A lignin-based hard carbon material, its preparation method, and its application in sodium-ion batteries

Abstract

This invention discloses a lignin-based hard carbon material, its preparation method, and its application in sodium-ion batteries. The invention uses phosphoric acid as an activator and lignin as a carbon source, preparing the lignin-based hard carbon material through low-temperature carbonization, washing, drying, and high-temperature carbonization. Low-temperature carbonization of the mixture of phosphoric acid and lignin can prepare porous carbon dominated by micropores. During high-temperature carbonization, with the rearrangement of carbon microcrystals and the growth of carbon layers, the micropores will close, forming a rich closed-pore structure in the hard carbon. This invention achieves the construction of high closed-pore volume in lignin-based hard carbon, providing abundant filling sites for sodium ions in the closed pores of the hard carbon, thereby achieving high specific capacity and plateau capacity of the hard carbon anode in sodium-ion batteries. The prepared hard carbon anode material has high reversible specific capacity, good rate performance, and cycle stability. This method is characterized by low cost and simple operation, realizing the high-value utilization of lignin and is conducive to large-scale production.

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 lignin-based hard carbon material. Background Technology

[0002] Sodium-ion battery anodes include alloy materials, metal oxides, organic compounds, and carbon materials. Hard carbon, a type of carbon material, consists of bent graphene sheets and amorphous carbon structures. These graphene sheets are randomly stacked to form numerous closed nanopores, providing abundant storage sites for sodium ions. The plateau capacity of hard carbon mainly comes from the filling of sodium ions in the closed pores (Chemistry of Materials 2022, 34, 3489-3500). Generally, the larger the closed pore volume, the higher the plateau capacity. Open pore structures often cannot provide active sites for sodium ion filling; therefore, porous carbon prepared by general activation methods cannot be used as a hard carbon anode material. Closed pores can be prepared by constructing channels in the hard carbon precursor. Chinese patent CN115893405A discloses a method for preparing hard carbon for sodium-ion battery anodes. This patent uses metal hydroxide as a pore-forming agent, which is ball-milled with lignin to obtain a mixture. The mixture is then carbonized at high temperature to prepare a metal hydroxide-activated lignin hard carbon material. This method, due to the addition of a highly active pore-forming agent, results in a precursor with an excessive amount of open-pore structure. Simultaneously, the activation temperature is higher than the precursor's decomposition temperature, leading to the formation of a stable carbon framework structure after activation. This is detrimental to the closure of the open pores during high-temperature carbonization. After carbonization at 1600℃, the specific surface area of ​​this hard carbon material reaches as high as 638 m². 2 g -1 Therefore, this lignin-based hard carbon material has a low reversible specific capacity (283 mAh g). -1 Closed pores can be achieved by constructing open channels in the hard carbon precursor and then closing them through post-processing techniques. The article (Carbon. 2018, 129, 85-94) reports a method for preparing hard carbon with low specific surface area and high closed pore volume by coating the surface of porous carbon with pitch. This method involves coating commercial porous carbon with pitch via solvent evaporation followed by high-temperature carbonization to prepare hard carbon with a relatively low specific surface area (24 m²). 2 g -1 (and high closed-pore volume hard carbon, the hard carbon anode has 293mAh g) -1 The platform capacity is determined by the size of the closed pores. This shows that the size of the closed pores determines the platform capacity of the hard carbon anode. Furthermore, the closed pore structure can be achieved by sealing the open pores.

[0003] The literature (Resources Chemicals and Materials. 2023 2, 245–251) prepared a hard carbon anode material via one-step carbonization of corn cob lignin. This hard carbon anode material suffers from a low closed-pore volume, failing to provide a large number of pore-filling active sites, thus limiting its reversible specific capacity (284 mAh g⁻¹). -1 The rate capability is relatively low, resulting in poor rate performance (at 1 Ag). -1 Its current density is 85.8 mAh g. -1 The aforementioned problems restrict the practical application of lignin-based hard carbon.

