A starch-based hard carbon material, a preparation method and application of gradient oxidation annealing thereof

Abstract

The application belongs to the technical field of sodium ion battery negative electrode materials, and provides a starch-based hard carbon material and a preparation method and application of gradient oxidation annealing thereof. The preparation method comprises the following steps: sequentially drying and sieving starch to obtain pretreated starch; performing ladder-type oxidation-annealing on the pretreated starch to obtain a precursor; performing carbonization treatment on the precursor to obtain a starch-based hard carbon material; the number of times of ladder-type oxidation-annealing is 2-10; and annealing to room temperature after each oxidation. The application constructs a multistage oxidation layer in the starch phase through ladder-type oxidation treatment; annealing treatment removes stress from starch particles, stabilizes the starch structure, relieves volume shrinkage, prevents starch melting, and is conducive to strengthening the oxidation efficiency of the next gradient oxidation treatment and enhancing the oxidation effect in the starch phase, so that the starch-based hard carbon material with spheroidized particles, inert surface and developed internal closed pore structure is obtained.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery anode material technology, and in particular to a starch-based hard carbon material and its preparation method and application via gradient oxidation annealing. Background Technology

[0002] Sodium-ion batteries (SIBs) operate on a similar principle to lithium-ion batteries (LIBs) and show great promise in applications requiring higher energy density and lower cost, such as grid-connected energy storage systems and low-speed electric vehicles. The success of LIBs has also enabled the development of cathode materials and electrolytes for SIBs. However, due to the large ionic radius of sodium and the thermodynamic instability of sodium intercalation compounds, graphite, a mature anode material for commercial LIBs, is unsuitable for sodium storage. Therefore, significant efforts are still needed to develop high-performance anode materials to realize the commercial application of SIBs.

[0003] Among recently reported anode materials, hard carbon (HCs) possesses advantages such as increased interlayer spacing, low operating potential, and a stable framework, making them considered the most promising choice for solid-state batteries (SIBs). Furthermore, the amorphous microstructure of hard carbon (e.g., short-range crystallites, defects, and internal voids) is Na₂O₃. + Storage provides sufficient active sites. Generally, the large specific surface area of ​​hydrocarbon-formed carbon anodes leads to poor first-cycle coulombic efficiency (ICE) because abundant defects and micropores on the carbon anode surface induce electrolyte decomposition, forming a thick solid electrolyte interphase (SEI) layer. In full cells, poor first-cycle coulombic efficiency of the anode results in additional sodium consumption in the cathode material, leading to lower energy density and higher production costs. Therefore, the practical application of hard carbon urgently requires achieving high first-cycle coulombic efficiency and high specific capacity.

[0004] Starch-based hard carbon materials are natural spherical biomass materials and excellent sodium-ion anode materials. However, the cumbersome synthesis process and low specific capacity at low voltage limit their application in sodium-ion batteries. To overcome these challenges, research teams have been exploring improved methods. For example, Li et al. successfully suppressed starch foaming and melting by extending the oxidation time, thereby improving the specific capacity of the material. Chen et al. used maleic anhydride to suppress starch foaming, preparing a hard carbon material with better performance. Wang et al. used CO2 etching technology to control the morphology of starch particles, preparing a hard carbon material with higher capacity. Although these methods have achieved some success in maintaining the spherical morphology of starch and improving capacity, they usually involve the use of oxidizing agents, which are complex and costly. Therefore, further optimizing the preparation method to maintain the spherical morphology and improve capacity remains an important direction for future research. Summary of the Invention

[0005] The purpose of this invention is to overcome the problem that the existing gas-phase oxidation technology has poor effect on starch pretreatment, resulting in insufficient hard carbon performance. It provides a starch-based hard carbon material and its preparation method and application of gradient oxidation annealing, so as to solve the problems of low capacity and broken spherical morphology of starch-based hard carbon materials in the prior art.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] This invention provides a method for preparing a starch-based hard carbon material, comprising the following steps:

[0008] 1) The starch is dried and sieved sequentially to obtain pretreated starch;

[0009] 2) The pretreated starch was subjected to stepwise oxidation-annealing to obtain the precursor;

[0010] 3) Carbonize the precursor to obtain starch-based hard carbon material;

[0011] The number of step oxidation-annealing cycles is 2 to 10; after each oxidation, the temperature is annealed to room temperature, and the cooling rate during the annealing process is 0.5 to 20 °C / min.

