Biomass hard carbon negative electrode material, preparation method thereof and sodium ion battery

Abstract

The invention provides a biomass hard carbon negative electrode material, a preparation method thereof and a sodium ion battery, and relates to the field of sodium ion batteries. The method comprises the following steps: mixing fir wood chips, a modifier and water, carrying out hydrothermal reaction, and carrying out solid-liquid separation to obtain modified wood chips; and under a protective atmosphere, carbonizing the modified wood chips to obtain the biomass hard carbon negative electrode material. The modifier comprises one or more of phosphoric acid, phosphorous acid, hydrofluoric acid, boric acid, phytic acid and hexafluorophosphoric acid. The preparation method has the advantages of low cost, no pollution, simple operation, large-scale expanded production and the like, and provides a new way and an effective measure for large-scale production of the sodium ion battery in the field of energy storage.

Description

Technical Field

[0001] The present application relates to the field of sodium ion batteries, and in particular to a biomass hard carbon negative electrode material, a preparation method thereof, and a sodium ion battery. Background Art

[0002] As the most widely used energy storage system, lithium-ion batteries (LIBs) dominate the power supply market for electric vehicles and portable electronic devices, thanks to their high energy density and long cycle life. However, the scarcity and uneven geographical distribution of lithium resources have severely hindered the large-scale application of LIBs in grid energy storage. Sodium shares similar physical and chemical properties with lithium, is abundant, and is relatively inexpensive. Benefiting from these advantages, sodium-ion batteries (SIBs) have been considered one of the most promising candidates for large-scale stationary energy storage. Compared to lithium ions, sodium ions have a much larger radius, making it difficult to simply select electrode materials from lithium-ion batteries for use in SIBs. Therefore, finding suitable host materials that provide sufficient space for the storage and transport of sodium ions is crucial. Recently, a number of layered oxides and polyanionic compounds have been identified as the primary cathode materials for SIBs. Regarding anode materials, commercial graphite lacks sufficient interlayer space to accommodate the larger sodium ions, resulting in low sodium storage capacity. In this regard, finding suitable anode materials for SIBs is crucial.

[0003] Among the many candidate negative electrode materials, hard carbon has made significant progress in academia and industry in recent years. This is due to its disordered graphitized structure, enlarged interlayer spacing, tunable pore structure, cost-effectiveness and wide range of precursor sources, making it a highly promising material for SIBs. However, the low first charge and discharge efficiency, low reversible capacity and poor cycle performance of hard carbon as a negative electrode material for sodium-ion batteries have seriously hindered its development. Therefore, to meet the application of hard carbon in practical batteries, the main challenge is how to increase the reversible capacity without sacrificing the first coulombic efficiency.

[0004] Biomass is one of the most commonly used precursors for synthesizing hard carbon. Due to their natural abundance, low cost, and environmental friendliness, biomass sources such as wood, banana peels, cotton, corn stalks, and silk have attracted significant attention as precursors for producing hard carbon. Therefore, a method for preparing anode materials using biomass as a carbon source is urgently needed to address these challenges. Summary of the Invention

[0005] The purpose of this application is to provide a biomass hard carbon negative electrode material and a preparation method thereof and a sodium ion battery to solve the above problems.

[0006] To achieve the above objectives, the present application provides a first aspect of a method for preparing a biomass hard carbon negative electrode material, comprising: Mixing fir wood chips, a modifier and water, performing a hydrothermal reaction, and performing solid-liquid separation to obtain modified wood chips; Carbonizing the modified sawdust under a protective atmosphere to obtain a biomass hard carbon negative electrode material; The modifier includes one or more of phosphoric acid, phosphorous acid, hydrofluoric acid, boric acid, phytic acid and hexafluorophosphoric acid.

[0007] Optionally, the mass ratio of the fir sawdust, the modifier and the water is 1:10-15:10-15.

[0008] Optionally, the temperature of the hydrothermal reaction is 120° C.-200° C., and the time is 5 h-30 h.

[0009] Optionally, the pH value of the hydrothermal reaction is greater than 0 and less than or equal to 2.

