A method and application for performance optimization of hard carbon based on fluorine atom doping strategy

Abstract

This invention provides a method and application for optimizing the performance of hard carbon based on a fluorine atom doping strategy, thereby improving the physicochemical properties of hard carbon materials and optimizing their electrochemical performance in sodium-ion batteries. Furthermore, this method effectively alters the electronic structure of the hard carbon electrode by selecting fluorine as the dopant source, thus controlling the solid electrolyte interphase (SEI) film derived on the electrode surface during charge and discharge, resulting in high mechanical stability and good ion conductivity. This invention also relates to methods for preparing and processing hard carbon precursors, as well as methods for optimizing the doping of hard carbon samples. Experiments demonstrate that the obtained fluorine-doped hard carbon material exhibits high capacity and excellent cycle stability. The method of this invention has significant practical implications for the commercial application of sodium-ion batteries.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery anode material technology, and particularly relates to performance optimization strategies for hard carbon materials and stable construction of solid electrolyte interface (SEI) films. Background Technology

[0002] Lithium-ion batteries, as an electrochemical energy storage device, boast the most mature industrial chain and technology, with significant potential for cost reduction. However, with the widespread application of lithium-ion batteries, the limited and uneven distribution of lithium resources is becoming increasingly prominent. Sodium-ion batteries, on the other hand, share high similarities with lithium-ion batteries in reaction mechanisms and battery structure. Their resource and cost advantages, excellent rate performance, and high recyclability make them more suitable for the construction of next-generation large-scale energy storage systems. Compared to traditional energy storage systems, sodium resources are abundant and widely distributed. Sodium-ion batteries are inexpensive, use readily available raw materials, are environmentally friendly, compatible with existing lithium-ion battery production equipment, offer good power characteristics, a wide temperature range adaptability, and good safety. For large-scale stationary energy storage devices with fewer site and environmental limitations, sodium-ion batteries are one of the more ideal choices.

[0003] To improve the electrochemical performance of energy storage devices, hard carbon anode materials for sodium-ion batteries have received increasing attention and application. Currently, in performance optimization research of hard carbon materials, heteroatom doping is widely used to enhance the transfer performance of sodium ions and electrons within the hard carbon material, thereby accelerating reaction kinetics. However, existing research on heteroatom doping strategies for hard carbon materials mainly focuses on changes in the intrinsic physicochemical properties of the material, with less attention paid to the regulation of the SEI film formed on the electrode surface. In traditional batteries, the SEI film formed on the surface of the hard carbon electrode plays an important protective role. However, the performance of the SEI film in existing technologies suffers from several problems, such as poor interfacial stability and slow ion transport. Therefore, a feasible method is needed to regulate the SEI film formed on the surface of the hard carbon electrode to improve battery performance and cycle stability.

[0004] To improve the performance of hard carbon, a method for preparing biomass hard carbon by lotus leaf pyrolysis has been reported (reference: Wang P, Qiao B, Du Y, et al. Fluorine-doped carbon particles derived from lotuspetioles as high-performance anode materials for sodium-ion batteries[J]. The Journal of Physical Chemistry C, 2015, 119(37):21336-21344.). This method can achieve in-situ fluorine doping. Through a series of characterization and electrochemical tests, it was confirmed that fluorine doping has a significant effect on improving the rate performance and cycle performance of biomass hard carbon materials. However, this method has two main limitations: first, it is only applicable to lotus leaf materials with their own fluorine content and cannot be applied to other types of hard carbon precursors; second, since the fluorine source comes from the biomass itself, the amount of fluorine doping cannot be precisely controlled. To overcome these limitations, this invention provides a new method that can solve the above problems. This method is not only applicable to different types of hard carbon precursors, but also enables precise control of the amount of fluorine doping, thus possessing the potential for widespread application in the field of sodium-ion batteries. Summary of the Invention

