CN116504971A - Hard carbon material, pole piece and electrochemical device - Google Patents

Abstract

The application relates to the technical field of electrochemical energy storage, in particular to a hard carbon material, a pole piece and an electrochemical device. The hard carbon material comprises a micropore structure, wherein micropores account for more than or equal to 65% of the total pore volume. The invention improves the specific capacity of the hard carbon cathode by improving the micropore content in the hard carbon material structure, especially the micropore content with the aperture of 0.5-1.8nm, thereby improving the energy density of the electrochemical device.

Description

Hard carbon materials, pole pieces and electrochemical devices

Technical Field

The present application relates to the field of electrochemical energy storage technology, and in particular to hard carbon materials, pole pieces and electrochemical devices.

Background Art

The rapid development of society has not only intensified the consumption of fossil fuels, but also caused serious environmental pollution. Therefore, it is particularly important to develop high-performance electrochemical energy storage devices and effectively utilize intermittent renewable energy (wind energy, tidal energy, solar energy, etc.). Although lithium-ion batteries have been widely used in many fields due to their high energy density, wide voltage range, no memory effect, green environmental protection and wide operating temperature range, their high cost and limited lithium resources are not conducive to their large-scale application in electric vehicles and static energy storage. In contrast, sodium resources are abundant in the earth's crust, and sodium-ion batteries have a similar working mechanism to lithium-ion batteries, so they are expected to become low-cost secondary batteries for large-scale applications in the future. However, unlike lithium ions, the larger radius of sodium ions means that graphite, the negative electrode material of commercial lithium-ion batteries, cannot be directly used as the negative electrode of sodium-ion batteries. Therefore, the development of high-performance negative electrode materials is conducive to promoting the commercial application of sodium-ion batteries.

Among many negative electrode materials, hard carbon has attracted widespread attention due to its good comprehensive performance. Although its sodium storage performance has been effectively improved after years of efforts, the energy density is still low and needs further improvement. Generally speaking, there are three types of pores in the structure of hard carbon: macropores (>50nm), mesopores (2-50nm) and micropores (<2nm). Since micropores can serve as active sites to store sodium or lithium ions, increasing their content can significantly increase the specific capacity of hard carbon negative electrodes, thereby increasing the energy density of electrochemical devices.

Summary of the invention

In view of this, the present invention provides a hard carbon material, a pole piece and an electrochemical device. The present invention improves the specific capacity of the hard carbon negative electrode by increasing the micropore content in the hard carbon material structure, thereby improving the energy density of the electrochemical device.

In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

The invention provides a hard carbon material, which comprises a microporous structure, wherein the volume percentage of the micropores in the total pores is ≥65%.

Since micropores can serve as active sites to store sodium ions or lithium ions, the abundant micropores in the hard carbon material of the present invention can increase the specific capacity of the hard carbon, thereby increasing the energy density of the electrochemical device.

Preferably, the volume percentage of micropores to total pores is 65% to 90%.

Preferably, the volume percentage of micropores to total pores is 66% to 72%.

Preferably, the volume of the micropores is ≥ 0.0015 cm ³ /g. Within this range, the hard carbon has abundant sodium or lithium storage sites, which can ensure its high capacity, thereby improving the energy density of the electrochemical device.

Preferably, the volume of the micropores is 0.0015 to 0.018 cm ³ /g.

More preferably, the volume of the micropores is 0.006 to 0.01 cm ³ /g.

Preferably, the pore size of the micropores under nitrogen isothermal adsorption and desorption test is 0.5 to 1.8 nm. Within this range, the micropores can serve as active sites to store sodium ions or lithium ions, which helps to improve the energy density of the electrochemical device.

Preferably, the pore size of the micropores under nitrogen isothermal adsorption-desorption test is 0.5-1.0 nm.

More preferably, the pore size of the micropores under nitrogen isothermal adsorption-desorption test is 0.5-0.7 nm.

Preferably, the total pore volume is ≥ 0.0017 cm ³ /g.

Preferably, the total pore volume is 0.003 to 0.02 cm ³ /g.

Preferably, the total pore volume is 0.009 to 0.014 cm ³ /g.

Preferably, the Raman spectrum of the hard carbon material has a D band and a G band, and the ratio of the peak intensity _ID of the D band to the peak intensity _IG of the G band is _ID / _IG ≤ 1.1. Within this range, the hard carbon has high conductivity and moderate defects, which is conducive to improving its initial coulombic efficiency and rate performance.

Preferably, the diffraction peak of the hard carbon material on the (002) crystal plane is located at 2θ=21.3°~25.6°, and the interlayer spacing is 0.35~0.42nm. Within this range, it is possible to ensure that the hard carbon stores more sodium/lithium ions and facilitates the rapid insertion and extraction of sodium/lithium ions, thereby improving its capacity and rate performance.

Preferably, the half-peak width of the (002) crystal plane diffraction peak is 2.3-15°. Within this range, the hard carbon material has a moderate particle size and high conductivity, which is beneficial to improving its rate performance and sodium and/or lithium storage capacity.