[0010] Optionally, the mass concentration of the modifier is 50%-90%.

[0011] Optionally, the carbonization temperature is 1000-1500° C., and the time is 1-10 h.

[0012] Optionally, before the hydrothermal reaction, the fir sawdust is further pretreated; The pretreatment comprises: subjecting the fir wood chips to ball milling, ultrasonication and drying in sequence.

[0013] Optionally, the ball milling time is 800 min-1500 min, and the rotation speed is 600 r / min-900 r / min; The ultrasound time is 60min-240min; The drying temperature is 50° C.-80° C., and the drying time is 12-24 hours.

[0014] The second aspect of the present application provides a biomass hard carbon negative electrode material, which is prepared by the above-mentioned method for preparing the biomass hard carbon negative electrode material.

[0015] The third aspect of the present application provides a sodium ion battery comprising the biomass hard carbon negative electrode material.

[0016] Compared with the prior art, the advantages of this application include: The preparation method of the biomass hard carbon negative electrode material provided by the present application is based on the structural characteristics and chemical properties of the fir wood chips themselves, and makes the modifier and the biomass raw material undergo esterification reaction with the fir wood chips in a hydrothermal environment, and reacts with some atoms or groups in the carbon structure, so that the originally connected atoms or groups are separated to form pores, breaking the ordered structure of the carbon layer; during hydrothermal treatment, a more stable three-dimensional structure is formed, and the addition of the modifier will introduce more oxygen-containing functional groups, which will decompose and produce gas during carbonization, promoting the formation of open pores. In the subsequent high-temperature carbonization process, due to the fir wood chips, the carbonization reaction is more stable. The hydrothermal treatment of sawdust and modifier produces a more stable structure, and the modifier will continue to work at this stage, causing a cross-linking reaction, which leads to the bending of the carbon layer; at the same time, the decomposition of the modifier and sawdust in the hydrothermal and subsequent high-temperature stages will also achieve phosphorus doping, which will promote the closure of open pores, form more closed-pore structures, and construct a closed-pore structure of derived hard carbon and a more stable SEI film; the long-range graphite-like layer produced by the decomposition of crystalline cellulose in the fir wood chip component can be used as a closed-pore wall structure, while amorphous hemicellulose and lignin have an inhibitory effect, preventing excessive graphitization of the carbon layer during high-temperature carbonization. This structure is very effective for Na + The rapid diffusion and storage kinetics of biomass hard carbon are very favorable, which can improve the capacity and rate performance of biomass hard carbon. The phosphorus element carried by fir wood chips can be used to dope the derived hard carbon. The defects generated by heteroatoms can introduce more active sites, improve surface properties, and promote the formation of a stable SEI film, thereby improving the initial charge and discharge efficiency and cycle capacity of the hard carbon. Under the combined action of modifiers and hydrothermal, the microstructure and surface properties of the derived hard carbon can be synergistically regulated, thereby improving the electrochemical performance of the hard carbon. This preparation method has the advantages of low cost, no pollution, simple operation, and large-scale production. It provides a new path and effective measure for the large-scale production of sodium-ion batteries in the field of energy storage.

[0017] The biomass hard carbon negative electrode material provided in the present application has a relatively closed-pore structure and realizes the doping of the P element.

[0018] The sodium ion battery provided in this application has excellent electrochemical performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope of the present application.

[0020] Figure 1 This is an SEM image of the biomass hard carbon negative electrode material provided in Example 1; Figure 2FT-IR images of the biomass hard carbon negative electrode materials provided in Examples 1-3 and Comparative Example 1; Figure 3 Raman spectra of the biomass hard carbon negative electrode materials provided in Examples 1-3 and Comparative Example 1; Figure 4 XRD patterns of the biomass hard carbon negative electrode materials provided in Examples 1-3 and Comparative Example 1. DETAILED DESCRIPTION

[0021] As used herein: "Prepared from" is synonymous with "comprising." As used herein, the terms "comprising," "including," "having," "containing," or any other variations thereof, are intended to cover a non-exclusive inclusion. For example, a composition, process, method, article, or apparatus that comprises the listed elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such composition, process, method, article, or apparatus.