[0005] This invention provides a method for optimizing the performance of hard carbon based on a fluorine atom doping strategy. The aim is to more effectively alter the electronic structure of hard carbon to improve its physicochemical properties and further promote the formation of a uniform, inorganic-rich SEI film on its surface, thereby optimizing the electrochemical performance of hard carbon materials in sodium-ion batteries. Through a systematic analysis of the influence of fluorine atom doping on the physicochemical properties of hard carbon materials, the positive role of appropriate fluorine atom doping in constructing a stable SEI film is revealed. Specifically, fluorine atom doping can lead to the formation of a NaF-rich SEI film on the surface of hard carbon materials. The NaF-rich SEI film can suppress the instability of the solid electrolyte interface and the excessive reduction reaction of the electrolyte, thereby improving the cycle stability of the battery. Experiments demonstrate the correlation between the electronic structure of the electrode and the properties of the SEI film, providing new ideas for doping optimization strategies and offering new design concepts for electrode materials with electrolytes exhibiting low reduction kinetics.

[0006] The present invention achieves the above objectives through the following technical solutions:

[0007] (1) The present invention selects fluorine, which has the highest electronegativity, as the doping source, which can more effectively change the electronic structure of the hard carbon electrode, thereby improving the physicochemical properties of the hard carbon material itself and its ability to reduce electrolyte, thus achieving the regulation of the SEI film.

[0008] (2) By doping hard carbon with fluorine, the present invention changes the composition and mechanical properties of the SEI film derived on the electrode surface during the charging and discharging process, which can make it have high mechanical stability and good ion conductivity.

[0009] (3) The present invention crushes and grinds the material after pre-firing, which can avoid serious agglomeration of carbon particles during carbonization to a certain extent and maintain the dispersion and uniformity of the material.

[0010] The specific plan is as follows:

[0011] A method for optimizing the performance of hard carbon based on a fluorine atom doping strategy includes the following steps:

[0012] (1) Crosslinking treatment of S1 precursor: First, weigh a certain amount of hard carbon precursor and crosslinking agent, mix them in a certain mass ratio, and then transfer them to a ball mill jar for ball milling until completely mixed.

[0013] (2) S2 Low-temperature pre-calcination: The mixture obtained by grinding in step (1) is pre-calcined at a low temperature of 100-300°C for 20-30 hours in an air atmosphere to obtain a black block;

[0014] (3) S3 Crushing and grinding: The black block obtained in step (2) is crushed and ground until it becomes a black powder with uniform size and particle size distribution of 10 to 100 micrometers.

[0015] (4) S4 High-temperature carbonization: The black powder obtained in step (3) is subjected to high-temperature carbonization. The temperature is raised to 1000-1600℃, the heating rate is set to 1-5℃ / min, and the temperature is held for 2-4 hours. Then the temperature is lowered to 200℃ at 1-5℃ / min. After cooling to room temperature in the furnace, the pure hard carbon sample before doping is taken out.

[0016] (5) S5 heat treatment fluorination: The pure hard carbon sample obtained in step (4) is ground and mixed with the doping source in a certain proportion to achieve a uniform mixing state. Then it is loaded into a burning boat, moved to a tube furnace, and heated to 600-900℃ in an inert gas atmosphere. The heating rate is set to 1-5℃ / min, and the temperature is held for 2-4 hours. Then it is cooled to 200℃ at 1-5℃ / min. After cooling to room temperature with the furnace, the fluorine-doped hard carbon sample can be taken out.

[0017] Furthermore, the hard carbon precursor mentioned in step (1) is preferably one of corn starch, wheat starch, or potato starch.

[0018] Furthermore, the crosslinking agent mentioned in step (1) is preferably one of maleic anhydride and diammonium hydrogen phosphate.

[0019] Furthermore, in step (1), the hard carbon precursor and the crosslinking agent are mixed in a certain mass ratio, specifically the mass ratio of the hard carbon precursor to the crosslinking agent is 3:1 to 5:1.

[0020] Furthermore, the doping source mentioned in step (5) is one of polyvinylidene fluoride or tetrafluoroterephthalic acid.

[0021] Furthermore, in step (5), the pure hard carbon sample and the dopant source are ground and mixed in a certain mass ratio, specifically, the mass ratio of the pure hard carbon sample to the dopant source is 1:1 to 1:3.

[0022] Furthermore, the inert gas mentioned in step (5) is either nitrogen or argon.