Under the above XRD characteristics, the hard carbon has an amorphous structure, which allows the hard carbon to have not only a large interlayer spacing but also good conductivity, which is beneficial to improving its sodium/lithium storage specific capacity and rate performance.

In a specific embodiment of the present invention, the hard carbon material further comprises a mesoporous structure.

Preferably, the volume of the mesopores is ≤0.0045 cm ³ /g, and the volume percentage of the mesopores in the total pores is ≤35%.

Preferably, the volume of the mesopores is 0.001 to 0.0043 cm ³ /g, and the volume percentage of the mesopores to the total pores is 20% to 35%.

More preferably, the volume of the mesopores is 0.0028 to 0.004 cm ³ /g, and the volume percentage of the mesopores to the total pores is 28% to 34%.

Preferably, the pore size of the mesopores under nitrogen isothermal adsorption-desorption test is 2.0-3.0 nm.

Preferably, the pore size of the mesopores under nitrogen isothermal adsorption-desorption test is 2.2-2.8 nm.

More preferably, the pore size of the mesopores under nitrogen isothermal adsorption-desorption test is 2.4-2.6 nm.

Preferably, the specific surface area of the hard carbon material is 1 to 41 m ² /g. Within this range, the hard carbon material structure can be guaranteed to contain abundant micropores, thereby increasing its sodium/lithium storage active sites and facilitating the improvement of its specific capacity. At the same time, a larger specific surface area can improve the wettability of the electrolyte, thereby improving the kinetic performance of the electrochemical device.

Preferably, the specific surface area of the hard carbon material is 5 to 30 m ² /g.

Preferably, the total pore volume of the hard carbon material is V (cm ³ /g), the specific surface area of the hard carbon material is B (m ² /g), the micropore volume percentage is A%, the radius of the micropore is r (nm), and A, B, V, and r satisfy any of the following relationships:

or,

When the above formula is met, the hard carbon material containing micropores can improve its sodium or lithium storage capacity and the energy density of the battery.

Preferably, the particle size D50 of the hard carbon material is 1-11 μm. Within this range, the hard carbon has good sodium/lithium storage kinetics and processing properties.

Preferably, the particle size D50 of the hard carbon material is 4 to 6 μm.

More preferably, the particle size D50 of the hard carbon material is 4.5 to 5.6 μm.

Preferably, the particle size D90 of the hard carbon material is 1.8-15 μm, and D100 is 3-27 μm. Within this range, the hard carbon has good sodium/lithium storage kinetics and processing properties.

Preferably, the particle size D90 of the hard carbon material is 4.2 to 11.8 μm, and D100 is 4.8 to 18.5 μm.

The present invention also provides a method for preparing the hard carbon material, comprising the following steps:

Step (1): calcining a carbon source to obtain a hard carbon precursor;

Step (2): mixing a hard carbon precursor with an activator, and subjecting the resulting mixture to low-temperature carbonization treatment in an inert atmosphere to obtain a first carbonized product;

Step (3): removing impurities from the first carbonized product, and performing a medium-temperature carbonization treatment in an organic atmosphere and an inert atmosphere to obtain a second carbonized product;

Step (4): In an inert atmosphere, subjecting the second carbonization product to a high-temperature carbonization treatment to obtain a hard carbon material.

Preferably, the temperature of low-temperature carbonization is 300-800° C., the heating rate is 0.5-5° C./min, and the holding time is 1-3 h.

Preferably, the temperature of low-temperature carbonization is 700-800° C., the heating rate is 2-4° C./min, and the holding time is 1.5-2.5 h.

Preferably, the cooling rate of the low-temperature carbonization is 0.5 to 5°C/min, preferably 2 to 4°C/min.

Preferably, the activator includes but is not limited to one or more of potassium carbonate, potassium bicarbonate, sodium carbonate, sodium bicarbonate, potassium hydroxide, water vapor, carbon dioxide, phosphoric acid, and zinc hydroxide.

Preferably, the mass ratio of the hard carbon precursor to the activator is (2.5-12.5):1.

Preferably, the medium-temperature carbonization temperature is 500-1200° C., the heating rate is 3-10° C./min, and the holding time is 0.2-1.5 h.

Preferably, the medium-temperature carbonization temperature is 1000-1200° C., the heating rate is 6-8° C./min, and the holding time is 0.5-1.5 h.

In an embodiment of the present invention, the medium-temperature carbonization temperature is higher than the low-temperature carbonization temperature.

Preferably, the high-temperature carbonization temperature is 1100-2100° C., the heating rate is 0.2-1.2° C./min, and the holding time is 2-5 h.

Preferably, the temperature of high-temperature carbonization is 1300-1500° C., the heating rate is 0.5-1.2° C./min, and the holding time is 2-4 h.

Preferably, the inert atmosphere includes at least one of nitrogen, helium, argon, xenon and radon.

In an embodiment of the present invention, in step (3), the substance generating the organic atmosphere is a volatile solvent, or a mixture of a volatile solvent and a non-volatile solvent.

In an embodiment of the present invention, the volatile solvent includes but is not limited to at least one of ethanol, methanol and acetone.