[0004] Therefore, lignin pretreatment can improve the interlayer spacing and closed pore volume of lignin-based hard carbon, which provides a large number of active sites for sodium ion intercalation and filling in lignin-based hard carbon, thereby improving the reversible specific capacity of hard carbon. Chinese patent CN110797533A discloses a lignin hard carbon microsphere and a hydrothermal preparation method. This patent involves adding inorganic or organic acids to a lignin solution for hydrothermal treatment, followed by high-temperature carbonization of the lignin hydrothermal product to obtain acid-treated lignin-based hard carbon. Pretreatment with inorganic or organic acids results in a derived hard carbon with a large interlayer spacing (greater than 0.37 nm), which is beneficial for sodium ion intercalation in the hard carbon. Although this hard carbon material has a large interlayer spacing, this method does not regulate the pore structure of the lignin precursor, because the specific surface area of ​​hydrothermally heated carbon under acidic conditions is often low (7.1–48.3 nm). 2 g -1 (Bioresource Technology 2015, 182, 336-344). Therefore, the reversible specific capacity of this hard carbon material (232.5 mAh g) -1 The pore volume is relatively low. Although physical activation (using water vapor or CO2 as activating agents) and chemical activation (using KOH, ZnCl2 and H3PO4 as activating agents) are widely used to manufacture activated carbon, the temperatures at which open channels are formed during the activation process are different, the stability of their carbon skeletons is different, and whether the open channels can be closed is also different. This is why it is difficult to obtain high closed pore volume. Summary of the Invention

[0005] To overcome the problem of insufficient specific capacity of lignin-based hard carbon, the primary objective of this invention is to provide a method for preparing lignin-based hard carbon materials with the assistance of phosphoric acid activation.

[0006] Another object of the present invention is to provide a phosphoric acid-activated assisted preparation of lignin-based hard carbon material obtained by the above preparation method.

[0007] Another object of the present invention is to provide the application of the above-mentioned phosphoric acid activation-assisted preparation of lignin-based hard carbon material in the anode of sodium-ion batteries.

[0008] The objective of this invention is achieved through the following technical solution:

[0009] Simultaneously, lignin-based hard carbon, possessing both large interlayer spacing and high closed-pore volume, is more conducive to providing more active sites for sodium storage, thereby improving the specific capacity, plateau capacity, and rate performance of hard carbon anodes. To this end, this invention mixes lignin and phosphoric acid in an aqueous solution to form a mixture, which serves as a precursor for lignin-based hard carbon. Lignin-based hard carbon is prepared through low-temperature carbonization, acid washing, drying, and high-temperature carbonization. During low-temperature carbonization, phosphoric acid acts as an activator, reacting with lignin to prepare lignin-based porous carbon dominated by micropores. During high-temperature carbonization, with the rearrangement of carbon microcrystals and the growth of carbon layers, the micropores close, forming a rich closed-pore structure in the hard carbon. The phosphoric acid-activated assisted preparation of lignin-based hard carbon materials of this invention can be used in electrochemical energy storage fields such as sodium-ion battery anodes.

[0010] A method for preparing a lignin-based hard carbon material includes the following steps:

[0011] (1) Disperse lignin in an aqueous solution of phosphoric acid, stir until homogeneous, and then dry to obtain a mixture of lignin and phosphoric acid;

[0012] (2) The mixture is carbonized at low temperature in a non-hydrothermal environment such as inert gas or air atmosphere, at a temperature of 250-450℃ for 0.5-5h, and then the precursor is obtained after washing and drying.

[0013] (3) The precursor is then carbonized at high temperature in an inert gas (nitrogen, etc.) atmosphere, at a temperature of 1200-1700℃ for 1-5 hours to obtain lignin-based hard carbon material.

[0014] Preferably, the phosphoric acid aqueous solution in step (1) has a mass concentration of 20–300 g / L. -1 The mass ratio of lignin to phosphoric acid is 9:1 to 1:9.