[0022] The conjunction "consisting of" excludes any unspecified element, step, or component. If used in a claim, this phrase renders the claim closed, excluding materials other than those described, except for conventional impurities associated therewith. When the phrase "consisting of" appears in a clause of the body of a claim, rather than immediately following the subject matter, it limits only the elements described in that clause; other elements are not excluded from the claim as a whole.

[0023] When an amount, concentration, or other value or parameter is expressed as a range, a preferred range, or a range defined by a series of upper preferred values ​​and lower preferred values, this should be understood as specifically disclosing all ranges formed by any pairing of any range upper limit or preferred value with any range lower limit or preferred value, regardless of whether the range is disclosed alone. For example, when a range of "1 to 5" is disclosed, the described range should be interpreted as including the range "1 to 4", "1 to 3", "1 to 2", "1 to 2 and 4 to 5", "1 to 3 and 5", etc. When a numerical range is described herein, unless otherwise stated, the range is intended to include its end values ​​and all integers and fractions within the range.

[0024] In these examples, parts and percentages are by mass unless otherwise indicated.

[0025] "Parts by mass" refers to the basic unit of measurement used to express the mass ratio of multiple components. One part can represent any unit of mass, such as 1g or 2.689g. For example, if we say that the parts by mass of component A are a parts and the parts by mass of component B are b parts, this means the ratio of the mass of component A to the mass of component B is a:b. Alternatively, we could say that the mass of component A is aK and the mass of component B is bK (K is an arbitrary number representing a multiplication factor). It's important to note that, unlike parts by mass, the sum of the parts by mass of all components is not limited to 100 parts.

[0026] "And / or" is used to indicate that one or both of the stated situations may occur, for example, A and / or B includes (A and B) and (A or B).

[0027] The first aspect of the present application provides a method for preparing a biomass hard carbon negative electrode material, comprising: Mixing fir wood chips, a modifier and water, performing a hydrothermal reaction, and performing solid-liquid separation to obtain modified wood chips; In some embodiments, solid-liquid separation includes filtration, and deionized water rinsing is not required during filtration, so that the modifier is partially retained in the solid mixture and continues to act during the subsequent carbonization process, resulting in a more open-pore structure in the modified sawdust. The two also undergo cross-linking, helping to close the open pores, which is very important for improving electrochemical performance. It should also be noted that direct carbonization of raw fir sawdust can easily lead to the ordered arrangement and restacking of carbon layers. This long-range microstructure is not conducive to the storage and migration of sodium ions. Carbonizing the modified sawdust under a protective atmosphere to obtain a biomass hard carbon negative electrode material; The modifier includes one or more of phosphoric acid, phosphorous acid, hydrofluoric acid, boric acid, phytic acid and hexafluorophosphoric acid.

[0028] Preferably, the modifying agent comprises phytic acid.

[0029] It should be noted that phytic acid has strong acid properties, and the chelation effect of phytic acid can remove metal impurities (such as Ca 2+ 、Fe 3+ ), reducing disordered etching during subsequent carbonization; hydrothermal treatment of fir wood chips and phytic acid can introduce oxygen-containing functional groups on the material surface. These functional groups are converted into active sites that are conducive to the formation of SEI film during the subsequent carbonization process, and preferentially react with solvent molecules (such as EC, DEC) and sodium salts (such as NaPF6) in the electrolyte to promote the formation of a more uniform and stable SEI film. Phytic acid and wood chips are also introduced into the hydrothermal pre-oxidation process to form phosphorus-containing carbon materials after carbonization. These components can catalyze the formation of SEI films rich in inorganic substances such as NaF and Na2CO3, and their mechanical strength and ionic conductivity are better than those of pure organic films.

[0030] It is also important to note that hydrothermal treatment can regulate the pore structure of the material, increasing the specific surface area and porosity, providing more sodium ion embedding sites, and optimizing electrolyte penetration and ion transport pathways, thereby helping to form a more uniform SEI film. The quality of the SEI film directly affects battery performance, so the combined hydrothermal pre-oxidation of phytic acid and fir wood chips optimizes the SEI film formation process and promotes improved sodium-ion battery performance.

