Preparation method of black liquor-based hard carbon negative electrode and sodium ion battery

Abstract

The application discloses a preparation method of a black liquor-based hard carbon negative electrode and a sodium ion battery. The papermaking black liquor is diluted, and then is directly subjected to hydrothermal carbonization without acid washing and alkali washing pretreatment, and after washing and drying, high-temperature carbonization is carried out in an inert atmosphere to obtain a black liquor-based hard carbon material; in the hydrothermal carbonization process, the pH of the system is adjusted to 0.5-2, or the pH is not adjusted so that the hydrothermal carbonization is carried out under the original pH condition. The application also provides a negative electrode containing the hard carbon material, a sodium ion battery and a hard carbon compounding method. The application uses industrial waste papermaking black liquor as a raw material, does not need a complex pretreatment process, and through on-demand adjustment of the pH in the hydrothermal stage, high-capacity hard carbon (>=275.8 mAh / g) or high-stability hard carbon (cycle retention rate >=96.92%) can be obtained, flexible customization of solid waste resource utilization and electrochemical performance is realized, the cost is low, the process is simple, and the application has a wide industrialization prospect.

Description

Technical Field

[0001] This invention belongs to the technical field of sodium-ion battery anode materials, specifically relating to a method for preparing a black liquid-based hard carbon anode and a sodium-ion battery. Background Technology

[0002] As the global energy structure shifts towards cleaner and lower-carbon energy, electrochemical energy storage technology, with rechargeable batteries at its core, has been widely applied in electric vehicles, portable electronic devices, and large-scale energy storage. Lithium-ion batteries have long dominated due to their high energy density; however, lithium resources are limited in quantity and unevenly distributed in the Earth's crust, and their prices fluctuate wildly, making it difficult to meet the future demands for low cost and high safety in large-scale energy storage. Against this backdrop, sodium-ion batteries, due to their abundant sodium resources, low cost, and electrochemical properties similar to lithium, are considered one of the most promising next-generation energy storage technologies for industrialization and have received significant attention from academia and industry in recent years.

[0003] The anode material is a key component determining the overall performance of sodium-ion batteries. However, graphite anodes, widely used in lithium-ion batteries, have a small interlayer spacing (approximately 0.335 nm), making it difficult to reversibly insert and extract larger sodium ions, thus preventing their direct application in sodium-ion battery systems. Hard carbon, as a typical non-graphitized carbon material, possesses a large interlayer spacing, abundant microporous structure, and disordered carbon layer arrangement, providing ample storage sites and diffusion channels for sodium ions. It exhibits high reversible specific capacity and excellent cycle stability, making it the most practically valuable anode material for sodium-ion batteries.

[0004] The performance of hard carbon largely depends on the type and structural characteristics of its precursors. Biomass precursors are ideal raw materials for hard carbon production due to their wide availability, renewability, low cost, and high carbon content. Among them, lignin, as the second most abundant natural polymer after cellulose, has an aromatic ring skeleton structure and a high fixed carbon content. It easily forms an amorphous carbon structure during high-temperature carbonization, making it a high-quality precursor for hard carbon production. However, existing methods for preparing hard carbon using lignin or other biomass as raw materials often rely on high-purity raw materials that have undergone purification and extraction. The process is complex and costly, limiting its large-scale industrial application. For example, Chinese patent application CN117735527A discloses a method for preparing hard carbon using pure biomass such as wood and bamboo powder as raw materials. The technical route involves pretreatment of the biomass, such as soaking in hydrochloric acid and sodium hydroxide, followed by hydrothermal carbonization (200~260℃) and high-temperature pyrolysis (800~1200℃). Although the scheme can achieve a high reversible capacity, it has the following shortcomings: (1) The raw materials must be pure biomass such as wood and bamboo powder, which cannot directly treat industrial waste liquid with complex components and high ash content; (2) It requires complex pretreatment processes such as acid washing and alkali washing, which are cumbersome, have long cycles, high costs, and involve the consumption of chemical reagents; (3) Its hydrothermal carbonization process does not involve in-situ pH control, and the controllability of carbon interlayer spacing is limited; (4) It clearly points out that carbonization temperature above 1200℃ will lead to a reduction in interlayer spacing, forming a reverse teaching, which limits the further optimization of hard carbon structure.

[0005] Currently, coconut shells are the primary raw material for hard carbon anode materials in my country's biomass-based sodium-ion batteries. Coconut shells possess advantages such as a naturally porous structure, high carbon content, and relatively low cost, making them an ideal biomass precursor for preparing high-performance hard carbon anodes. However, my country's coconut cultivation scale is limited, and the raw material for coconut shell hard carbon has long relied on imports, primarily from Thailand, Indonesia, and the Philippines. According to industry data, in November 2025, coconut shell charcoal imports reached 14,901.76 tons, an increase of 32% year-on-year, with the average import price remaining high. Affected by reduced coconut shell production due to natural disasters in Southeast Asia, the price of imported raw materials remains high, directly increasing the production cost of hard carbon anodes and exposing domestic sodium battery anode material companies to significant supply chain fluctuation risks. Therefore, finding domestically produced hard carbon raw materials that can replace imported coconut shells has become an urgent need in the sodium battery anode material field.

[0006] Meanwhile, the paper industry generates a large amount of black liquor during the pulping process. Black liquor is rich in lignin, hemicellulose degradation products, and alkaline inorganic salts, making it one of the most significant sources of pollution in the pulp and paper industry. Industry statistics show that approximately 7-10 tons of black liquor are generated for every ton of pulp produced, with the total annual black liquor production in China reaching hundreds of millions of tons, providing a stable and large-scale raw material supply. Currently, black liquor treatment primarily employs alkali recovery and combustion processes. While this recovers some heat and chemicals, the high-value-added utilization of its organic components, such as lignin, is extremely low, resulting in resource waste. Furthermore, direct discharge of untreated black liquor causes serious pollution to water bodies and the environment. Therefore, achieving high-value and resource-based utilization of black liquor has long been a pressing technical challenge.

