A hard carbon material and preparation method thereof and sodium ion battery

Abstract

The present invention relates to the field of battery materials, and in particular to a hard carbon material, a preparation method thereof, and a sodium ion battery. The preparation method comprises pre-burning and drying a biomass raw material under an inert atmosphere, then infiltrating it with a conductive liquid, and placing it in an alternating electric field for induction. After separation and drying, sintering is performed to obtain a hard carbon material. The hard carbon material prepared by this method is used as the negative electrode active material of a sodium ion battery. By introducing a large number of closed-pore structures inside the hard carbon material and performing surface modification, the reversible specific capacity and first coulomb efficiency of the sodium ion battery can be effectively improved, thereby improving the capacity and stability of the battery.

Description

Technical Field

[0001] The present invention relates to the field of battery materials, and in particular to a hard carbon material and a preparation method thereof, and a sodium ion battery. Background Art

[0002] Lithium-ion batteries have been widely used in energy storage systems due to their high power density and long cycle life, and they dominate the fields of electric vehicles and portable electronic products. However, the global reserves of lithium resources are limited and unevenly distributed, resulting in high raw material costs for lithium-ion batteries. Metallic sodium has similar physical and chemical properties to metallic lithium, and sodium resources are widely distributed and inexpensive. These advantages make sodium-ion batteries very suitable for large-scale energy storage, making sodium-ion batteries a hot topic in the field of battery development.

[0003] The electrode material of sodium-ion batteries is a key part of the battery, which determines the specific energy and service life of sodium-ion batteries. Among the reported negative electrode materials for sodium-ion batteries, hard carbon materials are considered to be the most likely negative electrode materials for sodium-ion batteries to be industrialized first due to their high sodium storage capacity, low cost, good conductivity, and low redox potential. However, sodium-ion batteries using hard carbon materials as negative electrode active materials have bottleneck problems such as poor first coulombic efficiency (ICE), which makes it difficult to meet application requirements. Summary of the invention

[0004] The object of the present invention is to provide a hard carbon material and a preparation method thereof which are helpful to improve the ICE of a sodium ion battery.

[0005] In order to achieve the above object, the present invention provides the following technical solutions:

[0006] A method for preparing a hard carbon material, comprising:

[0007] Pre-burning the biomass raw material under an inert atmosphere to obtain a dry first precursor;

[0008] Allowing the conductive liquid to soak the first precursor, and placing it in an alternating electric field for induction, and separating to obtain a dry second precursor;

[0009] The second precursor is sintered in an inert atmosphere to obtain a hard carbon material.

[0010] Optionally, the conductive liquid is an N-methylpyrrolidone solution containing a conductive solute, and the conductive solute is a conductive polymer compound, and the concentration of the conductive solute in the conductive liquid is any value between 3% and 20%.

[0011] Optionally, the conductive solute is polyaniline and / or polypyrrole, and the first precursor is immersed in the conductive liquid for any time between 0.5 h and 3 h.

[0012] Optionally, the intensity of the alternating electric field is any value between 1 V / cm and 20 V / cm, the frequency is any value between 10 Hz and 400 Hz, and the time for induction in the alternating electric field is any value between 10 min and 120 min.

[0013] Optionally, the pre-firing temperature is any value between 350° C. and 650° C., and the pre-firing time is any value between 1 h and 5 h.

[0014] Optionally, the sintering temperature is any value between 1000° C. and 1500° C., and the sintering time is any value between 2 h and 12 h.

[0015] Optionally, the first precursor is purified from the pre-burned material obtained after the pre-burning, and the purification includes washing with several acid solutions and / or alkali solutions for several times and then washing with water and drying, the acid solution is one or more of hydrochloric acid solution, sulfuric acid solution, nitric acid solution and acetic acid solution, and the concentration of the acid solution is any value between 1 mol / L and 10 mol / L, the alkali solution is sodium hydroxide solution and / or potassium hydroxide solution, and the concentration of the alkali solution is any value between 2 mol / L and 8 mol / L.

