Method for preparing polysaccharide derivative hard carbon negative electrode material by room temperature molten salt assisted joule heat and application thereof

Abstract

This invention discloses a method for preparing polysaccharide-derived hard carbon anode materials using room-temperature molten salt-assisted Joule heating. Polysaccharide compounds are mixed with room-temperature molten salt, and the mixture is subjected to molten salt heat treatment to obtain a uniformly morphologically uniform ultrafine carbon precursor. This precursor is then subjected to high-temperature carbonization using Joule thermal shock technology. By precisely controlling the Joule thermal shock time, a hard carbon anode material with a composite structure of short-range ordered graphite microregions and a uniformly sized closed-pore network is obtained. The room-temperature molten salt plays a morphology-regulating role during heat treatment, ensuring uniform precursor particles and a smooth surface, providing an ideal reaction interface for subsequent Joule heat treatment. The Joule thermal shock treatment, through ultrafast heating and time threshold control, effectively preserves structural defects while allowing closed pores to grow to the optimal size. This invention achieves synergistic optimization of high plateau capacity and high ramp capacity through the synergistic effect of room-temperature molten salt morphology regulation and Joule thermal time threshold. The process is simple, energy-efficient, and suitable for large-scale production.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery materials technology, and relates to a method and application for preparing polysaccharide-derived hard carbon anode materials by room temperature molten salt-assisted Joule heating. Background Technology

[0002] Sodium-ion batteries have great potential in large-scale energy storage due to the abundance and low cost of sodium resources. However, the radius of sodium ions (1.02 Å) is larger than that of lithium ions (0.76 Å), making it difficult to achieve reversible insertion and extraction in traditional graphite anodes. Hard carbon, due to its large interlayer spacing and abundant microporous structure, is considered an ideal anode material for sodium-ion batteries.

[0003] Polysaccharides (such as starch and sucrose) are widely available, renewable, and low-cost, making them ideal precursors for hard carbon production. Traditional hydrothermal methods combined with high-temperature pyrolysis are commonly used to prepare polysaccharide-derived hard carbon. However, traditional hydrothermal methods use only pure water as a medium, failing to fully utilize the morphological control of additives on the precursors. This leads to problems such as particle agglomeration, rough surfaces, and poor sphericity in the products. These morphological defects result in uneven stress distribution during subsequent heat treatment, random and disordered closed-cell nucleation and growth, ultimately leading to a chaotic closed-cell structure with a wide size distribution, failing to form a uniform and effective closed-cell network, and exhibiting poor batch-to-batch stability.

[0004] Room temperature molten salts (ionic liquids) have attracted attention in the field of materials synthesis due to their unique physicochemical properties—such as low vapor pressure, high ionic conductivity, good solubility, and designable structures. Unlike high temperature molten salts (such as NaCl and KCl, with melting points >800 °C), room temperature molten salts are liquid at room temperature and can be uniformly dispersed in polysaccharide matrices. As reaction media and morphology guiding agents, they can intercalate into the spaces between polysaccharide molecular chains, disrupting hydrogen bond networks and precisely controlling the nucleation and growth of carbon microspheres.

[0005] Furthermore, even if salt pretreatment improves the precursor morphology, traditional slow pyrolysis (several hours, thermodynamic equilibrium) still requires prolonged high temperatures to eliminate defects, leading to insufficient ramp capacity. Simultaneously, closed-cell coarsening and collapse further limit platform capacity. Joule thermal shock technology, through instantaneous (second-level) high-energy pulses, achieves non-equilibrium dynamic control, effectively suppressing long-range carbon atom migration, preserving structural defects, and precisely controlling closed-cell size through time thresholds.