[0031] In some embodiments, the mass ratio of the fir wood chips, the modifier, and the water is 1:10-15:10-15. Alternatively, the mass ratio of the fir wood chips, the modifier, and the water can be 1:10:10, 1:15:10, 1:10:15, 1:15:15, or any value between 1:10-15:10-15. It should be noted that when the mass ratio of the fir wood chips, the modifier, and the water is within the above-mentioned range, a sodium-ion battery anode material with excellent cycle and rate performance can be obtained, while other ratios result in poor cycle and rate performance.

[0032] In some embodiments, the hydrothermal reaction temperature is 120° C.-200° C., and the reaction time is 5 h-30 h.

[0033] Optionally, the temperature of the hydrothermal reaction can be 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, 180°C, 190°C, 200°C or any value between 120°C and 200°C, and the time can be 5h, 10h, 15h, 20h, 25h, 30h or any value between 5h and 30h.

[0034] It should be noted that within this temperature range, fir wood chips can exhibit the best electrochemical performance without any treatment, and within this temperature range, phytic acid will decompose, accompanied by thermal decomposition of phosphate groups and release of gas, which is conducive to the formation of open pores.

[0035] In some embodiments, the pH value of the hydrothermal reaction is greater than 0 and less than or equal to 2.

[0036] Optionally, the pH of the hydrothermal reaction can be 0.1, 0.5, 1, 1.5, 2, or any value between greater than 0 and less than or equal to 2. It should be noted that when the pH value of the hydrothermal reaction is greater than 0 and less than or equal to 2, compared with the hard carbon material obtained by high-temperature carbonization of the original precursor, not only the first charge and discharge efficiency, the first coulombic efficiency, the cycle performance and rate performance of the button cell are improved, but also the SEI layer on the electrode surface is more stable and thinner, the interfacial charge transfer kinetics is significantly enhanced, and higher diffusion kinetics are shown.

[0037] In some embodiments, the mass concentration of the modifier is 50%-90%.

[0038] Optionally, the mass concentration of the modifier can be 50%, 60%, 70%, 80%, 90% or any value between 50% and 90%.

[0039] In some embodiments, the carbonization temperature is 1000° C.-1500° C., and the time is 1-10 h.

[0040] Optionally, the carbonization temperature can be 1000°C, 1100°C, 1200°C, 1300°C, 1400°C, 1500°C or any value between 1000°C and 1500°C, and the time can be 1h, 2h, 4h, 6h, 8h, 10h or any value between 1-10h.

[0041] In some embodiments, the fir sawdust is further pretreated before the hydrothermal reaction. The pretreatment comprises: subjecting the fir wood chips to ball milling, ultrasonication and drying in sequence.

[0042] In some embodiments, the ball milling time is 800 min-1500 min, and the rotation speed is 600 r / min-900 r / min; Optionally, the ball milling time can be 800 min, 900 min, 1000 min, 1100 min, 1200 min, 1300 min, 1400 min, 1500 min or any value between 800 min and 1500 min, and the rotation speed can be 600 r / min, 700 r / min, 800 r / min, 900 r / min or any value between 600 r / min and 900 r / min; It should be noted that when the ball milling time and ball milling speed are within this setting range, the particle size of the pretreated material obtained is most suitable for ensuring that the fir wood chips can exert excellent electrochemical properties. If the speed and time are too low or too high, the obtained sodium ion battery hard carbon negative electrode material has an initial charge and discharge efficiency as low as 60%, and poor cycle performance and rate performance.

[0043] The ultrasound time is 60min-240min; Optionally, the ultrasound time can be 60 min, 120 min, 180 min, 240 min, or any value between 60 min and 240 min; The drying temperature is 50° C.-80° C., and the drying time is 12-24 hours.

[0044] Optionally, the drying temperature may be 50°C, 60°C, 70°C, 80°C or any value between 50°C and 80°C, and the drying time may be 12h, 18h, 24h or any value between 12 and 24h.

