A method for reconstructing hard carbon structure and improving its disorder and sodium ion battery with ultra-long cycle life

Abstract

The present invention discloses a method for reconstructing the structure of hard carbon and increasing its disorder, as well as a sodium-ion battery with an ultra-long cycle life. Commercial hard carbon is dried and placed in a graphite mold. The temperature is raised to 400-800°C and then maintained for 1-10 minutes. During this process, the vacuum is maintained at 0.05-0.001 Pa, a voltage of 3-4 V, a current of ~500 A, and a pressure of 10-50 MPa are applied. After the instrument cools to room temperature, the mold is removed, the powder is collected, sieved, and stored, thereby obtaining post-treated hard carbon. The present invention reconstructs the structure of the hard carbon by subjecting it to a rapid post-treatment method involving multiple fields of vacuum, Joule heating, and pressure, transforming it from a thick, relatively ordered graphene layer to a vortex layer structure composed of thin, locally curved, and more disordered graphite-like domains. The method of the present invention increases the disorder and interlayer spacing of the hard carbon material, thereby obtaining a hard carbon negative electrode for sodium-ion batteries with excellent cycle performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of performance optimization of sodium ion battery negative electrode materials, and specifically relates to a method for reconstructing a hard carbon structure and improving its disorder, and a sodium ion battery with an ultra-long cycle life. Background Art

[0002] Among the numerous anode materials for sodium-ion batteries (SIBs), hard carbon is considered the most mature and most likely to be commercialized. However, hard carbon still has disadvantages such as uncontrollable structure, low first coulombic efficiency (ICE), weak rate performance, and insufficient long-cycle stability. In particular, in commercial ester electrolytes with relatively lower costs, defect-rich hard carbon tends to form an uneven, thick, mechanically weak, and easily fractured solid electrolyte (SEI) layer on its surface, resulting in its performance in ester electrolytes being much weaker than that of ether electrolytes. These factors have, to a certain extent, restricted its commercialization process. Hard carbon is an amorphous material composed of randomly constructed graphene layers and has no fixed structure. The ideal hard carbon is constructed from completely disordered and few-layer graphene. However, the hard carbon currently prepared (including commercial hard carbon) inevitably contains some relatively ordered graphene layers with a large number of layers, which is not conducive to the storage and transport of sodium ions and will increase the sodium ion diffusion barrier. Summary of the Invention

[0003] The present invention aims to provide a method for reconstructing the hard carbon structure and improving its disorder, and a sodium-ion battery with an ultra-long cycle life. The method utilizes multi-field sintering to perform Joule heating post-treatment on the hard carbon to reconstruct the hard carbon structure, thereby improving its cycle performance.

[0004] To achieve the above object, the technical solution adopted by the present invention is:

[0005] A method for reconstructing a hard carbon structure and increasing its disorder comprises the following steps:

[0006] After drying the commercial hard carbon, place it in a graphite mold and heat it at 100-120℃min -1 The temperature is raised to the target temperature (400-800°C) at a heating rate and then kept warm for 1-10 minutes. During this process, the vacuum degree is maintained at 0.05-0.001Pa, the applied voltage is 3-4V, the current is 500A, and the pressure is 10-50MPa. After the instrument cools to room temperature, the mold is taken out to collect the powder, which is sieved and stored to obtain a post-treated sodium ion battery hard carbon negative electrode with excellent cycle performance.

[0007] The above method is universally applicable to various hard carbons.

[0008] Through the above, the structure of hard carbon is reconstructed, and the ordered graphene layer inside the hard carbon is further disordered, thereby greatly improving the sodium storage performance of hard carbon, which can be used to prepare sodium-ion batteries with ultra-long cycle life.

[0009] Sodium ion battery assembly: Mix the treated hard carbon, CMC and SP in a mass ratio of 80-85:5-10:10-15 and grind them evenly on a 1.2 cm 2 The working electrode was a copper foil with metallic sodium as the counter electrode. The electrolytes were 1M NaPF₆ dissolved in Diglyme and 1M NaPF₆ dissolved in a 1:1:1 (volume ratio) EC:DMC:EMC solution. Cell assembly was performed in a glove box under argon protection (oxygen and moisture levels were both below 1 ppm).

[0010] This paper proposes a rapid post-treatment method for hard carbon using multiple fields, including vacuum, Joule heating, and pressure, to restructure the hard carbon, transforming it from a thick, relatively ordered graphene layer to a vortex structure composed of thin, locally curved, and more disordered graphite-like domains. Molecular dynamics simulations and differential charge density distributions indicate that the electric field effectively enhances the interaction between ketones and carbonyl groups and the graphene layer, causing the graphene layer to twist and expand, thereby increasing the disorder and interlayer spacing of the hard carbon material. This is a versatile and efficient strategy for modifying various hard carbons within minutes. The optimized hard carbon negative electrode has excellent electrochemical performance and good electrolyte compatibility. Specifically, in ether-based electrolyte, at a high current density of 10C, the original hard carbon only worked for 280 cycles, while the reconstructed hard carbon can achieve an ultra-long cycle of 30,000 cycles, and its capacity retention rate is as high as 86.9%; in ester-based electrolyte, the capacity retention rate of different hard carbons after the same number of cycles can be significantly improved by 34-40%, and the reversible capacity and ICE are improved to varying degrees. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 Shown are the first-principles molecular dynamics simulation (left) and differential charge density distribution (right) of the hard carbon sample before (HC) and after (HC-P) treatment by the method of the present invention.