[0023] The fluorine-doped hard carbon material obtained by the above preparation method can be used as a negative electrode material in sodium-ion batteries, and has good commercial application prospects due to its excellent electrochemical performance.

[0024] The present invention has the following advantages and beneficial effects:

[0025] (1) The present invention uses starch as raw material to derive carbon material, which has unique advantages such as high capacity, easy availability and good consistency, which can greatly reduce the production cost of negative electrode material and improve its electrochemical performance.

[0026] (2) The fluorine-doped hard carbon material prepared by the method of the present invention has uniform particle size and can be used to derive an SEI film with high mechanical stability and good ion conductivity. It has excellent sodium storage performance, with a sodium storage specific capacity of not less than 400 mAh / g at 0.02 A / g and an initial coulombic efficiency of not less than 80%. Moreover, it can be stably cycled for 500 cycles at a high current density of 400 mA / g, and the cycle capacity retention rate is higher than 96%. Attached Figure Description

[0027] To more clearly illustrate the technical solution of the present invention, the accompanying drawings will be briefly described below. The drawings described below only relate to some embodiments of the present invention and are not intended to limit the present invention.

[0028] Figure 1 The diagram illustrates the steps of the hard carbon performance optimization method based on fluorine atom doping strategy provided by this invention.

[0029] Figure 2 This is a scanning electron microscope image of the fluorine-doped hard carbon material prepared in Example 1 of the present invention.

[0030] Figure 3The first charge-discharge curve of the fluorine-doped hard carbon material obtained in Example 1 of this invention applied to a sodium-ion battery.

[0031] Figure 4 This is a graph showing the charge specific capacity data of the fluorine-doped hard carbon material obtained in Example 1 of the present invention after 500 cycles in a sodium-ion battery.

[0032] Figure 5 Scanning electron microscope (SEM) image of the hard carbon material prepared in Comparative Example 1.

[0033] Figure 6 The first charge-discharge curve of the hard carbon material obtained in Comparative Example 1 applied to a sodium-ion battery is shown.

[0034] Figure 7 The graph shows the charge specific capacity data of the hard carbon material obtained in Comparative Example 1 when applied to a sodium-ion battery after 500 cycles.

[0035] Figure 8 The first charge-discharge curve of the fluorine-doped hard carbon material obtained in Comparative Example 2 when applied to a sodium-ion battery is shown.

[0036] Figure 9 The graph shows the charge specific capacity data of the fluorine-doped hard carbon material obtained in Comparative Example 2 when applied to a sodium-ion battery after 500 cycles.

[0037] Figure 10 The first charge-discharge curve of the fluorine-doped hard carbon material obtained in Comparative Example 3 when applied to a sodium-ion battery is shown.

[0038] Figure 11 The graph shows the charge specific capacity data of the fluorine-doped hard carbon material obtained in Comparative Example 3 when applied to a sodium-ion battery after 500 cycles. Detailed Implementation

[0039] The present invention is further illustrated below through specific embodiments, but these are not intended to limit the invention. Various modifications and improvements made by those skilled in the art based on the basic idea of ​​the present invention, as long as they do not depart from the basic idea of ​​the present invention, are within the scope of the present invention.

[0040] Figure 1 This diagram illustrates the steps of the hard carbon performance optimization method based on fluorine atom doping strategy provided by the present invention. The following examples demonstrate the preparation of fluorine-doped hard carbon materials following the steps of S1 precursor crosslinking treatment → S2 low-temperature pre-calcination → S3 crushing and grinding → S4 high-temperature carbonization → S5 heat treatment fluorination.

[0041] Example 1

[0042] (1) First, weigh a certain amount of wheat starch and mix it with diammonium hydrogen phosphate in a mass ratio of 5:1, then transfer it to a ball mill jar and ball mill until completely mixed.

[0043] (2) The mixture obtained in step (1) is pre-fired at 200°C for 24 hours in air atmosphere to obtain a black block;

[0044] (3) The black block obtained in step (2) is crushed and ground into a black powder with uniform size and particle size distribution of 10 to 100 micrometers;

[0045] (4) The black powder obtained in step (3) is subjected to high-temperature carbonization. The procedure is as follows: heat up to 1200℃, the heating rate is set to 2℃ / min, hold for 3 hours, then cool down to 200℃ at 5℃ / min, and after cooling to room temperature in the furnace, the pure hard carbon sample before doping is taken out.