[0006] Therefore, developing a method that combines room temperature molten salt-assisted morphology control with Joule heating time threshold control is of great significance for simultaneously improving the total capacity, plateau capacity, slope capacity and performance stability of hard carbon. Summary of the Invention

[0007] The purpose of this invention is to provide a method for preparing polysaccharide-derived hard carbon anode materials and their applications. By combining room temperature molten salt-assisted morphology control with Joule heating time threshold control, the ramp capacity is significantly improved while maintaining a high plateau capacity, achieving synergistic optimization of both.

[0008] The technical solution of the present invention: A method for preparing polysaccharide-derived hard carbon anode materials by room temperature molten salt-assisted Joule heating includes the following steps: (1) Pretreatment of polysaccharide compounds to obtain ultrafine carbon precursors; The pretreatment process involves mixing room temperature molten salt with polysaccharide compounds at a mass ratio of 6:60 to 6:100, heat-treating at 160-200 °C for 6-24 hours, and then washing and drying to obtain the carbon precursor.

[0009] (2) The carbon precursor obtained in step (1) is placed in a Joule heating device and heated to 1400-1800 ℃ at a heating rate of 100-500 ℃ / s under an inert atmosphere to perform Joule heat shock treatment for 10-120 s to obtain polysaccharide-derived hard carbon anode material.

[0010] The polysaccharide compound in step (1) is one or more of the following: corn starch, potato starch, cassava starch, wheat starch, sucrose, fructose, and agar.

[0011] The molten salt in step (1) is one or a mixture of two or more of the following: 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium hexafluorophosphate, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, N-butylpyridine tetrafluoroborate, N-butylpyridine bromide, tetrabutylammonium bromide, and tetrabutylphosphine bromide.

[0012] The inert gas mentioned in step (2) is helium, neon, or argon.

[0013] In one embodiment, the polysaccharide-derived hard carbon anode material is at 50 mA g -1 The reversible capacity at current density is 350-370 mAh g. -1 The platform capacity is 220-230 mAh g. -1 The ramp capacity is 130-150 mAh g. -1 .

[0014] The polysaccharide-derived hard carbon anode material obtained by the above preparation method can be used as an anode material for sodium-ion batteries.

[0015] The beneficial effects of this invention are: First, this invention utilizes the morphology control effect of room temperature molten salt during the heat treatment process to make the polysaccharide-derived carbon-carbon microsphere precursor particles uniform, smooth, and well-dispersible, avoiding the agglomeration and irregular morphology problems commonly found in traditional hydrothermal methods. This provides an ideal reaction interface for subsequent Joule heat treatment, which is beneficial for the uniform nucleation of closed pores.

[0016] Secondly, this invention employs Joule thermal shock technology for high-temperature carbonization. By controlling the ultra-fast heating and time threshold, a large number of structural defects are retained while suppressing the long-range migration of carbon atoms, thus obtaining a high ramp capacity. At the same time, by precisely controlling the Joule heating time (60 s optimal threshold), the closed pores are grown to the optimal size (1-2 nm), achieving a high plateau capacity.

[0017] Furthermore, compared with traditional pyrolysis, the polysaccharide-derived hard carbon anode material prepared in this invention maintains a high plateau capacity while significantly improving the ramp capacity, resulting in overall performance significantly superior to existing technologies. Specifically, the reversible capacity of the salt-FJH-1600-60s sample reaches 358.92 mAh g⁻¹. -1 Platform capacity 223.67 mAh g -1 Slope capacity 135.25 mAh g -1 The platform's capacity accounts for 62.3%.

[0018] Finally, the process of this invention is simple, green, and environmentally friendly. The room temperature molten salt can be recycled, the Joule heat treatment only takes seconds, the energy consumption is low, and it does not depend on a specific polysaccharide precursor, thus having good versatility and industrialization prospects. Attached Figure Description

[0019] Figure 1 The first charge-discharge curves of implementation cases 1, 2, 3, and 4 and comparative examples 1 and 2 are shown.

[0020] Figure 2 To implement the rate performance curves of Case 1, 2, 3, 4 and Comparative Examples 1, 2. Detailed Implementation

[0021] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and technical solutions.