In an embodiment of the present invention, the non-volatile solvent includes but is not limited to at least one of ethylene glycol, benzyl alcohol, dodecane, diformate, glycerol, ethyl benzoate, and N-methylpyrrolidone.

Preferably, in step (1), the calcination temperature is 300-500° C., and the calcination time is 0.5-5 h.

Preferably, the calcination temperature is 350-450° C., and the calcination time is 0.5-1.5 h.

In an embodiment of the present invention, in step (1), the carbon source includes but is not limited to one or more of glucose, epoxy resin, sucrose, starch, phenolic resin, polyvinyl pyrrolidone, walnut shell, coconut shell, mangosteen shell, almond shell, pecan shell, corn stalk, polyfurfuryl alcohol resin, polyacrylonitrile, coal, polyvinyl alcohol, polyvinyl chloride, peanut shell, cotton, buckwheat, lotus pod, asphalt, etc.

In an embodiment of the present invention, in step (3), the method for removing impurities in the first carbonized product comprises: washing the first carbonized product with an acidic solution, then washing with water until neutral, and then freeze-drying.

In an embodiment of the present invention, the solute of the acidic solution includes but is not limited to at least one of hydrochloric acid, sulfuric acid, nitric acid or acetic acid.

The present invention also provides a pole piece, which comprises the hard carbon material mentioned above. The hard carbon material is used as a negative electrode active material.

In a specific embodiment of the present invention, the above-mentioned electrode sheet is a negative electrode sheet.

In the embodiment provided by the present invention, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer coated on at least one side of the negative electrode current collector, and the negative electrode active material layer includes a negative electrode active material, a conductive agent and a binder. The negative electrode active material is the hard carbon material provided by the present invention, or a mixture of the hard carbon material provided by the present invention and other negative electrode active materials.

Preferably, the mass ratio of the negative electrode active material, the conductive agent, and the binder in the negative electrode active material layer is (70-100):(0.5-15):(0.5-15).

In a specific embodiment of the present invention, the mass ratio of the negative electrode active material, the conductive agent, and the binder is (70-90):(5-15):(5-15).

Preferably, the conductive agent is selected from one or more of conductive carbon black, carbon fiber, Ketjen black, acetylene black, carbon nanotubes, and graphene.

Preferably, the binder is selected from one or more of styrene-butadiene rubber, polyvinylidene fluoride, polyacrylic acid, polytetrafluoroethylene, and polyethylene oxide.

The present invention also provides an electrochemical device, which comprises the hard carbon material and/or pole piece.

Preferably, the electrochemical device is a sodium ion battery or a lithium ion battery.

In a specific embodiment provided by the present invention, the electrochemical device is a sodium ion battery.

In one embodiment of the present invention, the electrochemical device includes a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte.

In another embodiment of the present invention, an electrochemical device includes a counter electrode, a negative electrode sheet, a separator and an electrolyte.

In a specific embodiment provided by the present invention, the electrochemical device is a sodium ion battery, and the counter electrode is a metal sodium sheet.

In the embodiment provided by the present invention, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer coated on at least one side of the positive electrode current collector, and the positive electrode active material layer includes a positive electrode active material, a conductive agent and a binder.

Preferably, the mass ratio of the positive electrode active material, the conductive agent, and the binder in the positive electrode active material layer is (70-100):(0.5-15):(0.5-15).

In a specific embodiment of the present invention, the mass ratio of the positive electrode active material, the conductive agent, and the binder is (70-90):(5-15):(5-15).

Preferably, the positive electrode active material includes at least one of sodium transition metal oxides, polyanionic compounds, and Prussian blue compounds.

In the embodiments provided by the present invention, in the sodium transition metal oxide, the transition metal may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. The sodium transition metal oxide is, for example, Na _x MO ₂ , wherein M is one or more of Ti, V, Mn, Co, Ni, Fe, Cr, and Cu.

In the embodiments provided by the present invention, the polyanionic compound may be a compound having sodium ions, transition metal ions, and tetrahedral (YO ₄ ) ^n- anion units. The transition metal may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y may be at least one of P, S, and Si; and n represents the valence state of (YO ₄ ) ^n- .

In the embodiments provided by the present invention, the polyanionic compound may also be a compound having sodium ions, transition metal ions, tetrahedral (YO ₄ ) ^n- anion units, and halogen anions. The transition metal may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y may be at least one of P, S, and Si, and n represents the valence state of (YO ₄ ) ^n- ; and the halogen may be at least one of F, Cl, and Br.

In the embodiments provided by the present invention, the polyanionic compound may also be a compound having sodium ions, tetrahedral (YO ₄ ) ^n- anion units, polyhedral units (ZO _y ) ^m+ , and optional halogen anions. Y may be at least one of P, S, and Si, and n represents the valence state of (YO ₄ ) ^n- ; Z represents a transition metal, and may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce, and m represents the valence state of (ZO _y ) ^m+ ; and the halogen may be at least one of F, Cl, and Br.