[0016] Optionally, the biomass raw material includes one or more of coconut shell, bamboo, straw, sugar cane, and wood, and the particle size of the biomass raw material is less than or equal to 200 meshes.

[0017] In a second aspect, the present invention also provides a hard carbon material prepared by the above preparation method.

[0018] In a third aspect, the present invention further provides a sodium ion battery, comprising a negative electrode using the above-mentioned hard carbon material as an active material.

[0019] According to the first aspect of the present invention, the first precursor pre-fired into a porous whole is soaked in a conductive liquid, so that the voids inside the first precursor are filled with conductive liquid, and after the alternating electric field is induced, the conductive solute in the conductive liquid moves under the action of the electric field, driving the cellulose in the first precursor coated by it to bend and stack, thereby forming a closed-cell structure. After drying to obtain the second precursor, sintering is performed to solidify the dried conductive liquid. On the one hand, the pore openings can be filled and the openings can be converted into closed pores. On the other hand, the conductive polymer modifies the surface of the hard carbon material under the electric field condition, which can effectively reduce the edge defects of the graphene sheet in the hard carbon material and reduce the irreversible embedding of sodium ions in the hard carbon material. The closed-cell structure can effectively limit the expansion or contraction of the active material during the charge and discharge process, reduce the loss of active materials caused by structural damage, and effectively improve the reversible capacity and ICE of the hard carbon material. In addition, the closed-cell structure can also reduce the contact area between the hard carbon material and the electrolyte, reduce the irreversible loss of sodium ions caused by the formation of the interface SEI film, and further improve the ICE of the material.

[0020] According to the second aspect of the present invention, the hard carbon material is composed of randomly arranged graphite-like microcrystals, twisted graphene nanosheets, and pores generated by the stacking of the two. A large number of closed pores are introduced into the hard carbon material to improve the ability of the hard carbon material to resist its expansion or contraction during the charge and discharge process, and the defects existing at the edge of the internal graphene sheet are reduced by surface modification, thereby inhibiting the ability of the edge defects to irreversibly capture sodium ions, and reducing the occurrence of side reactions at the interface between the hard carbon and the electrolyte by forming a closed-pore structure, inhibiting the irreversible loss of capacity caused by the formation of the SEI film, so that the hard carbon material has a higher ICE.

[0021] According to the third aspect of the present invention, by using a hard carbon material with a large number of closed pores and surface modification as the active material of the negative electrode of the sodium ion battery, it helps to improve the cycle performance of the battery and make the battery have a higher specific capacity, first coulombic efficiency and capacity retention rate.

[0022] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention and implement it according to the contents of the specification, the following is a detailed description of the preferred embodiments of the present invention in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a flow chart of a method for preparing a hard carbon material as shown in Example 1 of the present invention;

[0024] Figure 2 This is a scanning electron microscope image of the hard carbon material shown in Example 1 of the present invention;

[0025] Figure 3 This is an electrochemical performance curve diagram of the sodium ion battery shown in Example 1 of the present invention;

[0026] Figure 4 This is an electrochemical performance curve of the sodium ion battery shown in Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0027] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0028] In addition, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0029] See also Figure 1 The method for preparing the hard carbon material to be protected by the present invention comprises:

[0030] S1. Pre-burning the biomass raw material under an inert atmosphere to obtain a dry first precursor.

[0031] S2. Allow the conductive liquid to soak the first precursor, place it in an alternating electric field for induction, and separate to obtain a dry second precursor.

[0032] S3. Sintering the second precursor in an inert atmosphere to obtain a hard carbon material.