[0046] (5) The pure hard carbon sample obtained in step (4) is optimized by doping. The pure hard carbon sample and tetrafluoroterephthalic acid are ground and mixed in a mass ratio of 1:2 to achieve a uniform mixture. The mixture is then loaded into a furnace boat, transferred to a tube furnace, heated to 600°C in an inert gas atmosphere at a heating rate of 3°C / min, held for 3 hours, and then cooled to 200°C at a rate of 5°C / min. After cooling to room temperature in the furnace, the fluorine-doped hard carbon sample is obtained.

[0047] (6) Apply the fluorine-doped hard carbon sample obtained in step (5) to the negative electrode of a sodium-ion battery and perform electrochemical tests.

[0048] (7) After 50 charge-discharge cycles in step (6), the battery was disassembled, and the electrodes were subjected to X-ray photoelectron spectroscopy (XPS) depth profiling to measure the NaF content in the SEI film formed on the electrode surface. The test results showed that the specific NaF content in the SEI film was 12%.

[0049] Figure 2 The scanning electron microscope (SEM) images of the fluorine-doped hard carbon material obtained in this embodiment show that the material has a uniform particle size and small particle size, ranging from 10 to 20 micrometers, which is beneficial for battery coating. Furthermore, the hard carbon particles are flattened, platelet-like in shape; this three-dimensional morphology can improve electrolyte wettability and ion transport rate, and also exhibits good electrical conductivity.

[0050] Figure 3 The first charge-discharge curve of the fluorine-doped hard carbon material obtained in this embodiment applied to a sodium-ion battery shows that the sodium-ion battery hard carbon anode material has a first coulombic efficiency of up to 85.11% and a reversible specific capacity of over 410 mAh / g.

[0051] Figure 4The graph shows the charge specific capacity data of the fluorine-doped hard carbon material obtained in this embodiment after 500 cycles in a sodium-ion battery. It can be seen that the hard carbon anode material for this sodium-ion battery can stably cycle for 500 cycles at a high current density of 400 mA / g, and the cycle capacity retention rate is higher than 96.9%. This is because the SEI film formed on the electrode surface has a high NaF content of 12%, which can suppress the instability of the solid electrolyte interface and the excessive reduction reaction of the electrolyte, thereby improving the battery's cycle stability.

[0052] Comparative Example 1

[0053] (1) First, weigh a certain amount of wheat starch and mix it with diammonium hydrogen phosphate at a mass ratio of 5:1, then transfer it to a ball mill jar and ball mill until completely mixed.

[0054] (2) The mixture obtained in step (1) is pre-fired at 200°C for 24 hours in air atmosphere to obtain a black block;

[0055] (3) The black block obtained in step (2) is crushed and ground into a black powder with uniform size and particle size distribution of 10 to 100 micrometers;

[0056] (4) The black powder obtained in step (3) is subjected to high-temperature carbonization. The procedure is as follows: heat up to 1200℃, set the heating rate to 2℃ / min, hold for 3 hours, and then cool down to 200℃ at 5℃ / min. After cooling to room temperature with the furnace, the pure hard carbon sample after high-temperature carbonization can be taken out.

[0057] (5) Apply the product obtained in step (4) to the negative electrode of a sodium-ion battery and perform electrochemical tests.

[0058] (6) After 50 charge-discharge cycles in step (5), the battery was disassembled, and the electrodes were subjected to X-ray photoelectron spectroscopy (XPS) depth profiling to measure the NaF content in the SEI film formed on the electrode surface. The test results showed that the specific NaF content in the SEI film was 6%.

[0059] Compared to Comparative Example 1, Example 1 contains specific measures for optimizing fluorine doping, which can increase the NaF content in its derived SEI film.

[0060] Figure 5 The scanning electron microscope image of the fluorine-doped hard carbon material obtained in this comparative example shows that its morphology is similar to that of the hard carbon obtained in Example 1, indicating that this fluorine doping strategy does not change the morphology of the hard carbon material.

