Hard carbon with sieving closed-pore structure as anode material for sodium-ion battery

Abstract

The invention discloses hard carbon with a molecular sieve type closed-pore structure and a preparation method thereof.The preparation method comprises the steps that a biomass mixture such as cotton and crop waste serves as a raw material, or a mixture containing 70-90% of cellulose, 9-20% of hemicellulose, 1-10% of lignin and the like serves as a raw material, calcination is conducted at the temperature of 300-600 DEG C, and a pre-carbonized precursor is obtained; and then cleaning the precursor with an alkali solution, and finally carrying out secondary calcination at 900-1400 DEG C to obtain a hard carbon product with a molecular sieve type closed pore structure.

Description

W O 2 02 6 / 08 59 17 A l l_ l_ l_ III III III H (12) INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT) (19) World Intellectual Property Organization International Bureau (43) International Publication Date 30 April 2026 (30.04.2026) (10) International Publication Number WO 2026 / 085917 Al WIPO I PCT (51) International Patent ClassiHcation: C01B 32 / 05 (2017.01) H01M10 / 054 (2010.01) H01M 4 / 583 (2010.01) H01M 4 / 133 (2010.01) (21) International Application Number: PCT / CN2024 / 129031 (22) International Filing Date: 31 October 2024 (31.10.2024) (25) Filing Language: English (26) Publication Language: English (30) Priority Data: 18 / 923,688 23 October 2024 (23.10.2024) US (71) Applicant: HONG KONG APPLIED SCIENCE AND TECHNOLOGY RESEARCH INSTITUTE COMPA­ NY LIMITED [CN / CN]; 5 / F, Photonics Centre, 2 Science Park East Avenue, Hong Kong Science Park, Shatin, N.T., Hong Kong (CN). (72) Inventors: XUE, Qi; 3202, Block 8, Jin Shan Fu, Jin- bi Road, Luohu District, Shenzhen,Guangdong 518000 (CN). FANG, Zhou; 4th Floor, Building 90, Shawei East Village, Fuqiang Road, Futian District, Shenzhen, Guang­ dong 518000 (CN). XIANG, Jing; Ma On Shan, Shatin, Hong Kong (CN). ZHENG, Zhong; 10H, 2# Baoruix- uan, Fumange, Ronghua Road, Baoshui District, Futian, Shenzhen, Guangdong 518000 (CN). ZHU, Bin; 508, 3# Tianyiyuan Tianzehuayuan, Futian District, Shenzhen, Guangdong 518000 (CN). (74) Agent: CHINA TRUER IP; Room 1104, Building 2, Excellence Meilin Central Plaza (North Area), No. 128 Zhongkang Road, Meidu Community, Meilin Street, Futian District, Shenzhen, Guangdong 518049 (CN). (81) Designated States (unless otherwise indicated, for every kind of national protection available) '. AE, AG, AL, AM, AO, AT, AU, AZ, BA, BB, BG, BH, BN, BR, BW, BY, BZ, CA, CH, CL, CN, CO, CR, CU, CV CZ, DE, DJ, DK, DM, DO, DZ, EC, EE, EG, ES, FI, GB, GD, GE, GH, GM, GT, HN, HR, HU, ID, IL, IN, IQ, IR, IS, ET, JM, JO, JP, KE, KG, (54) Title: HARD CARBON WITH SIEVING CLOSED-PORESTRUCTURE AS ANODE MATERIAL FOR SODIUM-ION BAT­ TERY FIG. 5 (57) Abstract: Hard caibon with sieving closed-pore structure and its preparation method are disclosed. Biomass mixture such as cotton, farm biowaste, or a mixture with 70-90% cellulose, 9-20% hemicellulose, and 1-10% lignin, etc., is applied as the raw material. This mixture is subjected to calcination at temperatures ranging from 300 to 600°C, resulting in a pre-caibonized precursor. The precursor is then washed with an alkaline solution. Finally, the precursor undergoes a second calcination at a temperature between 900 to 1400°C to yield the hard caibon product with sieving closed-pore structure. [Continued on next page] WO 2026 / 085917 Al l l||MIIIIM KH, KN, KP, KR, KW, KZ, LA, LC, LK, LR, LS, LU, LX MA, MD, MG, MK, MN, MU, MW, MX, MX MZ, NA, NG, NI, NO, NZ, OM, PA, PE, PG, PH, PL, PT, QA, RO, RS, RU, RW, SA, SC, SD, SE, SG, SK, SL, ST, SV, SY, TH, TJ, TM, TN, TR, TT, TZ, UA, UG, US, UZ, VC, VN, WS, ZA,ZM,ZW. (84)Designated States (unless otherwise indicated, for every kind of regional protection available)'. ARIPO (BW, CV, GH, GM, KE, LR, LS, MW, MZ, NA, RW, SC, SD, SL, ST, SZ, TZ, UG, ZM, ZW), Eurasian (AM, AZ, BY, KG, KZ, RU, TJ, TM), European (AL, AT, BE, BG, CH, CY, CZ, DE, DK, EE, ES, FI, FR, GB, GR, HR, HU, IE, IS, IT, LT, LU, LV, MC, ME, MK, MT, NL, NO, PL, PT, RO, RS, SE, SI, SK, SM, TR), OAPI (BF, BJ, CF, CG, CI, CM, GA, GN, GQ, GW, KM, ML, MR, NE, SN, TD, TG). Published: — with international search report (Art. 21(3)) WO 2026 / 085917 PCT / CN2024 / 129031 Hard Carbon with Sieving Closed-pore Structure as Anode Material for Sodium-ion Battery Qi XUE, Zhou FANG, Jing XIANG, Zhong ZHENG, Bin ZHU BACKGROUND OF THE INVENTION [1] The invention relates to hard carbon with sieving closed-pore structure and its fabrication method. [2] In recent years, sodium-ion batteries (SIBs) have gained significant attention as a promising alternative to lithium-ion batteries, particularly due to the abundantand widespread availability of sodium. However, the performance of SIBs depends heavily on the quality of the anode material, which must accommodate the larger ionic radius of sodium compared to lithium and permit rapid ion transport. [3] Hard carbon is widely used as active anode material in SIBs. Due to the low price, hard carbon is the best choice for the anode of SIBs. The structure of hard carbon includes a short carbon chain, a highly disordered structure, a long layer distance, and a closed-pore structure. Hard carbon is a form of non- graphitic carbon that is recognized for its amorphous structure with a degree of short-range order. This structure is particularly beneficial for energy storage applications due to its ability to host sodium ions. [4] Traditionally, hard carbon derived from various carbon sources has been utilized as an anode material for SIBs, but the lack of control over pore structure often leads to suboptimal electrochemical performance. Pore structure plays aneeded role in facilitating sodium ion diffusion and accommodating the volume changes during the charging and discharging cycles. 1 WO 2026 / 085917 PCT / CN2024 / 129031 SUMMARY OF THE INVENTION [5] In one aspect, hard carbon characterized by sieving closed-pore structure could provide surface and inner space, the structure further having a window to receive sodium during charging, wherein the sodium ion passes through the window and absorbs on the surface and occupies the inner space after charging. [6] In another aspect, a method is disclosed for the production of hard carbon. This method comprises a two-step heat treatment (calcining) of a biomass mixture under specific conditions to achieve desirable physical and chemical characteristics of the hard carbon product. Hard carbon with sieving closed-pore structures are formed by: calcining a biomass mixture having 70-90% cellulose, 9-20% hemicellulose, and 1-10% lignin at a temperature between 300-600°C to produce a pre-carbonizedprecursor; rinsing the pre-carbonized precursor with an alkali solution to adjust the pH to 6.0-8.0; and re-calcining the pre-carbonized precursor at a temperature between 900-1400°C to produce a sieving closed-pore hard carbon. [7] In a further aspect, an anode for a sodium-ion battery includes a hard carbon material characterized by sieving closed-pore structure. [8] In yet another aspect, a sodium-ion battery includes a cathode; an electrolyte coupled to the cathode; and an anode coupled to the electrolyte, the anode having a plurality of sieving dose-pore structures. [9] Implementations of the above aspects can include one or more of the following. The biomass mixture can be organic materials that are rich in carbon content. The biomass mixture serves as a precursor, which undergoes a heat treatment process or calcining. The calcining step enables the organic components of the biomass to be thermally decomposed to yield a carbon structure. The process of calcining is conducted inan inert atmosphere to prevent the oxidation of the carbonaceous material. The 2 WO 2026 / 085917 PCT / CN2024 / 129031 precursor is subjected to a temperature of 300-600°C. The temperature treatment is carried out with a ramping rate under inert gas at 2-10°C / min. This treatment leads to an initial decomposition and carbonization of the biomass. Following this, the material is washed by an alkali solution with a concentration between 0.05-1.0 mol / L to clean and activate the carbon surface. The second heat treatment is extended to a higher temperature of 900-1400’C. The temperature has a ramp rate of 2- 10°C / min in an inert gas atmosphere. During this re-calcination step further development of the hard carbon structure occurs as any remaining volatile materials are eliminated, enhancing the porosity of the hard carbon material. The resultant hard carbon material exhibits an optimized structure for the reversible insertion and extraction of sodium ions, resulting in high-performance SIBs.

[10] Advantages of the material may include one or more of the following. This method leverages the inherent structure of biomass, consisting of cellulose, hemicellulose, and lignin, to create a hard carbon precursor with a desirable pore framework upon calcination. By optimizing the calcination temperatures and employing post-processing rinsing and re-calcination steps, a hard carbon material is fabricated that possesses sieving closed-pore structure, predominantly with a target pore size distribution. This structure not only facilitates rapid ion transport but also provides structural integrity, enhancing the cycling stability of sodium-ion batteries. The method uses biomass as the starting material in a sustainable and cost- effective approach, given the biomass' natural abundance and renewable nature. The method recycles agricultural waste which promotes a greener manufacturing process while also addressing waste management issues. The biomass mixture employed in the disclosedmethod is sourced from agricultural waste, which provides an abundant and renewable feedstock for the production of hard carbon anode materials for sodium-ion batteries. By utilizing agricultural waste, the method not only offers a sustainable approach to battery manufacturing but also contributes to the management and reduction of agricultural waste, closing the loop in the agricultural lifecycle and promoting eco-friendly practices. 3 WO 2026 / 085917 PCT / CN2024 / 129031

[11] Another advantage is that an optimized structure for sodium storage is created, allowing for the development of sodium-ion batteries with improved capacity, rate performance, and cycling stability. This approach, therefore, represents a significant advancement in the design and synthesis of anode materials for next-generation energy storage technologies. The hard carbon, produced through a carefully controlled process of biomass calcination and chemical treatment, possesses sieving closed-pore structure well suitedas an anode material in sodium-ion batteries. Such a configuration has been found to be highly beneficial for the electrochemical performance of the material, specifically in regard to the kinetics of Na+ ion insertion and extraction.

[12] In other advantages, the method creates hard carbon with sieving closed-pore structure. The hard carbon material presents high surface area, proper pore distribution, and electrical conductivity which are key properties for achieving fast charge-discharge rates and high energy densities in batteries. These characteristics make the sieved closed-pore hard carbon an exemplary candidate for advanced energy storage applications, thereby aligning with the increasing demand for renewable energy solutions and the transition towards more sustainable energy storage technologies. 4 WO 2026 / 085917 PCT / CN2024 / 129031 BRIEF DESCRIPTION OF DRAWINGS

[13] Fig. 1 shows TEM image of the hard carbon in this invention and the illustration of a sodium layer dispersionafter charging.

[14] Fig. 2 shows exemplary XRD patterns of three hard carbon samples produced using the instant method.

[15] Fig. 3 shows exemplary Rama spectra patterns for three hard carbon samples produced using the instant method.

[16] Fig. 4 shows exemplary TEM images for three hard carbon samples produced using the instant method.

[17] Fig. 5 shows exemplary SIB performance curves. 5 WO 2026 / 085917 PCT / CN2024 / 129031 DETAILED DESCRIPTION OF THE INVENTION

[18] The present disclosure relates to hard carbon characterized by sieving closed-pore structure and a method to fabricate the mentioned hard carbon derived from a biomass mixture for use in sodium-ion batteries. This method optimizes the pore structure within the hard carbon to enhance Na+ ion insertion, improving the storage capacity and efficiency of the batteries.

[19] To obtain the desired hard carbon structure, suitable carbon-rich biomass is selected and prepared to ensure a high yield of carbon upon processing. Toprepare the biomass mixture, agricultural residues, which are otherwise considered to be by-products or waste from farming activities, are gathered. These may include but are not limited to, plant stalks, leaves, husks, shells, and other organic matter that is rich in cellulose. This material is initially processed, often through drying and grinding, to achieve a consistent particle size and homogenization for uniform calcination and chemical reactions throughout.

[20] The biomass is processed through a series of thermal and chemical treatments to transform it into hard carbon with properties conducive to energy storage in sodium-ion batteries. In one example, the biomass mixture is comprised of cellulose, hemicellulose, and lignin in proportions conducive to the effective conversion into hard carbon. Specifically, the cellulose content ranges from 70% to 90%, serving as the primary carbon source. Hemicellulose, a structural polysaccharide found in plant cell walls akin to cellulose,is present in an amount of 9% to 20%. Lignin, an organic polymer that provides structural support to vascular plantsand some algae, makes up 1% to 10% of the mixture. This specified composition ensures that the requisite properties for hard carbon formation are met, and that the resulting material possesses the desired characteristics for electrochemical applications.

[21] The above biomass mixture contains from 70-90% cellulose, 9-20% hemicellulose, and 1-10% lignin. The biomass mixture undergoes an initial calcination at 300-600°C, resulting in a precursor characterized by cellulose as a carbon chain, hemicellulose as a short carbon chain and pore structure, and lignin / other 6 WO 2026 / 085917 PCT / CN2024 / 129031 providing the pore structure. The precursor is then subjected to an alkaline with a concentration between 0.05-1.0 mol / L washing step then neutralize to pH of 6.0-8.0.

[22] Following this, a second calcination step is performed at 900-1400°C to produce the desired sievingclosed-pore hard carbon. The resulting hard carbon is suitable for use as an anode material in sodium-ion batteries.

[23] Upon acquisition of the biomass mixture, the process proceeds to the calcination of the mixture at a temperature controlled within the range of 300°C to 600°C. The calcination process induces thermal decomposition of the organic constituents in the biomass, facilitating the conversion of biomaterial into a pre-carbonized state. This precursor state denotes an intermediate step in the synthesis of hard carbon where the biomass has undergone substantial chemical changes due to heat exposure without reaching a fully carbonized stage.

[24] Following calcination, the pre-carbonized precursor generated from the biomass mixture is subjected to a rinsing phase. An alkali solution is employed for rinsing purposes to neutralize any residual acidic compounds that may be present within the pre-carbonized precursor and generate closed-pore structure. This process serves topurify the precursor and to optimize its chemical environment for subsequent conversion into hard carbon.

[25] Treatment with alkali prepares the hard carbon structure for efficient sodium-ion (Na+) insertion. When the precursor material is treated with alkali, a set of chemical reactions occurs, which modifies the carbon structure to provide carbon chains with different length. This improves the performance of sodium-ion batteries that use the hard carbon.

[26] Take NaOH as an example, the addition of NaOH to the carbonaceous precursor leads to a redox reaction wherein carbon (C) is oxidized, promoting the formation of sodium metal (Na), sodium carbonate 7 WO 2026 / 085917 PCT / CN2024 / 129031 (Na2CO3), and hydrogen gas (H2). This reaction contributes to the activation of the carbon surface, resulting in enhanced porosity and increased surface area conducive to Na+ ion diffusion and electrochemical activity.