[0033] The first precursor pre-fired into a porous whole is soaked in a conductive liquid, so that the voids inside the first precursor are filled with conductive liquid. After the alternating electric field is induced, the conductive solute in the conductive liquid moves under the action of the electric field, driving the cellulose in the first precursor covered by it to bend and stack, thereby forming a closed-cell structure. After drying to obtain the second precursor, sintering is performed to solidify the dried conductive liquid. On the one hand, the pore openings can be filled and the openings can be converted into closed pores. On the other hand, the conductive polymer modifies the surface of the hard carbon material under the electric field condition, which can effectively reduce the edge defects of the graphene sheet in the hard carbon material and reduce the irreversible embedding of sodium ions in the hard carbon material. The closed-cell structure can effectively limit the expansion or contraction of the active material during the charge and discharge process, reduce the loss of active material caused by structural damage, and effectively improve the reversible capacity and ICE of the hard carbon material. In addition, the closed-cell structure can also reduce the contact area between the hard carbon material and the electrolyte, reduce the irreversible loss of sodium ions caused by the formation of the interface SEI film, and further improve the ICE of the material.

[0034] In some embodiments, the conductive liquid is an N-methylpyrrolidone solution containing a conductive solute, and the conductive solute is a conductive polymer compound, and the concentration of the conductive solute in the conductive liquid is any value between 3% and 20%, for example, any value between 3%, 5%, 8%, 10%, 14%, 17% and 20%. The polymer material has a high viscosity, is easy to adhere to the surface of the material, and has a high density and strength after sintering, and can effectively fill gaps, support structures, and modify the surface of the material.

[0035] In some embodiments, the conductive solute is polyaniline and / or polypyrrole, and the immersion time of the first precursor in the conductive liquid is any value between 0.5h and 3h, for example, it can be any value between 0.5h, 1h, 1.5h, 2h, 2.5h and 3h, which helps the conductive liquid to fully infiltrate the interior of the first precursor.

[0036] In some embodiments, the intensity of the alternating electric field is any value in the range of 1 V / cm to 20 V / cm, such as any value in the range of 1 V / cm, 5 V / cm, 10 V / cm, 15 V / cm, and 20 V / cm; the frequency of the alternating electric field is any value in the range of 10 Hz to 400 Hz, such as any value in the range of 10 Hz, 100 Hz, 200 Hz, 300 Hz, and 400 Hz; the time for induction in the alternating electric field is any value in the range of 10 min to 120 min, such as any value in the range of 10 min, 30 min, 60 min, 90 min, and 120 min. By constraining the intensity, frequency, and treatment time of the alternating electric field, it is helpful to constrain the degree of bending and stacking of cellulose in the first precursor, thereby introducing closed pores in the hard carbon material.

[0037] In some embodiments, the pre-burning temperature is any value between 350°C and 650°C, for example, it can be any value between 350°C, 450°C, 450°C and 650°C; the time is any value between 1h and 5h, for example, it can be any value between 1h, 2h, 3h, 4h and 5h, so that the biomass raw material is pre-burned as a whole, and the cellulose inside is kept in an uncarbonized state, ensuring that the first precursor can be infiltrated by the conductive liquid and induced by the alternating electric field.

[0038] In some embodiments, the sintering temperature is any value between 1000°C and 1500°C, for example, it can be any value between 1000°C, 1100°C, 1200°C, 1300°C, 1400°C and 1500°C; the time is any value between 2h and 12h, for example, it can be any value between 2h, 4h, 6h, 8h, 10h and 12h, which helps to completely carbonize the second precursor to form a hard carbon material.

[0039] In some embodiments, the first precursor is purified from the pre-burned material obtained after pre-burning, and the purification includes washing with several acid solutions and / or alkali solutions for several times and then washing with water and drying, the acid solution is one or more of hydrochloric acid solution, sulfuric acid solution, nitric acid solution and acetic acid solution, and the concentration of the acid solution is any value between 1 mol / L and 10 mol / L, the alkali solution is sodium hydroxide solution and / or potassium hydroxide solution, and the concentration of the alkali solution is any value between 2 mol / L and 8 mol / L.