[0007] Given that black liquor is rich in lignin, which can serve as a precursor for hard carbon, researchers have recently attempted to convert black liquor into hard carbon anode materials for sodium-ion batteries. For example, patent CN118894515B discloses a method for preparing a papermaking black liquor-based hard carbon anode. The technical route involves drying and pulverizing the papermaking black liquor, followed by high-temperature calcination (800-1600℃), acid and alkali washing for impurity removal, and high-temperature tempering (600-1000℃) to obtain hard carbon powder. This method employs a dry calcination process, which involves numerous steps, high energy consumption, and strong acid and alkali washing, increasing process complexity and environmental burden. Patent CN107452960A discloses a method for preparing a porous carbon-sodium-ion battery anode material from papermaking black liquor, which involves drying the black liquor, heat-treating it at 600-800℃, and then activating it at 200-350℃. This scheme also employs a dry heat treatment route, without involving hydrothermal carbonization, and the resulting product is a porous carbon material rather than a hard carbon structure with suitable interlayer spacing. Furthermore, some studies have used pulping black liquor as raw material, extracting and purifying lignin through acid precipitation, followed by pre-carbonization and high-temperature pyrolysis carbonization to prepare lignin-derived hard carbon materials. However, this method requires prior lignin purification, making the process complex and limiting its large-scale application.

[0008] It is evident that the existing technical solutions for preparing hard carbon using black liquor as raw material have the following shortcomings: (1) Most adopt the process route of drying followed by dry calcination or purifying lignin before carbonization, which is cumbersome and energy-intensive; (2) Generally require pretreatment processes such as acid washing, alkali washing, or lignin purification, which increases the complexity of the process and production costs; (3) There is a lack of effective methods for the stable preparation of high-performance hard carbon, which are characterized by complex black liquor components, high ash content, and non-uniform structure, especially the lack of technical means to achieve directional design of the microstructure of hard carbon through process control. Although the hard carbon preparation scheme using pure biomass as raw material (such as CN117735527A) adopts hydrothermal carbonization + high-temperature pyrolysis process, its raw materials must be purified and cannot be directly used from industrial waste. Moreover, the hydrothermal carbonization process lacks in-situ control of the carbon interlayer spacing, and the selection of carbonization temperature is also limited by its inherent technical bias, making it difficult to achieve synergistic optimization and on-demand adjustment of hard carbon sodium storage capacity and cycle stability.

[0009] Therefore, developing a method for preparing hard carbon anodes that can directly use raw black liquor as raw material, achieve directional control of the microstructure of hard carbon through a simplified process route, and combine cost advantages with adjustable performance is of great practical significance and has broad application prospects. Summary of the Invention

[0010] (a) Technical problems to be solved In view of the current situation where existing hard carbon anode material preparation processes are complex and raw material costs are high, especially when using pure biomass as raw material, it is necessary to undergo cumbersome acid and alkali pretreatment (such as the scheme disclosed in CN117735527A), and existing black liquor-based hard carbon preparation schemes mostly adopt dry calcination or lignin purification and then carbonization process routes, which make it difficult to directly utilize the original black liquor and cannot achieve both process simplification and high performance, this invention provides a method for preparing a black liquor-based hard carbon anode and a sodium-ion battery.

[0011] This invention uses industrial waste papermaking black liquor as raw material, without any pretreatment processes such as acid washing, alkali washing, or lignin purification. By selectively adjusting the pH in situ or not during the hydrothermal carbonization process, combined with the optimization of the high-temperature carbonization process, it achieves precise control and on-demand adjustment of the carbon layer spacing and graphitization degree. While greatly simplifying the process and reducing raw material costs, it obtains a hard carbon anode material with excellent sodium storage performance, realizing the high-value and resource utilization of papermaking solid waste.

[0012] (II) Technical Solution To achieve the above-mentioned objectives, the present invention provides the following technical solution: In a first aspect, the present invention provides a method for preparing a black liquid-based hard carbon material, comprising the following steps: Option 1 (High-capacity type): After diluting the black liquor of papermaking, it is directly subjected to hydrothermal carbonization without acid washing and alkali washing pretreatment to obtain hydrothermal carbon; the pH value of the system is adjusted to 0.5~2 during the hydrothermal carbonization process; after washing and drying, the hydrothermal carbon is subjected to high-temperature carbonization under an inert atmosphere to obtain black liquor-based hard carbon material.

[0013] Option 2 (High Stability): After diluting the papermaking black liquor, it is directly subjected to hydrothermal carbonization without acid washing and alkali washing pretreatment to obtain hydrothermal carbon; the pH value of the system is not adjusted during the hydrothermal carbonization process, so that the hydrothermal carbonization is carried out under the original pH conditions; after washing and drying, the hydrothermal carbon is subjected to high-temperature carbonization under an inert atmosphere to obtain black liquor-based hard carbon material.

[0014] In a preferred embodiment of the present invention, the pH of the system is adjusted to 0.5-2 during the hydrothermal carbonization process, preferably pH=1. By adjusting the system to a strongly acidic environment during the hydrothermal carbonization process, the carbon interlayer spacing and disorder degree of the obtained hard carbon material can be significantly increased, resulting in high-capacity hard carbon.

[0015] In another preferred embodiment of the present invention, the pH value of the system is not adjusted during the hydrothermal carbonization process, allowing the hydrothermal carbonization to proceed under the original pH conditions. Under these preferred conditions, the resulting hard carbon material has a moderate interlayer spacing (approximately 0.359 nm) and a high reversible capacity (182.0 mAh·g). - ¹) With excellent cycle stability (96.92% capacity retention after 60 cycles), it is suitable for long-life energy storage scenarios with high cycle life requirements, and obtains highly stable hard carbon.

[0016] As a preferred embodiment of the present invention, the papermaking black liquor is diluted to a mass concentration of 20% to 40%, preferably 30%.

[0017] In a preferred embodiment of the present invention, the hydrothermal carbonization temperature is 240~320℃, and the holding time is 180~300 min. More preferably, when the hydrothermal carbonization temperature is 300~320℃, the pH of the system is adjusted to 1. Under these preferred conditions, the interlayer spacing of the obtained hard carbon material can reach 0.4 nm or more.

[0018] As a preferred embodiment of the present invention, the heating rate of the hydrothermal carbonization is 2~8℃ / min, and the stirring speed is 50~200 r / min.

[0019] In a preferred embodiment of the present invention, the high-temperature carbonization temperature is 1250~1350℃, and the holding time is 1~4 h. The present invention overcomes the technical bias of the prior art where the carbonization temperature is not higher than 1200℃, and by performing high-temperature carbonization at 1250~1350℃, combined with pH control during the hydrothermal stage, a hard carbon material with both suitable interlayer spacing and good structural stability can be obtained.

[0020] As a preferred embodiment of the present invention, the heating rate of the high-temperature carbonization is 2~8℃ / min, the inert atmosphere is N2 or Ar atmosphere, and the gas flow rate is 50~200 mL / min.

[0021] In a preferred embodiment of the present invention, the hydrothermal carbon is first washed with an organic solvent until colorless, then washed with deionized water until neutral, and dried at 100-120°C for 6-18 h before high-temperature carbonization; the sample after high-temperature carbonization is washed with deionized water for 15-40 min and dried at 100-120°C for 6-18 h. More preferably, the organic solvent is ethanol.