[27] Subsequently, the reaction of sodium metal with water, if present, generatesadditional NaOH and releases hydrogen gas. This step further influences the pore structure, creating an optimal environment that facilitates the insertion and de-insertion of Na+ ions during the charging and discharging cycles of the battery.

[28] Through these reactions, the alkali treatment modifies the biomass-derived precursor into a form of hard carbon with a sieving structure that enables the accommodation of Na+ ions, leading to improved cycling stability and energy capacity of the sodium-ion battery. The resulting material is ideal use as an anode in energy storage systems, particularly due to the high surface area, ideal pore size distribution, and the presence of functional groups bestowed by the alkali activation process.

[29] The action of the alkali solutions assists in the removal of organic impurities, as well as the etching of the carbon framework, which results in the enlargement or creation of pores that are needed for ion transport. The selection of the alkali isbased on the desired chemical affinity and the specific characteristics of the precursor being used.

[30] The selection of the alkali solution is not limited to sodium hydroxide (NaOH) but can be chosen from other suitable solutions such as potassium hydroxide (KOH), lithium hydroxide (LiOH) and ammonia (NH4OH). Each of these alkali solutions has unique properties that can impact the activation and pore development processes of the precursor material, hence affecting the structure and performance of the final hard carbon product. While NaOH is effective in enhancing pore structures and improving the accessibility of sodium ions, KOH is known for its ability to create larger pore volumes, whereas LiOH and NH4OH might incite different pore development dynamics owing to the ionic radius. 8 WO 2026 / 085917 PCT / CN2024 / 129031

[31] The rinsing not only cleanses the precursor but also contributes to the chemical activation of the carbon structure. Once the pre-carbonized precursor has beensufficiently rinsed, it is then subjected to further heat treatment under an inert atmosphere, such as nitrogen (N2) and argon (Ar) at a controlled heating rate, eventually reaching a targeted high temperature for the development of the desired hard carbon structure.

[32] The temperature elevation to 900-1400°C promotes the transformation of the activated precursor into the final form of hard carbon. It is at this stage that the unique pore structure becomes fixed, and the material acquires the high surface area and porosity necessary for effective sodium-ion insertion. The resulting substance, consisting of closed-pore hard carbon with pores smaller than 10 nm, is ideal for use as an anode material in sodium-ion batteries since it offers optimal pathways for ion transport, leading to enhanced battery performance in terms of energy density, lifespan, and charging rates. The sieving process serves to select particles of a specific size range, ensuring consistency in the material. Thesieving process also segregates closed-pore hard carbon particles that possess the ideal pore size, thereby ensuring high efficiency in Na+ ion insertion and extraction during battery operation.

[33] The second calcination step is preferably carried out in an inert atmosphere, such as nitrogen (N2), argon (Ar), etc., to prevent undesirable oxidation of the carbon material. The controlled processing environment and the specific temperature regimen ensures that the developed hard carbon presents a desirable pore distribution and integrity, ensuring consistency in electrochemical performance when deployed within sodium-ion batteries.

[34] This method of preparing the powdered and sequentially calcinated hard carbon is conducive for bulk synthesis, enabling large-scale production of anode material. This scalability is a significant consideration for the widespread adoption of sodium-ion battery technology in various applications, ranging from portable electronics to grid-scale energystorage systems. Moreover, the use of biomass as 9 WO 2026 / 085917 PCT / CN2024 / 129031 a starting material leverages renewable resources, hence contributing to the sustainability of the overall manufacturing process.

[35] The temperature for this second calcination is significantly higher than the initial calcination and is adjusted to lie within the range of 900°C to 1400°C. The increased temperature during re-calcination promotes the complete carbonization of the precursor material, resulting in the formation of hard carbon. The high-temperature treatment promotes the evolution of non-carbon atoms and supports the development of a graphitic-like structure, which is needed for achieving the desired electrical conductivity and crystallinity. It is during this phase that the desirable sieving dosed-pore structure is formed. These dosed pores are critical for the efficacy of the hard carbon when employed as an anode material, as they regulate the interaction between the hard carbonstructure and the sodium ions during the battery's charge and discharge cycles. The produced hard carbon with such refined microporosity demonstrates improved electrochemical performance when subsequently integrated into sodium-ion batteries, offering advantages in terms of energy storage capability, cycle life, and rate performance.

[36] In this patent, the methodology ensures the formation of a hard carbon structure with specific sieving closed-pore characteristics, making it suitable for use as an anode material in sodium-ion batteries. The diameter of sodium (Na+) is higher than lithium (Li+). Thus, the Na+ cannot be inserted into the graphite anode. Because of the short-disordered structure, the Na* ions can be: i) adsorbed on the surface of the hard carbon; ii) inserted into the hard carbon layers; and iii) aggregated inside closed-pore structure. Therefore, the Na+ amount is increased on the anode and generates positive impacts on the electrochemical property of SIBs. The TEMstructure of the hard carbon is shown in Fig. 1, and the sodium layer dispersion after charging is also illustrated in Fig. 1 where the Na* is (1) absorbed on the surface, (2) asserted into the layer, and (3) aggregated in the pores. The resulting structure has high coulombic efficiency, high specific capacity, resulting in increased battery life cycle. The sieved hard carbon is 10 WO 2026 / 085917 PCT / CN2024 / 129031 characterized by its uniformity in terms of pore distribution, which directly correlates to the efficiency of Na+ ion insertion during the battery charging process. This efficiency is particularly significant as the uniform and optimal pore size of less than 10 nm allows for improved ionic conductivity and charge­ discharge cycle stability, thereby enhancing the energy storage and lifespan of the battery cells.

[37] The sieving structure, which has undergone the process of selecting for pore sizes less than 10 nm, is visualized using a transmission electron microscope (TEM)image in Fig.l. The obtained image reveals the intricate details of the material's microstructure and provides direct evidence of the successful separation of closed-pore hard carbon particles based on their pore size. In this image, one can clearly discern the uniformity and distribution of pores within the carbon matrix, characteristic of the sieving structure.

[38] The closed-pore hard carbon structure is an integral component of the sodium-ion battery anode material. The closed pores provide discrete sites for sodium-ion insertion and storage within the hard carbon material. The confinement of Na+ ions in these pores facilitates efficient charge-discharge cycling by offering a stable environment that reduces the likelihood of detrimental reactions with electrolytes or other components of the battery. The sieve structure generated through the sieving step ensures that the majority of the pores are sized below 10 nm. The diagrams, therefore, display a hard carbon material where asignificant portion of the pores fall within this optimal size range, which is particularly advantageous for the kinetics of Na+ ion insertion and de-insertion during battery operation. Additionally, the diagrams delineate the result of the chemical and thermal treatments, including the use of NaOH which yields the chemical reaction NaOH + C -> Na + Na2CO3 + H2, and Na + H2O NaOH. These treatments contribute to developing the carbon's porosity and enhancing its electrochemically active surface area. 11 WO 2026 / 085917 PCT / CN2024 / 129031

[39] In one aspect concerning the Na+ ion insertion, it becomes evident that the sieving structure developed from the sieved closed-pore hard carbon enhances the electrochemical performance of sodium-ion batteries. The sieving structure is designed to selectively permit the insertion of Na+ ions during the charging process of the battery. The controlled insertion of Na+ ions into finely-tuned pore sizes, which are less than 10 nm in one example, ensuresefficient use of the available electrochemical surface within the sieving structure and maximizes charge storage capacity.

[40] Once inserted, Na+ ions are situated within the pores of the sieving structure, providing a stable and efficient pathway for ion transfer during battery operation. The sieving structure with inserted Na+ ions demonstrate a significant increase in performance when employed as an anode material in sodium- ion batteries, exhibiting superior charge capacity and enhanced cycle stability.

[41] The preparation of the hard carbon begins with selecting a suitable biomass mixture. This mixture is derived from organic matter that is carbonaceous in nature, capable of being transformed through thermal decomposition into a solid carbon form. To ensure the quality and consistency of the pore structure, the biomass is subjected to a calcination process. The initial calcination takes place at a temperature of around 300-600°C, during which volatile components of the biomassare expelled, leaving behind a carbon-rich residue.

[42] Following the initial calcination, the resultant carbonaceous material is mixed with an alkaline solution. The chemical reactions that occur during this stage not only activate the carbon material but also play a large role in pore development. Specifically, the interaction with alkali promotes the opening of pores within the carbon structure, prepping the material for subsequent processing.

[43] During the washing step, impurities that may have been introduced during the calcination process or inherent from the biomass precursor are removed, which can otherwise negatively impact the 12 WO 2026 / 085917 PCT / CN2024 / 129031 electrochemical performance of the hard carbon when utilized as an anode material in sodium-ion batteries.

[44] After washing and further cleaning, the carbon material undergoes a second calcination step, this time at a significantly higher temperature of 900-1400°C. It is during this re-calcination that thematerial consolidates into a form of hard carbon. Simultaneously, this treatment fine-tunes the pore structure, leading to the creation of closed pores that exhibit a distribution with a median size smaller than the exemplary 10 nm.

[45] Upon completion of the high-temperature treatment, the hard carbon is then sieved, a process that serves to isolate the fraction of the material possessing the desired pore size. This is a critical step, ensuring that the final product will have a consistent and uniform pore structure that can facilitate the targeted Na+ ion transport dynamics.

[46] Initially, the biomass mixture acts as a precursor to the closed-pore hard carbon, undergoing a calcination process at a reference temperature of 300-600°C. This stage eliminates volatile compounds and begins the carbonization of the biomass. The precursor is then subjected to an alkali treatment, which creates an activated carbon structure with the potential for desired pore formation. To further refinethe carbon structure, a re-calcination process takes place at a higher reference temperature of 900-1400°C. This step enhances the carbon matrix with the desirable traits for energy storage, namely, electrical conductivity and ion accessibility.

[47] The thermal treatment step at 900-1400’C engenders the elimination of any remaining non-carbon elements and promotes the growth of graphitic structures conducive to Na* ion storage. Furthermore, the rigor of this high-temperature regime assists in attaining a desirable level of graphitization and porosity, which are imperative characteristics for efficient ion transport and intercalation. The employment of an 13 WO 2026 / 085917 PCT / CN2024 / 129031 inert atmosphere, typically nitrogen, during the re-calcination safeguards the carbonaceous materials from oxidation and facilitates the development of a porous network needed for the targeted application.

[48] During the pre-carbonization stage, the temperature should stay below 600°C to preventexcessive ash formation which could diminish the quality of the final carbon product. Conversely, the temperature is kept above 300°C to ensure adequate thermal breakdown of the biomass components. This controlled thermal treatment is needed in breaking down the lignocellulosic structure and other organic compounds present in the biomass mixture, giving rise to a pre-carbonized material that possesses a porous matrix necessary for the subsequent activation and development of the final hard carbon structure.

[49] Following the pre-carbonization step, the resultant semi-carbonized biomass mixture is then immersed in a alkali solution, initiating a chemical activation process. The alkali not only cleanses the carbonized material of residual impurities but also creates additional porosity by etching away at the carbon framework. The reaction between alkali and the carbon elements leads to the formation of carbonate salt and hydrogen gas, further influencing the textural properties of thematerial.

[50] The activated material is then subject to a second phase of carbonization, referred to as re­ calcination, where it is heated to a temperature of approximately 900-1400“C. It is during this stage that the already developed porous carbon structure is transformed into a tightly-packed arrangement of graphene-like layers, known as hard carbon. This re-calcination at a higher temperature purifies the carbonaceous material by removing any remaining non-carbon elements and enhances electrical conductivity needed for battery anodes.

[51] The process enables narrow pore size distribution to be achieved, primarily featuring pores smaller than 10 nm in one case. This small pore size facilitates the insertion and de-insertion of sodium ions during the charge and discharge cycles of the battery. The sieving of the hard carbon to achieve this fine pore 14 WO 2026 / 085917 PCT / CN2024 / 129031 distribution involves mechanical separation techniques, capitalizing on the size differencebetween the desired porosity and larger, unusable pore structures.

[52] Finally, the sieved hard carbon, with its finely-tuned microstructure, is incorporated into a sodium- ion battery. This hard carbon serves as the anode where the intercalation of sodium ions occurs. The compact pore structure not only allows for efficient sodium-ion transport but also enhances the overall energy density and charge-discharge rates of the battery. This innovative approach combines sustainable raw materials with precise thermal and chemical treatments, culminating in an advanced energy storage solution that meets the growing needs for renewable and high-performance battery technology.

[53] Without alkali washing, soft and hard carbon mixture can arise. Also, when re-calcination temperature is higher than 1400°C, glassy carbon would be obtained.

[54] During the calcination process, and the subsequent re-calcination step. The majority of these trace impurities are reduced or eliminated. The elevatedtemperatures cause the volatilization and decomposition of many organic compounds and the potential transformation or oxidation of inorganic materials to form ash. This ash can be removed from the final product through various purification methods such as washing, filtering, or acid treatment.

[55] The precursor, after undergoing the initial heat treatment, is subjected to the chemical reactions with alkali which serves a dual purpose: it contributes to the activation of the carbon structure, facilitating the creation of porosity, and it also aids in the purification process by reacting with certain impurities to form soluble salts that can be washed away.

[56] One or more trace impurities are commonly found in natural sources of biomass, and these may include, but are not limited to, potassium ions (K+), calcium ions (Ca2+), and magnesium ions (Mg2+). The 15 WO 2026 / 085917 PCT / CN2024 / 129031 presence of these impurities can potentially influence the electrochemical characteristics ofthe hard carbon when applied as an anode material in sodium-ion batteries.

[57] Potassium ions (K+) may be present as a result of the biomass composition or as remnants from processing chemicals used during the extraction and purification of the carbon material. These ions can occupy active sites within the carbon lattice or reside within the micropores, potentially affecting the movement of Na+ ions and thus the storage capacity.

[58] Similarly, calcium ions (Ca2+) and magnesium ions (Mg2+) can be found integrated into the carbon structure. These multivalent impurities might affect the electrical conductivity and contribute to variations in pore structure due to their larger ionic size and possible interactions with the carbon matrices.

[59] While these impurities are trace in nature and constitute only a small fraction of the overall composition of the hard carbon, they can affect the overall performance of the material. Therefore, during the manufacturing process, steps may be takento either reduce the concentration of these impurities or to leverage their presence to optimize the electrochemical performance.

[60] Moreover, strategies to mitigate the influence of these trace impurities on the performance of the sodium-ion batteries could include additional purification processes, post-treatment modifications, or incorporation during controlled calcination stages to alter their state, distribution, or interaction with the Na+ ions.

[61] In one example, the removal of impurities is achieved by first washing the biomass mixture with an acidic solution. This acidic solution has a pH value carefully maintained between 3 and 6, ensuring that it is sufficiently acidic to dissolve and remove various impurities such as minerals, inorganic salts, and other non-carbonaceous components that are present within the biomass. The chosen pH range is critical to 16 WO 2026 / 085917 PCT / CN2024 / 129031 effectively purge the biomass of contaminants without causing excessive degradationof the organic material that is needed for the subsequent carbonization processes.

[62] In another embodiment, the washing procedure is conducted for a period ranging from 2 to 10 hours, depending on the type and level of impurities present in the biomass. This duration is selected to optimize the effectiveness of the acid wash while minimizing the risk of damaging the integrity of the carbon precursors within the biomass. Complete immersion of the biomass mixture in the acidic solution during this time allows for thorough interaction between the acid and the impurities, thus enhancing the overall deansing action.