In the embodiments provided by the present invention, the polyanionic compound is at least one of NaFePO ₄ , Na ₃ V ₂ (PO ₄ ) ₃ , NaM′PO ₄ F (M′ is one or more of V, Fe, Mn and Ni) and Na ₃ (VO _y ) ₂ (PO ₄ ) ₂ F _3-2y (0≤y≤1).

In the embodiments provided by the present invention, the Prussian blue compound may be a compound having sodium ions, transition metal ions and cyanide ions (CN ^- ). The transition metal may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce. The Prussian blue compound is, for example, Na _a Me _b Me' _c (CN) ₆ , wherein Me and Me' are each independently at least one of Ni, Cu, Fe, Mn, Co and Zn, 0<a≤2, 0<b<1, 0<c<1.

Compared with the prior art, the present invention has the following beneficial effects:

The hard carbon material of the present invention has abundant micropores, especially micropores with pore diameters between 0.5-1.8 nm, the volume of micropores is ≥0.005 cm ³ /g, and the volume percentage of micropores in total pores is ≥65%. Since micropores can be used as active sites to store sodium ions or lithium ions, the present invention improves the specific capacity of the hard carbon negative electrode by increasing the micropore content in the hard carbon material structure, especially the micropore content with pore diameters between 0.5-1.8 nm, thereby improving the energy density of the electrochemical device.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a pore size distribution diagram of hard carbon in Example 1; Pore Width represents pore width, and Pore Volume represents pore volume;

FIG2 is an XRD diagram of hard carbon of Example 1; Intensity represents peak intensity, and 2Theta represents 2θ;

FIG3 is a SEM image of hard carbon of Example 1;

Fig. 4 is a particle size distribution diagram of hard carbon of Example 1;

FIG5 is a diagram showing the cycle performance of the hard carbon negative electrode of Example 1.

DETAILED DESCRIPTION

The present invention discloses hard carbon materials, pole pieces and electrochemical devices. Those skilled in the art can refer to the content of this article and appropriately improve the process parameters to achieve the desired results. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in the present invention. The methods and applications of the present invention have been described through preferred embodiments, and relevant personnel can obviously modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit and scope of the present invention to implement and apply the technology of the present invention.

Terminology explanation:

Micropores refer to pores with a pore diameter of less than 2 nm. The radius of a micropore herein refers to half of the micropore diameter.

Mesopores refer to pores with a pore diameter between 2 and 50 nm.

Macropores refer to pores with a pore diameter greater than 50 nm.

Total pores refer to the collection of micropores, mesopores and macropores.

Detection method:

The particle size test was measured by Mastersize 3000 (Malvern 3000) using the laser method;

The pore size test and specific surface area test were performed using the NOVA Touch BET tester, and were measured using the ASAP 2460 nitrogen adsorption method;

Raman spectroscopy testing was performed using a Thermo Fisher Raman spectrometer;

XRD test was carried out using Shimadzu XRD-6100 X-ray diffractometer, the sample weight was 0.5 g/ ^cm2 , Cu Kα line was used as incident X-ray, the working voltage of X-ray source was 40 kV, the test power was 2 kW, 2θ was used as the horizontal coordinate and the unit was °, the signal intensity was used as the vertical coordinate, the test range was 10-80°, the scanning rate was 4°/min, and the data point interval was 0.02°.

The reagents, instruments or materials used in the present invention can be obtained through commercial channels.

The present invention will be further described below in conjunction with embodiments:

Example 1

(1) Weighing an appropriate amount of crushed mangosteen shells, placing them in a muffle furnace, calcining them at 410° C. for 1 h, then adding 1 g of the obtained black powder to 28.6 mL of a 3.5 mg/mL potassium bicarbonate aqueous solution, and after mixing evenly, rapidly freezing them with liquid nitrogen and freeze-drying them to remove the solvent water;

(2) the mixture obtained in step (1) was transferred to a tube furnace and carbonized at 750° C. for 2 h under nitrogen protection, and then cooled to room temperature. The heating and cooling rates of the process were both 3° C./min;

(3) washing the product obtained in step (2) several times with hydrochloric acid (5 mol/L), then washing it with deionized water until it is neutral, and then freeze-drying it;

(4) The dried product of step (3) is placed in a tube furnace, and a porcelain boat containing a mixture of ethanol and ethylene glycol is placed near the air inlet, and then argon is introduced and the temperature is raised to 1100°C at a rate of 7°C/min and kept at that temperature for 1 hour. After completion, the temperature is further raised to 1400°C at a rate of 0.5°C/min and kept at that temperature for 3 hours, and then cooled to room temperature to obtain the target sample.

As shown in Figure 1, the hard carbon material structure mainly contains micropores and mesopores, the pore size of the micropores is concentrated between 0.5-0.7nm, the pore size of the mesopores is mainly concentrated between 2.4-2.6nm, and the content of micropores is higher than that of mesopores. It can be seen that the hard carbon structure obtained in Example 1 contains abundant micropores, the pore volume and volume percentage are as high as ^0.01cm3 /g and 73% respectively, and the pore size is about 0.7nm.