[0022] The following comparative examples and embodiments demonstrate performance testing of a sodium-ion battery system assembled from various materials: Active material, conductive additive (Super P), and binder (PVDF) were mixed in an 8:1:1 ratio to form a slurry, which was then coated onto copper foil. The copper foil was dried in a vacuum oven at 60 °C for 12 h and then cut into electrode sheets with a diameter of 10 mm. In an argon-filled glove box (H₂O < 0.01 ppm and O₂ < 0.01 ppm), the electrode sheets, separator (glass fiber filter GF / D), and metallic sodium were assembled into a CR2032 button cell. 1 mol / L NaPF₆ in DME was used as the electrolyte. Charge-discharge curves (GCD) and cycle performance were obtained using a LAND-CT2001A battery tester (Wuhan, China). All electrochemical tests were performed at a voltage range of 0.01–2.5 V and a temperature of room temperature (25 °C).

[0023] Example 1 Corn starch and 1-butyl-3-methylimidazolium tetrafluoroborate were mixed at a mass ratio of 6:80 and solvothermal treated at 180 °C for 12 h. The molten salt was then removed by repeated water / ethanol washing, and the product was dried to obtain the precursor. The product was placed in a Joule heating apparatus and heated to 1600 °C at a rate of 300 °C / s under an argon atmosphere, followed by Joule heat shock treatment for 10 s to obtain a hard carbon material, denoted as Salt-FJH-1600-10s.

[0024] Example 2 The process is basically the same as in Example 1, except that the Joule heat shock treatment time is 30 s, resulting in the hard carbon material Salt-FJH-1600-30s.

[0025] Example 3 The process is basically the same as in Example 1, except that the Joule heat shock treatment time is 60 s, resulting in the hard carbon material Salt-FJH-1600-60s.

[0026] Example 4 The process is basically the same as in Example 1, except that the Joule heat shock treatment time is 120 s, resulting in the hard carbon material Salt-FJH-1600-120s.

[0027] Comparative Example 1 The precursor product obtained in step (1) of Example 1 was placed in a tube furnace and heated to 1600 °C at a heating rate of 5 °C / min under an argon (Ar) atmosphere. The temperature was held for 2 hours to obtain the hard carbon material salt-HC-1600.

[0028] Comparative Example 2 Corn starch and water were mixed at a mass ratio of 6:80 and hydrothermally treated at 180 °C for 12 hours. After drying, conventional pyrolysis (1600 °C, 2 h) was carried out without the addition of molten salt, in accordance with the method of Comparative Example 1, to obtain salt-free HC-1600.

[0029] Table 1

[0030] A detailed analysis was conducted on Examples 1-4 and Comparative Examples 1-2: Comparing the electrochemical performance of the salt-free HC-1600 and the salt-containing HC-1600, it can be seen that the plateau capacity increased from 204.07 mAh g⁻¹ to 221.04 mAh g⁻¹ after adding the molten salt. -1 This indicates that molten salt improves the closed-pore structure and enhances the capacity of the platform through morphology control.

[0031] In the salt-FJH series, as the time increased from 10 s to 60 s, the platform capacity increased sharply from 90.75 mAh g to 223.67 mAh g. -1 It reached a level comparable to traditional salt calcination; while the slope capacity gradually decreased from 149.61 to 135.25 mAh g. -1 When the time was extended to 120 seconds, the platform capacity decreased slightly to 198.04 mAh g. -1 The slope continued to drop to 127.11 mAh g -1 This non-monotonic change reveals the existence of a Joule heating time threshold (approximately 60 s): too short a time results in incomplete pore development (low plateau), while too long a time leads to pore coarsening (decreasing plateau); while the slope capacity decreases slowly with time, but the slope capacity of all Joule-heated samples is significantly higher than that of conventional salt-calcined samples (91.72 mAh g⁻¹). -1 This demonstrates that, under optimal processing time, Joule heating successfully preserves abundant defect sites and surface functional groups, and constructs closed-pore structures of suitable size (1-2 nm).