[0012] Figure 2 Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) images of HC and HC-P.

[0013] Figure 3 Electrochemical performances of HC and HC-P electrodes in ether-based electrolytes.

[0014] Figure 4 Electrochemical performances of HC and HC-P electrodes in ester-based electrolytes.

[0015] Figure 5 The electrochemical properties of four electrodes, SHC, SHC-P, KHC, and KHC-P, in ester-based electrolytes. DETAILED DESCRIPTION

[0016] Example 1

[0017] A method for reconstructing a hard carbon structure and increasing its disorder comprises the following steps:

[0018] (1) Commercial hard carbon (ATEC, Japan) was dried and placed in a graphite mold at a heating rate of 110 °C min -1 After reaching the target temperature (600℃), keep it warm for 5 minutes. During this process, maintain the vacuum degree at 0.05Pa, the applied voltage at 3.5V, the current at ~500A, and the pressure at 30MPa. After the instrument cools to room temperature, take out the mold to collect the powder, sieve it and store it to obtain a post-treated sodium ion battery hard carbon negative electrode with excellent cycle performance, which is named HC-P.

[0019] Example 2

[0020] The ATEC hard carbon in Example 1 was replaced with starch-based hard carbon, and the same operation process as in Example 1 was adopted. The sample was named SHC-P. The hard carbon before treatment was named SHC.

[0021] Example 3

[0022] The ATEC hard carbon in Example 1 was replaced with Kuraray hard carbon from Japan, and the same operation process as in Example 1 was adopted. The samples were named KHC-P. The hard carbon before treatment was named KHC.

[0023] Application Example 1

[0024] Application of the treated hard carbon of Examples 1-3 in sodium ion batteries

[0025] Sodium ion battery assembly: Mix hard carbon, CMC, and SP in a mass ratio of 80:10:10 and grind them evenly on a 1.2 cm 2 A copper foil was used as the working electrode, sodium metal was used as the counter electrode, and the electrolytes were 1M NaPF₆ dissolved in Diglyme and 1M NaPF₆ dissolved in a 1:1:1 (volume ratio) solution. Cell assembly was performed in a glove box under argon protection (oxygen and moisture levels were both below 1 ppm).

[0026] Figure 1Shown are the first-principles molecular dynamics simulation (left) and differential charge density distribution (right) of the hard carbon sample in Example 1 before (HC) and after (HC-P) treatment by the method of the present invention. The results show that under multi-field heating conditions, the parallel stacked graphene layers undergo a large deformation after 6 fs of conduction. In addition, theoretical calculations show that the interlayer spacing increases from 0.385 nm in the initial configuration to 0.399 nm in the final state. Considering the weak thermal field effect, the influence of the external electric field on the structural change is crucial. The electric field can cause polar effects on the entire molecular layer, especially polar ketones and carbonyl groups, in which the oxygen atoms are affected by the polarity of the electric field, which may cause the displacement of the electron cloud, thereby causing the deformation of the entire molecular layer. The differential charge density distribution calculation of C=O and C-OH groups under the action of the electric field verified this conjecture. The electric field strengthens the interaction between ketones, carbonyls and graphene layers, and promotes the distortion of the molecular structure to adapt to the new charge distribution.

[0027] Figure 2 The scanning electron microscope (SEM) and high-resolution transmission electron microscope (HRTEM) images of HC and HC-P. SEM (a, b) show that, unlike the distinct structure of HC particles, the carbon spheres in HC-P all show a mutually bonded morphology, similar to "cooked glutinous rice balls", and the fragmented particles are melted into one with the spheres again. HRTEM (c, d) shows that HC shows a higher degree of order, mainly composed of thick and numerous short-range ordered graphite-like domains, while HC-P shows a vortex layer structure composed of long-range, thin and locally distorted more disordered graphite-like domains. In addition, the interlayer spacing (d 002 ) is 0.385nm, while the d 002 is 0.395nm, which is consistent with the results of the previous theoretical calculations. This more curved and larger interlayer structure is more conducive to the Na + of embedding and disengagement.