[0015] Preferably, the phosphoric acid aqueous solution in step (1) has a mass concentration of 50–150 g / L. -1 The mass ratio of lignin to phosphoric acid is 3:1 to 1:3.

[0016] Preferably, the low-temperature carbonization in step (2) is carried out at a temperature of 250–420°C for 1 ± 0.5 h; the high-temperature carbonization in step (3) is carried out at a temperature of 1300–1500°C for 4 ± 0.5 h.

[0017] Preferably, the heating rate of the low-temperature carbonization in step (2) is 1 to 30°C / min; the heating rate of the high-temperature carbonization in step (3) is 0.1 to 30°C / min, and the flow rate of the inert gas is 2 to 100 mL / min.

[0018] Preferably, the water washing in step (2) refers to immersing the carbonized product in water and stirring for 12 to 36 hours, and then filtering it; the drying temperature in step (2) is 80 to 120°C and the time is 12 to 24 hours.

[0019] Preferably, the stirring temperature in step (1) is 25-100°C and the time is 1-5 hours; the drying temperature is 80-120°C and the time is 12-24 hours.

[0020] Preferably, the lignin in step (1) is at least one of alkali lignin, enzymatically hydrolyzed lignin, and lignin sulfonate. The enzymatically hydrolyzed lignin is the residue of enzymatically hydrolyzed lignin obtained from the bio-smelting process; the alkali lignin is alkali lignin extracted from papermaking black liquor; and the lignin sulfonate is lignin sulfonate extracted from sulfite pulping liquor.

[0021] Application of the lignin-based hard carbon material prepared by the method in sodium-ion batteries.

[0022] The method described in this invention enables the construction of high closed pore volume in lignin-based hard carbon, providing abundant filling sites for sodium ions in the closed pores of hard carbon, thereby achieving high specific capacity and plateau capacity of hard carbon anode in sodium-ion batteries.

[0023] The specific surface area of ​​the low-temperature phosphoric acid activated carbon material is 400-600 m². 2 g -1 It has a granular structure. The resulting hard carbon material has a low open pore structure, with a specific surface area of ​​less than 10 m². 2 g -1 The closed pore volume is 0.15-3.0 cm³. 3 g -1 The average pore size of the closed pores is 2.0-3.0 nm. This hard carbon at 1 mol L... -1 A sodium-ion half-cell assembled with NaPF6 / DME electrolyte was tested at a current density of 50 mAg. -1 The capacity is 320–371 mAh g. -1 .

[0024] This invention aims to use an environmentally friendly and inexpensive pore-forming agent to activate and create pores in inexpensive lignin at low temperatures, while simultaneously achieving pore sealing at high temperatures, thereby preparing a hard carbon anode material with high closed-pore volume. The activated carbon prepared by low-temperature activation with phosphoric acid is predominantly microporous. During high-temperature carbonization, as carbon microcrystals rearrange and carbon layers grow, the micropores close, forming a rich closed-pore structure. Simultaneously, phosphoric acid, as an activating agent, not only enables low-temperature pore creation but also removes impurities generated after activation through water washing, reducing preparation and purification costs. The hard carbon anode prepared by this method not only possesses a rich closed-pore structure but also exhibits excellent electrochemical performance.

[0025] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0026] (1) This invention utilizes lignin, which is abundant and inexpensive, as a carbon source. It prepares a hard carbon anode material with large interlayer spacing and high closed pore volume by activating it with phosphoric acid at low temperature and then carbonizing it at high temperature, thereby improving the electrochemical performance of lignin-based hard carbon.