[63] Following the acid wash, the biomass mixture undergoes a secondary washing step with water. This wash aims to remove the one or more acids that have been applied during the impurity removal process. It is imperative to conduct this step with care to ensure that all traces of the acidic solution are eliminated from the biomass mixture. The presence of residual acids couldinterfere with downstream processes, such as calcination, and negatively affect the properties of the developed hard carbon material.

[64] The water used for washing is typically at or near neutral pH, to counteract the acidity from the previous cleaning step. The biomass is thoroughly rinsed to ensure that no acidic remnants are retained. The washing with water not only neutralizes the biomass but also serves to further cleanse the material of any soluble impurities that may have been loosened by the acid wash but not wholly dissolved.

[65] Once the washing steps are completed, the biomass mixture is dried to remove the added moisture. The drying step serves to prepare the biomass for the high-temperature treatments that will follow, such as calcination and re-calcination, where temperatures of 300-600°C and 900-1400°C are employed to transform the biomass into a structured hard carbon material suitable for use in sodium-ion batteries. 17 WO 2026 / 085917 PCT / CN2024 / 129031

[66] Throughthese cleaning processes, the biomass mixture is conditioned to yield a precursor that is highly pure and conducive to producing a high-quality hard carbon material, featuring a controlled microstructure with dosed pores that are ideal for housing Na+ ions during the operation of sodium-ion batteries.

[67] One alternative embodiment uses an acidic solution to the treatment and optimization of the material's porosity and chemical properties. The acidic solution consists of one or more acids selected from the group comprising nitric acid (HNO3), hydrochloric acid (HCI), acetic acid (CH3COOH), sulfuric acid (H2SO4), and phosphate acid (H3PO4). This acidic solution serves multiple purposes in the treatment of the biomass precursor and the resulting hard carbon. Firstly, it acts as a cleansing agent, removing impurities and potentially catalyzing reactions that could impede the performance of the hard carbon as an electrode material. Typically, the use of the acidic solution is involved inan acid washing step, whereby the biomass mixture is submerged within the acidic medium. The selection of the acid or combination of acids depends on the desired chemical reactivity and the nature of the biomass utilized. The reaction with nitric acid often contributes to the introduction of nitrogen-doped sites within the carbon matrix, which can potentially enhance electronic conductivity and provide active sites for sodium-ion storage. Hydrochloric acid treatment is known to facilitate the removal of metallic impurities and assists in the opening of the carbon pore structure. Acetic acid, due to its relatively mild acidity, is used to modify the surface chemistry without overly etching the carbon framework. Lastly, sulfuric acid is known for its strong oxidizing capacity and can be employed to insert sulfonic acid groups onto the carbon surface, thereby increasing the hydrophilicity and possibly the ionic conductivity of the material. During the process of preparing the hard carbon,the biomass mixture is subjected to these acids under controlled conditions, which may vary in concentration, temperature, and duration of exposure depending on the desired properties of the final product. After the acid treatment, the mixture is thoroughly washed to remove residual acids and by­ products. A subsequent calcination step is applied, which aids in the development of a stable 18 WO 2026 / 085917 PCT / CN2024 / 129031 carbonaceous structure, often resulting in the after mentioned sieving structure characterized by a specific pore size that facilitates efficient sodium-ion insertion and extraction during the operation of the battery. The sieving structure obtained through this method can be optimized for the role of the anode in a sodium-ion battery by selectively capturing the hard carbon particles with desired pore sizes. The process of alkali treatment, calcination, and re-calcination ensures that the carbon material exhibits suitable electrical conductivity and stability tocycle sodium ions effectively. The integration of the sieving structure with inserted Na+ ions form a robust and efficient electrode for sodium-ion batteries, with the potential to deliver high energy density, long cycle life, and cost-effectiveness due to the use of abundant starting materials and scalable manufacturing processes.

[68] Moreover, the described method supports sustainable manufacturing practices as biomass, a renewable resource, is employed as the carbon source. The transformation of this raw material into an advanced energy storage solution is especially significant given the ongoing shift toward renewable energy systems and the necessity for compatible energy storage technologies.

[69] The pre-carbonized precursor can be subjected to a washing step with water. The washing step removes any loose dust and particulate matter that has not become chemically bonded to the precursor material during the pre-carbonization process. The process can involve agitation orsonication to enhance the cleaning action, followed by a filtration or decantation step to separate the solid carbonaceous material from the removed particulates suspended in the wash water.

[70] Once the pre-carbonized precursor has been thoroughly washed, it is exposed to rinsing with an alkali solution. The alkali treatment involves the use of the KOH, NaOH, LiOH, and NH4OH solutions, which reacts with the carbonaceous material. This step is needed, as it promotes the opening of the pores within the pre-carbonized structure, thereby facilitating the development of a network of accessible channels 19 WO 2026 / 085917 PCT / CN2024 / 129031 and cavities within the carbon matrix. The alkali rinsing not only aids in the functionalization of the carbon surface to improve its reactivity but also plays a significant role in pore development.

[71] The rinsing with the alkali solution is carried out under controlled conditions to ensure that the interaction between the alkali and the pre-carbonizedprecursor is sufficient to open up pores without damaging the overall structure of the carbon matrix. Parameters such as the concentration of the alkali solution, the contact time between the alkali and the precursor are carefully optimized to produce the desired pore size and distribution within the hard carbon that will be produced in subsequent steps.

[72] Following the alkali treatment, the precursor may undergo additional rinsing steps to remove any excess alkali as well as any salts formed as a result of the chemical reactions during the alkali treatment phase. The resultant hard carbon is expected to exhibit superior performance when utilized as an anode material in sodium-ion batteries due to its optimized structure for Na+ ion insertion and storage.

[73] In the production of the hard carbon composite, a biomass mixture is utilized as the primary source of carbon. This biomass mixture may be any kind of carbon-rich organic material that can serve as a precursor in the pyrolysisprocess. The choice of biomass affects the characteristics of the final hard carbon product.

[74] The precursor is treated through a calcination step under nitrogen atmosphere, which is performed by gradually increasing the temperature to 300-600°C at a rate of 2-10°C / min and maintaining the temperature for 2-10 hours. This temperature is chosen carefully to achieve initial carbonization while preserving the structural integrity of the biomass.

[75] The electrochemical property of the hard carbon is further improved by adjusting the sodium-ion insertion capabilities. This is achieved by modulating the pore size distribution and ensuring that the size of the pores is ideal for the insertion of Na+ or Li+ ions. During the charging of the battery, these ions are 20 WO 2026 / 085917 PCT / CN2024 / 129031 inserted into the pores of the hard carbon, and during discharging, the ions are extracted, which results in the flow of current in the battery. The uniformity in pore size achieved throughsieving allows for a more consistent and efficient insertion and extraction process, leading to a more stable battery with a higher energy capacity.

[76] The hard carbon composite is prepared through a meticulous process of calcination, washing, re­ calcination, and sieving to achieve desirable electrochemical properties for use in energy storage systems. The characteristics of the hard carbon, such as the specific pore structure tailored for ion insertion, provide an improved anode material for the next generation of lithium-ion and sodium-ion batteries. This enhanced performance results from the controlled microstructure, obtained from a sustainable biomass source, which leads to better ion storage capacity and the overall efficiency of the battery.

[77] The process further includes a washing step for the carbon powder precursor. After the initial calcination step at a temperature denoted by 300-600°C, the carbon powder precursor obtained from the biomass mixture is then purifiedthrough a washing procedure. This washing is executed with water, ensuring the removal of any soluble impurities, non-carbon elements, and ash content that may be present within the precursor material. Washing with water not only aids in purifying the carbonized product but also helps to expose and widen the pore structures, which are critical for the subsequent alkali treatment.

[78] Subsequent to the washing with water, the now cleaner and porous carbon powder is subjected to the alkali treatment as mentioned earlier. The alkaline environment create a network of more accessible channels within the carbon matrix. This chemical activation process promotes an increase in specific surface area, a feature desired for enhanced electrochemical performance.

[79] The washed and activated carbon powder is then directed to further thermal treatment wherein it is subjected to a temperature of 900-1400°C under an inert atmosphere. The purpose of this high- 21 WO 2026 / 085917 PCT / CN2024 / 129031temperature treatment, also referred to as re-calcination, is to improve the structural integrity of the hard carbon. This step contributes to the electrical conductivity of the final product.

[80] The alkali treatment is carefully conducted by immersing the carbon powder in an alkaline solution with a concentration ranging from 0.05 to 1.0 mol / L. The precise alkali concentration is chosen based on the desired level of porosity and the resultant electrochemical properties required for the specific application. The immersion is maintained for a duration of between 2 to 10 hours, a time frame selected to allow for adequate interaction between alkali and the carbon powder. During this period, alkali acts on the carbon material, etching away at specific sites and enlarging the interlayer spaces within the carbon domains. This etching not only increases the pore volume but also enhances the accessibility of the pores, making them more amenable to sodium ion insertion.

[81] With theincreased layer distance, the resultant pore structures exhibit improved ion diffusion dynamics which directly correlates with heightened battery performance. The strategic manipulation of layer distances and pore sizes aims at achieving a delicate balance between structural stability and ion transport efficiency. Consequently, a carbon structure with increased interlayer spacing provides a considerable advantage for energy storage in sodium-ion battery systems.

[82] Following the alkali treatment, the carbon material is thoroughly washed to remove any residual alkali that might have adhered to the surface. This washing step ensures that the integrity of the newly formed pore structures is not compromised by remnants of the treatment reagent. Thereafter, the carbon powder is dried to remove moisture that could potentially affect the measurement of the layer distances as well as the performance of the material in subsequent battery assembly and testing processes.

[83] It is through theeffective control of this treatment with alkali that a tailored porosity of the carbon material can be actualized, fostering the development of high-performance hard carbon anodes that are needed for advanced sodium-ion batteries. This innovative synthesis approach marks a significant 22 WO 2026 / 085917 PCT / CN2024 / 129031 enhancement over conventional hard carbon materials, pushing the boundaries of energy density, cycle life, and charging rate capabilities in next-generation sodium-ion batteries.

[84] The treated carbon powder, obtained post-calcination and chemical activation, is subjected to a cleaning process. This is executed by washing the powder with water until the pH of the mixture is neutral at 6.0-8.0. The purpose of this washing step is to remove any residual chemical reagents, particularly the unreacted alkali, and byproducts. Bringing the pH to neutrality is of paramount importance as it indicates that the excess alkali has been fully washed away, leaving behind a purecarbon material devoid of contaminants that could adversely affect the performance of the battery. This neutral pH is also indicative of a stable surface chemistry which is needed for the consistent electrochemical behavior of the hard carbon when used as an anode material in sodium-ion batteries. The resulting clean and neutral pH carbon powder exhibits a desirable microstructural architecture that promotes the efficient intercalation and deintercalation of sodium ions during the charge and discharge cycles of a battery. This process step is critical in ensuring that the physical and electrochemical properties of the hard carbon are maintained at optimum levels for subsequent use in energy storage devices. After the washing procedure, the cleaned carbon powder is dried at a suitable temperature to remove any remaining moisture, which could potentially interfere with the material's electrical conductivity and ion transport properties. Following the drying process, the carbon materialis now ready for further characterization or for direct incorporation into a battery cell as the anode component, where its performance can be evaluated in terms of capacity, cycle life, and overall efficiency in a sodium-ion battery system.

[85] The process for the production of hard carbon for sodium-ion batteries includes the step of rinsing the pre-carbonized precursor. After the biomass mixture has undergone initial calcination under inert gas to reach a temperature of 300-600°C for a period of 2-10 hours, creating the pre-carbonized precursor, it is subjected to a thorough rinsing process. This rinsing is performed using an alkali solution. The 23 WO 2026 / 085917 PCT / CN2024 / 129031 concentration of the alkali solution is carefully controlled to fall within a specified range, namely between 0.05 to 1.0 mol / L. The purpose of the alkali rinse is to effectively remove impurities and any remaining volatile matter that may be present within the pre-carbonized precursor. This step iscritical as it influences the structural characteristics of the resulting hard carbon.

[86] The duration of the alkali solution rinse is also a variable parameter and is maintained for a time frame ranging from 2 to 10 hours. The selection of the time period for rinsing is based upon the desired pore structure and chemical purity of the hard carbon. Extended rinsing times may lead to greater activation of the carbon structure, while shorter durations may be sufficient for certain precursor compositions and desired pore distributions.

[87] Following the alkali solution rinse, the pre-carbonized precursor undergoes an additional washing process with water. The washing with water is continued until the pH of the solution reaches approximately neutral (pH from 6.0 to 8.0). The water wash ensures the removal of any residual alkali that may remain from the previous process step. It also aids in neutralizing the carbon material, rendering it safe for handling and further processing. Thisthorough washing step is needed to prevent any undesired chemical reactions during subsequent heat treatments and to ensure the purity of the hard carbon.

[88] The pre-treated precursor, after being thoroughly rinsed and washed, is then subjected to calcination at higher temperatures of 900-1400°C, which aids in the formation of high-quality hard carbon. This hard carbon possesses an optimized closed-pore structure that is ideal for the insertion of sodium ions when used as an anode material in sodium-ion batteries. Through this systematic and precise treatment process, the precursor is transformed into an efficient and effective material with great potential for energy storage applications. The end result is a hard carbon anode material that can enhance the performance and longevity of sodium-ion batteries, thereby contributing to more sustainable and reliable energy storage solutions. 24 WO 2026 / 085917 PCT / CN2024 / 129031

[89] Fig. 2 shows exemplary XRD patterns of three hard carbonsamples produced using the instant method. The amorphous structures are obtained. There are no peaks belong to graphite indicating the successful synthesis of hard carbon. The peak positions are between 23.5 to 23.9°, which indicates a wide layer distance compared to graphite. The XRD pattern in Fig. 2 shows two broad peaks, which are characteristic of hard carbon. The absence of peaks from graphite indicates that the samples are highly amorphous. The first peak, centered at around 23.5 to 23.9°, is attributed to the (002) plane of hard carbon. The distance of the carbon layer is calculated to be 0.3414,0.3404 and 0.3411 nm, which indicates a wide layer distance compared to graphite. The second peak, centered at around 44.0°, is attributed to the (101) plane of hard carbon. The intensity of the (002) peak is typically greater than the intensity of the (101) peak.

[90] Fig. 3 shows exemplary Raman spectra patterns for three hard carbon samples produced using the instant method. TheRaman spectra of the three hard carbon embodiments show two main features: a broad D band at around 1350 cm'1 and a sharp G band at around 1580 cm'1. The D band is attributed to the presence of disorder and defects in the carbon structure, while the G band is attributed to the in­ plane stretching vibration of sp2-bonded carbon atoms. The intensity ratio of the D and G bands, Id / Ig, is a measure of the degree of disorder in the carbon structure. A higher Id / Ig ratio indicates a higher degree of disorder. The Id / Ig ratios of embodiment 1, embodiment 2 and embodiment 3 are 1.2549,1.2241 and 1.2424, respectively. The ratio is constantly higher than 1.000 confirming the hard carbon disorder short chain structure. The degree of disorder in the carbon structure is known to affect the performance of hard carbon electrodes. Hard carbon electrodes with a higher degree of disorder tend to have higher specific capacity and better rate capability.

[91] Fig. 4 shows exemplary TEM images for thethree hard carbon samples produced using the instant method. The method produces a sieving structure depicted through imaging techniques such as TEM, 25 WO 2026 / 085917 PCT / CN2024 / 129031 providing visual confirmation of the nanoscale pore size distribution within the hard carbon. The resulting hard carbon material with the sieving structure supports the ready insertion of Na+ ions upon charging of the battery. This feature is useful for high-efficiency sodium-ion batteries, where a reversible and rapid exchange of ions between the anode and cathode is required during charging and discharging cycles. As can be seen in Fig.4, the obtained hard carbon shows short, disordered carbon chains. The short chains construct closed pores structure with increased layer distance and spaces for Na+ insertion and storage.