[0045] The second aspect of the present application provides a biomass hard carbon negative electrode material, which is prepared by the above-mentioned method for preparing the biomass hard carbon negative electrode material.

[0046] The third aspect of the present application provides a sodium ion battery comprising the biomass hard carbon negative electrode material.

[0047] The embodiments of the present application will be described in detail below in conjunction with specific examples, but it will be understood by those skilled in the art that the following examples are merely illustrative of the present application and should not be considered as limiting the scope of the present application. In the examples, if specific conditions are not specified, the conditions are carried out according to conventional conditions or manufacturer recommendations. The reagents or instruments used are not specified by the manufacturer and are conventional products that can be purchased commercially.

[0048] Example 1 This embodiment provides a biomass hard carbon negative electrode material and a preparation method thereof. The specific preparation steps are as follows: S1: Take an appropriate amount of fir wood chips biomass raw material, put it into a ball mill, and mill it for 1000 minutes at 600 r / min. After sonication in deionized water for 2 hours, filter it and dry it in a 70℃ forced air drying oven for 12 hours. S2: 1g of fir wood chips, 10g of phytic acid (mass concentration of 90%), and 10g of ultrapure water were placed in a reagent bottle. After stirring and dispersing for 2h, the mixture was transferred to a stainless steel reactor lined with a polytetrafluoroethylene liner and placed in a blast drying oven. The temperature was raised to 180°C at 5°C / min. The reaction time was 24h. The pH value during the reaction was 1-2. After the reaction was completed, the filtered material was placed in a blast drying oven at 70°C and dried for 12h. S3: Grind the modified material obtained in step 2 in a mortar, put it into a crucible and then place it in a tubular furnace. Heat it to 1300°C at 5°C / min under an argon atmosphere, keep the temperature constant for 2 hours, and then cool it naturally to room temperature. The phytic acid hydroheated biomass hard carbon negative electrode material is recorded as PAH-CLHC-1300°C.

[0049] The SEM of the biomass hard carbon negative electrode material is shown in Figure 1 shown.

[0050] Example 2 The difference from Example 1 is that phytic acid is replaced by hexafluorophosphoric acid in a mass ratio of 1:1.

[0051] The biomass hard carbon negative electrode material prepared in this example is recorded as PA-CLHC-1300℃.

[0052] Example 3 The difference from Example 1 is that the pretreatment in step S1 is not performed.

[0053] The biomass hard carbon negative electrode material prepared in this example is recorded as H-CLHC-1300℃.

[0054] Comparative Example 1 The difference from Example 1 is that no phytic acid is added and no hydrothermal treatment is performed, and the carbonization is directly carried out at a high temperature of 1300°C.

[0055] The biomass hard carbon negative electrode material prepared in this comparative example is recorded as CLHC-1300℃.