[0027] (2) This invention uses phosphoric acid as a pore-forming agent for hard carbon precursors to activate and create pores in lignin within a relatively low temperature range. Compared with other activation and pore-forming methods, the phosphoric acid activator used in this invention completes activation and pore-forming before the pyrolysis and carbonization of the lignin precursor is completed, which is beneficial for the formation of closed pores during high-temperature carbonization. After activation, impurities generated after activation can be washed away by water, which reduces the purification cost in the preparation of hard carbon and is beneficial for large-scale production. The introduction of phosphoric acid activator not only increases the interlayer spacing of lignin-derived hard carbon but also prepares a large number of micropores, providing conditions for the formation of closed pores in hard carbon during high-temperature carbonization.

[0028] (3) The hard carbon anode prepared by this invention has the advantages of large interlayer spacing and high closed pore volume. The large interlayer spacing provides a channel for sodium ions to be inserted into the hard carbon anode, and the high closed pore volume provides a large number of active sites for sodium ions to fill the hard carbon anode. Thus, the lignin-based hard carbon prepared by phosphoric acid has a high plateau capacity, reversible specific capacity, excellent rate performance and cycle stability. This hard carbon material is conducive to commercial production. Attached Figure Description

[0029] Figure 1 This is the XRD pattern of lignin-based hard carbon (LPHC5-300-1300) from Example 1.

[0030] Figure 2 This is the Raman diagram of lignin-based hard carbon (LPHC5-300-1300) from Example 1.

[0031] Figure 3 This is the SAXS diagram of lignin-based hard carbon (LPHC5-300-1300) from Example 1.

[0032] Figure 4 This is a diagram showing the closed pore size distribution of lignin-based hard carbon (LPHC-300-1300) from Example 1.

[0033] Figure 5 Example 1 uses lignin-based hard carbon (LPHC5-300-1300) as the negative electrode in a sodium-ion battery at 0.05 Ag. -1 The first charge-discharge curve at current density.

[0034] Figure 6 Example 1 shows the curve of the specific capacity of lignin-based hard carbon (LPHC5-300-1300) used as the negative electrode of a sodium-ion battery, as a function of current density.

[0035] Figure 7 Example 1 uses lignin-based hard carbon (LPHC5-300-1300) as the negative electrode in a sodium-ion battery at 0.2 Ag. -1 Cyclic curves at current density.

[0036] Figure 8 Example 1 uses lignin-based hard carbon (LPHC5-300-1300) as the negative electrode in a sodium-ion battery, at 1Ag -1 Cyclic curves at current density.

[0037] Figure 9 This is a graph showing the specific capacity of lignin-based hard carbon (LHC-300-1300) used as the negative electrode in a sodium-ion battery, as a function of current density. Detailed Implementation

[0038] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the implementation of the present invention is not limited thereto.

[0039] Unless otherwise specified in the embodiments of this invention, the conditions shall be performed according to conventional conditions or conditions recommended by the manufacturer. All raw materials and reagents used, unless otherwise specified, are commercially available conventional products.

[0040] Example 1

[0041] (1) Prepare 100 mL of 10% phosphoric acid solution, add 10 g of alkali lignin to the phosphoric acid solution, and stir at 25 °C for 1 h until uniformly mixed.

[0042] (2) Place the mixed solution from step (1) in a 90°C oven for 24 hours to obtain the dried phosphate lignin mixture.

[0043] (3) The phosphate lignin mixture from step (2) is heated to 300°C and kept at that temperature for 1 hour in air at a heating rate of 5°C / min.

[0044] (4) The solid obtained after pyrolysis in step (3) was washed with ultrapure water at 100°C for 12 hours and dried in an oven at 90°C for 24 hours to obtain alkali lignin porous carbon, named LPC5-300.

[0045] (5) The alkali lignin porous carbon obtained in step (4) is heated to 1300℃ for 2h under an argon atmosphere and a gas flow rate of 50mL / min to obtain phosphate-activated assisted alkali lignin-based hard carbon, named LPHC-300-1300.