[0012] Preferably, the starch comprises corn starch, potato starch, rice starch, pea starch, or sorghum starch.

[0013] Preferably, the drying temperature in step 1) is 50–120°C, and the drying time is 6–48 h.

[0014] Preferably, in step 2), the temperature of the step oxidation is 120-300℃, the time of each oxidation is 1-12h, and the oxidation temperature increases with the number of oxidation cycles.

[0015] Preferably, the heating rate during the step-by-step oxidation process is 0.5–5 °C / min.

[0016] Preferably, the carbonization temperature in step 3) is 1100–2600°C, and the carbonization time is 1–8 h.

[0017] Preferably, the heating rate during the carbonization process is 0.5–10 °C / min.

[0018] The present invention also provides a starch-based hard carbon material prepared by the aforementioned preparation method.

[0019] The present invention also provides the application of the starch-based hard carbon material in sodium-ion batteries.

[0020] The beneficial effects of this invention include the following:

[0021] 1) This invention uses low-temperature pretreatment of starch to regulate moisture content, which can remove excess moisture from the surface of starch balls and prevent the starch balls from sticking together during heating, thus avoiding damage to their spherical shape. The starch is sieved to control particle size, which can increase the contact area between the starch balls and air, further construct oxygen diffusion channels, reduce the blockage of oxygen channels, and lay a good foundation for the establishment of the oxide layer.

[0022] 2) This invention constructs a multi-level oxide layer in the starch bulk phase through a step-by-step oxidation process. This step-by-step oxidation facilitates deeper oxide layer penetration and enhances the oxidation effect. Annealing removes stress from the starch particles, stabilizes the starch structure, mitigates volume shrinkage, and prevents starch melting, which improves the oxidation efficiency of the next step of the oxidation process. This results in a starch-based hard carbon material with spherical particles, an inert surface, and a well-developed closed-pore structure. Using the obtained carbon material as a negative electrode material for sodium-ion batteries, compared with carbon materials obtained through direct oxidation, it can simultaneously and significantly improve the first-cycle coulombic efficiency and capacity, and significantly enhance the low-voltage plateau capacity, demonstrating significant performance advantages.

[0023] 3) The preparation method of the present invention is simple and easy to implement, which solves the problems of complicated synthesis and serious pollution of starch-based hard carbon materials; the prepared starch-based hard carbon anode material has an excellent sodium storage capacity of 450 mAh / g when used in sodium-ion batteries, especially the low potential sodium storage capacity can reach 300 mAh / g at a current density of 20 mA / g. Attached Figure Description

[0024] Figure 1 and Figure 2 Here is a SEM image of the starch-based hard carbon material from Example 1;

[0025] Figure 3 SEM image of the starch-based hard carbon material in Comparative Example 3;

[0026] Figure 4 Charge-discharge curves of sodium-ion button batteries prepared from starch-based hard carbon materials in Example 1 at a current density of 20 mA / g.

[0027] Figure 5 Charge-discharge curves of sodium-ion button batteries prepared from starch-based hard carbon material for Comparative Example 1 at a current density of 20 mA / g.

[0028] Figure 6 The charge-discharge curves of the sodium-ion button battery prepared from the starch-based hard carbon material of Comparative Example 3 at a current density of 20 mA / g. Detailed Implementation

[0029] This invention provides a method for preparing a starch-based hard carbon material, comprising the following steps:

[0030] 1) The starch is dried and sieved sequentially to obtain pretreated starch;

[0031] 2) The pretreated starch was subjected to stepwise oxidation-annealing to obtain the precursor;

[0032] 3) Carbonize the precursor to obtain starch-based hard carbon material;

[0033] The number of step oxidation-annealing cycles is 2 to 10; after each oxidation, the temperature is annealed to room temperature, and the cooling rate during the annealing process is 0.5 to 20 °C / min.

[0034] In this invention, the starch includes corn starch, potato starch, rice starch, pea starch, or sorghum starch.

[0035] In this invention, the drying temperature in step 1) is preferably 50-120°C, more preferably 60-100°C, and even more preferably 80-90°C; the drying time is preferably 6-48 hours, more preferably 10-40 hours, and even more preferably 20-30 hours.