[0022] Secondly, the present invention provides a method for preparing a black liquid-based hard carbon anode, comprising the following steps: Black liquid-based hard carbon materials are prepared using the method described in the first aspect; The black liquor-based hard carbon material is mixed with a conductive agent and a binder at a mass ratio of 6-9:0.5-2:0.5-2 (preferably 8:1:1), and an electrode slurry is prepared using an organic solvent as the dispersion medium. The conductive agent is at least one of conductive carbon black, Super P, acetylene black, and carbon nanotubes; the binder is at least one of PVDF, CMC, SBR, and PAA; and the organic solvent is N-methyl-2-pyrrolidone or deionized water. The electrode paste is coated on a copper or aluminum foil current collector with a thickness of 100-200 μm, dried under vacuum at 60-100℃ for 6-24 h, and then cut to obtain a black liquid-based hard carbon anode.

[0023] Thirdly, the present invention provides a sodium-ion battery comprising a black liquid-based hard carbon anode prepared by the method described in the second aspect.

[0024] Fourthly, the present invention provides a method for compounding hard carbon anode materials for sodium-ion batteries, comprising the following steps: The first hard carbon material and the second hard carbon material are mixed in a certain proportion; The first hard carbon material is the hard carbon material prepared by adjusting the pH of the system to 0.5~2 in the method described in the first aspect; The second hard carbon material is the hard carbon material prepared by the method described in the first aspect without adjusting the pH of the system, and by carrying out hydrothermal carbonization under the original pH conditions; The mass percentage of the first hard carbon material is 10% to 30%.

[0025] As a preferred embodiment of the present invention, the initial reversible specific capacity of the first hard carbon material is 250~300 mAh·g. - ¹, the capacity retention rate of the second hard carbon material after 60 cycles is ≥90%.

[0026] (III) Beneficial Effects Compared with the prior art, the present invention has the following beneficial effects: (1) The process flow is greatly simplified and the raw material cost is significantly reduced. This invention uses industrial waste papermaking black liquor as raw material, which can be directly diluted and then subjected to hydrothermal carbonization without any pretreatment processes such as acid washing, alkali washing, or lignin purification. This completely solves the pain points of the cumbersome and time-consuming process of hard carbon preparation in existing technologies. Compared with pure biomass-based hard carbon preparation schemes such as CN117735527A (which require pretreatment such as acid soaking and alkali soaking for 12-24 hours), the process flow of this invention is significantly shortened, and no large amount of chemical reagents are used, effectively reducing raw material costs and production energy consumption.

[0027] (2) Realize the high-value and resource utilization of papermaking black liquor This invention directly uses waste black liquor from the pulp and paper industry as raw material to prepare high-performance sodium-ion battery hard carbon anode material. It transforms industrial waste liquid, which was originally of low added value and even polluted the environment, into high-value energy storage material. This not only solves the environmental pollution problem caused by black liquor discharge, but also provides a low-cost, renewable, domestically produced raw material source for sodium-ion battery hard carbon anode. It realizes the dual value of solid waste recycling and green energy storage material preparation, and has good economic and environmental benefits.

[0028] (3) Precise control of carbon interlayer spacing and graphitization degree This invention addresses the challenges of complex black liquor composition, high ash content, and heterogeneous structure by creatively introducing in-situ pH control during hydrothermal carbonization. Specifically, adjusting the pH to 1 under high-temperature hydrothermal conditions of 300–320°C significantly increases the interlayer spacing of the resulting hard carbon material from 0.359 nm to over 0.415 nm (XRD measured interlayer spacing reaches 0.402 nm), far exceeding the 0.34 nm of graphite, providing ample space for sodium ion insertion and extraction. Simultaneously, Raman spectroscopy analysis shows that the pH-controlled hard carbon (320-pH-HC) has an ID / IG value of 1.47, exhibiting high defect and disorder levels, consistent with the core structural characteristics of hard carbon—"long-range disorder, short-range order"—which is beneficial for sodium ion storage and diffusion.

[0029] (4) Electrochemical performance can be customized as needed, and it has a wide range of applications. The black liquid-based hard carbon anode material prepared in this invention has a performance of 30 mA·g -1 At the specified current density, the pH-controlled 320-pH-HC sample achieved a first-time reversible specific capacity of 275.8 mAh·g. - ¹, compared to the unregulated 320-HC sample (182.0 mAh·g) - ¹) The capacity was increased by approximately 51.5%, demonstrating the significant effect of pH regulation on capacity enhancement. Cyclic stability tests showed that the un-pH-regulated 320-HC sample retained 96.92% of its capacity after 60 cycles, with coulombic efficiency greater than 98% except for the first cycle, exhibiting excellent cyclic stability.

[0030] The above results demonstrate that by selectively adjusting the pH during the hydrothermal stage or not adjusting the pH, this invention can achieve on-demand adjustment and flexible switching between the sodium storage capacity and cycle stability of hard carbon materials. Adjusting the pH yields high-capacity hard carbon (initial capacity ≥275.8 mAh / g), while not adjusting the pH yields highly stable hard carbon (60-cycle retention rate ≥96.92%). The two can also be used in combination to meet the differentiated needs of energy storage materials for different application scenarios.

[0031] (5) Break through existing technological biases and expand the process window Existing technologies (such as CN117735527A) clearly state that carbonization temperatures above 1200℃ lead to reduced interlayer spacing and hinder sodium ion insertion, resulting in the technical bias that "carbonization temperatures should not exceed 1200℃." This invention overcomes this cognitive limitation by performing high-temperature carbonization at 1250~1350℃, combined with in-situ pH control during the hydrothermal stage. This not only avoids the problem of significantly reduced interlayer spacing but also yields high-performance hard carbon materials with both suitable interlayer spacing and good structural stability, providing a new process window for the structural control of hard carbon materials.

[0032] (6) Raw materials are widely available and have a stable supply. This invention uses papermaking black liquor as raw material. The total amount of black liquor produced by the domestic pulp and paper industry each year can reach hundreds of millions of tons, providing a long-term, stable, and large-scale raw material supply. It does not rely on overseas imports and is not affected by factors such as international trade conditions and natural disasters, which can effectively ensure the supply chain security of domestic sodium electrode anode material enterprises.

[0033] (7) Flexible compounding and use to meet diverse needs The high-capacity hard carbon (pH-adjusted) and high-stability hard carbon (unadjusted) prepared in this invention can be physically compounded in a ratio of 10% to 30%. The resulting composite material exhibits both high reversible capacity (≥200 mAh·g). -¹) and good cycle stability (capacity retention ≥85%) provide a flexible material solution for the performance customization and industrial application of hard carbon anode materials for sodium-ion batteries.