[92] The SIB anode in Fig. 1 features closed pores that are needed for efficient sodium ion storage, acting as the primary microstructure for accommodating Na+ ions. The closed porescreate a sieving effect that prevents the formation of a solid electrolyte interphase (SEI) inside the nanopores, which is beneficial for battery performance. The pore size and distribution are needed, in this example improving reversible capacity and initial Coulombic efficiency (ICE), while an upper limit of pore body diameter smaller than 10 nm ensures the reversibility of the plateau capacity.

[93] SIB cathodes can use materials such as sodium metal oxides or phosphates. The electrolyte in these batteries is a sodium salt dissolved in an organic solvent, and the anode composed of hard carbon characterized by sieving closed-pore structure helps prevent excessive electrolyte decomposition inside the pores. Sodium ion storage in hard carbon anodes occurs through multiple mechanisms, including insertion between carbon layers, adsorption on surfaces, and pore filling.

[94] There are several performance advantages associated with this process. Hard carbon anodes with optimized porestructures can achieve plateau capacities in various implementations of up to 300 mAh-g"1 and capacities of 382 mAh-g’1. The sieving closed-pore structure can significantly enhance the initial Coulombic efficiency, with some designs reaching up to ~85%. Additionally, the closed pores improve the capacity from the low-voltage platform and reduce electrolyte decomposition by minimizing undesired SEI formation inside the pores. By carefully controlling the pore structure, microcrystalline structure, and 26 WO 2026 / 085917 PCT / CN2024 / 129031 defects in hard carbon anodes, researchers aim to further improve the electrochemical performance of SIBs, making them a viable and cost-effective alternative to lithium-ion batteries.

[95] Various electrolytes can be used in the SIB. Organic Liquid Electrolyte with sodium salts dissolved in organic solvents can be used. NaCIO4-based organic liquid electrolytes are widely used due to their good compatibility with common cathode materials. Solidelectrolytes can also be used and include materials like Sodium Super Ionic Conductor (NASICON), which offer high ionic conductivity and are non-flammable, providing enhanced safety compared to liquid electrolytes. The electrolyte serves as a medium for sodium ions to move between the cathode and anode during charge and discharge cycles. It also participates in the formation of the solid electrolyte interphase (SEI) on the anode and the cathodic electrolyte interphase (CEI) on the cathode, which are needed for the battery's electrochemical performance.

[96] The SIBs can utilize various biomass materials as the starting point, including but not limited to organic farm materials and cotton. This raw material can be processed under controlled conditions, such as a gradual temperature increase in an inert atmosphere followed by washing with a alkali solution and additional heat treatment. Such meticulous control over the production process ensures the development of hard carbon anodes thatare we 11-suited for the next generation of energy storage systems.

[97] The biomass mixture contains one or more trace impurities which inherently exist due to the nature of the biomass sources from which the mixture is derived. The trace impurities commonly associated with biomass may include, but are not limited to, inorganic salts, metals, and non-carbon organic compounds that are present in minute quantities. These impurities can originate from the soil, water, or air that was in contact with the biomass during its growth and harvesting stages. The instant method enables the production of sieved closed-pore hard carbon which is specifically advantageous for use as anode material in lithium-ion and sodium-ion batteries. The method systematically employs the use of a biomass mixture as the starting material, which offers an environmentally friendly and cost-effective feedstock compared 27 WO 2026 / 085917 PCT / CN2024 / 129031 to fossil-fuel derived carbon sources. The biomass mixture issubjected to a calcination process at a temperature of 300-600°C to initiate the carbonization of the biomass. This is followed by a treatment with NaOH, wherein specific chemical reactions are carried out to activate the carbon structure and increase its porosity. The chemical reactions produce Na and Na2CO3 while releasing H2, and additionally, Na reacts with water to produce further NaOH. Once the re-calcined hard carbon is obtained, it is subjected to a sieving process. This step is critical as it ensures that the final product comprises exclusively those particles that possess an optimal pore size, specifically less than 10 nm. The diminutive size of these pores is instrumental in facilitating the insertion and diffusion of Na+ ions during the operation of the battery, which is needed for high efficiency and battery performance. This sieving step not only selects for pore size but also helps in achieving a uniform distribution of pores, which contributes to the homogeneity of thefinal anode material.

[98] The anode for the SIB includes a hard carbon material characterized by a sieving closed-pore structure with a surface and an inner space, the structure further having a window to receive sodium during charging, wherein the sodium ion passes through the window and absorbs on the surface and occupies the inner space after charging. The anode features closed pores that are needed for efficient sodium ion storage, acting as the primary microstructure for accommodating Na+ ions. The closed pores have tightened entrances creating a sieving effect that prevents the formation of a solid electrolyte interphase (SEI) inside the nanopores, which is beneficial for battery performance. The pore size and distribution are needed, with ultra-micropores improving reversible capacity and initial Coulombic efficiency (ICE), while an upper limit of pore body diameter less than 10 nm ensures the reversibility of the plateau capacity. The closed-pore structure not only enhancesthe reversible capacity but also contributes to a high initial Coulombic efficiency. For example, a closed pore enriched carbon anode can achieve a reversible capacity of 309.3 mAh / g with an initial Coulombic efficiency of 87.8%. The design of these dosed pores is needed for optimizing sodium storage, as they provide a stable environment for 28 WO 2026 / 085917 PCT / CN2024 / 129031 sodium ions during charge and discharge cycles, improving the overall cycle stability and energy density of the SIB.

[99] The sodium-ion battery includes a cathode; an electrolyte coupled to the cathode; and the above anode coupled to the electrolyte. The anode has the plurality of sieving close-pore structures with a surface and an inner space, the structure further having a window to receive sodium ions, wherein the sodium ion passes through the window and absorbs on the surface and occupies the inner space after charging. The closed pores are characterized by their small entrance size, which creates a sievingeffect. This design prevents the formation of a solid electrolyte interphase (SEI) inside the nanopores, a common issue with porous carbons that have larger surface area pores accessible to electrolytes. As a result, sieving carbons exhibit superior electrochemical performance compared to traditional porous carbon materials. For instance, while porous carbons may only achieve a reversible capacity of 39 mAh / g, sieving carbons can reach much higher capacities, such as 328 mAh / g at a current density of 50 mA / g.

[100] There are several performance advantages associated with this design. Hard carbon anodes with optimized pore structures can achieve plateau capacities of in various implementations up to 300 mAh-g"1 and reversible capacities of 382 mAh-g'1. The sieving closed-pore structure can significantly enhance the initial Coulombic efficiency, with some designs reaching up to 80.21%. Additionally, the closed pores improve the capacity from the low-voltage platform and reduceelectrolyte decomposition by minimizing undesired SEI formation inside the pores. By controlling the pore structure, microcrystalline structure, and defects in hard carbon anodes, the electrochemical performance of sodium-ion batteries can be a viable and cost-effective alternative to lithium-ion batteries.

[101] Fig. 5 shows more battery data, specifically the capacity and ICE of embodiment 1, embodiment 2 and embodiment 3. The obtained hard carbon shows a high discharge capacity above 240 mAh / g with ICE above 80%. The data is as follows: 29 WO 2026 / 085917 PCT / CN2024 / 129031 Charge Capacity (mAh / g) Discharge Capacity (mAh / g) ICE (%) Embodiment 1 321.80 276.24 85.8 Embodiment 2 380.22 296.40 78.0 Embodiment 3 287.30 246.33 85.7

[102] The structure of hard carbon is as follows: Short carbon chain; Highly disordered structure; Long layer distance; and Closed-pore structure. Because of the short-disordered structure, the Na+ ions can be adsorbed on the surface of the hard carbon; Insertedinto the hard carbon layers; and aggregated inside closed-pore structure. Thus, the Na+ amount is increased on the anode and generate positive impacts on the electrochemical property of SIBs.

[103] In one implementation of the processing of the biomass mixture, the pre-carbonized precursor obtained after the first calcination step can be subjected to grinding to produce a fine powder. This grinding can ensure homogeneity in particle size and morphology, as well as to increase the surface area of the precursor material. The increased surface area is critical to the performance of the hard carbon as it can potentially enhance the Na+ ion insertion capabilities by providing more accessible active sites for electrochemical reactions. The grinding process is carried out using a suitable grinding apparatus known to those skilled in the art, such as a ball mill, a jet mill, or a mortar and pestle, until the desired powder consistency is achieved. Care is taken to avoid contamination of theprecursor material, as impurities could influence the electrochemical properties of the final hard carbon product. Post grinding, the now powdered pre-carbonized precursor, with significantly reduced particle size, is directed to the second calcination step. It is here that the precursor undergoes a rigorous thermal treatment at a temperature of approximately 1300°C, as previously indicated. The transition to high temperatures aids in the expunging of any remaining inorganic content and facilitates the graphitization process. The resulting product from the second calcination is hard carbon with an improved degree of crystallinity, optimizing it for better 30 WO 2026 / 085917 PCT / CN2024 / 129031 electroconductivity and mechanical stability. Furthermore, with the NaOH treatment, the smaller particle size resulting from the grinding step allows for a more uniform insertion of Na+ ions into the sieving structure of the hard carbon. The sieving structure with Na+ ions inserted proves to haveenhanced electrical characteristics favorable for use as an anode material in sodium-ion batteries. The sieving structure effectively allows for the selective insertion of Na+ ions, ensuring that the battery operation is efficient and the charging and discharging rates are optimal.

[104] The prepared biomass mixture is subjected to a calcination process at a temperature of 450°C, during which the organic substances within the biomass decompose under the influence of heat in an inert nitrogen atmosphere. This thermal treatment is conducted with a specific heating rate of 5°C / min and maintained for a period of 3 hours, allowing gradual transformation of the biomass into a carbonized state while avoiding abrupt thermal shock that could damage the structural integrity of the material.

[105] Subsequently, to enhance the porosity and activate the resulting carbon, the material is treated with an aqueous solution of NaOH. This alkali treatment reacts with the carbonized biomass, as describedby the chemical equation wherein NaOH interacts with carbon to form sodium carbonate and hydrogen gas. This process also helps to widen the pore structure within the carbon matrix, creating a network of channels suitable for ion transport.

[106] Once NaOH treatment is complete, the precursor is subjected to a further increase in temperature, reaching 1300°C, under a nitrogen atmosphere. This re-calcination step serves to stabilize the hard carbon by removing any remaining non-carbon species and aligning the carbon domains to enhance electrical conductivity and structural resilience.

[107] The sieving of the hard carbon selectively removes particles based on pore sizes. It ensures that only those particles with an optimal pore diameter, specifically less than 10 nm, are utilized. These pores 31 WO 2026 / 085917 PCT / CN2024 / 129031 are ideal for accommodating Na+ ions during the charging process, thereby maximizing the charge storage capacity of the hard carbon.

[108] Finally, the resultantsieving structure with Na+ ions inserted exemplifies a material with a high density of accessible active sites for reversible Na+ intercalation, a critical attribute for high-performance sodium-ion battery anodes. Through this method, a sustainable and cost-effective pathway is established for converting agricultural waste into valuable components for the growing energy storage market, which is a significant step forward in the development of green technology solutions.

[109] The resulting hard carbon, with its densely populated, uniform, and dosed pores of approximately 10 nm, provides a unique structure that maximizes the accessible surface area for Na+ ion intercalation while also contributing to a stable and robust electrode material. This architecture contributes to a low strain during charge and discharge cycles, which not only enhances the battery's overall capacity but also its longevity and cycling stability,

[110] Acknowledging the variation in the biomass precursors and theaccompanying disparities in the re-calcination outcomes, the process parameters such as temperature hold time and heating rate can be optimized to yield hard carbon with specific microstructural characteristics tailored to the intended electrochemical application. Through this thermal treatment process, the realization of sieved closed- pore hard carbon is achieved, which is subsequently integrated into an anode that offers superior capacity retention and rate performance, addressing the increasingly exigent energy storage requirements.

[111] While specific embodiments and examples of the present invention have been illustrated and described, numerous modifications come to mind without significantly departing from the spirit of the invention, and the scope of protection is only limited by the scope of the accompanying claims. 32 WO 2026 / 085917 PCT / CN2024 / 129031 Claims 1. A hard carbon with sieving closed-pore structure, wherein a raw material to prepare the hard carbon is a biomassmixture which comprises 70-90% cellulose, 9-20% hemicellulose and 1-10% lignin, wherein a pore size of the sieving closed-pore structure is smaller than 10 nm. 2. A method for preparing hard carbon of claim 1, comprising: 1) calcining the biomass mixture at a temperature between 300-600°C to produce a pre-carbonized precursor; 2) rinsing the pre­ carbonized precursor with an alkali solution then adjusting the pH to 6.0-8.0; and 3) re-calcining the pre-carbonized precursor at a temperature between 900-1400°C to produce a sieving closed-pore hard carbon. 3. The method of claim 2, wherein the calcining biomass mixture is performed in an inert atmosphere. 4. The method of claim 2, wherein the temperature of the calcining biomass mixture is between 300- 600°C. 5. The method of claim 2, wherein the concentration of the alkali solution to rinse the pre-carbonized precursor is 0.05-1.0 mol / L. 6. The method of claim 2, wherein the alkali solution treatment is 2-10 hours. 7. The method of claim2, wherein the alkali solution treatment further comprises modulating the pH to 6.0-8.0. 8. An anode for a battery, comprising a hard carbon material characterized by a sieving closed-pore structure. 9. The anode of claim 8, wherein the sieving closed-pore structures are formed by: calcining a biomass mixture having 70-90% cellulose, 9-20% hemicellulose, and 1-10% lignin at a temperature between 300-600°C to produce a pre-carbonized precursor; rinsing the pre-carbonized precursor with an 33 WO 2026 / 085917 PCT / CN2024 / 129031 alkali solution to adjust the pH to 6.0-8.0; and re-calcining the pre-carbonized precursor at a temperature between 900-1400°C to produce the sieving dosed-pore structures. 10. A battery, comprising: a cathode; an electrolyte coupled to the cathode; and an anode coupled to the electrolyte, the anode having sieving close-pore structures. 11. The battery of claim 10, wherein the sieving closed-pore structures are formed by: calcining a biomass mixture at a temperaturebetween 300-600°C to produce a pre-carbonized precursor; rinsing the pre-carbonized precursor with an alkali solution to adjust the pH to 6.0-8.0; and re­ calcining the pre-carbonized precursor at a temperature between 900-1400°C to produce a sieving closed-pore hard carbon. 12. The battery of claim 11, wherein the biomass mixture comprises 70-90% cellulose, 9-20% hemicellulose, and 1-10% lignin. 34 WO 2026 / 085917 PCT / CN2024 / 129031 1 / 5 Na+ layer dispersion after charging FIG. 1 WO 2026 / 085917 PCT / CN2024 / 129031 2 / 5 (ne) Aiisueiui D is ta nc e (n m ) 0. 34 14 0. 34 04 0. 34 11 3^ o 2t he (d eg oo co CM ID CO CN IO cd CN CM co <-» ** c C ca) a> a) E E E ■o T5 bo o •O bo E E E LU LU UJ FI G . 2 WO 2026 / 085917 PCT / CN2024 / 129031 3 / 5 (ne) 々!sueiu| J? 1. 25 49 1. 22 41 1. 24 24 o CN 00■o C0 CO coc CN o CN(0 0 n N LO LO CO LO■O c IO co o oi co IO r*. CD CM c*> ** ’g *g 0) a> a) E E E ■O *□ ■o bo bo E E E LXJ LU LU 50 0 10 00 15 00 20 00 25 00 30 00 35 00 R am an S hi ft (c m "1) WO 2026 / 085917PCT / CN2024 / 129031 4 / 5 Vo lta ge (V v s. N a* / N a WO 2026 / 085917 PCT / CN2024 / 129031 5 / 5 FIG. 5 INTERNATIONAL SEARCH REPORT International application No. PCT / CN2024 / 129031 A. CLASSIFICATION OF SUBJECT MATTER C01B 32 / 05(2017.01)i; H01M4 / 583(2010.01)i; H01M 10 / 054(2010.01)i; H01M 4 / 133(2010.01)i According to International Patent Classification (IPC) or to both national classification and EPC B. FIELDS SEARCHED Minimum documentation searched (classification system followed by classification symbols) IPC: C01B 32 / -, H01M 4 / -, H01M 10 / - Documentation searched other than minimum documentation to the extent that such documents are included in the fields searched Electronic data base consulted during the international search (name of data base and, where practicable, search terms used) CNTXT, VEN, ENTXTC, CNKI, WEB OF SCIENCE: hard carbon, cotton, hemp, cellulose, hemicellulose, lignin, carboni+, alkali solution, sodium hydroxide, NaOH, sieving, closed, pore, battery, cell, anode C. DOCUMENTSCONSIDERED TO BE RELEVANT Category* Citation of document, with indication, where appropriate, of the relevant passages Relevant to claim No. X CN 118047370 A (QINGDAO JIALINXING TECHNOLOGY CO LTD) 17 May 2024 (2024-05-17) description, paragraphs