[0036] In step 2) of the present invention, the temperature of the step oxidation is preferably 120-300℃, more preferably 150-280℃, and even more preferably 200-260℃; the time for each oxidation is preferably 1-12h, more preferably 3-10℃, and even more preferably 5-8℃; the oxidation temperature increases with the number of oxidation cycles.

[0037] In the step oxidation process of the present invention, the heating rate is preferably 0.5 to 5 °C / min, more preferably 1 to 4 °C / min, and even more preferably 2 to 3 °C / min.

[0038] In this invention, the number of step-by-step oxidation-annealing cycles is preferably 4 to 8, more preferably 5 to 7; after each oxidation, the furnace is annealed to room temperature, and the cooling rate during the annealing process is preferably 1 to 15°C / min, more preferably 3 to 12°C / min, and even more preferably 5 to 10°C / min.

[0039] In this invention, the carbonization temperature in step 3) is preferably 1100-2600℃, more preferably 1400-2300℃, and even more preferably 1600-2000℃. The carbonization time is preferably 1-8h, more preferably 3-7h, and even more preferably 4-5h.

[0040] In this invention, during the carbonization process, the heating rate is preferably 0.5 to 10 °C / min, more preferably 1 to 8 °C / min, and even more preferably 3 to 5 °C / min.

[0041] The present invention also provides a starch-based hard carbon material prepared by the aforementioned preparation method.

[0042] The present invention also provides the application of the starch-based hard carbon material in sodium-ion batteries.

[0043] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0044] Example 1

[0045] Corn starch was dried at 80℃ for 24 hours and then passed through a 300-mesh sieve. The sieved starch underwent a four-stage oxidation-annealing process in a muffle furnace. Specifically, the process was as follows: oxidation was carried out at a rate of 2℃ / min to 200℃ for 3 hours, followed by annealing at a rate of 2℃ / min to room temperature. After the first oxidation-annealing, the starch was oxidized at 2℃ / min to 220℃ for 3 hours, followed by annealing at a rate of 2℃ / min to room temperature. After the second oxidation-annealing, the starch was oxidized at 2℃ / min to 240℃ for 3 hours, followed by annealing at a rate of 1℃ / min to room temperature. After the third oxidation-annealing, the starch was oxidized at 2℃ / min to 260℃ for 6 hours, followed by annealing at a rate of 1℃ / min to room temperature. This four-stage oxidation-annealing process yielded the precursor. The precursor was then carbonized at 1400℃ for 3 hours at a rate of 2℃ / min to obtain a starch-based hard carbon material.

[0046] Example 2

[0047] Corn starch was dried at 80℃ for 12 hours and then passed through a 300-mesh sieve. The sieved starch underwent a four-stage oxidation-annealing process in a muffle furnace: First, the temperature was increased to 120℃ at a rate of 2℃ / min for 12 hours, followed by annealing at a rate of 1℃ / min to room temperature. After the first oxidation-annealing, the starch was heated to 200℃ at a rate of 2℃ / min for 3 hours, followed by annealing at a rate of 1℃ / min to room temperature. After the second oxidation-annealing, the starch was heated to 240℃ at a rate of 2℃ / min for 3 hours, followed by annealing at a rate of 1℃ / min to room temperature. After the third oxidation-annealing, the starch was heated to 300℃ at a rate of 2℃ / min for 6 hours, followed by annealing at a rate of 1℃ / min to room temperature. The precursor was obtained after the four oxidation-annealing processes. The precursor was then carbonized at 1400℃ at a rate of 2℃ / min for 3 hours to obtain a starch-based hard carbon material.

[0048] Example 3

[0049] Corn starch was dried at 80℃ for 24 hours and then passed through a 300-mesh sieve. The sieved starch underwent a four-stage oxidation-annealing process in a muffle furnace. Specifically, the process was as follows: oxidation was carried out at a rate of 2℃ / min to 200℃ for 4 hours, followed by annealing at a rate of 1℃ / min to room temperature. After the first oxidation-annealing, the starch was oxidized at 2℃ / min to 230℃ for 3 hours, followed by annealing at a rate of 1℃ / min to room temperature. After the second oxidation-annealing, the starch was oxidized at 2℃ / min to 245℃ for 3 hours, followed by annealing at a rate of 1℃ / min to room temperature. After the third oxidation-annealing, the starch was oxidized at 2℃ / min to 265℃ for 5 hours, followed by annealing at a rate of 1℃ / min to room temperature. The precursor was obtained after the four oxidation-annealing processes. The precursor was then carbonized at 1300℃ for 3 hours at a rate of 2℃ / min to obtain a starch-based hard carbon material.