The XRD diagram of FIG2 shows that the diffraction peak of the (002) crystal plane of the hard carbon is located at 2θ=23°, indicating that its interlayer spacing is 0.386nm, and the half-peak width of the (002) crystal plane diffraction peak is 6.5°. In addition, the wider (002) crystal plane diffraction peak indicates its amorphous structure. It can be seen that the hard carbon obtained in Example 1 is an amorphous structure and has a larger interlayer spacing (0.386nm), indicating that it can store more sodium ions or lithium ions and has excellent rate performance.

The SEM image in Figure 3 shows that the hard carbon has a block-like structure.

Figure 4 shows that the hard carbon particle size is D50 = 4.8 μm. Its smaller particle size is beneficial to improving the rate performance and processing performance during the tableting process.

The specific characteristic parameters of the hard carbon in this example are shown in Table 1.

Example 2

The difference from Example 1 is the heating rate of high temperature carbonization. The specific preparation method is as follows:

(1) Weighing an appropriate amount of crushed mangosteen shells, placing them in a muffle furnace, calcining them at 410° C. for 1 h, then adding 1 g of the obtained black powder to 28.6 mL of a 3.5 mg/mL potassium bicarbonate aqueous solution, and after mixing evenly, rapidly freezing them with liquid nitrogen and freeze-drying them to remove the solvent water;

(2) the mixture obtained in step (1) was transferred to a tube furnace and carbonized at 750° C. for 2 h under nitrogen protection, and then cooled to room temperature. The heating and cooling rates of the process were both 3° C./min;

(3) washing the product obtained in step (2) several times with hydrochloric acid (5 mol/L), then washing it with deionized water until it is neutral, and then freeze-drying it;

(4) The dried product of step (3) is placed in a tube furnace, and a porcelain boat containing a mixture of ethanol and ethylene glycol is placed near the air inlet, and then argon is introduced and the temperature is raised to 1100°C at a rate of 7°C/min and kept at that temperature for 1 hour. After completion, the temperature is further raised to 1400°C at a rate of 1°C/min and kept at that temperature for 3 hours, and then cooled to room temperature to obtain the target sample.

The specific characteristic parameters of the hard carbon in this example are shown in Table 1.

Example 3

The difference from Example 1 is the heating rate of high temperature carbonization. The specific preparation method is as follows:

(1) Weighing an appropriate amount of crushed mangosteen shells, placing them in a muffle furnace, calcining them at 410° C. for 1 h, then adding 1 g of the obtained black powder to 28.6 mL of a 3.5 mg/mL potassium bicarbonate aqueous solution, and after mixing evenly, rapidly freezing them with liquid nitrogen and freeze-drying them to remove the solvent water;

(2) the mixture obtained in step (1) was transferred to a tube furnace and carbonized at 750° C. for 2 h under nitrogen protection, and then cooled to room temperature. The heating and cooling rates of the process were both 3° C./min;

(3) washing the product obtained in step (2) several times with hydrochloric acid (5 mol/L), then washing it with deionized water until it is neutral, and then freeze-drying it;

(4) The dried product of step (3) is placed in a tube furnace, and a porcelain boat containing a mixture of ethanol and ethylene glycol is placed near the air inlet, and then argon is introduced and the temperature is raised to 1100°C at a rate of 7°C/min and kept at that temperature for 1 hour. After completion, the temperature is further raised to 1400°C at a rate of 1.2°C/min and kept at that temperature for 3 hours, and then cooled to room temperature to obtain the target sample.

The specific characteristic parameters of the hard carbon in this example are shown in Table 1.

Example 4

The difference from Example 3 is the type of hard carbon precursor. The specific preparation method is as follows:

(1) Weighing an appropriate amount of phenolic resin, placing it in a muffle furnace, calcining it at 410° C. for 1 h, then adding 1 g of the obtained black powder to 28.6 mL of a 3.5 mg/mL potassium bicarbonate aqueous solution, and after mixing evenly, rapidly freezing it with liquid nitrogen and freeze-drying it to remove the solvent water;

(2) the mixture obtained in step (1) was transferred to a tube furnace and carbonized at 750° C. for 2 h under nitrogen protection, and then cooled to room temperature. The heating and cooling rates of the process were both 3° C./min;

(3) washing the product obtained in step (2) several times with hydrochloric acid (5 mol/L), then washing it with deionized water until it is neutral, and then freeze-drying it;

(4) The dried product of step (3) is placed in a tube furnace, and a porcelain boat containing a mixture of ethanol and ethylene glycol is placed near the air inlet, and then argon is introduced and the temperature is raised to 1100°C at a rate of 7°C/min and kept at that temperature for 1 hour. After completion, the temperature is further raised to 1400°C at a rate of 1.2°C/min and kept at that temperature for 3 hours, and then cooled to room temperature to obtain the target sample.

The specific characteristic parameters of the hard carbon in this example are shown in Table 1.