[0056] The chemical structure of the hard carbon precursor was studied by Fourier transform infrared spectroscopy (FT-IR) of the biomass hard carbon negative electrode material prepared in Examples 1-3 and Comparative Example 1. Figure 2 As shown, Example 1 (PAH-CLHC-1300℃) 1240 cm -1 The P=O peak intensity at 1050 cm-1 is significantly higher than that of PA-CL. -1 The POC peak near 1630 cm shifts to a lower wavenumber and its width increases, indicating that the hydrothermal environment promotes the high-density cross-linking of phytic acid and the matrix. -1 The sharpening of the C=C peak at 1240 cm-1 reveals the strengthening effect of hydrothermal treatment on the graphitization of carbon layers. This dual-functional structure, the combination of a highly conductive aromatic carbon skeleton and high-density phosphorus / oxygen active sites, enables PAH-CLHC-1 to have excellent electron conduction and ion adsorption capabilities after carbonization at 300°C, ultimately resulting in the best electrochemical performance. -1 A strong P=O stretching vibration peak appears at 1100–980 cm -1 A broad and strong POC / PO- superposition peak is formed in the range, and the peak at 3400 cm -1 The OH peak at 1630 cm-1 is significantly broadened. These features confirm that phytic acid is covalently cross-linked with cellulose via phosphate bonds (POC), successfully introducing phosphorus / oxygen active sites. This modification significantly improves the pseudocapacitive potential of the material. However, due to the limited cross-linking density and insufficient graphitization of the carbon skeleton, its electrochemical performance is still inferior to that of Example 1. The spectrum of Example 3 (H-CLHC-300℃) undergoes a key change, with the 1630 cm-1 peak at 1630 cm-1 being the peak at 1630 cm-1. -1 The C=C vibration peak at 1580 cm -1The appearance of conjugated carbonyl (C=O) shoulder peaks confirms that the hydrothermal reaction promotes the dehydration of hemicellulose and the formation of an aromatic carbon skeleton. This structure gives the material a certain degree of conductivity, but because no redox active groups are introduced, its electrochemical performance is only at a moderate level. The FT-IR spectrum of the original fir sawdust (CL) shows typical lignocellulose characteristics: 3400 cm - The strong broad peak at ¹ is attributed to the OH stretching vibration in cellulose / lignin, the weak peak near 2920 cm⁻¹ is attributed to the CH bond, and the peak at 1050 cm⁻¹ is attributed to the OH stretching vibration in cellulose / lignin. - The strong peak at ¹ reflects the COC vibration of the sugar ring. This structure indicates that CL lacks electrochemically active sites, resulting in a sparse conductive skeleton after carbonization and significantly limited electrochemical performance. The FT-IR spectrum of Comparative Example 1 (CLHC-1300℃) shows typical lignocellulose characteristics: 3400 cm -1 The strong broad peak at 2920 cm is due to the OH stretching vibration in cellulose / lignin. -1 The weak peak nearby is attributed to the CH bond, 1050 cm -1 The strong peak at reflects the COC vibration of the sugar ring. This structure shows that CLHC-1300℃ lacks electrochemically active sites, resulting in a sparse conductive skeleton after carbonization and significantly limited electrochemical performance.

[0057] The Raman spectra of the biomass hard carbon negative electrode materials prepared in Examples 1-3 and Comparative Example 1 are as follows: Figure 3 As shown, the surface defect state of the hard carbon material derived from fir wood chips after phytic acid-assisted hydrothermal pre-oxidation and high-temperature carbonization is detected. -1 and 1583cm -1 The two typical peaks at represent vibrational modes arising from disordered (D band) and graphitized (G band) carbon structures, respectively. The intensity ratio of the D band to the G band (ID / IG) is positively correlated with edge defects in carbon materials; a higher ratio generally indicates a higher number of edge defects. Figure 3The fitted Raman spectra show that Example 1 (PAH-CLHC-1300°C) has the lowest ID / IG value (1.76), indicating that carbon defects in this sample are effectively repaired after phytic acid-assisted hydrothermal preoxidation. This is likely due to the phytic acid-assisted hydrothermal process regulating the hard carbon structure and reducing disordered defects. Example 2 (PA-CLHC-1300°C) has the highest ID / IG value (1.90), which can be attributed to chemical interference caused by factors related to the preparation process (such as differences in pretreatment), resulting in a relatively high number of edge defects. The ID / IG values ​​of Example 3 (H-CLHC-300°C, ID / IG of 1.86) and Comparative Example 1 (CLHC-1300°C, D / IG of 1.81) fall between the two, reflecting the gradient of defect states in hard carbon materials after different pretreatments and carbonization. This demonstrates the role of phytic acid-assisted hydrothermal preoxidation in regulating defects in fir wood chip-derived hard carbon, which helps optimize the hard carbon structure and has positive implications for its applications, such as preventing irreversible loss of ion storage in energy storage.