[0046] X-ray diffraction (XRD) and Raman spectroscopy (Raman) patterns of the LPHC5-300-1300 hard carbon anode are shown below. Figure 1 and Figure 2 As shown, Figure 1 Two broad peaks appear near 22.5° and 44°, corresponding to the (002) and (100) crystal planes of the carbon material. Calculations show that the interlayer spacing of LPHC-300-1300 is 0.395 nm, the La value of the carbon material is 4.41 nm, and the Lc value is 0.73 nm. Ic is then calculated through fitting. D / I G The value is 1.90. This indicates that LPHC-300-1300 has a disordered carbon layer structure and a large interlayer spacing, which is conducive to sodium ion intercalation.

[0047] LPHC-300-1300, conductive carbon black, and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 8:1:1. After thorough mixing, the mixture was coated onto copper foil, dried, and rolled before being cut into circular electrode sheets with a diameter of 12 mm. The prepared electrode sheet was used as the working electrode, and a sodium metal sheet was used as the counter electrode. 1 mol L... -1 Sodium-ion half-cells were assembled using NaPF6 (with DME as the solvent) as the electrolyte, and the sodium-ion storage performance of LPHC-300-1300 was tested. DME is ethylene glycol dimethyl ether.

[0048] Figure 5 LPHC-300-1300 is used as the negative electrode in sodium-ion batteries at 0.05Ag. -1 The first charge-discharge curve at the current density. At 0.05Ag -1 At a current density of 568 mAh g, the LPHC-300-1300 exhibits a discharge specific capacity of 568 mAh g in the first cycle. -1 The charging specific capacity is 371mAh g. -1 Its first-round Coulomb efficiency was 65%.

[0049] Figure 6 The graph shows the specific capacity of LPHC-300-1300 as the negative electrode in a sodium-ion battery as a function of current density. (At 0.05Ag) -1 At a current density of 365 mAh g, the LPHC-300-1300 has a capacity of 365 mAh g. -1 The specific capacity, in 2Ag -1 At a current density, it has 216 mAh g -1 The specific capacity indicates that LPHC-300-1300 exhibits excellent rate performance when used as the anode in sodium-ion batteries.

[0050] Figure 7 and Figure 8 LPHC-300-1300 are used as the negative electrode in sodium-ion batteries, at 0.2Ag. -1 and 1Ag -1 Cyclic performance diagram at current density. At 0.2 Ag -1 At a current density of [value missing], the LPHC-300-1300 exhibits a 91% capacity retention after 400 cycles; at 1A g [value missing], [value missing] -1 At the specified current density, the LPHC-300-1300 exhibits a 72% capacity retention after 2000 cycles. This demonstrates the excellent cycle stability of the LPHC-300-1300 when used as the anode in sodium-ion batteries.

[0051] Example 2

[0052] (1) Prepare 100 mL of 2% phosphoric acid solution, add 18 g of alkali lignin to the phosphoric acid solution, and stir at 25 °C for 1 h until uniformly mixed.

[0053] (2) Place the mixed solution from step (1) in a 90°C oven for 24 hours to obtain the dried phosphate lignin mixture.

[0054] (3) The phosphate lignin mixture in step (2) is heated to 370°C and kept at 5°C / min for 1 hour in an air atmosphere.

[0055] (4) The solid obtained after pyrolysis in step (3) was washed with ultrapure water at 100°C for 24 hours and dried in an oven at 90°C for 24 hours to obtain alkali lignin porous carbon, named LPC1-370.

[0056] (5) The alkali lignin porous carbon obtained in step (4) is heated to 1500℃ for 2h under an argon atmosphere and a gas flow rate of 60mL / min, with a heating rate of 10℃ / min, to obtain alkali lignin-based hard carbon prepared with phosphoric acid activation, named LPHC1-370-1500.

[0057] Example 3

[0058] (1) Prepare 100 mL of 14% phosphoric acid solution, add 7 g of alkali lignin to the phosphoric acid solution, and stir at 25°C for 1 h until uniformly mixed.

[0059] (2) Place the mixed solution from step (1) in a 90°C oven for 24 hours to obtain the dried phosphate lignin mixture.