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[0023] to

[0027] ,

[0040] , Figures 1-2 1-12 A CN 118289733 A (GUILIN UNIVERSITY OF ELECTRONIC TECHNOLOGY) 05 July 2024 (2024-07-05) the whole document 1-12 A CN 116812906 A (HUNAN NAKE NEW MATERIAL CO LTD) 29 September 2023 (2023-09-29) the whole document 1-12 A CN 117003237 A (SHENZHEN JIANA ENERGY TECHNOLOGY CO LTD) 07 November 2023 (2023-11-07) the whole document 1-12 A CN 118239487 A (WUHAN UNIVERSITY OF SCIENCE AND TECHNOLOGY) 25 June 2024 (2024-06-25) the whole document 1-12 | / | Further documents are listed in the continuation of Box C. See patent family annex. * Special categories of cited documents: “T" later document published after the international filing date or priority “A” document defining the generalstate of the art which is not considered date and not in conflict with the application but cited to understand tbe to be of particular relevance principle or theory underlying the invention “D” document cited by the applicant in the international application “X” document of particular relevance; the claimed invention cannot be “E” earlier application or patent but published on or after the international considered novel or cannot be considered to involve an inventive step filing date when the document is taken alone “L” document which may throw doubts on priority claim(s) or which is “Y” document of particular relevance; the claimed invention cannot be cited to establish the publication date of another citation or other considered to involve an inventive step when the document is special reason (as specified) combined with one or more other such documents, such combination “0” document referring to an oral disclosure, use, exhibition or other being obvious to a person skilled in theart means document member of the same patent family “P” document published prior to the international filing date but later than the priority date claimed Date of the actual completion of the international search 26 June 2025 Date of mailing of the international search report 16 July 2025 Name and mailing address of the ISA / CN CHINA NATIONAL INTELLECTUAL PROPERTY ADMINISTRATION 6, Xitucheng Rd., Jimen Bridge, Haidian District, Bering 100088, China Authorized officer SHI,WeiLiang Telephone No. (+86) 010-62085011 Form PCT / ISA / 210 (second sheet) (July 2022) INTERNATIONAL SEARCH REPORT International application No. PCT / CN2024 / 129031 C. DOCUMENTS CONSIDERED TO BE RELEVANT Category* Citation of document, with indication, where appropriate, of the relevant passages Relevant to claim No. A CN 118373408 A (CHANGZHOU QUANT AN NEW ENERGY TECHNOLOGY CO LTD) 23 July 2024 (2024-07-23) the whole document 1-12 A CN 118561262 A (WUXI PANGU NEW ENERGY CO LTD, etc.) 30 August 2024 (2024-08-30) the wholedocument 1-12 A CN 114804065 A (CENTRAL SOUTH UNIVERSITY) 29 July 2022 (2022-07-29) the whole document 1-12 A CN 115064667 A (CENTRAL SOUTH UNIVERSITY) 16 September 2022 (2022-09-16) the whole document 1-12 A WO 2024207302 Al (GUANGDONG BRUNP RECYCLING TECHNOLOGY CO LTD, etc.) 10 Octoher 2024 (2024-10-10) the whole document 1-12 A US 2024336484 Al (HAYCARB PLC) 10 October 2024 (2024-10-10) the whole document 1-12 Form PCT / ISA / 210 (second sheet) (July 2022) INTERNATIONAL SEARCH REPORT Information on patent family members Patent document cited in search report Publication date (day / month / year) Patent family member(s) Publication date (day / month / year) CN 118047370 A 17 May 2024 None CN 118289733 A 05 July 2024 None CN 116812906 A 29 September 2023 None CN 117003237 A 07 November 2023 None CN 118239487 A 25 June 2024 None CN 118373408 A 23 July 2024 None CN 118561262 A 30 August 2024 None CN 114804065 A 29 July 2022 None CN 115064667 A 16 September 2022 None WO 2024207302 Al 10 October2024 None US 2024336484 Al 10 October 2024 KR 20240151114 A 17 October 2024 EP 4442641 Al 09 October 2024 International application No. PCT / CN2024 / 129031 Form PCT / ISA / 210 (patent family annex) (July 2022) (54)Title Hard carbon with a molecular sieve-type closed pore structure as an anode material for a sodium ion battery (57)Abstract The present invention discloses a hard carbon with a molecular sieve-type closed pore structure and a preparation method thereof. Using a biomass mixture, such as cotton, crop waste, as a raw material or a mixture containing 70-90% cellulose, 9-20% hemicellulose, and 1-10% lignin, etc. as a raw material, calcining in the range of 300-600 °C to obtain a pre-carbonized precursor, then washing the precursor with an alkali solution, and finally performing a second calcination in the range of 900-1400 °C to obtain a hard carbon product with a molecular sieve-type closed pore structure. (19)State Intellectual Property Office of the People's Republic of China (12)Patent application for invention (10)Publication number of the application CN 119604994 A (43)Publication date of the application 2025. 03. 11 (21)Application number 202481. 20% hemicellulose and 1-10% lignin, wherein the molecular sieve-type closed-pore structure has a pore size of less than 10 nm. 2. A method for preparing the hard carbon of claim 1, comprising: 1) calcining the biomass mixture at 300-600°C to produce a pre-carbonized precursor; 2) washing the pre-carbonized precursor with an alkaline solution and then adjusting the pH to 6.0-8.0; and 3) recalcining the pre-carbonized precursor at 900-1400°C to produce molecular sieve-type closed-pore hard carbon. 3. The method of claim 2, wherein the calcination of the biomass mixture is carried out in an inert gas atmosphere. 4. The method of claim 2, wherein the temperature at which the biomass mixture is calcined is between 300-600°C. 5. The method of claim 2, wherein the concentration of the alkaline solution used to wash the pre-carbonized precursor is 0.05-1.0 mol / L. 6. The method of claim 2, wherein the alkaline solution treatment time is 2-10 hours. 7. The method of claim 2, wherein the alkaline solution treatment further comprises adjusting the pH value to 6.0-8.0. 8. An anode of a battery, comprising: a hard carbon material having a molecular sieve-type closed-pore structure. 9. The anode of claim 8, wherein the molecular sieve-type closed-pore structure is formed by: calcining a biomass mixture having 70-90% cellulose, 9-20% hemicellulose and 1-10% lignin at 300-600°C to produce a pre-carbonized precursor; washing the pre-carbonized precursor with an alkaline solution to adjust the pH value to 6.0-8.0; and recalcining the pre-carbonized precursor at 900-1400°C to produce the molecular sieve-type closed-pore structure. 10. A battery, comprising: a cathode; an electrolyte coupled to the cathode; and an anode coupled to the electrolyte, the anode having a molecular sieve-type closed-pore structure. 11. The battery according to claim 10, wherein the molecular sieve-type closed-cell structure is formed by: calcining a biomass mixture at 300-600°C to produce a pre-carbonized precursor; washing the pre-carbonized precursor with an alkaline solution to bring the pH value to 6.0-8.0; and further calcining the pre-carbonized precursor at 900-1400°C to produce molecular sieve-type closed-cell hard carbon. 12. The battery according to claim 11, wherein the biomass mixture comprises 70-90% cellulose, 9-20% hemicellulose, and 1-10% lignin. 2 CN 119604994 A Specification 1 / 13 pages Hard Carbon with Molecular Sieve-Type Closed-Cell Structure as Anode Material for Sodium-Ion Batteries Background Art

[0001] The present invention relates to hard carbon with a molecular sieve-type closed-cell structure and a method for manufacturing the same.

[0002] In recent years, sodium-ion batteries (SIBs) have received widespread attention as a promising alternative to lithium-ion batteries, especially due to the abundance and wide availability of sodium. However, the performance of sodium-ion batteries largely depends on the quality of the anode material, which must accommodate the larger radius of sodium ions than lithium ions and allow for rapid ion transport.

[0003] Hard carbon is widely used as the active anode material for SIBs. Due to its low cost, hard carbon is the best choice for SIB anodes. The structure of hard carbon includes short carbon chains, highly disordered structure, long interlayer spacing, and closed-cell structure. Hard carbon is a non-graphite carbon with a certain degree of short-range order in its amorphous structure. Because this structure can accommodate sodium ions, it is particularly suitable for energy storage applications.

[0004] Traditionally, hard carbon from various carbon sources has been used as the anode material for SIBs, but the lack of control over the pore structure often leads to unsatisfactory electrochemical performance. The pore structure plays a necessary role in promoting sodium ion diffusion and accommodating volume changes during charge-discharge cycles.

[0005] On one hand, hard carbon is characterized by a molecular sieve-type closed-pore structure that provides surface and internal space. This structure also has a window for receiving sodium during charging, wherein sodium ions pass through the window and adsorb onto the surface after charging, occupying the internal space.

[0006] On the other hand, the present invention discloses a method for preparing hard carbon. This method includes a two-step heat treatment (calcination) of a biomass mixture under specific conditions to achieve ideal physical and chemical properties of the hard carbon product. Hard carbon with a molecular sieve-type closed-pore structure is formed by calcining a biomass mixture having 70-90% cellulose, 9-20% hemicellulose and 1-10% lignin at 300-600°C to generate a pre-carbonized precursor; washing the pre-carbonized precursor with an alkaline solution and adjusting the pH to 6.0-8.0; and recalcining the pre-carbonized precursor at 900-1400°C to generate molecular sieve-type closed-pore hard carbon.

[0007] In another aspect, the anode of the sodium-ion battery comprises a hard carbon material having a molecular sieve-type closed-pore structure.

[0008] In yet another aspect, the sodium-ion battery comprises a cathode; an electrolyte coupled to the cathode; and an anode coupled to the electrolyte, the anode having a plurality of molecular sieve-type closed-pore structures.

[0009] Implementations of the above aspects may include one or more of the following aspects. The biomass mixture may be a carbon-rich organic material. The biomass mixture, as a precursor, undergoes a heat treatment process or calcination. The calcination step thermally decomposes the organic components of the biomass, producing a carbon structure. The calcination process is carried out in an inert gas to prevent oxidation of the carbonaceous material. The precursor is subjected to a temperature of 300-600°C. The temperature is increased at a rate of 2-10°C / min in the inert gas. This treatment process results in the initial decomposition and carbonization of the biomass. Subsequently, a concentration of 0.05-1...The material is cleaned with an alkaline solution of 0 mol / L to clean and activate the carbon surface. The second heat treatment extends to a high temperature of 900-1400°C. The heating rate is 2-10 °C / min in an inert gas environment. In this recalcination step, the hard carbon structure is further developed as residual volatile substances are eliminated, thereby increasing the porosity of the hard carbon material. The resulting hard carbon material has an optimized structure that can be used for the reversible insertion and extraction of sodium ions, thereby producing high-performance SIBs.

[0010] The advantages of this material include one or more of the following. This method utilizes the inherent structure of biomass composed of cellulose, hemicellulose and lignin to form a hard carbon precursor with an ideal pore framework after calcination. By optimizing the calcination temperature and employing post-treatment cleaning and recalcination steps, the hard carbon material produced has a molecular sieve-type closed-pore structure with a target pore size distribution. This structure not only facilitates rapid ion transport but also provides structural integrity, enhancing the cycle stability of sodium-ion batteries. Given the natural abundance and renewability of biomass, this method uses biomass as a starting material, offering sustainability and cost-effectiveness. This method recycles agricultural waste, promoting a more environmentally friendly production process while addressing waste management issues. The biomass mixture used in the disclosed method is derived from agricultural waste, providing abundant renewable raw materials for the production of hard carbon anode materials for sodium-ion batteries. By utilizing agricultural waste, this method not only provides a sustainable battery manufacturing method but also helps manage and reduce agricultural waste, achieving a closed-loop agricultural lifecycle and promoting environmental practices.

[0011] Another advantage is the ability to create optimized structures for sodium storage, leading to the development of sodium-ion batteries with higher capacity, rate performance, and cycle stability. Therefore, this method represents a significant advancement in the design and synthesis of next-generation energy storage anode materials. Hard carbon, obtained through carefully controlled biomass calcination and chemical processing, possesses a molecular sieve-like closed-pore structure, making it highly suitable as an anode material for sodium-ion batteries. Studies have found that this structure is highly beneficial for improving the electrochemical performance of the material, particularly in the Na+ ion insertion and extraction kinetics.

[0012] Another advantage of this method is that it can produce hard carbon with a molecular sieve-type closed-pore structure. Hard carbon materials have high surface area, appropriate pore distribution, and conductivity, which are key characteristics for achieving fast charge / discharge rates and high energy density in batteries. These characteristics make molecular sieve-type closed-pore hard carbon a typical candidate material for advanced energy storage applications, thus meeting the growing demand for renewable energy solutions and the transition to more sustainable energy storage technologies.

[0013] Figure 1 shows a TEM image of the hard carbon of the present invention and a sodium layer dispersion after charging.

[0014] Figure 2 shows exemplary XRD patterns of three hard carbon samples prepared using the method of the present invention.

[0015] Figure 3 shows exemplary Raman spectra of three hard carbon samples prepared using the method of the present invention.

[0016] Figure 4 shows exemplary TEM images of three hard carbon samples prepared using the method of the present invention.

[0017] Figure 5 shows an example of SIB performance curves. Detailed Description

[0018] This disclosure relates to hard carbon characterized by a molecular sieve-type closed-pore structure, and a method for preparing said hard carbon, which is derived from a biomass mixture for use in sodium-ion batteries. The method optimizes the pore structure within the hard carbon to enhance the intercalation of Na+ ions, thereby improving the battery's storage capacity and efficiency.

[0019] To obtain the desired hard carbon structure, suitable carbon-rich biomass needs to be selected and prepared to ensure a high carbon yield during processing. To prepare the biomass mixture, agricultural residues, considered as byproducts or wastes of agricultural activities, need to be collected. These residues may include, but are not limited to, plant stems, leaves, shells, husks, and other cellulose-rich organic matter. These materials are typically subjected to preliminary treatments such as drying and grinding to achieve consistent particle size and homogenization, thereby enabling uniform calcination and chemical reactions.