Example 5

The difference from Example 3 is the type of hard carbon precursor and the high-temperature carbonization temperature. The specific preparation method is as follows:

(1) Weigh an appropriate amount of sucrose and place it in a muffle furnace. Calcined at 410° C. for 1 h, then added 1 g of the obtained black powder to 28.6 mL of a 3.5 mg/mL potassium bicarbonate aqueous solution. After mixing evenly, the mixture was quickly frozen with liquid nitrogen and freeze-dried to remove the solvent water.

(2) the mixture obtained in step (1) was transferred to a tube furnace and carbonized at 750° C. for 2 h under nitrogen protection, and then cooled to room temperature. The heating and cooling rates of the process were both 3° C./min;

(3) washing the product obtained in step (2) several times with hydrochloric acid (5 mol/L), then washing it with deionized water until it is neutral, and then freeze-drying it;

(4) The dried product of step (3) is placed in a tube furnace, and a porcelain boat containing a mixture of ethanol and ethylene glycol is placed near the air inlet, and then argon is introduced and the temperature is raised to 1100°C at a rate of 7°C/min and kept at that temperature for 1 hour. After completion, the temperature is further raised to 1300°C at a rate of 1.2°C/min and kept at that temperature for 3 hours, and then cooled to room temperature to obtain the target sample.

The specific characteristic parameters of the hard carbon in this example are shown in Table 1.

Example 6

The difference from Example 5 is the temperature and heating rate of medium-temperature carbonization and high-temperature carbonization. The specific preparation method is as follows:

(1) Weigh an appropriate amount of sucrose and place it in a muffle furnace. Calcined at 410° C. for 1 h, then added 1 g of the obtained black powder to 28.6 mL of a 3.5 mg/mL potassium bicarbonate aqueous solution. After mixing evenly, the mixture was quickly frozen with liquid nitrogen and freeze-dried to remove the solvent water.

(2) the mixture obtained in step (1) was transferred to a tube furnace and carbonized at 750° C. for 2 h under nitrogen protection, and then cooled to room temperature. The heating and cooling rates of the process were both 3° C./min;

(3) washing the product obtained in step (2) several times with hydrochloric acid (5 mol/L), then washing it with deionized water until it is neutral, and then freeze-drying it;

(4) The dried product of step (3) is placed in a tube furnace, and a porcelain boat containing a mixture of ethanol and ethylene glycol is placed near the air inlet, and then argon is introduced and the temperature is raised to 1000°C at a rate of 5°C/min and kept at that temperature for 1 hour. After completion, the temperature is further raised to 1500°C at a rate of 1°C/min and kept at that temperature for 3 hours, and then cooled to room temperature to obtain the target sample.

The specific characteristic parameters of the hard carbon in this example are shown in Table 1.

Comparative Example 1

The difference from Example 1 is the heating rate of high-temperature carbonization. The medium-temperature carbonization in step (4) does not include an organic atmosphere. The specific preparation method is as follows:

(1) Weighing an appropriate amount of crushed mangosteen shells, placing them in a muffle furnace, calcining them at 410° C. for 1 h, then adding 1 g of the obtained black powder to 28.6 mL of a 3.5 mg/mL potassium bicarbonate aqueous solution, and after mixing evenly, rapidly freezing them with liquid nitrogen and freeze-drying them to remove the solvent water;

(2) the mixture obtained in step (1) was transferred to a tube furnace and carbonized at 750° C. for 2 h under nitrogen protection, and then cooled to room temperature. The heating and cooling rates of the process were both 3° C./min;

(3) washing the product obtained in step (2) several times with hydrochloric acid (5 mol/L), then washing it with deionized water until it is neutral, and then freeze-drying it;

(4) The dried product of step (3) was placed in a tube furnace, argon was introduced, and the temperature was raised to 1100°C at a rate of 7°C/min and kept at that temperature for 1 hour. After completion, the temperature was further raised to 1400°C at a rate of 3°C/min and kept at that temperature for 3 hours, and then cooled to room temperature to obtain the target sample.

The specific characteristic parameters of the hard carbon of this comparative example are shown in Table 1.

Comparative Example 2

The difference from Example 1 is that no activator is added in step (1). The specific preparation method is as follows:

(1) Weigh an appropriate amount of crushed mangosteen shells and place them in a muffle furnace and calcine them at 410°C for 1 h;

(2) 1 g of the black powder obtained in step (1) was transferred to a tube furnace and carbonized at 750° C. for 2 h under nitrogen protection, and then cooled to room temperature. The heating and cooling rates of the process were both 3° C./min;

(3) washing the product obtained in step (2) several times with hydrochloric acid (5 mol/L), then washing it with deionized water until it is neutral, and then freeze-drying it;

(4) The dried product of step (3) is placed in a tube furnace, and a porcelain boat containing a mixture of ethanol and ethylene glycol is placed near the air inlet, and then argon is introduced and the temperature is raised to 1100°C at a rate of 7°C/min and kept at that temperature for 1 hour. After completion, the temperature is further raised to 1400°C at a rate of 0.5°C/min and kept at that temperature for 3 hours, and then cooled to room temperature to obtain the target sample.

The specific characteristic parameters of the hard carbon of this comparative example are shown in Table 1.