[0040] In some embodiments, the biomass raw material includes one or more of coconut shell, bamboo, straw, sugar cane, and wood, and the particle size of the biomass raw material is less than or equal to 200 mesh, for example, it can be 100 mesh, 150 mesh, 180 mesh, and 200 mesh, which helps to improve heat transfer efficiency, promote material diffusion and uniform mixing, and make the sintering process easier to control.

[0041] In the second aspect, the present invention also provides a hard carbon material prepared by the above-mentioned preparation method. The hard carbon material is composed of randomly arranged graphite-like crystallites, twisted graphene nanosheets, and pores generated by the stacking of the two. A large number of closed pores are introduced into the hard carbon material to improve the ability of the hard carbon material to resist its expansion or contraction during the charging and discharging process, and the defects existing at the edges of the internal graphene sheets are reduced by surface modification, thereby inhibiting the ability of the edge defects to irreversibly capture ions, and reducing the occurrence of side reactions at the interface between hard carbon and electrolyte by forming a closed-pore structure, inhibiting the irreversible loss of capacity caused by the formation of SEI film, so that the hard carbon material has a higher ICE, which is more suitable for the negative electrode of sodium ion batteries or lithium ion batteries.

[0042] In a third aspect, the present invention further provides a sodium ion battery, comprising a negative electrode using the hard carbon material as an active material. By using a hard carbon material having a large number of closed pores and having been surface-modified as the active material of the negative electrode of the sodium ion battery, it is helpful to improve the cycle performance of the battery, so that the battery has a higher specific capacity, first coulomb efficiency and capacity retention rate.

[0043] Please refer to the following examples for details.

[0044] Embodiment 1:

[0045] See also Figure 1 The method for preparing the hard carbon material shown in a preferred embodiment of the present application includes:

[0046] S1. Pre-burning the biomass raw material under an inert atmosphere to obtain a dry first precursor.

[0047] S2. Allow the conductive liquid to soak the first precursor, place it in an alternating electric field for induction, and separate to obtain a dry second precursor.

[0048] S3. Sintering the second precursor in an inert atmosphere to obtain a hard carbon material.

[0049] Wherein, step S1 comprises:

[0050] S101, taking 100 g of coconut shell powder with a particle size of 180 mesh as a biomass raw material, pre-burning it at 500° C. for 3 h under a nitrogen atmosphere, and cooling it at room temperature to obtain a pre-burned material.

[0051] S102, adding the pre-burned material into a prepared sulfuric acid solution with a concentration of 6 mol / L, acid-washing for 2 h under mechanical stirring, transferring to a sodium hydroxide solution with a concentration of 3 mol / L after filtration, alkaline-washing for 2 h under mechanical stirring, filtering and separating, washing with deionized water until the filtrate is neutral, and drying at 105°C to obtain the first precursor.

[0052] In step S2, 30 g of the first precursor is soaked in 30 mL of conductive liquid for 1 hour. The conductive liquid is a 10 wt% N-methylpyrrolidone (NMP) solution of polyaniline. The intensity of the alternating electric field is 10 V / cm, the frequency is 200 Hz, and the induction time is 60 min. After the induction is completed, the solid matter is separated and dried at 105 ° C to obtain the second precursor.

[0053] In step S3, the sintering atmosphere is nitrogen, the temperature is 1400° C., the time is 4 hours, and the hard carbon material is obtained after cooling to room temperature.

[0054] See also Figure 2 When observing the hard carbon material obtained in this embodiment under a scanning electron microscope, it can be seen that there are not a large number of micropores on the surface of conventional hard carbon materials. It can be seen that the openings of the micropores of the hard carbon material in this embodiment are closed to form a closed-pore structure.

