the Example; the carbon skeleton of Comparative Example 4 was too thick, which led to a decrease in its specific capacity per unit mass, and the first-cycle discharge specific capacity could only reach 429 mAh g. -1 The average attenuation rate per revolution is approximately 0.25%, which is significantly higher than that of the example.

[0071] Although the present invention has been illustrated and described with specific embodiments, it should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; those skilled in the art should understand that modifications can be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein, without departing from the spirit and scope of the present invention; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention; therefore, this means that all such substitutions and modifications that fall within the scope of the present invention are included in the appended claims.

Claims

1. A porous carbon, characterized in that, The porous carbon has micropores, mesopores and macropores, and has a continuous and interconnected honeycomb structure with connecting windows between the pore walls. The pore volume ratio of pores with a diameter < 5 nm is 50%~75%, the pore volume ratio of pores with a diameter of 5~55 nm is 10%~30%, and the pore volume ratio of pores with a diameter > 55 nm is 10%~30%.

2. The porous carbon according to claim 1, characterized in that, The cumulative pore volume of the porous carbon is 0.08~0.3 cm³. 3 g -1 Furthermore, the pore size distribution curve for pores with diameters ranging from 5 to 100 nm has a dv / dw ratio < 0.003 cm. 3 g -1 nm -1 .

3. The porous carbon according to claim 1 or 2, characterized in that, The porous carbon has a BET specific surface area of ​​200~400 m². 2 g -1 .

4. The method for preparing porous carbon according to any one of claims 1 to 3, characterized in that, Includes the following steps: S1. Dissolve sodium polyacrylate and sucrose in water and mix with KCl, then freeze-dry under vacuum to obtain a precursor dry gel; S2. The precursor dry gel is pretreated and carbonized under an inert atmosphere to obtain a primary carbonized product containing a salt template; S3. The primary carbonization product is washed to remove the KCl template and dried to obtain a porous carbon intermediate; S4. The porous carbon intermediate is mixed with an alkali, including KOH and / or NaOH, and then heated and activated under an inert atmosphere. After washing and drying, the porous carbon is obtained.

5. The method for preparing porous carbon according to claim 4, characterized in that, In step S1, at least one of the following characteristics is satisfied: (1) The mass ratio of sodium polyacrylate to sucrose is 1:1 to 9:1; (2) The mass ratio of sodium polyacrylate to KCl is 1:1 to 1:7; (3) The molecular weight of the sodium polyacrylate is 4 million to 5 million; (4) The temperature of the vacuum freeze drying is -40~-80℃ and the drying time is 24~48h.

6. The method for preparing porous carbon according to claim 4, characterized in that, In step S2, the temperature of the heating pretreatment is 400~500℃ and the pretreatment time is 0.5~1.5h; the temperature of the carbonization is 700~900℃ and the holding time is 2~4h.

7. The method for preparing porous carbon according to claim 4, characterized in that, In step S4, the mass ratio of the porous carbon intermediate to the alkali is 1:2 to 1:

6.

8. The method for preparing porous carbon according to claim 4, characterized in that, In step S4, the temperature of the heating activation treatment is 700~900℃, and the holding time is 1~2h.

9. A lithium-sulfur battery cathode material, characterized in that, Includes porous carbon and sulfur supported on the porous carbon; The porous carbon is the porous carbon according to any one of claims 1 to 3 or the porous carbon prepared by the method according to any one of claims 4 to 8.

10. A lithium-sulfur battery, characterized in that, Including the lithium-sulfur battery cathode material as described in claim 9.

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