[0060] (3) The phosphate lignin mixture in step (2) is heated to 350°C and kept at a heating rate of 5°C / min for 1 hour in an air atmosphere.

[0061] (4) The solid obtained after pyrolysis in step (3) was washed with ultrapure water at 100°C for 24 hours and dried in an oven at 90°C for 24 hours to obtain alkali lignin porous carbon, named LPC7-350.

[0062] (5) The alkali lignin porous carbon obtained in step (4) is heated to 1300℃ for 2h under an argon atmosphere and a gas flow rate of 60mL / min, with a heating rate of 10℃ / min, to obtain alkali lignin-based hard carbon prepared with phosphoric acid activation, named LPHC7-350-1300.

[0063] Example 4

[0064] (1) Prepare 100 mL of 30% phosphoric acid solution, add 10 g of sodium lignosulfonate to the phosphoric acid solution, and stir at 25 °C for 1 h until uniformly mixed.

[0065] (2) Place the mixed solution from step (1) in a 90°C oven for 24 hours to obtain the dried phospholipid mixture.

[0066] (3) The phospholipid lignin mixture in step (2) is heated to 420°C and kept at a heating rate of 5°C / min for 1 hour in an air atmosphere.

[0067] (4) The solid obtained after pyrolysis in step (3) was washed with ultrapure water at 100°C for 24 hours and dried in an oven at 90°C for 24 hours to obtain lignin porous carbon, named LSPC7.5-420.

[0068] (5) The lignin porous carbon obtained in step (4) is heated to 1300℃ for 2h under an argon atmosphere and a gas flow rate of 60mL / min at a heating rate of 5℃ / min to obtain lignin-based hard carbon prepared by phosphoric acid activation, named LSPHC7.5-420-1300.

[0069] Example 5

[0070] The difference between this embodiment and Embodiment 3 is that the low-temperature carbonization temperature is 250℃, and alkali lignin-based hard carbon is obtained, named LPHC7-250-1300.

[0071] (1) Prepare 100 mL of 14% phosphoric acid solution, add 7 g of alkali lignin to the phosphoric acid solution, and stir at 25°C for 1 h until uniformly mixed.

[0072] (2) Place the mixed solution from step (1) in a 90°C oven for 24 hours to obtain the dried phosphate lignin mixture.

[0073] (3) The phosphate lignin mixture from step (2) is heated to 250°C and kept at that temperature for 1 hour in air at a heating rate of 5°C / min.

[0074] (4) The solid obtained after pyrolysis in step (3) was washed with ultrapure water at 100°C for 24 hours and dried in an oven at 90°C for 24 hours to obtain alkali lignin porous carbon, named LPC7-250.

[0075] (5) The alkali lignin porous carbon obtained in step (4) is heated to 1300℃ for 2h under an argon atmosphere and a gas flow rate of 60mL / min, with a heating rate of 10℃ / min, to obtain alkali lignin-based hard carbon prepared with phosphoric acid activation, named LPHC7-250-1300.

[0076] Comparative Example 1

[0077] This comparative example is hard carbon prepared by direct carbonization of lignin.

[0078] (1) Add 20g of alkali lignin to 100mL of aqueous solution and stir at 25℃ for 1h until uniformly mixed.

[0079] (2) Place the mixed solution from step (1) in a 90°C oven for 24 hours to obtain dried alkali lignin.

[0080] (3) The alkali lignin in step (2) is heated to 300°C in air at a heating rate of 5°C / min and kept at that temperature for 1 hour.

[0081] (4) The solid obtained after pyrolysis in step (3) was washed with ultrapure water at 100°C for 24 hours and dried in an oven at 90°C for 24 hours to obtain alkali lignin porous carbon, named LC-300.

[0082] (5) The alkali lignin-derived carbon obtained in step (4) is heated to 1300℃ for 2h under an argon atmosphere and a gas flow rate of 60mL / min, with a heating rate of 10℃ / min, to obtain alkali lignin-based hard carbon, named LHC-300-1300.