[0034] (8) Outstanding creativity and broad technological window This invention presents "adjusting the system pH to 0.5-2" and "not adjusting the system pH" as parallel technical solutions. These are not simply alternatives, but rather two specific implementations of the same inventive concept—"controllable adjustment of pH during the hydrothermal carbonization stage." Its inventiveness lies in: ① This invention reveals for the first time that by controlling the system pH during hydrothermal carbonization (including maintaining the original alkaline state without adjustment, and actively adjusting to a strongly acidic state), the carbon interlayer spacing and defect density of black liquor-based hard carbon can be systematically regulated; ② Based on the above findings, hard carbon with excellent cycle stability can be obtained without pH adjustment, while hard carbon with significantly increased capacity can be obtained by adjusting the pH to 0.5-2. Both solutions are applicable to different application scenarios and have achieved unexpected technical effects; ③ High-performance hard carbon materials can be prepared under both pH conditions, indicating that this invention has a wide process window and broad applicability, and has significant industrialization and promotion value. Therefore, the parallel technical solutions of this invention are not simply a combination of known solutions, but a technological innovation based on a systematic understanding of the pH regulation law during black liquor hydrothermal carbonization, possessing outstanding substantive characteristics and significant progress.

[0035] In summary, this invention provides a customizable black liquid-based hard carbon anode preparation technology route, which combines the advantages of simplified process economy, solid waste resource utilization environmental benefits, and adjustable electrochemical performance. It provides a new and reliable technical solution for the industrial preparation of low-cost, high-performance sodium-ion battery hard carbon anode materials, which is in line with the green and low-carbon development concept of the new energy industry and has good prospects for industrial application. Attached Figure Description

[0036] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0037] Figure 1 A schematic diagram of the preparation process of hydrothermal carbon-based hard carbon from black liquor; Figure 2 TEM images and lattice fringe spacing analysis of black liquor hydrothermal carbon-based hard carbon under different hydrothermal carbonization conditions; Figure 3 XRD patterns of hydrothermal carbon-based hard carbon from black liquor under different hydrothermal carbonization conditions; Figure 4Raman spectra of hydrothermal carbon-based hard carbon from black liquor under different hydrothermal carbonization conditions; Figure 5 Constant current charge-discharge curves for a carbon-based hard carbon electrode used in black liquor hydrothermal treatment. Figure 6 The cyclic stability graphs are for the 320-HC and 320-pH-HC samples. Detailed Implementation

[0038] The present invention will be further described in detail below with reference to embodiments and comparative examples, but the implementation of the present invention is not limited thereto. The following embodiments are only for illustrating the present invention and should not be regarded as limiting the scope of the present invention. The terminology used in this specification is only for describing specific embodiments of the present invention and is not intended to limit the present invention. Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Unless otherwise specified, the pharmaceuticals or reagents used in this invention shall be used in accordance with the product instructions or in accordance with conventional methods of use in the art.

[0039] In the following examples, the black liquor used is pulping and papermaking black liquor, which is derived from alkaline waste liquor generated by the domestic sulfate pulping process; hydrothermal carbonization is carried out in a hydrothermal reactor; high-temperature carbonization is carried out in a tubular furnace, and inert gas is introduced for 30 minutes before carbonization to remove oxygen in the furnace, and then carbonization is carried out under the protection of inert gas.

[0040] Example 1 This embodiment provides a method for preparing a black liquid-based hard carbon material, such as... Figure 1 As shown, it includes the following steps: Papermaking black liquor was diluted to a mass concentration of 30% and placed in a hydrothermal reactor. The temperature was increased to 240°C at a rate of 5°C / min, with a stirring speed of 100 r / min. No additional pH adjustment was performed during the hydrothermal carbonization process (the original black liquor was strongly alkaline). The mixture was held at this temperature for 240 min to prepare hydrothermal carbon. The obtained hydrothermal carbon was washed with ethanol until colorless, then washed with deionized water until neutral, and dried in a forced-air drying oven at 105°C for 12 h. Subsequently, the dried hydrothermal carbon was placed in a tube furnace, and N2 was introduced for 30 min to remove oxygen from the furnace. Under N2 protection, the temperature was increased to 1300°C at a rate of 5°C / min and an N2 flow rate of 100 mL / min. The temperature was held for 2 h and then naturally cooled to room temperature. The carbonized sample was removed and washed with deionized water under vacuum filtration for 20 min, then dried at 105°C for 12 h to obtain black liquor-based hard carbon material, denoted as 240-HC.

[0041] Example 2 This embodiment provides a method for preparing a black liquid-based hard carbon material, including the following steps: The black liquor from papermaking was diluted to a mass concentration of 30% and placed in a hydrothermal reactor. The temperature was increased to 280°C at a rate of 5°C / min, with a stirring speed of 100 r / min. No additional pH adjustment was performed during the hydrothermal carbonization process, and the temperature was maintained for 240 min to prepare hydrothermal carbon. Subsequent washing, drying, high-temperature carbonization, and post-treatment steps were the same as in Example 1, yielding a black liquor-based hard carbon material, denoted as 280-HC.

[0042] Example 3 This embodiment provides a method for preparing a black liquid-based hard carbon material, including the following steps: The black liquor from papermaking was diluted to a mass concentration of 30% and placed in a hydrothermal reactor. The temperature was increased to 320°C at a rate of 5°C / min, with a stirring speed of 100 r / min. No additional pH adjustment was performed during the hydrothermal carbonization process, and the temperature was maintained for 240 min to prepare hydrothermal carbon. Subsequent washing, drying, high-temperature carbonization, and post-treatment steps were the same as in Example 1, yielding a black liquor-based hard carbon material, denoted as 320-HC.

[0043] Example 4 This embodiment provides a method for preparing a black liquid-based hard carbon material, including the following steps: The black liquor from papermaking was diluted to a mass concentration of 30% and placed in a hydrothermal reactor. The temperature was increased to 320°C at a rate of 5°C / min, with a stirring speed of 100 r / min. During the hydrothermal carbonization process, the pH of the system was adjusted to 1 using an acid solution (such as hydrochloric acid or sulfuric acid; hydrochloric acid was used in this example). The mixture was kept at this temperature for 240 min to prepare hydrothermal carbon. Subsequent washing, drying, high-temperature carbonization, and post-treatment steps were the same as in Example 1, yielding a black liquor-based hard carbon material, denoted as 320-pH-HC.

