Polyvinyl chloride-derived hard carbon materials, their preparation methods, applications, and batteries

Abstract

The present invention discloses a polyvinyl chloride-derived hard carbon material, a preparation method, an application thereof, and a battery. The method includes: heating and melting PVC powder and a thermoplastic resin, and obtaining substance A after cooling; then performing a pre-dehalogenation reaction on substance A under the protection of an inert gas to obtain a pre-dehalogenated sample; the mass ratio of the thermoplastic resin to the PVC powder is 5%-50%, the temperature of the pre-dehalogenation is 250-500°C, and the time is 1-5 h; in a reaction tube, covering an inorganic halide salt on the pre-dehalogenated sample, evacuating and sealing the reaction tube, and then performing carbonization, washing, and drying; the mass ratio of the inorganic halide salt to the pre-dehalogenated sample is (1.2-4.5):1, the temperature of the carbonization is not lower than the melting point of the inorganic halide salt, and the time is 5-20 h. The hard carbon material has a larger specific surface area, a larger interlayer spacing, and a more developed ion / electron conduction network, and has better rate performance and long cycle stability when applied to a battery.

Description

Technical Field

[0001] The present invention specifically relates to a polyvinyl chloride-derived hard carbon material, a preparation method, an application thereof, and a battery. Background Art

[0002] Worldwide, concerns about energy and environmental problems caused by the combustion and shortage of fossil fuel resources are increasing day by day, which has stimulated the interest in exploring efficient and renewable energy sources. For this purpose, various advanced electrochemical energy storage and conversion devices have been developed to achieve sustainable development. Although the working principles of energy storage and conversion systems are different, seeking suitable electrode materials with ideal components and structures is a key prerequisite for developing high-performance devices. Among the developed electrode materials, carbon-based materials have attracted much attention due to their advantages such as high conductivity, good chemical stability, and stable structure. Although carbon-based electrode materials have broad prospects, traditional carbon materials are all prepared from fossil fuels as synthetic raw materials, which are harmful to the environment and expensive. Therefore, efforts are being made to develop suitable precursors with rich resources and limited environmental impacts. From the perspective of sustainable development, using plastic waste as a precursor for carbon synthesis is a good solution to turn waste into treasure.

[0003] As the most typical chlorinated plastic, PVC is a thermoplastic polymerized from vinyl chloride monomers by free radical polymerization. One of the most promising practical applications of upgrading PVC waste is to prepare high-value carbon-based electrode materials for energy storage, such as hard carbon. As the negative electrode material of alkali metal ion batteries, the internal structure of hard carbon consists of a vortex layer structure composed of distorted and bent carbon layers, with a large interlayer spacing. Thanks to its unique turbulent layer structure, hard carbon has the advantages of high theoretical capacity, strong rate capability, and excellent cycling performance. However, it should be noted that the solid-state diffusion kinetics of lithium ions in the highly disordered structure of hard carbon is relatively slow, which may lead to low conductivity and insufficient ion diffusion ability, thus limiting the actual capacity and rate performance, and showing a low first-cycle Coulomb efficiency. The inherent inert electrochemical surface and disordered structure of hard carbon may cause slow diffusion of lithium ions and electrons, resulting in low rate capacity and poor long-cycle stability. Therefore, in order to effectively improve the rate performance and long-cycle stability, it is of great significance to design a suitable hard carbon interface structure to improve ion transport ability and provide an efficient diffusion network. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the defects of poor rate performance and long-cycle stability of PVC-based hard carbon materials in the prior art, and provide a polyvinyl chloride-derived hard carbon material, a preparation method, an application thereof, and a battery. The hard carbon material prepared by the present invention has a larger specific surface area, a larger interlayer spacing, and a more developed ion / electron conduction network, and has excellent electrochemical performance when applied to a battery, especially has better rate performance and long-cycle stability.

[0005] In the present invention, by first heating and melting PVC powder and thermoplastic resin, followed by low-temperature pre-dehalogenation and inorganic salt-assisted carbonization, the internal defect degree of hard carbon is effectively regulated, and a hard carbon material with a larger specific surface area, a larger interlayer spacing, a more developed ion / electron conduction network, and excellent electrochemical properties is prepared. Low-temperature pre-halogenation realizes partial dehalogenation of PVC. The rich active carbon chains on the partially dehalogenated polyvinyl chloride polymer and the active carbon chains of the thermoplastic resin aggregate, thus promoting the formation of subsequent highly conductive carbon. During the subsequent high-temperature carbon layer rearrangement assisted by inorganic halide salts, the HCl gas flow generated by the continuous dechlorination of the pre-dehalogenated PVC precursor impacts the internal structure, forming a dense internal disordered turbulent layer structure and closed pores; the presence of inorganic halide salt molten salt induces surface carbon layer rearrangement, forming an ordered graphene carbon layer. Inorganic halide salt crystals act as a graphene growth template and a capping agent, guiding carbon atoms to aggregate at grain boundaries and hindering the escape of HCl. Vertically oriented graphene layers grow with these carbon atoms as nucleation centers.