[0083] LHC-300-1300, conductive carbon black, and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 8:1:1. After thorough mixing, the mixture was coated onto copper foil, dried, and rolled before being cut into circular electrode sheets with a diameter of 12 mm. The prepared electrode sheet was used as the working electrode, and a sodium metal sheet was used as the counter electrode. 1 mol L... -1 Sodium-ion half-cells were assembled using NaPF6 (with DME as the solvent) as the electrolyte, and the sodium-ion storage performance of LHC-300-1300 was tested. DME is ethylene glycol dimethyl ether.

[0084] Figure 9 The graph shows the specific capacity of LHC-300-1300 as the negative electrode in a sodium-ion battery, as a function of current density. (At 0.05Ag) -1 At a current density of 265 mAh g, the LPHC-300-1300 has a capacity of 265 mAh g. -1 The specific capacity, in 2Ag -1 At a current density, it has 180 mAh g -1 The specific capacity indicates that the reversible specific capacity and rate performance of the lignin-based hard carbon decrease in the absence of phosphoric acid activator. The difference in specific capacity between LHC-300-1300 and LPHC-300-1300 is mainly due to plateau capacity, suggesting that phosphoric acid, acting as an activator, reacts with lignin to prepare a large number of micropores on the lignin precursor. These numerous micropores facilitate the formation of closed pores during high-temperature carbonization, thereby increasing the plateau capacity and specific capacity of the lignin-based hard carbon.

[0085] Comparative Example 2(1) Prepare 100 ml of 10% phosphoric acid solution, add 10 g of alkali lignin to the phosphoric acid solution, and stir at 25°C for 1 h until uniformly mixed.

[0086] (2) Place the mixed solution from step (1) in a 90°C oven for 24 hours to obtain the dried phosphate lignin mixture.

[0087] (3) The phosphate lignin mixture in step (2) is heated to 500℃ and held for 1h at a heating rate of 5℃ / min under an inert gas atmosphere.

[0088] (4) The solid obtained after pyrolysis in step (3) was washed with ultrapure water at 100°C for 12 hours and dried in an oven at 90°C for 24 hours to obtain alkali lignin porous carbon, named LPC5-500.

[0089] (5) The alkali lignin porous carbon obtained in step (4) is heated to 1300℃ for 2h under an argon atmosphere and a gas flow rate of 50mL / min to obtain phosphate-activated assisted alkali lignin-based hard carbon, named LPHC5-500-1300.

[0090] Comparative Example 3

[0091] (1) Prepare 100 mL of 19% phosphoric acid solution, add 1 g of alkali lignin to the phosphoric acid solution, and stir at 25°C for 1 h until uniformly mixed.

[0092] (2) Place the mixed solution from step (1) in a 90°C oven for 24 hours to obtain the dried phosphate lignin mixture.

[0093] (3) The phosphate lignin mixture in step (2) is heated to 500°C and kept at that temperature for 1 hour in air at a heating rate of 5°C / min.

[0094] (4) The solid obtained after pyrolysis in step (3) was washed with ultrapure water at 100°C for 12 hours and dried in an oven at 90°C for 24 hours to obtain alkali lignin porous carbon, named LPC9.5-500.

[0095] (5) The alkali lignin porous carbon obtained in step (4) is heated to 1700℃ for 6h under an argon atmosphere and a gas flow rate of 50mL / min to obtain phosphate-activated assisted alkali lignin-based hard carbon, named LPHC9.5-500-1700.

[0096] Comparative Example 4

[0097] This comparative example is porous carbon prepared by phosphoric acid activation.

[0098] (1) Prepare 100 mL of 10% phosphoric acid solution, add 10 g of alkali lignin to the phosphoric acid solution, and stir at 25 °C for 1 h until uniformly mixed.