[0061] Figure 6The first charge-discharge curve of the hard carbon material obtained in Comparative Example 1 applied to a sodium-ion battery shows that the hard carbon anode material for sodium-ion batteries has an initial coulombic efficiency of 84.45% and a reversible specific capacity of approximately 340 mAh / g.

[0062] Figure 7 The graph shows the charge specific capacity data of the hard carbon material obtained in Comparative Example 1 after 500 cycles in a sodium-ion battery. It can be seen that the hard carbon anode material of this sodium-ion battery has a significant decay trend under high current density of 400 mA / g, and its cycle capacity retention rate is 87.5%.

[0063] Comparative Example 2

[0064] (1) First, weigh a certain amount of wheat starch and mix it with diammonium hydrogen phosphate in a mass ratio of 5:1, then transfer it to a ball mill jar and ball mill until completely mixed.

[0065] (2) The mixture obtained in step (1) is pre-fired at 200°C for 24 hours in air atmosphere to obtain a black block;

[0066] (3) The black block obtained in step (2) is crushed and ground into a black powder with uniform size and particle size distribution of 10 to 100 micrometers;

[0067] (4) The black powder obtained in step (3) is subjected to high-temperature carbonization. The procedure is as follows: heat up to 1200℃, the heating rate is set to 2℃ / min, hold for 3 hours, then cool down to 200℃ at 5℃ / min, and after cooling to room temperature in the furnace, the pure hard carbon sample before doping is taken out.

[0068] (5) The pure hard carbon sample obtained in step (4) is optimized by doping. The pure hard carbon sample and tetrafluoroterephthalic acid are ground and mixed in a mass ratio of 2:1 to achieve a uniform mixture. The mixture is then loaded into a furnace boat, transferred to a tube furnace, heated to 600°C in an inert gas atmosphere at a heating rate of 3°C / min, held for 3 hours, and then cooled to 200°C at a rate of 5°C / min. After cooling to room temperature in the furnace, the fluorine-doped hard carbon sample is obtained.

[0069] (6) Apply the fluorine-doped hard carbon sample obtained in step (5) to the negative electrode of a sodium-ion battery and perform electrochemical tests.

[0070] Figure 8 The first charge-discharge curve of the hard carbon material obtained in Comparative Example 2 applied to a sodium-ion battery shows that the hard carbon anode material for sodium-ion batteries has an initial coulombic efficiency of 79.2% and a reversible specific capacity of approximately 340 mAh / g.

[0071] Figure 9The graph shows the charge specific capacity data of the hard carbon material obtained in Comparative Example 2 applied to a sodium-ion battery after 500 cycles. It can be seen that the hard carbon anode material of this sodium-ion battery has a significant decay trend under high current density of 400 mA / g, and its cycle capacity retention rate is only 35.1%.

[0072] Comparative Example 3

[0073] (1) First, weigh a certain amount of wheat starch and mix it with diammonium hydrogen phosphate in a mass ratio of 5:1, then transfer it to a ball mill jar and ball mill until completely mixed.

[0074] (2) The mixture obtained in step (1) is pre-fired at 200°C for 24 hours in air atmosphere to obtain a black block;

[0075] (3) The black block obtained in step (2) is crushed and ground into a black powder with uniform size and particle size distribution of 10 to 100 micrometers;

[0076] (4) The black powder obtained in step (3) is subjected to high-temperature carbonization. The procedure is as follows: heat up to 1200℃, the heating rate is set to 2℃ / min, hold for 3 hours, then cool down to 200℃ at 5℃ / min, and after cooling to room temperature in the furnace, the pure hard carbon sample before doping is taken out.

[0077] (5) The pure hard carbon sample obtained in step (4) is optimized by doping. The pure hard carbon sample and tetrafluoroterephthalic acid are ground and mixed in a mass ratio of 1:4 to achieve a uniform mixture. The mixture is then loaded into a furnace boat, transferred to a tube furnace, heated to 600°C in an inert gas atmosphere at a heating rate of 3°C / min, held for 3 hours, and then cooled to 200°C at a rate of 5°C / min. After cooling to room temperature in the furnace, the fluorine-doped hard carbon sample is obtained.