[0006] The present invention solves the above technical problems through the following technical solutions:

[0007] The present invention provides a method for preparing a polyvinyl chloride-derived hard carbon material, which includes the following steps:

[0008] (1) Heating and melting PVC powder and thermoplastic resin, and then cooling to obtain substance A; then carrying out a pre-dehalogenation reaction on the substance A under the protection of an inert gas to obtain a pre-dehalogenated sample; wherein, the thermoplastic resin is polypropylene and / or polyethylene, the mass ratio of the thermoplastic resin to the PVC powder is 5%-50%, the temperature of the pre-dehalogenation reaction is 250-500°C, and the time of the pre-dehalogenation reaction is 1-5 h;

[0009] (2) In a reaction tube, covering the pre-dehalogenated sample with an inorganic halide salt, evacuating and sealing the reaction tube, and then carrying out carbonization, washing and drying to obtain the polyvinyl chloride-derived hard carbon material; the mass ratio of the inorganic halide salt to the pre-dehalogenated sample is (1.2-4.5):1, the temperature of the carbonization is not lower than the melting point of the inorganic halide salt, and the time of the carbonization is 5-20 h.

[0010] In step (1), the K value of the viscosity number of the PVC powder can be 50-80, preferably 55-70.

[0011] In step (1), the melt index of the polypropylene can be 10-40 g / 10 min, for example 35 g / 10 min.

[0012] In step (1), the melt index of the polyethylene can be 10-40 g / 10 min, for example 25 g / 10 min.

[0013] In step (1), the mass ratio of the thermoplastic resin to the PVC powder is preferably 5%-20%, such as 10%.

[0014] In step (1), the heating and melting are generally carried out in a heating furnace. The heating furnace can be conventional in the art, such as a tube furnace. The heating and melting are generally carried out under an inert atmosphere (such as argon).

[0015] In step (1), the temperature of the heating and melting can be 100-140°C, such as 120°C.

[0016] In step (1), the time of the heating and melting can be 1-4 h, such as 2 h.

[0017] In step (1), the type of the inert gas can be conventional in the art, such as argon.

[0018] In step (1), the flow rate of the inert gas introduced can be 100-500 sccm, preferably 200-400 sccm, such as 300 sccm.

[0019] In step (1), the pre-dehalogenation reaction is generally carried out in a tube furnace.

[0020] In step (1), the temperature of the pre-dehalogenation reaction is preferably 300-400°C, such as 360°C.

[0021] In step (1), the time of the pre-dehalogenation reaction is preferably 1-3 h, such as 2 h.

[0022] In step (1), the rate of raising the temperature to the temperature of the pre-dehalogenation reaction is preferably 2-10°C / min, such as 5°C / min.

[0023] In step (1), after the pre-dehalogenation reaction, it generally further includes a process of naturally cooling to room temperature.

[0024] In step (1), after the pre-dehalogenation reaction, it is preferably necessary to wash and dry the pre-dehalogenation sample.

[0025] Among them, the solvent used for the washing can be conventional in the art, such as water and / or absolute ethanol. When the solvent used for the washing is a mixed solution of water and absolute ethanol, the volume ratio of water to absolute ethanol is preferably (1-5):1, such as 3:1.

[0026] In step (2), the reaction tube can be conventional in the art, such as a quartz glass test tube.

[0027] In step (2), after the evacuation, the vacuum degree of the reaction tube is preferably 10 -2Below mbar.

[0028] In step (2), the melting point of the inorganic halide salt is preferably not lower than 700 °C.

[0029] In step (2), the cations of the inorganic halide salt preferably include one or more of alkali metals, alkaline earth metals, and transition metals, more preferably alkali metals.

[0030] Among them, the alkali metals preferably include Na and / or K.

[0031] Among them, the alkaline earth metals preferably include one or more of Mg, Ca, Sr, and Ba.

[0032] Among them, the transition metals preferably include one or more of Fe, Co, Ni, Cu, and Zn.

[0033] In step (2), the anions of the inorganic halide salt preferably include one or more of Cl ions, Br ions, and I ions, more preferably Cl ions or Br ions.

[0034] In step (2), the inorganic halide salt is preferably one or more of NaCl, KCl, and NaBr.

[0035] In step (2), the mass ratio of the inorganic halide salt to the pre-dehalogenated sample is preferably (1.5 - 3.5):1, such as 2:1, 2.5:1, or 3.5:1.

[0036] In step (2), the carbonization temperature is preferably above the melting point of the inorganic halide salt by 10 °C and below 200 °C.

[0037] In step (2), the carbonization time is preferably 5 - 15 h, such as 10 h.