[0099] (2) Place the mixed solution from step (1) in a 90°C oven for 24 hours to obtain the dried phosphate lignin mixture.

[0100] (3) The phosphate lignin mixture in step (2) is heated to 300°C and kept at that temperature for 1 hour in air at a heating rate of 10°C / min.

[0101] (4) The solid obtained after pyrolysis in step (3) was washed with ultrapure water at 100°C for 12 hours and dried in an oven at 90°C for 24 hours to obtain alkali lignin porous carbon, named LPC5-300.

[0102] Comparative Example 5

[0103] (1) Prepare 100mL of 10% potassium hydroxide solution, add 10g of alkali lignin to the potassium hydroxide solution, and stir at 25℃ for 1h until uniformly mixed.

[0104] (2) Place the mixed solution from step (1) in a 90°C oven for 24 hours to obtain the dried composite.

[0105] (3) The complex in step (2) is heated to 700°C in air at a heating rate of 10°C / min and held for 1 hour.

[0106] (4) The solid obtained after pyrolysis in step (3) was washed with ultrapure water at 100°C for 12 hours and dried in an oven at 90°C for 24 hours to obtain porous carbon, which was named LKC5-700.

[0107] (5) The porous carbon obtained in step (4) is heated to 1100℃ for 2h under an argon atmosphere and a gas flow rate of 50mL / min to obtain alkali lignin-based hard carbon, named LKC5-700-1100.

[0108] The test current density for the reversible capacity in Table 1 is 50 mA / g. The specific surface area was obtained by fitting nitrogen adsorption-desorption data using the BET model. The diameter and volume of the closed pore were obtained by fitting the SAXS test results.

[0109] Table 1 compares the interlayer spacing, closed pore volume, specific capacity, and plateau capacity (test current density 50 mA / g) of hard carbon samples prepared by the methods of the examples and comparative examples.

[0110]

[0111] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. The application of a lignin-based hard carbon material in sodium-ion batteries, characterized in that, The preparation method of the lignin-based hard carbon material includes the following steps: (1) Disperse lignin in an aqueous solution of phosphoric acid, stir evenly, and dry to obtain a mixture of lignin and phosphoric acid, wherein the mass ratio of lignin to phosphoric acid is 9:1 to 1:3; the drying temperature is 80 to 120 °C and the time is 12 to 24 h; (2) The mixture is carbonized at low temperature in an inert gas or air atmosphere, at a temperature of 250-420 °C for 1 ± 0.5 h, and then washed with water and dried to obtain the precursor; the water washing refers to immersing the carbonized product in water at 100 °C and stirring for 12-36 h; the drying temperature is 80-120 °C and the time is 12-24 h. (3) The precursor is then carbonized at high temperature in an inert gas atmosphere, at a temperature of 1200-1700 °C for 1-5 h, to obtain lignin-based hard carbon material.

2. The application according to claim 1, characterized in that, The phosphoric acid aqueous solution in step (1) has a mass concentration of 20–300 g / L. −1 .

3. The application according to claim 2, characterized in that, The phosphoric acid aqueous solution in step (1) has a mass concentration of 50–150 g / L. −1 .

4. The application according to claim 1, 2, or 3, characterized in that, The high-temperature carbonization in step (3) is carried out at a temperature of 1300-1500 °C for 4 ± 0.5 h.

5. The application according to claim 4, characterized in that, The heating rate of the low-temperature carbonization in step (2) is 1 to 30 °C / min; the heating rate of the high-temperature carbonization in step (3) is 0.1 to 30 °C / min, and the flow rate of the inert gas is 2 to 100 mL / min.

6. The application according to claim 5, characterized in that, The stirring temperature in step (1) is 25-100 ℃ and the stirring time is 1-5 h.

7. The application according to claim 1, 2, or 3, characterized in that, The lignin mentioned in step (1) is at least one of alkali lignin, enzymatic hydrolyzed lignin, and lignin sulfonate.

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