[0050] Example 4

[0051] Corn starch was dried at 80℃ for 24 hours and then passed through a 300-mesh sieve. The sieved starch underwent a three-stage oxidation-annealing process in a muffle furnace: oxidation was carried out at a rate of 3℃ / min to 210℃ for 3 hours, followed by annealing at 3℃ / min to room temperature. After the first oxidation-annealing, the starch was oxidized at 2℃ / min to 230℃ for 3 hours, followed by annealing at 3℃ / min to room temperature. After the second oxidation-annealing, the starch was oxidized at 5℃ / min to 245℃ for 3 hours, followed by annealing at 3℃ / min to room temperature. The precursor was obtained after the three oxidation-annealing processes. The precursor was then carbonized at 1400℃ at a rate of 2℃ / min for 3 hours to obtain a starch-based hard carbon material.

[0052] Example 5

[0053] Potato starch was dried at 80℃ for 24 hours and then passed through a 100-mesh sieve. The sieved starch underwent four oxidation-annealing treatments in a muffle furnace. The specific process was as follows: oxidization was carried out at a rate of 5℃ / min to 215℃ for 3 hours, followed by annealing at a rate of 1℃ / min to room temperature. After the first oxidation-annealing, the starch was oxidized at 2℃ / min to 230℃ for 3 hours, followed by annealing at a rate of 1℃ / min to room temperature. After the second oxidation-annealing, the starch was oxidized at 5℃ / min to 245℃ for 3 hours, followed by annealing at a rate of 1℃ / min to room temperature. After the third oxidation-annealing, the starch was oxidized at 5℃ / min to 265℃ for 6 hours, followed by annealing at a rate of 1℃ / min to room temperature. After four oxidation-annealing treatments, a precursor was obtained. The precursor was carbonized at 2000℃ for 3 hours at a rate of 2℃ / min to obtain a starch-based hard carbon material.

[0054] Comparative Example 1

[0055] Corn starch was dried at 85℃ for 20 hours, and then passed through a 300-mesh sieve. The sieved starch was then oxidized in a muffle furnace. Specifically, the temperature was increased to 200℃ at a rate of 1℃ / min for 15 hours to obtain a precursor. The precursor was then carbonized at 1400℃ at a rate of 1℃ / min for 3 hours to obtain a starch-based hard carbon material.

[0056] Comparative Example 2

[0057] Corn starch was dried at 85℃ for 20 hours, and then passed through a 300-mesh sieve. The sieved starch was then oxidized in a muffle furnace. Specifically, the temperature was increased to 260℃ at a rate of 1℃ / min for 15 hours to obtain a precursor. The precursor was then carbonized at 1400℃ at a rate of 1℃ / min for 3 hours to obtain a starch-based hard carbon material.

[0058] Comparative Example 3

[0059] Corn starch was dried at 80℃ for 24 hours and then passed through a 300-mesh sieve. The sieved starch underwent a four-stage oxidation-non-annealing process in a muffle furnace. Specifically, the temperature was increased to 200℃ at a rate of 5℃ / min for 3 hours. After the first oxidation, the starch was heated to 230℃ at a rate of 2℃ / min for 3 hours. After the second oxidation, the starch was heated to 240℃ at a rate of 5℃ / min for 3 hours. After the third oxidation, the starch was heated to 260℃ at a rate of 5℃ / min for 6 hours. The precursor was obtained after the four-stage oxidation-non-annealing process. The precursor was then carbonized at 1400℃ at a rate of 2℃ / min for 3 hours to obtain a starch-based hard carbon material.