[0020] Biomass is converted into hard carbon through a series of heat and chemical treatments, and its properties are favorable for energy storage in sodium-ion batteries. In one example, the biomass mixture includes cellulose, hemicellulose, and lignin in proportions favorable for efficient conversion into hard carbon. Specifically, the cellulose content is 70% to 90%, which is the main carbon source. Hemicellulose is a structural polysaccharide found in plant cell walls, similar to cellulose, and its content is 9% to 20%. Lignin is an organic polymer that provides structural support for vascular plants and certain algae, and accounts for 1% to 10% of the mixture. This specific composition ensures that the necessary properties for hard carbon formation are met and that the resulting material has the properties required for electrochemical applications.

[0021] The above biomass mixture contains 70-90% cellulose, 9-20% hemicellulose, and 1-10% lignin. The biomass mixture is initially calcined at 300-600°C to obtain a precursor characterized by cellulose as a carbon chain, hemicellulose as a short carbon chain and a porous structure, with lignin / other substances providing the porous structure. The precursor is then washed in an alkali solution at a concentration of 0.05-1.0 mol / L and neutralized to a pH of 6.0-8.0.

[0022] Next, a second calcination is performed at 900-1400°C to produce the desired molecular sieve-type closed-pore hard carbon. The resulting hard carbon is suitable for use as an anode material in sodium-ion batteries.

[0023] After obtaining the biomass mixture, the process involves calcining the mixture at a temperature range of 300°C to 600°C. The calcination process causes thermal decomposition of the organic components in the biomass, promoting the conversion of the biomaterial to a pre-carbonized state. This precursor…The precursor state is an intermediate step in the synthesis of hard carbon. During this process, biomass undergoes significant chemical changes due to heating, but does not reach the stage of complete hard carbon formation.

[0024] After calcination, the pre-carbonized precursor generated from the biomass mixture is cleaned. Cleaning is performed using an alkaline solution to neutralize any residual acidic compounds that may be present in the pre-carbonized precursor and to produce a closed-cell structure. This process purifies the precursor and optimizes its chemical environment for subsequent conversion into hard carbon.

[0025] Alkaline treatment prepares the hard carbon structure for effective insertion of sodium ions (Na+). When the precursor material is treated with an alkali, a series of chemical reactions occur, thereby altering the carbon structure and forming carbon chains of different lengths. This improves the performance of sodium-ion batteries using hard carbon.

[0026] Taking NaOH as an example, adding NaOH to the carbonaceous precursor causes a redox reaction, where carbon (Na+) is oxidized, promoting the formation of metallic sodium (Na), sodium carbonate (Na2CO3), and hydrogen (H2). This reaction promotes the activation of the carbon surface, thereby increasing porosity and surface area, which is beneficial for the diffusion of Na+ ions and electrochemical activity.

[0027] Subsequently, the reaction of metallic sodium with water (if present) produces additional NaOH and releases hydrogen gas. This step further affects the pore structure, creating an optimal environment for the insertion and release of Na+ ions during battery charge-discharge cycles.

[0028] Through these reactions, alkaline treatment modifies the biomass-derived precursor into a hard carbon form with a molecular sieve structure, thereby accommodating Na+ ions and improving the cycle stability and energy capacity of sodium-ion batteries. This material is well-suited for use as an anode in energy storage systems, especially because of its high surface area, ideal pore size distribution, and functional groups generated during alkaline activation.

[0029] The alkaline solution helps remove organic impurities and etch the carbon skeleton, thereby enlarging or forming the pores required for ion transport. The choice of alkaline depends on the desired chemical affinity and the specific characteristics of the precursor used.

[0030] The choice of alkaline solution is not limited to sodium hydroxide (NaOH), but other suitable solutions can also be selected, such as potassium hydroxide (KOH), lithium hydroxide (LiOH), and ammonia (NH4OH). Each alkaline solution has unique properties that affect the activation and pore development process of the precursor material, thereby affecting the structure and performance of the final hard carbon product. NaOH can effectively enhance the pore structure and improve the accessibility of sodium ions, while KOH is known for producing larger pore volumes, while LiOH and NH4OH may produce different pore development dynamics due to their different ionic radii.

[0031] Cleaning not only cleans the precursor, but also helps the chemical activation of the carbon structure. After thorough cleaning, the pre-carbonized precursor is further heat-treated at a controlled heating rate under an inert atmosphere such as nitrogen (N2) and argon (Ar) to finally reach the target high temperature to form the desired hard carbon structure.

[0032] Increasing the temperature to 900-1400°C promotes the conversion of the activated precursor into the final form of hard carbon. It is at this stage (page 4 / 13 of specification 5 CN 119604994 A) that the unique pore structure is fixed, and the material obtains the high surface area and porosity required for efficient sodium ion insertion. This material, composed of closed-pore hard carbon with pores less than 10 nm, is ideally suited for use as an anode material in sodium-ion batteries because it provides an optimal ion transport path, thereby improving the battery's performance in terms of energy density, lifespan, and charge rate. The molecular sieve structure formation process produces particles within a specific size range, ensuring material consistency. The molecular sieve structure formation process also separates closed-pore hard carbon particles with ideal pore sizes, thereby ensuring efficient Na+ ion insertion and extraction during battery operation.

[0033] The second calcination is preferably carried out in an inert gas such as nitrogen (N2) or argon (Ar) to prevent undesirable oxidation of the carbon material. Controlled processing environment and specific temperature regulation ensure that the formed hard carbon has ideal pore distribution and integrity, thereby ensuring the consistency of electrochemical performance when used in sodium-ion batteries.

[0034] This method of preparing powdered hard carbon and calcining it sequentially is conducive to batch synthesis, thereby enabling large-scale production of anode materials. This scalability is an important consideration for the widespread application of sodium-ion battery technology in various applications, from portable electronics to grid-scale energy storage systems. In addition, the use of biomass as a starting material makes full use of renewable resources, thus contributing to the sustainability of the entire production process.

[0035] The temperature of the second calcination is significantly higher than that of the first calcination and is adjusted to the range of 900°C to 1400°C. The increase in temperature during the second calcination promotes the complete carbonization of the precursor material, thereby forming hard carbon. The high-temperature treatment promotes the evolution of non-carbon atoms and helps to form a graphite-like structure, which is necessary to achieve ideal conductivity and crystallinity. It is at this stage that the ideal molecular sieve closed-pore structure is formed. When hard carbon is used as an anode material, these closed pores are crucial to its effectiveness because they can modulate the interaction between the hard carbon structure and sodium ions during battery charge-discharge cycles. Hard carbon with such fine micropores exhibits better electrochemical performance when subsequently integrated into sodium-ion batteries, offering advantages in energy storage capacity, cycle life, and rate performance.

[0036] In this patent, the method ensures the formation of a hard carbon structure with specific molecular sieve closed-pore characteristics, making it suitable for use as an anode material for sodium-ion batteries. Sodium (Na+) has a larger diameter than lithium (Li+). Therefore, Na+ cannot be embedded in graphite anodes. Due to the short disordered structure, Na+ ions can: i) adsorb on the surface of hard carbon; ii) embed in the hard carbon layer; iii) aggregate within the closed-pore structure. Therefore, the increased amount of Na+ on the anode has a positive impact on the electrochemical performance of SIBs. The TEM structure of hard carbon is shown in Figure 1.The dispersion of the sodium layer after charging is also shown in Figure 1, where Na+ (1) is adsorbed on the surface, (2) is embedded in the carbon layer, and (3) is aggregated in the pores. The resulting structure has high coulombic efficiency and high specific capacity, thereby extending the battery life. The characteristic of molecular sieve-type hard carbon is that the pore distribution is uniform, which is directly related to the insertion efficiency of Na+ ions during battery charging. This efficiency is particularly important because uniform and optimal pore size of less than 10 nm can improve ion conductivity and charge-discharge cycle stability, thereby improving the energy storage and life of the battery.

[0037] The transmission electron microscope (TEM) image in Figure 1 shows the pore structure with a pore size of less than 10 nm that forms the molecular sieve structure. The obtained image reveals the complex details of the material's microstructure and provides direct evidence of the successful acquisition of molecular sieve-type closed-pore hard carbon particles with uniform pore size. In this image, we can clearly see the uniformity and distribution of pores inside the carbon matrix, which is a characteristic of molecular sieve-type structures.

[0038] Closed-pore hard carbon structure is an indispensable component of sodium-ion battery anode materials. Closed pores provide discrete locations for the insertion and storage of sodium ions in hard carbon materials. Na+ ions are confined within these pores, providing a stable environment and reducing the likelihood of harmful reactions with the electrolyte or other battery components, thus contributing to improved charge-discharge cycle efficiency. The resulting molecular sieve-like structure ensures that most pores are smaller than 10 μm in size. Therefore, most of the pores in the hard carbon material shown in the figure are within this optimal size range, which is particularly advantageous for the kinetics of Na+ ion insertion and extraction during battery operation. Furthermore, the figure also depicts the results of chemical and thermal treatments, including the chemical reactions NaOH + C → Na + Na2CO3 + H2, and Na + H2O → NaOH, produced using NaOH. These treatments contribute to increasing the porosity and electrochemically active surface area of ​​the carbon.

[0039] In one aspect concerning Na+ ion insertion, molecular sieve-type closed-pore hard carbon clearly improves the electrochemical performance of sodium-ion batteries. The molecular sieve-like structure is designed to selectively allow Na+ ion insertion during battery charging. Controlled embedding of Na+ ions into finely tuned pore sizes (in one example, less than 10 nm) ensures efficient utilization of the available electrochemical surface within the zeolite structure and maximizes charge storage capacity.

[0040] Once embedded, Na+ ions reside within the pores of the zeolite-type closed-pore structure, providing a stable and efficient pathway for ion transfer during battery operation. The zeolite-type closed-pore structure with embedded Na+ ions exhibits significantly improved performance when used as an anode material in sodium-ion batteries, demonstrating excellent charge capacity and enhanced cycle stability.

[0041] The preparation of hard carbon begins with selecting a suitable biomass mixture. This mixture is derived from carbonaceous organic matter and is capable of...Biomass is converted into solid carbon through thermal decomposition. To ensure the quality and consistency of the pore structure, biomass needs to undergo a calcination process. The initial calcination temperature is about 300-600°C. During the calcination process, volatile components in the biomass are discharged, leaving carbon-rich residues.

[0042] After the initial calcination, the resulting carbonaceous material is mixed with an alkaline solution. The chemical reaction that occurs at this stage not only activates the carbon material but also plays an important role in the pore development process. Specifically, the interaction with the alkali promotes the opening of pores within the carbon structure, preparing the material for subsequent processing.

[0043] In the cleaning step, impurities that may have been introduced during the calcination process or inherent impurities in the biomass precursor are removed. Otherwise, when hard carbon is used as the anode material for sodium-ion batteries, these impurities will negatively affect the electrochemical performance of the hard carbon.

[0044] After cleaning and further purification, the carbon material will undergo a second calcination at a much higher temperature, reaching 900-1400°C. It is during this recalcination process that the carbon material solidifies into hard carbon. Simultaneously, this treatment effectively generates a porous structure, forming closed pores with a median pore size of less than 10 nm.

[0045] After high-temperature treatment, the hard carbon produces a molecular sieve-like structure. This process aims to create the desired porous structure in the material. This is a crucial step to ensure that the final product has a consistent and uniform molecular sieve-like closed-pore structure, thereby promoting the transport kinetics of the target Na+ ions.

[0046] Initially, the biomass mixture serves as a precursor for closed-pore hard carbon, and is calcined at a reference temperature of 300-600 °C. This stage eliminates volatile compounds and initiates the carbonization of the biomass. The precursor is then treated with alkali to initially form the desired porous carbon structure. To further refine the carbon structure, recalcination is required at a higher reference temperature of 900-1400 °C. This step enhances the energy storage properties of the carbon matrix, namely its conductivity and ion accessibility.

[0047] The heat treatment step at 900-1400°C eliminates any residual non-carbon elements and promotes the formation of a graphitic structure favorable for Na+ ion storage. Furthermore, this stringent high temperature helps achieve the desired graphitization and porous structure, which are essential characteristics for efficient ion transport and intercalation. The use of an inert gas (typically nitrogen) during the recalcination process prevents oxidation of the carbonaceous material and facilitates the formation of the porous network required for the target application.

[0048] During the pre-carbonization stage, the temperature should be maintained below 600°C to prevent excessive ash formation, which would reduce the quality of the final carbon product. Conversely, the temperature should be maintained above 300°C to ensure sufficient thermal decomposition of the biomass components. This controlled heat treatment is necessary when decomposing the lignocellulose structure and other organic compounds in the biomass mixture to produce a pre-carbonized material with a porous matrix, which is essential for subsequent activation and formation of the final hard carbon structure.

[0049] After the pre-carbonization step, the resulting semi-carbonized biomass mixture is immersed in an alkaline solution to initiate the chemical activation process. The alkali not only removes residual impurities from the carbonized material but also generates additional porosity by etching the carbon skeleton. The reaction between the alkali and carbon elements forms carbonates and hydrogen, further affecting the material's texture properties.

[0050] The activated material then enters a second stage of carbonization (called recalcination), which is heated to a temperature of approximately 900-1400°C. In this stage, the already formed porous carbon structure is transformed into a tightly packed layer similar to graphene, i.e., hard carbon. This recalcination at a higher temperature removes residual non-carbon elements, thereby purifying the carbonaceous material and enhancing the conductivity required for the battery anode.

[0051] This process can achieve a narrow pore size distribution, in one case being that the pore size is mainly less than 10 nm. This small pore size is beneficial for the insertion and extraction of sodium ions during battery charge-discharge cycles. By utilizing the size difference between the desired porosity and the larger, unusable pore structure, a molecular sieve structure is produced through processing to achieve this reasonable closed-pore distribution.

[0052] Finally, this molecular sieve-type micro-closed-pore hard carbon is loaded into a sodium-ion battery. This hard carbon serves as the anode, in which sodium ions are embedded. The dense pore structure not only enables efficient sodium ion transport but also improves the overall energy density and charge / discharge rate of the battery. This innovative approach combines sustainable raw materials with precise thermal and chemical treatments, ultimately forming an advanced energy storage solution that meets the growing demand for renewable and high-performance battery technologies.

[0053] Without alkaline washing, a mixture of soft and hard carbon is produced. Furthermore, when the recalcination temperature is above 1400 °C, glassy carbon is produced.

[0054] During the calcination process and subsequent recalcination steps, most of these trace impurities are reduced or eliminated. High temperatures can cause the volatilization and decomposition of many organic compounds, and inorganic substances may be transformed or oxidized to form ash. Ash can be removed from the final product through various purification methods (such as washing, filtration or acid treatment).

[0055] After initial heat treatment, the precursor will react chemically with alkali. Alkali has a dual role: it helps to activate the carbon structure and promotes the formation of porosity; it can also react with certain impurities to form soluble salts that can be washed away, thereby aiding the purification process.

[0056] One or more trace impurities are usually present in the natural sources of biomass. These impurities may include, but are not limited to, potassium ions (K+), calcium ions (Ca2+) and magnesium ions (Mg2). When hard carbon is used as the anode material of sodium-ion batteries, the presence of these impurities may affect its electrochemical characteristics.

[0057] The presence of potassium ions (K+) may be a result of the biomass composition or may be due to the extraction and purification process of carbon materials.Residues from the processing chemicals used. These ions may occupy active sites within the carbon lattice or reside in micropores, affecting the movement of Na+ ions and thus impacting storage capacity.