[0038] When the inorganic salt is NaCl, the carbonization temperature is preferably 810 - 1000 °C, such as 820 °C, 850 °C, 900 °C, or 950 °C.

[0039] When the inorganic salt is NaBr, the carbonization temperature is preferably 780 - 900 °C, such as 800 °C.

[0040] When the inorganic salt is KCl, the carbonization temperature is preferably 800 - 1000 °C, such as 820 °C.

[0041] In step (2), the rate of heating up to the carbonization temperature is preferably 2 - 10 °C / min, such as 5 °C / min.

[0042] In step (2), the solvent used for washing can be conventional in the art, such as water and / or absolute ethanol. When the solvent used for washing is a mixed solution of water and absolute ethanol, the volume ratio of water to absolute ethanol is preferably 1:(1 - 5), such as 1:3.

[0043] In step (2), it is preferably followed by grinding after drying.

[0044] The present invention also provides a polyvinyl chloride-derived hard carbon material prepared by the preparation method as described above.

[0045] In the present invention, the outer part of the polyvinyl chloride-derived hard carbon material is preferably coated with a continuous few-layer graphene carbon layer. The few-layer graphene-like carbon layer generally refers to a material composed of 2 - 10 layers of graphene nanosheets. The number of layers of the few-layer graphene carbon layer is preferably less than 5.

[0046] In the present invention, the graphite layer spacing of the polyvinyl chloride-derived hard carbon material can be 0.352 - 0.40 nm, preferably 0.352 - 0.38 nm, such as 0.354 nm, 0.358 nm or 0.361 nm. The graphite layer spacing generally refers to the layer spacing d calculated according to the XRD test results through Bragg's law. 002 。

[0047] In the present invention, the pore size distribution range of the polyvinyl chloride-derived hard carbon material can be 7.5 - 15 nm, preferably 8 - 12 nm, such as 8.2 nm, 9 nm or 10.5 nm.

[0048] In the present invention, the specific surface area of the polyvinyl chloride-derived hard carbon material can be 4.8 - 15 m 2 / g, preferably 5 - 10 m 2 / g, such as 5.8 m 2 / g, 6.5 m 2 / g or 7.5 m 2 / g.

[0049] In the present invention, the I D / I G is preferably 0.94 - 1.05, such as 0.948, 0.965 or 0.991, more preferably 0.955 - 0.98. The I D / I G refers to the intensity ratio of the D peak and the G peak.

[0050] The present invention also provides an application of the polyvinyl chloride-derived hard carbon material as described above in a battery.

[0051] The present invention also provides a battery, which includes the polyvinyl chloride-derived hard carbon material as described above.

[0052] In the present invention, the battery is preferably a lithium-ion battery or a potassium-ion battery.

[0053] Based on the common general knowledge in the art, the above preferred conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention.

[0054] The reagents and raw materials used in the present invention are all commercially available.

[0055] The positive and progressive effects of the present invention are as follows:

[0056] (1) The hard carbon material prepared by the present invention has a larger specific surface area, a larger interlayer spacing and a more developed ion / electron conduction network, and exhibits excellent rate performance and long cycle stability as the negative electrode material of lithium-ion batteries and potassium-ion batteries, especially having better rate performance and long cycle stability;

[0057] (2) The carbon yield of the hard carbon material prepared by the present invention is as high as 26%-33%;

[0058] (2) The preparation method of the present invention is simple, low in cost and conducive to industrial production;

[0059] (3) The present invention provides a new solution for the upgrading and utilization of waste halogen plastics, promotes the expansion of the electrode material design of lithium-ion batteries and potassium-ion batteries to the waste plastic resource library, improves the resource utilization efficiency and realizes sustainable development. Description of the Drawings

[0060] Figure 1 Part (a) is the HRTEM image of the hard carbon material prepared in Example 1; Figure 1 Part (b) is the FFT image of the hard carbon material prepared in Example 1. Detailed Embodiments

[0061] The present invention will be further illustrated by the following examples, but the present invention is not limited to the scope of the described examples. The experimental methods without specific conditions noted in the following examples are carried out according to conventional methods and conditions, or selected according to the product specifications.