[0078] (6) Apply the fluorine-doped hard carbon sample obtained in step (5) to the negative electrode of a sodium-ion battery and perform electrochemical tests.

[0079] Figure 10 The first charge-discharge curve of the hard carbon material obtained in this comparative example applied to a sodium-ion battery shows that the hard carbon anode material for sodium-ion batteries has an initial coulombic efficiency of 77.4% and a reversible specific capacity of approximately 266 mAh / g.

[0080] Figure 11 The graph shows the charge specific capacity data of the hard carbon material obtained in this comparative example after 500 cycles in a sodium-ion battery. It can be seen that the hard carbon anode material of this sodium-ion battery has a significant decay trend under high current density of 400 mA / g, and its cycle capacity retention rate is only 27.1%.

[0081] Example 2

[0082] (1) First, weigh a certain amount of wheat starch and mix it with diammonium hydrogen phosphate in a mass ratio of 5:1, then transfer it to a ball mill jar and ball mill until completely mixed.

[0083] (2) The mixture obtained in step (1) is pre-fired at 300°C for 24 hours in air atmosphere to obtain a black block;

[0084] (3) The black block obtained in step (2) is crushed and ground into a black powder with uniform size and particle size distribution of 10 to 100 micrometers;

[0085] (4) The black powder obtained in step (3) is subjected to high-temperature carbonization. The procedure is as follows: heat up to 1000℃, set the heating rate to 2℃ / min, hold for 3 hours, then cool down to 200℃ at 5℃ / min, and after cooling to room temperature in the furnace, take out the pure hard carbon sample before doping.

[0086] (5) The pure hard carbon sample obtained in step (4) is optimized by doping. The pure hard carbon sample and tetrafluoroterephthalic acid are ground and mixed in a mass ratio of 1:2 to achieve a uniform mixture. The mixture is then loaded into a furnace boat, transferred to a tube furnace, heated to 700°C in an inert gas atmosphere at a heating rate of 3°C / min, held for 3 hours, and then cooled to 200°C at a rate of 5°C / min. After cooling to room temperature in the furnace, the fluorine-doped hard carbon sample can be obtained.

[0087] (6) Apply the product obtained in step (5) to the negative electrode of a sodium-ion battery and perform electrochemical tests.

[0088] Example 3

[0089] (1) First, weigh a certain amount of corn starch and mix it with diammonium hydrogen phosphate in a mass ratio of 5:1, then transfer it to a ball mill jar and ball mill until completely mixed.

[0090] (2) The mixture obtained in step (1) is pre-fired at 300°C for 24 hours in air atmosphere to obtain a black block;

[0091] (3) The black block obtained in step (2) is crushed and ground into a black powder with uniform size and particle size distribution of 10 to 100 micrometers;

[0092] (4) The black powder obtained in step (3) is subjected to high-temperature carbonization. The procedure is as follows: heat up to 1600℃, set the heating rate to 5℃ / min, hold for 4 hours, then cool down to 200℃ at 5℃ / min, and after cooling to room temperature in the furnace, take out the pure hard carbon sample before doping.

[0093] (5) The pure hard carbon sample obtained in step (4) is optimized by doping. The pure hard carbon sample and tetrafluoroterephthalic acid are ground and mixed in a mass ratio of 1:2 to achieve a uniform mixture. The mixture is then loaded into a furnace boat, transferred to a tube furnace, heated to 600°C in an inert gas atmosphere at a heating rate of 5°C / min, held for 4 hours, and then cooled to 200°C at a rate of 5°C / min. After cooling to room temperature in the furnace, the fluorine-doped hard carbon sample is obtained.

[0094] (6) Apply the product obtained in step (5) to the negative electrode of a sodium-ion battery and perform electrochemical tests.

[0095] Example 4

[0096] (1) First, weigh a certain amount of wheat starch and mix it with diammonium hydrogen phosphate in a mass ratio of 3:1, then transfer it to a ball mill jar and ball mill until completely mixed.