[0044] Example 5 This embodiment provides a method for preparing a black liquid-based hard carbon material, including the following steps: Papermaking black liquor was diluted to a mass concentration of 20% and placed in a hydrothermal reactor. The temperature was increased to 300℃ at a rate of 2℃ / min, with a stirring speed of 50 r / min. During hydrothermal carbonization, the pH of the system was adjusted to 1, and the temperature was maintained for 300 min to prepare hydrothermal carbon. The obtained hydrothermal carbon was washed with ethanol until colorless, then washed with deionized water until neutral, and dried at 100℃ for 18 h. Subsequently, the dried hydrothermal carbon was placed in a tube furnace, and Ar was introduced for 30 min to remove oxygen from the furnace. Under Ar protection, the temperature was increased to 1250℃ at a rate of 2℃ / min and an Ar flow rate of 50 mL / min, and maintained for 4 h before naturally cooling to room temperature. The carbonized sample was removed, washed with deionized water under vacuum filtration for 15 min, and then dried at 100℃ for 18 h to obtain black liquor-based hard carbon material.

[0045] Example 6 This embodiment provides a method for preparing a black liquid-based hard carbon material, including the following steps: Papermaking black liquor was diluted to a mass concentration of 40% and placed in a hydrothermal reactor. The temperature was increased to 280℃ at a rate of 8℃ / min, with a stirring speed of 200 r / min. During hydrothermal carbonization, the pH of the system was adjusted to 2, and the temperature was maintained for 200 min to prepare hydrothermal carbon. The obtained hydrothermal carbon was washed with ethanol until colorless, then washed with deionized water until neutral, and dried at 120℃ for 6 h. Subsequently, the dried hydrothermal carbon was placed in a tube furnace, and N2 was introduced for 30 min to remove oxygen from the furnace. Under N2 protection, the temperature was increased to 1350℃ at a rate of 8℃ / min and an N2 flow rate of 200 mL / min, and maintained for 1 h before naturally cooling to room temperature. The carbonized sample was removed, washed with deionized water under vacuum filtration for 40 min, and then dried at 120℃ for 6 h to obtain black liquor-based hard carbon material.

[0046] Example 7 This embodiment provides a method for preparing a black liquid-based hard carbon anode, including the following steps: Using the black liquid-based hard carbon materials prepared in Examples 1-6 as active materials, the black liquid-based hard carbon materials, Super P conductive agent, and PVDF binder were mixed at a mass ratio of 8:1:1. N-methyl-2-pyrrolidone (NMP) was used as the dispersion medium to prepare a slurry, which was then stirred in a stirrer for 5 min until the slurry was homogeneous. The slurry was coated onto a copper foil current collector with a thickness of 150 μm and vacuum dried at 80°C for 12 h to obtain the black liquid-based hard carbon negative electrode. The coated copper foil was then cut into 12 mm diameter circular negative electrode sheets for battery assembly as needed.

[0047] Example 8 This embodiment provides a method for preparing a black liquid-based hard carbon anode, which differs from Example 7 only in that: The mass ratio of the black liquor-based hard carbon material, conductive agent, and binder is 6:1.5:2.5. The conductive agent is conductive carbon black, the binder is CMC, the dispersion medium is deionized water, the current collector is aluminum foil, the coating thickness is 100 μm, and the drying conditions are vacuum drying at 60°C for 24 h. The remaining steps are the same as in Example 7.

[0048] Example 9 This embodiment provides a method for preparing a black liquid-based hard carbon anode, which differs from Example 7 only in that: The mass ratio of the black liquid-based hard carbon material, conductive agent, and binder is 9:2:1. The conductive agent is a mixture of acetylene black and carbon nanotubes (mass ratio 1:1), the binder is a mixture of SBR and PAA (mass ratio 1:1), the dispersion medium is NMP, the current collector is copper foil, the coating thickness is 200 μm, and the drying conditions are vacuum drying at 100°C for 6 h. The remaining steps are the same as in Example 7.

[0049] Example 10 This embodiment provides a method for assembling a sodium-ion battery, including the following steps: Using the black liquid-based hard carbon anode sheet prepared in any of Examples 7-9 as the working electrode, a sodium metal sheet as the counter electrode, and glass fiber filter paper as the separator, the electrolyte is a mixed solvent of 1 M NaPF6 dissolved in EC:DEC = 1:1 (volume ratio). All components are assembled in an argon-protected glove box according to conventional sodium-ion battery assembly processes to obtain a sodium-ion half-cell. Depending on the actual application requirements, the black liquid-based hard carbon anode of this invention can also be paired with suitable cathode materials (such as layered oxides, Prussian blue analogs, or polyanionic cathode materials) to assemble a sodium-ion full cell.

[0050] Comparative Example 1 This comparative example provides a method for preparing a hard carbon material, which differs from Example 3 only in that the high-temperature carbonization temperature is 1200℃, while the remaining steps are the same as in Example 3. This comparative example is used to illustrate the effect of a high-temperature carbonization temperature of 1200℃ and the 1300℃ of this invention on the properties of the prepared hard carbon material.

[0051] Comparative Example 2 This comparative example provides a method for preparing hard carbon materials, which differs from Example 4 only in that the pH adjustment is performed before hydrothermal carbonization. Specifically, the papermaking black liquor is first diluted to a mass concentration of 30%, and the pH is adjusted to 1 with hydrochloric acid before being placed in a hydrothermal reactor and subjected to hydrothermal carbonization at 320°C for 240 minutes. Subsequent steps are the same as in Example 4. This comparative example illustrates the effect of the timing of pH adjustment (pre-hydrothermal adjustment vs. in-situ adjustment during hydrothermal treatment) on the performance of hard carbon materials.

[0052] Comparative Example 3 This comparative example follows the method described in Example 1 of Chinese patent application CN117735527A, using wood and bamboo powder as raw material to prepare hard carbon anode material. The specific steps are as follows: Take 5 g of wood and bamboo powder and place it in a forced-air drying oven. Dry it at 105℃ for 12 h to remove moisture. After soaking in a 6 M hydrochloric acid solution for 6 hours, remove and dry for 12 hours; The acid-treated wood and bamboo powder was soaked in a 1 M NaOH solution for 12 h, then filtered and dried. The obtained material was placed in a reaction vessel, 70 mL of deionized water was added, and hydrothermal carbonization reaction was carried out at 240℃ for 1 hour. After cooling to room temperature, the material was filtered and dried. The obtained hydrothermal carbon was placed in a tube furnace and heated to 1000℃ at a heating rate of 5℃ / min under N2 atmosphere. The temperature was maintained for 2 h for pyrolysis. After cooling to room temperature, the carbon was taken out, ground and pulverized to obtain hard carbon anode material.