[0032] like Figure 2 As shown, the salt-FJH-1600-60s sample was tested at 50 to 5000 mA g. -1 Within the current density range, its reversible specific capacities are 354.3, 324.9, 307.9, 286.7, 263.0, 224.2 and 138.2 mAh g, respectively. -1 It is significantly superior to the traditionally calcined salt-HC-1600 sample, exhibiting excellent rate performance.

[0033] The salt-containing FJH-1600-60s sample exhibited the best performance among all samples, with a reversible capacity of 358.92 mAh g⁻¹.-1 Platform capacity 223.67 mAh g -1 (with 221.04 mAh g of salt-1600) -1 (Basically unchanged), slope capacity 135.25 mAh g -1 (Compared to 82.72 mAh g of salt-HC-1600) -1 (63.5% increase). This demonstrates the synergistic effect of molten salt morphology control and Joule heating time threshold, which significantly improves ramp capacity while maintaining high plateau capacity, successfully solving the technical challenge of balancing ramp and plateau in traditional processes.

Claims

1. A method for preparing polysaccharide-derived hard carbon anode materials by room temperature molten salt-assisted Joule heating, characterized in that, Includes the following steps: (1) Pretreatment of polysaccharide compounds to obtain ultrafine carbon precursors; (2) The carbon precursor obtained in step (1) is subjected to Joule thermal shock treatment in an inert atmosphere at a temperature of 1400-1800 ℃ for 10-120 s.

2. The method for preparing polysaccharide-derived hard carbon anode materials by room temperature molten salt assisted Joule heating according to claim 1, characterized in that, In step (1), the pretreatment method is to mix polysaccharide compounds with molten salt at a mass ratio of 6:60 to 6:100, heat treat with molten salt at 160-200 °C for 6-24 hours, and obtain carbon precursor after washing with water / ethanol and drying.

3. The method for preparing polysaccharide-derived hard carbon anode materials by room temperature molten salt assisted Joule heating according to claim 1, characterized in that, In step (1), the molten salt is one or a mixture of two or more of the following: 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium hexafluorophosphate, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, N-butylpyridine tetrafluoroborate, N-butylpyridine bromide, tetrabutylammonium bromide, and tetrabutylphosphine bromide.

4. The method for preparing polysaccharide-derived hard carbon anode materials by room temperature molten salt assisted Joule heating according to claim 1, characterized in that, In step (1), the polysaccharide compound is one or more of the following: corn starch, potato starch, cassava starch, wheat starch, sucrose, fructose, and agar.

5. The method for preparing polysaccharide-derived hard carbon anode materials by room temperature molten salt assisted Joule heating according to claim 1, characterized in that, In step (2), the inert atmosphere is helium, neon or argon.

6. The method for preparing polysaccharide-derived hard carbon anode materials by room temperature molten salt assisted Joule heating according to claim 1, characterized in that, In step (2), the heating rate of the Joule thermal shock treatment is 100-500 ℃ / s.

7. The polysaccharide-derived hard carbon anode material obtained by the preparation method according to any one of claims 1-6, characterized in that, The polysaccharide-derived hard carbon anode material possesses a composite structure with uniform morphology, short-range ordered graphite microdomains, and uniformly sized closed-pore networks; the polysaccharide-derived hard carbon anode material exhibits performance at 50 mA g. -1 The reversible capacity at current density is 350-370 mAh g. -1 The platform capacity is 220-230 mAh g. -1 The ramp capacity is 130-150 mAh g. -1 .

8. The application of the polysaccharide-derived hard carbon anode material according to claim 7 in sodium-ion batteries.

9. A sodium-ion battery, characterized in that, It includes the polysaccharide-derived hard carbon anode material as described in claim 7.

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