[0097] (2) The mixture obtained in step (1) is pre-fired at 100°C for 24 hours in air atmosphere to obtain a black block;

[0098] (3) The black block obtained in step (2) is crushed and ground into a black powder with uniform size and particle size distribution of 10 to 100 micrometers;

[0099] (4) The black powder obtained in step (3) is subjected to high-temperature carbonization. The procedure is as follows: heat up to 1000℃, set the heating rate to 2℃ / min, hold for 3 hours, then cool down to 200℃ at 5℃ / min, and after cooling to room temperature in the furnace, take out the pure hard carbon sample before doping.

[0100] (5) The pure hard carbon sample obtained in step (4) is optimized by doping. The pure hard carbon sample and polyvinylidene fluoride are ground and mixed in a mass ratio of 1:1 until they are uniformly mixed. The mixture is then loaded into a sintering boat, transferred to a tube furnace, heated to 600°C in an inert gas atmosphere at a heating rate of 3°C / min, held for 3 hours, and then cooled to 200°C at a rate of 5°C / min. After cooling to room temperature in the furnace, the fluorine-doped hard carbon sample is obtained.

[0101] (6) Apply the fluorine-doped hard carbon sample obtained in step (5) to the negative electrode of a sodium-ion battery and perform electrochemical tests.

[0102] Example 5

[0103] (1) First, weigh a certain amount of wheat starch and mix it with diammonium hydrogen phosphate in a mass ratio of 4:1, then transfer it to a ball mill jar and ball mill until completely mixed.

[0104] (2) The mixture obtained in step (1) is pre-fired at 200°C for 24 hours in air atmosphere to obtain a black block;

[0105] (3) The black block obtained in step (2) is crushed and ground into a black powder with uniform size and particle size distribution of 10 to 100 micrometers;

[0106] (4) The black powder obtained in step (3) is subjected to high-temperature carbonization. The procedure is as follows: heat up to 1500℃, heat up at a rate of 1℃ / min, hold for 2 hours, then cool down to 200℃ at a rate of 1℃ / min, and after cooling to room temperature in the furnace, take out the pure hard carbon sample before doping.

[0107] (5) The pure hard carbon sample obtained in step (4) is optimized by doping. The pure hard carbon sample and tetrafluoroterephthalic acid are ground and mixed in a mass ratio of 1:2 to achieve a uniform mixture. The mixture is then loaded into a furnace boat, transferred to a tube furnace, heated to 600°C in an inert gas atmosphere at a heating rate of 3°C / min, held for 3 hours, and then cooled to 200°C at a rate of 5°C / min. After cooling to room temperature in the furnace, the fluorine-doped hard carbon sample is obtained.

[0108] (6) Apply the fluorine-doped hard carbon sample obtained in step (5) to the negative electrode of a sodium-ion battery and perform electrochemical tests.

[0109] Example 6

[0110] (1) First, weigh a certain amount of wheat starch and mix it with diammonium hydrogen phosphate in a mass ratio of 5:1, then transfer it to a ball mill jar and ball mill until completely mixed.

[0111] (2) The mixture obtained in step (1) is pre-fired at 200°C for 24 hours in air atmosphere to obtain a black block;

[0112] (3) The black block obtained in step (2) is crushed and ground into a black powder with uniform size and particle size distribution of 10 to 100 micrometers;

[0113] (4) The black powder obtained in step (3) is subjected to high-temperature carbonization. The procedure is as follows: heat up to 1000℃, set the heating rate to 1℃ / min, hold for 2 hours, then cool down to 200℃ at 1℃ / min, and after cooling to room temperature in the furnace, take out the pure hard carbon sample before doping.

[0114] (5) The pure hard carbon sample obtained in step (4) is optimized by doping. The pure hard carbon sample and tetrafluoroterephthalic acid are ground and mixed in a mass ratio of 1:3 to achieve a uniform mixture. The mixture is then loaded into a furnace boat, transferred to a tube furnace, heated to 900°C in an inert gas atmosphere at a heating rate of 1°C / min, held for 2 hours, and then cooled to 200°C at a rate of 1°C / min. After cooling to room temperature in the furnace, the fluorine-doped hard carbon sample is obtained.

[0115] (6) Apply the fluorine-doped hard carbon sample obtained in step (5) to the negative electrode of a sodium-ion battery and perform electrochemical tests.