[0060] Sodium-ion button batteries were prepared using the starch-based hard carbon materials from the examples and comparative examples, respectively. The specific steps were as follows: A mixture of starch-based hard carbon material, sodium carboxymethyl cellulose, and carbon black in a mass ratio of 8:1:1 was coated onto copper foil to obtain an electrode sheet. This electrode sheet served as the working electrode, and the sodium sheet served as the counter electrode. The sodium-ion button batteries were then assembled in a glove box. Electrochemical performance tests were performed on the sodium-ion button batteries.

[0061] The charge and discharge values ​​of the sodium-ion coin cells prepared in the examples and comparative examples at a current density of 20 mA / g are shown in Table 1.

[0062] Table 1 shows the charge-discharge performance of the sodium-ion coin cells prepared in the examples and comparative examples at a current density of 20 mA / g.

[0063] sample Specific charging capacity (mAh / g) at a current density of 20mA / g First week Coulomb efficiency / % Example 1 410 88 Example 2 405 87 Example 3 389 86 Example 4 370 87 Example 5 380 89 Comparative Example 1 45 28 Comparative Example 2 325 80 Comparative Example 3 275 78

[0064] SEM image of the starch-based hard carbon material in Example 1 is shown below. Figure 1 and Figure 2 As shown, the SEM image of the starch-based hard carbon material in Comparative Example 3 is as follows. Figure 3 As shown. By Figure 1 It can be seen that the starch-based carbon spheres exhibit good monodispersity and maintain the natural spherical morphology of starch. In contrast, in the starch-based hard carbon material of Comparative Example 3, the original spherical structure of starch collapsed and broke, indicating that the gradient oxidation annealing technology has a significant advantage in maintaining the structure and morphology of starch-based hard carbon materials.

[0065] The charge-discharge curves of the sodium-ion button battery prepared from the starch-based hard carbon material in Example 1 at a current density of 20 mA / g are shown in the figure below. Figure 4 As shown, its capacity is as high as 300 mAh·g at below 0.1V. -1 Furthermore, the coulombic efficiency reached 88% in the first week. The charge-discharge curves of the sodium-ion button battery prepared from the starch-based hard carbon material in Comparative Example 1 at a current density of 20 mA / g are shown below. Figure 5 As shown, its low-voltage capacity is only 20 mAh·g -1 The capacity and first-week coulombic efficiency were only 35%, indicating that gradient annealing technology has a significant advantage in improving reversible capacity and first-week coulombic efficiency. The charge-discharge curves of the sodium-ion button battery prepared from the starch-based hard carbon material in Comparative Example 3 at a current density of 20 mA / g are shown in the figure. Figure 6 As shown, its low-voltage capacity is only 150 mAh·g. -1 The capacity and first-week coulomb efficiency were only 76%, indicating that the annealing process played a promoting role in increasing reversible capacity and first-week coulomb efficiency.

[0066] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a starch-based hard carbon material, characterized in that, It includes the following steps: 1) The starch is dried and sieved sequentially to obtain pretreated starch; 2) The pretreated starch was subjected to stepwise oxidation-annealing to obtain the precursor; 3) The precursor is carbonized to obtain starch-based hard carbon material; The number of step oxidation-annealing cycles is 3 to 10; after each oxidation, the temperature is annealed to room temperature, and the cooling rate during the annealing process is 0.5 to 20 °C / min; the oxidation temperature increases with the number of oxidation cycles. Step 3) The carbonization temperature is 1300~2600℃.

2. The preparation method according to claim 1, characterized in that, The starch includes corn starch, potato starch, rice starch, pea starch, or sorghum starch.

3. The preparation method according to claim 1 or 2, characterized in that, Step 1) The drying temperature is 50~120℃, and the drying time is 6~48h.

4. The preparation method according to claim 3, characterized in that, In step 2), the temperature for the step-by-step oxidation is 120~300℃, and the time for each oxidation is 1~12h.

5. The preparation method according to claim 4, characterized in that, During the step-by-step oxidation process, the heating rate is 0.5~5℃ / min.

6. The preparation method according to claim 1 or 5, characterized in that, The carbonization process in step 3) takes 1 to 8 hours.

7. The preparation method according to claim 6, characterized in that, During the carbonization process, the heating rate is 0.5~10℃ / min.

8. The starch-based hard carbon material prepared by the preparation method according to any one of claims 1 to 7.

9. The application of the starch-based hard carbon material according to claim 8 in sodium-ion batteries.

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