[0062] The raw material information used in the following examples and comparative examples is shown in Table 1:

[0063] Table 1

[0064]

[0065] Example 1

[0066] (1) Pre-dehalogenation:

[0067] 1 g of PVC and 0.1 g of polypropylene were heated and melted in a heating furnace (under an argon atmosphere) at a heating temperature of 120 °C for 2 h, and substance A was obtained after cooling;

[0068] Substance A was loaded into a quartz crucible and placed in a tube furnace for pre-dehalogenation. It was heated to 360 °C under a gas flow of 300 sccm argon and held for 120 minutes, with a heating rate of 5 °C / min. -1 After naturally cooling to room temperature, the obtained block was ground into powder, washed thoroughly with a mixed liquid of deionized water and absolute ethanol with a volume ratio of 3:1, and dried to obtain the prepared pre-dehalogenated sample;

[0069] (2) Carbonization: After weighing an appropriate amount of the pre-dehalogenated sample powder and loading it into a quartz glass test tube, solid NaCl was weighed and directly covered on the pre-dehalogenated sample powder. Under a vacuum condition (the vacuum condition is below 10 -2 mbar), the upper end of the quartz tube was melted and sealed with a hydrogen-oxygen flame for vacuum sealing. Among them, the mass ratio of NaCl to the pre-dehalogenated sample was 2:1; Subsequently, the quartz tube was placed in a muffle furnace and carbonized at 820 °C for 10 h with a heating rate of 5 °C / min. After the carbonization reaction ended, it was cooled to room temperature and taken out. After breaking the quartz glass tube, the obtained material was washed with a mixed solution of water and ethanol (volume ratio 1:3) to remove the NaCl flux, dried, and the obtained material was ground into powder and collected to obtain the polyvinyl chloride-derived hard carbon material.

[0070] Example 2

[0071] Compared with Example 1, except that the mass ratio of NaCl to the pre-dehalogenated sample in step (2) was adjusted to 3:1, the rest of the operations and conditions were the same as those in Example 1.

[0072] Example 3

[0073] Compared with Example 1, except that the carbonization temperature in step (2) was adjusted to 900 °C, the rest of the operations and conditions were the same as those in Example 1.

[0074] Example 4

[0075] Compared with Example 1, except that NaCl in step (2) was replaced by sodium bromide and the carbonization temperature was adjusted to 800 °C, the rest of the operations and conditions were the same as those in Example 1.

[0076] Example 5

[0077] Compared with Example 1, except that NaCl in step (2) was replaced by potassium chloride, the rest of the operations and conditions were the same as those in Example 1.

[0078] Example 6

[0079] Compared with Example 1, except that the polypropylene in step (1) was replaced with polyethylene, the remaining operations and conditions were the same as those in Example 1.

[0080] Comparative Example 1

[0081] Commercial hard carbon BHC-550 (purchased from Chengdu Baisige Technology Co., Ltd.).

[0082] Comparative Example 2

[0083] Compared with Example 1, except that NaCl was not added in step (2), the remaining operations and conditions were the same as those in Example 1.

[0084] Comparative Example 3 (directly carbonizing PVC)

[0085] Weigh an appropriate amount of PVC powder and put it into a quartz tube. After evacuating and sealing, place the quartz tube in a muffle furnace and carbonize it at 820 °C for 10 h with a heating rate of 5 °C / min. After the carbonization reaction is completed, cool it to room temperature and take it out. After breaking the quartz glass tube, wash the obtained material with a mixed liquid of deionized water and absolute ethanol with a volume ratio of 3:1, dry it, grind the obtained material into powder and collect it to obtain the hard carbon material.

[0086] Comparative Example 4

[0087] Compared with Example 1, except that the mass ratio of NaCl to the pre-dehalogenated sample in step (2) was adjusted to 5:1, the remaining operations and conditions were the same as those in Example 1.

[0088] Comparative Example 5

[0089] Compared with Example 1, except that the carbonization temperature in step (2) was adjusted to 700 °C, the remaining operations and conditions were the same as those in Example 1.

[0090] Comparative Example 6

[0091] Compared with Example 1, except that polypropylene was not added, the remaining operations and conditions were the same as those in Example 1.

[0092] Effect Example

[0093] (1) Morphology Characterization

[0094] The prepared polyvinyl chloride-derived hard carbon material and the pre-dehalogenated sample were subjected to morphology characterization. According to the SEM electron microscope observation results, the pre-dehalogenated sample was a loose random agglomerated particle with a fluffy surface and no special morphology, and after NaCl-assisted high-temperature carbonization, hard carbon blocky particles with clear edges were formed. Through the HRTEN diagram and FFT diagram ( Figure 1)It was further observed that the prepared hard carbon material is externally coated with a continuous few-layer (n < 5) graphene carbon layer, and the internal structure is a composite hard carbon structure with a disordered turbulent structure.

[0095] (2) XRD characterization

[0096] According to the XRD test results, two broad characteristic peaks appear near 25° and 42° in the hard carbon materials prepared in Example 1, Example 3, and Comparative Example 5, corresponding to the diffraction of the (002) and (100) crystal planes in a typical disordered amorphous carbon structure, confirming that the hard carbon material is a typical disordered hard carbon. As the carbonization temperature increases, the peak corresponding to the (002) crystal plane near 25° further broadens, and the peak position shifts to the left, and the peak intensity gradually weakens, indicating an increase in the degree of structural disorder and an increase in the interlayer spacing of the in-plane curved carbon layers. The interlayer spacing d was calculated by Bragg's law 002 increased from 0.351 nm in Comparative Example 5 to 0.354 nm in Example 1 and then to 0.361 nm in Example 3, while the interlayer spacing of Comparative Example 1 was only 0.347 nm. The increase in the interlayer spacing also proves that during the high-temperature carbonization process assisted by NaCl, the generated HCl cannot overflow in time, and the HCl gas enclosed inside the NaCl molten salt continuously impacts the internal structure of the hard carbon material during carbonization, promoting the generation of more dense and disordered structures inside, providing a driving force for the expansion of the interlayer spacing.