[0116] Table 1: First-round test results of Examples 1-6 and Comparative Examples 1-3

[0117]

[0118] Note: Cycle capacity retention = Charge specific capacity after n cycles / Charge specific capacity after activation and stabilization in the first three cycles

[0119] As shown in Table 1, Example 1 exhibits the best electrochemical performance optimization, with a significantly higher discharge mass specific capacity than other examples and comparative examples. This indicates that fluorine doping can provide additional sodium storage sites for hard carbon materials, thereby improving capacity. Furthermore, Example 1 demonstrates the highest capacity retention rate after 500 cycles, specifically 96.9%. The higher NaF content in the SEI film of Example 1 compared to Comparative Example 1 suggests that fluorine doping, which promotes the formation of a NaF-rich SEI film, can suppress the instability of the solid electrolyte interface and the excessive reduction reaction of the electrolyte, thus improving battery cycle stability. In addition, comparing Comparative Examples 2 and 3 reveals that excessively low or high fluorine doping levels fail to effectively suppress interfacial instability, resulting in poor cycle stability. This confirms the positive role of appropriate fluorine atom doping in constructing a stable SEI film.

[0120] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they are also considered as the content disclosed in the present invention.

Claims

1. A method for performance optimization of hard carbon based on fluorine atom doping strategy, characterized in that, It comprises the following steps: (1) precursor crosslinking treatment: the hard carbon precursor is mixed with the crosslinking agent, and then moved to a ball mill tank for ball milling until completely mixed; (2) low-temperature pre-burning: the mixture obtained in step (1) is pre-burned at low temperature in an air atmosphere to obtain a black block; (3) crushing and grinding: the black block obtained in step (2) is crushed and ground to a uniform size, and the particle size distribution of the black powder is 10-100 microns; (4) high-temperature carbonization: the black powder obtained in step (3) is subjected to high-temperature carbonization, and after the furnace chamber is cooled to room temperature, the pure hard carbon sample before doping is obtained; (5) heat treatment fluorination: the pure hard carbon sample obtained in step (4) is mixed with a doping source, and after being mixed uniformly, it is loaded into a boat and moved to a tube furnace for calcination. After the furnace chamber is cooled to room temperature, the fluorine-doped hard carbon sample is obtained; In step (5), the pure hard carbon sample and the doping source are mixed at a mass ratio of 1:1 to 1:3; the doping source is one of polyvinylidene fluoride and tetrafluoro terephthalic acid; In step (5), the calcination operation is as follows: under an inert gas atmosphere, the temperature is raised to 600-900℃ at a rate of 1-5℃ / min, and then held for 2-4 hours, and then cooled to 200℃ at a rate of 1-5℃ / min. The inert gas is selected from one of nitrogen and argon.

2. The method for performance optimization of hard carbon based on fluorine atom doping strategy according to claim 1, characterized in that, The hard carbon precursor in step (1) is selected from one of corn starch, wheat starch, and potato starch.

3. The method for performance optimization of hard carbon based on fluorine atom doping strategy according to claim 1, characterized in that, The crosslinking agent in step (1) is selected from one of maleic anhydride and diammonium hydrogen phosphate.

4. The method for performance optimization of hard carbon based on fluorine atom doping strategy according to claim 1, characterized in that, In step (1), the hard carbon precursor and the crosslinking agent are mixed at a mass ratio of 3:1 to 5:

1.

5. The method for performance optimization of hard carbon based on fluorine atom doping strategy according to claim 1, characterized in that, The low-temperature pre-burning temperature in step (2) is 100-300℃, and the pre-burning time is 20-30 hours.

6. The method for performance optimization of hard carbon based on fluorine atom doping strategy according to claim 1, characterized in that, The high-temperature carbonization operation in step (4) is as follows: the temperature is raised to 1000-1600℃ at a rate of 1-5℃ / min, and then held for 2-4 hours, and then cooled to 200℃ at a rate of 1-5℃ / min.

7. The fluorine atom-doped hard carbon prepared by the method of any one of claims 1-6.

8. Use of the fluorine atom-doped hard carbon according to claim 7 in a sodium-ion battery, characterized in that, Used as a negative electrode material for sodium ion batteries.

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