[0053] Performance testing methods (I) Microstructure Characterization 1. Transmission Electron Microscopy (TEM) Testing: The black liquid-based hard carbon materials prepared in Examples 1-4 were dispersed in anhydrous ethanol, ultrasonically dispersed, and then dropped onto a copper mesh support film. The microstructure and carbon layer lattice fringes of the samples were observed using a transmission electron microscope. Fourier transform was used to fit local regions, and the interlayer spacing of the carbon materials was measured. The test results are as follows: Figure 2 As shown.

[0054] 2. X-ray diffraction (XRD) test: The hard carbon materials prepared in Examples 1-4 were subjected to phase analysis using an X-ray diffractometer. The scanning range was 10°~70°, and the scanning speed was 2° / min. The interlayer spacing of the carbon materials was calculated using the Bragg equation based on the position of the (002) diffraction peak. The test results are as follows: Figure 3 As shown.

[0055] 3. Raman spectroscopy test: The hard carbon materials prepared in Examples 1-4 were tested using a Raman spectrometer with an excitation wavelength of 532 nm and a scanning range of 500-2500 cm⁻¹. -1 By analyzing the D peak (~1350 cm⁻¹) -1 ) and G peak (~1590cm) -1 A fitting was performed to calculate the ID / IG value to evaluate the degree of disorder and defects in the carbon material. The test results are as follows: Figure 4 As shown.

[0056] (II) Electrochemical performance testing 1. Constant current charge-discharge test: The hard carbon materials prepared in Examples 1-4 were assembled into sodium-ion half-cells according to the method in Example 7, and the cells were tested using a battery testing system at 30 mA·g. -1 Constant current charge-discharge tests were performed at current density with a voltage window of 0.01~3V (vs. Na). + / Na), record the charge / discharge specific capacity and coulombic efficiency for the first three cycles. Test results are as follows: Figure 5 As shown.

[0057] 2. Cyclic stability test: Two hard carbon samples with high reversible capacity, 320-HC (Example 3) and 320-pH-HC (Example 4), were selected and assembled into sodium-ion half-cells according to the above method. Cyclic stability was tested at 30 mA·g. -1 Multiple charge-discharge cycles were performed at the current density, and the capacity retention and coulombic efficiency were recorded after 60 cycles. The test results are as follows: Figure 6 As shown.

[0058] 3. Comparative performance test: The hard carbon materials prepared in Comparative Examples 1-3 were assembled into sodium-ion half-cells using the same method, and the performance was tested at 30 mA·g. -1 A constant current charge-discharge test was performed at the current density, the first reversible specific capacity was recorded, and compared with the embodiment of the present invention.

[0059] Performance test results I. Microstructure Analysis 1. TEM morphology and interlayer spacing analysis Figure 2 TEM images and lattice fringe spacing analysis diagrams of black liquor hydrothermal hard carbon prepared under different hydrothermal conditions in Examples 1-4 are presented. Figure 2 It can be seen that the samples under the four preparation conditions all exhibit a curved and disordered honeycomb structure, without the continuous and regular graphite lattice stripe structure of graphitized carbon material, and only locally visible short-range ordered pseudo-graphite microcrystalline stripes. Figure 2 (Marked by the orange box in the middle), which is completely consistent with the core structural characteristics of hard carbon materials: "long-range disorder and short-range order." Furthermore, a striped amorphous carbon matrix can be observed, rich in nanoscale pores and structural defects, similar to the characteristic closed-pore structure of hard carbon. Figure 2 (Indicated by the red circle in the middle), this structure provides ample storage space for sodium ions.

[0060] After performing Fourier transform fitting on the local regions, the interlayer spacing of the four hard carbon materials was obtained as follows: 0.370 nm for 240-HC, 0.369 nm for 280-HC, 0.359 nm for 320-HC, and 0.415 nm for 320-pH-HC, all greater than the theoretical graphite spacing of 0.34 nm. Among them, the interlayer spacing of 320-pH-HC is greater than 0.4 nm, providing excellent conditions for the insertion and diffusion of sodium ions. Furthermore, from 240-HC to 320-HC, as the hydrothermal carbonization temperature increased from 240℃ to 320℃, the interlayer spacing gradually decreased from 0.370 nm to 0.359 nm, indicating that the increase in hydrothermal temperature led to an increased graphitization tendency of the carbon materials during the subsequent high-temperature carbonization process. When pH=1 was introduced to regulate the 320-pH-HC under the same hydrothermal conditions of 320℃, the interlayer spacing was significantly increased to 0.415 nm, indicating that the acidic hydrothermal environment can effectively inhibit the orderly stacking of carbon layers and significantly increase the interlayer spacing.

[0061] 2. XRD Structure Analysis Figure 3 The images show the XRD patterns of black liquor hydrothermal carbon-based hard carbons prepared under different hydrothermal conditions in Examples 1-4. Figure 3 As can be seen, all four hard carbon samples exhibit a broad (002) characteristic diffraction peak near 2θ≈22°~24° and a weaker (100) diffraction peak near 2θ≈43°. Both peaks are characterized by broadening and low intensity, indicating that the obtained carbon material is a typical amorphous hard carbon structure. The interlayer spacing of the four hard carbon materials was calculated according to the Bragg equation, and the results were as follows: 0.375 nm for 240-HC, 0.366 nm for 280-HC, 0.365 nm for 320-HC, and 0.402 nm for 320-pH-HC. The XRD calculation results are similar to the TEM scale measurement results, further verifying the above interlayer spacing variation law. Among them, the interlayer spacing of 320-pH-HC reaches 0.402 nm, which is significantly larger than the other three samples. This higher interlayer spacing provides sufficient space for sodium ion insertion and extraction, which can effectively improve the sodium storage capacity of sodium-ion batteries.

[0062] 3. Raman spectral analysis Figure 4 The images show the Raman spectra of black liquor hydrothermal carbon-based hard carbons prepared under different hydrothermal conditions in Examples 1-4. Figure 4It is evident that the ID / IG values ​​reflect the degree of disorder and defects in carbon materials. By fitting the D and G peaks, the ID / IG values ​​of the four hard carbon samples were calculated as follows: 1.25 for 240-HC, 1.26 for 280-HC, 1.26 for 320-HC, and 1.47 for 320-pH-HC, all greater than 1. This further indicates that the carbon materials prepared in this study have high defects and disorder, consistent with the basic characteristics of hard carbon. Among them, the ID / IG value of 320-pH-HC is significantly higher than that of the other three samples (1.47 vs 1.25~1.26), indicating that the acidic hydrothermal environment can significantly increase the degree of disorder and reduce the degree of graphitization of carbon materials. This is consistent with the results of TEM and XRD interlayer spacing tests, jointly confirming the key role of pH regulation in inhibiting the ordered stacking of carbon layers and increasing interlayer spacing and defect density.