[0028] Table 1 Structure and composition analysis of HC and HC-P

[0029]

[0030] Table 1 shows the structure and composition analysis of HC and HC-P. 002 The value is consistent with the HRTEM structure. Raman spectroscopy was used to further analyze the structure and defects of the sample. The D1 band (I D1 ) and G band (I G ) intensity ratio is usually used to examine the disorder degree of carbon materials. The results show that the I D1 / I GThe ratios are 1.12 and 1.49, respectively. The larger the ratio, the more disordered the hard carbon structure. Then, the surface chemical composition of the sample was determined by XPS, and it was found that the material was composed of C and O. After post-treatment, due to the action of thermal field and electric field, the oxygen functional groups decomposed, resulting in a decrease in oxygen content. Finally, the pore structure of the hard carbon material was studied using the N2 (77K) adsorption-desorption isotherm. Both samples showed a typical type IV isotherm, indicating that the carbon material has rich micropores and mesoporous structures. According to the BET model, the specific surface areas of HC and HC-P are 47.21 and 11.54m, respectively. 2 g -1 This indicates that after post-treatment, the specific surface area decreases due to micropore closure. A lower specific surface area can reduce the contact between the hard carbon material and the electrolyte, thereby reducing the formation of the SEI and helping to improve ICE. At the same time, the pore size distribution is mainly concentrated in the range of 2 to 8 nm. With post-treatment and increasing temperature, the open pore volume gradually decreases. This pore size evolution can be attributed to the reduction of functional groups and the stronger interaction between stacked graphite domains at higher sintering temperatures.

[0031] Figure 3 The electrochemical performance of HC and HC-P electrodes in ether-based electrolytes. Figure 3 As shown in a, the reversible capacity of HC is 309.1 mAh g -1 , ICE was 88.3%, while HC-P showed a higher reversible capacity (327.7 mAh g -1 ) and ICE (91.2%). At different current densities (0.1, 0.2, 0.5, 1, 2, 3, 5, 10C) ( Figure 3 b), the reversible capacities of HC are 309.1, 301.3, 294.3, 280.5, 232.4, 170.6, 80.4, and 57.5 mAh g -1 In comparison, HC-P exhibits relatively excellent rate performance, which are 327.7, 321.9, 314.8, 306.7, 294.3, 280.8, 253.0, and 157.5 mAh g -1 When the current density gradually increases from 0.1C to 2C, the capacity difference between the two begins to become significant. When it reaches 5C, the capacity of HC-P can reach 3.15 times that of HC. Equally important is the long-cycle performance. HC stops working after 280 cycles at 10C, while HC-P can still achieve a retention rate of 86.9% after 30,000 cycles, with a decay rate of only 0.0004% per cycle. Figure 3 c), is one of the materials with the best lifespan and rate capability among the current hard carbon negative electrodes.

[0032] Figure 4The electrochemical performance of HC and HC-P electrodes in ester-based electrolytes. At 0.1C, the reversible capacity and ICE of HC are 295.3 mAh g -1 and 84.7%, while the post-treated HC-P showed 326.9 mAh g -1 In terms of cycle stability, after 50 cycles at 0.2C, the capacity retention rate of HC is only 41.3%, while that of HC-P can reach 79.8%, which is nearly 38.5% higher than that of HC.

[0033] Figure 5 The electrochemical performance of four electrodes, SHC, SHC-P, KHC, and KHC-P, in an ester-based electrolyte is demonstrated to demonstrate the versatility of this method across various hard carbons. Both SHC-P and KHC-P exhibit improvements in reversible capacity and ICE. More notably, in terms of cycling stability, after 50 cycles at 0.2C, SHC-P exhibits a 39.5% increase in capacity retention compared to SHC, while KHC-P exhibits a 34.2% increase compared to KHC.

Claims

1. A method for reconstructing a hard carbon structure and increasing its disorder, characterized in that: The following steps are involved: After drying commercial hard carbon, place it in a graphite mold, heat it to 400 ~ 800 ° C and keep it warm for 1 ~ 10 minutes. During this process, maintain a vacuum degree of 0.05 ~ 0.001 Pa, apply a voltage of 3 ~ 4 V, a current of 500 A, and a pressure of 10 ~ 50 MPa. After the instrument cools to room temperature, remove the mold to collect the powder, sieve it, and store it to obtain the post-treated hard carbon.

2. The method for reconstructing a hard carbon structure and increasing its disorder according to claim 1, wherein: Heating rate: 100~120℃ min -1 .

3. A hard carbon obtained by the method according to claim 1 or 2.

4. Use of the hard carbon as claimed in claim 3 in sodium ion batteries.

5. The use according to claim 4, characterized in that The sodium ion battery assembly method is as follows: hard carbon, CMC, and SP are mixed and ground in a mass ratio of 80-85:5-10:10-15 and then evenly applied on a 1.2 cm 2 Copper foil was used as the working electrode, metallic sodium was used as the counter electrode, and the electrolytes were 1 mol / L NaPF6 dissolved in DIGLYME solution and 1 mol / L NaPF6 dissolved in EC:DMC:EMC=1:1:1 solution. The battery assembly was carried out in a glove box under argon protection, and the oxygen and moisture contents were both less than 1 ppm.

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