[0055] The obtained hard carbon material, acetylene black and sodium carboxymethyl cellulose (CMC) were weighed in a mass ratio of 90:5:5, and ground in a mortar to mix them evenly. The uniform slurry obtained after grinding was placed on the current collector copper foil, and it was evenly coated with a scraper to form a film of uniform thickness, dried, and disassembled into pole pieces. The metal sodium sheet was used as the counter electrode, and the solute was a solution of propylene carbonate and diethyl carbonate mixed in a volume ratio of 1:1 and a sodium hexafluorophosphate solution with a concentration of 1 mol / L as the electrolyte, and a CR2032 button battery was assembled in an argon atmosphere glove box.

[0056] Correctly install the assembled button cell in the battery fixture of the test equipment, perform charge and discharge tests, and draw the electrochemical performance curve. Figure 3 , it can be seen that the battery has a high specific capacity, first coulombic efficiency and capacity retention rate.

[0057] Embodiment 2:

[0058] The difference between this embodiment and the first embodiment is that:

[0059] In step S101, the biomass raw material is bamboo powder, the pre-burning atmosphere is argon atmosphere, the temperature is 650° C., and the pre-burning time is 1 hour.

[0060] In step S102, the surface is acid-washed with a hydrochloric acid solution having a concentration of 1 mol / L for 4 hours, and then alkaline-washed with a sodium hydroxide solution having a concentration of 8 mol / L for 2 hours.

[0061] In step S2, the conductive solute is polypyrrole, and the immersion time is 2 hours. The frequency of the alternating electric field is 400 Hz, and the induction time is 10 minutes.

[0062] In step S3, the sintering temperature is 1300° C. and the sintering time is 3 hours.

[0063] Embodiment three:

[0064] The difference between this embodiment and the first embodiment is that:

[0065] In step S101, the biomass raw material is corn stalk powder, the pre-burning atmosphere is argon atmosphere, the temperature is 350° C., and the pre-burning time is 4 hours.

[0066] In step S102, the substrate is acid-washed with an acetic acid solution having a concentration of 5 mol / L for 6 hours, and then alkaline-washed with a potassium hydroxide solution having a concentration of 2 mol / L for 2 hours.

[0067] In step S2, the volume of the conductive liquid is 40 ml, the concentration is 5 wt%, the soaking time is 3 h, the intensity of the alternating electric field is 1 V / cm, the frequency is 400 Hz, and the induction time is 120 min.

[0068] In step S3, the sintering time is 6 hours.

[0069] Embodiment 4:

[0070] The difference between this embodiment and the first embodiment is that:

[0071] In step S101, the biomass raw material is poplar wood powder, and the pre-burning temperature is 500°C.

[0072] In step S102, the substrate is acid washed for 2 hours with a sulfuric acid solution having a concentration of 10 mol / L, without alkaline washing.

[0073] In step S2, the intensity of the alternating electric field is 15 V / cm, the frequency is 10 Hz, and the induction time is 120 min.

[0074] In step S3, the sintering temperature is 1500°C.

[0075] Embodiment five:

[0076] The difference between this embodiment and the first embodiment is that:

[0077] In step S101, the biomass raw material is sugarcane powder, the temperature is 550°C, and the pre-burning time is 5 hours.

[0078] In step S102, the substrate is acid-washed for 2 hours using a 3 mol / L nitric acid solution, and then alkaline-washed.

[0079] In step S2, the concentration of the conductive liquid is 3 wt %, the conductive solute is polypyrrole, and the immersion time is 1 hour.

[0080] In step S3, the sintering temperature is 1000° C. and the sintering time is 12 h.

[0081] Comparative Example 1:

[0082] The difference between this comparative example and Example 1 is that no alternating electric field induction is performed. The button cell obtained in this comparative example is subjected to charge and discharge tests, and an electrochemical performance curve is plotted. Figure 4 It can be seen that the specific capacity, first coulombic efficiency and capacity retention rate of the button battery obtained in this comparative example are lower than those of the button battery obtained in Example 1.

[0083] Comparative Example 2:

[0084] The only difference between this comparative example and the first embodiment is that no conductive liquid immersion is performed.