[0063] II. Electrochemical Performance Analysis 1. Constant current charge and discharge performance Figure 5 (a) to (d) represent 240-HC, 280-HC, 320-HC, and 320-pH-HC at 30 mA·g, respectively. -1 The first three cycles of constant current charge-discharge test curves at current density. From Figure 5 It can be seen that all four hard carbon electrodes exhibit typical charge-discharge characteristics of a ramp region + plateau region, consistent with the "intercalation-filling-adsorption" multiple mechanism of sodium storage in hard carbon materials. The initial reversible specific capacities of the four hard carbon samples are as follows: 240-HC is 152.5 mAh·g. - ¹, 280-HC is 152.6 mAh·g - ¹, 320-HC has a capacity of 182.0 mAh·g - ¹, 320-pH-HC is 275.8 mAh·g - ¹. The above results indicate that the reversible capacity of black liquor hydrothermal carbon-based hard carbon gradually increases with increasing pre-hydrothermal carbonization temperature (240℃→280℃→320℃); while under acidic hydrothermal carbonization pretreatment conditions (320-pH-HC), the reversible capacity further increases by approximately 51.5% compared to 320-HC, reaching 275.8 mAh·g. - ¹. This indicates that the introduction of acidity can further improve reversible capacity, mainly due to the increased interlayer spacing (0.402 nm) and defect density (ID / IG=1.47) achieved by pH regulation, which provides more storage sites and more unobstructed diffusion channels for sodium ions.

[0064] 2. Cyclic stability To further investigate the cycling stability of black liquor hydrothermal carbon-based hard carbon, two hard carbon samples with high reversible capacity, 320-HC and 320-pH-HC, were selected and tested at 30 mA·g.-1 Multiple charge-discharge cycles were performed at the current density. For example... Figure 6 As shown, after 60 cycles, the 320-HC sample maintained a reversible capacity retention of 96.92%, with a coulombic efficiency greater than 98% except for the first cycle, demonstrating excellent cycling stability. In contrast, the 320-pH-HC sample maintained a capacity retention of only 66.13% after 60 cycles; although its initial capacity was higher, its capacity decayed rapidly. These results indicate that the high reversible capacity of 320-pH-HC can be attributed to its large interlayer spacing (>0.4 nm) and abundant defect structures. However, during repeated charge-discharge cycles, with the repeated insertion and extraction of sodium ions, the excessively large interlayer spacing may cause irreversible expansion or contraction of the hard carbon interlayer spacing, disrupting the local ordered structure and resulting in rapid capacity decay.

[0065] The above results demonstrate that the present invention can directionally regulate the microstructure of hard carbon materials by adjusting the pH value during the hydrothermal carbonization process.

[0066] When no additional pH adjustment is required (such as 320-HC), the resulting hard carbon interlayer spacing is moderate, combining high capacity (182.0 mAh / g) with excellent cycle stability (96.92%), making it suitable for long-life energy storage scenarios. When acidic in-situ regulation is introduced (such as 320-pH-HC), the interlayer spacing of hard carbon is significantly expanded (>0.4 nm). Although the structural rearrangement effect under high rate or long cycle leads to a decrease in capacity retention, its extremely high reversible capacity (275.8 mAh / g) makes it irreplaceable in low cycle frequency scenarios with stringent energy density requirements.

[0067] 320-pH-HC is not limited to single-use applications; it can be used as a high-capacity additive in combination. In industry, high-capacity materials with slightly poor cycling performance are often physically combined (doped) with materials that have high cycling performance but low capacity.

[0068] For example, mixing 10%~30% of 320-pH-HC (275.8 mAh / g) with 320-HC (182.0 mAh / g) can easily increase the capacity of the composite material to over 200 mAh / g, while only slightly reducing the cycle life (from 96.9% to over 85%), still meeting the standards for power batteries. 320-pH-HC plays the role of a "capacity booster" in this compounding process.

[0069] This fully demonstrates that the present invention provides a customizable hard carbon preparation technology path, which can flexibly switch between the two application directions of 'high stability' and 'high capacity' of hard carbon anodes through simple pH control.

[0070] 3. Comparative Analysis The only difference between Comparative Example 1 and Example 3 is the high-temperature carbonization temperature: 1200℃ (Comparative Example 1) vs. 1300℃ (Example 3). Under the same hydrothermal conditions (320℃, no pH adjustment), the initial reversible specific capacity of the hard carbon material carbonized at 1200℃ is approximately 165~175 mAh·g. - ¹, slightly lower than 182.0 mAh·g in Example 3 - ¹ This indicates that carbonization at 1300℃ helps improve the electronic conductivity and structural order of carbon materials, thereby increasing their sodium storage capacity.

[0071] The only difference between Comparative Example 2 and Example 4 is the timing of pH adjustment—Comparative Example 2 adjusted the pH of the black liquor to 1 before hydrothermal carbonization, while Example 4 adjusted the pH to 1 in situ during hydrothermal carbonization. The results show that the interlayer spacing of the hard carbon material obtained in Comparative Example 2 is only about 0.37–0.38 nm, and the initial reversible specific capacity is about 200–210 mAh·g. - ¹, all significantly lower than in Example 4 (interlayer spacing 0.402 nm, capacity 275.8 mAh·g). - ¹) This indicates that in-situ pH control during the hydrothermal process is the key to achieving a significant increase in interlayer spacing, rather than simply changing the initial acidity or alkalinity of the raw materials.

[0072] Comparative Example 3: The wood-bamboo powder-based hard carbon anode material prepared according to the method in Example 1 of CN117735527A has an initial reversible specific capacity of 344.55 mAh·g. - ¹ (Based on the data in Table 1 of this patent), higher than the 275.8 mAh·g of Example 4 of this invention. - ¹. However, it should be noted that: (1) The raw material of Comparative Example 3 is a selected pure biomass (wood and bamboo powder), which requires complex pretreatment processes such as soaking in hydrochloric acid for 6 h and soaking in NaOH for 12 h. In contrast, the present invention directly uses industrial waste black liquor as raw material, omitting all acid and alkali pretreatment steps, and greatly simplifies the process; (2) The present invention realizes the high-value utilization of waste from the papermaking industry, and also has environmental benefits; (3) The present invention still has room to further improve capacity by further optimizing hydrothermal conditions and carbonization process.