[0058] The XRD patterns of the biomass hard carbon negative electrode materials prepared in Examples 1-3 and Comparative Example 1 are as follows: Figure 4 As shown, the graphitization information of the carbon sample was obtained. The graphitization structure information of the hard carbon materials of Examples 1-3 and Comparative Example 1 was analyzed by X-ray diffraction (XRD). Comparing the curves, the (002) peak of Example 1 (PAH-CLHC-1300℃) is relatively sharp and has high intensity, while the (002) peaks of Example 3 (H-CLHC-300℃), Comparative Example 1 (CLHC-1300℃), and Example 2 (PA-CLHC-1300℃) are broadened and have lower intensity. The (002) peak corresponds to the carbon interlayer stacking structure. The characteristics of the (002) peak of Example 1 (PAH-CLHC-1300℃) indicate that its carbon layer stacking is relatively regular and the interlayer spacing is small; while the peak shape changes of other samples reflect that the difference in pretreatment leads to an increase in the disorder of the carbon layer and the possible expansion of the interlayer spacing, which is related to the regulation of the carbon structure by pretreatments such as phytic acid and hydrothermal treatment during material preparation. Combined with the (100) peak (reflecting the ordered arrangement of the carbon hexagonal ring plane), the (100) peak of Example 1 (PAH-CLHC-1300℃) is relatively clear, further proving that its carbon structure has a high degree of order. Phytic acid synergistic hydrothermal pretreatment may promote the local ordered arrangement of the carbon layer. The differences in the (100) peaks of the other samples reflect the effects of different pretreatments on the structural regularity of the carbon hexagonal ring plane, which together indicate that pretreatment regulates the crystal structure of hard carbon materials by changing the order of the carbon layer, providing a structural basis for the correlation of electrochemical properties.

[0059] Comparative Example 2 The difference from Example 1 is that phytic acid is not added, and the mass ratio of fir wood chips to deionized water is 1:15.

[0060] Comparative Example 3 The difference from Example 1 is that the mass of the modifier is 4 g, and the pH value of the hydrothermal reaction is 3-4.

[0061] Comparative Example 4 The difference from Example 1 is that the carbonization temperature is 1700°C and the time is 2 hours.

[0062] Comparative Example 5 The difference from Example 1 is that no hydrothermal treatment is performed.

[0063] Comparative Example 6 The difference from Example 1 is that the fir wood chips are replaced by coconut shells.

[0064] The biomass hard carbon negative electrode materials prepared in the above examples and comparative examples were subjected to performance tests, and the specific test results are shown in Table 1.

[0065] Table 1 Electrochemical performance test

[0066] analyze: Based on the data from Example 1 and Comparative Examples 1, 2, and 5, both hydrothermal pre-oxidation of fir wood chips and treatment with a modifier can improve the electrochemical performance of sodium-ion hard carbon anode materials. The two can synergistically enhance the electrochemical performance of sodium-ion hard carbon anode materials, achieving a synergistic enhancement superior to either treatment alone. These data demonstrate that hydrothermal pre-oxidation of phytic acid mixed with fir wood chips can significantly improve the electrochemical performance of sodium-ion hard carbon anode materials, achieving successful modification.

[0067] It can be concluded from Example 1 and Example 2 that phytic acid can improve the electrochemical performance of fir wood chip derived hard carbon, and hexafluorophosphoric acid can also achieve the effect of enhancing the electrochemical performance, which illustrates the scalability of this method and provides a certain idea for the development of biomass-derived hard carbon in sodium ion battery negative electrode materials. By comparing Example 1 with Example 3, it can be found that the treatment of the precursor plays a vital role in the electrochemical performance of the derived hard carbon, and the particle size of the biomass-derived hard carbon plays an indispensable role in its electrochemical performance. It can be seen from Examples 1 and 2 and Comparative Example 3 that the pH value during the reaction process also has a certain influence on the electrochemical performance of fir wood chip derived hard carbon. Acidic conditions can promote the hydrolysis of cellulose and hemicellulose, have an important influence on the structural construction of the precursor, and affect the formation of the closed-pore structure of the subsequent high-temperature carbonized material.