[0085] Comparative Example 3:

[0086] The only difference between this comparative example and the first embodiment is that the conductive liquid immersion and alternating electric field induction are not performed.

[0087] The electrochemical properties of the button cells obtained in the embodiments of the present invention and the comparative examples were respectively tested, and the parameters obtained by the tests were recorded and organized into the following Table 1.

[0088]

[0089] From the data in Table 1, it can be seen that the first discharge specific capacity of the sodium ion battery using the hard carbon material prepared in each embodiment as the negative electrode active material is greater than 320 mAh / g, the first coulomb efficiency is greater than 92%, and the capacity retention rate after 300 cycles is greater than 96%; while in each comparative example, the first discharge specific capacity is less than 280 mAh / g, the first coulomb efficiency is less than 74%, and the capacity retention rate after 300 cycles is less than 89%. This shows that the hard carbon material and its preparation method applied for protection in the present invention can effectively solve the bottleneck problems such as low reversible specific capacity and poor first coulomb efficiency faced by biomass hard carbon materials, thereby improving the capacity and stability of sodium ion batteries.

[0090] By comparing Example 1 with the comparative examples, it can be seen that the impregnation with conductive liquid alone has little effect on the performance of the hard carbon material, and the alternating electric field induced treatment alone has very limited effects on the improvement of the specific capacity and the first coulombic efficiency of the hard carbon material, indicating that the synergistic effect of conductive liquid impregnation and alternating electric field induced treatment is the key to improving the performance of hard carbon.

[0091] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0092] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A method for preparing a hard carbon material, characterized in that: include: Pre-burning the biomass raw material under an inert atmosphere to obtain a dry first precursor; The first precursor is infiltrated with a conductive liquid, and is placed in an alternating electric field for induction, and a dried second precursor is obtained by separation, wherein the conductive liquid is an N-methylpyrrolidone solution containing a conductive solute, and the conductive solute is a conductive polymer compound, the intensity of the alternating electric field is any value in the range of 1 V / cm to 20 V / cm, the frequency is any value in the range of 10 Hz to 400 Hz, and the induction time in the alternating electric field is any value in the range of 10 min to 120 min; The second precursor is sintered in an inert atmosphere to obtain a hard carbon material.

2. The preparation method according to claim 1, characterized in that The concentration of the conductive solute in the conductive liquid is any value between 3% and 20%.

3. The preparation method according to claim 2, characterized in that: The conductive solute is polyaniline and / or polypyrrole, and the first precursor is immersed in the conductive liquid for any time between 0.5 hours and 3 hours.

4. The preparation method according to claim 1, characterized in that: The pre-calcination temperature is any value between 350° C. and 650° C., and the pre-calcination time is any value between 1 hour and 5 hours.

5. The preparation method according to claim 1, characterized in that: The sintering temperature is any value between 1000° C. and 1500° C., and the sintering time is any value between 2 h and 12 h.

6. The preparation method according to claim 1, characterized in that: The first precursor is obtained by purifying the pre-burned material obtained after the pre-burning, and the purification includes washing with several acid solutions and / or alkali solutions for several times and then washing with water and drying, the acid solution is one or more of hydrochloric acid solution, sulfuric acid solution, nitric acid solution and acetic acid solution, and the concentration of the acid solution is any value between 1mol / L and 10mol / L, the alkali solution is sodium hydroxide solution and / or potassium hydroxide solution, and the concentration of the alkali solution is any value between 2mol / L and 8mol / L.

7. The preparation method according to claim 1, characterized in that: The biomass raw material includes one or more of coconut shell, bamboo, straw, sugar cane and wood, and the particle size of the biomass raw material is less than or equal to 200 meshes.

8. A hard carbon material, characterized in that: The method is prepared according to any one of claims 1 to 7.

9. A sodium ion battery, characterized in that: A negative electrode comprising the hard carbon material as claimed in claim 8 as an active material.

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