[0073] III. Overall Conclusion In summary, this invention utilizes industrial waste papermaking black liquor as raw material and, through a process route of "direct dilution → hydrothermal carbonization (in-situ pH control) → high-temperature carbonization," successfully prepares a sodium-ion battery anode material with typical hard carbon structural characteristics without any pretreatment processes such as acid washing, alkali washing, or lignin purification. This process route significantly simplifies the preparation process, effectively reduces raw material costs and production energy consumption, and simultaneously achieves high-value and resource-based utilization of papermaking black liquor, providing a novel approach to solving the problem of wastewater pollution in the papermaking industry.

[0074] This invention enables targeted regulation of the microstructure of hard carbon materials by adjusting the pH value during the hydrothermal carbonization process, thereby achieving on-demand adjustment between sodium storage capacity and cycle stability. When no additional pH adjustment is required (e.g., 320-HC), the resulting hard carbon material exhibits a moderate interlayer spacing (approximately 0.359 nm) and a high reversible capacity (182.0 mAh·g). - ¹) With excellent cycle stability (capacity retention rate of 96.92% after 60 cycles), it is suitable for long-life energy storage scenarios with high requirements for cycle life. When acidic in-situ regulation (pH=1) is introduced (e.g., 320-pH-HC), the interlayer spacing of the resulting hard carbon material is significantly increased to 0.415 nm (XRD measured 0.402 nm), the degree of disorder is greatly improved (ID / IG reaches 1.47), and the reversible specific capacity reaches 275.8 mAh·g for the first time. - ¹ Compared to the unadjusted sample, the energy density is increased by approximately 51.5%, demonstrating a significant advantage in applications with stringent energy density requirements. Furthermore, this high-capacity hard carbon material can also serve as a "capacity booster," being physically combined with long-cycle hard carbon (e.g., by adding 10%–30%) to effectively improve overall energy density while maintaining high cycling stability, providing a flexible material solution for industrial applications.

[0075] Furthermore, this invention overcomes the technical bias in the prior art that "carbonization temperatures above 1200℃ will lead to a reduction in interlayer spacing," by raising the high-temperature carbonization temperature to 1250~1350℃. Combined with in-situ pH control during the hydrothermal stage, it further optimizes the structural order and electronic conductivity of carbon materials while ensuring suitable interlayer spacing, thus expanding the process window for structural control of hard carbon materials.

[0076] In summary, this invention provides a customizable black liquid-based hard carbon anode preparation technology route, which combines the advantages of simplified process economy, solid waste resource utilization environmental benefits, and adjustable electrochemical performance. It provides a new and reliable technical solution for the industrial preparation of low-cost, high-performance sodium-ion battery hard carbon anode materials, which is in line with the green and low-carbon development concept of the new energy industry.

[0077] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a black liquid-based hard carbon material, characterized in that, Includes the following steps: After diluting the black liquor from papermaking, it is directly subjected to hydrothermal carbonization without acid washing and alkali washing pretreatment to obtain hydrothermal carbon. The hydrothermal carbon is washed, dried, and then carbonized at high temperature under an inert atmosphere to obtain black liquor-based hard carbon material. The pH value of the system is adjusted to 0.5~2 during the hydrothermal carbonization process.

2. The method for preparing black liquid-based hard carbon material according to claim 1, characterized in that, The papermaking black liquor is diluted to a mass concentration of 20%~40%; The hydrothermal carbonization temperature is 240~320℃, and the holding time is 180~300 min; The high-temperature carbonization temperature is 1250~1350℃, and the holding time is 1~4 h.

3. The method for preparing black liquid-based hard carbon material according to claim 2, characterized in that, The heating rate of the hydrothermal carbonization is 2~8℃ / min, and the stirring speed is 50~200 r / min; The heating rate for high-temperature carbonization is 2~8℃ / min, the inert atmosphere is N2 or Ar, and the gas flow rate is 50~200mL / min.

4. The method for preparing black liquid-based hard carbon material according to claim 1, characterized in that, Before high-temperature carbonization, the hydrothermal carbon is first washed with an organic solvent until colorless, then washed with deionized water until neutral, and dried at 100~120℃ for 6~18h; the sample after high-temperature carbonization is washed with deionized water for 15~40 min and dried at 100~120℃ for 6~18h.

5. The method for preparing black liquid-based hard carbon material according to claim 2, characterized in that, When the hydrothermal carbonization temperature is 300~320℃, the pH of the system is adjusted to 1.

6. A method for preparing a black liquid-based hard carbon material, characterized in that, Includes the following steps: After diluting the black liquor from papermaking, it is directly subjected to hydrothermal carbonization without acid washing and alkali washing pretreatment to obtain hydrothermal carbon. The hydrothermal carbon is washed, dried, and then carbonized at high temperature under an inert atmosphere to obtain black liquor-based hard carbon material. The system pH value is not adjusted during the hydrothermal carbonization process, so that the hydrothermal carbonization is carried out under the original pH conditions.

7. A method for preparing a black liquid-based hard carbon anode, characterized in that, Includes the following steps: Black liquid-based hard carbon materials are prepared using the method described in any one of claims 1 to 6; The black liquid-based hard carbon material is mixed with a conductive agent and a binder in a certain proportion, and an electrode slurry is prepared by using an organic solvent as a dispersion medium; the mass ratio of the black liquid-based hard carbon material, the conductive agent, and the binder is 6~9:0.5~2:0.5~2. The electrode slurry is coated onto the current collector, dried, and cut to obtain a black liquid-based hard carbon anode. The conductive agent is at least one of conductive carbon black, Super P, acetylene black, and carbon nanotubes; the binder is at least one of PVDF, CMC, SBR, and PAA; and the organic solvent is N-methyl-2-pyrrolidone or deionized water. The electrode slurry coating thickness is 100~200 μm, and the drying is vacuum drying at 60~100℃ for 6~24 h. The current collector is a copper foil or an aluminum foil.

8. A sodium-ion battery, characterized in that, It includes a black liquid-based hard carbon anode prepared by the method of claim 7.

9. A method for compounding hard carbon anode materials for sodium-ion batteries, characterized in that, Includes the following steps: The first hard carbon material and the second hard carbon material are mixed in a certain proportion; The first hard carbon material is the hard carbon material prepared by adjusting the pH of the system to 0.5-2 in the method described in any one of claims 1 to 5; The second hard carbon material is the hard carbon material prepared by hydrothermal carbonization under the original pH conditions without adjusting the pH of the system in the method described in claim 6; The mass percentage of the first hard carbon material is 10% to 30%.

10. The compounding method according to claim 9, characterized in that, The initial reversible specific capacity of the first hard carbon material is 250~300 mAh·g. - ¹, the capacity retention rate of the second hard carbon material after 60 cycles is ≥90%.

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