Comparative Example 3

The difference from Example 1 is that the medium-temperature carbonization step is not included, and the heating rate of the high-temperature carbonization is different. The specific preparation method is as follows:

(1) Weighing an appropriate amount of crushed mangosteen shells, placing them in a muffle furnace, calcining them at 410° C. for 1 h, then adding 1 g of the obtained black powder to 28.6 mL of a 3.5 mg/mL potassium bicarbonate aqueous solution, and after mixing evenly, rapidly freezing them with liquid nitrogen and freeze-drying them to remove the solvent water;

(2) the mixture obtained in step (1) was transferred to a tube furnace and carbonized at 750° C. for 2 h under nitrogen protection, and then cooled to room temperature. The heating and cooling rates of the process were both 3° C./min;

(3) washing the product obtained in step (2) several times with hydrochloric acid (5 mol/L), then washing it with deionized water until it is neutral, and then freeze-drying it;

(4) The dried product of step (3) is placed in a tube furnace, and a porcelain boat containing a mixture of ethanol and ethylene glycol is placed near the air inlet. Argon gas is then introduced and the temperature is increased to 1400°C at a rate of 7°C/min and maintained for 3 hours. After cooling to room temperature, the target sample is obtained.

The specific characteristic parameters of the hard carbon of this comparative example are shown in Table 1.

Comparative Example 4

The difference from Example 1 is that the medium-temperature carbonization step is not included. The specific preparation method is as follows:

(1) Weighing an appropriate amount of crushed mangosteen shells, placing them in a muffle furnace, calcining them at 410° C. for 1 h, then adding 1 g of the obtained black powder to 28.6 mL of a 3.5 mg/mL potassium bicarbonate aqueous solution, and after mixing evenly, rapidly freezing them with liquid nitrogen and freeze-drying them to remove the solvent water;

(2) the mixture obtained in step (1) was transferred to a tube furnace and carbonized at 750° C. for 2 h under nitrogen protection, and then cooled to room temperature. The heating and cooling rates of the process were both 3° C./min;

(3) washing the product obtained in step (2) several times with hydrochloric acid (5 mol/L), then washing it with deionized water until it is neutral, and then freeze-drying it;

(4) The dried product of step (3) is placed in a tube furnace, and a porcelain boat containing a mixture of ethanol and ethylene glycol is placed near the air inlet. Argon gas is then introduced and the temperature is increased to 1400°C at a rate of 0.5°C/min and maintained for 3 hours. After cooling to room temperature, the target sample is obtained.

The specific characteristic parameters of the hard carbon of this comparative example are shown in Table 1.

Comparative Example 5

The difference from Example 1 is that no activator is added in step (1), and the medium-temperature carbonization in step (4) does not include an organic atmosphere. The specific preparation method is as follows:

(1) Weigh an appropriate amount of crushed mangosteen shell and place it in a muffle furnace and calcine it at 410°C for 1 h;

(2) 1 g of the black powder obtained in step (1) was transferred to a tube furnace and carbonized at 750° C. for 2 h under nitrogen protection, and then cooled to room temperature. The heating and cooling rates of the process were both 3° C./min;

(3) washing the product obtained in step (2) several times with hydrochloric acid (5 mol/L), then washing it with deionized water until it is neutral, and then freeze-drying it;

(4) The dried product of step (3) was placed in a tube furnace, argon was introduced, and the temperature was raised to 1100°C at a rate of 7°C/min and kept at that temperature for 1 hour. After completion, the temperature was further raised to 1400°C at a rate of 0.5°C/min and kept at that temperature for 3 hours, and then cooled to room temperature to obtain the target sample.

The specific characteristic parameters of the hard carbon of this comparative example are shown in Table 1.

Assembly of sodium-ion batteries

The hard carbon material obtained in step (4) of the above embodiment and comparative example was premixed with conductive carbon black and polyvinylidene fluoride at a mass ratio of 8:1:1, and then an appropriate amount of N-methylpyrrolidone was added and mixed evenly, and then coated on an aluminum foil, and transferred to a vacuum drying oven for baking for 12 hours (120°C). After completion, the punched pole piece, the metal sodium sheet and the glass fiber separator were assembled into a button battery, and the solvent of the electrolyte used was a mixture of ethylene carbonate and diethyl carbonate (volume ratio of 1:1), the solute was 1M NaClO ₄ , and the additive was 5% fluoroethylene carbonate.

Sodium storage performance test

The sodium ion batteries assembled in Examples 1 to 6 and Comparative Examples 1 to 5 were placed on a Land battery tester for electrochemical performance testing at a voltage range of 0 to 2 V. The current density of constant current charge and discharge was 25 mA/g, and the current density of rate performance testing was 50 mA/g, 100 mA/g, 200 mA/g, and 400 mA/g.