[0058] Similarly, calcium (Ca2+) and magnesium (Mg2+) ions may also be incorporated into the carbon structure. These multivalent impurities may affect conductivity and cause changes in pore structure due to their large ionic size and potential interaction with the carbon matrix.

[0059] Although these impurities are trace amounts, comprising only a small fraction of the overall composition of hard carbon, they can affect the overall performance of the material. Therefore, measures can be taken during manufacturing to reduce the concentration of these impurities or to utilize their presence to optimize electrochemical performance.

[0060] Furthermore, strategies to mitigate the impact of these trace impurities on sodium-ion battery performance may include additional purification processes, post-treatment improvements, or incorporation during controlled calcination to alter their state, distribution, or interaction with Na+ ions.

[0061] In one example, impurity removal is achieved by first washing the biomass mixture with an acidic solution. The pH of this acidic solution is strictly controlled between 3 and 6 to ensure that its acidity is sufficient to dissolve and remove various impurities present in the biomass, such as minerals, inorganic salts, and other non-carbonaceous components. The selected pH range is crucial for effectively removing impurities from the biomass without causing excessive degradation of the organic matter required for subsequent carbonization processes. 8 CN 119604994 A Specification 7 / 13 pages

[0062] In another embodiment, the washing process lasts from 2 to 10 hours, depending on the type and content of impurities in the biomass. This duration is chosen to optimize the acid washing effect while minimizing the risk of damaging the integrity of the carbon precursors in the biomass. During this period, the biomass mixture is completely immersed in the acidic solution, allowing sufficient interaction between the acid and the impurities, thereby enhancing the overall cleaning effect.

[0063] After acid washing, the biomass mixture undergoes a second water wash. The purpose of the water wash is to remove one or more acids used in the impurity removal process. This step must be carried out with care to ensure that all traces of acidic solution in the biomass mixture are removed. The presence of residual acid may interfere with downstream processes such as calcination and negatively affect the performance of the prepared hard carbon material.

[0064] The water used for rinsing is typically neutral or near-neutral pH to counteract the acidity from the previous rinsing step. The biomass is thoroughly rinsed to ensure no acidic residue remains. Water rinsing not only neutralizes the biomass but also further removes any soluble impurities that have been loosened but not completely dissolved by acid washing.

[0065] After the rinsing step, the biomass mixture is dried to remove any added moisture. The drying step prepares the biomass for subsequent high-temperature treatments (such as calcination and recalcination), at temperatures of 300-600°C.°C and 900-1400°C are used to convert biomass into structured hard carbon materials suitable for use in sodium-ion batteries.

[0066] Through these cleaning processes, the biomass mixture is conditioned into a high-purity precursor, which is beneficial for producing high-quality hard carbon materials characterized by a molecular sieve-type closed-pore microstructure, ideal for accommodating Na+ ions during sodium-ion battery operation.

[0067] Another embodiment uses an acidic solution to treat and optimize the porosity and chemical properties of the material. The acidic solution consists of one or more acids selected from nitric acid (HNO3), hydrochloric acid (HCl), acetic acid (CH3COOH), sulfuric acid (H2SO4), and phosphoric acid (H3PO4). These acidic solutions have multiple functions in treating the biomass precursor and the resulting hard carbon. First, it acts as a cleaning agent, removing impurities and potentially catalyzing reactions that hinder the performance of hard carbon as an electrode material. Typically, the use of acidic solutions involves an acid washing step, i.e., immersing the biomass mixture in an acidic medium. The choice of acid or combination of acids depends on the desired chemical reactivity and the properties of the biomass used. Reaction with nitric acid generally facilitates the introduction of nitrogen-doped sites into the carbon matrix, potentially enhancing electronic conductivity and providing active sites for sodium ion storage. Hydrochloric acid treatment is known to remove metallic impurities and help open the carbon pore structure. Acetic acid, due to its relatively mild acidity, can be used to alter surface chemistry without excessively etching the carbon skeleton. Finally, sulfuric acid, known for its strong oxidizing power, can be used to embed sulfonic acid groups into the carbon surface, thereby increasing the material's hydrophilicity and potential ionic conductivity. In the preparation of hard carbon, the biomass mixture is treated with these acids under controlled conditions; the concentration, temperature, and contact time vary depending on the desired properties of the final product. After acid treatment, the mixture is thoroughly washed to remove residual acid and byproducts. This is followed by a calcination step, which helps form a stable carbonaceous structure, typically resulting in a molecular sieve-type closed-pore structure (described later) with specific pore sizes that facilitate the efficient insertion and extraction of sodium ions during battery operation. The molecular sieve-type closed-pore structure obtained by this method can optimize the role of the sodium-ion battery anode by selectively capturing hard carbon particles with the desired pore size. The alkali treatment, calcination, and recalcination processes ensure that the carbon material has appropriate conductivity and stability, thereby effectively cycling sodium ions. The combination of the molecular sieve-type closed-pore structure and the embedded Na+ ions forms a robust and efficient sodium-ion battery electrode with the potential for high energy density, long cycle life, and cost-effectiveness due to the use of abundant starting materials and scalable manufacturing processes.

[0068] Furthermore, the method supports sustainable production practices because it utilizes biomass, a renewable resource, as a carbon source. Given the current shift to renewable energy systems and the necessity of compatible energy storage technologies, transforming this raw material into advanced energy storage solutions is particularly important. 9CN 119604994 A Specification 8 / 13 • Page

[0069] The pre-carbonized precursor can be cleaned with water. The cleaning step removes any loose dust and particulate matter that has not chemically bonded to the precursor material during the pre-carbonization process. This process may include stirring or sonication to enhance the cleaning action, followed by filtration or decantation to separate the solid carbonaceous material from the particles suspended in the cleaning water. [OOM] After thorough cleaning, the pre-carbonized precursor is then cleaned with an alkaline solution. The alkaline treatment includes the use of KOH, NaOH, LiOH, and NH4OH solutions, which react with the carbonaceous material. This step is necessary because it promotes the opening of pores in the pre-carbonized structure, thereby promoting the development of accessible channels and cavity networks in the carbon matrix. Alkaline cleaning not only helps to functionalize the carbon surface and improve its reactivity, but also plays an important role in pore development.

[0071] Alkaline solution cleaning is performed under controlled conditions to ensure that the interaction between the alkali and the pre-carbonized precursor is sufficient to penetrate the pores without damaging the overall structure of the carbon matrix. Parameters such as the concentration of the alkaline solution and the contact time between the alkali and the precursor are carefully optimized to produce the desired porous structure and distribution in the hard carbon produced in subsequent steps.

[0072] After alkali treatment, the precursor can undergo an additional cleaning step to remove excess alkali and salts formed by the chemical reactions during the alkali treatment stage. When used as an anode material in sodium-ion batteries, the hard carbon is expected to exhibit excellent performance due to its optimized structure, which facilitates the insertion and storage of Na+ ions.

[0073] In the production process of the hard carbon composite material, a biomass mixture is used as the primary source of carbon. This biomass mixture can be any type of carbon-rich organic material and can serve as a precursor in the pyrolysis process. The choice of biomass affects the properties of the final hard carbon product.

[0074] The precursor was calcined in a nitrogen atmosphere, gradually heated to 300-600 °C at a rate of 2-10 °C / min, and held at this temperature for 2-10 hours. This temperature was carefully selected to achieve preliminary carbonization while maintaining the structural integrity of the biomass.

[0075] The electrochemical properties of hard carbon were further improved by adjusting the sodium ion intercalation capability. Specifically, the pore size distribution was adjusted to ensure that the pore size was suitable for the intercalation of Na+ or Li+ ions. During battery charging, these ions are intercalated into the pores of hard carbon, and during discharge, the ions are extracted, thereby generating current in the battery. The achieved uniformity of pore size makes the intercalation and extraction processes more consistent and efficient, thus making the battery more stable and with higher energy capacity.

[0076] The preparation of hard carbon composite materials requires fine processes such as calcination, washing, recalcination, and sieving to achieve ideal electrochemical performance for use in energy storage systems. The characteristics of hard carbon, such as the specific pore structure suitable for ion intercalation, provide a basis for the next generation of lithium...Ion and sodium-ion batteries offer better anode materials. This performance improvement is attributed to the controlled microstructure obtained from sustainable biomass sources, thereby improving ion storage capacity and overall battery efficiency.

[0077] The method also includes a cleaning step for the carbon powder precursor. After an initial calcination step at a temperature of 300-600T, the carbon powder precursor obtained from the biomass mixture is purified by a washing step. Water washing ensures the removal of any soluble impurities, non-carbon elements, and ash that may be present in the precursor material. Water washing not only helps to purify the carbonized product but also helps to expose and expand the pore structure, which is crucial for the subsequent alkaline treatment.

[0078] After water washing, the cleaned porous carbon powder is alkaline treated as described above. The alkaline environment creates a more accessible network of channels in the carbon matrix. This chemical activation process promotes an increase in specific surface area, which is exactly what is needed to improve electrochemical performance.

[0079] The washed activated carbon powder is then subjected to further heat treatment, placed at a temperature of 900-1400°C under an inert gas atmosphere. This high-temperature treatment (also known as recalcination) aims to improve the overall structural properties of the hard carbon. This step helps to improve the conductivity of the final product.

[0080] The alkali treatment is carried out by immersing the carbon powder in an alkali solution with a concentration of 0.05 to 1.0 mol / L. The precise alkali concentration is selected based on the desired porous structure and the electrochemical performance required for the specific application. The immersion time is maintained between 2 and 10 hours, a time period chosen to allow sufficient interaction between the alkali and the carbon powder. During this period, the alkali acts on the carbon material, etching specific areas and expanding the interlayer space within the carbon domain. This etching not only increases the pore volume but also improves the pore structure's permeability, making it easier for sodium ions to insert.

[0081] As the interlayer spacing increases, the ion diffusion kinetics of the pore structure are improved, which is directly related to the improvement of battery performance. Strategic treatment of interlayer spacing and pore size aims to achieve a delicate balance between structural stability and ion transport efficiency. Therefore, carbon structures with increased interlayer spacing offer significant advantages in energy storage for sodium-ion battery systems.

[0082] After alkali treatment, the carbon material is thoroughly cleaned to remove any residual alkali that may adhere to the surface. This cleaning step ensures that the integrity of the newly formed pore structure is not affected by residual treatment reagents. The carbon powder is then dried to remove moisture that may affect interlayer spacing measurements and the material's performance during subsequent battery assembly and testing.

[0083] By effectively controlling the alkali treatment, customized porosity of the carbon material can be achieved, facilitating the development of high-performance hard carbon anodes required for advanced sodium-ion batteries. Compared to conventional hard carbon materials, this innovative synthesis method significantly improves the next...Energy density, cycle life, and charging rate of sodium-ion batteries.

[0084] The treated carbon powder obtained after calcination and chemical activation is cleaned. The cleaning method is to wash the carbon powder with water until the pH of the mixture is neutral at 6.0-8.0. The purpose of this cleaning step is to remove any residual chemical reagents, especially unreacted alkali and byproducts. It is crucial to achieve a neutral pH value because this indicates that excess alkali has been completely washed away, leaving pure carbon material free of contaminants that may adversely affect battery performance. This neutral pH value also indicates stable surface chemistry, which is essential for maintaining stable electrochemical performance when hard carbon is used as an anode material in sodium-ion batteries. The resulting clean and pH-neutral carbon powder has an ideal microstructure that promotes efficient insertion and extraction of sodium ions during battery charge-discharge cycles. This process step is crucial to ensuring that the physical and electrochemical properties of hard carbon are maintained at an optimal level for subsequent use in energy storage devices. After the cleaning process, the cleaned carbon powder is dried at an appropriate temperature to remove any residual moisture that may affect the conductivity and ion transport properties of the material. After the drying process, the carbon material can be further characterized or directly installed in the battery cell as an anode element for performance evaluation of its capacity, cycle life, and overall efficiency in a sodium-ion battery system.

[0085] The production process of hard carbon for sodium-ion batteries includes the step of cleaning the pre-carbonized precursor. The biomass mixture is initially calcined under an inert gas at a temperature of 300-600°C for 2-10 hours to form the pre-carbonized precursor, which then needs to be thoroughly cleaned. An alkaline solution is used for cleaning. The concentration of the alkaline solution is strictly controlled within a specified range, i.e., between 0.05 and 1.0 mol / L. The purpose of alkaline cleaning is to effectively remove any impurities and any remaining volatile substances that may be present in the pre-carbonized precursor. This step is crucial because it affects the structural characteristics of the resulting hard carbon.

[0086] The duration of alkaline cleaning is also a variable parameter, ranging from 2 to 10 hours. The choice of cleaning time depends on the desired pore structure and the chemical purity of the hard carbon. Extending the cleaning time may make the carbon structure more activated, while a shorter cleaning time may be sufficient for some precursor components and desired pore distribution.

[0087] After cleaning with alkaline solution, the pre-carbonized precursor is washed with water again. The water washing continues until the pH of the solution is close to neutral (between pH 6.0 and 8.0). Water washing ensures the removal of any residual alkali from the previous processing step. It also helps to neutralize the carbon material, making it safe to handle and further process. A thorough water washing step is required to prevent any unwanted chemical reactions during subsequent heat treatment and to ensure the purity of the hard carbon.

[0088] After thorough cleaning and water washing, the pretreated precursor is calcined at a high temperature of 900-1400T.This helps to form high-quality hard carbon. This hard carbon has an optimized closed-cell structure, which is very suitable for sodium ion intercalation when used as an anode material for sodium-ion batteries. Through this systematic and precise processing, the precursor is transformed into a highly efficient material with great potential for energy storage applications. The resulting hard carbon anode material can improve the performance and lifespan of sodium-ion batteries, thereby contributing to providing more sustainable and reliable energy storage solutions.

[0089] Figure 2 shows exemplary XRD patterns of three hard carbon samples prepared using the method of the present invention. An amorphous structure was obtained. There are no peaks belonging to graphite, indicating that the hard carbon synthesis was successful. The peak positions are between 23.5 and 23.9°, indicating that the interlayer spacing is wider than that of graphite. The XRD patterns in Figure 2 show two broad peaks, which are characteristic of hard carbon. The absence of the graphite peak indicates that the sample is highly amorphous. Due to the (002) plane of hard carbon, the center of the first peak is approximately 23.5 to 23.9°. The calculated carbon interlayer spacings are 0.3414, 0.3404, and 0.3411 nm, respectively, indicating that the carbon interlayer spacing is wider than that of graphite. Due to the (101) plane of hard carbon, the center of the second peak is located at approximately 44.0°. The intensity of the (002) peak is generally greater than that of the (101) peak.

[0090] Figure 3 shows exemplary Raman spectra of three hard carbon samples prepared using the method of the present invention. The Raman spectra of the three hard carbon examples show two main features: a broad D band of approximately 1350 cm⁻¹ and a sharp G band of approximately 1500 cm⁻¹. The D band is due to the presence of disorder and defects in the carbon structure, while the G band is due to the in-plane stretching vibrations of sp² bonded carbon atoms. The intensity ratio of the D band and the G band, ID / IG, is a measure of the degree of disorder in the carbon structure. A higher ID / IG ratio indicates a higher degree of disorder. The ID / IG ratios of Examples 1, 2, and 3 were 1.2549, 1.2241, and 1.2424, respectively. This ratio consistently exceeded 1.000, confirming the disordered short-chain structure of the hard carbon. It is known that the degree of disorder in carbon structure affects the performance of hard carbon electrodes. Hard carbon electrodes with higher disorder tend to have higher specific capacity and better rate performance.