[0097] In addition, according to the XRD test results of Examples 1-2 and Comparative Example 4, as the amount of NaCl added increases, the (002) characteristic peak intensity near 25° in the hard carbon material becomes stronger, and the peak width narrows, but the peak position does not shift. According to the test results in Table 2, for Examples 1-2 and Comparative Example 4, as the amount of NaCl fed increases, the I D / I G value first decreases and then increases, and Comparative Example 4 shows the largest I D / I G value, which also proves from the side that as the amount of NaCl sealing agent increases, its sealing effect is better, making it more difficult for the generated HCl gas to escape during the pyrolysis process of the pre-dehalogenated sample, and the destructive effect on the internal structure is stronger. Therefore, the degree of disorder of the disordered turbulent structure inside Comparative Example 4 is higher.

[0098] (3) Raman, nitrogen adsorption-desorption characterization test, FT-IR spectral test

[0099] The hard carbon materials prepared in the examples and comparative examples, as well as the pre-dehalogenated sample prepared in Example 1, were subjected to Raman, nitrogen adsorption-desorption characterization tests, and FT-IR spectral tests according to the conventional operations in the art.

[0100] According to the Raman test results, the ID / IG peak intensity ratios of the pre-dehalogenated samples prepared in Example 1, Comparative Example 1, Comparative Example 5, Example 1, and Example 3 are 1.067, 1.048, 0.910, 0.965, and 0.991, respectively. The intensity ratio of the D peak and the G peak (ID / IG) can be used to understand the degree of defects in the material. The lower the ratio, the lower the defects. This indicates that with the introduction of the NaCl flux, the degree of defects is significantly reduced compared to the commercial hard carbon BHC550. In addition, as the carbonization temperature increases, the HCl gas generated during the carbonization of the pre-dehalogenated sample has a greater destructive effect on the structure, so the degree of defects in the hard carbon material at the corresponding temperature is higher and the internal structure is more disordered, which is consistent with the XRD results.

[0101] Table 2 Structural parameters of the materials

[0102]

[0103] In addition, the HCl gas generated during the carbonization of the pre-dehalogenated sample also has a greater pore-forming effect on the inside of the hard carbon material. According to the test results in Table 2, as the carbonization temperature increases, the specific surface area and pore size of the hard carbon material increase. These results show that under vacuum conditions, as the temperature increases, the escape rate of the gas generated by the dechlorination reaction is faster, and the impact of the gas on the internal structure is stronger, making the hard carbon material have more microstructures. Therefore, the degree of disorder increases and the pore structure develops, which is beneficial for using the hard carbon material as an energy storage material in the future, providing more active sites for ion storage and increasing the specific capacity of the material.

[0104] According to the FT-IR spectrum, it can be seen that there are C-Cl stretching vibration peaks between 600 - 640 cm -1 in the PVC sample, H-CCl deformation vibration peaks between 1180 - 1350 cm -1 and Cl-C- deformation vibration peaks between 2906 - 2967 cm -1 and a series of characteristic peaks of chlorohydrocarbons. After pre-dehalogenation, the C-Cl peak near 600 - 640 cm -1 and the Cl-C- peak near 2906 - 2967 cm -1 are significantly weakened. At the same time, the H-CCl deformation vibration peak between 1180 - 1350 cm -1 is transformed into the deformation vibration peak of -CH 2 . At the same time, the emergence of a new C=C bending vibration peak near ~1670 cm -1 is observed. The changes in these peak positions all indicate that after the low-temperature pre-dehalogenation reaction, chlorine has been partially removed, and the sp 2Intermediates exist in the molecular chains of the pre-dehalogenated samples. After subsequent high-temperature carbonization, all the characteristic peaks of chlorohydrocarbons and C═C peaks in the hard carbon materials obtained in Example 1 disappear, indicating the completion of the dehalogenation reaction. At the same time, due to the growth and rearrangement of the carbon layer, under the impact of the HCl gas flow, the C═C olefin intermediates are difficult to maintain, forming a disordered turbulent layer structure. In addition, from the Raman test results of PVC, the pre-dehalogenated samples, and the hard carbon materials, it is found that after the pre-dehalogenation, the pre-dehalogenated samples have already produced some sp 2 hybridized carbon intermediates, showing D peaks and G peaks. By calculating the intensity ratio of the D peak to the G peak of the pre-dehalogenated sample precursor, it is 1.067, which is higher than 0.965 of the hard carbon material. This shows that through the subsequent carbonization process, through the removal of Cl and the rearrangement of the carbon layer, the defects in the hard carbon are significantly reduced. The removal of the Cl-related characteristic peaks in the hard carbon materials and the optimization of the surface carbon defect level further confirm the formation of the unique structure of the outer-layer graphene carbon layer and the inner-layer turbulent disordered carbon.