Table 1 Micropore percentage and particle size of hard carbon

Table 2 Sodium storage performance of hard carbon

As shown in FIG1 , the structure of the hard carbon obtained in Example 1 contains abundant micropores, and the pore volume and volume percentage thereof are as high as 0.01 cm ³ /g and 73%, respectively, and the pore diameter is about 0.7 nm. Since the smaller micropores can be used as active sites to store sodium ions and/or lithium ions, the hard carbon negative electrode exhibits a high specific capacity: the specific capacity after 60 cycles at a current density of 25 mA/g is 478.3 mAh/g ( FIG5 ), indicating that after 60 cycles of the hard carbon negative electrode, the specific capacity still has a charge capacity of 478.3 mAh/g at a current density of 25 mA/g, and the capacity is basically not attenuated.

The specific capacities at current densities of 50 mA/g, 100 mA/g, 200 mA/g and 400 mA/g are 463 mAh/g, 442.2 mAh/g, 412.1 mAh/g and 367.2 mAh/g, respectively (Table 2).

At the same time, it can be found from Table 1 that the construction of micropore-rich hard carbon requires the simultaneous experience of low-temperature carbonization (activator required), medium-temperature carbonization and high-temperature carbonization. The higher heating rate of medium-temperature carbonization and the lower heating rate and carbonization temperature of high-temperature carbonization are beneficial to increasing the content of micropores in the hard carbon material structure.

The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims (10)

1. A hard carbon material, characterized in that the hard carbon material includes a microporous structure, and the micropores account for a volume percentage of ≥65% of the total pores. 2. The hard carbon material according to claim 1, characterized in that the micropores account for 65% to 90% of the total pores by volume; Preferably, the volume percentage of the micropores in the total pores is 66%-72%. 3. The hard carbon material according to claim 1, characterized in that, the volume of the micropores is ≥0.0015 cm ³ /g; and/or, the pore diameter of the micropores under the nitrogen isothermal adsorption-desorption test is 0.5 ~1.8nm; and/or, total pore volume > 0.0017cm ³ /g; Preferably, the volume of the micropores is 0.0015-0.018 cm ³ /g; More preferably, the volume of the micropores is 0.006-0.01 cm ³ /g; Preferably, the pore size of the micropores under the nitrogen isothermal adsorption-desorption test is 0.5-1.0 nm; More preferably, the pore size of the micropores under the nitrogen isothermal adsorption-desorption test is 0.5-0.7 nm; Preferably, the total pore volume is 0.003-0.02 cm ³ /g; More preferably, the total pore volume is 0.009-0.014 cm ³ /g. 4. hard carbon material according to claim 1, is characterized in that, the Raman collection of illustrative plates of described hard carbon material has D band and G band, between the peak intensity ID of _D band and the peak intensity I _G of G band The ratio I _D / _IG ≤ 1.1; And/or, the diffraction peak of the hard carbon material on the (002) crystal plane is located at 2θ=21.3°～25.6°; And/or, the interlayer spacing of the (002) crystal plane is 0.35-0.42 nm; And/or, the half-height width of the (002) crystal plane diffraction peak is 2.3-15°. 5. hard carbon material according to claim 1, is characterized in that, described hard carbon hard carbon material also comprises mesoporous structure; Preferably, the volume of mesopores is ≤0.0045cm ³ /g, and the volume percentage of mesopores in the total pores is ≤35%; Preferably, the volume of mesopores is 0.001-0.0043 cm ³ /g, and the volume percentage of mesopores in the total pores is 20%-35%; More preferably, the volume of mesopores is 0.0028-0.004 cm ³ /g, and the volume percentage of mesopores in the total pores is 28%-34%; Preferably, the pore diameter of the mesopores under the nitrogen isothermal adsorption-desorption test is 2.0-3.0 nm; Preferably, the pore diameter of the mesopores under the nitrogen isothermal adsorption-desorption test is 2.2-2.8 nm; More preferably, the pore diameter of the mesopores under the nitrogen isothermal adsorption-desorption test is 2.4-2.6 nm. 6. The hard carbon material according to claim 1, characterized in that, the specific surface area of the hard carbon material is 1-41 m ² /g; Preferably, the specific surface area of the hard carbon material is 5-30 m ² /g. 7. hard carbon material according to claim 1, is characterized in that, the total pore volume of described hard carbon material is marked as V (cm ³ /g), and the specific surface area of hard carbon material is marked as B (m ² /g ), the volume percentage of micropores is denoted as A%, the radius of micropores is denoted as r (nm), and A, B, V, r satisfy any of the following relational expressions: or, 8. The hard carbon material according to any one of claims 1-7, characterized in that, the particle size D50 of the hard carbon material is 1-11 μm; and/or, the particle size D90 of the hard carbon material is 1.8-15 μm; and/or, the particle size D100 of the hard carbon material is 3-27 μm; Preferably, the particle size D50 of the hard carbon material is 4-6 μm; More preferably, the particle size D50 of the hard carbon material is 4.5-5.6 μm. 9. A pole piece, characterized in that the pole piece comprises the hard carbon material according to any one of claims 1-8. 10. An electrochemical device, characterized in that the electrochemical device comprises the hard carbon material according to any one of claims 1-8 and/or the pole piece according to claim 9; Preferably, the electrochemical device is a sodium ion battery or a lithium ion battery.