[0091] Figure 4 shows exemplary TEM images of three hard carbon samples prepared using the method of the present invention. This method produces a molecular sieve-type closed-pore structure, which is depicted by imaging techniques such as TEM, providing visual confirmation of the nanoscale pore size distribution within the hard carbon. The resulting hard carbon material with a molecular sieve-type closed-pore structure supports rapid Na+ ion insertion during battery charging. This characteristic is very useful for high-efficiency sodium-ion batteries because reversible and rapid ion exchange is required between the anode and cathode during charge-discharge cycles. As can be seen from Figure 4, the obtained hard carbon exhibits short and disordered carbon chains. The short chains construct a closed-pore structure, increasing the interlayer spacing and space for Na+ insertion and storage.

[0092] The SIB anode in Figure 1 has closed pores, which are necessary for the effective storage of sodium ions, serving as the primary microstructure for accommodating Na+ ions. The resulting closed-pore structure prevents the formation of a solid electrolyte interphase (SEI) within the nanopores, which is beneficial for battery performance. In this example, pore size and distribution are necessary to improve reversible capacity and initial coulombic efficiency (ICE), while an upper limit of less than 10 nm in pore diameter ensures the reversibility of the plateau capacity.

[0093] The SIB cathode can use materials such as sodium metal oxides or phosphates. The electrolyte in these batteries is a sodium salt dissolved in an organic solvent, and the anode is composed of hard carbon with a molecular sieve-type closed-pore structure, which helps prevent excessive decomposition of the electrolyte within the pores. Sodium ion storage in the hard carbon anode occurs through a variety of mechanisms, including intercalation between carbon layers, surface adsorption, and pore filling.

[0094] This process has several associated performance advantages. Hard carbon anodes with optimized pore structures can achieve plateau capacities up to 300 mAh·g⁻¹ and capacities up to 382 mAh·g⁻¹ in various implementations. Molecular sieve-type closed-pore structures can significantly improve initial coulombic efficiency, with some designs reaching approximately 85%. Furthermore, closed pores enhance the capacity of the low-pressure plateau and reduce electrolyte decomposition by minimizing unwanted SEI formation within the pores. By carefully controlling the pore structure, microcrystalline structure, and defects of the hard carbon anode, researchers aim to further improve the electrochemical performance of SIBs, making them a viable and cost-effective alternative to lithium-ion batteries.

[0095] Various electrolytes can be used in SIBs. Organic liquid electrolytes, such as NaClO4-based organic liquid electrolytes, which dissolve sodium salts in organic solvents, are widely used due to their good compatibility with common cathode materials. Solid electrolytes, including materials such as sodium superionic conductors (NASICON), are also used, exhibiting high ionic conductivity and non-flammability, offering greater safety compared to liquid electrolytes. During charge-discharge cycles, the electrolyte acts as a medium for the movement of sodium ions between the cathode and anode. It also participates in the formation of the solid electrolyte interphase (SEI) at the anode and the cathodic electrolyte interphase (CEI) at the cathode, which are essential for the electrochemical performance of the battery.

[0096] The SIB can utilize a variety of biomass materials as a starting point, including but not limited to organic farm products and cotton. Such raw materials can be processed under controlled conditions, such as by gradually heating in an inert gas, followed by washing with an alkaline solution and further processing.External heat treatment. This meticulous control over the production process ensures the development of hard carbon anodes that are highly suitable for next-generation energy storage systems.

[0097] The biomass mixture contains one or more trace impurities that are inherent to the nature of the biomass from which the mixture originates. Trace impurities typically associated with biomass may include, but are not limited to, trace amounts of inorganic salts, metals, and non-carbon organic compounds. These impurities may originate from soil, water, or air that comes into contact with the biomass during the growth and harvesting stages. This method is capable of producing molecular sieve-type closed-pore hard carbon, which is particularly advantageous for use as an anode material in lithium-ion and sodium-ion batteries. This method systematically uses biomass mixtures as starting materials, providing an environmentally friendly and cost-effective feedstock compared to fossil fuel-derived carbon sources. The biomass mixture is calcined at a temperature of 300-600T to initiate the carbonization of the biomass. It is then treated with NaOH, in which specific chemical reactions are carried out to activate the carbon structure and increase its porosity. The chemical reactions produce Na and Na2CO3, while releasing Na+, and Na reacts with water to produce further NaOH. Recalcination yields hard carbon with a molecular sieve structure. This step is crucial because it ensures that the final product contains only particles with optimal pore sizes (specifically less than 10 nm). The small size of these pores helps facilitate the insertion and diffusion of Na+ ions during battery operation, which is essential for high efficiency and battery performance. This molecular sieve formation process not only selects the pore size but also helps achieve a uniform distribution of pores, which contributes to the homogeneity of the final anode material.

[0098] The anode of the SIB comprises a hard carbon material characterized by a molecular sieve-type closed-pore structure with surface and internal spaces, which also has a window structure for receiving sodium during charging, where sodium ions pass through the window and adsorb onto the surface, and occupy the internal space after charging. The anode has the pore structure required for efficient sodium ion storage, serving as the primary microstructure for accommodating Na+ ions. The closed pores have tight entrances, creating a sieving effect and preventing the formation of a solid electrolyte interface (SEI) film within the nanopores, which is beneficial for battery performance. Pore size and distribution are essential; ultramicropores improve reversible capacity and initial coulombic efficiency (ICE), while an upper limit of less than 10 nm in pore diameter ensures the reversibility of plateau capacity. Closed-pore structures not only improve reversible capacity but also contribute to improved initial coulombic efficiency. For example, a carbon anode rich in closed pores can achieve a reversible capacity of 309.3 mAh / g and an initial coulombic efficiency of 87.8%. These closed-pore designs are necessary for optimizing sodium storage because they provide a stable environment for sodium ions during charge-discharge cycles, thereby improving the overall cycle stability and energy density of the sodium-ion battery.

[0099] A sodium-ion battery includes a cathode; an electrolyte coupled to the cathode; and the aforementioned anode coupled to the electrolyte. The anode has multiple molecular sieve-type closed-pore structures with surface and internal spaces, and the structure also has windows for receiving sodium ions, wherein sodium...Ions pass through the window and adsorb onto the surface, occupying the internal space after charging. The closed-pore structure is characterized by its small inlet size, resulting in a molecular sieve structure. This design prevents the formation of a solid electrolyte interface (SEI) within the nanopores, a common problem with porous carbon due to its large surface area pores that allow electrolyte ingress. Therefore, this molecular sieve-type hard carbon exhibits superior electrochemical performance compared to conventional porous carbon materials. For example, porous carbon can only achieve a reversible capacity of 39 mAh / g, but molecular sieve-type hard carbon can achieve much higher capacities, such as 328 mAh / g at a current density of 50 mA / g.

[0100] This design offers several performance advantages. Hard carbon anodes with optimized pore structures can achieve plateau capacities up to 300 mAh·g⁻¹ and reversible capacities up to 382 mAh·g⁻¹ in various embodiments. The molecular sieve-type closed-pore structure can significantly improve the initial coulombic efficiency, with some designs reaching up to 80.21%. Furthermore, closed-pore design improves the capacity of the low-pressure plateau and reduces electrolyte decomposition by minimizing the formation of undesirable SEI at 12 / 13 K within the pores. By controlling the pore structure, microcrystalline structure, and defects in the hard carbon anode, the electrochemical performance of sodium-ion batteries can serve as a viable and cost-effective alternative to hammer-ion batteries.

[0101] Figure 5 shows more battery data, particularly the capacity and ICE of Examples 1, 2, and 3. The resulting hard carbon exhibits a high discharge capacity of over 240 mAh / g and an ICE greater than 80%. The data are as follows: Charging gap (mAh / g) Discharge capacity (nAh / g) ICE (%) Example 1 321.80 276.24 85.8 Example 2 380.22 296.40 78.0 Example 3 287.30 246.33 85.7

[0102] The structure of hard carbon is as follows: short carbon chains; highly disordered structure; long interlayer spacing; and closed-pore structure. Due to the short disordered structure, Na+ ions can be adsorbed on the surface of hard carbon; embedded in the hard carbon layers; and aggregated in the closed-pore structure. Therefore, the amount of Na+ on the anode increases and has a positive effect on the electrochemical performance of SIBs.

[0103] In one embodiment of biomass mixture processing, the pre-carbonized precursor obtained after the first calcination step can be ground to produce fine powder. This grinding can ensure the uniformity of particle size and morphology, as well as increase the surface area of ​​the precursor material. The increased surface area is crucial for the performance of hard carbon because it can potentially enhance Na+ ion intercalation capacity by providing more accessible electrochemically reactive sites. The grinding process is carried out using suitable grinding equipment known to those skilled in the art, such as ball mills, jet mills, or mortars and pestles, until the desired powder consistency is achieved.Temperature. Care should be taken to avoid contamination of the precursor material, as impurities can affect the electrochemical properties of the final hard carbon product. After grinding, the pre-carbonized precursor, which has a significantly reduced particle size, is guided to the second calcination step. As noted earlier, here the precursor undergoes a rigorous heat treatment at a temperature of approximately 1300°C. The high-temperature transition helps to remove any residual inorganic matter and promotes the graphitization process. The product produced by the second calcination is a hard carbon with higher crystallinity, which gives it better electrical conductivity and mechanical stability. In addition, the smaller particle size produced by the grinding step through NaOH treatment allows Na+ ions to be more uniformly intercalated into the molecular sieve-like structure of the hard carbon. The molecular sieve-like closed-pore structure intercalated with Na+ ions has been shown to have enhanced electrical properties and is suitable for use as an anode material in sodium-ion batteries. This structure effectively allows for the selective intercalation of Na+ ions, ensuring efficient battery operation and optimal charge and discharge rates.

[0104] The prepared biomass mixture is calcined at a temperature of 450°C, during which the organic matter in the biomass is thermally decomposed in an inert nitrogen gas. The heat treatment was carried out at a specific heating rate of 5°C / min and maintained for 3 hours, allowing the biomass to gradually transform into a carbonized state while avoiding sudden thermal shock, as this could damage the structural integrity of the material.

[0105] Next, in order to increase porosity and activate the resulting carbon, the material was treated with an aqueous NaOH solution. This alkaline treatment reacts with the carbonized biomass, as described in the chemical equation, where NaOH interacts with carbon to form sodium carbonate and hydrogen. This process also helps to expand the pore structure within the carbon matrix, thereby forming a channel network suitable for ion transport.

[0106] After the NaOH treatment, the precursor was further heated to 1300°C under nitrogen gas. This recalcination step stabilizes the hard carbon by removing any residual non-carbon material and adjusting the carbon domains to enhance electrical conductivity and structural resilience.

[0107] The hard carbon was sieved to selectively remove particles according to pore size. It ensures that only particles with optimal pore sizes (especially pore sizes less than 10 nm) are used. These pores are well-suited to accommodate Na+ ions during charging, thereby maximizing the charge storage capacity of the hard carbon.

[0108] Finally, the resulting molecular sieve structure with intercalated Na+ ions embodies a material with a high density of accessible active sites (14 CN 119604994 A specification page 13 / 13), enabling reversible Na+ intercalation, a key property of high-performance sodium-ion battery anodes. This method establishes a sustainable and cost-effective pathway to transform agricultural waste into a valuable component of the growing energy storage market, representing an important step in the development of green technology solutions.

[0109] The resulting hard carbon possesses a dense, uniform pore structure of approximately 10 nm, providing a unique structure that maximizes…Increase the accessible surface area for Na+ ion insertion, and also contribute to the formation of a stable and robust electrode material. This structure helps to reduce the strain during the charge-discharge cycle, which can not only improve the overall capacity of the battery, but also enhance its lifespan and cycle stability.

[0110] Considering the variations in the biomass precursor and the resulting differences in the re-calcination results, the process parameters (such as temperature holding time and heating rate) can be optimized to obtain tailored hard carbon with specific microstructural features to meet the expected electrochemical applications. Through this heat treatment process, a molecular sieve-type closed-pore hard carbon is achieved, which is then integrated into the anode, providing excellent capacity retention and rate performance to meet the increasingly urgent energy storage requirements.

[0111] Although specific embodiments and examples of the present invention have been illustrated and described, many modifications can still be conceived without significantly departing from the spirit of the present invention. The scope of protection of the present invention is only limited by the scope of the appended claims. 15 CN 119604994 A Description of Drawings 1 / 5’ page Q3) Dispersion of Na+ layer after charging Figure 1 16 CN 119604994 A Description of Drawings 2 / 5 page Cm) g interesting W E 0. 34 14 0. 34 04 o co 2 T- co CXI co CM c*i CM 04 1闺 德 feK Figure 2 ( « 0 Z 0 9 O t o co 17 CN 119604994 A Description of Drawings 3 / 5 M ( n.B) S® Figure 3 J? 1. 25 49 丨 § CsJ V— s s cxi o C>Jo fe 5 s itp. s o JO 00 CT» 一Q ▼— 卜 s rsi s i i feK £ B g feK l ^ g feK I G - § ) Absolute Zhaoqiu Zhao 18 CN 119604994 A Description of Drawings 4 / 5 page Figure 4 19 CN 119604994 A Description of Drawings 5 / 5 page Figure 5 ( W N / ^ N > ) Ha call 20

Claims

1. A hard carbon with a molecular sieve type closed-pore structure, wherein the raw material for preparing the hard carbon is a biomass mixture, which contains 70-90% cellulose, 9-20% hemicellulose and 1-10% lignin, and the pore size of the molecular sieve type closed-pore structure is less than 10nm.

2. A method for preparing the hard carbon according to claim 1, comprising: 1) calcining the biomass mixture at 300-600° C. to produce a pre-carbonized precursor; 2) washing the pre-carbonized precursor with an alkaline solution and then adjusting the pH value to 6.0-8.0; and 3) calcining the pre-carbonized precursor at 900-1400° C. to produce molecular sieve type closed-cell hard carbon.

3. The method according to claim 2, wherein the calcining of the biomass mixture is carried out in an inert gas.

4. The method according to claim 2, wherein the temperature of calcining the biomass mixture is between 300-600°C. 5 . The method according to claim 2 , wherein the concentration of the alkaline solution for washing the pre-carbonized precursor is 0.05-1.0 mol / L.

6. The method according to claim 2, wherein the alkaline solution treatment time is 2-10 hours.

7. The method according to claim 2, wherein the alkaline solution treatment further comprises adjusting the pH value to 6.0-8.

0.

8. An anode of a battery, comprising: A hard carbon material with a molecular sieve-type closed-pore structure.

9. The anode according to claim 8, wherein the molecular sieve type closed-pore structure is formed by: calcining a biomass mixture having 70-90% cellulose, 9-20% hemicellulose and 1-10% lignin at 300-600°C to produce a pre-carbonized precursor; washing the pre-carbonized precursor with an alkaline solution to adjust the pH value to 6.0-8.0; and calcining the pre-carbonized precursor again at 900-1400°C to produce the molecular sieve type closed-pore structure.

10. A battery comprising: cathode; an electrolyte coupled to the cathode; and an anode coupled to the electrolyte, the anode having a molecular sieve type closed-pore structure.

11. The battery according to claim 10, wherein the molecular sieve type closed-pore structure is formed by: calcining a biomass mixture at 300-600°C to produce a pre-carbonized precursor; washing the pre-carbonized precursor with an alkaline solution to wash the pH value to 6.0-8.0; and calcining the pre-carbonized precursor again at 900-1400°C to produce a molecular sieve type closed-pore hard carbon.

12. The battery of claim 11, wherein the biomass mixture comprises 70-90% cellulose, 9-20% hemicellulose and 1-10% lignin.

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