[0105] (4) The carbon yield of the few-layer graphene-coated polyvinyl chloride-derived hard carbon material prepared by the two-step method is as high as 26% - 33%, which is much higher than the carbon yields of other reported polyvinyl chloride-derived carbons. Therefore, from the perspectives of environment and economy, converting polyvinyl chloride waste plastics into high-value carbon-based materials by the two-step pyrolysis method not only promotes resource recovery but also provides the possibility for sustainable development.

[0106] (5) Electrochemical performance test of lithium-ion batteries

[0107] The prepared samples were electrochemically measured by assembling CR-2016 type coin cells. The working electrode sheet was prepared by coating the mixed slurry on a copper foil current collector, then drying it overnight in a vacuum oven at 85 °C, and then cutting it into a circular sheet with a diameter of 1 cm × 1 cm. The slurry was composed of active material (the prepared hard carbon material), super conductive carbon black (SuperP), and binder PVDF dispersed in NMP and configured in a weight ratio of 8:1:1. The coin cells were fabricated in an argon-filled glove box, using a lithium metal sheet as the negative electrode, a single layer of polypropylene (PP) as the battery separator, and the KB046 electrolyte was 1 mol L -1 LiClO 4 dissolved in a mixed solution of EC / DEC (volume ratio 1:1), supplemented with 5 wt% vinylene carbonate (FEC) additive. The charge / discharge tests were carried out on a LAND CT2001A battery test system at a temperature of 20 °C. The voltage range for the rate performance and long-term cycling performance was 0.001 to 2.5 V vs. Li / Li + (1C = 250 mAh g -1 ).

[0108] Table 3 First-cycle charge and discharge data of the hard carbon materials obtained under constant current charge and discharge at 0.5C

[0109]

[0110] According to the results in Table 3, by comparing Examples 1-2 with Comparative Example 4, it can be seen that when the mass ratio of NaCl to the pre-dehalogenated sample is not within the range of (1.2-4.5):1, the specific capacity and the first-cycle Coulombic efficiency of the prepared hard carbon materials are relatively low; by comparing Example 1, Example 3 with Comparative Example 5, it can be seen that when the carbonization temperature is lower than the melting point of the salt, both the initial reversible specific capacity and the first-cycle Coulombic efficiency of the prepared hard carbon materials are relatively low.

[0111] Set the rate test procedure to 0.5C, 1C, 2C, 5C, 10C, 20C, 30C, 40C, 50C and 60C (1C = 250 mAh g -1 ), after a complete rate test, the current returns to 0.5C again. The reversible capacities of the hard carbon material negative electrode prepared in Example 1 at 0.5C, 1C, 2C, 5C, 10C, 20C, 30C, 40C, 50C and 60C currents are 396.0, 362.7, 335.3, 297.1, 267.8, 229.9, 208.5, 191.5, 176.3 and 163.4 mAh g -1 respectively. When the current suddenly returns to 0.5C, the reversible capacity of the hard carbon material prepared in Example 1 quickly recovers to 392.1 mAh g -1 , showing good cycle reversibility. The ratio of the reversible capacity of the hard carbon material prepared in Example 1 at 5C to that at 0.1C is 75%, which is much greater than 60%, indicating that the hard carbon material prepared in Example 1 still has excellent rate performance when evaluated from an industrial level. In contrast, the corresponding reversible capacities of Comparative Example 1, Comparative Example 2, Comparative Example 3, Comparative Example 5 and Example 3 at 60C are 30.7, 31.0, 31.1, 33.8 and 34.5 mAh g -1 respectively.

[0112] From the first-cycle charge and discharge behaviors of Comparative Example 1 and Examples 2-3, it can be found that: using the same precursor carbon source, introducing NaCl molten salt as a graphene template and a capping agent is indispensable for the formation of the unique structure of the hard carbon material. Although the PVC-derived pyrolytic carbon removes Cl heteroatoms through high-temperature treatment to reduce the defect degree of the carbon material and prepare highly conductive carbon, therefore, Examples 1 and 2-3 all have a first-cycle Coulombic efficiency significantly superior to that of the commercial hard carbon BHC550 in Comparative Example 1. However, due to the lack of the surface carbon layer rearrangement process assisted by NaCl, the solid-state diffusion process inside the materials in Comparative Examples 2-3 is restricted, so they show limited rate capacity.