[0068] Comparative Example 6 shows that, although coconut shell is also a plant, the electrochemical performance of hard carbon derived from sawdust using the same method is inferior. The core reason lies in the fundamental differences in initial composition and microstructure between coconut shell and fir sawdust, which lead to distinct reaction behaviors during the critical hydrothermal pretreatment stage. Consequently, they fail to form the hard carbon microstructure unique to the fir sawdust route, which is crucial for sodium storage performance. Specifically, coconut shell naturally has an extremely high lignin content (typically >40%) and an extremely dense, rigid fibrous structure (derived from its protective function as a nut shell). This high lignin content and high density severely hinder the uniformity and efficiency of the hydrothermal carbonization process: the high content of cross-linked lignin is difficult to fully degrade and dissolve, and the dense structure restricts the penetration of water molecules and the diffusion of reactants. As a result, coconut shell cannot form a uniform, spherical carbon microsphere precursor rich in oxygen-containing functional groups during the hydrothermal stage, unlike fir sawdust (which has a higher proportion of cellulose and hemicellulose as its main components and a softer, looser structure). In contrast, coconut shell hydrothermal carbon tends to form precursors with complex structures, uneven sizes, irregular shapes (a mix of flakes, blocks, and fibers), and inherently high density. This undesirable precursor structure, during subsequent high-temperature carbonization, evolves more easily into a carbon skeleton with small interlayer spacing, insufficient or poorly distributed closed pores, and a relatively high overall degree of graphitization (or locally highly disordered but poorly connected). This structure cannot effectively provide the rapid diffusion channels and sufficient embedding / adsorption sites (especially closed-pore sodium storage) required for sodium ions, ultimately making its key electrochemical performance indicators, such as specific capacity, first efficiency, and rate performance, difficult to match those of fir sawdust-based hard carbon.

[0069] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some or all of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present application.

[0070] Furthermore, those skilled in the art will appreciate that although some embodiments herein include certain features included in other embodiments but not others, combinations of features from different embodiments are intended to be within the scope of this application and to form distinct embodiments. For example, in the claims above, any of the claimed embodiments may be used in any combination. The information disclosed in this background section is intended solely to enhance understanding of the overall background of this application and should not be construed as an admission or any implication that such information constitutes prior art known to those skilled in the art.

Claims

1. A method for preparing a biomass hard carbon negative electrode material, characterized in that: include: Mixing fir wood chips, a modifier and water, performing a hydrothermal reaction, and performing solid-liquid separation to obtain modified wood chips; Carbonizing the modified sawdust under a protective atmosphere to obtain a biomass hard carbon negative electrode material; The modifier includes one or more of phosphoric acid, phosphorous acid, hydrofluoric acid, boric acid, phytic acid and hexafluorophosphoric acid.

2. The method for preparing a biomass hard carbon negative electrode material according to claim 1, characterized in that: The mass ratio of the fir wood chips, the modifier and the water is 1:10-15:10-15.

3. The method for preparing a biomass hard carbon negative electrode material according to claim 1, characterized in that: The temperature of the hydrothermal reaction is 120° C.-200° C., and the time is 5 h-30 h.

4. The method for preparing a biomass hard carbon negative electrode material according to claim 1, wherein: The pH value of the hydrothermal reaction is greater than 0 and less than or equal to 2.

5. The method for preparing a biomass hard carbon negative electrode material according to claim 1, wherein: The mass concentration of the modifier is 50%-90%.

6. The method for preparing a biomass hard carbon negative electrode material according to claim 1, wherein: The carbonization temperature is 1000-1500° C., and the time is 1-10 hours.

7. The method for preparing a biomass hard carbon negative electrode material according to claim 1, characterized in that: Before the hydrothermal reaction, the fir wood chips are also pretreated; The pretreatment comprises: subjecting the fir wood chips to ball milling, ultrasonication, and drying in sequence.

8. The method for preparing a biomass hard carbon negative electrode material according to claim 7, characterized in that: The ball milling time is 800 min-1500 min, and the rotation speed is 600 r / min-900 r / min; The ultrasound time is 60min-240min; The drying temperature is 50° C.-80° C., and the drying time is 12-24 hours.

9. A biomass hard carbon negative electrode material, characterized in that: The biomass hard carbon negative electrode material is prepared by the preparation method of any one of claims 1 to 8.

10. A sodium ion battery, characterized in that: Including the biomass hard carbon negative electrode material according to claim 9.

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