[0113] Table 4 Long cycle stability of Example 1 and Comparative Example 1

[0114]

[0115] According to the results in Table 4, regardless of the different current densities, the hard carbon materials prepared in Example 1 showed the highest capacity retention rate. Benefiting from its unique structure, the hard carbon material electrode has the best electrochemical performance: the few-layer graphene carbon layer promotes the rapid transport of ions / electrons, significantly improving the reaction kinetics; the disordered turbulent structure is beneficial to maintaining the structure of the hard carbon material electrode during the electrochemical cycling process, reducing the fragmentation of the electrode. Under the action of NaCl-assisted graphitization, the conductivity of the hard carbon material is improved, and the pore structure is developed, which is conducive to the repeated insertion / extraction of lithium ions. Therefore, from the environmental and economic perspectives, the development of high-value and functional carbon-based materials is realized by preparing few-layer graphene-coated polyvinyl chloride-derived hard carbon through NaCl-assisted stepwise pyrolysis, providing a new idea for the upgrading and utilization of polyvinyl chloride waste plastics.

[0116] (6) Electrochemical performance test of potassium ion batteries

[0117] The prepared samples were electrochemically measured by assembling CR-2016 coin cells. The working electrode sheet was made by coating the mixed slurry on a copper foil current collector, then drying it overnight at 85 °C in a vacuum oven, and then cutting it into a circular sheet with a diameter of 1 cm × 1 cm. The slurry was composed of active material (prepared hard carbon material), super conductive carbon black (SuperP), and binder PVDF dispersed in NMP in a weight ratio of 8:1:1. The coin cells were fabricated in an argon-filled glove box, using potassium metal sheet as the negative electrode, polypropylene (PP) monolayer as the battery separator, and LB046 electrolyte was 1 mol L -1 KPF 6 dissolved in a mixed solution of EC / DEC (volume ratio 1:1), supplemented with 5 wt% fluoroethylene carbonate (FEC) additive. The charge / discharge tests were carried out on a LAND CT2001A battery test system at a temperature of 20 °C. The voltage range for rate performance and long-term cycling performance was 0.001 to 2.5 V vs. K / K + (1C = 250 mAh g -1 )

[0118] By assembling K / K + half-cells, the potassium ion storage performance of the hard carbon material prepared in Example 1 was studied, which showed 312.2 / 478.6 mAh g in the first charge / discharge at 0.5C -1The capacity of the first stage is 65.2%, while that of Comparative Example 1's commercial BHC550 is 265.8 / 373.2 mAh g -1 The capacity of the first stage is 71.2%. The reversible capacities of Example 1 at 0.5C, 1C, 2C, 5C, and 10C are 333.4, 294.1, 256.4, 205.3, and 168.2 mAh g respectively -1 . Even at a high current density of 20C, the specific capacity of Example 1 can still be maintained at 128.7 mAh g -1 , maintaining 76.3% of the reversible capacity at a 10C current density. The specific capacity of BHC550 and the graphite electrode of Comparative Example 1 at 20C is 67.2 mAh g -1 . When the current density is suddenly reset back to 0.5C, the specific capacities of Example 1 and BHC550 of Comparative Example 1 recover to 297.4 and 203.3 mAh g respectively -1 . The excellent high-rate performance of potassium ions in Example 1 can be attributed to the fact that the few-layer graphene layers in the hard carbon material facilitate the storage and transportation of potassium ions during the potassium intercalation / deintercalation process. The hard carbon material maintains a high reversible capacity of 234.5 mAh g after cycling 100 times at a 1C current -1 . After cycling 300 times, the capacity retention rate is 74.27%. The capacity retention rate of Example 1 after 800 cycles at 5C is 68%, and the average capacity decay rate per cycle is 0.04%. Therefore, when using NaCl-assisted preparation of ordered graphene-coated polyvinyl chloride-derived hard carbon as the anode of a potassium-ion battery, it can have both high capacity, high-rate performance, and good cycle stability.

[0119] Although the specific embodiments of the present invention have been described above, those skilled in the art should understand that this is only for illustration. The protection scope of the present invention is defined by the appended claims. Without departing from the principles and essence of the present invention, those skilled in the art can make various changes or modifications to these embodiments, but these changes and modifications all fall within the protection scope of the present invention.

Claims

1. A method for preparing a polyvinyl chloride derived hard carbon material, characterized in that: It includes the following steps: (1) PVC powder and thermoplastic resin are heated and melted, and then cooled to obtain substance A; then the substance A is subjected to a pre-dehalogenation reaction under the protection of an inert gas to obtain a pre-dehalogenated sample; wherein the thermoplastic resin is polypropylene and / or polyethylene, the mass ratio of the thermoplastic resin to the PVC powder is 5%-50%, the temperature of the pre-dehalogenation reaction is 250-500° C., and the time of the pre-dehalogenation reaction is 1-5 hours; (2) In a reaction tube, an inorganic halogen salt is covered on the pre-dehalogenated sample, and the reaction tube is evacuated and sealed, followed by carbonization, washing and drying to obtain the polyvinyl chloride-derived hard carbon material; the mass ratio of the inorganic halogen salt to the pre-dehalogenated sample is (1.2-4.5):1, the carbonization temperature is not lower than the melting point of the inorganic halogen salt, and the carbonization time is 5-20h.

2. The method for preparing a polyvinyl chloride-derived hard carbon material according to claim 1, wherein: Step (1) satisfies one or more of the following conditions: (1) The heating and melting temperature is 100-140° C.; (2) The heating and melting time is 1-4 hours; (3) The temperature of the pre-dehalogenation reaction is 300-400° C.; (4) The time of the pre-dehalogenation reaction is 1-3h.

3. The method for preparing a polyvinyl chloride-derived hard carbon material according to claim 1, characterized in that: The preparation method meets one or more of the following conditions: (1) The viscosity number K value of the PVC powder is 50-80; (2) The melt index of the polypropylene is 10-40 g / 10 min; (3) The melt index of the polyethylene is 10-40 g / 10 min; (4) The mass ratio of the thermoplastic resin to the PVC powder is 5%-20%.

4. The method for preparing a polyvinyl chloride derived hard carbon material according to claim 1, characterized in that: The inorganic halogen salt satisfies one or more of the following conditions: (1) The melting point of the inorganic halogen salt is not less than 700°C; (2) The cation of the inorganic halide salt includes one or more of alkali metals, alkaline earth metals and transition metals; (3) the anions of the inorganic halide salt include one or more of Cl ions, Br ions and I ions; (4) The mass ratio of the inorganic halogen salt to the pre-dehalogenated sample is (1.5-3.5):

1.

5. The method for preparing a polyvinyl chloride-derived hard carbon material according to claim 4, characterized in that: The alkali metal includes Na and / or K.

6. The method for preparing a polyvinyl chloride-derived hard carbon material according to claim 4, characterized in that: The alkaline earth metal includes one or more of Mg, Ca, Sr and Ba.

7. The method for preparing a polyvinyl chloride derived hard carbon material according to claim 4, characterized in that: The transition metal includes one or more of Fe, Co, Ni, Cu and Zn.

8. The method for preparing a polyvinyl chloride-derived hard carbon material according to claim 4, characterized in that: The anion of the inorganic halogen salt is a Cl ion or a Br ion.

9. The method for preparing a polyvinyl chloride-derived hard carbon material according to claim 4, characterized in that: The inorganic halogen salt is one or more of NaCl, KCl and NaBr.

10. The method for preparing a polyvinyl chloride derived hard carbon material according to claim 1, characterized in that: In step (2), the carbonization temperature is 10° C. or higher and 200° C. or lower than the melting point of the inorganic halogen salt; And / or, the carbonization time is 5-15h.

11. The method for preparing a polyvinyl chloride-derived hard carbon material according to claim 10, characterized in that: When the inorganic halogen salt is NaCl, the carbonization temperature is 810-1000°C.

12. The method for preparing a polyvinyl chloride derived hard carbon material according to claim 10, characterized in that: When the inorganic halogen salt is NaBr, the carbonization temperature is preferably 780-900°C.

13. The method for preparing a polyvinyl chloride derived hard carbon material according to claim 10, characterized in that: When the inorganic halogen salt is KCl, the carbonization temperature is preferably 800-1000°C.

14. A polyvinyl chloride derived hard carbon material, characterized in that: The hard carbon material is prepared according to the method for preparing a polyvinyl chloride-derived hard carbon material according to any one of claims 1 to 13.

15. The polyvinyl chloride derived hard carbon material according to claim 14, characterized in that The exterior of the polyvinyl chloride derived hard carbon material is coated with a continuous few-layer graphene carbon layer.

16. The polyvinyl chloride derived hard carbon material according to claim 14, characterized in that The polyvinyl chloride derived hard carbon material satisfies one or more of the following conditions: (1) The graphite interlayer spacing of the polyvinyl chloride derived hard carbon material is 0.352-0.40 nm; (2) The pore size distribution range of the polyvinyl chloride derived hard carbon material is 7.5-15 nm; (3) The specific surface area of ​​the polyvinyl chloride derived hard carbon material is 4.8-15 m 2 / g; (4) I of the polyvinyl chloride-derived hard carbon material D / I G It is 0.94-1.

05.

17. Use of the polyvinyl chloride derived hard carbon material according to any one of claims 14 to 16 in a battery.

18. A battery, characterized in that: It comprises the polyvinyl chloride derived hard carbon material as claimed in any one of claims 14 to 16.

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